摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(8), 1153-1156 Adsorption in Energetically Heterogeneous Slit-like Pores : Comparison of Density Functional Theory and Computer Simulations G. Chrniel, L. Lajtar and S. Sok&owski*t Computer Laboratory, Faculty of Chemistry, MSC University,20031 Lublin, Poland A. Patrykiejew Department of Chemical Physics, Faculty of Chemistry,MCS University,20031 Lublin, Poland We present a comparison of density functional theory results with Monte Carlo calculations of adsorption in narrow, slit-like pores with energetically heterogeneous walls. The calculations have been carried out assuming Gaussian distribution of the adsorption energy and random topography of adsorbing sites. We have found a reasonable agreement between theoretical predictions and computer simulations.A system of spherical particles inside a slit formed by a pair of parallel walls in equilibrium with a bulk fluid is one of the simplest of the models of confined fluids that have been inten- sively studied in recent years. lL1z The investigated models have included slits with simple hard walls,'*2*'0.' ' slits with attractive, but energetically uniform walls, interacting via a potential, which is a function of the wall-particle distan~e~-~,'and slits with crystalline walls built of regularly arranged atom^.^.^,^.'^ All these three types of systems have been investigated by means of computer simula-tions,2,3,5-11,14 and the first two models have also been studied by using theoretical approaches, based either on inte- gral equation^'*^^'^*'^ or on application of density functional theory.3*4,93 ' It is well known that the energetic heterogeneity of adsorb- ing surfaces plays a significant role in adsorption.Classical studies concerning that problem have been usually based on the so-called 'integral adsorption eq~ation'.'~ Unfortunately, this equation did not always provide reliable information concerning the real influence of energetic heterogeneity on the thermodynamics of adsorbed films and for this reason development of new approaches is de~irable.l~-~ Recently, an increasing number of papers have reported computer modelling of gas adsorption on non-uniform adsorb-ents.9.16.19-24 In our previous works "*' we have initiated investigations of adsorption on energetically non-uniform surfaces by using a density functional method.The theoretical results obtained, however, have not been tested against computer simulations. Therefore, the primary goal of this paper is to compare the results of computer simulations with the theoretical predic- tions of density functional theory. Calculations We considered a simple fluid, i.e. the interaction energy between the molecules is pairwise additive and depends on the scalar distance between the molecules. The cut Lennard- Jones (12,6) function has been assumed as the pair potential u(r)= {;[(u/r)l2 -(~/r)~];for r < 2.5~ (1); for r < 2.50 We used o as a unit of length and the calculations were carried out for ~T/E0.8 and 1.5.= Each pore wall of size XL = 11 and YL= 6J3 has been built of hexagonally arranged atoms placed at a distance c one from another.Periodic boundary conditions in the X and Y directions have been applied. t Also at: HLRZ-KFA Julich, 5160 Julich, Germany. The interaction of adatoms with the adsorbent has been calculated by summing the Lennard-Jones (12-6) energies for gas atom-solid atom interactions The energy parameter, characterizing a single surface atom, ,~,E~~ has been drawn according to a Gaussian distribution X(E,s)* (4) In each case we used kT/E,, = 0.8, sg = 50 and kT/A = 0.7. Random topography (cf ref. 13), i.e. quite random assignment of the values of cgs to the wall atoms, has been assumed and the final adsorption results have been evaluated by taking averages over six random configurations. We used the GCEMC implementation due to Adams." Each step of this method consists of an attempted movement of a randomly chosen particle followed by either an at-tempted deletion of a randomly chosen particle or an attempted creation of a particle in a randomly chosen posi- tion inside the pore. We report the results us.relative activity, a, defined as a = exp[,u/kT -3 in A/a] (5) where A is the DeBroglie wavelength. Usually, more than 2 x lo6 initial steps were discarded and the final results have been obtained taking averages over at least 3 x lo6 sub-sequent steps. The local densities evaluated from Monte Carlo simula- tions have been compared with the results of density func- tional theory.This theory is based on a variational principle for the Helmholtz energy, F, and its practical implementation requires the construction of an approximate expression for F[p(r)], being a functional of the local density. Among various schemes proposed in the literature,26 one of most widely used is the so-called weighted density approximation proposed by Tarazona et ul.27,28 The approach starts from the definition of the grand poten- tial, Sl i2 = F + idrp(r)[v(r) -p] (6) The functional F is divided into two parts, representing the contributions due to repulsive, F, ,and attractive forces, FA, between the molecules. The former are modelled by hard spheres with suitably chosen diameter d and the latter are treated in the mean-field approximation.The Helmholtz- energy contribution, F,, is calculated by introducing a smoothed density function, F(r) P(r)= dr'p(r')wC I r -r' I, p(r)l (7)s where w is a weight function W(I, P) = wow + Wl(I)P + W2(I)P2 (8) and the coefficients wo, w1 and w2can be found in ref. 27. The Helmholtz energy takes the form F = s drp(r)(kT[ln p(r)A3 -11 +f[P(r)]} dr dr'p(r)p(r')u,( I r -r' I ) (9) where The Helmholtz-energy density of hard spheres,f, has been calculated from the Carnahan-Starling equation*' f(P)/kT= v(4 -3vMl -vI2 (11) where q = nd3p/6 and the hard-sphere diameter has been assumed to be equal to cr. The equilibrium density profile minimizes the grand poten- tial Q, thus the local density is evaluated from the condition We recall also that the excess adsorption isotherm, r, is defined as = dr[P(r) -Pbl (13)s where Pb is the density of a bulk fluid at a given temperature and chemical potential.The dependence of the bulk gas density on chemical potential (activity) was calculated by using the bulk counterpart of the theory. The minimum condition [eqn. (12)] can be rewritten in the following form 0 = In &)A3 +fC&)I + u(rl)-P (14) which is an integral equation for the density profile. This equation has been solved by using an iterational procedure with the grid size along each axis equal to 0.05. Because the local density is a function of three variables, the solution of the density profile equation is rather time-consuming. Fortu- nately, the form of eqn.(14) makes possible the application of a parallel algorithm for calculating subsequent iterational solutions on parallel computers. This method was used in our work. Results and Discussion Fig. 1 compares adsorption isotherms r, evaluated from density functional theory and from computer simulations. Density functional theory predicts the existence of a capillary condensation loop with the adsorption state at Pb = 0.001 95 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0 0.000 0.002 0.004 Pb Fig. 1 Adsorption isotherms at slit-like pores. The points (diamonds) are the results of computer simulations and the lines have been obtained from density functional calculations.The calculations were carried out for H = 5, kT/&= kT/&,,= 0.8. and the desorption step at Pb = O.OOO91. The equilibrium capillary condensation point is located at Pb = 0.001 53. Except for the phase-transition region, the agreement between theory and computer simulation is rather satisfac- tory. In particular, we observe a very good coincidence between computer simulations and theory at higher bulk gas densities. The Monte Carlo simulations predict no first-order capil- lary condensation for this system. According to the per- formed simulations, the filling of the pore with a dense, condensed liquid is smooth and reversible. A sharp, but con- tinuous adsorption step is located at Pb % 0.014. Note, however, that in the case of the pore with walls built of iden- tical atoms, interacting via a potential [eqn.(3)] with ~T/E~,= 0.768, the phenomenon of capillary condensation is evident and the adsorption jump is at Pb = 0.00172.30 Instantaneously, for pore walls built of identical atoms, density functional theory predicts capillary condensation at Pb = 0.00208, the adsorption step at Pb = 0.00293 and the desorption step at Pb = 0.001 27.30 Thus, the heterogeneity causes the hysteresis loop to become narrower. The observed discrepancy between theory and computer simulation is a direct consequence of neglect of statistical fluctuations by mean-field theory. It is known3' that in systems with random impurities any phase transition is smeared and rounded.Therefore, the observed finite slope of the adsorption isotherm is a manifestation of this effect. The structure of the adsorbed fluid has been characterized by one-particle distribution functions, p(r). Representative examples of the local densities, evaluated from density func- tional theory and from computer simulations are displayed in Fig. 2 and 3. Fig. 2 shows some selected local densities obtained from density functional theory and averaged over the y coordinate, (p(x, y, z)),. We see that the density profiles inside the pore with H = 5, filled with a gas-like phase, exhibit only one single maximum at each of the walls, and the capillary con- densation is connected with the filling of the two inner layers inside the pore.We can also realize that energetical hetero- geneity plays a more important role at lower bulk gas den- sities (activities). Indeed, the profile displayed in Fig. 2(a) indicates large variations in the density along the surface. The relative insensitivity of the liquid-like profile presented in Fig. 2(b) on energetical heterogeneity means that for a dense adsorbed film averaging along one direction is sufficient for smoothing the density distribution. Obviously, changes in the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 2 Density profiles averaged over y coordinates, (p(x, y, z)),, resulting from density functional theory. The calculations were per- formed for H = 5, kT/c = kT/Egs= 0.8 and for pb = 0.001 (a) and 0.002(b).full density distribution n(x, y, z) for the liquid-like phase still remain correlated with changes in the adsorbing potential; in particular, in both Fig. 2(a) and (b) we can distinguish sub- sequent minima and maxima, connected with the crystalline arrangement of the solid wall atoms. Fig. 3(a)-(c)show a comparison of the Monte Carlo and density functional theory profiles p,(z) = (n(x, y, z)),,,, where (0 --),,,denotes an unweighted average over the coor- dinates x and y parallel to the pore walls. The calculations have been performed for two pore widths and for two strengths of fluid-fluid interaction. In each case the topo- graphical model of the surface is the same. In comparison with computer simulations, density functional theory leads to a slightly less pronounced oscillatory character of the density profiles.In all cases, however, the positions of the subsequent local density minima and maxima are well preserved. In general, we can state that even for a quite dense adsorbed film, the agreement between the density functional calcu- lations and Monte Carlo results is satisfactory. Fig. 4 compares the Monte Carlo density profiles p,(z) averaged over six different random topographical distribu- tions of energies, cgs, with the profiles obtained for a model pore with all wall atoms being identical. The differences between density profiles, obtained for the liquid like as well as for the gas-like phases, are really small. This indicates that adsorption on random heterogeneous surfaces can be described by considering a suitably defined model homoge- neous surface. This conclusion remains in agreement with the results of our previous workI5 on the application of the 1155 2 h 3c: 1 0 0 1 2 3 4 5 z .2i ;I c 0 2 4 6 a Fig. 3 Density profiles, p,(z). The points denote the results of Monte Carlo simulations and the lines have been evaluated from density functional calculations. The subsequent parts have been evaluated for: (a)H = 5, Tk/E = 0.8 and Pb = 0.002; (b) H = 5, Tk/ E = 1.5 and pb = 0.535; (c) H = 8, Tk/E = 0.8 and pb = 0.004. In each case kT/c, = 0.8. density functional method to description of monolayer gas adsorption on heterogeneous surfaces, in which we have emphasized that adsorption on random heterogeneous sur- faces can be modelled by an effective 'average' homogeneous surface.In this paper we have demonstrated that the classical density functional method can be successfully applied to the description of adsorption in pores with energetically non- uniform walls. Therefore, we hope that further studies on the application of density functional theory, including its lattice 1156 31 h-.!3 2' t'i 0 1 2345 L Fig. 4 Comparison of the Monte Carlo density profiles evaluated for the pore with heterogeneous walls (solid lines) and with walls built of identical vertices (dashed lines and points). In the latter case the energy parameter in eqn. (3) was constant and equal to Tk/&,= 0.8.The calculations were performed for Tk/c = 0.8, H = 5 and for pb = 0.0008(lower curves) and 0.004 (upper curves). counterpart, may be very useful in explaining the microscopic structure of adsorbed layers and for the description of surface phase transitions. The paper was supported by KBN under Grant No. 303.077.05. References 1 Y. Zhou and G. Stell, MoZ. Phys., 1989,66,767. 2 S. Sokolowski and J. Fischer, J. Chem. Phys., 1991,93,6787. 3 B. K. Peterson, K. E. Gubbins, G. S. Heffelfinger, U. M. B. Marconi and F. van Swol, J. Chem. Phys., 1988,88,6487. 4 P. C. Ball and R. Evans, Mol. Phys., 1988,63, 159. 5A. Papadopoulou, F. van Swol and U. M. B. Marconi, J. Chem. Phys., 1992, 97, 6942. J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 6 S. Sarman, J. Chem. Phys., 1990,92,4447. 7 M. Schoen, D. J. Diestler and J. H. Cushman, J. Chem. Phys., 1987,87,5464. 8 A. Delville and S. Sokolowski, J. Phys. Chem., 1993,97,6261. 9 S. Sokolowski, Phys. Rev. A, 1991, 44, 3732; Mol. Phys., 1992, 75, 1301. 10 R. Pospiiil, A. Malijevskj. and J. Pech, Mol. Phys., 1993, 78, 1461. 11 M. Lupowski and F. van Swol, J. Chem. Phys., 1990,93,737. 12 S. Sokolowski and J. Fischer, J. Chem. SOC., Faraday Trans., 1993,89, 789. 13 W. Rudzinski and D. Everett, Adsorption of Gases on Heter- ogeneous Solid Surfaces, Academic Press, London, 199 1. 14 U. Heinbuch and J. Fischer, Phys. Rev. A, 1989,40, 1144. 15 L. Lajtar and S. Sokolowski, J. Chem. SOC., Faraday Trans. 2, 1992,88,2545.16 G. Chmiel, A. Patrykiejew, W. Riysko and S. Sokolowski, Phys. Rev. B, in the press. 17 G. Chmiel, K. Karykowski, A. Patrykiejew, W. Riysko and S. Sokolowski, Mol. Phys., in the press. 18 V. A. Bakayev and W. A. Steele, J. Chem. Phys., 1993,98,9922. 19 M. J. Bojan and W. A. Steele, Sur$ Sci., 1988,199, L395. 20 J. Cortes and P. Araya, J. Chem. Phys., 1991,95,7741. 21 A. Patrykiejew, Langmuir, in the press. 22 E. Kozak, L. Lajtar, A. Patrykiejew and S. Sokolowski, Physica A, in the press. 23 J. M. D. McElroy and K. Raghavan, J. Chem. Phys., 1990, 93, 2068. 24 R. D. Kaminsky and P. A. Monson, J. Chem. Phys., 1991, 95, 2936. 25 D. J. Adams, MoZ. Phys., 1975,29, 307. 26 Inhomogeneous Fluids, ed. D. Henderson, M. Dekker, New York, 1992. 27 P. Tarazona, MoZ. Phys., 1984,52,81. 28 P. Tarazona, M. B. Marconi and R. Evans, Mol. Phys., 1987,60, 573. 29 N. F. Carnahan and K. E. Starling, J. Chem. Phys., 1969, 51, 635. 30 K. Karykowski, W. Riysko, A. Patrykiejew, S. Sokolowski, Thin Solid Films, submitted. 31 I. Morgenstern, K. Binder and R. M. Hornereich, Phys. Rev. B, 1981,23,287. Paper 4/00361F; Received 20th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001153

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(8), 1157-1160 Characterization of Supported-palladium Catalysts by Deuterium NMR Spectroscopy Tsong-huei Chang Department of Chemical Engineering, Ming Hsin Engineering College, Hsin Feng, Hsinchu, Taiwan 30434, Republic of China Cheu Pyeng Cheng and Chuin-tih Yeh Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China Alumina-supported Pd, Rh and Pd-Rh bimetallic alloys have been prepared by the method of incipient wetness. Sorption of deuterium by these samples was investigated by means of 'H NMR spectroscopy and uptake mea- surements at room temperature. Deuterium chemisorbed on palladium and rhodium atoms was characterized by 2H NMR peaks with chemical shifts of -12 and -170 ppm, respectively, from monometallic samples under a deuterium pressure of 1 Torr or less.These two characteristic peaks remained for the bimetallic samples and their relative intensities unambiguously revealed the surface composition of the bimetallic crys- tallites. Deuterium atoms were absorbed or weakly chemisorbed at greater pressures of deuterium. The weakly sorbed deuterium atoms exchanged with the strongly chemisorbed deuterium atoms, indicated by the coalescence of the 2H NMR characteristic peaks of the bimetallic samples. The position of the coalesced peak depended on the composition of the alloy crystallites. Palladium is a versatile noble metal which has been used not only as an active ingredient of hydrogenation and hydro- genolysis catalysts,' but also as a material for hydrogen storage., The versatility of palladium arises mainly from its ability both to chemisorb hydrogen on the surface of its crys- tallites and to absorb hydrogen into the bulk.Interactions between deuterium and palladium have attracted much atten- tion because of possible applications in cold-fusion. Many analytical techniques, for instance hydrogen solu- bilit~,~-" heat of absorption,' ' magnetic susceptibility,' ,-16 X-ray and neutron scattering' and NMR spectroscopy,'* have been used to study the interactions of palladium and hydrogen. Among these techniques, NMR is distinguished for possibly probing the chemical binding between hydrogen and metals.'9-23 Sheng and Gay24 studied the adsorption of hydrogen on Pd/SiO, with proton NMR, and Vannice and co-workers have reported the absorption of deuterium on the same catalyst., In our recent report,26 deuterium adsorbed on Rh/Si02 samples is classified into three categories, i.e.rigidly adsorbed (D,), adsorbed but possessing mobility (D,) and weakly adsorbed (D,) (may be pumped away by evac- uation in a high-vacuum system for 5 min at room temperature), according to 'H NMR observations. D, and D, are strongly adsorbed deuterium atoms with different mobility. D, is attributed to those deuterium atoms that are unobservable in the 'H NMR spectra because they are adsorbed rigidly to the metal surface and their peaks become broad because of excessive quadrupole interaction.D, is comparatively mobile and observable with 2H NMR. The latter form is gradually converted into D, with decreasing system temperature. We report here an investigation on the sorption of deuterium by supported palladium and supported Pd-Rh alloys by means of ,H NMR. Experimental Alumina-supported Pd, Rh and Pd-Rh alloys with various metal loadings were prepared by the method of incipient wetness. Chlorides of palladium and rhodium (both Merck) were dissolved in deionized water and impregnated drop-by- drop into y-alumina (Merck) with constant agitation. The resulting pastes were dried for 4 h at 383 K and calcined for 4 h at 723 K. A calcined sample (2.0 g) was sealed in a Pyrex tube (external diameter 10 mm) connected to a high-vacuum system, reduced in flowing hydrogen for 1 h at 573 K, degassed (to evacuate adsorbed hydrogen completely) at the same temperature until a pressure of 5 x lo-' Torr was reached, and characterized either with an adsorption iso- therm of deuterium in the vacuum system at room tem-perature or by means of 2H NMR in the Pyrex tube sealed with deuterium at a predetermined pressure.All NMR spectra presented in this paper were recorded at room tem- perature with a Bruker MSL-200 spectrometer ;the operating frequency was 30.72 MHz at 4.7 T magnetic field. A solid echo technique (9O,-z-9Oy-z) with a 90" pulse (6 ps), a z delay (20 ps) and a repetition interval (0.2 s) was employed. A CH,OD-CH,OH solution (2.88% v/v) was used as an exter- nal standard for calibration of chemical shift.Cr(acac),, a relaxation reagent, was added to the methanol solution to ensure complete relaxation of the deuterium in the standard solution within the repetition interval 0.2 s. Results and Discussion Pd/Al,O, Fig. 1 presents three sorption isotherms of deuterium obtained volumetrically from samples of 1, 1.5 and 3% Pd/A1,0,, respectively, at 298 K. Each isotherm is divided into a chemisorption region (less than 20 Torr) and an absorption region, i.e. 2Pd, + D, e2Pd,D (1) 2Pdb + D2=2PdbD (2) subscripts 's' and 'b' denote the atoms on the surface and in the bulk of the palladium crystallites, respectively. In the che- misorption region, the uptake increased sharply with increas- ing pressure of D,.The rate of deuterium uptake gradually slowed and reached a plateau at about 20 Torr. The uptake in this region is attributed to deuteriums chemisorbed on the surface of palladium particles. The observed plateau in this region indicates chemisorption to the extent of one saturated monolayer. The uptake of the isotherm increased steeply and then became level again upon further increase of the deute- rium pressure to 90 and 200 Torr, respectively. This J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 40 0.0 1 I I I 0 100 200 300 D, pressure/Torr Fig. 1 Volumetric adsorption isotherms at 298 K of deuterium on samples of alumina-supported palladium with loadings (%) of 1.5 (O),3.0 (0)and 1.0 (A),respectively increment comes from the absorption of deuterium into the bulk of the palladium cry~tallites.~~ The dispersion of the palladium crystallites in these samples is estimated from the observed uptake at the first plateau of their isotherm.The dispersion was determined to be about 0.22, 0.57 and 0.48 for samples having 1.0, 1.5 and 3.0%Pd/Al,O, ,respectively. The extent of deuterium uptake has a significant effect on the observed 'H NMR spectrum of sorbed deuterium. Two deuterium peaks are identified in each spectrum of Fig. 2 for the 3% Pd/A1,0, catalyst. The sharp peak at 0 ppm is due to CH,OD of the external standard. The broad peak is assigned to deuteriums sorbed (including both chemisorption and absorption) by the palladium in the sample. The position and width of latter peak depended on deuterium uptake.Fig. 3 relates the variation of the chemical shift of the peak observed on 2H NMR with the deuterium uptake determined volumetrically. Resembling the uptake isotherm of Fig. 1, the I/ 200 100 0 -100 -200 6 Fig. 2 Effect of deuterium uptake (D :Pd ratio, measured volumetrically) on the 'H NMR spectrum of deuterium sorbed on Pd/AI,O, (3.0%). The sharp line at 0 ppm is due to the external standard (see text). D :Pd = 0.245 (a), 0.330 (b),0.445 (c), 0.501 (4, 0.740 (e). -1 0 6 I-60 -11( I I I 0.5 1.o deuterium uptake, D/Pd Fig. 3 Effect of deuterium uptake on the chemical shift of sorbed Pd/A1,0, (3.0%) observed in Fig. 2. (0)deuterium. (0) Pd/Al,O, (1.5%). The plateau at a chemical shift of -12 ppm indicates mono- layer adsorption.observed chemical shift also showed two plateaux corre-sponding to chemisorption and absorption, respectively, upon increasing deuterium uptake. The peak of sorbed deute- rium shifted dramatically from -78 to -20 ppm under sub- monolayer coverage (0 < 0.6), and gradually became level at -12 ppm upon increase of the coverage to monolayer chemi- sorption (0 = 1). This shift, which resembled an earlier obser- ~ation,~~,~'is attributed to the consumption of the spin density of Pd crystallites during formation of chemical bonds between adsorbed deuterium and surface palladium atoms. Above monolayer coverage, the peak associated with sorbed deuterium shifted further downfield due to a Knight shift'' upon increase of the deuterium uptake and became level at +25 ppm when the absorption became ~aturated.~' Concur-rently, the peak width abruptly decreased because of rapid exchange between chemisorbed and absorbed deuterium atoms.28 Rh/Al, O3 An overpressure of deuterium has no significant effect on the width of the 2H NMR peak for deuterium adsorbed on an Rh/A120, sample (1.5%).Nevertheless, the peak position shown in Fig. 4 shifted gradually from -180 to -100 ppm with increasing deuterium pressure from 10 to 600 Torr. We speculate that weakly adsorbed deuterium, either multi-adsorbed (i.e. two deuterium atoms coadsorbed on a single surface rhodium atom) or located at the interface between Rh crystallites and the alumina support, is the main reason for this observed shift. Pd-Rh/Al,O, Fig.5 illustrates the effect of deuterium pressure on the 2H NMR spectrum of a Pd-Rh/Al,O, sample (6%, with J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 100 0 -100 -200 -300 -400 6 Fig. 4 Effect of deuterium overpressure on the 2H NMR spectrum for deuterium adsorbed on Rh/Al,O, (1.5%, dispersion 80%). PDz/Torr= 10 (a),50 (b),300 (c),600 (4. Pd : Rh = 1 :1). Two distinct peaks, at -12 and -170 ppm, respectively, were observed after this sample was pretreated by means of reduction by deuterium at 573 K and light evac- uation (5 min in the vacuum system at room temperature to desorb weakly sorbed deuterium). These two peaks are assigned to mobile deuterium chemisorbed on the palladium atoms (-12 ppm) and rhodium atoms (170 ppm), respec- tively, according to the correlations obtained in Fig. 3 and 4.An energy barrier (for migration of adsorbed deuterium from one metal atom to adjacent metal atoms) evidently prevents the following migration exchange from proceeding at room temperature: Pd-D + Rh Pd + Rh-D (3) Since mobile deuterium adsorbed on Pd-Rh crystallites maintains its characteristic peak position, the surface com- position of the alloy, can be estimated if the influence of undetectable deuterium is ignored. The relative intensities of the two peaks in Fig. 5(a) indicate that most deuterium is adsorbed on palladium. A segregation of palladium to the surface of the Pd-Rh bimetallic alloy crystallites is therefore indicated.The surface segregation of palladium in a Pd-Rh film was shown by electron probe microanalysis upon hydro- gen reduction on a surface at 573 K.29 As deuterium gas at 100 or 300 Torr was introduced to this bimetallic sample, the two peaks in Fig. 5(a)coalesced in Fig. 5(b)and (c) to form a new peak located at about -50 ppm. Obviously, from the coalescence, the migration exchange rep- resented by reaction (3) became important with increased pressure of deuterium. The energy barrier for migrating mobile deuterium [reaction (3)] might have been removed by the presence of weakly adsorbed deuterium under these con- ditions. That this exchange occurred indicates that an alloy phase of Pd and Rh was formed on the sample during the reduction pretreatment.We conclude from the observed coalescence that weakly adsorbed deuterium definitely modi- fied the properties, such as the reactivity, of adsorbed deute- rium. Fig. 6 depicts the effect of deuterium overpressure on the 'H NMR spectrum of deuterium adsorbed on a sample of Pd-Rh/Al,O, (9%, Pd : Rh = 2 : 1). The mobile deuterium adsorbed on palladium (peak near -12 ppm) is distinguished 1 I I I I I I 1 200 0 -200 -400 200 0 -200 -400 6 6 Fig. 5 Effect of deuterium overpressure on the 2H NMR spectrum Fig. 6 Effect of deuterium overpressure on the 'H NMR spectrum for deuterium sorbed on the Pd-Rh/A120, bimetallic catalyst (6%, for deuterium sorbed on Pd-Rh/Al,O, bimetallic catalyst (9%, Pd : Rh = 1 : 1).PDz/Torr= 0 (a),100 (b),300 (c). Pd : Rh = 2 : 1). PDz/Torr= 0 (a),100 (b),300 (c). 1160 in Fig. 6(a) from those adsorbed on rhodium (peak near -160 ppm) after weakly adsorbed deuterium was pumped away by light evacuation. Introduction of gaseous deuterium (100 Torr) to this sample produced two peaks at -60 and +25 ppm, respectively. The existence of two peaks under these conditions is explained in terms of deuterium sorbed on metal crystallites with two distinct phases. The broad peak located at -60 ppm is assigned to deuterium adsorbed on an alloy phase with a Pd : Rh ratio of close to 1 :1 because this chemical shift is very similar to that found in Fig. 5(b)and (c). The sharp line located at +25 ppm is assigned to deuterium sorbed by pure palladium (or a palladium-rich phase). The existence of these two peaks is consistent with a surface migration mechanism [reaction (3)], instead of a conceivable mechanism through the gas phase. Moreover, the presence of pure palladium crystallites on this sample has been confirmed by the deuterium adsorption isotherm.Fig. 7 shows that the sample with Pd : Rh = 2 : 1 has an absorption uptake at around a deuterium pressure of 100 Torr. This characteristic feature of the palladium phase, however, is absent in the iso- therm for the sample with Pd :Rh = 1 :1. An advantage of the use NMR to study adsorption over the conventional isotherm measurement is that the former technique may provide information on binding between adsorbates and adsorbents.The 2H NMR results presented in this paper reveal five features. (i) In the absence of weakly sorbed deuterium (upon light evacuation treatment after a high-pressure adsorption), mobile chemisorbed deuterium atoms on palladium and rhodium crystallites are character- ized by their 'H NMR, with chemical shifts of -12 and -170 ppm, respectively. (ii) Under high deuterium pressure (>30 Torr), excess deuterium penetrates the palladium crys- tallites. These absorbed deuterium atoms exchange rapidly at ambient temperature with deuterium atoms chemisorbed on the crystallites, resulting in a sharp line at +25 ppm at satu- rated absorption. (iii) In the absence of weakly sorbed deute- rium, deuterium chemisorbed on the palladium atoms of Pd-Rh bimetallic crystallites is distinguished from that adsorbed on rhodium atoms by their characteristic peaks (with chemical shifts of -12 and -170 ppm, respectively).Accordingly, the average surface composition of the two ele- ments on these crystallites may be estimated from the relative 5 0.2 0.3 5 n if3 I I0.0I J 0.0 0 100 200 300 D, pressure/Torr Fig. 7 Adsorptions isotherms (298 K) of deuterium sorbed on the Pd-Rh bimetallic catalysts: (0)6% Pd-Rh/Al,O,; (0)9% Pd-Rh/Al,O 3 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 intensities of the peaks. A surface enrichment of Pd over Rh was thus suggested for a supported bimetallic sample with a Pd :Rh ratio of 1 :1.(iv) Under greater pressures of deute- rium, deuterium sorbed on the bimetallic sample loses its characteristic nature because of exchange between these deu- terium sites. (v) Palladium alloys well with rhodium on the supported bimetallic catalyst with a Pd : Rh ratio of 1 :1. When this ratio exceeds unity, two metallic phases with dif- ferent compositions were found; one has a Pd : Rh ratio of about unity, whereas the other contains almost pure palla- dium. We appreciate the financial support of this study the Nation- al Science Council of the Republic of China. References 1 F. A. Lewis, The Palladium Hydrogen System, Academic Press, London, 1967. 2 G. Alefeld and J. Volkl, Hydrogen in Metals (II), Application Oriented Properties, Springer Verlag, New York, 1978, ch.3. 3 S. E. Jones, E. P. Palmer, J. B. Czirr, D. L. Decker, G. L. Jesen, J. M. Thorne, S. F. Taylor and J. Rafelski, Nature (London), 1989,338,737. 4 T. Gram, Philos. Trans. R. SOC.London, 1966,156,415. 5 T. B. Flanagan, Engelhard Id.Tech. Bull., 1966,7,9. 6 H. Z. Brodosky, Z. Phys. Chem., N.F., 1965,44,129. 7 H. Z. Brodosky and E. Poeschel, 2. Phys. Chem. N.F., 1965,44, 143. 8 C. Wagner, Chem. Phys., 1951,19,626. 9 R. Burch and R. G. Buss, J. Chem. SOC.,Faraday Trans. 1, 1975, 71,913. 10 R. Burch and R. G. Buss, J. Chem. SOC.,Faraday Trans. I, 1975, 71,922. 11 C. Foo, C. Hong and F. D. Manchester, J. Phys. F, 1971,2,323. 12 S. Ladas, R. A. Dalla Betta and M. Boudart, J. Catal., 1978, 53, 356.13 L. H. Reyerson and A. Solbakken, Advances in Chemistry Series, Am. Chem. SOC., Washington, DC, 1961, no. 33. 14 W. Trzebiatowski and H. Kubicka, Z. Chem., 1963,3, 262. 15 H. Kubicka, J. Catal., 1966, 5, 39. 16 R. W. Zuehlike, J. Chem. Phys., 1966,45,411. 17 G. Alefeld and J. Volkl, Hydrogen in Metals (I), Basic Properties, Springer Verlag, New York, 1978, ch. 3,4. 18 G. Alefeld and J. Volkl, Hydrogen in Metals (I), Basic Properties, Springer Verlag, New York, 1978, ch. 9. 19 R. E. Norberg, Phys. Rev., 1952,86,745. 20 W. Z. Spalthoff, 2.Phys. Chem., 1972,76,760. 21 R. M. Cotts, Ber. Bunsenges. Phys. Chem., 1972,76,760. 22 R. R. Aron, H. G. Bohn and H. Lutgemier, Solid State Commun., 1974,14, 1203. 23 E. F. W. Seymour, R. M. Cotts and W. D. Williams, Phys. Reo. Lett., 1975,35, 165. 24 T. C. Sheng and I. D. Gay, J. Catal., 1982,77, 53. 25 A. A. Chen, A. J. Benesi and M. A. Vannice, J. Catal., 1989, 119, 14. 26 (a) T. C. Chang, C. P. Cheng and C. T. Yeh, J. Phys. Chem., 1991,%, 5239; (b)1992,%, 4151; (c)J. Catal., 1992, 138, 457. 27 G. Chen, W. T. Chou and C. T. Yeh, Appl. Catal., 1983,8,389. 28 T. C. Sheng and I. D. Gay, J. Catal., 1981,71, 119. 29 H. Muraki, M. Sobukawa and A. Isogai, 1990, SOC. Automotive Eng. paper 900610. Paper 3/06571E; Received 3rd November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001157

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(8), 1161-1170 Spectroscopic Characterization of Magnesium Vanadate Catalysts Part 1.-Vibrational Characterization of Mg,(VOJ2 Mg2V20, and MgV20, Powders Guido Busca* and Gabriele Ricchiardi lstituto di Chimica, Facolta di lngegneria, Universita P.le Kennedy, 1-16129 Genova, Italy D. Siew Hew Sam Elf Atochem, Centre de Recherche de l'Est BP 1005, F-57501 Saint-Avold Cedex, France Jean-Claude Volta lnstitut de Recherches sur la Catalyse, CNRS,Avenue A. Einstein, F49626 Villeurbanne Cedex, France The IR and Raman spectra of Mg orthovanadate Mg,(VO,), , Mg pyrovanadate Mg,V,O, and Mg metavanadate MgV,O, powders are reported, described and discussed on the basis of the crystal structures of these phases and of the optical activity of the fundamental vibrational modes predicted with the use of the correlation method.The IR-active combinations are also discussed. The structural and vibrational features of these compounds are discussed in relation to those of other V-based oxidic catalytic materials. The magnesium oxide-vanadium oxide system has been the object of attention recently because it presents promising catalytic activity in the oxidative dehydrogenation of hydro- carbons, such as alkanes (propane and butanel-') and ethyl- benzene.6 Processes allowing the production of light alkenes (ethene, propene and butenes) and butadiene by oxy-dehydrogenation of the corresponding alkanes are becoming very attractive because of the availability of very cheap, light alkanes from natural gas.Our research groups have been previously involved in investigations concerning other vanadia-based catalysts extensively used in industry in partial oxidation processes such as the V,O,-P,O, systems for maleic anhydride syn- thesis from b~tane~'~ and V,O,-TiO, systems for phthalic anhydride synthesis from o-~ylene.~.' A fundamental investi- gation of the structure and chemistry of these systems can allow an interpretation of the role of the components addi- tional to V205 (phosphorus, titania and magnesium), in gov- erning activity and selectivity. Attempts in this direction have already been proposed.' ',12 Several authors have investigated the solid-state chemistry of the MgO-V205 ~ystem.'~-'~Mo st of them'3.14*'7 con- sider that only three stable compounds appear in this phase diagram : the orthovanadate, Mg,(VO,), , the pyrovanadate, Mg,V,O, ,and the metavanadate, MgV206 .Moreover, both Mg,V,O, and MgV,06 exhibit the phenomenon of poly- m~rphism.'~.''.'~ In the present paper a bulk vibrational characterization of V-Mg-0 catalysts is reported. The materials investigated here appear to X-ray diffraction (XRD) analysis to be almost pure Mg orthovanadate [Mg,(VO,),], Mg pyrovanadate (Mg,V,O,) and Mg metavanadate (MgV206).3 The catalytic activity of these materials has also already been described., Experimental Catalyst Preparation The three pure magnesium vanadates were obtained by VMgO precursors generated by adding an appropriate amount of Mg(OH), to a basic aqueous solution (1% NH,OH) containing NH,VO,.The solid was then dried under vacuum at 373 K and immediately calcined at 823 K for 6 h to avoid any carbonation. Mg orthovanadate, Mg3(V0,), , was then prepared from the 60VMgO precursor (58.5% V205 w/w) by calcination in air for 49 h at 898 K, 60 h and 913 K, 15 h at 1023 K and 15 h at 1073 K. Mg pyrovanadate, Mg2V207, was prepared from the 69VMg0 precursor (66.4% V205 w/w) by calcina- tion in air for 6 h at 923 K and 6 h at 973 K. The metavana- date, MgV206, was prepared from the 82VMg0 precursor (79.8% V,O, w/w) by calcination in air for 6 h at 873 K and 24 h at 973 K. The XRD analyses (Siemens goniometer equipped with a quartz front monocromator, Cu-Ka radiation) showed quite pure phases except for Mg metavanadate, which presented traces of Mg pyr~vanadate.~ Spectroscopic Measurements The IR spectra were recorded using a Nicolet Magna 750 Fourier-transform instrument.The skeletal spectra in the region above 400 cm- 'were recorded with KBr pressed discs and with a KBr beam splitter, while those in the far-infrared (FIR) region (400-50 cm-') were recorded using the powder deposited on polyethylene discs, and with a 'solid substrate' beam splitter. The spectra of the overtone region (above lo00 cm-') were recorded using pressed discs of the pure powders, outgassed at 673 K in a heatablebiquid-nitrogen-cooledcell connected to a conventional gas-handling system. The laser Raman spectra were recorded on a Dilor Omars 89 spectrophotometer equipped with an intensified photo- diode array detector.The emission line at 514.5 nm from an Ar+ ion laser (Spectra Physics, mod. 124) was used for excita- tion. The power of the incident beam on the samples were 36 mW. The aquisition time was adjusted according to the intensity of the Raman scattering. 100 spectra were accumu- lated in order to improve the signal-to-noise ratio. The wave- number values obtained from the spectra were accurate to within about 2 cm-'.To reduce both thermal and photo- degradation of samples, the laser beam was scanned on the sample surface by means of a rotatory lens. The scattered light was collected in back-scattering geometry. Results The structures of the three Mg vanadates, deduced from liter- ature are shown schematically in Fig.1,2 and 3. In the case of the pyrovanadate (Mg2V,07), according to the XRD pattern and to ref. 17, the structure has been assumed Fig. 1 Scheme of the structure of Mg,(V04),, from ref. 18, On the right: the coordination of the vanadate ion. Symbols :black spheres, V; white spheres, Mg; grey spheres, oxygen. b Fig. 2 Scheme of the structure of monoclinic Co,V,O, from ref. 20, assumed to be isostructural with monoclinic Mg,V,O,. On the right: pairs of V,O:- ions. Symbols: black spheres, V; white spheres, Mg; grey spheres, oxygen. [010] t Fig. 3 Scheme of the structure of MgV,O,, from ref. 21. On the right: structure of the [(V,0,)2-], sheets.Symbols: black spheres, V; white spheres, Mg; grey spheres, oxygen. to be the same as that of Co,V,O, ,2o both being monoclinic (space group P2Jc =Cz,). In Table 1 the V-0 distances present in the relevant structures are summarized, also including those of triclinic Mg,V,O, l9 and of V,O,, 22 for the sake of completeness. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 V-0 distances (A)in metal vanadates (from ref. 18-22) compound ref. I I1 I11 IV V VI Mg3(V0,), Mg,V,O,triclinic Co,V,O, monoclinic 18 19 I I1 20 I I1 1.70" 1.63' 1.68" 1.63' 1.69" 1.70" 1.70" 1.71" 1.71" 1.70" 1.72" 1.71d 1.74d 1.72" 1.70" 1.81b 1.82' 1.78' 1.85' 1.84' 2.87/ 2.44/ MgV,O, v205 21 22 1.66" 1.58' 1.67" 1.77' 1.85h 1.88' 1.85' 1.88' 2.11' 2.02' 2.57g 2.83' "V-0-Mg, terminal oxygen coordinating two bivalent cations.V-O-Mg, terminal oxygen coordinating three bivalent cations. V-0-Mg terminal oxygen coordinating one bivalent cation. Asymmetric V,O-Mg,: like a but acting as weak fifth ligand to another V atom. 'Nearly symmetric V,O-Mg. V-0-V nearly symmetric bridge. Asymmetric V,O-Mg: like c but acting as weak sixth ligand to another V atom. Asymmetric V,O-V. V--. ..V terminal oxygen acting as weak sixth ligand to another V atom. In pyrovanadates the two V atoms (I and 11) are not equiva- lent. The FTIR, FT-FIR and laser-Raman spectra (1200-50 cm-') are summarized in Fig. 4, 5, 6 and 7. The positions of the observed bands are summarized in Table 2. Interpreta- tion of the spectra was achieved using the correlation method, as reported in ref.23 and 24. The Orthovanadate, MgJVO,,), The orthorhombic magnesium orthovanadate (Fig. 1) belongs to the Di8 =Cmca space group,18 with a =6.053 A, b =11.442 1,c =8.330 8, and four molecular units per unit cell. The crystallographic unit cell contains two Bravais cells, 1200 1000 800 600 400 200 waven umberjcm- ' Fig. 4 FTIR (a), FT-FIR (b) and laser Raman (c) spectra of Mg3(V04)2 powder -0.9 0.8-0.7-0.6-0.5-0.4-0.3: 4 ............................................. 1200 1000 800 600 400 200 ' wavenumber/cm - Fig. 5 FTIR (a), FT-FIR (b) and laser Raman (c) spectra of Mg,V,O, powder J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 0.60 0.55 0.50 0 0.45 $% 0.40 0.35 0.30 1200 1000 800 600 400 200 wavenumber/cm-' Fig. 6 FTIR (a), FT-FIR (b) and laser Raman (c) spectra of MgV,O, powder. The peaks marked with stars belong to Mg,V,O, impun ties. I I al Cm e 0 2400 2200 2000 1800 1600 1400 1200 wavenumber/cm-' Fig. 7 FTIR spectra in the overtone region: Mg3(V0,), (a), Mg,V,O, (b),MgV,O, (c)and V,O, (d)powders each one containing two molecular units, i.e. 26 atoms. Accordingly, 78 total modes are expected, three of which cor- respond to the acoustical modes and 75 are vibrational models. Two different types of octahedrally coordinated Mg ions are present while the tetrahedrally coordinated V atoms are all equivalent. The VOi- ions have C, symmetry, although approaching a symmetric tetrahedral configuration.According to factor group analysis we can obtain the fol- lowing irreducible representation for the optical modes of Mg,(VO,),: + 8AJinactive) + 12B,,(IR) + 11B2,(1R)+ 8B,,(IR) to which the acoustic modes (Blu + B,, + B,,) should be added. Consequently, we expect 31 IR-active modes and 36 Raman-active modes. The vibrational structure of Mg,(VO,), can be discussed by considering covalently bonded VO: -vanadate ions as molecular units placed in C, symmetry sites, coordinated through ionic bonds to Mg2+ cations. For each VOZ-molecular ion, nine internal vibrational modes are expected, which corresponds to 36 modes for the entire Bravais cell.The assignments of the remaining 39 optical modes are sum- marized in Table 3. The free V0:- ion is tetrahedral (T', point group) and cor- respondingly the following irreducible representation is valid : The A, mode is the symmetric stretching vl, while the E mode is the symmetric bending v2 . The two F, modes corre- spond to the asymmetric stretching (v,) and the asymmetric deformation (v,). In our case, one oxygen atom is bonded to V with a longer bond than the other three, so the symmetry is near C3",the two F, modes being split into A, + E. When the symmetry is lowered to C,, the site symmetry in our case, the degeneracies are completely broken and the irreducible representation changes to the following: rCsVOa= 6A' + 3A" where all modes are both IR- and Raman-active.Under the factor group D,, each A' mode gives rise to A,(R) + B3,(R)+ B,,,(IR) + B,,(IR) modes, while each A" mode gives rise to B,,(R) + B,,(R) + A,(inactive) + B,,(IR). According to these correlations, the symmetric stretching of the V0:-ions (v,, A, in Td point group) gives rise in the Mg orthovanadate to four modes [A,(R) + B3,(R) + B,,,(IR)+ B2,(1R)], while the asymmetric stretching (v3, F, in & point group) gives rise to 12 modes [2A,(R) + 2B,,(R)+ 2B1,(IR) + 2B,,(IR) + B2,(R) + B,,(R) + AJinactive) + B3J1R)]. Accordingly, also the number and activities of the vibrational modes arising from the asymmetric and sym- metric deformation modes of the VOZ-entity, as well as from the other lattice modes, can be predicted.The distribu- tion of the fundamental modes of Mg,(VO,), is summarized in Table 3. According to the above discussion, we can attempt some assignments of the observed IR and Raman spectra of Mg orthovanadate. In the region above 500 cm-' we expect the presence of bands arising from the stretchings of VO, tetra-hedra. The symmetric stretching (v,) of the isolated orthova- nadate ion is expected and is reported to correspond to the strongest Raman peak, quoted at 827 cm- ' for the free ion in aqueous solution.25 For the free ion this band is IR-inactive. According to the above discussion, under coupling of the four VO, unities in the Bravais cell, this mode is expected to split into two strong Raman peaks and two weak IR bands.So, we assign the two strongest Raman peaks in the spectrum of Mg orthovanadate, at 862 and 827 cm-', to the two com- ponents of the symmetric stretching mode. The strongest peak at higher frequency is assigned to the totally symmetric A, mode, the other being assigned to the B,, mode, i.e. to a mode that is symmetric with respect to the x, z symmetry plane and antisymmetric with respect to the x, y and y, z planes. It seems reasonable to assign the weak but sharp IR peak at 833 cm- to the two corresponding IR active modes with B,, and B,, symmetries, that are symmetric with respect to the x, y and x, z symmetry planes, respectively, and anti- symmetric with respect to the others. These modes could be superimposed upon each other, and are almost coincident with the Raman-active B,, mode, assuming that the three symmetry planes are equivalent, which is, obviously, not true.The frequency of these three modes, near 830 cm-', and also the 'centre of gravity' of the four components assigned to modes arising from v1 (837 cm- ') are very near to the wave- number of v1 of the unperturbed vanadate ion (827 cm- '). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Position of the observed vibrational bands for the Mg vanadate powders Mg,(V04)2 Mg2V20, MgV20, IR Raman assignment IR Raman assignment IR Raman assignment fundamentals 975 sh 968 VO str. 915 897 sh 917 923i881 sh 880 sh 888 861 840 VO, str. 840 sh asVOV str." VO, str. 833 827 818 730 sh 724 w 770 sh 690 vw 690 695 shI687 862 610 br sh 668 VOV str.655 575 570 620 575 sh 552 485 462 473 473 439 440 430 440 448 410 415 41 1 402 403 394 39 1 379 377 383 370 35 1 362 354 350 br 332 336 344 325 sh 335 320 330 3 14 316 308 vw 302 sh 305 302 309 291 290 285 282 286 275 V04 def. 268 VO, def. 268 248 245 br + lattice 242 243 + lattice 235 vw 228 205 200 220 216 212 204 198 198 198 174 190 181 171 165 156 145 155 150 149 145 136 137 130 131 122 113 IR combination 1933 975 + 948 1910 968 + 948 1867 655 + 655 + 552 732 + 695 + 440 1790 915 + 881 1790 917 + 873 1780 923 + 888 923 + 840 1720 862 + 861 1694 873 + 818 1672 861 + 827 845 + 840 1615 845 + 770 1347 862 + 485 1430 818 + 620 1408 731 + 695 1210 630 + 575 1208 695 + 523 1116 569 + 575 1117 731 + 383 str., stretch; def., deformation; sh, sharp; br, broad; as, asymmetric; vw, very weak.Very asymmetric VOV bridges. Table 3 Distribution and assignments of the fundamental modes of orthorhombic Mg3(V04), vo4 origin symmetry activity total acoustical optical lattice librational internal v1 v2 v3 v, 10 10 3 1 6 2 2 8 8 3 2 3 1 1 7 7 2 2 3 1 1 11 11 4 1 6 2 2 8 8 3 2 3 1 1 13 12 5 1 6 2 2 12 11 4 1 6 2 2 9 8 3 2 3 1 1 78 75 27 12 36 12 12 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The asymmetric stretching mode (v,) corresponds to the strongest IR band for the isolated V0:-ion, and is also Raman-active, although weak, at 780 ern-'.25 This mode is expected to give rise to five strong IR bands and six weak Raman peaks in our case. We must consequently assign the two very strong IR bands, both showing multiplicity, centred at 861 and 687 cm-', with components at 915, 740 and 610 em-' to the IR-active modes arising from v3. The weak Raman peaks at 897, 881 and 724 em-' should be assigned to three of the six expected weak Raman-active modes arising from the splitting of the asymmetric stretching mode, v3. To attempt an identification of these modes we recall that among the four oxygen atoms that are bonded to vanadium, only two are equivalent, according to the C, site symmetry. One of the two in-plane V-0 bonds is definitely longer than the others. So, we propose that the main cause of splitting of the asymmetric stretching mode is internal to the vanadate ion, due to the non-equivalence of the V-0 bonds.This allows us to explain why the splitting of v3 is much stronger than that of v,, which is only due to the crystal structure effect, i.e. to the copresence of four equivalent units in the Bravais cell, whose vibrations are coupled. As shown in Table 4, if one V-0 bond is longer than the others (C," symmetry) the triply degenerate v3 mode (F, in & point group) splits into two components (A, + E). If only two oxygens are equivalent, as in our case (C, site symmetry), the E mode further splits into A' + A", and the A, mode becomes A'.The last mode is expected to fall at frequencies even higher than the symmetric stretching mode. The A" modes arising from the E mode in C,, can be expected at lower frequency. Each A' mode under D,, point group produces four components [A,(R) + B,,(R) + B,,(IR) + B3,(1R)]. So, we assign the IR bands at 915 and 861 em-' to the B,,+ B,, modes and the Raman shoulders at 897 and 881 em-' to the A, + B,, modes, all arising from the high-frequency A' component of v,. The 'centre of gravity' of both pairs of bands is near 888 cm-l, which can be assigned to this com- ponent of the asymmetric stretching mode, modified by lattice effects. As a consequence, we can assign the Raman peak at 724 em-' and an extremely weak feature that can be envisaged near 680 em-' to the remaining four Raman-active modes arising from the asymmetric stretching of the shorter V-0 bonds, probably superimposed in pairs, although it is pos- sible that some of them are too weak to be detected, and lie at lower frequency.The IR bands at 740, 687 em-' are assigned to the corresponding IR-active components of the asymmetric stretching mode, a third one probably being superimposed on them and undetectable. The centre of gravity of these modes is near 720-700 cm-', which can be taken as the value of the asymmetric stretching mode under C,, symmetry, modified by crystal effects. A summary of our assignments for the stretching modes is given in Table 4. Even more complex would be the assignment of the many peaks observed in both IR and Raman spectra below 500 em-',to the deformation modes of vanadate ions and to the rotational and translational lattice modes.We will limit our- selves to the observation that we detect at least 15 Raman and 15 IR bands in the region 500-100 cm-', in contrast to the expected 28 Raman-active and 24 IR-active modes. This may be due to the superimposition of different components. The IR spectrum is apparently composed of a weakly split medium-intensity band at 485,473 ern-and of a very broad absorption centred near 400 em-',on which at least 13 com-ponents are superimposed. The former split absorption is assigned to a component arising from the v4 asymmetric deformation mode, also IR-active for the symmetric V0:- anion, while the latter broad adsorption agrees, because of its position and broadness, with the assignment to lattice modes mainly involving stretching modes of MgO, octahedra.,, From our tentative assignments, we can attempt some con- clusions.Comparison of the spectra we report here with those reported by other authors for the same ~ompo~nd~-~~~~~~~shows good agreement, but with non-negligible differences. For example, we do not detect at all of the weak IR component reported both by Hanuza et d6and by Baran and AymoninoZ7 at 962 cm-', which is likely to be due to an impurity arising from Mg2V,07 (see below). Simi- larly, the Raman spectrum of Mg3(V04), reported by Owen and Kung28 clearly shows additional peaks at 948 and 900 em-', also likely to be due to Mg,V,O, impurities.More- over, the interpretation of the spectra of Mg3(V04), we propose here shows some marked differences from the pre- vious based on an imperfect knowledge of the struc- ture. Comparison of our spectra with those reported for the compounds Ca,(VO,),, Sr,(VO,), and Ba,(V04),,2i27929*30 Table 4 Scheme for assignment of the dundamental stretching bands of orthorhombicMg3(VOJ2 (an-I) and their IR-active combinations site symmetry factor group 915 + 881 1790 IR 862 + 861 1720 IR 861 + 827 1672 IR 862 + 485 1347 IR 915 IR 897 R 881 R 862 R 861 IR 833, IR 827 R 740 IR 724 R 687 IR (690) R 1166 which are not isostructural with Mg3(V04), , clearly points to the effect of the crystal structure and symmetry on the vibrational spectra.This makes unreliable simple correlations on the position of bands with, e.g. catalytic activity, for non- isostructural compounds, such as those proposed in ref. 2 and 31. The Pyrovanadate, Mg,V,O, Mg pyrovanadate is polymorphic, showing at least three dif- ferent structures at ordinary pressure. 14*' According to Clark and Morley,', the form stable at room temperature is monoclinic (Fig. 3), belonging to the P2,/c = c;h space group, with a = 6.605 8,, b = 8.415 8, and c = 9.487 8, and /3 = 100.61", and with four molecules per unit cell. Although the structure of this form has not been refined, it is thought to be isostructural with Ni and Co divanadates, whose struc- tures have been studied in detail.20 Mg,V,O, transforms near 1000 K to a triclinic form, whose structure has been refined by Gopal and Ca1v0.l~ A further phase transform- ation occurs at even higher temperatures.14v1 , Triclinic magnesium pyrovanadate belongs to the Pi = Ci space group with a = 13.767 A, b = 5.414 8, and c = 4.912 A, a = 81.42", /3 = 100.61", y = 130.33", and with two molecular units per unit cell. The triclinic structure of Mg,V,O,, as well as that of several other bivalent divanadates, can be termed thortveitite-like, being closely related to that of the mineral thortveitite, Sc,Si,O, (C2/m = cih space group), because in both cases sheets of M,O, plyanions are present. However, the structure of triclinic Mg,V,O, differs in many respects from that of thortveitite owing to the geometry of the M-0-M bridge of the M,O, anions (linear in thortvei- tite and bent in Mg pyrovanadate) and because of the coordi- nation of the vanadium ions, which is approaching five-fold owing to weak interactions with the oxygens of the nearest pyrovanadate ions. The structure of monoclinic Mg,V,O, , assumed to be the same as that of monoclinic C0,V207, as proposed by Clark and Morley,17 is not thortveitite-like because the M207 entities do not form sheets.The unit cell of the monoclinic structure contains 44 atoms, and consequently, 129 optical modes are expected. The V,074-ions are bent, with a nearly eclipsed cis orienta-tion of the oxygens of the VO, tetrahedra.So, the ion sym- metry is near C,,.Factor group analysis for the unit cell of monoclinic Mg,V,07 leads to the following irreducible rep- resentation : rapt = 33Ag(R)+ 33Bg(R)+ 32A,(IR) + 31B,(IR) to which the acoustic modes A, + 2B, should be added. The unit cell of the triclinic structure contains 22 atoms, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 and consequently, 63 optical modes are expected, which are either IR- (A,) or Raman-active (BJ. The V20,4-ions are bent, with a nearly staggered trans orientation of the oxygens of the VO, units. So, the ion symmetry is near C,. Factor group analysis for the unit cell of triclinic Mg,V,O, leads to the following irreducible representation: rap, = 33Ag(R)+ 30A,(IR) to which the acoustic modes 3A, should be added.Since the volume of the monoclinic unit cell is nearly double that of the triclinic unit cell (both primitive), the two structures could, in principle, be distinguishable by vibra- tional spectroscopies, because for the former structure the number of bands should be nearly double that for the latter. However, if the A,B coupling for the monoclinic compound is sufficiently small, only half of the bands appear because they are superimposed in pairs. So, the two structures are indistin- guishable by vibrational spctroscopy. This is probably what occurs for our monoclinic Mg,V,O, , where nearly 30 IR and Raman components, instead of 66 Raman and 63 IR, are observed. In Table 5 the assignments of the vibrational modes to lattice modes and internal vibrations of the pyrovanadate groups are also reported for both structures.These groups approach C,, symmetry in the monoclinic structure, for which the following irreducible representation is obtained : rc.vv207 = 7A,+ 4A2 + 6B1+ 4~, Among these 21 vibrational modes of isolated V,07 ions, 17 are IR-active (A,, B, and B, symmetry modes) and 14 are Raman-active (A,, B, and B,). Under site symmetry C,, all modes become A and all split into Ag(R) + B,(R) + A,(IR)+ BJIR) under the factor grup Ci. Of these 21 modes, six are terminal V-0 stretchings, 12 terminal bendings, two bridge stretchings and one bridge bending. Note that of the six terminal V-0 bonds, one each side is particularly short, while the other four are equivalent to each other and to the shorter three bonds of the orthovandate ion in Mg3(V04), (see Table 1).To attempt an assignment of the observed bands we can again divide the spectrum at 500 cm-'. Above this frequency we expect only V-0 stretching modes. Above this frequency we detect eight well resolved IR bands and seven well resolv- ed Raman peaks. It seems reasonable to assign the IR bands at 668 cm- and at 575 cm-',as well as the Raman bands at 620 and 570 cm- to the asymmetric and symmetric stretch- ing modes of the V-0-V bridge. This assignment agrees with that of Pedregosa et dJ2The higher-frequency com- ponents of these bands in both the IR and Raman spectra are clearly split (690, 668 cm-' in IR, 630, 620 cm-' in Raman) Table 5 Distribution and assignments for the fundamental modes of monoclinic and triclinic Mg,V,O, v20, terminal V-0 v-0-v symmetry activity total acoustical optical lattice librational internal stretch bend stretch bend monoclinic, S.G.P2Jc A, B,A, Bu R R IR IR 33 33 33 33 33 33 32 31 9 9 8 7 21 21 21 21 6 6 6 6 12 12 12 12 total 132 129 33 84 24 48 triclinic, S.G. Pi A,A, R IR 33 33 33 30 9 6 21 21 6 6 12 12 total 66 63 15 42 12 24 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 and this can be taken as evidence for the A,-B, and A,-B, splittings, respectively. The bands in the region 1000-700 cm-’ should be due to terminal V-0 stretching modes. In both the IR and Raman spectra we observe two sharp bands just above 900 cm-’, but their intensity ratio is inverted in the two spectra.More- over, this pair of bands is stronger than the bands in the region 900-700 cm-’ in the Raman spectrum, while the reverse is found for the IR spectrum. It seems reasonable to assign the strongest Raman mode (902 cm-’) to the totally symmetric stretch, which has the character of a symmetric stretch of the two shortest V-0 bonds. Consequently, the weaker mode at higher frequency (948 cm-’) can be assigned to the asymmetric stretching of the two shorter V-0 bonds. The same modes origmate the IR bands at 917 and 968 cm-’,respectively, whose intensity ratio is, accordingly, the inverse of that in the Raman case. The weak splitting of the band at 968 cm-’ can be further evidence for the weak A,-B, splitting. The remaining bands, located in the 930-700 cm-region, are assigned to stretching modes of the longer terminal V-0 bonds.Five components can be distinguished in the IR spec- trum, as expected assuming no A,-B, splitting, while only two are found in the Raman spectrum, perhaps owing to the very weak strength of some of them. The region below 500 cm-’ shows a split IR band at 462, 439 cm-’, similar to that observed at 485 and 473 cm-’ in the case of the orthovanadate, assigned to a deformation mode of the VO, entities. The components in the region 450- 100 cm- ’ are assigned to lattice modes and to deformations of the pyrovanadate ions. According to this interpretation of the spectrum in the V-0 stretching region, we have very weak A,-B, and A,-B, splittings, while A,A, and B,-B, splittings are sig- nificant (up to 40 cm-’), as evidenced by the separation of the IR and Raman modes.This might be due to the particu- lar association in centrosymmetric pairs of the pyrovanadate ions in the structure of monoclinic Mg2V20, (see Fig. 3”), whose vibrations are consequently strongly coupled. Note that the IR spectrum we report here for Mg,V,O, is consistent, although not entirely correspondent, with that dis- cussed by Pedregosa et aL3’ and with that reported by Pate1 et but both the IR and Raman spectra are definitely different from those reported by Hanuza et a/.,, which corre- spond to a mixture of phases. On the other hand, the Raman spectra reported by Stencel” for a-Mg,V,O, (monoclinic polymorph) and by Hardcastle and Wachs3’ for #?-Mg2V20, (triclinic polymorph) are instead due to mixtures of a-Mg,V,O, and MgV,O,.The IR spectrum we report here does not compare well with those of the other alkaline-earth- metal pyrovanadates” which, in fact, are not isostructural with monoclinic MgV,O, . The Metavanadate MgV,O, Monoclinic magnesium metavanadate (Fig. 5) is isomorphous with the brannerite mineral having the formula (Th,U)Ti,O, : both belong to the C2/m = Czh space group,2’ with two molecular units per crystallographic cell. The unit cell dimen- sions are a = 9.279 A,b = 3.502 A,c = 6.731 A,#? = 111.77”. This phase transforms near 535°C into a ‘pseudo-brannerite’-type form, and at high pressure into a ‘cou1ombite’-type form.The smallest Bravais cell of the bran- nerite structure contains one molecule only, i.e. nine atoms. Accordingly, 27 total modes are expected of which three are acoustical modes and 24 optical modes. The structure con- sists of both Mg2+ ions and V5+ ions in octahedral coordi- nation, and of three different types of oxygen atom, each tricoordinated. VO, octahedra are linked by three edges, forming infinite anionic [(V,0,)2 -1, layers parallel to the (001) face. Inside the layers, zig-zag chains of edge-shared VO, octahedra may be distinguished along the [OlO] direc-tion. Factor group analysis allows us to obtain the following irreducible representation for the optical modes of Mg meta- vanadate: rapt = 8A,(R) + 4B,(R) + 4A,(IR) + 8B,(IR) to which the acoustic modes (A, + 2B,) should be added.To divide the optical modes into lattice and ‘internal’ V-0 vibrations with known physical meaning, we must divide the vibrations associated with motions of Mg ions (A, + 2B,) from those associated with internal vibrations of the [(v206)”], polymeric layered molecular anion lying in the (020) plane of the unit cell (with Mg ions assumed to be at the corners). For this layer, 21 internal vibrations are expected, whose distribution among symmetry species, obtained by the difference spectra, is reported in Table 6. The three oxygen atoms of each VO, unit, two of which are present in the smallest Bravais cell, are of three different types.O(1) is bonded to a single V with a short V-0 bond (1.666 A),but also bridges two Mg ions; O(I1) bridges two V atoms very asymmetrically with one very short (1.671 A) and one very long (2.671 A)V-0 bond, but also coordinates one Mg ions; O(II1) triply bridges V atoms with bonds of inter- mediate length. To simplify the structure, we can neglect the sixth weakest coordination at the vanadium ions, so that the O(I1) atoms become terminally bonded to V atoms like O(I), and the zig-zag chains of edge-shared V06 octahedra along the [OlO] direction are separated from each other. The structure of one chain is reported in Fig. 5. In this view, the 24 optical modes of the structure are divided into 20 internal vibrations, one rotational mode along the b axis of the [v,06]z-chains and three lattice translational modes.The vibrations associated with the weak bonds we have artifically ‘broken’ become rotations of the chains. Each V ion is now pentacoordinated, with two short ter- minal V-0 bonds located in the symmetry plane of the unit cell, parallel to the (010) plane, and three longer V-OV, bonds with O(II1) atoms in a plane parallel to the [OlO] direction. We can now divide the vibrations associated with VO, units with short V-0 bonds from those associated with the motions of the O(II1) atoms in the (V,O,), layers. Our 24 total modes are now constituted by three lattice vibrations, one rotation of the V,O, chains, eight modes of the V,O, ‘network’ involving O(II1) forming a chain along the b axis, and finally, six out-of-plane deformations, two in-plane defor- mations and four stretchings of VO, short-bond units.The symmetry species and activity of these modes can be found in Table 6. The observed spectra of the Mg metavanadate are compa- ratively simpler than those discussed above for the ortho- and meta-vanadate, owing to the smaller size of the unit cell. The Raman spectrum shows 12 very well resolved bands, just as expected. The IR spectrum is much less well resolved, but also shows almost 12 components, as expected. However, we also observe in the IR spectrum weak components arising from Mg,V,O, impurities. Above 800 cm-’ we observe two Raman bands and two IR bands, which are assigned to the stretching modes of the VO, units.The strongest Raman mode at 923 cm- ’ certainly arises from the symmetric stretching mode of V02 ,while that at 836 cm-’ arises from the asymmetric mode. By analogy, the IR band at 888 cm- ’ is due to the IR-active component arising from the symmetric stretching mode, while that at 840 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 6 Assignments for the fundamental optical modes of monoclinic MgV'O, (VO,)+ a cv202Ia symmetry activity total acoustical optical Mg [V,06]'- (internal) stretch ip op int. rot. 4% R 8 0 8 0 8 2 1 1 4 0 B* A,B" R IR IR 4 5 10 0 1 2 4 4 8 0 1 2 4 3 6 0 0 2 0 0 1 2 2 1 1 1 2 1 0 0 total 27 3 24 3 21 4 2 6 8 1 The vibrations of the planar polymeric macroanion [v,06]'-have been divided into those of the bent (VO,)' units and those of the [V,O,] polymeric zig-zag chains.ip, in-plane deformations of V02 units (short bonds); op, out-of-plane deformations of VO, units (short bonds). cm-' is assigned to the IR-active mode arising from the asymmetric stretching. This agrees with the stronger intensity of the former band. We can recall that only one of the V-0 bonds is a true terminal bond (although the oxygen is coordi- nated to two Mg ions), the other being a very asymmetric V-0-V. An alternative, more rigorous assignment for the peaks at 923 cm-' (Raman) and at 888 cm-' (IR) is to the asymmetric and symmetric stretchings of the two true termin- al V-0 bonds, one per VO, unit. The IR and Raman peaks, almost coincident near 840 cm-',are consequently assigned to the stretching of the very asymmetric V-0-V bridging system, involving O(I1).In the region 800-500 cm-' we observe two peaks in the Raman spectrum at 731 and 523 cm-', and a complex IR absorption with the most intense bands at 655 and 552 cm- ', and probably two other components at 695 and 620 cm-'. These bands are assigned to the stretching motions of the V20, network with triply bridging O(II1) atoms. These assignments find confirmation from the assignments of the IR and Raman spectra of V205.34*35In total, six modes are observed, as expected. At lower frequencies, a band almost coincident in the IR and Raman spectra is observed (430 cm-' in IR, 440 cm-' in Raman).This band can be assigned to the two components of the in-plane deformation mode of the VO, unit. Note that the IR spectrum of the Mg metavanadate differs significatly from those of the orthovanadate and of the pyro- vanadate (as well as those of many Mg2+ compounds) because of the absence of a strong, broad absorption in the 450-350 cm-' region. This can be related to the observation, reported by Mocala and Ziolk~wski,~~ that in this compound Mg ions occupy more space than usual, probably because of the particular lack of elasticity of the layered macroanion. So, the MgO, octahedra are unusually expanded, and the Mg-0 bond order is unusually low.We found less well defined absorptions at lower frequencies than usual, as expected. The IR spectrum we report here does not compare well The spectrum of Mg3(V0,), is composed of a triplet at 1790, 1720 and 1697 cm-' and of a further strong band at 1347 cm-'. The triplet clearly corresponds to the com-bination modes of the bands arising from the splitting of v1 (symmetric stretching of the V0:- anion) as well as of the highest-frequency components of v3 (asymmetric stretching of VO:-), while the band at 1347 cm-' could be due to a com- bination of the Raman-active mode at 862 cm- ',arising from the symmetric stretching vl, which the IR mode at 485 cm- ', probably arising from the deformation mode v4. Note also an additional very weak, sharp band at 1964 cm- ',which is due to a surface vibration (see Part 2 of this series).37 The spectrum of the pyrovanadate shows a very character- istic sharp doublet at 1933, 1910 cm-'.These two com-ponents necessarily arise from the two crossed combinations of the two superimposed Raman-active and two weakly resolved IR-active modes arising from the stretchings of the shorter V-0 bonds. The theoretical values of these modes are 1923 and 1916 cm- '. The strong band in the intermediate region, composed of at least three bands, is due to com- binations of VO, terminal stretchings. The broad, weak band near 1430 cm-' is assigned to a combination involving a V-0-V asymmetric stretching and a VO, stretching. Finally, the two strong harmonics at 1210 and 1116 cm-' can be assigned to crossed combinations of the Raman- and IR-active V-0-V stretching modes.The overtone region of the spectrum of Mg metavanadate shows, besides weak components arising from Mg2V,0, impurities, five main bands. The low-frequency pair of bands looks similar to that already discussed for Mg,V,O, and also observed for V205, although at higher frequencies in this case (1276, 1200 cm-'). These bands, absent for the orthova- nadate, should arise from modes involving bridging oxygens. Also, the band near 1400 cm-' certainly involves VO, stretchings, while the band at 1780 cm-' is assigned to com- binations of terminal V-0 stretchings. The highest-frequency component, instead, cannot be assigned to a binary combination.The most reasonable assignment is to a ternary combination of IR-active modes, or of two Raman modes with those reported previously for the same c~mpound~?~~ and with those of other alkaline-earth-metal metavana-dates,29 while the Raman spectrum corresponds entirely with that reported by Sten~el.'~ IR Spectra in the 'First Overtone' Region The IR spectra of pressed discs of the pure Mg vanadate powders (and of V205 for comparison) in the region of the first overtones of the skeletal vibrations are reported in Fig. 7. Since all of the structures we are dealing with are centro- symmetric, all IR-active first harmonic bands should be assigned to binary combinations of one Raman-active (or inactive) mode and one IR-active (or inactive) mode.The positions of the observed harmonic bands and tentative assignments are reported in Table 2. plus one IR mode. In Fig. 7 the spectrum in the overtone region of V205 is also reported. The two strong bands at 1276 and 1200 cm- ' and the shoulder near 1350 cm-' can be assigned to com- binations of the fundamental mode~~~,~ arising from stretch- ings of the V-0-V and VO, entities at 820 cm- ' (IR), 703 cm-' (R), 600 cm-' (IR) and 528 cm-' (R) in the following manner: 703 cm-' (R) + 520 cm-' (IR) = 1123 cm-'; 703 cm-' (R)+600 cm-' (IR)= 1303 cm-'; 528 cm-' (R) + 820 cm-' (IR) = 1348 cm-'. At higher frequencies two strong bands are observed at 2020 and 1975 cm-'. In pre- vious publications by Busca et u1.38,39 a weak shoulder in the middle was envisaged, but is probably non-existent.This doublet, considered to be a triplet, was erroneously attributed to the summation and combination modes of two V-0 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 stretchings. In reality, four short V=O bonds are present in the Bravais cell of V,O, (Prnrnn = Dii space group, 2 = 2). Consequently, four V=O fundamental stretching modes are present, two of which are Raman-active (Ag, Blg, almost coincident near 994 cm-’ 34) and two IR-active (Bz,, 1035 cm-’, B3,, 995 cm-’). The IR spectrum in the overtone region is consequently constituted by four combinations, superimposed in pairs, expected near 2027 and at 1990 cm-’, both B,, + B3,, and just found at 2020 and 1975 cm-’.In conclusion, the IR spectra in the ‘first overtone’ region are characteristic of the single phases as well as of the struc- tural units they contain, and can be used to detect the state and the phase purity of the active catalyst phases during IR adsorption experiments using pure powder pressed discs, as already proposed.38 The observed harmonic bands can be assigned according to the assignments of the fundamental modes, and allow us to propose some correlations. Note that the stretching modes of short terminal V-0 bonds give rise to sharp strong combination modes, in the region 2050-1800 cm-’. Broader and multiple combination bands are found in the region 1900-1500 cm-’ arising from the different asym- metric and symmetric stretching modes of terminal VO, entities.Single and triple bridges, V-0-V and V30, give rise to strong and relatively sharp bands in the region 1500-1OOO cm-’. The strength of these combination modes is related to the covalency of the V-0 bonds. When only low- oxidation-state cations are involved in similar structures, these modes are very weak or even absent. Discussion The analyses of the structures of the stable compounds in the MgO-V,O, system show a progressive modification of the coordination sphere of vanadium. While the coordination at vanadium is an asymmetrically distorted octahedron with one very short V-0 bond and one very long bond in V’O,, in the case of magnesium metavanadate the structure is similar but with the shortest bond being longer (two nearly equivalent V-0 bonds) and the longest bond shorter (Table 1).In the triclinic magnesium pyrovanadate, vanadium is nearly pentacoordinated, with a fairly symmetric tetrahedron entertaining a fifth weak coordination. The situation is similar in the monoclinic magnesium pyrovanadate (assumed to be isostructural with monoclinic C0,V207) where the exis- tence of a fifth coordination at vanadium is also suspected.22 In the case of the magnesium orthovanadate the structure is definitely tetrahedral, which implies a further expansion of the shorter V-0 bonds. This confirms the existence of vanadylic nature in V,O,, of a VO, unit in MgV,O, (although one of the terminal oxygens actually bridges to another V ion), and the presence of four almost equivalent bonds in Mg3(V04), . In Mg,V,07 the ‘tetrahedra’ are more asymmetric.This situation looks similar to that observed in aqueous solution, where at low pH the VO; ion and polyoxovana- dates with nearly octahedrally coordinated vanadium are observed, which convert to isolated tetrahedral pyrovana-dates and later to orthovanadate ions with increasing PH.~’ Therefore, this progressive evolution observed in the solid state for the VMgO (MgO-V,O,) system can be attributed to the increasing basic character of the compound caused by increasing the nominal MgO content. The extension to the solid state of this conclusion valid for oxovanadium species in solution, provides a potentially useful concept when real catalysts and materials are taken into consideration.Accord- ingly, we note that vanadia compounds mixed with acidic components (as in the cases of V,O,-MOO, and V,O,-P,O, catalysts) have vanadium in a nearly octa-hedral vanadylic coordination, in contrast with Mg orthova- nadate where it is definitely tetrahedral. The same concept is useful when the surface species on vanadia catalysts sup- ported on metal oxides are considered. One can expect that the acid-base character of the metal oxide strongly influences the nature of the surface species in this sense. This concept agrees with the observation that basic dopants on V,O,-TiO, decrease the vanadylic character of the surface vanadium oxide species, with a decreasing V=O bond order4’ and a significant effect on the catalytic behaviour.The IR and Raman spectra presented and discussed here represent reference data which allow a better structural char- acterization of real catalysts, generally polyphasic, as well as of materials where other techniques (e.g. XRD) cannot give definite information, such as highly amorphous catalysts, MgO-V,O, glasses:’ and ‘monolayer’-type supported catalysts. In particular, comparison with the present data was useful for the identification of surface species observed on vanadia-titania catalysts.43 Laser Raman experiments were performed at Ecole Centrale de Lyon. We thank Dr. R. Olier for his kind assistance. Part of this work was supported by MURST (Rome, Italy). References 1 M.A. Chaar, D. Patel, M. C. Kung and H. H. Kung, J. Catal., 1987,105,483. 2 K. Seshan, H. M. Swaan, R. H. H. Smits, J. G. VanOmmen and J. R. H. Ross, in New Developments in Selective Oxidation, ed. G. Centi and F. Trifiro, Elsevier, Amsterdam, 1990, p. 505. 3 D. Siew Hew Sam, V. Soenen and J. C. Volta, J. Catal., 1990, 123,417. 4 D. Bhattacharyya, S. K. Bej and M. S. Rao, Appl. Catal. A, General, 1992,87,29. 5 R. Burch and E. M. Crabb, Appl. Catal. A, General, 1993, 100, 111. 6 J. Hanuza, B. Jezowska-Trzebiatowska and W. Oganowski, J. Mol. Catal., 1985,29, 109. 7 G. Busca, F. Cavani, G. Centi and F. Trifiro, J. Catal., 1986, 99, 400. 8 J. C. Volta, K. Bere, Y. J. Zhang and R. Olier, in Catalytic Selec- tiue Oxidation, ed. T. Oyama and J.Hightower, American Chemical Society, Washington, DC, 1993, pp. 217-230. 9 G. Busca, in ref. 8, pp. 168-182. 10 V,0,-Ti02 Eurocat standard catalyst, ed. G. C. Bond and J. C. Vedrine, Catal. Today, in the press. 11 P. M. Michalakos, M. C. Kung, I. Jahan and H. H. Kung, J. Catal., 1993, 140, 226. 12 G. Busca, G. Ramis and V. Lorenzelli, in New Deuelopments in Selective Oxidation, ed. V. Cortes Corberan and J. L. G. Fierro, Elsevier, Amsterdam, in the press. 13 R. Kohlmuller and J. Perraud, Bull. SOC. Chim. Fr., 1964,3, 645. 14 R. Wollast and A. Tazairt, Silicates Ind., 1969,34,42. 15 E. I. Speranskaya, Inorg. Muter., Engl. Trans., 1971,7, 1611. 16 R. C. Kerby and J. R. Wilson, Can. J. Chem., 1973,51, 1032. 17 G. M. Clark and R.Morley, J. Solid State Chem., 1976, 16,429. 18 N. Krishnamakhari and C. Calvo, Can. J. Chem., 1971,49,1630. 19 R. Gopal and C. Calvo, Acta Crystallogr., Sect. B, 1974, 30, 249 1. 20 E. E. Sauerbrei, R. Faggiani and C. Calvo, Acta Crystallogr., Sect. B,1974,30, 2907. 21 H. N. Ng and C. Calvo, Can. J. Chem., 1972,50,3619. 22 H. G. Bechman, F. R. Ahmed and W. H. Z. Barnes, 2. Kristal-logr., 1961, 115, 110. 23 W. G. Fateley, F. R. Dollish, N. T. McDevitt and F. F. Bentley, Infrared and Raman Selection Rules for Molecular and Lattice Vibration: The Correlation Method, Wiley, New York, 1972. 24 J. C. Decius and R. M. Hexter, Molecular Vibrations in Crystals, McGraw-Hill, New York, 1977. 25 J. M. Stencel, Raman Spectroscopy for Catalysis, Van Nostrand, New York, 1990. 26 P. Tarte, Spectrochim. Acta, 1962, 18,467. 27 E. J. Baran and P. J. Aymonino, 2. Anorg. Allg. Chem., 1969, 365, 211. 1170 28 0.S. Owen and H. H. Kung, J. Mol. Catal., 1993,79,265. 29 T. Dupuis and V. Lorenzelli, J. Therm.Anal., 1969, 1, 15. 30 P. Tarte and J. Thelen, Spectrochim. Acta, Part A, 1972, 28, 5. 31 F. D. Hardcastle and I. E. Wachs, J. Phys. Chem., 1991,%, 5031. 32 J. C. Pedregosa, E. J. Baran and P. J. Aymonino, 2. Anorg. Allg. Chem., 1974,404,308. 33 D. Patel, M. Kung and H.H. Kung, in Proc. 8th Int. Congr. Catal., Chemical Institute of Canada, Calgary, 1988, p. 1554. 34 T. R. Gilson, 0.F. Bizri and N. Cheetham, J. Chem. SOC., 1973, 291. 35 C. Sanchez, J. Livage and G. Lucazeau, J. Raman Spectrosc., 1982, 12, 68. 36 K. Mocala and S. Ziolkowski, J. Solid State Chem., 1987, 69, 299. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 37 G. Ramis, G. Busca and V. Lorenzelli, J. Chem. SOC., Farday Trans., 1994,!40, in the press. 38 G. Busca and J. C. Lavalley, Spectrochim. Acta, Part A, 1986,42, 443. 39 G. Busca, G. Ramis and V. Lorenzelli, J. Mol. Catal., 1989, 50, 231. 40 W. P. Griffth and P. J. B. Lesniak, J. Chem. SOC.A, 1969, 1066. 41 G. Ramis, G. Busca and F. Bregani, Catal. Lett., 1993, 18,299. 42 A. Tsuzuki, K. Kani, K. Watari and Y. Torii, J. Muter. Sci., 1993,28,4063. 43 L. Lietti, P. Forzatti, G. Ramis and G. Busca, Appl. Catal. B, Environmental, 1993,3, 13. Paper 3/06456E; Received 28th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001161

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(8), 1171-1175 Mossbauer Study of Oxygen-deficient Zn"=bearing Ferrites (Zn,Fe,-,O,-~ 0 Ix 5 1) and their Reactivity toward CO, Decomposition to Carbon Masahiro Tabata, Kazuhiro Akanuma, Takayuki Togawa, Masamichi Tsuji and Yutaka Tamaura" Department of Chemistry, Research Center for Carbon Recycling & Utilization, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152,Japan Oxygen-deficient Zn'l-bearing ferrites (Zn,Fe,-,O,-, , 0 <x < 1, 6 > 0) have been synthesized and studied for their reactivity in the decomposition of CO, to carbon at 300 "C. They were prepared by reducing 2n"-bearing ferrites with H, gas at 300°C.The oxygen-deficient 2n"-bearing ferrites consisted of a single phase of a spinel-type structure which was oxygen-deficient compared with stoichiometric composition.Their lattice con- stants were larger than those of the corresponding stoichiometric spinel. Decomposition of CO, to carbon was accompanied by an oxidation of the oxygen-deficient 2n"-bearing ferrite. The amount of carbon deposited on the solid decreased when the Zn content in the 2n"-bearing ferrite increased. The decrease in decomposition rate is due to changes in the electron conductivity according to the Zn content in the Znl'-bearing ferrite. These changes may contribute to its reactivity for decomposition of the CO, to carbon. We have reported the synthesis of an oxygen-deficient mag- netite and its reactivity in the decomposition of CO, to carbon at 300°C.' The oxygen-deficient magnetite is a non- stoichiometric magnetite with spinel structure.It can be obtained by reducing magnetite with H, at 300 "C. Generally, magnetite exhibits deviations from stoichiometry with an excess of oxygen. However, the present magnetite is non- stoichiometric due to oxygen deficiency. This non-equilibrium oxygen-deficient magnetite was found to decompose CO, to carbon at around 300°C. The reaction was carried out by means of a batch system. In this reaction, a decrease in the amount of CO, injected was accompanied by an evolution of CO. No CO, or CO was evolved after completion of the reaction. No other gases were observed during the reaction. The reduction of CO, was due to the incorporation of the oxygen of CO, into the oxygen-deficient site of magnetite during the deposition of carbon. More recently, Mn"-bearing ferrites with the same spinel structure were studied in order to determine the effect of sub- stitution of divalent metal ions into magnetite on its reacti- vity towards decomposition at CO,.,p3 The oxygen-deficient Mn"-bearing ferrite could be synthesized by H, reduction of Mn"-bearing ferrites at 300°C.These ferrites were found to cause decomposition of CO,. The amount of deposited carbon was small compared with that obtained by the reac- tion with oxygen-deficient magnetite. An increase in the Mn content in Mn"-bearing ferrite resulted in a decrease in the amount of deposited carbon. This could be explained by the difference in electron conductivity between Mn"-bearing ferrite and magnetite.In the present paper, we have studied the synthesis of oxygen-deficient Zn"-bearing ferrites (ZnxFe3-xO, -d, 0 < x < l), their Mossbauer spectra and their reactivities for C02 decomposition. The effects of the level of substitution of metal ions and the difference in reactivity for CO, decompo-sition are also discussed. Experimental Materials All of the chemicals employed were of analytical grade, and distilled water was used for preparation of the solution. FeSO, -7H,O, ZnSO, 7H2O and NaOH were supplied by 1 Wako Chemical Industries, Ltd. Preparation of Magnetite and Zn"-bearing Ferrite Magnetite and Zn"-bearing ferrites (Zn,Fe3 -x04-d, 0 < x < 1) were synthesized by oxidation in air of aqueous suspensions of Fe" and Zn" mixed hydr~xide.~ The requisite quantities of FeSO, .7H,O and ZnSO, 7H20 were dis- solved in C0,-and 0,-free water (4 dm3) prepared by passing nitrogen gas through distilled water for a few h. The solution was adjusted to pH 10 by adding 3 mol dm-3 NaOH solution. Then, air was passed through the alkaline suspension for 6 h at 65°C. The reaction pH was kept con- stant at pH 10 by adding NaOH solution. The product was collected by decantation, washed with distilled water and acetone successively, and then dried in UQCUO at 65°C. The dried samples were placed in a quartz tube and heated in an N, stream for 1-3 h at 300"C. The products were identified by means of X-ray diffracto- metry with Cu-Ka radiation (Rigaku model RNT-2000 diffractometer) and Mossbauer spectroscopy. The lattice con- stants of the samples were calculated by extrapolating the values of a, vs.the Nelson-Riley function, cos2 8/ sin 8 + cos28/8, to zero using the least-squares All of the Mossbauer spectra were recorded at room tem- perature with a 57C0source diffused in metallic Rh, which was oscillated in constant acceleration mode. The spectra were calibrated with a thin absorber comprising an a-Fe foil. The chemical compositions of the products were determined by induced coupled plasma atomic emission spectroscopy (ICP-AES) ( Seiko Instruments model SPS 7000) for analysis of the Zn2 + and Fetotal contents, and colorimetry' (Hitachi Model Photospectrometer 124) with 2,2'-bipyridine for the Fe2 and Fetotal molar ratio.The surface areas of the samples + were determined by the BET method using N, adsorption (Yuasa Ionics model Quantasorb). Reactivity of Oxygen-deficient Magnetite and ZnII-bearing Ferrite towards CO, Decomposition Magnetite or Zn"-bearing ferrite (1.00 g) was placed in a quartz cell (20 mm in diameter and 200 mm long), as in pre- vious The cell was heated to 300°C in an electric furnace while the reaction cell was evacuated with an oil rotary pump. After evacuating the reaction cell for 5 min at 300 "C, H, gas was passed through the ferrite at a flow rate of 0.018 dm3 min-' for 2 h at 300°C. After the H,-reduction process, the reaction cell was evacuated and CO, gas (2 x dm3 or 8.32 x lo-' mol) was introduced with a microsyringe (recorded as a zero reaction time).The internal gas species were determined by gas chromatography with a thermal conductivity detector (TCD) (Shimadzu model GC-gA, using Porapak Q and molecular sieve 13X as adsorbents). The solid sample was quenched after the reac- tion by quickly placing the reaction cell into a refrigerant of ice. The solid phases of the reduced samples were analysed using Mossbauer spectroscopy. The solid phases of the sample before and after the CO, decomposition reactions were identified by X-ray diffractometry with Cu-Ka radi- ation. The amounts of carbon deposited on the samples were measured using an elemental analyser (Perkin-Elmer model 2400 CHN).The surface of the quenched samples after decomposition of CO, was analysed by means of FTIR spec- troscopy (Shimadzu model FT-IR 8500) with the KBr disc technique. Results and Discussion Characterization of Magnetite and Zn"-bearing Ferrite Only peaks of the cubic spinel structure appeared in the X-ray diffraction (XRD) patterns of the prepared magnetite and Zn"-bearing ferrites. The lattice constants, chemical com- positions and BET surface areas of the samples are shown in Table 1. The surface area was smallest for a Zn :Fe mole ratio of 0.0 : 1 (magnetite) and largest for a Zn : Fe mole ratio of 0.495 : 1. The lattice constant increased as Zn" content in the reaction solution increased. Chemical analysis showed that Zn" content in the sample increased with an increase in the Zn" content in the reaction solution.These findings confirm the Fe" ions in the spinel ferrite were replaced by Zn" ions and that the amount of Zn" ions on a lattice point increased with increasing Zn :Fe mole ratio of the reaction solution. The accuracy of the lattice constant of the sample with a Zn :Fe mole ratio of 0.495 : 1 was low, owing to the smaller crystal grain size compared with that of the other samples. The chemical compositions of the samples with Zn :Fe mole ratios of 0 : 1 (magnetite) and 0.0964 : 1 showed deviations from the stoichiometric composition. These com- positions could be expressed as 0.70Fe,0,-O.30y-Fe,O3 and 0.264ZnFe,04-0. 588Fe ,O4-0.224y-Fe20 , respectively, assuming that the compounds are solid solutions of stoichio- metric Zn"-bearing ferrite, ZnxFe3 -x04, and maghemite, y-Fe,03 .The samples with Zn, Fe mole ratios of 0.334: 1 and 0.495 :1 showed nearly stoichiometric compositions. Characterization of Oxygen-deficient Magnetite and Zn"- bearing Ferrite The XRD patterns of the H,-reduced magnetite and Zn"- bearing ferrites showed only peaks assigned to the cubic spinel structure. The lattice constants of these compounds increased upon reduction with H, for 2 h (Table 1). The lattice constant increased as Zn" content in the oxygen- deficient Zn"-bearing ferrite increased. These lattice constants J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 were larger than those of the corresponding spinels of stoi- chiometric composition. All of the reduced samples were oxygen-deficient compared with the stoichiometric composi- tion.These changes in chemical composition indicate that some Fe3+ ions in the Zn"-bearing ferrites were reduced to Fe2+ ions. These ferrites are unstable in air and easily oxi- dized at room temperature. Qian et aL9 have studied the sta- bility of Zn"-bearing ferrite with a Zn :Fe mole ratio of 0.5 : 1 in an H, atmosphere, and found that the Zn" ferrite decreased in weight in an H, flow while heating at a rate of 30°C h-'. They reported that the reduction of Zn" ferrite took place at 495°C where the solid phase gave a mixture of a-Fe and ZnO. Our recent study on the reduction of Zn" ferrite, ZnFe,O,, at 300°C '* showed that Zn" ferrite was reduced to a mixture of ZnO and FeO (Wustite) after 12 h H, reduction at a flow rate of 0.20 dm3 min-'.Therefore, the H,-reduced Zn"-bearing ferrite prepared in the present study was the oxygen-deficient Zn"-bearing ferrite. This is a new metastable phase of Fe-Zn oxide. We could thus prepare oxygen-deficient Zn"-bearing ferrites with spinel structure. Miissbauer Spectra Magnetite The Mossbauer spectra of magnetite are similar to those reported previo~sly.~ The spectrum gave the usual two sextets. A sextet with a smaller isomer shift was assigned to the Fe3+ ions in the tetrahedral site (site A) and the other was assigned to the Fe2+ and Fe3+ ions in the octahedral site (site B).The Mossbauer parameters are presented in Table 2 The site A to site B area ratio (AJAB) was 0.85, which is larger than that of stoichiometric magnetite. The larger value is due to the presence of Fe3+ ions which do not contribute to the electron hopping between B sites. The absorption due to these Fe3+ ions does not contribute to the site B spectra but on to the site A spectra." The Mossbauer patterns and the parameters of oxygen- deficient magnetite were similar to that of the unreduced magnetite (Table 2).3 However AJAB was 0.537, which is larger than that when the composition is stoichiometric (0.5). Since no other phase was observed in the XRD and Moss- bauer spectrum of the oxygen-deficient magnetite, the excess Fe2+ ions must be one of the constituents of the oxygen- deficient magnetite.These excess ions may be distributed among the interstices of the spinel structure. Thus, the increase in AJAB can be ascribed to the contribution of Fe2+ ions in the tetrahedral interstices which have migrated from the B sites. Unreduced Zn"-bearing Ferrite Mossbauer spectra of the Znn-bearing ferrite with a Zn :Fe mole ratio of 0.0964 : 1 (a), 0.334 : 1 (b) and 0.495 : 1 (c) are shown in Fig. 1. The parameters in Table 2 were evaluated by fitting the full spectra to two sextet subspectra with Lorentz- ian lineshapes. The spectrum of sample (a) consisted of a sharper sextet with a smaller isomer shift and a broader one with a larger isomer shift. The sextet with lower isomer shift Table 1 Chemical compositions, lattice constants and surface areas of Zn"-bearing ferrites before and after H2 reduction chemical composition lattice constant, ao/nm before reduction after reduction 0.8391 0.8399 0.8406 0.8409 0.8420 0.8435 0.8449 0.8450 Zn :Fe 0: 1 0.0964: 1 0.334 : 1 0.495 : 1 before reduction Fe3.0004.09 zn0.264Fe2.7404.08 zn0.752Fe2.2504.00 zn0.995Fe2.0104.00 after reduction Fe3.0003.93 zn0.264Fe2.7403.97 zn0.752Fe2.2503.94 Zn0.995Fe2.0103.96 surface area/m2 g-' 8.07 16.63 13.00 29.45 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Mossbauer parameters of magnetite and Zn"-bearing ferrites before and after H, reduction isomer shift/mm s -quadrupole splitting/mm s -magnetic hyperfine field/MA m -Zn : Fe AJAB A B before reduction after reduction 0: 1{ 0.850 0.537 0.290 0.273 0.662 0.662 0.0964: 1 before reduction after reduction 0.458 0.311 0.307 0.295 0.593 0.603 0.334: 1 before reduction after reduction a a 0.495 : 1 before reduction after reduction 0.352 0.357 Spectrum could not be decomposed to one sextet and one doublet.may correspond to the site A spectrum and the other one to the site B spectrum. AJAB of the subspectra was 0.458. If all of the Fe ions in site B contribute only to the site B sub-spectrum, AJAB should be 0.368. This may be caused by some Fe3+ ions in site B. If the Zn"-bearing ferrite is not a solid solution of stoichiometric Zn"-bearing ferrite and maghemite (y-Fe203) but a mixture of them, the chemical composition of the sample with a Zn: Fe mole ratio of 0.0964 : 1 can be expressed as 0.851Zn0.,loFe2~6904~oo-0.2~~y-Fe2~3. A,/AB of the sub- spectra can be calculated according to the pair-wise localized electron-hopping model of Daniel and Rosencwaig l1 using -10 -5 0 5 10 velocity/mm s -Fig.1 Mossbauer spectra of Zn"-bearing ferrite with a Zn : Fe mole ratio of (a)0.0964 : 1, (b)0.334 : 1 and (c)0.495 : 1. (-) Least-squares fit. A B A B 0.0252 -0.0256 -38.86 -36.80 0.000392 -0.00352 -38.90 -36.85 0.0371 -0.0101 -38.54 -35.46 0.0272 0.00926 -38.30 -35.52 0.414 0.394 the follow equation (all Fe ions in site A of Zno~31Fe2~6904~oo) + (all Fe ions in y-Fe203) AJAB = (1)all Fe ions in site B of Zno~31Fe2~6904~oo The calculated AJAB was 0.664, which is larger than the empirical value of 0.458.Hence, the Zn"-bearing ferrite is not a physical mixture of stoichiometric Zn"-bearing ferrite and y-Fe,O, , but the solid solution. The spectrum of the sample with a Zn : Fe mole ratio of 0.334 : 1 comprised a broad sextet and a sharp doublet [Fig. l(b)]. The latter was ascribed to a quadrupole splitting. The broad sextet could not be resolved and the Mossbauer parameters are not given in Table 2. The spectrum of sample (c) showed only a quadrupole splitting. The appearance of quadrupole splitting patterns for samples (b) and (c) may be due to domain-wall oscillations, as suggested by Srivastava et a1.12.13 Oxygen-deficient Zn"-bearing Ferrite The Mossbauer spectra of the H,-reduced sample with Zn : Fe mole ratios of (a') 0.0964 : 1, (b')0.334 : 1 and (c') 0.495 : 1 are shown in Fig.2. The spectra were similar to that of the unreduced Zn"-bearing ferrite, indicating that the syn- thesized H,-reduced Zn"-bearing ferrite contains no metallic Fe component. The spectra of sample (a')could be separated into two sextets with Lorentzian lineshapes. The site B spec-trum became sharp after H, reduction. This indicates an increase in the number of Fe ions which take part in the elec- tron hopping between the B sites. The isomer shift, quadru- pole splitting and magnetic hyperfine field were nearly equal to those of the unreduced samples (Table 2).This suggests that the Fe2+ : Fe3+ mole ratio in site B is not changed. AJAB was 0.311, which was smaller than that of the unre- duced sample (0.458) and that of the sample with stoichio- metric composition (0.368).Therefore, the amount of Fe ions in site A decreased and the amount of Fe ions in site B increased, while the Fe2+ : Fe3+ mole ratios in site B remained constant. Therefore, some of the Fe3+ ions in site A were allowed to migrate to site B, and some of the migrant Fe3+ ions were reduced to Fe2+ ions. The pattern of migra- tion during the formation of oxygen-deficient Zn"-bearing ferrite may be different from that of oxygen-deficient magne- tite. As mentioned above, Fe ions in site B of magnetite will migrate to site A.This difference may be due to the presence of Zn ions in site A. The Mossbauer spectrum of sample (b') showed a broad sextet and a sharp doublet (Fig. 2). The shape of the pattern was very similar to that of the unreduced sample. The spectra 1174 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 10 I -10 -5 0 5 10 velocity/mm s-l Fig. 2 Mossbauer spectra of oxygen-deficient ZnII-bearing ferrite with a Zn : Fe mole ratio of (a') 0.0964: 1 (b') 0.334: 1 and (c') 0.495 : 1. (-) Least-squares fit. could not be fitted to one Lorentzian sextet and one quadru- pole splitting pattern. The details are not clear and need to be studied. The Mossbauer spectrum of sample (c')shows only a quad- rupole splitting (Fig. 2). The spectra could be analysed as a single quadrupole splitting.The Mossbauer parameters were nearly equal to that of the unreduced Zn"-bearing ferrite. This indicates that changes in the Fe2+ : Fetota, do not affect the parameters in the region of Zn : Fe, where their spectra consist only of quadrupole splitting. CO, Decomposition with Oxygen-deficient Zn"-bearing Ferrites Decomposition of CO, was studied on sample (Q') at 300°C (Fig. 3). The initial CO, content in the reaction cell was 8.2 kPa. The CO, content decreased and became <lo% of the initial amount of CO, injected after 30 min and decreased to 30% after 4 h. On the other hand, CO was formed imme- diately after injection of CO, and its content increased up to 10% of the injected CO,. The amount of CO gradually decreased and became <3% of the initial CO, amount after 4 h.No other gases were observed during the reaction. The lattice constant of the ferrite was restored to that prior to H,-reduction after CO, decomposition. Fig. 4 and 5 show the time variations of the gas content during the reaction of CO, with samples (b')and (c'),respectively. A similar pattern for decrease in CO, and evolution of CO could be seen for 0 0 b 0" 0,." 0 100 200 ti me/mi n Fig. 3 Changes in the composition of CO, (0) duringand CO (0) the CO, decomposition reaction with oxygen-deficient Zn"-bearing ferrite (Zn : Fe-0.0964 : 1) at 300°C both samples. The amount of CO evolved was small with sample (c'). Carbon particles were observed after dissolving the reacted ferrites.The amount of carbon deposited was determined by dissolving reacted ferrites in HC1 solution, 0.874, 0.478 and 0.312 mg for samples (a'), (b') and (c'), respectively. The amounts of carbon were small compared with those esti- mated from the gas content. No peaks for CO and carbon- ates were observed in the IR spectrum of the ferrite after C02 0""b----0 0 0 100 200 time/min Fig. 4 Changes in the composition of CO,(O) and CO (0)during the CO, decomposition reaction with the oxygen-deficient Zn"- bearing ferrite (Zn : Fe = 0.334 : 1)at 300 "C I" I b - IY n 4-l "n 0 100 200 time/m i n Fig. 5 Changes in the composition of CO, (0)and CO (0)during the CO, decomposition reaction with oxygen-deficient Zn"-bearing ferrite (Zn : Fe = 0.495 : 1) at 300°C J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 decomposition. The carbonate compound (CaCO,) cannot be detected by our equipment when the C : ferrite weight ratio is below 0.001 : 1 on a weight basis. Therefore, the amount of carbon deposited after CO, decomposition could not be detected by means of IR spectroscopy, because the maximum amount of carbon deposited is 1.00 mg in the present experi- ment. In situ surface and bulk investigations of the sample during the CO, decomposition reaction will provide informa- tion on the intrinsic physicochemical properties of the ferrite. Further work is in progress. The smaller recovery of carbon can be explained as follows.Two types of carbon deposited on the metallic Ni catalyst have been rep~rted.'~ The c1 form of carbon is considered to consist of isolated carbon atoms on the surface and the p form is assigned to polymerized carbon. In the present work, at least two types of carbon were probably deposited on the surface. The elementary carbon could not be collected in the present procedure of dissolving the sample in HCl solution, because it would be washed out. Therefore, the reaction of oxygen-deficient Zn"- bearing ferrite with CO, may be expressed as the following stepwise processes : physical adsorption [eqn. (I)], chemical decomposition of adsorbed CO, to toad and Olattice[eqn. (11)] and further decomposition of adsorbed CO to carbon Csurface and Olattice (eqn. (WI.c02 c02 ad 'O2 ad Oad + Olattice (11) COad +Csurface + Olattice (111) The relationship between the amount of carbon deposited and the Zn :Fe shows that an increase in Zn content resulted in a lowering of the amount of deposited carbon [Fig. 6(a)]. This is in good agreement with the change in electron con- ductivity calculated from the electric resistivity data by Dobson et a1." [Fig. qb)].This can be interpreted in forms 0 1.2 EiTc,.-g 0.8 n -0 0.4 0.0 0.0 0.2 0.4 Zn/Fe c I E T3r 4-.-.-> c, $2 U s 1 4-Q, Q, 00.0 0.2 0.4 Zn/Fe Fig. 6 Relationship between Zn : Fe mole ratio and (a)the amount of deposited carbon after CO, decomposition, (b) the electric conduc- tivity, based mainly on the data of Dobson et ~1.'~ .; of the relationship between Zn content in Zn"-bearing ferrite and the charge-transfer process.The electric resistivity increases slowly with a Zn : Fe mole ratio of up to about 0.25 and rapidly increases at a Zn : Fe mole ratio of >0.25 : 1. The variation of the electric resistivity is in agreement with the variation of the Mossbauer spectra with Zn : Fe mole ratio." As noted above, two sextet peaks were observed in a region where the mole ratio of Zn :Fe was <0.25, i.e. rapid electron hopping among B sites occurs. In the region of Zn : Fe > 0.25 : 1, a quadrupole splitting became the main feature of the absorption spectrum. Conclusion Oxygen-deficient Zn"-bearing ferrites were synthesized by reducing Zn"-bearing ferrites with H, gas for 2 h at 300°C.Their lattice constants (ao)were larger than those of the unre- duced forms. The Mossbauer spectrum of the sample with a Zn : Fe mole ratio of 0.0994 after H, reduction showed a sharpening of the B site spectrum. For the samples with Zn : Fe = 0.334 : 1 and 0.495 : 1, the Mossbauer spectra were slightly changed by H, reduction. These oxygen-deficient Zn"-bearing ferrites have been found to be reactive towards CO, decomposition at 300°C. The CO, oxygen was incorp- orated into the solid phase, accompanied by the deposition of carbon on the surface of the oxygen-deficient Zn"-bearing ferrite. CO, may be decomposed to carbon with the concomi- tant formation of CO.The amount of carbon deposited decreased as the mole ratio of Zn : Fe increased. The amount of deposited carbon decreased in the case of the reaction with samples of higher Zn :Fe mole ratio. This relationship between the amount of carbon deposited and the Zn content can be understood in terms of the electron conductivity of the Zn"-bearing ferrite. The present work was partially supported by a Grant-in-Aid for Scientific Research No. 03203216 from the Ministry of Education, Science and Culture, Japan. M. T. and K. A. are grateful for grants from Fellowships of the Japan Society for the Promotion of Science for Japanese Junior Scientists. References 1 Y. Tamaura and M. Tabata, Nature (London), 1990,346,255. 2 M. Tabata, Y. Nishida, T. Kodama, K. Mimori, T. Yoshida and Y. Tamaura, J. Muter. Sci., 1993,28,971. 3 M. Tabata, K. Akanuma, K. Nishizawa, T. Yoshida, M. Tsuji and Y. Tamaura, J. Muter. Sci., 1993,243,6753. 4 T. Kanzaki, J. Nakajima, Y. Tamaura and T. Katsura, Bull. Chem. SOC. Jpn., 1981,54,135. 5 J. B. Nelson and D. P. Riley, Proc. Phys. SOC. London, 1945, 57, 160. 6 A. Taylor and H. Sinclair, Proc. Phys. SOC. London, 1945, 57, 126. 7 R. H. Geiss, Adu. X-Ray Anal., 1961, 5, 71. 8 I. Iwasaki, T. Katsura, T. Ozawa, M. Yoshida, M. Mashima, H. Harashima and B. Iwasaki, Bull. Volcanol. SOC. Jpn. Ser. ZZ, 1960,5, 9. 9 Y-T, Qian, R. Kershaw, S. Soled, K. Dwight and A. Wold, J. Solid State Chem., 1984, 52, 211. 10 T. Kodama, M. Tabata, K. Tominaga, T. Yoshida and Y. Tamaura, J. Muter. Sci., 1993,243, 547. 11 J. M. Daniel and A. Rosencwaig, J. Phys. Chem. Solids, 1969, 30, 1561. 12 C. M. Srivastava, S. N. Shringi and R. G. Srivastava, Phys. Rev. B, 1976,14,2041. 13 C. M. Srivastava and M. J. Patni, J. Magn. Reson., 1974, 15,359. 14 J. G. McCarty and H. Wise, J. Catal., 1979,57,406. 15 D. C. Dobson, J. W. Linnett and M. M. Rahman, J. Phys. Chem. Solids, 1970, 31, 2727. Paper 3/0479OC; Received 9th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001171

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(8), 1177-1182 Activation of Surface Lattice Oxygen in the Oxidation of Carbon Monoxide on Silica Yasuyuki Matsumura and John B. Moffat* Department of Chemistry and Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 Keiji Hashimoto Osaka Municipal Technical Research Institute, Joto-ku, Osaka 536,Japan Silica prepared by the sol-gel method from ethyl orthosilicate catalyses carbon monoxide oxidation at 850 K or above while no such activity is found with a commercial silica prepared from inorganic reagents. The activity of the silica prepared by the sol-gel method is increased by repetition of the carbon monoxide oxidation reaction. Labelled reactions with '*O, provide evidence to suggest that most of the lattice oxygen species on the surface of the silica participate in the reaction.After evacuation at 950 K or above, radical species identified as Si-0- -0--Si are generated on the silica. Si-0--Si radical species are apparently formed during the reaction with the radical electron originating from the Si-0-0--Si species in the initial stage of the reaction. Adsorp- tion of carbon monoxide and oxygen on the Si-0--Si species is proposed to cause formation of carbon dioxide and 0-which will react with carbon monoxide and provide the radical electron to the lattice oxygen species, that is, Si-0-Si and Si-0-0-Si. The present work shows that the lattice oxygen of silica can be involved in oxidation if the oxygen is activated by radical species such as 0-, which have been proposed as active species in catalytic oxidation processes.Inorganic substances with high surface areas are frequently employed as supports for catalytically active materials to increase the number of active sites and improve the catalytic properties. Hence, clarification of the effect of the support is of great importance in the design of good catalysts. Amorp- hous silica is a representative support with high surface area and is not infrequently used in studies of oxidation processes, such as the partial oxidation of methane.'-20 It is known that not only catalytic activity but also product selectivity often changes with the nature of the support, especially in methane oxidation where the contribution of the support to the reac- tion is not yet understood2'.'' although the structure of amorphous silica is well known.23 Although there are no effectively strong acid-base sites on its ~urface,'~-~~ silica itself often functions as a ~atalyst.~,~,~~-~~Work in this laboratory and elsewhere has shown that silica can catalyse the oxidation of methane; however, the products generated depend on the oxidant ~~ed.~*~*~~-~~ The product distribu- tion from the oxidation of methane varies with the sample of silica even though the samples are apparently similar in com- p~sition.~,~~-~~Alth ough the surface area of silica has been suggested as an important factor in its catalytic activity,29 changes in the product distribution cannot be accounted for by changes in surface area alone and thus special sites for oxidation apparently exist or are formed during the reaction.Silica prepared by the sol-gel method from ethyl orthosili- cate yields a high selectivity to carbon dioxide in the oxida- tion of methane compared with a commercial silica prepared from inorganic compounds, although the surface areas of the products prepared by these two methods are similar.32 Hence there are features of these two catalysts which are distinguish- able and the identification of such differences may assist in the clarification of the mechanism of oxidation. In the present paper, carbon monoxide oxidation, which is stoichiometrically one of the simplest oxidation processes, has been studied on silica prepared by these two methods to provide information on the differences between these two materials.Somewhat surprisingly, almost all of the lattice oxygen atoms on the surface of silica prepared by the sol-gel method appear to be reactive to carbon monoxide. The lattice oxygen is probably activated by radical oxygen species such as 0-,which has often been suggested as the active oxygen species in oxidation processes. Thus, the lattice oxygen on silica employed as a support for oxidation cata- lysts apparently is an active participant in the oxidation process. Experimental 0.2 mol of ethyl orthosilicate [Kanto Chemical Corp. (G.R. Grade)] was hydrolysed to silica gel with 0.05 dm3 of 5 mol dm-3 nitric acid solution.The hydrogel obtained was dried at CQ. 400 K in air, washed with water and heated at 720 K in air for 5 h in order to remove residual hydrocarbons or nitric acid. Analysis by atomic absorption spectroscopy showed that this sample (denoted Si02-0) has no observable impu- rities. For comparison purposes, a commercial silica sample prepared from inorganic compounds (Cariact- 15) was obtained from Fuji-Davison Chemical Ltd. The BET surface areas of these samples in powdered form were 245.1 and 177.9 m2 g- ',respectively. The oxidation of carbon monoxide was carried out in a static reactor of dead volume ca. 0.1 dm3 equipped with a Baratron pressure gauge which enabled continuous recording of the pressure in the reactor.The catalysts were held in a quartz tube of 6 mm inner diameter. No reaction was observed without the catalyst even at 950 K. The reactant gas contained carbon monoxide and oxygen in a molar ratio of 2 : 1. Tests showed that the reactant gases were well dispersed in the reactor below a pressure of 100 Pa. The reaction pro- ducts were analysed with a mass spectrometer (ULVAC MSQ-150A). Labelled oxygen (' *02, 98.58%) was obtained from Sen Saclay and was used without further purification. Electron paramagnetic resonance (EPR) spectra were recorded with a JEOL JES-RE2X spectrometer at 9.15 GHz (modulation, 100 kHz, 0.02 mT; time constant, 1 s) usually at 100 K. The microwave power was kept at 0.01 mW to prevent saturation of the spectra.The g value was determined with diphenylpicryl hydrazyl (DPPH, g = 2.0036). The silica 1178 sample (0.05 g) was placed in a quartz EPR tube in which pretreatment and activation of the sample was carried out. Surface analysis by X-ray photoelectron spectroscopy (XPS) was carried out using a Shimadzu ESCA 750 spec- trometer. The sample was mounted with adhesive tape in air just after the pretreatment or activation and set into the spec- trometer. Charge correction of the XPS data was accom-plished by assuming that the binding energy of the C 1s peak is 284.6 eV. IR spectra were recorded with a Nicolet 5DX FTIR spec- trometer. The silica sample (0.01 g) was pressed into trans- lucent self-supporting wafers and placed into an IR cell, equipped with KBr windows, which allowed pretreatment in uucuo and activation of the sample in a separate portion of the cell assembly.Results Oxidationof Carbon Monoxide The experiments involving the oxidation of carbon monoxide were generally performed, under a given set of conditions, in a sequential fashion. Carbon monoxide and oxygen were introduced to Si0,-0 preheated in vacuo (ca. low3Pa) at 950 K for 0.5 h. Although the activity of the catalyst evac- uated at 950 K for 0.5 h was very low in the first carbon monoxide oxidation [Fig. l(a)], the catalyst preheated at 950 K for 5 h in uucuo exhibited significantly higher activity in the initial experiment [Fig. l(b)]. When the catalyst was preheat- ed at 1050 K for 0.5 h in vacuo, the reaction rate increased abruptly after an induction period of 1 min [Fig.l(c)]. Evac- uation at 950 K for 0.5 h followed by preheating at 950 K for 0.5 h under carbon monoxide (50 Pa) or oxygen (25Pa) did not activate the catalyst. On the other hand, Cariact-15 showed a slight activity at 950 K even after evacuation at 1050 K for 0.5 h or after several experiments following pre- heating at 950 K for 0.5 h in uucuo (not shown). Following activation by subjecting the catalysts to use in a minimum of six carbon monoxide oxidation steps at 950 K, Si0,-0 catalysed the reaction even at 850 K (Fig. 2). The reaction rate was initially slow, although subsequently the rate increased abruptly. For example, the initial rate of carbon dioxide formation was 0.02 mmol min-' g-' at 950°C and the rates at 5 and 10% conversion were 0.06 and 0.07 mmol min-' g-', respectively.The rate at a conversion higher than ca. 10% decreased discernibly with decrease in h ,\" 40 v C .-E > 2 20 0 0 1 2 3 ti me/mi n Fig. 1 Initial activity of silica prepared by the sol-gel method in the oxidation of carbon monoxide. Catalyst : SO,-0, 0.020 g, evac- uated at (a)950 K for 0.5 h, (b) at 950 K for 5 h, (c) at 1050 K for 0.5 h. initial pressure of carbon monoxide, 50 Pa; oxygen, 25 Pa. Reac- tion temperature, 950 K. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2o0 L0 2 4 6 8 ti me/min Fig. 2 Transformation of carbon monoxide to carbon dioxide over silica prepared by the sol-gel method.Catalyst; Si0,-0, 0.0052 g. Initial pressure of carbon monoxide, 50 Pa; oxygen, 25 Pa. Prior to the runs shown here, the catalysts were activated by more than six runs at 950 K. Reaction temperatures of (a) 850, (b) 900 and (c) 950 K are shown. the pressure of the reactant. The apparent activation energy was calculated as ca. 90 kJ mol-' from Arrhenius plots for the rate at 10% conversion. A liquid-nitrogen trap was usually employed to remove carbon dioxide from the reac- tion gas, but no significant change was observed without the trap. The reaction proceeded stoichiometrically while a small amount of hydogen was detected in the reaction products of the initial two reactions. No reaction was observed without oxygen gas at 950 K (Pco = 50 Pa) over the activated cata- lyst.When Si0,-0 was heated with oxygen (Po2 = 25 Pa) for 0.5 h following preheating at 950 K for 0.5 in uacuo, no formation of carbon oxides was observed. Oxidation of Carbon Monoxide with Labelled Oxygen In order to obtain further information on the reaction, carbon monoxide oxidation on Si0,-0 was studied with 1802 contained in the reactant mixture. The catalyst was pre- heated in uucuo for 0.5 h at 950 K. The reaction was repeated on the same aliquot of catalyst after evacuation at 950 K for 0.2 h after each run. The internal pressure was <0.1 Pa when the reaction system was isolated before the reaction. The rate of carbon dioxide production during the first experiment was very low but increased with successive experiments up to a run number of 8 (Fig.3). A small amount of hydrogen was detected in experiments 1 and 2. Note that a considerable quantity of Cl60, was formed in the reaction while the quantity of 1602 in the reactant gas was very small. The accumulated amount of C60,in runs 1-10 was 1.0 mmol g-' and that of Cl8O, and Cl8O was 0.3 mmol g-'. No formation of dioxygen containing l6O was observed in the runs in which 1802was employed as an oxidant. EPR Spectra After evacuation of Si0,-0 at 950 K for 0.5 h, weak but sharp peaks were observed at 326-327 mT in the EPR spec- trum [Fig. 4(a)], and the peak intensity was decreased to ca. 1/3 of the initial intensity after introduction of the reactants (carbon monoxide, 50 Pa; oxygen, 25 Pa) at 950 K for 0.1 h (not shown).The intensity of EPR peaks increased after the next reaction at 950 K and the intensity of the peaks was 4/3 of that in Fig. 4(a) (not shown). The EPR peaks were signifi- cantly intensified with the third reaction at 950 K [Fig. 4(b)]. Before the measurement the sample was evacuated at room J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.10. r I n cn 0.08, I .-C E 5 0.06 E E.-. 0.04 C .-4-;0.02 5+ 0 1 2 3 4 5 6 7 8 9 10111 run number Fig. 3 Formation rates for products of the reaction between CO and 1802 over silica prepared by the sol-gel method. Catalyst: Si0,-0, 0.0047 g. Initial pressure of carbon monoxide, 50 Pa; oxygen, 25 Pa.Reaction temperature, 950 K. The rates were deter- mined by division of products yield by period of the reaction; each run was stopped at conversions below 60%. Solid bar, Cl6O,; cross hatched bar, C'60180; hatched bar, CEO,; open bar, C'*O. Run 11 was carried out with 1602 instead of "02. temperature for 0.1 h. Each reaction was carried out after evacuating at 950 K for 0.2 h following the previous reaction. No significant change in the EPR spectrum was observed on the sample either with or without evacuation at 950 K for 1 h after the reactions. The peaks were also evident in the EPR spectrum obtained at 450 K [Fig. qc)], although the shape of the peaks was broader than that observed at 100 K. The peak intensity increased by a factor of ca.two when the sample was evacuated at 1050 K for 0.5 h compared with the sample evacuated at 950 K for 0.5 h, but there was no significant change between the samples evacuated at 950 K for 0.5 h and 5 h (not shown). No peaks were observed in the spectrum for the sample evacuated at 750 h for 1 h. The shape of the spec- trum was reproduced by simulation using a Lorentzian func- tion [the result is shown in Fig. 4(b) with open circles]. Simulation using a Gaussian function was unsuccessful. The values for g1 and gI1were determined as 2.0023 and 2.0027, respectively, by simulation using a Lorentzian function (linewidth for gl, 0.025 mT; gI1, 0.035 mT). The radical species detected was so stable that the same EPR spectrum was observed after introduction of carbon monoxide, oxygen, hydrogen or air to the silica sample at room temperature, although heating the sample with one of these gases, with the exception of hydrogen, at 950 K for 0.2 h resulted in dimin- ishing the peaks.The EPR peaks appeared again after evac- uation at 950 K for 0.5 h following contact with oxygen or carbon monoxide at 950 K but the peak intensity was similar to that observed in the spectrum of the sample evacuated at 950 K for 0.5 h. No such EPR peaks were detected with Cariact- 15. XPS Analyses In order to obtain information on the air-stable radical species detected by recording EPR spectra, XPS analyses of the silica samples were performed. In the spectrum for Si0,-0 evacuated at 950 K for 0.5 h, the main peaks for Si 2p and 0 1s were present at 104.1 and 533.6 eV, respectively, although small shoulders were also detected [Fig.5(a)]. After three consecutive reactions with 50 Pa of carbon monoxide and 25 Pa of oxygen at 950 K followed by evacuation at 950 K for 0.2 h to ca. Pa, the spectrum for the sample showed a definite shoulder at 101 eV for Si 2p and a small shoulder at 532 eV for 0 Is, while no shift was observed with the main peaks [Fig. 5(b)]. IR Spectra After evacuation of Si0,-0 at 950 K for 0.5 h, the IR absorption band attributed to hydroxy groups was clearly observed at 3747 cm-' (not shown).33 Three consecutive reactions with 50 Pa of carbon monoxide and 25 Pa of oxygen (160,) at 950 K were carried out over the sample.After the final reaction the sample was cooled and evacuated at room temperature for 0.5 h while evacuation at 950 K for 0.2 h was carried out subsequent to the first and second reac- tions. No absorption bands in the range of 1400-4000 cm- attributed to adsorption species were observed after the final reaction and the intensity of the band at 3747 cm-' was almost the same as observed just after the pretreatment. 0 1s Si 2p -1 I I I 326.0 326.2 326.4 326.6 326.8 magnetic field/mT Fig. 4 EPR spectra of the silica prepared by the sol-gel method recorded at 100 K: (a) evacuated at 950 K for 0.5 h; (b)activated by oxidation of carbon monoxide at 950 K three times; (c) recorded at 450 K. Open circles are the results of computer simulations.540 535 530 110 105 100 95 binding energy/eV Fig. 5 XPS bands of the silica prepared by the sol-gel method for 0 1s and Si 2p: (a)evacuated at 950 K for 0.5 h; (b) activated by oxidation of carbon monoxide at 950 K for three times. The inten- sities of the spectra were normalized by using the atomic sensitivity factors of Si 2p (0.17) and 0 1s (0.63). Model and Results of Molecular Orbital Calculations In order to obtain further information on the nature of the active sites, ab initio Hartree-Fock calculations (Gaussian-86)34 were performed using the STO-3G basis set on the model compounds for the surface species of silica. The models used were (HO,),SiOSi(O,H), (labelled SiOSi), (HO,),SiO -Si(O,H), (labelled SiO -Si), (HO,),SiOOSi(O,H), (labelled SiOOSi), and (H0,),SiOO-Si(0,H)3 (labelled SiOO- Si) (Fig.6). Geometry optimization was performed on the Si-0-Si and Si-0-0 angles and the Si-0 and 0-0 bond lengths. The remaining parameters were held constant (Table 1). The internuclear separation between the two Si atoms in model SiOSi was calculated, after optimization, as 0.302 nm while that in model SiO-Si was 0.342 nm (Table 2). On the actual surface of silica, movement of the silicon atoms is restricted by the surrounding atoms. Hence, the SiO-Si model SiOSi model SQSi Fig. 6 Model compounds for Si-0-Si and Si-0-0-Si oxygen bridges Table 1 Structural parameters fixed during optimization bond lengthlnm angleldegrees Si-0, 0.163 0-Si-0, 109.5 0,-H 0.960 Si-0,-H 115.0 Table 2 Structural parameters optimized" bond length/nm andeldegrees model Si-0 Si-Si 0-0 Si-0-Si Si-0-0 SiOSi 0.159 0.302 144.5 SiO-Si 0.184 0.342 136.9 SiO-Si(S) 0.185 (0.302) 109.4 SiOOSi 0.168 0.396 0.141 104.8 SiOO-Si 0.163 0.345 0.197 78.1 SiOO-Si(S) 0.164 (0.396) 0.190 96.2 ~~~~ ~ Values in parentheses were fixed during optimization.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 radical species would be expected to have a structure similar to that of SiOSi. To provide a more realistic model for the radical species, only the position of the bridge oxygen in SiO-Si was optimized and the positions of the other atoms were fixed as in the model SiOSi. The bond length of Si-0 in this model, SiO-Si(S), was approximately equal to that in SiO-Si but significantly longer than that in SiOSi (see Table 2).The total energy of SiO-Si(S) was almost the same as that of SiO-Si (Table 3). The internuclear separation between the two Si atoms in model SiOOSi was 0.396 nm, significantly longer than that in model SiOO-Si (0.345 nm). Hence, a calculation was carried out with model SiOOSi(S) in which only the position of the bridge oxygen atoms was optimized and the positions of the remaining atoms were fixed. The cal- culated bond lengths of Si-0 and 0-0 were very close to those of SiOO-Si (see Table 2). The results of the calcu- lations are summarized in Table 3. The binding energies were obtained from the orbital energies corresponding to the 0 1s and Si 2p orbitals.The atomic spin densities for Si and 0 in model SiO- Si(S) were calculated as 0.33 and 0.80, respec-tively, while those in model SiOO-Si(S) were -0.07 and 0.65, respectively. Discussion Reaction Mechanism of Carbon Monoxide Oxidation In the experiments with labelled oxygen as oxidant significant quantities of C160, (1.0 mmol accumulated in runs 1-10 of Fig. 3) are produced. Since the catalyst is evacuated at 950 K to <0.1 Pa prior to the reaction, the quantity of adsorbed species on the surface is expected to be small and the number of l60included in these residual species should be negligible. Although the formation of Cl6O, could result from the dis- proportionation of carbon monoxide, the total amount of C1802 and Cl80 was only 0.3 mmol and is considerably smaller than that of C1602.In addition, the amount of PO, and Cl80 formed in each run increases with run number and is not proportional to the amount of C160,. Thus, the preponderance of the l60which participates in the oxidation of carbon monoxide evidently originates from the lattice oxygen of Si0,-0. The number of oxygen atoms on the surface of Si0,-0 is estimated as 6 mmol g- based on its surface area and the density of silica; therefore, approx- imately 12% of the lattice l60atoms on the surface are esti- mated to be consumed in the labelled reaction. With the last run (run 11 in Fig. 3), in which the reactant gas did not contain 1802, 8% of the carbon dioxide produced was C160180,showing that the lattice oxygen on the surface of Si0,-0 is replaced with l80.Since dioxygen containing '*O is not formed in the previous runs, the surface oxygen replacement is not the result of exchange between the lattice oxygen and gas-phase oxygen but rather occurs during the oxidation of carbon monoxide.In the early runs in which the Table 3 Calculated parameters for oxygen bridges binding energy/eV charge atomic bond population model energy/eV 0 1s Si 2p 0 Si Si-0 0-0 SiOSi -29707.0 548 111 -0.69 + 1.42 0.282 -SiO -Si -29697.9 539 103 -0.49 + 1.06 0.063 -SiO -Si(S) -29697.9 540 103 -0.44 + 1.03 0.055 -SiOOSi -31714.5 55 1 112 -0.35 + 1.40 0.224 SiOO- Si -31708.8 539 105 -0.53 + 1.30 0.241 -0.001 SiOO-Si(S) -31708.5 539 105 -0.52 + 1.28 0.202 O.OO0 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 number of the lattice lS0on Si0,-0 is considered to be small, ca. one half of the carbon dioxide produced is Cl60, while the oxidant was "O,, suggesting that oxidation of one carbon monoxide with lattice oxygen accompanies oxidation of another carbon monoxide with gas-phase oxygen. Since one half of the carbon dioxide produced evidently results from the reaction between carbon monoxide and the lattice oxygen, it is estimated that 16% of the lattice oxygen atoms which can react with carbon monoxide are replaced with ''0 in the catalyst of the last run. Although the number is higher than the number of surface lattice l60consumed in the labelled reactions (12%), the numbers are approximately identical; therefore, it appears that most of the lattice oxygen atoms on the surface of Si0,-0 participate in the oxidation process.Since lattice oxygen atoms on the surface of silica are gen- erally considered to be inactive, an activation process appar- ently occurs during the reaction. The oxygen atom in carbon monoxide is not exchanged with one from either 0, or the surface of silica because in the labelled reaction formation of C"0 is not accompanied by formation of oxygen molecules containing l6O. Furthermore, no Cl80 is formed by intro- duction of Cl60 and I6O2 on Si0,-0 which contains an appreciable amount of l80as lattice oxygen. This implies that surface atomic oxygen species such as 0-,which readily react with carbon monoxide,' 5*36 are present.The adsorption species appear to be consistent with the product distribution. Assuming that the atomic oxygen species is 0-,an electron must remain on the surface of silica after the reaction between 0-and carbon monoxide to form carbon dioxide. The atomic bond population of Si-0 in Si-O--Si is cal- culated as 0.06, significantly lower than in Si-0-Si (see Table 3), suggesting that the Si-O--Si group, if extant, will be reactive. Hence the reactions : CO + 0-+ Si-0-Si -+ CO, + Si-O--Si (1) CO + 0, + Si-O--Si --r Si-OCO + O,--Si (2) Si-OCO + O,--Si -+ CO, + 0-+ Si-0-Si (3) are expected to occur. In these reactions, one molecule of carbon monoxide reacts with a lattice oxygen and another reacts with oxygen originating from dioxygen.Radical Species on the Surface Since the number of the radical species on Si0,-0 is decreased by contact with carbon monoxide or oxygen at 950 K, the generation of the radical is not due to reduction or oxidation of the surface. The quantity of residual carbon in the silica sample is believed to be very small since carbon oxides were not observed after heating the sample in oxygen. No regeneration of the EPR peaks is expected after heating the sample with oxygen if the residual carbon is responsible for the EPR peaks. Since the EPR peaks appear again after evacuation of the sample at 950 K, residual carbon is evi- dently not the radical site. It is known that y-irradiation of silica can result in the cleavage of Si-0 bonds and consequently the generation of some radical centre^.'^'^^ Griscom et al.reported that annealing of phosphorus-doped silica glass at ca. 800-1100 K generates S centres which are attributed to E' type defects such as (OSi,)Si' and/or (0,Si)Si' irrespective of the irradia- ti~n.~~The centre gives a sharp EPR singlet at a g value of 2.0030. However, no such EPR peak was observed in the spectrum for silica prepared by the sol-gel method from ethyl orthosilicate after annealing at 850 K followed by y-irradiati~n.~'Moreover, the lineshape for the S centre is not L~rentzian.'~The EPR peaks for the E' centre are broad and 1181 also not L~rentzian.~~,~'Thus, the radical species in Si0,-0 cannot be attributed to these radical centres.Since the binding energies for the shoulders attributed to radical species in the XP spectra are lower than those for the main peaks (see Fig. 5), the silicon and oxygen atoms constituting the radical species are presumably more negative than those on the usual silica ~urface.~' This suggests that electrons are captured by the precursors of the radical species. The afore- mentioned Si-O--Si species are consistent with this sug- gestion. It is known that quantitative values for energy cannot be obtained by calculation using the STO-3G basis set, while the result will be qualitatively correct when com- paring similar models.42 The binding energies of 0 1s and Si 2p calculated for Si-0 -'-Si are significantly lower than those for Si-0-Si, while the charge on the oxygen atom is calculated to be less negative than in Si-0-Si (see Table 3).However, the g values for the EPR spectra of the radical species do not correspond to those for the 0-spe~ies.'~.~~.~' It has been reported that Si-0-0 species are formed during y-irradiation on the silica prepared by the sol-gel method, but the EPR spectra observed in such studies bear little or no resemblance to those obtained in the present However, the observation provides evidence for the presence of Si-0-0-Si species on the silica prepared by the sol-gel method. Silica prepared from the sol-gel method has been suggested to contain three-fold siloxane rings.45 Since the ring is strained,46 the Si-0-Si oxygen bridge must be less stable than usual and Si-0-0-Si species replaced with the one-oxygen bridge in the three-fold ring, would be expected to be more stable than those contain- ing the usual oxygen bridge. The total energy calculated for model SiOO-Si(S) is higher than that of model SiOOSi by 6 eV while the energy of model SiO-Si(S) is higher than that of model SiOSi by 9 eV (see Table 3).The result suggests that Si-0-O--Si may be formed in a similar manner to that shown in eqn. (1)during the reaction. Since the binding ener- gies of 0 1s and Si 2p calculated for Si-O-O--Si are significantly lower than those for Si-0-0-Si or Si-0-Si, Si-0-O--Si is also consistent with the results from XPS. The sharp EPR peaks observed are well simulated with a Lorentzian function, showing exchange narrowing of the EPR peaks.47 Since the oxygen atoms in the Si-0- -O--Si species are expected to be equivalent (see results of calculations), transposition of the radical, i.e.Si-O-O--Sie Si-0--0-Si can take place. Thus, the radical species observed on the silica sample are most prob- ably Si-0-O--Si species. Activation of the Silica Surface Although the activity of Si0,-0 in the first of a series of consecutive reactions is increased by preheating at 950 K for 5 h (in contrast with the sample pretreated at 950 K for 0.5 h), the intensities of the EPR peaks are not increased by the same treatment. Hydrogen is formed in the early reactions over Si0,-0 pretreated at 950 K for 0.5 h (Fig.3), while there is no change in hydroxy groups on the surface after the reaction. Hence, it is believed that a small quantity of molec-ular water adsorbed on the surface may suppress the reac- tion. Since, after the first reaction the intensities of the EPR peaks are reduced, it is hypothesized that water interacts with and deactivates the surface radical species. In the case of Si0,-0 evacuated at 1050 K for 0.5 h, the rate of the initial reaction increases after a relatively long induction period, suggesting that the water adsorbed still remains on the surface. The appearance of the sharp EPR peaks just after evacuation of Si0,-0 at 950 K provides evidence for the formation of the Si-O-O--Si radical species on the 1182 surface. Since the intensity of the EPR peaks increases after evacuation at 1050 K, the thermal process Si-0-0-Si + e -+ Si-0-O--Si (4) is expected to have occurred.The presence of surface water apparently does not affect this process since Si0,-0 evac-uated at 950 K for 5 h produces EPR peaks which are similar to those observed for the sample evacuated for 0.5 h while the activity under the former conditions is significantly higher. Since the EPR spectrum is regenerated by evacuation at 950 K after diminishing of the spectrum by contact with carbon monoxide or oxygen at 950 K, the radical is appar- ently restored. Hence, the thermal process [eqn. (4)] is expected to occur on the silica during carbon monoxide oxi- dation. The reaction, CO + 0-+ Si-0-0-Si -+ CO, + Si-0-O--Si (5) would lead to an increase in the number of Si-0-O--Si sites, if 0-is generated in reactions (1)-(3).Summary Catalytic carbon monoxide oxidation takes place over silica prepared by the sol-gel method from ethyl orthosilicate at a reaction temperature of 850 K or above, while no such activ- ity is found with a commercial silica prepared from inorganic reagents. When the silica prepared by the sol-gel method is evacuated at 950 K for 0.5 h before the reaction, the catalytic activity in the first reaction of the series is low, but is increased by repetition of the reaction. Labelled reactions with provide evidence to support the contention that most of the lattice oxygen species on the surface of the silica prepared by the sol-gel method participate in the reaction.From the product distribution of the labelled reaction, the formation of Si-O--Si surface species and atomic oxygen species such as 0-during the reaction appears to be pos- sible. After evacuation at 950 K or above, radical species identified as Si-0-O--Si are generated on the silica while the g1 and gll values for the EPR spectrum are 2.0023 and 2.0029, respectively. Although the radical is stable under carbon monoxide, oxygen, hydrogen or air at room tem-perature, its stability diminishes by heating in one of these gases, with the exception of hydrogen, at 950 K. The number of radical species is increased by repeating the reaction, but decreases in the first reaction after evacuating at 950 K for 0.5 h.Activation of Si-0-Si oxygen bridges on the surface of silica to generate Si-O--Si radical species appears to occur during the reaction. It is supposed that in the initial stage of the reaction the radical electron on the oxygen bridge originates from the Si-0-O--Si species formed in the thermal process. The water adsorbed on the surface may trap the electron and prevent the reaction. Adsorption of carbon monoxide and oxygen on the Si-O--Si radical species can cause formation of carbon dioxide and 0-which will react with carbon monoxide to provide the radical elec- tron for the lattice oxygen species, that is, Si-0-Si and Si-0- 0- Si. 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ISSN:0956-5000    

DOI:10.1039/FT9949001177

出版商:RSC    

年代:1994

数据来源: RSC

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J. CHEM. SOC. FARADAY TRANS., 1994, 90(8), 1183-1189 Oxidation of Carbon Monoxide on LaMn, -xCuxO, Perovskite-type Mixed Oxides Hiroyuki Yasuda, Yoshiko FujiwaraJ Noritaka MizunoS* and Makoto Misono* Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113,Japan Catalytic oxidation of carbon monoxide has been investigated over a series of LaMn,-,Cu,O, (x = 0-0.5) and La,CuO, catalysts having perovskite-type and perovskite-related structures. LaMn, -,Cu,O, catalysts were pre- pared by a freeze-drying method and showed uniform compositions. The oxidation states of Cu and Mn of the perovskite catalysts as well as several other properties relating to the reactivity of oxygen, such as temperature- programmed desorption of oxygen and carbon monoxide, the reducibility of the catalysts and the adsorption of carbon monoxide and dioxide were measured.La, -,Sr,MnO, (y = 0.2-1 .O) catalysts were also studied for com- parison. A remarkable synergistic effect on the catalytic activities for the oxidation was found with Mn and Cu. The synergistic effect and the deactivation process were discussed based on the above properties. The effect was attributed to the combination of two functions, namely the activation of oxygen by Mn oxide and that of carbon monoxide by Cu ions, and the deactivation towards adsorption of carbon dioxide. In the perovskite structure, ABO, , where the total charge on the cations is +6e and the ionic radius of the B-site cation is much smaller than those of the A-site cation and oxide ion, it is possible to control the valency of B-site ions and the amount of oxygen vacancies by changing the A- or B-site ions or by partial substitution with ions of different valency without affecting the fundamental structure.'-, The enhance- ment of the catalytic activity by A-site substitution has been studied extensively, but little is known on the effect of B-site element sub~titution.~-~ Zhang et a/., reported that the activ- ity of La,~4Sr,~,Mn,&o,~203 for the oxidation of butane was about five times higher than that of LaMnO, .However, we found that substitution of Co by Mn in LaCoO, led to only slight enhancement of the catalytic activity for the oxi- dation of propane when the catalyst composition was made uniform by freeze-drying.' Carbon monoxide (CO) oxidation catalysts have been used to clean up industrial, automotive and domestic emis-sions.'0-'2 For example, La,,,Ce,,,Co0,, a perovskite-type mixed oxide, is commercially used to oxidize CO and organic compounds in the emission from ovens by utilizing its high thermal stability, low cost and high catalytic activity.', We have also reported that La,~,Sr,,,CoO, shows higher cata- lytic activity for the oxidation of CH, and C,H, than Pt/Al,O,. 14,15 It has been reported that LaMn,~,Cu,,,O, shows a high catalytic activity for the oxidation of CO.', We have also steady-state activities, and discussed possible reasons for the pronounced synergistic effect and the deactivation based on the observed properties of catalysts.Experimental Catalysts LaMn, -,Cu,03 (x = 0-0.5) catalysts were prepared by freeze-drying solutions containing the component metal ace- tates.' LaMn,~,Cu,~,O, was also prepared from the metal nitrates in the same manner. La,CuO, and La,-,Sr,MO, (M = Co, Mn, y = 0.2-1.0) catalysts were prepared by evapo- rating the mixed acetates solutions, as described previously. " LaFe,,,Cu,~,O, and LaCo,~,Cu,~,O, catalysts were pre-pared via a citrate proce~s.'~ Each precipitate or precursor obtained was first decomposed in air at 573 K for 3 h and then calcined in air at 1123 K for 2-5 h. In this paper they are abbreviated by La,~,Sr,M,~,Cu,O, (M = Mn, Co, Fe), although the actual compositions may generally be non-stoichiometric with respect to oxygen.5% Pt/Al,O, and 0.7% Pt/A1,0, promoted by CeO, were commercially obtained. The crystal structure of the prepared catalysts was deter- mined by powder X-ray diffraction (XRD) (Rigaku Denki, Rotaflex, RU-200) using Cu-Kcr radiation. The specific surface areas were measured by means of the BET method using N, adsorption at 77 K after the pretreatment of the reported that the initial catalytic activity of L~M~,~,CU,~~O, samples in an He stream at 573 K. for the oxidation of CO was very high owing to a remarkable synergistic effect of Mn and Cu (ca. 102-103 times), although the activity declined rapidly with reaction time.' However, little is known of the mechanism of the synergistic effect and the catalyst deactivation.Hence, the elucidation of the syn- ergistic effect and the inhibiting effect of CO, may be of inter- est from the viewpoint of catalytic chemistry. In the present paper, we have extended our previous work," by measuring the steady-state catalytic activity and attempting to correlate the activity with the properties related to the adsorption and the reactivity of oxygen. As described below, we confirmed the synergistic effect for the Present address: Toshiba Co., Shin-isogo-cho, Isogo-ku, Yoko-hama 235, Japan.1Present address : Catalysis Research Center, Hokkaido Uni-versity, Sapporo 060, Japan. XPS Measurements Self-supporting discs (about 100 mg , 1 cm in diameter) were used, and the spectra were recorded with a JEOL JPS-90SX spectrometer using an Mg-Ka source (1253.6 eV).The pres- sure in the chamber was kept in the range 10-8-10-9 Torr. The binding energies were corrected by using the value of 285.0 eV for the C 1s peak resulting from carbon contami- nation. The surface atomic ratios of the samples were calcu- lated based on the equation described in the previous paper,,' using the integrated intensities of the La 3d,,,, Mn 2p3,, ,Cu 2p3,, and 0 1s photoelectron lines. Approximately the same La : Mn : Cu ratio was obtained when the peak intensities of the La 3d,/,, Mn 2p1,, and Cu 2p,/, lines were used. The oxidation states of copper on the surface of LaMn, -,Cu,03 catalysts were estimated from the ZsaJZmain ratios (Imsinand Isatare the peak intensities of the main and satellite signals of Cu 2p3!,, respectively) and the Cu L,VV Auger peak, as in the previous paper.,' Reduction of Catalysts by CO The reduction of the catalyst by CO was conducted by a pulse method at 573 K as in the previous study." The cata-lysts (25-50 mg) were heated in an 0, stream for 1 h at 673 K and then cooled to 573 K in 0, prior to the reaction.The flow rate of carrier gas (He) was 30 cm3 min- and the size of each pulse was 0.1 cm3. Products were analysed by gas chro- matography using a silica gel column. Adsorption of CO The adsorption of CO was measured volumetrically in a closed recirculation system. The catalysts (0.3-0.5 g) were evacuated at 773 K for 2 h and exposed to CO at 298 K.The equilibrium pressure was ca. 110 Torr. The amount of CO uptake was determined by the pressure decrease measured with a Baratron pressure gauge, giving the total amount of CO adsorbed. After evacuation of the sample for 1 h at 298 K, the amount of uptake was measured again. This was used to determine the amount of reversible adsorption. The amount of irreversible adsorption of CO was the difference between the total amount of CO adsorbed and the amount of reversible adsorption. Temperature-programmed Desorption (TPD) of 0,,CO, and CO TPD of O,, CO, and CO was carried out with a flow system using He as a carrier gas, as in the previous study.18 The oxygen impurity in He was removed by a molecular sieve 5A trap kept at 77 K.Prior to each run, the sample (ca. 0.5 g) was pretreated in an 0, stream (30 cm3 min-') at 1123 K for 1 h and was cooled to 298 K in 0,. In the case of TPD of CO, or CO, the sample was further pretreated in pure CO, or CO streams (30 cm3 min-') at 298 K for 30 min after the pretreatment with 0,. The temperature of the sample was raised from 298 to 1123 K at a constant rate of 20 K min-' in an He stream (30 cm3 min- '), and the gases desorbed were detected by use of a quadrupole mass spectrometer (NEVA, NAG-531). The rates and amounts of O,, CO, and CO desorbed were calculated from the concentration of the eluent gas. The reproducibility of the TPD curves was confirmed by repeating the TPD run after the same pretreatments.Catalytic Oxidation of CO The catalytic oxidation of CO was carried out at 473-873 K in a fixed-bed flow reactor at atmospheric pressure by feeding a gas mixture of CO (1.3%), 0, (1.3%) and N, (balance) at a flow rate of ca. 200 cm3 min- ' over a mixture of 3-20 mg of catalyst and 200 mg of Sic (GHSV = 1.8 x lo6 h-' for 10 mg of LaMnO~,CuO~,O,) after pretreatment of the catalyst in an 0, stream at 873 K for 1 h. The gas composition was analysed by gas chromatography using molecular sieve 5A and Porapak Q columns. Results Structures and Specific Surface Areas of Catalysts The XRD patterns of LaMn,-,Cu,O, (x = 0-0.5) and LaM,~,Cu,~,O, (M = Co and Fe) are shown in Fig. 1. The crystal structures and the specific surface areas of catalysts prepared in this study are summarized in Table 1. The XRD J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I I I 1 30 40 50 28/d egrees Fig. 1 XRD powder patterns of LaMn, -,Cu,O, (x = 0-0.5) and LaMo,6Cuo,,0, (M = Co and Fe). (a) LaMnO,; (b) LaMn0.8Cu0.203; (') LaMn0.6Cu0.40~~ (4 LaMn0,5Cu0,50,;LaCoo,6Cuo,,0, and (f)LaFeo.6Cuo,,0,. Bar: lo3 counts s-l. patterns of LaMn, -,Cu,03 (x = 0-0.4), La, -,Sr,MnO, (y = 0.2-0.4) and La,~,Sr,~,CoO, catalysts showed only the perovskite-type structure. The XRD patterns of La,CuO, and SrMnO, showed the orthorhombic K,NiF,-type struc-ture and the four-layer hexagonal SrMnO, structure,21 respectively. Additional phases such as La,CuO, and CuO were observed for LaMn,~,Cu,~,O, and LaM,~,Cu,~,O, (M = Co and Fe) besides the perovskite phase, and a small amount of SrMnO, phase was present for La,~,Sr,~,MnO,.The structures of LaMn, -,Cu,O, catalysts were rhombo- hedral for x = 0-0.2 and cubic for x = 0.3-0.5, while those of La, -,Sr,MnO, (y = 0.2-0.6) were orthorhombic. Table 1 Structures and surface areas of LaM,-,Cu,O, (M = Mn, Co, Fe; x = 0-OS), La,CuO, and La,-,Sr,MO, (M = Mn, Co; y = 0.2-1.0) catalysts surface area catalyst" /m' g-' structure LaMnO, 3.1 P(Wb LaMn0.8Cu0.203 3.8 P(R) LaMn0.7Cu0.303 4.2 P(C)f LaMn0.6Cu0.403 2.3 P(C) LaMn0.6Cu0.403(N)d 0.8 P(C) + unidentified phase (tr.)e 2.8 P(C) + CuO(tr.) + La2Cu0,(0.14) La,CuO, 1.2 K(OIf5.0 P(R) + La,CuO,(tr.)LaCo0.6Cu0.403 LaFe0.6Cu0.403 4.9 P(C) + La,Cu0,(0.25) La0.8Sr0.2Mn03 8.5 P(0) La0.6Sr0.4Mn03 9.6 P(0) La0.4Sr0.6Mn0 3 7.6 P(0) + SrMnO, SrMnO, 1.4 SrMnO,(HB) Lao.8Sro.2CoO 3 2.8 P(C) " Calcined at 1123 K unless noted otherwise. P(R),rhombohedra1 perovskite phase. P(C), cubic perovskite phase. Prepared from the mixed-metal nitrates and calcined at 1373 K. The numbers in par- entheses are the intensity ratios of the impurity phase to the per- ovskite phase; tr., trace. K(O), orthorhombic K2NiF, phase. H, hexagonal. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The specific surface areas of catalysts calcined at 1123 K varied from 1.2 m2 g-' for La,CuO, to 9.6 m2 g-' for La0.6Sr0.4Mn03 . Surface Properties of LaMn, -xCux03 (x = 0-0.5) and La,CuO, Catalysts The binding energies of the La 3d,,, , Mn 2p3,, ,Cu 2p3,, and 0 1s peaks, together with the IsaJlmainratios of Cu 2p3,,, for the LaMn,-,Cu,O, (x = 0-0.5) and La,CuO, catalysts are shown in Table 2.The XP spectra have intense shake-up satellite peaks in the Cu 2p regions. The IsaJZmainratios of Cu 2p3,, were between 0.50 (x = 0.5) and 0.62 (x = 0.3). These values agreed generally with the value of 0.53 reported for CuO.,, Only one L,VV Auger peak assignable to Cu2+ was observed for LaMn, -,CuXO3 (x = 0.3-0.5) and La,CuO, in the range 917.2-917.8 eV, while the Auger peak could not be detected for LaMn,~,Cuo~,O, probably due to the very weak intensity. The binding energies of Mn 2p3,, peaks for LaMn,-,Cu,O, were in the range 641.8-642.2 eV.The 0 1s spectra of LaMn,-,Cu,O, consisted of two peaks around 529 and 531 eV, as in the literat~re.~~ The former peak is assigned to the lattice oxide ions, while the latter peak has been assigned to hydro~ide,~~,~~ adsorbed water,,, adsorbed ~xygen~~.~~and/or carbonate oxygen.27 The surface compositions of LaMn, -,CuXO3 (x = 0-0.5) and La,CuO, catalysts determined by XPS are summarized in Table 3, together with those of the bulk calculated from the quantities of starting materials. It has previously been con- firmed by elemental analysis that the La : Mn atomic ratio in LaMnO, was 0.99 : The surface compositions of LaMn, -xCu,03 catalysts prepared from the metal acetates agreed fairly well with those of the bulk, except that the copper content on the surface was slightly greater than that in the bulk for x = 0.5.When LaMno&u0.60, was prepared from the metal nitrates, the La : Mn : Cu ratio of the surface was 1.0: 0.62: 0.68, different from that of the bulk Table 2 XPS binding energies (eV) of LaMn,-xCux03 (x = 0-0.5) and La,CuO," 0 641.8 834.9 531.4 529.2 0.2 934.2 0.58 642.1 834.6 531.0 529.5 0.3 934.3 0.62 642.2 834.4 531.0 529.4 0.4 934.2 0.55 642.2 834.1 531.1 529.3 0.5 934.2 0.50 641.8 835.0 531.6 529.3 La,CuO, 934.3 0.52 835.3 531.6 529.3 'The binding energies were corrected by using the value of 285.0 eV for the C 1s peak resulting from carbon contamination. Error limits are k0.3 eV. Intensity ratios of the satellite peak to the main peak of Cu 2p,/, .Table 3 Surface compositions of LaMn, -xCux03 (x = 0-0.5) and La,CuO, surface compositionb bulk composition X0 La Mn Cu 0 La Mn Cu 0 0 0.19 0.15 0.66 0.20 0.20 0.60 0.2 0.17 0.13 0.03 0.67 0.20 0.16 0.04 0.60 0.3 0.16 0.13 0.09 0.63 0.20 0.14 0.06 0.60 0.4 0.17 0.11 0.09 0.63 0.20 0.12 0.08 0.60 0.5 0.18 0.06 0.14 0.62 0.20 0.10 0.10 0.60 La,CuO, 0.23 0.12 0.65 0.20 0.14 0.60 LaMn, -xCu,03 (x = 0-0.5) catalysts were prepared by freeze-drying the mixed acetates solutions. Estimated by XPS peak intensity using the inte- grated areas of the La 3d,/, ,Mn 2p3/,,Cu 2p3,, and 0 1s photoelectron lines. 1185 (La : Mn : Cu = 1.0 :0.60 : 0.40), suggesting that the per-ovskite phase was not formed uniformly in this case.Reduction of LaMn, -xCux03by CO The reduction of catalyst by CO pulses in the absence of oxygen reflects the reducibility of the catalyst, that is, the oxi- dation power of the catalyst. The results of reduction of LaMn,-,Cu,O, (x = 0-0.5) by CO pulses at 573 K are shown in Fig. 2. The product was only CO,. The values in Fig. 2 are the amounts of CO, formed by the first pulse of CO per unit surface area. The reducibility of LaMn, -xCu,03 increased with x, reached a maximum at x = 0.4, and then decreased. The amount of CO, formed by the first pulse over LaMn,~,Cu,~,O, was 2.5 times the surface monolayer of oxygen, which was calculated on the assumption that the concentration of the surface oxide ion is 8.0 x 10l8 ions m-' according to ref.26. Adsorption of CO The amounts of irreversible adsorption of CO at 298 K on the LaMn, -,CuXO3 (x = 0-OS), La,CuO, and La, -,Sr,MnO, (y = 0.2-0.6) catalysts are shown in Fig. 3. As for LaMn, -,Cu,03, the amounts increased with x, reached 4.0 1 r I / I L I I 1 I I0.0 I J 0.0 0.2 0.4 0.6 X Fig. 2 Reduction of LaMn,-xCu,O, (x = 0-0.5) by the first CO pulse at 573 K. CO pulse size, 0.1 cm3. b 0.0 0.2 0.4 0.6 La2Cu04 x, Y Fig. 3 Amount of irreversible adsorption of CO on LaMn, -xCux03, La,CuO, and La,-,Sr,MnO,. (0)LaMn, -,Cu,03 (x = 0-0.5) and La,CuO,; (0)La, -,Sr,MnO, (y = 0.2-0.6); adsorption temperature, 298 K. 1186 a maximum at x = 0.4, and then decreased. The amounts of CO adsorbed varied in parallel with the reducibility of the catalysts by CO.On the other hand, the variation of the amount of CO adsorption was small for La,-,Sr,MnO, and the amount on La,~,Sr,~,MnO, was much smaller than that on LaMn,~,Cu,~,O,. A similar variation was observed for the amount of total or reversible adsorption of CO. TPD of Oxygen, CO, and CO Fig. 4 shows the TPD profiles of oxygen from LaMn, -,Cu,03 (x = 0-0.5) and La,CuO, in the tem-perature range 298-1123 K. Broad and small peaks appeared in a relatively low temperature range (473-823 K) for LaMn, -,Cu,03 (x = 0-0.5). Only small peaks were observed for La,CuO, . The amounts of oxygen desorbed below 823 K are summarized in Table 4.It was shown pre- viously that this amount for La, -,Sr,MnO, correlated well with the catalytic activity for the complete oxidation of propane." The highest value was obtained for LaMn,~,Cu,~,O,, but the amount was only 0.4 times that of a surface monolayer of oxygen.These trends are similar to those noted for the La,-,Sr,MnO, system,I8 and are in con- trast with those of the Co and Fe system^.^^,^^ Above 823 K, LaMnO, showed one large peak at 1060 K, which has been assigned to the desorption of the excess oxygen of LaMnO,,, accompanied by the reduction of Mn4+ to Mn3+.l8 Similar large peaks or ascents were observed for LaMn, -xCu,03 (x = 0.2-0.5). These results also resemble those found for the La, -,Sr,MnO, system.18 r& 1.0 I I 5 0.8 EL 0.6 i-II ternperature/K Fig.4 0, TPD profiles from LaMn,-,Cu,O, (x = 0-0.5) and La,CuO,. (-) LaMnO,; (--) LaMn,&u,,,O,; (-* -) LaMn,,,Cu,,,O,; (----) LaMn,,,Cu,~,O,; (-..-) LaMn,~,Cu,.,O,; and (. . .* .) La,CuO,. Table 4 Degrees of deactivation, amounts of 0, desorbed in the TPD of 0, and amounts of CO, desorbed in the TPD of CO, for LaMn, -xCu,03 (x = 0-0.5) and La,CuO, amount of OZb amount of CO,' X deactivation' degree of /lo-, mol g-' (<823 K) /lo-, mol m-' (<1123 K) 0 0.7 0.5 3.0 0.2 1.5 2.9 0.3 2.1 2.8 0.4 0.6 5.8 5.8 0.5 0.4 15.4 5.9 La,CuO, 0.2 4.5 9.2 Ratio of the steady-state rate to the initial rate at 473 K. The rate after 1 min was used as a measure of the initial rate. Amount of 0, desorbed in the range 298-823 K in the TPD experiment of 0,.Amount of CO, desorbed in the range 298-1123 K in the TPD experiment of CO, . J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 4.0 II II 3.0 r I!' ' & I i11 + 4.01 C .-4-g 3.0 v) -0 2.0 1 .o 0.0 273 473 673 873 1073 ternperature/K Fig. 5 CO, TPD profiles from LaMn,-,Cu,O, (x = 0-0.5) and La,CuO,. (-) LaMnO,; (--) LaMn,~,Cu,,,O,; -)(-a LaMn,,,Cu,,,O,; (-- - -) LaMn,,,Cu,.,O,; (-.--) LaMn,~,Cu,,,O,; 1 *and (. * .) La,CuO,. The TPD profiles of CO, are shown in Fig. 5. Several peaks, including two major ones at around 400 and 550 K, were observed for LaMn, -,Cu,03 (x = 0-0.5) and La,CuO, . Only a small amount of CO, was desorbed above 973 K.Tejuca et aL31 have assigned the peaks at around 390 and 540 K of LaMnO, to the desorption of a monodentate and bidentate carbonate, respectively. The amounts of CO, desorbed from 298 to 1123 K are summarized in Table 4. The amounts increased above x = 0.3. The coverages of CO, cal-culated from the data in Table 4, assuming that the cross- section of CO, is 17 were 0.29 and 0.94 for LaMn,. ,CU~.~O~ and La,CuO, ,respectively. The TPD profile of CO from LaMn,~,Cu,~,O, is shown in Fig. 6. For the desorption of CO, one peak and one ascent were observed at around 400 K and above 750 K, respec-tively. In addition to CO, CO, and 0, were also desorbed: 0, was desorbed in a similar way to the desorption in the 0, TPD (which was shown in Fig.4), but the amount of 0, desorbed below 823 K was smaller. As for CO,, only one desorption peak was observed at around 420 K. Similar results were obtained for x = 0, 0.3 and 0.5. The amounts of CO, CO, and 0, desorbed in CO TPD from LaMn,-,Cu,O, (x = 0-0.5) are summarized in Table 5. The differences between the data in Fig. 3 and Table 5 are due mainly to the different pretreatment (evacuation and oxidation). Catalytic Oxidation of CO When a reactant gas was introduced onto LaMn, -,Cu,O, (x = 0-0.5) and La,CuO, catalysts at 473 K after the pretreatment of 0, at 873 K for 1 h, the conversion decreased greatly in the initial stage and reached a constant value after ca. 2 h. It was confirmed for LaMn,,,Cu,~,O, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 5.0 Y N 4.0 -0f 3.0 C .-'i1.0 p -0 0.0 273 473 673 a73 1073 temperature/K Fig.6 CO TPD profiles from LaMno,,Cuo,,03. (-) Desorption of CO; (---) desorption of CO,; (-* -) desorption of 0, in CO TPD; and (-.. . .) desorption of 0,in 0,TPD. that the conversions (<40%) at the steady state were pro- portional to the weight of catalyst. The degrees of deactiva- tion expressed by the ratios of the steady-state rate to the initial rate (after 1 min) at 473 K are shown in Table 4. The ratio decreased in the following order, LaMnO, z LaMn,~,Cu,,,O, > LaMn,~,Cu,~,O, > La,CuO,. A similar order of the deactivation has previously been found by the experiment using a recirculation system.l7 When the reaction temperature was raised to 873 K and then lowered, little hysteresis was observed for the conversion at the steady state for LaMn, -xCux03 (x = 0-OS), indicating the absence of irreversible deactivation of the catalysts. The apparent activation energies for the oxidation of CO on LaMn, -xCu,03 were 58 (x = 0), 43 (x = 0.2), 49 (x = 0.3), 55 (x = 0.5) and 48 (La,CuO,) kJ mol- ',respectively, and are in general agreement with the value of 52 kJ mol- ' reported for La,~,Sr,~,CoO,. l4 The steady-state rates of LaMn,-,Cu,O, (x = 0-0.5) and La,CuO, for the oxidation of CO at 573 K are shown in Fig. 7. Note that the ordinate is in the logarithmic scale. The rate showed a maximum at LaMn,,,Cu,~,O,, and the rate was about 25 and 15 times greater than those of LaMnO, and La,CuO, ,respectively.A similar trend was also observed at 473 K. Thus, the pronounced synergistic effect was confirmed for the steady-state activity, although, owing to the deactiva- tion, the magnitude of the effect was smaller than that for the initial activity reported previou~ly.'~ The steady-state rates of La,-,Sr,MnO, (y = 0.2 -1.0) at 573 K are also shown in Fig. 7. Upon Sr substitution, the rate also increased and reached a maximum at y = 0.6. However, the increase in the activity by Sr substitution was much smaller than that obtained by the Cu substitution. In Table 6, the steady-state CO oxidation rate of LaMn,~,Cu,,,O, is compared with those of several other catalysts, which have been reported to have high catalytic Table 5 Amounts of CO, CO, and 0, desorbed in the TPD of CO for LaMn, -xCu,03 (x = 0-0.5) x amount of CO" mol m-' amount of COZb/lop6mol m-' amount of 0,' mol g-' 0 1 .o 1.9 0.1 0.3 1.4 2.0 co.1 0.4 2.2 4.2 1.4 0.5 1.5 3.3 8.6 a Amount of CO desorbed in the range 298-1123 K in the CO TPD experiment.Amount of CO, desorbed in the range 298-1123 K in the CO TPD experiment. Amount of 0, desorbed in the range 298-823 K in the CO TPD experiment. 100 -E-N I 1 I I I I Id 0.0 0.2 0.4 0.6 La2Cu04 SrMn03 x, Y Fig. 7 Rates of CO oxidation over LaMn,-,Cu,O,, La,CuO, and La, -,Sr,MnO, at steady state. (0)LaMn, -xCu,03 (x = 0-0.5) and La,CuO,; (0)La, _,Sr,MnO, (y = 0.2-1.0);reaction temperature, 573 K.activities for the oxidation of CO. Among them, Ce0,- promoted Pt catalyst and La-Co perovskite-type mixed oxide are used commercially for the oxidation of CO, hydro- carbons et~.'~,~~Note that the activity of the LaMn,~,Cu,,,O, catalyst was the highest among the cata- lysts tested, even higher than those of the Ce0,-promoted Pt catalyst and La,~,Sr,~,CoO, . Furthermore, the calculated turnover frequency of LaMn,~,Cu,~,O, C1.4 molecules s-' (surface Mn and Cu atom)-'] was about five times higher than that of the Pt-Ce/Al,O, catalyst C0.31 molecules s-l (surface Pt atom)- '3 at 473 K. LaMn,~,Cu,,,O, prepared from an aqueous solution of metal acetates was about four times more active than LaMn,,,Cu,~,O, prepared from an aqueous solution of metal nitrates (both were prepared by a freeze-drying method).Discussion Structure and Surface Properties of Catalysts LaMn, -,CuXO3 prepared by the present freeze-drying method had the single perovskite phase in the range 0 6 x 6 0.4, and small amounts of La,CuO, and CuO were additionally observed for x = 0.5. Gallagher et ~1.'~reported the formation of the single phase in the range x = 0-0.6 by the calcination at 1273-1373 K in 0,. Rojas et uLt3 also reported the single perovskite phase up to x = 0.6 for Table 6 Rates of CO oxidation at 473 K catalyst conversion (yo) rate/cm3 min-' g-' 2.2 (2.8)" 19 (1.4)b LaMn0.6Cu0.403 1.6 (4.3) 9.5 (0.57) La0.8Sr0.2C003 La0.4Sr0.6Mn03 2.0 (6.0) 8.2 (0.18) 2.6 (10.1) 6.4 (0.21) LaCo0.6Cu0.403 2.8 (10.0) 7.0(0.24)LaFe0.6Cu0.403 0.5% Pt/Al,O,' 3.8 (0.11) 5% Pt/Al,O, 1.4(5.0) 6.8 (0.02) 0.7% Pt/Ce-A1 ,03 6.0 (10.2) 15 (0.31) a The numbers in parentheses are the catalyst weights (mg).The numbers in parentheses are the turnover frequencies calculated by assuming that the concentration of the surface transition-metal ions of the perovskite-type mixed oxide is 2.67 x 1OI8 ions m-' and that Pt is fully dispersed (dispersion = l),according to ref. 26,33 and 34, respectively. Unit: molecules s-' (transition metal or Pt atom)-'. 'Taken from ref. 16. samples prepared by the decomposition of the amorphous citrate complex. In the present work, the range of the single perovskite phase was somewhat narrower, probably owing to the relatively lower calcination temperature (1123 K) or dif- ferent reactivity of the precursors. As shown in Table 3, the surface compositions of the ele- ments of LaMn, -,CuXO3 (x = 0-OS), which were prepared from the metal acetates by a freeze-drying method, and La,CuO, agreed well with those of the bulk.Therefore, the surface properties of these LaMn, -,Cu,O, catalysts would reflect well those of the bulk. On the other hand, the use of the metal nitrates for freeze-drying gave less satisfactory agreement between the surface and bulk compositions as described previously.' Therefore, the greater activity of LaMn,~,Cu,~,O, prepared from metal acetates is probably due to its more nearly uniform composition between the surface and the bulk.The binding energies of Cu 2p3,, of the LaMn,-,Cu,O, (x = 0.2-0.5) catalysts were 934.2-934.3 eV, in agreement with the value for La,CuO,, in which the oxidation number of copper was two. It is known that Cu+ gives no satellite peaks in the Cu 2p regions, while Cu2+ has intense shake-up satellite peaks. The XP spectra of LaMn,-,Cu,O, (x = 0.2-0.5) had intense satellite peaks and the ZsaJZmain ratio of Cu 2p3,, and the Cu L,VV Auger peak agreed with those report- ed for Cu0.22936 These results show that copper ions on the surface of LaMn, -,Cu,03 (x = 0.2-0.5) are present as Cu2 +. On the other hand, in the case of Mn it is difficult to dis- tinguish the oxidation state from the binding It has been reported that LaMnO, perovskite has an oxidative non-stoichiometry, LaMnO,., (or defects at A and B sites), that is, a mixed valency of Mn3+ and Mn4+.38 When Mn3+ (or Mn4+) is partly substituted for Cu2+ in LaMnO,, the concentration of Mn4 + should increase and/or the concentra- tion of excess oxygen should decrease. Rojas et aL2, have reported, on the basis of the results of H, reduction, that the non-stoichiometry (6) of oxygen in LaMn, -,Cu,03 +d decreased with the increase of x up to 0.6, and manganese ions were present almost always as Mn4+ for 0.4 d x < 0.6. Therefore, the structural changes of LaMn, -,Cu,03 from rhombohedra1 (x = 0-0.2) to cubic (x = 0.3-0.5) phase could be explained; distortion due to the Jahn-Teller effect of Mn3+ is released by the oxidation of Mn3+ to Mn4+ upon Cu2+ substitution. Thus, the valency control of Mn by Cu substitution was achieved, as in the case of La, -,Sr,MnO, .Synergistic Effect for the Oxidation of CO As shown in Fig. 7, a pronounced synergistic effect was found for the coexistence of Mn and Cu of LaMn,-,Cu,O, cata-lysts for the oxidation of CO (more than 10 times enhancement). The effect was even greater for the initial activ- ity,17 owing to the absence of deactivation by CO, which is discussed in the next section. The catalytic activity for the oxidation of CO also increased by the substitution of Sr for La in LaMnO, . However, the increase in the activity for the Sr substitution was much less. We have previously reported that the amounts of 0, desorbed from La,-,Sr,MnO, below 823 K increased from y = 0 to y = 0.6 and this trend was well correlated with their catalytic activities for the complete oxidation of C,H, .A similar trend was observed for the desorption of 0, from LaMn, -,Cu,O, catalysts (Fig. 4 and Table 4). The amounts of 0, desorbed became much smaller for CO TPD, owing to the consumption of oxygen by oxidation of CO to CO, (Fig. 6 and Table 5). These results suggest that the 0, desorbed below 823 K for the Mn systems is very reactive for the oxi- dation of CO and therefore probably reflects the intrinsic J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 nature of the Mn oxide system or oxide ion in the neighbour- hood of the Mn ion.No such 0, desorption was observed for La,CuO, ,supporting this idea. Part of the enhancing effect of the Cu substitution of LaMnO, may thus be ascribed to the increased reactivity of the oxide ion of LaMnO, . However, the increase in the cata- lytic activity caused by the Cu2+ substitution was much greater than the increase in the activity caused by the Sr2+ substitution for La, -,Sr,MnO, ,although the increases in 0, desorption were very similar for both cases. The additional enhancement may be explained by the influence of the Cu ion on CO activation as discussed below. For LaMn, -,Cu,03, the amount of CO adsorption increased with x, reached a maximum at x = 0.4 and then decreased (Fig. 3), in parallel with the change of the reducibility measured by CO reduction (Fig.2). The parallel change indicates that the CO adsorption is an important factor controlling the reducibility of LaMn, -xCu,03 cata-lysts. Similarly, the increase of CO adsorption with the amount of Cu up to x = 0.4 suggests that Cu is the active site for CO adsorption. The idea is further supported by the much lower increase in the CO adsorption for Sr2+ substitut- ion (Fig. 3), although 0, TPD changed similarly for Cu and Sr substitution. The decrease in the activity from x = 0.4to 0.5 in Fig. 7 may be due to the formation of La,CuO,, which was less active for CO adsorption, as shown in Fig. 3. Therefore, it is very probable that the synergistic com- bination of the two properties, that is, the activation of 0, by Mn as in La,-,Sr,MnO, and of CO by Cu ion, brought about the great increase of the catalytic activity of LaMn, -,Cu,O,.The fact that the increase was much less for the oxidation of propane" also supports the specific activa- tion of CO by the Cu ion. Thus, the coexistence of Mn and Cu in the neighbourhood is essential for the enhancement of the catalytic activity. In other words, the role of Cu in LaMn,-,Cu,O, catalysts is the valency control of Mn, as in the case of Sr substitution for La in LaMnO, , and the activa- tion of CO. Inhibiting Effect of CO, The ratios of the steady-state rate (after ca. 2 h) to the initial rate decreased in the order of LaMnO, x LaMn,,,Cu,,,O, > LaMn,~,Cu,~,O, > La,CuO, (Table 4).This order is nearly the reverse order of the amounts of CO, adsorption; LaMnO, < LaMn,~,Cu,~,O, x LaMn,,,Cu,~,O, < La,CuO, (Table 4). The larger was the amount of CO, adsorption, the more greatly the activity decreased. This correlation indicates that the deactivation is caused by CO, adsorption. Similar results were obtained for the reaction carried out in the recirculation system; the rate of CO oxidation was expressed by eqn. (l).' The rate constant (k) reached a maximum at x = 0.4, while the b value was in the order of La,CuO, z LaMn,~,Cu,~,O, > LaMn,~,Cu,,,O, > LaMn,.,Cu,,,O, > LaMn,~,Cu,~,O, > LaMnO,. This order was in general agreement with the orders of the extent of the catalyst deacti- vation and the amount of CO, adsorption measured by CO, TPD, as shown in Table 4.In addition, the initial rate of LaMn, -,Cu,03 declined very rapidly and was greatly sup- pressed by the preadsorption of CO,. These results also support that the catalyst deactivation was caused by the adsorption of CO, . As shown in Fig. 5 and Table 4, the amount of CO, desorbed generally increased with the substitution of Cu, and the significant desorption of CO, from La,CuO, needed high J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1189 temperatures such as above 570 K. Thus, the inhibiting effect of COz observed in LaMn, -xCux03 is mainly caused by the presence of Cu. Here it must be remarked that the pro- nounced synergistic effect observed in the study cannot be explained by the difference in the degree of this inhibiting 15 16 17 T.Nakamura, M. Misono and Y. Yoneda, J. Catal., 1983, 83, 151. P. K. Gallagher, D. W. Johnson Jr. and E. M. Vogel, J. Am. Ceram. SOC., 1977,60,28. N. Mizuno, Y. Fujiwara and M. Misono, J. Chem. SOC., Chem. Commun., 1989,316. effect, since the degree of deactivation was not in parallel with 18 T. Nitadori, S. Kurihara and M. Misono, J. Catal., 1986, 98, the steady-state catalytic activity (Fig. 7). As discussed in the previous section, the high activity most probably resulted from the synergistic effect of the Mn and Cu components; the ability for both the activation of 0, by Mn and of CO by Cu. 19 20 221. H-M. Zhang, Y. Teraoka and N. Yamazoe, Chem. Lett., 1987, 665. N. Mizuno, M. Yamato, M.Tanaka and M. Misono, Chem. Mater., 1989, 1, 232. This work was supported in part by a Grant-in-Aid for Scien- 21 22 T. Negas and R. S. Roth, J. Solid State Chem., 1970,1,409. D. C. Frost, A. Ishitani and C. A. Mcdowell, Mol. Phys., 1972, tific Research from the Ministry of Education, Science and Culture of Japan. 23 24,861. M. L. Rojas, J. L. G. Fierro, L. G. Tejuca and A. T. Bell, J. Catal., 1990, 124,41. 24 B. J. Tan, K. J. Klabunde and P. M. A. Sherwood, J. Am. Chem. References 1 2 3 4 5 6 7 8 9 10 11 12 13 R. J. H. Voorhoeve, in Advanced Materials in Catalysis, ed. J. J. Burton and R. L. Garton, Academic Press, New York, 1977, p. 129. L. G. Tejuca, J. L. G. Fierro and J. M. D. Tascon, Ah. Catal., 1989,36,237. M. Misono, in Future Opportunities in Catalytic and Separation Technology, ed.M. Misono, Y. Moro-oka and S. Kimura, Else- vier, Amsterdam, 1990, p. 13. N. Yamazoe and Y. Teraoka, Catal. Today, 1990,8,175. D. W. Johnson, Jr., P. K. Gallagher, F. Schrey and W. W. Rhodes, Am. Ceram. SOC. Bull., 1976,55520. H-M. Zhang, Y. Teraoka and N. Yamazoe, Hyomen Kagaku, 1987,8, 23. Y. Teraoka, T. Nobunaga and N. Yamazoe, Chem. Lett., 1988, 503. Y. Teraoka, H. Fukuda and S. Kagawa, Chem. Lett., 1990,l. T. Ichiki and M. Misono, 50th National Meeting of the Chemical Society of Japan, Tokyo, April, 1985, Abstr. No. 1Y25. M. Haruta, N. Yamada, T. Kobayashi and S. Iijima, J. Catal., 1989,115,301. S. D. Gardner, G. B. Hoflund, B. T. Upchurch, D. R. Schryer, E. J. Kielin and J. Schryer, J. Catal., 1991, 129, 114. M. Misono and N. Nojiri, Appl. Catal., 1990,64, 1. K. Tabata and M. Misono, Catal. Today, 1990,8,249. 25 26 27 28 29 30 31 32 33 34 35 36 37 38 SOC.,1991, 113, 855. L. G. Tejuca, A. T. Bell, J. L. G. Fierro and M. A. Pena, Appl. Surf Sci., 1988,31, 301. M. A. Pena, J. M. D. Tascon, J. L. G. Fierro and L. G. Tejuca, J. Colloid Interface Sci., 1987, 119, 100. E. A. Lombardo, K. Tanaka and I. Toyoshima, J. Catal., 1983, 80,340. N. Mizuno, M. Tanaka and M. Misono, J. Chem. SOC., Faraday Trans., 1992,88, 91. T. Nakamura, M. Misono and Y. Yoneda, Bull. Chem. SOC.Jpn., 1982,55, 394. T. Nitadori and M. Misono, J. Catal., 1985,93,459. L. G. Tejuca, A. T. Bell, J. L. G. Fierro and J. M. D. Tascon, J. Chem. SOC., Faraday Trans. I, 1987,83,3149. J. M. D. Tascon and L. G. Tejuca, J. Chem. SOC., Faraday Trans. I, 1981, 77, 591. R. Barth, R. Pitchai, R. L. Anderson and X. E. Verykios, J. Catal., 1989, 116,61. H. C. Yao, Appl. Surf: Sci., 1984, 19, 398. H. C. Yao and Y. F. Yu Yao, J. Catal., 1984,86,254. G. Schon, Surf. Sci., 1973,35,96. M. Oku, K. Hirokawa and S. Ikeda, J. Electron Spectrosc. Relat. Phenom., 1975,7,465. B. C. Tofield and W. R. Scott, J. Solid State Chem., 1974, 10, 183. 14 T. Nakamura, M. Misono, T. Uchijima and Y. Yoneda, Nippon Kagaku Kaishi, 1980, 1679. Paper 3/07031J; Received 26th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001183

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(8), 1191-1196 Inelastic Neutron Scattering Study of NH,Y Zeolites Wim P. J. H. Jacobs* and Rutger A. van Santen Schuit Institute of Catalysis, Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology,P.O.Box 513,5600 MB Eindhoven, The Netherlands Herve Jobic lnstitut de Recherches sur la Catalyse, 2 a venue Albert Einstein, 69626 Villeurbanne Cedex, France An inelastic neutron scattering study of NH,Y and CsNH,Y zeolites is presented from 2 to 350 meV (16-2800 cm-'). The spectra are interpreted in terms of translational and librational motions of ammonium ions, ammonia and water molecules. For the hydrated samples the translational modes of water species are observed at 12 meV and librational modes are found above ca.50 meV. The translational and librational modes of the ammon- ium ions also depend on the location of the cations. Ammonium species give rise to (ion-lattice) translational modes at 10-15 meV and at 15-25 meV for species localised in sodalite cages and supercages, respectively. The corresponding librational modes are observed at ca. 8 meV and in the region 3-6 meV, respectively. Strongly hindered librations are observed at 50-70 meV and 30-50 meV, respectively, for the ions in the two different cages. Low-frequency as well as high-frequency librational modes of the ammonium species may occur caused by the presence of different ammonium species. Differences in reorientational motions are observed for hydrated zeolites and for reammoniated zeolites.For the latter, a stronger interaction of the ammonium ions with the zeolitic lattice is present and the low-frequency librational modes are shifted to higher energy-transfer values. When increasing the loading with ammonia, librational and translational motions of ammonia species could be observed. Again, a heterogeneity in the reorientational barriers is present, indicating the presence of differ- en t ammonia species. Zeolites are frequently used as part of commercial catalysts.' Owing to the presence of channels and pores containing cata- lytically active sites, these materials are very efficient in obtaining high conversions and selectivities for various indus- trially important reactions. The active sites can consist of acidic protons, the Brsnsted sites, Lewis acid sites or small metal clusters. In zeolite synthesis, generally the Brnrnsted sites are not created immediately. Depending on the type of zeolite synthesized, calcination procedures have to be used in order to remove templating molecules.Ion-exchange pro- cedures are also used to produce zeolites containing ammon- ium ions. The ammonium form of the zeolite is converted to the acidic form by heating at elevated temperature in order to desorb ammonia, leaving a proton behind at a Brsnsted site.2 Because of the importance for catalysis, ammonium-containing zeolites have been studied extensively in the past3,, Adsorption of ammonia on the hydrogen form of zeolites has been used to study quantitatively and qualit- atively the acidic properties by, e.g.IR ~pectroscopy.~-' The desorption of ammonia has been studied by temperature- programmed desorption and related methods, leading to reported energies of activation for the desorption process of 40-190 kJ mol-' for various These energies cor- respond to the rate-limiting step in the desorption process, the transfer of a proton from an ammonium ion to the zeo- litic lattice. This step is energetically very unfavourable, causing diffusion effects to be of minor importance for zeo- lites with small cry~tallites.'~ At present, an ab initio quantum chemical description of the proton-transfer process is being developed. 14~24In order to evaluate the rate con- stant for desorption or the rate constant for proton transfer theoretically, the thermodynamic partition functions of the ground and excited states in the rate-limiting step, as well as the translational and librational motions of the ammonium ions inside the zeolitic lattice, have to be known.From spec- troscopic studies in the far-IR region, information about the translational modes of the ammonium ions in zeolites has been Quasi-elastic neutron scattering (QENS) and inelastic neutron scattering (INS) can be used to study the librational motions of the ammonium ions and their translational modes with respect to the oxygen atoms of the zeolite walk2' INS is especially sensitive for hydrogen-containing compounds. The intensities in the neutron spectra are then proportional to the number of hydrogen atoms present in the sample.30 In this paper we will present an inelastic neutron scattering study of Y zeolites containing ammonium ions.We will discuss the changes in the INS spectra during dehydration and deammoniation of the samples and after subsequent adsorption of ammonia. Experimental The starting material was the sodium form of zeolite Y (Si :A1 = 2.8 :1) which was converted to NH,Y by nine-fold ion-exchange with NH,N03 at 353 K. Chemical analysis indicated that almost 100% of the sodium ions were replaced by ammonium ions. A zeolite with different composition was obtained from NH,Y (LZ-Y62, Si : A1 = 2.4 :1) after four- fold ion-exchange with NH,NO, at 353 K followed by a three-fold exchange with CsNO, at 295 K. The composition of this sample as obtained by chemical analysis was cs,5(NH4) i8Na3 [(SiO,) I 36(A102)561.For the pretreatment of the samples 13 g of hydrated zeolite were placed in a glass ampoule. Each sample was first evacuated at 323 K for several h. Then the temperature was increased at a rate of 0.2 K min-' to the desired temperature level. After evacuation at this temperature for several h the sample was cooled to room temperature. For three of the samples at this stage anhydrous ammonia (99.98% purity) was adsorbed. Finally, the ampoule was sealed. A more detailed summary of the pretreatment conditions is given in Table 1. The ammonium content of the dehydrated samples was obtained from chemical analysis.The amount of ammonia adsorbed was determined volumetrically. These results are also listed in Table 1. 1192 Table 1 Sample properties and measurement conditions adsorbed remaining ammonia ammonium (NH, per (NH: persample treatment unit cell) unit cell) 1 NH,Y no -51 2 NH,Y INS: 24 h at 373 K -51 3 NH,Y INS: 24 h at 473 K -36 -4 NH,Y INS: 16 h at 623 K 24 -5 NH,Y INS: 16 h at 623 K 124 -6 NH,Y INS: 16 h at 623 K 180 7 NH,Y FTIR: 1 h at 723 K 8 CsNH,Y no 9 CsNH,Y INS: 60 h at 323 K 10 CsNH,Y FTIR: 1 h at 723 K The crystallinity of the samples was checked by XRD experiments before the treatment in the glass ampoules and after the INS measurements. These experiments showed that there was no severe damage of the crystal structure by the pretreatment conditions.IR spectra were recorded by a Bruker IFS 113v FTIR spectrometer. In these experiments self-supporting samples of 3-8 mg cm-2 were evacuated for 1 h at 723 K. After cooling energy transfer/cm -' 0 200 400 600 800 1000 1200 7 6 5 34 ti G3 2 1 0 0 25 50 75 100 125 150 energy transferlmev Fig. 1 Inelastic neutron scattering spectra from 2 to 150 meV for (a) NH,Y desorbed at 373 K (sample 2), (b) 473 K (sample 3), (c) differ-ence spectrum (sample 2 -sample 3), and (d) CsNH,Y desorbed at 323 K (sample 9) energy transfer/cm -' 800 1200 1600 2000 2400 2800 I-------1.50 r------7 183 219 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 energy transfer/cm- ' 200 400 600 800 1000 12009,0 ~---r--.I --r--F----1,---42 6 h 8 ci v m3 0 0 25 50 75 100 125 150 energy transferlmev Fig.3 Inelastic neutron scattering spectra from 2 to 150 meV for NH,Y readsorbed with increasing amounts of ammonia: (a) sample 4, (b) sample 5 and (c) sample 6. The spectrum for sample 4 has been multiplied by a factor of 3. to room temperature, spectra were recorded at a resolution of 1 cm-'. The INS measurements in the region from 2 to 350 meV were performed on the time-of-flight spectrometer TFXA (ISIS, Chilton, UK). This spectrometer has an energy-transfer range from 2 meV to 1 eV with a resolution of AE/E < 2%.31 The partly dehydrated and deammoniated samples were transferred from the glass ampoules to aluminium sachets for the measurements, while keeping them in an inert atmo-sphere.The advantage of this procedure is a reduction of the background scattering, but the disadvantage is that the amount of sample in the neutron beam can vary slightly. For the reammoniated samples an aluminium vacuum-tight cell was used instead of the sachets in order to enable adsorption of ammonia. The measurements were carried out at 20 K and the spectra obtained are corrected for the background of the empty cryostat or the empty aluminium container. Results Dehydrated Ammonium Zeolites In Fig. 1 the INS spectra for the region below 150 meV are presented for the samples evacuated at different temperatures (samples 2, 3 and 9 of Table 1).The spectrum of sample 2 [Fig. l(a)] contains sharp peaks near 4, 6 and 8 meV which energy transfer/cm- ' 800 1200 1600 2000 2400 2800 I --~I--1 '--+ X>,.AAL4\, --1 -0.00 Ib\ --l--__L (a) L L--._-L-I0.00 100 150 200 250 300 350100 150 200 250 300 350 energy transferlmev energy transfer/meV Fig. 2 Inelastic neutron scattering spectra from 100 to 350 meV for Fig. 4 Inelastic neutron scattering spectra from 100 to 350 meV for (a) NH,Y desorbed at 373 K (sample 2) and (b) CsNH,Y desorbed at NH,Y readsorbed with increasing amounts of ammonia: (a) sample 323 K (sample 9) 4, (b) sample 5 and (c) sample 6 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 are also present with approximately equal intensity in the spectrum for sample 3 [Fig.l(b)]. A feature found at 12 meV is almost absent in the spectrum for sample 3 and a broad band can be observed at 21 meV for sample 2 which is con- verted into a band with a reduced intensity at 19 meV for sample 3. Finally, the spectrum of sample 2 is characterised by a broad band from 30 to 70 meV, with a maximum at 46 meV and shoulders near 43 and 55 meV. The intensity above 70 meV is mainly due to multiphonon excitations. After evac- uation at 473 K (sample 3) the intensity of the broad band has decreased and peaks at 31, 41, 46 and 58 meV can be observed. Fig. l(c) is the difference spectrum: sample 2 -sample 3. This spectrum is characterized by a band from ca. 10 to 30 meV, with maxima near 13 and 23 meV. A strong band is observed between 30 and 70 meV, with a maximum at 46 meV and a shoulder near 53 meV.Furthermore, we can observe a very weak band from 70 to 150 meV, with a broad maximum near 85 meV. The spectrum of sample 9 [Fig. 1(4]contains sharp bands at 4 and 8 meV and a broader band at 12 meV. The band at 4 meV has a a reduced intensity when compared with the spectra for samples 2 and 3. The feature near 6 meV is missing for sample 9. At 21 meV, a small shoulder can be observed and a broad band is located between 40 and 120 meV. Furthermore, the spectra are characterised by a peak at 183 meV and a peak at 213 meV with a shoulder extending to 350 meV (Fig. 2). Reammoniated Zeolites The results for the reammoniation experiments are presented in Fig. 3 for the region 2-150 meV.Sample 4 is characterised by a broad band extending between 10 and 70 meV with maxima at 14, 31 and ca. 43 and 53 meV. A peak near 133 meV is also clearly visible and a very weak, broad feature may be observed at 21 1 meV (Fig. 4). After adding more ammonia (sample 5 and 6) peaks at 7, 14, 20 and 42 meV develop. For sample 5 additional peaks can be found at 28, 46 and 57 meV and also shoulders at 23 and 65 meV can be found here. For sample 6 shoulders can be distinguished at 56 and 66 meV. The spectra for samples 5 and 6 contain peaks at both 183 and 219 meV. Discussion From the results presented in Table 1 it can be concluded that during evacuation of NH,Y at 373 K desorption of ammonia is negligible.After evacuation at 473 K, 71% of the original amount of ammonium ions are still present. In the INS spectra a decrease of approximately a factor of two in the intensity is observed subsequently for the samples evac- uated at 373 and 473 K. This decrease can be explained par- tially by the presence of adsorbed water in the samples. After evacuation at 473 K the amount of water present in the zeo- litic cavities is negligible. Hence, the features in the difference spectrum [Fig. l(c)] correspond to adsorbed water, with con- tributions from ammonium species. For the CsNH,Y zeolite similar arguments can be used. A negligible amount of ammonia is desorbed during the heat treatment at 323 K. The INS spectrum contains contribu- tions due to adsorbed water because of the low desorption temperature used.This sample contains caesium ions located in the supercages and ammonium ions located in the sodalite cage^.^^.^^ The caesium ions are too large to penetrate into the sodalite cages, whereas the ammonium ions can enter these cages. Consequently, the IR spectrum of the caesium- 1.20 1 3100 3300 3500 3700 3900 wavenumber/cm-’ Fig. 5 Room-temperature IR spectra for the samples NH,Y (a) and CsNH,Y (b)after evacuation at 723 K exchanged sample recorded after deammoniation at 723 K in uucuo [Fig. 5(b)] shows only the low-frequency (LF) hydroxy-group stretching band (3550 cm-’), which is assigned to protons pointing into the sodalite cage.33.34 The high-frequency (HF) hydroxy-group stretching band (3650 m-’), which is due to protons pointing into the supercage, is almost completely absent.(According to ’H NMR experiments, these protons are rather immobile. In the absence of residual ammonium ions or water molecules their residence time at the lattice oxygen atoms is at least lop4s.~’This is very large compared with the hydroxy-group stretching vibration. Therefore, the IR signal corresponds to the energetically most favourable distribution of protons among the different sites.) The NH,Y sample, on the other hand, possesses both the LF and HF hydroxy-group bands after deammoniation [Fig. 5(a)]. A comparison of the INS spectra of these two zeolites can be used to differentiate between ammonium species in supercages and those localised in sodalite cages.Finally, we note, from the composition of the samples in Table 1, that only the INS spectra for samples 3, 4, and 9 may contain very weak contributions due to the presence of acidic protons. In a previous INS study, acidic Y zeolites have been inve~tigated.~~,~’ The main features in the INS spectra for acidic Y zeolites are due to the in-plane (135 meV) and out-of-plane (52 meV) bending modes of the acidic hydroxy groups. Dehydrated Ammonium Zeolites When comparing the INS spectra of NH,Y after desorption at 373 and 473 K, one observes a decrease in intensity for the peak at 21 meV and for a band extending from 30 to 70 meV, which is related to the shoulders at 43 and 55 meV [Fig. l(a)].Furthermore, the peak at 12 meV has almost completely disappeared after evacuation at 473 K. Some of these features may be assigned to water species present inside the zeolite. Increasing the desorption temperature from 373 to 473 K results in the desorption of water species that are responsible for part of the features in the difference spectrum [Fig. l(c)]. The other part in this spectrum is due to ammonium ions which are desorbed as ammonia, leaving behind an acidic Brernsted site. Several authors have studied water adsorbed in zeolites and other porous materials using INS? 8-42 Their results indicate that the translational modes of water species are located below 50 meV. The librational modes are found at higher energy-transfer values, typically in the region from 50 to 150 meV.Furthermore, owing to the inverse dependence of the intensity on the effective mass for the translations and librations, the former are expected to have the lower inten- ~ity.~~Using these considerations and the observed behav- iour upon evacuation at elevated temperatures, the features at 13 and 23 meV [Fig. l(c)] could both originate from trans- lational motions of water species. After evacuation at 473 K, a peak at 19 meV remains, together with a weak shoulder at 12 meV [Fig. l(b)]. These remaining features can be assigned to the translational modes of ammonium ions, which is in agreement with results obtained from far-IR measure-where the IR-active translational modes are observed in the range 10-25 meV.So it is not possible to discriminate between the translational modes of the water and ammonium ions. Apart from the IR-active translational modes of the ammonium ions, translational modes may also be present that are not observable in the far-IR spectrum. These modes are due to the parallel motion of the ammonium ions to the zeolite walls. The corresponding vibrational frequencies are not known, but may be very low. Furthermore, translational modes of cations can have a weak intensity in the far-IR spectrum, whereas these modes are more pronounced in the INS spectra. So, compared with results from far-IR spectros- copy, the translational modes for the ammonium ions need not be restricted within the region 10-25 meV.However, since the intensities of the features observed below 10 meV and above 25 meV are relatively high, the main contribution here comes from librational modes. Peaks near 4,6 and 8 meV are found for samples 2 and 3 with comparable intensities. From the composition of these samples it has to be concluded that these features are due to ammonium ions. Furthermore, since these bands are much more intense than the translational modes of the ammonium ions (12 and 19 meV), they have to be assigned mainly to librational modes. The region from 30 to ca. 70 meV contains contributions from the librational motions of ammonium species with a possible contribution from water for the sample evacuated at 373 K. For the sample evacuated at 473 K the contribution due to water species is negligible.Hence, we assign the fea- tures at 31,41, 46 and 58 meV to restricted librational modes of ammonium ions. This assignment is supported by the observation of similar features for the reammoniated sample (4)[Fig. 3(a)], though less well resolved. The differences in the region 30-150 meV for samples 2 and 3 are caused by the different water and ammonium contents. The librational modes of water are present above ca. 50 meV. Hence, part of the shoulder at 53 meV and the weak, broad band at 85 meV in the difference spectrum [Fig. l(c)] may be assigned to librational modes of water species. The broad band from 30 to 70 meV and centred at 46 meV in the difference spectrum [Fig. l(c)] is observed at an energy-transfer value that is too low to allow assignment of this band to water librations exclusively.Therefore, we assign part of this band to libra- tions of weakly bound ammonium ions. The maxima at 46 and 58 meV may be related to more strongly bound ammon- ium ions, since they are still observed after evacuation at 473 K. So, owing to the overlap of the librational modes of ammonium and water species, the modes are not resolved and the shoulders at 43 and 55 meV result, as observed in Fig. l(a). Except for the weaker modes at 31 and 41 meV, the librational modes of the ammonium ions observed for sample 3 [Fig. l(b)] are much more intense than the corresponding translational modes, which is to be expected.43 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Different research groups have studied ammonium-containing compounds using Values ranging from 6 to 48 meV were found for the librational modes of ammon- ium ions in inorganic salts, which is in the range of our results, except for the feature near 58 meV. These values cor- respond to activation energies for reorientation ranging from 1 to 200 kJ mol-1.43 For ammonium ions in zeolite rho, a librational energy of 10-14 meV was found, corresponding to a very low reorientational barrier of ca. 3.4 kJ mol-’.29 So we will assign the features between 3 and 8 meV in our spectra to librations of ammonium ions, having a weak reori- entational barrier. However, librations of ammonium ions with a high reorientational barrier are also present.From the influence of desorption temperature on the inten- sities of the various bands we can conclude that the modes at 4, 6 and 8 meV, the relatively narrow band at 46 meV and probably also the shoulder at 58 meV are related to ammon- ium ions which are relatively firmly bonded to the zeolitic lattice. The broad band from 30 to 70 meV, also centred near 46 meV [Fig. l(c)], contains contributions due to weakly bound ammonium species. According to Ozin et aL2’ the ammonium ions in Y zeo-lites are located at the exchangeable cation positions SI,, S,, and S,,,(Fig. 6).In zeolites, the ammonium ions depart from tetrahedral symmetry,’ because electrostatic interactions with the negatively charged zeolite lattice deform the ammonium ions. Ab initio quantum chemical calculations indicate that these interactions occur by bonding of two or three protons to the zeolite oxygen atom~.’~~’’ We can therefore conclude that two types of reorientational modes can be discriminated for the different ammonium species present.Slightly hindered librations are found from 3 to 8 meV. For freely rotating ammonium ions a value of 1.5 meV can be ~alculated.~’ Librational modes of ammonium ions reorientating in a stronger potential field are located in the region from 30 to sodalite cage Fig. 6 Faujasite structure: different oxygen atoms are numbered 1 to 4. Cation positions are indicated by roman numerals. For position I11 two slightly different positions 111’ and 111” can be discriminated which differ in location with respect to the adjacent four-rings: for position 111‘ the cation interacts with four oxygen atoms [O(l) and 0(4)]; for position 111” the main interaction is via one O(4) oxygen atom and weaker interactions are via two O(2)oxygen atoms.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ca. 70 meV. For zeolite rho the features in the region 38-45 meV were attributed to ammonium-coupled pore-opening modes of the zeolite framew~rk,'~ but this work shows that we also have to assign these features to librational modes of ammonium ions. Finally, the spectrum for sample 2 contains peaks at 183 and 213 meV which are due to the two N-H bending modes of the ammonium ion. The shoulder extending to 350 meV can be assigned to recoil scattering of weakly bonded ammonium ions or water molecules.From the spectrum for CsNH,Y [Fig. l(4] it is imme- diately obvious that the librational modes of the ammonium species at 4, 6 and in the region of 30-46 meV are missing. Hence, we assign these modes to reorientation of ammonium species located in the supercage. The mode near 8 meV can be assigned to ammonium species located in the sodalite cage. The region above ca. 45 meV is partly masked by the librational modes of water. The translational mode at 21 meV is almost completely absent for this sample, but the mode at 12 meV still remains. .Using results from far-IR spectroscopy, Ozin et ~1 have ~ assigned bands found at 24 and 19 meV to translational modes of ammonium ions located in the supercages (SIIor SIII).However, for a translational mode of ammonium ions observed at 11 meV a definite assignment could not be given: the ions responsible for this mode are located at site SI,,but there may also exist a contribution due to ions at site SIIIr.When comparing these assignments with our results for the hydrated samples we can conclude that ammonium ions localised in supercages give rise to translational modes in the region between 15 and 25 meV, because these modes are absent for the CsNH,Y zeolite. The modes observed in the region 10-15 meV have then to be assigned to the trans- lational motions of ammonium ions in the sodalite cages. The lower values are probably due to the more symmetric environment of the sodalite cages.In this region translational modes of water species are also observed, e.g. in an INS study of water adsorption on H-mordenite features at 7.5 and 13 meV are attributed to translations of water and hydronium species.42 Therefore, we cannot uniquely assign the trans- lational mode near 12 meV to either water or ammonium ions in the sodalite cages. The modes found at 4 and 8 meV are not much affected by increasing the desorption tem-perature, so these modes are not assigned to water. Therefore, these modes represent exclusively the librational motion of ammonium ions. The decrease in intensity for these modes upon increasing the desorption temperature is due to a reduced population of ammonium ions.It is also not likely that the mode observed in the region between 15 and 25 meV contains contributions from water molecules because it is missing in the spectrum for CsNH,Y. Rearnrnoniated Zeolites From the spectrum of the partly reammoniated sample 4 it is clear that the slightly hindered librations of the ammonium ions (4, 6 and 8 meV), which were present in the water- containing samples, are now missing. A broad band is left between CQ. 10 and 70 meV containing contributions due to translational and librational motions. A clear distinction between translational and librational motion is not observed, but from the preceding part it is known that the translational motions of ammonium ions in zeolites are located between 10 and 25 meV.Furthermore, we expect the intensity for the translational motions to be much less than that for the libra- tional motions. So we attribute the peaks at 14, 31 and the broad band around 50 meV to librations of ammonium ions, while masking the less intense features due to translational motions. Therefore, this region contains librational and translational modes belonging to different ammonium species; however, these modes are not resolved. Compared with the dehydrated samples the reammoniated zeolite has higher barriers for reorientation of the ammonium ions, since the librational modes below 10 meV are apparently shifted to higher energy-transfer values. These findings are in agreement with 'H, 29Si, 27Al and I4N NMR experiments on hydrated and dehydrated ammonium-containing zeolite^.^ 5349 These experiments indicate a stronger interaction of the ammonium ions with the zeolitic lattice for the water-free reammoniated zeolites compared with the dehydrated ammonium zeolites.QENS29 and 'H NMR relaxation measurement^^^ on ammonium zeolites obtained after desorption at elevated temperatures indicated a low barrier for the reorientational motion of the ammonium ions. This is probably due to the fact that only the librations with the lowest barrier for reori- entation are observed. When comparing our results for the librational modes with literature values for the librational modes of ammonium ions in inorganic salts and their corre- ~sponding reorientational barrier^,^ we expect values from 2 to 65 kJ mol-' for the reorientational barriers for the differ- ent ammonium species in water-free reammoniated zeolites.The peaks observed at 180 and 211 meV are present both in the dehydrated and reammoniated samples. These features are assigned to the two N-H bending modes of the ammon- ium ion. Superimposed on these modes we observe a weak broad band which is due to recoil scattering of weakly bonded ammonium ions or ammonia molecules. For both the dehydrated and reammoniated zeolites weak features can be found in the region from 120 to 170 meV. For the samples containing no acidic sites (samples 5 and 6), these bands are due to multiphonon excitations. However, for partly dehy- drated or partly reammoniated samples a small contribution due to the in-plane bending modes of the acidic hydroxy groups may be pre~ent.~~.~~ When adding extra ammonia, new features develop in the region below 100 meV and the total intensity increases.Most of the features observed for sample 4 remain visible, although weakly. These features are superimposed on a spectrum which looks quite similar to that of solid ammonia,52 where energy-transfer values below 24 meV were assigned to trans- lational motions and values between 24 and 70 meV to libra- tional motions of ammonia molecules. We can adopt a similar assignment for the region above 30 meV for our spectra. The intensity observed below 30 meV is too high to be of purely translational character.We think that a libra- tional contribution of ammonia molecules must be present here, indicating a larger heterogeneity in reorientational bar- riers compared with solid ammonia. This is probably due to librational modes of different ammonia species, resulting in low-frequency (below 30 mev) and high-frequency (above 30 meV) librational modes. This situation is very similar to that for samples 2 and 3. It is likely that differences in location and hydrogen-bonding interactions between the ammonium and ammonia species are now responsible for the presence of the different librational modes. Conclusion An INS study of hydrated CsNH,Y and of NH,Y after various dehydration, deammoniation and reammoniation treatments is presented.Together with the literature results, the spectra can be analysed in terms of translational and librational motions of ammonium, water and ammonia species. Water and ammonium species localised in the super- cages and the sodalite cages give rise to different translational and librational motions. For the hydrated samples, water 1196 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 species give rise to a translational mode at 12 meV and libra- tional modes above CQ. 50 meV. The ammonium species in the dehydrated zeolites give rise to translational modes observed between 10 and 15 meV for species localised in the sodalite cages and between 15 and 25 meV for species local- 12 13 14 15 R. D. Shannon, R. H. Staley, A. J. Vega, R. X. Fischer, W. H.Baur and A. Auroux, J. Phys. Chem., 1989,93,2019. E.Dima and L. V. C. Rees, Zeolites, 1990,10,8. E. H.Teunissen, F. B. van Duijneveldt and R. A. van Santen, J. Phys. Chem., 1992,%, 366. E.H. Teunissen, R. A. van Santen, A. P. J. Jansen and J. B. van ised in the supercages. The librational modes of the ammon- ium ions are found in the regions between 3 and 8 meV and between 30 and 70 meV. Our results indicate a large heterogeneity in reorientational barriers for the ammonium ions, which is caused by the dif- ferent positions of the ammonium ions in the zeolite resulting 16 17 18 Duijneveldt, J. Phys. Chem., 1993,97,203. W. J. Mortier, J. Sauer, J. A. Lercher and H. Noller, J. Phys. Chem., 1984,88,905. E.Kassab, K.Seiti and M. Allavena, J. Phys. Chem., 1988, 92, 6705.M. Allavena, K. Seiti, E. Kassab, G. Ferenczy and J. G. hgyan, Chem. Phys. Lett., 1990, 168,461. in different interactions with the zeolite walls. The slightly hindered librations (near 4 and 6 meV) are due to species located in the supercage, while the librations at 8 meV are assigned to species located in the sodalite cage. The features at 30-50 meV are ascribed to librations of ammonium species in the supercages. The ammonium ions located in the sodalite 19 20 21 22 23 J. Sauer, Acta Phys. Chem., 1985,31, 19. J. Sauer, J. Phys. Chem., 1987,91, 2315. J. Sauer, C. M. Kolmel, J-R. Hill and R. Ahlrichs, Chem. Phys. Lett., 1989, 164, 193. J. Sauer, J. Mol. CataI., 1989,54, 312. A. G. Pel'menshchikov, V. I. Pavlov, G. M. Zhidomirov and S. Beran, J. Phys.Chem., 1987,91, 3325. cages give rise to librations which can be observed between 50 and 70 meV. Therefore, in general, the librations of the ammonium ions in the sodalite cages (CsNH,Y) are observed at higher energy-transfer values than those localised in the supercages (NH,Y). This implies a more strongly hindered rotation for the ammonium ions in the small sodalite cages. 24 25 26 A. G. Pel'menshchikov, E. A. Paukshtis, N. U. Zhanpeisov, V.I. Pavlov and G. M. Zhidomirov, React. Kinet. Catal., 1987, 33, 423. M. D. Baker, G. A. Ozin and J. Godber, Catal. Rev. Sci. Eng., 1985,27, 591. M. D. Baker, J. Godber and G. A. Ozin, J. Am. Chem. SOC., 1985,107,3033. When the ammonium form is obtained by reammoniation of the proton form of the zeolite, generally higher barriers for the reorientational motions of the ammonium ions are observed.These librations give rise to peaks at 14 and 31 meV and a broad band around 50 meV. The low-frequency librations are not found and the weaker translational modes 27 28 29 G. A. Ozin, M. D. Baker, K. Helwig and J. Godber, J. Phys. Chem., 1985,89,1846. G. A. Ozin, M. D. Baker, J. Godber and C. J. Gil, J. Phys. Chem., 1989,93,2899. T.J. Udovic, R. R. Cavanagh, J. J. Rush, M. J. Wax, G. D. Stucky, G. A. Jones and D. R. Corbin, J. Phys. Chem., 1987,91, 5968. are masked by librational modes. When increasing the loading of ammonia, features due to ammonia clusters become visible in the spectra. The translational motions of these species are observed below 30 meV and the librational motions can be found in the region 5-70 meV, again indicat- ing high and low barriers for reorientation.This heter- ogeneity in reorientational barriers is due to differences in ammonia location resulting in different interactions with the zeolite walls and with neighbouring ammonia or ammonium species. 30 31 32 33 34 35 36 37 S. 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ISSN:0956-5000    

DOI:10.1039/FT9949001191

出版商:RSC    

年代:1994

数据来源: RSC