摘要:

J. CHEM.SOC. FARADAY TRANS., 1994, 90(5), 787-795 DRIFT and Mass Spectrometric Experiments on the Chemistry and Catalytic Properties of Small Ir Clusters at the Surfaces of Polycrystalline a-Al,O, L. Basini* and A. Aragno Snamprogetti SPA.Research Laboratories, Via Maritano 26,20097San Donato Milanese (MI), Italy The DRIFT and mass spectra collected during eight high-temperature, high-pressure experiments are described and discussed. Relationships between the occurrence of aggregation-disaggregation reactions of Ir species on a low-surface-area polycrystalline a-Al,O, and the formation of gaseous reaction product have been defined. Reactions in which carbidic oxidic and carbonyl species are interconverted have been produced and their role in CO and CO, hydrogenation is discussed.The results are compared with those previously found for Rh on a-alu m i na. This work is part of an extensive research programme dedi- cated to the high-temperature, high-pressure (HTHP) surface chemistry of small aggregates and/or monatomic species of Rh, Ir and Ru. Previously' we have described the chemistry of Rh'(CO), surface species. These were formed uia a reaction between Rh,(CO)', and the active surface sites of M,O, a-Al,O, and (30,(Rh content 0.1%); no other ligands, with the exception of the surface atoms and of the CO groups, were present. Here we present a study on Ir-containing a-A1203samples (Ir content 0.25 wt.%) which have been pre- pared oia a reaction between Ir,(CO),, and the surface of a-Al,O, .The anisotropic conditions and the 'degree of disorder' of the extremely mutable environment of the surfaces are responsible for the difficulties in describing the chemistry of the noble metals at the surfaces of polycrystalline oxides. However, many surface complexes have been successfully and selectively produced and characterized2-14 by organometallic chemistry methods, spectroscopic experiments and molecular mechanics modelling. Many small metal clusters appear to be stabilised in the cages or supercages of zeolite^,^*'^*'^ by con- finement in the cages and/or by mechanisms such as proton anchoring.''-' However, extensive aggregation-disaggregation reactions occur when the noble metal cluster species are anchored on low-surface-area polycrystalline oxides.Some clusters are formed only in reaction conditions as a result of a delicate balance between thermodynamic and kinetic processes. This work does not aim to produce, stabilize and charac- terize well defined surface species, but, (i) to investigate the HTHP formation and transformation processes of Ir surface species; (ii) to relate the molecular features discovered at the surfaces to the products of the solid-gas interactions and (iii) to discuss the analogies and the differences between Ir and Rh chemistry.' It also attempts to describe, at the molecular level, the elementary steps of complex reactions such as the hydrogenation of CO and CO, . Experimental Sample Preparation An anhydrous THF solution of Ir4(CO)12 (Strem) under CO atmosphere was dropped into a slurry of a-Al,O, (Aldrich 99.999 wt.%) dispersed in the same solvent.The purity and crystalline structure of a-Al,O, was checked by optical emis- sion arc spectroscopy and XRD analysis. The surface area measured by the BET method was ca. 10 m2 g-'. After a few hours the solid species was filtered in a CO atmosphere and dried under vacuum at 25°C. The preparation procedures and the insertion of the samples into the DRIFT cell were performed in moisture-free and C0,-free environments. The samples contained 0.25% Ir ; this amount corresponds approximately to a monolayer of Ir, carbonyl clusters with volume 3905 A3 which is the volume of the +P(CH2C6H,XC6H,),] [HIr,(CO),,] -Cluster.' Conse-quently, less than a monolayer of [HIr,(CO), ,]-anions [which are formed at the surface after the chemisorption of the Ir,(CO),,] is present at the surface.The DRIFT spectra of the materials obtained, used at point A (see Fig. 1, later) of each experiment, showed CO stretching bands at 2065 (sh), 2024 (s), 1990 (sh), 1958 (vw) and 1731 (m) cm-l. These spectra indicate that the Ir4(CO)1, clusters have been modi- fied after interaction with the surface as will be discussed below. Apparatus The reactivity experiments were performed in an HTHP cell equipped with two ZnSe windows, one for the incoming radi- ation and the other for the diffuse reflected radiation. The equipment has been described previously'*'s and allows the collection of DRIFT spectra in flowing gaseous environments between 25 and 500°C and between 0.1 and 5 MPa. The present experiments were performed at 0.1 MPa.The HTHP cell was inserted into the sample compartment of a Nicolet 2OSXC spectrometer equipped with an MCT detector. Spectra were recorded with a resolution of 4 cm-'.The cell exit line was linked to a quadrupole mass spectrometer (UTI) through a two-orifice pressure-reduction sampling stage with differential pumping between the orifices. The composition of the exit line was examined with selected peak monitoring with both time and with temperature, and with repeated scans between 1 and 60 u. High-purity (Strem electronic grade) He, H,, 0,, CO, CO, gases were used and the He and H, streams were further purified with OM1 filters (Li and Co compounds which react with H,O, CO, CO,, 0, impurities). Each gas line was equipped with dedicated mass- flow-meters and controllers. A laser Raman spectrometer (Dilor triple monocromator spectrometer) equipped with a multichannel detector with 512 diodes (Omars) was used to record spectra of the A samples (see Fig.1, later). The best spectra were achieved in a backscattering configuration with a slit width of 80 pm and with a confocal microscope. Under these microsampling con- ditions the S/N ratio was significantly improved, probably owing to the elimination of stray light coming from outside Experiment 1 A B co, C 25-500°C 25-500-c He H coAcExperiment 2 A 2 ~ B 2-oc C -D 25-50 "C He Experiment 3 A -He co2 __c CExperiment 5 A 2zc25-500°C He H co -BExperiment6 A c D 25-500°C 25-!XO"c 25-500s He 02 coExperiment 7 A --cdD 25400% ~500~25-5oO"c He H2-c COZ+H& Experiment8 A -25500°C * 25-500"c 25-500"c Fig.1 Scheme of the main steps of the experimental sequences the focal volume. The 514.5 nm line of an Ar' ion laser (Spectra Physics, Model 2020) with a power ranging from 5 to 50 mW was used to excite the Raman scattering. Experimental Sequences The eight experimental sequences are shown schematically in Fig. 1. The first step in each experiment was a thermal treat- ment in flowing He. This was performed both to investigate the temperature-induced reactions in an inert atmosphere and to obtain, at point B, the same type of bare Ir cluster.The materials obtained at point B were reacted with CO or with CO-H,, Experiments 1-4, or CO, and CO2-HZ, Experiments 5-8, after reducing (H,) or oxidising (0,) thermal treatments. The heating was always carried out in a flow of He; in each run the temperature was stabilised for 10 min at 25, 100, 200, 300, 400,500°C to collect the DRIFT spectra and to introduce pulses of gaseous reactants. Mass spectra were recorded continuously during the experiments. Results and Discussion The Ir4(CO)12 cluster is transformed into other cluster species after liquid-solid chemisorption. The tetrametallic cluster has only linearly bonded CO groups whose stretching vibrations absorb IR radiation at the frequencies reported in Table 1.The peak maxima of the species obtained at point A (see Fig. 2), particularly the peak at 1730 cm-', indicate the formation of clusters with bridged CO groups such as HIr,(CO);, ,' 7T24i25 or the Ir6(CO)16 red isomer with CO face-bridged groups26 and the Ir6(CO)Ii 27,28 anion. These species were interconverted by the reactions (1)-(4) both in Table 1 CO stretching frequencies of Ir4(CO)12 (cm-') THF solution' 2110 (w), 2067 (vs), 2026 (mw) KBr pellet and crystalline formb 2111 (w), 2086 (sh), 2056 (s),2040 (sh), 2040 (sh), 2020 (m), 2004 (sh) y-Al,O, Ic SiO, ,d Na-Ye 2086 (sh), 2054 (s), 2008 (sh) 'Ref. 19. Ref. 20. 'Ref. 21 and 22. Ref. 23. Ref. 8. J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 4.01 I Y r' 0.5 0.0 V I1 2100 2000 1900 1800 wavenum ber/cm- ' Fig. 2 (a)Spectrum of the species obtained at point A; (b) magnifi-cation of the multilplet in the 2100-1900 cm-' range homogeneous solution27 and at the surface of polycrystalline materials.**' 9*27--2 Ir4(CO)12+ OH--+ Ir,(CO),,CO,H-(1) OH-+ Ir,(CO),,CO,H -+ HIr,(CO);, + HCO, (2) 3HIr,(CO)r1 + 30H-+21r6(CO):; + 3c0, + 3H2 (3) Ir6(CO):5 + CO + OH-+Ir6(CO)16+ H, + 0,-(4) Reactions (1)-(4) require a basic environment in homoge- neous solution. They have been reported to occur also at the surfaces of MgO" and in the cages of the Na-X and Na-Y zeolite^.'*^^-^^ The formation of clusters with bridged-bonded CO species suggests that the ol-Al,O, used in the present work has OH-containing surface sites able to activate this chemistry.The comparisons between the DRIFT spectrum of species A and the IR spectra of the three candidate species indicate strongly that the anion HIr,(CO);, has been formed. However, the broadness of the IR bands (which is consistent with the interaction of the metal complex with slightly differ- ent surface sites) prevents a definitive assignment of the IR bands. A laser-Raman spectrum of a sample containing 3 wt.% Ir was also recorded (the higher loading of Ir was necessary because of the lower intensity of the Raman signals compared with IR signals). The sample was prepared by the same procedure by depositing Ir4(CO)12 on the surface of the a-Al,O,.The spectrum is shown in Fig. 3 and shows sharp peaks that are assigned to carbonyl stretching bands of HIr,(CO);, at 2110, 2035, 2010 and 2001 cm-'.9 Conse-727 1 I cnc h 1 127 1 2200 2100 2000 1900 wavenum ber/cm -' Fig. 3 Carbonyl Raman scattering of a sample prepared by depos- iting Ir4(CO)12 at the surface of a-Al,O, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 quently, the species obtained at point A will be represented by this tetrametallic anion. The Ir carbonyl species were highly stabilised at the surfaces and could not be extracted with a THF solution of [PPNl[Cl]. We assume that a strong anchoring is achieved because the low amount of Ir4(CO)12 molecules are allowed to react selectively with the more sta- bilising surface sites.The low frequency of the band at 1730 crr-' is consistent with a strong ion-pairing effect between surface A1 atoms and the bridged carbonyl groups of the anionic clusters.24 Experiment 1 The clusters obtained at point A were oxidized and disaggre- gated during the first heating in helium. This is inferred by the formation at 200°C of a doublet at 2055 and 1981 cm-'. The band positions (see Fig. 4) indicate that IrI(CO), mono- metallic surface species were formed (the two bands are assigned to symmetric and anti-symmetric CO stretching). The band shapes and the intensity ratios, also suggest that the low-frequency peak gains contributions from the vibra- tions of other carbonyl cluster species. The intensity of the doublet at 2055 and 1981 cm-' is strongly reduced at temperatures higher than 300°C but no other absorption bands, with the exception of a new peak at 1720 cm-', were formed.We assign this new peak (which is also present at 500°C when all the other absorption features in the 1600-2000 cm-' range have disappeared) to stretching vibrations of a hydrogencarbonate species. It is also worth noting that heating in He produced the species responsible for the 1720 cm-' band only on the Ir/a-A1203 sample; the same or an analogous band was not detected on the Rh/a- A1,0, sample or on pure a-Al,O, . Eqn. (5) is chosen to rep- resent the temperature-induced oxidation reaction of the species produced at point A. Other indications on the chem- istry which occurred during the first experimental step will be given after the discussion of Experiment 2.HIr,(CO);, + 30H--+ 4Ir'(co), + 3HCO; (5) The formation of gaseous CO,, revealed by mass spec- trometry as well as the reduction of the OH stretching band intensities and the formation of a peak at 1720 cm-' in the DRIFT spectra, sustains the formalism, eqn. (5). After cooling, a new thermal cycle was performed by intro- ducing 5 min CO pulses (flow rate 60 ml min-') into the cell at 50, 100, 200, 300,400 and 500°C. Peaks at 2055 and 1981 cm-' assigned to IrVCO), and a new band at 2045 cm-' were detected after the first two CO pulses at 50-100°C (see Fig. 4 and Table 2). This new band is assigned to vibrations of CO groups linearly bonded to Ir clusters and indicates the 2400 2000 1600 2400 2000 1600 wavenumber/cm -' wavenumber/cm-' Fig.4 DRIFT spectra in the co-stretching range recorded during Experiment 1. (a)-@) recorded during the first thermal cycle, in flowing helium, at (a) 25, (b) 100, (c) 200, (d) 300, (e) 400 and (f)500 "C and (9)on cooling to 50 "C.(h)-(n) recorded during the second heating cycle, with 5 min CO pulses, at (h) 50, (i) 100, 0')200, (k)300, (0 400 and (m)500"C and (n) on cooling to 50 "C. occurrence of CO-induced aggregation reactions of the monometallic species. At increasing temperatures the doublet assigned to IrVCO), disappeared, while the peak assigned to linearly bonded CO groups remains up to 500°C and a shoulder at 1960 cm- can also be distinguished at increasing temperatures. The formation of gaseous CO, was also detected.To summarise: the reaction between the II-,(CO)~, and the surface of the a-Al,O, produced HIr,(CO);, species, reac- tions (1)-(4) are suggested to be involved in this process. During the heating to 500°C in He, the surface clusters were first disaggregated to form IrYCO)2 species, gaseous CO, and Table 2 CO stretching frequencies measured at the peak maxima obtained in the last step of Experiments 1-3 Experiment 1 Experiment 2 Experiment 3 T/"C CO pulse He flow CO pulse He flow CO pulse He flow 50 2070, 1995 2060, 1995 2080 (sh), 2057 2046 2063, 1995 2062, 1995 100 200 2070, 1995 2070, 1995 2055, 1994 2070, 2050, 1995 2080 (sh), 2057 2052 2049 2046 300 2070,2050, 1995 2070, 2036, 1995 2048 2040 400 500 50 2040, 1995 (sh) 2035, 1974 (sh) 2067 2030, 1975 (sh) 2025, 1970 (sh) 2049, 1963 (sh) 2046 2032 2060 2035 2030 2050 carbonate surface species [see eqn.(5)]. Subsequently the monometallic species were decarbonylated. During the second heating the interactions between the bare clusters and gaseous CO molecules caused the reaggregation of the mono- metallic Ir complexes into new clusters. Experiment 2 Thermal treatment in He was followed by a second heating of the decarbonylated sample in a flowing H, atmosphere. At temperatures between 100 and 300°C a new weak band, centred near 2050 cm-', was detected and this then disap- peared at temperatures higher than 400°C while CH, and H,O were desorbed.The experiment was repeated by using deuterium instead of hydrogen and the same results were achieved (see Fig. 5). The absence of any clearly detectable isotopic shift excludes the possibility that the band could be related to Ir-H or Ir-D stretching vibrations. The purifi- cation system (see Experimental) and the analysis of the com- position of the D, and H, gases also excluded the possibility that the bands could be originated by CO impurities. Analo- gous indications were obtained while studying the reactivity of Rh species at the surfaces of a-Al,O, or MgO' during Experiments 1 and 2. We interpret these findings according to ref. 1, assuming: (i) the formation of carbidic and oxidic species by thermal dissociation of the CO ligands of the HIr,(CO);, clusters during the first experimental step and (ii) the H, (D,)-induced high-temperature reaggregation of the carbidic and oxidic species to produce new hydridocarbonyl clusters.Examples of well defined carbidic carbonyl clusters come from the studies on 0s surface chemistry which demon- strate the possibility of selectively synthesizing the [Os,C(CO), ,I2-and the [Osl,C(CO),,]2-carbidic clusters from OS,(CO),, under CO or H,-CO atmo~pheres.~~Evi-dence for the production of reactive oxidic species, comes from IR and XPS experiment^^'*^' which demonstrated the formation of Rh-oxygen adducts during the oxidative , , i' d 3800 2800 1800 21'00 1900 waven u m be r/cm -' wavenumber/cm-' Fig.5 Spectra, showing the OD and CO stretching ranges, re-corded in Experiment 2 at (a) 50°C in He flow and at (b) 50, (c) 100, (6)200, (e) 300, (f)400 and (9)500"Cin D, flow J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 decarbonylation of RhYCO), surface species. However at 400°C the carbonyl stretching bands disappeared and CH, and H,O were detected. In the third thermal cycle the recarbonylation reactions were carried out by introducing into the cell CO pulses as in Experiment 1. At 50"C, after the first CO pulse, a broad and asymmetric band at 2050 cm-' formed (see Fig. 6);this band became sharper and gradually shifted to 2030 cm-' with increasing temperatures up to 500 "C. The sharpening also reveals a shoulder at 1960 cm-(see Fig.6 and Table 2). The high-frequency band moves back to 2050 cm-' after cooling to 50°C. The spectrum produced at the end of the thermal cycle can be overlapped with the spectrum obtained at 50°C at the beginning of the cycle; this suggests that during the thermal cycle the CO pulses caused only reversible modifi- cations of the Ir aggregates. Comparison between Experi- ments 1 and 2 also indicates that carbonyl bands with the same shapes and positions were detected at temperatures above 300°C. This suggests that the same or a similar 'state of aggregation' is reached and stabilized at high temperatures in the presence of CO on both hydrogen-treated and untreated materials. When Experiments 1 and 2 were per- formed on the Rh-containing a-Al,O, the evolution of the spectral features indicated a different, continuous and irre- versible modification of the state of aggregation of the Rh clusters.' In Schemes 1 and 2 of Fig.7 we show the reactivity fea- tures that can be inferred from the results of Experiments 1 and 2 performed on the Ir- and Rh-containing a-Al,O, . Experiment 3 The Ir surface species obtained at point B were treated under CO atmosphere as described earlier. Initially, the spectra of the IrYCO), and of other carbonyl clusters described in Experiment 1 were produced and subsequently (after 30 min) i i d'i 2400 2000 1600 wavenumber/cm-' Fig. 6 Spectra recorded in Experiment 3 after CO pulses at (a) 50, (b) 100, (c) 200, (6)300, (e) 400 and (f)500°C and after cooling to 50 "C J.CHEM. SOC. FARADAY TRANS., 1994, VOL 90 Experiment 1 1r4(c0)1Z Rh4(C0)12 I CO a-A1,03 t 25-7 00"C.( C 25-500 "C, CO ii r Rh'jCO), + C02 Ir,(CO),+CO~ 20@100 "C 1 Rh,(CO), + COz I CO I a-A1203 species A Rhi (C O), Ir'(CO)2+ [Ir,C(CO)Y] + co, Ir,C + [Osl Rh,C + [OJ [HlrJ + CH, + H20J 1 [HRh,,,?] + CH4 + tiLO 25-500 "C, CO 25-500 "C. C<l1 1 (I Irxw), species D [HRh"p(co),J Fig. 7 Scheme 1: reactions detected during Experiment 1 on Ir on r-A1203(this work) and Rh on r-A1203,ref. 1. Scheme 2: reactions detected during Experiment 2 on Ir and Rh on r-Al,O I two bands at 2050 (s) and 1995 (m) cm- revealed the forma- tion of 1r4(C0)12 species (see Fig.8). The aggregative effect of the CO atmosphere on Ir1(C0), producing Ir4(CO)l ,has been observed already at the surfaces of ;,-A1,0,33 and in the cages of Na-Y zeolite' and this reaction was expected also !n this case. The subsequent heating at 100°C in 0, sharpened these two bands and the high-frequency peak was snifted slightly up to 2063 cm-'. Desorption of CO, was also seen in the exit line during the oxidative treatment. The sharpening of the bands is consistent with an 0,-induced dispersion of aggregates of Ir4(CO)12 molecules as already described in ref 23. The simultaneous desorption of CO, suggests that the carbidic species are oxidised. A CO pulse introduced into the cell, after cooling at 50°C did not modify the absorption fe't-tures of the DRIFT spectra.A different reaction was revealed when the experiment was performed on the RhIr-Al,O, sample. The Rh carbonyl clus- 79 I __-___~ ---.-.--/' 2400 2200 2000 1800 1600 wavenumber cm ' Fig. 8 Spectra recorded during Expenment 3. (a)Freshly prepared sarnple at 2S'C in He flow; (b) at 50 C after a decarbonylative thermal treatment in He and 30 min of reaction in CO atmosphere, (c)-( f)in flowing 0, at 1oO*C,(9)at 25 C after a S min ('0pulse ters were completely disaggregated at 50 -C in oxygen into Rb'(CO), and at 100"C a complete decarbonylation was achieved to produce a rhodium-oxygen adduct which in CO at 50 C gave again Rh'(CO), and gaseous CO, .' In Fig. 9 the reactions obtained with the Ir- and Rh-containing x-Al,O, art.composed. 1r4 CO),, + no rem on! 5o-1 00 "C.0,2 + RhACO), 4\rHspecies E Rh'(C;O), Fig 9 Reaction scheme detected for Experiment 3 on Ir on r-Al,O, (thi, work) and Rh on r-A1203samples (ref 1) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Experiment 4 The results obtained in the first two steps of Experiment 2 were reproduced here. In the third step 10 min CO-H, pulses (flow rate 60 ml min-') were introduced into the cell at 25, 100, 200, 300, 400 and 500°C. Broad bands were detected in the DRIFT spectra at each temperature investigated both in a flowing CO-H, mixture and in an He flow (see Fig. 10 and Table 3). The position of the peaks was always shifted 2400 2000 1600 2400 2000 1600 wavenumber/cm-' wavenumber/cm-' Fig.10 Spectra recorded during Experiment 4. (a) The decarbony- lated and hydrogen-treated sample at 50°C (b),(4,(f),(h)and (j)in CO-H, flow at 50, 100, 200, 300 and 400°C; (c), (e), (g), (i), (k) and (I) after switching to an He flow. Table 3 CO stretching frequencies revealed during Experiments 4 and 8 ~ ~~~ Experiment 4 Experiment 8 CO + H, CO, + H, T/"C pulse He flow pulse He flow 50 2010(b) 2010(b) 2003 2000 100 2010(b) 2010(b) 2017 2003 200 2032 (b) 2025 (b) 2023 2027 300 400 2033 (b) 2026(b) 2021 (b) 1993 (b) 2025 2025 2027 2025, 1970 (sh) 500 2020 (b) 1985 (b) 2026, 1978 (sh) 2028, 1966 CO pulse He flow CO pulse He flow 50 2061 2040 2052, 1965 (sh) 2050, 1965 (sh) upwards in the presence of the CO-H, mixture.The broad- ness of the bands indicates that many different aggregates are formed, and the upward frequency shift is interpreted as a consequence of the increased CO coverage of large Ir aggre- gates which cause : (i) enhancement of the dipole-dipole inter- actions between chemisorbed molecules ;34-36 (ii) decrease in the Ir-CO bond strength and increase in the IrC=O bond strength.37 The reaction between CO and H, produced CH,, H,O and CO, at temperatures >300"C. The presence of CO, is due to the occurrence of the water-gas shift reaction. Analogous results were achieved when Experiment 4 was per- formed with Rh/a-Al,O,, however, in this case, the formation of CH, ,H,O and CO, was first detected at 200 "C. Experiment 5 The interaction between CO, (5 min pulses, flow rate 60 ml min-') and the decarbonylated material obtained at point B, produced a chemical reaction at 300 "C.At this temperature a broad peak, centred near 2030 cm-', was detected in flowing CO, (see Fig. 11). We observed that on switching the CO, flow to He the peak was increased in intensity and sharpened. This result was also found in Experiment 7 (see later) where we observed that at 300°C a carbonyl band was stabilized after the CO, pulse only in an He atmosphere and disap- peared on restoration of the CO, flow. To explain these effects we tentatively propose, in accord with ref. 1, that Ir- oxygen adducts Irz(0), , Ir carbonyl clusters Ir-CO and Ir bare clusters Ir" are produced and interconverted as a result of temperature-induced aggregation reactions and oxygen- induced disaggregation reactions.The reaction cycle in which atomic surface oxygen species [O,] are also involved, is rep- resented by the following scheme. co2 The position, the intensity and the shape of the IR bands, recorded at 400 and 500°C are reported in Table 4 and Fig. 11. After cooling to 50°C absorptions at 2031 cm-' with shoulders at 2045 and 1962 cm-' were detected. The experi- ment was concluded by introducing, at 50"C, a CO pulse into the cell. This produced species with one IR band at 2054 cm-' and a shoulder at 2003 cm-' while a small amount of Table 4 CO stretching frequencies measured at the peak maxima obtained in the last step of Experiments 5-7 v(CO)/cm - Experiment 5 Experiment 6 T/"C C0,pulse He flow CO, pulse He flow 50 - - - - 100 - - - - 200 - - - - 300 - 2030, 2003 (sh) 2032, 1970 (sh) 2022, 1967 (sh) 400 2008 2023 2029, 1969 (sh) 2029, 1069 (sh) 500 2027 2023 2024, 1971 (sh) 2007 (w) CO pulse He flow CO pulse He flow 50 2056, 2005 2054,2003 2070 2043 Experiment 7 CO, pulse He flow 2060 (vw), 1995 (vw) 2060 (vw), 1995 (vw) 2060 (vw), 1995 (vw)2060 (vw), 1995 (vw) 2060 (vw), 1995 (vw)2025 2029 2029 2029 2028 2024 2022 CO pulse He flow 2072, 1998 2062, 1998 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2600 2400 2200 2000 1800 2600 2400 2200 2000 1800 wavenumber/cm-' wave n umber/cm -' Fig. 11 Spectra recorded during experiment 5.(a) The thermally decarbonylated sample; (b), (4,(f), (h), (j)and (I) during 5 min CO, pulses at 50, 100, 200, 300, 400 and 500°C; (c),(e), (g), (i), (k) and (m) in He flow after the CO, pulses; (n)the cooled sample; (c)during the final CO pulse; (p) after the CO pulse. CO, was desorbed. The formation of the CO, is interpreted by assuming that reactive oxygen species remained at the sur- faces together with Ir carbonyl clusters after the breaking of the one of the CO, carbon-oxygen bonds. Experiment 5 was performed' on Rh/a-Al,03 which showed a much lower reactivity towards the CO,; in that case a weak carbonyl stretching band formed only at 500°C and quickly disappeared under flowing He.The treatment with the CO pulse of the cooled sample produced gaseous CO, and the RhVCO), species. We concluded that inter- action with the CO, caused the oxidative disaggregation of the surface Rh clusters. Experiment 6 A decarbonylated sample was produced at point C as already described in Experiments 2 and 4. The following interaction with the CO, pulses at 300°C and 400°C produced an asym- metric peak at 2030 cm-' with a shoulder near 1971 cm-' (see Fig. 12 and Table 4). The peak was broadened and weakened at 500°C by switching the CO, flow to He. We stress that the band intensities were not enhanced in He flow as found in Experiments 5 and 7 (see later). After cooling at 50"C,a very weak absorption feature remained, centred at 2020 cm-'.This band was enhanced in intensity and shifted to 2043 cm-l when a CO pulse was introduced into the cell at 50°C and the formation of some gaseous CO, was detected. Comparison with the results of the previous experi- ment indicates that the CO, reductive chemisorption reac- tions were enhanced on the hydrogen pre-treated samples (see Table 4 and Fig. 11 and 12) but this effect is much less relevant for Ir than for the Rh/or-Al,O, sample.' In the last 2600 2400 2200 2000 1800 2600 2400 2200 2000 1800 wavenumber/cm-' wavenumber/cm-' Fig. 12 Spectra recorded during Experiment 6. (a) Thermally decarbonylated and hydrogen-treated sample; (b), (4,(f),(h), (j)and. (I) during 5 min CO, pulses at 50, 100, 200, 300,400 and 500°C; (c), (e),(g), (i), (k)and (m)in He flow after the CO, pulses; (n) the cooled sample; (0)during the final CO pulse; (p) after the CO pulse.case, at temperatures as low as 50"C, hydridocarbonyl com- plexes were detected. The enhancement in the CO, disso-ciative chemisorption on the hydrogen pre-treated samples can be interpreted by a metal-mediated reaction between a surface hydride species and CO,. This mechanism is consis- tent with the CO, insertion reactions into the M-H bonds of hydridocarbonyl complexes which occur in homogeneous liquid solution^.^^-^^ Solymosi and co-workers have shown that the CO, chemisorption reactions are enhanced on Pt, Pd, and Rh single crystals by reducing the work function of the metals with a chemical deposition of K.45-48The same effect should not be involved in the observed hydrogen- induced reactivity enhancement since H, adsorption on the same metals increases their work fun~tion.~~-~~ Experiment 7 The oxidative treatment in flowing oxygen at 100°Cdid not inhibit the high-temperature reactivity with the CO, pulses.A broad CO stretching band at 2025 cm-'was first detected at 200°C by switching the CO, flow to He (see Fig. 13 and Table 4). This phenomenon, noted in Experiment 5, is more evident when the interaction with CO, follows oxidative treatment. At higher temperatures, up to 500 "C, the carbonyl stretching bands remain in both flowing CO, and He. A sub-sequent CO pulse introduced at 50°C into the cell produced gaseous CO, and clusters with CO stretching at 2062, and 1998 (sh) cm-'.Bands with the same position and shape were detected during Experiment 3 and were assigned to Ir4(CO)1, species. In these same conditions the Rh surface species' showed a greater mobility and the carbonyl clusters were stabilized between 200 and 500°C only after switching the CO, flow to He. 2600 2400 2200 2000 1800 2600 2400 2200 2000 1800 wavenumber/cm-' waven u m ber/cm -' Fig. 13 Spectra recorded during Experiment 7. (a) Spectrum of the thermally decarbonylated and oxygen-treated sample; (b),(4,(f),(h), (j)and (0 during 5 min CO, pulses at 50, 100, 200, 300, 400 and 500 "C;(c),(e),(g), (i), (k)and (m)in He flow after the CO, pulses; (n) the cooled sample; (0) during the final CO pulse; (p) after the CO pulse.J---i 2400 2200 2000 1800 1600 2400 2200 2000 1800 1600 wavenumber/cm-' wavenumber/cm-' Fig. 14 Spectra recorded during Experiment 8. (a) Spectrum of the thermally decarbonylated and hydrogen-treated sample; (b),(4,(f), (h),(j),(0 and (n) at 50, 100, 200, 300,400 and 500°C in C0,-H,; (c), (e),(g) (i),(k),(m)and (0) at the same temperatures in He; @) during the final 5 min CO pulse; (4) the CO pulse. J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Experiment 8 A broad carbonyl stretching band centred near 1981 cm-' originated at 50°C during a 10 min pulse of the CO2-H, atmosphere and persisted in He flow. At increasing tem- peratures the bands were sharpened and one peak at 2025 cm-'and shoulders at 1970 and 2060 cm-' could be distin- guished up to 500°C.The bands detected in flowing C0,-H2 atmosphere and in He atmosphere had the same shape, posi- tion and intensity. The sharpening at increasing temperatures and the absence of any shift of the IR bands is interpreted by the selective formation of carbonyl clusters. After cooling, the peak became even sharper with a maximum at 2045 cm-' and the shoulders were shifted to 1961 and 2070 cm-' (see Fig. 14 and Table 4). When a CO pulse was introduced into the cell at room temperature desorption of CO, was observed, but the IR spectra were not modified. The same effects were also produced when Experiment 8 was performed on the Rh/a-Al,O, sample.' The occurrence of the CO, che- misorption reaction at 50°C with the formation of carbonyl clusters was not observed in Experiments 5,6 and 7.This can only be the consequence of the presence of H, in the gaseous environment. We assume that the reductive chemisorption of CO, occurred via a metal-mediated interaction with a hydrido surface species; this mechanism should be more effec- tive in the presence of gaseous hydrogen since the oxygen species produced by the breaking of the C-0 bonds can react to form OH bonds and water molecules. Conclusions DRIFT and mass spectra gathered during eight experiments gave insight on some undisclosed aspects of Ir surface chem- istry. In the following we summarize the main conclusions of the work.Formation and Reactivity of Carbidic and Oxidic Species and Metbanation Reactions Several assume that during CO hydrogenation processes, such as Fischer-Tropsch and methanation reac- tions, carbidic, oxidic and carbonyl species are simulta-neously present at the surfaces and are interconverted in stationary conditions. Our results sustain this hypothesis since there is a clear experimental evidence for the following steps in the CO hydrogenation process represented by eqn. (6)-(9), i.e. (i) the formation of carbidic and oxidic species from the thermal dissociation of C-0 bonds of carbonyl clusters; (ii) the H, (D,)-induced reaggregation of the carbidic and oxidic species into new carbonyl clusters stabilized up to 300°C; (iii) the hydrogenation (deuteriation) of the new clus- ters into bare Ir aggregates and gaseous CH, (CD,) and H20 (Dm Ir-(CO) +Ir-C + 0, (6) Ir-C + 0, + H, +HIr-(CO) (7) HIr-(CO) + H, +Ir + CH, + H,O (8) Ir + CO +Ir-(CO) (9) Surface, highly reactive, oxygen species, 0,, were also formed during CO, reductive chemisorption and were detected because of their ability to oxidize gaseous CO mol- ecules at 25 "C.We consider that these surface oxygen species were able to disaggregate and to oxidize carbonyl Ir clusters during Experiments 5 and 7. This disaggregative and oxida- tive effect should be counter-balanced by a temperature-induced reductive and aggregative effect in a flowing He environment (see Scheme 1 in Fig. 7). J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The hydrogen pretreatment moderately enhanced the C02 dissociative chemisorption which was first observed at 200 "C. However, carbonyl clusters were detected at 50 "C in a C0,-H, environment, much earlier than the products of the methanation reaction which were detected at 300°C. The low-temperature reactivity is explained by assuming a metal- mediated insertion of a surface hydrido species into a CO bond of the CO, followed by a reaction between atomic oxygen and hydrogen to produce surface OH groups and water. The hydrogenation of the surface carbonyl groups has a higher activation energy and is the rate-limiting step in the CO, methanation reaction. Ir Cluster Species at the Surfaces Tetrametallic Ir clusters were highly stabilized and strongly anchored to the surface of a-Al,O, .HIr4(CO)F1 is the candi- date species formed at point A after the chemisorption of Ir4(CO)12. The anion could not be extracted with a THF solution of PPN Cl. 1r4(C0)12 surface species were repro- duced after the interaction of a thermally decarbonylated sample with CO and were stabilized at 100°C in a flowing 0, environment. In many cases the IR bands of the carbonyl species were enhanced in intensity and sharpened at increas- ing temperatures thus indicating the selective formation of several types of small carbonyl clusters. The only exceptions to this general behaviour have been found in Experiment 4, when the interaction with H,-CO mixture was studied.In this case very broad bands were originated and were related to the unselective formation of large Ir aggregates. We are grateful to Dr. M. Marchionna and Dr. A. Vetere for helpful and stimulating discussions. References 1 L. Basini, M. Marchionna and A. Aragno, J. Phys. Chem., 1992, 96,9431. 2 Metal Clusters in Catalysis, ed. B. C. Gates, L. Guczi and H. Knozinger, Surface Science and Catalysis, Elsevier, Amsterdam, 1986, vol. 29. 3 H. H. Lamb and B. C. Knozinger, Angew. Chem., Znt. Ed. Eng., 1988,27, 1127. 4 Surface Organometallic Chemistry: Molecular Approaches to Surface Catalysis, ed. J-M. Basset, B. C. Gates, J. P. Candy, A. Chaplin, M. Leconte, F. Quignard and C. Santini, Kluwer, Dor- drecht, 1988. 5 L-F. Rao, A.Fukuoka, N. Kosugi, H. Kuroda and M. Ichikawa, J. Phys. Chem., 1990,94,5317. 6 P. Dufour, C. Houtman, C. C. Santini, J-M. Basset, L. Y. Hsu and S. G. Shore, J. Am. Chem. SOC., 1992,114,4248. 7 H. F. Van't Blick, J. B. A. Vanzon, T. Huizinga, D. D. Konis-berger and R. Prins, J. Phys. Chem., 1985,89,4783. 8 S. Kawi, J. R. Chang and B. C. Gates, J. Am. Chem. SOC., 1993, 115,4830. 9 G. Hestl, N. D. Trian Trafillou, H. Knozinger and B. C. Gates, J. Phys. Chem., 1993,97,666. 10 G-D. Lei and W. M. H. Sachtler, J. Catal., 1993,140,601. 11 Z. Zhang, B. Lerner, G-D. Lei and W. M. H. Sachtler, J. Catal., 1993,140,481. 12 Z. 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ISSN:0956-5000    

DOI:10.1039/FT9949000787

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 797-802 Photocatalysts with Tunnel Structures for Decomposition of Water Part 1.-BaTi,O, ,a Pentagonal Prism Tunnel Structure, and its Combination with various Promoters Yasunobu Inoue," Yoshihiro Asai and Kazunori Sat0 Department of Chemistry, Nagaoka University of Technology, Nagaoka, Niigata 940-21,Japan Photocatalysts have been prepared by impregnating barium tetratitanate, BaTi,O, , which has a pentagonal-prism tunnel structure, with aqueous solutions of CoCI, , Ni(NO,), , In(NO,), , RuCI, , H,lrCI, or H,PtCI, , and then activating with either reduction or oxidation. For decomposition of water under irradiation with light from a Xe lamp, reduction caused low photocatalytic activities. Oxidation of RuCI,-impregnated BaTi,O, at 848 K led to active photocatalysts which produce H, and 0, in the correct stoichiometric ratio.The photocatalytic activity increased with an increase in the amount of loaded Ru until it reached 1 wt.%, and remained nearly constant up to 3 wt.%. The X-ray photoelectron spectra showed that the active Ru was in a tetravalent state, forming RuO, on the BaTi,O, surface. UV diffuse reflectance spectra showed that BaTi,O, had a threshold of absorption of light at around 410 nm which reached a maximum at 320 nm. From high-resolution transmission electron microscopy and microanalysis the spherical RuO, particles of 1.4-3.0 nm in diameter were found to be dis- persed uniformly on the regular lattice of BaTi,O, . It is concluded that the pentagonal-prism tunnel structure of BaTi,O, has a 'nest' effect and is responsible for the high dispersion of RuO, particles, which leads to the high photocatalytic activity.In the development of photocatalysts using transition-metal oxides with high efficiency for the decomposition of water, it is important to design photocatalysts which promote the for- Experimental mation of photoexcited electrons and holes and the transfer Barium tetratitanate, BaTi,O, (BTO), was prepared by cal- of the charges to the adsorbed reactants. In this respect, it is cining a mixture of BaCO, and TiO, in air. Barium carbon- required that the oxides producing photoexcited charges are ate of reagent grade was obtained from Nakarai Chemicals combined suitably with metals or metal oxides which can act Ltd.Three kinds of TiO, were used: one from Junsei Chemi- as promoters. cal Co. Ltd., and the others were TYP-0511 and TYP-1011 In previous studies,'*2 we have shown that sodium hexa- from Nippon Soda Co. Ltd. which were in the form of titanate, Na2Ti601 3, becomes active for photodecomposition spheres and had narrow distributions with an average par- of water on combination with ruthenium oxide. The hexa- ticle size of 0.5 pm and 1 pm, respectively. Barium tetra- titanate is one of the Wadsley-Andersson type oxides3 in titanates prepared by using these forms of TiO, are referred which the octahedra share an edge at one level in linear to as BTO(J), BTO(T5) and BTO(T10), respectively. In order groups of three, giving a tunnel structure characterized by a to obtain BTO suitable for photocatalysts, the temperature of wide space corresponding to three octahedra.We have calcination was varied. The formation of BTO was confirmed pointed out that the photocatalytic activity is closely associ- by X-ray powder diffraction patterns obtained with a Rigaku ated with the presence of the tunnel structure. Recently, we Denki RAD I11 diffractometer. The UV diffuse reflectance have found4 that barium tetratitanate, BaTi,O,, is a prom- spectra were recorded on a JASCO UNIDEC 600 spectrom-ising oxide for development as a photocatalyst for the decom- eter. position of water. This oxide has an orthorhombic structure Barium tetratitanate was impregnated to incipient in which the coordination octahedra around the titanium wetness with aqueous solutions of CoCl, (Nakarai), atoms are not oriented parallel to each other, in contrast to Ni(NO,), (Nakarai), In(N03), (Junsei), RuCl, (Tanaka Na,Ti60, 3, such that a pentagonal-prism tunnel structure is Kikinzoku Kogyo, Ltd.), H,IrC16 (Tanaka Kikinzoku) or f~rrned.~.~ H,PtCI, (Soekawa Chemical Co.Ltd.). The impregnated The present study was aimed at establishing an efficient BTO was dried at 363 K and then subjected to either photocatalyst system for the decomposition of water using reduction or oxidation at various temperatures between 553 BaTi,O,. For this, optimum conditions for the synthesis of and 923 K. The amounts of promoters loaded were described BaTi,Og were investigated, and then the combinations with in terms of wt.% of metal and were maintained in the range various metals (Co, Ni, In, Ru, Ir and Pt) and their oxides 0-3.0 wt.%.which could be expected to have promoting effects were For the characterization of the photocatalysts, X-ray examined. The combination of BaTi,O, with oxidized ruthe- photoelectron spectra were recorded on a JEOL JPS-1OOSX nium was found to produce a promising photocatalytic spectrometer with an. Mg-Ka source. High-resolution elec- system, and the photocatalysts with different amounts of tron microscopic images of BTO and the photocatalysts were loaded Ru were prepared and subjected to different activa- obtained at 200 kV with a JEOL JEM-2010 electron trans- tion conditions. The effects of wavelength of light on the mission microscope. The distributions and sizes of promoters photocatalytic activity and photocurrents were investigated.loaded on BTO surfaces were measured, together with ele- In an attempt to reveal relationships between the structures mental analysis for the constituent elements at definite areas of the photocatalysts and the photocatalytic acitivity, the of the microscopic image. photocatalysts were characterized by X-ray powder diffraction, A closed gas-circulation system was used for the photo- X-ray photoelectron spectroscopy, UV diffuse reflection spec- catalytic reaction. The powdered photocatalyst (about 250 troscopy, and high-resolution transmission electron micros- mg) in a quartz reaction cell filled with 20 cm3 of distilled and COPY.deionized pure water was irradiated through a water filter J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 with an Xe lamp operated at 400 W. The reaction conditions were almost the same as those described previously.2 Hydro- gen and oxygen evolved in the gas phase were analysed by a gas chromatograph which was directly connected to the reac- tion system. Quantum yield is an important parameter to evaluate the efficiency of photocatalysts, but it is difficult to determine accurately the number of photons absorbed by photo- catalysts which are in the form of powders dispersed in water. In the present work, the ratio of H atoms produced to photons introduced to water from an Xe lamp was taken as a measure of photocatalytic activity and was represented by a term of catalytic efficiency, Q,.Monochromatic light, of dif- ferent wavelengths, was produced using a Ritsu MC-1ON monochromator, and the rate of hydrogen evolution was monitored as a function of wavelength. The number of photons emitted from an Xe lamp under the same conditions was determined by a chemical actinometer using K3[Fe(C,O&I ' 3H20.' Results Fig. 1 shows the photocatalytic activity of H,PtCl,-impregnated BTO(T10) which was subjected to reduction in H, at 773 K for 2 h. Initially, both H, and 0, were produced but the activity decreased remarkably. Fig. 2 shows the activ- ity of an RuCl,-impregnated BTO(T 10) photocatalyst which was activated either by reduction at 773 K for 2 h or by oxidation at 848 K for 7 h.Reduction brought about pro- duction of H, and O,, but the amount of 0, was about 30% less than that required by stoichiometry. Oxidation caused not only about a ten-fold larger production of H,, but also an improvement in the H, and 0, production ratio; the value off, defined as the ratio of (2 times the amount of 0,) to the amount of H, produced, was 0.96, which is quite close to unity. Fig. 3 compares the effects of various promoters (Co, Ni, In, Ru, Ir and Pt), combined with BTO upon either reduction or oxidation, on the photocatalytic activity. For reduction, which produces a metallic state, the photocatalytic activities were quite low, although 0, was evolved in addition to H, for Ru-, Ir-, or Ni-deposited BTO.Thefvalues were as low as 0.6-0.8. With oxidation, about six-fold higher activity was obtained for Ir, whereas there were no significant increase in activity with other metals apart from Ru. These results indi- cate that the combination of BTO with oxidized Ru and also oxidized Ir may result in a useful photocatalytic system. 150 -E =L 100 3U 2 n + c 3 0 50 0 2 4 6 8 time/h Fig. 2 Water decomposition on photocatalysts prepared by reduction or oxidation of RuC1,-impregnated BTO(T10). Reduction at 773 K for 2 h in a hydrogen atmosphere: H, (0);0, (y).Oxida-tion at 848 K for 7 h in air: H, (0);0, (a).1 wt.% loading as Ru metal. BTO was prepared at different calcination temperatures and employed as a photocatalyst after combination with oxi- dized Ru.As shown in Fig. 4, when BTO(T10) was used as starting material, the activity was nearly constant for calcina- tion temperatures between 1223 and 1373 K, and then " Co Ni In Ru Ir Pt (b) -20 --15--r 05s-10-. 5-0 L-U-0 2 4 6 8 Co Ni In Ru time/h Fig. 3 Effects of promoters on photocatalytic activity: H, (0);0, Fig. 1 Photoassisted water decomposition on H,PtCl,-impregrated (D). After impregnation, photocatalysts were activated by (a) BTO(T10). Pt was reduced at 773 K in an H, atmosphere: H, (0);reduction at 773 K for 2 h in a hydrogen atmosphere and (b) oxida-0,(0)-tion at 848 K for 7 h in air. 1 wt.% loading as metal. J. CHEM. SOC.FARADAY TRANS., 1994, VOL,. 90 \\-\ T,K Fig. 4 Effect of calcination temperatures of BTO on photocatalytic activity. BTO(T10); H, (01,0, (0):BTO(J); H, (A), 0, (A).BTO was impregnated with RuCI, (aq) and oxidized at 848 K; 1 wt.O/oRLI. decreased to approximately half at 1473 K. A similar change tiioc nhcnrxinA in thn coon nF RTT\(T\ In the X-ray powder diffraction patterns of BTO(T10), the but a small extra peak was observed at 20 = 31.85" with cal- cination temperatures between 1223 and 1273 K. This peak was assigned to the (1 10) plane of BaTiO, and it disappeared upon calcination above 1373 K. The surface area decreased with increasing calcination temperature, with values of 2.1 (1273), 1.1 (1373) and 0.9 m2 g-' (1473 K) for BTO(J) and 0.57 (1273) and 0.59 m2 g-' (1473 K) for BTO(T10), and 17 (1273 K) for BTO(T5). The scanning electron microscopic observation of BTO calcined at 1273 K showed that BTO(T10) consisted of rugged spherical particles (average size of 1-3 pm), whereas BTO(T5) had smaller particles of 0.5-1 pm with quite smooth surfaces.BTO(J), BTO(T10) and BTO(T5) calcined at 1273 K were combined with oxidized Ru and the photocatalytic activity compared; this was 20-30% higher for BTO(T10) than for BTO(J) and BTO(T5). Calcination at 1473 K resulted in the disappearance of the rugged structures of the BTO(T10) particles, and, as shown in Fig. 4, the photocatalytic activity decreased at this tem-perature. Fig. 5 shows the UV diffuse reflectance spectra of BTO.The absorption had a threshold wavelength at around 310 nm and reached a maximum at 320 nm. A shoulder was observed at around 380 nm for each oxide, which was signifi- cant for BTO(T10). A sample of BTO(T10) was pressed into a disc (20 mm in diameter and 0.5 mm thick), and a pair of the transparent thin Au electrodes were attached in parallel on the surface. On applying a dc voltage of 5 V, the surface photocurrents were measured in air as a function of wave-length of light from an Xe lamp. The photocurrents were gen- erated initially at 400 nm, increased markedly at wavelengths shorter than 400 nm and attained a constant level at around 310 nm. This change corresponded to that observed in the light-absorption characteristics.Fig. 6 shows changes in the photocatalytic activity Kith oxidation temperatures of RuC1,-impregnated BTO(TI 0). The threshold temperature for activation was 500 K. The activity increased with increasing temperature, reached a maximum at 848 K, and then decreased with higher oxida- tion temperatures. The production of hydrogen and oxygen 200 300 400 500 600 wavelength 'nm Fig. 5 UV diffuse reflectance spectra of BTO: BTO(J) (-), BTO(T10) (---) and BTO(T5) (- -1 was observed over the oxidation temperatures examined; the fvalue was 0.8 below 653 K, attained the stoichiometric ratio between 753 and 848 K, and then decreased to 0.8 above 953 K. Note that H, and 0, production was facilitated by oxidation and reduction was not required in the present photocatalytic system.For the active photocatalyst prepared by RuC1,-impregnation and then oxidation at 848 K, X-ray photoelec- tron spectra showed that a peak due to an Ru 3d,,, level appeared at 280.6 eV. This binding energy was the same as that found for the RuO,/Na,Ti,O, catalyst activated by Oxidation at 773 K after R~Cl,-impregnation.~ This value uas comparable with 280.7 eV for commercially available R u0, ,thus indicating that the oxidation state of Ru was +4 and the active photocatalyst was composed of a combination of BTO and RuO,. A small peak due to the C1 2p level was observed. As for the stability of RuO,/BTO(TlO) photocatalyst, there uas little deterioration of the photocatalytic activity with 600 800 1000 TiK Fig.6 Effects of oxidation temperature of RuC1,-impregnated BTO(T10) on photocatalytic activity: H, (O),0, (0);1 wt.% Ru irradiation over 30 h. A photocatalyst employed for the cata- lytic reaction was stored in water without irradiation. After three months, the photocatalytic acitivity was examined in fresh water and it was found to be unchanged. After impregnation of BTO(T10) with different amounts of Ru and then oxidation at 848 K, the photocatalytic activities were examined as a function of the amount of Ru. As shown in Fig. 7, the activity of Ru02/BTO(T10) increased with increasing amounts of Ru, levelled off at 1 wt.% and remained constant up to 3 wt.%. The stoichiometric pro- duction of H, and 0, was observed over the entire range examined. The figure also shows the activity changes with amount of Ru on BTO(J).There were similarities in the activ- ity changes between BTO(T10) and BTO(J), although the activity of the former was higher on average by a factor of 1.3. The effect of the amount of Ir deposited on BTO(T10) upon the photocatalytic activity is shown in Fig. 8. The activ- ity increased up to 1 wt.%, passed through a maximum, and then decreased considerably. High-resolution electron transmission microscopic obser- vations of BTO(T10) gave lattice images arrayed regularly up to the topmost surface. Fig. 9 shows the transmission micro- scopic image of a 1 wt.% RuO,/BTO(TlO) photocatalyst which was activated by oxidation at 848 K.Well distributed spherical dark spots of 1.4-3.0 nm in diameter could be seen on the regular BTO lattice image. An energy analysis of the characteristic X-ray peak was carried out for a small region of the dark spots, and it was demonstrated by microanalysis that they were composed of elemental ruthenium. Fig. 10 shows the catalytic efficiency, Q,, for a 1 wt.% Ru02/BTO(T10) photocatalyst as a function of wavelength of incident irradiation. The value of Q, at 360 nm was ca. 1% and increased with shorter wavelength. Measurements were repeated for photocatalysts prepared in different batches; the values obtained were 5-6% at 340 nm and 10 & 3% at 330 nm. The change in Q, with wavelength corresponded to that in absorption characteristics observed in the UV diffuse reflectance spectra.0-1 2 3 RU (wt.%) Fig. 7 Photocatalytic activity as a function of amounts of Ru deposited. BTO(T10): H, (O),0, (a);BTO(J):H, (A), 0, (A).Oxi-dation at 848 K for 7 h in air. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 20! A c I r-0 E, 102 0 1 2 Ir (wt.Y0) Fig. 8 Photocatalytic activity of BTO(T10) as a function of amounts of Ir deposited: H, (O),0, (0).1 wt.% Ir; oxidation at 848 K in air after impregnation with H,IrCl, (aq). Fig. 9 A high-resolution transmission electron microscopic image of RuO,/BTO(TlO). A white arrow shows one example of an RuO, particle. 1 wt.% Ru; oxidation at 848 K in air after impregnation with RuC1, (aq). 15 0 -2 10 me-v 0" -5 I I0 I 300 320 340 360 wavelengthlnm Fig.10 Values of catalytic efficiency Q, for RuO,/BTO(TlO) as a function of wavelength. 1 wt.% Ru; oxidation at 848 K in air after impregnation with RuCl, (as). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Discussion The X-ray diffraction patterns show that a small fraction of BaTiO, existed in BTO(T10) prepared by calcination at tem-peratures between 1223 and 1273 K and disappeared with calcination above 1373 K. The photocatalytic activities were almost the same for the photocatalysts using BTO(T10) pre- pared at 1223-1373 K, as shown in Fig. 4. Invariance in the photocatalytic activities was also observed for the BTO(J) photocatalysts prepared between 1273 and 1373 K. Thus, BaTiO, can be considered to have a negligible effect on photocatalysis. The BTO(T10) photocatalyst was 20-30% active, compared to those of BTO(T5) and BTO(J).As demonstrated by scanning electron microscopic images, the activity differences seem to be closely correlated with the presence of the rugged surface structures on BTO(Tl0). As for the effects of the promoters, Ni, Ru and Ir were able to produce oxygen not only on reduction but also on oxida- tion, whereas no oxygen was evolved for Go, In and Pt irre- spective of the treatment. For Ni-, Ru- and Ir-deposited photocatalysts which were activated by reduction, the activity was significantly lower and the f values were also lower than the stoichiometric ratio. These characteristics were improved by oxidation, in particular, for Ru and Ir metals.The X-ray photoelectron spectra showed the presence of tetravalent Ru ions by oxidation of Ru at 848 K, which indicates the forma- tion of RuO, . In the electrochemical decomposition of water, the overpotential for oxygen production was 0.22 V for ruthenium oxide electrodes and 0.28 V for iridium oxide elec- trodes in 1 mol dm- KOH solutions. These values are lower than 0.66 V for platinized Pt and 1.32 V for Pt electrode^.^ Thus, the hypothesis that the contribution of the metals and oxides deposited on BTO should decrease the overpotential for oxygen production accounts for the trend of the promoter effects shown in Fig. 3. The role of RuO, in 0, evolution has been reported by several whereas the evolu- tion of hydrogen from an RuO, surface has also been demon- ~trated.’~*’’It is a matter of controversy whether or not RuO, acts as an oxidation site.However, from the role of RuO, in decreasing the overpotential for oxygen production and the fact that the appearance of the high photocatalytic activity is constantly accompanied by the oxygen evolution, it is likely that the RuO, surface becomes an active site for the transfer of holes to adsorbed OH-species. It is of particular interest that photocatalytic activity of Ru0,JBTO occurred without any reduction. The fact that deterioration of the activity was hardly observed is related to activation in an oxidising atmosphere. Thus, it is unlikely that oxygen atoms produced in the decomposition of water are incorporated into the lattice of BTO surface, which has been thought to be one of the reasons for the lack of oxygen as a product in conventional Pt-TiO, photocatalysts.The total number of hydrogen atoms produced after 30 h irradia- tion is 550 times larger than that of the exposed Ti ions which was calculated from the surface area and the unit cell dimensions. The catalytic efficiency Q,of 10 f3% at 330 nm was higher by a factor of 2.5 than that of the RU02/Na2Ti601 photocatalyst obtained in our previous study., The value is similar to that (at 330 nm) obtained by Domen et al. for the initial stage of water decomposition on the NiO(O.1 wt.%)/Rb4Nb60, photocatalyst’6 (ca. 10%) and much higher than that for NiO(O.1 wt.%)/K,Nb,O, l7 (3.5%).Thus it is evident that the combination of RuO, and BTO has high potential for photocatalysis. As shown in the high-resolution transmission electron microscopic image, the spherical RuO, particles of 1.4-3.0 nm in diameter were uniformly distributed over the BTO surface. The dependence of photocatalytic activity on the amount of Ru deposited exhibited a nearly linear increase in 801 the low concentration of Ru until levelling off occurred at 1 wt.% Ru. These results suggest that an increase in number rather than growth of the RuO, particles is responsible for the activity increase below 1 wt.% Ru. There was a maximum for the dependence of the activity on the amount of Ir. It is not yet clear why there is a drastic decrease in the activity above 1 wt.%, but is seems likely that the complete oxidation of Ir to form IrOz at 848 K becomes dificult with increasing Ir, which lowers the effects of the promoter.The main feature of the structure of BTO is the presence of the pentagonal-prism tunnel structure, as shown in Fig. 1 l(a). Although most of the RuO, particles were considerably larger than the space of one pentagonal-prism tunnel, it is considered that the uniform distribution of the small RuO, particles is strongly associated with the characteristic struc- ture of the tunnel. The pentagonal-prism space provides the form of a ‘nest’, i.e., a concave site with a ridge. This unique structure substantially prevents RuO, particles from aggre- gating and growing into larger particles.An example of nest- egg type incorporation which accommodates a 1.4 nm particle in the tunnel space is shown in Fig. ll(b). In this model, it is anticipated that strong interactions between the small RuO, particle and the surrounding TiO, octahedra are produced, because of larger interfacial contact. This facilitates the formation of photoexcited electrons and holes and then the transfer to the adsorbed species. It is certain that the pres- ence of the tunnel structure causes the distortion of Ti06 octahedra, which appears to be closely associated with effi- cient production of photoexcited charges. A study to eluci- date a relationship between photoexcitation and the tunnel structure is in progress.=L X 0 0 Fig. 11 Schematic projection of BaTi,O, (a) and a model (b) of a ‘nest’ effect of a pentagonal-prism tunnel in the RuO,/BTO photo-catalyst. 802 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 This work was supported by the Kajima Foundation’s Research Grant and a Grant-in-Aid for Scientific Research on Priority Area for the Ministry of Education, Science and Culture of Japan. We thank the EM Application Laboratory of JEOL Ltd. for helpful observation of high-resolution transmission electron microscopic images. 9 10 11 12 M. Morita, C. Iwakura and H.Tamura, Electrochim. Acta, 1978, 23, 331 and ref. therein. J. M. Lehn, J. P. Sauvage and R. Ziessel, Noun J. Chim., 1979, 3, 423. T. Kawai and T. Sakata, Chem.Phys. Lett., 1980,72,87. D. Duonghong, E. Borgarello and M. Gratzel J. Am. Chem. SOC., 1981,103,4685. References Y. Inoue, T. Kubokawa and K. Sato, J. Chem. SOC., Chem. Commun., 1990,1298. Y. Inoue, T. Kubokawa and K. Sato, J. Phys. Chem., 1991, 95, 13 14 15 G. Blondeel, A. Harriman, G. Porter, D. Urwin and J. Kiwi, J. Phys. Chem., 1983,87,2629. E. Amouyal, P. Keller and A. Moradpour, J. Chem. SOC., Chem. Commun., 1980,1019. T. Sakata, K. Hashimoto and T. Kawai J. Phys. Chem., 1984,88, 4095. 5214. S. Anderson and A. D. Wadsley, Acta Crystallogr., 1962, 15, 194. Y.Ionue, T. Niiyama, Y. Asai and K. Sato, J. Chem. SOC., Chem. Commun., 1992, 579. F. W. Harrison, Acta Crystallogr., 1956,9, 198. K. Lukaszewicz, Procz. Chem., 1957,31,1111. 16 17 K. Sayama, A. Tanaka, K. Domen, K. Maruya and T. Onishi, J. Catul., 1990, 124, 541. A. Kudo, A. Tanaka, K. Domen, K. Maruya, K. Aika and T. Onishi J. Catal., 1988, 111,67. C. G. Hatchard and C. A. Parker, Proc. R. SOC.London, A 1956, 235, 518. W. Hofmeister, E. Tillmanns and W. H. Bauer, Acta Crystal- logr., Sect. C, 1984,40, 1510; U. Balachandra and N. G. Eror, J. Am. Ceram. SOC.. 1982.65. C-54. Paper 3/052015;Receiued 31st August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000797

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 803-807 Oxygen Exchange between Magnesium Oxide Surface and Carbon Dioxide Hideto Tsuji, Tetsuya Shishido, Akie Okamura, Yunzhi Gao, Hideshi Hattori' and Hideaki Kita Division of Materials Science Graduate School of Environmental Earth Science Hokkaido University, Sapporo 060,Japan The isotopic distribution of carbon dioxide desorbed from an MgO surface containing adsorbed '80-labelled carbon dioxide (C'802)has been measured by temperature-programmed desorption (TPD), in order to study acid-base pair sites on the surface. Three desorption peaks, differing in both isotopic distribution and tem- perature of desorption, were observed. The desorption peak in the temperature range 300-400 K (region I) is due to C'60180and Cl8O, in a ratio of nearly 1 :1.The desorption peak in the range 400-600 K (region II) is composed of Cl6O,, C' 60'80and Cl8O,,among which C"0, was most dominant. The third peak appeared in the temperature range 600-1000 K (region Ill). This peak consists mostly of C1602. For most of the adsorbed species, the surface Mg2+ contributes to CO, adsorption. It is concluded that the adsorbed CO, undergoes multiple oxygen exchange with the surface while migrating over it. Based on IR measurements of the adsorbed CO, , it is suggested that migration takes place during heating of the sample to 473 K in the TPD run. Carbon dioxide is frequently used as a probe molecule for investigating the basic properties of metal oxide surfaces in different methods such as IR and TPD.In the TPD of adsorbed CO, ,the concentration of the basic sites is reflected in the peak areas of the TPD plot, and the strength of the basic sites in the temperature at which the CO, desorption peak appears.lW3 In the IR study of adsorbed CO,, carbon-ate species of different types such as unidentate carbonates, bidentate carbonates, carbonate ions and hydrogen-carbonates are formed, depending on the adsorption condi- tions and the surface ~tructure.~ On a well degassed MgO at an elevated temperature (pretreatment temperature) in a vacuum. The surface areas were 166 and 90 m2 g-' for the MgO samples prepared at 973 and 1273 K, respectively. The '80-labelled carbon dioxide was supplied by Icon and its iso- topic purity was 99%.For the TPD experiments, the Mg(OH), sample (100 mg) was placed in an adsorption vessel and pretreated at 973 K or 1273 K in a vacuum for 3 h (ca. 10-3 Pa). After cooling to room temperature, a known amount of C1'0, was intro- duced into the vessel. The residual pressure was negligible. Practically all of the CO, introduced into the vessel was surface, unidentate and bidentate carbonates are f~rrned.~.~ adsorbed on the MgO. The TPD was run from room tem- Fukuda and Tanabe6 reported that at a high coverage of adsorbed CO, , unidentate carbonates are predominant, while at a low coverage, bidentate carbonates become pre- dominant. Recently, Yanagisawa et a/.' have reported that oxygen exchange between adsorbed CO, and the MgO surface takes place to a considerable extent.They observed a TPD desorp- tion peak consisting mainly of Cl6O, and Cl60l8O after C"0, adsorption on MgO, and suggested that the adsorbed C'80, interacts with the peroxy ion [('60s)22-] on a defect in the MgO surface. We have observed essentially the same phenomena in the TPD study of adsorbed Cl80,. The occurrence of oxygen exchange between CO, and the oxide surface indicates that CO, interacts not only with the basic sites of 02-,but also with the acidic sites of surface metal cations. Therefore, elucidation of the exchange mecha- nisms is expected to reveal the surface acid-base properties. The isotope distribution reflects the variety of adsorption sites and acid-base properties on the surface.In the present paper, we report a detailed TPD study of the MgO surface interacting with C1802, as well as an IR study of the adsorbed CO, . Experimental The MgO sample was prepared from commercially available MgO (Merck). Impurity levels in the MgO were at most 0.2 wt.% for Na and 0.035 wt.% for the other cations. The MgO powder (Merck) was soaked in distilled water and hydrated at ambient temperature for 24 h. After evaporation of water, the resulting magnesium hydroxide was dried at 373 K for 24 h, and used as a precursor of the MgO sample. The MgO sample was obtained by thermal decomposition of Mg(OH), perature to 1073 K at a heating rate of 10 K min-'. The desorbed gases were analysed by mass specrometry, using an Anelva AQAl00R quadrupole mass spectrometer. A small quantity of argon was continuously introduced into the system, and each peak in the mass spectrum was normalized to the argon peak intensity. For the IR experiments, an Mg(OH), disk was placed in an in situ IR cell, and pretreated similarly.A known amount of CO, was introduced into the cell. The sample with adsorbed CO, was heated in a vacuum, increasing the temperature by 100 K increments up to 973 K. After evacuating the sample at each step for 30 min, the sample was cooled to room tem- perature and an IR spectrum was recorded on a Jasco FTIR 5300 spectrometer. Results and Discussion TPD plots for desorption of each type of isotopically labelled carbon dioxide from the MgO sample pretreated at 973 K are shown in Fig.1. The concentration of the adsorbed carbon dioxide was 410 pmol g- which is equal to one mol- ecule of carbon dioxide per 67 A'. This concentration is close to that of the CO, remaining on the MgO after exposure to 20 Torr CO,, followed by evacuation at room temperature. In the case of TPD measurements without admission of CO, ,no significant desorption peaks were observed. The plot for the total CO, can be divided into three regions in terms of both desorption temperature and isotopic distribution. The first region (region I) ranges from room temperature to 400 K, the second one (region 11) from 400 to 600 K, and the third (region 111)above 600 K. In region I, the desorbed CO, comprised C1802 and C'60'80 in a ratio of close to 1 : 1.The detection of I I 1, I1 I 200 400 600 800 1000 1200 desorption temperature/K Fig. 1 TPD plots for C"0, adsorbed on MgO pretreated at 973 K. (-) Total CO,, (--) Ci802, (---) C'60180, (---) Cl60,. CO, concentration, 410 pmol g- '. (Peak heights are relative to that of Ar.). C160180just above room temperature indicates that the oxygen exchange occurs at room temperature. In region 11, C'60180 was dominant, though considerable fractions of Cl6O, and Cl80, were included. In region 111, the desorbed CO, was composed mostly of C1602, a small fraction of C160180being included. The desorption of C1802 was scarcely observed. The appearance of three regions in the TPD plot indicates the existence of three kinds of adsorption site, differing in adsorption strength for CO, .Since the isotopic distribution for each region was different, a mixing of the CO, adsorbed on different kinds of site does not take place upon adsorption at room temperature. Note that the classification of the surface adsorption sites becomes much clearer by the use of isotopic CO, and measurement of the isotopic distribution. On the well degassed MgO surface, two types of CO, adsorption were reported on the basis of the IR st~dy.~,~ One type of adsorbed species was unidentate carbonate and the other bidentate carbonate. In the form of unidentate carbon- ate, the adsorbed species is bound to the surface through a bond between the C in CO, and the surface 0 as shown in Fig.2(a). A single adsorption-desorption will not involve oxygen exchange between CO, and the MgO surface. In the form of bidentate carbonate, the adsorbed species is bound to the surface through two bonds: one between the C in CO, and the surface 0, and the other between an 0 in CO, and Mg as shown in Fig. 2(b). From the bidentate formed upon adsorption of C1*02,C180, will be desorbed if bonds I1 and I11 break, and C'60'80 will be desorbed if bonds I and IV break. This mechanism was proposed by several workers8-l * for the oxygen-exchange reaction between CO, and oxide surfaces. The desorbed CO, will be a J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1:1 mixture of C'60'80 and C"0, upon TPD if the l8O bound to Mg is equivalent to the l6O originating from the MgO surface.In region I, the desorbed CO, was a nearly 1: 1 mixture of ClaO, and C'60'80.This suggests that a major part of the adsorbed species desorbed in region I is in the form of biden- tate carbonate, and that the two C=O bonds [bonds I and I1 in Fig. 2(b)] are equivalent. In regions I1 and 111, the major isotopic form of CO, desorbed was Cl60,. If C"0, were adsorbed on one pair of Mg-0 sites in the form of bidentate carbonate, C1602 should not have been included in the desorbed CO,. There-fore, it is suggested that for the desorption of CO, in regions I1 and 111, processes other than simple adsorption-desorption of CO, on one pair of MgO sites are involved. There is no region in which Cl8O, is dominant over C160'80 and Cl6O,.Thus, the fraction of CO, adsorbed as unidentate carbonate is quite small. This indicates that for most adsorbed species bond formation between Mg2+ and the oxygen of CO, is involved in adsorption. On a thoroughly degassed MgO surface, coordinatively unsaturated Mg2+ cations are exposed,' '*12 and exhibit Lewis-acidic nature.13 Molecular orbital calculations have also pointed out that the coordinatively unsaturated Mg2 + cations can be regarded as Lewis-acid sites and that the acidity of the Mg2+ becomes stronger as the coordination number of 02-to Mg2+ decrease^.'^ In fact, amm~nia,'~*'~ hydrogen' 7-20 and hydrocarbons' 1-23 undergo heterolytic dissociation upon adsorption on the Mg2+-02- acid-base pair sites of MgO pretreated at a high temperature.We believe that CO, is also adsorbed on Mg2+-02- pair sites in the form of bidentate carbonate with an acid-base bifunc- tional interaction, though CO, does not seem to undergo heterolytic dissociation. It is certain that bond formation between the Mg2+ acid sites and the oxygen of CO, plays a role in oxygen exchange with the MgO surface during the adsorption-desorption cycle. Fig. 3 shows the TPD plot for each type of isotopically labelled CO, after C"0, was adsorbed on the MgO sample pretreated at 1273 K. The concentration of adsorbed CO, was 410 pmol g-'. Compared with the TPD plot in Fig. 1, the desorbed amounts increase in regions I and 11, and decrease in region 111. It is obvious that the number of surface sites which adsorb CO, strongly has decreased.The changes are considered to be due to the decrease in surface area of the MgO. The isotopic distribution and temperature range of each region, however, remain unchanged. In the case 200 400 600 800 1000 1200 desorption temperature/K Fig. 2 Structures of (a) surface unidentate and (b)surface bidentate Fig. 3 TPD plots for C'*O, adsorbed on MgO pretreated at 1273 carbonate, formed by adsorption of CO, on well degassed MgO K. All details as Fig. 1. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 of the sample pretreated at 973 K, small amounts of OH groups remain on the MgO surface, and the OH groups are completely eliminated by pretreatment at 1273 K.24*2sThere-fore, it is suggested that the I60 atoms incorporated into the desorbed CO, originate from lattice 0,-, and not from the surface OH groups.Lower-coordinated ion-pair sites, expressed as MgEc-O&, become exposed on the MgO surface by increasing the pretreatment temperature,' and the catalytic properties change with pretreatment temperature.26 In par- ticular, Mgfg -0:; ion-pair sites for which unsaturation of the coordination is most extensive are considered to be revealed at pretreatment temperatures above 973 K. The reactivity of the surface ions may increase as the coordination number becomes lower because the protrusion of wavefunc- tions is greatly enhanced at edges and corners on the surface of partially ionic crystals.27 However, the CO, desorption temperature was the same for the MgO samples pretreated at 1273 and 973 K.This indicates that the pretreatment above 973 K does not enhance the adsorption strength of MgO towards CO, . The molecular orbital calculation studyI4 pointed out that the basic strength of the 0,-ions is enhanced in the following manner; fewer Mg2+ cations are coordinated to the central 0,-ion in the basic sites and more 0,-ions are coordinated to the Mg2+ cations adjacent to the central 0,-ion. Our observation for the adsorption strength of CO, also suggests that the basicity of the 0,-ion is not strongest for MgSb-O&. We wish to correlate each desorption region of CO, in the TPD plot with the surface structure. The results obtained in the present study reveal the rearrangement of the surface Mg-0 bonds by interaction with CO,.To correlate the desorption peak in the TPD plot of adsorbed CO, with the surface structure, such a rearrange- ment of the surface Mg-0 should be taken into account. To investigate the CO, desorption in regions I1 and I11 in more detail, smaller amounts of C1802were adsorbed on the MgO sample pretreated at 973 K, and subjected to TPD measurements. TPD plots for each isotopic CO, are shown in Fig. 4 and 5. The concentrations of adsorbed CO, were 210 pmol g-', corresponding to one molecule per 134 A2 for Fig. 4, and 41 pmol g-', corresponding to one molecule per 670 A2 for Fig. 5. In Fig. 4, two peaks are observed. They correspond to the two peaks in regions I1 and I11 in Fig. 1.By reduction of the adsorbed amount of CO, to about a half, the peak in region I was removed. The main isotopic CO, species were C160180 81 I 0;6t.-0, 24 n$ 1 ILP ~ ~~~~~~~ 200 400 600 800 1000 1200 desorption temperature/K Fig. 4 TPD plots for C1802 adsorbed on MgO pretreated at 973 K. As Fig. 1 except CO, concentration,210 pmol g-I. 805 1.2 1.a N 0 0.8 E .P 0.6 E A3 0.4 a QJ .--0.2 2 0 200 400 600 800 1000 1200 desorption temperature/K Fig. 5 TPD plots for C'802 adsorbed on MgO pretreated at 973 K. As Fig. 1, except CO, concentration, 41 pmol g-'. for region 11, and Cl6O, for region 111.The fraction of C"0, was small for region 11, and not appreciable for region 111.Extensive 0 exchange between adsorbed CO, and the surface 0 should take place. The exchange is more extensive for the CO, desorbed in region 111. Extensive 0 exchange was more clearly demonstrated by further reduction of the amount of adsorbed C1802.In Fig. 5 where one C1802 molecule per 670 A2 was adsorbed, only one peak appeared in the TPD plot of the total CO,. This peak coincides with the peak in region I11 (Fig. 1). The coin- cidence of the peak in Fig. 5 with the region I11 peak in Fig. 1, and the disappearance of the peaks in regions I and I1 indicate that CO, is selectively desorbed from the strong adsorption sites when a small amount of CO, was admitted. The desorbed CO, consisted of 90% Cl6O, and 10% C160180,with no appreciable amount of C1802.Ito" has reported that multiple oxygen exchange between CO, and the lattice oxygen of MgO takes place by repetitive adsorption-desorption of CO, molecules. However, multiple oxygen exchange was observed in the presence of gas-phase CO, (> 1 kPa) at a reaction temperature of around 1123 K. The reaction conditions of his study were greatly different from those of the present study. In the conditions for Fig. 5, the partial pressure of CO, after admission of one molecule per 670 A' was negligible, which is evidenced by the absence of a desorption peak below 500 K. Therefore, it is excluded that the multiple 0 exchange between CO, and the surface 0 occurs through repetitive adsorption-desorption of CO, molecules.As the mechanism for the multiple oxygen exchange between CO, and the surface oxygen atoms, Yanagisawa et d7 proposed that the adsorbed C180, interact with the surface peroxy ions [( l60s);-] to form tentative intermediate rolling carbonate ions. Takaishi and Endoh,' also proposed the rolling three-fold coordinated carbonate as a mechanism for the oxygen exchange between CO, and the framework oxygen of zeolite A. Both of these mechanisms would result in the desorbed CO, containing at least 17% CI8O, even if four C-0 bonds are formed (with equivalent bonds in the tentative intermediates), and therefore, cannot explain the absence of C"0, and the extensive desorption of CI6O, in region 111, as observed in the present study.To explain the absence of C1802, the adsorbed species must encounter several surface oxygen atoms before desorption. We propose that extensive oxygen exchange results from the adsorbed CO, migrating over the surface without leaving the surface. During the migration over the surface, the adsorbed CO, forms bidentate carbonate and undergoes 0 exchange with the surface 0 atoms. The proposed processes are shown in Fig. 6. There are at least two ways, processes (I) and (11), for the adsorbed car- bonate species to migrate over the surface. In process I, carbon dioxide rolls over the surface in such a way that the free oxygen atom in the bidentate carbonate approaches the adjacent Mg atom on the surface. Three C-0 bonds do not break during the migration in this process.In process (11), the carbon atom approaches the adjacent 0 atom on the surface. One of the C-0 bonds breaks when the C atom forms a bond to the adjacent 0 atom. In process (I), the carbonate species always contains two l80atoms while migrating over the surface. Therefore, repe- tition of process (I) will result in the exchange of one oxygen atom, but not the exchange of two oxygen atoms in the desorbed CO,. The repetition of process (11) will also result in the exchange of one oxygen atom in the desorbed CO,. The free oxygen atom (0')is always away from the surface and not exchanged with the surface oxygen atom. For evolu- tion of Cl60, both processes (I) and (11) should be involved, assuming that replacement of the surface Mg-0 bond by the bond between 0 in CO, and surface Mg occurs upon desorption.The extensive formation of I60-rich carbonate species must be caused by repetition of processes (I) and (11). The occurrence of processes (I)or (11)seems to depend on the surface structure of MgO. A difference in the probabilities of processes I and I1 results in a difference in the isotopic dis- tribution in the desorbed CO, . In addition to processes (I) and (11), a further process, (111), should be considered. This process is essentially the same as the mechanism proposed for the oxygen exchange between bidentate carbonate and oxide surface as described earlier.*-'* The carbonate species could migrate over the surface over a long distance by a combination of process (111) with processes (I) and (11) without leaving the surface if process (111) exists.The Mg-0 sites from which CO, molecules are desorbed in regions I1 and I11 should be different from those on which CO, molecules are adsorbed. In other words, the CO, mol-ecule is adsorbed on a certain site on the MgO surface and desorbed from a different sites. On the CO, migration path, the surface Mg-0 bonds break and reform as CO, migrates over. 05 0"cP process (II) O$+7 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 It is not certain at this moment whether the migration takes place at room temperature or during heating in the TPD run. IR spectra of adsorbed Cl6O, were measured after heating the C' 60,adsorbed sample at different temperatures for 30 min in a vacuum, and are shown in Fig.7. The spectra, with subtraction of the spectrum of MgO pretreated at 973 K (background), are shown in the range from 800 to 2000 cm-'. The concentration of the adsorbed CO, was one molecule per 670 A', and is the same as the TPD run shown in Fig. 5. No peaks assigned to CO, weakly adsorbed onto cationic sites were observed in all spectra. It is confirmed that the weakly adsorbed linear species of CO, is not involved in the processes of the migration. At room temperature adsorption, two bidentate carbonates were observed. The peaks appeared at 1668, 1320, 1005 and 849 cm-' and those at 1630, 1277, 955 and 833 cm-' are assigned to bidentate carbonates.6 The broad peaks at 1517 and 1398 are assigned to unidentate carbonate.6 These peaks were not changed by evacuation at room temperature.By elevating the temperature to 373 K [Fig. 7(b)], the spectrum was changed. The peaks at 1630, 1277, 955 and 833 cm-' which are assigned to one bidentate carbonate disappeared and those at 1666, 1323, 1007 and 850 cm-' increased in intensity. As the temperature was raised, the spectrum grad- ually changed to give sharper peaks and the peak positions were slightly shifted. At 573 K [Fig. 7(d)], the peaks assigned to unidentate carbonate converted to bidentate carbonate and four peaks assigned to bidentate carbonate were posi- tioned at 1655, 1331, 1036 and 856 cm-'. These peaks increased in intensity up to 573 K and decreased greatly on 10.1 (f) IIII IIII I09 o~c-p'o process (III) 08-Cfl Fig.6 Proposed processes for the mechanism of migration of surface bidentate carbonate J. CHEM. SOC. FARADAY TRANS., 1994. VOL. 90 evacuation at 673 K for 30 min. The changes in the shdpe and position of the peaks were most extensive when the tem- perature was raised from room temperature to 473 K. Above 473 K. the change in the spectrum was small. It is, therefore. suggested that the bidentate carbonate formed on room tem- perature adsorption of CO, migrates over the surface as the temperature is raised in the TPD run. The migration occurs mostly in the temperature range from room temperature to 473 K. Conclusions The results are summarized as follows: (1) Oxygen exchange between adsorbed CO, and the MgO surface was found on TPD analysis by using C'*O, as adsorbate.(2)The oxygen exchange occurs at room temperature. (3) Most of the CO, molecules are released from the MgO surface through the adsorbed species in the form of bidentate carbonate. Not only 02-basic sites but also adjacent Mg" acidic sites participate in CO, adsorption. (4)The extensive desorption of C160, found above 600 K on TPD after adsorption of C"02 is explained by migration of adsorbed carbonate species. The migration takes place during heating of the sample up to 473 K in the TPD run. The concept of the adsorption of CO, on acid-base pair sites in the form of bidentate carbonate will be applicable to other systems.References G. Zhang, H. Hattori and K. Tanabe, Appl. Catal., 1988,36. 139. R. Philipp, K. Omata, A. Aoki and K. Fujimoto. J. Catal.. 1992, 134,422. A. L. McKenzie, C. T. Fishel and R. J. Davis. J. Catal., 1992. 138, 547. L. H. Little, Infrared Spectra of Adsorbed Species, Academic press, New York, 1966. S. J. Gregg and J. D. Ramsay, J. Chem. SOC. A, 1970,2784. Y. Fukuda and K. Tanabe, Bull. Chem. SOC.Jpn., 1973,46, 16!6 Y. Yanagsawa, H. Shimodama and A. Ito, J. Chem. Soc., Chem. Commun., 1992,610. J. B. Pen, J. Phys. Chem., 1975. 79, 1582. C. Gensse, T. F. Anderson and J. J. Friplat. J. Phys Chem.. 1980. 84, 3562. T. Ito. J. Chem. Soc., Faradar Trans. 1, 1982, 78, 1603. E. Garrone, A. Zecchina and F. S. Stone, Philos.May. B. 1980. 42, 683. S. Coluccia, in Proc. Symp. Adsorption and Catalysis on Oxide SurfLlces, London. 1984, ed. M. Che and G. C. Bond, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 1985, p. 59. A. Zecchina and F. S. Stone, J. Catal., 1986, 101, 227. H. Kawakami and S. Yoshida, J. Chem. SOC.,Faraday Trans. 2. 1984.80,92 1. S. Coluccia, E. Garrone and E. Borello. J. Chem. Soc., Faraday Trans. 1, 1983, 79, 607. E. Borello, S. Coluccia and E. Garrone, J. Catal., 1985,93, S31. S. Coluccia and A. J. Tench, in Proc. 7th In?. Congr. Catal.. Tokj.0, 1980, ed. T. Seliyama and K. Tanabe, Elsevier, Amster- dam. 1981, Part B, p. 1154. T. Ito, T. Sekino, N. Moriai and T. Tokuda, J. Chem. Soc, Faraday Trans. 1, 1981,77, 2181. S. Coluccia, F. Boccuzzi, G. Ghiotti and C. Morterra, J. Chem. Soc., Faraday Trans. 1, 1982,78,2111. T. Ito. T. Murakami and T. Tokuda, J. Chem. Soc.. Faraday Trans. 1, 1983, 79, 913. M. Utiyama, H. Hattori and K. Tanabe, J. Cutal., 1078, 53. 237. E. Garrone and F. S. Stone, in Proc. 8th Int. C'onyr. C'atal.. Berlin. 1984, Verlag Chemie. Dechema, Berlin, 1984, vol. 3, p. 441. M. F. Hoq, I. Nieves and K. J. Klabunde, J. Catul., 1990, 123, 349. P. J. Anderson, R. F. Horlock and J. F. Oliver, Trans. Faraday Soc., 1965,61. 2754. S. Coluccia, L. Marchese, S. Lavagnino and M. Anpo, Spectro-chim. Acca, Part A, 1987,43, 1573. H. Hattori, Muter. Chem. Phys.. 1988, 18, 533. C. Satoko, M. Tsukada and H. Adachi, J. Phys. SOC.Jpn., 1978, 45, 1333. T. Takaishi and A. Endoh, J. Chem. Soc.. Faradar Trans. 1. 1987. 83, 41 1. Paper 3/05168D; Receiued 26th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000803

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 809-814 Catalytic Studies with Dealuminated Y Zeolite Part 2.-Disproportionation of Toluene Nigel P. Rhodes and Robert Rudham* Department of Chemistry, University of Nottingham, Nottingham,UK NG7 2RD Toluene disproportionation has been studied on a series of hydrothermally dealuminated Y zeolite catalysts, both in the ‘as prepared ’, unextracted state and following extensive extraction of non-framework aluminium with aqueous Na,H,EDTA (H,EDTA = ethylenediamine tetracetic acid). Extraction had negligible effect on the frame- work aluminium content, which ranged from 13-25 Al atoms per unit cell, but reduced the non-framework alu- minium to an average of five Al atoms per unit cell. The initial activity at 673 K for both unextracted and extracted catalysts, using a microreactor in a continuous flow system, arose from a limited concentration of strong Br~nsted-acid sites.These were generated by synergic interaction between framework hydroxy groups and non-framework aluminium species. After 4-20 h on stream a pronounced maximum in the activity was observed with all catalysts. Since considerable coke had formed at this stage, ‘catalytically active coke’ was considered to be the seat of reaction, with an active site concentration directly related to the total Bransted acidity of the freshly activated zeolite. Two mechanisms, involving either proton addition to, or hydride ion abstraction from, the reactant toluene molecule, are suggested; the contribution these each make depends on the extent of reaction with respect to complete poisoning by coke.Early studies of the disproportionation of toluene to benzene and dimethylbenzene on Y zeolites, made in pulse-flow microreactors at 623-673 K, established that Brnmsted-acid centres were the seat of catalytic a~itivity.’-~ However, a maximum in the activity of HY generated from NH,Y at a temperature above that for complete NH, evolution,’ the high initial activity of AlHY compared with HY,2 and the small fraction of the total acidity active with stabilised Y zeo-lite~,~pointed to the need for superacid sites to effect reac- tion. Such superacidity can be achieved by synergic interaction between the framework hydroxy groups of Br~nsted acidity with the extra-framework Lewis-acid centres associated with charge-balancing cations containing A1 or with polymeric oxoaluminium More recent studies of toluene disproportionation, using continuous-flow micro- reactors, include comparisons of the activity of unstabilised6 and ~tabilised~.~Y zeolites with those of HZSM-5 and HZSM-11.Ultrastable Y was found’ to be considerably more active than HZSM-5 of similar framework aluminium content under the same reaction conditions at 723 K. Further measurements8 with ultrastable Y led to the conclusion that the concentration and acid strength of the active centres is regulated by the non-framework aluminium content. The fall in activity for toluene disproportionation on Y zeolites with pulse n~mber,~.~ time-~n-stream,~-’ is or accompanied by the increasing retention of carbonaceous compounds, termed ‘coke’, within the zeolite pore system.At temperatures where the disproportionation reaction predomi- nates, the coke consists of polyaromatic molecules of size approaching that of the zeolite supercages.’ Albeit with HZSM-5, studies show that both benzene and toluene can form tricyclic aromatics in a reaction where biphenyl and diphenylmethane are the primary condensation products. lo The present paper reports an investigation of toluene dis- proportionation on the same series of hydrothermally dealu- minated Y zeolites previously used in studies of ethylbenzene disproportionation. ’ Activity measurements at 673 K were made with ‘as prepared ’ catalysts and again following extrac- tion of non-framework aluminium with Na,H,EDTA.Since extraction had minimal effect on the framework structure, this permitted the catalytic effects of non-framework alu- minium to be assessed for a reaction which demands very strong acid sites. Unlike previous ~tudies,~*~*~-’ the activity passed through a maximum after 4-20 h on stream; this is attributed to the formation of coke which is catalyticalby active. Experimental Catalysts The catalysts were prepared by steaming samples from a single batch of highly exchanged NH,Y zeolite at six tem- peratures between 823 and 1073 K.” Non-framework alu- minium was extracted by treatment with aqueous Na,H,EDTA followed by aqueous NH,Cl.The unextracted catalysts are designated HYSTx where x is the final steaming temperature, and extracted catalysts are designated HYSTxEX. Our earlier paper” gives details of the determi- nation of the framework aluminium content per unit cell, AlF ,by infrared spectroscopy, and of the non-framework alu- minium content per unit cell, AlNF, from the difference between AlF and the total aluminium content per unit cell, Al,, derived from bulk Si : A1 ratios. Values of AlF, AlNF and the acidity per gram are given in Table 1 for the twelve dealu- minated catalysts, the starting material NH,YLS and a highly exchanged sample NH,Y. Further information on the properties of these catalysts, including specific micropore and mesopore volumes, has previously been given.’ ‘ Catalytic Activity Measurements The majority of activity measurements of toluene dispro- portionation were made using a continuous-flow micro-reactor system operating at a total pressure of 1 atmt, where the catalyst was in the form of a thin layer of powder sup-ported on a disc of sintered silica fused to the walls of a tubular reaction vessel.Accurately weighed catalyst samples corresponding to ca. 20 mg of dry zeolite were activated within the silica reaction vessel at 673 K, unless stated other- wise, for 16 h in a 40 cm3 min-’ flow of helium. Dispro- portionation was followed as a function of time-on-stream with a 30 cm3 min-’ flow of toluene (884 Pa) in helium passing through the catalyst bed. Samples of the product 1 atm = 101 325 Pa.810 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Al,/atom catalyst per unit cell NH,Y 56 NH,YLS 40 HYST823 24 HYST823(H2)0 24 HYST823EX 25 HYST823EX(H2)” 25 HYST873 22 HYST873EX 22 HYST923 18 HY ST923EX 20 HYST973 16 HYST973EX 17 HYST1023 15 HYST1023EX 16 HYST1073 13 HYST 1073EX 13 Table 1 Catalysts and their activities for toluene disproportionation at 673 K rB+DMB/10’7molecule g-’ s-l total conversion of carbon on Al,,/atom acidity toluene to B + DMB spent per unit cell /mmol g-’ initial maximum molecules g-’ catalyst/wt.% molecules of toluene to B + DMB/molecules of toluene to coke 6 12 16 21 21 31 12 19 9 15 8 14 9 15 7 14 0 3.69 3.89 13 2.52 2.71 41 1.29 8.02 41 1.29 6.88 5 1.74 3.06 5 1.74 2.10 44 1.06 5.41 6 1.46 2.60 49 0.67 2.04 4 1.28 1.72 52 0.58 1.54 6 1.17 1.30 54 0.54 1.28 5 1.12 1.03 56 0.49 1.19 4 0.95 0.71 8.84 0.56 11.8 12.38 1.60 15.7 10.59 1.99 15.1 8.72 2.72 15.2 12.78 3.27 17.9 10.91 4.78 17.8 7.24 1.54 15.0 11.02 2.87 18.2 4.44 1.10 15.2 9.45 2.43 18.7 3.25 1.03 15.6 8.91 2.41 18.9 2.89 1.13 15.4 8.56 2.65 19.3 2.24 0.93 15.3 5.62 2.12 17.7 (I Experiments with H, carrier gas.stream were injected automatically at controlled intervals into a Pye Unicam GCD gas chromatograph, with flame ion- isation detection, coupled to a Pye Unicam PU48 11 comput- ing integrator.The product mixture was separated in a 1.5 m column of 10% Apiezon L on diatomite C at 373 K. p-and rn-dimethylbenzene isomers remained unresolved at this tem- perature, but frequent measurements of overall activity were considered to be of greater interest than individual rates for the three dimethylbenzene isomers. For all the catalysts studied, toluene disproportionation was accompanied by the formation of carbonaceous residues which eventually totally poisoned the catalysts. Following purging with helium for 30 min at 673 K all spent cataysts were allowed to cool to room temperature in the same gas and were subsequently analysed for carbon content using a Perkin-Elmer 204B elemental anal yser.To measure the effects of temperature on reaction rate, a constant surface cleanliness is required if meaningful results are to be obtained. To this end, activities were determined with the same mass of catalyst under pulse-flow conditions, such that the total passage of reactant necessary to construct a ten-point Arrhenius plot corresponded to less than 1 min of continuous reactant-mixture flow. Results and Discussion Time-on-stream Effects The activity of the zeolite catalysts for toluene dispro-portionation at 673 K was determined as function of time-on- stream, with initial measurements after 0.15 h and then at intervals until total catalyst poisoning had taken place. Activ- ities expressed as the sum of the rates of benzene (B) and dimethylbenzene (DMB) formation rB + DMB against time-on- stream are presented for unextracted catalysts in Fig.1 and for Na,H,EDTA-extracted catalysts in Fig. 2. The figures show that all samples exhibited a progressive increase in activity, although for some of the more active catalysts this was proceeded by an initial decay in activity over the first 2 h. The activity was found to reach a maximum at between 4 and 20 h on stream; the time at which this occurred increased with the initial steaming temperature. The follow- ing deactivation proceeded more rapidly than the activation. Values for rB+DMBafter 0.15 h on stream and at the maxima are given in Table 1. Ratios of the rate of formation of benzene to that of dimethylbenzene IB/~D~were approx-imately unity throughout the reaction, which is consistent with ‘clean’ disproportionation.Further alkylation of dimethylbenzene to trimethylbenzene only occurred in trace amounts, probably due to a low partial pressure of dimethyl- benzene, since the extent of toluene conversion rarely exceeded 10%. However, after catalytic runs, a very small quantity of relatively involatile material was found to have condensed within the reaction vessel in the cold zone beyond the furnace. Mass spectrometric detection of parent ions at rn/z 178 and 192, together with appropriate UV-VIS spectra for solutions in hexane, showed the condensed material to be a mixture of anthracene and methylanthracene. Total numbers of toluene molecules converted to benzene and dimethylbenzene, determined by the graphical integration of Fig.1 and 2, are given in Table 1. Consideration of Table 1 shows that extraction of non-framework aluminium from material steamed at any one temperature invariably ~~~,decreased the initial value for I~+ but invariably increased the maximum value and the total conversion to benzene and dimethylbenzene. -10 .-Iv) .-b 8’i time/h Fig. 1 The activity of unextracted, dealuminated Y zeolite for toluene disproportionation as a function of time-on-stream at 673 K : 0, HYST823; a, HYST873; A, HYST923; A, HYST973; V, HYST1023; V,HYST1073 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 time/h Fig. 2 The activity of Na,H,EDTA-extracted, dealuminated Y zeolite for toluene disproportionation as a function of time-on-stream at 673 K: 0,HYST823EX; 0, HYST873EX; A, HYST923EX; A,HYST973EX; V, HYST1023EX; V,HYST1073EX Although separation of 0-,m-and p-dimethylbenzene was incomplete in the present experiments, a (m+ p)/o composi- tion ratio of ca. 3 was observed for all catalysts, but increased close to the full time-on-stream. Since the thermodynamic equilibrium distribution at 673 K gives a (rn + p)/o ratio of 3.2, there is no evidence for product-shape selectivity favour- ing p-dimethylbenzene, except when pore filling with carbon- aceous residues was almost complete.Carbonaceous Residue Formation Total carbon contents of the spent catalysts following the time-on-stream experiments are given in Table 1.To assess the rate of carbonaceous residue or coke formation, a series of experiments were conducted with HYST823. The standard catalyst procedure was followed, but after a given time-on- stream at 673 K the reactant flow was stopped and the cata- lyst purged with helium at that temperature before cooling and determining the carbon content. In this way the progres- sive formation of carbonaceous residue was monitored at seven times before poisoning was complete; the results, together with rB+DMB for HYST823, are given in Fig. 3. The sigmoid curve shows that the minimum activity occurs at ca. 12 c I v) -0- r Im8 v)- 3 al- ? I- r 0 -4--. z 0 +m L. L i 1 \ ,,” Fig.3 Carbonaceous residue formation and catalytic activity of IHYST823 as a function of time-on-stream at 673 K: 0, ~+ 0,~ carbon content following termination of reaction 811 10% of the carbon content associated with total poisoning, whilst the maximum activity occurs at ca. 55% of that carbon content. This suggests that reaction on catalytically active coke is responsible for the peak in activity occurring after ,a reaction time of ca. 4.8 h. From Table 1, the mean ratio of the carbon content of fully poisoned extracted catalysts to that of the corresponding unextracted catalysts is 1.21 f0.03. This is closer to the corresponding mean ratio of micropore volumes, 1.21 f0.05, than to that of mesopore volumes, 2.13 f0.27 (data from Table 2 of ref.11) reinforcing the view9*” that carbon associated with poisoning resides within micropores,. The final column of Table 1 presents ratios of the number of toluene molecules converted to benzene and dimethylbenzene to the calulated number converted to coke. In all cases, coke formation is suppressed by the extraction of non-framework aluminium. The activity of HYST823 and HYST823EX for toluene dis- proportionation was determined as a function of time-on -stream at 673 K with either He or H, as the carrier gas in the reactant mixture. The results presented in Fig. 4 show that using H, almost doubles the time required to attain the maximum activity that was observed with He carrier. Although the maximum value for rB+DMB is lower with H,., Table 1 shows that the total toluene coversion and the ratio of toluene molecules converted to benzene and dimethyl- benzene to those converted to coke are both appreciably higher with H,.Activation Temperature Effects The results presented above follow activation at 673 K, which should largely avoid structural dehydroxylation’ and so maximise the Brsnsted-acid site concentration. Any Lewis- acid sites, which may be involved in synergic relationships with Brsnsted-acid sites, should thus be associated with exist- ing non-framework aluminium species. The framework Si : A1 ratios of the present dealuminated Y zeolites lay between 7 and 14, giving the lattice greater thermal stability than the HY zeolites (Si : A1 = 2.5 : 1) where increases in activity fol- lowed activation above 673 K 2 v 3 Nevertheless, measurements were made to assess the effect of activation temperatures above 673 K on toluene disproportionation activity.In Fig. 12 Ic 0 10 time/h Fig. 4 The activity of HYST823 and HYST823EX for toluene dis- proportionation as a function of time-on-stream at 673 K, with either ~ He or H, as the carrier gas: 0,HYST873 with He; 0, HYST823~; with H,; A, HYST923EX with He; A,HYST823EX with H, -8-I hi -U. O-0-I I I 800 900 activation temperature/K Fig. 5 The activity of HYST873 and HYST873EX for toluene dis- proportionation at 673 K as a function of activation temperature: 0, HYST873 initial activity; 0, HYST873 maximum activity; A, HYST873EX initial activity; A,HYST873EX maximum activity 5, a progressive decrease in both the initial and maximum activity of HYST873 is observed as the activation tem-perature is increased from 673 to 949 K.Any synergic increase in activity of framework hydroxy-group sites, through an inductive effect with Lewis sites generated during dehydroxylation, appears to be negated by an overall decrease in the concentration of Brernsted-acid sites. Table 1 shows that AlNF > AlF for HYST873, suggesting that any further generation of Lewis acidity makes no appreciable increase to the acid strength of the framework hydroxy groups. However, when a similar series of measurements were made with HYST873EX, the initial activity increased with activation temperature, passed through a maximum at cu. 823 K before decreasing as the temperature approached 949 K.However, the maximum in activity fell with activation temperature, though not as smoothly as HYST873. Two factors may account for the different behaviour of the initial activity in HYST873 and HYST873EX. First, HYST873EX has been cation exchanged with NHf, but only the strongest Brsnsted-acid sites, which release NH, above 673 K, are catalytically active. This is in contradistinction to HYST873, which is totally in the hydrogen form after the hydrothermal method of preparation. Secondly, since AlN, = 6 for HYST873EX, it is necessary to generate further non-framework aluminium, or the associated Lewis acidity, to enhance the catalytic a~tivity.'*'~*'~ Both catalysts show a fall in the maximum activity with increasing activiation tem- perature.This difference in behaviour from the initial activity, may reflect that the reaction proceeds on catalytically active coke at this stage. The mean values for the carbon content of the spent catalysts from these experiments, 15.1 f0.1 wt.% for HYST873 and 18.0k0.2 wt.% for HYST873EX, are in excellent agreement with the corresponding values given in Table 1. Temperature Dependence of Catalytic Activity Experiments to determine the effect of temperature on toluene disproportionation were made under pulse-flow con- ditions using catalysts previously activated at 673 K. Activity measurements were made with each catalyst sample at ten temperatures in the range 553-723 K, selected such that the extent of reaction did not exceed 6% for any one pulse of J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 reactant. Where this is so, the reactor is operating in the dif- ferential mode and the response of the gas chromatograph is directly proportional to the amount of each product. It follows that meaningful activation energies can be obtained from Arrhenius plots of the area of either the benzene or dimethylbenzene peaks given by the computing integrator. Activation energies obtained this way for all 12 dealuminated Y zeolites are given in Table 2; the probable error on each value does not exceed & 3 kJ mol- '. No significant difference in the mean activation energy can be seen between that deter- mined from benzene production (E, = 7t.6 & 3.9 kJ mol-') and that determined from dimethylbenzene production (E, = 72.3 f5.5 kJ mol-').More importantly, there is no signifi- cant difference between the mean value for unextracted cata- lysts (E, = 70.5 f4.7 kJ mol-') and that for extracted catalysts (E, = 72.8 f4.7 kJ mol-'). It follows that differ- ences in activity between catalysts probably arise from differ- ences in the concentration of active centres rather than from differences in energetics. Activity Dependence on Acid Site Concentration The initial activity and maximum activity of both unex-tracted and extracted dealuminated Y zeolites at 673 K is given as a function of the Brernsted-acid site concentration in Fig.6. These concentrations, given in Table 1, were pre- viously determined' ' by the temperature-programmed desorption of NH, ; they decrease with increasing steaming temperature and are invariably greater following extraction of non-framework aluminium. Considering initial activities, it is evident from Fig. 6 that the fraction of Brernsted-acid sites that are effective in catalysis is greater for the unextracted zeolites than for the extracted zeolites. This we attribute to synergic interactions between the Brsnsted sites and extra- structural hydroxoaluminium species; for unextracted zeolites AlN, > AlF, whilst for extracted zeolites Al,, < AlF where the average value for AlNF is 5. Considering the maximum in activity, a single linear plot incorporates points for both extracted and unextracted zeolites.Accepting that the reac- tion proceeds on catalytically active coke, it appears that all, or a constant fraction of, the acid sites originally available as hydroxy groups generate active sites. Mechanistic Considerations At this stage it is appropriate to consider possible mecha- nisms for toluene disproportionation on the present catalysts. A dealkylation-alkylation mechanism, as previously pro- posed for ethylbenzene disproportionation,' ' is inapplicable Table 2 Activation energies for toluene disproportionation activation energy/kJ mol -catalyst from benzene formation from dimethylbenzene formation HYST823 75 77 HYST823EX 70 71 HYST873 71 77 HYST873EX 73 76 HYST923 64 62 HYST923EX 76 77 HYST973 69 72 HYST973EX 67 61 HYST1023 66 68 HY ST1023EX 76 77 HYST1073 71 74 HYST1073EX 75 75 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 15 -I u1 5 1 2 acid concentration/mrnol g -' Fig. 6 The ctivity for toluene disproportionation at 673 K sa function of acid-site concentration: 0,initial activity and 0, maximum acitivity of unextracted HYSTx catalysts; A, initial activ- ity and A, maximum activity of extracted HYSTxEX catalysts due to the instability of the CH; carbocation.'6 Fig. 7 pre-sents a possible mechanism, where protonation of a toluene molecule is followed by transfer of the methyl group via a bimolecular transition complex. The dimethylbenzene carbo- cation must then transfer a proton directly, or indirectly viu the zeolite framework, to another toluene molecule to con- tinue reaction.This mechanism explains the role of strong Brsnsted-acid sites in protonating the aromatic ring, but does not account for the formation of fused-ring aromatics which act as both the precursors and components of ~oke.~*'*-'~ I Fig. 7 Scheme for toluene disproportionation by protonation of toluene to form a toluene carbocation gas phase or physically electrostatically boundadsorbed f 0^01"'' \ .H' \ H II I f- t H2C+*CH3 Fig. 8 Scheme for toluene disproportionation by hydride ion abstraction from toluene to form a benzyl cation The second possible mechanism, presented in Fig.8, requires the formation of a diarylmethane intermediate.I8 Initial generation of the benzyl cation requires hydride ion abstraction from the methyl group of the toluene molecule, which is most likely to occur by protonation of a C-H bond of the methyl group followed by loss of molecular hydrogen. The diarylmethane carbocation may then lose a proton to form a discrete intermediate, where subsequent protonation of the methyl-phenyl bond leads to benzene and the dimethylbenzene carbocation. Alternatively, rearrangement of the diarylmethane carbocation may yield the same products directly. This scheme also provides a route for coke forma- tion starting from the diarylmethane intermediate.For example, aromatisation of the ortho-substituted isomer to anthracene, by facile proton and hydride ion-transfer ~teps,~,'~followed by further addition steps to yield the complex polycyclic aromatics that constitute coke. The coke formed within the zeolite participates in the mechanism associated with the peak in activity observed about 4-20 h on stream. The sensitivity of the timing of the peak to structural and extrastructural aluminium content and to the use of He or H, as the carrier gas, emphasises the importance of the chemical constitution of the coke. In the first mechanism, Fig. 7, the unsaturated coke may participatc in the proton transfer from the protonated dimethylbenzene to a further toluene molecule, so that the proton is recycled without return to the zeolite lattice.The presence of coke is thus effectively promoting the acidity of the framework hydroxy groups, so that maximum activity is more indicative of the total number of Brsnsted-acid sites than upon the number of superacid sites associated with the initial activity. Such an explanation would also account for the high maximum activities achieved by NH,Y and NH,YLS, which initially possess a high concentration of weak Brernsted-acid sites due to the low Si: A1 ratio of the framework. In the second mechanism, Fig. 8, the dimethylbenzene carbocation may extract a hydride ion from a polyatomatic coke mol- ecule, thus forming dimethylbenzene and a coke carbocation. Subsequent hydride abstraction from a toluene molecule by the coke carbocation forms the benzyl cation which con- tinues the reaction.Chen et al.” have suggested that a radical mechanism may be involved in the formation of benzyl cations during the disproportionation of toluene over HY and other zeolites. Using spectroscopic techniques they detected the presence of both benzyl radicals and diarylme- thane carbocations; a good corrlation between dispro-portionation activity and the ability of the zeolite to form diarylmethane carbocations was observed. It follows that coke may participate in a radical mechanism for benzyl cation formation in the present work, where abstraction of a hydrogen atom to form a benzyl radical is followed by elec- tron transfer.Whatever the mechanism for benzyl cation for- mation on coke might be, it should result in a maximum disproportionation activity directly related to the total number of Brsnsted-acid sites. Conclusions The initial activity for toluene disproportionation at 673 K, with both unextracted and extracted HY zeolites, is catalysed by a limited concentration of strong Brsnsted-acid sites. These arise from synergic interaction between framework hydroxy groups and extrastructural aluminium species. Two mechanisms, involving either proton addition or hydride ion abstraction from the reactant toluene molecule, are proposed and it is probable that both contribute to the reaction. The clear maximum in activity for toluene dispro-portionation presently observed after 4-20 h on stream is not known to have been previously reported.The time to achieve this maximum rate is sensitive to both the catalyst and carrier gas (He or H,) used and occurs after considerable coke formation. Reaction is associated with ‘catalytically active coke’, where the concentration of active sites is directly related to the total Brsnsted-acid site concentration of the freshly activated zeolite. Both mechanisms considered for the initial reaction are applicable, but, since the activity even- tually falls to zero through complete coking, the second mechanism producing the diarylmethane intermediate must occur in part. Considering the activity on a unit weight basis, the initial activities of unextracted catalysts are consistently greater than those of extracted catalysts.However, the maximum activities of the extracted catalysts are the greater, so that the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 total toluene conversion to useful products prior to complete poisoning by coking is greater with extracted catalysts. This points to possible catalytic advantages to be gained from the partial extraction of non-framework aluminium from hydro- thermally treated Y zeolites. The authors thank Unilever Research, Port Sunlight, for pro- viding experimental facilities, Mr. K. Griffn for experimental assistance and Dr. G.B. Gill for helpful discussions. They also thank the SERC for a maintenance grant to N.P.R. References 1 H. A. Benesi, J. Catal., 1967,8, 368.2 K. M. Wang and J. H. Lunsford, J. Catal., 1972,24, 262. 3 P. A. Jacobs, H. E. Leeman and J. B. Uytterhoeven, J. Catal., 1974,33, 31. 4 C. Miradatos and D. Barthomeuf, J. Chem. SOC., Chem. Commun., 1981, 39. 5 R. A. Beyerlein, G. B. McVicker, L. N. Yacullo and J. J. Ziemiak, J. Phys. Chem., 1988,92, 1967. 6 P. Beltrame, P. L. Beltrame, P. Carniti, L. Forni and G. Zuretti, Zeolites, 1985,5, 400. 7 V. Mavrodinora, V. Penchev, U. Lohse and H. Stach, Zeolites, 1989,9, 197. 8 V. Mavrodinora, V. Penchev, U. Lohse and T. Gross, Zeolites, 1989,9, 203. 9 P. Magnoux, C. Canaff, M. Machado and M. Guisnet, J. Catal., 1992,134,286. 10 J. R. Anderson, Q-N. Dong, Y-F. Chang and R. J. Western, J. Catal., 1991, 127, 113. 11 N. P. Rhodes and R. Rudham, J. Chem. SOC., Faraday Trans., 1993,89,2551. 12 S-B. Liu, S. Prasad, J-F. Wu, L-J. Ma, T-C. Yang, J-T. Chiou, J-Y. Chang and T-C. Tsai, J. Catal., 1993, 142, 664. 13 L. Kubelkova, S. Beran, A. Malecka and V. M. Mastikhin, Zeo-lites, 1989,9, 12. 14 A. G. Ashton, S. Batmanian, D. M. Clark, J. Dwyer, F. R. Fitch, A. Hinchcliffe and F. J. Machado, Catalysis by Acids and Bases, ed. B. Imelik, C. Naccache, C. Coudurier, Y. Ben Taarit and J. C. Vedrine, Elsevier, Amsterdam, 1985, p. 101. 15 J. H. Lunsford, in Fluid Catalytic Cracking II: Concepts in Cata- lyst Design, American Chemical Society, Washington, DC, 1991, p. 1. 16 H. Pines, The Chemistry of Catalytic Hydrocarbon Conversions, Academic Press, New York, 1981. 17 E. G. Derouane, Catalysis by Acids and Bases, ed. B. Imelik, C. Naccache, C. Coudurier, Y.Ben Taarit and J. C. Vedrine, Else- vier, Amsterdam, 1985, p. 221. 18 H. Pines and J. T. Arrigo, J. Am. Chem. SOC., 1958,80,4369. 19 F. Chen, G. Coudurier and C. Naccache, Zeolites: Facts, Figures, Future, ed. P. A. Jacobs and R. A. van Santen, Elsevier, Amsterdam, 1989, p. 1387. Paper 3/06675D; Received 8th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000809

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994. 9@5), 815-81h FARADAY COMMUNICATIONS Acoustic Effects on Catalytic Activities of Cu and Pd Thin Films combined with Piezoelectric Lead Strontium Zirconium Titanate Activated by Low-frequency Voltage Yasunobu lnoue Department of Chemistry, Nagaoka University of Technology, Nagaoka , Niigata 940-21,Japan A piezoelectric material with a dynamic radial-extensional lattice displacement has been used as a substrate for thin film Cu and Pd catalysts. Acoustic effects were found to bring about a marked increase in activity at a resonance frequency. Recently, we have demonstrated that the surface acoustic waves (SAWs) of Rayleigh and shear horizontal leaky types which are produced on ferroelectric single crystals of LiNbO, and LiTaO, have marked effects on the enhancement of activity of thin film Pd and Cu catalysts deposited on the SAW propagation path.'--6 The activity increases by the SAWs are closely associated with the forced lattice displace- ment of the catalyst surfaces and the generation of high elec- tric fields.Another method for producing forced lattice displacement of crystals is piezoelectric bulk lattice distortion which is caused by low-frequency (LF) voltage applied to electrodes deposited on the front and back planes of poled piezoelectric crystals. An interesting feature is that the lattice displacement takes place to a remarkable extent at a resonance frequent!,. On the basis of our preliminary work, we have predicted that there exist acoustic effects in which the application of LF voltage to the piezoelectric crystal as a catalyst substrate leads to the activation of a thin film catalyst deposited thereon.' The present study aims to verify the presence of the acoustic effects which are considered to permit in situ control of reaction rates while catalytic reactions proceed.A lead strontium zirconium titanate sample, Pbc,,,, Sro,05Zro,53Tio~4,03(referred to as PSZT). was prepared in the form of a disc 20 mm in diameter and 0.2 mm in thick- ness. Silver electrodes were deposited on the front and back planes of the disc. Copper or palladium, as catalyst. was deposited by an evaporation method at a thickness of SO nm. sufficient to cover the Ag electrodes completely.LF voltage was introduced to the electrodes in the range 0-15 V (peak-to-peak voltage) with a frequency of 60--110 kHz. In order to find a resonant frequency, admittance of the samples was measured with a network analyser at reaction temperature. The catalytic oxidation of ethanol was examined with a gas-circulating reaction system, and the products were analysed with a gas chromotograph. A small CA thermocouple was attached to the catalyst surface to monitor the surface tem- peratures. Fig. 1 shows the formation of acetaldehyde by oxidation of ethanol on Cu at 383 K and on Pd at 353 K. After the reac- tion rate reached a stationary level, LF voltage was applied at a frequency of 86.1 kHz. The reaction rate on Cu imme-diately increased by a factor of 4.7.The increased rate was maintained as long as LF voltage was imposed and returned to the original rate with turn-off. The figure also shows the result when a constant voltage was applied instead of LF voltage. In this case, there was little increase in the reaction rate. For the Pd catalyst, an increase in the reaction rate occurred with LF voltage on. but the increase was 7O(Il less lhan that with the Cu catalyst. As shown in Fig. 1, the tem- perature of the catalyst surface was raised with LF voltage m, but decreased to the original level after 10 min, since the :atalyst temperature was accurately controlled by an electric "urnace Note that the increased catalytic activity was main- rained independently of the short-period fluctuation of the ,u r face temper at ure.Fig. 2 shows an increase in the activity of the Cu catalyst with increasing LF voltage. No saturation of the activity was lbserved over the voltage range used. Fig. 3 shows Arrhenius plots of the reaction rate on the Cu catalyst. With LF voltage jn the temperature dependence varied and the activation :nergy decreased from 55 to 29 kJ mol-', indicating that mhancement of the catalytic activity is related to intrinsic ,urface pheomena rather than to a thermal effect. Fig. 4 shows the ratio of activity changes with the Cu cata- :>st as a function of frequency. There was little change below off v cu 2 4 t 'h Fig. 1 Acetaldehyde formation from ethanol oxidation on applica- tion of LF and dc voltage.(0)Cu catalyst,f= 86.1 kHz, LF voltage 1 l,F)= 15 V, T = 383 K; (0) 1 = 353 K; (a) Pd catalyst, f= 86.1 kHz. V,, = 15 V, Cu catalyst, dc voltage CV,,) = 15 V, T = 383 K. 816 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I 0 5 10 15 20 VLF/v Fig. 2 Relative activity, ron/roff,as a function of V,,. Cu catalyst, T = 383 K,f= 86.1 kHz. r,, , activity with V,, = 15 V; roff,activity without LF voltage (V,, = 0). 65 kHz. With increasing frequency, the activity increased markedly, particularly above 80 kHz, reaching a maximum at 86.1 kHz and then decreasing sharply with higher fre- quencies. Measurements of admittance of the catalyst-depositing PSZT sample showed that the resonance frequency was 86.0 kHz which is in good agreement with the frequency at which the catalytic activity increased.Since the resonance causes an extraordinary radial-extensional dis- placement, it is evident that the displacement of lattice atoms of the catalyst at the resonance mode may be associated with the activity increase. Recently, Boronin and co-workers' showed that the work function of a Pt(100) surface combined with piezoelectric quartz varies at the eigenfrequency. In X-ray photoelectron sputter depth profile analysis of the Cu catalyst used for the catalytic reaction at 393 K, an X-ray induced L,M,,,M,,, Auger peak due to Cu' was observed not only at the surface but also in the bulk, indicating that the total layer of the Cu catalyst was oxidized to form Cu,O 2.0 1.o c k 0.5-0 E,s 0.1 'Q 2.4 2.6 2.8 103 KIT Fig.3 Arrhenius plots for the reaction on a Cu catalyst. (a)VLF = V,,15 V,f= 86.1 kHz, (0)= 0. 5 c I I I I I I 60 80 100 120 frequency/kHz Fig. 4 r,,,/rOff,as a function of frequency. Cu catalyst, T = 383 K, V,, = 15 V. A broken line shows admittance. r,, ,activity with V,, = 15 V, roff,activity with V,, = 0. during the catalytic reaction. Thus, it is likely that differences between the semiconducting and metallic characters are associated with a larger increase in the catalytic activity with LF voltage for Cu than for Pd (Fig. l), since the lattice dis- placement is considered to influence the electron density at the surface of semiconducting oxide catalysts, which is similar to that observed in the SAW Application of dc voltage in place of LF voltage had little effect on activation of the Cu catalyst.From these results, it is evident that the res- onance mode of lattice displacement plays an important role in the activation of surfaces, the efficiency of which is strongly associated with the arrangements of atoms and electronic structures at the catalyst surface. This work was supported by the Sumitomo Foundation. The author thanks Mr. T. Kamoshida for measurements of the catalytic activity. References 1 Y. Inoue, M. Matsukawa and K. Sato, J. Am. Chem. SOC., 1989, 111,8965. 2 Y. Inoue and M. Matsukawa, J. Chem. SOC., Chem. Commun., 1990,296. 3 Y. Inoue, M. Matsukawa and K. Sato, J. Phys. Chem., 1992, 96, 2222. 4 Y. Inoue, M. Matsukawa and H. Kawaguchi, J. Chem. SOC., Faraday Trans., 1992,88,2923. 5 Y.Inoue and M. Matsukawa, Chem. Phys. Lett., 1992,198,246. 6 Y. Inoue and Y.Watanabe, Catal. Today, 1993. 7 T. Kamoshida, K. Sat0 and Y. Inoue, Proc. 11th Surface Science Conference, The Surface Science Society of Japan, Tokyo, 1991, p. 49. 8 V. N. Brezhnev, A. I. Boronin, V. P. Ostanin, V. S. Tupikov and A. N. Belyaev, Chem. Phys. Lett., 1992, 191, 379. Communication 3/07479J; Received 21st December, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000815

出版商:RSC    

年代:1994

数据来源: RSC