摘要:

J O U R N A L O F C H E M I S T R Y Materials The eVect of iron on the crystalline phases formed upon thermal decomposition of Mg–Al–Fe hydrotalcites Jose� Maria Ferna�ndez,a Maria Angeles Ulibarri,a Francisco M. Labajosb and Vicente Rives*b aDepartamento de Quý�mica Inorga�nica e Ingenierý�a Quý�mica, Facultad de Ciencias, Universidad de Co�rdoba, Co�rdoba, Spain bDepartamento de Quý�mica Inorga�nica, Universidad de Salamanca, Salamanca, Spain Received 26th June 1998, Accepted 5th August 1998 Layered double hydroxides (LDH) containing MgII, FeIII, and AlIII in the brucite-like layers and interlayer carbonate (with a constantMII/MIII ratio but varying AlIII/FeIII ratios) have been prepared and characterised by Xray diVraction, thermal analysis, FT-IR and UV–VIS/diVuse reflectance spectroscopies, temperature-programmed reduction and specific surface area assessment through low temperature adsorption of N2.AnMg,Al–LDH, but with intercalated hexacyanoferrate(III), has been also prepared and characterised, in which simultaneous formation of the carbonate analogue did not occur. Thermal decomposition in air at 450 and 750 °C leads to MgO and poorly crystallised MgFe2O4 spinel (crystallinity increasing with the iron content), while for the hexacyano-containing sample, crystallization only is observed after calcination at 900 °C.This diVerent behaviour has been related to the initial location of the iron ions. to a solution containing Mg2+, Al3 + and Fe3+ nitrates in Introduction selected concentrations (ranging from 0.3 M to 0.066 M) to Layered double hydroxides (LDH) are also known as yield solids with diVerent Al/Fe molar ratios, but with a anionic clays, as they show a structure electrically opposite constant Mg/(Al+Fe) molar ratio of 351, until a pH of 10 to that shown by clays.Their general formula is was reached, this value being maintained at constant pH (10) [MII1-xMIIIx(OH)2][Am-]x/m·nH2O.The commonest type are with a Dosimat 275 (Metrohm) coupled to a pH-meter model the hydrotalcite group of minerals, the structure of which 691 (also from Metrohm). When addition was complete, the consists of brucite-like layers [formed by edge-sharing mixture was further magnetically stirred for 2 h. The suspen- Mg(OH)6 octahedra] with partial MgII/AlIII isomorphic substi- sion was submitted to hydrothermal treatment at autogenous tution, the electrical balance being attained with carbonate pressure at 120 °C in a Teflon-lined stainless steel bomb for anions located, together with water molecules, in the interlayer. 24 h. The solid was then filtered and washed until nitrate and Substitution of the layer cations is very easy, and the interlayer sodium ions were completely absent.anion can be easily changed as well, thus giving rise to a In order to prepare the hexacyanoferrate(III)-containing continuous growing family of new layered materials with sample, aqueous solutions were prepared in deionised water important applications as catalysts or catalyst precursors, previously boiled to remove dissolved CO2; nitrogen was sensors, anion scavengers, etc.1–3 continuously bubbled through the water until it reached room On thermal decomposition, these materials lead to mixed temperature, after which bubbling was continued for 15 min oxides (also known as ‘non-stoichiometric spinels’), and it has at room temperature. Fifty ml of a 0.05 M K3[Fe(CN)6] been shown4 that the nature/structure of the solids obtained solution were placed in a three-necked round-bottom flask (and their applications) depend on the starting LDH which is and, while magnetically stirred, 50 ml of a solution containing decomposed.Mg2+ and Al3+ nitrates (0.3 M and 0.1 M, respectively) were The aim of the present work was to analyze how the added dropwise from a separation funnel. The same Dosimat presence of iron and its concentration in the brucite-like layers, and pH-meter cited above were used to maintain the pH at a or in the interlayer anion, may lead to solids, after calcination, value of 8.All of the processes were carried out at 60 °C. The with diVerent structures. We have recently reported4,5 on the suspension was left to settle overnight at room temperature preferential formation of Mg2V2O7 or MgV2O6, with diVerent and the precipitate washed with preboiled water heated to local environments around the V5+ cations, from hydrotalcite- 60 °C by magnetic stirring and centrifugation, and the solid like precursors, when starting from materials containing Mg2+, was left to dry in an oven at 80 °C in the open air.Al3+ and V3+ in the brucite-like layers and carbonate in the After characterization, the samples were calcined for 2 h in interlayer, or from materials containing Mg2+ and Al3+ in the air at 450 or 750 °C, in order to obtain mixed oxides.It has layers and decavanadate, V10O286-, in the interlayer. In been previously shown6 that the decomposition of the the present case, diVerences in location of Fe3+ cations in Mg–Al–Fe hydrotalcites is almost complete at 450 °C, and octahedral or tetrahedral sites, or diVerent degrees of crystalonly residual hydroxyl groups are further removed between linity, could be expected when Fe was in the brucite-like layer, 450 and 750 °C.Labelling of the samples is given in Table 1; or forming an anionic complex in the interlayer. Actually, it the calcination temperature is indicated in °C.While samples has been found that the crystallinity of the solids formed (or the calcination temperature required to obtain crystalline M2 to M5 and all calcined samples were ochreous, sample solids) depends on the precise nature of the starting LDH. M6 was light yellow. Experimental Techniques Sample preparation Elemental chemical analysis for metals was carried out by atomic absorption spectroscopy (AAS) using a Perkin-Elmer All chemicals were from Merck.For the carbonate-containing samples, a 1 M NaOH aqueous solution was added dropwise 3100 apparatus after dissolution of the samples in dilute HCl. J. Mater. Chem., 1998, 8(11), 2507–2514 2507Table 1 Labelling of the samples and elemental chemical analysis in the layers, Mg2+/(Al3++Fe3+), was 2.9751 for samples results M2 to M5; for sample M6 the Mg2+/Al3+ ratio was 3.1751; these two values are acceptably close to the expected value Sample %Mga %Ala %Fea Mg/(Al+Fe)b Mg/Alb Al/Feb existing in the starting solutions (351).On the other hand, the Al/Fe ratio for sample M6 was 2.7351. The expected value M2 24.38 5.47 8.11 2.88 1.40 M3 24.10 3.74 9.79 3.15 0.79 for this ratio was 351, as the negative charge of the interlayer M4 22.39 2.74 11.99 2.91 0.47 anion should balance the positive charge due to Al3+ in the M5 22.86 — 17.92 2.93 — layers, and so, the anion existing in this sample [hexacyanofer- M6 18.73 6.56 4.99 3.17 2.73 rate(III )] being trivalent, a value of 351 would be expected.M2–450 28.68 6.12 8.74 3.08 1.45 If hexacyanoferrate(III ) is the only anion existing in sample M3–450 26.90 4.50 11.49 2.96 0.81 M6, then the ratio between the weight percentages of C and M4–450 26.19 3.14 13.51 3.01 0.48 N would be 0.8651; however, the experimental value was M5–450 27.46 — 19.37 3.25 — 0.9351, indicating that a slight excess of carbon exists.From M6–450 23.76 8.26 5.56 3.19 3.09 the FT-IR results (see below) the presence of a small amount M2–750 32.15 7.29 10.21 2.92 1.48 of carbonate species, probably adsorbed on the external surface M3–750 28.02 5.04 11.91 2.88 0.88 of the crystallites, can be concluded, thus explaining this M4–750 27.04 3.58 14.62 2.82 0.51 finding, as formation of a co-product containing intercalated M5–750 32.62 — 23.48 3.19 — M6–750 25.38 8.93 6.38 3.16 2.90 carbonate was not observed (see powder X-ray diVraction results below).On the other hand, the molar N/Fe ratio is aWeight percentage. bMolar ratio, x:1. very close to the expected value (651). Powder X-ray diVraction Carbon and nitrogen were analyzed in sample M6 in a Perkin Elmer 2400 CHN analor the original samples are shown in Fig. 1. Powder X-ray diVraction (PXRD) diagrams were recorded They are all similar, with sharp bands at low 2h values, on a Siemens D500 instrument, using graphite-filtered Cu-Ka corresponding to the higher order 001 reflections of a layered radiation (l=1.54050 A° ); the instrument was equipped with a material.The sharp peaks recorded for sample M3 close to DACO-MP microcomputer, and software DiVract-AT was 2h=38 and 45° superimposed to broader maxima (also used to analyze the data, identification of existing crystalline recorded in the same positions for the other samples), and phases being concluded from comparison with JCPDS diVrac- that recorded at 2h=65° are due to the Al sample holder.The tion files. In some cases, where small amounts of sample were positions of the harmonics are coincident for samples M2, available, an Al sample holder was used, and thus, sharp, M3, M4, and M5, while for sample M6 these harmonics are intense diVraction peaks due to the holder were recorded, but shifted towards lower 2h values (i.e., larger spacings).these peaks were unambiguously identified. Assuming a 3R polytypism,9 the first (from low 2h values) DiVerential thermal analysis (DTA) and thermogravimetric peaks can be indexed as (003), (006), and (009), and from analysis (TG) were recorded on Perkin-Elmer DTA1700 their positions, the values for parameter c have been calculated and TGS-2 instruments, respectively, using flowing air (Table 2) as c=[d(003)+2 d(006)+3 d(009)].(60 ml min-1) at a heating rate of 12 °Cmin-1. The values for parameter a (which coincides with the average Fourier-transform infrared spectra (FT-IR) were recorded cation–cation distance in the brucite-like layers) are also using the KBr pellet technique on a Perkin-Elmer FTIR-1730 included in Table 2, and have been calculated from the position instrument; one hundred scans were averaged in order to of the peak due to planes (110), which is the first peak of the improve the signal-to-noise ratio, and the nominal resolution doublet recorded close to 2h=60°, as a=2 d(110).It should was 4 cm-1. be noted that for samples M2 to M5, a steady increase in a is Ultraviolet–visible (UV–VIS) spectra were recorded observed. This is due to the progressive Al3+/Fe3+ substitution following the diVuse reflectance (DR) technique in a Shimadzu (see elemental chemical analysis data in Table 1), and the UV-240 instrument, using MgO as reference and a slit of 5 nm.larger ionic radius of Fe3+ (78.5 pm in high spin, octahedral Specific surface area and porosity of the samples were coordination) than Al3+ (67.5 pm in octahedral coordidetermined on a Gemini instrument from Micromeritics. The nation).10 This is also the reason of the lower a value for samples were previously degassed at 125 °C for 2 h with sample M6, where the only trivalent cation in the layers is Al3+.nitrogen in a Micromeritics FlowPrep 060 apparatus. The The slight diVerences in the values of parameter c for adsorption–desorption isotherms (-196 °C) were analyzed using literature software.7 Temperature-programmed reduction (TPR) runs were performed in a TPR/TPD 2900 instrument from Micromeritics, using a 5% H2/Ar (vol.) mixture to reduce the samples.Amounts of samples of ca. 15 mg were used, and the gas flow, sample weight and heating rate were chosen in order to attain good resolution of the reduction peaks.8 The gas, at the reactor exit, was passed through a cold trap (melting isopropanol ) to retain vapours and condensable gases before entering the detector.Results and discussion Elemental chemical analysis The results obtained for Mg, Al and Fe are given in Table 1; the calculated MII/MIII and Al/Fe ratios are also given. C and N were analyzed for sample M6, obtaining values of 6.3 and Fig. 1 Powder X-ray diVraction profiles for samples M2 to M6. (*) 6.8%, respectively. The average MII/MIII ratio, i.e., the ratio signals due to the Al sample holder.The traces have been displaced vertically for clarity. between the molar fraction of divalent and trivalent cations 2508 J. Mater. Chem., 1998, 8(11), 2507–2514Table 2 Summary of X-ray diVraction results, specific surface area determination and temperature-programmed reduction Sample c/A° a/A° SBET/m2 g-1 H2/Fea M2 23.52 3.08 59 1.36 M3 23.73 3.09 68 1.68 M4 23.82 3.10 70 1.42 M5 23.67 3.11 48 1.25 M6 33.36 3.06 143b — M2–450 123 1.40 M3–450 145 1.56 M4–450 138 1.63 M5–450 81 1.67 M6–450 123 1.68 M2–750 101 1.12 M3–750 113 1.22 M4–750 117 1.43 M5–750 16 1.35 M6–750 110 1.28 aMolar ratio, x:1.b128 m2 g-1 surface area equivalent to adsorption on micropores, and 15 m2 g-1 external surface area.samples M2 to M5 [the higher value for sample M6 is due to the presence of hexacyanoferrate(III) instead of carbonate] cannot, however, be easily related to particular diVerences in the samples, as the small changes observed could be due to small diVerences in the hydration degree of the interlayer. From the thickness of the brucite-like layers, 4.8 A° ,3 the interlayer space for the carbonate-containing samples is close to 3 A° , corresponding to carbonate anions with their molecular Fig. 2 Powder X-ray diVraction profiles for samples M2 to M6 plane parallel to the brucite-like layers. calcined at (upper) 450 and (lower) 750 °C for 2 h. (*) signals due to With regards to sample M6, it should be stressed that no the Al sample holder. The traces have been displaced vertically peak has been recorded which could be ascribed to the presence for clarity.of a co-product corresponding to carbonate-interlayered hydrotalcite. This result is extremely important, as in most of the papers previously reported in the literature on hexacyano- nation, as the MII/MIII ratio in the spinel is equal to 0.551, and so crystallization of MIIO is always observed.19,20 ferrate-containing layered double hydroxides,11–13 coformation of a carbonate–LDH, together with that of the Additionally, some diVraction peaks of MgO are recorded almost coincident with diVraction peaks of spinels.hexacyanoferrate form, is usually observed. The gallery height [from the spacing for planes (003), 11.12 A° , and the thickness With this, taking into account the nature of the cations existing in our samples, the following phases could be formed: of the brucite-like layers, 4.8 A° ] was 6.32 A° .The size of the Fe(CN)63- anion is close to 11 A° along the C4 axis, 8.7 A° MgO, MgAl2O4, MgFeAlO4,MgFe2O4. The presence of MgO is concluded in all ten cases from the two intense peaks at along the C2 axis, and 6.5 A° along the C3 axis.14 This means that the anion should be oriented with its C3 axis (that joining 2.10 and 1.48 A° (ca. 2h=43 and 63°, respectively), and the excess in MgO above the stoichiometric amount required to parallel faces of the octahedron) perpendicular to the brucitelike layers and that, even so, some stress and distorsion should form any Mg-containing spinel. From the positions of the peaks in the PXRD diagram of exist.Alternatively, grafting of the anion to the brucite-like layers, in a similar way to that previously described for several sample M5-750 (where the sharpest peaks are recorded), excluding the peaks coincident with those of MgO, the cell vanadates intercalated in LDHs,15,16 could be claimed. The PXRD diagrams of the samples calcined at 450 and dimension for the spinel formed can be calculated as 8.402 A° .The reported values21 for other spinels are 8.083 A° (MgAl2O4, 750 °C are included in Fig. 2. Again, sharp peaks due to the Al sample holder are recorded in some cases. With regard to JCPDS file 21-1152), 8.320 A° (MgFeAlO4, JCPDS file 11-9), 8.387 A° (MgFe2O4, JCPDS file 36-398), and 8.396 A° (Fe3O4, the maxima of the samples, these are extremely broad and their positions roughly coincide for all five samples calcined JCPDS file 19-629).From comparison between the reported and the calculated values it can be concluded that the spinel at 450 °C, while for samples calcined at 750 °C the PXRD diagram of sample M6-750 is rather similar to those recorded formed in our samples should be MgFe2O4. We want to stress that the Al sample holder behaves in our case as a sort of for the samples calcined at 450 °C; on the contrary, for samples M2-750 to M5-750 a progressive increase in the intensity of ‘internal reference’, for better definition of the positions of the peaks.In addition, formation of Fe3O4 can be tentatively new, sharper peaks (not recorded for samples calcined at 450 °C) is observed.assumed, if Fe3+�Fe2+ reduction could take place during calcination in air. For a hydrotalcite structure to remain stable, the MII/MIII ratio should be larger than 151.2 Calcination of hydrotalcites So, the PXRD data for the calcined samples can be summarized as follows: calcination at 450 °C gives rise, in all leads to removal of volatile interlayer anions and hydroxyl groups, and formation of the corresponding oxides, usually cases, to formation of ill-crystallized MgO, and a small amount of a MgFe2O4 spinel.When the calcination temperature is MIIO and the MIIMIII2O4 spinel, although observation of diVraction peaks due to the crystalline spinel depends on the increased to 750 °C the presence of the spinel is more evident, especially in the case of sample M5-750 (that is, that without nature of the metal cations and on the calcination temperature (e.g., crystalline Mg,Al spinel is only detected after calcination aluminium and with the largest Fe content), while for sample M6-750 (prepared from the hexacyanoferrate precursor) the at ca. 900–1000 °C).17,18 The pure spinel cannot be obtained, unless redox processes simultaneously occur during calci- diagram is almost coincident with that recorded for the same J.Mater. Chem., 1998, 8(11), 2507–2514 2509sample, but calcined at 450 °C, indicating that crystallization of the spinel has not been favoured, or it is not detected due to the low Fe content (see Table 1). This conclusion can be easily reached from comparison of the intensity of the peak close to 2h=57° (d=1.615 A° ), corresponding to planes (511) or (333) of MgFe2O4.Its intensity increases steadily from sample M2-750 to sample M5-750, but the peak is absent in the diagram of sample M6-750. As for calcination of a Mg,Al hydrotalcite in this same temperature range, no crystalline phase containing Al has been observed.22,23 In order to gain insight into the formation of crystalline phases, selected samples have been calcined at higher temperatures or for longer periods of time.When sample M4 is calcined at 750 °C for 4, 8 or even 24 h, instead of 2 h as used for the sample whose PXRD diagram is shown in Fig. 2, the only eVect observed is a slight sharpening of the diVraction peaks due to the spinel. The eVect is even less evident for sample M5 calcined for these periods of time at 750 °C, as in this sample, calcination for 2 h is enough to form the spinel, as shown by the sharp peaks recorded (Fig. 2). With regards to sample M6, calcination at 750 °C for even 24 h has only minor eVects on the PXRD diagram, and only peaks due to MgO, and broad, ill-defined peaks due to the MgFe2O4 spinel, are again recorded. However, when sample M6 was calcined at higher temperatures, the changes were rather drastic.Fig. 3 includes the PXRD diagrams for this sample calcined at 450, 750, 900, Fig. 4 Thermogravimetric (dotted lines) and diVerential thermal (solid 1000, and 1100 °C for 2 h. From 900 °C upwards, the diVraclines) analyses for samples M5 (upper traces) and M6 ( lower traces). tion peaks due to the spinel are clearly observed, although some of them coincide with peaks due to MgO. Thus, we can conclude that the lack of detection of peaks surface of the particles, and water molecules from the interlayer due to the spinel in sample M6-750 is not due to the low space, and amounting to ca. 15% of the initial weight of the sensitivity of the technique, as calcination at higher tempera- sample. The second weight loss is almost completed at ca.ture gives rise to samples where the peaks due to the spinel 450 °C, and corresponds to removal of hydroxyl groups from are clearly detected. In other words, the lack of formation of the brucite-like layers, as well as of volatile species from the the MgFe2O4 spinel in sample M6-750, even though the spinel interlayer anions (i.e., CO2 from interlayer carbonate), as is formed in all other samples calcined for 2 h at this same concluded in previous studies25 on a Mg,Al–carbonate hydrotemperature, is undoubtedly related to the diVerent location talcite.The weight loss above 450 °C amounts to ca. 1–2% of of the Fe(III ) ions in these two series of samples: in the layers the initial sample weight, and is usually ascribed to removal (samples M2 to M5) or in the interlayer space, as hexacyano- of strongly held hydroxyl groups.From the elemental chemical ferrate (sample M6). composition of the starting solid (Table 1) and the total weight loss up to 750 °C, assuming formation of mixed oxides (MgO, DiVerential thermal analysis (DTA) and thermogravimetric Fe2O3 and Al2O3, or any combination of these) at the highest analysis (TG) temperature reached, the interlayer water content can be calculated, thus providing the whole formula of the starting Representative TG and DTA curves for selected samples are layered materials (Table 3).The behaviour shown by sample shown in Fig. 4. Weight loss starts from room temperature M6 is slightly diVerent and even though decomposition was and is completed at ca. 750 °C. Two steps are observed, as is essentially complete at the same temperature (about 750 °C), usual for hydrotalcites.24 The first one, up to ca. 200–250 °C, intermediate decomposition steps can be observed; unfortu- corresponds to removal of water physisorbed on the external nately, we were unable to analyze the gases evolved during decomposition, to assess the diVerent decomposition steps.The maximum number of water molecules hosted in the interlayer space of a carbonate-containing hydrotalcite can be easily calculated.26,27Water molecules and interlayer carbonate anions can be close-packed in the interlayer region, as hydroxyl groups are in the brucite-like layers. We can assume the size of a water molecule to be coincident with that of a hydroxyl group and about one third of that of a carbonate anion.If these are located with their molecular plane parallel to the Table 3 Formulae of the samples prepared Sample Formulaa M2 [Mg0.74Fe0.11Al0.15(OH)2][CO3]0.13 0.70H2O M3 [Mg0.76Fe0.13Al0.11(OH)2][CO3]0.12 0.69H2O M4 [Mg0.75Fe0.17Al0.08(OH)2][CO3]0.13 0.69H2O M5 [Mg0.75Fe0.25(OH)2][CO3]0.13 0.61H2O Fig. 3 Powder X-ray diVraction profiles for sample M6 calcined for M6 [Mg0.76Al0.24(OH)2][Fe(CN)6]0.08 0.83H2O 2 h at the temperatures given (in °C). (*) signals due to the Al aThe values have been rounded to the nearest 0.01. sample holder. 2510 J. Mater. Chem., 1998, 8(11), 2507–2514layers (as concluded from the width of the interlayer space, of two superimposed bands for samples M2 and M5, a very sharp band (also recorded for sample M6) at 1381 cm-1, and from the spacing determined by PXRD), then the maximum number of water molecules would be 2-(3x/2) per hydrotal- a broader band at 1374 cm-1.This last band is due to mode n3 of the interlayer carbonate anions. The shift from the cite ‘formula’, where x stands for the molar fraction of trivalent cations in the brucite-like layers.In our case x#0.25, and position reported30 for free carbonate is due to restricted freedom and hydrogen bonding (as concluded from the broad then up to 1.6 water molecules can be located. The experimental value is lower, probably because the preferred orientations absorption just above 3000 cm-1) in the interlayer region. This band could be argued to be present (although very much maximize hydrogen bonding. The DTA curves, shown in Fig. 4, are qualitatively weaker) in the spectrum of sample M6. However, other experimental data here described (absence of PXRD diVraction coincident with thospreviously reported for diVerent hydrotalcite- like materials,23,24,28 with two endothermic eVects corre- maxima close to 7.8 A° , characteristic of carbonate-interlayered hydrotalcites, and the lack of the FT-IR absorption slightly sponding to the weight losses above described. Despite the diVerences between the samples studied, the eVects are recorded above 3000 cm-1) strongly suggest that interlayer carbonate anions do not exist in this sample, and so this band could be at almost coincident temperatures, 205–230 and 405 °C, and should correspond to the two weight losses recorded in the due to the presence of carbonate species weakly adsorbed on the external surface of the crystallites, taking into account the TG curves.In all cases, a shoulder at ca. 365 °C is also recorded. Between both eVects, an exothermic eVect is observed strong basiticy of these solids and that the samples are exposed to atmosphere during manipulation to record the spectra.The at ca. 310 °C for sample M6, although it is absent in the curves for the other samples. It is clearly an exothermic eVect, and sharp band at 1381 cm-1 is due to a nitrate impurity existing in the KBr used to prepare the discs as it is also present in not an instrumental artifact (the other DTA curves also show a ‘maximum’ in this position), because the signal grows above the pure KBr discs.Another set of bands is recorded between ca. the baseline of the curve. We tentatively ascribe this eVect to combustion of the cyanide ligands under the oxidizing atmos- 2200–2000 cm-1 for sample M6. This constitutes a ‘window’ in the spectral range, where, in the samples here studied, only phere used during the DTA. the bands due to the C–N stretching of cyanide groups are expected.For hexacyanoferrate, the exact position of the band FT-IR spectroscopy depends on the oxidation state of iron,31 and for reference This technique has been used mainly to identify the interlayer potassium hexacyanoferrate(III) the band was recorded at anions in the samples studied. 2118 cm-1. This band is the most intense one recorded in this The spectra for samples M2, M5, and M6 are shown in range, but, in addition, a weak band is recorded at 2042 cm-1, Fig. 5. These samples have been selected as, due to their with even weaker shoulders at 2089 and 2060 cm-1. The nCN chemical composition, diVerences among their FT-IR spectra band of potassium hexacyanoferrate(II) is recorded at are expected to be more evident than those for samples M3 2044 cm-1, suggesting that a partial Fe3+�Fe2+ reduction and M4.has taken place in the sample studied here. This reduction The broad band centered around 3500 cm-1 is due to the process has been previously claimed by several authors stretching mode of hydroxyl groups, both those in the brucite- to occur upon intercalation of hexacyanoferrate(III) in like layers and from the interlayer water molecules; the broad- diVerent hydrotalcites, as well as oxidation of hexacyanoness of the band indicates that hydrogen bonds with a wide ferrate(II).11,12,32–36 It has been also reported that the range of strength exist.Hydrogen bonding of interlayer water Fe3+�Fe2+ reduction can take place under high pressure,37,38 molecules to interlayer carbonate anions has been claimed23,29 and, actually, the sample was submitted to high pressure to to be the origin of the broad, very weak shoulder recorded prepare the KBr disc.However, such multiple absorptions in slightly above 3000 cm-1, absent in the spectrum of sample this wavenumber range were also recorded when the spectrum M6, in agreement with the lack of interlayer carbonate in this was recorded by the DRIFTS technique (diVuse reflectance sample.The medium band at 1636 cm-1 is due to the defor- IR FT spectroscopy), without application of any sort of mation mode of water molecules, and the bands below pressure. Nevertheless, if such a reduction is assumed (and 1000 cm-1 are due to M–O vibration modes; the presence of hexacyanoferrate anions are known to be outer-sphere elecdi Verent amounts of Mg2+, Al3+ and Fe3+ in the brucite-like tron-transfer reductants or oxidants39,40), the origin of the layers in the diVerent samples would account for the diVerent other two weaker bands at 2089 and 2060 cm-1 can be easily relative intensities of these bands in the diVerent spectra. explained.According to Jones,41 the A1g, Eg and T1u nCN The absorption around 1370–1390 cm-1 is clearly composed modes required by the Oh point group for Fe(CN)64- are recorded at 2094, 2062 and 2044 cm-1 in aqueous solution, the first two bands being infrared-forbidden, but in the interlayer space of the hydrotalcite surely become partially activated by a decrease in symmetry, here being recorded at 2089 and 2060 cm-1, respectively.If grafting has occured [as it could be concluded from a calculated gallery height smaller than the size of the Fe(CN)63- moiety along the C3 axis], the decrease in symmetry would be much more drastic and the spectrum much more complicated. So, we may conclude that partial Fe3+�Fe2+ reduction has taken place because of the stress generated in the hexacyanoferrate(III ) anion between the close brucite-like layers. Ultraviolet–visible/diVuse reflectance spectroscopy The results obtained for samples M2 to M5 (as well as for the corresponding calcined samples) were rather similar, but discussion will be centered on sample M5, with the highest iron content.As expected, the behaviour shown by sample M6 Fig. 5 FT-IR spectra of samples M2, M5, and M6.Inset: spectrum of sample M6 in the 2175–1975 cm-1 range. was diVerent. J. Mater. Chem., 1998, 8(11), 2507–2514 2511increase from sample M2 to sample M4, but decreases for sample M5. These diVerences can be readily related to the diVerent crystallinity of the samples, as concluded from the sharpness and half-width of the main diVraction maxima recorded in the PXRD patterns.The value for sample M6 is extremely large, and its adsorption–desorption isotherm corresponds to type I in the IUPAC classification, characteristic of adsorption on microporous solids. These findings are similar to those previously reported by Cavalcanti et al.34 for hexacyanoferrate-containing hydrotalcites. According to these results, the nitrogen molecules are not able to enter into the interlayer space of the carbonate-containing hydrotalcites, but enter into the interlayer of the hexacyanoferrate hydrotalcites.The reason for this diVerent behaviour should be in the population of the interlayer space, and probably in the size of the gallery (ca. 3 A° for the carbonate-containing samples, but 6.32 A° for sample M6). Anions and water molecules exist in the interlayer Fig. 6 UV–VIS/DR spectra of sample M6 and of potassium hexabetween the brucite-like sheets. The role of the anions is to cyanoferrate(II ) and hexacyanoferrate(III ). balance the electric positive charge of the layers (originated by theM2+/M3+ exchange) and, as the formal negative charge The spectrum for sample M5 shows a broad absorption of the anions increases, a lower number of anions are required extending from 700 nm to lower wavelength, centered at ca.for a given positive charge to be balanced. In our case, the 400 nm, and with a sharp absorption close to 300 nm. As M2+/M3+ ratio is ca. 351 in all cases, but the formal charge Mg2+ and Al3+ have d0 configurations and Fe3+ has a d5 of the anions is -2 for samples M2 to M5 and -3 for sample high-spin configuration (due to weak field oxide ligands), the M6.With this, despite the larger size of the hexacyanoferrate expected absorptions should be exclusively due to charge anion, more room would be available to host nitrogen moltransfer processes from the oxide ligands to the Fe3+ ions. ecules in sample M6 than in samples M2 to M5. Moreover, When the sample is calcined at 450 °C the absorption becomes the swelling of the structure upon incorporation of the larger broader, with only minor changes in the spectrum of the hexacyanoferrate anion (from gallery heights ca. 3 A° for sample calcined at 750 °C. No importaerence is, however, carbonate-containing hydrotalcite to 6.32 A° for the hexacyanoobserved in the colour (brown) of the sample prior to or after ferrate form) will also facilitate nitrogen insertion into the calcination.interlayer space. The spectrum recorded for sample M6 (the sample is yellow) When the samples are calcined at 450 and 750 °C, the is shown in Fig. 6. For comparison, the spectrum of commercial layered structure is destroyed (see PXRD data above) and the potassium-hexacyanoferrate(III ) and -hexacyanoferrate(II) diVerences in the specific surface areas (Table 2) cannot be (existing in this sample, according to the FT-IR results above related to the layered structure, but simply to the diVerent discussed), are included in the same figure.[Fe(CN)6]3- shows crystallinity of the samples. For samples M2 to M5, an increase a broad absorption centered at 460 nm, and a structured is observed from the values for the original samples to those absorption at 315 nm.On the other hand, [Fe(CN)6]4- shows for the samples calcined at 450 °C, due to formation of less an absorption at ca. 330 nm. The origin of these bands has crystalline, mostly amorphous phases; among those samples been discussed for model hexacyanometalate compounds by calcined at 450 °C, the PXRD peaks of sample M5-450 are Gray and co-workers.42,43 In the hexcacyanoferrate(III ) com- much sharper than for the other samples calcined at 450 °C, pound there is a spin-allowed transition (d5 configuration, low thus indicating a more crystalline material, i.e., with a lower spin octahedral coordination), responsible for the absorption specific surface area, as experimentally observed.On heating around 450 nm, while absorptions at lower wavelengths and at 750 °C, the values decrease from those obtained at 450 °C in the hexacyanoferrate(II ) compound (d6 configuration, low in most of the cases, the behaviour shown by sample M5-750 spin octahedral coordination) are due to ligand/metal charge (without aluminium) being noticeable, with a specific surface transfer processes.The spectrum of sample M6 shows two area of 16 m2 g-1 only, probably due to formation of the well absorptions at 460 and 315 nm, coinciding with those of the crystallized spinel structure, see PXRD results in Fig. 2. Again, [Fe(CN)6]3- species, but the ‘valley’ between these two the behaviour shown by sample M6 is singular, with specific absorptions, close to 330 nm, is less pronounced, this probably surface areas close to 110–120 m2 g-1 in agreement with the being due to the presence of [Fe(CN)6]4-. Altogether, these rather amorphous structure of this material, as concluded results further confirm that the structure of the hexacyanofer- from PXRD measurements, and of the same order as those rate moiety is mostly maintained after incorporation into the for samples M2 to M4, even after calcination at 750 °C.interlayer space of the hydrotalcite material. Upon calcination, the colour of samples M6-450 and M6- 750 becomes brown, and their spectra are almost coincident Temperature-programmed reduction with those recorded for the other samples calcined at the same The technique was used in order to analyze the way Fe3+ is temperatures.These results indicate that, upon calcination and reduced in the samples. However, it should be taken into destruction of the layered structure, the local environment of account that the sample is being decomposed simultaneously the iron ions becomes almost the same (if not identical) in with the reduction as the temperature is increased during the all samples. TPR runs, and so the results obtained cannot be simply related to reduction of cations as they were in the original materials.45 Specific surface area measurements On the other hand, it has been observed in some cases that reduction has not been completed (i.e., the curve does not The specific surface areas of the samples, as determined from the nitrogen adsorption isotherms at -196 °C, are given in recover the baseline) even at the maximum temperature attainable by the instrument, and so a full quantitative analysis has Table 2.For original samples M2 to M5 the isotherms belong to type II in the IUPAC classification,44 and correspond to not been performed, although hydrogen consumptions are given in Table 3. unrestricted adsorption. The specific surface area values 2512 J.Mater. Chem., 1998, 8(11), 2507–2514to a broad peak at 758 °C, it shows also a ‘negative’ peak at 404 °C, undoubtedly due to removal of cyanide ligands during decomposition. It should be remembered that the TPR curve is obtained from a chromatographic analysis of the gas after the sample (where the concentration of H2 has decreased because of reduction), and thus, the presence of other gases (from cyanide decomposition) would account for unexpected changes in the conductivity.Thermogravimetric analysis has shown that these ligands are removed below 450 °C, and, unfortunately, reduction takes place in the same temperature range. The curves for the samples calcined at 450 °C are included in Fig. 7B. A sharp reduction peak is again recorded at 410±6 °C for samples M2-450 to M5-450, together with a weaker, broader feature at higher temperatures; reduction was not complete at 850 °C.However, for sample M6-450, two reduction peaks were recorded at 432 and 719 °C, and it should be noted that the relative intensities of the high/low reduction peaks is reversed for sample M6-450, if compared with the other samples.DiVerent peaks in a TPR profile where a single cation is reduced can be ascribed to consecutive reduction steps (e.g., Fe3+�Fe2+, and Fe2+�Fe0), or to reduction of diVerent species (e.g., reducible crystals diVering in their size or dispersion, or cations in diVerent environments) in a single step, or even a mixing of both. In the first case, the ratio between the integrated areas of the peaks (directly related to hydrogen consumption) should be constant (e.g., for Fe3+�Fe2+ and Fe2+�Fe0, it should be 0.551).So, in the present case, the change in relative intensities of the peaks recorded for sample M6-450 (ex-hexacyanoferrate) should correspond to the presence in this sample of species with a diVerent dispersion/ reducibility than in the samples prepared from hydrotalcites with Fe3+ ions in the brucite-like layers.The behaviour shown by the samples calcined at 750 °C, Fig. 7C, is rather similar to those of the samples calcined at 450 °C: A fairly sharp peak, now at 464±4 °C, followed by a broader, sometimes structured, peak extending from ca. 600–800 °C. On the contrary, two peaks (with reversed relative intensities) were recorded for sample M6-750, centered at 412 and 778 °C, a similar profile to that recorded for sample M6-450.Conclusions In this paper, we have prepared hyrotalcite-like materials with a given MII/MIII ratio (MII=Mg; MIII=Al, Fe), but varying the ratio between two trivalent cations (Al/Fe) in the layers. We have also prepared a sample with the same structure, but containing hexacyanoferrate(III) in the interlayer space, without simultaneous co-formation of a carbonate-intercalated Fig. 7 Temperature-programmed reduction profiles of samples M2, material, although hexacyanoferrate(II) is partially formed. M5, and M6. (A) Original samples; (B) calcined at 450 °C; (C) calcined The solids have been characterised, and their thermal behavat 750 °C. iour analyzed. It has been found that, despite thermal decomposition in air leading in all cases to a mixture of MgO and MgFe2O4, the crystallinity depends both on the calcination The curves for samples M2, M5 and M6 are shown in Fig. 7A. While for sample M2 a single reduction peak, with a temperature, and on the Al/Fe ratio and on the initial location of the FeIII ions: at 450 °C all samples are mostly amorphous, maximum at 416 °C (close positions are observed for the other carbonate-containing hydrotalcites, and the peak becomes and poorly crystallized MgO is formed.At 750 °C, additional crystallization of MgFe2O4 is observed in the ex-carlightly broader), is recorded, a second, broader, peak, incomplete for sample M5, is recorded for the other carbonate- samples, and especially in the absence of Al; however, in the ex-hexacyanoferrate sample the crystalline phases existing are containing samples.The molar H2/Fe ratio was in all cases fairly close to the expected value of 1.551, for total reduction the same as after calcining at 450 °C. Prolonging the calcination time from 2 to 24 h or raising the calcination temperature has from Fe3+ to Fe0 (PXRD analysis of the residue after the TPR run of sample M4 indicates formation of metallic Fe).only minor eVects on the crystallinity of the species formed. For the ex-hexacyanoferrate sample, however, spinel MgFe2O4 However, such a ratio was only 1.2551 for sample M5, in whose TPR profile the baseline has not been recovered even formation is only detected above 900 °C. These results show that the initial location of the FeIII ions at 850 °C, the maximum temperature attainable by our experimental system for TPR analysis.is of paramount importance in determining the nature of the crystalline phases formed, depending on the calcination tem- The profile for sample M6 is completely diVerent. In addition J. Mater. Chem., 1998, 8(11), 2507–2514 251320 J. M.Ferna�ndez, C. Barriga, M. A. Ulibarri, F. M. Labajos and perature, and thus will hopefully determine their use as cata- V. Rives, J. Mater. Chem., 1994, 4, 1117. lysts or magnetic materials. 21 JCPDS: Joint Committee on Powder DiVraction Standards, International Centre for DiVraction Data, Pennsylvania, 1977. Authors thank Dr. B.Macý�as and Mr. A.Montero (University 22 F. Rey, V.Forne�s and J. M. Rojo, J. Chem. Soc., Faraday Trans., of Salamanca), and Ms. F. Pe� rez-Taboada (University of 1992, 88, 2233. 23 F. M. Labajos, V. Rives and M. A. Ulibarri, J. Mater. Sci., 1992, Co�rdoba) for their assistance in obtaining some of the exper- 27, 1546. imental results. Financial support from Junta de Andalucý�a 24 L. Pesic, S. Salipurovic, V. Markovic, W. Kagunya and W.Jones, (group FQM-214) and Ministerio de Educacio�n y Ciencia J. Mater. Chem., 1992, 2, 1069. (PB96-1307-C03) is also acknowledged. 25 M. del Arco, C. Martý�n, I. Martý�n, V. Rives and R. Trujillano, Spectrochim. Acta, Part A, 1993, 49, 1575. 26 S. Miyata, Clays Clay Miner., 1975, 23, 369. References 27 S. K. Yun and T. J. Pinnavaia, Chem.Mater., 1995, 7, 348. 28 M. A. Ulibarri, M.J. Herna�ndez and J. Cornejo, Thermochim. 1 A. de Roy, C. Forano, K. El Malki and J. P. Besse, in Expanded Acta, 1987, 113, 79. Clays and Other Microporous Solids, eds. M. L. Occelli and 29 B. C. Kruissink, L. van Reidjen and J. R. H. Ross, J. Chem. Soc., H. E. Robson, Van Nostrand Reinhold, New York, 1992, p. 108. Faraday Trans. 1, 1981, 77, 649. 2 F. Trifiro` and A. Vaccari, in Comprehensive Supramolecular 30 K.Nakamoto, Infrared and Raman Spectra of Inorganic and Chemistry, eds. J. L. Atwood, J. E. D. Davies, D. D. MacNicol, Coordination Compounds, John Wiley & Sons, New York, 4th F. Vo� gtle, J.-M. Lehn, G. Alberti and T. Bein, Pergamon-Elsevier edn., 1986. Science, Oxford, 1996, vol. 7, p. 251. 31 L. Tosi and J. Danon, Inorg. Chem., 1964, 3, 150. 3 F. Cavani, F. Trifiro` and A. Vaccari, Catal. Today, 1991, 11, 1. 32 S. Idemura, E. Suzuki and Y. Ono, Clays Clay Miner., 1989, 4 F. Kooli, I. Crespo, C. Barriga, M. A. Ulibarri and V. Rives, 37, 553. J. Mater. Chem., 1996, 6, 1199. 33 K. A. Carrado, A. Kostapapas and S. L. Suib, Solid State Ionics, 5 F. M. Labajos, V. Rives, P. Malet, M. A. Centeno and 1988, 26, 77. M. A. Ulibarri, Inorg. Chem., 1996, 35, 1154. 34 F. A. P. Cavalcanti, A. Schutz and P. Biloen, in Preparation of 6 R. Trujillano, Ph.D. Thesis, University of Salamanca, Spain, 1997. Catalysts IV, eds. B. Delmon, P. Grange, P. A. Jacobs and 7 V. Rives, Adsorpt. Sci. Technol., 1991, 8, 95. G. Poncelet, Elseviere, Amsterdam, 1982, p. 165. 8 P. Malet and A. Caballero, J. Chem. Soc., Faraday Trans., 1988, 35 S. Miyata, Clays Clay Miner., 1988, 31, 305. 84, 2369. 36 I. Crespo, C. Barriga, V. Rives and M. A. Ulibarri, Solid State 9 A. S. Bookin and V. A. Drits, Clays Clay Miner., 1993, 41, 551. Ionics, 1997, 101–103, 729. 10 J. E. Huheey, E. A. Keiter and R. I. Keiter, Inorganic Chemistry: 37 A. R. Champion and H. G. Drickamer, J. Chem. Phys., 1967, Principles of Structure and Reactivity, Harper Collins, New York, 47, 2592. 4th edn., 1993. 38 H. A. Larsen and H. G. Drickamer, J. Phys. Chem., 1967, 61, 11 M. J. Holgado, V. Rives, M. S. Sanroma�n and P. Malet, Solid 1299. State Ionics, 1996, 92, 273. 39 M. Gordon, L. L. Williams and N. Sutin, J. Am. Chem. Soc., 12 H. C. B. Hansen and C. B. Koch, Clays Clay Miner., 1994, 42, 170. 1961, 83, 2061. 13 J. D.Wang, G. Senette, Y. Tian and A. Clearfield, Appl. Clay Sci., 40 E. Pelizzetti, E. Mentasti and C. Baiocchi, J. Phys. Chem., 1976, 1995, 10, 103. 80, 2979. 14 S. Kikkawa and M. Koizumi, Mater. Res. Bull., 1982, 17, 191. 41 L. H. Jones, Inorg. Chem., 1963, 2, 777. 15 C. Depe`ge, L. Bigey, C. Forano, A. de Roy and J. P. Besse, J. Solid 42 H. B. Gray and N. A. Beach, J. Am. Chem. Soc., 1963, 85, 2922. State Chem., 1996, 126, 314. 43 J. J. Alexander and H. B. Gray, J. Am. Chem. Soc., 1968, 90, 4260. 16 M. Me�ne�trier, K. S. Han, L. Guerlou-Depourgues and G. Delmas, 44 K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, Inorg. Chem., 1997, 36, 2441. R. Pierotti, J. Rouquerol and T. Sieminiewska, Pure Appl. Chem., 17 M. del Arco, V. Rives and R. Trujillano, Stud. Surf. Sci. Catal., 1985, 57, 603. 1994, 87, 507. 45 V. Rives, M. A. Ulibarri and A. Montero, Appl. Clay Sci., 1995, 18 T. Sato, H. Fujita, T. Endo, M. Shimala and A. Tsunashima, 10, 83. React. Solids, 1988, 5, 219. 19 M. A. Ulibarri, J. M. Ferna�ndez, F. M. Labajos and V. Rives, Chem. Mater., 1991, 3, 626. Paper 8/04867C 2514 J. Mater. Chem.

ISSN:0959-9428    

DOI:10.1039/a804867c

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Synthesis, structure, electrical and magnetic properties of the new nonstoichiometric perovskite phase, Ca2MnNbOc Angela Kruth,a Mitsuharu Tabuchi,b Ulrich Guthc and Anthony R. Westa aUniversity of Aberdeen, Department of Chemistry, MestonWalk, Aberdeen, UK AB24 3UE bOsaka National Research Institute, 1-8-31 Midorigaoka, Ikeda, Osaka 563, Japan cUniversity of Greifswald, Department of Chemistry, Institute of Physical Chemistry, Soldtmannstraße 23, 17489 Greifswald, Germany Received 26 June 1998, Accepted 6th August 1998 The new phase Ca2MnNbOc has an orthorhombic, GdFeO3 structure with B site disorder of Mn and Nb.The oxygen content c varies from 5.86 to 6.00 depending on heat treatment conditions. Treatment in high pressure O2 is required for full oxidation.Magnetic susceptibility measurements indicate mixed valence (2+, 3+) ofMn for c=5.87 and Curie–Weiss paramagnetism at high temperatures. SQUID measurements indicate spin glass-like behaviour below 15 K. Ac impedance shows semiconducting behaviour, E=0.31(2) eV. On heating in O2-poor atmospheres, resistive surface layers and grain boundary eVects are seen, which point to a p-type conduction mechanism.stack together with a layer of A-atoms in between. A homolo- Introduction gous series, AnBnO3n+2 forms, with n=4 in this case, but in Perovskites and perovskite-related oxides oVer a wide range other phases n-values range from 1 to 6.12–14 The brownmillerof interesting electrical properties, e.g. giant magnetoresistance, ite structure of Ca2Fe2O5 and Ca2FeAlO5 is derived from superconductivity and oxide ion conductivity. Mixed oxide perovskite with ordering of oxygen vacancies.15,16 The oxygen ion–electron conducting perovskites have possible uses as vacancies give tetrahedral coordination of iron resulting in electrode materials in oxygen sensors and solid-oxide fuel cells alternate sheets of FeO6 octahedra and FeO4 tetrahedra. and may have major advantages over porous noble-metal CaMnO3-d also has oxygen vacancies, in the compositional electrodes because they are stable to temperatures well above range 0d0.5, which order at certain compositions; Mn has 1250 °C1 and charge-transfer reactions can take place every- mixed 3+/4+ valence and occupies a mixture of octahedra where at the electrode/gas interface, rather than at the triple and square-pyramids which form ordered intergrowth strucjunctions: electrolyte/electrode/gas.In some non-stoichio- tures where the proportion of square-pyramidal Mn3+ relative metric perovskites, electrochemical reactions occur at specific to octahedral Mn4+ increases with d. In CaMnO2.5, d=0.5, vacant surface sites;2–4 these materials exhibit catalytic activity the Mn is 3+ and in five-fold square-pyramidal coordination for processes such as the oxidation of CHx and NOx.only, which is distinct from brownmillerite which contains a Perovskites based on elements of the first transition series mixture of tetrahedra and octahedra.17,18 display a wide variety of defect-related phenomena which have On replacing Fe3+ in brownmillerite, Ca2Fe2O5, by Nb5+, their origin in the unfilled 3d electron shell.The relative ease an extensive oxygen-deficient, cubic perovskite solid solution, with which electrons can be removed from or added to some Ca2Fe2-xNbxOc: 0.45x0.65 was obtained.19 Its oxygen metals, especially Fe and Mn, means that they often exist in content is variable, e.g. 5.55c5.92 for x=0.6. It has high numerous valence states, depending on temperature and electronic conductivity and probably also has high oxide ion oxygen partial pressure; this also gives rise to a potentially conductivity since the oxygen content, c, readjusts rapidly on large number of phases, which themselves may deviate from varying either temperature or oxygen partial pressure and exact stoichiometry in terms of both cation and oxygen could therefore have uses as a mixed conducting electrode in content.5 Non-stoichiometry was until recently explained in devices involving gas–solid interfacial reactions.terms of point defect equilibria.6,7 For most compounds with This project is based on the Ca2Fe2O5–CaMnO3–Ca2Nb2O7 sizeable deviations (>1%) in stoichiometry, however, the con- system with the initial objective of synthesising perovskitecept of point defect equilibria is inadequate. Alternative models related phases with variable oxygen content, high mixed elecare needed which involve ordering of point defects or new tronic and oxide ion conductivities and stability under both structural features which eliminate point defects.8 Defects in reducing and oxidising conditions over large temperature perovskite oxides can arise from cation deficiency in either A ranges. We attempt to control the oxidation state of Fe, Mn or B sites or oxygen deficiency; the resulting vacancies often and associated oxygen content by both adjusting the niobium form ordered superstructures.concentration at B-cation sites and post-reaction heat treat- Tofield and Scott9 suggested three possible models to ments at diVerent oxygen pressures.Initial investigations were accommodate oxygen excess in the perovskite structure: carried out on the binary joins, Ca2Fe1.4-xMnxNb0.6Oc and (i ) introduction of interstitial oxygen at either (D00) or (BBB) Ca2Mn2-xNbxOc. As a first step, a variety of characterisation positions of the cubic unit cell; this, however, is energetically studies on the new phase, Ca2Mn2-xNbxOc with x=1.0, were unfavourable; (ii ) creation of cation vacancies at A and/or B made and are described here.sites, leaving a perfect oxygen sublattice and (iii ) formation of new, oxidised perovskite-related phases. For example, in LaMnO3+l and La2TiCoO6+l, oxygen excess is accommo- Experimental dated by vacancies in both A and B sites.10,11 In Cu2Nb2O7 Starting materials CaCO3 (99%, AnalaR) and Nb2O5 (99.9%, and La2Ti2O7 with large oxygen excess, new perovskite-related Aldrich) were dried at 200 and 600 °C, respectively. MnO2 structures form: the parent, cubic perovskite is sliced parallel to (110) to give slabs of composition (An-1BnO3n+2)2 which (99%, Aldrich) was used directly from the bottle since a J.Mater. Chem., 1998, 8(11), 2515–2520 2515thermogravimetry (TG) study confirmed its stoichiometry, were determined by ac impedance spectroscopy over the frequency range 0.03 Hz to 13 MHz using a Hewlett-Packard during a two-stage decomposition to Mn2O3 at ca. 400–700 °C and Mn3O4 at ca. 900–1000 °C. Appropriate quantities of 4192A LF Impedance Analyser.Pellets, 8 mm diameter and 1–2 mm thick, were prepared by cold-pressing powders at ca. MnO2, CaCO3 and Nb2O5 giving ca. 5 g total weight were mixed in acetone for at least 10 minutes, dried and heated in 200 MPa, sintered in air overnight at 1400 °C and quenched to room temperature. To modify the oxygen content, pellets Pt boats at 900 °C for 12 h, 1200 °C for 20 h and at 1200 °C to 1350 °C in steps of 50 °C for periods of 20 h each, with were heated at 1100 °C for 5 h and slowly cooled to room temperature (2 K min-1) in either O2 or N2.One pellet was regrinding between each heating period. Samples were finally heated at 1400 °C for 72 h. annealed at 600 °C and ca. 40 atm O2. Electrodes were attached by coating pellet faces with a thin InGa (151) alloy layer and To modify the oxygen stoichiometry, samples were heated at diVerent temperatures in air, O2 or Ar and either quenched gold strips held to the sample by pressure.or cooled slowly to room temperature. AMorris High Pressure Furnace, HPS-3210 was used to anneal samples, wrapped in Results and discussion Au foil, under high O2 pressure.Two experimental conditions were used which gave pressures at 600 °C of either ca. 40 or The new phase, Ca2MnNbOc, was obtained on reaction of the oxides in a stepwise heating programme; phase-pure ca. 60 atm. After heating at 600 °C for 10 h, samples were cooled slowly under pressure. samples were obtained finally after heating at 1400 °C, 72 h. This new phase was easily recognised since its XRD pattern Oxygen contents and the oxidation state of Mn were determined from a combination of TG and magnetic measure- is very similar to that of Ca2FeNbO619 and, therefore, it also has an orthorhombic, GdFeO3 structure.ments. By TG, the oxygen contents were determined from the weight loss on reduction in 10% H2–90% N2 using a Stanton Oxygen contents, c, were determined by H2-reduction TG; typical traces are shown in Fig. 1.In all cases, no reduction Redcroft TG-DTA 1500 instrument. A Shimadzu MB-3 magnetic balance was used to measure magnetic susceptibility occurred up to at least 400 °C but reduction was complete by 950–1150 °C. Samples with higher oxygen contents (c,d ) values which were converted to eVective magnetic moments and then compared with theoretical values obtained from the started to lose weight at lower temperatures than the others.Although weight losses could be determined accurately for ‘spin-only’ formula: (NH4)2Mn(SO4)2·6H2O was used as calibration standard. SQUID measurements were carried out all samples, in order to convert these to variations in oxygen content, it was necessary to know the stoichiometry of either below 100 K with magnetic fields of 5000 and 10 Oe. Phase identity, stability and purity were determined by the final, fully reduced state or that of at least one of the starting compositions.The assumption was made initially that powder X-ray diVraction, Ha�gg-Guinier camera, Cu-Ka1 radiation. For indexing and lattice parameter refinement, a Philips the H2-reduction would aVect only the oxidation state of Mn and that the final state in all cases would be Mn2+, correspond- DiVractometer PW1710 and Stoe Stadi P software were used, KCl internal standard, 20 to 80° 2h, Cu-Ka1 radiation.A Stoe ing to an overall final stoichiometry of ‘Ca2MnNbO5.5’; H2 reduction of CaMnO3-d had shown the product to be Stadi P transmission diVractometer was used for structural studies (Rietveld refinement) of the fully oxidized phase, CaMnO2, containing Mn2+17 and we assumed a similar behaviour for our samples.Using this assumption, the starting Ca2MnNbO6 with data collected over the range 8 to 113° 2h and step width 0.02°. The refinement was carried out using compositions were evaluated. Two samples heated in high pressure O2 had c values close to 6, viz. 6.02(3) after 40 atm the package PFSR (pattern–fitting structure–refinement) with programs CDF, CDE, RVI and RVR. First, the lattice param- O2 at 600 °C and 5.97(3) after 60 atm O2; within errors, it is concluded that these were fully oxygenated with c=6.00. An eters and halfwidths of the reflections were refined, e.g. lattice constants, 2h zero point, halfwidth parameters and back- oxygen content of 6 is the highest that can be expected for this structure type; a large number of phases with analogous ground.After convergence of profile refinement, the structural parameters were refined, e.g. overall scale factor, atomic formulae and GdFeO3 structure are known and all have an oxygen content of 6. This result therefore further supports coordinates and isotropic thermal vibration parameters.DTA was used to look for any phase transitions over the range 25 the correctness of the initial assumption concerning the stoichiometry of the H2-reduced samples. to 1300 °C in air, at heating and cooling rates of 8 K min-1. Electrical properties in air over the range -60 to 200 °C The oxygen content of Ca2MnNbOd varied according to the Fig. 1 Weight loss and corresponding oxygen content, c, during H2 reduction of samples prepared under diVerent heating conditions: (a) slowly cooled from 1100 °C in Ar, (b) quenched from 1400 °C in air, (c) slowly cooled from 1100 °C in O2 and (d) heated at ca. 40 atm O2 and 600 °C; during heating 2 and cooling & cycle. 2516 J. Mater. Chem., 1998, 8(11), 2515–2520Table 1 X-Ray powder diVraction data for Ca2MnNbO5.87 orthorhombic; a=5.4527(5) A° , b=5.5622(5) A° , c=7.7062(8) A° h k l Int.d(obs)/A° d(calc)/A° 1 1 0 30 3.8933 3.8938 0 0 2 14 3.8540 3.8531 1 1 1 3 3.4753 3.4753 0 2 0 25 2.7808 2.7811 1 1 2 100 2.7388 2.7388 2 0 0 25 2.7267 2.7263 2 1 0 2 2.4467 2.4481 1 2 1 2 2.3597 2.3586 2 1 1 3 2.3333 2.3332 1 0 3 2 2.3252 2.3236 1 1 3 2 2.1441 2.1442 1 2 2 2 2.0834 2.0839 Fig. 2 Influence of annealing temperature in air on oxygen content, 2 1 2 2 2.0653 2.0663 c, in Ca2MnNbOc . 2 2 0 25 1.9462 1.9469 0 0 4 13 1.9265 1.9266 2 2 1 3 1.8873 1.8876 1 3 0 4 1.7552 1.7554 final heat treatment conditions; lowest values were obtained 2 2 2 5 1.7375 1.7377 at high temperatures, e.g. 5.87 in air at 1400 °C and 5.86 in 1 1 4 6 1.7266 1.7268 Ar at 1200 °C.The oxygen content as a function of temperature 1 3 1 2 1.7111 1.7115 in air is shown in Fig. 2. The data were obtained on samples 1 3 2 12 1.5973 1.5974 quenched from diVerent temperatures and show that oxygen 0 2 4 8 1.5837 1.5837 3 1 2 22 1.5758 1.5764 content varies over the range ca. 400 to 700 °C but is essentially 2 0 4 16 1.5731 1.5734 constant at higher and lower temperatures. 1 3 3 1 1.4487 1.4493 Indexed powder XRD data for one composition, 0 4 0 2 1.3906 1.3905 Ca2MnNbO5.87, are given in Table 1. The data index on an 2 2 4 10 1.3694 1.3694 orthorhombic unit cell with a Ó2, Ó2, 2 relation to a cubic 4 0 0 3 1.3626 1.3632 perovskite-like subcell. Lattice parameters show a small 3 2 3 2 1.3094 1.3091 4 1 1 1 1.3052 1.3049 increase in c with decreasing c from 7.683 A° for c=6.00 to 3 3 0 2 1.2977 1.2979 7.706 A° for c=5.87; a and b change little with c. 4 0 2 2 1.2856 1.2851 Ca2MnNbO6 for Rietveld refinement was synthesised in 60 2 4 0 1 1.2389 1.2387 atm oxygen. The starting model was the orthorhombic per- 3 3 2 3 1.2301 1.2300 ovskite-related GdFeO3 structure.20 A Pearson VII profile 2 4 1 3 1.2234 1.2230 function with exponent m=2.0 was applied.The profile and 0 4 3 6 1.2224 1.2229 1 1 6 3 1.2194 1.2197 structural parameters allowed to refine were scale factor, 2h 2 4 2 4 1.1793 1.1793 zero point, cell constants, background, halfwidth polynomial, 4 2 2 4 1.1663 1.1666 atomic positions for all atoms and thermal vibration param- 0 2 6 1.1660 eters for Ca and Mn/Nb. After convergence, R-factors were: 3 3 3 4 1.1584 1.1584 R( p)=0.0343, R(wp)=0.0443 and R(I,hkl)=0.1332.Final 1 3 5 1.1582 values for atomic positions and thermal parameters are given 0 4 4 2 1.1279 1.1275 4 0 4 2 1.1129 1.1128 in Table 2 and Table 3 shows calculated CaMO and 1 5 2 4 1.0489 1.0488 Nb/MnMO bond lengths. The XRD pattern for Ca2MnNbO6 2 4 4 2 1.0421 1.0419 and the diVerence between observed and calculated profiles 1 3 6 2 1.0367 1.0365 are shown in Fig. 3. 4 2 4 6 1.0331 1.0331 Magnetic measurements were carried out for 5 1 2 4 1.0310 1.0311 Ca2MnNbO5.87; Fig. 4(a) shows the field dependence of mag- 3 1 6 1.0307 netization at 83 and 293 K between 1.8 and 12.5 kOe. No spontaneous magnetization was observed down to 83 K. The magnetic susceptibility was 4.17×10-5 cm3 g-1, which is Fe analogue structures, A2FeXO65A=Ca, Sr, Ba; X=Nb, Ti, Sb.21–25 suYciently large to ignore the contribution of closed shell diamagnetism, 3.33×10-7 cm3 g-1.Temperature dependence Ac impedance data were recorded isothermally at temperatures in the range 200 to 400 K for pellets that had of the inverse molar susceptibility, xm-1, reveals a Curie– Weiss paramagnetism down to 83 K, Fig. 4(b), with meff= been given a range of post-sinter heat treatments. Two general patterns of behaviour were seen. Samples heat-treated in O2- 5.341(6) mB and h=+21 K. The positive Weiss temperature suggests ferromagnetic coupling below 83 K. The observed meff rich atmospheres showed the simplest response, Fig. 6 (a), (b); the Z vs. Z¾ complex plane plot showed a main arc and a value is intermediate between the spin only values of Mn2+, meff,Mn2+=Ó35 mB=5.92 mB, and Mn3+, meff,Mn3+=Ó24 mB= much smaller, poorly-resolved, low frequency arc.Z, M spectra showed essentially coincident peaks at high frequency 4.90 mB, suggesting that this sample is a Mn2+/3+ mixed valence compound. This iconsistent with the TG value for with associated capacitance 8–10 pF, indicative of bulk or intragranular response.The low frequency Z shoulder peak the oxygen content, c=5.87, which gives a 26574 ratio for Mn2+5Mn3+. had a capacitance of ca. 10 nF, attributable to either a thin surface layer or an electrode–sample interfacial eVect. Hence, SQUID measurements made for two applied fields over the temperature range 4–100 K are shown in Fig. 5.Above 20 K, the impedance data were dominated by the bulk response of the sample and there is little evidence of resistive grain the data sets are essentially superposable but diverge dramatically at lower temperature. In particular, the zero field cooled boundary eVects (which would have capacitances in the range 0.1 to 1 nF). data at 10 Oe show a sharp magnetisation maximum at 15 K which transforms to an almost temperature-independent mag- For samples heated in air or N2/Ar, the impedance response was more complex, Fig. 6(c), (d). The Z vs. Z¾ plots showed netisation in field cooled data. Such features are strongly indicative of spin glass-like behaviour of the kind reported in clear evidence of several overlapping arcs, but also dramatic J. Mater. Chem., 1998, 8(11), 2515–2520 2517Table 2 Values for atomic coordinates and thermal vibration parameters for Ca2MnNbO6 in Rietveld refinement WyckoV Atom position Occ. x/a y/b z/c Uij Ca 4c 1.0 -0.0074(11) 0.0050(5) 0.2500 0.0150(1) Mn 4b 0.5 0.5000 0.0000 0.0000 0.0025(4) Nb 4b 0.5 0.5000 0.0000 0.0000 0.0025(4) O(1) 4c 1.0 0.0915(15) 0.4684(19) 0.2500 0.0030 O(2) 8d 1.0 -0.2946(14) 0.2947(12) 0.04531(8) 0.0030 Table 3 Bond distances, D, and bond angles, a, for Ca2MnNbO6 CaMO D/A° OMMn/NbMO a/° CaMO(1) 2.303(10) (1×) O(2)M Mn/NbMO(2) 87.5(3) CaMO(2) 2.358(7) (2×) O(2)M Mn/NbMO(2) 89.0(3) CaMO(1) 2.416(11) (1×) O(2)M Mn/NbMO(1) 89.3(3) CaMO(2) 2.618(8) (2×) O(2)M Mn/NbMO(1) 90.7(3) CaMO(2) 2.700(7) (2×) O(2)M Mn/NbMO(1) 91.0(3) O(2)MMn/NbMO(1) 92.5(3) Mn/NbMO D/A° Mn/NbMO(1) 1.992(2) (2×) Mn/NbMO(2) 1.999(7) (2×) Mn/NbMO(2) 2.013(7) (2×) Pbnm (no. 62); a=5.445 A° , b=5.555 A° , c=7.683 A° . R( p)=0.0343, R(wp)=0.0443, R(I,hkl)=0.1332. Fig. 5 SQUID measurement for Ca2MnNbO5.87: at 10 (&; %) and 5000 Oe ($; #). changes occurred on polishing the pellet surfaces prior to attaching electrodes (c). The bulk response was again seen at highest frequencies in the M spectra, as the low frequency tail of a peak at 107 Hz (d), but the corresponding Z spectra and therefore the Z vs.Z¾ plots were completely dominated by large, lower frequency impedances. The sensitivity of the impedance response to polishing suggests that resistive surface layers are present, even after polishing; however, there is a large impedance, attributable to grain boundaries from its capacitance value of ca. 0.2 nF, which dominates the overall sample impedance. The conclusion from these observations is that resistive grain boundaries and surface layers appear on heating samples in less oxidising atmospheres. In order to assess possible variation in bulk resistance as a Fig. 3 Rietveld refinement: (a) observed pattern and (b) diVerence consequence of diVerent heat treatments, data for the frebetween observed and calculated patterns. quency maxima in the M spectra are compared in Fig. 7. A major advantage of comparing fmax data for a range of samples is that fmax is independent of sample geometry: R and C are both influenced by geometry, but inversely to each other and therefore, the geometrical terms cancel in fmax.26 In addition, C values vary little from sample to sample for the bulk response of similar-sized pellets. Hence fmax, given by 2pfmaxRC=1, provides a direct measure of R-1 and hence of s.The data for four diVerent heat treatments fall on a single straight line with activation energy 0.31(2) eV. This suggests that the bulk conduction mechanism, the conducting species and the magnitude of the conductivity are essentially independent of oxygen content for this range of compositions.Given the complete absence of any electrode polarisation eVects in the low frequency region of the impedance response and the low value of the activation energy, it is concluded that the conduction species are electrons rather than ions, and therefore are associated with the mixed valency of Mn.Conductivity data were extracted from the fmax data. These are plotted in Arrhenius format in Fig. 8, together with total pellet conductivity values, obtained from the low frequency intercepts of the Z vs. Z¾ plots on the Z¾ axis. The pellet Fig. 4 Magnetic data for Ca2MnNbO5.87: (a) field dependence of magnetization and (b) Curie–Weiss plot. conductivity values show a large variation, unlike the bulk 2518 J.Mater. Chem., 1998, 8(11), 2515–2520Fig. 6 (a) Complex impedance plane and (b) combined M, Z spectroscopic plot at 252 K for Ca2MnNbO6, annealed in O2 at 40 atm, 600 °C; (c) complex impedance plane and (d) combined M, Z spectroscopic plot at room temperature for Ca2MnNbO5.87, quenched in air from 1400 °C; $ polished pellet, #, unpolished pellet.values which are almost independent of sample history. For pellets treated in O2-rich atmospheres, the grain boundary resistance was relatively small and the total and bulk conductivities are almost coincident, Fig. 8. For the samples treated in air or N2, however, very large grain boundary resistances are evident, Fig. 6(c), (d), and these dominate completely the pellet conductivities.In such O2-poor atmospheres, the samples lose O2 from their surfaces by the reaction 2O2-�O2+4e Since the surface/grain boundary conductivity decreases as a consequence, it is concluded that the conduction mechanism, in these regions at least, is p-type. The similarity in activation energy for the bulk and total conductivities in unreduced samples, Fig. 8 (slow cool from 1100 °C, 1 atm O2 and from 600 °C, 40 atm O2), indicates that a similar conduction mechanism occurs in both bulk and grain boundary regions and Fig. 7 Arrhenius plots of bulk (filled symbols) and total (open therefore that the bulk conductivity is also p-type. The insensi- symbols) conductivities of Ca2MnNbOc , prepared by diVerent heat tivity of the bulk conductivity to sample history indicates that treatments; &, %: slow cool from 1100 °C in nitrogen; ,, (: the number of p-type carriers is high, associated with the high quenched from 1400 °C in air; $, #: slow cool from 1100 °C in 1 atm oxygen; +, 6: slow cool from 600 °C in 40 atm oxygen.concentration of Mn3+ ions. Conclusions Ca2MnNbO6 has the orthorhombic GdFeO3 structure. This is not surprising given the existence of similar structures in Ca2FeNbO6,27 Sr2CrTaO628 and Pb2ScNbO6.29 The similar sizes of Mn3+ and Nb5+ permit disorder of these elements over the octahedral B sites.Although Mn3+ is Jahn–Teller active, there is no evidence of distortions in the (Mn,Nb)O6 octahedra, presumably because there is insuYcient Mn present to exert a cooperative distortion. The possible existence of oxygen vacancies in the GdFeO3 structure is often speculated upon but is not well documented.Here, we find oxygen contents as low as 5.87 in samples heated Fig. 8 Arrhenius plots of bulk (filled symbols) and total (open at high temperatures and in atmospheres of low oxygen partial symbols) conductivities of Ca2MnNbOc, prepared by diVerent heat pressure. We believe that the oxygen content may be reduced treatments: &, %: slow cool from 1100 °C in nitrogen; ,, (: further on H2 reduction, although in fully reduced (to Mn2+ quenched from 1400 °C in air; $, #: slow cool from 1100 °C in 1 atm oxygen; +, 6: slow cool from 600 °C in 40 atm oxygen.at c=5.50) samples, the products are multiphase and therefore, J. Mater. Chem., 1998, 8(11), 2515–2520 2519Chem., Proceedings of the Second European Conference, Veldhoven, the oxygen content can not be reduced as low as 5.50; further ed.J. Schoonman, Elsevier, Amsterdam, 1982, p. 247. work on this is in progress. 4 S. Shin, Y. Hatakeyama, K. Ogawa and. Shimomura, Mater. The defect structure in the oxygen-deficient samples is not Res. Bull., 1979, 14, 133. known, but it must involve reduction in coordination of some 5 A.Atkinson, Adv. Ceram., 1987, 23, 3. of the (Nb,Mn)O6 octahedra. NbO6 octahedra are not easily 6 F. A. Kroger, The chemistry of the imperfect crystal, North- Holland, Amsterdam, 1964. reduced and oxygen loss may, therefore, be confined to the 7 G. G. Libowitz, Prog. Solid State Chem., 2, Pergamon Press, MnO6 octahedra, as occurs with CaMnO3 which exhibits a Oxford, 1965.coordination number of 5 for Mn on oxygen loss. Interestingly, 8 C. N. R. Rao, J. Gopalakrishnan and K. Vidyasagar, Indian it is diYcult to fully oxygenate Ca2MnNbOc and an oxygen J. Chem. A, 1984, 23, 265. content of 6.00 is achieved only under high oxygen pressures; 9 B. C. Tofield and W. R. Scott, J. Solid State Chem., 1974, 10, 183.this may reflect a reluctance of the Jahn–Teller active Mn3+ 10 C. N. R. Rao, A. K. Cheetham and R. Mahesh, Chem. Mater., 1996, 8, 2421. ion to occupy an octahedral site that is constrained to be 11 R. Mahesh, K. R. Kannon and C. N. R. Rao, J. Solid State undistorted by the adjacent NbO6 octahedra. Chem., 1995, 114, 294. Ca2MnNbOc provides another example of spin glass-like 12 R.Portier, A. Carpy, M. Fayard and J. Galy, Phys. Status Solidi behaviour, similar to that seen in A2FeXO6: A=Ca, Sr, Ba; A, 1975, 30, 683. X=Nb, Ti, Sb.21–25 It is a p-type hopping semiconductor with 13 M. Hervieu, F. Studer and B. Raveau, J. Solid State Chem., 1977, an activation energy, 0.31 eV, that is largely independent of 22, 273. 14 K. Scheunemann and H. K. Muller-Buschbaum, J.Inorg. Nucl. oxygen content and appears to be associated with the Mn3+ Chem., 1975, 37, 1875; 2261. ions that are present in high concentration. In reduced samples, 15 J. Berggren, Acta Chem. Scand., 1977, 25, 3616. there appears to be preferential oxygen loss from sample 16 J. C. Grenier, M. Pouchard and P. Hagenmuller, Structure surfaces and grain boundaries, as evidenced by a dramatic Bonding, 1981, 47, 1.decrease in conductivity in these regions. This is probably 17 K. R. Poeppelmeier, M. E. Leonowitz and J. M. Longo, J. Solid because the concentration of Mn3+ ions is greatly reduced, in State Chem., 1982, 44, 89. 18 K. R. Poeppelmeier, M. E. Leonowitz, J. C. Scalon, J. M. Longo favour of Mn2+ ions, and hence the p-type carrier concenand W. B. Yelon, J.Solid State Chem., 1982, 45, 71. tration is small. The bulk of the samples is largely uninfluenced 19 J. A. Chavez-Carvayar, T. C. Gibb and A. R. West, J. Mater. by this eVect (although the conductivity of the sample heated Chem., 1996, 6, 1957. in N2 is reduced, which may signal the outset of a significant 20 S. Geller and E. A.Wood, Acta Crystallogr., 1956, 9, 563. reduction in p-type carrier concentration in the bulk for this 21 R. Rodrý�guez, A. Ferna�ndez, A. Isalgue�, J. Rodrý�guez, A. Labarta, sample). Whilst it is widely accepted that oxidation/reduction J. Tejada and X. Obradors, J. Solid State Phys., 1985, 18, L401. 22 T. C. Gibb, P. D. Battle, S. K. Bollen and R. J. Whitehead, processes generally commence at surfaces, the present material J. Mater. Chem., 1992, 2, 111. provides a particularly clear example of the oxygen concen- 23 T. C. Gibb, J. Mater. Chem., 1993, 3, 441. tration gradients that must often occur; this eVect is readily 24 T. C. Gibb, A. J. Herod and N. Peng, J. Mater. Chem., 1995, 5, 91. apparent here because of the sensitivity of the electrical 25 P. D. Battle, T. C. Gibb, A. J. Herod, S.-H. Kim and P. H. Munns, properties to the degree of reduction. J. Mater. Chem., 1995, 5, 865. 26 J. T. S. Irvine, D. C. Sinclair and A. R. West, Adv. Mater., 1990, 2, 132. References 27 J. A. Chavez-Carvayar, PhD Thesis, Aberdeen, 1995. 28 G. Patrat, M. Brunel and F. DeBergevin, J. Phys. Chem. Solids, 1 B. A. Boukamp, M. P. van Dijk, K. J. de Vries and 1976, 37, 285. A. J. Burggraaf, Adv. Ceram., 1987, 23, 447. 29 F. Galasso and W. Darby, Inorg. Chem., 1965, 4, 71. 2 G. Bronoel, J. C. Grenier and J. Reby, Electrochim. Acta, 1980, 25, 1015. 3 J. C. Grenier, M. Pouchard and P. Hagenmuller, Solid State Paper 8/04865G 2520 J. Mater. Chem., 1998, 8(11), 2515&nd

ISSN:0959-9428    

DOI:10.1039/a804865g

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Preparation of a TiO2 based multiple layer thin film photocatalyst Atsuo Yasumori,* Kenichi Ishizu, Shigeo Hayashi and Kiyoshi Okada Department of Inorganic Materials, Tokyo Institute of Technology, Tokyo 152-8552, Japan. E-mail: ayasumor@ceram.titech.ac.jp Received 30th April 1998, Accepted 20th August 1998 ATiO2 thin filmsemiconductor is expected to show high photocatalytic activity because of the short diVusion distances of photoexcited electrons and holes to the surface as well as the short spatial separation of reducing and oxidizing sites.Themultiple layer photocatalyst, reported herein, which consists of a TiO2 thin film, platinumelectrode and porous alumina substrate, was prepared by spin-coating a TiO2 sol and sputter-coating of a platinum electrode on a porous substrate.After adequate heat treatment, thismulti-layered photocatalyst showed a high eYciency forH2 generation from ethanol aqueous solution under UV illumination as compared with platinized TiO2 fine particles. Research into photocatalytic reactions mediated by metal oxide semiconductors received a further boost after the work of Fujishima and Honda on the photoelectrolysis of H2O using TiO2 electrodes.1 Since that study was reported, a considerable amount of work involving metal oxide semiconductor photocatalysts has been done.To date, no substance superior to TiO2, however, has been found. TiO2 is especially attractive because of its high photocatalytic activity and chemical stability in aqueous solution under light irradiation.Recently, two branches involving the study of the TiO2 photocatalyst have emerged especially for the purposes of solving environmental issues. One branch involves the use of highly dispersed fine particles in a porous material,2–4 and the other is concerned with TiO2 thin films.5–10 Most TiO2 thin films are prepared by coating a substrate with a TiO2 sol, thereby producing a thin film consisting of TiO2 particles.Generally, this methodology often results in a porous TiO2 film. The fine particles that make up the film have several advantages, for example, large specific surface areas, short diVusion distance to the surface for the photo-generated electrons and holes, and quantum size eVects for particles of <10 nm in diameter.11However, some disadvantages of fine particles include Fig. 1 Schematic illustration of a TiO2 based multiple layer thin film problems associated with the close proximity of the redox sites photocatalyst. due to small particle sizes, i.e., the progress of reverse reactions between the products (oxidants and reductants) on the surfaces of particles is hard to inhibit. Experimental Nonetheless, an attractive photocatalytic system for spatially separating the respective oxidation and reduction sites is an The TiO2 thin film was prepared by a sol–gel method.The unsupported thin film system consisting of a semiconductor flowchart for the preparation of the TiO2 sol precursor is and an appropriate metal electrode. This system is also shown in Fig. 2. This procedure was adapted from the method expected to minimize diVusion distances to the surface for the reported by Sakka and Kamiya.16 Reagent grade titanium photogenerated charge carriers.The fabrication of such a thin tetraisopropoxide (3.98 g, TTIP, Wako Pure Chemicals) was film system on the nanometer or mesoscopic scale is obviously stabilized with 0.33 ml of acetylacetone (AcAc, Wako) and very challenging.There are some reports on TiO2 thin film 130 ml of isopropanol (PriOH, Wako). Stabilization of TTIP photocatalysts supported on metal electrodes12–14 or on porous ensures that during hydrolysis only isopropoxyl groups are substrate.15 However, it is not clear whether these photocata- removed, and consequently this prevents the rapid growth of lytic systems were intended to spatially separate photo- the TiO2 particles.Stabilized TTIP was hydrolyzed with 0.98 g generated electrons and holes or not. of acetic acid (Wako), 0.76 ml of distilled water and 10 ml of In this work, a photocatalyst comprising a TiO2 thin film PriOH under an argon atmosphere resulting in a transparent and platinum electrode was fabricated on a silica gel coated yellowish TiO2 sol.The molar ratio of TTIP5AcAc5H2O was porous alumina substrate as schematically illustrated in Fig. 1. 150.2353. The particle sizes in the TiO2 sol were <10 nm in In this multiple layer system, UV light irradiation is expected diameter as observed by transmission electron microscopy. to induce the following redox reaction; oxidation on the A flowchart for the fabrication of the thin film photocatalyst surface of the TiO2 thin film and reduction on the metal is shown in Fig. 3. An alumina membrane filter (Whatman, electrode. For the reduction reaction, the oxidant is expected Anodisk 25) was used as the porous substrate. This filter is to migrate to the platinum electrode surface via the porous ca. 60 mm in thickness and has an asymmetric pore structure with pores of diameter of 0.02 mm and 0.2 mm.The surface of substrate and porous silica gel layer. J. Mater. Chem., 1998, 8(11), 2521–2524 2521the sample was heat-treated at a selected temperature in the range 300 to 700 °C. The surface and the cross-section of the thin film were observed by scanning electron microscopy (Hitachi S-2050 SEM and JEOL JCM-890S FE-SEM).In order to evaluate the thickness of the TiO2 layer, the TiO2 layer was distinguished from the platinum and silica layers by both secondary electron image (SEI) and back scattered electron image (BEI) observations of the sample cross section. Crystalline phases of the samples were identified by X-ray diVractometry (Rigaku Geigerflex System, Cu-Ka radiation). The photocatalytic activity of the sample was evaluated by measuring the photogeneration rate of hydrogen from an aqueous solution of ethanol.The simplified model of the photoredox system in an aqueous solution of ethanol mediated by the multilayered photocatalyst is also shown in Fig. 1. Ethanol is oxidized to acetaldehyde, acetic acid or CO2.17 Protons are reduced to hydrogen molecules on the platinum electrode.The photocatalyst was immersed in 500 ml of a 20 vol% aqueous ethanol solution under an Ar atmosphere. Illumination of the photocatalyst (TiO2 film side) was carried out using a 300 W Xe lamp (ILC Tech., LX300F). A water Fig. 2 Flowchart of the preparation of the TiO2 sol. cell was used as an infrared filter. The photogenerated hydrogen was detected by gas chromatography (Shimazu GC-6A with a thermal conductivity (TCD) cell and a 2 m long molecular sieve 5A column).Argon was used as the carrier gas. A platinized commercial TiO2 powder (Nihon Aerosil P- 25, anatase, particle diameter 10–50 nm) was used as a reference sample. Platinization of the powder was carried out by a photodeposition method in an H2PtCl6 aqueous ethanol solution.2 Results and discussion The XRD patterns of as-prepared samples showed only a halo indicating that the TiO2 layer was amorphous.Upon heattreatment at >300 °C for 4 h, a diVraction peak corresponding to the (101) plane of anatase appeared on the pattern and the crystalline phase of all heat-treated samples was predominantly anatase. The anatase crystallites were highly oriented along the (101) plane.The crystallite size was calculated by Sherrer’s equation assuming that there was no distortion in the crystal. As the temperature of heat treatment was increased, there was a parallel gradual increase in the peak intensities and crystallite size of anatase from ca. 10 nm (at 330 °C) to ca. 15 nm (at 500 °C). SEM images revealed that the surface (TiO2 film side) of the sample fired at 450 °C was devoid of any surface texture, i.e., it was very smooth and flat.Images of the cross section of this sample are shown in Fig. 4. The two SEI images show the pore structures of the alumina substrate and the overlayers. In the BEI image, the bright part was identified as the platinum layer. Clearly these images exhibit the expected structure of the multi-layered thin film system.We estimated the thickness Fig. 3 Flowchart for the fabrication of the TiO2 thin film of the TiO2 layer in all samples from the SEI+BEI image. photocatalyst. The change of TiO2 film thickness with the number of coating applications is shown in Fig. 5. It is apparent that film thickness increased in proportion to the number of coating applications.the porous substrate was coated with a silica sol (Nissan Chemical, Snowtex N) in order to make the surface smooth Each coating application produced a film about 10 nm thick. This film thickness corresponds to the crystallite size as thereby preventing the Pt and TiO2 fine particles from going through the substrate. This silica sol contains 20–21 wt.% of estimated from the XRD measurement.This shows that the TiO2 sol particles maintained their dispersive condition, with silica and the colloidal particles are 10–20 nm in diameter. The sol was diluted to 10 vol% with reagent grade ethanol the subsequent production of a dense and flat TiO2 thin film. The photocatalytic activity of the photocatalyst was and the coating was made just once a low speed spin coater (Thomas TM-701, ca. 300 rpm). After drying at 60 °C and evaluated by monitoring the rate of hydrogen evolution from an aqueous ethanol solution as already outlined in the calcining at 300 °C, platinum was sputter-coated on the silica gel layer. The platinum surface was subsequently coated with Experimental section. The hydrogen generation rate per unit surface area for the eight-coated sample as a function of firing the TiO2 sol, dried at 60 °C for 10 min and calcined at 300 °C for 10 min.This coating procedure for the TiO2 sol was temperature is shown in Fig. 6. The rate increased fairly steeply up to 400 °C and decreased gradually thereafter. The increase repeated until the desired film thickness was obtained. Finally, 2522 J. Mater. Chem., 1998, 8(11), 2521–2524activity above 450 °C are the eVects of the structural changes of the silica gel layer and TiO2 layer upon polycondensation or sintering among SiO2 or TiO2 fine particles.In the former case, the gel obtained from Snowtex N is known to sinter above 800 °C from its company’s catalogue. Further, in our previous photocatalytic work on the silica gel containing anatase fine articles, the photocatalytic activity also decreased above 500 °C, though the crystalline phase was unchanged at this temperature and the porous structure of the material was retained up to 800 °C.2 Considering the above information and our observations the porous structure of silica gel layer was also unchanged up to 800 °C, therefore, the silica gel layer is not responsible for the decrease of photocatalytic properties.A structural change of the TiO2 layer would result in grain growth and/or decrease of the concentration of surface OH groups and lattice defects such as Ti3+ and might aVect the photocatalytic activity.18,19 In order to investigate the change of fine structure of the thin film, precise observations of the surface and the cross-section of the thin film are required.However, at present, it is diYcult to observe such precise fine structure of the film by use of our SEM system. As mentioned already, there was no evidence of grain growth from the results of XRD measurement. In order to explain the eVect of firing Fig. 4 SEM images of the cross-section of the TiO2 thin film temperature on catalytic activity, further investigations on photocatalyst.properties of the thin films such as the number of OH groups on the TiO2 surface and the concentration and mobility of charge carriers, are therefore necessary. Fig. 7 shows the changes in hydrogen generation rate with film thickness for a sample heat-treated at 450 °C for 4 h. The hydrogen generation rate is presented as the rate per unit surface area as well as per unit mass of the TiO2 sample.The sample mass was evaluated assuming the density of an anatase crystal (3.54 g cm-3) based on the assumption that the film was dense. When the thickness of the film was less than ca. 60 nm, the rate was very low. This may be due to the formation of island textures of TiO2 on the platinum electrode in the early stages of spin coating.It is plausible that this texture induces the reverse reaction between photogenerated active oxidant and reductant on platinum, and consequently, hampers hydrogen evolution. Further coating applications pro- Fig. 5 Change of the TiO2 film thickness with the number of coating duced a uniform TiO2 film all over the platinum surface applications. resulting in a homogeneous spatial separation of reaction sites.As the film thickness was increased to >60 nm, the rate increased proportionally with the film thickness, while the rate per unit mass was almost constant and was also independent of film thickness. The photogeneration rate per unit mass was about eight times that of the reference sample of platinized TiO2 fine particles. These results strongly suggest that a similar number of photogenerated electrons per unit TiO2 mass diVuse to the surface of the platinum electrode regardless of the depth of the film.A potential gradient at the bottom level of the Fig. 6 Change in H2 generation rate with firing temperature. in H2 generation rate up to 400 °C is considered to be due to the crystallization of amorphous TiO2 into the photoactive anatase phase.However, it is not clear as to why the rate decreased above 450 °C considering that the crystalline phase of the thin film was not photoinactive rutile but was still anatase. Also, a sharp growth in anatase crystallite size was not observed. Changes in crystallite size aVect the band gap energy and the charge potential thereby influencing the overall photocatalytic properties.Fig. 7 Changes in H2 generation rate with thickness of the TiO2 thin film. Other possible causes of the decrease of photocatalytic J. Mater. Chem., 1998, 8(11), 2521–2524 2523conduction band and at the top level of the valence band is problems on the reduction site in this material are not so important as oxidative processes, such as the removal of probably formed across the TiO2 layer macroscopically, organic substances from polluted water and NOx from the because the front and reverse of the TiO2 (n-type semiconatmosphere by photo-oxidation, would be the main appli- ductor) layer are in contact with the solution and platinum cations of this catalyst. electrode, respectively.Therefore, photoexcited electrons Although poisoning or fouling of the surface by dense diVuse to the platinum electrode and holes diVuse to the pollutants is not completely avoidable in this material, our surface of TiO2 thin film along the potential gradients.Since fabricated system has potential for application to the removal the film was dense and consequently had few recombination of pollutants from the environment in comparison to finely sites such as surface defects, the separation of charge carriers divided particulate photocatalysts in porous media.occurred eYciently. Furthermore, the multiple layered structure on the porous substrate prohibited the reverse reaction between photoproduced oxidant and reductant thus achieving References a high photocatalytic activity as evidenced by the results 1 A.Fujishima and K. Honda, Bull. Chem. Soc. Jpn., 1971, 44, 1148. obtained in this study. 2 A. Yasumori, K. Yamazaki, S. Shibata and M. Yamane, J. Ceram. Soc. Jpn., 1994, 102, 702. 3 Y. Zhang, J. C. Crittenden, D. W. Hand and D. L. Perram, Conclusions Environ. Sci. Technol., 1994, 28, 435. 4 J. Harrmann and J. Mansot, J. Catal., 1990, 121, 340. A multilayered TiO2 thin film photocatalyst was prepared by 5 Y-M.Gao, H-S. Shen, K. Dwight and A.Wold, Mater. Res. Bull., sol–gel and sputtering methods. Compared with the reference 1992, 27, 1023. platinized TiO2 powder sample, the heat-treated photocatalyst 6 M-C. Lu, G-D. Roam, J-N. Chen and C. P. Huang, J. Photochem. Photobiol., 1993, 76, 103. showed a high eYciency for H2 generation from an aqueous 7 I. Sopyan, S.Murasawa, K. Hashimoto and A. Fujishima, Chem. solution of ethanol. The high catalytic activity of this thin film Lett., 1994, 723. system was achieved by a judicious arrangement of the TiO2 8 M. A. Aguado, M. A. Anderson and C. G. Hill, Jr., J. Mol. Catal., film and the platinum electrode on a porous substrate. This 1994, 89, 165. mesoscopic scale structure resulted in the reduction of the 9 A.Fernandez, G. Lassaletta, V. M. Jimenez, A. Justo, diVusion distances of charge carriers and the separation of A. R. Gonzalez-Elipe, J. M. Herrmann, H. Tahiri and Y. Ait- Ichou, Appl. Catal. B: Environ., 1995, 7, 49. redox reaction sites. 10 Y. Paz, Z. Luo, L. Rabenberg and A. Heller, J. Mater. Res., 1995, Most photocatalytic systems for environmental use will be 10, 2842.fabricated with TiO2 fine particles and porous support mate- 11 M. Anpo, T. Shima, S. Kodama and Y. Kubokawa, J. Phys. rials in commercial or industrial applications. In these systems, Chem., 1987, 91, 4305. micropores in the system act as adsorbents of pollutants; 12 D. Kim and M. Anderson, Environ. Sci. Technol., 1994, 28, 479. 13 S. Sato, H. Koshiba, H. Minakami, N.Kakuta and A. Ueno, however, poisoning or fouling easily occurs by the adsorption Catal. Lett., 1994, 26, 141. of not only the pollutant but also some components in the 14 C. Natarajan and G. Nogami, J. Electrochem. Soc., 1996, 143, reaction system. 1547. Because our fabricated material has a smooth surface TiO2 15 M. Takahashi, K. Mita and H. Toyuki, J. Mater. Sci., 1989, 24, layer (i.e. oxidation sites), the likelihood of fouling is relatively 243. low on this side. On the other hand, the platinum surface (i.e. 16 S. Sakka, K. Kamiya, K. Makita and Y. Yamamoto, J. Non- Cryst. Solids, 1984, 63, 223. reduction site) is located on the bottom of porous substrate. 17 K. Kato, Bull. Chem. Soc. Jpn., 1992, 65, 34. However, the thickness of the substrate is only ca. 60 mm, 18 Y. Oosawa and M. Gratzel, J. Chem. Soc., Faraday Trans. 1, 1988, therefore, the adsorbates are readily removed by washing or 84, 197. burning away. Further, since the redox reaction sites are 19 K. Kobayakawa, Y. Nakazawa, M. Ikeda, Y. Sato and spatially separated in our material, we need only to use one A. Fujishima, Phys. Chem., 1990, 94, 1439. reaction surface of the fabricated material for an adequate reaction system and good arrangement of apparatus. Any Paper 8/03265C 2524 J. Mater. Chem., 1998, 8(11), 2521–2524

ISSN:0959-9428    

DOI:10.1039/a803265c

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials A study of Mn–Ti oxide powders and their behaviour in propane oxidation catalysis Fabio Milella,*a Jose� Manuel Gallardo-Amores,a,b Marco Baldic and Guido Buscaa* aIstituto di Chimica, Facolta` di Ingegneria, Universita`, P.le J.F. Kennedy, I-16129, Genova, Italy bDepartamento de Quý�mica Inorga�nica, Universidad Complutense, Ciudad Universitaria, E-28040-Madrid, Spain cDipartimento di Ingegneria Idraulica e Ambientale, Universita`, via Ferrata 1, I-27100 Pavia, Italy Received 28th May 1998, Accepted 23rd July 1998 Mn–Ti mixed oxides with composition Ti1-xMnxOy (x=0, 0.1, 0.2, 0.5, 0.8, 0.9 and 1) have been prepared.A Mn–TiO2 monolayer type sample has also been prepared by impregnation, for comparison. Manganese is found to speed up the anatase-to-rutile phase transition, more clearly in the impregnated sample, while titanium tends to slightly hinder the thermodynamically reversible hausmannite-to-bixbyite phase transition upon cooling.The catalytic activity of all samples in propene oxidation decreases by increasing the Ti content. Conversely, the catalytic activity in propane oxidation shows a maximum at intermediate composition.tion with the required amount of manganese acetate to an 1 Introduction aqueous suspension of TiO2 (Degussa). Then, it was stirred Manganese based mixed oxides, such as perovskite-type at 373 K until total removal of water, and treated thermally manganites,1 Mn-containing b-aluminas,2 Mn-containing spi- as above. nels,3 Mn oxides supported on alumina as powders4 and on Nitrates and residual organic compounds were decomposed, cordierite monoliths5 and the pure oxides MnO2,6–8 Mn2O39,10 in air, in an electronically controlled furnace at 723 K for 4 h.and Mn3O411 have been proposed as cheap, environmentally The heating and cooling rate before and upon calcination was friendly and active catalysts for volatile organic compound 40 K min-1.(VOC) and methane total oxidation processes. They are, XRD spectra were recorded on a Philips PW 1710 however, less active than the more expensive and environmen- diVractometer (Cu-Ka radiation, Ni filter; 45 kV, 35 mA) and tally demanding catalysts based on noble metals.5 To increase cell parameters calculated using dedicated least square the activity of oxidation catalysts, supporting the active phase software.FTIR spectra were recorded using a Nicolet Magna on oxide carriers or mixing it with other oxides is sometimes 750 Fourier Transform instrument. For the region useful. In particular, TiO2-anatase is reported to activate V2O5 4000–350 cm-1 a KBr beam splitter has been used with a catalysts for several oxidation reactions12,13 and vanadia– DTGS detector.For the FIR region (600–50 cm-1) a ‘solid titania based catalysts are used industrially for alkyl aromatic substrate’ beam splitter and a DTGS polyethylene detector oxidations14,15 as well as for reducing NOX with ammonia in were used. KBr pressed disks (IR region) or polyethylene the selective catalytic reduction (SCR) process.16,17 Titania pressed disks and samples deposed on Si disks (FIR region) was also reported to activate molybdena and tungsta based were used.catalysts for oxidation18 and MoS2 and WS2 sulfide catalysts FT-Raman spectra were recorded using a Bruker RFS100 for hydro-treating.19 The nature of such activating eVects is Instrument, with an Nd-YAG laser (1064 nm), using 30 mW still under debate, being either due to the ability of anatase to laser power, 2000 scans and 4 cm-1 resolution.DiVuse reflecdisperse appropriately the active phases or to optimise their tance spectra in the range 2500–200 nm were obtained with a acid–base properties,20 or due to electronic eVects.21,22 Jasco V-570 spectrophotometer at room temperature using a Supporting or mixing Mn oxides with titania seemed to be polymer as reference.BET surface areas were measured with a reasonable option to try to improve Mn oxides for oxidation a conventional volumetric instrument by nitrogen adsorption catalysis and to further study the eVect of oxide supports with respect to supported oxide catalysis. Mn–Al oxides have been deeply investigated as SCR catalysts.23,24 Mn–Ti oxides are industrially produced as inorganic pigments25 and are reported to behave as fairly active and selective catalysts in the oxidation of NH3 to N2.26 In the present paper we will summarise our results on the preparation, characterisation and testing of Mn–Ti complex oxides. 2 Experimental The preparation of the mixed oxide samples, TixMn1-xOy (x=0, 0.1, 0.2, 0.5, 0.8, 0.9, 1), was carried out by mixing carefully Mn(CH3COO)3·2H2O (Acros, 98%) and Ti[OCH(CH3)2]4 (Aldrich, 97%) hydrolysing with water, then drying the gel at 393 K for several hours.The subscript x in the notation corresponds to the atomic fraction of Ti in the whole metal content. Another sample was synthesised by a Fig. 1 XRD patterns of the powders after calcination at 773 K. conventional impregnation method, adding an aqueous solu- J.Mater. Chem., 1998, 8(11), 2525–2531 2525at liquid nitrogen temperature. DTA–TG experiments were performed in air, with a Setaram TGA 92-12 apparatus, from room temperature to 1273 K, with heating and cooling rates of 10 K min-1. Catalytic tests were carried out at atmospheric pressure in a continuous flow tubular glass reactor. Variable amounts of catalysts calculated to have the same exposed total surface area (1.83 m2) were loaded in the form of fine powder (60–70 mesh) mechanically mixed with a predetermined amount of inert, low surface area, material (quartz) to avoid preferential gas flow paths and hot spots in the catalytic bed.The total gas flow was 330 ml min-1 and the feed composition was ca. 1.5% of hydrocarbon in oxygen-containing helium.The hydrocarbon/oxygen molar ratio in the feed was 156. The reactants and the reaction products were analysed using two on-line gas chromatographs (HP 5890), working in diVerent analysis conditions in order to give a better resolution of inorganic and organic species. 3 Catalyst characterisation 3.1 Structural characterisation of the mixed oxide catalysts after calcination at 773 K Fig. 1 shows the XRD powder patterns of the mixed oxide samples after calcination at 773 K. The observed crystal phases with the measured unit cell parameters are summarised in Table 1. The Mn oxide sample is constituted by the random tetragonal spinel phase a-Mn3O4 (hausmannite). This phase alone is also found in the sample Ti0.1Mn0.9 and is present in the patterns of all samples up to Ti0.8Mn0.2 where it is still Fig. 2 FTIR/FTFIR skeletal spectra of the powders after calcination detected in traces. From Ti0.2Mn0.8 the TiO2-anatase phase is at 773 K. (a) TiO2, (b) Ti0.9Mn0.1, (c) Ti0.8Mn0.2, (d) Ti0.5Mn0.5, also observed, and is the only phase detectable in the sample (e) Ti0.2Mn0.8, (f ) Ti0.1Mn0.9, (g)Mn3O4, (h) 6%Mn-TiO2 (Degussa), Ti0.9Mn0.1.Pure titania is constituted by anatase with small (i) TiO2 (Degussa). amounts of brookite. Traces of rutile are detected for Ti0.5Mn0.5 only. The cell volume of the anatase phase appears to increase with dissolution of Mn in the samples Ti0.9Mn0.1 typical of spinels20,29 and reported explicitly for hausmannand Ti0.8Mn0.2. Conversely, the volume of the hausmannite ite,20,30 for pure Mn oxide [Fig. 3(a)] and for samples up to phase appears to decrease in the samples Ti0.2Mn0.8 and Ti0.8Mn0.2 [Fig. 3(b)–(e)]. In the Raman spectrum of Ti0.5Mn0.5 suggesting dissolution of Ti in Mn3O4. Ti0.2Mn0.8 the main peak of anatase21 at 142 cm-1 starts to The skeletal IR and FIR spectra clearly show the typical be present. For the sample Ti0.9Mn0.1 only the peaks of anatase absorptions of the hausmannite spinel phase for the pure Mn are found at 638, 512, 395, 322, 236 and 142 cm-1, while for oxide [Fig. 2(g)] with main bands at 611, 524, 421, 245, 165 pure TiO2 traces of brookite (450 and 359 cm-1) are detectable and 124 cm-1, in good agreement with the results reported by within the pattern of anatase.21 Thus, both vibrational spectro- Lutz et al.27 Theabsorptions are also observed in the scopic techniques fully confirm the data arising from XRD spectra of the Ti-containing samples up to Ti0.5Mn0.5 summarised in Table 1.[Fig. 2(d)–(f )], although traces of these absorptions can also The specific surface areas, reported in Table 1, show an be found for Ti0.8Mn0.2. In parallel, the broad absorptions of almost monotonic decrease upon increasing Mn content. TiO2 (anatase)28 are clearly found already for Ti0.2Mn0.8 [Fig. 2(a)–(c)]. In the case of Ti the complexity in the range 3.2 Study of the thermal stability of mixed oxide catalysts 600–400 cm-1 is attributed to the presence of some traces of brookite.21 DTA and XRD studies. The DTA curves for mixed oxide catalysts are shown in Fig. 4. The observed crystal phases with Similarly, the Raman spectra show a peak at 655 cm-1, Table 1 XRD data for TixMn1-xOy samples at 773 K Cell parameters/A° Sample Tcalc./K XRD phase(s) a c Volume/A° 3 SBET/m2 g-1 Ti 773 Anatase 3.777(2) 9.460(6) 135.0 94 (Brookite) Ti0.9Mn0.1 773 Anatase 3.790(1) 9.446(5) 135.6 83 Ti0.8Mn0.2 773 Anatase 3.790(1) 9.493(4) 136.4 53 (Hausmannite) Ti0.5Mn0.5 773 Anatase 3.809(6) 9.434(27) 136.9 42 Hausmannite 5.763(1) 9.429(5) 313.0 (Rutile) Ti0.2Mn0.8 773 Hausmannite 5.761(0) 9.427(3) 313.0 25 (Anatase) Ti0.1Mn0.9 773 Hausmannite 5.768(0) 9.446(1) 314.3 18 Mn 773 Hausmannite 5.759(1) 9.443(5) 314.3 11 2526 J.Mater. Chem., 1998, 8(11), 2525–2531Table 2 XRD data for TixMn1-xOy samples at 1273 K. Cell parameters/A° Tcalc./ XRD Volume/ Sample K phase(s) a c A° 3 Ti 1273 Rutile 4.592(0) 2.959(0) 62.4 Ti0.9Mn0.1 1273 Rutile 4.589(0) 2.957(0) 62.3 Pyrophanite 5.139(0) 14.277(5) 326.5 Ti0.8Mn0.2 1273 Rutile 4.585(0) 2.956(0) 62.1 Pyrophanite 5.125(1) 14.270(5) 324.6 (Bixbyite) Ti0.5Mn0.5 1273 Pyrophanite 5.136(1) 14.279(3) 326.2 Rutile 4.596(5) 2.943(9) 62.1 (Bixbyite) Ti0.2Mn0.8 1273 Bixbyite 9.425(5) 9.425(5) 837.2 Pyrophanite 5.118(2) 14.229(24) 322.8 (Rutile) Ti0.1Mn0.9 1273 Bixbyite 9.425(1) 9.425(1) 837.2 (Rutile) Mn3O4 1273 Hausmannite 5.761(0) 9.450(2) 313.6 Mn2O3 a Bixbyite 9.430(2) 9.430(2) 838.6 aReference sample.the measured unit cell parameters after DTA are summarised in Table 2. At the end of the DTA cycle, the Mn oxide is still in the form of Mn3O4 (hausmannite), although with a large decrease in the crystal size.By contrast the sample Ti0.1Mn0.9 only shows features that must be attributed to a-Mn2O3 (bixbyite) with small traces of TiO2-rutile. A further phase, the mixed oxide MnTiO3 (pyrophanite) present at a trace level in Ti0.2Mn0.8, is the predominant phase in Ti0.5Mn0.5 and is a minor phase for the sample Ti0.8Mn0.2. In the latter sample rutile is the major phase.Unit cell parameters of rutile slightly decrease by increasing Mn content in the mixed phase powder, in agreement with the behaviour for MnO2–TiO2 solid solu- Fig. 3 FT-Raman spectra of the powders after calcination at 773 K. (a) Mn3O4, (b) Mn0.9Ti0.1, (c) Mn0.8Ti0.2, (d) Mn0.5Ti0.5, tions.31 By contrast, bixbyite parameters decrease with respect (e) Mn0.2Ti0.8, (f )Mn0.1Ti0.9, (g) TiO2 (its intensity value is 10 times to a pure reference sample, possibly indicating that some greater than for the others).titanium is dissolved in it. For TiO2 [Fig. 4(a)] the DTA curve shows a sharp exothermic peak at 1013 K, due to the anatase-to-rutile transition, 32,33 preceded by a broad exothermic feature in the range 873–973 K due to anatase sintering.34 The anatase-to-rutile phase transition is shifted to ca. 963 K in catalysts Ti0.9Mn0.1 and Ti0.8Mn0.2 [Fig. 4(b) and (c)], and is associated with a very small weight loss detectable in the TG curve (Table 3). A pronounced endothermic peak associated to a weight loss is observed additionally at 1203 K in the DTA curve (Table 3) of catalysts Ti0.9Mn0.1 and Ti0.8Mn0.2. According to XRD analysis of the samples after the DTA runs up to 1073 and Table 3 TG data on TixMn1-xOy samples Temperature/ Weight Weight Sample K loss (%) gain (%) TiO2 873–973 — — 1013 — — Ti0.9Mn0.1Oy 963 — — 1203 0.65 — Ti0.8Mn0.2Oy 963 — — 1203 0.92 — Ti0.5Mn0.5Oy 873–963 0.80 — 963 — 0.45 1203 2.61 — Ti0.2Mn0.8Oy 863–1023 — 0.90 1203 0.45 — Ti0.1Mn0.9Oy 863–1057 — 1.70 1203 0.30 — Mn3O4 863 — 2.05 1057 1.40 — TiO2 (Degussa) 1083 — — Fig. 4 DTA curves of the powders after calcination at 773 K. (a) TiO2, 6%Mn-TiO2 (Degussa) 973 — 0.15 (b) Ti0.9Mn0.1, (c) Ti0.8Mn0.2, (d) Ti0.5Mn0.5, (e) Ti0.2Mn0.8, (f) 1213 0.35 — Ti0.1Mn0.9, (g) Mn3O4. J. Mater. Chem., 1998, 8(11), 2525–2531 25271273 K, these features are due to the reactions first giving rise to Mn2O3 from Mn3O4 (in fact Mn3O4 is metastable with respect to Mn2O3 below ca. 1253 K)35 and later producing MnTiO3 (pyrophanite), according to the following stoichiometry: TiO2+1/2 Mn2O3�MnTiO3+1/4 O2 In the case of Ti0.5Mn0.5 [Fig. 4(d)] we again find the features due to the anatase-to-rutile phase transition (963 K) and pyrophanite formation (1203 K). However, we also find a complex situation with weight loss and an exothermic phenomenon, in the range 873–973 K followed by a weight gain during the anatase-to-rutile transition phase.In agreement with the above experiments, during the exothermic anatase sintering, Mn3+ oxide species are segregated as Mn3O4, with oxygen loss. Later Mn3O4 is oxidised to Mn2O3 in the range 973–1013 K before reacting with TiO2 to give pyrophanite. In fact, the only way to explain a weight gain is that part of Mn is oxided.For the samples with a large content of Mn, no phase transition of anatase is observable while the endothermic peak associated with pyrophanite formation is still observed. Another endothermic peak due to the Mn2O3-to-Mn3O4 phase transition is found at 1057 K mainly for pure Mn oxide. In fact, according to the thermodynamic phase diagram,33 Mn3O4 is thermodynamically stable above 1253 K.In agreement with this the TG–DTA (Table 3) runs for Mn3O4 show first a weight gain at 863 K (metastable-to-stable phase transition) and later a weight loss due to the thermodynamically driven Fig. 5 UV–VIS spectra of the powders after calcination at 773 K. inverse reaction [Fig. 4(g)]. (a) TiO2 (Degussa), (b) 6%Mn-TiO2 (Degussa), (c) TiO2, (d) These data show that Mn favours the anatase-to-rutile phase Ti0.9Mn0.1, (e) Ti0.8Mn0.2, (f ) Ti0.5Mn0.5.transition. Previous studies showed that other cations such as Cu2+36 and V5+32 also favour anatase sintering and the anatase-to-rutile phase transition. Conversely, we reported that Mo6+, Con +,33 W6+,37 and Si4+38 tend to hinder both these phenomena.We observed that this behavior is predominantly found when the cations are impregnated at the anatase surface. We interpreted these data by a sintering-induced phase transition mechanism.33 The data presented here fully agree with previous observations, indicated that Mnn+ behaves similarly to V5+ and Cu2+. On the other hand, Ti seems to hinder slightly the thermodynamically reversible hausmanniteto- bixbyite phase transition upon cooling. 3.3 Electronic characterisation of mixed oxide catalysts UV–VIS diVuse reflectance spectra of TiO2, Ti0.9Mn0.1, Ti0.8Mn0.2 and Ti0.5Mn0.5 are shown in Fig. 5. The electronic spectra of the TiO2 samples [Fig. 5(a) and (c)] correspond to those reported previously for similar samples39,40 and other oxide binary systems such as Ti–Sr41 and Ti–Al.42 This spectrum is characterized by a strong absorption edge in the range 200–400 nm with two main absorptions at ca. 220 and 305 nm which are attributed an O 2p�Ti 3d charge-transfer transition.43 In the samples containing Mn, additional absorption appears at higher wavelengths, i.e. in the visible region. As a result, the absorption at 305 nm increases in intensity progressively and its position shifts towards higher wavelengths, up to 345 nm for the sample Ti0.8Mn0.2 [Fig. 5(e)] before falling to ca. 320 nm for Ti0.5Mn0.5 [Fig. 5(f )]. Meanwhile, the absorption at 220 nm is shifted to 255 nm for same samples. Above 400 nm a broad tail appears which becomes more predominant upon increasing the Mn content, with the appearance of higher wavelength components as shoulders at 570 and 760 nm, which is more evident for the sample Ti0.8Mn0.2.These two components become indistinguishable when the Mn content is 0.5. Fig. 6 UV–VIS spectra of the powders after calcination at 773 K. UV–VIS spectra of samples with greater Mn content are (a) Ti0.5Mn0.5, (b) Ti0.2Mn0.8, (c) Ti0.1Mn0.9, (d) Mn3O4, (e) Mn2O3 (reference sample). shown in Fig. 6, together those of the pure manganese oxides, 2528 J. Mater. Chem., 1998, 8(11), 2525–2531hausmannite and bixbyite (the latter calcined at 1073 K). In these spectra, the absorption at 255 nm decreases in intensity without shifting till it almost disappears, while the other at 320 nm becomes the predominant component in the range 320–305 nm in the spectrum of Mn3O4 [Fig. 6(d)]. As for the components in the region above 400 nm, at least three clear shoulders can be observed at 460, 565 and 740 nm for the Mn–Ti mixed oxides, the last decreasing in intensity with the Mn content. This component disappears in the Mn3O4 electronic spectrum, which however also shows a very broad absorption in the NIR region (see insert in Fig. 6). The a- Mn2O3 spectrum is formed by three bands at ca. 370, 485 and 755 nm. In the a-Mn2O3 structure, Mn3+ ions (d4) occupy octahedral Fig. 7 DTA curves of impregnated samples after calcination at 773 K. sites and, if highly symmetric, a single spin-allowed absorption (a) TiO2 (Degussa), (b) 6%Mn-TiO2 (Degussa). band in the d–d transition region is expected similarly to [Mn(H2O)6]3+ at 500 nm.44 However, a distortion of the octahedral coordination sphere can give rise to a diVerent and TiO2 (Degussa) samples.The TiO2 (Degussa) run is characterised by an exothermic peak near 1083 K without splitting of d levels and so other d–d transitions can occur.45,46 Thus, the absorptions in the bixbyite spectrum can be assigned, appreciable weight loss, as reported in Table 3, due to the anatase-to-rutile phase transition.The addition of Mn pro- in order of increasing wavelength, to a O2-AMn3+ chargetransfer transition, to superimposed 5B1gA5B2g and 5B1gA5Eg duces a notable shift of this peak up to 973 K, indicating that the presence of Mn favours the anatase-to-rutile phase trans- crystal-field d–d transitions, and to a 5B1gA5A1g crystal-field d–d transition, respectively.45,46 ition, not only in the mixed oxide samples but also if impregnated on the surface.Also, a new endothermic peak appears The interpretation of the hausmannite spectrum is complex because it shows random cation distribution in the spinel at 1203 K, which can be associated with pyrophanite formation. structure. However, some considerations should be taken into account: for instance, Mn2+ (d5) d–d transitions are expected The FTIR spectra (Fig. 2) of the Degussa support and of the Mn containing sample prepared from it appear to be very to be weak in both octahedral and tetrahedral sites, since they are, in principle, both spin and orbitally forbidden.47 similar and both show the strong absorptions of TiO2.The spectrum in the FIR region does not reveal appreciable traces According to previous studies48–51 the absorption band at 255 nm in the spectrum of Mn3O4 is associated with a of Mn oxide phases.The electronic spectra of the TiO2 (Degussa) support and O2-�Mn2+ charge-transfer transition and that at 320 nm with O2-�Mn3+ charge-transfer. In the near-IR region, a of the impregnated catalyst are compared in Fig. 5(a) and (b). When Mn is added, a significant absorption above 400 nm broad band with a maximum at ca. 1750 nm is only found in the hausmannite spectrum. Previous studies45,46,52 have into the visible region is observed in addition to that of the TiO2 edge. Components can be found near 440 nm, just at the reported that this band in the spinel structure can be assigned to a 5E�5T2 d–d transition of octahedral Mn3+ and its major lower energy side of the TiO2 gap transition, and in the 750 nm region.As discussed above, by comparison with the Mn2O3 energy with respect to that of the crystal field Do is due to a distortion from octahedral coordination. spectrum, these new features are likely to be due to octahedral Mn3+ species. In this case, however, the TiO2 band gap does In the UV–VIS spectra of Mn–Ti mixed oxide samples, the variation of Mn content explains clearly the decrease of not seem to be substantially modified, in contrast with what occurs for mixed oxide samples.This can be associated to the intensity and the shifting of the bands, since these features relate to TiO2 charge-transfer transitions. The component near existence, in the case of mixed oxides only, of Mn species dissolved in the TiO2 bulk. 460 nm and the broad absorption centred around 750–760 nm can be related to the absorptions present in the spectrum of a-Mn2O3 at ca. 485 and 755 nm, suggesting that the majority 4 Catalytic tests of manganese is Mn3+ in octahedral coordination. However, according to previous studies the apparent shift of the absorp- All Mn-containing samples are found to be active in the catalytic oxidation of both propane and propene.Propene tion edge in the range near 400 nm in the sample Ti0.9Mn0.1 (which only shows the anatase phase with, possibly, dissolved oxidation is total in all cases giving rise almost exclusively to CO2 with very small amounts of CO (selectivities always below Mn ions, according to XRD) could also be associated to transitions of Mn4+ in the TiO2 phase.49 The other weak and 5%) and negligible traces of ethylene (<0.1% selectivity).However, the sample activity measured by using the same sharp component near 565 nm can be tentatively assigned to the 6A1�4T1 forbidden d–d transition of Mn2+, either in an catalyst surface areas and flow rates, is very sensitive to composition (Fig. 8). Conversion above 97% is achieved on octahedral or tetrahedral site.47 bixbyite at 540 K while on hausmannite such levels of conversion are attained above 600 K. On mixed and supported 3.4 Characterisation of the impregnated catalyst after Mn–Ti oxides 6% or lower conversions are obtained at 700 K. calcination at 773 K However, comparison of the activities at lower temperatures and conversions (Fig. 8), where the kinetic regime is chemical Similar experiments have been performed on a sample prepared by impregnation of a commercial TiO2 support, from Degussa (as shown by the calculated activation energies of 19 kcal mol-1), shows similar behaviour for the two Mn (anatase+30% rutile mixture), for comparison. The loaded Mn amount (6% wt./wt.) was calculated to be approximately oxides.This apparently contradicts a previous study from our group, where Mn3O4 was reported to be significantly more that needed to cover the overall support surface with a complete ‘monolayer’. In the XRD powder pattern of a sample active than a-Mn2O3.10 However, in that case we measured the activity of samples diVerent than those described here, and after calcination at 773 K, anatase and rutile phases are present with relative ratios very similar to those of pure support TiO2, characterised by very diVerent surface area and pre-treatment temperatures. and traces of the bixbyite phase.53 Fig. 7 compares DTA curves of the 6%Mn–TiO2 (Degussa) The catalytic activities of both Mn–Ti mixed oxides and of J. Mater. Chem., 1998, 8(11), 2525–2531 2529Fig. 10 Product reaction upon propane oxidation as a function of the Fig. 8 Propene conversion over manganese and titanium oxides as a reaction temperature over 6%Mn-TiO2 (Degussa). (2) O2 (conv.), function of the reaction temperature. (2) Mn3 O4, (&) Mn2 O3, (1) CO2 (sel.), (&) C3H8 (conv.), (%) C3H6 (sel.), (#) CO (sel.), (+) Ti0.1Mn0.9, (1) Ti0.2Mn0.8, (%) Ti0.5Mn0.5, (#) 6%Mn-TiO2 (6) C2H4 (sel.).(Degussa). sample Ti0.5Mn0.5 and a maximum in selectivity to propane for Ti0.2Mn0.8. Conclusions Conclusions from the above data are as follows. 1 The addition of Ti to Mn oxide the formation of the thermodynamically stable phase a-Mn2O3 (bixbyite) with respect to that of the metastable phase (at room temp.) Mn3O4 (hausmannite). 2 Ti apparently enters the bixbyite phase in small amounts. 3 Mn addition, both in the samples of mixed oxides and the supported catalyst, favours the anatase-to-rutile phase transitions.This eVect is particularly evident for impregnation, in agreement with the sintering-induced phase-transition eVect, previously proposed by some of us.33 4 Mn enters the TiO2 anatase phase in small amounts, in Fig. 9 Propane conversion over manganese and titanium oxides as the samples of mixed oxides. a function of the reaction temperature. (2) Mn3 O4, (&) Mn2 O3, 5 The pyrophanite phase MnTiO3 is produced by heating (+) Ti0.1Mn0.9, (1) Ti0.2Mn0.8, (%) Ti0.5Mn0.5, ($) Ti0.8Mn0.2, (6) Ti0.9Mn0.1, (#) 6%Mn-TiO2 (Degussa). Mn-mixed oxides above 1200 K. 6 The surface areas of mixed Mn–Ti oxides tend to decrease upon increasing the Mn content. the supported catalysts are by far lower than those of the pure 7 Characterisation of a supported Mn-TiO2 catalyst shows Mn oxides.Temperatures of 580 K are needed to obtain 10% that Mn oxide species are well dispersed, with a-Mn2O3 propene conversion for the samples Ti0.1Mn0.9 and Ti0.2Mn0.8, (bixbyite) particles only detectable in traces.while the same conversion is attained only above 620 K for the 8 UV–VIS spectra suggest that surface Mn oxide species on sample Ti0.5Mn0.5 and by the supported sample. TiO2 are mainly constituted of octahedrally coordinated The conversion of propane is, as expected, lower than that of Mn3+ species. propene in all cases, under the same conditions (Fig. 9). In this 9Mixed and supported Mn–Ti oxides are active and selective case, however, oxydehydrogenation to propene is predominant catalysts for the total oxidation of propene to CO2.By at very low conversion and still remains significant up to total contrast, they give rise to substantial partial oxidation products propane conversion. CO2 is the predominant product when like propene and CO upon oxidation of propane after partial propane conversion becomes significant while CO is also formed conversion.later. Traces of ethylene are also observed. The behaviour of the 10 Mixing Mn oxides with, or supporting on, TiO2 strongly supported catalyst 6% Mn-TiO2 (Degussa) is shown in Fig. 10, deactivates them for propene oxidation. As for propane and can be regarded as representative of all Mn-containing oxidation, mixed Mn–Ti oxides are by far less active than samples.A similar behaviour has been described previously for a-Mn2O3 (bixbyite) but can be more active than Mn3O4 bulk Mn oxides.10,11 By comparing again the activities at lower (hausmannite). conversions (Fig. 9), we find that the catalytic activity of a- 11 The activation eVect of TiO2 on vanadia, molybdena and Mn2O3 (bixbyite) is in this case far higher than that of Mn3O4 tungsta oxidation catalysts does not occur on Mn oxides.This (hausmannite).Mn3O4 is, however, more selective towards pro- can be interpreted assuming that this eVect is not due to the pene at similar conversion, giving yields of propene of the order ability of TiO2 anatase to disperse the active phase (occurring of 5% under these conditions.Again CO is produced in very in both cases), but rather to diVerent electronic eVects that small amounts (selectivity <5%), CO2 being predominant. distinguish oxides of d0 cations (e.g. vanadia, molybdena and The mixed oxide samples show catalytic activity higher than tungsta) from the oxides of d-electron containing cations. Mn3O4 although lower than for Mn2O3. The catalytic activity of the supported catalyst does not diVer significantly from those Acknowledgements of mixed oxide catalysts, and also the sample Ti0.9Mn0.1 shows significant oxidation activity.The trends for propane conversion Part of this work has been supported by NATO activity and propene selectivities are quite diYcult to rationalise (CRG-960316).J. M. G. A. acknowledges MEC for a FPI grant. for Mn–Ti mixed oxides, with a maximum in activity for the 2530 J. Mater. Chem., 1998, 8(11), 2525–2531Escribano and P. Piaggio, J. Chem. Soc., Faraday Trans., 1994, References 90, 3181. 29 M. I. Baraton, G. Busca, V. Lorenzelli and R. J. Willey, J. Mater. 1 P. E. Marti, M. Maciejewski and A. Baiker, Appl. Catal., B: Sci. Lett., 1994, 13, 275.Environ., 1994, 4, 225. 30 M. C. Bernard, A. Hugot-Le GoV, B. Vu Thi and S. Cordoba de 2 G. Groppi, M. Bellotto, C. Cristiani and P. Forzatti, Appl. Catal., Torresi, J. Electrochem. Soc., 1993, 140, 3065. A: General, 1993, 104, 101. 31 M. Valigi and A. Cinimo, J. Solid State Chem., 1975, 12, 135. 3 H. G. Lintz and K. Wittstock, Catal. Today, 1996, 29, 457. 32 G. Oliveri, G.Ramis, G. Busca and V. Sanchez Escribano, 4 U. S. Ozkan, R. F. Kueller and E. Moctezuma, Ind. Eng. Chem. J. Mater. Chem., 1993, 3, 1239. Res., 1990, 29, 1136. 33 J. M. Gallardo Amores, V. Sanchez Escribano and G. Busca, 5 J. Carno� , M. Ferrandon, E. Bjo�rnbom and S. Ja�ra�s, Appl. Catal., J. Mater. Chem., 1995, 5, 1245. A: General, 1997, 155, 265. 34 J. L. He�brard and P.Nortier, J. Am. Cem. Soc., 1990, 73, 79. 6 G. K. Boreskov, B. I. Popov, V. N. Bibin and E. S. Kozishnikova, 35 R. Mtselaar, R. E. J. Van Tol and P. Piercy, J. Solid State Chem., Kinet. Katal., 1968, 9, 796. 1981, 38, 335; S. E. Dorris and T. O. Mason, J. Am. Ceram. Soc., 7 J. E. Germain and R. Perez, Bull. Soc. Chim. Fr., 1972, 4683. 1988, 71, 379. 8 C. Lahousse, A. Bernier, A.Gaigneaux, P. Ruiz, P. Grange and 36 J. M. Gallardo Amores, V. Sanchez Escribano, G. Busca and B. Delmon, in 3rd World Congress on Oxidation Catalysis, ed. V. Lorenzelli, J. Mater. Chem., 1994, 4, 965. R. K. Grasselli, S. T. Oyama, A. M. GaVney and J. E. Lyons, 37 G. Ramis, G. Busca, C. Cristiani, L. Lietti, P. Forzatti and Elsevier, Amsterdam, 1997, p. 777. F. Bregani, Langmuir, 1992, 8, 1744. 9 J. E. Germain and R. Perez, Bull. Soc. Chim. Fr., 1972, 541; 2042. 38 Li Yi, G. Ramis, G. Busca and V. Lorenzelli, J. Mater. Chem., 10 M. Baldi, V. Sanchez-Escribano, J. M. Gallardo-Amores, 1994, 4, 1755. F. Milella and G. Busca, Appl. Catal., B: Environ., 1998, 17, L175. 39 H. Bevan, S. V. Dawes and R. A. Ford, Spectrochim. Acta, 1958, 11 M. Baldi, E. Finocchio, F. Milella and G.Busca, Appl. Catal., B: 13, 43. Environ., 1998, 16, 43. 40 T. R. N. Kutty, R. Vivekanan and P. Murugaraf, Mater Chem. 12 P. J. Gellings in Catalysis, The Royal Society of Chemistry, Phys., 1988, 19, 533. London, 1985, vol. 7, p. 105. 41 T. R. N. Kutty and M. Avulaithai, in Properties and Applications 13 Vanadia catalysts for selective oxidation of hydrocarbons and their of Perovskite-type Oxides, ed.L. G. Tejuca and J. L. G. Fierro, derivatives, ed. B. Grzybowska-Swierkosz, F. Trifiro� and J. C. Marcel Dekker, New York, 1993, p. 307; L. G. J. DeHaart, Vedrine, Special issue of Appl. Catal., A: General, 1997, 157, A. J. DeVries and G. Blasse, J. Solid State Chem., 1985, 59, 291; 1–426. J. M. Gallardo-Amores, V. S. Escribano, M. Daturi and G. Busca, 14 V.Nikolov, D. Klissurski and A. Anastasov, Catal. Rev. Sci. Eng., J. Mater. Chem., 1996, 6, 879. 1991, 33, 319. 42 G. Pacheco-Malago� n, A. Garcý�a-Bo� rquez, D. Coster, A. Sklyarov, 15 C. R. Dias, M. F. Farinha-Portela and G. C. Bond, Catal. Rev. S. Petit and J. J. Fripiat, J. Mater. Res., 1995, 10, 1264; Sci. Eng., 1997, 39, 169. A. Gutierrez, M. Trombetta, G. Busca and J.Ramirez, 16 P. Forzatti and L. Lietti, Heterog. Chem. Rev., 1996, 3, 33. Microporous Mater., 1997, 12, 79. 17 G. 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Sanchez Paper 8/03994A J. Mater. Chem., 1998, 8(11), 2525–2531 2531

ISSN:0959-9428    

DOI:10.1039/a803994a

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials In situ X-ray absorption spectroscopic studies at the cobalt K-edge on an Al2O3-supported rhenium-promoted cobalt Fischer–Tropsch catalyst. Comparing reductions in high and low concentration hydrogen Arild Moen,a David G. Nicholson,a Magnus Rønningb and Hermann Emerichc aDepartment of Chemistry, Norwegian University of Science and Technology, N-7055 Trondheim, Norway bDepartment of Industrial Chemistry, Norwegian University of Science and Technology, N-7055 Trondheim, Norway cSwiss-Norwegian Beamline, European Synchrotron Radiation Facility BP 220, F-38043 Grenoble, France Received 5th June 1998, Accepted 10th August 1998 In situ XAFS spectroscopic studies have been carried out at 450 °C on the hydrogen reduction of a rheniumpromoted Co3O4/Al2O3 catalyst.Reductions carried out using 100% hydrogen and 5% hydrogen in helium gave diVerent results. Whereas the reduction using dilute hydrogen yielded bulk-like metallic cobalt particles (hcp or fcc), the reaction with pure hydrogen led to a more dispersed system with smaller cobalt metal particles (<40 A° ) the crystal form of which could not be established so that the recently reported metastable nonclose-packed bodycentred cubic form cannot be excluded.Reoxidation of a similar catalyst in water-containing gas mixtures has been reported in the literature; it is suggested that the diVerent outcome in the case of the 100% hydrogen protocol may be due to a similar mechanism. This would involve the in situ water produced by the reduction with reoxidation–reduction of cobalt metal particles in the water vapour–hydrogen mixture.However, this mechanism cannot be established by the present study. Additionally, in both reduction protocols a small fraction (3–4 wt.%) of the cobalt content is randomly dispersed over the tetrahedral vacancies of the alumina support with Co–O bond lengths of 1.96±0.01 A° .This dispersion occurs during reduction and not calcination.The cobalt in these sites cannot be reduced at 450 °C, a temperature that is too low to permit formation of the spinel CoAl2O4. X-Ray absorption spectroscopy (XAS) is a useful method for part involves the diVusion of cobalt ions into the structure of characterising local structural features of selected elements in c-Al2O3 where they occupy vacant octahedral and/or tetraheterogeneous catalysts.Although the technique is actually a hedral positions. This apparently leads to two main types of bulk technique it is also valuable for studying chemical reac- cobalt-containing phase: (a) cobalt bonded to the alumina tions that take place at the surfaces of heterogeneous catalysts support in the form of a surface phase that is resistant to because the overall contribution to the XAS signal arises from reduction and (b) the easily reduced spinel Co3O4 which the significant proportion of active sites that are highly dis- dominates above a certain cobalt concentration (3–4%).A persed over a large number of surfaces. Indeed, XAS is major factor determining catalytic activity is the influence of particularly suited for characterising such systems because promotors; e.g.there is a two-fold increase in the rates of their inherent lack of long range order precludes study by X- hydrogenation of carbon monoxide in the presence of platinum ray diVraction. In addition, XAS is capable of giving infor- and rhenium promoters.28,29 mation on interactions between the support and the metal We report here the results of in situ XAS experiments on catalyst, an aspect which is relevant to this work since such the reduction with dilute hydrogen (5% in helium) at 450 °C interactions are known to influence activity and selectivity.1–3 of the rhenium-promoted Co/Al2O3 (cobalt content 26 wt.%).This paper is concerned with an alumina-supported cobalt This is an extension of previous work35 in which the catalyst catalyst used in the Fischer–Tropsch process.In this process was reduced by 100% hydrogen at the same temperature. We linear high-molecular weight aliphatic hydrocarbons are syn- also report here the results of experiments carried out at lower thesised by catalytically hydrogenating carbon monoxide.4–30 temperatures (200 °C) on the same catalyst and the results of A comprehensive review of the literature dealing with this the reduction at 450 °C of a similar system but one with a system and its variants is given by Holmen et al.31 much lower cobalt content (4.6%).XAS has been used previously to study several modifications of Co/Al2O3 catalysts. Probably the earliest study was that Experimental reported by Greegor et al.32 Fay et al.24 used the method to characterise a boron-modified system, and HuVman et al.33 Synthesis used XAS in an in situ study of potassium-promoted cobalt Essentially following the procedure described in the catalysts supported on alumina and silica during reduction in literature29(but in our case with a triple cobalt loading) the hydrogen at 200 °C with subsequent reaction in a synthesis alumina-supported cobalt catalyst with 26% cobalt loading gas mixture.The latter support was also used by Takeuchi (analysis by atomic absorption spectroscopy) was prepared by et al.34 Recently, Moen et al.35 in a preliminary report described the incipient wetness impregnation of c-alumina (sieved to an in situ XAS study at the Co K-edge on the reduction of 40–50 mesh) with aqueous solutions of Co(NO3)2·6H2O and the rhenium-promoted Co/Al2O3 catalyst (cobalt content HReO4(calculated to give 1.0 wt.% rhenium). The resulting 26 wt.%) at 450 °C using 100% hydrogen.From these studies it appears that catalyst preparation in material was dried overnight at 120 °C and then calcined at J. Mater. Chem., 1998, 8(11), 2533–2539 2533500 °C for 8 h.A sample of the same system but with a much EXAFS data analysis. The data were corrected for dark currents, converted to k-space, summed and background sub- lower cobalt loading (4.6%) was prepared and treated similarly.29 tracted to yield the EXAFS function xobsi(k) using the EXCALIB and EXBACK programs.38 Model fitting was Hydrogen reduction of the catalysts was carried out in situ (see below).carried out with EXCURV90 using curved-wave theory and ab initio phase shifts.38,39 The edge positions were determined from the first inflection points (after any pre-edge features) of X-Ray absorption data the derivative spectra. The k3 weighting scheme compensates for the diminishing XAS data were collected using the facilities of the bending photoelectron wave at higher k.The curve-fitting was per- magnet Swiss-Norwegian Beamline (SNBL) at the European formed on data that had been Fourier filtered over a wide Synchrotron Radiation Facility (ESRF), Grenoble, France. range (1.0–25.0 A° ). This filter removes low-frequency contri- Spectra were obtained on station EH1 (SNBL-ESRF) at the butions to the EXAFS below 1 A° , but does not smooth the cobalt K-edge (l=1.6086 A° ; energy=7 709 eV).spectrum (i.e. the noise is not removed). The ranges for the A channel-cut silicon(111) monochromator with an Fourier transformations were 3–14 A° -1. unfocussed beam was used to scan the X-ray spectra. The The model compounds, cobalt Tutton salt,40 cobalt alumin- beam currents ranged from 80–130 mA at 6.0 GeV.Higherate41 and cobalt(II) oxide42 were used to check the validity of order harmonics (ca. two orders of magnitude) were rejected the ab initio phase shifts and establish the general parameter by means of a gold-coated mirror angled at 7.3 mrad from a AFAC (proportion of absorption causing EXAFS) and VPI beam of size 0.6×4.7 mm which was defined by the slits in (allows for inelastic scattering of the photoelectron).38 The the station.The maximum resolution (DE/E) of the Si(111) cobalt spinel (Co3O4), which contains both cobalt(II ) and bandpass is 1.4×10-4. cobalt(III),43 was used as a reference. Gas ion chamber detectors with their gases at ambient temperature and pressure were used for measuring the intensities of the incident (I0) and transmitted (It) X-rays.The detector Results and discussion gases were as follows: I0, detector length 17 cm, 100% N2; It, We have previously shown35 that the XAS of the unreduced length 31 cm, 35% Ar, 65% N2. material (sample A) is indistinguishable from that of the reference compound Co3O4. Clearly, the method of preparation, together with the comparatively high cobalt loading, Data collected at the Co K-edge.The spectral energy calibration was checked by measuring the spectrum of a Co- yields a material in which Co3O4 is the dominant phase and that this phase is distributed throughout sample A in the form foil (thickness 5 mm; the energy of the first inflection point being defined as the edge 7 709 eV; accurate calibrations are of relatively large (>100 A° ) crystallites.This conclusion is consistent with the sizes (140–190 A° ) derived from the line particularly important for the pre-edge and XANES regions where the need for comparisons between the diVerent spectra broadening observed in the X-ray diVractogram.29 The reduction at 450 °C with 5% hydrogen was monitored make it necessary to define the absolute energies of the spectral features; for the EXAFS the energy is relative to the individual by a series of quick short scans taken just before and after the edge.The intensity of the white line (characteristic of Co3O4) edges which are therefore defined as zero). The energy scale calibration of each spectrum was carried out using an rapidly decreased with time and on the basis of this indicator the reduction was essentially completed within approximately in-house program.The XAS of the catalysts, before and after reduction, and 15 min, although the process was continued for a further 6 h. Fig. 1 shows the XAS of sample A (26 wt.% loading of of the model compounds CoAl2O4, CoO, the cobalt Tutton salt, [NH4]2[Co(H2O)6][SO4]2, and Co3O4 were also meas- cobalt) reduced by 100% and 5% hydrogen (to give samples B and C, respectively) and sample D (4.6 wt.% loading) ured.The amounts of material in the samples were calculated from element mass fractions and the absorption coeYcients of reduced in 5% hydrogen; the spectrum of bulk cobalt metal is also shown. The X-ray absorption near edge structure the constituent elements36 above the absorption edge to give an absorber optical thickness of 1.5 absorption lengths.The (XANES) regions for the model/reference compounds and samples A, B, C, D are depicted in Fig. 2. The model/reference well-powdered samples of the model and reference compounds were mixed with boron nitride so as to give a sample thickness compounds, chosen for their tetrahedral and octahedral cobalt environments, are represented by the following compounds: of ca. 1.0 mm and placed in aluminium sample holders and held in place by Kapton tape. Co3O4/Al2O3 destined for the CoAl2O4 (tetrahedral CoII(O)4); [NH4]2[Co(H2O)6][SO4]2 (distorted octahedral CoII(O)6); CoO (octahedral CoII(O)6); in situ reduction was ground and sieved (7–125 mm) and mixed with the requisite amount of boron nitride or graphite to and Co3O4 (tetrahedral CoII(O)4 plus octahedral CoII(O)6).Fig. 3 contains the first derivatives of the edge region because achieve the desired absorber thickness. (The experiment using graphite was to establish whether that material aVects the they are useful for highlighting characteristic features about the edges (particularly the pre-edge features) and establishing product; the spectra of the samples diluted with boron nitride and graphite were similar.) their energies.35,44 A comparison of the XAS of cobalt metal (as a foil with The catalyst was then loaded into a Lytle in situ reactor-cell37 and reduced in a mixture of H2 (5%) in He (purity; 99.995%: the face centred cubic or hexagonal close-packed structure, their spectra are similar35) and sample B in Fig. 1 reveals that flow rate 60 ml min-1) by heating from room temperature to 450 °C and maintaining at that temperature for 6 h.there are significant diVerences between them. Yet, the spectrum of sample C closely resembles that of the cobalt metal. The same procedure was repeated for other samples but with the diVerence that the hydrogen flow was started at The edge regions of bulk cobalt metal and sample B are also diVerent, as emphasised by the first derivatives (Fig. 3). 450 °C. Similar measurements were also carried out at 200 °C and for 13 h. The samples are designated as follows. Sample Evidently, sample B, unlike sample C, is not composed of bulk metal particles. Hence, reduction by 100% and 5% A: the catalyst containing 26% cobalt (shown below to be mainly Co3O4/Al2O3).Sample B: produced by reducing sample hydrogen yields products (B and C) with diVerent physical characteristics. A in 100% hydrogen at 450 °C.35 Sample C: prepared by reducing sample A in 5% hydrogen at 450 °C. Sample D was The most prominent features in the XAS of sample B are the pre-edge peak at 7709 eV and the absence (or considerable prepared by reducing the catalyst containing 4.6% cobalt in 5% hydrogen at 450 °C.reduction) of the white line so characteristic of Co3O4, 2534 J. Mater. Chem., 1998, 8(11), 2533–2539Fig. 3 The first derivatives of the XANES spectra of samples A, B, C, D and the reference/model compounds. The positions of the preedge peaks are marked ($). CoAl2O4, CoO, cobalt Tutton salt and its precursor, sample A. The white line is also absent in the XAS of bulk cobalt metal which suggests that sample B contains a high degree of reduced material consistent with the estimate (80%) for reducibility of rhenium-promoted Co3O4/Al2O3 under the same Fig. 1 Normalised XAS of samples B, C, D and the reference cobalt conditions.29 metal. Pre-edge peaks The XAS at the K-edge of certain valence states of some transition compounds often contain an electronically interesting pre-edge feature a few eV below the edge.This feature is useful because it yields structural and electronic information, especially when combined with the extended X-ray absorption fine structure (EXAFS) region of the same spectrum.44,45 The peak results from the absorption process 1sA3d and for solid macromolecular materials (such as CoO, CoAl2O4 and Co3O4 ) the final state(s) are unoccupied bands.The transition probability (intensity) is related to the symmetry (A1AT2 for T d symmetry) and to the occupancy of the 3d shell. For a given occupancy, the transition is most intense when the first coordination shell lacks inversion symmetry. In the case of the cubic point groups this applies to tetrahedral (T d) environments but not to octahedral (Oh) symmetry; although for the latter point group a considerably weaker preedge feature does actually occur (despite being forbidden by the centre of symmetry).This is because the crystallographic point group represents a static model derived by time-averaging the asymmetric vibrations within the molecule whereas the XAS reacts to the individually and constantly changing structures of the local environment as vibrations momentarily Fig. 2 Normalised Co K-edge XANES of the samples and reference materials. eliminate the centre of symmetry. An example is the spectrum J. Mater. Chem., 1998, 8(11), 2533–2539 2535(Fig. 2 and 3) of CoO, with its very weak pre-edge feature the bond length of a tetrahedral CoII(O)4 environment the latter appears to be somewhat shorter than that yielded by the (better revealed in the derivative of the spectrum), in which the octahedrally coordinated cobalt is in an Oh environment.44 EXAFS of the bulk cobalt metal reference (2.48±0.02 A° , Debye–Waller type factor (2s2)=0.013 A° 2).Apai et al.47 For symmetries lower than Oh, as in the distorted octahedrally coordinated cobalt environment (Co(H2O)62+) in the reported that metal–metal distances in very small metal particles are contracted relative to those in the bulk, an obser- Tutton salt, [NH4]2[Co(H2O)6 ][SO4]2,40 the intensity is somewhat enhanced, although still comparatively weak.vation that has subsequently been reported by others.48,49 Although a shortened Co–Co distance would accompany an By contrast, the pre-edge peaks associated with tetrahedral cobalt environments are more intense in accordance with the edge shift a more detailed study that focuses on this particular aspect is required before the final conclusion can be drawn.noncentrosymmeric Td point group. This is exemplified in the spectrum of CoAl2O4 (Fig. 2 and 3). These figures also show XAS of sample C the pre-edge region for the spinel Co3O4 which has one third cobalt(II) in tetrahedral sites and two thirds Co(III) in octa- The XAS spectrum (Fig. 5 shows the magnitude of the Fourier hedral sites. The pre-edge feature is composed of the tetratransforms of the EXAFS region) of sample C shows that hedral peak and the very weak (forbidden) octahedral peak, reduction at 450 °C in 5% hydrogen produces bulk-like cobalt the overall peak intensity being reduced because the proportion metal (hcp or fcc) particles with only a minor fraction of of the cobalt content in tetrahedral sites is now only one third cobalt being incorporated into the alumina support (the the total cobalt content and not unity as in CoAl2O4 (but nonmetallic phase); the latter is discussed below.see below). XAS of sample D XAS of sample B It is evident from the full XAS spectrum (Fig. 1) and the Fig. 4 shows the XANES regions of samples A and B and EXAFS region (Fig. 6) that the major cobalt-containing com- bulk cobalt metal. The edge position of sample B is at a higher ponent consists of cobalt incorporated into the alumina sup- energy (7714 eV) than that in bulk cobalt metal.The pre-edge port. This cobalt fraction is not reduced by hydrogen at peak at 7710 eV in sample B serves both as a convenient 450 °C. The pre-edge peak shows that the cobalt environment control of the edge energy and as an important diagnostic is tetrahedral CoII(O)4 the EXAFS yielding a Co–O distance feature for tetrahedral CoII(O)4 environments. Another sigof 1.96±0.01 A° that is consistent with this.43 nificant feature in the sample B spectrum is the absence of the white line. As shown in the same figure, this is also character- The eVects of using concentrated versus dilute hydrogen istic of bulk metallic cobalt.Since the edge energy of bulk metallic cobalt is 7 709 eV, any significant contribution from This study shows that the main reduction product of sample that metal in a composite spectrum of two-or-more phases A at 450 °C using 100% hydrogen (sample B) diVers from that would obscure this pre-edge peak.This does not occur in sample B because the edge is actually moved to a higher energy. The significance of the edge shift, the pre-edge peak and a considerably reduced EXAFS amplitude have been discussed in terms of small metal particles (<40 A° ) and the consequent deviation from bulk properties.35 The eVect of particle size on the EXAFS amplitude is significant for dimensions below ca. 30 A° .26 Recent studies on the size determination by EXAFS confirm this.46 It is therefore apparent that a minor fraction of cobalt in sample B is sited in the tetrahedral environment that we know is present from the pre-edge peak.The distances extracted35 from the EXAFS are 1.92±0.01 A° (Debye–Waller type factor (2s2)=0.011 A° 2) for the Co(O)4 tetrahedral site (which is close to the distance extracted from the cobalt-containing support in sample D, see below) and 2.49±0.01 A° (Debye–Waller type factor (2s2)=0.023 A° 2) due to Co–Co backscattering. Whereas the former distance corresponds to Fig. 5 Fourier transforms of samples B and C (top and bottom, Fig. 4 Normalised Co K-edge XANES regions of samples A and B respectively). The amplitudes of the peaks for sample B are much reduced compared with bulk cobalt metal (centre). and the reference cobalt metal (bulk). 2536 J. Mater. Chem., 1998, 8(11), 2533–2539ing gas mixtures. They found that unpromoted and rheniumpromoted catalysts behave diVerently, with cobalt in the latter being more easily reoxidised and the resulting oxidised phase being more easily reduced.Other results in the literature also show that water has an eVect; the introduction of water to Fischer–Tropsch reactions that are catalysed by variants of the Co/Al2O3 system aVect the rate of reaction and distribution of the reaction products.51,52 Turning to the present experiments, since in situ water (ca. 2 mg) is produced during the reduction some degree of reoxidation of the freshly reduced cobalt metal would fit in with the results of Hilmen et al.50 If the action of in situ water is responsible for the diVerent outcomes of 100% and 5% hydrogen reduction at 450 °C then it seems likely that the partial pressures of water must be highest for the former and hence more eVective in reoxidising metal particles which are again reduced by hydrogen. Further experiments are necessary to ascertain whether this suggested mechanism is viable and also to establish whether a reoxidation–reduction process increases the metal dispersion by breaking up the initially large cobalt particles into smaller particles.The nonmetallic phase Information concerning the amount of cobalt incorporated into the alumina support is forthcoming from the XAS of samples C and D with 26% and 4.6% cobalt (as noted above). The data for the latter show that the amount of metallic cobalt produced is small. In order to estimate the fraction of cobalt that is incorporated into the alumina support the spectra of the high- and low-cobalt samples were first normalised and then a series of summed spectra were constructed by adding the low-cobalt spectrum (the major component in the support being cobalt incorporated into the support) to progressively increasing numbers of cobalt-metal spectra until the actual spectrum of the high-cobalt sample was closely reproduced.The best simulation was obtained with the fractions cobalt metal5low-cobalt catalyst being in the ratio 851.For the 26% loading this translates into ca. 3% cobalt being incorporated into the alumina support. (If the small fraction of metallic cobalt present in sample D (0.77%, see below) is taken into account then this figure would be slightly higher.) In order to check this result, the following alternative procedure was used.On comparing the pre-edge regions of samples C and D (Fig. 2) it is evident that the pre-edge peak assigned to tetrahedral Co(O)4 environments is obscured by the dominating contribution from metallic cobalt in the former (the position of the edge for cobalt metal being the same as that for the pre-edge peak.) Consistent with the low-cobalt metal content in sample D, the pre-edge peak in the spectrum of that material can be discerned because it is only partially merged into the metal edge.The spectrum of the nonmetallic phase was separated from the observed spectrum by subtracting the minor cobalt metal spectral contribution from the spectrum of sample D. The appropriately weighted cobaltmetal spectrum was obtained by multiplying the normalised cobalt-metal spectrum by a series of factors and subtracting each resulting spectrum from the actual spectrum of D until Fig. 6 Fourier transforms of sample A and sample D together with those of the spinel reference compounds. a spectrum (and its derivative) was obtained which exhibited a pre-edge peak that closely matched the same feature in CoAl2O4. A good match was generated by subtracting the cobalt-metal obtained using 5% hydrogen in helium (sample C). An explanation for this diVerence is sought in a mechanism that we spectrum weighted by the fraction 0.167 from the observed composite spectrum.This corresponds to a metal content of suggest may be connected with the reduction of this catalyst. Although the present results cannot establish this mechanism 0.77% cobalt for the 4.6% cobalt loading in sample D, the low-cobalt catalyst.(The same procedure has been described the concentration of hydrogen must be the key factor here. The background for the mechanism is provided by Hilmen recently in ref. 44.) Hence, the nonmetallic phase contributes 3.8% to the total cobalt loading of the catalysts. This can be et al.50 who used temperature programmed reduction and gravimetry to show that cobalt is reoxidised in water-contain- rationalised in terms of the structure of c-alumina.The sup- J. Mater. Chem., 1998, 8(11), 2533–2539 2537port, c-Al2O3, has a defect spinel structure in which not all of alumina lattice28 but the present study clearly excludes the presence of CoAl2O4 in all of the reduced samples (B, C and the cation sites are occupied, i.e.Al21Hh22IO32, where h designates vacant tetrahedral and octahedral sites.53 If cobalt D) (see Fig. 3 and 5) because the Co,Co distance in that compound is considerably longer (2.83 A° ).41 Instead, it is is distributed as Co(II) over the vacant tetrahedral sites in alumina (one third of the total vacant sites) then this would evident that only a small fraction of the total cobalt contents of samples B and C are randomly dispersed over vacant yield AlIII21HCoIIU~V O32U~V , with the cobalt content being 4.5%, a value which is close to that found (3.8%) in the nonmetallic tetrahedral sites of the alumina support.This conclusion is supported by an X-ray diVraction study57 which also phase.Thus, the data for the high and low-cobalt loadings are shows that CoAl2O4 is not formed and additionally by the observation28 that much higher calcination temperatures consistent with 3–4% cobalt being randomly dispersed over vacant tetrahedral sites of the alumina support. Fig. 7 shows (>1200 °C) are needed to form CoAl2O4 from mixtures of Co3O4 and Al2O3. the EXAFS and the magnitude of the Fourier transform together with the parameters obtained from the least-squares refinement.The Co–O bond distance (1.96±0.01 A° ) is typical Reduction at 200 °C for tetrahedral coordinated cobalt(II),44 a similar value is also HuVman et al.33 reported that reduction of potassium- obtained for sample B (see above) and the pre-edge peak in promoted Co3O4/Al2O3 at 200 °C yields cobalt-metal particles that material’s spectrum is consistent with some of the cobalt in which the coordination numbers are less than those for being incorporated in the support in a similar manner.bulk cobalt metal. On this basis they estimated the average The XANES spectrum and its derivative of the particle size to be as small as 10–20 A° although they doubted cobalt-incorporated support (designated sample D-Co and this estimate noting that the actual size must be larger.Unlike obtained by removing the cobalt metal component from the the present work, no other cobalt-containing phase was spectrum of sample D) are shown in Fig. 2 and 3. There are detected. The present study also diVers in another respect since marked similarities with the XANES of CoAl2O4. Like the it shows that the rhenium-promoted Co3O4/Al2O3 catalyst is latter the derivative spectrum is also consistent with a minor not reduced at 200 °C even when reacted with hydrogen for amount of cobalt entering some of the vacant octahedral sites as long as 13 h.of the bearer.44 It has been assumed that CoAl2O4 is the phase generated when cobalt occupies the vacant tetrahedral positions of the The role of rhenium In a temperature-programmed reduction (TPR) study of the mechanism of rhenium promotion of alumina-supported cobalt Fischer–Tropsch catalysts, Holmen et al.31 reported that direct contact between rhenium and cobalt particles does not appear to be necessary for promotion.It is suggested that the mechanism by which rhenium promotes the reduction of Co3O4 is by hydrogen spillover.(The influence that rhenium exerts with regard to reduction and oxidation is mentioned above.) They also reported that cobalt diVuses into the support during the reduction and not during calcination which agrees with our findings. The increased dispersion that they found can account for the fact that the reduced rhenium-promoted catalyst relative to the unpromoted catalyst is consistent with the small particle sizes found in this XAS study.Conclusion XAS shows that high temperature reduction (450 °C) by 100% hydrogen of rhenium-promoted Co3O4/Al2O3 yields sample B which contains highly dispersed cobalt in the form of small (<40 A° ) metal particles (Co–Co distance 2.49±0.01 A° ) together with a minor fraction of cobalt atoms that are randomly spread over the tetrahedral vacancies of the alumina support (Co(II )–O distance 1.92±0.01 A° ).The contribution to the XAS from this phase was isolated by reducing a catalyst in which essentially all of the cobalt entered the alumina support (sample D). These results are consistent with those of Holmen et al.29–31 who found that the metal particles constitute Fig. 7 (Top) k3-Weighted experimental and least-squares fitted 78% of the cobalt content. Also in agreement is the finding EXAFS of sample D adjusted for 0.77% cobalt metal (see main text). that cobalt diVuses into the support during reduction and not (Bottom) The magnitude of the Fourier transform (FT). The solid during calcination and that this phase is resistant to reduction line shows the experimental data and the broken line represents the at 450 °C and is not the spinel CoAl2O4. calculated EXAFS with its corresponding FT using a single Co–O A particularly interesting feature of the XANES of sample shell.The final round of refinement yielded the following: distance B is the 3–4 eV shift of the K-edge to higher energy relative RCoMO=1.963(3) A° , coordination number N=3.2(2), and Debye– Waller-like factor A=2s2=0.0010(9) A° 2, and the refined correction to bulk cobalt metal.This, together with the reduced EXAFS to the threshold energy E0=21.98 eV for an R-factor=54.3%. The amplitudes, is attributed to an enhanced dispersion of cobalt standard deviation in the last significant digit as calculated by in the reduced material relative to the large bulk-like crystallites EXCURV90 is given in parentheses.However, note that such estimates of Co3O4 the calcined product. This has been attributed to of precision (which reflect statistical errors in the fitting) overestimate diminished shielding of the 1s electrons by the valence electrons the accuracy. The estimated errors for distances are 0.01 A° at R<2.5 A° which attends this heightened dispersion.35 The higher disper- with 20% accuracy for N and A, although the accuracy for these is increased by refinements using k1 vs.k3 weighting.44 sion and smaller particle sizes in sample B expresses the 2538 J. Mater. Chem., 1998, 8(11), 2533–253923 C. Bai, S. Soled, K. Dwight and A. Wold, J. Solid State Chem., positive eVect that rhenium-promotion has on the reducibility 1991, 91, 148. of sample A. 24 M. J. Fay, A. Procter, D. P. HoVmann, M. Houalla and No diVerence was observed when graphite was mixed with D. M. Hercules, Appl. Spectrosc., 1992, 46, 345. the catalyst instead of boron nitride. It is therefore clear that 25 E. A. Blekkan, H. Holmen and S. Vada, Acta Chem. Scand., 1993, carbon does not play a role here in transforming the metal 47, 275. 26 M. Shirai, T. Inoue, H. Onishi, K. Asakura and Y. Iwasawa, from the hexagonal or face-centred cubic structures to the J. Catal., 1994, 145, 159. metastable body-centred form (ref. 35 and refs. therein). 27 A. Kogelbauer, J. C.Weber and J. G.Goodwin, Catal. Lett., 1996, 34, 259. 28 P. G. Dimitrova and D. R. Mehandjiev, J. Catal., 1994, 145, 356.Support from the Nansen Foundation, the Norwegian 29 S. Vada, A. HoV, E.A° dnanes, D. Schanke and A. Holmen, Top. Research Council (including a NATO Postdoctoral Fellowship Catal., 1995, 2, 155. to A. Moen) and VISTA-Statoil is much appreciated. The 30 D. Schanke, A. M. Hilmen, E. Bergene, K. Kinnari, E. Rytter, preliminary work (contribution No. 98-9) was carried out at E.A° dnanes and A. Holmen, Catal. Lett., 1995, 34, 269. the Swiss-Norwegian Beamline (SNBL) for which we thank 31 A. M. Hilmen, D. Schanke and A. Holmen, Catal. 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ISSN:0959-9428    

DOI:10.1039/a804261f

出版商:RSC    

年代:1998

数据来源: RSC