Satohiro Yoshida

X-ray absorption fine structure analysis of local structure of CeO2–ZrO2 mixed oxides with the same composition ratio (Ce/Zr=1)

Abstract Three types of CeO2–ZrO2 (Ce:Zr=1:1 molar ratio) compounds with different oxygen storage/release capacities (OSCs) were characterized by means of the Ce K-edge and Zr K-edge X-ray absorption fine structure (XAFS). In order to investigate the relationship between the OSC and local structure, the quantitative EXAFS curve-fitting analysis was applied. By enhancing the homogeneity of the Ce and Zr atoms in the CeO2–ZrO2 solid solution, the OSC performance increased. Especially, the atomically homogeneous Ce0.5Zr0.5O2 solid solution exhibited the highest OSC among these CeO2–ZrO2 samples. Additionally, the local oxygen environment around Ce and Zr was remarkably modified by enhancing the homogeneity of the CeO2–ZrO2 solid solution. It was postulated that the enhancement of the homogeneity of the CeO2–ZrO2 solid solution and the modification of the oxygen environment would be the source for the OSC improvement.

X-ray absorption (EXAFS/XANES) study of supported vanadium oxide catalysts. Structure of surface vanadium oxide species on silica and γ-alumina at a low level of vanadium loading

X-Ray absorption spectroscopy has been employed to clarify the environmental structure around vanadium atoms in silica-supported and γ-alumina-supported vanadium oxide catalysts. Catalysts containing 2.8% of vanadium by weight were prepared with NH4VO3 and VO(acac)2 as impregnation agents. X.p.s. (V 2p3/2) of the catalysts showed that the vanadium atoms in these catalysts are pentavalent. E.s.r. signals from V4+ in the reduced catalysts indicated that paramagnetic VO4–4 and (VO)2+ ions are generated by the reduction on silica and alumina, respectively. Although the dispersion of vanadium oxide in the catalysts prepared with VO(acac)2 solution was found to be higher than those prepared with NH4VO3, the XANES and EXAFS spectra do not exhibit a significant difference for the two types of the catalyst, indicating that the dominant surface species are the same at such a low level of vanadium loading. Analysis of the XANES spectra suggests that VO4 tetrahedra are the dominant species on alumina and the vanadates on silica are square pyramidal; part of the vanadium species is present as V2O5 microcrystallites. EXAFS spectra of the catalysts and their Fourier-transforms show that VO4 units are isolated on alumina, and that the majority of vanadates on silica are polymeric. These findings by EXAFS/XANES spectroscopy indicate that VO4 on silica and VO5(or VO6) on γ-alumina, as detected by e.s.r., are only a minority species on each support even at a low level of loading of vanadium.

Photo-enhanced reduction of carbon dioxide with hydrogen over Rh/TiO2

Photoreduction of gaseous carbon dioxide to carbon monoxide with hydrogen over Rh-loading TiO2 was investigated. As the reaction proceeded even in the dark but was enhanced under irradiation, the reaction should be considered as “photo-enhanced reaction”. When Rh in Rh/TiO2 was strongly reduced to be in a fully metallic state, the activity was lowered and the main product changed from CO to CH4. A similar effect was obtained by changing the loading amount of Rh: when the loading amount of Rh was raised, the activity was lowered and the main product shifted from CO to CH4. The results of an X-ray absorption spectroscopic analysis showed that the ratio of Rh in a metallic state to that in an oxidized stale increased along with the increase of the amount of Rh loading on Rh/TiO2. The photocatalytic activity was higher as Rh was richer in the component of metallic and oxidized state.

The support effect on propane combustion over platinum catalyst: control of the oxidation-resistance of platinum by the acid strength of support materials

Abstract The support effect on the low temperature propane combustion over supported platinum catalyst was studied by using a series of metal oxides as support materials: MgO, La 2 O 3 , ZrO 2 , Al 2 O 3 , SiO 2 , SiO 2 –Al 2 O 3 , and SO 4 2− –ZrO 2 . The catalytic activity varied with the support materials, and the platinum supported on more acidic support material showed higher activity. The support effect on the oxidation state of platinum was investigated by Pt L II - and L III -edge XANES. In the oxidizing atmosphere, platinum on the acidic support materials was less oxidized than that on the basic one, indicating that the oxidation-resistance of platinum is enhanced with the increase in the acid strength of support materials. This support effect in the oxidizing atmosphere is entirely different from that in the reducing atmosphere; the electron-deficiency of platinum increases with the increase in the acid strength of support materials under the reducing atmosphere. The relation between the catalytic activity and the oxidation state of platinum clearly indicates that the variation of the catalytic activity with the support materials come from the variation of the oxidation state of platinum, and that support materials affect the catalytic activity through the control of the oxidation state of platinum. These results suggest that the acid strength of support materials is an important factor for the design of the active supported platinum catalyst for the reaction under the oxidizing atmosphere.

Analysis of XANES for identification of highly dispersed transition metal oxides on supports

XANES of vanadium and niobium oxide on silica or alumina have been analyzed quantitatively by a deconvolution technique. Based on the results for reference compounds, local structures of supported vanadium and niobium species were identified. The composition was estimated from difference spectra for the samples which consisted of two kinds of species.

Preparation of highly dispersed niobium oxide on silica by equilibrium adsorption method

Abstract Two series of Nb 2 O 5 /SiO 2 catalysts are prepared by an equilibrium adsorption method (type A) and a conventional evaporation to dryness method (type E). The loading amounts of type A catalysts were low ( ~ 0.2 wt% as Nb 2 O 5 ) and those of type E catalysts were varied from 0.1 to 10 wt%. UV/VIS diffuse reflectance spectra of the catalyst samples show that micro particles of Nb 2 O 5 ) are present on the catalysts with high-loading and monomeric or oligomeric niobate species are present on the catalysts with low-loading. By analyzing luminescence spectra of the low-loading samples, we conclude that the type A catalyst contains monomeric NbO 4 tetrahedra and the type E catalyst oligomeric NbO 4 tetrahedra. Propene photo-oxidation on type A catalysts yields propene oxide selectively, formed on monomeric NbO 4 , whereas the photo-oxidation on the type E catalyst with low-loading yields propanal selectively. Propanal is the product in decomposition of propene oxide on oligomeric NbO 4 tetrahedra.

Photoreduction of CO2 with H2 over ZrO2. A study on interaction of hydrogen with photoexcited CO2

By the analysis of the kinetic isotope effect and the effect of reaction temperature, and by EPR, the reaction between hydrogen and photoexcited CO2 species in the photoreduction of CO2 by H2 over ZrO2 was investigated. As the yield of products decreased when D2 was used as a reductant instead of H2, the rate-determining step of the reaction was associated with hydrogen. The reaction was enhanced at elevated temperature, suggesting that the reaction rate was strongly influenced by a thermal process. The EPR study showed the formation of the CO2− anion radical on irradiation of ZrO2 with adsorbed CO2. The CO2− radical had a long life and was stable in the dark for at least 60 min. However, on contact with hydrogen, the CO2− radical was suppressed within 30 min, indicating the reaction of CO2 with hydrogen in the dark. It was concluded that hydrogen was activated in the dark to react with the photoexcited CO2− radical.

Photoreduction of carbon dioxide by hydrogen over magnesium oxide

Magnesium oxide was found to show activity for the reduction of carbon dioxide to carbon monoxide under photoirradiation using hydrogen as a reductant. Fourier transform infrared spectroscopy was applied to a study of the reaction mechanism by detection and identification of the surface species arising during the photoreaction. The formation of surface formate ion was observed during the photoreaction. Since CO was produced from the surface formate in the presence of CO2 under irradiation, the surface formate was a reaction intermediate which acted as a reductant and converted another CO2 molecule to CO. The correlation of the reaction activity with the amount of introduced CO2 indicated that adsorbed carbonate was reduced by H2 to the surface formate, and that the surface formate also reduced the adsorbed carbonate to CO. The IR spectra showed the difference in the adsorption form of CO2 between the adsorbed carbonate reduced by H2 to the surface formate and that reduced by the surface formate to CO.

TiO2/SiO2 photocatalysts at low levels of loading: preparation, structure and photocatalysis

Abstract The effect of precursor on the dispersion and photocatalytic performance of titanium oxide supported on silica has been investigated. The catalysts were prepared by a simple impregnation method with three kinds of titanium complexes of different ligands (dipyvaroylmethanato (DPM), acetylacetonato (acac), isopropoxide (O- i Pr)) abbreviated to D-TS, A-TS, and I-TS. The UV/Vis diffuse reflectance, Raman, XAFS spectroscopic study showed that surface titanate of D-TS is isolated TiO 4 species, and that of I-TS is a mixture of rutile and anatase crystallites. The surface state of A-TS is the intermediate between D-TS and I-TS. TiO 2 /SiO 2 samples were more active than bulk TiO 2 and partial oxidation selectivity of TiO 2 /SiO 2 samples are higher than that of TiO 2 in photo-oxidation of propane. Products distribution over TiO 2 /SiO 2 was drastically varied with the surface titanates. The activity for photo-assisted selective catalytic reduction (SCR) of NO with ammonia was found for D-TS.

Catalysis by amorphous metal alloys. Part 1.—Hydrogenation of olefins over amorphous Ni–P and Ni–B alloys

Amorphous Ni–P (Ni81P19) and Ni–B (Ni62B38) alloys have been prepared by the rapid quenching method. The untreated alloys are inactive for the hydrogenation of olefins, but successive pretreatments of the alloys with the dilute HNO3, oxygen and hydrogen bring about greater catalytic activity than that possessed by the crystalline alloys. Measurements of the ESCA spectra of the surfaces of the alloys have indicated that the alloys become active by the partial oxidation of the nickel and phosphorus or boron on the surface. Buta-1,3-diene is hydrogenated to give isomeric butenes of constant composition, and butenes isomerise rapidly to give a similar composition to that obtained on the hydrogenation of buta-1,3-diene. The hydrogenation of buta-1,3-diene is first and zeroth order with respect to the partial pressures of hydrogen and buta-1,3-diene, respectively. These results are explained by assuming that the hydrogen and the olefins are not competing for adsorption on the same sites and that hydrogen is activated on electron-deficient nickel atoms formed by electron transfer from nickel to the oxidised species.