Tsunehiro Tanaka

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.

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 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.

Reaction mechanism in the photoreduction of CO2 with CH4 over ZrO2

The surface species arising during the photoreduction of carbon dioxide with methane over zirconium oxide is observed by infrared spectroscopy. Two definite species have been expected to exist on the surface during the photoreaction. One has been supposed to be a reaction intermediate and decomposed to CO at around 623 K, and the other has not been decomposed even at 673 K which should be a carbonaceous residue. From the resemblance of the IR spectral features, the latter species is assigned to the surface acetate ion. Several properties of the former species are found to be quite similar to those of the surface formate ion, which is a reaction intermediate of the photoreduction of CO2 by H2 over ZrO2. The former species is therefore assigned to the surface formate, which is also supposed to be the reaction intermediate of the photoreaction between CO2 and CH4. The existence of another carbonaceous residue than the surface acetate is suggested. As no IR bands assigned to the C–H vibration are observed in the spectrum of the carbonaceous residues, the other residue is supposed to be a highly carbonaceous species. The EPR spectrum indicates the photoexcitation of adsorbed CO2 to the CO2− anion radical, and the interaction of the CO2− radical with CH4 in the dark. On the basis of these results, a possible reaction mechanism in this reaction is proposed.

Identification and reactivity of a surface intermediate in the photoreduction of CO2 with H2 over ZrO2

Zirconium oxide is active for photoreduction of gaseous carbon dioxide to carbon monoxide with hydrogen. A stable surface species arises under the photoreduction of CO2 on zirconium oxide and is identified as surface formate by IR spectroscopy. Adsorbed CO2 is converted to formate by photoreaction with hydrogen. The surface formate is a true reaction intermediate since CO is formed by the photoreaction of formate and CO2; surface formate works as a reductant of carbon dioxide to yield carbon monoxide. The dependence on the wavelength of irradiation light shows that bulk ZrO2 is not the photoactive species. When ZrO2 adsorbs CO2 a new band appears in the photoluminescence excitation spectrum. The photoactive species in the reaction between CO2+H2 which yields HCOO- is presumably formed by the adsorption of CO2 on the ZrO2 surface.

Generation of acid sites on silica-supported rare earth oxide catalysts: Structural characterization and catalysis for α-pinene isomerization

Silica-supported rare earth oxide catalysts (Ln/SiO2; Ln=La, Ce, Pr, Sm, Eu, Tb, Yb and Y), loading amounts of which were 3.4 mmol g (support)-1, were characterized by α-pinene isomerization, temperature-programmed desorption (TPD), Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), thermogravimetric-differential thermal analysis (TG-DTA) and Raman spectroscopy. In the lanthanoid series, the catalytic activity increased with atomic number from 57La to 70Yb, except for Ce. All the Ln/SiO2 catalysts, except for Ce, were amorphous. On the surface of the catalyst, Ln–O–Si and Ln–O–Ln linkages formed, the ratio of which varied with the loaded element. The ratio of Ln–O–Si linkage increases with stronger affinity among LnOn units and SiO4 tetrahedra, and the affinity depends on the size of Ln3+. With increasing ratio of Ln–O–Si to Ln–O–Ln linkage, the catalytic activity increases. Silica-supported yttrium oxide catalyst, the trivalent ion radius of which is quite similar to that of ytterbium, exhibited the same activity as that of Yb/SiO2. Raman spectroscopic characterization revealed that excess loading of Yb atoms on SiO2-support block Yb–O–Si linkage to form Yb2O3 fine particles. When Yb/SiO2 was pretreated at 1273 K, fine ytterbium silicate crystallites formed. Ln–O–Si linkage without a long-ranged ordering structure was the active site for α-pinene isomerization. Introduction