Yoshiteru Yazawa

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.

The support effect on platinum catalyst under oxidizing atmosphere: improvement in the oxidation-resistance of platinum by the electrophilic property of support materials

Abstract The support effect on platinum catalyst for low temperature propane combustion was investigated by using a series of platinum catalysts with different acid strengths support materials and different platinum dispersions. The catalytic activity of platinum catalyst varied with both the acid strength of support materials and the platinum dispersion. On each support material, the turnover frequency increased with the decrease in platinum dispersion. This variation in the turnover frequency is attributed to the variation in the oxidation state of platinum, since it was clarified by Pt L II and L III -edge XAFS that platinum with small dispersion is less oxidized than that with large dispersion. The turnover frequency also varied with the acid strength of support materials; the turnover frequency increased with the increase in the acid strength of support materials at the same dispersion. The Pt L II and L III -edge XAFS investigation clearly indicated that platinum on the acidic support material is less oxidized than that on basic material. Thus, it is clearly shown that the support materials affect the catalytic activity of platinum through the control of the oxidation-resistance of platinum by its electrophilic/electrophobic property, demonstrating that the electrophilic/electrophobic property of support materials is an important factor for the design of platinum catalyst under the oxidizing atmosphere.

The additive effect on propane combustion over platinum catalyst: control of the oxidation-resistance of platinum by the electronegativity of additives

Abstract The additive effect on the low temperature propane combustion over the supported platinum catalyst was investigated by using Pt/Al 2 O 3 with a series of additives. The catalytic activity of platinum catalyst drastically varied with the kind of additives, and increased with the increase in the electronegativity of additives. The additive with large electronegativity enhanced the catalytic activity compared with the original Pt/Al 2 O 3 and vice versa. The additive effect on the oxidation state of platinum was investigated by Pt L II - and L III -edge XANES. In the oxidizing atmosphere, the oxidation state of platinum varied with the electronegativity of additives to a more or less oxidized state compared with original Pt/Al 2 O 3 : the oxidation of platinum was more depressed by the additives with larger electronegativity, indicating that the oxidation-resistance of platinum is more enhanced with the increase in the electronegativity of additives. Although the electron deficiency of platinum increases with the increase in electronegativity of the additives in the reducing atmosphere, the additive effect in the oxidizing atmosphere is entirely different from that in the reducing one. The relation between the catalytic activity of platinum and its oxidation state indicates that the variation in the catalytic activity comes from the variation in the oxidation state, and that the additives affect the catalytic activity through the control of oxidation state of platinum. This additive effect is similar to the support effect on platinum catalyst for propane combustion found in the previous studies; platinum on more acidic support materials has higher oxidation-resistance, and the catalytic activity of supported platinum can be improved through the control of the oxidation state of platinum by support materials. These results clearly reveal that the electrophilic/electrophobic properties of the support materials and additives is one of the key factors for the control of the activity of platinum catalyst used under the oxidizing atmosphere.

Kinetic study of support effect in the propane combustion over platinum catalyst

The support effect on the low-temperature propane combustion over platinum catalysts was investigated by kinetic study. The catalytic activity of supported platinum catalysts varied with the support material, and Pt/SiO2–Al2O3 showed much higher activity than Pt/ZrO2, as already reported. The reaction order for oxygen was negative and that for propane was positive. The reaction order for propane and oxygen also greatly depended on the support material: Pt/ZrO2 gave anomalous reaction orders, i.e., −2.9 for oxygen and 3.4 for propane. Further, oxidized Pt/ZrO2 showed a long-term change of the catalytic activity with time-on-stream, compared with oxidized Pt/SiO2–Al2O3. From these results, it was concluded that high catalytic activity of platinum on acidic support is attributed to high ability to maintain the metallic state of platinum with high oxidation-resistance and high reducibility of platinum oxide.

Pd–Pt bimetallic catalyst supported on SAPO‐5 for catalytic combustion of diluted methane in the presence of water vapor

The examination of the combustion of a trace amount of methane over Pd‐ion‐exchanged silicoaluminophosphate‐5 (Pd‐SAPO‐5) at low temperature in the presence of water vapor reveals that water vapor strongly depresses methane combustion and causes a significant decrease in catalytic activity with time. The newly prepared bimetallic catalyst Pd–Pt‐SAPO‐5, on the other hand, shows higher performance for methane combustion under similar conditions.

XANES study of the support effect on the state of platinum catalysts

Pt I~u and Ln-edges XANES clarified the oxidation state of platinum catalysts supported on various metal oxides. The acidbase property seemed one of the most dominant factors to control the electron state of platinum. On the reduced samples, there was a tendency that acidic support reduced the 5d-electron density of platinum, while the oxidized samples on acidic support contained more metallic platinum. The latter tendency agreed well with high activity of platinum catalysts on acidic supports toward propane combustion in oxidizing condition, and it was explained by a hypothetical mechanism for prevention of oxidation of platinum by acidic supports.

Acid strength of support materials as a factor controlling catalytic activity of noble metal catalysts for catalytic combustion

The support effect on the low temperature catalytic combustion of propane over noble metal catalysts was studied by using a series of metal oxides as support materials. The catalytic activity varied with the acid strength of the support materials; The acidic oxide is superior as the support for platinum catalyst, while palladium on the support with the moderate acid strength showed the highest catalytic activity. This difference in the support effect on the activity between platinum and palladium catalysts arises from the difference in the oxidation state which shows the highest activity, though the support effect on the oxidation state is common to both metals: Both platinum and palladium were prevented from the oxidation on the acidic support, and the highest activity was observed on metallic state for platinum and partially oxidized state for palladium. This investigation proposes that the acid strength of support materials is an effective parameter for the design of the active noble metal catalyst for the catalysis in the oxidizing atmosphere.

XAFS study of the additive cation effect on the oxidation-resistance of platinum catalyst

The additive cation effect on the oxidation-resistance of platinum supported on alumina was investigated by Pt L(III) and L(II)-edge XAFS. The white line intensity at Pt L(III)-edge of oxidized catalysts decreased with an increase in the electronegativity of the additive cation in the same way as the support effect previously reported: The addition of electrophilic cations make the supported platinum less oxidized state compared with the original Pt/Al2O3 and vice versa. Thus, it was revealed that the addition of more electrophilic cations provides the higher oxidation-resistance to platinum catalyst, even when platinum is supported on basic oxides.

XANES and XPS Analyses of Silicon Irradiated by Deuterium Ions

XANES and X-ray photoelectron spectra (XPS) using synchrotron radiation as a X-ray source were applied for analysis of silicon crystalline irradiated by 5 keV deuterium ions. Si K-edge XANES spectra recorded either by a photocurrent or electron yield modes clearly showed a new absorption peak in addition to the absorption due to surface SiO2. In Si Is XPS spectra, on the contrary, no clear peak or shoulder was detected except SiO2 species. Taking into account that Si K-edge XANES reflects the electron transition from Si 1s to np (empty bound state), the new absorption peak is very likely caused by the generation of localized electronic bond between Si and D in the vicinity of the damaged region in the irradiated Si.