Shin-ichi Komai

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 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.

Solid superacid as the support of a platinum catalyst for low-temperature catalytic combustion

Abstract The effect of the acidic property of support materials on the activity of a platinum catalyst in the low-temperature catalytic combustion of propane was examined by using a series of single metal oxides, mixed oxides and SO 2− 4 -doped oxides as the support materials. It was revealed that the catalytic activity strongly depends on the acidic property of the support materials: the activity is higher on the support having stronger acidity. SO 2− 4 -doped oxides, known as solid superacids, are superior support materials giving high activity at low temperatures.

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.

Activity of palladium loaded on zeolites in the combustion of methane

Activity of Pd loaded on zeolite for the methane combustion was examined using various kinds of mordenite and ZSM-5 zeolites with different compositions of aluminum and cation. The activity strongly depended on the composition as well as on the structure of zeolites. Among tested catalysts, the highest and durable activity was found on the catalyst which was prepared from an ion exchange with palladium amine complex on the H-type siliceous mordenite such as Pd/H-mordenite (Si/Al2=145). Reaction orders with respect to the partial pressures of methane and oxygen indicated that the oxidized form of palladium was the active species. Characterization using TEM and XPS showed the high dispersion of palladium in the framework of zeolite. TPR using methane as a reductant showed that the high activity of Pd on the siliceous H-mordenite could be explained qualitatively and quantitatively; i.e., Pd was affected by the qualitative alteration caused by the induced effects by Al and Na cations, whereas the number of active oxygens on Pd was quantitatively enhanced in the zeolite, particularly mordenite. The micropores of zeolite may have a role in preventing the sintering into large particles to stabilize the activity.

Catalyst effectiveness factor of cobalt-exchanged mordenites for the selective catalytic reduction of NO with hydrocarbons

The influence of intracrystalline diffusion on the selective catalytic reduction of NO was examined by using cobalt-exchanged mordenites with different crystal sizes. In the NO-CH4-O2 reaction, the reaction rate increased with cobalt exchange level, but the rate over a large crystal mordenite was slightly lower than that over a small crystal mordenite. In the NO-C3H8-O2 reaction, where the reaction rate was higher than that of the NO-CH4-O2 reaction, the effect of crystal size was more significant. Furthermore, the reaction rate over the small crystal increased with increasing cobalt exchange level, whereas the rate over the large crystal decreased with increasing cobalt exchange level. The effectiveness factor calculated from these results was below unity, indicating that the volume of zeolite crystal was not fully utilized because of the diffusion limitations.

Structure and catalytic properties of Ga-MFI in propane aromatization

Abstract The structure–activity relationship in the dehydrogenation steps in propane-aromatization was examined by using MFI-type gallosilicate catalysts containing various amounts of gallium and calcined at various temperatures. The characterization by TPD of ammonia and TEM indicated that, with increasing gallium content, both the zeolitic acidity and the extraframework Ga 2 O 3 increased, and that, with increasing calcination temperature, the number and the particle size of extraframework Ga 2 O 3 species increased while the strong acidity decreased. The reactions of propane and cyclohexane were conducted to evaluate the catalytic activity, especially in the dehydrogenation steps in propane-aromatization, and it was found that the activity was strongly influenced by the calcination temperature. In the case of high gallium content, the conversions of propane and cyclohexane decreased with the calcination temperature, while in the case of low gallium content the conversions increased. The former result was attributed to the decrease in the acid amount due to the extraction of gallium from the MFI-framework, and the latter result to the increase in the number and the size of extraframework Ga 2 O 3 particles.

COH2 titration for the determination of super small metal surface area

A novel method is proposed for the determination of super small metal surface areas of supported metal catalysts. The method gives the amount of adsorbed CO by detecting CH4 formed during the titration of preadsorbed CO with H2 at elevated temperatures. The method was verified by using 0.1% and 0.5% PtAl2O3 catalysts which could be characterized by other methods. It was shown that the experimental procedure satisfies the assumptions on which the method is based and that the dispersions of both catalysts determined by the proposed method agree well with those obtained by other methods. Then the method was applied to PtAl2O3 catalysts with very low Ft content (to 0.0007%, 7 ppm), and the optimum experimental condition was established for super low metal content. The dispersions thus determined were reasonable, in the range from 80 to 100%, supporting the validity of the method.

Structure-sensitive reaction over calcium oxide — decomposition of nitrous oxide

Decomposition of nitrous oxide (N2O) was examined by using CaO catalysts having various surface areas, and structure sensitivity in the title reaction was discussed. The reaction rate of N2O rapidly decreased with time-on-stream and then became almost constant. Temperature programmed desorption (TPD) after the reaction showed that produced oxygen is irreversibly adsorbed on CaO surface as a poisoning molecule. Photoluminescence spectra suggested that highly unsaturated sites were readily poisoned with adsorbed oxygen. The strong dependence of the initial activity and steady state activity on CaO surface area was observed, i.e., the reaction rate per surface area in the steady state increased linearly with the surface area, but that at the initial state depended more strongly on the surface area. Thus, it was found that N2O decomposition over CaO is a structure-sensitive reaction demanding coordinately unsaturated sites. The highly unsaturated sites are extremely active for this reaction, but readily poisoned with oxygen. On the other hand, moderately and poorly unsaturated sites are responsible for the continuous catalytic activity of N2O decomposition. The structure-sensitive dependence was rationalized by assuming the number of unsaturated sites having different coordination number, such as plane, edge, and corner sites, on geometric model of CaO crystallites.

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.