Seiichiro Imamura

Effect of cerium on the mobility of oxygen on manganese oxides

Addition of a small amount of cerium (3 and 5 mol.%) remarkably affected the oxidation state of manganese oxide. Manganese oxide without cerium absorbed oxygen in the temperature region from 600 to 900 K to form Mn2O3, while Mn2O3 was not formed in the presence of cerium. In the low temperature region (< 700 K) the valence of Mn was 3.2 in the presence of cerium while it was 2.85 in the absence of cerium. It was concluded that cerium provided oxygen to manganese oxide at low temperature and, on the contrary, withdrew oxygen at high temperatures. This behavior well explained the catalytic action of manganese-cerium composite oxide in the oxidation of CO and in the decomposition of N2O.

State of Ru on CeO2 and its catalytic activity in the wet oxidation of acetic acid

Abstract The relationship between the state of Ru on CeO 2 and catalytic activity in the wet oxidation of acetic acid was investigated for Ru/CeO 2 catalysts prepared by different methods. The temperature programmed reduction (TPR) experiments of Ru/CeO 2 showed that the oxygen species of RuO 2 was reduced at different temperatures depending upon the methods of preparation. Ru species reduced at low temperatures could not be observed by TEM and XRD. It was concluded that RuOCe bonds in the well-dispersed Ru species are highly fragile and its mobile oxygen is the active species in the wet oxidation.

Oxidation of carbon monoxide catalyzed by manganese-silver composite oxides

The oxidation of carbon monoxide on Mn/Ag composite oxides was carried out and their catalytic action was investigated. The composite oxides had much higher activities than either the Mn or the Ag single oxide catalysts. The Ag in the composite catalyst remained in the oxidized state in air even at calcination temperatures as high as 400/sup 0/C, as long as its content was less than 50 mol%, whereas it lost lattice oxygen in the absence of Mn at the same temperature. The TPD and TG analyses showed that the composite oxides had active lattice oxygen belonging to Ag which was easily liberated on heating in the absence of oxygen. It was found, however, that Ag still retained the oxidized states even after the desorption of its active lattice oxygen, but reduction of Mn occurred instead. These facts suggested that the active oxygen on Ag was mainly consumed in the oxidation of CO, and Mn served as an oxygen carrier; i.e., vapor phase oxygen was first incorporated into the composite catalyst through Mn and, then, was transferred to the reduced Ag. This concerted action of Mn and Ag in the composite catalyst provided high activity in the oxidation of CO.

Decomposition of methanol on Pt-loaded ceria

Abstract Methanol was decomposed to carbon monoxide and hydrogen on supported precious metals. Among the precious metals examined the performance of Pt was found to be the best, and ceria was the best support. Pt/CeO2 decomposed methanol completely at 230°C with 99.2% and 94.6% of selectivities to H2 and CO, respectively. TEM, ESCA, and XAFS analyses showed that Pt interacted very strongly with ceria, indicating the possibility of the formation of Pt–O–Ce bond and the penetration of Pt into the bulk ceria. The reaction mechanism was discussed on the basis of the kinetic analysis.

Combustion activity of Ag/CeO2 composite catalyst

Oxidation of methane and carbon monoxide was carried out on silver/ceria composite oxide prepared by co-precipitation, and the behavior of the lattice oxygen of silver was investigated. In the high temperature oxidation of methane, silver(I) oxide decomposed and aggregated to large particles of metallic silver and rapidly deactivated. On the other hand, silver retained high activity during the low temperature oxidation of carbon monoxide. It was found, however, that ceria accelerated the desorption of the lattice oxygen of silver, which was assumed to be related to the well known oxygen storage function of ceria. Although silver tends to be converted more easily to metallic state in the presence of ceria, the function of ceria of sustaining the dispersed state of silver will help to maintain the high activity of the latter in the oxidation of carbon monoxide.

Oxidative decomposition of formaldehyde on silver-cerium composite oxide catalyst

Silver-cerium composite oxide was active for low temperature oxidative decomposition of formaldehyde. Its high activity was due partly to the high dispersion of active silver on the CeO2. The surface oxygen of this composite catalyst was removed more easily than those on single component Ag2O or CeC2, which also seemed to contribute to the high activity of this catalyst. IR analysis revealed that formaldehyde was decomposed both on silver-cerium composite catalyst and CeO2 in the presence of oxygen to produce methoxide, dioxymethylene, and/or polyoxymethylene even at room temperature. In addition bi-carbonate was formed on silver-cerium composite catalyst and formate was produced on CeO2. These intermediates suffered further oxidation at higher temperatures (373 and 423 K) easily on silver-cerium composite catalyst, whereas degradation of them was rather difficult on CeO2.

Effect of the composition of titania–silica on its physical and catalytic properties

The effect of the composition of titania–silica on its physical and catalytic properties has been investigated. There were two types of Ti species: segregated TiO2 and isolated Ti species which interacted with Si—O groups. As the Ti content decreased, the grain size of TiO2 in it decreased and the proportion of the isolated Ti species seemed to increase. The Ti content affected the oxidation–reduction properties of the titania–silica. The electron-transfer steps in the oxidation of CO and to form the 7,7,8,8-tetracyanoquinodimethane radical anion were suppressed on the titania–silica with low Ti content. This phenomenon was explained by a size quantization effect of TiO2. The bandgap between the valence and conduction bands of the TiO2 was widened as its grain size decreased, and its electron-accepting and -donating abilities decreased. Titania–silica with a low Ti content (with small TiO2 grains) had low redox ability. Acid sites were produced by the combination of Ti and Si. The Ti content also affected the strength of these acid sites. It was assumed that titania–silica with high Ti content could easily accommodate electrons donated by bases or proton sources into its large TiO2 grain and exhibited strong acidity. Titania–silica with low Ti content had mainly weak acid sites due to the low electron-withdrawing ability of its small TiO2 grains. It was also speculated that the isolated Ti species decomposed hydrogen peroxide via a non-radical path. Hydrogen peroxide decomposed more effectively on the titania–silica with lower Ti content which had higher proportion of isolated Ti species.

Effect of tetrahedral Ti in titania–silica mixed oxides on epoxidation activity and Lewis acidity

The states of Ti in titania–silica mixed oxides have been studied by varying the Ti content. EXAFS analyses indicated that the amount of tetrahedral Ti species increased with a decrease in the Ti content reaching a maximum at 10 to 20 mol% of Ti while octahedrally coordinated Ti predominated in the high Ti content region (Ti 50 mol%). Tetrahedral Ti species catalyse the eposidation of oct-1-ene and cyclohexene using tert-butyl hydroperoxide as an oxidant. Lewis acid sites of the titania–silica also originated from the tetrahedral Ti species. Both epoxidation activity and Lewis acidity of the titania–silica were well explained by the coordinative unsaturation of the tetrahedral Ti site.

Strong interaction between rhodium and ceria

Rh was loaded on various supports including ceria by a colloid deposition method. The average particle size of the starting Rh colloid was about 50 A. When it was deposited on alumina or zinc oxide, Rh particles were clearly observed by a TEM analysis. However, no TEM image of Rh was obtained on ceria which was prepared by calcination at 550°C. ESCA and XRD analyses revealed that Rh penetrated into the ceria and only a small amount remained on the surface. On the other hand Rh was present as metallic particles on the surface of ceria which was calcined at 950°C. The ceria calcined at a low temperature (550°C) had high surface area; thus, this ceria had high surface energy and could dissolve Rh. Although the amount of Rh exposed on the surface of ceria was small, the Rh in this state was highly active in decomposing methanol.

Decomposition of N2O on Rh/CeO2/ZrO2 composite catalyst

Nitrous oxide (N2O) was decomposed over Rh supported on ceria/zirconia (Ce/Zr) composite oxide, and the effects of the composition of the oxide and of its calcination temperature on the catalytic performance of Rh were investigated. Ceria was fragile against high temperature calcination, while addition of Zr remarkably increased its thermal stability to retain high surface area even at the calcination temperature of 900°C. Rh was supported on these oxides and was calcined at 550°C. The Rh supported on the composite oxide with a Ce/Zr molar ratio of 7/3 which had been calcined at 900°C exhibited the highest activity. TEM and ESCA analyses revealed that the Rh strongly interacted with the oxide, and there was a possibility that a part of Rh even dissolved into its bulk. The Rh exposed to the surface of the composite oxide in a highly dispersed state exhibited the high catalytic activity. However, when the calcination temperature of the composite oxide (Ce/Zr molar ratio of 7/3) was increased to 1200°C, its surface area decreased remarkably and the supported Rh was present in an aggregated state. The Rh in this state had only low catalytic activity despite its high surface concentration.