Mitsutaka Okumura

Chemical vapor deposition of gold on Al2O3, SiO2, and TiO2 for the oxidation of CO and of H2

In order to clarify the effect of metal oxide support on the catalytic activity of gold for CO oxidation, gold has been deposited on SiO2 with high dispersion by chemical vapor deposition (CVD) of an organo-gold complex. Comparison of Au/SiO2 with Au/Al2O3 and Au/TiO2, which were prepared by both CVD and liquid phase methods, showed that there were no appreciable differences in their catalytic activities as far as gold is deposited as nanoparticles with strong interaction. The perimeter interface around gold particles in contact with the metal oxide supports appears to be essential for the genesis of high catalytic activities at low temperatures.

Hydrogenation of 1,3-butadiene and of crotonaldehyde over highly dispersed Au catalysts

When Au is deposited as nano-particles on select metal oxides, it exhibits surprisingly high catalytic activity for many oxidation reactions. Therefore, there is also the possibility to improve the activities of Au catalysts for hydrogenation using the appropriate preparation methods like the gas-phase grafting method (GG) and the deposition precipitation method (DP). In this work, we investigated the hydrogenation of 1,3-butadiene and of crotonaldehyde over Au catalysts prepared by GG and DP and discussed the structure sensitivity of these reactions. From these experiments, it was found that the catalytic activities for the hydrogenation of 1,3-butadiene over Au catalysts was almost structure insensitive in terms of the size effect of Au particles and the influence of metal oxides supports and the crotonaldehyde hydrogenation over Au catalysts was slightly sensitive to the selection of the support in the view point of the product selectivity.

The influence of the support on the activity and selectivity of Pd in CO hydrogenation

Abstract Catalytic hydrogenation of carbon monoxide was studied over Pd supported on Al2O3, SiO2, TiO2, and ZrO2. Both the activity and the selectivity of the catalysts were strongly affected by the nature of the support. As a major product, Pd/Al2O3 produces dimethyl ether, Pd/SiO2 favors methanol formation, Pd/TiO2 produces CH4, and Pd/ZrO2 yields methanol. Higher CO conversions over Pd/ZrO2 and Pd/TiO2 were ascribed to the presence of cationic palladium species formed through the metal–support interaction. Significant dimethyl ether formation over Pd/Al2O3 was attributed to the acidity of the support metal oxide. The catalytic natures of Pd catalysts were discussed based on the results of catalyst characterization by XPS, TEM, and temperature-programmed reduction and desorption.

The reactivities of dimethylgold(III)β-diketone on the surface of TiO2: A novel preparation method for Au catalysts

Abstract A preliminary study has been made on the reactivities of dimethylgold(III)β-diketone with a solid surface using two types of TiO 2 powder, anatase and amorphous, in order to develop a novel gas-phase method for the preparation of highly dispersed gold catalysts. The organogold compound was adsorbed at 33 °C on the surface of TiO 2 that had been pretreated under a variety of conditions and was decomposed into metallic gold particles by calcination in air at different temperatures. The despersion of gold particles and the catalytic activity of Au/TiO 2 samples for CO oxidation were as high as those obtained by the liquid-phase deposition-precipitation method. Moreover, the chemical vapor deposition method was found to be potentially more advantageous in depositing gold particles that are smaller than 2 nm in diameter.

DFT studies of interaction between O2 and Au clusters. The role of anionic surface Au atoms on Au clusters for catalyzed oxygenation

Abstract Density functional and ab initio molecular orbital (MO) calculations have been carried out for the AuO, AuO2, AuO2−, and the Au13O2 clusters. The results show that the surface atoms of the icosahedral Au13 cluster are negatively charged and the anionic Au atoms in the cluster have a stronger interaction with O2 than a neutral Au atom. These findings suggest that the negatively charged surface Au atoms on the surface of the Au clusters are the active sites for oxygenation on the supported and unsupported Au catalysts.

Epoxidation of propylene over gold catalysts supported on non-porous silica

Abstract Non-porous silica was used as a carrier of Au/Ti-SiO2 catalysts for the direct vapor-phase epoxidation of propylene in the copresence of H2 and O2, because the smooth surfaces were expected to facilitate the desorption of propylene oxide (PO). Their catalytic performances were inferior to those of Au/TS-1 and Au/Ti-MCM catalysts, however, an interesting feature was that propylene epoxidation over Au/Ti-SiO2 appreciably depended on calcination temperature of the Ti-SiO2 support. Higher calcination temperature benefits PO production at reaction temperatures above 100°C. IR and HRTEM analyses showed that the interaction between titanium oxides and silica was intensified to form more isolated TiO4 units with tetrahedral coordination on the surfaces of Ti-SiO2 supports with an increase in heat treatment temperature. These isolated TiO4 sites incorporated in silica surface layers are assumed to be an important structural factor for the selective deposition of Au particles and the selective PO formation.

Methanol synthesis from carbon monoxide and hydrogen catalyzed over Pd/CeO2 prepared by the deposition–precipitation method

Ceria‐supported palladium catalysts prepared by the deposition–precipitation method are highly active for the methanol synthesis from carbon monoxide and hydrogen in comparison with the catalyst prepared by the conventional impregnation method. Analyses by EXAFS show that palladium particles can be dispersed very well on the surface of ceria by both the methods, implying that the higher activity of the catalysts prepared by deposition–precipitation is not simply due to the particle size of palladium. Cationic palladium species are present in the samples prepared by deposition–precipitation after reduction with hydrogen at 300 °C, suggesting that the active species are produced by strong contact between palladium particles and the support.

Influence of palladium precursors on methanol synthesis from CO hydrogenation over Pd/CeO2 catalysts prepared by deposition–precipitation method

Abstract The impact of palladium precursors on the catalytic behavior of Pd/CeO 2 catalysts was studied for methanol synthesis through hydrogenation of carbon monoxide. Catalysts prepared from palladium chloride and palladium acetate showed much higher overall catalytic activities than those prepared using palladium nitrate. Palladium precursors did not have a significant influence on the distribution of products, very similar selectivities of methanol and methane were obtained. The palladium precursor, or more specifically its anion, had an effect on the final palladium particles, and therefore affected the interaction between Pd and ceria, which would cause different reaction results. The catalytic deactivation with time-on-stream was interpreted in terms of the growth of Pd particles under reaction conditions.

Vapor-phase epoxidation of propene using H2 and O2 over Au/Ti-MCM-41 and Au/Ti-MCM-48

Vapor-phase epoxidation of propene using H2 and O2 over homogeneously dispersed gold particles deposited by deposition-precipitation (DP) on the supports Ti-MCM-41 (hexagonal) and Ti-MCM-48 (cubic) is studied at a space velocity of 4000 h−1. cm3/g(cat.) and 100°C or 150°C. Better performance of Ti-MCM-48 over Ti-MCM-41 in the propene epoxidation reaction was observed. Influence of the various parameters were investigated: in the case of Ti-MCM-48 support, Si/Ti ratio, precipitating agent for Au deposition, Au loading, calcination temperature and in the case of Au/Ti-MCM-41 the influence of CsCl addition as a promoter. GC-MS investigation of the desorbed species from the used catalyst revealed the presence of acidic as well as oligomeric species accumulated on the catalyst surface responsible for the catalyst deactivation. Based on the above experimental results, a probable reaction mechanism is explained.

Deposition of gold nanoparticles on silica by CVD of gold acethylacetonate

Although Au/SiO2 catalysts prepared by liquid phase methods are poorly active for the oxidation of CO and of H2, the catalysts prepared by chemical vapor deposition of organo gold complex exhibit remarkably enhanced catalytic activities. In order to investigate the surface processes occurring during the adsorption and decomposition of dimethyl gold acetyl acetonate [Me2Au(acac)] on the surface of SiO2, FT-IR, TG-TDA analyses, and the catalytic activity measurements for CO and H2 oxidation were carried out. Ab initio DFT (Density Functional Theory) calculations were also carried out to estimate the adsorption state of the gold precursor. The results of the above experiments indicated that the gold precursor was adsorbed weakly through the interaction of the electron-rich oxygen in acetylacetonate with the OH group on the surface of SiO2 and was decomposed into metallic particles during the following calcination process.