Susumu Tsubota

Photoassisted hydrogen production from a water-ethanol solution: a comparison of activities of AuTiO2 and PtTiO2

Abstract A comparison of the photocatalytic activity for H 2 generation between AuTiO 2 and PtTiO 2 has been made. The deposition of Au and Pt was carried out using TiO 2 powders in aqueous suspensions containing HAuCl 4 ·4H 2 O or H 2 PtCl 6 ·6H 2 O by deposition-precipitation (DP), impregnation (IMP), photodeposition (FD) and, in the case of Au, by mixing TiO 2 with colloidal gold suspensions (MIX). The main reaction products obtained from the irradiation of an aqueous 5 M C 2 H 5 OH suspension containing AuTiO 2 or PtTiO 2 were hydrogen, methane, carbon dioxide and acetaldehyde. Small amounts of acetic acid were also detected. The overall activity of Au samples was generally about 30% lower than that of Pt samples. The activity of Au samples strongly depended on the method of preparation and decreased in the order AuTiO 2 -FD>AuTiO 2 -DP>AuTiO 2 -IMP>AuTiO 2 -MIX. The activities of the platinum samples were less sensitive to the preparation method and decreased in the order PtTiO 2 -FD>PtTiO 2 -DP≈PtTiO 2 -IMP. Gold and platinum precursors calcined in air at 573 K showed the highest activity towards H 2 generation, followed by a decline in activity with increasing calcination temperature. The H 2 yield was found to be dependent on the metal content on TiO 2 and showed a maximum in the ranges 0.3–1 wt.% Pt and 1–2 wt.% Au. The exposed surface area of gold had only a small influence on the rate of hydrogen generation. On the other hand, the rate of H 2 production was strongly dependent on the initial pH of the suspension. pH values in the range 4–7 gave better yields, whereas highly acidic and basic suspensions resulted in a considerable decrease in the H 2 yield.

The influence of the preparation methods on the catalytic activity of platinum and gold supported on TiO2 for CO oxidation

The influence of the preparation methods on the catalytic activity for CO oxidation was markedly large for Au-TiO2 and negligible for Pt-TiO2 catalysts. Platinum and gold were deposited on TiO2 by deposition-precipitation (DP), photodeposition (FD) and impregnation (IMP). The DP method gave the most active catalysts for both Pt and Au. Gold catalysts prepared by DP were active at temperatures below 273 K and showed a much greater activity than Pt catalysts.

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.

Effect of calcination temperature on the catalytic activity of Au colloids mechanically mixed with TiO2 powder for CO oxidation

In order to elucidate the role of the contact structure between gold and metal oxide support in low-temperature CO oxidation, a mechanical mixture of colloidal gold with TiO2 powder was prepared and calcined at different temperatures. The sample calcined at 473 K, which is composed of spherical gold particles with a mean diameter of 5.1 nm and TiO2 powder, is poorly active for CO oxidation at temperatures up to 473 K. The catalytic activity appreciably increases with an increase in calcination temperature up to 873 K even though gold particles grow to larger ones, reaching a level with almost the same turnover frequency as that of Au/TiO2 prepared by a deposition–precipitation method.

Catalysis by Gold Nanoparticles: Epoxidation of Propene

Supported nano-gold catalysts have attracted rapidly growing interest due to their potential applicability to various reactions of industrial and environmental significance. In this article, we focus on the advances related to the direct vapor-phase oxidation of propene to propene oxide in the presence of molecular oxygen and hydrogen over gold catalysts supported on Ti-incorporated silica materials prepared by different methods. The importance of catalyst preparation and pretreatment method, nature of support material, Au particle size and loading amount is emphasized. Possibilities to enhance the catalyst performance by using promoters and by silylation are also discussed.

Low-temperature catalytic combustion of methanol and its decomposed derivatives over supported gold catalysts

Abstract Gold can be compared favorably with Pd and Pt in the catalytic combustion of CH 3 OH, HCHO and HCOOH when it is deposited on some reducible metal oxides (α-Fe 2 O 3 , TiO 2 , etc.). While the supported gold catalysts are less active in H 2 oxidation, they exhibit much higher activities in CO oxidation. For Au/TiO 2 , the effect of catalyst preparation was further investigated. Since the activity for CO oxidation of the gold catalysts is not depressed but enhanced by moisture, they are practically applicable to CO removal from air at room temperature. Gold supported on manganese oxide is especially effective in the selective CO removal from hydrogen, indicating its potential applicability to polymer electrolyte fuel cells using the reformed gas of methanol.

Preparation of nanometer gold strongly interacted with TiO2 and the structure sensitivity in low-temperature oxidation of CO

Abstract Gold can be deposited on TiO2 as hemispherical fine particles with diameters smallerthan 4nm by deposition precipitation. Gold hydroxide precipitates with high and homogeneous dispersion on TiO2, most probably on specific surface sites, in the pH range 6 to 10. The calcination of TiO2 with Au(OH)3 in reducing gas atmospheres, such as H2 and CO led to smaller gold particles than in air. In the case of Au/TiO2 samples prepared by deposition precipitation, the catalytic activity for low-temperature CO oxidation was very high and could be observed even at temperatures below 0°C. Physically mixed Au/TiO2 samples though of much lower catalytic activity, showed gradual improvement with increasing calcination temperature. An increase in calcination temperature not only caused particle coagulation but also brought about a stronger interaction with the TiO2 support. The above results therefore indicate that the catalytic activity of Au/TiO2 is sensitive to the structure of the perimeter interface between Au and TiO2.

Selective oxidation of propylene over gold deposited on titanium-based oxides

Gold supported on titanium-based metal oxides can assist the selective partial oxidation of propylene at temperatures from 313 K to 573 K in a gas containing both H2 and O2. The preparation method was found to be crucial in controlling the selectivities. In general, impregnation and chemical vapor deposition methods do not produce selective catalysts. Only the deposition-precipitation method makes gold selective to propylene oxide or propanal, suggesting that a strong contact between the gold particles and the titanium ion sites on the support is important. The effect of changing the support was also dramatic; the use of the anatase form of TiO2 and Ti-MCM-41 results in propylene oxide production, while the rutile structure of TiO2 caused complete oxidation to CO2. Microporous crystalline titanium silicates such as TS-1, TS-2, and Ti-β zeolite make gold relatively selective to propanal and of the three TS-1 gives the highest selectivity. These results indicate that the oxidation of propylene in the copresence of H2 must involve the surface of the supports and that the reaction takes place at the interface perimeter around the gold particles.

Adsorption of CO on gold supported on TiO2

Abstract The adsorption of CO on TiO2 supported gold has been investigated both under a constant pressure static system and under a closed recirculation system with liquid nitrogen cooled trap. Au TiO 2 with 3.3 wt% loading and 3.5 nm mean particle diameter of Au was prepared by deposition-precipitation. Adsorption of CO on Au TiO 2 was 90% reversible and satisfied the Langmuir isotherm. The amount of CO2 produced during CO adsorption agreed well with the amount of irreversible CO adsorption. TiO2 powder produced only 1/185th the amount of CO2 at 273 K compared to gold powder, even though the amount of CO adsorbed per unit surface area was similar to that of gold powder. The results indicate that a reaction between oxygen adsorbed on the surface of small gold particles with CO is one of the major reaction pathways in forming CO2.

Hydrogenation of CO2 over gold supported on metal oxides

Abstract Gold highly dispersed on a variety of metal oxides were prepared by coprecipitation and deposition-precipitation methods. The hydrogenation of CO 2 on supported gold catalysts was investigated at temperatures between 150 and 400° C and a pressure of 8 atm. The methanoi yields reached a maximum at temperatures between 200 and 300 ° C, depending on the support oxides. The highest yield and selectivity towards methanol was obtained on Au/ZnO at 250°C. At 200°C Au/Fe 2 O 3 was the most active for methanol synthesis, exhibiting activity almost comparable to that of the conventional Cu/ZnO catalyst with the same metal content. Gold supported on TiO 2 was so active in reducing CO 2 to CO that the conversion was close to equilibrium. Over Au/ZnO as well as over Cu/ZnO, CO 2 could be hydrogenated to methanol at lower temperatures than CO.