Wen-Jie Shen

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.

Interaction between palladium and the support in Pd/CeO2 prepared by deposition-precipitation method and the catalytic activity for methanol decomposition

Abstract Adsorption of carbon monoxide on cerium oxide at room temperature can proceed over 3 wt.% Pd/CeO 2 prepared by a deposition–precipitation method. The quantity of carbon monoxide adsorbed is largely excessive to the content of palladium in the sample while the amount of carbon monoxide adsorbed on 3 wt.% Pd/CeO 2 prepared by the conventional impregnation technique is small. The catalyst prepared by deposition–precipitation is highly active for the methanol decomposition to carbon monoxide and hydrogen at 160°C–220°C compared with the impregnated sample. Analyses by XPS show presence of cationic palladium species in the former catalyst after the reaction while only metallic species can be found in the latter, appearing that there is strong interaction between the palladium species and the support in the catalyst prepared by deposition–precipitation.

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.

Low-temperature methanol decomposition to carbon monoxide and hydrogen catalysed over cationic palladium species in Pd/CeO2

Methanol can be selectively decomposed to carbon monoxide and hydrogen at a reaction temperature as low as 433 K over ceria-supported palladium catalysts prepared by deposition–precipitation and by impregnation methods, with the former method resulting in the higher catalytic activity. Cationic palladium species can be present in the catalyst prepared by deposition–precipitation even after reduction with hydrogen at 773 K. On the other hand, metallic palladium is the major species in the impregnated catalyst. A largely excessive amount of carbon monoxide over the palladium content is adsorbed at room temperature on the deposition–precipitation samples when reduced at 573 K, however, this phenomenon does not occur with the impregnation method. This suggests that the cationic palladium species enhance the transfer of carbon monoxide from the palladium sites to the ceria surface at room temperature. The adsorption strength of carbon monoxide on the cationic species is probably weaker than on the metallic surface, this may be advantageous in the methanol decomposition, which is suppressed in the presence of carbon monoxide.

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.

Catalytic Activity of Palladium Supported on Mesoporous Zirconium Oxide in Low-Temperature Methanol Decomposition

Methanol decomposition to hydrogen and carbon monoxide can be effectively catalyzed at 160–220°C over palladium supported on mesoporous zirconium oxide by the deposition–precipitation method. The electronic state and the particle size of palladium in the catalyst are very similar to those of palladium supported on non-porous zirconium oxide, but the palladium surface area of the former sample is smaller than that of the latter. However, the activity of the mesoporous catalyst is significantly higher than that of the non-porous catalyst, implying a promotional effect of the mesoporous structure.

Catalytic methanol decomposition over palladium deposited on mesoporous cerium oxide

Ultrafine palladium particles can be deposited on mesoporous cerium oxide by a deposition–precipitation method. After reduction with hydrogen at 300 °C, palladium on the mesoporous compound is cationic with a valence close to +1 whereas the particles have a metallic structure. The catalytic activity in selective methanol decomposition to hydrogen and carbon monoxide at 180 °C is significantly higher than that of palladium supported on non-porous cerium oxide, suggesting that the mesoporous structure is advantageous to the reaction. When the palladium content is high, part of mesopores are probably choked with large palladium particles, which will cause saturation of the activity.

Methanol Decomposition and Synthesis Over Palladium Catalysts

Palladium is generally active in the catalytic decomposition and synthesis of methanol. The activity of palladium is largely affected by its support. Cerium oxide is an effective support especially when the catalyst is prepared by the coprecipitation or depositionprecipitation method which leads to a strong interaction between palladium and the support. Cationic palladium species formed in the catalyst are probably active species for both decomposition and synthesis while formation of Pd-O-Ce bonding is rather important in the synthesis reaction.

Methanol Synthesis from Carbon Monoxide and Hydrogen over Ceria-Supported Copper Catalyst Prepared by a Coprecipitation Method

High catalytic activity in the synthesis of methanol from carbon monoxide and hydrogen can be produced with ceria-supported copper catalysts prepared by a coprecipitation method. The activity at 468 K is comparable with that produced with commercial copper-zinc catalysts at 503-523 K, while it is still unstable. Although the reaction atmosphere is reductive, metallic copper particles on cerium oxide are oxidized during the reaction and the catalyst is activated. Hence, formation of the copper oxide species is indispensable for the appearance of the high catalytic activity.

A Comparative Study of Palladium and Copper Catalysts in Methanol Synthesis

Catalytic activity of copper supported on cerium oxide (Cu/CeO2) in methanol synthesis from carbon monoxide and hydrogen at 473 K is similar to that of ceria-supported palladium (Pd/CeO2). Both catalysts contain 0.3 mmol g-1 of the active metals and the activities on a mole basis of the active metals are significantly higher than that of a commercial copper catalyst. Analyses bt EXAFS suggest that the particle size of copper in Cu/CeO2 is similar to that of palladium in Pd/CeO2. The activity of copper supported on silica is very low even at 523 K although the particle size of copper is close to that in Cu/CeO2. Hence, cerium oxide promotes the activity of copper as can be seen in Pd/CeO2.