Adsorption Studies on Clean and Sulphur-Poisoned Supported Copper Catalysts

[摘要] The adsorption of methanol, carbon monoxide, and carbon dioxide has been studied in a pulse flow system on "clean" and hydrogen sulphide poisoned alumina-supported copper, a commercial copper/zinc oxide/alumina catalyst, zinc oxide, and alumina surfaces. Adsorption of hydrogen sulphide was also studied, while the interaction of methanol and carbon monoxide with "clean" surfaces of the above catalysts was investigated by FTIR. The catalytic decomposition of methanol on copper-based catalysts resulted in the formation of carbon monoxide, carbon dioxide and hydrogen. The formation of carbon dioxide was attributed to the interaction of the product carbon monoxide with the unreduced oxide on the catalyst surface. Use of a carbon monoxide flow as a reducing agent to ensure the complete reduction of copper-based catalysts surfaces, showed that carbon monoxide and hydrogen are the major products of the decomposition reaction. The reaction of methanol on zinc oxide also resulted in the formation of carbon monoxide, carbon dioxide and hydrogen. Interaction of carbon monoxide with the oxide surface was responsible for the formation of carbon dioxide, as was evident from the change in the oxide colour from grey to blue during methanol interaction, suggesting a conversion of the oxide to a metallic zinc. At higher reaction temperatures and increased methanol exposure, trace amounts of methane were formed. On an alumina surface, methanol interacts dissociatively to form carbon monoxide and methane as major products, together with small amounts of, carbon dioxide and water. Sulphur was found to have a great influence on the catalytic activity. The presence of small concentrations of H2S on the catalyst surface greatly reduced the catalytic activity towards methanol decomposition, but no change in the catalyst selectivity was observed due to this poisoning effect. 14C-labelled carbon monoxide and carbon dioxide were used to study the extent of the adsorption and the effect of sulphur poisoning on copper-based catalysts. The results showed that carbon monoxide interacts with copper-based catalysts forming a strongly held carbonate species, due to the presence of unreduced oxide. Desorption experiments show two adsorption peaks corresponding to a relatively weakly adsorbed and strongly held species. No significant carbon monoxide adsorption was observed on the oxide surfaces. C-carbon dioxide interacts with the catalyst surfaces in a similar way to that of carbon monoxide; significant adsorption occurs on the oxide surfaces. Preadsorbed sulphur reduces the extent of the adsorption of both carbon monoxide and carbon dioxide, by weakening the adsorbate-metal bond, as indicated by the shift of the desorption peaks to lower temperatures. 35S-H2S strongly adsorbs on all surfaces. The adsorption is irreversible on Cu/Al2O3, Cu-ZnO/Al2O3 and ZnO, with zinc oxide bulk zinc sulphide is formed. Trace amounts of water are found to increase the ability of the oxide to adsorb H2S. FTIR investigations of the interaction of methanol shows that, on copper catalysts, methoxy species are formed, while on zinc oxide the methoxy species decompose to form a carbonato species as well as carbon monoxide and carbon dioxide. On alumina bands due to methoxy, formate, CO, CO2 and water are observed. The methoxy species decomposes to a formate species upon being heated.

[发布机构] University:University of Glasgow

[关键词] Inorganic chemistry