A Mechanistic Study of Methanol Decomposition over Cu/SiO_2, ZrO_2/SiO_2, and Cu/ZrO_2/SiO_2

摘要

The interaction of methanol with Cu/SiO_2, ZrO_2/SiO_2, and Cu/ZrO_2/SiO_2 has been investigated by in situ infrared spectroscopy and temeprature programmed desorption and reaction with the aim of understanding the nature of the species and the mechanism involved in methanol decomposition. In the case of Cu/SiO_2, methanol exposure results in the formation of methoxide species on Cu and SiO_2 at 323 K. In the absence of methanol in the gas phase, methoxide species on Cu are dehydrogenated to give formaldehyde starting at ～340 K and are oxidized ot formate species at ～373 K, which then decomposes to CO_2 and H_2 at ～390 K. When methanol is present in the gas phase, methyl formate, CO, and H_2, resulting from the decomposition of methyl formate, are formed in addition to the above reactions. When zrO_2/SiO_2 or Cu/ZrO_2/SiO_2 is expwsed to methanol, the majority ofhte surface species observed are associated with zirconia. Methanol adsorption on either ZrO_2/SiO_2 or CuZrO_2/SiO_2 leads methoxide formation on zirconia and SiO_2 at 323 K. In the presence of Cu, methoxide species on zirconia are dehydrogenated to give formaldehyde starting at ～325 K, which is oxidized to formate species on zirconia at ～373 K, which decompose to CO, CO_2, and H_2 at ～400 K. Methyl formate and dimethyl ether are also observed to form above 460 K. In the absence of Cu, methoxide species on zirconia decompose only very slowly at 523 K. Metallic Cu is proposed to provide sites on which hydrogen atoms formed during the dehydrogenation of species located on zirconia can recombine efficiently and desorb as H_2. Thus, methanol decomposition over Cu/ZrO_2/SiO_2 is envisioned to occur primarily on ZrO_2, with the primary funciton of Cu being the removal of hydrogen.