Formation and decomposition of formate intermediates on supported Cu catalysts exposed to CO–CO2–H2 mixtures: evidence for a precursor state

Abstract

A silica-supported Cu catalyst has been investigated by Fourier-transform infrared spectroscopy in the adsorption of a CO–CO₂–H₂ reaction mixture. The results show that a Cu formate species is not formed at 325 K after exposure to a pulse of the reaction mixture. After the adsorbates have been flushed from the infrared cell, no bands were observed which could be attributed to Cu formate or stable carbonate or bicarbonate-type species. Exposure of the catalyst to the CO–CO₂–H₂ reaction mixture at 425 K resulted in infrared bands characteristic of a bridged Cu formate species. The same species could be produced by heating a sample which had been exposed to the CO–CO₂–H₂ mixture at 325 K. These results are not consistent with previously published work, in which it has been claimed that Cu formate is formed after exposure to CO₂ and H₂ at low temperatures. It is proposed that there is a precursor state on the Cu surface which converts into formate on mild heating. This precursor state does not appear to have any characteristic infrared bands and so its structure is unknown. Possible types of precursor state are considered.