High-Pressure CO Adsorption on Cu-Based Catalysts: Zn-Induced Formation of Strongly Bound CO Monitored by ATR-IR Spectroscopy

CO adsorption at 1 MPa on Cu-Zn stearate colloids and supported Cu catalysts was studied in situ by attenuated total reflection infrared (ATR-IR) spectroscopy. Subsequent to thorough reduction by H-2, the IR band at 2110-2070 cm(-1) due to linearly adsorbed CO on dean metallic Cu was always observed, initially on all Cu catalysts. During the exposure of Zn-containing samples to CO at high pressure, a new IR band at ca. 1975 cm(-1) appeared in addition and increased in intensity even at room temperature. The detailed analysis of the IR, spectra showed that the new IR band at ca. 1975 cm-1 was not related to coadsorbed carbonate/formate-like species, but to the content of Zn in the samples. This IR band was found to be more stable than that at 2110-2070 cm(-1) during purging with inert gas. It disappeared quickly in synthetic air, pointing to a strongly reduced state of the Zn-containing Cu catalysts achieved during high-pressure CO exposure. It is suggested that CO can reduce ZnO to Zn in the presence of Cu, resulting in the formation of a CuZnx surface alloy. As the CO species with the characteristic IR band at ca. 1975 cm(-1) binds more strongly to this CuZnx alloy than the linearly adsorbed CO to pure Cu, it is suggested to be adsorbed on a bridge site.