ABSTRACT
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 clean 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 CuZn(x) surface alloy. As the CO species with the characteristic IR band at ca. 1975 cm(-1) binds more strongly to this CuZn(x) alloy than the linearly adsorbed CO to pure Cu, it is suggested to be adsorbed on a bridge site.
ABSTRACT
The adsorption of methanol on pure ZnO and Au-decorated ZnO nanoparticles and its thermal decomposition monitored by temperature-programmed desorption (TPD) experiments and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), both applied under continuous flow conditions in fixed bed reactors, is reported. Two distinguishable methoxy species are formed during methanol adsorption on ZnO differing in the C-O stretching bands. During the subsequent TPD experiments two different H(2) peaks are observed, indicating the conversion of methoxy into formate species. By applying different heating rates, activation energies of 109 kJ mol(-1) and 127 kJ mol(-1) for the selective oxidation of the two methoxy species are derived. Correspondingly, the methoxy decomposition results in two distinguishable formate species, which are identified by the asymmetric and symmetric OCO stretching bands on pure ZnO and Au/ZnO. Based on the decreased intensities of the OH bands during methanol adsorption, which are specific for the various ZnO single crystal surfaces, on the different reactivities of these surfaces, and on the formate FTIR bands observed on ZnO single crystal surfaces, the two methoxy and the corresponding formate species are identified to be adsorbed on the exposed less reactive non-polar ZnO(10 10) surface and on the highly reactive polar ZnO(000 1) surface. The simultaneous formation of H(2), CO, and CO(2) at about 550-600 K during the TPD experiments indicate the decomposition of adsorbed formate species. The CO/CO(2) ratio decreases with increasing Au loading, and a broad band due to electronic transitions from donor sites to the conduction band is observed in the DRIFT spectra for the Au-decorated ZnO nanoparticles. Thus, the presence of the Au nanoparticles results in an enhanced reducibility of ZnO facilitating the generation of oxygen vacancies.
