Decomposition of methanol, formaldehyde, and formic acid on nonpolar (101̄0), stepped (505̄1), and (0001) surfaces of ZnO by temperature-programmed decomposition

The decompositions of methanol, formaldehyde, and formic acid were studied on a nonpolar (101̄0), a stepped (505̄1), and a Zn polar (0001) surface by temperature-programmed decomposition. The decomposition products on the (101̄0) and the (505̄1) surfaces were similar, but the coverage and the amount of products were consistently higher on the (505̄1) surface. On these two surfaces, methanol decomposed in two pathways. In one pathway, dissociatively adsorbed methanol decomposed into methane and adsorbed oxygen at 150 °C. In the other pathway, the methoxide was oxidized to a surface formate-like species which decomposed at 380 °C into CO, CO₂, H₂, and H₂O. Formaldehyde and formic acid also decomposed via the surface formate. Thus the decomposition of methanol and formaldehyde was accompanied by surface reduction. No coverage dependence of product distribution was observed. On the Zn polar (0001) surface, methanol decomposed in two competitive pathways via a common formaldehyde intermediate. In one pathway, dehydrogenation of formaldehyde to CO occurred. In the other pathway, oxidation by lattice oxygen to a formate intermediate occurred which eventually decomposed into CO, CO₂, H₂, and H₂O above 400 °C. The competition between dehydrogenation and oxidation depended on the coverage such that dehydrogenation was more favored for higher coverages. Within the dehydrogenation pathway, the selectivity for formaldehyde versus CO was lower for higher coverages. Within the oxidation pathway, the selectivity for CO versus CO₂ increased with higher coverages. Formaldehyde and formic acid also decomposed via the formate intermediate. The desorption of all decomposition products was reaction-limited except for water which was desorption-limited. The results indicated that the (0001) surface is more metallic in its behavior than the (101̄0) and (505̄1) surfaces. Comparison of the desorption temperatures of different compounds suggests that at room temperature, alcohols adsorb molecularly on the (0001) surface, but dissociatively on the other two surfaces. On the (0001), comparison among methanol, ethanol, and isopropanol suggests that the ease of dehydrogenation parallels the strength of the α_cH bond.