Structure sensitivity in methanol decomposition on ZnO single-crystal surfaces

Decomposition of methanol-d₄ was studied using temperature-programmed decomposition spectroscopy on the nonpolar flat (101̄0), stepped (505̄1) and (404̄1), and the polar (0001) ZnO surfaces. The (505̄1) and (404̄1) planes are surfaces with (0001) steps and (101̄0) terraces. CD₄, CO, CO₂, and D₂ were the decomposition products on the nonpolar stepped and flat surfaces. The ratios of these products were similar on these surfaces. The fraction of desorbed product being decomposition products increased in the order stoichiometric (101̄0) < reduced (101̄0) < (505̄1) < (404̄1). The activity of the stepped surfaces decreased slightly after oxygen pretreatment of the surface. The results indicated that steps and anion vacancy defects are the active sites for methanol decomposition. The activity of the polar (0001) surface was similar to that of the stepped surfaces. However, the products were different. CD₂O and D₂O, but no CD₄ were observed in addition to CO, CO₂ and D₂. The results indicated that methanol decomposition on ZnO is a structure-sensitive reaction. Implication of these observations on the reverse reaction of methanol synthesis on ZnO and CuZn oxide is discussed.