Methanol decomposition on zinc-oxide single crystal surfaces

Catalytic studies using single crystal metal surfaces have provided information such as kinetics and reaction mechanisms, and their relationship with the surface composition and structure. However, there are very few similar studies on single crystal oxide surfaces. We report here the kinetic study using oxide single crystal surfaces at ambient pressure.; Methanol decomposition was studied on the Zn-polar and the O-polar surface of ZnO. Experiments were performed with a gold-plated batch reactor. The sample was heated by focusing a beam of light onto the surface in order to minimize the background activity. A UV filter was used to remove light of above 3 eV. The experiments were conducted between 10 and 100 torr of methanol and 250 and 300{dollar}spcirc{dollar}C on the Zn-polar surface and between 20 and 100 torr of methanol and 260 and 300{dollar}spcirc{dollar}C on the O-polar surface. Under these experimental conditions, the O-polar surface was inactive. On the Zn-polar surface, the reaction was shown to be zeroth order in the pressure range studied and methanol was decomposed at a rate of 1.3 {dollar}times{dollar} 10{dollar}sp{lcub}13{rcub}pm{dollar} 22% molecules/cm{dollar}sp2{dollar}-sec at 270{dollar}spcirc{dollar}C. The major carbon product detected was CO. A small amount of hydrogen was also detected but quantitative measurements were not made. Water was not detected ({dollar}<{dollar}1%). Formaldehyde and carbon dioxide were looked for but not found. The rate of methanol decomposition increased with increasing temperature and the activation energy was 33.1 {dollar}pm{dollar} 2 kcal/mole.; Dandy studied the same reaction on powder ZnO and reported that the major products were CO and H{dollar}sb2{dollar}, and small amount of CO{dollar}sb2{dollar} (34). The reaction was zeroth order between 10 and 35 torr and the activation energy was 37.2 kcal/mole. Tawarah et al. studied the reaction below 100 mtorr and reported an activation energy of 30 kcal/mole (35). These data are consistent with our observations on the Zn-polar surface and we concluded that single crystal oxide surfaces can be good model catalysts for powder samples.