Catalytic decomposition of methanol on ZnO single-crystal surfaces at low and near-atmospheric pressures

The catalytic decomposition of methanol at 0 to 50 mPa and 1 to 13 kPa was studied on three ZnO single-crystal surfaces: a Zn-polar (0001), a stepped nonpolar (5051) and an O-polar (0001) surface. The low-pressure study was conducted in an ultrahigh vacuum chamber in which methanol was directed onto the surfaces with a doser tube. Up to 425 °C, the Zn-polar surface was at least five times more active than the other two surfaces, and the stepped nonpolar surface was marginally more active than the O-polar surface. The near-atomospheric pressure study was conducted in a microreactor at 250-300 °C. No measurable activities were found on the O-polar (0001) surface, whereas the activities of the Zn-polar (0001) and the stepped nonpolar (5051) surfaces were comparable. The reaction rate increased with increasing methanol pressure up to about 27 mPa beyond which the reaction became zeroth order in methanol. In the zeroth order region, the activation energy was 138-161 kj/mole for the Zn-polar surface. These values and the turnover frequencies were comparable to values obtained on powder ZnO samples under near-atmospheric pressures. In the first-order region, the activation energy decreased as the temperature was increased. The major carbon-containing product observed was formaldehyde at low pressures and CO at near-atmospheric pressures. Differences between the results obtained at the two different pressures and the importance of surface defects generated by reduction of the surfaces were discussed.