Reaction of Methanol with Cu(111) and Cu(111) + O(ads)

摘要

The reactive chemistry of methanol on the Cu(111) surface, both with and without preadsorbed oxygen atoms, is investigated between 190 and 700 K. The clean Cu(111) surface is inert, and molecularly adsorbed methanol, the only stable surface species identified on this surface, desorbs at about 210 K. Various trends are examined as a function of oxygen coverage from the clean surface to the saturation oxygen coverage (approximately 0.45 0 atom/Cu atom). The capacity of the surface to adsorb methanol (190 K), and the formaldehyde yield (approx. 400-450 K) are both maximized when the oxygen coverage is about 0.26 O atom/Cu atom. Trends in the yield of other products, and the temperature for decomposition of the stable methoxy intermediate are examined. Also, the rate of methoxy decomposition is limited by C-H bond breaking as evidenced by a deuterium kinetic isotope effect (C-H versus C-D). A minor decomposition path for methanol on O+Cu(111) involves CO2 formation probably via a formate surface intermediate. Preadsorbed oxygen serves as an acceptor of the methanol hydroxyl hydrogen, enabling facile methanol conversion to methoxy at low oxygen coverage for T > or = 190 K. However, at high oxygen coverage (Theta > or = 0.26 O atom/Cu atom) oxygen inhibits surface reactivity. A two-dimensional model which defines three types of surface sites is used to explain the general trend of methanol reactivity as a function of oxygen coverage.