A secondary ion mass spectrometry study of the catalytic oxidation of methanol on Cu(110)

Formaldehyde formation via the oxidation of methanol has been studied on a Cu(110) surface by static secondary ion mass spectrometry (SIMS). Previous work by temperature programmed thermal desorption (TPTD or TPD) suggests enhanced methanol adsorption at 180 K after preadsorption of oxygen and the formation of a methoxide intermediate (CH₃O) species. Upon heating, this intermediate decomposes and formaldehyde is desorbed above 365 K. Using isotopically labeled oxygen-18 and deuterated methanol (CH₃OD), SIMS analysis suggests the presence of D₂^l8O, CH₃OD, CH₂0, and H by observation of their Cu cations (CuD₂^l8O⁺, CuCH₃OD⁺, cuch₂0⁺, and Cu₂H⁺) after methanol adsorption at 178 K. By the use of SIMS and thermal desorption SIMS (TDSIMS), methanol is observed to initially react at oxygenated sites to form water by reaction of its hydroxyl hydrogen and surface oxygen. The methoxide intermediate then reorients itself into a transition state which, during the collision process, breaks apart to form surface hydrogen and formaldehyde. These results suggest SIMS may be capable of assuming a limited role in fundamental catalysis studies. However, its dependence on molecular cations as the means to observe surface species will limit its use to systems that can efficiently produce such clusters.