DECOMPOSITION OF METHANOL ON OXYGEN-MODIFIED FE(100) SURFACES .2. PREADSORBED OXYGEN AS POISON, SELECTIVITY MODIFIER AND PROMOTER

Decomposition of methanol (CH3OH) on the Fe(100) surface modified by low temperature adsorption of oxygen has been studied, using high resolution electron energy loss spectroscopy (HREELS) and temperature programmed reaction spectroscopy (TPRS). Fe(100) surfaces studied were modified by adsorption of O2 at 113 K, and methanol decomposition as a function of oxygen coverage was monitored. The effect of pre-heating the oxygen overlayers on the methanol decomposition was also examined. Decomposition of methanol on these O-modified surfaces passes through a methoxy (-OCH3) intermediate. The thermal stability of methoxy increases in the presence of pre-adsorbed oxygen. At low coverage, atomic oxygen occupies four-fold hollow sites. In this case, the effect of oxygen on the methanol decomposition is similar to that observed previously on the annealed O-modified surfaces. At higher oxygen coverage, a more weakly bound non-hollow site oxygen also exists on the surface, which reacts with hydroxyl (-OH) hydrogen of the CH3OH, promoting the formation of methoxy. At high oxygen coverage (close to saturation coverage at 113 K), decomposition of methanol results in the formation of formaldehyde (H2CO), without production of carbon monoxide (CO). This is very different from the decomposition of methanol on the clean Fe(100) surface, where decomposition leads to the formation of CO without H2CO. The effect of oxygen modification is discussed in terms of changing relative probabilities of competing reaction pathways. © 1990.