EVIDENCE FOR LIGAND-EXCHANGE IN IRON PENTACARBONYL ADSORBED ON NI(100) SURFACES

The chemistry of iron pentacarbonyl on clean and carbon monoxide predo sed Ni(100) surface was studied by using temperature programmed desorp tion (TPD) and X-ray photoelectron (XPS) spectroscopies. The thermal a ctivation of adsorbed Fe(CO)5 below 25% of a monolayer results in tota l decomposition to Fe and CO, but at higher coverages the probability for decomposition is reduced at the expense of both the desorption of some molecular Fe(CO)5 below 200 K and the development of a new desorp tion peak in the CO TPD around 290 K. Based on the results from our TP D and XPS experiments, the new CO TPD peak is proposed to be associate d with the formation of one or more iron carbonyl surface intermediate s. The data from additional coadsorption experiments indicate that tho se intermediates are capable of undergoing ligand exchange reactions w ith isotopically labeled (CO)-C-13 but not of reversing all the way to Fe(CO)5 as on Pt(111). The energetics of the surface reactions involv ed is discussed.