Reactivity studies of ethylene, benzene and cyclohexane on carbide-modified Mo(110) using high resolution X-ray photoelectron spectroscopy

Carbide-modified molybdenum surfaces have been proposed as noble metal-free catalysts for a variety of reactions, among them the dehydrogenation of hydrocarbons, with a reactivity towards that of Pt-group metals. Herein, we study and compare the adsorption and reaction of ethylene, benzene and cyclohexane on a monolayer carbide on Mo(110) in situ using synchrotron radiation-based high-resolution X-ray photoelectron spectroscopy (HR-XPS). During adsorption at low temperature of ethylene, we find two different molecular species that are related to different adsorption sites. Upon heating the saturated ethylene layer, ethylidyne formation starts at 180 K. Benzene also adsorbs molecularly at low temperature; upon heating, acetylide starts to form at 240 K followed by the formation of a smaller methylidyne fraction at 350 K. At higher temperatures, 325 K for C₂H₄ and 380 K for C₆H₆, decomposition to carbon sets in. The XPS results are in good agreement with the temperature programmed desorption (TPD) measurements. Near edge X-ray absorption fine structure (NEXAFS) measurements provide additional information on the adsorption geometry of the intact molecules. The comparison of the decomposition pathways on monolayer carbide with Pt(111) and Rh(111) shows an enhanced reactivity for C/Mo(110), as indicated by significantly lower onset temperatures for ethylene and benzene; interestingly, for cyclohexane, the opposite behavior is observed, that is, no decomposition occurs on C/Mo(111), whereas dehydrogenation to benzene was found on Pt(111).