Adsorption of CO on ultrathin Cr layers on Ru(0 0 0 1)

Abstract

We studied the adsorption of CO on ultrathin Cr layers on Ru(0 0 0 1) as a function of Cr coverage and deposition temperature. The preparation of the Cr layers was characterized by Auger electron spectroscopy, low energy electron diffraction, and the subsequent CO adsorption itself. At 90 K Cr grows statistically on Ru(0 0 0 1). For Cr deposition at 700 K, a pseudomorphic monolayer is formed; thereafter Cr grows in three dimensional islands and the formation of a highly ordered bcc(1 1 0) Cr phase is observed. For Cr deposition at 1000 K we propose the formation of a stable surface alloy. Temperature programmed desorption of CO after dosing onto the Cr layers prepared at 700 and 1000 K indicates an enhanced reactivity of the layers towards CO dissociation upon heating as compared to Ru(0 0 0 1). In both cases a high temperature CO desorption state (above 700 K) is observed that is attributed to recombinative desorption of CO; its intensity increases up to a nominal Cr coverage of 1 ML. For the Cr layers prepared at 700 K, this state is observed already at the lowest Cr coverage studied (0.12 ML); for 1 ML Cr, 47% of the saturated CO layer decompose upon heating. For the Cr layers prepared at 1000 K (surface alloy) significant decomposition is observed only for Cr coverages larger than 0.5 ML; a maximum fraction of 18% of the saturated CO layer decomposes upon heating. Thus, the Cr layer prepared at 700 K is more reactive towards CO dissociation than the one prepared at 1000 K.