Adsorption of CO on Cr2O3(0001)

The adsorption of CO molecules on the Cr-terminated (0001) surface plane of Cr2O3 is studied theoretically by means of quantum chemical cluster calcul ations and experimentally by thermal desorption spectroscopy and infrared r eflection absorption spectroscopy. The combination of these approaches yiel ds a detailed picture of the CO/Cr2O3(0001) adsorption. in the low coverage limit, CO is strongly bound (chemisorbed) and occupies a O-3-hollow positi on, i.e. it is adsorbed above a threefold hollow site in the first full oxy gen layer, with the CO axis oriented along a line connecting two Cr ions at the surface and tilted strongly against the surface normal. CO molecules i n this position desorb at 175 K, which corresponds to an adsorption energy of 45 kJ/mol, and exhibit a blue shift of the CO stretching frequency of ab out 35 cm(-1) with respect to CO in the gas phase (from 2143 to 2178 cm(-1) ). At higher coverages, a second desorption peak at 105 K is found which is accompanied by a small red shift of -10 cm(-1) of the CO stretching freque ncy. This adsorption state corresponds to a physisorption of CO with an ads orption energy of 28 kJ/mol and is tentatively assigned to CO molecules ads orbed parallel to the surface above one O2- anion in the oxygen layer. (C) 2001 Elsevier Science B.V. All rights reserved.