Geometric Structure and Desorption Kinetics of CO on the Cr2O3(0001)/Cr(110) Surface
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Understanding the interaction of simple molecules such as carbon monoxide on metal oxide surfaces is important for many industrial applications. For instance, Cr2O3 is an industrial catalyst used for the reduction on nitrogen oxide compounds and for the polymerization of various hydrocarbons. Previous groups have studied the adsorption of CO on the Cr2O3(0001)/Cr(110) surface and found that it lies flat on the surface instead of an upright geometry, which is observed on most other metal and metal oxide surfaces. In addition, there is only one published measurement of the activation energy for desorption of CO from Cr2O3(0001), and this value is approximately twice as high as the value calculated using quantum chemical cluster theory. To determine the geometric structure of the clean and CO covered Cr2O3(0001)/Cr(110) surface, we have performed temperature dependent low energy electron diffraction measurements of these surfaces. Our diffraction measurements reveal an unreconstructed clean Cr2O3(0001)/Cr(110) surface over a temperature range of room temperature to 120 K. The diffraction pattern of the surface at a saturation coverage of CO shows only a slight increase in diffuse background with no overlayer reconstruction being observed. This is consistent with previous reports that CO molecules are easily desorbed by low energy electrons. To determine the desorption kinetics of CO from the Cr2O3(0001)/Cr(110) surface, we have performed temperature programmed desorption measurements. At a crystal temperature of 120 K, it was found that a saturation coverage of CO was obtained for a CO dose of ~1 L. The maximum of the desorption curve was observed to shift to higher temperature by only 4 K as the coverage approached saturation, which indicates a 1st order kinetics with an attractive adsorbate-adsorbate interaction. The activation energy for CO desorption was measured to be 49.2 ±2.6 kJ/mol using the Redhead method with heating rates ranging from 5 °C/min to 50 °C/min.