Quantitative determination of reaction products by in-line thermal desorption spectroscopy: The system methanol∕Pd(111)

Abstract

An evaluation procedure for the quantitative determination of thermal desorption products based on in-line mass spectrometry is described in detail. This procedure is, in particular, applicable for species which are pumped with high speed and, therefore, yield small signals in conventional mass spectrometry. The evaluation method was applied to the adsorption system methanol∕Pd(111). After dosing a Pd(111) surface at 140K with methanol up to saturation of the chemisorbed layer, desorption of methanol, hydrogen, carbon monoxide, and water can be observed. The amount of all of the reaction products was determined quantitatively. The total number of adsorbed methanol was found to be 2.6×1014MeOH∕cm2, which is a lower limit, because coadsorbed water and hydrogen partially block adsorption sites on the surface. While 62% of the adsorbed methanol desorbs associatively, 38% of the adsorbed methanol dehydrogenate, resulting in desorption of hydrogen and carbon monoxide. No indication for C–O bond scission could be observed when methanol was adsorbed on the clean Pd(111) surface.