Desorption studies of hydrogen and carbon monoxide from nickel surfaces using thermal desorption spectroscopy

Abstract

The adsorption of hydrogen and carbon monoxide at room temperature on nickel samples was studied with thermal desorption spectroscopy. Desorption from nickel foil was studied using a mass spectrometric method in an ultrahigh vacuum system, while desorption from nickel powder was studied with a microreactor system using a thermal conductivity detector. Two hydrogen desorption peaks are observed at low heating rates (less than 3 K/s). These peaks correspond to the β1 and β2 states observed in single crystals studies. An activation energy of 7–10 kcal/mol is obtained for the low temperature peak (β1 state), assuming first order kinetics, while an energy of 28–30 kcal/mol is obtained for the higher temperature peak (β2 state) with second order kinetics. Desorption spectra using the two different experimental methods are very similar, and the kinetic data derived from the spectra agree well. When the heating rate is high (larger than 12 K/s) only one hydrogen desorption peak is observed, indicating that experimental resolution decreases for very fast heating rates. Kinetic analysis of this spectra yields an activation energy similar to the β2 state. The two H2 desorption features (β1 and β2) obtained for Ni powder and foil agree well with previously reported studies on Ni single crystals, indicating that the presence of the two desorption states is insensitive to surface structure. The activation energy and order of the desorption for the β2 state obtained in this work is in general agreement with values reported for single crystals studies. The activation energy and order for the β1 state is in agreement with some studies performed with Ni single crystals, largely due to the assumption in prior work that desorption proceeds as a second order process for this state. Three carbon monoxide desorption peaks are observed for low heating rates (less than 3 K/s). Presumably, the two low temperature peaks which are associated with adsorbed molecular CO correspond to on-top and bridge site adsorption. The activation energy for the weakest bonding (on-top site/dominant peak) was obtained and corresponded to 6–10 kcal/mol. The activation energy for dissociated CO (high temperature peak) corresponded to 30–36 kcal/mol. Our ability to resolve the two types of adsorbed molecular CO is lost when very fast heating rates are used (12 K/s).