Temperature-programmed desorption of CO from Ni/SiO2 Application of analytic techniques for strongly interacting adsorbates

Abstract

The interactions of CO with a Ni/SiO2 catalyst were studied using the temperature-programmed desorption (TPD) technique. Programmed heating following adsorption led to desorption of both CO and CO2. C and O were found to remain on the catalyst surface after TPD of CO; both were removed by subsequent H2 treatment. Four CO desorption peaks, designated β*, β1, β2, and β′, were assigned to a surface-type carbonyl, linear-bonded CO, bridge-bonded CO, and the recombination of C and O adatoms, respectively. CO2 desorption spectra showed a single desorption peak, P2, when the catalyst weight was low. A new chemical pathway, revealed by the appearance of another peak, P1, was opened for a higher weight of sample. The formation of P1 and P2 followed first-order and second-order kinetics, respectively.The strong readsorption properties of CO within the catalyst bed have made direct determination of kinetic parameters difficult. In a limited range of temperatures, however, a similarity of the CO desorption spectra to results from unsupported Ni was found. Numerical simulation of the TPD process for a flow system has shown that the differential bed assumption is reasonable, i.e. a uniform distribution of the adsorbate within the bed during desorption is approximated. With these observations serving as a basis, the catalyst weight, the amount adsorbed, and the heating rate were varied to obtain desorption energies by analysis of the TPD data for both CO and CO2.