Energetic Mapping of Ni Catalysts by Detailed Kinetic Modeling

Abstract

Temperature-programmed desorption (TPD) of CO has been performed on supported and unsupported nickel catalysts. The unsupported Ni catalyst consists of a Ni(14 13 13) single crystal which has been studied under ultrahigh vacuum conditions. The desorption energy for CO at low CO surface coverage was found to be 119 kJ/mol, and the binding energy of C to the Ni(111) surface of the crystal was 703 kJ/mol. The supported catalysts consist of nickel supported on hydrotalcite-like compounds with three different Mg2+/Al3+ ratios. The experimental results show that for the supported Ni catalysts TPD of CO leads to desorption of both CO and CO2, with the latter being dominant. Dissociation of CO takes place, and considerable amounts of residue C are left on the surface. The residue C is removed by temperature-programmed oxidation (TPO). The results show that a low Mg2+/Al3+ ratio in the hydrotalcite precursor seems to result in more steplike sites, kinks, and defects for carbon monoxide dissociation. A detailed kinetic modeling of the TPO results based on elementary reaction steps has been conducted to give an energetic map of supported Ni catalysts. Experimental results from the ideal Ni surface fit nicely with literature values, providing useful information for identifying active sites on supported Ni catalysts.