Structural and Catalytic Properties of Model Supported Nickel Catalysts

published in Aduonce ACS Abstracts, January 1, 1994. 0022-36S4/94/2098-124S%04.S0/0 nation at present. Martini2 and Goodmani3 have suggested that the activity of different crystal planes toward hydrogenolysis is responsible for the observed variations in activity with metal particle size. Single-crystal surface science studies have shown these structure-sensitive reactions to proceed more rapidly over the (100) crystal plane compared to the (1 11) facet.13 A direct experimental verification of the single-crystal results using supported metal catalysts has not been made due to the difficulties encountered with oxide support materials in surfacescience studies and the inability to characterize the precise morphology of small metal clusters with traditional catalyst characterization techn ique~ . '~ In contrast to alkane hydrogenolysis, carbon monoxide methanation is structure insensitive and exhibits little variation in specific activity with change in particle size.]' The close correspondence of specific activity, pressure effects, and apparent activation energies among studies over high surface area supported n i ~ k e 1 , ' ~ J ~ nickel single crystals,' ',l7-l9 and nickel supported on model thin-film aluminaZo is evidence of the structure insensitivity of the methanation reaction. Single-crystal kinetic studies have shown the mechanism of methanation to involve dissociation of CO and the subsequent hydrogenation of the resulting carbon species. The active surface carbon species is a carbidic type which his generally formed for all methanation ~ata1ysts.l~ The removal rate of carbon by hydrogen is in delicate balance with the formation of surface carbide. In this paper, we investigated the structural and kinetic properties of model silica-supported nickel catalysts which are used for alkane hydrogenolysis and CO methanation. Metal particle size and kinetic properties have been studied using a combination of ultrahigh-vacuum (UHV) surface science techniques and elevated pressure kinetic measurements. The results are compared with measurements from traditional supported nickel catalysts and nickel single crystals. Experimental Section The experiments were carried out in two UHV surface analysis chambers. One chamber is equipped with Auger electron spectroscopy (AES), quadrupole mass spectrometry (QMS), gas and metal dosing capabilities, and a Mattson Cygnus 100 FTIR spectrometer for IR single reflection-absorption spectroscopy (IRAS) studies. The details of the apparatus have been described elsewhere.z1 The IRAS spectra were acquired at an incident angleof 8S0 and were averages of 128 scans at 4-cm-' resolution. All spectra were taken at a base pressure of less than Torr.