Structure and reactivity investigations on supported bimetallic Au{single bond}Ni catalysts used for hydrocarbon steam reforming

The addition of small quantities of gold to the surface of supported nickel catalysts has been described as a means to retard carbon formation during hydrocarbon steam reforming. Calculations by others have indicated that gold locates at the most catalytically active (step and edge) sites that also serve as nucleation sites for carbon formation. In this paper, we describe experiments to characterize the Ni{single bond}Au interactions on bimetallic Au{single bond}Ni/MgAl₂O₄ catalysts at various Ni and Au loadings. The catalyst structure was investigated using EXAFS/XANES spectroscopy and adsorption-desorption measurements with H₂ and N₂O. Evidence for surface alloy formation is provided in the Ni K-edge and Au L_III-edge EXAFS measurements of Au-promoted 8.8% Ni/MgAl₂O₄, especially at Au loadings ≤ 0.2   wt%. At higher Au concentrations, there is evidence of a combination of alloy and segregated Au species. H₂ chemisorption and N₂O temperature-programmed desorption (TPD) measurements showed a significant decrease in total surface sites, or surface site reactivity, on Au-modified Ni/MgAl₂O₄ catalyst. The XANES structure is consistent with perturbation of the electronic structure of both the Ni and Au atoms as a result of alloy formation. TGA studies with steam/n-butane feed confirmed the ability of Au to retard coke deposition under low S/C reforming conditions, although carbon formation was not fully suppressed. When testing for methane steam reforming, a lower initial activity and deactivation rate resulted from Au promotion of the Ni catalyst. However, both catalysts showed a declining activity with time. The lack of a direct correlation between the surface characterization results and catalytic activity is most likely a result of decreasing effectiveness of the surface alloy with increasing temperature.