Adsorption and catalytic properties of Pd/SiO2, Cu/SiO2, and Pd-Cu/SiO2 systems. III, Carbon monoxide and benzene hydrogenation over Pd-Cu/SiO2 catalysts

Abstract The addition of copper to a well dispersed 2.48% Pd/SiO 2 catalyst gave reasonably homogeneous, stable bimetallic systems under mild reaction conditions. Benzene hydrogenation exhibited a relatively constant activation energy around 11.5 kcal mol -1 (1 cal=4.1868 J) and a reversible activity maximum with temperature which shifted to higher values with increasing copper content. The turnover frequency (TOF) decreased non-linearly with copper content and implied the active site was an ensemble of three surface palladium atoms. Activation energies for methane did not vary with copper content when palladium was present, and TOFs at 0.1 MPa changed little as copper was added although a weak maximum was obtained with 42% Cu. This behavior showed that methanation is a structure-insensitive reaction over palladium. At 1.5 MPa, another weak maximum was observed for methane formation and a sharper maximum for methanol synthesis was obtained near a composition of 59% Cu. Selectivity to methanol increased wit hincreasing copper content. At 1.5 MPa and 548K these SiO 2 -supported catalysts sintered significantly during the first 24 h on stream. This tended to increase methanol selectivity, but it made the determination of actual metal surface compositions much more difficult.