Study of sites and species during CO hydrogenation over silica-supported Co–Pd catalysts. Relation to performance in the process

Supported bimetallic catalysts of (Co + Pd)/SiO2 system were studied in carbon monoxide hydrogenation. Comparative analysis showed that depending on precursor treatment mode the catalysts ranged in different rows of activity in CO conversion and selectivity to methane. Samples of best performance were obtained after pretreatment in Ar flow. Highly selective catalysts were synthesized by reduction at 450 °C determining low metal dispersion, high extent of alloying, and agglomeration. A low H2,100C/COstrong adsorbed gas ratio was ascribed to a great amount of bimetallic particles and concerned with a diminished number of sites for multiply bonded CO species. Metal dispersion was low due to large Co particles, which enhanced CO dissociation and hydrogenation to CH4. In presence of bimetallic particles the reaction CO + 3H2 = CH4 + H2O was hampered. A decreased H2O formation influenced the WGS reaction. Catalyst samples activated at higher temperatures had better selectivity. During the process, formation of bidentate carbonate species was registered. It was supposed that palladium impeded creation of the latter species and following decomposition to CO2. Active catalyst samples were prepared by reduction at 300 °C leading to higher unreduced cobalt quota and metal dispersion, and decreased alloy particle formation. Higher H2,100C/COstrong ratio values were assigned to pure Co and Pd particle segregation, i.e. availability of sites for multiply bonded CO species favoring a higher activity in CO dissociation and further hydrogenation. A higher amount of CO species on these samples was conducive to CH4 formation, but also to CO2 production. The latter reaction was facilitated by unreduced cobalt.