SSITKA analysis of CO hydrogenation on Zn modified cobalt catalysts

Cobalt and rhenium promoted cobalt catalysts with particle sizes in the range 7–40 nm, supported on different modified aluminas, were investigated for CO hydrogenation. Steady-state isotopic transient kinetic analysis (SSITKA) was carried out at 483 K, 1.85 bar, and H2/CO/inert = 15/1.5/33.5 Nml/min. A loading-dependent adverse effect of zinc was found, and loss in methanation activity could be explained by an increased surface residence time of CHx (kCH4 ≡ 1/τCHx) and decreased selectivity to CH4. The surface concentration of CO (NCO) was largely unaffected by Zn and thus uncorrelated to activity. The relative effect of Zn on hydrocarbon formation decreased with increasing chain length, indicating that hydrogenation was more strongly affected than chain propagation. In situ measurements prior to and during CO hydrogenation suggested that the surface was largely covered by monomeric carbon species experiencing competitive adsorption from other species. As expected, no particle size effect was observed.