STUDY OF PT-PROMOTED COBALT CO HYDROGENATION CATALYSTS

The influence of small amounts of Pt on supported Co catalysts has bee n investigated by several methods. Catalysts containing 9 wt.% Co and 0 or 0.4 wt.% Pt on Al2O3 or SiO2 were prepared by coimpregnation. At T = 483 K, P = 1 bar, and H-2/CO = 7, CO hydrogenation rates (based on weight of cobalt) for Pt-promoted catalysts were 3-5 times higher tha n those on their unpromoted analogues, The selectivity was not influen ced by the presence of Pt. TPR-studies have shown that the presence of Pt strongly influences the reducibility of the catalysts, with TPR-pe aks shifting to lower temperatures for all catalysts. This effect is m ost pronounced on the Al2O3-supported catalysts, where the highly disp ersed and otherwise difficult to reduce surface cobalt oxides are read ily reduced at normal reduction temperatures (623 K) in the presence o f Pt. The dispersion of metallic cobalt on Pt promoted catalysts incre ased compared to that on the unpromoted catalysts, with Al2O3-supporte d catalysts showing the largest effect due to the reduction of highly dispersed surface cobalt oxide. Apparent turnover numbers (based on H- 2-chemisorption) were higher for Pt-promoted catalysts than for their unpromoted analogues by a factor of similar or equal to 2. Decoupling coverage and intrinsic reactivity effects on the apparent turnover num ber by the use of steady-state isotopic transient kinetic analysis rev ealed essentially constant true turnover numbers on all catalysts. Thu s, the increased apparent turnover numbers on Pt-promoted catalysts ar e due to a higher coverage of reactive intermediates. (C) 1995 Academi c Press, Inc.