Catalytic consequences of platinum deposition order on cobalt-based Fischer–Tropsch catalysts with low and high cobalt oxide dispersion

Abstract

In this study, the effect of the presence of Pt in supported Co-based Fischer–Tropsch catalysts is investigated. To this end, the effect of the addition of 0.1 wt% Pt to the formulation of Al₂O₃-supported catalysts containing “large” (≈20 nm) or “small” (≈10 nm) Co₃O₄ crystallites is presented. The cobalt particle size can be changed by the presence of diethylene glycol (DEG) in Co-nitrate precursor solution. Pt is deposited on the support by following either the sequential deposition order (SDO), according to which Pt is impregnated onto the calcined Co-catalyst, or the reverse sequential deposition order (RSDO), according to which Pt is impregnated onto the support before Co. We show that despite the small amount of Pt used and regardless of the initial Co₃O₄ crystallite size and of the deposition order of Pt and Co, the catalyst reducibility is significantly improved when Pt is added to the catalyst formulation. Indeed, Pt favors the reduction of the smallest Co oxide particles, which are hardly reducible in the unpromoted samples, thus shifting the average Co⁰ particle size towards smaller values. In particular, in the case of “large” Co₃O₄ particles, Pt has a larger effect when added according to the RSDO method, very likely due to the stronger interaction that is established between Pt and Co. In comparison, the sample with smaller Co₃O₄ particles (prepared with DEG) shows a higher reducibility when prepared according to the SDO method, due to the presence of a significant fraction of small and difficult to reduce Co₃O₄ particles. The effect of Pt on the catalytic performance depends on a complex interplay involving the catalyst reducibility, number of Co metal sites and size of the Co⁰ particle aggregates. Pt strongly improves the performance of the samples originating from the larger cobalt oxide crystallites but on the other hand decreases the reactivity of the samples from the smaller particles. This is due to the fact that the small Co⁰ sites formed in the presence of Pt are in the size-range where the FTS is known to be structure sensitive and the intrinsic activity (TOF) is reduced.