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Reaction analysis of potassium promotion of Ru-catalyzed CO hydrogenation using steady-state isotopic transients

1992; Elsevier BV; Volume: 137; Issue: 1 Linguagem: Inglês

10.1016/0021-9517(92)90136-6

1090-2694

T.E. Hoost, James G. Goodwin,

Catalysis and Oxidation Reactions

Abstract The effects of alkali on catalyst activity and deactivation during CO hydrogenation have been studied in the past mostly based on an available surface-metal atoms approach. However, such an approach cannot easily distinguish to what extent the modifier brings about changes in surface concentrations of reaction intermediates or affects site activity during steady-state reaction. Steady-state isotopic transient analysis (SSITKA) with carbon tracing was used to decouple the effects of potassium on the methane-producing sites during steady-state CO hydrogenation over Ru/SiO 2 catalysts having modifier loadings of up to (K/Ru) atom = 0.2. The SSITKA results indicate that, during steady-state CO hydrogenation, carbidic carbon evolved into methane via a high-reactivity (C 1α ) and a low-reactivity (C 1β ) trajectory. With increasing amounts of K + the average “true” intrinsic turnover frequency ( k ) of both of these carbidic pools decreased, as did their steady-state surface abundance. Relative to C 1β , the C 1α pool was affected to a slightly greater extent, both in terms of its reactivity and abundance. It is likely that potassium was able to strengthen the carbon-metal interaction which made hydrogenation of the carbon adlayer more difficult, resulting in smaller methane-destined pools of active surface carbon. With time-on-stream, deactivation by deposition of inactive carbon did not significantly affect the product distribution or the methane rate constant at the prevailing K + doping levels; instead, deactivation was due to a loss in the steady-state abundance of carbon-containing surface intermediates exiting as methane. Implications of the role of potassium during steady-state CO hydrogenation in influencing the active metal surface, the carbidic adlayer, and the latter's transformation into unreactive carbon are addressed.