REACTION ANALYSIS OF POTASSIUM PROMOTION OF RU-CATALYZED CO HYDROGENATION USING STEADY-STATE ISOTOPIC TRANSIENTS

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/SiO2 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 (C1α) and a low-reactivity (C1β) 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 C1β, the C1α 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. © 1992.