CO hydrogenation over alumina‐supported sulfide cluster catalysts

Abstract

Bimetallic Mo-Fe and Mo-Co sulfide clusters were anchored on AI,O, and used for CO hydrogenation. In addition to methane, significant amounts of dimethyl ether were produced. The reaction orders obtained from power rate laws for methanation indicated that the surfaces of the catalytic ensembles were not completely saturated by CO, in contrast to the observations for most conventional CO hydrogenation catalysts. A kinetic analysis and parameter estimation was performed to identify the rate-determining step for methanation. A catalytic cycle was postulated that could account not only for the formation of methane and higher hydrocarbons, but also for dimethyl ether as a primary product. Introduction Objectives and relevanceOne of the greatest challenges in heterogeneous catalysis is to elucidate how the nature of the active site influences catalytic behavior. The adsorption of reactant molecules, the formation of reaction intermediates, and ultimately the product distribution depend on the composition and structure of the catalytic surface sites. Catalysts derived from structurally well defined organometallic clusters are very attractive model systems insofar as they are amenable to detailed spectroscopic characterization and evaluation of the intrinsic kinetics that control their catalytic behavior. These types of catalysts may also provide a link between homogeneous catalysis, where the catalytic complexes are easily identified, and heterogeneous catalysis, where the surfaces tend to be quite irregular.The objective of this study was to investigate the CO hydrogenation activities of catalysts prepared from organometallic sulfide clusters containing Mo and either Fe or Co. Although the late transition metals are the most active catalysts for CO hydrogenation, they tend to be nonselective, with product distributions following the Anderson-Schulz-Flory relationship. The late transition metals are also extremely sensitive to poisoning by sulfur, which is a common impurity in synthesis gas. Molybdenum, on the other hand, in its oxide as well as its sulfide form exhibits remarkable tolerance to deactivation in H,S and has proven activity for CO hydrogenation. Furthermore, the addition of Fe or Co to Mo appears to beneficially effect catalyst selectivity. Consequently, discrete organometallic clusters con- taining Mo, S, and Fe (or Co) represent attractive precursor compounds for the preparation of active, sulfur tolerant C O hydrogenation catalysts. An attempt was made to identify the rate-determining step for methanation on these cluster-derived catalysts. To this end, the commonly accepted sequences of elementary reaction steps for the two most important methanation mechanisms, the CO insertion mechanism and the carbide mechanism, were evaluated. Finally, a catalytic cycle has been proposed to account for the total product distribution, including hydrocarbons and oxygenates. Significant prior workTypical commercial catalysts are prepared either by coprecipitation or by sequential adsorption of suitable precursor salt ...

Production of methane from carbon monoxide and carbon dioxide in a plasma-catalytic combined reactor system

“…Previous studies that have dealt with mechanistic aspects of catalytic CO conversion can be classifi ed into two categories: carbide mechanism and CO insertion mechanism [8,18,19].…”

“…Meanwhile, the CO insertion mechanism assumes molecular adsorption of CO and hydrogenation of adsorbed CO to give an oxygenated intermediate, which further reacts with adsorbed hydrogen to form methane [18]. The important elementary steps accounting for the CO insertion mechanism are summarized below.…”

“…On the other hand, Alstrup [8] obtained good agreements between experimental data and kinetic model by postulating the hydrogenation reaction between CH * and H * as the rate-determining step. Thomson et al [18] suggested that the rate-determining step is either the hydrogenation of CH * for the carbide mechanism or the hydrogenation of HCO * for the CO insertion mechanism. Whatever elementary reaction acts as the rate-determining step, nonthermal plasma may enhance the rate of methane production, because the adsorption characteristics of the molecules excited by the plasma should be different from those of ground-state molecules and the plasma can help dissociate adsorbed molecules through various processes like direct electron impact.…”