The reactivity of adsorbed carbon on vanadium promoted rhodium catalysts

Rhodium/vanadium catalyst, promoter action, adsorbed carbon, metal-carbon bond strength, carbon chain growth, quantum theoretical calculations The hydrogenation reactivity of surface carbon deposited by CO decomposition was investigated for a rhodium-vanadium catalyst. It appeared that the rate of methanation of reactive surface carbon is decreased by vanadium. The reactivity towards C2+ hydrocarbons is enhanced by vanadium. The relation between stronger adsorbed carbon atoms and the formation of higher hydrocarbons is discussed. ASED calculations upport he proposal that changes in metal-carbon bond strength ave a significantly larger effect on the rate of methanation than on carbon chain growth. 1

Reactivity of carbon monoxide on vanadium-promoted rhodium catalysts as studied with transient techniques

The effect of vanadium promotion on the kinetics of CO methanation on silica-based rhodium catalysts was probed. Transient model experiments employing temperature-programmed surface reaction spectroscopy and pulse surface reaction rate analysis were used to unravel changes in rates of elementary reaction steps. CO dissociation is the rate-limiting step in the overall methanation reaction. The rate of CO dissociation is found to be enhanced by vanadium. The activation energy is lowered from 90 kJ/mol for Rh/SiO2 to 65 kJ/mol for the promoted catalyst. The activation energy for CO dissociation is not dependent on the hydrogen partial pressure, while it is a strong function of the CO surface coverage. CO adsorption at temperatures above 250°C results in very reactive surface carbonaceous intermediates. Even at -15°C methane formation was observed upon hydrogenation. This reactive surface carbon can be incorporated in higher hydrocarbons. This process is favoured by vanadium promotion