Temperature-programmed surface reaction as a means of characterizing supported-metal catalysts and probing their surface reactivity

Abstract

A series of ruthenium–copper bimetallic catalysts on silica were prepared by the co-impregnation method. The amount of ruthenium was maintained constant at 1 wt.%. Copper loadings varied from 0.01 to 0.1 wt.%. Catalysts were characterized using surface area measurements (BET), CO adsorption followed by temperature-programmed surface reaction (TPSR) of the pre-adsorbed CO with hydrogen to form mainly methane. TPSR profiles (methane profiles) were used to calculate CO uptake, dispersion, metal surface area and crystallite size. Peak temperature, T_max, was taken as a measure of the reactivity of the surface for the methanation reaction. CO uptake was observed to increase with copper addition, which could be attributed to some kind of ligand effect. Peak temperature was seen to shift to higher temperatures with increasing copper content, indicative of a decline in the rate of the methanation reaction with copper addition. It is suggested that this could be due to a decrease in the dissociation rate of CO on the bimetallic catalysts. TPSR profiles were also used to calculate the activation energies, E_a, of the methanation reaction over the catalysts used using three different methods, namely Redhead; Chan, Aris and Weinberg (CAW); and Arrhenius plots based on lineshape analysis of the resulting TPSR profiles. It is significant that, regardless of the method used for estimating E_a, no appreciable change in its value is observed with increasing copper content, which might indicate that no change in the mechanism of reaction occurs with copper addition.

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