Determination of Concentration of Active Sites on a Ni/γ-Al₂O₃ Catalyst with Methanation Reactions of Carbon Monoxide and Carbon Dioxide

Abstract

The Concentration of active sites on a Ni/γ-Al₂O₃ catalyst was determined with methanation reactions of CO and CO₂ and compared with the amount of CO chemisorbed on the catalyst at 25°C. In CO and CO₂ methanation at around 200°C by using a pulse technique, the pronounced tailing of chromatographic peaks of CH₄ produced was observed. As CH₄ giving the tailing part was produced after the passing away of the gaseous reactant with the carrier gas, H₂, over the catalyst, the vacant sites formed after the desorption of CH₄ remained unoccupied by the reactant. This suggested that the concentration of active sites could be evaluated from the amount of CH₄ produced when the amount of the reactant pulsed was large enough to cover the Ni surface. In CO methanation the amount of CH₄ produced reached constant as the amount of CO pulsed was increased. The concentration of active sites on the catalyst determined from this constant value agreed well with the amount of CO chemisorbed at 25°C. On the other hand, in CO₂ methanation because of the weak adsorption of CO₂ on the catalyst the amount of produced CH₄ increased as the amount of CO₂ pulsed was increased. The nature of active sites was discussed with these experimental results and considerations of the effect of thiophene poisoning on the methanation.