Kinetic analysis of supported Ni-catalyzed CO2/CH4 reactions using photoacoustic spectroscopy

Abstract

CO₂ photoacoustic spectroscopy was employed to analyze kinetically the CO₂/CH₄ reaction catalyzed by 14 wt% Ni/Al₂O₃ and 14 wt% Ni/TiO₂. The catalytic reaction was carried out in the temperature range of 673–923 K at various partial pressures of CO₂ and CH₄ (40 Torr total pressure) in a closed-circulating reactor system. The CO₂ photoacoustic signal, measured by using a differential photoacoustic cell, was recorded as a function of reaction time. Under these conditions, Al₂O₃ and TiO₂ used as supports do not promote the reaction as noted by the lack of changes in the CO₂ photoacoustic signal. Reactions run in the presence of H₂-reduced supported Ni catalysts are associated with significant time dependent changes in the CO₂ photoacoustic signal, while processes carried out in the presence of unreduced catalysts do not. Changes in the CO₂ photoacoustic signal at early reaction times provide precise data for the rate of CO₂ disappearance. The rate of CO₂ disappearance is observed to increase with increasing temperature in the range of 673–923 K. Apparent activation energies for CO₂ consumption were calculated to be 15.4 kcal mol⁻¹ for the Ni/Al₂O₃- and 14.3 kcal mol⁻¹ for the Ni/TiO₂-catalyzed reactions. Reaction orders, determined from initial rates of CO₂ disappearance at 873 K, were found to be 0.48 in CH₄ and 0.45 in CO₂ for the Ni/Al₂O₃-promoted process, and 0.38 in CH₄ and 0.32 in CO₂ for the Ni/TiO₂-catalyzed reaction. The results of this effort were compared with those reported previously and were used to construct a mechanism for the low pressure CO₂/CH₄ reaction.