Photoacoustic investigations of CO₂-CH₄ reaction catalyzed by nickel particles embedded into SBA15 mesopores

A photoacoustic spectroscopy technique was employed to the kinetic study of the CO₂/CH₄ reaction catalyzed by Ni particles embedded into the mesochannels of SBA15. The catalytic CO₂/CH₄ reaction was performed in a closed-circulating reactor system at various partial pressures of CO₂ and CH₄ (40 Torr total pressure) in the temperature range of 500-700 °C. The CO₂ photoacoustic signal that varied with the concentration of CO₂ during the catalytic reaction was recorded as a function of time by using a differential photoacoustic cell. Under the reaction conditions, the CO₂ photoacoustic measurements showed the SBA15 compound used as support to be inactive for the reaction. The CO₂/CH₄ reaction carried out in the presence of the H₂-reduced Ni/SBA15 catalyst showed significant time-dependent changes in the CO₂ photoacoustic signal, while the reaction performed in the presence of the as-prepared Ni/SBA15 catalyst did not. The CO₂ photoacoustic signal obtained at early reaction times provided precise data of the CO₂ disappearance rate. The rate of CO₂ disappearance was observed to increase with increasing temperature in the range of 500-700 °C. The apparent activation energy for the CO₂ consumption in the Ni/SBA15 catalyzed reaction was calculated to be 6.2 kcal/mol. Reaction orders, determined from initial rates of CO₂ disappearance at various P_{C O2}'s and P_{C H4}'s at 700 °C, were found to be 0.28 for CH₄ and 0.39 for CO₂, respectively. The kinetic results were compared with those previously reported and were used to infer a catalytic reaction mechanism for the CO₂/CH₄ reaction at low pressures.