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

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, Al203 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.