The role of the Boudouard and water–gas shift reactions in the methanation of CO or CO$_2$ over Ni/γ-Al$_2$O$_3$ catalyst

The Boudouard and the water–gas shift reactions were studied at different temperatures between 453 and 490 K over a Ni/γ-Al$_2$O$_3$ catalyst in a Carberry batch reactor using various mixtures of CO, H$_2$ and CO$_2$. The activity of the Boudouard reaction was found to be low, compared to the water–gas shift reaction, and diminished over time, suggesting that the temperature was too low for significant activity after an initiation period of CO adsorption. Furthermore, the rate of the Boudouard reaction has been reported to decrease in the presence of H$_2$O and H$_2$. The water–gas shift reaction was found to be the main reaction responsible for the production of CO$_2$ in a mixture of CO, H$_2$ and H$_2$O in the batch reactor. The ratio of the total amount of CO consumed to the total amount of CO$_2$ produced showed that the catalyst was also active towards hydrogenation, where the rate of the hydrogenation reaction was very much faster than the water–gas shift reaction. The resulting ratio of $\textit{p}$$_{H_2}$ to $\textit{p}$$_{CO}$ was found to be extremely low, probably leading to the production of long-chain hydrocarbons. The stoichiometry of the overall reaction was such that for every mole of CO$_2$ produced, 1.5 mol of CO was consumed in the batch reactor. Kinetic studies were performed in the batch reactor. An Eley-Rideal mechanism was found to provide a good agreement with the experimental results over a wide range of partial pressures of steam and CO.JYL was funded by the Cambridge International Scholarship Scheme. The Cambridge Philosophical Society, the Lundgren Research Award and Corpus Christi College are also gratefully thanked for contributing to the support of his PhD studies. We are grateful for the assistance of Dr A.P.E. York, Johnson Matthey Technology Centre, Sonning Common, Reading RG4 9NH, United Kingdom, for his valuable input to this research.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.ces.2016.06.04