Kinetics behavior of Co/Ni-ordered mesoporous alumina for the CO methanation

Abstract

Abstract The severe requirement of a higher activity at lower temperatures and a longer stability at higher temperatures evokes a great challenge for the development of an industrially viable catalyst for the CO methanation reaction. In this work, the Co-Ni bimetallic catalysts were synthesized via post-impregnating the cobalt precursor within the mesoporous channel of Ni-embedded ordered mesoporous alumina (Ni-OMA). The low-temperature activity and high-temperature stability of Co/Ni-OMA for the CO methanation were significantly regulated by easily tuning the ratio of free Co and confined Ni species. The optimal catalyst of 8Co/15Ni-OMA showed a high activity with the CH4 formation rate of 126\xa0mol kgcat−1 h−1 at a temperature of as low as 300\xa0°C and a long-term durability for a time-on-stream of 200\xa0h without an observable deactivation under the conditions of 600\xa0°C and an extremely high GHSV of 180000\xa0mL g−1 h−1. Kinetics results reveal that the apparent activation energy of the CO methanation over 8Co/15Ni-OMA (100.2\xa0kJ mol−1) was clearly lower than that over 15Ni-OMA (124.0\xa0kJ mol−1) or 15Co-OMA (131.8\xa0kJ mol−1). In the absence of mass transport and heat transfer limitations, three microkinetics models were developed following the H-assisted CO dissociation and Langmuir-Hinshelwood mechanism, which the H-assisted CO dissociation, the hydrogenation of surface carbon species (C*) or surface CH3* species are proposed as the rate-determining step, respectively. The kinetics behaviors over 15Ni-OMA and 8Co/15Ni-OMA are matched well with all of the kinetics models, indicating the same elementary sequence and rate-determining step. In the case of 15Co/OMA, the CH4 formation rate was predicted very well by the kinetics models derived from the stepwise hydrogenation of surface carbon species as the rate-determining step, and the kinetics model based on the H-assisted CO dissociation as the rate-determining step could be ruled out, indicating that the rate for the H-assisted CO dissociation rate is faster than that of the following stepwise hydrogenation. Based on the discrimination of different kinetics models, Ni species confined within OMA matrix were proposed as the dominant active sites for catalyzing the CO methanation, while the post-impregnated Co was acted as a promoter for the H-assisted CO dissociation. As a result, an enhanced low-temperature activity was achieved over the optimal 8Co/15Ni-OMA catalyst.