The Power-to-Gas (PtG) process offers the opportunity to store fluctuating renewable energy in form of chemical energy by hydrogenating carbon oxides into methane. In addition, potential carbon point sources often consist of CO/CO2 (COx) mixtures. Hence, reactor design requires kinetic models valid for unsteady-state operation and a broad spectrum of feed gas compositions. In order to provide the required experimental data basis for derivation of kinetic expressions valid under transient conditions, the dynamic response of a continuously operated fixed-bed methanation reactor is studied by applying periodic step-changes in the feed composition. The obtained results are evaluated based on a simple reactor model, providing the molar flow rate exchanged between the gas bulk and the solid surface for CO, CO2, CH4, and H2O. The results further reveal that the transient kinetic processes at the catalyst surface strongly affect the reactor response under reaction conditions of technical relevance.

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Methanation is capable of chemical storage of fluctuating renewable energy, which requires fundamental understanding of the dynamics of the kinetic processes taking place on the catalyst surface. In order to elucidate the impact of the catalyst support on the dynamics, Co-based model catalysts were synthesized via double flame spray pyrolysis (DFSP) differing only in the support materials. In particular, Co nanoparticles of identical particle size distributions are supported on SiO 2 , TiO 2 and SiO 2 -TiO 2 mixtures by independent formation of the support and the active metal by means of DFSP. The transient catalyst behavior was studied by step changes of the feed gas composition and the analysis of the reactor response with high temporal resolution, according to the periodic transient kinetics method. It was revealed that H 2 O adsorption strongly depends on the support with increasing sorption capacity from SiO 2 , via SiO 2 -TiO 2 to TiO 2 . The storage of H 2 O on the support induces a sorption-enhancement effect and thereby promotes the transient CH 4 formation rate for Co/TiO 2 catalysts. Transient experiments before and after a steady-state operation period under CO 2 methanation reveal an unchanging reaction mechanism, while changes in selectivity towards CH 4 and CO are observed together with a certain degree of deactivation. Hence, the support material was identified to play a major role in activity, selectivity and deactivation behavior for supported Co catalysts in CO 2 methanation. This publication is part of a joint Special Collection with ChemElectroChem on "Catalysts and Reactors under Dynamic Conditions for Energy Storage and Conversion (DynaKat)". Please check our homepage. for more articles in the collection