Spatially resolved operando investigations of the CO$_2$ methanation reaction using DRIFTS and gas phase analysis

Power-to-X (PtX) processes using renewable electricity for hydrogen production plus non-fossil CO$_2$ will be an important part of the future energy system relying also on e.g. synthetic methane. The fluctuating nature of renewable energies, however, represents a challenge since it can propagate all the way through the PtX process chain. For stationary operation with well-defined feed and process conditions, the CO$_2$ methanation reaction is well understood. However, for tuning catalyst properties and process parameters for transient operation, deeper insights based on fundamental operando studies is of crucial importance. In this contribution, we present a unique reactor setup enabling spatially resolved operando investigations of both, gas phase composition and species adsorbed on a catalyst surface using gas chromatography and infrared spectroscopy (DRIFTS). The performance of an industrial Ni/Al$_2$O$_3$ methanation catalyst was investigated as a function of reaction temperature (100–450° C) and space velocity (1–20 ml$_N$ mg$^{-1}$ min$^{-1}$). The spatially resolved DRIFTS (SRD) reactor was modeled, and the flow profile and concentration gradients were simulated using CFD, indicating that the gas
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sampling does well represent the actual concentration along the reactor coordinate, which was additionally validated by benchmarking to a catalytic plate reactor reference system. The results presented and discussed not only verify the ability of the SRD reactor setup for in-depth investigation but are also in line with the methanation mechanism discussed in literature. Further, the results suggest that the formation of carbonyls which readily react to methane can be seen as the rate-determining step for this type of catalyst.