Alkali Promotion in the Formation of CH4 from CO2 and Renewably Produced H-2 over Supported Ni Catalysts

Abstract

In Power‐to‐Methane (PtM) plants, the renewable electricity supply can be stabilized by using green electrons to produce H2 via H2O electrolysis, which is subsequently used to hydrogenate CO2 into CH4. In this work PtM is studied in a cascade fashion, from simulated solar light to methane production in an all‐in‐one setup, which was newly developed for this work. This setup was used to assess the effects of H2 stream purity on the activity of Ni/SiO2 catalysts in CO2 methanation. An activity effect in downstream methanation is shown to be onset by aerosols that evolve from the electrochemical splitting of water. Small amounts of K are shown to affect CH4 production positively, but only if they are deposited in situ, via KOH aerosols. K‐doped Ni/SiO2 catalysts prepared in an ex situ manner, by impregnation with a KOH solution, showed a decrease in activity, while the same amount of KOH was deposited. Operando FT‐IR spectroscopy reveals that increased back‐donation to CO‐containing intermediates and carbonates formation likely causes catalyst deactivation in ex situ samples as often reported in literature for Ni/SiO2 catalysts. The mechanism for in situ promotion is either an increased rate in the hydrogenation of CHx (X=0–3) fragments, or a more facile water formation or desorption as CO‐containing reaction intermediates are unaffected by in situ promotion. These results are relevant to PtM from a fundamental standpoint explaining the effect of potassium on nickel methanation, but also from a practical standpoint as the presented effect of in situ promotion is difficult to achieve via standard synthesis methods.