Alkali Promotion in the Formation of CH₄ from CO₂ and Renewably Produced H₂ over Supported Ni Catalysts

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 CH_x (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.