The influence of alkali and alkaline earth metals on RU/YSZ catalyst for catalytic and electrochemical promotion of Carbon Dioxide Methanation

Carbon dioxide (CO₂) is the major contributor of global greenhouse gas (GHG) emissions, leading to an increase in the global temperature. CO₂ utilisation is a sensible approach to mitigate the CO₂ emission while conserving the carbon resources. One of the ways to utilise CO2 is through hydrogenation process which allows efficient conversion of CO₂ into value added fuels like methane (CH4). In this study, catalytic hydrogenation of CO₂ into CH4 and carbon monoxide (CO) was investigated over a ruthenium catalyst film supported on 8 mol% yttria-stabilised zirconia pellet (Ru/YSZ), externally modified with several alkali and alkaline earth metal species i.e. sodium (Na), potassium (K), calcium (Ca) and magnesium (Mg) at low coverage (0.16%) between 250-400°C in a single chamber reactor. A stronger promotion of the catalytic activity and product selectivity of the methanation reaction was observed in the Ru/YSZ system modified with alkali metals i.e. Na and K. Of these, a higher catalytic activity was observed for Ru/YSZ modified with K at low coverage (0.16%), attributed to the smaller Ru crystallite size which results in higher Ru catalyst dispersion. The Ru-K/YSZ system at varied K coverage (0.16-16%) was further investigated in terms of the reaction kinetics. The calculated order of methanation reaction with respect to H2 and CO₂ concentrations indicated an Eley-Rideal mechanism for the Ru sample, in which one of the reactants (CO₂) needs to adsorb, while the other reacts directly from the gas phase, but this changed at higher potassium coverage (16%), to a Langmuir-Hinshelwood mechanism where reaction depends on both reactants’ adsorption as both have weak chemisorption. The increase in the activation energy with higher potassium coverage may be due to the increase in the intrinsic activity barrier on the catalyst because of the presence of metal species which blocked the active surface sites. On the other hand, a higher potassium coverage was in favour of CO production possibly due to weaker adsorption of CO₂. Electrochemical promotion of CO₂ methanation mainly shows an inverted-volcano type promotional behaviour with strong non-faradaic rates except for CO production at 16% K (volcano type). A synergistic effect, in which reaction rate is higher than that caused by addition of K and polarisation can be observed at 16% K, indicating that both K and oxygen species spillover onto the catalyst react favourably to some extent to promote CO₂ methanation.