Na-Doping Promotes CO and Hydrocarbon Formation over Ru/m-ZrO2 during CO2 hydrogenation at 20 Bar

Hybrid Na-doped 1%Ru/m-ZrO₂ catalysts consisting of reverse water-gas shift (RWGS) and Fischer-Tropsch synthesis functions were tested at 20 bar, 300°C, SV of 80,000 mL/g_cat∙h, and H₂/CO₂ = 3:1. Secondary reaction of CO, produced from RWGS, occurs on Ru metal on-top sites to produce CH₄. Addition of Na plays an important role in mitigating this undesired reaction. Although increasing Na-doping from 2.5%Na to 5%Na slightly decreased CO₂ conversion from 14% to 10%, selectivity was remarkably improved. CH₄ selectivity decreased from 60% to 21%, while CO selectivity virtually doubled from 36% to 71%. Infrared spectroscopy experiments at 1 atm and H₂/CO₂ = 3:1 showed that addition of Na increases surface basicity and attenuates Ru activity. The increased basicity promotes adsorption of CO₂, weakens the formate CH bond increasing CO selectivity during RWGS, and hinders methanation on Ru on-top sites. At 20 bar, the suppression of methanation allows the probability for carbon chain growth to be enhanced decrease in the reduction rate during TPR and TPR-EXAFS/XANES at the Ru K-edge suggests that Na is in direct contact with Ru. Following H₂ reduction, all catalysts displayed spectra of reduced Ru metal; there was no significant alteration of the white line intensity, suggesting that electronic modification of the catalyst is not due to charge transfer, but rather an effect of enhanced basicity by Na addition. Higher surface basicity increases the bond strength between catalyst surface and the -CO₂ functional group of formate, resulting in weakening of the formate C-H bond.