Effect of sodium promotion on zinc-ferrite catalysts for CO and CO2 co-conversion: Insights from in situ and theoretical studies

This study investigates the influence of sodium (Na) on the co-conversion of CO and CO₂ over Na-promoted zinc-ferrite catalysts. Using a combination of H₂-TPR, CO-TPR, in situ Raman spectroscopy, calorimetry, and density functional theory (DFT) calculations, we demonstrate that Na significantly modifies catalyst performance. Even at low Na levels (Na/Fe = 0.06), CO conversion is accelerated by up to 120 times compared to CO₂, while methanation is suppressed and olefin production is enhanced. However, excess Na leads to graphite formation and coke deposition. In situ Raman analysis reveals the transformation of Fe oxides into Fe carbides in Na-doped catalysts, while calorimetry indicates a reduced strength in the exothermicity associated with suppressed methanation and enhanced C₂+ hydrocarbon formation. DFT calculations further demonstrate that high Na content strengthens adsorption, particularly for CO₂ and 1-hexene, increasing the likelihood of coke formation. These findings emphasize the key role of Na in modifying catalytic activity and offer valuable insights for optimizing CO/CO₂ co-conversion processes.