Influence of sodium-modified Ni/SiO2 catalysts on the tunable selectivity of CO2 hydrogenation: Effect of the CH4 selectivity, reaction pathway and mechanism on the catalytic reaction
2021
Wu, Hung-Chi | Chen, Tse-Ching | Wu, Jia-Huang | Pao, Chih-Wen | Chen, Ching-Shiun
CO₂ hydrogenation over Ni/SiO₂ catalysts with and without Na additives was investigated in terms of the catalytic activity, selectivity of CO₂ methanation and reaction mechanism. Na additives could cause the formation of Na₂O species that might deposit on the Ni surface of Ni/SiO₂ (NiNaₓ/SiO₂). When the Ni metal is partially covered with Na₂O species, a highly positive charge on the Ni metal could occur compared to the original Ni/SiO₂ catalyst. The addition of Na to the Ni/SiO₂ catalyst could influence selectivity toward CO formation. The adsorbed formic acid is the major intermediate on the Ni/SiO₂ catalyst during CO₂ hydrogenation. The formic acid species might decompose into adsorbed CO complexes in the forms of linear CO, bridged CO and multibonded CO. CH₄ formation should be ascribed to the hydrogenation of these adsorbed CO complexes. The Ni/SiO₂ catalyst with the Na additive might have very weak ability for H₂ and CO adsorption, thus making it difficult for CO methanation to occur. The hydrogen carbonate species adsorbed on the NiNaₓ/SiO₂ catalysts were proposed to be the key intermediate, and they might decompose to CO or be hydrogenated to form CH₄.
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