Ni-In Synergy in CO₂ Hydrogenation to Methanol

Indium oxide (In₂O₃) is a promising catalyst for selective CH₃OH synthesis from CO₂ but displays insufficient activity at low reaction temperatures. By screening a range of promoters (Co, Ni, Cu, and Pd) in combination with In₂O₃ using flame spray pyrolysis (FSP) synthesis, Ni is identified as the most suitable first-row transition-metal promoter with similar performance as Pd-In₂O₃. NiO-In₂O₃ was optimized by varying the Ni/In ratio using FSP. The resulting catalysts including In₂O₃ and NiO end members have similar high specific surface areas and morphology. The main products of CO₂ hydrogenation are CH₃OH and CO with CH₄ being only observed at high NiO loading (≥75 wt %). The highest CH₃OH rate (∼0.25 g_MeOH/(g_cat h), 250 °C, and 30 bar) is obtained for a NiO loading of 6 wt %. Characterization of the as-prepared catalysts reveals a strong interaction between Ni cations and In₂O₃ at low NiO loading (≤6 wt %). H₂-TPR points to a higher surface density of oxygen vacancy (O_v) due to Ni substitution. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electron paramagnetic resonance analysis of the used catalysts suggest that Ni cations can be reduced to Ni as single atoms and very small clusters during CO₂ hydrogenation. Supportive density functional theory calculations indicate that Ni promotion of CH₃OH synthesis from CO₂ is mainly due to low-barrier H₂ dissociation on the reduced Ni surface species, facilitating hydrogenation of adsorbed CO₂ on O_v.