Antiover-Reduction of Ni/In₂O₃ Nanocatalysts by Atomic Layer Deposition of Al₂O₃ Films for Durable CO₂ Hydrogenation to Methanol

Abstract

The supported Ni/In2O3 catalysts are of great interest in CO2 hydrogenation, but the formation of the In0 or Ni–In alloy phases due to over-reduction would lead to rapid catalyst deactivation. Herein, thin Al2O3 films were deposited on the Ni/In2O3 surface by atomic layer deposition (ALD) as an antireduction agent, which enables the In2O3 catalyst to maintain a significant abundance of active In3+ species even during high-temperature H2 treatment (400 °C, 2 h), thereby effectively resisting the deactivation caused by over-reduction of In2O3. Various characterization methods confirm the antiover-reduction effect of ALD-Al2O3 films, namely, inhibiting the formation of In0 and In3Ni2 alloys during high-temperature H2 treatment and facilitating the reoxidation of low-valence In to form the active In2O3–x phase in the presence of a CO2 atmosphere. As treated with H2 at 400 °C, the 5Al2O3/Ni/In2O3-R400 catalyst exhibited an STYMeOH of 7.39 gMeOH h–1gNi–1 and a methanol selectivity of 64% (reaction conditions: 3 MPa, 300 °C, and 12,000 mL gcat–1h–1). Particularly, the STYMeOH increased by 11.5 times as compared with that of the Ni/In2O3-R400 catalyst without coating the ALD-Al2O3 layer (0.59 gMeOH h–1gNi–1). Density functional theory (DFT) calculations validate the effectiveness of specific charge transfer tendencies in suppressing the over-reduction of high-valence In species. Through the analysis of the crystal orbital Hamilton population (COHP) and projected density of states (PDOS) electronic structures of the adsorbate species, the Al2O3/Ni/In2O3 catalyst is shown to significantly enhance CO2 activation and further catalytic reactions. Accordingly, this study unveils the deactivation mechanism of In2O3-based catalysts due to over-reduction and provides a method to regulate their reduction level by depositing ALD-Al2O3 films on the In2O3-based catalysts.