Reversal of methanation-oriented to RWGS-oriented Ni/SiO2 catalysts by the exsolution of Ni2+ confined in silicalite-1

Abstract

Investigation of catalytic hydrogenation of CO₂ to CO via the reverse water–gas shift (RWGS) was undertaken using Ni/SiO₂-based catalysts. Among the array of catalysts tested, the Ni/SiO₂ catalyst derived from the reduction of silicalite-1-encapsulated, ligand-protected Ni²⁺ (Ni_0.2@S-1-red) exhibited promising performance. This catalyst demonstrated a CO₂ conversion rate approaching the equilibrium conversion of RWGS, a selectivity for CO exceeding 99%, and a high space time yield of CO (9.7 mol g_Ni⁻¹ h⁻¹). The outcomes observed can be attributed to several factors, such as the highly dispersed Ni⁰ and Ni^δ+ species, as well as the presence of bridging oxygen of the Ni–O–Si structure, on which CO₂ can be adsorbed moderately. The moderately bonded CO₂ on Ni_0.2@S-1-red allows for the efficient desorption of its reduced intermediate, i.e. *CO, resulting in the generation of gaseous CO at a rapid rate, consequently preventing its deep hydrogenation to CH₄. Complementary Density Functional Theory (DFT) calculations were performed and revealed that CO molecules have poor adsorption and higher adsorption energy on the Ni@S-1 surface compared to the S-1 surface. This supports the rapid desorption of *CO and the observed high selectivity of CO. Moreover, the structure–activity correlation analysis further supports the claim of Ni_0.2@S-1-red as a promising RWGS catalyst.