Unlocking dynamic intermetallic synergy: Ir/Ni alloy nanoparticles catalyze CO2 hydrogenation to formic acid in ionic liquid environments

Abstract

The Bimetallic Ir/Ni alloy nanoparticles (∼2.0 nm) are prepared in 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIm.NTf₂) ionic liquid, which have a unique architecture, featuring an Ir-rich alloy core surrounded by a Ni-rich alloy. The prepared bimetallic NPs exhibited efficient activity for the hydrogenation of CO₂ to formic acid (1.02 M) with TOFs of 151.7 h⁻¹ as compared to their counterparts; Ir NPs (TOF 38.5 h⁻¹) and Ni NPs (TOF 0) in 1-n-butyl-3-methylimidazolium acetate (BMIm.OAc) IL solutions. This synergistic metal dilution effect arises from the inter-particle heterogeneities present in our Ir/Ni alloy NPs, causing alterations in surface structure and composition with an increase in the number of d-band holes. It offers a diverse population of active sites and tunes the adsorbate migration between reactive intermediates. The BMIm.OAc IL representes multifunctional roles, acting as a buffer and creating an IL-solvent cage around the NPs, reactants, and reactive intermediates. This alters the entropy by increasing the number of microstates, overcoming thermodynamic limitations. The synchrotron X-ray photoelectron spectroscopy and soft-XANES analyses reveal that the Ir/Ni NPs have variable electron density, the NPs surface have lower electron density than those at inner regions, which enhance the number of valence vacancies. The dilution of iridium with nickel in our Ir/Ni NPs attributes the abundance of d-band holes and O-p holes to the high oxidation state at the surface, and increases the presence of lattice vacancies. These distinct characteristics of NPs significantly boost the FA formation as compared to their counterparts. DFT calculations confirmed that the formation of FA follows the hydrogenation of HCO₃* reactive intermediates.