Silica-supported nickel catalysts synthesized via the molten salt method for reverse water-gas shift: Impact of chlorine and bromine halides

Nickel catalysts supported on silica were synthesized via a molten salt method (MSM) using Na- and K-based salts with Cl⁻ or Br⁻ counterions, and evaluated in low-temperature reverse water–gas shift (RWGS) reaction. Despite similar Ni nanoparticle sizes, the presence of residual salts significantly influenced catalyst performance by altering the electronic properties of Ni and the nature of surface carbonates. X-ray absorption spectroscopy (XAS) revealed negatively charged Ni species (Ni δ⁻ ), particularly in Br-containing samples. H 2 -temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) confirmed Ni-Cl and Ni-Br interactions, and the latter showed a higher extent. In-situ infrared studies indicated that Br-based catalysts suppressed the formation of bidentate carbonate (b-*CO 3 ), a spectator that passivates active sites, and instead favored monodentate carbonate (m-*CO 3 ), leading to higher CO 2 conversions. The r-Ni@Na 1.9 K 3.3 Br(5)/SiO 2 catalyst achieved stable CO 2 conversion ( ca. 35 %) with 100 % CO selectivity and 100-hour durability. These results highlight the importance of halide identity in modulating Ni–salt interactions and reaction pathways for RWGS.