Synergistic Ni−Fe Nanoalloy Catalysis for CO2-Assisted Tar Reforming: Towards Sustainable Syngas Production via Toluene as a Model Compound

Abstract

Efficient conversion of biomass tar, together with effective fixation and valorization of carbon dioxide (CO2), remains a major challenge in renewable energy. Addressing this challenge requires a fundamental understanding of the atomic-scale mechanisms governing CO2-assisted catalytic tar reforming over bimetallic catalysts, which remain poorly understood despite extensive reports on macroscopic catalytic performance. Herein, multiscale characterization is integrated with density functional theory (DFT) calculations to unravel the synergistic interplay between Ni-Fe alloy and HZSM-5 acid sites in CO2-activated toluene cracking, a representative biomass tar model compound. Three previously unrecognized mechanistic insights are revealed: (i) CO2 modulates the electronic structure of Ni through Fe-mediated charge transfer, as evidenced by XPS binding energy shifts; (ii) the Boudouard reaction proceeds with an activation barrier of 1.98 eV, quantitatively accounting for the observed 66% reduction in coke deposition; and (iii) CO2 shifts the reaction pathway toward monocyclic aromatic hydrocarbons by lowering the Gibbs free energy of polycyclic aromatic hydrocarbon (PAH) condensation, resulting in a 22% increase in selectivity. Experimentally, the Ni-Fe/HZSM-5 catalyst achieves a syngas yield of 323.3 mL/g-toluene at 900 °C with a hydrogen selectivity of 56.2%, while catalyst stability is enhanced more than threefold through coke removal via the Boudouard reaction. Comprehensive characterization combined with DFT calculations further demonstrates that Fe promotes the dispersion of active Ni sites and enhances CO2 activation capability. Collectively, this work establishes a mechanistic blueprint for the rational design of coke-resistant bifunctional catalysts and provides fundamental mechanistic insights into the in situ tar removal and hydrogen production during biomass gasification, extending the applicability of CO2-assisted reforming strategies beyond biomass tar to broader hydrocarbon conversion processes.