Interface of Ni-MgCr₂O₄ Spinel Promotes the Autothermal Reforming of Acetic Acid through Accelerated Oxidation of Carbon-Containing Intermediate Species

Abstract

Autothermal reforming (ATR) is an effective route for hydrogen production from acetic acid (HAc) derived from biomass. Ni-based catalysts are promising candidates for ATR due to their high activity, but coke formation hinders their practical application. To tackle this issue, a series of Ni-Mg-Cr catalysts with supports of Cr2O3 or MgCr2O4 were prepared by the sol–gel method and evaluated in ATR. The results indicated that as compared to the Ni-Cr2O3 catalyst, the Ni0.25Mg0.75CrO3.5±δ catalyst with MgCr2O4 support presented higher catalytic performance: the conversion rate of acetic acid was stable near 100%, with hydrogen yield reaching 2.64 mol-H2/mol-HAc during a 40 h ATR test, while there was no obvious coking. It was found that Mg modification was prone to constituting a stable MgCr2O4 spinel support with a high specific surface area for adsorption and transformation of HAc; however, for catalysts with excessive Mg addition, namely, Ni0.43Mg2.56CrO4.5±δ and Ni0.69Mg5.31CrO7.5±δ, low reactivity was found and was linked to constraining of Ni from the solid solution of Mg(Ni)O. Density functional theory (DFT) calculations reveal that during the ATR process, Ni4-MgCr2O4 presents a low energy barrier for the overall transformation path and a high stabilization of reaction intermediates; furthermore, as compared to Ni4-Cr2O3, oxidation of C* species by O* and OH* is significantly accelerated on Ni4-MgCr2O4 due to the considerably decreased energy barriers, thus eliminating carbon deposition and improving catalytic activity.