Initial Stages of Hydrogen-Enhanced Methane Reduction of Nickel Oxide

This study investigates the initial stages of hydrogen-enhanced methane (CH4) reduction of nickel oxide (NiO) under reduced gas pressure (below 15 kPa) at 1050 °C. An experimental setup to investigate the reduction process enabled rapid (10–100 ms) reaction termination and quenching of samples to probe these early stages. The effects of hydrogen (H2) pretreatment on NiO and the reduction with H2 + CH4 mixtures with a low H2 content are investigated at nearly zero degrees of conversion. The results are compared to reductions by H2 or CH4 under similar conditions. Even brief H2 pretreatment or low H2 content in the H2 + CH4 mixture significantly alters the nucleation and kinetics of CH4 reduction. In the absence of H2, during CH4 reduction, Ni nuclei appear in selected areas on NiO and grow into the NiO bulk, forming a porous structure. High-resolution electron microscopy investigations suggest that H2 facilitates rapid Ni nucleation on the entire surface of NiO crystals, while it does not change the growth mechanism. Continuous reduction leads to significant morphological evolution of Ni, creating nanoscale grains and porous structures. CH4 decomposition on nanoscale Ni accumulates carbon on the pore walls, and high carbon solubility in the newly reduced Ni lattice accelerates the overall reduction reaction. This finding is crucial for understanding the dynamics of NiO reduction in industrial applications, offering insights into optimizing the process for enhanced efficiency.