Systematic study on the Ni exsolution behavior of NiAl₂O₄ catalysts for steam methane reforming

The NiAl₂O₄ spinel exhibited a metal exsolution phenomenon to form Ni nanoparticles and nonstoichiometric Ni_1−xAl₂O_4−x without structural collapse in a reducing atmosphere. The Ni exsolution behavior of NiAl₂O₄ was strongly affected by the reducing conditions, such as the reduction temperature and holding time during the reduction. The average particle size and amount of the exsolved Ni metal increased with increase in both the reduction temperature and the holding time. The re-dissolution oxidation reaction of the exsolved Ni into Ni_1−xAl₂O_4−x to form NiAl₂O₄ occurred reversibly over 900 °C. The exsolved Ni nanoparticles were socketed into the surface of the reduced NiAl₂O₄ matrix and well dispersed without any agglomeration. Owing to this microstructure leading to an extension of the catalytic active site and suppression of agglomeration, the Ni-exsolved NiAl₂O₄ exhibited better steam methane reforming performance compared with the commercial Ni/Al₂O₃ catalyst. In addition to the improvement in catalytic activity, the Ni-exsolved NiAl₂O₄ showed strong tolerance for the formation of carbon due to a smaller driving force for carbon diffusion resulting from the nano-socketed structure.