Effect of initial nickel particle size on stability of nickel catalysts for aqueous phase reforming

The deactivation behavior by crystallite growth of nickel nanoparticles on various supports (carbon nanofibers, zirconia, SiC, α-Al₂O₃ and γ-Al₂O₃) was investigated in the aqueous phase reforming of ethylene glycol. Supported Ni catalysts of ∼10 wt% were prepared by impregnation of carbon nanofibers (CNF), ZrO₂, SiC, γ-Al₂O₃ and α-Al₂O₃. The extent of the Ni nanoparticle growth on various support materials follows the order CNF ∼ ZrO₂ > SiC > γ-Al₂O₃ >> α-Al₂O₃ which sequence, however, was determined by the initial Ni particle size. Based on the observed nickel leaching and the specific growth characteristics; the particle size distribution and the effect of loading on the growth rate, Ostwald ripening is suggested to be the main mechanism contributing to nickel particle growth. Remarkably, initially smaller Ni particles (∼12 nm) supported on α-Al₂O₃ were found to outgrow Ni particles with initially larger size (∼20 nm). It is put forward that the higher susceptibility with respect to oxidation of the smaller Ni nanoparticles and differences in initial particle size distribution are responsible for this behavior.