Reduction Kinetics of Nickel Species Supported on Silica

The reaction mechanism of reduction processes of NiO supported on silica has been clarified by means of the time-resolved dispersive XAFS (DXAFS) technique. The reduction of NiO shows two-phase changes. The first reaction corresponds to the reduction of NiO to the metallic Ni particle, and the sintering of the formed Ni particles occurs in the second step. The reduction process is composed of the surface reduction of the NiO particle and the successive oxygen diffusion from the particle core to the surface. The first-order kinetics suggests that the latter is the rate-determining step. The conditional rate constants indicate that the diffusion of oxygen atoms is relatively faster for the lower Ni loading.