Nickel Speciation and Methane Dry Reforming Performance of Ni/Ce<sub><i>x</i></sub>Zr<sub>1–<i>x</i></sub>O<sub>2</sub> Prepared by Different Synthesis Methods
Yimeng Lyu, Jennifer N. Jocz, Rui Xu, Eli Stavitski, Carsten Sievers
Abstract
Ceria–zirconia-supported Ni catalysts (Ni/Ce0.83Zr0.17O2 or Ni/CZ) are prepared by dry impregnation, strong electrostatic adsorption, coprecipitation (CP), and combustion synthesis (CS). The nature and abundance of Ni species in these samples are characterized by X-ray adsorption spectroscopy, temperature-programmed reduction, and CO chemisorption. The bulk synthesis methods (i.e., CP and CS) produce Ni cations that are incorporated into the CZ lattice forming mixed-metal oxides with Ni3+ species at low Ni content. The formation of mixed-metal oxides increases the reducibility of CZ and increases the abundance of active surface oxygen. All NiO/CZ catalysts are active for methane dry reforming and retain some of their activity at a steady state. The initial methane conversion correlates linearly with the fraction of accessible Ni after reduction. The predominant path of catalyst deactivation strongly depends on the structure of the catalyst and, thus, on the synthesis method used. All catalysts experience agglomeration of Ni particles under reaction conditions. Improving the Ni dispersion to isolated species embedded in a support does not improve resistance to Ni particle growth. Coke formation is inversely related to the concentration of active surface oxygen. The dominant deactivation mechanism for catalysts made by CS is the encapsulation of Ni particles by the support.