Oscillatory Behaviour of Ni Supported on ZrO₂ in the Catalytic Partial Oxidation of Methane as Determined by Activation Procedure

Ni/ZrO₂ catalysts, active and selective for the catalytic partial oxidation of methane to syngas (CH₄-CPO), were prepared by the dry impregnation of zirconium oxyhydroxide (Z_hy) or monoclinic ZrO₂ (Z_m), calcination at 1173 K and activation by different procedures: oxidation-reduction (ox-red) or direct reduction (red). The characterization included XRD, FESEM, in situ FTIR and Raman spectroscopies, TPR, and specific surface area measurements. Catalytic activity experiments were carried out in a flow apparatus with a mixture of CH₄:O₂ = 2:1 in a short contact time. Compared to Z_m, Z_hy favoured the formation of smaller NiO particles, implying a higher number of Ni sites strongly interacting with the support. In all the activated Ni/ZrO₂ catalysts, the Ni-ZrO₂ interaction was strong enough to limit Ni aggregation during the catalytic runs. The catalytic activity depended on the activation procedures; the ox-red treatment yielded very active and stable catalysts, whereas the red treatment yielded catalysts with oscillating activity, ascribed to the formation of Ni^δ+ carbide-like species. The results suggested that Ni dispersion was not the main factor affecting the activity, and that active sites for CH₄-CPO could be Ni species at the boundary of the metal particles in a specific configuration and nuclearity.