Structural effect of ZrO₂ on supported Ni-based catalysts for supercritical water gasification of oil-containing wastewater

Supercritical water gasification (SCWG) is a promising technology to treat oil-containing wastewater. This study reports a Ni/ZrO₂ catalyst with mixed phase of tetragonal ZrO₂ and monoclinic ZrO₂ synthesized by a one-step sol-gel method, which shows good catalytic activity and stability in SCWG of oil mixture. The evolution of particle size, crystal structure, and pore size caused by increasing calcination temperature is demonstrated by X-ray diffraction and transmission electron microscopy. When calcined at 700 °C, Ni/ZrO₂ shows superior catalytic activity and stability in SCWG reaction, with a high carbon gasification efficiency of 91.3% at 500 °C. The characterizations of fresh and spent catalysts suggest that Ni/ZrO₂ catalyst with the fraction of monoclinic ZrO₂ around 75% would be highly active and stable in SCWG reaction. In addition, two sets of optimal gasification conditions of SCWG of oil are obtained by varying operating parameters, which provides guidance for different concentrations of oil-containing wastewater treatment.