Phase-dependent catalytic performance of sol-gel synthesized ZrO2 in supercritical water gasification of formaldehyde

The catalytic performance of ZrO2 samples synthesized via sol-gel (SG) and calcined at 500 degrees C (ZrO2-500-SG) and 1400 degrees C (ZrO2-1400-SG), as well as commercial ZrO2 (ZrO2-C), was investigated for the supercritical water gasification (SCWG) of formaldehyde (CH2O). ZrO2-500-SG, featuring a mixed tetragonal-monoclinic phase and nanometric particle size, exhibited the highest hydrogen selectivity, achieving CH2O conversion of 91 % and carbon gasification efficiency (CGE) of 72 % at 400 degrees C, 30 min, and 10 wt.% CH2O. In contrast, ZrO2-1400-SG and ZrO2-C, predominantly monoclinic with larger particle sizes, favored methane formation while showing lower hydrogen yields. The enhanced activity of ZrO2-500-SG is attributed to abundant oxygen vacancies, high structural defect density, and active surface oxygen species, as confirmed by O2-TPD and H2-TPR analyses. The stability of the catalysts was demonstrated by thermal resistance (TGA), retention of crystallographic phases and microstructures after SCWG, and consistent catalytic performance over multiple reactions. Reaction conditions including temperature, residence time, and CH2O concentration were found to strongly influence product distribution, lattice strain, and particle morphology, highlighting the interplay between structural properties and catalytic behavior.