Transition Metal (Fe, Cu, Ni and Co) Decorated CeO2x-ZrO2(100-x) Mesoporous Structure for Enhanced Low-Temperature Oxidation of Formaldehyde

CeO2x-ZrO2(100-x) mixed oxides (x = 0, 5, 15, 25 and 50 wt.%) were successfully synthesized via the sol–gel method and further promoted with 5 wt.% of different transition metals (Ni, Fe, Cu, and Co) through impregnation. XRD analysis confirmed the formation of a tetragonal Ce-Zr solid solution at low ceria loadings (5–25 wt.%), while the Ce50Zr50 sample showed coexistence of tetragonal and cubic solid-solution phases, consistent with the CeO2-ZrO2 phase diagram; crystallite size decreased systematically with increasing ceria content. Physisorption measurements revealed a mesoporous texture for all solids, with pore size and pore shape characteristics varying with ceria content, and specific surface areas increasing with increasing ceria loading. The catalysts were evaluated for formaldehyde (HCHO) oxidation. Complete HCHO conversion was achieved from 50 °C over pure ZrO2 and Ce50Zr50, whereas the Ce5Zr95, Ce15Zr85 and Ce25Zr75 samples showed markedly lower activity at this temperature, revealing a non-monotonic dependence of catalytic activity on ceria content. The Ce50Zr50 support showed high CO2 selectivity, indicating near-total mineralization of HCHO and reduced formation of by-products. The incorporation of Cu and Co into this optimal support maintained a high CO2 selectivity, comparable to that of the bare support, while reducing the formation of minor by-products; the Fe-containing catalyst showed lower activity and selectivity. Oxygen storage capacity (OSC) measurements confirmed the redox ability of the cerium-containing solids; however, since pure ZrO2, which showed no measurable OSC, was also highly active and selective, catalytic performance cannot be attributed to OSC alone and is more likely governed by a combination of textural (surface area, porosity) and structural (defect/vacancy density) properties. HRTEM analysis showed that all samples contained similar ceria–zirconia support crystallites, which were well defined, highly crystalline, and exhibited a particle size distribution of approximately 5–15 nm. H2-TPR and XPS analyses provided insights into the presence and nature of metal species on the support surfaces.