Urea–nitrate combustion synthesis of CuO/CeO₂ nanocatalysts toward low-temperature oxidation of CO: the effect of Red/Ox ratio

The low-temperature catalytic oxidation of carbon monoxide (CO) is the key process to overcome incomplete combustion of fossil fuels for both commercial (e.g., lowering automotive emissions) and domestic (e.g., indoor air quality improvement) applications. So the development of cost-effective and high-active catalysts made from non-noble metals is an actual material science challenge. Ce–Cu–O oxide system is one of the most prospective to investigate due to the feasibility of the Langmuir–Hinshelwood dual-site oxidation mechanism, and, in our previous work, it was shown that solution combustion synthesis using the urea as fuel in stoichiometric ratio with metal nitrates is a most reasonable fuel choice toward the synthesis of CuO/CeO₂ nanocatalysts for effective CO oxidation. In this work, the effect of the Red/Ox (urea to nitrates or U/N) ratio on the catalytic oxidation activity of resulting CuO/CeO₂ composites is in the spotlight. Series of CuO/CeO₂ nanocatalysts were successfully synthesized via the urea–nitrate combustion method with U/N ratios from 0.2 to 1.4 and then were applied for CO oxidation. The samples before and after catalysis were characterized by EDX, SEM, PXRD, N₂-ASA, H₂-TPR, etc. It was shown that the U/N ratio strongly affects the qualitative and quantitative X-ray phase composition (c-CeO₂, m-CuO, and am-CuO, in general), CeO₂ and CuO crystallite sizes (4.9–63.0 nm and 14.7–28.3 nm, correspondently), specific surface area (4–46 m²/g) and porosity (0.009–0.033 cm³/g), as well as the morphology of nanocomposites and their catalytic characteristics (t_50% = 113–199 °C and t_100% = 137–348 °C). The sample synthesized at U/N = 0.2 shows the best oxidative performance (t_50% = 113 °C and t_100% = 137 °C) explained by the highest surface (46 m²/g), the lowest CeO₂ crystallite size (4.9 nm), and the lowest degree of agglomeration of nanocrystals in the CuO/CeO₂ composite. Besides, cyclic and water resistivity tests of this sample confirm the high stability of its catalytic activity in the CO oxidation process due to the close conjugation of the CeO₂ and CuO components in the resulting nanocomposite, confirmed by the H₂-TPR analysis results. Thus, it was shown that the optimization of Red/Ox ratio in the urea–nitrate combustion process contributes to obtaining the stable CuO/CeO₂ nanocatalyst with gradual conversion of CO at quite low temperatures (t_i ~ 50 °C). We believe that the subsequent reduction of CuO loading in the Ce–Cu–O system allows creating a cost-effective and stable composite nanocatalyst with high activity even at room temperature.