Soot Combustion over Cu–Co Spinel Catalysts: The Intrinsic Effects of Precursors on Catalytic Activity

In this work, a series of CuCo2O4-x (x = N, A and C) catalysts were synthesized using different metal salt precursors by urea hydrothermal method for catalytic soot combustion. The effect of CuCo2O4-x catalysts on soot conversion and CO2 selectivity in both loose and tight contact mode was investigated. The CuCo2O4-N catalyst exhibited outstanding catalytic activity with the characteristic temperatures (T10, T50 and T90) of 451 &deg:C, 520 &deg:C and 558 &deg:C, respectively, while the CO2 selectivity reached 98.8% during the reaction. With the addition of NO, the soot combustion was further accelerated over all catalysts. Compared with the loose contact mode, the soot conversion was improved in the tight contact mode. The CuCo2O4-N catalysts showed better textural properties compared to the CuCo2O4-A and CuCo2O4-C, such as higher specific surface areas and pore volumes. The XRD results confirmed that the formation of a CuCo2O4 crystal phase in all catalysts. However, the CuO crystal phase only presented in CuCo2O4-N and CuCo2O4-A. The relative contents of Cu2+, Co3+ and Oads on the surface of CuCo2O4-x (x = N, A and C) catalysts were analyzed by XPS. The CuCo2O4-N catalyst displayed the highest relative content of Cu2+, Co3+ and Oads. The activity of catalytic soot combustion showed a good correlation with the order of the relative contents of Cu2+, Co3+ and Oads. Additionally, the CuCo2O4-N catalyst exhibited lower reduction temperature compared to the CuCo2O4-A and CuCo2O4-C. The cycle tests clarified that the copper&ndash:cobalt spinel catalyst obtained good stability. In addition, based on the Mars&ndash:van Krevelen mechanism, the process of catalytic soot combustion was described combined with the electron transfer process and the role of oxygen species over CuCo2O4 spinel catalysts.