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

In this work, a series of CuCo(2)O(4)-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 CuCo(2)O(4)-x catalysts on soot conversion and CO(2) selectivity in both loose and tight contact mode was investigated. The CuCo(2)O(4)-N catalyst exhibited outstanding catalytic activity with the characteristic temperatures (T(10), T(50) and T(90)) of 451 °C, 520 °C and 558 °C, respectively, while the CO(2) 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 CuCo(2)O(4)-N catalysts showed better textural properties compared to the CuCo(2)O(4)-A and CuCo(2)O(4)-C, such as higher specific surface areas and pore volumes. The XRD results confirmed that the formation of a CuCo(2)O(4) crystal phase in all catalysts. However, the CuO crystal phase only presented in CuCo(2)O(4)-N and CuCo(2)O(4)-A. The relative contents of Cu(2+), Co(3+) and O(ads) on the surface of CuCo(2)O(4)-x (x = N, A and C) catalysts were analyzed by XPS. The CuCo(2)O(4)-N catalyst displayed the highest relative content of Cu(2+), Co(3+) and O(ads). The activity of catalytic soot combustion showed a good correlation with the order of the relative contents of Cu(2+), Co(3+) and O(ads). Additionally, the CuCo(2)O(4)-N catalyst exhibited lower reduction temperature compared to the CuCo(2)O(4)-A and CuCo(2)O(4)-C. The cycle tests clarified that the copper–cobalt spinel catalyst obtained good stability. In addition, based on the Mars–van Krevelen mechanism, the process of catalytic soot combustion was described combined with the electron transfer process and the role of oxygen species over CuCo(2)O(4) spinel catalysts.