Kinetic Parameters of Soot Oxidation Catalyzed by Nanosized ZnO-CeO2 Solids

In the recent decades, the change in the air quality caused by pollutant emissions that are harmful to human health and to the environment has been a subject of an increasing concern. Among these pollutants, particulate emissions arising from the incomplete combustion of diesel deserve a special attention due to many adverse health effects, especially on the respiratory system. In view of this scenario, several studies have focused on practical ways to overcome this problem through the development of devices able to mitigate the emission of particulate matter in engines operating in diesel cycles. In this sense, catalytic filters are designed to reduce the amount of soot generated in the diesel combustion through the retention or low-temperature oxidation of carbonaceous materials. The construction of catalytic converters usually involves the use of several metals and metal oxides and rare earth-based compounds are a promising class for such systems. Particularly, cerium oxide is a widely employed catalyst due to its stability, high oxygen lability and redox behavior arising from its non-stoichiometric structure. In addition, the modification of CeO2 with other metals usually enables the formation of defects and vacancies, which are directly associated to the formation and diffusion of reactive oxygen species. This, in turn, results in a more efficient soot oxidation at low temperatures, thus allowing for the use of less expensive metals and lower amounts of loaded compounds for the construction of the catalytic converters. Prior to the development of the final catalysts and the composition of the ceramic substrates, which are commonly placed in the engines exhaust tubes, the evaluation of the catalytic properties in model conditions is proved to be highly valuable. In these cases, generally, thermal analyses of mixtures of soot standards and solid catalysts provide essential information about the involved mechanisms, energies and temperatures. Recently, it has been shown that the addition of zinc oxide to CeO2 solids leads to a large enhancement of the stability and catalytic ability of such compounds in the decomposition of soot generated in diesel engines. Therefore, this communication presents the evaluation of the synergistic effects of the addition of ZnO on the catalytic ability of CeO2 nanoparticles for soot oxidation. For this, the determination of the kinetic activation parameters (i.e., activation energies, enthalpies, entropies and Gibbs free energies of activation, and pre-exponential factors) can be readily obtained from thermal analysis experiments, which involved on the oxidation of a soot model (Printex-U, Degussa) in the presence of the prepared catalysts in nonisothermal transient experiments.