Catalytic Soot Combustion-General Concepts and Alkali Promotion

In this Review, we discuss catalytic systems for soot combustion. The focus is on alkali-containing materials since they exhibit the most promising and platinum group metal (PGM) free solutions, providing highly efficient catalysts at a low price. The wide range of experimental conditions used for catalyst evaluation and parameters commonly used to compare different materials was scrutinized. The presented synthetic summary of the classical and long-researched materials reveals that the most active catalysts, which may fulfill the low-temperature activity apart from PGM-based systems, are only those containing alkalis as beneficial dopants. The alkali promotion by surface doping or nanostructuration of earth abundant transition metal oxides emerged as an effective way to develop effective soot oxidation catalytic materials; therefore, the physical nature of alkali promotion is thoroughly discussed. A detailed presentation of the state of the art three-dimensional macroporous materials is presented, as these catalysts combine beneficial catalytic activity with morphological features designed for enhancing the contact between the catalyst and the soot particles. Finally, a set of reactivity descriptors for the alkali-promoted catalysts (work function, alkali desorption energy, and oxygen availability) was proposed to foster the rational design of catalytic materials for soot oxidation.