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.