H2-SCR over Low Loaded Platinum-Based Catalysts: Investigations in the Reaction Pathways

The pathways and mechanistic aspects of H2-SCR over precious metal-based catalysts is still under debate. This study focusses on low loaded platinum-based catalysts (0.07–0.3%) in a large temperature range (50–500 °C), with special focus on (i) the role of NH3 as a possible intermediate species, (ii) the origin of the undesired N2O emission and (iii) the platinum sites involved in the H2-SCR deNOX reactions. Up to 60 °C, the N2O selectivity was close to 100%, with no influence of the presence of oxygen in the 50–100 °C temperature range. Ammonia formation was observed at relatively low temperatures (from 60 °C), but its reactivity was then limited. All these low temperature reactions were associated with the same platinum sites, probably a mix of edge and face sites. The maximum outlet NH3 was observed around 100 °C and the role of the NH3-SCR in the whole H2-SCR process appeared very limited. On the contrary, the ammonia oxidation by O2, which started near 120 °C, significantly contributed to the H2-SCR process and appeared responsible for the second N2O emission peak (150–500 °C). This reaction did not imply the same platinum sites and appears mainly dependant on the platinum particle size.