Enhancing the Performance of Pd/zeolite-based H2-SCR Catalysts: The Role of Noble Metal Loading, Promoter Addition, and Combination with a Conventional Fe-BEA NH3-SCR Catalyst

Abstract

The direct utilization of hydrogen (H₂) as a reductant in the selective catalytic reduction (SCR) of nitrogen oxides (NO_x) presents a promising strategy for mitigating emissions from hydrogen-fueled internal combustion engines (H₂-ICEs). Despite recent advancements, H₂-SCR suffers from a narrow operational temperature window and limited product selectivity. In this study, Pd/TiO₂-HY catalysts with varying palladium loadings were synthesized, characterized, and evaluated for their catalytic performance. A strong correlation between NO conversion and H₂ activation was observed, particularly at low temperatures, with higher Pd content enhancing pollutant abatement. The incorporation of tungsten oxide (WO₃) as a promoter improved catalytic activity and suppressed ammonia (NH₃) formation under both dry and humid conditions, albeit at the cost of increased nitrous oxide (N₂O) emissions. Alternatively, vanadia (V₂O₅) addition reduced secondary emissions but compromised NO_x conversion efficiency. A bifunctional catalyst system combining Pd/TiO₂-HY with a conventional NH₃-SCR catalyst, Fe-BEA, achieved high NO conversion and with more than 90% N₂ selectivity between 180–300 °C also superior product selectivity. Spatially resolved concentration profiles in monolithic samples revealed that Fe-BEA mediates N₂O suppression and facilitates the rapid consumption of NH₃ species formed in situ over Pd/TiO₂-HY. These findings underscore the potential of advanced catalyst combinations to overcome current limitations of H₂-SCR and pave the way for cleaner hydrogen-based combustion technologies.