Novel Catalysts Development for NOx Reduction from H2 Internal Combustion Engines

Doctoral thesis, 2025

As a clean energy source, hydrogen contains no carbon and thus produces no greenhouse gas CO₂ during combustion. This has generated significant interest and development in applying hydrogen internal combustion engines (H₂-ICE) to heavy-duty vehicles. To address the NO_x emissions resulting from the combustion process, NO selective reduction by H₂ (H₂-SCR) has been considered a promising aftertreatment solution. This work focuses on developing efficient H₂-SCR catalysts while minimizing the formation of N₂O as a byproduct, with additional consideration given to water tolerance, hydrothermal stability, and operating temperature range.

The H₂-SCR reaction process over Pt/SSZ-13 was first investigated. The results revealed that 0.5 wt% Pt/SSZ-13 exhibited the highest catalytic performance; higher hydrogen concentrations favored N₂ selectivity; and SSZ-13 as a support provided better hydrothermal stability compared to BEA. However, the H₂-SCR reaction on Pd/SSZ-13 was clearly inhibited by water vapor. In the mechanistic study using in situ DRIFTS experiments, the presence of NH₄⁺ as a reaction intermediate was confirmed.

Pt/SSZ-13 was found to catalyze H₂-SCR only at low temperatures (~100 °C). To broaden the operating window and improve N₂ selectivity, Pd/SSZ-13 and Ir/SSZ-13 were studied. The NO conversion followed Pt > Pd > Ir, while N₂ selectivity showed the opposite trend. Their catalytic temperature windows covered low, medium, and high-temperature ranges, respectively. By combining the three catalysts, the effective working range of H₂-SCR was extended. For Ir/SSZ-13, reduction pretreatment and replacing SSZ-13 with BEA doubled its activity. Afterwards, reduction pretreatment and higher H₂ concentration were applied to further improve the performance of the catalyst series.

The latter work focused on Pd catalysts, due to their balanced activity and over 80% N₂ selectivity. Studies on Pd/SSZ-13 and Pd/BEA showed that Pd/BEA contained a larger amount of ionic Pd, while SSZ-13 had more small Pd oxide particles. A six-step protocol and operando XAFS were used to study Pd redox and coordination changes during reaction, with DFT-assisted XANES analysis. Subsequently, to improve the performance of Pd catalysts, various supports were examined (Al₂O₃, CeO₂, TiO₂, 5% CeO₂/Al₂O₃, 10%CeO₂/Al₂O₃, 20%CeO₂/Al₂O₃, BEA, and SSZ-13). Pd/TiO₂ showed the best activity and significant NH₃ formation. A kinetic model including mass transfer effects was established, showing that external mass transfer suppressed rapid H₂ oxidation and indirectly promoted H2-SCR.

The unique performance of TiO₂ was attributed to the strong metal-support interaction (SMSI) with the active centers. To investigate this further, Pd catalysts supported on TiO₂, ZrO₂, SSZ-13, and Mn-doped TiO₂ were studied and compared. The results confirmed that Mn doping significantly improved the performance of Pd/TiO₂. This improvement was attributed to that Mn effectively mitigated the overly strong SMSI; increasing the proportion of reduced-state Pd species; and optimized surface adsorption properties for H atoms, thereby enhancing hydrogen spillover. The enhanced hydrogen spillover also reduced the volcano-shaped NO conversion profile typically observed for Pd catalysts. The water concentration was increased from 5% to 12% to evaluate the water tolerance of the catalysts. It was found that Pd oxides exhibited weaker water resistance compared to SSZ-13, but still maintained relatively high catalytic activity using 12% water. Finally, since the Pd/MnTiO₂ catalyst generated the most NH₃ as a byproduct, a Cu/SSZ-13 catalyst was placed downstream to fully consume the NH₃ and further reduce NO. As a result, the overall NO conversion using 5% water increased from 60% to 80%.