Novel catalysts development for NOx reduction from H2 combustion engines

Licentiate thesis, 2024

Hydrogen, as a clean energy carrier, burns without producing any carbon emissions. Hydrogen internal combustion engines (H2-ICE) are considered an alternative for the transition from conventional fossil-fuel vehicles. However, the development of effective aftertreatment catalysts remains necessary to address the inevitable NOx emissions from internal combustion engines. This thesis covers the performance of various active metals supported on zeolites as catalysts for the selective reduction of NO by H2 (H2-SCR), as well as the investigation of the influence of various reaction conditions and the related reaction mechanisms. The results of the thesis work are reported in two manuscripts, namely Paper I and II.
Paper I focus on catalysts with Pt supported on SSZ-13 zeolite. Their performance is evaluated with varying H2/NO feed ratios (5/10/15), Pt loadings (0.5/1.0/2.0 wt%), and with and without water co-feeding. Activity tests showed that Pt catalysts have high catalytic activities for H2-SCR at low temperatures (<150 ℃), but only NO oxidation occurs at high temperatures. The reaction process is complex and includes multiple reactions such as competing H2 oxidation and NO oxidation, as well as other side reactions. The selectivity for the by-product N2O cannot be ignored and it is even greater than that for N2 at temperatures less than 120 ℃. Higher H2 concentration promotes N2 selectivity, and the more exothermic H2 oxidation aids NO oxidation to occur at lower temperatures. The 0.5 wt% Pt catalyst was found to possess the highest N2 selectivity in the loading studies, not only because fewer active sites attenuate the competing hydrogen oxidation reaction, but also it possesses a smaller particle size and higher dispersion, and a larger portion of Pt in its metallic state. Water had a strong inhibitory effect on H2-SCR at low temperatures, with a significant reduction in N2 generation compared to anhydrous environments. It was found in in-situ DRIFTS experiments that nitrosyl species weakly adsorbed on the catalyst and could be easily removed; H2 is absorbed on the Pt active sites and is activated to interact with nitrates. Meanwhile, NH4+ ions were formed during the reaction and could play a role as reaction intermediates to assist in the reduction of NO. Simultaneous entry of NO and H2 induces a faster reaction than sequential entry and affects the binding of surface species, especially NO. The introduction of O2 to the NO+H2 mixture generates more nitrite (NO2-) species on the catalyst. 
Paper II focuses on extending the range of reaction temperatures for H2-SCR. It was found that Pt/SSZ-13, Pd/SSZ-13, and Ir/SSZ-13 catalyze the H2-SCR at three temperature intervals, low, medium, and high temperatures, respectively. Also, nitrogen selectivity increases sequentially with temperature. Combining them in series in the order of iridium-palladium-platinum is an interesting option. Due to the low reactivity of Ir, two strategies, a reduction pretreatment and replacement of the support with BETA zeolite, were examined and found to be successful in increasing its nitrogen generation. Similarly, the difference in H2-SCR reaction properties on the three active centres was investigated using in-situ DRIFTS measurements, and it was found that the Ir had the strongest NO adsorption, which was one of the reasons for its weak activity. No H2-SCR reaction occurred based on observation from the spectra at 200 ℃. After the reduction pretreatment, NO adsorption was weakened due to the metal present more in its metallic state and this contributed to the occurrence of H2-SCR.