3D printed Pt-ZSM5/Geopolymer as catalyst for H2-SCR

The reduction of CO2 emissions is crucial in mitigating global warming and emissions by cars play a key role. Compared with hydrocarbons and NH3, hydrogen is considered a clean fuel because its combustion only generates water. Hydrogen can run on a lean mixture and, under certain conditions, nitrogen oxides can be also produced mainly driven by the thermal formation mechanism. The use of a catalyzed exhaust after-treatment system with hydrogen as a reducing agent has the great advantage that only one fuel needs to be carried on board both for propulsion and NOx reduction, thus avoiding the device for storage and injection of urea, as for the more common NH3-SCR. In recent years, great efforts have been paid to understanding the relationship between the structure and performance of H2-SCR catalysts. Supported noble metals are the most active catalysts for H2-SCR in the low-temperature range (100-200°C) whereas non noble metals catalysts are active at temperatures higher than 200 °C. Selectivity represents a crucial issue also for H2-SCR. The competitive reaction of hydrogen with the excess oxygen contained in the lean exhaust gas must be limited and the potential formation of climate-damaging N2O avoided as much as possible. The use of a proper support for the active metal can largely affect the catalytic performance. Zeolites as support for Pt provided an enhanced activity due to their confinement property. Moreover, Pt/H-ZSM-5 showed excellent SO2 and H2O resistance due to the stabilization of Pt nanoparticles in the structure. The superior H2-SCR activity of Pt-H-ZSM-5 was attributed to the synergistic effect between hydrogen activation and NOx adsorption. It was found that Pt species outside the surface of HZSM-5 existed as metallic Pt0 species, which played an important role in activating H2, whereas Pt species inside the zeolitic channels is present as oxidized Pt2+ species, which acted as actives sites for NO adsorption. At low temperatures, H from the dissociative hydrogen adsorption on Pt sites migrates from Pt to the surface of the zeolite and then reacted with the adsorbed NOx species to form N2. At high temperatures, NO2 and NH4+ species are important active intermediates to form N2 for the H2-SCR reaction . Migration of active species could be greatly enhanced in materials with hierarchical structure characterized by a distribution of pore sizes ranging from macro- to micro-pores like in zeolite/geopolymer composites. Similar materials showed excellent performance in NH3-SCR when the zeolite was exchanged with copper and they can be formed as 3D-printed monoliths showing a good mechanical resistance allowing their use in mobile applications. The introduction of up to 60% zeolite into the macro-meso-porous matrix allows to significantly increase the amount of active phase in the catalytic reactor compared to conventional washcoated monoliths. In this work, we set out to investigate a structured catalyst for H2-SCR made by incorporating up to 60% wt. of a commercial ZSM-5 (SiO2/Al2O3=25) powder into a 3D printed geopolymeric matrix and introducing platimum as the active element from a H2PtCl6. The catalyst was characterized by XRD, SEM, N2 physisorption, FTIR, H2-TPR and NH3-TPD analysis and tested for the H2-SCR reaction in a lab-scale rig equipped with continuous analyzers for all possible nitrogen oxides. A reference Pt-ZSM-5 powder was also characterized and tested in order to demonstrate the effect of the hierarchical structure generated by the co-presence of macro, meso and micropores in the 3D monolith.