Effect of sulfur poisoning on the performance and active sites of Cu/SSZ-13 catalyst

Interface Induced by Hydrothermal Aging Boosts the Low-Temperature Activity of Cu-SSZ-13 for Selective Catalytic Reduction of NOx

Hitherto, sulfur poisoning and hydrothermal aging have still been the challenges faced in practical applications of the Cu-SSZ-13 catalyst for the selective catalytic reduction (SCR) of NOx from diesel engine exhaust. Here, we elaborately design and conduct an in-depth investigation of the synthetic effects of hydrothermal aging and SO2 poisoning on pristine Cu-SSZ-13 and Cu-SSZ-13@Ce0.75Zr0.25O2 core@shell structure catalysts (Cu@CZ). It has been discovered that Cu@CZ susceptible to 750 °C with 5 vol % H2O followed by 200 ppm SO2 with 5 vol % H2O (Cu@CZ-A-S) could still maintain nearly 100% NOx conversion across the significantly wider temperature region of 200-425 °C, which is remarkably broader than that of the Cu-SSZ-13-A-S (300-400 °C) counterpart. The experimental results show that the hydrothermal aging process results in the migration of highly active Cu species within the cage of Cu-SSZ-13 to the CZ surface, forming CuO/CZ with abundant interfaces, which significantly enhances the adsorption and subsequent activation of NO, leading to the generation of reactive N2O3 and HONO intermediates. Moreover, density functional theory (DFT) calculations reveal that the H of the HONO* species can function as Brønsted acid sites, effectively adsorbing NH3 to generate the active NH4NO2* intermediate, which readily decomposes into N2 and H2O. Furthermore, this pathway is the rate-determining step with an energy barrier of 0.93 eV, notably lower than that of the "standard SCR" pathway (1.42 eV). Therefore, the formation of the new CuO/CZ interface profoundly boosts the low-temperature NH3-SCR activity and improves the coresistance of the Cu@CZ catalyst to sulfur poisoning and hydrothermal aging.

Insights into SO2 Poisoning Mechanisms of Fresh and Hydrothermally Aged Cu-KFI Catalysts for NH3-SCR Reaction

The complex poisoning of Cu-KFI catalysts by SO₂ and hydrothermal aging (HTA) was investigated. The low-temperature activity of Cu-KFI catalysts was restrained by the formation of H₂SO₄ and then CuSO₄ after sulfur poisoning. Hydrothermally aged Cu-KFI exhibited better SO₂ resistance than fresh Cu-KFI since HTA significantly reduced the number of Brønsted acid sites, which were considered to be the H₂SO₄ storage sites. The high-temperature activity of SO₂-poisoned Cu-KFI was basically unchanged compared to the fresh catalyst. However, SO₂ poisoning promoted the high-temperature activity of hydrothermally aged Cu-KFI since it triggered CuOx into CuSO₄ species, which was considered as an important role in the NH₃-SCR reaction at high temperatures. In addition, hydrothermally aged Cu-KFI catalysts were more easily regenerated after SO₂ poisoning than fresh Cu-KFI on account of the instability of CuSO₄.

Utah Wintertime Measurements of Heavy-Duty Vehicle Nitrogen Oxide Emission Factors

There have only been a few wintertime studies of heavy-duty vehicle (HDV) NO_x emissions in the United States, and while they have observed increased emissions, fleet characterization to identify the cause has been lacking. We have collected wintertime measurements of NO_x emission factors from 1591 HDVs at a Utah Port of Entry in December 2020 that includes individual vehicle identification. In general, NO_x emission factors for 2011 and newer chassis model year HDV are significantly higher than those for 2017 spring measurements from California. The newest chassis model year HDV (2017-2021) NO_x emission factors are similar, indicating no significant emission deterioration over the 5 year period, though they are still approximately a factor of 3 higher than the portable emission measurement on-road enforcement standard. We estimate that ambient temperature increases NO_x emissions no more than 25% in the newer HDV, likely through reductions in catalyst efficiencies. NO_x emissions increase to a significantly higher level for the 2011-2013 chassis model year vehicles, where within the uncertainties, they have emissions similar to older precontrol vehicles, indicating that they have lost their NO_x control capabilities within 8 years. MOVES3 modeling of the Utah fleet underpredicted mean NO_x emissions by a factor of 1.8 but the MOVES3 estimate is helped by including a larger fraction of high-emitting glider kit trucks (new chassis with pre-emission control engines) than found in the observations.

Improvement of Alkali Metal Resistance for NH3-SCR Catalyst Cu/SSZ-13: Tune the Crystal Size

To improve the alkali metal resistance of commercial catalyst Cu/SSZ-13 for ammonia selective catalytic reduction (NH3-SCR) reaction, a simple method to synthesize Cu/SSZ-13 with a core–shell like structure was developed. Compared with smaller-sized counterparts, Cu/SSZ-13 with a crystal size of 2.3 μm exhibited excellent resistance to Na poisoning. To reveal the influence of the crystal size on Cu/SSZ-13, physical structure characterization (XRD, BET, SEM, NMR) and chemical acidic distribution (H2-TPR, UV-Vis, Diethylamine-TPD, pyridine-DRIFTs, EDS) were investigated. It was found that the larger the crystal size of the molecular sieve, the more Cu is distributed in the crystal core, and the less likely it was to be replaced by Na to generate CuO. Therefore, a 2.3 μm sized Cu/SSZ-13 well-controlled the reactivity of the side reaction NH3 oxidation and the generation of N2O. The result was helpful to guide the extension of the service life of Cu/SSZ-13.

Highly Efficient NO Abatement over Cu-ZSM-5 with Special Nanosheet Features

Conventional Cu-ZSM-5 and special Cu-ZSM-5 catalysts with diverse morphologies (nanoparticles, nanosheets, hollow spheres) were synthesized and comparatively investigated for their performances in the selective catalytic reduction (SCR) of NO to N₂ with ammonia. Significant differences in SCR behavior were observed, and nanosheet-like Cu-ZSM-5 showed the best SCR performance with the lowest T₅₀ of 130 °C and nearly complete conversion in the temperature range of 200-400 °C. It was found that Cu-ZSM-5 nanosheets [mainly exposed (0 1 0) crystal plane] with abundant mesopores and framework Al species were favorable for the formation of high external surface areas and Al pairs, which influenced the local environment of Cu. This motivated the preferential formation of active copper species and the rapid switch between Cu²⁺ and Cu⁺ species during NH₃-SCR, thus exhibiting the highest NO conversion. In situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) results indicated that the Cu-ZSM-5 nanosheets were dominated by the Eley-Rideal (E-R) mechanism and the labile nitrite species (NH₄NO₂) were the crucial intermediates during the NH₃-SCR process, while the inert nitrates were more prone to generate on Cu-ZSM-5 nanoparticles and conventional one. The combined density functional theory (DFT) calculations revealed that the decomposition energy barrier of nitrosamide species (NH₂NO) on the (0 1 0) crystal plane of Cu-ZSM-5 was lower than those on (0 0 1) and (1 0 0) crystal planes. This study provides a strategy for the design of NH₃-SCR zeolite catalysts.

Insights into sulfur poisoning and regeneration of Cu-SSZ-13 catalysts: in situ Cu and S K-edge XAS studies

The temperature during sulfur poisoning affects the relation between total sulfur content and the fraction of sulfur-free copper in poisoned and regenerated Cu-SSZ-13 catalysts.

New Insight into the In Situ SO2 Poisoning Mechanism over Cu-SSZ-13 for the Selective Catalytic Reduction of NOx with NH3

To reveal the nature of SO2 poisoning over Cu-SSZ-13 catalyst under actual exhaust conditions, the catalyst was pretreated at 200 and 500 °C in a flow containing NH3, NO, O2, SO2, and H2O. Brunner−Emmet−Teller (BET), X-ray diffraction(XRD), thermo gravimetric analyzer (TGA), ultraviolet Raman spectroscopy (UV Raman), temperature-programmed reduction with H2 (H2-TPR), temperature-programmed desorption of NO+O2 (NO+O2-TPD), NH3-TPD, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), and an activity test were utilized to monitor the changes of Cu-SSZ-13 before and after in situ SO2 poisoning. According to the characterization results, the types and generated amount of sulfated species were directly related to poisoning temperature. Three sulfate species, including (NH4)2SO4, CuSO4, and Al2(SO4)3, were found to form on CZ-S-200, while only the latter two sulfate species were observed over CZ-S-500. Furthermore, SO2 poisoning had a negative effect on low-temperature selective catalytic reduction (SCR) activity, which was mainly due to the sulfation of active sites, including Z2Cu, ZCuOH, and Si-O(H)-Al. In contrast, SO2 poisoning had a positive effect on high-temperature SCR activity, owing to the inhibition of the NH3 oxidation reaction. The above findings may be a useful guideline to design excellent SO2-resistant Cu-based zeolite catalysts.