The impact of mild hydrothermal aging (HTA) on low-temperature (150-200 degrees C) standard-SCR is investigated using transient response methods and transient kinetic analysis. We decouple the reduction and the oxidation half-cycles (RHC and OHC) of the standard-SCR redox mechanism to study them independently. While the RHC rates are essentially unaffected, OHC is inhibited by mild HTA. By equating the estimated rate expressions (both 2nd order in Cu sites, OHC with O2 as the sole oxidant), we predict exactly the steady-state low-temperature standard-SCR performance in terms of both NO conversion and bed-average Cu redox state, as well as the detrimental effect of mild HTA on the DeNOx efficiency. We also demonstrate that the DeNOx activity of the aged catalyst can be precisely restored by incrementing the oxygen partial pressure in proportion to the drop of the OHC rate constant, as predicted by our simple two-reaction model. These findings offer valuable insights into the design of next-generation urea-SCR exhaust gas aftertreatment (EGA) systems featuring enhanced cold-start performance and durability under aging conditions.

Transient Response Methods (TRMs) coupled with kinetic analysis are powerful tools to investigate the mechanism of catalytic reactions. In this work we apply them to study the redox chemistry of low temperature Standard-SCR and N2O formation over a model Cu-CHA catalyst. Two monolith samples with different washcoat loadings were tested using transient protocols varying temperature, space velocity, pressure and feed composition. Kinetic analysis of NO conversions and oxidized Cu fractions confirmed the dual-site mechanisms of the reduction (RHC) and oxidation (OHC) half-cycles, with quadratic dependences on the CuII and CuI species, respectively. N2O formation was also successfully predicted by incorporating an unselective CuII reduction (RHC_N2O) pathway in the redox model. Eventually, a global regression on the whole dataset provided calibrated rate parameters of the three reactions: RHC, OHC and RHC_N2O. The resulting redox model accurately describes the steady-state DeNOx activity and selectivity across the whole range of investigated conditions.

Reducing NO to N2 in diesel engine exhaust is typically accomplished using a Cu/CHA catalyst. Although commercially available, Cu/CHA catalysts are still susceptible to degradation in performance via sulfur poisoning. In this study, we used different approaches to sulfur poisoning. We compared the results from these approaches to a fresh catalyst, one that had only undergone hydrothermal aging (HTA), i.e. no exposure to S, and an engine aged sample. For S poisoning, the catalysts were first exposed to SO2 at 400 degrees C then a desulfation phase at 575 degrees C. We tested samples after 1 cycle, 5 cycles and 100 cycles. Multiple cycles of SO2 exposure and desulfation led to an accumulation of S, and loss in multinuclear Cu species as well as reducible Cu. These factors led to a decrease in NOx conversion as well. Correlations were examined between the different Cu sites and NOx reduction, with the best being related to dynamic dimers. Most of the results suggest residual sulfate species (ZCuxSOy). The results demonstrate that a single sulfur exposure and regeneration process did not represent an engine-aged catalyst, in extent of SCR performance and in changes to Cu speciation.

Fe-exchanged zeolite catalysts have recently raised interest due to their higher de-N2O activity compared to Cu-CHA. However, no methodology is currently available to assess their active species and redox properties under real conditions. Herein we investigate Fe-CHA catalysts coupling Transient Response Methods, previously used to investigate Cu-CHA, and Operando FT-IR, with the aim to gain insights in the Fe active species and their behavior under different reactive conditions. Since Fe is well reported in literature to undergo a redox cycle between oxidized and reduced forms, the analysis was divided in two sections focused on reducing and oxidizing methods. From H2-TPR and NO+NH3 reductive protocols, different responses compared to Cu-CHA were observed, evidencing that oxidation of Fe-CHA is less effective. Concerning oxidative protocols, NO2 adsorption/desorption was identified as a key protocol for the titration of active Fe species, as NO2 was stored as nitrate every two iron sites regardless of the speciation. This result was supported by Operando FT-IR tests. The NO2 ads/des protocol was replicated on pre-reduced catalysts, revealing the strong ability of NO2 to oxidize Fe sites, redisperse them and use them as storage sites. Finally, transient N2O decomposition runs, followed by NO+NH3 reduction or TPD, were used to probe Fe-CHA sites. These experiments confirmed the strong oxidizing power of N2O, close to NO2 and superior to O2, showing also how the stability of the stored oxygen, strictly linked with the auto-reduction of the catalysts, as well as the Fe oxidation state, were dependent on the N2O decomposition temperature.

Spatially resolved Mass Spectrometry (Spaci-MS) is a powerful tool for the study of chemical processes over honeycomb monolith catalysts, the commercial configuration of catalytic converters for pollutant abatement from combustion exhausts. A limit of this technology lies in the intrinsic perturbation of the measurements caused by the mass spectrometry probe: the insertion of the capillary affects the flow distribution in the probed monolith channel due to the incremented flow resistance, altering the catalyst contact time and thus the measured catalytic activity. In this work, we study the effect of the probe on the spatially resolved low temperature DeNOx activity of a model Cu-CHA NH3-SCR catalyst. Transient tests were performed to calibrate a redox model of the investigated catalyst, incorporating Cu site reduction (RHC, CuII -> CuI) and oxidation (OHC, CuI -> CuII) kinetics. Then, the simulated axial evolution of NO conversion (validated by tests performed in a conventional flow reactor at increasing space velocity) was compared to spatially resolved data obtained in a Spaci-MS rig to capture the effect of the probe. Based on this comparison, a simple analytical formula was developed to directly assess a priori the effect of the diameter, sampling flowrate and eccentricity of the Spaci-MS capillary probe. The equation was successfully validated against both CFD simulations and the experimental NH3-SCR activity data.

Transient Response Methods (TRM) are applied to investigate the reduction half-cycle (RHC) of the Standard SCR reaction over a state-of-the-art Cu-SSZ-13 catalyst at temperatures ranging from 135 to 350°C. We assess the effects of hydrothermal aging (HTA), water feed content and temperature on the reaction redox chemistry. By testing three samples with different aging levels, we reveal the CuII reduction rate to be unaffected by HTA in the whole T-range under investigation. Kinetic analysis assesses the inhibiting role of water on the RHC, consistent with literature. As long as enough NH3 is stored on the catalyst surface, reduction transients are accurately predicted assuming a second-order dependence of the reduction rate on the copper(II) fraction, also at temperatures exceeding 200°C. Remarkably, under wet conditions the low-T CuII reduction mechanism remains the same at least up to 300°C.

Pd-exchanged chabazite (Pd-CHA) catalysts show NO adsorption and desorption features which comply well with the requirements for low-T passive NOx adsorber (PNA) applications. An earlier work based on transient adsorption tests investigated the NO storage pathway on Pd-CHA, a still debated topic in the literature. Such research highlighted a Pd-redox mechanism (Pd2+ <-> Pd+) underlying the NO storage chemistry over these systems. CO and NO were capable of reducing Pd2+ at low temperatures, and the newly formed Pd+ acted as the main NO storage site. Increasing temperatures activated a Pd-oxidation process, which reduced the fraction of Pd+ sites, and consequently the NO storage, but was inhibited by H2O. Herein we challenge quantitatively such a scheme relying on transient kinetic analysis. We show that a simple redox kinetic model of NO + CO storage on Pd-CHA, based on the above, reproduces the main features of the species evolution and of the NO storage observed under variable operating conditions over Pd-CHA samples with two Pd-loadings, thus lending support to the proposed Pd-redox chemistry.

Hydrothermal aging (HTA) conditions are commonly reached in Diesel engine NH3-SCR architectures based on Cu-CHA catalysts, altering the catalyst performance due to irreversible transformations of the catalytic sites. Relying on transient response methods (TRMs), we study two Cu-SSZ-13 samples with different HTA extents. The following conclusions apply: i) the Cu2+content is invariant with HTA; ii) the ZCu2+(OH)- population decreases concurrent to the increase of the Z2Cu2+ fraction and to iii) the loss of Br & oslash;nsted acidity. This evidence agrees with the simple stoichiometry proposed for mild HTA processes: ZCu2+(OH)-+ZH+-+Z2Cu2++H2O. Furthermore, TRMs permit to observe additional phenomena which comply with a low-T Cu2+-pair mediated reduction pathway recently proposed. The results from a comprehensive set of simple experimental flow reactor methods consolidate the proposed aging mechanism, strengthen the SCR reduction half cycle (RHC) mechanistic understanding and showcase TRMs as a viable alternative to more complex techniques for the characterization of CuCHA catalysts.

The nuclearity of Cu2+ species active in the low-temperature Reduction Half Cycle (RHC) of NOx Selective Catalytic Reduction with NH3 over Cu-CHA catalysts is controversial. In the past, transient CO to CO2 oxidation protocols have been used to titrate binuclear Cu2+ species, and identified NH3-solvated ZCu2+(OH)- ions as their precursors. However, the prior results relied on asymptotic extrapolation due to the very slow CO oxidation kinetics. We herein present the results from a prolonged (24 h) CO titration experiment under dry conditions over an industrial Cu-SSZ-13 catalyst: the results demonstrate the conversion of all ZCu2+(OH)- ions to binuclear Cu2+ complexes and agree well with the extrapolation of a shorter (90 min) experiment, based on the assumption of the CO oxidation rate being second order in ZCu2+(OH)-. These outcomes confirm the adequacy of short CO oxidation tests to titrate ZCu2+(OH)- ions and support a Cu2+ pair mediated RHC pathway.

A transient kinetic approach was applied to independently investigate the oxidation half-cycle (OHC) and reduction half-cycle (RHC) of NH3-selective catalytic reduction (SCR) at low temperature (150-200 degrees C). Three model Cu-exchanged chabazite (Cu-CHA) samples with fixed Cu loading (similar to 1.8% w/w) and different silica-to-alumina ratios (SARs = 10-17-25) were investigated under dry and wet conditions. We confirmed the following: (i) OHC proceeds via second- and first-order kinetics in Cu-I and O-2, respectively, with O-2 (+H2O) alone able to completely reoxidize Cu-I sites; (ii) RHC proceeds via second- and first-order kinetics in Cu-II and NO, respectively, according to a Cu/NO = 1:1 stoichiometry. Notably, coupling RHC and OHC rates resulted in an accurate prediction of the steady-state standard SCR conditions, providing the consistent closure of the SCR redox chemistry. Unexpectedly, we revealed the impact of H2O to vary depending on the catalyst formulation. At high SAR, water inhibits the RHC and promotes the OHC. As a result, a limited impact was observed on steady-state deNO(x) activity, while the Cu-oxidation state was significantly enhanced by H2O. With decreasing SAR, however, the H2O effect on RHC gradually shifts from inhibition to promotion, while the OHC is always promoted. At fixed water content, we revealed the RHC rate to decrease with decreasing Al density, with minor influence observed on the OHC; as a result, a lower deNO(x) activity was observed upon increasing SAR. Remarkably, the application of transient kinetic analysis to decouple RHC and the OHC greatly facilitated the identification of complex H2O and SAR effects on the global SCR redox chemistry.

The undesired production of N2O during NH3-SCR reactions is investigated over a reference commercial Cu-CHA catalyst. Steady-state experiments performed in the 150–500 °C temperature range exhibit a bimodal trend in the N2O formation profile, confirming the existence of two different reaction mechanisms occurring at low and high temperatures. Focusing on a low-to-medium T-range, N2O production, usually ascribed to NH4NO3 formation and decomposition, increases with the NO2/NOx ratio. However, an excess of NO2 leads to a decrease in the N2O release due to ammonium nitrate deposition and catalyst clogging phenomena. Steady-state and dynamics experiments show the promoting effect of both NH3 feed concentration and NH3 storage on N2O production at T > 200 °C. Surprisingly, N2O decreases with increasing NH3/NOx ratio at lower temperature. A novel approach based on the strategic injection of NH3 is also applied to mitigate the N2O formation while maintaining high deNOx activity. Remarkably, complete NOx conversion and 11

We employ UV/Vis Diffuse Reflectance spectroscopy directly coupled with a packed bed flow reactor to extract quantitative kinetic information. We use as a show-case the CuII/CuI redox dynamics during the reduction half cycle of the NH3-Selective Catalytic Reduction (SCR) on Cu-CHA catalysts. Our measurements enable quantification of the fraction of oxidized Cu, reconstructed by Multivariate Curve Resolution (MCR) together with monitoring of the gas-phase evolution during the reaction. These data both on the dynamics of the gas-phase and of the active site oxidation state have been used to assess the reduction half cycle rate equation and estimate the rate constant. Our results in terms of reaction orders and kinetic constant are in line with previous findings in the literature. Overall, our results demonstrate that the combined analysis of the UV spectra and of the gas-phase dynamics provides converging and unparalleled kinetic insight: this approach effectively resolves ambiguities concerning RHC kinetics and mechanism. More in general, this work provides evidence that operando spectroscopy can be used to extract quantitative kinetic information on catalytic cycles.

The investigation of the ZCu 2+ (OH) − and Z 2 Cu 2+ ions modifications during NH 3 -SCR on Cu–CHA catalysts is a key aspect to clarify the still-debated low-T redox SCR mechanism. In previous works, the dry transient CO oxidation protocol has been employed to identify the generation of dinuclear Cu 2+ structures under conditions representative of the low-T SCR–RHC: NH 3 solvation promotes the inter-cage mobility and coupling of ZCu 2+ (OH) − , acting as the catalytic centers for the CO oxidation process, while Z 2 Cu 2+ results inactive. Herein the same protocol, with pre-stored NH 3 , has been applied to a set of Cu–CHA catalysts with variable Cu loading (0.7–2.4% w/w) but fixed Si/Al: an increasing Cu content produced a net positive effect on the CO 2 production, coherent with a growing ZCu 2+ (OH) − population, while a further enhancement was observed in the presence of H 2 O. The analysis of the integral CO 2 production enabled to predict the maximum CO conversion, corresponding to the titration of the whole ZCu 2+ (OH) − content for each catalyst under dry condition, verifying the initial mechanism. Conversely, in the presence of water, the analysis evaluated an asymptotic titration of the total catalyst Cu 2+ contents. This finding permits to generalize a recent study where combined TRMs, DFT and FTIR were used to probe the complete reversible Cu 2+ sites hydrolysis and pairing in the presence of H 2 O and NH 3 , thus activating the participation of Z 2 Cu 2+ species, too. These results also highlight the versatility and effectiveness of the CO oxidation protocol as a multi-purpose technique to study the Cu 2+ ions in Cu–CHA catalysts.

The Front Cover illustrates a tug-of-war played between two concomitant but opposite effects brought forth by the presence of H2O in the Reduction Half-Cycle (RHC) of NH3-SCR, leading to a reduction in both the rate and its apparent activation energy with respect to dry conditions. In their Research Article, M. Maestri, E. Tronconi and co-workers show that such phenomena are a consequence of enthalpic stabilization and additional entropic penalties of the TS brought forth by the presence of H2O in the cage of Cu-CHA. This result thus provides a theoretical understanding of the kinetic role of H2O in the RHC, highlighting the importance of the molecular scale description of the reaction environment in voids of molecular dimensions. Image credit: Gabriele Contaldo and Lia Tagliavini. More information can be found in the Research Article by M. Maestri, E. Tronconi and co-workers.

Dispersion corrected density functional theory calculations show that the presence of H2O in the Reduction Half-Cycle (RHC) of NH3-SCR affects the free energy of the kinetically-relevant transition state (TS) leading to a reduction in the rate and activation energy with respect to dry conditions. In particular, H2O enthalpically stabilizes the kinetically-relevant TS by 20 kJ mol(-1) with respect to the dry counterpart. Such enthalpic stabilization vanishes when van der Waals (vdW) interactions are excluded from the calculations, thus showing the preeminent role of non-specific dispersion forces in the reduction of the activation enthalpy. At the same time, the enthalpic stabilization is more than compensated by the additional entropy losses of the TS brought forth by the presence of H2O in the CHA cage. Calculated enthalpy and entropy changes with respect to the dry case agree quantitatively with the experimental measurements and reflect the modified reacting environment in the presence of H2O. As a result, this study provides theoretical underpinnings on the mechanistic role of H2O in the RHC and, on a more general basis, highlights the importance of the molecular scale description of the reaction environment in voids of molecular dimensions.

The reduction (RHC: CuII-* CuI) and oxidation (OHC: CuI-* CuII) half-cycles of low-temperature (150-200 degrees C) NH3-selective catalytic reduction (SCR) were investigated by transient response methods over a model Cu-CHA catalyst (Cu loading = 1.8% w/w; silica to alumina ratio = 25). In line with previous findings: i) RHC proceeded via second-order kinetics in CuII and with an equimolar stoichiometry between CuII reduced and NO consumed; ii) complete reoxidation of reduced NH3- solvated Cu sites was achieved by exposing the catalyst to O2 + H2O only, no other species (e.g., NO) were found to be relevant to the OHC pathway. Dedicated kinetic tests highlighted a second order dependence of the OHC rate on CuI and a first-order dependence on O2. Coupling the RHC and OHC kinetic models, independently developed from transient tests, resulted in an accurate description of Standard SCR turnovers, predicting steady-state NO conversion, N2 formation, and bed-average Cu-oxidation states, as well as light-off and extinction transients closely consistent with experimental measurements. These results demonstrate the rational dissection of the complex steady-state SCR reaction network into two redox half-cycles and provide a stoichiometrically and kinetically consistent closure of the Standard SCR redox cycle over Cu-CHA catalysts.

Periodic Open Cell Structures (POCS) have been considered as possible catalytic substrates, thanks to recent developments in the Additive Layer Manifacturing (ALM) technique. In this work, the impact of the POCS cell shape on mass transfer has been investigated through CFD simulations. An algorithm able to increase the wet surface of a simple cubic POCS has been developed, keeping constant the mean pore diameter and with negligible porosity variations. Catalytic performances of the substrates have been evaluated, suggesting the possibility to enhance the mass transfer properties of the POCS up to that of foams by continuous randomical transformation of its shape. A dedicated CFD multi-region solver has been used for this purpose, accounting for heat/mass transfer and catalytic reactions. Moreover, a fluid-dynamic approach has been proposed in order to estimate the value of tortuosity, investigating its dependency on the POCS randomization level and its effect on the flow field.(c) 2022 Elsevier Ltd. All rights reserved.

Notwithstanding the high efficiency of the modern NH 3 -SCR technology in reducing NOx from Diesel engines exhausts, their removal is still a concern in view of the forthcoming application of more stringent environmental guidelines. Actually, a large fraction of the total NOx emitted by the engine derives from transient engine operations, especially at low temperature. In this context, herein we systematically investigate the effect of oxygen feed content variation under Standard SCR reaction conditions over a reference Cu-CHA catalyst. Transient Response Methods (TRM) have been applied to mimic lean to rich and rich to lean transients. Steady state experiments reveal that the higher the O 2 feed content, the higher the NO x conversion and N 2 O formation, with N 2 O turning out to be very sensitive to oxygen concentration changes. Additionally, through TRM runs, it is possible to obtain quantitative information on the average fraction of oxidized Cu sites (Cu II /Cu tot ) at the Standard SCR steady state. Particularly, the NOx conversion and the Cu II fraction increase both with temperature and with O 2 feed content. Hence, the higher the oxidized copper fraction, the higher the N 2 O formation in the Standard SCR reaction.

As the state-of-the-art catalyst for the selective catalytic reduction (SCR) of NOx from lean-burn engines, Cu-exchanged chabazite zeolite (Cu-CHA) has been a spotlight in environmental catalysis because of its preeminence in DeNOx performance and hydrothermal stability. The microscopic cycling of active Cu cations between Cu-II and Cu-I in response to dynamic, macroscopic reaction conditions dominates SCR catalysis over Cu-CHA zeolites. In such cycling, Cu cations are solvated by gas-phase reactants, e.g., NH3, under low-temperature (LT) conditions, conferring peculiar mobility to Cu-NH3 complexes and making them act as mobilized entities during LT-SCR turnovers. Such motions provide LT-SCR-a typical heterogeneous catalytic process-with homogeneous features over Cu-CHA, but, differently from conventional homogeneous catalysis, the motions are tethered by electrostatic interactions between Cu cations and conjugate Al centers. These features affect distinctly the LT-SCR redox chemistry on Cu-CHA, resulting in, for example, the involvement of two Cu-I-diamines in activating O-2 and reoxidizing Cu-I to Cu-II (oxidation half-cycle, OHC). The kinetically relevant reduction half-cycle (RHC) that reduces Cu ll to Cu l is far less understood particularly within the context of such linked homo- and heterogeneous catalysis. Here, we focus on the LT-RHC chemistry over Cu-CHA and summarize observations from a series of recent, dedicated works from our group, benchmarking these findings against those closely relevant in the literature. We thus attempt to reconcile and rationalize results informed from independent, multitechnique evidence and to further progress mechanistic insights into LT-SCR catalysis, especially in the context of dynamic interconversion between mono- and binuclear Cu sites.