Mobility and Reactivity of Cu+ Species in Cu-CHA Catalysts under NH3-SCR-NOx Reaction Conditions: Insights from AIMD Simulations
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SUBMITTER: Millan R
PROVIDER: S-EPMC8549050 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Cu2+ ions (ZCu2+ (OH)- , Z2 Cu2+ ) are regarded as the NH3 -SCR (SCR=selective catalytic reduction) active site precursors of Cu-exchanged chabazite (CHA) which is among the best available catalysts for the abatement of NOx from Diesel engines. During SCR operation, copper sites undergo reduction (Reduction half-cycle, RHC: Cu2+ →Cu+ ) and oxidation (Oxidaton half-cycle, OHC: Cu+ →Cu2+ ) semi cycles, whose associated mechanisms are still debated. We recently proposed CO oxidation to CO2 as an effective method to probe the formation of multinuclear Cu2+ species as the initial low-T RHC step. NH3 pre-adsorption determined a net positive effect on the CO2 production: by solvating ZCu2+ (OH)- ions, ammonia enhances their mobility, favoring their coupling to form binuclear complexes which can catalyze the reaction. In this work, dry CO oxidation experiments, preceded by modulated NH3 feed phases, clearly showed that CO2 production enhancements are correlated with the extent of Cu2+ ion solvation by NH3 . Analogies with the SCR-RHC phase are evidenced: the NH3 -Cu2+ presence ensures the characteristic dynamics associated with a second order kinetic dependence on the oxidized Cu2+ fraction. These findings provide novel information on the NH3 role in the low-T SCR redox mechanism and on the nature of the related active catalyst sites.
| S-EPMC9471058 | biostudies-literature
Project description:The dynamic nature of the copper cations acting as active sites for selective catalytic reduction of nitrogen oxides with ammonia is investigated using a combined theoretical and spectroscopic approach. Ab initio molecular dynamics simulations of Cu-CHA catalysts in contact with reactants and intermediates at realistic operating conditions show that only ammonia is able to release Cu+ and Cu2+ cations from their positions coordinated to the zeolite framework, forming mobile Cu+(NH3)2 and Cu2+(NH3)4 complexes that migrate to the center of the cavity. Herein, we give evidence that such mobilization of copper cations modifies the vibrational fingerprint in the 800-1000 cm-1 region of the IR spectra. Bands associated with the lattice asymmetric T-O-T vibrations are perturbed by the presence of coordinated cations, and allow one to experimentally follow the dynamic reorganization of the active sites at operating conditions.
| S-EPMC7720274 | biostudies-literature
Project description:Selective catalytic reduction (SCR) of NOx by ammonia is one of the dominant pollution abatement technologies for near-zero NOx emission diesel engines. A crucial step in the reduction of NOx to N2 with Cu zeolite NH3-SCR catalysts is the generation of a multi-electron donating active site, implying the permanent or transient dimerization of Cu ions. Cu atom mobility has been implicated by computational chemistry as a key factor in this process. This report demonstrates how variable temperature 1H NMR reveals the Cu induced generation of sharp 1H resonances associated with a low concentration of sites on the zeolite. The onset temperature of the appearance of these signals was found to strongly correlate with the NH3-SCR activity and was observed for a range of catalysts covering multiple frameworks (CHA, AEI, AFX, ERI, ERI-CHA, ERI-OFF, *BEA), with different Si/Al ratios and different Cu contents. The results point towards universal applicability of variable temperature NMR to predict the activity of a Cu-zeolite SCR catalyst. The unique relationship of a spectroscopic feature with catalytic behavior for zeolites with different structures and chemical compositions is exceptional in heterogeneous catalysis.
| S-EPMC10537069 | biostudies-literature
Project description:To gain insight into the underlying mechanisms of catalyst durability for the selective catalytic reduction (SCR) of NOx with an ammonia reductant, we employed scanning transmission X-ray microscopy (STXM) to study Cu-exchanged zeolites with the CHA and MFI framework structures before and after simulated 135 000-mile aging. X-ray absorption near-edge structure (XANES) measurements were performed at the Al K- and Cu L-edges. The local environment of framework Al, the oxidation state of Cu, and geometric changes were analyzed, showing a multi-factor-induced catalytic deactivation. In Cu-exchanged MFI, a transformation of CuII to CuI and Cux Oy was observed. We also found a spatial correlation between extra-framework Al and deactivated Cu species near the surface of the zeolite as well as a weak positive correlation between the amount of CuI and tri-coordinated Al. By inspecting both Al and Cu in fresh and aged Cu-exchanged zeolites, we conclude that the importance of the preservation of isolated CuII sites trumps that of Brønsted acid sites for NH3 -SCR activity.
| S-EPMC7522683 | biostudies-literature
Project description:Ce-Mn/TiO2 catalyst prepared using a simple impregnation method demonstrated a better low-temperature selective catalytic reduction of NO with NH3 (NH3-SCR) activity in comparison with the sol-gel method. The Ce-Mn/TiO2 catalyst loading with 20% Ce had the best low-temperature activity and achieved a NO conversion rate higher than 90% at 140-260°C with a 99.7% NO conversion rate at 180°C. The Ce-Mn/TiO2 catalyst only had a 6% NO conversion rate decrease after 100 ppm of SO2 was added to the stream. When SO2 was removed from the stream, the catalyst was able to recover completely. The crystal structure, morphology, textural properties and valence state of the metals involving the novel catalysts were investigated using X-ray diffraction, N2 adsorption and desorption analysis, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive spectroscopy, respectively. The decrease of NH3-SCR performance in the presence of 100 ppm SO2 was due to the decrease of the surface area, change of the pore structure, the decrease of Ce4+ and Mn4+ concentration and the formation of the sulfur phase chemicals which blocked the active sites and changed the valence status of the elements.
| S-EPMC5882715 | biostudies-literature
Project description:The reduction of NO x emissions has become one of the most important subjects in environmental protection. Cu-containing SSZ-13 is currently the state-of-the-art catalyst for the selective catalytic reduction of NO x with ammonia (NH3-SCR-DeNO x ). Although the current-generation catalysts reveal enhanced activity and remarkable hydrothermal stability, still open challenges appear. Thus, this review focuses on the progress of Cu-containing SSZ-13 regarding preparation methods, hydrothermal resistance and poisoning as well as reaction mechanisms in NH3-SCR-DeNO x . Remarkably, the paper reviews also the progress of Cu-containing SSZ-13 in the selective ammonia oxidation into nitrogen and water vapor (NH3-SCO). The dynamics in the NH3-SCR-DeNO x and NH3-SCO fields make this review timely.
| S-EPMC9450943 | biostudies-literature
Project description:A series of Fe α Cu1-α TiO x catalysts with variable Cu doping amounts was directly synthesized by the sol-gel method and their catalytic performances were tested for the selective catalytic reduction of NO with ammonia. The highest activity was achieved on Fe0.9Cu0.1Ti catalyst. NO conversion was above 80% and N2 selectivity exceeded 90% on this catalyst in the temperature range of 200-375 °C. High NO and NH3 oxidation activities facilitated the high NH3-SCR activities of the catalysts in the low temperature range, while too strong NH3 oxidation ability resulted in the decline of NH3-SCR activity. DFT calculations based on the Fe and Cu co-doping TiO2 model showed that the barrier of NH3 activation is dramatically reduced as compared to pure Fe doping. This is due to the lowered p-band of lattice O. However, such activated O will also strongly decrease the barrier for the dissociation of NH2 to NH species, which will lead to the formation of N2O. Both Brønsted and Lewis acid sites over Fe0.9Cu0.1Ti catalyst are involved in the NH3-SCR reaction. The adsorption of NO x is strong in the low temperature range, and large amounts of nitrates were decomposed on the catalyst surface in the high temperature range.
| S-EPMC9080720 | biostudies-literature
Project description:The small pore zeolite chabazite (SSZ-13) in the copper exchanged form is a very efficient material for the selective catalytic reduction by ammonia (NH3) of nitrogen oxides (NOx) from the exhaust of lean burn engines, typically diesel powered vehicles. The full mechanism occurring during the NH3-SCR process is currently debated with outstanding questions including the nature and role of the catalytically active sites. Time-resolved operando spectroscopic techniques have been used to provide new level of insights in to the mechanism of NH3-SCR, to show that the origin of stable Cu(I) species under SCR conditions is potentially caused by an interaction between NH3 and the Cu cations located in eight ring sites of the bulk of the zeolite and is independent of the NH3-SCR of NOx occurring at Cu six ring sites within the zeolite.
| S-EPMC6413821 | biostudies-literature
Project description:Commercial Cu-exchanged small-pore SSZ-13 (Cu-SSZ-13) zeolite catalysts are highly active for the standard selective catalytic reduction (SCR) of NO with NH3. However, their activity is unexpectedly inhibited in the presence of NO2 at low temperatures. This is strikingly distinct from the NO2-accelerated NOx conversion over other typical SCR catalyst systems. Here, we combine kinetic experiments, in situ X-ray absorption spectroscopy, and density functional theory (DFT) calculations to obtain direct evidence that under reaction conditions, strong oxidation by NO2 forces Cu ions to exist mainly as CuII species (fw-Cu2+ and NH3-solvated CuII with high CNs), which impedes the mobility of Cu species. The SCR reaction occurring at these CuII sites with weak mobility shows a higher energy barrier than that of the standard SCR reaction on dynamic binuclear sites. Moreover, the NO2-involved SCR reaction tends to occur at the Brønsted acid sites (BASs) rather than the CuII sites. This work clearly explains the strikingly distinctive selective catalytic behavior in this zeolite system.
| S-EPMC9360435 | biostudies-literature
Project description:To understand the degradation mechanism of the copper-ion-exchanged SSZ-13 (Cu-SSZ-13) is of high significance for rationally designing a zeolitic catalyst for ammonia-selective catalytic reduction of NO x (NH3-SCR). In this work, we focused on an Al-rich Cu-SSZ-13 and studied its structural degradation under hydrothermal conditions through a set of characterization techniques, including in situ X-ray diffraction (XRD), pair distribution function analysis and transmission electron microscopy-energy dispersive X-ray analysis (TEM-EDX). The results indicated that the chabazite structure tends to contract in the c direction upon hydrothermal treatment and consequently leads to the collapse of the four-membered ring. Such a structure change then results in the movement of isolated Cu2+ species from the face of the double six-membered ring to its center, which damages the structure further. However, the larger rings (6MRs and 8MRs) partially remain during the structure degradation, which possibly explains that some of the isolated Cu2+ species are alive even when the XRD-detectable crystallinity completely loses. The particle-by-particle observations through TEM-EDX analysis suggested that the occurrence of structural degradation differs remarkably from one individual particle to another. In general, particles with smaller size, having a lower Si/Al ratio and a higher Cu/Al ratio, tend to degrade easily. These results offer a thorough understanding of the structural degradation of Cu-SSZ-13 from the microscopic point of view and point out that the uniformity in composition and particle size of the zeolites plays a critical role in the early-stage degradation.
| S-EPMC6648293 | biostudies-literature