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Ammonia-mediated selective catalytic reduction (NH 3 -SCR) is currently the key approach to abate nitrogen oxides (NO x ) emitted from heavy-duty lean-burn vehicles. The state-of-art NH 3 -SCR catalysts, namely, copper ion-exchanged chabazite (Cu-CHA) zeolites, perform rather poorly at low temperatures (below 200 °C) and are thus incapable of eliminating effectively NO x emissions under cold-start conditions. Here, we demonstrate a significant promotion of low-temperature NO x reduction by reinforcing the dynamic motion of zeolite-confined Cu sites during NH 3 -SCR. Combining complex impedance-based in situ spectroscopy (IS) and extended density-functional tight-binding molecular dynamics simulation, we revealed an environment- and temperature-dependent nature of the dynamic Cu motion within the zeolite lattice. Further coupling in situ IS with infrared spectroscopy allows us to unravel the critical role of monovalent Cu in the overall Cu mobility at a molecular level. Based on these mechanistic understandings, we elicit a boost of NO x reduction below 200 °C by reinforcing the dynamic Cu motion in various Cu-zeolites (Cu-CHA, Cu-ZSM-5, Cu-Beta, etc.) via facile postsynthesis treatments, either in a reductive mixture at low temperatures (below 250 °C) or in a nonoxidative atmosphere at high temperatures (above 450 °C).

The low-temperature mechanism of chabazite-type small-pore Cu-SSZ-13 zeolite, a state-of-the-art catalyst for ammonia-assisted selective reduction (NH 3 -SCR) of toxic NO x pollutants from heavy-duty vehicles, remains a debate and needs to be clarified for further improvement of NH 3 -SCR performance. In this study, we established experimental protocols to follow the dynamic redox cycling ( i.e ., Cu II ↔ Cu I ) of Cu sites in Cu-SSZ-13 during low-temperature NH 3 -SCR catalysis by in situ ultraviolet-visible spectroscopy and in situ infrared spectroscopy. Further integrating the in situ spectroscopic observations with time-dependent density functional theory calculations allows us to identify two cage-confined transient states, namely, the O 2 -bridged Cu dimers ( i.e. , μ-η 2 :η 2 -peroxodiamino dicopper) and the proximately paired, chemically nonbonded Cu I (NH 3 ) 2 sites, and to confirm the Cu I (NH 3 ) 2 pair as a precursor to the O 2 -bridged Cu dimer. Comparative transient experiments reveal a particularly high reactivity of the Cu I (NH 3 ) 2 pairs for NO-to-N 2 reduction at low temperatures. Our study demonstrates direct experimental evidence for the transient formation and high reactivity of proximately paired Cu I sites under zeolite confinement and provides new insights into the monomeric-to-dimeric Cu transformation for completing the Cu redox cycle in low-temperature NH 3 -SCR catalysis over Cu-SSZ-13.

Reduction of harmful nitrogen oxides (NO x ) from diesel engine exhausts is one of the key challenges in environmental protection, and can be achieved by NH 3 -assisted selective catalytic reduction (NH 3 -SCR) using copper-exchanged chabazite zeolites (i.e. Cu-CHA, including Cu-SSZ-13 and Cu-SAPO-34) as catalysts. Understanding the redox chemistry of Cu-CHA in NH 3 -SCR catalysis is crucial for further improving the NO x reduction efficiency. Here, a series of Cu-SSZ-13 catalysts with different Cu ion exchange levels were prepared, thoroughly characterized by different techniques such as X-ray diffraction, diffuse reflectance ultraviolet-visible spectroscopy and temperature-programmed desorption using NH 3 as a probe molecule, etc., and tested in NH 3 -SCR reactions under steady-state conditions. In situ studies by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), supplemented with density-functional theory calculations, provided solid evidence for the formation of ammonium ion (NH 4 + ) intermediates resulting from the reduction of Cu 2+ to Cu + by co-adsorbed NH 3 and NO molecules on Cu-SSZ-13. Catalytic relevance of the NH 4 + intermediates, as demonstrated by an increase of NO conversion over Cu-SSZ-13 pre-treated in NH 3 /NO atmosphere, can be attributed to the formation of closely coupled Cu + /NH 4 + pairs promoting the Cu + re-oxidation and, consequently, the overall NH 3 -SCR process. This study thus paves a new route for improving the NH 3 -SCR efficiency over Cu-CHA zeolite catalyst., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Zeolites have been found to be promising sensor materials for a variety of gas molecules such as NH3, NOx, hydrocarbons, etc. The sensing effect results from the interaction of the adsorbed gas molecules with mobile cations, which are non-covalently bound to the zeolite lattice. The mobility of the cations can be accessed by electrical low-frequency (LF; mHz to MHz) and high-frequency (HF; GHz) impedance measurements. Recent developments allow in situ monitoring of catalytic reactions on proton-conducting zeolites used as catalysts. The combination of such in situ impedance measurements with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), which was applied to monitor the selective catalytic reduction of nitrogen oxides (DeNOx-SCR), not only improves our understanding of the sensing properties of zeolite catalysts from integral electric signal to molecular processes, but also bridges the length scales being studied, from centimeters to nanometers. In this work, recent developments of zeolite-based, impedimetric sensors for automotive exhaust gases, in particular NH3, are summarized. The electrical response to NH3 obtained from LF impedance measurements will be compared with that from HF impedance measurements, and correlated with the infrared spectroscopic characteristics obtained from the DRIFTS studies of molecules involved in the catalytic conversion. The future perspectives, which arise from the combination of these methods, will be discussed. [ABSTRACT FROM AUTHOR]

The phase-pure zeolite Y with a high n (Si)/ n (Al) ratio is obtained with the cyclic polyether 15-Crown-5 as a structure-directing agent. In copper-exchanged for this new zeolite outperformed other Cu-containing zeolite Y counterparts during NH 3 -SCR-DeNO x with more than 80% NO conversion and more than 85% N 2 selectivity in the temperature range of 100 – 500 °C. The presence of Cu+ species located in sodalite cages with elevated redox cyclability guarantees high activity and N 2 selectivity up to 400 °C over CuY-Crown. Above 400 °C, a high activity and N 2 selectivity are related to the accessibility of chemisorbed NH 3 to the gaseous NO during the NH 3 -SCR-DeNO x. [Display omitted] • Zeolite Y with a high n (Si)/ n (Al) ratio is obtained using 15-Crown-5. • CuY-Crown outperformed other CuY counterparts in NH 3 -SCR-DeNO x. • Cu+ species located in sodalite cages guarantee high activity and N 2 selectivity. [ABSTRACT FROM AUTHOR]

The performance of Cu-chabazite (Cu-CHA) zeolite, the state-of-art catalyst for the ammonia-assisted selective reduction (NH 3 -SCR) of toxic NO x pollutants from heavy-duty diesel vehicles, is insufficient at low reaction temperatures and needs to be improved for meeting stringent emission regulations. Here, we demonstrate that the auto-reduction of isolated Cu sites (i.e. Cu2+[OH]→Cu+) in Cu-CHA could promote the low-temperature NO x abatement efficiency in NH 3 -SCR. Combining in situ spectroscopy, steady-state/transient kinetic measurements and computational simulations, we unveiled that Cu auto-reduction, which was driven by non-oxidative thermal activation, weakened the Cu tethering to CHA framework and thus increased the Cu mobility, particularly, in the presence of NH 3. More importantly, auto-reduction-induced charge redistribution favored NO activation to form intimately interacted Cu+···NO+ pairs, which enabled alternative and highly efficient pathways leading to drastically promoted NO abatement at temperatures below 250 °C. These mechanistic findings shed new light on tuning the speciation of Cu single-sites to promote NO x reduction catalysis over Cu-zeolites. [Display omitted] • Low-temperature NH 3 -SCR promoted by Cu auto-reduction in Cu-CHA zeolite. • Auto-reduction weakened Cu tethering to zeolite framework. • Auto-reduction led to a higher Cu mobility in NH 3 -containing atmosphere. • Highly reactive Cu+-NO+ pairs formed on auto-reduced Cu-CHA in NH 3 -SCR. • The promoting effect of Cu auto-reduction in NH 3 -SCR is universal for Cu-zeolites. [ABSTRACT FROM AUTHOR]

In order to meet the legislative emission requirements for NO x -containing exhaust gases, SCR catalysts, in particular zeolites, are used. To improve catalysts and the catalytic processes, an in-depth understanding of the reaction mechanisms is required as well as an analysis of the physicochemical properties of the SCR catalysts, preferably in real-time. Here, we introduce a setup combining impedance spectroscopy and infrared spectroscopy in diffuse reflection mode for in situ measurements on zeolites under SCR-related conditions. This setup allows for the first time to simultaneously monitor both, the proton conductivity of zeolites and the vibration modes of the molecules involved in the catalytic conversion of NO by NH 3 . We studied both, pure and Fe-promoted H-form zeolites, as sensors and model catalysts at the same time, and found out that weakly bound NH 3 is dominating the proton conductivity of both zeolites in a temperature range below the desorption temperature of NH 3 . When a part of the weakly bound NH 3 is consumed by the SCR reaction, proton conductivity and thus the sensing effect gets dominated by strongly bound NH 3 . This allows applying impedance spectroscopy to assess the degree of NH 3 loading and the state of the SCR conversion process in zeolite catalyst. [ABSTRACT FROM AUTHOR]

Abstract: We compare the characteristic features obtained in TPD and in impedance measurements on the desorption of ammonia from zeolite H-ZSM-5. We show that the ammonia desorption peaks observed by TPD correspond to characteristic changes in the proton transport mechanism. While at low temperature an almost temperature independent Grotthus-like proton conductivity in the presence of ammonia is observed, the on-set of the low temperature desorption peak in TPD coincides with a change in the transport mechanism to a vehicle-mediated transport of protons. Furthermore, at higher temperature, the desorption of ammonia from the Brønsted acid sites, as indicated by the high temperature peak in TPD, causes again a characteristic change in the temperature dependent conductivity, reflected by a decreasing conductivity with increasing temperature in an intermitted range, which then turns over again to simply activated proton hopping. [Copyright &y& Elsevier]

Abstract: We present a humidity sensor based on H-ZSM-5 type zeolite that is suitable to detect traces of humidity (∼10–110ppmV) under harsh conditions, e.g. reducing atmosphere (H2) and high temperature (up to 600°C). By means of complex impedance spectroscopy (IS) we show that the zeolite sensor responds linearly towards minimal changes in humidity. Therefore this result indicates that the zeolite sensor is capable to detect traces of humidity in processes where high temperatures in a hydrogen environment are required. [Copyright &y& Elsevier]

Abstract: The acid properties of H-ZSM-5 zeolites with different SiO2/Al2O3 ratios (30, 50, 80, 150, 280 and 1000) were examined by means of the temperature programmed desorption of ammonia (NH3-TPD). Different pretreatments together with a comparison of different curve-fitting methods after desorption of ammonia were used to differentiate between different adsorption sites. 27Al-MAS-NMR spectroscopy was applied to distinguish the framework (AlF) from the extra-framework aluminum (AlEF). Both techniques reveal that the concentration of Brønsted acid sites is lower than expected when considering the aluminum content. The strength of the Brønsted acid sites, determined by a theoretical model assuming the free readsorption of ammonia, is found to be in the range of ΔH =129–161kJmol−1 and shows variations in the acid strength to be less than σ =12kJmol−1. [Copyright &y& Elsevier]