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Phosphorus (P) stemming from biodiesel and/or lubricant oil additives is unavoidable in real diesel exhausts and deactivates gradually the Cu-SSZ-13 zeolite catalyst for ammonia-assisted selective catalytic NO x reduction (NH 3 -SCR). Here, the deactivation mechanism of Cu-SSZ-13 by P-poisoning was investigated by ex situ examination of the structural changes and by in situ probing the dynamics and redox of Cu active sites via a combination of impedance spectroscopy, diffuse reflection infrared Fourier transform spectroscopy, and ultraviolet-visible spectroscopy. We unveiled that strong interactions between Cu and P led to not only a loss of Cu active sites for catalytic turnovers but also a restricted dynamic motion of Cu species during low-temperature NH 3 -SCR catalysis. Furthermore, the Cu II ↔ Cu I redox cycling of Cu sites, especially the Cu I → Cu II reoxidation half-cycle, was significantly inhibited, which can be attributed to the restricted Cu motion by P-poisoning disabling the formation of key dimeric Cu intermediates. As a result, the NH 3 -SCR activity at low temperatures (200 °C and below) decreased slightly for the mildly poisoned Cu-SSZ-13 and considerably for the severely poisoned 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]

Zeolites are in use as catalyst materials in the selective catalytic reduction of NO x with NH 3 as reducing agent. An initial NH 3 -adsorption on the catalyst is essential for the reaction with NO and NO 2 to occur. Due to the adsorbed NH 3 , the proton conductivity of the zeolite increases markedly. This change of the electrical properties of the catalyst material can be monitored in a contactless way and in operando using a radio frequency-based method to determine the ammonia loading on the catalyst. In this work, the ammonia storage is examined on a copper exchanged Chabazite Cu-SSZ-13, a typical SCR catalyst as it is in use for serial applications, at different temperatures and various feed ratios. [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]

The development of more efficient zeolite catalysts for selective catalytic reduction of NO x by NH 3 (NH 3 -SCR) requires insights into both reaction mechanism and real-time state of the catalyst ( e.g. , the storage level of NH 3 ). Here, we show that impedance spectroscopy (IS) can be applied to sense electrically the uptake of NH 3 into proton-conducting, copper-promoted zeolite catalysts, i.e. , into Cu-ZSM-5 and Cu-SAPO-34. Comparative investigations indicate that Cu-SAPO-34, as compared to Cu-ZSM-5, showed high sensitivity to the change of NH 3 concentration at a broader temperature range. While both zeolites performed similarly for direct monitoring of the SCR conversion of stored NH 3 at temperatures above 350 °C, Cu-SAPO-34 showed a better performance than Cu-ZSM-5 at lower temperatures. A simultaneous IS and diffuse reflection infrared Fourier transform spectroscopy (IS-DRIFTS) study revealed that NH 4 + or NH 4 NO 3 intermediates form on the two zeolite catalysts under SCR-related conditions and affect the proton conductivity at low temperatures, thus influencing significantly the monitoring of SCR conversion of stored NH 3 . The correlation of integral electrical responses with molecular processes, achieved by our simultaneous IS-DRIFTS studies, not only clarifies the origin of the sensing mechanism of zeolite catalysts at a molecular level, but also provides a new perspective to understand the NH 3 -SCR mechanism over metal-promoted zeolites at low reaction temperatures. [ABSTRACT FROM AUTHOR]

Metal(IV) phosphates such as zirconium phosphate (ZrP) are well known proton conductors based on the mobility of protons localized at Brønsted-acid sites. By introduction of mesoporous silica into the interlayer region, these compact materials are transformed into Porous Phosphate Heterostructures (PPH). Here we studied the electrical properties of PPH and show that the proton conductivity is supported by ammonia, which does not occur neither in ZrP nor in the mesoporous silica. This demonstrates that transforming the layer structure of ZrP into a mesoporous hybrid material leads to a new ammonia gas sensing material. This concept might be promising for the development of new gas sensor materials in general. [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: 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]