Cu-CHA zeolite-based catalysts for the selective catalytic reduction of NOx in exhaust diesel gas: addressing the issue of Sulfur Stability

Project Information

CHASS

Grant agreement ID: 955839

DOI

10.3030/955839

Project closed

EC signature date 5 March 2021

Start date 1 June 2021

End date 31 May 2025

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 1 093 793,04

EU contribution

€ 1 093 793,04

Coordinated by

UNIVERSITA DEGLI STUDI DI TORINO
Italy

Periodic Reporting for period 2 - CHASS (Cu-CHA zeolite-based catalysts for the selective catalytic reduction of NOx in exhaust diesel gas: addressing the issue of Sulfur Stability)

Reporting period: 2023-06-01 to 2025-05-31

Summary of the context and overall objectives of the project

Emission of nitrogen oxides (NOx) is an important contributor to air pollution world-wide. The main sources of NOx emissions are power plants and combustion engines. To reduce NOx emissions from vehicles, catalytic exhaust systems are used. The focus of CHASS is on freight transport, an important driving force of EU economy but also a contributor to emissions, with consequences on citizen’s health and social costs.
NH3-SCR (Ammonia Selective Catalytic Reduction) is an important technology to mitigate unwanted NOx emissions from diesel engines, using Cu-zeolites as catalysts. Cu-CHA shows an excellent performance in the low temperature range (150-300 °C)., which is important in view of the development of engines with improved fuel efficiency.

Despite the superior stability of Cu-CHA, repeated exposures to high temperature and the harsh environment in exhaust systems still cause deactivation, i.e. the performance deteriorates with time. Small amounts of sulphur dioxide, a common component in diesel exhaust gas, can result in deactivationat low temperature. As deactivation may cause malfunction, the applicability of Cu-CHA requires ultra-low Sulphur diesel. Even then, exhaust systems must be designed to handle possible deactivation.

The CHASS project has generated knowledge to enhance the performance of Cu-zeolite for the abatement of NOx by NH3-SCR, with the following specific objectives:
- Understanding the interaction of sulphur oxides with Cu-CHA at different operating conditions at the atomic level.
- Determine the influence of sulphur oxides on the reaction.
- Understanding the processes leading to hydrothermal aging of Cu-zeolites at the atomic level.
- Characterization and identification of the critical atomic structures responsible for hydrothermal aging.
- Determine the influence of hydrothermal aging on the deactivation by sulphur oxides.
- Development of kinetic model(s) for activity, deactivation, and performance of Cu-zeolites for NH3-SCR, including the effects of sulphur oxides and hydrothermal aging, based on atomistic first principles data, applicable for commercial exhaust systems.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Density functional theory (DFT) calculations coupled to in situ spectroscopies (X-ray absorption, Diffuse Reflectance UV-Vis, infrared and ex situ Resonant Raman) have shed light on the mechanism by which SO2 interacts with Cu ions and deactivate the catalyst.
The deactivation of Cu-CHA catalysts in powder was measured by exposing SO2 in different conditions (alone, with O2, H2O, NO, NH3 etc.) after different pretreatments, developing a descriptive model for the deactivation by SO2.
The evolution of Al and Cu atoms during dealumination in H-CHA and Cu-CHA has been described by DFT calculations, ab initio thermodynamics and microkinetic modeling. A DFT based kinetic model describes accurately the inhibiting effect of water on the reaction.
Fresh and hydrothermally treated H-CHA and Cu-CHA with different composition have been characterized with structural and spectroscopic techniques to understand the evolution of Cu and Al sites in the aging process. Quantitative titration methods have been optimized for the determination of Cu sites evolution along hydrothermal ageing treatments.
Catalytic tests have been carried out on monoliths, exposing hydrothermal aged catalyst to SO2 cycling experiments followed by regeneration. Moreover, the effect of H2O in combination with SO2 on the catalytic activity of Cu-CHA has been investigated and rationalized.
Water is always present in the NH3-SCR mixture, but so far no systematic studies about its effect on activity have been reported. By combining activity measurements and DFT-based microkinetic modeling, we have developed an atomic-level insight into the mechanism for inhibition of the NH3-SCR reaction by water over Cu-CHA catalysts and established a clear relationship between the reaction order and the partial pressure of water
Molecular dynamics simulations with a machine-learning force field augmented with long-range Coulomb interactions have been carried out to study the influence of Al distribution on the diffusion mechanisms and pairing of [Cu(NH3)2]+ complexes in Cu-CHA.
A two-sites kinetic model has been developed, based on the knowledge generated in the project about the mechanism of interaction of SO2 and Cu sites, characterization and catalytic data. The model can describe the regeneration process for different gas compositions.
The results have been published in 11 papers in peer reviewed Journals (2 manuscripts under review or submitted), communicated in 1 workshop, 13 International and 2 National conferences, for a total of 18 oral presentations and 7 posters. Two oral presentations will be given at 2 international conferences in June and July 2025. 3 colloquia on related topics have been organized by the PhD students, with high attendance by stakeholders.
The project and its results have been communicated in 4 press releases (one of which has been relaunched many times), 2 popular articles, 1 video and through CHASS website and social media

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The results by DFT calculations suggest that the key mechanism for low temperature sulphur dioxide deactivation is of physical origin and that the catalyst can be regenerated by exposure to high temperatures. This model does not explain why catalysts are deactivated at very low sulphur uptake. Our in situ spectroscopic studies indicate a chemical effect of sulphur dioxide. The proposed two step reaction mechanism explains how small sulphur uptake can cause consistent deactivation. Moreover, the combination of a physical and chemical effect could be the key to explaining the reasons for the observed reversible and irreversible catalyst deactivatio.
We have adapted an approach where deactivation is quantified, without any prior assumptions, where deactivation is described in terms of the sensitivity of the catalyst for SO2. The impact of SO2 on the activity is strongest at low SO2 uptakes, after which the sensitivity declines linearly with increasing uptake.
We have explained the different hydrothermal stability of Cu-CHA and H-CHA by quantum mechanical calculations showing that the introduction of copper into the zeolite structure increases the barriers for dealumination compared to the H–CHA. Moreover, the formation of copper-bound aluminum species is thermodynamically favorable compared to the formation of extraframework Al species.
Our molecular dynamics calculations allowed us to obtain unique information inaccessible to first-principle calculations and experiments, about the mobility of charged ions as a function of Al density and distribution.
This understanding form a solid basis for the development of a kinetic model for activity, deactivation, and performance of Cu-zeolites for NH3-SCR, including the effects of sulphur oxides and water.
Given that we have begun to phase out fossil fuels over the coming decades, we will probably not see major changes in the development of new catalysts. However, the outcome of CHASS help to optimise exhaust systems to get the most out of the materials that we already have. A successful introduction of hydrogen combustion engines, which have 3-4 times more water vapor in the exhaust, may ignite the development of new SCR catalyst materials. All the knowledge we develop on the current exhaust systems will then give us a strong head-start in the development of such new catalysts.

Species formed in Cu-CHA cage by sulphur poisoning in NH3-SCR reaction