Periodic Reporting for period 1 - 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: 2021-06-01 to 2023-05-31
Emission of nitrogen oxides (NOx) is an important contributor to air pollution. The main sources of NOx emissions are power plants and combustion engines, where road transportation is an important representative. To reduce NOx emissions from vehicles, catalytic exhaust systems have been developed. Notwithstanding the expected shift to electric vehicles, long haul transportation still relies on diesel engines and stricter legislation on air quality and greenhouse gas emissions cannot be attained without an improvement in diesel engine technology.
The focus of CHASS is on freight transport, an important driving force of EU economy but also a contributor to emissions, with important 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, employing Cu-zeolites as catalysts. Cu-CHA (a Cu exchanged zeolite with the chabazite topology) shows an excellent performance in the low temperature range (150-300 °C). The development of engines is currently focused on improved fuel efficiency, and, therefore, the temperature in diesel exhausts systems will become lower. Consequently, the performance of SCR catalysts in the temperature range 150-300 ℃ will be an important parameter for application in future diesel engines.
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 deactivation between 150 and 300 ℃, where Cu-zeolites otherwise show their superior performance. 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 aims at generating knowledge to enhance the performance of Cu-zeolite materials 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.
The focus of CHASS is on freight transport, an important driving force of EU economy but also a contributor to emissions, with important 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, employing Cu-zeolites as catalysts. Cu-CHA (a Cu exchanged zeolite with the chabazite topology) shows an excellent performance in the low temperature range (150-300 °C). The development of engines is currently focused on improved fuel efficiency, and, therefore, the temperature in diesel exhausts systems will become lower. Consequently, the performance of SCR catalysts in the temperature range 150-300 ℃ will be an important parameter for application in future diesel engines.
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 deactivation between 150 and 300 ℃, where Cu-zeolites otherwise show their superior performance. 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 aims at generating knowledge to enhance the performance of Cu-zeolite materials 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.
The research activity carried out in CHASS consists of a combination of quantum-chemical calculations with advanced spectroscopic methods, kinetic measurements, and modelling on catalysts. At this stage, we are addressing separately the deactivation by sulphur oxides poisoning and that caused by dealumination (hydrothermal aging).
We have used density functional theory (DFT) to investigate on an atomic scale how sulphur dioxide deactivates Cu-CHA during low-temperature NH3-SCR reaction conditions. We have calculated the reaction of sulphur dioxide with intermediates that are suggested to be present during low-temperature NH3-SCR, particularly the peroxo diamino dicopper(II) complex, that has been suggested to be a key intermediate in the reaction. Our calculations show that sulphur dioxide reacts strongly with this complex, thus suggesting that this reaction is responsible for the deactivation forming tetraamino dicopper(II) sulphate. We have studied the interaction of tetraamino dicopper(II) sulphate with NO and ammonia, two molecules that are present during NH3-SCR, finally yielding ammonium sulphate and ammonium hydrogen sulphate, which cause a physical blocking of the reaction.
The reaction of sulphur dioxide with Cu-CHA pre-treated in different conditions has been followed in situ by X-ray absorption spectroscopy (XAS), Diffuse Reflectance UV-Vis, Infrared and ex situ Raman spectroscopies. XAS and UV-Vis show that sulphur dioxide preferentially interacts with the peroxo diamino dicopper(II) complex, causing a breaking of the dimeric complex and a partial reduction from Cu(II) to Cu(I). Vibrational spectroscopies confirm the formation of ammonium (and/or hydrogen) sulphate, predicted by DFT calculations.
Deactivation has been studied by measuring the effect of Cu content and Si/Al ratio of the catalysts on sulphur dioxide uptake and catalytic activity after formation of the peroxo diamino dicopper(II) complex. We find that the effect of sulphur dioxide on deactivation diminishes with increasing its uptake. A formalism/method to model the effect of sulphur dioxide on activity has been developed.
DFT has also been used to investigate the dealumination of H-CHA (Cu-free) with different Si/Al ratios, and to calculate vibrational fingerprints of fresh and aged catalyst. A set of H-CHA and Cu-CHA with different Si/Al ratios and Cu contents have been characterized as such and after an aging treatment. The results show that aging does not affect crystallinity but causes a local damage of the structure, with increase of surface area and pore volume, especially in the absence of Cu. Dealumination causes a crucial loss of Brønsted sites, with formation of silanols in H-CHA and defective OH groups able to protonate ammonia in Cu-CHA catalysts.
We have used density functional theory (DFT) to investigate on an atomic scale how sulphur dioxide deactivates Cu-CHA during low-temperature NH3-SCR reaction conditions. We have calculated the reaction of sulphur dioxide with intermediates that are suggested to be present during low-temperature NH3-SCR, particularly the peroxo diamino dicopper(II) complex, that has been suggested to be a key intermediate in the reaction. Our calculations show that sulphur dioxide reacts strongly with this complex, thus suggesting that this reaction is responsible for the deactivation forming tetraamino dicopper(II) sulphate. We have studied the interaction of tetraamino dicopper(II) sulphate with NO and ammonia, two molecules that are present during NH3-SCR, finally yielding ammonium sulphate and ammonium hydrogen sulphate, which cause a physical blocking of the reaction.
The reaction of sulphur dioxide with Cu-CHA pre-treated in different conditions has been followed in situ by X-ray absorption spectroscopy (XAS), Diffuse Reflectance UV-Vis, Infrared and ex situ Raman spectroscopies. XAS and UV-Vis show that sulphur dioxide preferentially interacts with the peroxo diamino dicopper(II) complex, causing a breaking of the dimeric complex and a partial reduction from Cu(II) to Cu(I). Vibrational spectroscopies confirm the formation of ammonium (and/or hydrogen) sulphate, predicted by DFT calculations.
Deactivation has been studied by measuring the effect of Cu content and Si/Al ratio of the catalysts on sulphur dioxide uptake and catalytic activity after formation of the peroxo diamino dicopper(II) complex. We find that the effect of sulphur dioxide on deactivation diminishes with increasing its uptake. A formalism/method to model the effect of sulphur dioxide on activity has been developed.
DFT has also been used to investigate the dealumination of H-CHA (Cu-free) with different Si/Al ratios, and to calculate vibrational fingerprints of fresh and aged catalyst. A set of H-CHA and Cu-CHA with different Si/Al ratios and Cu contents have been characterized as such and after an aging treatment. The results show that aging does not affect crystallinity but causes a local damage of the structure, with increase of surface area and pore volume, especially in the absence of Cu. Dealumination causes a crucial loss of Brønsted sites, with formation of silanols in H-CHA and defective OH groups able to protonate ammonia in Cu-CHA catalysts.
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 where ammonium hydrogen sulphate decomposes. The suggested mechanism provides atomistic understanding of sulfur poisoning of Cu-CHA during NH3-SCR. However, the in situ spectroscopic study also indicates a chemical effect of sulphur dioxide, proved by changes in the oxidation state of Cu. The combination of a physical and chemical effect could be the key to explaining the reasons for the observed reversible and irreversible catalyst deactivation, not yet understood in the literature.
We have adapted an approach where deactivation is quantified, without any prior assumptions, as proposed earlier for a different reaction. The proposed formulation is also the basis for a description of the NOx conversion as a function of sulphur dioxide content, which can be implemented in a kinetic model.
The final aim of CHASS is the development of kinetic model(s) for activity, deactivation, and performance of Cu-zeolites for NH3-SCR, including the effects of sulphur oxides.
The understanding at an atomic level of poisoning of Cu-CHA catalysts by sulphur dioxide form a solid basis for the development of such a model. In the automotive industry, there is a clear demand for such models from the engine- and car manufacturers, as it will most probably become an industry standard to have sufficient (modeling) tools to describe the impact of sulphur dioxide on the performance of Cu-CHA catalysts for NH3-SCR.
We have adapted an approach where deactivation is quantified, without any prior assumptions, as proposed earlier for a different reaction. The proposed formulation is also the basis for a description of the NOx conversion as a function of sulphur dioxide content, which can be implemented in a kinetic model.
The final aim of CHASS is the development of kinetic model(s) for activity, deactivation, and performance of Cu-zeolites for NH3-SCR, including the effects of sulphur oxides.
The understanding at an atomic level of poisoning of Cu-CHA catalysts by sulphur dioxide form a solid basis for the development of such a model. In the automotive industry, there is a clear demand for such models from the engine- and car manufacturers, as it will most probably become an industry standard to have sufficient (modeling) tools to describe the impact of sulphur dioxide on the performance of Cu-CHA catalysts for NH3-SCR.