Metal-exchanged zeolites for NH3-SCR applications - Activity and Deactivation studies

Emissions of nitrogen oxides (NOX) formed during the burning process in internal combustion engines is a major contributor to global air pollutions. One effective way to reduce NOX in lean environments, i.e. oxygen excess is selective catalytic reduction with ammonia (NH3-SCR). Metal-exchanged zeolites have proven to be active as SCR catalysts, where copper and iron are the most common metals. When using metal-exchanged zeolites in exhaust aftertreatment systems, several challenges arise. Resistance towards hydrothermal deactivation and chemisorption of impurities on the active sites of the catalyst are two of the more important challenges. Temperatures between 600-700oC can be seen during regeneration of the particulate filter, which usually is placed upstream close to the SCR catalyst in the exhaust aftertreatment system, and therefore hydrothermal stability of the metal-exchanged zeolite is crucial. Furthermore, high tolerance against catalyst poisons which originate from (bio-) fuels and lubricating oils is desired, where phosphorous and potassium are among the more important poisons. In this thesis thermal and chemical deactivation of iron-exchanged zeolite BEA as SCR catalyst is experimentally studied with special focus paid on the active iron species. Based on the experimental results a kinetic model is developed to predict the decreased activity of the catalyst after deactivation. Several characterization techniques are used to evaluate and correlate structural changes in the catalyst with the decreased activity. Catalysts are prepared and characterized using BET, XPS, XRD, TPD, in-situ FTIR and UV-Vis. The catalytic performance of the samples is measured using a flow-reactor system. It is concluded that the hydrothermal deactivation of Fe-BEA is a result of migration of isolated iron species forming iron cluster inside the zeolite pores and iron particles located on the external surface of the zeolite crystals. Further, it is shown that the growth of iron clusters and particles can be partially reversed by high temperature hydrogen treatment. The chemical deactivation due to phosphorous exposure is the result of formation of metaphosphates replacing hydroxyl groups on the active isolated iron species. Furthermore, the chemical deactivation of Fe-BEA by potassium is concluded to be due to exchange and loss of active isolated iron species in the zeolite forming smaller iron clusters inside the zeolite pores. A kinetic model where different iron species are included was developed based on the hydrothermal deactivation experiments and validated using phosphorous and potassium exposed samples. By fitting and fix the kinetic parameters towards a fresh sample, the decreased SCR activity can be predicted by just decreasing the number of active iron sites, representing loss of active iron species due to hydrothermal treatment and poisoning. The effect of gas atmosphere during solid-state ion-exchange of copper-zeolites was studied as well. It is concluded that copper becomes highly mobile due to formation of copper-ammine complexes in presence of NH3 after reduction of CuII to CuI by adding NO in the exposing gas during the solid-state ion-exchange. Copper-exchanged zeolites could be prepared by exposing physical mixtures of copper-oxides with zeolites to NO and NH3 at as low temperature as 250oC. Finally, the ammonia formation during the rich period of NOX storage and reduction (NSR) cycles was studied using kinetic modeling for the possibility of combining NSR and SCR catalysts in the exhaust aftertreatment system. It is concluded that the formation of ammonia is due to stored NOX and hydrogen from the gas in the first half of the catalyst. However, it was further concluded that the formation of ammonia is delayed due to formation of N2O from stored NOX and formed NH3

Deactivation of a Vanadium-Based SCR Catalyst Used in a Biogas-Powered Euro VI Heavy-Duty Engine Installation

We have investigated how the exhaust gases from a heavy-duty Euro VI engine, powered with biogas impact a vanadium-based selective catalytic reduction (SCR) catalyst in terms of performance. A full Euro VI emission control system was used and the accumulation of catalyst poisons from the combustion was investigated for the up-stream particulate filter as well as the SCR catalyst. The NO(x)reduction performance in terms of standard, fast and NO2-rich SCR was evaluated before and after exposure to exhaust from a biogas-powered engine for 900 h. The SCR catalyst retains a significant part of its activity towards NO(x)reduction after exposure to biogas exhaust, likely due to capture of catalyst poisons on the up-stream components where the deactivation of the oxidation catalyst is especially profound. At lower temperatures some deactivation of the first part of the SCR catalyst was observed which could be explained by a considerably higher surface V4+/V(5+)ratio for this sample compared to the other samples. The higher value indicates that the reoxidation of V(4+)to V(5+)is partially hindered, blocking the redox cycle for parts of the active sites

Deactivation of a Pd/Pt Bimetallic Oxidation Catalyst Used in a Biogas-Powered Euro VI Heavy-Duty Engine Installation

The reduction of anthropogenic greenhouse gas emissions is crucial to avoid further warming of the planet. We investigated how effluent gases from a biogas powered Euro VI heavy-duty engine impact the performance of a bimetallic (palladium and platinum) oxidation catalyst. Using synthetic gas mixtures, the oxidation of NO, CO, and CH4\ua0before and after exposure to biogas exhaust for 900 h was studied. The catalyst lost most of its activity for methane oxidation, and the activity loss was most severe for the inlet part of the aged catalyst. Here, a clear sintering of Pt and Pd was observed, and higher concentrations of catalyst poisons such as sulfur and phosphorus were detected. The sintering and poisoning resulted in less available active sites and hence lower activity for methane oxidation

Metal-exchanged zeolites for NH3-SCR applications - Activity and Deactivation studies

Emissions of nitrogen oxides (NOX) formed during the burning process in internal combustion engines is a major contributor to global air pollutions. One effective way to reduce NOX in lean environments, i.e. oxygen excess is selective catalytic reduction with ammonia (NH3-SCR). Metal-exchanged zeolites have proven to be active as SCR catalysts, where copper and iron are the most common metals. When using metal-exchanged zeolites in exhaust aftertreatment systems, several challenges arise. Resistance towards hydrothermal deactivation and chemisorption of impurities on the active sites of the catalyst are two of the more important challenges. Temperatures between 600-700oC can be seen during regeneration of the particulate filter, which usually is placed upstream close to the SCR catalyst in the exhaust aftertreatment system, and therefore hydrothermal stability of the metal-exchanged zeolite is crucial. Furthermore, high tolerance against catalyst poisons which originate from (bio-) fuels and lubricating oils is desired, where phosphorous and potassium are among the more important poisons. In this thesis thermal and chemical deactivation of iron-exchanged zeolite BEA as SCR catalyst is experimentally studied with special focus paid on the active iron species. Based on the experimental results a kinetic model is developed to predict the decreased activity of the catalyst after deactivation. Several characterization techniques are used to evaluate and correlate structural changes in the catalyst with the decreased activity. Catalysts are prepared and characterized using BET, XPS, XRD, TPD, in-situ FTIR and UV-Vis. The catalytic performance of the samples is measured using a flow-reactor system.It is concluded that the hydrothermal deactivation of Fe-BEA is a result of migration of isolated iron species forming iron cluster inside the zeolite pores and iron particles located on the external surface of the zeolite crystals. Further, it is shown that the growth of iron clusters and particles can be partially reversed by high temperature hydrogen treatment. The chemical deactivation due to phosphorous exposure is the result of formation of metaphosphates replacing hydroxyl groups on the active isolated iron species. Furthermore, the chemical deactivation of Fe-BEA by potassium is concluded to be due to exchange and loss of active isolated iron species in the zeolite forming smaller iron clusters inside the zeolite pores.A kinetic model where different iron species are included was developed based on the hydrothermal deactivation experiments and validated using phosphorous and potassium exposed samples. By fitting and fix the kinetic parameters towards a fresh sample, the decreased SCR activity can be predicted by just decreasing the number of active iron sites, representing loss of active iron species due to hydrothermal treatment and poisoning.The effect of gas atmosphere during solid-state ion-exchange of copper-zeolites was studied as well. It is concluded that copper becomes highly mobile due to formation of copper-ammine complexes in presence of NH3 after reduction of CuII to CuI by adding NO in the exposing gas during the solid-state ion-exchange. Copper-exchanged zeolites could be prepared by exposing physical mixtures of copper-oxides with zeolites to NO and NH3 at as low temperature as 250oC.Finally, the ammonia formation during the rich period of NOX storage and reduction (NSR) cycles was studied using kinetic modeling for the possibility of combining NSR and SCR catalysts in the exhaust aftertreatment system. It is concluded that the formation of ammonia is due to stored NOX and hydrogen from the gas in the first half of the catalyst. However, it was further concluded that the formation of ammonia is delayed due to formation of N2O from stored NOX and formed NH3

Fundamental deactivation mechanisms of Fe-BEA as NH3-SCR catalyst - Experimental studies and kinetic modeling

Due to globalization and an increasing transport sector the interest for more fuel efficient combustion engines operating under lean conditions has increased. The products formed during the burning process in internal combustion engines are major contributants to global air pollution, where carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOX) are the major toxic components regulated in many countries. One effective way to reduce nitrogen oxides in lean environments is selective catalytic reduction with ammonia (NH3-SCR). Metal-exchanged zeolites have in this connection proven to be very active and promising catalysts for NOX reduction. Several challenges arise when using these materials in exhaust aftertreatment systems for vehicles. One problem is thermal deactivation due to the high-temperature conditions in connection with the regeneration of the particulate filter which in addition to the SCR catalyst is an important component in the aftertreatment system. In this licentiate thesis, the thermal stability of iron-based zeolite beta, Fe-BEA, as NH3-SCR catalyst is evaluated with several different experimental techniques. Based on the experimental results a kinetic model is developed to describe the kinetics and the fundamental deactivation mechanisms for Fe-BEA after hydrothermal treatment with focus on the dynamics of the active iron sites. Cordierite supported Fe-BEA samples were hydrothermally treated at 600 and 7000C for 3-100 h to capture the effect of time and temperature on the ageing process. The samples were characterized with BET, XPS, XRD and NH3-TPD. The catalytic performance of the samples with respect to NO and NH3 oxidation, and NOX reduction (NH3-SCR) was studied by flow reactor experiments. The catalytic performance was correlated with structural changes of the zeolite and the iron phases. The results showed that the NOX reduction at low temperatures is more sensitive to changes in the oxidation state of iron caused by the hydrothermal ageing than at higher temperatures. Furthermore, a maximum in activity for NO oxidation and an increased oxidation state of iron indicate Fe2O3 particle growth. This was further investigated with DRIFT spectroscopy which showed that the change of the nature of the iron species in Fe-BEA proceeds in two steps; (i) milder ageing results in a decreased amount of isolated iron species due to migration, and (ii) more sever aging results in a continuous migration and formation of larger iron oxide particles. High surface coverage of ammonia inhibits the SCR reaction. However, the inhibition effect is not significantly affected by the hydrothermal treatment. Furthermore, the possibility to regenerate the catalyst by treatment with hydrogen was investigated. The H2-treatment showed a reversed trend compared to the hydrothermally treated samples. Increased NOX reduction at low temperatures was observed indicating that the H2-treatment results in the formation of isolated iron species in the zeolite.A kinetic ageing model was developed based on the experimental results for H-BEA and Fe-BEA. The ageing model describes the experiments well for both H-BEA and Fe-BEA, before and after hydrothermal treatment, by decreasing the density of active sites. Furthermore, the model showed that the spillover rate of ammonia, inhibiting the NOX reduction is independent of the site density and depends only on the fraction of free sites, indicating that a constant number of Br\uf8nsted sites buffer each active iron site is constant and unaffected by the hydrothermal treatment

Fundamental deactivation mechanisms of Fe-BEA as NH3-SCR catalyst - Experimental studies and kinetic modeling

Due to globalization and an increasing transport sector the interest for more fuel efficient combustion engines operating under lean conditions has increased. The products formed during the burning process in internal combustion engines are major contributants to global air pollution, where carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOX) are the major toxic components regulated in many countries. One effective way to reduce nitrogen oxides in lean environments is selective catalytic reduction with ammonia (NH3-SCR). Metal-exchanged zeolites have in this connection proven to be very active and promising catalysts for NOX reduction. Several challenges arise when using these materials in exhaust aftertreatment systems for vehicles. One problem is thermal deactivation due to the high-temperature conditions in connection with the regeneration of the particulate filter which in addition to the SCR catalyst is an important component in the aftertreatment system. In this licentiate thesis, the thermal stability of iron-based zeolite beta, Fe-BEA, as NH3-SCR catalyst is evaluated with several different experimental techniques. Based on the experimental results a kinetic model is developed to describe the kinetics and the fundamental deactivation mechanisms for Fe-BEA after hydrothermal treatment with focus on the dynamics of the active iron sites. Cordierite supported Fe-BEA samples were hydrothermally treated at 600 and 7000C for 3-100 h to capture the effect of time and temperature on the ageing process. The samples were characterized with BET, XPS, XRD and NH3-TPD. The catalytic performance of the samples with respect to NO and NH3 oxidation, and NOX reduction (NH3-SCR) was studied by flow reactor experiments. The catalytic performance was correlated with structural changes of the zeolite and the iron phases. The results showed that the NOX reduction at low temperatures is more sensitive to changes in the oxidation state of iron caused by the hydrothermal ageing than at higher temperatures. Furthermore, a maximum in activity for NO oxidation and an increased oxidation state of iron indicate Fe2O3 particle growth. This was further investigated with DRIFT spectroscopy which showed that the change of the nature of the iron species in Fe-BEA proceeds in two steps; (i) milder ageing results in a decreased amount of isolated iron species due to migration, and (ii) more sever aging results in a continuous migration and formation of larger iron oxide particles. High surface coverage of ammonia inhibits the SCR reaction. However, the inhibition effect is not significantly affected by the hydrothermal treatment. Furthermore, the possibility to regenerate the catalyst by treatment with hydrogen was investigated. The H2-treatment showed a reversed trend compared to the hydrothermally treated samples. Increased NOX reduction at low temperatures was observed indicating that the H2-treatment results in the formation of isolated iron species in the zeolite.A kinetic ageing model was developed based on the experimental results for H-BEA and Fe-BEA. The ageing model describes the experiments well for both H-BEA and Fe-BEA, before and after hydrothermal treatment, by decreasing the density of active sites. Furthermore, the model showed that the spillover rate of ammonia, inhibiting the NOX reduction is independent of the site density and depends only on the fraction of free sites, indicating that a constant number of Br\uf8nsted sites buffer each active iron site is constant and unaffected by the hydrothermal treatment

Impact of Thermal and Chemical Ageing of Fe-BEA SCR Catalyst on NOx Conversion Performance

Emissions of nitrogen oxides (NOx) from heavy-duty diesel engines are subject to more stringent environmental legislation. Selective catalytic reduction (SCR) over metal ion-exchanged zeolites is in this connection an efficient method to reduce NOx. Understanding durability of the SCR catalyst is crucial for correct design of the aftertreatment system. In the present paper, thermal and chemical ageing of Fe-BEA as NH3-SCR catalyst is studied. Experimental results of hydrothermal ageing, and chemical ageing due to phosphorous and potassium exposure are presented. The catalyst is characterized by flow reactor experiments, nitrogen physisorption, DRIFTS, XRD, and XPS. Based on the experimental results, a multisite kinetic model is developed to describe the activity of the fresh Fe-BEA catalyst. Furthermore, the model can predict deactivation of the catalyst well by decreasing the number of active sites, representing loss of active iron sites due to migration or chemical blockage of the sites. By performing a systematic study of different deactivation mechanisms, a deactivation expression for the active sites can be formulated

Impact of Thermal and Chemical Ageing of Fe-BEA SCR Catalyst on NOx Conversion Performance

Emissions of nitrogen oxides (NOx) from heavy-duty diesel engines are subject to more stringent environmental legislation. Selective catalytic reduction (SCR) over metal ion-exchanged zeolites is in this connection an efficient method to reduce NOx. Understanding durability of the SCR catalyst is crucial for correct design of the aftertreatment system. In the present paper, thermal and chemical ageing of Fe-BEA as NH3-SCR catalyst is studied. Experimental results of hydrothermal ageing, and chemical ageing due to phosphorous and potassium exposure are presented. The catalyst is characterized by flow reactor experiments, nitrogen physisorption, DRIFTS, XRD, and XPS. Based on the experimental results, a multisite kinetic model is developed to describe the activity of the fresh Fe-BEA catalyst. Furthermore, the model can predict deactivation of the catalyst well by decreasing the number of active sites, representing loss of active iron sites due to migration or chemical blockage of the sites. By performing a systematic study of different deactivation mechanisms, a deactivation expression for the active sites can be formulated