Abstract The accurate description of NH 3 adsorption/desorption from the catalyst surface is the ... more Abstract The accurate description of NH 3 adsorption/desorption from the catalyst surface is the basis for the correct description of the NH 3SCR catalytic chemistry. Currently, Temkin-type kinetics are widely and successfully used to describe the NH 3 adsorption/desorption ...
Abstract A numerical model describing the ammonia based SCR process of NO X on zeolite catalysts ... more Abstract A numerical model describing the ammonia based SCR process of NO X on zeolite catalysts is presented. The model is able to simulate coated and extruded monoliths. The development of the reaction kinetics is based on a study which compares the activity of ...
It is widely recognised that diesel engine vehicles are fated to significantly increase their wor... more It is widely recognised that diesel engine vehicles are fated to significantly increase their worldwide market penetration, even in countries like the United States where the present market share is not as significant as that of gasoline engines. This is mainly due to the fact that diesel engines are inherently more thermodynamically efficient than petrol engines and thus they offer the prospect of reducing emissions of carbon dioxide as well [1,2]. However, diesels produce higher emissions of nitrogen oxides (NO X ) and particulates. The tail-pipe emissions levels that can be achieved depend upon both the engine-out emissions and the performance of the emissions control system. Improvements in combustion and/or alternative fuels can lead to lower NOx emissions, but it is widely accepted that in order to meet future legislative emissions standards (Euro6 for light duty vehicles and EuroVI for Heavy Duty engines, as well as the US 2010 Bin5), the employment of after treatment systems will be required [1,2]. Indeed, diesel particulate filters (DPFs) will be needed to achieve the PM emission levels regardless of the system used to reduce NOx. But, most importantly, the significant CO 2 reduction (i.e. the improved fuel consumptions) that will be also dictated by future regulations will imply a drastic decrease in the average temperature profile of the exhaust gases; in such conditions, the catalytic removal of NO x is extremely challenging. In fact, a significant portion of the new test cycles (both the NEDC, New European Driving Cycle, and the WHTC, World Harmonized Transient Cycle), is characterized by a very low exhaust temperature profile [2]. Currently, there are two main after-treatment technologies under consideration as potentially compliant with such strict limits: the Lean NOx Trap systems, which are used with direct injection gasoline and diesel engines, and the NH 3 /urea Selective Catalytic Reduction (SCR) for diesel engines. Lean NO x Traps were first developed and put into the market by Toyota to remove NO x from vehicles equipped with lean burn engines . NO x are stored under lean conditions in the form of nitrites and nitrates that are reduced to nitrogen during a short rich excursion. LNT materials typically consist of a NO x storage component, such as an alkaline earth metal oxide (e.g. Ba), and of a noble metal (e.g. Pt) that catalyzes the oxidation of NO x , CO and of hydrocarbons and the reduction of stored NO x as well. In spite of the fact that these catalysts are being used at the commercial scale, common agreement on the mechanisms of the storage of NO x and of their subsequent reduction is still lacking . SCR was the European motor industry's main technology of choice to meet Euro4 and Euro5 emissions requirements for heavy-duty diesel engines and during the last year it has been announced by some manufacturers for light-duty applications in the US and more recently in Europe, as well [1]. An SCR system is designed to catalytically reduce NO x emissions in the oxygen rich environment of diesel exhaust by injecting urea as reducing agent . A Diesel Oxidation Catalyst is also present in the system configuration, upstream of the SCR converter, to partially convert NO to NO 2 ; this permits the flow entering the SCR reactor to contain significant amounts of NO 2 in addition to NO, and thus the onset of the SCR deNOx reactions can occur at lower temperature, in comparison to the case where most of NOx is made of NO alone. During the last few years, both LNT and SCR catalytic processes have been widely investigated in our labs: this paper will report on the main results of these studies and in particular it will analyse the chemistry and the reaction pathways operating for the reduction of NOx in both the processes, in order to highlight the similarities/differences between them. A systematic study of the reduction by hydrogen under near isothermal conditions of NOx species adsorbed on model Pt-Ba/Al 2 O 3 LNT catalysts was performed: it was found that the reduction process is not initiated by the thermal decomposition of the stored NO x ad-species, but a catalytic pathway involving Pt is instead operating. It was also shown that such a catalytic pathway is composed of two consecutive steps in which NH 3 is formed as an intermediate upon reaction of nitrates with H 2, and further reacts with nitrates to produce selectively N 2 . The reactions involved in the NH 3 -NO/NO 2 SCR reacting system were investigated by an extensive set of various unsteady state experiments performed over both vanadium based and zeolite based commercial catalysts: the bulk of results led to the proposal of an original global mechanistic scheme of the different reactions that compose the complete NH 3 -NO/NO 2 SCR reacting system. In such a scheme, a key role is played by nitrate species, which are formed by NO 2 disproportion onto the catalyst surface and are then reduced by NO to nitrites. These species lead then to nitrogen upon reaction with ammonia . Accordingly, in both LNT and SCR chemistries, the reduction of NOx involves nitrite and nitrate surface species, which are selectively reduced to nitrogen by ammonia, either formed as intermediate or supplied as a reactant.
SCR has become the technology of choice for the abatement of NOx emissions from heavy-duty vehicl... more SCR has become the technology of choice for the abatement of NOx emissions from heavy-duty vehicles, and it has been recently introduced also in passenger cars. The current challenge for Diesel vehicles both in Europe and in the US markets is the achievement of a substantial reduction of CO 2 emissions while bringing NOx emissions below the upcoming stringent Euro VI and US Tier 2 Bin 5 limits 1 . As the new test cycles require catalysts active at very low temperature, the "Fast" SCR reaction used in present SCR-based after treatment converters to enhance the low-T DeNOx activity, plays and will play even more in the near future a critical role in the development and optimization of SCR systems 2 . Different metal exchanged (Fe and Cu) commercial zeolite based catalysts are investigated in this study under experimental conditions representative of real applications, to analyse each individual reaction step of the complete NO/NO 2 + NH 3 + O 2 SCR reacting system. The identification of the SCR kinetic mechanisms, and of the role of the related surface species, as well as the development of a detailed kinetic model will be herein presented.
Abstract an extensive investigation of the chemistry, the mechanism and the kinetics of the NO-NO... more Abstract an extensive investigation of the chemistry, the mechanism and the kinetics of the NO-NO2/NH3 SCR reactions for mobile applications was performed over a commercial Vanadium-based catalyst. On the basis of the collected results a Mars-Van Krevelen ...
Urea/NH 3 -SCR is presently considered a key technology for the control and abatement of NO emiss... more Urea/NH 3 -SCR is presently considered a key technology for the control and abatement of NO emissions from Diesel engines; the presence of a DOC catalyst upstream from the SCR unit warrants significant amounts of NO 2 entering the SCR reactors together with NO. In this work new zeolite-based catalysts promoted by Fe and by Cu were investigated: reactivity tests were performed to allow a quantitative comparison, whereas systematic mechanistic experiments were run to elucidate the major reaction pathways, with particular focus on the NO 2 /NH 3 reacting system and on formation of N 2 O. The bulk of information was then used to develop a dynamic kinetic model in close agreement with all the details of the SCR catalytic chemistry.
Transient methods, which consist in imposing perturbations to the reacting system (typically step... more Transient methods, which consist in imposing perturbations to the reacting system (typically step changes in the inlet reactant concentration) while analyzing the outlet transient response, are being more and more applied to study the kinetics and the mechanism of heterogeneous catalytic processes. In the past such techniques (Transient Response Method, TRM) were extensively applied in our labs to the study of the Selective Catalytic Reduction (SCR) of NO by NH 3 . In this paper we present the results obtained by the kinetic modeling of the TRM applied to the study of the storage of NOx under lean conditions over a model Pt-Ba/Al 2 O 3 catalyst sample. These catalytic systems, known as "NO x storage-reduction catalysts", were recently proposed to reduce NO x in "lean burn" engines [3] according to a cyclic process during which NO x are adsorbed under excess oxygen by a NO x -storage component (e.g. an alkaline metal oxide), and are then reduced during a short "rich" period to N 2 and H 2 O by a noble metal (e.g. Pt). Although these catalysts are currently used, even if only at a demonstrative scale, a lot of aspects concerning their mechanistic and kinetic features are not completely clarified yet. In this paper we present the results of a study aimed at investigating the NO x storage function and the related kinetics over a model Pt-Ba/Al 2 O 3 catalyst sample. NO step changes in the reactor inlet concentration were imposed, while analyzing the outlet transient response. In parallel, a kinetic model recently reported in the literature has been adapted to our catalytic system and to our experimental conditions: its simulations are herein compared to our experimental results.
Thermal sintering of SCR-DeNO x catalysts was studied by calcining at increasing temperature samp... more Thermal sintering of SCR-DeNO x catalysts was studied by calcining at increasing temperature samples of a commercial catalyst and testing them under operating conditions of industrial relevance. Results were analyzed by means of a complete model of the SCR ...
The Selective Catalytic Reduction by NH 3 /urea is a world-wide established after treatment techn... more The Selective Catalytic Reduction by NH 3 /urea is a world-wide established after treatment technology for the removal of NO x in the stack gases of power plants and of other stationary sources. Typical SCR catalysts operate between 300 and 400 • C, however for some applications SCR systems with reasonable DeNO x activity at temperatures as low as 180 • C are required nowadays.It was recently shown that over both V 2 O 5 -WO 3 /TiO 2 and Fe-ZSM-5 commercial SCR catalysts the addition of aqueous solutions of NH 4 NO 3 to a NO-NH 3 containing feed stream results in the occurrence of the "Enhanced SCR" (E-SCR) reaction (2NH 3 + 2NO + NH 4 NO 3 → 3N 2 + 5H 2 O), characterized by superior NO reduction efficiencies in the 200-350 • C range. In this paper we present a systematic experimental study of the E-SCR reaction over a commercial V-based Haldor Topsøe catalyst. The effects of three operating variables, namely space velocity (18-75 k h −1 ), temperature (180-250 • C) and ammonium nitrate feed content (0-100% of stoichiometric feed), are addressed in order to identify the best process conditions in view of the specific application of this new technology to stationary SCR installations.
In this paper the results obtained in our previous studies on the reduction of nitrates by CO ove... more In this paper the results obtained in our previous studies on the reduction of nitrates by CO over PtK/Al2O3 and PtBa/Al2O3 catalysts are compared and discussed in order to define the chemistry that leads to nitrogen evolution during the regeneration of the different lean ...
... ZSM-5 catalyst. Maria Pia Ruggeri a , Antonio Grossale a , Isabella Nova a , Enrico Tronconi ... more ... ZSM-5 catalyst. Maria Pia Ruggeri a , Antonio Grossale a , Isabella Nova a , Enrico Tronconi a , Corresponding Author Contact Information , E-mail The Corresponding Author , Hana Jirglova b , Zdenek Sobalik b. a, Laboratory ...
We review in the present paper the key mechanistic features of the NH 3 -NO/NO 2 Fast SCR reactio... more We review in the present paper the key mechanistic features of the NH 3 -NO/NO 2 Fast SCR reaction over vanadium and Fe-zeolite commercial catalysts: this is the main reaction occurring in the urea-SCR technology, that is currently considered the best technique for the abatement of NOx in the exhausts of Diesel engines. A detailed investigation by means of dynamic reactive experiments led to the proposal of an original scheme which accounts for stoichiometry, selectivity and intrinsic kinetics of the global SCR process: NO 2 forms surface nitrites and nitrates via dimerization and disproportionation/heterolytic chemisorption, NO reduces nitrates to nitrites, and NH 3 enables the rapid and selective decomposition of nitrites to nitrogen via formation of unstable ammonium nitrite.
The selective catalytic reduction (SCR) technology, based on the reaction between nitrogen oxides... more The selective catalytic reduction (SCR) technology, based on the reaction between nitrogen oxides in the flue gases and NH 3 /urea, has become one of the most promising technological solutions for the abatement of NO x emissions from Diesel vehicles.1 First ...
In the present work we investigate the adsorption of NO 2 onto a Fe-and a Cu-promoted zeolite SCR... more In the present work we investigate the adsorption of NO 2 onto a Fe-and a Cu-promoted zeolite SCR catalyst in the absence of gaseous water, with a particular focus on the effect of the catalyst red-ox state. For both catalytic systems, results from the analysis of the gas phase during step changes of the NO 2 concentration, combined with literature FTIR information, emphasize an important role of the catalyst red-ox state in the dynamics of NO 2 adsorption: the molar ratio of released NO to adsorbed NO 2 is close to 1/3 in the case of oxidized catalysts, in line with a two-steps mechanism leading to the formation of surface nitrates, but is greater when the catalyst is partially reduced. Furthermore the initial catalyst red-ox state influences the amount of nitrates that can be adsorbed on the catalyst surface, the prereduced sample exhibiting a greater storage capacity. The Cu-zeolite catalysts are able to store a greater amount of nitrates and are more easily reduced in comparison with the Fe-zeolite.
Abstract The accurate description of NH 3 adsorption/desorption from the catalyst surface is the ... more Abstract The accurate description of NH 3 adsorption/desorption from the catalyst surface is the basis for the correct description of the NH 3SCR catalytic chemistry. Currently, Temkin-type kinetics are widely and successfully used to describe the NH 3 adsorption/desorption ...
Abstract A numerical model describing the ammonia based SCR process of NO X on zeolite catalysts ... more Abstract A numerical model describing the ammonia based SCR process of NO X on zeolite catalysts is presented. The model is able to simulate coated and extruded monoliths. The development of the reaction kinetics is based on a study which compares the activity of ...
It is widely recognised that diesel engine vehicles are fated to significantly increase their wor... more It is widely recognised that diesel engine vehicles are fated to significantly increase their worldwide market penetration, even in countries like the United States where the present market share is not as significant as that of gasoline engines. This is mainly due to the fact that diesel engines are inherently more thermodynamically efficient than petrol engines and thus they offer the prospect of reducing emissions of carbon dioxide as well [1,2]. However, diesels produce higher emissions of nitrogen oxides (NO X ) and particulates. The tail-pipe emissions levels that can be achieved depend upon both the engine-out emissions and the performance of the emissions control system. Improvements in combustion and/or alternative fuels can lead to lower NOx emissions, but it is widely accepted that in order to meet future legislative emissions standards (Euro6 for light duty vehicles and EuroVI for Heavy Duty engines, as well as the US 2010 Bin5), the employment of after treatment systems will be required [1,2]. Indeed, diesel particulate filters (DPFs) will be needed to achieve the PM emission levels regardless of the system used to reduce NOx. But, most importantly, the significant CO 2 reduction (i.e. the improved fuel consumptions) that will be also dictated by future regulations will imply a drastic decrease in the average temperature profile of the exhaust gases; in such conditions, the catalytic removal of NO x is extremely challenging. In fact, a significant portion of the new test cycles (both the NEDC, New European Driving Cycle, and the WHTC, World Harmonized Transient Cycle), is characterized by a very low exhaust temperature profile [2]. Currently, there are two main after-treatment technologies under consideration as potentially compliant with such strict limits: the Lean NOx Trap systems, which are used with direct injection gasoline and diesel engines, and the NH 3 /urea Selective Catalytic Reduction (SCR) for diesel engines. Lean NO x Traps were first developed and put into the market by Toyota to remove NO x from vehicles equipped with lean burn engines . NO x are stored under lean conditions in the form of nitrites and nitrates that are reduced to nitrogen during a short rich excursion. LNT materials typically consist of a NO x storage component, such as an alkaline earth metal oxide (e.g. Ba), and of a noble metal (e.g. Pt) that catalyzes the oxidation of NO x , CO and of hydrocarbons and the reduction of stored NO x as well. In spite of the fact that these catalysts are being used at the commercial scale, common agreement on the mechanisms of the storage of NO x and of their subsequent reduction is still lacking . SCR was the European motor industry's main technology of choice to meet Euro4 and Euro5 emissions requirements for heavy-duty diesel engines and during the last year it has been announced by some manufacturers for light-duty applications in the US and more recently in Europe, as well [1]. An SCR system is designed to catalytically reduce NO x emissions in the oxygen rich environment of diesel exhaust by injecting urea as reducing agent . A Diesel Oxidation Catalyst is also present in the system configuration, upstream of the SCR converter, to partially convert NO to NO 2 ; this permits the flow entering the SCR reactor to contain significant amounts of NO 2 in addition to NO, and thus the onset of the SCR deNOx reactions can occur at lower temperature, in comparison to the case where most of NOx is made of NO alone. During the last few years, both LNT and SCR catalytic processes have been widely investigated in our labs: this paper will report on the main results of these studies and in particular it will analyse the chemistry and the reaction pathways operating for the reduction of NOx in both the processes, in order to highlight the similarities/differences between them. A systematic study of the reduction by hydrogen under near isothermal conditions of NOx species adsorbed on model Pt-Ba/Al 2 O 3 LNT catalysts was performed: it was found that the reduction process is not initiated by the thermal decomposition of the stored NO x ad-species, but a catalytic pathway involving Pt is instead operating. It was also shown that such a catalytic pathway is composed of two consecutive steps in which NH 3 is formed as an intermediate upon reaction of nitrates with H 2, and further reacts with nitrates to produce selectively N 2 . The reactions involved in the NH 3 -NO/NO 2 SCR reacting system were investigated by an extensive set of various unsteady state experiments performed over both vanadium based and zeolite based commercial catalysts: the bulk of results led to the proposal of an original global mechanistic scheme of the different reactions that compose the complete NH 3 -NO/NO 2 SCR reacting system. In such a scheme, a key role is played by nitrate species, which are formed by NO 2 disproportion onto the catalyst surface and are then reduced by NO to nitrites. These species lead then to nitrogen upon reaction with ammonia . Accordingly, in both LNT and SCR chemistries, the reduction of NOx involves nitrite and nitrate surface species, which are selectively reduced to nitrogen by ammonia, either formed as intermediate or supplied as a reactant.
SCR has become the technology of choice for the abatement of NOx emissions from heavy-duty vehicl... more SCR has become the technology of choice for the abatement of NOx emissions from heavy-duty vehicles, and it has been recently introduced also in passenger cars. The current challenge for Diesel vehicles both in Europe and in the US markets is the achievement of a substantial reduction of CO 2 emissions while bringing NOx emissions below the upcoming stringent Euro VI and US Tier 2 Bin 5 limits 1 . As the new test cycles require catalysts active at very low temperature, the "Fast" SCR reaction used in present SCR-based after treatment converters to enhance the low-T DeNOx activity, plays and will play even more in the near future a critical role in the development and optimization of SCR systems 2 . Different metal exchanged (Fe and Cu) commercial zeolite based catalysts are investigated in this study under experimental conditions representative of real applications, to analyse each individual reaction step of the complete NO/NO 2 + NH 3 + O 2 SCR reacting system. The identification of the SCR kinetic mechanisms, and of the role of the related surface species, as well as the development of a detailed kinetic model will be herein presented.
Abstract an extensive investigation of the chemistry, the mechanism and the kinetics of the NO-NO... more Abstract an extensive investigation of the chemistry, the mechanism and the kinetics of the NO-NO2/NH3 SCR reactions for mobile applications was performed over a commercial Vanadium-based catalyst. On the basis of the collected results a Mars-Van Krevelen ...
Urea/NH 3 -SCR is presently considered a key technology for the control and abatement of NO emiss... more Urea/NH 3 -SCR is presently considered a key technology for the control and abatement of NO emissions from Diesel engines; the presence of a DOC catalyst upstream from the SCR unit warrants significant amounts of NO 2 entering the SCR reactors together with NO. In this work new zeolite-based catalysts promoted by Fe and by Cu were investigated: reactivity tests were performed to allow a quantitative comparison, whereas systematic mechanistic experiments were run to elucidate the major reaction pathways, with particular focus on the NO 2 /NH 3 reacting system and on formation of N 2 O. The bulk of information was then used to develop a dynamic kinetic model in close agreement with all the details of the SCR catalytic chemistry.
Transient methods, which consist in imposing perturbations to the reacting system (typically step... more Transient methods, which consist in imposing perturbations to the reacting system (typically step changes in the inlet reactant concentration) while analyzing the outlet transient response, are being more and more applied to study the kinetics and the mechanism of heterogeneous catalytic processes. In the past such techniques (Transient Response Method, TRM) were extensively applied in our labs to the study of the Selective Catalytic Reduction (SCR) of NO by NH 3 . In this paper we present the results obtained by the kinetic modeling of the TRM applied to the study of the storage of NOx under lean conditions over a model Pt-Ba/Al 2 O 3 catalyst sample. These catalytic systems, known as "NO x storage-reduction catalysts", were recently proposed to reduce NO x in "lean burn" engines [3] according to a cyclic process during which NO x are adsorbed under excess oxygen by a NO x -storage component (e.g. an alkaline metal oxide), and are then reduced during a short "rich" period to N 2 and H 2 O by a noble metal (e.g. Pt). Although these catalysts are currently used, even if only at a demonstrative scale, a lot of aspects concerning their mechanistic and kinetic features are not completely clarified yet. In this paper we present the results of a study aimed at investigating the NO x storage function and the related kinetics over a model Pt-Ba/Al 2 O 3 catalyst sample. NO step changes in the reactor inlet concentration were imposed, while analyzing the outlet transient response. In parallel, a kinetic model recently reported in the literature has been adapted to our catalytic system and to our experimental conditions: its simulations are herein compared to our experimental results.
Thermal sintering of SCR-DeNO x catalysts was studied by calcining at increasing temperature samp... more Thermal sintering of SCR-DeNO x catalysts was studied by calcining at increasing temperature samples of a commercial catalyst and testing them under operating conditions of industrial relevance. Results were analyzed by means of a complete model of the SCR ...
The Selective Catalytic Reduction by NH 3 /urea is a world-wide established after treatment techn... more The Selective Catalytic Reduction by NH 3 /urea is a world-wide established after treatment technology for the removal of NO x in the stack gases of power plants and of other stationary sources. Typical SCR catalysts operate between 300 and 400 • C, however for some applications SCR systems with reasonable DeNO x activity at temperatures as low as 180 • C are required nowadays.It was recently shown that over both V 2 O 5 -WO 3 /TiO 2 and Fe-ZSM-5 commercial SCR catalysts the addition of aqueous solutions of NH 4 NO 3 to a NO-NH 3 containing feed stream results in the occurrence of the "Enhanced SCR" (E-SCR) reaction (2NH 3 + 2NO + NH 4 NO 3 → 3N 2 + 5H 2 O), characterized by superior NO reduction efficiencies in the 200-350 • C range. In this paper we present a systematic experimental study of the E-SCR reaction over a commercial V-based Haldor Topsøe catalyst. The effects of three operating variables, namely space velocity (18-75 k h −1 ), temperature (180-250 • C) and ammonium nitrate feed content (0-100% of stoichiometric feed), are addressed in order to identify the best process conditions in view of the specific application of this new technology to stationary SCR installations.
In this paper the results obtained in our previous studies on the reduction of nitrates by CO ove... more In this paper the results obtained in our previous studies on the reduction of nitrates by CO over PtK/Al2O3 and PtBa/Al2O3 catalysts are compared and discussed in order to define the chemistry that leads to nitrogen evolution during the regeneration of the different lean ...
... ZSM-5 catalyst. Maria Pia Ruggeri a , Antonio Grossale a , Isabella Nova a , Enrico Tronconi ... more ... ZSM-5 catalyst. Maria Pia Ruggeri a , Antonio Grossale a , Isabella Nova a , Enrico Tronconi a , Corresponding Author Contact Information , E-mail The Corresponding Author , Hana Jirglova b , Zdenek Sobalik b. a, Laboratory ...
We review in the present paper the key mechanistic features of the NH 3 -NO/NO 2 Fast SCR reactio... more We review in the present paper the key mechanistic features of the NH 3 -NO/NO 2 Fast SCR reaction over vanadium and Fe-zeolite commercial catalysts: this is the main reaction occurring in the urea-SCR technology, that is currently considered the best technique for the abatement of NOx in the exhausts of Diesel engines. A detailed investigation by means of dynamic reactive experiments led to the proposal of an original scheme which accounts for stoichiometry, selectivity and intrinsic kinetics of the global SCR process: NO 2 forms surface nitrites and nitrates via dimerization and disproportionation/heterolytic chemisorption, NO reduces nitrates to nitrites, and NH 3 enables the rapid and selective decomposition of nitrites to nitrogen via formation of unstable ammonium nitrite.
The selective catalytic reduction (SCR) technology, based on the reaction between nitrogen oxides... more The selective catalytic reduction (SCR) technology, based on the reaction between nitrogen oxides in the flue gases and NH 3 /urea, has become one of the most promising technological solutions for the abatement of NO x emissions from Diesel vehicles.1 First ...
In the present work we investigate the adsorption of NO 2 onto a Fe-and a Cu-promoted zeolite SCR... more In the present work we investigate the adsorption of NO 2 onto a Fe-and a Cu-promoted zeolite SCR catalyst in the absence of gaseous water, with a particular focus on the effect of the catalyst red-ox state. For both catalytic systems, results from the analysis of the gas phase during step changes of the NO 2 concentration, combined with literature FTIR information, emphasize an important role of the catalyst red-ox state in the dynamics of NO 2 adsorption: the molar ratio of released NO to adsorbed NO 2 is close to 1/3 in the case of oxidized catalysts, in line with a two-steps mechanism leading to the formation of surface nitrates, but is greater when the catalyst is partially reduced. Furthermore the initial catalyst red-ox state influences the amount of nitrates that can be adsorbed on the catalyst surface, the prereduced sample exhibiting a greater storage capacity. The Cu-zeolite catalysts are able to store a greater amount of nitrates and are more easily reduced in comparison with the Fe-zeolite.
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Papers by Isabella Nova