Ammonia SCR Research Articles

Synthesis of Pd0.01FexCe(1−x)/2Zr(1−x)/2Oy catalysts and their catalytic performance for ammonia production by passive SCR reaction

The passive ammonia SCR system has been considered as a novel after-treatment method to reduce the emission of NOx and produce ammonia for downstream SCR.

High Surface Area VOx/TiO2/SBA-15 Model Catalysts for Ammonia SCR Prepared by Atomic Layer Deposition

The mode of operation of titania-supported vanadia (VOₓ) catalysts for NOx abatement using ammonia selective catalytic reduction (NH₃-SCR) is still vigorously debated. We introduce a new high surface area VOₓ/TiO₂/SBA-15 model catalyst system based on mesoporous silica SBA-15 making use of atomic layer deposition (ALD) for controlled synthesis of titania and vanadia multilayers. The bulk and surface structure is characterized by X-ray diffraction (XRD), UV-vis and Raman spectroscopy, as well as X-ray photoelectron spectroscopy (XPS), revealing the presence of dispersed surface VOx species on amorphous TiO₂ domains on SBA-15, forming hybrid Si–O–V and Ti–O–V linkages. Temperature-dependent analysis of the ammonia SCR catalytic activity reveals NOx conversion levels of up to ~60%. In situ and operando diffuse reflection IR Fourier transform (DRIFT) spectroscopy shows N–Hstretching modes, representing adsorbed ammonia and -NH₂ and -NH intermediate structures on Bronsted and Lewis acid sites. Partial Lewis acid sites with adjacent redox sites are proposed as the active sites and desorption of product molecules as the rate-determining step at low temperature. The high NOₓ conversion is attributed to the presence of highly dispersed VOₓ species and the moderate acidity of VOₓ supported on TiO₂/SBA-15.

High Surface Area VOx/TiO2/SBA-15 Model Catalysts for Ammonia SCR Prepared by Atomic Layer Deposition

The mode of operation of titania-supported vanadia (VOx) catalysts for NOx abatement using ammonia selective catalytic reduction (NH3-SCR) is still vigorously debated. We introduce a new high surface area VOx/TiO2/SBA-15 model catalyst system based on mesoporous silica SBA-15 making use of atomic layer deposition (ALD) for controlled synthesis of titania and vanadia multilayers. The bulk and surface structure is characterized by X-ray diffraction (XRD), UV-vis and Raman spectroscopy, as well as X-ray photoelectron spectroscopy (XPS), revealing the presence of dispersed surface VOx species on amorphous TiO2 domains on SBA-15, forming hybrid Si–O–V and Ti–O–V linkages. Temperature-dependent analysis of the ammonia SCR catalytic activity reveals NOx conversion levels of up to ~60%. In situ and operando diffuse reflection IR Fourier transform (DRIFT) spectroscopy shows N–Hstretching modes, representing adsorbed ammonia and -NH2 and -NH intermediate structures on Bronsted and Lewis acid sites. Partial Lewis acid sites with adjacent redox sites are proposed as the active sites and desorption of product molecules as the rate-determining step at low temperature. The high NOx conversion is attributed to the presence of highly dispersed VOx species and the moderate acidity of VOx supported on TiO2/SBA-15.

Comparison of Vehicle Aged SCR Catalyst on a Particulate Filter (SCRF) with Oven Aged Equivalent

To meet forthcoming Euro 6 Diesel engines NOx emissions legislation; highly efficient after-treatment systems are required. The use of urea or ammonia SCR is a well-established solution for high NOx abatement for diesel engines. The combination of a Cu-Zeolite SCR catalyst and a diesel particulate filter has been widely investigated in recent years to respond to the European NOx emissions regulations especially for those on light-duty vehicles. The after-treatment systems are preferred to be durable during the entirety of vehicle use. Indeed, the SCRF system must withstand the temperature resulting from the active regeneration of the particulate filter continuously. A 160,000 km vehicle aged SCRF catalyst which has endured more than 91 hours at temperatures superior to 600°C during the regeneration of the particulate filter was compared to an SCRF catalyst aged in the laboratory oven for 16 hours at 800°C with 10% H2O. The vehicle aged SCR has not only been exposed to the hydrothermal ageing generated by the diesel particulate regeneration but also has seen chemical elements from urea decomposition, soot accumulation, engine oil and fuel residues. In this paper, deeper comprehension of the behaviour the SCRF catalyst during vehicle ageing was exposed. It is evident that the loss of NOx activity is not only due to the regeneration of the particulate filter but also to the possible contamination from the urea injection and the elements present in the diesel fuel and engine oil. XRF results demonstrate that the concentrations of poisoning elements coming from the engine oil or fuel are concentrated mainly at the outlet of vehicle aged SCRF catalyst. Also, results reveal that the poisoning elements have affected the SCR activity of the vehicle aged SCRF catalyst. Finally, it was possible to establish a correlation between a 160,000 km vehicle aged SCRF catalyst and an equivalent SCRF aged for 16 hours at 800°C.

NH3-TPD methodology for quantifying hydrothermal aging of Cu/SSZ-13 SCR catalysts

We identified a new acidic feature in Cu/SSZ-13 catalysts, related to the presence of two distinct desorption peaks in the NH3-TPD profiles, and to the changes in their relative intensity in response to mild hydrothermal aging (Luo et al., 2017). In this work, we have translated those findings into a sensitive laboratory diagnostic methodology for quantifying cumulative aging exposure, based on the ratio of the two NH3-TPD peaks. We have also addressed some practical aspects of applying this methodology to real-world aged samples, by decoupling the effects of sulfur poisoning and of the catalyst loading variability. The developed method yielded a simple kinetic model of the Cu/SSZ-13 catalysts hydrothermal aging, and some useful insights into the practically relevant aspects of the aging process. In particular, we have shown that the aging is cumulative, and that the sequence of the exposure steps does not matter. One other important consequence of the developed model is related to the steep temperature dependence of the aging process (Ea ≈ 168 kJ/mol). As a result, the majority of the on-engine operation exposure, typically below ∼400 °C, has no appreciable impact on the catalyst aging. In contrast, such aging is almost entirely induced by the periodic high-temperature events, such as those related to various system regeneration regimes. We have verified that observation by a 1000-h aging test at 400 °C.Finally, we expect that the developed method of diagnosing and quantifying cumulative hydrothermal age can be applicable to other types of catalysts which include Cu/SSZ-13 in their formulation, including ammonia oxidation catalysts (AMOX) and SCR on filter (SCRF), since it is based on the intrinsic properties of this metal-exchanged zeolite.

Hierarchical Vanadia Model Catalysts for Ammonia Selective Catalytic Reduction

There is an ongoing debate on the structure of titania supported vanadia used for catalytic partial oxidation reactions as well as for abatement of nitrogen oxide (NOx) by ammonia selective catalytic reduction (NH3-SCR). To gain further insight into the essential features of the catalyst composition of the surface we introduce a new class of vanadia model catalysts based on monodispersed silica particles as a platform. The evaluation of the influence of different catalyst components (TiO2, WO3) and different vanadia synthesis approaches (incipient wetness, ion-exchange) is facilitated by the hierarchical structure of the catalysts as well as the controlled synthesis approach. Catalysts tested under industrial ammonia SCR conditions show a strong dependence on the catalyst composition. In particular, the presence of crystalline titania and tungsta leads to a strong increase in NO conversion. Detailed catalyst characterization by X-ray diffraction and by visible Raman, UV Raman, UV–Vis and X-ray photoelectron spectroscopies provides insight into the bulk structure, the surface composition as well as the vanadia surface structure of the catalysts. The potential of UV resonance Raman spectroscopy for structural analysis of vanadia model catalysts for SCR is highlighted. Differences in catalytic activity are discussed in light of the structural results and the available literature.

Radio-Frequency-Based Urea Dosing Control for Diesel Engines with Ammonia SCR Catalysts

Radio-Frequency-Based Urea Dosing Control for Diesel Engines with Ammonia SCR Catalysts

Aging of Commercial Diesel Oxidation Catalysts: A preliminary Structure/Reactivity Study

This study focuses on the catalytic behavior of the diesel oxidation catalyst using different aging characteristics of road mileage in order to improve the efficiency of an ammonia SCR system on an after-treatment line composed by a DOC + DPF + SCR. The studied catalyst is a commercial diesel oxidation catalyst (Pt/Pd/Al2O3) provided by Continental. Hydrothermal aging were performed on carrots of monolith in varying the temperature of treatment. These carrots were characterized, X-ray diffraction, transmission electron microscopy coupled by EDS and specific surface measurement (SBET). The catalytic measurements consisted of series of temperature programmed surface reactions. To be close to real driving conditions, our experimental setup comprised a pre-heater in which the gases were heated up to 500 °C. The goal of this study is to understand the evolution of the active phase with the aging and to correlate these aging results with the corresponding overall catalytic activity and their impact on the reactions involved in the DOC with respect to CO, hydrocarbons and NO.

HERFD-XANES and XES as complementary operando tools for monitoring the structure of Cu-based zeolite catalysts during NOx-removal by ammonia SCR

In this article, we demonstrate the potential of hard X-ray techniques to characterize catalysts under working conditions. Operando high energy resolution fluorescence detected (HERFD) XANES and valence to core (vtc) X-ray emission spectroscopy (XES) have been used in a spatially-resolved manner to study Cu-zeolite catalysts during the standard-SCR reaction and related model conditions. The results show a gradient in Cu oxidation state and coordination along the catalyst bed as the reactants are consumed. Vtc-XES gives complementary information on the direct adsorption of ammonia at the Cu sites. The structural information on the catalyst shows the suitability of X-ray techniques to understand catalytic reactions and to facilitate catalyst optimization.

Comparison of Cu/BEA, Cu/SSZ-13 and Cu/SAPO-34 for ammonia-SCR reactions

In this study, the ammonia-SCR process was investigated using 2.5wt.% Cu/BEA, 2.6wt.% Cu/SAPO-34 and 3.1wt.% Cu/SSZ-13. Several reactions such as NO oxidation, ammonia oxidation, standard SCR, fast SCR and NO2 SCR were studied to understand the effect of zeolite type. It was found that the small-pore zeolites/silicoaluminophosphates with CHA structure (Cu/SAPO-34 and Cu/SSZ-13) exhibited higher SCR activity at 150°C and lower selectivity toward N2O formation during standard SCR conditions than Cu/BEA. However, formation of ammonium nitrate species during fast SCR conditions at 150°C occurred over Cu/CHA catalysts, which resulted in a gradual decrease of the NOx conversion. Such blocking was also observed over Cu/BEA, albeit to a minor extent. The ammonium nitrate formation and its decomposition temperature regimes resulted in that the Cu/BEA was catalytically more active at lower temperature than either Cu/SAPO-34 or Cu/SSZ-13 during fast SCR conditions. Additionally, our results show that the ammonium nitrate species were more stable on the small-pore zeolites than on Cu/BEA. Comparing the two Cu/CHA catalysts, Cu/SAPO-34 and Cu/SSZ-13, it was found that ammonia oxidation at high temperatures and ammonia SCR at 150°C was higher on Cu/SAPO-34. Further, TPR experiments showed that Cu in Cu/SAPO-34 is more easily reduced compared to Cu/SSZ-13. This can facilitate the redox processes and can thereby be a reason for the higher activity at 150°C for Cu/SAPO-34. In addition, Cu/SAPO-34 forms less N2O and this might be a result of the formation of more stable ammonium nitrates. To conclude, the choice of the type of zeolite/silicoaluminophosphates affects the activity and selectivity of the different steps in the SCR process.

Preparation, characterization and NH3–SCR activity of FeVO4 supported on TiO2–WO3–SiO2

A series of FeVO4 catalysts supported on TiO2–WO3–SiO2 based oxides were prepared and studied in the ammonia SCR reaction. Their characteristics have been studied by X-ray powder diffraction, X-ray fluorescence, surface area measurements, Mössbauer spectroscopy and temperature programmed methods. It is shown that homogeneity of starting FeVO4 powder is one of the parameters that governs the activity of the final catalytic material. Stoichiometric or Fe/V rich compositions can be obtained by varying the pH of precipitation; this affects interaction with the support and thermal stability, which in turn modifies SCR activity. Homogeneous FeVO4 shows the highest activity and thermal stability which might be correlated with the characteristics of Fe3+OV5+ bond and the acidity of the VO moiety. Both vanadium and iron excess promote phase evolution of the support with formation of rutile and consequent surface area loss at much lower temperature.

Mixed iron–erbium vanadate NH3-SCR catalysts

A series of SCR catalysts of mixed iron–erbium vanadates supported on TiO2–WO3–SiO2 were prepared and their reduction, textural, structural and morphological properties characterized by temperature programmed reduction, X-ray powder diffraction, B.E.T. methods and transmission electron microscopy. The influence of Fe/Er ratio in the vanadate as well as the effect of aging conditions on their activity in ammonia SCR reaction was investigated. SCR activity at medium/low temperature was found to correlate directly with the Fe loading of the catalyst, with supported FeVO4 resulting the most active material. This behavior might be correlated with the characteristics of Fe3+OV5+ bond and the acidity of the VO moiety. Supported FeVO4 suffers a strong deactivation following thermal aging due to rutile formation in TiO2-based support and consequent collapse of surface area. The effect of Er is that of blocking transformation to rutile, thus enhancing activity after thermal treatment. Mixed composition FexEr1−xVO4 with x=0.5 represents the best compromise between activity and stability and are potential candidates for deNOx process when a higher stability is requested.

Reaction pathways and mechanism for ammonia formation and removal over palladium-based three-way catalysts: Multiple roles of CO

In-situ ammonia generation over a three-way catalyst is an important part of the “passive ammonia SCR” technology being developed at GM Global R&D as a low-cost alternative to the current lean NOx emission control technologies. The purpose of this laboratory reactor study is to clarify the reaction pathways and to identify/quantify the key controlling factors for the formation and destruction of ammonia over Pd-based three-way catalysts under realistic feedstream conditions simulating the exhaust from a lean gasoline engine operating at fuel-rich air–fuel ratios. In addition to the “full-feed” experiments, we also conducted some “simple-feed” experiments to better understand the NH3 formation process (via the NO+H2 reaction with and without CO) as well as the possible removal process of the NH3 formed (via the NH3+O2 reaction or NH3 decomposition). Of particular interest in this study are the multiple roles that CO plays in both the formation and destruction of NH3. It is also found that the presence of Rh and/or Ce in a Pd-based three-way catalyst decreases the amount of NH3 formation significantly under fuel-rich (net-reducing) conditions of practical interest.

Highly dispersed iron species created on alkali-treated zeolite for ammonia SCR

Alkaline treatment using sodium hydroxide was introduced to obtain a hierarchical pore structure in H–ZSM-5 zeolite. Fe-exchanged zeolite catalysts were prepared by impregnation on the original and alkali-treated zeolites, and were evaluated for NOx reduction by NH3, NO oxidation, and NH3 oxidation reactions. The highly dispersed iron species as active sites can be obtained by controlling the pore structure and particle size of zeolite. Therefore, the Fe/ZSM-5 catalyst treated mildly by sodium hydroxide before iron exchange, which contains amounts of highly dispersed Fe species, obtains over 80% NOx conversion at a wide temperature range of 250–500°C.

Hysteresis effect study on diesel oxidation catalyst for a better efficiency of SCR systems

This study focuses on the behaviour of the diesel oxidation catalyst in order to improve the efficiency of an ammonia SCR system on an after-treatment line composed by a DOC+DPF+SCR. The studied catalyst is a commercial diesel oxidation catalyst (Pt/Pd/Al2O3) provided by Renault. For this study, the catalytic phase was extracted from a monolith and different ageings in temperature were performed. The catalysts were characterized by elementary analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM) coupled by EDS and X-ray photoelectrons spectroscopy (XPS). The specific surface area was also calculated using a specific apparatus. Main physicochemical properties and elementary analysis of the catalysts were performed in order to determine the noble metal contents. The catalytic measurements consisted on temperature programmed surface reaction (TPSR). The heating rate was chosen equal to 10°Cmin−1. The TPSR were used to determine light off temperature of each pollutant present in the exhaust gases. To be close to industrial conditions, our experimental setup is composed by a heater where the gases are heated up to 500°C. The goal of this study is to study the hysteresis effect with different gas composition and with different catalysts state (ageing, sample/powder).

Improved high temperature stability of NH3-SCR catalysts based on rare earth vanadates supported on TiO2[sbnd]WO3[sbnd]SiO2

The ammonia SCR activity of a series of rare earth vanadates supported over TiO2WO3SiO2 was investigated after thermal treatment in the temperature range 700–750°C and their properties studied by XRD, TPR, BET and HRTEM analysis. It was found that the thermal stability of the catalyst is remarkably higher than that observed in traditional vanadia-based materials, with the rare earth vanadate being able to suppress the unfavorable transformation of anatase to rutile and the consequent surface area drop. The deNOx activity after aging was higher than that observed over V2O5/TiO2WO3SiO2 and dependent on the nature of the rare earth element. A group of elements comprising Tb, Er, Dy, Sm and Gd showed excellent resistance to deactivation and high activity after aging while a second group with Ce, La, Nd and Pr was more sensitive to thermal treatment and less active. This was correlated to their position in the periodic table and to their ability to alter the characteristics of the VO bond and the acid/base surface properties.

Mechanistic investigation of hydrothermal aging of Cu-Beta for ammonia SCR

The selective catalytic reduction of NOx with NH3 over a Cu-BEA catalyst was studied after hydrothermal aging between 500 and 900°C. The corresponding catalyst was characterized using XPS and XRD techniques in the aging interval of 500, 700 and 800°C. No structural changes during the aging process were observed. However, the oxidation state of copper changed during aging and more Cu2+ was formed. We suggest that one of the deactivation mechanisms is the decrease of the Cu+ species. The NO oxidation and NH3 oxidation activity was decreased with increasing aging temperature. Further, we observed that the ammonia oxidation was decreased faster compared to the SCR reactions at low aging temperatures. The experiments from the calorimeter as well as from the ammonia TPD investigations indicate a trend towards more weakly bound ammonia with higher aging temperatures. From the results of the SCR experiments using different NO2/NOx ratios and ammonia oxidation experiments we suggest that most of the N2O is coming from side reactions of the SCR mechanism and not from reactions between NH3 and O2 alone. Interestingly, we observe that after the 900°C aging a quite large activity remained for the case with 75% NO2/NOx ratio. The N2O production shows a maximum at 200°C, but increases again at higher temperatures. However, the N2O formed at low temperature is decreased after hydrothermal aging while the high temperature N2O is increased. We propose that the different reactions examined in this work do not all occur on the same type of sites, since we observe different aging trends for some of the reactions.

The effect of zirconia additive on the activity and structure stability of V2O5/WO3-TiO2 ammonia SCR catalysts

The NH3-SCR activities of zirconia containing V2O5/WO3-TiO2 catalysts have been studied before and after aging treatment. On fresh catalysts, the addition of zirconia increases high temperature activity and N2 selectivity but decreases low temperature activity. Interestingly, zirconia prominently enhances the NH3-SCR activities of aged catalysts, with 10wt% ZrO2 containing catalyst exhibits the highest NO conversions and N2 selectivity in the temperature range of 150–550°C. The catalysts were characterized by XRD, SEM, XPS, DRIFTS and NH3-TPD. SCR kinetic studies were also conducted to understand the mechanistic features of the zirconia modified catalysts. The main results show that the addition of zirconia inhibits the shrinking of catalyst surface area and the growth of TiO2 crystallite size, and therefore improves the thermal stability of V2O5/WO3-TiO2 catalysts. These advantages of zirconia additive can essentially benefit the catalysts performance with high temperature durability.

An operando IR study of the unburnt HC effect on the activity of a commercial automotive catalyst for NH3-SCR

A commercial zeolite-based catalyst for NOx removal via ammonia SCR has been studied in a synthetic flow very close to a real automotive exhaust mixture. Particular attention has been paid to reproduce effective temperatures and space velocities. The impact of the hydrocarbon presence towards the NH3-SCR efficiency has been investigated through the use of a mixture of representative hydrocarbons, i.e. propene, toluene and decane. It has been found that both decane and toluene produce a carbonaceous deposit, partially hindering the reaction by pore blocking. However, it also appears that decane is the most inhibiting compound for the deNOx activity due to a competitive adsorption with ammonia on the active sites, notably at low temperature.

Investigation of the electrode effects in mixed potential type ammonia exhaust gas sensors

Mixed potential ammonia exhaust gas sensors provide a high capability for applications in harsh environments when appropriate means are conducted to stabilize the electrodes catalytic activity. The discussed sensor utilizes oxygen ion conducting yttria stabilized zirconia and gold electrodes, one of which is covered with an SCR active film to establish ammonia sensitivity and selectivity. The used SCR catalyst is well-known and developed especially for ammonia SCR exhaust gas aftertreatment systems. Electrochemical half-cell measurements vs. a Pt reference indicate that both electrode configurations are electro catalytically active for ammonia oxidation, but the activity is more pronounced at the additional SCR catalyst layer. The results of the half-cell tests including ammonia dependent voltage and oxygen interference agree very well with the performance of the ammonia sensor device. The behavior of the catalyst electrode cover is characterized separately by activity measurements of powders. The sensing mechanism combines electrochemical reactions at the three phase boundary and chemical reactions at the catalyst material.