NH3-SCR Process Research Articles

Mn/beta and Mn/ZSM-5 for the low-temperature selective catalytic reduction of NO with ammonia: Effect of manganese precursors

Two series of Mn/beta and Mn/ZSM-5 catalysts were prepared to study the influence of how different Mn precursors, introduced to the respective parent zeolites by wet impregnation, affected the selective catalytic reduction (SCR) of NO by NH3 across a low reaction temperature window of 50–350 °C. In this study, the catalysts were characterized using N2 adsorption/desorption, X-ray diffraction, X-ray fluorescence, H2 temperature-programmed reduction, NH3 temperature-programmed desorption and X-ray photoelectron spectroscopy. As the manganese chloride precursor only partially decomposed this primarily resulted in the formation of MnCl2 in addition to the presence of low levels of crystalline Mn3O4, which resulted in poor catalytic performance. However, the manganese nitrate precursor formed crystalline MnO2 as the major phase in addition to a minor presence of unconverted Mn-nitrate. Furthermore, manganese acetate resulted principally in a mixture of amorphous Mn2O3 and MnO2, and crystalline Mn3O4. From all the catalysts screened, the test performance data showed Mn/beta-Ac to exhibit the highest NO conversion (97.5%) at 240 °C, which remained >90% across a temperature window of 220–350 °C. The excellent catalytic performance was ascribed to the enrichment of highly dispersed MnOx (Mn2O3 and MnO2) species that act as the active phase in the NH3-SCR process. Furthermore, together with a suitable amount of weakly acidic centers, higher concentration of surface manganese and a greater presence of surface labile oxygen groups, SCR performance was collectively enhanced at low temperature.

Analysis of Mn valence in the Mn based catalyst for NH3-SCR process

A series of MnWOx/TiO2 catalysts were prepared by liquid phase deposition. The MnWOx/TiO2 catalyst was characterized by N2 physical adsorption, X-ray diffraction, H2 programmed temperature reduction, transmission electron microscopy and X-ray electron spectroscopy, and their NH3-SCR performance were tested. The effect of Mn valence on the NH3-SCR performance of MnWOx/TiO2 catalyst was analyzed and discussed. The results show that the active components are uniformly dispersed on the surface, and the average valence of manganese are different. The average valence state of manganese in Mn3WOx/TiO2 catalyst is the highest and the activity at low temperature is the best. On the contrary, the average valence of manganese in MnWOx/TiO2 catalyst is the lowest while the N2 selectivity is the best, which means that high valence of manganese is beneficial to its low temperature activity while low valence is favorable for its N2 selectivity.

Synthesis of CuO catalyst derived from HKUST-1 temple for the low-temperature NH3-SCR process

CuO catalyst was prepared from HKUST-1 temple through a pyrolysis process, noted as CuO-p. Its morphology with octahedral shape was verified to be inherited from the HKUST-1 and however essentially difference with CuO-c prepared by coprecipitation method. Moreover, CuO-p (15.4m2/g) obtained by HKUST-1 pyrolysis has a bigger surface area than that (CuO-c, 3.4m2/g) obtained by coprecipitation. The CuO samples and HKUST-1 were tested for low-temperature selective catalytic reduction (SCR) of NO with NH3 as reducing agent. The CuO-p catalyst derived from HKUST-1 exhibited the superior deNOx activity with respect to CuO-c. The further calcination at 600°C under an N2 atmosphere is beneficial for improving the specific surface area and achieving an ideal SCR performance (TOF of 3.45×10−5s−1 and ∼100% selectivity towards N2 at 270°C). This work expands the potential applications of MOFs as catalysts.

Fe–Mn Mixed Oxide Catalysts Synthesized by One-Step Urea-Precipitation Method for the Selective Catalytic Reduction of NO x with NH3 at Low Temperatures

A range of Fe–Mn mixed oxide catalysts, FeαMn1−αO x (α = 1, 0.25, 0.33, 0.50, 0 mol%) were prepared via one-step urea-precipitation method and applied to the selective catalytic reduction (SCR) of nitric oxide (NO x ) with NH3. The Fe0.33Mn0.66O x catalyst showed the highest activity in the NH3-SCR process within a broad operation temperature range (75–225 °C) with 90% NO x conversion. Series of characterization have been taken to investigate the physical and chemical properties of the catalysts. The BET results evidenced that the doping of iron species increased the surface area of the catalyst effectively, and this structure could provide more acid sites and active sites on the surface of catalysts for SCR reaction. X-ray powder diffraction results and Raman spectroscopy indicate that the active Mn and Fe species were in poorly crystalline or amorphous states which could increase lattice defects and oxygen vacancies on the surface of Fe0.33Mn0.66O x catalyst. The X-ray photoelectron spectra results suggested that more surface-adsorbed oxygen (OA), Fe3+, Mn4+ species existed on the surface of the Fe0.33Mn0.66O x catalyst compared with that of MnO x and FeO x catalysts, which is favorable to the NH3-SCR performance. Analysis by in situ Fourier transform infrared spectroscopy (FTIR) suggested that Fe-doping can enhance the absorption and the activation ability of NO which could promote the catalytic performance in the SCR process. Catalyst characterization Powder X-ray diffraction (XRD) was carried out on a PAN alytical powder X-ray diffractometer (Model EMPYREAN) with a monochromatic Cu Kα1 radiation (λ = 0.154056 nm) within a 2θ range of 10°–90° in the step of 0.02° at room temperature. The Brunauer–Emmett–Teller (BET) surface areas of the samples were computed by physical adsorption of N2 at − 196 °C using a NOVA 1200 (Quanta Chrome). The samples were pretreated at 300 °C for 5 h in a vacuum state previous to BET measurement. The high-resolution transmission electron microscopy (HR-TEM) determinations were conducted on a JEOL-2100 microscope. The Raman spectrums of samples were computed by a confocal Raman microscope (Labram DILOR) equipped with an Olympus BX-41 microscope (objective ×100) and TEcooled CCD detector (Andor), in a back-scattering configuration using an He–He laser (532 nm excitation line) with power of 510 mW at a sample and spectral resolution of 0.8 cm−1. X-ray photoelectron spectra (XPS) over the samples were by captured Al-Kα radiation (1486.7 eV). Binding energies of Mn 2p and O 1s were calibrated using C 1s (BE = 285.0 eV) as a standard. The in situ FTIR experiments of NO adsorption over MnO x and Fe0.33Mn0.66O x catalysts were performed on a VERTEX70-FTIR. Prior to NO adsorption, the samples (approximately 20 mg each) was pretreated at 250 °C in the He atmosphere for 1 h. The spectrum of NO adsorption at different temperatures were selected over each catalyst. The reaction condition was controlled as follows: 500 ppm NO and He as balance, and the flow rate was 100 mL/min.

Remarkable enhancement of the selective catalytic reduction of NO at low temperature by collaborative effect of ethanol and NH3 over silver supported catalyst

The NOx selective catalytic reduction (SCR) is extensively studied as an effective process for air pollutants abatement from lean burn and Diesel vehicles. In the implemented Urea-SCR technology, the NO2/NOx ratio is a key parameter that limits the deNOx efficiency at low temperature (175–250°C). We demonstrate that co-feeding of ammonia and ethanol on a Ag/Al2O3 catalyst enables a drastic enhancement of the NOx conversion at temperatures below 200°C using only NO as NOx (standard SCR condition). Even if NO2 is provided at low temperature by the NO oxidation over Ag/Al2O3 in presence of EtOH, the NOx conversion improvement is not only due to a direct reaction between NH3 and NOx, but mainly attributed to the availability of hydrogen H* species resulting from EtOH oxidation (similar to a H2 assisted NH3-SCR process). Due to the presence of remaining NH3 and NO2 (formed over Ag/Al2O3 catalyst), further deNOx efficiency improvement was obtained at low temperature by addition of a NH3-SCR catalyst (WO3/Ce-Zr). The critical dependence of the SCR process on the Diesel Oxidation Catalyst (DOC) efficiency at low temperature is thus avoided.

Relationship between the WO3 states and reaction pathway over CeO2-ZrO2-WO3 catalysts for selective catalytic reduction of NO with NH3

The CeO2-ZrO2-WO3 catalysts were investigated for the selective catalytic reduction of NOx by NH3 (NH3-SCR). The results indicated that monodentate nitrate species generated from amorphous WO3, and these species could not be observed over CeO2-ZrO2-WO3 with the presence of Ce2(WO4)3 compounds. The formation of amorphous W species resulted in the abundant of Ce3+ and oxygen vacancies. Besides, the amorphous W species could enhance the surface acidity and NO oxidation ability. Nitrate species (bridging nitrate and monodentate nitrate), coordinated NH3 and NH4+ all participated in the NH3-SCR process. The NH3-SCR reaction pathway of CeO2-ZrO2-WO3 mainly followed Langmuir-Hinshelwood mechanism.

Inhibition effect of HBr over a commercial V2O5-WO3/TiO2 catalyst in a NH3-SCR process

Although HBr promotes Hg0 oxidation over the V2O5-WO3/TiO2 catalyst, it possibly influences the catalyst DeNOx activity. NH3-SCR experiments were conducted over a commercial V2O5-WO3/TiO2 catalyst in the presence of HBr. NO conversion obviously decreases in the low temperature range (i.e., <300°C). XPS analysis suggests that HBr reacted with the surface vanadium species, increasing the amount of Br species and V4+ species. In-situ FTIR study reveals that HBr adsorbed on the catalyst surface and subsequently reacted with V=O species which served as active sites in the SCR reaction, thus resulting in the inhibition effect for the DeNOx activity.

Manganese-rich MnSAPO-34 molecular sieves as an efficient catalyst for the selective catalytic reduction of NO x with NH3: one-pot synthesis, catalytic performance, and characterization.

Manganese-rich MnSAPO-34 molecular sieves were prepared by one-pot synthesis method for NO x abatement using the ammonia-selective catalytic reduction (NH3-SCR) technology and characterized using ICP, BET, XRD, FE-SEM, H2-TPR, NH3-TPD, XPS, and DR UV-Vis analyses. The experimental results indicate that the Mn content and chemical state, as well as the surface acidity, of the MnSAPO-34 molecular sieves significantly enhance their DeNO x efficiency at low temperatures (ca. 200-300°C). The manganese-rich MnSAPO-34 was synthesized using a combination of triethylamine and diisopropylamine as the structural directing agents and high Mn loading (n(MnO)/n(P2O5)=0.4). The resulting catalyst exhibits the highest activity among all of the samples with a NO x conversion value of nearly 95% and a N2 selectivity that is higher than 90% at 220-400°C. In addition, this catalyst presents higher NO x conversion than the conventional V2O5-WO3/TiO2 catalysts and other SAPO-based catalysts below 300°C. Furthermore, the analytical results indicate that the manganese species in the catalyst are mainly in the form of a framework Mn(IV), which could play a significant role in the NH3-SCR process as the specific active species. The results suggest that controlling the types and content of the organic amine templates and variations in the surface acidity of the catalysts may significantly enhance the SCR activity at lower temperatures.

Activating effect of cerium in hydrotalcite derived Cu\u2013Mg\u2013Al catalysts for selective ammonia oxidation and the selective reduction of NO with ammonia

Hydrotalcite originated mixed metal Cu–Mg–Al oxide system was doped with various amounts of cerium (0.5 or 3.0 wt%) and tested in the role of catalysts for the selective catalytic oxidation of ammonia to dinitrogen (NH3-SCO) and the selective catalytic reduction of NO with ammonia (NH3-SCR). The activating effect of cerium was observed in both studied processes. However, the CeO2 loading is a very important parameter determining catalytic performance of the studied samples. It was shown that an introduction of cerium into Cu–Mg–Al mixed oxide resulted in its significant activation in the low-temperature NH3-SCR process, independently of the CeO2 loading and a decrease in the efficiency of the NO reduction at higher temperatures, which was more significant for the catalyst with the lower cerium content. In the case of the NH3-SCO process, the introduction of cerium into Cu–Mg–Al mixed oxide resulted in the activation of the low temperature reaction, which was more intensive for the catalyst with lower cerium content. These effects were related to the presence of cerium in the form of crystallites of various size and therefore their different reducibility.

Synthesis of Bimetallic MOFs MIL-100(Fe-Mn) as an Efficient Catalyst for Selective Catalytic Reduction of NO x with NH3

In this study, we have successfully synthesized bimetallic MOFs of MIL-100(Fe-Mn) for SCR of NOx with NH3 via a hydrothermal method The catalyst were characterized by XRD, EDX, N2 adsorption(BET), SEM, in situ FT-IR and XPS. MIL-100(Fe-Mn) catalyst exhibited higher NOx conversion than either of its monometallic counterparts of MIL-100(Fe) and MIL-100(Mn). MIL-100(Fe-Mn) displayed excellent low-temperature activity, which achieved a maximum NOx conversion of 96 % at 260 °C. Moreover, MIL-100(Fe-Mn) showed a satisfactory stability and a good capacity of resisting SO2 and H2O. During NH3-SCR process, a slightly raised NO x conversion (approximately 7 %) was observed in the presence of H2O and SO2, which could be attributed to the formed sulfate species that act as new acid sites on the surface of the catalyst. Combined with the in situ FT-IR results, it was concluded that the SCR reaction of the MIL-100(Fe-Mn) followed Eley–Rideal (E–R) mechanism. Schematic diagram of SCR reaction

Temperature effect on the morphology and catalytic performance of Co-MOF-74 in low-temperature NH3-SCR process

Co-MOF-74 materials were synthesized at different temperatures using solvothermal method. The effect of the synthetic temperature on the structure and low temperature NH3-SCR catalytic performance of Co-MOF-74 was systematically investigated. Co-MOF-74 synthesized at 100°C has flower-like morphology, while hexagonal prism structures could be obtained at other temperatures. Co-MOF-74 with flower-like morphology has a larger specific surface area and pore volume compared with the hexagonal prism ones. The highest catalytic activity was obtained at 200°C under the optimal synthesis temperature, which was attributed to the combination of special morphology, the highest specific surface area and Co content.

Study on the mechanism of NH3-selective catalytic reduction over CuCexZr1–x/TiO2

Copper-cerium-zirconium catalysts loaded on TiO2 prepared by a wet impregnation method were investigated for NH3-selective catalytic reduction (SCR) of NOx. The reaction mechanism was proposed on the basis of results from in situ diffuse reflectance infrared transform spectroscopy (DRIFT). When NH3 is introduced, ammonia bonded to Lewis acid sites is more stable over CuCe0.25Zr0.75/TiO2 at high temperature, while Bronsted acid sites are more important than Lewis acid sites at low temperature. For the NH3+NO+O2 co-adsorption, NH3 species occupy most of activity sites on CuCe0.25Zr0.75/TiO2 catalyst, and mainly exist in the forms of NH4 + (at low temperature) and NH3 coordinated (at high temperature), playing a crucial role in the NH3-SCR process. Two different reaction routes, the L-H mechanism at low temperature ( 200°C), are presented for the SCR reaction over CuCe0.25Zr0.75/TiO2 catalyst.

Density function theoretical and experimental study of NH3 + NOx adsorptions on MnOx/TiO2 surface

The adsorption of NO and NH3 gaseous molecules on Mn3O4/TiO2 catalysts is studied based on density function theory and the corresponding experiments for low-temperature SCR of NOx by ammonia. The SCR performances of the Mn3O4/TiO2 catalysts prepared by joint precipitation method were systematically carried out in the temperature range of 90–300°C. The results show that nitrogen oxide can first adsorb on the manganese-terminal to form nitrates species, and then NH3 are more inclined to adsorb on Brønsted acid sites. Bridging nitrate and bidentate nitrate can be turned into monodentate nitrates, and NH4+ was formed when there is ammonia and water molecules because the adsorption and dissociation of the water on the surface of the catalyst is the main source of hydroxyl groups. It is indicated that L-H mechanism plays a major role in NH3-SCR process.

Use of a µ-Scale Synthetic Gas Bench for Direct Comparison of Urea-SCR and NH3-SCR Reactions over an Oxide Based Powdered Catalyst

The selective catalytic reduction (SCR) of NOx by NH3 has been extensively studied in the literature, mainly because of its high potential to remediate the pollution of diesel exhaust gases. The implementation of the NH3-SCR process into passenger cars requires the use of an ammonia precursor, provided by a urea aqueous solution in the conventional process. Although the thermal decomposition and hydrolysis mechanisms of urea are well documented in the literature, the influence of the direct use of urea on the NOx reduction over SCR catalysts may be problematic. With the aim to evaluate prototype powdered catalysts, a specific synthetic gas bench adjusted to powdered material was developed, allowing the use of NH3 or urea as reductant for direct comparison. The design of the experimental setup allows vaporization of liquid urea at 200 °C under 10 bar using an HPLC pump and a micro injector of 50 μm diameter. This work presents the experimental setup of the catalytic test and some remarkable catalytic results towards further development of new catalytic formulations specifically dedicated to urea-SCR. Indeed, a possible divergence in terms of DeNOx efficiency is evidenced depending on the nature of the reductant, NH3 or urea solution. Particularly, the evaluated catalyst may not allow an optimal NOx conversion because of a lack in ammonia availability when the urea residence time is shortened. This is attributed to insufficient activity of isocyanic acid (HNCO) hydrolysis, which can be improved by addition upstream of an active solid for the hydrolysis reaction such as ZrO2. Thus, this µ-scale synthetic gas bench adjusted to powdered materials enables the specific behaviour of urea use for NOx reduction to be demonstrated.

Study of M-ZSM-5 nanocatalysts (M: Cu, Mn, Fe, Co …) for selective catalytic reduction of NO with NH3: Process optimization by Taguchi method

A series of different transition metals (V, Co, Cr, Mn, Fe, Ni, Cu and Zn) promoted H-ZSM-5 catalysts were prepared by impregnation method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The catalytic activity of these catalysts was evaluated for the selective catalytic reduction (SCR) of NO with NH3 as reductant in the presence of oxygen. The results revealed that the catalytic activity of Cu-ZSM-5 nanocatalyst for NO conversion to N2 was 80% at 300°C, which was the best among various promoted metals. Design of experiments (DOEs) with Taguchi method was employed to optimize NH3-SCR process parameters such as NH3/NO ratio, O2 concentration, and gas hourly space velocity (GHSV) over Cu-ZSM-5 nanocatalyst at 250 and 300°C. Results showed that the most important parameter in NH3-SCR of NO is O2 concentration; followed by NH3/NO ratio and GHSV has little importance. The NO conversion to N2 of 63.1% and 94.86% was observed at 250°C and 300°C, respectively under the obtained optimum conditions.

IR studies of Fe modified ZSM-5 zeolites of diverse mesopore topologies in the terms of their catalytic performance in NH3-SCR and NH3-SCO processes

The desilication and direct synthesis route techniques were used to prepare mesostructured ZSM-5 zeolites. Iron was introduced to purely microporous and to hierarchical analogs by two-fold ion-exchange procedure using a Fe(NO3)3 solution. The results of the catalytic studies of the NH3-SCR process showed that the Fe-exchanged ZSM-5 sample prepared by direct synthesis route with amphiphilic organosilanes as a mesopore-directing agents presented the highest catalytic activity in the low temperature range comparing to the other catalyst studied. Moreover, the Fe-catalysts revealed high catalytic performance in the NH3-SCO process. High catalytic activity of the studied samples was related to high concentration of mononuclear Fe3+ cations with pseudo-Td or Oh coordination, guaranteed by the enhanced mesopore area of zeolite support, as well as with high acidity of the zeolite itself. The facility of the transport of the reactants to and from the active surface sites seems to be also ensured by highly developed system of mesopores. IR studies of adsorption forms both of nitrogen monoxide and ammonia as well as the products of their conversion gave insight into the NH3-SCR and NH3-SCO reaction mechanisms.

Effects of mercury oxidation on V2O5–WO3/TiO2 catalyst properties in NH3-SCR process

The effects of mercury oxidation on V2O5–WO3/TiO2 SCR catalyst's physical and chemical properties have been investigated. Both fresh catalyst and mercury exposed catalyst have been examined by BET, XRD, XPS and catalytic activity measurements. Mercury oxidation promoted the V5+ species transforming to the V4+ species and consumed the lattice oxygen on the surface of catalyst. In addition, the NO conversion of mercury exposed catalyst decreased in the range of 200°C to 300°C. It suggested a competitive relationship between gaseous NH3 and adsorbed mercury on the catalyst surface in that temperature range.

Experimental Microkinetic Approach of De-NOx by NH3 on V2O5/WO3/TiO2 Catalysts. 1. Individual Heats of Adsorption of Adsorbed NH3 Species on a Sulfate-Free TiO2 Support Using Adsorption Isobars

The present study is dedicated to the development of experimental procedures allowing the measurement of the individual heats of adsorption of adsorbed NH3 species on a sulfate-free TiO2 solid (P25 from Degussa). This solid has been selected because it is frequently used as support of V2O5– or/and WO3–TiO2 model catalysts for the understanding of the surface processes implicated in the selective catalytic reduction of NOx by NH3 (NH3-SCR). Two original analytical procedures denoted adsorption equilibrium infrared spectroscopy (AEIR) and temperature-programmed adsorption equilibrium (TPAE) (developed in previous works) were applied. These methods are based on (a) the experimental measurement of the change in the adsorption equilibrium coverage of the individual adsorbed species in isobar conditions and (b) the comparison of the experimental data to an adsorption model. It is shown that in the ranges of the ammonia partial pressures and reaction temperatures of the NH3-SCR process, only two adsorbed NH3 species on Lewis sites (Ti+δ) are detected on the solid dehydrated at 673 K. These species noted NH3ads-L1 and NH3ads-L2 are differentiated by their δs NH3 IR bands at 300 K (1149 and 1228 cm–1, respectively), whereas their δas IR bands are at the same position (1596 cm–1). The AEIR and TPAE methods indicate that the heats of adsorption of the NH3ads-L1 and NH3ads-L2 species (noted EL1(θ) and EL2(θ) accuracy ± 5 kJ/mol) vary linearly with their respective coverages θ from EL1(1) = 56 kJ/mol to EL1(0) = 105 kJ/mol and from EL2(1) = 105 kJ/mol to EL2(0) = 160 kJ/mol. These values are compared to (a) isosteric heat of adsorption provided by the Clausius–Clapeyron method and (b) literature data using temperature-programmed desorption, microcalorimetry, and DFT calculations. Forthcoming articles show that the simplicity of the analytical procedures allows studying the impact of the presence of sulfate and VOx/WOy depositions over TiO2 on the nature and heats of adsorption of adsorbed NH3 species.

In situ DRIFTS investigation on the SCR of NO with NH3 over V2O5 catalyst supported by activated semi-coke

Abstract 3 wt.% of V2O5 is loaded onto the activated semi-coke (V2O5/ASC) via impregnation method and used for low temperature selective catalytic reduction (SCR) of NOx with NH3. The prepared V2O5/ASC catalyst yields an over 90% NO conversion rate with excellent N2 selectivity at 250 °C with a space velocity of 12,000 h−1. The adsorption state of different species and reaction behaviors under various conditions are systematically examined with in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). It is evidenced that NH3 and NO are absorbed on different active sites over the V2O5/ASC catalyst. At the reaction temperature (200 °C) in this study, NH3 is mainly absorbed on the Lewis acid sites provided by V2O5, and NO is mainly absorbed on the active sites originated from the support ASC. In addition, the NH3-SCR process takes place according to two pathways, including reaction between the coordinated NH3 and gaseous NO (E–R mechanism), and reaction between the absorbed NO2 and coordinated NH3 (L–H mechanism). The latter one plays a primary role for the improved low-temperature SCR performance of V2O5/ASC catalyst.

Recent NH3-SCR Mechanism Research over Cu/SAPO-34 Catalyst

The preliminary mechanism research of NH3-SCR and NH3 oxidation over Cu/SAPO-34 catalyst is explored. The XRD, SEM, in situ EPR, NH3-SCR, NH3 oxidation test, DRIFTs, and the kinetic tests were performed for the bulk characterization, catalytic activity measurement, and the mechanism estimation. The NH3-SCR result showed that Cu/SAPO-34 revealed excellent activity during 120–600 °C, while the NH3 oxidation appeared above 300 °C and caused the decline of NO conversion. The order of various reactants (NO, NH3, and O2 for NH3-SCR and NH3, O2, for NH3 oxidation) was estimated by the kinetic tests to explain their behaviors during the NH3-SCR process. The NH3-SCR presents strong dependence on adsorbed NH3 species on Cu/SAPO-34, but the NH3 oxidation does not. Furthermore, the EPR experiment proved that the isolated Cu2+ species are the active sites and the ammonia nitrites species are the intermediate for NH3-SCR over Cu/SAPO-34 catalysts. The different behavior of adsorbed NH3 species in NH3-SCR and NH3 oxidat...