Fast Selective Catalytic Reduction Research Articles

Uncovering the Dinuclear Mechanism of NO2-Involved NH3-SCR over Supported V2O5/TiO2 Catalysts.

Commercial vanadium oxide catalysts exhibit high efficiency for the selective catalytic reduction (SCR) of NO with NH3, especially in the presence of NO2 (i.e., occurrence of fast NH3-SCR). The high-activity sites and their working principle for the fast NH3-SCR reaction, however, remain elusive. Here, by combining in situ spectroscopy, isotopic labeling experiments, and density functional theory (DFT) calculations, we demonstrate that polymeric vanadyl species act as the main active sites in the fast SCR reaction because the coupling effect of the polymeric structure alters the elementary reaction step and effectively avoids the high energy barrier of the rate-determining step over monomeric vanadyl species. This study unveils the high-activity dinuclear mechanism of the NO2-involved SCR reaction over vanadia-based catalysts and provides a fundamental basis for developing high-efficiency and low V2O5-loading SCR catalysts.

Nitration-Promoted Vanadate Catalysts for Low-Temperature Selective Catalytic Reduction of NOX with NH3.

Vanadium-based catalysts have been commercially used in selective catalytic reduction (SCR), owing to their high catalytic activity and effectiveness across a wide temperature range; however, their catalytic efficiency decreases at lower temperatures under exposure to SOX. This decrease is largely due to ammonium sulfate generation on the catalyst surface. To overcome this limitation, we added ammonium nitrate to the V2O5-WO3/TiO2 catalyst, producing a V2O5-WO3/TiO2 catalyst with nitrate functional groups. With this approach, we found that it was possible to adjust the amount of these functional groups by varying the amount of ammonium nitrate. Overall, the resultant nitrate V2O5-WO3/TiO2 catalyst has large quantities of NO3- and chemisorbed oxygen, which improves the density of Brønsted and Lewis acid sites on the catalyst surface. Furthermore, the nitrated V2O5-WO3/TiO2 catalyst has a high NOX removal efficiency and N2 selectivity at low temperatures (i.e., 300 °C); this is because NO3- and chemisorbed oxygen, generated by nitrate treatment, facilitated the occurrence of a fast SCR reaction. The approach outlined in this study can be applied to a wide range of SCR catalysts, allowing for the development of more, low-temperature SCR catalysts.

Mechanism study of Na poisoning on Ce/TiO2(001) surface based on adsorption of reaction species and elementary reaction pathways

The presence of alkali metal Na in flue gas can significantly impair the reactivity of Ce/TiO2 (CT) SCR (Selective Catalytic Reduction) catalyst. This study investigates the deactivation mechanism of Na on the Ce/TiO2 catalyst using Density Functional Theory (DFT) calculations, focusing on the adsorption of surface species and the pathway of elementary reaction. Due to the high affinity between Na atom and the catalyst surface, Na poisoning readily occurs on Ce/TiO2 (001) surface. The presence of Na on the catalyst surface hampers hydrogenation, oxygen vacancy formation, and NH3 adsorption, while also reducing the catalyst's acidity and reducibility. Additionally, Na impedes the NH3 dehydrogenation reaction, as well as the formation and decomposition of NH2NO and the dissociation of O2 on the catalyst surface, thereby inhibiting the NH3-SCR reaction. The inhibition of NO2 formation further affects the fast SCR reaction, which may explain the low-temperature deactivation of the catalyst.

Performance characterization of a selective catalytic reduction on filter aftertreatment system in a heavy-duty diesel engine tested on a laboratory dynamometer

Increasingly stringent emission regulations call for improvement in the current emissions control technology in heavy-duty diesel engines. In the existing aftertreatment system utilized in diesel engines, there is already a challenge with space and volume, and an attempt to add more components would lead to the consumption of more space and impracticable. The Selective Catalytic Reduction on Filter (SCRF)-an integration of Selective Catalytic Reduction (SCR) and Diesel Particulate Filter (DPF) in a single component, offers space saving and favorable thermal inertia opportunities. However, literatures are conflicting on the effect of the SCR and DPF activities on the operations of the SCRF, as both require NO2 for fast SCR reaction, and passive soot oxidation respectively. This paper presents a detailed experimental characterization of a Silicon-Carbide (Si–C) coated with Copper-Zeolite (Cu-Ze) catalyst SCRF after-treatment component in a heavy-duty diesel engine, evaluated on a laboratory dynamometer. The effect of particulate matter loading on the SCRF activities in steady state, and transient operations was investigated, with emphasis on the simultaneous utilization of NO2 between the passive soot oxidation activities and SCR reactions-which is a determinant of the NOx conversion efficiency. The SCRF was loaded from 0 g/L up to 5 g/L while passive regeneration and NOx conversion activities were investigated over steady and transient cycles. The investigation has shown that both NOx conversion efficiency and passive soot oxidation increase with particulate matter loading in the SCRF system of a heavy-duty diesel engine.

Preparation of coal fly ash supported high-temperature SCR catalyst by a novel alkali-acid combined activation

Coal fly ash (CFA) is an alternative carrier of selective catalytic reduction (SCR) catalyst, but its reactivity is usually poor. Herein, the alkali-acid combined activation was proposed to prepare the CFA-supported high-temperature SCR catalyst. The results revealed that 1.2 g/gCFA of NaOH and 0.6 g/gCFA of H2SO4 was the optimal CFA activation condition. The NaOH first destroyed the quartz crystal over the CFA, generating the Si-OH groups. Subsequently, H2SO4 enhanced the extraction of aluminates and enriched the acid sites, which promoted the standard SCR. Besides, the generated porous fragments facilitated the dispersion of Fe3+ species and improved the fast SCR.

Redox on Mn-Ce interface and its effects on low temperature selective catalytic reduction for NOx removal

The objective of this work was to clearly elucidate the interactions on Mn-Ce interface and understand how they affect low temperature selective catalytic reduction (SCR) for NOx removal. In this work, three Mn-Ce catalysts in similar particle size and morphology with different Mn-Ce interface areas were prepared. MnO2@CeO2-F had the tightest and largest amount of Mn-Ce interfaces compared with MnO2@CeO2-P and MnO2-CeO2-M, and also exhibited the highest NO conversion degree at 100–150 °C as well as the strongest activity of Mn species and lowest generation of N2O. The TG-DTG, XPS and electrochemical impedance spectroscopy results proved the redox of Ce4+ + Mn3+ ↔ Ce3+ + Mn4+ on the Mn-Ce interface, and MnO2@CeO2-F had the highest Mn4+, Ce3+, Oα concentrations. The results of the study on the redox reactions during the NH3-SCR process provided powerful evidence of the Mn4+/Mn3+ and Ce3+/Ce4+ recycles that are important for low temperature NH3-SCR process. Moreover, the Mn-Ce interface was directly proved to benefit the surface acid sites, redox ability, and activity of the adsorbed intermediates. Both the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reaction mechanisms were observed in the SCR reaction with prepared Mn-Ce materials, where “fast SCR” can be observed with MnO2@CeO2-F. Moreover, the Mn-Ce interface also lowers the activation energy of the low-temperature SCR process, with MnO2@CeO2-F having the lowest activation energy. The results from this work offered powerful evidences for the interactions of Mn/Ce redox at the Mn-Ce interface and elucidated clear understanding of the effects of Mn-Ce redox on low-temperature SCR process, which is expected to promote the NOx removal process.

NO Selective Catalytic Reduction over Atom-Pair Active Sites Accelerated via In Situ NO Oxidation.

Selective catalytic reduction (SCR) of NOx with NH3 is the most efficient technology for NOx emissions control, but the activity of catalysts decreases exponentially with the decrease in reaction temperature, hindering the application of the technology in low-temperature SCR to treat industrial stack gases. Here, we present an industrially practicable technology to significantly enhance the SCR activity at low temperatures (<250 °C). By introducing an appropriate amount of O3 into the simulated stack gas, we find that O3 can stoichiometrically oxidize NO to generate NO2, which enables NO reduction to follow the fast SCR mechanism so as to accelerate SCR at low temperatures, and, in particular, an increase in SCR rate by more than four times is observed over atom-pair V1-W1 active sites supported on TiO2(001) at 200 °C. Using operando SCR tests and in situ diffuse reflectance infrared Fourier transform spectra, we reveal that the introduction of O3 allows SCR to proceed along a NH4NO3-mediated Langmuir-Hinshelwood model, in which the adsorbed nitrate species speed up the re-oxidation of the catalytic sites that is the rate-limiting step of SCR, thus leading to the enhancement of activity at low temperatures. This technology could be applicable in the real stack gas conditions because O3 exclusively oxidizes NO even in the co-presence of SO2 and H2O, which provides a general strategy to improve low-temperature SCR efficacy from another perspective beyond designing catalysts.

Metal-Doped Mesoporous MnO2-CeO2 Catalysts for Low-Temperature Pre-Oxidation of NO to NO2 in Fast SCR Process

Selective catalytic reduction (SCR) is an effective system for treating nitrogen oxides (NOx; mainly NO), and fast SCR requires the equimolar reactants of NO and NO2. This study focused on catalysts for oxidizing 50% of NO to NO2. A series of catalysts composed of a variety of components, such as mesoporous mMnO2-nCeO2 as carrier catalysts (m:n = 9:1 and 7:3) and transition metals (e.g., Fe, Co, Ni, Cu, and Cr), were synthesized and characterized using N2 adsorption, in situ XRD, TEM, and XPS. All samples had a mesoporous structure with pore size around 8 nm. XPS results demonstrated that addition of cerium ion increased the surface area and provided oxygen vacancy due to the formation of Ce3+ within the structure. NO oxidation activity was tested using a feed (205~300 ppm NO and 6% O2) that simulated typical flue gas conditions. Doped mesoporous mMnO2–nCeO2 has higher NO oxidation activity than pristine mMnO2–nCeO2. The doped mMnO2-nCeO2 catalyzed 50% of NO to NO2 at between 140 and 200 °C resulting in an equivalent amount of NO and NO2. Among the transition metals, Cu, Ni, Co, Fe, and Cr have the highest to lowest oxidation activity, respectively. The precatalytic oxidation of NO can potentially be combined with the current SCR system without changes to existing equipment and can be applied to the exhaust gas treatment for de-NOx.

Investigation of intrinsic catalytic mechanism for NO oxidation to NO2 in CeO2 used for NO removal

Redox properties of CeO2 materials have a major impact on the NO oxidation to NO2, which is a crucial step for selective-catalytic-reduction (SCR) because NO2 can trigger the “Fast SCR” reaction. Despite decades of investigations, the mechanism of NO oxidation to NO2 at the CeO2 surface is still under debate, the controversy is whether the key oxidants are the CeO2 catalyst’s high-valence metal ions or the O2 molecules (in the reactant gas) which could be activated by the reducing CeO2 surface oxygen vacancies (Ovac)? We perform density-functional-theory (DFT) simulations and synthesize CeO2 catalysts for in situ spectroscopy experiments, transient-reaction-analysis (TRA) experiments, and isotopic experiments, to reveal the NO oxidation mechanism and achieve atomic-level understanding. Our results show that Ovac at the CeO2 surface strongly interacts with O2 gas, leading to the formation of a key oxidizing intermediate O* (“*” means an adsorbed state). TRA experiments, isotopic experiments, and in situ spectroscopy results further provide evidence that O* oxidizes NO to NO2. Given the theoretical–experimental-joint results, we demonstrate that the effective approach, in terms of utilizing the intrinsic catalytic properties of CeO2, should be simultaneously optimizing the surface Ovac concentration and the Ovac’s activity for O2-dimer activation, which provides atomic-scale insights for a rational design of NO-conversion catalysts.

Synergistic Effects of Keggin-Type Phosphotungstic Acid-Supported Single-Atom Catalysts in a Fast NH3-SCR Reaction

Fast selective catalytic reduction of nitrogen oxide with ammonia (NH3-SCR) (2NH3 + NO2 + NO → 2N2 + 3H2O) has aroused great interest in recent years because it is inherently faster than the standard NH3-SCR reaction (4NO + 4NH3 + O2 → 4N2 + 6H2O). In the present paper, the mechanism of the fast NH3-SCR reaction catalyzed by a series of single-atom catalysts (SACs), M1/PTA SACs (PTA = Keggin-type phosphotungstic acid, M = Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, and Pt), has been systematically studied by means of density functional theory (DFT) calculations. Molecular geometry and electronic structural analysis show that Jahn-Teller distortion effects promote an electron transfer process from N-H bonding orbitals of the NH3 molecule to the symmetry-allowed d orbitals (dxy and dx2-y2) of the single metal atom, which effectively weakens the N-H bond of the adsorbed NH3 molecule. The calculated free energy profiles along the favorable catalytic path show that decomposition of NH3 to *NH2 and *H species and decomposition of *NHNOH into N2 and H2O have high free energy barriers in the whole fast NH3-SCR path. A good synergistic effect between the Brønsted acid site (surface oxygen atom in the PTA support) and the Lewis acid site (single metal atom) effectively enhances the decomposition of NH3 to *NH2 and *H species. M1/PTA SACs (M = Ru, Rh, Pd, and Pt) were found to have potential for fast NH3-SCR reaction because of the relatively small free energy barrier and strong thermodynamic driving forces. We hope our computational results could provide some new ideas for designing and fabricating fast NH3-SCR catalysts with high activity.

Mechanistic assessment of NO oxidative activation on tungsten-promoted ceria catalysts and its consequence for low-temperature NH3-SCR

Ceria has absorbed extensive interests in functioning as catalysts for low-temperature (LT) selective catalytic reduction (SCR) of NOx due to its abundant surface oxygen species and salient redox property. The mechanistic interpretations of how such redox property contributes to its LT-SCR activity, however, still remain debated. Here, we use a model tungsten-promoted ceria catalyst (WO3/CeO2), known as preeminent in LT-SCR reactions, and combine steady-state kinetic, structural characteristic and in situ spectroscopic experiments with theoretical treatments to reveal the mechanistic connections between NO oxidative activation and LT-SCR turnovers. We show that, compared with NO oxidation to NO2, NO oxidative activation to nitrite intermediates is both kinetically and thermodynamically more favorable; surface nitrate species are not detected in the present case, indicating their more difficult formation than nitrites. All these results thus suggest the prevalence of NO activation to nitrites over NO to nitrates or oxidation to NO2 and subsequent occurrence of fast SCR in the WO3/CeO2-catalyzed LT-SCR reaction. These findings progress the understanding of LT-SCR reaction mechanism on CeO2-based catalysts and convey a detailed perspective of different reaction intermediates and their consequences for LT-SCR turnovers, which may contribute to the rational design of catalysts towards further improved LT-SCR activity.

Porous washcoat structure in CeO2 modified Cu‐SSZ‐13 monolith catalyst for NH3‐SCR with improved catalytic performance

AbstractA series of CeO2 modified Cu‐SSZ‐13 monolith catalysts were prepared by embedding CeO2 into the washcoat of Cu‐SSZ‐13 monolith catalyst through solvent combustion method. These CexCu‐SSZ‐13 catalysts were studied in the selective catalytic reduction (SCR) of NO with NH3, among which the Ce2Cu‐SSZ‐13 catalyst exhibited the best low‐temperature activity, hydrothermal stability, and sulfur resistance. The physicochemical properties of the catalysts were characterized using multiple methods. Results showed that the acidity, redox capacity, and ammonia adsorption capacity significantly enhanced after CeO2 modification, thus leading to the high performance of Ce2Cu‐SSZ‐13 catalyst. Furthermore, the introduction of CeO2 induced the fast SCR reaction by promoting the oxidation of NO to NO2. Analog calculation suggested that the porous structure generated via solvent combustion in the washcoat effectively increased the diffusion rate of reaction. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFT) analysis showed that Brønsted acid sites were the main active center and the reaction followed Eley–Rideal mechanism.

One-pot synthesis of CrαMnβCeTiOx mixed oxides as NH3-SCR catalysts with enhanced low-temperature catalytic activity and sulfur resistance

A series of CrαMnβCeTiOx catalysts were prepared using one-pot precipitation method and evaluated for the selective catalytic reduction (SCR) of NO with ammonia. Among the catalysts prepared, the Cr0.006Mn0.05CeTiOx catalyst exhibited the highest low-temperature activity and widest activity window (＞90% NO conversion at 160–425 °C) as well as outstanding sulfur resistance.The catalysts were characterized using XRD, NH3/NO/SO2 + O2-TPD, XPS, in situ DRIFT, and so on.The amorphous Cr0.006Mn0.05CeTiOx catalyst had a larger specific surface area, high ammonia adsorption and strong acidity, resulting in an improved catalytic activity. A high ratio of surface adsorption oxygen and Ce3+ promoted the formation of NO2, a key intermediate gas in the fast SCR reaction.Furthermore, in situ DRIFT showed that the SCR reaction followed the Eley Rideal and Langmuir Hinshelwood mechanisms.Moreover, the synergetic effect of Cr and Mn inhibited the accumulation of hydrogen sulfate or sulfate considerably.

Reaction Pathways of Standard and Fast Selective Catalytic Reduction over Cu-SSZ-39.

Cu-SSZ-39 exhibits excellent hydrothermal stability and is expected to be used for NOx purification in diesel vehicles. In this work, the selective catalytic reduction (SCR) activities in the presence or absence of NO2 were tested over Cu-SSZ-39 catalysts with different Cu contents. The results showed that the NOx conversion of Cu-SSZ-39 was improved by NO2 when NO2/NOx = 0.5, especially for the catalysts with low Cu loadings. The kinetic studies showed two kinetic regimes for fast SCR from 150 to 220 °C due to a change in the rate-controlling mechanism. The activity test and diffuse reflectance infrared Fourier transform spectra demonstrated that the reduction of NO mainly occurred on the Cu species in the absence of feed NO2, and when NO2/NO = 1, NO could react with NH4NO3 on the Brønsted acid sites in addition to undergoing reduction on Cu species. Thus, NO2 can promote the SCR reaction over Cu-SSZ-39 by facilitating the formation of surface nitrate species.

Mechanism of NH3–Selective Catalytic Reduction (SCR) of NO/NO2 (Fast SCR) over Cu-CHA Zeolites Studied by In Situ/Operando Infrared Spectroscopy and Density Functional Theory

Mechanism of NH₃–Selective Catalytic Reduction (SCR) of NO/NO₂ (Fast SCR) over Cu-CHA Zeolites Studied by <i>In Situ/Operando</i> Infrared Spectroscopy and Density Functional Theory

Study on the Mechanism of Selective Catalytic Reduction of NOx by NH3 over Mn-Doped CoCr2O4

In this paper, the first-principles calculation method based on density functional theory is used to calculate the structure of CoCr2O4 and Mn-doped Mn0.1Co0.9Cr2O4 low-refractive-index surface (100) and the adsorption model of NH3 and other molecules. Moreover, the process of the NOx removal reaction with NH3 was studied in detail. The results showed that the adsorption energy of NH3 and the amount of Mulliken charge transfer increased after Mn doping. The projected density of states indicates that the interactions between the adsorbed molecule and the substrate are stronger. It is worth noting that the doped Mn sites are also favorable sites for catalytic reactions. Based on the calculation results, we know that NH3 molecules are easily adsorbed on the surface of the catalyst and then dehydrogenated to produce NH2, NH, and other products. The next major step is the reaction of NH2 with gaseous NO to form an intermediate product of NH2NO, which is then decomposed into N2 and H2O. However, the route of N2O as an intermediate is energetically infeasible. Among them, NH3 dehydrogenation is a rate-determining step, and the activation energy barrier is 1.01 eV. However, under aerobic conditions, the activation energy barrier (0.71 eV) from NH3 to NH2 is significantly reduced. In addition, the adsorbed NO reacts with the active O atoms to generate NO2 on the surface, and NO2 can undergo a “fast selective catalytic reduction” process. To sum up, doping manganese on the surface of the CoCr2O4 catalyst can improve the catalytic activity of denitration, and the Mn0.1Co0.9Cr2O4 catalyst has a good selectivity.

Intensification of NO x Conversion over Activated Coke by Ozone Oxidation for Sintering Flue Gas at Low Temperatures.

Denitration (De-NOx) over activated cokes (ACs) for sintering flue gas needs intensification. Gaseous reactions in a gas mixture containing NO, NO2, and NH3, with the effect of O2 concentration and moisture, were taken into consideration in the study of NOx conversion over ACs. Experimental studies on NOx conversion with and without NH3 over ACs were conducted using a fixed-bed reactor at 100 °C. The results demonstrated that moisture significantly affected NOx removal over ACs, especially the NO2 conversion. Under dry conditions, a disproportionation reaction of NO2 over ACs dominated NOx conversion with no NH3, whereas apparent fast selective catalytic reduction (SCR) over the ACs was observed in the presence of NH3. Regardless of the presence of absence of NH3 in wet mixtures, NO2 adsorption on ACs via the disproportionation route dominated the NOx conversion. Increasing the NO2/NO ratio in the simulated flue gas enhanced the NOx conversion rate over ACs. −C(ONO2) deposition on ACs generated by the disproportionation route inhibited NOx conversion with time. O3 oxidation was found to be efficient in increasing the NO2/NO ratio and intensifying the NOx conversion compared with commercially direct NH3-SCR over ACs. Increasing the temperature and decreasing the gas hourly space velocity can promote NOx conversion over ACs after O3 oxidation. NO oxidized with O3 coupled with NH3 spray and continuous regeneration of ACs is a potential method for removing NOx from sintering flue gas.

Impact of Water Vapor on the Performance of a Cu-SSZ-13 Catalyst under Simulated Diesel Exhaust Conditions

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;Cu-SSZ-13 selective catalytic reduction (SCR) catalysts are broadly applied in diesel aftertreatment systems for the catalytic conversion of oxides of nitrogen (NO + NO&lt;sub&gt;2&lt;/sub&gt;). Diesel exhaust contains a wide range of water vapor concentrations depending on the operating condition. In this study, we evaluate the impact of water vapor on the relevant SCR catalytic functions including NO&lt;sub&gt;x&lt;/sub&gt; conversion, NO oxidation, NH&lt;sub&gt;3&lt;/sub&gt; oxidation, and N&lt;sub&gt;2&lt;/sub&gt;O formation under both standard and fast SCR conditions. Reactor-based experiments are conducted in the presence and absence of water vapor. Results indicate that water vapor can have both a positive and negative impact on low temperature NO&lt;sub&gt;x&lt;/sub&gt; conversion for standard SCR reaction. At low inlet NO&lt;sub&gt;x&lt;/sub&gt; concentrations, the presence of water vapor has a negative effect on NO&lt;sub&gt;x&lt;/sub&gt; conversion, whereas, at high inlet NO concentrations, water vapor improves NO&lt;sub&gt;x&lt;/sub&gt; conversion. At high temperatures, water vapor has a beneficial effect on NO&lt;sub&gt;x&lt;/sub&gt; conversion at all NO&lt;sub&gt;x&lt;/sub&gt; inlet concentrations evaluated. Experiments under fast SCR conditions reveal that water vapor improves low temperature NO&lt;sub&gt;x&lt;/sub&gt; conversion mainly by minimizing the accumulation of ammonium nitrate, as evidenced with temperature programmed desorption (TPD) results. Furthermore, NO oxidation by ammonium nitrate formed under fast SCR conditions is shown to be significantly faster in the presence of water vapor.&lt;/div&gt;&lt;/div&gt;

Selective oxidation of NH3 in a Pt/Al2O3@Cu/ZSM-5 core-shell catalyst: Modeling and optimization

Modeling and optimization is presented of the Pt/Al2O3@Cu/ZSM-5 core–shell (CS) catalyst introduced in our recent study in which we successfully demonstrated the synthesis and application of the CS catalyst as an Ammonia Slip Catalyst (ASC). The commercial ASC converts NH3 to N2 using a dual-layer architecture comprising a bottom oxidation catalyst layer (supported Pt) and a top selective catalytic reduction (SCR) of NOx layer (Fe- or Cu-exchanged zeolite). The Pt/Al2O3@Cu/ZSM-5 particle, which emulates the dual-layer architecture, was shown to have superior performance due in part to an enhanced Pt activity and dense zeolite shell. A 1 + 1 D heterogeneous fixed-bed reactor containing the core–shell catalyst is developed in this study to predict the CS catalyst performance data, advance the understanding, and enable optimization. Incorporating independent kinetic models for Pt-catalyzed NH3 oxidation and Cu/ZSM-5 catalyzed NOx reduction, the reactor model predicts all of the experimental trends. The Cu/ZSM-5 kinetics are tuned using independently measured data for NH3 uptake/desorption, NH3 and NO oxidation, and standard, fast, and NO2 SCR. The particle-scale model segregates the SCR reactions from the parallel process of intracrystalline diffusion of reacting species using independently-estimated species diffusivities from diffusion-limited NH3 oxidation on a Pt/Al2O3@Na/ZSM-5. The reactor model is validated for CS catalysts having a 0.5 and 1.2 μm thick Cu/ZSM-5 shell. The tuned model is used to determine the minimum Pt loading and Cu/ZSM-5 shell thickness that achieves prescribed NH3 conversion and N2 selectivity targets. A Pt loading as low as 0.02 wt% along with 0.83 μm thick Cu/ZSM-5 is predicted to give a NH3 conversion of 70% at 250°C and a N2 selectivity at 500°C of 90%.

The role of copper in the enhanced performance of W/Ti catalysts for low-temperature selective catalytic reduction

The influence of copper (Cu) on the performance of a selective catalytic reduction (SCR) WOx/TiO2 catalyst was investigated. The properties of the catalysts were investigated by Brunauer-Emmett-Teller (BET) analysis, Raman spectroscopy, temperature programmed desorption (TPD), temperature programmed reduction (TPR), in-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTs), and X-ray photoelectron spectroscopy (XPS). The addition of Cu led to an interaction between the octahedral WOx species in W/TiO2 catalysts, which increased the redox capacities of the catalysts, thereby enhancing the low-temperature performance of the catalysts at ≤350 °C. In contrast, the addition of Cu above certain content (~1.0 wt%) resulted in the generation of polymerized or bulk CuOx, which inhibited the enhancement of low-temperature performance. Furthermore, the increased capacity for oxidation resulted in increased rates of NH3 oxidation, which reduced the performance of the catalyst in the high-temperature region at temperatures exceeding 500 °C. The adsorption characteristics of the W-Cu/TiO2 catalyst following the addition of Cu exhibited a decrease in Brønsted-acid sites while an increase in Lewis-acid sites. Moreover, by inhibiting the production of adsorption NOx species, such as monodentate nitrate, while inducing physical adsorption of NOx, resulted in the generation of NO2(ad), thereby promoting the ‘Fast SCR’ on the catalyst.