Rate Of Selective Catalytic Reduction Research Articles

Enhancing SO2-shielding effect and Lewis acid sites for high efficiency in low-temperature SCR of NO with NH3: Reinforced electron-deficient extent of Fe3+ enabled by Ti4+ in Fe2O3

An ideal catalyst that can be used for the selective catalytic reduction (SCR) of NO at low temperature should have a high activity and ability to resist SO2. Herein, Ti-doped Fe2O3 (Ti-Fe2O3) nanoparticles that extraordinarily convert NO (T90, 200 °C) and highly resist SO2 meet this requirement. In the presence of 100 ppm SO2, the optimal Ti-Fe2O3 can convert 90% NO at 220 °C for 24 h. With the addition of Ti4+, the electrons around Fe3+ migrate toward Ti4+, strengthening the electron-deficient tendency of Fe3+. Therefore, electron-donating NH3 is preferentially adsorbed by Fe3+, endowing Ti-Fe2O3 with the ability to resist SO2 and exceptionally tolerate SO2 at low temperature. Besides, based on the strength of the embedded Ti4+, a large number of Brønsted acid sites are converted to Lewis acid sites to speed the reaction rate of SCR below 210 °C. High anti-SO2 capacity and adequate Lewis acid sites maximize the efficiency of Ti-Fe2O3 in reducing NO.

Effect of Exhaust Gas Recirculation Combined with Selective Catalytic Reduction on NO x Emission Characteristics and Their Matching Optimization of a Heavy-Duty Diesel Engine.

Exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) have become important technologies to reduce the NOx emission of heavy-duty diesel engines and meet the increasingly stringent emission regulations. This paper studied the effect of EGR combined with SCR on the NOx emission characteristics of a heavy-duty diesel engine based on the engine bench test. The results showed that the NO reduction rate of EGR-coupled SCR increased with the increase of engine load, and the effect was no longer significant when the NO reduction rate exceeded a certain limit under the same working conditions. EGR combined with SCR has little effect on NO2 emission reduction, and the increase of engine speed can significantly improve the efficiency of the NO2 reduction rate at 75 and 100% load. 25% opening of the EGR valve (OEV) and 50% OEV have very similar effects on the NOx reduction rate when the engine speed is at a low level. Compared with low engine speeds, increased OEV or ammonia NOx molar ratio (ANR) had a more obvious effect on the NOx reduction rate at high engine speeds. SCR combined with low valve-opening EGR had a more significant effect on the NOx reduction rate. The increase of OEV led to the increase of fuel consumption rate, but the effect on the fuel consumption rate decreased gradually with the increase of diesel engine speed. Meanwhile, this study optimized the matching relationship between OEV and ANR based on the data of the genetic algorithm, which provides a theoretical research method and application basis for diesel engine-matching of EGR and SCR.

Temperature dependence of Cu(I) oxidation and Cu(II) reduction kinetics in the selective catalytic reduction of NOx with NH3 on Cu-chabazite zeolites

The selective catalytic reduction (SCR) of NOx with NH3 over Cu-exchanged chabazite (CHA) zeolites at low temperatures (<523 K) occurs via a redox mechanism in which O2 oxidizes two NH3-solvated CuI sites, and NO and NH3 react together to reduce NH3-solvated CuII sites. Increasing the Cu ion density in Cu-CHA zeolites increases the kinetic relevance of CuII reduction relative to CuI oxidation during SCR at fixed gas pressures, given the dual-site requirement of CuI oxidation but the single-site requirement of CuII reduction. Apparent activation energies (Eapp) measured at fixed gas pressures increase with Cu ion density, at first glance suggesting that CuI oxidation has a lower Eapp value than CuII reduction, assuming mean-field kinetic behavior wherein barriers are independent of Cu site density. Here, steady-state SCR rates were measured at varying O2 pressures (1–60 kPa) and temperatures (446–501 K) to isolate Eapp values for CuI oxidation and CuII reduction on Cu-CHA samples of varying Cu ion density (0.065–0.35 Cu per 103 Å3), revealing instead that Eapp values depend on Cu density for both CuI oxidation and CuII reduction steps. Eapp values for CuI oxidation increase monotonically with Cu density in the full range studied, while Eapp values for CuII reduction are invariant with Cu density except at the lowest Cu densities (0.065–0.10 Cu per 103 Å3). Moreover, the small differences between Eapp values for CuI oxidation and CuII reduction indicate that their kinetic relevance depends only weakly on temperature for a given Cu-CHA sample. These findings sharply contrast the conclusions from previously reported Eapp data measured at fixed gas pressures and interpreted using mean-field kinetic descriptions, and illustrate the importance of measuring rate data over wide ranges of reaction conditions and sample compositions to more precisely describe the non-mean-field kinetic behavior of NH3-solvated Cu ions that become mobilized in zeolites during low-temperature SCR catalysis.

Solvation and Mobilization of Copper Active Sites in Zeolites by Ammonia: Consequences for the Catalytic Reduction of Nitrogen Oxides.

ConspectusCopper-exchanged chabazite (Cu-CHA) zeolites are catalysts used in diesel emissions control for the abatement of nitrogen oxides (NOx) via selective catalytic reduction (SCR) reactions with ammonia as the reductant. The discovery of these materials in the early 2010s enabled a step-change improvement in diesel emissions aftertreatment technology. Key advantages of Cu-CHA zeolites over prior materials include their effectiveness at the lower temperatures characteristic of diesel exhaust, their durability under high-temperature hydrothermal conditions, and their resistance to poisoning from residual hydrocarbons present in exhaust. Fundamental catalysis research has since uncovered mechanistic and kinetic features that underpin the ability of Cu-CHA to selectively reduce NOx under strongly oxidizing conditions and to achieve improved NOx conversion relative to other zeolite frameworks, particularly at low exhaust temperatures and with ammonia instead of other reductants.One critical mechanistic feature is the NH3 solvation of exchanged Cu ions at low temperatures (<523 K) to create cationic Cu-amine coordination complexes that are ionically tethered to anionic Al framework sites. This ionic tethering confers regulated mobility that facilitates interconversion between mononuclear and binuclear Cu complexes, which is necessary to propagate SCR through a Cu2+/Cu+ redox cycle during catalytic turnover. This dynamic catalytic mechanism, wherein single and dual metal sites interconvert to mediate different half-reactions of the redox cycle, combines features canonically associated with homogeneous and heterogeneous reaction mechanisms.In this Account, we describe how a unified experimental and theoretical interrogation of Cu-CHA catalysts in operando provided quantitative evidence of regulated Cu ion mobility and its role in the SCR mechanism. This approach relied on new synthetic methods to prepare model Cu-CHA zeolites with varied active-site structures and spatial densities in order to verify that the kinetic and mechanistic models describe the catalytic behavior of a family of materials of diverse composition, and on new computational approaches to capture the active-site structure and dynamics under conditions representative of catalysis. Ex situ interrogation revealed that the Cu structure depends on the conditions for the zeolite synthesis, which influence the framework Al substitution patterns, and that statistical and electronic structure models can enumerate Cu site populations for a known Al distribution. This recognition unifies seemingly disparate spectroscopic observations and inferences regarding Cu ion structure and responses to different external conditions. SCR rates depend strongly on the Cu spatial density and zeolite composition in kinetic regimes where Cu+ oxidation with O2 becomes rate-limiting, as occurs at lower temperatures and under fuel-rich conditions. Transient experiments, ab initio molecular dynamics simulations, and statistical models relate these sensitivities to the mobility constraints imposed by the CHA framework on NH3-solvated Cu ions, which regulate the pore volume accessible to these ions and their ability to pair and complete the catalytic cycle. This highlights the key characteristics of the CHA framework that enable superior performance under low-temperature SCR reaction conditions.This work illustrates the power of precise control over a catalytic material, simultaneous kinetic and spectroscopic interrogation over a wide range of reaction conditions, and computational strategies tailored to capture those reaction conditions to reveal in microscopic detail the mechanistic features of a complex and widely practiced catalysis. In doing so, it highlights the key role of ion mobility in catalysis and thus potentially a more general phenomenon of reactant solvation and active site mobilization in reactions catalyzed by exchanged metal ions in zeolites.

Effects of dioxygen pressure on rates of NOx selective catalytic reduction with NH3 on Cu-CHA zeolites

At low temperatures (<523 K), the selective catalytic reduction (SCR) of NO with NH3 on Cu-exchanged zeolites occurs via elementary steps catalyzed by NH3-solvated Cu ions, which are reactive intermediates in a CuII/CuI redox cycle. SCR rates are typically measured under “standard” reaction conditions, in which O2 is the oxidant and present at partial pressures (~10 kPa O2) that cause both single-site CuII reduction and dual-site CuI oxidation to behave as kinetically relevant steps. As a result, “standard” SCR rates (per g) transition from a second-order to a first-order dependence on isolated Cu content (per g) among Cu-CHA zeolites with increasing Cu content and thus spatial density (Si/Al = 15, Cu/Al = 0.08–0.37), as dual-site CuI oxidation steps limit SCR rates to lesser extents. On a given Cu-CHA catalyst, SCR rates (per Cu, 473 K) show a Langmuirian dependence on dioxygen pressure when varied widely (0–60 kPa O2), enabling the isolation of first-order or zero-order kinetic regimes with respect to O2. These kinetic regimes respectively correspond to limiting conditions in which either CuI oxidation or CuII reduction becomes the dominant kinetically relevant step, consistent with in operando X-ray absorption spectra. First-order rate constants (per Cu) increase approximately linearly with Cu density, reflecting the dual-site requirement of O2-assisted CuI-oxidation steps. Zero-order rate constants (per Cu) increase more gradually with Cu density, in part reflecting the increasing fraction of isolated Cu ions that are able to form binuclear intermediates and thus participate in SCR turnovers. Combining steady-state and transient kinetic data with in operando spectra provides a methodology to quantitatively describe the rate dependences of SCR reduction and oxidation processes on Cu-zeolite properties such as Cu ion density as shown here, and others including the zeolite framework topology and its density and distribution of Al atoms.

Single-atom catalysts reveal the dinuclear characteristic of active sites in NO selective reduction with NH3

High-performance catalysts are extremely required for controlling NO emission via selective catalytic reduction (SCR), and to acquire a common structural feature of catalytic sites is one key prerequisite for developing such catalysts. We design a single-atom catalyst system and achieve a generic characteristic of highly active SCR catalytic sites. A single-atom Mo1/Fe2O3 catalyst is developed by anchoring single acidic Mo ions on (001) surfaces of reducible α-Fe2O3, and the individual Mo ion and one neighboring Fe ion are thus constructed as one dinuclear site. As the number of the dinuclear sites increases, SCR rates increase linearly but the apparent activation energy remains almost unchanged, evidencing the identity of the dinuclear active sites. We further design W1/Fe2O3 and Fe1/WO3 and find that tuning acid or/and redox properties of dinuclear sites can alter SCR rates. Therefore, this work provides a design strategy for developing improved SCR catalysts via optimizing acid-redox properties of dinuclear sites.

Experimental assessment of the bifunctional NH3-SCR pathway and the structural and acid-base properties of WO3 dispersed on CeO2 catalysts

A bifunctional pathway for selective catalytic reduction (SCR) with NH3 was established using a mixture of WO3/ZrO2 possessing strong acidity and CeO2 possessing moderate redox activity. The physical mixture (tight contact) of WO3/ZrO2 (4.2 W/nm2) and CeO2 provided enhanced SCR reactivity but did not improve the redox property assessed by NO oxidation. No improvement of the SCR conversion, however, was observed for mixtures of individual pellets (loose contact), suggesting the requirement of submicrometer-level proximity of the acidic sites to the redox centers. WO3/CeO2 (5.3 W/nm2) with intermediate acidic strength had higher SCR and NO oxidation rates than the aforementioned physical mixture of WO3/ZrO2 and CeO2. The weakly basic sites (OH species) on CeO2 were replaced stoichiometrically with strongly acidic sites on the WO3 domains along with an increase in the W density (0–24.4 W/nm2), and monoclinic WO3 crystallites then formed at approximately 10 W/nm2. The dispersed WO3 was in a distorted octahedral environment in all WO3/CeO2 samples. The intrinsic SCR reactivity was controlled by only the W surface density; the SCR rate (per surface area) increased within the polytungstate submonolayer region (<5–10 W/nm2) and then nearly reached a plateau. The NO oxidation rate also increased with an increase of the W density in a similar trend to the SCR rate, which is indicative of the formation of redox-active centers, rendering the synergetic enhancement of SCR reactivity. These findings and conclusions reached here provide useful guidance for new bifunctional strategies to achieve practical SCR performance.

A study on chemisorbed oxygen and reaction process of Fe–CuOx/ZSM-5 via ultrasonic impregnation method for low-temperature NH3-SCR

Fe–CuOx/ZSM-5 catalyst was prepared for low-temperature selective catalytic reduction (SCR) of NOx with ammonia by ultrasonic impregnation method. The catalyst were characterized by HRTEM, XRD, H2-TPR, XPS, in situ DRIFTS and tested for their activities in a fixed-bed reactor. The NOx conversion over Fe(0.67)–Cu/ZSM-5 sample reached 98% within the temperature range from 180 to 360°C. Ultrasonic impregnation can promote active component to highly disperse into the channel of ZSM-5, greatly improving low-temperature SCR performance compared to conventional impregnation according to characterization of XRD, HRTEM. XPS results showed that chemisorbed oxygen had catalytic oxidation activity against NO and NH3, the proportion of chemisorbed oxygen was affected by molar ration of iron and copper. When the molar ratio of iron and copper was 2:1, the maximum chemisorbed oxygen by XPS and the minimum reduction temperature of 263°C by H2-TPR were obtained. In situ DRIFTS results revealed that catalyst had oxidation activity against NO and NH3 under oxygen-free condition due to action of chemisorbed oxygen, reaction rate of SCR is much faster than the reaction between adsorbed ammonia and oxygen, nitrate as a kind of intermediate was produced by reaction of NH3 and NO in the SCR process.

Hybrid catalysts—an innovative route to improve catalyst performance in the selective catalytic reduction of NO by NH3

The selective catalytic reduction (SCR) of NO by NH3 was investigated over mechanical mixtures consisting of an oxidation catalyst and Fe-ZSM-5 as a component active for SCR. Oxidation components used included several metal oxides MOx, where M is Mn, Mn–Ce, Mn–Cr, Mn–Cu, Ce–Zr, Mn/Ce–Zr. By comparing SCR rates and selectivities obtained with these mixtures (“hybrid catalysts”) with those of their individual components, significant, sometimes drastic synergistic effects between both components could be established. Mn-based oxidation components, which provide high SCR activity on their own, were improved with respect to selectivity towards N2 by the presence of Fe-ZSM-5. A strong synergy with clearly improved N2 selectivity remained after the SCR activity of the Mn-containing phases was suppressed by thermal ageing. With the Ce–Zr oxidation component, lower activities were achieved, however at very high selectivity. The measurement of NO oxidation rates could not prove the basic idea of the oxidation catalyst providing NO2 and enabling Fe-ZSM-5 to react the remaining NO via a fast SCR path. While the trends concerning synergy and NO oxidation activity were generally parallel, the NO oxidation activity of Fe-ZSM-5 in the absence of NH3 exceeded or equaled that of most oxidation components which provided significant synergetic effects. For proving the mechanism, data on NO2 formation in the presence of NH3 are needed, which can be made available only by kinetic modeling as the NO/NH3/O2 system prefers the SCR route over NO oxidation. With Cu- and Cr-containing systems, unexpected changes in selectivity (N2O formation) were noted upon thermal stress, the reasons of which remain unclear.

Magnetic field effects on selective catalytic reduction of NO by NH3 over Fe2O3 catalyst in a magnetically fluidized bed

Selective catalytic reduction (SCR) of NO from simulated flue gas by ammonia with Fe 2 O 3 particles as the catalyst was performed using a magnetically fluidized bed (MFB). X-ray diffraction (XRD) spectroscopy and Brunauer–Emmett–Teller (BET) method were used to analyze Fe 2 O 3 catalyst. Important effects of magnetic fields were observed in the SCR of NO by ammonia over Fe 2 O 3 catalyst. The apparent activation energies of SCR were reduced by external magnetic fields, and the SCR activity of Fe 2 O 3 catalyst was improved with the magnetic fields at low temperatures. Thus the scope of temperature with high efficiency of NO removal was extended from 493–523 K to 453–523 K by magnetic fields. Magnetic fields of 0.01–0.015 T were suggested for NO removal on Fe 2 O 3 catalyst with MFB. The results suggested that the magnetoadsorption of NO onto Fe 2 O 3 surface together with NH 2 and NO free radicals effects induced by the external magnetic fields both acted to improve the rate of SCR of NO on Fe 2 O 3 catalyst. On the other hand, magnetic field effects were also attributed to improved gas–solid contact in MFB.

FTIR study of the selective catalytic reduction of NO 2 with ammonia on nanocrystalline NaY and CuY

In this study, the selective catalytic reduction (SCR) of NO 2 to N 2 and O 2 with ammonia at 298 K on nanocrystalline NaY, Aldrich NaY and nanocrystalline CuY was investigated using in situ Fourier transform infrared (FTIR) spectroscopy. It was determined that the kinetics of SCR were 30% faster on nanocrystalline NaY compared to Aldrich NaY. The superior performance of the nanocrystalline zeolite was attributed to an increase in external surface reactivity. External surface sites, which include silanol groups and extra framework alumina (EFAL), gave rise to differences in the adsorption of NO 2 and NH 3 on nanocrystalline NaY compared to commercial NaY. Copper cation-exchanged nanocrystalline Y resulted in an additional increase in the rate of SCR as well as distinct NO 2 and NH 3 adsorption sites associated with the copper cation. This is the first study of a transition metal cation-exchanged nanocrystalline zeolite and its potential use as a catalyst in the SCR of nitrogen oxides.

Thermal stability and catalytic activity of flame-made silica–vanadia–tungsten oxide–titania

Vanadia (0.9 or 2wt.%) and silica (0–5wt.%) doping of flame-made tungsten oxide–titania nanostructured catalyst powders (anatase, 100m2/g, 10wt.% WO3) is investigated. The effect of dopants on structural and chemical properties of these powders were analyzed by nitrogen adsorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), transmission electron microscopy (TEM) and Raman spectroscopy. After calcination for 20h at 700°C in air, the thermally most stable composite powder conserved its specific surface area (SSA) to 90m2/g and its anatase content to 96wt.%. Tungsten oxide and vanadia form thin polymeric layers (∼1nm) on the surface of the titania support. Adding silica improves the thermal and crystal stability of the catalysts even at higher reactor temperatures. As a result both NO conversion and the rate of selective catalytic reduction (SCR) with NH3 were increased.

Selective catalytic reduction of nitric oxide by ethylene in the presence of oxygen over Cu 2+ ion-exchanged pillared clays

Cu 2+ ion-exchanged pillared clays are substantially more active than Cu 2+-ZSM-5 for selective catalytic reduction (SCR) of NO by hydrocarbons. More importantly, H 2O (or SO 2) has only mild effects on their activities. First results on Cu 2+-exchanged TiO 2-pillared montmorillonite were reported by this laboratory (Yang and Li, Ref. [1]), that showed overall activities two to four times higher than Cu 2+-ZSM-5. A delaminated pillared clay was subjected to Cu 2+ ion-exchange and studied for SCR by C 2H 4 in this work. The Cu 2+ ion-exchanged delaminated Al 2O 3-pillared clay yielded substantially higher SCR rates than both Cu 2+-exchanged TiO 2-pillared clay and Cu 2+-ZSM-5 at temperatures above 400°C. The peak NO conversion was 90% at 550°C and at a space velocity of 15,000 h −1 (with O 2 = 2%). The peak temperature decreased as the concentration of O 2 was increased. The macroporosity in the delaminated pillared clay was partially responsible for its higher peak temperatures (than that for laminated pillared clays). At 1000 ppm each for NO and C 2H 4, the NO conversion peaked at 2% O 2 for all temperatures. H 2O and SO 2 caused only mild deactivation, likely due to competitive adsorption (of SO 2 on Cu 2+ sites and H 2O on acid sites). The high activity of Cu 2+-exchanged Al 2O 3-pillared clay was due to a unique combination of the redox property of the Cu 2+ sites and the strong Lewis acidity of the pillared clay. The suggested mechanism involved NO chemisorption (in the presence of O 2) on Cu 2+ O Al 3+-on the pillars, and C 2H 4 activation on the Lewis acid sites to form an oxygenated species.