Global Kinetic Study of NO Reduction by NH3 over V2O5–WO3/TiO2: Relationship between the SCR Performance and the Key Factors

Investigation on the reduction in unburned ammonia and nitrogen oxide emissions from ammonia direct injection SI engine by using SCR after-treatment system

Currently generated nitrogen oxides (NOx) and unburned ammonia (NH3) can be converted into nitrogen and moisture that are harmless to the human body and environment using selective catalytic reduction (SCR). The concentrations of NOx and unburned NH3 emitted from the ammonia combustion engines are significantly higher than those emitted by engines using existing hydrocarbon fuels. In this study, ammonia, a representative carbon-free fuel, was used in spark ignition engines for existing passenger vehicles to identify the trends in exhaust gases emitted from engines and conduct experiments on after-treatment strategies to reduce NOx and unburned NH3. The addition of oxygen significantly maximized the conversion efficiency of the SCR after-treatment system by changing the concentration of both NOx and NH3 in the exhaust gas.

Plasma-assisted manipulation of vanadia nanoclusters for efficient selective catalytic reduction of NOx

Supported nanoclusters (SNCs) with distinct geometric and electronic structures have garnered significant attention in the field of heterogeneous catalysis. However, their directed synthesis remains a challenge due to limited efficient approaches. This study presents a plasma-assisted treatment strategy to achieve supported metal oxide nanoclusters from a rapid transformation of monomeric dispersed metal oxides. As a case study, oligomeric vanadia-dominated surface sites were derived from the classic supported V2O5-WO3/TiO2 (VWT) catalyst and showed nearly an order of magnitude increase in turnover frequency (TOF) value via an H2-plasma treatment for selective catalytic reduction of NO with NH3. Such oligomeric surface VOx sites were not only successfully observed and firstly distinguished from WOx and TiO2 by advanced electron microscopy, but also facilitated the generation of surface amide and nitrates intermediates that enable barrier-less steps in the SCR reaction as observed by modulation excitation spectroscopy technologies and predicted DFT calculations.

Hydrothermal Aging Mechanism and Modeling for SCR Catalysts

Based on the activity evaluation and characterization test, we explored the hydrothermal aging mechanism of a vanadium-based SCR catalyst and constructed a dual-site hydrothermal aging kinetic model in this study. The vanadium-based catalyst contains Brønsted acidic sites and Lewis acidic sites, which show different sensitivities to hydrothermal aging, and the reduction of active sites is the main reason for the NOx conversion efficiency reduction after hydrothermal aging. The ammonia storage capacities of both sites have a high correlation coefficient with the NOx conversion efficiency. Based on the method of NH₃-TPD curve peak resolution, we quantified the transformations of the two active sites and established the relationship between the site density, the aging temperature, and the duration to determine the aging factor. Then, a hydrothermal aging kinetic model was constructed, and the parameter identification and verification of the model were carried out through flow reactor experiments. The results show that the model constructed in this study can accurately reflect the catalyst activity after hydrothermal aging.

Mechanistic studies on the conversion of NO gas on urea-iron and copper metal organic frameworks at low temperature conditions: in situ infrared spectroscopy and Monte Carlo investigations

Nitrogen oxide (NOx) emissions from high-temperature combustion processes under fuel-lean conditions continue to be a challenge for the energy industry. Selective catalytic reduction (SCR) is possible using metal oxides and zeolites. There is still a need to identify catalytic materials that are efficient in reducing NOx to environmentally benign nitrogen gas at temperatures lower than 200 °C. Metal-organic frameworks (MOFs) have emerged as a class of highly porous materials with unique physical and chemical properties. This study is motivated by the lack of systematic investigations on SCR using MOFs under industrially relevant conditions. Here, we investigate the extent of NO conversion with two commercially available MOFs, Basolite F300 (Fe-BTC) and HKUST-1 (Cu-BTC), mixed with solid urea as a source for the reductant, ammonia gas. For comparison, experiments were also conducted using cobalt ferrite (CoFe2O4) as a non-porous counterpart to relate its reactivity to those obtained from MOFs. Fourier-transform infrared spectroscopy (FTIR) was utilized to identify the gas and surface species in the temperature range of 115–180 °C. Computational analysis was performed using Monte Carlo simulations to quantify the adsorption energies of different surface species. The results show that the rate of ammonia production from the in situ solid urea decomposition was higher using CoFe2O4 than Fe-BTC and Cu-BTC and that there was very limited conversion of NO on the mixed solid urea-MOF systems due to site blocking. The main conclusions from this study are that MOFs have limited ability to convert NO under low-temperature conditions and that surface regeneration requires additional experimental steps.

Safe disposal of deactivated commercial selective catalytic reduction catalyst (V2O5-MoO3/TiO2) as a low-cost and regenerable sorbent to recover gaseous elemental mercury in smelting flue gas

The reduction of Hg emissions from non-ferrous metal smelting was proposed in the Minamata Convention. Regenerable sulfureted MoO₃/TiO₂, which displayed excellent performance in capturing gaseous Hg⁰, was once developed by us to recover gaseous Hg⁰ in smelting flue gas (SFG) for centralized control. Recently, a large amount of spent commercial selective catalytic reduction catalysts (for example V₂O₅-MoO₃/TiO₂) mostly deactivated by CaSO₄ was formed, creating a need for their safe disposal. As the main constituent of deactivated V₂O₅-MoO₃/TiO₂ is MoO₃/TiO₂, deactivated V₂O₅-MoO₃/TiO₂ was sulfureted to capture gaseous Hg⁰ from SFG for its safe disposal and the effects of V₂O₅ and CaSO₄ on Hg⁰ adsorption onto sulfureted MoO₃/TiO₂ were investigated. Although the capturing capacity of sulfureted MoO₃/TiO₂ moderately decreased after the impregnation of V₂O₅ and CaSO₄, sulfureted deactivated V₂O₅-MoO₃/TiO₂ still displayed excellent performance and reproducibility in gaseous Hg⁰ capture. Meanwhile, the cost performance of sulfureted deactivated V₂O₅-MoO₃/TiO₂ for Hg⁰ capture was outstanding as deactivated V₂O₅-MoO₃/TiO₂ needs to be safely disposed. Therefore, deactivated V₂O₅-MoO₃/TiO₂ can be sulfureted as a regenerable and low-cost sorbent that is effective in recovering gaseous Hg⁰ from SFG, as well as being a cost-effective and environmentally friendly method for the safe disposal of spent V₂O₅-MoO₃/TiO₂.

Starch bio-template synthesis of W-doped CeO2 catalyst for selective catalytic reduction of NOx with NH3: influence of ignition temperature

A novel tungsten-doped CeO₂ catalyst was fabricated via the sweet potato starch bio-template spread self-combustion (SSC) method to secure a high NH₃-SCR activity. The study focuses on the influence of ignition temperature on the physical structure and redox properties of the synthesized catalyst and the catalytic performance of NO_x reduction with NH₃. These were quantitatively examined by conducting TG-DSC measurements of the starch gel, XRD analysis for the crystallites, SEM and TEM assessments for the morphology of the catalyst, XPS and H₂-TPR measurements for the distribution of cerium and tungsten, and NH₃-TPD assessments for the acidity of the catalyst. It is found that the ignition temperature shows an important role in the interaction of cerium and tungsten species, and the optimal ignition temperature is 500 °C. The increase of ignition temperature from 150 °C is beneficial to the interactions of species in the catalyst, depresses the formation of WO₃, and refines the cubic CeO₂ crystallite. The sample ignited at 500 °C shows the biggest BET surface area, the highest surface concentration of Ce species and molar ratio of Ce³⁺/(Ce³⁺+Ce⁴⁺), and the most abundant surface Brønsted acid sites, which are the possible reasons for the superiority of the NH₃-SCR activity. With a high GHSV of 200,000 mL (g h)^-1 and the optimal ignition temperature, Ce₄W₂O_z-500 can achieve a steadily high NO_x reduction of 80% or more at a lowered reduction temperature in the range of 250~500 °C.

Outstanding performance of CuO/Fe–Ti spinel for Hg0 oxidation as a co-benefit of NO abatement: significant promotion of Hg0 oxidation by CuO loading

Conversion of gaseous Hg⁰ to soluble Hg²⁺ using selective catalytic reduction (SCR) catalysts with gaseous HCl as an oxidant as a co-benefit of NO abatement is widely used for resolving Hg pollution from coal-burning power plants.

Effect of iron minerals during coaling on the transformation of NO in the presence of NH3: Take pyrite as an example

Pyrite, a naturally occurring mineral, can be found extensively in coal. The change in the pyrite structure that occurs during coaling process, the ability of the pyrite-derived α-Fe₂O₃ to convert NO in the presence of NH₃ before catalyst bed and the kinetic study were investigated in this work. The pyrite-derived α-Fe₂O₃ was obtained by calcining at 500, 600, 700, 800 °C and was characterized by the X-ray diffraction (XRD), N₂ physisorption, the X-ray photoelectron spectrometer (XPS), the scanning electron microscope (SEM), UV-visible near-infrared spectroscopy (UV-vis DRS), the temperature-programmed desorption of ammonia (NH₃-TPD) and the in situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS). The results indicated that the α-Fe₂O₃ derived from natural pyrite exhibited an affirmative effect on NO conversion in the presence of NH₃ at reaction temperatures of 200-450 °C, particularly at 350 °C, the pyrite-derived α-Fe₂O₃ displayed the best efficiency for the NO conversion. In addition, the formed sulfate derived from the oxidation of pyrite enhanced the NO conversion at the temperature of 300-450 °C, while hinder the NO conversion at 200-275 °C. The in-situ DRIFTS and kinetic studies demonstrated that both the Eley-Rideal and Langmuir-Hinshelwood mechanism contributed to the selective catalytic reduction (SCR) of NO when the reaction temperature was over 200 °C, while selective catalytic oxidization (CO) happened over 300 °C. This study favored the understanding of the NO behavior in flue gas pipeline after sprawling NH₃ and the mechanism of NO conversion before the catalyst bed.

CeO2 grafted with different heteropoly acids for selective catalytic reduction of NOx with NH3

The CeO₂ catalysts grafted with heteropoly acid (i.e., HPA) could enhance their catalytic performance for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). In comparison to HSiW/CeO₂, HPMo/CeO₂, and commercial V₂O₅-WO₃/TiO_x catalysts, HPW/CeO₂ catalysts showed the best SCR performance. XPS and DRIFTS demonstrated that the amount of HPA on HPW/CeO₂ was more than those on HSiW/CeO₂ and HPMo/CeO₂. H₂-TPR results indicated that reducibility of HPMo/CeO₂ was stronger than those of HSiW/CeO₂ and HPW/CeO₂, resulting in the high-temperature performance loss. According to kinetic results, below 250 °C, k_SCR-ER and k_SCR-LH of HPW/CeO₂ were higher than those of HSiW/CeO₂, meanwhile k_side of both HSiW/CeO₂ and HPW/CeO₂ were low. Therefore, HPW/CeO₂ had the better SCR performance than HSiW/CeO₂. As NH₃ was completely consumed, SCR activity depended on the ratio of SCR reaction in the consumption of NH₃. The selectivity of SCR reaction, NSCR reaction, and C-O reaction of HSiW/CeO₂ were almost the same as those of HPW/CeO₂ above 250 °C, resulting in the NO_x conversion of HPW/CeO₂ was basically the same as that of HSiW/CeO₂ above 250 °C. Due to the lowest k_SCR-ER and k_SCR-LH, and highest k_side, NO_x conversion of HPMo/CeO₂ was the worst compared to HSiW/CeO₂ and HPW/CeO₂ catalysts.

Novel Synergistic Effect of Fe and Mo in FeMoSx/TiO2 for Recovering High Concentrations of Gaseous Hg0 from Smelting Flue Gas: Reaction Mechanism and Kinetics

There is a high demand for developing a more effective and environment-friendly technology to substitute the complicated and hazardous Boliden-Norzink technology for recovering gaseous Hg⁰ from smelting flue gas. In this work, a low-cost and reproducible sorbent (FeMoS_x/TiO₂) was developed to recover gaseous Hg⁰ from smelting flue gas. FeMoS_x/TiO₂ exhibited a superior ability for capturing high concentrations of Hg⁰, with an adsorption rate of 72.2 μg g^-1 min^-1 and a capacity of 41.8 mg g^-1 at 60 °C. These were generally larger than the sums of those of FeS_x/TiO₂ and MoS_x/TiO_2. The kinetic model of Hg⁰ adsorption by FeS_x/TiO₂, MoS_x/TiO₂, and FeMoS_x/TiO₂ were constructed according to the adsorption mechanism. Then, the structure-activity relationship of FeMoS_x/TiO₂ for Hg⁰ capture was determined by comparing the kinetic parameters. The intrinsic adsorption of Hg⁰ by MoS_x/TiO₂ (i.e., physically adsorbed Hg⁰ was oxidized by MoS₃ to HgS) was inhibited marginally after FeS_x was incorporated. However, another Hg⁰ adsorption route (i.e., physically adsorbed Hg⁰ was oxidized by FeS₂ to HgS) appeared on FeMoS_x/TiO₂. Its rate was significantly higher than that of FeS_x/TiO₂. Thus, a novel synergistic effect of Fe and Mo in FeMoS_x/TiO₂ for Hg⁰ capture was observed.

NH3-SCR of NO with novel active, supported vanadium-containing Keggin-type heteropolyacid catalysts

Supported vanadium-substituted Keggin polyoxometalates (POMs) were applied as catalysts for the selective catalytic reduction of NO using NH₃ as reductant (NH₃-SCR).

Sulfation effect of Ce/TiO2 catalyst for the selective catalytic reduction of NO x with NH3: mechanism and kinetic studies

Ceria-based catalysts are competitive substitutes for the commercial SCR catalysts due to their high SCR activity and excellent redox performance. For a better understanding of the SO2 poisoning mechanism over ceria-based catalysts, the sulfation effect of the Ce/TiO2 catalyst on the SCR activity over a wide reaction temperature range was systematically studied via comprehensive characterizations, in situ DRIFT studies and kinetic studies. The results demonstrated that the NO conversion at 150 °C is significantly inhibited by the formation of cerium sulfites/sulfates due to the inhibited redox properties and excessive adsorption of NH3, which restrict the dissociation of NH3 to NH2, resulting in a much lower reaction rate of E-R reaction over the sulfated Ce/TiO2 catalyst. With the increase in the reaction temperature, the reaction rate of the E-R reaction significantly increased due to the improved redox properties and weakened adsorption of NH3. Moreover, the rate of the C-O reaction over the sulfated Ce/TiO2 catalysts is obviously lower than that of the fresh Ce/TiO2 catalyst. The promotion of NO conversion over the sulfated catalyst at 330 °C is attributed to both the increase in the reaction rate of E-R reaction and the inhibition of the C-O reaction.

Balance between Reducibility and N2O Adsorption Capacity for the N2O Decomposition: CuxCoy Catalysts as an Example

Cu_xCo_y (CuO-Co₃O₄ mixed oxides) catalysts were prepared via co-precipitation for the N₂O decomposition reaction. They exhibited a higher N₂O decomposition activity than that of pure CuO and Co₃O₄ because of the balance of the redox property and N₂O adsorption capacity. Co₃O₄ presented a large number of surface oxygen vacancies, increasing the N₂O chemical adsorption as "□-Co-ON₂" on the catalyst surface, whereas CuO was dispersed around Co₃O₄ and presented high reducibility on the interface of Co₃O₄-CuO_x for the N-O break of N₂O, healing oxygen vacancies by leaving one oxygen atom in the vacancy. Based on kinetic studies, the rate constant of N₂O decomposition was related to the number of surface vacancy sites ([Mⁿ⁺]) and the rate of N-O break (k₃), whereas the rate-determining step is the N-O break. Therefore, the N₂O decomposition rate is first order to the N₂O concentration. Overall, both the density functional theory calculations and kinetic results indicate that the quantities of adsorption and activation sites derived from the interaction between Co and Cu (dual-function mechanism) were accounted for the excellent N₂O decomposition performance of Cu_xCo_y catalysts.

Effect of Transition Metal Additives on the Catalytic Performance of Cu–Mn/SAPO-34 for Selective Catalytic Reduction of NO with NH3 at Low Temperature

The adsorption of NO, NH3, H2O, and SO2 gaseous molecules on different transition metal oxides was studied based on density function theory (DFT), and three better-performing transition metal elements (Fe, Co, and Ce) were selected. Cu–Mn/SAPO-34 catalysts were prepared by impregnation method and then modified by the selected transition metals (Fe, Co, and Ce); the SO2 resistance experiments and characterizations including Brunner−Emmet−Teller (BET), X-ray Diffraction (XRD), Scanning Electronic Microscopy (SEM), and thermal gravity analysis (TG)-differential thermal gravity (DTG) before and after SO2 poisoning were conducted. The results showed that the deactivation of the Cu–Mn/SAPO-34 catalyst is ascribed to the deposition of lots of ammonium sulfates on the surface, depositing on the active sites and inhibiting the adsorption of NH3. After the modification of Fe, Co, and Ce oxides, the SO2 resistance of the modified Cu–Mn/SAPO-34 catalyst was significantly enhanced due to the less formation of ammonium sulfates. Among all these modified Cu–Mn/SAPO-34 catalysts, the Cu–Mn–Ce/SAPO-34 exhibited the highest SO2 resistance owing to the decreased decomposition temperature and the trapper of ceria for capturing SO2 to form Ce(SO4)2, further inhibiting the deposition of ammonium sulfates.

Optimal Design of a Tower Type SCR-deNOx Facility for a 1000 MW Coal-Fired Power Plant Based on CFD Simulation and FMT Validation

Selective catalytic reduction (SCR) is one of the most efficient methods to reduce NOx emissions from coal-fired power plants. This paper deals with an optimal design tower type SCR-deNOx facility for a 1000 MW coal-fired power plant. Combined with computational fluid dynamics (CFD), the configuration of the baffles geometry was studied with spatial constraints. Flow field was regulated at the ammonia injection grid (AIG) with the dual aim of reducing difficulties in implementing the non-uniformed ammonia (NH3) injection strategy and achieving a more homogeneous distribution at the catalyst entrance. A flow model test (FMT) was carried out at a laboratory scale to verify the design results. The results of the flow model test are in good agreement with the computational fluid dynamics. It is indicated that small-sized baffles are recommended for installation at the upstream side of the facility as the optimal design and ability to regulate the flow field at the ammonia injection grid makes it an effective way to deal with spatial constraints. This paper provides a good reference for optimizing the tower type SCR-deNOx facilities with spatial constraints.

Mechanistic Study of Selective Absorption of NO in Flue Gas Using EG-TBAB Deep Eutectic Solvents

The selective absorption of NO in flue gas has been investigated using a series of deep eutectic solvents (DESs) as novel denitrifying agents. The EG-TBAB DESs used in this work are composed of a hydrogen donor ethylene glycol (EG) and a parent salt tetrabutylammonium bromide (TBAB). Effects of DES composition (EG:TBAB molar ratio), operation temperature, residence time, and O₂ concentration in the flue gas on denitrification performances of EG-TBAB DESs have been investigated. The highest denitrification efficiency and capacity were achieved using EG to TBAB molar ratio of 50:1 at an operation temperature of 50 °C. The O₂ partial pressure in the flue gas showed no noticeable effects on NO absorption in EG-TBAB DESs. EG-TBAB DESs maintain high denitrification stability after five absorption-desorption cycles. The calculated absorption equilibrium constant ( K⁰) and Henry's law constant ( H) showed that EG-TBAB DESs exhibited high absorption capacity for NO molecules, indicating that they are applicable in industrial denitrification processes. The kinetics analysis of NO absorption in EG-TBAB DESs indicated that EG-TBAB DESs could effectively absorb NO and the absorption of NO was strongly influenced by mass transfer.

The effects of H2O on a vanadium-based catalyst for NH3-SCR at low temperatures: a quantitative study of the reaction pathway and active sites

The effects of H₂O on the adsorption amounts of NO, NO₂, and NH₃, NH₃-SCR reaction pathway and active site distribution over V₂O₅/WO₃–TiO₂ at low temperatures were quantitatively studied by the TRM and TPSR.

The Role of Fe2O3 Species in Depressing the Formation of N2O in the Selective Reduction of NO by NH3 over V2O5/TiO2-Based Catalysts

Promotion of 2.73% Fe2O3 in an in-house-made V2O5-WO3/TiO2 (VWT) and a commercial V2O5-WO3/TiO2 (c-VWT) has been investigated as a cost effective approach to the suppression of N2O formation in the selective catalytic reduction of NO by NH3 (NH3-SCR). The promoted VWT and c-VWT catalysts all gave a significantly decreased N2O production at temperatures >400 °C compared to the unpromoted samples. However, such a promotion led to the loss in high temperature NO conversion, mainly due to the oxidation of NH3 to N-containing gases, particularly NO. Characterization of the unpromoted and promoted catalysts using X-ray diffraction (XRD), NH3 adsorption-desorption, and Raman spectroscopy techniques could explain the reason why the promotion showed much lower N2O formation levels at high temperatures. The addition of Fe2O3 to c-VWT resulted in redispersion of the V2O5 species, although this was not visible for 2.73% Fe2O3/VWT. The iron oxides exist as a highly-dispersed noncrystalline α-Fe2O3 in the promoted catalysts. These Raman spectra had a new Raman signal that could be tentatively assigned to Fe2O3-induced tetrahedrally coordinated polymeric vanadates and/or surface V-O-Fe species with significant electronic interactions between the both metal oxides. Calculations of the monolayer coverage of each metal oxide and the surface total coverage are reasonably consistent with Raman measurements. The proposed vanadia-based surface polymeric entities may play a key role for the substantial reduction of N2O formed at high temperatures by NH3 species adsorbed strongly on the promoted catalysts. This reaction is a main pathway to greatly suppress the extent of N2O formation in NH3-SCR reaction over the promoted catalysts.

Elemental Mercury Oxidation over Fe-Ti-Mn Spinel: Performance, Mechanism, and Reaction Kinetics

The design of a high-performance catalyst for Hg⁰ oxidation and predicting the extent of Hg⁰ oxidation are both extremely limited due to the uncertainties of the reaction mechanism and the reaction kinetics. In this work, Fe-Ti-Mn spinel was developed as a high-performance catalyst for Hg⁰ oxidation, and the reaction mechanism and the reaction kinetics of Hg⁰ oxidation over Fe-Ti-Mn spinel were studied. The reaction orders of Hg⁰ oxidation over Fe-Ti-Mn spinel with respect to gaseous Hg⁰ concentration and gaseous HCl concentration were approximately 1 and 0, respectively. Therefore, Hg⁰ oxidation over Fe-Ti-Mn spinel mainly followed the Eley-Rideal mechanism (i.e., the reaction of gaseous Hg⁰ with adsorbed HCl), and the rate of Hg⁰ oxidation mainly depended on Cl^• concentration on the surface. As H₂O, SO₂, and NO not only inhibited Cl^• formation on the surface but also interfered with the interface reaction between gaseous Hg⁰ and Cl^• on the surface, Hg⁰ oxidation over Fe-Ti-Mn spinel was obviously inhibited in the presence of H₂O, SO₂, and NO. Furthermore, the extent of Hg⁰ oxidation over Fe-Ti-Mn spinel can be predicted according to the kinetic parameter k_E-R, and the predicted result was consistent with the experimental result.

Controllable synthesis of Ce-doped α-MnO2for low-temperature selective catalytic reduction of NO

On account of the highly active exposed Mn atom on α-MnO₂with a mesoporous channel, the catalyst exhibits high performance for NH₃-based selective catalytic reduction of NO.

Facile fabrication of hollow tubular mixed oxides for selective catalytic reduction of NOx at low temperature: a combined experimental and theoretical study

The 1D nanowire or hollow tubular structure of various transition metal oxides can be tuned by controlling heating rates.