Selective Catalytic Reduction Catalyst Research Articles

Recent advances in heighten sulfur resistance of SCR catalysts: A review

NOx removal by selective catalytic reduction (SCR) technology is a research hotspot in the field of environmental catalysis, and this method is dominated by catalysts. However, denitrification catalyst is easy to be polluted by the presence of SO2, which seriously restricts its practical industrial application. This review focuses on the latest domestic and foreign research results and advancement in improving sulfur resistance of deNOx catalysts, reveals the sulfur poisoning mechanism and regeneration process, as well as introduces the positive role of quantum chemistry in the field of sulfur resistance. In view of the questions set forth in this review, the future development direction of deNOx catalysts is prospected, which provides valuable scientific guidance for the design and development of efficient and practical sulfur resistant deNOx catalysts.

Cu- and Fe-speciation in a composite zeolite catalyst for selective catalytic reduction of NOx: insights from operando XAS

Cu-SAPO-34 (Cu-CZC) and Fe-mordenite (Fe-MOR) and their mechanical mixture (50 : 50) have been exhaustively investigated by means of operando X-ray absorption spectroscopy under NH3-SCR conditions.

High-temperature vanadium-free catalyst for selective catalytic reduction of NO with NH3 and theoretical study of La2O3 over CeO2/TiO2

SCR catalysts of the La2O3–CeO2/TiO2 series for de-NOx with NH3 were prepared and optimized. The Ce10La2 catalyst has a great NO conversion efficiency, good N2 selectivity, good SO2 resistance, and good anti-aging properties at high temperature.

Decision tree analysis on the performance of zeolite-based SCR catalysts

Urea-based selective catalytic reduction (SCR) is a promising method for removing NOx emissions. In the urea-based SCR method, zeolite-based catalysts are popularly used owing to their applicability over a wide range of temperatures compared to other catalysts. However, they still have several drawbacks such as inferior performance at low temperatures and thermal instability. This has subsequently led to numerous studies and experiments on the development of superior catalysts. While substantial experimental data exist on zeolite-based catalysts, extracting useful information from the data with a simple literature search is difficult owing to not only the amount of data but also complex correlations between feature, including preparation variables such as doped-metal loading and operational variables such as reaction temperature. Recently, extracting insights from a large database has become possible by utilizing machine learning tools. Among them, the decision tree can extract insights on the synthesis of the catalysts as the results derived from the models are intuitive and easy to interpret, unlike those from conventional discriminant machine learning models. In this study, classification models are obtained by training decision tree models with literature data on zeolite-based urea SCR catalysts, particularly the Beta and ZSM-5 types, and several experimental heuristics are extracted from the derived models.

Lithium cuprate, a multifunctional material for NO selective catalytic reduction by CO with subsequent carbon oxide capture at moderate temperatures

Li2CuO2 was evaluated as a possible catalyst for the NO selective catalytic reduction (NO SCR) by CO.

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.

Environment-Friendly Preparation of Adsorptive TiO <sub>2</sub>/SiO <sub>2</sub> Core-Shell Nanospheres and V2o5 Microrods from Spent SCR Catalysts by Ultrasonic-Coupled Alkali Leaching Process

Spent selective catalytic reduction (SCR) catalysts comprising of valuable V2O5 and TiO2 are usually disposed as hazardous waste because of the toxic V2O5 ingredients. Therefore, recovery of V2O5 and TiO2 from spent SCR catalysts has highly environmental benefits as well as economic value. In this study, an ultrasonic-coupled alkali leaching process was developed for environment-friendly preparation of adsorptive TiO2/SiO2 core-shell nanospheres and V2O5 microrods from spent SCR catalysts. The results show that the synergy of ultrasonic and alkali leaching significantly enhance the separation of V2O5 from spent SCR catalysts, and the addition of NaOH decreases by 70% compared with the traditional roasting-leaching process. The as-prepared adsorptive TiO2/SiO2 core-shell nanospheres exhibit superior adsorptive and photocatalytic performance for 2,4-dinitrophenol removal, and 99.9% of 2,4-dinitrophenol was oxidized within 120 min. In addition, high-purity V2O5 microrods with high H2 S removal rate were efficiently prepared from low-grade filtrate using a hydrothermal precipitation method, which was superior to the traditional NH4VO3 precipitation and solvent extraction process. This study provides a potential method for controlling the environmental pollution of V2O5 and recycling of valuable adsorptive TiO 2 /SiO 2 core-shell nanospheres and V2O5 microrods from spent SCR catalysts.

Insights into Reaction Conditions for Selective Catalytic Reduction of NOx with a MnAl Oxide Catalyst at Low Temperatures

ABSTRACT MnOx catalysts have been proven to have superior activity at low temperatures (LT) for selective catalytic reduction (SCR) of NOx. In this study, spherical mesoporous MnAlx catalysts with different molar ratios were prepared using a simple precipitation method. In addition, their NOx catalytic performance, along with relevant main factors (reaction temperature, molar ratio, particle size, space velocity, and O2 content), was investigated in a simulated fixed bed reactor. The relationship between the main factors and DeNOx efficiency was evaluated by analyzing two parameters: the NOx conversion rate and N2 selectivity. The results showed that the MnAl0.1 catalyst had the optimal low temperature DeNOx performance and had a relatively stable NOx conversion rate of more than 92% in a space velocity range of 14331 h–1 to 23885 h–1, where smaller catalyst particle size led to higher NOx conversion rate and N2 selectivity as high as 100%. In addition, the appropriate oxygen concentration was found to increase NOx conversion and N2 selectivity. This work would be beneficial to further optimization of LT SCR catalyst systems for practical applications.

Mechanism of the hydrocarbon resistance of selective catalytic reduction catalysts supported on different zeolites

Cu–Mn/SAPO-34 demonstrated excellent HC resistance, NOx complexes and NH3 species could be formed and SCR reaction proceed. While on Cu–Mn/ZSM-5, the active site was occupied by acrylate leading to SCR reaction blocked.

Optimization of SCR inflow uniformity based on CFD simulation

Abstract The inflow uniformity before selective catalytic reduction (SCR) catalyst carrier is a major issue for DeNO x capability of diesel engine after-treatment. Through the construction of the numerical model and CFD simulation of six perforated plate variations with different structural and positional characteristics, the influence of perforated plates on the uniformity of the airflow velocity at the inlet of the SCR catalyst carrier was analyzed. Comparison of different perforated plate variations shows that the encircling flow is a major hindrance to achieve higher inflow uniformity. Enclosed flow passage can remove the encircling flow and increase inflow uniformity at the cost of increased pressure drop. Rational layout of the perforated plate can achieve uniformity increase, while decrease pressure drop. High-velocity exhaust coupled with larger holes can improve both uniformity and pressure drop. The uniformity index increased from 97.6% of the original design to 98.7% of the optimized design, while pressure drop increased from 11.20 to 12.09 kPa. Weighing the relationship between inflow uniformity and pressure drop is an issue worthy of attention.

Active sites adjustable phosphorus promoted CeO2/TiO2 catalysts for selective catalytic reduction of NOx by NH3

Highly-efficient CeO2-TiO2-based catalyst is an attractive alternative to traditional V2O5-based catalysts for selective catalytic reduction of NOx by NH3 (NH3-SCR). Herein, we construct an efficient and stable P-doped CeO2/TiO2 (yCeTi-Px) catalyst by dispersing CeO2 on strongly acidic and highly stable mesoporous P-doped TiO2 (Ti-Px). The characteristic structure of Ti-Px causes highly dispersed CeO2, strong active component-support interaction, and abundant surface active oxygen and acid sites on CeTi-P catalyst. Therefore, yCeTi-Px catalyst exhibits much higher catalytic activity than yCeTi catalyst at temperature of 200–450 °C. The yCeTi-Px catalysts show high activity in the presence of H2O and strong resistance to H2O + SO2 at medium-high temperature, implying that they have great industrial application prospects. Different P/Ti ratio and CeO2 loading content were investigated. Excessive P doping can block the Ce4+/Ce3+ redox couple, and suppress redox sites on yCeTi-Px catalyst. Additionally, increasing CeO2 content can reduce the NH3 capacity of yCeTi-Px catalyst. By controlling the P/Ti ratio and CeO2 loading content, surface acid sites and redox sites on yCeTi-Px catalyst can be balanced to obtain the best catalytic activity. This work demonstrates a feasible method in adjusting acid sites and redox sites on CeTi catalyst, which ultimately contributes to the new design of CeO2-TiO2-based catalyst.

Simulation Study of Ammonia Storage Characteristics of Selective Catalytic Reduction for Diesel Engines

Non-road machinery is very important, but non-road machinery has problems such as working under complex operating conditions. Agricultural machinery is an important type of non-road machinery, which mainly relies on diesel engines due to their high fuel efficiencies. The exhaust temperature of agricultural machinery changes drastically under different working conditions. This article aims to research on pollution control of non-road machinery. Among the exhaust after treatment technologies, selective catalytic reduction (SCR) technology is a very important method to reduce NOx. In this study, a series of tests was conducted to investigate the effects of temperature and exhaust gas mass flow on the NOx conversion and dynamic ammonia storage space distribution characteristics in simulation models. The genetic algorithm, a kind of global optimization algorithm that can avoid local optimization, was used in the parameter identification process. The simulation results showed that the ammonia storage capacity of SCR catalyst decreases as the temperature of SCR increases. With the decrease in temperature, the NH3 desorption rate gradually decreased. An uneven distribution of ammonia storage similar to the shape of ɛ was observed in the simulation and this kind of uneven distribution is obvious at low temperature. The effective area of the catalyst is near the inlet of the catalyst. This work presents an approach to help figure out the details in the ammonia storage process. This work provides information about the dynamic ammonia storage characteristics and guide for better SCR design.

The microscopic oxidation mechanism of NH3 on CuO(111): A first-principles study

Understanding the oxidation of ammonia (NH3) over CuO surface and then the formation routes of N2 and NOx is rather crucial to provide a favorable direction for the rational design of high-performance Cu-based oxygen carriers in chemical looping combustion (CLC) and CuO-containing catalysts in selective catalytic reduction (SCR). This study aims to investigate the reaction mechanisms of nitrogen-containing species using density functional theory (DFT) calculations. The potential dehydrogenation pathway is identified as NH3* → NH2* + H* → NH(1)* + 2H* → N(2)* + 3H*, and the rate-determined step is the NH2* dehydrogenation. Additionally, we consider 10 dominating elementary reactions for the formation of N2, NO, NO2 and N2O; two skeletal schemes of the NH3 oxidation under low or high temperature conditions are then proposed. Under the low temperature condition of SCR, the majority of gaseous N2 comes from the Eley–Rideal reaction between NH2* fragment and gaseous NO, while the lateral recombination of N* to form N2 might play a more crucial role under the high temperature condition of CLC. The high temperature and surface adsorbed oxygen provide positive impacts on the yield of gaseous NO and NO2, respectively. Finally, the effects of O2 and H2O on the fate of nitrogen during heterogeneous reactions have also been determined.

Insight into the promotion mechanism of activated carbon on the monolithic honeycomb red mud catalyst for selective catalytic reduction of NOx

Insight into the promotion mechanism of activated carbon on the monolithic honeycomb red mud catalyst for selective catalytic reduction of NOx

Effect of Calcination Temperature on the Activation Performance and Reaction Mechanism of Ce-Mn-Ru/TiO2 Catalysts for Selective Catalytic Reduction of NO with NH3.

In this study, anatase TiO2-supported cerium, manganese, and ruthenium mixed oxides (CeOx–MnOx–RuOx/TiO2; CMRT catalysts) were synthesized at different calcination temperatures via conventional impregnation methods and used for selective catalytic reduction (SCR) of NOx with NH3. The effect of calcination temperature on the structure, redox properties, activation performance, surface-acidity properties, and catalytic properties of the CMRT catalysts was investigated. The results show that the CMRT catalyst calcined at 350 °C exhibits the most efficient low-temperature (<120 °C) denitration activity. Moreover, the selective catalytic oxidation (SCO) reaction of ammonia is intensified when the reaction temperature is >200 °C, which leads to a decrease in the N2 selectivity of the CMRT catalysts and further results in an increase in the production of NO and N2O byproducts. X-ray photoelectron spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy show that the CMRT catalyst calcined at 350 °C contains more Ce4+, Mn4+, Ru4+, and lattice oxygen, which greatly improve the catalyst’s ability to activate NO that promotes the NH3-SCR reaction. The Run+ sites of the CMRT catalyst calcined at 250 °C are the competitive adsorption sites of NO and NH3 molecules, while those of the CMRT catalyst calcined at 350 and 450 °C are active sites. Both the Langmuir–Hinshelwood (L–H) mechanism and the Eley–Rideal (E–R) mechanism occur on the surface of the CMRT catalyst at the low reaction temperature (100 °C).

Theoretical insights into the poisoning effect of Na and K on α-Fe2O3 catalyst for selective catalytic reduction of NO with NH3

The poisoning effect of potassium (K) and sodium (Na) on the α-Fe2O3 catalyst in the ammonia-selective catalytic reduction (NH3-SCR) for the NO reduction was investigated by the density functional theory calculation. The results show that, in the presence of alkali metals, the strength of the Lewis acid site on the surface is not affected, while the strength of the Brønsted acid site is significantly inhibited, which thus decreases the de-NOx performance of the α-Fe2O3 catalyst. Both K and Na enhance the NOx (NO and NO2) adsorption capacity and inhibit the O2 adsorption capacity of the α-Fe2O3 catalyst. These data indicate that NO and NH3 form competitive adsorption, and the ‘Fast-SCR’ reaction is inhibited in the alkali metal poisoned systems. In addition, the alkali metals don't affect the dehydrogenation of NH3, but inhibit the formation of H2O which is the rate determination step of NH3-SCR for α-Fe2O3 catalyst.

Effects of hydrogen addition into liquefied petroleum gas reductant on the activity of Ag–Ti–Cu/Cordierite catalyst for selective catalytic reduction system

In this study, low temperature activity of Ag–Ti–Cu/Cordierite catalyst was investigated with liquefied petroleum gas (LPG) and hydrogen-liquefied petroleum gas (H2-LPG) mixture as reductant. The selective catalytic reduction (SCR) catalyst was synthesized by impregnation method and characterized by Brunauer-Emmett-Teller (BET), Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) analyzes. BET analysis of the catalyst revealed surface area as 12.89 m2/g. Silver (Ag), titanium (Ti) and copper (Cu) nanoparticles were observed on the catalyst surface with SEM analysis. XRD analysis showed high dispersion of catalytic elements. The SCR performance tests were carried out at 170–270 °C temperature range, 30,000 h−1 and 40,000 h−1 space velocities, 1 kW, 2 kW, 3 kW and 4 kW engine loads with diesel engine real exhaust gas sample. NOx conversion efficiency increased significantly in the presence of H2, especially at low exhaust temperatures. The maximum NOx conversion ratio was obtained as 89.53% with H2-LPG reductant at 270 °C, 4 kW engine load and 30,000 h−1 space velocity.

Quantitative Cu Counting Methodologies for Cu/SSZ-13 Selective Catalytic Reduction Catalysts by Electron Paramagnetic Resonance Spectroscopy

Two Cu/SSZ-13 selective catalytic reduction (SCR) catalysts with distinct Si/Al ratios and isolated Z2Cu and ZCuOH distributions are prepared for in situ electron paramagnetic resonance (EPR) spectroscopic studies. These in situ studies include dehydration, titration of dehydrated samples with NO+O2 and NH3, titration of NH3-saturated samples with NO+O2, and finally steady-state standard NH3-SCR reaction. During dehydration, EPR-active hydrated ZCuOH loses H2O ligands and becomes EPR-silent due to a pseudo Jahn–Teller effect; a portion of ZCuOH also undergoes autoreduction to ZCu(I) species, a process that also induces EPR invisibility. During NO+O2 treatment of dehydrated samples, ZCu(I) species are oxidized to Cu(II)–NO3– species, regaining EPR visibility. During NH3 titration, EPR-silent dehydrated ZCuOH can also regain EPR visibility by coordinating with NH3 ligands. During NO+O2 titration of NH3-saturated samples, EPR-active Cu contents first decrease due to Cu(II) reduction to Cu(I) and then increase due to Cu(II)–NO3– species formation. However, the Cu(II)–NO3– formation chemistry is substantially slower for the Si/Al = 36 catalyst. In steady-state SCR studies, the EPR-active content decreases with increasing temperature in the kinetically controlled low-temperature regime and becomes largely invariant in the mass transfer-limited regime. Importantly, Cu sites in the SCR more active Si/Al = 6 catalyst display substantially higher EPR visibility than the SCR less active Si/Al = 36 catalyst at any reaction temperatures tested. The higher Cu loading for the former catalyst is believed to be the key for this difference.

The promoting mechanism of in situ Zr doping on the hydrothermal stability of Fe-SSZ-13 catalyst for NH3-SCR reaction

Fe-SSZ-13 has been proven to be a highly efficient catalyst for the ammonia selective catalytic reduction (NH3-SCR) of NOx, especially at medium and high reaction temperatures. Nevertheless, the hydrothermal stability of Fe-SSZ-13 catalyst is inferior, limiting its application in vehicle exhaust purification. Impressively, it is found that the hydrothermal stability of Fe-SSZ-13 can be remarkably enhanced (as high as 30 % improvement at 300 °C) by in situ introduction of Zr and Fe into initial gels via one-pot synthesis in this work. A small amount of Zr addition (0.31 wt.%) is beneficial for improving the relative crystallinity of Fe-SSZ-13. The characterization results (obtained by 27Al NMR, NH3-TPD, UV–vis, XPS and in situ DRIFTS) revealed that Zr doping by a one-pot method would not only enhance the dispersion of active isolated Fe ions but also inhibit the migration and agglomeration of isolated Fe ions during hydrothermal aging, improving the stability of the catalyst. Additionally, the introduction of Zr generates new hydroxy groups and Lewis acid sites, facilitating the formation of reaction intermediates, such as -NH2, -HNO2, -NO2, etc., and promoting the SCR reaction especially over the aged catalyst.

SO2 Poisoning and Recovery of Copper-Based Activated Carbon Catalysts for Selective Catalytic Reduction of NO with NH3 at Low Temperature

A series of materials based on activated carbon (AC) with copper deposited in various amounts were prepared using an incipient wetness impregnation method and tested as catalysts for selective catalytic reduction of nitrogen oxides with ammonia. The samples were poisoned with SO2 and regenerated in order to analyze their susceptibility to deactivation by the harmful component of exhaust gas. NO conversion over the fresh catalyst doped with 10 wt.% of Cu reached 81% of NO conversion at 140 °C and about 90% in the temperature range of 260–300 °C. The rate of poisoning with SO2 was dependent on Cu loading, but in general, it lowered NO conversion due to the formation of (NH4)2SO4 deposits that blocked the active sites of the catalysts. After regeneration, the catalytic activity of the materials was restored and NO conversion exceeded 70% for all of the samples.