SCR Catalyst Research Articles

Poisoning mechanism of different Cd precursors on Fe-Ce/TiO2 catalyst for selective catalytic reduction of NOx with NH3

As one of the most harmful heavy metals, Cd can present in many chemical forms in real flue gas and may deactivate the SCR catalysts, but the poisoning mechanism of different Cd species over the Fe-Ce/TiO2 catalysts remains unclear. Therefore, in our work, Fe-Ce/TiO2 catalyst is prepared and doped with Cd(NO3)2, CdCl2, and CdSO4, respectively, aiming to investigate the effect of different Cd precursors on the catalytic performance, physicochemical properties, and poisoning mechanisms of Fe-Ce/TiO2 catalyst. The results demonstrate that different Cd precursors exert different influences on the catalytic activity of the catalyst and the deactivation sequence is CdSO4 <Cd(NO3)2 <CdCl2. The doping of Cd decreases the specific surface area, promotes the grain growth of active species, and results in the aggregation of surface active species to varying degrees. Although the total H2 consumption of Cd poisoned catalysts is enhanced, they are not originated from more amounts of surface active species and thus it does not contribute to the redox cycles and catalytic activity of catalysts. The doping of Cd(NO3)2 decreases the surface acidity of catalysts, while CdCl2 and CdSO4 promote surface acidity, but the adsorbed NH3 over CdCl2 poisoned catalyst is unreactive. In addition, both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms are involved in the SCR reaction over the Fe-Ce/TiO2 catalyst and different Cd poisoning makes no change to the reaction mechanism.

NOx reduction against alkali poisoning over Ce(SO4)2-V2O5/TiO2 catalysts by constructing the Ce4+–SO42− pair sites

Commercial V2O5-based catalysts have been successfully applied in NH3 selective catalytic reduction (NH3-SCR) of NOx from power stations, but their poor alkali-resistance restrains the wider application in nonelectrical industries. In this study, NOx reduction against alkali poisoning over V2O5/TiO2 is greatly improved via Ce(SO4)2 modification. It has been originally demonstrated that Ce4+–SO42− pair sites play crucial roles in improving NOx reduction against alkali poisoning over V2O5/TiO2 catalysts. The strong interaction between V species and Ce sites of Ce4+–SO42− pairs triggers the reaction between NH4+species and gaseous NO via Eley-Rideal (E-R) reaction pathway. After K-poisoning, the SO42− sites of Ce4+–SO42− pairs as protective sites strongly bond with K and thus maintain the high reaction efficiency via the E-R reaction pathway. This work demonstrates an effective strategy to enhance NOx reduction against alkali poisoning over catalysts via constructing Ce4+–SO42− pair sites, contributing to developing alkali-resistant SCR catalysts for practical application in nonelectrical industries.

Recent advances in Pb resistance over SCR catalysts: Reaction mechanisms and anti-inactivation measures

The study of SCR catalysts for nitrogen oxides reduction with NH3 had recently attracted much attention. As a typical heavy metal element, Pb was prone to seriously damage the denitrification performance of SCR catalysts in flue-gas environment. This work systematically reviewed the mechanisms of Pb poisoning and anti-deactivation measures reported these years for SCR catalysts. The mechanisms of Pb poisoning were summarized from both physical and chemical aspects, including the specific surface area, redox ability and surface acidity of the catalysts, and the corresponding reaction paths of the catalysts were also considered during the SCR process. In the meantime, this paper reviewed the anti-deactivation mechanisms and measures currently used to solve the problem of lead poisoning for SCR catalysts. The main methods included adding active substances to improve the redox ability and surface acidity of the catalysts, using ions or anti-poisoning sites to preferentially bind to Pb species, or using the structural characteristics of the carriers to prevent Pb from contacting with active substances. The concrete measures included the modification of catalysts with metal oxides and solid super acids, the change of carriers’ structure, and the dual effect of carriers and active materials. Finally, a prospect of lead poisoning for further research on SCR catalysts was proposed.

Three-dimensionally ordered macroporous (3DOM) structure promoted the activity and H2O poisoning resistance of CeMn/3DOM-TiO2 catalyst in NH3-SCR

NH3-SCR is an effective mean of NOx removal in the non-electric industry, however, the high activation temperature and poor H2O resistance of SCR catalysts posed a barrier to its application. In this work, a series of three-dimensionally ordered macroporous (3DOM) catalysts were synthesized via a colloidal crystal template (CCT) method, and various characterizations were carried out to explore the physicochemical property of catalysts. The experiment results reveal that Ce0.2Mn0.2/3DOM-TiO2 catalyst presents the excellent low-temperature catalytic activity of nearly 100% at 100 °C. Furthermore, the enhanced H2O resistance is achieved, certified by the unaffected NO remove at 150 °C in the participation of 15 vol% H2O. The characterizations results exhibit that the improved dispersion of the active component and enhanced redox ability are conducive to the low-temperature catalytic activity. N2 adsorption and desorption experiments indicate that catalyst with 3DOM support possesses a larger pore diameter and specific surface area, which may weaken the condensation of H2O in the microporosity of catalysts and improved the H2O resistance of the catalyst. In situ DRIFTS results manifest that Ce0.2Mn0.2/3DOM-TiO2 catalyst could not only absorb more NH3 and generate more surface-active sites, but inhibit the competitive adsorption between H2O and SCR reactants.

Efficient NOx Reduction against Alkali Poisoning over a Self-Protection Armor by Fabricating Surface Ce2(SO4)3 Species: Comparison to Commercial Vanadia Catalysts.

Resolving severe deactivation by alkali metals for selective catalytic reduction of NOx with NH3 (NH3-SCR) is challenging. Herein, surface Ce2(SO4)3 species as a self-protection armor originally exhibited antipoisoning of potassium over ceria-based catalysts. The self-protection armor was also effective for other alkali (Na), alkali-earth (Ca), and heavy (Pb) metals, considerably resolving the deactivation of ceria-based SCR catalysts in practical applications. The catalytic activity tests indicated that the presence of ∼0.8 wt % potassium did not deactivate sulfated CeO2 catalysts, yet commercial V2O5-WO3/TiO2 catalysts almost lost the NOx conversions. Potassium preferably bonded with surface sulfates to form K2SO4 accompanied with the majority of surface Ce2(SO4)3 over sulfated CeO2 catalysts, but preferably coupled with active vanadia to generate inactive KVO3 species over V2O5-WO3/TiO2 catalysts. Such an active Ce2(SO4)3 species facilitated the adsorption and reactivity of NH3 and NOx, enabling ceria catalysts to maintain high catalytic efficiency in the presence of potassium. Conversely, the introduction of potassium into V2O5-WO3/TiO2 catalysts caused a considerable loss of surface acidity, hindering catalyst reactivity during the SCR reaction. The self-protection armor of Ce2(SO4)3 species may open a promising pathway to develop efficient ceria-based SCR catalysts with strong antipoisoning ability.

Optimization and comprehensive mechanism of environment-friendly bimetal oxides catalysts for efficient removal of NO in ultra-low temperature flue gas

The catalytic purification of NOx at ultra-low temperature (less than100 °C) is still a significant challenge so far. In this work, a new UT-USCR (ultra-low temperature SCR with urea) technology scheme is adopted to directly utilize the amino –NH2 in urea to carry out the catalytic reduction of NOx, thus realizing the efficient removal of NOx at 50 ∼ 100 °C and saving the energy consumption required for urea decomposition in the industrial NH3-SCR process. Specifically, novel binary UT-USCR nanocatalysts preloaded with reducing agent were successfully synthesized by simple solvent evaporation-induced assembly method and systematically optimized. The catalysts were characterized in detail by FESEM, EDS, TEM, XRD, XPS, AAS, TPD, H2-TPR, in-situ DRIFTS, etc. The activity results showed that the optimized catalyst could achieve over 95 % NOx removal at 50 ∼ 100 °C and had good cycle stability. Through the research path of “exclusion-verification-thermodynamic theoretical analysis-transient &in-situ experimental analysis”, the rate-limiting step and specific reaction path on the surface of UT-USCR catalyst were expounded in detail for the first time. The specific synergistic path between the two active metal elements was revealed and the modified catalyst enhanced the chemical adsorption of NO, promoted the direct reaction of NO with reductant urea and the formation of NO2, thus promoting the UT-USCR reaction. More interestingly, the SCR catalyst in this work is a new type of catalyst that does not require acid sites, and the research of UT-USCR catalyst can provide ideas for the future development of anti-poisoning SCR catalysts in environments prone to acid site poisoning.

Catalytic performances of Cu-ZK-5 zeolites with different template agents in NH3-SCR

Cu-ZK-5 catalyst can be successfully synthesized by using inorganic templates with low cost and organic templates with low toxicity, respectively. Based on this, this paper has conducted research on the small-pore ZK-5 zeolite (KFI topology) to explore its feasibility as a diesel vehicle SCR catalyst. The T90 and T50 of Cu-ZK-5 catalysts prepared by organic template were 180–550 ℃ and 145 ℃, respectively. Meanwhile, the Cu-ZK-5 (org.) possesses better hydrothermal stability and better SO2-resistance ability. The main reasons are that the Cu-ZK-5 (org.) had higher crystallinity, more surface acid sites and isolated Cu2+ species.

Burner based thermal management approach utilizing in-exhaust burner technology with a CDA equipped engine

Commercial vehicles require fast aftertreatment heat-up to move the SCR catalyst into the most efficient temperature range to meet upcoming NOX regulations while minimizing CO2. One solution to this challenge is to add a fuel burner upstream of the con`ventional heavy-duty diesel aftertreatment system. The focus of this paper is to optimize a burner based thermal management approach. The objective included complying with CARB’s 2027 low NOX emissions standards for on-road heavy duty diesel engines. This was accomplished by pairing the burner system with cylinder de-activation on the engine and/or a light-off SCR sub-system. A system solution is demonstrated using a heavy-duty diesel engine with an aged aftertreatment system targeted for 2027 emission levels using various levels of controls. The baseline layer of controls includes cylinder deactivation to raise the exhaust temperature more than 100°C in combination with elevated idle speed to increase the mass flowrate through the aftertreatment system. The combination of operating the fuel burner, cylinder deactivation and elevated idle speed (during cold start) allows the aftertreatment system to heat up in a small fraction of the time demonstrated by today’s systems. Performance was quantified over the cold FTP, hot FTP, low load cycle (LLC) and the U.S. beverage cycle. The improvement in NOX reduction and the CO2 savings over these cycles are highlighted.

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.

Revealing the effect of crystal size on the high-temperature hydrothermal stability of Cu/SSZ-13 NH3-SCR catalysts

The large-sized Cu/SSZ-13 having a lower amount of Al–OH keeps a relative complete CHA structure after aging. The complete structure is beneficial to the re-dispersion process of copper species and SCR performance during aging.

Modeling of the Rates of Evaporation and Decomposition of Urea on SCR Catalyst

Modeling of the Rates of Evaporation and Decomposition of Urea on SCR Catalyst

Mechanistic insight into low temperature SCR by ceria–manganese mixed oxides incorporated into zeolites

The low temperature activity of zeolite-based SCR catalysts can be improved through incorporation of a metal oxide. The metal oxide catalyzes NO oxidation and prevents NH4NO3 deposits, while the zeolite phase provides NH3 adsorption sites.

Active sites of NO selective catalytic reduction over V2O5–WO3/TiO2

Heterodinuclear vanadium–tungsten sites are the most active catalytic sites of conventional V2O5–WO3/TiO2 catalysts in the SCR reaction for efficient NO emission control. Thus, we propose a new strategy to improve conventional SCR catalysts.

Ultra-high hydrothermal stability of Ce-based NH3-SCR catalyst for diesel engines: A substitute for Cu zeolites

Hydrothermal stability is one of the important bottlenecks limiting the application of NH3-SCR catalysts in NOx abatement. Different acid groups are used to modify CeO2 which can greatly increase the number of surface oxygen vacancies and acid sites, thus improving the redox properties and acidity of Ce-based catalysts. Therefore, all the Ce-PO43-, Ce-SO42- and Ce-WO3 catalysts demonstrate excellent catalytic activities. However, only the Ce-PO43- catalyst shows superior hydrothermal stability and achieves > 80 % NOx conversion between 325 °C and 475 °C after hydrothermal treatment at 800 °C and 850 °C. The hydrothermal process resulted in the decomposition of sulfates and the sintering of WO3, leading to the decrease of oxygen vacancies and the weakness of acid sites. Nevertheless, the Ce-PO43- catalyst could maintain these properties due to the ultra-high stable structure P-Ce-Ov, which has the potential to replace the commercial Cu-SSZ-13. It provides a strategy to improve the hydrothermal stability of metal oxide SCR catalysts to satisfy the application in removing NOx from vehicles.

NO&lt;sub&gt;x&lt;/sub&gt; Reduction Characteristics of Ship Power Generator Engine SCR Catalysts according to Cell Density Difference

NO&lt;sub&gt;x&lt;/sub&gt; Reduction Characteristics of Ship Power Generator Engine SCR Catalysts according to Cell Density Difference

Mass transfer of multi-pollutants over titania-based SCR catalyst: A molecular dynamics study

Mass transfer can significantly affect the SCR process which is designed for NOx removal. However, it is still challenging to characterize the transport of gaseous species involved in the process, especially at nano-scale. This work probes into the application of non-equilibrium molecular dynamics (NEMD) simulations to study the mass transfer of multi-pollutants over a titania-based catalyst. A dual control-volume (DCV) model was proposed to simulate transport of typical gaseous molecules (e.g. NO, NH3 and SO2). The impacts of temperature, pore width, hydroxyl sites and competitive diffusion on diffusivity of objective molecules were studied in details. The results showed that temperature and surface sites could affect NH3 more significantly than NO and SO2, yet the influence of surface sites was strongly size-dependent. The reduction in NH3 diffusivity caused by the presence of surface sites decreased from 32.37 % to 2.97 % when the pore width grows from 25 Å to 75 Å. The competitive transport between NH3 and SO2 has also mitigated the impacts of surface sites on both molecules.

Elimination of PCDD/Fs over Commercial Honeycomb-Like Catalyst of V2O5-MoO3/TiO2 at Low Temperature: From Laboratory Experiments to Field Study

With the need for ultra-low emissions and the strict regulation of PCDD/Fs from MSWI plants, traditional SCR catalysts have been applied to remove PCDD/Fs. In this study, we compared one typical commercial V2O5-MoO3/TiO2 catalyst’s performance in removing PCDD/Fs under laboratory and industrial conditions. Various characterization methods like XRF, XPS, BET, and H2-TPR were applied to analyze the catalyst’s properties. The laboratory results showed that the adsorption could significantly affect the removal at low temperatures. The RE on PCDD/Fs was 59.4% (55.0% for toxicity RE), 88.5% (90.3%), and 78.0% (76.0%) at 160 °C, 180 °C, and 200 °C, respectively, showing that 180 °C is the most suitable operation temperature for this V2O5-MoO3/TiO2 catalyst. The field study was conducted at 180 °C, and the results revealed that the competition between water vapor and the interaction of SO2 could lower the RE. However, comparisons between laboratory and field conditions showed that this V2O5-MoO3/TiO2 catalyst still showed good stability, with only a 6.8% drop.

Understanding the temperature-dependent H2O promotion effect on SO2 resistance of MnOx-CeO2 catalyst for SCR denitration

Understanding how H2O affects SO2 tolerance of SCR catalysts at different working temperatures is of great importance. Herein, we reported that H2O addition at ultra-low temperature of 100 °C induced a significant promotion on SO2 tolerance of MnOx-CeO2 catalysts. Sole SO2 led to serious activity loss from 100% to 20%, while H2O coexistence alleviated the deactivation and an admirable activity of 60% was reserved. Combining experimental characterizations with DFT calculation, we demonstrated that H2O promoted hydroxyl formed on defect sites, which promoted more sulfite formation and retarded sulfates with strong electron-withdrawing capacity disturbing active Mn sites. In comparison with sulfates, sulfites exhibited less interference in adsorbed species, ensuring the occurrence of Eley-Rideal and Langmuir-Hinshelwood mechanisms. The present study discloses the unique temperature-dependent effect of H2O on SO2 tolerance, which is expected to deepen our understanding on the role of H2O in modulating surface species and catalytic behaviors of SCR catalysts.

Effective Mixer Design an Important Factor In SSCR Systems for Reduction of NOx from Exhaust of Diesel Engines

In recent decades, the environment has been seriously polluted by the hazardous exhaust components of diesel engines. The international community, which is dedicated to preserving the harmony between nature and humanity, has taken this seriously and imposed strict regulations on Diesel engine manufacturers regarding the quantity of exhaust components from Diesel engines that may apply to the standards of EURO-VI. The SCR technology attempted to reduce the problem somewhat, but the associated problems of solid particle formation on the pipe walls, ammonia slip, and incomplete NOx reduction led to the development of new technology - solid selective catalytic reduction. The use of solid ammonium salt for ammonia generation has shown better results in NOx reduction and reduction of solid particle formation compared to SCR. However, it was not possible to fully resolve the ammonia slip issue. A uniform flow rate of ammonia through the SCR catalyst can reduce NOx efficiently. In this paper, the role of mixer design in achieving a uniform flow rate of ammonia is investigated in detail. The results show that an optimized mixer design leads to efficient reduction of NOx and thus reduces ammonia slip to a great extent. When the mixer is placed near the ammonia injection point, the most homogeneous ammonia distribution is achieved for flow through the SCR catalyst.

Application progress of small-pore zeolites in purifying NOx from motor vehicle exhaust

• Small-pore SSZ-13 and SSZ-39 zeolites have unique advantages in purifying NOx. • Pd/zeolites as PNAs present the superior low-temperature NOx adsorption activity. • Non-precious metal Co/zeolite is an ideal substitute for Pd/PNAs to reduce the cost. • Cu/zeolites possess the excellent high-temperature NH3-SCR catalytic activity. • Fe/Mn/Ce doped Cu/zeolites can broaden the NH3-SCR active temperature window. Lean-burn mode results in the high NOx emission of diesel vehicles despite its high fuel economy. NOx can be effectively removed by the SCR unit during normal operation, however, elimination during the cold start period needs to be addressed soon. Passive NOx adsorber (PNA) was proposed to adsorb and store NOx emitted during the cold start period. With the increase of exhaust gas temperature during normal operation, the adsorbed NOx will be quickly released and efficiently converted to N 2 when flowing through the downstream SCR catalyst. After years’ investigation, small-pore zeolites such as CHA and AEI have been found to possess superior hydrothermal stability and NOx selectivity compared with medium and large pore zeolites, and the former’s eight-membered ring structure effectively exclude large hydrocarbons from entering the zeolite channels, making them play a prominent role in NOx purification via both PNA and SCR units. SSZ-13 and SSZ-39 zeolites, which belong to CHA and AEI topologies respectively, has been confirmed to exhibit excellent low-temperature NOx adsorption performance and high-temperature SCR denitrification performance. In the field of purifying NOx from the diesel vehicle exhaust, the review simultaneously focused on the application of SSZ-13 and SSZ-39 zeolites in NOx purification from two aspects of PNA and SCR in recent decades. In addition, the unique advantages of their modified form of loading Pd/Co as PNAs, or loading Cu doped with Fe/Mn/Ce as SCR catalysts were systematically clarified, in order to provide some advanced guidance for the researchers in the purification of NOx from the diesel engine exhaust.