NH3-SCR reaction mechanisms of NbO /Ce0.75Zr0.25O2 catalyst: DRIFTS and kinetics studies

One-Pot Synthesis of Ce-SSZ-39 Zeolite with Performance in the NH3-SCR Reaction

Cu-SSZ-39 zeolite with 8-membered rings is regarded as a very promising catalyst in the NH3-SCR reaction, but its hydrothermal stability still remains to be improved. One of the solutions to promote hydrothermal stability is the insertion of rare earth elements in the product. Nevertheless, normal ion exchange of rare earth elements limits their contents in the zeolite product due to their large hydrated ionic radius and alkaline environment under hydrothermal conditions. Herein, we for the first time present a new method for the one-pot synthesis of Ce-SSZ-39 zeolite under solvent-free conditions. The key to success is the use of Ce-FAU zeolite as a precursor. The obtained product shows good crystallinity, sheet-like morphology, large BET surface area, and 4-coordinated Al species. Detailed investigations illustrate that Ce species in the Cu/Ce-SSZ-39 zeolite micropore can prevent the dealumination and thus formation of CuAlOx species during hydrothermal aging at 850 °C for 16 h, giving the excellent hydrothermal stability and thus showing the excellent catalytic performance in the NH3-SCR reaction. One-pot synthesis of Ce-SSZ-39 zeolite with excellent catalytic performance might open a new door for developing very efficient selective catalytic reduction (SCR) catalysts in near future.

Phosphotungstic Acid as a Dechlorination Agent Collaborates with CeO2 for Synergistic Catalytic Elimination of NOx and Chlorobenzene

The development of efficient technologies for the synergistic catalytic elimination of NOx and chlorinated volatile organic compounds (CVOCs) remains challenging. Chlorine species from CVOCs are prone to catalyst poisoning, which increases the degradation temperature of CVOCs and fails to balance the selective catalytic reduction of NOx with the NH3 (NH3-SCR) performance. Herein, synergistic catalytic elimination of NOx and chlorobenzene has been originally demonstrated by using phosphotungstic acid (HPW) as a dechlorination agent to collaborate with CeO2. The conversion of chlorobenzene was over 80% at 270 °C, and the NOx conversion and N2 selectivity reached over 95% at 270-420 °C. HPW not only allowed chlorine species to leave as inorganic chlorine but also enhanced the Bro̷nsted acidity of CeO2. The NH4+ produced in the NH3-SCR process can effectively promote the dechlorination of chlorobenzene at low temperatures. HPW remained structurally stable in the synergistic reaction, resulting in good water resistance and long-term stability. This work provides a cheaper and more environmentally friendly strategy to address chlorine poisoning in the synergistic reaction and offers new guidance for multipollutant control.

Regulating the Selectivity of Nitrate Photoreduction for Purification or Ammonia Production by Cooperating Oxidative Half-Reactions

The removal and conversion of nitrate (NO3-) from wastewater has become an important environmental and health topic. The NO3- can be reduced to nontoxic nitrogen (N2) for environmental remediation or ammonia (NH3) for recovery, in which the tailoring of the selectivity is greatly challenging. Here, by construction of the CuOx@TiO2 photocatalyst, the NO3- conversion efficiency is enhanced to ∼100%. Moreover, the precise regulation of selectivity to NH3 (∼100%) or N2 (92.67%) is accomplished by the synergy of cooperative redox reactions. It is identified that the selectivity of the NO3- photoreduction is determined by the combination of different oxidative reactions. The key roles of intermediates and reactive radicals are revealed by comprehensive in situ characterizations, providing direct evidence for the regulated selectivity of the NO3- photoreduction. Different active radicals are produced by the interaction of oxidative reactants and light-generated holes. Specifically, the introduction of CH3CHO as the oxidative reactant results in the generation of formate radicals, which drives selective NO3- reduction into N2 for its remediation. The alkyl radicals, contributed to by the (CH2OH)2 oxidation, facilitate the deep reduction of NO3- to NH3 for its upcycling. This work provides a technological basis for radical-directed NO3- reduction for its purification and resource recovery.

Boosting the catalytic performance of Cu-SAPO-34 in NOx removal via hydrothermal treatment

Phosphate ions promoted Cu-SAPO-34 (P-Cu-SAPO-34) were prepared using bulk CuO particles as Cu2+ precursor by a solid-state ion exchange technique for the selective catalytic reduction of NOx with NH3 (NH3-SCR). The effects of high temperature (H-T) hydrothermal aging on the NOx removal (de-NOx) performance of Cu-SAPO-34 with and without phosphate ions were systematically investigated at atomic level. The results displayed that both Cu-SAPO-34 and P-Cu-SAPO-34 presented relatively poor NOx removal activity with a low conversion (< 30%) at 250-500°C. However, after H-T hydrothermal treatment (800°C for 10 hr at 10% H2O), these two samples showed significantly satisfied NOx elimination performance with a quite high conversion (70%-90%) at 250-500°C. Additionally, phosphate ions decoration can further enhance the catalytic performance of Cu-SAPO-34 after hydrothermal treatment (Cu-SAPO-34H). The textural properties, morphologies, structural feature, acidity, redox characteristic, and surface-active species of the fresh and hydrothermally aged samples were analyzed using various characterization methods. The systematical characterization results revealed that increases of 28% of the isolated Cu2+ active species (Cu2+-2Z, Cu (OH)+-Z) mainly from bulk CuO and 50% of the Brønsted acid sites, the high dispersion of isolated Cu2+ active component as well as the Brønsted acid sites were mainly responsible for the accepted catalytic activity of these two hydrothermally aged samples, especially for P-Cu-SAPO-34H.

Selective Synergistic Catalytic Elimination of NOx and CH3SH via Engineering Deep Oxidation Sites against Toxic Byproducts Formation

NOx and CH3SH as two typical air pollutants widely coexist in various energy and industrial processes; hence, it is urgent to develop highly efficient catalysts to synergistically eliminate NOx and CH3SH. However, the catalytic system for synergistically eliminating NOx and CH3SH is seldom investigated to date. Meanwhile, the deactivation effects of CH3SH on catalysts and the formation mechanism of toxic byproducts emitted from the synergistic catalytic elimination reaction are still vague. Herein, selective synergistic catalytic elimination (SSCE) of NOx and CH3SH via engineering deep oxidation sites over Cu-modified Nb-Fe composite oxides supported on TiO2 catalyst against toxic CO and HCN byproducts formation has been originally demonstrated. Various spectroscopic and microscopic characterizations demonstrate that the sufficient chemisorbed oxygen species induced by the persistent electron transfer from Nb-Fe composite oxides to copper oxides can deeply oxidize HCOOH to CO2 for avoiding highly toxic byproducts formation. This work is of significance in designing superior catalysts employed in more complex working conditions and sheds light on the progress in the SSCE of NOx and sulfur-containing volatile organic compounds.

Insights into Nb doping effects on the catalytic activity and SO2 tolerance of Mn-Cu/BCN catalyst for low-temperature NH3-SCR reaction

Biochar-based supported denitration catalysts have shown tremendous potential in reducing NOx, while improving low-temperature NH3-SCR catalytic activity and SO2 tolerance still faces great challenges. In this work, Mn7-Cu3/BCN and Mn7-Cu3-Nbx/BCN catalysts were prepared by one-step wet impregnation. The enhanced effect of Nb doping on the catalytic performance and SO2 tolerance over the Mn7-Cu3/BCN catalyst was evaluated in the temperature range of 75-275 °C. The denitrification activity test showed that the introduction of an appropriate amount of Nb increased the catalytic activity and N2 selectivity of the catalyst. The NO conversion of Mn7-Cu3-Nb0.05/BCN with an optimum doping ratio of 0.05 wt% Nb was higher than 94% at 150-275 °C. The characterization results indicated that the introduction of Nb enhanced the interaction between the active components MnOx and CuOx, accelerated the electron transfer between elements, and thus improved the Mn4+/Mnn+ and Oα/(Oα+Oβ+Oγ) proportions and redox performance. On the other hand, Nb modification increased the number of weakly acidic sites, which was beneficial for the adsorption and activation of the reducing agent NH3 under low-temperature conditions. Meanwhile, Nb could significantly improve the SO2 poisoning resistance of the Mn7-Cu3/BCN-S catalyst when SO2 was added to the reaction system. The NO conversion of Mn7-Cu3-Nb0.05/BCN remained above 75% after a 13.5 h reaction under 100 ppm SO2 and 5 vol% H2O at 225 °C. By combining experimental characterization results with DFT calculation results, we effectively confirmed that Mn7-Cu3-Nb0.05/BCN had good sulfur resistance, mainly because Nb could effectively inhibit the formation of manganese sulfate and promote the decomposition of ammonium bisulfate.

Revealing the Dynamic Behavior of Active Sites on Acid-Functionalized CeO2 Catalysts for NOx Reduction

Unraveling the dynamics of the active sites upon CeO₂-based catalysts in selective catalytic reduction of nitrogen oxides by ammonia (NH₃-SCR) is challenging. In this work, we prepared tungsten-acidified and sulfated CeO₂ catalysts and used operando spectroscopy to reveal the dynamics of acid sites and redox sites on catalysts during NH₃-SCR reaction. We found that both Lewis and Brønsted acid sites are needed to participate in the catalytic reaction. Notably, Brønsted acid sites are the main active sites after a tungsten-acidified or sulfated treatment, and the change of Brønsted acid sites significantly affects the NO_x removal. Moreover, acid functionalization promotes the cerium species cycle between Ce⁴⁺ and Ce³⁺ for the NO_x reduction. This work is critical to deeply understanding the natural properties of active sites, and it also provides new insights into the mechanism for NH₃-SCR over CeO₂-based catalysts.

Improvement of Cu-SAPO-34 hydrothermal stability by tuning P/Al ratio for selective catalytic reduction of NO by NH3

Cu-SAPO-34 is a promising catalyst for abatement of NO via selective catalytic reduction with NH₃ (NH₃-SCR), but its hydrothermal stability needs to be enhanced. In this work, the Cu-SAPO-34 catalysts with different P/Al ratios of 0.8, 1.0 and 1.2 were prepared, and the temperature window with NO conversion >90% (T₉₀) for all catalysts were similar (160-570 °C). The T₉₀ of Cu-SAPO-34 with P/Al of 0.8 dramatically decreased (220-470 °C) after hydrothermal treatment, and interestingly, the catalysts with high P/Al ratios (1.0 and 1.2) remained high activity. The T₉₀ of the aged catalysts with P/Al of 1.2 was 155-525 °C. The characterizations showed that the increase of P/Al ratio not only enhanced the crystallinity but also enlarged the grain size of catalysts, which were conducive to the zeolite framework stability. Moreover, the Cu-SAPO-34 with large grain size facilitated the conversion of CuO to isolated Cu²⁺ ions as well as inhibited the aggregation of Cu species. Furthermore, the large grain sized catalysts provided more acid sites, and thus, the catalysts presented excellent hydrothermal stability. In situ DRIFTS analysis confirmed the existence of both Langmuir-Hinshelwood and Eley-Rideal pathway over the catalyst with a P/Al ratio of 1.2. This work provided a facile method to promote the hydrothermal stability of Cu-based zeolite catalysts.

Self-Defense Effects of Ti-Modified Attapulgite for Alkali-Resistant NOx Catalytic Reduction

Nowadays, the serious deactivation of deNO_x catalysts caused by alkali metal poisoning was still a huge bottleneck in the practical application of selective catalytic reduction of NO_x with NH₃. Herein, alkali-resistant NO_x catalytic reduction over metal oxide catalysts using Ti-modified attapulgite (ATP) as supports has been originally demonstrated. The self-defense effects of Ti-modified ATP for alkali-resistant NO_x catalytic reduction have been clarified. Ti-modified ATP with self-defense ability was obtained by removing alkaline metal cation impurities in the natural ATP materials without destroying its initial layered-chain structure through the ion-exchange procedure, accompanied with an obvious enrichment of Brønsted acid and Lewis acid sites. The self-defense effects embodied that both ion-exchanged Ti octahedral centers and abundant Si-OH sites in the Ti-ion-exchange-modified ATP could effectively anchor alkali metals via coordinate bonding or ion-exchange process, which induced alkali metals to be immobilized by the Ti-ion-exchange-modified ATP carrier rather than impair active species. Under this special protection of self-defense effects, Ti-ion-exchange-modified ATP supported catalysts still retained plentiful acidic sites and superior redox ability even after alkali metal poisoning, giving rise to the maintenance of sufficient NH_x and NO_x adsorption and the subsequent efficient reaction, which in turn resulted in high NO_x catalytic reduction capacity of the catalyst. The strategy provided new inspiration for the development of novel and efficient selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) catalysts with high alkali resistance.

Synergistic Catalytic Elimination of NOx and Chlorinated Organics: Cooperation of Acid Sites

The synergistic catalytic removal of NO_x and chlorinated volatile organic compounds under low temperatures is still a big challenge. Generally, degradation of chlorinated organics demands sufficient redox ability, which leads to low N₂ selectivity in the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR). Herein, mediating acid sites via introducing the CePO₄ component into MnO₂/TiO₂ NH₃-SCR catalysts was found to be an effective approach for promoting chlorobenzene degradation. The observation of in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) and Raman spectra reflected that the Lewis acid sites over CePO₄ promoted the nucleophilic substitution process of chlorobenzene over MnO₂ by weakening the bond between Cl and benzene ring. Meanwhile, MnO₂ provided adequate Brønsted acid sites and redox sites. Under the cooperation of Lewis and Brønsted acid sites, relying on the rational redox ability, chlorobenzene degradation was promoted with synergistically improved NH₃-SCR activity and selectivity. This work offers a distinct pathway for promoting the combination of chlorobenzene catalytic oxidation and NH₃-SCR, and is expected to provide a novel strategy for synergistic catalytic elimination of NO_x and chlorinated volatile organic compounds.

Insight into the promoting effect of support pretreatment with sulfate acid on selective catalytic reduction performance of CeO2/ZrO2 catalysts

In this paper, sulfated ZrO₂ were synthesized via precipitation and impregnation method, and the promoting effects of support sulfation on selective catalytic reduction (SCR) performance of CeO₂/ZrO₂ catalysts were investigated. The results revealed that sulfated ZrO₂ could significantly enhance the SCR activity of CeO₂/ZrO₂ catalysts in a wide temperature range. Especially when S/Zr molar ratio was 0.1, CeO₂/ZrO₂-0.1S catalyst exhibited a large operating temperature window of 251 ∼ 500 °C and its N₂ selectivity was 100 % in the temperature range of 150 ∼ 500 °C. Moreover, CeO₂/ZrO₂-0.1S catalyst possessed a superior low-temperature activity over 0.1S-CeO₂/ZrO₂ catalyst. After exposing to 100 ppm SO₂ for 15 h, a high NO conversion efficiency of CeO₂/ZrO₂-0.1S catalyst (90.7 %) could still be reached. The characterization results indicated that ZrO₂ treated with a proper dosage of sulfate acid was beneficial to enlarge the specific surface area greatly. Sulfated ZrO₂ was also in favor of promoting the transformation of CeO₂ from crystalline state to highly-dispersed amorphous state, and inhibiting the transformation of ZrO₂ from tetragonal to monoclinic phase. It could also enhance the total surface acidity greatly with an increase in both Brønsted acid sites and Lewis acid sites, thus significantly improving NH₃ adsorption on catalyst surface. Besides, the promoting effect of support sulfation on SCR performance of CeO₂/ZrO₂ catalysts was also related with the enhanced redox property, higher Ce³⁺/(Ce³⁺+Ce⁴⁺) ratio and abundant surface chemisorbed labile oxygen. The in-situ DRIFTS results implied that nitrate species coordinated on the surface of CeO₂/ZrO₂-0.1S catalyst could participate in the Selective catalytic reduction with ammonia (NH₃-SCR) reactions at either medium or high temperature, suggesting that both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms might be followed in SCR reactions.

Insight on the anti-poisoning mechanism of in situ coupled sulfate over iron oxide catalysts in NOx reduction

In situ coupled sulfate uniquely migrated to the surface of iron oxide catalysts to capture metal poisons and thus maintained efficient adsorption and activation of NH3 and NOx reactants.

The promoting effect of support pretreatment with sulfate acid on the Ca resistance of a CeO2/ZrO2 catalyst for NH3-SCR of NOx with NH3

CaSO4 was formed through the reaction between S and Ca to relieve the effect of Ca-poisoning on the catalyst.

Promotional mechanism of enhanced denitration activity with Cu modification in a Ce/TiO2-ZrO2 catalyst for a low temperature NH3-SCR system

This study aims to investigate the enhanced low temperature denitration activity and promotional mechanism of a cerium-based catalyst through copper modification. In this paper, copper and cerium oxides were supported on TiO2-ZrO2 by an impregnation method, their catalytic activity tests of selective catalytic reduction (SCR) of NO with NH3 were carried out and their physicochemical properties were characterized. The CuCe/TiO2-ZrO2 catalyst shows obviously enhanced NH3-SCR activity at low temperature (<300 °C), which is associated with the well dispersed active ingredients and the synergistic effect between copper and cerium species (Cu2+ + Ce3+ ↔ Cu+ + Ce4+), and the increased ratios of surface chemisorbed oxygen and Cu+/Cu2+ lead to the enhanced low-temperature SCR activity. The denitration reaction mechanism over the CuCe/TiO2-ZrO2 catalyst was investigated by in situ DRIFTS and DFT studies. Results illustrate that the NH3 is inclined to adsorb on the Cu acidic sites (Lewis acid sites), and the NH2 and NH2NO species are the key intermediates in the low-temperature NH3-SCR process, which can explain the promotional effect of Cu modification on denitration activity of Ce/TiO2-ZrO2 at the molecular level. Finally, we have reasonably concluded a NH3-SCR catalytic cycle involving the Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism, and the former mechanism dominates in the NH3-SCR reaction.

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 NO_x 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 NO_x conversion of Cu-SSZ-39 was improved by NO₂ when NO₂/NO_x = 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.

Influence of phosphorus on the NH3-SCR performance of CeO2-TiO2 catalyst for NOx removal from co-incineration flue gas of domestic waste and municipal sludge

Domestic waste and municipal sludge are two major solid hazardous substances generated from human daily life. Co-incineration technology is regarded as an effective method for the treatment of them. However, the emitted NO_x-containing exhaust with high content of phosphorus should purified strictly. CeO₂-TiO₂ is a promising catalyst for removal of NO_x by NH₃-SCR technology, but the effect of phosphorous in the exhaust is ambiguous. Therefore, the effect of phosphorus on NH₃-SCR performance and physicochemical properties of CeO₂-TiO₂ catalyst was investigated in our present work. It was found that phosphorus decreased the NH₃-SCR activity below 300 °C. Interestingly, it suppressed the formation of NO_x and N₂O caused by NH₃ over-oxidation above 300 °C. The reason might be that phosphorus induced Ti⁴⁺ to migrate from CeO₂-TiO₂ solid solution and form crystalline TiO₂, which led to the destruction of Ti-O-Ce structure in the catalyst. So, the transfer of electrons between Ti and Ce ions, the relative contents of Ce³⁺, and surface adsorbed oxygen, as well as the redox performance were limited, which further inhibited the over-oxidation of NH₃. In addition, phosphorus weakened the NH₃ adsorption on Lewis acid sites and the adsorption performance of NO + O₂, while increased the Brønsted acid sites. Finally, the reaction mechanism over CeO₂-TiO₂ catalyst did not change after introducing phosphorus, L-H and E-R mechanisms co-existed on the surface of the catalysts.

Revealing the effect of paired redox-acid sites on metal oxide catalysts for efficient NOx removal by NH3-SCR

Understanding the nature of active sites on metal oxide catalysts in the selective catalytic reduction (SCR) of NO by NH₃ (NH₃-SCR) is a crucial prerequisite for the development of novel efficient NH₃-SCR catalysts. In this work, two CeO₂-based SCR catalyst systems with diverse acidic metal oxides-CeO₂ interfaces, i.e., Nb₂O₅-CeO₂ (Nb₂O₅/CeO₂ and CeO₂/Nb₂O₅) and WO₃-CeO₂ (WO₃/CeO₂ and CeO₂/WO₃), were prepared and used to reveal the relationship between NH₃-SCR activity and surface acidity/redox properties. In combination with the results of the NH₃-SCR activity test and various characterizations, it was found that the NH₃-SCR performance of Nb₂O₅-CeO₂ and WO₃-CeO₂ catalysts was highly dependent on the strong interactions between the redox component (CeO₂) and acidic component (Nb₂O₅ or WO₃), as well as the amount of paired redox-acid sites. From a quantitative perspective, an activity-surface acidity/redox property relationship was proposed. For both Nb₂O₅-CeO₂ and WO₃-CeO₂ catalysts systems operated at the more concerned low-temperature range (200 °C), the NH₃-SCR activity in low NO_x conversion region (< 40%) was mainly dominated by the surface acidity of catalysts, while the NH₃-SCR activity in high NO_x conversion region (> 40%) was more determined by redox properties.

NO oxidation performance and kinetics analysis of BaMO3 (M=Mn, Co) perovskite catalysts

Perovskite is an efficient and emerging catalyst for NO oxidation. In this study, BaMnO₃ and BaCoO₃ perovskite catalysts were synthesized by the sol-gel method, and their catalytic oxidation performances of NO were studied. The catalytic performances indicated that BaMnO₃ and BaCoO₃ perovskites had the highest NO oxidation activities with the NO conversions of 78.2% at 350 °C and 84.3% at 310 °C, respectively. The high activities of BaMnO₃ and BaCoO₃ perovskite catalysts were related to the abundant surface adsorption oxygen (O_A = 76.21% and 78.57%, respectively) and the high concentration of Mn⁴⁺ (Mn⁴⁺/Mn = 66.95%) and Co³⁺ (Co³⁺/Co = 63.8%). Moreover, the results of FT-IR and kinetics revealed that NO and O₂ adsorbed on the surface of samples and combined with the B-O band to form bidentate nitrate and bridging nitrate, which eventually was converted into NO₂. The kinetics analysis revealed that the NO oxidation reaction followed the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. In addition, the activation energies were 36.453 kJ/mol for BaMnO₃ and 30.081 kJ/mol for BaCoO₃, implying that BaMnO₃ and BaCoO₃ provide low-cost and efficient catalysts, which can be comparable to Pt noble metal catalysts.

Boosting the Alkali/Heavy Metal Poisoning Resistance for NO Removal by Using Iron-Titanium Pillared Montmorillonite Catalysts

It is still a big challge to improve the alkali and heavy metal resistance of deNO_x catalysts for selective catalytic reduction (SCR) of NO_x with NH₃. In this study, a novel catalyst developed by pillaring montmorillonite with iron and titanium (Fe-Ti-MMT) was proposed. It is quite interesting that high resistance to alkaline and heavy metals has been demonstrated by using Fe-Ti-MMT catalysts. It has been demonstrated that the specific pillaring synthesis procedure and further addition of the Ti pillared sites greatly contributed to the wide active temperature window and enhanced the resistance to alkali and heavy metal. The higher ratio of active Fe²⁺ species, more active acid sites, and enhanced ammonia adsorption indicated the remarkable activity as well as K and Pb resistance. Moreover, the K and Pb poisons would promote the generation of active adsorbed NO_x species on the Fe-Ti-MMT but induce the formation of stable inactive ones on that of Fe-MMT, which greatly tuned the reaction pathways and improved the reaction rate for Ti modified Fe pillared MMT catalysts. The strategy of incorporating Ti into the Fe pillared MMT catalysts strongly provides a novel inspiration for keeping excellent NH₃-SCR performance in the presence of alkali/heavy metal for NO_x removal.

Alkali-Resistant NOx Reduction over SCR Catalysts via Boosting NH3 Adsorption Rates by In Situ Constructing the Sacrificed Sites

Currently, improving the alkali resistance of vanadium-based catalysts still remains as an intractable issue for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). It is generally believed that the decrease in adsorbed NH_x species deriving from the declined acidic sites is the chief culprit for the deactivation of alkali-poisoned catalysts. Herein, alkali-resistant NO_x reduction over SCR catalysts via boosting NH₃ adsorption rates was originally demonstrated by in situ constructing the sacrificed sites. It is interesting that the adsorbed NH_x species largely decrease while the NH₃ adsorption rate is well kept over the V₂O₅/CeO₂ catalyst by in situ constructing the sacrificed sites. The SCR activity could be maintained after alkali poisoning because in situ constructed SO₄^2- groups would prefer to be combined with K⁺ so that the specific V═O species can endow K-poisoned V₂O₅/CeO₂ with high adsorption rate of NH₃ and high reactivity of NH_x species. This work provides a new viewpoint that NH₃ adsorption rate plays more decisive roles in the performance of alkali-poisoned catalysts than the amount of NH₃ adsorption and enlightens an alternative strategy to improve the alkali-resistance of catalysts, which is significant to both the academic and industrial fields.

Self-Protected CeO2-SnO2@SO42-/TiO2 Catalysts with Extraordinary Resistance to Alkali and Heavy Metals for NOx Reduction

Reducing the poisoning effect of alkali and heavy metals over ammonia selective catalytic reduction (NH₃-SCR) catalysts is still an intractable issue, as the presence of K and Pb in fly ash greatly hampers their catalytic activity by impairing the acidity and affecting the redox properties of the catalysts, leading to the reduction in the lifetime of SCR catalysts. To address this issue, we propose a novel self-protected antipoisoning mechanism by designing SO₄^2-/TiO₂ superacid supported CeO₂-SnO₂ catalysts. Owing to the synergistic effect between CeO₂ and SnO₂ and the strong acidity originating from the SO₄^2-/TiO₂ superacid, the catalysts show superior catalytic activity over a wide temperature range (240-510 °C). Moreover, when K or/and Pb are deposited on SO₄^2-/TiO₂ catalysts, the bond effect between SO₄^2- and Ti-O would be broken so that the sulfate in the bulk of SO₄^2-/TiO₂ superacid support would be induced to migrate to the surface to bond with K and Pb, thus prohibiting poisons from attacking the Ce-Sn active sites, and significantly boosting the resistance. Hopefully, this novel self-protection mechanism derived from the migration of sulfate in the SO₄^2-/TiO₂ superacid to resist alkali and heavy metals provides a new avenue for designing novel catalysts with outstanding resistance to alkali and heavy metals.

Unraveling the Unexpected Offset Effects of Cd and SO2 Deactivation over CeO2-WO3/TiO2 Catalysts for NOx Reduction

It is challenging for selective catalytic reduction (SCR) of NO_x by NH₃ due to the coexistence of heavy metal and SO₂ in the flue gas. A thorough probe into deactivation mechanisms is imperative but still lacking. This study unravels unexpected offset effects of Cd and SO₂ deactivation over CeO₂-WO₃/TiO₂ catalysts, potential candidates for commercial SCR catalysts. Cd- and SO₂-copoisoned catalysts demonstrated higher activity for NO_x reduction than a Cd-poisoned catalyst but lower than that for an SO₂-poisoned catalyst. In comparison to SO₂, Cd had more severe effects on acidic and redox properties, distinctly decreasing the SCR activity. After sulfation of Cd-poisoned catalysts, SO₄^2- preferentially bonded with the surface CdO and released CeO₂ active sites poisoned by CdO, thus reserving the highly active CeO₂-WO₃ sites and maintaining a high activity. The sulfation of Cd-poisoned catalysts also provided more strong acidic sites, and the synergistic effects between the formed cerium sulfate and CeO₂ contributed to the high-temperature SCR performance. This work sheds light on the deactivation mechanism of heavy metals and SO₂ over CeO₂-WO₃/TiO₂ catalysts and provides an innovative pathway for inventing high-performance SCR catalysts, which have great resistance to heavy metals and SO₂ simultaneously. This will be favorable to academic and practical applications in the future.

Highly efficient WO3-FeOx catalysts synthesized using a novel solvent-free method for NH3-SCR

WO₃-FeO_x catalysts with various WO₃ contents were synthesized through a facile solvent-free method, satisfying the selective catalytic reduction of NO (NH₃-SCR). Strikingly, the optimum 30 %WO₃-FeO_x catalyst with the largest surface area exhibited the most outstanding catalytic activity, achieving the nearly 100 % NO_x removal efficiency in a wide temperature window between 225-500 °C, which was better than that of Fe-W series catalysts reported in other studies. In addition, Raman and XPS results proved that the introduction of WO₃ altered the electronic environment of Fe₂O₃, inducing the formation of Fe₃O₄ (Fe²⁺) and surface adsorbed oxygen. In situ DRIFTS demonstrated that the interaction between WO₃ and Fe₂O₃ not only promoted the adsorption capacity of NH₃ on the catalyst, but also contributed to the formation of adsorbed NO_x species. NO_x reduction reaction on WO₃-FeO_x catalyst proceeded via the Eley-Rideal and Langmuir-Hinshelwood mechanism synchronously. All of these factors, jointly, accounted for the superior catalytic activity and N₂ selectivity of WO₃-FeO_x catalysts.

Promotion effect of cerium doping on iron–titanium composite oxide catalysts for selective catalytic reduction of NOx with NH3

Doping with a suitable amount of Ce enhances the SCR performance of FeTi catalysts.

Low Temperature NH3-SCR over Mn-Ce Oxides Supported on MCM-41 from Diatomite

A series of MCM-41 molecular sieves with different molar ratio of template to silicon were synthesized through hydrothermal synthesis method by using cetyltrimethylammonium bromide (CTAB) as the template, diatomite as the silicon source. By using impregnation method, the Mn-Ce/MCM-41 SCR molecular sieve-based catalysts were prepared. The results observed that when the molar ratio of template to silicon was 0.2:1, the MCM-41 as catalyst carrier has the highest surface area and largest pore volume, it also presented typically ordered hexagonal arrays of uniform channels. The denitration catalytic material based on this carrier has a high number of Lewis acidic sites, and the denitration efficiency can reach more than 93%.

The balance of acidity and redox capability over modified CeO2 catalyst for the selective catalytic reduction of NO with NH3

The effect of acidity and redox capability over sulfuric acid-modified CeO₂ catalysts were studied for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). The deposition of sulfate significantly enhanced the catalytic performance over CeO₂. NO_x conversion over 4H₂SO₄/CeO₂ at 230-440 °C was higher than 90%. The strong redox capability of CeO₂ could result in unselective NH₃ oxidation and decrease high temperatures catalytic activity and N₂ selectivity. The deposition of sulfate increased the acidity and weakened the redox capability, and then increased the high temperature NO_x conversion and N₂ selectivity. An appropriate level of acidity also promoted the activity at 190-250 °C over ceria-based catalysts, and with further increase in the acidity, the SCR activity decreased slightly. Weak redox capability lowered the low-temperature catalytic activity. Excellent SCR activity requires a balance of acidity and redox capability on the catalysts.

Development of cerium-based catalysts for selective catalytic reduction of nitrogen oxides: a review

Nitrogen oxides (NO_X) are major pollutants of the atmosphere, and selective catalytic reduction of nitrogen oxides using ammonia as a reductant (NH₃-SCR) is an effective method to remove nitrogen oxides.

NO oxidative activity of mesoporous LaMnO3 and LaCoO3 perovskite nanoparticles by facile molten-salt synthesis

LaMnO₃ and LaCoO₃ perovskite nanoparticles have sprung up as the PGM-free catalysts for NO oxidation.

Effect of iron loading on the performance and structure of Fe/ZSM-5 catalyst for the selective catalytic reduction of NO with NH3

A series of Fe/ZSM-5 catalysts with different Fe contents were prepared by impregnation method. The catalysts were characterized by TEM, XRD, H₂ temperature-programed reduction (H₂-TPR), temperature-programed desorption of ammonia (NH₃-TPD), and in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), and the catalytic activity test was also carried out on selective catalytic reduction (SCR) denitration device. Results showed that the single metal iron-supported ZSM-5 catalyst has high deNO_x activity in the medium-high temperature range, and the optimal loading of Fe active component is 10 wt%; the deNO_x efficiency over 80% at the range of 350-450 °C and 431 °C reaches the maximum of 96.91%. Iron species can be finely dispersed on the surface of the carrier as amorphous oxides, and the crystalline structure of zeolite is retained. The significant redox performance, highly dispersed nanoparticles, and rich Lewis acid sites on the surface of catalyst are favorable for the SCR denitration reaction. Fe/ZSM-5 10 wt% catalyst has rich Lewis acid sites and less B acid sites and Lewis acidic sites play an important role during the reaction. Only Eley-Rideal (E-R) mechanism existed during the NH₃-SCR reaction process, and there is no denitration reaction being accomplished by L-H mechanism at 150 °C.