Alkali and Phosphorus Resistant Zeolite-like Catalysts for NOx Reduction by NH3

Synergistic catalytic removal of NOx and chlorobenzene by a combination punch of Lewis and Bronsted acid and redox sites

Multi-pollutant control of nitrogen oxides (NOx) and chlorinated aromatics in industrial flue by synergistic catalysis is still a huge challenge. Tailoring well-defined interfacial structures of multi-component heterogeneous catalysts has become an effective strategy for facilitating reactions involving multiple reactants. Here, a coupling of copper and tin oxide with particle-particle heterostructure supported on H-ZSM5 is designed to achieve a high-performance catalyst for NOx and chlorobenzene synergistic elimination. Experimental and theoretical calculation (DFT) studies show that the particle-particle coupling Janus heterostructure induced Sn-O-Cu interfaces. The strong electronic interaction improves the interfacial charge redistribution and mediates the activated interfacial oxygen, supporting redox (R) sites for the redox reaction cycle. Together with the abundant intrinsic Lewis (L) acid sites from CuOx and Brønsted (B) acid sites from the H-ZSM-5 interface, a combination punch of ideal L-B-R sites was constructed for the synergistic catalysis of NOx reduction and chlorobenzene oxidation. The designed Sn-Cu/H-ZSM5 catalyst exhibits significant low-temperature synergistic catalytic activity, a wide temperature window, robust long-term stability, and excellent water resistance, which outperforms Sn/H-ZSM5 and Cu/H-ZSM5. Moreover, in situ infrared spectra of serial transient reactions evidenced that the NOx reduction reaction promotes chlorobenzene oxidation. This novel strategy of regulating the overall L acid, B acid, and redox properties to fabricate balanced L-B-R sites via interfacial engineering provides a distinct strategy for facilitating the synergistic abatement of NOx and chlorinated aromatics.

NH3-Induced Challenges in CO2 Hydrogenation over the Cu/ZnO/Al2O3 Catalyst

Gas sources rich in CO2 derived from biomass/waste gasification, anaerobic digestion, or industrial carbon capture often contain impurities such as H2S, H2O, and NH3, which can significantly hinder catalyst performance. Here, we show the role of NH3 on the reverse water-gas shift (RWGS) reaction over a commercial Cu/ZnO/Al2O3 catalyst, examining its effects on both the catalytic activity and the catalyst structure. We found that NH3 reversibly decreases CO2 conversion immediately by suppressing carbonate hydrogenation and CO desorption. This effect intensifies with an increase in NH3 concentration but decreases at higher temperatures. However, prolonged exposure (over 100 h) to RWGS conditions in the presence of 1.4% NH3 leads to near-total and irreversible deactivation of the Cu/ZnO/Al2O3 catalyst. Under NH3 exposure, the catalyst loses Cu+ sites on the surface, causing a spatial separation of Cu and ZnO. Finally, to address this challenge, we propose a novel strategy to mitigate NH3 inhibition by decomposing NH3 into N2 and H2.

Challenges and Perspectives of Environmental Catalysis for NO x Reduction

Environmental catalysis has attracted great interest in air and water purification. Selective catalytic reduction with ammonia (NH3-SCR) as a representative technology of environmental catalysis is of significance to the elimination of nitrogen oxides (NO x ) emitting from stationary and mobile sources. However, the evolving energy landscape in the nonelectric sector and the changing nature of fuel in motor vehicles present new challenges for NO x catalytic purification over the traditional NH3-SCR catalysts. These challenges primarily revolve around the application limitations of conventional industrial NH3-SCR catalysts, such as V2O5-WO3(MoO3)/TiO2 and chabazite (CHA) structured zeolites, in meeting both the severe requirements of high activity at ultralow temperatures and robust resistance to the wide array of poisons (SO2, HCl, phosphorus, alkali metals, and heavy metals, etc.) existing in more complex operating conditions of new application scenarios. Additionally, volatile organic compounds (VOCs) coexisting with NO x in exhaust gas has emerged as a critical factor further impeding the highly efficient reduction of NO x . Therefore, confronting the challenges inherent in current NH3-SCR technology and drawing from the established NH3-SCR reaction mechanisms, we discern that the strategic manipulation of the properties of surface acidity and redox over NH3-SCR catalysts constitutes an important pathway for increasing the catalytic efficiency at low temperatures. Concurrently, the establishment of protective sites and confined structures combined with the strategies for triggering antagonistic effects emerge as imperative items for strengthening the antipoisoning potentials of NH3-SCR catalysts. Finally, we contemplate the essential status of selective synergistic catalytic elimination technology for abating NO x and VOCs. By virtue of these discussions, we aim to offer a series of innovative guiding perspectives for the further advancement of environmental catalysis technology for the highly efficient NO x catalytic purification from nonelectric industries and motor vehicles.

Unlocking Mixed-Metal Oxides Active Centers via Acidity Regulation for K&SO2 Poisoning Resistance: Self-Detoxification Mechanism of Zeolite-Confined deNOx Catalysts

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) is an efficient NOx reduction strategy, while the denitrification (deNOx) catalysts suffer from serious deactivation due to the coexistence of multiple poisoning substances, such as alkali metal (e.g., K), SO2, etc., in industrial flue gases. It is essential to understand the interaction among various poisons and their effects on the deNOx process. Herein, the ZSM-5 zeolite-confined MnSmOx mixed (MnSmOx@ZSM-5) catalyst exhibited better deNOx performance after the poisoning of K, SO2, and/or K&SO2 than the MnSmOx and MnSmOx/ZSM-5 catalysts, the deNOx activity of which at high temperature (H-T) increased significantly (>90% NOx conversion in the range of 220-480 °C). It has been demonstrated that K would occupy both redox and acidic sites, which severely reduced the reactivity of MnSmOx/ZSM-5 catalysts. The most important, K element is preferentially deposited at -OH on the surface of ZSM-5 carrier due to the electrostatic attraction (-O-K). As for the K&SO2 poisoning catalyst, SO2 preferred to be combined with the surface-deposited K (-O-K-SO2ads) according to XPS and density functional theory (DFT) results, the poisoned active sites by K would be released. The K migration behavior was induced by SO2 over K-poisoned MnSmOx@ZSM-5 catalysts, and the balance of surface redox and acidic site was regulated, like a synergistic promoter, which led to K-poisoning buffering and activity recovery. This work contributes to the understanding of the self-detoxification interaction between alkali metals (e.g., K) and SO2 on deNOx catalysts and provides a novel strategy for the adaptive use of one poisoning substance to counter another for practical NOx reduction.

Selective Reduction of Aqueous Nitrate to Ammonium with an Electropolymerized Chromium Molecular Catalyst

Nitrate (NO3-) is a common nitrogen-containing contaminant in agricultural, industrial, and low-level nuclear wastewater that causes significant environmental damage. In this work, we report a bioinspired Cr-based molecular catalyst incorporated into a redox polymer that selectively and efficiently reduces aqueous NO3- to ammonium (NH4+), a desirable value-added fertilizer component and industrial precursor, at rates of ∼0.36 mmol NH4+ mgcat-1 h-1 with >90% Faradaic efficiency for NH4+. The NO3- reduction reaction occurs through a cascade catalysis mechanism involving the stepwise reduction of NO3- to NH4+ via observed NO2- and NH2OH intermediates. To our knowledge, this is one of the first examples of a molecular catalyst, homogeneous or heterogenized, that is reported to reduce aqueous NO3- to NH4+ with rates and Faradaic efficiencies comparable to those of state-of-the-art solid-state electrocatalysts. This work highlights a promising and previously unexplored area of electrocatalyst research using polymer-catalyst composites containing complexes with oxophilic transition metal active sites for electrochemical nitrate remediation with nutrient recovery.

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.

Balancing acid and redox sites of phosphorylated CeO2 catalysts for NOx reduction: The promoting and inhibiting mechanism of phosphorus

The role of phosphorus in metal oxide catalysts is still controversial. The precise tuning of the acidic and redox properties of metal oxide catalysts for the selective catalytic reduction in NO_x using NH₃ is also a great challenge. Herein, CeO₂ catalysts with different degrees of phosphorylation were used to study the balance between the acidity and redox property by promoting and inhibiting effects of phosphorus. CeO₂ catalysts phosphorylated with lower phosphorus content (5 wt%) exhibited superior NO_x reduction performance with above 90% NO_x conversion during 240-420 °C due to the balanced acidity and reducibility derived from the highest content of Brønsted acid sites on PO₄^3- to adsorb NH₃ and surface adsorbed oxygen species. Plenty of PO₃^- over CeO₂ catalysts phosphorylated with the higher phosphorus content (≥ 10 wt%) significantly disrupted the balance between the acidity and the redox property due to the reduced acid/redox sites, which resulted in the less active NO_x species. The mechanism of different structural phosphorus species (PO₄^3- and PO₃^-) in promoting or inhibiting the NO_x reduction over CeO₂ catalysts was revealed. This work provides a novel method for qualitative and quantitative study of the relationship between acidity/redox property and activity of catalysts for NO_x reduction.

Unique Compensation Effects of Heavy Metals and Phosphorus Copoisoning over NOx Reduction Catalysts

Selective catalytic reduction (SCR) of NO_x from the flue gas is still a grand challenge due to the easy deactivation of catalysts. The copoisoning mechanisms and multipoisoning-resistant strategies for SCR catalysts in the coexistence of heavy metals and phosphorus are barely explored. Herein, we unexpectedly found unique compensation effects of heavy metals and phosphorus copoisoning over NO_x reduction catalysts and the introduction of heavy metals results in a dramatic recovery of NO_x reduction activity for the P-poisoned CeO₂/TiO₂ catalysts. P preferentially combines with Ce as a phosphate species to reduce the redox capacity and inhibit NO adsorption. Heavy metals preferentially reduced the Brønsted acid sites of the catalyst and inhibited NH₃ adsorption. It has been demonstrated that heavy metal phosphate species generated over the copoisoned catalyst, which boosted the activation of NH₃ and NO, subsequently bringing about more active nitrate species to relieve the severe impact by phosphorus and maintain the NO_x reduction over CeO₂/TiO₂ catalysts. The heavy metals and P copoisoned catalysts also possessed more acidic sites, redox sites, and surface adsorbed oxygen species, which thus contributed to the highly efficient NO_x reduction. This work elaborates the unique compensation effects of heavy metals and phosphorus copoisoning over CeO₂/TiO₂ catalysts for NO_x reduction and provides a perspective for further designing multipoisoning-resistant CeO₂-based catalysts to efficiently control NO_x emissions in stationary sources.

NOx Reduction over Smart Catalysts with Self-Created Targeted Antipoisoning Sites

Selective catalytic reduction of NO_x in the presence of alkali (earth) metals and heavy metals is still a challenge due to the easy deactivation of catalysts. Herein, NO_x reduction over smart catalysts with self-created targeted antipoisoning sites is originally demonstrated. The smart catalyst consisted of TiO₂ pillared montmorillonite with abundant cation exchange sites to anchor poisoning substances and active components to catalyze NO_x into N₂. It was not deactivated during the NO_x reduction process in the presence of alkali (earth) metals and heavy metals. The enhanced surface acidity, reducible active species, and active chemisorbed oxygen species of the smart catalyst accounted for the remarkable NO_x reduction efficiency. More importantly, the self-created targeted antipoisoning sites expressed specific anchoring effects on poisoning substances and protected the active components from poisoning. It was demonstrated that the tetrahedrally coordinated aluminum species of the smart catalyst mainly acted as self-created targeted antipoisoning sites to stabilize the poisoning substances into the interlayers of montmorillonite. This work paves a new way for efficient reduction of NO_x from the complex flue gas in practical applications.

Alkali and Heavy Metal Copoisoning Resistant Catalytic Reduction of NOx via Liberating Lewis Acid Sites

The catalyst deactivation caused by the coexistence of alkali and heavy metals remains an obstacle for selective catalytic reduction of NO_x with NH₃. Moreover, the copoisoning mechanism of alkali and heavy metals is still unclear. Herein, the copoisoning mechanism of K and Cd was revealed from the adsorption and variation of reaction intermediates at a molecular level through time-resolved in situ spectroscopy combined with theoretical calculations. The alkali metal K mainly decreased the adsorption of NH₃ on Lewis acid sites and altered the reaction more depending on the formation of the NH₄NO₃ intermediate, which is highly related to NO_x adsorption and activation. However, Cd further inhibited the generation of active nitrate intermediates and thus decreased the NO_x abatement about 60% on potassium-poisoned CeTiO_x catalysts. Physically mixing with acid additives for CeTiO_x catalysts could significantly liberate the active Lewis acid sites from the occupation of alkali metals and relieve the high dependence on NO_x adsorption and activation, thus recovering the NO_x removal rate to the initial state. This work revealed the copoisoning mechanism of K and Cd on Ce-based de-NO_x catalysts and developed a facile anti-poisoning strategy, which paves a way for the development of durable catalysts among alkali and heavy metal copoisoning resistant catalytic reduction of NO_x.

SO2- and H2O-Tolerant Catalytic Reduction of NOx at a Low Temperature via Engineering Polymeric VOx Species by CeO2

Selective catalytic reduction (SCR) of NO_x over V₂O₅-based oxide catalysts has been widely used, but it is still a challenge to efficiently reduce NO_x at low temperatures under SO₂ and H₂O co-existence. Herein, SO₂- and H₂O-tolerant catalytic reduction of NO_x at a low temperature has been originally demonstrated via engineering polymeric VO_x species by CeO₂. The polymeric VO_x species were tactfully engineered on Ce-V₂O₅ composite active sites via the surface occupation effect of Ce, and the obtained catalysts exhibited remarkable low-temperature activity and strong SO₂ and H₂O tolerance at 250 °C. The strong interaction between Ce and V species induced the electron transfer from V to Ce and tuned the SCR reaction via the E-R pathway between the NH₄⁺/NH₃ species and gaseous NO. In the presence of SO₂ and H₂O, the polymeric VO_x species had not been hardly influenced, while the formation of sulfate species on Ce sites not only promoted the adsorption of NH₄⁺ species and the reaction between gaseous NO and NH₄⁺ but also facilitated the decomposition of ammonium bisulfate through weakening the strong bond between HSO₄^- and NH₄⁺. This work provided a new strategy for SO₂- and H₂O-tolerant catalytic reduction of NO_x at a low temperature.

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.

Time-resolved in situ DRIFTS study on NH3-SCR of NO on a CeO2/TiO2 catalyst

Ce–Ti catalysts were considered as a promising replacement for V–Ti based catalysts for selective catalytic reduction (SCR) of nitrogen oxides (NO and NO2) with NH3.

Inhibition Effect of Phosphorus Poisoning on the Dynamics and Redox of Cu Active Sites in a Cu-SSZ-13 NH3-SCR Catalyst for NOx Reduction

Phosphorus (P) stemming from biodiesel and/or lubricant oil additives is unavoidable in real diesel exhausts and deactivates gradually the Cu-SSZ-13 zeolite catalyst for ammonia-assisted selective catalytic NO_x reduction (NH₃-SCR). Here, the deactivation mechanism of Cu-SSZ-13 by P-poisoning was investigated by ex situ examination of the structural changes and by in situ probing the dynamics and redox of Cu active sites via a combination of impedance spectroscopy, diffuse reflection infrared Fourier transform spectroscopy, and ultraviolet-visible spectroscopy. We unveiled that strong interactions between Cu and P led to not only a loss of Cu active sites for catalytic turnovers but also a restricted dynamic motion of Cu species during low-temperature NH₃-SCR catalysis. Furthermore, the Cu^II ↔ Cu^I redox cycling of Cu sites, especially the Cu^I → Cu^II reoxidation half-cycle, was significantly inhibited, which can be attributed to the restricted Cu motion by P-poisoning disabling the formation of key dimeric Cu intermediates. As a result, the NH₃-SCR activity at low temperatures (200 °C and below) decreased slightly for the mildly poisoned Cu-SSZ-13 and considerably for the severely poisoned Cu-SSZ-13.

Alkali-Resistant Catalytic Reduction of NOx via Naturally Coupling Active and Poisoning Sites

Releasing the poisoning effect of alkali metals over catalysts is still an intractable issue for selective catalytic reduction (SCR) of NOx with ammonia. The presence of K in fly ash always dramatically suppressed catalytic activity by impairing acidity and redox properties, leading to severe reduction of lifetime for SCR catalysts. Herein, alkali-resistant NOx reduction over TiO2-supported Fe2(SO4)3 catalysts was originally demonstrated via naturally coupling active and poisoning sites. Notably, TiO2-supported Fe2(SO4)3 catalysts expressed admirable NOx conversion and K resistance within a quite broad temperature window of 200-500 °C. The catalysts with more conserved sulfate species revealed that sulfate groups preferred to migrate from the bulk phase to surface, thus effectively binding with K poisons to release the damage on iron active sites. Because of protection effects of migrated sulfates and closely coupling effects with Fe active sites, NH3 and NO adsorption amounts and rates were well maintained. In this way, Fe metal sites and sulfate species closely coupled together on a self-preserved TiO2-supported Fe2(SO4)3 catalyst played essential roles as highly active sites and unique poisoning sites. This work paves a new way to design SCR catalysts with superior alkali resistance that are more reliable in practical deNOx application.

Insights into the co-doping effect of Fe3+ and Zr4+ on the anti-K performance of CeTiOx catalyst for NH3-SCR reaction

A novel K-resistant Fe³⁺ and Zr⁴⁺ co-doped CeTiO_x catalyst was first prepared by co-precipitation method for the ammonia-selective catalytic reduction (NH₃-SCR) of NO_x. On the premise of retaining the outstanding catalytic activity of CeTiO_x catalyst, Fe³⁺ and Zr⁴⁺ co-doping efficiently improves its K-resistance with superior NO_x conversion up to 84% after K-poisoning. Specially, the grain growth during the second calcination after K poisoning is successfully inhibited by Fe³⁺ and Zr⁴⁺ co-doping. Consequently, the large specific surface area with increased acid sites and efficiently retained reducibility over K-poisoned FeZrCeTiO_x catalyst are realized, which prompt NH₃ activation and NO oxidation, further benefit NH₃-SCR. Besides, NH₃-SCR reaction over CeTiO_x and FeZrCeTiO_x catalysts follows a possible L-H mechanism, and K-poisoning makes no change to it. Finally, a reasonable anti-K poisoning mechanism of FeZrCeTiO_x catalyst is proposed: the excellent K-resistance is attributed to part of Fe and Zr are sacrificed to form Fe-O-K and Zr-O-K species protecting the active site Ce-O-Ti from K-poisoning, as well as the additional reducibility and surface acidity brought from Fe-O species with Zr prompting its uniform distribution.

Phosphoric Acid Modification of Hβ Zeolite for Guaiacol Hydrodeoxygenation

Regulating the acid property of zeolite is an effective strategy to improve dehydration of intermediate alcohol, which is the rate-determining step in hydrodeoxygenation of lignin-based phenolic compounds. Herein, a commercial Hβ (SiO2/Al2O3 = 25) was modified by phosphoric acid, and evaluated in the catalytic performance of guaiacol to cyclohexane, combined with Ni/SiO2 prepared by the ammonia evaporation hydrothermal (AEH) method. Incorporating a small amount of phosphorus had little impact on the morphology, texture properties of Hβ, but led to dramatic variations in acid property, including the amount of acid sites and the ratio of Brønsted acid sites to Lewis acid sites, as confirmed by NH3-TPD, Py-IR, FT-IR and 27Al MAS NMR. Phosphorus modification on Hβ could effectively balance competitive adsorption of guaiacol on Lewis acid sites and intermediate alcohol dehydration on Brønsted acid sites, and then enhanced the catalytic performance of guaiacol hydrodeoxygenation to cyclohexane. By comparison, Hβ containing 2 wt.% phosphorus reached the highest activity and cyclohexane selectivity.

Improved NOx Reduction over Phosphate-Modified Fe2O3/TiO2 Catalysts Via Tailoring Reaction Paths by In Situ Creating Alkali-Poisoning Sites

The deactivation issue arising from alkali poisoning over catalysts is still a challenge for the selective catalytic reduction of NO_x by NH₃. Herein, improved NO_x reduction in the presence of alkaline metals over phosphate-modified Fe₂O₃/TiO₂ catalysts has been originally demonstrated via tailoring the reaction paths by in situ creating alkali-poisoning sites. The introduction of phosphate results in the partial formation of iron phosphate species and makes the catalyst to mainly exhibit the characteristics of FePO₄, which is responsible for the widened temperature window and enhanced alkali resistance. The tetrahedral [FeO₄]/[PO₄] structures in iron phosphate act as the Brønsted acid sites to increase the catalyst surface acidity. In addition, the formation of an Fe-O-P structure enhances the redox ability and increases surface adsorbed oxygen. Furthermore, the created phosphate groups (PO₄^3-) serving as alkali-poisoning sites preferentially combine with potassium so that iron species on the active sites are protected. Therefore, the enhanced NH₃ species adsorption capacity, improved redox ability, and active nitrate species remaining in the phosphate-modified Fe₂O₃/TiO₂ catalyst ensure the de-NO_x activity after being poisoned by alkali metals through the Langmuir-Hinshelwood reaction pathway. Hopefully, this novel strategy could provide an inspiration to design novel catalysts to control NO_x emission with extraordinary resistance to alkaline metals.

Enhanced activity of vanadia supported on microporous titania for the selective catalytic reduction of NO with NH3: Effect of promoters

Vanadium oxide-based catalysts are considered a promising catalyst for selective catalytic reduction (SCR) of NO with NH₃, which is an effective NO_x removal technology. As environmental issues have garnered more attention, however, improvements to vanadium-based SCR catalysts are strongly required. In a previous study, we found that vanadium oxide on microporous titania as a support (V/MPTiO₂) has certain advantages, such as improved thermal stability and more suppressed N₂O formation, over the use of conventional nanoparticle titania (DT-51) as a support. In this study, widely used promoters, such as W, Sb, and Mo, were added to V/MPTiO₂ to investigate whether they have promoting effects on V/MPTiO₂ as well. Among these promoters added catalysts, the W and Mo were found to have significant promoting effects on the enhancement of deNO_x activities at low temperatures, while the addition of Sb to V/MPTiO₂ tended to have a negative effect on the SCR activity. Based on the characterizations, including laser Raman, H₂-temperature programmed reduction (H₂-TPR), and in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) analysis, we found that the addition of W and Mo increased the degree of polymerization in V/MPTiO₂, which generated more reactive vanadia species. Hence, such changes, resulting from the addition of W and Mo promoters to V/MPTiO₂, yielded enhanced catalytic activity at low temperatures.

Highly Efficient NO Abatement over Cu-ZSM-5 with Special Nanosheet Features

Conventional Cu-ZSM-5 and special Cu-ZSM-5 catalysts with diverse morphologies (nanoparticles, nanosheets, hollow spheres) were synthesized and comparatively investigated for their performances in the selective catalytic reduction (SCR) of NO to N₂ with ammonia. Significant differences in SCR behavior were observed, and nanosheet-like Cu-ZSM-5 showed the best SCR performance with the lowest T₅₀ of 130 °C and nearly complete conversion in the temperature range of 200-400 °C. It was found that Cu-ZSM-5 nanosheets [mainly exposed (0 1 0) crystal plane] with abundant mesopores and framework Al species were favorable for the formation of high external surface areas and Al pairs, which influenced the local environment of Cu. This motivated the preferential formation of active copper species and the rapid switch between Cu²⁺ and Cu⁺ species during NH₃-SCR, thus exhibiting the highest NO conversion. In situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) results indicated that the Cu-ZSM-5 nanosheets were dominated by the Eley-Rideal (E-R) mechanism and the labile nitrite species (NH₄NO₂) were the crucial intermediates during the NH₃-SCR process, while the inert nitrates were more prone to generate on Cu-ZSM-5 nanoparticles and conventional one. The combined density functional theory (DFT) calculations revealed that the decomposition energy barrier of nitrosamide species (NH₂NO) on the (0 1 0) crystal plane of Cu-ZSM-5 was lower than those on (0 0 1) and (1 0 0) crystal planes. This study provides a strategy for the design of NH₃-SCR zeolite catalysts.

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.

Selective catalytic oxidation of NH3 over noble metal-based catalysts: state of the art and future prospects

The state of the art and future prospects for selective catalytic oxidation of NH₃ over noble metal-based catalysts are presented.