Tailored Alkali Resistance of DeNOx Catalysts by Improving Redox Properties and Activating Adsorbed Reactive Species

A comprehensive review of deactivation and modification of selective catalytic reaction catalysts installed in cement kilns

After the ultralow emission transformation of coal-fired power plants, cement production became China's leading industrial emission source of nitrogen oxides. Flue gas dust contents at the outlet of cement kiln preheaters were as high as 80-100 g/m3, and the calcium oxide content in the dust exceeded 60%. Commercial V2O5(-WO3)/TiO2 catalysts suitable for coal-fired flue gas suffer from alkaline earth metal Ca poisoning of cement kiln flue gas. Recent studies have also identified the poisoning of cement kiln selective catalytic reaction (SCR) catalysts by the heavy metals lead and thallium. Investigation of the poisoning process is the primary basis for analyzing the catalytic lifetime. This review summarizes and analyzes the SCR catalytic mechanism and chronicles the research progress concerning this poisoning mechanism. Based on the catalytic and toxification mechanisms, it can be inferred that improving the anti-poisoning performance of a catalyst enhances its acidity, surface redox performance-active catalytic sites, and shell layer protection. The data provide support in guiding engineering practice and reducing operating costs of SCR plants. Finally, future research directions for SCR denitrification catalysts in the cement industry are discussed. This study provides critical support for the development and optimization of poisoning-resistant SCR denitrification catalysts.

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.

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.

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.

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.

Adjustment of operation temperature window of Mn-Ce oxide catalyst for the selective catalytic reduction of NOx with NH3

In order to enhance the catalytic activity of Mn-Ce oxide catalyst for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR), W was introduced as a promoter. With the doping of W, the NO_x conversion over Mn₃CeO_x catalyst above 150 °C was increased, and the N₂O production was significantly decreased. Even in the present of water vapour, Mn₃CeW_0.3O_x still showed a good SCR activity. H₂-TPR and XPS results suggested that the doping of tungsten could inhibit the charge imbalance and reducibility, which would inhibit NO oxidation to NO₂ over Mn₃CeO_x. As a result, the NO_x conversion below 150 °C over Mn₃CeW_0.3O_x was slightly lower than that over Mn₃CeO_x. Since the NO_x production and the NH₃ conversion during the NH₃ oxidation of Mn₃CeO_x were inhibited after the doping of W, the NO_x conversion above 150 °C over Mn₃CeW_0.3O_x was higher than that over Mn₃CeO_x. The transient reaction demonstrated that the doped W species on Mn₃CeW_0.3O_x could inhibit the N₂O produced by the Langmuir-Hinshelwood mechanism. Kinetic study proved that ν_SCR over Mn₃CeW_0.3O_x was obviously higher that over Mn₃CeO_x, ν_NSCR and ν_C-O over Mn₃CeO_x were much higher than those of Mn₃CeW_0.3O_x, which were consistent with the SCR activity.

Recent advances in layered double hydroxides (LDHs) derived catalysts for selective catalytic reduction of NOx with NH3

In recent years, layered double hydroxides (LDHs) derived metal oxides as highly efficient catalysts for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) have attracted great attention. The high dispersibility and interchangeability of cations within the brucite-like layers make LDHs an indispensable branch of catalytic materials. With the increasingly stringent and ultra-low emission regulations, there is an urgent need for highly efficient and stable low-medium temperature denitration catalysts in markets. In this contribution, we have critically summarized the recent research progress in the LDHs derived NH₃-SCR catalysts, including their ability for NO_x removal, N₂ selectivity, active temperature window, stability and resistance to poisoning. The advantages and defects of various types of LDHs-derived catalysts are comparatively summarized, and the corresponding modification strategies are discussed. In addition, considering the importance of the catalyst's resistance to poisoning in practical applications, we discuss the poisoning mechanism of each component in flue gases, and provide the corresponding strategies to improve the poisoning resistance of catalysts. Finally, from the perspective of practical applications and operation cost, the regeneration measures of catalysts after poisoning is also discussed. We hope that this work can give timely technical guidance and valuable insights for the applications of LDHs materials in the field of NO_x control.

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.

Selective catalytic reduction of NOx with NH3 over TiO2 supported metal sulfate catalysts prepared via a sol–gel protocol

Metal sulfate catalysts exhibited high SO₂ tolerance in the NH₃-SCR reaction. The NH₃-SCR reaction mechanism on metal sulfate catalysts should follow the Eley–Rideal mechanism.