DeNOx Catalysts Research Articles

Insight into the sulfur resistance of manganese oxide for NH3-SCR: Perspective from the valence state distributions

The multiple valence states distributions (VSDs) endowed the catalyst with excellent low temperature de-NOx activity, while little is known about whether the SO2 resistance can be affected by VSDs modulation. In this context, three model manganese oxide catalysts (MnO2, Mn2O3 and Mn3O4) with different valence state distributions were fabricated and evaluated in the NH3-SCR reaction under the presence of SO2. The results show that the three catalysts exhibited a valance-state-dependent SO2 resistance (Mn3O4 > Mn2O3 > MnO2) under the SCR atmosphere with the presence of 100 ppm SO2 at 100 °C within 7 h. Thermal regeneration analysis revealed the overall deactivation was dominated by the remarkably different reversible activity loss proportions, which were found proportionally correlated to the superficial Mn4+/Mnn+ content of the catalysts. Further characterizations indicated the Mn3O4 catalyst with more basic sites and less bulk-like sulfate formation could be the reason for its more slowly deactivation within the limited test period. However, the overall number of basic sites on MnOx is still critically insufficient for longer SO2 resistance test. This work clarified the deactivations details of MnOx from the perspective of valence state distributions, which can provide references for the design of better low temperature SO2-tolerant de-NOx catalyst.

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

Nowadays, the serious deactivation of deNOx catalysts caused by alkali metal poisoning was still a huge bottleneck in the practical application of selective catalytic reduction of NOx with NH3. Herein, alkali-resistant NOx 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 NOx 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 NHx and NOx adsorption and the subsequent efficient reaction, which in turn resulted in high NOx catalytic reduction capacity of the catalyst. The strategy provided new inspiration for the development of novel and efficient selective catalytic reduction of NOx with NH3 (NH3-SCR) catalysts with high alkali resistance.

Structure–activity relationship and the inhibitory effect of sulfur dioxide and water on nitrous oxide formation in selective catalytic reduction of nitrogen oxides by ammonia over hollow Co3O4@CoMn2O4 catalyst

Hollow structures have attracted great interest in many areas for their diverse applications. In this work, a new catalyst with an open and hollow structure (Co3O4@CoMn2O4) is designed for selective catalytic reduction of nitrogen oxides by ammonia (NH3-SCR). The as-prepared hollow-structured catalyst provides a high surface area and has thin shells. Owing to its structural benefits, this catalyst exhibited enhanced nitrogen oxides (NOx) removal activity and better resistance to water and sulfur dioxide than cobalt manganate nanoparticles. It also has proved that both the Eley-Rideal and Langmuir-Hinshelwood mechanisms are present in the NH3-SCR process in this catalyst. The improved nitrogen selectivity after the addition of water and sulfur dioxide occurs owing to the inhibition of nitrous oxide formation through the Eley-Rideal and Langmuir-Hinshelwood mechanisms. The deep insight into the structure–activity relationship and the influence of water and sulfur dioxide on nitrogen selectivity provide a new perspective for constructing high-performance de-NOx catalysts.

Promotional effect of phosphorus addition on improving the SO2 resistance of V2O5-MoO3/TiO2 catalyst for NH3-SCR of NO

The oxidation of SO2 into SO3 promote the formation of NH4HSO4 (ABS), which can cover surface active sites catalyst, and then leading to the decrease of NH3-SCR De-NOx activity. In this work, phosphorus species were loaded on the V2O5-MoO3/TiO2 De-NOx catalyst to enhance its SO2 resistance via inhibiting the SO2/SO3 conversion. XRD and N2-adsorption results showed that phosphorus has little impact on the structure and pore property of the catalyst. The results of XPS, H2-TPR, and NH3-TPD analyses illustrated that suitable amount of phosphorus (1%) increases the content of polymeric vanadate and chemisorbed oxygen (Oα) of the catalyst, which is benefit to the SCR process. Phosphorus species decreases the SO2 absorbability and SO2 oxidation activity of V-Mo-P/Ti catalysts, thus it exhibits higher NH3-SCR activity than that of the P-free catalyst at presence of SO2 due to the lower SO2/SO3 conversion and fewer amount of NH4HSO4 deposition.

SO2-Induced Alkali Resistance of FeVO4/TiO2 Catalysts for NOx Reduction.

Selective catalytic reduction of nitrogen oxides with ammonia (NH3-SCR) is an efficient NOx abatement strategy, but deNOx catalysts suffer from serious deactivation due to the coexistence of multiple poisoning substances such as K, SO2, etc. in the flue gas. It is essential to understand the interaction among various poisons and their effects on NOx abatement. Here, we unexpectedly identified the K migration behavior induced by SO2 over K-poisoned FeVO4/TiO2 catalysts, which led to alkali-poisoning buffering and activity recovery. It has been demonstrated that the K would occupy both redox and acidic sites, which severely reduced the reactivity of FeVO4/TiO2 catalysts. After the sulfuration of the K-poisoned catalyst, SO2 preferred to be combined with the surface K2O, lengthened the K-OFe and K-OV, and thus released the active sites poisoned by K2O, thereby preserving an increase in the activity. As a result, for the K-poisoned catalyst, the conversion of NOx increased from 21 to 97% at 270 °C after the sulfuration process. This work contributes to the understanding of the specific interaction between alkali metals 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.

Hydrometallurgical process development to recycle valuable metals from spent SCR deNOX catalyst

Spent catalyst, containing vanadium and tungsten oxide in a TiO2 glass fiber matrix, pose a risk of environmental contamination due to the high toxicity of its metal oxides if leached into the soil when disposed in landfills. Due to the increasing demand of metals and the continuous depletion of primary resources there is an growing necessity for recycling and reprocessing of spent catalysts and other secondary metal sources for environmental and economical reasons. Study of spent SCR catalyst soda roasting process with dissolved NaOH compared with the usual NaOH dry roasting and its influence in the subsequent water leaching. After optimization, the ideal parameters are roasting using a 0.4 ratio of NaOH/spent SCR catalyst in solution for 2 h at 973 K and de-ionized water leaching for 30 min, at 298 K with a pulp density of 30%. The research results show an important reduction of the roasting temperature and leaching time during the processing of spent SCR catalyst obtaining a 95.4% W and 80.2% V leaching efficiency liquor. Silicon compounds are one of the main impurities leached alongside the valuable metals and in this work, the silicon compounds leached are reduced significantly with the aim of avoiding the de-silication post-processing of the leach liquor. The main advantage of the proposed process is the increase of the leaching efficiency of vanadium and tungsten with a minimization of silicon impurities in a shorter time regardless of the leaching temperature.

One-step synthesis by redox co-precipitation method for low-dimensional Me-Mn bi-metal oxides (Me=Co, Ni, Sn) as SCR DeNOx catalysts.

In this research, one-step synthesis of redox co-precipitation method (using sodium lauryl sulfate, KMnO4, and metal precursor) was well applicable in universally preparing low-dimensional Me-MnOx nanosheet catalysts with different metal doping (Me=Co, Ni, or Sn). NH3-SCR activity was explored to the relationship with structure morphology and physio-chemical properties via the characterization techniques of SEM, XRD, XPS, H2-TPR, and NH3-TPD. It was found that Ni-MnOx has a relatively poor activity at low-down temperature but was improved as the reaction temperature rising. Co-MnOx presented a relatively stable catalytic activity of which the NOx conversion rate can be maintained 80~90% in a wide temperature window of 100-250 °C with relatively better N2 selectivity. Compared with Co- or Ni-modified MnOx, Sn-MnOx catalyst has an excellent low-temperature catalytic activity (93% NOx conversion at 100 °C) that was maintained > 80% before 200 °C but with poor selectivity to N2. Due to its nanosheet-structured solid solution structure, Sn-MnOx promoted the interaction between MnOx and SnO2 with the increased contents of adsorbed oxygen and also the numbers of surface Lewis acid sites, which integrally promoted the NH3-SCR reaction at low temperature and also contributed to an acceptable resistances to water and sulfur. High content of adsorbed oxygen was beneficial to improve the catalytic activity at lower temperatures, while the electron cycle interaction of different metal valence ions will play a more important role with the increase of reaction temperature.

1-04 微粉炭ボイラー用脱硝触媒上のSi化合物析出特性

1-04 微粉炭ボイラー用脱硝触媒上のSi化合物析出特性

Structured NSR-SCR hybrid catalytic technology: Influence of operational parameters on deNOx activity

An intermediate approximation using structured reactors between the study of deNOx catalysts at laboratory scale and in motor bench has been addressed. The hybrid NSR (NOx Storage and Reduction) and SCR (Selective Catalytic Reduction) technology has been analyzed for Pt-Ba-K/Al2O3 and Cu-SAPO-34 catalysts structured in monolithic form. In order to analyze the influence of the way of driving on deNOx efficiency, operational parameters such as NO inlet concentration, space velocity in terms of total flow rate, temperature, and the concentration of reductant agent, using H2 and C3H6, have been modified. The coupled NSR-SCR configuration is more efficient than the single NSR alternative, since the former used the ammonia produced in NSR monolith to reduce the non-adsorbed NOx through the SCR process over the system placed downstream. However, NH3 production depended on NO concentration, temperature, and reductant agent content in the process, limiting the promoting effect at high temperatures and NO concentrations, and low hydrogen concentration. N2 selectivity increased when temperature increased, and NO concentration and hydrogen content decreased. On the other hand, NOx to N2 reduction efficiency of the combined NSR-SCR system was not affected by variations in space velocity, with a maximum range studied up to 90,000 h−1. Based on these results, the location of the catalytic system should be adjusted in order to work in the 250–350 °C temperature range, where conversion and N2 selectivity values were around 70% and 95%, respectively. On the other hand, the hydrogen concentration of 2% and 3% can be suitable for the efficient operation of the system, since as reductant agent increased the ammonia formation and the surface regeneration also increased. The substitution of H2 by propylene as reducing agent caused a decrease in the amount of ammonia produced in the NSR configuration, which limits the improvement of deNOx activity of the double NSR-SCR configuration, and overproduces CO2, CO and N2O through parallel reactions as reverse water gas shift or C3H6 oxidation, which could poison the system and require emission control through other technologies.

Separation of Vanadium and Tungsten from Spent SCR DeNOX Catalyst by Ion-exchange Column

SCR 탈질 폐촉매로부터 바나듐과 텅스텐은 소다배소-수침출 공정을 통해 얻은 침출액으로부터 분리/회수하여 얻을 수 있다. 본 연구에서는 강염기성 음이온교환수지인 Lewatit monoplus MP 600을 사용하여 연속식 이온교환칼럼에서 수용액에 용해되어 있는 바나듐과 텅스텐의 흡·탈착 거동을 알아보고, 바나듐/텅스텐 분리를 위한 연속식 이온교환칼럼 운전조건을 제시하고자 하였다. 수용액 pH 8.5에서 단일 성분 수용액으로 연속식 흡착실험을 수행한 결과, 흡착용량은 바나듐 44.75 mg/(g of resin)과 텅스텐 64.92 mg/(g of resin)으로 바나듐보다 텅스텐의 흡착용량이 크게 나타났으며 이는 이온교환수지에 흡착되는 이온의 전하수가 바나듐 보다는 텅스텐이 작기 때문이라고 사료된다. 텅스텐이 흡착된 이온교환수지에 바나듐 함유 수용액이 공급됨에 따라 이온교환수지에 흡착되었던 텅스텐이 바나듐과 교환되며 탈착되는 거동을 보였으며, 이로부터 MP 600에 대하여 바나듐이 텅스텐보다 친화도(affinity)가 높음을 알 수 있었다. SCR 탈질 폐촉매 침출액과 동일한 농도의 바나듐과 텅스텐 혼합용액으로 pH 8.5에서 연속식 실험을 수행한 결과 바나듐의 흡착 용량은 48.72 mg/(g or resin)으로 공급량의 80%가 흡착된 반면 텅스텐의 경우 이온교환수지에 흡착된 양이 거의 0에 근접하며 바나듐과 텅스텐의 분리가 효과적으로 이루어졌다. 바나듐이 흡착된 이온교환수지로부터 2M HCl를 15 mL/h로 공급하여 97.7%의 바나듐을 99%의 순도로 탈착시킬 수 있었다. 탈착용액으로부터 염화암모늄을 침전제로 사용하여 90℃에서 암모늄폴리바나데이트 형태로 93%의 바나듐을 회수하였다.

Synthesis of Macro-Porous De-NOx Catalysts for Poly-Tetra-Fluoro-Ethylene Membrane Bag Filter.

This study examined the effects of the porosity of catalytic bag-filter materials for applications to the SNCR (selective noncatalytic reduction)-SCR (selective catalytic reduction) hybrid process for highly treating nitrogen Oxides (NOx) in the exhaust gas of a combustion process. A V₂O5/TiO₂ catalyst was dispersed in a PTFE (poly-tetra-fluoro-ethylene) used as the catalytic bag-filter material to remove particulate matter and nitrogen oxides contained in the combustion exhaust gas. Macroporous alumina was added into a V₂O5/TiO₂-dispersed PTFE to improve the catalytic activity of V₂O5/TiO₂ dispersed in the PTFE material. In this study, the textural properties and denitrification performances of the V₂O5/TiO₂-dispersed PTFE materials were examined according to the addition of macro-porous alumina. When the denitrification catalyst was solely dispersed in the PTFE material, the catalyst inside the PTFE backbone had low gas-solid contact efficiency owing to the low porosity of the PTFE materials, resulting in low denitrification efficiency. On the other hand, the catalytic activity of V₂O5/TiO₂ dispersed inside the macro-porous PTFE material was significantly enhanced by adding macro-porous alumina into the PTFE matrix. The enhanced textural properties of the macro-porous PTFE material where V₂O5/TiO₂ was uniformly dispersed proved the facilitated diffusion of combustion exhaust gas into the PTFE material.

Experimental and Theoretical Studies of Sonically Prepared Cu–Y, Cu–USY and Cu–ZSM-5 Catalysts for SCR deNOx

The objective of our study was to prepare Y-, USY- and ZSM-5-based catalysts by hydrothermal synthesis, followed by copper active-phase deposition by either conventional ion-exchange or ultrasonic irradiation. The resulting materials were characterized by XRD, BET, SEM, TEM, Raman, UV-Vis, monitoring ammonia and nitrogen oxide sorption by FT-IR and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). XRD data confirmed the purity and structure of the Y/USY or ZSM-5 zeolites. The nitrogen and ammonia sorption results indicated that the materials were highly porous and acidic. The metallic active phase was found in the form of cations in ion-exchanged zeolites and in the form of nanoparticle metal oxides in sonochemically prepared catalysts. The latter showed full activity and high stability in the SCR deNOx reaction. The faujasite-based catalysts were fully active at 200–400 °C, whereas the ZSM-5-based catalysts reached 100% activity at 400–500 °C. Our in situ DRIFTS experiments revealed that Cu–O(NO) and Cu–NH3 were intermediates, also indicating the role of Brønsted sites in the formation of NH4NO3. Furthermore, the results from our experimental in situ spectroscopic studies were compared with DFT models. Overall, our findings suggest two possible mechanisms for the deNOx reaction, depending on the method of catalyst preparation (i.e., conventional ion-exchange vs. ultrasonic irradiation).

Selective Catalytic Reduction of NO by NH3 over Mn–Cu Oxide Catalysts Supported by Highly Porous Silica Gel Powder: Comparative Investigation of Six Different Preparation Methods

In this study, Mn-based catalysts supported by highly porous silica gel powder (SSA up to 470 m2·g−1 and total pore volume up to 0.8 cm3·g−1) were prepared by six different methods in liquid solutions (electroless metal deposition, stepwise addition of a reducing agent, wet impregnation, incipient wetness impregnation, urea hydrolysis, and ammonia evaporation) and tested for selective catalytic reduction of NOx with ammonia (NH3-SCR de-NOx). Prior to the activity test all the catalysts prepared were characterized by ICP-OES, SEM, EDX mapping, XPS, XRD and N2 adsorption techniques to provide the comprehensive information about their composition and morphology, investigate the dispersion of active components on the carrier surface, identify the chemical forms and structural properties of the catalytically active species of the catalysts prepared. The results revealed that all the methods applied for preparation of SCR de-NOx catalysts can ensure the uniform distribution of Mn species on the carrier surface, however as it is typical for preparation techniques in a liquid phase the significant reduction in SSA and pore volume along with increasing the loading was observed. Considering both the physicochemical properties and the catalytic performance of the catalysts the least effective preparation method was shown to be ammonia evaporation, while the most attractive techniques are incipient wetness impregnation and electroless metal deposition.

Single-atom iron as a promising low-temperature catalyst for selective catalytic reduction of NOx with NH3: A theoretical prediction

Selective catalytic reduction of NOx with NH3 (NH3-SCR) is a dominant technology to reduce NOx (deNOx). However, there are two shortcomings for commercial deNOx catalysts (vanadium-titanium-based metal oxides), such as poor low-temperature efficiency and toxicity. Thus, it is urgent to develop environmentally friendly low-temperature catalysts with high deNOx efficiency. Therefore, we firstly proposed a single-atom iron coordinated with four N atoms (Fe1-N4) as a novel low-temperature NH3-SCR catalyst, due to its high coordination unsaturation and safety. The detailed reaction mechanisms are revealed via density functional theory calculations and microkinetic modeling. Seven possible reaction pathways were found in the NH3-SCR reaction. Different from metal oxides, special intermediates such as N2H and NHNO are found in the reaction pathway. The dominant pathway of the NH3-SCR reaction over the Fe1-N4 catalyst is a three-step process including NO oxidation, NO2 reduction, and NHNO decomposition. The suitable temperature window of the Fe1-N4 catalyst is <430 K due to its relatively low energy barrier of 0.99 eV. Different from other metal oxide catalysts, the fast oxidation of NO on Fe1-N4 catalyst significantly promotes the reaction rate of NH3-SCR on Fe1-N4 catalyst. Based on its outstanding performance, we believe that single Fe atom catalyst can provide new insights to design novel catalysts for NH3-SCR.

Cost-effective fast-synthesis of chabazite zeolites for the reduction of NOx

The costly synthesis of conventional CHA zeolites for ammonia selective catalytic reduction of nitrogen oxides (NH3-SCR of NOx) has hindered their widespread applications. In this study, highly crystalline CHA zeolites (CHA-Y and CHA-B) were successfully synthesized from low-cost NaY and Al-rich beta precursors and alternately low cost or low content structure directing agents through the inter-zeolite conversion route. After ion exchange with Cu, the Cu−CHA-Y and Cu−CHA-B showed superior performance in the NH3-SCR of NOx reaction, which can be attributed to their high surface area, abundant acidic sites. The in situ analysis by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed the reaction intermediates of NH3-SCR of NOx over the low-cost prepared Cu−CHA catalysts. The as- prepared low-cost Cu−CHA catalysts are promising candidates for the reduction of NOx conforming to the economic production of deNOx catalysts.

Effect of sulfation on the activity and alkali resistance of TiO2-supported cerium-based deNOx catalysts

Effect of sulfation on the activity and alkali resistance of TiO2-supported cerium-based deNOx catalysts

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

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

Seven Modern Trends in the DeNOx Catalyst Development

Seven Modern Trends in the DeNOx Catalyst Development

Deactivation of V2O5−WO3/TiO2 DeNOx Catalyst under Commercial Conditions in Power Production Plant

Nitrogen dioxide is one of the most dangerous air pollutants, because its high concentration in air can be directly harmful to human health. It is also responsible for photochemical smog and acid rains. One of the most commonly used techniques to tackle this problem in large combustion plants is selective catalytic reduction (SCR). Commercial SCR installations are often equipped with a V2O5−WO3/TiO2 catalyst. In power plants which utilize a solid fuel boiler, catalysts are exposed to unfavorable conditions. In the paper, factors responsible for deactivation of such a catalyst are comprehensively reviewed where different types of deactivation mechanism, like mechanical, chemical or thermal mechanisms, are separately described. The paper presents the impact of sulfur trioxide and ammonia slip on the catalyst deactivation as well as the problem of ammonium bisulfate formation. The latter is one of the crucial factors influencing the loss of catalytic activity. The majority of issues with fast catalyst deactivation occur when the catalyst work in off-design conditions, in particular in too high or too low temperatures.

排煙脱硝技術（ハニカム）の変遷

排煙脱硝技術（ハニカム）の変遷