Catalysts For Reduction Of NOx Research Articles

Superior catalytic performance within H2O-vapor of W-modified CoMn2O4/TiO2 catalyst for selective catalytic reduction of NOx with NH3

Improving the resistance to H2O is crucial to promote industrial applications of Mn-based catalysts for selective catalytic reduction of NOx by NH3 (NH3-SCR). In this work, we reported the CoMn2O4/TiO2 catalyst, and W was introduced as a promoter to enhance the resistance to H2O at low-temperature. CoMn2O4/W-TiO2 catalyst exhibited remarkable H2O-resistance of above 95% NOx conversion and nearly 100% N2 selectivity at 150–250 °C in the presence of 10 vol% H2O. Further, the role of tungsten and the effect of water on the structure and chemical properties of CoMn2O4/W-TiO2 catalyst were revealed by various techniques of BET, XRD, Raman, H2-TPR, NH3-TPD, XPS and in-situ DRIFTS experiments. The superior performance was attributed to unique spinel structure, mesoporous structure, highly dispersed tungsten species, greater surface acidity. The electron cycle among Mn, Co, W and Ti was beneficial to maintain the Mn3+/Mn4+ and Co3+ at high concentrations and enhance lattice oxygen mobility. The SCR reaction mainly followed the Eley-Rideal (E-R) mechanism over two catalysts, but the poor water resistance of CoMn2O4/TiO2 was attributed to the suppression of NH3 activation and E-R mechanism, despite NH3 adsorption was enhanced. Both Lewis and Brønsted acid sites were created by the incorporation of tungsten in the presence of H2O, and enhanced NH3 adsorption, promoting E-R reaction pathway. Besides, N2 selectivity was significantly enhanced by H2O, which could be due to a decrease in the redox activity of both catalysts. This study opens up a new avenue for designing efficient and environment-friendly NH3-SCR catalysts and looks promising for practical application.

Influence of Coke Deposition by Propene Poisoning on Cu-Ssz-13 Catalyst for Selective Catalytic Reduction of Nox with Nh3

Influence of Coke Deposition by Propene Poisoning on Cu-Ssz-13 Catalyst for Selective Catalytic Reduction of Nox with Nh3

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.

Ordered Nanostructure Catalysts Efficient for NOx Storage/Reduction (NSR) Processes

NOx emissions in the atmosphere can cause various environmental problems, which should be strictly controlled and regulated. Furthermore, because of the limited amount of crude oil resources in the world and severe global warming, the development of fuel-efficient vehicles has long been desired. Accordingly, efficient NOx storage and reduction catalysts have been developed over the decades, called NSR (NOx storage/reduction) catalysts. In the present article, recent advances in NSR catalysts which possess ordered nanostructures will be summarized, including our noble Pt/KNO3/K-titanate nanobelt (KTN), Pt-KNO3/CeO2 and Pt-KNO3/ZrO2 catalysts, as well as nanoporous Ni-phosphate (VSB-5) and Co-substituted VSB-5 catalysts.

Comparative Study of SO2 and SO2/SO3 Poisoning and Regeneration of Cu/BEA and Cu/SSZ-13 for NH3 SCR

Two copper-exchanged zeolites, Cu/SSZ-13 and Cu/BEA, were studied as catalysts for the selective reduction of NOx by NH3 (NH3-SCR). Their activities for standard SCR (NOx = NO) and fast SCR (NOx = 50% NO + 50% NO2) were measured before and after sulfur poisoning at 250 °C. The effect of 30 ppm SO2 and a mixture of 24 ppm SO3 + 6 ppm SO2 was evaluated. The repetition of subsequent activity measurements served as regeneration method in SCR conditions. SO2 deactivated Cu/SSZ-13 whereas Cu/BEA was only moderately affected. SO3 led to stronger deactivation of both catalysts than SO2. However, also for this case, the Cu/BEA was significantly less affected than Cu/SSZ-13, even though Cu/BEA contained larger amount of stored sulfur. One possible reason for this could be the large pores of Cu/BEA, where the sulfur species possibly resulted in less sterical hindrance than in the small pore SSZ-13 structure. NH3 temperature-programmed desorption (NH3-TPD) showed no loss of storage sites upon sulfur treatment and subsequent regeneration. Partial activity recovery was observed after a period in SCR conditions at 400 °C and 500 °C. Temperature at 300 °C was insufficient to regenerate the catalysts. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of NO adsorption suggested that SO2 interacts with the ZCuOH sites on Cu/SSZ-13, causing the strong poisoning.

High-temperature selective catalytic reduction of NO with NH3: Optimization of ZrO2 and WO3 complex oxides

The catalyst for selective catalytic reduction of NOx at high temperatures is significantly demanded by current gas-fired exhaust purification. In this work, the complex oxides composed of two different structures of ZrO2-WO3 and WO3/ZrO2 with zirconium as the main component were prepared for NH3-SCR of NO at high temperatures by blending method and equal volume impregnation method, respectively. Combined with XRD, NH3-TPD, FE-SEM, BET and in situ DRIFTS characterization, the effects of component optimization of two kinds of ZrO2 and WO3 complex oxides on the performance for NH3-SCR of NO were mainly studied. The optimal compatibility of ZrO2 and WO3 complex oxides was determined, and the corresponding N2 selectivity and anti-interference ability of water vapor and SO2 were further investigated. Results showed that the performance of ZrO2-WO3 prepared by blending method for NH3-SCR of NO was significantly better than that of WO3/ZrO2 prepared by impregnation method. The ZrO2-WO3 containing 20% WO3 has the best performance in NH3-SCR of NO with NO conversion higher than 90% and N2 selectivity higher than 90% at 450–650 °C, and the performance was not affected by SO2, slightly affected by competitive adsorption of water vapor at 450 °C and not disturbed by water vapor above 500 °C. Further investigation showed that the NH3-SCR of NO over the ZrO2-WO3 reaction followed both L-H and E-R mechanisms. Characterization analysis revealed that ZrO2-WO3 has finer microscopic particles, larger specific surface area, higher acid amount and stronger acid strength than WO3/ZrO2.

Effect of pre-oxidation process on V2O5/AC catalyst for the selective catalytic reduction of NOx with NH3.

Activated coke-based catalysts have attracted extensive attention in denitration by selective catalytic reduction by NH3 (NH3-SCR), due to their excellent catalytic performance at low temperature. In the paper, the V2O5/AC catalyst was prepared by the impregnation method to investigate the effect of pre-oxidation process on its NH3-SCR activity. Activity test results show that the V2O5/AC catalyst with 4-h pre-oxidation exhibits the best NOx removal efficiency, which reaches the NOx conversion is over 75% in the range of 200-240 °C and exhibits an excellent resistance to SO2 and H2O. Characterization results demonstrate that the V4+ was oxidized by oxygen molecule during pre-oxidation process, which contributes to the formation of V5+ ions and surface-active oxygen species. The surface-active oxygen species are conducive to promoting the "fast SCR" reaction; thus, the pre-oxidized process can contribute to the superior NH3-SCR performance for V2O5/AC catalyst at low temperature.

Migration mechanism on Cu species in CHA-type catalyst for the selective catalytic reduction of NOx during hydrothermal treatment

CHA-type aluminosilicate zeolites, have aroused widespread concern recently due to their remarkable physicochemical properties. Meanwhile, Cu-based CHA-type catalysts have been commercialized for desirable low-temperature activity in selective catalytic reduction of NOx. In this work, thermal aging (TA) and hydrothermal aging (HTA) samples were prepared to investigate the respective effects of heat and water on Cu-based CHA-type catalysts during hydrothermal aging treatment. Impressively, TA sample preformed greater SCR activity in low and high temperature range, but HTA sample exhibited a slight decrease compared to the TA sample. The characterization data (obtained by XPS, H2-TPR and in situ DRIFTS techniques) revealed that high temperature promoted the transformation from ZCu(OH) to Z2Cu while high temperature and water facilitated the migration from CuO to Cu2+ during hydrothermal aging. The increase of isolated Cu2+could improve the adsorption capacity of Lewis acid sites at low temperature. However, the gradual drop for the adsorption capacity of Lewis acid sites with the rise of aging temperature indicated the reduction in acid strength after thermal and hydrothermal aging. Additionally, addition of water aggravated the dealumination and reduced the conversion of NOx in the temperature range from 300 °C to 450 °C. While the dealumination occurring at Lewis sites was greatly weakened due to the protection of interaction between Cu and support.

Effect of the modification of alumina supports with chloride on the structure and catalytic performance of Ag/Al2O3 catalysts for the selective catalytic reduction of NOx with propene and H2/propene

The effect of the modification of an alumina support with chloride on the structure and the catalytic performance of Ag/Al2O3 catalysts (SA) was investigated for the selective catalytic reduction (SCR) of NO using C3H6 or H2/C3H6 as reductants. The Ag/Al2O3 catalyst and Cl−-modified Ag/Al2O3 catalysts (SA-Cl) were prepared by a conventional impregnation method and characterized by X-ray diffraction, Brunauer-Emmett-Teller isotherm analysis, electron probe microanalysis, transmission electron microscopy, UV-Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and hydrogen temperature-programmed reduction. The catalytic activities in the C3H6-SCR and H2/C3H6-SCR reactions were evaluated, and the reaction mechanism was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy and synchrotron vacuum ultraviolet photoionization mass spectroscopy (SVUV-PIMS). We found that Cl− modification of the alumina-supported Ag/Al2O3 catalysts facilitated the formation of oxidized silver species (Agnδ+) that catalyze the moderate-temperature oxidation of hydrocarbons into partial oxidation products (mainly acetate species) capable of participating in the SCR reaction. The low-temperature promoting effect of H2 on the C3H6-SCR (“hydrogen effect”) was found to originate from the enhanced decomposition of strongly adsorbed nitrates on the catalyst surface and the conversion of these adsorbed species to –NCO and –CN species. This “H2 effect” occurs in the presence of Agnδ+ species rather than the metallic Ag0 species. A gaseous intermediate, acrylonitrile (CH2CHCN), was also identified in the H2/C3H6-SCR reaction using SVUV-PIMS. These findings provide novel insights in the structure-activity relationship and reaction mechanisms of the SA-catalyzed HC-SCR reaction of NO.

Can NOx reduction by CO react over carbon‐based single‐atom catalysts at low temperatures? A theoretical study

AbstractFirst principles studies combined with the microkinetic analysis were performed to study the reliability and reaction mechanisms of single‐atom‐doped graphene materials in catalyzing NOx reduction with CO. By screening the 3d transition metals (Sc‐Zn) and group IV elements (Si and Ge), it was found that the Ti‐ and Co‐doped graphene sheets (TiGr and CoGr), respectively, exhibited excellent catalytic activities in the NO/NO2‐to‐N2O and the N2O‐to‐N2 processes at a relatively low temperature (450 K). Therefore, the tandem catalyst (TiGr + CoGr) can be a promising catalyst in NOx reduction with CO. It was further revealed that the combination of adsorption energy and electronegativity was a good descriptor to predict the activation energies. The obtained results provide useful information for rational design of carbon‐based single‐atom catalysts for NOx reduction by CO at low temperatures.

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 NOx 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 CuII ↔ CuI redox cycling of Cu sites, especially the CuI → CuII 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.

Redox and acid properties of MnV2Ox/TiO2 catalysts synthesized by assistance of microwave for NO selective catalytic reduction by ammonia

Supported manganese vanadate catalysts for selective catalytic reduction of NOx by ammonia (NH3-SCR) with wide temperature window were fabricated by assistance of microwave. Vanadate dispersed well on the surface of TiO2, giving an enhanced activity in a wide temperature range. The activity over the catalyst at low temperatures enhanced as vanadate loading increased, deriving from the promoting redox property and the lowering stability of adsorbed NH3 species. The in situ DRIFT results indicated that NH3-SCR reaction over the well dispersed vanadate catalyst follow the “Eley-Rideal” (E-R) mechanism for the effective adsorption of NH3. The results showed the potential applications of this kind of catalyst for NOx elimination in some certain conditions.

Wood-inspired high-performing hierarchical porous Ce0·7Zr0·3O2 catalyst for low-temperature selective catalytic reduction of NOx by NH3

Inspired by natural wood, novel Ce0·7Zr0·3O2 catalyst with hierarchical porous structure, W–CeZr, was fabricated using wood template method by utilizing catheter pores and pits in poplar. W–CeZr catalyst was then used for selective catalytic reduction of NO with NH3 for the first time. Compared with conventional Ce0·7Zr0·3O2 catalyst (C–CeZr), W–CeZr catalyst mimicked biological microstructure of wood and exhibited much greater low-temperature activity than C–CeZr, i.e., NO conversion for W–CeZr was 100% in the range of 250–350 °C, while that for C–CeZr was only 58.1% at 250 °C. Based on a series of characterization techniques, this effect can be attributed to an increase in surface area (m2·g−1), enrichment of Ce3+, and enrichment of surface-active oxygen species by 133.7%, 60%, and 59.5% compared to C–CeZr, respectively. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy results show that more activated reactants were present on the surface of W–CeZr because of its stronger surface acidity, as well as redox and oxygen storage abilities.

Revealing active species of CePO4 catalyst for selective catalytic reduction of NOx with NH3

Revealing the active species of the catalyst is conducive to the design of more efficient catalyst. Herein, we tried to demonstrate the roles of amorphous and crystalline structures on CePO4 catalyst during selective catalytic reduction (SCR) of NOx by NH3. Higher calcination temperature promotes the transfer of amorphous structure to crystalline structure on the surface of CePO4. Both amorphous and crystalline CePO4 species on CePO-X samples can provide acid sites for NH3 adsorption, but the former can provide more acid sites. The superior redox property of surface amorphous CePO4 species contributes to its high NH3-SCR activity at low temperature, but it also leads to the decrease of high temperature (>350 °C) NH3-SCR activity due to the oxidation of NH3. In contrast, crystalline CePO4 species shows high activity only at high temperature because of its poor redox property. Therefore, it can be inferred that amorphous and crystalline structures on CePO4 catalyst can be the efficient active species of NH3-SCR at low and high temperature, respectively.

The role of Si coordination structures in the catalytic properties and durability of Cu-SAPO-34 as NH3-SCR catalyst for NOx reduction

Si coordination structures have been proven to greatly influence the ammonia-selective catalytic reduction (NH3-SCR) catalytic properties and the hydrothermal stability of Cu-based silicoaluminophosphate-form catalysts. However, the role of various Si coordination structures in the NH3-SCR reaction over Cu-SAPO-34 catalyst remains unknown. Herein, a batch of Cu-SAPO-34 samples with various Si contents was synthesized via a one-pot method to study the role of Si coordination structures in the NH3-SCR catalytic properties and hydrothermal stability. Cu/34–2 with the highest proportion of Si(xOAl) (x = 1~3) structures exhibits remarkable durability with 90% NO reduction efficiency within 200~450 °C even after a hydrothermal aging treatment at 850 °C. In contrast, Cu/34–1 and Cu/34–4 with the highest proportions of Si(4OAl) and Si(0OAl) structures, respectively, are significantly deactivated by the same hydrothermal treatment. To better understand this phenomenon, the relationship between the Si coordination structures and SCR performance is established using characterization techniques and kinetics measurements. Results reveal that a high content of Si(4OAl) and Si(0OAl) is detrimental to the hydrothermal stability of Cu-SAPO-34 catalyst. However, Si(xOAl) (x = 1~3) structures are conducive to the stabilization of isolated Cu2+, thus enhancing the stability to severe hydrothermal treatment.

Sm-MnOx catalysts for low-temperature selective catalytic reduction of NOx with NH3: Effect of precipitation agent

A series of Sm–Mn mixed oxide catalysts were prepared via precipitation using various precipitants, namely Na2CO3 (NH4)2CO3, and NH3·H2O, and evaluated for the selective catalytic reduction (SCR) of NOx with NH3 at low temperatures. Various characterisation techniques were used to determine the physicochemical properties of the catalysts, and it is found that their catalytic performance is greatly influenced by the nature of the precipitation agent used. It is found that Sm0.1Mn–Na2CO3 and Sm0.1Mn-(NH4)2CO3 exhibit superior catalytic performance in the SCR reaction to that of Sm0.1Mn–NH3·H2O due to an abundance of surface acid sites, high surface concentration of Mn4+, and high NO oxidation capacity. From in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) analysis, we conclude that the Sm–Mn catalysts follow both Eley-Rideal and Langmuir–Hinshelwood mechanisms, and that the Eley-Rideal mechanism is dominant at elevated temperatures.

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.

Insight into the homogenous and heterogeneous transformation behavior of arsenic on commercial V2O5-WO3-TiO2 and novel γ-Fe2O3 catalysts during selective catalytic reduction of NOx

Effect of arsenic on the activity of V2O5-WO3-TiO2 and γ-Fe2O3 catalysts for selective catalytic reduction of NOx with ammonia were studied in this work, and the more important objective was to reveal the heterogeneous and homogeneous reactions about arsenic on V2O5-WO3-TiO2 and γ-Fe2O3 catalysts during arsenic poisoning process. It was discovered that oxygen enhanced the deactivation of V2O5-WO3-TiO2 caused by arsenic. However, the increased oxygen exhibited a contrast impact on arsenic poisoning process of γ-Fe2O3 catalyst. On the other hand, temperature suppressed deactivation effect of arsenic both on V2O5-WO3-TiO2 and γ-Fe2O3. Further concluded from the results of arsenic content in poisoned catalysts, it was found that promotion effect of oxygen on the oxidation of trivalent arsenic in gas side intensified the combination between vanadium and arsenic while inhibited reactions between iron and arsenic. Besides, the aggravation effect of temperature rising on the decomposition of gaseous pentavalent arsenic weakened the combination between gaseous arsenic and vanadium on V2O5-WO3-TiO2 catalyst, while facilitated the combination between gaseous arsenic and non-active iron sites on γ-Fe2O3. Moreover, after trivalent arsenic combined on the surface of catalyst, adsorbed oxygen and pentavalent vanadium competed for providing vacant electron sites to proceed the oxidation of combined trivalent arsenic.

Unraveling SO2-tolerant mechanism over Fe2(SO4)3/TiO2 catalysts for NOx reduction

Developing low-temperature SO2-tolerant catalysts for the selective catalytic reduction of NOx is still a challenging task. The sulfation of active metal oxides and deposition of ammonium bisulfate deactivate catalysts, due to the difficult decomposition of the as-formed sulfate species at low temperatures (<300 °C). In recent years, metal sulfate catalysts have attracted increasing attention owing to their good catalytic activity and strong SO2 tolerance at higher temperatures (>300°C); however, the SO2-tolerant mechanism of metal sulfate catalysts is still ambiguous. In this study, Fe2(SO4)3/TiO2 and Ce2(SO4)3/TiO2 catalysts were prepared using the corresponding metal sulfate salt as the precursor. These catalysts were tested for their low-temperature activity and SO2 tolerance activity. Compared to Ce2(SO4)3/TiO2, Fe2(SO4)3/TiO2 showed significantly better low-temperature activity and SO2 tolerance. It was demonstrated that less surface sulfate species formed on Fe2(SO4)3/TiO2 and Ce2(SO4)3/TiO2. However, the presence of NO and O2 could assist the decomposition of NH4HSO4 over Fe2(SO4)3/TiO2 at a lower temperature, endowing Fe2(SO4)3/TiO2 with better low-temperature SO2 tolerance than Ce2(SO4)3/TiO2. This study unraveled the SO2-tolerant mechanism of Fe2(SO4)3/TiO2 at lower temperatures (<300 °C), and a potential strategy is proposed for improving the low-temperature SO2-tolerance of catalysts with Fe2(SO4)3 as the main active component or functional promoter.

Enhanced performance of iron-cerium NOx reduction catalysts by sulfuric acid treatment: The synergistic effect of surface acidity and redox capacity

CeO2-based catalysts have raised extensive attention in selective catalytic reduction of NO with NH3 but the poor T90 operation window limits their industrial application. In present work, FeOx-CeO2 mixed oxides were prepared by a sol-gel method for NH3-SCR, and the effects of sulfuric acid treatment on the catalytic performance were systematically investigated. The results showed that sulfuric acid treatment significantly improved the high temperature activity. Compared to the FeOx-CeO2 sample, the T90 operation window of FeOx-CeO2-S was greatly broadened from 200−300 °C to 250−450 °C. Various characterizations demonstrated that sulfuric acid introduction increased redox capacity and also enhanced surface acidity. The synergistic effect of these two parameters was closely associated with the enhanced performance. Further investigations revealed that the sulfuric acid modification favored the formation of active intermediate species for N2 generation, while inhibited the formation of N2O and NOx, thus contributing to the high catalytic activity and N2 selectivity.