SO2 Durability Research Articles

Promotional effects of Me (Sb, La, Ce, Mo) additives on the NH3-SCR activity and SO2 durability of V2O5-WO3/TiO2 catalysts

In this study, we investigated the effects of incorporating Sb, La, Ce, and Mo additives into V2O5-WO3/TiO2 catalysts with high vanadium content for NH3-SCR. The addition of Sb, La, or Ce to the V-site of the V4/W5/G5 catalyst led to improved reaction activity, which was attributed to an increase in the nonstoichiometric V4+(3+) ratio and enhanced surface adsorbed oxygen species. Fast-SCR due to NO2 oxidation resulted in increased the reaction rates. Conversely, Mo addition reduced reaction rates. Durability against SO2 improved after incorporating Sb, La, Ce, or Mo, which was attributed to SO2 adsorption inhibition by Sb and Mo, and SO2 consumption by La and Ce forming sulfate species. Sb, Ce, and Mo additives facilitated ABS (ammonium bisulfate, (NH4)HSO4) decomposition at lower temperatures compared to the V4/W5/G5 catalyst, with distinctive NH3 and SO2 desorption profiles. Particularly, Mo addition induced NH3 desorption from ABS decomposition at a low temperature of 177 °C.

Simultaneous removal NO and toluene over the Sb enhanced MnOx catalysts

xSb-Mn (Sb enhanced MnOx) catalysts with different Sb contents were synthesized by co-precipitation method for the simultaneous removal of NO and toluene. Among them, 5 %Sb-Mn catalyst showed superior NO and toluene conversions, and the NO conversion could reach 90 % in 200–350 °C, toluene conversion could reach 100 % in 200–400 °C. The effects of Sb on MnOx catalyst were explored by XRD, N2 adsorption–desorption, SEM, XPS, which suggested that the appropriate Sb doping could improve the surface area of catalyst and increase the dispersity of active component on catalyst. Moreover, Sb could enhance the surface acidity, redox ability, and chemically adsorbed oxygen content over catalyst, enhancing the catalytic performance. Meanwhile, the Sb significantly improved the SO2 and H2O durability of catalyst, which duo to Sb species inhibiting the oxidation of SO2 by Mn, enhancing the SO2 durability of catalyst. Moreover, the reaction mechanism of 5 %Sb-Mn for simultaneous removal of NO and toluene was analyzed by in-situ DRIFTS. Furthermore, the effect of NH3-SCR process on toluene oxidation was investigated. Those results showed that MnOx and 5 %Sb-Mn catalysts follow the E-R mechanism. Besides, NH3 would react with active oxygen species before toluene, which inhibit further oxidation of toluene intermediates. Notably, the Sb species contributed to reducing the restrain of NH3 on toluene oxidation process.

Insight into the mechanisms of activity promotion and SO2 resistance over Fe-doped Ce-W oxide catalyst for NOx reduction

Ceria-based catalysts for the selective catalytic reduction of NOx with NH3 (NH3-SCR) are always subject to deactivation by sulfur poisoning. In this study, Fe-doped Ce-W mixed oxides, which were synthesized by the co-precipitation method, improved the SCR activity and SO2 durability at low temperatures of undoped Ce-W oxides. The improved low-temperature activity was mainly due to the enhancement of redox properties at low temperatures and more active oxygen species, together with the adsorption and activation of more abundant NOx species, facilitating the “fast SCR” reaction. In the presence of SO2, doping with Fe species effectively prevented sulfate deposition on the CeW catalyst, due to the interaction between Fe, Ce, and W species inducing electron transfer among different metal sites and altering the electron distribution. The competitive adsorption behavior between NO and SO2 was changed by Fe doping, in which the adsorption and oxidation of SO2 were restrained. Besides, the elevated NO oxidation accelerated the decomposition of ammonium bisulfate, causing the SCR reaction to not be greatly suppressed. Hence, Fe-doped Ce-W oxides catalysts showed excellent sulfur resistance. This study provides an in-depth understanding of efficient Ce-based catalysts for SO2-tolerance strategies.

Unravelling the promotional effect of Nb and Mo on VOx-based catalysts for NOx reduction with NH3

A series of VxNbMoTi for the selective catalytic reduction (SCR) of NO by NH3 have been synthesized to improve the SCR performance at low temperatures and the SO2 durability of vanadium-based catalysts. In this study, Nb and Mo were selected as promoters for vanadium-based catalysts in the SCR reaction, and the introduction of an appropriate amount of Nb-Mo enhanced the activity and SO2 durability at low temperatures. With the addition of Nb and/or Mo, physicochemical analysis was performed to characterize of the catalysts. The introduction of Nb-Mo into VTi increased the total amount of NH3 adsorption and improved the Vn+ (n = 3, 4) fraction. In addition, the number of oxygen vacancies and the amount of labile oxygen increased. Thus, introducing Nb-Mo into VTi increased SCR performance and enhanced resistance to SO2 and H2O in the presence of SO2 and H2O by reducing the affinity between SO2 and the catalytic surface.

Enhanced SO2 and H2O resistance of MnTiSnOy composite oxide for NH3-SCR through Sm modification

NOx is one of the main sources of air pollution, and the abatement of NOx emission has aroused increasing attention. The NH3-SCR technology is mature and widely used for controlling NOx emissions from fixed sources. In this work, SmxMnTiSnOy was synthesized by a mixed solvothermal synthesis. The NH3-SCR performance of SmxMnTiSnOy was tested and the H2O and SO2 resistance of Sm0MnTiSnOy, Sm0.1MnTiSnOy, and Sm0.2MnTiSnOy catalysts (5 %H2O, 25 ppm SO2) were tested at 250 °C. The physicochemical properties of SmxMnTiSnOy were studied by XRD, FESEM, TEM, BET, XPS, H2-TPR, NH3-TPD and In-situ DRIFTS. The experimental results reveal that Sm0.2MnTiSnOy has a better low-temperature NH3-SCR performance and splendid H2O and SO2 durability. Moreover, appropriate Sm doping increases the specific area and enhances the acidity of the catalyst’s surface. The In-situ DRIFTS results suggest that the adsorption and activation of NH3 are of primary importance in the NH3-SCR reaction. Besides, a large number of weak acid sites on Sm0.2MnTiSnOy are conducive to adsorb and activate NH3 and good redox ability facilitates the activation of bidentate nitrate, respectively. Meanwhile, Sm doping could transfer electrons from the Sm species to the Mn species, which inhibits the formation of Mn sulfate.

New insight into the role of Mo–Sb addition towards VMoSbTi catalysts with enhanced activity for selective catalytic reduction with NH3

The effect of adding Mo and/or Sb to a VTi catalyst on its activity and durability in the selective catalytic reduction (SCR) of nitrogen oxide by NH3 was investigated. The addition of an appropriate amount of Mo–Sb significantly improved the catalyst specific surface area, enhanced the number of Brønsted acid sites, and adjusted the surface species composition. The catalytic activity improved as the Vn+ (n = 4, 3) fraction increased due to the formation of functional groups on the catalyst surface. The redox properties, amount of active labile oxygen, and number of surface oxygen vacancies were improved by the presence of V2MoO8. This species also played an important role in improving the SCR performance and water/sulfur durability of the catalysts due to its sufficient acidity enhancement and reduction of SO2 adsorption. The addition of Mo–Sb likely enhanced catalyst SO2 durability because it reduced SO2 adsorption by inhibiting the reaction between terminal oxygen (V = O) and SO2(g).

NH3-SCR performance and SO2 resistance comparison of CeO2 based catalysts with Fe/Mo additive surface decoration

Fe2O3 and/or MoO3 surface decorated CeO2 were designed and synthesized by in situ deposition and incipient wetness impregnation to adjust the SCR catalytic performance and SO2 resistance of CeO2. The addition of iron oxide slightly improved low temperature SCR activity of CeO2-Fe2O3, but resulted in a fast decline of NOx removal efficiency at higher temperatures, which was due to the enhanced NH3 oxidation by surface redox capacity improvement. Surface acidity instead of redox capacity was the main limiting factor for SCR activity of CeO2 and CeO2-Fe2O3. Meanwhile, the introduction of MoO3 increased NOx conversion of CeO2-MoO3 and CeO2-Fe2O3-MoO3 dramatically in a broad temperature range. The increase of catalysts surface acid sites promoted their SCR catalytic performance through Eley-Rideal mechanism. SO2 resistance test indicated the different surface characteristics for the four catalysts resulting from the composition disparity leaded to different deactivation behaviors at 250 °C. The deposition of ammonium sulfate species accounted for the main activity loss of CeO2 and CeO2-Fe2O3, while the sulfation of redox sites was main deactivation reason for CeO2-MoO3 and CeO2-Fe2O3-MoO3. The interaction between SO2 and CeO2/CeO2-Fe2O3 catalysts changed the surface active sites for SCR reaction essentially and MoO3 could inhibited surface sulfation and ammonium sulfate species deposition. The variation behavior of NOx conversion during SO2 resistance test depended on the reaction temperature. All four catalysts show the usability in the presence of SO2 when the reaction temperature is between 300 °C and 400 °C. Considering the complexity of preparation process, catalytic performance and SO2 durability, CeO2-MoO3 catalyst is a better choice for substituting the traditional V-Ti catalyst.

Effects of different methods of introducing Mo on denitration performance and anti-SO2 poisoning performance of CeO2

Cerium-based catalysts are very attractive for the catalytic abatement of nitrogen oxides (NOx) emitted from stationary sources. However, the main challenge is still achieving satisfactory catalytic activity in the low-temperature range and tolerance to SO2 poisoning. In the present work, two series of Mo-modified CeO2 catalysts were respectively obtained through a wet impregnation method (Mo-CeO2) and a co-precipitation method (MoCe-cp), and the roles of the Mo species were systematically investigated. Activity tests showed that the Mo-CeO2 catalyst displayed much higher NO conversion at low temperature and anti-SO2 ability than MoCe-cp. The optimal Mo-CeO2 catalyst displayed over 80% NO elimination efficiency even at 150 °C and remarkable SO2 resistance at 250 °C (nearly no activity loss after 40 h test). The characterization results indicated that the introduced Mo species were highly dispersed on the Mo-CeO2 catalyst surface, thereby providing more Brønsted acid sites and inhibiting the formation of stable adsorbed NOx species. These factors synergistically promote the selective catalytic reduction (SCR) reaction in accordance with the Eley-Rideal (E-R) reaction path on the Mo-CeO2 catalyst. Additionally, the molybdenum surface could protect CeO2 from SO2 poisoning; thus, the reducibility of the Mo-CeO2 catalyst declined slightly to an adequate level after sulfation. The results in this work indicate that surface modification with Mo species may be a simple method of developing highly efficient cerium-based SCR catalysts with superior SO2 durability.

The role of molybdenum on the enhanced performance and SO2 resistance of V/Mo-Ti catalysts for NH3-SCR

The influence of molybdenum on the selective catalytic reduction (SCR) performance and SO2 durability of V/Mo-Ti catalysts was investigated. The characteristics of the catalysts were investigated by Brunauer-Emmett-Teller (BET) surface area analysis, field-emission transmission electron microscopy (FE-TEM), Raman spectroscopy, NH3-temperature programmed desorption (NH3-TPD), in-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTs), SO2 temperature-programmed desorption (SO2-TPD), and X-ray photoelectron spectroscopy (XPS). The V/Mo-Ti catalysts exhibited improved SCR performances and SO2 resistances at 250 °C compared to those of V/W/Ti catalysts. The improved catalytic activity was shown to be imparted by amorphous MoOx. The formation of amorphous MoOx species with the addition of molybdenum to titanic acid was found to be more significant than that by the addition of molybdenum to crystalline TiO2. The increased density of NH3-adsorption and Brønsted-acid sites by addition of molybdenum was found to have a positive effect on catalyst performance. Since the reaction between SO2 and VOx was suppressed by the addition of molybdenum, the area of the SO2-TPD peak due to desorption of adsorbed SO2 from the catalyst surface was reduced. This decrease in the amount of adsorbed SO2 improved the SO2 resistances of the catalysts. The V/Mo-Ti catalysts exhibited increasing activities and SO2 resistances with increasing Mo6+ ratio (and Mo6+ atoms/cm3). Furthermore, they exhibited suppressed formation of ammonium sulfate salts due to decreased SO2 adsorption, as the reaction of terminal VO groups and adsorbed SO2 is inhibited by the addition of molybdenum. Thus, the V/Mo-Ti catalysts exhibited better catalytic activities and SO2 durability than those of V/W/Ti catalysts.

Promotional effect of antimony on the selective catalytic reduction NO with NH3 over V-Sb/Ti catalyst

ABSTRACTThe effect of antimony on the selective catalytic reduction (SCR) performance and SO2 durability of V-Sb/Ti was investigated. The physicochemical characteristics of catalyst were characterized by various techniques, including Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction (XRD), NH3/SO2-temperature programmed desorption (TPD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs), X-ray photoelectron spectroscopy (XPS), and H2-temperature programmed reduction (H2-TPR). The V-Sb/Ti catalyst showed excellent activity in the range 200–300°C (compared with V/Ti), with an optimum achieved for 2 wt.% antimony. The total amount of acidic sites and NH3 adsorption characteristics did not affect the catalytic efficiency. The Sb5+ fraction was highest for V-2.0Sb/Ti and exhibited a positive correlation with the V4+ fraction. This phenomenon is related to the effect of synergistic between vanadium and antimony, promoting the conversion of V5+ to V4+ by Sb5+. Increasing the V4+ fraction in V-Sb/Ti increased the catalytic activity, which was mainly attributed to enhanced catalyst re-oxidation capability due to the addition of antimony. Furthermore, the addition of antimony delayed the adsorption of SO2 onto the V-Sb/Ti catalyst surface, improving the resistance to this gas. Therefore, the addition of antimony to V/Ti improved NOx conversion and SO2 durability.

Sulfur-poisoning and thermal reduction regeneration of holmium-modified Fe-Mn/TiO2 catalyst for low-temperature SCR

AbstractThe effect of SO2 on the low-temperature SCR activity and the thermal reductive regeneration for holmium-modified Fe-Mn/TiO2 catalyst were investigated by activity assessment and various characterization methods. The deposition of ammonium sulfate ((NH4)2SO4) on catalyst surface and the sulfuration of active component (MnSO4) were proved to be the main causes for the deactivation in the presence of SO2. The catalyst Fe0.3Ho0.1Mn0.4/TiO2 exhibited superior SO2 durability when the concentration of SO2 was lower than 0.04%, and the catalytic activity could markedly recover with the termination of sulfur-poisoning source. The deactivation behavior was irreversible when the concentration of SO2 was increased to 0.1% but the deactivated catalyst could be regenerated after thermal reduction (350°C) for 60 min by 5% NH3. The microstructure and physicochemical properties could be significantly restored after the thermal reductive regeneration. Moreover, the NOx conversion could return to about 80%.

SO42−–Mn–Co–Ce supported on TiO2/SiO2 with high sulfur durability for low-temperature SCR of NO with NH3

The catalysts SO42−Mn–Co–Ce/TiO2/SiO2 were investigated for the low-temperature SCR of NO with NH3 in the presence of SO2. An excellent SO2 durability at low temperature was obtained with the catalyst used TiO2/SiO2 as support and modified with SO42−. The catalyst sulfated with 0.1mol/L H2SO4 solution and then calcined at 300°C exhibited the best NOx conversion efficiency of 99.5% at 250°C in the presence of 50ppm SO2. The conversion efficiency did not decrease after repeatedly used for 8 times.

Selective catalytic reaction of NOx with NH3 over Ce–Fe/TiO2-loaded wire-mesh honeycomb: Resistance to SO2 poisoning

Ce–Fe/TiO2/Al2O3/wire-mesh honeycomb catalyst (Ce–Fe/WMH) exhibited a high activity for the selective catalytic reduction (SCR) of NOx in SO2-containing gases. Analysis of fresh and spent catalysts by XRD, BET, XPS, TG and FTIR indicated that Ce sites were sulfated preferentially over Ce–Fe/WMH during the SCR reaction in the presence of SO2, which could increase the amount of surface active oxygen species. On the other hand, the formation of surface hydroxyls due to the hydration of SO42− could supply more Brønsted acid sites to adsorb NH3 in the form of NH4+. These two factors played significant roles in the good SO2 durability of Ce–Fe/WMH. In addition, the sulfation of CeO2 on catalyst surface might approach a stable state upon certain amount of SO2 in reactant gas. The reaction mechanism study showed that only Eley–Rideal reaction pathway between ionic NH4+, coordinated NH3 and gaseous NO dominated in the reaction; the Langmuir–Hinshelwood reaction pathway was cut off by the sulfation, resulting in the rapid decrease of NOx conversion in the early period of SCR reaction in the presence of SO2.

Improvement of the Activity of γ-Fe2O3 for the Selective Catalytic Reduction of NO with NH3 at High Temperatures: NO Reduction versus NH3 Oxidization

Lignite is widely used as the fuel for coal-fired power plants, and its flue gas temperature is about 50–100 °C higher than others. V2O5–WO3/TiO2 is extremely restricted in the selective catalytic reduction (SCR) of NO from the coal-fired power plants burning lignite due to the drop of NOx conversion, low N2 selectivity, and volatility of vanadium pentoxide at high temperatures. Therefore, a more environmental-friendly SCR catalyst with excellent SCR activity and better N2 selectivity at 350–450 °C should be developed for this application. In this work, sulfated Fe–Ti spinel catalyst was developed for the SCR of NO from the coal-fired power plants burning lignite. The drop of NOx conversion at high temperatures was mainly related to the simultaneous occurrence of the catalytic oxidization of NH3 to NO during the SCR reaction. Ti was incorporated into γ-Fe2O3 to decease the oxidization ability of Fe3+ on the surface, and the sites for −NH2 adsorption and the active components for −NH2 oxidization were separated after the sulfation to decrease the probability of the collision between −NH2 adsorbed and Fe3+ on the surface. They both inhibited the catalytic oxidization of NH3 to NO over γ-Fe2O3. However, the SCR reaction over γ-Fe2O3 was simultaneously restrained after the incorporation Ti and the sulfation. Therefore, NOx conversion over γ-Fe2O3 at high temperatures depended on the ratio of NH3 conversion through the catalytic oxidization of NH3 to NO to that through the SCR reaction. This ratio decreased after the incorporation of Ti, and it further decreased after the sulfation, resulting in an obvious promotion of NOx conversion at high temperatures. Therefore, sulfated Fe–Ti spinel showed excellent SCR activity, N2 selectivity, and H2O+SO2 durability at 300–450 °C, which was suitable for the application in the coal-fired power plants burning lignite.

Improvement of Water-, Sulfur Dioxide-, and Dust-Resistance in Selective Catalytic Reduction of NOx with NH3 Using a Wire-Mesh Honeycomb Catalyst

A novel V2O5/WO3/TiO2/Al2O3/wire-mesh honeycomb (WMH) catalyst was prepared for selective catalytic reduction (SCR) of NOx with NH3. The resistances to H2O, SO2, and dust were investigated for the WMH catalyst, which were compared with those for ceramic honeycomb (CH) catalysts. The results showed that the WMH catalyst kept above 95% NOx conversion in the broad temperature window (250–425 °C) and provided nearly 92% NOx conversion during H2O and SO2 durability test, which might be attributed to the unique three-dimensional structure. Furthermore, the WMH catalyst could provide nearly 90% NOx conversion during 40 h dust exposure experiment owing to the little dust deposition of 2.9 g/m2, whereas the amount of dust deposited on the CH catalyst with the same cell density reached 6.7 g/m2, which resulted in a decrease of the NOx conversion from 72% to 58%.

CeO2–WO3 Mixed Oxides for the Selective Catalytic Reduction of NO x by NH3 Over a Wide Temperature Range

A series of cerium-tungsten oxide catalysts was prepared by the co-precipitation method and was evaluated for the selective catalytic reduction of NO x by ammonia (NH3-SCR) over a wide temperature range. These catalysts were characterized by BET, XRD, XPS and H2-TPR analyses. The experimental studies demonstrated that, among cerium-tungsten oxides, CeO2–WO3 with a Ce/W molar ratio of 3/2 exhibited the best activity toward NH3-SCR reactions, N2 selectivity and SO2 durability over a broad temperature range of 175–500 °C at a space velocity of 47,000 h−1. The strong interaction between Ce and W could be the main factor leading to the high activity of the CeO2–WO3 mixed oxide catalyst. A series of cerium-tungsten oxide catalysts was prepared by the co-precipitation method and was evaluated for the selective catalytic reduction of NO x by ammonia (NH3-SCR) over a wide temperature range. These catalysts were characterized by BET, XRD, XPS and H2-TPR analyses. The experimental studies demonstrated that, among cerium-tungsten oxides, CeO2–WO3 with a Ce/W molar ratio of 3/2 exhibited the best activity toward NH3-SCR reactions, N2 selectivity and SO2 durability over a broad temperature range of 175–500 °C at a space velocity of 47,000 h−1. The strong interaction between Ce and W could be the main factor leading to the high activity of the CeO2–WO3 mixed oxide catalyst.