Showing papers on "Selective catalytic reduction published in 2012"

A superior Ce-W-Ti mixed oxide catalyst for the selective catalytic reduction of NOx with NH3

Abstract: A superior Ce-W-Ti mixed oxide catalyst prepared by a facile homogeneous precipitation method showed excellent NH3-SCR activity and 100% N2 selectivity with broad operation temperature window and extremely high resistance to space velocity, which is a very promising catalyst for NOx abatement from diesel engine exhaust. The excellent catalytic performance is associated with the highly dispersed active Ce and promotive W species on TiO2. The introduction of W species could increase the amount of active sites, oxygen vacancies, and Bronsted and Lewis acid sites over the catalyst, which is also beneficial to improve the low temperature activity by facilitating “fast SCR” reaction and enhance both of the high temperature activity and N2 selectivity simultaneously by inhibiting the unselective oxidation of NH3 at high temperatures.

Confirmation of Isolated Cu2+ Ions in SSZ-13 Zeolite as Active Sites in NH3-Selective Catalytic Reduction

Abstract: NH3-Selective Catalytic Reduction (NH3-SCR) is a widely used technology for NOx reduction in the emission control systems of heavy duty diesel vehicles. Copper-based ion exchanged zeolites and in particular Cu-SSZ-13 (CHA framework) catalysts show both exceptional activity and hydrothermal stability for this reaction. In this work, we have studied the origin of the SCR activity of Cu-SSZ-13 as evidenced from a combination of synchrotron-based and laboratory techniques. Synchrotron-based in situ XAFS/XRD measurements were used to provide complementary information on the local copper environment under realistic NH3-SCR conditions. Crucial then to the catalytic activity of Cu-SSZ-13 is the local environment of the copper species, particularly in the zeolite. Cu-SSZ-13 contains mononuclear Cu2+ species, located in the face of the double-6-ring subunit of the zeolite after calcination where it remains under reaction conditions. At lower temperatures (with low activity), XAFS and XRD data revealed a conformatio...

Superior activity of MnOx-CeO2/TiO2 catalyst for catalytic oxidation of elemental mercury at low flue gas temperatures

Abstract: TiO 2 supported Mn-Ce mixed oxides (Mn-Ce/Ti) synthesized by an ultrasound-assisted impregnation method were employed to oxidize elemental mercury (Hg 0 ) at low temperatures in simulated low-rank (sub-bituminous and lignite) coal combustion flue gas and corresponding selective catalytic reduction (SCR) flue gas. The catalysts were characterized by BET surface area analysis, X-ray diffraction (XRD) measurement and X-ray photoelectron spectroscopy (XPS) analysis. The combination of MnO x and CeO 2 resulted in significant synergy for Hg 0 oxidation. The Mn-Ce/Ti catalyst was highly active for Hg 0 oxidation at low temperatures (150–250 °C) under both simulated flue gas and SCR flue gas. The dominance of Mn 4+ and the presence of Ce 3+ on the Mn-Ce/Ti catalyst were responsible for its excellent catalytic performance. Hg 0 oxidation on the Mn-Ce/Ti catalyst likely followed the Langmuir–Hinshelwood mechanism, where reactive species on catalyst surface react with adjacently adsorbed Hg 0 to form Hg 2+ . NH 3 consumed the surface oxygen and limited the adsorption of Hg 0 , hence inhibiting Hg 0 oxidation over Mn-Ce/Ti catalyst. However, once NH 3 was cut off, the inhibited mercury oxidation activity could be completely recovered in the presence of O 2 . This study revealed the possibility of simultaneously oxidizing Hg 0 and reducing NO x at low flue gas temperatures. Such knowledge is of fundamental importance in developing effective and economical mercury and NO x control technologies for coal-fired power plants.

The influence of silicon on the catalytic properties of Cu/SAPO-34 for NOx reduction by ammonia-SCR

Abstract: The effect of Si content in SAPO-34 on NO selective catalytic reduction over Cu/SAPO-34 catalyst was investigated. Three fresh catalysts with different Si contents, prepared by the same procedure, showed different NO conversions from 120 °C to 600 °C. These catalysts were characterized in detail by various techniques (ICP, XRD, SEM, NMR, NH 3 -TPD, H 2 -TPR and EPR). The NH 3 -TPD results indicate that the number of acid sites in Cu/SAPO-34 catalyst increases with increasing Si contents and the SCR activities of the samples correlate well with the number of acidic sites at low temperatures (200 °C). The EPR and H 2 -TPR results show that both the Si and the Al contents affect the number of isolated Cu 2+ ions in Cu/SAPO-34 catalysts. The catalysts aged at 750 °C for 12 h showed higher SCR activities than those of the untreated samples, consistent with the higher numbers of the isolated Cu 2+ ions.

Low-Temperature Selective Catalytic Reduction of NOx with NH3 over Fe–Mn Mixed-Oxide Catalysts Containing Fe3Mn3O8 Phase

Abstract: Novel Fe–Mn mixed-oxide catalysts were prepared for the low-temperature selective catalytic reduction (SCR) of NOx with ammonia in the presence of excess oxygen. It was found that Fe(0.4)–MnOx catalyst showed the highest activity, yielding 98.8% NOx conversion and 100% selectivity of N2 at 120 °C at a space velocity of 30 000 h–1. XRD results suggested that a new crystal phase of Fe3Mn3O8 was formed in the Fe–MnOx catalysts. TPR and Raman data showed that there was a strong interaction between the iron oxide and manganese oxide, which is responsible for the formation of the active center―Fe3Mn3O8. Intensive analysis of fresh, used, and regenerated catalysts by XPS revealed that electron transfer between Fen+ and Mnn+ ions in Fe3Mn3O8 may account for the long lifetime of the Fe(0.4)–MnOx catalyst. In addition, the SCR activity was suppressed a little in the presence of SO2 and H2O, but it was reversible after their removal.

DRIFT Studies on the Selectivity Promotion Mechanism of Ca-Modified Ce-Mn/TiO2 Catalysts for Low-Temperature NO Reduction with NH3

Abstract: Manganese-based catalysts have shown excellent low-temperature selective catalytic reduction (SCR) activity for NOx removal. However, they would always produce a high amount of more toxic byproduct, N2O. Ca modification has been reported to be able to promote N2 selectivity for SCR catalysts, but the mechanism is still not clear. In this study, a series of Ca-modified Ce0.02Mn0.4/TiO2 SCR catalysts were utilized for mechanism study. In terms of DRIFT analysis, the addition of Ca had significantly reduced the formation of NH on catalyst surface, which limited its reaction with NO to form N2O. Furthermore, Ca addition had also caused a deceased formation of NO2, reducing its reaction with NH3 to form N2O, further increasing the N2 selectivity of the catalysts. The results present herein could be beneficial to the development of efficient low-temperature SCR catalysts, particularly for Mn-based SCR catalysts.

Alkali Metal Poisoning of a CeO2–WO3 Catalyst Used in the Selective Catalytic Reduction of NOx with NH3: an Experimental and Theoretical Study

Abstract: The alkali metal-induced deactivation of a novel CeO(2)-WO(3) (CeW) catalyst used for selective catalytic reduction (SCR) was investigated. The CeW catalyst could resist greater amounts of alkali metals than V(2)O(5)-WO(3)/TiO(2). At the same molar concentration, the K-poisoned catalyst exhibited a greater loss in activity compared with the Na-poisoned catalyst below 200 °C. A combination of experimental and theoretical methods, including NH(3)-TPD, DRIFTS, H(2)-TPR, and density functional theory (DFT) calculations, were used to elucidate the mechanism of the alkali metal deactivation of the CeW catalyst in SCR reaction. Experiments results indicated that decreases in the reduction activity and the quantity of Bronsted acid sites rather than the acid strength were responsible for the catalyst deactivation. The DFT calculations revealed that Na and K could easily adsorb on the CeW (110) surface and that the surface oxygen could migrate to cover the active tungsten, and then inhibit the SCR of NO(x) with ammonia. Hot water washing is a convenient and effective method to regenerate alkali metal-poisoned CeW catalysts, and the catalytic activity could be recovered 90% of the fresh catalyst.

Enhancement of Catalytic Activity Over the Iron-Modified Ce/TiO2 Catalyst for Selective Catalytic Reduction of NOx with Ammonia

Abstract: A series of iron-modified Ce/TiO2 catalysts with different Fe/Ti molar ratios were prepared by an impregnation method and used for selective catalytic reaction (SCR) of NOx with NH3. The Fe–Ce/TiO2 catalyst with a Fe/Ti molar ratio of 0.2 had good low-temperature activity and sulfur-poisoning resistance compared with the Ce/TiO2 catalyst. The introduction of Fe could increase the amount of Ce3+ and chemisorbed oxygen species on the catalyst surface and thereafter generate more ionic NH4+ and in situ formed NO2, respectively. In addition, the dispersion of cerium oxide could be improved by the addition of iron, and no visible phase of iron oxide could be observed at low Fe/Ti molar ratios (≤0.2). All of these factors played significant roles in the enhanced catalytic activity, especially the low-temperature activity. Furthermore, mechanisms of the SCR reaction and the SO2 poisoning of the Fe(0.2)–Ce/TiO2 catalyst were studied using in situ diffuse reflectance infrared Fourier transform spectroscopy. Coordi...

The Effect of Copper Loading on the Selective Catalytic Reduction of Nitric Oxide by Ammonia Over Cu-SSZ-13

Abstract: The effect of Cu loading on the selective catalytic reduction of NOx by NH3 was examined over a series of Cu ion-exchanged (20–80%) SSZ-13 zeolite catalysts. High NO reduction efficiencies (80–95%) were obtained over all catalyst samples between 250 and 500 °C, and at the gas hourly space velocity of 200,000 h−1. Both NO reduction and NH3 oxidation activities under these conditions were found to increase slightly with increasing Cu loading at low temperatures. However, NO reduction activity was suppressed with increasing Cu loadings at high temperatures (>500 °C) due to excess NH3 oxidation. The optimum Cu ion exchange level appears to be ~40–60% since higher than 80% NO reduction efficiency was obtained over 50% Cu ion-exchanged SSZ-13 up to 600 °C. The NO oxidation activity of Cu-SSZ-13 was found to be low regardless of Cu loading, although it was somewhat improved with increasing Cu ion exchange level at high temperatures. During the “fast” SCR (i.e., NO/NO2 = 1), only a slight improvement in NOx reduction activity was obtained for Cu-SSZ-13. Regardless of Cu loading, near 100% selectivity to N2 was observed; only a very small amount of N2O was produced even in the presence of NO2. The apparent activation energies for NO oxidation and NO SCR were estimated to be ~58 and ~41 kJ/mol, respectively. .

Mn-Fe/ZSM5 as a low-temperature SCR catalyst to remove NOx from diesel engine exhaust

Abstract: A Mn–Fe/ZSM5 catalyst has been developed for removing NOx from diesel engine exhausts and its excellent low-temperature SCR activity and N 2 selectivity demonstrated in comparison with other representative SCR catalysts including CuZSM5 and a Cu-based commercial catalyst (COM). The well-dispersed MnO 2 and the high NH 3 adsorption capacity of the Mn–Fe/ZSM5 catalyst have been identified as the primary sources for its high deNOx activity for NH 3 /SCR. Hydrothermal stability and durability of the Mn–Fe/ZSM5 catalyst have been examined and compared to those of the CuZSM5 and COM catalysts. The hydrothermal stability of the catalyst improved upon the increase of Mn content and/or the addition of Er, the latter of which helps to stabilize the dispersion of MnOx on the catalyst surface during hydrothermal aging. The deNOx activity of the Mn–Fe/ZSM5 and its Er-promoted counterpart was less affected by HC poisoning, C 3 H 6 poisoning in particular, compared to the CuZSM5 and COM catalysts, mainly due to the excellent C 3 H 6 oxidation activity of MnO 2 . No poisoning of the Mn-based ZSM5 and CuZSM5 catalysts has been observed upon the addition of 2 wt.% of K + and Ca 2+ to their surface, primarily due to the high NH 3 adsorption capacity of the ZSM5 support, whereas the COM catalyst has been severely deactivated by the deposition of K + and Ca 2+ . The deNOx activity of the Mn-based ZSM5 catalyst, particularly the Er-promoted one, was less affected by SO 2 compared to the CuZSM5 and COM catalysts, although it was hardly regenerated at 500 °C. Formation of MnSO 4 on the catalytic surface appears to be the primary cause for the deactivation of the Mn-based ZSM5 catalysts in the presence of SO 2 in the feed gas stream.

Recent advances in the selective catalytic reduction of NOx by hydrogen in the presence of oxygen

Abstract: Selective catalytic reduction of NOx by hydrogen (H2-SCR) in the presence of oxygen has received much attention as a potential technology for reducing NOx emissions. A lot of research has been done in order to understand the reaction mechanism of H2-SCR and some possible mechanisms have been proposed. These mechanisms can be classified into two categories: NO adsorption/dissociation mechanisms and oxidation–reduction mechanisms. Based on the discussion of the reaction mechanism, the influence of the nature of the noble metal, catalyst support, catalyst preparation method, promoters and reaction conditions (including the presence of H2 and O2, water, sulfur, CO and CO2) on the catalytic performance of some H2-SCR catalysts has been discussed. Finally, future research directions in the area of H2-SCR have been proposed.

Hydrolysis and thermolysis of urea and its decomposition byproducts biuret, cyanuric acid and melamine over anatase TiO2

Abstract: A major problem in the selective catalytic reduction of NOx with urea (urea-SCR) is the formation of urea decomposition byproducts, which can be suppressed by the use of TiO2 as a hydrolysis catalyst. Temperature programmed desorption and reaction (TPD/R) experiments with urea, biuret, triuret, cyanuric acid and melamine on TiO2-coated and inert cordierite monoliths were used to investigate the reaction network of urea byproduct formation and decomposition over anatase TiO2. All investigated compounds were found to be catalytically hydrolyzed over TiO2. Biuret was directly hydrolyzed to urea in one step, whereas melamine hydrolyzed step-wise via ammeline and ammelide to cyanuric acid. Finally, cyanuric acid completely hydrolyzed to ammonia and carbon dioxide. The formation of byproducts was strongly favored in the absence of water. A reaction network was developed for the uncatalyzed and catalytic decomposition of urea, showing the most important reactions of urea, isocyanic acid, biuret, triuret, cyanuric acid, ammelide, ammeline and melamine under low-temperature operating conditions in SCR systems. Our results support the approach of using a special hydrolysis catalyst for urea decomposition or of catalytic coatings on exhaust pipes to avoid byproduct formation.

Selective catalytic reduction of NOx on combined Fe- and Cu-zeolite monolithic catalysts: Sequential and dual layer configurations

Abstract: Iron and copper-based zeolites are effective catalysts for the lean selective catalytic reduction (SCR) of NOx with NH3. Cu-zeolites are more active at lower temperatures (≤350 °C) while Fe-zeolites are more active at higher temperatures (≥400 °C). The effectiveness of a catalytic system comprising Fe- and Cu-based zeolites was examined for the standard (NO + O2 + NH3) and fast (NO + NO2 + NH3) SCR reactions. Experiments carried out with in-house and commercial Fe- and Cu-zeolite monoliths of varying lengths quantified their relative SCR activities. The commercial Cu-zeolite achieved complete NOx conversion for the standard SCR at 250 °C while the commercial Fe-zeolite achieved high NOx conversion at higher temperatures (≥400 °C) where it out-performed the Cu-zeolite. Subsequently, three configurations of combined Fe and Cu-zeolite catalysts were compared: • “Sequential brick” catalyst comprising Fe-zeolite and Cu-zeolite monolith. • “Mixed washcoat” catalyst comprising a washcoat layer having equal mass fractions of Fe- and Cu-zeolites. • “Dual layer” catalyst comprising monolith coated with individual layers of Fe- and Cu-zeolites of different thicknesses and mass fractions. The sequential brick design with Fe-zeolite brick followed by a Cu-zeolite brick gave a higher conversion than the Cu/Fe sequence of equal loadings with the Fe(33%)/Cu(67%) achieving the highest NOx conversion over a wide range of temperatures. The mixed washcoat catalyst achieved NOx conversion that was nearly an average of the individual Fe-only and Cu-only catalysts. The dual layer catalyst with a thin Fe-zeolite (33% of the total washcoat loading) layer on top of a thicker Cu-zeolite layer (67%) resulted in very high NOx removal efficiencies over a wide temperature range for both the standard and fast SCR reactions. The performance of this dual-layer system was comparable to the series arrangement of Fe and Cu-bricks. The Cu-zeolite on Fe-zeolite dual layer catalyst was not nearly as effective for the same loadings. The Fe/Cu dual layer catalyst also exhibited superior performance for the fast SCR reaction. The results are interpreted in terms of the activities of each catalyst for SCR and ammonia oxidation. An assessment of the extent of washcoat diffusion limitations shows that the dual layer configuration is superior to the sequential brick configuration. The existence of an optimal loading distribution of the Fe- and Cu-zeolite catalysts as well as other intangible benefits of the dual layer SCR catalyst are discussed.

Promotional effect of F-doped V2O5–WO3/TiO2 catalyst for NH3-SCR of NO at low-temperature

Abstract: A F-doped V 2 O 5 –WO 3 /TiO 2 has been developed for low-temperature selective catalytic reduction (SCR) of NO with NH 3 . The aim of this novel design was to improve the activity of a catalyst with low WO 3 loading. Analysis by PL spectra, XRD, XPS and EPR showed that F doping improved the interaction of WO 3 with TiO 2 by oxygen vacancies to facilitate the formation of W 5+ that was important to improve the formation of superoxide ions. The experimental results showed that NO conversion could be improved by F doping and V 1 W 3 TiF 1.35 showed the highest NO removal efficiency in SCR reaction at low temperatures.

A review of selective catalytic reduction of nitrogen oxides with hydrogen and carbon monoxide

Abstract: The selective reduction of NO with hydrogen (H2-SCR) and CO (CO-SCR) over platinum group metal catalysts in the presence of O2 is overviewed. In the case of H2-SCR, Pt and Pd show high activity at low temperatures. The acidity of the support material greatly affects NO reduction activity and selectivity to N2/N2O. Although the activity of Ir and Rh for H2-SCR is low, coexisting SO2 in the reaction gas considerably promotes NO reduction. The best support for Ir and Rh is SiO2. Li and Zn additives improve the activity of Ir/SiO2 and Rh/SiO2, respectively, by maintaining the active reduced metal state. For CO-SCR, on the other hand, Ir is almost the only active metal species. Coexisting SO2 is also essential for CO-SCR on Ir/SiO2 to occur. The role of SO2 for both H2-SCR and CO-SCR on Ir/SiO2 is to keep Ir in the form of the catalytically active Ir metal state. The additions of WO3 and Nb2O5 considerably promote the activity of Ir/SiO2 for CO-SCR, catalyzing CO-SCR even in the absence of SO2. Ir metal interacting strongly with W oxide is the active species on WO3-promoted Ir/SiO2. Furthermore, the addition of Ba improves the performance of Ir/WO3/SiO2 catalyst.

Low absorption vitreous carbon reactors for operando XAS: a case study on Cu/Zeolites for selective catalytic reduction of NO(x) by NH3.

Abstract: We describe the use of vitreous carbon as an improved reactor material for an operandoX-ray absorption spectroscopy (XAS) plug-flow reactor. These tubes significantly broaden the operating range for operando experiments. Using selective catalytic reduction (SCR) of NOx by NH3 on Cu/Zeolites (SSZ-13, SAPO-34 and ZSM-5) as an example reaction, we illustrate the high-quality XAS data achievable with these reactors. The operando experiments showed that in Standard SCR conditions of 300 ppm NO, 300 ppm NH3, 5% O2, 5% H2O, 5% CO2 and balance He at 200 °C, the Cu was a mixture of Cu(I) and Cu(II) oxidation states. XANES and EXAFS fitting found the percent of Cu(I) to be 15%, 45% and 65% for SSZ-13, SAPO-34 and ZSM-5, respectively. For Standard SCR, the catalytic rates per mole of Cu for Cu/SSZ-13 and Cu/SAPO-34 were about one third of the rate per mole of Cu on Cu/ZSM-5. Based on the apparent lack of correlation of rate with the presence of Cu(I), we propose that the reaction occurs via a redox cycle of Cu(I) and Cu(II). Cu(I) was not found in in situSCR experiments on Cu/Zeolites under the same conditions, demonstrating a possible pitfall of in situ measurements. A Cu/SiO2 catalyst, reduced in H2 at 300 °C, was also used to demonstrate the reactor's operando capabilities using a bending magnet beamline. Analysis of the EXAFS data showed the Cu/SiO2 catalyst to be in a partially reduced Cu metal–Cu(I) state. In addition to improvements in data quality, the reactors are superior in temperature, stability, strength and ease of use compared to previously proposed borosilicate glass, polyimide tubing, beryllium and capillary reactors. The solid carbon tubes are non-porous, machinable, can be operated at high pressure (tested at 25 bar), are inert, have high material purity and high X-ray transmittance.

Detailed modeling of the evaporation and thermal decomposition of urea‐water solution in SCR systems

Abstract: This work aims to develop a multi-component evaporation model for droplets of urea-water-solution (UWS) and a thermal decomposition model of urea for automotive exhausts using the Selective Catalytic Reduction (SCR) systems. In the multi-component evaporation model, the influence of urea on the UWS evaporation is taken into account using a NRTL activity model. The thermal decomposition model is based on a semi-detailed kinetic scheme accounting not only for the production of ammonia (NH3) and isocyanic acid (HNCO), but also for the formation of heavier solid by-products (biuret, cyanuric acid and ammelide). This kinetics model has been validated against gaseous data as well as solid-phase concentration profiles obtained by Lundstroem et al. (2009) and Schaber et al. (2004). Both models have been implemented in IFP-C3D industrial software in order to simulate UWS droplet evaporation and decomposition as well as the formation of solid by-products. It has been shown that the presence of the urea solute has a small influence on the water evaporation rate, but its effect on the UWS temperature is significant. In addition, the contributions of hydrolysis and thermolysis to urea decomposition have been assessed. Finally, the impacts of the heating rate as well as gas-phase chemistry on urea decomposition pathways have been studied in detail. It has been shown that reducing the heating rate of the UWS causes the extent of the polymerization to decrease because of the higher activation energy.