Showing papers on "Selective catalytic reduction published in 2010"

DRIFT Study on Cerium−Tungsten/Titiania Catalyst for Selective Catalytic Reduction of NOx with NH3

Abstract: CeO(2)/TiO(2) and CeO(2)-WO(3)/TiO(2) catalysts prepared by impregnation method assisted with ultrasonic energy were investigated on the selective catalytic reduction (SCR) of NO(x) (NO and NO(2)) by NH(3). The catalytic activity of 10% CeO(2)/TiO(2) (CeTi) was greatly enhanced by the addition of 6% WO(3) in the broad temperature range of 200-500 °C, the promotion mechanism was proposed on basis of the results of in situ diffuse reflectance infrared transform spectroscopy (DRIFT). When NH(3) was introduced into both catalysts preadsorbed with NO + O(2), SCR would not proceed except for the reaction between NO(2) and ammonia. For CeO(2)/TiO(2) catalysts, coordinated NH(3) linked to Lewis acid sites were the main adsorbed ammonia species. When NO + O(2) was introduced, all the ammonia species consumed rapidly, indicating that these species could react with NO(x) effectively. Two different reaction routes, L-H mechanism at low temperature ( 200 °C), were presented for SCR reaction over CeO(2)/TiO(2) catalyst. For CeO(2)-WO(3)/TiO(2) catalysts, the Lewis acid sites on Ce(4+) state could be converted to Bronsted acid sites due to the unsaturated coordination of Ce(n+) and W(n+) ions. When NO + O(2) was introduced, the reaction proceeded more quickly than that on CeO(2)/TiO(2). The reaction route mainly followed E-R mechanism in the temperature range investigated (150-350 °C) over CeO(2)-WO(3)/TiO(2) catalysts. Tungstation was beneficial for the formation of Ce(3+), which would influence the active sites of the catalyst and further change the mechanisms of SCR reaction. In this way, the cooperation of tungstation and the presence of Ce(3+) state resulted in the better activity of CeO(2)-WO(3)/TiO(2) compared to that of CeO(2)/TiO(2).

Copper Coordination in Cu-SSZ-13 and Cu-SSZ-16 Investigated by Variable-Temperature XRD

Abstract: Nitrogen oxides (NOx) are a major atmospheric pollutant produced through the combustion of fossil fuels in internal combustion engines. Copper-exchanged zeolites are promising as selective catalytic reduction catalysts for the direct conversion of NO into N2 and O2, and recent reports have shown the enhanced performance of Cu-CHA catalysts over other zeolite frameworks in the NO decomposition of exhaust gas streams. In the present study, Rietveld refinement of variable-temperature XRD synchrotron data obtained for Cu-SSZ-13 and Cu-SSZ-16 is used to investigate the location of copper cations in the zeolite pores and the effect of temperature on these sites and on framework stability. The XRD patterns show that the thermal stability of SSZ-13 is increased significantly when copper is exchanged into the framework compared with the acid form of the zeolite, H-SSZ-13. Cu-SSZ-13 is also more thermally stable than Cu-SSZ-16. From the refined diffraction patterns, the atomic positions of atoms, copper locations a...

Structure-Activity Relationship of Iron Titanate Catalysts in the Selective Catalytic Reduction of NOx with NH3 †

Abstract: A series of iron titanate catalysts, FeaTibOx, with different Fe−Ti molar ratios are synthesized via a facile coprecipitation method and tested for the selective catalytic reduction (SCR) of NOx wi...

Origination of N2O from NO reduction by NH3 over β-MnO2 and α-Mn2O3

Abstract: Selective catalytic reduction (SCR) of NO with NH3 was studied on β-MnO2 and α-Mn2O3 catalysts at 150 °C and the formation of N2O from SCR was mainly investigated. The activity evaluation showed that the rates of both NO conversion and N2O formation per unit surface area on β-MnO2 were much higher than the corresponding values on α-Mn2O3, while two catalysts gave same generated rate of N2. Transient reactions of NO with NH3 showed that N2O predominantly originated from direct reaction of NO and NH3 via an Eley–Rideal mechanism. β-MnO2 gave higher generated rate of N2O in transient reaction of NH3 and more desorption amount of N2O in temperature-programmed desorption of NH3 than α-Mn2O3. The results of temperature-programmed reduction of H2 exhibited that oxygen species on β-MnO2 are more active than α-Mn2O3. Therefore, β-MnO2 had higher selectivity to N2O in SCR reaction than α-Mn2O3, predominantly resulting from higher activated capability to NH3, and β-MnO2 can cleave more N–H bonds in NH3 molecules to give more adsorbed nitrogen atom species, which reacted with gaseous NO to form more N2O.

Selective catalytic reduction of NO with NH3 over iron titanate catalyst: Catalytic performance and characterization

Abstract: A novel iron titanate catalyst prepared by conventional co-precipitation method showed excellent activity, N-2 selectivity and H2O/SO2 durability in the selective catalytic reduction (SCR) of NO with NH3. The influence of precursors and preparation methods on the catalyst structure and activity was comprehensively investigated. Iron titanate catalyst prepared using titanium sulfate as Ti precursor was favorable for the high activity and selectivity, comparing with that using titanium tetrachloride as precursor and Fe2O3/TiO2 loaded type catalyst. Especially, the best iron titanate catalyst showed good activity in a temperature window of 200-350 degrees C with the NOx conversion above 90% in the absence of H2O, which was 50-150 degrees C lower than those of other known Fe-based catalysts. Iron titanate crystallite with specific Fe-O-Ti structure was found to be the main active phase. The interaction between iron and titanium species in atomic scale led to an enhancement of oxidative ability of Fe3+, which was beneficial to the SCR reaction. (C) 2010 Elsevier B.V. All rights reserved.

DRIFT Study of the SO2 Effect on Low-Temperature SCR Reaction over Fe−Mn/TiO2

Abstract: SO2 would deactivate the low-temperature SCR (selective catalytic reduction) catalysts and reduce NO removal. In this study, Fe(0.1)−Mn(0.4)/TiO2 prepared by sol−gel method was selected to carry out the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) investigation for revealing the mechanism of the SO2 effect on the SCR reaction. The DRIFT spectroscopy showed that SO2 could be adsorbed on the surface of the catalyst as the bidentate mononuclear sulfate. This type of sulfate would retard the formation of NO complex on the surface of catalyst, resulting in the decrease of NO adsorption. For NH3 adsorption, the adsorption of SO2 had little effect on the coordinated NH3, but would increase the amount of NH4+ because of the formation of new Bronsted acid sites. Therefore, besides the deposition of ammonium sulfates, the competitive adsorption between SO2 and NO on the active sites of the catalysts also contributed to the poisoning effect of SO2 on the SCR reaction. When sulfate was ...

Sulfur poisoning resistant mesoporous Mn-base catalyst for low-temperature SCR of NO with NH3

Abstract: Mesoporous MnO2-Fe2O(3)-CeO2-TiO2 was prepared with sot-gel method and demonstrated to have good low-temperature activity and sulfur-poisoning resistance for selective catalytic reduction (SCR) of NO with NH3 in SO2-containing gases. In comparison with this, the catalyst with the same composition but made according to the conventional impregnation method exhibited obviously lower SO2-poisoning resistance and selectivity to the formation of N-2 in the SCR reactions. FFIR analysis of the spent catalysts after SCR reactions for 16 h and 60 h in a SO2-contaning gas demonstrated that there was little difference in the amount of deposited ammonium sulfate over the mesoporous catalyst between the two cases. The mesopore channels existing in the mesoporous catalyst enabled probably a dynamic balance between the formation and decomposition of ammonium sulfate in SCR reactions. This concern was justified through comparing the N-2 adsorptions and XPS spectra for the catalysts made with the impregnation and sot-gel methods. The article clarified as well the facilitation effects of introducing Ce and Fe into the mesoporous catalyst on activity, selectivity and SO2-poisoning resistance. Crown Copyright 2009 Published by Elsevier B.V. All rights reserved.

The poisoning effect of Na+ and Ca2+ ions doped on the V2O5/TiO2 catalysts for selective catalytic reduction of NO by NH3

Abstract: Deactivation of the supported V2O5/TiO2 catalysts due to deposition of alkali and alkaline earth metal salts in fly ashes is one of the most important problems in the selective catalytic reduction (SCR) of NOx by NH3. In this work, the poisoning effect of Na+ and Ca2+ ions doped on the V2O5/TiO2 catalysts for selective catalytic reduction of NO by NH3 has been studied. It has been shown that the Na+ and Ca2+ ions doped can poison the V2O5/TiO2 catalyst and the Na+ ions exhibit greater poisoning effect than Ca2+ ions. The NO rate constant k on the Na+-V2O5/TiO2 catalyst decreased very sharply from 97.1 to 35.0 cm3 g−1 s−1 with increasing Na/V molar ratio from 0.00 to 0.20. In comparison, k on the Ca2+-V2O5/TiO2 catalyst remained almost unchanged when Ca/V molar ratio was below 0.05. Further increasing Ca/V molar ratio to 0.20 resulted in an obvious decrease of k from 101.8 to 46.3 cm3 g−1 s−1. It was suggested that Na+ ions combined strongly with dispersed vanadia species, neutralized the Bronsted acid sites and reduced their reducibility. In comparison, Ca2+ ions slightly affected the Bronsted acid sites of catalyst. Besides, the dispersed vanadia species on the Ca2+-V2O5/TiO2 catalyst had higher reducibility than Na+-V2O5/TiO2 catalyst. The different poisoning effects of Na+ and Ca2+ ions doped on the V2O5/TiO2 catalyst were correlated not only to the surface acidity but also to the reducibility of catalyst.

Iron-doped Mn-Ce/TiO2 catalyst for low temperature selective catalytic reduction of NO with NH3.

Abstract: The catalysts of iron-doped Mn-Ce/TiO2 (Fe-Mn-Ce/TiO2) prepared by sol-gel method were investigated for low temperature selective catalytic reduction (SCR) of NO with NH3. It was found that the NO conversion over Fe-Mn-Ce/TiO2 was obviously improved after iron doping compared with that over Mn-Ce/TiO2. Fe-Mn-Ce/TiO2 with the molar ratio of Fe/Ti = 0.1 exhibited the highest activity. The results showed that 96.8% NO conversion was obtained over Fe(0.1)-Mn-Ce/TiO2 at 180°C at a space velocity of 50,000 hr−1. Fe-Mn-Ce/TiO2 exhibited much higher resistance to H2O and SO2 than that of Mn-Ce/TiO2. The properties of the catalysts were characterized using X-ray diffraction (XRD), N2 adsorption, temperature programmed desorption (NH3-TPD and NOx-TPD), and X-ray photoelectron spectroscopy (XPS) techniques. BET, NH3-TPD and NOx-TPD results showed that the specific surface area and NH3 and NOx adsorption capacity of the catalysts increased with iron doping. It was known from XPS analysis that iron valence state on the surface of the catalysts were in Fe3+ state. The doping of iron enhanced the dispersion and oxidation state of Mn and Ce on the surface of the catalysts. The oxygen concentrations on the surface of the catalysts were found to increase after iron doping. Fe-Mn-Ce/TiO2 represented a promising catalyst for low temperature SCR of NO with NH3 in the presence of H2O and SO2.

Unexpected CO2 Splitting Reactions To Form CO with N-Heterocyclic Carbenes as Organocatalysts and Aromatic Aldehydes as Oxygen Acceptors

Abstract: The catalytic reduction of carbon dioxide to carbon monoxide under mild conditions using aromatic aldehydes as reductants and NHCs as organocatalysts was developed. This carbon dioxide splitting reaction provides a new method for metal-free carbon dioxide reduction and steps forward in utilizing carbon dioxide as a renewable “green” source under mild conditions. On the other hand, this reaction also shows a new economical way to oxidize aromatic aldehydes under mild conditions with carbon dioxide and could be applied in pharmaceutical synthesis.

High-throughput study of the effects of inorganic additives and poisons on NH3-SCR catalysts. Part II: Fe–zeolite catalysts

Abstract: The influence of phosphorus, alkaline and alkaline earth metals, chromium and copper on the catalytic activity and selectivity of a V2O5–WO3/TiO2 catalyst for the selective catalytic reduction (SCR) of nitrogen oxides with ammonia has been studied. These components are put through a catalytic aftertreatment system of a diesel engine as impurities of biodiesel (K, Na, P), urea solution (K, Na, Ca, Mg) and abrasion of the engine (Cr, Cu). The catalysts were exposed to corresponding nitrates or ammonium compounds in diluted solution by wet impregnation or deposition of inorganic aerosol particles. The second approach allows a more realistic investigation using catalysts and reaction conditions close to mobile application. Hereby, the effects of single catalyst poisons have been investigated. Furthermore, the influence of combinations of poisons using Design of Experiments (DOE) has been determined in case of impregnated catalysts. Physical and chemical characterization methods focusing on NH3-TPD, penetration profiles and chemical analysis have been carried out to gain insight into the extent and the mechanism of deactivation. Both impregnated and aerosol deactivated catalysts show a strong poisoning effect of alkaline metals due to a reduced ammonia adsorption capacity. This effect could be weakened by a simultaneous doping of phosphates or sulphates. Chromium and copper are moderate catalyst poisons but have the ability to increase the N2O production.

Characterization of Nb-Containing MnOx−CeO2 Catalyst for Low-Temperature Selective Catalytic Reduction of NO with NH3

Abstract: The reactivity in the selective catalytic reduction of the individual and binary components of NbOx−MnOx−CeO2 catalysts has been studied with coated cordierite monoliths in the temperature range of 150−450 °C. FTIRS, DRIFTS, TA, XRD, BET, and XAS have been used to elucidate the structural and catalytic properties. The results confirmed the contribution of the manganese oxides, particularly to the low-temperature NO-to-NO2 oxidation reaction. The significant increase of the surface acidity as a result of niobium oxide addition has been established. The data obtained revealed also the strong interaction between the manganese and niobium catalytic active sites. This phenomenon leads to a very good distribution of the oxidizing and acidic sites in the catalyst structure and also diminishes the unselective NH3 oxidation at higher temperatures. However, in order to keep the low-temperature catalytic activity, an excess of manganese relative to the niobium content is needed.

Mechanism of Propene Poisoning on Fe-ZSM-5 for Selective Catalytic Reduction of NOx with Ammonia

Abstract: Application of Fe-zeolites for urea-SCR of NOx in diesel engine is limited by catalyst deactivation with hydrocarbons. In this work, we investigated the effect of propene on the activity of Fe-ZSM-5 for selective catalytic reduction of NOx with ammonia (NH3-SCR), and proposed a deactivation mechanism of Fe3+ active site blockage by propene residue. The NO conversion decreased in the presence of propene at various temperatures, while the effect was not significant when NO was replaced by NO2 in the feed, especially at low temperatures (<300 °C). The surface area and pore volume were decreased due to carbonaceous deposition. The site blockage was mainly on Fe3+ sites on which NO was to be oxidized to NO2. The activity for NO oxidation to NO2 was significantly inhibited on a propene poisoned catalyst below 400 °C. The adsorption of NH3 on the Bronsted acid sites to form NH4+ was not hindered even on the propene poisoned catalyst, and the amount of absorbed NH3 was still abundant and enough to react with NO2 ...

Influence of co-cations on the formation of Cu+ species in Cu/ZSM-5 and its effect on selective catalytic reduction of NOx with NH3

Abstract: The influence of H+ and Na+ co-cations on the formation of Cu species and on NO reduction activity of Cu/ZSM-5 catalyst is investigated. 3.5Cu/0.6NaZSM-5 catalyst was significantly more active for NOx reduction with NH3 compared with 2.6Cu/HZSM-5. Catalyst characterization indicated that, the type of co-cations present in the catalyst tailors copper species redox properties, NO2 formation rate and acidity of the catalyst. Copper species in NaZSM-5, is relatively easily reducible than when present in HZSM-5. UV–vis and FTIR characterization showed the presence of a higher amount of Cu+ species in 3.5Cu/0.6NaZSM-5 compared to 2.6Cu/HZSM-5. The easily reducible copper along with Cu+ species in 3.5Cu/0.6NaZSM-5 seems to activate oxygen which reacts with NO to form NO2. The Na+ co-cations can further capture the thus produced NO2 to form intermediate surface nitrite/nitrate species, which improves the NOx conversion. The acidity measured by NH3-TPD and FTIR is needed for SCR, but not crucial for NOx conversion.

The impact of urea on the performance of metal exchanged zeolites for the selective catalytic reduction of NOx: Part I. Pyrolysis and hydrolysis of urea over zeolite catalysts

Abstract: Urea-SCR over metal exchanged zeolites is one of the leading catalytic technologies to abate NO x emissions in diesel exhaust. Ideally, urea injected into the diesel exhaust upstream of the SCR catalyst decomposes only to the gaseous products CO 2 and NH 3 , where the latter gas can react with NO x emissions to form harmless N 2 and H 2 O. However, solid by-products can be formed as well, and if deposited on the catalyst harm the long-term catalytic performance. In order to identify the impact of various urea decomposition products on the catalytic activity, we studied the pyrolysis and hydrolysis of neat urea and of urea over different zeolites (H-Y, Cu-Y, H-Beta, Na-Beta, and Fe-Beta). The experiments were run in dry and steam-containing N 2 between 20 and 750 ° C by using simultaneous thermogravimetric analysis (TGA), differential thermoanalysis (DTA), and online GC/MS evolved gas analysis. Solid intermediate products at different decomposition temperatures were identified by means of ATR-FTIR and luminescence spectroscopy. As for neat urea, CO 2 , NH 3 and HNCO could be detected as major gaseous products. At 270 ° C significant amounts of cyanuric acid and ammelide and at 500 ° C of melem and melon were identified as solid intermediates. Above 625 ° C, all solid residues decomposed to cyanogen and isocyanic acid. Furthermore, it could be shown clearly that the investigated zeolites significantly accelerate the pyrolysis of urea and cyanuric acid, and the hydrolysis of HNCO, by shifting the decomposition processes to lower temperatures and by inhibiting the formation of solid by-products. In addition, the presence of steam in the feed gas can prevent even further the formation of solid residues and the high temperature adsorption of gaseous products.

Low temperature selective catalytic reduction of NO by activated carbon fiber loading lanthanum oxide and ceria

Abstract: Due to the contribution to air pollution, the controlling of NO discharge needs further studies. In this paper, loaded catalysts of 10–40% (w/w) CeO2/ACF were prepared by impregnation method and used for the selective catalytic reduction of NO with ammonia in the presence of O2. Such catalysts were characterized by surface area measurement (BET), thermo gravimetric (TG) analysis and Fourier transform infrared (FTIR) spectroscopy. The catalytic activity of 10–40% CeO2/ACF at different temperatures, and the catalytic stability at 200 °C were studied. Moreover, the experimental results were compared with those of previous studied catalysts. The results show that the 10% CeO2/ACF and 20% La2O3/ACF can yield higher NO conversion and maintain higher catalytic activity at higher temperature than others. 10% CeO2/ACF yields about 70% NO conversion in the SCR of NO at 150 °C, and meanwhile 20% La2O3/ACF and 10% CeO2/ACF yield over 90% NO conversion when the temperatures are higher than 350 °C. The catalytic activity and the catalyst stability of 10% CeO2/ACF and 20% La2O3/ACF are both higher than those of many reported catalysts. Based on the catalytic and characterizing results, such as BET, TG and FTIR, it can be found that the SCR of NO mechanism of loaded catalysts is different from those of ACF and HNO3/ACF. ACF-C is the catalyst and reducing agent in the SCR of NO of ACF and HNO3/ACF, while the metal oxides loaded by ACF are the catalytic centers, NH3 is the main reducing agent in the SCR of loaded catalysts.

A Kinetic Model for the Selective Catalytic Reduction of NOx with NH3 over an Fe-zeolite Catalyst

Abstract: The selective catalytic reduction of NOx with ammonia over all Fe-zeolite catalyst was investigated experimentally and a transient kinetic model was developed. The model includes reactions that describe ammonia storage and oxidation, NO oxidation, selective catalytic reduction (SCR) of NO and NO2, formation of N2O, ammonia inhibition and ammonium nitrate formation. The model call account for a broad range of experimental conditions in the presence of H2O, CO2 and O-2 at temperatures from 150 to 650 degrees C. The catalyst stores ammonia at temperatures up to 400 degrees C and shows ammonia oxidation activity from 350 degrees C. The catalyst is also active for the oxidation of NO to NO2 and the oxidation reaches equilibrium at 500 degrees C. The SCR of NO is already active at 150 degrees C and the introduction of equal amounts of NO and NO2 greatly enhances the conversion of NOx at temperatures up to 300 degrees C. The formation of N2O is negligible if small fractions of NO2 are fed to the reactor, but a significant amount of N2O is formed at high NO2 to NO ratios. An ammonia inhibition oil the SCR of NO is observed at 200 degrees C. This kinetic model contains 12 reactions and is able to describe the experimental results Well. The model was validated using short transient experiments and experimental conditions not used in the parameter estimation and predicted these new conditions adequately.

Oxidation of gaseous elemental mercury by hydrochloric acid over CuCl2/TiO2-based catalysts in SCR process

Abstract: CuCl 2 /TiO 2 -based catalysts were examined to investigate the role of copper chloride for the oxidation of gaseous elemental mercury in selective catalytic reduction (SCR) process. CuCl 2 on CuCl 2 /TiO 2 catalyst was decomposed releasing Cl by calcination at high temperatures and restored to its original form by being exposed to gas phase HCl, reversibly. The activity for mercury oxidation was significantly increased with the increase of CuCl 2 loading and HCl concentration. CuCl 2 /TiO 2 catalysts revealed high activity for mercury oxidation even in the absence of HCl. This suggests that mercury oxidation could occur via a Mars–Maessen mechanism by which adsorbed or weakly bound Hg 0 would react with Cl in CuCl 2 that is replenished from gas phase HCl. However, the activity of CuCl 2 -loaded catalysts for NO removal considerably decreased with the increase of temperature above 300–350 °C, which may be due to the ability of CuCl 2 for NH 3 oxidation in SCR reaction.