Showing papers on "Selective catalytic reduction published in 2008"

The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal‐Exchanged Zeolite Catalysts

Abstract: An overview is given of the selective catalytic reduction of NOx by ammonia (NH3‐SCR) over metal‐exchanged zeolites. The review gives a comprehensive overview of NH3‐SCR chemistry, including undesired side‐reactions and aspects of the reaction mechanism over zeolites and the active sites involved. The review attempts to correlate catalyst activity and stability with the preparation method, the exchange metal, the exchange degree, and the zeolite topology. A comparison of Fe‐ZSM‐5 catalysts prepared by different methods and research groups shows that the preparation method is not a decisive factor in determining catalytic activity. It seems that decreased turnover frequency (TOF) is an oft‐neglected effect of increasing Fe content, and this oversight may have led to the mistaken conclusion that certain production methods produce highly active catalysts. The available data indicate that both isolated and bridged iron species participate in the NH3‐SCR reaction over Fe‐ZSM‐5, with isolated species being the ...

Exhaust purification device of internal combustion engine

Abstract: An internal combustion engine wherein an HC treatment catalyst (12) having the function of adsorbing the HC in the exhaust gas is arranged upstream of the NOx selective reducing catalyst (14), an urea aqueous solution fed from the reducing agent feed valve (15) is arranged upstream of the HC treatment catalyst (12), urea, and NOx contained in the exhaust gas, and HC adsorbed on the HC treatment catalyst 12 are reacted with each other to form intermediate products having cyano groups, oximes, and amino groups, and these intermediate products are sent to the NOx selective reducing catalyst (14).

Low-Temperature Selective Catalytic Reduction of NO with NH3 over Ti0.9M0.1O2-δ (M = Cr, Mn, Fe, Co, Cu)

Abstract: The catalysts, Ti0.9M0.1O2-δ (M = Cr, Mn, Fe, Co, Cu), were synthesized in anatase phase by solution combustion. Selective catalytic reduction (SCR) of NO with NH3 was investigated over these catalysts. The reaction occurred at the lowest temperature over Ti0.9Mn0.1O2-δ, but the selectivity for N2 was highest over Ti0.9Fe0.1O2-δ. Therefore, both Mn and Fe were substituted in TiO2 (Ti0.9Mn0.05Fe0.05O2-δ). The reaction occurred at low temperature with a high selectivity over this catalyst. In order to understand the reaction mechanism and the nature of the active sites, temperature programmed desorption (TPD) of NH3 and hydrogen uptake studies were carried out. The relation between the Lewis acid sites and SCR window and the relation between Bronsted acid sites and low temperature was established. The order of the SCR reaction with respect to NO, NH3, and O2 was also investigated. It was also shown that the N2 selectivity of the SCR reaction has a strong inverse correlation with the oxidation of ammonia.

Effect of transition metals addition on the catalyst of manganese/titania for low-temperature selective catalytic reduction of nitric oxide with ammonia

Abstract: To retard the sintering, a series of transition metals were added to the low-temperature SCR catalysts based on Mn/TiO 2 , and activity of these catalysts was investigated. It was found that the transition metal had significant effects on the catalytic activity. With the addition of transition metals, more NO could be removed at lower temperature. The temperature of 90% NO conversion could decrease to 361 K by using Fe(0.1)–Mn(0.4)/TiO 2 . The results of X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction spectra (EDS) indicated that manganese oxides and titania could be better dispersed in the catalyst, and higher catalytic activity was obtained. From X-ray photoelectron spectrum (XPS) it could be known that solid solution was formed among the transition metal, manganese oxides and titania. With the formation of this solid solution, the Brunauer–Emmett–Teller (BET) area and pore volume increased. Furthermore, the in situ diffuse reflectance infrared transform spectroscopy (DRIFT) results showed that by using these catalysts, more NO could be oxidized to NO 2 and nitrate, and then reacted with NH 3 . Therefore, the catalytic activity was greatly improved by the addition of transition metals.

A kinetic model for ammonia selective catalytic reduction over Cu-ZSM-5

Abstract: Kinetic modeling, in combination with flow reactor experiments, was used in this study for simulating NH 3 selective catalytic reduction (SCR) of NO x over Cu-ZSM-5. First the mass-transfer in the wash-coat was examined experimentally, by using two monoliths: one with 11 wt.% wash-coat and the other sample with 23 wt.% wash-coat. When the ratio between the total flow rate and the wash-coat amount was kept constant similar results for NO x conversion and NH 3 slip were obtained, indicating no significant mass-transfer limitations in the wash-coat layer. A broad range of experimental conditions was used when developing the model: ammonia temperature programmed desorption (TPD), NH 3 oxidation, NO oxidation, and NH 3 SCR experiments with different NO-to-NO 2 ratios. 5% water was used in all experiments, since water affects the amount of ammonia stored and also the activity of the catalyst. The kinetic model contains seven reaction steps including these for: ammonia adsorption and desorption, NH 3 oxidation, NO oxidation, standard SCR (NO + O 2 + NH 3 ), rapid SCR (NO + NO 2 + NH 3 ), NO 2 SCR (NO 2 + NH 3 ) and N 2 O formation. The model describes all experiments well. The kinetic parameters and 95% linearized confidence regions are given in the paper. The model was validated with six experiments not included in the kinetic parameter estimation. The ammonia concentration was varied from 200 up to 800 ppm using NO only as a NO x source in the first experiment and 50% NO and 50% NO 2 in the second experiment. The model was also validated with transient experiments at 175 and 350 °C where the NO and NH 3 concentrations were varied stepwise with a duration of 2 min for each step. In addition, two short transient experiments were simulated where the NO 2 and NO levels as well as NO 2 -to-NO x ratio were varied. The model could describe all validation experiments very well.

Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils, and urea solution: I. Catalytic studies

Abstract: The influence of the combustion products of different lubrication oil additives (Ca, Mg, Zn, P, B, Mo) and impurities in Diesel fuel (K from raps methyl ester) or urea solution (Ca, K) on the activity and selectivity of vanadia-based SCR catalysts were investigated. Standard V2O5/WO3–TiO2 catalysts coated on metal substrates (400 cpsi) were impregnated with water soluble compounds of these elements and calcined at 400 and 550 °C, in order to investigate the chemical deactivation potential of different elements and combinations of them. It was found that potassium strongly reduced the adsorption equilibrium constant K N H 3 of ammonia. At small ammonia concentrations in the feed, only part of the active sites were covered with ammonia resulting in a reduced SCR reaction rate. At high ammonia concentrations, the surface coverage and SCR reaction rate increased, but high SCR activity at concurrent low ammonia emissions was impossible. Calcium caused less deactivation than potassium and did not affect the ammonia adsorption to the same extent, but it lowered the intrinsic SCR reaction rate. Moreover, deactivation by calcium was much reduced if counter-ions of inorganic acids were present (order of improvement: SO42− > PO43− > BO33−). Zinc was again less deactivating than calcium, but the positive effect of the counter-ions was weaker than in case of calcium. The degree of N2O production at T > 500 °C, which is typical for V2O5/WO3–TiO2 catalysts, was not influenced by the different compounds, except for molybdenum, which induced a small increase in N2O formation.

Development of silica/vanadia/titania catalysts for removal of elemental mercury from coal-combustion flue gas.

Abstract: SiO2/V2O5/TiO2 catalysts were synthesized for removing elemental mercury (Hg0) from simulated coal-combustion flue gas. Experiments were carried out in fixed-bed reactors using both pellet and powder catalysts. In contrast to the SiO2-TiO2 composites developed in previous studies, the V2O5 based catalysts do not need ultraviolet light activation and have higher Hg0 oxidation efficiencies. For Hg0 removal by SiO2-V2O5 catalysts, the optimal V2O5 loading was found between 5 and 8%, which may correspond to a maximum coverage of polymeric vanadates on the catalyst surface. Hg0 oxidation follows an Eley-Rideal mechanism where HCI, NO, and NO2 are first adsorbed on the V2O5 active sites and then react with gas-phase Hg0. HCI, NO, and NO2 promote Hg oxidation, while SO2 has an insignificant effect and water vapor inhibits Hgo oxidation. The SiO2-TiO2-V2O5 catalysts exhibit greater Hg0 oxidation efficiencies than SiO2-V2O5, may be because the V-O-Ti bonds are more active than the V-O-Si bonds. This superior oxidation capability is advantageous to power plants equipped with wet-scrubbers where oxidized Hg can be easily captured. The findings in this work revealed the importance of optimizing the composition and microstructures of SCR (selective catalytic reduction) catalysts for Hg0 oxidation in coal-combustion flue gas.

Co3O4 based catalysts for NO oxidation and NOx reduction in fast SCR process

Abstract: Reaction activities of several developed catalysts for NO oxidation and NO x (NO + NO 2 ) reduction have been determined in a fixed bed differential reactor. Among all the catalysts tested, Co 3 O 4 based catalysts are the most active ones for both NO oxidation and NO x reduction reactions even at high space velocity (SV) and low temperature in the fast selective catalytic reduction (SCR) process. Over Co 3 O 4 catalyst, the effects of calcination temperatures, SO 2 concentration, optimum SV for 50% conversion of NO to NO 2 were determined. Also, Co 3 O 4 based catalysts (Co 3 O 4 -WO 3 ) exhibit significantly higher conversion than all the developed DeNO x catalysts (supported/unsupported) having maximum conversion of NO x even at lower temperature and higher SV since the mixed oxide Co-W nanocomposite is formed. In case of the fast SCR, N 2 O formation over Co 3 O 4 -WO 3 catalyst is far less than that over the other catalysts but the standard SCR produces high concentration of N 2 O over all the catalysts. The effect of SO 2 concentration on NO x reduction is found to be almost negligible may be due to the presence of WO 3 that resists SO 2 oxidation.

Selective catalytic reduction of NO with NH3 at low temperatures over iron and manganese oxides supported on mesoporous silica

Abstract: A series of catalysts of iron–manganese oxide supported on mesoporous silica (MPS) with different Mn/Fe ratio were studied for low-temperature selective catalytic reduction (SCR) of NO with ammonia in the presence of excess oxygen. Effects of amounts of iron–manganese oxide and calcination temperatures on NO conversion were also investigated. It was found that the Mn–Fe/MPS with Mn/Fe = 1 at the calcination temperature of 673 K showed the highest activity. The results showed that this catalyst yielded 99.1% NO conversion at 433 K at a space velocity of 20,000 h −1 . H 2 O has no adverse impact on the activity when the SCR reaction temperature is above 413 K. In addition, the SCR activity was suppressed gradually in the presence of SO 2 and H 2 O, while such effect was reversible after heating treatment.

Manganese Oxide/Titania Materials for Removal of NOx and Elemental Mercury from Flue Gas

Abstract: A novel catalyst for low temperature selective catalytic reduction (SCR) using CO as reductant, MnO x supported on titania, has been shown to be effective for both elemental mercury capture and low temperature SCR. In low temperature (200 °C) SCR trials using an industrially relevant space velocity (50 000 h −1) and oxygen concentration (2 vol %), nearly quantitative reduction of NO x was obtained using CO as the reductant. Fresh catalyst used as an adsorbent for elemental mercury from an inert atmosphere showed remarkable mercury capture capacity, as high as 17.4 mg/g at 200 °C. The catalyst effectively captured elemental mercury after use in NO x reduction. Mercury capture efficiency was not affected by the presence of water vapor. Mercury capacity was reduced in the presence of SO 2. Manganese loading and bed temperature, which influence surface oxide composition, were found to be important factors for mercury capture. X-ray photoelectron spectroscopy (XPS) results reveal that the mercury is present in...

Impacts of Halogen Additions on Mercury Oxidation, in A Slipstream Selective Catalyst Reduction (SCR), Reactor When Burning Sub-Bituminous Coal

Abstract: This paper presents a comparison of impacts of halogen species on the elemental mercury (Hg(0)) oxidation in a real coal-derived flue gas atmosphere. It is reported there is a higher percentage of Hg(0) in the flue gas when burning sub-bituminous coal (herein Powder River Basin (PRB) coal) and lignite, even with the use of selective catalytic reduction (SCR). The higher Hg(0)concentration in the flue gas makes it difficult to use the wet-FGD process for the mercury emission control in coal-fired utility boilers. Investigation of enhanced Hg(0) oxidation by addition of hydrogen halogens (HF, HCl, HBr, and HI) was conducted in a slipstream reactor with and without SCR catalysts when burning PRB coal. Two commercial SCR catalysts were evaluated. SCR catalyst no. 1 showed higher efficiencies of both NO reduction and Hg(0) oxidation than those of SCR catalyst no. 2. NH3 addition seemed to inhibit the Hg(0) oxidation, which indicated competitive processes between NH3 reduction and Hg(0) oxidation on the surface of SCR catalysts. The hydrogen halogens, in the order of impact on Hg(0) oxidation, were HBr, HI, and HCl or HF. Addition of HBr at approximately 3 ppm could achieve 80% Hg(0) oxidation. Addition of HI at approximately 5 ppm could achieve 40% Hg(0) oxidation. In comparison to the empty reactor, 40% Hg(0) oxidation could be achieved when HCl addition was up to 300 ppm. The enhanced Hg(0) oxidation by addition of HBr and HI seemed not to be correlated to the catalytic effects by both evaluated SCR catalysts. The effectiveness of conversion of hydrogen halogens to halogen molecules or interhalogens seemed to be attributed to their impacts on Hg(0) oxidation.

Mercury Oxidation over the V2O5(WO3)/TiO2 Commercial SCR Catalyst

Abstract: Mercury oxidation by hydrochloric acid over the V{sub 2}O{sub 5}(WO{sub 3})/TiO{sub 2} commercial SCR catalyst was investigated. Both fresh and aged catalysts with honeycomb structure, which were exposed to a coal combustion flue gas in a coal-fired boiler for over 71 000 h. were examined. The aged catalysts were characterized by X-ray and SEM-EDX analysis to examine the presence of ash deposition on the surface. The mercury oxidation rate was enhanced by increasing HCl concentrations and inhibited strongly by the presence of NH{sub 3}. This behavior could be explained by a kinetic model assuming that HCl competes for the catalyst active sites against NH{sub 3}. As the catalyst operation time increased, the mercury oxidation rate was observed to decrease considerably in the presence of NH{sub 3} while NO reduction rate was apparently nearly unchanged. By examining aged catalysts, deposits stemming from fly ash and SO{sub 2}/SO{sub 3} were observed to accumulate continuously on the catalyst surface. The ash deposited on the surface may partially block the active catalyst sites and decrease their number. The decrease of the number of active sites on the catalyst surface caused NH{sub 3} to remain unreacted in the honeycomb catalyst. The decrease of the Hg{supmore » 0} oxidation rate was caused by the inhibition effect of NH{sub 3} remaining in the catalyst.« less

Heterogeneous Mercury Reaction on a Selective Catalytic Reduction (SCR) Catalyst

Abstract: The heterogeneous mercury reaction mechanism, reactions among elemental mercury (Hg0) and simulated flue gas across laboratory-scale selective catalytic reduction (SCR) reactor system was studied. The surface of SCR catalysts used in this study was analyzed to verify the proposed reaction pathways using transmission electron microscopy with energy dispersive X-ray analyses (TEM-EDX) and X-ray photoelectron spectroscopy (XPS). The Langmuir–Hinshelwood mechanism was proven to be most suitable explaining first-layer reaction of Hg0 and HCl on the SCR catalyst. Once the first layer is formed, successive layers of oxidized mercury (HgCl2) are formed, making a multi-layer structure.

Catalytic reduction of NO by CO over NiO/CeO2 catalyst in stoichiometric NO/CO and NO/CO/O2 reaction

Abstract: A new catalyst composed of nickel oxide and cerium oxide was studied with respect to its activity for NO reduction by CO under stoichiometric conditions in the absence as well as the presence of oxygen. Activity measurements of the NO/CO reaction were also conducted over NiO/γ-Al 2 O 3 , NiO/TiO 2 , and NiO/CeO 2 catalysts for comparison purposes. The results showed that the conversion of NO and CO are dependent on the nature of supports, and the catalysts decreased in activity in the order of NiO/CeO 2 > NiO/γ-Al 2 O 3 > NiO/TiO 2 . Three kinds of CeO 2 were prepared and used as support for NiO. They are the CeO 2 prepared by (i) homogeneous precipitation (HP), (ii) precipitation (PC), and (iii) direct decomposition (DP) method. We found that the NiO/CeO 2 (HP) catalyst was the most active, and complete conversion of NO and CO occurred at 210 °C at a space velocity of 120,000 h −1 . Based on the results of surface analysis, a reaction model for NO/CO interaction over NiO/CeO 2 has been proposed: (i) CO reduces surface oxygen to create vacant sites; (ii) on the vacant sites, NO dissociates to produce N 2 ; and (iii) the oxygen originated from NO dissociation is removed by CO.

SCR on low thermal mass filter substrates

Abstract: Provided are selective catalytic reduction (SCR) filters that effectively provide simultaneous treatment of particulate matter and NOx. Provided also are methods for reducing NOx concentration and particulate matter in a diesel engine exhaust by using the SCR filters. The SCR filter can include a fiber matrix wall flow filter comprising a plurality of non-woven inorganic fibers and a chabazite molecular sieve SCR catalyst on the fiber matrix wall flow filter. By combining a fiber matrix wall flow filter with a chabazite molecular sieve SCR catalyst, high catalyst loading can be achieved without causing excessive back pressure across the filter when implemented in emission treatment systems.

NOx storage and reduction with H2 on Pt/BaO/Al2O3 monolith : Spatio-temporal resolution of product distribution

Abstract: The regeneration of a model Pt/BaO/Al 2 O 3 monolith catalyst was studied with hydrogen as the reductant to elucidate the reaction pathways to molecular nitrogen and ammonia. NO x storage and reduction experiments (NSR) were conducted with a 2 cm length monolith for a wide range of feed conditions. The NSR experiments were replicated for a series of monoliths of progressively decreasing length, enabling the construction of spatio-temporal profiles of reactant and product concentrations. The results show that there are two primary competing routes to the desired N 2 product; specifically a direct route from the reduction of stored NO x by H 2 (H 2 + NO x → N 2 ) or by a sequential route through NH 3 (H 2 + NO x → NH 3 ; NH 3 + NO x → N 2 ). A comparison between H 2 and NH 3 as reductant feeds during NSR revealed H 2 is a more effective reductant in terms of NO x conversion for temperatures below approximately 230 °C. At higher temperatures (230–380 °C), the regeneration of stored NO x is feed-limited and the difference between the reductants H 2 and NH 3 is found to be small with H 2 being a slightly superior reductant. Experimental measurements of the traveling front velocity are compared with a simple feed-limited model that assumes complete consumption of H 2 as stored NO x is depleted. At lower temperatures the regeneration is limited by chemical processes at the Pt/Ba interface. The findings are pieced together to establish a phenomenological description of the spatio-temporal features of the lean NO x trap with hydrogen as the reductant.

Selective Reduction of NO with CO Over Titania Supported Transition Metal Oxide Catalysts

Abstract: A series of transition metal oxides promoted titania catalysts (MO x /TiO2; M = Cr, Mn, Fe, Ni, Cu) were prepared by wet impregnation method using dilute solutions of metal nitrate precursors. The catalytic activity of these materials was evaluated for the selective catalytic reduction (SCR) of NO with CO as reductant in the presence of excess oxygen (2 vol.%). Among various promoted oxides, the MnO x /TiO2 system showed very promising catalytic activity for NO + CO reaction, giving higher than 90% NO conversion over a wide temperature window and at high space velocity (GHSV) of 50,000 h−1. It is remarkable to note that the catalytic activity increased with oxygen, up to 4 vol.%, under these conditions leading primarily to nitrogen. Our TPR studies revealed the presence of mixed oxidation states of manganese on the catalyst surface. Characterization results indicated that the surface manganese oxide phase and the redox properties of the catalyst play an important role in final catalytic activity.

Catalytic reduction of NH4NO3 by NO: Effects of solid acids and implications for low temperature DeNOx processes

Abstract: Ammonium nitrate is thermally stable below 250 °C and could potentially deactivate low temperature NO x reduction catalysts by blocking active sites It is shown that NO reduces neat NH 4 NO 3 above its 170 °C melting point, while acidic solids catalyze this reaction even at temperatures below 100 °C NO 2 , a product of the reduction, can dimerize and then dissociate in molten NH 4 NO 3 to NO + + NO 3 − , and may be stabilized within the melt as either an adduct or as HNO 2 formed from the hydrolysis of NO + or N 2 O 4 The other product of reduction, NH 4 NO 2 , readily decomposes at ≤100 °C to N 2 and H 2 O, the desired end products of DeNO x catalysis A mechanism for the acid catalyzed reduction of NH 4 NO 3 by NO is proposed, with HNO 3 as an intermediate These findings indicate that the use of acidic catalysts or promoters in DeNO x systems could help mitigate catalyst deactivation at low operating temperatures (

Combination of V2O5/WO3−TiO2, Fe−ZSM5, and Cu−ZSM5 Catalysts for the Selective Catalytic Reduction of Nitric Oxide with Ammonia

Abstract: Cordierite monoliths coated with V2O5/WO3−TiO2, Fe−ZSM5, and Cu−ZSM5 catalysts were combined to investigate the possibility of combining the advantages of the different single catalysts for the selective catalytic reduction of nitric oxide with ammonia (NO SCR) while minimizing their drawbacks. Selected combinations of two of the three above-mentioned catalysts were connected in series such that the volume of each catalyst was halved in order to maintain the total space velocity constant. The combinations V2O5/WO3−TiO2 followed by Fe−ZSM5 and the reversed catalyst order achieved markedly lower NOx reduction efficiencies (DeNOx) than the pure vanadia-based catalyst. Fe−ZSM5 applied downstream of V2O5/WO3−TiO2 did not reduce the N2O formed over the V-based catalyst at temperatures above 450 °C, as expected from the known N2O decomposition and N2O SCR activities of Fe−ZSM5. When V2O5/WO3−TiO2 was mounted downstream of Fe−ZSM5, ammonia slip was observed even at small NOx reduction efficiencies. The combinatio...

High temperature catalyst and process for selective catalytic reduction of nox in exhaust gases of fossil fuel combustion

Abstract: A process for producing a stable high-temperature catalyst for reduction of nitrogen oxides in combustion exhaust gases at operating temperatures from 300°C to over 700°C without the need for exhaust dilution. A zeolite material is steam-treated at a temperature and duration sufficient to partially de-aluminize the zeolite to approximately a steady state, but not sufficient to fully collapse its chemical structure. Iron is added to the zeolite material. The zeolite material is calcined at a temperature, humidity, and duration sufficient to stabilize the zeolite material. Examples and specifications for ranges, order, and durations of steaming, calcining, and other steps are provided.

System and method for selective catalytic reduction control

Abstract: A selective catalytic reduction (SCR) catalyst control system and method for an engine is disclosed. Urea injection to an SCR catalyst is determined based on an SCR catalyst model, which determines a value of stored NH 3 in the SCR catalyst based on the NO x engine emission value, the SCR catalyst temperature, the quantity of urea supplied to the SCR catalyst and a pre-determined efficiency of conversion of NO x gases. A target value of stored NH 3 and the value of stored NH 3 in the SCR catalyst is then used to determine a stored NH 3 differential, which is then used to calculate urea injection.