NOx Reduction Activity Research Articles

Power-bench demonstration of the Pt-catalysed C3H6-SCR of NOx in a diesel exhaust

The SCR of NOx by C3H6 using a 0.1 wt.% Pt/Al2O3 catalyst has been successfully performed under real diesel gas streams in a power bench. The influence of some experimental variables affecting NOx, CO, and THC conversions, such as hydrocarbon concentration, reaction temperature and space velocity effect, are discussed. 96% NOx conversion has been reached under optimum conditions. The highest NOx conversion level is maintained in the range 250–325 °C but slightly higher temperature (350 °C) is desired to ensure a stable temperature and complete CO and THC removal. An optimum THC concentration is found (1500 ppm THC), higher THC concentration having a negative effect in the NOx conversion. From experiments performed at different GHSV, the reactor volume required for total NOx removal in a real exhaust stream has been estimated to be 6.5 L. Additionally, the effect of CeO2 on platinum behaviour has been studied, concluding that it enhances N2 formation as NOx reduction product with respect to N2O, but lowers the platinum activity for NOx reduction and CO and THC oxidation.

Diesel exhaust-gas reforming for H2 addition to an aftertreatment unit

The work described in this paper has been undertaken as part of the design of an integrated system comprising a diesel engine, an exhaust-gas fuel reformer and a NOx aftertreatment unit. The exhaust-gas reformer is used to provide hydrogen-rich reformate to the NOx aftertreatment unit, containing a hydrocarbon-SCR catalyst, in order to improve its NOx reduction activity at low exhaust-gas temperatures. The reformer configuration and operating parameters have been examined in order to optimise the performance of the hydrocarbon-SCR catalyst, which is promoted by the presence of H2 but inhibited by CO. The length of the catalyst bed inside the reformer is a key factor in determining the extent to which the water-gas shift reaction can contribute to the reforming process, and therefore strongly influences the proportions of CO and H2 in the reformate. However, it is also necessary for the reactant ratios at the reformer inlet to be controlled in response to changes in the engine operating conditions. In practice, this means that the rate of fuel addition to the reformer needs to be optimised for different exhaust gas compositions and space velocities.

Synthesis of nanostructured lean-NO x catalysts by direct laser deposition of monometallic Pt-, Rh- and bimetallic PtRh-nanoparticles on SiO2 support

Monometallic Pt and Rh and bimetallic PtRh catalysts with a highly dispersed noble metal weight loading of ca. 1 wt% were produced via the direct deposition of nanoparticles on different SiO2 supports by means of pulsed ultra-violet (248 nm) excimer laser ablation of Pt, Rh bulk metal and PtRh alloy targets. Backscattered electron microscopy (BSE), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) were employed to characterize the deposited nanoparticles, which were found to exhibit narrow size distribution centred around 2.5 nm. The catalytic activities for lean NOx reduction of the monometallic and bimetallic catalyst samples were investigated in a flow reactor setup in the temperature range 100–400°C using a test gas mixture representative of oxygen rich diesel engine exhaust gas. For comparison a Rh/SiO2 reference catalyst prepared by a conventional impregnation method was also tested. Further experiments were performed in which PtRh nanoparticles were deposited on a Rh/SiO2 reference catalyst sample to study the possibility for controlled modification of its activity. The catalytic activity measurements revealed that among the samples solely prepared by laser deposition the PtRh–SiO2 nanoparticle catalyst showed the highest activity for NOx reduction at low temperatures 100–300°C. In addition, it could be demonstrated that the initially low NOx reduction activity and the N2 selectivity of the Rh/SiO2 reference catalyst sample for temperatures below 250°C can be enhanced by post laser deposition of PtRh nanoparticles.

ANN modeling applied to NO x reduction with octane in a new microreactor

A silver/alumina catalyst was tested for its NO x reduction activity during oxygen-rich conditions and during variation in the input parameters (nitric oxide, octane and oxygen). The experimental data using the microreactor was investigated by means of artificial neural networks (ANN).

Synergic effect of Pd/γ-alumina and Cu/ZSM-5 on the performance of NO x storage reduction catalyst

NO x reduction with a combination of catalysts, Pd catalyst, NO x storage reduction (NSR) catalyst and Cu/ZSM-5 in turn, was investigated to elucidate for the high NO x reduction activity of this catalyst combination under oxidative atmosphere with periodic deep rich operation. The catalytic activity was evaluated using the simulated exhaust gases with periodically fluctuation between oxidative and reductive atmospheres, and it was found that the NO x reduction activity with this catalyst combination was apparently higher than that of the solely accumulation of these individual activities, which was caused by the additional synergic effect by this combination. The Pd catalyst upstream of the NSR catalyst improved NO x storage ability by NO2 formation under oxidative atmosphere. The stored NO x was reduced to NH3 on the NSR catalyst, and the generated NH3 was adsorbed on Cu/ZSM-5 downstream of the NSR catalyst under the reductive atmosphere, and subsequently reacted with NO x on the Cu/ZSM-5 under the oxidative atmosphere.

Poisoning effect of sulphate on the selective catalytic reduction of NOx by C3H6 over Ag-Pd/Al2O3

The Ag-Pd/Al2O3 catalyst showed a higher NOx conversion than Ag/Al2O3 in the selective catalytic reduction (SCR) of NOx by C3H6 especially at temperatures ranging from 573 to 773K. However, the presence of SO2 in the feed gas decreased the activity of NOx reduction over Ag-Pd/Al2O3 more greatly than that over Ag/Al2O3. The sulphate species formed on the poisoned Ag-Pd/Al2O3 and Ag/Al2O3 were characterized by BET, inductively couple plasma optical emission spectrometer (ICP-OES), XRD, temperature programmed desorption (TPD) and in situ diffuse reflectance infra-red fourier transform spectroscopy (DRIFTS) methods. Two kinds of sulphate species were found on the SO2 poisoned Ag-Pd/Al2O3 catalyst, and the formation of nitrate was blocked by the surface sulphate species. The addition of Pd into Ag/Al2O3 promoted the formation of sulphate on the Ag sites in the presence of SO2, which was considered to be the possible reason for the weak sulphur resistance of Ag-Pd/Al2O3.

Reactor and in situ FTIR studies of Pt/BaO/Al2O3 and Pd/BaO/Al2O3 NOx storage and reduction (NSR) catalysts

The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1wt.% Pt/20wt.% BaO/Al2O3 and 1wt.% Pd/20wt.% BaO/Al2O3 NOx storage reduction (NSR) catalysts. At temperatures between 250 and 350°C, the Pd/BaO/Al2O3 catalyst exhibits higher overall NOx reduction activity. Limited amounts of N2O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1wt.% Pt/Al2O3 and Pd/Al2O3 catalysts demonstrate that under these conditions Pd exhibits a higher activity for the oxidation of both propylene and NO. Furthermore, in situ FTIR studies conducted under identical conditions suggest the formation of higher amounts of surface nitrite species on Pd/BaO/Al2O3. The IR results indicate that this species is substantially more active towards reaction with propylene. Moreover, its formation and reduction appear to represent the main pathway for the storage and reduction of NO under the conditions examined. Consequently, the higher activity of Pd can be attributed to its higher oxidation activity, leading both to a higher storage capacity (i.e., higher concentration of surface nitrites under “lean” conditions) and a higher reduction activity (i.e., higher concentration of partially oxidized active propylene species under “rich” conditions). The performance of Pt and Pd is nearly identical at temperatures above 375°C.

Catalytic performance of Ag/Al 2O 3-C 2H 5OH-Cu/Al 2O 3 system for the removal of NOx from diesel engine exhaust

The selective catalytic reduction (SCR) of NOx by C 2H 5OH was studied in excess oxygen over Ag/Al 2O 3 catalysts with different Ag loadings at lab conditions. The 4% Ag/Al 2O 3 has the highest activity for the C 2H 5OH-SCR of NOx with a drawback of simultaneously producing CO and unburned THC in effluent gases. An oxidation catalyst 10% Cu/Al 2O 3 was directly placed after the Ag/Al 2O 3 to remove CO and unburned THC. Washcoated honeycomb catalysts were prepared based on the 4% Ag/Al 2O 3 and 10% Cu/Al 2O 3 powders and tested for the C 2H 5OH-SCR of NOx on a diesel engine at the practical operating conditions. Compared with the Ag/Al 2O 3 powder, the Ag/Al 2O 3 washcoated honeycomb catalyst (SCR catalyst) has a similar activity for NOx reduction by C 2H 5OH and the drawback of increasing the CO and unburned THC emissions. Using the SCR + Oxi composite catalyst with the optimization of C 2H 5OH addition, the diesel engine completely meets EURO III emission standards.

NOx reduction behavior on alumina with discharging nonthermal plasma in simulated oxidizing exhaust gas

The NOx reduction activity on γ-alumina was significantly improved by discharging nonthermal plasma in a simulated oxidizing exhaust gas under conditions of a high space velocity. On the other hand, those on a Pt-loaded catalyst and Cu-ZSM-5 could not be improved. The discharging nonthermal plasma converted NO to NO2 and hydrocarbons to partial-oxidized hydrocarbons, such as aldehydes, under the oxidizing conditions. From the relationship between the NOx reduction activities and the properties of several alumina catalysts, it was found that the NOx reduction activity was correlated with the number of acid sites on the alumina based on the NH3 adsorption measurement. Therefore, it is concluded that the NOx reduction occurred at the acid sites of the alumina, and the discharging nonthermal plasma improves the NOx reduction activity by NO2 formation and partial-oxidation of hydrocarbons. These results suggest that a catalytic reaction assisted by the discharging nonthermal plasma could be a promising technology for NOx reduction in lean-burn and diesel exhaust gases even under the conditions of high space velocity. Copyright © 2006 Society of Chemical Industry

Ir-based additives for NO reduction and CO oxidation in the FCC regenerator: Evaluation, characterization and mechanistic studies

Ir-based additives, developed to reduce NO and CO emitted during the regeneration of spent fluid catalytic cracking (FCC) catalysts were characterized to correlate physicochemical properties with catalytic performance. Support, metal loading and the state of the metal significantly affected the catalytic performance. Increasing the Ir loading or using a Ce-promoted γ-alumina (CPBase) support results in the formation of larger Ir particles. Local reduction of iridium oxide surface in such particles leads to coexisting Ir and Ir2O phases being very beneficial for the catalytic activity.NO reduction and CO oxidation take place thermally at 700°C. Increasing the O2 concentration in the feed favors CO oxidation at the expense of NO reduction. With 500ppmIr/CPBase and 1000ppmIr/CPBase additives, complete NO reduction and CO oxidation is achieved in the presence of 40% excess oxygen. Higher oxygen excess, however, reduces or eliminates the NOx reduction activity of these materials. IR studies suggest that NO reduction by CO proceeds on Ir/alumina additives via the dissociative adsorption of NO, the formation of NCO species on Ir and their migration to the alumina support, where N2 and CO2 are formed. IR spectroscopy indicates that Ce modifies the Ir surface enhancing the CO oxidation and enabling NO reduction via the NO2 formation.

05/00329 Catalytic exhaust gas fuel reforming for diesel engines — effects of water addition on hydrogen production and fuel conversion efficiency: Tsolakis, A. and Megaritis, A. International Journal of Hydrogen Energy, 2004, 29, (13), 1409–1419

In the presented work the selective catalytic reduction (SCR) of NOx in a real diesel engine exhaust gas (O2 present) from the engine operating at different conditions with and without H2 and CO additions were studied. The tests were carried out using real diesel engine exhaust gas over 1%Pt supported on alumina (Al2O3). The catalyst exhibits good NOx reduction activity at a narrow temperature range of 200 to 300 ∘C when there is only a HC present. The maximum NOx reduction of around 60% was achieved at temperature of 260 ∘C. Although, the engine operating with EGR improves the percentage of NOx converted in the SCR system due to increased HC:NOx ratio and reduced NOx concentration in the engine exhaust gas, the number of NOx-ppm reduced over the catalyst was reduced. The cause of this effect is not yet clear, but there are evidences that this attributes to (a) lower NOx coverage on the catalyst surface, which in turn makes its reduction by HC less probable and (b) the increased soot emissions which are blocking part of the catalyst active sites that are active in reducing NOx. Hydrogen addition expands the SCR activity window towards lower temperatures (100–300 ∘C) without affecting the maximum NOx conversion. In contradiction to H2 the CO addition is favourable to the H2 oxidation reaction and the poisoning of the catalysts active sites and the good low temperature NOx reduction activity cannot be seen. The incorporation of a mini-exhaust gas-reformer on-board a vehicle to provide the H2 in the SCR reactor will require catalyst design and reactor engineering to maximise H2 production and eliminate CO with the minimum penalty in the fuel economy.

Catalytic Properties of Ag/Al2O3Catalysts for Lean NOxReduction Processes and Characterisation of Active Silver Species

A study of the lean NOx reduction activity employing different reductants over Ag/Al2O3 samples prepared from reverse microemulsions or impregnation with EDTA-complexes is presented. A multitechnique approach is employed for characterisation of the samples and/or processes taking place in the course of the NOx-SCR reaction with propene and propane. Results by in situ-DRIFTS reveal that, for the propene reductant, silver provides a new path for hydrocarbon activation involving generation of adsorbed acrylate species as a partially oxidised active intermediate, in line with previous proposals for other non-noble metal systems. It is shown, mainly on the basis of XAFS studies, that active silver species are related to well dispersed silver aluminate-like phases with tetrahedral local symmetry and a relatively high disorder in the oxygen first shell.

Comparative NOxReduction Behavior of Pt, Pd, and Rh Supported Catalysts in Simulated Exhaust Gases as a Function of Oxygen Concentration

NO x reduction activity on Pt and Pd catalysts had a maximum for S value as stoichiometry number at a fixed temperature, and the S value at the maximum NO x conversion increased with decreasing temperature. NO x conversion on Rh catalyst increased with decreasing S value, but independent of temperature. As for the effect of HC on NO x reduction behavior, it was concluded that, for Pt and Pd catalysts, HC adsorbs strongly on the catalysts surface to cause the self-inhibition. Increasing O2 concentration lead to oxidation of HC, but decreased the value of NO/O2 ratio. The balance point of the two factors generated a maximum NO x conversion. For Rh catalyst, the strongly adsorbed oxygen is more reactive with decreasing S value, and thus NO x conversion is increased.

The influence of surface acidity on NO2 reduction by propane under lean conditions

This investigation focuses on the influence of surface acidity on the continuous catalytic reduction of NOx by propane under lean conditions. The acidity was varied by studying noble metal free alumina, silica and co-precipitated aluminium-silicates with varying alumina-silica ratio. The catalysts were characterised by isopropylamine temperature programmed desorption (TPD), with respect to NOx reduction (NO2 and propane) and by Fourier Transform Infrared (FTIR) spectroscopy. The isopropylamine TPD was used to determine the sample acidity. The amount of Bronsted acid sites increased with increasing amount of alumina. The sample activity for NOx reduction with propane was found to correlate with the Bronsted-site density. Furthermore, no N2O was formed during the activity studies. The FTIR measurements implied formation of isocyanate species on the surface of the catalyst sample when NO/NO2, propane and oxygen were present in the gas phase. FTIR data also reveal an increasing amount of adsorbates on the surface with increasing amount of alumina in the sample. Moreover, with propane present, NO-species were consumed to a higher extent over samples containing a larger amount of alumina. Our results suggest a connection between the Bronsted-site density and the NOx reduction with propane under lean conditions.

In situ ammonia generation as a strategy for catalytic NOx reduction under oxygen rich conditions.

Under highly oxygen rich conditions palladium/alumina catalysts exhibit strongly enhanced NOx reduction activity in the presence of CO:H2 mixtures due to in situ generation of ammonia via the formation and subsequent hydrolysis of isocyanate (NCO) species.

Artificial neural network modelling applied on NO x reduction with octane in excess oxygen over Ag/Al 2 O 3

A silver/alumina catalyst was tested for its NOx reduction activity during oxygen-rich conditions and during variation in the input parameters, which were nitric oxide, octane and oxygen. The experimental data were investigated by means of artificial neural networks.

Characterization and catalytic tests of Au/MFI prepared by sublimation of AuCl3 onto HMFI

Au/MFI was prepared by sublimation of AuCl3 onto HMFI. The oxidation state of the gold in the as-synthesized sample is Au3+. Upon heating in He or O2, it transforms into gold metal and electron-deficient gold particles which agglomerate to larger Au particles. Reduction of Au3+ with CO to Au+ is accompanied by formation of an Au+(CO) complex. The carbonyl ligand has a remarkable stabilization effect on Au+ ions in zeolite cages. Strong IR bands show the CO vibration, and also the strong perturbation of the T–O–T vibrations of the zeolite lattice. [Au+(CO)]/MFI has a characteristic XRD pattern and is stable up to 200 °C. Au/MFI catalyzes the decomposition of N2O to N2 even in the presence of 3% O2. With hydrocarbons or NH3 as the reductant, it has some NOx reduction activity, but these catalysts deactivate at higher temperature even in inert gas atmosphere.

Activity and N2 Selectivity of Sol–Gel Prepared Pt/Alumina Catalysts for Selective NOx Reduction

We prepared 1 and 2 wt% Pt/Al2O3 catalysts by a single-step sol–gel process and tested their activity and N2 selectivity for NOx reduction with propene. Our results show that when compared to the impregnation catalysts, the single-step sol–gel catalysts have superior N2 selectivity both in terms of percentage of NO converted to N2 (∼70% or greater) and comparable activity. All of the catalysts tested show a minimum in N2 selectivity at the point of maximum NOx conversion. Both the value of the minimum selectivity and the NOx conversion are functions of oxygen concentration in the feed. The N2 selectivity at peak conversion exhibits a minimum at ∼6.8% O2. Increasing the feed oxygen concentration beyond 6.8% has many beneficial effects including broadening of the temperature window, suppression of NO2 formation and increasing the N2 selectivity without significantly changing the maximum NOx conversion. The sol–gel catalysts show remarkable robustness after exposure to high temperatures, SO2, and long time exposure to reactants.

Selective Catalytic Reduction of Nitric Oxide with Ethylene on Copper Ion-Exchanged Al−MCM-41 Catalyst

Selective catalytic reduction (SCR) of NOx by ethylene was investigated on copper and/or cerium ion-exchanged Al−MCM-41 in the presence of excess oxygen. Ce−Al−MCM-41 showed little activity, but Cu−Al−MCM-41 and cerium-promoted Cu−Al−MCM-41 (i.e., Ce−Cu−Al−MCM-41) were found to be active in this reaction. Higher NOx conversions to N2 were obtained on Ce−Cu−Al−MCM-41 as compared with Cu−Al−MCM-41. The maximum activity of Ce−Cu−Al−MCM-41 was close to that of Cu−ZSM-5, but the former had a wider temperature window. TPR results indicated that only isolated Cu2+ and Cu+ ions were detected in the Cu2+-exchanged Al−MCM-41 samples, which may play an important role in the selective catalytic reduction of NOx to N2. After some cerium ions were introduced into Cu−Al−MCM-41, Cu2+ in the molecular sieve became more easily reducible by H2. This may be related to the increase of catalytic activity of NOx reduction by ethylene.

Properties and catalytic activity of NOx reduction of alumina-titania catalysts prepared by sol-gel method

The alumina-titania catalysts were prepared from various alumina and titania sources by sol-gel method, which were metal alkoxide and metal alkoxide modified with organic groups. Specific surface area, pore size distribution, solid acidity and catalytic activity of NO reduction for the alumina-titania catalysts depended on the alumina and titania sources. The alumina-titania catalyst prepared from metal alkoxide for alumina source and metal alkoxide modified with organic groups for titania source exhibited higher activity of NO reduction than the other alumina-titania catalysts. Catalytic activity of NO reduction for the alumina-titania catalysts depended on specific surface area and solid acidity. It was suggested that solid acidity of the alumina-titania catalysts depended on the coordination structure of Al atoms and the homogeneity of alumina and titania components.