HC-SCR Reaction Research Articles

Effect of the modification of alumina supports with chloride on the structure and catalytic performance of Ag/Al2O3 catalysts for the selective catalytic reduction of NOx with propene and H2/propene

The effect of the modification of an alumina support with chloride on the structure and the catalytic performance of Ag/Al2O3 catalysts (SA) was investigated for the selective catalytic reduction (SCR) of NO using C3H6 or H2/C3H6 as reductants. The Ag/Al2O3 catalyst and Cl−-modified Ag/Al2O3 catalysts (SA-Cl) were prepared by a conventional impregnation method and characterized by X-ray diffraction, Brunauer-Emmett-Teller isotherm analysis, electron probe microanalysis, transmission electron microscopy, UV-Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and hydrogen temperature-programmed reduction. The catalytic activities in the C3H6-SCR and H2/C3H6-SCR reactions were evaluated, and the reaction mechanism was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy and synchrotron vacuum ultraviolet photoionization mass spectroscopy (SVUV-PIMS). We found that Cl− modification of the alumina-supported Ag/Al2O3 catalysts facilitated the formation of oxidized silver species (Agnδ+) that catalyze the moderate-temperature oxidation of hydrocarbons into partial oxidation products (mainly acetate species) capable of participating in the SCR reaction. The low-temperature promoting effect of H2 on the C3H6-SCR (“hydrogen effect”) was found to originate from the enhanced decomposition of strongly adsorbed nitrates on the catalyst surface and the conversion of these adsorbed species to –NCO and –CN species. This “H2 effect” occurs in the presence of Agnδ+ species rather than the metallic Ag0 species. A gaseous intermediate, acrylonitrile (CH2CHCN), was also identified in the H2/C3H6-SCR reaction using SVUV-PIMS. These findings provide novel insights in the structure-activity relationship and reaction mechanisms of the SA-catalyzed HC-SCR reaction of NO.

Germany's supreme court rules in Solvay's favour on patent case with Neo

Methanol has been shown to promote the hydrocarbon selective catalytic reduction of NOx with octane and toluene over 2 wt% Ag/Al2O3 catalyst for the first time. In order to understand its role in the reaction fast transient kinetic methods and in situ DRIFTS analysis have been used. The catalytic activity tests showed that the addition of methanol to the HC-SCR reaction results in a significant improvement in the low temperature activity of a Ag/Al2O3 catalyst, despite the fact that methanol on its own is not reactive for the HC-SCR reaction. This promotional effect of methanol is dependent on the concentration of added methanol and is not necessarily associated with a higher concentration of reductant in the SCR feed. The fast transient kinetic analysis has shown that at each temperature the addition of methanol enhances the conversions of both NOx and octane and the production of N2 with high selectivity in comparison with those observed with n-octane or toluene alone. This phenomenon is similar to the effect of H2 which may be associated with the release of hydrogen and ammonia during the transient switches at 250 and 300 °C. Together with the fast transient experiments, the DRIFTS results showed that NCO species are formed when introducing methanol to the n-octane-SCR feed while CN species are removed/consumed from the surfaces of the Ag catalyst. These NCO species formed by adding methanol may play a vital role in promoting the catalytic activity of NOx reduction and methanol itself can be an in situ source for hydrogen formation, which subsequently enhances the SCR reaction.

PM and NOx reduction characteristics of LNT/DPF+SCR/DPF hybrid system

The purpose of study is to investigate effects and differences when applying a hybrid system of LNT/DPF+SCR/DPF catalysts and LNT/DPF in diesel engine. A lean NOx trap (LNT) catalyst was prepared using an impregnation method, and a selective catalytic reduction (SCR) catalyst was prepared through an ion-exchange method. Each catalyst was dip-coated in an advanced cordierite-diesel particulate filter (AC-DPF). Its de-NOx and de-PM characteristics were analyzed using a model gas test-rig. As a result, NOx conversion of the hybrid system was higher than that of single LNT/DPF system. Because of NO2 and NH3, which is formed in a LNT/DPF in the rich air-fuel ratio duration is used a reductant for SCR/DPF catalyst of the hybrid system. Along with SCR/DPF, an increase in NOx conversion occurs through HC-SCR reaction because fuel as a dosing reductant is used not only for LNT/DPF but also for SCR/DPF. For de-PM characteristics, the light-off temperature 50, which is 50% of the oxidizing temperature of the PM, is 598 °C in the hybrid system and 627 °C in LNT/DPF. Moreover, the rate of PM oxidation over the hybrid system is higher than that of LNT/DPF because the oxidizing power of SCR/DPF was additionally added into the hybrid system.

Investigating the promotional effect of methanol on the low temperature SCR reaction on Ag/Al2O3

Methanol has been shown to promote the hydrocarbon selective catalytic reduction of NOx with octane and toluene over 2wt% Ag/Al2O3 catalyst for the first time. In order to understand its role in the reaction fast transient kinetic methods and in situ DRIFTS analysis have been used. The catalytic activity tests showed that the addition of methanol to the HC-SCR reaction results in a significant improvement in the low temperature activity of a Ag/Al2O3 catalyst, despite the fact that methanol on its own is not reactive for the HC-SCR reaction. This promotional effect of methanol is dependent on the concentration of added methanol and is not necessarily associated with a higher concentration of reductant in the SCR feed. The fast transient kinetic analysis has shown that at each temperature the addition of methanol enhances the conversions of both NOx and octane and the production of N2 with high selectivity in comparison with those observed with n-octane or toluene alone. This phenomenon is similar to the effect of H2 which may be associated with the release of hydrogen and ammonia during the transient switches at 250 and 300°C. Together with the fast transient experiments, the DRIFTS results showed that NCO species are formed when introducing methanol to the n-octane-SCR feed while CN species are removed/consumed from the surfaces of the Ag catalyst. These NCO species formed by adding methanol may play a vital role in promoting the catalytic activity of NOx reduction and methanol itself can be an in situ source for hydrogen formation, which subsequently enhances the SCR reaction.

Evolution of unburnt hydrocarbons under “cold-start” conditions from adsorption/desorption to conversion: On the screening of zeolitic materials

The general purpose of this work is to examine the relative ability of some well-selected zeolitic materials for the reduction of HC emissions generated within the Diesel “cold-start” period, i.e. when the work temperature of the Diesel Oxidation Catalyst (DOC) has not been reached. More peculiarly, this study is focused on the chemical, textural and structural parameters of zeolites influent on the elimination, namely by adsorption, of unburnt HC (propene, toluene and decane) in presence of potential inhibitors (H2O, CO, NO). Simulated “cold-start” conditions consisted in the rapid heating of the pre-treated zeolite sorbent/catalyst under the whole gas mixture from 35 to 530°C. The quantity of trapped HC and those converted to COx by oxidation were measured in function of the temperature, as well as the amount of NOx converted by the HC-SCR reaction. The interpretation of the HC emission profiles in close relation with the porous and acidic (through FTIR of adsorbed pyridine) properties of the corresponding zeolites allowed to gain insight onto the relative contributions of the pore topology, the pore size and the acid strength. For some selected zeolites, several consecutive cold-start cycles were performed in order to assess their stability.

Insights into the influence of the Ag loading on Al2O3 in the H2-assisted C3H6-SCR of NOx

The addition of H2 has been reported to promote drastically the selective catalytic reduction of NOx by hydrocarbons (HC-SCR). Yet, the influence of the Ag loading on the H2-promoted HC-SCR has been the subject of a very limited number of investigations. The H2-HC-SCR earlier studies reported mostly on Ag/Al2O3 samples containing about 2wt% Ag, since this particular loading has been shown to provide optimum catalytic performances in the HC-SCR reaction in the absence of H2. The present study highlights for the first time that the H2-C3H6-SCR catalytic performances of Ag/Al2O3 samples improved in the 150–550°C temperature domain as the Ag loading (Ag surface density: x (Ag/nmAl2O32)) decreased well below 2wt%. A detailed kinetic study of H2-C3H6-SCR was performed in which the reaction orders in NO, C3H6 and H2, and the apparent activation energies were determined for the reduction of NOx to N2 on a Ag(x)/Al2O3 catalysts series, for which Ag was found to be in a highly dispersed state by TEM and HAADF-STEM. Remarkably, changes in these kinetic parameters were found to occur at an Ag surface density close to 0.7 Ag/nmAl2O32 (Ag loading of 2.2wt%) coinciding with the changes observed earlier in the NOx uptakes of the Al2O3 supporting oxide [18]. Interpretation of the activity and kinetic data led us to conclude that the H2-C3H6-SCR reaction proceeds via the activation of H2 and C3H6 on Ag species and their further reaction with NOx adspecies activated on the Al2O3 support. The unexpected higher catalytic performances of the Ag samples with the lower Ag surface densities was attributed to the higher concentration of active sites on the Al2O3 supporting oxide able to chemisorb NOx species, in agreement with the NOx uptake data. The kinetic data obtained for Ag surface densities lower than 0.7 Ag/nmAl2O32 also suggest that the interaction between NOx and C3H6 adspecies would be rate determining in the C3H6-SCR process.

Environmentally-benign catalysts for the selective catalytic reduction of NOxfrom diesel engines: structure–activity relationship and reaction mechanism aspects

Selective catalytic reduction of NOx using NH3 or hydrocarbons (NH3-SCR or HC-SCR) in oxygen-rich exhaust from diesel engines remains a major challenge in environmental catalysis. The development of highly efficient, stable and environmentally-benign catalysts for SCR processes is very important for practical use. In this feature article, the structure-activity relationship of vanadium-free catalysts in the NH3-SCR reaction is discussed in detail, including Fe-, Ce-based oxide catalysts and Fe-, Cu-based zeolite catalysts, which is beneficial for catalyst redesign and activity improvement. Based on our research, a comprehensive mechanism contributing to the performance of Ag/Al2O3 in HC-SCR is provided, giving a clue to the design of a catalytic system with high efficiency.

Sulphated TiO 2 for selective catalytic reduction of NOx by n-decane

The selective catalytic reduction of NO by n-decane (C 10H 22-SCR), with or without water (1.5 vol.%), has been studied over sulphated titania catalysts (∼2 wt% S). Such sulphated materials have been explored as candidates for HC-SCR by decane with process temperatures >600 °C. The nature of the functional groups from residual reaction products and the thermal stability of sulphated compounds were characterized by mass spectroscopy, FTIR analysis and temperature programmed desorption experiments. It was observed that hydrocarbons (C x H y ), aldehyde (RCHO), acid (RCOOH), ester (RCOOR) and primary and secondary amine (RNH 2, R 2NH) functional groups were present at the catalyst surface. For the sample not tested under HC-SCR reaction, the sulphate compounds (SO 4 2−, SO 3 2−) were stable up to 600 °C under He-TPD whereas for samples tested under the HC-SCR reaction, sulphate species were released above 400 °C. This might be due to the exothermic oxidation reaction of deposited carbon compounds, characterized by ex situ analysis, with oxygen supplied by the sulphated support. This effect was confirmed by mixing the catalyst with soot particles. Lastly, we propose a reaction scheme for the HC-SCR of NO by decane over sulphated titania.

Kinetic behaviour of HC-SCR over Ag/alumina catalyst using a model paraffinic second generation biodiesel compound

The effect of longer paraffins on the mechanism of the HC-SCR reaction over a 1.91wt.% Ag/alumina catalyst was investigated by kinetic studies. Hexadecane (C16H34) was chosen as a model compound as it is also a representative molecule for a second generation biodiesel consisting of only long-chain paraffins. The kinetic behaviour of the catalytic reduction of NOx was examined at steady-state conditions in the temperature range 250–550°C (50°C ramping) and by using the following gas concentrations: PNO=100, 250, 500, 750 or 1000ppm, Phexadecane=31, 94, 188, 281 or 375ppm and PO2=1.5, 3, 4.5, 6 or 9vol.%. Results showed that in the temperature range 250–425°C high hexadecane concentration had an inhibiting effect on the NO reduction. At temperatures above 350°C the apparent reaction orders for hexadecane with respect to hexadecane increased to close or above 1. Reaction orders towards NO were close to 0.55 indicating that NO adsorption on the catalyst surface is stronger than hexadecane adsorption. Based on the experimental data it is proposed that small clusters alone cannot be the active sites for HC-SCR over Ag/Al2O3 but the important requirement for high activity over the catalyst is the local concentrations of hydrocarbon and NO on the interface of silver and the support.

HC-SCR reaction pathways on ion exchanged ZSM-5 catalysts

Metal cation (metal = Cu, In and La) ion exchanged ZSM-5 zeolites as catalysts for the NO selective reduction by propane and propene in excess oxygen. The surface reactions of HC-SCR over catalysts were investigated through in situ DRIFTS method. For C 3H 8-SCR, adsorbed nitrate species (–NO 3) were observed as main reaction intermediates and they could react with gaseous propane to produce N 2, H 2O and CO 2. While for C 3H 6-SCR, adsorbed amine species (–NH 2) were observed as main reaction intermediates and they could react with NO or NO 2 to produce the final products. The different reaction pathways for C 3H 8-SCR and C 3H 6-SCR over catalysts were proposed based on the DRIFTS results and the main factors controlling the activities of catalysts were discussed in details. The competing adsorption between NO–O 2 and HC–O 2 on the Brønsted acid sites of catalysts was responsible for the different reaction pathways in HC-SCR.

NO x SCR by decane and propylene on Pt + Cu/Zr-pillared clays in realistic feeds: Performance and mechanistic features versus structural specificity of nanosized zirconia pillars

Pt + Cu-loaded ZrPILC tested in the reactions of NO x selective catalytic reduction by propylene and decane in realistic feeds with a high content of water, oxygen and an admixture of SO 2 demonstrated promising performance at high space velocities (up to 100 000 h −1) in the temperature range of 150–400 °C. A strong promoting effect of water, oxygen and the catalyst presulfation on the degree of NO x conversion into N 2 was revealed. The specificity of these catalyst action in realistic feeds was explained by taking into account the textural, structural and surface features of zirconia-pillared clays as well as the mechanism of NO x HC SCR reaction including participation of strongly bound species in key stages.

Mechanistic Study of Lean NO2 Reduction by Propane Over HZSM-5 in the Presence of Water

This study focuses on the mechanism of lean NO2 redn. by propane in the presence of water, over an acidic zeolite (HZSM-5). Fourier Transform IR spectroscopy measurements with NO2 and propane in excess oxygen show formation of surface bound NO+, isocyanate, unsatd. hydrocarbons and traces of amine species. Upon addn. of water the isocyanate species disappear and amine species are formed. Hence, it seems likely that the isocyanate species are hydrolyzed to amine species, which are possible reaction intermediates in the HC-SCR reaction over HZSM-5.

Continuous reduction of NO with octane over a silver/alumina catalyst in oxygen-rich exhaust gases: combined heterogeneous and surface-mediated homogeneous reactions

An Ag/alumina catalyst was prepared and tested for its HC(octane)-SCR activity using a gas mixture simulating exhaust gas from diesel and lean-burn vehicles. The catalyst showed excellent NO to N 2 activity, having a maximum conversion of approximately 90% at 450 °C and an average conversion of 66% in the temperature window 300–600 °C. During the high conversion of NO, formation of CO was recorded. A commercial oxidation (“cleanup”) catalyst was placed after the Ag/alumina in order to remove the formed CO as well as unburnt hydrocarbons. This resulted, however, in a drastic drop of NO conversion, which was more pronounced when the distance was short between the two catalysts. When the distance between the two catalysts was increased to 33 mm (maximum value) the conversion was almost unaffected. This raised the question how nitrogen, once formed from nitrogen oxides, can seemingly disappear over the oxidizing catalyst at temperatures far too low to make this phenomenon thermodynamically feasible. In this study we propose that the HC-SCR reaction mechanism over Ag/alumina involves a gas-phase reaction between species created over the catalyst surface, and that this reaction is dependent on the residence time behind the catalyst.

Selective catalytic reduction of NO x by propane on Co-loaded zeolites

To ascertain the reason of the superiority of cobalt ion-exchanged BEA (Co-BEA) in the selective catalytic reduction of NO x by hydrocarbons (HC-SCR) using propane, the dependence of the catalytic performance of Co-loaded zeolite (Co-zeolite) in the HC-SCR reaction using propane on zeolite species was examined using IR and catalytic activity tests. The order of catalysts according to their initial activity in the absence of sulfur dioxide (SO 2) was BEA>MFI>MOR≫FER≫FAU. In the presence of SO 2, only Co-BEA exhibited stable activity, whereas the activities of Co-MOR and Co-MFI rapidly decreased, although no tendency to clogging of micropores nor decrease in dispersion of Co were observed. On the other hand, there was no obvious difference in the IR spectra of adsorbed NO x species between Co-BEA and Co-MFI. Since in the presence of SO 2 the crystal size effect on catalytic performance was observed even with Co-BEA, intracrystalline diffusion is considered to control the overall reaction rate under such conditions. Thus, it is concluded that the micropore structure of BEA, advantageous to intracrystalline diffusion, is considered to be one of the reasons for the superiority of Co-BEA in the HC-SCR reaction using propane.