Catalytic clean-up of emissions from small-scale combustion of biofuels

A knitted silica-fibre was prepared and used as support for combustion catalysts. Different Pd–MeO and Pt–MeO (Me=Ni, Co, Cu and Mn) catalysts were prepared, and their catalytic activities were investigated in the conversion of gas mixtures consisting of methane, ethene, naphthalene (model PAH), carbon monoxide, carbon dioxide, nitrogen and water vapour in the temperature range 150–800°C. Combinations of Pd–Ni and Pt–Ni were found to result in decreased light-off temperatures in methane combustion. The Pd–Ni/silica-fibre catalyst exhibited a light-off temperature in methane combustion of ca 220°C lower than that obtained over the Pd/silica-fibre catalyst. Deactivation of the catalysts was observed by subjecting the catalysts to reaction mixture flow at 800°C for 6 h. For the Pd-containing catalysts, the deactivation was considered to be due to both support and metal sintering as well as changes in the nature of the Pd–O species. The catalysts were characterised by N₂-adsorption, H₂-adsorption, O₂–TPD and H₂–TPR.     

Effects of electrodeposited Co and Co–P catalysts on the hydrogen generation properties from hydrolysis of alkaline sodium borohydride solution

Co and Co–P catalysts electroplated on Cu in sulfate based solution without or with an addition of H₂PO₂⁻ ions were developed for hydrogen generation from alkaline NaBH₄ solution. The microstructures of the Co and Co–P catalysts and their hydrogen generation properties were analyzed as a function of cathodic current density and plating time during the electrodeposition. An amorphous Co–P electrodeposit with micro-cracks was formed by electroplating in the sulfate based solution containing H₂PO₂⁻ ions. It was found that the amorphous Co–P catalyst formed at 0.01 A/cm² exhibited 18 times higher catalytic activity for hydrolysis of NaBH₄ than did the polycrystalline Co catalyst. The catalytic activity of the electrodeposited Co–P catalyst for hydrolysis of NaBH₄ was found to be a function of both cathodic current density and plating time, that is, parameters determining the concentration of P in the Co–P catalyst. Especially, Co–13 at.% P catalyst electroplated on Cu in the Co–P bath at a cathodic current density of 0.01 A/cm² for 1080 s showed the best hydrogen generation rate of 954 ml/min g-catalyst in 1 wt.% NaOH + 10 wt.% NaBH₄ solution at 30 °C.     

Catalytic combustion of methane over bimetallic Pd–Pt catalysts: The influence of support materials

The effect of support material on the catalytic performance for methane combustion has been studied for bimetallic palladium–platinum catalysts and compared with a monometallic palladium catalyst on alumina. The catalytic activities of the various catalysts were measured in a tubular reactor, in which both the activity and stability of methane conversion were monitored. In addition, all catalysts were analysed by temperature-programmed oxidation and in situ XRD operating at high temperatures in order to study the oxidation/reduction properties.

The activity of the monometallic palladium catalyst decreases under steady-state conditions, even at a temperature as low as 470 °C. In situ XRD results showed that no decomposition of bulk PdO into metallic palladium occurred at temperatures below 800 °C. Hence, the reason for the drop in activity is probably not connected to the bulk PdO decomposition.

All Pd–Pt catalysts, independently of the support, have considerably more stable methane conversion than the monometallic palladium catalyst. However, dissimilarities in activity and ability to reoxidise PdO were observed for the various support materials. Pd–Pt supported on Al₂O₃ was the most active catalyst in the low-temperature region, Pd–Pt supported on ceria-stabilised ZrO₂ was the most active between 620 and 800 °C, whereas Pd–Pt supported on LaMnAl₁₁O₁₉ was superior for temperatures above 800 °C. The ability to reoxidise metallic Pd into PdO was observed to vary between the supports. The alumina sample showed a very slow reoxidation, whereas ceria-stabilised ZrO₂ was clearly faster.     

A structured Co–B catalyst for hydrogen extraction from NaBH4 solution

A structured Co–B catalyst has been developed to produce hydrogen from an alkaline NaBH₄ solution. The catalyst was prepared by chemical reduction of Co precursors coated on a Ni foam support. The effects of catalyst preparation conditions on activity of the catalyst were investigated. The active catalyst was amorphous in structure and contains boron with a Co/B molar ratio of 1.5–2.8. With increasing the heat treatment temperature, the catalyst showed a maximum activity to hydrogen generation at approximately 250 °C. Adhesion of the catalyst to the support was also enhanced by heat treatment at 300–400 °C. The catalysts were successfully applied in both a batch reactor and a flow reactor for continuous generation of hydrogen.     

Abatement of volatile organic compounds: oxidation of ethanal over niobium oxide-supported palladium-based catalysts

Niobium-supported palladium-based catalysts (Pd, Pd–Cu and Pd–Au) were employed in the oxidation of ethanal. The catalysts were prepared according to original methods by either multi-steps (anchoring of complexes, calcination and reduction) or one-step (photoassisted reduction) procedures. The oxidation of ethanal was carried out in gas phase in a dynamic-differential reactor at 300 °C at atmospheric pressure. The activity/selectivity of the catalysts depend on (i) the catalyst preparation; (ii) the presence of a second metal. Addition of Au or Cu decreases the catalysts deactivation and the best performance in total oxidation was obtained with Pd–Au/Nb₂O₅ prepared by photoassisted reduction. As shown by in situ IR spectroscopy of adsorbed CO, this peculiarity may be ascribed to Au→Pd electron donation, which prevents the surface oxidation of palladium particles.     

Uncatalyzed and catalyzed NO and N2O reaction using various catalysts and binary barium mixtures supported on activated carbon

The kinetics of the reaction of NO and N₂O with activated carbon, without catalyst, and impregnated with precursor salts of Mg, Mn, Ba, Pb, Cu, Ni, Fe and Co and their binary mixtures was investigated. The conversion of NO and N₂O was studied (300°C–900°C) using a TGA apparatus and a fixed-bed reactor. The reactor effluents were analyzed using a GC/MS on line. The best synergetic effects were observed for samples doped with Ba + Pb, Ba + Fe, Ba + Cu, Ba + Mn, and Cu + Mn. Adsorption of NO and N₂O (20°C–100°C) increased considerably in the presence of catalysts, but binary mixtures had no synergistic effects. In situ XRD was an useful tool for interpreting catalyst behavior and identifying phases during reaction conditions.     

A comparative study of TiO2 supported noble metal catalysts for the oxidation of formaldehyde at room temperature

The TiO₂ supported noble metal (Au, Rh, Pd and Pt) catalysts were prepared by impregnation method and characterized by means of X-ray diffraction (XRD) and BET. These catalysts were tested for the catalytic oxidation of formaldehyde (HCHO). It was found that the order of activity was Pt/TiO₂  Rh/TiO₂ > Pd/TiO₂ > Au/TiO₂  TiO₂. HCHO could be completely oxidized into CO₂ and H₂O over Pt/TiO₂ in a gas hourly space velocity (GHSV) of 50,000 h⁻¹ even at room temperature. In contrast, the other catalysts were much less effective for HCHO oxidation at the same reaction conditions. HCHO conversion to CO₂ was only 20% over the Rh/TiO₂ at 20 °C. The Pd/TiO₂ and Au/TiO₂ showed no activities for HCHO oxidation at 20 °C. The different activities of the noble metals for HCHO oxidation were studied with respect to the behavior of adsorbed species on the catalysts surface at room temperature using in situ DRIFTS. The results show that the activities of the TiO₂ supported Pt, Rh, Pd and Au catalysts for HCHO oxidation are closely related to their capacities for the formation of formate species and the formate decomposition into CO species. Based on in situ DRIFTS studies, a simplified reaction scheme of HCHO oxidation was also proposed.     

Cloth catalysts for water denitrification: II. Removal of nitrates using Pd–Cu supported on glass fibers

The use of glass fibers in the form of woven cloth (GFC), as a new type of catalytic support, was studied for the reduction of aqueous nitrate solutions using a Pd/Cu–GFC catalyst. The activity (per gram Pd) and selectivity to nitrogen were found to be comparable with those found for Pd–Cu catalysts supported on the other carriers. The maximal initial removal activity was found for a catalyst with a Pd/(Pd+Cu) ratio of 0.81. The corresponding activity was 0.7 mmol min⁻¹ (g_Pd)⁻¹, and the selectivity was 97 mol% at 25°C and pH 6.5 for initial nitrate concentration of 100 mg l⁻¹. The selectivity to nitrogen declined at high conversions of nitrate and high pH.     

A comparison study on non-thermal plasma-assisted catalytic reduction of NO by C3H6 at low temperatures between Ag/USY and Ag/Al2O3 catalysts

A comparison study was carried out on non-thermal plasma (NTP)-assisted selective catalytic reduction (SCR) of NO_x by propene over Ag/USY and Ag/Al₂O₃ catalysts. Ag/USY was almost inactive in thermal SCR while it showed obvious activities in NTP-assisted SCR at 100 °C–200 °C. Although the NO_x conversion over Ag/Al₂O₃ was also enhanced at 300 °C–400 °C by the assistance of NTP, it was ineffective below 250 °C. The intermediates over Ag/USY and Ag/Al₂O₃ were investigated using in situ DRIFTS method. It was found that key intermediates in HC-SCR, such as NCO, CN, oxygenates and some N-containing organic species were enriched after the assistance of NTP. The differences in the behaviors of above intermediates were not found between these two kinds of catalysts. However, some evidences suggested that different properties of the absorbed NO_x species resulted in the distinction of SCR reactions over Ag/USY and Ag/Al₂O₃. TPD profiles of Ag/Al₂O₃ showed that nitrates formed over the catalyst were quite stable at low temperatures, which might occupy the active sites and were unfavorable to SCR reactions. The nitrates over Ag/USY were unstable, among which the unidentate nitrate species is probably contributed to the SCR reactions at low temperatures.     

Effect of NaBH4 concentration on the characteristics of PtRu/C catalyst for the anode of DMFC prepared by the impregnation method

PtRu/C catalysts were prepared using an aqueous co-impregnation method with NaBH₄ as a reducing agent. In order to investigate the effect of the reducing agent concentration, metal ions were reduced in different NaBH₄ concentrations for which the molar ratios of NaBH₄ to metal ions were controlled to 1, 2, 5, 15, 50, and 250. The electrochemical properties were studied by cyclic voltammetry in a 0.5 M H₂SO₄ solution. The surface compositions and oxidation states of the catalysts were observed by X-ray photoelectron spectroscopy (XPS). According to the X-ray diffraction (XRD) results, Pt (fcc) peak shifts were observed and crystal sizes were calculated. The electro-catalytic activities of the prepared catalysts for methanol electro-oxidation were estimated using linear sweep voltammetry. Unit cell tests were carried out to compare the direct methanol fuel cell performances. The NaBH₄ concentration was found to affect the dispersion and the surface composition of the prepared PtRu particles. Optimum molar ratios of NaBH₄ to metal ions were 5 and 15 for methanol electro-oxidation.     

Oxidation of H2S on activated carbon KAU and influence of the surface state

Properties of the oxidized activated carbon KAU treated at different temperatures in inert atmosphere were studied by means of DTA, Boehm titration, XPS and AFM methods and their catalytic activity in H₂S oxidation by air was determined. XPS analysis has shown the existence of three types of oxygen species on carbon catalysts surface. The content of oxygen containing groups determined by Boehm titration is correlated with their amount obtained by XPS. Catalytic activity of the KAU catalysts in selective oxidation of hydrogen sulfide is connected with chemisorbed charged oxygen species (O_3.1 oxygen type with BE 536.8–537.7 eV) present on the carbons surface.

Formation of dense sulfur layer (islands of sulfur) on the carbons surface and removal of active oxygen species are the reason of the catalysts deactivation in H₂S selective oxidation. The treatment of deactivated catalyst in inert atmosphere at 300 °C gives full regeneration of the catalyst activity at low temperature reaction but only its partial reducing at high reaction temperature. The last case is connected with transformation of chemisorbed charged oxygen species into CO groups.

The KAU samples treated in flow of inert gas at 900–1000 °C were very active in H₂S oxidation to elemental sulfur transforming up to 51–57 mmol H₂S/g catalyst at 180 °C with formation of 1.7–1.9 g S_x/g catalyst.     

CoMo/MgO–Al2O3 supported catalysts: An alternative approach to prepare HDS catalysts

Three different supports were prepared with distinct magnesia–alumina ratio x = MgO/(MgO + Al₂O₃) = 0.01, 0.1 and 0.5. Synthesized supports were impregnated with Co and Mo salts by the incipient wetness method along with 1,2-cyclohexanediamine-N,N,N′,N′-tetraacetic acid (CyDTA) as chelating agent. Catalysts were characterized by BET surface area, Raman spectroscopy, SEM-EDX and HRTEM (STEM) spectroscopy techniques. The catalysts were evaluated for the thiophene hydrodesulfurization reaction and its activity results are discussed in terms of using chelating agent during the preparation of catalyst. A comparison of the activity between uncalcined and calcined catalysts was made and a higher activity was obtained with calcined MgO–Al₂O₃ supported catalysts. Two different MgO containing calcined catalysts were tested at micro-plant with industrial feedstocks of heavy Maya crude oil. The effect of support composition was observed for hydrodesulfurization (HDS), hydrodemetallization (HDM), hydrodeasphaltenization (HDAs) and hydrodenitrogenation (HDN) reactions, which were reported at temperature of 380 °C, pressure of 7 MPa and space-velocity of 1.0 h⁻¹ during 204 h of time-on-stream (TOS).     

Preparation and characterization of inexpensive heterogeneous catalysts for air pollution control: Two case studies

Relatively inexpensive heterogeneous catalysts for two reactions of great importance in air pollution control, NO reduction and VOC combustion, were prepared and characterized. Apart from their common practical goal and the frequent need for simultaneous removal of air pollutants, these reactions share a similar redox mechanism, in which the formulation of more effective catalysts requires an enhancement of oxygen transfer.

For NO reduction, supported catalysts were prepared by adding a metal (Cu, Co, K) using ion exchange (IE) and incipient wetness impregnation (IWI) to chars obtained from pyrolysis of a subbituminous coal. The effects of pyrolysis temperature, between 550 and 1000 °C, on selected catalyst characteristics (e.g., BET surface area, XRD spectrum, support reactivity in O₂) are reported. For IE catalysts, the surface area increased in the presence of the metals while the opposite occurred for IWI catalysts. For the Co-IE catalysts, the highest surface area was obtained at 700 °C. The XRD results showed that, except for Cu (which exhibited sharp Cu⁰ peaks), the catalysts may be highly dispersed (or amorphous) on the carbon surface. For the C–O₂ reaction the order of (re)activity was K  Co > Cu for IE catalysts and K > Cu > Co for IWI catalysts. For NO reduction the orders were K > Co > Cu (IE catalysts) and Cu > Co > K (IWI catalysts). In all cases the catalytic (re)activity for NO reduction was lower than that exhibited for the C–O₂ reaction. The K-IE and Cu-IWI catalysts appeared to be the most promising ones, although further improvements in catalytic activity will be desirable. Some surprising results regarding CO and CO₂ selectivity are also reported, especially for Co catalysts.

In VOC combustion, the effect of the nature of ion B (Fe and Ni) on the partial substitution of ion A (Ca for La) in ABO₃ perovskites (e.g., LaFeO₃ and LaNiO₃) and on their catalytic activity was studied. The perovskite-type oxides were characterized by means of surface area measurements, XRD, temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR). The effect of partial substitution of La³⁺ by Ca²⁺ was more significant for the La_1−xCa_xFeO₃ perovskites. In this case, the electronic perturbation is compensated by an oxidation state increase of part of Fe³⁺ to Fe⁴⁺. The TPD results revealed that, at higher substitution levels, oxygen vacancies are also formed to preserve electroneutrality. For the La_1−xCa_xNiO₃ perovskites, the characterization results showed no evidence of large differences in electronic properties as calcium substitution increases. The La_1−xCa_xNiO₃ perovskites exhibited lower activity than the simple LaNiO₃ perovskite, whereas for the La_1−xCa_xFeO₃ substituted perovskites the most active catalyst (exhibiting the lowest ignition temperature) was obtained at the highest substitution level, La_0.6Ca_0.4FeO₃.

The performance of both groups of catalysts is briefly discussed in terms of redox processes, in which the interplay between oxygen transfer and electron transfer requires further elucidation for the improvement of catalytic activity.     

Ion-exchanged pillared clays for selective catalytic reduction of NO by ethylene in the presence of oxygen

Ion-exchanged pillared clays (PILCs) were studied as catalysts for selective catalytic reduction (SCR) of NO by ethylene. Three most important pillared clays, Al₂O₃-PILC (or Al-PILC), ZrO₂-PILC (or Zr-PILC) and TiO₂-PILC (or Ti-PILC), were synthesized. Cation exchanges were performed to prepare the following catalysts: Cu–Ti-PILC, Cu–Al-PILC, Cu–Zr-PILC, Cu–Al–Laponite, Fe–Ti-PILC, Ce–Ti-PILC, Ce–Ti-PILC, Co–Ti-PILC, Ag–Ti-PILC and Ga–Ti-PILC. Cu–Ti-PILC showed the highest activities at temperatures below 370°C, while Cu–Al-PILC was most active at above 370°C, and both catalysts were substantially more active than Cu-ZSM-5. No detectable N₂O was formed by all of these catalysts. H₂O and SO₂ only slightly deactivated the SCR activity of Cu–Ti-PILC, whereas severe deactivation was observed for Cu-ZSM-5. The catalytic activity of Cu–Ti-PILC was found to depend on the method and amount of copper loading. The catalytic activity increased with copper content until it reached 245% ion-exchange. The doping of 0.5 wt% Ce₂O₃ on Cu–Ti-PILC increased the activities from 10% to 30% while 1.0 wt% of Ce₂O₃ decreased the activity of Cu–Ti-PILC due to pore plugging. Cu–Ti-PILC was found to be an excellent catalyst for NO SCR by NH₃, but inactive when CH₄ was used as the reducing agent. Subjecting the Cu–Ti-PILC catalyst to 5% H₂0 and 50 ppm SO₂ at 700°C for 2 h only slightly decreased its activity. TPR results showed that the overexchanged (245%) PILC sample contained Cu²⁺, Cu⁺ and CuO. The TPR temperatures for the Cu–Ti-PILC were substantially lower than that for Cu-ZSM-5, indicating easier redox on the PILC catalyst and hence higher SCR activity.     

Non-steady activity during methane combustion over Pd/Al2O3 and the influences of Pt and CeO2 additives

Methane combustion over Pd/Al₂O₃ catalysts with and without added Pt and CeO₂ in both oxygen-rich and methane-rich mixtures at temperatures in the range 250–520°C has been investigated using a temperature-programmed reaction procedure with on-line gas analysis (FTIR). During the temperature loop under oxygen-rich conditions, there was an appreciable hysteresis in the activity of unmodified Pd/Al₂O₃, which was greatly enhanced over Pd–Pt/Al₂O₃. Over both catalysts the hysteresis was reversed under slightly methane-rich atmospheres, and as temperature was reduced, a sudden collapse or fluctuations in activity were shown respectively over Pd–Pt/Al₂O₃ and Pd/Al₂O₃. Such non-steady behaviour was almost eliminated over Pd/Al₂O₃–CeO₂. Under a very narrow range of conditions and over a Pd/Al₂O₃ packed bed, oscillation of methane combustion was observed.     

Comparison of activities in selective catalytic reduction of NOx by C3H8 over Co/MFI, Fe/MFI, and H/MFI zeolite catalysts

The Co/MFI(SiO₂/Al₂O₃ = 30) were prepared by a precipitation method with NaOCl in alkali solutions exhibited high activities to N₂ at 250 °C for the selective catalytic reduction (SCR) of NO_x. These catalysts showed two UV–vis bands at 700 and 400 nm, indicating the presence of octahedral Co(III) as well as tetrahedral Co(II). The high SCR activity over such Co(III, II)/MFI(30) seems to come from Co(III)---O moieties. The Co(II)MFI(30) catalysts prepared from Co(II)Cl₂ exhibited low SCR activities due to the presence of tetrahedral Co(II) ions in MFI. Less CO formation occurred over Co/MFI catalysts. The Fe/MFI(30) catalyst exhibited high activity due to the presence of some Fe---O species in MFI but more amount of CO were produced during SCR. H/MFI(30) catalyst exhibited a good SCR activity. However, more amount of carbonaceous deposits were produced on it. The correlation between acid concentration and SCR activity was discussed over H/MFIs.     

CoBr2-MnBr2 containing catalysts for catalytic oxidation of p-xylene to terephthalic acid

Catalytic oxidation of p-xylene (PX) to terephthalic acid (TA) was studied with catalysts containing cobalt acetate, manganese acetate, CoBr₂ and MnBr₂. The catalysts contain neither highly corrosive hydrogen bromide nor other metal ions, and have the advantage of easy catalyst recovery. The effects of Br/Co atomic ratio, reaction time and temperature, PX concentration, oxygen pressure, and catalyst concentration on PX conversion and product/intermediate yields were investigated. The catalyst system had a suitable reaction temperature of 100 °C, which was much lower than the commercial process temperature (175–225 °C). The maximum product (TA) yield was 93.5%, obtained at a Br/Co atomic ratio of three. Higher Br concentration resulted in the lower TA yield, which was ascribed to the benzylic bromide formation. The synthesis of TA could be adequately described as four reaction steps in series (PX → p-tolualdehyde → p-toluic acid → 4-carboxybenzaldehyde → TA), with a pseudo-first-order rate equation for each step, and the third step was rate-limiting. The rate constant ratios (k_j/k₃, j = 1 → 4) obtained at 100 °C were similar to the k_j/k₃ values reported earlier for cobalt acetate/manganese acetate/HBr catalysts in a range of 185–191 °C.     

Hydrodehalogenation with sonochemically prepared Mo2C and W2C

The catalytic hydrodehalogenation (HDH) of several halogenated organic compounds was performed over sonochemically prepared molybdenum carbide and tungsten carbide at low temperatures (T=200–300 °C). Both catalysts were selective, active, and stable for all substrates tested. Benzene was observed as the major product for the HDH of fluoro-, chloro-, bromo- and iodobenzene, with activities following the trend of C₆H₅F>C₆H₅Cl>C₆H₅Br>C₆H₅I. For the HDH of chlorofluorocarbons (CFCs), Cl was selectively removed over F and alkanes were the major product. In the liquid phase, the HDH of polychlorinated and polybrominated biphenyls resulted in biphenyl as the only product. The HDH of substrates bearing aliphatic C–Cl bonds occurs faster than those with arene C–Cl bonds. Time-on-stream studies of HDH of chlorobenzene show high stability for the sonochemically prepared catalysts, with half-lives as long as 600 h.     

Co from partial reduction of La(Co,Fe)O3 perovskites for Fischer–Tropsch synthesis

Small Co clusters (d<10 nm) supported over mixed La–Co–Fe perovskites were successfully synthesized. These catalysts were active for Fischer–Tropsch (FT). Depending on the Co to Fe ratios the mixed perovskite exhibited two different forms: the rhombohedral phase of LaCoO₃ is maintained for the mixed perovskite when x>0.5, the orthorhombic phase of LaFeO₃ is found for x<0.5. Interestingly only one of these structures is active for the FT reaction: the orthorhombic structure. This is most likely due to the capacity of this material to maintain its structure even with a high number of cation vacancies. These cations (mostly Co) were on purpose extracted and reduced. Magnetic measurements clearly showed their metallic nature. Rhombohedral Co–Fe mixed perovskites (x≥0.5) cannot be used as precursors for Fischer–Tropsch catalysts: their partial reduction only consists in a complete reduction of Co³⁺ into Co²⁺.

The partial reduction of orthorhombic perovskites (x<0.5) leads to active Fischer–Tropsch (FT) catalysts by formation of a metal phase well dispersed on a cation-deficient perovskite. The FT activity is related to the stability of the precursor perovskite. When initially calcined at 600 °C, a maximum of 8.6 wt.% of Co⁰ can be extracted from LaCo_0.40Fe_0.60O₃ (compared to only 2 wt.% after calcination at 750 °C). The catalyst is then composed of Co⁰ particles of 10 nm on a stable deficient perovskite LaCo_0.05³⁺Fe_0.60³⁺O_2.40. Catalytic tests showed that up to 70% in the molar selectivity for hydrocarbons was obtained at 250 °C, 40% of which was composed of the C₂–C₄ fraction.     

NOX removal in excess oxygen by plasma-enhanced selective catalytic reduction

In the off-gases of internal combustion engines running with oxygen excess, non-thermal plasmas (NTPs) have an oxidative potential, which results in an effective conversion of NO to NO₂. In combination with appropriate catalysts and ammonia (NH₃-SCR) or hydrocarbons (HC-SCR) as a reducing agent, this can be utilized to reduce nitric oxides (NO and NO₂) synergistically to molecular nitrogen.

The combination of SCR and cold plasma enhanced the overall reaction rate and allowed an effective removal of NO_X at low temperatures. Using NH₃ as a reducing agent, NO_X was converted to N₂ on zeolites or NH₃-SCR catalysts like V₂O₅–WO₃/TiO₂ at temperatures as low as 100–200 °C. Significant synergetic effects of plasma and catalyst treatment were observed both for NH₃ stored by ion exchange on the zeolite and for continuous NH₃ supply.

Certain modifications of Al₂O₃ and ZrO₂ have been found to be effective as catalysts in the plasma-assisted HC-SCR in oxygen excess. With an energy supply of about 30 eV/NO-molecule, 500 ppm NO was reduced by more than half at a temperature of 300 °C and a space velocity of 20 000 h⁻¹ at the catalyst. The synergistic combinations of NTP and both NH₃- and HC-SCR have been verified under real diesel engine exhaust conditions.