The selective catalytic reduction of nitrogen oxides by methane on noble metal-loaded sulfated zirconia

The selective catalytic reduction of NO_x by methane on noble metal-loaded sulfated zirconia (SZ) catalysts was studied. Ru, Rh, Pd, Ag, Ir, Pt, and Au-loaded sulfated zirconia catalysts were compared with the intact sulfated zirconia. For the NO–CH₄–O₂ reaction, Ru, Rh, Pd, Ir, and Pt showed promotion effect on NO_x reduction, while for the NO₂–CH₄–O₂ reaction, only Rh and Pd showed promotion effect. Over intact and Rh, Pd, Ag, and Au-loaded sulfated zirconia, NO_x conversion in NO₂–CH₄–O₂ reaction was significantly higher than that in NO–CH₄–O₂ reaction, while clear difference was not observed over Ru, Ir, and Pt-loaded sulfated zirconia. Comparison of [NO₂]/([NO]+[NO₂]) in the effluent gases in NO–O₂ and NO₂–O₂ reactions showed that Ru, Ir, and Pt has high activity for NO oxidation under the reaction conditions. These facts suggest that effects of these metals toward NO_x reduction by methane can be categorized into the following three groups: (i) low activity for NO oxidation to NO₂, and high activity for NO₂ reduction to N₂ (Pd, Rh); (ii) high activity for NO oxidation to NO₂, and low activity for NO₂ reduction to N₂ (Ru, Ir, Pt); (iii) low activity for both reactions (Ag, Au). To confirm these suggestions, combination of these metals were investigated on binary or physically-mixed catalysts. The combination of Pd or Rh with Pt or Ru gave high activity for the selective reduction of NO_x by methane.     

Determination of active palladium species in ZSM-5 zeolite for selective reduction of nitric oxide with methane

The active site in ZSM-5 zeolite-supported palladium, which shows the catalytic activity for NO reduction with methane as a reducing agent, has been investigated qualitatively and quantitatively by means of NO chemisorption and NaCl titration, comparing with PdO supported on silica. Palladium species in 0.4 wt.% Pd loaded H-ZSM-5 can adsorb NO equimolarly after calcination at 773 K, and almost all the NO was desorbed at around 673 K, while the palladium species on PdO/SiO₂ hardly adsorbed NO. The palladium species in Pd(0.4)/H-ZSM-5 are ion-exchangeable with Na⁺ in NaCl solution, indicating that they exist in a cationic state of an isolated Pd²⁺. This method for quantitative analysis of the isolated Pd²⁺ cations is named as ‘NaCl titration’. The amount of the isolated Pd²⁺ cationic species increased with increasing palladium content on Pd/H-ZSM-5, and PdO co-existed above 1 wt.%. The amount of the isolated Pd²⁺ cation was unchanged after the reaction of NO₂–CH₄, NO₂–CH₄–O₂, or CH₄–O₂ at 673 K, while the adsorbed amount of NO per the Pd²⁺ as determined by NO-TPD decreased after the NO₂–CH₄–O₂ reaction. It was found by NaCl titration that the catalytic activity of Pd/H-ZSM-5 for NO₂–CH₄–O₂ reaction increased with increasing amount of the isolated Pd²⁺ cationic species up to 0.7 wt.%, while the increase in the amount of PdO led to decrease in selectivity towards NO₂ reduction. The palladium species that are active and selective for NO reduction with CH₄ will be proposed.     

Relative rates of various steps of NO–CH4–O2 reaction catalyzed by Pd/H-ZSM-5

Reaction mechanism of the reduction of nitrogen monoxide by methane in an oxygen excess atmosphere (NO–CH₄–O₂ reaction) catalyzed by Pd/H-ZSM-5 has been studied at 623–703 K in the absence of water vapor, in comparison with the mechanism for Co-ZSM-5. Kinetic isotope effect for the N₂ formation in NO–CH₄–O₂ vs. NO–CD₄–O₂ reactions was 1.65 at 673 K and decreased with a decrease in the reaction temperature. In addition, H–D isotopic exchange took place significantly in NO–(CH₄+CD₄)–O₂ reaction. These results are in marked contrast with the case of Co-ZSM-5, for which the C–H dissociation of methane is the only rate-determining step, and show that the C–H dissociation is slow but not the only rate-determining step in the case of Pd/H-ZSM-5.

A reaction scheme was proposed, in which the relative rates of the three steps ((i)–(iii) below) vary depending on the reaction conditions.

Further, in contrast to Co-ZSM-5, NO_x–CH₄–O₂ reaction was much slower than CH₄–O₂ reaction for Pd/H-ZSM-5; the presence of NO_x retards the reaction of CH₄ over the latter catalyst, while it accelerates the reaction over the former. It is suggested that CH₄ is activated directly by the Pd atoms in the case of Pd/H-ZSM-5, but by NO₂ strongly adsorbed on Co ion for Co-ZSM-5. The reaction order of the NO–CH₄–O₂ reaction with respect to NO pressure was consistent with this mechanism; 1.05 for Pd/H-ZSM-5 and 0.11 for Co-ZSM-5.     

On the promotional effect of Pd on the propene-assisted decomposition of NO on chlorinated Ce0.68Zr0.32O2

The selective catalytic reduction (SCR) of NO_x assisted by propene is investigated on Pd/Ce_0.68Zr_0.32O₂ catalysts (Pd/CZ), and is compared, under identical experimental conditions, with that found on a Pd/SiO₂ reference catalyst. Physico-chemical characterisation of the studied catalysts along with their catalytic properties indicate that Pd is not fully reduced to metallic Pd for the Pd/CZ catalysts. This study shows that the incorporation of Pd to CZ greatly promotes the reduction of NO in the presence of C₃H₆. These catalysts display very stable deNO_x activity even in the presence of 1.7% water, the addition of which induces a reversible deactivation of about 10%. The much higher N₂ selectivity obtained on Pd/CZ suggests that the lean deNO_x mechanism occurring on these catalysts is different from that occurring on Pd⁰/SiO₂. A detailed mechanism is proposed for which CZ achieves both NO oxidation to NO₂ and NO decomposition to N₂, whereas PdO_x activates C₃H₆ via ad-NO₂ species, intermediately producing R-NO_x compounds that further decompose to NO and C_xH_yO_z. The role of the latter oxygenates is to reduce CZ to provide the catalytic sites responsible for NO decomposition. The proposed C₃H₆-assisted NO decomposition mechanism stresses the key role of NO₂, R-NO_x and C_xH_yO_z as intermediates of the SCR of NO_x by hydrocarbons.     

Catalytic reduction of NOx by hydrocarbons over Co/ZSM-5 catalysts prepared with different methods

Co/ZSM-5 catalysts were prepared by several methods, including wet ion exchange (WIE), its combination with impregnation (IMP), solid state ion exchange (SSI) and sublimation (SUB). FTIR results show that the zeolite protons in H-ZSM-5 are completely removed when CoCl₂ vapor is deposited. TPR shows peaks for Co²⁺ ions at 695–705°C and for Co₃O₄ at 385–390°C, but a peak in the 220–250°C region appears to indicate Co²⁺ oxo-ions.

The catalysts have been tested for the selective reduction of NO_x with iso-C₄H₁₀ under O₂-rich conditions and in the absence of O₂, both with dry and wet feeds. A bifunctional mechanism appears to operate at low temperature: oxo-ions or Co₃O₄ clusters first oxidize NO to NO₂, which is chemisorbed as NO_y (y≥2) and reduced. In this modus operandi catalyst SUB shows the highest N₂ yield 90% near 390°C for dry and wet feeds. It is found to be quite stable in a 52 h run with a wet feed. In contrast, the WIE catalyst, which mainly contains isolated Co²⁺ ions and has poor activity below 400°C, excels at T>430°C. This and the observation that, at high temperature, NO is reduced in O₂-free feeds over Co/MFI catalysts, suggest that NO can be reduced over Co²⁺ ions without intermediate formation of NO₂.

The bifunctional mechanism at low temperature is supported by the fact that a strongly enhanced performance is obtained by mixing WIE with Fe/FER, a catalyst known to promote NO₂ formation.     

CH4-SCR of NO over Co and Pd ferrierite catalysts: Effect of preparation on catalytic performance

Selective catalytic reduction of NO with methane (CH₄-SCR) in the presence of oxygen excess and water vapour was studied over two bimetallic cobalt/palladium-based FER catalysts, which differ on the order of introduction of metal ions. H₂-TPR and UV–vis analysis showed that the simple change in the order of addition of metals to catalyst, gives rise to totally diverse species (Co²⁺ ions, Co oxides, Co-oxo cations and Pd species) both in type and quantity but also in location within zeolite framework. Experiments of TPD and TPSR of NO and NO₂ provided important information to establish a relation between the various active sites formed on both catalysts and their function in the reaction mechanism. The importance of NO₂ in the mechanism of NO reaction with CH₄ and O₂ was explored and the catalyst with a higher capacity to retain adsorbed NO₂ is the less active for deNO_x. The preparation of a bimetallic catalyst active for NO reduction must provide the proximity between Co and Pd species, and the presence of Co-oxo cations together with palladium species seem to be essential. Furthermore, a suitable amount of cobalt oxides must exist in order to originate NO₂ that is the main reaction intermediate. Nevertheless, an excessive amount of these Co species can lead to an increase of adsorbed NO₂, which reduces the rate of the reaction of some of the mechanism steps.     

Mn/MFI catalyzed reduction of NOx with alkanes

Mn/MFI catalysts were prepared by different methods and probed as catalysts for the catalytic reduction of NO_x with CH₄ or iso-butane in a gas flow containing excess O₂. Mn/MFI with high manganese loading was obtained by solid state ion exchange (SSI). The intensity of an IR band at 957 cm⁻¹, which is due to the perturbation of zeolite lattice vibrations by Mn ions attached to cage walls is proportional to the Mn content of the catalysts. Conversely, the intensity of the 3610 cm⁻¹ band, assigned to Brønsted acid sites decreases linearly with the Mn loading. A catalyst obtained by exchanging Na/MFI with an aqueous solution of Mn acetate is found most active for NO_x reduction with methane. Transport by surface diffusion of Mn ions from MnI₂ to exchange positions in MFI is more efficient than their transport through the gas phase. High NO conversion over proton-free catalysts indicates that protons are not instrumental in NO_x reduction over Mn/MFI.     

NO reduction by CH4 over Pd/Co-sulfated zirconia catalysts

A series of sulfated zirconia supported Pd/Co catalysts was synthesized by the sol–gel method and examined for NO_x reduction by methane. The NO conversion increased up to a Co/S ratio of 0.43, and then decreased at a higher Co loading (Co/S = 0.95). Sulfate content was also essential for obtaining high selectivity to molecular nitrogen. A catalyst loaded with 0.06 wt.% Pd, 2.1 wt.% Co and 2.1 wt.% S (Pd/Co-SZ-2) exhibited remarkable performance under lean conditions and displayed stability in a long-term durability test using a synthetic reaction mixture containing 10% water vapor. This catalyst exhibited the highest sulfur retention most probably as cobalt sulfide. Besides, the catalytic oxidation of NO to NO_y groups was confirmed by FT-IR, in agreement with the general mechanism for the SCR of NO by hydrocarbons. In the absence of oxygen in the feed stream, the catalyst was highly active for NO reduction with methane. IR stretching bands assigned to N₂O and adsorbed nitro groups were identified upon adsorbing NO on Pd/Co-SZ-2. This indicates that under rich conditions disproportionation of NO to N₂O and NO₂ occurs and confirms that the formation of NO₂ species is an essential step for NO reduction by CH₄.     

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

Reaction activities of several developed catalysts for NO oxidation and NO_x (NO + NO₂) reduction have been determined in a fixed bed differential reactor. Among all the catalysts tested, Co₃O₄ 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 Co₃O₄ catalyst, the effects of calcination temperatures, SO₂ concentration, optimum SV for 50% conversion of NO to NO₂ were determined. Also, Co₃O₄ based catalysts (Co₃O₄-WO₃) 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, N₂O formation over Co₃O₄-WO₃ catalyst is far less than that over the other catalysts but the standard SCR produces high concentration of N₂O over all the catalysts. The effect of SO₂ concentration on NO_x reduction is found to be almost negligible may be due to the presence of WO₃ that resists SO₂ oxidation.     

The bifunctional reaction pathway and dual kinetic regimes in NOx SCR by methane over cobalt mordenite catalysts

The pathway for selective reduction of NO_x by methane over Co mordenite cataysts has been studied by comparing the rates of the individual reactions (NO oxidation, CH₄ oxidation, NO₂ reduction) with that of the combined reaction (NO + O₂ + CH₄). Co⁽⁺²⁾ was exchanged into H-MOR and Na-MOR to give catalysts with different metal loading and number of support protons. Additionally, exchanged Co⁽⁺²⁾ ions were precipitated with NaOH to produce dispersed cobalt oxide on Na-MOR. The NO oxidation rate is the same for ion exchanged Co⁽⁺²⁾ ions in H-MOR and Na-MOR, but the rate of Co⁽⁺²⁾ ions is much lower than that of cobalt oxide. NO oxidation equilibrium is obtained only for those catalysts with high metal loading, cobalt oxide or run at low GHSV. Under the conditions of selective catalytic reduction, methane oxidation by O₂ is low for all catalysts. The turnover frequency of Co on Na-MOR, however, is higher than that on H-MOR. The rate of NO₂ reduction to N₂ is directly proportional to the number of support acid sites and independent of the amount of Co. Comparison of the rates and selectivities for the individual reactions with the combined reaction of NO + O₂ + CH₄ indicates that there are two types of catalysts. For the first, the NO oxidation is in equilibrium and the rate determining step is reduction of NO₂. For these catalysts, the rate (and selectivity) for formation of N₂ is identical from NO + O₂ + CH₄ and NO₂ + CH₄. These catalysts have high metal loading and few acid sites. Nevertheless, the rate of N₂ formation increases with increasing number of protons. For the second type of catalyst, NO oxidation is not in equilibrium and is the rate limiting step. For these catalysts the rate of N₂ formation increases with increasing metal loading. Neither catalyst type, however, is optimized for the maximum formation of N₂. By using a mixture of catalysts, one with high NO oxidation activity and one with a large number of Brønsted acid sites, the rate of N₂ is greater than the weighted sum of the individual catalysts. The current results support the proposal that the pathway for selective catalytic reduction is bifunctional where metal sites affect NO oxidation, while support protons catalyze the formation of N₂.     

The activity and characterization of sol–gel Sn/Al2O3 catalyst for selective catalytic reduction of NOx in the presence of oxygen

Catalytic performance of Sn/Al₂O₃ catalysts prepared by impregnation (IM) and sol–gel (SG) method for selective catalytic reduction of NO_x by propene under lean burn condition were investigated. The physical properties of catalyst were characterized by BET, XRD, XPS and TPD. The results showed that NO₂ had higher reactivity than NO to nitrogen, the maximum NO conversion was 82% on the 5% Sn/Al₂O₃ (SG) catalyst, and the maximum NO₂ conversion reached nearly 100% around 425 °C. Such a temperature of maximum NO conversion was in accordance with those of NO_x desorption accompanied with O₂ around 450 °C. The activity of NO reduction was enhanced remarkably by the presence of H₂O and SO₂ at low temperature, and the temperature window was also broadened in the presence of H₂O and SO₂, however the NO_x desorption and NO conversion decreased sharply on the 300 ppm SO₂ treated catalyst, the catalytic activity was inhibited by the presence of SO₂ due to formation of sulfate species (SO₄²⁻) on the catalysts. The presence of oxygen played an essential role in NO reduction, and the activity of the 5% Sn/Al₂O₃ (SG) was not decreased in the presence of large oxygen.     

Catalytic reduction of NOx with H2/CO/CH4 over PdMOR catalysts

Conversion of NO_x with reducing agents H₂, CO and CH₄, with and without O₂, H₂O, and CO₂ were studied with catalysts based on MOR zeolite loaded with palladium and cerium. The catalysts reached high NO_x to N₂ conversion with H₂ and CO (>90% conversion and N₂ selectivity) range under lean conditions. The formation of N₂O is absent in the presence of both H₂ and CO together with oxygen in the feed, which will be the case in lean engine exhaust. PdMOR shows synergic co-operation between H₂ and CO at 450–500 K. The positive effect of cerium is significant in the case of H₂ and CH₄ reducing agent but is less obvious with H₂/CO mixture and under lean conditions. Cerium lowers the reducibility of Pd species in the zeolite micropores. The catalysts showed excellent stability at temperatures up to 673 K in a feed with 2500 ppm CH₄, 500 ppm NO, 5% O₂, 10% H₂O (0–1% H₂), N₂ balance but deactivation is noticed at higher temperatures. Combining results of the present study with those of previous studies it shows that the PdMOR-based catalysts are good catalysts for NO_x reduction with H₂, CO, hydrocarbons, alcohols and aldehydes under lean conditions at temperatures up to 673 K.     

Effect of addition of Zn on the catalytic activity of a Co/HZSM-5 catalyst for the SCR of NOx with CH4

The selective catalytic reduction (SCR) of NO_x by methane in the presence of excess oxygen was studied on a Zn-Co/HZSM-5 catalyst. It was found that the addition of Zn could improve effectively the selectivity of methane towards NO_x reduction. When prepared by a coimpregnation method, the Zn-Co/HZSM-5 catalyst showed much higher catalytic activity than the two catalysts of a Zn/Co/HZSM-5 and Co/Zn/HZSM-5 prepared by the successive impregnation method. It is considered that there exists a cooperative effect among the Zn, Co and zeolite, which enhances the reduction of NO to NO₂ reaction and the activation of methane.     

An examination of the role of plasma treatment for lean NOx reduction over sodium zeolite Y and gamma alumina Part 1. Plasma assisted NOx reduction over NaY and Al2O3

The role of plasma processing on NO_x reduction over γ-alumina and a basic zeolite, NaY was examined. During the plasma treatment NO is oxidized to NO₂ and propylene is partially oxidized to CO, CO₂, acetaldehyde, and formaldehyde. With plasma treatment, NO as the NO_x gas, and a NaY catalyst, the maximum NO_x conversion was 70% between 180 and 230 °C. The activity decreased at higher and lower temperatures.

As high as 80% NO_x removal over gamma alumina was measured by a chemiluminescent NO_x meter with plasma treatment and NO as the NO_x gas.

For both catalysts a simultaneous decrease in NO_x and aldehydes concentrations was observed, which suggests that aldehyde may be important components for NO_x reduction in plasma-treated exhaust.     

Effect of Pt on the water resistance of Co-zeolites upon the SCR of NOx with CH4

The objective of this work was to study the promotional effect of Pt on Co-zeolite (viz. mordenite, ferrierite, ZSM-5 and Y-zeolite) and Co/Al₂O₃ on the selective catalytic reduction (SCR) of NO_x with CH₄ under dry and wet reaction stream. After being reduced in H₂ at 350°C, the PtCo bimetallic zeolites showed higher NO to N₂ conversion and selectivity than the monometallic samples, as well as a combination of the latter samples such as mechanical mixtures or two-stage catalysts. After the same pretreatment, under wet reaction stream, the bimetallic samples were also more active. Among the other catalysts studied with 5% of water in the feed, (NO = CH₄ = 1000 ppm, O₂ = 2%), the NO conversion dropped to zero over Co_2.0Mor at 500°C and GHSV = 30,000 h⁻¹, whereas it is 20% in Pt_0.5Co_2.0Mor. In Pt/Co/Al₂O₃ the NO_x conversion dropped below 5% with only 2% of water under the same reaction conditions. The specific activity given as molecules of NO converted per total metal atom per second were 16.5 × 10⁻⁴ s⁻¹ for Pt_0.5Co_2.0Fer, 13 × 10⁻⁴ s⁻¹ for Pt_0.5Co_2.0Mor, 4.33 × 10⁻⁴ s⁻¹ for Pt_0.5Co_2.0ZSM-5 and 0.5 × 10⁻⁴ s⁻¹ for Pt/Co/Al₂O₃. The Y-zeolite-based samples were inactive in both mono and bimetallic samples. The species initially present in the solid were Pt° and Co°, together with Co²⁺ and Pt²⁺ at exchange positions. Co° seems not to participate as an active site in the SCR of NO_x. Those species remained after the reaction but some reorganization occurred. A synergetic effect among the different species that enhances both the NO to NO₂ reaction, the activation of CH₄ and also the ability of the catalyst to adsorb NO, could be responsible for the high activity and selectivity of the bimetallic zeolites.     

Influence of support acid pretreatment on the behaviour of CoOx/γ-alumina monolithic catalysts in the CH4-SCR reaction

The effect of treatment with different mineral acids (H₂SO₄, H₃PO₄, HNO₃ and HCl) on the activity of monolithic CoO_x/γ-Al₂O₃ catalysts in the reduction of nitric oxide with methane in the presence of oxygen (CH₄-SCR of NO_x) was studied. Their behaviour in the methane oxidation reaction in both the presence and absence of NO_x was determined in order to interpret the results in terms of intrinsic activity and competition between both processes. Depending on the nature of the acid used, significant differences were observed in the catalytic activities which were related to the textural states, surface acidities and the nature of the detected species. The best results were obtained after treatment with H₂SO₄, which increased the activity towards NO_x elimination compared to the other catalysts. This behaviour was attributed not only to an increase in surface acidity but also to the stabilisation of the active Co²⁺ species, thus avoiding the formation of Co₃O₄ spinel that is responsible for the strongly adsorbed NO_x species that lead to NO₂ formation which increase the rate of the undesired methane oxidation reaction at high temperatures.     

Multifunctional catalysts for de-NOx processes: The case of H3PW12O40·6H2O-metal supported on mixed oxides

The role of a multifunctional catalyst for de-NO_x process has been investigated. The NO_x storage capacity of H₃PW₁₂O₄₀·6H₂O (HPW) was improved by the presence of a noble metal (Pt, Rh or Pd). Both HPW and noble metal were deposited on a specific support (based on Zr–Ce or Zr–Ti). The presence of noble metal in several oxidation states, as evidenced by TPR and IR, involves the possibility of forming different catalytic sites: (i) M⁰ (zero-valent metal) and perhaps (ii) (metal–H)^δ+ from specific interactions between noble metal and the HPW proton. Supports were also able to adsorb and activate NO_x and to generate cationic catalytic sites (M^x+). These cationic sites seem to be the clue for their important activity toward NO_x reduction. This catalyst presents an outstanding resistance to SO₂ poisoning which can be related to NO and NO₂ absorption mechanism in HPW. The use of alternating short cycles of lean/rich mixtures allows us optimising the performance of this catalytic system in terms of both NO_x reduction capacity and NO_x storage efficiency: up to 48 and 84%, respectively (with a 2% CO + 1% H₂ mixture for reducing). Experimental results sustain two hypotheses: first, HPW-metal-support catalyst includes several (independent) catalytic functions required for a de-NO_x process to occur and second, the formation of oxygenate active species must be indispensable for NO_x reduction into nitrogen.     

Alumina- and titania-based monolithic catalysts for low temperature selective catalytic reduction of nitrogen oxides

The selective catalytic reduction of NO+NO₂ (NO_x) at low temperature (180–230°C) with ammonia has been investigated with copper-nickel and vanadium oxides supported on titania and alumina monoliths. The influence of the operating temperature, as well as NH₃/NO_x and NO/NO₂ inlet ratios has been studied. High NO_x conversions were obtained at operating conditions similar to those used in industrial scale units with all the catalysts. Reaction temperature, ammonia and nitrogen dioxide inlet concentration increased the N₂O formation with the copper-nickel catalysts, while no increase was observed with the vanadium catalysts. The vanadium-titania catalyst exhibited the highest DeNO_x activity, with no detectable ammonia slip and a low N₂O formation when NH₃/NO_x inlet ratio was kept below 0.8. TPR results of this catalyst with NO/NH₃/O₂, NO₂/NH₃/O₂ and NO/NO₂/NH₃/O₂ feed mixtures indicated that the presence of NO₂ as the only nitrogen oxide increases the quantity of adsorbed species, which seem to be responsible for N₂O formation. When NO was also present, N₂O formation was not observed.     

The combustion of carbon particulates using NO/O2 mixtures: The influence of SO4 and NOx trapping materials

The activity of several catalysts are studied in the soot combustion reaction using air and NO/air as oxidising agents. Over Al₂O₃-supported catalysts NO_(g) is a promoter for the combustion reaction with the extent of promotion depending on the Na loading. Over these catalysts SO₄²⁻ poisons this promotion by preventing NO oxidation through a site blocking mechanism. SiO₂ is unable to adsorb NO or catalyse its oxidation and over SiO₂-supported Na catalysts NO_(g) inhibits the combustion reaction. This is ascribed to a competition between NO and O₂. Over Fe-ZSM-5 catalysts the presence of a NO_x trapping component does not increase the combustion of soot in the presence of NO_(g) and it is proposed that this previously reported effect is only seen under continuous NO_x trap operation as NO₂ is periodically released during regeneration and thus available for soot combustion. Experiments during which the [NO]_(g) is varied show that CO, rather than an adsorbed carbonyl-like intermediate, is formed upon reaction between NO₂ (the proposed oxygen carrier) and soot.