Contribution to the mechanism of NO reduction by CO over Pt/NaX zeolite

The reaction of NO + CO was studied over Pt/NaX prepared by the decomposition of [Pt(NH₃)₄]²⁺. The decomposition was carried out via calcination followed by reduction, by vacuum decomposition, and by decomposition in hydrogen, by ways which are known to lead to the formation of Pt clusters of different sizes and location. The NO reduction by CO was studied under static conditions for longer (20–30 min) and shorter (100 s) time intervals, and the reaction was followed by temperature programmed decomposition (TPD) of species adsorbed during the preceding isothermal reactions. The effect of various NO/CO ratios and of added oxygen was examined. The reactions of N₂O + CO were compared with those of NO + CO. The increasing size of Pt clusters enhances the reduction of NO by CO, but it is complicated at lower reaction temperatures (below 230°C) by the poisoning of active Pt centres, especially by adsorbed CO. Smaller Pt clusters exhibit higher preference towards NO adsorption from NO + CO mixtures than the larger Pt clusters. The incomplete reduction of NO to N₂O proceeds under our experimental conditions below 230°C, and is accompanied by the formation of adsorbed species. N₂O formation is enhanced by the increased NO/CO ratio and by the addition of oxygen. The reduction of nitrous oxide occurs much slower than that of nitric oxide, and therefore N₂O could play a role only as a surface intermediate in the CO + NO reaction.     

High thermal conductive β-SiC for selective oxidation of H2S: A new support for exothermal reactions

This study aims at synthesizing a new by substituting 1 atom% Pd²⁺ in ionic state in TiO₂ in the form of Ti_0.99Pd_0.01O_1.99 with oxide-ion vacancy. The catalyst was synthesized by solution combustion method and was characterized by XRD and XPS. The catalytic activity was investigated by performing CO oxidation, hydrocarbon oxidation and NO reduction. A reaction mechanism for CO oxidation by O₂ and NO reduction by CO was proposed. The model based on CO adsorption on Pd²⁺ and dissociative chemisorption of O₂ in the oxide-ion vacancy for CO oxidation reaction fitted the experimental for CO oxidation. For NO reduction in presence of CO, the model based on competitive adsorption of NO and CO on Pd²⁺, NO chemisorption and dissociation on oxide-ion vacancy fitted the experimental data. The rate parameters obtained from the model indicated that the reactions were much faster over this catalyst compared to other catalysts reported in the literature. The selectivity of N₂, defined as the ratio of the formation of N₂ and formation of N₂ and N₂O, was very high compared to other catalysts and 100% selectivity was reached at temperature of 350 °C and above. As the N₂O + CO reaction is an intermediate reaction for NO + CO reaction, it was also studied as an isolated reaction and the rate of the isolated reaction was less than that of intermediate reaction.     

Low temperature decomposition of nitrous oxide over Fe/ZSM-5: Modelling of the dynamic behaviour

The kinetics of N₂O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with low content of iron (<400 ppm) under transient and steady-state conditions was investigated in the temperature range of 250–380 °C. The catalysts were prepared from the HZSM-5 with Fe in the framework upon steaming at 550 °C followed by thermal activation in He at 1050 °C. The N₂O decomposition began at 280 °C. The reaction kinetics was first order towards N₂O during the transient period, and of zero order under steady-state conditions. The increase of the reaction rate with time (autocatalytic behaviour) was observed up to the steady state. This increase was assigned to the catalysis by adsorbed NO formed slowly on the zeolite surface from N₂O. The formation of NO was confirmed by temperature-programmed desorption at temperatures >360 °C. The amount of surface NO during the transient increases with the reaction temperature, the reaction time, and the N₂O concentration in the gas phase up to a maximum value. The maximum amount of surface NO was found to be independent on the temperature and N₂O concentration in the gas phase. This leads to a first-order N₂O decomposition during the transient period, and to a zero-order under steady state. A kinetic model is proposed for the autocatalytic reaction. The simulated concentration–time profiles were consistent with the experimental data under transient as well as under steady-state conditions giving a proof for the kinetic model suggested in this study.     

Activity and stability of Ag–alumina for the selective catalytic reduction of NOx with methane in high-content SO2 gas streams

In this work, we investigated the activity and stability of Ag–alumina catalysts for the SCR of NO with methane in gas streams with a high concentration of SO₂, typical of coal-fired power plant flue gases. Ag–alumina catalysts were prepared by coprecipitation–gelation, and dilute nitric-acid solutions were used to remove weakly bound silver species from the surface of the as prepared catalysts after calcination. SO₂ has a severe inhibitory effect, essentially quenching the CH₄-SCR reaction on this type catalysts at temperatures <600 °C. SO₂ adsorbs strongly on the surface forming aluminum and silver sulfates that are not active for CH₄-SCR of NO_x. Above 600 °C, however, the reaction takes place without catalyst deactivation even in the presence of 1000 ppm SO₂. The reaction light-off coincides with the onset of silver sulfate decomposition, indicating the critical role of silver in the reaction mechanism. SO₂ is reversibly adsorbed on silver above 600 °C. While alumina sites remain sulfated, this does not hinder the reaction. Sulfation of alumina only decreases the extent of adsoption of NO_x, but adsorption of NO_x is not the limiting step. Methane activation is the limiting step, hence the presence of sulfur-free Ag–O–Al species is a requirement for the reaction. Strong adsorption of SO₂ on Ag–alumina decreases the rates of the reaction, and increases the activation energies of both the reduction of NO to N₂ and the oxidation of CH₄, the latter more than the former. Our results indicate partial contribution of gas phase reactions to the formation of N₂ above 600 °C. H₂O does not inhibit the reaction at 625 °C, and the effect of co-addition of H₂O and SO₂ is totally reversible.     

Oxidative gas phase carbonylation of methanol to dimethyl carbonate over chloride-free Cu-impregnated zeolite Y catalysts at elevated pressure

Incipient wetness impregnation of zeolite Y with copper(II) nitrate solution and inert activation at 650 °C led to active catalysts for the oxidative carbonylation of methanol to dimethyl carbonate in the gas phase. Activities were measured under elevated pressure (0.4–1.6 MPa) with feed compositions of CO/MeOH/O₂ = 40/20/6–1.5 vol.% (balanced by N₂) over zeolite Y loaded with 10–17 wt.% copper. It could be shown that inert activation at 650 °C enhanced the activity, and that Cu loading of 14–17 wt.% gave the best performance. By combined XRD, TEM, TPR and DRIFT characterization it was found that the inert activation initiated dispersion of crystalline CuO, auto-reduction of Cu²⁺ to Cu⁺ and redistribution of copper ions with enrichment inside the supercages of the zeolite. The O₂ content of the feed was found to control the selectivity to dimethyl carbonate. Dimethyl carbonate selectivities of 70–75% were achieved within the temperature range of 140–170 °C at an O₂ content of 1.5 vol.%. This allowed space-time yields of dimethyl carbonate up to 632 g l_cat⁻¹ h⁻¹ at methanol conversions of 5–12%. Formation of the main side product, dimethoxymethane, was surprisingly affected by CO, which is not in line with suggested reaction pathways. A mechanism is proposed including formation of surface carbonate structures as common intermediate.     

Reaction of nitric oxide and nitric oxide + carbon monoxide with amorphous Fe-Co-B alloy powders

The activity of amorphous Fe---Co---B alloy powder was investigated for the decomposition and the reduction of nitrogen monoxide. The transient response technique and a fixed bed reactor were applied to study the interactions of the Fe---Co---B alloy with two gas mixtures: NO + Ar at 353 and 573 K and NO + CO + Ar at 333–573 K. Moessbauer spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to study the state of the initial sample and the samples utilized in both gas mixtures. It is shown that the amorphous Fe---Co---B alloy powder has an activity for the direct decomposition of nitric oxide to nitrous oxide and nitrogen at a high gas space velocity (26 000 h⁻¹). Oxygen from the decomposed nitric oxide poisons the surface for the formation of nitrogen. In the presence of carbon monoxide (a NO + CO + Ar gas mixture) nitric oxide is reduced to nitrous oxide at 333–353 K and fully reduced to nitrogen at 373–573 K. The quantities of the carbon dioxide formed are not equal to the values expected from the stoichiometry of the NO + CO reaction. Probably, the interaction of carbon monoxide with the adsorbed oxygen (left on the surface by the decomposed nitric oxide) enhances the rate of nitric oxide decomposition to nitrous oxide and nitrogen. The rate limiting steps for both reactions of nitric oxide decomposition, as indicated by the transient response data, change with increasing temperature. The data from the Moessbauer spectroscopy and the X-ray photoelectron spectroscopy (XPS) studies have shown that the amorphous Fe---Co---B alloy powder undergoes phase changes under the conditions of both, the NO + Ar and the NO + CO + Ar gas mixtures. Boron migrates to the surface of globules and serves the accumulation of oxygen by the formation of B₂O₃ (or B(OH)₃).     

Thermal stability of tungsten carbide in 7 mol.% calcia–zirconia solid solution matrix heat treated in argon

Zirconia polycrystals stabilised with 7 mol.% CaO containing 10 vol.% WC particles (Ca-PSZ/WC) were obtained by using zirconia nanopowder and WC micropowder. Cold isostatically pressed samples were pressureless sintered in argon at 1350–1950 °C. The influence of the sintering temperature and the incorporation of WC particles on the phase composition and mechanical properties of the composites were studied. Decomposition of WC due to the reaction with the zirconia matrix was found. W₂C and metallic tungsten were detected as decomposition products when heat treated below 1750 °C. At higher temperatures, ZrC is formed. The mechanism of WC decomposition was discussed. The zirconia polycrystals modified with in situ formed W and W₂C inclusions showed a bending strength of 417 ± 67 MPa, a fracture toughness of 5.2 ± 0.3 MPa m^0.5 and a hardness of 14.6 ± 0.3 GPa.     

Influence of the pre-treatment on the structure and reactivity of Ir/γ-Al2O3 catalysts in the selective reduction of nitric oxide by propene

The selective catalytic reduction (SCR) of nitric oxide by propene over Ir/Al₂O₃ under lean-burn conditions (1000 vpm NO, 2000 vpm C₃H₆, 500 vpm CO, 10 vol.% O₂) was studied. The activity was shown to be strongly enhanced after exposure of the catalyst at 600°C under the reaction mixture, irrespective of the oxidising or reducing pre-treatment. Simultaneously, the Ir dispersion decreased from 78 to 10%. The influence of each component of the reaction mixture on the activation process was examined. The presence of both CO and O₂ was found to be necessary to activate Ir/Al₂O₃ while NO would not be. In situ FT-IR results revealed that initially fully oxidised Ir particles partially reduced in the feed to form Ir⁰ reduced surface sites (νCO at 2060 cm⁻¹) which adsorbed CO up to 350–400°C. The activation under reactants was related to the formation of these sites. The presence of reduced (or partially reduced) Ir sites, possibly siting at the surface of IrO₂ particles and stabilised by CO adsorption, was proposed to be responsible for the SCR activity.     

An examination of the role of plasma treatment for lean NOx reduction over sodium zeolite Y and gamma alumina Part 2: Formation of nitrogen

NO_x reduction with NO₂ as the NO_x gas in the absence of plasma was compared to plasma treated lean NO_x exhaust where NO is converted to NO₂ in the plasma. Product nitrogen was measured to prove true chemical reduction of NO_x to N₂. With plasma treatment, NO as the NO_x gas, and a NaY catalyst, the maximum conversion to nitrogen was 50% between 180 and 230 °C. The activity decreased at higher and lower temperatures. At 130 °C a complete nitrogen balance could be obtained, however between 164 and 227 °C less than 20% of the NO_x is converted to a nitrogen-containing compound or compounds not readily detected by gas chromatograph (GC) or Fourier transform infrared spectrometer (FT-IR) analysis. With plasma treatment, NO₂ as the NO_x gas, and a NaY catalyst, a complete nitrogen balance is obtained with a maximum conversion to nitrogen of 55% at 225 °C.

For γ-alumina, with plasma treatment and NO₂ as the NO_x gas, 59% of the NO_x is converted to nitrogen at 340 °C. A complete nitrogen balance was obtained at these conditions. As high as 80% NO_x removal over γ-alumina was measured by a chemiluminescent NO_x meter with plasma treatment and NO as the NO_x gas.

When NO is replaced with NO₂ and the simulated exhaust gases are not plasma treated, the maximum NO_x reduction activity of NaY and γ-alumina decreases to 26 and 10%, respectively. This is a large reduction in activity compared to similar conditions where the simulated exhaust was plasma treated. Therefore, in addition to NO₂, other plasma-generated species are required to maximize NO_x reduction.     

Deactivation of a Fe-ZSM-5 catalyst during the selective catalytic reduction of NO by n-decane: An operando DRIFT study

The selective catalytic reduction (SCR) of NO by n-decane was investigated on a Fe-ZSM-5 prepared by the FeCl₃ sublimation method. NO conversion profiles versus temperature were followed in both temperature programmed surface reaction (TPSR, 10 °C min⁻¹) and steady state experiments. A higher NO conversion with a maximum of ca. 80% at 400 °C is observed in the course of the TPSR tests. This phenomenon has been attributed to strong adsorption of n-decane which protects the active sites against the poisoning. Indeed, in steady state experiments at 390 °C the strong decrease of activity as a function of time on stream is due to the polymerisation of conjugated nitriles. This study indicates that long chain alkanes are not the most adequate reductants of NO for high temperature SCR applications. Moreover, due to an easier polymerization of conjugated nitriles on iron zeolites (stronger Fe Lewis sites), this type of catalyst seems less attractive than Cu-zeolite catalysts for the SCR of NO by hydrocarbons in this respect.     

Selective catalytic reduction of NOx in lean-burn engine exhaust over a Pt/V/MCM-41 catalyst

The activities of Pt supported on various metal-substituted MCM-41 (V-, Ti-, Fe-, Al-, Ga-, La-, Co-, Mo-, Ce-, and Zr-MCM-41) and V-impregnated MCM-41 were investigated for the reduction of NO by C₃H₆. Among these catalysts, Pt supported on V-impregnated MCM-41 showed the best activity. The maximum conversion of NO into N₂+N₂O over this Pt/V/MCM-41 catalyst (Pt=1 wt.%, V=3.8 wt.%) was 73%, and this maximum conversion was sustained over a temperature range of 70 °C from 270 to 340 °C. The high activity of Pt/V/MCM-41 over a broad temperature range resulted from two additional reactions besides the reaction occurring on usual supported Pt, the reaction of NO with surface carbonaceous materials, and the reaction of NO occurring on support V-impregnated MCM-41. The former additional reaction showed an oscillation characteristic, a phenomenon in which the concentrations of parts of reactant and product gases oscillate continuously. At low temperature, some water vapor injected into the reactant gas mixture promoted the reaction occurring on usual supported Pt, whereas at high temperature, it suppressed the additional reaction related to carbonaceous materials. Five-hundred parts per million of SO₂ added to the reactant gas mixture only slightly decreased the NO conversion of Pt/V/MCM-41.     

Catalytic oxidation of HCN over a 0.5% Pt/Al2O3 catalyst

The adsorption of HCN on, its catalytic oxidation with 6% O₂ over 0.5% Pt/Al₂O₃, and the subsequent oxidation of strongly bound chemisorbed species upon heating were investigated. The observed N-containing products were N₂O, NO and NO₂, and some residual adsorbed N-containing species were oxidized to NO and NO₂ during subsequent temperature programmed oxidation. Because N-atom balance could not be obtained after accounting for the quantities of each of these product species, we propose that N₂ and was formed. Both the HCN conversion and the selectivity towards different N-containing products depend strongly on the reaction temperature and the composition of the reactant gas mixture. In particular, total HCN conversion reaches 95% above 250 °C. Furthermore, the temperature of maximum HCN conversion to N₂O is located between 200 and 250 °C, while raising the reaction temperature increases the proportion of NO_x in the products. The co-feeding of H₂O and C₃H₆ had little, if any effect on the total HCN conversion, but C₃H₆ addition did increase the conversion to NO and decrease the conversion to NO₂, perhaps due to the competing presence of adsorbed fragments of reductive C₃H₆. Evidence is also presented that introduction of NO and NO₂ into the reactant gas mixture resulted in additional reaction pathways between these NO_x species and HCN that provide for lean-NO_x reduction coincident with HCN oxidation.     

A catalytic NOx reduction system using periodic steps, lean and rich operations

Catalytic lean NO reduction system in periodic two steps, lean/rich operations has been investigated over Rh-based catalysts. The investigation was done using a pulse reaction. In the reaction, after H₂ or CO was pulse-injected for a moment to achieve reducing conditions, the highly dispersed Rh catalyst could catalyze NO reduction at 200–400 °C in lean conditions for 1 min. Furthermore, NO was effectively reduced over the highly dispersed Rh/β-zeolite after exposure to the gas composed of 3% of O₂, 40 ppm of SO₂ and balance of He at 300 and 400 °C for 24 h.     

Preferential oxidation of carbon monoxide over Pt, Au monometallic catalyst, and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate

The objective of this paper was to study a preferential oxidation (PROX) of carbon monoxide over monometallic catalysts including Pt, Au and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate. Single step sol–gel method (SSG) and impregnation on sol–gel method (ISG) were chosen for the preparation of the catalysts. The characteristics of these catalysts were investigated by X-ray diffractometer (XRD), Brunauer–Emmet–Teller (BET) method, transmission electron microscope (TEM), scanning electron microscope (SEM) and temperature-programmed reduction (TPR). The XRD patterns of the catalysts showed only the peaks of ceria crystallite and no metal peak appeared. From TEM images, the active components were seen to be dispersed throughout the ceria support. The TPR patterns of PtAu/CeO₂ catalyst prepared by SSG showed the reduction peaks were within a low temperature range and therefore, the catalysts prepared by SSG exhibited excellent catalytic activity for preferential oxidation of CO. Bimetallic Pt–Au catalyst improved the activity (90% conversion and 50% selectivity at 90 °C) because of the formation of a new phase. When the metal content of (1:1) PtAu/CeO₂ catalyst prepared by SSG was increased, the CO conversion did not change much while the selectivity decreased in the low temperature range (50–90 °C). The CO conversion increased with increasing W/F ratio. The presence of CO₂ and H₂O had a negative effect on CO conversion and selectivity due to blocking of carbonate and water on active sites.     

Reaction and characterization studies of an industrial Cr-free iron-based catalyst for high-temperature water gas shift reaction

Activity and stability of an industrial Cr-free iron-based catalyst (NBC-1) for high-temperature water gas shift (WGS) reaction were studied in a fixed-bed reactor under 350 °C, 1 atm, H₂O:gas = 1:1 and 3000 h⁻¹ (dry-gas basis). Physical properties of the NBC-1 catalyst before and after the WGS reaction, the desorption behavior of H₂O, CO, CO₂ and H₂, and surface reaction over the catalyst were characterized by BET, X-ray diffraction (XRD), Mössbauer emission spectroscopy (MES), temperature programmed desorption (TPD) and temperature programmed surface reaction (TPSR). The NBC-1 catalyst is active and has excellent thermo-stability even after pretreatment at a high temperature of 530 °C. Its activity and thermo-stability are comparable to those of an UCI commercial Fe-Cr catalyst, C12-4. XRD and MES studies show that iron in the fresh NBC-1 catalyst is present as γ-Fe₂O₃, most of which is converted to Fe₃O₄ during reduction and reaction. Results of TPD demonstrate that adsorbed CO₂ and CO cannot exist on the NBC-1 surface beyond the temperature of 300 °C while higher temperatures (>400 °C) are required to completely desorb H₂O. A redox mechanism of WGS on the NBC-1 surface is proposed based on the TPD and TPSR observations.     

Low-temperature H2-SCR of NO on a novel Pt/MgO-CeO2 catalyst

The selective catalytic reduction of NO by H₂ under strongly oxidizing conditions (H₂-SCR) in the low-temperature range of 100–200 °C has been studied over Pt supported on a series of metal oxides (e.g., La₂O₃, MgO, Y₂O₃, CaO, CeO₂, TiO₂, SiO₂ and MgO-CeO₂). The Pt/MgO and Pt/CeO₂ solids showed the best catalytic behavior with respect to N₂ yield and the widest temperature window of operation compared with the other single metal oxide-supported Pt solids. An optimum 50 wt% MgO-50wt% CeO₂ support composition and 0.3 wt% Pt loading (in the 0.1–2.0 wt% range) were found in terms of specific reaction rate of N₂ production (mols N₂/g_cat s). High NO conversions (70–95%) and N₂ selectivities (80–85%) were also obtained in the 100–200 °C range at a GHSV of 80,000 h⁻¹ with the lowest 0.1 wt% Pt loading and using a feed stream of 0.25 vol% NO, 1 vol% H₂, 5 vol% O₂ and He as balance gas. Addition of 5 vol% H₂O in the latter feed stream had a positive influence on the catalytic performance and practically no effect on the stability of the 0.1 wt% Pt/MgO-CeO₂ during 24 h on reaction stream. Moreover, the latter catalytic system exhibited a high stability in the presence of 25–40 ppm SO₂ in the feed stream following a given support pretreatment. N₂ selectivity values in the 80–85% range were obtained over the 0.1 wt% Pt/MgO-CeO₂ catalyst in the 100–200 °C range in the presence of water and SO₂ in the feed stream. The above-mentioned results led to the obtainment of patents for the commercial exploitation of Pt/MgO-CeO₂ catalyst towards a new NO_x control technology in the low-temperature range of 100–200 °C using H₂ as reducing agent. Temperature-programmed desorption (TPD) of NO, and transient titration of the adsorbed surface intermediate NO_x species with H₂ experiments, following reaction, have revealed important information towards the understanding of basic mechanistic issues of the present catalytic system (e.g., surface coverage, number and location of active NO_x intermediate species, NO_x spillover).     

Catalytic activity of pitch-based activated carbon fiber of large surface area heat-treated at high temperature and its regeneration for NO–NH3 reaction at ambient temperatures

The catalytic activity of a pitch-based activated carbon fiber (ACF) of very large surface area (OG-20A) was studied for NO–NH₃ reaction in a flow reactor at ambient temperatures. The ACF exhibited the highest activity in wet as well as dry gas among heat-treated ACFs so far examined by the present authors. The calcination at 1100°C was essential to exhibit the highest activity especially in wet gas. Although high humidity always retarded the reaction very markedly, its retardation was very much emphasized against NO of low concentration around 10 ppm. Sufficient amount of OG-20A-H1100 (3 g) allowed complete removal of 10–200 ppm NO by reduction and adsorption for initial 6 h even at least in wet gas at 25–30°C depending on NO concentration. The removal conversion decreased gradually for several hours following to the stationary one. The reactivity of adsorbed NO and NH₃ was examined in air to regenerate the period of complete NO removal over the ACF. The regeneration at 30°C was found optimum after the removal reaction at 25 or 30°C to provide the same period of complete removal by 3 h, leaking minimum amounts of adsorbed NO and NH₃. A higher reaction temperature of 35°C shortened the period of complete NO removal, and the successive regeneration at 30°C by 3 h failed in the complete NO removal in the second run. Oxygen appears necessary to regenerate the activity through enhancing the reaction of adsorbed NO and NH₃. NH₃ in the regeneration gas appears to inhibit the reaction of adsorbed species, increasing NH₃ leak.     

Photocatalytic activity of anatase powders for oxidation of methylene blue in water and diluted NO gas

Two series of samples were prepared from titanium tetraisopropoxide (TTIP) through hydrolysis with different aging times, 10 min and 24 h, at room temperature and then annealed at different temperatures. Photocatalytic activities of these samples for methylene blue (MB) in water and for diluted NO gas were examined. For MB decomposition, rate constant k was determined from linear relation between logarithm of relative concentration of MB remained in water and UV irradiation time. For NO, its decomposition fraction after 10 h irradiation was determined. Difference in aging time gave different crystalline state of the precipitates, anatase phase with poor crystallinity after 10 min aging, but amorphous state after 24 h aging. These two precipitates showed different changes in crystallinity and in photoactivity for MB and NO with annealing temperature. Maximum rate constant k for MB decomposition was obtained at around 0.6° of full width at half maximum intensity (FWHM) of 101 diffraction line. For NO gas, a maximum decomposition fraction was obtained at FWHM of around 1.6°. Different crystallinity of anatase was shown to be required for the decomposition of MB in water and NO gas, high crystallinity of anatase phase for the former but poor crystallinity for the latter.     

The NO selective reduction on the La1−xSrxMnO3 catalysts

La_1−xSr_xMnO₃ (x=0, 0.1, 0.3, 0.5, 0.7) perovskite-type oxides (PTOs) were prepared by coprecipitation under various calcination temperature, and their performances for the NO reduction were evaluated under a simulated exhaust gas mixture. The X-ray diffraction (XRD) and thermogravimetric analysis were carried out to find the formation process of the perovskite. The NO reduction rate under different reaction temperature, the concentration of oxygen and the presence of hydrocarbon were observed by the input/output analysis. In the presence of 10% excess oxygen, the catalyst La_0.7Sr_0.3MnO₃ calcined at 900 °C showed a NO reduction rate of 61% at 380 °C. The study of the reaction curves showed that C₃H₈ could act as the reducer for the NO reduction below 400 °C. The NO reduction is highly affected by increasing the O₂ concentration from 0.5 to 10%, especially at high temperatures when oxygen becomes more competitive than NO on the oxidation of C₃H₈, leading to a decrease of the NO reduction from 100% to zero at 560 °C.     

In situ FTIR characterization of the adsorption of CO and its reaction with NO on Pd-based FCC low NOx combustion promoters

The adsorption of CO and its reaction with NO in the 400–600 °C temperature range on Ceⁿ⁺/Na⁺/γ-Al₂O₃ and Pdⁿ⁺/Ceⁿ⁺/Na⁺/γ-Al₂O₃ type materials used commercially as FCC additives were monitored by FTIR spectroscopy. Exposure of both types of samples to CO leads to the formation of carboxylates and carbonates. The concentration of these species was higher in samples containing Pd, indicating that palladium catalyzes their formation. The Pdⁿ⁺ cations initially present in these samples undergo partial reduction to form metallic Pd in the presence of CO even at room temperature. More complete reduction of Pd, along with some aggregation, was observed after exposure to CO at elevated temperatures. Exposure of both types of samples to NO/CO mixtures in the 400–600 °C temperature range leads to the formation of surface isocyanate species. Both Na⁺ and Ceⁿ⁺ promote the formation of such NCO species. However, surface isocyanate species were formed with substantially higher rates in the presence of palladium. The formation of the isocyanate species strongly correlates with changes observed in the ν_OH region, indicating that hydroxyls actively participate in the surface chemistry involved and are capable of protonating the NCO species. The isocyanates are also reactive towards O₂ and NO yielding CO₂ and N₂. These results suggest that isocyanates are possibly involved as intermediates in the CO–NO reaction over the materials examined.