Direct synthesis of H2O2 from H2 and O2 over Pd/H-beta catalyst in an aqueous acidic medium: Influence of halide ions present in the catalyst or reaction medium on H2O2 formation

The influence of different halide ions present in the catalyst or reaction medium on the performance of Pd/H-beta catalyst in the direct H₂O₂ synthesis in an aqueous acidic (0.03 M H₃PO₄) reaction medium at 27 °C and atmospheric pressure has been thoroughly investigated. The results showed a strong influence of both the bulk Pd oxidation state in the catalyst and the halide ions added to the reaction medium on the performance of the catalyst in the H₂ to H₂O₂ oxidation, H₂O₂ decomposition/hydrogenation reactions. The different ammonium halides impregnated reduced Pd/H-beta catalyst calcined in inert (N₂) and oxidizing (air) gaseous atmospheres also revealed that the bulk Pd oxidation state and nature of the halide ions present in the catalyst together control the overall performance of the catalyst in the H₂O₂ formation reaction. The presence of halide ions in reaction medium or in the catalyst significantly changes the selectivity for H₂O₂ formation in the direct H₂O₂ synthesis. Bromide ions are found to remarkably enhance the H₂O₂ selectivity in the direct H₂O₂ synthesis irrespective of the Pd oxidation state in the catalyst. The promoting action of Br⁻ is attributed mainly to the large decrease in the H₂O₂ decomposition and hydrogenation activities of the catalyst and also inhibition for the non-selective H₂-to-water oxidation over the catalyst.     

Selective oxidation of propane over PMoV?M/α‐Al2O3 (M: Co,Fe, or Ni)

Alumina supported phosphovanodomolybdic acid and alumina supported phosphovanodomolybdic acid‐transition metal ions (M: Fe³⁺, Co²⁺, or Ni²⁺) were prepared by impregnation. The thermal decomposition, in situ at 400°C, of supported catalysts showed the formation of V₂O₅, P₂O₅, MoO₃ and MoO₃, CoMoO₄, (Mo_0.3V_0.7)₂O₅ phases, on the alumina surface, in the presence of H₄PMo₁₁VO₄₀/α‐Al₂O₃ and H₄PMo₁₁VO₄₀? Co/α‐Al₂O₃, respectively. The catalytic activity of alumina‐supported catalysts was evaluated in the reaction of propane oxidation at 380 and 400°C. The addition of transition metal increases the conversion and changes the reaction products distribution. The reaction conditions (temperature and propane/oxygen ratio) have also modified the behaviour of the studied catalysts.     

Direct Oxidation of H2 to H2O2 and Decomposition of H2O2 Over Oxidized and Reduced Pd-Containing Zeolite Catalysts in Acidic Medium

Both the conversion and H₂O₂ selectivity (or yield) in direct oxidation of H₂-to-H₂O₂ (using 1.7 mol% H₂ in O₂ as a feed) and also the H₂O₂ decomposition over zeolite (viz. H-ZSM-5, H-GaAlMFI and H- ) supported palladium catalysts (at 22 °C and atmospheric pressure) are strongly influenced by the zeolite support and its fluorination, the reaction medium (viz. pure water, 0.016 M or 1.0 M NaCl solution or 0.016 M H₂SO₄, HCl, HNO₃, H₃PO₄ and HClO₄), and also by the form of palladium (Pd⁰ or PdO). The oxidized (PdO-containing) catalysts are active for the H₂-to-H₂O₂ conversion and show very poor activity for the H₂O₂ decomposition. However, the reduced (Pd⁰-containing) catalysts show higher H₂ conversion activity but with no selectivity for H₂O₂, and also show much higher H₂O₂ decomposition activity. No direct correlation is observed between the H₂-to-H₂O₂ conversion activity (or H₂O₂ selectivity) and the Pd dispersion or surface acidity of the catalysts. Higher H₂O₂ yield and lower H₂O₂ decomposition activity are, however, obtained when the non-acidic reaction medium (water with or without NaCl) is replaced by the acidic one.     

Direct formation of H2O2 from H2 and O2 and decomposition/hydrogenation of H2O2 in aqueous acidic reaction medium over halide-containing Pd/SiO2 catalytic system

Formation of H₂O₂ from H₂ and O₂ and decomposition/hydrogenation of H₂O₂ have been studied in aqueous acidic medium over Pd/SiO₂ catalyst in presence of different halide ions (viz. F⁻, Cl⁻ and Br⁻). The halide ions were introduced in the catalytic system via incorporating them in the catalyst or by adding into the reaction medium. The nature of the halide ions present in the catalytic system showed profound influence on the H₂O₂ formation selectivity in the H₂ to H₂O₂ oxidation over the catalyst. The H₂O₂ destruction via catalytic decomposition and by hydrogenation (in presence of hydrogen) was also found to be strongly dependent upon the nature of the halide ions present in the catalytic system. Among the different halides, Br⁻ was found to selectivity promote the conversion of H₂ to H₂O₂ by significantly reducing the H₂O₂ decomposition and hydrogenation over the catalyst. The other halides, on the other hand, showed a negative influence on the H₂O₂ formation by promoting the H₂ combustion to water and/or by increasing the rate of decomposition/hydrogenation of H₂O₂ over the catalyst. An optimum concentration of Br⁻ ions in the reaction medium or in the catalyst was found to be crucial for obtaining the higher H₂O₂ yield in the direct synthesis.     

Reduction of oxygen by hydroxylammonium salt or hydroxylamine over supported Au nanoparticles for in situ generation of hydrogen peroxide in aqueous or non-aqueous medium

Reaction of O₂ with hydroxylamine or its salts over a number of supported gold catalysts containing Au nanoparticles (at 10–70 °C) has been studied at atmospheric pressure for the in situ generation of H₂O₂ (required for organic oxidation reactions in the synthesis of fine/specialty chemicals) in aqueous (water) or non-aqueous medium. Hydrogen peroxide in high yields with harmless by-products (viz. water and nitrogen) can be generated in situ by the reduction of O₂ by hydroxylammonium sulfate (or chloride) or hydroxylamine using the supported gold catalysts particularly Au/Gd₂O₃, Au/La₂O₃ and Au/MgO, in aqueous (water) or non-aqueous (viz. methanol) medium at close to ambient conditions. The reduction of O₂ by hydroxylammonium salt to H₂O₂, however, requires preneutralization of the salt by alkali; in the absence of the neutralization, only water is formed in the reaction.     

Direct H2-to-H2O2 oxidation over highly active/selective Br–F–Pd/Al2O3 catalyst in aqueous acidic medium: Influence of process conditions on the H2O2 formation

The influence of the O₂/H₂ mole ratio in the gaseous feed and also those of other reaction conditions [viz. concentration of H₃PO₄ (0–5 mol/dm³), temperature (5–50 °C), gas (H₂ and O₂) space velocity (5.8–23.4 h^?1) and reaction time (0.1–8 h)] on the H₂O₂ formation in the H₂-to-H₂O₂ oxidation over the Br(1 wt%)–F(1 wt%)–Pd(5 wt%)/Al₂O₃ catalyst in an aqueous acidic (H₃PO₄) medium have been thoroughly investigated. The effects of the O₂/H₂ ratio, reaction temperature and acid concentration on the destruction of H₂O₂ by its decomposition and/or hydrogenation reactions over the catalyst in the acidic reaction medium have also been studied. The net H₂O₂ formation (H₂O₂ yield) over the catalyst passed through a maximum with increasing the acid concentration, the temperature or the O₂/H₂ feed ratio. However, it decreased markedly with increasing the gas space velocity or the reaction period. The H₂O₂ decomposition and hydrogenation activities of the catalyst increased appreciably with increasing the reaction temperature and decreased with increasing the acid concentration. The H₂O₂ destruction during the H₂-to-H₂O₂ oxidation increased with increasing the concentration of H₂ (relative to that of O₂) due to the increased H₂O₂ hydrogenation rate over the catalyst. The net H₂O₂ formation in the H₂-to-H₂O₂ oxidation decreased sharply with increasing the initial amount of H₂O₂ present in the reaction mixture. The presence of H₂O₂ and the higher H₂/O₂ ratios have detrimental effects on the net formation of H₂O₂.     

Color-tunable up-conversion luminescence of Zn(AlxGa1-x)2O4:Yb3+,Tm3+,Er3+ powders

Color-tunable up-conversion powder phosphors Zn(Al_xGa_1-x)₂O₄: Yb³⁺,Tm³⁺,Er³⁺ were synthesized via high temperature solid-state reaction. Also, the morphological and structural characterization, up-conversion luminescent properties were all investigated in this paper. In brief, under the excitation of a 980?nm laser, all powders have same emission peaks containing blue emission at 477?nm (attributed to ¹G₄→³H₆ transition of Tm³⁺ ions), green emission at 526?nm and 549?nm (attributed to ²H_11/2→ ⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transition of Er³⁺ ions respectively), red emission at about 659?nm and 694?nm (attributed to ⁴F_9/2→⁴I_15/2 transition of Er³⁺ ions and ³F₃→³H₆ transition of Tm³⁺ ions, respectively), which are not changed after the doping of Al³⁺ ions. However, the doping of Al³⁺ ions can enhance the up-conversion luminescent intensity and efficiency, while the emission color of as-prepared powder phosphors can be tunable by controlling the doping amount of Al³⁺ ions. Taking Zn(Al_0.5Ga_0.5)₂O₄:Yb,Tm,Er as the cut-off value, the emissions have clear blue-shift firstly and then show obvious red-shift with the increasing doping of Al³⁺ ions. Stated thus, pink emission in ZnAl₂O₄:Yb,Tm,Er, purplish pink emission in ZnGa₂O₄:Yb,Tm,Er and Zn(Al_0.9Ga_0.1)₂O₄:Yb,Tm,Er, purple emission in Zn(Al_0.1Ga_0.9)₂O₄:Yb,Tm,Er and Zn(Al_0.3Ga_0.7)₂O₄:Yb,Tm,Er, purplish blue emission in Zn(Al_0.7Ga_0.3)₂O₄:Yb,Tm,Er, blue emission in Zn(Al_0.5Ga_0.5)₂O₄:Yb,Tm,Er can be observed, which confirm the potential applications of as-prepared Zn(Al_xGa_1-x)₂O₄:Yb³⁺,Tm³⁺,Er³⁺ powder phosphors in luminous paint, infrared detection and so on.     

TWO-PHASE WITTIG REACTION OF SYNTHESIZING LIQUID CRYSTAL INTERMEDIATE BY ACTIVE QUATERNARY PHOSPHONIUM SALT

In this work, (4-methyl)methyl benzoatyltriphenylphosphonium bromide (BrPPh₃CH₂C₆H₄COOCH₃, MBPB), which was prepared from the reaction of (4-bromomethyl)methyl benzoate BrCH₂(C₆H₄)COOCH₃ and triphenyl phosphine (PPh₃) in a CH₂Cl₂/H₂O two-phase medium in the laboratory, acted as the active reagent to synthesize methyl {4-[4-(nonyloxy)-styryl]} benzoate in Z- and E-form (n-C₉H₁₉O(C₆H₄)CH=CH(C₆H₄)COOCH₃, MNSB). The product is a useful liquid crystal intermediate used in industries. Through the active reagent, MNSB is synthesized from the reaction of 4-nonyloxybenzoic aldehyde (n-C₉H₁₉O(C₆H₄)CHO, NAOD) in an alkaline solution of NaOH/organic solvent two-phase medium. The reaction is largely enhanced in the presence of MBPB compound as the active reagent in the Wittig reaction. Under most reaction conditions, this reaction is favorable for the production of E-form MNSB product rather than forming the Z-form MNSB product. A kinetic model was developed and a pseudo-first-order rate law was found to be sufficient to express the kinetic behaviors of the reaction. Effects of the reaction conditions, including the agitation speed, the temperature, the reactant concentration, the organic solvents, the sodium hydroxide concentration, and the 4-nonyloxybenzoic aldehyde concentration on the conversion of NAOD and the reaction rate were investigated in detail. Rational explanations to the observed results have been made.     

Oxidation of cyclopentene catalyzed by phosphotungstic quaternary ammonium salt catalysts

A series of phosphotungstic quaternary ammonium salts, Q₃ (PW₁₂O₄₀) and Q₃(PW₄O₁₆) [Q = (C₅H₅)N⁺(C₁₆H₃₃), (C₁₆H₃₃)N⁺(CH₃)₃, (C₄H₉)₄N⁺, and (CH₃)₄N⁺], were used as the catalysts in oxidation of cyclopentene. The catalysts [(C₅H₅)N(C₁₆H₃₃)]₃(PW₄O₁₆) and [(C₁₆H₃₃)N(CH₃)₃]₃(PW₄O₁₆) showed high catalytic activity in the selective oxidation of cyclopentene while using H₂O₂ (50%) as an oxidant and 2-propanol as a solvent. The oxidation products mainly consisted of glutaraldehyde, cis-1,2-cyclopentanediol and trans-1,2-cyclopentanediol. The above-mentioned two catalysts were dissolved completely in the reaction medium during the catalysis process and precipitated themselves from the reaction system after reaction, showing the characteristics of reaction-controlled phase-transfer catalysis. The types of quaternary ammonium cations and the phosphotungstic anions in phosphotungstic quaternary ammonium salts affected catalytic activity.     

Reaction products of triammonium pyrophosphate in different Indian soils

Studies were undertaken on the isolation and identification of reaction products of triammonium pyrophosphate (TPP), the major non-orthophosphate constituent of ammonium polyphosphate newly introduced in India, in representative soils of the alfisol, oxisol, entisol, mollisol and vertisol groups of India. Saturated solution of TPP were reacted with soils for periods of 30 minutes and one day with corresponding precipitation times of 15 days, three months and one year to isolate reaction products which were identified by X-ray diffraction technique, infra-red spectroscopy and chemical analyses. Six reaction products, namely, Ca(NH₄)₂P₂O₇ · H₂O, Mg(NH₄)₂P₂O₇ · 4H₂O, Ca(NH₄)₄H₂(P₂O₇)₂, Ca₃(NH₄)₄H₆(P₂O₇)₄·3H₂O, FeNH₄P₂O₇ and NH₄Al_0·33 Fe_0·67P₂O₇ were identified in different soils; Ca(NH₄)₂P₂O₇·H₂O and Mg(NH₄)₂P₂O₇ · 4H₂O occurring in abundance in most soils. Significant hydrolytic degradation of pyrophosphate reaction products to orthophosphate was not observed.Complementary studies where TPP in solid form was applied to soil, and reaction formed at and around the site of TPP placement were identified after six weeks incubation also showed the formation of Ca(NH₄)₂P₂O₇ · H₂O and Mg(NH₄)₂P₂O₇ · 4H₂O in the soils examined.     

ION EXCHANGE PROPERTIES OF BiO(NO3) 0.5H2O TOWARDS FLUORIDE IONS

ABSTRACT

We studied the ion exchange behavior of the inorganic anion exchanger BiO(NO₃)0.5H₂O with regard to fluoride ions. The ion exchange reaction was rapid at pH 1, 6.6, and 12. The mechanisms of ion exchange reactions at pH 1 and pH 2-12 were studied in a solution with fluoride ions excess to BiO(NO₃)0.5H₂O. A mixture of the β-phase and an unknown phase was produced in the solution at pH 1. BiOF was produced at pH 2-12. Fluoride ions did not react at pH 13, due to the decomposition of BiO(NO₃) 0.5H₂O at pH 13 to yield Bi₂O₃ (major) and Bi₂O₂CO₃ (minor). The structure of the reaction products depended on the solution pH, mole ratios of BiO(NO₃)0.5H₂O to F’, and the reaction time. We observed that BiO(NO₃)0.5H₂O is capable of removing 99% of the fluoride ions from the solution at pH 1-12 under optimal conditions. The ion exchange reaction of BiO(NO₃)0.5H₂O with fluoride ions was studied under the co-existence of both Cl and Br^?at pH 1, 6.6, 12, and 13. The order of decreasing affinity was found to be (Br^?, Cl^?)> F^?. The reaction product was not a simple mixture of BiOCl, BiOBr, and bismuth oxide fluorides, but an unknown compound.     

Graft copolymerization of polyfurfuryl alcohol and cellulose

Viscosity of polyfurfuryl alcohol (PFA) and addition of hydroquinone exert a strong positive effect on grafting polyfurfuryl alcohol to cellulose fiber with H₂O₂/Fe²⁺ oxidant in an aqueous slurry at pH 2. Composition of the atmosphere and the nature of acids used to adjust pH does not influence the reaction, with the exception of phosphoric acid, which negatively influences polymer loading. Conducting the reaction in nonaqueous media inhibits grafting. K₂S₂O₈, Ce(SO₄)₂, and K₂Cr₂O₇ perform as well as H₂O₂/Fe²⁺, whereas H₂O₂/thiourea dioxide, H₂O₂/thiourea, NH₄VO₃, KMnO₄, and KNO₃ give inferior polymer loads; Mn³⁺/acetylacetonate and azobisisobutyronitrile are ineffective. A small improvement in polymer loading is obtained by gradual addition of the oxidant to the reaction slurry.     

Mixed Valence Compounds of Vanadium II. Properties and Formation Mechanism

A second mixed valence compound of vanadium was obtained by decomposition of vanadyl (IV)bis (acetylacetonate) by fluoride ions in a nonpolar solvent. The compound has the formula [(C₂H₅)₄N]₄[V₁₀O₂₆F₂H₂] i.e. two oxygens of the previous compound [(C₂H₅)₄N]₄[V₁₀O₂₈H₄] were replaced by fluoride ions. The thermal stability, electronic and ESR spectra are described. The properties of this compound are very similar to the compound obtained by addition of a halide ion (chloride, bromide or iodide) to vanadyl(IV) bis (acetylacetonate) in the presence of another metal complex. The mechanism of formation of these compounds is discussed.     

NUV light induced visible emission in Er3+-activated NaSrLa(MoO4)O3 phosphors for green LEDs and thermometer

Er³⁺-activated NaSrLa(MoO₄)O₃ phosphors were synthesized by a traditional solid-state reaction technique, which exhibited bright green emissions ascribing to the (²H_11/2, ⁴S_3/2) → ⁴I_15/2 transitions of Er³⁺ ions under 377 nm excitation. The luminescence intensity increased with increasing the Er³⁺ ion concentration and achieved its maximum value when the doping concentration was 4 mol%. Moreover, the critical distance was estimated to be 25.32 Å, and the dipole-dipole interaction played a significant role in NR energy transfer between Er³⁺ ions in NaSrLa(MoO₄)O₃ host lattices. At a forward bias current of 100 mA, the Light Emitting Diode (LED) device emitted a bright green emission with the color coordinate of (0.2547, 0.5996) that can be observed by the naked eye. Besides, based on the thermally coupled levels of ²H_11/2 and ⁴S_3/2, the temperature sensing performances of the prepared phosphors in the temperature range of 303-483 K were studied using the fluorescence intensity ratio technique. The maximum sensor sensitivity was about 0.0150 K⁻¹ when the temperature was 483 K, and the Er³⁺ ion concentration largely influenced the sensor sensitivity of studied samples. Furthermore, the prepared phosphors exhibited excellent water resistance and thermal stability behavior. These characteristics demonstrated that the Er³⁺ activated NaSrLa(MoO₄)O₃ phosphors were dual-functional materials for solid-state illumination and non-contact temperature measurement.     

A novel 12-molybdovanadate nanocluster: Synthesis,structure investigation and its application as an efficient heterogeneous sulfoxidation catalyst

Tetraethylammonium salt of a new Keggin-type 12-molybdovanadate nanocluster, [(C₂H₅)₄N]₄[VMo₁₂O₄₀] (1) was synthesized via reaction between sodium tungstate, ammonium vanadate and tetraethylammonium bromide in acidic medium. Compound 1 was characterized by X-ray crystallography, FT-IR, UV–Vis and cyclic voltammetry and then applied as an efficient heterogeneous catalyst to oxidation of various organosulfides to sulfoxides with H₂O₂ at room temperature with 81–100% conversion and 60–99% selectivity. Nanocluster 1 was also shown to display excellent recyclability – it can be reused more than 10 times.     

Influence of surfactants on the electroreduction of oxygen to hydrogen peroxide in acid and alkaline electrolytes

The effect of surfactants on the electroreduction of O₂ to H₂O₂ was investigated by cyclic voltammetry and batch electrolysis on vitreous carbon electrodes. The electrolytes were either 0.1 M Na₂CO₃ or 0.1 M H₂SO₄ at 295 K, under 0.1 MPa O₂. Electrode kinetics and mass transport parameters showed the influence of surfactants on the O₂ electroreduction mechanism. The cationic surfactant (Aliquat 336^®, tricaprylmethylammonium chloride), at mM levels, increased the standard rate constant of O₂ electroreduction to H₂O₂ 15 times in Na₂CO₃ and 1900 times in H₂SO₄, to 1.8 × 10^–6 m s^–1 and 9.9 × 10^–10 m s^–1, respectively. This effect on the reaction rate might be due to an increase of the surface pH, induced by the Aliquat 336^® surface film. The nonionic (Triton X-100) and anionic (sodium dodecyl sulfate) surfactants retarded the O₂ electroreduction, presumably by forming surface structures, which blocked the access of O₂ to the electrode. Ten hour batch electrosynthesis experiments performed at 300 A m^–2 superficial current density, 0.1 MPa O₂, 300 K, on reticulated vitreous carbon (30 ppi), showed that compared to the values obtained in the absence of surfactant, mM concentrations of Aliquat 336^® increased the current efficiency for peroxide from 12% to 61% (0.31 M H₂O₂) in 0.1 M Na₂CO₃ and from 14% to 55% (0.26 M H₂O₂) in 0.1 M H₂SO₄, respectively.     

The Nature of Active Sites of Co/Al2O3 for the Selective Catalytic Reduction of NO with C2H4

The effects of Co loading and calcination temperatures on the catalytic activity of Co/Al₂O₃ for selective catalytic reduction (SCR) of NO with ethylene in excess oxygen were investigated. Co/Al₂O₃ showed high and low activities when calcined at high (800 °C) and low (350 °C) temperatures, respectively. The formation and dispersion of cobalt species for catalysts calcined at 350 and 800 °C as well as for Al₂O₃ were studied by XRD, UV–vis and FTIR spectra. Combined with DRIFTS results of ad-species and reaction experiments, it allowed us to correlate the catalytic activity with active sites of Co/Al₂O₃, and the catalytic functions of active cobalt species and support were clarified. Co₃O₄ species contributed to the oxidation of NO to various nitrates and of C₂H₄ to reactive formate species, even in the absence of O₂, whereas the side reaction of ethylene combustion occurred simultaneously when excess oxygen was present. Tetrahedral Co²⁺ ions in CoAl₂O₄, which acted as the active sites, were responsible for the reaction between formate and nitrate species to form organic nitro compound.     

Effect of reductants in N2O reduction over Fe-MFI catalysts

We investigated the effect of reductants over ion-exchanged Fe-MFI catalysts (Fe-MFI) based on the catalytic performance in N₂O reduction in the presence and absence of an oxygen atmosphere. In the case of N₂O reduction with hydrocarbons (CH₄, C₂H₆, and C₃H₆) in the presence of excess oxygen, the order of N₂O contribution was as follows: CH₄ > C₂H₆ > C₃H₆. This indicates that CH₄ is a more efficient reductant than C₂H₆ and C₃H₆. The TOFs of N₂O decomposition and the N₂O reduction by various reductants (H₂, CO, CH₄) in the absence of oxygen increased with increasing Fe/Al ratio (Fe/Al⩾0.15), wheras the TOFs were lower and constant in the range of Fe/Al⩽0.10. Temperature-programmed reduction with hydrogen (H₂-TPR) showed that the catalysts with a higher Fe/Al ratio were reduced more easily than those with a lower Fe/Al ratio. Temperature-programmed desorption of O₂ (O₂-TPD) showed that oxygen was desorbed at lower temperatures over the catalysts with a higher Fe/Al ratio. As the result of extended X-ray absorption fine structure (EXAFS) analysis, only mononuclear Fe species were observed over Fe(0.10)-MFI after treatment with N₂O or O₂. On the other hand, binuclear Fe species and mononuclear Fe species were observed over Fe(0.40)-MFI after treatment with N₂O or H₂. More reducible Fe species, which gave lower-temperature O₂ desorption, can be due to Fe binuclear species. Since the N₂O reduction with reductants proceeds via a redox mechanism, the reducible binuclear Fe species can exhibit higher activity. Furthermore, CH₄ can be oxidized by N₂O more easily than can H₂ and CO, although it is generally known that the reactivity of methane is very low.     

NO reduction by CH4over La2O3: temperature‐programmed reaction and in situ DRIFTS studies

The chemistry between NO _x species adsorbed on La₂O₃ and CH₄ was probed by temperature‐programmed reaction (TPR) as well as in situ DRIFTS. During NO reduction by CH₄ in the presence of O₂, NO ₃ ^- does not appear to activate CH₄, thus either an adsorbed O species or an NO ₂ ^- species is more likely to activate CH₄. In the absence of O₂, a different reaction pathway occurs and NO^- or (N₂O₂)^2- species adsorbed on oxygen vacancy sites seem to be active intermediates, and during NO reduction with CH₄ unidentate NO ₃ ^- , which desorbs at high temperature, behaves as a spectator species and is not directly involved in the catalytic sequence. Because reaction products such as CO₂ or H₂O as well as adsorbed oxygen cannot be effectively removed from the surface at lower temperatures, steady‐state catalytic reactions can only be achieved at temperatures above 800 K, even though formation of N₂ and N₂O from NO was observed at much lower temperature during the TPR experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selectivity-determining role of C3H8/NO ratio in the reduction of nitric oxide by propane in presence of oxygen over ZSM5 zeolites

The reaction of (NO + C₃H₈ + O₂) can result in selective formation of NO₂ over H-ZSM5, Cu,H-ZSM5, Ag,H-ZSM5, and Li,H-ZSM5 catalysts when the concentrations of NO and O₂ are 0.1 and 9%, SV > 60,000 h⁻¹ (typical for automotive exhausts), and C₃H₈/NO > 1. Despite stoichiometric excess of reductant hydrocarbon below this limit, the in situ formed NO₂ does not react with C₃H₈, thus conversion of NO to N₂ is negligible. NO can be reduced by C₃H₈ selectively to N₂ only when C₃H₈/NO ≧ 1. Contrary to many suggestions the reaction temperature, concentration of oxygen, space velocity, and type of exchange ions have minor influence on the selectivity for N₂. These parameters affect the rates of reactions (NO + ₂), (C₃H₈ + NO_x) and (C₃H₈ + O₂), therefore they also affect the production of N₂ in the HC-SCR process, but only when the ratio of C₃H₈/NO permits. The metal-exchanged zeolites were prepared in situ by solid-state ion exchange from H-ZSM5. Despite the low degree of copper exchange (63%), Cu,H-ZSM5 produces substantially more N₂ than H-ZSM5, Ag,H-ZSM5, or Li,H-ZSM5. However, the selectivity for N₂ is lowest over Cu,H-ZSM5, which also produces considerable NO₂ in the reaction of (NO + C₃H₈ + O₂) even at C₃H₈/NO ≧ 1. Contrary to prior findings, the catalytic activity of Cu,H-ZSM5 for the oxidation of NO by O₂ to NO₂ in absence of hydrocarbon was comparable to that of H-ZSM5 at high space velocities (2.3 l g⁻¹ min⁻¹). By replacing 30 and 40% of the protons of H-ZSM5 by Ag⁺ and Li⁺ ions in Ag,H-ZSM5 and Li,H-ZSM5, respectively, the catalytic activity for this reaction becomes negligible at temperatures ≧100°C. Some mechanistic consequences of these experimental observations are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.