Kinetic evaluation of mechanistic models for O2 release from ZSM-5-supported [Cu2+ –O–Cu2+] ions by thermal reduction or chemical interaction with impinging N2O molecules

For a series of oxidized Cu-ZSM-5 catalysts which were characterized in the catalytic amounts of the oxygen-bridged Cu²⁺-dimers, [Cu²⁺–O–Cu²⁺], activation energies required for the reduction of the Cu²⁺-dimer species by O₂ release were determined using the temperature-programmed experiments of thermal O₂ desorption (TPD) and N₂O decomposition reaction. The activation energy for the thermal reduction of the Cu²⁺-dimers during the TPD decreased linearly with increasing molar number of the Cu²⁺-dimers available on the ZSM-5, suggesting that the energy barrier of the O₂ formation via a Langmuir-Hinshelwood (LH) mechanism increased in proportion to the distance between the two Cu²⁺-dimers in the nearest neighbor. Activation energies of thermal O₂ release were comparable to the literature-reported binding energies of the differently spaced Cu²⁺-dimers. It was also revealed that the activation energy of O₂ release during the temperature programmed N₂O decomposition reaction over an oxidized catalyst was generally low as compared to that in the TPD, and that the degree of reduction of the Cu²⁺-dimers was much greater in the N₂O decomposition reaction than in the TPD at the same temperatures. These beneficial effects N₂O decomposition on the reduction of the Cu²⁺-dimers were discussed in respect of the removal mechanism of the Cu²⁺-dimer bridged oxygen.     

Highly Efficient Fe-silicalite Zeolite in Direct Propane Ammoxidation with N2O and O2

A novel process for the direct ammoxidation of propane over steam-activated Fe-silicalite at 723–823 K is reported. Yields of acrylonitrile (ACN) and acetonitrile (AcCN) below 5% were obtained using N₂O or O₂ as the oxidant. Co-feeding N₂O and O₂ boosts the performance of Fe-silicalite compared to the individual oxidants, leading to AcCN yields of 14% and ACN yields of 11% (propane conversions of 40% and products selectivity of 25–30%). The beneficial effect of O₂ on the propane ammoxidation with N₂O contrasts with other N₂O-mediated selective oxidations over iron-containing zeolites (e.g. hydroxylation of benzene and oxidative dehydrogenation of propane), where a small amount of O₂ in the feed dramatically reduces the selectivity to the desired product. It is shown that the productivity of ACN and especially AcCN, expressed as mol product h⁻¹ kg_cat⁻¹, is significantly higher over Fe-silicalite than over active propane ammoxidation catalysts reported in the literature. Our results open new perspectives to improve the performance of alkane ammoxidation catalysts.     

The photocatalytic reduction of nitrous oxide with propane on lead(II) ion-exchanged ZSM-5 catalysts

UV irradiation of the Pb²⁺/ZSM-5 catalyst prepared by an ion-exchange method in the presence of N₂O leads to the decomposition of N₂O into N₂. This reaction is found to be dramatically enhanced by the addition of propane to produce N₂ and oxygen-containing compounds such as ethanol or acetone. UV light effective for the reaction lies in wavelength regions shorter than 250 nm where the absorption band of the Pb²⁺ ion ([Xe] 4f¹⁴5d¹⁰6s² [Xe] 4f¹⁴5d¹⁰6s¹6p¹) exists, indicating that the excited state of the isolated Pb²⁺ ions plays a significant role in this decomposition of N₂O both in the absence and the presence of propane, and the role of propane is found to be a capture of oxygen atoms formed by the decomposition of N₂O.     

The Temperature-Programmed Desorption of Oxygen from an Alumina-Supported Silver Catalyst

The associative desorption kinetics of O₂ from a 15 wt% Ag/-Al₂O₃ atalyst were studied under atmospheric pressure in a microreactor set-up by performing temperature-programmed desorption (TPD) experiments. Saturation with chemisorbed atomic oxygen (O*) was achieved by dosing O₂ for 1 h at 523 K and at atmospheric pressure followed by cooling in O₂ to room temperature. The TPD spectra showed almost symmetric O₂ peaks centred above 500 K, indicating associative desorption of O₂ from Ag metal surface sites. By varying the heating rates from 2 to 20 K min^-1, the O₂ TPD peak maxima were found to shift from 508 to 542 K, respectively. A microkinetic analysis of these TPD traces yielded an activation energy for desorption of 149±2 kJ mol^-1 and a corresponding pre-exponential factor of 2×10¹²±1×10¹² s^-1 in good agreement with the kinetic parameters reported for O₂ desorption under UHV conditions from Ag(111) and Ag(110) single crystal surfaces.     

Adsorbate-assisted NO decomposition in NO reduction by C3H6 over Pt/Al2O3 catalysts under lean-burn conditions

The rates and product selectivities of the C₃H₆-NO-O₂ and NO-H₂ reactions over a Pt/Al₂O₃ catalyst, and of the straight, NO decomposition reaction over the reduced catalyst have been compared at 240C. The rate of NO decomposition over the reduced catalyst is seven times greater than the rate of NO decomposition in the C₃H₆-NO-O₂ reaction. This is consistent with a mechanism in which NO decomposition occurs on Pt sites reduced by the hydrocarbon, provided only that at steady state in the lean NO _x reaction about 14% of the Pt sites are in the reduced form. However, the (extrapolated) rate of the NO-H₂ reaction at 240C is about 10⁴ times faster than the rate of the NO decomposition reaction thus raising the possibility that NO decomposition in the former reaction is assisted by H_ads. It is suggested that adsorbate-assisted NO decomposition in the C₃H₆-NO-O₂ reaction could be very important. This would mean that the proportion of reduced Pt sites required in the steady state would be extremely small. The NO decomposition and the NO-H₂ reactions produce no N₂O, unlike the C₃H₆-NO-O₂ reaction, suggesting that adsorbed NO is completely dissociated in the first two cases, but only partially dissociated in the latter case. It is possible that some of the associatively adsorbed NO present during the C₃H₆-NO-O₂ reaction may be adsorbed on oxidised Pt sites.     

Comparative IR diffuse reflectance study of fundamental C=O stretching and combination C=O plus M-C stretching vibrational modes of CO linearly adsorbed on Ir/Al2O3 catalyst

Incorporation of ZrO₂ into a solid solution with CeO₂ strongly enhances the reducibility of the Ce⁴⁺ in the metallized samples and favours an effective NO decomposition over the reduced catalysts.     

Research in Nanosciences – Great Opportunity for Catalysis Science

Au/MFI was prepared by sublimation of AuCl₃ onto HMFI. The oxidation state of the gold in the as-synthesized sample is Au³⁺. Upon heating in He or O₂, it transforms into gold metal and electron-deficient gold particles which agglomerate to larger Au particles. Reduction of Au³⁺ with CO to Au⁺ is accompanied by formation of an Au⁺(CO) complex. The carbonyl ligand has a remarkable stabilization effect on Au⁺ ions in zeolite cages. Strong IR bands show the CO vibration, and also the strong perturbation of the T–O–T vibrations of the zeolite lattice. [Au⁺(CO)]/MFI has a characteristic XRD pattern and is stable up to 200°C. Au/MFI catalyzes the decomposition of N₂O to N₂ even in the presence of 3% O₂. With hydrocarbons or NH₃ as the reductant, it has some NO_x reduction activity, but these catalysts deactivate at higher temperature even in inert gas atmosphere.     

SO2 and NO removal from flue gas over V2O5/AC at lower temperatures — role of V2O5 on SO2 removal

Supporting V₂O₅ onto an activated coke (AC) has been reported to significantly increase the AC's activity in simultaneous SO₂ and NO removal from flue gas. To understand the role of V₂O₅ on SO₂ removal, V₂O₅/AC is studied through SO₂ removal reaction, surface analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) techniques. It is found that the main role of V₂O₅ in SO₂ removal over V₂O₅/AC is to catalyze SO₂ oxidation through a VOSO₄-like intermediate species, which reacts with O₂ to form SO₃ and V₂O₅. The SO₃ formed transfers from the V sites to AC sites and then reacts with H₂O to form H₂SO₄. At low V₂O₅ loadings, a V atom is able to catalyze as many as 8 SO₂ molecules to SO₃. At high V₂O₅ loadings, however, the number of SO₂ molecules catalyzed by a V atom is much less, due possibly to excessive amounts of V₂O₅ sites in comparison to the pores available for SO₃ and H₂SO₄ storage.     

Gas transport properties of 6FDA-TAPOB hyperbranched polyimide membrane

Physical and gas transport properties of the hyperbranched polyimide prepared from a triamine, 1,3,5-tris(4-aminophenoxy)benzene (TAPOB), and a dianhydride, 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), were investigated and compared with those of linear-type polyimides with similar chemical structures prepared from diamines, 1,4-bis(4-aminophenoxy)benzene (TPEQ) or 1,3-bis(4-aminophenoxy)benzene (TPER), and 6FDA. 6FDA-TAPOB hyperbranched polyimide exhibited a good thermal stability as well as linear-type analogues. Fractional free volume (FFV) value of 6FDA-TAPOB was higher than those of the linear-type analogues, indicating looser packing of molecular chains attributed to the characteristic hyperbranched structure. It was found that increased resistance to the segmental mobility decreases the gas diffusivity of 6FDA-TAPOB, in spite of the higher FFV value. However, 6FDA-TAPOB exhibited considerably high gas solubility, resulting in high gas permeability. It was suggested that low segmental mobility and unique size and distribution of free volume holes arising from the characteristic hyperbranched structure of 6FDA-TAPOB provide effective O₂/N₂ selectivity. It is concluded that the 6FDA-TAPOB hyperbranched polyimide has relatively high permeability and O₂/N₂ selectivity, and is expected to apply to a high-performance gas separation membrane.     

Spinels as cathodes for the electrochemical reduction of O2 and NO

Spinels were synthesised and investigated as electro-catalyst for the electrochemical reduction of oxygen and nitric oxide using cyclic voltammetry and cone shaped electrodes. The following four spinels were investigated; CoFe₂O₄, NiFe₂O₄, CuFe₂O₄ and Co₃O₄. The composition CuFe₂O₄ revealed the largest difference in activity between reduction of oxygen and the reduction of nitric oxide, the activity being highest for the reduction of nitric oxide. The material is probably not stable when polarised cathodically. However it seems that the electrode material can be regenerated upon oxidation. NiFe₂O₄ is also more active for the reduction of nitric oxide than for the reduction of oxygen, whereas the cobalt containing spinels have a higher activity for the reduction of oxygen than for the reduction of nitric oxide.     

Investigation of flue-gas treatment with O3 injection using NO and NO2 planar laser-induced fluorescence

Direct ozone (O₃) injection is a promising flue-gas treatment technology based on oxidation of NO and Hg into soluble species like NO₂, NO₃, N₂O₅, oxidized mercury, etc. These product gases are then effectively removed from the flue gases with the wet flue gas desulfurization system for SO₂. The kinetics and mixing behaviors of the oxidation process are important phenomena in development of practical applications. In this work, planar laser-induced fluorescence (PLIF) of NO and NO₂ was utilized to investigate the reaction structures between a turbulent O₃ jet (dry air with 2000 ppm O₃) and a laminar co-flow of simulated flue gas (containing 200 ppm NO), prepared in co-axial tubes. The shape of the reaction zone and the NO conversion rate along with the downstream length were determined from the NO-PLIF measurements. About 62% of NO was oxidized at 15d (d, jet orifice diameter) by a 30 m/s O₃ jet with an influence width of about 6d in radius. The NO₂ PLIF results support the conclusions deduced from the NO-PLIF measurements.     

N2O decomposition over [Fe]-ZSM-5 and Fe-HZSM-5 zeolites

The N₂O decomposition over an [Fe]-ZSM-5 and an Fe-HZSM-5 zeolite was studied. We found that framework incorporated iron species were much more active than Fe(III) introduced as framework charge countercations by ion exchange (TOF at 0.1 vol% N₂O:1.47 × 10^–4 at 280°C for [Fe]-ZSM-5 vs. 2.58 × 10^–4 at 468°C for Fe-HZSM-5). The higher activity of [Fe]-ZSM-5 was attributed to the uniqueness of framework iron species. Both [Fe]-ZSM-5 and Fe-HZSM-5 zeolites showed enhanced activity in the presence of excess oxygen. This is in sharp contrast to ruthenium exchanged zeolites which showed strong oxygen inhibiting effect on the rate of N₂O decomposition.     

Possible role of nitrite/nitrate redox cycles in N2O decomposition and light-off over Fe-ZSM-5

Surface nitrite/nitrate redox cycles were proposed to explain light-off behavior that was observed during the decomposition of N₂O over Fe-ZSM-5. Further study has demonstrated that while the nitrite/nitrate model can explain the original observations as an isothermal, mechanistic phenomenon, the light-off behavior is thermal, and not a mechanistic effect. Nonetheless, a pathway involving nitrite/nitrate redox cycles appears to be more consistent with experimental observation than the simple two-step pathway involving cation redox cycles. In particular, the nitrite/nitrate pathway can explain the effect of added NO upon the reaction kinetics and the reported isotopic product composition when unlabeled N₂O reacts over an oxygen-labeled catalyst. Further, a nitrite/nitrate pathway is consistent with the steady-state kinetics as well as published thermal desorption and infrared spectroscopic results.     

Isotopic Study of N2O Decomposition on an Ion-Exchanged Fe-Zeolite Catalyst: Mechanism of O2 Formation

N₂O decomposition on an ion-exchanged Fe-MFI catalyst has been studied using an ¹⁸O-tracer technique in order to reveal the reaction mechanism. N₂ ¹⁶O was pulsed onto an ¹⁸O₂-treated Fe-MFI catalyst at 693 K, and the O₂ molecules produced were monitored by means of mass spectrometry. The ¹⁸O fraction in the produced oxygen had almost half the value of that on the surface oxygen, and ¹⁸O¹⁸O was not detected. The result shows that O₂ formation proceeds via the Eley–Rideal mechanism (N₂ ¹⁶O + ¹⁸O(a) N₂ + ¹⁶O¹⁸O).     

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.     

Reduction of NO by CO on Cu/ZrO2/Al2O3 catalysts: Characterization and catalytic activities

ZrO₂, γ-Al₂O₃ and ZrO₂/γ-Al₂O₃-supported copper catalysts have been prepared, each with three different copper loads (1, 2 and 5 wt%), by the impregnation method. The catalysts were characterized by nitrogen adsorption (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR) with H₂, Raman spectroscopy and electronic paramagnetic resonance (EPR). The reduction of NO by CO was studied in a fixed-bed reactor packed with these catalysts and fed with a mixture of 1% CO and 1% NO in helium. The catalyst with 5 wt% copper supported on the ZrO₂/γ-Al₂O₃ matrix achieved 80% reduction of NO. Approximately the same rate of conversion was obtained on the catalyst with 2 wt% copper on ZrO₂. Characterization of these catalysts indicated that the active copper species for the reduction of NO are those in direct contact with the oxygen vacancies found in ZrO₂.     

Activation of Supported Pd Metal Catalysts for Selective Oxidation of Hydrogen to Hydrogen Peroxide

Catalytic activity of supported Pd metal catalysts (Pd metal deposited on carbon, alumina, gallia, ceria or thoria) showing almost no activity in the liquid-phase direct oxidation of H₂ to H₂O₂ (at 295 K) in acidic medium (0.02 M H₂SO₄) can be increased drastically by oxidizing them using different oxidizing agents, such as perchloric acid, H₂O₂, N₂O and air. In the case of the Pd/carbon (or alumina) catalyst, perchloric acid was found to be the most effective oxidizing agent. The order of the H₂-to-H₂O₂ conversion activity for the perchloric-acid-oxidized Pd/carbon (or alumina) and air-oxidized other metal oxide supported Pd catalysts is as follows: Pd/alumina < Pd/carbon < Pd/CeO₂ < Pd/ThO₂ < Pd/Ga₂O₃. The H₂ oxidation involves lattice oxygen from the oxidized catalysts. The catalyst activation results mostly from the oxidation of Pd metal from the catalyst producing bulk or sub-surface PdO. It also caused a drastic reduction in the H₂O₂ decomposition activity of the catalysts. There exists a close relationship between the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity in the oxidation process and the H₂O₂ decomposition activity of the catalysts; the higher the H₂O₂ decomposition activity, the lower the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity.     

UV/H2O2氧化联合CaO吸收脱除NO的传质-反应动力学

在实验室规模的光化学反应器中,基于实验研究﹑动力学理论以及双膜理论,研究了UV/H₂O₂氧化联合CaO吸收(UV/H₂O₂-CaO工艺)脱除燃煤烟气中NO的传质-反应动力学。分析了NO吸收的传质-反应过程,明确了NO吸收过程的主要控制步骤和强化措施,测定了关键的动力学参数,推导了NO吸收过程的理论模型。结果表明:在实验范围内,NO吸收速率随着NO浓度的增加几乎呈线性增加。随着H₂O₂浓度和CaO浓度的增加,NO的吸收速率均呈现先增加后变缓的趋势。UV/H₂O₂-CaO工艺脱除NO是一个拟一级快速反应过程,强化气相主体扰动﹑增加气液接触面积和提高NO分压可有效提高NO的吸收速率。NO吸收速率方程的计算值和实验值具有较好的一致性。     

Dehydrogenation of propane in the presence of N2O over In2O3―Al2O3 mixed oxide catalysts

Dehydrogenation of propane coupled with N₂O over a series of binary In₂O₃―Al₂O₃ mixed oxides was investigated. In contrast to the poor performance for sole N₂O decomposition, a remarkable synergy was identified between N₂O decomposition and propane dehydrogenation. Among the catalysts tested, the In₂O₃―Al₂O₃ sample containing a 20 mol% In₂O₃ showed the highest activity for propane dehydrogenation in the presence of N₂O. Moreover, stability far superior to those of the conventional iron-based materials was observed, attributable to the moderate surface acidity of the In―Al―O composite. The essential role of N₂O is suggested to generate active oxygen species facilitating propane dehydrogenation.