Higher Oxidation State Responsible for Ozone Decomposition at Room Temperature over Manganese and Cobalt Oxides: Effect of Calcination Temperature

The heterogeneous catalytic decomposition of ozone was investigated over unsupported manganese and cobalt oxide at room temperature. All catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption (Brunauer–Emmet–Teller method), H₂-temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS). The catalytic activity test indicated that these oxides had a good activity on ozone conversion meanwhile the catalysts remained highly active over time under reaction conditions. The treated temperature of the catalyst had a significant impact on the performance of ozone abatement and the samples treated at lower temperature showed higher activity. The surface area decreased obviously when developing the calcination temperature and H₂-TPR results demonstrated that much higher oxidation state of metal ions and active oxygen species were maintained on the surface under low treated temperature. XPS analysis showed that there were higher oxidation states of metal ions (Mn⁴⁺ and Co³⁺) and adsorbed oxygen species on the surface of catalysts treated at lower temperature, both of which play a significant role in ozone decomposition. However, the activity of manganese oxide was higher than that of cobalt oxide and the possible reason for this phenomenon was discussed.     

Investigating the Influence of Fe Speciation on N<Subscript>2</Subscript>O Decomposition Over Fe–ZSM-5 Catalysts

The influence of Fe speciation on the decomposition rates of N₂O over Fe–ZSM-5 catalysts prepared by Chemical Vapour Impregnation were investigated. Various weight loadings of Fe–ZSM-5 catalysts were prepared from the parent zeolite H-ZSM-5 with a Si:Al ratio of 23 or 30. The effect of Si:Al ratio and Fe weight loading was initially investigated before focussing on a single weight loading and the effects of acid washing on catalyst activity and iron speciation. UV/Vis spectroscopy, surface area analysis, XPS and ICP-OES of the acid washed catalysts indicated a reduction of ca. 60% of Fe loading when compared to the parent catalyst with a 0.4 wt% Fe loading. The TOF of N₂O decomposition at 600 °C improved to 3.99?×?10³ s^?1 over the acid washed catalyst which had a weight loading of 0.16%, in contrast, the parent catalyst had a TOF of 1.60?×?10³ s^?1. Propane was added to the gas stream to act as a reductant and remove any inhibiting oxygen species that remain on the surface of the catalyst. Comparison of catalysts with relatively high and low Fe loadings achieved comparable levels of N₂O decomposition when propane is present. When only N₂O is present, low metal loading Fe–ZSM-5 catalysts are not capable of achieving high conversions due to the low proximity of active framework Fe³⁺ ions and extra-framework ɑ-Fe species, which limits oxygen desorption. Acid washing extracts Fe from these active sites and deposits it on the surface of the catalyst as Fe_xO_y, leading to a drop in activity. The Fe species present in the catalyst were identified using UV/Vis spectroscopy and speculate on the active species. We consider high loadings of Fe do not lead to an active catalyst when propane is present due to the formation of Fe_xO_y nanoparticles and clusters during catalyst preparation. These are inactive species which lead to a decrease in overall efficiency of the Fe ions and consequentially a lower TOF.     

The Benefits of Small Quantities of Nitrogen in the Oxygen Feed to Ozone Generators

This paper reports the ozone generation in pulsed multichannel dielectric barrier discharge. The influence of nitrogen addition (0.1%–10%) on ozone concentration and ozone generation efficiency in nitrogen–oxygen gas mixtures is studied. Results show that adding 0.1% N₂ would not seriously increase the ozone production. Meanwhile, 1% N₂ content exhibits the highest ozone production efficiency in low SIE (J/L, defined as the ratio of power to gas flow rate) region (0–200 J/L) while adding 0.3% N₂ would lead to the highest ozone generation efficiency in high SIE region (300–800 J/L). The increase of ozone production induced by N₂ addition is more significant in low SIE region compared with that in high SIE region. At 100 J/L, ozone production efficiency increases 26.9% to 201.6 g/kWh with 1% N₂ addition when compared with that in oxygen. At 18 J/L, the observed maximum ozone generation efficiency reaches 252 g/kWh at 1.3 g/Nm³ with 1% N₂ addition. An increase of ozone production can be obtained with 0.3%–2% N₂ addition in all explored SIE ranges.     

CO hydrogenation over potassium-promoted Fe/carbon catalysts

The effects of potassium on the catalytic behavior in CO hydrogenation over K-promoted Fe/carbon catalysts having low K/Fe ratios were investigated. Even though the doses of potassium were low the promotional effects were pronounced, especially on the olefin-to-paraffin ratio, and theC ₃ toC ₄ olefin selectivities of the K-promoted catalysts were as high as 51 to 66 mol%. Over the catalysts having no or low potassium content the olefin-to-paraffin ratio and the ratio of the CO₂ formation rate to the rate of CO conversion to hydrocarbons remained roughly the same regardless of temperature, while over the K-promoted catalysts having higher potassium content they increased with temperature. Formation of significant amounts of filamentous carbon was observed in the K-promoted catalysts; however, the carbon deposition did not appear to affect the inherent activity and selectivity of the K-promoted catalysts.     

Effects of ozone treatment of carbon support on Pt-Ru/C catalysts performance for direct methanol fuel cell

This research is aimed to increase the activity and utilization of Pt-Ru alloy catalysts and thus to lower the catalyst loading in anodes for methanol electrooxidation. The Pt-Ru/C catalysts were prepared by chemical reduction. The support of Vulcan XC-72 carbon black was pretreated by ozone at different temperatures for different times. The specific surface area of the samples was evaluated by the standard BET method. The surface concentrations of oxygen were determined by XPS. The results showed that the surface concentrations of oxygen on the carbon were first decreased and then increased with pretreating times, and the specific surface area of the carbon was decreased with pretreating times at the same temperature. The specific surface area was increased with increasing temperature, and the surface concentration of oxygen was first decreased and then increased with increasing temperature for the same pretreating time. Pt-Ru/C catalysts supported by untreated and O₃ treated carbon black were characterized and tested for methanol electrooxidation. X-ray diffraction (XRD) was used to characterize the influence of carbon treated with ozone on Pt-Ru/C catalysts. It was found that the catalysts were composed only of f.c.c. Pt-Ru alloy particles without metallic Ru or Ru oxide. Cyclic voltammetry (CV) and Tafel curves were used for methanol electrooxidation on Pt-Ru/C catalysts in a solution of 0.5 mol/L CH₃OH and 0.5 mol/L H₂SO₄, showing that the catalytic activity of Pt-Ru/C catalysts supported by ozone treated carbon was higher than that by the untreated one. The ozone treatment time and temperature, which affect the performance of Pt-Ru/C catalysts, were discussed. Electrochemical measurements showed that the catalysts supported by the carbon after ozone treatment for 6 min at 140 °C had the best performance.     

Oxygen reduction by Fe-based catalysts in PEM fuel cell conditions: Activity and selectivity of the catalysts obtained with two Fe precursors and various carbon supports

Fe-based catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cell conditions have been prepared by adsorbing two Fe precursors on various commercial and developmental carbon supports. The resulting materials have been pyrolyzed at 900 °C in an atmosphere rich in NH₃. The Fe precursors were: iron acetate (FeAc) and iron tetramethoxy phenylporphyrin chloride (ClFeTMPP). The nominal Fe content was 2000 ppm (0.2 wt.%). The carbon supports were HS300, Printex XE-2, Norit SX-Ultra, Ketjenblack, EC-600JD, Acetylene Black, Vulcan XC-72R, Black Pearls 2000, and two developmental carbon black powders, RC1 and RC2 from Sid Richardson Carbon Corporation. The catalyst activity for ORR has been analyzed in fuel cell tests at 80 °C as well as by cyclic voltammetry in O₂ saturated H₂SO₄ at pH 1 and 25 °C, while their selectivity was determined by rotating ring-disk electrode in the same electrolyte. A large effect of the carbon support was found on the activity and on the selectivity of the catalysts made with both Fe precursors. The most important parameter in both cases is the nitrogen content of the catalyst surface. High nitrogen content improves both activity towards ORR and selectivity towards the reduction of oxygen to water (4e⁻ reaction). A possible interpretation of the activity and selectivity results is to explain them in terms of two Fe-based catalytic sites: FeN₂/C and FeN₄/C. Increasing the relative amount of FeN₂/C improves both activity and selectivity of the catalysts towards the 4e⁻ reaction, while most of the peroxide formation may be attributed to FeN₄/C. When FeAc is used as Fe precursor, iron oxide and/or hydroxide are also formed. The latter materials have low catalytic activity for ORR and reduce O₂ mainly to H₂O₂.     

Synthesis of doped MnOx/diatomite composites for catalyzing ozone decomposition

The wide applications and unintended generation of ozone stimulate the exploration of catalysts for ozone decomposition. Herein, manganese oxides loaded on porous ceramic beads were prepared via a simple thermolysis method. Doping the composite with Ni, Fe, and Al could tune the oxygen vacancies in the manganese oxide, as evidenced by the X-ray photoelectron spectroscopy (XPS) analysis. The average valance of Mn could also be altered by the dopants. The catalytic properties of the composites doped with Fe, Ni, and Al have been significantly improved. And the Ni doped MnO_x with the highest density of oxygen vacancy exhibits the best catalytic activity. The prepared composite catalyst can easily be incorporated into filters for air circulation.     

Cation Vacant Fe3?x?y V x �� y O4 Spinel-Type Catalysts for the Oxidation of Methanol to Formaldehyde

The potential of Fe?CV-oxide catalysts for use in methanol oxidation is explored. Our results show that although FeVO₄ is active and selective for formaldehyde (FA) formation, it is not completely stable towards volatilization under reaction conditions. Attempts to stabilize Fe?CV-oxide were made using titania, alumina and silica supports. However, we observe that although some stabilization is achieved using titania and alumina, the supported catalysts are sensitive to volatilization considering the relatively low content of active oxide. Compared with supported V-oxide, the results show that iron causes stabilization of vanadium decreasing its volatility. Considering the observation that the neat FeVO₄ restructures to form a spinel-type phase under influence of the catalysis, we prepared a series of cation vacant spinel-type Fe_3?x?y V _x ?? _y O₄ catalysts with various V/Fe ratio and consequent number of cation vacancies ??. Opposed to the activity, which is rather constant irrespectively of the vanadium content, the selectivity to FA passes through a maximum of about 90% for Fe/V = 14. A spinel-type phase with the composition Fe_2.62V_0.19??_0.20O₄ was prepared and subsequently preoxidized to different degree. It is observed that the spinel-type structure is stable and that the oxidation of vanadium and iron is balanced by an increasing number of cation vacancies. Moreover, irrespectively of the original degree of preoxidation, it is found that in methanol oxidation a steady state is reached where all samples are equally active and selective and have the same composition both in the bulk and at the surface. The results clearly demonstrate that the spinel-type catalysts are phase-stable, nonvolatile and flexible in that the cations can change oxidation state retaining the same basic structure type and Fe/V ratio.     

Preparation of MnOx/TiO2 ultrafine nanocomposite with large surface area and its enhanced toluene oxidation at low temperature

The TiO₂ support materials were synthesized by a chemical vapor condensation (CVC) method and the subsequent MnO_x/TiO₂ catalysts were prepared by an impregnation method. Catalytic oxidation of toluene on the MnO_x/TiO₂ catalysts was examined with ozone. These catalysts had a smaller particle size (9.1 nm) and a higher surface area (299.5 m² g⁻¹) compared to MnO_x/P25-TiO₂ catalysts. The catalysts show high catalytic activity with the ozone oxidation of toluene even at low temperature. As a result, the synthesized support material by the CVC method gave more active catalyst.     

NaY分子筛担载FeSO4催化剂用于氨气还原NOx的性能

研究了分子筛担载FeSO₄催化剂在SCR脱硝反应中的催化性能和反应机理。实验结果表明,在同等工况下分子筛担载FeSO₄后的催化剂具有更好的物理结构,与纯FeSO₄相比脱硝率可提高将近20%。经Mossbauer谱分析,催化剂制备过程中Fe²⁺转化为Fe³⁺,其具体存在形式为Fe (OH)SO₄与Fe₂O(SO₄)₂,前者催化脱硝效果优于后者。原位红外分析结果表明,吸附在分子筛担载催化剂表面的氨与气相中的NO反应,Fe离子是吸附及发生催化氧化还原反应的活性中心。与钒、钛系相似文献   

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.     

Oxygen evolution over Ag/FexAl2−xO3 (0.0 ≤ x ≤ 2.0) catalysts via N2O and H2O2 decomposition

In this paper, a comparative study between nitrous oxide and hydrogen peroxide decomposition over a series of catalysts prepared via the combustion of silver, aluminum, and iron nitrates (with different aluminum: iron ratios). Urea was used as a combustion fuel. The calcinations were affected at the 400–700 °C temperature range. The produced catalysts were characterized by using XRD and SEM analyses. The obtained results revealed that silver metal supported on Al₂O₃ and/or Fe₂O₃ represent the major constituents of all the calcinations products, i.e. Ag/Fe_xAl_2−xO₃. However, two different interfaces are involved in the two test reactions, all the catalysts were able to decompose both reactants yielding oxygen as a joint product. Meanwhile, it was found nitrous oxide destruction activity increases with decreasing both silver particles size and iron content in the catalysts substrate. On the contrary, increasing iron content in the different catalyst was found to enhance hydrogen peroxide decomposition activity. Moreover, a synergic effect was observed for the catalysts having Al:Fe ratio of 0.5:1.5.     

The influence of bimetallic catalyst composition on single-walled carbon nanotube yield

We show that the yield of single-walled carbon nanotubes (SWCNTs) grown with bimetallic catalysts is a strong function of their atomic-scale composition. A series of compositionally-tuned Ni_xFe_1?x bimetallic catalysts with a constant mean diameter of 2.0 nm are used to catalyze the growth of nanotubes via a floating catalyst method. Increasing the Fe content in the catalysts is found to lower the fraction of SWCNTs in the collected as-grown product. Based on a simple surface-to-volume model, these results are explained by the higher carbon solubility of Fe compared to Ni which results in a larger amount of carbon precipitation and the formation of multi-walled tubes when the nanotubes are nucleated from catalysts with high Fe content. Overall, our study demonstrates that the size and composition of bimetallic catalysts must be precisely controlled to obtain high yields of SWCNTs for large-scale production.     

Application of iron-based cathode catalysts in a microbial fuel cell

Catalysts for the oxygen reduction reaction (ORR) in a microbial fuel cell (MFC) were prepared by the impregnation on carbon black of Fe^II acetate (FeAc), Cl–Fe^III tetramethoxyphenyl porphyrin (ClFeTMPP), and Fe^II phthalocyanine (FePc). These materials were subsequently pyrolyzed at a high temperature. The ORR activity of all Fe-based catalysts was measured at pH 7 with a rotating disk electrode (RDE) and their performance for electricity production was then verified in a continuous flow MFC. Catalysts prepared with FeAc and pyrolyzed in NH₃ showed poor activity in RDE tests as well as a poor performance in a MFC. The ORR activity and fuel cell performance for catalysts prepared with ClFeTMPP and FePc and pyrolyzed in Ar were significantly higher and comparable for both precursors. The iron loading was optimized for FePc-based catalysts. With a constant catalyst load of 2 mg cm⁻² in a MFC, the highest power output (550–590 mW/m²) was observed when the Fe content was 0.5–0.8 wt%, corresponding to only 0.01–016 mg Fe/cm². A similar power output was observed using a Pt-based carbon cloth cathode containing 0.5 mg Pt/cm². Long-term stability of the Fe-based cathode (0.5 wt% Fe) was confirmed over 20 days of MFC testing.     

Methane decomposition over ceria modified iron catalysts

The catalytic behaviour of ceria supported iron catalysts (Fe–CeO₂) was investigated for methane decomposition. The Fe–CeO₂ catalysts were found to be more active than catalysts based on iron alone. A catalyst composed of 60 wt.% Fe₂O₃ and 40 wt.% CeO₂ gave optimal catalytic activity, and the highest iron metal surface area. The well-dispersed Fe state helped to maintain the active surface area for the reaction. Methane conversion increased when the reaction temperature was increased from 600 to 650 °C. Continuous formation of trace amounts of carbon monoxide was observed during the reaction due to the oxidation of carbonaceous species by high mobility lattice oxygen in the solid solution formed within the catalyst. This could minimise catalyst deactivation caused by carbon deposits and maintain catalyst activity over a longer period of time. The catalyst also produced filamentous carbon that helped to extend the catalyst life.     

Selective low temperature hydroxylation of isobutane by molecular oxygen catalyzed by an iron perhaloporphyrin complex

Irontetrakis(pentafluorophenyl)-octabromoporphyrinato complexes have been synthesized for the first time and shown to have unprecedented catalytic activity for the reaction of molecular oxygen with isobutane to givetert-butyl alcohol. This is the first report of the use of a perhaloporphyrin complex for mild, selective air-oxidation of an alkane and extends the trend of increased activity with halogen substitution established previously. Replacing the eight -(pyrrolic) hydrogens in Fe(TPPF₂₀) complexes with bromines gives catalysts having twice the room temperature air-oxidation activity of the Fe(TPPF₂₀) complexes. Room temperature reaction of isobutane with oxygen catalyzed by Fe(TPPF₂₀6-Br₈)Cl produces 190 moles product per mole catalyst per hour with over 90% selectivity to the alcohol. The catalyst activity is unchanged after 74 hours.     

An investigation into the effect of Cs promotion on the catalytic activity of NiO in the direct decomposition of N2O

A series of Cs promoted NiO catalysts have been prepared and tested for direct decomposition of N₂O. These catalysts are characterized by BET surface area, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption of N₂O (TPD-N₂O) and X-ray photo electron spectroscopy (XPS). The Cs promoted NiO catalysts exhibit higher activity for the decomposition of N₂O compared to bulk NiO. The catalyst with Cs/Ni ratio of 0.1 showed highest activity. The enhancement in catalytic activity of the Cs promoted catalysts is attributed to the change in the electronic properties of NiO. The characterization techniques suggest weakening of Ni–O bond thereby the desorption of oxygen becomes more facile during the reaction. The Cs promoted NiO catalyst is effective at low reaction temperature and also in the presence of oxygen and steam in the feed stream. IICT Communication No: 070523.     

Dependence of n-Butane Activation on Active Site of Vanadium Phosphate Catalysts

The nature and the role of oxygen species and vanadium oxidation states on the activation of n-butane for selective oxidation to maleic anhydride were investigated. Bi–Fe doped and undoped vanadium phosphate catalysts were used a model catalyst. XRD revealed that Bi–Fe mixture dopants led to formation of α_II-VOPO₄ phase together with (VO)₂P₂O₇ as a dominant phase when the materials were heated in n-butane/air to form the final catalysts. TPR analysis showed that the reduction behaviour of Bi–Fe doped catalysts was dominated by the reduction peak assigned to the reduction of V⁵⁺ species as compared to the undoped catalyst, which gave the reduction of V⁴⁺ as the major feature. An excess of the oxygen species (O^2?) associated with V⁵⁺ in Bi–Fe doped catalysts improved the maleic anhydride selectivity but significantly lowering the rate of n-butane conversion. The reactive pairing of V⁴⁺-O^? was shown to be the centre for n-butane activation. It is proposed that the availability and appearance of active oxygen species (O^?) on the surface of vanadium phosphate catalyst is the rate determining step of the overall reaction.     

Role of tungsten in supported Pt-W reforming catalysts Part I. Modification of platinum activity and selectivity by addition of tungsten

Bimetallic supported Pt-W catalysts are studied for 3-methylhexane reforming. An increase in the activity and selectivity in aromatization is found for Pt-low W/Al₂O₃ catalysts compared to the classical monometallic Pt/Al₂O₃ catalyst. Changes in activity and selectivity, for bimetallic catalysts, are attributed to tungsten moderator interaction effects between platinum and support which modify the metallic particle sizes. These changes are observed on Pt-high W/Al₂O₃ and Pt-W/SiO₂ catalysts. Superficial carbide formation and modification of the hydrogen chemisorption may be proposed to explain the reactivity of Pt-W catalysts under low hydrogen pressure.     

Preparation and Characterization of SnO2-Based Composite Metal Oxides: Active and Thermally Stable Catalysts for CH4 Oxidation

A series of SnO₂-based catalysts modified by Fe, Cr and Mn were prepared by the combination of redox reaction and co-precipitation methods, and applied to catalytic CH₄ oxidation. The modified catalysts show generally higher activity than the unmodified SnO₂. XRD analysis indicates that Fe, Cr and Mn cations could be incorporated into the lattice of rutile SnO₂ (cassiterite) to form solid solution structure. As a result, more reducible and active oxygen species was formed in the samples, as substantiated by the H₂-TPR results. Moreover, the specific surface areas of the modified catalysts are much higher than that of pure SnO₂ and their crystallite sizes are smaller, indicating they are more resistant to thermal sintering. Indeed, the high specific surface areas and the formation of more active oxygen species in the modified samples are believed to be the predominant reasons leading to their enhanced CH₄ oxidation activity. Eventually, it is noted that SnCrO displays not only remarkable CH₄ oxidation activity, but also potent resistance to SO₂ and water deactivation, which makes it a promising catalyst with the potential to be applied in some real CH₄ oxidation processes.