The dynamic states of silica-supported metal oxide catalysts during methanol oxidation

The in situ Raman spectra of silica-supported metal oxide catalysts (containing surface metal oxide species of V, Nb, Cr, Mo, W and Re) were measured during methanol oxidation. Stable surface methoxy species were found to form via reaction with the surface Si---OH groups for all the catalysts investigated. The surface Si---OH groups were also titrated by the surface metal oxide species and the surface concentration of Si---OH groups decreases with increasing metal oxide loading. The stable surface M---OCH₃ were only found for the V₂O₅/SiO₂ catalysts. The surface metal oxide species were all influenced by the methanol oxidation reaction. The surface rhenium oxide species were removed from the silica surface and the surface molybdenum oxide species were partially agglomerated to crystalline β-MoO₃ particles by the formation of Re-methoxy and Mo-methoxy complexes. The surface niobium oxide and tungsten oxide species were partially reducing by the net reducing methanol oxidation environment. In situ Raman spectra for the CrO₃/SiO₂ catalysts could not be obtained due to the formation of reduced chromium oxide species during methanol oxidation which gave rise to sample fluorescence. The in situ Raman observations provided a fundamental basis for understanding the selectivity patterns of the silica-supported metal oxide catalysts during methanol oxidation. However, the mechanism by which the silica support ligands activate the redox properties of the surface metal oxide species is not completely understood.     

Dynamic processes on vanadium phosphorous oxides for selective alkane oxidation

The use of complementary physicochemical tools (XRD, Raman spectroscopy, XPS, ³¹P NMR, and electron microscopy techniques), sometimes used in in situ conditions has allowed to evidence the dynamic processes occurring during the oxidation of light alkanes on the vanadium phosphorus oxide (VPO) system. The transformations of the VPO system in the course of the oxidation of n-butane to maleic anhydride and of the oxidation of propane to acrylic acid are contrasted in connection with the evolution of the catalytic performances.     

In situ Raman spectroscopy studies of bulk and surface metal oxide phases during oxidation reactions

Bulk V-P-O and model supported vanadia catalysts were investigated with in situ Raman spectroscopy during n-butane oxidation to maleic anhydride in order to determine the fundamental molecular structure-reactivity/selectivity insights that can be obtained from such experiments. The in situ Raman studies of the bulk V-P-O catalysts provided information about the bulk crystalline phases, the hemihydrate precursor and its transformation to vanadyl pyrophosphate. However, the Raman experiments could not provide any molecular structural information about the amorphous and surface phases also present in this bulk metal oxide catalyst because of the strong Raman scattering from the crystalline phases. In contrast, in situ Raman studies of the model supported vanadia catalysts, where the active phase is present as a two-dimensional surface metal oxide overlayer, provided new insights into this important hydrocarbon oxidation reaction. In addition, the surface properties of the supported vanadia catalysts could be molecularly engineered to probe the role of various functionalities upon the structure-reactivity/selectivity relationship of n-butane oxidation to maleic anhydride. These fundamental studies revealed that the oxidation of n-butane required only one surface vanadia site and that the critical rate determining step involved the bridging V---O---P or V---O-support bonds. The selective oxidation of n-butane to maleic anhydride could occur over one surface vanadia site as well as multiple adjacent surface vanadia sites, but the reaction is more efficient with multiple sites. The n-butane oxidation TOF increased with the introduction of both surface Brönsted and Lewis acid sites, but only the surface Lewis acid sites increased the maleic anhydride selectivity.     

Manganese oxide catalyzed methane partial oxidation in trifluoroacetic acid: Catalysis and kinetic analysis

Direct oxidation of methane to methanol has been studied for decades, and has yet to be commercialized. Three years ago, UOP LLC, a Honeywell Company, started a government co-sponsored project (NIST/ATP Award 70NANB4H3041) for selective liquid phase oxidation of methane to methanol. Recently we have discovered an efficient methane oxidation by manganese oxide. When used as stoichiometric oxidant, quantitative metal oxide-based yield was observed for methane oxidation. The spent catalyst activity can be 100% regenerated with air under basic conditions. A high methane-based yield (36%) with high selectivity (>95%) was achieved when manganese oxide was used in catalytic amount in the presence of air for methane oxidation. Our online GC analysis showed that catalytic methane oxidation occurs with two-stage reaction kinetics with constant reaction rate at the active stage, which can be explained by a low steady-state active catalyst concentration as observed by in situ UV–vis spectrometer.     

Catalytic vapour phase epoxidation of propene with nitrous oxide as an oxidant II. Development of a kinetic model

The vapour phase epoxidation of propene with nitrous oxide (N₂O) was experimentally investigated in a fixed bed reactor using a CsO_x/FeO_y/SiO₂ catalyst over a broad range of residence times. The influence of feed composition on the conversion and product distribution was determined for the reactants propene, propylene oxide (PO), propionaldehyde (PA), and N₂O. The experimental results were used to derive a formal kinetic model to describe the reactions in a network deduced in the first part of this publication [Thömmes, T., Zürcher, S., Wix, A., Reitzmann, A., Kraushaar-Czarnetzki, B., 2007. Catalytic vapour phase epoxidation of propene with nitrous oxide as an oxidant: I. Reaction network and product distribution. Applied Catalysis A 318, 160-169]. Self-inhibition of the propene conversion was observed, and an inhibition of the PO conversion through PO isomerisation products. The N₂O concentration has almost no effect on the conversion of propene and PO, but the PA conversion is accelerated significantly through N₂O. Propene related and N₂O related PO selectivities display opposed dependencies from reactant concentration. The modelling results indicate that coke is predominantly formed from oxygenated products. It is the key issue for future developments to prevent the fast consecutive conversion of PO.     

Electrochemical oxidation of organic pollutants in water at metal oxide electrodes: A simple theoretical model including direct and indirect oxidation processes at the anodic surface

The electrochemical oxidation of organics in water at metal oxide electrodes was investigated with the aim to discuss the correlations between the instantaneous current efficiency ICE and operative conditions by considering both the hypothesis of a direct oxidation process and of an indirect process mediated by adsorbed hydroxyl radicals or chemisorbed “oxygen”, in order to explicit the main differences expected between these cases. Thus, a simple theoretical model was discussed, as an extension of previous studies of Comnnellis and co-workers which were focused on indirect oxidation paths [C. Comninellis, Electrochim. Acta 39 (1994) 1857; O. Simond, V. Schaller, Ch. Comninellis, Electrochim. Acta, 42 (1997) 2009], concerning both the cases of mass transfer control and oxidation reaction control and mixed kinetic regimes. A very good agreement, between theoretical predictions and experimental data pertaining to the electrochemical oxidation of oxalic and formic acid at IrO₂–Ta₂O₅, was observed.     

Promotion of palladium-based catalysts on metal monolith for partial oxidation of methane to syngas

Four different modifications of alumina were prepared for use as the support for a Pd catalyst used for the partial oxidation of methane to syngas. The catalysts were washcoated on a metallic monolith in order to determine their activities at high gas flow rates. Compared with the Pd/Al₂O₃ catalyst, enhanced partial oxidation activities were observed with the Pd/CeO₂/Al₂O₃, Pd/CeO₂/BaO/Al₂O₃ and Pd/CeO₂/BaO/SrO/Al₂O₃ catalysts. The palladium particles were better dispersed in the presence of CeO₂ and SrO. Adding BaO, CeO₂ and BaO–CeO₂ to γ-Al₂O₃ prevented the transformation of the alumina phase during the 3-day aging process at 1000 °C, providing the support with some level of thermal stability. The addition of small amounts of SrO to the CeO₂/BaO/Al₂O₃ support enhanced the thermal stability of the Pd particles and minimized their sintering. The triply promoted Pd catalyst studied in this work was effective in carrying out partial oxidation at high temperatures, with BaO and CeO₂ promoting the thermal stability of the support, CeO₂ and SrO dispersing the Pd particles and SrO anchoring the Pd particles strongly to the support. The composition of the catalyst which gave both the highest partial oxidation activity and the best thermal stability was Pd(2)/CeO₂(23)/BaO(11)/SrO(0.8)/Al₂O₃.     

Qualitative aspects of ethylene oxide oxidation in a well-stirred flow reactor

The slow oxidation of ethylene oxide in a stirred-flow reactor was studied, and accurate, fast-response thermocouples were used for the measurement of temperature rises due to the oxidation. The effects of mixture strength, ambient temperature and total pressure on the temperature rise are illustrated and discussed. Comparisons are made with results previously reported for ethylene oxide when studied in closed vessels.     

Solid acidity of metal oxide monolayer and its role in catalytic reactions

Such metal oxide as SO₄²⁻, MoO₃, WO₃, and V₂O₅ spread readily on supports like SnO₂, ZrO₂, and TiO₂ due to the different properties between acid and base oxides to generate the acid site on the monolayer. Number, strength, and structure of the acid site were characterized by temperature-programmed desorption (TPD) of ammonia principally, together with various physico-chemical techniques, and its role for catalytic reactions was studied. Approximately, one to two acid sites were stabilized on 1 nm² of the surface, which consisted of four to eight metal atoms. The limit in surface acid site density was estimated on the monolayer based on the concept of solid acidity on zeolites. Sequence of the metal oxide to show the strong acidity was, SO₄²⁻>WO₃>MoO₃>V₂O₅, and for the support oxide to accommodate the monolayer, SnO₂>ZrO₂>TiO₂>Al₂O₃. From these combinations, the metal oxide monolayer to show the adequate strength of acid site could be selected. Brønsted acidity was observed often, however, the Lewis acidity was prevailing on the reduced vanadium oxide. The structure of acid site, Brønsted or Lewis acid site, thus depended on the oxidation state. Relationship of the profile of solid acidity with various catalytic activities was explained. The solid acid site on the monolayer will possibly be applied to environment friendly technologies.     

A comparison of H2 addition to 3 ms partial oxidation reactions

We compare the effects of adding large amounts of H₂ to 3 ms partial oxidation reactions, ethane to ethylene, propane to olefins, and methane and ammonia to HCN. It is found that H₂ can be safely added at the 2/1 H₂/O₂ stoichiometry in the presence of these fuels without any homogeneous reactions, flames, or explosions. For all of these systems the addition of H₂ increases the selectivities to the desired products while strongly decreasing CO and CO₂. Addition of H₂ forces water formation near the front face of the catalyst which consumes O₂ and allows dehydrogenation processes to dominate.     

Steady-state and dynamic modeling of indirect partial oxidation of methane in a wall-coated microchannel

Steady and dynamic characteristics of catalytic indirect partial oxidation (combined total oxidation and steam reforming) of methane to hydrogen in a wall-coated microchannel are investigated using computational techniques. Steady-state behavior is initially modeled using a two-dimensional axisymmetrical wall-coated reactor model. Considering the small channel diameter, adiabatic operation and negligible transport resistances, response of the microchannel is also investigated using a one-dimensional pseudohomogeneous tubular reactor model. Simulations of the microchannel are carried out using both models for different feed conditions ranging between 1.89 and 2.24 for CH₄/O₂ and 1.17–2.34 for H₂O/CH₄. Outcomes from both models are found to be close, allowing the use of the low-cost one-dimensional model in dynamic simulations. Analysis of transients during the system start-up indicate that steady state is reached between 100 and 120 s depending on the feed composition. Product temperature and flow rates obtained from steady-state and dynamic simulations are found to be close with some differences arising from the finite difference-based numerical method used to solve partial differential equations of the dynamic model. Dynamic responses of the microchannel to several disturbances in the feed are analyzed. The response to a step increase in the inlet oxygen flow rate (decrease of CH₄/O₂ from 2.24 to 1.89) is the elevation of temperature by ca. 100 K, which in turn leads to ca. 33% in hydrogen yield, and the time to reach the new steady state is around 90 s. If the disturbance involves an increase in inlet steam flow, temperature and hydrogen yield decrease in time to a local minimum within 10 s and then gradually increase to the subsequent steady state within 50 s ending up with net reductions of ca. 1.6% and 9%, respectively.     

On the composition and structure of thin layers of titanium oxide on platinum surfaces

This work reviews the available data about the structure and composition of thin layers of titanium oxide on the surface of platinum. These systems can be prepared by vapor deposition and subsequent oxidation of Ti on a massive Pt substrate or by oxidation at low pressure of the Pt₃Ti alloy. For both types of substrate, the XPS data show that two distinct Ti states in the oxide layer can be detected. The structure and the composition of the oxide layer varies depending on the substrate composition and on the conditions of preparation. On pure Pt, the preferential formation of a defective TiO_2–x oxide was observed. On Pt₃Ti, the oxide film a layer was formed of TiO of thickness of the order of a single atomic layer and of crystallites of nearly stoichiometry TiO₂.     

甲烷部分氧化制合成气反应中床层热点问题研究进展

综述了甲烷部分氧化制合成气反应中催化剂床层热点问题,包括热点产生的原因,热点位置的测定,热点温度的影响因素,以及热点问题的解决方法,对于保护催化剂和反应器,降低反应的危险性起到借鉴作用.     

Effects of methane and oxygen on decomposition of nitrous oxide over metal oxide catalysts

Catalytic activities of various metal oxides for decomposition of nitrous oxide were compared in the presence and absence of methane and oxygen, and the general rule in the effects of the coexisting gases was discussed. The reaction rates of nitrous oxide were well correlated to the heat of formation of metal oxide, i.e., a V-shaped relationship with a minimum at −ΔH_f⁰ around 450 kJ (O mol)⁻¹ was observed in N₂O decomposition in an inert gas. In the case of metal oxides having the heat of formation lower than 450 kJ (O mol)⁻¹, CuO, Co₃O₄, NiO, Fe₂O₃, SnO₂, In₂O₃, Cr₂O₃, the activities were strongly affected by the presence of methane and oxygen. On the other hand, the activities of TiO₂, Al₂O₃, La₂O₃, MgO and CaO were almost independent. The reaction rate of nitrous oxide was significantly enhanced by methane. The promotion effect of methane was attributed to the reduction of nitrous oxide with methane: 4N₂O+CH₄→2N₂+CO₂+2H₂O. The activity was suppressed in the presence of oxygen on the metal oxides having lower heat of formation. On the basis of Langmuir–Hinshelwood mechanism, the effect of oxygen on nitrous oxide decomposition was rationalized with the strength of metal–oxygen bond.     

Transient kinetics of toluene partial oxidation over V/Ti oxide catalysts

Transient kinetics in the toluene oxidation over V/Ti oxide catalysts prepared by grafting and impregnation have been compared. V⁴⁺ cations are supposed to be the sites for the formation of electrophilic oxygen species participating in deep oxidation. Another oxygen species (probably nucleophilic) present on the oxidised catalyst surface are responsible for benzaldehyde formation. Selectivity of 80–100% can be obtained during the initial period of the reaction on the grafted catalysts in the presence of gaseous oxygen and during the interaction of toluene (without O₂ in the mixture) with partially reduced catalysts.     

Solid state chemistry of bulk mixed metal oxide catalysts for the selective oxidation of propane to acrylic acid

Syntheses of Mo–V–Sb–Nb–O bulk materials, which are candidate catalyst systems for the selective oxidation of propane to acrolein and acrylic acid, were made using soluble precursor materials. The products were characterized by X-ray powder diffraction and Raman spectroscopic studies. The objectives of this work were to explore the utility of liquid phase automated synthesis for the preparation of bulk mixed metal oxides, and the identification of the oxide phases present in the system. This is the first published study of the phase composition for these materials. After calcination of these bulk oxides under flowing nitrogen at 600°C, and using stoichiometric ratios of Mo–V–Sb–Nb (1:1:0.4:0.4) and Mo–V–Sb–Nb (3.3:1:0.4:0.4) it was demonstrated that a mixture of phases were obtained for the syntheses. X-ray powder diffraction studies distinguished SbVO₄, Mo₆V₉O₄₀, MoO₃, and a niobium-stabilized defect phase of a vanadium-rich molybdate, Mo_0.61–0.77V_0.31–0.19Nb_0.08–0.04O_x, as the major phases present. Complementary data were provided by the Raman spectroscopic studies, which illustrated the heterogeneity of the phases present in the mixture. Raman also indicated bands attributable to the presence of phases containing terminal M=O bonds as well as M–O–M polycrystalline phases. Previous studies on this system have identified SbVO₄ and niobium-stabilized vanadium molybdate species as the active phases necessary for the selective oxidation of alkanes.     

Kinetic parameters for anodic oxidation of hypochlorite ion on Pt and Pt oxide electrodes in alkaline solution

Kinetic parameters for the anodic oxidation of hypochlorite ion have been determined by means of normal pulse voltammetry by using a platinum disk as the working electrode. By using the working electrode that formed an oxide film by electrochemical pretreatment, the effect of the lattice oxygen of the surface oxide on the reaction was also examined. The measurement results were analyzed by the classical method, and then the analytical results were evaluated by digital simulation. The normal pulse voltammogram of the hypochlorite ion showed quasi-reversible oxidation waves. The apparent rate constant was calculated to be 5.0-8.1 × 10⁻⁴ cm s⁻¹, depending on the electrode surface state. At the low-concentration range of <4.0 mg Cl dm⁻³, the oxidation current was concentration dependent at the cathodically polarized electrode, while it became independent after the anodic polarization.     

Electrochemical behaviour of metal hexacyanoferrate converted to metal hydroxide films immobilized on indium tin oxide electrodes—Catalytic ability towards alcohol oxidation in alkaline medium

In this work, we demonstrate a simple method to modify indium tin oxide (ITO) electrodes in order to perform electro-catalytic oxidation of alcohols in alkaline medium. Metal hexacyanoferrate (MHCF) films such as nickel hexacyanoferrate (NiHCF) and copper hexacyanoferrate (CuHCF) were successfully immobilized on ITO electrodes using an electrochemical method. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the structural and morphological aspects of MHCF films. Cyclic voltammetry (CV) was used to study the redox properties and to determine the surface coverage of these films on ITO electrodes. Electrochemical potential cycling was carried out in alkaline medium in order to alter the chemical structure of these films and convert to their corresponding metal hydroxide films. SEM and XPS were performed to analyze the structure and morphology of metal hydroxide modified electrodes. Electro-catalytic oxidation ability of these films towards methanol and ethanol in alkaline medium was investigated using CV. From these studies we found that metal hydroxide modified electrodes show a better catalytic performance and good stability for methanol oxidation along with the alleviation of CO poisoning effect. We have obtained an anodic oxidation current density of ∼82 mA cm⁻² for methanol oxidation, which is at least 10 fold higher than that of any metal hydroxide modified electrodes reported till date. The onset potential for methanol oxidation is lowered by ∼200 mV compared to other chemically modified electrodes reported. A plausible mechanism was proposed for the alcohol oxidation based on the redox properties of these modified electrodes. The methodology adapted in this work does not contain costlier noble metals like platinum and ruthenium and is economically viable.     

Solvent-free partial oxidation of cyclohexane to KA oil over hydrotalcite-derived Cu-MgAlO mixed metal oxides

A highly efficient and stable hydrotalcite-derived Cu-MgAlO catalyst was developed for the partial oxidation of cyclohexane with molecular oxygen. The physical-chemical properties of Cu-MgAlO catalysts were studied, and the results indicated that the copper component had been successfully introduced into the hydrotalcite unit layer structure. The catalytic reaction results showed that copper as the active species could activate C-H bond and effectively promote the decomposition of cyclohexyl hydroperoxide (CHHP) to the mixture of cyclohexanol and cyclohexanone (KA oil). 8.3% of cyclohexane conversion and 82.9% of selectivity for KA oil were obtained over 9%Cu-MgAlO catalyst at 150℃ with 0.6 MPa of oxygen pressure for 2 h. Especially, its catalytic performance was still stable after five runs.