Oxidation State Of Catalyst Research Articles

Platinum Supported on NbRuyOz as Electrocatalyst for Ethanol Oxidation in Acid and Alkaline Fuel Cells

Platinum supported on a mixed metal oxide, NbRu{sub y}O{sub z} (8Nb:1Ru), was evaluated as an electrocatalyst for the ethanol oxidation reaction (EOR) in 0.1 M HClO{sub 4} and 1 M KOH. The support was synthesized from a liquid precursor solution of metal chlorides that was aerosolized and thermally decomposed into a powder via the spray pyrolysis (SP) process. Two samples were of primary interest: 30%Pt deposited onto the support by dry impregnation and 60%Pt as part of the precursor solution that underwent in situ SP Pt dispersion. TEM, SEM, and XRD were used to confirm morphology and deposition of Pt. XPS and XAS studies confirmed elemental distribution and oxidation state of Pt catalyst. In situ IRRAS studies in 0.1 M HClO{sub 4} show that these electrocatalysts are capable of facilitating the complete oxidation pathway of EOR, involving scission of the C-C bond and CO oxidation.

Solar Light Active Photodegradation of Phenol over a FexTi1−xO2−yNy Nanophotocatalyst

This article details the preparation, characterization, and catalytic activity evaluation of an efficient mesoporous FexTi1−xO2−yNy nanophotocatalyst (FeNT) for the degradation of phenol under direct solar light illumination. For fabrication of the FeNT, Fe doped TiO2 (FeT) was first prepared by a nonhydrolytic sol−gel route and N doping was subsequently performed by heating the resultant materials under a flow of NH3. The samples were analyzed by XRD, SEM, UV-vis DRS, FTIR, PL, and XPS to investigate the structural, electronic and optical features. FeNT samples exhibit about 1.5 fold higher direct solar light induced photocatalytic activity than the FeT catalyst and N doped TiO2 (NT). A noticeable red shift of the absorption edge and ca. 1.2 eV band gap narrowing is confirmed by UV-vis DRS. XPS has been applied to characterize the element oxidation state and near surface composition of catalysts. The presence of Fe ion facilitates the surface charge transfer process. Overall, the synergistic effect of ir...

How to switch from a tip to base growth mechanism in carbon nanotube growth by catalytic chemical vapour deposition

The catalytic growth of carbon nanotubes is performed at 580 °C with iron catalyst using a hot-wall CVD technique. It is shown that the growth mode can be switched from ‘tip’ to ‘base’ growth mode by an insertion of plasma pre-treatment of catalyst at room temperature before growth. To understand this phenomenon, the oxidation state of catalyst is compared using XPS before and after the pre-treatment. Growth mechanisms induced by different oxidation states of catalyst are proposed to explain the switching of the growth mode. A thermodynamic calculation shows the relation between induced growth mode and resulting CNT diameter.

A REACTION-EXTRACTION-REGENERATION SYSTEM FOR HIGHLY SELECTIVE OXIDATION OF BENZENE TO PHENOL

The reaction-extraction-regeneration system for the liquid-phase oxidation of benzene to phenol in the benzene-water-oxygen system was investigated. Phenol was extracted in the extractor to reduce the concentration of phenol in the benzene phase. As vanadium catalyst was oxidized to inactive species after the oxidation reaction, the regenerator was installed in the system to reduce the oxidation state of vanadium catalyst from V4+ or VO2+ to the active V3+ under H2 flow. The effects of various operating parameters including concentration of VCl3 catalyst, O2 and H2 flow rates, benzene bubble size, pH, surface area of Pt regeneration catalyst, the metal species, and amount of ascorbic acid were investigated. Ascorbic acid was employed as a reducing agent for helping reduce the V4+ form to the active form and therefore improving the activity of vanadium catalyst. VCl3 catalyst concentration of 10 mol/m3 with pH of 3–4 in the reactor and Pt surface area of 0.05 m2 in the regenerator showed optimal conditions for the system.

Mechanism of Cis-Dihydroxylation and Epoxidation of Alkenes by Highly H2O2 Efficient Dinuclear Manganese Catalysts

In the presence of carboxylic acids the complex [Mn(IV)2(micro-O)3(tmtacn)2]2+ (1, where tmtacn = N,N',N''-trimethyl-1,4,7-triazacyclononane) is shown to be highly efficient in catalyzing the oxidation of alkenes to the corresponding cis-diol and epoxide with H2O2 as terminal oxidant. The selectivity of the catalytic system with respect to (w.r.t.) either cis-dihydroxylation or epoxidation of alkenes is shown to be dependent on the carboxylic acid employed. High turnover numbers (t.o.n. > 2000) can be achieved especially w.r.t. cis-dihydroxylation for which the use of 2,6-dichlorobenzoic acid allows for the highest t.o.n. reported thus far for cis-dihydroxylation of alkenes catalyzed by a first-row transition metal and high efficiency w.r.t. the terminal oxidant (H2O2). The high activity and selectivity is due to the in situ formation of bis(micro-carboxylato)-bridged dinuclear manganese(III) complexes. Tuning of the activity of the catalyst by variation in the carboxylate ligands is dependent on both the electron-withdrawing nature of the ligand and on steric effects. By contrast, the cis-diol/epoxide selectivity is dominated by steric factors. The role of solvent, catalyst oxidation state, H2O, and carboxylic acid concentration and the nature of the carboxylic acid employed on both the activity and the selectivity of the catalysis are explored together with speciation analysis and isotope labeling studies. The results confirm that the complexes of the type [Mn2(micro-O)(micro-R-CO2)2(tmtacn)2]2+, which show remarkable redox and solvent-dependent coordination chemistry, are the resting state of the catalytic system and that they retain a dinuclear structure throughout the catalytic cycle. The mechanistic understanding obtained from these studies holds considerable implications for both homogeneous manganese oxidation catalysis and in understanding related biological systems such as dinuclear catalase and arginase enzymes.

Oxidation of methanol on Ru catalyst: Effect of the reagents partial pressures on the catalyst oxidation state and selectivity

In situ core level photoelectron spectroscopy and mass spectrometry have been utilized to study the methanol oxidation on a model RuO 2 catalyst at pressures ranging from 10 −6 to 10 −1 mbar. The experiments were carried out varying the O 2/CH 3OH molecular mixing ratio from 0.25 to 3.3 and the reaction temperature from 350 to 720 K. The Ru 3d 5/2 and O 1s core level spectra were used to characterise the dynamic changes in the Ru oxidation state by exposing the oxide pre-catalyst to different reagents partial pressures and temperatures. Full oxidation to CO 2 + H 2O or partial oxidation to CO + H 2O + H 2 have been observed in the whole pressure range for specific reaction conditions, which preserve the oxide catalyst state or reduce the oxide to metallic Ru. The selective oxidation to formaldehyde is observed only at pressures in the 10 −1 mbar range, catalyzed by a RuO x surface oxide formed by partial reduction of the oxide pre-catalyst.

Structure and Oxidation State of Silica-Supported Manganese Oxide Catalysts and Reactivity for Acetone Oxidation with Ozone

Silica-supported manganese oxide catalysts with loadings of 3, 10, 15, and 20 wt % (as MnO2) were characterized with use of X-ray absorption spectroscopy and X-ray diffraction (XRD). The edge positions in the X-ray absorption spectra indicated that the oxidation state for the manganese decreased with increasing metal oxide loading from a value close to that of Mn2O3 (+3) to a value close to that of Mn3O4 (+2(2)/3). The XRD was consistent with these results as the diffractograms for the supported catalysts of higher manganese oxide loading matched those of a Mn3O4 reference. The reactivity of the silica-supported manganese oxide catalysts in acetone oxidation with ozone as an oxidant was studied over the temperature range of 300 to 600 K. Both oxygen and ozone produced mainly CO2 as the product of oxidation, but in the case of ozone the reaction temperature and activation energy were significantly reduced. The effect of metal oxide loading was investigated, and the activity for acetone oxidation was greater for a 10 wt % MnOx/SiO2 catalyst sample compared to a 3 wt % MnOx/SiO2 sample.

Gold catalysts for pure hydrogen production in the water–gas shift reaction: activity, structure and reaction mechanism

The production of hydrogen containing very low levels of carbon monoxide for use in polymer electrolyte fuel cells requires the development of catalysts that show very high activity at low temperatures where the equilibrium for the removal of carbon monoxide using the water-gas shift reaction is favourable. It has been claimed that oxide-supported gold catalysts have the required high activity but there is considerable uncertainty in the literature about the feasibility of using these catalysts under real conditions. By comparing the activity of gold catalysts with that of platinum catalysts it is shown that well-prepared gold catalysts are significantly more active than the corresponding platinum catalysts. However, the method of preparation and pre-treatment of the gold catalysts is critical and activity variations of several orders of magnitude can be observed depending on the methods chosen. It is shown that an intimate contact between gold and the oxide support is important and any preparative procedure that does not generate such an interaction, or any subsequent treatment that can destroy such an interaction, may result in catalysts with low activity. The oxidation state and structure of active gold catalysts for the water-gas shift reaction is shown to comprise gold primarily in a zerovalent metallic state but in intimate contact with the support. This close contact between small metallic gold particles and the support may result in the "atoms" at the point of contact having a net charge (most probably cationic) but the high activity is associated with the presence of metallic gold. Both in situ XPS and XANES appear unequivocal on this point and this conclusion is consistent with similar measurements on gold catalysts even when used for CO oxidation. In situ EXAFS measurements under water gas shift conditions show that the active form of gold is a small gold cluster in intimate contact with the oxide support. The importance of the gold/oxide interface is indicated but the possible role of special sites (e.g., edge sites) on the gold clusters cannot be excluded. These may be important for CO oxidation but the fact that water has to be activated in the water gas shift reaction may point towards a more dominant role for the interfacial sites. The mechanism of the water gas shift reaction on gold and other low temperature catalysts has been widely investigated but little agreement exists. However, it is shown that a single "universal" model is consistent with much of the experimental literature. In this, it is proposed that the dominant surface intermediate is a function of reaction conditions. For example, as the temperature is increased the dominant species changes from a carbonate or carboxylate species, to a formate species and eventually at high temperatures to a mechanism that is characteristic of a redox process. Similar changes in the dominant intermediate are observed with changes in the gas composition. Overall, it is shown that reported variations in the kinetics, structure and reaction mechanism for the water gas shift reaction on gold catalysts can now be understood and rationalised.

Electrochemical promotion of the oxidation of propane on Pt/YSZ and Rh/YSZ catalyst-electrodes

The effect of electrochemical promotion (EP) or non-faradaic electrochemical modification of catalytic activity (NEMCA) was studied in the catalytic reaction of the total oxidation of propane on Pt and Rh films deposited on Y2O3-stabilized-ZrO2 (or YSZ), an O 2) conductor, in the temperature range 420–520 � C. In the case of Pt/YSZ and for oxygen to propane ratios lower than the stoichiometric ratio it was found that the rate of propane oxidation could be reversibly enhanced by application of both positive and negative overpotentials (‘‘inverted volcano’’ behavior), by up to a factor of 1350 and 1130, respectively. The induced rate increase Dr exceeded the corresponding electrochemically controlled rate I/2F of O 2) transfer through the solid electrolyte, resulting in absolute values of the apparent faradaic efficiency K=Dr/(I/2F) up to 2330. The Rh/YSZ system exhibited similar EP behavior. Abrupt changes in the oxidation state of the rhodium catalyst, accompanied by changes in the catalytic rate, were observed by changing the O2 to propane ratio and catalyst potential. The highest rate increases, by up to a factor of 6, were observed for positive overpotentials with corresponding absolute values of faradaic efficiency K up to 830. Rate increases by up to a factor of 1.7 were observed for negative overpotentials. The observed EP behavior is explained by taking into account the mechanism of the reaction and the effect of catalyst potential on the binding strength of chemisorbed reactants and intermediates and on the oxidative state of the catalyst surface.

05/00769 Effects of support and additive on oxidation state and activity of Pt catalyst in propane combustion

05/00769 Effects of support and additive on oxidation state and activity of Pt catalyst in propane combustion

XPS investigations of VPO catalysts under reaction conditions

The surface of vanadium phosphorus oxide (VPO) catalysts was investigated by (in situ) X-ray photoelectron spectroscopy (XPS) under reaction conditions. Two differently prepared VPO samples with similar catalytic activities showed different spectral behaviour while the catalytic conditions were changed. The vanadium surface oxidation state of both catalysts was found to have the same value close to 4 under reaction conditions, while the oxidation state of vanadium in deeper layers differed significantly. The experimental results suggest that in VPO the catalytically active species located in the topmost surface layers (up to 1 nm depth) are only weakly related to the structure of deeper layers. Based on our results we suggest that the deeper layers act as a substrate material only and can be different from the surface.

Effects of support and additive on oxidation state and activity of Pt catalyst in propane combustion

The effects of support and additive on the oxidation state and catalytic activity of Pt catalyst in the low temperature propane combustion were systematically investigated on Pt/MgO, Pt/Al 2O 3 and Pt/SiO 2–Al 2O 3. The catalytic activity varied much with both support materials and additives. The catalyst on the more acidic support showed higher activity, and the catalytic activity on every support materials increased as the electronegativity of additives increased, while some additives decreased the activity. The oxidation state of platinum, estimated by white line intensity of Pt L III-edge XANES spectrum, also varied with the support and additives, and additives with higher electronegativity greatly prevented the platinum from its oxidation under oxidising atmosphere. Among almost all the catalysts with various supports and various additives, a clear relationship was observed between the oxidation state of platinum and the catalytic activity; the more metallic platinum showed higher activity. Thus, it was concluded that the total electrophilic/electrophobic property derived from those of the support and additive controls the oxidation state of platinum, which intensively affects the catalytic activity; i.e. higher electrophilic property provides less oxidised platinum, resulting in high catalytic activity. The mechanism of this effect was also discussed on the basis of thermochemical data, and it was proposed that the electrophobic materials promote the noble metal oxidation since the noble metal oxo-anion such as PtO α δ− is more stabilised with electrophobic cation.

Interaction of N 2O (as gas dope) with nickel molybdate catalysts during the oxidative dehydrogenation of propane to propylene

We report effects brought about by the introduction of N 2O as gas dope in the reaction feed during the oxidative dehydrogenation of propane to propylene on nickel molybdate catalysts. The addition of N 2O modifies the catalytic performances. At low concentration (300 ppm) of N 2O, the conversion of propane decreases, the yield in propylene increases and the selectivity to propylene is improved. Decreases in the oxygen consumption, in the yield and in the selectivity in CO 2 were also observed. N 2O induces the reduction of Mo at the surface of the catalyst. This is supposed to occur through an inhibition of the adsorption of O 2 because of the presence of N 2O at the surface of the catalyst, which thus slows down the reoxidation rate of the catalyst. In presence of N 2O, catalytic sites thus work in a more reduced state. At higher concentrations of N 2O (1–10% in volume), the selectivity in propylene decreases, the oxygen consumption and the selectivity to CO 2 decrease further. The catalyst is more deeply reduced and carbonaceous products is formed. These observations are interpreted as being due to the inhibition effect evoked at low concentration but additionally to a direct reduction of the catalytic surface brought about by N 2O. Propylene is produced from propane in presence of pure N 2O. But this probably occurs only under transitory conditions as N 2O probably needs the presence of molecular oxygen to produce propylene durably. This work shows that adjusting the concentration of promoters, like N 2O and CO 2, could be a useful tool to modulate the oxidation state of oxide catalysts at work during the catalytic reaction, and thus their catalytic performances.

The CuCl2/Al2O3 Catalyst Investigated in Interaction with Reagents

Alumina supported CuCl2, the basic catalyst for ethylene oxychlorination, has been investigated by UV-Vis spectroscopy, EPR, EXAFS and XANES in a wide range (0.25-9.0 wt%) of Cu concentration. We have evidenced that, at low Cu content, the formation of a surface aluminate species takes place. The formation of this surface copper aluminate stops at 0.95 wt% Cu / 100 m2; at higher Cu concentrations excess copper chloride precipitates directly from solution during the drying step forming an highly dispersed CuCl2.H2O, phase, overlapping progressively the surface aluminate. Depletion tests and IR spectroscopy of adsorbed NO have demonstrated that the latter is the only active phase. A complete catalytic cycle has then been performed on CuCl2/Al2O3 catalyst. EPR, XANES and EXAFS, have been used to demonstrate that the ethylene oxychlorination reaction: C2H4 + 2HCl + ½ O2 --> C2H4Cl2 + H2O follows a three steps mechanism: (i) reduction of CuCl2 to CuCl (2CuCl2 + C2H4 --> C2H4Cl2 + 2CuCl), (ii) oxidation of CuCl to give an oxychloride (2CuCl + ½ O2 --> Cu2OCl2) and (iii) closure of the catalytic circle by rechlorination with HCl, restoring the original CuCl2 (Cu2OCl2 + 2HCl --> 2CuCl2 + H2O). Finally, we have shown that time resolved, in situ, spectroscopy is a very promising technique to investigate the interplay between catalyst activity and oxidation state of copper.

Organic Synthesis in Ionic Liquids: The Stille Coupling

[figure: see text] The Stille coupling reaction has been performed in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM BF4), a room-temperature ionic liquid (RTIL). Use of this solvent system allows for facile recycling of the solvent and catalyst system, which can be used at least five times with little loss in activity. An interesting preference in starting catalyst oxidation state for use with aryl bromides and aryl iodides was observed.

Solid electrolyte potentiometry aided study of reactions on oxidic catalysts — results and perspectives

Combined kinetic and potentiometric measurements have been used in the investigation of partial oxidations of organic compounds on oxidic catalysts of increasing complexity. The response of the catalyst to variations of the gas phase composition consists of abrupt phase transitions in the case of propene oxidation on copper oxides and to the existence of a large ‘concentration window’ in the case of acrolein oxidation on a multicomponent oxide where the oxidation state of the catalyst is kept constant at values favourable for high selectivity. Solid electrolyte potentiometry is shown to be an excellent tool to probe in situ the oxidation state of catalysts under working conditions.

Oxidation state of bimetallic PdCu catalysts during liquid phase nitrate reduction

The oxidation state and the structural properties of Al2O3-supported bimetallic PdCu catalysts during the catalytic reduction of KNO3 carried out in the aqueous phase were investigated by X-ray absorption spectroscopy. Under reaction conditions the noble metal component (Pd) was in a reduced state, while the less noble metal (Cu) was found to be partially oxidized. A PdCu phase was formed in the bimetallic catalysts, which appears to be located in small domains on the surface of Pd rich particles.

TEM Study of the Microstructural Modifications of an Alumina-Supported Palladium Combustion Catalyst

An α-alumina-supported Pd catalyst was studied for the catalytic combustion of methane. Changes in catalyst activity due to variations in the oxygen partial pressure were correlated with corresponding changes in the catalyst oxidation state, particle size, and morphology. At a temperature of 1070 K, the apparent catalyst activity increased with time under the reaction conditions where decomposition of the PdO phase occurs. The formation of large, faceted crystallites of metallic Pd, 200–300 nm in size, was observed by TEM during this process. The increase in activity with increasing particle size is counterintuitive and suggests that metallic Pd surfaces formed under reaction conditions are highly reactive. Since the activation energy for methane oxidation over Pd metal is greater than that over PdO, our results can only be explained by an increase in the pre-exponential factor for this reaction on metal surfaces. To gain insight into the changes in morphology, a model Pd/α-alumina catalyst was subjected to similar thermal treatments. It was found that decomposition of PdO leads to the formation of large Pd metal particles. Upon cooling, these Pd particles re-oxidize by the formation of rough polycrystalline PdO particles rather than by the formation of uniform layers of Pd oxide on the Pd metal. Our observations suggest a mechanism to explain the phenomenon of Pd catalyst activation observed in our previous research and by other research groups.

Acid Strength of Support Materials as a Factor Controlling Oxidation State of Palladium Catalyst for Propane Combustion

The support effect on the low temperature catalytic combustion of propane over palladium catalyst was studied by using a series of metal oxides as support materials: MgO, ZrO2, Al2O3, SiO2, SiO2–ZrO2, SiO2–Al2O3, and SO42−–ZrO2. The catalytic activity varied with the kind of the support materials; a support material with moderate acid strength gave maximum conversion. In order to discuss more details, 5 wt% Pd/ZrO2, Pd/Al2O3, and Pd/SiO2–Al2O3 were subjected to catalytic testing under the various conditions, and the catalysts after the test were characterized by XRD and XPS. The activity changed with the oxygen concentration, and the concentration providing the maximum activity depended on the support material. The maximum activity was obtained at high concentration on Pd/SiO2–Al2O3, at moderate concentration on Pd/Al2O3, and at low concentration on Pd/ZrO2. This sequence corresponded to that of acid strength of the support materials measured by Hammett indicators. The relation between the oxidation state of palladium and its catalytic activity under the various mixtures indicated that the catalytic activity on each catalyst depends on the oxidation state of palladium, and that partially oxidized palladium is effective for propane combustion. Moreover, palladium on acidic support kept the metallic state even under high oxygen concentration, indicating that acidic support gives palladium the resistibility against oxidation. These results suggest one guideline for the design of active palladium catalyst; an acidic support must be chosen under the mixture of the higher oxygen concentration and vice versa.

ESR studies on oxidation state of titanocene and zirconocene catalysts

An on-line electron spin resonance (ESR) technique was applied to investigate the oxidation states of the metallocene catalysts CpTiCl 3 , CpZrCl 3 , Cp 2 TiCl 2 , and Cp 2 ZrCl 2 . These metallocene catalysts were activated by modified methylaluminoxane (MMAO). It was found that the titanocene catalysts (CpTiCl 3 and Cp 2 TiCl 2 ) were readily reduced to the trivalent state while the zirconocene catalysts (CpZrCl 3 and Cp 2 ZrCl 2 ) were quite stable with respect to reduction. The concentrations of the trivalent species Ti(III) and Zr(III) showed linear relationships with the concentrations of metallocene catalyst precursors. However, their slopes were always smaller than unity indicating the existence of bimetallic interactions of the active sites. The ESR detectable Ti(III) and Zr(III) concentrations initially increased with the MAO/catalyst ratio and then leveled off after an 800-1000 Al/catalyst molar ratio. The deactivation processes were followed as a function of aging time over a range of temperature (25-100°C). The decay curves strongly depended on aging temperature with higher temperature giving faster decay rates.