Oxidation State Of Pd Research Articles

Thermal Stable Pd/Ce0.2Y0.8O2-δ Catalysts for CO and CH4 Oxidation

A series of Pd/Ce0.2Y0.8O2-δ catalysts were prepared and tested for CO and CH4 oxidation. It is found that the Pd/Ce0.2Y0.8O2-δ catalysts are thermal stable, and the catalytic activity enhances with increasing calcination temperature. The enhancement of the activity is due to the decomposition of surface carbonate species and the increase in average oxidation state of Pd.

Characterization and oxidation states of Cu and Pd in Pd–CuO/ZnO/ZrO2 catalysts for hydrogen production by methanol partial oxidation

Abstract Copper and zinc oxide based catalysts prepared by coprecipitation were promoted with palladium and ZrO2, and their activity and selectivity for methanol oxidative reforming was measured and characterized by N2O decomposition, X-ray absorption spectroscopy, BET, X-ray photoelectron spectroscopy, X-ray diffraction, and temperature programmed reduction. Addition of ZrO2 increased copper dispersion and surface area, with little effect on activity, while palladium promotion significantly enhanced activity with little change of the catalytic structure. A catalyst promoted with both ZrO2 and palladium yielded hydrogen below 150 °C. EXAFS results under reaction conditions showed that the oxidation state of copper was influenced by palladium in the catalyst bulk. A palladium promoted catalyst contained 90% Cu0, while the copper in an unpromoted catalyst was 100% Cu1+ at the same temperature. Palladium preferentially forms an unstable alloy with copper instead of zinc during reduction, which persists during reaction regardless of copper oxidation state. A 100-h time on stream activity measurement showed growth in copper crystallites and change in copper oxidation state resulting in decreasing activity and selectivity. A kinetic model of the reaction pathway showed that palladium and ZrO2 promoters lower the activation energy of methanol combustion and steam reforming reactions.

An Operando Spectroscopic Investigation of the NO/H2 Reaction on LaCoO3 and Pd-modified LaCoO3 — Influence of O2 on Catalyst Performances and Structure of Adsorbed Species

The catalytic reduction of NO by hydrogen has been studied over palladium-based catalysts supported on LaCoO3. Particular attention has been paid to the observation of chemisorbed N-containing species from operando infrared spectroscopic experiments during the NO/H2 reaction in the presence and in the absence of excess oxygen. X-ray photoelectron spectroscopy coupled to a catalytic chamber was used to examine parallel changes in oxidation state and chemical environments of Co and Pd after exposure to various reactive conditions. Complementary atomic information were obtained from the examination of the N 1s photopeak and compared to those obtained from IR observations. Dinitrosyl species adsorbed on oxidic cobalt species were observed at low temperature, whereas nitrate/nitrite species and NHx species predominate at high temperature. The presence of dinitrosyl species has been discussed for elucidating surface changes in the presence of oxygen and related modifications in the selectivity behavior of prere...

Direct oxidation of H2 to H2O2 over PdO/Al2O3 catalysts in aqueous acidic medium: Influence on H2O2 formation of Pd loading, calcination temperature and reduction of catalyst and presence of halide anions

Effects of the Pd loading (0.2–10 wt%), calcination temperature (400–900 °C) and reduction of PdO/Al2O3 on its performance in the H2-to-H2O2 oxidation (with H2/O2 = 1:1) in an aqueous 0.1 M H3PO4 medium, in the absence and presence of different halide anions, under ambient conditions have been investigated. The effect of calcination temperature on the H2O2 formation varied depending upon the Pd loading. The influence of the different halide anions was found to depend strongly on the nature of halide anion (F−, Cl−, Br− or I−) and the oxidation state of Pd and its loading in the catalyst.

Electrochemical behaviour of palladium electrode: Oxidation, electrodissolution and ionic adsorption

Abstract In electrochemistry, Pourbaix diagrams also known as potential-pH diagrams map out the regions of stability of metals as well as the regions of possible existence of stable compounds. The stable states other than the metallic one are macroscopic compounds, typically oxides and hydroxides, or their hydrated forms. Yet, several noble metals possess the ability to form thin surface oxides, to adsorb anions, and to adsorb or to absorb hydrogen in the potential range of water stability. Palladium is a unique noble metal owing to its ability to adsorbed and to absorb H, as well as to form compact and stable surface oxides. Anodic polarization of Pd results in the formation of a surface oxide. Thus, a great majority of anodic electrode processes takes place not at a metallic surface but at a surface covered with an oxide layer. Although thin surface oxides reveal metallic conductivity, the presence of O-containing species affects the electron transfer kinetics and the adsorption behaviour of reactants, intermediates, and products. Thus, the electrocatalytic properties of Pd electrodes, and the mechanism and kinetics of the reaction under consideration are strongly affected by the oxide layer, its thickness, chemical composition (Pd oxidation state and nature of O-containing species) and 3D structure, degree of oxide hydration, and electronic properties. This contribution presents an overview of the current understanding of electro-oxidation of Pd in aqueous acidic and basic electrolytes. In particular, it describes the formation of Pd oxides under various experimental conditions and discusses their chemical and physical nature. It examines the reduction of O-containing species present on Pd electrodes as well as the adsorption of anions and cations. Since Pd can undergo electrodissolution that results in material loss, the process is analyzed in relation to experimental parameters. Recent developments in the electrochemical behaviour of single-crystal Pd electrodes are discussed. Electrochemical parameters such as the potential of zero charge, potential of zero total charge and potential of zero free charge are of importance to the structure of the electric double layer; thus, their values are discussed in relation to the absence/presence of Pd surface oxides. Finally, various experimental procedures commonly used to determine the real surface area of Pd electrodes are outlined and compared.

Start Smart

Palladium(0) complexes are widely used as homogeneous catalysts for formation of carbon-carbon, carbon-oxygen, and carbon-nitrogen bonds. In general, the active catalysts are too unstable to store, and so precursors [often in the Pd(II) oxidation state] are prepared with stabilizing ligands that dissociate under the reaction conditions. However, the mechanisms and efficiency whereby these precursors transform into active catalysts have largely gone unaddressed, as has the potentially inhibitory effect of the stabilizing ligands left behind in the reaction solution. Biscoe et al. undertook a more careful approach by synthesizing a stable Pd(II) precursor complex resembling a reaction intermediate along the catalytic cycle. In three efficient steps, they appended a cyclometalated phenyl ring with a tethered chelating amine group to the Pd center. Exposure of this precatalyst to basic reaction conditions in the presence of aryl chlorides and amines led to rapid liberation of the protective ligand as an inert dihydroindole, leaving the resultant Pd(0) complex free to proceed with a similar C-N coupling cycle of the bulk reagents. In comparison with traditional precatalysts, these complexes dramatically accelerated coupling reactions (in one case from 4 days to 4 hours), allowing loadings below 1 mol % and reaction temperatures at or below 25°C for sensitive substrates. The absence of interfering precatalyst ligands also facilitated clear mechanistic studies. — JSY

Activity and selectivity for hydroisomerisation of n-decane over Ni impregnated Pd/H-mordenite catalysts

Nickel impregnated Pd/H-mordenite catalysts with different Ni contents were synthesised and tested for hydroisomerisation of n-decane with flow H2 gas. The XRD analysis of the catalysts showed the decrease of intensity with increase in Ni addition. The average metal particle size, measured by TEM, is also increased by increasing the Ni loading up to 0.2 wt.%. The acidity of the catalysts measured by TPD of ammonia revealed the ion-exchange of the some acid sites by Ni2+. The oxidation states of Pd and Ni were identified by XPS. The reducibility of H-mordenite supported Pd, Ni and Ni-Pd catalysts was studied by TPR. The catalytic activities of Ni containing and Ni free Pd/H-mordenite catalysts were studied and found that addition of Ni up to 0.2 wt.% over 0.1 wt.% Pd/H-mordenite, increased the n-decane conversion and isomerisation selectivity. As the Ni content exceeded 0.2 wt.%, the conversion and cracked products are found to decrease and increase. Further Ni containing Pd/H-mordenite showed increased sustainability and favoured the protonated cyclopropane (PCP) intermediate mechanism in n-decane isomerisation. It has been concluded that Ni impregnated Pd/H-mordenite showed good activity towards n-decane isomerisation where isomerized products mixture showed better octane number.

Microwave-assisted cloud point extraction of Rh, Pd and Pt with 2-mercaptobenzothiazole as preconcentration procedure prior to ICP-MS analysis of pharmaceutical products

New approaches for acceleration of cloud point extraction of Rh, Pd and Pt by microwave and ultrasound irradiation have been investigated. 2-Mercaptobenzothiazole and Triton X-100 were used as ligand and non-ionic surfactant, respectively. Microwave irradiation substantially increased the procedure efficiency. In contrast no significant effect was observed when a micellar solution was treated with ultrasound energy. The acceleration of complex formation reactions by reducing agents (KI and SnCl2) has also been studied. The use of SnCl2 was a prerequisite for quantitative extraction of Rh. Since the extraction kinetics of platinum depends on its oxidation state and presence of Pd, the addition of SnCl2 equalized the platinum extraction behavior. For the first time the whole cloud point procedure was successively accomplished in a microwave system. 10 min of irradiation were sufficient for gravimetrical phase separation and quantitative extraction of platinum group metals. As a result the procedure time was shortened by a factor of 9 in comparison to conventional hot plate cloud point extraction. The surfactant-rich phase was analyzed after dilution with 1 M HCl by continuous nebulization to ICP-MS. Quantitative recoveries and good plasma tolerance were reported. Detection limits of 1 ng.l−1 (Rh), 5 ng.l−1 (Pd), and 6 ng.l−1 (Pt) were achieved regarding a preconcentration factor of 8. The microwave-assisted cloud point extraction has been successfully applied for simultaneous determination of trace amounts of platinum group metals in pharmaceutical products (Tritace, Vivace, Laprilen and Enalapril).

Hydrogenation of nitrotoluene using palladium supported on chitosan hollow fiber: Catalyst characterization and influence of operative parameters studied by experimental design methodology

The strong affinity of chitosan for metal ions and more specifically for precious metals such as palladium and platinum has focused the interest on using this biopolymer as a support for catalytic metals. The manufacturing of hollow chitosan fibers, softly cross-linked with glutaraldehyde, followed by palladium sorption at pH 2 in HCl solutions and further reduction using hydrogen gas, opened the route for the design of a new continuous catalytic system. This material was used for the hydrogenation of nitrotoluene, which was converted into o-toluidine, in methanol solutions. The substrate was circulated inside the lumen of the fiber, while the hydrogen donor (hydrogen gas) was maintained at constant pressure in the outlet compartment of the reactor. Several parameters (substrate concentration, metal content in the fiber, and flow rate) have been tested for their impact on catalytic performance, measured by the turnover frequency (TOF), conversion yield or o-toluidine production, using a surface response methodology for the optimization of the process. Metal content in the fiber revealed a critical parameter; the influence of this parameter was extensively studied through the structural characterization of the fibers using XPS analysis (oxidation state of Pd), X-ray diffraction analysis (size of Pd crystals), TEM analysis (size and distribution of Pd crystals), and diffusion profiles (porosity) in order to correlate catalytic performance to fiber characterization.

Decomposition and/or hydrogenation of hydrogen peroxide over Pd/Al2O3 catalyst in aqueous medium: Factors affecting the rate of H2O2 destruction in presence of hydrogen

Hydrogen peroxide destruction by its decomposition and/or hydrogenation to water in the presence of H2 over Pd(5 wt.%)/Al2O3 catalyst in an aqueous reaction medium, similar to that used in the H2-to-H2O2 oxidation, has been thoroughly investigated at different reaction conditions. The H2O2 destruction is strongly influenced by the oxidation state of Pd in the catalyst and also by the presence of different halide anions (viz. F−, Cl−, Br− and I−) in the acidic aqueous medium or in the catalyst, depending upon the concentration of halide anions. The cations associated with halide anions have, however, a little or no influence on the H2O2 destruction. The iodide anions strongly poisoned the catalyst, even at their very low concentration. The fluoride anions have only a small effect on the H2O2 destruction. The chloride or bromide anions drastically inhibit the rapid H2O2 decomposition, but promote the slower H2O2 hydrogenation. The H2O2 destruction reactions are strongly influenced by the halide anion and acid concentrations. The presence of acid (protons) plays a very important role in drastically reducing the H2O2 destruction, particularly in the presence of chloride anions. Both in the presence and absence of Cl− or Br− anions, the H2O2 destruction activity of the catalyst is markedly reduced because of the oxidation of Pd but it is drastically increased due to the presence of H2. A plausible reaction mechanism for the rapid H2O2 decomposition and slower H2O2 hydrogenation reactions, prevailing under the different reaction conditions, has also been discussed.

Direct synthesis of H2O2 from H2 and O2 and decomposition/hydrogenation of H2O2 in an aqueous acidic medium over halide-modified Pd/Al2O3 catalysts

Abstract Direct synthesis of H 2 O 2 from its elements was carried out in an acidic aqueous reaction medium over halide-modified oxidized and reduced Pd/Al 2 O 3 catalysts under very mild conditions (at 27 °C and atmospheric pressure). The halide ions were introduced into the catalyst by incorporating halide ions into supported Pd/γ-Al 2 O 3 catalyst or via depositing halide ions on the support (γ-Al 2 O 3) prior to Pd deposition. The H 2 O 2 decomposition and hydrogenation over the corresponding catalysts were also carried out under the reaction conditions similar to those employed for the H 2 O 2 synthesis in order to elucidate the factors strongly affecting the H 2 O 2 yield/selectivity in the direct H 2 O 2 process. The performance of halide-modified Pd/Al 2 O 3 catalysts in the direct H 2 O 2 synthesis revealed that halide insertion in the catalyst system prior to or after Pd deposition on the support had comparable qualitative effect on the H 2 O 2 formation. Both the Pd oxidation state and the nature of the halide ions had strong influences on the H 2 conversion (in direct H 2 O 2 synthesis process) and H 2 O 2 decomposition and/or hydrogenation reaction. While the effect of Pd oxidation state on the H 2 O 2 formation was significant for the catalytic system containing F − and Cl − ions, the influence of the Pd oxidation state was found less important for the catalyst system containing Br − ions; the H 2 O 2 formation selectivity increased significantly due to the presence of Br − ions, irrespective of the Pd oxidation state. The nature of the H 2 O 2 destruction pathway (i.e. hydrogenation and/or decomposition) in the presence of hydrogen over halide-modified Pd/Al 2 O 3 catalysts was found to be strongly dependent upon the nature of the halide ions incorporated in the catalyst during halide-modification of the catalyst.

Direct oxidation of H2 to H2O2 over Pd/Ga2O3 catalyst under ambient conditions: Influence of halide ions added to the catalyst or reaction medium

The formation of H2O2 from H2 and O2 and H2O2 decomposition/hydrogenation have been investigated in an acidic aqueous reaction medium over halide-assisted Pd/Ga2O3 catalyst system. The halide ions were introduced in the catalytic system by directly adding the halide ions to the reaction medium or by indirectly through the catalyst. The H2O2 formation selectivity was found to be influenced significantly with the addition of halide ions to the catalytic system. The H2O2 formation and H2O2 decomposition/hydrogenation studies over halide-modified Pd/Ga2O3 catalysts (pretreated under different gaseous atmosphere viz. air and nitrogen) revealed that bulk Pd oxidation state and nature of halide ions incorporated in the catalyst cooperatively promote the selective H2O2 formation. The decomposition and hydrogenation H2O2 on the Pd/Ga2O3 catalyst were also found strongly depend upon the nature of the halide ions introduced in the catalytic system. The selectivity for the conversion of H2 to H2O2 was enhanced markedly due to the presence of Br− ions in the catalytic system, irrespective of the bulk palladium oxidation state (PdO or Pd0) in the catalyst. The beneficial role of Br− ions is attributed to the inhibition of the parallel H2 to water oxidation and the consecutive H2O2 decomposition and hydrogenation reactions over the catalyst.

Direct oxidation of H2 to H2O2 over Pd/CeO2 catalyst under ambient conditions: Influence of halide ions

Abstract Direct oxidation of H 2 by O 2 to H 2 O 2 and decomposition/hydrogenation of H 2 O 2 have been investigated over Pd/CeO 2 catalyst in presence of different halide ions. All reactions were carried out in an aqueous acidic (0.03 M H 3 PO 4 ) medium at room temperature (25 °C) and atmospheric pressure. When different halide ions were added directly to the reaction medium or incorporated in the catalyst, the activity/selectivity of Pd/CeO 2 catalyst in the H 2 to H 2 O 2 oxidation changed significantly, depending upon the nature of the halide ions. Different ammonium halides impregnated reduced Pd/CeO 2 catalyst calcined under inert (flowing nitrogen) and oxidizing (static/flowing air) gaseous atmospheres revealed that bulk oxidation state of Pd and the nature of the halide ions incorporated into the catalyst cooperatively control the activity/selectivity of catalyst in H 2 to H 2 O 2 oxidation. Both H 2 O 2 decomposition and hydrogenation were found to affect strongly depending upon the nature of the halide ions present in the reaction medium or in the catalyst. Among the different halides, bromide was found the most effective promoter for enhancing the H 2 O 2 yield/selectivity in H 2 to H 2 O 2 oxidation, irrespective of the Pd oxidation state in the catalyst.

Influence of nature/concentration of halide promoters and oxidation state on the direct oxidation of H2 to H2O2 over Pd/ZrO2 catalysts in aqueous acidic medium

The nature/concentration of halide promoters and influence of the Pd oxidation state on the promoted reaction system has been investigated on the direct H2O2 process over a 2.5 wt.% Pd/ZrO2 catalyst in an aqueous acidic reaction medium. The oxidation state of Pd had a profound influence on the H2O2 synthesis process. Interestingly, the nature of the halide determined the magnitude/type of influence the Pd oxidation state exerted on the overall process. While the effect of the oxidation state on the H2O2 yields was large for the reaction systems containing F− or no halide, the effect was significantly smaller for the reaction systems containing Br− and Cl−. The nature of the halide also strongly influenced the H2O2 synthesis process. Br− strongly enhanced the H2O2 yields, while F− had a negative influence on the H2O2 yields. The ability of the halides to enhance the H2O2 process was found to strongly depend on its propensity to suppress the secondary H2O2 decomposition reaction. The influence of Br− and Cl−concentration studies revealed that the optimum halide concentration for the direct H2O2 synthesis process was dependent on the nature of the halide. While the maximum in H2O2 yields for the Br− containing reaction medium corresponded to a concentration of ∼0.9 mmol/dm3 (KBr) the maximum for the Cl− containing solution was obtained at ∼1.5 mmol/dm3 (KCl). Such knowledge is crucial from the viewpoint of optimization (catalyst/reaction system screening studies) of the direct H2O2 process. The qualitative trends (H2O2 selectivity/yield) observed in case of the incorporated halide catalysts were similar to those observed with halides in reaction medium over the Pd/ZrO2 catalyst.

Catalytic reduction of N 2O and NO 2 with methane over sol–gel palladium-based catalysts

Pd/TiO 2 and Gd-doped Pd/TiO 2 prepared through a ‘one-pot’ sol–gel method are shown to be active for the reduction of both N 2O and NO 2 using CH 4 as a reducing agent under excess O 2 conditions. In previous studies these catalysts were shown to effectively reduce NO with CH 4, however activity was strongly dependant on the oxidation state of Pd. Through the electropositive nature of Gd, Pd was maintained in metallic state under higher oxygen concentrations compared to the Gd-free catalysts. Characterization through XPS and in situ DRIFTS studies indicate that in the reduction of NO 2, even under excess oxygen, a significant portion of the surface Pd is maintained in the active metallic state.

Direct synthesis of H 2O 2 from H 2 and O 2 over Pd/H-beta catalyst in an aqueous acidic medium: Influence of halide ions present in the catalyst or reaction medium on H 2O 2 formation

The influence of different halide ions present in the catalyst or reaction medium on the performance of Pd/H-beta catalyst in the direct H 2O 2 synthesis in an aqueous acidic (0.03 M H 3PO 4) reaction medium at 27 °C and atmospheric pressure has been thoroughly investigated. The results showed a strong influence of both the bulk Pd oxidation state in the catalyst and the halide ions added to the reaction medium on the performance of the catalyst in the H 2 to H 2O 2 oxidation, H 2O 2 decomposition/hydrogenation reactions. The different ammonium halides impregnated reduced Pd/H-beta catalyst calcined in inert (N 2) and oxidizing (air) gaseous atmospheres also revealed that the bulk Pd oxidation state and nature of the halide ions present in the catalyst together control the overall performance of the catalyst in the H 2O 2 formation reaction. The presence of halide ions in reaction medium or in the catalyst significantly changes the selectivity for H 2O 2 formation in the direct H 2O 2 synthesis. Bromide ions are found to remarkably enhance the H 2O 2 selectivity in the direct H 2O 2 synthesis irrespective of the Pd oxidation state in the catalyst. The promoting action of Br − is attributed mainly to the large decrease in the H 2O 2 decomposition and hydrogenation activities of the catalyst and also inhibition for the non-selective H 2-to-water oxidation over the catalyst.

Direct oxidation of H 2 to H 2O 2 over Pd-based catalysts: Influence of oxidation state, support and metal additives

The influence of oxidation state (reduced and oxidized), support (ZrO 2, Ga 2O 3, CeO 2, SiO 2, H-β, ThO 2, CeO 2–ZrO 2, BPO 4 and Pd/Al 2O 3) and precious metal additives (Au, Pt, Rh and Ru) on the direct H 2O 2 synthesis has been investigated over Pd-based catalysts. For all the supports investigated herein, the oxidized Pd catalysts showed significantly superior H 2O 2 yields as compared to their reduced counterparts. The effect of the Pd oxidation state was found to be more important for determining the H 2O 2 yields than the particle size and surface area properties of the catalyst system. An excellent correlation was observed between the H 2O 2 selectivity and H 2O 2 decomposition activity of the oxidized Pd catalysts. The oxidized Pd catalysts showed almost an order of magnitude lower H 2O 2 decomposition activity than the reduced catalysts. The H 2O 2 yield in the H 2 to H 2O 2 oxidation passed through a maximum with increase in the Au concentration. Although a similar effect was also observed in case of Pt, Au was found to be a superior promoter for the direct H 2O 2 synthesis process. Addition of Rh and Ru was found to be detrimental for the H 2O 2 yields; these results can be explained in terms of increased H 2O 2 decomposition activity and/or enhanced H 2 to H 2O reaction activity in their presence.

The effects of Fe on the oxygen storage and release properties of model Pd–Rh/CeO 2–Al 2O 3 three-way catalyst

This work attempts to gain fundamental information about the role of Fe in the chemical deactivation of a three-way catalyst (TWC), with Fe originating from materials used in the construction of automotive engines and exhaust pipe systems. Topics of particular interest are Fe's role as a chemical poison of the oxygen storage and release properties of a model TWC (1 wt% Pd–Rh/20 wt% CeO 2–Al 2O 3), its effects on the redox properties of Pd and Rh, and its effects on NO and CO chemisorption; these topics are investigated for the first time here. Oxygen storage capacity (OSC) measurements in the 500–850 °C range indicated that OSC increased after the addition of 0.1–0.3 wt% Fe in the model catalyst. It can be concluded that iron acted as an oxygen storage component under oxidizing conditions through the process Fe → FeO → Fe 3O 4 (FeO⋅Fe 2O 3) → Fe 2O 3, and not as a chemical poison of the oxygen storage and release properties of the solid. However, when the amount of deposited iron increased to 0.4 wt%, the OSC of the catalyst deteriorated but still remained slightly greater than that measured over the catalyst uncontaminated with Fe catalyst. H 2 temperature-programmed desorption and reduction and X-ray photoelectron spectroscopy (XPS) studies performed on the oxidized and reduced Fe-contaminated samples indicated that iron (0.1–0.3 wt%) was deposited on both noble metals (Pd and Rh) and support, causing the development of electronic interactions that influenced the oxidation states of Pd and Rh and enhanced the oxygen chemisorption properties of ceria. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies proved that iron deposited to a level of 0.2 wt% significantly favors the formation of a superoxide ( O 2 − ) species on CeO 2. The latter species was found to desorb completely at temperatures above 250 °C. However, at the 0.4 wt% level, O 2 − formation was minor. In situ DRIFTS chemisorption studies of CO and NO performed at 25 °C also provided important information about the electronic interactions between Pd and Fe in the model Pd–Rh–Fe/20 wt% CeO 2–Al 2O 3 TWC, confirming the XPS results.

Direct synthesis of hydrogen peroxide on zirconia-supported catalysts under mild conditions

Palladium catalysts supported on SO 4 = -, Cl −-, F −-, and Br −-doped zirconia were tested for the direct synthesis of hydrogen peroxide under very mild (1 bar and 20 °C) and nonexplosive conditions. The catalysts were characterized by thermogravimetric/differential scanning calorimetry analysis, N 2 physisorption, and temperature-programmed reduction before and after catalytic tests to investigate the oxidation state of the metal. The catalytic tests were carried out in different solvents, and the effect of the Pd oxidation state was ascertained. The best catalytic results were observed in methanol, using H 2/O 2 mixtures containing a large excess of oxygen and using the sulfate-doped zirconia catalyst. Surface-oxidized Pd 0 catalysts showed high catalytic activity and the highest selectivity.

Active control of catalysis and product selectivity by a fuel cell system

A new concept to control catalysis and catalytic reaction through partial oxidation of alkenes with O2 is described. Oxidation of alkenes was studied by alkene/Pd-anode/H3PO4-electrolyte/cathode/O2 fuel cell (FC). An idea based on electrocatalysis and electrochemical reactions to control reaction rates and product selectivity was proposed and proven through the oxidation of propylene, Wacker and π-allyl oxidation. The oxidation rate and the product selectivity to the Wacker and the π-allyl oxidations could be controlled by changing electrode potentials. We could active control oxidation states of Pd on the anode, Pd(II) or Pd(0), during the oxidation from outer circuit. The oxidation states of Pd on the anode decided the product selectivity.