Catalytic Activity Of Pt Research Articles

Catalytic Activity of Pt(II) Complexes in Addition of Tetrachloromethane to Alkenes

The use of catalytic systems based on Cu(I) and Co(II) complexes for addition of tetrachloromethane to alkenes was reported previously [1, 2]; CCl4, as a rule, was taken in a 5 10-fold molar excess. In this work we discovered that platinum(II) phosphine complexes also exhibit a catalytic activity in this reaction, and with these complexes the reaction can be performed with only a slight excess of CCl4 : CCl4 : 1-hexene = 1.3 : 1.0. For example, when a 1 : 1 : 1 (by volume, at 25 C) mixture of CCl4, 1-hexene, and acetonitrile is heated to 67 140 C in the presence of Pt(II) complexes (10 3 10 2 M), an addition product, 1,1,1,3-tetrachloroheptane, is formed in a yield of up to 35% (all the experiments were performed in sealed degassed evacuated ampules). As catalysts we used dichlorobis(triphenylphosphine)platinum(II), dichlorobis[tri(m-tolyl)phosphine] platinum(II), dimethylbis(triphenylphosphine)platinum(II), diiodobis(triphenylphosphine)platinum(II), and other compounds. The reaction can also be performed with other alkenes (1-heptene, cyclohexene). The results of these experiments will be reported in the subsequent papers.

Sulfur Poisoning of Bi- and Trimetallic γ-Al2O3-Supported Pt, Re, and Sn Catalysts

Bifunctional metal/acid catalysts with Pt, Pt−Re, Pt−Sn, and Pt−Re−Sn as metallic functions supported on chlorided γ-alumina were prepared by successive impregnation or by coimpregnation of aqueous solutions of the metal precursors. They were tested in cyclohexane dehydrogenation (400 °C, 1 atm, weight hourly space velocity = 10, and H2/CH = 30) with an intermediate step of sulfur poisoning (1, 3, 5, or 7 ppm S). The addition of Sn and/or Re produces a decrease in the catalytic activity of Pt. Both this activity decrease and the increase in sulfur poisoning follow the order trimetallic > bimetallic > monometallic catalysts. The catalysts prepared by coimpregnation are more affected than those prepared by successive impregnations. In the last case, the activity and poisoning depend on the order of addition of the metal precursors during preparation.

On the kinetics of oxygen reduction on platinum stepped surfaces in acidic media

Oxygen reduction on platinum single crystals has been studied in perchloric and sulfuric acid media using a hanging meniscus rotating disk electrode configuration. The surfaces studied belong to the [0 1 1] zone and can be classified in two different series, i.e., surfaces with terraces with (1 1 1) symmetry and (1 0 0) steps and surfaces with (1 0 0) terraces and (1 1 1) steps. In sulfuric acid media, the highest catalytic activity is found for the Pt(2 1 1) electrode whereas the Pt(1 1 1) electrode exhibits an anomalously low j 0 value. Conversely, the extrapolated value for the catalytic activity of Pt(1 1 1) obtained from the series of stepped surfaces having (1 1 1) terraces is much higher than that obtained experimentally. The comparison with the results obtained in perchloric acid solutions indicates that the anomalously low catalytic activity of the Pt(1 1 1) electrode in sulfuric acid solutions is related to the formation of an ordered adlayer of specifically adsorbed (bi)sulfate anions. For practical purposes, the effect of the step density and the (bi)sulfate specific adsorption is small, except for the Pt(1 1 1) electrode.

IMPURITY AND SUPPORT EFFECTS ON SURFACE COMPOSITION AND CO+NO REACTIONS OVER Pt–Rh/CeO2 NANOPARTICLES: A COMPARATIVE STUDY

A Monte Carlo simulation technique has been used to investigate the effect of sulphur impurity and ceria support on the surface composition and catalytic activity of Pt–Rh /ceria nanocatalysts. For temperatures in the range of 400–1300 K the surface Pt concentration decreases with increase in sulphur coverage. The Pt concentration at the base of the nanocatalyst in contact with the support increases with metal-support interaction. However, for weak metal-support interaction (Vms<-0.01 eV ) the surface Pt concentration slightly increases with metal-support interaction. Overall, sulphur is found to influence the surface composition much more strongly than the metal-support interaction. The MC-simulated surface composition results have been used to study the energetics of the CO+NO reactions over Pt–Rh/CeO 2 catalysts. It is found that the CO 2 formation rate decreases with increase in sulphur coverage and marginally increases with metal-support interaction up to Vms=-0.01 eV . All the results qualitatively agree with the experimental results.

Physicochemical Investigations of the Basicity of the Cation Exchanged ETS-10 Molecular Sieves

Alkali-metal exchanged M-ETS-10 (M= Li, Na, K, Rb, Cs, and Ba) molecular sieves have been investigated by different techniques such as X-ray diffraction, FTIR, TPD, and XPS. It is found that the overall Lewis basicity in M-ETS-10 increases as the cation is changed from Li to Cs in the alkali cation series. FTIR spectra and TPD of adsorbed CO2 indicate the presence of different types of adsorbed CO2 species. XPS studies reveal an increase in the extent of destabilization of O 2p valence levels from Li-ETS-10 to Cs-ETS-10 systems. A strong overlap is observed between Cs 5p and O 2p valence bands. Catalytic activity of Pt supported on M-ETS-10 for n-hexane conversion to benzene increases with the basicity of the support and the same is correlated with the FTIR, TPD, and XPS results.

Comparison of n-pentane reforming over Pt supported on amorphous and γ-Al2O3

The n-pentane reforming activity of Pt supported on nonhydrolytic amorphous Al2O3 (Pt/NH–Al2O3), was investigated and compared to the catalytic activity of Pt supported on crystalline γ-Al2O3. The Pt was introduced by (a) impregnation with either a solution of H2PtCl6 in water or a solution of platinum acetylacetonate (PtAcac) in toluene; (b) in situ introduction of a Pt precursor, either PtBr4 or cis-bis(benzonitrile)platinum dichloride, before gelation of the NH alumina. The rate-controlling step in the reforming of n-pentane for both amorphous and crystalline aluminas was found to be the reaction on the alumina acidic sites. The Pt/γ-Al2O3 catalysts exhibit higher conversions of n-pentane and higher selectivity to isopentane, than the corresponding amorphous alumina samples. After 1.5 h at 400 °C, the highest conversion of the γ-Al2O3-based catalysts was ∼47% with 20.3% selectivity to isopentane. The highest conversion of the NH–Al2O3-based catalysts under the same conditions was only ∼26% with 13.6% selectivity to isopentane. The high intrinsic Cl content (2.6 wt%) of the amorphous alumina was found to have a minor effect on the activity of the alumina, compared to the activity of the more ordered γ-alumina. Catalysts prepared by impregnation of the NH alumina with aqueous chloroplatinic acid, exhibited higher conversions compared to catalysts prepared by impregnation of the NH alumina with a solution of PtAcac in toluene. This result occurred in spite of the lower surface area and lower Pt dispersions of the chloroplatinic acid-impregnated catalysts, and is explained by the formation of microcrystalline surface structures and existence of surface chlorine.

Improvement of electrical characteristics of silicon oxynitride layers by a platinum method

The improvement of the electrical properties of silicon oxynitride layers formed by nitridation of silicon dioxide layers using the electron impact plasma method has been achieved by a platinum (Pt) method which includes the Pt deposition on the oxynitride layers followed by heat treatment at 300 °C in an oxygen atmosphere. A leakage current density and the density of oxide fixed positive charges induced by the plasma treatment are markedly decreased by the Pt treatment. Although the thickness of the oxynitride layers is not changed and the nitrogen concentration is decreased by the Pt treatment, the dielectric constant increases after the Pt treatment probably due to the elimination of defect states. It is concluded that the improvement of the electrical characteristics results from the injection of oxygen ions (O −), produced by the catalytic activities of Pt, into the oxynitride layers, and the elimination of defect states by a reaction with the O − ions.

Effect of a platinum membrane on the sensing properties of materials based on thin and thick tin dioxide films

A possible way to improve the selectivity of tin dioxide sensors is to deposit a membrane with specific properties above the sensing element. The purpose of this paper is to investigate the effect of a platinum membrane on the sensing properties of thin (chemical vapour deposition) and thick (screen-printing technology) SnO 2 films under gases (air, diluted pollutant gases CO, CH 4 or C 2H 5OH). The measurement of the catalytic activity of Pt has been investigated. At above 500 °C, this metal is very efficient for the total oxidation of CO and C 2H 5OH, but CH 4 does not react. As a consequence, it should be possible to develop a selective sensor to CH 4 in presence of C 2H 5OH and CO as interfering gases. Firstly, we tried to correlate this catalytic activity to the sensing detection properties of the thin SnO 2 film+Pt structure. The membrane strongly decreases the sensor response under alcohol, and does not affect the CH 4 one, which is in agreement with catalytic tests. In CO's case, at high temperature (500 °C), although Pt is very efficient with regard to CO oxidation, it does not strongly decrease the sensor response. To explain this result, a mechanism based on the contact between Pt particles and thin SnO 2 film is proposed. Hence, in order to decrease the CO's response, Pt membrane has to be insulated from SnO 2 film. A new structure, thick SnO 2 film+thick insulator coating (SiO 2)+Pt, has been tested. A good correlation between catalytic tests and the effects on the sensor response is obtained. However, contrary to thin film device, a good correlation is also obtained in the thick SnO 2 film+Pt structure when a contact between Pt and SnO 2 is materialised. In thick-film structures, no insulator coating seems to be necessary.

The influence of a platinum membrane on the sensing properties of a tin dioxide thin film

A possible way to improve the selectivity of tin dioxide sensors is to deposit a membrane with specific properties above the sensing element. The purpose of this paper is to investigate the effect of a platinum membrane on the sensing properties of a chemical vapour deposition (CVD) SnO 2 thin film under gases (air, pollutant gases CO, CH 4 or C 2H 5OH). Electrical measurements have shown that the platinum membrane thickness has to be limited to avoid a short-circuit of the SnO 2. For a small platinum layer thickness (10 nm) the conductance of SnO 2 film decreases with increasing platinum thickness. This effect is attributed to additional oxygen species on SnO 2, resulting from an enhanced oxygen dissociation by Pt. Concerning the influence on the selectivity, the measurement of the catalytic activity of Pt has been investigated. Above 500 °C, this metal is very efficient for the total oxidation of CO and C 2H 5OH, but CH 4 does not react. We tried to correlate this catalytic activity to the sensing detection properties of the SnO 2+Pt structure. Firstly, the membrane strongly decreases the sensor response under alcohol, which is true to catalytic tests. In CO case, on one hand, at high temperature (500 °C), although Pt is very efficient in regards of CO oxidation, it does not strongly decrease the sensor response. A mechanism is proposed, based on oxygen exchange between Pt particles and SnO 2 film during CO oxidation. On the other hand, at temperature lower than 150 °C, Pt membrane induces a strong increase of the conductance under CO. We have shown that this electrical phenomenon only results from Pt particles not from SnO 2 thin film. Finally, the CH 4 response is slightly decreased which is not true to catalytic properties of Pt.

Isomerization of n-alkanes on Pt/WO 3–SO 4/ZrO 2 systems

The role of the electronic state of Pt and acidic properties of anion-promoted support in n-alkane isomerization on Pt-containing sulfate- and tungstate-promoted zirconia (Pt/SZR and Pt/WZR) was studied. The addition of Pt with further oxidation and mild reduction is found to enhance the catalytic activity in n-hexane isomerization 10 times compared with metal-free system. The mechanism of the action of the pretreatment steps is proposed. Effect of the calcination temperature on the catalytic activity in n-alkane isomerization and physico-chemical properties of the catalysts is also evaluated. The catalytic activity of Pt promoted combined sulfated–tungstated zirconia (Pt/SWZR) catalysts in n-hexane and n-pentane isomerization decreases in the following order (the calcination temperature is shown in parentheses): Pt/ SZR (625 ° C)> Pt/ WSZR (625 ° C)> Pt/ WSZR (800 ° C)≈ Pt/ WZR (800 ° C) The properties of WZR (800 °C) and SWZR (800 °C) are connected with the surface WO 4 groups. The presence of sulfate species in Pt/SZR (625 °C) and Pt/WSZR (625 °C) is responsible for the strong acidic properties of these catalysts. The performance of classical sulfated and tungstated catalysts is slightly improved by the combination of anions.

Electrochemical characteristics of Pt–WO 3/C and Pt–TiO 2/C electrocatalysts in a polymer electrolyte fuel cell

The electrochemical characteristics and the catalytic activity of Pt–tungsten oxide and Pt–titanium oxide catalysts in polymer electrolyte fuel cell were investigated by cyclic voltammetry and in fuel cells. Fuel cell performances for those catalysts at 80°C with humidified hydrogen and oxygen increased with increasing added oxide content in the catalysts up to a certain content. Adsorption characteristics for hydrogen and oxygen on the surface of platinum were greatly influenced by added oxide content in the catalysts. It was also obtained that the electrochemically active surface area of these catalysts was influenced by their oxide content.

Effect of Ba Addition on Catalytic Activity of Pt and Rh Catalysts Loaded on γ-Alumina

The catalytic activity of Pt catalyst loaded on γ-alumina was improved by Ba addition in simulated automotive exhaust gases. On the other hand, the result of Rh catalyst was the opposite. From the results of the partial reaction orders in C3H6–O2 reaction and TPR, it was concluded that the Ba addition to Pt catalyst suppressed the hydrocarbon chemisorption on the Pt catalyst and therefore allowed the catalytic reaction to proceed smoothly. On the other hand, Ba addition to Rh catalyst caused such a strong oxygen adsorption on Rh that rejected the hydrocarbon adsorption and suppressed the reaction.

Catalytic activity of Pt and tungstophosphoric acid supported on MCM-41 for the reduction of NO x in the presence of water vapor

The catalytic activity of tungstophosphoric acid and Pt loaded MCM-41 for the catalytic reduction of NO x with propene in the presence of water vapor was studied. Pt/MCM-41 was found to be most active, the loading with H 3PW 12O 40 led to an improved selectivity to N 2 formation and to an enhanced activity in the presence of water vapor. Hydrated tungstophosphoric acid generated additional sorption sites for C 3H 6 and led to a higher local concentration of the reducing agent on the catalyst surface, which was found to increase the activity of the Pt and H 3PW 12O 40 loaded catalysts in the presence of water vapor.

The effects of SO2 on the oxidation of CO and propane on supported Pt and Au catalysts

The catalytic activities of Pt and Au supported on TiO2 were compared with respect to the oxidation of CO and propane. While the Au catalysts showed higher activities for CO oxidation, the Pt catalysts were more active for propane combustion. A strong de-activation of the CO oxidation activity by SO2 was observed only over the TiO2-supported Au catalyst, indicating that SO2 can block the active sites for CO oxidation over Au catalysts. The results are consistent with a model in which the perimeter sites have a special role in the CO oxidation reaction over Au catalysts.

Fabrication of Poly(diphenylsilylenemethylene) and Poly(diphenylsiloxane) Thin Films Using Fine Metal Particles

A new polymerization technique based on the use of sputtered metal particles for the thermal ring-opening polymerization of 1,1,3,3-tetraphenyl-1,3-disilacyclobutane (TPDC) was developed. This method facilitates the synthesis of polydiphenylsilylenemethylene (PDPhSM) thin films which are difficult to make by conventional methods because of their insolubility and high melting point. TPDC was first evaporated on silicon substrates and then exposed to metal particle deposition by sputtering prior to heat treatment. The catalytic activities of Pt, Pt/Pd, Au, Cu, and Ag particles were examined. The chemical structure of the film was basically the same, whatever the kind of metal used. However, the polymerization efficiency and film crystallinity depended greatly on the kind of metal under the same sputtering and heating conditions. The parts of the initial TPDC films that were not subjected to metal sputtering were entirely evaporated from the films without any polymerization. This phenomenon allowed us to mak...

Electrocatalytic properties of conducting polymer based composite film containing dispersed platinum microparticles towards oxidation of methanol

We demonstrate the usefulness of poly-N-methylpyrrole as a matrix for the fabrication of a composite film containing spatially dispersed platinum and highly reactive Ru oxo centers in a form of polynuclear ruthenium(III, IV) oxide/cyanoruthenates. It comes from scanning electron micrography that spherical Pt microparticles are randomly dispersed in the film. Cyclic voltammetry has been employed to characterize electrocatalytic oxidation of methanol at Pt-free and platinized composite films at ambient (20°C) and increased (60°C) temperatures. The presence of ruthenium(III, IV) oxide/cyanoruthenate species in the composite film increases the catalytic activity of Pt towards methanol oxidation.

A kinetic analysis of distinct reaction pathways in methanol electrocatalysis on Pt(111)

Accumulation of partial decomposition products during methanol electrocatalysis influences both the product distribution and kinetics of reaction. To quantify the catalytic activity of Pt(111) to methanol oxidation, we performed chronoamperometric experiments at 0.6 V RHE for varying durations (0.03–300 s). Subsequently, the coverage of accumulated residue was evaluated from linear sweep voltammetry performed in methanol-free electrolyte. A straightforward mass and charge balance allowed for calculation of the yields of partial and complete oxidation products. From these time-dependent data, we extract kinetic and mechanistic information. In the first 30 ms of reaction, only partial decomposition products form on the surface without significant CO 2 production. In contrast, for reaction periods longer than 5 s, CO 2 production is greatly favored over residue formation. Time-dependent methanol reaction data and complementary CO oxidation measurements at 0.6 V RHE show that a serial reaction path, involving adsorbed residue, is inadequate to explain the observed rate of CO 2 production. As an alternative, we propose a dual path mechanism for methanol electrocatalysis when CO is the most abundant species. We find that a parallel pathway, not involving adsorbed CO, is responsible for the majority (greater than 80%) of CO 2 produced. We present quantitative kinetic information regarding the rates of residue accumulation and CO 2 production; we also briefly discuss the over-all mechanistic implications of our results for methanol electrocatalysis on Pt(111).

Characterization of structure and catalytic activity of Pt–Sn catalysts supported in Al 2O 3, SiO 2 and TiO 2

In this work we characterized two types of Pt–Sn bimetallic catalysts: a `model' catalyst made by sequentially evaporating the two metals on a planar SiO 2 film deposited on a 3 mm gold grid and a series of `real' catalysts prepared by the impregnation method on SiO 2, TiO 2 and Al 2O 3 and were called `real' catalysts. Lattice spacings from high resolution electron micrographs exhibited differences between the `model' and the `real' catalysts, implying that the preparation method affects the formation of different chemical phases. We observed the formation of core-shell morphologies in both `model' and `real' catalysts, which implies the segregation and formation of different phases at the surface of the particles. The `real' catalysts were also studied with X-ray diffraction to compare the chemical phases formed with the HREM measurements. We used dehydrogenation of cyclohexane as a test reaction to study the catalytic behaviour of the Pt when we add Sn. Catalysts at different atomic concentrations were prepared in order to observe the changes in activity and selectivity.

Gas-Phase Hydroxyl Radical Generation by the Surface Reactions over Basic Metal Oxides

Hydroxyl radical desorption in the heterogeneous catalytic reactions of water or hydrogen with oxygen was examined by laser-induced fluorescence spectroscopy. The catalytic activities of Pt, Al2O3, and basic metal oxides (MgO, CaO, SrO, BaO) supported on Al2O3 were studied in the pressure range 0.1−10 Torr and the temperature range 1100−1300 K. In the case of OH generation from water, the catalytic activities of Pt and all oxides except Al2O3 were very high and the OH concentration reached the equilibrium value within a residence time of 4 ms. In the case of hydrogen oxidation, differences in the catalytic behavior were clearly observed. The surface reaction mechanisms and the effects of Al2O3 and MgO on gas-phase ignition are discussed.

Non-Faradaic electrochemical modification of the catalytic activity of Pt using a CaZr0.9In0.1O3-α proton conductor

The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) was investigated for the case of C2H4 oxidation on a Pt polycrystalline catalyst film also acting as a working electrode in a galvanic cell of the type: $$C_2 H_4 ,O_2 ,CO_2 ,H_2 O,Pt|CaZr_{0.9} In_{0.1} O_{3 - \alpha } |Au,C_2 H_4 , O_2 ,CO_2 ,H_2 O$$