Ion-dipole interactions between adsorbed CO and support cations in Pt/K-LTL

The adsorption of carbon monoxide on a non-acidic Pt/K-LTL catalyst has been studied by diffuse reflectance and transmission IR spectroscopy. The CO spectrum is strongly dependent on the experimental conditions. Adsorption on the small Pt clusters in the presence of water gives linear-CO bands between 2060 and 1990 cm^–1 and a bridging-CO band around 1800 cm^–1. In the absence of water, the linear bands are red shifted to about 1940 and 1720 cm^–1, respectively. The frequency shift is attributed to an ion-dipole interaction between adsorbed CO and support cations. The ion-dipole interaction is screened by the adsorbed water leading to a smaller red shift in the CO stretching frequency.     

Alkylation of benzene with ethane over platinum-loaded zeolite catalyst

Alkylation of benzene with ethane was carried out using various zeolite catalysts at temperature ranges of 400–550°C. Loading of platinum onto zeolite greatly enhanced the yield of ethylbenzene. Among the zeolites tested, H-ZSM5 and H-MCM22 showed catalytic activities. By contrast, mordenite did not yield ethylbenzene. Moderate acid strength distribution is the key factor of zeolite catalysts. Optimum catalyst and conditions for this reaction are as follows. The platinum-loaded H-ZSM5 catalyst containing 6.8 wt% Pt, at a reaction temperature of 500°C, afforded ethylbenzene and styrene formation rates of 14.2 and 0.8 mmolh^–1g-cat^–1, respectively (benzene-based yields 7.3 and 0.4%). In the alkylation of benzene with ethane over platinum-loaded H-ZSM5, ethene was initially formed from ethane over the metallic platinum. Then the alkylation proceeded over the acid sites of H-ZSM5.     

In situ FTIR and UV/VIS spectroscopic study of the generation of anionic Pt-carbonyl complexes within a faujasite matrix

The temporal generation of anionic platinum-carbonyl complexes in platinum ionexchanged zeolites X and Y by reductive carbonylation at 10⁵ Pa and 363 K is monitored by in situ UV/VIS and FTIR spectroscopy. A monomer [Pt₃(CO)₆]^2–, exhibiting bands at 318 and 456 nm in the UV/VIS spectra and at 1790 and 2025 cm^–1 in the FTIR spectra, is the only platinum/species formed in NaX. The monomer as well as oligomers are generated in NaY, where the formation of the latter species is due to the stronger acidity in the NaY as compared to NaX. The decomposition of the complexes results in the generation of Pt clusters of the size 1 nm.     

Strong modification of Pt-CO interaction caused by alloying with chromium in Pt-Cr/HZSM-5 catalysts

The catalysts Pt/HZSM-5 and Pt-Cr/HZSM-5 are characterized by FTIR spectroscopy using CO as a probe molecule, and by transmission electron microscopy. The major fraction of the metal phase detected in the micrographs exhibits particles in the range of 1–3 nm uniformly distributed in the zeolite crystals. Nanodiffraction analysis of single particles confirmed the formation of fcc Pt-Cr alloy with lattice constanta = 0.386 nm. The infrared spectra at 300 K exhibit bands assigned to CO linearly bonded to zeolite-hosted metal particles at 2089 cm^–1 for Pt/HZSM-5, and at 2120 and 2092 cm^–1 for Pt-Cr/HZSM-5. The temperature increase to 610 K resulted in a strong shift of the 2089 cm^–1 band for Pt/HZSM-5 by 25 cm^–1 and a slight shift of the corresponding bands for Pt-Cr/HZSM-5 by 6 cm^–1. The differences are referred to different electron backdonation capacities of the CO binding Pt surface atoms of the metal particles. The lower capacity for the Pt-Cr/HZSM-5 sample is due to alloying in the metal particles.     

Correlation between CO frequency and Pt coordination number. A DRIFT study on supported Pt catalysts

The infrared stretching frequencies of linear-bonded carbon monoxide on face, edge and corner atoms have been identified with four platinum catalysts. The metal particles were supported on different absorbentia and each having a different average particle size, ranging from about 10 to 200 Å. The values of the various absorption bands correlate linearly with the number of nearest neighbours of the different surface sites (n in C _n ). Effects of an interaction between Pt clusters and support on the linear-CO stretching frequency have not been observed. This empirical correlation between and C_n, and the advantages ofin situ Diffuse Reflectance Fourier transform Infrared Spectroscopy in combination with derivative spectrometry opens the possibility to monitor the dispersity of supported platinum in a simple and relatively quick way.     

A study of molecular orientation in one-way drawn poly(ethylene terephthalate) films by means of polarized infra-red spectroscopy and birefringence measurements

Summary Polarized infrared spectroscopy and birefringence measurements have been used to study the changes in molecular orientation which occur on homogeneous hot (T=80°C) and cold (T=20°C) drawing of poly(ethylene terephthalate) (PET) sheets. Excellent agreement was obtained between the infrared orientation functions for absorption bands associated with 1507 cm^–1, 1580 cm^–1 and 1615 cm^–1, C-C stretching mode of the benzene ring, and average orientation functions obtained from optical birefringence. It was confirmed that the Gauche conformer of the ethylene glycol residue in the amorphous phase could be oriented perpendicularly to the draw direction (_p<4). The development of molecular orientation along the specimen axis in the necked sample confirms the entangled nature of the polymer chains in which the natural draw-ratio corresponds to the response of a network stretched up to its maximum of extensibility.     

Monitoring of molybdenum H-ZSM5 catalyst preparation by in situ ultraviolet Raman spectroscopy

In situ ultraviolet Raman spectroscopy was used to follow the preparation of Mo/H-ZSM5 catalysts by thermal treatment of a physical mixture of MoO₃ and H-ZSM5. The Raman spectra recorded from 300 to 1800 cm^–1 during the thermal treatment showed that MoO₃ spread over the zeolite support to form dispersed molybdenum oxide species. Curve-fitting analysis of the overall spectral profile for the samples after treatment at 600°C in O₂ showed the presence of three spectral components at 962, 868 and 815 cm^–1. The band at 815 cm^–1 was assigned to the zeolite support. The most intense band at 962 cm^–1 could be attributed to terminal Mo=O groups of isolated tetrahedrally coordinated monomers and the weaker band at 868 cm^–1 to bridging Mo–O–Mo groups of dimeric species. These last two assignments are not definitive and may be due to other unique structures of oxides of molybdenum on the zeolite framework.     

Infrared spectroscopy and microcalorimetry studies of CO adsorption on sulfated zirconia supported platinum catalysts

Carbon monoxide adsorption has been investigated on Pt particles supported on a high surface area zirconia and sulfated zirconias. The accessibility of the Pt surface determined from the comparison of H₂ chemisorption and transmission electron microscopy depends on two parameters: the temperature of treatment in air used to dehydroxylate sulfated zirconia, and the temperature of reduction. An oxidative pretreatment at 823 K yields a poor accessibility of Pt (0.03 < H/Pt < 0.05) whatever the temperature of reduction, whereas a Pt dispersion of 0.6 can be obtained by oxidation at 673 K followed by a mild reduction at 473 K. FTIR spectroscopy of adsorbed CO on Pt/ZrO₂ shows besides the normal linear species at 2065 cm^–1, a band at 1650 cm^–1 which is attributed to CO bridged between Pt and Zr atoms. On Pt/ZrO₂-SO ₄ ^2– , all bridged species tend to disappear, as well as the dipole-dipole coupling andv _CO is shifted by 57 cm^–1 to higher frequencies. These results are attributed to sulfur adsorption on Pt which decreases the electron back-donation from Pt to the 2 ^* antibonding orbital of CO. The lower initial heat of CO adsorption observed on Pt/ZrO₂-SO^4/2– supports this proposal.     

HREM imaging of small Pt clusters dispersed in Y-zeolites

High resolution electron microscopy (HREM) has been used to study dispersed Pt clusters in NaY- and USY-zeolites. All the samples contained 0.8% Pt and were reduced at temperatures of 300 ° C, 500 ° C and 650 ° C. The size of the Pt clusters ranged from a few å up to 30 å. When the incident electron beam was sufficiently strong, it caused some of the extremely small metal clusters to sinter. This was usually accompanied by zeolite damage. This in-situ sintering must be taken into consideration when interpreting the particle size distribution results obtained solely by TEM, especially for particles that are smaller than 10 å. The minimum phase contrast imaging condition is demonstrated to be more appropriate than optimum defocus for detecting the extremely small Pt clusters inside the zeolite structures.     

Partial oxidation of methane to syngas over Co/MgO catalysts. Is it low temperature?

Co/MgO catalysts with high Co-loading (>28 wt%) are able to initiate the reaction of methane with oxygen at temperatures around 500 °C. High conversions of methane ( 70%) and very high selectivities for hydrogen and carbon monoxide ( 90%) are obtained at very high reactant gas space velocities (10⁵–10⁶ h^–1). The temperature of the catalyst at the conditions of partial oxidation of methane to form syngas was found to be extremely high (1200–1300 °C); it is about 600–850 °C higher than that previously reported by others. At these temperatures, high temperature homogeneous reactions may prevail. It is suggested that combustion of methane to carbon dioxide occurs on the catalyst with major heat release and that methane and water, respectively methane and carbon dioxide are reformed thermally in an endothermic reaction leading to syngas.     

Characterization of the Pt-C bond of CO adsorbed by small Pt particles in a NaX zeolite by IR diffuse reflectance spectroscopy

The unusual properties of small (1–2 nm) Pt particles within a zeolite support are documented by diffuse reflectance infrared spectroscopy of the metal-carbon bond of linearly adsorbed CO. The wave number of the metal-carbon stretching frequency is found to be about 60 cm^–1 higher compared to larger particles (2–3 nm) or to the bulk metal. The stronger bond is attributed to a negative charge on the small metal particles.     

Chemical reactivity of CO and CO2 at a Cu(110)-Cs surface

Carbon dioxide and carbon monoxide undergo reactive chemisorption with cesium modified Cu(110) and Cu(110)-O surfaces and via the anionic intermediate CO ₂ ^– (a) form a surface carbonate. The CO ₂ ^– (a) species was characterised by VEELS and XPS at low temperature (80 K) and the surface carbonate at 295 K. For cesium modified Cu(110) surfaces chemisorption of carbon monoxide gives rise to electron energy loss peaks (v _co) as low as 1450 cm^–1 at 295 K.     

Effects of Water and Ion-Exchanged Counterion on the FTIR Spectra of ZSM-5. II. (Cu+–CO)-ZSM-5: Coordination of Cu+–CO Complex by H2O and Changes in Skeletal T–O–T Vibrations

The 2157 cm^–1 (strong) and 2108 cm^–1 (very weak) (CO) IR bands due to Cu⁺–CO in ZSM-5 zeolite with ¹²C and ¹³C isotopes, respectively, are reversibly red-shifted by subsequent adsorption of H₂O at 293 K. On the contrary, the locally perturbed internal (T–O–T) asymmetric stretching framework vibration [ _as ^int (TOT)(Cu⁺–CO)=965 cm^–1] is reversibly blue-shifted. The courses of the band shifts revealed notable features. Charge transfers from water to Cu⁺ ions, changes in coordination spheres of Cu⁺(CO)(H₂O) _n aqua complexes and secondary (solvent-like) effects were considered to explain the results.     

Preparation of Pt–Ru bimetallic anodes by galvanostatic pulse electrodeposition: characterization and application to the direct methanol fuel cell

The electrodeposition of platinum and ruthenium was carried out on carbon electrodes to prepare methanol anodes with different Pt/Ru atomic ratios using a galvanostatic pulse technique. Characterizations by XRD, TEM, EDX and atomic absorption spectroscopy indicated that most of the electrocatalytic anodes consisted of 2 mg cm^–2 of Pt–Ru alloy particles with the desired composition and with particle sizes ranging from 5 to 8 nm. Electrochemical tests in a single DMFC show that these electrodes are very active for methanol oxidation and that the best Pt/Ru atomic ratio in the temperature range used (50–110 °C) is 80:20. The influence of the relaxation time t _off was also studied and it appeared that a low t _off led to smaller particle sizes and higher performances in terms of current density and power density.     

Do not forget the electrochemical characteristics of the membrane electrode assembly when designing a Proton Exchange Membrane Fuel Cell stack

The membrane electrode assembly (MEA) is the key component of a PEMFC stack. Conventional MEAs are composed of catalyzed electrodes loaded with 0.1–0.4 mg_Pt cm⁻² pressed against a Nafion^® membrane, leading to cell performance close to 0.8 W cm⁻² at 0.6 V. Due to their limited stability at high temperatures, the cost of platinum catalysts and that of proton exchange membranes, the recycling problems and material availability, the MEA components do not match the requirements for large scale development of PEMCFs at a low cost, particularly for automotive applications.Novel approaches to medium and high temperature membranes are described in this work, and a composite polybenzimidazole–poly(vinylphosphonic) acid membrane, stable up to 190 °C, led to a power density of 0.5 W cm⁻² at 160 °C under 3 bar abs with hydrogen and air. Concerning the preparation of efficient electrocatalysts supported on a Vulcan XC72 carbon powder, the Bönnemann colloidal method and above all plasma sputtering allowed preparing bimetallic platinum-based electrocatalysts with a low Pt loading. In the case of plasma deposition of Pt nanoclusters, Pt loadings as low as 10 μg cm⁻² were achieved, leading to a very high mass power density of ca. . Finally characterization of the MEA electrical properties by Electrochemical Impedance Spectroscopy (EIS) based on a theoretical model of mass and charge transport inside the active and gas diffusion layers, together with the optimization of the operating parameters (cell temperature, humidity, flow rate and pressure) allowed obtaining electrical performance greater than 1.2 W cm⁻² using an homemade MEA with a rather low Pt loading.     

Spatially (nanometer) controlled hydrogenation and oxidation of carbonaceous clusters by the platinum tip of a scanning tunneling microscope operating inside a reactor cell

Hydrocarbon clusters formed by the thermal decomposition of propylene on Pt(111) were rehydrogenated or oxidized with nanometer spatial resolution using the platinum tip of a scanning tunneling microscope (STM) at 300 K in atmospheric pressures of H₂ or O₂. The reaction rate shows a strong dependence on the oxygen or hydrogen pressures and on the tip-surface separation. The reaction stops when the Pt tip becomes contaminated with carbon, after the removal of 10⁷–10⁸ carbon atoms, but can be regenerated by removing material from the tip by application of a voltage pulse. Dissociative adsorption of H₂ and O₂ on the tip, followed by transfer of atoms to the surface is the proposed mechanism of these tip-catalyzed reactions.     

Deep desulphurization of diesel fuels on bifunctional monolithic nanostructured Pt-zeolite catalysts

The preparation of Pt-zeolite catalysts, including choice of the noble metal precursor and loading (1.0–1.8 wt.%), was optimized for maximizing the catalytic activity in thiophene hydrodesulphurization (HDS) and benzene hydrogenation (HYD). According to data obtained by HRTEM, XPS, EXAFS and FTIR spectroscopy of adsorbed CO, the catalysts contained finely dispersed Pt nanoparticles (2–5 nm) located on montmorillonite and zeolite surfaces as: Pt⁰ (main, ν_CO = 2070–2095 cm⁻¹), Pt^δ+ (ν_CO = 2128 cm⁻¹) and Pt²⁺ (ν_CO = 2149–2155 cm⁻¹). It was shown that the state of Pt depended on the Si/Al zeolite ratio, montmorillonite presence and Pt precursor. The use of H₂PtCl₆ as the precursor (impregnation) promoted stabilization of an oxidized Pt state, most likely Pt(OH)_xCl_y. When Pt(NH₃)₄Cl₂ (ion-exchange) was used, the Pt⁰ and hydroxo- or oxy-complexes Pt(OH)₆²⁻ or PtO₂ were formed. The addition of the Ca-montmorillonite favoured stabilization of Pt^+δ. The Cl⁻ ions inhibit reduction of oxidized Pt state to Pt particles. The Pt-zeolite catalyst demonstrated high efficiency in ultra-deep desulphurization of DLCO. The good catalyst performance in hydrogenation activity and sulphur resistance can be explained by the favourable pore space architecture and the location and the state of the Pt clusters. The bimodal texture of the developed zeolite substrates allows realizing a concept for design of sulphur-resistant noble metal hydrotreating catalyst proposed by Song [C. Song, Shape-Selective Catalysis, Chemicals Synthesis and Hydrocarbon Processing (ACS Symposium Series 738), Washington, 1999, p. 381; Chemtech 29(3) (1999) 26].     

Reaction Coupling of Ethylbenzene Dehydrogenation with Nitrobenzene Hydrogenation

The chemical equilibrium for the coupling of ethylbenzene dehydrogenation with nitrobenzene hydrogenation, to produce styrene and aniline simultaneously, has been calculated on the basis of the Soave–Redlich–Kwong equation of state. The dehydrogenation of ethylbenzene in the presence of nitrobenzene over the catalysts -Al₂O₃, ZSM-5, activated carbon and platinum supported on activated carbon has been carried out at 400 °C. The effects of Pt loading and the pretreatment of the catalysts have been investigated. It has been revealed that the conversion of ethylbenzene can be greatly improved by the reaction coupling due to the elimination of the hydrogen produced in the reaction by the hydrogenation of nitrobenzene. Platinum supported on the activated carbon has been suggested as a suitable catalyst. The best results with ethylbenzene conversion of 33.8% and styrene selectivity of 99.2% were obtained over Pt(0.02 wt%)/AC at 400 °C. Moreover, such process is also energetically favored since the necessary process heat to drive the ethylbenzene dehydrogenation can be provided by the coupling with the exothermic nitrobenzene hydrogenation reaction.     

Review in Applied Electrochemistry. Number 54 Recent Developments in Polymer Electrolyte Fuel Cell Electrodes

Since the 1980s there has been a significant lowering of the platinum loading of polymer electrolyte fuel cell electrodes from about 4–10 mg cm^–2(platinum black) to about 0.4 mg cm^–2 or even less (carbon supported platinum), by the introduction of ionomer (liquid Nafion^®) impregnated gas diffusion electrodes, extending the three-dimensional reaction zone. From the 1990s to the present studies have been carried out to decrease the loss of performance during cell operation due both to the presence of liquid water causing flooding of the catalyst layer and mass transport limitations and to the poisoning of platinum by the use of reformed fuels. This review deals with the developments in electrode configuration going from dual layer to three layer electrodes. The preparation methods, the characteristics and the optimal composition of both diffusion and reactive layers of these electrodes are described. The improvement in the performance of both CO tolerant anodes and cathodes with enhanced oxygen reduction by Pt alloying is also discussed.     

Ship-in-Bottle synthesis of Pt9-Pt15 carbonyl clusters inside NaY and NaX zeolites,in-situ FTIR and EXAFS characterization and the catalytic behaviors in13CO exchange reaction and NO reduction by CO

[Pt₉(CO)₁₈]^2–/NaY (orange-brown, 2056 and 1798 cm^–1), [Pt₁₂(CO)₂₄]^2–/NaY (dark-green, 2080 and 1824 cm^–1 and [Pt₁₅(CO)₃₀]^2–/NaX (yellow-green, 2100 and 1865 cm^–1) were stoichiometrically synthesized by the reductive carbonylation of [Pt(NH₃)₄]²⁺/NaY, Pt²⁺/NaY and Pt²⁺/NaX, respectively. The IR bands characteristic of their linear carbonyls shift to higher frequencies whereas the bridging CO bands to lower frequencies, compared with those on the external zeolites and in solution. In-situ FTIR studies suggested that the subcarbonyl species such as PtO(CO) and Pt₃(CO)₃(₂ –CO)₃ are formed as the proposed intermediates towards [Pt₁₂(CO)₂₄]^2–/NaY in the reductive carbonylation of Pt²⁺/NaY.¹³CO exchange reaction preceded with the different intrazeolite Pt carbonyl species in the following order of activity at 298–343 K: Pt₃(CO)₃(₂ –CO)₃/NaY PtO(CO)/NaY>[Pt₉(CO)₁₈]^2–/NaY >[Pt₁₂(CO)₂₄]^2–/NaY. Pt-L₃-edge EXAFS measurment for these synthesized samples demonstrated that they are consistent with the Pt carbonyl clusters having trigonal prismatic Pt₉ and Pt₁₂ frameworks infered to a series of the Chini complexes such as [NEt₄]₂[Pt₃(CO)₆] _n ( _n = 3–5). The intrazeolite Pt₉ and Pt₁₂ carbonyl clusters exhibited higher cataytic activity in NO reduction by CO towards N₂ and N₂O at 473 K, compared with those on the conventional Pt/Al₂O₃ catalysts. The mechanism of intrazeolite Pt₉-Pt₁₅ carbonyl cluster formation are discussed in terms of the intrazeolite basicity and acidity.On leave from National Laboratory for Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 129 Street, China.