Average Platinum Particle Size Research Articles

Sintering resistance of Pt-based oxidation catalyst via constructing Pt/MnOx interface

It is a challenge to improve the catalytic activity and hydrothermal stability of platinum-based catalyst in concurrently oxidizing CO/C3H6/NO. The construction of Pt/MnOx interface over Pt-based diesel oxidation catalyst via overlayer deposition to form partial-embedded structure, inhibiting the aggregation of platinum nanoparticles in Ostwald Ripening process. Even after hydrothermal aging, the average platinum particle size is equal to that of the fresh catalyst via traditional impregnation method. The electronic-transferring from platinum to manganese results in a weaker CO adsorption ability on Pt species and an improved oxygen activation ability with the help of the redox cycle of Pt and MnOx, as well as the formation of more active nitrate species via in situ DRIFTs. Measurement on the activity shows the lower apparent activation energy, so that the light-off temperature of CO/C3H6/NO on the partial-embedded catalyst via overlayer deposition method decreases by around 20 °C and 40% NO maximum conversion still keeps after severely aging comparing with the traditional catalyst. The work enlightens us the importance of interface sites in stabilizing platinum atoms and creating oxidize active sites.

A novel electrochemical method for the preparation of Pt/C nanostructured materials

An original method for the preparation of Pt/C materials dispersed by means of platinum electrodeposition on the carbon particles which are in the form of suspension has been proposed. The materials were studied by XRD, cyclic voltammetry, SEM and TEM electron microscopy. The obtained Pt/C materials contain from 9 to 14 wt% Pt. The average size of platinum particles is from 7 to 15 nm; electrochemically active surface area of Pt/C is from 15 to 29 m2 g−1 (Pt). The resulting materials exhibit catalytic activity in the oxygen electroreduction reaction which predominantly flows through the 4-electron mechanism.

Degradation of Pt/C electrocatalysts having different morphology in low-temperature PEM fuel cells

The electrochemical degradation of platinum–carbon catalysts with different morphology has been studied under model conditions in low-temperature proton exchange membrane fuel cells. It has been found that catalysts with an average size of platinum nanoparticles ranging from 2 to 3 nm uniformly distributed over the carbon support exhibit the best current–voltage characteristics; however, they have also the highest degradation rate. It is shown that the main cause of Pt/C electrocatalyst degradation consists of both the detachment of small platinum particles from the carbon support and the recrystallization of platinum, leading to an increase in the average particle size. On the contrary, the catalysts having the initial average size of platinum particles ranging from 3 to 4 nm show a considerable stability in current–voltage characteristics even after 10000 cycles of accelerated degradation.

Electrocatalysis of oxygen reduction reaction on Nafion/platinum/gas diffusion layer electrode for PEM fuel cell

In this work a new membrane electrode based on Pt-coated Nafion membrane was fabricated. Chemical deposition process was used to coat platinum on Nafion 117 membrane and then Pt-coated Nafion membrane was hot pressed on gas diffusion layer (GDL) to make new membrane electrode. The electrochemical and chemical studies of the Pt-coated Nafions were investigated by electrochemical techniques, X-ray diffraction and scanning electron microscopy. The electrochemical results indicated that as the concentration of H 2PtCl 6 increased, the oxygen reduction reaction rate increased until the concentration was reached where the reduction reaction was limited by the problem of mass transport. The electrochemical results for oxygen reduction reaction showed that the new electrode which prepared by plating Nafion membrane with 0.06 M H 2PtCl 6 in electroless plating solution, has a higher performance than other electrodes. The XRD results showed that the average platinum particle size of the best sample was about 3 nm. The loading of platinum for this electrode was 0.153 mg cm −2.

Fabrication of nanocomposites based on carbon nanotubes containing Pt nanoparticles and TiO2

Using conical multiwalled carbon nanotubes (CNTs), we have prepared Pt/CNT and Pt/TiO2/CNT nanocomposites with an average platinum particle size of 3–5 nm, Pt/Ti molar ratio on the surface in the range 3.5–4, and C/Pt = 21–22. Titania was deposited onto the CNTs through titanium tetrachloride (TiCl4) hydrolysis. Platinum particles were produced by reducing chloroplatinic acid (H2PtCl6) with sodium borohydride (NaBH4) in the presence of CNTs. The composition and structure of the composites have been studied using X-ray photoelectron spectroscopy, electron microscopy, X-ray diffraction, and thermogravimetry. The materials have been tested as catalysts for hydrogen oxidation and oxygen reduction. The results demonstrate that the modification of Pt/CNT with titania enhances the catalytic activity of the material.

Synthesis and electrocatalytic activity of platinum nanoparticle/carbon nanotube composites

Pt/CNT nanocomposite materials with an average platinum particle size of 3–5 nm and platinum content of 13–28 wt % have been prepared by reducing chloroplatinic acid, H2PtCl6, in the presence of conical carbon nanotubes. The effect of synthesis conditions on the average platinum particle size, total platinum content, and surface composition of the nanocomposites has been studied using X-ray photoelectron spectroscopy, IR spectroscopy, electron microscopy, X-ray diffraction, and thermogravimetry. The materials have been tested as catalysts for hydrogen oxidation and oxygen reduction. Their performance has been assessed by cyclic and steady-state voltammetric techniques. The structure and composition effects on the electrocatalytic properties of the nanocomposites are discussed.

Platinum nanoparticles on carbon nanomaterials with graphene structure as hydrogenation catalysts

Carbon nanomaterials with graphene structure (single- and multiwall nanotubes and nanofibers) after oxidizing by a mixture of sulfuric and nitric acids and presumable introducing of carboxyl groups can be used as carrying agents of hydrogenation catalysts. Platinum in a concentration which should not exceed 10 wt % can be fixed using H2PtCl6 as a precursor in presence of an organic base. Catalysts based on these nanomaterials with the average size of platinum particles 6–8 nm exceed in activity the Pt/C catalyst with the size of platinum particles 65–70 nm, but are inferior to catalysts based on fullerene black with the average size of platinum particles 3–4 nm.

Uniformly dispersed platinum nanoparticles loading on porous carbons without using reduction agents

Platinum nanoparticles loading on carbon nanotube, activated carbon, and activated carbon fiber was carried out by impregnation of hexachloro palatinate ion(IV) from hydrogen hexachloro platinate hydrate [H 2PtCl 6·5.7H 2O] dissolved solution without using reduction agents, and heating the hexachloro platinate(IV) impregnated carbons up to 800 °C. When the initial platinum content was controlled to 1000 ppm in the solution, the adsorption capacities of hexachloro platinate(IV) on carbon nanotube, activated carbon and activated carbon fiber were 24%, 47%, and 76%, respectively at the equilibrium state. The adsorption isotherm type of hexachloro platinate(IV) on carbon nanotube was two-step linear and quite different from Langmuir model of activated carbon fiber due to the uniformly developed cylindrical pore structure and size distribution. The average platinum particle size on porous carbons was under 2 nm by heating the hexachloro platinate(IV) up to 400 °C in spite of non-using reduction agents, while the average size increased due to the agglomeration of some particles by heating them up to 800 °C. Therefore, uniformly distributed platinum nanoparticles loading on porous carbons can be obtained from simple impregnation of hexachloro palatinate ion(IV) from solution and heating it up to 400 °C.

Synthesis of perfluorosulfonate ionomer-stabilized platinum colloid and its application in direct methanol fuel cells

Pt colloid was synthesized by the reduction of hexachloroplatinic acid with formaldehyde in the presence of a stabilizer (a cation-exchanged polymer - perfluorosulfonate ionomer (PFSI) or Teflon) or without any stabilizers. The average size of platinum particles with a stabilizer, characterized by a transmission electron microscopy and X-ray diffraction, is in a nanometer range in comparison with the particle agglomeration without any stabilizers. Highly dispersive 40 wt. % Pt/C catalyst obtained from the PFSI-stabilized Pt colloid shows much better performance than its counterpart with Teflon as a stabilizer or without any stabilizer according to the cyclic voltammetric (CV) characterization and the single direct methanol fuel cell (DMFC) performance testing. It is strongly suggested that the proton-conducting ionomer could smooth the reaction pathway by facilitating the proton transport, while Teflon inhibit the active sites, though both of them can inhibit the particle growth. The improved fuel cell performance is attributed to the small particles and the unhindered transport of protons/electrons ascribed to the intimate contact of Pt and proton-conducting ionomer.

Different Sized Platinum Nanoparticles Supported on Carbon: An XPS Study on These Methanol Oxidation Catalysts

Different sized platinum nanoparticles on carbon supports have been prepared using PtCl4 (catalyst Ia) and H2PtCl6 (catalyst IIa) as starting materials and 1-hexanethiol as a surfactant and then heated to 200 °C (catalyst Ib and catalysts IIb), 300 °C (catalyst Ic and catalysts IIc), and 400 °C (catalyst Id and catalysts IId) for 4 h under argon gas. All the catalysts showed a face-centered cubic (fcc) crystal structure as determined by X-ray diffraction. X-ray diffractograms and transmission electron microscopy results reveal that the platinum nanoparticles are homogeneously dispersed on the carbon support, exhibit narrow particle size distribution, and show no appreciable aggregation. The average platinum particle size as determined from XRD data was found to be ∼2.00, ∼2.56, ∼4.23, ∼4.52, ∼2.13, ∼2.77, ∼4.29, and ∼4.62 nm for catalysts Ia−d and IIa−d, respectively. X-ray photoelectron spectra of all the catalysts indicated that most (>70%) of the platinum nanoparticles have an oxidation state of zero and a small amount (<30%) have a +4 oxidation state with binding energies of 71.2−72.2 (Pt 4f7/2) and 74.3−75.5 eV (Pt 4f7/2), respectively. Increasing the temperature during the heat treatment process results in an enlargement of platinum particle size, a positive shift in the binding energies of Pt(0) and Pt(IV), and a decrease in catalytic activity in the methanol oxidation reaction.

Preparation and Characterization of Pt/Superfine Mesocarbon Microbead Powers Electrocatalysts

Superfine mesocarbon microbead powders (SFMCMBs) as the new supports for platinum electrocatalysts were first investigated. The Pt∕SFMCMB electrocatalysts were prepared by an impregnation-reduction method, with hexachloroplatinic acid as the platinum precursor and formaldehyde as the reducing agent. The catalysts were characterized with x-ray diffraction (XRD), field emission gun transmission electron microscope (TEM), and electrochemical analysis. TEM photos showed the platinum particles were dispersed uniformly on the surface of SFMCMBs and there existed a little aggregation of platinum particles in the Pt∕SFMCMB catalysts. The TEM photos showed the existence of the platinum on the supports where the average platinum particle size were 4-6nm. The electrochemical analysis proved that SFMCMBs are excellent candidates to be used as the support of platinum electrocatalyst for methanol electrochemical oxidation as the potential catalyst candidate for direct methanol fuel cells (DMFCs).

Study of the reaction mechanisms between Pt(acac) 2 and alumina surface sites : Application to a new refilling technique for the controlled variation of the particle size of Pt/Al 2O 3 catalysts

A new preparation technique is presented allowing the controlled variation of the average size of platinum particles in small steps of 0.2–0.5 nm in the size range between 1 and 2 nm. The particle growth is achieved by refilling, i.e. by increasing the metal loading of a parent catalyst in a separate preparation step. The technique makes use of the selective decomposition of a platinum complex, platinum bis-acetylacetonate, Pt(acac) 2, on reduced platinum particles. Anchoring of the complex on the surface of the support and thus appearance of new small particles is completely avoided by appropriate surface treatments developed on the basis of a study of the reaction mechanisms between Pt(acac) 2 and the support. Three types of surface sites could be identified in this study by their specific reactivity towards Pt(acac) 2. A model is presented describing the nature of these sites, the occurrence of which is correlated with the degree of dehydroxylation of the alumina surface. The efficiency of the new technique in increasing the particle size while reducing to a minimum any broadening of the initial narrow size distribution is demonstrated. The results are discussed in terms of particle shapes and two- and three-dimensional growth mechanisms.

On the determination of platinum particle size in carbon-supported platinum electrocatalysts for fuel cell applications

Using a conventional laboratory diffractometer, small angle X-ray scattering (SAXS) was used to determine the average platinum particle size in samples of carbon with a range of platinum loadings. The results obtained were compared with those obtained from wide-angle X-ray diffraction (WAXS) studies. SAXS was more effective than WAXS for determining the average platinum particle size in samples where the grains were so small that the resultant diffraction peaks in the WAXS profiles were too broad to accurately determine the peak width for use in the Scherrer equation.

Hydrodechlorination of CCl 4 over Pt/Al 2O 3: effects of platinum particle size on product distribution

The impregnated platinum catalysts showed various platinum particle sizes depending on the nature of the platinum precursors (Pt(NH 3) 2(NO 2) 2 versus H 2PtCl 6) and on the pH of the Al 2O 3 suspension. The average platinum particle size increased with decrease in pH of the suspension in case of Pt(NH 3) 2(NO 2) 2, but this trend was vice versa for H 2PtCl 6. The product distribution in hydrodechlorination (HDC) of CCl 4 varied greatly with the platinum particle size; the larger the platinum particle size was, the higher was the selectivity to CHCl 3. To elucidate the origin of this platinum particle size effect on product distribution, CO chemisorption, NH 3 and CO 2 temperature-programmed desorption (TPD), high resolution transmission electron microscopy (HRTEM), temperature-programmed surface reaction (TPSR), Fourier-transformed-infrared spectra (FT-IR) and X-ray absorption fine structure (XAFS) experiments were carried out. The formation of completely dechlorinated CH 4 was favorable owing to the strong chemisorption of CCl 4 on the small platinum particles characterized by low surface coordination numbers and by an electron-deficient property. The nature of carbonaceous species formed on platinum surface at the beginning of reaction also varied greatly with platinum particle sizes and changes of electronic state of platinum particles affected catalytic activity and products’ distribution.

Characterization of Bimetallic Pt–Sn/Al2O3Catalysts: Relationship between Particle Size and Structure

Pt–Sn/Al2O3bimetallic catalysts have been prepared using organometallic precursors and characterized. Two modelsdepending on average platinum particle size are proposed inorder to describe these solids. The catalyst obtained from ahighly dispersed monometallic precursor may be described asPt/SnOx/Al2O3with platinum probably in interaction with tinoxide. When the monometallic precursor has low dispersion, thebimetallic system may be described asPtSn0+SnOx/Al2O3.

Propene hydrogenation on low-temperature reduced Pt/TiO. Effects of TiO phases and H treatment on specific catalytic activity

TiO2 gel obtained by hydrolysis of titanium tetra-n-butoxide was heated at various temperatures, providing poorly crystallized TiO2, disordered anatase and a well crystallized anatase—rutile mixture. Platinum dispersion was obtained on these TiO2 supports by the reduction of platinum—allyl species with H2 at 278 K. These catalysts and a Pt/SiO2-gel sample were used for propene hydrogenation at various temperatures and H2/propene molecular ratios. Fresh and run catalysts were subjected to X-ray diffraction analysis for the study of extant phases and determination of average size of platinum particles. In all cases, results obtained for catalysts as prepared or after H2 treatment at 493 K indicated that the ordinary hydrogenation mechanism was maintained but that important changes of specific catalytic activity were observed, depending on H2 treatment and nature of the support. TiO2 morphology was assumed to be responsible for the specific activity decrease in Pt on the rutile—anatase mixture and for the opposite effect found for platinum on anatase.

Propene hydrogenation on low-temperature reduced Pt/TiO 2. Effects of TiO 2 phases and H 2 treatment on specific catalytic activity

TiO 2 gel obtained by hydrolysis of titanium tetra- n-butoxide was heated at various temperatures, providing poorly crystallized TiO 2, disordered anatase and a well crystallized anatase—rutile mixture. Platinum dispersion was obtained on these TiO 2 supports by the reduction of platinum—allyl species with H 2 at 278 K. These catalysts and a Pt/SiO 2-gel sample were used for propene hydrogenation at various temperatures and H 2/propene molecular ratios. Fresh and run catalysts were subjected to X-ray diffraction analysis for the study of extant phases and determination of average size of platinum particles. In all cases, results obtained for catalysts as prepared or after H 2 treatment at 493 K indicated that the ordinary hydrogenation mechanism was maintained but that important changes of specific catalytic activity were observed, depending on H 2 treatment and nature of the support. TiO 2 morphology was assumed to be responsible for the specific activity decrease in Pt on the rutile—anatase mixture and for the opposite effect found for platinum on anatase.

Dispersion changes and reactivity of platinum on silica during thermal treatment under vacuum and at low oxygen pressure

The aggregation of platinum in thin films deposited onto quartz glass substrates in a high vacuum and heated in vacuum (10 -9–10 -7 Torr) and at low oxygen pressures (10 -6–10 -5 Torr) at temperatures up to 823 K was investigated by means of resistance measurements and electron microscopy and diffraction. The break-up of an initially continuous (or quasi-continuous) film into separate platinum crystallites was observed at temperatures between 500 and 600 K. The growth of the crystallites in both high vacuum and oxygen is accompanied by interaction of the platinum with the SiO 2 substrate and formation of platinum oxide (PtO 2) respectively. The formation of PtO 2 and the depletion of platinum in the film probably occur in pure oxygen, and this leads to a substantially lower average platinum particle size than that obtained in vacuum after identical thermal treatment.

The effect of inorganic constituents of the support on the characteristics of carbon-supported platinum catalysts

Two commercial activated carbons and the corresponding demineralized samples have been used as catalyst supports for platinum, in order to learn about the role played by the inorganic constituents of the carbons on the catalyst characteristics. CO and H 2 chemisorption and X-ray line broadening have been used to determine metal dispersion and mean particle size. The results indicate that the inorganic matter of the carbon plays an important role in the stabilization of the average platinum particle size by making sintering more difficult. H 2-C 2H 4 titrations have also been used to determine the platinum particle size; the results found are in good agreement with H 2 chemisorption, especially for catalysts supported on carbons with larger micropore volume and lower meso and macropore volume.

Effects of particle size on the desorption behaviour of hydrogen from silica-supported platinum catalysts

Temperature-programmed desorption experiments on hydrogen over Pt/SiO2catalysts show that the activation energy for the desorption of hydrogen, which desorbed between 60 and 3000 °C, increases with an increase in the average platinum particle size.