Small Platinum Particles Research Articles

Constructing g-C3N4/Cd1−xZnxS-Based Heterostructures for Efficient Hydrogen Production under Visible Light

Two types of photocatalysts, 1%Pt/Cd1−xZnxS/g-C3N4 (x = 0.2–0.3) and Cd1−xZnxS/1%Pt/g-C3N4 (x = 0.2–0.3), were synthesized by varying the deposition order of platinum, and a solid solution of cadmium and zinc sulfides onto the surface of g-C3N4. The characterization of photocatalysts showed that, for 1%Pt/Cd1−xZnxS/g-C3N4, small platinum particles were deposited onto a solid solution of cadmium and zinc sulfides; in the case of Cd1−xZnxS/1%Pt/g-C3N4, enlarged platinum clusters were located on the surface of graphitic carbon nitride. Based on the structure of the photocatalysts, we assumed that, in the first case, type II heterojunctions and, in the latter case, S-scheme heterojunctions were realized. The activity of the synthesized samples was tested in hydrogen evolution from triethanolamine (TEOA) basic solution under visible light (λ = 450 nm). A remarkable increase in hydrogen evolution rate compared to single-phase platinized 1%Pt/Cd1−xZnxS photocatalysts was observed only in the case of ternary photocatalysts with platinum located on the g-C3N4 surface, Cd1−xZnxS/1%Pt/g-C3N4. Thus, we proved using kinetic experiments and characterization techniques that, for composite photocatalysts based on Cd1−xZnxS and g-C3N4, the formation of the S-scheme mechanism is more favorable than that for type II heterojunction. The highest activity, 2.5 mmol H2 g−1 h−1, with an apparent quantum efficiency equal to 6.0% at a wavelength of 450 nm was achieved by sample 20% Cd0.8Zn0.2S/1% Pt/g-C3N4.

Size effect of Pt nanoparticles in acid-assisted soot oxidation in the presence of NO

Pt/Al2O3 catalysts with mean Pt particle size ranged from 2.7 to 7.1 nm were synthesized by chemical reduction method, and the sulfated counterparts were prepared by impregnation of sulfuric acid. The turnover frequency of platinum for soot oxidation under loose contact conditions in a feed flow containing NO and O2 are positively correlated with the size of platinum. The sulfated Pt/Al2O3 exhibits higher catalytic activity for soot oxidation in the presence of NO despite their reduced ability for NO2 production. Such a contradiction is more significant for those catalysts with smaller platinum particles. Herein, the catalysts were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), inductive coupled plasma (ICP) emission spectrometry, CO chemisorption, thermogravimetric analysis (TGA), NH3 temperature-programmed desorption (NH3-TPD), NO temperature-programmed oxidation (TPO) and NOx temperature-programmed desorption (TPD). Possible effect of Pt particle size for the catalytic oxidation of soot in the presence of NO was presented based primarily on the promoted NO2 transfer efficiency onto the soot pushed by the acidic catalysts.

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.

Size effect in the oxidation of platinum nanoparticles on graphite with nitrogen dioxide: An XPS and STM study

The interaction of NO2 with model catalysts prepared by platinum evaporation onto the surface of highly oriented pyrolytic graphite has been investigated at room temperature and a pressure of 3 × 10−6 Torr by X-ray photoelectron spectroscopy and scanning tunneling microscopy. In the catalyst containing only small (<2.5 nm) platinum particles, these particles oxidize to PtO and PtO2. The action of NO2 on the graphite support and on the graphite-supported Pt catalyst causes graphite oxidation. The oxygen concentration in the model catalyst is higher than on the support. This is supposed to be due to the spillover of oxygen atoms from platinum particles to graphite.

Platinum deposited on TiO2-C and SnO2-C composites for methanol oxidation and oxygen reduction

Electrocatalytic activities and stabilities of Pt supported on SnO2-carbon and TiO2-carbon as electrocatalysts were examined for methanol oxidation and oxygen reduction reactions. The Pt/oxide-C catalysts were synthesized by photo-deposition method at RT. The samples prepared were characterized by X-ray diffraction, TEM analysis, cyclic and lineal voltammetry, CO stripping and chronoamperometry techniques. Electrochemical activities of Pt/oxide-C for methanol oxidation reaction (MOR) and methanol tolerance for oxygen reduction were compared with those of Pt/C (Pt-Etek). The MOR activity of Pt/SnO2-C was enhanced over those of the Pt/TiO2-C and Pt/C at different concentrations due to the small platinum particles size. Pt-Sn interaction produce changes in the platinum electronic properties that improve the electrochemical activity towards the methanol oxidation and stability to the methanol presence. The SnO2-C composite appear to be a promising support material that promote electrochemical reactions and stabilize catalytic particles in direct alcohol fuel cells.

ChemInform Abstract: Recently Published Work on EUROPT-1, a 6% Pt/SiO2 Reference Catalyst

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Dispersion Improvement of Platinum Catalysts Supported on Silica, Silica-Alumina and Alumina by Titania Incorporation and pH Adjustment

Platinum catalysts supported on silica, silica-alumina and alumina supports were prepared using four different cationic and anionic platinum precursors in order to examine the contribution of the precursors and supports to the platinum dispersion. The pH adjustment of the precursor suspension to obtain an opposing surface charge of the support to that on the platinum precursor ions considerably improved the platinum dispersion by strengthening their adsorption on the support. However, exceptional platinum dispersion was achieved, regardless of the precursor and support, by simultaneous pH adjustment and titania incorporation on the supports. The increase in the interaction between the platinum precursor and the titania-incorporated support raised the dispersion and stability of the platinum catalysts with high-loading, even after reduction at 400 °C. The physico-chemical states of platinum and titanium were investigated by XRD, TEM, XAFS spectroscopy and CO adsorption technique. The platinum catalysts supported on the titania-incorporated supports with pH adjustment exhibited highly dispersed platinum particles ranging in size from 1 to 3 nm. They also exhibited high catalytic activity in the oxidation of carbon monoxide because of its incorporated titania and partially oxidized, small platinum particles.

Size-specific catalytic activity of platinum clusters enhances oxygen reduction reactions

Colloidal platinum nanoparticles with diameters of 2-5 nm on carbon supports are currently regarded as the best catalysts for the oxygen reduction reaction. However, the particle size is limited by the conventional preparation methods that are used to synthesize small platinum particles; the inherent activity of ultrasmall nanoparticles has not yet been revealed. We present a practical synthesis for ultrafine subnanometre platinum clusters using a spherical macromolecular template with no disorder in molecular weight or structure. The template, a phenylazomethine dendrimer, offers control of the number of metal complexes in an assembly through stepwise complexation, allowing the complexes to accumulate in discrete nano-cages. Subsequent reduction of Pt(IV) chloride to Pt(0) results in the formation of platinum clusters composed of a defined number of atoms. As a result of exceptionally small particle size, the clusters exhibit very high catalytic activity for the four-electron reduction of oxygen molecules.

Fabrication of platinum particles by intense, femtosecond laser pulse irradiation of aqueous solution

Platinum nano-particles were directly fabricated through molecular dissociation process induced by tightly focused femtosecond laser pluses in hydrogen hexachloroplatinate (IV) hexahydrate (H 2PtCl 6·6H 2O) aqueous solution. After irradiation, it was found that UV–visible absorption spectrum of the solution was changed and a large number of small platinum particles were synthesized. The variation of absorbance in the spectrum and the yield the particle synthesis depended on the concentration of the solution. The most effective condition for particle synthesis was 0.1 wt%. Black precipitates of platinum particles were observed in the bottom of the vessel after irradiation in some cases. The dispersibility of the particles was greatly improved by the addition of polyvinylpyrrolidone (PVP) as a dispersant, and no precipitates were found in the solution with 1.0 × 10 −2 wt% PVP. Consequently, highly monodispersed platinum nano-particles with the mean size of about 3 nm were successfully fabricated.

Synthesis of very highly dispersed platinum catalysts supported on carbon xerogels by the strong electrostatic adsorption method

Highly dispersed Pt/carbon xerogel catalysts are obtained by applying the “strong electrostatic adsorption” (SEA) of hexachloroplatinic acid to carbon xerogels (PZC = 9.4) and platinum tetraammine chloride to oxidized carbon xerogels (PZC = 2.4). After the reduction step, all these Pt/carbon xerogel catalysts display a very high level of metal dispersion: very small platinum particles (1.1–1.3 nm) are observed by TEM. Pt particle sizes obtained by CO chemisorption are in good agreement with TEM micrographs, which shows that the metal is accessible to reactants. These Pt/carbon xerogel catalysts are very active for the hydrogenation of benzene into cyclohexane.

Modeling of single catalyst particle in cathode of PEM fuel cells

A microscopic catalyst model is proposed for predicting the local mass diffusion of a single catalyst particle in the cathode catalyst layer. The numerical model is employed to obtain a geometric description of the active layer. The cathode catalyst particle model proposed here is based on the microstructure of the catalyst layer. The catalyst particle is treated as many small platinum particles (1–10.0 nm) embedded in the larger carbon particle support (30–100 nm). This assembly is surrounded by an ionomer film with thickness ranging from 0.5 nm to 10.0 nm. The modeling results confirm that the platinum particle size, platinum loading and ionomer thickness can each play an important role on local mass and charge transport in the PEM fuel cell catalyst particle agglomerate. The local spherical diffusion, reactant distribution and electrochemical kinetics are strongly influenced by particle size, platinum loading and ionomer thickness.

Hydrodechlorination of CCl4 over Pt/γ-Al2O3 prepared from different Pt precursors

Abstract The platinum particle size on γ-Al2O3 prepared from different platinum precursors such as Pt(NH3)2(NO2)2, H2PtCl6, and K2PtCl6, and its effect on hydrodechlorination (HDC) of CCl4 with the variation of calcination temperatures was investigated. It concomitantly affects the products distribution and catalytic stability in HDC of CCl4. To verify the effects of platinum particle size with different platinum precursors on the product distribution, the catalysts have been characterized by HRTEM and CO chemisorption, FT-IR, and TP methods (TPR, TPD and TPSR). The catalysts with small platinum particles, which possess low coordination number and electron-deficient character, favor the complete dechlorination of CCl4 and produce CH4 more selectively. It could be due to the strong adsorption strength of CCl4 or the decreased activation energy of surface intermediates on small platinum particles. FT-IR studies reveal that the maximum peak position of linear-bonded CO shifted to the higher frequency with the increase of platinum particle size in all catalysts prepared from three different platinum precursors. While the selectivity to CH4 increased with the decrease in the platinum particle size, the total amount of carbonaceous species on the platinum particles was enhanced. The larger the platinum particle, the higher the selectivity to CHCl3 was obtained in all tested catalysts under the non-deactivating condition.

Stability of platinum particles on a carbon substrate investigated by atomic force microscopy and scanning electron microscopy

To investigate the stability of platinum particles, a flat carbon substrate coated with platinum nano-particles was made as a model of PEMFC cathode catalyst. Changes of the platinum particles caused by potential retention in acid solution were observed by FE-SEM. Stability of the carbon substrate without platinum was also observed by AFM. The position of the AFM and FE-SEM observation before and after each potential retention was identical so that morphological change could be traced like in situ experiments. Behaviors of each particle, i.e. disappearance, migration and aggregation, was observed after potential retentions. Statistical analysis of the images revealed that small platinum particles on the model electrode decreased in number, especially at high potentials and under an oxygen atmosphere.

Surface Water-Assisted Preferential CO Oxidation on Pt/CeO2 Catalyst

The production of clean hydrogen is a key requirement for a future hydrogen economy, in general, and, specifically, for the application of proton exchange membrane fuel cells (PEMFC). Here, we focus on one of the essential purification methods, the so-called “PROX” reaction, the preferential oxidation of traces of CO in a large hydrogen excess. Small platinum particles on a reducible support like ceria are effective to remove CO from hydrogen feed. The paper specifically addresses the mechanism of the PROX reaction on a Pt/CeO2 catalyst using in situ experimentation with time-resolved and temperature-programmed diffuse reflectance infrared spectroscopy. Surface species (carbonates, formates, carbonyls, hydroxyls, and adsorbed water) present under reaction conditions are identified, and correlations of their abundance with catalytic performance allow the discrimination between mechanistically relevant species (intermediates) and spectator species. The following scenario is proposed: hydrogen initially adsorbed on platinum spills over to the support, leading to ordered vacancy formation in the ceria bulk as well as hydroxylation and hydration of the surface. CO is mainly adsorbed in on-top orientation on metallic platinum. The linear relationship between the amount of adsorbed water (H2Oads) and the CO2 production indicates that the hydrated ceria supplies an oxidizing agent at the metal/support interface reacting with the nearby surface carbonyls on the Pt particles yielding CO2. Moreover, adsorbed water also blocks hydrogen oxidation because of desorption hindrance. From the correlations in the presented results, an intelligent PROX catalyst can be formulated, providing a guideline for future developments.

Catalyst Preparation by Electroless Plating and Hydrogen Direct Reduction Methods: Practical Application for a High Cell Performance PEFC

AbstractHigh performance Pt catalysts for polymer electrolyte fuel cells (PEFCs) were prepared by both the electroless plating and hydrogen direct reduction methods. The catalysts were then characterized by cyclic voltammetry (CV), transmission electron microscopy (TEM), and cell performance testing. Both methods exhibited superior cell performance compared to the conventional impregnation preparation method. The catalysts that consisted of too many large platinum particles showed remarkably poor cell output compared to the catalysts with smaller platinum particles. Platinum particles less than 2 nm in diameter exhibited an excellent power density per unit of Pt catalyst loading.

The deposition of ultrafine platinum particles on carbon black by surface ion exchange—increase in loading amount

Carbon black-supported platinum electrocatalysts were prepared by an ion-exchange technique. The ion-exchange characteristics were investigated from the view point of the platinum loading amount related to the amount of carboxylic acid on the surface of the carbon blacks and the kind of platinum precursor salt. The platinum loading amount was up to 37.4 wt.% using this technique. TEM observation showed that extremely small platinum particles were deposited in a highly dispersed state over the surface of the carbon black. The platinum loading amount could be further increased through a procedure of repetitive ion exchange and reduction.

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.

The Influence of Pt Oxide Formation and Pt Dispersion on the Reactions NO 2⇔NO+1/2 O 2 over Pt/Al 2O 3 and Pt/BaO/Al 2O 3

Flow reactor experiments and XPS measurements were used to investigate the observed decrease in activity of Pt/Al 2O 3 and Pt/BaO/Al 2O 3 catalysts during reaction conditions. The reactions studied were NO ( g) +1/2O 2( g) ⇔NO 2( g) . The influence of platinum dispersion on activity and on the activity decrease with time was also studied. When the Pt/Al 2O 3 catalyst was exposed to NO 2 (350°C), the NO 2 dissociation rate decreased. This activity decrease with time was also studied with XPS and it is suggested that it was due to platinum oxide formation. Further, when the platinum dispersion was increased, the platinum activity decreased and the reason may be that small platinum particles more easily form oxides. According to XPS measurements platinum supported on BaO/Al 2O 3 gave a higher concentration of platinum oxide after NO 2 exposure compared to what was found for Pt supported on only Al 2O 3. Flow reactor experiments also showed that Pt/BaO/Al 2O 3 was less active, both for oxidation of NO and for reduction of NO 2, than Pt/Al 2O 3 was and one possible explanation for this is the fact that platinum on BaO/Al 2O 3 forms more oxides.

Quantum Size Effects in Catalysis by TiO2/Platinum: The Switch from Partial Oxidation to Partial Hydrogenation of Styrene

Within a restricted range of metal loadings, the presence of small Pt particles on a hydroxylated fully oxidized TiO2 surface leads to the appearance of unusual chemical behavior. The chemisorption strength of styrene is markedly increased and the partial oxidation activity intrinsic to the hydroxylated titania is replaced by partial hydrogenation activity: styrene is converted to ethylbenzene instead of acetophenone. UPS and X-ray photoelectron spectroscopy data indicate that in this regime of platinum loading the Pt particles are subject to the so-called quantum size effect (QSE) and must therefore be of nanoscopic dimensions. The appearance of hydrogenation activity in the presence of these very small platinum particles may be rationalized in terms of a QSE/bifunctional catalysis mechanism involving water dissociation on the Pt followed by Ha spill-over over at the Pt/titania boundary where captured, strongly bound, styrene molecules undergo hydrogenation.

Investigation of Methanol Oxidation by a Model Catalyst Electrode Method.

A model catalyst method has been adopted to clarify the effect of particle size of the Pt metals supported on carbon materials that are used for the electrocatalytic oxidation of methanol in an acidic solution at 60oC. The model catalyst electrodes, Pt/GC, were prepared by vacuum deposition of Pt on a mirror-polished surface of non-porous glassy carbon (GC) to avoid the influence of the carbon substrates pores on the catalytic properties. The electrocatalytic activity of the electrodes evaluated both by cyclic voltammetry and chronoamperometry in 0.5 M H2SO4 showed that the specific activity of the methanol oxidation increased with an increase in the Pt particle size. The oxidation of CO on the Pt/GC electrodes, which was examined to elucidate the size effect, indicated that smaller platinum particles need higher energy for the oxidation of carbon monoxide. The size effect on the specific activity was discussed in terms of the exposed crystal planes and characteristic valence band structure of the Pt particles supported on carbon.