Dispersed Platinum Particles Research Articles

Pt Catalysts Prepared via Top-down Electrochemical Approach: Synthesis Methodology and Support Effects

The synthesis of Pt nanoparticles and catalytically active materials using the electrochemical top-down approach involves dispersing Pt electrodes in an electrolyte solution containing alkali metal cations and support material powder using an alternating pulsed current. Platinum is dispersed to form particles with a predominant crystallographic orientation of Pt(100) and a particle size of approximately 7.6±1.0 nm. The dispersed platinum particles have an insignificant content of PtOx phase (0.25±0.03 wt.%). The average formation rate was 9.7±0.5 mg cm−2 h−1. The nature of the support (carbon material, metal oxide, carbon-metal oxide hybrid) had almost no effect on the formation rate of the Pt nanoparticles as well as their crystallographic properties. Depending on the nature of the support material, Pt-containing catalytic materials obtained by the electrochemical top-down approach showed good functional performance in fuel cell technologies (Pt/C), catalytic oxidation of CO (Pt/Al2O3) and electrochemical oxidation of methanol (Pt/TiO2-C) and ethanol (Pt/SnO2-C).

Ceria–Zirconia Supported Platinum Catalysts for Water Gas Shift Reaction: Influence of Support Composition

The study is presented on the influence of the composition of a ceria-zirconia support on the structure and the activity in water gas shift reaction of platinum catalysts (Pt/Ce0.75Zr0.25O2 и Pt/Ce0.4Zr0.5Y0.05La0.05O2). The structure diagnostics of the samples were performed using high-resolution transmission electron microscopy, powder X-ray diffraction, CO chemisorption and X-ray atomic pair distribution function method. It was shown that the catalysts contain highly dispersed platinum particles not exceeding 2 nm in size. Platinum particles supported on Ce0.75Zr0.25O2 are smaller due to the higher specific surface area of the support. The catalysts Pt/Ce0.75Zr0.25O2 and Pt/Ce0.4Zr0.5Y0.05La0.05O2 proved to have similar efficiency while having the same platinum content. It was assumed that the catalysts supported on Ce0.4Zr0.5Y0.05La0.05O2 demonstrate a slightly higher turnover frequency per platinum surface atom, but it is likely compensated by the difference in the supported metal particle size.

Construction of hierarchical core-shell Pt/USY@MSA bifunctional catalysts for hydroisomerization of n-hexadecane

Hydroisomerization of long-chain n-alkanes is an indispensable process in the upgrading of fuel oil and lubricants to meet the state of art regulations. In this work, a novel core-shell structural material comprising USY zeolite core and mesoporous aluminosilicate (MSA) shell, denoted as USY@MSA, was fabricated by virtue of a desilication and recrystallization approach. Comprehensive characterizations demonstrate that the USY@MSA composite exhibits a hierarchical pore architecture and a gradient acidity. Due to the confinement effect of the mesoporous aluminosilicate shell, highly dispersed platinum particles were selectively deposited on the shell, generating a nanoscale distance with acidic sites of USY zeolite core. In the hydroisomerization of n-hexadecane, the Pt/USY@MSA composite catalyst outperforms the conventional Pt/USY and desilicated Pt/USY-ds catalysts, effectively suppressing the cracking side reactions and producing higher selectivity to branched isomers.

Investigation into the roles of different oxygen species in toluene oxidation over manganese-supported platinum catalysts

With the aim to elucidate the roles of different oxygen species, the catalytic oxidation of toluene over Pt/Mn3O4 synthesized by a facile reduction method was investigated. Compared with the Pt/Mn3O4-C using the commercial Mn3O4 as the support, Pt/Mn3O4-R was found to possess more surface oxygen vacancies, exhibit better redox properties and dispersion of platinum particles, leading to its higher activity for toluene oxidation (T90 = 183 ℃). Through toluene-TPD, toluene-TPSR and in-situ DRIFTS, it was found that toluene oxidation over Pt/Mn3O4-R catalyst might follow the Mars–van Krevelen (MVK) mechanism and the pathway of toluene oxidation over Pt/Mn3O4-R was possible toluene → benzoate → carbonate → CO2. The lattice oxygen species on the surface of catalysts was directly involved in the oxidation of toluene and played an important role in the formation of key intermediate such as carbonate, resulting in the difference of the reaction mechanism and pathway between Pt/Mn3O4-R and Pt/Mn3O4-C. In addition, the degradation of the surface carbonate was demonstrated to be the rate-controlling step at 180℃ to 200℃.

Methanol Steam Reforming over ZnPt/MoC Catalysts: Effects of Hydrogen Treatment

Molybdenum carbide-based catalysts have been utilized widely in hydrogen production from methanol steam reforming (MSR) because of their high selectivity to H₂ and low reaction temperature. However, the catalytic performance and stability for MSR are highly vulnerable to the method and condition of catalyst preparation. Here, ZnPt/MoC catalysts synthesized by different processing techniques were used for the production of hydrogen from MSR at low temperature with emphasis on the influence of hydrogen treatment on the catalytic performance and stability. These ZnPt/MoC catalysts were analyzed fully using several characterization techniques. From the results, it was found that reducing the ZnPt/MoC catalyst precursor before the carburization facilitated the formation of the α-MoC₁ – ₓ phase and enhanced the platinum dispersion on the catalyst surface, which resulted in increased catalytic performance and stability for MSR. Furthermore, after treating with flowing hydrogen at 200–400 °C, more α-MoC₁ – ₓ phase and highly dispersed platinum particles were obtained in the modified catalysts. The optimal ZnPt/MoC-p3–200 catalyst exhibited outstanding catalytic performance for MSR and extremely low CO selectivity at low reaction temperature. This study provides a perspective on the design and synthesis of stable and highly active molybdenum carbide-based catalysts for MSR.

Enhancement of the Electrochemical Properties of an Open-Pore Graphite Foam with Electrochemically Reduced Graphene Oxide and Alternating Current Dispersed Platinum Particles

This paper aimed to improve the electrochemical activity of a pitch-derived open-pore graphite foam (GF) by an electrochemical coating of reduced graphene oxide (RGO) and platinum particles without significantly affecting its 3D-structure. RGO was synthesized using cyclic voltammetry (CV) from a 3 g L−1 GO and 0.1 M LiClO4 solution. For the electrodeposition of Pt particles, an alternating current method based on electrochemical impedance spectroscopy (EIS) was used. A sinusoidal voltage from a fixed potential Ei was varied following a selected amplitude (ΔEac = ± 0.35 V) in a frequency range of 8 Hz ≤ fi ≤ 10Hz, where i = 500. This method proved its efficiency when compared to the traditional CV by obtaining more highly electroactive coatings in less synthesis time. For samples’ characterization, physical measures included permeability, pressure drop, and nitrogen adsorption isotherms. The electrochemical characterization was performed by CV. The surface morphology and chemical composition were examined using field emission electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDX), respectively. RGO improved the electron transfer rate constant of GF, and a more homogeneous coating distribution of reduced size Pt particles was obtained.

EFFECT OF DURATION OF OXYCHLORINATION ON DEGREE OF ACCESSIBILITY FOR CATALYSIS OF PLATINUM CENTERS OF PLATINUM-RHENIUM REFORMING CATALYST

The work presents the results of a chemisorption analysis of a platinum-rhenium catalyst on an alumina support after regeneration and reduction with hydrogen. Adsorption-desorption diagrams were obtained by stepwise-pulsed chemisorption of carbon monoxide on reforming catalyst samples. With an increase in the number of carbon monoxide injections from 1 to 4, the catalyst sample is poisoned, and subsequent desorption peaks indicate the termination of the interaction. With an increase in the time of oxychlorination, the CO/Pt ratio in the carrier volume increases linearly. The effect of the oxychlorination process on the chemisorption of CO and the subsequent availability of platinum nanoparticles for catalysis has been shown. The absorption on freshly prepared platinum-rhenium catalyst samples reaches a CO/Pt molar ratio of about 0.4. The results show that the duration of oxychlorination for 16–20 h allows us to achieve the value of the ratio CO/Pt, which is in the range of 0.4-0.5. This indicates that the availability of platinum centers in its composition reaches the level of a fresh catalyst, and, on the other hand, taking into account a slight excess of this ratio, we can assume that some of the Re atoms participate in the absorption of CO molecules. The presence of finely dispersed platinum particles in the composition of the regenerated catalyst was confirmed by IR spectroscopy. The analysis of catalyst samples on an IR spectrometer in the frequency range of 1900-2200 cm-1 revealed a rather wide absorption band with a pronounced extremum at 2060 cm-1. In this frequency range, there is another, slightly pronounced extremum at 2149 cm-1. However, for samples with a short duration of oxychlorination, it did not appear. An absorption band with an extremum of 2060 cm-1 can be attributed to linear vibrations of adsorbed CO molecules on the surface of particles of metallic platinum.

The Role of the Stage of Metal Complex Deposition on Support in the Properties Formation of Supported Platinum Catalysts

The subject of this review is related to the development of research on the mechanism of interaction of platinum complex compounds, primarily, H2[PtCl6], with alumina supports and the role of this interaction in properties formation of the corresponding platinum catalysts. Based on the study of the composition of Pt(IV) chloride complexes adsorbed on the γ-Al2O3 surface, it was proposed that the complexes should be quantitatively discriminated as inner- and outer-sphere complexes in accordance with the nature of their binding with the support. The chloride ligands were found to be dominant in the bound outer-sphere Pt(IV) complexes, and adsorption of the complexes on the surface occurs due to the protonation of the hydroxyl groups of the support without any significant changes in the composition and geometry of the [PtCl6]2– complex anion. The binding of Pt(IV) in the outer-sphere complexes leads (after their reduction with hydrogen) to the formation of Pt particles with a relatively low dispersion containing platinum atoms mainly in a nearly Pt0 state, which are characterized by hydrogenating and dehydrogenating activity. The inner-sphere platinum(IV) complexes are the hydrolyzed forms of the chloride complexes formed by the replacement of chloride ligands by the hydroxyl groups of the support. Their reduction forms finely dispersed platinum particles containing a significant fraction of atoms in the ionic form, which increases the activity of the Pt/Al2O3 catalyst in dehydrocyclizations of n-alkanes. Several approaches were proposed that make it possible to vary the ratio between the outer- and inner-sphere platinum(IV) complexes (increasing the degree of hydrolysis of the complexes, or varying the nature of the surface OH groups of the alumina support) and hence to modify the properties of the catalyst. The possibility of selective synthesis of outer- and inner-sphere platinum(IV) complexes bound with other supports was shown for the Pt/MgAlOx system. Using the structural features of layered aluminum-magnesium hydroxides makes it possible to bind platinum(IV) in the complexes of the given type at high selectivity and to change the electronic state of Pt atoms in Pt particles and their activity in alkane dehydrogenations without changing the chemical composition of the catalyst.

Design of new nanostructured thermostable TiO2 of anatase structure doped with alumina

Titania doped with 1-10 wt.% alumina was used as a thermostable support in the preparation of Pt/(Al2O3-TiO2) catalysts for the oxidation of CO. Pure titania (anatase) is metastable and irreversibly converts to rutile phase at 750 °C. However, in TiO2 doped with alumina, the temperature of the phase transition is above 950 °C. The stability of the anatase phase in Pt/(Al2O3-TiO2) catalysts at high temperatures ensures the maintenance of a high specific surface area and dispersion of platinum particles, which retain their high catalytic activity. This is due to the formation of a nanocrystalline structure of anatase in titania doped with alumina, unlike pure titania, in which a well-ordered regular anatase structure is formed.

(Invited) Interfacial Functionalization of Carbon Nanostructures: From Effective Charge Propagation and Storage to Enhancement of Electrocatalytic and Bioelectrocatalytic Properties

Of particular interest to the preparation of advanced materials is synthesis and characterization of carbon nanostructures (e.g. nanotubes) and noble metal nanoparticles, their stabilization (e.g. through self-assembly), as well as organization into two-dimensional arrays and controlled fabrication (e.g. through the sequential attraction) into three-dimensional network films. They can form nanosized materials with well-defined composition, structure and thickness. The interfaces can be also highly functionalized, and they can exhibit specific catalytic or unique electronic, charge storage, optical and sensing properties. We explore here the ability of inorganic structures to stabilize and derivatize metal and carbon nanostructures. Among inorganic systems, polyoxometallates of molybdenum and tungsten are attractive since they can not only adsorb irreversibly on solid surfaces but also exhibit reversible stepwise multielectron transfer reactions. The concept of the layer-by-layer formation of hybrid (organic-inorganic) assemblies composed of anionic polyoxometallate-protected carbon nanotubes (or metal nanoparticles) and ultra-thin films of positively charged conducting polymers (e.g. such as polyaniline or PEDOT) will be described and discussed here. The resulting novel composite materials have been fabricated as thin or moderately thick (µm level) films on electrode surfaces. As evidenced from STM and scanning electron microscopy, their morphology is still granular but the structure is fairly dense. Further, they are characterized by fast dynamics of charge propagation. Obviously, this research is of importance to the construction of effectively operating charge storage devices (capacitors), charge mediators (e.g. in bioelectrochemistry), molecular electronic systems and electrocatalysis. In the latter case, polyoxometallates can also be applied to stabilize and link Pt-Ru, Pt-Sn and various alloyed Pt-based nanoparticles. It is apparent from diagnostic cyclic voltammetric, rotating disk voltammetric and chronoamperometric measurements that such systems exhibit attractive properties towards electroreduction of oxygen or oxidation of alcohols (ethanol, methanol). Here, it is possible that addition of polytungstate or molybdate clusters to ruthenium or tin hydroxo species at the catalytic interface results in activating effect on dispersed platinum particles. An alternate explanation may involve a possibility of electronic effects and/or different morphologies of the catalytic films in the presence and absence of polyoxometallate.

Origin of the different behavior of some platinum decorated nanocarbons towards the electrochemical oxidation of hydrogen peroxide

The electrochemical behavior of different platinum-decorated nanocarbons (Pt@C) towards the oxidation of hydrogen peroxide (H2O2) was investigated. Three different types of nanocarbons were considered: i) carbon black, ii) dahlia-like carbon nanohorns and iii) carbon nanotubes, which included both commercial (single-wall and multi-wall) and laboratory prepared (multi-wall) samples. Shape and size distribution of the platinum nanoparticles and morphology of the nanocarbons were analyzed by transmission electron microscopy. Their nanostructure was investigated by micro-Raman spectroscopy, while elemental composition of the samples and chemical bonding states were studied by X-ray photoelectron spectroscopy. Electrochemical behavior towards H2O2 oxidation was evaluated by means of cyclic voltammetry modifying the working screen-printed carbon electrode surface with the prepared Pt@C nanocomposites. Data obtained suggest that the size and dispersion of the Pt nanoparticles play a key role in increasing the sensitivity towards H2O2 detection. Thanks to the presence of smaller and more dispersed platinum particles and of a greater amount of platinum hydroxide, acting as intermediary in the H2O2 oxidation process, Pt@dahlia-like carbon nanohorns result to be the most promising platform for the development of H2O2 electrochemical sensors.

Increased conversion and selectivity of 4-nitrostyrene hydrogenation to 4-aminostyrene on Pt nanoparticles supported on titanium-tungsten mixed oxides

A catalyst series consisting in platinum nanoparticles photodeposited on pure titania and on W/Ti mixed oxides, these latter prepared by the sol-gel method, were tested in the hydrogenation of 4-nitrostyrene. A remarkable increase in the reaction rate occurred when the catalyst support contained tungsten, with a parallel boosting in the selective reduction of the nitro group. With the selective W-containing catalysts, the reaction proceeded at constant rate (zero order rate law), while the tungsten-free catalyst showed a rate-dependence on the 4-nitrostyrene concentration (positive order reaction). The presence of tungsten in the support is beneficial not only because a higher surface area is obtained, thanks to the stabilization of anatase owing to the presence of tungsten, but also because it allows the photodeposition of smaller, better dispersed platinum particles, on which the adsorption of the aromatic part of 4-nitrostyrene is less favored. Tungsten not only substitutes titanium in the titania lattice, as revealed by HAAF-STEM analysis, but it is also present as WOx species partly covering the Pt nanoparticles photodeposited on the mixed oxide support, as revealed by an in depth distribution XPS analysis. This accounts for the progressively lower performance observed with increasing tungsten content in the catalysts, the highest conversion and selective hydrogenation of the 4-nitrostyrene nitro group having been achieved on the catalyst with a 1% W/Ti molar ratio.

Kinetic features of the size effect in methane oxidation over Pt/γ-Al2O3 catalysts

The correlation between the kinetic parameters of methane oxidation and the size of supported platinum nanoparticles was studied. The reaction order with respect to methane appeared to differ from unity while that for water was significantly negative. An impact of carbon dioxide was not found and the order with respect to oxygen was suggested to be equal to zero at an excess of oxygen in the mixture. The numerically optimized kinetic equations were proposed for each catalyst based on the results of fifty experiments. According to the most realistic models, competitive adsorption of methane and water affected considerably the reaction over the catalysts with the highly dispersed platinum particles (1.3 nm in size). In the case of the catalysts with the large platinum particles (10–16 nm in size), the competitive methane adsorption did not influence the process. The apparent activation energies for the process ranged between 65 and 90 kJ/mol and depended slightly on the average size of the supported platinum nanoparticles.

Organic Solvent’s Effect in the Deposition of Platinum Particles on MWCNTs for Oxygen Reduction Reaction

We reported the synthesis of platinum particles anchored on the surface of multiwall carbon nanotubes (MWCNTs). The synthesis of platinum particles was carried out by microemulsion method using hexadecyltrimethylammonium bromide (CTAB) as surfactant to enhance the dispersion of platinum particles in hexane (C6), heptane (C7), and octane (C8) solutions. The effects of the microemulsion dispersion medium on the synthesis of platinum particles on MWCNTs (MWCNT/Pt) hybrid materials and their catalytic activities of the oxygen reduction reaction (ORR) in HClO4were investigated. The anchored platinum particles showed good dispersion on carbon nanotubes surface with the average particle sizes of2.65±0.60,2.89±0.68, and0.97±0.29 nm for the dispersion medium of C6, C7, and C8, respectively. The experimental results of ORR experiments indicated a relationship between the size and the dispersion media of the platinum particles; also the catalytic activity of the anchored platinum on MWCNT particles strongly depends on the dispersion medium employed in the microemulsion process.

Platinum catalyst formed on carbon nanotube by the in-liquid plasma method for fuel cell

In-liquid plasma was generated in the carbon nanotube (CNT) dispersion fluid using platinum electrodes. The generated plasma spattered the surface of the platinum electrodes and dispersed platinum particles into the CNT dispersion. Therefore, the platinum nanoparticles were successfully formed on the CNT surface in the dispersion. The platinum nanoparticles were applied to the proton exchange membrane fuel cell (PEMFC) as a catalyst. The electrical power of 108mW/cm2 was observed from the fuel cell which was assembled with the platinum catalyst formed on the CNT by the in-liquid plasma method.

Platinum supported on highly-dispersed ceria on activated carbon for the total oxidation of VOCs

Catalysts consisting in platinum supported on cerium oxide highly dispersed on activated carbon, with a Pt loading of 1wt.% and ceria loadings of 5, 10 and 20wt.% have been prepared by impregnation method and characterized by several techniques (N2 adsorption at 77K, ICP, XRD, H2-TPR and XPS). Their catalytic behavior has been evaluated in the total oxidation of ethanol and toluene after reduction at 473K. The obtained results show that the prepared catalysts have better performances than platinum supported on bulk CeO2. The best catalytic performance was obtained for 10wt.% ceria loading, likely due to an optimum synergistic interaction between highly dispersed cerium oxide and platinum particles. Pt-10Ce/C achieves total conversion of ethanol and toluene to CO2 at 433K and 453K, respectively, and shows no deactivation during a test for 100h. Under humid conditions (relative humidity, RH, of 40 and 80%), the activity was only slightly influenced due to the hydrophobic character of the activated carbon support, which prevents the adsorption of water.

Synthesis of highly dispersed platinum particles on carbon nanotubes by an in situ vapor-phase method

Highly dispersed Pt nanoparticles were prepared on functionalized multi-walled carbon nanotubes (f-MWCNTs) using a simple in situ vapor-phase method. The method consisted in two-step procedure in which an initial mixture of Pt precursor (Pt-acac) and f-MWCNTs was heated in a quartz tube reactor, first at 180°C and then at 400°C. Fourier transform infrared spectroscopy (FTIR–ATR), thermal gravimetric analysis (TGA) and X-ray diffraction (XRD) were used to follow the chemical and structural transformations of mixture components during heating steps. The functionalization of MWCNTs with HNO3/H2SO4 solution resulted in formation of surface carbonyl groups. The FTIR–ATR and XRD results indicated that individual Pt-acac withstood heating at 180°C, whereas it was dissociated when heated in contact with f-MWCNTs at the same temperature. Thus, the functional carbonyl groups were found to be responsible for assisted decomposition of Pt-acac at 180°C. Since carbonyl groups served as reaction sites for decomposition of Pt-acac, the resulting particles were highly and homogeneously dispersed on the surface of MWCNTs even the relatively high metallic loading of 27wt%. TEM observations revealed that crystalline Pt particles exhibit narrow size distribution with a mean size of 2.3nm.

Ice-Crystallization Kinetics in the Catalyst Layer of a Proton-Exchange-Membrane Fuel Cell

Nucleation and growth of ice in the catalyst layer of a proton-exchange-membrane fuel cell (PEMFC) are investigated using isothermal differential scanning calorimetry and isothermal galvanostatic cold-starts. Isothermal ice-crystallization rates and ice-nucleation rates are obtained from heat-flow and induction-time measurements at temperatures between 240 and 273 K for four commercial carbon-support materials with varying ionomer fraction and platinum loading. Measured induction times follow expected trends from classical nucleation theory and reveal that the carbon-support material and ionomer fraction strongly impact the onset of ice crystallization. Conversely, dispersed platinum particles play little role in ice crystallization. Following our previous approach, a nonlinear ice-crystallization rate expression is obtained from Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. A validated rate expression is now available for predicting ice crystallization within water-saturated catalyst layers. Using a simplified PEMFC isothermal cold-start continuum model, we compare cell-failure time predicted using the newly obtained rate expression to that predicted using a traditional thermodynamic-based approach. From this comparison, we identify conditions under which including ice-crystallization kinetics is critical and elucidate the impact of freezing kinetics on low-temperature PEMFC operation. The numerical model illustrates that cell-failure time increases with increasing temperature due to a longer required time for ice nucleation. Hence, ice-crystallization kinetics is critical when induction times are long (i.e., in the “nucleation-limited” regime for T > 263 K). Cell-failure times predicted using ice-freezing kinetics are in good agreement with the isothermal cold-starts, which also exhibit long and distributed cell-failure times for T > 263 K. These findings demonstrate a significant departure from cell-failure times predicted using the thermodynamic-based approach.

Enhancement of OSC property of Zr rich ceria–zirconia by loading a small amount of platinum

The OSC properties and catalytic activities of Pt/ceria–zirconia catalysts with a small amount of platinum and cerium were investigated so as to reduce the usage of rare metals. In the case of Pt/CZ catalysts calcined at 800°C, the nano-sized platinum particles highly dispersed on the surface of CZ support. The platinum particles sintered via the calcination at 1000°C, whereas the mean diameters of Pt particles in Pt/CZ with small Pt loadings were maintained in nano-scale size. The enhancement of oxygen release properties due to Pt loadings was confirmed through H2-TPR measurements. On the other hand, the activities for CO oxidation were also improved by the loading of dispersed platinum particles. Moreover, the temperature of total CO oxidation was correlated with the oxygen release temperature. It was suggested that the release of lattice oxygen in ceria–zirconia support should be indispensable for CO oxidation reaction. This study revealed the effectivity of a small amount of Pt loadings and Zr rich CZ support.

Preparation of Dispersed Spherical Platinum Particles with Controlled Size and Internal Structure

Uniform dispersed spherical platinum particles were precipitated by reducing Pt(IV) hexaammine ([Pt(NH3)6]4+) complex ions with L-ascorbic acid in the presence of polymeric dispersants. By varying the nature and the amount of dispersing agent the average diameter of the Pt spheres could be adjusted between 200 nm and 800 nm. Electron microscopy and X-ray diffraction (XRD) evaluations revealed that the final Pt particles were the result of an irreversible aggregation of small (~6 nm) nanoparticles. The size of the constituent crystallites was controllably increased through a subsequent heat treatment process without affecting the shape or the dispersion of the Pt spheres. The method described represents a versatile and cost-effective route for producing Pt powders at the sub-micrometre or micrometre scale with controlled crystallinity for thick film electronic applications.