Shape Of Pt Nanoparticles Research Articles

Electrochemical Top-Down Synthesis of Nanostructured Electrocatalysts for Polymer Electrolyte Membrane Fuel Cells

Nanostructured catalysts are a promising class of materials to meet the requirements for the energy transition away from fossil fuels towards alternative, carbon-free, energy carriers, such as hydrogen. These catalyst materials boost the efficiency of energy conversion devices, such as fuel cells or electrolyzers, by accelerating their fundamental electrochemical reactions. Bottom-up synthetic routes for nanostructured materials often involve multiple synthesis steps and the use of surfactants, reducing agents or stabilizers. This results in complex and extensive synthesis protocols.[1] In recent years, a novel top-down synthesis approach to form metal nanoparticles has been established, in which bulk metal wires are immersed in an electrolyte and subsequently subjected to a high alternating potential. This leads to the generation of nanoparticles dispersed in the electrolyte.[2] There is no need for reducing agents, surfactants, or precursor solutions. The complete synthesis can be performed in one pot involving one main step with consequent washing and drying of the nanoparticles.In this study, we synthesized Pt-based nanoparticles supported on Vulcan® carbon support (Pt/C) and tested their electrochemical activity towards the oxygen reduction reaction (ORR) for polymer electrolyte membrane fuel cells (PEMFCs). Firstly, we elucidated the influence of synthesis parameters such as potential amplitude, frequency, electrolyte composition, and concentration on the size and shape of Pt nanoparticles.[3] We continued with the synthesis of more complex nanostructures, such as Pt alloy nanoparticles. This introduces strain and ligand effects at the surface of the nanoparticles, which increases the electrocatalytic activity for the ORR. We demonstrated the facile, simple, and surfactant-free synthesis of highly active PtxPr/C[4] and PtxCu/C, as two examples for Pt alloy catalysts for the ORR, and give an outlook on other systems, such as PtxGd/C.

Preparation of Pt/ZIF-7 hybrid porous material and its application in hydrogen fuel cell

Hydrogen fuel cell vehicles are the most ideal low-carbon and environmentally friendly new energy vehicles, but the catalyst for hydrogen fuel cells requires the use of precious metal platinum. At present, the catalysts for hydrogen fuel cells mainly rely on imports, greatly limiting the development of hydrogen fuel cell vehicles in China. We used a simple and energy-saving room temperature stirring method to in-situ load polyvinyl pyrrolidone modified Pt onto ZIF-7, so that the surface of ZIF crystal can continuously absorb PVP modified Pt nanoparticles while growing. PVP is a non-ionic surfactant, which can not only stabilize Pt nanoparticles in polar solvents, but also control the size and shape of Pt nanoparticles as a capping agent. This can not only avoid the agglomeration of Pt nanoparticles to keep their original characteristics, but also effectively control the spatial distribution of Pt nanoparticles in ZIF-7. The catalysts Pt/ZIF-7 were prepared, with corresponding Pt content of 0.052%, 0.26%, 0.52%, and 1.04%, respectively. Subsequently, we will apply the prepared Pt/ZIF-7 catalyst to hydrogen fuel cells and test its catalytic performance.

The influence of dielectric permittivity of water on the shape of PtNPs synthesized in high-pressure high-temperature microwave reactor

In this paper, a novel method for the synthesis of Pt nanoparticles (PtNPs) using a microwave autoclave reactor is proposed. For benchmarking, the obtained results are compared with the traditional, batch method. A novel process window is proposed, which is the application of high-temperature and high-pressure. The main finding is that this only brings advantage, when the ionic strength of the system is enough low. It is explained, that at high pressure and high temperature, water behaves like only a slightly polar solvent, approaching a subcritical state. This reduces the electrostatic stabilization of the particles. Moreover, a change in the Pt particle shape is observed under high pressure and temperature conditions, suggesting that additional physical–chemical processes are involved.

Confinement Impact for the Dynamics of Supported Metal Nanocatalyst.

Supported metal nanoparticles play key roles in nanoelectronics, sensors, energy storage/conversion, and catalysts for the sustainable production of fuels and chemicals. Direct observation of the dynamic processes of nanocatalysts at high temperatures and the confinement of supports is of great significance to investigate nanoparticle structure and functions for practical utilization. Here, in situ high-resolution transmission electron microscopy photos and videos are combined with dynamics simulations to reveal the real-time dynamic behavior of Pt nanocatalysts at operation temperatures. Amorphous Pt surface on moving and deforming particles is the working structure during the high operation temperature rather than a static crystal surface and immobilization on supports as proposed before. The free rearrangement of the shape of Pt nanoparticles allows them to pass through narrow windows, which is generally considered to immobilize the particles. The Pt particles, no matter what their sizes, prefer to stay inside nanopores even when they are fast moving near an opening at temperatures up to 900 °C. The porous confinement also blocks the sintering of the particles under the confinement size of pores. These contribute to the continuous high activity and stability of Pt nanocatalysts inside nanoporous supports during a long-term evaluation of catalytic reforming reaction.

First-Principles Calculation of Pt Surface Energies in an Electrochemical Environment: Thermodynamic Driving Forces for Surface Faceting and Nanoparticle Reconstruction.

Platinum is a widely used catalyst in aqueous and electrochemical environments. The size and shape of Pt nanoparticles and the faceting (and roughness) of extended Pt surfaces change during use in these environments due to dissolution, growth, and reconstruction. Further, many Pt nanoparticle synthesis techniques are carried out in an aqueous environment. The surface structures formed are impacted by the relative surface energies of the low index facets in these environments. Density functional theory is used to calculate the surface energy of the low index facets of platinum as a function of electrochemical potential and coverage of adsorbed hydrogen, hydroxide, oxygen, and the formation of surface oxide in an aqueous environment. Whereas Pt(111) is the lowest energy bare surface in vacuum, the strong adsorption of hydrogen to Pt(100) at low potentials and of hydroxide to Pt(110) and oxygen to Pt(100) at high potentials drives these surfaces to be more stable in an electrochemical environment. We experimentally conditioned a polycrystalline platinum electrode by cycling the potential and find a growth in the total area as well as in the fraction of 110 and 100 sites, which are lower in energy at potentials where dissolved Pt is deposited or surface oxide is reduced. Further, we find that the lower surface energy of Pt(100) at low potentials may play a role in the growth of tetrahexahedral nanoparticles seen on square wave cycling of spherical Pt nanoparticles. Wulff constructions are presented as a function of Pt electrode potential.

Shape-Controlled Synthesis of Pt Nanopeanuts.

Exploring the novel shape of Pt nanoparticles is one of the most useful ways to improve the electrocatalytic performance of Pt in fuel cells. In this work, the Pt nanopeanuts consisting of two nanospheres grown together have been fabricated through a two-step polyol method. The high resolution scanning electron microscope (SEM) images and energy dispersive x-ray (EDX) spectrum collected at adjacent part point out the Pt nanopeanut is apparently different from the two physical attached nanospheres. To understand the growth mechanism of this nanopeanut, the final products in different synthesis situations are studied. The results indicate the interesting morphology of Pt nanopeanuts mainly benefit from the chemical reagent (FeCl3) while the size and homogeneity are greatly affected by the temperature. Furthermore, the electrocatalytic activity of the Pt nanopeanuts has also been demonstrated here. Our two-step synthesis of Pt nanopeanuts not only enlarges the group of Pt nanoparticles, but also provides a beneficial strategy for the synthesis of novel metal nanoparticles.

Synthesis of sub-nanosized Pt particles on mesoporous SBA-15 material and its application to the CO oxidation reaction.

In this work, we show that the size and shape of Pt nanoparticles in SBA-15 can be controlled through vacuum and air calcination. The vacuum-calcination/H2-reduction process is used to thermally treat a 0.2 wt% Pt(4+)/SBA-15 sample to obtain small 2D clusters and single atoms that can significantly increase Pt dispersion in SBA-15. Compared with thermal treatments involving air-calcination/H2-reduction, which result in ∼4.6 nm rod-like Pt particles, vacuum-calcination/H2-reduction can dramatically reduce the size of the Pt species to approximately 0.5-0.8 nm. The Pt particles undergoing air-calcination/H2-reduction have poor conversion efficiency because the fraction of terrace sites, the major sites for CO oxidation, on the rod-like Pt particles is small. In contrast, a large amount of low-coordinated Pt sites associated with 2D Pt species and single Pt atoms in SBA-15 is effectively generated through the vacuum-calcination/H2-reduction process, which may facilitate CO adsorption and induce strong reactivity toward CO oxidation. We investigated the effect of vacuum-calcination/H2-reduction on the formation of tiny 2D clusters and single atoms by characterizing the particles, elucidating the mechanism of formation, determining the active sites for CO oxidation and measuring the heat of CO adsorption.

Adhesion of Pt Nanoparticles Supported on γ-Al2O3 Single Crystal

A model catalyst system of Pt nanoparticles (2–5 nm) dispersed on γ-Al2O3 (111) was prepared to study the support effects on nanoparticles shape and structure by cross-sectional high-resolution electron microscopy. A dewetting equilibrium shape of Pt nanoparticles was observed with a low-index plane orientation relation: Pt(111)[211]∥γ-Al2O3(111)[211] and Pt(100)[011]∥γ-Al2O3(111)[211]. Lattice-matching epitaxy of Pt(110)∥γ-Al2O3(110) was found across their interface to maintain a minimum interface strain during particle growth. To quantitatively interpret the interfacial energy and adhesion energy of the nanoparticles, the Wulff–Kaischew theorem was applied by taking into account the γ-Al2O3 support modifications to the equilibrium shape of nanoparticles. The present approach quantitatively solved the adhesion energy of the particle/support and could help to interpret nanoparticles equilibrium shape and stability.

First-Principles Prediction of the Equilibrium Shape of Nanoparticles Under Realistic Electrochemical Conditions

A first-principles model of the electrochemical double layer is applied to study surface energies and surface coverage under realistic electrochemical conditions and to determine the equilibrium shape of metal nanoparticles as a function of applied potential. The potential bias is directly controlled by adding electronic charge to the system, while total energy calculations and thermodynamic relations are used to predict electrodeposition curves and changes in surface energies and coverage. This approach is applied to Pt surfaces subject to hydrogen underpotential deposition. The shape of Pt nanoparticles under a cathodic scan is shown to undergo an octahedric-to-cubic transition, which is more pronounced in alkaline media due to the interaction energy of the pH-dependent surface charge with the surface dipole.

Pt/TiO2 composite nanoparticles synthesized by electron beam irradiation for preferential CO oxidation

This paper describes a novel synthesis method of stabilizer-free Pt/TiO2 composite nanoparticles using electron beam irradiation. The chemical compositions were analyzed by inductively coupled plasma-atomic emission spectroscopy. The microstructures of the samples were observed by using transmission electron microscope. Pt nanoparticles with the sizes of 2–4nm were deposited on TiO2 without any use of stabilizers. The concentrations of Pt ions and 2-propanol notably affected the size and shape of Pt nanoparticles. Their reactions of preferential CO oxidation were measured in temperature region from 60 to 140°C. The Pt/TiO2 catalyst with spherical Pt nanoparticles exhibited a 67% of CO conversion rate and 100% of selectivity at a low temperature of 60°C.

Roughening of Pt nanoparticles induced by surface-oxide formation

Using density functional theory (DFT) and thermodynamic considerations we studied the equilibrium shape of Pt nanoparticles (NPs) under electrochemical conditions. We found that at very high oxygen coverage, obtained at high electrode potentials, the experimentally-observed tetrahexahedral (THH) NPs consist of high-index (520) faces. Since high-index surfaces often show higher (electro-)chemical activity in comparison to their close-packed counterparts, the THH NPs can be promising candidates for various (electro-)catalytic applications.

High Structure Sensitivity of Vapor-Phase Furfural Decarbonylation/Hydrogenation Reaction Network as a Function of Size and Shape of Pt Nanoparticles

Vapor-phase transformations of furfural in H(2) over a series of Pt nanoparticles (NPs) with various particle sizes (1.5-7.1 nm size range) and shapes (rounded, cubes, octahedra) encapsulated in poly(vinylpyrrolidone) (PVP) and dispersed on MCF-17 mesoporous silica were investigated at ambient pressure in the 443-513 K temperature range. Furan and furfuryl alcohol (FFA) were two primary products as a result of furfural decarbonylation and hydrogenation reactions, respectively. Under conditions of the study both reactions exhibited structure sensitivity evidenced by changes in product selectivities, turnover rates (TORs), and apparent activation energies (E(A)'s) with Pt particle size and shape. For instance, upon an increase in Pt particle size from 1.5 to 7.1 nm, the selectivity toward FFA increases from 1% to 66%, the TOR of FFA production increases from 1 × 10(-3) s(-1) to 7.6 × 10(-2) s(-1), and E(A) decreases from 104 kJ mol(-1) to 15 kJ mol(-1) (9.3 kPa furfural, 93 kPa H(2), 473 K). Conversely, under the same experimental conditions the decarbonylation reaction path is enhanced over smaller nanoparticles. The smallest NPs (1.5 nm) produced the highest selectivity (96%) and highest TOR values (8.8 × 10(-2) s(-1)) toward furan formation. The E(A) values for decarbonylation (∼62 kJ mol(-1)) was Pt particle size independent. Furan was further converted to propylene via a decarbonylation reaction, but also to dihydrofuran, tetrahydrofuran, and n-butanol in secondary reactions. Furfuryl alcohol was converted to mostly to 2-methylfuran.

The Influence of Rh Addition on the Catalytic Activity of Cubic Pt Nanocrystals Supported on Alumina for NO/CH4 Reaction

The effect of the Rh addition to the well defined cubic (≈70%) Pt nanocrystals of around 13 nm supported on alumina was investigated for NO/CH4 reaction. The impact of size and shape of Pt nanoparticles on the catalytic activity were also analyzed by comparing the results with a conventionally prepared catalyst.

Shape Control of Pt Nanoparticles

A preparative method to control the shape of platinum (Pt) nanoparticles in the presence of sodium polyacrylate (PAA) or poly(N-vinyl-2-pyrrolidone) (PVP) is described. Regardless of the kind of protective polymer used, the dominant shape of Pt nanoparticles was controlled by changing the reduction rate of Pt4+ ions. Tetrahedral particles predominated by using H2 reduction of H2[PtCl6]. Methanol reduction generated mainly truncated octahedral particles. It seems that the slow H2 reduction of Pt4+ ions favorably leads to the formation of tetrahedral Pt nuclei enclosed with four {111} planes that have the lowest surface energy. The truncated octahedral nuclei are formed by the faster reduction with methanol. The selective growth of the {111} planes takes place at a lower polymer concentration and results in the generation of cubic nanoparticles.