Colloidal Syntheses of Shape- and Size-Controlled Pt Nanoparticles for Electrocatalysis

Electron transfer in heterojunction catalysts

Heterojunction catalysis, the cornerstone of the modern chemical industry, shows potential to tackle the growing energy and environmental crises. Electron transfer (ET) is ubiquitous in heterojunction catalysts, and it holds great promise for improving the catalytic efficiency by tuning the electronic structures or building internal electric fields at interfaces. This perspective summarizes the recent progress of catalysis involving ET in heterojunction catalysts and pinpoints its crucial role in catalytic mechanisms. We specifically highlight the occurrence, driving forces, and applications of ET in heterojunction catalysis. For corroborating the ET processes, common techniques with measurement principles are introduced. We end with the limitations of the current study on ET, and envision future challenges in this field.

The Effect of the Reducing Sugars in the Synthesis of Visible-Light-Active Copper(I) Oxide Photocatalyst

In the present work, shape tailored Cu₂O microparticles were synthesized by changing the nature of the reducing agent and studied subsequently. d-(+)-glucose, d-(+)-fructose, d-(+)xylose, d-(+)-galactose, and d-(+)-arabinose were chosen as reducing agents due to their different reducing abilities. The morpho-structural characteristics were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and diffuse reflectance spectroscopy (DRS), while their photocatalytic activity was evaluated by methyl orange degradation under visible light (120 min). The results show that the number of carbon atoms in the sugars affect the morphology and particle size (from 250 nm to 1.2 µm), and differences in their degree of crystallinity and photocatalytic activity were also found. The highest activity was observed when glucose was used as the reducing agent.

From platinum atoms in molecules to colloidal nanoparticles: A review on reduction, nucleation and growth mechanisms

Platinum (Pt) is one of the most studied materials in catalysis today and considered for a wide range of applications: chemical synthesis, energy conversion, air treatment, water purification, sensing, medicine etc. As a limited and non-renewable resource, optimized used of Pt is key. Nanomaterial design offers multiple opportunities to make the most of Pt resources down to the atomic scale. In particular, colloidal syntheses of Pt nanoparticles are well documented and simple to implement, which accounts for the large interest in research and development. For further breakthroughs in the design of Pt nanomaterials, a deeper understanding of the intricate synthesis-structures-properties relations of Pt nanoparticles must be obtained. Understanding how Pt nanoparticles form from molecular precursors is both a challenging and rewarding area of investigation. It is directly relevant to develop improved Pt nanomaterials but is also a source of inspiration to design other precious metal nanostructures. Here, we review the current understanding of Pt nanoparticle formation. This review is aimed at readers with interest in Pt nanoparticles in general and their colloidal syntheses in particular. Readers with a strongest interest on the study of nanomaterial formation will find here the case study of Pt. The preferred model systems and characterization techniques used to perform the study of Pt nanoparticle syntheses are discussed. In light of recent achievements, further direction and areas of research are proposed.

Electrocatalysis of Oxygen Reduction Reaction on Shape-Controlled Pt and Pd Nanoparticles-Importance of Surface Cleanliness and Reconstruction

Shape-controlled precious metal nanoparticles have attracted significant research interest in the recent past due to their fundamental and scientific importance. Because of their crystallographic-orientation-dependent properties, these metal nanoparticles have tremendous implications in electrocatalysis. This review aims to discuss the strategies for synthesis of shape-controlled platinum (Pt) and palladium (Pd) nanoparticles and procedures for the surfactant removal, without compromising their surface structural integrity. In particular, the electrocatalysis of oxygen reduction reaction (ORR) on shape-controlled nanoparticles (Pt and Pd) is discussed and the results are analyzed in the context of that reported with single crystal electrodes. Accepted theories on the stability of precious metal nanoparticle surfaces under electrochemical conditions are revisited. Dissolution, reconstruction, and comprehensive views on the factors that contribute to the loss of electrochemically active surface area (ESA) of nanoparticles leading to an inevitable decrease in ORR activity are presented. The contribution of adsorbed electrolyte anions, in-situ generated adsorbates and contaminants toward the ESA reduction are also discussed. Methods for the revival of activity of surfaces contaminated with adsorbed impurities without perturbing the surface structure and its implications to electrocatalysis are reviewed.

Particle Size-Controlled Growth of Carbon-Supported Platinum Nanoparticles (Pt/C) through Water-Assisted Polyol Synthesis

A water-assisted control of Pt nanoparticle size during a surfactant-free, microwave-assisted polyol synthesis of the carbon-supported platinum nanoparticles (Pt/C) in a mixture of ethylene glycol and water using (NH₄)₂PtCl₆ as the Pt precursor is demonstrated. The particle size was tuned between ∼2 and ∼6 nm by varying either the H₂O volume percent or the Pt precursor concentration during synthesis. The electrochemical surface area (ECSA) and the oxygen-reduction reaction activity obtained for the Pt/C electrocatalyst show a catalytic performance competitive to that of the state-of-the-art commercial Pt/C electrocatalysts used for polymer electrolyte membrane fuel cell electrodes (ECSA: ∼70 m²/g; half-wave potential for oxygen reduction reaction: 0.83 V vs reversible hydrogen electrode). The synthesized Pt/C electrocatalysts show durability equivalent to or better than that of the commercial Pt/C. The durability was found to improve with increasing particle size, with the ECSA loss values being ∼70 and ∼55% for the particle sizes of 2.1 and 4.3 nm, respectively. The study may be used as a route to synthesize Pt/C electrocatalysts from a convenient and economic Pt precursor (NH₄)₂PtCl₆ and avoiding the use of alkaline media.

Synthesis and Catalytic Properties of Modified Electrodes by Pulsed Electrodeposition of Pt/PANI Nanocomposite

In this study, the modification of glassy carbon electrodes by potentiostatic pulsed deposition of platinum nanoparticles and potentiostatic pulsed polymerization of polyaniline nanofibers was investigated. During the preparation of the nano-composite materials, the control of the potentiostatic pulsed deposition and potentiostatic pulsed polymerization parameters, such as pulse potential, pulse width time, duty cycle, and platinum precursor concentration allowed the optimization of the size, shape, and distribution of the deposited Pt nanoparticles. It is noteworthy that the polymerization method, cyclic voltammetry method, or potentiostatic pulsed polymerization method show an important effect in the morphology of the deposited polyaniline (PANI) film. The obtained modified electrodes, with highly uniform and well dispersed platinum nanoparticles, exhibit good electrocatalytic properties towards methanol oxidation.

Shape-controlled metal nanoparticles for electrocatalytic applications

The application of shape-controlled metal nanoparticles is profoundly impacting the field of electrocatalysis. On the one hand, their use has remarkably enhanced the electrocatalytic activity of many different reactions of interest. On the other hand, their usage is deeply contributing to a correct understanding of the correlations between shape/surface structure and electrochemical reactivity at the nanoscale. However, from the point of view of an electrochemist, there are a number of questions that must be fully satisfied before the evaluation of the shaped metal nanoparticles as electrocatalysts including (i) surface cleaning, (ii) surface structure characterization, and (iii) correlations between particle shape and surface structure. In this chapter, we will cover all these aspects. Initially, we will collect and discuss about the different practical protocols and procedures for obtaining clean shaped metal nanoparticles. This is an indispensable requirement for the establishment of correct correlations between shape/surface structure and electrochemical reactivity. Next, we will also report how some easy-to-do electrochemical experiments including their subsequent analyses can enormously contribute to a detailed characterization of the surface structure of the shaped metal nanoparticles. At this point, we will remark that the key point determining the resulting electrocatalytic activity is the surface structure of the nanoparticles (obviously, the atomic composition is also extremely relevant) but not the particle shape. Finally, we will summarize some of the most significant advances/results on the use of these shaped metal nanoparticles in electrocatalysis covering a wide range of electrocatalytic reactions including fuel cell-related reactions (electrooxidation of formic acid, methanol and ethanol and oxygen reduction) and also CO2 electroreduction.

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A Review on Recent Developments and Prospects for the Oxygen Reduction Reaction on Hollow Pt-alloy Nanoparticles

Due to their interesting electrocatalytic properties for the oxygen reduction reaction (ORR), hollow Pt-alloy nanoparticles (NPs) supported on high-surface-area carbon attract growing interest. However, the suitable synthesis methods and associated mechanisms of formation, the reasons for their enhanced specific activity for the ORR, and the nature of adequate alloying elements and carbon supports for this type of nanocatalysts remain open questions. This Review aims at shedding light on these topics with a special emphasis on hollow PtNi NPs supported onto Vulcan C (PtNi/C). We first show how hollow Pt-alloy/C NPs can be synthesized by a mechanism involving galvanic replacement and the nanoscale Kirkendall effect. Nickel, cobalt, copper, zinc, and iron (Ni, Co, Cu, Zn, and Fe, respectively) were tested for the formation of Pt-alloy/C hollow nanostructures. Our results indicate that metals with standard potential -0.4<E<0.4 V (vs. the normal hydrogen electrode) and propensity to spontaneously form metal borides in the presence of sodium borohydride are adequate sacrificial templates. As they lead to smaller hollow Pt-alloy/C NPs, mesoporous carbon supports are also best suited for this type of synthesis. A comparison of the electrocatalytic activity towards the ORR or the electrooxidation of a CO_ads monolayer, methanol or ethanol of hollow and solid Pt-alloy/C NPs underlines the pivotal role of the structural disorder of the metal lattice, and is supported by ab initio calculations. As evidenced by accelerated stress tests simulating proton-exchange membrane fuel cell cathode operating conditions, the beneficial effect of structural disorder is maintained on the long term, thereby bringing promises for the synthesis of highly active and robust ORR electrocatalysts.

Electrochemical valorisation of glycerol

The worldwide glycerol stocks are increasing; to make the biodiesel industry sustainable economically, this chemical could be used as a secondary primary raw material. Electric energy or hydrogen and added-value-chemical cogeneration becomes more and more an important research topic for increasing economical and industrial interests towards electrochemical technologies. Studies on glycerol electrooxidation for fuel or electrolysis cell applications are scarce. The valorisation of glycerol is generally performed by organic chemistry reactions forming, for example, esters, glycerol carbonates, ethers, acetals or ketals. Glycerol oxidation is made up of complex pathway reactions that can produce a large number of useful intermediates or valuable fine chemicals with presently limited market impact due to expensive production processes. Many of these chemical oxidation routes lead to significant amounts of undesired by-products, and enzymatic processes are limited. Converse to classical heterogeneous processes, electrocatalytic oxidation processes can be tuned by controlling the nature, composition and structure of the electrocatalyts as well as the electrode potential. Such control may lead to very high selectivity and activity, avoiding or limiting product separation steps. The coupling of glycerol oxidation to produce chemicals with the oxygen reduction reaction in a fuel cell or water reduction reaction in an electrolysis cell on Pt-free catalysts results either in coproduction of electrical energy or hydrogen for energy storage.

Evidence of an Eley-Rideal mechanism in the stripping of a saturation layer of chemisorbed CO on platinum nanoparticles

The oxidative stripping of a saturation layer of CO(chem) was studied on platinum nanoparticles of high shape selectivity and narrow size distribution. Nanospheres, nanocubes, and nano-octahedrons were synthesized using the water-in-oil microemulsion or polyacrylate methods. The three shapes allowed examination of the CO(chem) stripping in relation to the geometry of the nanoparticles and presence of specific nanoscopic surface domains. Electrochemical quartz crystal nanobalance (EQCN) measurements provided evidence for the existence of more than one mechanism in the CO(chem) stripping. This was corroborated by chronoamperometry transient for a CO(chem) saturation layer at stripping potentials of E(strip) = 0.40, 0.50, 0.60, and 0.70 V. The first mechanism is operational in the case of CO(chem) stripping at lower E(strip) values; it proceeds without adsorption of anions or H(2)O molecules and corresponds to desorption of a fraction of CO(chem) in the form of a prepeak in voltammograms or in the form of an exponential decay in chrono-amperometry (CA) transients. The second mechanism is operational in the desorption of the remaining CO(chem) at higher E(strip) values and gives rise to at least two voltammetric peaks or two CA peaks. Analysis of the experimental data and modeling of the CA transients lead to the conclusion that the stripping of a saturation layer of CO(chem) first follows an Eley-Rideal mechanism in the early stage of the process and then a Langmuir-Hinshelwood mechanism.