Electrochemical Behavior and Shape Evolution of Structured Pd Nanoparticles in Alkaline Media─Influence of Electrochemically Absorbed Hydrogen.

We report on the electrochemical behavior and shape evolution of Pd nanocubes (Pd NCs) and Pd nanooctahedrons (Pd NOs) with an average size of 9.8 and 6.9 nm, respectively, in aqueous alkaline medium in the potential range of the underpotential deposition of H (UPD H) and H absorption. While the Pd NCs and Pd NOs remain stable in the potential region of the UPD H, H absorption and desorption of absorbed H (Habs) induce structural changes to the Pd NPs, as indicated by the results of electrochemical measurements and identical location-transmission electron microscopy (IL-TEM) analyses. Because both Pd NCs and Pd NOs are known to be stable in the potential region of H absorption and Habs desorption in acidic medium and maintain their structure, the irreversible structural changes are attributed to their interfacial interaction with the aqueous alkaline medium. In the alkaline medium, the nanoparticle surface/electrolyte interfacial structure plays an essential role in the mechanism of Habs desorption that is observed at higher potentials than that in the acidic medium. Hydrogen desorption is substantially hindered due to the structure of the water network adjacent to the Pd nanoparticles or the interaction between hydrated cations and adsorbed OH on the nanoparticle surface, resulting in the trapping of a small amount of H (incomplete Habs desorption). It is proposed that H trapping and associated structural strain lead to the deformation of the Pd nanoparticles and the loss of their initial structure.

Characterization of platinum nanoparticles for fuel cell applications by single particle inductively coupled plasma mass spectrometry.

The most effective utilization of platinum (Pt) in fuel cells is achieved through the use of nanoparticles (NPs) that offer a large electrochemically active surface area. Because the stability of NPs decreases as they become smaller, their size and size distribution must be known in order to optimize the catalysts' durability, while offering high catalytic activity. Single particle inductively coupled plasma mass spectrometry (spICPMS) can quantify the mass of metallic NPs suspended in aqueous medium, which can then be converted into a size if the NPs' shape, density and composition are known. In this study, for the first time, spICPMS was compared to transmission electron microscopy (TEM) for the characterization of 10 nm Pt NPs. After verifying the accurate sizing of commercial Pt NPs with diameters of 30, 50 and 70 nm, spICPMS with solution calibration was applied to laboratory-synthesized 10 nm Pt NPs possessing a near spherical shape and 10 ± 2 nm diameter according to TEM. The same NPs were also analyzed by spICPMS with Pt size calibration using Pt NPs standards. Irrespectively of the calibration strategy, spICPMS measured the entire population of 659 Pt NPs (6-65 nm), while TEM analyzed the 500 Pt NPs that appeared isolated in the field of view (6-18 nm). Analysis of the size distribution histograms revealed that the modal and mean diameters were respectively 10 and 11 ± 2 nm using solution calibration, and 12 and 13 ± 2 nm using particle size calibration. Both of these mean diameters are in agreement with the TEM measurements according to a Student's t-test at the 95% confidence level, demonstrating that spICPMS, with a size detection limit of 6 nm, can accurately quantify 10-nm Pt NPs while at the same time analyzing the entire sample.

Pd-Shaped Nanoparticles Modified by Gold ad-Atoms: Effects on Surface Structure and Activity Toward Glucose Electrooxidation.

Palladium nanoparticles (Pd-NPs) with controlled distributions of sizes and shapes (nanospheres-Pd-NS-, nanocubes -Pd-NC-, and nanooctahedrons -Pd-NO-) are synthesized by wet chemistry methods and characterized by TEM/HRTEM. The surfaces of Pd-NPs are modified by spontaneous adsorption of gold and characterized by cyclic voltammetry in acidic medium. It is shown that the modification of Pd-NPs by dipping in HAuCl4 solutions of different concentrations allows controlling the surface coverage by gold. It is also shown that the modification of Pd-NPs surfaces involves first the formation of PdAu surface alloys. For higher coverages, both PdAu surface alloys and pure Au structures are formed. The activity toward the glucose electrooxidation reaction is determined by linear scan voltammetry (LSV). Higher activity is observed on pure Pd-NC presenting extended (100) surfaces than on Pd-NO with mainly (111) surface orientation and on Pd-NS without preferential surface orientation, both these latter Pd-NPs displaying almost the same activity. The modification of the surface by spontaneous adsorption of gold greatly improves the activity of all Pd-NPs. However, Au-modified Pd-NC materials remain the most active catalysts. PdAu surface alloys seem to be involved in the improvement of the catalytic activity at low potentials, although the role of pure gold structures on Pd-NPs toward the enhancement of the catalytic activity cannot be excluded for high gold coverage. The study allows a better understanding of the material structure/electrocatalytic behavior relationship.

Selective Electrooxidation of Glycerol Into Value-Added Chemicals: A Short Overview.

A comprehensive overview of the catalysts developed for the electrooxidation of glycerol with the aim of producing selectively value-added compounds is proposed in the present contribution. By presenting the main results reported in the literature on glycerol electrooxidation in acidic and alkaline media, using different kinds of catalytic materials (monometallic catalysts based on platinum group metals and non-noble metals, multimetallic alloys, or modification of surfaces by adatoms, etc.) and under different experimental conditions, some general trends concerning the effects of catalyst composition and structure, of reaction medium and of the electrode potential to enhance the activity for the glycerol oxidation reaction and of the selectivity toward a unique value-added product will be presented and discussed. The objective is to provide a guideline for the development of electrochemical systems which allow performing the electrooxidation of glycerol at the rate and selectivity as high as possible.

Interfacial structure of atomically flat polycrystalline Pt electrodes and modified Sauerbrey equation.

The electrochemical quartz-crystal nanobalance (EQCN) measures in situ mass changes associated with interfacial electrode processes. Real electrodes are not atomically flat, thus their surface roughness affects the conversion of frequency variations (Δf) to mass changes (Δm) associated with electrochemical processes. Here, we analyze Δm associated with the electrochemical H adsorption/desorption and surface oxide formation/reduction on Pt electrodes of gradually increasing surface roughness using the EQCN and cyclic-voltammetry in an aqueous H2SO4 solution. These two interfacial processes are ideal to probe changes in the electrochemically active surface area. The surface roughness of Pt-coated resonators is fine-tuned through Pt electrodeposition and examined using atomic force microscopy. The results acquired using Pt electrodes of increasing roughness factor (1.61 ≤ R ≤ 13.0) reveal a linear relationship between Δm and R. Extrapolation of this relationship to R = 1.00 leads to the determination of Δm associated with H adsorption/desorption and oxide formation/reduction on an atomically flat polycrystalline Pt electrode. The values of Δm associated with these processes are analyzed in terms of the number of H, O, water, and ionic species interacting with each Pt atom of the electrode surface. We find that the charge densities associated with these electrochemical processes and mass variations do not scale up by the same factor. This leads to a modified version of the Sauerbrey equation for Pt electrodes, which takes into account the intrinsic surface roughness.

Octahedral palladium nanoparticles as excellent hosts for electrochemically adsorbed and absorbed hydrogen.

We report new results for electrochemical H adsorption on and absorption in octahedral palladium nanoparticles (Pd-NPs) with an average tip-to-tip size of 7.8 nm and a narrow size distribution. They reveal a very high H loading of 0.90 that cannot be achieved using bulk Pd materials or larger NPs; this behavior is assigned to a combination of two factors: their small size and face morphology. Temperature-dependent cyclic voltammetry (CV) studies in the range of 296 to 333 K reveal unique features that are attributed to electrochemical H adsorption, H absorption, and H2 generation. The CV features are used to prepare H adsorption and absorption isotherms that are then used in thermodynamic data analysis. Modeling of the experimental results demonstrates that, upon H adsorption and absorption, Pd-NPs develop a core-shell-skin structure, each with its unique H loading. The electrochemical results obtained for octahedral Pd-NPs are compared to analogous data obtained for cubic Pd-NPs with a similar size as well as for larger cubic Pd-NPs and bulk materials under gas-phase conditions.

Electrochemical behavior of unsupported shaped palladium nanoparticles.

The potential range in which hydrogen electro-adsorption, electro-absorption, and evolution reaction occur is examined in an acidic medium using cyclic-voltammetry (CV) and Pd nanoparticles with controlled size and shape distributions. The three processes give rise to unique features in CV profiles and are observed in distinct potential ranges. This behavior is not observed for bulk Pd materials and arises due to the nanoscopic nature of the Pd materials.

Self-supported Pd(x)Bi catalysts for the electrooxidation of glycerol in alkaline media.

Highly active self-supported PdxBi catalysts are synthesized by the sacrificial support method. Self-supported PdxBi catalysts have a porous nanostructured morphology with high surface areas (in the range from 75 to 100 m(2) g(-1)), making PdxBi a state-of-the-art catalyst. Pd4Bi displays the highest activity toward glycerol oxidation. In situ Fourier transform infrared spectroscopy highlights the unique catalytic behavior of self-supported PdxBi materials due to their particular structure and morphology. The confinement of reactants and intermediates in pores acting as nanoreactors is responsible for the high selectivity as a function of the electrode potential: aldehyde and ketone at low potentials, hydroxypyruvate at moderate potentials, and CO2 at high potentials. Moreover, the selectivity depends on the electrode history: it is different for the positive potential scan direction than for the reverse direction, where the catalyst becomes selective toward the production of carboxylates.

One-step synthesis and chemical characterization of Pt-C nanowire composites by plasma sputtering.

Plasma increases activity: A one-step synthesis of Pt-C nanowire composites using a plasma co-deposition method is reported. Electrodes with a very low Pt loading can be obtained. Pt particles with sizes ranging from 1 to 2 nm are decorating the columnar carbon nanostructures because of strong interactions. The composite microstructure is responsible for a very high metal utilization rate as exemplified by reactions occurring in fuel cell electrodes.

Pt particles functionalized on the molecular level as new nanocomposite materials for electrocatalysis.

A nanocomposite material consisting of platinum nanoparticles surrounded by an ionic conducting polymer dispersed on carbon Vulcan XC72 was synthesized. The aim of this nanocomposite material is to translate the triple-phase boundary to a molecular level in electrochemical systems involving a polymer electrolyte. The ionic conducting polymer is a poly(styrenesulfonic acid) (PSSA, or PSSNa in its sodium form) synthesized by atom-transfer radical polymerization. The polymer has a terminal thiol group to ensure bonding with platinum nanoparticles. The nanocomposite material (Pt-PSSA/C) exhibited thermal stability up to 160 °C and electrochemical stability up to 1 V versus RHE. Compared to a Pt/C catalyst, the nanocomposite catalyst has a lower active surface area but comparable catalytic activity for the oxygen reduction reaction. Furthermore, this nanocomposite material exhibits similar behavior in a fuel cell active layer without Nafion as a classical Pt/C catalyst with Nafion included in the active layer.

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.

Electro-oxidation of CO(chem) on Pt nanosurfaces: solution of the peak multiplicity puzzle.

An understanding of the oxidation of chemisorbed CO (CO(chem)) on Pt nanoparticle surfaces is of major importance to fuel cell technology. Here, we report on the relation between Pt nanoparticle surface structure and CO(chem) oxidative stripping behavior. Oxidative stripping voltammograms are obtained for CO(chem) preadsorbed on cubic, octahedral, and cuboctahedral Pt nanoparticles that possess preferentially oriented and atomically flat domains. They are compared to those obtained for etched and thermally treated Pt(poly) electrodes that possess atomically flat, ordered surface domains separated by grain boundaries as well as those obtained for spherical Pt nanoparticles. A detailed analysis of the results reveals for the first time the presence of up to four voltammetric features in CO(chem) oxidative stripping transients, a prepeak and three peaks, that are assigned to the presence of surface domains that are either preferentially oriented or disordered. The interpretation reported in this article allows one to explain all features within the voltammograms for CO(chem) oxidative stripping unambiguously.

Insights into the effects of functional groups on carbon nanotubes for the electrooxidation of methanol.

Functionalized carbon nanotubes were used as a support for PtCo nanoparticles. Their performance as electrocatalysts for the electrooxidation of methanol was evaluated by cyclic voltammetry and in situ FTIR reflectance spectroscopy. The onset potentials for both the electrooxidation of methanol and the production of CO(2) shifted to less positive values for catalysts prepared with more oxygen groups on the support. Furthermore, the production of CO(2) was higher on catalysts prepared with functionalized carbon nanotubes. The functional groups play two different but complementary roles. On the one hand, they help to stabilize smaller PtCo particles of ca. 3 nm. On the other hand, they provide the -OH groups necessary for the total oxidation of methanol to CO(2) at potentials less positive than on nonfunctionalized supports. Remarkably, the consumption of carboxylic acid groups along with the production of water is observed in the infrared spectra of the functionalized supports recorded during the electrooxidation of methanol. This observation suggests that the -OH groups of the support can also react with methanol, forming water and an ester.

Modification of carbon substrates by aryl and alkynyl iodonium salt reduction.

Different carbon materials were modified using iodonium ion reduction creating radicals, which after reaction with carbon surfaces formed grafted layers of molecules. Several molecules (4-bromophenyl, 4-fluorophenyl, 6-chlorohexyne, and 4-bromobutyne) were grafted on glassy carbon and Vulcan XC72 carbon substrates. Carbon substrates were shown to be free of halogen atoms; therefore, the quantification of the grafted groups containing halogen atoms was facilitated. The grafting of the different molecules was first electrochemically studied on glassy carbon electrodes using cyclic voltammetry, in order to determine the reduction potential of the corresponding iodonium ions. Voltammetric study using Fe(CN)(6)(4-) and Fe(CN)(6)(3-) probe molecules and XPS characterization were also used to evidence the effectiveness of grafting from iodonium ion reduction reaction. Reduction potentials were found in the range from -0.9 V vs SCE to -1.0 V vs SCE, lower than those for corresponding diazonium ion reduction reaction on glassy carbon (close to -0.3 V vs SCE). Therefore, grafted layers from iodonium ions were carried out on carbon Vulcan XC72 powder using NaBH(4) as reducing agent. Functionalized carbon powders were characterized by elemental analysis, thermogravimetric analysis, and X-ray photoelectron spectroscopy to evidence the presence of grafted molecules on the materials. However, low grafting yields were obtained. Then, several synthesis parameters were studied to optimize the grafting reactions, such as the control of the addition of reactants and their concentrations, leading to increase the surface concentration by a factor 2. At last, according to XPS measurements the grafting of alkinyliodonium ions led to very low surface concentrations (0.5 wt % for 6-chlorohexyne), whereas elemental analysis and TGA indicate ca. 2.4 wt % and ca. 5 wt %, respectively.

Improvement of the platinum nanoparticles-carbon substrate interaction by insertion of a thiophenol molecular bridge.

The effect of thiophenol layer grafted on carbon for platinum catalyst stabilization was studied. The grafted layer was prepared by reduction of 4-thiophenoldiazonium ions in the presence of Vulcan XC72 substrate. The grafted layer was characterized by elemental analysis, thermogravimetric analysis coupled with mass spectrometry, and X-ray photoelectron spectroscopy. Platinum nanoparticles prepared by the "water in oil" microemulsion method were then deposited on modified substrates and bare Vulcan XC72. The platinum stability improvement was characterized by in situ X-ray diffraction and electrochemical aging. These experiments enabled to evidence a lower crystallite growth during heat treatment under hydrogen atmosphere and a lower active surface area loss for platinum particles deposited on modified substrates compared to those deposited on bare Vulcan XC72. This stability improvement can be attributed to a better interaction between platinum particles and carbon substrate due to the thiophenol molecular bridge.

Electrochemical derivatization of carbon surface by reduction of in situ generated diazonium cations.

The derivatization of a glassy carbon electrode surface was achieved by electrochemical reduction of several in situ generated diazonium cations. The diazonium cations were synthesized in the electrochemical cell by reaction of the corresponding amines with NaNO2 in aqueous HCl. The versatility of the method was demonstrated by using six diazonium cations. This deposition method, which involves simple reagents and does not require the isolation and purification of the diazonium salt, enabled the grafting of covalently bounded layers which exhibited properties very similar to those of layers obtained by the classical derivatization method involving isolated diazonium salt dissolved in acetonitrile or aqueous acid solution. Cyclic voltammetry and electrochemical impedance spectroscopy carried out in aqueous solutions containing electroactive redox probe molecules such as Fe(CN)6(3-/4-) and Ru(NH3)6(3+) confirmed the barrier properties of the deposited layers. The chemical composition of the grafted layers was determined by X-ray photoelectron spectroscopy and surface coverage in the range 3 x 10(-10) to 6 x 10(-10) mol cm(-2) was estimated for films grown in our experimental conditions.