José Solla-Gullón

Electrochemical characterisation of platinum nanoparticles prepared by microemulsion: how to clean them without loss of crystalline surface structure

Abstract A new process to clean nanoparticles of platinum obtained by a water-in-oil (W/O) microemulsion without loss of crystalline structure was studied. Polyethyleneglycol–dodecylether (BRIJ®30) was the surfactant used for the preparation of the platinum particles. Pt(111), Pt(110), Pt(100) and polyoriented platinum electrodes were used to check the effectiveness of this new procedure. Preliminary results obtained with nanoparticles of platinum are also presented. The voltammetric profiles of platinum particles show the adsorption states associated with (110), (100) and (111) domains. The ratio Pt SURF /Pt TOT was found to be 0.1 for an upper potential limit of 1 V and 0.12 for an upper potential limit of 1.5 V.

Imaging Structure Sensitive Catalysis on Different Shape-Controlled Platinum Nanoparticles

The structure sensitive catalytic activity for oxygen reduction reaction (ORR) on shape-controlled Pt nanoparticles (NPs) is directly imaged using scanning electrochemical microscopy (SECM). We synthesize and compare four types of Pt NPs: spherical, cubic, hexagonal, and tetrahedral-octahedral. Our SECM images show the hexagonal Pt NPs displaying the highest activity for ORR in two acid electrolytes. Meanwhile, cubic and tetrahedral-octahedral NPs drastically change their activity depending on specific adsorption of the different anions in solution. The NPs morphology produces predominant crystallographic planes at the surface of these shape-controlled Pt NPs, which are responsible for their different catalytic activity. Our results translate the studies on Pt single crystal electrodes present in the literature into Pt NPs that are useful as a catalyst in real fuel cells.

Electrochemical characterisation of platinum-palladium nanoparticles prepared in a water-in-oil microemulsion

Abstract A detailed electrochemical characterisation of platinum-palladium nanoparticles prepared by reduction with hydrazine of H 2 PtCl 6 and K 2 PdCl 4 in a water-in-oil microemulsion of water/polyethylenglycol-dodecylether (BRIJ ® 30)/ n -heptane is reported. XPS, AAS and UV–vis experiments were carried out to determine the atomic composition of the bimetallic nanoparticles obtained in the whole composition range. Cyclic voltammograms of clean nanoparticle alloys were obtained after a controlled decontamination procedure of their surfaces. CO adsorption–oxidation and dissociative adsorption of formic acid and methanol were used as test reactions to check the electrocatalytic behaviour of the bimetallic nanoparticles. FTIRS experiments of adsorbed CO were also carried out giving information on the relative amount of linearly and bridge-bonded CO, which is known to depend on the surface distribution of the two elements. Experimental results show how the electrochemical adsorption behaviour and electrocatalytic properties of the particles are under the control of their composition and, in a case studied in this work, of their size.

Synthesis and electrochemical decontamination of platinum-palladium nanoparticles prepared by water-in-oil microemulsion

The synthesis and electrochemical decontamination of platinum-palladium nanoparticles prepared by reduction with hydrazine of H 2 PtCl 6 and K 2 PdCl 4 in a water-in-oil microemulsion of water/poly(ethylene glycol)-dodecyl ether (BRIJ 30)/n-heptane is reported. X-ray photoelectron spectroscopy experiments were carried out to determine the composition of the nanoparticles obtained. Due to the presence of surfactant molecules coating on the nanoparticles, a decontamination procedure was developed. The decontamination procedure applied allows a cleaning of the surface of the nanoparticles without modification or damage for the crystalline surface structure as a result of synthesis conditions. The influence of the classical electrochemical activation or cleaning of the surface on the electrochemical behavior of the nanoparticles is also compared. The results obtained show a deep modification of the electrocatalytic behavior of the nanoparticles after the electrochemical activation treatment. They demonstrate that this activation process should not he applied in studies where the influence of the size, surface structure, and surface composition of the nanoparticles on their electrocatalytic and catalytic properties are studied.

Significantly Enhancing Catalytic Activity of Tetrahexahedral Pt Nanocrystals by Bi Adatom Decoration

Tetrahexahedral Pt nanocrystals (THH Pt NCs) bounded by high-index facets possess a high density of active sites and display therefore a higher catalytic activity in comparison with those enclosed by low-index facets. In the current communication, we report, for the first time, the decoration of THH Pt NC surfaces by using Bi adatoms and have demonstrated that the catalytic activity of the Bi decorated THH Pt NCs toward HCOOH electrooxidation has been drastically enhanced in comparison with bare THH Pt NCs. It has also been revealed that the catalytic activity of Bi decorated THH Pt NCs for all coverages investigated always exhibits a higher catalytic activity that is about double that of Bi decorated Pt nanospheres. The study is of great importance regarding both fundamentals and applications.

Role of surface defect sites: from Pt model surfaces to shape-controlled nanoparticles

In the present paper, preferentially oriented (111) Pt nanoparticles (mostly octahedral and tetrahedral, namely {111}Pt nanoparticles) have been characterized and compared with a Pt(554) single-crystal electrode as their voltammetric features are quite similar in 0.5 M H2SO4. The anion and Bi adsorption behaviours suggest that the {111}Pt nanoparticles contain relatively wide hexagonal domains and also isolated sites which could adsorb solely hydrogen. Bi step decoration has been successfully extended to modify the defects of {111}Pt nanoparticles without blocking terrace sites. CO charge displacement has been applied to determine the potential of zero total charge (pztc) of non-decorated and Bi decorated surfaces. It has found that the positive shift of pztc on defect-decorated {111}Pt nanoparticles is not so significant in comparison with that of Pt(554) due to the relative short mean length of (111) domains on the {111}Pt nanoparticles. CO stripping demonstrates that {111}Pt nanoparticles exhibit higher reactivity toward CO oxidation. This reflects the role of the defect sites in nanoparticles, evidenced by the disappearance of the “pre-wave” in the stripping voltammogram once the defects were blocked by Bi. The stripping peaks shift to higher potential on Bi decorated surfaces, indicating the active role of both steps and defects for CO oxidation. By comparing the CO stripping charge and the change in hydrogen adsorption charge of surfaces with and without Bi decoration, including reasonable deconvolution, the local CO coverage on defect and terrace sites were obtained for the first time for the {111}Pt nanoparticles, and the results are in good agreement with those obtained on Pt(554). Chronoamperometry studies show tailing in all current–time transients of CO oxidation on all surfaces studied. The kinetics of CO oxidation can be satisfactorily simulated by a modified Langmuir–Hinshelwood model, demonstrating that CO oxidation on all studied surfaces follows the same mechanism.

Electrochemical and electrocatalytic behaviour of platinum–palladium nanoparticle alloys

The electrochemical and electrocatalytic behaviour of Pt/Pd nanoparticles prepared in water-in-oil microemulsion was reported. The catalytic activity of the nanoparticles was studied by using the reactions of dissociative adsorption of methanol and formic acid. The use of these surface probe reactions allowed the detection of palladium at the surface of the nanoparticles. The electrochemical stability of the particles was also investigated by voltammetry and electrochemical quartz crystal microbalance (EQCM). We shown that EQCM technique may be quantitatively used to correlate mass and area modifications when the electrochemical conditions produce corrosion of the elements of the alloy.

Synthesis of Pt Nanoparticles in Water-in-Oil Microemulsion: Effect of HCl on Their Surface Structure.

The synthesis of shape-controlled nanoparticles is currently a hot research topic. However, from an applied point of view, there is still a lack of easy, cheap, and scalable methodologies. In this communication we report, for the first time, the synthesis of cubic platinum nanoparticles with a very high yield using a water-in-oil microemulsion method, which unlike others, such as the colloidal method, fulfills the previous requirements. This shape/surface structure control is determined by the concentration of HCl in the water phase of the microemulsion. The results reported here show that the optimal HCl percentage in the water phase is about 25% to obtain the highest amount of cubic nanostructures. Ammonia electro-oxidation is used as a surface structure sensitive reaction to illustrate HCl surface structure effects. Moreover, in situ electrochemical characterization has been performed to study the nanoparticle surface structure.

Enhanced electrocatalytic activity of Au@Cu core@shell nanoparticles towards CO2 reduction

The development of technologies for the recycling of carbon dioxide into carbon-containing fuels is one of the major challenges in sustainable energy research. Two of the main current limitations are the poor efficiency and fast deactivation of catalysts. Core–shell nanoparticles are promising candidates for enhancing challenging reactions. In this work, Au@Cu core–shell nanoparticles with well-defined surface structures were synthesized and evaluated as catalysts for the electrochemical reduction of carbon dioxide in neutral medium. The activation potential, the product distribution and the long term durability of this catalyst were assessed by electrochemical methods, on-line electrochemical mass spectrometry (OLEMS) and on-line high performance liquid chromatography. Our results show that the catalytic activity and the selectivity can be tweaked as a function of the thickness of Cu shells. We have observed that the Au cubic nanoparticles with 7–8 layers of copper present higher selectivity towards the formation of hydrogen and ethylene; on the other hand, we observed that Au cubic nanoparticles with more than 14 layers of Cu are more selective towards the formation of hydrogen and methane. A trend in the formation of the gaseous products can be also drawn. The H2 and CH4 formation increases with the number of Cu layers, while the formation of ethylene decreases. Formic acid was the only liquid species detected during CO2 reduction. Similar to the gaseous species, the formation of formic acid is strongly dependent on the number of Cu layers on the core@shell nanoparticles. The Au cubic nanoparticles with 7–8 layers of Cu showed the largest conversion of CO2 to formic acid at potentials higher than 0.8 V vs. RHE. The observed trends in reactivity and selectivity are linked to the catalyst composition, surface structure and strain/electronic effects.

Identical Location Transmission Electron Microscopy Imaging of Site-Selective Pt Nanocatalysts: Electrochemical Activation and Surface Disordering.

We have employed identical location transmission electron microscopy (IL-TEM) to study changes in the shape and morphology of faceted Pt nanoparticles as a result of electrochemical cycling; a procedure typically employed for activating platinum surfaces. We find that the shape and morphology of the as-prepared hexagonal nanoparticles are rapidly degraded as a result of potential cycling up to +1.3 V. As few as 25 potential cycles are sufficient to cause significant degradation, and after about 500-1000 cycles the particles are dramatically degraded. We also see clear evidence of particle migration during potential cycling. These finding suggest that great care must be exercised in the use and study of shaped Pt nanoparticles (and related systems) as electrocatlysts, especially for the oxygen reduction reaction where high positive potentials are typically employed.