Ethanol oxidation on shape-controlled platinum nanoparticles at different pHs: A combined in situ IR spectroscopy and online mass spectrometry study

Ethanol oxidation on different shape-controlled platinum nanoparticles at different pHs was studied using electrochemical, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and, especially, Differential Electrochemical Mass Spectrometry (DEMS) techniques, the latter giving interesting quantitative information about the products of ethanol oxidation. Two Pt nanoparticle samples were used for this purpose: (100) and (111) preferentially oriented Pt nanoparticles. The results are in agreement with previous findings that the preferred decomposition product depends on surface structure, with COads formation on (100) domains and acetaldehyde/acetic acid formation on (111) domains. However, new information has been obtained about the changes in CHx and CO formation at lower potentials when the pH is changed, showing that CHx formation is favored against the decrease in CO adsorption on (100) domains. At higher potentials, complete oxidation to CO2 occurs from both CHx and CO fragments. In (111) Pt nanoparticles, the splitting of Csingle bondC bond is hindered, favoring acetaldehyde and acetate formation even in 0.5 M H2SO4. C1 fragments become even less when the pH increases, being nearly negligible in the highest pH studied.This work has been financially supported by the MCINN-FEDER (Spain) and Generalitat Valenciana through projects CTQ 2013-44083-P and PROMETEO/2014/013, respectively

Structure, surface chemistry and electrochemical de-alloying of bimetallic PtxAg100-x nanoparticles: Quantifying the changes in the surface properties for adsorption and electrocatalytic transformation upon selective Ag removal

We investigated the structure and surface chemistry of bimetallic PtAg nanoparticles, which were prepared by a water-in-oil microemulsion synthesis procedure, as well as the changes therein induced by electrochemical (surface) de-alloying, by a combination of electrochemical and in situ FTIR and online differential electrochemical mass spectrometry (DEMS) measurements. Based on transmission electron microscopy the resulting nanoparticles have a narrow size distribution with a mean diameter of around 6 nm, and increasing Ag contents lead to a broadening of the particle size distribution and a loss of the preferential formation of {100} facets observed for Pt nanoparticles. Evaluation of the hydrogen adsorption and CO oxidation characteristics and of the CO adsorption properties (IR) reveals detailed information on the surface composition of the bimetallic nanoparticles before and after electrochemical de-alloying, including also the distribution of Pt and Ag atoms in the surface. Furthermore, relevant information about the modifications in the chemical properties of Pt atoms/sites induced by the presence of Ag is extracted. For the as-prepared nanoparticles we find Ag surface enrichment, while de-alloying results in a core-shell structure with a PtAg core and a Pt shell, whose chemical properties are close to, but not identical, with those of polycrystalline Pt. More general, the study demonstrates that Hupd measurements are not suitable to identify the Pt surface content in such kind of bimetallic nanoparticles.This work was supported by the Deutsche Forschungsgemeinschaft via the project BE 1201/17-1 and by the Ministerio de Economía y Competitividad (project CTQ2013-44083-P) and Generalitat Valenciana (project PROMETEOII/2014/013)

Bifunctional versus defect‐mediated effects in electrocatalytic methanol oxidation

It is widely accepted that MeOH and CO electroxdation is enhanced on bimetallic Pt−Ru electrodes, where Ru provides oxygen species at lower overpotential than Pt, decreasing the effect of surface poisoning by CO. By investigating surfaces with different Pt−Ru compositions and structures, the authors show that bimetallic Pt−Ru sites on these electrodes are almost inactive to MeOH oxidation, whereas pure Pt defect sites play an important role. The most prominent and intensively studied anode catalyst material for direct methanol oxidation fuel cells consists of a combination of platinum (Pt) and ruthenium (Ru). Classically, their high performance is attributed to a bifunctional reaction mechanism where Ru sites provide oxygen species at lower overpotential than Pt. In turn, they oxidize the adsorbed carbonaceous reaction intermediates at lower overpotential; among these, the Pt site-blocking carbon monoxide. We demonstrate that well-defined Pt modified Ru(0001) single crystal electrodes, with varying Pt contents and different local PtRu configurations at the surface, are unexpectedly inactive for the methanol oxidation reaction. This observation stands in contradiction with theoretical predictions and the concept of bifunctional catalysis for this reaction. Instead, we suggest that pure Pt defect sites play a more critical role than bifunctional defect sites on the electrodes investigated in this work

Electrodeposition of a Pt Monolayer Film: Using Kinetic Limitations for Atomic Layer Epitaxy

A new and facile one-step method to prepare a smooth Pt monolayer film on a metallic substrate in the absence of underpotential deposition-type stabilizations is presented as a general approach and applied to the growth of Pt monolayer films on Au. The strongly modified electronic properties of these films were demonstrated by in situ IR spectroscopy at the electrified solid–liquid interface with adsorbed carbon monoxide serving as a probe molecule. The Pt monolayer on Au is kinetically stabilized by adsorbed CO, inhibiting further Pt deposition in higher layers

Formic Acid Electrooxidation on Noble-Metal Electrodes: Role and Mechanistic Implications of pH, Surface Structure, and Anion Adsorption

The influence of the electrolyte pH on formic acid (HCOOH) electrooxidation is investigated on both polycrystalline Pt and Au electrodes and on single-crystalline Au electrodes in perchloric and sulfuric acid-based electrolytes. On Au electrodes, the potentiodynamic oxidation currents are found to depend, in a nonlinear way, on the electrolyte pH in a bell-shaped relation, with a maximum of the catalytic activity at the pKa of HCOOH. On polycrystalline Pt electrodes, this feature is not observed; the catalytic activity increases steadily with increasing pH up to a pH value of approximately 5, which is followed by a plateau until pH 10, in contrast with recent observations [J. Joo, T. Uchida, A. Cuesta, M. T. M. Koper, M. Osawa, J. Amer. Chem. Soc.­ 2013, 135, 9991–9994]. In addition, for Au surfaces, the reaction is only weakly influenced by the electrode surface structure, whereas for Pt, structural effects are known to be considerable. Anion effects, in contrast, are much stronger for the reaction on Au electrodes compared to Pt electrodes. Also, it is shown that Pt-group-metal-free Au electrodes do not oxidize molecular hydrogen under reaction conditions. The results are discussed in relation to findings in previous mechanistic studies. Most importantly, the activity on both electrodes is closely correlated with the concentration of HCOO−, and for Au correlates with both HCOO− and HCOOH concentrations. Based on these results, a number of mechanistic proposals put forward in earlier studies must be discarded, and examples for mechanisms compatible with these results are discussed.This work was supported by the Ministerio de Economía y Competitividad (MINECO) (Spain) (project EUI2009–04176) and by the Deutsche Forschungsgemeinschaft (BE 1201/17–1)

De l'électrolyte liquide à l'électrolyte solide, vers l'incorporation des catalyseurs PtNi nanostructurés en assemblage membrane électrodes

National audienc

De l'électrolyte liquide à l'électrolyte solide, vers l'incorporation des catalyseurs PtNi nanostructurés en assemblage membrane électrodes

National audienc

Further Insights into the Formic Acid Oxidation Mechanism on Platinum: pH and Anion Adsorption Effects

The influence of the electrolyte pH in the formic acid oxidation reaction on a polyoriented Pt electrode in the presence of different anions, including sulfate, perchlorate, phosphate and chloride, in different concentrations has been investigated, using cyclic voltammetric measurements. The curves of the peak currents in the negative scan direction vs. the pH in pure sulfate and perchlorate solutions are very similar. For these solutions, the maximum oxidation currents increase steadily with increasing pH up to pH ≈ 5, followed by a plateau until pH 10. This suggests that the reaction proceeds via a similar reaction mechanism, in which the concentration of HCOO− anions in solution plays a key role. For phosphate or chloride containing solutions, in contrast, the maximum oxidation currents show a bell-shaped pH-oxidation current correlation, whose exact shape depends on the anion concentration. We suggest that in these cases the pH-current relation is modified by competing specific adsorption of anions which act as site blocking spectator species. These results will be discussed in relation with compatible mechanistic proposals.This work has been financially supported by the MICINN (Spain) (projects CTQ2013-44083-P and CTQ2013-48280-C3-3-R) and Generalitat Valenciana (project PROMETEOII/2014/013, FEDER), as well as by the Deutsche Forschungsgemeinschaft via the project BE 1201 / 17-1

Preferentially-shaped PtNi/C ORR nanocatalysts - Challenges towards their integration in MEA

International audienceDue to strain and ligand effects, the simultaneous presence of concave and convex surfaces and their highly-defective nanostructure (atomic vacancies, grain boundaries), highly defective hollow PtNi/C electrocatalysts have proven to enhance remarkably the oxygen reduction reaction (ORR) kinetics [1,2,8,9]. Likewise, PtNi aerogel [3], jagged PtNi nanowires [4,5] feature both high concentration of structural defects and enhanced ORR activity. On the other hand, inspired from single crystal studies, nanostructured octahedral-shaped PtNi/C electrocatalysts exhibiting only Pt(111) facets are among the most active ORR electrocatalysts [6,7]. This presentation will focus on structure-activity-stability relationships of these two classes of materials. A special emphasis will be given to the comparison of their behavior in model conditions (liquid electrolyte, RDE configuration) and in more realistic ones (liquid electrolyte, GDE configuration) before addressing their integration in MEA. Finally, a comparison of the durability of this library of materials will be discussed

Preferentially-shaped PtNi/C ORR nanocatalysts - Challenges towards their integration in MEA

International audienceDue to strain and ligand effects, the simultaneous presence of concave and convex surfaces and their highly-defective nanostructure (atomic vacancies, grain boundaries), highly defective hollow PtNi/C electrocatalysts have proven to enhance remarkably the oxygen reduction reaction (ORR) kinetics [1,2,8,9]. Likewise, PtNi aerogel [3], jagged PtNi nanowires [4,5] feature both high concentration of structural defects and enhanced ORR activity. On the other hand, inspired from single crystal studies, nanostructured octahedral-shaped PtNi/C electrocatalysts exhibiting only Pt(111) facets are among the most active ORR electrocatalysts [6,7]. This presentation will focus on structure-activity-stability relationships of these two classes of materials. A special emphasis will be given to the comparison of their behavior in model conditions (liquid electrolyte, RDE configuration) and in more realistic ones (liquid electrolyte, GDE configuration) before addressing their integration in MEA. Finally, a comparison of the durability of this library of materials will be discussed