Monitoring of CO Binding Sites on Stepped Pt Single Crystal Electrodes in Alkaline Solutions by in Situ FTIR Spectroscopy.

The site geometry preference of CO binding on stepped Pt single crystals in alkaline solution was investigated by in situ FTIR spectroscopy. The surfaces of the Pt single crystals consisted of different width (111) terraces, interrupted by (110) or (100) monatomic steps. Experiments carried out with CO adsorbed exclusively on the top of the steps revealed that only linearly bonded CO formed on the (110) steps, while two CO binding geometries (linear and bridge) were observed on the (100) steps. On one hand, for CO adsorbed only on the steps, the positions of the bands corresponding to linearly bonded CO were similar, regardless of the density of steps, suggesting the existence of an interaction between COads only along the line of the steps. On the other hand, for full CO coverage, the CO stretching frequencies and the geometry of bound CO were sensitive to the width of the (111) terraces and the step orientations. Consequently, the CO binding sites favored linearly bonded CO for surfaces consisting of shorter (111) terraces and (110) steps. Bridge-bonded CO was favored on surfaces consisting of shorter (111) terraces interrupted by (100) steps. In order to understand the origin of the preference of CO binding sites, the results were compared to the corresponding behavior in acid media, which revealed that, in addition to the effect inherent to the Pt surface, the charge on the metal side in an aqueous environment should be taken into consideration. The analysis suggested that the CO adlayers formed at full coverage in acidic and alkaline media had different structures. On the other hand, the structure of the layer of CO adsorbed only at the steps was independent of pH.

Determination of Specific Electrocatalytic Sites in the Oxidation of Small Molecules on Crystalline Metal Surfaces.

The identification of active sites in electrocatalytic reactions is part of the elucidation of mechanisms of catalyzed reactions on solid surfaces. However, this is not an easy task, even for apparently simple reactions, as we sometimes think the oxidation of adsorbed CO is. For surfaces consisting of non-equivalent sites, the recognition of specific active sites must consider the influence that facets, as is the steps/defect on the surface of the catalyst, cause in its neighbors; one has to consider the electrochemical environment under which the "active sites" lie on the surface, meaning that defects/steps on the surface do not partake in chemistry by themselves. In this paper, we outline the recent efforts in understanding the close relationships between site-specific and the overall rate and/or selectivity of electrocatalytic reactions. We analyze hydrogen adsorption/desorption, and electro-oxidation of CO, methanol, and ammonia. The classical topic of asymmetric electrocatalysis on kinked surfaces is also addressed for glucose electro-oxidation. The article takes into account selected existing data combined with our original works.

Unraveling the Nature of Active Sites in Ethanol Electro-oxidation by Site-Specific Marking of a Pt Catalyst with Isotope-Labeled 13CO.

This works deals with the identification of preferential site-specific activation at a model Pt surface during a multiproduct reaction. The (110)-type steps of a Pt(332) surface were selectively marked by attaching isotope-labeled 13CO molecules to them, and ethanol oxidation was probed by in situ Foureir transfrom infrared spectroscopy in order to precisely determine the specific sites at which CO2, acetic acid, and acetaldehyde were preferentially formed. The (110) steps were active for splitting the C-C bond, but unexpectedly, we provide evidence that the pathway of CO2 formation was preferentially activated at (111) terraces, rather than at (110) steps. Acetaldehyde was formed at (111) terraces at potentials comparable to those for CO2 formation also at (111) terraces, while the acetic acid formation pathway became active only when the (110) steps were released by the oxidation of adsorbed 13CO, at potentials higher than for the formation of CO2 at (111) terraces of the stepped surface.

Mobility and Oxidation of Adsorbed CO on Shape-Controlled Pt Nanoparticles in Acidic Medium.

The knowledge about how CO occupies and detaches from specific surface sites on well-structured Pt surfaces provides outstanding information on both dynamics/mobility of COads and oxidation of this molecule under electrochemical conditions. This work reports how the potentiostatic growth of different coverage CO adlayers evolves with time on both cubic and octahedral Pt nanoparticles in acidic medium. Data suggest that during the growth of the CO adlayer, COads molecules slightly shift toward low coordination sites only on octahedral Pt nanoparticles, so that these undercoordinated sites are the first filled on octahedral Pt nanoparticles. Conversely, on cubic Pt nanoparticles, adsorbed CO behaves as an immobile species, and low coordinated sites as well as (100) terraces are apparently filled uniformly and simultaneously. However, once the adlayer is complete, irrespectively of whether the CO is oxidized in a single step or in a sequence of different potential steps, results suggest that COads behaves as an immobile species during its oxidation on both octahedral and cubic Pt nanoparticles.

Formic acid electrooxidation on thallium-decorated shape-controlled platinum nanoparticles: an improvement in electrocatalytic activity.

Thallium modified shape-controlled Pt nanoparticles were prepared and their electrocatalytic activity towards formic acid electrooxidation was evaluated in 0.5 M sulfuric acid. The electrochemical and in situ FTIR spectroscopic results show a remarkable improvement in the electrocatalytic activity, especially in the low potential region (around 0.1-0.2 V vs. RHE). Cubic Pt nanoparticles modified with Tl were found to be more active than the octahedral Pt ones in the entire range of Tl coverages and potential windows. In situ FTIR spectra indicate that the promotional effect produced by Tl results in the inhibition of the poisoning step leading to COads, thus improving the onset potential for the complete formic acid oxidation to CO2. Chronoamperometric experiments were also performed at 0.2 V to evaluate the stability of the electrocatalysts at constant potential. Finally, experiments with different concentrations of formic acid (0.05-1 M) were also carried out. In all cases, Tl-modified cubic Pt nanoparticles result to be the most active. All these facts reinforce the importance of controlling the surface structure of the electrocatalysts to optimize their electrocatalytic properties.