Potential modulated reflectance spectroscopy of Pt(111) in acidic and alkaline media: cyanide adsorption

Improved Rate for the Oxygen Reduction Reaction in a Sulfuric Acid Electrolyte using a Pt(111) Surface Modified with Melamine

The feasible commercialization of alkaline, phosphoric acid and polymer electrolyte membrane fuel cells depends on the development of oxygen reduction reaction (ORR) electrocatalysts with improved activity, stability, and selectivity. The rational design of surfaces to ensure these improved ORR catalytic requirements relies on the so-called "descriptors" (e.g., the role of covalent and noncovalent interactions on platinum surface active sites for ORR). Here, we demonstrate that through the molecular adsorption of melamine onto the Pt(111) surface [Pt(111)-M_ad], the activity can be improved by a factor of 20 compared to bare Pt(111) for the ORR in a strongly adsorbing sulfuric acid solution. The M_ad moieties act as "surface-blocking bodies," selectively hindering the adsorption of (bi)sulfate anions (well-known poisoning spectator of the Pt(111) active sites) while the ORR is unhindered. This modified surface is further demonstrated to exhibit improved chemical stability relative to Pt(111) patterned with cyanide species (CN_ad), previously shown by our group to have a similar ORR activity increase compared to bare Pt(111) in a sulfuric acid electrolyte, with Pt(111)-M_ad retaining a greater than ninefold higher ORR activity relative to bare Pt(111) after extensive potential cycling as compared to a greater than threefold higher activity retained on a CN_ad-covered Pt(111) surface. We suggest that the higher stability of the Pt(111)-M_ad interface stems from melamine's ability to form intermolecular hydrogen bonds, which effectively turns the melamine molecules into larger macromolecular entities with multiple anchoring sites and thus more difficult to remove.

Update on current state and problems in the surface tension of condensed matter

The dual concept of surface energy formally allows application of Gibbs thermodynamics to the surface tension of solids and is unlimited using the classical Lippmann equation for solids that is shown to contradict all available in situ experimental data. At present, the generalized Lippmann equation is believed to be the most universal, since the classical Lippmann equation, the Shuttleworth and Gokhshtein equations could be derived from it. Lately it was evaluated in two opposite ways: the first--the experimental verification of the Gokhshtein equation supports correctness of the generalized Lippmann and Shuttleworth equations; the second--the incompatibility of the Shuttleworth equation with Hermann's mathematical structure of thermodynamics makes invalid all its corollaries, including the generalized Lippmann and Gokhshtein equations. Both approaches are shown here to be incorrect, since the Gokhshtein equation cannot be correctly derived from any of the above-mentioned equations. The Frumkin derivation of the first and second Gokhshtein equations follows from one thermodynamic relationship general for the surface tension of both solid and liquid electrodes. The classical Lippmann equation is also derived from this general relationship as a particular case of the second Gokhshtein equations. We have considered the hierarchy of these equations and discussed the straightforward application of the classical Lippmann equation for solids with an account for elasticity of the surface structured layers of liquids. The partial charge transfer during anion adsorption cannot be measured in electrochemical experiments or reliably estimated by quantum-chemical and DFT calculations. However, it is directly involved in the adsorbate charge that is experimentally accessible by in situ contact electric resistance technique. We present the first quantitative evaluation of charge transfer during halides adsorption on silver from aqueous solutions in dependence on the electrode potential.

Cyclic voltammetry, FTIRS, and DEMS study of the electrooxidation of carbon monoxide, formic acid, and methanol on cyanide-modified Pt(111) electrodes

We have used cyanide-modified Pt(111) electrodes, in combination with cyclic voltammetry (CV), Fourier transform infrared spectroscopy (FTIRS), and differential electrochemical mass spectrometry (DEMS), to investigate the oxidation of formic acid and methanol on Pt electrodes. Since CO is the poison intermediate formed during the oxidation of both formic acid and methanol, we have previously characterized the CO adlayer on cyanide-modified Pt(111) electrodes. Poison formation on cyanide-modified Pt(111) is nearly completely inhibited in the case of formic acid and methanol, the corresponding electro-oxidation reaction proceeding, hence, exclusively through the reactive intermediate pathway. These results suggest that, in the oxidation of formic acid and methanol, the formation of adsorbed CO would require the presence of at least three contiguous Pt atoms.

Oxidation of adsorbed CO on Pt(111) in CO-saturated perchloric acid aqueous solutions: simultaneous in situ time-resolved reflectance spectroscopy and second harmonic generation studies

Simultaneous normalized differential reflectance spectroscopy (DeltaR/R) and second harmonic generation (SHG) has been employed to follow, independently, OH and adsorbed CO (CO(ads)) on a single Pt(111) microfacet in CO-saturated aqueous perchloric acidic solutions during voltammetric cycles, leading to the oxidation of CO(ads) and subsequent readsorption of CO on the surface. The results obtained are consistent with the disruption of the radical19 x radical19R19.1 degrees phase just prior to the oxidation of adsorbed CO.

At Least Three Contiguous Atoms Are Necessary for CO Formation during Methanol Electrooxidation on Platinum

We have used cyanide-modified Pt(111) electrodes to investigate the size and geometry of the minimum atomic ensemble necessary for the oxidation of methanol on Pt electrodes. Poison formation on cyanide-modified Pt(111) is completely inhibited, the corresponding electrooxidation reaction proceeding, hence, exclusively through the reactive intermediate pathway. These results suggest that formation of adsorbed CO would require the presence of at least three contiguous Pt atoms.