Improved understanding of the fundamental processes leading to degradation of platinum nanoparticle electrocatalysts is essential to the continued advancement of their catalytic activity and stability. To this end, the oxidation of platinum nanoparticles is simulated using a ReaxFF reactive force field within a grand-canonical Monte Carlo scheme. 2-4 nm cuboctahedral particles serve as model systems, for which electrochemical potential-dependent phase diagrams are constructed from the thermodynamically most stable oxide structures, including solvation and thermochemical contributions. Calculations in this study suggest that surface oxide structures should become thermodynamically stable at voltages around 0.80-0.85 V versus standard hydrogen electrode, which corresponds to typical fuel cell operating conditions. The potential presence of a surface oxide during catalysis is usually not accounted for in theoretical studies of Pt electrocatalysts. Beyond 1.1 V, fragmentation of the catalyst particles into [Pt 6 O 8 ] 4− clusters is observed. Density functional theory calculations confirm that [Pt 6 O 8 ] 4− is indeed stable and hydrophilic. These results suggest that the formation of [Pt 6 O 8 ] 4− may play an important role in platinum catalyst degradation as well as the electromotoric transport of Pt 2+/4+ ions in fuel cells.

The interface between liquid water and the Pt(111) metal surface is characterized structurally and thermodynamically via reactive MD simulations within the ReaxFF framework. The formation of a distinct buckled adsorbate layer and subsequent wetting layers is tracked via the course of the water's density and the distribution of the H 2 O molecules with increasing distance to the metal surface. Hereby, also the two-phase thermodynamics (2PT) method has been utilized for studying the course of entropy as well as the translational, rotational, and vibrational entropic contributions throughout the Pt(111)|H 2 O interface. A significant reduction of the entropy compared to the bulk value is observed in the adsorbate layer (S = 31.05 ± 2.48 J/mol K) along with a density of 3.26 ± 0.06 g/ cm 3 . The O−O interlayer distribution allows for direct tracing of the water ordering and a quantified comparison to the ideal hexagonal adlayer. While the adsorbate layer at the Pt surface shows the occurrence of hexagonal motifs, this near-order is already weakened in the wetting layers. Bulk behavior is reached at 15 Å distance from the Pt(111) metal. Introducing an electric field of 0.1 V/Å prolongs the ordering effect of the metal surface into the liquid water. ■ METHODS2PT Method: Formalism. The 2PT algorithm begins by calculating the total VACF C(t) as the mass weighted sum of the respective atom VACFs c j k

In order to study the time-dependent behavior of catalytic systems during operation, we have developed a grand canonical molecular dynamics approach based on the ReaxFF reactive force-field framework. After describing the details of the implementation, the capabilities of this method are demonstrated by studying the gas-phase water formation from oxygen and hydrogen on platinum catalysts during the steady state where we discuss the effects of the surface structure as well as the importance of kinetics. The approach presented here can be extended to other dynamic (catalytic) systems, providing a framework for exploring catalytic and electrocatalytic processes, in particular, allowing studies on the effects of reaction conditions on a system’s behavior, characteristics, and stability.

The design of active and stable Pt-based nanoscale electrocatalysts for the oxygen reduction reaction (ORR) plays the central role in ameliorating the efficiency of proton exchange membrane fuel-cells towards future energy applications. On that front, theoretical studies have contributed significantly to this research area by gaining deeper insights and understanding of the ongoing processes. In this work, we present an approach capable of characterizing differently-shaped platinum nanoparticles undergoing thermally-and adsorbate-induced restructuring of the surface. Further, by performing ReaxFF-Grand Canonical Molecular Dynamics simulations we explored the water formation on these roughened ("realistic") nanoparticles in a H 2 /O 2 environment. Taking into consideration the coverage of oxygen-containing intermediates and occurring surface roughening the nanoparticles' activities were explored. Hereby, we succeeded in locally resolving the water formation on the nanoparticles' surfaces, allowing an allocation of the active sites for H 2 O production. We observed that exposed, low-coordinated sites as well as pit-shaped sites originating from roughening of vertices and edges are most active towards H 2 O formation.

The interface between an electrode and an electrolyte is the location where electro- chemical processes for countless technologically important applications occur. Though its high relevance and the intense efforts devoted to its elucidation, an atomic-level description of the interfacial structure and especially the dynamics of the electric double layer is still amiss. Here, we present reactive force field molecular dynamics simulations of electrified Pt(111)|water interfaces, shedding light on the orientation of water molecules in the vicinity of the Pt(111) surface, considering the influence of potential, adsorbates and ions simultaneously. We obtain a shift of the water’s preferred orientation in the surface oxidation potential region, breaking with the so far proclaimed strict correlation to the free charge density. Further, the course of the entropy and the intermolecular ordering in the interfacial region complements the characterization. Our work contributes to the ongoing understanding process of electric double layers and in particular of the structure of the electrified Pt(111)|water interface and aims at providing insights into electrochemical processes occurring there.