Dataset Information

Effect of Step Density and Orientation on the Apparent pH Dependence of Hydrogen and Hydroxide Adsorption on Stepped Platinum Surfaces.

ABSTRACT: The effect of the alkali-metal cation (Li⁺, Na⁺, K⁺, and Cs⁺) on the non-Nernstian pH shift of the Pt(554) and Pt(533) step-associated voltammetric peak is elucidated over a wide pH window (1-13), through computation and experiment. In conjunction with our previously reported study on Pt(553), the non-Nernstian pH shift of the step-induced peak is found to be independent of the step density and the step orientation. In our prior work, we explained the sharp peak as due to the exchange between adsorbed hydrogen and hydroxyl along the step and the non-Nernstian shift as a result of the adsorption of an alkali-metal cation and its subsequent weakening of hydroxyl adsorption. Our density functional theory results support this same mechanism on Pt(533) and capture the effect of alkali-metal cation identity and alkali cation coverage well, where increasing electrolyte pH and cation concentration leads to increased cation coverage and a greater weakening effect on hydroxide adsorption. This work paints a consistent picture for the mechanism of these effects, expanding our fundamental understanding of the electrode/electrolyte interface and practical ability to control hydrogen and hydroxyl adsorption thermodynamics via the electrolyte composition, important for improving fuel cell and electrolyzer performance.

SUBMITTER: McCrum IT

PROVIDER: S-EPMC6150671 | biostudies-literature | 2018 Jul

REPOSITORIES: biostudies-literature

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Effect of Step Density and Orientation on the Apparent pH Dependence of Hydrogen and Hydroxide Adsorption on Stepped Platinum Surfaces.

McCrum Ian T IT Chen Xiaoting X Schwarz Kathleen A KA Janik Michael J MJ Koper Marc T M MTM

The journal of physical chemistry. C, Nanomaterials and interfaces 20180703 29

The effect of the alkali-metal cation (Li+, Na+, K+, and Cs+) on the non-Nernstian pH shift of the Pt(554) and Pt(533) step-associated voltammetric peak is elucidated over a wide pH window (1-13), through computation and experiment. In conjunction with our previously reported study on Pt(553), the non-Nernstian pH shift of the step-induced peak is found to be independent of the step density and the step orientation. In our prior work, we explained the ...[more]

PMID: 30258524

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Project description:The adsorption of amyloidogenic peptides, amyloid beta 1-40 (A?1-40), alpha-synuclein (?-syn), and beta 2 microglobulin (?2m), was attempted over the surface of nano-gold colloidal particles, ranging from d = 10 to 100 nm in diameter (d). The spectroscopic inspection between pH 2 and pH 12 successfully extracted the critical pH point (pHo) at which the color change of the amyloidogenic peptide-coated nano-gold colloids occurred due to aggregation of the nano-gold colloids. The change in surface property caused by the degree of peptide coverage was hypothesized to reflect the ?pHo, which is the difference in pHo between bare gold colloids and peptide coated gold colloids. The coverage ratio (?) for all amyloidogenic peptides over gold colloid of different sizes was extracted by assuming ? = 0 at ?pHo = 0. Remarkably, ? was found to have a nano-gold colloidal size dependence, however, this nano-size dependence was not simply correlated with d. The geometric analysis and simulation of reproducing ? was conducted by assuming a prolate shape of all amyloidogenic peptides. The simulation concluded that a spiking-out orientation of a prolate was required in order to reproduce the extracted ?. The involvement of a secondary layer was suggested; this secondary layer was considered to be due to the networking of the peptides. An extracted average distance of networking between adjacent gold colloids supports the binding of peptides as if they are "entangled" and enclosed in an interfacial distance that was found to be approximately 2 nm. The complex nano-size dependence of ? was explained by available spacing between adjacent prolates. When the secondary layer was formed, A?1-40 and ?-syn possessed a higher affinity to a partially negative nano-gold colloidal surface. However, ?2m peptides tend to interact with each other. This difference was explained by the difference in partial charge distribution over a monomer. Both A?1-40 and ?-syn are considered to have a partial charge (especially ?+) distribution centering around the prolate axis. The ?2m, however, possesses a distorted charge distribution. For a lower ? (i.e., ? <0.5), a prolate was assumed to conduct a gyration motion, maintaining the spiking-out orientation to fill in the unoccupied space with a tilting angle ranging between 5° and 58° depending on the nano-scale and peptide coated to the gold colloid.

Project description:1. The pH-dependence is considered of a reaction between E and S that proceeds through an intermediate ES under "Briggs-Haldane' conditions, i.e. there is a steady state in ES and [S]o greater than [E]T, where [S]o is the initial concentration of S and [E]T is the total concentration of all forms of E. Reactants and intermediates are assumed to interconvert in three protonic states (E equilibrium ES; EH equilibrium EHS; EH2 equilibrium EH2S), but only EHS provides products by an irreversible reaction whose rate constant is kcat. Protonations are assumed to be so fast that they are all at equilibrium. 2. The rate equation for this model is shown to be v = d[P]/dt = (kcat.[E]T[S]o/A)/[(KmBC/DA) + [S]o], where Km is the usual assembly of rate constants around EHS and A-D are functions of the form (1 + [H]/K1 + K2/[H]), in which K1 and K2 are: in A, the molecular ionization constants of ES; in B, the analogous constants of E; in C and D, apparent ionization constants composed of molecular ionization constants (of E or ES) and assemblies of rate constants. 3. As in earlier treatments of this type of reaction which involve either the assumption that the reactants and intermediate are in equilibrium or the assumption of Peller & Alberty [(1959) J. Am. Chem. Soc. 81, 5907-5914] that only EH and EHS interconvert directly, the pH-dependence of kcat. is determined only by A. 4. The pH-dependence of Km is determined in general by B-C/A-D, but when reactants and intermediate are in equilibrium, C identical to D and this expression simplifies to B/A. 5. The pH-dependence of kcat./Km, i.e. of the rate when [S]o less than Km, is not necessarily a simple bell-shaped curve characterized only by the ionization constants of B, but is a complex curve characterized by D/B-C. 6. Various situations are discussed in which the pH-dependence of kcat./Km is determined by assemblies simpler than D/B-C. The special situation in which a kcat./Km-pH profile provides the molecular pKa values of the intermediate ES complex is delineated.

Dataset Information

Effect of Step Density and Orientation on the Apparent pH Dependence of Hydrogen and Hydroxide Adsorption on Stepped Platinum Surfaces.

Publications

Effect of Step Density and Orientation on the Apparent pH Dependence of Hydrogen and Hydroxide Adsorption on Stepped Platinum Surfaces.

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