Bromide Adsorption on Pt(111) over a Wide Range of pH: Cyclic Voltammetry and CO Displacement Experiments

Abstract

Bromide adsorption on Pt (111) is investigated by means of cyclic voltammetry and CO displacement experiments at different pH values. In acidic pH bromide adsorption is strongly overlapped with hydrogen desorption process. However, as the pH increases, hydrogen adsorption process displaces towards negative potentials while bromide adsorption remains nearly in the same potential region. In consequence, both processes decouple at higher pH values. The structural transition from Pt(111)-(1×1) to Pt(111)(3×3)-4Br is pH independent, in the SHE scale, and not observed for pH > 9.1. Values of pztc are extracted from the combination of voltammetric and CO displacedcharges. An alternative approach to obtain charge curves is based on the coincidence of the curves the structural transition characteristic of the bromide adlayer completion.Pztc values obtained from different approaches with and without bromide are compared and their dependence on pH discussed. A thermodynamic analysis is carried out to obtain hydrogen Gibbs excess and charge number from the Esin Markov analysis.or "commensurate" depending on what type of interaction is stronger, as pointed out by Lucas et al. 15 On the one hand, if the interaction between nearby adatoms is stronger, the structural adlayer will be "incommensurate". On the other hand, if the adatomsubstrate interaction is stronger, then structural adlayer will be "commensurate".Bromide adsorption on Pt(111) was subject of investigation by Hubbard's group [16][17][18] providing precious information about the characterization of the formed adlayers.UHV measurements by LEED and Auger spectroscopy were employed to characterize the adlayer, and these works opened the way for further similar investigations. 3,19-22 The Pt(111)-Br ads adlayer was also characterized by STM 3,23 and surface X-ray scattering. 19 The bromide adlayers can be formed either from bromide solutions or bromide vapors. 3 A structural transition at 0.25 V vs. RHE in 0.1 M HClO 4 was observed. 3,18,23 It was first proposed that this feature corresponds to the change from a Pt(111)-(3×3)-4Br adlattice at high potentials to a Pt(111)-(4×4)-7Br adlattice at low potentials, 18 but Itaya et al. observed an incommensurate Pt(111)-(1×1) structure instead of Pt(111)-(4×4)-7Br. 23 Independently of the technique used for the characterization of the Pt(111)-Br ads adlayer, it was shown that the Pt(111)-(3×3)-4Br structure is dense and the bromine adatoms are arranged in a closed-packed hexagonal layer, where the Br-Br distance is close to van der Waals diameter of bromine. They pointed out also that these adlayers are ordered preferentially along the directions of the substrate densest rows. 3,[16][17][18]24 Lucas et al. 19 proposed similar observations, although in these studies they observed that the (3×3) adlayer structure was incommensurate, whereas Orts et al. 3 proposed it as commensurate.In the present work, the knowledge about bromide adsorption on Pt(111) is extended by performing cyclic voltammetry and CO displacement experiments...

Understandings on the Inhibition of Oxygen Reduction Reaction by Bromide Adsorption on Pt(111) Electrodes at Different pH Values

“…In this work, the effect of bromide adlayers on Pt(111) toward the ORR is thoroughly investigated at different pH values. The cyclic voltammetric profiles recorded in the absence of oxygen and with different concentrations of bromide are in good agreement with previous results in phosphate buffer solutions, 34 pointing out that bromide adsorption is strong enough to be independent of the presence of other species with less adsorption energy. The ORR measurements in the presence of bromide anions show that the limiting current densities decrease to nearly half the theoretical value for very acidic pH values, but as the solution pH is increased, this difference diminishes and finally vanishes at pH = 3.…”

“…As the concentration is increased the peaks become sharper and more intense and they shift toward more negative potentials as the formation of a close-packed bromide adlayer becomes thermodynamically more favorable. As can be seen in this figure, bromide adsorption shifts (in the RHE scale) to higher potential values as the pH increases, but when the profiles are plotted vs. the SHE scale, the spike for the order-disorder transition of the bromide layer appears at constant potential, 27,32,34 implying that bromide adsorption is a pH-independent process. Regarding the adsorption strength of bromide, it can be seen that the blank profiles for solutions containing 10 −2 M KBr are practically identical to those measured for solutions prepared with phosphate buffers at pH values >3.…”

“…This new signal shows a spike associated to the transition between a Pt(111)-(3 × 3)-4Br adlattice at positive potentials and a Pt(111)-(1 × 1) adlayer at negative potentials. 27,32,34 It is worth mentioning that for 10 −4 M KBr the peaks are less sharp and not symmetrical. This is because the adsorption of bromide for this concentration is still controlled by mass transport and the equilibrium coverage is not reached in the time scale of the 50 mV s −1 sweep.…”

Dissolution of Platinum Single Crystals in Acidic Medium

“…Comparing to the CVs taken in the bulk glass cell, the current densities are slightly diminished and some changes in the CV profiles are identified, which is likely due to small amounts of contaminations in this setup with increased complexity. However, we believe this to have a minimal impact on irreversible oxide formation and reduction and the subsequent dissolution, as has been previously suggested, while contaminations≥300 ppm typically cause severe changes in the CVs . Furthermore, the high reproducibility of the dissolution experiments suggests relatively clean surfaces.…”

In Operando Visualization of the Electrochemical Formation of Liquid Polybromide Microdroplets

“…Zuschriften ure 1e), and we hypothesize that the interaction is due to adsorption. [19] AB rs ignal was detected using time-of-flight secondary-ion mass spectrometry (TOF-SIMS) after charging and washing away the droplets with de-ionized (DI) water to support the hypothesis ( Figure S2). Thec orresponding voltage profile for the first full cycle (charge and discharge) at 20 mA( 0.066 mA cm À2 )i ss hown in Figure 1f.W ei ntentionally used ac arbon electrode because the Zn anode did not lead to successful visualization:e lectrochemically generated Br 2 crosses over and reacts with Zn, which delays the appearance of phase-separated polybromide droplets (see Figure S3, and Supplementary Movies S2 and S3 with Zn foils as the counter electrodes).…”