C. Gutiérrez

The Role of Bridge‐Bonded Adsorbed Formate in the Electrocatalytic Oxidation of Formic Acid on Platinum

The oxidation of formic acid (HCOOH) on platinum electrodes has been extensively investigated as a model electrocatalytic reaction. It is generally accepted that HCOOH is oxidized to CO2 through a dual-pathway mechanism: one pathway (the main pathway) involves a fast reaction via a reactive intermediate and the second pathway includes a step in which a poisoning species is formed. This species, which is oxidized to CO2 at high potentials, has been identified as adsorbed CO, which is formed by dehydration of HCOOH. Adsorbed hydroxycarbonyl (COOHads) has long been assumed to be the reactive intermediate in the main pathway, but the spectroscopic detection of this species has not been reported to date. By using surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATRSEIRAS), Miki et al. observed that formate is adsorbed in a bridge-bonded configuration on Pt electrodes during HCOOH oxidation. On the basis of systematic time-resolved ATR-SEIRAS analysis of the oxidation dynamics, Samjesk et al. suggested that adsorbed formate (HCOOads) is a reactive intermediate in the main pathway and its decomposition to CO2 is the rate-determining step (rds). The adsorbed formate is in equilibrium with HCOOH in the bulk solution and the reaction pathway (formate pathway) can be represented by Equation (1)

Electrooxidation of 2,4-dichlorophenol and other polychlorinated phenols at a glassy carbon electrode

The electrooxidation of 2,4-dichlorophenol (2,4-DCP) on a rotating GC disk electrode has been studied by cyclic voltammetry and chronoamperometry at different pH values. A dual mechanism where one pathway yields a quinone-like species and the other one leads to insoluble polymers that passivate the electrode surface is proposed. From a comparative study of the electrooxidation of several DCPs at pH 2.2, it is concluded that for the same number of chlorine atoms the ortho isomer is oxidized at a lower potential than the para isomer. The anodic peak potential for electrooxidation of the chlorophenols (CPs) on glassy carbon increases with increasing pKa, with a slope of 35 mV (pKa unit)−1, illustrating the correlation of one chemical parameter of the CPs, namely their acidity constant, with their electrochemical reactivity.

Electro-oxidation of chlorophenols at a gold electrode

Abstract The electrooxidation of chlorophenols (CPs) with one to five chlorine atoms at gold electrodes was studied by cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). The results obtained indicate that the oxidation of CPs at gold electrodes depends on the pH, the phenol concentration, the number of chlorine atoms in the aromatic ring, and the position of these Cl atoms with respect to the phenolic OH. At low scan rates and/or high CP concentrations, for mono- and di-CPs the phenoxi radicals condense forming polymers, or at least oligomers, that precipitate at the electrode surface, the resulting film behaving as an insulator that passivates the gold electrode. On the contrary, at high potential scan rates and/or low CP concentrations the film can be porous enough for charge transfer to continue.

Adsorbed formate: the key intermediate in the oxidation of formic acid on platinum electrodes.

The electrooxidation of formic acid on Pt and other noble metal electrodes proceeds through a dual-path mechanism, composed of a direct path and an indirect path through adsorbed carbon monoxide, a poisoning intermediate. Adsorbed formate had been identified as the reactive intermediate in the direct path. Here we show that actually it is also the intermediate in the indirect path and is, hence, the key reaction intermediate, common to both the direct and indirect paths. Furthermore, it is confirmed that the dehydration of formic acid on Pt electrodes requires adjacent empty sites, and it is demonstrated that the reaction follows an apparently paradoxical electrochemical mechanism, in which an oxidation is immediately followed by a reduction.

Electrooxidation of 2-chlorophenol on polyNiTSPc-modified glassy carbon electrodes

Abstract The electrooxidation of 2-chlorophenol (2-CP) on poly-nickel tetrasulfophthalocyanine (polyNiTSPc) modified glassy carbon (GC) and indium tin oxide (ITO) electrodes was studied by cyclic voltammetry and in situ UV–Vis transmission spectroscopy. The polyNiTSPc films were characterized by XPS and AFM, both before and after reaction with 2-CP. The results indicate that NiTSPc polymerizes at both GC and ITO electrodes, apparently retaining its phthalocyanine structure in which the Ni atoms play a role similar to that in Ni(OH) 2 films. In the oxidation of 2-CP at polyNiTSPc-modified electrodes the electrode fouling by a layer of polymerized 2-CP oxidation products (polyCP) is weaker than on the bare GC electrode, probably because the polyCP layer formed on polyNiTSPc–ITO is very rough, as indicated by AFM, thus allowing the passage of CP molecules to the polyNiTSPc film.

The Passive Film on Iron at pH 1–14 A Potential‐Modulated Reflectance Study

The passive film on iron has been studied by potential-modulated reflectance (PMR) spectroscopy over a wide range of pH (1-14) and polarization potentials

An electrochemical and UV-visible potential-modulated reflectance study of the electrooxidation of carbon monoxide on oxide-free smooth platinum: Part 1. Results in 0.5 M HClO4

Abstract The electrooxidation of CO on oxide-free smooth Pt in 1 M NaOH has been studied by electrochemical techniques and by UV-Visible Potential-Modulated Reflectance Spectroscopy (PMRS). The influence of the potential at which CO was first admitted to the electrolyte, Eadm on the electrocatalytic activity of Pt for CO oxidation depended on the electrochemical technique used, viz. stationary or potential sweep, since the poisoning CO species formed at high Eadm was desorbed after cathodic polarization of the electrode for a few minutes. Voltammograms in CO-saturated 1 M NaOH showed three anodic peaks, the most negative of which corresponded to the oxidation of dissolved CO, and the other two to oxidation of surface species. Apparently, the simultaneous presence of three peaks in the voltammogram under these conditions has not been reported before. A third surface peak was detected from voltammograms for CO-free solution; it did not appear in CO-saturated solution because it was hidden under the peak due to dissolved CO. Although all three surface species blocked the adsorption of hydrogen, only the CO species which is oxidized at the most positive voltammetric peak acted as a poison for CO electrooxidation, in the same way as in the acidic medium. The PMR spectrum of chemisorbed CO shows a maximum at 280 nm, which has been assigned to bridge-bonded CO, although perhaps mixed with linear CO.

Adsorption and electrooxidation of carbon monoxide on polycrystalline platinum at pH 0.3-13

Abstract Over the whole pH range 0.3–13 electrooxidation of dissolved CO on polycrystalline Pt was found to depend dramatically on the CO dosing potential, occurring at about 0.6 or 0.9 V for CO dosing potentials lower or higher, respectively, than about 0.4 V versus the reversible hydrogen electrode. Only in the former case a small anodic peak at about 0.5 V appeared in stripping voltammograms of chemisorbed CO, suggesting that dissolved CO electrooxidation at 0.6 V takes place on a small fraction of Pt sites that are free from CO. These findings are in agreement with literature reports for pH 0.3 and 14. The IR bands of both bridge and linear CO disappeared at about the same potential for each pH value, showing that at least the main peak of chemisorbed CO corresponded to both linear and bridge CO. As for the small CO stripping peak, it corresponded mostly, if not exclusively, to bridge CO. The ratio of bridge to linearly chemisorbed CO increased with increasing pH, and the stretching frequency of both bridge and linear CO decreased with increasing pH. Both trends were attributed to the exclusion by chemisorbed CO of water and its ions from the inner part of the electrochemical double layer, which would render the point of zero charge of the CO-covered metal independent of pH. At all pH values the intensity of the band of linear CO increased with increasing potential up to 0.4 V, while that of bridge CO decreased. This behaviour is in agreement with literature reports for Pt(100), Pt(111) and Rh(111), and is attributed to a decreasing back-donation from the metal d-orbitals to the antibonding 2π* MO of chemisorbed CO, which would favour a decrease of the coordination degree of CO.

Adsorption and electro-oxidation of carbon monoxide, methanol, ethanol and formic acid on osmium electrodeposited on glassy carbon

The activity of Os electrodeposited on glassy carbon (Os/GC) for the electro-oxidation of carbon monoxide, methanol, ethanol and formic acid has been studied. CO chemisorbed on Os/GC in its linear (on-top) form, as determined by in situ Fourier-transform infrared spectroscopy (FTIRS). The typical reflectance increase produced by the chemisorption of CO on thin metal films electrodeposited on glassy carbon was observed. Dissociative chemisorption of the above C1 organic compounds led to linearly chemisorbed CO. The electrolyte electroreflectance spectrum of Os/GC in CO-saturated 1 M HClO4 showed the same maximum at 270 nm already reported for Ru, Rh, Pd, Pt and Au. This maximum was assigned to an electronic transition from the Fermi level of the metal to the antibonding 2π* level of chemisorbed CO.

Quantitative Study of Non‐Covalent Interactions at the Electrode–Electrolyte Interface Using Cyanide‐Modified Pt(111) Electrodes

Cations at the outer Helmholtz plane (OHP) can interact through non-covalent interactions with species at the inner Helmholtz plane (IHP), which are covalently bonded to the electrode surface, thereby affecting the structure and the properties of the electrochemical double layer. These non-covalent interactions can be studied quantitatively using cyanide-modified Pt(111) electrodes.