Surface coordination chemistry of platinum studied by thin-layer electrodes. Adsorption, orientation, and mode of binding of aromatic and quinonoid compounds

Catechol adsorption on graphene nanoplatelets: isotherm, flat to vertical phase transition and desorption kinetics

The adsorption of catechol (1,2-dihydroxybenzene) on graphene nanoplatelets (GNPs) is investigated electrochemically and spectroscopically. The reversible adsorption of catechol on GNPs is Langmuirian with an adsorption constant of (0.2 ± 0.002) mM^-1 at low adsorbate concentrations (≤100 mM). At higher concentrations (>100 mM) the adsorption of catechol on GNPs is shown to undergo a flat to vertical concentration driven phase transition. The kinetics of desorption are measured with a single particle electrochemical technique. The study of individual impacts allows the determination of the rate of catechol desorption from GNPs to be k = 0.08 ± 0.01 s^-1 with first order kinetics. The method provides a powerful and efficient generic approach to study adsorption and, importantly, desorption of molecules on nanomaterials, as well as giving insight into the modification process.

C-H activation and metalation at electrode surfaces: 2,3-dimethyl-1,4-dihydroxybenzene on Pd(pc) and Pd(111) studied by TLE, HREELS and DFT

Previous studies, based on thin-layer electrochemistry (TLE), in situ scanning tunneling microscopy (EC-STM), high-resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) computations, on the chemical adsorption of hydroquinone from aqueous solutions onto atomically smooth Pd (and Pt) electrode surfaces indicated two modes of attachment that depended upon the solution concentration. At low activities, the diphenol was oxidatively chemisorbed as benzoquinone in a flat orientation, suggestive of a Pd(2,3,5,6-η-C6H4O2) surface complex; at higher concentrations, vertical chemisorption was effected via two C-H bond activations (or metalations) at the 2 and 3 ring positions, evocative of an o-phenylene organopalladium compound. We have extended the work to 2,3-dimethyl-1,4-dihydroxybenzene on Pd(pc) and Pd(111) electrodes to probe the effect of two methyl substituents on only one side of the diphenol ring. Surface coverage and adsorbed-molecule cross section data from TLE and HREELS measurements revealed non-random concentration-dependent adsorbate orientations similar to the oxidative chemisorption of hydroquinone: flat at low concentrations and edgewise at elevated concentrations. The DFT results suggested that, for the flat structure, surface coordination is via the two double bonds of the quinone ring as in [Pd(2,3,5,6-η)-2,3-dimethyl-p-quinone]. For the edge-vertical orientation, a structure analogous to an o-phenylene compound is generated in which C-H bonds at the 5 and 6 ring positions are activated and then metalated. DFT-simulated HREELS spectra helped identify the observed peaks that distinguish the surface-coordinated quinone from the surface-metalated diphenol.

Electrochemical Reduction of Quinones in Different Media: A Review

The electron transfer reactions involving quinones, hydroquinones, and catechols are very important in many areas of chemistry, especially in biological systems. The therapeutic efficiency as well as toxicity of anthracycline anticancer drugs, a class of anthraquinones, is governed by their electrochemical properties. Other quinones serve as important functional moiety in various biological systems like electron-proton carriers in the respiratory chain and their involvement in photosynthetic electron flow systems. The present paper summarizes literatures on the reduction of quinones in different solvents under various conditions using different electrochemical methods. The influence of different reaction conditions including pH of the media, nature of supporting electrolytes, nature of other additives, intramolecular or intermolecular hydrogen bonding, ion pair formation, polarity of the solvents, stabilization of the semiquinone and quinone dianion, catalytic property, and adsorption at the electrode surface, are discussed and relationships between reaction conditions and products formed have been presented.

Electrocatalytic hydrogenation and oxidation of aromatic compounds studied by DEMS: Benzene and p-dihydroxybenzene at ultrathin Pd films electrodeposited on Au(hkl) surfaces

Differential electrochemical mass spectrometry (DEMS) was used to investigate the electrocatalytic hydrogenation and oxidation of benzene and p-dihydroxybenzene (hydroquinone, H(2)Q) chemisorbed on ultrathin Pd films electrodeposited at Au(332) and Au(111) surfaces. At low sub-monolayer coverages on Au(332), the Pd ad-atoms preferentially adsorb on the step sites. At higher (but still sub-monolayer) coverages, Pd also occupies terrace sites; on Au(111), only terrace sites are available. The electrochemical reactivities of the subject compounds at the Pd-decorated steps and terraces were then compared. No hydrogenation products were detected by DEMS for either benzene or H(2)Q, but a considerable degree of electrodesorption of (intact) benzene occurred near the hydrogen-evolution region (HER). Anodic oxidation of both compounds yielded only CO(2) as the volatile (DEMS-detectable) product, although it took up to three anodic cycles to attain exhaustive (complete) oxidation. The anodic oxidation of benzene was also accompanied by potential-induced desorption of (unreacted) benzene particularly in the case of the stepped surface. Electrodesorption at the HER was more facile at the terrace sites than from the step sites; the opposite was true for electrodesorption at the more positive potential. Contrary to hydrogen, which does not adsorb at such monoatomic rows, both aromatic species adsorb when the nominal Pd coverage just corresponds to step decoration.