Squaric acid adsorption and oxidation at gold and platinum electrodes

Abstract

The adsorption and oxidation of squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione, H 2 C 4 O 4 , SQA) at platinum and gold electrodes were studied spectroelectrochemically in perchloric acid solutions. Voltammetric experiments demonstrate that reversible adsorption takes place at gold electrodes in the double-layer region. As a difference with platinum electrodes, no dissociative adsorption processes leading to the blocking of the electrode surface are detected. ATR-SEIRAS experiments show potential-dependent adsorbate bands at potentials below 1.20 V RHE that, according to DFT calculations, can be assigned to adsorbed squarate. For platinum electrodes, the potential-dependent adsorption of squarate anions is coupled with the oxidative stripping of adsorbed carbon monoxide, which is formed upon dissociative SQA adsorption. Bonding of squarate species to the platinum and gold surfaces involves two oxygen atoms in a bidentate configuration, with the molecular plane perpendicular to the metal surface. The ATR-SEIRA spectra obtained for gold electrodes in the SQA oxidation region show bands for adsorbed bicarbonate anions formed from dissolved carbon dioxide molecules. In the case of platinum, distinct bands are observed for adsorbed oxidation products which probably are formed upon opening of the SQA ring.

Spectroelectrochemical and Density Functional Theory Study of Squaric Acid Adsorption and Oxidation at Gold Thin Film and Single Crystal Electrodes

“…The electrochemical reactivity of oxocarbons [26][27][28][29][30][31][32][33][34][35][36][37][38] , including that of squaric acid [26][27][28][29]34,35,37,38 and some of its derivatives 39 , has been described in water 26,27,[29][30][31][32][33][34][35]37,38 and in non-aqueous solvents 28,36,39 . In aqueous solutions, most of the studies on the electrochemical oxidation of H 2 SQ 26,27,29,34,35,38 and other oxocarbons 26,[30][31][32][33] , are related to their behaviour at platinum electrodes. It has been proved that oxocarbon electrooxidation at this electrode material is characterized by parallel reaction paths involving, in addition to their direct oxidation, their dissociative adsorption to for...…”

“…In aqueous solutions, most of the studies on the electrochemical oxidation of H 2 SQ 26,27,29,34,35,38 and other oxocarbons 26,[30][31][32][33] , are related to their behaviour at platinum electrodes. It has been proved that oxocarbon electrooxidation at this electrode material is characterized by parallel reaction paths involving, in addition to their direct oxidation, their dissociative adsorption to form adsorbed carbon monoxide 27,[29][30][31][32][33][34][35]38 . This process leads to an effective blocking of the electrode surface for the hydrogen adsorption-desorption processes and is witnessed by the detection of C-O stretching bands in the in situ infrared spectra collected in the oxocarbon-contaning solutions 29,[31][32][33][34][35]38 .…”

“…It has been proved that oxocarbon electrooxidation at this electrode material is characterized by parallel reaction paths involving, in addition to their direct oxidation, their dissociative adsorption to form adsorbed carbon monoxide 27,[29][30][31][32][33][34][35]38 . This process leads to an effective blocking of the electrode surface for the hydrogen adsorption-desorption processes and is witnessed by the detection of C-O stretching bands in the in situ infrared spectra collected in the oxocarbon-contaning solutions 29,[31][32][33][34][35]38 . In the case of squaric acid, the formation of adsorbates different from CO has been detected spectroscopically in a narrow potential window just above the onset of the oxidative stripping of the CO adlayer 34,38 .…”

“…In the case of squaric acid, the formation of adsorbates different from CO has been detected spectroscopically in a narrow potential window just above the onset of the oxidative stripping of the CO adlayer 34,38 . The formed adspecies was tentatively identified as adsorbed squarate anions 34 , this assignment being later confirmed by preliminary DFT (Density Functional Theory) calculations 38 .…”

“…In a recent paper 38 we have presented new results regarding the electrochemical behavior of squaric acid at gold and platinum thin film electrodes. ATR-SEIRA (Surface Enhanced Infrared Reflection Absorption Spectroscopy under Attenuated Total Reflection conditions) spectra collected for the gold electrode showed potential-dependent bands in the double layer potential region related to the presence of adsorbates coming from squaric acid.…”