In situ FTIR studies on the effect of temperature on the electro-oxidation of small organic molecules at the Ru(0001) electrode

Abstract

The adsorption and electro-oxidation of formaldehyde, formic acid and methanol at the Ru(0001) electrode in perchloric acid solution have been studied as a function of temperature, potential and time using in situ FTIR spectroscopy, and the results interpreted in terms of the surface chemistry of the Ru(0001) electrode and compared to those obtained during our previous studies on the adsorption of CO under the same conditions. It was found that no dissociative adsorption or electro-oxidation of methanol takes place at Ru(0001) at potentials <900 mV vs. Ag/AgCl, and at all three temperatures employed, 10, 25 and 50 °C. However, both formaldehyde and formic acid did undergo dissociative adsorption, even at −200 mV, to form linear (CO_L) and 3-fold-hollow (CO_H) binding CO adsorbates. In contrast to the adsorption of CO, it was found that increasing the temperature to 50 °C markedly increased the amount of CO adsorbates formed on the Ru(0001) surface from the adsorption of both formaldehyde and formic acid. On increasing the potential, the electro-oxidation of the CO adsorbates to CO₂ took place via reaction with the active (1 × 1)-O oxide. Formic acid was detected as a partial oxidation product during formaldehyde electro-oxidation. At all three temperatures employed, it was found that adsorbed CO species were formed from the adsorption of both formic acid and formaldehyde, and were oxidised to CO₂ faster than was observed in the experiments involving CO adsorbed from CO(g), suggesting a higher mobility of the CO adsorbates formed from the adsorption of the HCOOH and HCHO. At potentials >1000 mV, both the oxidation of formic acid to CO₂ and the oxidation of formaldehyde to both CO₂ and formic acid were significantly increased, and the oxidation of methanol to CO₂ and methyl formate was observed, all of which were attributed to the formation of an active RuO₂ phase on the Ru(0001) surface.