Methanol Dehydrogenation on Pt Electrodes: Active Sites and Role of Adsorbed Spectators Revealed through Time-Resolved ATR-SEIRAS

Abstract

The dynamics of methanol dehydrogenation to adsorbed CO (COad) on polycrystalline Pt in the hydrogen underpotential adsorption (Hupd) region has been investigated under potentiostatic conditions using time-resolved surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS) after a potential step. Our experiments reveal that the electrooxidation of methanol to COad is possible at potentials at least as negative as 0.01 V versus reversible hydrogen electrode and occurs nearly exclusively at (111)- and (100)-oriented defect sites, whereby (111)-oriented defects show higher activity unless blocked by spectator species. Under conditions in which, due to a combination of low methanol concentration and sufficiently negative potential, the formation of COad is very slow, we have been able to determine the singleton frequency of linearly bonded COad (COL) on Pt (2002 cm–1 ± 2 between 0.01 and 0.06 V). Under these conditions, the slow increase in the coverage of COad (θCO), as revealed by the increase of the integrated intensity of the bands corresponding to COL and bridge or multiple-bonded COad (COB/COM), is not accompanied by an increase in the COL stretching frequency (νCOL), which initially remains constant and then increases sharply, an effect that repeats itself several times, giving rise to a staircase-like increase of νCOL before it starts to increase monotonously in parallel with θCO. We attribute this behavior to the progressive population of terraces by COad diffusing from the defect sites where it has been formed. We also detected under these conditions a band at 1677 cm–1, which we attribute to adsorbed formyl (HCOad) and which we suggest is the last intermediate in the oxidation leading from methanol to COad. Our experiments also allow the direct spectroscopic determination of the rate of formation of COad at the limit of zero coverage (θCO → 0), from which Tafel plots revealing the potential dependence of the reaction rate when θCO → 0 can be drawn. These plots, together with the potential dependence of the time elapsed between the first observation in spectra of COB/COM and that of COL, provide evidence of the important role played by adsorbed spectators in determining the reaction rate.