Analysis of adsorbed intermediates and determination of surface potential shifts by dems

Differential electrochemical mass spectroscopy (DEMS) is used to elucidate the strongly bound intermediates of small organic molecules. The oxidation state of the adsorbates is determined by correlating the Faradaic charge of the electrodesorption peak with the amount of product formed. Isotope labelling yields information about the structure of the adsorbate. Thus, the intermediates of formic acid and methanol and also reduced CO2, all need 3 electrons to be oxidized to CO2. The adsorbates contain one C. one O, and H bound to the C. Therefore in all these cases the adsorbate is HCOad. In analogy, the intermediate of ethanol is characterized as CH3COad. Since the ion current of the product measured with the mass spectrometer is proportional to the Faradaic current only, the capacitive current can be calculated that flows while electrodesorption occurs. This allows the shift of the pzc caused by the adsorbate to be estimated. In the case of CO adsorbed on Pt a shift of 250 ± 40 mV was found.