Role of germanium in promoting the electrocatalytic reduction of nitrate on platinum: An FTIR and DEMS study

We have studied the role of germanium in the electrocatalysis of nitrate reduction on polycrystalline platinum by a combination of electrochemical measurements, in situ FTIR spectroscopy and differential electrochemical mass spectrometry (DEMS). The FTIR measurements indicate that adsorbed germanium is not elemental germanium but rather a germanium hydride adlayer, at least in the potential region where nitrate reduction takes place. Interestingly, there is no effect of germanium on the electrocatalytic nitrate reduction in perchloric acid solutions for potentials more positive than 0.2 V (vs RHE). For potentials below 0.2 V, there is a clear rate enhancement for nitrate reduction in perchloric acid. In sulphuric acid solution, there is a clear promoting effect of germanium, but the overall rate is always lower than in perchloric acid. These results suggest that the effect of germanium is rather related to removing inhibitors from the surface (sulfate, hydrogen) than to directly promoting nitrate reduction itself. If present on the surface, germanium does have a significant influence on the product distribution of nitrate reduction. Both hydroxylamine (some 15–25%) and a small amount of gaseous nitric oxide were observed, whereas in the absence of germanium only ammonia is produced. This change in selectivity is attributed to a “third body” effect of adsorbed germanium, inhibiting N–O bond breaking by limiting the number of free sites needed for the adsorbed NO to lie flat on the surface, thereby promoting hydroxylamine formation and nitric oxide desorption.