Correlation between the catalytic activity of polyoxometallates and the special features of their tunnel and optical spectra

The tunnel spectra of phosphomolybdic acid, a classic heteropoly acid with a Keggin anion, were measured in ultrahigh-vacuum experiments with the use of scanning tunnel microscopy. The dependences of the resonance characteristics of the spectra, “negative differential resistances,” on the vacuum gap, material of contacts, and field polarity were studied. An earlier unknown mechanism of the formation of these characteristics was described. The mechanism included the action of strong electric fields in scanning tunnel microscope nanocontacts and a low degree of the delocalization of Keggin anion peripheral electrons. Strong electric fields (∼10 7 V/cm) characteristic of spectroscopic measurements with the use of scanning tunnel microscopes could break exchange bonds in heteropoly acids and their derivatives. This produced spectroscopic effects of interest for catalysis and nanoelectronics.