A SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE STUDY OF DEUTERIUM ADSORBED ON RHODIUM SILICA - EXCHANGE BETWEEN MOBILE AND WEAKLY ADSORBED DEUTERIUM

The chemisorption of deuterium on 5% Rh/SiO2 was studied by solid-state NMR spectroscopy with a 200-MHz spectrometer. The observed peak position and width depend on D2 pressure and temperature. The adsorbed deuterium can be divided into three categories: rigidly adsorbed (D(r)), mobile (D(m)), and weakly adsorbed (D(w)). D(r) is not detectable by NMR spectroscopy because of excessive quadrupole broadening. D(m) shows a NMR peak at -120 ppm and is converted to D(r) on temperature lowering. D(w) is characterized by a sharp signal at 0 ppm at 170 K. A downfield shift of the NMR signal upon increasing the D2 pressure is explained by a fast exchange between D(m) and D(w). The exchange mechanism was established as purely associative. And the exchange rate is (4.8 +/- 0.4) x 10(4) THETA-1 s-1 at 295 K with an activation energy of (16.6 +/- 1.9) kJ mol-1. Studies with a 300-MHz instrument further support the exchange model. The amounts of D(r), D(m), and D(w) are D2 overpressure dependent, D(m) remains relatively constant, D(w) increases, but but D(r) decreases with pressure. The major releaxation mechanism of adsorbed deuterium is the nuclear quadrupole interaction.