Mass Spectrometric Studies of Oxygen Isotope Exchange Reactions with Oxide Surfaces

Isotopic exchange reactions with the surface of amorphous silica and crystalline quartz were studied mass spectrometrically. A mixture of approximately 2%18o2 in neon was used as the test gas, and was passed over the surface of the oxides. Measurements were taken as a function of temperature and residence time, for kinetic studies. These studies clearly show a second order rate law for the surfaces examined. The rate constant on amorphous silica was 2.2xl02L/mol/sec. The energy of activation was 35.3 kcal/mol and the frequency factor was l.35xl09L/mol/sec. On the crystalline quartz the rate constant was 2.0xl02L/mol/sec. The energy of activation was 65.2 kcal/mol and the frequency factor was 4.52xl02L/mol/sec. The postulated mechanism for a bimolecular reaction would occur at temperatures above that required for dehydroxylation. This allows chemisorption of a molecule of 18o2 to form a bridged six-membered ring upon which a subsequent collision of a second 18o2 molecule results in an oxygen isotope exchange.