Nitric oxide/nitrogen isotope exchange on tungsten

Abstract

The formation of ¹⁴N¹⁵N from a mixture of NO and ¹⁵N¹⁵N in the presence of a tungsten filament is measurable only at filament temperatures in excess of 1600 K, some 700 K above the temperature at which a mixture of ¹⁴N¹⁴N and ¹⁵N¹⁵N reacts to give ¹⁴N¹⁵N. The temperature difference is attributed to the inhibiting effect of adsorbed oxygen derived from the NO. At the highest filament temperatures (> 1800 K) most (> 80 %) of the NO is decomposed. Under these conditions the rate of production of ¹⁴N¹⁵N is greater than the rate calculated from the measured partial pressures of ¹⁴N¹⁴N (produced by the decomposition of NO) and of ¹⁵N¹⁵N, using the known efficiency of the filament as a nitrogen-isotope equilibration catalyst. This effect may be due to an enhanced surface concentration of ¹⁴N atoms resulting from the dissociative adsorption of NO. An order-of-magnitude calculation of the sticking probability of NO needed to account for the observations is in agreement with the value deduced from measurements of the rate of decomposition of NO by the hot filament.