Stable isotope fractionation of nitrous oxide during thermal decomposition and reduction processes

Kinetic isotope effects in nitrous oxide thermal decomposition and reduction reactions were investigated in flowing N2O/He and N2O/He/H-2 systems. The intramolecular distribution of stable isotopes, isotopomer, was measured and evaluated using an isotope ratio mass spectrometer equipped with a modified collector system. Thermal decomposition and reduction of nitrous oxide were carried out using a flow tube reactor made of quartz glass in the temperature range of 1173-1373 K. Both thermal decomposition and hydrogen reduction of nitrous oxide proceeded by first-order kinetics. Isotopomer ratios (delta(15)N(bulk), delta(15)N(alpha), delta(15)N(beta), and delta(18)O) of residual nitrous oxide were found to follow a Rayleigh model based on batch distillation. Residual nitrous oxide was depleted slightly in N-15 during thermal decomposition but slightly enriched in N-15 during hydrogen reduction. Regarding the nitrogen isotopomers, results showed that the central nitrogen atom, N-alpha, was enriched in N-15 during both destruction reactions of nitrous oxide. Although the N2O destruction reactions proceeded at high temperature, large isotopic fractionations of nitrogen isotopomers were observed. Enrichment factors for nitrous oxide as a result of thermal decomposition and reduction reactions were evaluated.