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

Abstract

Kinetic isotope effects in nitrous oxide thermal decomposition and reduction reactions were investigated in flowing N2O/He and N2O/He/H2 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 (δ15Nbulk, δ15Nα, δ15Nβ, and δ18O) of residual nitrous oxide were found to follow a Rayleigh model based on batch distillation. Residual nitrous oxide was depleted slightly in 15N during thermal decomposition but slightly enriched in 15N during hydrogen reduction. Regarding the nitrogen isotopomers, results showed that the central nitrogen atom, Nα, was enriched in 15N 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.