Temperature dependence of isotopic fractionation in the CO2 -O2 isotope exchange reaction

Abstract

Rationale: Oxygen isotope exchange between O₂ and CO₂ in the presence of heated platinum (Pt) is an established technique for determining the δ¹⁷ O value of CO₂ . However, there is not yet a consensus on the associated fractionation factors at the steady state.

Methods: We determined experimentally the steady-state α¹⁷ and α¹⁸ fractionation factors for Pt-catalyzed CO₂ -O₂ oxygen isotope exchange at temperatures ranging from 500 to 1200°C. For comparison, the theoretical α¹⁸ equilibrium exchange values reported by Richet et al. (1997) have been updated using the direct sum method for CO₂ and the corresponding α¹⁷ values were determined. Finally, we examined whether the steady-state fractionation factors depend on the isotopic composition of the reactants, by using CO₂ and O₂ differing in δ¹⁸ O value from -66 ‰ to +4 ‰.

Results: The experimentally determined steady-state fractionation factors α¹⁷ and α¹⁸ are lower than those obtained from the updated theoretical calculations (of CO₂ -O₂ isotope exchange under equilibrium conditions) by 0.0024 ± 0.0001 and 0.0048 ± 0.0002, respectively. The offset is not due to scale incompatibilities between isotope measurements of O₂ and CO₂ nor to the neglect of non-Born-Oppenheimer effects in the calculations. There is a crossover temperature at which enrichment in the minor isotopes switches from CO₂ to O₂ . The direct sum evaluation yields a θ value of ~0.54, i.e. higher than the canonical range maximum for a mass-dependent fractionation process.

Conclusions: Updated theoretical values of α¹⁸ for equilibrium isotope exchange are lower than those derived from previous work by Richet et al. (1997). The direct sum evaluation for CO₂ yields θ values higher than the canonical range maximum for mass-dependent fractionation processes. This demonstrates the need to include anharmonic effects in the calculation and definition of mass-dependent fractionation processes for poly-atomic molecules. The discrepancy between the theory and the experimental α¹⁷ and α¹⁸ values may be due to thermal diffusion associated with the temperature gradient in the reactor.