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

Rationale: Oxygen isotope exchange between O-2 and CO2 in the presence of heated platinum (Pt) is an established technique for determining the delta O-17 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 alpha(17) and alpha(18) fractionation factors for Pt-catalyzed CO2-O-2 oxygen isotope exchange at temperatures ranging from 500 to 1200 degrees C. For comparison, the theoretical alpha(18) equilibrium exchange values reported by Richet et al. (1997) have been updated using the direct sum method for CO2 and the corresponding alpha(17) values were determined. Finally, we examined whether the steady-state fractionation factors depend on the isotopic composition of the reactants, by using CO2 and O-2 differing in delta O-18 value from -66 parts per thousand to +4 parts per thousand. Results: The experimentally determined steady-state fractionation factors alpha(17) and alpha(18) are lower than those obtained from the updated theoretical calculations (of CO2-O-2 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 O-2 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 O-2. The direct sum evaluation yields a theta value of similar to 0.54, i.e. higher than the canonical range maximum for a mass-dependent fractionation process. Conclusions: Updated theoretical values of alpha(18) for equilibrium isotope exchange are lower than those derived from previous work by Richet et al. (1997). The direct sum evaluation for CO2 yields theta 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 alpha(17) and alpha(18) values may be due to thermal diffusion associated with the temperature gradient in the reactor.