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

Abstract

Rationale: Oxygen isotope exchange between O 2 and CO 2 in the presence of heated platinum (Pt) is an established technique for determining the δ 17 O value of CO 2 . However, there is not yet a consensus on the associated fractionation factors at the steady state. Methods:We determined experimentally the steady-state α 17 and α 18 fractionation factors for Pt-catalyzed CO 2 -O 2 oxygen isotope exchange at temperatures ranging from 500 to 1200 C. For comparison, the theoretical α 18 equilibrium exchange values reported by Richet et al. (1997) have been updated using the direct sum method for CO 2 and the corresponding α 17 values were determined. Finally, we examined whether the steady-state fractionation factors depend on the isotopic composition of the reactants, by using CO 2 and O 2 differing in δ 18 O value from À66 ‰ to +4 ‰. Results:The experimentally determined steady-state fractionation factors α 17 and α 18 are lower than those obtained from the updated theoretical calculations (of CO 2 -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 CO 2 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 CO 2 to O 2 . 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 α 18 for equilibrium isotope exchange are lower than those derived from previous work by Richet et al. (1997). The direct sum evaluation for CO 2 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 α 17 and α 18 values may be due to thermal diffusion associated with the temperature gradient in the reactor.

Exploring the potential of Δ17O in CO2 for determining mesophyll conductance

“…The Δ 17 O of CO 2 was determined using the CO 2 –O 2 exchange method ( Barkan et al 2015 ; Adnew et al 2019 ). A detailed description of the CO 2 –O 2 exchange system at Utrecht University is given in Adnew et al (2019 , 2022) . Equal amounts of CO 2 and O 2 were mixed in a quartz reactor containing a platinum sponge catalyst at the bottom and heated at 750 °C for 2 h. After isotope equilibration, the CO 2 was trapped at liquid nitrogen temperature, while the O 2 was collected with 1 pellet of 5 Å molecular sieve (1.6 mm, Sigma-Aldrich, USA) at liquid nitrogen temperature.…”

“…where δ 17 O = ( 17 R sample − 17 R reference )/ 17 R reference and δ 18 O = ( 18 R sample − 18 R reference )/ 18 R reference . Theoretical considerations suggest that λ varies from 0.5000 to 0.5305 for different mass-dependent isotope fractionation processes, but recent studies have reported λ values even outside this range ( Hayles et al 2017 ; Adnew et al 2022 ; Hayles and Killingsworth 2022 ). Deviations from Equation ( 3 ) are quantified as Δ 17 O, and in this study, we used the linearized definition for Δ 17 O.…”

A Dual Inlet System for Laser Spectroscopy of Triple Oxygen Isotopes in Carbonate‐Derived and Air CO2

“…Ellis and Passey (2023) built upon this methanation‐fluorination technique, but the initial step of their method involves the high‐temperature conversion of the oxygen in the analyzed material, for example, carbonates, to CO. Isotope exchange between CO 2 and O 2 over hot platinum is another established protocol to obtain high‐precision Δ′ 17 O data (Adnew et al., 2022; Mahata et al., 2013). Limitations of these methods are that they are laborious and involve multiple fractionation steps, which can affect the accuracy of the results.…”