Adnew Getachew Agmuas, Workman Evelyn, Janssen Christof, Röckmann Thomas
Institute for Marine and Atmospheric Research Utrecht (IMAU), Physics Department, Utrecht University, Utrecht, The Netherlands.
Laboratoire d'Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères (LERMA), Sorbonne Université, Observatoire de Paris, Université PSL, Paris, France.
Rapid Commun Mass Spectrom. 2022 Jun 30;36(12):e9301. doi: 10.1002/rcm.9301.
Oxygen isotope exchange between O and CO in the presence of heated platinum (Pt) is an established technique for determining the δ O value of CO . However, there is not yet a consensus on the associated fractionation factors at the steady state.
We determined experimentally the steady-state α and α fractionation factors for Pt-catalyzed CO -O oxygen isotope exchange at temperatures ranging from 500 to 1200°C. For comparison, the theoretical α equilibrium exchange values reported by Richet et al. (1997) have been updated using the direct sum method for CO and the corresponding α values were determined. Finally, we examined whether the steady-state fractionation factors depend on the isotopic composition of the reactants, by using CO and O differing in δ O value from -66 ‰ to +4 ‰.
The experimentally determined steady-state fractionation factors α and α are lower than those obtained from the updated theoretical calculations (of CO -O 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 and CO 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 to O . 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.
Updated theoretical values of α for equilibrium isotope exchange are lower than those derived from previous work by Richet et al. (1997). The direct sum evaluation for CO 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 and α values may be due to thermal diffusion associated with the temperature gradient in the reactor.
在加热的铂（Pt）存在下，O与CO之间的氧同位素交换是一种确定CO的δO值的成熟技术。然而，关于稳态下相关的分馏系数尚未达成共识。
我们通过实验确定了在500至1200°C温度范围内Pt催化的CO - O氧同位素交换的稳态α和α分馏系数。为了进行比较，使用CO的直接求和方法更新了Richet等人（1997年）报道的理论α平衡交换值，并确定了相应的α值。最后，我们通过使用δO值在-66‰至+4‰之间不同的CO和O，研究了稳态分馏系数是否取决于反应物的同位素组成。
实验确定的稳态分馏系数α和α分别比从更新后的理论计算（平衡条件下的CO - O同位素交换）获得的值低0.0024±0.0001和0.0048±0.0002。这种偏差不是由于O和CO同位素测量之间的尺度不兼容，也不是由于计算中忽略了非玻恩 - 奥本海默效应。存在一个交叉温度，在该温度下，次要同位素的富集从CO切换到O。直接求和评估得出的θ值约为0.54，即高于质量依赖分馏过程的标准范围最大值。
平衡同位素交换的α的更新理论值低于Richet等人（1997年）先前工作得出的值。CO的直接求和评估得出的θ值高于质量依赖分馏过程的标准范围最大值。这表明在多原子分子的质量依赖分馏过程的计算和定义中需要考虑非谐效应。理论与实验α和α值之间的差异可能是由于与反应器温度梯度相关的热扩散。
