Isotopic homogenization and scrambling associated with oxygen isotopic exchange on hot platinum: studies on gas pairs (O2, CO2) and (CO, CO2)

Abstract

The catalytic exchange between O₂ and CO₂ on hot platinum leads to isotope scrambling in CO₂ and homogenization of the oxygen isotopes in the two phases (O₂ and CO₂) even though the two gases could be widely different in isotope ratios from each other (e.g., about 45‰ in δ¹⁸O) before the exchange. After the exchange, the δ¹⁸O values of the gases become close to each other (within 2‰) depending on the time and temperature. The clumped-isotope analysis of the post-exchange CO₂ samples shows that the Δ₄₇ values (relative to the pre-exchange values) decrease with increase in temperature from near zero (at ∼25 °C) to about −0.8 ± 0.1‰ (at ∼700 °C). The low value of −0.8‰ equals the values typically obtained for pure CO₂ heated to 1000 °C inside quartz tubes suggesting a scrambled state. If the catalytic exchange between CO₂ and O₂ is associated with isotopic scrambling (or random mixing) a microscopic mechanism of isotopic mixing can be inferred. It seems that CO₂ and O₂ dissociate in the adsorbed state on platinum (or quartz surfaces) and the product CO molecules and O atoms mix uniformly while doing a random walk on the hot surface and produce two reservoirs (CO and O) where the three oxygen isotopes are randomly distributed. This can produce the observed internal isotopic scrambling in CO₂ when CO and O recombine to form CO₂ molecules on the platinum surface and desorb. Experiments using CO–CO₂ gas pairs over hot platinum also show homogenization showing exchange of CO as a molecular entity and supporting the suggested mechanism.