Product-Specific Isotope Effects as Constraints on Branched Catalytic Reaction Networks

Rates and selectivities alone often cannot distinguish between competing mechanisms in branched catalytic reactions. Here, we use GC–IRMS at natural carbon-isotope abundance to measure the isotope compositions of both CO and CH4 formed during CO2 hydrogenation over SiO2-supported Ni and Co nanoparticles. Comparing the isotope effects of both products, rather than considering either product alone, allows us to test whether their pathways share intermediates and where the isotope-sensitive steps occur. An isotope-resolved carbon balance accounts for finite conversion, and a simple branching model separates isotope effects arising before and after the product pathways split. Across changes in selectivity, catalyst identity, and Ni particle size, the isotope effects of CO and CH4 change together and remain separated by an approximately constant difference. This behavior is inconsistent with fully independent pathways. Instead, it supports a dominant shared intermediate pool that is most plausibly derived from CO. Methane formation shows an additional normal isotope effect relative to CO formation, indicating that the stronger isotope-sensitive chemistry occurs during the C–O activation and hydrogenation steps after the pathways split. The isotope pattern remains similar across the Ni particle sizes, while Co shows a somewhat larger methane-side contribution. Paired product isotope measurements therefore provide an independent test of catalytic reaction networks that complements kinetics, spectroscopy, transient experiments, and theory.