The high sensitivity and mass resolution of proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS) has enabled real-time measurements of trace analytes in the fields of atmospheric science and heterogeneous catalysis. Here, I will show how PTR-TOF-MS can be used for the operando measurement of C1-C3 products from electrochemical CO₂ reduction (EC-CO₂R) and, prospectively, for ¹³C isotope enrichment.
Current research aimed at scaling up EC-CO₂R employs reactors with gas diffusion electrodes (GDEs). When a Cu electrocatalyst is used, the gas phase products are a mixture of gas and volatilized liquid phase C1-C4 products along with H₂ at sub-ppm to percent levels entrained in CO₂. PTR-TOF-MS is well suited for measurements in this environment. Sampling is accomplished in a minimally perturbing manner by placing the inlet probe of the PTR-TOF-MS in close proximity to the GDE surface. While H₂, CO, and CH₄ cannot be detected, most other products can be detected at sub-ppm levels either by proton transfer or by electron transfer from trace amounts of O₂⁺ from the source. Operando measurements are used for mechanism evaluation in two ways. First, by slowly scanning the potential to more negative values, the onset potentials for volatile minor and major products can be precisely determined. Secondly, the high mass resolution allows us to track the degree of ¹³C-¹²C isotope fractionation (δ¹³C) through the chemical network. As the elementary steps in CO₂R are highly irreversible we expect and observe discrimination against ¹³C in the products (negative values of δ¹³C). Moreover, as we expect the largest (primary) kinetic isotope effect to at the C-C coupling steps, we are able group products sharing a given pathway by their δ¹³C values.
Surprisingly, we find that discrimination against ¹³C in EC EC-CO₂R is very large. We observe δ¹³C values for the C2 products of up to -240‰, which are significantly larger than for biological CO₂ fixation in photosynthesis and for Fischer-Tropsch synthesis of hydrocarbons. As a result, unreacted ¹²CO₂ exiting the reactor is enriched in ¹³C. Using a natural abundance feed, we experimentally demonstrate enriching of the ¹³C fraction from its natural abundance of 1.1% to ~1.3% in a single pass reactor and propose a scalable and economically attractive process to yield isotopes of a commercially relevant purity.

Joel Ager

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