Tracking oxygen atoms in electrochemical CO oxidation - Part II: Lattice oxygen reactivity in oxides of Pt and Ir

Electrochemical oxidation of carbon monoxide (CO oxidation) is often used as a model reaction to investigate the surface of metallic electrocatalysts, most notably in CO stripping experiments. In this report, we use chip-based  electrochemistry mass spectrometry with 18O isotope-labeled oxides Pt and Ir to investigate the involvement of lattice oxygen in the electrochemical oxidation of water (the oxygen evolution reaction, OER), adventitious carbon, and CO.

For Pt, we find that the labeled oxygen from Pt18Ox is incorporated into the CO2 resulting from CO oxidation at the potential at which oxygen at the electrode surface is reduced to hydroxyl (∗OH), confirming that ∗OH is the reactive species in the Langmuir-Hinshelwood electrochemical oxidation of CO.

For Ir we find that lattice oxygen in Ir18O2 is similarly involved in electrochemical CO oxidation, but only if it is first activated by a reductive sweep. The labeled CO2 signal is transient, indicating that activated lattice oxygen provides the “ignition sites” for the Langmuir-Hinshelwood electrochemical oxidation of CO on Ir.

We also confirm the previously reported result that electrochemically prepared, amorphous, Ir18Ox incorporates much more lattice oxygen in O2 evolved during OER than does rutile Ir18O2, but we also quantify the amount and show that in all cases the labeled O2 is a very small portion of the total O2 evolved, and that more lattice oxygen is released in CO2 when oxidizing CO and adventitious carbon than is released in O2 when oxidizing water.

Through these results, we demonstrate that EC-MS in concert with isotope labeling and CO as a probe molecule can provide insight into lattice oxygen reactivity, extending the utility of CO oxidation to the study of noble metal oxides used in e.g. PEM electrolyzer anodes.