Isotopic evidence for the tangled mechanism of the CO-PROX reaction over mixed and bare cobalt spinel catalysts

Abstract

The catalytic performance of the bare Co₃O₄ and mixed cobalt-spinel catalysts (M_xCo_3−xO₄; M = Cr, Mn, Fe, Ni, Cu, Zn) in the CO-PROX process was investigated in the temperature-programmed surface reaction (TPSR) mode using ¹⁸O₂ as an oxidant. The developed heuristic approach, where the prototype isotopic compositions of the reaction products (C¹⁶O¹⁸O, C¹⁶O₂, C¹⁸O₂, H₂¹⁶O and H₂¹⁸O), inferred from a conceivable molecular course of the postulated catalytic scenarios, are confronted with the experimental data allows for delineation of the CO-PROX reaction mechanism. For this purpose, in addition to mixed spinels, several intentionally labeled isotopic ¹⁸O₂/Co₃¹⁸O₄, ¹⁶O₂/Co₃¹⁸O₄, ¹⁸O₂/Co₃¹⁶O₄ reference CO-PROX systems were examined. It was shown that the catalytic turnovers of CO and H₂ result from various combinations of the generic intrafacial Mars van Krevelen and suprafacial Langmuir–Hinshelwood/Eley–Rideal patterns, where the formation of surface carbonates as a common key intermediate allows for successful reproduction of the observed variation of the isotopic composition of CO₂ and H₂O with the selectivity. The mechanistic proposals were substantiated by DFT+U and ab initio thermodynamic modeling, corroborated by IR studies, which provided the requisite theoretical background for the dual role of the carbonate species as intermediates or spectators in the CO-PROX reaction, depending on their mode of attachment on the catalyst surface.