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

The catalytic performance of the bare Co 3 O 4 and mixed cobalt-spinel catalysts (M x Co 3− x O 4 ; M = Cr, Mn, Fe, Ni, Cu, Zn) in the CO-PROX process was investigated in the temperature-programmed surface reaction (TPSR) mode using 18 O 2 as an oxidant. The developed heuristic approach, where the prototype isotopic compositions of the reaction products (C 16 O 18 O, C 16 O 2 , C 18 O 2 , H 2 16 O and H 2 18 O), 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 18 O 2 /Co 3 18 O 4 , 16 O 2 /Co 3 18 O 4 , 18 O 2 /Co 3 16 O 4 reference CO-PROX systems were examined. It was shown that the catalytic turnovers of CO and H 2 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 CO 2 and H 2 O 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. The catalytic performance of the bare Co 3 O 4 and mixed cobalt-spinel catalysts (M x Co 3− x O 4 ; M = Cr, Mn, Fe, Ni, Cu, Zn) in the CO-PROX process was investigated in the temperature-programmed surface reaction (TPSR) mode using 18 O 2 as an oxidant.