Catalytic performance of mixed MxCo3−xO4 (M = Cr, Fe, Mn, Ni, Cu, Zn) spinels obtained by combustion synthesis for preferential carbon monoxide oxidation (CO-PROX): insights into the factors controlling catalyst selectivity and activity

A series of mixed cobalt spinel catalysts (MxCo3−xO4 (M = Cr, Fe, Mn, Ni, Cu, Zn)) was synthesized and tested in the CO-PROX reaction and in sole CO oxidation and H2 oxidation as references. The catalysts were thoroughly characterized by XRF, XRD, XPS, Raman, and IR spectroscopy, by TEM/STEM/EDX and with SEM techniques. Depending on the u parameter value (oxygen centricity between the tetra- and octahedral cations), two types of mixed spinel catalyst with distinctly different catalytic behavior in the CO-PROX reaction were distinguished. The A-type spinels (Co, Ni, Cu) with u ∼ 0.2625 exhibited overall higher activity in CO and H2 oxidation and lower selectivity in the high-temperature range (T > 180 °C), whereas B-type spinels (Cr, Fe, Mn) with u < 0.2625 were less active yet more selective in the high-temperature range. The work function of the spinel catalysts was found to be a useful concise parameter in accounting for their CO-PROX performance, supporting the proposed categorization of the mixed spinels. Two heuristic descriptors, EO2p + kΔ|<χM> − χO| and ΔEM–O = (EM3d − EO2p), based on the position of the oxygen 2p (EO2p) and the metal 3d (EM3d) band centers and on the average metal <χM> and oxygen (χO) electronegativity difference, were proposed for the rationalization of the mixed spinel performance in terms of their intrinsic electronic properties. It was established that the activity depends on the volcano-type fashion of the ΔEM–O values, whereas the selectivity is correlated with the EO2p + kΔ|<χM> − χO| parameter in a monotonous fashion. The unique behavior of the Zn–Co spinel results from the lifting of the O2p center above the 3d band center of the redox Co3+ cations, in contrast to the other mixed spinel catalysts.