Catalytic performance of mixed M x Co 3− x O 4 (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

• Published in: Catalysis science & technology 2022-04, Vol.12 (8), p.2446-2461
• Main Authors: Hudy, Camillo, Długosz, Olga, Gryboś, Joanna, Zasada, Filip, Krasowska, Aneta, Janas, Janusz, Sojka, Zbigniew
• Format: Article
• Language: English
• ISSN: 2044-4753; 2044-4761
• DOI: 10.1039/D2CY00388K

A series of mixed cobalt spinel catalysts (M x Co 3− x O 4 (M = Cr, Fe, Mn, Ni, Cu, Zn)) was synthesized and tested in the CO-PROX reaction and in sole CO oxidation and H 2 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 H 2 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, E O2p + k Δ|< χ M > − χ O | and Δ E M–O = ( E M3d − E O2p ), based on the position of the oxygen 2p ( E O2p ) and the metal 3d ( E M3d ) band centers and on the average metal < χ M > and oxygen ( χ O ) 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 Δ E M–O values, whereas the selectivity is correlated with the E O2p + k Δ|< χ M > − χ O | 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 Co 3+ cations, in contrast to the other mixed spinel catalysts.