Understanding Oxygen Activation on Metal- and Nitrogen-Codoped Carbon Catalysts

Multidoped carbons are often used for oxygen activation catalysis, both in heterogeneous catalysis and electrocatalysis. Identifying their catalytic sites is crucial for developing better catalysts. We now report an in-depth study into O₂ activation on an important class of materials: carbons codoped by nitrogen and 10 different metals (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, and Pb). To identify catalytic sites, we studied their composition and structure (both bulk and surface) by a wide range of techniques, including temperature-programmed reduction, X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy, and N₂ sorption porosimetry. The O₂ activation step was studied by electrochemical oxygen reduction. To assign active sites, the electrocatalytic activity, selectivity, and stability were correlated to material composition and to known reactivity pathways. Two types of sites for O₂ activation were identified and assigned for each multidoped material: (1) particles of partially reduced metal oxides and (2) metal-nitrogen clusters embedded into the carbon matrix. The detailed assignment correlates to activity in alcohol oxidation through similar volcano plots and leads to practical suggestions for catalyst development.