Spin Selection Rule in Single-Site Catalysis of Molecular Oxygen Adsorption on Transition-Metal Phthalocyanines

Development of nonprecious catalysts for O 2 reduction reactions based on N4 macrocyclic transition-metal (TM) complexes demands a comprehensive understanding of the microscopic nature of O 2 interaction with the central TM atoms. Yet to date, the underlying mechanism that dominates the adsorption ability and thus the reactivity of O 2 remains elusive. Here we systematically investigate the adsorption strengths of O 2 on the central TM atoms in monolayer and bilayer films of cobalt and iron phthalocyanine molecules supported on the Au(111) surface, using low-temperature scanning tunneling microscopy in combination with first-principles calculations. Comparing the experimental results with the theoretic calculations, we find that spin conservation is a prerequisite in ensuring effective adsorption of O 2 on the central TM atoms, in addition to the expectation of the d-band center theory based on the charge degree of freedom of the electrons. We expect this concerted principle of spin and charge to be broadly applicable to other chemical processes as well.