NO Decomposition on Singly and Doubly (K and Li)-Doped CoO Nanocubes─Establishing Key Factors Governing Redox Behavior of Catalysts.

The intimate mechanism of NO decomposition on bare and redox-tuned CoO nanocubes (achieved by single (Li or K) and double (Li and K) doping) was elucidated. The catalysts synthesized by the hydrothermal method were characterized by X-ray electron absorption fine structure measurements, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and Kelvin Probe techniques. TPSR and steady-state isothermal catalytic tests reveal that the NO turnover frequencies are critically sensitive to the work function of the catalysts, adjusted purposely by doping. For the catalysts obtained by one-pot hydrothermal synthesis, lithiation of the CoO nanocubes leads to the formation of {Li', Co·} species, decreasing steadily the work function and the activity, while for the catalysts prepared by postsynthesis impregnation, formation of {Li', Co' Co··} species leads to a volcano-type dependence of the catalytic activity and the work function in parallel. The beneficial effect of potassium was discussed in terms of mitigation of surface potential buildup due to the accumulation of ionosorbed oxygen intermediates (surface electrostatics), which hinders the interfacial electron transfer. Analysis of the catalytic activity response to the redox tuning of CoO, substantiated by DFT calculations, allowed for a straightforward conceptualization of the redox nature of the NO decomposition in terms of the lineup of frontier orbitals of the NO/NO and O/O reactants with the surface DOS structure and the resultant molecular orbital interactions. The positions of the virtual bonding 3π(NO)-α-3d and the occupied 2π(O)-α-3d states relative to the Fermi energy level play a crucial role in the regulation of the forward and backward interfacial electron transfer events, which drive the redox process.