Since that the standard BE values of metallic Cu0 and Cu+ are
almost the same as each other, the metallic Cu0 and Cu+ species
cannot be distinguished directly according to their BE values. In
order to ascertain the chemical states of the copper species present
in the reduced catalysts, the Wagner plots are drawn and shown in
Fig. 2f. The kinetic energy (KE) of the Auger transition and the Cu
2p3/2 binding energy of the photoemission were on Y and X axes,
respectively. The data for Cu, Cu2O, and CuO reference samples
were taken from Refs. [24,25]. Fig. 2f shows that the Auger parameters
of the reduced catalysts fell on the line of metallic copper,
certifying that the copper species present in the reduced catalysts
were metallic Cu0, which was consistent with the conclusion
obtained by XRD analysis