Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 237

7. RATE LIMITING STEP
A zero order reaction rate with respect to dioxygen concentration suggests
that the rate limiting step is Au(III) reduction by CEES, not reoxidation of
Au(I) by dioxygen. For futher proof of the rate limiting step, we determined
whether Au(I) and/or Au(III) was the dominant redox state of Au during the
main reaction. This was assessed by measuring the absorbance for Au(III)
after the induction period as function of CEES and concentrations, while
all other reactants and conditions were kept constant. The electronic
absorption spectrum (350 - 450 nm) of Au-complexes during the main
reaction is very similar to that of conventional isolable square-planar Au(III)
complexes for is colorless
After the induction period, the
observed spectra and their intensity remained unchanged and were
independent of both reactants ([CEES] from 0.05-0.37 M and from 50-
100%) within experimental error. Thus, the aggregate Au species during
catalytic CEESO formation was almost entirely in the form of Au(III).
Based on the two definitive results described above, it became obvious that
the reduction of Au(III) species and not the oxidation of Au(I) species is rate
limiting in these aerobic oxidation reactions.