We report the performance
of Co(II) and Cu(II) coordination complexes
on H2O2 activation. The heterogeneous catalysts
containing aliphatic amine, N-heterocycle, and/or
carboxylic acid ligands in hydrogel materials coordinated with Co(II)
or Cu(II) were used in this study. These complexes were characterized
by solid-state NMR, X-ray photoelectron spectroscopy (XPS), and X-ray
fluorescence techniques in order to quantify the superficial and bulk
metal ion centers together with the aim of elucidating the ligands
involved in the uptake of Co and Cu ions. The release of free radicals
on H2O2 activation and the identity of reactive
oxygen species were studied by spin trapping using DMPO in electron
spin resonance (ESR) experiments. The Co(II) complex/H2O2 systems produced O2, anion superoxide (O2
•–), and hydroxyl radical (OH•), which diffused into the solution at the time that
a decrease in pH was detected. A possible catalytic mechanism would
involve the Co(II)/Co(III) redox couple, according to XPS results.
In the same way, the Cu(II) complex/H2O2 systems
produced O2 and OH•, with evidence of
Cu(II)/Cu(I) redox cycle. For these catalytic systems, there was no
direct evidence of intermediary reactive species. The identity of
the ligands played a crucial role in the efficiency of catalytic activation.
In addition, in the absence of H2O2, the dissolved
O2 was activated by most of the complexes tested, releasing
only OH•.