Revealing *OOH key intermediates and regulating H 2 O 2 photoactivation by surface relaxation of Fenton-like catalysts.

Hydrogen peroxide (H 2 O 2 ) molecules play important roles in many green chemical reactions. However, the high activation energy limits their application efficiency, and there is still huge controversy about the activation path of H 2 O 2 molecules over the presence of *OOH intermediates. Here, we confirmed the formation of the key species *OOH in the heterogeneous system, via in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), isotope labeling, and theoretical calculation. In addition, we found that compared with *H 2 O 2 , *OOH was more conducive to the charge transfer behavior with the catalyst and the activation of an O-O bond. Furthermore, we proposed to improve the local coordination structure and electronic density of the YFeO 3 catalyst by regulating the surface relaxation with Ti modification so as to reduce the activation barrier of H 2 O 2 and to improve the production efficiency of •OH. As a result, the kinetics rates of the Fenton-like (photo-Fenton) reaction had been significantly increased several times. The •OH free radical activity mechanism and molecular transformation pathways of 4-chloro phenol (4-CP) were also revealed. This may provide a clearer vision for the further study of H 2 O 2 activation and suggest a means of designing catalysts for efficient H 2 O 2 activation.