*2.2.3.1. Supported noble metal catalyst*

Various noble metals (Ru, Pt, Rh, Ir, and Pd) and some metal oxides (Cu, Mn, Co, Cr, V, Ti, Bi, and Zn) have traditionally been used as heterogeneous catalysts in CWAO. Several studies have ranked catalysts according to their activity. Imamura and his colleagues ranked noble metal and metal oxide catalysts according to their total organic carbon conversion achieved in 1 h, during the oxidation of polyethylene glycol at 200°C and pH of 5.4 [57]. They found the following order: Ru = Rh = Pt > Ir > Pd > MnO.
