Confirming the Formation of Hydroxyl Radicals in the Catalytic Decomposition of H2O2 on Metal Oxides Using Coumarin as a Probe

Hydrogen peroxide can be catalytically decomposed to O-2 and H2O on metal oxide surfaces in contact with aqueous solutions containing H2O2. The initial step in this process has been proposed to be the formation of surface-bound hydroxyl radicals which has recently been verified using tris as a radical scavenger. Here, we make use of the unique fluorescent product 7-hydroxycoumarin formed in the reaction between hydroxyl radicals and coumarin to probe the formation of surface-bound hydroxyl radicals. The experiments clearly show that 7-hydroxycoumarin is formed upon catalytic decomposition of H2O2 in aqueous suspensions containing ZrO2-particles and coumarin, thereby confirming the formation of surface-bound hydroxyl radicals in this process. The results are quantitatively compared to results on the same system using tris as a probe for hydroxyl radicals. The effects of the two probes on the system under study are compared and it is concluded that coumarin has a significantly lower impact on the system.