Surface Reactions of H2O2, H2, and O2 in Aqueous Systems Containing ZrO2

In radiolysis of water, three molecular products are formed (H2O2, O-2, and H-2). It has previously been shown that aqueous hydrogen peroxide is catalytically decomposed on many oxide surfaces and that the decomposition proceeds via the formation of surface-bound hydroxyl radicals. In this work, we have investigated the behavior of aqueous H-2 and O-2 in contact with ZrO2. Experiments were carried out in an autoclave with high H2 pressure and low O-2 pressure (40 and 0.2 bar, respectively). In the experiments the concentration of H-abstracting radicals was monitored as a function of time using tris(hydroxymethyl)aminomethane (Tris) as scavenger and the subsequent formation of formaldehyde to probe radical formation. The plausible formation of H2O2 was also monitored in the experiments. In addition, density functional theory (employing the hybrid PBE0 functional) was used to search for reaction pathways. The results from the,experiments show that hydrogen-abstracting radicals: are formed in the aqueous H2O2-system in contact with solid ZrO2. Formation of H2O2 is also detected, and the time dependent production of hydrogen-abstracting radicals follows the time-dependent H2O2 concentration, strongly:indicating that the radicals are produced upon catalytic decomposition of H2O2. The DFT study implies that H2O2 formation proceeds via a pathway where HO2 is a key intermediate. It is interesting to note that all the stable molecular products from aqueous radiolysis are precursors of quite intriguing radical reactions at water/oxide interfaces.