Evaluation of the O2 and pH Effects on Probes for Surface Bound Hydroxyl Radicals

Abstract

Interfacial reactions between H2O2 and metal oxides are important in several fields but are yet not fully understood. Recently, tris(hydroxymethyl)aminomethane (Tris) was used as a probe in detecting the intermediate hydroxyl radical (HO•) during such process via the formation of formaldehyde (CH2O). In this work, we evaluate two probes (methanol and Tris) for detection of surface bound HO• by investigating the O2 and pH effects on the production of formaldehyde. Moreover, we also examine the pH effect on the production of formaldehyde from Tris in the catalytic decomposition of H2O2 on ZrO2. The influence of O2 and pH on the yield of formaldehyde under homogeneous conditions was studied via γ-radiolysis of water. The solution was either deoxygenated or saturated with gas containing 20% O2, and the pH was ranging from 7.0 to 9.0. In the γ-radiolysis experiment, O2 shows a strong impact on the yield of formaldehyde: 14–68% for methanol and 16–29% for Tris. However, during the catalytic decomposition of H2O2, O2 only enables a 30% enhancement of the production of CH2O when using Tris as the scavenger. While for methanol, the O2 effect is almost negligible, and the production of CH2O from Tris is much higher than that from methanol. For practical reasons, only Tris was studied when evaluating the pH effect. A significant increase in the production of formaldehyde is observed by increasing pH during γ-radiolysis of water while an even more pronounced pH-dependent increase is observed in the catalytic decomposition of H2O2 on ZrO2. The former indicates that the scavenging yield is base-catalyzed while the latter indicates that the formation of HO• is also base-catalyzed. On the basis of the observed effects of O2 and pH, we propose a mechanism for the production of formaldehyde from Tris. The mechanism accounts for the observed impacts of O2 and pH on the yield of formaldehyde.