Peroxide-Modified Titanium Dioxide: a Chemical Analog of Putative Martian Soil Oxidants

Abstract

Hydrogen peroxide chemisorbed on titanium dioxide (peroxide-modified titanium dioxide) is investigated as a chemical analog to the putative soil oxidants responsible for the chemical reactivity seen in the Viking biology experiments. When peroxide-modified titanium dioxide (anatase) was exposed to a solution similar to the Viking labeled release (LR) experiment organic medium, CO2 gas was released into the sample cell headspace. Storage of these samples at 10 °C for 48 hr prior to exposure to organics resulted in a positive response while storage for 7 days did not. In the Viking LR experiment, storage of the Martian surface samples for 2 sols (∼49 hr) resulted in a positive response while storage for 141 sols essentially eliminated the initial rapid release of CO2. Heating the peroxide-modified titanium dioxide to 50 °C prior to exposure to organics resulted in a negative response. This is similar to, but not identical to, the Viking samples where heating to approximately 46 °C diminished the response by 54–80% and heating to 51.5 apparently eliminated the response. When exposed to water vapor, the peroxide-modified titanium dioxide samples release O2 in a manner similar to the release seen in the Viking gas exchange experiment (GEx). Reactivity is retained upon heating at 50 °C for three hours, distinguishing this active agent from the one responsible for the release of CO2 from aqueous organics. The release of CO2 by the peroxide-modified titanium dioxide is attributed to the decomposition of organics by outer-sphere peroxide complexes associated with surface hydroxyl groups, while the release of O2 upon humidification is attributed to more stable inner-sphere peroxide complexes associated with Ti4+ cations. Heating the peroxide-modified titanium dioxide to 145 °C inhibited the release of O2, while in the Viking experiments heating to this temperature diminished but did not eliminated the response. Although the thermal stability of the titanium-peroxide complexes in this work is lower than the stability seen in the Viking experiments, it is expected that similar types of complexes will form in titanium containing minerals other than anatase and the stability of these complexes will vary with surface hydroxylation and mineralogy.