The Role of Titanium Dioxide (TiO₂) in the Production of Perchlorate (ClO₄^–) from Chlorite (ClO₂^–) and Chlorate (ClO₃^–) on Earth and Mars

Abstract

The widespread presence of perchlorate (ClO 4 − ) on Mars has significant implications for the alteration or destruction of indigenous organic compounds that may have been or still be present on Mars. The intermediary products of the UV-driven production of ClO 4 − include oxychlorines (ClO x ) such as chlorite (ClO 2 − ), chlorate (ClO 3 − ), and chlorine-dioxide (ClO 2 ) gas. The objective of this study was to start with ClO 2 − or ClO 3 − under Mars ambient and vary temperature, humidity, and UV wavelengths in order to isolate the reaction pathways leading to ClO 4 − . We also investigated the role of titanium dioxide (TiO 2 ) as a catalyst for these reactions. We show here that the production of ClO 4 − from ClO 2 − and ClO 3 − proceeds through different pathways. The ClO 2 − is rapidly converted to stable levels of ClO 3 − and Cl − , suggesting that the amount present on Mars will likely be very low compared to other ClO x . We also observed that temperature does not affect ClO 4 − production when starting with NaClO 2 but causes a decrease in ClO 4− production when starting with NaClO 3 , and production of ClO 4 − using UV > 300 nm with O 2 present does not involve an ozone (O 3 ) pathway. We have also shown that adding TiO 2 to the SiO 2 /ClO x mixture has a catalytic effect in the production of ClO 4 − under terrestrial conditions but shows primarily a shielding effect under Mars ambient; ClO 3 − is stable under Mars ambient even in the presence of TiO 2 and is not affected by temperature or humidity. Finally, it was shown that water is necessary for generation of ClO 2 (g) during perchlorate production from either NaClO 2 or NaClO 3 . This suggests that production of ClO 2 might be occurring on Mars in areas where ice can provide increased humidity levels.

Stratospheric Gas‐Phase Production Alone Cannot Explain Observations of Atmospheric Perchlorate on Earth

“…Laboratory experiments demonstrated that perchlorate production is possible via aqueous‐phase oxidation of Cl x O y species or gas‐solid‐plasma interactions (Carrier & Kounaves, 2015; Dasgupta et al., 2005; Estrada et al., 2021; Jackson et al., 2018; Kang et al., 2006, 2008, 2009; D. Liu & Kounaves, 2019; Rao, Mohan, et al., 2012; Wu et al., 2018). Some of these reactions could potentially occur on aerosols, in clouds, and/or land surfaces.…”

“…Oxygen isotopes suggest that atmospheric ozone (O 3 ) is involved in the formation of perchlorate sampled from the Atacama desert of Chile and Death Valley in the US (Bao & Gu, 2004; Jackson et al., 2010). Laboratory experiments have studied various perchlorate‐production mechanisms, such as aqueous reaction of oxychlorine (Cl x O y ) in the presence of ozone and/or UV light (Dasgupta et al., 2005; Estrada et al., 2021; Kang et al., 2006, 2008, 2009), solid‐gas interactions between chloride and ozone (Estrada et al., 2021; Jackson et al., 2018; Kang et al., 2008), photo‐catalyzed electrochemistry (Carrier & Kounaves, 2015; D. Liu & Kounaves, 2019), and plasma chemistry during electric discharge (Dasgupta et al., 2005; Rao, Mohan, et al., 2012; Wu et al., 2018). However, the contribution of these mechanisms to the occurrence of non‐synthetic perchlorate on Earth remains elusive.…”

Degradation of Amino Acids on Mars by UV Irradiation in the Presence of Chloride and Oxychlorine Salts

“…A 0.5 g sample was placed in each vial during the experiment with a resulting sample layer thickness of ~ 1 mm. The samples were configured as previously described (Liu and Kounaves, 2019).…”

“…During this process several oxychlorine intermediates such as hypochlorite (ClO -), chlorite (ClO 2 -), chlorate (ClO 3 -), and chlorine dioxide (ClO 2 ) gas, as well as radicals such as • OCl, • Cl, O 2and • OH are also likely generated (Catling et al, 2010). Recently, it was confirmed that the photochemical perchlorate formation pathway can exist without ozone (O 3 ) and that a highly oxidizing oxychlorine gaseous intermediate, most likely ClO 2 gas, was present in this pathway (Liu and Kounaves, 2019;Jain et al, 2017).…”

Trace Perchlorate Quantification in Small-Volume, High-Chloride Solutions: A Method Supporting Experimental Investigation of Perchlorate Generation on Mars

“…Efforts to explain the presence of ClO 4 – on Mars include hypotheses involving atmospheric reactions between oxidants such as ozone and volatile chlorine (Cl) species , or gas–solid phase reactions involving ozone and Cl-bearing minerals. − As an alternative to these hypotheses, two studies , have demonstrated that ClO 4 – and other oxychlorines such as chlorate (ClO 3 – ) and chlorite (ClO 2 – ) can be photochemically produced and degraded in the presence of illuminated semiconducting minerals (e.g., anatase and rutile) under Earth- and Mars-like conditions.…”