Thermal and non-thermal processes involving water on Apollo lunar samples and metal oxide powders
Poston, Michael Joseph
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Water is of interest for understanding the formation history and habitability of past and present solar system environments. It also has potential as a resource - when split to its constituent oxygen and hydrogen - both in space and on the Earth. Determining the sources, evolution, and eventual fate of water on bodies easily reachable from Earth, especially Earth's moon, is thus of high scientific and exploration value to the private sector and government space agencies. Understanding how to efficiently split water with solar energy has potential to launch a hydrogen economy here on Earth and to power spacecraft more sustainably to far away destinations. To address the fundamental interactions of water with important surfaces relevant to space exploration and technology development, temperature programmed desorption (TPD) and water photolysis experiments under well controlled adsorbate coverages have been carried out and are described in detail in this thesis. TPD experiments under ultra-high vacuum (UHV) conditions were conducted on lunar surrogate materials and genuine lunar samples brought to Earth by the Apollo program. The TPD's were conducted to determine the desorption activation energies of water chemisorbed directly to the powder surfaces, knowledge of which can improve existing models of water evolution on Earth's moon and aid in interpreting data collected by spacecraft-based investigations at the Moon. The TPD experiments of molecular water interacting with two lunar surrogates (micronized JSC-1A and albite) in ultra-high vacuum revealed water desorption during initial heating to 750 K under ultra-high vacuum. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) indicated possible water formation during the initial heating via recombinative desorption of native hydroxyls above 425 ± 25K. Dissociative chemisorption of water (i.e., formation of surface hydroxyl sites) was not observed on laboratory time scales after controlled dosing of samples (initially heated above 750 K) with 0.2 - 500 L exposures of water. However, pre-heated samples of both types of surrogates were found to have a distribution of molecular water chemisorption sites, with albite having at least twice as many as the JSC-1A samples by mass. A fit to the TPD data yields a distribution function of desorption activation energies ranging from ~0.45 eV to 1.2 eV. Using the fitted distribution function as an initial condition, the TPD process was simulated on the timescale of a lunation. A preview of these results and their context was published in Icarus (2011) 213, 64, doi: 10.1016/j.icarus.2011.02.015 by lead author Charles Hibbitts and the full treatment of the results from the TPD on lunar surrogates (presented here in Chapter 2) has been published in the Journal of Geophysical Research – Planets (2013) 118, 105, doi: 10.1002/jgre.20025 by lead author Michael J Poston. The desorption activation energies for water molecules chemisorbed to Apollo lunar samples 72501 and 12001 were determined by temperature programmed desorption (TPD) experiments in ultra-high vacuum. A significant difference in both the energies and abundance of chemisorption sites was observed, with 72501 retaining up to 40 times more water (by mass) and with much stronger interactions, possibly approaching 1.5 eV. The dramatic difference between the samples may be due to differences in mineralogy, surface exposure age, and contamination of sample 12001 with oxygen and water vapor before it arrived at the lunar sample storage facility. The distribution function of water desorption activation energies for sample 72501 was used as an initial condition to mathematically simulate a TPD experiment with the temperature program matching the lunar day. The full treatment of the TPD results from these two lunar samples (presented here in Chapter 3) has been submitted with the title "Water chemisorption interactions with Apollo lunar samples 72501 and 12001 by ultra-high vacuum temperature programmed desorption experiments" to Icarus for publication in the special issue on lunar volatiles by lead author Michael J Poston. A new ultra-high vacuum system (described in Chapter 4) was designed and constructed for planned experiments examining the possible formation of hydrated species, including water, from interaction of solar wind hydrogen with oxygen in the lunar regolith and to examine the effects of the active radiation environment on water adsorption and desorption behavior on lunar materials. This system has been designed in close collaboration with Dr. Chris J Bennett. An examination of a unique system for water photolysis - zirconia nanoparticles for hydrogen production from water with ultra-violet photons - was performed to better understand the mechanism and efficiency of water splitting on this catalyst. Specifically, formation of H₂ from photolysis of water adsorbed on zirconia (ZrO₂) nanoparticles using 254 nm (4.9 eV) and 185 nm (6.7 eV) photon irradiation was examined. The H₂ yield was approximately an order of magnitude higher using monoclinic versus cubic phase nanoparticles. For monoclinic particles containing 2 monolayers (ML) of water, the maximum H₂ production rate was ~0.4 µmole hr⁻¹ m⁻² using 185 + 254 nm excitation and a factor of 10 lower using only 254 nm. UV reflectance reveals that monoclinic nanoparticles contain fewer defects than cubic nanoparticles. A H₂O coverage dependence study of the H₂ yield is best fit by a sum of interactions involving at least two types of adsorbate-surface complexes. The first dominates up to ~0.06 ML and is attributed to H₂O chemisorbed at surface defect sites. The second dominates at coverages up to a bilayer. H₂ formation is maximum within this bilayer and likely results from efficient energy transfer from the particle to the interface. Energy transfer is more efficient for the monoclinic ZrO₂ nanoparticles and likely involves mobile excitons. These results (presented in Chapter 5) have been submitted with the title "UV Photon-Induced Water Decomposition on Zirconia Nanoparticles" for publication in the Journal of Physical Chemistry C by lead author Michael J Poston. This paper has been reviewed and will be accepted after minor modification.