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Surface and Bulk Thermal Dehydroxylation of FeOOH Polymorphs
Xiaowei Song1, Jean-François Boily2
1Department of Molecular Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft , Faradayweg 4-6 D-14195 Berlin, Germany.
The Journal of Physical Chemistry. A
|July 19, 2016
Summary
This study investigated the thermal decomposition of iron oxyhydroxides (FeOOH) to iron oxides. Surface interactions and HCl pre-treatment influenced dehydroxylation temperatures, revealing insights into FeOOH behavior under thermal stress.
Area of Science:
- Materials Science
- Geochemistry
- Mineralogy
Background:
- Iron oxyhydroxides (FeOOH) are common minerals with diverse polymorphs (e.g., akaganéite, lepidocrocite, goethite).
- Understanding their thermal decomposition is crucial for applications in catalysis, environmental remediation, and geochemistry.
- Bulk and surface dehydroxylation processes can differ significantly, impacting material transformation pathways.
Purpose of the Study:
- To investigate the bulk and surface thermal decomposition mechanisms of synthetic akaganéite, lepidocrocite, and goethite.
- To determine the influence of HCl pre-treatment on the dehydroxylation temperatures of these FeOOH polymorphs.
- To elucidate the role of surface hydroxyl groups and coordination environments in thermal decomposition.
Main Methods:
- Utilized temperature-programmed desorption (TPD) coupled with Fourier transform infrared (FTIR) spectroscopy and quadrupole mass spectrometry.
- Analyzed submicron-sized particles of akaganéite (β-FeOOH), rod- and lath-shaped lepidocrocite (γ-FeOOH), and goethite (α-FeOOH).
- Heated samples in a vacuum between 30-400 °C, monitoring OH vibrational modes and H2O(g) release.
Main Results:
- Dehydroxylation temperatures varied among FeOOH polymorphs, with lath lepidocrocite and akaganéite dehydroxylating at lower temperatures than rod lepidocrocite and goethite.
- HCl pre-equilibration increased dehydroxylation temperatures for most minerals, attributed to dissolution and strengthened hydrogen bonding in akaganéite.
- Surface singly-coordinated hydroxyl groups released H2O(g) below 125 °C, while doubly- and triply-coordinated groups dehydroxylated congruently with bulk OH groups.
Conclusions:
- Differentiated bulk and surface thermal decomposition pathways for FeOOH polymorphs.
- Demonstrated the significant impact of surface chemistry and pre-treatment on thermal stability.
- Provided critical insights into the behavior of iron oxyhydroxides under thermal gradients relevant to natural and technological environments.
