Surface and Bulk Thermal Dehydroxylation of FeOOH Polymorphs
2016 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 120, no 31, 6249-6257 p.Article in journal (Refereed) Published
In this study, bulk and surface thermal decomposition of synthetic iron oxyhydroxides to iron oxides was followed using the temperature programmed desorption (TPD) technique. Submicron-sized akaganeite beta-FeOOH), rod- and lath-shaped lepidocrocite (gamma-FeOOH), and goethite (alpha-FeOOH) particles were heated in vacuo in the 30-400 degrees C range, and their OH vibrational modes were monitored by Fourier transform infrared (FTIR) spectroscopy while H2O(g) release was monitored by quadrupole mass spectrometry. Peak thermal dehydroxylation temperatures were larger in the order of lath lepidocrocite (200 degrees C) < akaganeite (200/260 degrees C) < rod lepidocrocite (268 degrees C) < goethite (293 degrees C). Pre-equilibration of these particles to aqueous solutions of HCl increased dehydroxylation temperatures of all minerals except goethite by 13-40 degrees C. These shifts were explained by (1) the dissolution of particles or regions of particles of lower degree of crystallinity by HCl, as well as (2) the strengthening of the hydrogen bond environment in the akaganeite bulk. The latter is a means of facilitating H2O(g) formation via interactions between two adjacent OH groups. Strongly analogous forms of interactions at the FeOOH particle surfaces were also shown to facilitate the release of singly coordinated (-OH) hydroxo groups to the gas phase at temperatures lower than 125 degrees C, thus creating OH vacancies that may be actively involved in the transfer of bulk to surface OH groups during thermal dehydroxylation. Doubly- (mu-OH) and triply- (mu(3)-OH) coordinated hydroxo groups were however resilient to exchange under those conditions, and their dehydroxylation was strongly congruent with that of bulk OH groups. By resolving the bulk and surface thermal decomposition of FeOOH polymorphs, this work provides clearer insight into the fate of these materials in natural and technological settings where important thermal gradients are commonplace.
Place, publisher, year, edition, pages
2016. Vol. 120, no 31, 6249-6257 p.
IdentifiersURN: urn:nbn:se:umu:diva-126369DOI: 10.1021/acs.jpca.6b04294ISI: 000381452200015PubMedID: 27426101OAI: oai:DiVA.org:umu-126369DiVA: diva2:1034465