title: Chromium Reaction Mechanisms for Speciation using Synchrotron in-Situ High-Temperature X-ray Diffraction.
abstract: We use in situ high-temperature X-ray diffraction (HT-XRD), ex-situ XRD and synchrotron X-ray absorption near edge structure spectroscopy (XANES) to derive fundamental insights into mechanisms of chromium oxidation during combustion of solid fuels. To mimic the real combustion environment, mixtures of pure eskolaite (Cr(3+)2O3), lime (CaO) and/or kaolinite [Al2Si2O5(OH)4] have been annealed at 600-1200 °C in air versus 1% O2 diluted by N2. Our results confirm for the first time that (1) the optimum temperature for Cr(6+) formation is 800 °C for the coexistence of lime and eskolaite; (2) upon addition of kaolinite into oxide mixture, the temperature required to produce chromatite shifts to 1000 °C with a remarkable reduction in the fraction of Cr(6+). Beyond 1000 °C, transient phases are formed that bear Cr in intermediate valence states, which convert to different species other than Cr(6+) in the cooling stage; (3) of significance to Cr mobility from the waste products generated by combustion, chromatite formed at >1000 °C has a glassy disposition that prevents its water-based leaching; and (4) Increasing temperature facilitates the migration of eskolaite particles into bulk lime and enhances the extent to which Cr(3+) is oxidized, thereby completing the oxidation of Cr(3+) to Cr(6+) within 10 min.
authors.affiliation: †Department of Chemical Engineering, Monash University, GPO Box 36, Clayton, Victoria 3800, Australia.
authors.forename: Fiona
authors.initials: F
authors.lastname: Low
authors.affiliation: ‡Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia.
authors.forename: Justin
authors.initials: J
authors.lastname: Kimpton
authors.affiliation: §School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria 3800, Australia.
authors.forename: Sasha
authors.initials: S
authors.lastname: Wilson
authors.affiliation: †Department of Chemical Engineering, Monash University, GPO Box 36, Clayton, Victoria 3800, Australia.
authors.forename: Lian
authors.initials: L
authors.lastname: Zhang
mesh_terms: D000388:Air; D002857:Chromium / Q000737:chemistry*; D017608:Chromium Compounds / Q000737:chemistry; D005742:Gasoline; D006358:Hot Temperature; D010084:Oxidation-Reduction; D017356:Synchrotrons; D013997:Time Factors; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
chemical_list: D017608:Chromium Compounds; D005742:Gasoline; D002857:Chromium; C023600:chromic oxide