Hematites Precipitated in Alkaline Precursors: Comparison of Structural and Textural Properties for Methane Oxidation

Abstract

Hematite (α-Fe2O3) catalysts prepared using the precipitation methods was found to be highly effective, and therefore, it was studied with methane (CH4), showing an excellent stable performance below 500 °C. This study investigates hematite nanoparticles (NPs) obtained by precipitation in water from the precursor of ferric chloride hexahydrate using precipitating agents NaOH or NH4OH at maintained pH 11 and calcined up to 500 °C for the catalytic oxidation of low concentrations of CH4 (5% by volume in air) at 500 °C to compare their structural state in a CH4 reducing environment. The conversion (%) of CH4 values decreasing with time was discussed according to the course of different transformation of goethite and hydrohematites NPs precursors to magnetite and the structural state of the calcined hydrohematites. The phase composition, the size and morphology of nanocrystallites, thermal transformation of precipitates and the specific surface area of the NPs were characterized in detail by X-ray powder diffraction, transmission electron microscopy, infrared spectroscopy, thermal TG/DTA analysis and nitrogen physisorption measurements. The results support the finding that after goethite dehydration, transformation to hydrohematite due to structurally incorporated water and vacancies is different from hydrohematite α-Fe2O3. The surface area SBET of Fe2O3_NH-70 precipitate composed of protohematite was larger by about 53 m2/g in comparison with Fe2O3_Na-70 precipitate composed of goethite. The oxidation of methane was positively influenced by the hydrohematites of the smaller particle size and the largest lattice volume containing structurally incorporated water and vacancies.

Photocatalytic and Cathode Active Abilities of Ni-Substituted α-FeOOH Nanoparticles

“…Thermogravimetric (TG) and differential thermal analysis (DTA) curves for Ni x Fe 1−x OOH NPs with ‘x’ 0, 0.10, 0.20, 0.30, 0.40, and 0.50 are shown in Figure 2 a,b, respectively. The weight loss at temperatures up to approximately 140 °C corresponds to the release of H 2 O molecules adsorbed on the surface of Ni x Fe 1−x OOH NPs crystals or trapped within the interstitial spaces between them [ 19 , 20 ]. This low-temperature weight loss increased in the Ni x Fe 1−x OOH NPs samples with the increased Ni content, which can be explained by more adsorbed water in the NiFeOOH NPs samples with a large surface area [ 20 ].…”

“…The weight loss at temperatures up to approximately 140 °C corresponds to the release of H 2 O molecules adsorbed on the surface of Ni x Fe 1−x OOH NPs crystals or trapped within the interstitial spaces between them [ 19 , 20 ]. This low-temperature weight loss increased in the Ni x Fe 1−x OOH NPs samples with the increased Ni content, which can be explained by more adsorbed water in the NiFeOOH NPs samples with a large surface area [ 20 ]. Furthermore, different molar fractions of incorporated Ni 2+ ions and different crystal facets in Ni-doped Ni x Fe 1−x OOH NPs crystals can also affect the amount of adsorbed water in Ni x Fe 1−x OOH NPs samples.…”

“…Furthermore, different molar fractions of incorporated Ni 2+ ions and different crystal facets in Ni-doped Ni x Fe 1−x OOH NPs crystals can also affect the amount of adsorbed water in Ni x Fe 1−x OOH NPs samples. The weight loss at temperatures between approximately 150 and 290 °C can be attributed to the dehydroxylation of the surface hydroxyl groups [ 18 , 19 , 20 ]. This weight loss also increases in the Ni x Fe 1−x OOH NPs with a higher Ni content due to the larger surface area.…”

“…The calculated average size of α-Ni x Fe 1−x OOH NPs by the Scherrer equation from the FWHM of the (110) diffraction line is summarized in Table 1. [19,20]. This low-temperature weight loss increased in the Ni x Fe 1−x OOH NPs samples with the increased Ni content, which can be explained by more adsorbed water in the NiFeOOH NPs samples with a large surface area [20].…”

“…[19,20]. This low-temperature weight loss increased in the Ni x Fe 1−x OOH NPs samples with the increased Ni content, which can be explained by more adsorbed water in the NiFeOOH NPs samples with a large surface area [20]. Furthermore, different molar fractions of incorporated Ni 2+ ions and different crystal facets in Ni-doped Ni x Fe 1−x OOH NPs crystals can also affect the amount of adsorbed water in Ni x Fe 1−x OOH NPs samples.…”