Photocatalytic hydrogen production using FeTiO3 concentrates modified by high energy ball milling and the presence of Mg precursors

Ilmenite (FeTiO3) concentrates were modified by high energy ball milling (HEBM) at different times (1, 2, and 3 h) and in the presence of MgO or metallic Mg at different contents (0.5, 1.0, and 3.0 wt%), with the aim of obtaining a low-cost and highly available photocatalyst with enhanced performance towards H-2 production. FeTiO3 concentrates were obtained from ilmenite-rich black sand by gravimetric concentration followed by wet electromagnetic and dry magnetic separation. HEBM was performed to insert Mg into FeTiO3 structure and to simultaneously reduce the particle size. The insertion of Mg intended to shift the conduction band of the materials to more negative potentials, in spite of the band-gap widening. XRD patterns of the milled samples showed a decrease in the intensity of ilmenite peaks without any displacement, indicating slight amorphization without significant changes in the crystalline structure. However, the increase in the intensity of hematite peaks suggests substitutional doping according to the proposed solid-state reactions. Although thermodynamic analyses showed that doping with metallic Mg should be more favorable, doping with MgO appears to be kinetically favored due to the physical characteristics of the precursor, as it was revealed by XRD, XPS, Raman spectroscopy, SEM-EDS, and UV-Vis DRS results. Deconvolution of high-resolution XPS spectra corresponding to Mg1s exhibited the presence of an additional component peak for the sample milled with metallic Mg, in comparison to that with MgO. This result evidenced the formation of a new phase, suggesting that part of the metallic Mg was not inserted into FeTiO3, as it was confirmed by Raman spectroscopy. UV-Vis DRS analyses showed an increase in the band-gap of the samples milled in the presence of Mg precursors (from 2.51 to 2.55-2.59 eV) which were attributed to slight modifications of the conduction band due to the insertion of Mg. The performance towards hydrogen production under UV irradiation was improved from 240.5 mu mol g(-1) h(-1) (unmilled sample) to 255.3 mu mol g(-1) h(-1) (2 h milled in the absence of Mg precursor), 296.0 mu mol g(-1) h(-1) (2 h milled with 1.0 wt% MgO), and 265.2 mu mol g(-1) h(-1) (2 h milled with 1.0 wt% metallic Mg). This improvement was attributed mainly to the insertion of Mg, and the consequent modification of the band structure, rather than the modification in the surface area.