Hydrothermic Reduction of Rutile-Ilmenite Mineral Producing an Oxyhydride η-Ti 2 FeO 0.2 H 2.8 : Towards In-Situ Hydrogen Production and Storage

: As an alternative to the physical storage of hydrogen as compressed gas or liquid hydrogen requiring high-pressure tanks and cryogenic temperatures, the material-based storage of hydrogen in solids involves hydrogen uptake and release from the surface of adsorbents or within interstitials of hydrides. We report a hydrothermic reduction of rutile-ilmenite mineral into hydrogen-rich fibrous products, η-Ti 2 FeO 0.2 H 2.8 , in an ethanol-water system at 120°C for 4 hrs. As part of a project to generate hydrogen from water-ethanol system using advanced catalysts containing graphene oxide (GO) as carbon source, a system of 62.5 μg graphene oxide per g of rutile-ilmenite mineral was employed in a concentration of 50 mg/mL of ethanol-water solution. As well as in the original mineral, XRD of thermal annealed mineral between 500 and 800°C showed no hydride or phase change in rutile-ilmenite. With hydrothermal treatment of GO/rutile-ilmenite (50 mg/mL) in ethanol-water (1:1 v/v) at 120°C, a hydrogen-rich ferrotitanium hydride phase was formed, and there was a change in morphology from plate-like and granular particles into fibrous structures. Like the release of hydrogen by its ‘carriers’ (e.g., CaH 2 , NH 4 BH 4 , NaBH 4 , NH 3 , formic acid), it is anticipated that hydrogen was generated from the ethanol-water system in-situ, which reduced the rutile-ilmenite mineral into a hydride. EDX results showed that the reduction affected specifically the oxides of Fe and aluminosilicates in the mineral. The study demonstrated a possibility of in-situ hydrogen generation and storage via low-temperature graphene oxide hydrothermic reduction of rutile-ilmenite mineral in an ethanol-water system.


INTRODUCTION
Several hydrogen storage materials were developed but are either unstable at the required temperature or tend to show low hydrogen carrying capacity.For example, ferrotitanium alloys are well-known hydrogen-storage materials (Ma et al., 2006).Whereas rutile is a stable form of titania that could be a product of anatase irreversible transformation at elevated temperatures (Hanaor and Sorrell, 2011).The hydrothermal reduction of ilmenite or its alloy in alkaline media has been reported in a few studies.Simpraditpan et al., (2013) observed the formation of nanofibers using 72-hr hydrothermal treatment of ilmenite ore in an alkaline solution of 10 M NaOH.Their XRD result showed a hydride of the mineral was formed after the hydrothermal treatment, which was identified as a layered titanate H2TixO2x+1 structure.There was a complete transformation into nanofibers morphology.
In contrast, a study of hydrothermal treatment of rutilequartz concentrates at 200°C for 1 hr in 5 M NaOH only obtained a partial transformation into fiber-like structures.XRD showed no hydride formation (Zanaveskin et al., 2014).While some studies attribute the formation of amorphous phases or nanofibers to the dissolution of impurities such as ilmenite, silica/quartz, and others (Simpraditpan et al., 2013;Zanaveskin et al., 2014), the selected area EDS in the work of Zanaveskin et al., (2014) indicates that the formed rod-like phases may not be due to ilmenite but to aluminosilicate impurities including some Ti and C in the mineral.Since there are other impurities such as MgO, CaO, ZnO and others in the samples (Simpraditpan et al., 2013), it therefore becomes complicated to identify the specific compound responsible for the reduction of ilmenite-rutile mineral into hydrides.
Reduction of ilmenite can occur in any favourable reducing medium.Taguchi et al., (2020) recently observed the formation of amorphous iron hydrides in aqueous solutions at ambient conditions.The two forms, nanowires and granular, undergo transformation to crystalline α-Fe by heat treatment at 600°C.The nanowire hydride exhibits a hydrogen content of 0.10 wt%, while the granular iron hydride of 0.22 wt%.In their study, an aqueous NaBH4 solution was added to the Fe2+ solution and no other chemical was involved.It therefore implies that the chemical hydrides such as NaBH4 can reduce the inorganic soluble species e.g., layered clusters into rod-like morphologies via hydrogenation.However, minerals are complicated for understanding of the specific reducing agents due to the presence of several potential reductants.Using carbon sources as reducing agent in place of hydrides, the reduction of ilmenite or ferrotitanium alloy in a hydrogen formation medium such as the ethanol-water system involving no alkali combines an in-situ hydrogen production, and hydrogen storage via metal oxides reduction.Though, there is a task of identifying whether the reduction is caused by carbon or by some hydrides or hydrogen formed in situ., such system is unique in creating a highly reducing environment as found in the formation of oxyhydrides (Kobayashi et al, 2012).
The carbothermal reduction of metal oxides into their metallic form has been known for decades (Karlsson et al., 2018).Carbon is a strong reducing agent.For example, the use of carbonaceous materials such as graphite and CH4 (C: ZnO molar ratios of 0 to 1) as reducing agents were employed to significantly lower the reduction temperature of ZnO and avoid recombination (Osinga et al, 2004;Wieckert and Steinfeld, 2002).Recent studies in this direction have used carbohydrate biomass such as glucose and cellulose as carbon source to cause the reduction of metal oxides and salts partly into their metallic forms under hydrothermal conditions of 250°C with and without NaOH (Zhou et al, 2022).Apart from their use as reducing agents for metal oxides, graphene oxide was shown to double the catalytic hydrogen production of ZnO/ZnS using sacrificial reagents that show a trend of efficiencies as follows: ethanol > methanol > isopropanol > ethylene glycol (Gultom et al, 2019).
In this study, the reduction of rutile-ilmenite mineral in an ethanol-water system at 120°C was found to transform the morphologies of the mineral into nanofibers which are hydrogen-rich hydrides that may find application as hydrogen storage materials.Based on EDX results before and after hydrothermal process, the reduction was seen to affect mainly the iron oxides and also aluminosilicates (or Al2O3 and SiO2) in the original mineral.The dominant effect of reduction on ilmenite can be implied from the hydrogen occupation of oxygen vacancies of iron oxides in the formed hydride, η-Ti2FeO0.2H2.8.

B. Carbo-hydrothermal Process
Graphene oxide nanomaterial was prepared from zinccarbon electrodes of used batteries using electrochemical exfoliation method and was purified to obtain the dried powder (Mohammed et al., 2023).A priori, size reduction and sieving of the rutile-ilmenite ore was carried out to obtain ≤50 μm and ≤100 μm particles and possible phase changes were investigated by its calcination at temperatures of 500, 600, 700, and 800°C.For the hydrothermal process, precisely 10 g of the ore (≤50 μm particles) was mixed with 100 mL of deionized water and 25 mL of graphene suspension (2 mg/mL).The mixture was sonicated for about 15 min, and the resulting solution was separated by centrifugation.The wet composite was dispersed in 200 mL of a deionized water/ethanol mixture (1:1 v/v), placed in the autoclave reactor and thermally treated at 120°C for 4 h.The precipitate was allowed to cool to room temperature.The resulting powdered mixture was filtered, washed with deionized water several times, and dried at 60°C for 2 h.

C. Characterizations
Graphene oxide powder was characterized by transmission electron microscopy (TEM), Energy dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD).Xray fluorescence (XRF) analysis of the rutile-ilmenite mineral was carried out.The mineral and its hydrothermal product were characterized using high-resolution scanning electron microscopy (HRSEM/EDX) and X-ray diffraction (XRD).The single crystal XRD patterns were acquired at room temperature (25°C) and scanning from 8° to 80° at a step of 0.034° using Cu Kα radiation wavelength of 1.5406 Å.

A. Structure and Composition of Graphene Oxide
Table 1 shows that the graphene oxide powder used in the hydrothermal process has EDX atomic % composition of C (87.52), O (11.7), Si (0.11) and Cl (0.66), and has C/O atoms ratio of 7.48.There were trace amounts of silicon and chlorine in the graphene oxide powder which were introduced from carbon electrodes and the synthesis procedure.As quartz should contain nearly fifty percent by weight each of Si and O, the relatively high 11.7 wt% of O in the EDX indicates that some of the as-prepared graphene sheets were oxidized.Quartz-based constituents may not be responsible for hydride formation because earlier studies of hydrothermal (200°C) treatment of rutile-quartz concentrate in NaOH solution had noticed no hydride formation (Zanaveskin et al., 2014).
The EDX spectrum in Figure 1(a) supports the elemental compositions provided in Table 1.In addition, the SEM in Figure 1(b) and the acquired TEM images in Figure 2 showed that the graphene oxide is multi-layered with several layers of nanosheets.The X-ray diffractogram in Figure 3 was also used to confirm the graphene nanosheet (200).It also showed the possible presence of graphite particles (002).Natural graphite/graphite flake possesses a (002) diffraction peak at 26.6°, i.e., d-interlayer spacing distance: ~0.334 nm.After the exfoliation process, the (002) peak gradually shifts from 26.6° to a smaller scattering angle, indicating the presence of graphene nanosheets (Hsieh and Hsueh, 2016).XRD analysis shows the structural changes of graphitic structure which is related to the interplanar expansion.The XRD spectrum of graphite has four intensive peaks.These peaks are at 12.9 o (001) plane, 26.correspond to ( 100) and ( 004) crystal planes indicate an increase in interlayer spacing in graphitic structures.These peaks can give ideas about the presence of graphene.Therefore, XRD analysis confirms the formation of graphene nanosheets.In the diffractogram, the graphitic (002) plane, and the two peaks at (100) and ( 004) planes are due to graphene.Other peaks are identified as quartz (SiO2).
As shown in Figure 5, the XRD of the original rutileilmenite mineral reveals that it contained both rutile and anatase phases of titania, including ilmenite and quartz phases.
Thermal treatments at 500, 600, 700 or 800°C transformed the anatase phases of the original ore into rutile phases.However, neither the original rutile-ilmenite ore nor its thermal annealed products in air furnace contain or form hydrides.As shown in Figures 6-8, XRD of the thermally treated rutile mineral revealed that it remained as rutile and ilmenite phases up to 800°C with no obvious influence of particle sizes on their thermal behaviour within 500-800°C temperature that was studied.The only peak differences can be attributed to variations in quartz contents of the samples.
The absence of anatase phases in the thermally treated minerals at 500°C, 600°C, 700°C, and 800°C, which shows that anatase was transformed into the rutile phases is supported by the fact that anatase transforms irreversibly to rutile at elevated temperatures (Hanaor and Sorrell, 2011).
Accordingly, this transformation did not occur during carbo-hydrothermal treatment in the ethanol-water system at the lower temperature of 120°C for 4 hrs.Anatase phases in the original mineral were retained in the reduced product that was obtained after the hydrothermal process.This is supported by the X-ray diffractograms shown in Figure 9 in the next section.

C. Carbo-hydrothermal Formation of FeTi2O0.2H2.8
The in-situ generation of hydrogen with simultaneous reduction of rutile-ilmenite ore into its ferrotitanium hydride, η-FeTi2O0.2H2.8 was confirmed using XRD, SEM and SEM-EDX.As revealed in the diffraction patterns of rutile-ilmenite mineral and its reduced form containing graphene oxide given in Figures 9(a) and 9(b), dominant peaks of anatase/rutile titania were obvious.One notes that the peak shift toward lower 2θ (degree) position i.e., higher d-spacing (Figure 9a), could be due to the increase in strain and lattice distortion by hydrogen incorporation into the ferrotitanium alloy.
In Figure 9(a), XRD patterns exhibited strong diffraction peaks at 32°, 36° and 54° indicating TiO2 in the rutile phase.All peaks are in good agreement with the standard spectrum (JCPDS no.: 88-1175 and 84-1286).In the XRD of both the original mineral, and reduced products of the hydrothermal process, characteristics peak due to the distinct phases of rutile (TiO2), ilmenite (FeTiO3), anatase (TiO2), and quartz-SiO2 (or carbon-graphite) are identifiable.XRD of the reduced product contains a new phase, the η-phase Ti2FeO0.2H2.8, in addition to the phases due to rutile, anatase, ilmenite and carbon/graphite which were present in the starting mineral.This is an oxyhydride of FeTi2Ox (ferropseudobrookite) which was not found in the initial mixture.Equivalent products i.e., hydride of Ti4Fe2Ox, η-phase hydride Zr4Fe2O0.6H5.4,and BaTiO3−xHx, possessing cubic Ti2Ni structure had been reported (Eklöf-Österberg et al, 2018;Fjellvåg et al, 2019;Stioui et al, 1981).The formation of this new, hydrogenated phase from FeTi2O3 confirms that hydridation occurred during the hydrothermal process.Though the actual crystal lattice structure of   Ti2FeO0.2H2.8 is not yet resolved, its strongest peak intensity was at a d-spacing of 2.28 Å, and its lattice constant of 9.12 Å is close to that found in FeTi2O3.
This structure is certainly different from that obtained by Yang et al., (2003).It has been shown in similar compounds that lattice structure is preserved on hydrogenation (Stioui et al, 1988).The oxygen-stabilized compound has 2.2 atomic % of oxygen, and it is believed that the iron atom is coordinating, surrounded by oxygen and hydrogen atoms (Aubertin et al, 1984).
In the ICDD database, the new oxyhydride was referenced to a material defined with the compound name, 'iron titanium oxide hydride' (JCPDS#00-036-1385).It was represented in the database by the chemical formula: Ti2FeO0.2H2.8 and empirical formula: FeH2.8O0.2Ti2.The compound's eta (η) phase-modified state could have regular platelet morphology

Ru.
Ru. Ru.Il.Long et al., (2009).In Figures 11(a-d) and 12(a-d), a comparison of the HRSEM images of the mineral with its hydrothermally reduced product as obtained in the present study shows that the only additional features present in the hydrothermal product were the ≤ 200 nm sized tubular and fibrous structures and some sheets.These are supposed to be the new oxyhydride phase which was also distinctly identified in the XRD pattern.

Il
Based on the reaction conditions involved, the X-ray diffraction, and the structural features in the HRSEM images, a few reaction schemes may be proposed in the formation of η-Ti2FeO0.2H2.8.One possible mechanism is the leaching of ilmenite (FeTiO3) in acidic media, which could take place around 80-150°C in the formation of rutile in autoclaves at atmospheric pressure (Habashi, 2016;Janssen et al, 2010).In this scheme, the reductive dissolution of ilmenite occurs when (organic) anions adsorbed at the surface weaken the Fe 3+ -O bonds, along the zones of weakness in the original ilmenite crystal or an intricate branching network of fractures in the ilmenite (Janssen et al, 2010;Schwertmann, 1991).Structural alteration begins at the crystal surface by initial enrichment in 18 O isotope.Fracturing may have been driven by volume changes associated with the dissolution of ilmenite and simultaneous reprecipitation of the product phases from an interfacial solution.The rutile inherits crystallographic information from the parent ilmenite, a highly porous product is formed, the original morphology of ilmenite is preserved, and significant depletion in Fe and relative enrichment of Ti could be observed (Janssen et al, 2010).The reduction of the highly insoluble iron (III) oxides into the soluble iron (II)

(d)
oxides could occur in the ethanol-water system, a weak acid medium.By this or another reducing system, Eqns.1(a, b), hydrogen is introduced in the structure.Oxyhydride formation by reduction: (1b) Another possible mechanism involves the formation of metal hydride i.e., TiH2 from the metal-steam reaction at the solid-vapor interface with exfoiled anatase TiO2, and/or reaction between anatase and ethanol, Eqns.(2a, b).The mechanism of exfoiled anatase from the bulk was proposed in the formation of titanate nanotubes (Nakahira et al, 2010).This is followed by the reaction between ilmenite (FeTiO3) and the metal hydride (TiH2) to form ferropseudobrrokite (FeTi2O3), Eqns.(3a, b).The formation of ferropseudobrookite from ilmenite was due to the inter-diffusion/interaction of (organic) anions with ilmenite-rutile-anatase system.Then, there occurs the reduction of FeTi2O3 with H2 from metal hydride into FeTi2O3-xHx in which Fe has an oxidation state of II or I (Kim et al, 2020), according to Eqns. (2a, b).
Metal hydride formation: Anatase-steam: Formation of metastable FeTi2O0.2 (an oxygen-deficient compound) Ferropseudobrookite formation: Ferropseudobrookite formation: (3b) The hydride FeTi2O0.2H2.8 has higher hydrogenation capacity compared to the deuteride Ti4Fe2OD2.25 which has its 16c site completely occupied by oxygen (Zavaliy et al, 2007).A hypothetical structure of the oxyhydride FeTi2O0.2H2.8 with Fe as the central coordinating atom would show a hydrogento-oxygen atomic ratio of 1: 12, which is equivalent to H/O ratio of 0.23: 2.77 as found in the empirical formula of the oxyhydride.
In addition, there may be carbon deposits due to reactions from ethanol or similar species.In Eqn.(2a), the volume changes caused by inter-diffusion/interaction of steam with the solid rutile-ilmenite-anatase system permitted the formation of TiH2, and then Eqn.(3a) ferropseudobrookite (FeTi2O3), which was then reduced into FeTi2O3-xHx.Ferropseudobrookite could be stable up to 1068°C, and its equilibrium with metallic iron always contains trivalent titanium (Simons and Woermann, 1978).The reduction in Eqn.(3a) can take place, forming a compound such as FeTi2O3-x, in which Fe has an oxidation state of II or I (Kim et al, 2020).The ease of hydride formation by Ti4M2Ox, where (M = Fe, Co, Ni; 0 ≤  ≤ 1), e.g., Ti4Fe2Ox has been reported (Aubertin et al, 1984).
Such structures -Zr4Fe2Ox and Ti4Fe2Ox (0.38 ≤  ≤ 1) need at least 6 atomic% oxygen to stabilize the structure.Stioui et al (1988) found that deuterium occupied three different sites in Ti4Fe2OD2.25.Also, in FeTi2O0.2H2.8, the hydrogen and oxygen share the same lattice site and the ferrous centers in the original paramagnetic ilmenite coordinate the complex.If the path of hydrogen absorption/uptake by the oxides was involved, catalytic materials (such as oxidized graphene) may cause the ethanol steam reforming at the temperature condition to make hydrogen available according to Eqns.2).A comparison of the oxygen-stabilized Ti-containing hydrides obtained in this study to those of some previous studies is presented in Table 3.
IV. CONCLUSION The hydrothermic process involving a low-temperature (120°C) reduction of rutile-ilmenite mineral in an ethanolwater medium produced a ferrotitanium oxyhydride which may be useful for hydrogen storage.An oxyhydride FeTi2O0.2H2.8 conforming to the formula FeTi2O3-xHx with a likely high hydrogen content in the interstitials was obtained as a reduced form of the original mineral.Formation of the ferrotitanium hydride suggests that hydrogen may be generated from an ethanol-water system at 120°C using the graphene oxide/rutile-ilmenite composite as a catalyst system.Otherwise, a highly reducing environment could be responsible for the formation of the oxyhydride. b.

Figure 3 :
Figure 3: XRD of graphene oxide nanopowder showing evidence of graphene nanosheets with the presence of graphite.

Figure 7 :
Figure 7: Effects of heat treatment on mesh sub-100 μm rutile mineral particles in air furnace.