Ultra-small and highly dispersive iron oxide hydroxide as an efficient catalyst for oxidation reactions: a Swiss-army-knife catalyst

Ultra-small and highly dispersive (< 10 nm) iron oxide hydroxide is characterized by some methods. The compound is an efficient and stable catalyst for alcohol oxidation, organic sulfide oxidation, and epoxidation of alkenes in the presence of H2O2. The electrochemical oxygen-evolution reaction of the iron oxide hydroxide is also tested under acidic, neutral, and alkaline conditions. In the presence of the iron oxide hydroxide, excellent conversions (75–100%) and selectivities of substrates (92–97%), depending on the nature of the sulfide, were obtained. Benzylalcohols having electron-donating and-withdrawing substituents in the aromatic ring were oxidized to produce the corresponding aldehydes with excellent conversion (65–89%) and selectivity (96–100%) using this iron oxide hydroxide. The conversion of styrene and cyclooctene toward the epoxidation in the presence of this catalyst are 60 and 53%, respectively. Water oxidation for the catalysts was investigated at pH 2, 6.7, 12, and 14. The onset of OER at pH 14 is observed with a 475 mV overpotential. At 585 mV overpotential, a current density of more than 0.18 mA/cm2 and a turnover frequency of 1.5/h is observed. Operando high-resolution visible spectroscopy at pH 14, similar to previously reported investigations, shows that Fe(IV)=O is an intermediate for water oxidation.


Results
The highly dispersive iron oxide hydroxide (1) is pure (99.5% trace metals basis) and highly dispersed (20 wt% in water).
X-ray powder diffraction (XRD) is a fast analytical method for phase identification of a crystalline material and information on unit cell dimensions. The analyzed material should be finely ground, homogenized, and the average bulk composition is determined. X-ray diffraction is a common technique for the study of crystal structures and atomic spacing.
X-ray diffraction works based on constructive interference of monochromatic X-rays and a crystalline sample. X-ray is generated by a cathode ray tube, filtered to the monochromatic radiation, collimated to concentrate, and focused on the sample. The interaction of the ray with the sample produces constructive interference, Bragg's Law, where (Eq. 1): This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, processed, and counted. By scanning the sample through a range of 2θ angles, all possible diffraction directions of the lattice should be attained due to the random orientation of the powdered material. According to the conversion of the diffraction peaks to d-spacings, the identification of the crystalline material is possible because each crystalline material has a set of unique d-spacings.
XRD showed that 1 was not crystalline, but the attributed weak peaks for δ-FeOOH ( Fig. 1b) 25 . The size of the nanoparticle was 2-8 nm based on DLS (Fig. 1c). 1 was also highly monodisperse in size. We found that 1 was stable at least for three years when stored at the pH range (3.0-4.0) and 2-8 °C without any aggregation or agglomeration (Fig. 1c).
UV-Vis spectrum of 1 showed a broad peak at 300-400 nm, which was due to ligand to metal charge transfer (Fig. 1d). However, Sherman et al. suggested that the ligand to metal charge transfer transitions in Fe(III) oxides and silicates occur at higher energies than those suggested by others and that the visible region is an intense Fe(III) ligand field as well as Fe(III)-Fe(III) pair transitions 26 . They suggested that both types of these transitions are Laporte and spin-allowed via the magnetic coupling of adjacent Fe(III) cations 26 .
A scanning electron microscopy (SEM) is a type of electron microscopy that provides images of a sample by scanning the surface with the electron beam. The interaction of electrons with the surface atoms in the sample forms signals with information on the surface topography and composition of the sample. Samples are investigated in high vacuum in a conventional SEM or wet conditions or environmental SEM. SEM images of 1 showed small nanoparticles ( Fig. 2a and b). A high monodispersity of particles was observed in SEM images ( Fig. 2a and  b). However, the resolution of a SEM is about 10 nm (nm), which is limited by the width of the electron beam and the interaction volume of electrons in a sample. Thus, tiny particles in Fe oxide are not clearly detectable by SEM (Fig. 2c). EXD-Mapping and EDX spectrum showed high dispersity for Fe and O on the surface of 1 ( Fig. 2d; Fig. S2).
Transmission electron microscopy (TEM) is a microscopy method in which a beam of electrons is transmitted through a sample to form an image. The sample should have an ultrathin section less than 100 nm thick or a suspension on a grid. In TEM, an image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. The resolution of a TEM is 25-50 times greater than SEM. In TEM images for 1, amorphous and tiny nanoparticles (< 10 nm) and high monodispersity were observed ( Fig. 3a; (1) n = 2d sin θ  S3). HRTEM images showed a crystal lattice spacing of 2.5 Å, corresponding to (100) plan of δ-FeOOH (Fig. 3b). After the methylphenyl sulfide oxidation (next section), the catalyst showed no change in the morphology, phase, or size ( Fig. 3c and d), which show the catalyst is stable.

Discussion
Catalytic performance of 1. The study of catalytic performance began with an effort to optimize the reaction conditions for sulfide oxidation. Methylphenylsulfide as a model substrate, and H 2 O 2 as a green oxidant were used to optimize the sulfoxide production (Table 1). Water, as a standard "green" solvent, was selected for all oxidation reactions. In the absence of a catalyst as a blank experiment (entry 1), a trace amount of products was observed, which indicated that the presence of a catalyst is crucial. By continuously increasing the catalyst amount from 22.5 to 67.4 μL (entries 2 − 4), a significant increase in the conversion was observed. As indicated in Table 1 (entries 5-7), a substantial decrease in conversion was observed by reducing the amount of oxidant.
To examine the substrate scope of the reaction, the optimized reaction conditions were then extended to a range of different sulfides, including dialkylsulfides, cyclic sulfide, benzylalkylsulfide, and dialkylsulfides, with H 2 O 2 as a green oxidant (Table 2). After the methylphenyl sulfide oxidation, the catalyst showed no change in the morphology, phase, or size (Fig. 3).  www.nature.com/scientificreports/ Similar to the oxidation of methylphenylsulfide, excellent catalytic activities, and selectivities were obtained for all the sulfides tested (entries 1-6). Excellent conversions (75-100%) and selectivities of substrates (92-97%), depending on the nature of the sulfide, were obtained for all cases. The diversity of oxidation reaction catalyzed by 1 was also extended to oxidation of various alcohols. Benzylalcohols having electron-donating and -withdrawing substituents in the aromatic ring were oxidized to produce the corresponding aldehydes with excellent conversion (65-89%) and selectivity (96-100%) (entries 7-10). Furthermore, steric hindrance had little effect on the reaction yields because of the ortho substituents in the benzylalcohols (entries 8 and 9). Secondary alcohols such as 1-indanol and cyclohexanol could be converted to the corresponding ketones in 80% and 58% conversion, respectively (entries 11 and 12). These results encouraged us to check the epoxidation reaction of several alkenes in the presence of 1, but the catalytic epoxidation of styrene and cyclooctene were found to be less efficient than that of sulfide/alcohol oxidation (entries 13 and 14).
According to the nature of the oxidation products, the mechanism of reactions was proposed. In the presence of H 2 O 2 , after the formation of Fe(IV)=O center, the reaction of organic substrates to a Fe(IV)=O could be proposed as the mechanism for the oxidation reactions in the presence of 1 (Fig. S4). To show the advantage and performance of the present catalytic system in comparison with lately reported protocols, we compared the results of the benzyl alcohol oxidation in the presence of other nano-iron oxide catalysts 12 . As shown in Table 3, in contrast to previously reported systems, the catalytic system presented in this paper does not suffer from the severe reaction conditions, such as using a large amount of catalyst, long reaction time, and high reaction temperature.
Oxygen-evolution reaction (OER). OER of the catalyst in the stable potential ranges was investigated for the catalyst at pH 2, 6.7, 12, and 14 (Fig. 4). The onset of OER in the presence of a trace amount of 1 (≈ 1 mg (11.2 μmol), see ESI for details) using fluorine-doped tin oxide coated glass electrode (FTO) at pH 2 was observed at 1.56 V (throughout the remaining sections, all potentials are reported vs. Ag/AgCl (KCl (3 M) reference electrode) with 660 mV overpotential (Fig. 4a). At 1100 mV overpotential, a current density of more than 1.5 mA/cm 2 and a turnover frequency of 1.35/h was observed. FTO showed low activity toward OER under the same conditions. The onset of OER in the presence of 1 at pH 6.7 was observed at 1.32 V with a 690 mV overpotential. At 870 mV overpotential, a current density of more than 1.45 mA/cm 2 and a turnover frequency of 1.3/h were observed (Fig. 4b). The onset of OER in the presence of 1 at pH 12 was observed at 0.96 V with a 640 mV overpotential. At 680 mV overpotential, a current density of more than 1.0 mA/cm 2 and a turnover frequency of 0.9/h was observed (Fig. 4c). Finally, the onset of OER in the presence of 1 at pH 14 was observed at 0.67 V with a 470 mV overpotential. At 580 mV overpotential, a current density of more than 0.18 mA/cm 2 and a turnover frequency of 1.5/h was observed (Fig. 4d).
To compare oxygen-evolution activity and to find the reaction mechanism of electrocatalysts, a Log(A/ cm 2 )/overpotential or Tafel plot is generally considered. Using the Tafel method, the sensitivity of the current to the applied potential is plotted, which provides information about the rate-determining steps. The Log(A/ cm 2 )/overpotential or Tafel plots were recorded for 1 in all the stated conditions (Fig. 4e). Tafel slopes are often influenced by electron and mass transports, gas bubbles, etc. The slopes of Tafel plots for 1 at pH 2.0, 6.7, 12.0, and 14.0 using FTO were 361.5, 203.9, 114.0 and 124.2 mV•decade −1 , respectively, which suggests the electron transfer to the electrode is the rate-determining step. At pH 14 because of the production of FeO 4 2− at higher potential, a different range was selected (Fig. 4). Table S1 shows a comparison of different metal-oxide based catalysts toward OER.
In the next step, an operando high-resolution visible spectroscopy was applied for a very thin and transparent FeOOH covered FTO. For Mn, Co, Ni, Fe, and Cu oxides, the changes in the oxidation state of the redox-active metal can be detected by the changes in the absorption in UV/Vis area. Such electrochromic character has been reported for materials based on metal oxides and related binary oxyhydroxides deposited on transparent substrate electrodes (ITO or FTO glass), where a broadband absorption was recorded upon oxidation of redoxactive metal centers [27][28][29] .
For FeOOH, the operando high-resolution visible spectroscopy showed no peak below 0.53 V, but at 0.53 V a small peak at 475 nm was recorded; at higher potentials, in addition to this peak, other peaks at 560 and 660 nm were also observed. In our setup, the counter electrode was separated from the working electrode by a small salt-bridge to inhibit reaction hydrogen or other reductants to high-valent intermediates in the operando highresolution visible spectroscopy (for setup see Fig. S5).  [30][31][32][33] . The reported mechanisms include acid-base and radical coupling mechanisms which in both the formation of Fe(IV)=O group is critical (Fig. 5c). A nucleophilic attack on Fe(IV)=O occurs by OH or H 2 O groups in acid-base mechanism 31 while the radical coupling mechanisms include the reaction of two neighboring Fe(IV)=O groups 32 4 mmol), at room temperature. b The GC conversion (%) is measured relative to the starting substrate. c Selectivity to sulfoxide = (sulfoxide %/ (sulfoxide% + sulfone%)) × 100; Selectivity to benzaldehyde = (aldehyde%/(aldehyde% + carboxylic acid%)) × 100; Selectivity to epoxide = (epoxide%/(epoxide% + aldehyde%)) × 100; TON = mol product/mol catalyst; TOF = TON/time of reaction (4 h).
All the above-mentioned experiments showed that the ultra-small and highly dispersive iron oxide hydroxide was an efficient catalyst for many oxidation reactions.
Such ultra-small and highly dispersive iron oxide could be investigated to be a bridge between homogeneous and heterogeneous catalysis 12 . Among different nanomaterials, ultra-small particles (< 10 nm) show even different properties and activities than bigger particles (10-100 nm) 12,33 .
Importantly, such small iron oxides from impurity or formed by the decomposition of iron complexes can catalyze many oxidation reactions. On the other hand, such species should be carefully checked in the presence of many metal complexes since even for many pure metal complexes, the ligands are not usually stable under the harsh condition of reactions and the formation of such active metal oxides are possible [35][36][37][38][39][40][41][42][43][44][45][46] . Although an Mn oxide-based catalyst is used by Nature to oxidize water, nanosized Fe oxide shows promising activity toward OER 47,48 .

Conclusions
Ultra-small iron oxide hydroxide (< 10 nm) was characterized by a number of methods. These methods showed that iron oxide was δ-FeOOH. Using this iron oxide, excellent conversions (75-100%) and selectivities of substrates (92-97%), depending on the nature of the sulfide, were obtained for the sulfide-oxidation reaction. The iron oxide was also applied to the oxidation of various alcohols. Benzylalcohols having electron-donating and -withdrawing substituents in the aromatic ring was oxidized to produce the corresponding aldehydes with excellent conversion (65-89%) and selectivity. A moderated activity for the epoxidation of styrene and cyclooctene was also found. A trace amount of the iron catalyst showed OER under acidic, neutral, and alkaline conditions. The Table 3. Oxidation of organic substrates with different nano-iron oxide catalysts. Methods. Reagents and solvents were purchased from commercial sources and were used without further purification. Ultra-small iron oxide hydroxide (FeOOH) (1) (< 10 nm) was purchased from Sigma-Aldrich Company. H 2 O 2 (20%) was purchased from Merck Company. TEM was carried out with FEI Tecnai G 2 F20 transmission electron microscope (TF20 200 kV). SEM and EDX were carried out with VEGA\TESCAN-XMU. The X-ray powder patterns were recorded with a Bruker D8 ADVANCE diffractometer (CuK α radiation). Electrochemical experiments were performed using an EmStat 3+ device from the PalmSens Company (Netherlands). For the electrochemical investigation of iron oxide catalytic behavior in water oxidation, a three-electrode cell was used. The cell was contained Ag|AgCl as a reference electrode, Pt as a counter electrode, and fluorinedoped tin oxide (Sigma-Aldrich Company, FTO) as a working electrode. The electrochemical determination was performed in bufferic phosphate solution in three different pHs (2.0, 6.7, 12.0 and 14.0). KOH was added to a solution of phosphoric acid (0.25 M) and adjusted pH in 2, 6.7, and 12.0. 5.0 µL of iron oxide mixture (20% by weight) was spread on the 1.0 cm 2 of FTO surface. The mixture on the electrode dried at 60˚C and then 10 µL of Nafion was used to fix the solids on FTO. This electrode was placed in the cell and cyclic voltammetry at different pHs was performed. For comparison, oxygen-evolution reaction (OER) at the same pHs and surface of FTO electrode without iron oxide was determined. Thermodynamic potentials for OER in various pHs were calculated by Eqs. (2) and (3). Overpotential was calculated by Eq. (3).
General procedure for the oxidation reaction. For all oxidation experiments, we used a standard procedure. To a solution of a substrate (0.2 mmol), and 1 in water (Sigma-Aldrich Company, 67.4 μM; 1 mL), H 2 O 2 (Merck Company, 0.4 mmol) was added as an oxidant. After four hours, water (5 mL) was added, and the resulting mixture was extracted with EtOAc (2 × 5 mL). The collected organic phases were dried with anhydrous CaCl 2 and the extract was also concentrated down to 1.0 mL by distillation in a rotary evaporator at room temperature. Then, a sample (2 μL) was taken from the solution and was monitored by GC. Assignments of the products were made by comparison with authentic samples.
(2) E eq = 1.23−0.0592 pH Figure 5. Operando high-resolution visible spectroscopy for Fe oxide covered FTO as working electrode (a, b) in KOH (pH 14). Each amperometry and its related high-resolution visible spectroscopy is in the same color (see Fig. S4 for setup). Two schematic proposed mechanisms for OER by Fe oxide under alkaline conditions (c).