Iron oxychloride(FeOCl) is a typical iron-based material with layered structure. It was discovered in the 1930s, and since the 1970s, has been extensively studied as a unique host in the field of supramolecular intercalation chemistry. The modification of FeOCl is much more flexible and easier than traditional iron(hydr)oxides due to the layered structure. In 2013, our group reported that FeOCl has excellent Fenton-like activity, which shows the promising potentialities for practical applications. Inspired by the properties of FeOCl and encouraged by our result, FeOCl and its intercalation compounds have recently attracted significant attention in catalysis and energy storage. In this review, the characteristics of pristine FeOCl and its intercalation compounds are summarized, especially intercalation-induced crystal and electronic structure changes. And then we focus on the contributions made by these materials toward advanced oxidation processes(AOPs), selective oxidation, electrode materials and other fields. Finally, challenges and future perspectives are reviewed in terms of structural design and the improvement of stability.
Layered materials are a class of materials with a thin layer structure on the atomic scale, and the layers are connected by electrostatic force or Van der Waals force. The typical representatives are layered double hydroxides (such as hydrotalcite), graphite, transition metal dichalcogenide (such as MoS2), metal halides (such as ZrCl) and metal oxyhalides (FeOCl, BiOCl)[1⇓~3]. For the layered compounds with van der Waals force, the van der Waals force is a kind of intermolecular force with weak strength, which makes them have abundant modulation space.At present, these layered compounds have shown great application potential in the fields of electrochemical energy storage, catalysis and semiconductor devices, which traditional solid materials can not match. The development and modification of new layered materials have become an important research field of materials science[4,5].
FeOCl is a typical layered material. As early as the 1930s, German scientist Sch Schäfer discovered FeOCl in partially pyrolyzed FeCl3·6H2O[6]. Later, people also found the existence of FeOCl in the marine natural environment, for example, the surface of cultural relics salvaged on the seabed is often associated with FeOCl, and its proper removal is very important for the underwater cultural relics and anti-corrosion work. With the rise of supramolecular chemistry in the 1970s, FeOCl, as an interesting inorganic host, was widely studied in the field of organic-inorganic intercalation chemistry, forming the first research climax of FeOCl materials (Fig. 1A)[7]. However, during this period, there was little research on the application of FeOCl except for a small amount of research on the use of FeOCl as a cathode material for lithium-ion batteries[8]. In 2013, our research team found that FeOCl is an excellent solid Fenton catalyst, which can activate H2O2 to generate hydroxyl radicals (HO ·) with a capacity 1 to 3 orders of magnitude higher than other iron (hydrogen) oxides to achieve the degradation of pollutants in water[9]. Since then, FeOCl has been widely studied as a catalyst for water treatment, and researchers have done a lot of work on its structural modification (morphology optimization, exfoliation, intercalation, etc.), engineering (loading), and reaction system enhancement (photo-assisted, ultrasound-assisted, etc.). In addition, FeOCl also showed excellent catalytic activity in other oxidation systems, such as benzene hydroxylation[10]. In addition to catalytic applications, FeOCl has also made some progress in the research of rechargeable lithium ion batteries and supercapacitor electrode materials[11,12]. FeOCl can also be used as a precursor, a template or an interfacial cross-linking layer for the construction of other nanomaterial composite structures[13,14].
According to our statistics, since 2013, the number of research papers on the application of FeOCl system has exceeded 40, and shows an increasing trend year by year (Figure 1b). However, the current review of FeOCl is not good enough, except that Whittingham (winner of the Nobel Prize in Chemistry in 2019) summarized the structure and intercalation of FeOCl in his monograph Intercalation Chemistry in 1982, no one has involved this interesting material in the past 40 years[7]. In 2019, Whittingham won the Nobel Prize in Chemistry for his research on intercalation chemistry in lithium-ion batteries, which once again aroused the enthusiasm of many scientists for the research of layered materials. FeOCl, as a kind of layered materials with unique properties but little research at present, undoubtedly has rich potential for development.
Materials scientists have accumulated a lot of research experience around classical layered materials, such as MoS2 and graphene, and have a deeper understanding of their controllable preparation, structural modification and related applications. Compared with these classical systems, FeOCl has the advantages of high environmental abundance of the main Fe element (ranking fourth in the earth's crust), flexible valence changes, and non-toxic and harmless, which are difficult for other layered materials to have. Although FeOCl has shown great research potential, our understanding of its properties is still relatively shallow, and there is a lack of systematic combing of its existing research, which limits the further development of FeOCl materials. A comprehensive understanding of the structure of FeOCl and a summary of the existing research are necessary to promote the further development of its application. In this review, we first summarize the research on the structure of FeOCl and its structural modification represented by intercalation reaction, which mainly focused on the period from 1970 to 2000. In addition, we summarized the various applications of FeOCl since 2013, and prospected its further application development from the perspective of structural design.
2 Preparation and Structure of FeOCl
2.1 Preparation of FeOCl
2.1.1 FeOCl bulk material
At present, the preparation of FeOCl bulk materials with good crystallinity mainly includes two methods: Partial pyrolysis and Chemical vapor transport[15][16]. The partial pyrolysis method mainly uses anhydrous ferric chloride (FeCl3) or hexahydrate ferric chloride (FeCl3·6H2O) as the precursor, and the ferric chloride reacts with its own crystal water or water molecules in the air by heating, and then generates FeOCl by HCl removal. The reaction equation can be expressed by (1) or (2):
The chemical vapor transport method uses anhydrous ferric chloride and iron oxide (hematite, α-Fe2O3) as precursors, which need to be fully mixed under anhydrous and oxygen-free conditions under the condition of excessive anhydrous ferric chloride, and the mixed solid is roasted.During the roasting process, anhydrous ferric chloride will be gasified, and topological transformation will occur at the gas-solid interface of iron oxide to form FeOCl3, and then FeOCl will be formed through the decomposition and structural rearrangement of FeOCl3. Due to the inhomogeneity of the reaction system, chlorine may also be released during the rearrangement of the FeOCl3structure (Cl2). The reaction equations can be expressed by (3) and (4):
The morphology and structure of FeOCl prepared by the two methods are quite different.The FeOCl prepared by the partial pyrolysis method has smaller size and more irregular morphology, while the FeOCI prepared by the chemical vapor deposition method has more regular structure and higher crystallinity. Therefore, although the latter process is cumbersome, it has received more attention in intercalation chemistry and some application studies. It is generally believed that FeOCl prepared by partial pyrolysis method has poor stability, but in some recent application studies, FeOCl materials with good stability have been prepared by controlling certain preparation conditions (such as calcination temperature, time, atmosphere, etc.)[17]. Many mechanism problems (such as phase transformation behavior, crystal growth behavior, etc.) Involved in the preparation of FeOCl bulk materials are not well understood at present, and the empirical change of preparation conditions is still the main means to optimize the preparation method. In addition, some other preparation methods, such as solution impregnation-calcination and liquid pulsed laser ablation (LAL), have also been developed in recent studies, and we will introduce them in the application study.
2.1.2 FeOCl two-dimensional material
As a typical layered material, FeOCl can also be prepared by exfoliation or vapor phase growth like other layered materials (such as graphite and MoS2) in theory. However, due to the fact that the research of FeOCl materials is still in its infancy, only a few studies have been carried out on the preparation and properties of two-dimensional FeOCl materials. During the preparation of FeOCl by vapor phase growth (chemical vapor migration), the size and thickness of FeOCl layers are affected by the mixing ratio of precursors, heating rate and reactor volume. Our research group tried to control the partial pressure of FeCl3 by changing the reactor volume during the preparation of FeOCl, and found that the growth of (010) crystal plane of FeOCl could be inhibited by increasing the partial pressure of FeCl3 (decreasing the volume size), but the preparation of two-dimensional FeOCl nanosheets could not be achieved[18]. Although there is no successful precedent for the preparation of two-dimensional FeOCl nanosheets by direct vapor phase growth, the preparation of two-dimensional FeOCl nanosheets from FeOCl bulk materials by liquid phase exfoliation has been reported. Similar to the traditional exfoliation method of Van der Waals layered materials, Zhang et al. And Schoop et al. Successfully prepared FeOCl nanosheets with few layers by ultrasonic exfoliation and n-butyl lithium intercalation exfoliation, respectively.Compared with the bulk FeOCl, the H2O2, activation behavior and electromagnetic properties of the prepared material have significant changes, which will be further introduced in the corresponding application research section[17,98]. In a word, although FeOCl is a new type of layered material, it has similar properties with traditional Van der Waals layered materials, and can be designed and prepared by using the development ideas of other materials.
2.2 Structure of FeOCl
Goldsztaub had measured the crystal structure of FeOCl in the 1930s, but there were some errors in the unit cell parameters due to the poor crystallinity of the material[19]. Since then, in the development of Mossbauer spectroscopy (Mossbauer spectroscopy), due to the strong nuclear fourth-order interaction, FeOCl has been used as an excellent material to determine the nuclear fourth-order moment value of Mossbaer 57Fe, and its crystallographic parameters have been studied[20,21]. In 1970, Lind re-determined the crystal structure of FeOCl by single crystal X-ray diffraction, and obtained more accurate structural information, which has been used up to now[22]. In the subsequent study of intercalation chemistry, people will also determine the structural parameters of the original FeOCl in the process of studying the intercalation products. Although there are some differences in the obtained figures, the differences are often not significant. The crystal of FeOCl belongs to the orthorhombic system with the space group of Pmnm(D2h13), and the unit cell parameters are a = (3.780 ± 0.005) Å, B = (7.917 ± 0.005) Å, and C = (3.302 ± 0.001) Å. The typical structure is shown in Figure 2. In FeOCl, each Fe atom is coordinated by four O atoms and two Cl atoms, forming a cis cis-[FeO4Cl2] octahedron. The octahedrons are connected by O — O or O — Cl to form a bilayer, and a layer of Cl atoms is closely packed outside the bilayer. Due to the Van der Waals interaction of the Cl atoms in the layer, FeOCl has a layered structure.
The bond lengths between Fe — O, Fe — Cl and Fe — Fe are the most important structural parameters to understand the reactivity of FeOCl. The Fe — O bond lengths of FeOCl are 1.964 and 2.100 100 Å, which are similar to those of γ-FeOOH and α-Fe2O3, but shorter than those of garnet and perovskite. The Fe-Cl bond length of 2.368368Å is similar to the Fe-Cl ionic bond length in the FeCl3 and much longer than that in the [FeCl2(H2O)4]+, indicating that the ionicity may be stronger. Due to a series of differences in the electronegativity, atomic interaction radius and bonding ability of O and Cl in the Z direction, the spatially extended coplanar octahedral layer is distorted compared with the γ-FeOOH, α-FeOOH and α-Fe2O3 layers, which is reflected by the bond angles of O-Fe-Cl and O-Fe-O on both sides of the (001) plane being 172. 39 ° and 148. 48 °, respectively, while the bond angles of the above iron oxides in similar topological directions are equal, both being about 160 °. The bond angle between Fe-O along the (001) plane is not significantly different from that in the α-Fe2O3. The distance between Cl and Cl in the epitaxy is 3.6868 Å, which is basically twice the Van der Waals interaction radius of Cl, forming a typical Van der Waals layer. In addition to X-ray diffraction, microscopic imaging techniques have also played an important role in understanding the crystal structure of FeOCl. In 1995, Pieczko and Breen studied FeOCl by atomic force microscopy for the first time. The surface images showed that FeOCl had a smooth surface in the imaging area (10 μm × 10 μm), and the atomic resolution images showed that the terminal Cl atoms had obvious Van der Waals layer structure. The measurement of the periodic structure of the surface atoms was basically consistent with the XRD crystallographic data[23]. In addition, there are obvious surface defects on the surface of FeOCl in some regions, with height differences of 0.70 nm and 1.31 nm, which may correspond to the layer height of FeOCl, but some holes with a depth of several microns are also observed, which may be caused by the reaction of FeOCI in air atmosphere or the damage caused by electron beam irradiation.
Since the 1970s, the electronic and magnetic properties of FeOCl have been studied by Mossbauer spectroscopy and theoretical calculations. According to the Mossbauer study, the characteristic signal of Fe atom in FeOCl at 300 K is basically the same as that of the asymmetric high-spin Fe3+. At low temperature, the ordering of FeOCl exhibits antiferromagnetism, but the ordering temperature is reported to be (68 ± 2) K, (85 ± 1) K or (90 ± 1) K.In addition to the influence of early measurement techniques on the results, the differences of these experimental values may reflect the influence of crystallinity, crystal orientation, impurity composition and air stability on the structure and physical properties of FeOCl[24][25][26]. The sensitivity of FeOCl to water can be further reflected by the difference in its Mossbauer lattice temperature, which is (327 ± 6) K for the FeOCl sample prepared by strictly controlling the water and humidity of the precursors, and (267 ± 5) K for the sample prepared without these methods[26]. At room temperature, the molar susceptibility χm of FeOCl is 3.86×10-3 or 3.26×10-3, and the value of χm decreases gradually with the decrease of temperature, but no obvious magnetic anomaly is found after the ordering temperature. The antiferromagnetic structure is nonlinear and has two unequal Fe3+[27]. In FeOCl, the splitting of the Fe d orbital into the eg and the p orbital filling of the half-filled t2g;O affects the coupling between two adjacent eg and t2g, thus affecting the paramagnetic direct exchange coupling and the antiferromagnetic super exchange coupling. The coupling strength is directly affected by the Fe-O-Fe bond length and bond angle in the structure. The electron spin directions in FeOCl are parallel to its b-axis and c-axis, respectively, and the time-dependent spin relaxation occurs due to the competition with the diamagnetic super-exchange coupling in the van der Waals layer[25]. FeOCl exhibits semiconducting properties, and its resistivity is about 107Ω. The intercalation of electron donor and Lewis base can reduce its impedance in different degrees[7]. The electronic structure of FeOCl and the processes of electron transfer and electron transition between FeOCl and intercalation guest were further revealed by Mossbauer spectroscopy, X-ray absorption and other methods combined with band model, cluster model and charge transfer model[28,29][30,31][32]. The information of density of States (DOS) and local density of States (PDOS) indicates that the electron density near the Fermi level of FeOCl is mainly contributed by Fe-3d orbitals, so the electron transfer behavior (such as intercalation reaction) mainly affects the Fe atom. The COOP plot shows that the molecular orbitals near the Fermi level are the weak bonding orbitals of Fe and Cl, while the orbitals above the Fermi level are the anti-bonding orbitals of Fe and O, which are easily occupied by the electrons transferred from the guest during the intercalation process[33].
3 FeOCl intercalation modified
3.1 Intercalation mechanism
The study of intercalation mechanism of layered compounds began with the classical layered transition metal oxides (such as MoS2). Because most of the intercalation guests have lone pair electrons, that is, they have Lewis base characteristics, the early studies believed that the intercalation reaction was caused by the Lewis acid-base interaction between the host and the guest[29]. In the intercalation structure, the lone-pair electrons of the guest species overlap with the dz2 unoccupied orbitals of the host metal atoms to form new chemical bonds. However, with the development of characterization techniques such as NMR, neutron diffraction and Mossbauer spectroscopy, it has been shown that the lone pair electrons of the guest species are arranged parallel to the layer direction during the intercalation process, and can not form bonds with the dz2 orbitals of the host metal, and the reduction of the host metal has been observed in the intercalation compounds[32,34⇓ ~36]. Therefore, researchers have come to believe that the redox reaction between the host and guest species is the cause of intercalation. A new charge transfer model is proposed to explain the intercalation process, in which the delocalized electrons in the valence band of the guest species induce the reduction of the host skeleton species, and then interact with the guest cations or protonated species to form stable intercalation compounds. Since then, many studies and characterizations of FeOCl intercalation process have also confirmed that FeOCl intercalation follows the same redox mechanism based on charge transfer as other layered compounds (Figure 3), and FeOCl is more oxidizing, and the intercalation process is more intense and rapid[29].
3.2 Intercalation Structure Evolution of FeOCl
For FeOCl, due to the intercalation of guest species and the reduction of the host (usually the framework Fe atom), the crystal structure, electronic properties and magnetic properties of FeOCl intercalated compounds are often greatly changed compared with the original FeOCl. Crystallographic diffraction, Mossbauer spectroscopy, synchrotron radiation X-ray absorption spectroscopy and theoretical calculations provide a wealth of information about the intercalation behavior of FeOCl and the changes in the properties of FeOCl intercalated compounds, which greatly increases the depth of understanding of this material.
Due to the large size of the intercalation guest, the main effect on the crystal structure of FeOCl is the interlayer expansion perpendicular to the layer direction (b-axis). It has been reported in the literature that the intercalation of almost all guest species, except Li ion, leads to an increase in the interlayer spacing of FeOCl. This expansion of the interlayer spacing can be observed directly by XRD and has also been observed in some localized electron diffraction (ED) results[37]. In addition to the change of the interlayer spacing, the guest intercalation can also cause the change of the unit cell parameters (a, C) in the FeOCl layer, which is mainly due to the further distortion of the coordination field environment caused by the reduction of Fe (Ⅲ) and the coordination of the guest species with the terminal Cl atom[37]. In addition, due to the existence of n-slip planes perpendicular to the layer direction of FeOCl, the intercalation process may cause the sliding of the layer structure of FeOCl, destroy its structural symmetry and change its space group. For example, FeOCl intercalated with tetrathiafulvalene exhibits body-centered symmetry, and its space group becomes Immm[36,38]; The FeOCl space group of the methoxide intercalation changes to Amma and so on[39]. However, it should be noted that it is difficult to accurately assign the space group of FeOCl intercalation compounds in the early stage (before the 21st century) due to the poor X-ray diffraction ability for light elements and the deterioration of crystals after intercalation. In recent years, with the construction of large scientific facilities such as synchrotron radiation X-ray source and spallation neutron source, the role of synchrotron radiation X-ray diffraction, neutron diffraction and other techniques in material characterization is increasing, and their spatial resolution is greatly enhanced compared with traditional diffraction techniques, which can obtain high-intensity and high-quality diffraction data for structure analysis. In addition, some auxiliary devices for testing (such as full process inert atmosphere sample preparation) are also improving day by day. These techniques have greatly improved the success rate and reliability of structural analysis of FeOCl intercalation compounds, and greatly promoted the understanding of their crystallographic structures.
In addition to the crystallographic evolution of FeOCl intercalation compounds, the changes in their electronic and magnetic properties have also been extensively studied. Despite the difference of guest species, the results of Mossbauer spectroscopy showed that the FeOCl intercalation process was accompanied by the reduction of Fe (Ⅲ), and the Fe (Ⅱ) ratio was between 0.10 and 0.13, which was almost independent of the guest species. The results show that FeOCl intercalated compounds exhibit magnetic ordering at low temperature. The isomer shifts of Fe (Ⅲ) species are consistent with those of pure FeOCl, and the quadrupole splitting of Fe (Ⅱ) resonance is observed. With the increase of temperature, the proportion of Fe (Ⅱ) decreases and the quadrupole splitting of Fe (Ⅲ) resonance weakens. According to the change of the characteristic parameters of Mossbauer spectra of Fe (Ⅲ) and Fe (Ⅱ), it can be considered that there is a rapid electron exchange in the average electron environment around the Fe sites, which is realized by the electron transition between different valence Fe sites. The activation energy of electron transition is about 5.0 ± 0.9 kJ/mol according to the results of temperature change experiments[20,29,32]. Although Mossbauer spectroscopy has made a great contribution to the study of the charge transfer process of FeOCl intercalation, it is difficult to accurately describe and quantify the electron transfer process due to the limitation of the time scale (~10-8s) of Mossbauer effect. In recent years, synchrotron radiation X-ray absorption and emission spectra have been used to explore the charge transfer process of FeOCl, which can characterize the electronic transition behavior on ~10-16s time scales. The Fe (Ⅱ) ratio of the FeOCl intercalated with Na and polyaniline is about 0.25, which is much higher than the result of Mossbauer spectroscopy[31].
3.3 FeOCl intercalation compound
At present, a variety of compounds (such as amines, metallocenes, alkali metal cations, alkoxy compounds (alkoxides) and other heterocyclic compounds) have been used as guest species in the intercalation of FeOCl.They have different structures and chemical properties, so the intercalation process and the properties of the intercalated compounds are also very different, which will be introduced one by one below.
3.3.1 Amine intercalation compound
Amine compounds, such as aniline, pyridine, pyrrole and so on, are the most common Lewis bases, which have been widely studied in the intercalation reaction of FeOCl, and are the most widely studied and important FeOCl intercalation guest. Maeda et al. prepared a variety of amine intercalated FeOCl by soaking a series of amine compounds such as aniline with FeOCl in ethanol. The amine species acted as electron donors between the layers and were oxidized to form polymers (such as polyaniline (PANI))[40]. They found that the acidity of the guest species had a significant impact on the synthesis of intercalated compounds, and only amines with acid dissociation constants (pKa) greater than 5 could be effectively intercalated into the FeOCl layers. Wu et al studied the intercalation polymerization of aniline with FeOCl in detail, and characterized the intercalation process and the stability of the product in air atmosphere in detail[41]. The molar ratio of Fe (Ⅱ)/Fe (Ⅲ) of PANI intercalated FeOCl is about 1/9. The results of X-ray single crystal diffraction show that the insertion of aniline induces the formation of 2a × 2C superstructure of FeOCl. The weight average molecular weight (Mw) and number average molecular weight (Mn) of PANI were about 6 100 and 3 500, respectively, which were much lower than those of PANI synthesized by conventional open polymerization method (Mw was about 69 000 and Mn was about 7 700). PANI had shorter chain length and narrower chain length distribution in the interlayers. The temperature-dependent 1H NMR spectra showed that the rotation of benzene ring in PANI was obviously restricted in the interlayers. Based on the characterization of the crystal structure of PANI intercalated FeOCl and the distribution of PANI in the interlayer, and combined with the previous study of one-dimensional electron density, Wu et al. Proposed that the lone pair electrons of aniline would be transferred to the conduction band of FeOCl during the intercalation process, which would lead to the polymerization of aniline in the interlayer, and the coordination of amino groups with Cl atoms at the end of FeOCl would promote the stabilization of PANI coordination in the interlayer (Figure 4). When PANI intercalated FeOCl was exposed in air, it gradually disintegrated and transformed into a mixture of PANI and β-FeOOH. The polymerization, guest-host coordination and conformational transformation of aniline in the FeOCl interlayer have significant effects on the electronic and magnetic properties of FeOCl. Prassides et al. Measured the phonon spectra of PANI and PANI intercalated FeOCl by neutron time-of-flight technology, indicating that PANI will form a p-type semiconductor between layers, and its conduction behavior is affected by the main body of FeOCl[42]. Hwang et al. studied the magnetic structure change of PANI intercalated FeOCl by high-resolution neutron diffraction. They found that PANI intercalation blocked the magnetic correlation between adjacent FeOCl layers, so that the Fe spin in the main body of FeOCl showed the characteristics of a coupled quasi-two-dimensional magnetic system, but PANI intercalation had no significant effect on the ordering temperature of Fe spin[43].
In addition to aniline, the intercalation of heterocyclic aromatic amines such as pyridine and pyrrole has also been studied. Their reaction behavior with FeOCl is similar to that of aniline. The solvent environment of the reaction process has a great impact on the intercalation process. Generally, the intercalation products are relatively stable in the absence of solvents (amine vapor) or organic solvents (alcohols, acetonitrile), while the aqueous solution environment will greatly hinder the intercalation reaction and lead to the disintegration of the intercalated FeOCl structure (formation of γ-FeOOH)[44]. The molar ratio of these heterocyclic amines to FeOCl is generally less than 1/3, but Dai Yaodong et al. Have prepared 8-hydroxyquinoline (HQ) intercalated FeOCl by hydrothermal method, in which the molar ratio of HQ to FeOCl is as high as 0.78/1[45]. In addition, the intercalation of heterocyclic amines may have a greater impact on the stability of FeOCl. Yamada et al studied the thermal stability of guest species such as 2-vinylpyridine in FeOCl. The results showed that these intercalated compounds began to decompose below 423 K, which was much lower than the thermal decomposition temperature of pure FeOCl (773 K), and the decomposition products were Fe2O3, Fe3O4 and FeCl2[46]. Kanamaru et al. Have characterized pyridine intercalated FeOCl by EPR, and the structure shows that there is an obvious characteristic peak at G = 2.003, which corresponds to the transfer of N lone pair electrons from pyridine to the conduction band of FeOCl.In other studies, the charge transfer between host and guest and the polymerization of guest species were also observed by IR and Mooney spectroscopy, which indicated that the intercalation mechanism of heterocyclic amines was similar to that of aniline[47][48].
As the most classical FeOCl intercalation guest, the structure of FeOCl intercalated by amine compounds is relatively stable, and the basic physical and chemical properties and layer structure of FeOCl are significantly changed by intercalation, which mainly leads to the increase of conductivity, the generation of magnetic anisotropy and the reduction of framework Fe (Ⅲ) species.In addition, the polymerization of amine species in the interlayer can also affect the conductivity of the intercalated compounds, which makes the amine-intercalated FeOCl system play an important role in the study of FeOCl intercalated compounds and have unique advantages in the development and design of various functional materials and devices.
3.3.2 Metallocene intercalation compound
Metallocene is a kind of organometallic compound formed by the coordination of transition metal and cyclopentadiene. In the current research, it has been found that many kinds of metallocenes, such as ferrocene, nickelocene, ruthenium, cobalt and chromium, can be inserted into the FeOCl interlayer, among which ferrocene is the most widely studied. Metallocene intercalation can significantly improve the conductivity of FeOCl, for example, the conductivity of ferrocene intercalated FeOCl is about 4 orders of magnitude higher than that of the original FeOCl[49]. The crystal structure of FeOCl was also changed by metallocene intercalation. Halbert et al. Found that the interlayer spacing of FeOCl intercalated by ferrocene and cobaltocene was expanded by about 55 Å, and the layer structure was shifted to body-centered orthorhombic system, but there was no evidence that the metallocene was ordered between the layers[50]. The results of Mossbauer spectroscopy showed that the effects of different metallocene intercalations on the chemical environment of Fe sites in FeOCl were basically the same.Fe (Ⅱ) was observed in the main body of FeOCl, and some Fe sites were only coordinated by Fe (Ⅲ), while at least one Fe (Ⅱ) was observed in other Fe sites[51]. The Neel temperature of ferrocene intercalated FeOCl is 75 K. Above 245 K, Fe (Ⅱ) and Fe (Ⅲ) show delocalization characteristics and obvious electronic transition occurs. In the range of 100 ~ 245 K, the environment of the Fe site is an intermediate state between the delocalization at high temperature and the localization at low temperature[51]. Sch Schäfer-Stahl et al. Found that only Fe (Ⅲ) signal was shown in the Mooney spectrum of ferrocene intercalated FeOCl at room temperature (295 K), which may indicate that the terminal Cl atomic layer of FeOCl may have a shielding effect on the electron transition, thus hindering the electron transfer to the Fe (Ⅲ) site at room temperature[49]. The characterization of ferrocene intercalated FeOCl by Siu et al. Using Mossbauer spectroscopy, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) showed that ferrocene was completely oxidized during the intercalation process, resulting in a significant low spin-spin relaxation of the host Fe atom[52].
The effect of the intercalation of metallocene species on the structure and physical properties of FeOCl is similar to that of amine species.However, due to the possible formation of electronic or magnetic interactions (electronic transition, magnetic coupling, etc.) Between the metal center of metallocene and the Fe species in FeOCl, the understanding of the property changes is more complicated. In addition, the metallocene center in the interlayer may also become the active site of the catalytic reaction, so the detection and design of the interlayer environment should be paid special attention to in the metallocene intercalated FeOCl system.
3.3.3 Alkali metal cation intercalation compound
The intercalation of alkali metal cations is generally achieved by the reaction of FeOCl with the reducing salts of alkali metal cations (alkoxide, iodate, etc.).In the intercalation process, the reducing agent induces the partial reduction of Fe (Ⅲ) in FeOCl, and the alkali metal cations are intercalated into the interlayer as charge compensation. The intercalation reaction equation is as follows (taking iodide as an example):
Maguire and Banewicz et al. Have prepared alkali metal cation intercalated FeOCl by the reaction of alkali metal cation methoxide and iodide with FeOCl. The molar ratio of alkali metal cations to FeOCl is in the range of 0.16 ~ 0.13 ∶ 1[53,54]. Banewicz et al. Also found that the interlayer spacing of the intercalated compounds of Na+ and K+ with smaller ionic diameters was even wider than that of the intercalated compounds of Rb+ and Cs+ with larger ionic diameters, which may be due to the formation of solvent shells between the alkali metal cations with smaller ionic diameters and water or methanol. Sagua et al. Studied the intercalation of n-butyl lithium with FeOCl, and found that although the intercalated FeOCl still had semiconductor properties, its conductivity was improved by 2 to 3 orders of magnitude[37]. The crystal structure of FeOCl was greatly changed by the intercalation of Li, and a long-period superstructure was induced, but the structure of the whole intercalated compound was not uniform, which may be related to the intercalation and diffusion behavior of n-butyl lithium between layers.
Alkali metal species are quite different from amine species and metallocene in size and structure, and the stability of intercalated compounds is directly related to the size and solvation of alkali metal cations. Due to the single coordination form of alkali metal cations, it is easy to obtain a clear crystallographic structure, which makes it convenient to detect and understand the interlayer environment of FeOCl, which is conducive to the development of the interlayer environment of FeOCl to achieve the corresponding functional applications.
3.3.4 Alkoxy intercalation compound
The intercalation of alkoxy compounds has been systematically studied by Choy et al., who achieved this process by reacting methoxide and ethoxide with FeOCl[39]. XAS characterization shows that the symmetry of the six-coordinated structure of Fe in FeOCl is greatly improved by intercalation, which is close to the coordination structure of wurtzite (γ-FeOOH). Therefore, they believe that the alkoxyl compounds are not simply intercalated into the FeOCl layers, but a local substitution reaction occurs, and some alkoxyl groups replace Cl atoms to coordinate with Fe, which also indicates that the intercalation of guest species may lead to the weakening of Fe — Cl bonds at the end of FeOCl. Kikkawa et al. soaked methanol and FeOCl at 110 ℃ for 6 d, and also prepared the methoxy intercalation substituted compound, which still basically maintained the layered structure, but the interlayer spacing was extended to 9.9797Å[55]. The appearance of C — O vibration and the disappearance of O — H vibration in the IR characterization proved the occurrence of local substitution. Under the same preparation method, the intercalation substitution reaction can also occur by replacing methanol with ethylene glycol. The interlayer spacing of the original intercalation product is 14.55 Å, but it shrinks to 10.9898 Å after washing with acetone, indicating that the intercalation substitution structure is not stable[56].
In general, the local displacement reaction of FeOCl caused by alkoxyl compounds is quite different from that of other intercalation guests, which may be related to the nature of the alkoxyl species itself (for example, the alkoxyl groups exist in the form of organic anions and have a certain basicity), which still needs further understanding.
3.3.5 Other heterocyclic compound
Kauzlarich et al. studied the intercalation reactions of various organic sulfur compounds, such as tetrathiafulvalene (TTF), tetramethyltetrathiafulvalene (TMTTF), tetrathionaphthalene (TTN) and tetrathiacetracene (TTT), with FeOCl. The molar ratio of the guest to FeOCl varies from 1/8.5 (TTF) to 1/23 (TTT) depending on the size of the guest species[57]. XRD patterns showed that all the organic sulfides were perpendicular to the FeOCl layer except for TMTTF, which was parallel to the FeOCl layer. The FeOCl intercalated by organic sulfur still has semiconductor characteristics, but the conductivity is UNK 1 times of that of the original FeOCl. IR and XAES characterizations show that these guest species exist in the form of free radical cations in the interlayer, which have a certain inclination angle with the layer structure of FeOCl. Bringley et al. Studied the intercalation of various polycyclic aromatic hydrocarbons (perylene, tetracene, perylene, etc.) In FeOCl, and found that these polycyclic aromatic hydrocarbons were parallel to the FeOCl layer structure after intercalation, and IR characterization showed that they existed in the form of organic cations between the layers.It is speculated that each PAH molecule transfers one unit of electron to FeOCl, and the conductivity of these intercalated compounds is also significantly increased by about 105 times compared with the original FeOCl[58,59].
3.3.6 Summary of FeOCl intercalation system
The intercalation mechanism of FeOCl and typical FeOCl intercalation systems are summarized in fig. 5. The redox reaction between FeOCl host and guest species is a typical feature of the FeOCl intercalation process, and is also the driving force for the intercalation process, which is mainly determined by the potential of the redox couple between Fe (Ⅲ)/Fe (Ⅱ) in FeOCl and the guest species. Accompanied by the redox reaction, the guest species intercalate into the FeOCl layers to form intercalation compounds. Both the crystallographic structure and the intrinsic electromagnetic properties of FeOCl vary significantly depending on the size and physicochemical properties of the guest species, as described in detail above. The changes in the properties of FeOCl caused by intercalation provide new opportunities for the development of its applications. Although FeOCl intercalation systems have been reported in a large number of literatures, these studies are mainly focused on the field of basic supramolecular chemistry, lacking application-oriented structure-property understanding. In addition, these studies mainly focused on the middle and late 20th century, lacking the dynamic evolution information of FeOCl intercalation structure based on in-situ characterization methods, which needs to be paid attention to and solved in future studies.
4 Application of FeOCl and Its Intercalation Compound
4.1 Fenton oxidation catalyst
4.1.1 Pure phase FeOCl system
Fenton Oxidation is the oxidation of pollutants by generating highly oxidative reactive oxygen species (ROS), such as hydroxyl radicals (HO-), through the activation of Advanced Oxidation Process by Fe-based materials.As a major Advanced Oxidation Process (AOP), Fenton oxidation is of great significance in the fields of industrial wastewater and soil remediation. In 2013, our research group first discovered that FeOCl has excellent Fenton activity. Under typical Fenton reaction conditions (pH = 4, room temperature), its ability to activate H2O2 to generate HO · is 1 to 3 orders of magnitude higher than that of traditional iron (hydrogen) oxides, and even higher than that of homogeneous Fenton system (FeClO4-H2O2) (Table 1). It has good degradation effect on benzene, nitrobenzene and other refractory organic pollutants[9]. Based on the characterization of high-resolution TEM and selected-area electron diffraction (SAED), we believe that the unique (O-Fe-Cl) or (O-Fe-O) linear structure on the surface of FeOCl has a high distribution of coordination unsaturated Fe sites, which can effectively promote the activation of H2O2 by single electron transfer. After our study, Sun et al optimized the preparation of FeOCl based on partial pyrolysis and prepared FeOCl nanosheets[60]. They found that FeOCl nanosheets could maintain good Fenton activity under neutral conditions, and their activity could be restored by simple hydrochloric acid treatment after multiple reactions. Based on EXAFS and other characterizations, they proposed that the surface Fe configuration of FeOCl nanosheets could effectively inhibit the formation of reactive inert high-valence Fe intermediates and promote the rapid cyclic transformation of Fe (III) and Fe (II) (Fig. 6).
Recently, Wang et al studied the decomposition and transformation mechanism of H2O2 on the surface of FeOCl in detail by DFT calculation[61]. Their research is based on the (100) crystal plane of FeOCl and simulates the acidic environment by modulating the surface H atom coverage. The calculation results show that it is difficult to activate the (100) crystal plane H2O2 without H atom coverage to form HO.On the other hand, the (100) crystal plane H2O2 with H atom coverage can be activated to form HO.on the [Fe (Ⅱ) -O-Fe (Ⅱ)] structure, but can not be desorbed from the surface. The [Fe (Ⅱ) -O-Fe (Ⅲ)] structure on FeOCl (100) surface is firstly induced by the synergistic effect of protonation of FeOCl and dehydrogenation of H2O2 only in the case of partial H coverage. In this structure, the H2O2 is easily activated on the Fe (Ⅲ) site, and the generated HO · is easily desorbed. This reaction path may reflect the real activation of H2O2 on FeOCl surface.
In addition to the traditional Fenton system, the enhancement of external energy (light radiation, ultrasound) is also an important way to enhance the activity of Fenton system. El Shafei et al. Have investigated the Fenton activity of various metal oxychlorides, such as FeOCl, using o-nitrophenol (2-NP) as a model pollutant under ultraviolet (UV) and ultrasound (US) conditions[62]. The activity of FeOCl is the highest in this series of catalytic materials, and the effect of strengthening conditions on the reaction activity is in the order of UV + US > UV > US. In the later study, they also optimized the reaction conditions of this system with nitrobenzene (NB) as a model pollutant, and found that the pollutant could be degraded efficiently at a low ratio of H2O2∶ to pollutant (30.86: 1, molar ratio), which enhanced the industrial application value of this system[63]. Li et al. Also studied the photocatalytic Fenton activity of pure FeOCl[64]. Rhodamine B (RhB) can be degraded in a wide pH range (3 ~ 9) under visible light irradiation. The quantitative analysis showed that the generation rate of the photo-Fenton system was 30% higher than that of the traditional Fenton system. However, in general, pure FeOCl is a semiconductor with a narrow band gap (~ 1.9 eV), and the separation efficiency of electron-hole pairs is low, which limits the further development of its photo-Fenton system. Researchers generally use other photocatalytic materials to modify FeOCl to improve its photo-Fenton performance, which will be described in the following part.
4.1.2 FeOCl exfoliation modification system
Peeling the layered FeOCl into a few-layer or single-layer structure can effectively increase the number of exposed Fe active sites, and the reaction activity of the Fe sites may also change, which can improve the Fenton activity of the material. Zhang et al. Obtained Fe-layered FeOCl by sonication in acetonitrile solvent, which showed more excellent Fenton activity than bulk FeOCl. They found that fe-layered FeOCl surface Fe sites had lower valence state than bulk FeOCI by DFT calculation, which may make fe-layer FeOCI more effective in activating H2O2[17]. After that, Zhang et al. developed a "heating-rapid annealing" preparation method based on the partial pyrolysis method, in which few-layer FeOCl nanosheets were prepared by directly heating and stripping the FeCl3·6H2O, and the prepared FeOCl nanosheets had a FeOCl/iron hydroxide composite structure, which had a higher specific surface area (34.65 m2·g-1) compared with the bulk FeOC1[65]. The FeOCl/Fe (OH) 2 can achieve effective degradation of dye pollutants under neutral and visible-light-assisted conditions.Mechanism studies suggest that the high Fenton activity of FeOCl/Fe (oh) 2 is due to the fact that part of the Cl on the surface of FeOCl is replaced by OH, which promotes the formation and desorption of HO ·.
4.1.3 FeOCl composite material modified system
In view of the poor photocatalytic activity of pure FeOCl system, the introduction of highly active photocatalytic materials to construct composite system and improve the separation efficiency of electron-hole pairs of FeOCl is an important means to promote its photocatalytic activity. Zhang et al. Developed a variety of FeOCl photocatalytic systems based on heteroatom doping and heterojunction construction. In the study of heteroatom doping, they designed three systems of rare earth elements (La and Y) doping, Sn4+ doping and Ce3+ doping. They successfully introduced heteroatoms into the lattice substitution sites of FeOCl by partial pyrolysis using chloride salts of heteroatoms as precursors[66⇓~68]. Under the condition of photo-Fenton, the three doped materials showed better degradation activity than the original FeOCl with phenol and ibuprofen as model pollutants. DFT calculations show that the doping of heteroatoms effectively changes the bond lengths of Fe — O and Fe — Cl, and even successfully induces the breaking of Fe — Cl bonds in the Sn4+ system, which changes the chemical environment of Fe sites on the surface of FeOCl, reduces the charge, and promotes the direct conversion of Fe (Ⅲ) and Fe (Ⅱ).
Heterojunction structure is a special interface structure formed by the superposition of semiconductor material layers and layer structures. The formation of heterojunction structure can also effectively improve the separation efficiency of electron-hole pairs of FeOCl, thereby improving its photocatalytic activity. Zhang et al. developed three heterojunction systems of graphene oxide (GO), reduced graphene oxide (rGO) and polyoxometalate (POM), and Xie et al. developed g-C3N4 heterojunction system[69⇓~71][72]. These FeOCl heterojunction systems also showed excellent catalytic activity in the active test with rhodamine B (RhB) and phenol as pollutants, and the leaching of Fe was greatly reduced due to the protection of the FeOCl layer structure by the heterojunction structure. The characterization based on photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) shows that the heterojunction system has faster interfacial charge transfer and more efficient photo-generated carrier separation ability than the FeOCl system.DFT characterization also shows that the charge of Fe atoms at the interface of the heterojunction is lower than that of the original FeOCl system, which indicates that the Fe atoms at the interface have a more efficient charge transfer rate with the H2O2, which is the reason for the improvement of Fenton activity.
In addition, Liu et al introduced carbon quantum dots (CDots) with high photochemical activity on the surface of FeOCl by chemical embedding method to construct FeOCl/CDots system[73]. CDots, as an active site for electron-hole pair separation, can induce O2 in solution to generate H2O2 in situ via a two-electron transfer pathway (O2→HOO·→H2O2), which is then activated by FeOCl to generate HO · to achieve pollutant degradation (Figure 7). In the case of using 4-chlorophenol (4-CP) as a model pollutant, the degradation rate and mineralization rate of this system are 4 times and 6 times higher than those of using FeOCl alone. The success of this system enriches the oxidation activation pathway of FeOCl and deepens the understanding of the redox reactivity of Fe sites on the surface of FeOCl.
4.1.4 FeOCl intercalation compound system
Based on the understanding that the geometry and electronic structure of FeOCl may be significantly affected by the intercalation modification, thus affecting its catalytic performance, our group has recently developed a polyaniline (PANI) intercalated FeOCl system[74]. We first prepared FeOCl-PANI materials with different intercalation time in acetonitrile solvent, and the gel permeation chromatography (GPC) characterization showed that the degree of polymerization (molecular weight) of PANI increased with the increase of intercalation time. FeOCl-PANI with different intercalation time was used in Fenton degradation of 2-methoxyphenol (2-MeOP) and other pollutants under acidic (pH = 4) and neutral conditions, and it was found that the reaction activity was higher than that of the original FeOCl system. It is more noteworthy that the intercalation time of FeOCl-PANI and its reaction activity showed an opposite trend under acidic and neutral conditions, that is, the material with the shortest intercalation time (4 H) had the highest reaction activity under acidic conditions, and the reaction activity decreased with the increase of intercalation time. However, the material with the shortest intercalation time had the lowest activity under neutral condition, and the activity increased with the increase of intercalation time, and the material with the longest intercalation time (12 d) had the highest activity. Based on EXAFS, Mossbauer spectroscopy, DMPO-EPR and adsorption experiments, the reaction characteristics of FeOCl-PANI system were revealed, and the intrinsic reactivity of Fenton reaction was very high under acidic conditions.The reaction system is mainly controlled by diffusion, and the hydrogen bonding interaction between the pollutants and the PANI between the layers inhibits the diffusion of the pollutants to the reaction sites. The longer the intercalation time is, the higher the polymerization degree of PANI is, the more unfavorable the diffusion of the pollutants is, so the lower the activity is. Under neutral conditions, the intrinsic reactivity of Fenton reaction is low, and the reaction is mainly controlled by the intrinsic reactivity. The higher the degree of polymerization of PANI, the more the number of open Fe sites formed by coordination with Cl atoms in the interlayer, so the higher the activity (Figure 8). This system is the first case to explore the structure and Fenton reactivity of FeOCl interlayer space, which may have some inspiration for the functional development and utilization of FeOCl interlayer space.
4.1.5 FeOCl engineering system
FeOCl shows excellent and highly modulated Fenton activity in the previous studies, and many researchers have modified it or coupled it with specific processes according to the actual industrial application needs, and achieved good results. In order to improve the practicability of FeOCl materials in the actual industrial sewage treatment conditions, we developed a method of impregnation and calcination to achieve the immobilization of FeOCl. The impregnation and calcination method is a preparation method based on the partial pyrolysis method, which is mainly used in the preparation of supported FeOCl materials[75,76]. The specific preparation process comprises the following steps of: directly impregnating a carrier component with an aqueous solution, an alcoholic solution or a molten (80-100 deg C) solution of anhydrous ferric chloride (FeCl3) or ferric chloride hexahydrate (FeCl3·6H2O), and then heating at a high temperature to generate a FeOCl phase. In this process, the phase transformation process of FeOCl is the same as that of partial pyrolysis, but due to the interaction with the carrier, the stability of FeOCl can be effectively improved, and the structural disintegration or leaching of active components can be avoided.
Using the above method, we successfully supported FeOCl on the surface of mesoporous silica (SBA-15), and the loading amount of Fe was 10 wt%. The prepared FeOCl/SBA-15 still has excellent Fenton activity while maintaining cycle stability, and an elementary reaction model (Formula 6) is established for the decomposition of H2O2 in this system:
The kinetic constants in the formula can be fitted by the experimental data, and the calculated Thiele modulus of the decomposition H2O2 is in the range of 10-5~10-6cm2/s, which indicates that the reaction is a surface-controlled process. Furthermore, we established a semi-empirical kinetic equation for this system with BPA as the model pollutant, covering the effects of various reaction parameters on the degradation of BPA, and optimized the reaction parameters of this system by response surface methodology.
Since then, Sun et al. Loaded FeOCl on polyvinylidene fluoride (PVDF) ultrafiltration membrane and developed a "separation-degradation" synergistic pollutant treatment system for organic pollutants[77]. FeOCl was uniformly coated on the PVDF membrane, and the average particle size was about 1. 5 nm.The membrane system had good hydrophilicity and self-cleaning ability. FeOCl/PVDF can maintain excellent Fenton performance under neutral conditions, and can realize the degradation of BPA with high activity. Li et al. Developed a composite catalytic system of FeOCl and MoS2. The synergistic catalysis of Mo (Ⅳ) to Mn (Ⅵ) can effectively promote the reduction of Fe (Ⅲ) to Fe (Ⅱ) in FeOCl, thereby improving the efficiency of H2O2 activation. The degradation of simulated pollutant methylene blue (MB) can be achieved at a low dose of H2O2 (0.6 mM)[78].
Based on this, they designed a continuous flow filtration system based on a layered membrane coated with FeOCl/MoS2. The pollutant water can flow in a non-linear form in the pores of the membrane, which increases the contact area between the catalyst and the pollutant, and enables the pollutant to be efficiently degraded in a steady state mode of continuous flow (Figure 9). Recently, Liu et al. developed an electro-Fenton device based on a FeOCl-functionalized porous carbon nanotube filter, which has shown excellent performance in both tetracycline degradation and Sb (Ⅲ) detoxification studies. The process operates in a continuous flow mode and has higher degradation kinetics than a batch system[79,80].
4.2 Other advanced oxidation process (AOP) catalyst
In addition to the Fenton oxidation system using H2O2 as oxidant, FeOCl can also catalyze the activation of other peroxides to achieve the degradation of organic matter, and the research of these AOP systems has gradually attracted attention in recent years. Li et al. Found that FeOCl can effectively activate percarbonate (PC) and persulfate (PS) under visible light catalysis to achieve the degradation of RhB, in which the PS system has obvious catalytic activity in a wide pH range (3 ~ 10)[81,82]. Detection of reactive oxygen species (ROS) showed that HO- and superoxide anion (O2-·) were the main ROS for pollutant degradation. Xie et al. Studied the activation of peroxomonosulfate (PMS) by FeOCl, and found that this system had good degradation activity for many emerging pollutants such as Imipramine (an antidepressant) and many refractory organic compounds such as nitrobenzene under nearly neutral reaction conditions[83]. The detection of reactive oxygen species showed that there were two kinds of ROS in the system, namely sulfate radical ( ·) and HO ·, but the concentration of · in the system was higher, which was the main ROS for the degradation of pollutants. In addition, some impurity ions (such as bicarbonate, chloride, etc.) In the reaction system may also participate in the chain reaction of PMS activation to promote the formation of ROS. Considering that these ions are very common in real water, FeOCl-PMS system may have obvious application value in the remediation of natural water environment. On this basis, Li et al. Introduced photocatalysis into the activation system of PMS, and realized the rapid cycling of Fe (Ⅲ)/Fe (Ⅱ) through visible light excitation of photogenerated electrons, which extended the pH application range of FeOCl/PMS system to alkaline environment (pH = 11), further enhancing the application significance of this system[84].
4.3 Selective oxidation catalyst
FeOCl has shown excellent reactivity and development potential in AOP systems such as Fenton oxidation in the field of environmental catalysis. These AOP systems mainly rely on the formation of high ROS to achieve deep oxidative degradation of organic pollutants. In addition, controlling the intensity of the oxidation process to selectively oxidize the target reactant to a specific product is called a selective oxidation system, which is very important in the field of industrial catalysis such as industrial chemical synthesis. Elmetwally et al. studied the selective oxidation of benzene to phenol based on FeOCl[10]. They prepared FeOCl nanoparticles with a particle size of ~ 50 nm based on small-sized Fe2O3 precursors by chemical vapor transport (Figure 10 a). The prepared FeOCl nanoparticles were tested for the hydroxylation of benzene in acetic acid as the reaction solvent.By adjusting the process parameters such as reaction time, catalyst/oxidant dosage and reaction temperature, it was found that the conversion of benzene was about 40% and the selectivity of phenol was close to 100% under the optimal reaction conditions. They believe that the abundant Fe (II) sites on the surface of FeOCl can effectively activate the H2O2 to generate HO · in acetic acid solvent, and the HO · reacts directly with benzene to generate phenol (Fig. 10 B).
4.4 Catalyst for organic synthesis
In addition to its application in AOPs and selective oxidation catalytic systems, FeOCl has also been reported in the study of some organic synthesis catalytic systems. Selvakannan et al. grafted FeOCl nano-species onto the silanol groups on the surface of ordered mesoporous material MCM-41 by thermal condensation method[85]. Surface FeOCl, as a Lewis acid site, can effectively catalyze the polycondensation of naphthol, amide and aldehydes, and the reaction product, alkyl naphthol, is an important pharmaceutical intermediate. Mohammadi et al. Prepared a SiO2 supported BiOCl/FeOCl catalyst (BiOCl/FeOCl/SiO2) by impregnation-calcination using BiCl3 and FeCl3·6H2O as precursors, which can catalyze the synthesis of 2-aryl-1H-benzimidazoles from benzaldehyde and o-phenylenediamine under solvent-free conditions. The catalyst can be recycled for at least three times without significant loss of activity[86].
4.5 Lectrode material
4.5.1 Battery electrode material
Electrode materials are another major application area of FeOCl besides catalysts. In the 1970s, Exxon Research & Engineering Co. First proposed the possibility of using FeOCl as a cathode material for lithium-ion batteries in a patent it published[8]. In the example of the patent, FeOCl was mixed with carbon black (10 wt%) and polytetrafluoroethylene (10 wt%), pressed onto a stainless steel grid at room temperature and fired at 300.d egree. C. to prepare an electrode grid. The open circuit voltage of the lithium-ion battery composed of this material and metal Li anode is 2. 96 V, and it can be recharged and discharged 100 times at a discharge rate of 1 ~ 4 mA. In 1989, Takehara et al. Also studied FeOCl as a cathode material for lithium batteries. The discharge capacity of this system was about three times that of traditional manganese dioxide materials. However, in the charge-discharge test, it was found that the stability of FeOCl was poor. During the discharge process, Li would insert into the layers of FeOCl, eventually causing the disintegration of the FeOC1structure and decomposing into α-Fe, LiCl and Li2O[87]. Since then, they have also tested the charge and discharge process of aniline intercalated FeOCl, and found that aniline intercalated FeOCl will become γ-FeOOH and aniline derivatives (polymers or oligomers) during the charge and discharge process, in which aniline derivatives will undergo redox, which also indicates the instability of the material[88].
Because of the instability of FeOCl in the cathode material of lithium ion batteries, there has been no further report on its use as an electrode material for nearly 20 years. This silence was broken in 2013, and the emergence of a new type of chloride ion battery (CIB) provided new opportunities for the application of FeOCl materials[89]. Because oxychlorides are more stable in the cell system than transition metal chlorides, the metal oxychloride/metal system also exhibits a larger Gibbs free energy change during chloride ion transfer, resulting in a larger electromotive force, and the overall reaction of this cell can be expressed as follows:[11]
Zhao et al. First studied the application of FeOCl as a cathode material in CIB, and the discharge capacity of the first cycle was 158 mAh/G, and the discharge capacity was stable at 60 mAh/G in the next 30 cycles (Fig. 11)[11]. However, the volume shrinkage (-58.6%) and expansion (141.7%) of FeOCl are much higher than those of BiOCl (shrinkage: -24.7%, expansion: 32.7%), which indicates that the microstructure of FeOCl may need to be modified in practical applications. They also tried to develop an all-solid-state FeOCl/Li system CIB with poly (ethylene oxide) (PEO) as the chloride ion transfer matrix, and achieved success[90].
Although CIB based on pure FeOCl has been successfully constructed, the actual discharge capacity of FeOCl is low due to its poor conductivity and slow charge transfer and mass transfer. In order to improve the conductivity of FeOCl, Zhao et al prepared FeOCl/carbon composite electrodes by mixing FeOCl with various carbon materials (carbon nanotubes, carbon black, graphene) by mechanical ball milling, respectively. The addition of graphene improved the electrochemical performance of FeOCI electrode significantly, and its reversible discharge capacity increased to 184 mAh/G[91]. They also calculated and compared the electrostatic potential energy of different Cl sites in FeOCl lattice through DFT calculation, and preliminarily revealed the phase transition mechanism of FeOCl during charging and discharging. Since then, they have developed a variety of FeOCl composite carbon-based material systems, such as ordered mesoporous carbon CMK-3 supported FeOCl (FeOCl/CMK-3) prepared by vacuum impregnation-partial pyrolysis method, and polypyrrole (PPy) coated FeOC1 material (FeOC1/PPy) prepared by partial pyrolysis and gas phase polymerization method[92,93]. The reversible discharge capacities of FeOCl/CMK-3 and FeOCl/PPy were 202 mAh/G and 187 mAh/G, respectively.
In addition to considering the combination of FeOCl with other materials, Zhao et al. Also investigated the application of FeOCl in CIB from the perspective of intercalation modification[94]. They designed a FeOCl-polyaniline system (FeOCl-PANI) electrode with aniline intercalation. PANI intercalation promotes the charge transfer of FeOCl and stabilizes the interlayer structure of FeOCl, which significantly improves the electrochemical performance of FeOC1. With the increase of intercalation time, the reversible discharge capacity of FeOCl-PANI was improved gradually, and the maximum (6 d intercalation) reached 120 mAh/G (50 cycles), and the capacity retention rate reached 82%, which was much higher than 42% of the original FeOCl.
4.5.2 Lectrode material for supercapacitor
FeOCl has also been used in the development of anode materials for asymmetric supercapacitors (ASCs). Wan et al. Prepared FeOCl/CDCA materials by mixing FeOCl and porous cellulose-derived carbon-based aerogel (CDCA) through mechanical ball milling. The introduction of CDCA significantly enhanced the mechanical stability and charge storage kinetics of FeOCl[12]. FeOCl/CDCA can provide an ultra-high specific surface area capacitance of up to 1618 mF·cm-2(647 F·g-1) at a current of 2 mA·cm-2, while the capacitance loss is less than 10% after 10,000 cycles in a voltage range of -1 to 0 V in a 1 M Na2SO4 electrolyte, showing extremely high stability. The ASC composed of FeOCl/CDCA and MnO2 cathode exhibits high energy and power density (289μWh·cm-2, 1.8 mW·cm-2) as well as excellent rate capability and cycle stability. In this system, the layered stack structure of FeOCl has large specific surface area, high theoretical capacitance value and good mechanical stability, while the highly conductive and porous CDCA can be used as an electrolyte system to shorten the ion diffusion path between ions and layered FeOCl (Figure 12), which determines the excellent electrochemical performance of FeOCl/CDCA in ASC.
4.6 Other application
In addition to being used as catalyst and electrode material, FeOCl also has some applications in other fields. Sun et al. Have prepared two-dimensional FeOCl nanosheets by ultrasonic exfoliation of FeOCl, showing a good adsorption effect on Pb (Ⅱ) in water environment, with an adsorption capacity of 409. 71 mg/G, which is higher than that of most current adsorption materials, and the adsorption equilibrium time is only one third of that of bulk FeOCl[95]. HRTEM and DFT calculations show that the (110) crystal plane is mainly exposed in the two-dimensional FeOCl nanosheets, and the adsorption energy of Pb (Ⅱ) on this crystal plane is lower than that on other crystal planes of FeOCl, which can form a more stable adsorption structure and is conducive to the adsorption of Pb (Ⅱ). Wang et al. Successfully prepared FeOCl nanosheets from FeCl3 solution by pulsed laser ablation (LAL), and then uniformly modified the surface of FeOCl nanosheets with Au nanoparticles[96]. Due to the selective adsorption of HCl by Au, the Au/FeOCl composite nano-material can be used as a chemical resistor to realize the sensing of HCl in air with high sensitivity and selectivity at room temperature, and can be applied to the detection of HCl in industrial sites. The main principle of the method is to use the high energy pulse laser generated in a short time to cause the phase transition of the precursor components. This method has been widely used in the synthesis of micro/nanostructured compounds[97]. Here, due to the directionality and high energy of the pulsed laser, we can realize the phase transformation of FeOCl more controllably, that is, the crystal size and composition of Au/FeOCl materials can be effectively adjusted by changing the concentration of the FeCl3 solution, which is the most obvious advantage over the traditional partial pyrolysis or chemical vapor transport method. However, the laser device requires specific equipment and high energy consumption, the reaction conditions are more stringent, and it is difficult to scale up. These shortcomings limit the further application of the method in the preparation of FeOCl. Schoop et al. achieved the exfoliation of large size (0.5 ~ 1.0 μm) FeOCl 2D nanosheets by n-butyllithium intercalation and precise control of the humidity of the system in methanol solvent, and AFM showed that the thickness was only ~ 2.4 nm (Fig.Magnetic susceptibility measurements show that FeOCl 2D nanosheets have the same antiferromagnetic properties as bulk FeOCl, but the temperature dependence of magnetic susceptibility follows the Curie-Weiss law, with a lower Curie temperature and a magnetoresistance setback (Fig. 13C, 13d)[98]. As a novel two-dimensional antiferromagnet, the few-layer FeOCl nanosheet can be applied to the development of spintronic devices.
In addition, FeOCl can also be used as a precursor or template to induce the synthesis of other materials. Li et al. First prepared bulk FeOCl by chemical vapor deposition, then ultrasonically exfoliated it into FeOCl nanosheets, and finally used solvothermal reduction method to induce phase transformation of FeOCl to obtain Fe3O4 nanobelts[99]. The prepared Fe3O4 nanobelt has high magnetic saturation value (~53 emu·g-1) and magnetic anisotropy (6.6×104erg·cm-3), can be uniformly dispersed in most solvent environments, and has wide application prospect in biomedical materials. It can also be prepared by phase inversion using FeOCl α-Fe2O3,Frydrych et al. And Marinovi Marinović-Cincovi Cincović et al. Successfully realized this transformation process by high temperature calcination and liquid phase aging, respectively[100][101]. Yang et al. prepared FeOCl nanosheet arrays (NSAs) on fluorine-doped tin oxide glass substrates by chemical vapor deposition (CVD), and then converted them into porous α-Fe2O3NSAs by calcination-annealing (Figure 14). The potential of this material is as high as 1.23 V (vs RHE), which is three times that of ordinary α-Fe2O3 and can be used for photocatalytic water dissociation[13]. Xu et al. Successfully prepared ultrathin polypyrrole (PPy) nanosheets using FeOCl as a template through interlayer confinement in-situ polymerization-acid washing, which can be used as a photothermal agent in the second infrared region for medical treatment processes such as tumor ablation[14].
5 Conclusion and prospect
In recent years, various application researches based on FeOCl have shown a vigorous development trend, among which the catalyst research is the most rapidly developing field of FeOCl.Various AOP systems based on the activation of H2O2 and other peroxides have been widely developed, and some selective oxidation systems have also been gradually developed, which reflects the in-depth understanding of the activation behavior of oxidants on the surface of FeOCl and the progress of control methods. In addition, the application of FeOCl as an electrode material in CIB and ACS is also increasing, and it has become the second largest application research direction after catalysts. However, we must also see that the increasingly vigorous application research of FeOCl is inseparable from a large number of basic research in its early stage. The controllability of FeOCl preparation and the adjustability of its layer structure are the basis for the development of its application system. The in-depth understanding of the preparation methods (chemical vapor transport method, partial pyrolysis method, etc.) And the crystallographic structure of FeOCl enables us to effectively control the morphology and size of FeOCl.The understanding of the intercalation behavior of FeOCl and the development of various intercalation systems enable us to effectively change the crystallographic and electronic structures of FeOCl through different intercalation processes, because different intercalation guests have different geometric sizes and charges. In addition, the intercalation of different organic (metal) guest species will form organic-inorganic hybrid interfaces with specific coordination structures between FeOCl layers, and the terminal Cl atoms will coordinate with the guest species in different structures, thus significantly affecting the chemical environment of Fe — Cl bonds and the spatial distribution of FeOCl layers[102,103]. Based on this, we can directionally design the geometric and electronic structures of surface Fe sites and the interlayer structure of FeOCl, so as to develop more efficient and practical catalysts or electrode materials with excellent electrochemical properties and cycle stability.
However, there are still some important problems to be solved in the study of FeOCl system. Firstly, we need to develop more controllable FeOCl preparation technology to realize the preparation of FeOCl with specific crystal face (specific growth direction), which requires us to have a deeper understanding of the phase transformation process of FeOCl in the preparation process. Secondly, we need to have a deeper understanding of the geometry and electronic structure of FeOCl, so as to realize the precise regulation of its surface Fe site reactivity (catalytic field) or the redox performance of FeOCl structure (electrode material field).Controlled intercalation modification is the most promising means to achieve this goal, but it may also need to be combined with advanced characterization techniques or theoretical calculations. Thirdly, due to the layered structure of FeOCl and the ionic bonding characteristics of Fe-Cl bonds, its stability is slightly worse than that of general iron (hydrogen) oxides, and it is necessary to develop a highly stable FeOCl system.This requires us to clarify the structural evolution behavior of FeOCl under in-situ working conditions, and to solve the problem on the basis of improving the understanding of the first two points, such as developing supported FeOCl materials, inducing the formation of highly stable crystal planes, and strengthening the interlayer Van der Waals force by intercalation and other means.
In general, although we have a certain understanding of the physical and chemical properties of FeOCl, these understandings are still relatively simple, especially how to establish the relationship between the structure of FeOCl and its properties is still controversial and uncertain, so we need to do more in-depth research. FeOCl is a new kind of Fe-based nanomaterials, and there are still many interesting but arduous tasks around its scientific research and application development, but we believe that with the gradual deepening of its understanding.Especially, the further research on the intercalation behavior of FeOCl and the basic properties and applications of its intercalation compounds will make great contributions to the fields of environment and energy.
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