N. V. Maksimova

Anodic Oxidation of Graphite in 10 to 98% HNO3

Systematic studies into the anodic oxidation of natural fine-particle graphite in 10 to 98% HNO3are described. The concentration ranges of the formation of stage I–IV graphite nitrates are determined. Below CHNO3 = 55%, no graphite intercalation compounds are detected by x-ray diffraction. The electrochemical oxidation of graphite in dilute HNO3yields graphite oxide phases. Their formation seems to be preceded by intercalation and hydrolysis, which occur in parallel. The properties of the oxidation and hydrolysis products are shown to depend strongly on the process parameters.

Synthesis of ternary intercalation compounds in the Graphite-HNO3-R (R = H2SO4, H3PO4, CH3COOH) systems

The reactions of stage II–IV graphite nitrates with concentrated H2SO4 , H3PO4 , and CH3COOH were studied at graphite : acid weight ratios from 3 : 1 to 1 : 1. The results demonstrate that the reactions in question follow different paths. In the graphite nitrate–H2SO4system, the reaction decreases the stage index and yields a ternary graphite intercalation compound. The contents of intercalated HNO3and H2SO4are controlled by the amount of H2SO4and the stage index of the parent graphite nitrate. The reaction between graphite nitrate and H3PO4leads to partial replacement of HNO3by H3PO4 , increasing the identity period without changes in the stage index. The results for the graphite nitrate–CH3COOH system provide no direct evidence for the formation of an intercalation compound with HNO3and CH3COOH. It is shown that varying the nature and amount of the second intercalate species opens up possibilities for preparing oxidized graphite with controlled physicochemical properties.

Effect of graphite nitrate exfoliation conditions on the released gas composition and properties of exfoliated graphite

We have studied the fundamental aspects of the exfoliation of intercalation compounds of natural flake graphite with nitric acid and proposed a technique for qualitative and quantitative in situ analysis of the gases forming during graphite nitrate exfoliation. We have examined the effects of the graphite nitrate exfoliation atmosphere, temperature, and time on the released gas composition and the properties of the resulting exfoliated graphite.

Intercalation of Graphite in the Ternary Systems C–HNO3–R (R = H2O, CH3COOH, H3PO4 , H2SO4)

The reactions between highly oriented pyrolytic graphite and HNO3–R (R = H2O, CH3COOH, H3PO4 , H2SO4) solutions were studied by x-ray diffraction and potentiometry. The results demonstrate that the nature of component R has a crucial effect on the intercalation process and phase composition of the reaction products. The ability to form ternary graphite intercalation compounds (GICs) depends on the acidity of R. It is shown that CH3COOH, a weak protic Brönsted acid (pKα = 4.76), does not form cointercalation compounds when graphite is treated chemically or electrochemically in HNO3–CH3COOH solutions. H3PO4 , a weak Brönsted acid (pKα = 2.12) forms ternary intercalation compounds. Stage II–IV ternary GICs with HNO3 and H3PO4 (di = 8.05 Å) were for the first time synthesized and investigated. H2SO4 , a strong Brönsted acid (pKα = –2.8), forms stage I cointercalation compounds (Ic = 8.02 Å), independent of the HNO3 content (5–95 wt %) of the oxidizing mixture. The potential of the HNO3–H2SO4 solutions was found to be EAg/AgCl = 1.39 V, independent of the HNO3 : H2SO4 ratio. The main relationships in the ternary systems were shown to be similar to those for the formation of binary GICs with acids. There is a perfect correlation between the redox potential of the HNO3–R (R = H2O, CH3COOH, H3PO4 , H2SO4) solutions and the stage index of the resulting GIC. The concentration ranges of GIC formation in nonaqueous HNO3 solutions were extended substantially. The behavior of stage I–IV graphite nitrates in different solvents (H2O, CH3COOH, H3PO4 , and H2SO4) was studied. Based on the experimental results, mechanisms of the processes in the systems studied were proposed.

Synthesis of Intercalation Compounds in the Graphite–HNO3–H3PO4 System

The highly oriented pyrolytic graphite–HNO3–H3PO4system was studied by x-ray diffraction and potentiometry at different acid concentrations. The results demonstrate that chemical and electrochemical intercalation in the graphite–98% HNO3–85% H3PO4system yields graphite nitrate, a binary graphite intercalation compound (GIC). H3PO4is shown to have an ambiguous effect on the concentration ranges of different stages of graphite nitrate, shifting them to lower HNO3concentrations as compared to the graphite–HNO3–H2O system. In the graphite–98% HNO3–100% H3PO4system, a stage II ternary GIC is obtained, with an intercalate layer thickness di≃ 4.7 Å. Stages II–VI of this GIC were prepared via exchange reaction between graphite nitrate and 100% H3PO4 . A mechanism for the formation of the ternary GIC is proposed. The synthesis of the cointercalated GIC is likely to involve two steps: in the first step, graphite nitrate is formed; subsequent reaction in the intercalate layer leads to partial replacement of solvated HNO3by H3PO4molecules.

Potentials of graphite nitrate formation during spontaneous and electrochemical graphite intercalation

Earlier and new results on spontaneous and electrochemical intercalation reactions of highly oriented pyrolytic graphite and nitric acid in a wide range of HNO3 concentrations are summarized and analyzed. The oxidizing capacity of the solution is shown to determine the extent of intercalation and the stage index of the resulting graphite intercalation compound (GIC), as in the graphite–H2SO4 system. The final potential of graphite nitrate after spontaneous intercalation coincides with the potential EAg/AgCl in HNO3 . During the reaction of graphite with an HNO3 solution, the potential of graphite nitrate varies monotonically, in contrast to the steplike variation inE in the C–H2SO4–oxidant system. The behaviors of Brönsted acids which can and cannot spontaneously intercalate into graphite are compared. It is shown that the anodic polarization of graphite in HNO3 offers the possibility of controlling the potential and stage index of the resulting GIC by varying the HNO3 concentration and current. During anodic polarization in 75–98% HNO3 at I= 30–100 μA, the interplay between the spontaneous and electrochemical oxidation processes leads to the formation of stage II graphite nitrate, irrespective of the charge passed, and notably reduces the intercalation rate. This effect is interpreted in terms of the intercalation mechanism and sorption processes. The data on the anodic polarization of graphite at small currents point to fundamental differences in electrochemical behavior between the intercalants that can (HNO3) and cannot (H2SO4) spontaneously intercalate into graphite. The concentration ranges and potentials of the formation of stage I and II graphite nitrates via anodic oxidation in HNO3 are determined. The electric current is shown to influence the potential of formation of the stage I GIC: the minimal potential of graphite nitrate formation in 98% HNO3 is EAg/AgCl = 1.34 V (I= 500 μA). The potentialities of the spontaneous and electrochemical intercalation reactions for the controlled synthesis of graphite nitrate with a particular stage index are compared.

Fire protection performance of oxidized graphite modified with boric acid

We have studied intercalation in the graphite-H2SO4-H3BO3-K2Cr2O7 system with the acids in the weight ratio 1: 0, 12: 1, 6: 1, or 3: 1. The results demonstrate that, with decreasing sulfuric acid (active intercalant) concentration in solution, the stage number of the resultant graphite intercalation compounds increases. Boric acid modification raises the oxidation onset temperature of oxidized graphite by 200–300°C and improves its fire protection performance, with little or no effect on the bulk density of exfoliated graphite.

Radiotracer Diagnostics of Graphite Intercalation Compounds with Sulfuric and Acetic Acids

The electrochemical intercalation of graphite in H2SO4–CH3COOH solutions is investigated by the radiotracer method in combination with autoradiography. The results attest to the formation of a stage I ternary intercalation compound in the solutions containing 60 and 80 wt % H2SO4 and stage II graphite bisulfate in the solutions containing 20 and 40 wt % H2SO4. The hydrolysis of the ternary compound leads to partial removal of acetic acid from the graphite host. Heat treatment of hydrolyzed samples results in complete deintercalation of the acids. The presence of both acetic and sulfuric acids in the synthesized compounds is confirmed by thermogravimetry combined with mass spectrometry and Fourier-transform IR spectroscopy.

Preparation of Exfoliated Graphite Modified with Magnesium Ferrite

A magnetic sorbent based on exfoliated graphite modified with magnesium ferrite has been prepared by impregnating oxidized graphite in a mixed solution of FeCl3 and Mg(NO3)2, followed by heat treatment of the impregnated oxidized graphite in air. X-ray diffraction and Mössbauer spectroscopy results demonstrate that the structure of the magnesium ferrite is an inverse spinel with a degree of inversion of 0.59. The saturation magnetization of the magnesium ferrite-containing exfoliated graphite is 16.1 emu/g, whereas its oil sorption capacity is as high as 54 g/g. Compaction of the exfoliated graphite to a density of 0.03 g/cm3 reduced its sorption capacity to 26 g/g. Further increasing the density of the material led to a considerable decrease in its sorption capacity.

Gas permeability of graphite foil modified with iron, cobalt, and nickel oxides

Graphite foils containing iron(III), cobalt(II), and nickel(II) oxides have been prepared via the impregnation of oxidized graphite in aqueous FeCl3, Co(NO3)2, and Ni(NO3)2 solutions, followed by exfoliation via thermal shock and pressing of the resultant exfoliated graphite. The materials thus prepared have been characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. We have studied the gas transport properties of the modified graphite foil samples and observed a considerable increase in the nitrogen and hydrogen permeabilities of the metal oxide-modified graphite foils relative to the unmodified graphite foil. The thermal properties of the samples have been studied by thermogravimetry and differential scanning calorimetry. The results demonstrate that their oxidation onset temperature reaches 570°C.