Intercalation of graphite in the ternary systems C-HNO3-R (R = H2O, CH3COOH, H3PO4, H2SO4)

Abstract

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 Bronsted acid (pK α = 4.76), does not form cointercalation compounds when graphite is treated chemically or electrochemically in HNO3–CH3COOH solutions. H3PO4 , a weak Bronsted acid (pK α = 2.12) forms ternary intercalation compounds. Stage II–IV ternary GICs with HNO3 and H3PO4 (d i = 8.05 A) were for the first time synthesized and investigated. H2SO4 , a strong Bronsted acid (pK α = –2.8), forms stage I cointercalation compounds (I c = 8.02 A), independent of the HNO3 content (5–95 wt %) of the oxidizing mixture. The potential of the HNO3–H2SO4 solutions was found to be E Ag/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.