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SOPHIA R. SU AND DANIEL W. OBLAS GTE Laboratories, Inc., Waltham, Massachusetts,

02254,

USA

ABSTRACT Gas-phase intercalation of graphite by nitric acid is a one-step process. The weight uptake of the sample is a function of nitric acid vapor pressure. A pure second stage compound was formed when the HNO 3 reservoir was maintained 0 at 17 C. Only a fourth stage compound was formed when the acid was kept at 0°C. The product gases due to intercalation and gas species evolved during deintercalation were analyzed by mass spectrometry. NO2 and H2 0 were the major components detected from the intercalation product gases. A small amount of oxygen was also present. The existence of 02 is probably due to the photo-chemical decomposition of nitric acid. As such, the photodecomposition of nitric acid is not a contributory factor in the intercalation chemistry.

INTRODUCTION Nitric acid intercalated graphite is a widely studied system. It is straightforward to prepare, and though it is more stable in air than most other intercalation compounds, it is still difficult to characterize chemically. It is known that the enhanced electrical conductivity of the graphite is due to the fractional charge transfer between the graphite and the nitric acid intercalant. In addition, at equilibrium, approximately 3 to 5 neutral nitric acid molecules are also absorbed into the graphite lattice, as well. Various chemical mechanisms have been proposed describing the nature of the nitric acid intercalation into graphite. Forsman, et. al. [1] have identified the brown gas evolved during the intercalation process as NO2 by chemiluminescence techniques. However, no NO2 was detected in the absence of graphite. They also reported that the intercalation was inhibited by water vapor and oxygen but not by nitrogen. The reaction mechanism is then proposed as follows: 2 HNO 3 -) H2 0 + NO- + NO2 + (graphite surface)

(1)

The graphite is oxidized by nitronium ions NO2 + prior to the intercalation of NO. Water vapor inhibits the reaction by causing the equilibrium to shift to the left of equation (1). The suggested role of oxygen however is more complex involving the capture of lattice electrons to minimize the reaction of NO1 with the graphite lattice. They concluded that there are 4.5 neutral nitric acid molecules for each NO. In a slightly different approach, Loughlin, et. al. [2] used optical absorption techniques to measure the gas species evolved during the intercalation process. In addition to observing the increase in the NO2 partial pressure with intercalation, they also detected the N2 0 4 dimer. The proposed reaction scheme followed that discussed by Forsman, et. al. [3], but they concluded that 4 neutral molecules of HNO 3 intercalate with every NO. . Although there is close agreement for some of the foregoing data, there are ambiguous points. For example, it is well known that pure nitric acid undergoes photochemical decomposition above its meltinq point according to the following

Hat. Res.

Soc. Symp. Proc. Vol.

20 (1983) @Elsevier Science