ИСТИНА

The aim of this work was to determine the effect of heat treatment on the composition and structure of microwave exfoliated graphite oxide (MEGO). Heat treatment was carried out by Knudsen Effusion Mass Spectrometry (KEMS) using nickel effusion cell. Positive ions formed by electron ionization of molecules (Eion = 75 eV) were analyzed by mass/charge ratio, m/e, on a MI 1201 magnetic mass spectrometer (r = 200mm, R = 500). At first, a long (~ 5 hour) pumping of the ion source area to a high vacuum was carried out. After the signal at the mass number m/e = 18 (H2O+) reached the level corresponding to the “background” of the device, the cell was gradually heated to the final annealing temperature, Tf = 573 K or Tf = 1073 K. At each intermediate temperature, signals were measured at the mass numbers m/e = 18 (H2O+), 28 (CO+), 32 (O2+), 44 (CO2+), 48 (SO+), and 64 (SO2+). As a result of heat treatment, two samples were obtained: low-temperature, MEGO/573, and high-temperature, MEGO/1073. Formation conditions (temperature, K / annealing time, hour) of sample MEGO/573: 523 / 2.5, 573 / 8.0; sample MEGO/1073: 573 / 11.7, 673 / 5.0, 773 / 6.3, 873 / 6.0, 973 / 8.5, 1073 / 23.0. During multi-stage MEGO annealing to final temperature of Tf = 1073 K, the signals at m/e = 32 (O2+), 48 (SO+) and 64(SO2+) remained permanent and low-intensity, weakly dependent on the presence of the sample in cell. Taking into account these features, these signals were considered as "background" for all experiments. On the contrary, for m/e = 18 (H2O+), 28 (CO+), 44 (CO2+) signal intensities in the experiment with the MEGO sample are much higher than without. The formation of the MEGO/573 sample at Tf = 573 K occurred at stationary fluxes of H2O, CO, and CO2, and the level of these fluxes indicated that their sources remain in the product after the end of the 8-hour annealing. During the formation of the MEGO/1073 sample, the H2O flux decreased with temperature increasing, and at T>773 K remained at a level close to the “background”. The analysis of CO+ and CO2+ signal dependencies on the annealing time showed that the most intense release of carbon oxides occurred in the initial annealing periods (first 5 hours) at temperatures of 873 K, 973 K and 1073 K. After the initial period, the fluxes decreased markedly, and the annealing passed into the stationary state. At Tf = 1073 K the level of stationary flows of CO and CO2 went beyond the sensitivity limit of the device. During the annealing of MEGO, no release of oxygen O2 and sulfur dioxide SO2 was detected. Annealing of MEGO at Tf = 573 K (MEGO/573) is accompanied by the release of H2O, CO, and CO2 molecules, and does not lead to the complete removal of water and oxygen-containing groups from the sample. The release of water molecules reaches a certain minimum level at a temperature of 773K, and then changes little. Intensive evolution of carbon oxides CO and CO2 at temperatures of 873–1073 K is completed by reducing their fluxes to a level beyond the sensitivity limits of the mass spectrometer. However, the absence of measurable stationary flows of CO and CO2 rather indicates the attainment of a certain minimum oxygen content for the given heat treatment conditions in the final product MEGO/1073. The prevalence of CO oxide in the gas phase corresponds to the ratio of pressures of carbon oxides under equilibrium conditions: C(s) + CO2 = 2CO