THERMAL KINETICS OF OXIDATION OF GRAPHENE–MICRO   POWDERS IN OXYGEN FLOW 

Abstract

The oxidizing properties of graphene micro powders in a purified oxygen flow were measured (v_O2 = 5.21 · 10^–4 mol · sec^–1 = сonst). The graphene powders were certified by the manufacturer (Center for Advanced 2D Materials, Innovation Development Laboratories, National University of Singapore). The following methods were used to test the powders: fractional sequential oxidation over time at set temperature maxima of the CO₂ release rate and gradual heating, like that provided by DTA, at a speed of ~20 K/min. The amount of extracted CO₂ (as C% wt.), was recorded every minute. The repeated sequence of fractionated temperature oxidation of parallel graphene samples determined the characteristic decomposition (oxidation) of three fractions at temperatures of 953, 1003, and 1043 ± 10 K. The oxidation results showed that the graphene powders were satisfactorily homogeneous, and their oxidation proceeded in the same way as in the transverse, conical and tubular carbon nanofibers at 923, 973 and 1013 ± 10 K. Three similar morphological components of graphene particles-micro were visually identified in scanning electron microscopy images as planar, conical, and tubular particles-micro. The average sizes of graphene microparticles were three orders of magnitude larger than the average diameters of the same fibrous nanoforms. The oxidation mechanism for carbon nano-and micro forms in air has two states. The first stage involves peripheral oxidation of graphene particles (wait state): absorption of oxygen molecules on the surface of 2D graphene (≥234 K), migration to the edge (perimeter) carbon atoms, recombination of oxygen molecules with perimeter carbon, and saturation of broken carbon bonds with bridged oxygen (2–) bonds. The second stage (thermo-kinetic state): CO cleavage in heating and oxidation of CO to CO₂ (oxidation, decay, combustion). The wait state is maintained by the thermo-kinetic process. The sizes of microparticles and the specific concentration of edge perimeter pairs of carbon and oxygen atoms on the periphery of graphene platelets influence the oxidation rate and oxidation temperature of powder fractions. The shift of oxidation temperatures for morphological graphene forms in comparison with fibrous nanoforms is +(30–50 K) on average. The procedure for purification of graphene powders promotes the transition of the most active planar particles into rolled forms. The kinetic temperature dependence for the oxidation of purified graphene represents a step S-shaped curve with saturation. The initial rates of carbon oxidation, v_oxC = 4.57 · 10^–8 mol × sec^–1, were recorded at 823 K. In entry into the exponential range of measurements (873–983 K), the oxidation reaction rates v_oxC of graphene powder-micro increase from 9.99 · 10^–8 to 1.5 · 10^–6 mol · sec^–1 and the oxidation reaction rate constants k_oхC from 1.91 · 10^–4 to   1.51 · 10^–3. Activation characteristics are E_a.oxC = 168 ± 10 kJ · mol^–1 and frequency characteristics are A₀ = 6.06 · 10⁵ to 7.40 · 10⁶ sec^{– 1}. The oxidation rate varied from 1.92 · 10^–6 to 1.06 · 10^–6 mol · sec^–1 after the inflection point at 983 K and up to 1073 K and from 8.89 · 10^–7 to 3.30 ·10^–8 mol · sec^–1 at 1093 K. Subsequently, up to 1123 K, the carbon oxidation rate was zero when the sample burned completely. The known theoretically calculated activation energy of graphite oxidation is 172 kJ · mol^–1. The experimental results are within the theoretical value. Preliminary measurements for multiwalled nanotubes and intragranular inclusions of graphite and free nanosized carbon (onions, graphite platelets, conical fibers, and tubes) of commercial boron carbide powders B_15–xC_x are close to the above data.