2.2. Thermostabilization and carbonization
The Alcell lignin fibers with and without Pt (ALFPt and ALF, respectively) were thermostabilized in a tubular furnace un-der an oxidizing atmosphere (150 cm3 STP/min of air). The fi-bers were heated from room temperature up to 200 °C at a heating rate of 0.05 °C/min, maintaining the final temperature (200 °C) for 36 h. The stabilized fibers with and without Pt (ALFSPt and ALSF, respectively) were subsequently carbonized in the same tubular furnace with a flow of N2 (150 cm3 STP/ min, Air Liquide, AlphagazÒ 1, purity of 99.999%, with amounts of oxygen and water lower than 2 and 3 ppm, respectively) at different temperatures, from 600 to 1000 °C. The heating rate in the carbonization stage was 10 °C/min. Carbon fibers obtained with and without Pt were denoted by ALCFPt and ALCF, respectively, followed by the carbonization
temperature (°C). The carbon fibers with higher platinum load, those obtained by electrospinning of a more concen-trated lignin/ethanol/platinum acetyl acetonate solution with weight ratio 1:1:0.004 and carbonized at 900 °C, are denoted as
ALCFPt-900b.
2.3. Characterization
Carbonization of the Alcell lignin fibers was studied in a thermogravimetric system (CI Electronics). The amounts of CO and CO2 evolved from the carbonization experiments were monitored by NDIR analyzers (Siemens; ULTRAMAT 22 model). A mass spectrometer (OmnistarTM, Pfeiffer Vacuum) was used to analyze the evolved gas concentrations of H2 and H2O. The surface morphology was studied by scanning electron microscopy (SEM) using a JSM 840 JEOL microscope working at 25 kV voltage and by transmission electron microscopy (TEM) in a Philips CM200 microscope at an accelerating voltage of 200 kV. Particle size distribution of the fibers with high platinum content was obtained by counting between 100 and 200 particles. The size distribution was fitted to a normal distribution. From the size distribution, we deter-mined the number average diameter dn ¼nidi=ni [25]. Platinum particle size (supposing spherical particles) and dispersion (D) are related by D = 108/dn (nm) [26]. Raman spectra were recorded with a RENISHAW micro-Raman system using an Ar+laser at 514 nm as the excitation source with a spectral resolution of 2 cmÿ1.
The porous structure was characterized by N2 adsorption desorption at ÿ196 °C and by CO2 adsorption at 0 °C, carried out in a Micromeritics ASAP2020 and a Quantachrome Autosorb-1 apparatus, respectively. Samples were previously out-gassed for 8 h at 150 °C under vacuum. From the N2 adsorption/desorption isotherm, the apparent surface area (ABET) was calculated by applying the BET equation, and the microporevolume (Vt) and the external surface area (At) were calculated using the t-method. The narrow micropore volume (VDR) was estimated using the expression of the Dubinin–Rad-ushkevich adsorption isotherm, as well as characteristic energy (E0) and optimized Dubinin–Astakhov exponent (n) of the Dubinin Astakhov equation. The three parameters were obtained from the CO2 adsorption isotherm.
The surface chemistry of the samples was studied by X-ray photoelectron spectroscopy (XPS) analysis. XPS analyses of the carbons were obtained using a 5700C model Physical Electronics apparatus with Mg Ka radiation (1253.6 eV). For the analysis of the XPS peaks, the C1s peak position was set at 284.4 eV and used as reference to locate the other peaks. The fitting of the XPS peaks was done by least squares using Gaussian–Lorentzian peak shapes. The oxidation resistance of the carbon fibers was evaluated using thermogravimetric analysis (TG) performed in a CI Electronics MK2 balance un-der N2 and air flow (150 cm3 STP/min) from room temperature to 900 °C at a heating rate of 10 °C/min with a sample weight
of about 10 mg.