Evaluation of activated carbons based on olive stones as catalysts during hydrogen production by thermocatalytic decomposition of methane

Catalytic decomposition of methane over carbon materials has been intensively studied as an environmental approach for CO2-free hydrogen production without further by-products except hydrogen and valuable carbon. In this work, we will investigate the catalytic activity of activated carbons based on olive stones prepared by two different processes. Additionally, the effect of three major operational parameters: temperature, weight of catalyst and flow rate of methane, was determined. Therefore, a series of experiments were conducted in a horizontal-flow fixed bed reactor. The outflow gases were analysed using a mass spectrometer. The textural, structural and surface chemistry properties of both fresh and used activated carbons were determined respectively by N2 gas adsorption, X-Ray Diffraction and Raman and Temperature Programmed Desorption. The results reveal that methane decomposition rate increases with temperature and methane flow however it decreases with catalyst weight. The two carbon samples exhibit a high initial activity followed by a rapid decay. Textural characterization of the deactivated carbon presents a dramatic drop of surface area, pore and micropore volumes against an increase of average pore diameter confirming that methane decomposition occurs mainly in micropores. XRD characterization shows a turbostratic structure of fresh samples with more graphitization in deposed carbon explaining the lowest activity at the end of reaction. Raman spectra reveal the domination of the two bands G and D which varying intensities affirm that the different carbons tend to organise in aromatic rings. Finally the surface chemistry qualitatively changes greatly after methane dissociation for CAGOC unlike CAGOP but quantitatively a small difference is observed which indicates that these functionalities may have a role in this heterogeneous reaction but cannot be totally responsible. Among the two catalysts tested, CAGOC has the highest initial methane decomposition rate but CAGOP is the most stable one.