Thermocatalytic pyrolysis of methane for hydrogen production: Kinetic studies on Fe/Al2O3 catalysts and time-on-stream studies on biochar catalysts

Currently, fossil fuels serve as the primary energy source for humanity. Nevertheless, they are finite resources and their widespread consumption contributes significantly to CO2 emissions, which leads to severe consequences like global warming and climate change. Renewable energy sources are currently unreliable for decarbonization due to their variable outputs. A promising solution for the ecological transition is hydrogen, which acts as an energy carrier with low environmental impact. Various production technologies exist for hydrogen, primarily differentiated by their materials, costs, and emissions. At present, “grey hydrogen”, mainly produced via steam methane reforming (SMR), dominates the market and generates significant CO2 emissions. Future production aims for “green hydrogen” from water electrolysis using renewable energy, which is expected to remain costly and insufficient in the medium-term. In the interim, Methane Thermal Decomposition (TDM) could be a viable technology that produces “turquoise hydrogen” and solid carbon without CO2 emissions, but it requires temperatures above 1000 °C, resulting in harsh and costly operating conditions. Different types of catalysts can be employed to enhance reaction kinetics and lower these temperatures, the most common being metal and carbon catalysts. Metal catalysts provide better overall conversions and lower temperatures for the reaction, whereas carbon catalysts are more affordable and do not require catalyst regeneration. This study focuses on the thermocatalytic pyrolysis of methane over Fe/Al2O3 catalyst and biochars. An iron-based catalyst (60% wt.) supported on alumina (40% wt.) was synthesized using the Wet Impregnation (WI) method. Biochars from the pyrolysis of vine stems and branches were subjected to different upgrading processes to obtain activated chars. Also, two physicochemical characterization techniques were used to investigate chemical, structural and morphological properties of Fe/Al2O3 catalyst: nitrogen physisorption (BET method) and X-ray diffraction (XRD). Kinetic tests were performed on the iron-based catalyst, while time-on-stream tests were also conducted on carbon catalysts. An activation energy of 119,7 KJ/mol was calculated for the methane pyrolysis over the Fe/Al2O3 catalyst. Kinetic parameters are useful for the modelling of different types of reactors suitable for the methane pyrolysis process (such as fluidized-bed reactors), with the goal of regenerating the catalyst and recovering the carbon produced. At the same time, the performance of carbon catalysts and their deactivation in relation to their structure and upgrading processes were also assessed, highlighting that. biochar activated with CO2 demonstrates superior peak conversion overall compared to biochar activated with N2.