Catalytic and non-catalytic chemical kinetics of hydrocarbons cracking for hydrogen and carbon materials production

In the perspective of decarbonizing the chemical and energy industries, the production of hydrogen from the thermal and thermo-catalytic pyrolysis of hydrocarbons has been highlighted as a potentially effective route. Indeed, the "turquoise" hydrogen pathway enables to fixate the carbon content of the feedstock in high-performance, durable and recyclable materials entirely avoiding CO2 emissions. Such multiphase processes include chemical vapor deposition (CVD) and infiltration (CVI), diamond synthesis and the catalytic production of carbon nanotubes (CNTs). All the above involve a homogeneous gas-phase environment where feedstock cracking occurs together with heterogeneous interactions between gas-phase fragments and an active carbon (preform) or catalyst surface. Mechanistic models provide insights into gas-phase product distribution, untangling the competition between amorphous carbon formation (soot) in the gas-phase and structured carbon deposition. Once the models are developed and validated, kinetic analyses allow the identification of optimal conditions by relating process parameters (temperature, pressure, feedstock, catalyst morphology, etc.) to feedstock conversion, product yields and quality. This chapter provides an overview of relevant chemical kinetics aspects starting systematically from the description of reaction classes and reference rate parameters used to describe homogeneous gas-phase cracking. Then the formation mechanisms of polycyclic aromatic hydrocarbons and soot is addressed. The latter is a critical aspect as soot formation competes with the synthesis of highly structured and organized carbon materials through heterogeneous mechanisms. These are then discussed specifically focusing on CVD/CVI processes, diamond growth mechanisms and carbon nanotubes synthesis.