Kinetic modelling of methane pyrolysis over a carbonaceous catalyst

Thermocatalytic decomposition (TCD) of methane over carbonaceous catalysts represents a promising pathway for CO2-free hydrogen production, yielding a commercially relevant solid carbon co-product suitable for battery electrode and construction applications Despite extensive work on carbon-based catalysts, no previous study has integrated homogeneous C2, C3 gas phase chemistry with transient Langmuir-Hinshelwood surface deposition kinetics in a single 1D Plug Flow Reactor transient framework for CBp2000. This thesis addresses that gap. A laboratory-scale fixed-bed quartz reactor was operated under validated kinetic regime conditions (Weisz-Prater criterion Φ = 0.013, Carberry number Ca = 1.3 · 10−5) across high temperatures of 850 ◦C to 900 ◦C, total flow rates of 72 to 130 Nml min−1 (referenced at 273.15 K and 101325 Pa), and a varying inlet CH4 concentration between 0.3-0.7 at near atmospheric pressure (1.6-2 bar), using 0.1 g of CBp2000 catalyst (CABOT®) mixed with 0.9 g of SiC. Product speciation was performed by online GC and GCxGC, enabling carbon and hydrogen balances closure. Experimental results identified three distinct temporal phases of catalyst evolution: rapid micropore occlusion (Phase 1), quasi steady-state heterogeneous activity (Phase 2), and autocatalytic regeneration of deposited carbon (Phase 3). A transient Lumped Kinetic Model (LKM) was developed comprising four homogeneous gas phase reactions and four heterogeneous Langmuir-Hinshelwood surface reactions, formulated as a system of coupled partial differential equations including catalyst deactivation function and porosity evolution. Parameter estimation was performed via a two-stage optimization strategy combining a global search (PatternSearch) with a local gradient-based refinement (Fmincon) on 12 identifiable parameters. Obtaining a surface methane decomposition activation energy ofEa,S3 = 253.5 kJ mol−1, a reduction of approximately 170 kJ mol−1 relative to homogeneous pyrolysis (Ea ≈ 420 kJ mol−1), and a deactivation exponent β = 0.73. The model reproduces the outlet molar flow rates of CH4 and H2 within ±10% across Phases 1 and 2, and captures the correct order of magnitude for C2 and C3 intermediates. While the macroscopic autocatalytic regrowth of Phase 3 falls outside the current framework’s boundaries. These results provide a quantitative transient description of CBp2000 activity, demonstrating that accurate hydrogen yield prediction over extended time on stream requires the explicit coupling of gas phase intermediates with surface deposition kinetics.