Thermodynamic insights into carbon allotropes-guided hydrogen production in methane dry reforming

Methane dry reforming (DRM) has been recognized as a promising pathway for sustainable hydrogen production while greenhouse gases are simultaneously mitigated through CO₂ utilization. However, hydrogen yield and catalyst performance are significantly impacted by carbon deposition during the process. While graphite is typically assumed as the sole carbon species in previous studies, a comprehensive thermodynamic analysis incorporating multiple carbon allotropes - including graphite, amorphous carbon (AC), filamentous carbon (FC), single-wall (SWCNT), and multi-wall carbon nanotubes (MWCNT) - is presented in this work. Gibbs free energy minimization reveals distinct structure-dependent effects on hydrogen yield. Hydrogen production is most strongly influenced by MWCNT formation, which enhances H₂O formation at low temperatures. In contrast, amorphous carbon has minimal impact. Novel phase diagrams are established to correlate operating parameters with selective carbon formation, whereby hydrogen yield is demonstrated to be maximized at temperatures above 1200K where all carbon structures are suppressed. Additionally, hydrogen production is shown to be enhanced by up to 40% compared to conventional conditions when lower pressures (1–5 atm) are coupled with higher CO₂/CH₄ ratios, primarily attributed to minimized MWCNT formation. These findings can be utilized to enable more precise control of DRM processes, thereby advancing the development of optimized hydrogen production systems.