Carbon formation pathways and kinetic implications in methane dry reforming on nickel revealed by spatio-temporal operando Raman spectroscopy

Carbon formation pathways and their kinetic implications during Methane Dry Reforming (MDR) reaction on Ni-based catalysts were investigated by integrating catalytic tests under kinetic control with spatio-temporal operando Raman spectroscopy. This approach revealed the influence of operating conditions on the surface dynamics and resulting gas-phase behavior over unconventionally long time-on-stream tests, typically lasting 24 h. From the combination of gas-phase composition analysis, semi-quantitative information from G (≈ 1580 cm −1 ) and D (≈ 1350 cm -1 ) bands integration in the Raman spectra, and carbon quantification via CO₂-regeneration cycles, we propose the coexistence of two distinct carbon species with different time-scale origins and impacts on catalyst activity: (I) an early deposited graphitic fraction primarily coming from CH 4 , which is responsible for a rapid loss of catalytic activity, and (II) a more defective carbon, formed via carbon diffusion into Ni lattice and associated with carbon structures commonly reported in the literature as tip-growth filaments or encapsulating species, which dictates the long-term deactivation dynamics. These insights provide a consistent picture of both the nature and impact of deposited carbon under varying CH 4 and CO 2 concentrations, CO₂/CH₄ ratios, temperatures, and space velocities . Finally, these mechanistic observations are systematically compared with those previously established for Rh-based catalysts under analogous conditions using the same experimental setup, revealing fundamental mechanistic distinctions between transition-metal and noble-metal systems primarily associated with a different extent of carbon diffusion. The non ideality of Ni imposes severe limitations on long-term stability, whereas a suitable choice of operating conditions for Rh can ensure extended performance over time.