Kinetic consequences of carbon deposition in CH₄ steam and dry reforming on Rh via operando Raman spectroscopy and microkinetic analysis

This study presents a comparative investigation of Methane Steam Reforming (MSR) and Methane Dry Reforming (MDR) over a Rh/α-Al₂O₃ catalyst, with specific focus on the role of carbon deposition in catalyst deactivation. Catalytic tests combined with time-resolved operando Raman spectroscopy reveal that carbon accumulation is strongly affected by the co-reactant-to-CH₄ ratio and significantly more pronounced under MDR conditions. Although MSR exhibits similar deactivation trends, these occur at lower co-reactant concentrations, highlighting the superior effectiveness of water in suppressing carbon formation. To rationalize these observations, a detailed C₁-based microkinetic model was employed to simulate the complete set of experiments. The model incorporates an activity parameter representing the available Rh surface area per reactor volume. Using the initial Rh active surface of the fresh catalyst (α₀) as a constant input led to a systematic overestimation of CH₄ conversion, up to + 30 %, for tests affected by carbon deposition. Therefore, a TOS-dependent α parameter was introduced to account for the progressive loss of active surface due to carbon coverage. The resulting α/α₀ trends are fully consistent with both spectroscopic and kinetic evidence of carbon formation, confirming the link between surface carbon accumulation and catalyst deactivation. Finally, the model yielded spatial profiles of C* coverage consistent with the decreasing carbon trends observed along the reactor axis via spatially resolved operando Raman analysis. These results confirm the mechanistic role of C* as a carbon precursor and demonstrate that the model effectively captures both the macroscopic performance and the microscopic complexity of the reforming environment.