Effect of metal nature and operating conditions on activity, deactivation pathway and reaction mechanism of Al₂O₃-supported catalysts in methane dry reforming

Dry reforming of methane (DRM) is considered a promising strategy for producing valuable syngas by utilizing two greenhouse gases. However, despite the high DRM activity of supported metal catalysts, the process presents limited applicability due to catalyst deactivation phenomena. In this study, the effect of metal nature (Rh, Ru, Ir, Pt, Re, Ni) on the activity, deactivation pathway and reaction mechanism of Al2O3-supported catalysts was investigated. The DRM activity order of the Pt group metals (PGMs) compared with Ni was found to be Pt < Re < Ni < Ir ∼ Rh < Ru, confirming Ru as the most active one. On the other hand, Rh presents higher values of H2/CO ratio and more sufficient stability compared to Ru, as shown by 40 h time-on-stream experiments. Although Rh/Al2O3 exhibits higher tendency to coke formation, Ru/Al2O3 is more prone to metal particles sintering, which seems to be the main cause of Ru deactivation. Reducing the Weight Gas Hourly Space Velocity (WGHSV) results in increased reactants conversion, but has limited impact on product distribution (i.e., the H₂/CO ratio). In situ DRIFTS studies indicated a bi-functional DRM reaction pathway according to which CH₄ is activated on metal surface, forming CHx species and H atoms, while CO₂ is mainly activated on the support surface forming carbonate species. Both CHx and carbonates, ultimately, lead to the formation of formates, which appear to be active intermediates that are, eventually, converted to CO. The reaction cycle closes with the recombination of part of H atoms, producing H2.