(98f) Identification of Highly Selective Surface Pathways for Methane Dry Reforming Using Mechano-Chemical Synthesis of Pd-CeO2

As natural gas production is projected to increase in tandem with a global effort to curb emissions, the valorization of methane while simultaneously decreasing carbon dioxide emissions has attracted substantial interest. The dry reforming of methane (DRM) is the one-to-one reaction between methane and carbon dioxide to generate carbon monoxide and hydrogen (DRM, CH₄ + CO₂ ⇋ 2H₂ + 2CO, ΔH^o_298K = 247 kJ/mol), i.e., syngas, which is an essential chemical building block to higher value products. The main challenge to DRM lies in the high thermodynamic stability of CH₄ and CO₂ and the endothermic nature of the reaction, where the activation of the methane requires high operating temperatures. To this end, ceria-based catalysts have been explored due to their stability under high operating temperatures and their ability to stabilize active metal centers in addition to serving as an adsorption site for CO₂. Furthermore, mechanochemical milling of metal precursors onto ceria has been shown to be a promising method to prepare highly selective and active catalysts for oxidation reactions.

The DRM reaction mechanism was explored on mechanochemically prepared Pd/CeO₂ catalysts (PdAcCeO₂M), which yield unique Pd-Ce interfaces, where PdAcCeO₂M has a distinct reaction mechanism and higher reactivity for DRM relative to traditionally synthesized impregnated Pd/CeO₂ (PdCeO₂IW). In-situ characterizations and DFT calculations revealed that the enhanced chemistry of PdAcCeO₂M can be attributed to the presence of a carbon-modified Pd⁰ and Ce^4+/3+ surface arrangement, where distinct surface species are sustained exclusively under reaction conditions. This unique arrangement leads to highly selective and distinct surface reaction pathways that prefer the direct oxidation of CH_x to CO, identified on PdAcCeO₂M using isotope DRIFTS and highlighting for the first time linear Pd-CO species bound on metallic and C-modified Pd, leading to adsorbed HCOO [1595 cm^-1] species as key DRM intermediates, stemming from associative CO₂ reduction.