(658c) Tuning the Hydrogenation of CO2 to CH4 over Mechano-Chemically Prepared Palladium Supported on Ceria

One promising route to mitigating CO₂ is using point source CO₂ capture technologies to create high purity CO₂ that can be upgraded to value added products, such as methane. Due to the high thermodynamic stability of CO₂, catalytic routes are often required to convert CO₂ into high value products. The thermo-catalytic approach is often favored due the robust nature of the process, which can be readily scaled to demand and easily integrated into existing point source CO₂ sites. However, while several advances have been made in the mechanistic understanding of CO₂ hydrogenation, there is still need for detailed spectroscopic evidence that unambiguously link changes in the surface to shifts in the reaction mechanism.

CO₂ methanation reactivity, reaction mechanism, and surface structure were investigated on a mechanochemically prepared Pd/CeO₂ catalyst (PdAcCeO₂-M), where an oxidative pretreatment (-o) increased methane yield by a factor of two compared to a reductive pretreatment (-h). Methanation rates were maintained for over 48 hours and further increased upon oxidative regeneration treatments. The surface species of both PdAcCeO₂-M-o and PdAcCeO₂-M-h were explored via in situ CO₂ and CO hydrogenation DRIFTS, where CO hydrogenation effectively models the dissociative CO₂ mechanism (CO₂ à CO à CH₄). Additionally, isotopically labeled CO₂ hydrogenation showed that bridged formate was the primary intermediate and the influence of *OH/O_vac was critical for methanation. PdAcCeO₂-M-o yielded distinct Pd-CO adsorption and the absence of monodentate carbonate at ~ 1400 cm^-1, while AP-XPS showed that PdAcCeO₂-M-o yielded a unique Pd^δ+ contribution at 335.9 eV. By gaining insights from various in-situ spectroscopic techniques, and by breaking the CO₂ hydrogenation mechanism into piecewise steps, a deeper understanding of the direct CO₂ reduction towards methane and CO over mechanochemically prepared Pd/CeO₂ catalysts was obtained.