Palladium-catalyzed thermo, photo, and electrocatalytic CO₂ conversion to methanol and formaldehyde: A review of mechanistic pathways using synthetic, biogas, and fossil-derived CO2

The rapid growth of atmospheric greenhouse gases makes it necessary to introduce sustainable approaches, such as the sequestration of CO₂, to mitigate CO₂ emissions and reduce dependence on depleting fossil energy sources. The direct conversion of CO₂ into valuable products such as methanol and formaldehyde is a promising sustainable route to the key building blocks of industrial chemistry, towards a sustainable pathway toward carbon neutrality, and supports the transition to a circular carbon economy. Palladium (Pd), a leading transition metal catalyst, is superior at CO₂ activation and hydrogenation. Its applications in tandem systems, with co-catalysts like copper, silver, or platinum, Pd-based systems exhibit excellent activity, selectivity, and stability. These Pd–MₓXᵧ bimetallic catalysts with X = a, secondary dopant or metal are best suited for accelerating the reduction of CO₂ to methanol, a liquid fuel with high density and wide applications in energy storage, transport, and chemical industries. This review depicts the evaluation of recent advancements in Pd-based catalysts for the process of CO₂-to-methanol and formic acid conversion via thermocatalytic, photocatalytic, and electrocatalytic pathways. Emphasis is placed on reaction mechanisms, intermediate stabilization, and the stabilization of key intermediates, offering insights into the design principles that govern catalytic efficiency and selectivity. Furthermore, the rational design of catalysts is greatly enhanced by advanced techniques such as X-ray absorption spectroscopy (XAS) and density functional theory (DFT), which together elucidate the intricate relationships between structure, functional groups, and catalytic performance, leading to efficient CO₂ conversion technologies.