PdAg Nanoparticles Supported on Na ₂ Ti ₃ O ₇ for CO ₂ Hydrogenation into Formic Acid: Expanding Catalyst Design from Powder to a Metal 3D-Printed Self-Catalytic Reactor

Formic acid (FA: HCOOH) has emerged as a promising hydrogen storage material because of its high hydrogen storage density and ease of handling. Thus, a practical and rational heterogeneous catalyst for FA synthesis via liquid-phase CO2 hydrogenation is highly expected. In this study, we first prepared PdAg alloy nanoparticles (NPs) supported on Na2Ti3O7 via simple ion-exchange followed by chemical reduction. These powdered specimens promote the efficient selective hydrogenation of CO2 to give FA even under mild reaction conditions. The attack of the dissociated H atom on the HCO3– intermediate was found to be the rate-determining step, which is facilitated by PdAg alloy NPs with a low Pd/Ag ratio. The insights obtained from the powdered specimens were subsequently applied to the design of a self-catalytic reactor fabricated using metal three-dimensional (3D)-printing technology. Oxidation and hydrothermal treatment under alkaline conditions of cylindrical-shaped Ti-based catalytic reactors produced Na2Ti3O7 nanofibers on their surface, which further enabled the deposition of PdAg NPs via ion-exchange followed by chemical reduction, and facilitated the liquid-phase CO2 hydrogenation into FA. We also investigated the influence of 3D-printing scan strategies on the microstructure and catalytic performance by changing the laser scan direction.