Effect of Tertiary Amine Selection on CO₂ to Formic Acid Hydrogenation with the Au-np Catalyst

Over the past decades, the production and storage of molecular hydrogen has been identified as a key solution to the current global warming crises. Here, formic acid has gained considerable traction as a possible liquid organic hydrogen carrier. One of the possible production methods is the direct hydrogenation of CO₂. However, the reaction is highly unfavorable from a thermodynamic point of view and can be made slightly favorable using tertiary amines. The addition of tertiary amines leads to adduct formation, which drives the reaction toward the product side through reduction of the formic acid activity in the bulk liquid. However, currently, the competitiveness of the process is severely hindered by the challenging and energy-intensive formic acid purification due to the formation of azeotropes between formic acid and the low-boiling tertiary amines. Additionally, the reaction rates are hampered by the limited solubility of the adduct and nonbinding amine leading to the necessity of additional solvents. Steric hindrance and the pK_a of the tertiary amine were identified as key parameters influencing both the observed kinetic rates and the CO₂ conversion. The usage of solventless polar amines such as diethylethanolamine allowed for FA productivity up to 5× that of the benchmark triethylamine system. Catalyst deactivation of the Au/TiO₂ catalyst was observed for all amines studied within this work, and the deactivation mechanism was shown to be sintering of the Au nanoparticles with no significant leaching, morphological changes, or oxidation of the Au species observed.