Pd/TiO₂ and Surface Hydrogen: A Dual Driver for n-Butyraldehyde Aldol Condensation and Selective Hydrogenation of the...

The integrated aldol condensation of butyraldehyde and selective hydrogenation of its product on Pd/TiO₂ in situ is a novel approach for synthesizing 2-ethylhexanoic acid. Understanding the catalytic reaction mechanism is essential for enhancing the performance of Pd/TiO₂, and Density Functional Theory (DFT) was utilized to explore the fundamental principles. The main aldol condensation pathway involved α-H dissociation, C-C coupling, β-O hydrogenation, and stepwise dehydration, with β-O hydrogenation as the rate-determining step (0.90 eV). The rate-determining steps for aldol condensation side reactions were the dissociation of formyl-H and β-H, with barriers of 3.50 and 1.41 eV, respectively. The rate-determining barrier for C=C hydrogenation on Pd sites was 1.25 eV, lower than that for C=O hydrogenation (1.44 eV), indicating that C=C hydrogenation is kinetically more favorable and selective. The broadening of the projected density of states (PDOS) near the Fermi level for C=C confirmed strong π-d hybridization, facilitating Pd electron injection into the C=C π* orbital and promoting hydrogenation. In contrast, the narrowed PDOS of C=O indicated weak interaction with Pd, inhibiting electron injection and suppressing its hydrogenation. Moreover, comparing rate-determining steps revealed that elevating surface hydrogen concentration facilitated the main reactions (β-O and β-C hydrogenation) and inhibited side reactions (formyl-H and β-H dissociation). Therefore, Pd/TiO₂ and surface hydrogen constituted a dual driver that catalyzed the aforementioned process. This work provides a critical theoretical foundation for designing high-performance Pd/TiO₂ catalysts and optimizing process conditions.