Aligning H2 Activation with Functional-Group Polarity: K+-Promoted Switchable Hydrogenation on Pd/TiO2.

Rationally aligning H2 activation modes with the polarity of substrate functional groups is central to chemoselective heterogeneous hydrogenation but remains poorly understood. Here we show that K+ doping of Pd/TiO2 (Pd/KTiOx) enables concerted control of H2 dissociation and C═O activation, resulting in a switchable hydrogenation chemoselectivity. Pristine Pd/TiO2 selectively hydrogenates the C═C bond of maleic anhydride to succinic anhydride with 99% selectivity, whereas Pd/KTiOx hydrogenates both C═C and C═O bonds to γ-butyrolactone with 99% selectivity. On both catalysts, H2 undergoes homolytic cleavage on Pd nanoparticles to generate H· species responsible for C═C hydrogenation. Introduction of K+ at the Pd-TiO2 interface strengthens K-O electrostatic interactions and increases the electron density on Pd, which together lower the barrier for heterolytic H2 cleavage at Pd-O sites and generate polarized Hδ--Hδ+ pairs. These heterolytic hydrogen species preferentially activate polar C═O bonds, thus redirecting the hydrogenation pathway. This work establishes alkali-metal promotion as a strategy to tune the balance between homolytic and heterolytic H2 activation and, in turn, to control functional-group-dependent hydrogenation selectivity in heterogeneous catalysis.