Reactive Hydrogen Species Behaviors on Pd/TiN: In Situ SERS Guided Regulation for Chemoselective Hydrogenation

Uncovering the H₂ dissociation and H-spillover behaviors of the generated reactive hydrogen species on catalyst surfaces is crucial for achieving efficient and chemoselective hydrogenation. However, those behaviors remain largely elusive given the challenges of directly observing H atoms, the smallest atoms in the element table. Herein, we explored the Pd site-specific D₂ cleavage and the H-spillover behaviors of dissociated D on the titanium nitride (TiN) support via in situ surface-enhanced Raman spectroscopically (SERS). Besides facilitated forming Pd single atom (Pd₁), fully exposed Pd cluster (Pd_n) and Pd nanoparticles, dissociated D₂ on Pd sites (indicated by ν_Pd-D at 1,800 cm^-1), spillover to TiN, and formation of the N─D bond (indicated by ν_N-D at 2,400 cm^-1) were spectroscopically traceable. Combined with density functional theory (DFT) calculation, Pd_n is identified as the most favorable site for the provision of reactive H through hydrogen spillover. Moreover, we propose ν_Pd-D and the ν_N-D/ν_Pd-D ratio as the index to relative amounts of Pd₁ and Pd_n sites, respectively, and develop an in situ SERS-based method to study the synthesis-structure-activity relationship of Pd/TiN catalyst with an optimized structure for chemoselective hydrogenation.