Pd Supported Catalysts with Intrinsic Surface Electropositive Sites for Improved Selective Hydrogenation of Cinnamaldehyde

Uniform-spherical Pd nanoparticles (NPs) supported catalysts were prepared by a mild-temperature chemical reduction method. Pd colloidal suspension was wet-impregnated on various supports, P25-TiO₂, SiO₂, and γ-Al₂O₃. In XPS, asymmetric Pd 3d_5/2 peak reveals % surface concentration of Pd²⁺ and Pd⁰ species. The surface Pd²⁺/Pd⁰ ratio on the catalyst surface varied between ~1 to 0.15 depending on strong-metal support interactions (SMSI) inferred from XPS and H₂-TPR studies. A linear correlation between Pd²⁺/Pd⁰ ratio and turnover frequency (TOF) was observed, with 1% Pd/P25-TiO₂ showing the highest TOF/selectivity with Pd²⁺/Pd⁰ ratio ~1.0, whereas 1% Pd/γ-Al₂O₃ showed the lowest TOF/selectivity with lowest Pd²⁺/Pd⁰ ratio 0.15. Interestingly, H₂-TPR reveals PdH decomposition peaks along with the Ti⁴⁺ reduction peak, and XPS Ti 2p of 1% Pd/P25-TiO₂ indicates the presence of Ti³⁺ in TiO₂ lattice, which may have generated due to H₂-spillover from Pd to P25-TiO₂. Hence, we observed excellent COL selectivity (~90%) and 100 % conversion with 1.5% Pd/P25-TiO₂ catalyst. Excellent COL selectivity may be ascribed to small Pd NPs (~3 nm) with intrinsic surface electropositive sites (Pd²⁺) created by partial reduction on the catalyst surface along with SMSI. These electropositive sites (Pd²⁺) promote preferential C=O adsorption. On the other hand, post-reduced catalyst in H₂ @300 °C (1% Pd/P25-TiO₂-PRH₂) with large Pd NPs (~7 nm) showed significant selectivity loss (>50 %), which confirm significance of small Pd NPs with electropositive sites.