Pd Supported Catalysts with Intrinsic Surface Electropositive Sites for Improved Selective Hydrogenation of Cinnamaldehyde
Preprints are manuscripts made publicly available before they have been submitted for formal peer review and publication. They might contain new research findings or data. Preprints can be a draft or final version of an author's research but must not have been accepted for publication at the time of submission.
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 3d5/2 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.