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

Abstract

<p>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-TiO<sub>2</sub>, SiO<sub>2</sub>, and γ-Al<sub>2</sub>O<sub>3</sub>.<sub> </sub> In XPS, asymmetric Pd 3d<sub>5/2</sub> peak reveals % surface concentration of Pd<sup>2+</sup> and Pd<sup>0 </sup>species.<sup> </sup> The surface Pd<sup>2+</sup>/Pd<sup>0</sup> ratio on the catalyst surface varied between ~1 to 0.15 depending on strong-metal support interactions (SMSI) inferred from XPS and H<sub>2</sub>-TPR studies. A linear correlation between Pd<sup>2+</sup>/Pd<sup>0</sup> ratio and turnover frequency (TOF) was observed, with 1% Pd/P25-TiO<sub>2</sub> showing the highest TOF/selectivity with Pd<sup>2+</sup>/Pd<sup>0</sup> ratio ~1.0, whereas 1% Pd/γ-Al<sub>2</sub>O<sub>3 </sub>showed the lowest TOF/selectivity with lowest Pd<sup>2+</sup>/Pd<sup>0</sup> ratio 0.15. Interestingly, H<sub>2</sub>-TPR reveals PdH decomposition peaks along with the Ti<sup>4+</sup> reduction peak, and XPS Ti 2p of 1% Pd/P25-TiO<sub>2</sub> indicates the presence of Ti<sup>3+</sup> in TiO<sub>2</sub> lattice, which may have generated due to H<sub>2</sub>-spillover from Pd to P25-TiO<sub>2</sub>. Hence, we observed excellent COL selectivity (~90%) and 100 % conversion with 1.5% Pd/P25-TiO<sub>2 </sub>catalyst. Excellent COL selectivity may be ascribed to small Pd NPs (~3 nm) with intrinsic surface electropositive sites (Pd<sup>2+</sup>) created by partial reduction on the catalyst surface along with SMSI. These electropositive sites (Pd<sup>2+</sup>) promote preferential C=O adsorption. On the other hand, post-reduced catalyst in H<sub>2 </sub>@300 °C (1% Pd/P25-TiO<sub>2</sub>-PRH<sub>2</sub>) with large Pd NPs (~7 nm) showed significant selectivity loss (>50 %), which confirm significance of small Pd NPs with electropositive sites. </p>