High-quality 2D-hexagonal ordered mesoporous titania with well-crystallized framework structure and high surface area was successfully synthesized, in a reproducible way, using Pluronic F127, Pluronic P123, Synperonic F108 and CTAB as...

Abstract. Palladium nano particles in tetrahedral (Td), octahedral (Oh) and spherical (Sp) shapes, supported on hydrotalcite, were synthesized and characterized by XRD and TEM. Td shaped Pd nano particles, with maximum number of exposed (111) planes of Pd, display maximum turn over frequency (TOF) / intrinsic activity for hydrogenation of cinnamaldehyde, indicating the importance of active site geometry. Oh & Sp shaped Pd nano particles that expose both (111) and (100) planes have relatively lower intrinsic activity. For a given reaction, it is essential that appropriate active site geometry, as dictated by the shape/morphology of nano particles be made available. Synthesis and stabilization of such shape controlled nano particles is of prime importance in developing catalysts with higher activity.Keywords: Pd nano particles, shape control, active site geometry, cinnamaldehyde, hydrogenation, TOF. IntroductionSupported metal catalysts have been extensively studied, as they display high activity, selectivity, stability under mild reaction conditions as required in many catalytic reactions [1]. A number of factors are known to influence the performance of supported metal catalysts [2], namely, the metal loading, nature of metals, crystallite size, metal surface morphology, nature of the support, addition of second/promoter elements [3], the metal-support interactions and method of preparation [4]. Of these, the crystallite size and morphology of the metal particles are the two important factors affecting the number and nature/geometry of the active sites, which in turn influence activity and selectivity respectively. Nano size metal particles with different sizes and shapes have different degrees of dispersion, exposed facets and fractions of atoms located at different corners, edges, and defects, each having specific activity/selectivity. Thus, for a given reaction, the catalytic activity of particles with different morphology can vary significantly. Semagina and Kiwi-Minsker [5] compared the activity of nano size Pd hexagons and spheres for hydrogenation of 2-methyl-3-butyn-2-ol and observed that nano spheres with maximum amount of exposed (111) planes displayed highest TOF. Role of shaped Pd nano crystals in controlling activity & selectivity for various reactions by influencing the facets, and relative populations of atoms in edges, corners and planes has been reviewed in detail by Zhang et al [6]. Wang et al. [7] reported that silica supported Pd nano particles in octahedral shape (with predominantly (111) planes) was more active for CO oxidation compared to cubic Pd (with (100) facet) while the facet activity pattern was exactly reverse in the case of silica supported Pt. Hence, studies on the preparation, characterization and performance evaluation of shape controlled supported metal catalysts has been area of tremendous importance both from basic as well as applied aspects of catalysis. In the present work, we have explored the effect of Pd nano particles in tetrahedral (Td), octahedral (Oh) and spherical ...

<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-TiO₂, SiO₂, and γ-Al₂O₃. In XPS, asymmetric Pd 3d_5/2 peak reveals % surface concentration of Pd²⁺ and Pd⁰species. 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-Al₂O₃showed 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-TiO₂catalyst. 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. </p>

<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-TiO₂, SiO₂, and γ-Al₂O₃. In XPS, asymmetric Pd 3d_5/2 peak reveals % surface concentration of Pd²⁺ and Pd⁰species. 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-Al₂O₃showed 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-TiO₂catalyst. 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. </p>