Synthesis of hydrotalcite-supported shape-controlled Pd nanoparticles by a precipitation–reduction method

Abstract

A Pd catalyst supported on a layered double hydroxide (LDH or hydrotalcite-like material) has been synthesized in a one step reaction using a precipitation–reduction method in which hydrolysis of hexamethylenetetramine was used to both precipitate the LDH by virtue of the resulting increase in pH and provide formaldehyde which reduces the Pd 2+ precursor to Pd 0. The resulting Pd nanoparticles were mostly tetrahedral in shape, and were highly dispersed on the surface of the LDH. After introducing a capping agent during the synthesis, the morphology of the Pd particles changed to truncated octahedral, although a similar high degree of dispersion was obtained. Compared with a conventional impregnated catalyst composed of pseudo-spherical Pd particles, catalysts prepared by the new method showed both higher activity and selectivity in the hydrogenation of acetylene. The enhanced activity is due to the specific morphology of the Pd particles, which results in their higher dispersion, while the higher selectivity is attributed to the Pd–C phase formed in the catalyst during the reaction. Furthermore, compared with the truncated octahedral Pd particles enclosed by (1 1 1) and (1 0 0) facets, the tetrahedral particles with only (1 1 1) facets exposed showed higher ethylene selectivity, suggesting that the Pd (1 1 1) facet is the preferred facet in the selective hydrogenation of acetylene to ethylene.