Characterization of Pt Nanoparticles Encapsulated in Al₂O₃ and Their Catalytic Efficiency in Propene Hydrogenation

Pt nanoparticles supported in nanoporous Al 2 O 3 catalyst are prepared by reduction of K 2 PtCl 4 solution using H 2 in the presence of Al 2 O 3 and poly(acrylic acid) as capping material. After thorough washing with water to remove Pt nanoparticles located on the external surface of the Al 2 O 3 and drying at 70 °C for 12 h, they were used in propene hydrogenation to evaluate catalytic activity as measured by the value of the activation energy in the temperature range between 30 and 90 °C. The Pt nanoparticles are characterized by using transmission electron microscopy (TEM). The particles in Pt/Al 2 O 3 are found to be encapsulated and uniformly dispersed inside the Al 2 O 3; however, the size and shapes are not clearly seen. After extraction of the Pt nanoparticles from the Al 2 O 3 channels by using an ethanol-diluted HF solution, various shapes such as truncated octahedral, cubic, tetrahedral, and spherical with a size around 5 nm are observed. The encapsulated particles have various shapes but are smaller in size than those prepared in K 2 PtCl 4 solution with polyacrylate in the absence of Al 2 O 3 . Using FT-IR studies, the capping material initially used in Pt/Al 2 O 3 is not found in the Al 2 O 3 channels. This might be due to the fact that the polymer (average MW 2100) is too large to be accommodated within the Al 2 O 3 pores. The nanopores of Al 2 O 3 have several roles in the synthesis of these nanoparticles. It allows for uniform dispersion and encapsulation of Pt nanoparticles. It controls the Pt sizes with narrow distribution that is determined by the pore dimension (5.8 nm). It protects against metal particle aggregation and produces various shapes even in the absence of the capping material. Using these Pt nanoparticles, the catalysis of hydrogenation of propene gas was studied. The initial rates, reaction order, rate constants, and activation energy for the hydrogenation are determined by use of mass spectrometric techniques. The activation energy is found to be 5.7 kcal/mol, which is about one-half that previously reported for catalysis by Pt metal deposited in SiO 2 and TiO 2 synthesized by using H 2 PtCl 6 and Pt(allyl) 2 by impregnation method.