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

Abstract

Pt nanoparticles supported in nanoporous Al2O3 catalyst are prepared by reduction of K2PtCl4 solution using H2 in the presence of Al2O3 and poly(acrylic acid) as capping material. After thorough washing with water to remove Pt nanoparticles located on the external surface of the Al2O3 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/Al2O3 are found to be encapsulated and uniformly dispersed inside the Al2O3; however, the size and shapes are not clearly seen. After extraction of the Pt nanoparticles from the Al2O3 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 K2PtCl4 solution with polyacrylate in the absence of Al2O3. Using FT-IR studies, the capping material initially used in Pt/Al2O3 is not found in the Al2O3 channels. This might be due to the fact that the polymer (average MW 2100) is too large to be accommodated within the Al2O3 pores. The nanopores of Al2O3 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 SiO2 and TiO2 synthesized by using H2PtCl6 and Pt(allyl)2 by impregnation method.

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