Highly active and reusable silica-aerogel-supported platinum–cobalt bimetallic catalysts for the dehydrogenation of ammonia borane

Abstract

In this study, a series of platinum–cobalt bimetallic catalysts supported on mesoporous silica aerogel (SAG), referred to as PCx/SAG, x = 1–6, were prepared by facile chemical reduction and a simple, efficient microwave-assisted method using Co and Pt precursors. The structure, morphology, and chemical composition of a representative catalyst, PC3/SAG, with molar ratio of Pt : Co = 0.27 : 0.73, was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission transmission electron microscopy (FE-TEM), inductively coupled plasma emission spectroscopy (ICP), as well as adsorption–desorption isotherm measurements. All of the as-prepared Pt–Co bimetallic catalysts were applied for the generation of hydrogen from an aqueous solution containing 0.33 wt% ammonia borane (AB, NH₃BH₃) at a metal to AB molar ratio (M/AB) of 0.05 at 303 K. The results indicated that among the prepared catalysts, PC3/SAG exhibited the highest catalytic efficiency for the hydrolysis of AB. As compared with that observed for the corresponding mixture of Pt/SAG and Co/SAG, the time required to complete hydrogen generation using the PC3/SAG catalyst was reduced by approximately 5.6 times. A synergistic catalytic effect was observed from PC3/SAG, leading to a high turnover frequency (123.1 mol H₂ per min per mol metal) and a low activation energy (30.2 kJ mol⁻¹). Furthermore, after reusing the catalyst for 5 cycles, the H₂/NH₃BH₃ molar ratio by the dehydrogenation of aqueous NH₃BH₃ by PC3/SAG retained an ideal value of 3.0, and complete hydrolysis was achieved each time in less than 3 min. These results indicate that the bimetallic catalyst PC3/SAG prepared herein is highly efficient for the generation of hydrogen from an aqueous AB solution.