The microstructural refinement and performance improvement of a nanoporous Ag/CeO2 catalyst for NaBH4 oxidation

摘要

In this paper, Cu and Ce were added to melt-spun Al-Ag precursor alloys to refine the microstructures of nanoporous Ag and Ag/CeO2 composite catalysts for NaBH4 oxidation. After the precursor alloys were dealloyed in 20% NaOH, calcined in air and corroded again in 50% NaOH, Ag2Al in the precursor alloys was completely removed, and refined nanoporous Ag could be obtained; from this process, the finest microstructures were exhibited by Al84Ag8Cu8. When more than 0.3% Ce was added to the Al84Ag8Cu8 ribbons, a refined nanoporous Ag material that consisted of CeO2 nanorods interspersed between Ag ligaments was obtained. Electrochemical measurements indicated that the catalytic properties were clearly increased due to the Cu addition to the Al-Ag alloy. After Ce was added to the Al84Ag8Cu8 ribbons, the catalytic properties of the resulting material were further improved. In regard to melt-spun Al84Ag8Cu8Ce0.5, the obtained nanoporous Ag/CeO2 presented the best properties, and its current density was 2.5 times that of Al84Ag8Cu8, 3.1 times that of Al90Ag8Cu2 and 2.3 times that of Ag/Ce from the Al79Ag15Ce6 precursor alloy without Cu. It was believed that the core-shell structure composed of Ag and Cu-rich phases formed during dealloying could limit the diffusion of Ag and prevent the coarsening of Ag ligaments. Thus, the refined microstructures could provide a large specific surface or additional active sites for the catalytic reaction. Strong interactions resulted from the many interfaces between the Ag ligaments and interspersed CeO2 nanorods, and the more effective utilization of Ag was due to the decomposition of Ag2Al; this result was the key reason for the clear improvement in catalytic performance.