Temperature-dependent hydrogen storage mechanism in palladium nanoparticles decorated on multi-walled carbon nanotubes

Abstract

In this study, Palladium (Pd) nanoparticles (NPs) are decorated on the multi-walled carbon nanotubes (MWCNTs) using the chemical reduction method. In situ extended x-ray absorption fine structure (EXAFS) spectroscopy of Pd NPs decorated on MWCNTs is carried out in different temperatures from 77 K up to 540 K to investigate the local structure around the absorber atom Pd, under molecular hydrogen, H2, pressure, and in a vacuum. The Pd K-edge EXAFS gives a deeper insight into the structural properties of Pd-MWCNT compounds under in-situ H2 treatment at various temperatures. EXAFS data show three mechanisms of the interaction of H2 with Pd: the first mechanism involves physical absorption of H2 at low temperatures from 110 to 190 K, the second mechanism includes the formation of PdHx compound at 300 K, and the third mechanism is attributed to the decomposition of PdHx compound at above 400 K. Furthermore, X-ray photoelectron spectroscopy (XPS) results verify the reduction of the palladium oxide after interaction of the sample with molecular hydrogen and the stability of the catalyst under several cycles of annealing and cooling in hydrogen.