Enhanced IR absorption of CO adsorbed on Pd nanoparticles embedded in the mesoporous molecular sieve SBA-15

Abstract

Enhanced IR absorption (EIRA) of CO adsorbed on Pd nanoparticles embedded in the mesoporous molecular sieve SBA-15 was discovered in the current paper. Palladium ions (Pd 2+) were introduced into the mesoporous molecular sieve SBA-15 through impregnation of the sieve into palladium chloride solution followed by calcinations. The Pd 2+ ions embedded in SBA-15 were reduced firstly to small primary Pd particles (Pd 0) by cyclic potential scans between 0.00 and −0.40 V (SCE). Pd nanoparticles (Pd 0 n ) were formed ultimately by adsorption and desorption of CO on Pd 0-embedded SBA-15, in which CO served to induce migration and coalescence of the Pd 0. Cyclic voltammograms of electrodes of both Pd 0 and Pd 0 n display characteristic features of surface processes of hydrogen adsorption–desorption, which are different from those of a bulk Pd electrode showing mainly bulk processes of hydrogen absorption. Cyclic voltammetric studies demonstrated that electrochemical reactions might be carried out easily in SBA-15 that is composed of an orderly arrangement of hexagonal channels of size 10 nm, ascribed to a small resistance of ion diffusion and electron transition in the Pd 0 n -embedded SBA-15 system. In situ FTIR results revealed that IR absorption of CO adsorbed on Pd nanoparticles embedded in the SBA-15 has been enhanced 11-fold, and the full-width at half-maximum of the CO bands is significantly increased. Nevertheless, the Stark tuning rate of the IR band of bridge bonded CO is determined to be only 24 cm −1 V −1, which is much smaller than the value (42 cm −1 V −1) measured for the same species on a bulk Pd electrode. The present study is of importance to reveal the particular IR optical properties of metal nanoparticles confined in molecular sieves.