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

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 (Pd2+) were introduced into the mesoporous molecular sieve SBA-15 through impregnation of the sieve into palladium chloride solution followed by calcinations. The Pd2+ ions embedded in SBA-15 were reduced firstly to small primary Pd particles (Pd0) by cyclic potential scans between 0.00 and −0.40 V (SCE). Pd nanoparticles (Pd0n) were formed ultimately by adsorption and desorption of CO on Pd0-embedded SBA-15, in which CO served to induce migration and coalescence of the Pd0. Cyclic voltammograms of electrodes of both Pd0 and Pd0n 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 Pd0n-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.