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 (Pd-0) by cyclic potential scans between 0.00 and -0.40 V (SCE). Pd nanoparticles (Pd-n(0)) 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 PdO and Pd-n(0) 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-n(0)-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. (C) 2003 Elsevier B.V. All rights reserved.