The adsorption of CO over monomer, dimer and trimer of copper deposited on a&3bond; Si-O-center dot defect on silica surface was studied using the density functional theory formalism (DFT). The results indicate that the CO adsorption strength follows this order: Cu1 > Cu2 >= Cu3. With respect to the CO vibrational frequencies, whereas for closed-shell systems (Cu1/SiO2, Cu3/SiO2 and Cu2O) the CO molecule shows, in comparison with free CO, the lower frequency shifts, for the open-shell system (Cu2/SiO2) it produces the higher frequency shift. This behavior could be related with the variation of polarizabilities due to the interaction of CO with Cu /SiO2. The signal at 2130 cm(-1) experimentally observed for reduced Cu/SiO2 catalysts could be assigned to the CO adsorption on a partially electropositive atomic Cu linked to a &3bond; Si-O-center dot site. (c) 2005 Elsevier B.V All rights reserved.