Revisiting the Evolution of IR Spectra of CO Adsorbed on Au Nanoparticles Supported on Non-reducible Supports.

Gold surfaces and nanoparticles have commonly been investigated by IR spectroscopy using carbon monoxide as a molecular probe. An evolution of the IR spectra with time under CO was reported in some cases and tentatively assigned to geometrical effects (e.g. surface reconstruction or particle restructuring) or electronic effects, e.g. gold becoming negatively charged following support reduction. We report here an investigation of the evolution of the IR spectrum of CO adsorbed at 50 °C in the case of Au nanoparticles supported on non-reducible silica and alumina supports. The evolution of the carbonyl IR band signal, proposed to arise mostly from Au particle restructuring, was clearly observed. In addition, the formation of hydrogenocarbonates species on the alumina reemphasized that Au nanoparticles are somewhat active for CO dissociation via Boudouard reaction near room temperature, leading to some deposition of carbon. Water could not prevent restructuring, while O₂ prevented irreversibly the reconstruction, which could even be reversed when O₂ was introduced on an already restructured sample. It is hypothesized that the role of O₂ would be to remove the deposited elemental carbon that otherwise stabilize reconstructed Au. These results emphasize the complexity of the parameters affecting the restructuration of Au surfaces induced by CO and the difficulty in interpreting IR spectra of CO adsorption on Au-based materials. [ABSTRACT FROM AUTHOR]