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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - OSCAR (Operando study of the catalytic activity of gold nanoparticles by Surface Differential Reflectivity Spectroscopy (SDRS): from plasmonic response to morphology)

Teaser

Summary

The outstanding catalytic activity of gold nanoparticles (NPs) on metal-oxide supports has attracted enormous attention not only on a fundamental point of view but also for technological applications related to the demand of a new generation of more efficient and more selective catalysts. The puzzling fact is that extended faces of gold are inert while particles below a few nanometers in size become active. Following the early finding of an increase of the activity per gold atom when the NP size decreases, numerous studies have been devoted to the determination of the relationship between NP morphology, support nature and their catalytic activity. The OSCAR project has coped this issue by combining a battery of surface science techniques for exploring the model systems of Au/TiO2(110).

Work performed

OSCAR has demonstrated that, amongst the relevant parameters controlling the gold NP catalytic activity, there are the charge transferred from the metal-oxide support to the NP and the type of NP growth mode (nucleation vs growth process). As a further step, the effect on the charged state of Au/TiO2 caused by the exposure to reactive gases (CO and O2) has been evaluated by X-ray photoemission spectroscopy (XPS) after reactions. It suggests that O2 principally interacts with Ti sites located on the bare surface and not with those already interacting with Au NPs. During the project, the development of existing techniques and of new approach methods has been also achieved. The validation of surface differential reflectivity spectroscopy (SDRS) for studying NP growth in vacuum has been attained by comparing SDRS data to the results obtained by other in situ complementary techniques. In addition, the groundwork for extending the use of SDRS in reactive conditions has been laid by establishing the technical specifications required for running it in near ambient pressures. As a further step, an innovative method that allows to control defect type in TiO2 substrates have been applied for the first time for growing Au NPs. It showed that a larger quantity of charge can be transferred at interface with respect to the use of traditional preparation methods. These results are of high importance because they indicate that the present method may potentially increase the catalytic performances of the Au/TiO2(110) interface, and more in general, the properties of the oxide-based interfaces. In order to extend the results of Au/TiO2(110) to titania phase mixtures and nanostructures, the structural and electronic properties of TiO2 anatase and thin films have been explored. For this purpose, an original method based on Ti LMV Auger spectra taken by XPS has been developed. It allows at laboratory to quantify polymorph and exposed facets in titania phase mixtures. The present quantification method opens interesting perspectives for the characterization of metal-oxide phases and orientations, particularly for thin films which may escape the sensitivity of X-ray diffraction measurements. The formation of titania ultrathin films on polar ZnO substrates has also been studied. Titania films have been grown by evaporating metal Ti on ZnO in ultra-high vacuum followed by annealing treatments. It has been demonstrated for the first time that the polarity of the surface plays a crucial role in the interface reactivity and thermal stability of the film. Depending on the Zn- and O- termination of the (0001) polar surface, Ti forms TiO2 or (Ti, Zn, O) compounds, respectively, with a much lower activation temperature on the O-terminated surface.

Final results

The results of OSCAR constitute a relevant contribution to the state of the art of metal/oxide catalytic systems. They showed for the first time that charge transfer at interface and growth mode are relevant parameters for the catalytic properties of gold NPs. These results are important not only for comprehending the Au/TiO2(110) interface but also to encourage new experiments for revising the current understanding of catalytic systems based on other metal/metal-oxides interfaces. The techniques and approach methods developed in OSCAR represent a highly efficient way to evaluate the factors determining the catalytic activity and the means to improve it, which are essential for advances towards practical use. They constitute a further step between fundamental research and final applications. In addition, the high-level technical expertise obtained by the completion of OSCAR (in particular the expertise on varied complementary techniques, the deep knowledge in surface science topics, the dissemination skills and the ability in research management) is envisaged to be highly beneficial for future recruitment and career development of the fellow.