Size-dependent strong metal-support interaction in TiO2 supported Au nanocatalysts

Abstract

The strong metal-support interaction (SMSI) has long been studied in heterogonous catalysis on account of its importance in stabilizing active metals and tuning catalytic performance. As a dynamic process taking place at the metal-support interface, the SMSI is closely related to the metal surface properties which are usually affected by the size of metal nanoparticles (NPs). In this work we report the discovery of a size effect on classical SMSI in Au/TiO₂ catalyst where larger Au particles are more prone to be encapsulated than smaller ones. A thermodynamic equilibrium model was established to describe this phenomenon. According to this finding, the catalytic performance of Au/TiO₂ catalyst with uneven size distribution can be improved by selectively encapsulating the large Au NPs in a hydrogenation reaction. This work not only brings in-depth understanding of the SMSI phenomenon and its formation mechanism, but also provides an alternative approach to refine catalyst performance.

Fig. 1. In situ DRIFT spectra of CO adsorption over Au/TiO₂ with different particle size.

a Au-3 nm, b Au-7 nm, c Au-9 nm, and d Au-13 nm. The notation of -fresh represents the as-synthesized samples, the notation of -HX represent samples reduced by 10 vol% H₂/He under different temperatures, and the notation of -HX-O400 represent samples reoxidized at 400 °C after reduction.

Fig. 2. Representative HRTEM images of different samples.

a Au-3 nm, b Au-3 nm-H600, c Au-7 nm, d Au-7 nm-H500, e Au-9 nm, f Au-9 nm-H400, g Au-13 nm, and h Au-13 nm-H400.

Fig. 3. EELS mapping analysis of Au-3 nm-H400 sample.

a Survey image for the EELS mapping test, b the EELS spectrum image, c the EELS elemental map for Ti, d the corresponding high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image, and e the extracted EELS spectra (background subtracted) of the selected positions (red mark) in (b), as also marked by yellow squares in (d). The yellow lines in (a–c) mark the interface of Au NP and TiO₂.

Fig. 4. Establishment of the thermodynamic equilibrium model.

a Representative HRTEM image of Au-3 nm-H400. b Schematic of encapsulation of Au particle (in a spherical segment) by TiO₂ support. r (black arrow) represents the radius of Au particle, and γ_Au (orange arrow), ${\gamma }_{{\mathrm{TiO}}_{2-x}}$ (red arrow), and γ_int (blue arrow) are the surface tensions of the Au particle, TiO₂ support, and the interface between them. The thick and thin dashed red line in (b) indicate the outline of the encapsulation layer and the outline of the original TiO₂ support, respectively.