113HIGHLIGHTS 2020

PLATINUM NANOPARTICLES LIVING ON THE EDGE A combination of high-resolution electron microscopy, electron tomography, in-situ X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations applied to the study of Pt/γ-Al2O3 catalysts reveals the preferential localisation of Pt nanoparticles on the edges of alumina crystallites, which seem to be thermodynamically as favourable as the crystallite surfaces.

Platinum supported on chlorinated γ-Al2O3 is a typical catalyst used in the naphtha reforming process. This bifunctional catalyst contains simultaneously metallic (Pt) and acidic sites (Brønsted hydroxyls boosted by chlorine). It is generally accepted that the mean distance between the two types of sites is one of the main parameters controlling the selectivity of the catalyst, and which consequently requires optimisation. This work focuses on the quantitative assessment of the localisation of Pt on the support.

The studied systems contain small amounts of Pt (between 0.3 and 1 wt. %) and were studied as-prepared (in an oxidised state) and after activation under H2 (in the metallic phase) according to a well-defined IFPEN protocol. The electron microscopy analysis in the high- resolution and 3D modes were performed at IPCMS, the XAS spectra were acquired at beamline BM16 [1], and the quantum simulations at DFT level were undertaken at IFPEN, France. For the XAS analysis of the reduced catalysts, a dedicated operando reactor [2] allowed in-situ re-reduction prior to the acquisition procedure. In the case of the oxidised samples (Figure 96), the Pt-based phase was found to be mostly in single-atom oxide complex, the formation of clusters being noticeable only at high Pt loading. The analysis of the EXAFS data confirms the highly localised electron microscopy findings for all the analysed specimens: excepting the oxygen and chlorine neighbour atoms, no significant contribution of Pt neighbours can be deduced.

In contrast, for the reduced samples (Figure 97), the Pt phase forms well-defined nanoparticles with an average size of about 0.9 nm. These electron microscopy findings were confirmed by the quantitative XAS analysis, in which an average Pt-Pt coordination number of three was obtained for both low and high

loadings. Another feature is the occurrence of a single-atom population that can be directly visualised on the high-resolution images, the strong anchoring of such Pt species to the support being one of the factors responsible for the low k-signal in the XAS spectra.

3D analyses by electron tomography in the high- angle annular dark-field mode (HAADF) were performed on the reduced catalysts and showed that at least 70% of nanoparticles are located on the edges of the support crystallites or on edge-like features. Based on NMR experiments and DFT calculations, the first atomistic model of alumina edge sites was established, showing that hydroxyls located at edges are preferentially exchanged with chlorine [3]. A synergistic effect based on the stabilisation of the Pt phase at these sites, due to the presence of chlorine, may explain the preferential location of the particles, as illustrated by the DFT model of the Pt13 cluster on the chlorinated edge (insets of Figure 97a). This DFT model is also compatible with EXAFS. The average distances between Pt nanoparticles were also quantified using a customised geometrical analysis [4] of the 3D reconstructions. A direct comparison of the as- obtained values (9 and 16 nm for the lowest and highest loadings, respectively) shows that the inter-particular distance can be finely tuned by varying the Pt metal loading.

This multi-selective study on industrially relevant catalysts consisting of Pt-based phase supported on chlorinated γ-alumina showed, for the first time, that Pt particles are located mostly on the edges of the crystallites and are uniformly distributed through the support aggregate. The role of chlorine to stabilise the Pt close to the

Fig. 96: a) High-resolution image, in HAADF mode in scanning transmission

electron microscopy (STEM), of oxide catalyst 0.3%Pt/γ-Al2O3-Cl; the white

arrows highlight the presence of single atoms. b) Wavelet transform analysis of

Pt L3-edge k2 weighted EXAFS data, for the oxidised 0.3%Pt/γ-Al2O3-Cl catalyst.