Bifunctional heterogeneous catalysts consisting of metallic sites and acidic sites are used in various processes of petroleum refining and the impact of active site proximity in catalytic efficiency was expressed as an “intimacy criterion”. In particular, platinum supported on chlorine doped γ-Al2O3 is the traditional catalyst for catalytic naphtha reforming, at the core of the present work. In this work, a multi-technique approach, including HR-HAADF-STEM, electron tomography, high field MAS NMR, HERDF-XANES, EXAFS, and DFT calculations, was applied to determine the location of the metallic and acidic sites on the Al2O3 support crystallites for Pt/γ-Al2O3-Cl catalysts and to estimate the distance between such sites. The samples, representative of the commercial materials, were synthesized so as to vary the distance between the two types of sites as a function of three key parameters: %Pt, %Cl and Al2O3 crystallite size and shape.
By building the first DFT model of crystallite edge between the (110) and (100) surfaces, an improved 1H NMR peak assignment was proposed and it was found that µ1-OH on the edge are preferably exchanged with chlorine. EXAFS confirmed that oxide samples are populated by single-atoms. STEM analysis showed that reduced Pt nanoparticles of size close to 0.9 nm and around 15% of Pt is in the form of single-atoms, which are stabilized by Cl. By electron tomography it was showed that, Pt nanoparticles are mostly located on the edges of alumina crystallites and that the quantified Pt inter-particles distance is directly tuned through Pt loading. We propose a first geometrical model of the location of Pt and Cl in the catalyst. Catalytic testing in n-heptane reforming showed that the molar Pt/Cl ratio is an adequate descriptor of metallic to acid balance.