Development of custom made bimetallic alloy model systems based on platinum – rhodium for heterogeneous catalysis

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The current study extends the insight into solid-state diffusion in alloys of the Platinum Group Metals (PGMs). Thereby, we pave the way for tailor making surface compositions of binary platinum (Pt) – rhodium (Rh) model catalysts, with deposited rhodium layers on the surface of Pt single crystals (SCs) as the starting point. Platinum single crystals with (100), (110) and (111) orientations were studied by scanning electron microscopy (SEM) and energy dispersive X – ray spectroscopy (EDS). In addition, X – ray Photoelectron spectroscopy (XPS) characterization was carried out on Pt(110). The crystallographic orientation has a prominent effect on diffusion characteristics. The slowest moving Kirkendall – plane was observed for Pt(110) at xK = 250 nm from reference point x0, while, the fastest for Pt(100) was at xK = 450 nm, indicating that the inter-diffusion rate with respect to orientation increases in the sequence Pt(110) < Pt(111) < Pt(100). The diffusion coefficient of Rh into Pt is of the order ∼10−17–10−18 m2/s, whereas it was slightly less than ∼ 10−19 m2/s for Pt into Rh; albeit comparable to the vacancy diffusion rate from Pt into Rh. Our experimental findings show good agreement with calculated inter-diffusion coefficients using Darken analysis. A theoretical direction is proposed for improved fitting of data.