Determination of Nanoparticle Size by Measuring the Metal–Metal Bond Length: The Case of Palladium Hydride

Formation and properties of Pd nanoparticles have been investigated in situ in the temperature range between 186 and 483 K by X-ray absorption fine structure (XAFS) spectroscopy and by ex situ electron microscopy. Results of the experiments conducted under hydrogen and helium atmospheres were compared, in order to illuminate and explain the changes in bond length of Pd particles as a result of hydride formation. By contrasting the data obtained at these two atmospheres we were able to isolate the effects of both the finite particle size and hydrogen absorption on Pd–Pd bond length, obtain hydrogen intake values, and, as a result, estimate the particle size. The calculated average particle size agreed with the electron microscopy data. These results open up new opportunities for in situ size investigations by XAFS of catalytic Pd particles in reaction conditions, when catalyst fragmentation, shape change, and/or coarsening may occur.