Contraction and relaxation of interatomic distances in small platinum particles from extended X-ray absorption fine structure (EXAFS) spectroscopy

A sample consisting of a small Pt particle encased in Y zeolite with a mean dia. of 12 A was used to show how extended X-ray absorption fine structure spectroscopy (intrinsic resolution 0.02 A) can provide new information on the structure of small Pt particles used as catalysts. The prereduced sample was introduced into a special EXAFS cell which was evacuated at 400 deg C; absorption spectra were measured at 120 deg K after introduction of 50 torr He or 50 torr H2. Main peaks of the Fourier transforms of the absorption spectra were attributed to Pt--Pt nearest neighbour distances. For a sample covered with He the Pt--Pt distance was shortened by 0.12 A with respect to the bulk, but it was nearly the same as in the bulk when the sample was covered with H. It was concluded that H chemisorption induced a relaxation of the Pt--Pt distance. This relaxation concerned the 42 surface atoms but was also effective between the surface layer and the 12 atoms in the inner shell. 9 ref.--E.J.S.