Extended X-ray absorption fine structure studies of platinum-tin catalysts

Extended X-ray absorption fine structure (EXAFS) studies at 140 K, along with studies of X-ray absorption edges themselves, were conducted on a Pt-Sn/Al/sub 2/O/sub 3/ catalysts that had previously been exposed to hydrogen at high temperatures (775-795 K). For comparison, similar studies were conducted on a Pt-Sn/SiO/sub 2/ sample. For the Pt-Sn/Al/sub 2/O/sub 3/ catalyst the extent of coordination of tin to platinum atoms or to other atoms of tin is small, but the extent of coordination to oxygen is substantial. An analysis of the energy shift of the K absorption edge of the tin indicates that the tin is not present as Sn/sup 4 +/. The EXAFS data, coupled with the absorption edge data, suggest that the tin is largely present as Sn/sup 2 +/. There appears to be a significant interaction between the platinum and tin, since the platinum is more highly dispersed on alumina when tin is present. After consideration of all the data, it seems reasonable to regard the catalysts as if it consisted essentially of platinum clusters dispersed on an alumina that has Sn/sup 2 +/ present at the surface. The EXAFS data on the Pt-Sn/SiO/sub 2/ sample lead to very different conclusions. In thismore » case, most of the tin has an oxidation state of zero, and the material consists predominantly of bimetallic entities of platinum and tin on silica.« less