Evidence for direct observation by Mossbauer spectroscopy of surface tin atoms in platinum-tin particles

Sn-119 Mbssbauer spectroscopy at 77 K was used to observe surface tin atoms on PtSnx/SiO2 catalysts. Two families of PtSnx/SiO2 catalysts with fully reduced Pt-(0) and Sn-(0) atoms were prepared following the Surface Organo-metallic Chemistry on Metal strategy. These two families of catalysts exhibit about the same Sn/Pt composition, but very different metallic particle diameters, each with narrow particle size distribution, so that, the ratio between surface tin atoms and "bulk" tin atoms was significantly different for the two families. In addition to the well known "Pt-Sn" bulk alloy with small quadripole splitting, a new Pt-Sn species with large quadripole splitting and isomer shift in the range of Sn-(0) was observed. The relative proportion of this species with respect to "Pt-Sn" bulk alloy is greater for small particles than for the larger ones, which suggests that this species is a surface tin atom present in a "Pt-Sn" alloy.