Mossbauer characterization of carbon supported ruthenium-tin catalysts

Sn-119 and Ru-99 Mossbauer spectroscopy was used to investigate catalysts of ruthenium and tin supported on activated carbon (Ru-Sn/C) containing 2 wt % ruthenium and having Sn:(Sn+Ru) ratios between zero and 0.4. Samples were subjected to different reducing and oxidizing treatments, and suitable precursor products were studied. Four major components in the Sn-119 Mossbauer spectra are attributed to oxidic Sn(LI) and Sn(IV) and to Ru-Sn and Ru-Sn-O-x species forming on the surface of ruthenium metal particles. In addition to this, the Sn-119 spectra reveal the presence of minor amounts of Ru3Sn7. While some of the features of the 99Ru Mossbauer spectra cannot be interpreted unambiguously, the presence of tin leads to a substantial increase of the Lamb-Mossbauer factor of the metallic ruthenium particles, which shows that tin strengthens the attachment of the particles to the support even at very low tin loadings. None of the ruthenium-tin phases on the catalysts can be clearly identified as those responsible for the development of the catalytic activity and selectivity. The observed close contact between the two metals, however, appears to be decisive for the formation of catalytically active sites during the induction period at the beginning of the reaction.