Mossbauer characterization of carbon supported ruthenium-tin catalysts

Sn-119 and Ru-99 Mossbauer spectroscopy was used to investigate catalysts o f 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 wer e subjected to different reducing and oxidizing treatments, and suitable pr ecursor products were studied. Four major components in the Sn-119 Mossbaue r 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 additi on to this, the Sn-119 spectra reveal the presence of minor amounts of Ru3S n7. While some of the features of the 99Ru Mossbauer spectra cannot be inte rpreted unambiguously, the presence of tin leads to a substantial increase of the Lamb-Mossbauer factor of the metallic ruthenium particles, which sho ws 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 catalyst s 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 catalytic ally active sites during the induction period at the beginning of the react ion.