Characterization and infrared study of the effect of Cr, Mo and W on carbon deposition on platinum/alumina

Deposition of increasing amount of Cr, Mo and W ions on gamma-Al2O3 leads t o a decrease of the surface area on the catalysts. Additional introduction of Pt leads to the increase of the latter except for 0.3 wt% Pt-2 wt% Cr/Al 2O3 sample. Low-loaded Pt-Cr/Al2O3 samples have the surface CrOOH groups and Pt6+ ions. However, high-loaded sample show evidence of the presence of alpha- and ga mma-Cr2O3 as well as hydrous chromium oxide. Both high- and low-loaded Pt-Mo/Al2O3 samples contain Pt6+-Mo surface speci es. High-loaded samples are characterized by high thermal stability of surf ace species. High-loaded Pt-W/Al2O3 samples contain W6+ surface species but evidence for the presence of Pt6+ ions is ambiguous. The results show that ethylene deposition on low-loaded samples yield two s trong bands with maxima near 1580 and 1460 cm(-1) in the FT-IR spectra. The se bands, identical to those observed for chemisorbed acetic acid were assi gned to the asymmetric and symmetric stretching vibrations of C-O bonds in carboxylate structure. Moreover, for Pt 2 wt% Mo/Al2O3 sample the bands ass igned to the conventional coke were also noticed. For high-loaded samples the bands attributed to carboxylate were only obser ved for Pt-Cr/Al2O3. However, the FT-IR spectra of all samples from this se ries showed bands assigned to alkylnaphthalenes and polyphenylenes. In conclusion, we can suggest that in low-loaded Pt-Me/Al2O3 samples coke h as a more carboxylate character whereas for high-loaded samples coke is mor e aromatic. (C) 1999 Elsevier Science B.V. All rights reserved.