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

Deposition of increasing amount of Cr, Mo and W ions on γ-Al2O3 leads to 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/Al2O3 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 α- and γ-Cr2O3 as well as hydrous chromium oxide. Both high- and low-loaded Pt–Mo/Al2O3 samples contain Pt6+–Mo surface species. High-loaded samples are characterized by high thermal stability of surface 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 strong bands with maxima near 1580 and 1460 cm−1 in the FT-IR spectra. These bands, identical to those observed for chemisorbed acetic acid were assigned to the asymmetric and symmetric stretching vibrations of C–O bonds in carboxylate structure. Moreover, for Pt 2 wt% Mo/Al2O3 sample the bands assigned to the conventional coke were also noticed. For high-loaded samples the bands attributed to carboxylate were only observed for Pt–Cr/Al2O3. However, the FT-IR spectra of all samples from this series showed bands assigned to alkylnaphthalenes and polyphenylenes. In conclusion, we can suggest that in low-loaded Pt–Me/Al2O3 samples coke has a more carboxylate character whereas for high-loaded samples coke is more aromatic.