Using acetylene and ethylene as probe molecule, acetylene hydrogenation on Pd-Ag/Al₂O₃ and Pd/Al₂O₃ catalysts were studied by in-situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS). Chemisorptions properties of catalysts were characterized by C₂H₂ adsorption, C₂H₂ and H₂ alternate adsorption and co-adsorption. The results showed the different adsorption properties between acetylene and ethylene over Pd-Ag/Al₂O₃ or Pd/Al₂O₃. In addition, hydrogenation of acetylene would create the carbonaceous layer on the catalysts surface, the carbonaceous layer was made up of long-chain alkane which has similar infrared spectra of green oil and has great effect on the selectivity properties of catalyst. There is a correlation between the amount of carbonaceous layer and the adsorption order of acetylene and hydrogen.

Key words: DRIFTS, ethyne, hydrogen, metal catalyst, Pd-Ag/Al₂O₃