Microcalorimetric and infrared spectroscopic studies of CO and C2H4 adsorption on Pd/SiO2 and Pd-Ag/SiO2 catalysts

Pd/SiO2 and Pd-Ag/SiO2 catalysts prepared by incipient wetness impregnation method were studied by using the techniques of microcalorimetric adsorptio n and infrared spectroscopy for the adsorption of H-2, CO and ethylene at r oom temperature. It is found that most sites of H-2 adsorption on Pd-Ag(1:1 )/SiO2 and Pd-Ag(1:4)/SiO2 are weaker than those on Pd/SiO2 though the init ial heat of H2 adsorption on Pd/SiO2 is not affected by Ag. The initial hea t of 135 kJ mol(-1) for CO adsorption on Pd/SiO2 is the average heat produc ed by both bridged and linearly bonded CO on Pd while the initial heat of 1 17 kJ mol(-1) on Pd-Ag(1:1)/SiO2 and 90 kJ mol(-1) on Pd-Ag(1:4)/SiO2 are c ontributed by the linearly bonded CO on Pd sites modified by Ag with differ ent Pd/Ag atomic ratios. These results suggest that the Pd-Ag/SiO2 catalyst s may have well mixed Pd and Ag atoms in the Pd-Ag clusters. Strong interac tions between the two types of atoms were observed. Ethylene adsorption on Pd/SiO2 at room temperature produces adsorbed ethylidyne and atomic hydroge n with the initial heat of 175 kJ mol(-1). Addition of Ag may inhibit the f ormation of ethylidyne and promote the formation of pi -species for ethylen e adsorption in Pd-Ag/SiO2 catalysts. The formation of the pi- and di-sigma species for ethylene adsorption on the Pd-Ag(1:1)/SiO2 catalyst at room te mperature generates the initial heat of 100 kJ mol(-1). The initial heat of 74 kJ mol(-1) produced for the adsorption of ethylene on the Pd-Ag(1:4)/Si O2 at room temperature may be attributed to the formation of only pi -speci es on Pd with more surrounding Ag atoms. This work demonstrates how the nat ure and bond strength of the surface species formed on the adsorption of et hylene on Pd may be altered by using Ag as a modifier. (C) 2001 Elsevier Sc ience B.V. All rights reserved.