THE INFLUENCE OF PARTICLE-SIZE ON CO ADSORPTION ON PD ALUMINA MODEL CATALYSTS

The adsorption of CO on Pd(111) and small Pd particles deposited on an alumina support has been studied by means of temperature programmed desorption (TPD) using a molecular beam doser. The supported Pd particles with an average size of 27 and 2.5 nm were prepared by electron bombardment induced thermal evaporation. The CO desorption data were obtained for the Pd particles as well as for a Pd(111) single crystal. Major differences in the Pd particle and single crystal data were observed. On the 2.5 nm Pd particle surface different CO adsorption states were found. The decay in CO adsorption capacity during repeated adsorption/desorption cycles caused by simultaneous accumulation of surface carbon from partial CO dissociation was monitored. It is shown that surface carbon formation reduces the value of desorption activation energy for the adsorption state with higher binding energy. The calculated activation energy of desorption shows that CO desorption kinetics is particle size dependent.