THERMAL-DESORPTION SPECTROSCOPY OF PALLADIUM AND COPPER ON SILICA

Thermal desorption spectroscopy of palladium and copper films grown on clean silica substrates was performed using CO2 laser heating. After cleaning the surface by high temperature heating, a controlled, low coverage dose of metal atoms was deposited on the substrate. Temperature ramping was achieved using a constant laser power, the value of which depended on the nature of the metal and substrate as well as the substrate size. At high temperatures (above 1025 K for palladium and above 975 K for copper), metal films vaporize and desorption spectra provide information about the nature of the metal deposit and metal-support interaction. With increasing coverage of palladium on silica, a positive temperature shift in the leading edge of desorption was seen. At higher coverages, above about 2 x 10(15) atoms cm-2, a common leading edge appears and zero-order kinetic analysis gave E(act) values between 3.9 and 4.3 +/- 0.1 eV which can be compared with the value of 3.83 eV for DELTA-H(vap)(1200 K) for palladium metal. Similar coverage-dependent properties were not seen for copper on silica; instead. a common desorption leading edge appeared down to submonolayer coverages. Zero-order analysis at about 1 x 10(15) atoms cm-2 gave an E(act) of 3.3 +/- 0.1 eV. which is comparable with the value of 3.44 eV for DELTA-H(vap)(1100 K) for copper metal.