Changes induced on the surfaces of small Pd clusters by the thermal desorption of CO

Abstract

The stability and adsorption/desorption properties of supported Pd crystallites less than 5 nm in size were studied by Auger electron spectroscopy and repeated flash thermal desorption of CO. The Pd particles were grown epitaxially on heat‐treated, UHV‐cleaved mica at a substrate temperature of 300°C and a Pd impingement flux of 4×1013 atoms/cm2 s. Auger analysis allowed i n s i t u measurement of relative particle disperion and contamination, while FTD monitored the CO desorptionproperties. The results show that significant changes in the adsorptionproperties can easily be detected. Changes in the Pd Auger signal and the desorption spectrum during the first few thermal cycles are believed to be due to particle coalescence and facetting. The rate of this change is dependent on the temperature and duration of the desorption. Significant reductions in the amplitude of the desorptions peak occur during successive CO desorptions are attributed to increases of surface carbon, induced by the desorption of CO (an effect not observed on large single crystals). The contamination process could be reversed by heat treatment in oxygen or hydrogen.