Carbon segregation to the Pd(111) single crystal surface as seen in carbon monoxide thermal desorption spectra

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Abstract
Carbon monoxide adsorption was studied on the Pd(111) surface with submonolayer carbon coverages. It is shown that the carbon monoxide desportion spectrum can be used in the detection of a carbon coverage much below the detectability level of AES. Although the estimation of the absolute carbon coverage is subjected to quite a bit uncertainty it seems that a few percent of a monolayer of carbon manifests clearly in the desorption spectrum at low carbon monoxide coverages. With the aid of CO desorption spectra the temperature dependence of the surface segregation of carbon was investigated at temperatures from 450 to 1300 K with the mean C/Pd atomic ratio in the bulk ranging from 1×10−6 to 7×10−6. At any temperature below 835 K carbon does not appear on the surface. At higher temperatures carbon segregates at the surface very quickly and within a temperature range depending on the C/Pd ratio. Carbon that has been segregated on the surface can be easily redissolved even at temperature as low as 450 K. The behavior of carbon is explained in terms of formation and decomposition of the carbon-rich palladium phase PdC0.13. Some implications for cleaning Pd(111) crystals are also shortly discussed.

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