SURFACE-CHEMISTRY OF CYANOGEN ON PT(111) - ISOTOPIC EXCHANGE AND COADSORPTION WITH CO

The adsorption of C2N2 on Pt(111) has been studied by thermal desorption mass spectrometry using 12C214N2 and 13C215N2 under various adsorption conditions and by work function measurements. Thermal desorption spectra revealed three desorption peaks (α, β1, β2) in close agreement with results on other Pt single crystal surfaces. No isotopic exchange was observed in the α peak and it is attributed therefore to molecularly adsorbed C2N2. Isotopic exchange between coadsorbed 12C214N2 and 13C215N2 was found in the β peaks depending on the experimental conditions. Whereas the β2 state showed isotopic exchange irrespective of simultaneous or consecutive adsorption of 12C214N2 and 13C215N2, no isotopic exchange was detected after consecutive adsorption in the β1 state once preformed at the surface. These results are discussed in terms of dissociative adsorption of C2N2 yielding CN particles into the β2 state and of the association of CN units into a more complex adsorbate layer (β1) at higher adsorbate densities. An overall work function increase was observed during C2N2 adsorption yielding Δφ = 370 mV at saturation. An opposite surface dipole is suggested for the α and β states. Coadsorption of C2N2 and CO was followed by thermal desorption and work function measurements. Whereas the C2N2-saturated Pt(111) surface can accommodate a considerable amount of CO, CO preadsorption drastically reduces the subsequent C2N2 uptake. The results indicate no reaction and little interaction between C2N2 and CO in the mixed adlayer. © 1979.