Surface chemistry of cyanogen on Pt(111): Isotopic exchange and coadsorption with CO

Abstract

The adsorption of C 2N 2 on Pt(111) has been studied by thermal desorption mass spectrometry using 12C 2 14N 2 and 13C 2 15N 2 under various adsorption conditions and by work function measurements. Thermal desorption spectra revealed three desorption peaks (α, β 1, β 2) 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 C 2N 2. Isotopic exchange between coadsorbed 12C 2 14N 2 and 13C 2 15N 2 was found in the β peaks depending on the experimental conditions. Whereas the β 2 state showed isotopic exchange irrespective of simultaneous or consecutive adsorption of 12C 2 14N 2 and 13C 2 15N 2, no isotopic exchange was detected after consecutive adsorption in the β 1 state once preformed at the surface. These results are discussed in terms of dissociative adsorption of C 2N 2 yielding CN particles into the β 2 state and of the association of CN units into a more complex adsorbate layer ( β 1) at higher adsorbate densities. An overall work function increase was observed during C 2N 2 adsorption yielding Δφ = 370 mV at saturation. An opposite surface dipole is suggested for the α and β states. Coadsorption of C 2N 2 and CO was followed by thermal desorption and work function measurements. Whereas the C 2N 2-saturated Pt(111) surface can accommodate a considerable amount of CO, CO preadsorption drastically reduces the subsequent C 2N 2 uptake. The results indicate no reaction and little interaction between C 2N 2 and CO in the mixed adlayer.