Direct observation of C2N2 uptake and dissociation on Pd(110) by fast high-resolution XPS

The full parameter space of C2N2 adsorption and reaction on Pd(110) from 80 to 800 K has been mapped out by fast high-resolution XPS performed at the SuperESCA beamline of the ELETTRA synchrotron radiation facility in Trieste. Both C 1s and N 1s photoemissions hale been followed continuously during exposure and on ramping the temperature, thus allowing direct observation of adsorption and of the dissociation of adsorbed species. The fraction of C2N2 dissociated to CN increases from zero at 80 K to 100% at 220 K. Between these two extremes, dissociation is favoured at low coverages with the amount of, coexisting intact C2N2 species decreasing with temperature. The dissociation threshold is extrapolated to 90-100 K. Whereas only one surface configuration is detected for CN, two molecular C2N2 monolayer species may be distinguished, which are distinct by way of their core level binding energies and associated with C2N2 in symmetric (majority species) and less symmetric (minority species) adsorption geometries. The energetic parameters of the dissociation reaction have been estimated using the quantitative coverage information from temperature-programmed XPS and a simple model for the reaction kinetics. (C) 1998 Elsevier Science B.V. All rights reserved.