Hreels of Cyanogen on Pd(111)

Abstract

Cyanogen (NCCN) adsorbs molecularly on Pd(111) between 120 and 250 K. The C-C stretch vibration (105 meV) persists when the sample is heated to 250 K. Above 250 K, cyanogen is dissociated; three peaks are observed: Pd-CN (37 meV), and CN stretch modes (225, 237 meV) that are attributed to successively filled adsorption sites. The CN stretch losses are weak, approximately 0.1% of the elastic peak intensity. No further fragmentation of CN is observed. At 120 K, cyanogen adsorbs in multilayers. At 1xE-7 torr constant cyanogen back pressure, the CN stretch vibrations (261, 283 meV) are 0.1% of the elastic intensity. At monolayer coverage, the lattice (bending) modes are dominant (30, 60 meV). At room temperature, Pd(111) converts NCCN to HCN by the reaction CN(ads) + H(ads) = HCN(ads) where the HCN is rehybridized to a CN double bond (190 meV). Comparison of HREEL spectra of cyanogen with those of the CN ring compound s-triazine make the possibility of polymerization of CN to a “paracyanogen” unlikely.