Adsorption and decomposition of HCN on a polycrystalline Pt foil studied by TPD, MBRS, XPS and FT-RAIRS

Abstract

The adsorption and decomposition of HCN on a polycrystalline Pt foil have been studied by temperature-programmed desorption (TPD), molecular beam reactive scattering (MBRS), X-ray photoelectron spectroscopy (XPS) and FT reflection–absorption IR spectroscopy (FT-RAIRS). Adsorption of HCN at 100 K yields two distinct CN-containing species for sub-monolayer coverages. The first is identified as a weakly adsorbed molecular HCN species whilst the second species is identified as CN(ads), formed by the dissociative chemisorption of HCN. Higher exposures of HCN at 100 K result in the additional formation of physisorbed multilayers of HCN, which are observed to desorb below 200 K. Adsorption of HCN at 300 K still yields CN(ads) and H(ads), with some of the latter desorbing as H2 at around 400 K, but also produces a new surface intermediate. The most likely structure for this intermediate is identified by FT-RAIRS as a trans-HCNH surface species. The near coincident desorption of HCN and H2 observed in the temperature range 400–500 K is then attributed to the decomposition of this HCNH intermediate and subsequent rapid recombination reactions. Above 650 K the desorption of C2N2 is observed owing to the surface combination of CN(ads) species.