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.