Kinetics of HCN Decomposition on the Pt(111) Surface by Time-Dependent Infrared Spectroscopy

Abstract

Time-dependent reflection absorption infrared spectroscopy has been used to investigate the kinetics of HCN decomposition on the Pt(111) surface over the temperature range of 120 to 135 K. At these low temperatures, HCN bonds at an atop site with the HCN axis perpendicular to the surface, which gives rise to an intense C–H stretch at ∼3300 cm^–1. Further support for this HCN adsorption geometry is obtained through HCN/CO coadsorption experiments in which both molecules are seen to compete for the atop sites. The disappearance of the C–H stretch peak of HCN at low temperatures is indicative of dissociation to produce adsorbed H and CN. When the decrease in HCN coverage is followed for a sufficiently long time, the data deviate from the expected first-order rate law, and the temperature dependence of the rate constant deviates from the Arrhenius form. Over a more restricted coverage range, simpler behavior is observed, and an activation energy for HCN dissociation of 0.33 eV is obtained