AVS 56th International Symposium & Exhibition
    Surface Science Tuesday Sessions
       Session SS2-TuM

Paper SS2-TuM6
Kinetics of HCN Decomposition and CNH2 Formation on Pt(111)

Tuesday, November 10, 2009, 9:40 am, Room N

Session: Reactions on Metals and Bimetallics
Presenter: X. Hu, University of Illinois at Chicago
Authors: X. Hu, University of Illinois at Chicago
M. Trenary, University of Illinois at Chicago
R.J. Meyer, University of Illinois at Chicago
Correspondent: Click to Email
The kinetics of HCN (hydrogen cyanide) decomposition to H and CN and the subsequent hydrogenation of CN to the CNH2 species on the Pt(111) surface were studied with time-resolved reflection absorption infrared spectroscopy (RAIRS). The use of RAIRS for kinetic studies can uniquely provide important information on the mechanisms of surface reactions, particularly when these reactions occur below the temperature at which products desorb from the surface. In addition, from the analysis of kinetic data, activation energies for elementary surface reactions on well-defined systems can be determined and thereby provide important experimental benchmarks for comparison with the results of state-of-the-art theoretical calculations. Hydrogen cyanide adsorbs on Pt(111) at a temperature of 85 K to give an intense CH stretch vibration at 3300 cm-1. The time dependent decrease of this peak was monitored at temperatures between 120 and 135 K, the temperature range where the C-H bond of HCN breaks. The decrease in CH stretch peak intensity appears to follow first order kinetics and is attributed to both C-H bond breaking and HCN desorption. The CNH2 species is characterized by RAIRS peaks at 3370 cm-1 (N-H stretch), 1566 cm-1 (NH2 scissors), and 1324 cm-1 (C-N stretch). The increase in coverage of the CNH2 species was monitored by following the time dependence of the 1566 cm-1 peak, the most intense mode of CNH2, in the temperature range of 210 to 230 K. The CNH2 formation rate is found to follow second order kinetics. The spectra show the appearance of an intermediate species with a peak at 3347 cm-1, the intensity of which initially increases with time, reaches a maximum, and then decreases as CNH2 is formed. This peak is identified as the N-H stretch of an HNC surface intermediate. Through kinetic modeling of the time dependence of the coverages of the HCN, HNC, and CNH2 species, rate constants were derived at each temperature and activation energies were obtained from Arrhenius plots. The experimental results were then compared with the results of density functional theory (DFT) calculations on the stability and structures of the intermediates. Activation barriers for each step of the reaction were also obtained from the DFT calculations. The calculations confirm that CNH2 is the energetically favored product of the reaction and that CNH2 has the structure implied by the RAIR spectra. The calculations also predict that HNC is a stable surface intermediate. However, the structures of both adsorbed HCN and HNC as implied from the RAIRS data disagrees with the optimized structures obtained from the DFT calculations.