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Surface Chemistry of NCO Formed from HNCO on Pt(111)

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journal contribution
posted on 25.12.2008, 00:00 by James E. Jones, Michael Trenary
The surface chemistry of isocyanate, NCO, and its precursor, isocyanic acid, HNCO, on Pt(111) was studied with the techniques of reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). Isocyanate is known to form on platinum-group metals in the course of the catalytic reduction of NO by carbon-containing molecules, such as CO or simple hydrocarbons. It is therefore important to establish its stability and reaction chemistry on well-characterized platinum surfaces. Exposure of the Pt(111) surface at 90 K to isocyanic acid leads to both molecularly adsorbed HNCO, as well as its dissociation products, H and NCO. At 90 K and low coverages, molecularly adsorbed HNCO exhibits peaks at 1338, 2227, and 3357 cm−1 due to the NCO symmetric stretch, the NCO asymmetric stretch, and the NH stretch, respectively. Isocyanate has a markedly lower asymmetric NCO stretch of 2113 cm−1 at low coverages, which shifts to higher values at higher coverages. The decomposition of HNCO is largely completed when the surface is heated to 150 K. As the temperature is increased from 150 to 300 K, a series of HNCO and NCO decomposition products are identified with RAIRS, including CO, NH, NH2, NH3, H2O, OH, and NO. These reaction products indicate that NCO dissociation can occur at both the N−C and C−O bonds. Dissociation at the C−O bond is also indicated by observation of HCN desorption with TPD.