Adsorption and Dissociation of HCN on the Pt(111) and Pt(112) Surfaces

The interaction of HCN with the Pt(111) and Pt(112) surfaces has been studied with temperature programmed desorption (TPD), scanning kinetic spectroscopy (SKS), LEED and work function measurements (Δφ) in the temperature range 87 to 1000 K. The only desorption products observed were H2, C2N2 (cyanogen) and HCN. Labelling studies utilizing D13CN + HCN revealed that HCN desorbs molecularly in three different low temperature states, while higher temperature β states were due to recombination of H(a) + (CN)(a). Different β-state desorption behavior was observed on Pt(112) which may be due to competitive desorption effects. Evidence suggests that the highest temperature molecular desorption state of HCN may act as a precursor to dissociation. The high step density Pt(112) surface was found to be more effective in promoting dissociation of HCN. The following sequence of LEED patterns was observed on Pt(111) with decreasing coverage of HCN beginning with monolayer coverage: (3×3) → c(4×2) → (2×2) → (1×1). Both HCN(a) and CN(a) were found to give an overall work function decrease on Pt(111) yielding values of -1.2 and -0.8 V, respectively at saturation. LEED measurements in conjunction with TPD and Δφ results suggest that HCN is bonded in a linear fashion at saturation with the nitrogen bonded to Pt. After dissociation, CN(a) is likely bonded with the molecular axis parallel to the Pt surface.