Effect of Sn on the Reactivity of Cu Surfaces

Periodic, density functional theory (DFT-GGA) calculations, using PW91 (self-consistently) and RPBE functionals, have been employed to determine preferred binding sites, adsorbate structures, and binding energies for the adsorption of atomic (H, N, O, S, and C), molecular (NO and CO), and radical (OH) species on Cu(111) and CuSn(0001) alloy surfaces. Our results indicate the following order in the binding energies from the least to the most strongly bound: NO \u3c CO \u3c H \u3c OH \u3c N \u3c O \u3c S \u3c C for Cu-terminated CuSn(0001). On Cu(111), the corresponding relative order of adsorbates from the least strongly bound to the most strongly bound is CO \u3c NO \u3c H \u3c OH \u3c N \u3c O \u3c S \u3c C. On the Sn-terminated CuSn(0001) surface, CO does not adsorb and the relative order of adsorbates from the least strongly bound to the most strongly bound is NO \u3c H \u3c OH \u3c N \u3c S \u3c O \u3c C. For all adsorbates, the binding on Cu-terminated CuSn(0001) is stronger than on Cu(111), resulting from a combination of electronic and strain effects caused by the addition of Sn to Cu. CO dissociation is endothermic on Cu-terminated CuSn(0001) and Cu(111) surfaces, while CO oxidation is exothermic on these surfaces. OH dissociation is endothermic on all three surfaces. On all surfaces studied, thermodynamics of NO decomposition are much more favorable than those of CO and OH dissociation on the corresponding surfaces. Our microcalorimetric studies of the interaction of NO with Cu/SiO2 and Cu6Sn5/SiO2 samples give initial heats of 270 (2.80 eV) and 130 (1.35 eV) kJ/mol, respectively. These values correspond to the decomposition of NO to give adsorbed oxygen plus gaseous N2 on Cu/SiO2 and adsorbed oxygen plus gaseous N2O on the Sn-terminated phase of Cu6Sn5/SiO2