Coadsorption of nitrogen with Cu, Ag and Au on W{100}: the role of metal adatoms in controlling surface reactivity

Abstract

Structural transformations arising from coadsorption of nitrogen with Cu, Ag and Au on W{100} have been investigated using thermal desorption spectroscopy (TDS), ion scattering spectroscopy (ISS) and low-energy electron diffraction (LEED). The surface chemistry observed is shown to depend critically upon the sequence of adsorption (metal deposition followed by nitrogen chemisorption or vice versa). The uptake of nitrogen by thin films of Cu, Ag and Au deposited on W{100} at 300 K is found to decrease linearly with increasing metal loading until at exactly one monolayer (1 × 10¹⁵ atoms cm^–2), all nitrogen chemisorption ceasses. Pre-annealing the metal overlayer at 800 K causes all nitrogen chemisorption to stop after only 0.5 monolayer coverage. It is proposed that island growth of a two-dimensional surface alloy formed between the tungsten substrate and metal overlayer at elevated temperatures can lead to the complete removal of all reactive centres for nitrogen chemisorption at coverages of 0.5 monolayers and above. Adsorption of nitrogen on mixed ensemble sites (those consisting of both tungsten atoms and metal adatoms) is inhibited by an activation barrier, in agreement with previous studies of nitrogen on Pt/W{100} surface alloys and H₂ on Cu, Ag, Au/W{100}. In contrast, the adsorption of metal adatoms on W{100} predosed with 5 × 10¹⁴ N atoms cm^–2 causes some displacement of nitrogen atoms from c(2 × 2) to (1 × 1) symmetry and new low-temperature nitrogen desorption states corresponding to nitrogen adatoms bonded to both tungsten and Cu, Ag or Au. ISS and LEED have been used to probe the selvedge in order to deduce changes in the adsorption site for nitrogen as a function of temperature and metal loading. Possible implications for these materials as novel catalysts in the Haber process are discussed.