Gold-nickel surface alloy chemistry with oxygen and carbon monoxide

Abstract

Key to low temperature carbon monoxide oxidation on gold-nickel random surface alloys is molecularly (rather than atomically) adsorbed oxygen, and weakly bound carbon monoxide. To characterize these adsorbates, saturation coverages of each diatomic on Au-Ni(111) held at 80 K are separately measured via high-resolution electron energy loss spectroscopy, as a function of increasing gold coverage. The progression from adsorptive to non-adsorptive behavior is rationalized by constructing two mutually consistent adsorption models for O₂ and CO that account for the alloy d-band, the surface atom centers' moiré pattern arrangement, and the associated subsurface reconstruction with triangular misfit dislocation loops. Surface alloying with Au is central to the existence and stability of peroxo- and/or superoxo-like species unknown on Ni(111) and presently observed at 743, 856, and 957 cm⁻¹, corresponding to O₂ adsorption sites of type pseudo-3-fold fcc/hcp, degenerate-pseudo-2-fold fcc/hcp and bridge, and pseudo-3-fold bridge, respectively. No new Au-Ni adsorption sites for CO result from alloying. CO adsorption occurs at Ni-bridge, Ni-atop, and Au-atop sites detectable at 1860-1960, 2060-2110, and 2150-2170 cm⁻¹, respectively. These contrasting behaviors originate from differences in the O₂ and CO frontier orbitals' electron configuration, and hence, the surface-adsorbate densities of states near the Fermi level. The vibrational loss features' center frequencies and intensities also reflect the alloy morphology. Distinct molecular O₂ species are related to distinct regions of an effective alloy unit cell. Evidence of the surface reconstruction is also detectable in a weighted average of the CO center frequencies, and the adsorbed CO configurational entropy. CO adsorption sites behave nearly independently of each other due to an approximate equivalence between the effect of CO lateral interactions and d-band lowering with Au. With these insights, further experiments involving O₂ followed by CO exposures on Au-Ni(111) enable better characterization of the CO oxidation mechanism and reaffirm the central role of molecular O₂ in CO₂ production.