CO adsorption on the Ni2Pb/Ni(1 1 1) surface alloy: A DFT study

Abstract

Structural and electronic properties of the Pb/Ni(111) overlayer and the Ni2Pb/Ni(111) surface alloy have been investigated within a DFT-PBE approach in order to determine its reactivity towards adsorption of CO molecules. This work has been motivated by a photoemission study of CO adsorption on Pb/Ni(111) surface phases [V. Matolín et al., Phys. Rev. B 74 (2006) 075416] indicating that Pb adatoms inhibit CO adsorption in a purely geometrical way by site blocking at Ni(111), whereas surface alloying has a poisoning effect of the Ni–CO bond weakening. In general, our DFT computations confirm experimental findings for the Pb/Ni(111) overlayer, as the very high activation barrier of about 2eV due to the presence of Pb adatoms makes the CO chemisorption virtually impossible. For the Ni2Pb/Ni(111) surface alloy, we show that CO can bind to Ni atoms in the on-top position, and this process occurs to be exothermic with the energy gain of 0.35 eV per CO molecule. Dramatic reduction of the computed adsorption energy with respect to the pure Ni(111) substrate is in apparent agreement with experiment. However, it follows from our simulations that the CO adsorption process is accompanied by a substantial rearrangement of Ni atoms within the Ni2Pb surface alloy layer. Taking into account the associated deformation energy in the overall energetic balance yields nearly the same interaction energy between the CO molecules and the Ni atoms for the alloyed and the pure Ni(111) substrate, so the Ni–CO bond appears not to be weakened. The experimentally observed suppression of CO adsorption upon the alloy formation can be explained by a notable increase of the activation barrier for CO chemisorption from about 0.1 eV for the pure Ni(111) to roughly 0.5 eV for the Ni2Pb/Ni(111) surface alloy, affecting the corresponding reaction rate.