Interaction of CO and NO with PdCu(111) surfaces

A theoretical study of the structural parameters, interaction energies, and bonding mechanism of CO and NO to a Pd center located in two copper-rich bimetallic PdCu(111) surfaces and several coordination positions of the Pd(111) surface is reported. For CO, the bonding nature is predominantly covalent, and the analysis of bonding nature variations through the series is used to interpret the experimentally observed decrease of the CO/PdCu interaction energy with the increase in copper percentage of the alloy, the insensitivity of the C-O stretch frequency to the composition of the central Pd environment, and the linear correlation observed between the CO desorption energy and the X-ray photoelectron spectroscopy (XPS) core-level shift. For NO, the nature of the interaction varies from nearly covalent for pure Pd to a mixture of covalent plus ionic for copper-rich alloys; this parallels the expected growth of the ionic component of the bond with the decrease of the work function in going from Pd to Cu. A correlation between the N-O stretch frequency shift and the copper content of the binary systems is also found.