A theoretical study of the structural parameters, interaction energies
, and bonding mechanism of CO and NO to a Pd center located in two cop
per-rich bimetallic PdCu(111) surfaces and several coordination positi
ons 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 de
crease of the CO/PdCu interaction energy with the increase in copper p
ercentage of the alloy, the insensitivity of the C-O stretch frequency
to the composition of the central Pd environment, and the linear corr
elation observed between the CO desorption energy and the X-ray photoe
lectron spectroscopy (XPS) core-level shift. For NO, the nature of the
interaction varies from nearly covalent for pure Pd to a mixture of c
ovalent plus ionic for copper-rich alloys; this parallels the expected
growth of the ionic component of the bond with the decrease of the wo
rk function in going from Pd to Cu. A correlation between the N-O stre
tch frequency shift and the copper content of the binary systems is al
so found.