Site selectivity in chemisorption of C on Pd(211)

Abstract

Site selectivity in chemisorption of C on Pd(211) Results of ab initio electronic structure calculations of C on Pd(211), a vicinal of Pd (111), show a hierarchy of adsorption sites with adsorption energy ranging from −8.21 to − 5.46 eV (0.33 ML coverage) and scaling almost linearly with the effective coordination at the site. In the most preferred site, C atoms sit almost under the Pd step atoms and influence drastically multilayer relaxations and surface registry of Pd(211). The adsorption energy at this site is −9.10 eV for 0.17 ML coverage. The local densities of electronic states indicate strong hybridization between C p states and neighboring Pd d states leading to the formation of strong covalent C u Pd bonds. The depletion of the electronic density of states of the Pd surface atoms at the Fermi level suggests a poisoning effect of C which is found to be coverage dependent. We compare the changes in the electronic structure of Pd(211) on C adsorption with those on S adsorption.

C and S induce changes in the electronic and geometric structure of Pd(533) and Pd(320)

“…This effect is expected to poison catalytic activity of these surfaces. We have compared the results obtained for C and S chemisorption on Pd(533) with those obtained earlier for Pd(211) [14]. These two stepped surfaces have similar structures, but Pd(533) has one atomic chain wider terrace.…”

“…They form bonds with SC and CC atoms and build extra S -TC2 bonds which induce upward displacement of the TC2 atoms (layer 3) and hence an increase in δd 3,4 . Interestingly, our recent calculations performed for Pd(211) 6 reveal a similar response of the surface lattice to S and C adsorption [14]: the separation between SC and BNN is substantially increased upon C adsorption and and TC is displaced upwards upon S adsorption. Note that Pd(211) has one less chain of atoms on its terrace as compared to that of Pd(533) Lattice relaxation upon S and C adsorption has also been studied for the kinked Pd (320) surface.…”

“…In one of the earlier studies [13], it was suggested that the depletion of the local density of states at the Fermi-level [N a (E F )] (a denotes the atom contributing to LDOS) upon S adsorption can cause poisoning of surface reactivity. Computational studies of S adsorption on some Pd surfaces [14,15,16] also show a reduction of N a (E F ) due to a strong hybridization of the p states of S with the d states of Pd. Several experimental studies also focus on the poisoning effect of S on metal surfaces [17,18,19].…”

“…Although the rate of CO dissociation during catalytic oxidation is low, small amount of C can atomically adsorb on the catalyst surface. Our recent results [14,15] show that C atoms adsorbed on Pd stepped surfaces suppress substantially N a (E F ) of the neighboring surface atoms which may be taken as indication of poisoning. Atomic carbon adsorption on Pd surface and its poisoning effect were also reported for the case of catalytic vinyl acetate synthesis [20].…”

“…Finally, examination of the relative effect of C and S on Pd(320) will provide the basis for comparison of results on stepped surfaces with fcc(110) terrace geometry to those with fcc(111) terraces [14]. Since the nonequivalent atoms on a vicinal surface, in the presence of an adsorbate, account for a complex and inhomogeneous system, these studies are expected to have implications also for the characteristics of nanoparticles which contain a range of undercoordinated sites in complex environments.…”