AVS2010 Session SS1-TuM: Catalysis on Metal and Alloy Surfaces

Sulfur adsorption on Ag(100) is the object of this investigation because sulfur affects reshaping and decay of Ag nanostructures on this, and other, surfaces. We have studied sulfur on Ag(100) using STM. Consistent with prior LEED work [1-2], we find two structures that coexist at room temperature: a p(2x2) chemisorbed phase, and a (√17x√17)R14^o reconstruction, the latter being the phase with higher sulfur coverage. As sulfur coverage increases, sulfur atoms replace Ag in the surface plane to form the √17, resulting in ejection of Ag and development of √17 islands on the terraces. DFT supports a model in which 5 Ag atoms are ejected per unit cell. In STM, the dominant local motif of the √17 reconstruction consists of rectangular groups of (primarily) four sulfur atoms, very similar to sulfur on Cu(100) in the √17 phase [3]. At room temperature, the √17 islands are dynamic, and can develop extensions of disordered material that link and island, transiently, to another √17 island or domain.

References

[1] G. Rovida and F. Pratesi, Surf. Sci. 104(2-3), 609 (1981).

[2] M. P. Sotto and J. C. Boulliard, Surf. Sci. 162(1-3), 285 (1985).

[3] M. L. Colaianni and I. Chorkendorff, Phys. Rev. B 50(12), 8798 (1994).

Using high-resolution and time-dependent x-ray photoelectron spectroscopy (XPS) adsorption and reaction processes can be followed in-situ. From the data, adsorbed species as well as reaction intermediates and products can be determined and quantitatively analyzed. Time-dependent data allow for the determination of kinetic parameters.

We are applying this method to pyridine molecules adsorbed on flat and stepped Pt surfaces. Using a regularly stepped Pt(355) surface with (111) oriented terraces which are five atomic rows wide and (111) oriented monatomic steps, C 1s and N 1s core level data are recorded during adsorption at various temperatures. As additional parameter, the free Pt terrace width is varied by deposition of Ag atoms on the surface. At 300 K, they form rows along the step edges, thus effectively reducing the terrace width. Following the core level data while increasing the temperature with a fixed heating rate, temperature-programmed XPS data allow to follow the thermal reaction of the adsorbed species. The experiments have been performed at the synchrotron radiation facility BESSY II in Berlin .

Literature reports a temperature- and coverage-dependent reorientation of the pyridine molecules on Pt111) [1]. While at 300 K an upright molecule is proposed, the situation changes for low temperatures from flat-lying at low to upright at higher coverages. In line with these reports, we observe distinct changes in the core level binding energies and intensities. While at 200 K only one strong N 1s feature is observed for all coverages, there is a change from two features to one for 300 K. Here, the remaining feature is observed at a higher binding energy. Temperature-dependent data show the change from the low to the high energy feature. This is accompanied by respective changes in the C 1s data, thus suggesting again temperature- and coverage-dependent changes.

The presence of steps and especially their decoration by Ag changes the situation quite drastically. This manifests itself in changed binding energies and reaction schemes. From a comprehensive analysis a scheme for adsorption and thermal reaction of pyridine on Platinum surfaces is derived.

[1] S. Haq, D.A. King, J. Phys. Chem. 100 (1996) 16957.

Palladium and its alloys play a central role in a wide variety of industrially important applications such as hydrogen reactions, separations, storage devices, and fuel cell components. The exact mechanisms by which many of these processes operate have yet to be discovered. Low-temperature scanning tunneling microscopy (STM) has been used to investigate the atomic-scale structure of Pd/Au and Pd/Cu bimetallics created by depositing Pd on both Au(111) and Cu(111) at a variety of surface temperatures. We demonstrate that individual isolated Pd atoms in an inert Cu matrix are active for the dissociation of hydrogen and subsequent spillover onto Cu sites. The same mechanism does not operate for Pd in Au(111) surfaces, however, because the spillover is thermodynamically unfavorable. These results demonstrate the powerful influence an inert substrate has on the catalytic activity of individual Pd atoms supported in its surface. In addition, differential conductance spectroscopy has enabled the electronic structure of these active sites to be measured and quantitatively compared to that of the host metal.

Bimetallic materials have shown great promise for the development of superior catalysts. The recent surge of new interest in catalysis by gold has led researchers to investigate the effects of adding gold to other metals. While mechanisms underlying the alloying effect are still not understood in detail, recent evidence suggests that the enhanced reactivity of bimetallic catalysts can be attributed to a combination of metal-metal interactions (ligand effect) and unique mixed-metal surface sites (ensemble effect). The ability to accurately predict the arrangements of constituent atoms in a surface alloy is indispensable to unraveling the roles played by the ensemble and ligand effects in the performance of bimetallic model catalysts. We have developed a scheme to predict the equilibrium arrangement of atoms in surface alloys at finite temperatures. It is based on the Ising model and is capable of reproducing DFT-predicted total energies to within no more than a few meV per surface atom. We have used it successfully to predict monomer and dimer concentrations in Au-Pd and Au-Pt fcc (111) surface alloys. The scheme will be presented in detail, as well as what we have learned about the effects of temperature and composition on ensemble formation in both fcc (111) and (100) surface alloys, including the size and shape distributions of larger ensembles. We will also discuss how the atomic arrangements affect the reactivity of gold-based alloy surfaces, particularly towards oxidation of hydrogen and carbon monoxide.

Pd/Fe alloy catalysts have attracted attention in PEM fuel cell research because they have been found to be comparable to Pt electrocatalysts in oxygen reduction reaction (ORR) kinetics at the cathode. Higher electrocatalytic activity is also found when these bimetallic nanoparticles act as a support of Pt or Pd monolayers. The mechanism of enhancing ORR kinetics with this alloy is not well understood. We report here on the structure and segregation at a clean and oxidized Pd₃Fe(111) alloy using LEED, LEIS, XPS, and STM. Pd starts to segregate at the surface at 600 K and reaches a coverage of 0.9 monolayers at 1100 K. STM reveals a complex structure with 0.17 monolayers of Pd adatoms on top of the outmost alloy layer, which is different from other well-studied binary alloy systems. The experimental observation of this unusual Pd structure is supported by DFT calculations. Adsorption of oxygen reverses the segregation trend and causes Fe atoms to accumulate on the surface. Fe at the surface is oxidized by oxygen and initially protects Pd oxidation. Oxygen adsorption at different temperatures caused the formation of nanclusters at lower temperatures (300-500 K) and several well-organized surface structures at higher temperatures. Such studies of structure, segregation and oxidation of these model Pd/Fe alloy catalysts may help explain the origin of enhanced ORR kinetics using these alloys.

Gold-based bimetallic alloys have been found to significantly increase catalytic efficiency, compared to their monometallic counterparts in various reactions including low temperature oxidation of CO, direct synthesis of H₂O₂ from H₂ and O₂, and production of vinyl acetate monomers. Recent experimental and theoretical studies have suggested that the reactivity of bimetallic catalysts can be governed by creation of unique mixed-metal surface sites (ensemble effect) and/or electronic structure change by metal-metal interactions (ligand effect), while mechanisms underlying the alloying effect still remain unclear. In this talk, we will present the recent results of our first principles calculations on the mechanisms of CO oxidation and oxygen reduction reaction (ORR) on AuPd and AuPt alloy surfaces. Using periodic density functional theory calculations, we find that the rate of CO oxidation can be a strong function of Pd/Pt ensembles, while the ensemble effect is likely to be more important in the AuPd case. In addition, we find that ORR tends to be promoted on PdAu alloys, compared to the pure Pd case. This work provides valuable hints on the importance of the interplay of the ensemble and ligand effects in determining the catalytic activity of Au-based bimetallic catalysts particularly toward CO oxidation and ORR.