CO adsorption on (111) and (100) surfaces of the Pt sub 3 Ti alloy. Evidence for parallel binding and strong activation of CO

The CO adsorption on a 40 atom cluster model of the (111) surface and a 36 atom cluster model of the (100) surface of the Pt3Ti alloy was studied. Parallel binding to high coordinate sites associated with Ti and low CO bond scission barriers are predicted for both surfaces. The binding of CO to Pt sites occurs in an upright orientation. These orientations are a consequence of the nature of the CO pi donation interactions with the surface. On the Ti sites the orbitals donate to the nearly empty Ti 3d band and the antibonding counterpart orbitals are empty. On the Pt sites, however, they are in the filled Pt 5d region of the alloy band, which causes CO to bond in a vertical orientation by 5 delta donation from the carbon end

Initial stages of Pt growth on Ge(001) studied by scanning tunneling microscopy and density functional theory

We have studied the initial stages of submonolayer Pt growth on the Ge(001). We have observed several stable and meta-stable adsorption configurations of Pt atoms at various temperatures. Calculations indicate relatively high binding energies of Pt atoms onto the Ge lattice, at different adsorption sites. Our results show that through-the-substrate bonding (concerted bonding) of two Pt atoms is more favored on Ge(001) surface then a direct Pt-Pt bond. Both our experiments and calculations indicate the breaking of Ge-Ge bonds on the surface in the vicinity of Pt adsorbates. We have also observed the spontaneous generation of 2 + 1 dimer vacancy defects at room temperature that cause the ejection of Ge atoms onto the surface. Finally we have studied the diffusion of Pt atoms into the bulk as a result of annealing and found out that they get trapped at subsurface sites

Adsorption and Diffusion of Pt and Au on the Stoichiometric and Reduced TiO2 Rutile (110) Surfaces

A comparative first principles pseudopotential study of the adsorption and migration profiles of single Pt and Au atoms on the stoichiometric and reduced TiO2 rutile (110) surfaces is presented. Pt and Au behave similarly with respect to (i) most favorable adsorption sites, which are found to be the hollow and substitutional sites on the stoichiometric and reduced surfaces, respectively, (ii) the large increase in their binding energy (by ~1.7 eV) when the surface is reduced, and (iii) their low migration barrier near 0.15 eV on the stoichiometric surface. Pt, on the other hand, binds more strongly (by ~2 eV) to both surfaces. On the stoichiometric surface, Pt migration pattern is expected to be one-dimensional, which is primarily influenced by interactions with O atoms. Au migration is expected to be two-dimensional, with Au-Ti interactions playing a more important role. On the reduced surface, the migration barrier for Pt diffusion is significantly larger compared to Au.Comment: 3 figures, 1 table, submitted to PR

Ab initio study of element segregation and oxygen adsorption on PtPd and CoCr binary alloy surfaces

The segregation behavior of the bimetallic alloys PtPd and CoCr in the case of bare surfaces and in the presence of an oxygen ad-layer has been studied by means of first-principles modeling based on density-functional theory (DFT). For both systems, change of the d-band filling due to charge transfer between the alloy components, resulting in a shift of the d-band center of surface atoms compared to the pure components, drives the surface segregation and governs the chemical reactivity of the bimetals. In contrast to previous findings but consistent with analogous PtNi alloy systems, enrichment of Pt atoms in the surface layer and of Pd atoms in the first subsurface layer has been found in Pt-rich PtPd alloy, despite the lower surface energy of pure Pd compared to pure Pt. Similarly, Co surface and Cr subsurface segregation occurs in Co-rich CoCr alloys. However, in the presence of adsorbed oxygen, Pd and Cr occupy preferentially surface sites due to their lower electronegativity and thus stronger oxygen affinity compared to Pt and Co, respectively. In either cases, the calculated oxygen adsorption energies on the alloy surfaces are larger than on the pure components when the more noble components are present in the subsurface layers

Interactive Surface Chemistry of CO2 and NO2 on Metal Oxide Surfaces: Competition for Catalytic Adsorption Sites and Reactivity

Cataloged from PDF version of article.Interactive surface chemistry of CO2 and NO2 on BaOx/Pt(111) model catalyst surfaces were investigated via X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) techniques with a particular emphasis on the competition between different adsorbates for the catalytic adsorption sites and adsorbate-induced morphological changes. After NO2 adsorption, nitrated BaO x/Pt(111) surfaces do not reveal available adsorption sites for CO2 at 323 K, irrespective of the presence/absence of exposed Pt sites on the surface. Although NO2 adsorption on thick BaO x(>10MLE)/Pt(111) overlayers at 323 K leads to the formation of predominantly nitrate species, NO2 adsorption on the corresponding carbonated surface leads to the formation of coexisting nitrates and nitrites. The presence of carbonates on BaOx/Pt(111) overlayers does not prevent NO2 uptake. Carbonated BaOx(1.5 MLE)/Pt(111) surfaces (with exposed Pt sites) obtained via CO2 adsorption can also further interact with NO2, forming surface nitrate/nitrite species, accompanied by the transformation of surface carbonates into bulk carbonate species. These results suggest that the nitrate formation process requires the presence of two adjacent unoccupied adsorption sites. It is apparent that in the presence of both NO2 and CO2, carbonate species formed on Lewis base (O2-) sites enable the formation of nitrites on Lewis acid (Ba2+) sites. Thermal aging, nitration, and carbonation have a direct impact on the morphology of the two-/three-dimensional (2D/3D) BaO x aggregates on Pt(111). While thermal aging in vacuum leads to the sintering of the BaOx domains, nitration and carbonation results in redispersion and spreading of the BaOx domains on the Pt(111) substrate. © 2013 American Chemical Society.

Coverage-dependent molecular tilt of carbon monoxide chemisorbed on Pt{110}: A combined LEED and DFT structural analysis

The adsorption of carbon monoxide on the Pt{110} surface at coverages of 0.5 ML and 1.0 ML was investigated using quantitative low-energy electron diffraction (LEED IV) and density-functional theory (DFT). At 0.5 ML CO lifts the reconstruction of the clean surface but does not form an ordered overlayer. At the saturation coverage, 1.0 ML, a well-ordered p(2×1) superstructure with glide line symmetry is formed. It was confirmed that the CO molecules adsorb on top of the Pt atoms in the top-most substrate layer with the molecular axes tilted by ±22° with respect to the surface normal in alternating directions away from the close packed rows of Pt atoms. This is accompanied by significant lateral shifts of 0.55 Å away from the atop sites in the same direction as the tilt. The top-most substrate layer relaxes inwards by −4% with respect to the bulk-terminated atom positions, while the consecutive layers only show minor relaxations. Despite the lack of long-range order in the 0.5 ML CO layer it was possible to determine key structural parameters by LEED IV using only the intensities of the integer-order spots. At this coverage CO also adsorbs on atop sites with the molecular axis closer to the surface normal (b10°). The average substrate relaxations in each layer are similar for both coverages and consistent with DFT calculations performed for a variety of ordered structures with coverages of 1.0 ML and 0.5 ML