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Hoffmann F.M.,BMCC CUNY | Hrbek J.,Brookhaven National Laboratory | Ma S.,Brookhaven National Laboratory | Park J.B.,Brookhaven National Laboratory | And 3 more authors.
Surface Science | Year: 2015

Low-coordinated sites are surface defects whose presence can transform a surface of inert or noble metal such as Au into an active catalyst. Starting with a well-ordered Au(111) surface we prepared by ion sputtering gold surfaces modified by pits, used microscopy (STM) for their structural characterization and CO spectroscopy (IRAS and NEXAFS) for probing reactivity of surface defects. In contrast to the Au(111) surface CO adsorbs readily on the pitted surfaces bonding to low-coordinated sites identified as step atoms forming (111) and (100) microfacets. Pitted nanostructured surfaces can serve as interesting and easily prepared models of catalytic surfaces with defined defects that offer an attractive alternative to vicinal surfaces or nanoparticles commonly employed in catalysis science. © 2015 Elsevier B.V.

Hoffmann F.M.,BMCC CUNY | Hoo Y.S.,SUNY | Cai T.H.,Brookhaven National Laboratory | White M.G.,SUNY | Hrbek J.,Brookhaven National Laboratory
Surface Science | Year: 2012

We present here a study of the interaction of triruthenium dodecacarbonyl Ru 3(CO) 12 with gold surfaces using time-evolved and temperature-programmed infrared reflection absorption spectroscopy (IRAS) and STM. Ru 3(CO) 12 exhibits drastically different adsorption/desorption behavior on high-index surfaces of gold in comparison to the smooth Au(111) surface. On the smooth Au(111) surface, the adsorption of Ru 3(CO) 12 at 200 K is observed to be molecular and reversible with the molecule's Ru 3-plane oriented essentially perpendicular to the surface in the first and second layer. In the multilayer (> 3 ML), the molecule is oriented parallel (or moderately inclined) to the surface. On high-index gold surfaces, prepared by partial annealing of rough gold films, the molecules dissociate. Vibrational spectra reveal dissociation of carbonyl to Ru and CO at elevated temperature (> 250 K) with the formation of CO covered Ru-islands and the subsequent desorption of CO from Ru-islands. Increasing amounts of CO observed with increasing surface roughness demonstrate that the rate of Ru 3(CO) 12 dissociation is related directly to the surface roughness of the gold surface. STM images reveal at low coverage the formation of 2-D islands of carbonyl fragments with lateral sizes of 1 to 1.5 nm and at higher coverage the formation of larger 3-D islands of 1 to 3 layers and lateral sizes above 10 nm. © 2012 Elsevier B.V. All rights reserved.

Mudiyanselage K.,Brookhaven National Laboratory | Xu F.,Brookhaven National Laboratory | Xu F.,State University of New York at Stony Brook | Hoffmann F.M.,BMCC CUNY | And 4 more authors.
Physical Chemistry Chemical Physics | Year: 2015

Adsorbate-driven morphological changes of pitted-Cu(111) surfaces have been investigated following the adsorption and desorption of CO and H. The morphology of the pitted-Cu(111) surfaces, prepared by Ar+ sputtering, exposed a few atomic layers deep nested hexagonal pits of diameters from 8 to 38 nm with steep step bundles. The roughness of pitted-Cu(111) surfaces can be healed by heating to 450-500 K in vacuum. Adsorption of CO on the pitted-Cu(111) surface leads to two infrared peaks at 2089-2090 and 2101-2105 cm-1 for CO adsorbed on under-coordinated sites in addition to the peak at 2071 cm-1 for CO adsorbed on atop sites of the close-packed Cu(111) surface. CO adsorbed on under-coordinated sites is thermally more stable than that of atop Cu(111) sites. Annealing of the CO-covered surface from 100 to 300 K leads to minor changes of the surface morphology. In contrast, annealing of a H covered surface to 300 K creates a smooth Cu(111) surface as deduced from infrared data of adsorbed CO and scanning tunnelling microscopy (STM) imaging. The observation of significant adsorbate-driven morphological changes with H is attributed to its stronger modification of the Cu(111) surface by the formation of a sub-surface hydride with a hexagonal structure, which relaxes into the healed Cu(111) surface upon hydrogen desorption. These morphological changes occur ∼150 K below the temperature required for healing of the pitted-Cu(111) surface by annealing in vacuum. In contrast, the adsorption of CO, which only interacts with the top-most Cu layer and desorbs by 200 K, does not significantly change the morphology of the pitted-Cu(111) surface. © the Owner Societies 2015.

Mudiyanselage K.,Brookhaven National Laboratory | Yang Y.,SUNY | Hoffmann F.M.,BMCC CUNY | Furlong O.J.,National University of San Luis | And 4 more authors.
Journal of Chemical Physics | Year: 2013

The interaction of atomic hydrogen with the Cu(111) surface was studied by a combined experimental-theoretical approach, using infrared reflection absorption spectroscopy, temperature programmed desorption, and density functional theory (DFT). Adsorption of atomic hydrogen at 160 K is characterized by an anti-absorption mode at 754 cm-1 and a broadband absorption in the IRRA spectra, related to adsorption of hydrogen on three-fold hollow surface sites and sub-surface sites, and the appearance of a sharp vibrational band at 1151 cm-1 at high coverage, which is also associated with hydrogen adsorption on the surface. Annealing the hydrogen covered surface up to 200 K results in the disappearance of this vibrational band. Thermal desorption is characterized by a single feature at ∼295 K, with the leading edge at ∼250 K. The disappearance of the sharp Cu-H vibrational band suggests that with increasing temperature the surface hydrogen migrates to sub-surface sites prior to desorption from the surface. The presence of sub-surface hydrogen after annealing to 200 K is further demonstrated by using CO as a surface probe. Changes in the Cu-H vibration intensity are observed when cooling the adsorbed hydrogen at 180 K to 110 K, implying the migration of hydrogen. DFT calculations show that the most stable position for hydrogen adsorption on Cu(111) is on hollow surface sites, but that hydrogen can be trapped in the second sub-surface layer. © 2013 AIP Publishing LLC.

Mudiyanselage K.,Brookhaven National Laboratory | Luo S.,State University of New York at Stony Brook | Kim H.Y.,Chungnam National University | Yang X.,Brookhaven National Laboratory | And 7 more authors.
Catalysis Today | Year: 2015

Mixed-metal oxides exhibit novel properties that are not present in their isolated constituent metal oxides and play a significant role in heterogeneous catalysis. In this study, a titanium-copper mixed-oxide (TiCuO x ) film has been synthesized on Cu(111) and characterized by complementary experimental and theoretical methods. At sub-monolayer coverages of titanium, a Cu2O-like phase coexists with TiCuO x and TiO x domains. When the mixed-oxide surface is exposed at elevated temperatures (600-650K) to oxygen, the formation of a well-ordered TiCuO x film occurs. Stepwise oxidation of TiCuO x shows that the formation of the mixed-oxide is faster than that of pure Cu2O. As the Ti coverage increases, Ti-rich islands (TiO x ) form. The adsorption of CO has been used to probe the exposed surface sites on the TiOx-CuO x system, indicating the existence of a new Cu+ adsorption site that is not present on Cu2O/Cu(111). Adsorption of CO on Cu+ sites of TiCuO x is thermally more stable than on Cu(111), Cu2O/Cu(111) or TiO2(110). The Cu+ sites in TiCuO x domains are stable under both reducing and oxidizing conditions whereas the Cu2O domains present on sub-monolayer loads of Ti can be reduced or oxidized under mild conditions. The results presented here demonstrate novel properties of TiCuO x films, which are not present on Cu(111), Cu2O/Cu(111), or TiO2(110), and highlight the importance of the preparation and characterization of well-defined mixed-metal oxides in order to understand fundamental processes that could guide the design of new materials. © 2015 Elsevier B.V.

Baber A.E.,Brookhaven National Laboratory | Yang X.,Brookhaven National Laboratory | Kim H.Y.,Center for Functional Nanomaterials | Kim H.Y.,Chungnam National University | And 13 more authors.
Angewandte Chemie - International Edition | Year: 2014

The oxidation of CO is the archetypal heterogeneous catalytic reaction and plays a central role in the advancement of fundamental studies, the control of automobile emissions, and industrial oxidation reactions. Copper-based catalysts were the first catalysts that were reported to enable the oxidation of CO at room temperature, but a lack of stability at the elevated reaction temperatures that are used in automobile catalytic converters, in particular the loss of the most reactive Cu+ cations, leads to their deactivation. Using a combined experimental and theoretical approach, it is shown how the incorporation of titanium cations in a Cu2O film leads to the formation of a stable mixed-metal oxide with a Cu+ terminated surface that is highly active for CO oxidation. Positively active: Copper oxide based structures were the first that could catalyze the oxidation of CO at room temperature. Their deactivation, however, is facile, because the required Cu+ state cannot be preserved under the reaction conditions. The addition of the right amount of titanium leads to mixed CuTiOx films that are thermally and chemically stable and more active CO oxidation catalysts than pure copper oxide materials. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Paulius D.,University of the Virgin Islands | Torres D.,BMCC CUNY | Illas F.,University of Barcelona | Archibald W.E.,University of the Virgin Islands
Physical Chemistry Chemical Physics | Year: 2014

Self-assembled monolayers on Au(111) have outstanding chemical, electrical, and optical properties, and Au adatoms seem to play a key role in these properties. Still, the fundamental understanding of adatom transport inside the self-assembled structure is very thin. In this paper we use first-principles calculations to reveal new details about the migration mechanism of Au adatoms in the presence of a CH3S self-assembled structure on Au(111). We study the inclusion of Au adatoms inside a well-packed (√3 × √3)-R30°-CH3S self-assembled lattice and present atomistic models supporting adatom migration by means of a hopping mechanism between pairs of CH3S species. Our calculations reveal that the transport of Au adatoms is slowed down inside the molecular network where the kinetic barrier for adatom migration is larger than on the clean Au surface. We attribute the hindered mobility of Au adatoms to the fact that adatom transport involves the breaking and making of Au-S bonds. Our results form a basis for further understanding the role played by defect transport in the properties of molecular assemblies. This journal is © the Partner Organisations 2014.

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