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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 | 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.

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.

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.

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