Session A2: Focus Session: Surface Chemistry and Catalysis I

Surface Chemistry of PdO(101)

The formation of palladium oxide (PdO) is thought to be responsible for the exceptional activity of supported Pd catalysts toward the complete oxidation of alkanes under oxygen-rich conditions. In this talk, I will discuss our investigations of the surface chemical properties of a PdO(101) thin film, focusing particularly on the adsorption and selective activation of alkanes. We find that $n$-alkanes adsorb relatively strongly on the PdO(101) surface by forming $\sigma $-complexes along rows of coordinatively-unsaturated Pd atoms, and that this adsorbed state acts as the precursor for initial C-H bond cleavage. I will discuss characteristics of the binding and activation of alkane $\sigma $-complexes on PdO(101) as determined from both experiment and density functional theory calculations. I will also discuss elementary processes involved in adsorbate oxidation on PdO(101) and make comparisons with the chemical reactivity of other late transition metal oxides.   

Inelastic neutron scattering (INS) studies of hydrogen spillover on pure and Pd decorated metal oxides

Recent INS and quasielastic neutron scattering (QENS) measurements of the interaction of H2 with pure and metal decorated metal oxides(MOs) will be discussed. These materials find widespread use as energy materials e.g.as oxidation and hydrogenation catalysts. These studies are aimed at revealing the microscopic details of the process(es) that underlie ``hydrogen spillover'' to identify what, if any, role it plays in the catalytic cycle. Hydrogen spillover refers to the diffusion of hydrogen from a surface capable of disassociating H2, onto an adjoining surface. This diffusing hydrogen may possess an electron capable of pairing with an unpaired free radical electron on an adjacent surface. Many catalysts consist of nm sized metal clusters supported on high surface area MOs and many catalytic reactions involve hydrogen. Recent INS observations show surface OH formation on MOs supports occurs only when the metal catalyst is present even at low temperatures. Spectral signatures in both the rotational and vibrational portions of the INS signals underscore this behavior. QENS data establishes that translational diffusion is significant at T\textless 40 K. Behavior on various support materials will be highlighted.   

Simple Molecules Adsorption Studies on Highly Epitaxial -Pure Phase- Delafossite CuFeO$_{2}$ Thin Films

Carbon dioxide (CO$_{2})$ and hydrogen (H2) adsorption studies on CuFeO$_{2}$ thin films grown on Al$_{2}$O$_{3}$ (00.1) substrates were performed in ultrahigh vacuum using thermal programmed desorption (TPD). Growth of pure phase Delafossite CuFeO$_{2}$ thin films on Al$_{2}$O$_{3}$ (00.1) substrates by pulsed laser deposition was systematically investigated as a function of growth temperature and oxygen pressure. CO$_{2}$ and H$_{2}$ TPD were performed on CuFeO$_{2}$ -grown at 600$^{\circ}$C and in 0.1mTorr pressure- indicating chemisorption of both gases on the oxide surface. TPD with a temperature ramp of 50 K/s showed a CO$_{2}$ peak at 573 K and H$_{2}$ peak at 823 K. The chemisorption of CO$_{2}$ and H$_{2}$ on the CuFeO$_{2}$ surface is relevant to the potential use of this material in photocatalytic applications for H$_{2}$ production and/or CO$_{2}$ conversion.   

Computational Database of Metal-Oxide Surface Reactivities for Catalyst Design

We study surface reactivity of low index facets of MO, MO2 and ABO3 oxide groups using small atoms and molecules (O, OH, CO, NO, CH3, NH3). The computed database of adsorption and activation energies will be used to identify possible correlations with other quantities such as surface energies or electronic structure in order to establish scaling relations for future high-throughput screening efforts. A comparison will be made between DFT functionals of various levels of accuracy, e.g., GGA, GGA+U, GGA+vdW and GGA-hybrid, meta-GGA and hybrid meta-GGA, and compared to available experiments. This effort is part of the ``Predictive Theory of Transition Metal Oxide Catalysis'' funded through the DOE Materials Genome Project.   

Density functional theory investigation of NO$_2$ and SO$_2$ adsorption on isolated and anatase-supported BaO clusters

BaO is the most commonly used storage component in NO$_x$ storage and reduction catalysts (NSR). TiO$_2$ has recently been suggested by several authors as a promising support material, with increased sulphur tolerance when compared with traditional supports such as $\gamma$-Al$_2$O$_3$. The optimization of NSR catalysts requires knowledge of the interaction between the storage and support components. In this talk, we present a DFT investigation of the electronic and structural properties of NO$_2$ and SO$_2$ adsorption on isolated and anatase-supported (BaO)$_n$ (n=1,2,4,6,8,9) clusters. Generally, supported BaO clusters are found to display better tolerance towards sulphur poisoning compared to both bare BaO (100) surface and supported BaO overlayers.   

\textit{In-situ} NAP XPS studies of dissociative water adsorption on GaAs(100) surfaces

In current semiconductor-based technology it is important to design and fabricate new materials in order to achieve specific well-defined properties and functionalities. Before such systems can be applied they first need to be understood, refined and controlled. Therefore, a basic knowledge about molecule/semiconductor surface interfaces is essential. In the present work dissociative water adsorption on the GaAs(100) surface is monitored using X-ray Photoelectron Spectroscopy (XPS) performed \textit{in situ} under near ambient conditions. Firstly, the crystal surface is exposed to water vapor pressures ranging from UHV to 0.5 kPa. At elevated pressures an increase of oxygenation and hydroxylation of Ga surface atoms has been observed in the Ga2p XPS spectra. Moreover, intense signals obtained from molecularly adsorbed water molecules or water molecules adsorbed \textit{via} hydrogen bond to surface OH groups have been also observed in the O1s spectra. Finally, the crystal surface is annealed up to 700 K at water vapor pressure of 0.01 kPa, which leads to desorption of physisorbed water molecules and further increase of surface oxidation.   

Fundamental mechanistic studies in formic acid decomposition on transition metal surfaces

Formic acid (HCOOH) is a simple molecule that is an abundant product of biomass processing and can serve as an internal source of hydrogen for oxygen removal and upgrading of biomass to chemicals and fuels. In addition, HCOOH can be used as a fuel for low temperature direct fuel cells. We present a systematic study of the HCOOH decomposition reaction mechanism starting from first-principles and including reactivity experiments and microkinetic modeling. In particular, periodic self-consistent Density Functional Theory (DFT) calculations are performed to determine the stability of reactive intermediates and activation energy barriers of elementary steps. Pre-exponential factors are determined from vibrational frequency calculations. Mean-field microkinetic models are developed and calculated reaction rates and reaction orders are then compared with experimentally measured ones. These comparisons provide useful insights on the nature of the active site, most-abundant surface intermediates as a function of reaction conditions and feed composition. Trends across metals on the fundamental atomic-scale level up to selectivity trends will be discussed. Finally, we identify from first-principles alloy surfaces, which may possess better catalytic properties for selective dehydrogenation of HCOOH than monometallic surfaces, thereby guiding synthesis towards promising novel catalytic materials.   

Autocatalytic dissociation of water at stepped transition metal surfaces

By means of density functional theory calculations, we investigate the adsorption and dissociation of water clusters on flat and stepped surfaces of several transition metals: Rh, Ir, Pd, Pt, and Ru. We find that water binds preferentially to the edge of the steps than to terrace sites, so that isolated clusters or one-dimensional water wires can be isolated by differential desorption. The enhanced reactivity of metal atoms at the step edge and the cooperative effect of hydrogen bonding enhance the chances of partial dissociation of water clusters on stepped surfaces. For example, water dissociation on Pt and Ir surface turns from endothermic at terraces to exothermic at steps. The interpretation of water dissociation is achieved by analyzing changes in the electronic structure of both water and metals, especially focusing on the interaction between the lone-pair electrons of water and the d-band of the metals [1]. The shift in the energetics of water dissociation at steps is expected to play a prominent role in catalysis and fuel cells reactions, as the density of steps at surfaces could be an additional parameter to design more efficient anode materials or catalytic substrates. \\[4pt] [1] D. Donadio, L.M. Ghiringhelli, and L. Delle Site, J. Am. Chem. Soc. 134, 19217 (2012).   

Trimethyltin Mediated Formation of Covalent Gold-Carbon Bonds

Covalent Au-C bond formation via trimethyltin (SnMe$_{\mathrm{3}})$ precursors has been hypothesized based on recent single-molecule conductance measurements$^{\mathrm{1}}$. Here, we provide spectroscopic evidence for the formation of Au-C bonds using a trimethylbenzyltin precursor on Au(110) and Au(111) surfaces. From X-Ray photoemission spectroscopy (XPS), we find that the precursor molecule cleaves on both surfaces at temperatures as low as 200K. As substrate temperature rises to 300K, shifts in the C1s and Sn3d XPS spectra indicate the formation of Au-Benzyl and Au-SnMe$_{\mathrm{3}}$ moieties on the surface. Near-edge X-Ray photoemission spectroscopy (NEXAFS) of the Au-Benzyl system on Au(110) shows a new unoccupied state near E$_{\mathrm{Fermi}}$ accompanied by a broadened lowest unoccupied molecular orbital (LUMO). That these features are missing in Au(111) suggests that the Au-C bond formation occurs preferentially on under-coordinated gold surfaces. Lastly, we use core-hole-clock resonant photoemission to understand the dynamics of charge transfer from this broadened LUMO to the underlying Au substrate, and find evidence for sub-femtosecond charge transfer [1] JACS 133, 17160--17163 (2011)   

Epicatalysis: Bending the third principle of catalysis

A standard principle of traditional catalysis -- that a catalyst cannot alter the final thermodynamic equilibrium of a reaction -- can fail in low-pressure, heterogeneous gas-surface reactions [1]. Kinetic theory for this {\em epicatalysis} is presented, and two well-documented experimental examples are shown: surface ionized plasmas and hydrogen dissociation on refractory metals. This phenomenon should be observable over a wide range of temperatures and pressures, and for a broad spectrum of heterogeneous reactions. By transcending some constraints of equilibrium thermodynamics, epicatalysis might provide new control parameters and synthetic routes for reactions, and enable product streams boosted in thermochemical energy or desirable species. Recent experiments involving hydrogen dissociation on tungsten and rhenium indicate that steady-state nonequilibria can be be maintained between competing epicatalysts within a single blackbody cavity, challenging thermodynamic expectations.\\[4pt] [1] Sheehan, D.P., Phys. Rev. E 88, 032125 (2013).\\[0pt] [2] Sheehan, D.P., D.J. Mallin, J.T. Garamella, and W.F. Sheehan, Found. Phys., in review (2013).