Catherine Stampfl

Structure and stability of a high-coverage (1x1) oxygen phase on Ru(0001)

The formation of chemisorbed O phases on Ru(0001) by exposure to O{sub 2} at low pressures is apparently limited to coverages {Theta}{le}0.5. Using low-energy electron diffraction and density-functional theory we show that this restriction is caused by kinetic hindering and that a dense O overlayer ({Theta}=1) can be formed with a (1{times}1) periodicity. The structural and energetic properties of this new adsorbate phase are analyzed and discussed in view of attempts to bridge the so-called {open_quote}{open_quote}pressure gap{close_quote}{close_quote} in heterogeneous catalysis. It is argued that the identified system actuates the unusually high rate of oxidizing reactions at Ru surfaces under high oxygen pressure conditions. {copyright} {ital 1996 The American Physical Society.}

Catalysis and corrosion: the theoretical surface-science context

NNumerous experiments in ultra-high vacuum as well as (T ¼ 0K ,p ¼ 0) theoretical studies on surfaces have been performed over the last decades in order to gain a better understanding of the mechanisms, which, for example, underlie the phenomena of catalysis and corrosion. Often the results achieved this way cannot be extrapolated directly to the technologically relevant situation of finite temperature and high pressure. Accordingly, modern surface science has realized that bridging the so-called pressure gap (getting out of the vacuum) is the inevitable way to go. Of similar importance are studies in which the temperature is changed systematically (warming up and cooling down). Both aspects are being taken into account in recent experiments and ab initio calculations. In this paper we stress that there is still much to learn and important questions to be answered concerning the complex atomic and molecular processes which occur at surfaces and actuate catalysis and corrosion, although significant advances in this exciting field have been made over the years. We demonstrate how synergetic effects between theory and experiment are leading to the next step, which is the development of simple concepts and understanding of the different modes of the interaction of chemisorbed species with surfaces. To a large extent this is being made possible by recent developments in theoretical methodology, which allow to extend the ab initio (i.e., starting from the selfconsistent electronic structure) approach to poly-atomic complexes with 10,000 and more atoms, time scales of seconds, and involved statistics (e.g., ab initio molecular dynamics with 10,000 and more trajectories). In this paper we will 1. sketch recent density–functional theory based hybrid methods, which bridge the length and time scales from those of electron orbitals to meso- and macroscopic proportions, and 2. present some key results on properties of surfaces, demonstrating their role in corrosion and heterogeneous catalysis. In particular we discuss • the influence of the ambient gas phase on the surface structure and stoichiometry, • adsorbate phase transitions and thermal desorption, and • the role of atoms’ dynamics and statistics for the surface chemical reactivity.

Theory of doping and defects in III-V nitrides

Doping problems in GaN and in AlGaN alloys are addressed on the basis of state-of-the-art first-principles calculations. For n-type doping we find that nitrogen vacancies are too high in energy to be incorporated during growth, but silicon and oxygen readily form donors. The properties of oxygen, including DX-center formation, support it as the main cause of unintentional n-type conductivity. For p-type doping we find that the solubility of Mg is the main factor limiting the hole concentration in GaN. We discuss the beneficial e⁄ects of hydrogen during acceptor doping. Compensation of acceptors by nitrogen vacancies may occur, becoming increasingly severe as x increases in Al x Ga 1~x N alloys. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ji; 71.55.Eq

First-principles theory of surface thermodynamics and kinetics

In this Letter, with the aim to improve upon this approach, we combine state-of-the-art procedures of (i) microscopic theories, i.e., DFT electronic structure calculations and (ii) macroscopic phenomenological approaches, i.e., lattice gas and rate equations, and Monte Carlo schemes. On doing this, we present a consistent first-principles-based approach for calculation of the thermodynamic and kinetic properties of an adsorbate, such as heats of adsorption, temperature programmed desorption (TPD) spectra, and the surface phase diagram. We have chosen the system of oxygen at Ru(0001) for which detailed structural [4 ‐9], thermodynamic [10], and kinetic data [11,12] exist. We will show that, with the present approach, a realistic description of these physical properties is indeed feasible.

Theoretical study of O adlayers on Ru(0001)

Recent experiments performed at high pressures indicate that ruthenium can support unusually high concentrations of oxygen at the surface. To investigate the structure and stability of high coverage oxygen structures, we performed density functional theory calculations, within the generalized gradient approximation, for O adlayers on Ru(0001) from low coverage up to a full monolayer. We achieve quantitative agreement with previous low energy electron diffraction intensity analyses for the (2x2) and (2x1) phases and predict that an O adlayer with a (1x1) periodicity and coverage of 1 monolayer can form on Ru(0001), where the O adatoms occupy hcp-hollow sites.

Anomalous behavior of Ru for catalytic oxidation: A theoretical study of the catalytic reaction CO + 1/2 O_2 -> CO_2

Recent experiments revealed an anomalous dependence of carbon monoxide oxidation at Ru(0001) on oxygen pressure and a particularly high reaction rate. Below we report density functional theory calculations of the energetics and reaction pathways of the speculated mechanism. We will show that the exceptionally high rate is actuated by a weakly but nevertheless well bound (1{times}1)-oxygen adsorbate layer. Furthermore, it is found that reactions via scattering of {ital gas-phase} CO at the oxygen covered surface may play an important role. Our analysis reveals, however, that reactions via {ital adsorbed} CO molecules (the so-called Langmuir-Hinshelwood mechanism) dominate. {copyright} {ital 1997} {ital The American Physical Society}

Stability and morphology of cerium oxide surfaces in an oxidizing environment: A first-principles investigation

We present density functional theory investigations of the bulk properties of cerium oxides (CeO2 and Ce2O3) and the three low index surfaces of CeO2, namely, (100), (110), and (111). For the surfaces, we consider various terminations including surface defects. Using the approach of “ab initio atomistic thermodynamics,” we find that the most stable surface structure considered is the stoichiometric (111) surface under “oxygen-rich” conditions, while for a more reducing environment, the same (111) surface, but with subsurface oxygen vacancies, is found to be the most stable one, and for a highly reducing environment, the (111) Ce-terminated surface becomes energetically favored. Interestingly, this latter surface exhibits a significant reconstruction in that it becomes oxygen terminated and the upper layers resemble the Ce2O3(0001) surface. This structure could represent a precursor to the phase transition of CeO2 to Ce2O3.

Adsorption of Xe atoms on metal surfaces: New insights from first-principles calculations

The adsorption of rare gases on metal surfaces serves as the paradigm of weak adsorption where it is typically assumed that the adsorbate occupies maximally coordinated hollow sites. Density-functional theory calculations using the full-potential linearized augmented plane wave method for Xe adatoms on Mg(0001), Al(111), Ti(0001), Cu(111), Pd(111), and Pt(111), show, however, that Xe prefers low coordination on-top sites in all cases. We identify the importance of polarization and a site-dependent Pauli repulsion in actuating the site preference and the principle nature of the rare-gas atom-metal surface interaction.

Atomistic description of oxide formation on metal surfaces: the example of ruthenium

The microscopic process of the formation of oxides on metal surfaces is barely known or understood. Usingdensityfunctional theory we studied the oxidation of Ruð 0001 Þ: from the initial oxygen adsorption, subsequent O incorporation into the metal, aggregation of sub-surface islands, to the transition to the oxide film. Along the atomistic pathway several metastable precursor configurations are identified. It is argued that their properties and the metastabilities in the surface-oxide formation process will have important consequences for the discernment and molecular modelingof catalysis. 2002 Elsevier Science B.V. All rights reserved.

Metastable precursors during the oxidation of the Ru(0001) surface

The interaction of metals with our oxygen-rich atmosphere leads to the oxidation of the metal surfaces. Although this is common knowledge, little is known about the microscopic processes that actuate this oxide formation. Roughly speaking, the reaction sequence may be divided into the initial dissociation of O2 and O chemisorption, followed by oxide nucleation, and finally the growth of the formed oxide film. In this scheme, particularly the transition from a twodimensional on-surface O adlayer to a three-dimensional surface-oxide nucleus has hitherto barely been addressed. Based on a host of density-functional-theory ~DFT! calculations, we present an atomistic pathway for the oxide formation on the Ru~0001! surface. 1 The situation for this surface is in fact unique, as both the initial O chemisorption on the metal and the finally resulting RuO 2(110) oxide patches were already characterized experimentally on an atomic level. 2,3 Bridging this detailed knowledge of the initial and final state of the oxidation, we predict that after the completion of a full monolayer of chemisorbed O on Ru~0001!, the incorporation of O into the lattice leads to the formation of two-dimensional subsurface O islands between the first and second substrate layer. This implies that domains are formed that have a local (1 31) periodicity, and that can be described as a trilayered Oad-Ru-Osub film on top of Ru~0001!. Further O incorporation also occurs between the first and second substrate layer, saturating the underlying metal and almost completely decoupling the O-Ru-O trilayer. The ongoing oxidation results in the successive formation of more of these O-Ru-O trilayers, which at first remain in a loosely coupled stacking sequence. Once a critical film thickness is exceeded, this trilayer stack unfolds into the experimentally reported RuO2(110) rutile structure. 3