M. Verónica Ganduglia-Pirovano

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.

In Situ and Theoretical Studies for the Dissociation of Water on an Active Ni/CeO2 Catalyst: Importance of Strong Metal–Support Interactions for the Cleavage of O–H Bonds†

Water dissociation is crucial in many catalytic reactions on oxide-supported transition-metal catalysts. Supported by experimental and density-functional theory results, the effect of the support on OH bond cleavage activity is elucidated for nickel/ceria systems. Ambient-pressure O 1s photoemission spectra at low Ni loadings on CeO2 (111) reveal a substantially larger amount of OH groups as compared to the bare support. Computed activation energy barriers for water dissociation show an enhanced reactivity of Ni adatoms on CeO2 (111) compared with pyramidal Ni4 particles with one Ni atom not in contact with the support, and extended Ni(111) surfaces. At the origin of this support effect is the ability of ceria to stabilize oxidized Ni(2+) species by accommodating electrons in localized f-states. The fast dissociation of water on Ni/CeO2 has a dramatic effect on the activity and stability of this system as a catalyst for the water-gas shift and ethanol steam reforming reactions.

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

Hydrogen activation, diffusion, and clustering on CeO2(111): A DFT+U study

We present a comprehensive density functional theory+U study of the mechanisms underlying the dissociation of molecular hydrogen, and diffusion and clustering of the resulting atomic species on the CeO2(111) surface. Contrary to a widely held view based solely on a previous theoretical prediction, our results show conclusively that H2 dissociation is an activated process with a large energy barrier ~1.0 eV that is not significantly affected by coverage or the presence of surface oxygen vacancies. The reaction proceeds through a local energy minimum--where the molecule is located close to one of the surface oxygen atoms and the H-H bond has been substantially weaken by the interaction with the substrate--, and a transition state where one H atom is attached to a surface O atom and the other H atom sits on-top of a Ce(4+) ion. In addition, we have explored how several factors, including H coverage, the location of Ce(3+) ions as well as the U value, may affect the chemisorption energy and the relative stability of isolated OH groups versus pair and trimer structures. The trimer stability at low H coverages and the larger upward relaxation of the surface O atoms within the OH groups are consistent with the assignment of the frequent experimental observation by non-contact atomic force and scanning tunneling microscopies of bright protrusions on three neighboring surface O atoms to a triple OH group. The diffusion path of isolated H atoms on the surface goes through the adsorption on-top of an oxygen in the third atomic layer with a large energy barrier of ~1.8 eV. Overall, the large energy barriers for both, molecular dissociation and atomic diffusion, are consistent with the high activity and selectivity found recently in the partial hydrogenation of acetylene catalyzed by ceria at high H2/C2H2 ratios.

In Situ Investigation of Methane Dry Reforming on Metal/Ceria(111) Surfaces: Metal-Support Interactions and C−H Bond Activation at Low Temperature

Studies with a series of metal/ceria(111) (metal = Co, Ni, Cu; ceria = CeO2) surfaces indicate that metal–oxide interactions can play a very important role for the activation of methane and its reforming with CO2 at relatively low temperatures (600–700 K). Among the systems examined, Co/CeO2(111) exhibits the best performance and Cu/CeO2(111) has negligible activity. Experiments using ambient pressure X-ray photoelectron spectroscopy indicate that methane dissociates on Co/CeO2(111) at temperatures as low as 300 K—generating CHx and COx species on the catalyst surface. The results of density functional calculations show a reduction in the methane activation barrier from 1.07 eV on Co(0001) to 0.87 eV on Co/CeO2(111), and to only 0.05 eV on Co/CeO2@x(111). At 700 K, under methane dry reforming conditions, CO2 dissociates on the oxide surface and a catalytic cycle is established without coke deposition. A significant part of the CHx formed on the Co /CeO2@x(111) catalyst recombines to yield ethane or ethylene. Natural gas can transform the energy landscape of the world since it is a cheap and abundant fuel stock and a good source of carbon for the chemical industry. Methane (CH4) is the primary component of natural gas but is difficult to convert to upgraded fuels or chemicals because of the strength of the C@H bonds in the molecule (104 kcalmol@1) and its non-polar nature. Enabling low-temperature activation of CH4 is a major technological objective. It is known that enzymes, such as the CH4 monooxygenase, and some copperand zinc-based inorganic compounds can activate C@H bonds near room temperature. In recent studies, we found that a Ni/CeO2(111) system activates CH4 at room temperature as a consequence of metal–support interactions. The dry reforming of CH4 with CO2 (DRM; [Eq. (1)]): CH4 þ CO2 ! 2 COþ 2 H2 ð1Þ then takes place at a moderate temperature of about 700 K. Over this surface, Ni and O sites of ceria (CeO2) work in a cooperative manner during the dissociation of the first C@H bond in CH4. We pondered whether this useful phenomenon might be seen with other admetal/CeO2 combinations. Herein, we compare the behavior of Co, Ni, and Cu on CeO2(111) using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), kinetic testing, and theoretical calculations based on density functional theory. The deposition of small amounts of Co (< 0.3 ML) on a CeO2(111) film at 300 K produced a partial reduction of the oxide surface and adsorbed Co/CoOx species (Supporting Information, Figure S1). Upon annealing from 300 to 700 K, most of the Co transformed into Co (Supporting Information, Figure S2). This particular type of metal/oxide surface was exposed to CH4 at 300, 500, and 700 K. Figure 1 shows C 1s XPS spectra collected before and after exposing a Co/CeO2(111) surface to 1 Torr of CH4 at 300 K for 5 minutes. The strong peak near 285 eV is attributed to CHx groups formed by the partial dissociation of CH4 on the metal/oxide interface. 6] This peak was not seen when a pure CeO2(111) substrate was exposed to CH4 at 300 K. In Figure 1 there is a second strong peak near 289.5 eV. This likely corresponds to a COx species. [5, 6] Some of the CH4 molecules fully dissociated, producing C atoms that eventually reacted with oxygen atoms of the CeO2 to yield COx species. The intensity of the C 1s peak for the CHx species increased with Co coverage up to 0.15–0.2 ML, and then decreased at higher admetal coverages. Thus, small clusters of Co on CeO2 are the best for C@H bond activation. The dissociative adsorption of CH4 on the Co/CeO2(111) surface at room temperature did not induce a change in the oxidation state of Co or Ce. Such changes [*] Dr. Z. Liu, Dr. R. M. Palomino, Dr. D. C. Grinter, Dr. S. D. Senanayake,

In situ Investigation of Methane Dry Reforming on M-CeO2(111) {M= Co, Ni, Cu} Surfaces: Metal-Support Interactions and the activation of C-H bonds at Low Temperature

Studies with a series of metal/ceria(111) (metal=Co, Ni, Cu; ceria=CeO2 ) surfaces indicate that metal-oxide interactions can play a very important role for the activation of methane and its reforming with CO2 at relatively low temperatures (600-700 K). Among the systems examined, Co/CeO2 (111) exhibits the best performance and Cu/CeO2 (111) has negligible activity. Experiments using ambient pressure X-ray photoelectron spectroscopy indicate that methane dissociates on Co/CeO2 (111) at temperatures as low as 300 K-generating CHx and COx species on the catalyst surface. The results of density functional calculations show a reduction in the methane activation barrier from 1.07 eV on Co(0001) to 0.87 eV on Co2+ /CeO2 (111), and to only 0.05 eV on Co0 /CeO2-x (111). At 700 K, under methane dry reforming conditions, CO2 dissociates on the oxide surface and a catalytic cycle is established without coke deposition. A significant part of the CHx formed on the Co0 /CeO2-x (111) catalyst recombines to yield ethane or ethylene.

Nonuniform temperature dependence of the reactivity of disordered VOx/κ+Al2O3(001) surfaces : A density functional theory based Monte Carlo study

Periodic density functional theory (DFT) calculations concerning VO(x)/kappa-Al(2)O(3)(001) surfaces are used for parametrizing Monte Carlo simulations performed on a mesoscopic scale surface sample (41.95x48.80 nm(2)). New structural and chemical information are then obtained that are not accessible from DFT calculations: segregation, short- and long-range ordering, and the effect of the temperature on the reducibility. The reducibility of the surface is investigated for locating chemical potential regions where the catalyst is especially reactive for oxidation reactions. Comparison with the V(2)O(5)(001) surface at catalytic conditions (800 K, 1 bar) is performed. The reducibility exhibits an unexpectedly strong temperature dependence.

Direct Conversion of Methane to Methanol on Ni-Ceria Surfaces: Metal–Support Interactions and Water-Enabled Catalytic Conversion by Site Blocking

The transformation of methane into methanol or higher alcohols at moderate temperature and pressure conditions is of great environmental interest and remains a challenge despite many efforts. Extended surfaces of metallic nickel are inactive for a direct CH4 → CH3OH conversion. This experimental and computational study provides clear evidence that low Ni loadings on a CeO2(111) support can perform a direct catalytic cycle for the generation of methanol at low temperature using oxygen and water as reactants, with a higher selectivity than ever reported for ceria-based catalysts. On the basis of ambient pressure X-ray photoemission spectroscopy and density functional theory calculations, we demonstrate that water plays a crucial role in blocking catalyst sites where methyl species could fully decompose, an essential factor for diminishing the production of CO and CO2, and in generating sites on which methoxy species and ultimately methanol can form. In addition to water-site blocking, one needs the effects of metal-support interactions to bind and activate methane and water. These findings should be considered when designing metal/oxide catalysts for converting methane to value-added chemicals and fuels.