Reactivity Of Oxide Surfaces Research Articles

Tuning the surface reactivity of oxides by peroxide species

The Mars-van Krevelen mechanism is the foundation for oxide-catalyzed oxidation reactions and relies on spatiotemporally separated redox steps. Herein, we demonstrate the tunability of this separation with peroxide species formed by excessively adsorbed oxygen, thereby modifying the catalytic activity and selectivity of the oxide. Using CuO as an example, we show that a surface layer of peroxide species acts as a promotor to significantly enhance CuO reducibility in favor of H2 oxidation but conversely as an inhibitor to suppress CuO reduction against CO oxidation. Together with atomistic modeling, we identify that this opposite effect of the peroxide on the two oxidation reactions stems from its modification on coordinately unsaturated sites of the oxide surface. By differentiating the chemical functionality between lattice oxygen and peroxide, these results are closely relevant to a wide range of catalytic oxidation reactions using excessively adsorbed oxygen to activate lattice oxygen and tune the activity and selectivity of redox sites.

Visualization of Water-Induced Surface Segregation of Polarons on Rutile TiO2(110).

Water-oxide surfaces are ubiquitous in nature and of widespread importance to phenomena like corrosion as well as contemporary industrial challenges such as energy production through water splitting. So far, a reasonably robust understanding of the structure of such interfaces under certain conditions has been obtained. Considerably less is known about how overlayer water modifies the inherent reactivity of oxide surfaces. Here we address this issue experimentally for rutile TiO2(110) using scanning tunneling microscopy and photoemission, with complementary density functional theory calculations. Through detailed studies of adsorbed water nanoclusters and continuous water overlayers, we determine that excess electrons in TiO2 are attracted to the top surface layer by water molecules. Measurements on methanol show similar behavior. Our results suggest that adsorbate-induced surface segregation of polarons could be a general phenomenon for technologically relevant oxide materials, with consequences for surface chemistry and the associated catalytic activity.

Structure-Dependent Dissociation of Water on Cobalt Oxide.

Understanding the correlation between structure and reactivity of oxide surfaces is vital for the rational design of catalytic materials. In this work, we demonstrate the exceptional degree of structure sensitivity of the water dissociation reaction for one of the most important materials in catalysis and electrocatalysis. We studied H2O on two atomically defined cobalt oxide surfaces, CoO(100) and Co3O4(111). Both surfaces are terminated by O2- and Co2+ in different coordination. By infrared reflection absorption spectroscopy and synchrotron radiation photoelectron spectroscopy we show that H2O adsorbs molecularly on CoO(100), while it dissociates and forms very strongly bound OH and partially dissociated (H2O) n(OH) m clusters on Co3O4(111). We rationalize this structure dependence by the coordination number of surface Co2+. Our results show that specific well-ordered cobalt oxide surfaces interact very strongly with H2O whereas others do not. We propose that this structure dependence plays a key role in catalysis with cobalt oxide nanomaterials.

Controlling oxide surface dipole and reactivity with intrinsic nonstoichiometric epitaxial reconstructions

The composition and reconstruction of oxide surfaces can be deterministically controlled via ambient conditions. We demonstrate that such intrinsic alterations can have a crucial effect on the surface dipole and reactivity, even for surfaces with the same crystallographic plane. The surface dipole potential drops of ${\mathrm{BaTiO}}_{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{SrTiO}}_{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{LaFeO}}_{3}$, and ${\mathrm{TiO}}_{2}$ surfaces with various reconstructions and compositions are shown to vary by as much as 5 V, leading to significant variation of the band edge positions at these surfaces. These variations are shown to correlate with the calculated oxygen binding energy, demonstrating how oxide surface reactivity can be substantially manipulated using environmental changes.

ChemInform Abstract: Chemical Modification of Nanocrystalline Tin Dioxide for Selective Gas Sensors

AbstractReview: 133 refs.

A chemical approach to understanding oxide surfaces

Chemical bonding has often been ignored in favor of physics based energetic considerations in attempts to understand the structure, stability, and reactivity of oxide surfaces. Herein, we analyze the chemical bonding in published structures of the SrTiO3, MgO, and NiO surfaces using bond valence sum (BVS) analysis. These simple chemical bonding theories compare favorably with far more complex quantum mechanical calculations in assessing surface structure stability. Further, the coordination and bonding of surface structures explains the observed stability in a readily comprehensible manner. Finally, we demonstrate how simple chemical bonding models accurately predict the adsorption of foreign species onto surfaces, and how such models can be used to predict changes in surface structures.

The Role of Electrochemical Phenomena in Scanning Probe Microscopy of Ferroelectric Thin Films

Applications of piezoresponse force microscopy and conductive atomic force microscopy to ferroelectric thin films necessitate understanding of the possible bias-induced electrochemical reactivity of oxide surfaces. These range from reversible ionic surface charging (possibly coupled to polarization) and vacancy and proton injection to partially reversible vacancy ordering, to irreversible electrochemical degradation of the film and bottom electrode. Here, the electrochemical phenomena induced by a biased tip are analyzed and both theoretical and experimental criteria for their identification are summarized.

Tuning the Reactivity of Oxide Surfaces by Charge‐Accepting Adsorbates

Tuning the Reactivity of Oxide Surfaces by Charge‐Accepting Adsorbates

Second-harmonic generation and theoretical studies of protonation at the water/α-TiO 2 (1 1 0) interface

The pH of zero net surface charge (pH pzc) of the α-TiO 2 (1 1 0) surface was characterized using second-harmonic generation (SHG) spectroscopy. The SHG response was monitored during a series of pH titrations conducted at three NaNO 3 concentrations. The measured pH pzc is compared with a pH pzc value calculated using the revised MUltiSIte Complexation (MUSIC) model of surface oxygen protonation. MUSIC model input parameters were independently derived from ab initio calculations of relaxed surface bond lengths for a hydrated surface. Model (pH pzc 4.76) and experiment (pH pzc 4.8 ± 0.3) agreement establishes the incorporation of independently derived structural parameters into predictive models of oxide surface reactivity.

Photoelectron spectroscopic and electronic structure studies of CH(2)O bonding and reactivity on ZnO surfaces: steps in the methanol synthesis reaction.

Adsorption of CH(2)O on ZnO(0001) has been investigated using XPS, NEXAFS, variable-energy photoelectron spectroscopy (PES), and density functional theory (DFT) calculations. CH(2)O is chemisorbed on the (0001) surface at 130 K. Its C1s XPS peak position at 292.7 eV and NEXAFS sigma shape resonance at 302.6 eV are consistent with an eta(1) bound surface geometry. Geometry optimized DFT calculations also indicate that CH(2)O is bound to the Zn(II) site in an eta(1) configuration through its oxygen atom. The variable-energy PES of the eta(1) bound CH(2)O/ZnO(0001) complex exhibits four valence band features at 21.2, 16.4, 13.8, and 10.7 eV below the vacuum level providing an experimental and theoretical description of this surface interaction. Annealing the ZnO(0001)/CH(2)O surface complex to 220 K decomposes the chemisorbed CH(2)O, producing formyl (291.5 eV), methoxide (290.2 eV), and formate (293.6 eV) intermediates. Thus this reaction coordinate involves the conversion of an oxygen bound formaldehyde to a carbon bound formyl species on ZnO(0001). Only formate is formed on the ZnO(100) surface. DFT is used to explore surface intermediates and the transition state in the methanol synthesis reaction (MSR). The bonding interactions of H(2), CO, CH(3)O(-), HCO(-), and trans-HCOH to the ZnO(0001) surface are elucidated using geometry optimization. H(2) was found to be heterolytically cleaved on the ZnO(0001) surface, and carbon monoxide, formyl, and methoxide are calculated to be eta(1) bound. These results are consistent with observed metal oxide surface reactivity where heterolytic bond cleavage is dominant. The oxygen atom in the bound formyl was found to be activated for attack by a proton. This results in the planar eta(1) bound trans-HCOH surface species. The transition state in the gas phase rearrangement of trans-HCOH to formaldehyde was calculated to have a barrier of 31 kcal/mol. The correlation diagram for this rearrangement in the gas phase indicates that configuration interaction at the crossing of two levels helps to lower the barrier. A transition state calculation was also performed for this rearrangement on the ZnO(0001) surface. The surface transition state geometry is significantly different than the gas phase. The surface geometry is no longer planar (23 degrees dihedral angle) and is displaced parallel to the surface. Interaction with the Zn(II) site at the crossing of surface bound CH(2)O and trans-HCOH levels further lowers the barrier to rearrangement relative to gas phase by 9 kcal/mol. The rearrangement of trans-HCOH (carbon bound) to CH(2)O (oxygen bound) on ZnO(0001) was calculated to be the overall barrier of the MSR reaction.

Orbital-band interactions and the reactivity of molecules on oxide surfaces: from explanations to predictions

The surface chemistry of oxides is relevant for many technological applications: catalysis, photoelectrolysis, electronic-device fabrication, prevention of corrosion, sensor development, etc. This article reviews recent theoretical works that deal with the surface chemistry of oxides. The account begins with a discussion of results for the adsorption of CO and NO on oxides, systems which have been extensively studied in the literature and constitute an ideal benchmark for testing the quality of different levels of theory. Then, systematic studies concerned with the behavior of adsorbied alkali metals and sulfur-containing molecules are presented. Finally, a correlation between the electronic and chemical properties of mixed-metal oxides is analyzed and basic principles for designing chemically active oxides are introduced. Advances in theoretical methods and computer capabilities have made possible a fundamental understanding of many phenomena associated with the chemistry of molecules on oxide surfaces. Still many problems in this area remain as a challenge, and the approximate nature of most theoretical methods makes necessary a close coupling between theory and experiment. Following this multidisciplinary approach, the importance of band-orbital interactions for the reactivity of oxide surfaces has become clear. Simple models based on band-orbital mixing can explain trends found for the interaction of many adsorbates with oxide surfaces. These simple models provide a conceptual framework for modifying or controlling the chemical activity of pure oxides and for engineering mixed-metal oxides. In this respect, theoretical calculations can be very useful for predicting the best ways for enhancing the reactivity of oxide systems and reducing the waste of time, energy and materials characteristic of an empirical design.

NH 3 probing of the surface acidity of passive films on chromium

The acid–base properties of passive films formed on chromium have been investigated with a probe molecule. The adsorption of NH 3, a Lewis base, has been studied by X-ray photoelectron spectroscopy (XPS) on passive films formed on Cr(1 1 0) in 0.5 M H 2SO 4. A single N 1s signal resulted from the exposure of the passive film to gaseous ammonia (25°C, pNH 3=20 mbar ). The comparison with NH 3 probing of dry oxide films reveals that the oxide surface reactivity is markedly reduced by the layer of OH. The Lewis acid sites of the oxide surface are not directly available on the passive film. The data indicate that NH 3 reacts with Brönsted acid sites consisting of hydrogen termination of the hydroxyl or hydroxide groups on the passive film surface. The implications of these findings for the interactions of chloride ions with passive films (passivity breakdown and initiation of pitting) are discussed.

ChemInform Abstract: Active Sites on Oxides: From Single Crystals to Catalysts

Abstract Metal oxides are ubiquitous in heterogeneous catalysis, serving as catalysts, as catalyst supports, and as modifiers and promoters, among other roles. The surface science approach to understanding the structure and reactivity of metal oxide surfaces has blossomed during the preceding decade. We consider here important concepts drawn from catalysis by metal oxides and their connections to reaction pathways and principles of oxide surface reactivity revealed by experimental and theoretical studies of well-defined oxide surfaces. The applications of reaction mechanisms and site requirements from surface science studies of carboxylic acid chemistry, in particular, to emerging catalysts for dehydration, coupling, and reduction reactions provide important examples of contributions to heterogeneous catalysis from oxide surface science. We also consider relationships between oxide surface reactivity and the physical and electronic properties of oxides as represented by characteristics such as electronegativity, bond energies, ionicity, Madelung potential, and polarizability. This analysis highlights the need for reaction data on well-defined single-crystal surfaces in order to distinguish structural and electronic effects when comparing patterns of catalytic activity and selectivity among different metal oxides. This review of the connetions between metal oxide single-crystal surfaces and high surface area catalysts (or their components) demonstrates the potential of surface science approaches to elucidate the chemical and physical bases for catalysis by metal oxides, in pursuit of the goal of catalyst design.

Reactivity of sulphur dioxide with Cu(100) surface investigated by infrared reflection absorption spectroscopy: role of structure and oxidation level

Interaction of sulphur dioxide (SO2) with the Cu(100) surface has been investigated by infrared reflection absorption spectroscopy (IRAS). Results have been compared with data obtained previously under similar conditions on the Cu(110) surface in order to make clear the role of the surface structure on the adsorption process. Auger electron spectroscopy and XPS were used to control the surface chemical state before and after SO2 interaction. On Cu(100), the dissociation of SO2 is observed for low exposure, and is then followed by the formation of SOx molecular species (x≥2). On oxygen-induced reconstructed Cu(100), sulphites and sulphates are formed by interaction with SO2. Sulphites and sulphates are oxygen-bound to the surface, resulting from nucleophilic attack of the sulphur atoms of impinging SO2 molecules by the surface oxygen atoms. On the oxidized Cu(100) surface, SO2 is only converted to sulphites. The less ‘oxidizing’ character of a superficial oxide layer compared to an oxygen adsorbed layer is correlated to the coordination and interatomic distances of the surface oxygen atoms. The nature of the adsorbed species resulting from the interaction of SO2 with copper strongly depends on the structure and on the chemical composition of the surface. These data confirm that this molecule is a good probe of the metallic and oxide surface reactivity. Copyright © 2000 John Wiley & Sons, Ltd.

Reactivity of oxide surfaces modified by palladium atoms

We report CO adsorption studies on modified surfaces of Al 2O 3 and MgO. The oxides were bulk crystals (α-Al 2O 3 and MgO) or thin films (γ-Al 2O 3); they were sown by palladium atoms. By using static secondary ion mass spectroscopy (SSIMS), the surfaces were in situ characterized with respect to metal coverage. We found that upon deposition at room temperature, 3D palladium clusters or chains are formed on alumina depending on the oxide stoichiometry. On MgO cleavage, 2D islands grow. We show that the work function of metal deposit varies in the 8–6 eV range during the early stages of deposition. The adsorption of CO probe molecules on ultrathin Pd deposits was studied, results on their chemical reactivity are discussed with respect to the morphology of the metal clusters.

Structure and reactivity of oxide surfaces: new perspectives from scanning tunnelling microscopy

Scanning tunnelling microscopy (STM) has emerged in the past few years as a uniquely powerful tool for the investigation of oxide surfaces. STM is capable of real-space imaging of periodic structures with atomic resolution and of characterising local atomic arrangements associated with defect sites. The power of the technique is further enhanced by the ability to probe filled and empty density of states profiles at specific atomic sites by monitoring variations in tunnelling currents with applied voltage. The interpretation of STM images is not always unambiguous, a question of central importance being whether oxygen or metal ions appear as maxima in the images. The necessity for adequate sample conductivity imposes some constraints on the applicability of the technique, although developments in instrumentation and in techniques for sample preparation are helping to overcome these limitations. The range and limitations of the technique are illustrated by reference to work on tungsten oxides, titanium dioxide and iron oxides. The review concludes with a discussion of recent developments in the study of molecular adsorbates.

Dependence of the Lattice Parameter of Magnesium Oxide on Crystallite Size

Lattice parameters were measured on MgO specimens prepared in air between 450° and 1200°C. The lattice parameter, a, decreases with increase in preparation temperature, Tp, and with increase in crystallite size. A hydroxide layer is present on the MgO particles. If MgO is prepared in vacuum, a increases as Tp increases and as crystallite size, D, increases. It is concluded that a dilatant volume stress is imposed by the hydroxide layer. It is also shown that the oxide surface reactivity toward water is linked to the deviation from perfect order ensuing from low preparation temperatures.