Exoemission accompanying electron interactions in complex copper-cerium oxide catalysts and synergism phenomena

The formation of the electronic structure of the surface of complex copper-cerium oxide catalysts with different copper concentrations was studied using the exoemission methods at different stages of preparation. The introduction of copper enhances the emissivity of CeO₂, and the number of charges emitted from the catalyst surface exceeds the emission activity of the starting CeO₂ and NuO components. The synergism phenomenon in exoemission is compared with synergism in the catalysis of CO oxidation by these systems. The problem of electron interactions between the components of the complex oxides is discussed. The electron interactions are caused by the electron transitions at the interface and result in an increase in both the emission of weakly bound electrons and catalytic activity in the oxidation of CO.     

Electronic Metal-Support Interaction-Modified Structures and Catalytic Activity of CeOx Overlayers in CeOx/Ag Inverse Catalysts

Electronic metal-support interactions (EMSIs) of oxide-supported metal catalysts strongly modifies the electronic structures of the supported metal nanoparticles. The strong influence of EMSIs on the electronic structures of oxide overlayers on metal nanoparticles employing cerium oxides/Ag inverse catalysts is reported herein. Ce₂O₃ overlayers were observed to exclusively form on Ag nanocrystals at low cerium loadings and be resistant to oxidation treatments up to 250 °C, whereas CeO₂ overlayers gradually developed as the cerium loading increased. Ag cubes enclosed by {001} facets with a smaller work function exert a stronger EMSI effect on the CeO_x overlayers than Ag cubes enclosed by {111} facets. Only the CeO₂ overlayers with a fully developed bulk CeO₂ electronic structure significantly promote the catalytic activity of Ag nanocrystals in CO oxidation, whereas cerium oxide overlayers with other electronic structures do not. These results successfully extend the concept of EMSIs from oxide-supported metal catalysts to metal-supported oxide catalysts.     

Cu/CdCO3 catalysts for efficient electrochemical CO2 reduction over the wide potential window

High efficiency and low-cost catalyst-driven electrocatalytic CO₂ reduction to CO production are of great significance for energy storage and development. The severe competitive hydrogen evolution reaction occurs at large negative potential window limits the achievement of the target product from CO₂ at high efficiency. Here, we successfully prepared Cu_x/CdCO₃ composite catalyst rich in interfaces, in which achieved high CO Faraday efficiency exceeded 90% in a wide potential window of 700 mV and highest value up to 97.9% at −0.90 V vs. RHE. The excellent performance can be ascribed to the positive contribution of Cu_x/CdCO₃, which maintains a suitable high local pH value during electrochemical reduction, thus inhibiting the competitive hydrogen evolution reaction. Moreover, the compact structure between Cu and CdCO₃ ensures fast electron transfer both inside catalysts and interface, thus speeding up the reaction kinetics of CO₂ to CO conversion. Theoretically calculations further prove that the combination of Cu and CdCO₃ provides the well-defined electronic structure for intermediates adsorption, significantly reducing the reaction barrier for the formation of CO. This work provides new insights into the design of efficient electrochemical CO₂ reduction catalysts for inhibiting hydrogen evolution by adjusting the local pH effect.     

Platinum-Tin/Tin Oxide/CNT Catalysts for High-Performance Electrocatalytic Ethanol Oxidation

Ethanol is a promising liquid clean energy source in the energy conversion field. However, the self-poisoning caused by the strongly adsorbed reaction intermediates (typically, CO) is a critical problem in ethanol oxidation reaction. To address this issue, we proposed a joint use of two strategies, alloying of Pt with other metals and building Pt/metal-oxide interfaces, to achieve high-performance electrocatalytic ethanol oxidation. For this, a well-designed synthetic route combining wet impregnation with a two-step thermal treatment process was established to construct PtSn/SnO_x interfaces on carbon nanotubes. Using this route, the alloying of Pt−Sn and formation of PtSn−SnO_x interfaces can simultaneously be achieved, and the coverage of SnO_x thin films on PtSn alloy nanoparticles can be facilely tuned by the strong interaction between Pt and SnO_x. The results revealed that the partial coverage of SnO_x species not only retained the active sites, but also enhanced the CO anti-poisoning ability of the catalyst. Consequently, the H−PtSn/SnO_x/CNT-2 catalyst with an optimized PtSn−SnO_x interface showed significantly improved performances toward the ethanol oxidation reaction (825 mA mg_Pt⁻¹). This study provides deep insights into the structure-performance relationship of PtSn/metal oxide composite catalysts, which would be helpful for the future design and fabrication of high-performance Pt-based ethanol oxidation reaction catalysts.     

Structural and spectroscopic characterization of Mo1−xWxO3−δ mixed oxides

Mo_1−xW_xO₃ oxides with different cationic fraction (x=0.2, 0.5 and 0.8) and, for comparison purposes, pure MoO₃ and WO₃ were prepared. Along with textural and structural characterizations, absorbance FT-IR, diffuse reflectance UV-vis-NIR and EPR spectroscopies were employed to study the changes in the electronic properties of these materials passing from Mo_1−xW_xO₃ in oxidizing atmosphere to Mo_1−xW_xO_3−δ in reducing conditions. XRD analysis showed that the Mo-W mixed oxides are constituted by two or three crystalline phases, whose abundance and composition are well characterized by structural refinement with the Rietveld method. Only the sample with the highest Mo content (x=0.2) shows a predominant mixed phase and also a superior ability to lose oxygen with respect to the other mixed oxides. The oxygen loss in the reduced oxides induces the formation of defects with electronic levels in the band gap of the material, in particular, electrons trapped in oxygen vacancies and/or at cationic sites (polarons). While the nature of defect sites induced in the mixed and in the pure oxides is similar, the photo-ionization energies, the ratio between surface and bulk defects and the stability of the defects in oxygen at increasing temperature are peculiar of each mixed oxide.     

Catalytic properties of Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> prepared using a template in the oxidation of CO

The catalytic activity in CO oxidation of Ce _x Zr_1–x O₂ double oxides prepared using pine sawdust and cetyltrimethylammonium bromide (CTAB) as templates is compared. It is found by means of SEM and the low-temperature adsorption of N₂ that biomorphic oxides reproduce the macropore structure of the template. It is shown via XRD and Raman spectroscopy that all samples contained mixed ceria-zirconia oxide. The double oxides form a cubic phase with a lattice of the fluorite type at a ratio of Ce: Zr = 4, regardless of the nature of the template; when Ce: Zr = 1, the biomorphic mixed oxide forms a tetragonal phase. According to Raman spectroscopy and XRD it was shown that the distortion of the oxygen sublattice is higher in biomorphic samples. Energy dispersive analysis shows that Ca impurities were present in the biomorphic samples, introducing additional distortions in the lattice of double oxide and leading to the formation of anionic vacancies. It is found that when Ce: Zr = 4, the conversion of CO on biomorphic oxide in the range of 100–350°C is higher than that observed for Ce _x Zr_1–x O₂ (CTAB); reducing the Ce: Zr ratio in the biomorphic sample to 1 results in a marked decrease in CO conversion at 100–200°C. It is concluded that these differences are due to changes in the mobility of the lattice oxygen.     

MXene-Regulated Metal-Oxide Interfaces with Modified Intermediate Configurations Realizing Nearly 100% CO2 Electrocatalytic Conversion

Electrocatalytic CO₂ reduction via renewable electricity provides a sustainable way to produce valued chemicals, while it suffers from low activity and selectivity. Herein, we constructed a novel catalyst with unique Ti₃C₂T_x MXene-regulated Ag−ZnO interfaces, undercoordinated surface sites, as well as mesoporous nanostructures. The designed Ag−ZnO/Ti₃C₂T_x catalyst achieves an outstanding CO₂ conversion performance of a nearly 100% CO Faraday efficiency with high partial current density of 22.59 mA cm⁻² at −0.87 V versus reversible hydrogen electrode. The electronic donation of Ag and up-shifted d-band center relative to Fermi level within MXene-regulated Ag−ZnO interfaces contributes the high selectivity of CO. The CO₂ conversion is highly correlated with the dominated linear-bonded CO intermediate confirmed by in situ infrared spectroscopy. This work enlightens the rational design of unique metal-oxide interfaces with the regulation of MXene for high-performance electrocatalysis beyond CO₂ reduction.     

Photoelectron spectroscopy and diffraction of surface nanoscale NbO/Nb(110) structures

X-ray photoelectron spectroscopy and diffraction (XPS and XPD) are applied to analyze oxygen-induced surface structures on the Nb(110) face formed due to oxygen segregation from the crystal bulk on thermal annealing to 2000 K in vacuum and/or oxygen adsorption in situ at temperatures above 1100 K. The Nb3d, O1s electronic states and valence band spectra of the NbO _x /Nb(110) surface are studied by XPS, and the results are compared with data for NbO, NbO₂, and Nb₂O₅ oxides. It is shown that niobium atoms entering the composition of surface oxide structures on Nb(110), from the standpoint of the nearest environment and chemical bond, are similar to metal states in NbO. The NbO _x layer thickness is estimated to be 0.5 nm. Two chemically inequivalent oxygen states are distinguished on Nb(110), which are, presumably, atomic chemisorbed oxygen on the parts of the clean surface of the Nb monolayer with hexagonal packing and oxygen in the composition of NbO _x -like linear clusters on Nb(110). A model of the NbO _x /Nb(110) surface takes into account a distortion of the structure of NbO _x clusters: a periodic vertical shift of metal atoms in Nb-chains and changes in Nb-O bond angles. Original Russian Text Copyright ? 2009 by M. V. Kuznetsov, A. S. Razinkin, and E. V. Shalaeva __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 50, No. 3, pp. 536–543, May–June, 2009.     

Character of the electronic ground state and of charge-transfer excited states in ionic solids: An ab initio cluster model approach

The electronic structure of the ground electronic state and of some special charge-transfer excited states in ionic solids is examined from the ab initio cluster model approach. Different ab initio wave functions, including a frozen orbital approach, the Hartree–Fock self-consistent field, and multireference configuration interaction wave functions, are considered and analyzed using different theoretical techniques. We explicitly consider some alkaline–earth oxides such as CaO, a more difficult case such as A1₂O₃, a transition-metal oxide such as NiO, and a system with a more complicated structure such as KNiF₃. Analysis of ab initio wave functions in terms of valence bond components shows that all these compounds are largely ionic, thus supporting the simple picture arising from the ionic model. However, the nature of the excited states is more complex. Alkaline–earth oxides lowest excited states are essentially described as charge-transfer excitations dominated by a single resonant valence bond structure and the calculated energy difference is comparable to the experimental optical gap. In the case of A1₂O₃, the electronic spectra presents excitonic features and the local charge-transfer excitation excited states provide a reasonable representation of these phenomena. Finally, several different valence bond structures are present in the lowest electronic states of KNiF₃. © 1994 John Wiley & Sons, Inc.     

Interfacial chemistry in internally oxidized (Cu,Mg)-alloys

(Cu,Mg) alloys are internally oxidized at different oxygen chemical potential at 900°C. Oxidation scale microstructure is studied by SEM and TEM. MgO forms as large magnesia agglomerates without any special orientation relationship and isolated cubo-octahedral topotaxial MgO precipitates, the shape of which varies with decreasing oxygen activity from octahedral to cubic. The interfaces of the cubo-octahedral precipitates are studied in detail by CTEM, HREM and EELS. At the highest oxygen activity, important rigid-body contraction/expansion across the interface is found together with a strong modification in the interfacial electronic structure (compared to the adjacent bulk phases) indicating important hybridization of O 2p and Cu 3d states. Both suggest oxide bonding. At lower oxygen activity, interfaces show increasing structural disorder in the copper phase and microfaceting or terracing of the interfacial plane; the intensity of interfacial ELNES features associated to the O 2p and Cu 3d hybridization diminishes and finally disappears with decreasing oxygen activity. Changes with oxygen chemical potential in precipitate morphology, interface atomic and electronic structure are explained by Gibbs’ adsorption/desorption of excess oxygen to the interface. Adsorption isotherms are modeled for various configurations and compared to the experimental results.     

Smoothing and passivation of special Si(111) substrates: studied by SPV,PL, AFM and SEM measurements

Surface sensitive techniques, the field-modulated surface photovoltage, photoluminescence measurements, atomic force microscopy and scanning electron microscopy, were employed to yield detailed information on the influence of wet-chemical treatments on the preparation induced microroughness and electronic properties of wet-chemically passivated Si(111) substrates with special surface morphology. Stepped substrates with evenly distributed atomically flat terraces were prepared and passivated by thin oxide layers, which were used as a starting point for the subsequent H-termination after long storage in air. It was shown that their surface morphology and electronic properties do not degrade. Applying this preparation method to solar cell substrates with randomly distributed Si(111) pyramids, we achieved significantly lower densities of surface states and reduced recombination loss at a-Si:H/c-Si interfaces, compared with conventional pretreatments. The surface microroughness, the density of rechargeable states and the resulting recombination loss on a-Si:H/c-Si heterojunctions were found to be mainly influenced by two steps of surface pretreatment: firstly, the wet-chemical smoothing procedure of structured substrates and, secondly, the removal of native and wet-chemical oxides during the final etching in HF- or NH₄F- containing solutions. Figure After wet-chemical oxidation in H₂SO₄/H₂O₂ and storage in air     

Studies on the Surface Oxygen Species and the Catalytic Activity for CO Oxidation of La1-xSrxCo1-xMnxO3, a Perovskite-Type Oxides Catalyst

The surface oxygen distribution the active oxygen species for CO on the perovskite-type catalyst La_1-xSr_xCo_1-xMn_xO₃ and its catalytic oxidation activity with CO as probe were investigated by means of XRD, TPD and XPS in a continuous flow microreactor. Results showed that different adsorbed oxygen species and lattice oxygen were distributed on the catalyst surface. Meanwhile, the surface lattice oxygen of the oxides was reacting in the course of CO oxidation. This leads to the conclusion that, when x=0.6, the catalyst shows the best oxidative activity and lower starting temperature.     

Considerations of the intermediate oxides via XPS elemental quantitative analysis for the thickness measurements of ultrathin SiO2 on Si

The thicknesses of intermediate oxides at the interface between ultrathin SiO₂ and Si substrates have been measured via XPS elemental quantitative analysis for some SiO₂/Si(100) and SiO₂/Si(111) samples with the silicon oxide thickness less than 2 nm. The measurements involve XPS determination of the Si relative atomic ratio, calculation of Si atomic densities for the intermediate oxide, etc. and then the intermediate oxide thicknesses and the number of monolayers are obtained by referencing the thickness data from two international comparisons for these samples. The results show that the thickness of the intermediate oxides is in the range 0.14–0.16 nm with an average value of 0.15 nm. The number of monolayers for the intermediate oxides at the interface is less than one monolayer with an average value of 0.60. In the present work, there are a series of approximations. By making these approximations many parameters, including L and R₀, used in the conventional calculation method are removed to give a simpler equation, which is valid when the thicknesses of SiO₂ overlayer and the intermediate oxides are very small. This, therefore, appears to be a simple and quick method to obtain approximate oxide thicknesses of modest accuracy. The present work does not in any way replace or improve on Eqns ( 2 –6) cited in the text. Copyright © 2010 John Wiley & Sons, Ltd.     

A Tandem Catalyst with Multiple Metal Oxide Interfaces Produced by Atomic Layer Deposition

Ideal heterogeneous tandem catalysts necessitate the rational design and integration of collaborative active sites. Herein, we report on the synthesis of a new tandem catalyst with multiple metal‐oxide interfaces based on a tube‐in‐tube nanostructure using template‐assisted atomic layer deposition, in which Ni nanoparticles are supported on the outer surface of the inner Al₂O₃ nanotube (Ni/Al₂O₃ interface) and Pt nanoparticles are attached to the inner surface of the outer TiO₂ nanotube (Pt/TiO₂ interface). The tandem catalyst shows remarkably high catalytic efficiency in nitrobenzene hydrogenation over Pt/TiO₂ interface with hydrogen formed in situ by the decomposition of hydrazine hydrate over Ni/Al₂O₃ interface. This can be ascribed to the synergy effect of the two interfaces and the confined nanospace favoring the instant transfer of intermediates. The tube‐in‐tube tandem catalyst with multiple metal‐oxide interfaces represents a new concept for the design of highly efficient and multifunctional nanocatalysts.     

Gas-sensing properties of Fe2−xTixO3+γ (x = 0–1.4)

As part of a systematic study of mechanisms of response of semiconducting oxides as trace gas sensors, we have explored the behaviour of iron–titanium oxide solid solutions Fe_2−xTi_xO₃ (x = 0.1–1.4). The materials were single-phase for x = 0.1 with increasing proportions of a pseudobrookite second phase at higher degree of substitution. Unmodified, pure iron oxide does not show sensitivity to CO. A significant signal was developed for x = 0.1, that then diminished with increasing x and was lost for x = 1.4. Three effects have been deduced important for the gas response: significant surface segregation of Ti at low Ti content; grain growth inhibition and agglomeration into more massive, non-porous lumps as Ti content increased; and the appearance of a band-gap state associated with Fe(II) at higher Ti content. The effects of microstructure change have been analyzed by fitting the data to a simple 2-resistor model of gas-insensitive ‘grains’ in series with gas-sensitive ‘grain boundaries’. A Mars–van Krevelen type model for the response is presented, based on reactions at surface-segregated defect clusters, to develop and remove electrically-active surface trap states.     

Atomically flat single terminated oxide substrate surfaces

Scientific interest in atomically controlled layer-by-layer fabrication of transition metal oxide thin films and heterostructures has increased intensely in recent decades for basic physics reasons as well as for technological applications. This trend has to do, in part, with the coming post-Moore era, and functional oxide electronics could be regarded as a viable alternative for the current semiconductor electronics. Furthermore, the interface of transition metal oxides is exposing many new emergent phenomena and is increasingly becoming a playground for testing new ideas in condensed matter physics. To achieve high quality epitaxial thin films and heterostructures of transition metal oxides with atomically controlled interfaces, one critical requirement is the use of atomically flat single terminated oxide substrates since the atomic arrangements and the reaction chemistry of the topmost surface layer of substrates determine the growth and consequent properties of the overlying films. Achieving the atomically flat and chemically single terminated surface state of commercially available substrates, however, requires judicious efforts because the surface of as-received substrates is of chemically mixed nature and also often polar. In this review, we summarize the surface treatment procedures to accomplish atomically flat surfaces with single terminating layer for various metal oxide substrates. We particularly focus on the substrates with lattice constant ranging from 4.00 Å to 3.70 Å, as the lattice constant of most perovskite materials falls into this range. For materials outside the range, one can utilize the substrates to induce compressive or tensile strain on the films and explore new states not available in bulk. The substrates covered in this review, which have been chosen with commercial availability and, most importantly, experimental practicality as a criterion, are KTaO₃, REScO₃ (RE = Rare-earth elements), SrTiO₃, La_0.18Sr_0.82Al_0.59Ta_0.41O₃ (LSAT), NdGaO₃, LaAlO₃, SrLaAlO₄, and YAlO₃. Analyzing all the established procedures, we conclude that atomically flat surfaces with selective A- or B-site single termination would be obtained for most commercially available oxide substrates. We further note that this topmost surface layer selectivity would provide an additional degree of freedom in searching for unforeseen emergent phenomena and functional applications in epitaxial oxide thin films and heterostructures with atomically controlled interfaces.     

Nanostructure‐Directed Physisorption vs Chemisorption at Semiconductor Interfaces: The Inverse of the HSAB Concept

A concept, complementary to that of hard and soft acid–base interactions (HSAB‐dominant chemisorption) and consistent with dominant physisorption to a semiconductor interface, is presented. We create a matrix of sensitivities and interactions with several basic gases. The concept, based on the reversible interaction of hard‐acid surfaces with soft bases, hard‐base surfaces with soft acids, or vice versa, corresponds 1) to the inverse of the HSAB concept and 2) to the selection of a combination of semiconductor interface and analyte materials, which can be used to direct a physisorbed vs chemisorbed interaction. The technology, implemented on nanopore coated porous silicon micropores, results in the coupling of acid–base chemistry with the depletion or enhancement of majority carriers in an extrinsic semiconductor. Using the inverse‐HSAB (IHSAB) concept, significant and predictable changes in interface sensitivity for a variety of gases can be implemented. Nanostructured metal oxide particle depositions provide selectivity and complement a highly efficient electrical contact to a porous silicon nanopore covered microporous interface. The application of small quantities (much less than a monolayer) of nanostructured metals, metal oxides, and catalysts which focus the physisorbtive and chemisorbtive interactions of the interface, can be made to create a range of notably higher sensitivities for reversible physisorption. This is exemplified by an approach to reversible, sensitive, and selective interface responses. Nanostructured metal oxides developed from electroless gold (Au_xO), tin (SnO₂), copper (Cu_xO), and nickel (NiO) depositions, nanoalumina, and nanotitania are used to demonstrate the IHSAB concept and provide for the detection of gases, including NH₃, PH₃, CO, NO, and H₂S, in an array‐based format to the sub‐ppm level.     

Scanning Auger analysis for the clarification of oxidation and interdiffusion processes during laser ablation of YBa2Cu3O7-x on alloy/oxide substrates

Scanning Auger investigations are reported on layer systems consisting of a metallic substrate, an oxide buffer layer and the YBa₂Cu₃O_7?x (YBCO) high temperature superconductor. By bending the samples under UHV conditions the internal interfaces have been made accessible for the Auger analysis. The examination of the interfaces have shown that an oxidation of the substrate alloy had taken place during the deposition of the YBCO in spite of the buffer layer already being present. Additionally, segregated sulphur has been found on the substrate/buffer interface, which can lead to a loss of adhesion of the layer system. By line scan investigations on samples prepared by ball cratering a carbon enriched zone of approximately 50 nm thickness has been detected at the interface of buffer and YBCO.     

Adsorption activity and selectivity of the surface of InP-CdS system semiconductors with respect to toxic microimpurities

The mechanism of and rules governing the interaction of carbon oxides (CO and CO₂) and ammonia with the surface of binary (InP and CdS) and quaternary ((InP) _x (CdS)_{1 ? x} ) InP-CdS system components were studied by piezoelectric quartz microweighing and IR and Raman spectroscopy. The results obtained, acid-base and other physicochemical adsorbent properties, and the electronic structure of adsorbate molecules led us to conclude that the adsorption of the gases at temperatures above 297 K was predominantly chemical in nature and largely occurred according to the donor-acceptor mechanism with the participation of coordination unsaturated “metallic” atoms. The adsorbabilities of the gases on the same adsorbent under equal conditions were different and increased in the series a(CO₂) → a(NH₃) → a(CO). The acid-base, adsorption, and optical properties of adsorbents changed in parallel, which was indicative of the same origin of active adsorption centers and surface biographical states. This also allowed the adsorption activity of a surface to be predicted from its acid-base properties and the volumetric properties of the initial binary compounds and elementary components. The behaviors of (InP) _x (CdS)_{1 ? x} solid solutions and binary system components (InP and CdS) had common and special features. Surface characteristic-composition diagrams were used to find the system component most active with respect to CO, which was the (InP)_0.95(CdS)_0.05 solid solution. It was used to create a high-sensitivity and selectivity sensor.     

Synergetic effect in PdAu/CeO<Subscript>2</Subscript> catalysts for the low-temperature oxidation of CO

Gold-palladium catalysts supported on cerium oxide were synthesized with the double complex salts. X-ray photoelectron spectroscopy (XPS) and other physicochemical methods (TEM, TPR) were used to demonstrate that synthesis of highly active palladium catalysts requires the oxidative treatment stimulating the formation of a catalytically active surface solid solution Pd _x Ce_1?x O₂, which is responsible for the lowtemperature activity (LTA) in the reaction CO + O₂. In the case of gold catalysts, active sites for the lowtemperature oxidation of CO are represented by gold nanoparticles and its cationic interface species. Simultaneous deposition of two metals increases the catalyst LTA due to interaction of both gold and palladium with the support surface to form a Pd_1?x CexO₂ solid solution and cationic interface species of palladium and gold on the boundary of Pd-Au alloy particles anchored on the solid solution surface.