Molecular adsorption on V2O3(0001)/Au(111) surfaces

The adsorption of carbon monoxide (CO), propane (C₃H₈) and propene (C₃H₆) on V₂O₃(0001) films grown on Au(111) was studied by Temperature Programmed Desorption (TPD) and X-ray Photoelectron Spectroscopy (XPS). The “oxidized” surface (i.e., as prepared exhibiting V=O termination), the “reduced” surface (i.e., V=O groups being removed by electron irradiation), as well as the oxygen pre-covered reduced surface were investigated. Both TPD and XPS indicate that the oxidized surface has little affinity for CO adsorption, while the reduced surface readily binds CO (CO amount approx. 10 times higher). Accordingly, CO can be used to titrate the presence or absence of vanadyl oxygen (via adsorption on the vanadium atoms) but also of defects like surface oxygen vacancies. For propane and propene, desorption of the parent molecules was the major process, i.e., surface reactions were absent under the applied conditions. When oxygen was pre-adsorbed on the reduced surface, the adsorption properties resembled that of the oxidized surface, i.e., the vanadyl groups were (partially) re-established. TPD and XPS provide a handle to differentiate the binding sites on the V₂O₃ surface. Dedicated to Prof. Konrad Hayek.     

锆基MOF次级结构单元调控及轻烃吸附分离性能增强

从天然气中回收C₂/C₃轻烃组分具有重要的工业价值,吸附分离技术可在常温常压下实现轻烃的回收。对MOF材料进行次级结构单元（SBU）调控,可在继承其晶体结构和发达孔道的同时,优化孔道化学微环境并引入新的吸附位点。使用三嗪（TZ）取代Zr-TBAPy(NU-1000)SBU中的配位水分子,在其孔道内构筑对轻烃吸附质具有更强限域作用的碱性表面化学微环境,得到了高选择性的新型TZ@Zr-TBAPy吸附剂。TZ的引入在分子尺度上提高了孔道的表面粗糙度,同时强化对轻烃吸附质的限域作用,提高材料对烷烃的吸附容量和选择性。常温常压下,TZ@Zr-TBAPy对丙烷和乙烷的吸附容量分别为10.08和4.19 mmol?g^-1,比Zr-TBAPy提高了27%和9%,是目前国际上已报道的丙烷吸附容量最高的吸附剂之一。此外,丙烷/甲烷的IAST选择性为1518,是原材料的6.27倍;乙烷/甲烷的IAST选择性为11.7,比原材料提高了22%。更为重要的是,以TZ@Zr-TBAPy吸附剂为核心的固定床吸附过程可实现在常温常压天然气中乙烷和丙烷的一步分离回收。     

Atomistic Modeling of Effect of Mg on Oxygen Vacancy Diffusion in α‐Alumina

Oxygen diffusion plays an important role in grain growth and densification during the sintering of alumina ceramics and governs high‐temperature processes such as creep. The atomistic mechanism for oxygen diffusion in alumina is, however, still debated; atomistic calculations not being able to match experimentally determined activation energies for oxygen vacancy diffusion. These calculations are, however, usually performed for perfectly pure crystals, whereas virtually every experimental alumina sample contains a significant fraction of impurity/dopants ions. In this study, we use atomistic defect cluster and nudged elastic band (NEB) calculations to model the effect of Mg impurities/dopants on defect binding energies and migration barriers. We find that oxygen vacancies can form energetically favorable clusters with Mg, which reduces the number of mobile species and leads to an additional 1.5 eV energy barrier for the detachment of a single vacancy from Mg. The migration barriers of diffusive jumps change such that an enhanced concentration of oxygen vacancies is expected around Mg ions. Mg impurities were also found to cause destabilization of certain vacancy configurations as well as enhanced vacancy–vacancy interaction.     

Impermeability of boron nitride defect-sensitive monolayer with atomic-oxygen-healing ability

Hexagonal boron nitride (h-BN) is a new type of anti-corrosion coating. However, a large number of studies have shown that defects can significantly reduce the barrier property of h-BN. So how to repair the defect of h-BN is a problem to be solved. In this study, we used the density functional theory of the first principle to calculate the barrier properties of perfect h-BN and the h-BN with various vacancies. The study showed that monolayer h-BN has impermeability to all gas molecules, and vacancies can greatly reduce the barrier ability of h-BN to oxygen molecules. Fortunately, atomic oxygen can block the vacancy site of h-BN by chemical adsorption, thereby reducing the impact of vacancies on h-BN. Compared to the situation before the vacancy repaired, the barrier ability of h-BN to oxygen molecules was greatly increased after being repaired. In addition, the electron cloud density at vacancies after the repair (with) atomic oxygen was also greatly increased. Therefore, atomic oxygen can repair vacancies of h-BN and enhance the barrier ability of the defective h-BN. The research in this paper is of some significance. It is helpful to repair h-BN and evaluate corrosion resistance of h-BN in oxygen-rich or atomic-oxygen-rich environment.     

Methanol Adsorption on V2O3(0001)

Well ordered V₂O₃(0001) layers may be grown on Au(111) surfaces. These films are terminated by a layer of vanadyl groups which may be removed by irradiation with electrons, leading to a surface terminated by vanadium atoms. We present a study of methanol adsorption on vanadyl terminated and vanadium terminated surfaces as well as on weakly reduced surfaces with a limited density of vanadyl oxygen vacancies produced by electron irradiation. Different experimental methods and density functional theory are employed. For vanadyl terminated V₂O₃(0001) only molecular methanol adsorption was found to occur whereas methanol reacts to form formaldehyde, methane, and water on vanadium terminated and on weakly reduced V₂O₃(0001). In both cases a methoxy intermediate was detected on the surface. For weakly reduced surfaces it could be shown that the density of methoxy groups formed after methanol adsorption at low temperature is twice as high as the density of electron induced vanadyl oxygen vacancies on the surface which we attribute to the formation of additional vacancies via the reaction of hydroxy groups to form water which desorbs below room temperature. Density functional theory confirms this picture and identifies a methanol mediated hydrogen transfer path as being responsible for the formation of surface hydroxy groups and water. At higher temperature the methoxy groups react to form methane, formaldehyde, and some more water. The methane formation reaction consumes hydrogen atoms split off from methoxy groups in the course of the formaldehyde production process as well as hydrogen atoms still being on the surface after being produced at low temperature in the course of the methanol ?? methoxy + H reaction.     

A comparative theoretical study of Au,Ag and Cu adsorption on TiO<Subscript>2</Subscript> (110) rutile surfaces

The adsorption properties of Au, Ag and Cu on TiO₂ (110) rutile surfaces are examined using density functional theory slab calculations within the generalized gradient approximation. We consider five and four different adsorption sites for the metal adsorption on the stoichiometric and reduced surfaces, respectively. The metal-oxide bonding mechanism and the reactivity of metal atoms are also discussed based on the analyses of local density of states and charge density differences. This study predicts that Au atoms prefer to adsorb at the fourfold hollow site over the fivefold-coordinated Ti(5c) and in-plane and bridging O(2c) atoms with the adsorption energy of ≈0.6 eV. At this site, it appears that the covalent and ionic interactions with the Ti(5c) and the O(2c), respectively, contribute synergistically to the Au adsorption. At a neutral F _s ⁰ center on the reduced surface, Au binds to the surface via a rather strong ionic interaction with surrounding sixfold-coordinated Ti(6c) atoms, and its binding energy is much larger than to the stoichiometric surface. On the other hand, Ag and Cu strongly interact with the surface bridging O(2c) atoms, and the site between two bridging O(2c) atoms is predicted to be energetically the most favorable adsorption site. The adsorption energies of Ag and Cu at the B site are estimated to be ≈1.2 eV and ≈1.8 eV, respectively. Unlike Au, the interaction of Ag and Cu with a vacancy defect is much weaker than with the stoichiometric surface. °This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Surface anion vacancies on ceria: Quantum modelling of mutual interactions and oxygen adsorption

Quantum DFT + U calculations using periodic slab models are made to understand the characteristics of surface defects containing one, two or three anion vacancies at the CeO₂(1 1 1) surface. Several non-equivalent energy minima with different positions of localized Ce³⁺ ions can exist due to polaron-type distortions that influence the relative stability of Ce³⁺ in 6- and 7-fold coordinated sites. The calculated total energies do not confirm the tendency to vacancy association suggested by previous works. In the case of associated vacancies spontaneous outwards movement of one oxygen ion from a subsurface layer is observed. Oxygen adsorption on these defects leads to electron transfer to the O₂ molecule, forming peroxide species, except when only one electron is available in which case paramagnetic O₂⁻ species appear; the adsorption energies involved are computed and discussed. All these species appear asymmetrically coordinated to three surface Ce ions. The presence of excess electrons, as is the case when two or more vacancies associate, does not lead in general in these calculations to a barrierless complete reduction of the O₂ molecule, a process which would produce a pair of oxide ions; this is explained considering the relative energies of the orbitals involved. Thermal activation would be needed to complete this process.     

The significance of defect chemistry for the rate of gas–solid reactions: three examples

Three examples are revisited in which the reaction rate could be reliably correlated with point defect chemistry highlighting the role of point defects as acid–base active centers. In the case of dehydrohalogenation of tertiary butyl chloride, AgCl becomes increasingly active as heterogeneous catalyst, if AgCl is homogeneously or heterogeneously doped. By such a procedure the silver vacancy concentration is adequately increased. The oxygen incorporation into SrTiO₃ offers an example in which the surface mechanism in terms of adsorbed species, oxygen vacancies and electronic centers has been elucidated. Appropriate surface coatings give rise to significant catalytic effects. Increasing iron (acceptor) doping not only changes the point defect chemistry but also the nature of the rate determining step. Lastly, the electrocatalytic function of Sr-doped LaMnO₃ is considered as regards oxygen reduction reaction and O²⁻ incorporation into Y-doped ZrO₂ in the context of solid oxide fuel cells. Again the defect chemistry is of prime importance for the reaction rate.     

NO adsorption on Pd clusters. A density functional study

The interaction of NO with Pd clusters has been studied by means of the LCGTO-DF method. Metal cluster models (up to 13 atoms) with different size and geometry have been used to describe the atop, bridge and three-fold sites. The use of different model core potentials to increase the size of the cluster model treated and to save computational time has been discussed. The binding energies of N(1s), 4σ, 5σ and 1π electrons are calculated and compared directly to the experimental XPS and UPS data available. The NO is tilted with respect to the surface normal axis when adsorbed on top and bridge sites by about 52.6 and 46.7 degrees, respectively. On the two types of three-fold sites (hcp and fcc) the NO remains upright. The bending angle is very sensitive to the cluster size and affects the binding energies of N(1s), 4σ, 5σ and 1π orbitals. The NO adsorption energies on the different adsorption sites have been estimated using different cluster models. The vibrational frequencies have been calculated in the harmonic approximation and they are in reasonable agreement with the available experimental values. The cluster model approach is discussed in terms of its reliability to determine the adsorption energies and the favored site of adsorption. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ternary adsorption kinetics of gases in activated carbon

Ternary adsorption kinetic experiments of ethane (light species), propane (intermediate species) and n-butane in activated carbon are collected under various concentration combinations, temperatures and particle sizes. The effects of these parameters on the ternary adsorption dynamics are investigated. All the experimental data are compared with the predictions by a multicomponent heterogeneous macropore, surface and micropore diffusion (HMSMD) model recently proposed by Hu and Do (AIChE J (1993) 39 1628) using only single-component equilibrium and mass transfer parameters. The model can accurately predict the adsorption rates of ethane, propane and n-butane, but a small error in the calculation of the adsorbed amount of propane at ternary equilibrium is observed.     

甲烷、乙烷、丙烷和丁烷在中孔活性炭上的吸附平衡

以活性炭AC1为吸附剂,在体积法实验装置上分别测定了其对甲烷、乙烷、丙烷和丁烷4种气体的吸附等温线,吸附温度分别为283、293、303和313 K。利用77 K吸附氮气数据表征AC1,得到其比表面积为956 m²·g^-1,孔体积为1.36 mL·g^-1,孔径分布在1~5 nm,中孔的比例达到了61%。AC1对4种气体的吸附等温线均为I型等温线,分别采用Langmuir方程和Langmuir-Freundlich方程(简称L-F方程)对吸附平衡数据进行拟合,结果表明,L-F方程具有更好的拟合效果,为后序的多组分吸附平衡研究提供了基础数据。AC1对乙烷/丙烷的吸附平衡选择性系数在1.7~2.5,吸附选择性随吸附压力的增大而减小,吸附温度对吸附选择性无明显影响。     

Oxygen vacancy engineering on copper-manganese spinel surface for enhancing toluene catalytic combustion: A comparative study of acid treatment and alkali treatment

Copper-manganese spinel is a low-cost VOCs catalytic combustion catalyst with good performance. Oxygen vacancy has excellent properties for oxygen activation and VOCs dehydrogenation activation, which is beneficial for the catalytic combustion of VOCs. In this study, a large number of oxygen vacancies were introduced on the copper-manganese spinel surface by selective dissolution method (acid treatment and alkali treatment) for catalytic combustion of toluene. Furthermore, the effects of acid treatment and alkali treatment on the catalytic performance, oxygen vacancy amount, physical and chemical properties, and toluene catalytic combustion mechanism of copper-manganese spinel were studied. Both acid treatment and alkali treatment can produce large quantities of oxygen vacancies on the copper-manganese spinel surface. The generation of surface oxygen vacancies can greatly improve the catalytic combustion activity of copper-manganese spinel. At 240 °C, the combustion rate of toluene increased by 8.8 times for the acid-treated catalyst and 11.2 times for the alkali-treated catalyst. The numerous surface oxygen vacancies, Mn³⁺/Mn⁴⁺ at the ratio of 1.11 and appropriate acidity result in the alkali-treated catalyst exhibiting excellent catalytic activity and stability for toluene combustion. This strategy provides a new method to further improve catalytic combustion activity of copper-manganese spinel and a reference for the development of the surface oxygen vacancy engineering of transition metal oxides.     

Low‐Temperature Ionic Conductivity of an Acceptor‐Doped Perovskite: I. Impedance of Single‐Crystal SrTiO3

Low‐temperature conductivity mechanisms were identified in acceptor‐doped SrTiO₃ single crystals quenched from high temperatures under reducing conditions. Impedance spectroscopy measurements made on samples of the prototypical perovskite structure doped with iron provided a framework for creating a complete conductivity model for a well‐defined point defect system. The dominant conductivity mechanism in the room‐temperature range was identified as being controlled by oxygen vacancy hopping. The activation energy for oxygen vacancy migration, an often debated value in the perovskite community, is determined to lie within the range of 0.59–0.78 eV for the iron‐doped system with the bottom of this range approaching the intrinsic value for oxygen vacancy hopping in an undoped single crystal. At low temperatures, oxygen vacancies form defect complexes with iron impurities, and the observed range of activation energies is explained and modeled in terms of an oxygen vacancy trapping mechanism.     

Atomic Resolution Analysis of the Defect Chemistry and Microdomain Structure of Brownmillerite-Type Strontium Cobaltite

To investigate the fundamental aspects of vacancy ordering in oxygen-transporting ceramic membranes, we have performed atomic resolution analysis of individual domains in brownmillerite-type SrCoO_3−δ. Electron energy loss spectroscopy indicates that the Co valence state in adjacent planes can be 2+ and 4+. This charge localization is accompanied by oxygen deficiency and the formation of ordered octahedral and tetrahedral coordinated Co sites. At microdomain boundaries, Z -contrast images reveal a structural relaxation of the octahedral site with the reduction of the Co valence state from 4+ to 3+ and the incorporation of extra oxygen vacancies.     

Conversion of N2O to N2 on TiO2(1 1 0)

In this study, we examine the interaction of N₂O with TiO₂(1 1 0) in an effort to better understand the conversion of NO_x species to N₂ over TiO₂-based catalysts. The TiO₂(1 1 0) surface was chosen as a model system because this material is commonly used as a support and because oxygen vacancies on this surface are perhaps the best available models for the role of electronic defects in catalysis. Annealing TiO₂(1 1 0) in vacuum at high temperature (above about 800 K) generates oxygen vacancy sites that are associated with reduced surface cations (Ti³⁺ sites) and that are easily quantified using temperature programmed desorption (TPD) of water. Using TPD, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), we found that the majority of N₂O molecules adsorbed at 90 K on TiO₂(1 1 0) are weakly held and desorb from the surface at 130 K. However, a small fraction of the N₂O molecules exposed to TiO₂(1 1 0) at 90 K decompose to N₂ via one of two channels, both of which are vacancy-mediated. One channel occurs at 90 K, and results in N₂ ejection from the surface and vacancy oxidation. We propose that this channel involves N₂O molecules bound at vacancies with the O-end of the molecule in the vacancy. The second channel results from an adsorbed state of N₂O that decomposes at 170 K to liberate N₂ in the gas phase and deposit oxygen adatoms at non-defect Ti⁴⁺ sites. The presence of these O adatoms is clearly evident in subsequent water TPD measurements. We propose that this channel involves N₂O molecules that are bound at vacancies with the N-end of the molecule in the vacancy, which permits the O-end of the molecule to interact with an adjacent Ti⁴⁺ site. The partitioning between these two channels is roughly 1:1 for adsorption at 90 K, but neither is observed to occur for moderate N₂O exposures at temperatures above 200 K. EELS data indicate that vacancies readily transfer charge to N₂O at 90 K, and this charge transfer facilitates N₂O decomposition. Based on these results, it appears that the decomposition of N₂O to N₂ requires trapping of the molecule at vacancies and that the lifetime of the N₂O–vacancy interaction may be key to the conversion of N₂O to N₂.     

Simple group contribution theory for adsorption of alkanes in nanoporous carbons

This paper develops a simplified form of the group-contribution theory of Russell and LeVan (Chem. Eng. Sci. 51 (1996) 4025) for the prediction of adsorption equilibria of n-alkanes on activated carbon. Molecules are separated into elements or groups, and adsorbate group-adsorbent interactions are the determining factor for adsorption equilibrium. The adsorbent surface is considered to be heterogeneous with a single dimensionless energy distribution. Model parameters are determined for pure-component adsorption of methane, ethane, n-butane, and n-hexane on BPL activated carbon at multiple temperatures. The resulting parameters are then used to predict adsorption equilibrium of propane and n-octane at multiple temperatures and mixtures of methane and n-hexane and ethane and n-hexane. The model gives a more accurate prediction of mixture adsorption than the ideal adsorbed solution theory. The model is also used to describe methane, propane, and n-pentane adsorption on BAX activated carbon (Ind. Eng. Chem. Res. 40 (2001) 338) and methane, ethane, and propane on Columbia Grade L carbon (Ind. Eng. Chem. 42 (1950) 1315) at multiple temperatures. The model is shown to describe adsorption equilibria of alkanes on all three activated carbons with good accuracy and can also be used as a predictive tool for components on which no experimental data are available.     

Adsorption and diffusion of Pb(II) on the kaolinite(001) surface: A density-functional theory study

The adsorption and diffusion of Pb(II) atom on the hydroxylated (001) surface of kaolinite were investigated using density-functional theory within the generalized gradient approximation and a supercell approach. The coverage dependence of the adsorption structures and energetics was systematically studied for a wide range of coverage Θ [from 0.11 to 1.0 monolayers (ML)] and adsorption sites. The most stable among all possible adsorption sites was the two-fold bridge site followed by the one-fold top site, and the adsorption energy increased with the coverage, thus indicating the higher stability of surface adsorption and a tendency to the formation of Pb(II) islands (clusters) with increasing coverage. Moreover, the energy barrier for diffusion of Pb(II) atom between the one-fold top and the two-fold bridge adsorption sites on kaolinite(001) surface was 0.23 (0.31) eV, implying that the Pb(II) atom is prone to diffusing on kaolinite(001) surface. The other properties of the Pb(II)/kaolinite(001) system including the different charge distribution, the lattice relaxation, and the electronic density of states were also studied and discussed in detail.     

Nonstoichiometry in Undoped BaTiO3

Equilibrium electrical conductivity data for large-grained, poly crystalline, undoped BaTiO₃, as a function of temperature, 750° to 1000°C, and oxygen partial pressure, 10⁻²⁰< P _O2<10⁻¹ MPa, were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The latter are invariably present in sufficient excess to control the defect concentrations through the compensating oxygen vacancies, except under the most severely reducing conditions. Singly ionized oxygen vacancies play no discernible role in the defect chemistry of BaTiO₃ within this experimental range. The derived accidental acceptor content has a slight temperature dependence which may reflect some small amount of defect association. Deviation of the conductivity minima from the ideal shape yields a small P _O2-independent conductivity contribution, which is tentatively identified as oxygen vacancy conduction.     

Dissociative adsorption of small molecules at vacancies on the graphite (0 0 0 1) surface

The adsorption of molecular and atomic hydrogen as well as other molecules in the atmosphere on vacancies in the (0 0 0 1) graphite surface are investigated using density functional theory. Atomic hydrogen adsorbs with energies ranging from 4.7 to 2.3 eV. The validity of the model is confirmed by the good agreement between calculated vibrational spectra and those of high-resolution electron energy loss spectroscopy. It is shown that molecular hydrogen dissociates with a barrier of 1.1 eV on this model system. Water and oxygen also dissociate with respective barriers of 1.6 and 0.2 eV. Carbon dioxide and nitrogen have no interaction with the defect whereas carbon monoxide is incorporated into the vacancy with an activation energy of 1.5 eV. A comparison is made with the reactivity of graphene edges, both zigzag and armchair.     

Cathodoluminescence evaluation of the degradation of Mg,Ca and Dy Co-doped BaTiO3 Ceramics

Mg, Ca and Dy co-doped BaTiO₃ ceramics were prepared by conventional solid-state method. About 25% of grains possessed core-rim structure, which was caused by different Ca and Mg solutions. Cathodoluminescence, an efficient tool to obtain defect information with high spatial and spectral resolution, was applied to investigate the degradation mechanism of the ceramics. Oxygen vacancy concentration in the core is higher than that in the rim due to the higher Ca content in the rim. In the initial stage of degradation, oxygen vacancies mainly migrated within the core region. In the middle stage, oxygen vacancies migration in the grains with core-rim structure was confined to the grains, from the core to the rim. In contrast, oxygen vacancies in the grains without core-rim structure migrated across the grain boundaries to the cathodic region. Core-rim structure could delay oxygen vacancy migration to prolong the lifetime.