Surface dependence of CO2 adsorption on Zn2GeO4

An understanding of the interaction between Zn(2)GeO(4) and the CO(2) molecule is vital for developing its role in the photocatalytic reduction of CO(2). In this study, we present the structure and energetics of CO(2) adsorbed onto the stoichiometric perfectly and the oxygen vacancy defect of Zn(2)GeO(4) (010) and (001) surfaces using density functional theory slab calculations. The major finding is that the surface structure of the Zn(2)GeO(4) is important for CO(2) adsorption and activation, i.e., the interaction of CO(2) with Zn(2)GeO(4) surfaces is structure-dependent. The ability of CO(2) adsorption on (001) is higher than that of CO(2) adsorption on (010). For the (010) surface, the active sites O(2c)···Ge(3c) and Ge(3c)-O(3c) interact with the CO(2) molecule leading to a bidentate carbonate species. The presence of Ge(3c)-O(2c)···Ge(3c) bonds on the (001) surface strengthens the interaction of CO(2) with the (001) surface, and results in a bridged carbonate-like species. Furthermore, a comparison of the calculated adsorption energies of CO(2) adsorption on perfect and defective Zn(2)GeO(4) (010) and (001) surfaces shows that CO(2) has the strongest adsorption near a surface oxygen vacancy site, with an adsorption energy -1.05 to -2.17 eV, stronger than adsorption of CO(2) on perfect Zn(2)GeO(4) surfaces (E(ads) = -0.91 to -1.12 eV) or adsorption of CO(2) on a surface oxygen defect site (E(ads) = -0.24 to -0.95 eV). Additionally, for the defective Zn(2)GeO(4) surfaces, the oxygen vacancies are the active sites. CO(2) that adsorbs directly at the Vo site can be dissociated into CO and O and the Vo defect can be healed by the oxygen atom released during the dissociation process. On further analysis of the dissociative adsorption mechanism of CO(2) on the surface oxygen defect site, we concluded that dissociative adsorption of CO(2) favors the stepwise dissociation mechanism and the dissociation process can be described as CO(2) + Vo → CO(2)(δ-)/Vo → CO(adsorbed) + O(surface). This result has an important implication for understanding the photoreduction of CO(2) by using Zn(2)GeO(4) nanoribbons.     

CO2 adsorption on TiO2(101) anatase: a dispersion-corrected density functional theory study

Adsorption, diffusion, and dissociation of CO(2) on the anatase (101) surface were investigated using dispersion-corrected density functional theory. On the oxidized surface several different local minima were identified of which the most stable corresponds to a CO(2) molecule adsorbed at a five-fold coordinated Ti site in a tilted configuration. Surface diffusion is characterized by relatively small activation barriers. Preferential diffusion takes place along Ti rows and involves a cartwheel type of motion. The presence of a bridging oxygen defect or a surface interstitial Ti atom allows creation of several new strong binding configurations the most stable of which have bent CO(2) structures with simultaneous bonding to two surface Ti atoms. Subsurface oxygen vacancy or interstitial Ti defects are found to enhance the bonding of CO(2) molecules to the surface. CO(2) dissociation from these defect sites is calculated to be exothermic with barriers less than 21 kcal/mol. The use of such defects for catalytic activation of CO(2) on anatase (101) surface would require a mechanism for their regeneration.     

The mechanism of the water-gas shift reaction on Cu/TiO2(110) elucidated from application of density-functional theory

The Cu/TiO(2)(110) surface displays a great catalytic activity toward the water-gas shift reaction (WGSR), for which Cu is considered to be the most active metal on a TiO(2)(110)-supported surface. Experiments revealed that Cu nanoparticles bind preferentially to the terrace and steps of the TiO(2)(110) surface, which would not only affect the growth mode of the surface cluster but also enhance the catalytic activity, unlike Au nanoparticles for which occupancy of surface vacancies is favored, resulting in poorer catalytic performance than Cu. With density-functional theory we calculated some possible potential-energy surfaces for the carboxyl and redox mechanisms of the WGSR at the interface between the Cu cluster and the TiO(2) support. Our results show that the redox mechanism would be the dominant path; the resident Cu clusters greatly diminish the barrier for CO oxidation (22.49 and 108.68 kJ mol(-1), with and without Cu clusters, respectively). When adsorbed CO is catalytically oxidized by the bridging oxygen of the Cu/TiO(2)(110) surface to form CO(2), the release of CO(2) from the surface would result in the formation of an oxygen vacancy on the surface to facilitate the ensuing water splitting (barrier 34.90 vs. 50.49 kJ mol(-1), with and without the aid of a surface vacancy).     

Ab initio cluster calculations on the electronic structure of oxygen vacancies at the polar ZnO(0001) surface and on the adsorption of H2, CO, and CO2 at these sites

Oxygen vacancies at the polar O terminated (0001) surface of ZnO are of particular interest, because they are discussed as active sites in the methanol synthesis. In general, the polar ZnO surfaces are stabilized by OH groups, therefore O vacancies can be generated by removing either O atoms or OH or H2O groups from the surface. These defects differ in the number of electrons in the vacancy and the number of OH groups in the neighborhood. In the present study, the electronic structure and the adsorption properties of four different types of oxygen vacancies have been investigated by means of embedded cluster calculations. We performed ab initio calculations on F+ like surface excitations for the different defect types and found that the transition energies are above the optical band-gap, while F+ centers in bulk ZnO show a characteristic optical excitation at 3.19 eV. Furthermore, we studied the adsorption of CO2 and CO at the different defect sites by DFT calculations. We found that CO2 dissociates at electron rich vacancies into CO and an O atom which remains in the vacancy. At the OH vacancy which contains an unpaired electron CO2 adsorbed in the form of CO2-, while it adsorbed as a linear neutral molecule at the H2O defect. CO adsorbed preferentially at the H2O defect and the OH defect, both with a binding energy of 0.3 eV.     

Adsorption of CO2 on oxidized, defected, hydrogen and oxygen covered rutile (1 x 1)-TiO2(110)

Presented are initial, S(0) and coverage, Theta, dependent S(Theta), adsorption probability measurements of CO(2) as a function of impact energy, E(i) = 0.12-1.3 eV, adsorption temperature, T(s) = 85-300 K, hydrogen and oxygen pre-exposure, as well as density of defects, Gamma, as varied by annealing (T = 600-900 K) and Ar(+) ion sputtering (dose chi(Ar) at 600 eV at 85 K) of a rutile (1 x 1) TiO(2)(110) surface. The defect densities were qualitatively characterized by thermal desorption spectroscopy (TDS) of CO(2). The CO(2) TDS curves consisted of two structures that can be assigned to adsorption on pristine and oxygen vacancy sites, in agreement with earlier studies. S(0) decreased linearly with E(i) and was independent of T(s). The adsorption dynamics were dominated by the effect of precursor states leading to Kisliuk-like shapes over the E(i) and T(s) range studied. Oxygen vacancy sites reduced S(0) of CO(2). Preadsorbed oxygen blocked preferentially defect sites, which led to an increase in S(0). Hydrogen preadsorption results in physical site blocking with decreased S(0) as H-preexposure increased, while the shape of S(Theta) curves was conserved. In contrast to oxygen, hydrogen does not adsorb preferentially on defect sites. The adsorption probability data were parameterized by analytic functions (Kisliuk model) and by Monte Carlo simulations (MCSs).     

N2O decomposition on TiO2 (110) from dynamic first-principles calculations

We have carried out a systematic study of N(2)O dissociation on a TiO(2) (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have made use of both static and dynamic calculations in order to elucidate N(2)O decomposition mechanisms. We find that dissociation is not favorable on the stoichiometric surface. On the other hand, the presence of oxygen bridging vacancies make the N(2)O decomposition possible. The role of the defective surface is to provide electrons to the adsorbed molecule. We find two channels for decomposition, depending on whether the molecule is adsorbed with the O or the N end of the molecule on a vacancy. The first case is energetically downhill and proceeds spontaneously, leading to N(2) ejection from the surface and vacancy oxidation. The second case relies on the formation of an intermediate bridging configuration of the adsorbed molecule and is hindered by a small energy barrier. In this case, molecule breaking produces N(2) in the gas phase and leaves oxygen adatoms on the surface. We relate our results to recent experimental findings.     

Oxygen vacancy promoting catalytic dehydration of formic acid on TiO2(110) by in situ scanning tunneling microscopic observation

The catalytic dehydration reaction processes of formic acid on a TiO2(110) surface at 350 K have been studied to visualize reaction intermediates and their dynamic behaviors by scanning tunneling microscopy. Three types of configurations of adsorbed formates on the surface were identified by their shapes and positions in STM images. Successive STM observations revealed transformations among the three configurations, i.e., bridge formate on a 5-fold coordinated Ti4+ row, bridge formate on an oxygen vacancy site with an oxygen atom of formate and on a 5-fold coordinated Ti4+ ion and with the other formate oxygen atom, and a monodentate formate on an oxygen vacancy site with an oxygen atom of formate. The decomposition of the monodentate formate to carbon monoxide and hydroxyl was also imaged, which is a rate-determining step in the catalytic dehydration of formic acid. Combined with first-principle DFT calculations, the overall reaction processes of the catalytic dehydration of formic acid on the surface have been elucidated. Oxygen vacancies on the surface that can be produced by dehydration of two hydroxyls in situ under the catalytic reaction conditions are essential for the reaction.     

Chain structures of surface hydroxyl groups formed via line oxygen vacancies on TiO2(110) surfaces studied using noncontact atomic force microscopy

Structures of surface hydroxyl groups arranged on a reduced TiO2(110) surface that had line oxygen vacancies were studied using noncontact atomic force microscopy (NC-AFM). NC-AFM results revealed that by increasing the density of oxygen vacancies on the TiO2(110) surface, line oxygen vacancies were formed by removal of oxygen atoms in a bridge oxygen row on the TiO2(110) surface. After the TiO2(110) surface with the line oxygen vacancies was exposed to water, the surface showed hydroxyl chain structures that were composed of hydroxyl groups linearly arranged in a form of two rows on the line oxygen vacancies and on a neighboring bridge oxygen row. In-situ NC-AFM measurements of these surfaces exposed to water at room temperature revealed that hydroxyl chain structures were formed at the line oxygen vacancy. Annealing above 500 K was sufficient to remove the hydroxyl chain structures on the TiO2(110) surface and allowed line oxygen vacancies to reappear on the surface. The line oxygen vacancies are active sites for water dissociation. In conclusion, the formation of the hydroxyl chain structure suggests that the surface hydroxyl groups on a TiO2(110) surface can be controlled by preparing oxygen vacancy structures on the surface.     

石墨烯中的单空位缺陷对铂原子催化解离氧分子的影响

采用密度泛函理论中的B3LYP方法研究了石墨烯中的单空位缺陷对铂原子(Pt)催化解离O_2分子的影响.计算发现O_2分子首先通过[2+1]或[2+2]环加成作用吸附在以单空位缺陷石墨烯为载体的Pt上(Pt-SV),并以不同的路径进行解离,吸附能分别为-158.23和-152.45kJ/mol.由于石墨烯片上单空位缺陷的存在,O_2分子更容易吸附在单空位缺陷处的Pt上,并且O_2在Pt-SV上解离的能垒(130.25kJ/mol)也明显比在Pt-pristine上解离的能垒低(76.23kJ/mol).因此石墨烯上单空位缺陷的存在提高增加了Pt的催化能力.     

Adsorption, diffusion, and dissociation of molecular oxygen at defected TiO2(110): a density functional theory study

The properties of reduced rutile TiO2(110) surfaces, as well as the adsorption, diffusion, and dissociation of molecular oxygen are investigated by means of density functional theory. The O2 molecule is found to bind strongly to bridging oxygen vacancies, attaining a molecular state with an expanded O-O bond of 1.44 A. The molecular oxygen also binds (with somewhat shortened bond lengths) to the fivefold coordinated Ti atoms in the troughs between the bridging oxygen rows, but only when vacancies are present somewhere in the surface. In all cases, the magnetic moment of O2 is lost upon adsorption. The expanded bond lengths reveal together with inspection of electron density and electronic density of state plots that charging of the adsorbed molecular oxygen is of key importance in forming the adsorption bond. The processes of O2 diffusion from a vacancy to a trough and O2 dissociation at a vacancy are both hindered by relative large barriers. However, we find that the presence of neighboring vacancies can strongly affect the ability of O2 to dissociate. The implications of this in connection with diffusion of the bridging oxygen vacancies are discussed.     

Formation and diffusion of oxygen-vacancy pairs on TiO2(110)-(1x1)

We present the measurements for the diffusion of bridging oxygen vacancy (OV) crossover Ti rows via OV pairs (OVPs). Using a high-resolution scanning tunneling microscope (STM), we show that the OVs can be moved along the bridging oxygen rows driven by the STM tip at voltages higher than 3.0 V on TiO(2)(110)-(1x1) surface. It is found that the combination of OVPs leads to the formation of OVPs, which can diffuse crossover Ti rows under the mediation of OVs in adjacent bridging oxygen rows. The deduced diffusion activation energy for the diffusive OVPs from experiments is in agreement with first-principles calculations. The reaction activation energy of the OVPs with O(2) is lower than that of the OVs by 82 meV.     

Conversion of N2O to N2 on MgO （001） Surface with Vacancy： A DFT Study

The adsorption and decomposition of NzO at regular and defect sites of MgO (001) surface have been studied using cluster models embedded in a large array of point charges (PCs) by DFT/B3LYP method. The results indicate that the MgO (001)surface with oxygen vacancies exhibits high catalytic reactivity toward N2O adsorptive-decomposition. It is different from the regular MgO surface or the surface with magnesium vacancies.Much elongation of O—N bond of N2O after adsorption at oxy-gen vacancy site with O end down shows that O—N bond has been broken with concurrent production of N2, leaving a regu-lar site instead of the original oxygen vacancy site (F center ).The MgO (001) surface with magnesium vacancies hardly ex-hibits catalytic reactivity. It can be concluded that N2O dissoci-ation likely occurs at oxygen vacancy sites of MgO (001) sur-face, which is consistent with the generally accepted viewpoint in the experiments. The potential energy surface (PES) reflects that the dissociation process of N2O does not virtually need to surmount a given energy barrier.     

Observation of photocatalytic dissociation of water on terminal Ti sites of TiO2(110)-1 × 1 surface

The water splitting reaction based on the promising TiO(2) photocatalyst is one of the fundamental processes that bears significant implication in hydrogen energy technology and has been extensively studied. However, a long-standing puzzling question in understanding the reaction sequence of the water splitting is whether the initial reaction step is a photocatalytic process and how it happens. Here, using the low temperature scanning tunneling microscopy (STM) performed at 80 K, we observed the dissociation of individually adsorbed water molecules at the 5-fold coordinated Ti (Ti(5c)) sites of the reduced TiO(2) (110)-1 × 1 surface under the irradiation of UV lights with the wavelength shorter than 400 nm, or to say its energy larger than the band gap of 3.1 eV for the rutile TiO(2). This finding thus clearly suggests the involvement of a photocatalytic dissociation process that produces two kinds of hydroxyl species. One is always present at the adjacent bridging oxygen sites, that is, OH(br), and the other either occurs as OH(t) at Ti(5c) sites away from the original ones or even desorbs from the surface. In comparison, the tip-induced dissociation of the water can only produce OH(t) or oxygen adatoms exactly at the original Ti(5c) sites, without the trace of OH(br). Such a difference clearly indicates that the photocatalytic dissociation of the water undergoes a process that differs significantly from the attachment of electrons injected by the tip. Our results imply that the initial step of the water dissociation under the UV light irradiation may not be reduced by the electrons, but most likely oxidized by the holes generated by the photons.     

Photoluminescence of TiO2: effect of UV light and adsorbed molecules on surface band structure

The photoluminescence (PL) of TiO(2) at 529.5 nm (2.34 eV) has been found to be a sensitive indicator of UV-induced band structure modification. As UV irradiation occurs, the positive surface potential changes and shifts the depth of the depletion layer. In addition, reversible band bending due to the adsorption of the electron-donor NH(3) and CO molecules has been observed in measurements combining PL with FTIR surface spectroscopy. It has been found that the O(2) molecule acts in two ways: as a reversibly adsorbed electron-acceptor molecule and as an irreversibly adsorbed molecule that heals natural oxygen vacancy defects in the near-surface region.     

CO Preferential oxidation promoted by UV irradiation in the presence of H2 over Au/TiO2

The catalytic oxidation of CO was performed over Au/TiO(2) under UV irradiation in the presence of H(2) in different reaction systems. It was found that the introduction of H(2) enhanced the CO thermocatalytic oxidation in a CO pre-introduced system (CO/O(2)vs. CO/H(2)/O(2)), but suppressed that in an O(2) pre-introduced (O(2)/CO vs. O(2)/H(2)/CO) system. Although the CO oxidation in both CO/H(2)/O(2) and O(2)/H(2)/CO systems could be remarkably enhanced under UV irradiation, the oxidation of H(2) was suppressed under UV irradiation. It was proposed that the dissociative chemisorption H ([triple bond]Ti-H) at surface oxygen vacancy sites of TiO(2) could act as both the electron-acceptors for the photogeneration electrons and the electron-donors for the chemisorbed O(2) at TiO(2), and thus enhance the CO oxidation during the coinstantaneous process of thermocatalysis and photocatalysis. The suppression of H(2) thermocatalytic oxidation under UV irradiation might be ascribed to the electron transfer effect, i.e., the dissociative chemisorption H on Au (Au-H) could be desorbed at the H(2) molecule via accepting the photogenerated electrons from TiO(2).     

Charging of Au atoms on TiO2 thin films from CO vibrational spectroscopy and DFT calculations

Au atoms have been deposited on oxidized and reduced TiO2 thin films grown on Mo(110). The gold binding sites and the occurrence of Au-TiO2 charge transfer were identified by measuring infrared spectra as a function of temperature and substrate preparation. The results have been interpreted by slab model DFT calculations. Au binds weakly to regular TiO2 sites (De < 0.5 eV) where it remains neutral, and diffuses easily even at low temperature until it gets trapped at strong binding sites such as oxygen vacancies (De = 1.7 eV). Here, a charge transfer from TiO2 to Au occurs. Au(delta-)CO complexes formed on oxygen vacancies easily lose CO (De = 0.4 eV), and the CO stretching frequency is red-shifted. On nondefective surfaces, CO adsorption induces a charge transfer from Au to TiO2 with formation of strongly bound Audelta+CO complexes (De = 2.4 eV); the corresponding CO frequency is blue-shifted with respect to free CO. We propose possible mechanisms to reconcile the observed CO desorption around 380 K with the unusually high stability of Au-CO complexes formed on regular sites predicted by the calculations. This implies: (a) diffusion of AuCO complexes above 150 K; (b) formation of gold dimers when the diffusing AuCO complex encounters a Au atom bound to an oxygen vacancy (reduced TiO2) or a second AuCO unit (oxidized TiO2); and (c) CO desorption from the resulting dimer, occurring around 350-400 K.     

CO为还原剂同时还原SO2和NO的SnO2-TiO2固溶体催化剂Ⅱ.催化剂的物理化学性质

 用XRD, XPS, CO-TPR, NH3-TPD, SO2-TPD和IR等方法表征了SnO2-TiO2固溶体催化剂的物理化学性质. 不同配比的SnO2和TiO2均可形成均一的具有金红石结构的连续固溶体,其晶粒度比单纯的SnO2或TiO2的晶粒度小. SnO2-TiO2固溶体的比表面积随SnO2含量的增大呈火山形变化,说明在SnO2-TiO2固溶体中SnO2可阻止TiO2由锐钛矿型变为金红石型过程中比表面积的减小,而TiO2则提供了维持大表面的结构框架. SnO2倾向于在固溶体表面偏析,固溶体的表面氧含量高于单纯SnO2的表面氧含量而低于单纯TiO2的表面氧含量. SnO2, TiO2和SnO2-TiO2表面含有能被CO还原的吸附氧和晶格氧,被还原的SnO2, TiO2和SnO2-TiO2的表面晶格氧的数量仅占所有晶格氧的0.001%, 说明CO只使部分晶格氧还原并生成氧阴离子空穴. TiO2表面没有酸性, SnO2和SnO2-TiO2呈微弱酸性. 经CO还原的SnO2-TiO2上存在大量的强碱中心,说明SnO2和TiO2之间发生了协同作用. SnO2-TiO2固溶体的这些物化性质均十分有利于SO2+NO+CO的氧化还原反应.     

Molecular oxygen adsorption behaviors on the rutile TiO2(110)-1×1 surface: an in situ study with low-temperature scanning tunneling microscopy

A knowledge of adsorption behaviors of oxygen on the model system of the reduced rutile TiO(2)(110)-1×1 surface is of great importance for an atomistic understanding of many chemical processes. We present a scanning tunneling microcopy (STM) study on the adsorption of molecular oxygen either at the bridge-bonded oxygen vacancies (BBO(V)) or at the hydroxyls (OH) on the TiO(2)(110)-1×1 surface. Using an in situ O(2) dosing method, we are able to directly verify the exact adsorption sites and the dynamic behaviors of molecular O(2). Our experiments provide direct evidence that an O(2) molecule can intrinsically adsorb at both the BBO(V) and the OH sites. It has been identified that, at a low coverage of O(2), the singly adsorbed molecular O(2) at BBO(V) can be dissociated through an intermediate state as driven by the STM tip. However, singly adsorbed molecular O(2) at OH can survive from such a tip-induced effect, which implies that the singly adsorbed O(2) at OH is more stable than that at BBO(V). It is interesting to observe that when the BBO(V)s are fully filled with excess O(2) dosing, the adsorbed O(2) molecules at BBO(V) tend to be nondissociative even under a higher bias voltage of 2.2 V. Such a nondissociative behavior is most likely attributed to the presence of two or more O(2) molecules simultaneously adsorbed at a BBO(V) with a more stable configuration than singly adsorbed molecular O(2) at a BBO(V).     

O2-coverage-dependent CO oxidation on reduced TiO2(110): A first principles study

First principles periodic slab calculations based on gradient-corrected density functional theory have been performed to investigate CO oxidation on rutile TiO2(110) at varying O2 coverages (theta = 1, 2, and 3, where theta is defined as the number of O2 per oxygen vacancy). For each coverage we only present the reaction of CO with oxygen species in the most stable configuration. Our results show a significant variation in the oxidation activation energy with O2 coverage.     

First principles study of CO oxidation on TiO2(110): the role of surface oxygen vacancies

The reactivities of the stoichiometric and partially reduced rutile TiO2(110) surfaces towards oxygen adsorption and carbon monoxide oxidation have been studied by means of periodic density functional theory calculations within the Car-Parrinello approach. O2 adsorption as well as CO oxidation are found to take place only in the presence of surface oxygen vacancies (partially reduced surface). The oxidation of CO by molecularly adsorbed O2 at the O-vacancy site is found to have an activation energy of about 0.4 eV. When the adsorbed O2 is dissociated, the resulting adatoms can oxidize incoming gas-phase CO molecules with no barrier. In all studied cases, once CO is oxidized to form CO2, the resulting surface is defect-free and no catalytic cycle can be established.