Oxygen density dominated gas sensing mechanism originated from CO adsorption on the hexagonal WO3 (001) surface

Background oxygen play important role in the detection of gases on metal oxide surfaces. In this work, a new mechanism dominated by oxygen density has been proposed based on density functional theory (DFT) calculation of CO adsorption on the oxygen pre-absorbed and oxygen deficient hexagonal WO₃ (h-WO₃) (001) surface. Taking clean WO-terminated h-WO₃ (001) surface as the datum, we can define the O- and WO-terminated h-WO₃ (001) surfaces to be situations with surface oxygen density (denoted as d_O) of 1 and 0, respectively. And the oxygen density will be positive (1 > d_O > 0) for oxygen absorbed surfaces and negative (0 > d_O > ?1) for oxygen vacancy presented surfaces. More importantly, environmental oxygen concentration can be reflected directly by surface oxygen density. A positive correlation between environmental oxygen concentration (surface oxygen density) and sensing ability (charge transfer number) can be constructed based on the data of CO sensing on h-WO₃ (001) surfaces (Zhao et al., 2013; Tian et al., 2014). And these ideas obtained for CO on h-WO₃ can also be generalized to other gases and materials. The new proposed oxygen density dominated gas sensing mechanism, combined the two existing models of surface absorbed oxygen and oxygen vacancy together by the use of one physical quantity of oxygen density, will simplify the understanding of the effect of environmental oxygen on gas sensing largely. And the new findings here will provide substantial chances for controllable sensing by surface tuning.     

Structural and electronic properties of tungsten trioxides: from cluster to solid surface

Geometries and electronic structures of WO₃(001) surface and a series of stoichiometric (WO₃) _n clusters (n = 1–6) have been systematically investigated using first-principles density functional calculations. Six possible reconstructured models of WO₃(001) surface with cubic phase are explored, and the most stable configuration is the \( (\sqrt 2 \times \sqrt 2 )R45^{\circ} \) surface. The main feature of WO₃(001) surface is that the top of valence band is dominated by the 2p states of the bridging oxygen atom, rather than the top terminal oxygen. By comparing the geometrical parameters, from the structural point of view, the W₃O₉ cluster can be used as the smallest molecular prototype of the WO₃ surface. However, in terms of the electronic structure, only until W₆O₁₈, the cluster begins to appear the electronic feature of the WO₃(001) surface. This may be due to the reason that the W₆O₁₈ cluster and the top layer of WO₃(001) surface show similar “stoichiometry” if we treat two kinds of oxygen atoms as different “elements”. In addition, for the chemical reactivity, using BH₃ as a probe molecule, the W₆O₁₈ cluster also bears general resemblance to the WO₃(001) surface, and the bridging oxygen atoms in two systems are the preferred sites for the nucleophilic reaction. Therefore, our results indicate that the W₆O₁₈ cluster with a spherical buckyball structure can be viewed as the smallest molecular model to understand the properties such as catalytic activity of WO₃(001) surface.     

Opposite Face Sensitivity of CeO2 in Hydrogenation and Oxidation Catalysis

The determination of structure–performance relationships of ceria in heterogeneous reactions is enabled by the control of the crystal shape and morphology. Whereas the (100) surface, predominantly exposed in nanocubes, is optimal for CO oxidation, the (111) surface, prevalent in conventional polyhedral CeO₂ particles, dominates in C₂H₂ hydrogenation. This result is attributed to the different oxygen vacancy chemistry on these facets. In contrast to oxidations, hydrogenations on CeO₂ are favored over low‐vacancy surfaces owing to the key role of oxygen on the stabilization of reactive intermediates. The catalytic behavior after ageing at high temperature confirms the inverse face sensitivity of the two reaction families.     

Structure and chemical activity of transition metal and metal oxide catalysts: An insight from theoretical DFT studies

The present theoretical DFT study discusses the structure and chemical activity of transition metal and metal oxide catalysts within the well-known cluster approach. Selective oxidation of carbon monoxide on gold supported on titania (Au/TiO₂ (110)) as well as some key points in understanding the effect of non-metal doping on TiO2 with the aim to increase its photocatalytic functionality have been briefly discussed. It was shown that Au (with formal oxidation state equal to plus one) stabilized on water-assisted and vacancy containing TiO₂ (110) can explain selective oxidation of CO. Here binding of O₂ with the vacancy site is energetically preferable than its adsorption on an Au site. Conversely, CO adsorbs on an Au center of Au/TiO₂ (110) which is energetically much more profitable than its interaction with the oxygen vacancy site. Also, carbon and nitrogen doping on TiO₂ (110) leads to two different structures. Energetically most profitable is that carbon occupies an interstitial position in deep bulk while nitrogen replaces the protruded oxygen atom and forms a surface N-H group.     

Pd(Cu)在MgO(100)表面吸附CO和O2的理论研究

采用固体镶嵌势能模型和DFT/B3LYP方法研究了在Pd/MgO和Cu/MgO表面吸附CO和O₂分子的电子性质. 计算结果表明, 在完美MgO(100)表面Pd原子对CO和O₂的吸附能分别为206.5和84.8 kJ/mol, 因此可知Pd原子更容易吸附CO分子; 而当Pd原子附着于有氧缺陷的MgO表面时, 它对两种分子的吸附都非常弱. 相反, 附着于MgO表面的Cu原子对O₂分子的吸附更为有利, 其吸附能在140～155 kJ/mol之间. 研究结果还表明, 对于双分子吸附体系, 即CO＋CO, CO＋O₂, O₂＋O₂体系, 双分子之间的结合力可减小完美MgO表面上Pd原子与被吸附分子的相互作用, 使吸附能减少了46～96 kJ/mol. 而对于在MgO表面上的Cu原子, 只有O₂＋O₂ 体系使吸附能减少了大约50～71 kJ/mol.     

CeO2(110)表面上CO氧化反应的密度泛函理论研究

利用密度泛函理论系统研究了O2与CO在CeO2(110)表面的吸附反应行为. 研究表明, O2在洁净的CeO2(110)表面吸附热力学不利, 而在氧空位表面为强化学吸附, O2分子被活化, 可能是重要的氧化反应物种. CO在洁净的CeO2(110)表面有化学吸附与物理吸附两种构型, 前者形成二齿碳酸盐物种, 后者与表面仅存在弱的相互作用. 在氧空位表面, CO可分子吸附或形成碳酸盐物种, 相应吸附能均较低. 当表面氧空位吸附O2后(O2/Ov), CO可吸附生成碳酸盐或直接生成CO2, 与原位红外光谱结果相一致. 过渡态计算发现,O2/Ov/CeO2(110)表面的三齿碳酸盐物种经两齿、单齿过渡态脱附生成CO2. 利用扩展休克尔分子轨道理论分析了典型吸附构型的电子结构, 说明表面碳酸盐物种三个氧原子电子存在离域作用, 物理吸附的CO及生成的CO2电子结构与相应自由分子相似.     

Roles of Oxygen Vacancies of CeO2 and Mn-Doped CeO2 with the Same Morphology in Benzene Catalytic Oxidation

Mn-doped CeO₂ and CeO₂ with the same morphology (nanofiber and nanocube) have been synthesized through hydrothermal method. When applied to benzene oxidation, the catalytic performance of Mn-doped CeO₂ is better than that of CeO₂, due to the difference of the concentration of O vacancy. Compared to CeO₂ with the same morphology, more oxygen vacancies were generated on the surface of Mn-doped CeO₂, due to the replacement of Ce ion with Mn ion. The lattice replacement has been analyzed through XRD, Raman, electron energy loss spectroscopy and electron paramagnetic resonance technology. The formation energies of oxygen vacancy on the different exposed crystal planes such as (110) and (100) for Mn-doped CeO₂ were calculated by the density functional theory (DFT). The results show that the oxygen vacancy is easier to be formed on the (110) plane. Other factors influencing catalytic behavior have also been investigated, indicating that the surface oxygen vacancy plays a crucial role in catalytic reaction.     

Highly Efficient Low‐Temperature Plasma‐Assisted Modification of TiO2 Nanosheets with Exposed {001} Facets for Enhanced Visible‐Light Photocatalytic Activity

Anatase TiO₂ nanosheets with exposed {001} facets have been controllably modified under non‐thermal dielectric barrier discharge (DBD) plasma with various working gas, including Ar, H₂, and NH₃. The obtained TiO₂ nanosheets possess a unique crystalline core/amorphous shell structure (TiO₂@TiO_2?x), which exhibit the improved visible and near‐infrared light absorption. The types of dopants (oxygen vacancy/surface Ti³⁺/substituted N) in oxygen‐deficient TiO₂ can be tuned by controlling the working gases during plasma discharge. Both surface Ti³⁺ and substituted N were doped into the lattice of TiO₂ through NH₃ plasma discharge, whereas the oxygen vacancy or Ti³⁺ (along with the oxygen vacancy) was obtained after Ar or H₂ plasma treatment. The TiO₂@TiO_2?x from NH₃ plasma with a green color shows the highest photocatalytic activity under visible‐light irradiation compared with the products from Ar plasma or H₂ plasma due to the synergistic effect of reduction and simultaneous nitridation in the NH₃ plasma.     

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.     

Effect of hydrogen gas impurities on the hydrogen dissociation on iron surface

A small addition of oxygen to hydrogen gas is known to mitigate the hydrogen embrittlement (HE) of steels. As atomic hydrogen dissolution in steels is responsible for embrittlement, catalysis of molecular hydrogen dissociation by the steel surface is an essential step in the embrittlement process. The most probable role of oxygen in mitigating HE is to inhibit the reactions between molecular hydrogen and the steel surface. To elucidate the mechanism of such surface reaction of hydrogen with the steel in the presence of oxygen, hydrogen, and oxygen adsorption, dissociation, and coadsorption on the Fe(100) surface were investigated using density functional theory. The results show that traces of O₂ would successfully compete with H₂ for surface adsorption sites due to the grater attractive force acting on the O₂ molecule compared to H₂. The H₂ dissociation would be hindered on iron surfaces with predissociated oxygen. Prompted by the notable results for H₂ + O₂, other practical systems were considered, that is, H₂ + CO and CH₄. Calculations were performed for the CO chemisorption and H₂ dissociation on iron surface with predissociated CO, as well as, CH₄ surface dissociation. The results indicate that CO inhibition of H₂ dissociation proceeds via similar mechanism to O₂ induced inhibition, whereas CH₄ traces in the H₂ gas have no effect on H₂ dissociation. © 2014 Wiley Periodicals, Inc.     

Adsorption of atomic and molecular oxygen on the LaMnO3(001) surface: ab initio supercell calculations and thermodynamics

We present and discuss the results of ab initio DFT plane-wave supercell calculations of the atomic and molecular oxygen adsorption and diffusion on the LaMnO(3) (001) surface which serves as a model material for a cathode of solid oxide fuel cells. The dissociative adsorption of O(2) molecules from the gas phase is energetically favorable on surface Mn ions even on a defect-free surface. The surface migration energy for adsorbed O ions is found to be quite high, 2.0 eV. We predict that the adsorbed O atoms could penetrate the electrode first plane when much more mobile surface oxygen vacancies (migration energy of 0.69 eV) approach the O ions strongly bound to the surface Mn ions. The formation of the O vacancy near the O atom adsorbed atop surface Mn ion leads to an increase of the O-Mn binding energy by 0.74 eV whereas the drop of this adsorbed O atom into a vacancy possesses no energy barrier. Ab initio thermodynamics predicts that at typical SOFC operation temperatures (approximately 1200 K) the MnO(2) (001) surface with adsorbed O atoms is the most stable in a very wide range of oxygen gas pressures (above 10(-2) atm).     

Impact of Surface Defects on LaNiO3 Perovskite Electrocatalysts for the Oxygen Evolution Reaction

Perovskite oxides are regarded as promising electrocatalysts for water splitting due to their cost-effectiveness, high efficiency and durability in the oxygen evolution reaction (OER). Despite these advantages, a fundamental understanding of how critical structural parameters of perovskite electrocatalysts influence their activity and stability is lacking. Here, we investigate the impact of structural defects on OER performance for representative LaNiO₃ perovskite electrocatalysts. Hydrogen reduction of 700 °C calcined LaNiO₃ induces a high density of surface oxygen vacancies, and confers significantly enhanced OER activity and stability compared to unreduced LaNiO₃; the former exhibit a low onset overpotential of 380 mV at 10 mA cm⁻² and a small Tafel slope of 70.8 mV dec⁻¹. Oxygen vacancy formation is accompanied by mixed Ni²⁺/Ni³⁺ valence states, which quantum-chemical DFT calculations reveal modify the perovskite electronic structure. Further, it reveals that the formation of oxygen vacancies is thermodynamically more favourable on the surface than in the bulk; it increases the electronic conductivity of reduced LaNiO₃ in accordance with the enhanced OER activity that is observed.     

Radical electrophilicities in solvent

A series of Ti-doped SnO₂(110) surfaces with different oxygen vacancies have been investigated by means of first principles DFT calculations combined with a slab model. Three kinds of defective SnO₂(110) surfaces are considered, including the formations of bridging oxygen (O _b ) vacancy, in-plane oxygen (O _i ) vacancy, and the coexistence of O _b and O _i vacancies. Our results indicate that Ti dopant prefers the fivefold-coordinated Sn site on the top layer for the surface with O _b or O _i vacancy, while the replacement of sublayer Sn atom becomes the most energetically favorable structure if the O _b and O _i vacancies are presented simultaneously. Based on analyzing the band structure of the most stable configuration, the presence of Ti leads to the variation of the band gap state, which is different for three defective SnO₂(110) surfaces. For the surface with O _b or O _i vacancy, the component of the defect state is modified, and the reaction activity of the corresponding surface is enhanced. Hence, the sensing performance of SnO₂ may be improved after introducing Ti dopant. However, for the third kind of reduced surface with the coexistence of O _b and O _i vacancies, the sublayer doping has little influence on the defect state, and only in this case, the Ti doping state partly appears in the band gap of SnO₂(110) surface.     

MgO缺陷和不规则表面吸附CO的能带和电子结构研究

采用从头算程序优化MgO表面三种不同配位位置吸附CO构型,并用扩展休克尔紧束缚(EHT)晶体轨道方法对MgO的缺陷和不规则表面吸附CO的可能构型进行能带计算,讨论了能带结构及组成,不同构型吸附前后能带和成键性质的变化,以及吸附前后的电荷转移和吸附键的变化规律。研究结果发现,CO的C端更有利于在MgO固体表面的吸附,具有氧缺陷结构的MgO更有利于吸附分解CO。     

Isolated Platinum Atoms Stabilized by Amorphous Tungstenic Acid: Metal–Support Interaction for Synergistic Oxygen Activation

Oxygen activation plays a crucial role in many important chemical reactions such as oxidation of organic compounds and oxygen reduction. For developing highly active materials for oxygen activation, herein, we report an atomically dispersed Pt on WO₃ nanoplates stabilized by in situ formed amorphous H₂WO₄ out‐layer and the mechanism for activating molecular oxygen. Experimental and theoretical studies demonstrate that the isolated Pt atoms coordinated with oxygen atoms from [WO₆] and water of H₂WO₄, consequently leading to optimized surface electronic configuration and strong metal–support interaction (SMSI). In exemplified reactions of butanone oxidation sensing and oxygen reduction, the atomic Pt/WO₃ hybrid exhibits superior activity than those of Pt nanoclusters/WO₃ and bare WO₃ as well as enhanced long‐term durability. This work will provide insight into the origin of activity and stability for atomically dispersed materials, thus promoting the development of highly efficient and durable single atom‐based catalysts.     

Remarkable Enhancement of O2 Activation on Yttrium‐Stabilized Zirconia Surface in a Dual Catalyst Bed

Yttrium‐stabilized zirconia (YSZ) has been extensively studied as an electrolyte material for solid oxide fuel cells (SOFC) but its performance in heterogeneous catalysis is also the object of a growing number of publications. In both applications, oxygen activation on the YSZ surface remains the step that hinders utilization at moderate temperature. It was demonstrated by oxygen isotope exchange that a dual catalyst bed system consisting of two successive LaMnO₃ and YSZ beds without intimate contact drastically enhances oxygen activation on the YSZ surface at 698 K. It can be concluded that LaMnO₃ activates the triplet ground‐state of molecular oxygen into a low‐lying singlet state, thereby facilitating the activation of the O₂ molecule on the YSZ oxygen vacancy sites. This phenomenon is shown to improve the catalytic activity of the LaMnO₃‐Pd/YSZ system for the partial oxidation of methane.     

The surface dependence of CO adsorption on Ceria

An understanding of the interaction between ceria and environmentally sensitive molecules is vital for developing its role in catalysis. We present the structure and energetics of CO adsorbed onto stoichiometric (111), (110), and (100) surfaces of ceria from first principles density functional theory corrected for on-site Coulomb interactions, DFT+U. DFT+U is applied because it can describe consistently the properties of both the stoichiometric and reduced surfaces. Our major finding is that the interaction is strongly surface dependent, consistent with experiment. Upon interaction of CO with the (111) surface, weak binding is found, with little perturbation to the surface or the molecule. For the (110) and (100) surfaces, the most stable adsorbate is that in which the CO molecule bridges two oxygen atoms and pulls these atoms out of their lattice sites, with formation of a (CO(3)) species. This results in a strong modification to the surface structure, consistent with that resulting from mild reduction. The electronic structure also demonstrates reduction of the ceria surface and consequent localization of charge on cerium atoms neighboring the vacancy sites. The surface-bound (CO(3)) species is identified as a carbonate, (CO(3))(2-) group, which is formed along with two reduced surface Ce(III) ions, in good agreement with experimental infrared data. These results provide a detailed investigation of the interactions involved in the adsorption of CO on ceria surfaces, allowing a rationalization of experimental findings and demonstrate further the applicability of the DFT+U approach to the study of systems in which reduced ceria surfaces play a role.     

Highly Ordered Mesoporous Tungsten Oxides with a Large Pore Size and Crystalline Framework for H2S Sensing

An ordered mesoporous WO₃ material with a highly crystalline framework was synthesized by using amphiphilic poly(ethylene oxide)‐b‐polystyrene (PEO‐b‐PS) diblock copolymers as a structure‐directing agent through a solvent‐evaporation‐induced self‐assembly method combined with a simple template‐carbonization strategy. The obtained mesoporous WO₃ materials have a large uniform mesopore size (ca. 10.9 nm) and a high surface area (ca. 121 m² g^?1). The mesoporous WO₃‐based H₂S gas sensor shows an excellent performance for H₂S sensing at low concentration (0.25 ppm) with fast response (2 s) and recovery (38 s). The high mesoporosity and continuous crystalline framework are responsible for the excellent performance in H₂S sensing.     

Rationally designed/constructed MnOx/WO3 anode for photoelectrochemical water oxidation

The activity of WO₃ photoanode could be improved efficiently after loading MnO_x by photodeposition. The maximum photocurrent density of composite photoanode is achieved with a deposition time of 3 min, which is higher than that of pristine WO₃ photoanode around 40%.     

Adsorption of CO on Ru and Pt blacks and catalysts and the possibility of its utilization for the determination of the ruthenium-free surface

Electrochemical voltammetric curves on Ru and Pt blacks of a different surface area were measured in potential intervals 0.05–1.05 V in pure 0.5 M H₂SO₄ and after CO adsorption. It was proved that after the CO adsorption, the outset of ruthenium oxidation is shifted by about 150 mV towards the positive potentials, e.g. to the region of oxidation of adsorbed CO. This fact made possible the determination of a double-layer charging current of Ru electrodes and, subsequently, also the determination of the amount of adsorbed hydrogen on the Ru surface. An evaluation of the amount of CO and hydrogen adsorption showed that the ratio of adsorbed CO:H on the Pt surface was about 1:1, while on Ru electrodes this ratio was around twice as large. The amount of hydrogen adsorbed on Ru blacks depends on the preliminary preparation of the electrodes. The CO adsorption could also be employed in the determination of a charging current of electrode double-layers during voltammetric oxidation of adsorbed hydrogen on ruthenium supported on Al₂O₃, SiO₂, or TiO₂ carriers. However, a similar determination of hydrogen adsorbed on the tin-modified Ru catalysts is not very reliable.