A computational study of H2 dissociation and CO adsorption on the PtML/WC(0 0 0 1) surface

We studied computationally the relative stability of Pt_ML/WC(0 0 0 1) [pseudomorphic monolayer of Pt(1 1 1) on WC(0 0 0 1)] interfacial structures using a density functional slab model approach. The work of adhesion was calculated for six different interfacial structures, taking into account both W- and C-terminations of the carbide. The results show that the optimal interfacial structure of Pt_ML/WC(0 0 0 1) is the W-terminated WC(0 0 0 1) with Pt atoms adhesion on the hcp site (W-hcp). The nature of metal/carbide bonding for the W-hcp interfacial geometry was determined on the basis of the partial density of states (PDOS). Adsorption of atomic hydrogen and dissociation of the hydrogen molecule on the W-hcp Pt_ML/WC(0 0 0 1) was investigated and compared to that on Pt(1 1 1). It is found that the most favorable H₂ dissociation channels need similar activation energies of 5.28 and 4.93 kJ/mol on Pt_ML/WC(0 0 0 1) and Pt(1 1 1), respectively, with the release of considerable reaction energies. Furthermore, adsorption of CO on the W-hcp Pt_ML/WC(0 0 0 1) and Pt(1 1 1) was also investigated. The results indicate that Pt_ML/WC(0 0 0 1) is much less susceptible to CO poisoning than Pt(1 1 1), especially at the low coverage of CO.     

Adsorption and dissociation of H2 on the Cu2O(1 1 1) surface: A density functional theory study

Interactions of atomic and molecular hydrogen with perfect and deficient Cu₂O(1 1 1) surfaces have been investigated by density functional theory. Different kinds of possible modes of H and H₂ adsorbed on the Cu₂O(1 1 1) surface and possible dissociation pathways were examined. The calculated results indicate that O_SUF, Cu_CUS and O_vacancy sites are the adsorption active centers for H adsorbed on the Cu₂O(1 1 1) surface, and for H₂ adsorption over perfect surface, Cu_CUS site is the most advantageous position with the side-on type of H₂. For H₂ adsorption over deficient surface, two adsorption models of H₂, H₂ adsorbing perpendicularly over O_vacancy site and H₂ lying flatly over singly-coordinate Cu-Cu short bridge, are typical of non-energy-barrier dissociative adsorption leading to one atomic H completely inserted into the crystal lattice and the other bounded to Cu_CUS atom, suggesting that the dissociative adsorption of H₂ is the main dissociation pathway of H₂ on the Cu₂O(1 1 1) surface. Our calculation result is consistent with that of the experimental observation. Therefore, Cu₂O(1 1 1) surface with oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of H₂.     

Room temperature growth and properties of ZnO films by pulsed laser deposition

ZnO thin films were prepared by pulsed laser deposition at room temperature on glass substrates with oxygen pressures of 10-30 Pa. The structural, electrical, and optical properties of ZnO films were studied in detail. ZnO films had an acceptable crystal quality with high c-axis orientation and smooth surface. The resistivity was in the 10² Ω cm order for ZnO films, with the electron concentration of 10¹⁶-10¹⁷ cm⁻³. All the films showed a high visible transmittance ∼90% and a high UV absorption about 90-100%. The UV emission ∼390 nm was observed in the photoluminescence spectra. The oxygen pressures in the 10-30 Pa range were suitable for room temperature growth of high-quality ZnO films.     

Adsorption of oxygen molecular on pristine and defected SiC nanotubes

The structural and electronic properties of oxygen molecular adsorbed on the exterior surface of pristine and N_C or B_C defected (10,0) or (6,6) SiCNT have been investigated systematically using the first-principles projector-augmented wave potential within the density-functional theory under the generalized-gradient approximation. We find that for both pristine tubes the preferred adsorption sites of the O₂ molecule are above and nearly parallel to armchair Si-C bond whether physisorption or chemisorption. The strong chemical interaction between O₂ molecule and tube leads to not only a vanishing in magnetism of the O₂ molecule but also an outward relaxation of the underlying Si-C bond. The C atom substituted by N or B atom assists O₂ molecule adsorption above and nearly parallel to zigzag Si-N or Si-B bond as well as imparts a metallic character on the SiCNTs with higher concentration of the defects or a magnetism on the SiCNTs with lower concentration of the defects. Therefore, a combination of N or B doping followed by exposure to air may be an effective way to tune the electronic properties of the semiconducting SiCNTs. Furthermore, the lower binding energies for the pair of oxygen interstitials chemisorbed on N_C or B_C defected (10,0) or (6,6) SiCNT show that the oxygen molecule will dissociate to the pair of oxygen interstitials at the sidewall of N_C or B_C defected SiCNTs.     

Hydrogen Production by Chemical Looping Gasification of Corn Stalk Driven by a tert-Butanol Solution

The purpose of this work is to convert organic wastewater into oxidants(H₂O and CO₂) to promote biomass gasification during the chemical looping process to achieve high-H₂/CO-ratio syngas. A tert-butanol solution was selected as the model organic wastewater to generate enough H₂O and CO₂ to promote corn stalk chemical looping gasification(CLG). A series of CLG experiments was conducted at 850℃ under various degrees of oxygen excess(Ω). An Ω of approximately 0.9 led to the highest hydrogen yield and fixed carbon conversion compared with the other cases. Chemometrics and thermodynamic analysis further validated the possibility of corn stalk CLG using a tert-butanol solution. The results show that CLG of biomass-organic wastewater can both treat organic waste and promote chemical looping processes.     

Cobalt‐based Catalysts Modified Cathode for Enhancing Bioelectricity Generation and Wastewater Treatment in Air‐breathing Cathode Microbial Fuel Cells

To seek an efficient way to enhance the power output and wastewater treatment of microbial fuel cell (MFC), several cobalt‐based composites are successfully synthesized by a facile hydrothermal method under different pyrolysis temperature, and these composites are used as electrocatalyst in air‐breathing cathode of MFC. Different species of nitrogen atom are successfully grafted on the cobalt‐based composites and confirmed by physical and electrochemical analyses. In MFC tests, the maximum power density increases from 577.8 mW m^?2 to 931.1 mW m^?2 with pyrolysis temperature (except for 1000 °C). These electrochemical tests and high COD removal show that Co/N/C‐900 can rapidly transfer electron via a 2×2 e^? transfer pathway, mainly due to the exposure of large electrochemical active area and introduction of the defects of pyridinic?N and abundant oxygen vacancies. Although the power density of MFC with Co/N/C‐900 is 81.1 % of that of commercial Pt/C, the MFC with Co/N/C‐900 is more stable than that of Pt/C, and the power density for Co/N/C‐900 has only a 2.8 % decrease during 25‐cycles operation. The great electrocatalytic activity of the novel Co/N/C‐900 composite exhibits a superior outlook for scale‐up application of MFC in the future.     

Electrochemical Reduction and Oxidation of Six Natural 2′‐Deoxynucleosides at a Pyrolytic Graphite Electrode in the Presence or Absence of Ambient Oxygen

Recently we have demonstrated that pyrolytic graphite electrodes (PGE) offer a broad analytically useful potential window, ranging from about ?2.0 V to +1.6 V vs. Ag|AgCl|3 M KCl, which enables to use the PGE not only for anodic measurements, but also for direct reduction of nucleobases in DNA oligonucleotides. In this follow‐up study, we have focused on the electrochemical behavior of four 2′‐deoxynucleosides derived from adenine, guanine, cytosine, thymine, uracil, and 5‐methyl cytosine on the PGE. On one hand we have obtained analogous primary redox responses as previously with the oligonucleotides. On the other hand, significant differences were observed, particularly when considering secondary responses involving products of the primary conversions, suggesting involvement of different mechanisms. Further we have found that presence of the ambient oxygen in the electrolyte does not dramatically affect the redox signals of the nucleosides. This finding is in contrast with DNA responses measured at the mercury‐based electrodes, where deaeration prior to the measurements was necessary. We demonstrate that all studied nucleosides can be analyzed using a simple ex situ (medium exchange) procedure.     

Ion-Pair Dissociations of BrCN by Electron Impacts

Ion-pair dissociation is an important molecular process and frequently happens when the target molecule is pumped to its electronically superexcited states. In contrast to the experimental studies of photoexcitation ion-pair dissociation, there are some experimental challenges in the electron-impact ion-pair dissociation study, in particular, on determination of the energetic threshold. Here we report an experimental development for the ion-pair dissociation study by using the monochromized electron impacts. As an example, the threshold of BrCN→Br^?+CN⁺ is determined as 13.78 eV according to the appearance energy of CN⁺ signals, meanwhile, the time-sliced ion velocity image of CN⁺ is recorded at 16.09 eV and indicates an anisotropic distribution of the CN⁺ momentum.     

氧化镧表面反应性对甲烷和氧分子活化的影响

以氧化镧催化剂在甲烷氧化偶联(OCM)反应中的结构敏感性实验研究为基础, 采用周期性密度泛函理论(DFT)计算研究氧化镧(001), (110)和(100)3个晶面及OCM反应物分子甲烷和氧在其上的吸附、 活化和解离. 结果表明, 氧化镧(001), (110)和(100)3个晶面的表面能大小顺序为(110)>(100)>(001), 3个晶面的价带和导带间隙大小顺序为(110)<(100)<(001), 即(001)是3个晶面中最稳定的晶面, 而(110)则是最活泼的晶面. 甲烷分子在氧化镧(001), (110)和(100)晶面上的吸附很弱(0.03 eV), H—CH₃解离吸附能分别为2.16, 0.68和0.90 eV, 解离反应的难易性与晶面的活性顺序一致; 而氧分子在氧化镧(001), (110)和(100)晶面上的分子吸附能分别为-0.04, -0.31和-0.12 eV, 解离吸附能分别为1.22, 0.53和1.52 eV, 即氧化镧晶面结构对氧分子吸附具有明显的影响, 其中, (001)晶面上吸附最弱, (110)晶面上吸附最强, 以致O—O在(110)晶面上可以较低能垒(0.53 eV)解离, 形成亲电的过氧物种. 由于氧分子在氧化镧表面的吸附较甲烷分子强, 因此, 氧化镧在OCM反应中结构敏感性应与氧分子的吸附和活化密切相关. 甲烷和氧分子在氧化镧表面上活化的本质源自于电子自表面流向甲烷和氧分子的反键轨道, 且表面结构的改变会导致不同强度的电子流动驱动.     

氧还原反应前沿与进展

主要介绍氧还原反应的研究意义、反应机理以及研究现状。氧还原反应作为燃料电池的阴极反应,其能否高效进行将直接影响燃料电池的转化效率。目前,氧还原反应的反应机制仍存在较大争议,包括活性位点及反应步骤等。商业碳载铂虽然活性很高,然而其在实际应用中却会受到多方面限制。本文着重介绍了近些年报道的非金属及非贵金属催化剂。非金属及非贵金属催化剂在自然界中资源丰富、价格低廉、制备简单、导电性及稳定性良好,且不会被小分子毒化。所以,对非金属及非贵金属材料的氧还原研究可为新型能源装置的应用提供参考。