Strain facilitated small gas molecules sensing properties on Au doped SnSe2 monolayer

By the first-principles calculations, the sensitivity of CO, H₂O and NO adsorption on Au doped SnSe₂ monolayer surface is investigated. The results show that CO and H₂O molecules are physically adsorbed on Au doped SnSe₂ monolayer and act as donors to transfer 0.012 e and 0.044 e to the substrate, respectively. However, the NO molecule is chemically adsorbed on substrate and acts as an acceptor to obtain 0.116 e from the substrate. In addition, our results also show that the biaxial strain can effectively improve the adsorption energy and charge transfer of gas molecules adsorbed on the substrate surface. Also, the recovery time of desorbed gas molecules on the substrate surface is calculated, and the results indicate that the Au doped SnSe₂ is a perfect sensing material for detection and recovery of CO and NO under ?8% strain.     

The catalytic effect of the Au(111) and Pt(111) surfaces to the sodium borohydride hydrolysis reaction mechanism: A DFT study

In this research, hydrolysis mechanism of sodium borohydride (NaBH₄) have been studied theoretically on Au (111) and Pt (111) noble metal surfaces by periodic density functional theory calculations. Elementary reaction steps have been generated based on study of borohydride oxidation. Reaction intermediates which have plethora of hydroxyl (OH) radical(s) have been produced by decomposition of water molecule(s). In order to investigate surface effect, we have followed two different routes. The first route is that the atomic and molecular structures in the reaction steps have been optimized in 3-d box without a catalyst. At second one, they were interacted with the Au (111) and Pt (111) surfaces to compare relative behavior with reference to the non-catalytic medium. The relative energy diagrams were produced by relative energy differences which is useful to generate energy landscape using required/released energies in order to pursue the reaction. Three main peaks that means considerable energy changes have been observed to proceed the reaction in the non-catalytic medium. Then, changes in the energy differences depending on surfaces have been discussed. Although acquired relative energies are not within chemical accuracy, they are very successful to show the affect of the OH radical concentration to the potential energy diagram. Pt (111) surface have been found more reactive than Au (111) surface for Sodium Borohydride Hydrolysis reaction, as it is obviously coherent with the literature.     

Interaction of H2S with perfect and S-covered Ni(110) surface: A first-principles study

First-Principles study based on Density functional theory (DFT) calculations are employed to investigate the dissociative mechanism of H₂S adsorption and its dissociation on perfect, and sulfur covered Ni(110) surface. On both surfaces, we probe the site preference for H₂S, HS, H, and S adsorption mechanisms. The results indicate that H₂S is energetically adsorbed on their high symmetry adsorption sites with the preferred short-bridge (SB) site on both surfaces. Furthermore, we found that chemisorption of HS is stronger in contrast to H₂S at favorable short-bridge (SB) with a binding energy of −3.59 eV on perfect Ni(110) surface, and on S-covered Ni(110) surface at the favorable hollow site having a binding energy of −3.57 eV. In the first H₂S dehydrogenation, energy barriers for S–H bond breaking over the clean surface are 0.08–0.46 eV and a little bit higher on the S-covered surface are 0.1–0.78 eV, while in second dehydrogenation the energy barrier on a clean surface is 0.19 eV. For further detail, electronic densities of states and d-band center model are used to characterize the interaction of adsorbed H₂S with both surfaces. Hence, our results show that decomposition of H₂S over perfect and S-covered Ni(110) surface is exothermic and also an easy process. However, kinetically and thermodynamically, the subsistence of surface sulfur avoids the H–S bond breaking process.     

Electrocatalytic oxidation of sodium borohydride on metal ad-atom modified Au(111) single crystal electrodes in alkaline solution

The electrochemical oxidation of borohydride was investigated by using various ad-atom modified Au(111) electrodes in alkaline media in comparison to Au(111) single crystal, polycrystalline Au, Pt and Zn electrodes. The catalytic activity of gold towards borohydride oxidation has tended to increase in more alkaline media as reflected in the oxidation peak in the concentration range of NaOH (0.01-2.0 M) studied. Additional shift on the oxidation peak potential of borohydride on Pt and Zn ad-atom modified Au(111) electrodes was observed for both ad-atom modified electrodes to more negative potentials compared to that of bare electrodes, respectively. The ad-atom modified Au(111) electrodes surfaces do not only provide a superior electrical contact, but also accelerates electron transfer as proven by the increase in peak current and positive shift in the peak potential.     

New insights into the interfacial interactions of O2 and H2O molecules with PuH2 (110) and (111) surfaces from first-principles calculations

The interfacial reaction of highly active plutonium hydride in humid circumstance is of great interest in nuclear safe handling and storage, but it is poorly understood so far. In this paper, we first studied the O₂ adsorption on (110) and (111) surfaces of PuH₂ by first-principles DFT + U method. The results show that there are dissociative and non-dissociative adsorption of oxygen on the surfaces. We analyze the vibrational frequencies of non-dissociative oxygen adsorbed on the surfaces. It is found that the corresponding frequency of oxygen with bond length of 1.330–1.340 Å is 1094.8–1098.2 cm^?1. The corresponding frequency of oxygen with bond length of 1.448–1.500 Å is 726.3–905.2 cm^?1. It shows that non-dissociative oxygen could be considered as superoxide (O₂^?) or peroxide (O₂^2?) species. In order to expound the atomistic evolution process of oxidized surface exposed to moist air or corrosive solution, the interactions between H₂O molecules and the strongest oxygen adsorption structures were further explored. The results indicate that H₂O molecules could dissociate into OH groups and H atoms, then they were captured to create Pu–O and H–O bonds. This work could provide new insights into the adsorption morphology of oxygen on hydride surface and the interaction between oxide/hydride interface and water.     

The effect of Ti atom on hydrogenation of Al(111) surface: First-principles studies

First-principles calculations were performed to investigate hydrogen dissociation and subsequent diffusion over both clean and Ti-doped Al(111) surfaces. The calculations show that it is energetically favorable to dope the surface or subsurface layer of Al(111) with Ti atom. Through calculations on the detailed process associated with hydrogen dissociation and diffusion, we found that Ti doping will decrease the hydrogen dissociation barrier by about 0.6 eV. Additionally, the mobility of hydrogen atoms on surface will be easier if Ti atom is placed in subsurface layer instead of top surface layer. The present results further contribute towards understanding the improved kinetics observed in recycling of hydrogen in Ti-doped NaAlH₄.     

Synthesis of low-cost catalyst NiO(111) for CO2 hydrogenation into short-chain carboxylic acids

Reducing gaseous carbon dioxide to valuable chemicals and fuels by using gaseous hydrogen can decrease the concentration of greenhouse gases that contribute to global warming. Carbon dioxide conversion into fuels such as methane, methanol, and formic acid is a good hydrogen-storage method. In this paper, a comparative study of CO₂ conversion into formic and acetic acids on alumina-supported nickel oxide with and without the presence of carbon is reported. NiO (111) with high surface area was synthesized through a simple and one-pot fusion solid-state method at 550 °C and 700 °C. The synthesized catalysts were tested in carbon dioxide hydrogenation reaction in a batch slurry reactor at 130 °C and under mild pressure. Interestingly, the optimum condition of the reaction also successfully produced C₂ carboxylic acid in significant amounts. The highest levels of formic acid and acetic acid production were 8.13 and 7.63 mmol/L, respectively.     

Insights into the reaction mechanisms of methanol decomposition,methanol oxidation and steam reforming of methanol on Cu(111): A density functional theory study

Cu-based catalysts have been widely used for hydrogen production from methanol decomposition, methanol oxidation and steam reforming of methanol (MSR). In this study, we have systematically identified possible reaction paths for the thermodynamics and dynamics involved in the three reactions on a Cu(111) surface at the molecular level. We find that the reaction paths of the three reactions are the same at the beginning, where methanol scission is favourable involving O–H bond scission followed by sequential dehydrogenation to formaldehyde. Formaldehyde is an important intermediate in the three reactions, where direct dehydrogenation of formaldehyde to CO is favourable for methanol decomposition; for methanol oxidation, formaldehyde tends to react with oxygen to form dioxymethylene through C–H bond breaking and finally the end products are mainly CO₂ and hydrogen; for MSR, formaldehyde tends to react with hydroxyl to form hydroxymethoxy through formic acid and formate formation, followed by dissociation to CO₂. CH₂O formation from methoxy dehydrogenation is considered to be the rate-limiting step for the three reactions. In general, the thermodynamic and kinetic preference of the three reactions shows the order methanol oxidation > MSR > methanol decomposition. Methanol oxidation and MSR are faster than methanol decomposition by about 500 and 85 times at typical catalytic conditions (e.g., 523 K), respectively. The result may be useful for computational design and optimization of Cu-based catalysts.     

Density functional theory study on catalytic dehydrogenation of methylcyclohexane on Pt(111)

Density Functional Theory (DFT) method was used to study the step-by-step dehydrogenation of methylcyclohexane (MCH) to toluene on a Pt(111) surface to understand adsorption properties of the reactants, intermediates and the products involved. The results indicate that dehydrogenation occurs preferentially in the para position. Methylcyclohexane is a saturated molecule and its adsorption on the surface of Pt(111) falls into the category of physical adsorption. 4-methyl-cyclohexene and methyl-cyclohexadiene are the most likely dehydrogenation intermediates. The C–C bond on the six-membered ring has a significant shrinkage after the dehydrogenation reaction. The highest energy barrier of 32.46 kcal/mol is calculated for the first dehydrogenation step, which may potentially be the rate-determining step for the entire reaction network. These are consistent with the experimental results.     

A reversible hydrogen storage material of Li-decorated two-dimensional (2D) C4N monolayer: First principles calculations

Two-dimensional (2D) materials can be regarded as potential hydrogen storage candidates because of their splendid chemical stability and high specific surface area. Recently, a new dumbbell-like carbon nitride (C₄N) monolayer with orbital hybridization of sp³ is reported. Motivated from the above exploration, the hydrogen adsorption properties of Li-decorated C₄N monolayer are comprehensively investigated via first principles calculations based on the density functional theory (DFT). It is found that the Dirac points and Dirac cones exists in the Brillouin zone (BZ) from the calculated electronic structure and indicates the C₄N can be used as an excellent topological material. Also, the calculated phonon spectra demonstrate that the C₄N monolayer owns a strong stability. Moreover, the calculated binding energy of decorated Li atom is bigger than its cohesive energy and results in Li atoms disperse over the surface of C₄N monolayer uniformly without clustering. In addition, the Li₈C₄N complex can capture up to 24H₂ molecules with an optimal hydrogen adsorption energy of −0.281 eV/H₂ and achieves the hydrogen storage density of 8.0 wt%. The ab initio molecular dynamics (AIMD) simulations suggest that the H₂ molecules can be desorbed quickly at 300 K. This study reveals that Li-decorated C₄N monolayer can be served as a promising hydrogen storage material.     

Theoretical investigation of solvent effects on the selective hydrogenation of furfural over Pt(111)

It was well known that solvent effect plays a very important role in the catalytic reaction. There are many theoretical studies on the solvent effect in homogeneous catalysis while there are few theoretical studies on the solvent effect in the heterogeneous catalytic reaction and there has been no work to investigate the solvent effect on furfural transformation in heterogeneous catalysis. In the present work, both the density functional calculations and the microkinetic analysis were performed to study the selective hydrogenation of furfural over Pt(111) in the presence of methanol as well as toluene and compared with that in the gas condition. The present results indicated that the methanol can enhance the adsorption strength of furfural and other oxygen-containing reaction species due to its relatively strong polarity properties and this can be a main reason for solvent-induced high activity and selectivity. Another reason is that reaction paths study showed that the presence of methanol solvent makes the dehydrogenation of furfural less thermochemical due to the fact that furfural is more stabilized than that of dehydrogenation species, and methanol also has an inhibition effect on the dehydrogenation of furfural in the kinetic aspect, and further energetic span theory proves highest activity and selectivity for hydrogenation in methanol solvent of vapor, methanol and toluene. Moreover, microkinetic model simulation demonstrated that the activity and selectivity of hydrogenation in methanol is both higher than that in vapor and toluene. The much higher activity in methanol is due to the stabilized adsorbed reactants in the surface, which leads to a higher surface coverage of furfural. It might be proposed based on the present work that a solvent with relatively strong polarity may be favorable for the high selective hydrogenation of furfural.     

First-principles study of the H2 splitting processes on pure and transition-metal-doped Al (111) surfaces

By performing first-principles calculations, H₂ splitting processes on pure and transition metal (TM) atom substituted Al (111) surfaces were examined. Corrected reaction pathways with splitting energy barriers (0.99 eV) lower than those in previous studies (1.28 eV) were obtained. By further analyzing the H₂ splitting process on the 3d-TM-atom-doped Al (111) surface, the relationship of the catalysis effect and the electron donation-back donation process on TM 3d orbitals were examined in detail. Finally, to confirm the possibility of reducing the partially oxidized Al (111) surface with an H₂ molecule, the surface reduction process was studied by using the climb-image nudged elastic band (CI-NEB) method systematically.     

A UPD-spontaneous redox approach to the preparation of monolayer palladium on reduced graphene oxide-supported gold nanoparticles for ethanol electrooxidation in alkaline media

Monolayer Pd on electrochemical-reduced graphene oxide supported Au nanoparticles (m-Pd–Au/ERG) was designed by a simple and effective route, which produces electrocatalyst with a considerably low Pd loading and good electrocatalytic performance for ethanol oxidation. The as-prepared m-Pd–Au/ERG nanocompostie was characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results show that high-density m-Pd–Au nanoparticles are dispersed on the surface of ERG uniformly. The electrochemical activities of the as-prepared nanocomposites toward ethanol oxidation in alkaline media were investigated by using cyclic voltammetry and chronoamperometric technique. The results demonstrate that the m-Pd–Au/ERG catalyst shows much higher electrocatalytic activity, stronger tolerance to CO than the m-Pd–Au and b-Pd–Au/ERG catalysts for ethanol oxidation.     

Hydrogen adsorption and diffusion on doped Zr(0001) surfaces: A first-principles study

The hydrogen adsorption and diffusion behaviors on the clean and a series of element doped Zr(0001) surfaces are studied through first-principles calculations. Among the studied doping elements, Cu, Co, Y, and Mg prefer to substitute Zr on the topmost surface layer, Al, Pd, Ir, and Si are favored from topmost layer to several surface layers down, while Mo is not favored. Independent of the substitution energies, Mo, Co, and Ir induce a symmetry-breaking local distortion surface structure. Based on the obtained geometries, it is found that most dopants promote the hydrogen adsorption on their next nearest neighbor sites but hinder it on the nearest neighbor sites. Most of the dopants also promote both the hydrogen diffusion on the surface plane and the hydrogen penetration into the subsurface layers. The results indicate that element doping may facilitate the hydride nucleation in Zr alloys.     

First-principles study on the dehydrogenation properties and mechanism of Al-doped Mg2NiH4

Mg₂NiH₄, with fast sorption kinetics, is considered to be a promising hydrogen storage material. However, its hydrogen desorption enthalpy is too high for practical applications. In this paper, first-principles calculations based on density functional theory (DFT) were performed to systematically study the effects of Al doping on dehydrogenation properties of Mg₂NiH₄, and the underlying dehydrogenation mechanism was investigated. The energetic calculations reveal that partial component substitution of Mg by Al results in a stabilization of the alloy Mg₂Ni and a destabilization of the hydride Mg₂NiH₄, which significantly alters the hydrogen desorption enthalpy ΔH_des for the reaction Mg₂NiH₄ → Mg₂Ni + 2H₂. A desirable enthalpy value of ∼0.4 eV/H₂ for application can be obtained for a doping level of x ≥ 0.35 in Mg_2−xAl_xNi alloy. The stability calculations by considering possible decompositions indicate that the Al-doped Mg₂Ni and Mg₂NiH₄ exhibit thermodynamically unstable with respect to phase segregation, which explains well the experimental results that these doped materials are multiphase systems. The dehydrogenation reaction of Al-doped Mg₂NiH₄ is energetically favorable to perform from a metastable hydrogenated state to a multiphase dehydrogenated state composed of Mg₂Ni and Mg₃AlNi₂ as well as NiAl intermetallics. Further analysis of density of states (DOS) suggests the improving of dehydrogenation properties of Al-doped Mg₂NiH₄ can be attributed to the weakened Mg-Ni and Ni-H interactions and the decreasing bonding electrons number below Fermi level. The mechanistic understanding gained from this study can be applied to the selection and optimization of dopants for designing better hydrogen storage materials.     

The study of Au/MoS2 anode catalyst for solid oxide fuel cell (SOFC) using H2S-containing syngas fuel

Au/MoS₂ is a promising anode catalyst for conversion of all components of H₂S-containing syngas in solid oxide fuel cell (SOFC). MoS₂-supported nano-Au particles have catalytic activity for conversion of CO when syngas is used as fuel in SOFC systems, thus preventing poisoning of MoS₂ active sites by CO. In contrast to use of MoS₂ as anode catalyst, performance of Au/MoS₂ anode catalyst improves when CO is present in the feed. Current density over 600 mA cm⁻² and maximum power density over 70 mW cm⁻² were obtained at 900 °C, showing that Au/MoS₂ could be potentially used as sulfur-tolerant catalyst in fuel cell applications.     

Structural,elastic, phononic,optical and electronic properties investigation of two-dimensional XIS (X=Al,Ga, In) for photocatalytic water splitting

Two-dimensional (2D) materials have been widely developed due to their attractive properties. Here, by using density functional theory (DFT) calculations, for the first time, we explore potential applications of the novel XIS (X = Al, Ga, In) monolayer 2D materials on photocatalytic water splitting. A series of simulations were carried out to predict and study the structural, elastic, phononic, optical and electronic properties of 2D XIS materials. The results show that GaIS and InIS demonstrate low thermal conductivity. For optical properties, AlIS shows strong light absorption coefficients and refractive index only under ultraviolet (UV) light, while GaIS and InIS show stronger performance under both visible light and UV light with the band edge positions spanned the redox potential of water. The reasonable band positions and bandgaps make them promising photocatalysts for water splitting. This work reveals the potential applications of monolayer 2D XIS in thermal, electronic, and photocatalytic water splitting.     

First principles study of the ZrX2 (X = H,D and T) compounds

The Zr–H system is of particular importance for the solid state storage of hydrogen isotopes in the form of zirconium hydride. Here we report the structural, electronic, vibrational and thermodynamic properties of ZrH₂, ZrD₂ and ZrT₂ using density functional theory (DFT). The structural optimization was carried out by the plane-wave based pseudo-potential method under the generalized gradient approximation (GGA) scheme. The electronic structure of the ZrH₂ compound was illustrated explicitly. The vibrational, thermodynamic, and the effect of isotopes on ZrX₂ (X = H, D, T) compounds were evaluated by the frozen phonon method. Both the Raman and infrared active vibrational modes of ZrX₂ at Γ-point showed significant isotopic effect on ZrX₂ compounds. For example, the phonon energy gap between optical and acoustic modes reduces for ZrT₂ than ZrD₂ and ZrH₂. The formation energies of ZrX₂ compounds, including the ZPE contributions, were estimated to be −143.68, −147.87 and −150.01 kJ/(mole of compound) for X = H, D and T, respectively.     

Effects of gas diffusion layer (GDL) and micro porous layer (MPL) on cathode performance in microbial fuel cells (MFCs)

This study focused on novel cathode structures to increase power generation and organic substrate removal in microbial fuel cells (MFCs). Three types of cathode structures, including two-layer (gas diffusion layer (GDL) and catalyst layer (CL)), three-layer (GDL, micro porous layer (MPL) and CL), and multi-layer (GDL, CL, carbon based layer (CBL) and hydrophobic layers) structures were examined and compared in single-chamber MFCs (SCMFCs). The results showed that the three-layer (3L) cathode structures had lower water loss than other cathodes and had a high power density (501 mW/m²). The MPL in the 3L cathode structure prevented biofilm penetration into the cathode structure, which facilitated the oxygen reduction reaction (ORR) at the cathode. The SCMFCs with the 3L cathodes had a low ohmic resistance (R_ohmic: 26-34 Ω) and a high cathode open circuit potential (OCP: 191 mV). The organic substrate removal efficiency (71-78%) in the SCMFCs with 3L cathodes was higher than the SCMFCs with two-layer and multi-layer cathodes (49-68%). This study demonstrated that inserting the MPL between CL and GDL substantially enhanced the overall electrical conduction, power generation and organic substrate removal in MFCs by reducing water loss and preventing biofilm infiltration into the cathode structure.     

The effect of oxygen vacancies on the structure and electrochemistry of LiTi2(PO4)3 for lithium-ion batteries: A combined experimental and theoretical study

We report that a partially oxygen deficient LiTi₂(PO₄)₃ shows a much better rate capability as a cathode material for lithium-ion batteries compared to stoichiometric LiTi₂(PO₄)₃. A combination of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrochemistry, and first-principles calculations was used to determine and rationalize the structural and electrical changes that occur with different heat treatment atmospheres. XRD and XPS experiments confirmed that some Ti⁴⁺ transformed to Ti³⁺ in oxygen deficient LiTi₂(PO₄)₃ heat treated under N₂; Ti³⁺ was detected and the lattice parameter increased compared to that of LiTi₂(PO₄)₃. Electrical conductivity measurements indicated an increase in the electronic conductivity of nearly two orders of magnitude for the oxygen deficient LiTi₂(PO₄)₃ sample compared to LiTi₂(PO₄)₃. First-principles calculations suggest that the oxygen vacancies could be formed in LiTi₂(PO₄)₃ under oxygen-poor conditions, and this may significantly decrease the donor levels of other possible donor defects and thus improve the electronic mobility.