A theoretical study of the ability of 2D monolayer Au (111) to activate gas molecules

The adsorption and activation of gas molecules are investigated substantially in solid-gas heterogeneous catalysis. Here we investigated the interaction between gas molecules and unique two-dimensional monolayer Au (111) structure using density functional theory. It is found that CO₂, H₂O, N₂ and CH₄ molecules are weakly adsorbed on the surface with the adsorption energies between ?0.150 and ?0.250 eV due to van der Waals interaction. While CO, NO, NO₂, and NH₃ molecules are adsorbed more stably with the adsorption energies between ?0.300 and ?0.470 eV. Especially, the bond length of CO is stretched by 0.038 Å and the bond angle of NO₂ is obviously enlarged by 10.460°. The activation originates from the rearrangement of molecule orbitals and the orbitals hybridization between the partial orbitals of gas molecules and Au-5d orbitals. The fundamental analyses of adsorption mechanism and electronic properties may provide guidance for the applications of two-dimensional monolayer metal catalysis.PACSnumbers 73.22.-f, 73.61.-r     

A DFT study on the hydrogen storage performance of MoS2 monolayers doped with group 8B transition metals

The adsorption of hydrogen (H₂) molecules on MoS₂ monolayers doped with Fe, Co, Ni, Ru, Rh, Pd, Os, Ir or Pt was calculated via first-principle density functional theory (DFT). The H₂ was found to interact most strongly with the MoS₂ doped with Os with a higher adsorption energy of ?1.103 eV. Investigations of the adsorptions of two to five H₂ molecules on Os-doped MoS₂ monolayers indicate that there are at most four H₂ interacting stably with the substrate with a promising average adsorption energy of ?0.792 eV. Molecular dynamics simulations also confirmed that the four H₂ molecules can still be reasonably adsorbed and stored on the Os-doped MoS₂ monolayer with a comparable average adsorption energy of ?0.713 eV at 300 K. This study indicates that MoS₂ monolayer doped with Os is a promising substrate to interact strongly with H₂ and can be applied to effectively store H₂ at room temperature.     

Hydrogen evolution and oxygen evolution reactions of pristine and alkali metal doped SnSe2 monolayer

Many transition metal di-selenides such as MoSe₂ and WSe₂ show good catalytic activity on their edges with limited active orientations. These metal di-selenides are actively being used as target material for increasing the number of electrocatalytic active sites and in turn to improve the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities by increasing the ratio of edges to the basal plane. In present work, we have studied the activity of pristine and alkali atoms (Na, K and Ca) doped-SnSe₂ for HER and OER catalyst. The state-of-art density functional theory (DFT) based computations are performed for estimating the catalytic activity of the pristine and doped SnSe₂ by means of evaluating the adsorption and Gibbs free energies subjected to hydrogen and oxygen adsorption. Further, to get better prediction of adsorption energy on the individual catalytic surface, we have included the dispersion correction term to exchange-correlation functional. Results show that the pristine SnSe₂ is not a good HER catalyst when hydrogen is adsorbed on its basal plane. However, edge-sites show the good hydrogen adsorption and indicates that the edges of SnSe₂ are the most preferential site for hydrogen adsorption. As far as the catalytic activity of SnSe₂ with dopants is concerned, the Na-doped SnSe₂ among all shows the best catalytic activity over its edge-site; whereas K and Ca doped SnSe₂ show basal plane as preferred catalytic site. It is interesting to note that the disadvantage of low catalytic activity on basal plane of SnSe₂ can be improved by selective doping of alkali metals.     

Complex interaction of hydrogen with the monolayer TiS2 decorated with Li and Li2O clusters: an ab initio random structure searching approach

We have applied ab initio random structure searching to study the structure, stability and hydrogen storage properties of monolayer TiS₂ coated with Li and small Li₂O clusters. For the low Li covered system we found a complex adsorption mechanism: some hydrogen molecules were adsorbed due to polarization with Li, others due to polarization with S near the surface of TiS₂. The peculiarities of the interaction of the H₂ molecules with each other and the preferred adsorption sites allowed us to formulate a series of recommendations that can be useful when selecting the material for the most effective support. Moreover, the findings also show that the storage capacity of this system can reach up to 9.63 wt%, presenting a good potential as hydrogen storage material. As for the Li₂O clusters supported on TiS₂, we found that the polarization of the Li–O bond increases upon the adsorption of the Li₂O nanocluster. Moreover, the polarized Li–S bonds appear in addition to the already existing Li–O bonds. All this is possible due to the extraction of 1.46 electrons from the S atom of the substrate by O atom of the cluster, and should contribute to an increase in both the adsorption energy and the maximum capacity. The adsorption energies of H₂ for the systems studied here are within 0.11–0.16 eV/H₂ which is a recommended range for reversible hydrogen physisorption under standard test conditions. This study may stimulate experimental efforts to check the claims of high-capacity, stable and reversible hydrogen adsorption reported here.     

Computational insights of alkali metal (Li / Na / K) atom decorated buckled bismuthene for hydrogen storage

The mechanism of hydrogen molecule adsorption on 2D buckled bismuthene (b-Bi) monolayer decorated with alkali metal atoms was studied using density functional theory based first principles calculations. The decorated atoms Li, Na and K exhibited distribution on surface of b-Bi monolayer with increasing binding energy of 2.6 eV, 2.9 eV and 3.6 eV respectively. The adsorption of H₂ molecule on the slabs appeared stable which was further improved upon inclusion of van der Waals interactions. The adsorption behaviour of H₂ molecules on the decorated slabs is physisorption whereas the slabs were able to bind up to five H₂ molecules. The average adsorption energy per H₂ molecules are in range of 0.1–0.2 eV which is good for practical applications. The molecular dynamics simulation also confirmed the thermodynamic stabilities of five H₂ molecules adsorbed on the decorated slabs. The storage capacity values are found 2.24 wt %, 2.1 wt %, and 2 wt %, for respective cases of Li, Na and K atoms decorated b-Bi. The analysis of the adsorbed cases pointed to electrostatic interaction of Li and H₂ molecule. The adsorption energies, binding energies, charge analysis, structural stability, density of states, and hydrogen adsorption percentage specifies that the decorated b-Bi may serve as an efficient hydrogen storage material and could be an effective medium to interact with hydrogen molecules at room temperature.     

Hydrogen adsorption and storage behaviors of Li-decorated PdS2 monolayer: An extended tight-binding study based on DFT

The hydrogen adsorption and storage of the lithium-decorated PdS₂ monolayer at nano-size has been investigated by using extended tight-binding (GFN1-xTB) based on density functional theory (DFT). The calculation results demonstrate that the average adsorption energies of 1–5H₂ change in 0.47–0.20 eV/H₂ range which decreases with increasing of adsorbed hydrogen molecule number. The gravimetric density for hydrogen storage adsorption with 12Li atom and 60H₂ molecules of Li-decorated PdS₂ monolayer is about 6.98 wt% considered as possible application in hydrogen storage. The examination of the hydrogen store mechanism between the monolayer and hydrogen molecules is presented by polarization between Pd and H₂, which can be effect on the adsorption behavior.     

The competitive adsorption behavior of CO and H2 molecules on FeO surface in the reduction process

The competitive adsorption behavior of CO and H₂ molecules on FeO surface in the reduction process was investigated by TG (thermogravimetry) experiments and MD (molecular dynamics) simulation. The model based on the Langmuir adsorption equations was built to describe the adsorption ability of CO and H₂ molecules on the FeO surface. The results show that: oxygen atoms cooperate with iron atoms to have influences on the adsorption ability of CO molecules. When the Fe mole fraction is higher than the critical value of 0.57, the governed factor of carbon monoxide's adsorption is converted from oxygen atoms to iron atoms, but the hydrogen's adsorption is only governed by oxygen atoms on the FeO surface. The experimental results is offered by the explanations that there exist a attractive force between carbon atoms and iron atoms and carbon monoxide could be spontaneously adsorbed on the iron shell of FeO surface while the adsorption of hydrogen on the iron shell is unstable. The high temperature will weaken strength of carbon monoxide's adsorption because of a negative effect on the adsorption energy.     

Photocatalytic decomposition of formic acid and methyl formate on TiO2 doped with N and promoted with Au. Production of H2

The photo-induced vapor-phase decompositions of formic acid and methyl formate were investigated on pure, N-doped and Au-promoted TiO₂. Infrared (IR) spectroscopic studies revealed that illumination initiated the decomposition of adsorbed formate formed in the dissociation of formic acid and located mainly on TiO₂. The photocatalytic decompositions of formic acid and methyl formate vapor on pure TiO₂ occurred to only a limited extent. The deposition of Au on pure or doped TiO₂ markedly enhanced the extent of photocatalytic decomposition of formic acid. The main process was dehydrogenation to give H₂ and CO₂. The formation of CO occurred to only a very small extent. Addition of O₂ or H₂O to the formic acid decreased the CO level from ∼0.8% to ∼0.088%. Similar features were experienced in the photocatalytic decomposition of methyl formate, which dissociated in part to give surface formate. Experiments over Au deposited on N-doped TiO₂ revealed that the photo-induced decomposition of both compounds occurs even in visible light.     

Alkali and transition metal atom-functionalized germanene for hydrogen storage: A DFT investigation

In this work, we have performed density functional theory-based calculations to study the adsorption of H₂ molecules on germanene decorated with alkali atoms (AM) and transition metal atoms (TM). The cohesive energy indicates that interaction between AM (TM) atoms and germanene is strong. The values of the adsorption energies of H₂ molecules on the AM or TM atoms are in the range physisorption. The K-decorated germanene has the largest storage capacity, being able to bind up to six H₂ molecules, whereas the Au and Na atoms adsorbed five and four H₂ molecules, respectively. Li and Ag atoms can bind a maximum of three H₂ molecules, while Cu-decorated germanene only adsorbed one H₂ molecule. Formation energies show that all the studied cases of H₂ molecules adsorbed on AM and TM atom-decorated germanene are energetically favorable. These results indicate that decorated germanene can serve as a hydrogen storage system.     

Correlation between CO surface coverage and selectivity/kinetics for the preferential CO oxidation over Pt/γ-Al2O3 and Au/α-Fe2O3: an in-situ DRIFTS study

We present in-situ IR (DRIFTS) measurements on CO adsorption and preferential CO oxidation (PROX) in H₂-rich gas on Pt/γ-Al₂O₃ and Au/α-Fe₂O₃ catalysts at their envisaged operating temperatures of 200°C and 80°C, respectively, which in combination with kinetic data show that the underlying reason for the very different PROX reaction kinetics on these two catalysts is the difference in steady-state CO coverage. Whereas on the platinum catalyst this is always near saturation under reaction conditions, causing a negative reaction order (−0.4) and a p_CO-independent selectivity, the amount of adsorbed CO on the gold particles (indicated by an IR band at ∼2110 cm⁻¹) strongly depends on the CO partial pressure. From the position of the IR band of CO adsorbed on Au/α-Fe₂O₃, the steady-state coverages on the Au surface are shown to be significantly below saturation, with an upper limit of approximately θ_CO=0.2. Low reactant surface concentrations on Au explain the positive reaction order with respect to p_CO (+0.55 at 80°C) as well as the observed decoupling of the CO and H₂ oxidation rates, which results in a loss of selectivity with decreasing p_CO.     

Comparison between hydrogen production via H2S and H2O splitting on transition metal-doped TiO2 (101) surfaces as potential photoelectrodes

Doping is an effective way to engineer the electronic band structure of semiconductor materials and consequently their photocatalytic activity for hydrogen generation. In this work, periodic Density Functional Theory (DFT) was employed to compare the adsorption of H₂S and H₂O molecules on TiO₂(101) anatase surfaces compared to four transition metal-doped TiO₂(101) anatase surfaces; Cr⁴⁺-TiO₂, V⁴⁺-TiO₂, Mn⁴⁺-TiO₂, and Nb⁴⁺-TiO₂. The defect formation energy, molecular adsorption energy, hydrogen splitting energies, geometrical changes, electronic structure and charge transfer characteristics were investigated to determine and compare the changes in adsorption of H₂S and H₂O on the pristine vs. doped surfaces. The defect formation energy calculations revealed the Nb⁴⁺-TiO₂ surface resulted in the highest stability, smallest change in neighboring bond lengths and the highest dopant to surface charge transfer. However, upon H₂S and H₂O adsorption, the calculations concluded that the V⁴⁺-TiO₂ surface resulted in the most stable structure for adsorbed H₂S and lowest hydrogen splitting energy requiment compared to the other dopant metals and the lowest for H₂S vs H₂O, indicating its potential catalytic activity for facile dehydrogenation for industrial applications.     

NO mechanisms of syngas MILD combustion diluted with N2, CO2, and H2O

The NO mechanism under the moderate or intense low-oxygen dilution (MILD) combustion of syngas has not been systematically examined. This paper investigates the NO mechanism in the syngas MILD regime under the dilution of N₂, CO₂, and H₂O through counterflow combustion simulation. The syngas reaction mechanism and the counterflow combustion simulation are comprehensively validated under different CO/H₂ ratios and strain rates. The effects of oxygen volume fraction, CO/H₂ ratio, pressure, strain rate, and dilution atmosphere are systematically investigated. For all the MILD cases, the contribution of the prompt and NO-reburning routes to the overall NO emission is less than 0.1% due to the lack of CH₄ in fuel. At atmospheric pressure, the thermal route only accounts for less than 20% of the total NO emission because of the low reaction temperature. Moreover, at atmospheric pressure, the contribution of the NNH route to NO emission is always larger than 55% in the N₂ atmosphere. The N₂O-intermediate route is enhanced in CO₂ and H₂O atmospheres due to the increased third-body effects of CO₂ and H₂O through the reaction N₂ + O (+M) ? N₂O (+M). Especially in the H₂O atmosphere, the N₂O-intermediate route contributes to 60% NO at most. NO production is reduced with increasing CO/H₂ ratio or pressure, mainly due to decreased NO formation from the NNH route. Importantly, a high reaction temperature and low NO emission are simultaneously achieved at high pressure. To minimize NO emission, the reactions should be operated at high values of CO/H₂ ratios (i.e., >4) and pressures (e.g., P > 10 atm), low oxygen volume fractions (e.g., X_O2 < 15%), and using H₂O as a diluent. This study provides a new fundamental understanding of the NO mechanism of syngas MILD combustion in N₂, CO₂, and H₂O atmospheres.     

Li-decorated B2O as potential candidates for hydrogen storage: A DFT simulations study

Two-dimensional (2D) B₂O monolayer is considered as a potential hydrogen storage material owing to its lower mass density and high surface-to-volume ratio. The binding between H₂ molecules and B₂O monolayer proceeds through physisorption and the interaction is very weak, it is important to improve it through appropriate materials design. In this work, based on density functional theory (DFT) calculations, we have investigated the hydrogen storage properties of Lithium (Li) functionalized B₂O monolayer. The B₂O monolayer decorated by Li atoms can effectively improve the hydrogen storage capacity. It is found that each Li atom on B₂O monolayer can adsorb up to four H₂ molecules with a desirable average adsorption energy (E_ave) of 0.18 eV/H₂. In the case of fully loaded, forming B₃₂O₁₆Li₉H₇₂ compound, the hydrogen storage density is up to 9.8 wt%. Additionally, ab initio molecular dynamics (AIMD) calculations results show that Li-decorated B₂O monolayer has good reversible adsorption performance for H₂ molecules. Furthermore, the Bader charge and density of states (DOS) analysis demonstrate H₂ molecules are physically absorbed on the Li atoms via the electrostatic interactions. This study suggests that Li-decorated B₂O monolayer can be a promising hydrogen storage material.     

O-assisted and pristine Au-Pt(100) surfaces: A platform for adsorption and decomposition of H2O

Adsorption and decomposition of water (H₂O) over pristine and oxygen (O) assisted Au-Pt(100) surfaces were systematically explored using ab initio calculations based on density functional theory (DFT). To consider the long-range interaction, semi-empirical dispersion correction (D2) was included in all calculations. The most preferable adsorption sites for H₂O and its fragments such as OH, O, and H, over clean and O-assisted Au-Pt(100) surfaces were determined by examining adsorption energy with different configurations. Our calculations showed that the H₂O prefers top site over Au atom while OH, O, and H prefer to be adsorbed over bridge position. In present study, we determine the best possible co-adsorption sets for considered adsorbates. We further investigated the transition states, dehydrogenation process, and activation energies for extracting H from adsorbed H₂O over both pristine and O-aided Au–Pt(100) surfaces. It was found that the O promotes H₂O dissociation significantly by diminishing the barrier energy. The decisive role played by O in decomposing H₂O molecule is revealed in this work. Our study will further assist experimentalists in designing and synthesizing novel catalysts for dehydrogenation of H₂O in hydrogen production.     

Hydrogen adsorption on Ce/BNNT systems: A DFT study

The adsorption of H₂ on Ce-doped boron nitride nanotubes (BNNT) is investigated by using density functional theory. For the Ce/BNNT system, it is found that Ce preferentially occupies the hollow site over the BN hexagon. The results indicate that seven H₂ per Ce can be adsorbed and 5.68 wt% H₂ can be stored in Ce₃/BNNT system. Among nanotubes doped with metals, Ce exhibits the most favorable hydrogen adsorption characteristics in terms of the adsorption energy and the uptake capacity. Both hybridization of the Ce-5d orbital with the H-1s orbital and the polarization of the H₂ molecules contribute to the hydrogen adsorption. Ce clustering can be suppressed by preferential binding of Ce atoms on BNNT, which denotes that BNNT as a hydrogen storage substrate is better than CNT due to its heteropolar binding nature.     

Theoretical study of H2 separation performance of two-dimensional graphitic carbon oxide membrane

Membrane technology has been widely used for H₂ separation. In this paper, we theoretically explored the H₂ separation performance of graphitic carbon oxide (g-C₂O) monolayer. The van-der-Waals-corrected density functional theory (DFT) calculations demonstrate that g-C₂O monolayer is chemically inert to the studied gas molecules (H₂, CO₂, CO, N₂, and CH₄), and with a suitable pore size, the g-C₂O monolayer shows an exceptionally high selectivity for H₂/CO₂ (CO, N₂, and CH₄) in a wide range of temperatures. In addition, the molecular dynamics (MD) simulations yield a high H₂ permeance for the g-C₂O monolayer at room temperature. With excellent selectivity and ultrahigh permeance, the g-C₂O monolayer has great potential application in H₂ separation.     

The hydrogen evolution reaction on single crystal gold electrode

As one of the important candidate of power sources for the future, the research and production of hydrogen gas has a significant importance. In this article, the emphasis is on the influence of impurities on hydrogen evolution reaction, i.e., the influence of an addition of decacyclene, C₁₂H₃₅C₆H₄SO₄Na, CH₃CH₂OH, chromanone, H₂SO₄, HNO₃, 4,4′-biphenediol and 1,2,3,4-tetraphenyl-1,3-cyclopentadiene was studied by electrochemical impedance technique. The adsorption structure for some organics was measured by scanning tunneling spectroscopy techniques. Superstructure of adsorbed decacyclene on Au(111) surface was captured. The ordered adsorption structure of 4,4′-biphenyldiol on Au(111) and (100) was also observed. The addition of decacyclene has shown an opposite effects on hydrogen evolution for Au(111) and (100) surface, i.e., it inhibits the reaction at Au(100) but enhances the one at Au(111). The results show that the addition of C₁₂H₃₅C₆H₄SO₄Na and HNO₃, especially the latter, can improve the hydrogen evolution. In the article the adsorption structure and hydrogen evolution reaction have been studied in order to give some useful information about the relation between the adsorption structure and the properties. The purpose of this article is to attempt to find the relation between electrochemical performance and the adsorption structure, and to explore the effect of some additives.     

The effect of low concentrations of CO on H2 adsorption and activation on Pt/C: Part 2—In the presence of H2O vapor

CO affects H₂ activation on supported Pt in the catalyst layers of a PEMFC and significantly degrades overall fuel cell performance. This paper establishes a more fundamental understanding of the effect of humidity on CO poisoning of Pt/C at typical fuel cell conditions (80 °C, 2 atm). In this work, direct measurements of hydrogen surface concentration on Pt/C were performed utilizing an H₂-D₂ switch with Ar purge (HDSAP). The presence of water vapor decreased the rate of CO adsorption on Pt, but had very little effect on the resulting CO surface coverage on Pt_S (θ_CO) at steady-state. The steady-state θ_COs at 80 °C for Pt exposed to H₂ (PH₂=1 atm) and a mixture of H₂/H₂O (1 atm H₂, 10%RH) were 0.70 and 0.66 ML, respectively. Furthermore, total strongly bound surface hydrogen measured after exposure to H₂/H₂O was, surprisingly, the sum of the exchangeable surface hydrogen contributed by each component, even in the presence of CO. In the absence of any evidence for strong chemisorption of H₂O on the carbon support with/without Pt, this additive nature and seemingly lack of interaction from the co-adsorption of H₂ and H₂O on Pt may be explained by the repulsion of strongly adsorbed H₂O to the stepped-terrace interface at high coverages of surface hydrogen.     

The enhanced hydrogen-sensing performance of the Fe-doped MoO3 monolayer: A DFT study

A pure monolayer of orthorhombic MoO₃ and Fe-doped MoO₃ were constructed to study their hydrogen sensing properties through first-principle density functional theory (DFT) calculations. The results show that Fe can be stably doped into the MoO₃ monolayer with a high binding energy of −8.09 eV. Further calculations revealed that pure MoO₃ is insensitive to molecular oxygen or hydrogen. However, oxygen can be chemisorbed onto the doped Fe in the modified MoO₃ with a high adsorption energy of −0.807 eV, capturing approximately 0.2 e from the sensing material. The introduced hydrogen molecules tended to interact strongly with the pre-adsorbed O₂ molecule to form two H₂O, releasing 1.01 e back to the sensing material. There were 1.92 e released back to the MoO₃ doped with two Fe atoms during the sensing process which significantly enhancing the hydrogen sensing performance of the modified material. Our study indicates that doping MoO₃ with Fe atoms improved its hydrogen sensing performance and is a reasonable way to design effective gas sensing materials.     

The promoted effect of UV irradiation on preferential oxidation of CO in an H2-rich stream over Au/TiO2

The catalytic oxidation of CO over Au/TiO₂ in an H₂-rich stream was performed under UV irradiation. It is found that UV irradiation over Au/TiO₂ promotes the preferential oxidation of CO in the H₂-rich stream. The respective chemisorption of CO, H₂ and O₂ at Au/TiO₂ can be described as a process of forming –OH or H₂O species. UV irradiation over Au/TiO₂ enhances the chemisorption of CO but suppresses the chemisorption of H₂ both at TiO₂ and Au surface. It is proposed that the photogenerated electrons from TiO₂ will cause the change of the chemisorption of CO, H₂ and O₂ at Au/TiO₂, which promotes the preferential oxidation of CO in an H₂-rich stream.