Decrease In Adsorption Energy Research Articles

Ultra-stable ZnO nanobelts in electrochemical environments

The In-ZnO nanobelts present ultrahigh stability of morphology and electrical properties in the electrochemical environment. It can be attributed to the adsorption energy decrease in the unique lattice structure with doped indium atoms.

Wettability and hydrolytic stability of 3-aminopropylsilane coupling agent and phenol-urea-formaldehyde binder on silicate surfaces and fibers

The stability of phenol-urea-formaldehyde (PUF) binder and 3-aminopropylsilane (APS) on composite silicate materials (fibers and wafers) was studied with surface sensitive techniques (X-ray photoelectron spectroscopy (XPS) and streaming potential) through a wide range of humidity and temperature and ab initio modelling complemented the results. Behavior was compared for wettability properties, determined by vapor adsorption and contact angle analysis. APS and PUF, deposited on the silicate surfaces, decrease surface energy and wettability but water adsorption remains high, facilitating hydrolytic decomposition of the composite material. Deposited APS is unstable at T>50°C and 75% RH, while PUF is less sensitive to high humidity and temperature. Molecular dynamics confirmed APS sensitivity to humidity. Water adsorption and surface energy decrease, and material stability increases when a hydrophobization agent is applied to APS/PUF treated surfaces. The direct correlation between wettability and stability of PUF/APS/fiber composites can contribute in designing new materials with controlled hydrophobic properties.

Adsorption of acetylene on Sn-doped Ni(111) surfaces: a density functional study.

First-principle density functional theory calculations have been performed to investigate the adsorption of C2H2 on Ni(111) and Sn@Ni(111) at different coverages. At low coverage, the C2H2 molecule is strongly adsorbed on Ni(111) and the dissociation of the H atom is not favorable. Furthermore, the more the H atom dissociated, the more unstable the system is. However, the dissociation structure of C2H+H has the largest adsorption energy on Sn@Ni(111), indicating that the dissociation structure is more stable than molecular adsorbed C2H2. At moderate coverage, there is some repulsive interaction between two C2H2 molecules, inducing the decrease in adsorption energy. On Ni(111), the two C2H2 tend to adsorb separately, however, the dimer C4H4 has the largest adsorption energy on Sn@Ni(111). At high coverage, the trimer derivative benzene has the largest adsorption energy on both Ni(111) and Sn@Ni(111) surfaces. The adsorption energies of the formed benzene are very high on the two systems, even larger than those of three individual adsorbed C2H2.

Theoretical study of adsorption mechanism of heavy metals As and Pb on the calcite (104) surface

Heavy metal pollution has attracted much attention because it has long-term serious impact on the environment. Arsenic (As) and lead (Pb) are two kinds of heavy metals that are widely existing and highly toxic. Adsorption method has been proved to be an economical and efficient treatment to remove or reduce their toxicity. The adsorption of As and Pb atoms on the calcite (104) surface has been studied by density functional theory. The coverage dependence of the adsorption energetics and structures were comprehensively discussed at different adsorption sites for a wide range of coverage Θ from 0 to 1.0 monolayers (ML). Among all the adsorption sites, the one-fold top site cannot be adsorbed, and the three-fold hollow site was the most stable, followed by the two-fold bridge site. For the adsorption of As atom, the adsorption energy increased first and then decreased in the coverage range of 0–1.0 ML, indicating that the interaction between As atoms leaded to the decrease of adsorption energy. However, the adsorption energy of adsorbed Pb atom increased with the increase of coverage, implying that the Pb atoms tended to form clusters on calcite surface, thus improving adsorption stability. The calculation results showed that calcite minerals could be used to remove heavy metal atoms, which had high removal ability to As and Pb atoms. Besides, the lattice relaxation, different charge distribution and electron density of states for As or Pb / calcite (104) system were also investigated in depth.

Toward the design of efficient adsorbents for Hg2+ removal: Molecular and thermodynamic insights

AbstractA systematic DFT study was performed to evaluate the effect of oxygenated functional groups for Hg2+ adsorption in aqueous systems. This work includes several aspects usually neglected in many current works, namely, ground‐state multiplicity, solvation effects, establishment of thermodynamic parameters, atomic charge transfer, and modeling of infrared spectra. In addition, two carbonaceous models were studied to account for both the effect of the carbonaceous matrix and the oxygenated functional groups on the Hg2+ binding. Adsorption energies indicated that Hg2+ adsorption on the unsaturated model is favored in the following order: phenol > lactone > semiquinone > carboxyl, whereas for the saturated model, the Hg2+ adsorption energy decrease order is: carboxyl > semiquinone > lactone. Thermodynamic parameters confirmed that the adsorption process is spontaneous (unsaturated model), while the infrared spectra provided an insight at the atomic level about the experimentally reported bands. Our results contributed to a deeper understanding of the current experimental information on the effect of the surface functional groups on the Hg2+ adsorption over carbonaceous materials as different active sites can be present on oxygenated carbonaceous materials for metal adsorption. The results also create new ways to improve the performance of adsorption capability of mercury and other pollutants.

Molecular adsorption properties of CH4 with noble metals doped onto oxygen vacancy defect of anatase TiO2 (1 0 1) surface: First-principles calculations

The adsorption properties of methane molecules (CH4) on the surface of noble metals doped anatase TiO2 (1 0 1) were studied using the first-principles method based on density functional theory (DFT). The noble metals M (M = Rh, Pd, Ag, Os, Ir, Pt and Au) are doped on the bridge oxygen vacancy defects of the anatase TiO2 surface, and the results show that the doping formation energy of Ir and Os are the lowest, −6.75 eV and −6.34 eV, respectively, indicating relatively stable systems. The adsorption energy analysis shows that both oxygen vacancy modification system and Os atom doping system can improve the stability of CH4 adsorption. By analyzing the density of state and charge density difference of the system, the orbital hybridization and charge transfer are verified the main reasons for the decrease of adsorption energy. The results show that Os atom doped TiO2 is a potential methane sensor material.

Boron nitride nanotubes as containers for targeted drug delivery of doxorubicin

Using molecular dynamics simulations, the adsorption and diffusion of doxorubicin drug molecules in boron nitride nanotubes are investigated. The interaction between doxorubicin and the nanotube is governed by van der Waals attraction. We find strong adsorption of doxorubicin to the wall for narrow nanotubes (radius of 9 Å). For larger radii (12 and 15 Å), the adsorption energy decreases, while the diffusion coefficient of doxorubicin increases. It does, however, not reach the values of pure water, as adsorption events still hinder the doxorubicin mobility. It is concluded that nanotubes wider than around 4 nm diameter can serve as efficient drug containers for targeted drug delivery of doxorubicin in cancer chemotherapy.

Heterogeneous interfacial engineering of Pd/TiO2 with controllable carbon content for improved direct synthesis efficiency of H2O2

Series of heterogeneous interfacial engineered TiO2 (C-TiO2) with controllable carbon content were facilely synthesized by incipient-wet impregnation using glucose and subsequent thermal carbonization. The obtained C-TiO2 were used as catalytic supports to load Pd nanoparticles for H2O2 direct synthesis from H2 and O2. The as-prepared samples were systematically studied by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), air isothermal microcalorimeter, temperature-programmed reduction of H2 (H2-TPR), and so on. The catalytic results showed that H2O2 productivity and H2O2 selectivity of Pd/C-TiO2 firstly rose with increasing carbon content and then declined. Pd/C-TiO2 catalyst with 1.89 wt% of carbon content showed the best catalytic performance that had 61.2% of selectivity and 2192 mmol H2O2/gPd/h of productivity, which were significantly better than those of pristine Pd/TiO2 (45.2% and 1827 mmol H2O2/gPd/h). Various characterization results displayed that the carbon species were heterogeneously dispersed on TiO2 surface. Moreover, no obvious geometric transformation in supports and Pd nanoparticles were observed among different catalysts. The superficial hydrophobicity of Pd/C-TiO2 was gradually promoted with increasing carbon content, which led to the corresponding decrease in adsorption energy of H2O2 with catalysts. According to structure-performance relationship analyses, the heterogeneous interfacial engineering of carbon could maintain the interaction of Pd nanoparticles with TiO2 and simultaneously accelerate the H2O2 desorption. Both factors further determined the excellent H2O2 direct synthesis performance of Pd/C-TiO2.

A computational-experimental investigation on high ethylene selectivity in ethanol dehydration reaction found on WOx/ZrO2-activated carbon bi-support systems

The high ethylene selectivity exhibited on the zirconia-activated-carbon bi-support catalyst is investigated by experiment and density functional theory–based (DFT) analysis. This bi-support catalyst systems prepared by the physical mixing method for the tungsten catalyst show a significant increase in ethylene selectivity up to 90% compared to the zirconia single support system (~58%) during the ethanol dehydration reaction. Besides, the optimal percent weight ratio of zirconia to activated carbon, which results in the highest ethanol conversion is 50:50. The DFT–based analysis is used to investigate high ethylene selectivity in the bi-support system. It shows that the WO5/zirconia is the most stable model for the zirconia single-support tungsten catalyst represented by the zirconia (101) facet of the tetrahedral phase. The carbon atoms were added to the WO5/zirconia to model the tungsten catalyst on the bi-support system. The Bader charge analysis is carried out to determine the electron transfer in the catalyst. The bonding between ethylene and the WO5 active site on the catalyst is weakened when the system is bi-support, where the added carbon atoms on the catalyst in the ZrO2 region decrease the ethylene adsorption energy. Thus, the desorption and the selectivity of ethylene are promoted. The decrease in adsorption energy can be explained via the analysis of the projected density of states (PDOS) profiles of atom involving the adsorption. It was found that the added carbon in the ZrO2 region induces the electron transfer from the ethylene molecule to the surface, especially to the ZrO2 region. The depletion of the electron around the ethylene molecule weakens the bonds, thus, promote desorption. Hence, the advantages of using the bi-support system in the tungsten catalyst are that the catalyst exhibit (1) high conversion due to the zirconia support and (2) high ethylene selectivity due to the added carbon promoting the desorption of ethylene via the induction of electron from an ethylene molecule to surface.

Tuning Surface Structure of Pd3Pb/Pt n Pb Nanocrystals for Boosting the Methanol Oxidation Reaction.

Developing an efficient Pt‐based electrocatalyst with well‐defined structures for the methanol oxidation reaction (MOR) is critical, however, still remains a challenge. Here, a one‐pot approach is reported for the synthesis of Pd3Pb/PtnPb nanocubes with tunable Pt composition varying from 3.50 to 2.37 and 2.07, serving as electrocatalysts toward MOR. Their MOR activities increase in a sequence of Pd3Pb/Pt3.50Pb << Pd3Pb/Pt2.07Pb < Pd3Pb/Pt2.37Pb, which are substantially higher than that of commercial Pt/C. Specifically, Pd3Pb/Pt2.37Pb electrocatalysts achieve the highest specific (13.68 mA cm−2) and mass (8.40 A mgPt −1) activities, which are ≈8.8 and 6.8 times higher than those of commercial Pt/C, respectively. Structure characterizations show that Pd3Pb/Pt2.37Pb and Pd3Pb/Pt2.07Pb are dominated by hexagonal‐structured PtPb intermetallic phase on the surface, while the surface of Pd3Pb/Pt3.50Pb is mainly composed of face‐centered cubic (fcc)‐structured PtxPb phase. As such, hexagonal‐structured PtPb phase is much more active than the fcc‐structured PtxPb one toward MOR. This demonstration is supported by density functional theory calculations, where the hexagonal‐structured PtPb phase shows the lowest adsorption energy of CO. The decrease in CO adsorption energy and structural stability also endows Pd3Pb/PtnPb electrocatalysts with superior durability relative to commercial Pt/C.

Adsorption of small gas molecules on transition metal (Fe, Ni and Co, Cu) doped graphene: A systematic DFT study

Abstract We predict the CO2, NO, NO2 and SO2 gas molecule absorption and sensing performance of transition metal (Fe, Ni, Co and Cu) doped graphene by a systematic density functional theory (DFT) study. Our results demonstrate that graphene doped with different transition metal atoms produces completely different adsorption behaviors of small gas molecules originated from changes in the electronic structure of the systems under strain. Graphene doped with Fe atoms was the best platform for sensing NO2 gas molecules (NO2/Fe-MG). The NO2/Fe-MG system showed the best adsorption rate, the higher charge transfer and the shortest distance between the graphene platform and the gas molecule of all the calculated systems. As the strain increases, the adsorption energy and charge transfer decreases. So the NO2 gas molecule adsorption properties of Fe-MG without strain would help in guiding experimentalists to develop better materials based on graphene for efficient gas detection or sensing applications.

DFT STUDY OF METHYL (CH3) AND HYDROXYL (OH) ADSORPTION ON A GOLD (001) SURFACE

We used the DACAPO code with the GGA-PW91 approximation to study the adsorption of methyl (CH3) and hydroxyl (OH) for four- and five-layer gold (Au) (001) slabs. We have determined for each species the best binding site, adsorption energy, the change in the work function, surface energy, surface dipole moment, geometrical parameters and projected density of states (PDOS). We performed spin-unpolarized and spin-polarized DFT calculations for free and adsorbed CH3 and OH species. The most important point is that the spin polarization diminishes the adsorption energies but does not change the geometrical parameters. For the CH3 species, only the top site was found to be stable for different coverages. We found that during the optimization phase, the hollow and bridge sites were found to be unstable. In both cases the CH3 species moves toward the top site. We observe that the adsorption energy decreases when increasing the coverage. However, the OH species was stable in all investigated sites (top, bridge and hollow). We notice that the adsorption energy is dependent on the number of slab layers and the bridge is the best site in adsorption energy. The analysis of the calculated O PDOS of OH radical shows a mixing between the O orbitals and the Au bands.

An investigation of thin Zn films on 4H-SiC(0001)[sbnd]graphene

In this work, the growth of thin Zn films on 4H-SiC(0001)graphene and the effects of system annealing at various temperatures are presented. Metal growth on graphene is interesting to understand the grown morphology and metal intercalation, to use it to tune graphene properties. The properties such as chemical composition and surface structure were studied for deposited films and after annealing from 640 K to 1080 K. Using X-ray Photoelectron Spectroscopy and Low Energy Electron Diffraction techniques it was found that zinc interacts weakly with the substrate and does not change the XPS spectra bond energies but it intercalates, as seen from the extinction of the 6 × 6 diffraction spots. Moreover, in order to obtain complementary results, Density Functional Theory calculations were performed. The theoretical results for adsorption energy of Zn at various types of SiC surfaces can explain the preferred adsorption sites and the decrease of adsorption energy with Zn coverage presented in the experimental results.

CO2‐activation and enhanced capture by C6Li6: A density functional approach

AbstractIn this study, we propose a simple and yet effective approach for capture and storage of CO2 by C6Li6. C6Li6 possesses a planar star‐like structure, whose ionization energy is lower than that of Li atom and hence, it behaves as a superalkali. We have systematically studied the interaction of successive CO2 molecules with C6Li6 using long‐range dispersion corrected density functional ωB97xD/6‐311 + G(d) calculations. We notice that these interactions lead to stable C6Li6‐nCO2 complexes (n = 1‐6) in which the structure of CO2 moieties is bent appreciably (122‐125°) due to electron transfer from C6Li6, whose planarity is distorted only slightly (≤7°). This clearly suggests that the CO2 molecules can successfully be activated and captured by C6Li6. It has been also noticed that the bond‐length of CO2 in C6Li6‐nCO2 complexes increases monotonically whereas adsorption energy decreases, ranging 3.18‐2.79 eV per CO2 with the increase in n. These findings establish the potential of C6Li6 for capture and storage of CO2 molecules.

Influence of pH and grain size on physicochemical properties of biochar and released humic substances

The main goal of this study was to investigate the influence of pH and grain size of biochar on its physicochemical properties and on the quantity and quality of humic substances released from biochar. The studies were conducted on 5 biochar fractions and on the mixture of all fractions. Influence of grain size on physicochemical properties of biochar was characterized by analyzing changes in total carbon content, ash, density, total surface charge, average adsorption energy, specific surface area, content of functional groups and FTIR spectra. The effect of pH and grain size of biochar on releasing organic compounds were examined at pH of the extraction solution: 3, 5, 7 and 9 by analyzing aqueous extracts for changes in organic carbon content and for variation of humification degree coefficients. The results showed the grain size affected physicochemical properties of investigated material. The fraction <0.5 mm was characterized by the highest density, ash, content of functional groups and the lowest carbon content. Increasing grain diameter corresponded with decrease in average adsorption energy and with increase in the specific surface area. Only grain size, not pH of the solution, revealed clear effect on the quantity and quality of humic substances released from biochar.

Promoting effect of Ni on the structure and electronic properties of NixMo(1−x)S2 catalyst and benzene adsorption: A periodic DFT study

Aromatic hydrogenation (e.g., benzene) over MoS2-based catalyst harbours essential importance in hydrotreating process due to stringent environmental and fuel requirement. In this work, the promoting effect of Ni on structure and electronic properties of NixMo(1−x)S2(1 0 0) surface (x = 0, 0.25, 0.5, 1.0) and benzene adsorption has been investigated by DFT calculations using PW91-OBS and PW91 functions. It is found that the adsorption of benzene requires at least three coordinatively unsaturated sites (CUS), which can improve the activation of CC bond. The introduction of Ni facilitates the formation of stable CUS sites, and enhances the eletrophilicity of the Mo atoms for adsorption. Furthermore, the volcanic-shape relationship between Ni content and benzene adsorption ability is then established. With the increase of Ni percentage from 0 to 50%, the adsorption energy and electrons transferred from benzene molecules to the surface gradually increase due to the increased ligand effect for Mo atoms. However, overmuch Ni leads to the sharp decrease of benzene adsorption energy. Dispersion correction analysis indicate that the interplay between benzene and Ni is dominated by weak vdW interactions. The results reported herein are beneficial to the design of efficient NiMoS2 catalysts for hydrotreating.

The inhibition of hydrogen and oxygen recombination reaction by halogen atoms on over-all water splitting over Pt-TiO2 photocatalyst

Semiconductor photocatalysts for overall water splitting into H2 and O2 require metal cocatalyst, such as Pt, to catalyze H2 evolution efficiently. However, these metal cocatalysts can also catalyze hydrogen and oxygen recombination to form water. In this paper, we found that the pre-adsorbed halogen atom catalyst could inhibit the reverse reaction of water formation from H2 and O2 due to the decrease of adsorption energies of H2 and O2 on Pt. The adsorption energy decrease of H2 and O2 followed the order of F/Pt < Cl/Pt < I/Pt < Br/Pt. H2-TPD results exhibited similar dependence. This inhibition was achieved via the occupation of halogen atom on the Pt surface sites, and thereby the adsorption and activation of hydrogen and oxygen molecules were decreased. The occupation difference of halogen atoms are determined by radius of halogen ions, which further leads the different activity for H2 and O2 recombination. By inhibition of water formation reverse reaction, the over-all water splitting over various TiO2 photocatalysts has been achieved. Isotope experiments with D2O and H218O confirmed the over-all water splitting to H2 and O2. This study may help scientist to develop high-efficient photocatalyst for overall water splitting.

Effect of organic type and moisture on CO2/CH4 competitive adsorption in kerogen with implications for CO2 sequestration and enhanced CH4 recovery

Although research attentions for CO2 injection in gas-bearing reservoirs have been drawn to CO2 sequestration with enhanced gas recovery (CS-EGR), the microscopic competitive adsorption mechanism of methane (CH4) and carbon dioxide (CO2) considering the effect of organic type and moisture remains to be determined. In this work, we focus on the competitive adsorption behaviors of CH4 and CO2 on dry and moist realistic kerogen models of different organic types by performing combined molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations. The effects of organic type and moisture content on kerogen pore structures, moisture distribution and interaction between CH4/CO2 and kerogen surfaces are discussed in details. Simulation results show that CO2/CH4 adsorption capacity and adsorption selectivity are in the order of kerogen IA < IIA < IIIA, consistent with the sequence of enterable pore volume fraction (IA, 9.38%; IIA, 13.59%; IIIA, 28.88%). H2O molecules are preferentially adsorbed on the sulfur- and oxygen-containing groups at low moisture, and then migrate and aggregate into clusters in the middle of enterable pores at high moisture. The CO2/CH4 adsorption capacity decreases with increasing moisture content, while the CO2/CH4 adsorption selectivity, specific adsorption energy and CO2 isosteric heat decrease at the beginning, and then increase with the moisture content. Moisture has a bigger effect on the adsorption of CO2 than that of CH4. This study indicates that kerogen IIIA is the optimized organic type for CS-EGS due to its large and stable CO2 storage capacity. Despite its negative effect on gas adsorption capacity, moisture can potentially boost the displacement of CH4 by CO2 at certain moisture conditions. Results of this study lay the foundation for future optimization design of CS-EGR projects with application to coal and shale systems.

Silver cluster supported on nitrogen-doped graphene as an electrocatalyst with high activity and stability for oxygen reduction reaction

An Ag8 cluster deposited on three different types of nitrogen (N)-doped graphene was studied using density functional theory calculations with empirical pair potentials (DFT-D). Among the different kinds of N-doped graphene, the pyridinic-N3 (P-N3) type can act as the best anchor position to stabilize Ag8. In addition, it is found that supported Ag8 clusters show higher activity in oxygen reduction reaction compared to unsupported clusters due to significant decrease in O2 adsorption energy and higher charge transfer to O2. Electron transfer from Ag to O2 leads to the elongation of the OO bond, which facilitates the breaking of this bond in the oxygen reduction reaction. All results suggest that N-doped graphene support can play a significant role in the chemical reactivity of a Ag8 cluster in oxygen reduction reaction.

The investigation of adsorption and dissociation of H2O on Li2O (111) by ab initio theory

The adsorption and dissociation mechanism of H2O molecule on the Li2O (111) surface have been systematically studied by using the density functional theory calculations. The parallel and vertical configurations of H2O at six different symmetry adsorption sites on the Li2O (111) surface are considered. In our calculations, it is suggested that H2O can dissociate on the perfect Li2O surface, of which the corresponding adsorption energy is 1.118eV. And the adsorption energy decrease to be 0.241eV when oxygen atom of H2O bonds to lithium atom of the slab. The final configurations are sensitive to the initial molecular orientation. By Bader charge analysis, the charge transfer from slab to adsorbed H2O/OH can be found due to the downward shift of lowest-unoccupied molecular orbital. We also analyze the vibrational frequencies at the Brillouin Zone centre for H2O molecule adsorbed on the stoichiometric surface. Due to the slightly different structure parameters, the calculated values of the vibrational frequencies of hydroxyl group range from 3824 to 3767cm−1. Our results agree well with experimental results performed in FT-IR spectrum, which showed that an absorption peak of OH group appeared at 3677cm−1 at room temperature.