Differences In Adsorption Energies Research Articles

Prediction of the Crystal Growth Mechanism of Aspirin Using Molecular Simulations

Controlling the physicochemical properties of a drug formulation is important for proper drug efficacy, since in the gastrointestinal tract many drugs undergo dissolution, limiting their efficacy. Factors affecting a drug's physicochemical properties include its crystal habit. Therefore, we predicted the crystal habit by molecular simulation for the purpose of controlling crystal morphology. In this study, we used aspirin as a model compound. By performing simulations based on known crystal structure data, we trained the simulation algorithm to produce the cubic and plate-like morphologies of crystals actually obtained. By these methods, we showed that the crystal plane of the crystal form actually obtained coincides with the characteristic crystal plane obtained by simulation. Furthermore, to consider the influence of the crystallization solvent on crystal growth, we simulated adsorption of solvent molecules on characteristic crystal planes. The difference in adsorption energy of the solvent molecules prevents the aspirin molecules from attaching to the crystal plane. As a result, we concluded that the crystal habit was caused by the difference in growth rate of the crystal plane. By applying the methods developed in this research, the growth of crystal planes can be predicted by molecular simulation, making it possible to efficiently obtain crystal forms with optimal physical properties for drug development. We believe that further development of this approach will lead to dramatic decreases in the cost and duration of drug development.

Application of the training of density functional theory potentials within machine learning to adsorptions and reaction paths on Platinum surfaces

This study is conducted by making density functional theory (DFT) calculations within gpaw code and then trained the DFT potentials within a machine learning approaches of amp and ænet. Adsorbate molecules are chosen based on occurrences in fuel cells. The outcomes from the machine learned training of the potentials show differences in adsorption energies of only up to the order of 10−2 eV when trained sufficiently well. However, such small differences in energies do not seem to carry into a differences in electronic structures. It shows that (110) surface is better in facilitating a dissociation process of hydrogen and oxygen molecules into the respective ions with a barrier of dissociation of up to 0.75 eV and 1.91 eV, respectively. The relatively small adsorption energies of hydrogen and oxygen molecules, 0.82 eV and -0.10 eV, respectively, seem to be indicative of a favorable mobility of the respective ions within the electrolyte. Carbon monoxide adsorbs relatively stronger with adsorption energy of up to 1.80 eV. As a result, the formation of carbon dioxide and water molecules as a byproducts is strongly activated, but is exothermic, and seem to preferably take place on (100) and (111) surfaces. A weak adsorption energies of these byproduct molecules could also be indicative of a good mobility of the molecules within fuel cells, once formed. Training of the DFT potentials also appear to show differences in energies of reaction paths only to within the order of 10−1 eV.

Selective Electroreduction of Carbon Dioxide over SnO2 -Nanodot Catalysts.

The development of electrochemical CO2 conversion allows green carbon utilization. Formate and syngas are two typical products of electrochemical CO2 reduction, and the coproduction of these two products will maximize the energy efficiency of CO2 conversion. However, few works have successfully achieved the cogeneration of formate and syngas. This paper describes a novel strategy to maximize the efficiency of CO2 conversion through coproduction of formate and syngas on ultrasmall SnO2 nanodots (NDs) homogeneously anchored on carbon nanotubes (CNT#SnO2 NDs) electrodes. The CNT#SnO2 NDs not only decreased the adsorption energy of *OCHO but also reduced the adsorption energy difference of *COOH and *H. High energy efficiency toward formate and adjustable H2 /CO ratio were obtained over a broad potential window with long-term stability. In addition, CNT#SnO2 NDs and Ir foil were coupled together to construct an electrolyzer for electrochemical CO2 reduction reaction and oxygen evolution reaction (CO2 ERR-OER), which also produced formate and syngas with 24 h stability. A promising approach is presented for the electrochemical CO2 conversion in fuel production.

Oxygen Reduction Reaction Catalyzed by Pt3M (M = 3d Transition Metals) Supported on O-doped Graphene

Pt3M (M = 3d transition metals) supported on oxygen-doped graphene as an electrocatalyst for oxygen reduction was investigated using the periodic density functional theory-based computational method. The results show that oxygen prefers to adsorb on supported Pt3M in a bridging di-oxygen configuration. Upon reduction, the O–O bond breaks spontaneously and the oxygen adatom next to the metal–graphene interface is hydrogenated, resulting in co-adsorbed O* and OH* species. Water formation was found to be the potential-limiting step on all catalysts. The activity for the oxygen reduction reaction was evaluated against the difference of the oxygen adsorption energy on the Pt site and the M site of Pt3M and the results indicate that the oxygen adsorption energy difference offers an improved prediction of the oxygen reduction activity on these catalysts. Based on the analysis, Pt3Ni supported on oxygen-doped graphene exhibits an enhanced catalytic performance for oxygen reduction over Pt4.

What ultimately drives the wrapping, deprivation, and transfer of graphene nanosheets.

This study was devoted to investigating the interactions between graphene sheets (GNSs) and metal nanowires via molecular dynamics simulations. The simulation results indicated that the element type, shape, and size of the nanowires would affect their adhering behavior and help achieve the interesting wrapping, deprivation, and transfer phenomenon of a GNS. An appropriate length-width ratio of the GNS and nanowire contributed to the formation of a helical configuration of GNS out of nanowires under the action of van der Waals interactions. Importantly, some of the GNSs could spontaneously peel off from one nanowire and re-assemble a new spiral core-shell structure on another nanowire. This deprivation and re-assembly was associated with the definite elemental selectivity, which was determined by the adsorption energy of the GNSs on the nanowires. The adsorption energy difference drove the full or partial transfer of the GNSs. This study provides deep insights into the interactions between GNSs and nanowires.

Analysis of adsorption properties of SF6 decomposed gases (SOF2, SO2F2, SF4, CF4, and HF) on Fe-doped SWCNT: A DFT study

In order to develop a potential material for monitoring of SF6 decomposed gases online in insulated equipment, the Fe-doped single-walled carbon nanotube (Fe-SWCNT) was proposed and its gas sensing properties to five SF6 decomposed gases were investigated with DFT method. The adsorption structures, charge transfers, density of states (DOS), and frontier molecular orbital theory analysis have been performed. Calculations show that the doping of Fe enhances the conductivity of SWCNT and Fe acts as the active center for adsorption. The conductivity of zigzag Fe-(8, 0) SWCNT is increased with SOF2 adsorption. While the adsorption of SF4 distinctly decreases the conductivity of the system. For SO2F2 adsorption, the difference in adsorption energy of different configurations is small, but the HOMO-LUMO energy gaps vary widely. The adsorption of SO2F2 is weaker than that of SOF2 and SF4. For adsorption of CF4 and HF, the little electron transfer and the low adsorption energy indicate the weak interaction. The negligible decreases of conductivity confirms that the Fe-(8, 0) SWCNT is insensitivity for CF4 and HF detection. After SO2F2, SF4, CF4, and HF adsorbed onto armchair Fe-(5, 5) SWCNT, the conductivity of system is also changed.

Hydrogen Adsorption on Ir(111), Ir(100) and Ir(110)—Surface and Coverage Dependence

Hydrogen adsorption on the perfect Ir(111) as well as the metastable and unreconstructed Ir(100) and Ir(110) surfaces up to saturation coverage has been systematically computed using periodic density functional theory and ab initio atomistic thermodynamics for understanding the interaction mechanism of hydrogen on iridium surfaces. On the Ir(111) surface including van der Waals dispersion, hydrogen adsorption prefers the threefold hollow sites at low coverage and the top sites at high coverage; in agreement with the experiments (Phys. Rev. B 60 (1999) 14016). The computed adsorption energy and desorption temperature of hydrogen agree with the experiments [−0.57 (fcc-3H) and −0.53 (hcp-3H) vs. −0.55 eV; 180 and 325 K vs. 190 and 310 K, respectively]. On the Ir (100) surface, the bridge adsorption sites are preferred in the whole coverage range, in agreement with the LEED pattern (Phys. Rev. B 73 (2006) 75430), however, adsorption energy and desorption temperature are slightly overestimated by including van der Waals dispersion (−1.50 vs. −1.02 ± 0.15 eV; 470 vs. 425 – 389 K). On the Ir(110) surface, short-bridge sites are preferred at low coverage and the top sites become dominant at high coverage, and the calculated desorption temperatures are close to experiments by including van der Waals dispersion (210 and 365 K vs. 220 and 375 K). At low coverage, the different configurations of hydrogen adsorption on the Ir(111) have the similar energies, indicating their negligible repulsive interaction, while the Ir(100) and Ir(110) surfaces prefer regular line-shape adsorption configurations due to attractive interaction, and such adsorption configurations have not been observed experimentally. Our results show that differences in adsorption configurations and energies are associated with their differences in surface structures, and in turn explain the need of different methods in computing the adsorption properties on different surfaces. Such surface-dependent properties should also be possible on other metal surfaces.

WITHDRAWN: Adsorption of NO2 on monolayer MoS2 doped with Fe, Co, and Ni, Cu: A first-principles study

Abstract Here, we reported that Fe, Co, and Ni, Cu -doped MoS2 monolayers nanomaterials was a promising approach to improve their gas sensing performance. The adsorptions of NO2 gas molecule on Fe, Co, and Ni, Cu -doped MoS2 monolayer were studied in terms of adsorption energy, charge transfer, band structures, and charge density differences. The results suggested that the adsorption of NO2 gas molecules on the Fe-MoS2 monolayer was energetically favorable, which gives rise to the most stable configurations. Besides, the NO2 gas molecule were physisorbed on the monolayer surface due to the weak interaction between them and Fe, Co, and Ni, Cu -doped MoS2 monolayer. Most notably, the Fe -doped MoS2 monolayer might be a great candidate for low-cost, highly active, and stable catalysts and gas sensors, providing an avenue to facilitate the design of high active MoS2-based two-dimensional gas sensor.

Similarities and differences for atomic and diatomic molecule adsorption on the B-5 type sites of the HCP(101̅6) surfaces of Co, Os, and Ru from DFT calculations

The differences in relative adsorption energies for mono-atomic and diatomic prototype species (C,N,O,S,H,CO,NO,SO,CH,NH,H2,O2) relevant to catalytic processes such as Fischer-Tropsch and Ammonia Synthesis chemistry are investigated on the previously un-studied (101¯6) surface(s) of Co, Os, and Ru. Recent work in the literature has confirmed that catalytically relevant nanoparticles of HCP elements such as Co, Os, and Ru typically possess highly active ‘B5’ sites; unfortunately many early and extant theory and model-ing treatments of "stepped HCP surfaces" use ad-hoc created steps via manual deletion of atoms from an ideal HCP(0001) slab model. To date the differences in adsorption energies at various B5 step edge types, and any possible trends across the same type of B5 sites on various HCP catalyst species has not been thoroughly characterized. Our work in this manuscript uses the low energy (101¯6) Miller Index surface of Co, Os, and Ru which exposes 2 distinct and strongly adsorbing step edge sites, the B5B and B5A step edge which have been reported as relevant in the literature for Cobalt nanoparticle catalysis applications. Results from this study should be used to help further understand atomistic processes on the stepped surfaces of catalytically active HCP elements.

DFT-based study on H2S and SOF2 adsorption on Si-MoS2 monolayer

H2S and SOF2 gas are two typical decomposition products of SF6 gas under partial discharge condition. Based on the density functional theory (DFT), several doping structures of Si-MoS2 and adsorption structures of gas adsorbed Si-MoS2 were investigated. The electronic property, adsorption energy, charge transfer, and electron density difference were calculated to analyze the adsorption mechanism. Compared with the structure of pristine MoS2 monolayer, Si doping improves the surface activity of MoS2 monolayer. Moreover, the introduction of Si atom increases the effects of orbital hybridization between gas molecules and MoS2 monolayer, and promotes the charge transfer. Especially, SOF2 possesses a large adsorption energy with Si-MoS2 by crossing a small energy barrier. In addition, the good compatibility of Si-MoS2 with SF6 atmosphere, makes it a promising adsorbent for SF6 decomposition products removal.

Competitive adsorption of CO and H<sub>2</sub> on strained Fe(110) surface

In this work, the competitive adsorption behavior of H<sub>2</sub> and CO on strained Fe(110) are investigated by the first-principles method based on the spin-polarized density functional theory to study the hydrogen embrittlement of steels. The results show that the most stable adsorption site for CO is top site, and the orbital of CO molecule hybridizing with Fe 3p and 4s states illustrates a strong electronic interaction between them. The adsorption energy values of CO at the four calculated adsorption sites are more negative than those of H<sub>2</sub>, which favors the binding with Fe(110) surface. The potential energy variations for CO and H<sub>2</sub> molecules close to the surface are calculated. The attractive force of the Fe(110) surface acting on CO in 1.5–3 Å is greater than that acting on H<sub>2</sub>. The pre-adsorbed CO increases the dissociation energy barrier of H<sub>2</sub> from 0.08 eV to 0.13 eV but reduces the force between H<sub>2</sub> and surface. The surface tensile strain enhances the interaction between hydrogen and Fe(110), which, however, is reduced by the compressive strain. The opposite tendency is found in the adsorption of CO. The binding strength of CO is stronger than that of H<sub>2</sub> on the strained Fe(110) surface. The difference in adsorption energy between CO and H<sub>2</sub> decreases with tensile strain increasing. The effect of surface strain and partial pressure of CO gas phase on the surface coverage ratio of H atom are also calculated quantitatively based on thermodynamics at 298 K, with the partial pressure of H<sub>2</sub> set to be 10 MPa. The surface ratio of the H atom decreases with partial pressure of CO increasing. The hydrogen coverage drops nearly to zero when the partial pressure of CO reaches a certain value. This result reveals that CO can inhibit hydrogen adsorption on Fe surface. In the case where the surface ratio of hydrogen decreases to 1%, the corresponding CO partial pressures are 105 Pa, 1.1 × 10<sup>3</sup> Pa, 2.4 × 10<sup>5</sup> Pa on –2%, 0, 2% strained Fe(110) surface, respectively. High CO partial pressure is needed to suppress the hydrogen adsorption since the binding strength of CO is close to that of H<sub>2</sub> on the expanded surface.

Diffusion of Water and Carbon Dioxide and Mixtures Thereof in Mg-MOF-74

Self-diffusion coefficients for CO2, H2O, and mixtures thereof in Mg-MOF-74 have been determined using molecular simulations. The crystal structure was modeled in a rigid and flexible manner with the intent to identify the impact of the flexibility of the framework on the diffusivity. The results show that especially at low loadings the mobility of the molecules is enhanced in flexible lattices because of a small difference in adsorption energy and the enlarged equilibrium distance to the main adsorption sites. In mixtures the flexibility has the same influence; however, because of the presence of the secondary species, water is predominantly located in the close proximity of the magnesium-ion which effectively leads to a segregation effect from the more freely moving CO2.

Adsorption of O3, SO2 and SO3 gas molecules on MoS2 monolayers: A computational investigation

Owing to the wide surface area, two-dimensional materials such as MoS2 are being used for chemical sensing of harmful gas molecules, which reduce the concentration of these pollutants to an environmentally acceptable value. Using density functional theory calculations, the adsorptions of SO2, SO3 and O3 gas molecules on MoS2 monolayers were studied in terms of adsorption energy, charge transfer, band structures, and charge density differences. The results suggest that the adsorption of gas molecules on the MoS2 is energetically favorable, which gives rise to the most stable configurations. Besides, the gas molecules were physisorbed on the monolayer surface due to the weak interaction between them and MoS2. The OO and SO bonds of the adsorbed gas molecules were elongated after the adsorption process. This elongation of bond lengths is mostly attributed to the transfer of electronic density from the old bonds to the newly formed bonds between the molecules and MoS2. The band structure calculations indicate that the electronic properties of the MoS2 monolayers are altered upon the adsorption of gas molecules. Furthermore, the charge density difference calculations reveal the charge accumulation on the adsorbed molecules, which indicate the acceptor property of these gas molecules.

Predicting Surface Clustering at Ambient Conditions from Thermodynamic Data

Scanning tunneling microscopy (STM) has proved to be a prime tool to characterize the atomic structure of crystal surfaces under UHV conditions. With the development of high-pressure scanning tunneling microscopy (HP-STM), the scope of this technique has been largely extended, as new structures were found to occur under gas phase chemical potentials achieved under ambient conditions. Particularly interesting is the substantial restructuring of initially flat and stable surfaces into new orientations by formation of nanoclusters. Here we discuss the possible generality of this phenomenon by analyzing cases where atomically flat surfaces of certain transition metals undergo such changes in the presence of CO at room temperature (RT) while some remain unchanged. From our analysis we argue that such changes can be predicted from thermodynamic data published in the literature, particularly from the difference in adsorption energy on low- and high-coordination sites, like terrace and step sites, which can be ob...

Tuning the magnetic properties of hydrogenated bilayer graphene and graphene/h-BN heterostructures by compressive pressures

We investigated the electronic structure and magnetic properties of hydrogenated bilayer graphene and graphene/h-BN heterostructure as a function of applied uniaxial compression normal to the interface plane using van der Waals density functional theory. The H atoms preferred a cluster formation was found in the hydrogenated bilayer graphene and graphene/h-BN systems with a non-magnetic ground state. With increasing external pressure, the equivalence of two sublattices in graphene was broken because of the interaction between graphene and the underlying layer and the site dependent adsorption energy difference was greatly enhanced with the applied pressure. This resulted in a transition from an NM ground state to a ferromagnetic in hydrogenated bilayer graphene and graphene/h-BN heterostructure. Thus, we propose that the magnetic property can be manipulated by external pressure and that hydrogenated systems can be utilized for potential spintronics applications at low H concentrations.

Adsorption of NO on Fe3O4(111)

Adsorption of NO on Fe3O4(111) is studied by density functional theory (DFT) calculations. NO is preferably adsorbed atop the octahedral site which has a clearly higher adsorption energy than the tetrahedral site. The difference in adsorption energy correlates with differences in adsorption geometries and NO stretch vibrations. The results are in good agreement with temperature programmed desorption (TPD) and reflection-adsorption IR spectroscopy (RAIRS) measurements and provide an explanation of the observation of only one vibrational mode despite two distinct TPD features.

Achievement of two logical states through a polymer/silicon interface for organic-inorganic hybrid memory

A hysteresis loop of minority carrier lifetime vs voltage is found in polystyrenesulfonate (PSS)/Si organic-inorganic hybrid heterojunctions, implying an interfacial memory effect. Capacitance-voltage and conductance-voltage hysteresis loops are observed and reveal a memory window. A switchable interface state, which can be controlled by charge transfer based on an electrochemical oxidation/deoxidation process, is suggested to be responsible for this hysteresis effect. We perform first-principle total-energy calculations on the influence of external electric fields and electrons or holes, which are injected into interface states on the adsorption energy of PSS on Si. It is demonstrated that the dependence of the interface adsorption energy difference on the electric field is the origin of this two-state switching. These results offer a concept of organic-inorganic hybrid interface memory being optically or electrically readable, low-cost, and compatible with the flexible organic electronics.

Adsorption and Photodesorption of CO from Charged Point Defects on TiO2(110).

The adsorption and photochemistry of CO on rutile TiO2(110) are studied with scanning tunneling microscopy (STM), temperature-programmed desorption, and angle-resolved photon-stimulated desorption (PSD) at low temperatures. Site occupancies, when weighted by the concentration of each kind of adsorption site on the reduced surface, show that the adsorption probability is the highest for the bridging oxygen vacancies (VO). The probability distribution for the different adsorption sites corresponds to very small differences in CO adsorption energies (<0.02 eV). UV irradiation stimulates diffusion and desorption of CO at low temperature. CO photodesorbs primarily from the vacancies with a bimodal angular distribution, indicating some scattering from the surface, which also leads to photostimulated diffusion. Hydroxylation of VO's does not significantly change the CO PSD yield or the angular distribution, which suggests that photodesorption can be initiated by recombination of photogenerated holes with excess electrons localized near the charged point defect (either VO or bridging hydroxyl).

Investigation of water adsorption and dissociation on Au1/CeO2 single-atom catalysts using density functional theory

We examined the water adsorption and dissociation on ceria surfaces as well as ceria-supported Au single-atom catalysts using density functional theory calculations. Molecular and dissociative water were observed to coexist on clean CeO2 and reduced Au1/CeO2−x surfaces because of the small difference in adsorption energies, whereas the presence of dissociative water was highly favorable on reduced CeO2−x and clean Au1/CeO2 surfaces. Positively charged Au single atoms on the ceria surface not only provided activation sites for water adsorption but also facilitated water dissociation by weakening the intramolecular O–H bonds. In contrast, negatively charged Au single atoms were not reactive for water adsorption because of the saturation of Au 5d and 6s electron shells. This work provides a fundamental understanding of the interaction between water and single-atom Au catalysts.

Selective Hydrogen Adsorption in Graphene Rotated Bilayers.

The absorption energy of atomic hydrogen at rotated graphene bilayers is studied using ab initio methods based on the density functional theory including van der Waals interactions. We find that, due to the surface corrugation induced by the underneath rotated layer and the perturbation of the electronic density of states near the Fermi energy, the atoms with an almost AA stacking are the preferential ones for hydrogen chemisorption. The adsorption energy difference between different atoms can be as large as 80 meV. In addition, we find that, due to the logarithmic van Hove singularities in the electronic density of states at energies close to the Dirac point, the adsorption energy of either electron or hole doped samples is substantially increased. We also find that the adsorption energy increases with the decrease of the rotated angle between the layers. Finally, the large zero point energy of the C-H bond (∼0.3 eV) suggests adsorption and desorption of atomic hydrogen and deuterium should behave differently.