Stable Adsorption Modes Research Articles

CO2 dissociation and hydrogenation on pure and Ni-doped Fe(1 1 1). A DFT theoretical approach

Using the density functional theory, we have investigated the effect of Ni doping on the Fe(111) surface in two reactions involving CO2: its dissociation to CO and O, and the formation of HCOO. These competitive reactions are of great interest because they are the first ones occurring during CO2 hydrogenation reactions to obtain hydrocarbons. Three bimetallic surfaces were considered: Ni as a substituent in the first layer (Ni1L), in the second layer (Ni2L), and as an adatom (Niad). In all the cases, the presence of Ni inhibits CO2 adsorption in comparison with Fe(111). For Fe(111) and Ni1L-Fe(111), we have obtained an adsorption state where the CO2 molecule is particularly activated, being this configuration different from the most stable adsorption mode. On these surfaces, a two-step reaction was proposed; first, the migration from the most stable state to the activated geometry, and then its dissociation. On Fe(111), the two-step dissociation was found to be kinetically more favored than the direct mechanism. Among the bimetallic surfaces, only Niad-Fe(111) is more favorable kinetically for CO2 dissociation in comparison with Fe(111). Concerning the hydrogenation process to form HCOO, it was found that the reaction is inhibited on mixed Ni-Fe sites.

Interaction of amino acid functional group with water molecule on methane hydrate growth

Kinetic inhibitor-amino acid exhibits a promising advantage in preventing the hydrate formation. In this work, adsorption behaviors of amino acid including glycine, serine and valine on methane hydrate surface are investigated by the density functional theory (DFT). It is found that the nitrogen/oxygen and hydrogen atoms in hydrophilic groups of amino acid prefer to attach the hydrogen and oxygen atoms of water molecules in hydrate simultaneously to achieve a more stable adsorption mode. The adsorption energy of hydrophobic groups is lower than that of hydrophilic groups. Meanwhile, molecular dynamics (MD) simulations are carried out to analyze the inhibition effect of glycine, serine and valine. It is observed that the strongest interaction appears between the hydrophilic groups of serine and methane hydrate surface. The hydrophobic group-methyl of valine weakens the interaction of amino and carboxyl groups with water molecules, whereas increases the disturbance of water molecules.

Intrinsic mechanism of active metal dependent primary amine selectivity in the reductive amination of carbonyl compounds

For the catalytic reductive amination of carbonyl compounds, the kind of active metal used is the most important factor determining the catalytic selectivity in heterogeneous catalysis systems. However, a detailed understanding of the intrinsic mechanism is still lacking. In this work, by evaluating the reductive amination of butyraldehyde and the hydrogenation reaction of secondary imine on various metal catalysts, the competitive hydrogenation reactions of primary imine and secondary imine are proven to be the key factors determining primary amine selectivity. DFT calculations verify that Co, Ni, and Ru, rather than Fe, Rh, Pd, and Pt, tend to show high primary amine selectivity in terms of adsorption energy and activation energy. It should be noted that the different stable adsorption modes of secondary imine on metal surfaces (C&N adsorption mode on Fe, Co, Ni, Ru, and Rh, and N adsorption mode on Pd and Pt) are key factors affecting these reaction characteristics. Moreover, microkinetic simulation proves that the coadsorption of NH3 improves primary amine selectivity on Co, Ni, and Ru surfaces. Combining the theoretical and experimental results, it is clearly verified that the active metal determines the catalytic selectivity of the primary amine by affecting the competitive hydrogenation reactions of the primary imine and secondary imine.

The Density Functional Theory Study on the Adsorption and Dissociation of NO on Pd (111) Surface

Based on the calculation of density functional theory, the adsorption and dissociation of NO on Pd (111) surface were systematically studied. The results show that the most stable adsorption mode of NO was at fcc site with N-terminal downward, followed by hcp site and top site; Bader charge and charge density difference analysis show that there was obvious charge transfer between NO and Pd(111) surface; the analysis of density of states further indicates that there was obvious chemical interaction between them; the dissociation barrier of NO on Pd(111) surface is 2.45eV, which indicates that NO is easy to poison Pd metal surface. Finally, the two dissociation pathways of NO adsorbed on fcc site indicate that NO is very difficult to dissociate on Pd (111) surface.

Atomic arrangements, bond energies, and charge distribution on Si(0 0 1) surfaces with the adsorption of a Ge dimer by DFTB calculations

The atomic arrangements and electrical properties of SiGe nanomaterials are of significance when used in novel nanoelectronic devices and optoelectronic devices. Atomic arrangements, bond energies, and charge distribution of a Ge dimer adsorbed on Si(001) surfaces with different initial heights and inclination angles are investigated by using the DFTB algorithm. Eight different stable adsorption modes of the Ge dimer on the Si slab surface are demonstrated. The variations of the structural and electrical properties of the eight modes are elucidated. Different initial conditions caused stable adsorption of Ge dimer on the surface or caused two single Ge atoms from the Ge dimer to fragment. The Ge dimer, with different inclination angles, is adsorbed on the top site, bridge site, or hollow site of the Si dimer on the surface. The bond length of the Ge dimer is stretched, and the two Ge atoms lose charges. The further the Ge atom is from the surface, the more charges it loses. The different adsorption sites of the Ge dimer affect the properties of the Si dimer on the surface. The top adsorption site mode exerts a greater influence on the bond length and bond energy of the Si dimer, while the bridge adsorption site mode exerts a greater influence on the electron distribution of the Si dimer. The properties of the Ge monoatomic adsorption are akin to the case of a Ge dimer being adsorbed on the bridge site. The different adsorption sites and inclination angles cause the EGap values of the systems to differ, and the effect of the inclination angle on different sites also differs. The Ge dimer and Ge monoatom not only have similar effects on the properties of the Si dimer, but also exert the similar effects on EGap, suggesting that changes in the Si surface are the main contributors to the change in EGap.

Formaldehyde oxidation on Co-doped reduced CeO2(111): First-principles calculations

Catalytic oxidation is most promising among various possible ways of removal of Formaldehyde (HCHO). Here, DFT+U calculations have been utilized to explore theoretically three potential pathways of HCHO oxidation on Co doped reduced CeO2(111). Formation oxygen vacancy formation energies are calculated on developed CoxCe1-xO2(111) p(4 × 4) surface. Chemisorbed and physisorbed configurations of HCHO are both explored, where the most stable adsorption mode on CoxCe1-xO2(111) is comparable with previous reports by dioxymethylene configuration. Based on Langmuir-Hinshelwood mechanism in which O2 and HCHO are both co-adsorbed on the defective CoxCe1-xO2-δ(111) surface, resultant reaction products from three different initial HCHO co-adsorption modes with O2, are identified finally in detail within the frame of transition state theory. We predict that the reaction pathway of HCHO to CO proceeds with the lowest overall barrier on defective Co-doped CeO2(111). These findings provide clear insight into further exploration in formaldehyde activation processes.

Binding Mode of Malonic Acid on the IrO2 Surface

Malonic acid possessing two carboxylic acid anchoring groups is employed as the anchoring group to attach molecular dyes onto IrO2 nanoparticles to oxidize water in a dye-sensitized water splitting system. Yet, the detailed binding modes and nanoscopic structures of malonic acid on the IrO2 substrate are not thoroughly investigated due to the sophisticated structure. Previous researches assume that malonic acid interacts with the IrO2 nanoparticles via two carbonyl groups. In this study, we present first principles calculations on the interfacial structures of malonic acid/IrO2 and state that this assumption is incorrect. Instead, we propose that multiple stable adsorption modes are available at the molecule/IrO2 interface. The associated electronic properties corresponding to different binding modes are discussed.

Binding mode of malonic acid on IrO2 surface

Malonic acid, possessing two carboxylic acid anchoring groups, has been employed as the anchoring group to attach molecular dyes onto IrO2 nanoparticles to oxidize water in a dye-sensitized water splitting system. Yet, the detailed binding modes and the nanoscopic structures of malonic acid on IrO2 substrate have not been thoroughly investigated due to the sophisticated structure. Previous researches assume that the malonic acid interacts with the IrO2 nanoparticles via the two carbonyl groups. In this study, we performed first principles calculations on the interfacial structures of malonic acid/IrO2, and found that such assumption is incorrect. Instead, we propose that multiple stable adsorption modes are available at the molecule/IrO2 interface. The associated electronic properties corresponding to different binding modes are discussed.

Water dissociation at the Au/α-Fe2O3(0001) interface

The dissociative adsorption of water on a model catalyst formed by a Au5 cluster attached to the Fe-terminated (0001) surface of hematite (α-Fe2O3) was investigated within the density functional theory including an on-site Coulomb term (DFT + U). A flattened 2D-like structure was employed as supported gold particle. On clean hematite, the water molecule interacts with its O atom directly bound to a surface Fe ion. Conversely, in the most stable adsorption mode on Au5/hematite, it adsorbs in a multi-coordinated fashion at the metal-oxide interface and with one H atom oriented downward. Regarding the dissociative process, the isolated Au5 particle has a poor performance to activate one of the OH bonds (H2O → OH + H). However, when supported on hematite it becomes very active, having an activation barrier of only 0.09 eV. This process is even more favorable than on clean hematite. Thus, a very reactive site emerges at the metal-support interface. In this distinctive site, the water molecule is able to adsorb in a configuration (H-down) wherein one OH bond is strongly activated. An adsorbate-induced modification on the way that the flattened Au5 is anchored to the surface was observed, accompanied with changes in Au charges.

Study of the modes of adsorption and electronic structure of hydrogen peroxide and ethanol over TiO2 rutile (110) surface within the context of water splitting

While photocatalytic water splitting over many materials is favourable thermodynamically the kinetic of the reaction is very slow. One of the proposed reasons linked to the slow oxidation reaction rate is H2O2 formation as a reaction intermediate. Using Density Functional Theory (DFT) H2O2 is investigated on TiO2 rutile (110) surface to determine its most stable adsorption modes: molecular, (H)O(H)O − (a), partially dissociated, (H)OO − (a), and fully dissociated (a) − OO − (a). We then compare H2O2 interaction to that of a fast hole scavenger molecule, ethanol. Geometry, electronic structure, charge density difference and work function determination of both adsorbates are presented and compared using DFT with different functionals (PBE, PBE-D, PBE-U, and HSE + D). H2O2 is found to be strongly adsorbed on TiO2 rutile (110) surface with adsorption energies reaching 0.95 eV, comparable to that of ethanol (0.89 eV); using GGA PBE. The negative changes in the work function upon adsorption were found to be highest for molecular adsorption ( − 1.23 eV) and lowest for the fully dissociated mode ( − 0.54 eV) of H2O2. This may indicate that electrons flow from the surface to the adsorbate in order to make O(s)-H partially offset the overall magnitude of the oxygen lone pair interaction (of H2O2) with Ti4+ cations. Examination of the electronic structure through density of states (DOS) at the PBE level of computation, indicates that the H2O2 highest occupied molecular orbital (HOMO) level is not overlapping with oxygen atoms of TiO2 surface at any of its adsorption modes and at any of the computation methods. Some overlap is seen using the HSE + D computational method. On the other hand the dissociated mode of ethanol (ethoxides) does overlap with all computational methods used. The high adsorption energy and the absence of overlapping of the HOMO level of H2O2 with TiO2 rutile (110) surface may explain why water splitting is slow.

Charge and Geometrical Effects on the Catalytic N2O Reduction by Rh6– and Rh6+ Clusters

The catalytic conversion of nitrous oxide (N2O) is of crucial environmental relevance because this chemical compound is a greenhouse gas with an important contribution to climate change, even larger than CO2, depleting the ozone layer. Recently, reduction of N2O catalyzed by rhodium subnanoclusters has been the subject of intensive research, both experimental and theoretical, finding dependencies of reaction rate on the size and geometry and electronic structure of the cluster. In this work, the catalytic reduction mechanism of N2O by Rh6– and Rh6+ ionic clusters has been studied by means of density functional theory calculations within the zero-order-regular approximation (ZORA), which explicitly includes relativistic effects. The N2O + Rh6– and N2O + Rh6+ reaction pathways were approached starting from a comprehensive search of different stable adsorption modes; transition states were determined as well. We have obtained that the Rh6– anions present the lowest activation barriers without spin selectivit...

Coadsorption of Cinchona Alkaloids and Oxygen on Pt(111): A Theoretical Study

The adsorption (binding energy, configuration, and spatial orientation) of the cinchona alkaloids cinchonidine and cinchonine, which are used to bestow different chirality to catalytically active noble metal surfaces, has been studied in the presence of coadsorbed oxygen on a model Pt(111) surface using density functional theory. The influence of oxygen was analyzed at 1/36, 0.5, and 1 monolayer coverage, mimicking the adsorption behavior of the cinchona alkaloids on partially and fully oxidized surfaces. Both cinchona alkaloids are destabilized with increasing oxygen coverage; however, adsorption of these molecules on a fully oxidized Pt surface leads to a drastic increase of their stability. The influence of coadsorption with oxygen on the adsorption of the cinchona alkaloids is compared to that of the earlier reported coadsorption with hydrogen. In the absence of coadsorbed oxygen or hydrogen, the most stable adsorption mode is where the quinoline moiety of the cinchona alkaloids lays nearly parallel t...

Chemical properties of two-dimensional oxide systems: Adsorption of (WO3)3 clusters on CuWO4

A two-dimensional ternary oxide layer, Cu-tungstate (CuWO4) on Cu(110), has been tested as a substrate for the adsorption of (WO3)3 cluster molecules from the gas phase. Scanning tunneling microscopy/spectroscopy and X-ray photoelectron spectroscopy have been used to characterize the adsorbed species, while density functional theory has scanned the energy landscape for stable adsorption modes. The (WO3)3 clusters adsorb in preferential adsorption sites at low temperature (85K) as stable, only slightly perturbed units, as established both experimentally and theoretically. The preferred sites are the Cu surface rows in the [100] substrate direction, which feature elastic flexibility as a result of ultrasoft phonon modes and are a characteristic of this two-dimensional ternary oxide structure.

Theoretical prediction of simultaneous removal efficiency of ZnO for H2S and Hg0 in coal gas

The simultaneous removal mechanisms of H2S and Hg0 on the perfect and oxygen-deficient ZnO (101¯0) surfaces are systematically investigated using periodic density functional theory (DFT) calculations. The most stable adsorption modes of Hg0 and HgS are located. It is determined that Hg0 is weakly adsorbed on the two ZnO (101¯0) surface, while HgS is strongly bound to the surface, which implies that the oxidized form of mercury is in favor of removing mercury from coal gas. For the reaction, the dissociation of H2S is occurred firstly, which is almost not affected by the adsorbed Hg atom on the ZnO (101¯0) surface comparing with that on the clean ZnO (101¯0) surface. The dissociated S easily captures Hg0 in gas or adsorbed on the perfect surface leading to the formation of HgS, which shows that the perfect ZnO (101¯0) surface is efficient to remove H2S and Hg0 in coal gas simultaneously and H2S is indispensable to remove Hg0. However, the activation energies of the formation of HgS on the oxygen-deficient ZnO (101¯0) surface are higher than that on the perfect surface. So the removal efficiency of Hg0 on the oxygen-deficient ZnO (101¯0) surface is lower than that on the perfect surface.

Formamide adsorption over the TiO2 (110) surface: a theoretical study

The adsorption of monomeric formamide over the rutile TiO2 (110) surface has been studied by way of density functional theory. At a coverage of one formamide per two surface repeat units, there is very little difference in stability between molecular and dissociative adsorption configurations. At larger coverages, however, dissociative adsorption in a manner analogous to that observed for carboxylic acid becomes favoured. Stable adsorption modes for the initial steps of a formic acid-like decomposition route are also studied. In the presence of an oxygen vacancy, dissociatively adsorbed formamide bonds with either its N (or O) atom above the oxygen vacancy while the other atom (N or O) coordinates to a five-fold coordinated surface titanium atom. Both modes of adsorption are however found to be isoenergetic. Results provide a possible explanation for the experimentally observed nitriding of the TiO2 surface post adsorption. Density of States analyses indicated that the nature of interaction is mainly covalent with large degree of hybridisation between the formamide orbitals and surface states.

The adsorption of acrolein on a Pt (1 1 1): A study of chemical bonding and electronic structure

The adsorption of acrolein on a Pt (111) surface was studied using ab-initio and semiempirical calculations. Geometry optimization and densities of states (DOS) curves were carried out using the Vienna Ab-initio Simulation Package (VASP) code. We started our study with the preferential geometries corresponding to the different acrolein/Pt (111) adsorption modes previously reported. Then, we examined the evolution of the chemical bonding in these geometries, using the crystal orbital overlap population (COOP) and overlap population (OP) analysis of selected pairs of atoms. We analyzed the acrolein intramolecular bonds, Pt (111) superficial bonds and new moleculesurface formed bonds after adsorption. We found that PtPt bonds interacting with the molecule and acrolein CO and CC bonds are weakened after adsorption; this last bond is significantly linked to the surface. The obtained CPt and OPt OP values suggest that the most stable adsorption modes are η3-cis and η4-trans, while the η1-trans is the less favored configuration. We also found that C pz orbital and Pt pz and dz2 orbitals participate strongly in the adsorption process.

Adsorption of organic dyes on TiO2 surfaces in dye-sensitized solar cells: interplay of theory and experiment

First-principles computer simulations can contribute to a deeper understanding of the dye/semiconductor interface lying at the heart of Dye-sensitized Solar Cells (DSCs). Here, we present the results of simulation of dye adsorption onto TiO(2) surfaces, and of their implications for the functioning of the corresponding solar cells. We propose an integrated strategy which combines FT-IR measurements with DFT calculations to individuate the energetically favorable TiO(2) adsorption mode of acetic acid, as a meaningful model for realistic organic dyes. Although we found a sizable variability in the relative stability of the considered adsorption modes with the model system and the method, a bridged bidentate structure was found to closely match the FT-IR frequency pattern, also being calculated as the most stable adsorption mode by calculations in solution. This adsorption mode was found to be the most stable binding also for realistic organic dyes bearing cyanoacrylic anchoring groups, while for a rhodanine-3-acetic acid anchoring group, an undissociated monodentate adsorption mode was found to be of comparable stability. The structural differences induced by the different anchoring groups were related to the different electron injection/recombination with oxidized dye properties which were experimentally assessed for the two classes of dyes. A stronger coupling and a possibly faster electron injection were also calculated for the bridged bidentate mode. We then investigated the adsorption mode and I(2) binding of prototype organic dyes. Car-Parrinello molecular dynamics and geometry optimizations were performed for two coumarin dyes differing by the length of the π-bridge separating the donor and acceptor moieties. We related the decreasing distance of the carbonylic oxygen from the titania to an increased I(2) concentration in proximity of the oxide surface, which might account for the different observed photovoltaic performances. The interplay between theory/simulation and experiments appears to be the key to further DSCs progress, both concerning the design of new dye sensitizers and their interaction with the semiconductor and with the solution environment and/or an electrolyte upon adsorption onto the semiconductor.

Acetylacetone, an Interesting Anchoring Group for ZnO-Based Organic−Inorganic Hybrid Materials: A Combined Experimental and Theoretical Study

Acetylacetone (acacH) adsorption on ZnO (10-10) surface has been studied by a theoretical periodic approach using density functional theory. Two dissociative adsorption modes were investigated and compared to the most stable adsorption mode of formic acid. Acetylacetone appears as a suitable anchoring group for hybrid materials, with adsorption energies of the same order of magnitude as formic acid. IR spectra of the acac/ZnO systems were computed in order to determine the spectral signature of adsorption and, possibly, of each adsorption mode to follow the coordination of acac on ZnO at the experimental level. The results have been compared to Fourier transform infrared (attenuated total reflection-IR) experimental spectra. The present investigation points out the interest of acetylacetone as an anchoring group for the development of new ZnO-based functionalized hybrid layers for corrosion protection, light emitting diodes, photocatalytic systems, and dye-sensitized solar cells.

The adsorption of 1,3-butadiene on Pd/Ni multilayers: The interplay between spin polarization and chemisorption strength

The adsorption of 1,3-butadiene (BD) on the Pd/Ni(1 1 1) multilayers has been studied using the VASP method in the framework of the density functional theory (DFT). The adsorption on two different configurations of the Pd n /Ni m (1 1 1) systems were considered. The most stable adsorption sites are dependent on the substrate composition and on the inclusion or not of spin polarization. On Pd 1Ni 3(1 1 1) surface, di–π–cis and 1,2,3,4-tetra-σ adsorption structures are the most stable for non-spin polarized (NSP) and spin polarized (SP) levels of calculation, respectively. Conversely, on Pd 3Ni 1(1 1 1) surface, the 1,2,3,4-tetra-σ adsorption structure is the most stable for both NSP and SP levels, respectively. The magnetization of the Pd atoms strongly modifies the adsorption energy of BD and its most stable adsorption mode. On the other hand, as a consequence of BD adsorption, the Pd magnetization decreases. The smaller adsorption energies of BD and 1-butene on the Pd 1Ni 3(1 1 1) surface than on Pd(1 1 1) can be associated to the strained Pd overlayer deposited on Ni(1 1 1).

A comparative DFT study of atomic and molecular oxygen adsorption on neutral and negatively charged Pd xCu 3−x ( x = 0–3) nano-clusters

Adsorption of molecular and atomic oxygen on neutral and negatively charged Pd x Cu 3− x ( x = 0–3) nano-clusters have been studied by density functional theory. It has been observed that modes and energies of adsorption strongly depend on the charge and composition of the nano-clusters. The most stable adsorption mode for molecular oxygen on neutral Pd/Cu nano-clusters is to bridge Pd–Cu site (adsorption energy = −103.7 kJ mol −1) while on negatively charged nano-clusters bridging of Pd–Pd or Cu–Cu are more stable adsorption modes (adsorption energies = −140.9 and −172.9 kJ mol −1). Also, the most stable adsorption mode for atomic oxygen on neutral Pd/Cu nano-clusters is on the hollow site (adsorption energy = −111.9 kJ mol −1) while on negatively charged nano-clusters adsorption on top of Pd or Cu are the most stable adsorption modes (adsorption energies = −289.7 and −370.2 kJ mol −1). With increasing copper content the adsorption energy of molecular and atomic oxygen on neutral nano-clusters decrease while on negatively charged nano-clusters increase. Also, it has been shown that (I) increasing of copper content in nano-cluster and (II) adding of negative charge to nano-clusters weaken O–O bond which can increase the activity of the nano-cluster for dissociation of molecular oxygen. Moreover, it has been shown that dissociation of molecular oxygen on nano-clusters is exothermic and occurs more favorably on Pd/Cu nano-clusters compared to on Pd nano-cluster.