Chemisorption Bond Research Articles

Electrochemical promotion for hydrogen production via ethanol steam reforming reaction

In this work we have investigated for the first time the electrochemical activation of a catalyst for the ethanol reforming reaction. For that purpose, a Pt-KβAl2O3 electrochemical catalyst has been prepared, characterized and tested under ethanol reforming reaction conditions. The electrochemically supply of potassium ions under negative polarization step, strongly increased the hydrogen production rates leading to a reversible and controllable promotional effect. It has been attributed to the enhancement of the kinetic of ethanol dehydrogenation reaction, due to the strengthening of the chemisorptive bond of intermediate ethoxy molecules. It will increase the stability of this intermediate, thus favoring its formation, which initiates the ethanol reforming process. However, a large amount of carbonaceous species were formed on the catalyst surface during the negative polarization step that causes a continuous decrease in the catalytic activity under long polarization times. Under these conditions, the application of a catalyst potential of 2 V leads to a complete removal of the previous deposited carbonaceous molecules which allows further electrochemical activation steps. The obtained catalytic results have been supported by in-situ temperature programmed oxidation analysis and ex-situ Raman spectroscopy and Scanning Electron Microscopy. These techniques, in conjunction with the obtained catalytic results, have demonstrated the interest of the EPOC phenomenon for in-situ tuning the adsorption of reactants molecules on catalyst surface and its application in the hydrogen production technology, improving catalyst conversion and selectivity.

Quantum-Chemical Analysis of the Mechanism of Degradation of Binary Platinum Nanoclusters with Sulfur-Containing Compounds

We propose a quantum-chemical model of poisoning of binary Pt42Me13 platinum nanoclusters (where Me is a transition metal, namely, Fe, Co, or Ni) with core-shell structure by hydrogen sulfide and sulfur dioxide in media of low-temperature fuel cells. This model is based on the evaluation of the adsorption characteristics of interaction of H2S and SO2 molecules with the surfaces of nanoclusters. It is shown that their susceptibility to the formation of strong chemisorption bonds with H2S and SO2 molecules depends on the type of nanocluster core and the sites of adsorption on their surfaces. By using the quantum-chemical method of density functional, we establish the regularities of influence of the cores of binary nanoclusters of transition metals Fe, Co, or Ni on the geometric and energy characteristics of the adsorption of molecules. We also propose the mechanism of interaction of H2S and SO2 molecules with binary platinum nanoclusters based on the changes in the electronic properties of the surface platinum atoms or in the distribution of active adsorption centers on the surface of nanoclusters depending on the type of the core. The accumulated results confirm the prospects of the density functional method in the theoretical evaluation of the influence of chemical composition and the structure of binary nanoparticles on their properties in the reactions of low-temperature fuel cells.

Effect of elastic deformation and the magnetic field on the electrical conductivity of p-Si crystals

It is shown that at a deformation rate of 0.41 kg/min, the characteristic feature of the dependence of the surface resistance of the p-Si sample on the magnitude of its elastic deformation (R(σ)) is the reduction of the resistance during compression and unclamping. With the increase in the number of “compression-unclamping” cycles, the difference between the positions of the compression and unclamping curves decreases. The transformation of two types of magnetically sensitive defects occurs under the impact of a magnetic field on p-Si crystals. The defects are interrelated with two factors that cause the mutually opposite influence on the conductivity of the crystal. The first factor is that the action of the magnetic field decreases the activation energy of the dislocation holders, which leads to an increase in the electrical conductivity of the sample. The second factor is that due to the decay of molecules of oxygen-containing impurities in the magnetic field, the stable chemisorption bonds appear in the crystal that leads to a decrease in its conductivity. If the sample stays in the magnetic field for a long time, the one or the other mechanism predominates, causing a slow growth or decrease in resistance around a certain (averaged) value. Moreover, the frequency of such changes is greater in the deformed sample. The value of the surface resistance of p-Si samples does not change for a long time without the influence of the magnetic field.

Quantum chemical approach to atomic manipulation of chlorobenzene on the Si(111)- 7×7 surface: Resonance localization, vibrational activation, and surface dynamics

We present a cluster model to describe the localization of hot charge carriers on the Si(111)-$7\ifmmode\times\else\texttimes\fi{}7$ surface, which leads to (nonlocal) desorption of chlorobenzene molecules in scanning tunneling microscope (STM) manipulation experiments. The localized charge carriers are modeled by a small cluster. By means of quantum chemical calculations, this cluster model explains many experimental findings from STM manipulation. We show that the negative charge is mainly localized in the surface, while the positive one also resides on the molecule. Both resonances boost desorption: In the negative resonance the adatom is elevated; in the positive one the chemisorption bond between the silicon surface adatom and chlorobenzene is broken. We find normal modes promoting desorption matching experimental low-temperature activation energies for electron- and hole-induced desorption.

Combined Electrochemical Impedance Spectroscopy, X-ray Photoelectron Spectroscopy, and Atomic Force Microscopy study of the distinct behavior of L-cysteine-thiolate formed at p- and n-GaAs(100) electrodes in Sodium Dodecyl Sulfate solution

Electrochemical impedance spectroscopy (EIS), X-ray spectroscopy (XPS), and atomic force microscopy (AFM) have been used to investigate the effects of the sodium dodecyl sulfate (SDS) interaction with the L-cysteine-thiolate covered p- and n-GaAs(100) electrodes in H2SO4 solution. The potential-induced reversible proton transfer found to occur within the L-cysteine-thiolate layer in the simple acid solution during the cathodic as well as the anodic potential scans at the p-doped semiconductor electrode and only in the anodic scan at the n-doped one is suppressed in both cases. XPS data revealed that the anionic surfactant has a complex interaction with the L-cysteine-thiolate covered GaAs(100) electrodes, which implicates both electrostatic and chemical forces. The SO42− head group interacts electrostatically with the NH3+ group of the cysteine-thiolate but it is also chemically involved in a chemisorption bond with the available Ga atoms. The amount of Ga-bound SDS species is much more pronounced at the L-cysteine-thiolate/p-GaAs(100) than at the L-cysteine thiolate/n-doped GaAs(100) electrodes meaning that cysteine-thiolate is mainly formed at As sites in the first case and at Ga sites in the latter one. This outcome strengthens the previous conclusions concerning the role of the dopant in determining the bonding way of the L-cysteine on GaAs(100) surfaces.

Interaction of Adatoms and Molecules with Single-Layer Arsenene Phases

Recent studies have shown that arsenic can form single-layer phases in buckled honeycomb as well as symmetric washboard structures, named as arsenene. These structures are stable even in freestanding form and are nonmagnetic semiconductors in the energy range which is suitable for various electronic applications. In this study we investigated the adsorption of selected adatoms (H, Li, B, C, N, O, Al, Si, P, Cl, Ti, Ga, Ge, As, Se, and Sb) and physisorption of molecules (H2, O2, and H2O) to these two arsene phases. Since the interaction of these adspecies with arsenene are studied using large supercells, the coupling between adspecies is minimized, and hence our results can be interpreted to mimic the effects of isolated adatom or physisorbed molecule. It is found that the adatoms form strong chemisorption bonds and hence modify the atomic structure and physical properties locally. Some of the adatoms give rise to significant local reconstruction of the atomic structure. Electronic states of some adatoms b...

Density-functional-based tight-binding parameterization of Mo, C, H, O and Si for studying hydrogenation reactions on molybdenum carbide

Hydrogenation reactions catalyzed by transition-metal-containing nanoparticles represent an important type of reaction in chemical industry. However, the modeling of these reactions in their working conditions requires much longer simulation times than what could usually be achieved with ab initio or first-principle methods. To address this problem, in this work, the density-functional-based tight-binding (DFTB) method was parameterized for hydrogenation reactions on molybdenum carbide catalysts, involving the elements C, H, Mo, O and Si. The overall quality of the DFTB parameters was tested with band structure/molecular orbital energies, molecular/crystal structures, chemisorption bond strengths, hydrogen adsorption energies, hydrogenation reaction energies, molecular vibrational frequencies, energy barriers and the structures of the transition states for systems of interest. The parameterized DFTB method gave errors of <1.45 % for bond distances of hydrocarbons and 4.86 % for non-hydrocarbons. It could reproduce the structure and vibrational frequencies (with errors of about 100 cm−1) of selected hydrocarbon–molybdenum carbide complexes obtained from DFT calculations. Good agreement was reached between DFTB and DFT on the dissociative adsorption of hydrogen on the α-Mo2C (0001) surface. For most of the hydrogenation reactions examined, DFTB showed errors of ~2 kcal/mol compared to DFT/PBE, with a few exceptions of ~5 kcal/mol. It could also describe the reaction energies, the forward and reverse energy barriers and the transition-state structures for the benzene hydrogenation reaction on a Mo38C19 cluster.

Application of topological analysis of the electron localization function to the complexes of molybdenum carbide nanoparticles with unsaturated hydrocarbons

The catalytic transformation of the heavy aromatics in bitumen into lighter components is the key to the upgrading and refining of the oil sands. To understand the chemical bonding in molybdenum carbide nanoparticle (MCNP) catalysts and the chemisorption bonds between the MCNPs and unsaturated hydrocarbons, the topological analysis of the electron localization function was applied to various MCNPs and their complexes with unsaturated hydrocarbons. For some of the smaller complexes, comparisons are made with the atoms in molecules approach, including the calculation of delocalization indices. The results are interpreted in the Lewis bonding scheme. It was found that the Mo–C bonding can be highly ionic in cases like Mo8C12 and MoC but shows significant covalent character in Mo2C, Mo3C, and Mo28C14. The chemisorption bonds between hydrocarbons and the MCNPs involve electron sharing of various types with strong covalent character. The strong three– or four–center interactions determine the adsorption configurations of the hydrocarbons on the MCNPs. Derivatives of benzene show some different bonding features, which depend strongly on the substituent or the heteroatom.

How molecular is the chemisorptive bond?

Trends in adsorption energies as a function of transition metal differ for adsorbates that are attached atop a surface atom or are adsorbed onto a high coordination site. When adsorption onto early and late transition metals is compared variation in relative bond energies of adsorbates attached to different sites is large. A theoretical understanding is provided based on the analysis of the electronic structure of the respective chemical bonds. The electronic structure analysis is based on partial density of states (PDOS) and bond order overlap population densities from crystal orbital Hamiltonian population (COHP) calculations available from DFT electronic structure computations. This is complemented by calculations of Bader charge densities and electron density topology properties. Variation of the respective bond energies depends on the symmetry of the molecular orbitals that form the chemical bond. The key electronic structure parameters are the position of the Fermi level in the bonding or antibonding molecular orbital partial density of states region of the chemical bond and chemical bond polarity. These are very different for adsorbates adsorbed onto the same transition metal surface, but which have different coordination with surface metal atoms. The adsorption energies and the respective electronic structures of adatoms H, C and O and molecular fragments CHx (x = 1-3) are compared with those of the analogous molecules that contain a single transition metal atom. When adsorbed atop, trends in bond energies are remarkably similar to those of the corresponding molecules. The difference in bond energies of adsorbates and transition metal molecules, i.e. the embedding energy, is shown to consist of three contributions: quenching of the sometimes high molecular spin states, weakening of the adsorbate-surface interaction energy and weakening of the metal-metal atom bond energies next to the adsorbate. Conventional scaling rules of the interaction energies of adsorbed CHx (0 < x ≤ 3) fragments are satisfied only for adsorbates in high coordination sites. For the early transition metals a breaking of this rule is found for C and CH or N and NH when adsorbed atop a transition metal surface or when they are part of a transition metal molecule. The M-C bond energy is found to be only stronger than that of the M-CH bond as long as the Fermi level or the HOMO is located in the antibonding molecular orbital partial density of states of the chemical bond.

Localized surface plasmon resonance and surface enhanced Raman scattering responses of Au@Ag core-shell nanorods with different thickness of Ag shell.

The properties of the localized surface plasmon resonance (LSPR) and the surface enhanced Raman scattering (SERS) of the core-shell bimetallic nanostructures, that is the monodisperse Au@Ag core-shell nanorods with different thickness of Ag shell, are theoretically and experimental researched. The UV-vis-NIR absorption spectra of the Au@Ag core-shell nanorods are measured and displayed their blue-shifts of the longitudinal plasmon resonance peaks with increasing of Ag concentrations in the colloidal solution. And the absorption spectra of the Au@Ag core-shell nanorods are simulated by the Finite Element Method (FEM), which are in agreement with the experimental measurements and reveal their LSPR mechanism as the varying structures. In addition, Rhodamine 6G, as a Raman reporter molecule, is used to investigate SERS of gold nanorods and Au@Ag core-shell nanorods. It is found that Au@Ag core-shell nanorods have better SERS responses, comparing with those of Au nanorods, and their SERS intensities are increased with the increases of the Ag shell thickness, which demonstrate that the chemisorptive bond effect and the morphology of the nanoparticle play key roles to the SERS signals. It is significant to design the biosensor based on the properties of Au@Ag core-shell nanorods.

A quantum mechanical study of water adsorption on the (110) surfaces of rutile SnO₂ and TiO₂: investigating the effects of intermolecular interactions using hybrid-exchange density functional theory.

Periodic hybrid-exchange density functional theory calculations are used to explore the first layer of water at model oxide surfaces, which is an important step for understanding the photocatalytic reactions involved in solar water splitting. By comparing the structure and properties of SnO2(110) and TiO2(110) surfaces in contact with water, the effects of structural and electronic differences on the water chemistry are examined. The dissociative adsorption mode at low coverage (1/7 ML) up to monolayer coverage (1 ML) on both SnO2 and TiO2(110) surfaces is analysed. To investigate further the intermolecular interactions between adjacent adsorbates, monolayer adsorption on each surface is explored in terms of binding energies and bond lengths. Analysis of the water adsorption geometry and energetics shows that the relative stability of water adsorption on SnO2(110) is governed largely by the strength of the chemisorption and hydrogen bonds at the surface of the adsorbate-substrate system. However on TiO2(110), a more complicated scenario of the first layer of water on its surface arises in which there is an interplay between chemisorption, hydrogen bonding and adsorbate-induced atomic displacements in the surface. Furthermore the projected density of states of each surface in contact with a mixture of adsorbed water molecules and adsorbed hydroxyls is presented and sheds some light on the nature of the crystalline chemical bonds as well as on why adsorbed water has often been reported to be unstable on rutile SnO2(110).

Ga and As competition for thiolate formation at p-GaAs(1 1 1) surfaces

Self-assembled layers of 4,4′-thio-bis-benzene-thiolate spontaneously adsorbed on p-doped GaAs(111)A and GaAs(111)B electrodes were examined by EIS, XPS, and AFM investigations. XPS data provide evidence that (i) both As and Ga atoms are involved in the thiolate formation no matter which one is prevailing on the semiconducting surface and (ii) only one of the two thiol groups participates in the chemisorption bond. EIS and AFM results point to a more stable thiolate layer formed on the As-terminated surface than that formed on the Ga-terminated surface due to stronger self-assembling effects developed between the adsorbed species.

Structure and stability of weakly chemisorbed ethene adsorbed on low-index Cu surfaces: performance of density functionals with van der Waals interactions

We have investigated the performance of popular density functionals that include van der Waals interactions for the experimentally well-characterized problem of ethene (C2H4) adsorbed on the low-index surfaces of copper. This set of functionals does not only include three van der Waals density functionals—vdwDF-PBE, vdwDF-revPBE and optB86b-vdwDF—and two dispersion-corrected functionals—Grimme and TS–but also local and semi-local functionals such as LDA and PBE. The adsorption system of ethene on copper was chosen because it is a weakly chemisorbed system for which the vdW interactions are expected to give a significant contribution to the adsorption energy. Overall the density functionals that include vdW interactions increased substantially the adsorption energies compared to the PBE density functional but predicted the same adsorption sites and very similar C–C bonding distances except for two of the van der Waals functionals. The top adsorption site was predicted almost exclusively for all functionals on the (110), (100) and (111) surfaces, which is in agreement with experiment for the (110) surface but not for the (100) surface. On the (100) surface, all functionals except two van der Waals density functionals singled out the observed cross-hollow site from the calculated C–C bonding distances and adsorption heights. On the top sites on the (110) surface and the cross-hollow site on the Cu(100) surface, the ethene molecule was found to form a weak chemisorption bond. On the (111) surface, all functionals gave a C–C bonding distance and an adsorption height more typical for physisorption, in agreement with experiments.

Correlation of Surface Characteristics with Catalytic Performance of Potassium Promoted Pd/Al2O3 Catalysts: The Case of N2O Reduction by Alkanes or Alkenes

The present study aims at investigating the role of potassium (K) promoter on the surface and catalytic behavior of Pd/Al2O3 catalysts during N2O reduction by alkanes (CH4, C3H8) or alkenes (C3H6). In this context, the surface properties of un-promoted and K-promoted catalysts were evaluated by means of X ray photoelectron spectroscopy (XPS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption and FTIR-pyridine adsorption. The results reveal that both the electronic and structural properties of Pd entities can be substantially modified by potassium ions, which are in close proximity with catalyst surface without exhibiting a tendency to accumulate. This in turn results in significant modifications on reactants/intermediates chemisorption bonds, affecting the catalytic performance of K-promoted catalysts. Indeed, it was found that potassium strongly promotes the N2O reduction by propane or propene yielding notably lower N2O light off temperatures (ca. 100 °C) as compared to those obtained over un-promoted catalysts. On the contrary, a slight inhibition upon K-promotion was observed, when using CH4 as reducing agent, implying that the action of the K-promoter is strongly related with the reducing agent used and its’ relative interaction with the catalyst’s surface. The results are discussed in terms of the correlation between the surface chemistry modifications induced by electropositive promoters and the de-N2O performance of K-promoted catalysts.

Face-Dependent Bond Lengths in Molecular Chemisorption: The Formate Species on Cu(111) and Cu(110)

Many previous structural studies of molecular adsorbates on metal surfaces indicate that the local coordination and bonding is closely similar to that in organometallic compounds, implying that the metallic substrate has no significant influence. Here we show that such an influence is detectable for one model system, namely, the formate species, HCOO, adsorbed on the atomically rough and smooth (110) and (111) surfaces of Cu, leading to a statistically significant difference (0.09±0.05 Å) in the Cu-O chemisorption bond length. The effect is reproduced in density functional theory calculations.

Understanding the Interaction of the Porphyrin Macrocycle to Reactive Metal Substrates: Structure, Bonding, and Adatom Capture

We investigate the adsorption and conformation of free-base porphines on Cu(110) using STM, reflection absorption infrared spectroscopy, and periodic DFT calculations in order to understand how the central polypyrrole macrocycle, common to all porphyrins, interacts with a reactive metal surface. We find that the macrocycle forms a chemisorption bond with the surface, arising from electron donation into down-shifted and nearly degenerate unoccupied porphine π-orbitals accompanied with electron back-donation from molecular π-orbitals. Our calculations show that van der Waals interactions give rise to an overall increase in the adsorption energy but only minor changes in the adsorption geometry and electronic structure. In addition, we observe copper adatoms being weakly attracted to adsorbed porphines at specific molecular sites. These results provide important insights into porphyrin-surface interactions that, ultimately, will govern the design of robust surface-mounted molecular devices based on this important class of molecules.

ChemInform Abstract: The Effect of Chlorine on Hydrogen Chemisorption by Silica-Supported Ru Catalysts: A Proton NMR Study.

Silica-supported ruthenium catalysts prepared from the RuCl3 · 3H2O precursor contain residual chloride after reduction in flowing hydrogen. The amount of this chlorine was determined by X-ray fluorescence spectroscopy to be less than one monolayer on the ruthenium surfaces. Both volumetric hydrogen chemisorption and proton NMR show suppression of hydrogen chemisorption capacity associated with the presence of chlorine contamination, which can be removed effectively with hot water elution. The variations of the hydrogen chemisorption capacity with chlorine coverage suggests that chlorine would completely block and poison the Ru surfaces for hydrogen chemisorption at a Cl coverage of about 76%. The effect of residual chloride on the proton NMR resonance position and the spin-lattice relaxation time for the chemisorbed hydrogen indicates an electronic interaction between Cl adspecies and surface Ru, which results in the weakening of the H-Rus chemisorptive bond at the Ru surfaces. This reduced H-Rus interaction may be the cause for an increase in the ratio of the reversibly adsorbed hydrogen over the irreversibly adsorbed hydrogen with increasing Cl coverage. It is proposed that the mechanism for suppression of hydrogen chemisorption by chlorine involves both physical site blocking of surface Ru atoms and shortranged electronic interactions between Cl adatoms and neighboring surface Ru atoms.

ChemInform Abstract: NMR Spectroscopy as a Probe of Surfaces of Supported Metal Catalysts

The nuclear magnetic resonance (NMR) properties of bulk metals are usually dominated by a magnetic coupling between the nuclear spin and the spin of the conduction electrons. The coupling is proportional to the density of one-electron-states at the Fermi energy and results in NMR shifts that are large in comparison with the usual chemical shift range and also in a characteristic temperature dependence of the nuclear spin lattice relaxation rate. In systems that lack the translational invariance of the extended solid, these properties depend on the local electronic environment of the resonating nucleus (the local density of states). Examples of such systems are bulk random alloys and small metal particles, such as those in supported metal catalysts. In certain cases an adsorbate on a metal also acquires such metallic NMR properties; this often happens with carbon monoxide and probably also with hydrogen, the only adsorbates considered in this review. These metallic NMR characteristics are of interest in catalysis for several reasons. In investigations of the metal (typically 195 Pt), the local density of states on the surface is different from that in the bulk so that NMR spectroscopy can be used to determine the metal dispersion. The 1 H NMR shift of adsorbed hydrogen induced by the metal surface can be used to distinguish hydrogen on the metal from that on the support, e.g., in investigations of hydrogen spillover. The spin lattice relaxation rate of adsorbed 13 CO shows that the chemisorption bond has metallic character. On a theoretical level, the local density of electron states at the Fermi energy on metal surface sites plays a role in frontier orbital theories of chemisorption. This quantity can be measured by NMR of the metal for catalysts that have undergone various surface treatments, not necessarily under vacuum conditions, and the results can be qualitatively interpreted in a much simplified version of such a theoretical framework. This review provides a brief presentation of the relevant theory of metals and a thorough discussion of the metallic characteristics observed in the NMR spectroscopy of hydrogen, carbon monoxide, and platinum.

Role of Au−C Interactions on the Catalytic Activity of Au Nanoparticles Supported on TiC(001) toward Molecular Oxygen Dissociation

High-resolution photoemission and density functional calculations on realistic slab surface models were used to study the interaction and subsequent dissociation of O(2) with Au nanoparticles supported on TiC(001). The photoemission results indicate that at 150 K O(2) adsorbs molecularly on the supported gold nanoparticles, and upon heating to temperatures above 200 K the O(2) --> 2O reaction takes place with migration of atomic oxygen to the TiC(001) substrate. The addition of Au to TiC(001) substantially enhances the rate of O(2) dissociation at room temperature. The reactivity of Au nanoparticles supported on TiC(001) toward O(2) dissociation is much larger than that of similar nanoparticles supported either on TiO(2)(110) or MgO(001) surfaces, where the cleavage of O-O bonds is very difficult. Density functional calculations carried out on large supercells show that the contact of Au with TiC(001) is essential for charge polarization and an enhancement in the chemical activity of Au. Small two-dimensional particles which expose Au atoms in contact with TiC(001) are the most reactive. While O(2) prefers binding to Au sites, the O atoms interact more strongly with the TiC(001) surface. The oxygen species active during the low-temperature (<200 K) oxidation of carbon monoxide on Au/TiC(001) is chemisorbed O(2). Once atomic O binds to TiC(001), the chemisorption bond is so strong that temperatures well above 400 K are necessary to remove the O adatoms from the TiC(001) substrate by direct reaction with CO. The high reactivity of Au/TiC(001) toward O(2) at low-temperature opens the route for the transformation of alcohols and amines on the supported Au nanoparticles.

Quantum chemical modeling of hydroxide ion adsorption on group IB metals from aqueous solutions

Interactions between hydroxide ions and (001), (011), and (111) faces of Cu, Ag, and Au in the gas phase and in an infinitely diluted aqueous solution are nonempirically studied with the density functional method (DFT) with the use of B3LYP functional. Adsorption surfaces are modeled by n-atomic clusters (n = 10–18). The presence of solvent is taken into account in terms of the combined molecular-continuum model, and the effect of polarizable dielectric is estimated with the use of self-consistent solvent reaction field COSMO model. The composition and structure of hydrated hydroxide ion are found and the hydration energy is estimated in satisfactory agreement with the experimental data. Relative effects of the metal nature, crystal face orientation, and adsorption site on the adsorption characteristics are determined, and the geometrical parameters of the chemisorption bond and the degree of the partial charge transfer are estimated. A comparative analysis of the results obtained shows that the adsorption on-top position, which is energetically unfavorable in the gas phase, becomes prevailing in an aqueous solution, which can be explained by the increased stability of the on-top adsorption complex.