Partial Desorption Research Articles

A Comprehensive Study of Self-Assembled Monolayers of Anthracenethiol on Gold: Solvent Effects, Structure, and Stability

The formation and molecular structure of self-assembled monolayers (SAMs) of anthracene-2-thiol (AnT) on Au(111) have been characterized by reflection adsorption infrared spectroscopy, thermal desorption spectroscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption spectroscopy, scanning tunneling microscopy, and low energy electron diffraction. It is demonstrated that highly ordered monolayer films are formed upon immersion, but their quality depends critically on the choice of solvents and rinsing conditions. The saturated monolayer is characterized by a closed packed arrangement of upright standing molecules forming a (2 x 4)rect unit cell. At about 450 K a partial desorption takes place and the remaining molecules form a dilute (4 x 2)-phase with an almost planar adsorption geometry, while further heating above 520 K causes a thermally induced fragmentation. According to their different densities both phases reveal very diverse chemical reactivities. Whereas the saturated monolayer is stable and inert under ambient conditions, the dilute phase does not warrant any protection of the sulfur headgroups which oxidize rapidly in air.

Frontal analysis for characterizing the adsorption–desorption behavior of β-lactoglobulin on immunoadsorbents

High-performance frontal affinity chromatography was employed to study the adsorption–desorption kinetics characterizing the retention of β-lactoglobulin (β-LG) onto polyclonal anti-β-lactoglobulin (anti-β-LG) chromatographic supports. The adsorption and desorption processes were studied by analyzing two different elution fronts separated by a relatively long rinsing step. The method consists in performing two successive frontal injections of the protein. In between, the column was rinsed with a given volume of mobile phase (buffer alone). During this rinsing stage, a partial desorption may occur and a novel amount of protein could be adsorbed in the second frontal injection step. The whole process (first adsorption, possible desorption, and second adsorption) was simulated by a numerical procedure, in which the column was divided into a large number of slices. A model based on bi-Langmuir type kinetics was used to describe the adsorption of the protein on the support. The model assumes a non-uniform adsorbent with two types of binding sites. At equilibrium the adsorption isotherm is of the bi-Langmuir type. A global adsorption effect was considered which includes the effective binding process and the mass transfer resistances due to the transport to the binding site. Therefore, the column capacity and the kinetic parameters of the model (apparent adsorption and desorption rate constants) were determined from the best fit of the first and second adsorption fronts to the experimental ones. The other parameters of the model are the saturation capacities for the adsorption on each type of sites. The equilibrium affinity constants were estimated in a single experiment from the ratio of the apparent adsorption and desorption rate constants. The high values found (around 10 8 M −1) reveal a strong interaction of β-LG with the immunoadsorbent. Kinetic measurements were carried out at different flow rates. Both the apparent adsorption and desorption kinetics were faster at larger flow rates, indicating an important contribution of the mass transfer resistance in the stagnant fluid at the particle boundary. However, as expected, close values were found for the resulting equilibrium constants calculated from the ratio of the apparent adsorption and desorption rate constant determined at various flow rates.

EQCM analysis of Bi oxidation mechanism on a Pt electrode

The electro-oxidation behaviours of bismuth (Bi) species adsorbed on a platinum (Pt) anode were investigated using an in situ electrochemical quartz crystal microbalance analysis in the measurement of cyclic voltammetry (CV) and electrochemical impedance spectroscopy under potentiostatic mode. In the CV of Bi modified Pt, there were four distinct features in the current–potential curves during an anodic scan: (i) the adsorption of water molecules in acidic media, (ii) the formation of Bi oxide, (iii) ionization of Bi oxide and (iv) partial desorption of Bi species. During a cathodic scan, the Bi modified Pt surface recovered to its original state via the reduction of Bi oxide and re-deposition of Bi ion. Surface mass data with electrical charge change and impedance measurements of Bi modified Pt supported the electrocatalytic oxidation of bismuth species as the responsible mechanism.

Tertiary amide chemistry at the Ge(1 0 0)-2 × 1 surface

We have investigated the adsorption of several tertiary amides, including N, N-dimethylformamide, N, N-dimethylformamide- d 7, 1-methyl-2-pyrrolidinone, and N-methylcaprolactam, on Ge(1 0 0)-2 × 1 using multiple internal reflection Fourier transform infrared spectroscopy and density functional theory. At 310 K, all four tertiary amides were observed to selectively form a dative bond to the germanium surface through the oxygen atom. While previous work has shown that oxygen dative bonds are unstable near room temperature, tertiary amides exhibit delocalization of electron density from nitrogen to oxygen, which appears to increase the stability of the oxygen dative-bonded state. Partial desorption of these surface adducts on the timescale of minutes indicates weakly bound surface adducts with coverage dependent binding energies.

Water adsorption on O-covered Ru{0 0 0 1}: Coverage-dependent change from dissociation to molecular adsorption

When water is coadsorbed with oxygen at coverages above 0.25ML an intact water species is observed in high resolution X-ray photoelectron spectroscopy up to 220 K, which is significantly more stable than intact water on the clean surface. The presence of this species causes a shift in the O 1s binding energy of the pre-adsorbed oxygen, which indicates the formation of hydrogen bonds between the two adsorbates. Low coverages of oxygen induce partial dissociation and recombinative desorption in the same temperature range, which illustrates that desorption temperatures alone cannot be used to determine whether water is molecularly intact or not.

Sb-induced reconstructions on Si(113): Adatoms as the key elements

The adatom site is identified and determined as the main structural element of Sb-induced surface reconstructions of Si(113) by means of x-ray standing waves, x-ray photoelectron spectroscopy, and low-energy electron diffraction (LEED) in combination with density functional theory calculations. The analysis yields that the Sb adatom site can be found prevalently at Sb coverages below 1 ML. With increasing coverage, the relative amount of Sb residing in adatom sites is reduced in favor of mainly Sb tetramer formation, leading to a complex mixture of $(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructions at coverages above 1 ML. The observation of $(1\ifmmode\times\else\texttimes\fi{}2)$ LEED patterns can be explained by the formation of small antiphase domains of $(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructions. Upon annealing, partial desorption of Sb leads to the formation of a missing-row structure with a $(1\ifmmode\times\else\texttimes\fi{}n)$ LEED pattern.

Atomic structure of adsorbed sulfide on Cu(1 1 1) in acidic solution: In situ STM studies

Combined cyclovoltammetry (CV) and electrochemical scanning tunneling microscopy (ECSTM) studies on the electrochemical behavior of a Cu(1 1 1) single crystal electrode in a mixed solution of equal volumes of 0.5 mM H 2SO 4 and 1 mM Na 2S are presented. After an overview over all structural surface phases observed at different electrode potentials E, i.e. a chain-structure ( E = +200 to −150 mV versus RHE), a Moiré-structure ( E = −50 to −300 mV versus RHE) as well as a (√7 × √7)R19.1°-S-( E = −100 to −450 mV versus RHE) and a (2√7 × 2√7)R19.1°-S-structure ( E = −300 to −650 mV versus RHE), this paper focuses on a detailed description of the latter two. All phases are regular adsorbate structures on the surface and reversibly transform into each other. Except step alignment no reconstruction of the copper surface is observed. The unit cell of the √7-structure is rotated by ±19.1° with respect to that of the substrate lattice and contains, in contrast to a publication by Wang et al. (D. Wang, Q.M. Xu, L.J. Wan, C. Wang, C.L. Bai, Surf. Sci. 499 (2002) L159.), only one extra S-atom and therefore corresponds to a coverage of 0.286 (2/7) ML. Detailed atomistic models are proposed for the structure of the rotational domain boundaries in perfect agreement with the experimental observation. With decreasing electrode potential defects develop within the √7-structure due to partial sulfide desorption until a perfect 2√7-structure is formed. At given potential the creation and annihilation of defects occurs by an exchange process with the solution and not by intralayer hopping of adsorbed sulfur atoms.

Electrochemical Processes of Sulfide in NaCl Electrolyte Solutions on Mercury Electrode

AbstractThe electrochemical processes of sulfide in NaCl electrolyte solutions were studied using stripping cyclic voltammetry, differential capacity and chronoamperometric measurements. Some new effects were observed and are here discussed. At potentials around −0.1 to −0.3 V a pair of Faradaic adsorptive peaks, which can only be observed in the presence of chloride, is attributed to the formation of HgCl$\rm{ {_{4}^{2-}}}$ ion from the deposited layer of HgS. A second pair of peaks of purely capacitive character was observed at −0.4 V. This is the result of the reorientation and partial desorption or transformation of HgS layers. The formation and dissolution of HgS layers on mercury has been well investigated previously. We observed some new effects after depositing approximately ten layers of HgS and attributed this to the formation of polysulfides at positive potentials.

Electrochemical detection of lead ions via the covalent attachment of human angiotensin I to mercaptopropionic acid and thioctic acid self-assembled monolayers

An electrochemical sensor for the detection of lead ions is described which is made by modifying a gold electrode substrate with self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) or thioctic acid (TA) followed by covalent attachment of a lead binding peptide, human angiotensin I. Cyclic voltammetry of MPA–angiotensin modified gold electrodes complexed with lead displayed voltammograms with prominent lead peaks at E 0′, −0.29 V. A detection limit of 1 nM was achieved using Osteryoung square wave voltammetry. However, the electrodes were not stable over repeated electrochemical cycles due to partial electrochemical desorption of the SAM. The TA–angiotensin modified gold electrode showed greater stability and were able to be regenerated several times. Using Osteryoung square wave voltammetry for TA–angiotensin modified electrodes, lead concentrations down to 1.9 nM were detected. Although the detection limit of the TA–angiotensin modified electrode is higher than achieved with MPA–angiotensin, it is still well below Australian drinking water guidelines. Studies of interference effects on the Pb 2+ current showed Hg 2+ as a significant interferent but only at levels significantly greater than those found in natural waters.

Hydrodesulfurization of Selective Catalytic Cracked Gasoline

Hydrodesulfurization (HDS) reaction of catalytic cracked gasoline (CCG) on Co–Mo/γ-Al2O3 was investigated in detail to make clear the important factors for deep HDS of CCG. A CCG containing 229 ppm sulfur and 30.4 vol% olefins was used in this study. Eleven alkylthiophenes and 2 alkylbenzothiophenes, 3 alkylthiacyclopentanes, and 2 disulfides were identified in this CCG by means of GC-AED analyses. In the reaction at 220 °C and 1.6 MPa using a conventional flow reactor of bench pilot scale, these sulfur compounds were hydrodesulfurized, whereas thiols were produced from H2S and olefins. The reactions of thiophene HDS, isoolefin and n-olefin hydrogenation (HG) were studied to clarify the active sites on the catalyst. First, the effect of H2S on the reaction was examined. The HG of n-olefin as well as thiophene HDS was inhibited by H2S, while the HG of isoolefin was promoted. The effects of Co on these three reactions were also examined over the catalysts with different Co/(Co + Mo) ratios. Thiophene HDS was promoted by Co, while isoolefin HG was little affected and n-olefin HG was largely retarded. From these examinations, three types of active sites for thiophene HDS, isoolefin HG and n-olefin HG were proposed. Oligomers of isoolefins were found in the isoolefin hydrotreated product. The possibility of improving the HDS selectivity by carbonaceous deposit was investigated for HDS reactions of CCG and model compounds. The coking pretreatment was carried out on the catalyst and each reaction was examined. HDS selectivity (higher activity for HDS and lower activity for olefin HG) on CCGHDS was improved. Relative deactivation was in the following order, isoolefin HG > thiophene HDS > n-olefin HG. Pyridine modification (i.e. pyridine injection at 150 °C and partial pyridine desorption at 300 °C) was investigated on thiophene and olefins reaction. Thiophene HDS was little affected. Olefin HG and thiol production reaction were strongly inhibited. Improvement of HDS selectivity was observed in the reactions of CCG after pyridine modification. Improvement of HDS selectivity by pyridine modification was considered to result from the selective deactivation of the active sites for olefin reactions (hydrogenation and thiol production).

Activated adsorption of methane on Pt(1 1 1) —an in situ XPS study

We have investigated the activated adsorption of methane on Pt(1 1 1) by the combination of a supersonic molecular beam and in situ high-resolution X-ray photoelectron spectroscopy at the German synchrotron radiation facility BESSY II. On exposing the surface to a methane beam with kinetic energies between 0.30 and 0.83 eV, CH3 is formed as a stable species at 120 K; upon heating, at around 260 K the adsorbed methyl partly dehydrogenates to CH and partly recombines to methane, which desorbs. Upon adsorption at 300 K, CH is directly formed as a stable surface species. To verify the chemical identity of CH as an intermediate, we have also investigated the thermal evolution of a saturated ethylene layer. Upon heating, at ∼290 K partial ethylene desorption and the formation of ethylidyne is clearly observed in the spectra, as expected from the literature. From the binding energies and also from the vibrational signature of the C 1s spectra, an unequivocal assignment of the various surface species is possible. Measurements of the sticking coefficients of methane show that the saturation coverage at 120 K depends on the kinetic energy of the molecule; furthermore, the sticking coefficient for vibrationally excited molecules is strongly enhanced.

Underpotential Deposition of Hydrogen on Benzene-Modified Pt(111) in Aqueous H2SO4

The Pt(111) electrode is modified by an overlayer of C6H6 (ads) upon its cycling in the 0.05-0.80 V range in aq H2SO4 + 1 mM C6H6. The C6H6 (ads) overlayer significantly changes the underpotential-deposited H (H(UPD)) and anion adsorption, and cyclic-voltammetry (CV) profiles show a sharp cathodic peak and an asymmetric anodic one in the 0.05-0.80 V potential range. The C6H6 (ads) layer blocks the (bi)sulfate adsorption but facilitates the adsorption of one monolayer of H(UPD). Cycling of the benzene-modified Pt(111) in benzene-free aq 0.05 H2SO4 from 0.05 to 0.80 V results in a partial desorption of C6H6 (ads) and in a partial recovery of the CV profile characteristic of an unmodified Pt(111). The peak potential of the cathodic and anodic feature is independent of the scan rate, s (10 < or = s < or = 100 mV s(-1)), and the peak current density increases linearly with an increase of the scan rate. Temperature variation modifies the peak potential and current density but does not affect the charge density of the cathodic or anodic feature. Temperature-dependent studies allow us to determine the thermodynamic state function for the H(UPD) adsorption and desorption. Delta G degrees(ads)(H(UPD))assumes values from -4 to -12 kJ mol(-1), while has values from 9 to 14 kJ mol(-1). The values of delta Delta G degrees (delta Delta G degrees = delat Delta G degrees(ads) + delta Delta D degrees(des)) decrease almost linearly from 6 kJ mol(-1) at theta(H(UPD) --> 0 to 0 kJ mol(-1) at theta(H(UPD) --> 1. The nonzero values of delta Delta G degrees testify that the adsorbing and desorbing H(UPD) adatoms interact with an energetically different substrate. The lateral interactions changed from repulsive (omega = 29 kJ mol(-1) at theta(H(UPD) --> 0) to attractive (omega = -28 kJ mol(-1) at theta(H(UPD) --> 1) as the H(UPD) coverage increases. The values of delta S degrees(ads)(H(UPD)) increase from 19 to 56 J K(-1) mol(-1), while those of delta S degrees(des)(H(UPD)) decrease from 45 to -30 J K(-1) mol(-1) with an increase of H(UPD) coverage. The values of delta H degrees(des)(H(UPD)) and delta H degrees(des)(H(UPD)) vary from 0 to 27 kJ mol(-1). The Pt(111)-H(UPD) surface bond energy at the benzene-modified Pt(111) electrode falls in the 191-218 kJ mol(-1) range and is weaker than in the case of the unmodified Pt(111) electrode in the same electrolyte.

Structural investigation of amylose complexes with small ligands: inter- or intra-helical associations?

Highly crystalline amylose complexes with menthone (1) and linalool (2) were analysed by wide-angle X-ray diffraction and solid-state nuclear magnetic resonance (NMR). The complexes, after partial water desorption in a controlled atmosphere ( a w = 0.75), displayed a typical V–isopropanol structure, showing the presence of ligand inside or between the helices in the crystalline domains. Sequential washing of the powdered complexes with ethanol before and after desorption permitted probing the intra- and inter-helical inclusions. High resolution magic angle spinning (HRMAS) recordings were used to compare the chemical shifts of free and bound aroma which allowed a proposal that some hydrogen bonding is involved in the amylose complexing. Moreover, it showed that free aroma was completely removed by ethanol washing. Using cross polarization magic angle spinning (CPMAS) and X-ray scattering experiments, it was demonstrated that the V–isopropanol type was retained for linalool whatever the treatment used. On the contrary, the measurement shifts toward the V–6I amylose hydrate (V–h) type for menthone after ethanol washing before the desorption step, reflecting the disappearance of inter-helical associations between menthone and amylose. The stability of the complex prepared with linalool shows that this ligand is more strongly associated to amylose helices. The discrepancies observed in the chemical shifts attributed to carbons C1 and C4 in CPMAS spectra of V–isopropanol and V–h forms could be attributed either to a deformation of the single helix (with possible inclusion of the ligand inside) or to the presence of the ligand between helices (only water molecules are present in the V–h form).

Formation of a supramolecular gel between alpha-cyclodextrin and free and adsorbed PEO on the surface of colloidal silica: effect of temperature, solvent, and particle size.

Aqueous solutions of alpha-cyclodextrin (alpha-CD) complex spontaneously with poly(ethylene oxide) (PEO), forming a supramolecular structure known as pseudopolyrotaxane. We have studied the formation of the complex obtained from the threading of alpha-CD onto PEO, both free in solution and adsorbed on colloidal silica. The kinetics of the reaction were studied by gravimetric methods and determined as a function of temperature and solvent composition for the PEO free in solution. PEO was then adsorbed on the surface of colloidal silica particles, and the monomers were displaced by systematically varying the degree of complexation, the concentration of particles, and the molecular weight of the polymer. The effect of the size of the silica particles on the yield of the reaction was also studied. With the adsorbed PEO, the complexation was found to be partial and to take place from the tails of the polymer. The formation of a gel network containing silica at high degrees of complexation was observed. Small-angle X-ray and neutron scattering experiments were performed to study the configuration of the polymeric chains and confirmed the partial desorption of the polymer from the surface of the silica upon complexation.

Anomalous Phenomena in Oxygen Desorption from Tungsten and Their Mathematical Modeling

A model is developed that describes unusual phenomena in oxygen desorption from tungsten surfaces: (1) in temperature-programmed desorption (TPD) in the transfer from the linear regime of sample heating to the isothermal regime, a maximum of the desorption rate is observed; (2) in the stepped partial desorption of oxygen, the beginning of each subsequent TPD peak shifted toward higher temperatures; (3) the apparent activation energy of desorption increased in the latter case by a factor of ∼3 (from 280 to 920 kJ/mol). Monte Carlo modeling that takes into account surface migration of adsorbed oxygen atoms and their lateral interactions gave semiquantitative description of experimental data.

X-ray diffraction study on GaAs(0 0 1)- [formula omitted] surfaces under molecular-beam epitaxy conditions

The GaAs(0 0 1)-(2×4) reconstructed surface was investigated by in situ surface X-ray diffraction. X-ray diffraction patterns were measured with increasing substrate temperature within the β phase of GaAs(0 0 1)-(2×4) in a constant As flux of 5×10−7 Torr. At relatively low temperatures up to 545 ∘C, the observed X-ray diffraction patterns agree well with the β2(2×4) surface. However, a different X-ray diffraction pattern was obtained at temperatures close to the α phase, while the 2×4 periodicity still persisted. This change is explained by partial As-dimer desorption which results in a mixture of the β2(2×4) and α2(2×4) structures.

Conformational Switching of Molecular Brushes in Response to the Energy of Interaction with the Substrate

Cylindrical brush molecules adsorbed on a surface change their contour length and then switch conformation from rodlike to globular upon decrease of the surface energy of the substrate. The conformational changes result from partial desorption of poly(n-butyl acrylate) side chains as the surface pressure drops from 23.7 to 3.1 mN/m and the energy of interaction between the side chains and the substrate decreases from 89.7 mJ/m2 to 69.1 mJ/m2. At the lowest value of the interaction energy, one observes a coexistence of rodlike and globular molecules. This result is in agreement with the theoretical prediction of the rod-globule transition of surface confined brush molecules as a conformational phase transition of the first order.

The Study of Nitric Oxide Adsorption and the Mechanism of Surface “Explosions” in the Reaction of CO + NO on Pt(100) and Pd(110) Single Crystal Surfaces

High resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD) and temperature-programmed reaction (TPR) were used to study NO adsorption and the reactivity of COads and NOads molecules on Pd(110) and Pt(100) single crystal surfaces. Compared to the Pt(100)-(1 × 1) surface, the unreconstructed Pt(100)-hex surface is chemically inert toward NO dissociation into Nads and Oads atoms. When a mixed adsorbed COads + NOads layer is heated, a so-called surface “explosion” is observed when the reaction products (N2, CO2, and N2O) synchronously desorb in the form of sharp peaks with a half-width of 7-20 K. The shape specificity of TPR spectra suggests that the “vacancy” mechanism consists of the autocatalytic character of the reaction initiated by the formation an initial concentration of active sites due to partial desorption of molecules from the COads + NOads layer upon heating to high temperatures. Kinetic experiments carried out on the Pd(110) surface at a constant reaction pressure and a linear increase in the temperature confirm the explosive mechanism of the reaction NO + CO.

Heat capacity measurements of two-dimensional self-assembled hexadecanethiol monolayers on polycrystalline gold

The melting characteristics of hexadecanethiol two-dimensional self-assembled monolayers (SAMs) grown on polycrystalline Au surfaces are obtained via heat capacity measurements using nanocalorimetry at scanning rates of ∼30 000°C∕s. The analyzed amount of alkanethiol in the form of SAMs is typically in the 10−11 mol range. Melting occurs over a broad (full width on the half height ∼100°C) temperature range with a melting temperature of about 70°C, if the alkanethiol is partially desorbed from the Au surface during thermal treatment. SAMs in their as-deposited state without desorption have melting temperatures above 100°C. The heat of fusion is about 20kJ∕mol. Partial desorption of the alkanethiol is directly observed. The melting characteristics of three-dimensional SAMs on Au nanoparticles are comparable to earlier studies using conventional calorimetry.

Structural evolution of self-assembled monolayer of 1-mercapto-2-propanol on Au(1 1 1) in a N 2 flow: an electrochemical and STM study

The structural evolution of 1-mercapto-2-propanol layer on Au(1 1 1) during a treatment in a N 2 flow was investigated using STM and cyclic voltammetry. The pristine layer showed a striped structure of (2√3×√3) with coverage of 0.33. As the SAM was treated in the N 2 flow, the superlattice evolved sequentially and irreversibly to the structures of (√21×√3) and (√3×√3)R30° without any coverage change and ultimately to amorphous domains with coverage less than 0.33. In cyclic voltammetric studies, it was found that the coverage of 1-mercapto-2-propanol was 0.33 and that the N 2 treatment induced a partial desorption of the adsorbed thiol molecules. In addition to this, the reductive stripping potential shifted to the anodic direction, signaling that the SAM of 1-mercapto-2-propanol became unstable. The structural evolution and destabilization of the 1-mercapto-2-propanol layer on Au(1 1 1) was discussed in terms of the attractive force between the OH groups via hydrogen bonding and the repulsive force between the methyl groups.