Interaction of water with clean and oxygen precovered nickel surfaces

The adsorption of water on a Ni(111) single crystal surface, clean as well as precovered with oxygen, has been investigated with thermal desorption spectroscopy (TDS) and measurements of the adsorption-desorption equilibrium combined with XPS (X-ray photoelectron spectroscopy). The measurements have been carried out with water pressures up to 10^–5 mbar on surfaces, which have been either clean or precovered with oxygen. On the clean Ni(111) surface the first adsorbate layer with a maximum coverage of 0.42 ML (monolayers) has a desorption energy of 52 kJ/mol and a preexponential factor of desorption of 10¹⁶s^–1. A second water layer adsorbs with the desorption energy of the ice multilayer but with first order kinetics. On Ni(111) precovered with chemisorbed oxygen an additional state of molecular, more strongly bound water is found, but no dissociation. For higher oxygen precoverages where NiO islands are formed on the surface, also the water dissociation product OH is found adsorbed. On a sample covered with a closed NiO layer, adsorbed OH and molecular water in an energetically not well-defined state are found. High doses of water on oxygen-precovered Ni(111) induce a slow surface modification leading to water dissociation.     

Carbonate formation and decomposition on atomic oxygen precovered Au(111)

Experimental results supported by density functional theory calculations show carbonate formation and reaction on atomic oxygen precovered Au(111). Oxygen mixing is observed in temperature-programmed desorption measurements when a Au(111) precovered with 16O is exposed to isotopically labeled CO2 (C18O2). The presence of 16O18O is attributed to surface carbonate formation and decomposition at surface temperatures ranging from 77-400 K and initial oxygen coverages ranging from 0.18-2.1 ML. A reaction probability on the order of 10(-4) and an activation energy of -0.15+/- 0.08 eV are estimated for this reaction.     

A DFT study of H2O dissociation on metal‐precovered Fe (100) surface

The H₂O adsorption and dissociation on the Fe (100) surface with different precovered metals are studied by density functional theory. On both kinds of metal‐precovered surface, H₂O molecules prefer adsorb on hollow sites than bridge and top sites. The impurity energy difference is proportional to the adsorption energy, but the adsorbates are not sensitive to the adsorption orientation and height relative to the surface. The Hirshfeld charge analysis shows that water molecules act as an electron donor while the surface Fe atoms act as an electron acceptor. The rotation and dissociation of H₂O molecule occur on the Co‐ and Mn‐precovered surfaces. Some H₂O molecules are dissociated into OH and H groups. The energy barriers are about 0.5 to 1.0 eV, whose are consistence with the experimental data. H₂O molecules can be dissociated more easily at the top site on Co‐precovered surface 1 than that at bridge site on Mn‐precovered surface 2 because of the lower reaction barrier. The dispersion correction effects on the energies and adsorption configurations on Co‐precovered surface 1 were calculated by OBS + PW91. The dispersion contributions can improve a bit of the bond energy of adsorbates and weaken the hydrogen bond effect between adsorption molecules a little.     

Atomic hydrogen interaction with Ru(1010)

The interaction of atomic hydrogen with clean and deuterium precovered Ru(1010) was studied by means of temperature-programmed desorption (TPD) spectroscopy. Compared to molecular hydrogen experiments, after exposure of the clean surface to gas-phase atomic hydrogen at 90 K, two additional peaks grow in the desorption spectra at 115 and 150 K. The surface saturation coverage, determined by equilibrium between abstraction and adsorption reactions, is 2.5 monolayers. Preadsorbed deuterium abstraction experiments with gas-phase atomic hydrogen show that a pure Eley-Rideal mechanism is not involved in the process, while a hot atom (HA) kinetics describes well the reaction. By least-squares fitting of the experimental data, a simplified HA kinetic model yields an abstraction cross section value of 0.5 +/- 0.2 angstroms2. The atomic hydrogen interaction with an oxygen precovered surface was also studied by means of both TPD and x-ray photoelectron spectroscopy: oxygen hydrogenation and water production take place already at very low temperature (90 K).     

Adsorption and desorption of stearic acid self-assembled monolayers on aluminum oxide

Development of coatings to minimize unwanted surface adsorption is extremely important for their use in applications, such as sensors and medical implants. Self-assembled monolayers (SAMs) are an excellent choice for coatings that minimize nonspecific adsorption because they can be uniform and have a very high surface coverage. Another equally important characteristic of such coatings is their stability. In the present study, both the bonding mechanism and the stability of stearic acid SAMs on two aluminum oxides (single-crystal C-plane aluminum oxide (sapphire) and amorphous aluminum oxide (alumina)) are investigated. The adsorption mechanism is investigated by ex situ X-ray photoelectron spectroscopy and infrared (IR) spectroscopy. The results revealed that stearic acid binds to sapphire surfaces via a bidentate interaction of carboxylate with two oxygen atoms while it binds to alumina surfaces via both bidentate and monodentate interactions. Desorption kinetics of stearic acid self-organized on both aluminum oxide surfaces into water is explored by ex situ tapping mode atomic force microscopy, IR spectroscopy, and contact angle measurements. The results exhibit that the SAMs of stearic acid formed on sapphire are not stable in water and are continuously lost through desorption. Water contact angle measurements of SAMs that are immersed in water further indicate that the desorption rate of adsorbates from atomically smooth terrace sites is substantially faster than that of adsorbates from the sites of surface defects due to weaker molecular interaction with the smooth surface. A time-dependent desorption profile of SAMs grown on amorphous alumina reveals that contact angles decrease monotonically without any regional distinction, providing further evidence for the presence of adsorption sites with different types of affinity on the amorphous alumina surface.     

Solvent effects on two-dimensional molecular self-assemblies investigated by using scanning tunneling microscopy

Self-assembly structures investigated by using scanning tunneling microscopy (STM) at liquid/solid interface have been a topic of broad interest in surface science, molecular materials, molecular electronics. The delicate balance among the adsorbate–solvent, adsorbate–adsorbate, solvent–solvent interactions would give rise to the coadsorption or competitive deposition of solvent with adsorbate. The solvents at the interface enable dynamic absorption and desorption of the adsorbates leading to the controlled assembly of the molecular architectures. The solvent-induced polymorphism, coadsorption effect, as well as solvent effects on chirality and electronic structures are discussed in this report in view of the polarity, solubility and viscosity of the solvent, the hydrogen bonding formation between solute and solvent, and the solvophobic and solvophilic effects. The systematic studies on the solvent effects would shed light on better control of assembly structures for design of new molecular materials and molecular electronics.     

Adsorption and reaction of SO2 on clean and oxygen precovered Pd(100)--a combined HR-XPS and DF study

We studied the adsorption and reactivity of SO(2) on clean and oxygen precovered Pd(100) with high resolution X-ray photoelectron spectroscopy and density functional calculations. Upon adsorption at 120 K two different SO(2) species were detected, which were identified as upright-standing and flat-lying molecules by comparing the calculated core level shifts. In agreement with the relative stabilities determined by the calculations the intensities of the photoelectron spectra indicate that the majority species are upright-standing SO(2). Upon heating the quantitative analysis of the data indicates desorption of SO(3) and formation of atomic sulfur. On the oxygen precovered surface small amounts of SO(3) are formed already upon SO(2) adsorption at low temperatures. Upon heating stepwise oxidation of SO(2) to SO(3) and, eventually, to SO(4) is found. Two different SO(4) species were detected, which are assigned to SO(4) bound in the proximity of or remote from oxygen adatoms, according to core level shift estimates.     

Evidence for C-H...O=C bonding in coadsorbed aromatic-carbonyl systems on Pt(111)

The coadsorption of ethyl formate, acetone, and methyl pyruvate with benzene and naphthalene on Pt(111) was studied with reflection absorption infrared spectroscopy (RAIRS) and thermal desorption (TPD) measurements. Coadsorbed benzene or naphthalene are found to convert eta1- and eta2-states of ethyl formate and acetone into new states displaying slightly red-shifted carbonyl bands. Similarly, coadsorption converts the enediolate state of methyl pyruvate into a new adsorption geometry in which the carbonyl bands are silent. In each case, coadsorption of the aromatic leads to significantly modified carbonyl desorption spectra. The results suggest an attractive carbonyl-aromatic interaction that weakens or removes the direct interaction of the carbonyl function with the metal surface. The aromatic-carbonyl interaction is attributed to hydrogen bonding between C-H and C=O, enhanced by the chemisorption induced polarization of the aromatic.     

NO和O2在Pt(110)面上吸附的TDS和PEEM研究

采用热脱附和光电子发射普微镜研究了Ｏ２和ＮＯ在Ｐｔ（１１０）面上的吸附和共吸附。结果表明，室温条件下，ＮＯ在Ｐｔ（１１０）面上有端式和桥式两种不同的吸附方式。Ｐｔ（１１０）表面预吸附的原子态氧占据了ＮＯ进入了桥式吸附的活性粒，从而阻遏了ＮＯ在Ｐｔ表面发生解离反应必须经过的桥式吸附中间态的形式，进而降低了ＮＯ在Ｐｔ表面直接分散的能力。     

Reactions on alkali-modified low-index stepped copper surfaces

Interest in CO, CO₂ and H adsorption on alkali-promoted Cu surfaces stems from the promoting role of alkali metals in heterogeneous catalysis. Adsorption of an alkali metal on a Cu surface causes a substantial decrease in the work function. The change in the electronic structure of the surface has strong consequences for the adsorption and reactive properties of the Cu surfaces. Another important factor for the reaction yields is the corrugation of the surface. The influence of these two parameters, the alkali metal coverage and the corrugation, on the dissociation of CO and the reactions between low-molecular gases like H, CO and CO₂ has been investigated on a series of low-index and stepped Cu surfaces. Particularly the synthesis of formate HCOO⁻ from coadsorption of H and CO₂ has been studied. The experimental evidence for these findings is synchrotron radiation based measurements of valence band energy distributions, and work function measurements. It is demonstrated in this report that CO dissociates on the potassium-modified stepped surface, Cu(1 1 2), at 125 K. The dissociation process is conditioned by the presence of steps and the alkali metal coverage. Carbonate is formed via the process CO + CO →CO₂ + C and reaction with oxygen. Adsorption of atomic hydrogen in the presence of K gives rise to two H-1s-induced states in the valence band. The influence of temperature on the binding energies and the population of these states have been studied. Formate is synthesized when the alkali/Cu surface, precovered with hydrogen, is exposed to CO₂.     

Acetone and water on TiO2(110): H/D exchange

Isotopic H/D exchange between coadsorbed acetone and water on the TiO2(110) surface was examined using temperature programmed desorption (TPD) as a function of coverage and two surface pretreatments (O2 oxidation and mild vacuum reduction). Coadsorbed acetone and water interact repulsively on reduced TiO2(110) on the basis of results from the companion paper to this study, with water exerting a greater influence in destabilizing acetone and acetone having only a nominal influence on water. Despite the repulsive interaction between these coadsorbates, about 0.02 monolayers (ML) of a 1 ML d6-acetone on the reduced surface (vacuum annealed at 850 K to a surface oxygen vacancy population of 7%) exhibits H/D exchange with coadsorbed water, with the exchange occurring exclusively in the high-temperature region of the d6-acetone TPD spectrum at approximately 340 K. The effect was confirmed with combinations of d0-acetone and D2O. The extent of exchange decreased on the reduced surface for water coverages above approximately 0.3 ML due to the ability of water to displace coadsorbed acetone from first layer sites to the multilayer. In contrast, the extent of exchange increased by a factor of 3 when surface oxygen vacancies were pre-oxidized with O2 prior to coadsorption. In this case, there was no evidence for the negative influence of high water coverages on the extent of H/D exchange. Comparison of the TPD spectra from the exchange products (either d1- or d5-acetone depending on the coadsorption pairing) suggests that, in addition to the 340 K exchange process seen on the reduced surface, a second exchange process was observed on the oxidized surface at approximately 390 K. In both cases (oxidized and reduced), desorption of the H/D exchange products appeared to be reaction limited and to involve the influence of OH/OD groups (or water formed during recombinative desorption of OH/OD groups) instead of molecularly adsorbed water. The 340 K exchange process is assigned to reaction at step sites, and the 390 K exchange process is attributed to the influence of oxygen adatoms deposited during surface oxidation. The H/D exchange mechanism likely involves an enolate or propenol surface intermediate formed transiently during the desorption of oxygen-stabilized acetone molecules.     

Correlation and Calculation of Multicomponent Adsorption Equilibria Data Using a Generalized Adsorption Isotherm

Enhanced by the need for reliable and accurate data of multicomponent gas adsorption equilibria on porous solids like activated carbons or zeolites, a new method to measure and correlate coadsorption equilibria has been developed. This method is a combination of gravimetric or volumetric measurements of the total load of pure or multicomponent adsorbates (Staudt, 1994; Gregg and Sing, 1982) and a correlation and calculation procedure using a new adsorption isotherm (AI) (Keller, 1990). This AI is thermodynamically consistent and describes adsorbates with fractal dimension for single- or multicomponent systems and load dependent adsorption energies. This method allows calculation of partial loads of multicomponent coadsorption equilibria from pure component data and the total loads of the mixture adsorption equilibria. This will be demonstrated for binary and ternary adsorption equilibria of CH₄, C₂H₄ and C₂H₆ on activated carbon (Reich et al., 1980).     

Enhancement of O2 dissociation on Au111 by adsorbed oxygen: implications for oxidation catalysis

We show that the dissociation probability of O2 on the reconstructed, Au111-herringbone surface is dramatically increased by the presence of some atomic oxygen on the surface. Specifically, at 400 K the dissociation probability of O2 on oxygen precovered Au111 is on the order of 10(-3), whereas there is no measurable dissociation on clean Au111, establishing an upper bound for the dissociation probability of 10(-6). Atomic oxygen was deposited on the clean reconstructed Au111-herringbone surface using electron bombardment of condensed NO2 at 100 K. The dissociation probability for dioxygen was measured by exposing the surface to 18O2. Temperature programmed desorption (TPD) was used to quantify the amount of oxygen dissociation and to study the stability of the oxygen in all cases. Oxygen desorbs as O2 in a peak centered at 550 K with pseudo-first-order kinetics; i.e., the desorption peak does not shift with coverage. Our interpretation is that the coverage dependence of the activation energy for dissociation (deltaE(dis)) and/or preexponential factor (nu(d)) may be responsible for the unusual desorption kinetics, implying a possible energy barrier for O2 dissociation on Au111. These results are discussed in the context of Au oxidation chemistry and the relationship to supported Au nanoparticles.     

Photochemical desorption from chlorinated Si(100) and Si(111) surfaces — Mechanisms and models

Understanding the mechanisms controlling the anisotropy of microetching is particularly critical as the scale of semiconductor devices shrink. Defining complex, dynamic chemical systems such as halogen etching require microscopic measurements combining kinetics, dynamics, surface layer composition and micromorphology on prototypical surfaces. This study is concerned with two important variables in addition to spontaneous chemical etching, the role of electronic defects induced by high level doping in producing site-specefic reaction and the enhancement of etching by irradiation at low fluences.

Substitutional defects introduced by selective doping significantly influence the rate of chlorine etching by forming shallow electronic states that are ionized at room temperature¹. We have shown that chlorine sticking coeficients as well as laser-assisted etching are significantly affected by doping at very high dopant levels. Enhancement for n-type doping is consistent with the simple assumption that holes at the surface should enhance Si-Si surface bond breaking and in disagreement with the fact that heavily p-doped silicon has a higher chlorine sticking coefficient than n-doped material².

Carrier effects generated by photoirradiation with above bandgap photons are considerably more complex than simple doping. A depletion layer and associated electric field are set up at the surface and minority carriers are preferentially swept to the surface. The type of photocarrier present at the surface is determined by both the doping and the photoirradiation.

Using photoinduced etching of heavily doped Si(100) and Si(111) by chlorine at low laser fluences, we studied the mechanism of photostimulated desorption using core-level photoemission and time-of-flight measurements of the photoproducts². These results will be interpreted in terms of field-modified electron-hole transport together with carrier-modified chlorine adsorption and desorption.     

Desorption of oxygen and nitrogen from iron surfaces Application of X-ray photoelectron spectroscopy to surface analysis

The patterns of desorption of oxygen and nitrogen from metal surfaces were examined and applied in the determination of gaseous elements in metal samples. The abundance of gaseous elements on metal surfaces is controlled by thermal desorption and diffusion to or from the surface. A heat treatment suitable for the separation of adsorbed gases from those in the bulk sample is discussed. Preheating of the metal samples at ca. 300 degrees in a vacuum reduces the residual adsorbates to a minimum and retards the diffusion of the gaseous elements from the bulk to the surface.     

Carbon monoxide interaction with oxygenated nickel single-crystal surfaces studied by vibrational spectroscopy

High-resolution electron energy loss spectroscopy has been used to study the coadsorption of CO and O on Ni(1 1 1) and Ni(1 0 0) surfaces at 250 K. These vibrational measurements showed that with increasing surface coverage at 250 K, the preadsorbed O caused changes in the adsorption sites for the after-adsorbed CO molecules revealing, however, dissimilar adsite alterations for the two Ni single crystals. Also, a different behavior towards carbonate formation was found upon CO adsorption at 250 K on Ni(1 0 0) and Ni(1 1 1) surfaces precovered with O at 250 K.     

Theoretical study of the adsorption of oxygen on a Cu(100) surface and the coadsorption with alkali atoms

We performed a theoretical study of the adsorption of oxygen on a cluster model of the Cu(100) surface and also the surface coadsorbed with lithium and potassium atoms. The study showed that alkali coadsorption facilitates in a significant way the process of molecular adsorption, whereas the adsorption of atomic oxygen is only slightly modified. The alkali atoms on the copper surface produce an increase in the charge transfer toward the oxygen molecule, favoring the oxygen dissociation. The effect is greater for the potassium coadsorption. In addition, we found that the potassium coadsorption favored the dissociation and recombination processes by about 60 and 15%, respectively. In turn, the lithium coadsorption favored only the recombination process by about 50%. These results could be an important aspect for catalytic processes.From the Proceedings of the 28th Congreso de Quimicos Teóricos de Expression Latina (QUITEL 2002)     

The Adsorption of Oxygen and Coadsorption of CO and Oxygen on Structurally Well‐Defined PdAg Surface Alloys

The dissociative interaction of oxygen with structurally well‐defined monolayer Pd_xAg_1?x/Pd(111) surface alloys of different compositions, with well‐known distributions of the respective surface atoms (A. K. Engstfeld et al., Phys. Chem. Chem. Phys. 2012 , 14, 10754–10761), and the coadsorption of/reaction with CO on oxygen pre‐covered surfaces were studied by high‐resolution electron energy loss spectroscopy (HREELS) and temperature‐programmed desorption/reaction spectroscopy (TPD/TPR). The impact of geometric ensemble effects as well as electronic ligand and strain effects on the adsorption and reaction behaviour of the respective species on the bimetallic surfaces is elucidated and compared with related systems such as CO adsorption on similar surfaces and oxygen adsorption on a Pd₆₇Ag₃₃(111) bulk alloy surface. The data show a clear dominance of ensemble effects on the oxygen adsorption and CO coadsorption behaviour, with oxygen adsorption limited to threefold‐hollow sites on Pd₃ sites, while the combined electronic effects, as evident from modifications in the adsorption and reaction characteristics on the Pd sites, are small.     

Interaction of carbon oxides with the surface of catalysts containing iron and nickel nanoparticles

The interaction of individual carbon oxides and their mixture with the surface of monometallic catalysts that contain iron and nickel nanoparticles was investigated for the first time by thermoprogrammed desorption. It was found that both oxides are adsorbed molecularly and dissociatively. No competition between CO and CO₂ was observed upon the coadsorption of these oxides. It was shown that the preadsoption of hydrogen on the surface leads to a strengthening of the Me-CO bond, raises the activation energy of desorption, assists in the dissociative chemisorption of carbon monoxide, and leads to the appearance of hydrocarbons among the products of desorption.     

对甲苯磺酰胺和1．4－丁炔二醇在汞电极上的共吸附

用电毛细曲线、微分电容曲线、Ｓｔｕａｒｔ模型和自洽场分子轨道法（ＣＮＤＯ／２）研究对甲苯磺酰胺和１．４－丁炔二醇在汞电极上的共吸附特性。此共吸附本质上是一种物理吸附。在其最大的吸附电位附近,二者分别以侧卧与平卧方式吸附于汞表面,并在共吸附层中表现出静电斥力作用。当两者的浓度相等时,１．４－丁炔二醇的吸附占优势。