Chemical composition and reactivity of water on hexagonal Pt-group metal surfaces

The dissociation behaviour and valence-electronic structure of water adsorbed on clean and oxygen-covered Ru{0001}, Rh{111}, Pd{111}, Ir{111} and Pt{111} surfaces has been studied by high-resolution X-ray photoelectron spectroscopy with the aim of identifying similarities and trends within the Pt-group metals. On average, we find higher reactivity for the 4d metals (Ru, Rh, Pd) as compared to 5d (Ir, Pt), which is correlated with characteristic shifts in the 1b(1) and 3a(1) molecular orbitals of water. Small amounts of oxygen (< 0.2 ML) induce dissociation of water on all five surfaces, for higher coverages (> 0.25 ML) only intact water is observed. Under UHV conditions these higher coverages can only be reached on the 4d metals, the 5d metals are, therefore, not passivated.     

Influence of aggregation, defects, and contaminant oxygen on water dissociation at Cu(110) surface: a theoretical study

The DFT-PW91 slab model approach is employed to investigate the influence of aggregation, surface defects, and contaminant oxygen on water dissociation on Cu(110) at low temperatures. The dissociation barriers of water in various aggregate states are calculated in the range of 60-75 kJ/mol on the clean surfaces, in nice agreement with the experimentally determined values. It is revealed that the aggregation of water shows no propensity to reduce the activation barrier for the O-H bond breaking on Cu(110), at variance with the water chemistry on Ru(0001). The calculated activation energy on Cu(211) which is the most active stepped surface investigated is equal to the value on the (110) surface, indicating that the hydroxyl groups observed on Cu(110) at low temperatures may not stem from surface defects. The coadsorbed oxygen, whether as a "spectator" or a "participant," facilitates the water dissociation both kinetically and thermodynamically.     

First-principles study of C adsorption, O adsorption, and CO dissociation on flat and stepped Ni surfaces

The adsorption of atomic oxygen and carbon was studied with plane wave density functional theory on four Ni surfaces, Ni(110), Ni(111), Ni(210), and Ni(531). Various adsorption sites on these surfaces are examined in order to identify the most favorable adsorption site for each atomic species. The dependence of surface bonding on adsorbate coverage is also investigated. Adsorption energies and structural information are obtained and compared with existing experimental results for Ni(110) and Ni(111). In addition, activation barriers to CO dissociation have been determined on Ni(111) and Ni(531) by locating the transition states for these processes. Our results indicate that the binding energies of C are comparatively stronger on stepped surfaces than on flat surfaces, and the energy barriers associated with CO dissociation strongly favor reactions occurring near surface steps.     

CO Blocking of D2 Dissociative Adsorption on Ru(0001)

The influence of pre‐adsorbed CO on the dissociative adsorption of D₂ on Ru(0001) is studied by molecular‐beam techniques. We determine the initial dissociation probability of D₂ as a function of its kinetic energy for various CO pre‐coverages between 0.00 and 0.67 monolayers (ML) at a surface temperature of 180 K. The results indicate that CO blocks D₂ dissociation and perturbs the local surface reactivity up to the nearest‐neighbour Ru atoms. Non‐activated sticking and dissociation become less important with increasing CO coverage, and vanish at θ_CO≈0.33 ML. In addition, at high D₂ kinetic energy (>35 kJ mol^?1) the site‐blocking capability of CO decreases rapidly. These observations are attributed to a CO‐induced activation barrier for D₂ dissociation in the vicinity of CO molecules.     

Comparative study of water dissociation on Rh(111) and Ni(111) studied with first principles calculations

The dissociation and formation of water on the Rh(111) and Ni(111) surfaces have been studied using density functional theory with generalized gradient approximation and ultrasoft pseudopotentials. Calculations have been performed on 2x2 surface unit cells, corresponding to coverages of 0.25 ML, with spot checks on 3x3 surface unit cells (0.11 ML). On both surfaces, the authors find that water adsorbs flat on top of a surface atom, with binding energies of 0.35 and 0.25 eV, respectively, on Rh(111) and Ni(111), and is free to rotate in the surface plane. Barriers of 0.92 and 0.89 eV have to be overcome to dissociate the molecule into OH and H on the Rh(111) and Ni(111) surfaces, respectively. Further barriers of 1.03 and 0.97 eV need to be overcome to dissociate OH into O and H. The barriers for the formation of the OH molecule from isolated adsorbed O and H are found to be 1.1 and 1.3 eV, and the barriers for the formation of the water molecule from isolated adsorbed OH and H are 0.82 and 1.05 eV on the two surfaces. These barriers are found to vary very little as coverage is changed from 0.25 to 0.11 ML. The authors have also studied the dissociation of OH in the presence of coadsorbed H or O. The presence of a coadsorbed H atom only weakly affects the energy barriers, but the effect of O is significant, changing the dissociation barrier from 1.03 to 1.37 and 1.15 eV at 0.25 or 0.11 ML coverage on the Rh(111) surface. Finally, the authors have studied the dissociation of water in the presence of one O atom on Rh(111), at 0.11 ML coverage, and the authors find a barrier of 0.56 eV to dissociate the molecule into OH+OH.     

Enhanced low-temperature CO oxidation on a stepped platinum surface for oxygen pressures above 10(-5) Torr

The rate of CO oxidation has been characterized on the stepped Pt(411) surface for oxygen pressures up to 0.002 Torr, over the 100-1000 K temperature range. CO oxidation was characterized using both temperature-programmed reaction spectroscopy (TPRS) and in situ soft X-ray fluorescence yield near-edge spectroscopy (FYNES). New understanding of the important role surface defects play in accelerating CO oxidation for oxygen pressure above 10(-5) Torr is presented in this paper for the first time. For saturated monolayers of CO, the oxidation rate increases and the activation energy decreases significantly for oxygen pressures above 10(-5) Torr. This enhanced CO oxidation rate is caused by a change in the rate-limiting step to a surface reaction limited process above 10(-5) Torr oxygen from a CO desorption limited process at lower oxygen pressure. For example, in oxygen pressures above 0.002 Torr, CO(2) formation begins at 275 K even for the CO saturated monolayer, which is well below the 350 K onset temperature for CO desorption. Isothermal kinetic measurements in flowing oxygen for this stepped surface indicate that activation energies and preexponential factors depend strongly on oxygen pressure, a factor that has not previously been considered critical for CO oxidation on platinum. As oxygen pressure is increased from 10(-6) to 0.002 Torr, the oxidation activation energies for the saturated CO monolayer decrease from 24.1 to 13.5 kcal/mol for reaction over the 0.95-0.90 ML CO coverage range. This dramatic decrease in activation energy is associated with a simple increase in oxygen pressure from 10(-5) to 10(-3) Torr. Activation energies as low as 7.8 kcal/mol were observed for oxidation of an initially saturated CO layer reacting over the 0.4-0.25 ML coverage range in oxygen pressure of 0.002 Torr. These dramatic changes in reaction mechanism with oxygen pressure for stepped surfaces are consistent with mechanistic models involving transient low activation energy dissociation sites for oxygen associated with step sites. Taken together these experimental results clearly indicate that surface defects play a key role in increasing the sensitivity of CO oxidation to oxygen pressure.     

Microcalorimetry of oxygen adsorption on fcc Co{110}

The coverage dependent heats of adsorption and sticking probabilities for oxygen on fcc Co{110} have been measured at 300 K using single crystal adsorption calorimetry (SCAC). Initial adsorption is consistent with dissociative chemisorption at low coverage followed by oxide formation above 0.6 ML coverage. The initial heat of adsorption of 633 kJ mol(-1) is similar to heat values calorimetrically measured on other ferromagnetic metal surfaces, such as nickel and iron. As the coverage increases, the heat of adsorption and sticking probability drop very rapidly up to the onset of oxidation. As already observed for other oxygen-metal surface systems, strong lateral adatom repulsions are responsible for the transition from the chemisorption regime to oxide film formation at higher coverage. The heat of oxide formation at the onset is 475 kJ mol(-1), which is consistent with the formation of CoO crystallites. The oxide film formation is discussed in terms of nucleation and island growth, and the Mott-Cabrera mechanisms, the latter being evidenced by the relatively constant heat of adsorption and sticking probability in contrast to the nickel and iron oxidation cases.     

A computational study of H2 dissociation on silver surfaces: the effect of oxygen in the added row structure of Ag110

We studied computationally the activation of H(2) on clean planar (111), (110) and stepped (221) as well as oxygen pre-covered silver surfaces using a density functional slab model approach. In line with previous data we determined clean silver to be inert towards H(2) dissociation, both thermodynamically and kinetically. The reaction is endothermic by approximately 40 kJ mol(-1) and exhibits high activation energies of approximately 125 kJ mol(-1). However, oxygen on the surface, modeled by the reconstructed surface p(2 x 1)O/Ag(110) that exhibits -O-Ag-O- added rows, renders H(2) dissociation clearly exothermic and kinetically feasible. The reaction was calculated to proceed in two steps: first the H-H bond is broken at an Ag-O pair with an activation barrier E(a) approximately 70 kJ mol(-1), then the H atom bound at an Ag center migrates to a neighboring O center with E(a) approximately 12 kJ mol(-1).     

The CO formation reaction pathway in steam methane reforming by rhodium

Three different pathways toward CO formation from adsorbed CH and O are compared by quantum-chemical density functional theory (DFT) calculations for planar and stepped Rh surfaces. The conventional pathway competes with the pathway involving a formyl (CHO) species. This holds for both types of surfaces. The barrier for carbon-oxygen bond formation for the planar surface (180 kJ/mol) is substantially higher than that for the stepped surface (90 kJ/mol). The reaction path through intermediate formyl formation competes with direct formation of CO from recombination via adsorbed C and O atoms. Calculations are used as a basis for the analysis of the overall kinetics of the methane steam reforming reaction as a function of the particle size and the metal.     

A DFT study of the adsorption and dissociation of CO on Fe(100): influence of surface coverage on the nature of accessible adsorption states.

In the present article, we report adsorption energies, structures, and vibrational frequencies of CO on Fe(100) for several adsorption states and at three surface coverages. We have performed a full analysis of the vibrational frequencies of CO, thus determining what structures are stable adsorption states and characterizing the transition-state structure for CO dissociation. We have calculated the activation energy of dissociation of CO at 0.25 ML (ML = monolayers) as well as at 0.5 ML; we have studied the dissociation at 0.5 ML to quantify the destabilization effect on the CO(alpha3) molecules when a neighboring CO molecule dissociates. In addition, it is shown that the number and nature of likely adsorption states is coverage dependent. Evidence is presented that shows that the CO molecule adsorbs on Fe(100) at fourfold hollow sites with the molecular axis tilted away from the surface normal by 51.0 degrees. The asorprton energy of the CO molecule is -2.54 eV and the C-O stretching frequency is 1156 cm(-1). This adsorption state corresponds to the alpha3 molecular desorption state reported in temperature programmed desorption (TPD) experiments. However, the activation energy of dissociation of CO(alpha3) molecules at 0.25 ML is only 1.11 eV (approximately 25.60 kcal mol(-1)) and the gain in energy is -1.17 eV; thus, the dissociation of CO is largely favored at low coverages. The activation energy of dissociation of CO at 0.5 ML is 1.18 eV (approximately 27.21 kcal mol(-1)), very similar to that calculated at 0.25 ML. However, the dissociation reaction at 0.5 ML is slightly endothermic, with a total change in energy of 0.10 eV Consequently, molecular adsorption is stabilized with respect to CO dissociation when the CO coverage is increased from 0.25 to 0.5 ML.     

Simulation of crack propagation in alumina with ab initio based polarizable force field

We present an effective atomic interaction potential for crystalline α-Al(2)O(3) generated by the program potfit. The Wolf direct, pairwise summation method with spherical truncation is used for electrostatic interactions. The polarizability of oxygen atoms is included by use of the Tangney-Scandolo interatomic force field approach. The potential is optimized to reproduce the forces, energies, and stresses in relaxed and strained configurations as well as {0001}, {1010}, and {1120} surfaces of Al(2)O(3). Details of the force field generation are given, and its validation is demonstrated. We apply the developed potential to investigate crack propagation in α-Al(2)O(3) single crystals.     

Crystallographic and Morphological Sensitivity of N2 Activation over Ruthenium

Ruthenium (Ru) serves as a promising catalyst for ammonia synthesis via the Haber-Bosch process, identification of the structure sensitivity to improve the activity of Ru is important but not fully explored yet. We present here density functional theory calculations combined with micro-kinetic simulations on nitrogen molecule activation, a crucial step in ammonia synthesis, over a variety of hexagonal close-packed (hcp) and face-center cubic (fcc) Ru facets. Hcp \begin{document}$\left\{ {21\overline 3 0} \right\}$\end{document} facet exhibits the highest activity toward N\begin{document}$_2$\end{document} dissociation in hcp Ru, followed by the (0001) monatomic step sites. The other hcp Ru facets have N\begin{document}$_2$\end{document} dissociation rates at least three orders lower. Fcc \begin{document}$\{211\}$\end{document} facet shows the best performance for N\begin{document}$_2$\end{document} activation in fcc Ru, followed by \begin{document}$\{311\}$\end{document}, which indicates stepped surfaces make great contributions to the overall reactivity. Although hcp Ru \begin{document}$\left\{ {21\overline 3 0} \right\}$\end{document} facet and (0001) monatomic step sites have lower or comparable activation barriers compared with fcc Ru \begin{document}$\{211\}$\end{document} facet, fcc Ru is proposed to be more active than hcp Ru for N\begin{document}$_2$\end{document} conversion due to the exposure of the more favorable active sites over step surfaces in fcc Ru. This work provides new insights into the crystal structure sensitivity of N\begin{document}$_2$\end{document} activation for mechanistic understanding and rational design of ammonia synthesis over Ru catalysts.     

CHx hydrogenation on Co(0001): a density functional theory study

Hydrogenation is an important process in the Fischer-Tropsch synthesis. In this work, all the elementary steps of the hydrogenation from C to CH4 are studied on both flat and stepped Co(0001) using density functional theory (DFT). We found that (i) CH3 hydrogenation (CH3+H-->CH4) is the most difficult one among all the elementary reactions on both surfaces, possessing barriers of around 1.0 eV; (ii) the other elementary reactions have the barriers below 0.9 eV on the flat and stepped surfaces; (iii) CH2 is the least stable species among all the CHx(x=1-3) species on both surfaces; and (iv) surface restructuring may have little effect on the CHx(x=0-3) hydrogenation. The barriers of each elementary step on both flat and stepped surfaces are similar and energy profiles are also similar. The reason as to why CHx hydrogenation is not structure-sensitive is also discussed.     

Effect of steps on the decomposition of CH3O at PdZn alloy surfaces

The decomposition of methoxide (CH(3)O) on a PdZn alloy is considered to be the rate-limiting step of steam re-forming of methanol over a Pd/ZnO catalyst. Our previous density functional (DF) studies (Langmuir 2004, 20, 8068; Phys. Chem. Chem. Phys. 2004, 6, 4499) revealed only a very low propensity of defect-free flat (111) and (100) PdZn surfaces to promote C-H or C-O bond breaking of CH(3)O. Thus, we applied the same DF periodic slab-model approach to investigate these two routes of CH(3)O decomposition on PdZn(221) surfaces that expose Pd, (221)(Pd), and Zn, (221)(Zn), steps. C-H bond cleavage of CH(3)O is greatly facilitated on (221)(Pd): the calculated activation energy is dramatically reduced, to approximately 50 kJ mol(-1) from approximately 90 kJ mol(-1) on flat PdZn surfaces, increasing the rate constant by a factor of 10(8). The lower barrier is mainly due to a weaker interaction of the reactant CH(3)O and an enhanced interaction of the product CH(2)O with the substrate. The activation energy for C-O bond scission did not decrease on the (221)(Pd) step. On the (221)(Zn) step, the calculated reaction barriers of both decomposition routes are even higher than on flat surfaces, because of the stronger adsorption of CH(3)O. Steps (and other defects) appear to be crucial for methanol steam re-forming on Pd/ZnO catalyst; the stepped surface PdZn(221)(Pd) is a realistic model for studying the reactivity of this catalyst.     

Initial Subsurface Incorporation of Oxygen into Ru(0001): A Density Functional Theory Study

The adsorption and diffusion of oxygen on Ru(0001) surfaces as a function of coverage are systematically investigated by using density functional theory. A high incorporation barrier of low‐coverage adsorbed oxygen into the subsurface is discovered. Calculations show that the adsorption of additional on‐surface oxygen can lower the penetration barrier dramatically. The minimum penetration barrier obtained is 1.81 eV for a path starting with oxygen in mixed on‐surface hcp and fcc sites at an oxygen coverage of 0.75 ML, which should be regarded as close to 1 ML. Energy diagrams show that oxygen‐diffusion barriers on the surface and in the subsurface are much lower than the penetration barrier. Oxygen diffusion on the surface is an indispensable step for its initial incorporation into the subsurface.     

The effect of surface roughness on the adhesion of solid surfaces for systems with and without liquid lubricant

We present molecular dynamics results for the interaction between two solid elastic walls during pull-off for systems with and without octane (C(8)H(18)) lubricant. We used two types of substrate--flat and corrugated--and varied the lubricant coverage from approximately 1/8 to approximately 4 ML (monolayers) of octane. For the flat substrate without lubricant the maximum adhesion was found to be approximately three times larger than for the system with the corrugated substrate. As a function of the octane coverage (for the corrugated substrate) the pull-off force first increases as the coverage increases from 0 to approximately 1 ML, and then decreases as the coverage is increased beyond monolayer coverage. It is shown that at low octane coverage, the octane molecules located in the substrate corrugation wells during squeezing are pulled out of the wells during pull-off, forming a network of nanocapillary bridges around the substrate nanoasperities, thus increasing the adhesion between two surfaces. For greater lubricant coverages a single capillary bridge is formed. The adhesion force saturates for lubricant coverages greater than 3 ML. For the flat substrate, during pull-off we observe discontinuous, thermally activated changes in the number n of lubricant layers (n-1-->n layering transitions), whereas for the corrugated substrate these transitions are "averaged" by the substrate surface roughness.     

Methylthiolate on Au(111): adsorption and desorption kinetics

Low energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy and line of sight mass spectrometry have been used to study the adsorption and desorption of dimethyldisulfide (DMDS) on Au(111). At 300 K adsorption is dissociative, forming a chemisorbed adlayer of methylthiolate with a 1/3 ML, (sq rt 3 x sq rt 3)R30 degrees, structure. At 100 K adsorption is molecular, with dissociation to form the 1/3 ML (sq rt 3 x sq rt 3)R30 degrees methylthiolate structure occurring at 138-160 K. A physisorbed DMDS layer, with a coverage of 1/6 ML of DMDS, forms on top of the (sq rt 3 x sq rt 3)R30 degrees chemisorbed MT surface for T < or = 180 K, with multilayers forming for T < or = 150 K. In temperature programmed desorption, multilayers of DMDS desorbed with zero order kinetics and an activation energy of 41 kJ mol(-1); the physisorbed layer desorbed with first order kinetics, exhibiting repulsive lateral interactions with an activation energy which varied from 63 kJ mol(-1) (theta = 0) to 51 kJ mol(-1) (theta = 1); the chemisorbed methylthiolate layer desorbed associatively as DMDS via the physisorbed layer, the activation energy for the reaction, 2 methylthiolate --> physisorbed DMDS, exhibiting repulsive lateral interactions with an activation energy which varied from 65 kJ mol(-1) (theta = 0) to 61 kJ mol(-1) (theta = 1). The physisorbed disulfide layer explains the pre-cursor state adsorption kinetics observed in sticking probability measurement, while its relatively facile formation provides a mechanism by which thiolate self-assembled monolayers can become mobile at room temperature.     

Morphology‐Engineered Highly Active and Stable Ru/TiO2 Catalysts for Selective CO Methanation

Ru/TiO₂ catalysts exhibit an exceptionally high activity in the selective methanation of CO in CO₂‐ and H₂‐rich reformates, but suffer from continuous deactivation during reaction. This limitation can be overcome through the fabrication of highly active and non‐deactivating Ru/TiO₂ catalysts by engineering the morphology of the TiO₂ support. Using anatase TiO₂ nanocrystals with mainly {001}, {100}, or {101} facets exposed, we show that after an initial activation period Ru/TiO₂‐{100} and Ru/TiO₂‐{101} are very stable, while Ru/TiO₂‐{001} deactivates continuously. Employing different operando/in situ spectroscopies and ex situ characterizations, we show that differences in the catalytic stability are related to differences in the metal–support interactions (MSIs). The stronger MSIs on the defect‐rich TiO₂‐{100} and TiO₂‐{101} supports stabilize flat Ru nanoparticles, while on TiO₂‐{001} hemispherical particles develop. The former MSIs also lead to electronic modifications of Ru surface atoms, reflected by the stronger bonding of adsorbed CO on those catalysts than on Ru/TiO₂‐{001}.     

Kinetics and Equilibria for Complex Formation between Palladium(II) and Chloroacetate

Kinetics and equilibria for the formation of a 1:1 complex between palladium(II) and chloroacetate were studied by spectrophotometric measurements in 1.00 mol HClO₄ at 298.2 K. The equilibrium constant, K, of the reaction

CO_(2)加氢制甲酸理论研究及高效铁基催化剂设计北大核心CSCD

CO_(2)加氢制甲酸由于需同时活化惰性氢气及CO_(2)而富有挑战性,同时此过程原子经济性100%,具有很好的理论和现实研究价值,但文献中报道的活性较好的催化剂均为贵金属催化剂.为了开发活性更高的用于CO_(2)加氢制甲酸的铁基催化剂,我们采用理论计算方法研究了12种不同种类的PNP-Fe(PNP=2,6-(二-叔丁基-磷甲基)吡啶)化合物催化CO_(2)加氢制甲酸的过程.理论研究结果表明,CO_(2)加氢制甲酸反应过程包括H2活化及CO_(2)插入金属氢键两个步骤,H_(2)活化过程是整个反应的速控步骤.催化剂吡啶环上进行P原子取代可以显著降低H_(2)活化能垒.基于以上发现,我们设计了一种新颖的高效铁基催化剂,使用此催化剂催化CO_(2)加氢制甲酸反应,速控步骤能垒只有85.6 kJ/mol,催化活性与贵金属的比较接近.我们研究的12种铁基催化剂速控步骤能垒范围为85.6~126.4 kJ/mol,显示了配体良好的调控催化活性能力.