On the Importance of Gradient-Corrected Correlation for van der Waals Density Functionals

The van der Waals density functional (vdW-DF) family of exchange?Ccorrelation functionals is a promising step towards accounting for van der Waals interactions in density functional theory. This approach consists of a nonlocal correlation term in addition to semilocal generalized gradient approximation exchange and local density approximation correlation. It has proven useful for describing vdW bonded complexes but unfortunately deteriorates the prediction of solid-state properties such as bulk lattice parameters and cohesive energies, as compared to the underlying GGA functional. By considering a broad range of different condensed matter systems including weakly interacting complexes as well as strongly bonded molecules and bulk solids, we show that inclusion of gradient-corrected correlations in vdW-DF-type calculations may not only improve the accuracy for vdW bonded systems, but also amend vdW-DF deficiencies in predicting structural properties of solids. Based on this insight we construct a prototype vdW-DF which demonstrates high accuracy in describing the dispersive interactions responsible for benzene adsorption on the noble Au(111) surface.     

Theoretical study of CO adsorption on Au catalysts under environmental catalytic conditions

Density Functional Theory calculations with both standard GGA and hybrid functionals are performed on Au adatoms, steps, and low index surfaces with coordination numbers (CNs) varying from 3 to 9. The results are used to study adsorption thermodynamics and reactivity of CO on Au nanoparticles. We find that the hybrid functional improves calculated site preferences and predicts CO top site adsorption, regardless of the Au CN, in good agreement with experiments. The calculated adsorption energies vary monotonically with respect to Au CNs, and the results from the hybrid functional are around 20% smaller than the corresponding values from the GGA–PBE functional. A comparison with experimental adsorption energies suggests that these functionals may bound the true CO–Au interaction strength, and seven-coordinated Au atoms may be the active low-coordinated sites on many Au single crystal surfaces. However, thermodynamic analysis on Wulff-like Au particles at ambient temperatures shows that, even though the number of 6-coordinated corner Au atoms is much less than the number of 7-coordinated edge Au atoms and of higher-coordinated Au atoms, they are the dominant sites for CO adsorption on Au nanoparticles with sizes up to 10 nm. In addition, we find that CO adsorption is not influenced by the shape of Au nanoparticles, but the CO oxidation reaction may be.     

Creation of Low-Coordination Gold Sites on Au(111) Surface by 1,4-phenylene Diisocyanide Adsorption

The adsorption of CO on a saturated overlayer of 1,4-phenylene diisocyanide (PDI) adsorbed on a Au(111) surface at 300 K is studied using scanning tunneling microscopy (STM), density functional theory (DFT) calculations and reflection absorption infrared spectroscopy (RAIRS). The PDI forms closed-packed rows of gold-PDI chains by extracting gold atoms from the Au(111) substrate. They are imaged by STM and the structure calculated by DFT. The adsorption of CO is studied on the low-coordination gold sites formed on the PDI-covered surface where it adsorbs exhibiting a CO stretching frequency of 2004 cm^?1, consistent with adsorption on an atop site. It is found that CO is stable on heating the sample to ~150 K and is only removed from the surface by heating to ~180 K. Since low-coordination gold atoms are suggested to be the active catalytic sites on supported gold nanoclusters, ??embossing?? the surface to form similar low-coordination sites using PDI might offer a strategy for tailoring the catalytic activity of gold.     

Understanding the mechanism of hydrogen adsorption into metal organic frameworks

Hydrogen adsorption mechanism into MOF-5, a porous metal-organic framework (MOF) has been studied by density functional theory calculation. The selected functionals for the prediction of interaction energies between hydrogen and potential adsorption sites of MOF-5 were utilized after the evaluation with the various functionals for interaction energy of H₂C₆H₆ model system. The adsorption energy of hydrogen molecule into MOF-5 was investigated with the consideration of the favorable adsorption sites and the orientations. We also calculated the second favorable adsorption sites by geometry optimization using every combination of two first adsorbed hydrogen molecules. Based on the calculation of the first and the second adsorption sites and energies, it has been suggested that the hydrogen adsorption into MOF-5 follows a cooperative mechanism in which the metal sites initiate the propagation of the hydrogen adsorption on the whole frameworks. In addition, the interaction mode between the simple benzene ring with hydrogen is significantly changed when the benzene ring has been incorporated into the framework of MOF-5.     

In/Au(111)和Ir/Au(111)面上巴豆醛选择加氢的机理研究及比较

基于巴豆醛在M/Au（111）合金表面（M=In,Ir）垂直吸附的最稳定吸附结构,采用密度泛函理论对其不完全加氢的反应机理进行探究。从不同加氢机理下各基元反应的活化能、反应热计算以及构型变化分析中可知,巴豆醛在M/Au（111）面上均优先对距离合金表面较近的C＝O进行加氢,且以C为活性中心优先进行加氢为最优机理,其中第1步加氢反应的活化能较高,是该机理的控速步骤。反应物巴豆醛的O原子与合金的掺杂原子M形成较强的化学吸附,提高了M/Au（111）面对C＝O加氢的选择性。巴豆醛按最优机理加氢的基元反应中在In/Au（111）面上最高反应能垒为0.969 eV,比在Ir/Au（111）面的最高反应能垒1.332 eV低,因此认为In/Au合金对其不完全加氢有更好的催化活性。     

The adsorption of methyl nitrite on the Au(111) surface

The interaction of the methyl nitrite molecule (CH₃ONO) with the gold(111) surface has been studied by means of density functional calculations. The perfect Au(111) surface has been represented by a rather large cluster model, Au₂₂, that was in turn used to extract information about the preferred adsorption geometry of the CH₃ONO species. Vibrational frequencies and adsorption energy are also reported. The calculated adsorption energies are 31.2 kJ/mol with respect to gas phase cis-conformer and 35.1 kJ/mol with respect to trans-methyl nitrite, very close to the experimental adsorption energy of 33.5 kJ/mol. From the analysis of vibrational frequencies of gas phase and adsorbed species it is concluded that only the cis-conformer is present at the Au(111) surface.     

Making gold less noble

Self‐consistent density functional calculations for the adsorption of O and CO on flat and stepped Au(111) surfaces are used to investigate effects which may increase the reactivity of Au. We find that the adsorption energy does not depend on the number of Au layers if there are more than two layers. Steps are found to bind considerably stronger than the (111) terraces, and an expansive strain has the same effect. On this basis we suggest that the unusually large catalytic activity of highly‐dispersed Au particles may in part be due to high step densities on the small particles and/or strain effects due to the mismatch at the Au–support interface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Why Au and Cu Are More Selective Than Pt for Preferential Oxidation of CO at Low Temperature

Self-consistent, periodic density functional theory (DFT) calculations and micro-kinetic modeling are used to compare selectivity for the preferential oxidation of CO (PROX) with respect to H₂ based on studies of elementary reaction steps on the (111) facet of Au, Cu and Pt. The first step of H oxidation (OH formation) has a higher activation barrier than the second step (H₂O formation) on all three metal surfaces, indicating that OH formation competes with CO oxidation for the removal of trace amounts of CO from a typical reformate gas. The activation energy barrier for CO oxidation is found to be 0.18eV on Au(111), 0.82eV on Cu(111) and 0.96eV on Pt(111), whereas the barrier for OH formation is 0.90, 1.28 and 0.83eV respectively. A micro-kinetic model based on the DFT results shows that trends in the selectivity of these metals at different temperatures is due to (i) differences in the rate constants of the competitive CO and H oxidation reactions, and (ii) differences in the CO and H surface coverages. Our results explain why Au and Cu are more selective PROX catalysts compared to Pt at low temperatures. At higher temperatures, Pt and Cu lose some of their selectivity to CO oxidation, whereas the selectivity on Au decreases substantially primarily because of the significantly weaker CO adsorption.     

Self-assembly of thiolated cyanine aggregates on Au(111) and Au nanoparticle surfaces

Heptamethinecyanine J-aggregates display sharp, intense fluorescence emission making them attractive candidates for developing a variety of chem-bio-sensing applications. They have been immobilized on planar thiol-covered Au surfaces and thiol-capped Au nanoparticles by weak molecular interactions. In this work the self-assembly of novel thiolated cyanine (CNN) on Au(111) and citrate-capped AuNPs from solutions containing monomers and J-aggregates has been studied by using STM, XPS, PM-IRRAS, electrochemical techniques and Raman spectroscopy. Data show that CNN species adsorb on the Au surfaces by forming thiolate-Au bonds. We found that the J-aggregates are preferentially adsorbed on the Au(111) surface directly from the solution while adsorbed CNN monomers cannot organize into aggregates on the substrate surface. These results indicate that the CNN-Au interaction is not able to disorganize the large J-aggregates stabilized by π-π stacking to optimize the S-Au binding site but it is strong enough to hinder the π-π stacking when CNNs are chemisorbed as monomers. The optical properties of the J-aggregates remain active after adsorption. The possibility of covalently bonding CNN J-aggregates to Au planar surfaces and Au nanoparticles controlling the J-aggregate/Au distance opens a new path regarding their improved stability and the wide range of biological applications of both CNN and AuNP biocompatible systems.     

Mechanistic Insights in the Catalytic Synthesis of Vinyl Acetate on Palladium and Gold/Palladium Alloy Surfaces

The reaction pathways for the synthesis of vinyl acetate monomer (VAM) are explored on model palladium and gold–palladium alloy single crystal catalysts by combining experiments carried out in ultrahigh vacuum together with density functional theory calculations and Monte Carlo simulations. Previous work by Goodman has shown that both pure palladium and gold–palladium alloys catalyze VAM formation at high pressures, thereby paving the way for fundamental studies of the pathways for this reaction. The coverages of the reactants and products on the surface were found to play an important role in controlling both the reaction pathways and the selectivity. The high coverages on the catalyst under reaction conditions favor bond-forming reactions while inhibiting bond-breaking reactions. On Pd(111), the reaction is initiated by the coupling of ethylene and surface acetate species to form an acetoxyethyl-palladium intermediate, a bond-forming reaction. The high coverages also act to control the selectivity since VAM is stabilized on the crowded surface. The gold in model Au/Pd(111) and Au/Pd(100) alloys gold preferentially segregates to the surface. In the case of Au/Pd(111) alloys, there is a slightly repulsive interaction between the gold and palladium atoms, resulting in a larger proportion of isolated palladium sites than would be expected if they were randomly distributed, while the longer-range interactions on Au/Pd(100) lead to the formation of ordered surface structures and the existence of isolated palladium sites for gold coverages greater than 0.5 ML. Higher coverages of Au on the Au/Pd(111) and Au/Pd(100) alloys decrease the population of bridging Pd sites and thus increase Pd site isolation. This eliminates the larger Pd ensembles that that lead to the decomposition of VAM and ethylene thus increasing the reaction selectivity and weakens the adsorption of ethylene and acetate which enhances the rate of reaction. Higher coverages of Au, however, also suppress the activation of O₂ which decrease the rate of acid deprotonation thus resulting in optimal Au/Pd compositions.     

Density functionals with broad applicability in chemistry

Although density functional theory is widely used in the computational chemistry community, the most popular density functional, B3LYP, has some serious shortcomings: (i) it is better for main-group chemistry than for transition metals; (ii) it systematically underestimates reaction barrier heights; (iii) it is inaccurate for interactions dominated by medium-range correlation energy, such as van der Waals attraction, aromatic-aromatic stacking, and alkane isomerization energies. We have developed a variety of databases for testing and designing new density functionals. We used these data to design new density functionals, called M06-class (and, earlier, M05-class) functionals, for which we enforced some fundamental exact constraints such as the uniform-electron-gas limit and the absence of self-correlation energy. Our M06-class functionals depend on spin-up and spin-down electron densities (i.e., spin densities), spin density gradients, spin kinetic energy densities, and, for nonlocal (also called hybrid) functionals, Hartree-Fock exchange. We have developed four new functionals that overcome the above-mentioned difficulties: (a) M06, a hybrid meta functional, is a functional with good accuracy "across-the-board" for transition metals, main group thermochemistry, medium-range correlation energy, and barrier heights; (b) M06-2X, another hybrid meta functional, is not good for transition metals but has excellent performance for main group chemistry, predicts accurate valence and Rydberg electronic excitation energies, and is an excellent functional for aromatic-aromatic stacking interactions; (c) M06-L is not as accurate as M06 for barrier heights but is the most accurate functional for transition metals and is the only local functional (no Hartree-Fock exchange) with better across-the-board average performance than B3LYP; this is very important because only local functionals are affordable for many demanding applications on very large systems; (d) M06-HF has good performance for valence, Rydberg, and charge transfer excited states with minimal sacrifice of ground-state accuracy. In this Account, we compared the performance of the M06-class functionals and one M05-class functional (M05-2X) to that of some popular functionals for diverse databases and their performance on several difficult cases. The tests include barrier heights, conformational energy, and the trend in bond dissociation energies of Grubbs' ruthenium catalysts for olefin metathesis. Based on these tests, we recommend (1) the M06-2X, BMK, and M05-2X functionals for main-group thermochemistry and kinetics, (2) M06-2X and M06 for systems where main-group thermochemistry, kinetics, and noncovalent interactions are all important, (3) M06-L and M06 for transition metal thermochemistry, (4) M06 for problems involving multireference rearrangements or reactions where both organic and transition-metal bonds are formed or broken, (5) M06-2X, M05-2X, M06-HF, M06, and M06-L for the study of noncovalent interactions, (6) M06-HF when the use of full Hartree-Fock exchange is important, for example, to avoid the error of self-interaction at long-range, (7) M06-L when a local functional is required, because a local functional has much lower cost for large systems.     

Structure sensitivity in acetylene reactions over bimetallic Pd/W surfaces

Acetylene reactions over a Pd/W(111) surface are found to exhibit size effects on the nanometer scale. Upon annealing 1 ML Pd/W(111), pyramidal facets are formed having bcc(211) faces with nanometer dimensions. The facets grow as a function of annealing time and temperature. Temperature-programmed desorption (TPD) spectra of benzene and ethylene, formed reactively following acetylene adsorption, change as a function of relative facet size. Results are compared to spectra from a similar experiment performed on planar Pd/W(211). TPD of chemisorbed benzene from several different Pd/W surface morphologies are also presented. The data show that structure sensitivity is exhibited for reactively-formed ethylene and benzene, as well as for chemisorbed benzene.     

Gold nanoparticles on ceria: importance of O vacancies in the activation of gold

Synchrotron-based techniques (high-resolution photoemission, in-situ X-ray absorption spectroscopy, and time-resolved X-ray diffraction) have been used to study the destruction of SO₂ and the water-gas shift (WGS, CO + H₂O → H₂ + CO₂) reaction on a series of gold/ceria systems. The adsorption and chemistry of SO₂ was investigated on Au/CeO₂(111) and AuO _x /CeO₂ surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO₂(111) was 4–7 kcal/mol larger than on Au(111). However, there was negligible dissociation of SO₂ on the Au/CeO₂(111) surfaces. The full decomposition of SO₂ was observed only after introducing O vacancies in the ceria support. AuO _x /CeO₂ surfaces were found to be much less chemically active than Au/CeO₂(111) or Au/CeO_2−x (111) surfaces. In a separate set of experiments, in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy were used to monitor the behavior of nanostructured {Au + AuO _x }–CeO₂ catalysts under the WGS reaction. At temperatures above 250 °C, a complete AuO _x → Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO₂(111) single crystal corroborate that cationic Au^δ+ species cannot be the key sites responsible for the WGS activity at high temperatures. The active sites in {Au + AuO _x }/ceria catalysts should involve pure gold nanoparticles in contact with O vacancies of the oxide.     

Electrochemical preparation of pyronin Y thin films on gold substrates

Cyclic voltammetry, chronoamperometry, UV‐vis absorption spectroscopy, fluorescence spectroscopy, FTIR spectroscopy, and AFM techniques have been employed to investigate pyronin Y thin films formed on Au(111) substrates by electrochemical oxidation of pyronin Y monomer. The medium used in the electropolymerization was an anhydrous acetonitrile solution containing 0.1M TBAClO₄ as supporting electrolyte. Anodic electropolymerization potential (1450 mV) of pyronin Y has been obtained from cyclic voltammetry data. Solid‐state electropolymerization of pyronin Y was performed by the potential‐controlled electrolysis technique. Chronoamperometry studies indicate that the adsorption of pyronin Y takes place in an instantaneous three‐dimensional nucleation and growth mechanism which is accompanied by random adsorption. UV‐vis absorption and fluorescence spectra of the electrolysis solution as a function of electrodeposition time show the adsorption of insoluble pyronin Y films on Au electrode surface. FTIR‐specular reflectance of a polymer coated Au electrode reveals that there is a possible C? C coupling in the formation of polymeric pyronin Y structure. A well ordered polymeric chain structure of pyronin Y on Au(111) has been observed from AFM data. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Pt/Au(111)表面合金电化学稳定性的第一性原理研究

利用密度泛函理论(DFT)对Pt/Au (111)表面合金的电化学稳定性进行了初步研究。形成能计算结果表明,Au与Pt不易在块体中形成合金,但能在Pt (111)面形成表面合金。溶解电位计算结果进一步表明,Pt/Au (111)面上Pt原子的溶解电位与其第一近邻Au原子数有很好的线性关系,而Au对第二近邻及更远近邻的Pt溶解电位的影响可忽略。这些结果意味着可建立表面配位环境与表面原子溶解电位间的标度关系,为揭示表面合金的"结构-电化学稳定性"构型关系奠定了基础。     

Adsorption of nitrogen dioxide on polycrystalline gold

The adsorption of nitrogen dioxide (NO₂) on a polycrystalline Au surface was studied by temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). Three desorption states due to chemisorbed NO₂ were observed using TPD, with desorption activation energies,E _d , of 11,13, and 17 kcal/mol. The desorption energies reflect the heats of adsorption of NO₂ on the polycrystalline gold surface, since NO₂ adsorption is not an activated process. Desorption of physisorbed NO₂ from N₂O₄ multilayers was also seen at 130–140 K. The sticking probability of NO₂ at 120 K is independent of coverage indicating a strong influence of a precursor state in the adsorption kinetics. Vibrational spectra using HREELS show that chemisorbed NO₂ is molecularly adsorbed on the surface, probably as a Au O,O'-nitrito surface chelate. No evidence for the dissociation of NO₂ on Au was found using AES, TPD, or HREELS, even for large exposures of NO₂ at surface temperatures up to 500 K. Comparison of these results with those for NO₂ adsorption on a Au(111) surface is made. High energy sites, such as steps and kinks, and other crystal faces of Au can chemically bond NO₂ more tightly than occurs on Au(111), but the activation energy for dissociation of NO₂ at all of these sites exceeds 17 kcal/mol, and thus NO₂ adsorption is reversible on Au under low pressure conditions.     

Theoretical study of benzene,toluene, and dibromobenzene at a Si(111)7×7 surface

Theoretical Austin model 1 (AM1) calculations on the adsorption of benzene and toluene on Si(111)7×7 are presented. Both physisorbed and chemi-sorbed states have been calculated for up to three adsorbed molecules per half unit cell of the Si(111)7×7 surface. Secondly, theoretical calculations on the induced attachment of benzene as well as rationalization of the dynamics of the halogenation reaction of 1,2- and 1,4-dibromobenzene on Si(111)7×7 are reviewed. The main incentive for this study was the interpretation of recent experimental scanning tunneling microscopy (STM) results from the Toronto laboratory on a new electron-induced or photo-induced attachment process for benzene on Si(111), and, particularly, experimental results related to the thermal dissociative reactions of 1,2- and 1,4-dibromobenzene on a Si(111)7×7 surface. The central objective is to relate the reagent geometry in 1,2-dibromobenzene and 1,4-dibromobenzene to the Br-Br pair distance of dibrominated Si(111)7×7. For benzene, we propose a possible path for the conversion from the normal strained di-sigma-bound state (S) at Si(111) to a more strongly bound state (B) consisting of a phenyl plus an H-atom adsorbed species. For 1,2- and 1,4-dibromobenzene dibromination of silicon, evidence has been found for two mechanisms of reaction. One reaction pathway involves intermediate binding of the organic molecule on the Si surface through C-Si bonds, analogous to the benzene S structure. The second dynamical pathway involves intermediate binding through weak Br. Si attachment followed by formation of pairs of covalently-bound Br-Si. The outcomes from the two dynamical pathways are consistent with the observed STM patterns for pairs of Br-Si at Si(111) 7×7 due to the reaction of 1,2- and 1,4-dibromobenzene.     

Insights from Theory on the Relationship Between Surface Reactivity and Gold Atom Release

Density functional theory, informed by experimental studies, is used to investigate the interplay of surface morphology, the adsorption site of reactants, the nature of the interaction between adsorbates and the surface, the potential energy landscape for adsorbates on the surface, adsorbate coverage, temperature, and the dynamic evolution of these factors during adsorption and reaction. We summarize our current understanding of Au atom release on the (111) surface and the corresponding effects on adsorption and reactivity. Gold was selected for these investigations because of the recent intense interest in the activity of gold nanoparticles for several important catalytic reactions. Fundamental experimental studies on Au single-crystal surfaces have established that atomic O is extremely active for oxidation of CO and olefins, that the local bonding of O is an important factor in determining the reactivity and selectivity for oxidation, and that Au atom release is induced by electronegative adsorbates, such as O, Cl, and S. These experimental results guided our theoretical studies. Density functional theory is an extremely useful tool since it evaluates the energetics associated with the incorporation of gold into the adsorbate layer, while providing fundamental physical insight into the underlying cause of gold incorporation. We use our results from static DFT calculations along with ab initio molecular dynamics simulations to understand the effect of surface morphology on the activity of gold for CO oxidation. Our investigation of Au atom release and incorporation induced by electronegative atoms clearly illustrates the importance of using experiments in combination with theory to establish the importance of and the underlying reasons for metal atom release and the affect on bonding and reactivity.     

The bonding in thiolate protected gold nanoparticles from Au4f photoemission core level shifts

Density functional theory calculations are used to evaluate Au4f core level shifts of methyl thiolate protected Au(25), Au(102) and Au(144) nanoparticles. The shifts are found to provide sensitive fingerprints of the chemical environment. In particular, Au atoms in protective gold-thiolate complexes have higher binding energies than Au atoms with solely metal neighbors. The core level shifts for the nanoparticles are compared to the corresponding results for methyl thiolates adsorbed on Au(111) and implications for the understanding of the gold-sulfur bond is discussed.     

Hydrogen evolution during the oxidation of formaldehyde on Au:: The influence of single crystal structure and Tl-upd

Hydrogen evolution during formaldehyde oxidation in alkaline solution has been monitored by Differential Electrochemical Mass Spectrometry on Au(111) and polycrystalline gold. The current efficiency for hydrogen evolution increases with higher concentration and is in the same range on both, polycrystalline Au and Au(111) electrode. The onset potentials and half-wave potentials are higher on Au(111). Reaction orders for the faradaic current on the bare gold electrodes have been determined as 0.21 for higher and 0.76 for lower concentrations. Reaction orders for hydrogen evolution during formaldehyde oxidation are 1.4 times higher in each case. Tafel slopes in the range of 140–160 mV are found. This signifies that the first reaction step involving the formation of adsorbed hydrogen is largely determining the overall reaction rate. In the presence of thallium adlayers hydrogen evolution from formaldehyde oxidation is largely suppressed. On the thallium modified polycrystalline Au, formaldehyde oxidation is shifted for 100 mV to higher potentials where Tl is partially desorbed and hydroxide is coadsorbed on the modified surface. On thallium modified Au(111), a similar process takes place, but in the same potential region as the onset of formaldehyde oxidation on the bare surface and therefore the formaldehyde oxidation is only slightly shifted. Tafel slopes are decreased to 80 mV/dec in the presence of thallium. In the presence of adsorbed thallium, the first reaction step is in equilibrium, the coverage with adsorbed hydrogen is smaller and its recombination to H₂ is largely suppressed.