The tensile strengths of heterogeneous interfaces: a comparison of static and dynamic first-principles calculations

First-principles molecular dynamics (FPMD) simulations and static quantum chemical (QC) calculations are used to evaluate the tensile strengths, σ(c), of interfaces consisting of (0001) surfaces of α-Al(2)O(3) separated by small organic species. The evaluation of σ(c) with FPMD was achieved by performing simulations in which the simulation cell was extending in a direction normal to the fracture plane until rupture of the interface occurred. The static QC calculations employed an approach which treated fracture of the interface as a competition between uniform extension of the simulation cell and crack formation at the rupture site, which is analogous to that used in the construction of universal binding energy relationships. The results showed that the static QC calculations accurately reproduced the FPMD simulations with respect to tensile strength and the cell extension at which rupture occurred, provided that the rupture site employed in the static calculations matched the site at which rupture occurred during the FPMD simulations. A simple strategy for identifying the rupture site, even in complex systems containing many potential rupture sites, is proposed. Overall, the work extends the calculation of tensile strengths with static QC methods to highly heterogeneous interfaces, thus providing a computationally efficient alternative to demanding FPMD simulations for this purpose.     

Direct observation of the substitution effects on the hydrogen bridge dynamics in selected Schiff bases--a comparative molecular dynamics study

We have studied substituent effects on the properties of the intramolecular hydrogen bond of some ortho-hydroxy Schiff bases using density functional theory (DFT) based first-principle molecular dynamics (FPMD) and path integral molecular dynamics. The studied compounds possess a strong intramolecular hydrogen bond (r((O???N)) ≤ 2.6 A?), which can be tuned by substitution to either (i) enhance the basicity of the acceptor moiety by induction effects or (ii) decrease the hydrogen bond length through steric repulsion. DFT calculations and FPMD were employed to investigate structural and dynamical properties of the selected molecules, while quantum effects on the structural properties were assessed using path integral FPMD. The simulations were performed in vacuo and in the solid state to study the influence of the environment on the hydrogen bond and spectroscopic properties. We give computational support to the suggestion that induction effects are less effective to tune the intramolecular hydrogen bond properties of the discussed ortho-hydroxy Schiff bases than the steric or the environmental effects.     

Ab initio simulations of liquid electrolytes for energy conversion and storage

Understanding physicochemical properties of liquid electrolytes is essential for predicting and optimizing device performance for a wide variety of emerging energy technologies, including photoelectrochemical water splitting, supercapacitors, and batteries. In this work, we review recent progress and open challenges in predicting structural, dynamical, and electronic properties of the liquids using first-principles approaches. We briefly summarize the basic concepts of first-principles molecular dynamics (FPMD), and we discuss how FPMD methods have enriched our understanding of a number of liquids, including aqueous solutions, organic electrolytes and ionic liquids. We also discuss technical challenges in extending FPMD simulations to the study of liquid electrolytes in more complex environments, including the interface between electrolytes and electrodes, which is a key component in many energy storage and conversion systems.     

Li2C2, a High‐Capacity Cathode Material for Lithium Ion Batteries

As a typical alkaline earth metal carbide, lithium carbide (Li₂C₂) has the highest theoretical specific capacity (1400 mA h g^?1) among all the reported lithium‐containing cathode materials for lithium ion batteries. Herein, the feasibility of using Li₂C₂ as a cathode material was studied. The results show that at least half of the lithium can be extracted from Li₂C₂ and the reversible specific capacity reaches 700 mA h g^?1. The C?C bond tends to rotate to form C4 (C?C???C?C) chains during lithium extraction, as indicated with the first‐principles molecular dynamics (FPMD) simulation. The low electronic and ionic conductivity are believed to be responsible for the potential gap between charge and discharge, as is supported with density functional theory (DFT) calculations and Arrhenius fitting results. These findings illustrate the feasibility to use the alkali and alkaline earth metal carbides as high‐capacity electrode materials for secondary batteries.     

Pt- (and Co-) Promoted Molybdena-Alumina Catalysts: Analysis of the Impregnation Steps

The equilibrium parameters for the adsorption of Mo(VI) on gamma-Al(2)O(3) and of Co(II) and Pt(IV) on MoO(3)/gamma-Al(2)O(3) were determined. The adsorption isotherms were performed from aqueous solutions of the corresponding precursors on two different alumina supports. According to the classification given by Giles, L-type-shaped, subgroup 2, adsorption curves were found for the system Mo on gamma-Al(2)O(3), L-type, subgroup 1, for the Pt on MoO(3)/gamma-Al(2)O(3), and S-type for Co on the MoO(3)/gamma-Al(2)O(3) system. Numerical calculations were carried out for all the isotherms to find the equilibrium parameters. These constants are being used to model the development of Pt, Co, and Mo profiles on MoO(3)/gamma-Al(2)O(3) or gamma-Al(2)O(3) extrudates, respectively, which belong to the new generation of noble-metal-MoO(3)/gamma-Al(2)O(3)-supported catalysts to be used in oil-refining processes. Copyright 2001 Academic Press.     

Effects of bulk impurity concentration on the reactivity of metal surface: sticking of hydrogen molecules and atoms to polycrystalline Nb containing oxygen

Nonmetallic impurities segregated onto metal surfaces are able to drastically decrease the chemical reactivity of metals. In the present paper, effects of bulk impurities on the reactivity of metallic surfaces were investigated in a wide temperature range on an example of the sticking of hydrogen molecules and atoms to Nb [polycrystalline, with mainly (100)] containing solute oxygen. At all the investigated surface temperatures, T(S) (300-1400 K), we found the bulk oxygen concentration C(O) to have a strong effect on the integral probability, alpha(H(2) ), of dissociative sticking of H(2) molecules followed by hydrogen solution in the metal lattice: alpha(H(2) ) monotonically decreased by orders of magnitude with increasing C(O) from 0.03 to 1.5 at. %. The sticking coefficient alpha(H(2) ) was found to depend on T(S) but not on the gas temperature. The effect of C(O) on alpha(H(2) ) is explained by the presence of oxygen-free sites (holes in coverage) serving as active centers of the surface reaction in the oxygen monolayer upon Nb. In contrast to H(2) molecules, H atoms were found to stick to, and be dissolved in, oxygen-covered Nb with a probability comparable to 1, depending neither on C(O) nor on T(S). This proves that, unlike H(2) molecules, H atoms do stick to be dissolved mainly through regular surface sites covered by oxygen and not through the holes in coverage.     

Adsorption and dissociation of NH3 on clean and hydroxylated TiO2 rutile (110) surfaces: a computational study

The adsorption and dissociation of NH(3) on the clean and hydroxylated TiO(2) rutile (110) surfaces have been investigated by the first-principles calculations. The monodentate adsorbates such as H(3)N-Ti(a), H(2)N-Ti(a), N-Ti(a), H(2)N-O(a), HN-O(a), N-O(a) and H-O(a), as well as the bidentate adsorbate, Ti-N-Ti(a) can be formed on the clean surface. It is found that the hydroxyl group enhances the adsorption of certain adsorbates on the five-fold-coordinated Ti atoms (5c-Ti), namely H(2)N-Ti(a), HN-Ti(a), N-Ti(a) and Ti-N-Ti(a). In addition, the adsorption energy increases as the number of hydroxyl groups increases. On the contrary, the opposite effect is found for those on the two-fold-coordinated O atoms (2c-O). The enhanced adsorption of NH(x) (x = 1-2) on the 5c-Ti is due to the large electronegativity of the OH group, increasing the acidity of the Ti center. This also contributes to diminish the adsorption of NH(x) (x = 1-2) on the two-fold-coordinated O atoms (2c-O) decreasing its basicity. According to potential energy profile, the NH(3) dissociation on the TiO(2) surface is endothermic and the hydroxyl group is found to lower the energetics of H(2)N-Ti(a)+H-O(a) and HN-Ti(a)+2{H-O(a)}, but slightly raise the energetic of Ti-N-Ti(a)+3{H-O(a)} compare to those on the clean surface. However, the dissociation of NH(3) is found to occur on the hydroxylated surface with an overall endothermic by 31.8 kcal/mol and requires a barrier of 37.5 kcal/mol. A comparison of NH(3) on anatase surface has been discussed. The detailed electronic analysis is also carried out to gain insights into the interaction nature between adsorbate and surface.     

Adsorption of O2 on tubelike Au24 and Au24- clusters

The nondissociative adsorptions of O(2) on the neutral and anionic Au(24) have been studied using the density functional theory (DFT) in the generalized gradient approximation. Their geometrical structures are optimized by using a combination of the relativistic effective core potential and all-electron potential with scalar relativistic corrections. It is found that the adsorptions of O(2) on the tubelike Au(24) and Au(24) (-) are more stable than it on their space-filled counterparts. Mulliken population analysis shows that the O(2) adsorbed on the tubelike Au(24) and Au(24) (-) got more electrons than on the amorphous ones, which may be a reason why the O(2) can be adsorbed more easily on the former rather than on the latter. Compared with the previous DFT studies of O(2) adsorbed on small Au(n) (n< or =10) clusters, we have shown that the O(2) can also be adsorbed on the neutral even Au(24) with an adsorption energy compatible with that on the small neutral odd gold clusters, but the adsorption energy of O(2) on the anionic Au(24) (-) is lower than that on the small anionic Au(n) with even n. In all the optimized geometrical structures of the O(2)-adsorbed Au(24) and Au(24) (-) clusters, including both tubelike and amorphous ones, we found that O(2) prefers its two O atoms to be attached to two near gold atoms with the least coordination number rather than only one O atom to be attached to one gold atom.     

Adsorption and decomposition of cyclohexanone (C6H10O) on Pt(111) and the (2 × 2) and (√3 × √3)r30°-Sn/Pt(111) surface alloys

Adsorption and decomposition of cyclohexanone (C(6)H(10)O) on Pt(111) and on two ordered Pt-Sn surface alloys, (2 × 2)-Sn/Pt(111) and (√3 × √3)R30°-Sn/Pt(111), formed by vapor deposition of Sn on the Pt(111) single crystal surface were studied with TPD, HREELS, AES, LEED, and DFT calculations with vibrational analyses. Saturation coverage of C(6)H(10)O was found to be 0.25 ML, independent of the Sn surface concentration. The Pt(111) surface was reactive toward cyclohexanone, with the adsorption in the monolayer being about 70% irreversible. C(6)H(10)O decomposed to yield CO, H(2)O, H(2), and CH(4). Some C-O bond breaking occurred, yielding H(2)O and leaving some carbon on the surface after TPD. HREELS data showed that cyclohexanone decomposition in the monolayer began by 200 K. Intermediates from cyclohexanone decomposition were also relatively unstable on Pt(111), since coadsorbed CO and H were formed below 250 K. Surface Sn allowed for some cyclohexanone to adsorb reversibly. C(6)H(10)O dissociated on the (2 × 2) surface to form CO and H(2)O at low coverages, and methane and H(2) in smaller amounts than on Pt(111). Adsorption of cyclohexanone on (√3 × √3)R30°-Sn/Pt(111) at 90 K was mostly reversible. DFT calculations suggest that C(6)H(10)O adsorbs on Pt(111) in two configurations: by bonding weakly through oxygen to an atop Pt site and more strongly through simultaneously oxygen and carbon of the carbonyl to a bridged Pt-Pt site. In contrast, on alloy surfaces, C(6)H(10)O bonds preferentially to Sn. The presence of Sn, furthermore, is predicted to make the formation of the strongly bound C(6)H(10)O species bonding through O and C, which is a likely decomposition precursor, thermodynamically unfavorable. Alloying with Sn, thus, is shown to moderate adsorptive and reactive activity of Pt(111).     

环氧丙烷在金属氧化物表面上吸附的IR研究

环氧丙烷(PO)是一种重要的化工原料,可用于合成多种特殊化学品及材料[1].其中在某些化学品合成过程中,经常以均相酸或碱作催化剂,例如在合成有机溶剂丙二醇醚的过程中就用到了矿物酸或苛性碱.虽然均相酸或碱作催化剂有活性高、选择性好等优点,但同时存在产物与催化剂分离、腐蚀和废液处理等多种弊端,因此在一些反应过程中人们正积极探求用符合要求的多相催化法来代替均相催化法.A l2O3、ZnO和MgO分别具有酸性、两性和碱性并已用于多种催化反应中[2,3],本文通过IR光谱法研究了环氧丙烷在MgO,A l2O3和ZnO上的吸附活化态.在这些氧化物中…     

Investigations of V(2)O(5)-based LPG sensors

Sensor characteristics of V(2)O(5) dispersed on oxide supports such as Al(2)O(3), TiO(2) and ZrO(2) with respect to various gases and vapours including liquefied petroleum gas (LPG) have been investigated. Of all the systems studied, 20 mol% V(2)O(5) dispersed on ZrO(2) shows the highest sensitivity for LPG, the log sensitivity-log concentration (in ppm) plots being linear up to 1000 ppm or more. The sensitivity is not affected by humidity or recycling. Addition of P(2)O(5) to V(2)O(5) however destroys the sensitivity. Considering all aspects, 20 mol% V(2)O(5)/ZrO(2) is suggested for use as a practical LPG sensor. ESR spectroscopy indicates the formation of V(4+) species on exposure of V(2)O(5)/ZrO(2) or TiO(2) to LPG. In-situ high-temperature x-ray diffraction measurements show the formation of an unusual monoclinic form of VO(2) on exposure to LPG at 625 K which gets oxidized back to V(2)O(5) on exposure to air.     

Directed, selective insertion of single molecules into patterned self-assembled monolayers of alkanethiols with different chain lengths

Pd(ii) pincer adsorbate molecules (1) were inserted into self-assembled monolayers (SAMs) of alkanethiols with different chain lengths (C(8) to C(18)) on annealed gold substrates. Their presence was brought to expression by reaction of with Au nanoclusters bearing phosphine moieties (2). The surface-confined Au nanoclusters were observed only on the shorter chain SAMs (C(8)SH to C(16)SH) and not on C(18)SH SAMs. This is attributed to the longer chain length of C(18)SH preventing the insertion of pincer molecules. Microcontact printing (microCP) with C(18)SH on unannealed gold substrates and the subsequent immersion of the substrates into C(8)SH, C(10)SH, C(12)SH, or C(16)SH solutions, yielded a series of patterned SAMs that have areas of thiols of different chain lengths. Insertion of 1 followed by expression using 2, or insertion of 3 showed inserted molecules only in the shorter chain SAM areas. The absolute particle densities in the former case were higher than on the corresponding homogeneous SAMs on annealed substrates, probably due to larger numbers of defects in the SAMs on unannealed substrates.     

Photoinduced graft polymerization of 2-methacryloyloxyethyl phosphorylcholine on polyethylene membrane surface for obtaining blood cell adhesion resistance

Phospholipid polymer, poly[2-methacryloyloxyethyl phosphorylcholine (MPC)], was grafted with polyethylene (PE) membrane using photoinduced polymerization technique to make the membrane resistant to cell adhesion. The water contact angle on the PE membrane grafted with poly(MPC) decreased with an increase in the photopolymerization time. This decrease corresponded to the increase in the amount of poly(MPC) grafted on the PE surface. The same graft polymerization procedure was applied using other hydrophilic monomers, such as acrylamide (AAm), N-vinylpyrrolidone (VPy) and methacryloyl poly(ethylene glycol) (MPEG). These monomers were also polymerized to form grafted chains on the PE membrane, and the grafting was confirmed with X-ray photoelectron spectroscopy. Analysis of amount and distribution of plasma proteins at the plasma-contacting surface of the original and the modified PE membranes were analyzed using immunogold assay. The grafting of poly(MPC) and poly(VPy) on PE membrane reduced the plasma protein adsorption significantly compared with that on the original PE membrane. However, the PE membranes grafted with poly(AAm) or poly(MPEG) did not show any effects on protein adsorption. Platelet adhesion on the original and modified PE membranes from platelet-rich plasma was also examined. A large number of platelets adhered and activated on the original PE membrane. Grafting with poly(AAm) did not suppress platelet adhesion, but grafting with poly(MPC) or poly(VPy) on the PE membrane was effective in preventing platelet adhesion. It is concluded that the introduction of the phosphorylcholine group on the surface could decrease the cell adhesion to substrate polymer.     

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.     

Micro volume changes due to Pb(II) and Cu(II) sorption on amorphous Fe(III) hydroxide

Micro volume changes due to Pb(II) and Cu(II) sorption on amorphous Fe(III) hydroxide (AFH) were determined by a dilatometer at pH 4.50. Volume change is attributed to change in hydration status of dissolved and/or suspended substances. The volume of the system increased due to Pb(II) and Cu(II) sorption, suggesting that water molecules hydrated around Pb(II) or Cu(II) ions and AFH were released during sorption. Volume increases due to Pb(II) and Cu(II) sorption were smaller than those due to bulk precipitation of Pb and Cu hydroxides. Precipitation of Pb(II) and Cu(II) was not likely to occur at pH 4.50 in the presence of AFH. In conclusion, Pb(II) and Cu(II) formed an inner-sphere complex on AFH at pH 4.50, keeping hydrated water on the adsorbed species. Adsorbed Cu(II) kept more hydrated water than adsorbed Pb(II) on AFH.     

Am(III) adsorption on oxides of aluminium and silicon: effects of humic substances,pH, and ionic strength

The adsorption of Am(III) (total concentration 10(-9) mol/l) on alumina, silica, and hematite was studied by a batch technique. The effects of pH, ionic strength, and humic substances on the adsorption of Am(III) on alumina and silica were investigated, and the adsorption isotherms of Am(III) on alumina and silica at different pH values were determined. It was found that compared with the adsorption of Am(III) on alumina, the adsorbability of silica on the basis of mass is less, the relative adsorption rate on silica is slower, the sensitivity of adsorption on silica to ionic strength is less, the dependence of adsorption on silica on pH is gentler, and consequently that the adsorption characteristics of Am(III) on alumina and silica are distinctly different. The negative effect of fulvic acid on the adsorption on silica and the positive effect of humic acid on the adsorption on alumina were found. In contrast to the Am(III) adsorption on alumina and silica, a tremendously high adsorbability of Am(III) on hematite was found. The sequence of adsorbabilities of Am(III) on the basis of mass is Fe2O3 > Al2O3 > SiO2.     

On the clarification of the IR stretching vibration assignment of adsorbed N2 on Rh0 and Rhdelta+ surface atoms of supported Rh crystallites

The low-temperature adsorption of N(2) on Rh/SiO(2) samples of various particle-size distributions was followed by FTIR. The addition of O(2) pulses on Rh(0) surfaces saturated with chemisorbed N(2) allowed us to reassign stretching frequencies attributed originally to N(2)-Rh(0) to N(2)-Rh(delta+). The formation of the latter oxidized Rh species is assumed to be induced by an electron withdrawal from adsorbed oxygen species on Rh surface centers neighboring those onto which N(2) species are chemisorbed. The present work, thus, enables us to delimit ranges of frequencies for which the adsorption of N(2) can be considered to occur on either Rh(0) or Rh(delta+) centers for nu(N2) lower or higher than 2243 cm(-1), respectively. The N(2)-FTIR experiments performed on the studied catalysts also suggest a lattice plane selectivity for N(2) adsorption on metallic Rh planes of different natures which, to our knowledge, has not been reported yet for Rh.     

吸附原子电催化机理的研究——铂单晶(100)晶面上Pb、Cd吸附原子在甲酸电氧化中的助催化特性

In this work a modified adsorption method was used to form firstly a saturated Cd or Pb adlayer on platinum single crystal (100) surface, then its surface structure, the electrochemical stability and the desorption-readsorption CV properties were studied. The HCOOH oxidation has been used as a probe to determine the electrocatalytic propertics of Pt (100) with different surface concentration of Cd or Pb adatoms. The results show that Cd adatoms on Pt(100) inhibit both HCOOH dissociation adsorption (to form poisoning intermediates) and HCOOH oxidation via the reactive intermediate, and that Pb adatoms on Pt(100) inhibit only HCOOH disociation adsorption but catalyse HCOOH oxidation via the reactive intermediate, in this case the more Pb adatoms on Pt(100), the higher te catalytic effect. The highest enhanced catalytic effet has been found with the saturated Pb adlayer on Pt(100) (the coverage of Pb, calibrated by hydrogen adsorption, is then equal to 1.0), i.e. with a surface structure of C(2×2)-bridged Pb. The difference of influence to the hydrogen adsorption-desorption process and the different catalytic effect on HCOOH oxidation of Cd and Pb adatoms on Pt(100) were attributed to the intrinsic properties of Cd and Pd elements and also to their interaction with Pt(100). This results indicate that the electrocatalytic mechanism of Cd and Pb adatom on Pt(100) depends on the geometrical surface structure effect and, more importantly, on the electro structure effect during the interaction of adatoms with Pt(100) surface atoms.     

Enhanced selectivity towards O(2) and H(2) dissociation on ultrathin Cu films on Ru(0001)

The reactivity of Cu monolayer (ML) and bilayer films grown on Ru(0001) towards O(2) and H(2) has been investigated. O(2) initial sticking coefficients were determined using the King and Wells method in the incident energy range 40-450 meV, and compared to the corresponding values measured on clean Ru(0001) and Cu(111) surfaces. A relative large O(2) sticking coefficient (～0.5-0.8) was measured for 1 ML Cu and even 2 ML Cu/Ru(0001). At low incident energies, this is one order of magnitude larger than the value observed on Cu(111). In contrast, the corresponding reactivity to H(2) was near zero on both Cu monolayer and bilayer films, for incident energies up to 175 meV. Water adsorption on 2 ML Cu/Ru(0001) was found to behave quite differently than on the Ru(0001) and Cu(111) surfaces. Our study shows that Cu/Ru(0001) is a highly selective system, which presents a quite different chemical reactivity towards different species in the same range of collision energies.     

Effects of magnetite nanoparticles on the thermorheological properties of carrageenan hydrogels

The influence of magnetite (Fe(3)O(4)) nanoparticles on the rheological properties of kappa-, iota- and lambda-carrageenan gels has been investigated. Small amplitude oscillatory shear measurements were performed to study the effect of the presence of Fe(3)O(4) nanoparticles with particle sizes of ca. 10 nm on the gel properties, as a function of carrageenan type, carrageenan concentration and magnetite load. The formation of Fe(3)O(4) nanoparticles on the presence of biopolymer was observed to promote the gelation process and lead to stronger gels as indicated by an increase in the gel viscoelastic moduli and of the gelation temperature. This effect was more marked for kappa-carrageenan than for iota- and lambda-carrageenan and has been proposed to depend not only on Fe(3)O(4) concentration but also on the concentration of potassium ions. A mechanism based on the combined effect of Fe(3)O(4) nanoparticles and potassium ions was suggested, involving the adsorption of potassium ions on the negatively charged surface of the Fe(3)O(4) nanoparticles, thus leading to an increase of the potassium ion concentration within the "carrageenan cages" containing the magnetite. This would, therefore, promote more extensive biopolymer helical aggregation, thus resulting in the formation of a stronger kappa-carrageenan gel in the presence of Fe(3)O(4), as observed. Since iota- and lambda-carrageenan gels are known to be less sensitive to potassium ions concentration, the effect of precipitating Fe(3)O(4) within these biopolymers is reduced.