Nanoscopic modelling of the adhesion, indentation and fracture characteristics of metallic systems via molecular dynamics simulations

Large-scale molecular dynamics simulations have been performed on canonical ensembles to model the adhesion and indentation characteristics of 3-D metallic nano-scale junctions in tip-substrate geometries, and the crack propagation in 2-D metallic lattices. It is shown that irreversible flows in nano-volumes of materials control the behaviour of the 3-D nano-contacts, and that local diffusional flow constitutes the atomistic mechanism underlying these plastic flows. These simulations show that the force of adhesion in metallic nano-contacts is reduced when adsorbate monolayers are present at the metal—metal junctions. Our results are in agreement with the conclusions of very accurate point-contact experiments carried out in this field. Our fracture simulations reveal that at low temperatures cleavage fractures can occur in both an elemental metal and an alloy. At elevated temperatures, the nucleation of dislocations is shown to cause a brittle-to-ductile transition. Limiting crack propagation velocities are computed for different strain rates and a dynamic instability is shown to control the crack movement beyond this limiting velocity, in line with the recent experimental results.

Relevance of heterometallic binding energy for metal underpotential deposition

We present first-principles calculations for a number of metals adsorbed on several different metallic substrates. Some of these systems are very relevant in electrochemistry, especially in the field of underpotential deposition phenomena. The present studies reveal the existence of a relationship between the excess binding energy and the surface energy difference between substrate and adsorbate. Comparisons with experimental underpotential shifts show that excess binding energies are systematically underestimated. By analyzing experimental information on different systems, we conclude that this discrepancy between our vacuum calculations and experiments carried out in an electrolytic solution is likely to be due to anion adsorption and/or solvent effects.

Quantitative analysis of the reactivity of formate species seen by DRIFTS over a Au/Ce(La)O2 water–gas shift catalyst: First unambiguous evidence of the minority role of formates as reaction intermediates

The reactivity of the species formed at the surface of a Au/Ce(La)O2 catalyst during the water������¢���¯���¿���½���¯���¿���½gas shift (WGS) reaction were investigated by operando diffuse reflectance Fourier transform spectroscopy (DRIFTS) at the chemical steady state during isotopic transient kinetic analyses (SSITKA). The exchanges of the reaction product CO2 and of formate and carbonate surface species were followed during an isotopic exchange of the reactant CO using a DRIFTS cell as a single reactor. The DRIFTS cell was a modified commercial cell that yielded identical reaction rates to that measured over a quartz plug-flow reactor. The DRIFTS signal was used to quantify the relative oncentrations of the surface species and CO2. The analysis of the formate exchange curves between 428 and 493 K showed that at least two levels of reactivity were present. ������¢���¯���¿���½���¯���¿���½Slow formates������¢���¯���¿���½���¯���¿���½ displayed an exchange rate constant 10- to 20-fold slower than that of the reaction product CO2. ������¢���¯���¿���½���¯���¿���½Fast formates������¢���¯���¿���½���¯���¿���½ were exchanged on a time scale similar to that of CO2. Multiple nonreactive readsorption of CO2 took place, accounting for the kinetics of the exchange of CO2(g) and making it impossible to determine the number of active sites through the SSITKA technique. The concentration (in mol g������¢���¯���¿���½���¯���¿���½1) of formates on the catalyst was determined through a calibration curve and allowed calculation of the specific rate of formate decomposition. The rate of CO2 formation was more than an order of magnitude higher than the rate of decomposition of formates (slow + fast species), indicating that all of the formates detected by DRIFTS could not be the main reaction intermediates in the production of CO2. This work stresses the importance of full quantitative analyses (measuring both rate constants and adsorbate concentrations) when investigating the role of adsorbates as potential reaction intermediates, and illustrates how even reactive species seen by DRIFTS may be unimportant in the overall reaction scheme.

The dissociation of molecularly adsorbed CO and CN over the 4d transition metals: A universal relationship between the reaction barriers and the reaction enthalpies

Density functional theory calculations are used to study the stability of molecularly adsorbed CO and CN over transition metal surfaces. The minimum energy reaction pathways, corresponding reaction barriers (E-a), and reaction enthalpies (Delta H) for the dissociation of CO and CN to atomic products over the 4d transition metals from Zr to Pd have been determined. CO is found to be the more stable adsorbate on the right hand side of the period (from Tc onwards), whereas CN is the more stable surface species on the early metals (Zr, Nb and Mo). A single linear relationship is found to exist that correlates the barriers of both reactions with their respective reaction enthalpies. (c) 2006 Elsevier B.V. All rights reserved.

Resolution of an ancient surface science anomaly: Work function change induced by N adsorption on W{100}

For many decades it has been assumed that an adsorbate centered above a metal surface and with a net negative charge should increase the work function of the surface. However, despite their electronegativity, N adatoms on W{100} cause a significant work function decrease. Here we present a resolution of this anomaly. Using density functional theory, we demonstrate that while the N atom carries a negative charge, of overriding importance is a reduction in the surface overspill electron density into the vacuum, when that charge is engaged in bonding to the adatom. This novel interpretation is fundamentally important in the general understanding of work function changes induced by atomic adsorbates.

Softened C-H modes of adsorbed methyl and their implications for dehydrogenation: An ab initio study

To investigate the softening of CH vibrational frequencies and their implications for dehydrogenation of adsorbed hydrocarbons, an issue of scientific and technological importance, density functional theory calculations have been performed on the chemisorption and dehydrogenation of CH3 on Cu(111) and Pt(111) surfaces. By comparing these results with those of Ni(111) we find that the CH bonds of the adsorbate, when close enough, interact with metal atoms of the surface. It is this interaction and its associated lengthening and weakening of CH bonds that is the physical origin of mode softening. We rule out the possibility of a relationship between the mere presence of mode softening and dehydrogenation. We do show, however, that there is a clear relationship between the extent to which a surface can induce mode softening and the activation energy to dehydrogenation. In addition, periodic trends concerning the extent of mode softening are reproduced. (C) 2001 American Institute of Physics.

A density functional theory study of the surface relaxation and reactivity of Cu2O(100)

Density functional theory has been used to investigate the surface relaxation of Cu2O(100) and the adsorption of NO. The calculations indicate the formation of surface copper dimers on relaxation coupled with a large contraction of the spacing between the first and second layers. Local density of states for atoms in the top three layers shows that the third layer copper atoms have the greatest change in bonding character. Adsorption energies have been calculated for the N-down and O-down adsorption of NO on the Cu2O(100) surface. These indicate that N-down adsorption is favoured and that in this case NO-lattice oxygen interactions dominate the adsorbate structure. (C) 2000 Elsevier Science B.V. All rights reserved.

In-situ FT-IR spectroscopic studies of fuel cell electro-catalysis: From single-crystal to nanoparticle surfaces

The work presented in this article shows the power of the variable temperature, in-situ FT-IR spectroscopy system developed in Newcastle with respect to the investigation of fuel cell electro-catalysis. On the Ru(0001) electrode surface, CO co-adsorbs with the oxygen-containing adlayers to form mixed [CO+(2x2)-O(H)] domains. The electro-oxidation of the Ru(0001) surface leads to the formation of active (1x1)-O(H) domains, and the oxidation of adsorbed CO then takes place at the perimeter of these domains. At 20 degrees C, the adsorbed CO is present as rather compact islands. In contrast, at 60 degrees C, the COads is present as a relatively looser and weaker adlayer. Higher temperature was also found to facilitate the surface diffusion and oxidation of COads. No dissociation or electro-oxidation of methanol was observed at potentials below approximately 950mV; however, the Ru(0001) surface at high anodic potentials was observed to be very active. On both Pt and PtRu nanoparticle surfaces, only one linear bond CO adsorbate was formed from methanol adsorption, and the PtRu surface significantly promoted both methanol dissociative adsorption to CO and its further oxidation to CO2. Increasing temperature from 20 to 60 degrees C significantly facilitates the methanol turnover to CO2.

Changes in surface stress caused by the adsorption of an epitaxial metal monolayer

The adsorption of a metal monolayer on a foreign substrate generates a change in the surface stress. We calculate this change for a number of substrate/adsorbate systems using the embedded-atom method. The results are compared with those obtained from a continuum model. A cycle, in which the stretching of a substrate/adsorbate system is decomposed into several steps, helps in understanding the numerical results. (C) 2000 Elsevier Science B.V. All rights reserved.

Textile wastewater treatment using granular activated carbon adsorption in fixed beds

This work involved the treatment of industrial wastewater from a nylon carpet printing plant which currently receives no treatment and is discharged to sea. As nylon is particularly difficult to dye, acid dyes are required for successful coloration and cause major problems with the plant's effluent disposal in terms of color removal. Granular activated carbon Filtrasorb 400 was used to treat a ternary solution of acid dyes and the process plant effluent containing the dyes in a fixed-bed column system. Experimental data were correlated using the bed depth service time (BDST) model to previously published work by the authors for single dye adsorption. The results were expressed in terms of the BDST adsorption capacity, in milligrams of adsorbate per gram of adsorbent, and indicated that there was a 12-25% decrease iri adsorption capacity in the ternary system compared to the single component system; This reduction has been attributed to competitive adsorption occurring in the ternary component system. Dye adsorption from the process plant effluent showed an approximate 65% decrease in adsorption capacity compared to the ternary solution system. This has been attributed to interference caused by the other colorless textile effluent pollutants found in the process wastewater. A chemical oxygen demand analysis on these components indicated that the dyes accounted for only 14% of the total oxygen demand.

Hydrogen adsorption on functionalized nanoporous activated carbons

There is considerable interest in hydrogen adsorption on carbon nanotubes and porous carbons as a method of storage for transport and related energy applications. This investigation has involved a systematic investigation of the role of functional groups and porous structure characteristics in determining the hydrogen adsorption characteristics of porous carbons. Suites of carbons were prepared with a wide range of nitrogen and oxygen contents and types of functional groups to investigate their effect on hydrogen adsorption. The porous structures of the carbons were characterized by nitrogen (77 K) and carbon dioxide (273 K) adsorption methods. Hydrogen adsorption isotherms were studied at 77 K and pressure up to 100 kPa. All the isotherms were Type I in the IUPAC classification scheme. Hydrogen isobars indicated that the adsorption of hydrogen is very temperature dependent with little or no hydrogen adsorption above 195 K. The isosteric enthalpies of adsorption at zero surface coverage were obtained using a virial equation, while the values at various surface coverages were obtained from the van't Hoff isochore. The values were in the range 3.9-5.2 kJ mol(-1) for the carbons studied. The thermodynamics of the adsorption process are discussed in relation to temperature limitations for hydrogen storage applications. The maximum amounts of hydrogen adsorbed correlated with the micropore volume obtained from extrapolation of the Dubinin-Radushkevich equation for carbon dioxide adsorption. Functional groups have a small detrimental effect on hydrogen adsorption, and this is related to decreased adsorbate-adsorbent and increased adsorbate-adsorbate interactions.

Reactive dye adsorption onto a novel mesoporous carbon

A new mesoporous carbon (MCSG60) was developed using an inexpensive commercial mesoporous silica gel as a template and sucrose as the carbon source. The surface area, porosity and density of the carbon were determined. The material possesses a high specific surface area and pore volume accessible for most typical aqueous pollutants. The adsorbent material was tested in a batch dye adsorption system. The behaviour of three reactive dyes adsorbed onto MCSG60 was evaluated (Naphthol Blue Black, NBB; Reactive Black 5, RB5; and Remazol Brilliant Blue R, RBBR). The maximum adsorption capacities obtained for the dyes were: 270. mg/g for NBB; 270. mg/g for RB5; and 280. mg/g for RBBR. Kinetic studies indicated that the adsorption process onto the mesoporous carbon was rapid and that equilibrium was reached in less than 1. h for all the dye systems investigated. Further batch experiments showed MCSG60 successfully adsorbed the dyes over a wide pH range and at low adsorbate concentration. The adsorption potential of MCSG60 for dye removal was further evaluated using a fixed-bed adsorption column. © 2013 Elsevier B.V.

Chemisorption and decomposition of pyrrole on Pd{111}

XPS, TPD and HREEL results indicate that molecular pyrrole is a fragile adsorbate on clean Pd{111}. At 200 K and for low coverages, the molecule remains intact and adopts an almost flat-lying geometry. With increasing coverage, pyrrole molecules tilt away from the surface and undergo N-H bond cleavage to form strongly tilted pyrrolyl (C4H4N) species. In addition, a weakly bound, strongly tilted form of molecular pyrrole is observed at coverages approaching saturation. Heating pyrrole monolayers results in desorption of similar to 15% of the overlayer as molecular pyrrole and N-a+ C4H4Na recombination with formation of hat-lying pyrrole molecules. This strongly bound species undergoes decomposition to adsorbed CN, CHx and H, leading ultimately to desorption of HCN and H-2. The implications of these results for the production of pyrrole by a heterogeneously catalysed route are discussed.

Chemisorption and reactivity of furan on Pd{111}

XPS, HREELS, ARUPS and Delta phi data show that furan chemisorbs non-dissociatively on Pd{111} at 175 K, the molecular plane being significantly tilted with respect to the surface normal. Bonding involves both the oxygen lone pair and significant a interaction with the substrate. The degree of decomposition that accompanies molecular desorption is a strong function of coverage: similar to 40% of the adsorbate desorbs molecularly from the saturated monolayer. Decomposition occurs via decarbonylation to yield COa and H-a followed by desorption rate limited loss of H-2 and CO. It seems probable that an adsorbed C3H3 species formed during this process undergoes subsequent stepwise dehydrogenation ultimately yielding H-2 and C-a.

Quantum chemistry studies on electronic properties of CN adsorbed on silver electrodes

The electronic properties of CN adsorbed on Ag electrodes at different potentials have been studied by using the method of self-consistent-charge discrete variational Xa (SCC-DV-Xa) cluster calculations. It is shown that the binding of NCAg is dominated by both electrostatic and polarization effects derived from the charge of CN, and that the transfer of s charge from CN to silver is the largest donation contribution. The electrode potential influences this charge transfer process and the interaction between CN adsorbate and silver electrode. The results of quantum chemistry calculations fit well with the experimental data of in situ spectroscopic studies on the CN/Ag electrode systems. © 1991.