Methyl chemisorption on Ni(111) and C-H-M multicentre bonding:a density functional theory study

Density functional theory has been used to study the adsorption of CH3 on Ni(111). CH3 is found to adsorb strongly at all four high symmetry sites of the Ni(111) surface. Calculated adsorption energies of CH3 on the different sites are in the following order: hcp approximate to fcc>bridge>top. The bonding and structures of CH3 on the different sites are analysed in detail. An important factor, namely three-centre bonding between carbon, hydrogen and nickel which contributes to the 'soft' C-H vibrational frequency of CH3 on Ni(111), and may determine the preferred chemisorption site, is stressed. (C) 1999 Elsevier Science B.V. All rights reserved.

Density functional theory study of the interaction between CO and on a Pt surface:CO/Pt(111), O/Pt(111), and CO/O/Pt(111)

Ab initio total energy calculations within the Density Functional Theory framework were carried out for Pt(111), Pt(111)-p(2x2)-CO, Pt(111)-p(2x2)-O, and Pt(111)-p(2x2)-(CO+O) to provide an insight into the interaction between CO and O on metal surfaces, an important issue in CO oxidation, and also in promotion and poisoning effects of catalysis. The geometrical structures of these systems were optimized with respect to the total energy, the results of which agree with existing experimental values very well. It is found that (i) the local structures of Pt(111)-p(2x2)-(CO+O), such as the bond lengths of C-O, C-Pt, and O-Pt (chemisorbed O atom with Pt), are almost the same as that in Pt(111)-p(2x2)-CO and Pt(111)-p(2x2)-O, respectively, (ii) the total valence charge density distributions in Pt(111)-p(2x2)-(CO+O) are very similar to that in Pt(111)-p(2x2)-CO, except in the region of the chemisorbed oxygen atom, and also nearly identical to that in Pt(111)-p(2x2)-O, apart from in the region of the chemisorbed CO, and (iii) the chemisorption energy of CO on a precovered Pt(111)-p(2x2)-O and the chemisorption energy of O on a precovered Pt(111)-p(2x2)CO are almost equal to that in Pt(111)-p(2x2)-CO and Pt(111)-p(2x2)-O, respectively. These results indicate that the interaction between CO and chemisorbed oxygen on a metal surface is mainly shore range in nature. The discussions of Pt-CO and Pt-O bonding and the interaction between CO and the chemisorbed oxygen atom on Pt(111) are augmented by local densities of states and real space distributions of quantum states.

CO oxidation on Pt(111):An ab initio density functional theory study

CO oxidation on Pt(111) is studied with ab initio density functional theory. The low energy pathway and transition state for the reaction are identified. The key event is the breaking of an O-metal bond prior to the formation of a chemisorbed CO2 molecule. The pathway can be rationalized in terms of competition of the O and C atoms for bonding with the underlying surface, and the predominant energetic barrier is the strength of the O-metal bond.

HOLOGRAPHIC IMAGES OF IODINE ATOMS ON A SILVER SURFACE FROM ELECTRON-EMISSION PATTERNS

BARTON 1 has suggested that photoelectron interference patterns may be used directly as holograms to obtain atomic-resolution images of surface structures. Bulk structures have been obtained previously by this means from experimental patterns of high-energy Kikuchi(quasi-elastically scattered) and Auger electrons 2,3. Here we test the feasibility of this technique for determination of surface structures using Auger intensity patterns obtained 4,5 from iodine chemisorbed on a pseudomorphic silver monolayer on Pt{111}. By direct numerical holographic inversion, we obtain three-dimensional images which show that iodine adatoms are located in hollows of 3-fold symmetry on the surface. The images yield the site symmetry with good atomic resolution in the surface plane, but suffer from poor resolution along the Ag-I axis. We anticipate that data with better angular resolution obtained at low temperatures would improve the spatial resolution of such images.

Nonclassicality and criticality in symmetry protected magnetic phases

Quantum and global discord in a spin-1 Heisenberg chain subject to single-ion anisotropy (uniaxial field) are studied using exact diagonalisation and the density matrix renormalisation group (DMRG). We find that these measures of quantum non-classicality are able to detect the quantum phase transitions confining the symmetry protected Haldane phase and show critical scaling with universal exponents. Moreover, in the case of thermal states, we find that quantum discord can increase with increasing temperature.

Effect of solvent on the hydrogenation of 4-phenyl-2-butanone over Pt based catalysts

The hydrogenation of 4-phenyl-2-butanone over Pt/TiO2 and Pt/SiO2 catalysts has been performed in a range of solvents and it has been observed that the solvent impacted on the selectivity of ketone and aromatic ring hydrogenation as well as the overall TOF of the titania catalyst with no solvent effect on selectivity observed using the silica supported catalyst where ring hydrogenation was favored. For the titania catalyst, alkanes were found to favor ring hydrogenation whereas aromatics and alcohols led to carbonyl hydrogenation. A two-site catalyst model is proposed whereby the aromatic ring hydrogenation occurs over the metal sites while carbonyl hydrogenation is thought to occur predominantly at interfacial sites, with oxygen vacancies in the titania support activating the carbonyl. The effect of the solvent on the hydrogenation reaction over the titania catalyst was related to competition for the active sites between solvent and 4-phenyl-2-butanone.

A kinetic analysis methodology to elucidate the roles of metal, support and solvent for the hydrogenation of 4-phenyl-2-butanone over Pt/TiO2

The rate and, more importantly, selectivity (ketone vs aromatic ring) of the hydrogenation of 4-phenyl-2-butanone over a Pt/TiO2 catalyst have been shown to vary with solvent. In this study, a fundamental kinetic model for this multi-phase reaction has been developed incorporating statistical analysis methods to strengthen the foundations of mechanistically sound kinetic models. A 2-site model was determined to be most appropriate, describing aromatic hydrogenation (postulated to be over a platinum site) and ketone hydrogenation (postulated to be at the platinum–titania interface). Solvent choice has little impact on the ketone hydrogenation rate constant but strongly impacts aromatic hydrogenation due to solvent-catalyst interaction. Reaction selectivity is also correlated to a fitted product adsorption constant parameter. The kinetic analysis method shown has demonstrated the role of solvents in influencing reactant adsorption and reaction selectivity.

The role of low coverage sodium surface species on electrochemical promotion in a Pt/YSZ system

The effect of sodium-modification on the catalyst and electrocatalytic properties of a platinum catalyst supported on a YSZ solid electrolyte was studied. Increasing the sodium coverage on the catalyst surface appears to block some of the three-phase boundary (tpb) sites and reduces the rate of the charge transfer reaction. The promotion of the platinum surface reaction (ethylene oxidation) seems to a first approximation to be a function of the rate of oxygen supply or removal to or from the surface irrespective of whether this is contaminated by sodium or not (samples with sodium contamination require a higher overpotential to achieve the same current density as a clean sample because of poisoning in the tpb). At high negative polarisations (oxygen removed from the surface) the sodium contaminated samples show a significant increase in rate, possibly due to the decomposition of e.g. sodium hydroxides and carbonates. © 2012 Elsevier B.V.

Influence of impurities and catalyst surface characteristics on the oxygen charge transfer reaction in the Pt/YSZ system

Spillover processes (i.e. the migration of ionic species from the support to the catalyst and vice versa) are known to play a very important role in catalysis and electrocatalysis. These spillover processes can be influenced by impurities (pre-existing on the catalyst surface) and by the catalyst morphology that may differ as a result of the differences in catalyst manufacturing processes. This work investigates the influence of impurities present in three commercial platinum (Pt) precursors. The resulting platinum films studied here were supported on yttria-stabilised-zirconia (YSZ). It was found that the three different catalyst films contained a range of impurities (determined by ICP-OES) that appear to affect the oxygen charge transfer reaction as studied by cyclic voltammetry (CV). © 2012 Elsevier B.V.

Electrochemical promotion of a Pt catalyst supported on La 0.6Sr 0.4Co 0.2Fe 0.8O 3 - δ hollow fibre membranes

The use of wireless electrochemical promotion of catalysis (EPOC) of a Pt catalyst supported on a mixed ionic electronic conducting hollow fibre membranes is investigated. This reactor configuration offers high surface areas per unit volume and is ideally suited for scaled-up applications. The MIEC membrane used is the La _0.6Sr _0.4Co _0.2Fe _0.8O ₃ perovskite (LSCF) with a Pt catalyst film deposited on the outer surface of the LSCF membrane. Experimental results showed that after initial catalyst deactivation (in the absence of an oxygen chemical potential difference across the membrane) the catalytic rate can be enhanced by using an oxygen sweep and wireless EPOC can be used for the in situ regeneration of a deactivated catalyst. © 2012 Elsevier B.V.

Remote control of the activity of a Pt catalyst supported on a mixed ionic electronic conducting membrane

A novel configuration for the in situ control of the catalytic activity of a polycrystalline Pt catalyst supported on a mixed ionic electronic conducting (MIEC) substrate is investigated. The modification of the catalytic activity is achieved by inducing the reverse spillover of oxygen promoting species from the support onto the catalyst surface, thus modifying the chemisorptive bond energy of the gas phase adsorbed reactants. This phenomenon is known as Electrochemical Promotion of Catalysis (EPOC). In this work we investigate the use of a wireless system that takes advantage of the mixed ionic electronic conductivity of the catalyst support (internally short-circuiting the system) in a dual chamber reactor. In this wireless configuration, the reaction takes place in one chamber of the membrane reactor while introduction of the promoting species is achieved by the use of an appropriate sweep gas (and therefore control of the oxygen chemical potential difference across the membrane) on the other chamber. Experimental results have shown that the catalytic rate can be enhanced by using an oxygen sweep, while a hydrogen sweep can reverse the changes. Total rate enhancement ratios of up to 3.5 were measured. © 2008 Elsevier B.V. All rights reserved.

Comparative studies between classic and wireless electrochemical promotion of a Pt catalyst for ethylene oxidation

A comparative study between a classic and a wireless electrochemical promotion experiment was undertaken as a tool towards the better understanding of both systems. The catalytic modification of a platinum catalyst for ethylene oxidation was studied. The catalyst was supported on yttria-stabilised-zirconia (YSZ), a known pure oxide ion conductor, for the classic experiment and La _0.6Sr_0.4Co_0.2Fe_0.8O _3-δ-a mixed oxide ion electronic conductor-was used for the wireless experiment. The two systems showed certain similarities in terms of the reaction classification (in both cases electrophobic behaviour was observed) and the promotion mechanism. Significant difference was observed in the time scales and the reversibility of the induced rate modification. © 2008 Springer Science+Business Media B.V.

In situ modification of a Pt catalyst supported on a mixed ionic-electronic conducting membrane

The electrochemical promotion of a platinum catalyst for ethylene oxidation on a dual chamber membrane reactor was studied. The catalyst was supported on a La_0.6Sr_0.4Co_0.2Fe_0.803 membrane. Due the supporting membrane's electronic conductivity it is possible to promote the reaction by controlling the oxygen chemical potential difference across the membrane. Upon establishment of an oxygen potential difference across the membrane, oxygen species can migrate and spillover onto the catalyst surface, modifying the catalytic activity. Initial experiments showed an overall promotion of approximately one order of magnitude of the reaction rate of ethylene, under an oxygen atmosphere on the sweep side of the membrane reactor, as compared with the rate under an inert sweep gas. The reaction rate can keep its promoted state even after the flow of oxygen on the sweep side was interrupted. This behavior caused further promotion with every experiment cycle. The causes of permanent promotion and on demonstrating controllable promotion of the catalytic activity are presented. This is an abstract of a paper presented at the AIChE Annual Meeting (San Francisco, CA 11/12-17/2006).