Angelo Vargas

Università degli Studi di Palermo, Palermo, Sicily, Italy

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Publications (42)128.64 Total impact

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    ABSTRACT: The semiconducting nature of many metal oxides makes them interesting materials for numerous industrially and economically important applications. Wide band gap semiconducting metal oxides such as SnO2 and TiO2 are used, for example, in solar cells [1] or photocatalysis [2]. Moreover, their ability to change conductivity when gaseous molecules are reacting with the surface makes them particularly applicable for chemoresistive portable gas sensors [3] used, for example, in medical diagnostics (breath analysis). A main drawback of metal oxide materials both in photocatalysis and as gas sensors is the reaction of their surfaces with water vapor. In other words, changes in the relative humidity of the environment can significantly influence the performance of the metal oxide device. Experimental studies have shown recently that this major drawback can be overcome by co-synthesis of SnO2 with another oxide such as TiO2 [3]. In fact, flame spray synthesis of the latter led to the formation of Sn1-xTixO2 solid solutions with controlled surface properties and high sensitivity (e.g. to EtOH). The understanding of the synthesis process and the resulting formation of the rutile lattice, however, is still in progress. In this project, density functional theory within the Gaussian and plane waves formalism [4] has been used to study H2O adsorption mechanisms on rutile (110) surfaces of Ti-doped SnO2 in comparison to pure TiO2 and SnO2. The stability of surfaces with homogeneously distributed Ti atoms in the whole crystal was compared to that obtained with surface localized ones, showing that the localization at the surface is thermodynamically favored, in particular, when six-fold coordinated surface Sn atoms are substituted by Ti atoms. Furthermore, the adsorption properties of H2O on the surface of SnO2, TiO2 and of Sn1-xTixO2 have been investigated. The Sn1-xTixO2 solid solutions have been studied for x values from 0 to 20% where Ti atoms were distributed homogeneously on the surface and specifically on six-fold coordinated sites. For high H2O coverage (1 monolayer), the binding energy of dissociatively adsorbed H2O decreased monotonously with increasing Ti-surface content. At low coverage (1/12 monolayer), however, the binding energy showed remarkable dependency on the distribution of Ti atoms on the surface. Substitution of six-fold coordinated surface Sn-atoms with Ti led to a remarkable decrease of the binding energy. This study indicates that the presence of Ti surface atoms greatly affects the adsorption of water on the oxide surface and gives a possible explanation for the experimentally observed minimum of the cross-sensitivity to humidity of Sn1-xTixO2 nanoparticles at a total Ti content of 4.6 %mol [3]. Furthermore, these findings give fundamental information for the design of high-performing photocatalysts independent of the relative humidity. [1] J.F. Wager, Science 300 (2003), 1245. [2] A. Fujishima, K. Honda, Nature 238 (1972), 37. [3] A. Tricoli, M. Righettoni, S.E. Pratsinis, Nanotechnology 20 (2009), 315502. [4] G. Lippert, J. Hutter, M. Parrinello, Mol. Phys. 92 (1997), 477.
    12 AIChE Annual Meeting; 10/2012
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    ABSTRACT: Both associative and dissociative H(2)O adsorption on SnO(2)(110), TiO(2)(110), and Ti-enriched Sn(1-x)Ti(x)O(2)(110) surfaces have been investigated at low ((1)/(12) monolayer (ML)) and high coverage (1 ML) by density functional theory calculations using the Gaussian and plane waves formalism. The use of a large supercell allowed the simulation at low symmetry levels. On SnO(2)(110), dissociative adsorption was favored at all coverages and was accompanied by stable associative H(2)O configurations. Increasing the coverage from (1)/(12) to 1 ML stabilized the (associatively or dissociatively) adsorbed H(2)O on SnO(2)(110) because of the formation of intermolecular H bonds. In contrast, on TiO(2)(110), the adsorption of isolated H(2)O groups ((1)/(12) ML) was more stable than at high coverage, and the favored adsorption changed from dissociative to associative with increasing coverage. For dissociative H(2)O adsorption on Ti-enriched Sn(1-x)Ti(x)O(2)(110) surfaces with Ti atoms preferably located on 6-fold-coordinated surface sites, the analysis of the Wannier centers showed a polarization of electrons surrounding bridging O atoms that were bound simultaneously to 6-fold-coordinated Sn and Ti surface atoms. This polarization suggested the formation of an additional bond between the 6-fold-coordinated Ti(6c) and bridging O atoms that had to be broken upon H(2)O adsorption. As a result, the H(2)O adsorption energy initially decreased, with increasing surface Ti content reaching a minimum at 25% Ti for (1)/(12) ML. This behavior was even more accentuated at high H(2)O coverage (1 ML) with the adsorption energy decreasing rapidly from 145.2 to 101.6 kJ/mol with the surface Ti content increasing from 0 to 33%. A global minimum of binding energies at both low and high coverage was found between 25 and 33% surface Ti content, which may explain the minimal cross-sensitivity to humidity previously reported for Sn(1-x)Ti(x)O(2) gas sensors. Above 12.5% surface Ti content, the binding energy decreased with increasing coverage, suggesting that the partial desorption of H(2)O is facilitated at a high fractional coverage.
    Langmuir 12/2011; 28(2):1646-56. · 4.38 Impact Factor
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    ABSTRACT: The composition and thermodynamic stability of the (110) surface of Sn1-xTixO2 rutile solid solutions was investigated as a function of Ti-distribution and content up to the formation of a full TiO2 surface monolayer. The bulk and (110) surface properties of Sn1-xTixO2 were compared to that of the pure SnO2 and TiO2 crystal. A large supercell of 720 atoms and a localized basis set based on the Gaussian and plane wave scheme allowed the investigation of very low Ti-content and symmetry. For the bulk, optimization of the crystal structure confirmed that up to a Ti-content of 3.3at.%, the lattice parameters (a, c) of SnO2 do not change. Increasing further the Ti-content decreased both lattice parameters down to those of TiO2. The surface energy of these solid solutions did not change for Ti-substitution in the bulk of up to 20at.%. In contrast, substitution in the surface layer rapidly decreased the surface energy from 0.99 to 0.74J/m2 with increasing Ti-content from 0 to 20at.%. As a result, systems with Ti atoms distributed in the surface (surface enrichment) had always lower energies and thus were thermodynamically more favorable than those with Ti homogeneously distributed in the bulk. This was attributed to the lower energy necessary to break the TiO bonds than SnO bonds in the surface layer. In fact, distributing the Ti atoms homogeneously or segregated in the (110) surface led to the same surface energy indicating that restructuring of the surface bond lengths has minimal impact on thermodynamic stability of these rutile systems. As a result, a first theoretical prediction of the composition of Sn1-xTixO2 solid solutions is proposed.
    Surface Science 08/2011; 605(15):1473-1479. · 1.87 Impact Factor
  • Catalytic Methods in Asymmetric Synthesis: Advanced Materials, Techniques, and Applications, 07/2011: pages 291 - 321; , ISBN: 9781118087992
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    ABSTRACT: By means of ab initio molecular dynamics, we have investigated the molecular adsorption of acetaldehyde on Pt 13 nanoparticles in the presence of coadsorbed hydrogen on the surface of the metal particle. The acetaldehyde molecules exclusively interact with low-coordinated metal atoms of the nanoparticle while they remain inert toward direct interaction with adsorbed hydrogen, thus confirming the key role of the metal as intermediate binding site for hydrogenation. At room temperature within a time scale of picoseconds aldehyde–metal bonds are formed. Coadsorbed hydrogen decreases the reactivity of the aldehyde molecules toward the metal. Kinetically, the first adsorption modes to occur are of the η1 type, either via the oxygen or via the carbon atom. A tendency for double adsorption on the same metal site is observed. Upon addition of ammonia molecules to the simulation box, the interesting phenomenon of the conversion of η1 to η2 carbonyl bonding appears, mediated by the adsorption of the ammonia nitrogen to a platinum atom. This investigation highlights the richness of the interaction modes of a carbonyl group with a platinum nanoparticle, reached in the very brief time scale of a few picoseconds. In particular the adsorption modes of the aldehyde are modified by the presence of a second electron-donor molecule, such as another aldehyde molecule or an ammonia molecule, in the latter case even changing the adsorption mode of the carbonyl moiety from η1 to η2.
    The Journal of Physical Chemistry C. 05/2011; 115(21).
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    ABSTRACT: The presence of a tertiary amine plays a role in accelerating the catalysis of ketones hydrogenation on a platinum surface covered by hydrogen. Ab initio molecular dynamics has been applied to achieve an atomistic understanding of this base effect. The interaction of trimethylamine with hydrogen adsorbed on a platinum nanoparticle has been simulated for 22 ps revealing that hydrogen increases its oscillatory modes upon interaction with the base therefore its activation. Nonetheless in the case of trimethylamine, competing interactions appear in the form of skeletal hydrogen (from the methyl group) interacting with the metal particle. Such interactions distract the action of the base and predominate at the end of the simulation. The interaction of the trimethylamine molecule with hydrogen adsorbed on platinum is compared to the corresponding interaction of the quinuclidine moiety of cinchonidine, which is the most widely applied chiral modifier for the platinum catalyzed enantioselective hydrogenations. The behavior of the quinuclidine moiety of cinchonidine on a Pt(111) surface is shown to be similar to the one of trimethylamine, since it also interacts with surface hydrogen causing an increase of the oscillatory amplitudes of the Pt−H bonds. But the skeleton of the alkaloid and its fixed adsorption mode via the quinoline moiety keeps the quinuclidine base in position for interacting with surface hydrogen, resulting in a greater stability of interaction between the base and the surface. In fact the quinuclidine base cannot give secondary skeletal interactions due to its optimally constrained position with respect to the metal. The investigation sheds some light on the mechanism of hydrogen activation on metal surfaces by means of tertiary amines and underlines the originality of metal surface chemistry with respect to homogeneous chemistry.
    The Journal of Physical Chemistry C. 01/2011; 115(5).
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    ABSTRACT: The efficiency of propylamino functionalization of magnetic silica-coated FeOx nanoparticles prepared by different methods, including coprecipitation and flame aerosol synthesis, has been evaluated by attenuated total reflection infrared spectroscopy (ATR-IR) combined with a specific surface reaction, thus revealing the availability of the grafted functional groups. Large differences in the population of reactive groups were observed for the investigated materials, underlining the tight relation between the structure of nanoparticles and their suitability for organic functionalization. The materials possessed different core structure, surface area, and porosity, as evidenced by transmission electron microscopy and nitrogen adsorption–desorption isotherms. Grafting of aminopropyl groups using a standard procedure based on reaction with (3-aminopropyl)trimethoxysilane as source of the propylamino groups was performed, followed by classical dry analysis methods to determine the specific concentration of the organic functional groups (in mmol g−1 of material). ATR-IR spectroscopy in a specially constructed reactor cell was applied as wet methodology to determine the chemically available amount of such functional groups, showing that the materials possess largely different loading capacity, with a variability of up to 70% in the chemical availability of the organic functional group. The amount of (3-aminopropyl)trimethoxysilane used for functionalization was optimized, thus reaching a saturation limit characteristic of the material.Research highlights▶ Propylamino functionalization of magnetic silica-coated FeOx nanoparticles prepared by different methods, including coprecipitation and flame aerosol synthesis, shows different efficiency. ▶ ATR-IR spectroscopy combined with a specific surface reaction allows to determine specifically the chemically available aminopropyl groups on the surface. ▶ Elemental analysis gives an upper bound for the concentration of aminopropyl groups, while ATR-IR additionally provides information about the chemical availability of these groups. ▶ Depending on the structure and surface properties of the silica-coated FeOx nanoparticles results from elemental analysis and ATR-IR measurements can deviate significantly.
    Applied Surface Science 01/2011; 257(7):2861-2869. · 2.54 Impact Factor
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    ABSTRACT: Semiconducting metal oxides (e.g. SnO2, WO3 or TiO2) are used in a wide variety of applications such as transparent conductors or isolators, photo catalysis and solar cells. In particular, metal-oxide nanoparticles are highly performing materials in the field of solid state gas sensors. Recently, it was shown that WO3 can be used for selective detection of acetone already in particle per billion concentrations.[1] Nanosized SnO2,on the other hand, is an important sensing material for ethanol detection [2] and represents an interesting alternative to WO3. The major shortcoming of SnO2, however, is its poor reliability in the presence of variable relative humidity. Doping of SnO2 with other metal oxides is a promising way to overcome this shortcoming. It has in fact been shown that the selectivity of SnO2 nanoparticles towards ethanol in the presence of water vapor is drastically increased already at low Ti-doping content.[3] Here, the adsorption properties of H2O on the surface of SnO2, TiO2 and of Sn1-xTixO2 have been investigated using density functional theory (DFT) within the Gaussian and Plane Wave (GPW) formalism. The solid solutions Sn1-xTixO2 have been investigated for x values of 1.7% and 3.3%. Calculations have been coupled to temperature programmed desorption (TPD) experiments in order to identify surface species. This combined theoretical and experimental investigation indicates that the presence of titanium surface sites weakly bind surface water which is therefore promptly desorbed at the working temperature of the sensor (300C). This study thus provides a basis for an improved mechanistic understanding of metal oxide based solid state gas sensors. [1] Righettoni, M.; Tricoli, A.; Pratsinis, S.E. Anal. Chem. 2010, 82, 3581. [2] Tricoli, A; Graf, M; Mayer, F.; Kuehne, S.; Hierlemann, A.; Pratsinis, S.E. Adv. Mater. 2008, 20, 3005. [3] Tricoli, A.; Righettoni, M.; Pratsinis, S. E. Nanotechnology 2009, 20, 315502.
    2010 AIChE Annual Meeting; 11/2010
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    ABSTRACT: Metal oxide semiconductors with a rutile-phase such as TiO2 and SnO2 have been largely studied in the past due to their wide range of applications. For example, TiO2 is used as white pigment, UV-shield in sun creams, photo catalyst and in solar cells. Pure or doped SnO2, on the other hand, is employed as transparent isolating material, transparent conductive oxide (TCO) and it is the most utilized metal oxide for chemical gas sensing. In all of these applications the surface properties of the oxides play a major role. For instance, the adsorption of hydroxyl groups is directly correlated with the poor reliability of SnO2-based sensors in the presence of variable relative humidity. Recently, it has been shown that this major shortcoming can be overcome by co-synthesis of another oxide such as TiO2.[1] In fact, flame spray synthesis of the latter led to formation of Sn1-xTixO2 solid solutions with controlled surface properties and high sensitivity (e.g. to EtOH). However, the understanding of the synthesis process and the resulting formation of rutile lattice is still in progress. Using density functional theory (DFT) within the Gaussian and Plane Wave (GPW) formalism, rutile (110) surfaces of Ti-doped SnO2 have been studied and compared to pure TiO2 and SnO2 surfaces. [2] The stability of surfaces with homogeneously distributed Ti atoms in the whole crystal was compared to that obtained with surface localized ones. Furthermore, accumulations of Ti-impurities in the SnO2 bulk structure have been investigated including their effect on the rutile cell lattice constants (a and c) and on the formation energy .[3] [1] Tricoli, A.; Righettoni, M.; Pratsinis, S. E. Nanotechnology 2009, 20, 315502. [2] Bandura, A. V.; Kubicki, J. D.; Sofo, J. O. J. Phys. Chem. B 2008, 112, 11616. [3] Camargo, A. C.; Igualada, J.A.; Beltrn, A.; Llusar, R.; Longo, E.; Andrs, J. Chem. Phys. 1996, 212, 381.
    2010 AIChE Annual Meeting; 11/2010
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    ABSTRACT: The description of the conformational space generated by metal nanoparticles is a fundamental issue for the study of their physicochemical properties. In this investigation, an exhaustive exploration and a unified view of the conformational space of a gold nanocluster is provided using a Au 12 cluster as an example. Such system is characterized by coexisting planar/quasiplanar and tridimensional conformations separated by high-energy barriers. The conformational space of Au 12 has been explored by means of Born-Oppenheimer ab initio metadynamics, i.e., a molecular dynamics simulation coupled with a history dependent potential to accelerate events that might occur on a long time scale compared to the time step used in the simulations (rare events). The sampled conformations have complex, in general not intuitive topologies that we have classified as planar/quasiplanar or tridimensional, belonging to different regions of the free energy surface. Three conformational free energy basins were identified, one for the planar/quasiplanar and two for the tridimensional structures. At thermodynamic equilibrium, the planar/quasi-planar and tridimensional conformations were found to coexist, to be fluxional and to be separated by high-free-energy barriers. The comparison between the free energy and the potential energy revealed the relevance of the entropic contribution in the equilibrium distribution of the conformations of the cluster.
    Physical review. B, Condensed matter 05/2010; 81(17). · 3.77 Impact Factor
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    ABSTRACT: The Earth and the other terrestrial planets are composed of solid rocks, with the exception of the liquid core. Grain boundaries are therefore ubiquitous in earth science and the location of many phenomena of interest in geo-dynamics, channeling chemical and vacancy diffusion, functioning as a storage for volatiles, playing a key role in the high pressure and high temperature rheology of the rock. We propose preliminary results for a first implementation of a full an ab-initio molecular dynamics model of the dynamic of grain boundary sliding for solid crystalline silica, simulated in a standard periodic setting characterized by periodically repeated atomic cells. We analyzed three types of grain boundary. First a simple tilt in the crystal plane direction which induces a moderate weakening of the rock strength. Second a hydrated grain boundary, modeled through the saturation with hydrogen of the oxygen at the surface of each grain. Finally, a gel of water and silica placed at the boundary between the sliding grains. We explicitly quantify the order of magnitude of loss of strength during the entire process of sliding for each setup. In order to test the method accuracy, we consider first the full stress tensor directly extracted from a standard ab-initio MD simulation of a cell with one grain boundary at its center responding to a given external differential stress and PT conditions. This outcome is compared with a Metadynamics simulation in which the entire free energy landscape is extracted, as a function of the dominant prescribed leading variables of the system. We consider as meta-variable the relative position of the center of mass of the two grains, which allowed us to express the forces as the gradient of the free energy.
    AGU Fall Meeting Abstracts. 12/2009;
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    ABSTRACT: The structure and electronic properties of gold nanoparticles (Au 12, Au 13, Au 14, Au 15, Au 20, Au 34, and Au 55) have been investigated using Born-Oppenheimer ab initio molecular dynamic simulations of 50 to 80 ps in order to have an insight in the recently proposed fluxional character of nanosized gold. The dynamic changes in shape, symmetry, and atomic coordination of atoms within clusters, occurring in the time scale of picoseconds, which are characteristic of fluxionality, have been investigated for all the above systems at 300 K. Except for Au 20, all systems have been found to have fluxional properties. The extent and the type of fluxional behavior changed according to the number of atoms constituting the particle. At 300 K Au 12 and Au 13 rapidly generate several different topologies which cyclically interconvert. Au 14 shows a rotation of 8 external gold atoms around a core of six atoms. Au 15 is more rigid, but interestingly shows the interconversion between enantiomeric structures within the time scale of the simulation. Au 20 shows a high stability of the pyramidal topology and is the only one of the investigated systems not to show fluxionality within the assigned temperature and time scale. Au 34 and Au 55 show fluxionality of the outer shell and within the sampled time scale are able to change coordination of the outer shell atoms and thus open and close surface holes. For all the particles in study the structures forming the local minima were isolated and separately optimized, and the electronic properties of the thus obtained structures were analyzed.
    Physical Review B 11/2009; 80(19). · 3.66 Impact Factor
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    ABSTRACT: Characterization of organic functional groups grafted to magnetic nanoparticles often causes considerable difficulty because of lack of information concerning their accessibility and chemical reactivity. Classical solid-state techniques only allow gaining qualitative information and the determination of upper bounds of the total loading of organic moieties. To overcome this limitation we applied attenuated total reflection (ATR) infrared spectroscopy in a specially designed mechanically stirred in situ batch reactor-cell. Using the example of iron oxide magnetic nanoparticles embedded in a silica matrix that have been functionalized by grafting aminopropyl groups, we elucidated the accessibility and reactivity of amino groups. Reactivity tests using the transformation of the amine to benzaldimine revealed that less than 40% of the introduced amino groups reacted to the corresponding benzaldimine indicating that the theoretical loading provides only an upper bound of active functional groups. The reactivity of these groups has been followed by observing the transformation of the amine to the corresponding benzaldimine, revealing population density, accessibility, and reactivity of these groups. The present in situ technique facilitates the monitoring of the transformation of grafted functional groups and simultaneously provides quantitative and structural information when the material is suspended in a solvent system.
    Chemistry of Materials - CHEM MATER. 08/2009; 21(18).
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    ABSTRACT: A series of peptides, all containing the natural amino acid tryptophan (Trp), have been used as chiral surface modifiers for asymmetric hydrogenation reactions on alumina supported platinum catalysts. The surface chiral sites have been investigated using density functional theory calculations in order to elucidate the structure of the asymmetric environment produced by the adsorption of the peptides on the metal surface. As a test reaction, ketopantolactone (KPL) has been asymmetrically hydrogenated using the modified catalyst. Catalytic results were tested against a computational model to shed light on the phenomenon of chiral induction. The choice of Trp as the first member of the peptidic chain is due to its structural resemblance to cinchona alkaloids, already successfully used as modifiers for asymmetric hydrogenation reactions. The amino acidic moiety of Trp was elongated by addition of other natural amino acids via a peptidic bond, and the resulting peptides were tested as chiral modifiers. The indole moiety of Trp anchors the peptide by adsorbing parallel to the metal surface, while the amino acidic chain forms the chiral environment that promotes the asymmetric reaction. It is shown that Trp-based peptides greatly extend the class of chiral surface modifiers for heterogeneous enantioselective hydrogenations. Peptides are in fact in principle simple to prepare, give access to a huge variety of tridimensional structures able to form stable chiral surface sites, and allow introduction of functional groups through peptidic bond elongation. A preliminary screening carried out in this work using a relatively low number of peptides shows enantioselectivities up to 30% for a choice of standard test asymmetric reactions on platinum catalyst. The main features of the chiral environment created by the peptidic modifiers on the metal surface are described by means of first principle computational modeling. The enantioselectivity is interpreted using a computational docking model of KPL within the chiral site.
    Journal of Physical Chemistry C - J PHYS CHEM C. 08/2009; 113(34).
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    ABSTRACT: The structure sensitivity of enantioselective hydrogenations on chirally modified metals was investigated using Pt nanoparticles of different shapes. All three samples had an average particle size of 10 nm, but the fraction of dominantly cubic, cubooctahedral, and octahedral particles varied with decreasing {100} and increasing {111} faces in the same order. In the absence of chiral modifier the hydrogenation of ethyl pyruvate was independent of the shape of the Pt nanoparticles; variation of the specific reaction rates did not exceed the experimental error on all self-prepared catalysts and on a commercial Pt/Al(2)O(3) used as reference. Addition of cinchonidine or quinine induced a significant rate enhancement by a factor of 4-15, and the rate was always higher with quinine. Also, 72-92% ees were achieved, and the reaction was shape selective: both the rate and the ee increased with increasing Pt{111}/Pt{100} ratio. A similar correlation in the hydrogenation of ketopantolactone confirmed that decarbonylation or aldol-type side reactions of ethyl pyruvate were not the reason for structure sensitivity. A combined catalytic and theoretical study revealed that the probable origin of the particle shape dependency of enantioselective hydrogenation is the adsorption behavior of the cinchona alkaloid. DFT studies of cinchonidine interacting with Pt(100) and Pt(111) terraces indicated a remarkably stronger interaction on the former crystallographic face by ca. 155 kJ/mol. The higher adsorption strength on Pt(100) was corroborated experimentally by the faster hydrogenation of the homoaromatic ring of the alkaloid, which fragment interacts the strongest with Pt during its adsorption. Thus, an ideal catalyst for the hydrogenation of activated ketones contains dominantly Pt{111} terraces, which crystallographic face is more active and affords higher enantioselectivity, combined with the higher stability of the modifier.
    Journal of the American Chemical Society 08/2009; 131(34):12358-67. · 10.68 Impact Factor
  • Barbara Panella, Angelo Vargas, Alfons Baiker
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    ABSTRACT: A magnetic Pt/SiO2/Fe3O4 catalyst consisting of chirally modified platinum supported on silica coated magnetite nanoparticles was prepared using an easy synthetic route and successfully applied for the enantioselective hydrogenation of various activated ketones. The magnetic catalyst modified with cinchonidine showed a catalytic performance (activity, enantioselectivity) in the asymmetric hydrogenation of various activated ketones in toluene comparable to the best known Pt/alumina catalyst used for these reactions. The novel catalyst can be easily separated from the reaction solution by applying an external magnetic field and recycled several times with almost complete retention of activity and enantioselectivity.
    Journal of Catalysis - J CATAL. 01/2009; 261(1):88-93.
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    ABSTRACT: We present a study on structural and electronic properties of bulk platinum and the two surfaces (111) and (100) comparing the Gaussian and plane wave method to standard plane wave schemes, normally employed for density functional theory calculations on metallic systems. The aim of this investigation is the assessment of methods based on the expansion of the Kohn-Sham orbitals into localized basis sets and on the supercell approach, in the description of the metallicity of Pt. Electronic structure calculations performed at Gamma-point only on supercells of different sizes, from 108 up to 864 atoms, are compared to the results obtained for the unit cell of four Pt atoms where the k-point expansion of the wave function over Monkhorst-Pack grids up to (10x10x10) has been employed. The evaluation of the two approaches with respect to bulk properties is done through the calculation of the equilibrium lattice constant, the bulk modulus, and the total and the d-projected density of states. For the Pt(111) and Pt(100) surfaces, we consider the relaxation of the first layers, the surface energies, the work function, the total density of states, as well as the center and filling of the d bands. Our results confirm that the accuracy of two approaches in the description of electronic and structural properties of Pt is equivalent, providing that consistent supercells and k-point meshes are used. Moreover, we estimate the supercell size that can be safely adopted in the Gaussian and plane wave method in order to obtain the same reliability of previous theoretical studies based on well converged plane wave calculations available in literature. The latter studies, in turn, set the level of agreement with experimental data. In particular, we obtain excellent agreement in the evaluation of the density of states for either bulk and surface systems, and our data are also in good agreement with previous works on Pt reported in literature. We conclude that Gaussian and plane wave calculations, with simulation cells of 400-800 atoms, can be safely used in the study of chemistry related problems involving transition metal surfaces.
    The Journal of Chemical Physics 01/2009; 129(23):234703. · 3.12 Impact Factor
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    ABSTRACT: Docking of molecules to surface chiral sites generated by adsorbed chiral organic modifiers is a fundamental step in heterogeneous metal-catalyzed asymmetric hydrogenation. The understanding of such docking events is nonetheless limited by the technical difficulties in obtaining experimental submolecular information for complex adsorbed structures. In addition an accurate theoretical analysis is computationally very demanding due to the need of including the metal surface. The present investigation explores by means of density functional theory the supramolecular docking structures of ketopantolactone within the chiral sites formed by cinchonidine adsorbed on a platinum surface, a crucial step in the enantioselective hydrogenation of ketopantolactone over cinchonidine-modified platinum. The study is performed using periodic slabs exposing Pt(111) surfaces of (6 × 6) and (8 × 8) atoms with a depth of four layers and including complete relaxation of the first three. Twenty six docking sites are investigated thus covering the most complete configurational space until now. The implications of the physi- and chemisorption of ketopantolactone in the docking have been also explored, thus revealing the role of weak and strong adsorption of the substrate in the formation of precursor states for hydrogen uptake within a docking site. The calculations on the (8 × 8) periodic slab were performed in order to decouple the energy related to the adsorption of the substrate from the energy involved in the docking interactions with the adsorbed alkaloid. Such a surface is in fact apt to accommodate both the chiral surface site and the noninteracting substrate. The study revealed that the substrate likely approaches the chiral site either from solution or from the physisorbed state. The resulting scenario gives a basic understanding of the elements involved in the docking of substrates within the chiral sites formed by cinchona alkaloids on platinum, and constitutes a fundamental stage for the comprehensive clarification of enantioselectivity at chirally modified surfaces.
    The Journal of Physical Chemistry C 06/2008; 112(27). · 4.84 Impact Factor
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    ABSTRACT: The adsorption of naphthalene and quinoline on Pt(111), Pd(111) and Rh(111) surfaces is studied using density functional theory. The metal surfaces are simulated by means of large confined clusters and for Pt by means of a slab with periodic boundary conditions (PBC). Calculation parameters such as basis set convergence, basis set superposition error and effects of cluster relaxation and size are analyzed in order to assess the aptness of the cluster model. For all the metals, the preferred sites of adsorption are analyzed, thus revealing their different behaviors concerning structure and stability of adsorption modes. On Pt, the molecules have the richest theoretical configurational variety. Naphthalene and quinoline are found to adsorb preferentially on di-bridge[7] sites on the three metals, and Rh exhibits higher adsorption energies than Pt and Pd. Structural features of the adsorbed molecules are correlated to the calculated adsorption energies. The di-bridge[7] adsorption modes are studied in deeper detail decomposing the adsorption energies in two terms arising from molecular distortion and binding interaction to the metal. Molecular distortion is correlated to the HOMO-LUMO energy gap. The larger adsorption energies found for interactions with Rh result from the lower contribution of the distortion term. Binding interactions are described by analyzing the wave functions of naphthalene and quinoline adsorbed on a subunit of the large clusters in order to reduce the complexity of the analysis. Molecular orbitals are studied using concepts of Frontier Molecular Orbitals theory. This approach reveals that in the adsorption of naphthalene and quinoline on Pt and Pd, an antibonding state lies below the Fermi energy, while on Rh all antibonding states are empty, in agreement with the larger interaction energies. In addition, further insight is gained by projecting the density of states on the d band of the clean surfaces and of the adsorbed systems. This results in the rationalization of the structural features in terms of the concepts of electronic structure theory. The distributions of electronic density are described by means of Hirshfeld charges and isosurfaces of differential electron density. The net electron transfer from the metals to the molecules for most of the sites correlates with the trends of the adsorption energies.
    ChemPhysChem 03/2008; 9(3):401-13. · 3.35 Impact Factor
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    ABSTRACT: The adsorption behavior of cinchona-type chiral modifiers (β-isocinchonine, cinchonidine, and cinchonine) on model rhodium and platinum catalysts has been studied using attenuated total reflection infrared (ATR-IR) spectroscopy and density functional theory (DFT). The ATR-IR studies performed under conditions closely resembling those encountered during enantioselective hydrogenation as well as in the absence of hydrogen revealed significant differences in the adsorption mode of the modifiers, depending on the rotational flexibility of the modifier, the platinum group metal, and the presence of hydrogen. The more rigid β-isocinchonine showed preferential tilted adsorption on both metals, independent of the presence or absence of hydrogen. Cinchonidine and cinchonine were adsorbed on Pt predominantly with the quinoline ring nearly parallel to the surface, irrespective of the presence or absence of hydrogen, whereas on Rh in the presence of hydrogen only tilted species were observed. The latter behavior is attributed to the fast hydrogenation of the aromatic anchoring group (quinoline) on Rh that interferes with the adsorption process. DFT calculations confirmed that the energy difference between tilted and adsorbed species decreases in the case of the β-isocinchonine. This combined spectroscopic and theoretical investigation indicates a greater bias toward the tilting of the anchoring group in the case of β-iCN as compared to the natural occurring alkaloids cinchonidine and cinchonine. Such behavior explains the greater stability toward quinoline ring saturation observed for β-isocinchonine on rhodium-supported catalysts during enantioselective hydrogenation. This study thus helps define the delicate equilibrium between formation of chiral surface sites and their stability in strongly reducing conditions in terms of the structure and consequent adsorption behavior of the modifier molecule, as a function of the metal catalyst of choice.
    The Journal of Physical Chemistry C 02/2008; 112(10). · 4.84 Impact Factor

Publication Stats

164 Citations
128.64 Total Impact Points

Institutions

  • 2011
    • Università degli Studi di Palermo
      Palermo, Sicily, Italy
    • University of Zurich
      Zürich, Zurich, Switzerland
  • 2006–2011
    • ETH Zurich
      • Institute for Chemical and Bioengineering
      Zürich, Zurich, Switzerland
  • 2004–2006
    • École Polytechnique Fédérale de Lausanne
      • Institut des sciences et ingénierie chimiques
      Lausanne, VD, Switzerland
  • 2001–2005
    • Eawag: Das Wasserforschungs-Institut des ETH-Bereichs
      Duebendorf, Zurich, Switzerland