151 research outputs found
Mechanistic insights into CO2 activation via reverse water - Gas shift on metal surfaces
No full textBy
the means of density functional theory calculations, we find
that CO<sub>2</sub> activation via reverse water–gas shift
(r-WGS) follows different elementary steps on different metals (Pt,
Rh, Ni, Cu, Ag, and Pd). We relate these differences to the interactions
between the adsorbed oxygen and the metals, which strongly affect
the dissociation activation energy. In particular, CO<sub>2</sub> dissociation
is favored on metals that present high affinity toward oxygen. As
the O interaction with the metals weakens, CO<sub>2</sub> hydrogenation
becomes more favored at the expenses of the dissociation. We found
that the binding energy of oxygen scales almost linearly with the
difference between the activation energy of the two competing paths,
and therefore this quantity can be used as a simple descriptor to
discriminate which of the two mechanisms is dominant on different
metals. Such findings allow rationalization of the different catalytic
cycles reported in the literature for the r-WGS reaction on metal
surfaces
A first principles study of water oxidation catalyzed by a tetraruthenium-oxo core embedded in polyoxometalate ligands
No full textWe present a computational study addressing the catalytic cycle of a recently-synthesized all-inorganic homogeneous catalyst capable to promote water oxidation with low overpotential and high turnover frequency [Sartorel et al., J. Am. Chem. Soc., 2008, 130, 5006; Geletii et al., Angew. Chem., Int. Ed., 2008, 47, 3896]. This catalyst consists of a tetraruthenium-oxo core [Ru(4)O(4)(OH)(2)center dot(H(2)O)(4)](6+) capped by two polyoxometalate [SiW(10)O(36)](8-) units. The reaction mechanism underpinning its efficiency is currently under debate. We study a reaction cycle involving four consecutive proton-coupled electron transfer (PCET) processes that successively oxidize the four Ru(IV)-H(2)O units of the initial state (S(0)) to the four Ru(V)-OH centers of the activated intermediate (S(4)). The energetics of these electrochemical processes as well as the structural and electronic properties of the reaction intermediates are studied with ab initio Density Functional Theory (DFT) calculations. After characterizing these reaction intermediates in the gas phase, we show that the solvated tetraruthenate core undergoes a solvent-induced structural distortion that brings the predicted molecular geometry to excellent agreement with the experimental X-ray diffraction data. The calculated electronic properties of the catalyst are instead weakly dependent on the presence of the solvent. The frontier orbitals of the initial state as well as the electronic states involved in the PCET steps are shown to be localized on the tetraruthenium-oxo core. The reaction thermodynamics predicted for the intermediate reaction steps is in good agreement with the available cyclic voltammetry measurements up to S(3), but the calculated free energy difference between the initial and the activated state (S(0)/S(4)) turns out to be significantly lower than the thermodynamic limit for water oxidation. Since the oxidizing power of the S(0)/S(4) couple is not sufficient to split water, we suggest that promoting this reaction would require cycling between higher oxidation states
Sensitization of WO3 and SnO2 photoanodes with molecular dyes for water splitting
No full textDyes endowed with strongly oxidizing ground state oxidation potentials are exploited in dye sensitized photoelectrochemical cells for water splitting with the aim of characterizing their interfacial separation dynamics. In the first example [(N,N-Bis(2-(trimethylammonium)-ethylene) perylene 3,4,9,10-tetracarboxylic acid bisimide)-(PF6)2] [1] was observed to spontaneously adsorb onnanocrystalline WO3 surfaces via aggregation/hydrophobic forces. Under visible irradiation (λ > 435 nm), the excited state of the peylene underwent oxidative quenching by electron injection (kinj > 108 s-1) to WO3, leaving a strongly positive hole (Eox ≈ 1.7 V vs SCE), which allows to drive demanding photo-oxidation reactions in photoelectrochemical cells (PECs). The casting of IrO2 nanoparticles (NPs), acting as water oxidation catalysts (WOCs) on the sensitized electrodes, led to a 4-fold enhancement in photoanodic current, consistent with hole transfer from oxidized dye to IrO2 occurring on the microsecond time scale. In a second case study the charge transfer dynamics involving a new Ru(II) polypyridine complex developed to generate strongly oxidizing photoholes for water oxidation, was studied by electrochemical, photo electrochemical and spectroscopic means. Interestingly this species, loaded on TiO2, underwent a change in the injection mechanism in the presence of ascorbic acid, consistent with the reductive quenching of the MLCT excited state and injection from the photo generated reduced state. On the other hand the usual oxidative quenching was observed on SnO2 photo anodes, where the activation of IrO2 by the oxidized state of the sensitizer was observed. Once the interaction with suitable WOCs is optimized, these molecular designs may hold potentialities for the straightforward building of molecular level devices for solar fuel production
Seismic performance of suspended piping restraint installations
No full textThe damage observed during past earthquakes repeatedly showed the vulnerability of non-structural elements and their importance in the functionality of critical facilities, such as hospitals. Performance-based seismic design (PBSD) implies the harmonization of performances between structural and non-structural elements. To this aim, performance parameters for non-structural elements need to be evaluated through experimental and numerical studies. Among the multitude of non-structural typologies, the seismic performance of piping systems is of paramount importance in order to guarantee the immediate post-event functionality of critical facilities. Few research studies available in the literature have attempted to evaluate the performance parameters required to enable PBSD of piping systems and more specifically of suspended piping restraint installations. This paper summarizes the results of an experimental and numerical research project dealing with the evaluation of the seismic behaviour of suspended piping restraint installations. Four typologies of suspended trapeze piping restraint subassemblies with channel and rod bracing systems were tested under monotonic and cyclic loading to determine their hysteretic responses and failure modes. The results of the subassembly tests were used to calibrate simplified nonlinear numerical models useful to assess the seismic response of full-scale suspended piping layouts subjected to floor acceleration time-historie
Stability of intermediate states for ethylene epoxidation on Ag-Cu alloy catalyst: A first-principles investigation
No full textBy means of first-principles density functional theory combined with atomistic thermodynamics, we have investigated the stability of several intermediates in the ethylene epoxidation reaction catalyzed by Ag-Cu alloys. We studied the surface phase diagrams of the low-index facets of Ag-Cu as a function of temperature and partial pressures of oxygen and ethylene, considering ethylene to be either physisorbed or chemisorbed in oxametallacycle and ethylenedioxy forms. We find that at high ethylene partial pressure or low temperature ethylene adsorbs as ethylenedioxy on a thin CuO layer formed on top of the silver particle. Subsurface oxygen can be present on the (111) facet at the interface between the CuO layer and silver. At temperatures and pressures relevant for industrial applications, though, the catalyst is not covered by ethylene and on all facets a thin CuO layers forms. The oxametallacycle intermediate is not predicted to be stable under any conditions. We have also investigated the shape of the catalyst particles as a function of the copper loading and temperature, showing that the dominant facet under conditions relevant for practical applications is the (100)
Ag-Cu catalysts for ethylene epoxidation: Selectivity and activity descriptors
No full textAg-Cu alloy catalysts for ethylene epoxidation have been shown to yield higher selectivity towards ethylene oxide compared to pure Ag, the unique catalyst employed in the industrial process. Previous studies showed that under oxidizing conditions Cu forms oxide layers on top of Ag. Using first-principles atomistic simulations based on density functional theory, we investigate the reaction mechanism on the thin oxide layer structures and establish the reasons for the improved selectivity. We extend the range of applicability of the selectivity descriptor proposed by Kokalj et al. [J. Catal. 254, 304 (2008)], based on binding energies of reactants, intermediates, and products, by refitting its parameters so as to include thin oxide layer catalysts. We show that the selectivity is mainly controlled by the relative strength of the metal-carbon vs. metal-oxygen bonds, while the height of the reaction barriers mostly depend on the binding energy of the common oxametallacycle intermediate. (C) 2013 AIP Publishing LLC
First principles assessment of CO2 activation over metal catalysts
No full textLAUREA MAGISTRALEL’attivazione della CO2 su Platino, Rodio e Nichel è stata studiata nel dettaglio grazie all’impiego di un’analisi DFT a onde piane.
La valutazione dei cammini di reazione che coinvolgono la CO2 è di crescente interesse per le tecnologie emergenti volte a convertire la CO2, come il “Power to Gas”, la metanazione della CO2 e la Reverse Water Gas Shift nei processi di reforming a basso tempo di contatto.
L’avvincente tema legato alla riduzione delle emissioni di CO2 sugli impianti di generazione di potenza e la premente necessità d’indipendenza dai combustibili fossili ha mobilitato il settore di ricerca e sviluppo verso la produzione di energia elettrica da fonti rinnovabili. Un modo efficiente per accumulare energia entro un composto chimico può essere fondato sulla produzione di idrogeno, via elettrolisi dell’acqua. Tuttavia, questo composto non è né facilmente trasportabile né maneggiabile con sicurezza. È in questo contesto che si inserisce il progetto “Power to Gas”. L’idrogeno è convertito in un composto ad ampio campo di utilizzo (gas naturale sintetico), mediante la reazione catalitica di metanazione della CO2.
In questa tesi sono stati considerati quattro diversi cammini, e gli step rappresentativi che avvengono sulla superficie catalitica sono:
La decomposizione diretta: CO_2→CO+O
La reazione di Boudouard: CO_2+C→2CO
La reazione di Boudouard in condizioni di coking: CO_2+nC→2CO+(n-1)C
L’idrogenazione diretta: CO_2+H→COOH
A questo proposito, un approccio fondato sull’utilizzo della meccanica quantistica è stato selezionato come ideale per realizzare una modellazione teorica sulla scala atomica. In particolare l’impiego di metodi “first-principles”, come la Density Functional Theory, permette di comprendere nel dettaglio processi rilevanti, quali quelli coinvolti nella conversione e accumulo dell’energia. L’equazione di Schrödinger viene risolta nell’approssimazione di Kohn-Sham; sulla base del calcolo dell’energia totale del sistema studiato, è possibile estrarre informazioni indispensabili per la valutazione delle proprietà fondamentali di un processo eterogeneo, come calori di adsorbimento e proprietà geometriche dell’adsorbato. Mediante l’algoritmo CI-NEB si è ricercato il Transition State, calcolato le energie di attivazione, e correlato queste ultime grandezze ad altre proprietà caratteristiche dei catalizzatori metallici.
Questa analisi ha evidenziato una scarsa affinità della CO2 nei confronti del substrato metallico, mentre le altre specie chimiche coinvolte negli atti elementari, quali C, O, H, CO e COOH si adsorbono con forza su tutti e tre i metalli considerati. È stato osservato che su Platino, il meccanismo di attivazione avviene preferibilmente, da un punto di vista energetico, attraverso un processo idrogenazione del biossido di carbonio. Nel caso di catalizzatori a base di Rodio e Nichel il cammino reattivo favorito è quello che prevede la decomposizione diretta della CO2. Data la presenza di composti carboniosi, anche la possibile formazione di coke è stata considerata. Si è riscontrato che ad alto ricoprimento di carbonio le energie di attivazione, per la reazione di Boudouard crollano e appaiono scarsamente influenzate dalla tipologia di metallo.CO2 activation on Platinum, Rhodium and Nickel has been studied in detail by the mean of a periodic planewave DFT analysis.
The evaluation of the reaction paths involving CO2 is of increasing interest in emerging technologies that entail CO2 conversion, such as “Power to Gas”, CO2 methanation and Reverse Water Gas Shift for short contact time reforming processes.
The challenging issue related to the reduction of CO2 emissions in power plants and the pressing need for independence from fossil fuels mobilized the research and development areas towards the production of electricity from renewable sources. A successful method to store energy in a chemical way may be grounded on water electrolysis for hydrogen production. However, this compound is neither easily transportable nor manageable with ease. It is in this context that one inserts the "Power to Gas" project. The hydrogen is converted into a chemical with wide range of use (synthetic natural gas), by means of the catalytic reaction of CO2 methanation.
In this Thesis four pathways have been considered on different surface environments as representative steps for:
Direct decomposition: CO_2→CO+O
Boudouard reaction: CO_2+C→2CO
Boudouard in coking condition: CO_2+nC→2CO+(n-1)C
Direct hydrogenation: CO_2+H→COOH
In this regard, an approach based on the use of quantum mechanics has been selected as an ideal tool to make a theoretical modeling of the atomic scale. In particular, the adoption of "first-principles" methods, as Density Functional Theory, allows us to understand in detail the relevant processes such as those involved in energy conversion and storage. The Schrödinger equation is solved according to Kohn-Sham approximation; in relation to the calculation of the total energy of the examined system, it is possible to extract the essential information for the evaluation of the fundamental properties of an heterogeneous process, such as heats of adsorption and adsorbate’s geometric features. By means of the CI-NEB algorithm, the Transition State has been located, activation energies have been calculated, and these variables have been correlated with other characteristic properties of the metallic catalysts.
This analysis showed a poor affinity of CO2 towards metal substrates, while the other chemical species involved in the elementary steps, such as C, O, H, COOH and CO strongly adsorb on all the three considered metals. It was noticed that on Platinum, the mechanism of activation preferably occurs, in terms of energy, through the process of hydrogenation of carbon dioxide. In case of Rhodium and Nickel based catalysts, the favored reactive path is the one that concerns CO2 direct decomposition. Given the presence of carbonaceous compounds, also the possible coke formation has been considered. It was found that the activation energies for the Boudouard reaction with high carbon coating collapse and appear scarcely affected by the type of metal
QMMMW: A wrapper for QM/MM simulations with QUANTUM ESPRESSO and LAMMPS
No full textWe present QMMMW, a new program aimed at performing Quantum Mechanics/Molecular Mechanics (QM/MM) molecular dynamics. The package operates as a wrapper that patches PWscf code included in the QUANTUM ESPRESSO distribution and LAMMPS Molecular Dynamics Simulator. It is designed with a paradigm based on three guidelines: (i) minimal amount of modifications on the parent codes, (ii) flexibility and computational efficiency of the communication layer and (iii) accuracy of the Hamiltonian describing the interaction between the QM and MM subsystems. These three features are seldom present simultaneously in other implementations of QMMM. The QMMMW project is hosted by qe-forge at (http://qe-forge.org/gf/project/qmmmw/)
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