We report the calculation of Raman modes of thiophenol molecules adsorbed on a real gold surface. The calculated Raman spectra strongly depend on the absorption configuration of the molecule on the metallic surface, a feature that should be carefully taken into account in the interpretation of the surface enhanced Raman spectra (SERS). The calculated Raman spectra are compared with experimental SERS measurements, the best accordance being obtained for a tilted configuration of the absorbed molecule. The present study supports the necessary combination of computational approaches with SERS measurements to predict the type of molecular adsorption configurations on metallic surfaces.

In this study, the coordination sphere of copper in Cu-SSZ-13 as a catalyst for the selective catalytic reduction of NOx by ammonia is analyzed as a function of environmental parameters: temperature, partial pressure of water P(H2O), and partial pressure of ammonia P(NH3). By periodic density functional theory calculations, we obtain stability domains for variable loadings of water and ammonia (nH(2)O + mNH(3)with (m + n <= 6)) close to Cu-II ions, which are located at 6-membered ring (6MR) or 8-membered ring (8MR) of the zeolitic structure. Ab initio calculations and thermodynamic investigations were performed to build phase diagrams, with vibrational analysis, so as to provide Gibbs free energy, G, values. Copper located in the 8MR appears to be more reactive toward H2O and NH3 adsorption than the one in the 6MR because of a lower coordination number of copper at 8MR in the absence of adsorbates. Depending on the operating conditions, structures containing adsorbed water and ammonia as ligands at the metal site can simultaneously be stabilized. The most widespread coordination number of Cu-II is 4 even at m + n > 4. The theoretical predictions were validated by in situ X-ray absorption spectroscopy, in dehydration conditions and in two gas atmospheres: dry He with P(NH3) = 10(-3) bar (1000 ppm) and He with P(NH3) = 10(-3) bar and P(H2O) = 10(-2) bar. Trends in terms of ammonia desorption temperature as well as coordination numbers are well reproduced. Experimentally determined behaviors of Cu-I and Cu-II open new perspectives for the systematic computational investigation of the behavior of Cu-I in a H2O/NH3 atmosphere.

ADVERTISEMENT RETURN TO ISSUEPREVCommentNEXTComment on "A Computational Exploration of the CO Adsorption in Cation-Exchanged Faujasites"O. Cairon*View Author Information Chem. Des Pyrénées, 64160 Serres-Morlaàs, France*E-mail: [email protected]Cite this: J. Phys. Chem. C 2013, 117, 42, 22162–22164Publication Date (Web):September 25, 2013Publication History Received18 February 2013Revised9 September 2013Published online9 October 2013Published inissue 24 October 2013https://pubs.acs.org/doi/10.1021/jp401695khttps://doi.org/10.1021/jp401695karticle-commentaryACS PublicationsCopyright © 2013 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views343Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (166 KB) Get e-AlertscloseSUBJECTS:Adsorption,Cations,Fourier transform infrared spectroscopy,Metal clusters,Zeolites Get e-Alerts

Molecular simulations have been employed to explore at the microscopic. scale the adsorption of CO in two families of sodium exchanged faujasite, NaX and NaY. As a preliminary step, density functional theory calculations have been conducted to derive new sets of potential parameters for accurately describing the interactions between CO and the extra framework cations present within the supercage that act as preferential adsorption sites for the guest molecules. Two different parametrizations have been considered to discriminate the Na+ sites, Na-SII(+) and Na-SIII'(+), involved in the interactions with CO. On the basis of these forcefields, Grand Canonical Monte Carlo simulations were further realized to first predict the adsorption properties (isotherms and enthalpies) of these two types of faujasites up to high pressure. This was followed by a careful analysis of the microscopic mechanism in play along the whole adsorption process with a special emphasis on understanding the arrangements of CO in the vicinity of the Na+ whether they occupy SII or SIII' sites. These findings were discussed in light of the enthalpy profile obtained as a function of the loading and a. few experimental data available in the literature. Finally, complementary simulations were realized with mobile extra framework cations upon CO adsorption It was evidenced that Na-SII(+) can migrate toward SIII' sites in NaY, while in NaX, there is no cation redistribution within the supercage due to a steric hindrance. Such cation dynamics were shown to not drastically affect the adsorption properties of both Faujasites; however, this is a critical prerequisite to allow CO to form a double type interaction with both Na-SIII'(+) and Na-SII(+) via its C- and O-ends respectively, as predicted in cation exchanged zeolites using quantum chemical calculations.

The oxidative power toward CO of α-oxygen formed upon N2O dissociation over isolated and binuclear Fe/ZSM-5 zeolite is investigated by means of DFT calculations. The two α-sites [Fe–O]+ and [Fe–(μO)–(μOH)–Fe]+ exchanged in ZSM-5 were considered since their activity in the N2O decomposition was recently shown. Computed electronic properties, charge transfers and frequency analysis of α-oxygen and iron in [O–Fe–O]+ and [OFe–(μO)–(μOH)–FeO]+ suggest a FeII character for the isolated and FeIV for the binuclear Fe–ZSM-5 sites. Addition of CO on oxygen atoms reveals that along the oxidation reaction the valence state for the isolated iron is II and remains relatively constant while a clear change from IV to II is calculated for the binuclear iron. According to DFT calculations CO addition on the α-oxygen from the iron active sites induces a significant length increase of the Fe–α-oxygen bond. Whatever the α-sites, the addition of CO is strongly exothermic and leads to stable minima resembling an adsorbed CO2 on iron active site. This reactivity is in line with the well known high reactivity of α-oxygen and the rapid CO2 formation at low temperatures. Based on the calculated enthalpy values, the adsorption of CO is slightly more favourable on binuclear [OFe–(μO)–(μOH)–FeO]+ than over isolated [O–Fe–O]+ iron site. A comparison of the entropic parameters suggests the opposite with a stronger oxidative power of α-oxygen from isolated over those from binuclear iron site.

Microporous zeolite nanoparticles have been used as a host for the confinement of 3-hydroxyflavone (3-OHF), leading to stable fluorescent colloids that may constitute a first step in the development of future biomarkers. Prior to the dye inclusion, the host physicochemical properties (porosity and crystalline structure) have been extensively studied. In particular, conditions for obtaining a template-free host have been established. It has been shown that standard calcination procedures, required to reveal the micropores, cannot be straighforward applied since they leave a non-negligible concentration of template residues in the host. Afterward, structural and chemical characterization, as well as simulation of 3-OHF-loaded zeolite beta colloids enabled to determine the presence and location of [(3-OHF)Al] chelates in the zeolite micropores. Characterization of the dye loading by independent techniques involving elemental analysis, pore volume, and crystalline structure investigations highlighted the determinant impact of prior calcination treatments and the presence in the zeolite micropores of trapped solvent molecules simultaneously with the dye.

Transition metal ions in zeolites TMI-zeolite (TM=Fe, Co, Cu) attract great attention due to their potentialities as catalysts. In the recent years, the high efficiency of TMI-zeolites for the selective catalytic reduction (SCR) of contaminated flue gases has been demonstrated. It has been shown that the structure of the framework, the nature and location of extraframework cation species play a fundamental role in the process. Experimental results based on spectroscopies, as well as on reactivity studies have led to valuable insights about the structure of cationic sites as well as about the active species involved during the catalytic reactions. However, it is not sufficient to obtain all this information. This review reporting density functional theory (DFT) calculations shows that a molecular approach is very useful and has become an indispensable tool for the determination of the geometries, the electronic structures, the spectroscopic properties and the reactivity of TMI-zeolites.

A Born Oppenheimer molecular dynamic (BOMD) approach was chosen to study the interaction of water molecules with Cu-FAU models. We have compared the reactivity of Cu-I and Cu-II with the zeolite and compared the results to those calculated for Na. This Study shows a decrease of the coordination with time for Cu-I whereas there is not a significant change for Cu-II. BOMD shows also that, in the presence of water, Na+ may lead to easier cation exchange than for transition metal cations.

The selective catalytic reduction (SCR) of NO by NH3 in the presence of O-2 has been studied by operando DRIFT spectroscopy on four Cu-faujasite catalysts, Cu(0.12)Na-FAU, Cu(0.14)H-FAU, Cu(0.16)Ba-FAU, and Cu(0.38)Na-FAU. From steady-state SCR activity at 523, 623, and 723 K with NO/NH3/O-2/He (0.2/ 0.2/3.0/96.6, v/v), the feed was switched to NO/O-2/He (0.2/3.0/96.8), and after stabilization to NO/NH3/He (0.2/0.2/99.6). The aim was to simulate the SCR catalytic cycle, which obeys a redox couple Cu+/Cu2+. The changes of the Cu oxidation state were identified by following an IR band near 900 cm(-1), assigned to an internal vibration Of [Cu-O-Cu](2+), or to a modification of adjacent T-O-T framework vibrations induced by [Cu-O-Cu](2+). At the steady state of the SCR at 523 K, there is no band visible near 900 cm(-1) providing evidence that [Cu-O-Cu](2+) is reduced as Cul. Moreover, the redox cycle with NO+O-2 and NO+NH3 shows that the oxidation of Cu+ to [Cu-O-Cu](2+) by NO+O-2 takes much longer than the reduction of [Cu-O-Cu](2+) by NO+NH3 at this temperature. These experiments demonstrate that the oxidation is the ratedetermining step when the SCR is carried out at low temperature. At 723 K, DRIFT examinations of the SCR at steady state shows that the catalysts are mainly composed of Cu oxo species, for example, [Cu-O-Cu](2+) with the band near 900 cm(-1). On the other hand, the redox cycle with NO+O-2 and NO+NH3 indicates that the oxidation became much faster than reduction. Therefore, and in contrast with the reaction at 523 K, the reduction Of Cu2+ to Cu+ is the rate-determining step in the SCR at 723 K. The study of the redox properties of the various Cu-FAU catalysts by NO+O-2 and NO+NH3 demonstrates that the high activity at low temperature of Cu(0.38)Na-FAU and Cu(0.16)Ba-FAU can be mainly described in terms of a fine-tuning of Cu redox properties.

Detailed structures of zeolite catalysts, including Al and cation distribution, framework and catalytic site geometries, are not fully accessible from experiment. Since the magnetic properties of framework elements and extra-framework cations are strongly dependent on their environment, the combined use of magic angle spinning NMR or ESR techniques with quantum chemical calculations is very useful to establish the local structure around specific sites. General effects of Al and B substitution in the zeolite framework, coupled with H+ or Na+ counter-ions, on the Si-19, 27 Al and B-11 NMR spectra were studied, using a density functional theory (DFT)-based methodology, for a model of the zeolite mazzite. In agreement with experiment, the exchange of Na+ by H+ in the boron compound is accompanied by a change of B coordination from tetrahedral to trigonal, with a characteristic downfield shift of around 10 ppm, whereas the presence of water restores the tetrahedral boron and its NMR chemical shift. Further, ESR spectra of zeolites exchanged with open-shell cations provide useful data on the metal coordination and its reactivity compared with calculated model ESR properties. The ESR hyperfine coupling constants, calculated using DFT, for models of different Cu sites of a Cu(II)-Y zeolite, with and without H2O or NH3, show a clear correlation between the Cu spin population and its coordination, involving the participation of the zeolite framework in the reactivity. Copyright (C) 2004 John Wiley Sons, Ltd.

Quantum mechanical modeling of the properties of transition metal ions (TMI) in zeolites gives a picture of the material which corresponds to that of a large organometallic system in which the zeolite framework behaves as a multidentate ligand. The electron density is distributed among the whole system with highly delocalized frontier orbitals. Analyses of the electron density changes in CuZSM-5 and CuFAU models upon adsorption and desorption of donor or acceptor ligands point to a supermolecular behavior of the whole system where the zeolite framework acts as a reservoir of electronic charge. This molecular description of TMI-zeolites provides a rational explanation of various aspects of their catalytic behavior in the decomposition and selective catalytic reduction (SCR) of nitrogen oxides, such as the nature of the rate determining step and the positive influence of protons in the SCR of NO by NH3.

Electron paramagnetic resonance (EPR) experiments have shown that Cu(II) occupy different positions in the Y zeolite. We have studied the EPR properties of different cluster models in which copper is located at different cristallographic sites and different positions. The calculated results showed that the unpaired electron of copper is delocalized on the zeolite and that the hyperfine coupling constants depend linearly on the spin density of the models whatever are the structures and the calculation methods. These calculated constant values are close to experimental values when Cu(II) occupy ideal positions, ie. when it is four-coordinated at the center of a four or six-membered ring.

This chapter discusses modeling transition-metal cations in zeolites. Density functional theory has been used to study model clusters of zeolites Y and β containing Fe(II), Co(II), and Cu(II) TM cations. The calculations yield a charge of around +1 on TM, whatever are their coordination, size of the cluster, and aluminum (Al) distribution. The TM-zeolite system behaves as a supermolecule and that the zeolite is able to keep or release electrons when reactive molecules are incoming.

Model clusters of a Cu(II)-Y zeolite have been studied using a density functional theory based method in order to investigate the electronic properties of the metal site involved in the catalytic activity of this zeolite. This work has shown that different Si/Al ratios, sizes, and shapes of the models do not induce significant changes in the electronic propel-ties of the Cu site, whereas the influence of its coordination is emphasized. A large charge transfer (CT) from the zeolite to the metal has been found in the case of a formal Cu(II) cation. Comparable net charges on Cu are thus obtained for models of Cu(II) and Cu(I)-zeolite. However, these two systems differ by their frontier orbital patterns, which are characterized by a low lying empty orbital localized on the zeolite and on Cu in the case of Cu (II), whereas the lowest virtual molecular orbital of the Cu(I) model has a much higher energy. Addition of NH3 molecules to the Cu(II) model is accompanied by a rearrangement in the electron distribution of the whole system, underlying the non negligible role of the zeolite ill the catalytic process.

The selective catalytic reduction (SCR) by NH3 is the most important technology to control the emissions of NOx from stationary sources. A new mechanism is proposed for the SCR of NO on Cu-exchanged FAU catalysts which markedly differs from that occurring on the V2O5-TiO2 catalysts; NO takes part in the reoxidation of Cu+ to Cu2+, but not of V4+ to V5+. This conclusion was achieved by decomposing the catalytic cycle into the oxidation and reduction steps of Cu species by reactant mixtures composed of NO, O2, and NH3 and the comparison with the global SCR reaction. A quantum chemical (QM) modeling of the catalytic site has been untertaken, leading to a better knowledge of the Cu electronic structure.

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Molecular structures, energies, vibrational frequencies, and dissociation energies for Al(CH3)(3), Al(CH3)(3)(+), and [Al(CH3)(3)](2) have been studied using density functional and Moller-Plesset perturbation methods. The calculated properties are compared with the available experimental results. All the methods correctly describe the geometries of the neutral molecules. Density functional or MP2 (or MP4) methods provide similar ionization energies, whereas the dissociation energy of the dimer is more dependent on the methodology.

Molecules containing a benzene ring and an oxygen atom typically have two types of protonation sites: on the ring (where facile intramolecular hydrogen transposition from carbon to carbon probably takes place) or on an oxygen lone pair. Four aryl ethers are compared: the isomers phthalan (1, a cyclic benzylic ether) and coumaran (2, a cyclic phenyl ether), as well as isochroman (3) and isopropyl phenyl ether (iPrOPh). The proton affinities of 1–3 have been measured using FT-ICR techniques as 830, 855 and 838 kJ/mol, respectively. Comparison with model compounds and Hartree-Fock-based SCF calculations indicate that protonated phthalan (1H) and protonated isochroman (3H) have O-protonated structures. By contrast, the conjugate acids of coumaran and iPrOPh prefer ring-protonated structures. Acidification/neutralization experiments in the ICR, as well as MIKE spectra, demonstrate that chemical ionization of iPrOPh produces noninterconverting O- and ring-protonated forms. Metastable ion decompositions of protonated phthalan and protonated isochroman give evidence of separate decomposition pathways for both types of tautomers. Protonated coumaran exhibits complete randomization of hydrogen between oxygen and the ring, which is attributed to high barriers for expulsion of neutral fragments.