DFT investigations for the catalytic reaction mechanism of methane combustion occurring on Pd(ii)/Al-MCM-41.

In this work, a theoretical analysis was carried out on the mechanism of methane combustion occurring on single site palladium oxide species [Pd]2+ supported on Al-MCM-41 silica. Single site Pd-oxo and PdO2-superoxo structures were used to represent the active centers. Activation energies for all the elementary steps involved in the oxidation of methane into formaldehyde are presented. The competition of methane/methanol substrates on active sites was examined. It was found that the formation of methanol via the reaction of methane with the superoxo species, formed via the adsorption of O2 on reduced Pd(ii) centers, facilitates the production of the very active Pd-oxo catalytic sites.

Oxidation of Methane to Methanol over Single Site Palladium Oxide Species on Silica: A Mechanistic view from DFT.

A theoretical analysis was carried out on the mechanism of methane oxidation to methanol occurring on single site palladium oxide species [PdO]2+ supported on a model of Al-MCM-41 silica. Both 6- and 8-membered ring structures were considered to represent the support. The energy profile for each elementary reaction was determined from density functional theory calculations with the OPBE functional. The calculated overall activation energies are close to the experimental values. Our calculations confirm that spin inversion can play a significant role in decreasing the barrier heights for the pathways. Indeed, in this type of reactions we could show a crossing between singlet and triplet reaction paths. We showed that the mechanism for the C-H bond cleavage and for the formation of methanol has a radical nature. According to our results, the [PdO]2+ species located on a 8-membered ring of silica is more active than that deposited on a 6-membered ring. The calculated activation energies to cleave the methane C-H bond are 35 and 84 kJ/mol for the radical and ionic pathways, respectively. The activation barrier and the transition state geometry of this H-abstraction step are directly correlated with the optimal angle at which the substrate should approach the [Pd═O]2+ moiety, with the elongation of the Pd-Ooxo bond and finally with the energy of the π* acceptor orbital.

The mechanism of the initial step of germanosilicate formation in solution: a first-principles molecular dynamics study.

The condensation reactions between Ge(OH)4 and Si(OH)4 units in solution are studied to understand the mechanism and stable species during the initial steps of the formation process of Ge containing zeolites under basic conditions. The free energy of formation of (OH)3Ge-O-Ge-(OH)2O(-), (OH)3Si-O-Si-(OH)2O(-), (OH)3Ge-O-Si-(OH)2O(-) and (OH)3Si-O-Ge-(OH)2O(-) dimers is calculated with ab initio molecular dynamics and thermodynamic integration, including an explicit description of the water solvent molecules. Calculations show that the attack of the conjugated base (Ge(OH)3O(-) and Si(OH)3O(-)) proceeds with a smaller barrier at the Ge center. In addition, the formation of the pure germanate dimer is more favorable than that of the germano-silicate structure. These results explain the experimental observation of Ge-Ge and Si-Ge dimer species in solutions, with a few Si-Si ones.

Grafting trimethylaluminum and its halogen derivatives on silica: general trends for (27)Al SS-NMR response from first principles calculations.

(27)Al NMR is the method of choice for studying grafted Al species on a large area solid support, such as co-catalysts for α-olefin oligomerization involving mesoporous silica materials. Here, we show how to interpret the (27)Al solid-state NMR spectrum and parameters for various types of Al monomeric and dimeric alkyl and halogen compounds grafted on silica, based on the trends obtained from first-principles calculations. Since most alkylaluminum species tend to form dimers in the gas phase, we chose as prototypes both the AlMe3 monomer and the Al2Me6 dimer. On top of that the influence of chlorine substituents on the NMR parameters is explored considering all possible isomers. There are two main effects on the Al NMR parameters observed in the case of monomers: (i) the larger π-donating character of the ligands (from Me to Cl for example) leads to a decrease of the quadrupolar coupling constant CQ and (ii) the larger σ-attracting character of the ligand (from Cl to F for example) yields an upfield variation of the Al chemical shift δISO while in contrast CQ is increased. The same is true also in the case of dimeric species, with an additional specific effect. By (27)Al solid state NMR we can differentiate clearly between terminal and bridge positions for the substituents. The reason for this phenomenon is explained in terms of different natural localized MO (NLMO) contributions to the CQ parameter. This aspect is important because the surface sites for this type of system are expected to be mostly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands.

Triisobutylaluminum: bulkier and yet more reactive towards silica surfaces than triethyl or trimethylaluminum.

Triisobutylaluminum reacts with silica yielding three different Al sites according to high-field aluminum-27 NMR and first principle calculations: a quadruply grafted dimeric surface species and two incorporated Al(O)x species (x = 4 or 5). This result is in stark contrast to the bis-grafted species that forms during Et3Al silica grafting. Thus the isobutyl ligands, which render R3Al monomeric, lead to greater reactivity towards the silica surface.

γ-Alumina: the essential and unexpected role of water for the structure, stability, and reactivity of "defect" sites.

Combining experiments and DFT calculations, we show that tricoordinate Al(III) Lewis acid sites, which are present as metastable species exclusively on the major (110) termination of γ- and δ-Al(2)O(3) particles, correspond to the "defect" sites, which are held responsible for the unique properties of "activated" (thermally pretreated) alumina. These "defects" are, in fact, largely responsible for the adsorption of N(2) and the splitting of CH(4) and H(2). In contrast, five-coordinate Al surface sites of the minor (100) termination cannot account for the observed reactivity. The Al(III) sites, which are formed upon partial dehydroxylation of the surface (the optimal pretreatment temperature being 700 °C for all probes), can coordinate N(2) selectively. In combination with specific O atoms, they form extremely reactive Al,O Lewis acid-base pairs that trigger the low-temperature heterolytic splitting of CH(4) and H(2) to yield Al-CH(3) and Al-H species, respectively. H(2) is found overall more reactive than CH(4) because of its higher acidity, hence it also reacts on four-coordinate sites of the (110) termination. Water has the dual role of stabilizing the (110) termination and modifying (often increasing) both the Lewis acidity of the aluminum and the basicity of nearby oxygens, hence the high reactivity of partially dehyxdroxylated alumina surfaces. In addition, we demonstrate that the presence of water enhances the acidity of certain four-coordinate Al atoms, which leads to strong coordination of the CO molecule with a spectroscopic signature similar to that on Al(III) sites, thus showing the limits of this widely used probe for the acidity of oxides. Overall, the dual role of water translates into optimal water coverage, and this probably explains why in many catalyst preparations, optimal pretreatment temperatures are typically observed in the "activation" step of alumina.

Nature and structure of aluminum surface sites grafted on silica from a combination of high-field aluminum-27 solid-state NMR spectroscopy and first-principles calculations.

The determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While (27)Al solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadrupolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found.

The initial step of silicate versus aluminosilicate formation in zeolite synthesis: a reaction mechanism in water with a tetrapropylammonium template.

The initial step for silicate and aluminosilicate condensation is studied in water in the presence of a realistic tetrapropylammonium template under basic conditions. The model corresponds to the synthesis conditions of ZSM5. The free energy profile for the dimer formation ((OH)(3)Si-O-Si-(OH)(2)O(-) or [(OH)(3)Al-O-Si-(OH)(3)](-)) is calculated with ab initio molecular dynamics and thermodynamic integration. The Si-O-Si dimer formation occurs in a two-step manner with an overall free energy barrier of 75 kJ mol(-1). The first step is associated with the Si-O bond formation and results in an intermediate with a five-coordinated Si, and the second one concerns the removal of the water molecule. The template is displaced away from the Si centres upon dimer formation, and a shell of water molecules is inserted between the silicate and the template. The main effect of the template is to slow down the backward hydrolysis reaction with respect to the condensation one. The Al-O-Si dimer formation first requires the formation of a metastable precursor state by proton transfer from Si(OH)(4) to Al(OH)(4)(-) mediated by a solvent molecule. It then proceeds through a single step with an overall barrier of 70 kJ mol(-1). The model with water molecules explicitly included is then compared to a simple calculation using an implicit continuum model for the solvent. The results underline the importance of an explicit and dynamical treatment of the water solvent, which plays a key role in assisting the reaction.

Growth of a Pt film on non-reduced ceria: a density functional theory study.

The growth of platinum on non-reduced CeO(2) (111) surface is studied by means of calculations based on the density functional theory. Particles of increasing size are formed on the oxide surface by incorporating the platinum atoms one by one until multilayer films are obtained. The main conclusion is that platinum atoms tend to maximize the number of metallic bonds and to approach the situation of the bulk, hence preferring films to particles, particles to isolated atoms, and a three-dimensional growth to a two-dimensional one. The supported particles and the films exhibit a contraction of the Pt-Pt distances, with respect to those of the Pt bulk, in order to match the ceria lattice. The density of states projected on the film surface platinum atoms shows important differences in shape and energy (lower d-band center) compared to the Pt(111) reference surface, which could be the major reason for the observed changes in catalytic reactivity when deposited particles are compared with single crystal surfaces.

Promoter Effect of Early Stage Grown Surface Oxides: A Near-Ambient-Pressure XPS Study of CO Oxidation on PtSn Bimetallics.

The knowledge of the catalyst active phase on the atomic scale under realistic working conditions is the key for designing new and more efficient materials. In this context, the investigation of CO oxidation on the bimetallic Pt3Sn(111) surfaces by near-ambient-pressure X-ray photoelectron spectroscopy and density functional theory calculations illustrates how combining advanced methodologies allows the determination of the nature of the active phase. Starting from 300 K and 500 mTorr of oxygen, the progressive formation of surface oxides is observed with increasing temperature: SnO, PtO units first, and SnO2, PtO2 units afterward. For CO oxidation on the (2 × 2) surface, the activity gain is assigned to the build-up of ultrathin domains composed of SnO and SnO2 units. The formation of these early stage surface oxides is entirely supported by a density functional theory analysis. More generally, this study demonstrates how the catalyst surface oxidation and transformation can be better controlled by a relevant choice of environmental conditions.

Unravelling the mechanism of glycerol hydrogenolysis over rhodium catalyst through combined experimental-theoretical investigations.

We report herein a detailed and accurate study of the mechanism of rhodium-catalysed conversion of glycerol into 1,2-propanediol and lactic acid. The first step of the reaction is particularly debated, as it can be either dehydration or dehydrogenation. It is expected that these elementary reactions can be influenced by pH variations and by the nature of the gas phase. These parameters were consequently investigated experimentally. On the other hand, there was a lack of knowledge about the behaviour of glycerol at the surface of the metallic catalyst. A theoretical approach on a model Rh(111) surface was thus implemented in the framework of density functional theory (DFT) to describe the above-mentioned elementary reactions and to calculate the corresponding transition states. The combination of experiment and theory shows that the dehydrogenation into glyceraldehyde is the first step for the glycerol transformation on the Rh/C catalyst in basic media under He or H(2) atmosphere.

Polyoxometalate grafting onto silica: stability diagrams of H3PMo12O40 on {001}, {101}, and {111} ß-cristobalite surfaces analyzed by DFT.

The process of grafting H(3)PMo(12)O(40) onto silica surfaces is studied using periodic density functional theory methods. For surfaces with a high hydroxyl coverage, the hydroxyl groups are consumed by the polyoxometalate protons, resulting in water formation and the creation of a covalent bond between the polyoxometalate and the surface, and mostly no remaining acidic proton on the polyoxometalate. When the surfaces are partially dehydroxylated and more hydrophobic, after temperature pretreatment, less covalent and hydrogen bonds are formed and the polyoxometalate tends to retain surface hydroxyl groups, while at least one acidic proton remains. Hence the hydroxylation of the surface has a great impact on the chemical properties of the grafted polyoxometalate. In return, the polyoxometalate species affects the compared stability of the partially hydroxylated silica surfaces in comparison with the bare silica case.

Dinitrogen: a selective probe for tri-coordinate Al "defect" sites on alumina.

Dinitrogen selectively binds to tri-coordinate Al(III) sites of the (110) termination of γ- and δ-alumina, the "defects" responsible for the low temperature dissociation of methane. Similar observations on η-Al(2)O(3) and extra framework aluminium of microporous aluminosilicates also suggest the presence of Al(III) sites on these materials.

Nature of adhesion of condensed organic films on platinum by first-principles simulations.

Understanding the nature of the adhesion of an organic liquid on a metal surface is of paramount importance for elucidating the stability and chemical reactivity at these complex interfaces. However, to date, the morphology, layering and chemical properties at organic liquid metal interfaces have been rarely known. Using semi-empirical dispersion corrected density functional theory calculations and ab initio molecular dynamics simulations, we show that carbon tetrachloride and ethanol films confined to a platinum surface alter their intrinsic properties and exhibit interfacial reactivity. A few interface carbon tetrachloride (ethanol) molecules adsorb dissociatively (molecularly) on platinum thanks to the surrounding medium. The adsorption strength of the interfacial molecules is consequently increased in the condensed phase as compared to the gas phase. This remarkable effect is rationalized by an interaction energy decomposition model and an electrostatic potential analysis.

Stability of intermediates in the glycerol hydrogenolysis on transition metal catalysts from first principles.

The hydrogenolysis reaction catalyzed by a transition metal solid catalyst is a potential way to transform glycerol to 1,2-propylene glycol or 1,3-propylene glycol, two important chemicals. We explore the thermodynamic profile of this reaction from first principle simulation, comparing Ni, Rh and Pd catalysts modeled by (111) surfaces. The stability of adsorbed reactants, dehydrated intermediates, and hydrogenated propylene glycol is compared, with a special focus on the factors controlling the selectivity of the reaction. From a global thermodynamic view point, the formation of 1,2-propylene glycol is favored, and in addition the most stable intermediates in the gas phase (acetol and 1,2-aldol) lead to the formation of this product. The metal catalyst has three roles. First it stabilizes the dehydrated intermediates and renders the dehydration more exothermic. Second, the adsorption on the surface modifies the relative stability of the dehydrated intermediates, with implications on the reaction selectivity. Third it catalyses the hydrogenation step, leading to propylene glycol.

Reconstruction and stability of ß-cristobalite 001, 101, and 111 surfaces during dehydroxylation.

We analysed the dehydroxylation of 001, 101, and 111 ß-cristobalite surfaces using the periodic density functional theory method and established the OH density stability diagrams of these surfaces as a function of temperature and water partial pressure. Our calculations suggest that important surface reconstructions, involving SiO(2) unit migrations, are required to reach the experimentally measured values for hydroxyl coverage. Our thermochemical data, i.e., 3.7-5.2 OH nm(-2) in standard conditions and 1.4-2.6 OH nm(-2) at P = 10(-10) atm and T = 800 K, agree with the experimental values for amorphous silica and explain the trends observed, although some topological differences obviously exist between our periodic models and amorphous silica surfaces.

Restructuring of the Pt3Sn(111) surfaces induced by atomic and molecular oxygen from first principles.

The surface restructuring of Pt(3)Sn(111) induced by oxygen chemisorption is examined by means of density-functional theory calculations. Molecular and atomic oxygen chemisorption is investigated on the two available terminations of the bulk alloy--(2 x 2) and (square root(3) x square root(3))R30 degrees--these two surfaces differing by the tin content and the nature of chemical sites. An extensive geometric, energetic, and vibrational analysis is performed including the influence of oxygen coverage in the case of atomic adsorption. For molecular adsorption, regular structures have been obtained for both surfaces with a clear effect of tin on the stability of the adsorption forms. In contrast, for atomic adsorption, two oxygen chemical states are found. In particular, a peculiar surface restructuring, involving the formation of a network of SnO(2) species, appears for large oxygen coverage. However the two terminations present discrepancies for the restructuring mechanism all along the oxygen coverage increase. All these results are supported by a systematic vibrational analysis.