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Dilute Pd-in-Au alloys are valuable materials for selectively hydrogenating alkynes and isomerizing alkenes. By diluting Pd in a Au host, the selectivity toward semihydrogenated alkene isomers can be significantly enhanced and the unfavorable overhydrogenation to alkanes is suppressed. However, a detailed mechanistic study on the origin of the enhanced alkene selectivity over dilute alloy catalysts is still missing. Here, we combine experiment and theory to unravel the reaction mechanism, identifying rate-limiting and selectivity-controlling steps in 1-hexene hydrogenation over dilute Pd-in-Au catalysts. Using isotope-exchange hydrogenation experiments, we show that 1-hexene and hydrogen over a bimetallic Pd4Au96 in silica catalyst preferentially form 1-hexene isomers, (trans and cis) 2- and 3-hexene and only a small amounts of hexane. The reaction is consistent with a Horiuti-Polanyi mechanism, similar to a monometallic Pd nanoparticle catalyst. Computation of the free-energy profiles for 1-hexene hydrogenation and isomerization over a single Pd atom in a Au surface using first principles calculations indicated that the isomerization of 1-hexene to 2-hexene is energetically favorable due to the relatively large barrier for H2 dissociation preventing hydrogenation to n-hexane. Microkinetic modeling established that H2 dissociation on the single-atom Pd sites and H spillover from these sites onto the Au host are rate-limiting and key in steering the selectivity of dilute Pd-in-Au alloys toward the hexene isomers. The mechanistic insights from this study contribute to the rational design of optimized dilute alloy catalysts for selective alkene isomerization.

To understand the nature and structure of N-doping centers in carbon materials, we combine two-dimensional (2D) solid-state NMR experiments and chemical shift calculations for 15N, 13C, and 1H nucl...

Dimethyl methylphosphonate (DMMP) is used as a simulant for toxic nerve agents and pesticides, rendering the understanding of surface chemistry requisite to design effective materials for organopho...

We explore the structure and formation mechanism of the surface tungsten hydride complex supported on alumina, which is a catalyst for alkane and alkene metathesis and cross-metathesis. We show that the kinetics for the formation reaction of the hydride from the grafted alkylidyne–alkyl W complex strongly favors the creation of a metastable WVI trihydride, which is less coordinated by the alumina surface than the most stable isomer. The reaction network for the hydride formation involves hydrogenation of the W≡C or W═C bond, hydrogenolysis of W–C bonds, and a second grafting step. The creation of metastable surface complexes is proposed to be a key path for the design of active and selective catalysts.

Alloys of Ag and small amounts of Pd are promising as bifunctional catalysts, potentially combining the inherent selectivity of the noble Ag with that of the more reactive Pd. Stable PdAg surface a...

The reaction of CH3F on partially dehydroxylated Al2O3 yields isobutene and higher hydrocarbons along with dimethyl ether. In a combined experimental and computational study we show that the reaction starts through the initial formation of a methoxy intermediate observed experimentally and formed via an SN2 type process between surface O atoms and the CH3F molecule which undergoes C–F activation on the most acidic Lewis site of the 110 termination of the Al2O3 surface. Coordination of an additional CH3F molecule on adjacent Al Lewis acid sites generates methyl cation like species which abstract a hydride from the adjacent initially generated surface methoxy species yielding a “AlOCH2+” oxonium like transient structure and CH4. Both species can further react forming the first carbon–carbon bond by producing an ethoxy intermediate in a process very exoergic and with accessible energy barriers. This ethoxy intermediate can further react in successive exoergic steps with additional “activated” CH3F on Al sites leading to higher alkoxy species which are finally liberated as isobutene and higher hydrocarbons.

Density functional theory (DFT) and density functional perturbation theory (DFPT) were applied to study the structural, electronic, and optical properties of a (Na2–xCux)Ta4O11 solid solution to accurately calculate the band gap and to predict the optical transitions in these materials using the screened coulomb hybrid (HSE06) exchange-correlation formalism. The calculated density of states showed excellent agreement with UV–vis diffuse reflectance spectra predicting a significant red-shift of the band gap from 4.58 eV (calculated 4.94 eV) to 2.76 eV (calculated 2.60 eV) as copper content increased from 0 to 83.3%. The band gap narrowing in these materials, compared to Na2Ta4O11, results from the incorporation of new occupied electronic states, which are strongly localized on the Cu 3d orbitals, and is located within 2.16–2.34 eV just above the valence band of Na2Ta4O11. These new occupied states, however, possess an electronic character localized on Cu, which makes hole mobility limited in the semiconductor.

The stability of the near-surface region of palladium(111) under a pressure of acetylene and hydrogen is studied via a combination of density functional theory calculations and atomistic ab initio thermodynamics. Gibbs free energy maps give insight into the deposition of C in the near surface of Pd(111) due to acetylene fragmentation. Our results indicate that under working conditions Pd is able to incorporate large amounts of C that will be distributed equally among the top metal interlayers. We further explored factors such as the partial hydrogen pressure, temperature, and presence of strain and found that whereas high temperatures and large hydrogen pressures inhibit C accumulation, the presence of tensile strain enhances the dissolution of C. Additional calculations indicated that the accumulation and distribution of C in the near surface of Pd critically weaken the adsorption energy of ethylene, hence favoring its desorption and yielding a high selectivity for partial hydrogenation. We claim that th...

Using first principles calculations, we investigate the structural, electronic, optical, and energetic properties of S-doped anatase TiO2 bulk systems. To ensure accurate band gap predictions, we use the HSE06 exchange correlation functional, and the absorption spectra are obtained with density functional perturbation (DFPT) theory by employing HSE06. Various oxidation states (anionic and cationic) of sulfur are considered depending on the location in bulk TiO2: in interstitial position or in substitution for either oxygen or titanium atoms. Among the explored structures, two anionic and one cationic configurations induce an improved optical absorption response in the visible region as observed experimentally. Moreover, we undertake a thermodynamic analysis as a function of the chemical potential of oxygen and considering three relevant sulfur chemical doping agents (S2, H2S, and thiourea). It highlights that cationic configurations (S4+ and S6+) are strongly stabilized in a wide range of oxygen chemical ...

In this study, the nature of the hydroxyl groups present on γ-alumina, γ-Al2O3, pretreated at various temperatures has been reinvestigated by 1H NMR spectroscopy. The peaks are assigned by comparison between experimental and simulated spectra, in agreement with previous IR studies. The lowest chemical shifts δ correspond to OH groups strongly bound to the most acidic Al atoms (AlIV and AlV). High chemical shifts δ are assigned to OH groups making hydrogen bonds. A large range of values is found depending on the strength of these bonds. The structure of the surface complexes obtained by grafting Hf(CH2tBu)4, 1, on γ-Al2O3 at various pretreatment temperatures T (350, 500, 700 °C), referred to as 1-γ-Al2O3-(T), and of their thermolysis products has been determined, by a combined experimental (mass balance, in situ IR,) and theoretical (DFT calculations) study. These results unambiguously prove the presence of two kinds of neopentyl–metal bonds, Hf–CH2tBu and Al–CH2tBu for 1-γ-Al2O3-(500) and 1-γ-Al2O3-(700),...

Extension of the absorption properties of TiO2 photocatalytic materials to the visible part of the solar spectrum is of major importance for energy and cleaning up applications. We carry out a systematic study of the N-doped anatase TiO2 material using spin-polarized density functional theory (DFT) and the range-separated hybrid HSE06 functional. The thermodynamic stability of competitive N-doped TiO2 structural configurations is studied as a function of the oxygen chemical potential and of various chemical doping agents: N2, (N2 + H2), NH3, N2H4. We show that the diamagnetic TiO(2–3x)N2x system corresponding to a separated substitutional N species (with 2–4% N impurities) and formation of one-half concentration of O vacancies (1–2 atom %) is an optimal configuration thermodynamically favored by NH3, N2H4, and (N2 + H2) chemical doping agents presenting a dual nitrating–reducing character. The simulated UV–vis absorption spectra using the perturbation theory (DFPT) approach demonstrates unambiguously that...

The structure and stability of dimeric Mo(VI) oxide species on γ-alumina are investigated with a periodic DFT approach. A large number of Mo dimers of various geometries, located on the (100) and (110) surfaces of γ-Al2O3, are modeled. The most stable dimeric species on the (100) plane are mixed dioxo−monooxo or double-monooxo species that are 3-, 4-, or 5-fold bonded to the surface. In most cases, the molybdenum atoms are pseudotetrahedrally coordinated. In the case of the (110) surface, the most stable dimeric Mo(VI) forms are double-monooxo species, which are 6- or 5-fold bonded to the support. The coordination of the molybdenum atoms is most frequently a distorted square pyramidal, but distorted octahedral or distorted tetrahedral geometry also happens. The energetic stability of the dimeric Mo(VI) species, relative to the corresponding monomeric Mo(VI) centers, is determined. On the minority (100) surface, the complex−surface interaction is moderate and the dimeric Mo(VI) species are more stable than...

Anhydrous Keggin-type phosphorus heteropolyacids were deposited on partially dehydroxylated silica by using the surface organometallic chemistry (SOMC) strategy. The resulting solids were characterized by a combination of physicochemical methods including IR, Raman, 1D and 2D 1H, and 31P MAS NMR, electron microscopy experiments and density functional theory (DFT) calculations. It is shown that the main surface species is [≡Si(OH...H+)]2[H+]1[PM12O403−] where the polyoxometalate is linked to the support by proton interaction with two silanols. Two other minor species (10% each) are formed by coordination of the polyoxometalate to the surface via the interaction between all three protons with three silanol groups or via three covalent bonds formed by dehydroxylation of the above species. Comparison of the reactivity of these solids and of compounds prepared by a classical way shows that the samples prepared by the SOMC approach contain ca. 7 times more acid sites.

Electron-poor transition metal complexes are of high interest in polymerization or oligomerization, but they require the use of a Lewis acid cocatalyst in order to reach the cationic active structure. The structure of the surface complexes obtained by grafting Hf(CH2tBu)4, 1, on γ-alumina has been resolved by a combined experimental (mass balance analysis, labeling, in situ IR, NMR) and theoretical (DFT calculations) study. Thermolysis, oxidation, and hydrogenolysis reactions have unambiguously proved the presence of two kinds of neopentyl−metal bonds: Hf−CH2tBu and Al−CH2tBu. Three coexisting surface complexes have been fully characterized and quantified: a monoaluminoxy [(≡AlIVO)Hf(CH2tBu)3], a neutral bis-aluminoxy [(≡AlIVO)(AlsO)Hf(CH2tBu)2], and a zwitterionic bis-aluminoxy complex [(≡AlIVO)(AlsO)Hf(CH2tBu)2]+[(CH2tBu)Als]− in 40%, 26%, and 34% yield, respectively. In 13C NMR calculations the important effect of spin−orbit coupling has been underlined on the chemical shifts of the carbon atoms direct...

Selective alkyne hydrogenation in the presence of carbon−carbon double bond compounds, for which Pd is an excellent catalyst, is a strategically important large-scale industrial process. Although in palladium, functionality and structure are closely interrelated, knowledge of the structure of Pd is insufficient as the interaction with the chemical environment causes drastic compositional changes near the subsurface region: while unselective hydrogenation proceeds in the presence of a β-hydride phase, selective hydrogenation can be achieved only in the presence of a near-surface Pd−C phase. Here, we show from a combination of in situ prompt gamma activation analysis and ab initio simulations based on density functional theory that (i) the presence of the Pd−C phase created under selective conditions implies a strong change in the surface and subsurface stability of hydrogen, (ii) there is still a slower exchange of bulk and surface hydrogen, and (iii) the reaction rate for selective hydrogenation is indepe...

Dicarboxylic acids have been recently found to provide a significant improvement of the mechanical properties of hard materials such as bitumen when added at low concentration. To understand such properties, periodic DFT calculations combined with empirical van der Waals corrections have been performed on crystals of dicarboxylic acid molecules with various chain lengths. Our calculations allow us to understand the interactions governing the cohesive energy of the crystal and to qualitatively correlate diacid transition phase properties (fusion, vaporization, and sublimation) to their behavior when mixed in bitumen. The molecules are distorted because of structural constraints in the crystal packing and that distortion is found to be larger for odd diacids than for even diacids, explaining the usual odd−even alternation of their properties related to the solid state. In particular, the relative fusion temperature of diacids as a function of chain length is dominated by distortion effects. In contrast, mos...

This paper deals with the assignment of photoluminescence excitation spectra of hydroxylated MgO powders thanks to a combined theoretical and experimental approach. Experimentally, despite the numerous types of hydroxyl groups formed upon hydroxylation of bare MgO surfaces, only three types of luminescent species are observed. TD-DFT excitation energy calculations are performed on hydroxylated MgO clusters modeling planes, edges, corners, kinks, and divacancies. The results indicate that the photoluminescence excitation properties do not depend primarily on the coordination of oxygen of OLCH groups (LC= low coordination), which can be satisfactorily explained by theoretical calculations. The hydroxylation state of the surface can thus be more accurately followed by the thermal evolution of the bands characteristic of bare surface irregularities. In addition, OLCH groups are themselves directly implied in the excitation process only if they are located on convex areas of the surface and if L ≤ 3, L being d...

Photoluminescence excitation spectra of dehydroxylated MgO powders are assigned from a combined theoretical and experimental approach. Experimentally, a set of five species is identified, among which are two new species, by recording the spectra with enhanced resolution and at 77 K so as to minimize energy transfer along the surface. Performed on various clusters modeling terraces, edges, monatomic steps, corners, kinks, and divacancies, TD-DFT calculations show that the excitation energy depends not only on the coordination of surface ions, but also on the local topology (kinks versus corners for example). This, together with the fact that a single defect can be excited at several wavelengths, explains the complexity of the experimental spectra and the difficulty encountered so far for their assignment. Several groups of calculated excitation energies can be identified which enable to rationalize the assignment of the experimental spectra.

Complex 1H MAS NMR spectra of hydroxylated MgO powders have been assigned by combining DFT embedded cluster calculations and experiments using single pulse, Hahn-echo, and 2D NOESY like sequences. Chemical shifts calculations suggest the qualitative classification of protons into three main categories, characterized by different chemical shifts ranges. The highest chemical shifts (δH > −0.7 ppm) are proposed to be characteristic of hydrogen-bond donor OH groups (threefold O3C−H, fourfold O4C−H, and fivefold O5C−H localized on corners, edges, and in valleys respectively). The lowest chemical shifts (δH < −0.7 ppm) are associated to isolated and hydrogen-bond acceptor twofold O2C−H and onefold O1C−H, whereas the central signal at δH = −0.7 ppm would correspond to isolated O3C−H and O4C−H on kinks and divacancies. These assignments can be refined by considering dipolar interactions between vicinal protons observed thanks to the NOESY like sequence. It is thus shown that some hydrogen bond donor OH groups are...

Periodic density functional calculations are reported for NO dissociation on the Rh(221) and RhCu(221) surfaces. The analysis of the reaction pathways indicates that the presence of steps results not only in new adsorption sites for NO adsorption but also on the appearance of a completely new reaction pathway with activation energies which are smaller than those found previously for the regular Rh(111) surfaces. At low NO surface coverage and a small Cu density, the presence of Cu on Rh(221) does not change the reactivity of rhodium stepped surfaces toward NO dissociation, since the effective activation energy barrier is higher (1.32 eV for RhCu(221) versus 1.17 eV for Rh(221)). The essential reason is the increased metastability of the step sites. However, for stronger NO coverage or a higher Cu density, the stepped RhCu(221) surface is expected to exhibit an enhanced activity: either by increasing the occupancy of these metastable sites along the RhCu steps or by removing the possibility of highly stab...