Single-site and nano-confined photocatalysts designed in porous materials for environmental uses and solar fuels.

Silica-based micro-, meso-, macro-porous materials offer attractive routes for designing single-site photocatalysts, supporting semiconducting nanoparticles, anchoring light-responsive metal complexes, and encapsulating metal nanoparticles to drive photochemical reactions by taking advantage of their large surface area, controllable pore channels, remarkable transparency to UV/vis and tailorable physicochemical surface characteristics. This review mainly focuses on the fascinating photocatalytic properties of silica-supported Ti catalysts from single-site catalysts to nanoparticles, their surface-chemistry engineering, such as the hydrophobic modification and synthesis of thin films, and the fabrication of nanocatalysts including morphology controlled plasmonic nanostructures with localized surface plasmon resonance. The hybridization of visible-light responsive metal complexes with porous materials for the construction of functional inorganic-organic supramolecular photocatalysts is also included. In addition, the latest progress in the application of MOFs as excellent hosts for designing photocatalytic systems is described.

Phenylamine-functionalized mesoporous silica supported PdAg nanoparticles: a dual heterogeneous catalyst for formic acid/CO2-mediated chemical hydrogen delivery/storage.

A PdAg-based nanoparticle catalyst supported on the mesoporous silica material, SBA-15, modified with a weakly basic phenylamine functional group has been developed as a dual heterogeneous catalyst for the H2 delivery and H2 storage reactions mediated by formic acid and carbon dioxide.

A Brown Mesoporous TiO2-x /MCF Composite with an Extremely High Quantum Yield of Solar Energy Photocatalysis for H2 Evolution.

A brown mesoporous TiO2-x /MCF composite with a high fluorine dopant concentration (8.01 at%) is synthesized by a vacuum activation method. It exhibits an excellent solar absorption and a record-breaking quantum yield (Φ = 46%) and a high photon-hydrogen energy conversion efficiency (η = 34%,) for solar photocatalytic H2 production, which are all higher than that of the black hydrogen-doped TiO2 (Φ = 35%, η = 24%). The MCFs serve to improve the adsorption of F atoms onto the TiO2 /MCF composite surface, which after the formation of oxygen vacancies by vacuum activation, facilitate the abundant substitution of these vacancies with F atoms. The decrease of recombination sites induced by high-concentration F doping and the synergistic effect between lattice Ti(3+)-F and surface Ti(3+)-F are responsible for the enhanced lifetime of electrons, the observed excellent absorption of solar light, and the photocatalytic production of H2 for these catalysts. The as-prepared F-doped composite is an ideal solar light-driven photocatalyst with great potential for applications ranging from the remediation of environmental pollution to the harnessing of solar energy for H2 production.

Surface science approach to the solid-liquid interface: surface-dependent precipitation of Ni(OH)2 on α-Al2O3 surfaces.

Surface-dependent precipitation: The adsorption of Ni(II) complexes in aqueous solution on (0001) and (1102) α-Al(2)O(3) single-crystal surfaces has been studied (see the X-ray absorption spectra obtained for parallel and perpendicular polarization directions). The use of planar model systems emphasizes the crucial role of the Al(2)O(3) orientation for Ni dispersion with practical implications in catalyst preparation procedures.

Lipase-embedded silica nanoparticles with oil-filled core-shell structure: stable and recyclable platforms for biocatalysts.

Lipase enzyme was embedded within silica nanoparticles with oil-filled core-shell structure. The enzyme embedded within such architecture retained all of its activity and showed high catalytic performance both in water and in organic media with optimal stability and recyclability.

Structure of clean and hydrated α-Al2O3 (1102) surfaces: implication on surface charge.

Periodic DFT calculations coupled to a first-principle thermodynamic approach have allowed us to establish a surface phase diagram for the different terminations of the α-Al(2)O(3) (1102) surface in various temperature and water pressure conditions. Theoretical results are compared with previous experimental data from the literature. Under a wide range of temperature and water pressure (including ambient conditions) the most stable surface (denoted C2_1H(2)O in this work) is terminated with singly coordinated hydroxyls on four-fold coordinated aluminium (Al(4C)-µ(1)-OH) while most existing surface models are only considering six-fold coordinated surface Al atoms as in the bulk structure of alumina. The presence of more acidic Al(4C)-µ(1)-OH sites helps explain the low Point of Zero Charge (PZC) (between 5 and 6) determined from the onset of Mo oxoanions adsorption on (1102) single crystal wafers. It is also postulated that another termination (corresponding to the hydration of the non-polar, stoichiometric surface, stable in dehydrated conditions) may be observed in aqueous solution depending on the surface preparation conditions.

Synthesis and unique catalytic performance of single-site Ti-containing hierarchical macroporous silica with mesoporous frameworks.

Single-site Ti-containing hierarchical macroporous silica with mesoporous frameworks (Ti-MMS) was successfully prepared by a solvent evaporation method using organic surfactant and poly(methyl methacrylate) (PMMA) colloidal crystals as the template. The formation of a well-defined macroporous structure composed of mesoporous silica walls was characterized by SEM and TEM observations. The successful incorporation of tetrahedrally coordinated Ti oxide moieties within their frameworks was also confirmed by spectroscopic techniques such as UV-vis and XAFS measurements. Comparative studies revealed that Ti-MMS exhibited higher catalytic activities for the epoxidation of linear α-olefin compared to Ti-containing mesoporous silica without macropores (Ti-MS). The reaction rate was significantly enhanced on Ti-MMS depending on increases in the alkyl chain length of linear α-olefins. It was also found that Ti-MMS showed good catalytic performance in the selective epoxidation of methyl oleate, which is a kind of unsaturated fatty acid methyl ester (FAME), under acid-free reaction conditions with tert-butylhydroperoxide (TBHP) because of the advantages of the combination of hierarchical macroporous and mesoporous structures.

Basic reactivity of CaO: investigating active sites under operating conditions.

A set of CaO samples was prepared from thermal decomposition of several precursors, leading to very different surface properties. During storage, CaO samples rehydrated quickly but reversibly. Before characterization, the samples were pre-treated at 1023 K under nitrogen flow to obtain CaO as the active phase. Although this pre-treatment led to almost the same specific surface areas for all samples, their basic reactivity levels toward 2-methylbut-3-yn-2-ol conversion were different from one preparation to another. In contrast with the properties of MgO pre-treated at the same temperature, the basic reactivity of CaO correlates neither with the concentration of surface defects (exposing ions in low coordination) determined by photoluminescence nor with the deprotonation ability toward methanol. In order to identify the active sites on CaO pre-treated under nitrogen in the temperature range 673 K-1023 K, OH groups were quantified with (1)H NMR: the higher the surface density of OH groups, the higher the basic reactivity. Even after pre-treatment at 1023 K, after which only a few hydroxyls remain, the basic reactivity is governed by the remaining hydroxylation of the surface. The higher reactivity of OH groups of CaO compared to those of Ca(OH)(2) and MgO is discussed.

EXAFS spectroscopy as a tool to probe metal-support interaction and surface molecular structures in oxide-supported catalysts: application to Al2O3-supported Ni(II) complexes and ZrO2-supported tungstates.

EXAFS spectroscopy is shown as a tool of prime importance to probe the formation of metal-oxygen-support bonds and unravel the surface molecular structure in oxide-supported systems through two examples: (i) a molecular metal complex (Ni(II) bisglycinate) characterized after impregnation and drying on Al2O3, and (ii) a tungsten oxide nanophase characterized after deposition on zirconia and high temperature thermal treatment (tungstated zirconia catalysts, i.e. WOx/ZrO2). Unlike other spectroscopic techniques, EXAFS at the Ni K-edge proves that a modest thermal activation during the impregnation step triggers the grafting of nickel(II) bisglycinate onto the support: Al next-nearest neighbours are detected when the impregnation is carried out at 60 degrees C instead of room temperature. Characterization of WOx/ZrO2 catalysts shows the presence of W next-nearest neighbours around tungsten, with W-W distances distinctive of edge-shared WO6 octahedra only. The WOx overlayer can thus be described as bidimensional, nanometric slabs of 4 to 5 WO6 units on each side. In these slabs, W octahedra are interconnected to form a more condensed structure than the one present in bulk WO3 (in which linkage through corners exists). Moreover, EXAFS results conclusively demonstrate that the WOx overlayer is directly anchored to the ZrO2 surface by means of W-O-Zr bonds with a W-Zr distance of 3.14 A.

Infrared characterization of hydroxyl groups on MgO: a periodic and cluster density functional theory study.

The infrared OH stretching frequencies of the various types of hydroxyl groups on MgO surfaces have been calculated by periodic (VASP) and cluster (Gaussian) DFT simulations. Surface irregularities (mono and diatomic steps, corners, step divacancies, and kinks) have been considered to model the IR spectra of hydroxylated MgO powders. A good correspondence between calculated and experimental frequencies is obtained with the B3LYP functional. Hydrogen-bonding is the parameter which influences most the IR frequency of OH groups, followed by location of OH groups in concave or convex areas of the surface and then oxygen coordination. The evolution of experimental IR spectra upon evacuation at increasing temperature can be rationalized on the basis of calculated thermal stabilities of each kind of OH groups. A new model is finally proposed to help assign the experimental bands, in terms of hydrogen-bonding, local topology of the hydroxylated sites, and coordination of oxygen.

Transformations of gamma-alumina in aqueous suspensions 1. Alumina chemical weathering studied as a function of pH.

Hydration of gamma-Al2O3 is often reported to occur via the superficial transformation of the alumina surface into aluminum hydroxide-like layers. However, very little evidence has been given so far to support this hypothesis. It is demonstrated here by X-ray diffraction, TEM, electron diffraction, and solubility studies that a second process of hydration takes place that involves the dissolution of alumina and subsequent precipitation of well-shaped Al(OH)3 particles from supersaturated alumina aqueous solution. This process can be observed on a macroscopic scale (XRD, TEM) for any pH5, provided that the contact time between alumina and water exceeds 10 h. The least thermodynamically stable phase of aluminum hydroxide, bayerite, becomes favored compared with gibbsite when the pH of the solution is increased. It is assumed that the rate of formation of bayerite germs is greater than that of gibbsite due to variations in aluminum speciation in solution as a function of pH.

Functionalization of heteropolyanions-osmium and rhenium nitrido derivatives of Keggin- and Dawson-type polyoxotungstates: synthesis, characterization and multinuclear ((183)W, (15)N) NMR, EPR, IR, and UV/Vis fingerprints.

Reaction of K(10)[alpha(2)-P(2)W(17)O(61)] or K(10)[alpha(1)-P(2)W(17)O(61)] or [Bu(4)N][OsCl(4)N] in a water/methanol mixture, and subsequent precipitation with (Bu(4)N)Br provided [alpha(2)-P(2)W(17)O(61){Os(VI)N}](7-) and [alpha(1)-P(2)W(17)O(61){Os(VI)N}](7-) Dawson structures as tetrabutylammonium salts. Reactions of [(Bu(4)N)(4)][alpha-H(3)PW(11)O(39)] with either [ReCl(3)(N(2)Ph(2))(PPh(3))(2)] or [Bu(4)N][ReCl(4)N] are alternatives to the synthesis of [(Bu(4)N)(4)][alpha-PW(11)O(39){Re(VI)N}]. (183)W and (15)N NMR, EPR, IR, and UV-visible spectroscopies and cyclic voltammetry have been used to characterize these compounds and the corresponding [(Bu(4)N)(4)][alpha-PW(11)O(39){Os(VI)N}] Keggin derivative.

Remarkable effect of the preparation technique on the state of cobalt ions in BEA zeolites evidenced by FTIR spectroscopy of adsorbed CO and NO, TPR and XRD.

The state of cobalt in two BEA zeolites was studied by XRD, TPR, and FTIR spectroscopy using CO and NO as probe molecules. One of the samples, CoAlBEA (0.4 wt % of Co), was prepared by conventional ion exchange and the other, CoSiBEA (0.7 wt % Co), by a two-step postsynthesis method involving dealuminated SiBEA zeolite. The introduction of Co into SiBEA leads to an increase of unit cell parameters of the BEA structure and to the consumption of silanol groups in vacant T-sites of the dealuminated zeolite. In contrast, no structural changes are observed after incorporation of cobalt into AlBEA by ion-exchange. The reduction temperature of cobalt in CoSiBEA zeolite (1130 K), is much higher than for CoAlBEA and indicates a strong interaction of cobalt ions with SiBEA. Low-temperature CO adsorption on CoAlBEA results in (i) H-bonded CO, (ii) Co(3+)-CO adducts (2,208 cm(-1)) and (iii) a small amount of Co(2+)-CO complexes (2,188 cm(-1)). In agreement with these results, NO adsorption leads to the appearance of (i) NO(+) (2,133 cm(-1), formed with the participation of the zeolite acidic hydroxyls), (ii) Co(3+)-NO (1932 cm(-1)), and (iii) a small amount of Co(2+)(NO)(2) dinitrosyls (nu(s) = 1,898 and nu(as) = 1,814 cm(-1)). Low-temperature CO adsorption on CoSiBEA leads to formation of two kinds of Co(2+)-CO adducts (2,185 and 2,178 cm(-1)). No Co(3+) cations are detected. In line with these results, adsorption of NO reveals the existence of two kinds of Co(2+)(NO)(2) dinitrosyls (nu(s) = 1,888 and nu(as) = 1,808 cm(-1) and nu(s) = 1,878 and nu(as) = 1,799 cm(-1), respectively).