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.

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.

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.

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.

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.

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.

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.

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).

Revisiting acido-basicity of the MgO surface by periodic density functional theory calculations: role of surface topology and ion coordination on water dissociation.

Low-coordinated (LC) ions at the MgO surface (noted Mg2+LC and O2-LC with L = 1-5), located on monatomic and diatomic steps, corners, step divacancies, and kinks, have been modeled thanks to periodic density functional theory (DFT) calculations (VASP). Ions of lowest coordination induce the strongest surface geometry relaxation and the highest surface energies. The hydration energies of these sites and thermodynamic stabilities of the resulting surfaces were studied. The factors controlling the interaction strength between water and the surface are the possibility for the hydroxyl group to adopt a bridging geometry between two Mg2+ cations in concave areas of the surface, such as the bottom of the monatomic step, and at second order the surface atomic coordination, and especially the presence of three-coordinated ions. The Lewis basicity and acidity of O2-LC and Mg2+LC, respectively, increase as their coordination number decreases, which implies the same trend for the Brønsted basicity of the Mg2+-O2- pair toward water. However, this trend can be changed if pairs leading to the formation of bridging OH groups are involved, typically on monatomic steps or in step divacancies where O2C-H and O3C-H are obtained, respectively, instead of the expected O1C-H. Thanks to thermodynamic calculations, the state of the surface as a function of temperature can be determined at a given pressure, unraveling the roles of surface topology and ions coordination.

Spectroscopic and electrochemical study of the adsorption of [Co(en)2Cl2]Cl on gamma-alumina: influence of the alumina ligand on Co(III)/(II) redox potential.

UV-visible and Raman spectroscopies as well as electrochemical techniques have been used to characterize cis- and trans-[Co(III)(en)2Cl2]Cl (en=ethylenediamine) complexes and the gamma-alumina-supported cis-Co((III)) complex. It is shown that the electrochemical reduction of these complexes occurs according to a multistage mechanism involving two electrochemical steps, with the formation of a dimer that was characterized by UV-visible spectroscopy (intervalence band at 670 nm). The apparent standard redox potential for each step has been determined, and experimental results reveal that cis and trans complexes present similar electrochemical characteristics. It is also shown that the deposition of trans-[Co(III)(en)2Cl2]+ on gamma-alumina leads to an inner-sphere complex (ISC) in a cis configuration in which Cl- ligands are substituted by OH or O- surface groups of alumina. These changes in the coordination sphere of the complex induce a substantial decrease of its apparent redox potential since it is -0.63 V/SCE (saturated calomel electrode) for the gamma-alumina-supported cis-Co(III) complex, whereas values of -0.17 and -0.35 V/SCE were determined in dimethyl sulfoxide (DMSO) for the trans and cis precursor complexes, respectively.

Oxidation state of vanadium introduced in dealuminated beta zeolite by impregnation with VIVOSO4 solution: influence of preparation parameters.

Vanadium was introduced in dealuminated beta zeolite by impregnation with a VIVOSO4 aqueous solution at 353 K in air or argon (to prevent oxidation of VIV), leading to VSibeta and VSibeta-Ar zeolites, respectively. The samples were characterized by spectroscopy, XRD, and N2-physisorption. The oxidation state and environment of V in Sibeta zeolite depend on the preparation parameters (i.e., on the way the solid is recovered after impregnation and on the drying temperature). In solids recovered by centrifugation, washed with distilled water, and then dried overnight at 298 K in argon, vanadium is found as extra-lattice octahedral VIV ions as evidenced by EPR. In contrast, in solids not washed but directly dried overnight at 353 K in air or argon, vanadium is found in both cases as lattice tetrahedral VV ions. These ions are incorporated into vacant T sites associated with SiOH, SiO-, oxygen vacancies (OVs) or nonbridging oxygen (NBOs) defects as shown by diffuse reflectance UV-visible, 51V MAS NMR, FT-IR, and photoluminescence. The oxidation to VV ions is suggested to be due to an electron transfer from VO2+ to trigonal identical with Si+ defect sites followed by reaction of the resulting VO2+ ions with particular defects of vacant T sites. These processes occur already upon drying of V-impregnated Sibeta at 353 K. 51V MAS NMR allows detection of one kind of lattice tetrahedral V ions in VSibeta and two kinds in VSibeta-Ar. The formation of different kinds of tetrahedral V species is related to the presence in vacant T sites of Sibeta zeolite of different types of defect sites such as trigonal identical with Si+ defect or SiOH and SiO- groups.

Physicochemical and in situ photoluminescence study of the reversible transformation of oxide ions of low coordination into hydroxyl groups upon interaction of water and methanol with MgO.

The interaction of water and methanol with MgO samples with different distributions of oxide ions of low coordination has been investigated by physical techniques, particularly in situ photoluminescence. First, the three photoluminescence fingerprints of oxide ions vs their coordination number have been obtained for samples outgassed at 1273 K. By a pseudo quantitative approach, the relative distribution of the oxide ions of low coordination O(2-)LC (where LC = 3C, 4C, and 5C refer to tri-, tetra-, and pentacoordinated oxide ions, respectively) was determined and correlated with the shape and size of MgO particles determined by TEM and XRD. The photoluminescence of surfaces of MgO obtained after outgassing at increasing temperature or after interaction of water or methanol with a clean surface, i.e., obtained by outgassing at 1273 K, was then studied and evidenced three other photoluminescent species assigned to surface OH groups. The nature and mechanism of formation of the hydroxyls groups responsible for these new luminescent species are discussed in relation with their thermal stability and FTIR experiments.

A systematic study of the interactions between chemical partners (metal, ligands, counterions, and support) involved in the design of Al2O3-supported nickel catalysts from diamine-Ni(II) chelates.

1.5 Ni wt %/Al2O3 catalysts have been prepared by incipient wetness impregnation using [Ni(diamine)x(H2O)(6-2x)]Y2 precursors (diamine = 1,2-ethanediamine (en) and trans-1,2-cyclohexanediamine (tc); x = 0, 1, and 2; Y = NO3- and Cl-), to avoid the formation, during calcination, of difficult-to-reduce nickel aluminate. N2 was chosen for thermal treatment to help reveal and take advantage of the reactions occurring between Ni2+, ligands, counterions, and support. In the case of [Ni(en)2(H2O)2]Y2 salts used as precursors, in situ UV-vis and DRIFT spectroscopies show that after treatment at 230 degrees C Ni(II) ions are grafted to alumina via two OAl bonds and that the diamine ligands still remain coordinated to grafted nickel ions but in a monodentate way, bridging the cation with the alumina surface. With Y = Cl-, the chloride counterions desorb as hydrogen chloride, and hydrogen released upon decomposition of the en ligands is able to reduce a fraction of nickel ions into metal as evidenced by XPS. In contrast, with Y = NO3-, compounds such as CO or NO are formed during thermal treatment, indicating that nitrate ions burn the en ligands. After thermal treatment at 500 degrees C, a surface phase containing Ni(II) ions forms, characterized by XPS and UV-vis spectroscopy. Temperature-programmed reduction shows that these ions can be quantitatively reduced to the metallic state at 500 degrees C, in contrast with the aluminate obtained when the preparation is carried out from [Ni(H2O)6]2+, which is reduced only partly at 950 degrees C. On the other hand, a total self-reduction of nickel complexes leading to 2-5-nm metal particles is obtained upon thermal treatment via the hydrogen released by a hydrogen-rich ligand such as tc, whatever the Y counterion. An appropriate choice of the ligand and the counterion allows then to obtain selectively Ni(II) ions or a dispersed reduced nickel phase after treatment in N2, as a result of the reactions occurring between the chemical partners present on alumina.