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A series of photocatalysts with the [Ir(Mebib)(ppy)Cl] (Mebib: bis(N-methylbenzimidazolyl)pyridine, ppy: phenylpyridine) complex anchored to (3-aminopropyl)trimethoxysilane-modified mesoporous silica materials (SBA-15, MCM-48, and MCM-41) were successfully synthesized. At room temperature, the anchored Ir(III) complex exhibits a strong phosphorescence emission at nearly 530 nm due to triplet metal-to-ligand charge transfer (3MLCT) transitions. The intensity of the emission increases in the sequence of MCM-41 < MCM-48 < SBA-15, which follows the photocatalytic activity in the selective oxidation using molecular oxygen (O2) under visible light irradiation (λ > 420 nm). Scavenging experiments suggest that singlet oxygen (1O2) is the main reactive species. It is assumed that the enhanced excitation rate and quantum efficiency of the anchored Ir complex increase the energy-transfer from the excited 3MLCT to O2, leading to an increase in photooxidation activity.

Designing an active site: A new type of core–shell structured catalyst was prepared, with a uniform SiO2core supporting Pd nanoparticles (NPs) covered with a Ti‐containing mesoporous silica shell. Generated H2O2on the inner Pd NP site could interact with the Ti oxide moieties within the mesopores before dispersion to the solvent can occur, which ultimately enhances the oxidation activity for one‐pot oxidation of sulfide to sulfoxide.

Chloro(2,2′:6′,2′′-terpyridine)platinum(II) ([Pt(tpy)Cl]Cl) complex was successfully anchored to a series of (3-aminopropyl)trimethoxysilane-modified mesoporous silica materials (MCM-41, SBA-15, and MCM-48). Pt LIII-edge X-ray absorption fine structure (XAFS) measurements reveal that the Pt complex reacts with amino groups anchored on the mesoporous silica to create a new Pt−N bond. Upon anchoring, the nonemissive Pt(II) complex exhibits strong photoluminescence at room temperature, which is maximized near 530 nm due to ligand-centered (3LC) and/or metal-to-ligand charge transfer (3MLCT) transitions. The intensities of the emission increase in the order of MCM-41 < SBA-15 < MCM-48. In the case of MCM-48, the emission intensity is the highest at 0.42 wt % Pt loading, while concentration quenching is observed accompanied with a new emission due to the metal−metal-to-ligand charge-transfer (3MMLCT) transition at high Pt loading. These results correspond well with the photocatalytic activities in the selective oxidation of styrene derivatives using molecular oxygen (O2). It can be supposed that the enhanced excitation rate and quantum efficiency of the anchored Pt complex, due to the differences in nanoconfinement, increase the energy and/or electron transfer to O2, which ultimately enhances the photooxidation activity. The 3D-connected channel structure of the MCM-48 silica also accounts for the high photocatalytic activity, where the diffusion of O2toward the anchored Pt complex occurs smoothly compared to the one-dimensional MCM-41 and SBA-15 silicas, as demonstrated by the quenching rate constants obtained from Stern−Volmer plots.

Oxide ions in low coordination (O2−LC) play a major role in adsorption and catalysis properties of CaO powders. They also give specific photoluminescence features that could be used to characterize the distribution of these ions at the molecular level, but up to now, the photoluminescence of defective CaO powders was assigned incompletely. In this study, photoluminescence spectra of defective CaO powders prepared in various ways are recorded at 77 K with unprecedented resolution. A new feature is thus evidenced with an excitation at 320 nm and an emission at 490 nm. Combining infrared and photoluminescence spectroscopies, we show that this feature does not originate from remaining hydroxyl groups, but it completes the set of photoluminescence fingerprints of O2−LCions on CaO. A revisited assignment of this set is proposed, consistent with the dependence of transition energy on coordination numbers. An application example shows how the photoluminescence spectroscopy can be used as an original tool to compare surface morphologies of CaO samples at the molecular level and under in situ conditions.

The combined use of DR-UV−vis and Raman spectroscopies allows one to evidence three kinds of tetrahedral VVwith different structure in VxSiBEA zeolite. In all cases, vanadium species possess distorted tetrahedral configuration. At very low vanadium content (0.05 wt %), the majority of tetrahedral V(V) species are strongly distorted and have a nonhydroxylated pyramidal structure (SiO)3VO with VO stretching at 1054 cm−1whereas the remaining tetrahedral V(V) species are less distorted with a hydroxylated pyramidal structure (SiO)2(HO)VO with VO stretching at 1018 cm−1. In contrast, at higher vanadium contents (0.2−2 wt %), the main distorted V(V) species possess a nonhydroxylated pyramidal structure (SiO)3VO with VO stretching at 1033−1036 cm−1. Periodic density functional theory (DFT) calculations confirm the presence of at least two different types of V site in V-containing zeolites. The experimental Raman bands recorded for VxSiBEA with low and high V content were confirmed on the basis of DFT calculations related to seven vanadium model sites. The difficulty to assign the vibrations in V-containing silica materials is due to the overlap of the different critical vibrational fingerprints (VO, and Si−O−V vibration modes). Moreover, it was found that H-bond formation with a VO group is not energetically favorable.

Plasmonic photocatalyst: Anchoring the dye [Ru(bpy)3]2+(bpy=2,2′‐bipyridine) on the surface of Ag nanoparticles coated with a thin SiO2layer (see picture) afforded a photocatalyst whose phosphorescence emission and photoinduced oxidation activity in the selective liquid‐phase oxidation of styrene are efficiently enhanced through interaction with the localized surface plasmon resonance of the core Ag nanoparticles.

Nickel(II) nitrate is the nickel precursor generally selected to prepare oxide‐supported catalysts, due to its low cost, high solubility in water and decomposition at moderate temperatures. However, its decomposition produces NOx, and it is known to melt and redistribute over the support upon heating, leading to poorly dispersed metal particles after reduction and to their partial expulsion from the porous support system. The benefits of using NiIIacetate as an alternative to NiIInitrate are explored by investigating a series of model Ni catalysts (1–5 wt %) prepared from the mesoporous siliceous supports MCM‐41 and Al‐MCM‐41. The problems encountered with NiIInitrate, in terms of particle size and redistribution of nickel, are not observed with NiIIacetate. In the case of MCM‐41‐supported systems, metal particles are well dispersed and remain located within the porous support. The higher pH of the impregnation solution, favoring electrostatic interactions between nickel ions and the support, may account for the higher metal dispersion. The formation of silicate‐like species in strong interaction with the support does not appear prominent on siliceous MCM‐41 when nickel acetate is the precursor, but seems to be enhanced for higher Ni contents. Finally, Al‐MCM‐41 supports are more sensitive to the exposure to nickel acetate than their siliceous analogues because acetate ions are able to extract Al ions from the support, resulting in a partial disruption of the support organization and to the formation of nickel silicate species.

The adsorption geometries, energies, and vibrational frequencies of methanol on MgO defective surfaces have been calculated by periodic DFT simulations. The results are very comparable with those obtained with water and are also in very good accordance with microcalorimetry and infrared experiments. At low coverage, the dissociation is observed on all defects involving ions in low coordinations. Over and above the coordination number of surface ions, the adsorption energy is strongly governed by the surface topology: dissociation on confined sites gives rise to methoxy groups highly stabilized by bridging two or even three cations. The occurrence of such very strong sites on MgO powder is confirmed by microcalorimetry. The dissociation ability depends on the methanol coverage because it modifies the surface relaxation and the network of H bonds, resulting, for a given defect, in similar adsorption energies for molecular and dissociated species at high coverage. This explains why there are more strong sites (quantified by microcalorimetry) than dissociating sites (quantified by infrared).

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Richtig vergleichbarwerden Untersuchungen der Aktivität von Katalysatoren nur, wenn unter nichtdiffusionsbegrenzten Bedingungen gearbeitet wird. Andernfalls geben die Ergebnisse nur die Leistungsfähigkeit des Reaktor und/oder des Reaktionsdesigns wieder. Um falsche Schlüsse in der Titelreaktion (siehe Bild) zu vermeiden, werden in diesem Essay Vorschläge für ein besser standardisiertes Verfahren gemacht.

CrxSiBEA catalysts (x= 0.2, 1.0, and 3.4 Cr wt %) are prepared by a two-step postsynthesis method. The introduction of Cr ions in the framework of dealuminated SiBEA zeolite evidenced by XRD generates Brønsted acidic sites as shown by FTIR of pyridine used as molecular probe. In CrxSiBEA as prepared, chromium is mainly present as octahedral Cr(III) and in calcined CrxSiBEA (773 K, 3 h, flowing air) as distorted tetrahedral Cr(VI), as evidenced by DR UV−vis, TPR, and XPS. Upon outgassing of calcined CrxSiBEA at 773 K for 1 h (10−3Pa), part of Cr(VI) is reduced to distorted tetrahedral Cr(V), as shown by EPR. Isolated distorted tetrahedral Cr sites present in activated CrxSiBEA are responsible for their catalytic activity in the oxidative dehydrogenation (ODH) of propane, performed in the presence of air or N2O. The propane conversion increases with Cr content in the presence of air or N2O, while the selectivity toward propene depends on the reaction temperature and oxidant. In the presence of air, the selectivity toward propene is highest for 1.0 Cr wt %. The lower selectivity for the content of 3.4 Cr wt % may be the result of the presence of some amount of extra-framework octahedral Cr sites. The lower propane conversion and the higher selectivity toward propene in the presence of N2O probably depend on the oxidation state of Cr under reaction conditions and on the nature of surface oxygen. The presence of isolated tetrahedral chromium sites with oxidation state lower than (VI) on activated CrxSiBEA is essential for maintaining a high selectivity.

The method (Dzwigaj, S., et al. Chem. Commun. 1998, 87) proposed earlier to incorporate V(V) ions into the BEA zeolite framework at the solid−liquid interface from V precursors in aqueous solution has been successfully extended to the solid−gas interface and titanium, with TiCl4vapor as the precursor. The use of TiCl4vapor has the advantage to restrict the speciation of titanium to this single species and to lead to a significant amount of Ti (∼5 Ti wt %) determined by chemical analysis. The incorporation of Ti into the SiBEA zeolite framework is evidenced by XRD. The reaction of TiCl4vapor with H-bonded and terminal SiO−H groups of vacant T-atom sites is monitored by FTIR, 29Si MAS NMR, 1H−29Si CP MAS NMR and 1H MAS NMR. The presence of tetrahedral Ti(IV) as the main titanium species is evidenced by diffuse reflectance UV−vis. A possible pathway for the formation of framework tetrahedral Ti(IV) in TiSiBEA is proposed.

This chapter is an update of an earlier version (Adv. Catal.44, 119 (1999)) devoted to the applications of photoluminescence (PL) spectroscopy to the characterization of solid surfaces in relation to adsorption, catalysis, and photocatalysis. This chapter concerns highly dispersed or bulk oxide‐containing catalysts, including a discussion of the effects of pressure and temperature on PL spectra of various materials. The aim of this chapter is to describe recent advances in the identification of catalytically active sites by PL spectroscopy and other spectroscopic techniques, such as IR, EPR, XAS (XANES and EXAFS), UV–vis, and NMR. One can thus obtain precise information about the nature and local environment of transition metal ions in zeolites and oxide and hydroxide ions on MgO surfaces. PL appears particularly useful in the case of zeolites and supported samples with low metal loadings (e.g., vanadium‐containing Siβ zeolite). Indeed, PL spectroscopy in static and dynamic modes allows one to distinguish, in VSiβ zeolite, three kinds of tetrahedral V(V) species, the relative amounts of which depend on vanadium content and calcination/rehydration treatments. PL, which is far more sensitive than UV–vis and 51V NMR spectroscopies, gives electronic and vibrational (fine structure and energy) information about each type of vanadium. Moreover, dynamic PL allows investigation of the chemical nature and properties of catalytically active surface sites in the working state and their role in photocatalytic reactions. This chapter provides a description of the advantages of PL techniques to characterize acidic and basic sites by means of probe molecules, as well as various materials used in catalysis and electroluminescence, including a consideration of future directions of research involving PL spectroscopy.

The dealumination of BEA zeolite by treatment with concentrated nitric acid is evidenced by X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. Two-dimensional 27Al 3Q and 5Q magic-angle-spinning (MAS) NMR allow the detection of two kinds of tetrahedral Al atoms whose relative amounts depend on the Si/Al ratio and which correspond to two specific T-sites. 29Si MAS NMR and 1H MAS NMR measurements confirm these results. 29Si MAS NMR spectra evidence two resonances at around −114 and −111 ppm ascribed to Si sites of the Si(OSi)4environment of two different crystallographic sites. Moreover, the presence of Si atoms associated with hydroxyl groups is confirmed by a resonance at −102 ppm when 1H−29Si CP is applied. The Brønsted and Lewis acidic sites in dealuminated BEA zeolites are evidenced by FTIR spectra of adsorbed pyridine which show two kinds of bridging hydroxyl groups (Si−O(H)−Al) of different acid strength. 51V MAS NMR confirms the incorporation of vanadium atoms into vacant T-atom sites of a fully dealuminated SiBEA zeolite leading to two kinds of tetrahedral V(V) sites (δiso= – 708 and – 766 ppm), with a V=O double bond and linked by V−OSi bonds to the framework. The two types of tetrahedral V(V) sites are in line with the two kinds of tetrahedral Al sites initially present in the zeolite. Moreover, the two bands at 3620 and 3645 cm−1suggest that VSiBEA also contains V(V) sites with V(V)O−H groups, which exhibit Brønsted acidic character as shown by FTIR of adsorbed pyridine. Possible ways for the formation of tetrahedral V(V) in the BEA structure are proposed.

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 defined as the number of magnesium neighbors. Otherwise, the observed excitation modes of OH groups rather correspond to perturbations of excitation modes of the related dehydroxylated surface. The emission process is followed experimentally by lifetime measurements, which enable one to visualize energy transfer between surface species.

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 (H> −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 (H< −0.7 ppm) are associated to isolated and hydrogen-bond acceptor twofold O2C−H and onefold O1C−H, whereas the central signal at H−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 characterized by a lower chemical shift than expected from calculations and also contribute to the central signal. Calculated thermal stabilities and chemical shifts suggest that these protons correspond to O4C−H on monatomic steps. The final assignment is fully consistent with previous experimental results on CD3OH adsorption and quantitative analysis of the evolution of spectra with temperature. This study illustrates the synergism between experiments and theory, by comparison with the results obtained by either one.

It is shown for the first time that the photoassisted reduction of NO by CO into N2and N2O can occur on TiO2catalysts upon visible light irradiation ( > 380 nm) at room temperature. The selectivity of photoreduction of NO into N2reaches 90−95%. The CO2formed which predominantly remains on the surface can be quantitatively desorbed after completion of the photoreaction by heating TiO2to ∼500 K. The rates of NO consumption and product accumulation remain virtually constant upon successive admissions of the CO−NO mixture thus indicating a high stability of catalyst activity. It is found that the quantum yield of NO photoreduction by CO is considerably greater for visible light irradiation ( 405 436 nm) than for UV irradiation ( 365 nm). Experiments with 18O-enriched NO revealed that, under visible light irradiation, an intense oxygen isotopic exchange between NO and TiO2develops. The photocatalytic reaction requires the presence in nonstoichiometric TiO2-xof electron-donor centers (Ti3ions, F and Fcenters) able to absorb visible light. A reaction mechanism is proposed which includes the following stages:  (a) NO photoadsorption along two parallel routes, by e-capture from the electron-donor center by NO to yield adsorbed NO-species and by NO interaction with the hole O-to give adsorbed nitrite NO2-; (b) reduction of NO-into N2O and further into N2; and (c) photoreduction of adsorbed nitrite NO2-into N2O and further into N2by CO which regenerates the donor centers. It is assumed that the photoinduced oxygen heteroexchange proceeds via adsorbed nitrite complexes.

The state of nickel ions in two BEA zeolites was studied by X-ray diffraction (XRD), temperature-programmed reduction (TPR), and Fourier transform infrared (FTIR) spectroscopy using CO and NO as probe molecules. One of the samples, NiAlBEA (0.9 wt % of Ni), was prepared by conventional ion exchange, and the other, NiSiBEA (1.0 wt % Ni), was prepared by a two-step postsynthesis method involving dealuminated SiBEA zeolite. No structural changes are observed with AlBEA after ion exchange with nickel. In contrast, the incorporation of Ni 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 the NiAlBEA sample Ni shows three TPR peaks in the region of 600−1100 K, whereas one sharp peak at 650 K with a shoulder at 750 K is observed with the NiSiBEA sample. Adsorption of CO at 100 K on NiAlBEA results in formation of Ni2−CO species (2200 and 2214 cm-1). The latter are partially converted, at high coverages, to Ni2(CO)2dicarbonyls (2206−2204 cm-1). Reduction of the sample with CO leads to generation of Niions. The latter form, with CO, different carbonyl species that are in equilibrium:  (i) Ni−CO (2113 cm-1), (ii) Ni(CO)2(sat 2138 cm-1and asat 2095 cm-1), and (iii) Ni(CO)3(2157, 2122, and 2113 cm-1). The polycarbonyl structures were proven by 12CO−13CO coadsorption. In line with these results, NO adsorption on NiAlBEA leads to formation of two kinds of nitrosyl species (1905 and 1897 cm-1). Adsorption of CO at 100 K on NiSiBEA leads to formation of several kinds of Ni2−CO species observed in the 2196−2170 cm-1region. The intensity of the bands was low, suggesting the majority of incorporated nickel ions are coordinatively saturated or inaccessible and thus not able to form carbonyls. Negligible amounts of Niions were present on the activated sample and formed various (poly)carbonyls. In this case the reduction hardly affected the state of the sample. Most of the nickel ions in NiSiBEA were not able to coordinate NO; the others formed nitrosyl complexes (1872−1845 cm-1) which were much less stable (up to 323 K) than those observed with NiAlBEA.

The thermal reduction of Co/Al2O3catalysts prepared by impregnation of Co(II) nitrate generally leads to migration of cobalt ions into alumina with formation of cobalt aluminate. In order to prevent the latter, it is attempted to reduce cobalt by decomposition of supported precursor salts containing ethylenediamine (en), a procedure successfully applied earlier to nickel. Alumina is impregnated with an aqueous solution containing Co(II) nitrate and two or three equivalents of en. After drying, several cobalt compounds are detected on the support by X-ray diffraction, diffuse reflectance UV−vis spectroscopy and X-ray absorption spectroscopy:  crystals of Co(III)(en)3(NO3)3whatever the initial en/Co stoichiometry, Co(II) and Co(III)−en complexes, grafted or not, as well as a Co(II)(OH)2-like phase. The composition of these phases reveals that dramatic evolutions in cobalt speciation have taken place in the impregnating solution, driven by the oxidation of complexed cobalt by dissolved oxygen and by the precipitation of the Co(III) salt. Decomposition of these phases in an inert gas atmosphere does not lead to metal particles exclusively, as is the case of nickel, but to a mixture of oxidic and metallic phases. A treatment in hydrogen at 500 °C completes cobalt reduction, leading to 6−20 nm metal particles. This rather broad size distribution is linked to the wide range of cobalt species on the support, which stems itself from the coordination chemistry of cobalt bonded to ethylenediamine in aqueous solutions.

Reaction of K10[α2‐P2W17O61] or K10[α1‐P2W17O61] or [Bu4N][OsCl4N] in a water/methanol mixture, and subsequent precipitation with (Bu4N)Br provided [α2‐P2W17O61{OsVIN}]7−and [α1‐P2W17O61{OsVIN}]7−Dawson structures as tetrabutylammonium salts. Reactions of [(Bu4N)4][α‐H3PW11O39] with either [ReCl3(N2Ph2)(PPh3)2] or [Bu4N][ReCl4N] are alternatives to the synthesis of [(Bu4N)4][α‐PW11O39{ReVIN}]. 183W and 15N NMR, EPR, IR, and UV‐visible spectroscopies and cyclic voltammetry have been used to characterize these compounds and the corresponding [(Bu4N)4][α‐PW11O39{OsVIN}] Keggin derivative.

This contribution presents the molecular approach used to investigate the different preparation steps of important catalytic systems, based on Mo, Ni, and Co. The first preparation step consists of impregnating oxide supports by aqueous solutions of transition-metal complexes (TMCs), called precursor complexes, and involves the liquid-solid (wet) interface. The following steps consist of thermal activation treatments, involving the gas-solid (dry) interface. The molecular approach is based on (i) the use of the TMC to monitor and probe the nature of its own interactions with the oxide support, (ii) the selection of the most appropriate techniques to monitor such interactions, and (iii) the design of experiments to identify and isolate the key parameters controlling the "TMC-oxide support" interactions.

EPR studies using naturally abundant and 95Mo-enriched molybdenum have shown that reduced MoOx/SiO2catalysts with different dispersion of molybdenum provide a unique opportunity for molecular-level investigations of surface phenomena involved in the bonding to and activation of small molecules by oxide surfaces. Different types of electron transfer (ET) processes accompanying the activation of O2and N2O as well as the reactivity of O-and ⋅CH2OH transients with coadsorbed CH3OH and O2, respectively, have been identified. They include nondissociative and dissociative ET, surface intramolecular ET, and electroprotic transformation (ET coupled with proton transfer). In the case of methanol dehydrogenation it was found that if the reaction occurs on the MoOxcluster centers, ET exhibits a complementary character, in contrast to isolated Mo centers where noncomplementary ET accompanied by migration of the hydroxymethyl intermediate is observed.

In calcined and rehydrated VSiß zeolite prepared following a method described earlier, three kinds of tetrahedral VVspecies can be clearly identified by photoluminescence spectroscopy even at low V content, in contrast to 51V NMR and diffuse reflectance UV-Visible spectroscopies which do not allow them to be distinguished. A comparative study by photoluminescence and 51V NMR spectroscopies leads to the suggestion that there is a correlation between the vibrational energy of the V=O bond and the corresponding chemical shift of the 51V NMR signal.

In the light-off temperature range of methane total oxidation (250–600°C), two types of Pt/δ-Al2O3catalysts prepared from H2PtCl6or Pt(NH3)4(OH)2show different catalytic behaviors. Initially very active, fresh chlorine-free catalysts deactivate in isothermal conditions of reaction, while fresh chlorine-containing catalysts, initially poorly active, undergo an activation with time on stream. Consequences of dechlorination of chlorine-containing catalysts and of impregnation by various amounts of NH4Cl on both types of catalysts are examined. Chlorine is responsible for the inhibition observed on ex-H2PtCl6catalysts, which could be attributed, more specifically, to the chloride ions remaining at the platinum–support interface after reduction of platinum. After aging of the catalysts under reaction, sintering of the platinum particles is observed on all samples, regardless of their preparation mode. This phenomenon is linked to the presence of oxygen in excess in the gas phase and can account for the deactivation of chlorine-free catalysts. The water produced during the reaction removes the inhibiting chlorine on all chlorine-containing catalysts. The activity of the largest sintered particles is shown to vary with the nature of the fresh catalyst, the aged ex-H2PtCl6catalysts giving the best results: the molecular nature of the fresh catalyst would also affect the activity of the stable aged catalyst. Both the initial presence of chloride ions at the metal–support interface and the composition of the reacting mixture are key factors to this positive reconstruction.

A tungstated zirconia catalyst containing approximately enough tungsten oxide to give a theoretical monolayer on the zirconia support was characterised by Raman spectroscopy, temperature-programmed reduction, EPR and IR spectroscopies. The data show that treatment of the catalyst in H₂ at temperatures as low as 470 K led to partial reduction; EPR spectra indicated the formation of W⁵⁺ centres in two locations: in the bulk of tungstate clusters dispersed on the zirconia surface and at the surfaces of these clusters and consequently accessible to gas-phase reactants. When 1% n-pentane in N₂ flowed through a bed of particles of the catalyst in an EPR tube at 523 K for 20 min, the following surface species formed: W⁵⁺ centres (shown by EPR); organic radicals (shown by EPR); and OH groups (shown by IR). The results indicate that n-pentane undergoes a homolytic C–H bond cleavage reaction such as the following followed by one-electron transfer steps that yield surface W⁵⁺ ions, OH groups, and chain carriers in the catalytic isomerisation of n-pentane, inferred to be carbenium ions. These processes are considered to be non-catalytic redox initiation reactions that explain the promoter role of the tungsten in the catalyst.

This review deals with the applications of photoluminescence techniques to the study of solid surfaces in relation to their properties in adsorption, catalysis, and photocatalysis. After a short introduction, the review presents the basic principles of photoluminescence spectroscopy in relation to the definitions of fluorescence and phosphorescence. Next, we discuss the practical aspects of static and dynamic photoluminescence with emphasis on the spectral parameters used to identify the photoluminescent sites. In Section IV, which is the core of the review, we discuss the identification of the surface sites and the following: coordination chemistry of ions at the surface of alkaline–earth and zirconium oxides, energy and electron transfer processes, photoluminescence and local structure of grafted vanadium oxide, and photoluminescence of various oxide-based catalysts. In Section V, we present a summary of the close relatiohship between the dynamics of the photoluminescence and the reactivities of the catalysts. Sections VI and VII deal with the applications of photoluminescence to adsorption and catalysis and discuss the case of both bulk and supported oxides. Section VIII is devoted to photocatalysis, again on bulk and supported catalysts. Sections IX and X briefly review the application of photoluminescence to chemical detection and its relation with other techniques. The last section concludes the work covered in the review and gives the most likely future directions of research work involving photoluminescence spectroscopy.

Methane oligomerization to alkanes can be accomplished on supported platinum via a two-step procedure: formation of carbonaceous species on the metallic surface by methane adsorption, followed by hydrogenation of these species. Temperature-programmed oxidation (TPO) experiments performed after hydrogenation steps of various durations show that the hydrogenation of a carbonaceous deposit obtained at 300°C on the reference Pt/SiO2catalyst EUROPt-1 is not a fast process. Two groups of surface carbonaceous species have been characterized through their different reactivities toward oxygen, but at 300°C their reactivities toward hydrogen are similar. Among alkanes up to C5, methane is the main product of hydrogenation, corresponding to one-half of the surface carbon reactive toward hydrogen; linear and branched alkanes are produced from the other half of the reactive carbonaceous species. On EUROPt-1, mainly ethane andn-pentane are produced during the first minutes of reaction, while on a sintered catalyst the initial production inn-pentane is negligible. The release ofn-pentane during an intermediate purge with inert gas on EUROPt-1 shows that C–C bonds can form already during methane adsorption, leading to C5precursors on specific active sites of this catalyst maybe coordinately unsaturated platinum atoms. A model of formation of C5precursors is proposed by analogy with the organometallic chemistry of molecular hydrocarbon platinacycles. The subsequent production of alkanes (C2>C3>C4>C5) could be described through a statistical model of dynamic coupling between carbonaceous species involving hydrogen, rather than by hydrogenolysis of heavier carbonaceous species. However, this latter mechanism is likely to predominate for the production of C6–C8compounds.

The compositions of Pt complexes adsorbed on γ-alumina from acidic H2PtCl6solutions and their transformations upon drying, rewetting, and calcination were studied using 195Pt NMR, UV–Vis diffuse reflectance, EXAFS, and Raman spectroscopy. Two acidic H2PtCl6solutions (at pH 1.15 and 2.8) were used to impregnate unmodified and HCl treated Al2O3. On the basis of 195Pt NMR measurements, platinum is found to be adsorbed on alumina as [PtCl6]2−and [PtCl5(OH)]2−, the latter species being produced by deprotonation of [PtCl5(H2O)]−present in the initial solutions. Depending on the pH of the impregnating H2PtCl6solutions and alumina pretreatment, the 195Pt chemical shift (δPt) varies from −6.5 to 0.1 ppm for [PtCl6]2−and from 638 to 660 ppm for [PtCl5(OH)]2−(δPt[PtCl6]2−=0 ppm in the H2PtCl6solution at pH 1.15). The [PtCl6]2−/[PtCl5(OH)]2−ratio depends on the pH of the impregnating solutions, the alumina pretreatment, and the impregnation time. The adsorbed Pt anion complexes are held on the surface by electrostatic interaction with positively charged protonated hydroxyls. No marked grafting of the adsorbed Pt species occurs upon impregnating the unmodified or HCl treated aluminas with the H2PtCl6solution at pH 1.15, whereas a certain part of these species becomes grafted when unmodified alumina is brought into contact with the H2PtCl6solution at pH 2.8. After removal of physisorbed water by drying at 20°C, the NMR signals are no longer observed. The UV–Vis, Raman, and EXAFS data evidence a modification of the Pt coordination sphere upon drying at 20°C which can be explained in terms of a Cl−ligand exchange with neighboring alumina hydroxyls, i.e., grafting. The ligand exchange can be reversed by hydration of the dried solids accompanied by the reappearance of the initial NMR signals. Upon drying H2PtCl6/Al2O3at 90 or 150°C, the number of Cl and O atoms in the Pt coordination sphere does not change markedly, but the adsorbed Pt species become irreversibly grafted to the surface. No ligand exchange and no grafting of [PtCl6]2−adsorbed on the HCl treated alumina was found to occur upon drying at temperatures up to 150°C.

The superoxide radical anion O2-is both an important intermediate in heterogeneous catalytic oxidation and a useful probe for positive charges in ionic solids, such as metal oxides and zeolites. The paper illustrates the main circumstances under which stable superoxide anions are formed at surfaces: (i) direct surface–oxygen electron transfer; (ii) photoinduced electron transfer; (iii) surface intermolecular electron transfer; (iv) decomposition of hydrogen peroxide.

Abstract: La2O3 catalysts prepared at 923 K (La2O3-LT) and 1073 K (La2O3-HT) exhibit different photoluminescence properties due to notably different concentrations of ions in position of low coordination at the surface or coordinatively unsaturated surface sites (cus). The catalyst which exhibits the higher yields of photoluminescence due to the higher concentration of cus corresponds to the one which gives the higher C2+ selectivity in the oxidative methane coupling reaction.

EPR spectroscopy is a powerful tool to identify at a molecular level, the different steps of catalyst preparation, and of catalytic reactions:(i)Deposition of paramagnetic transition metal ions onto a support is monitored, and the coordination sphere of the metallic center is characterized by EPR.(ii)The catalyst is also characterized after activation (thermal oxidation or reduction):- the distribution among the different sites in zeolites can be determined;- the dispersion of the active phase may be appreciated;- the unsaturation degree of the active site may be evaluated using probe molecules such as water or13C enriched carbon monoxide.(iii)The catalytic mechanisms can be investigated by studying the elementary steps of the catalytic reaction, as illustrated for methanol oxidation over Mo/SiO2 catalysts whose EPR results have extended the reaction mechanism proposed on the basis of kinetic data. In addition, reaction intermediates may be isolated inquasi-in situ conditions as in the case of olefin oligomerization catalyzed by Ni/SiO2 systems.

Metals constitute a wide class of catalysts, and because catalysis occurs on the surface, there is an economic incentive, especially for precious metals, to obtain catalysts in the form of small metal particles. This, however, raises two main problems. One is fundamental in nature and addresses the question as to below which particle size the metallic properties are lost. The other is more practical and concerns the preparation and characterization of very small particles and their catalytic activity. The other problem concerns the practical ways of producing metals with particle sizes around the critical value calculated above. The easiest and most practical way to achieve this goal is to deposit the metal in low concentration on a high-surface-area support. The main purpose of using a carrier is, of course, to achieve a high dispersion of the metal and to stabilize it against sintering. In a number of reactions, however, the metal support is not inert and the overall process is actually a combination of two functions: that of the metal and that of the catalytically active support.

Cu2+ions supported onto SiO2(Cu2+/SiO2) prepared by an ion-exchange method are reduced to Cu+ions when the Cu2+/SiO2sample is evacuated above 573 K. Cu+/SiO2catalyst decomposes NO photocatalytically and stoichiometrically at 275 K. The excited state of the Cu+ions plays a significant role in the photocatalytic decomposition of NO on the Cu+/SiO2catalyst.

Substantial differences in photocatalytic reactivity were observed on powdered TiO2and ZnO, both Pt-loaded and unloaded, in the photocatalytic hydrogenation of CH3C≡CH and C3H6in the presence of H2O. Similar reactivity was observed, however, in the photocatalytic oxidation of i-C4H8and C3H6 by O2on these same catalysts. Photoluminescence of TiO2and ZnO in the presence and the absence of H2O and/or O2indicate that H2O causes a downward band bending to increase the efficiency of recombination of photogenerated electrons and holes, with a greater effect being observed on ZnO than on TiO2. These results, together with the direct detection of reaction intermediates by ESR, are rationalized by differences in the yields of the photogenerated intermediates, i. e., H (formed from trapped electrons) and OH (formed from trapped holes). The steady state concentrations of these intermediates depend on the degree of the positively-charged adducts on the surfaces of these oxides. The addition of O2causes an upward band bending of the oxide and suppresses recombination of photo-generated electrons and holes, by formation of negatively-charged surface adducts. With similar degrees of band bending, TiO2and ZnO produce almost the same photocatalytic reactivities as are observed for these oxides as dry systems.

The oxidative dimerisation of propene to 1,5-hexadiene has been investigated on Bi-Zn-O catalysts. The Bi48ZnO73 phase, observed in the catalysts calcined at 700 ° C is an active and selective catalyst for the formation of 1,5-hexadiene. The best catalytic performance (1,5-hexadiene selectivity ˜ 64%) has been obtained at 525 ° C, with a propene to oxygen ratio of 26, on a catalyst formed by Bi48ZnO73 with a small excess of ZnO.

The doping of lanthanum oxide with strontium maintains the good selectivity of the oxidative methane coupling for the catalysts prepared or calcined at high temperature (> 800 °C) by preserving the platelet shape of oxide particles.

ESR studies of the photoreduction of Ti-oxide supported onto porous Vycor glass showed that anchored samples exhibit only one type of Ti3+ions, which are in position of low coordination. On the other hand, impregnated catalysts show several types of Ti3+ions with different coordinations.

The fluorescence, fluorescence excitation, and time-resolved fluorescence spectra of pyrene molecules adsorbed on calcinated (773 K) Vycor glass (average pore diameter, 40 Å) have been investigated as a function of the amount of adsorbed pyrene. The results of the analyses of these spectra and multi-component decays of fluorescence suggest that there are three fluorescent species with different decay times on the surface of Vycor glass. Their lifetimes are determined to be 303–355, 100–145, and 19–48 ns. These decay components are reasonably assigned to the emissions of monomeric, dimeric, and excimer-like species in the order of decreasing the lifetimes. A 26600 cm-1peak appearing at the first stage of the time-resolved fluorescence spectra is assingned to the 0–0 hand of dimeric pyrene based on the results reported by Yamazaki et al. in the Langmuir-Blodgett monolayer.

The photocatalytic decomposition of NO into N2and CO2on Mo-MCM-41 was found to proceed efficiently in the presence of CO. In situ photoluminescence measurements demonstrated that this reaction proceeds in a redox cycle between alternating Mo6+and Mo4+ions. The yields of N2formation in the photocatalytic reaction correspond with the yields of the photoluminescence of the tetrahedrally coordinated Mo oxide species, indicating that the charge transfer excited triplet state of the tetrahedral Mo oxide species plays a significant role in this reaction, leading to the formation of N2and CO2with a good stoichiometry.

This chapter describes the experiences of John Turkevich. John Turkevich made major contributions to several fields of human endeavor. He wrote several authoritative publications and articles in popular magazines on Russian science, particularly chemistry. He also lectured on government and science at the famous Woodrow Wilson School at Princeton. He contributed to public service in an exceptional way, serving both his country and science. He was an expert in Soviet science and frequently advised the U.S. government and congressional committees on atomic energy and foreign affairs. He served on the Manhattan Project for the development of atomic energy, providing the first measurement and analysis of the infrared spectrum of uranium hexafluoride, a key step for its purification and isotopic enrichment. Professor Turkevich had a distinguished research career, starting in 1936 with the discovery in Sir Hugh Taylor's laboratory of the reforming of linear heptane on chromium oxide.

Breaking the impasse: Earlier this year Maschmeyer and Che outlined an approach for normalizing and comparing the apparent quantum yields (AQY) for hydrogen photogeneration catalyzed by semiconductor powders. In the plot of photocatalyst concentration versus AQY they proposed that the linear region was the most appropriate region for comparison. In his Correspondence Kisch refutes this proposal and states that the area at the onset of the plateau region (B) in a plot of catalyst concentration versus reaction rate is relevant. Maschmeyer and Che explain their rationale and conclude that the discussion has arisen because of different emphases in studying photocatalysis.

Cr-containing mesoporous silica molecular sieves (Cr-HMS) containing tetrahedrally coordinated isolated chromium oxide (chromate) moieties can operate as an efficient photocatalyst for the decomposition of NO and the partial oxidation of propane with molecular oxygen under visible light irradiation.

Investigations of the mesoporous Mo-MCM-41 molecular sieves have shown that they include the tetrahedrally coordinated Mo-oxide species, which exhibits a photoluminescence spectrum attributed to the radiative decay from the charge transfer excited state. Also, the photocatalytic reaction of NO into N2was found to proceed much more efficiently in the presence of propane than in the absence of propane. The dynamic quenching of the photoluminescence spectrum by the addition of NO or propane indicates that the excited state of the highly dispersed Mo-oxide species having a tetrahedral coordination plays a significant role in this reaction.

Review: 83 refs.