Proc. R. Soc. A (2012) 468, 2113–2128doi:10.1098/rspa.2012.0139
Published online 18 April 2012
Elaboration, characterization and propertiesof silica-based single-site heterogeneous
photocatalystsBY MICHEL CHE1,2,*, KOHSUKE MORI3 AND HIROMI YAMASHITA3
1Institut Universitaire de France, 103 Boulevard Saint-Michel,75005 Paris, France
2Laboratoire de Réactivité de Surface, Université Pierre et MarieCurie-Paris 6, CNRS-UMR 7197, Paris, France
3Division of Materials and Manufacturing Science, Graduate School ofEngineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
This study concerns single-site heterogeneous photocatalysts, composed of isolatedtransition metals ions (single-sites) dispersed on silica-based supports, which can be usedas photocatalysts. The first part gives the definition of single-site heterogeneous catalysts.The main difficulty to prepare the latter resides in the speciation of the key elements(metal of the catalytically active phase and oxygen of the solid support) and otherspecies (notably counter ions and ligands) the presence of which depends on the synthesismethod adopted. The best preparation methods as well as the ideal features of single-site heterogeneous photocatalysts are discussed before the study focuses on single-siteheterogeneous photocatalysts. Case studies are presented with photocatalysts involvingtitanium and chromium, and their photocatalytic reactions such as CO2 reduction withH2O, degradation of organic pollutants diluted in water, NO decomposition and selectiveoxidation of propane are also described.
1. Definitions of single-site homo- or heterogeneous catalysts
On the basis of Taylor’s seminal paper on ‘A theory of the catalytic surface’(Taylor 1925), Boudart (1997) distinguished two extreme cases of catalyst: onein which all surface metal centres are active but with different activities, and theother where all the metal centres are active but possess the same activity. Thelatter case corresponds to molecular catalysts or single-site catalysts (vide infra).Although Taylor (1925) referred mainly to metal surfaces, his conclusions couldbe extended to catalysts where the active phase (a transition metal oxide or ametal) is dispersed on a solid oxide. In such catalysts, e.g. VOx/Al2O3 or Pt/SiO2,the catalytically active element, hereafter referred to as metal centre, is V orPt, respectively.*Author for correspondence ([email protected]).
One contribution of 14 to a Special feature ‘Recent advances in single-site heterogeneous catalysis’.
The term single-site was coined quickly after zirconocene-based catalystsinvolving methylalumoxane were discovered to exhibit unprecedented activityand selectivity in olefin polymerization (Andresen et al. 1976; Sinn & Kaminsky1980). By contrast to heterogeneous Ziegler–Natta catalysts, polymerizationby such homogeneous metallocene-based catalysts takes place at a single typeof metal centre with a defined coordination environment (Brintzinger et al.1995), hence the name of single-site catalysts. This has allowed to correlate thestructure of the metallocene single-site to the physical and chemical propertiesof the polymer (molecular weight, stereochemical microstructure, crystallinity,mechanical characteristics, etc). With such catalytic systems, it has becomepossible to efficiently control regio- and stereo-regularities, molecular weightand weight distribution and comonomer incorporation. Both homogeneous(Coates 2000) and heterogeneous (Hlatky 2000) single-site olefin polymerizationcatalysts have been reviewed.
In the context of heterogeneous catalysts, the single-sites have been definedby Thomas et al. (2005; p. 6459) as follows: ‘consisting of one or more atoms,the single-sites are spatially isolated from one another, with no spectroscopic orother cross-talk, have the same energy of interaction with reactants, are well-characterized, in the same way as the single-sites are in homogeneous catalysts’.From the preceding discussion, one would expect all heterogeneous single-sites toexhibit the same catalytic activity and selectivity.
As mentioned earlier (Boudart 1997), the best example of heterogeneous single-sites is illustrated by zeolites such as H-ZSM-5 (MFI) for the catalytic cracking ofn-hexane (Haag et al. 1984). By contrast to X and Y acidic zeolite catalysts, thealuminium concentration in ZSM-5 is low enough to represent a dilute solution.It can nevertheless be varied over several orders of magnitude (from ca 20 toa few 10 000 ppm). For ZSM-5, the active sites are the protons associated withtetrahedral Al atoms and Haag et al. showed that the activity was proportionalto the number of Al atoms, in agreement with the homogeneity of active sites.The absolute turnover rates and numbers obtained were found to be comparablewith enzymatic turnover numbers. This is extensively discussed in a recent bookby Thomas (2012) who pioneered the field of single-site heterogeneous catalystsreferred to as SSHCs.
We now wish to discuss the difficulties encountered in the preparationof SSHCs.
2. Problems inherent to the preparation of single-site heterogeneous catalysts
(a) Speciation
In the case of heterogeneous catalysts, the preparation of which generally starts bythe initial deposition on an oxide support of a metal precursor of the catalyticallyactive phase from an aqueous solution, the obtaining of single-sites is a challenge.This mainly arises from problems related to the so-called speciation, referred to as‘the distribution of an element among defined chemical species in a given system’(Templeton et al. 2000; p. 1456) where chemical species are ‘the specific formof an element defined as to isotopic composition, electronic or oxidation state,and/or complex or molecular structure’.
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Figure 1. Speciation of Pt in the system H2PtCl6 (5 × 10−3 M) in acidic aqueous solutioninvestigated by 195Pt NMR at room temperature. The species, its chemical shift d (upper number)and intensity (italics) are given for each peak (after Boujday et al. 2003a). (Online versionin colour.)
The notion of speciation can be applied, for example, to an acidic aqueoussolution of H2PtCl6 (mostly used to prepare reforming catalysts). The 195PtNMR spectrum obtained (figure 1) qualitatively illustrates the speciation ofplatinum, i.e. its distribution among several octahedral Pt(IV) complexes. Here,the chemical shift is the fingerprint of the first coordination sphere of Pt(IV).This NMR spectrum is strongly altered when an oxide support, e.g. silica(Boujday et al. 2003a) or alumina (Shelimov et al. 1999), is introduced in theH2PtCl6 solution.
(b) Secondary speciation
While this definition is straightforward for species containing only one metalcentre, this is no longer the case for molecular species containing severalmetal centres. As a matter of fact, the same element can be distributedamong different defined environments in the same species. For instance, inthe polyoxometallate [Mo6O18(NNC6H4-m-NO2)]3− (Bustos et al. 2010) whichhas the well-known Lindquist structure (figure 2), i.e. an overall octahedronmade of six Mo octahedra (Gouzerh & Che 2006), molybdenum is presentin three different states characterized by specific 95Mo NMR chemical shifts:one Mo(VI) in axial position, four Mo(VI) in equatorial positions and oneMo(II) bonded to the NNC6H4-m-NO2 moiety. These data clearly indicatethat there exists a ‘secondary’ speciation of molybdenum within the single[Mo6O18(NNC6H4-m-NO2)]3− species. The same holds true for the ‘secondary’speciation of oxygen in the Lindquist structure [Mo6O19]3− (figure 2), one ofthe molecular models of oxide catalysts (Masure et al. 1989). In the absenceof adsorbed oxygen species (Che et al. 1974), there are three types of oxygen:six (black) in terminal molydenyl Mo=O position, 12 (red) in bridging M−O−Mposition and one (hidden) at the centre of the overall octahedron.
The notion of speciation can be also extended to the surface of an oxide. For thesake of simplicity, let us consider the surface of MgO, with the simple rock salt(NaCl) structure, where each ion is surrounded by six ions of opposite charge.Depending on experimental conditions, the surface of MgO exhibits a complex
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Figure 2. The [Mo6O19]n− Lindquist structure (Oh symmetry) is a compact arrangement ofsix-edge-shared MoO6 octahedra (also known for M = W, Nb, Ta). There are three types ofoxygen: terminal metal-oxo (black), bridging (red) and central (hidden at the centre of the overalloctahedron) (adapted from Gouzerh & Che 2006). (Online version in colour.)
distribution of surface hydroxyl groups and oxide ions of different basicities(Bailly et al. 2005). Even if surface hydroxyl groups have been carefully removedby high temperature outgassing, the speciation of oxygen remains complex withvarious concentrations of oxide ions in positions of low coordination (LC) O2−
LC,with LC = 5C, 4C, 3C for an oxide ion on a face, edge or corner, respectively(figure 3; Chizallet et al. 2007; p. 6445). The case of silica, see title of this study,is even more complex because of its amorphous nature with the surface exhibitingSiO4 tetrahedra linked via Si−O−Si bridges to form −(Si−O−)n cycles (n = 3–8)and silanol Si−OH groups of different types: isolated (I), geminal (II) or vicinal(III) (Boujday et al. 2003b; figure 4).
(c) Preparation methods
In addition to speciation problems related to the metal precursor solution andsolid oxide support, the two main components involved in catalysts preparation,a further complication comes from the preparation method adopted. As a matterof fact, when the latter does not involve a washing step, both counter ions andmetal precursor complexes remain on the support surface. Counter ions andligands can act, to different extent, as oxidizing (e.g. NO−
3 ) or reducing agent (e.g.ethylenediamine, en = NH2CH2CH2NH2), respectively, particularly upon dryingand further thermal treatment of the catalytic system (Negrier et al. 2005),making the obtaining of single-sites problematic.
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Figure 3. The speciation of oxygen illustrated at the surface (001) plane of MgO. Red circlesrepresent Mg2+ ions, and green circles represent oxide O2− ions. Surface defects (steps, kinks andcorners) provide sites for oxide ions in positions of low coordination O2−
LC, with LC = 3C, 4C, 5C(adapted from Chizallet 2007). (Online version in colour.)
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Figure 4. Model of the surface of amorphous silica showing −(Si−O−)n cycles with n = 3–8 andsilanol Si−OH groups of various types: isolated (I), geminal (II) and vicinal (III) (adapted fromBoujday et al. 2003a,b). (Online version in colour.)
In this context, the question now arises as how to prepare catalysts with allidentical and well-separated single-sites in view of the complexities describedearlier. One of the most elegant answers to this question has been givenby Thomas (2012; personal communication) with a strategy which can besummarized as follows: ‘To design new solid catalysts take high-area porous solidsof the appropriate kind and ‘sprinkle’ spatially isolated active centres over theentire (internal) surface area’. The reason of taking open-structure solids (suchas zeolitic or mesoporous materials) is that they offer large specific surface area,
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most of it being developed by internal cavities to favour catalytic reactions insuch confined space. In elegant studies, Thomas and co-workers demonstratedhow to graft appropriate catalytic entities such as metallocenes to the wallsof mesoporous silica (Maschmeyer et al. 1995). By varying the pretreatmenttemperature, the concentration of the grafting SiOH (silanol) sites can becontrolled, thereby offering the possibility to control in turn the concentrationof catalytic entities.
Catalysts preparation methods can be subdivided into three main categories:(i) those where single-sites are fixed by grafting/anchoring (Averseng et al. 2008)to a preformed oxide support; (ii) those where the support is manipulated beforegrafting/anchoring of single-sites takes place and (iii) those where the support andsingle-sites are prepared simultaneously. The various cases can be illustrated by:
— the grafting of metallocene to mesoporous silica following Thomas’strategy mentioned earlier (Maschmeyer et al. 1995);
— the two-step post-synthesis method (Dzwigaj et al. 1998) which consistsof the first step to remove aluminium from a zeolite with simultaneousformation of framework vacancies and silanol nests and; in the second step,to react the latter with a molecular complex, say TiCl4, so as to locatetitanium in the framework position previously occupied by aluminium.Thus, titanium ions are in the same framework environment and wellseparated; and
— the coprecipitation and sol–gel methods where both support and single-sites are formed together. The methods of type iii unfortunately lead tometal ions such as Mg2+
LC (LC = 5C, 4C, 3C) in various environments forthey will be distributed among various surface sites as discussed in thecase of their oxide ion analogues (here we consider the green spheres thatrepresent metal ions; figure 3).
(d) Grafting and anchoring
Because both grafting and anchoring are effective means to synthesize, we nowwish to give the proper definitions of these terms (Averseng et al. 2008).
Grafting and anchoring both refer to chemical bonding of a metal-containingmolecular species or a metal complex to a surface. Thus, physisorbed species(hydrogen-bonded, van der Waals-sorbed or entrapped species, etc.) or purelyorganic species are excluded. With this in mind:
— grafting refers to the situation where one/several group(s) of the supportsurface is/are chemically bonded to a metal centre, these groups being partof its inner coordination sphere and thus acting as ligands. The graftingresults in electron sharing between the surface and the metal centre—asituation that modifies the features of the metal centre (such as symmetry,coordination number, oxidation or even spin state);
— while anchoring refers to the situation where the metal centre of acomplex is connected to the surface by a linker via a series of covalentbonds. As a consequence, there cannot be any electron doublet sharedbetween the metal centre and the surface group. The ligand(s) involvedin anchoring is/are most often referred to as ‘linker(s)’, ‘tether(s)’ or
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‘spacer(s)’. In anchored species, the coordination sphere of the metal centreis not or little altered compared with the precursor complex in solution orin the gas phase.
These definitions allow to clearly distinguish between grafting and anchoring inthe case of mononuclear species, but this distinction becomes more difficult forpolynuclear species.
For those cases, the reader is referred to the reference (Averseng et al. 2008).
3. Elaboration and characterization of silica-based single-site heterogeneousmaterials as attractive photocatalysts
Among single-site heterogeneous catalysts, an important part concerns silicamatrices, for the latter allow to keep the unique coordination geometry of theirhomogeneous analogues. To design efficient (Shannon et al. 1998), the silicamatrix should (i) exhibit a large specific surface area, (ii) offer sufficiently largecavities and channels so as to favour the access of reactants to the single-sitesand the removal of products, thus avoiding diffusion problems (Maschmeyer &Che 2010), and (iii) be transparent to light. Silica-based zeolites and mesoporoussilicas are attractive materials because they possess such characteristics. It willbe shown thereafter that their suitable functionalization by transition metalcomplexes can transform them into unique and efficient photocatalysts.
The unique and fascinating properties of silica-based zeolites and mesoporoussilica have opened up new possibilities for many chemical and physical processes(Thomas 1999; Thomas & Sankar 2001; Yamashita et al. 2008). Because they aretransparent to UV–visible light, silica materials have often been functionalizedwith elements such as Ti, Cr, V, Mo and W (Yamashita et al. 1996, 2001,2002; Ikeue et al. 2001) whose corresponding bulk oxides are known to besuitable photocatalysts. These well-defined active centres have shown to be highlydispersed at the atomic level in tetrahedral-coordination and have been namedas ‘single-site heterogeneous photocatalysts’. The single-sites can be implantedinto silica matrices by various techniques, including hydrothermal synthesis,sol–gel method and chemical vapour deposition, and are stable, providing thesilica matrix keeps it porous structure. In the case of single-site heterogeneousphotocatalysts, excitation by light of Mn+–O2− pairs induces a metal-to-oxygenelectron transfer as follows (Yamashita & Anpo 2003):
[Mn+ − O2−] + hn → {M(n−1)+ − O−}∗, (3.1)
resulting in the formation of an exciton, i.e. an electron-hole pair constitutedby electron (M(n−1)+) and hole (O−) centres. This exciton is essentially free tomigrate through the lattice. When defects are present such as surface ions, theexciton is now bound to such defect and is equivalent to an excited electronicstate of the defect. These charge-transfer-excited states, i.e. the electron-holepair state which are quite close one to each other compared with the electronsand holes generated in titanium oxide (TiO2), exhibit unique activity in variousphotocatalytic reactions.
The steric and electronic properties of single-site heterogeneous photocatalystsplay an important role in determining the reaction rate as well as the
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Figure 5. Typical characterization techniques for investigating the local structure of single-siteheterogeneous catalysts.
selectivity. The detailed local atomic structures, including coordination geometryand oxidation state can be determined by means of various physico-chemicaltechniques (Che & Védrine 2012), such as X-ray absorption spectroscopy (XAS),UV–vis absorption, photoluminescence, electron paramagnetic resonance (EPR),Fourier-transform (FT)-IR and solid-state MAS NMR, in addition to densityfunctional theory calculations (figure 5). The hydrophilic–hydrophobic characterof the sample surface is also important in orienting the catalytic properties, andcan be evaluated by contact angle measurements.
It is important to stress the importance of the relationship existing betweenthe local structure of active single-sites and their photocatalytic propertiesbased on information obtained from XAS and photoluminescence measurements.XAS being element-specific is one of the most powerful tools to monitor thedynamics of electronic and structural features, particularly for non-crystallinesolids. Photoluminescence is not only powerful but also highly sensitive incharacterizing the nature of charge-transfer-excited states and their interactionswith gaseous reactants. The power of such spectroscopic characterization hasbeen validated in showing the influence of pore size, channel structures anddimensions, and the hydrophilic–hydrophobic character of silica-based zeolitesand mesoporous silicas on their catalytic properties.
4. Case study of titanium single-site heterogeneous photocatalysts
Ti-oxide was introduced in Y-zeolite (SiO2/Al2O3 = 5.5) by ion-exchange with anaqueous solution of titanium ammonium oxalate and noted Ti-oxide/Y-zeolite
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Figure 6. (a) Ti K-edge XANES and (b) FT-EXAFS spectra of TiO2 powder (a,a′) andTi-oxide/Y-zeolite (b,b′).
(1.1 wt% as TiO2). Figure 6 shows the Ti K-edge XANES spectra of TiO2 powder(P 25: anatase 80%, rutile 20%) and Ti-oxide/Y-zeolite. TiO2 gives rise toseveral well-defined pre-edge peaks assigned to Ti in octahedral symmetry. Bycontrast, Ti-oxide/Y-zeolite exhibits an intense single pre-edge peak at 4967 eVcorresponding to dipolar-allowed transitions from the 1 s to t2 molecular levelsbuilt from 3d and 4p metal orbitals and from a neighbouring orbital (Yamashitaet al. 1998a,b). This intense single pre-edge peak suggests that Ti-oxide moietiesin Ti-oxide/Y-zeolite are tetrahedral. In FT-EXAS spectra, the peak suggestiveof – (Ti−O−Ti)− oligomers was observed in TiO2, whereas Ti-oxide/Y-zeoliteshows a strong peak only at around 1.6 Å assigned to oxygen neighbours owingto Ti−O monomers. The inverse FT of the main peaks is well fitted using onlya Ti−O bond with a coordination number of 3.7 and an inter-atomic distance of1.78 Å. These are clear evidence of the presence of isolated Ti-oxide species.
The affinity for water and the photocatalytic properties of Ti-containingzeolites significantly depend on their hydrophilic–hydrophobic character(Yamashita et al. 2002; Kuwahara et al. 2008, 2009). The hydrophilic (Ti-BEA(OH)) and hydrophobic (Ti-BEA (F)) zeolites can be synthesized using OH− andF− media, respectively. The interaction of Ti-oxide moieties with H2O moleculescan be investigated in situ by XAS, because any change in the coordination of Tiaffects the Ti K-edge XANES region. As shown in figure 7, the addition of waterto Ti-BEA zeolites strongly decreases the intensity of the pre-edge peak andshifts it to higher energy, indicating that the coordination of Ti increases fromfour to five and finally to sixfold. The changes in peak intensity and positionare more pronounced for Ti-BEA (OH) than for Ti-BEA (F), suggesting that
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Figure 7. Plots of normalized height versus energy of the Ti pre-edge peak observed withTi-b(OH) (open squares with solid line) and Ti-b (F) (filled circles with solid line) zeolites inthe absence/presence of added H2O, respectively. The amount of added H2O is (A, a) 0, (B, b) 1.4,(C, c) 3.0, (D, d) 4.6 mmol g−1·cat. The inset shows the effect of adding H2O on the intensity andposition of the pre-edge peak observed in Ti K-edge XANES spectra of Ti-b (OH) (i) and Ti-b(F)(ii) zeolites. The amount of added H2O is 0, 1.4, 3.0, 4.6 mmol g−1·cat (from top to bottom).
water interacts more strongly with TiO2 moieties in the pores of hydrophilicTi-BEA (OH) than of hydrophobic Ti-BEA (F). Because the hydrophilic–hydrophobic character affects the accessibility of photocatalytic active sites towater, it is finally the important factor that determines photocatalytic activityand selectivity. For example, in the photocatalytic reduction of CO2 with H2Oto produce CH4 and CH3OH (Ikeue et al. 2001; Yamashita et al. 2002), thehigher activity for the formation of CH4 was observed with hydrophilic Ti-BEA
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Figure 8. DRUV–vis spectra of Ti-HMS (dashed line), bulk TiO2 (solid line) and Cr-HMS(dotted line).
(OH), and the higher selectivity for the formation of CH3OH was observed withthe hydrophobic Ti-BEA (F). Furthermore, these hydrophobic zeolite materialsexhibit the high ability for the adsorption of organic compounds diluted inwater. Hydrophobic Ti-BEA (F) is more active than hydrophilic Ti-BEA (OH) tophotocatalytically mineralize alcohols and organic halides diluted in water. Theseresults can be assigned to the larger affinity of organics for Ti-oxide moietiesdepending on the hydrophobic surface properties of F−-doped zeolites.
5. Case study of chromium single-site heterogeneous photocatalysts
Single-site heterogeneous photocatalysts containing Ti, V and Mo exhibit high-photocatalytic activity for various reactions under UV light but not under visiblelight. In order to use the most available and environment-friendly energy source—solar light—it becomes vital to develop photocatalysts that can efficiently operateunder visible light. This is the case of single-site Cr oxide moieties introducedin mesoporous silica (Cr-HMS) which not only show photocatalytic activityunder UV-light but also under visible light (Yamashita et al. 1999; Yamashita &Mori 2007). Figure 8 shows the diffuse reflectance UV–vis (DRUV–vis) spectra ofTi-HMS, bulk TiO2 and Cr-HMS. The Ti-HMS exhibits a sharp ligand-to-metal,O2− to Ti4+, charge-transfer band at around 220 nm in contrast to bulk TiO2powder which shows a wide absorption ranging from 400 to 200 nm, suggestingthe presence of extended Ti−O−Ti connectivity. Cr-HMS exhibits three distinctabsorption bands at around 250, 360 and 480 nm which can be assigned to O2−to Cr6+ charge transfer of tetrahedral coordinated Cr oxide moieties.
This assignment is confirmed by XAS spectra (figure 9). Bulk Cr2O3 exhibitsa weak pre-edge peak in the XANES spectra and an intense peak owing toneighbouring Cr atoms (Cr−O−Cr) in the FT-EXAS spectra, indicating that the
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Figure 9. (a) Cr K-edge XANES and (b) FT-EXAFS spectra of Cr2O3 (a,a′) and Cr-oxide/Y-zeolite (b,b′).
catalyst consists of a mixture of tetrahedral and octahedral Cr oxide species. Onthe other hand, Cr-HMS exhibits a sharp and intense pre-edge peak characteristicof tetrahedral Cr oxide with a chromyl Cr=O bond. In the FT-EXAFS spectrum,only a single peak owing to neighbouring oxygen atoms (Cr−O) is observed,showing that Cr ions are highly dispersed.
Cr-HMS catalysts exhibit a characteristic emission at 550–750 nm assigned tocharge-transfer processes related to tetrahedral Cr oxide moieties upon excitationat around 520 nm (figure 10a). This result suggests that Cr-HMS involvestetrahedral Cr oxide moieties with a chromyl Cr=O bond, in good agreement withXAS and UV–vis data. The addition of O2 to Cr-HMS leads to a quenching of thephosphorescence intensity, which depends on the amount of oxygen, suggestingthat the charge-transfer-excited state of isolated tetrahedral Cr oxide moieties,{Cr5+−O−}∗, easily interacts with O2 under visible light. More interestingly, afine structure owing to the vibration mode of Cr=O bond is also observed for Cr-HMS even upon addition of O2, suggesting that the energy separation betweenthe bands for the vibronic transition in the Cr=O bond. The open space offeredin Cr-HMS is large enough to avoid the formation of perturbation due to theneighbouring surface OH groups, thus keeps the original isolated tetrahedrallycoordinated Cr oxide moieties without perturbation within the mesopore. On theother hand, the band separation in the photoluminescence of the Cr-containingmicroporous silicalite-1 zeolite (CrS-1) is very vague (figure 10b). It can besuggested that the coordination geometry of the tetrahedrally coordinated Croxide moieties in CrS-1 might be distorted by the perturbation between Cr=O
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Figure 10. Photoluminescence spectra of (a) Cr-HMS; (b) CrS-1 and (c) their proposed surfacestructures and the effect of adding oxygen. (Online version in colour.)
bond and the neighbouring surface OH groups within zeolite cavities. Theproposed local structures of the various Cr oxide species are illustrated. Thepresence of this weak interaction could be observed only by the analysis ofthe photoluminescence.
As expected, isolated tetrahedral Cr oxide moieties in HMS are efficientphotocatalysts even under visible light with a high selectivity. For example, theCr-HMS showed photocatalytic activity for NO decomposition to produce N2,N2O and O2 even under visible light irradiation (l > 450 nm). Moreover, visiblelight irradiation of the Cr-HMS in the presence of propane and O2 also leads tothe photocatalytic oxidation of propane notably into acetone and acrolein.
6. Conclusions
This study deals with single-site heterogeneous photocatalysts, composed ofisolated transition metal ions (single-sites) dispersed on silica-based supports,which are active in photocatalysis. The study first defines what single-siteheterogeneous catalysts are. The difficulties inherent to their preparation are thenpresented which include the problems of speciation (the concept of secondaryspeciation is presented) related not only to the two main starting components(metal precursor solution and solid oxide support) but also to other speciespresent, depending on the synthesis method adopted. Three main types of method
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are given: (i) grafting/anchoring of single-sites on a preformed support, (ii) post-synthesis modification of the support followed by fixation of single-sites and(iii) simultaneous synthesis of single-sites and support. After description of theideal features to be conferred to single-site heterogeneous photocatalysts, thestudy deals with the best preparation methods and most suitable techniques tosynthesize and characterize silica-based single-site heterogeneous photocatalysts.The materials prepared involve titanium and chromium, and the photocatalyticreactions discussed include CO2 reduction with H2O, degradation of organicpollutants diluted in water, NO decomposition and selective oxidation of propane.
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