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Photoreactivity of TSP: A novel microporous titanosilicate
Microporous and Mesoporous Materials 101 (2007) 184–190 www.elsevier.com/locate/micromeso
Photoreactivity of TSP: A novel microporous titanosilicate Morag Murdoch, Rachel Yeates, Russell Howe
*
Chemistry Department, University of Aberdeen, Aberdeen, Scotland AB24 3UE, United Kingdom Received 9 August 2006; received in revised form 2 October 2006; accepted 11 October 2006 Available online 5 December 2006
Abstract The photoreactivity of TSP, a microporous titanosilicate with the pharmacosiderite structure consisting of nanoclusters of titania octahedra linked through silicate tetrahedra, has been studied by EPR spectroscopy. UV irradiation of TSP in the presence of O2 produces the EPR signal of holes trapped at oxide ion sites (O). This is attributed to the scavenging of electrons by O2. The trapped holes in TSP are much more stable than the corresponding species in TiO2. Reaction of O2 with O to form O 3 is also observed. Trapped holes can be seen as well when TSP is irradiated in the presence of adsorbed CO2, which can also act as an electron scavenger. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Pharmacosiderite; Titanosilicate; EPR; Hole trapping; Photoreactivity; Electron trapping
1. Introduction Photocatalysis using various forms of TiO2 or modified TiO2 is a rapidly expanding field, both in the area of environmental catalysis, and in more novel areas such as hydrogen generation from water, CO2 fixation and organic synthesis [1]. In all of these applications, improvement of catalyst performance requires an improved understanding of the many different factors that influence the photogeneration of holes and electrons in wide bandgap semiconductors and their subsequent reactivity. The ultimate objective in any photocatalyst is to effectively separate holes and electrons and allow their reaction with adsorbed reactants. EPR spectroscopy has been used by several groups to follow formation and reaction of holes and electrons in TiO2. Howe and Graetzel first showed that UV irradiation of hydrated anatase at 4.2 K in vacuo produced EPR signals of electrons trapped at titanium sites (Ti3+) and holes trapped at oxide ion sites (O) which were believed to be immediately sub-surface (the species observed may be represented as Ti4+–O–Ti4+–OH) [2]. In vacuo, the trapped holes and electrons were stable only under continuous irra*
Corresponding author. Tel.: +44 1224 272948. E-mail address: [email protected] (R. Howe).
1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.10.029
diation at 4.2 K, and recombination occurred on ceasing irradiation and/or raising the temperature. In the presence of oxygen however, the trapped electron signal was not seen and the trapped holes were stable to 77 K. This was attributed to the scavenging of electrons by oxygen, thereby inhibiting hole:electron recombination. Similar observations have been reported more recently by Berger et al. [3,4], Hurum et al. [5] and Ke et al. [6], although some differences in the behaviour observed may be attributed to differences in the type of TiO2 used (e.g. nanocrystalline or not, anatase purity, and extent of surface hydration). The effects of reducing particle size on the photoreactivity of TiO2 have attracted recent attention. Particle size has a pronounced effect on the visible–UV absorption characteristics of semiconductors when the particle size becomes comparable with the size of the exciton (the so-called Q-size effect), resulting in a pronounced blue shift in the absorption band edge with decreasing particle size (although Serpone has argued that direct bandgap transitions are an alternative explanation for many reported Q-size effects [7]). Decreasing particle size will increase the surface to volume ratio of the particles and hence enhance in principle the reactivity of photo-generated holes and electrons with adsorbed molecules, although surface sites may also act as recombination centres and thereby inhibit charge
M. Murdoch et al. / Microporous and Mesoporous Materials 101 (2007) 184–190
separation. The state of hydroxylation of the surface is another variable, as is the crystal structure (anatase versus rutile) and crystallinity of the TiO2. The high photocatalytic activity of Degussa P25 anatase for many reactions has been attributed to the presence of a significant rutile impurity in this material (20%), with the interface between the rutile and anatase components playing an important role [8]. The difficulty of separating out the different structural, surface chemical and electronic factors controlling the photoreactivity of polydisperse titania materials has inhibited understanding of these effects. We [9–13] and others [14– 18] have begun exploring the photoreactivity of titanosilicate zeolites which contain well-defined titania units as part of their frameworks. The microporosity and high surface area of such materials also offers in principle enhanced opportunity for interception of holes and electrons by adsorbed molecules. The titanosilicate ETS-10 contains one-dimensional titania nanorods surrounded by silicate tetrahedra, generating a three-dimensional microporous framework. Theoretical calculations confirm that these titania nanorods behave as quantum confined semiconductors [14–16]. However, the photocatalytic activity of ETS10 is strongly dependent on the presence of defects in the structure, which are necessary to expose titanium sites to adsorbed molecules within the pores or at the external surface [9,11,13,17,18]. In this study we describe our initial photoreactivity studies on a titanosilicate zeolite hereafter referred to as TSP, which is isostructural with the natural mineral Pharmacosiderite (KFe4(OH)4(AsO4)3 Æ 6H2O). TSP has the stoichiometry K3HTi4O4(SiO4)3 Æ nH2O, with the structure represented schematically in Fig. 1. In this structure, four face sharing TiO6 octahedra form a cubic cluster; the cubic clusters are corner-linked through silicate tetrahedra, forming an interconnected three-dimensional framework with a three-dimensional pore system of 8-ring channels, occupied by alkali metal cations and adsorbed water molecules (not shown in Fig. 1) [19]. In comparison with ETS-10, TSP contains titania clusters rather than nanorods, which might be expected to alter its photoreactivity. More importantly,
185
the titania nanoclusters in TSP are directly exposed to the internal pore structure, which is not the case with defectfree ETS-10. We report here studies by EPR spectroscopy of species formed when TSP is UV-irradiated in the presence of adsorbed molecules, and contrast its photoreactivity with that of conventional or nanocrystalline anatase. 2. Experimental TSP was prepared according to Ref. [20]. 1.5 g of fumed silica (Aerosol 200) was added to 3.57 g of titanium isopropoxide and 20 ml distilled water, the resulting mixture was then stirred continuously in a plastic beaker overnight. This resulted in the production of a thick white solution, which was subsequently centrifuged and the resulting solid washed twice with distilled water, with the supernatant being discarded in between washing steps. Distilled water was then added until the resulting gel-like solid’s total weight was 15 g. 7.8 ml of a 5.06 M potassium hydroxide solution was then added to the gel under continuous stirring. The solution was then placed in a Teflon lined steel autoclave in an oven at 200 °C for 48 h. The product was collected under suction filtration and washed with pure ethanol then dried in an oven at 60 °C overnight. Characterisation of the TSP was performed with X-ray powder diffraction (Bruker D8 Advance instrument, Cu Ka radiation), SEM (JEOL JSM5200 instrument, samples mounted on carbon disks and gold coated), TEM (JEOL 2011 EX operating at 200 kV, samples dispersed in isopropanol and deposited on a carbon coated copper grid), 29Si NMR (Varian Infinity plus 400 MHz instrument) and surface area measurements (nitrogen adsorption at 77 K in a Coulter SA3100 following vacuum outgassing at 523 K) were also undertaken. For EPR experiments, samples were loaded into vacuum cell fitted with a quartz sidearm, outgassed to the desired temperature at a vacuum of 105 mbar, then exposed to reactant gases before inserting into the EPR cavity. EPR spectra were measured with a Bruker ECS106 instrument (9.1 GHz, 5 mW microwave power) at 77 K, using an insert dewar. Subsequent processing of spectra was undertaken with Bruker WINEPR software. Ex situ irradiation was performed at room temperature using two Uvitec LI208G lamps (254 nm maximum output, stated total intensity of 1800 lW at 15 cm). In situ irradiations in the EPR cavity at 77 K used a 150 W Optosource xenon lamp focussed on the front grid of the EPR cavity (50% transmission). 17 O enriched O2 (36.8 at.% enrichment) was provided by Merck, Sharp and Dohme, Canada. 3. Results and discussion 3.1. Characterisation
Fig. 1. Schematic drawing of the structure of TSP.
Fig. 2 shows the X-ray powder diffraction pattern of TSP. This agrees well with published data for the
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M. Murdoch et al. / Microporous and Mesoporous Materials 101 (2007) 184–190
cluster), and this pre-treatment was used in all the experiments described below (unless otherwise stated). The UV–visible diffuse reflectance spectrum of TSP showed an absorption edge at around 310 nm, significantly blue shifted relative to anatase, as might be expected for nanometer sized titania clusters.
1600
Intensity
1200
800
3.2. Photoreactivity in oxygen 400
0
10
20
30
40
two-theta Fig. 2. X-ray powder diffraction pattern of TSP.
pharmacosiderite structure [19], and can be indexed on a simple cubic lattice (space group P 43m). The breadth of the peaks suggests however that the crystalline domain size is relatively small, and this is confirmed by TEM observation. Fig. 3 shows a typical TEM image. The sample comprises a collection of overlapping crystallites ca. 20–30 nm in diameter. SEM images (not shown) reveal that these nanocrystallites are aggregated into larger particles about 500 nm in diameter. Nitrogen adsorption measurements gave a Langmuir surface area of 170 m2 g1, and the 29Si NMR spectrum gave a Q0 (Si(OTi)4) resonance at 82 ppm relative to TMS. FTIR and thermogravimetric analysis measurements reveal that vacuum pre-treatment at around 400 K removes loosely adsorbed water from the pore system without causing any dehydroxylation (the unit cell contains one hydroxyl group per cubic titania
Fig. 3. High resolution TEM image of TSP.
Two different EPR signals were observed when TSP outgassed at 400 K was UV-irradiated in the presence of oxygen at room temperature. The first of these was seen most clearly when irradiation was performed in the presence of 20–40 mbar of O2 at room temperature for 1–2 h, and spectra recorded subsequently at 77 K (Fig. 4a). This signal has the line shape characteristic of an axial g-tensor, with gperpendicular = 2.018 and gparallel = 2.007. These parameters are characteristic of the O radical anion. For a simple ionic species in an axial crystal field, the g-tensor components are given by [21]: gparallel ge
gperpendicular ¼ ge þ 2k=DE
where k is the spin–orbit coupling constant, and DE the crystal field splitting of the O 2p orbitals. In a more covalent environment, the gparallel value shifts to lower field. Table 1 summarises g-tensor data for some O species reported in the literature. Most striking is the close similarity between the species seen here and the trapped hole species in TiO2 first described by Howe and Graetzel [2] and later seen by others [3–6]. In this case, trapping of positive holes at a titania lattice oxide ion forms a sub-surface O species. Such species should be distinguished from the O formed on oxide surfaces by dissociative adsorption of N2O, for example [21]. Given the sensitivity of the gperpendicular value to the crystal field splitting, and the close similarity of the gperpendicular values for O in TSP and anatase, it is likely that this trapped hole species in TSP is formed within the Ti4O4 clusters rather than at a bridging Ti–O–Si group. The second signal generated on irradiation of TSP in oxygen could be seen most clearly by brief evacuation of the gas phase oxygen following UV irradiation, then cooling thoroughly to 77 K before recording the spectrum at this temperature. As seen in Fig. 4b, this treatment left the O signal unchanged, but led to the appearance of a new almost isotropic signal at a g-value of about 2.007. This new signal shows a number of unusual characteristics. It is very strongly broadened by the presence of physisorbed oxygen. Readmission of oxygen to the EPR cell restored completely the spectrum shown in Fig. 4a. Secondly, the line width of the second signal is strongly temperature dependent. The signal intensity continued to develop on standing in liquid nitrogen for up to 30 min following evacuation at room temperature, as the sample slowly reached temperature equilibrium with its surroundings. This behaviour was not seen with the O species.
M. Murdoch et al. / Microporous and Mesoporous Materials 101 (2007) 184–190
(a)
(b)
3250
3270
3290
3310
3330
3350
gauss Fig. 4. EPR spectra measured at 77 K of TSP after (a) irradiation in 40 mbar of O2 at room temperature, and (b) subsequent evacuation at room temperature for 5 min.
A signal with parameters similar to this one seen on addition of oxygen to previously irradiated nanocrystalline
187
anatase was assigned by Berger et al. [4] to the ozonide ion O 3 formed by reaction of O2 with trapped holes. The ozonide ion has been studied in detail on a number of different oxide surfaces, and in oxygen containing inorganic salts [22]. Based on the Walsh diagram for a 19 electron species, a bent structure with an orthorhombic g-tensor is expected (Scheme 1), and in the known examples of this species, the g-tensor anisotropy is considerably greater than that seen here (Table 1). On the other hand, a so-called anomalous O 3 species has been reported in several instances which has an almost isotropic g-tensor (including that on TiO2) [22]. To determine the origin of the signals seen on irradiation of TSP in oxygen, the experiment was repeated in the presence of 17O enriched O2. The O signal observed (Fig. 5a) on irradiation in 17O enriched O2 is identical to that seen with 16O2, confirming that this species is formed by hole trapping at lattice oxide ions, and not from dissociation of O2. The second signal appearing on subsequent evacuation does however show additional hyperfine splitting due to a single I = 5/2 nucleus (6 line pattern centred on g = 2.007, with a hyperfine splitting of 75 G, Fig. 5b). The 6 lines should have equal intensities, but variations in line width (due, perhaps to non-rigidity of the species concerned) are commonly seen for 17O hyperfine lines in adsorbed oxygen radicals [22]. The observation of 17O hyperfine splitting confirms that the second species has its origins in gas phase O2. The level of 17O enrichment available prevented the observation of doubly labelled species, but the observation of a single set of 6 hyperfine lines means that either the species contains two equivalent 17O atoms, or (unlikely) that it results from dissociation of O2. The bent O 3 species formed by reaction of O2 with O on MgO or CaO surfaces [23,24] show two different 17O hyperfine couplings, as expected from the inequivalence of the two oxygen atoms originating from O2 (Scheme 1). A T-shaped species (Scheme 2), resulting from the sideways interaction of O2 with O, would on the other hand, result in a single 17O hyperfine
Table 1 EPR parameters of oxygen radicals O
gxx
gyy
gzz
Ref.
In TiO2 In TiO2
2.016 2.012
2.012 2.012
2.002 2.0046
[2] [4]
In TSP On MoO3–SiO2
2.018 2.019
2.018 2.019
2.007 2.002
This work [26]
On MgO O 3
2.042
2.042
2.0013
[27]
On MgO
2.017
2.010
2.0014 A (17O) = 82 G and 65 G
[23]
On CaO
2.010
2.0189
2.0031
[24]
On V2O5:SiO2
2.007
2.002 A (17O) = 78 G
1.998
[28]
On TiO2
1.9996
1.9996
2.0073
[4]
17
2.007 A ( O) = 75 G
In TSP In KClO4
1.9996
1.9996
This work 2.0057
[25]
188
M. Murdoch et al. / Microporous and Mesoporous Materials 101 (2007) 184–190
(a)
X5
(b)
3000
3100
3200
3300
3400
3500
3600
gauss Fig. 5. EPR spectra measured at 77 K of TSP after (a) irradiation in 40 mbar of 17O enriched O2 at room temperature, and (b) subsequent evacuation at room temperature for 5 min.
coupling. Such coupling has been reported for the anomalous O 3 species formed on supported vanadia catalysts [25], and the 17O coupling constant reported of 78 G is close to that seen in TSP.
O
-
O Os Scheme 1.
-
O
O Os
Scheme 2.
Further evidence as to the nature of the species seen in TSP comes from the temperature dependent line broadening and sensitivity to oxygen pressure. The broadening by oxygen indicates that the O 3 species in TSP is located in
sites accessible to the gas phase. An O 3 species with g-tensor components similar to those seen here was reported by Byberg to be formed in X-irradiated KClO4 [25]. The almost isotropic g-tensor of this species was accounted for by an exchange coupling to an O2 molecule, and the species was formally represented by Byberg as ½O 3 ; O2 . The ½O 3 ; O2 species was reported also to show a strongly temperature dependent line broadening above 120 K due to rapid spin–lattice relaxation, similar to that seen here for the O 3 species in TSP. Exchange coupling to oxygen of this kind may also be occurring with O 3 in TSP or on the other surfaces where an ‘‘anomalous’’ O 3 is formed [22], although further work is needed to test this hypothesis. Both of the radicals seen on irradiating TSP in oxygen are stable indefinitely on standing in oxygen at room temperature, and to evacuation at room temperature. Outgassing above 400 K was needed to remove the species. Although the exact structure of the anomalous O 3 species formed requires further investigation, we believe that both species are the result of hole trapping by oxide ions in the TSP structure. The O species formed in this way within the Ti4O4 clusters are not directly accessible to the gas phase, and the signal is not broadened by oxygen. Holes trapped at oxide ions adjacent to the 8-ring pores (possibly the Ti–O–Si groups) are accessible to O2 in the pores, and can therefore form the anomalous O 3 species by reaction with O2. This appears to be similar to the exchange coupled ½O 3 ; O2 species proposed by Byberg. The stability of the species in TSP is consistent with its location within the micropores and not at the external surface. Irradiation of TSP in vacuo at either 77 K or room temperature gave very weak signals of O and of Ti3+ only. Only in the presence of oxygen were the O and O 3 species generated in higher yields. As discussed in the TiO2 literature [2–6], this indicates that O2 is acting as an electron scavenger, thereby allowing higher concentrations of trapped holes (or their reaction products) to be formed. On anhydrous anatase surfaces, the superoxide ion O 2 is observed as a primary product of electron trapping by oxygen [3]. Superoxide ions are not formed in TSP samples pretreated at 393 K. Superoxide was also not observed as a primary product of electron trapping on fully hydroxylated anatase [2]. In both cases, this can be attributed to the absence of adsorption sites on which O 2 can be stabilised. In experiments with TSP activated at higher temperatures (e.g. 500 K) we have observed an additional EPR signal which appears to be O 2 , but further work is needed to confirm this assignment. Infrared experiments have shown that structural dehydroxylation occurs on outgassing above 400 K, which may generate sites able to stabilise O 2. Irradiation of TSP in oxygen at 77 K instead of room temperature does not give the O and O 3 signals described above, but they appear on subsequent warming to room temperature. Furthermore, if the sample is subsequently irradiated at 77 K, the O signal is diminished with increasing irradiation time. These preliminary results suggest that
M. Murdoch et al. / Microporous and Mesoporous Materials 101 (2007) 184–190
* (c)
* (b)
*
(a)
3250
3270
3290
3310
3330
3350
gauss Fig. 6. (a) EPR spectrum measured at 77 K of TSP after exposure to 50 mbar of CO2 at room temperature; (b) following UV irradiation at room temperature for 1 h; (c) following UV irradiation at room temperature for 2 h. Asterisk marks features due to a defect in the quartz sample tube.
the dynamics of hole and electron trapping are strongly temperature dependent, and further work is in progress to clarify these phenomena. 3.3. Photoreactivity in carbon dioxide Fig. 6 shows EPR spectra measured at 77 K following UV irradiation of TSP outgassed at 393 K in the presence of carbon dioxide at room temperature. The O signal which develops as a function of time is identical to that formed on irradiation in O2 (Fig. 4). However, in CO2 the second signal seen after irradiation in O2 and assigned to O 3 species was not observed. We presume that CO2 is functioning here, like O2, as an electron scavenger. One electron transfer to CO2 would form the CO 2 radical anion, which has been observed on MgO surfaces, for example [29]. This species was not seen here, possibly because of the absence of suitable sites on which it could be adsorbed, as for O 2. 3.4. Comparisons with anatase The photoreactivity of TSP activated at 393 K resembles in some respects that of fully hydroxylated anatase. The
189
ability of oxygen to scavenge electrons and allow a stable trapped hole signal to be observed is common to both materials. However, the trapped holes in TSP are considerably more stable than those in hydroxylated anatase (the O signal in anatase is stable in the presence of O2 only at 77 K). The thermal activation required to trap holes in TSP is unique to this material, as is the formation of an unusually stable anomalous ozonide radical. The microporosity of TSP would appear to contribute to the stability of both oxygen related radicals. The small size of the titania clusters in TSP may be another important factor. The stabilisation of trapped holes by CO2 (functioning as an electron scavenger) is also novel. Further work is in progress in our laboratory to characterise the photoreactivity of TSP subjected to higher temperature pre-treatments, and to investigate the effects of varying the exchangeable cations in the TSP structure on its photoreactivity. Although at first sight the 8-ring pore dimensions of TSP might be considered to restrict its applications as a photocatalyst, the nanocrystallinity of the material means that many adsorption sites are available also on the external surface and at pore openings where larger molecules may interact with photo-generated holes or electrons. Investigation of the photocatalytic properties of this novel small-pore zeolite system may therefore be worthwhile. Acknowledgments This work was supported by the Donors of the Petroleum Research Fund of the American Chemical Society. We thank Dr. Wuzong Zhou of the University of St Andrews for his assistance in obtaining the TEM images. References [1] O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004), and references therein. [2] R.F. Howe, M. Graetzel, J. Phys. Chem. 91 (1987) 3906. [3] T. Berger, M. Sterrer, O. Diwald, E. Knozinger, D. Panayatou, T.L. Thompson, J.T. Yates, J. Phys. Chem. B 109 (2005) 6061. [4] T. Berger, M. Sterrer, O. Diwald, E. Knozinger, ChemPhysChem 6 (2005) 2104. [5] D.C. Hurum, K.A. Gray, T. Rajh, M.C. Thurnauer, J. Phys. Chem. B 109 (2005) 977. [6] S.C. Ke, T.C. Wang, M.S. Wong, N.O. Gopal, J. Phys. Chem. B 110 (2000) 11628. [7] N. Serpone, D. Lawless, R. Kharautdinov, J. Phys. Chem. 99 (1995) 16646. [8] D.C. Hurum, A.G. Agrios, K.A. Gray, T. Rajh, M.C. Thurnauer, J. Phys. Chem. B 107 (2003) 4545. [9] R.F. Howe, Y.K. Krisnandi, Chem. Commun. (2001) 1588. [10] P.D. Southon, R.F. Howe, Chem. Mater. 14 (2002) 4209. [11] Y. Krisnandi, P. Southon, A. Adesina, R.F. Howe, Int. J. Photoenergy 5 (2003) 131. [12] Y. Krisnandi, E. Lachowski, R.F. Howe, Chem. Mater. 18 (2006) 928. [13] Y. Krisnandi, R.F. Howe, Appl. Catal. A 307 (2006) 62. [14] E. Borello, C. Lamberti, S. Bordiga, A. Zecchina, C. Otero Arean, Appl. Phys. Lett. B 71 (1997) 2319.
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[15] S. Bordiga, G. Turnes Palomino, A. Zecchina, G. Ranghino, E. Giamello, C. Lamberti, J. Chem. Phys. 112 (2000) 3859. [16] A. Damin, F.X. Llabres, I. Xamena, C. Lamberti, B. Civalleri, C.M. Zicovich-Wilson, A. Zecchina, J. Phys. Chem. B 108 (2004) 1328. [17] P. Calza, C. Paze, E. Pelizzetti, A. Zecchina, Chem. Commun. (2001) 2130. [18] F.X. Llabres i Xamena, P. Calza, C. Lamberti, C. Prestopino, A. Damin, S. Bordiga, E. Pelizzetti, A. Zecchina, J. Am. Chem. Soc. 125 (2003) 2264. [19] W.T.A. Harrison, E.G. Thurman, G.D. Stucky, Zeolites 15 (1995) 408. [20] E.A. Behrens, D.M. Poojary, A. Clearfield, Chem. Mater. 8 (1996) 1236.
[21] [22] [23] [24] [25] [26] [27] [28] [29]
M. Che, A. Tench, Adv. Catal. 31 (1982) 77. M. Che, A.J. Tench, Adv. Catal. 32 (1983) 1. N.B. Wong, J.H. Lunsford, J. Chem. Phys. 56 (1972) 2664. M.C. Paganini, M. Chiesa, F. Dolci, P. Martino, E. Giamello, J. Phys. Chem. B 110 (2006) 11918. J.R. Byberg, Chem. Phys. Lett. 56 (1978) 130. Y. Ben Taarit, J.H. Lunsford, Chem. Phys. Lett. 19 (1973) 348. N.B. Wong, J.H. Lunsford, J. Chem. Phys. 55 (1971) 3007. B.N. Shelimov, C. Naccache, M. Che, J. Catal. 37 (1975) 279. P. Meriaudeau, J.C. Vedrine, Y. Ben Tarrit, C. Naccache, J. Chem. Soc. Faraday Trans. 2 71 (1975) 736.
Photoreactivity of TSP: A novel microporous titanosilicate Morag Murdoch, Rachel Yeates, Russell Howe
*
Chemistry Department, University of Aberdeen, Aberdeen, Scotland AB24 3UE, United Kingdom Received 9 August 2006; received in revised form 2 October 2006; accepted 11 October 2006 Available online 5 December 2006
Abstract The photoreactivity of TSP, a microporous titanosilicate with the pharmacosiderite structure consisting of nanoclusters of titania octahedra linked through silicate tetrahedra, has been studied by EPR spectroscopy. UV irradiation of TSP in the presence of O2 produces the EPR signal of holes trapped at oxide ion sites (O). This is attributed to the scavenging of electrons by O2. The trapped holes in TSP are much more stable than the corresponding species in TiO2. Reaction of O2 with O to form O 3 is also observed. Trapped holes can be seen as well when TSP is irradiated in the presence of adsorbed CO2, which can also act as an electron scavenger. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Pharmacosiderite; Titanosilicate; EPR; Hole trapping; Photoreactivity; Electron trapping
1. Introduction Photocatalysis using various forms of TiO2 or modified TiO2 is a rapidly expanding field, both in the area of environmental catalysis, and in more novel areas such as hydrogen generation from water, CO2 fixation and organic synthesis [1]. In all of these applications, improvement of catalyst performance requires an improved understanding of the many different factors that influence the photogeneration of holes and electrons in wide bandgap semiconductors and their subsequent reactivity. The ultimate objective in any photocatalyst is to effectively separate holes and electrons and allow their reaction with adsorbed reactants. EPR spectroscopy has been used by several groups to follow formation and reaction of holes and electrons in TiO2. Howe and Graetzel first showed that UV irradiation of hydrated anatase at 4.2 K in vacuo produced EPR signals of electrons trapped at titanium sites (Ti3+) and holes trapped at oxide ion sites (O) which were believed to be immediately sub-surface (the species observed may be represented as Ti4+–O–Ti4+–OH) [2]. In vacuo, the trapped holes and electrons were stable only under continuous irra*
Corresponding author. Tel.: +44 1224 272948. E-mail address: [email protected] (R. Howe).
1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.10.029
diation at 4.2 K, and recombination occurred on ceasing irradiation and/or raising the temperature. In the presence of oxygen however, the trapped electron signal was not seen and the trapped holes were stable to 77 K. This was attributed to the scavenging of electrons by oxygen, thereby inhibiting hole:electron recombination. Similar observations have been reported more recently by Berger et al. [3,4], Hurum et al. [5] and Ke et al. [6], although some differences in the behaviour observed may be attributed to differences in the type of TiO2 used (e.g. nanocrystalline or not, anatase purity, and extent of surface hydration). The effects of reducing particle size on the photoreactivity of TiO2 have attracted recent attention. Particle size has a pronounced effect on the visible–UV absorption characteristics of semiconductors when the particle size becomes comparable with the size of the exciton (the so-called Q-size effect), resulting in a pronounced blue shift in the absorption band edge with decreasing particle size (although Serpone has argued that direct bandgap transitions are an alternative explanation for many reported Q-size effects [7]). Decreasing particle size will increase the surface to volume ratio of the particles and hence enhance in principle the reactivity of photo-generated holes and electrons with adsorbed molecules, although surface sites may also act as recombination centres and thereby inhibit charge
M. Murdoch et al. / Microporous and Mesoporous Materials 101 (2007) 184–190
separation. The state of hydroxylation of the surface is another variable, as is the crystal structure (anatase versus rutile) and crystallinity of the TiO2. The high photocatalytic activity of Degussa P25 anatase for many reactions has been attributed to the presence of a significant rutile impurity in this material (20%), with the interface between the rutile and anatase components playing an important role [8]. The difficulty of separating out the different structural, surface chemical and electronic factors controlling the photoreactivity of polydisperse titania materials has inhibited understanding of these effects. We [9–13] and others [14– 18] have begun exploring the photoreactivity of titanosilicate zeolites which contain well-defined titania units as part of their frameworks. The microporosity and high surface area of such materials also offers in principle enhanced opportunity for interception of holes and electrons by adsorbed molecules. The titanosilicate ETS-10 contains one-dimensional titania nanorods surrounded by silicate tetrahedra, generating a three-dimensional microporous framework. Theoretical calculations confirm that these titania nanorods behave as quantum confined semiconductors [14–16]. However, the photocatalytic activity of ETS10 is strongly dependent on the presence of defects in the structure, which are necessary to expose titanium sites to adsorbed molecules within the pores or at the external surface [9,11,13,17,18]. In this study we describe our initial photoreactivity studies on a titanosilicate zeolite hereafter referred to as TSP, which is isostructural with the natural mineral Pharmacosiderite (KFe4(OH)4(AsO4)3 Æ 6H2O). TSP has the stoichiometry K3HTi4O4(SiO4)3 Æ nH2O, with the structure represented schematically in Fig. 1. In this structure, four face sharing TiO6 octahedra form a cubic cluster; the cubic clusters are corner-linked through silicate tetrahedra, forming an interconnected three-dimensional framework with a three-dimensional pore system of 8-ring channels, occupied by alkali metal cations and adsorbed water molecules (not shown in Fig. 1) [19]. In comparison with ETS-10, TSP contains titania clusters rather than nanorods, which might be expected to alter its photoreactivity. More importantly,
185
the titania nanoclusters in TSP are directly exposed to the internal pore structure, which is not the case with defectfree ETS-10. We report here studies by EPR spectroscopy of species formed when TSP is UV-irradiated in the presence of adsorbed molecules, and contrast its photoreactivity with that of conventional or nanocrystalline anatase. 2. Experimental TSP was prepared according to Ref. [20]. 1.5 g of fumed silica (Aerosol 200) was added to 3.57 g of titanium isopropoxide and 20 ml distilled water, the resulting mixture was then stirred continuously in a plastic beaker overnight. This resulted in the production of a thick white solution, which was subsequently centrifuged and the resulting solid washed twice with distilled water, with the supernatant being discarded in between washing steps. Distilled water was then added until the resulting gel-like solid’s total weight was 15 g. 7.8 ml of a 5.06 M potassium hydroxide solution was then added to the gel under continuous stirring. The solution was then placed in a Teflon lined steel autoclave in an oven at 200 °C for 48 h. The product was collected under suction filtration and washed with pure ethanol then dried in an oven at 60 °C overnight. Characterisation of the TSP was performed with X-ray powder diffraction (Bruker D8 Advance instrument, Cu Ka radiation), SEM (JEOL JSM5200 instrument, samples mounted on carbon disks and gold coated), TEM (JEOL 2011 EX operating at 200 kV, samples dispersed in isopropanol and deposited on a carbon coated copper grid), 29Si NMR (Varian Infinity plus 400 MHz instrument) and surface area measurements (nitrogen adsorption at 77 K in a Coulter SA3100 following vacuum outgassing at 523 K) were also undertaken. For EPR experiments, samples were loaded into vacuum cell fitted with a quartz sidearm, outgassed to the desired temperature at a vacuum of 105 mbar, then exposed to reactant gases before inserting into the EPR cavity. EPR spectra were measured with a Bruker ECS106 instrument (9.1 GHz, 5 mW microwave power) at 77 K, using an insert dewar. Subsequent processing of spectra was undertaken with Bruker WINEPR software. Ex situ irradiation was performed at room temperature using two Uvitec LI208G lamps (254 nm maximum output, stated total intensity of 1800 lW at 15 cm). In situ irradiations in the EPR cavity at 77 K used a 150 W Optosource xenon lamp focussed on the front grid of the EPR cavity (50% transmission). 17 O enriched O2 (36.8 at.% enrichment) was provided by Merck, Sharp and Dohme, Canada. 3. Results and discussion 3.1. Characterisation
Fig. 1. Schematic drawing of the structure of TSP.
Fig. 2 shows the X-ray powder diffraction pattern of TSP. This agrees well with published data for the
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cluster), and this pre-treatment was used in all the experiments described below (unless otherwise stated). The UV–visible diffuse reflectance spectrum of TSP showed an absorption edge at around 310 nm, significantly blue shifted relative to anatase, as might be expected for nanometer sized titania clusters.
1600
Intensity
1200
800
3.2. Photoreactivity in oxygen 400
0
10
20
30
40
two-theta Fig. 2. X-ray powder diffraction pattern of TSP.
pharmacosiderite structure [19], and can be indexed on a simple cubic lattice (space group P 43m). The breadth of the peaks suggests however that the crystalline domain size is relatively small, and this is confirmed by TEM observation. Fig. 3 shows a typical TEM image. The sample comprises a collection of overlapping crystallites ca. 20–30 nm in diameter. SEM images (not shown) reveal that these nanocrystallites are aggregated into larger particles about 500 nm in diameter. Nitrogen adsorption measurements gave a Langmuir surface area of 170 m2 g1, and the 29Si NMR spectrum gave a Q0 (Si(OTi)4) resonance at 82 ppm relative to TMS. FTIR and thermogravimetric analysis measurements reveal that vacuum pre-treatment at around 400 K removes loosely adsorbed water from the pore system without causing any dehydroxylation (the unit cell contains one hydroxyl group per cubic titania
Fig. 3. High resolution TEM image of TSP.
Two different EPR signals were observed when TSP outgassed at 400 K was UV-irradiated in the presence of oxygen at room temperature. The first of these was seen most clearly when irradiation was performed in the presence of 20–40 mbar of O2 at room temperature for 1–2 h, and spectra recorded subsequently at 77 K (Fig. 4a). This signal has the line shape characteristic of an axial g-tensor, with gperpendicular = 2.018 and gparallel = 2.007. These parameters are characteristic of the O radical anion. For a simple ionic species in an axial crystal field, the g-tensor components are given by [21]: gparallel ge
gperpendicular ¼ ge þ 2k=DE
where k is the spin–orbit coupling constant, and DE the crystal field splitting of the O 2p orbitals. In a more covalent environment, the gparallel value shifts to lower field. Table 1 summarises g-tensor data for some O species reported in the literature. Most striking is the close similarity between the species seen here and the trapped hole species in TiO2 first described by Howe and Graetzel [2] and later seen by others [3–6]. In this case, trapping of positive holes at a titania lattice oxide ion forms a sub-surface O species. Such species should be distinguished from the O formed on oxide surfaces by dissociative adsorption of N2O, for example [21]. Given the sensitivity of the gperpendicular value to the crystal field splitting, and the close similarity of the gperpendicular values for O in TSP and anatase, it is likely that this trapped hole species in TSP is formed within the Ti4O4 clusters rather than at a bridging Ti–O–Si group. The second signal generated on irradiation of TSP in oxygen could be seen most clearly by brief evacuation of the gas phase oxygen following UV irradiation, then cooling thoroughly to 77 K before recording the spectrum at this temperature. As seen in Fig. 4b, this treatment left the O signal unchanged, but led to the appearance of a new almost isotropic signal at a g-value of about 2.007. This new signal shows a number of unusual characteristics. It is very strongly broadened by the presence of physisorbed oxygen. Readmission of oxygen to the EPR cell restored completely the spectrum shown in Fig. 4a. Secondly, the line width of the second signal is strongly temperature dependent. The signal intensity continued to develop on standing in liquid nitrogen for up to 30 min following evacuation at room temperature, as the sample slowly reached temperature equilibrium with its surroundings. This behaviour was not seen with the O species.
M. Murdoch et al. / Microporous and Mesoporous Materials 101 (2007) 184–190
(a)
(b)
3250
3270
3290
3310
3330
3350
gauss Fig. 4. EPR spectra measured at 77 K of TSP after (a) irradiation in 40 mbar of O2 at room temperature, and (b) subsequent evacuation at room temperature for 5 min.
A signal with parameters similar to this one seen on addition of oxygen to previously irradiated nanocrystalline
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anatase was assigned by Berger et al. [4] to the ozonide ion O 3 formed by reaction of O2 with trapped holes. The ozonide ion has been studied in detail on a number of different oxide surfaces, and in oxygen containing inorganic salts [22]. Based on the Walsh diagram for a 19 electron species, a bent structure with an orthorhombic g-tensor is expected (Scheme 1), and in the known examples of this species, the g-tensor anisotropy is considerably greater than that seen here (Table 1). On the other hand, a so-called anomalous O 3 species has been reported in several instances which has an almost isotropic g-tensor (including that on TiO2) [22]. To determine the origin of the signals seen on irradiation of TSP in oxygen, the experiment was repeated in the presence of 17O enriched O2. The O signal observed (Fig. 5a) on irradiation in 17O enriched O2 is identical to that seen with 16O2, confirming that this species is formed by hole trapping at lattice oxide ions, and not from dissociation of O2. The second signal appearing on subsequent evacuation does however show additional hyperfine splitting due to a single I = 5/2 nucleus (6 line pattern centred on g = 2.007, with a hyperfine splitting of 75 G, Fig. 5b). The 6 lines should have equal intensities, but variations in line width (due, perhaps to non-rigidity of the species concerned) are commonly seen for 17O hyperfine lines in adsorbed oxygen radicals [22]. The observation of 17O hyperfine splitting confirms that the second species has its origins in gas phase O2. The level of 17O enrichment available prevented the observation of doubly labelled species, but the observation of a single set of 6 hyperfine lines means that either the species contains two equivalent 17O atoms, or (unlikely) that it results from dissociation of O2. The bent O 3 species formed by reaction of O2 with O on MgO or CaO surfaces [23,24] show two different 17O hyperfine couplings, as expected from the inequivalence of the two oxygen atoms originating from O2 (Scheme 1). A T-shaped species (Scheme 2), resulting from the sideways interaction of O2 with O, would on the other hand, result in a single 17O hyperfine
Table 1 EPR parameters of oxygen radicals O
gxx
gyy
gzz
Ref.
In TiO2 In TiO2
2.016 2.012
2.012 2.012
2.002 2.0046
[2] [4]
In TSP On MoO3–SiO2
2.018 2.019
2.018 2.019
2.007 2.002
This work [26]
On MgO O 3
2.042
2.042
2.0013
[27]
On MgO
2.017
2.010
2.0014 A (17O) = 82 G and 65 G
[23]
On CaO
2.010
2.0189
2.0031
[24]
On V2O5:SiO2
2.007
2.002 A (17O) = 78 G
1.998
[28]
On TiO2
1.9996
1.9996
2.0073
[4]
17
2.007 A ( O) = 75 G
In TSP In KClO4
1.9996
1.9996
This work 2.0057
[25]
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(a)
X5
(b)
3000
3100
3200
3300
3400
3500
3600
gauss Fig. 5. EPR spectra measured at 77 K of TSP after (a) irradiation in 40 mbar of 17O enriched O2 at room temperature, and (b) subsequent evacuation at room temperature for 5 min.
coupling. Such coupling has been reported for the anomalous O 3 species formed on supported vanadia catalysts [25], and the 17O coupling constant reported of 78 G is close to that seen in TSP.
O
-
O Os Scheme 1.
-
O
O Os
Scheme 2.
Further evidence as to the nature of the species seen in TSP comes from the temperature dependent line broadening and sensitivity to oxygen pressure. The broadening by oxygen indicates that the O 3 species in TSP is located in
sites accessible to the gas phase. An O 3 species with g-tensor components similar to those seen here was reported by Byberg to be formed in X-irradiated KClO4 [25]. The almost isotropic g-tensor of this species was accounted for by an exchange coupling to an O2 molecule, and the species was formally represented by Byberg as ½O 3 ; O2 . The ½O 3 ; O2 species was reported also to show a strongly temperature dependent line broadening above 120 K due to rapid spin–lattice relaxation, similar to that seen here for the O 3 species in TSP. Exchange coupling to oxygen of this kind may also be occurring with O 3 in TSP or on the other surfaces where an ‘‘anomalous’’ O 3 is formed [22], although further work is needed to test this hypothesis. Both of the radicals seen on irradiating TSP in oxygen are stable indefinitely on standing in oxygen at room temperature, and to evacuation at room temperature. Outgassing above 400 K was needed to remove the species. Although the exact structure of the anomalous O 3 species formed requires further investigation, we believe that both species are the result of hole trapping by oxide ions in the TSP structure. The O species formed in this way within the Ti4O4 clusters are not directly accessible to the gas phase, and the signal is not broadened by oxygen. Holes trapped at oxide ions adjacent to the 8-ring pores (possibly the Ti–O–Si groups) are accessible to O2 in the pores, and can therefore form the anomalous O 3 species by reaction with O2. This appears to be similar to the exchange coupled ½O 3 ; O2 species proposed by Byberg. The stability of the species in TSP is consistent with its location within the micropores and not at the external surface. Irradiation of TSP in vacuo at either 77 K or room temperature gave very weak signals of O and of Ti3+ only. Only in the presence of oxygen were the O and O 3 species generated in higher yields. As discussed in the TiO2 literature [2–6], this indicates that O2 is acting as an electron scavenger, thereby allowing higher concentrations of trapped holes (or their reaction products) to be formed. On anhydrous anatase surfaces, the superoxide ion O 2 is observed as a primary product of electron trapping by oxygen [3]. Superoxide ions are not formed in TSP samples pretreated at 393 K. Superoxide was also not observed as a primary product of electron trapping on fully hydroxylated anatase [2]. In both cases, this can be attributed to the absence of adsorption sites on which O 2 can be stabilised. In experiments with TSP activated at higher temperatures (e.g. 500 K) we have observed an additional EPR signal which appears to be O 2 , but further work is needed to confirm this assignment. Infrared experiments have shown that structural dehydroxylation occurs on outgassing above 400 K, which may generate sites able to stabilise O 2. Irradiation of TSP in oxygen at 77 K instead of room temperature does not give the O and O 3 signals described above, but they appear on subsequent warming to room temperature. Furthermore, if the sample is subsequently irradiated at 77 K, the O signal is diminished with increasing irradiation time. These preliminary results suggest that
M. Murdoch et al. / Microporous and Mesoporous Materials 101 (2007) 184–190
* (c)
* (b)
*
(a)
3250
3270
3290
3310
3330
3350
gauss Fig. 6. (a) EPR spectrum measured at 77 K of TSP after exposure to 50 mbar of CO2 at room temperature; (b) following UV irradiation at room temperature for 1 h; (c) following UV irradiation at room temperature for 2 h. Asterisk marks features due to a defect in the quartz sample tube.
the dynamics of hole and electron trapping are strongly temperature dependent, and further work is in progress to clarify these phenomena. 3.3. Photoreactivity in carbon dioxide Fig. 6 shows EPR spectra measured at 77 K following UV irradiation of TSP outgassed at 393 K in the presence of carbon dioxide at room temperature. The O signal which develops as a function of time is identical to that formed on irradiation in O2 (Fig. 4). However, in CO2 the second signal seen after irradiation in O2 and assigned to O 3 species was not observed. We presume that CO2 is functioning here, like O2, as an electron scavenger. One electron transfer to CO2 would form the CO 2 radical anion, which has been observed on MgO surfaces, for example [29]. This species was not seen here, possibly because of the absence of suitable sites on which it could be adsorbed, as for O 2. 3.4. Comparisons with anatase The photoreactivity of TSP activated at 393 K resembles in some respects that of fully hydroxylated anatase. The
189
ability of oxygen to scavenge electrons and allow a stable trapped hole signal to be observed is common to both materials. However, the trapped holes in TSP are considerably more stable than those in hydroxylated anatase (the O signal in anatase is stable in the presence of O2 only at 77 K). The thermal activation required to trap holes in TSP is unique to this material, as is the formation of an unusually stable anomalous ozonide radical. The microporosity of TSP would appear to contribute to the stability of both oxygen related radicals. The small size of the titania clusters in TSP may be another important factor. The stabilisation of trapped holes by CO2 (functioning as an electron scavenger) is also novel. Further work is in progress in our laboratory to characterise the photoreactivity of TSP subjected to higher temperature pre-treatments, and to investigate the effects of varying the exchangeable cations in the TSP structure on its photoreactivity. Although at first sight the 8-ring pore dimensions of TSP might be considered to restrict its applications as a photocatalyst, the nanocrystallinity of the material means that many adsorption sites are available also on the external surface and at pore openings where larger molecules may interact with photo-generated holes or electrons. Investigation of the photocatalytic properties of this novel small-pore zeolite system may therefore be worthwhile. Acknowledgments This work was supported by the Donors of the Petroleum Research Fund of the American Chemical Society. We thank Dr. Wuzong Zhou of the University of St Andrews for his assistance in obtaining the TEM images. References [1] O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004), and references therein. [2] R.F. Howe, M. Graetzel, J. Phys. Chem. 91 (1987) 3906. [3] T. Berger, M. Sterrer, O. Diwald, E. Knozinger, D. Panayatou, T.L. Thompson, J.T. Yates, J. Phys. Chem. B 109 (2005) 6061. [4] T. Berger, M. Sterrer, O. Diwald, E. Knozinger, ChemPhysChem 6 (2005) 2104. [5] D.C. Hurum, K.A. Gray, T. Rajh, M.C. Thurnauer, J. Phys. Chem. B 109 (2005) 977. [6] S.C. Ke, T.C. Wang, M.S. Wong, N.O. Gopal, J. Phys. Chem. B 110 (2000) 11628. [7] N. Serpone, D. Lawless, R. Kharautdinov, J. Phys. Chem. 99 (1995) 16646. [8] D.C. Hurum, A.G. Agrios, K.A. Gray, T. Rajh, M.C. Thurnauer, J. Phys. Chem. B 107 (2003) 4545. [9] R.F. Howe, Y.K. Krisnandi, Chem. Commun. (2001) 1588. [10] P.D. Southon, R.F. Howe, Chem. Mater. 14 (2002) 4209. [11] Y. Krisnandi, P. Southon, A. Adesina, R.F. Howe, Int. J. Photoenergy 5 (2003) 131. [12] Y. Krisnandi, E. Lachowski, R.F. Howe, Chem. Mater. 18 (2006) 928. [13] Y. Krisnandi, R.F. Howe, Appl. Catal. A 307 (2006) 62. [14] E. Borello, C. Lamberti, S. Bordiga, A. Zecchina, C. Otero Arean, Appl. Phys. Lett. B 71 (1997) 2319.
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