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Interparticle electron transfer in methanol dehydrogenation on platinum-loaded titania particles prepared from P25
Catalysis Today 303 (2018) 327–333
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Interparticle electron transfer in methanol dehydrogenation on platinumloaded titania particles prepared from P25 Kunlei Wanga, Zhishun Weib, Bunsho Ohtania,b, Ewa Kowalskaa,b, a b
T
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Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Action spectra Interparticle electron transfer Platinum nanoparticles P25 Photocatalytic activity Titania
Commercial titania sample (P25) was homogenized first (HomoP25), and then crystalline phases (anatase and rutile) were isolated from it by chemical dissolution methods followed by samples’ purification by washing and annealing. The photocatalytic activities were tested for anaerobic methanol dehydrogenation in two reaction systems: (1) under UV/vis polychromatic irradiation with in situ platinum (Pt) deposition, and (2) under UV/vis monochromatic irradiation (action spectra) for ex situ platinum deposition. The properties of bare and modified titania samples were investigated by XRD, DRS, STEM, BET and zeta potential measurements. It was found that platinized rutile was more active than platinized anatase and platinized HomoP25, due to ability of photoabsorption of more photons (narrower band gap). Moreover, in HomoP25 sample, rutile was platinized first, probably due to positively charged surface allowing favorable adsorption of chloroplatinate ions. More than 10 times lower content of Pt (< 0.1 wt%) deposited on annealed samples (forming aggregates with an increased interface between titania NPs) than that on HomoP25 (2 wt%) resulted in similar level of photocatalytic activity, suggesting an interparticle electron transfer (IPET) between titania NPs inside one aggregates (one Pt NP was sufficient for one aggregate).
1. Introduction P25 (also known as AEROXIDE® TiO2 P 25, Evonik P25 or Degussa P25) is a commercial titania sample, produced by Nippon Aerosil Co., Ltd., and composed of two crystalline forms of titania: anatase and rutile, and amorphous phase [1]. P25 has been widely used as photocatalyst due to high photocatalytic activity in various photocatalytic reaction systems [1,2], such as decomposition of organic (e.g., phenols (nitro-, chloro-), dyes, acids (formic, oxalic, salicylic, hydroxybenzoic and humic), herbicides (isoproturon, simazine and propazine), pharmaceuticals, chloromethane, catechol, dioxane, aromatic aminocompounds [3–7]) and inorganic (arsenic [8]) compounds in water and wastewater, degradation of gas-phase pollutants (carbon tetrachloride, carbon monoxide, acetylene, ethanol, 2-propanol, acetone, trichloroethylence, NOx [9–13]), purification of indoor environment (mite allergens [14]), inactivation of microorganisms [15–18], selfcleaning surfaces (cotton textiles, cementitious and cellulose materials [19–21]), and for solar energy conversion [22]. Complete degradation (100%) of various compounds (often resulting in their mineralization) has been frequently reported, e.g., (i) textile dyes (Reactive Yellow 17 (RY17), Reactive Red 2 (RR2) and Reactive Blue 4 (RB4)) were
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completely decomposed (mineralized) on suspended P25 (1 g/L) during 3–12 h of UV irradiation, and during longer time (> 12 h) of solar radiation [3], ii) pharmaceutical drugs were more efficiently decomposed and mineralized on P25 than on Merck anatase under UV irradiation reaching complete mineralization for 22 h and 28 h, respectively [23], and (iii) P25 showed the highest photocatalytic activities among forty commercial titania photocatalysts for oxidative decomposition of acetic acid [24]. It should be pointed that P25 does not have the best surface properties (as shown in Table 1) among various titania photocatalysts [24], but usually it has better photocatalytic activities (for both oxidation and reduction reactions) than other photocatalysts even those with better surface properties (larger specific surface area, smaller crystallites, larger crystallinity, better morphology, e.g., faceted nanoparticles [25,26]). Therefore, high photocatalytic activity of P25 could not be simply explained by surface properties, and various reasons have been already proposed, such as (i) co-presence of some impurity in the crystal lattice of titania, e.g., Fe3+, which may retard charge carriers’ recombination [27], (ii) enrichment of titania surface with OH groups resulting in enhanced formation of hydroxyl radicals under irradiation [28–30] (iii), presence of amorphous titania (usually inactive form of titania due to fast recombination of charge carriers), which could
Corresponding author at: Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan. E-mail address: [email protected] (E. Kowalska).
http://dx.doi.org/10.1016/j.cattod.2017.08.046 Received 16 June 2017; Received in revised form 18 August 2017; Accepted 22 August 2017 Available online 26 August 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.
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Table 1 The characterization of isolated samples and HomoP25. Sample
HomoP25 ANA RUT
Crystal composition (XRD)
Crystallite size (XRD)
SSA (BET)
Particle size (STEM)
anatase (%)
rutile (%)
NC part (%)
anatase/nm
rutile/nm
/m2g−1
average/nm
mean/nm
78 86 0
14 1 98
8 13 2
25.3 23.7 –
39.6 – 39.5
52 61 28
27.5 23.3 40.2
22.5 19.5 32.4
NC–non-crystal, SSA–specific surface area.
2. Experimental
participate in transfer of charge carriers between different phases (anatase ↔ amorphous ↔ rutile (various combination of electrons and holes migrations between two or three phases have been proposed [31]), due to the existence of localized Anderson states at the band edges of the amorphous phase creating tails into the bandgap [31]), (iv) co-existence of anatase and rutile (e.g., the overlayer of rutile on an anatase crystal [31]) resulting in electron transfer between them, and thus retardation of charge carriers’ recombination. The last reason is the most often used for explanation of the high photocatalytic activity of P25 [2,31–33], where usually the transfer of electrons from anatase to rutile has been proposed, due to more negative potential of conduction band (CB) of anatase (larger bandgap) than rutile [34–41]. In contrast, some reports proposed opposite direction of electron transfer from rutile to anatase, e.g., migration of holes from anatase core to the rutile overlayer and adverse transfer of electrons [31], and transfer of electrons from rutile to anatase lattice trapping sites [2,42]. Although, the results of some investigations suggest the electron transfer between anatase and rutile in P25 (e.g., by EPR spectroscopy [2,42]), the reference samples of anatase and rutile have not been the same as those in P25. Therefore, it is impossible to definitely conclude that high photocatalytic activity of P25 is caused by transfer of charge carriers between crystalline phases. In our previous study, anatase and rutile crystalline particles were isolated from P25 and re-build P25 sample (R-P25) was prepared with the same anatase to rutile ratio as in P25 (anatase/rutile = 5.6). It was found that the activity of isolated-anatase was higher than that of P25 and R-P25 for oxidative decomposition of acetic acid and acetaldehyde; whereas the activity of isolated-rutile was the highest for water oxidation (oxygen liberation in the presence of silver sulfate) [1], suggesting that single crystalline phases (anatase and rutile) were responsible for high photocatalytic activity of P25, but not charge carriers’ migration. However, during methanol dehydrogenation (with in-situ platinum (Pt) photodeposition), the P25 and R-P25 showed higher photocatalytic activity than that of isolated-anatase and isolatedrutile, which could support the hypothesis of charge carriers’ transfer between anatase and rutile. Nevertheless, it must be pointed that in this reaction, Pt nanoparticles (NPs) are formed in situ as an electron collector for the hydrogen liberation [43–45]. It is thought that in this reaction not only the intrinsic properties of titania, but also the properties of Pt NPs are crucial for overall photocatalytic activity. For example, it was shown that efficiency of methanol dehydrogenation on octahedral anatase particles (OAPs, with one of the highest photocatalytic activities among other titania samples for oxidative decomposition of organic compounds and medium activities for methanol dehydrogenation) was significantly enhanced when distribution of Pt NPs on OAPs was improved (inhibition of Pt NPs aggregation with simultaneous decrease in the content of bare OAPs (without Pt NPs)) [46]. Therefore, methanol dehydrogenation on P25 and isolated crystalline phases was investigated in detail in the present study (with special emphasis on Pt-loading) to confirm or reject the possibility of charge carriers’ migration between anatase and rutile as the main reason of high photocatalytic activity of P25.
2.1. Sample preparation Since P25 is highly heterogeneous and sampling it (at different positions in the package) results in receiving various data of its composition (the content of anatase (73–85%), rutile (14–17%) and noncrystalline phase (0–13) [1]), P25 sample (AEROXIDE® TiO2 P 25 produced by Nippon Aerosil Co., Ltd) was homogenized first (HomoP25), i.e., 20 g of P25 was suspended in 0.2 L of Milli-Q water, mechanical stirred for 24 h and freeze-dried for 24 h (< 193 K and < 10 Pa; details in SI) [47,48]. Anatase particles (isolated-anatase) and rutile particles (isolated-rutile) were prepared from HomoP25 by procedures proposed in literature [49,50] with slight modifications (for sample purification). In brief, isolated-anatase sample (ANA) was obtained by suspending HomoP25 (6.0 g) in a mixture of hydrogen peroxide (H2O2, 30%, 200 mL) and ammonia (NH3, 25%, 6 mL), stirring for 15 h at 298 K, washing with water, freeze-drying and aerobic annealing for 2.5 h at 473 K. The isolated-rutile sample (RUT) was obtained from HomoP25 (9.0 g) by its suspending in an aqueous solution of hydrofluoric acid (10%, 200 mL), stirring for 15 h, washing with water and with alkaline solution (NaOH, 1 M), freeze-drying and aerobic annealing for 2.5 h at 473 K. Reference sample of HomoP25 annealed at 473 K (in the presence of air) was also prepared, and was named as HomoP25-200. For action spectrum (AS) analysis, Pt NPs were first deposited on HomoP25, ANA and RUT samples by photodeposition method. Pt photodeposition was carried out under UV/vis irradiation (400-W highpressure mercury arc, λ > 290 nm) on 500 mg of titania suspended in aqueous methanol (50 vol%, 25 mL). Aqueous solution of chloroplatinic acid (H2PtCl6) was added to the test tube (55-mL pyrex glass containing the suspension of titania) under continuous stirring (to enable uniform distribution of Pt NPs on titania). Then, air from the tube was purged by argon bubbling (ca. 15 min), the tube was sealed with a rubber septum, and the absence of oxygen was checked by gas-chromatography (TCDGC, Shimadzu GC-8A-IT). The suspension was irradiated under continuous stirring (1000 rpm) at 298 K in a thermostated water bath [51], and the amount of evolved hydrogen was checked each 15 min by TCDGC. After irradiation, the sample was centrifuged, washed with methanol (3 times) and water (5 times), and freeze-dried. 2.2. Characterization of samples Absorption properties of samples were measured by diffuse reflectance spectroscopy (DRS). The measurements were carried out on Jasco V-670 spectrophotometer equipped with a PIN-757 integrating sphere. The baseline was recorded using BaSO4 as a reference. The crystalline composition was investigated by XRD analysis on a Rigaku Intelligent X-ray diffraction system SmartLab equipped with: a sealed tube X-ray generator (a copper target; operated at 40 kV and 30 mA), a D/teX high-speed position sensitive detector system and an ASC-10 automatic sample changer. Data acquisition conditions were as follows: range: 10–90°, scan speed: 1.00° min−1 and scan step 0.008°. The obtained XRD patterns were analyzed by Rigaku PDXL, a crystal structure analysis package including Rietveld analysis, installed in a 328
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computer controlling the diffractometer. The crystallinity of the samples was evaluated by the internal standard method, where highly crystalline nickel oxide (NiO) was used as the standard [24]. In brief, 40 mg of NiO (20.0 wt%) was mixed thoroughly with a 160 mg of titania sample (80.0 wt%) by grinding in an agate mortar. The specific surface area (SSA) was measured by nitrogen adsorption at 77 K on titania (100–150 mg) using a NOVA-1200e analyzer by the Brunauer-Emmett-Teller (BET) equation. During the pretreatment (degasification), the samples were heated at 423 K for 1.5 h. Scanning transmission electron microscopy (STEM) was used for morphology examination. Titania powders were suspended in water by ultrasounds for a few minutes, several drops of the suspension were placed on a copper grid (Onken, type A, 150 μm), and then the grid was dried under vacuum overnight. More than 500 NPs of titania and 300 NPs of Pt were counted for evaluation of distribution of particles’ sizes. It should be pointed that for evaluation of Pt NPs’ size, only the width of single NP was considered, but not the length of formed aggregates (as shown in Fig. 5).
Fig. 1. XRD patterns of RUT, ANA and HomoP25.
The intensity of irradiation, measured by a Hioki 3664 optical power meter, was in the range of 6.19–7.63 mW. During the irradiation, a portion (0.2 mL) of the gas phase from the reaction mixture was withdrawn with a syringe and injected into a gas-chromatograph. Apparent quantum yield (Φapp) was calculated as the ration of the rate of electron consumption from the rate of hydrogen generation to the flux of incident photons, assuming that two photons are required.
2.3. Photocatalytic activity test The photocatalytic activities of samples were evaluated for methanol dehydrogenation on platinized titania samples by measurement of the amount of liberated hydrogen in two reaction systems, i.e., (1) under polychromatic irradiation (UV/vis), and (2) under monochromatic irradiation (action spectrum (AS) analysis). For polychromatic irradiation (1), the Pt NPs were photodeposited in situ under anaerobic conditions. The procedure was the same as that described for deposition of Pt on titania (point 2.1.), with the exception of: i) the volume of testing tubes (35 mL), ii) the volume of methanol (5 mL), iii) the amount of weighted titania (50 mg). For AS analysis (2), first Pt was deposited on titania (as described in point 2.1.) and then apparent quantum yield was measured, as follows: 30 mg of photocatalyst was suspended in 3 mL of aq. methanol (50 vol %), air was removed from a quartz cell by argon bubbling (ca. 15 min), and a cell was sealed with a rubber septum. Then, a cell was irradiated for 1 h using a diffraction grating type illuminator (JASCO CRM-FD) equipped with a 300-W xenon lamp (Hamamatsu Photonics C2578-02).
3. Results and discussion The homogenization of P25 sample (Table S1) was efficient and resulted in preparation of HomoP25 sample of uniform composition with the content of anatase, rutile and non-crystalline phases amounted to 78%, 14% and 8%, respectively, independently on the sampling position, as shown in Table S2. The isolation processes resulted in preparation of almost pure anatase and rutile samples, as shown in Fig. 1 and Table 1. The disappearance of anatase in XRD pattern of RUT confirmed that anatase was not stable in the HF solution, as first proposed by T. Ohno et al. [1,40,49,52]. Anatase disappearance was also observed by a decrease in specific surface area (SSA) from 52 m2/g (HomoP25) to 28 m2/g (RUT), because of its smaller particles/crystallites than those in rutile (Table 1, Fig. 2). Moreover, the isolation process did not change the original Fig. 2. STEM (SE mode) images of HomoP25 (A), RUT (B) and ANA (C) with distribution of NPs’ sizes (D).
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Fig. 3. (A) Dependence of photocatalytic activity on the amount of Pt loading, and (B) relative DRS of HomoP25, ANA and RUT.
overall activity in this reaction system. Therefore, the influence of Ptloading amount (0.005–2 wt%) on the photocatalytic activity was investigated, and the obtained data are shown in Fig. 3A. It was found that the amount of platinum highly influenced the photocatalytic activity. Obviously, with an increase in Pt-loading amount, the photocatalytic activities increased for all tested samples, confirming that Pt NPs are necessary for efficient hydrogen evolution. RUT sample showed the highest photocatalytic activity independently on loading amount of platinum, probably due to broader photoabsorption than that of anatase (narrower band-gap), as shown in Fig. 3B. Although, the lower level of conduction band (CB) of rutile has been reported as the main reason of its lower photocatalytic activity than that of anatase (poor reduction ability of photogenerated electrons) [70–72], the presence of Pt NPs (as a co-catalyst and an electron pool) compensates it, and probably the broader photoabsorption results in enhanced overall photocatalytic activity. For larger amount of loaded-platinum (2 wt%), the activities of all samples seemed to saturate (were almost the same), and slight decrease at the higher loadings are attributable to the shielding of light by Pt deposits. The saturation can be interpreted by the expectation that at least one Pt NP was deposited on each titania particle (all titania particles were modified), on the basis of assumption that only Pt-deposited particles were active for photocatalytic hydrogen evolution and the activity did not depend on the number of Pt NPs if at least one Pt NP was deposited, as was suggested previously [45]. In the lower Pt content range (0.005–0.2 wt%), the photocatalytic activities of RUT and ANA are significantly (few times) larger than that of HomoP25. It should be reminded that isolated samples were annealed at 473 K (for sample purification). Although this relatively low temperature should not cause NPs sintering (what was confirmed by STEM observations where only a few larger rutile NPs could be observed (Fig. 2)), it is proposed that the interface between NPs was increased allowing enhanced electron transfer between two (or more) titania NPs. In this regard, much lower amount of Pt could be sufficient for high photocatalytic activity, i.e., one Pt NP on one titania aggregate (as shown in Fig. 4). In other words, interparticle electron transfer (IPET) must occur within those aggregates/agglomerates formed by relatively low-temperature heating. Slight decrease in photocatalytic activity for larger than 0.2 wt% content of Pt for ANA and RUT is possibly caused by Pt aggregation (increase in Pt size with loading amount was observed, e.g., as shown for HomoP25 sample in Fig. 5) or “inner filter effect” (as Pt NPs are dark limiting photon absorption by titania). To confirm that enhanced photocatalytic activity could be obtained by titania NPs aggregation by annealing, HomoP25 was also annealed at the same temperature as ANA and RUT (473 K), and obtained data for this sample (HomoP25-200) are shown also in Fig. 3A. It was found, that lower amount of Pt was sufficient to cause the same photocatalytic activity for HomoP25-200, e.g., the same photocatalytic activities for both samples were obtained for 0.2 wt% (HomoP25-200) and 0.5 wt% (HomoP25) of Pt loading. By the annealing at 473 K in air, the activity of HomoP25-200 at the lower region of Pt-loading amount was slightly
rutile NPs present in HomoP25 since the full width at half maximum (FWHM) of rutile peaks in RUT was almost the same as that in HomoP25 (ca. 40-nm rutile crystallites). Similarly, the isolation of anatase was very efficient, suggesting that almost all rutile crystallites were dissolved in the mixture of hydrogen peroxide and ammonia. It was reported that titania treatment with alkaline solution of hydrogen peroxide resulted in formation of yellow Ti4+-H2O2 complex for both crystalline forms of titania, but dissolution of rutile seemed to be faster than that of anatase [50]. The partial dissolution of also anatase was confirmed by yellow coloration of isolated-anatase sample (before annealing, which was successfully used for sample purification) and slight decrease in crystallite size (Table 1). Although it was proved that single crystalline phases could be obtained efficiently, non-crystal part (NC) could not be removed completely (2% in RUT), and even an increase in its content was observed for ANA sample (from 8% in HomoP25 to 13%). It is proposed that titania crystallites dissolved during isolation process could re-form into amorphous titania particles (The possibility of other compounds/impurities was discarded by XPS analysis.). Nevertheless, amorphous titania should not influence the overall photocatalytic activity significantly due to the small content of NC in the final products (ANA, RUT), and its low photocatalytic activity (fast charge-carriers’ recombination) [53–55]. The microscopic observations confirmed that rutile NPs are larger than anatase NPs (Fig. 2). Moreover, the small content of larger NPs in HomoP25 suggests that all of them are composed of rutile phase (Fig. 2, Table 1). Interestingly, the average size of anatase (23.3 nm) in ANA sample and the average size of rutile (40.2 nm) in RUT sample are almost the same as respective crystallite sizes (23.7 nm and 39.5 nm). In addition, the average size of titania NPs in HomoP25 (27.5 nm) is practically the same as weighted average size of anatase and rutile crystallites (27.4 nm). Therefore, it has been concluded that multiphase particles (anatase-rutile, anatase-amorphous, rutile-amorphous) are not present in P25 in contrast to some previous reports (e.g., anatase with rutile overlayer [31]). Of course, an existence of several multiphase particles could not be completely rejected, but their content in the overall composition is negligible. It should be pointed that isolated samples possessed also some larger aggregates, which were not observed in HomoP25 sample, because of possible NPs sintering during annealing, which was used as post-treatment operations for sample purification. The photocatalytic activities of P25 and isolated samples were examined for methanol dehydrogenation. It is known that bare titania samples are practically inactive for hydrogen generation (Although the position of conduction band of titania is negative enough to reduce proton, the high activation overpotential for hydrogen evolution results in negligible activity.). Therefore, the metallic co-catalysts have been usually used for efficient hydrogen evolution, e.g. platinum [43,45,56,57], gold [58–62], silver [63–66], and copper [67–69]. It is thought that not only the properties of titania, but also the amount and the properties of Pt NPs (size and distribution) should influence the 330
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explanation of mechanism needs further study, which is now in progress. One of the most valuable methods to investigate the mechanism of photocatalytic reactions is action spectrum analysis. Action spectra of methanol dehydrogenation on bare and platinizedsamples (for 0.2 wt% of Pt) were investigated, and obtained data are shown in Fig. 6A. The necessity of Pt NPs’ presence was confirmed since bare ANA and bare RUT sample were practically inactive in this reaction system. At short UV range (350–365 nm), the apparent quantum efficiency (Φapp) of platinized samples correlated with the overall photocatalytic activity of those samples (Fig. 3A), i.e., Pt@RUT > Pt@ANA > Pt@HomoP25. Much lower photocatalytic activity of Pt@ HomoP25 was caused by insufficient amount of Pt in that sample (nonaggregated titania NPs), as has been discussed above. The shape of action spectra of Pt@ANA and Pt@RUT resembled respective absorption spectra (Fig. 3B and Fig. 6C). Interestingly, the photocatalytic activity of Pt@HomoP25 was higher than that of Pt@ANA at UV range of 380–395 nm. It should be reminded that at this UV range mainly rutile absorbs light (especially at 395 nm). Therefore, higher photocatalytic activity of Pt@HomoP25 than that of Pt@ANA indicates that rutile was platinized first, especially taking into consideration much lower content of rutile than anatase in HomoP25 (14% vs. 78%). To confirm this assumption, various composition of samples (physical mixtures) were prepared and tested, i.e., (a) 15% Pt(0.2 wt%)/RUT and 85% ANA; (b) 50% Pt(0.2 wt%)/RUT and 50% ANA; (c) 15% RUT and 85% Pt(0.2 wt %)/ANA; (d) 15% RUT and 85% Pt(0.1 wt%)/ANA; (e) 15% Pt(0.2 wt %)/RUT and 85% Pt(0.2 wt%)/ANA; (f) 15% Pt(0.1 wt%)/RUT and 85% Pt(0.1 wt%)/ANA. Firstly, larger content of platinized sample in the mixture resulted in enhanced photocatalytic activity, i.e., (b) > (a), (c) > (d), and (e) > (f). At short wavelengths, the mixtures of both platinized phases ((e) and (f)) showed higher photocatalytic activities than that of mixtures composed of one bare and one modified phase, which is not surprising since both platinized phases showed high photocatalytic activities (as shown in Fig. 6A). It should be pointed that all those mixtures had lower activities than that of Pt@
Fig. 4. Schematic drawing showing interparticle electron transfer in aggregated singlephase titania particles (Pt@ANA and Pt@RUT).
improved compared with that of P25, i.e., the amount of Pt deposits required for activation of titania particles was reduced by annealing, but was lower than those of ANA and RUT. This may be also explained by the aggregation/agglomeration of titania particles by the relatively low-temperature heating to enable photoexcited electrons transfer within aggregates/agglomerates (IPET), and the effective size of HomoP25-200 aggregates/agglomerates for IPET is smaller than those of ANA and RUT (Note that even though the heating temperature at the final stage of isolation of ANA and RUT was similar to that of HomoP25, ANA and RUT isolation processes could modify their surface structure affecting the size and connection of aggregate/agglomerates.). The
Fig. 5. (A-B) STEM observation of HomoP25 modified with 2 wt% of Pt NPs: (A) SEmode, and (B) TEM-mode, (C) distribution of Pt NPs’ sizes, and (D) XRD patterns for bare and modified titania.
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Fig. 6. Action spectra (A, B, D) and absorption spectra (C, D) of samples: (a) 15% Pt(0.2 wt%)/RUT and 85% ANA; (b) 50% Pt (0.2 wt%)/RUT and 50% ANA; (c) 15% RUT and 85% Pt(0.2 wt%)/ANA; (d) 15% RUT and 85% Pt(0.1 wt%)/ANA; (e) 15% Pt (0.2 wt%)/RUT and 85% Pt(0.2 wt%)/ANA; (f) 15% Pt(0.1 wt%)/RUT and 85% Pt (0.1 wt%)/ANA.
RUT. However, the Φapp of (e) sample (Φapp = 73%, containing platinized anatase and rutile) was higher than that of Pt@ANA (Φapp = 69%) confirming that rutile was more active phase in this system. The most interesting is the shape of action spectra of (a) and (b) samples containing only platinized rutile with the sharp peak at 395 nm. The explanation is similar to that used for explanation of higher photocatalytic activity of Pt@HomoP25 than Pt@ANA (Fig. 6A), i.e., at this wavelength only rutile can absorb light, whereas at shorter wavelength (380 nm) both phases absorb light. Therefore, a decrease in Φapp (at 380 nm) was caused by the competition between two phases: active (Pt@RUT) and inactive (ANA), about photon absorption. Interestingly, re-estimated absorption spectrum (from the DRS measurements of ANA and RUT (details in SI)) shows characteristic peak (Fig. 6C) observed at action spectrum. The characteristic peak in the spectrum is clearly caused by the difference in photoabsorption properties between anatase and rutile, due to the difference in the bandgap of rutile (3.0 eV) and anatase (3.2 eV) [41], i.e., i) at the wavelengths shorter than ca. 390 nm both anatase and rutile can absorb photons; ii) at 390–400 nm the absorption properties of anatase are decreasing, and iii) at the wavelength longer than ca. 400 nm only rutile can absorb photons. In addition, HomoP25 with larger content of platinum (0.5 wt %) was also tested and characteristic peak at 395 nm was obtained (Fig. 6D; the same as for (a) and (b) samples), confirming that rutile phase was platinized first. Therefore, it was shown that rutile is more active form of titania for methanol dehydrogenation in the presence of platinum as a co-catalyst. Moreover, at first rutile was platinized, despite larger content of anatase in P25. One of the possible reasons is preferable adsorption of Pt precursor (hexachloroplatinum(IV) ions) on rutile particles in P25. As a general trend, it has been often reported that rutile particles tend to exhibit higher photocatalytic activity for deposition of metal such as silver than anatase particles [73,74]. Another possible reason is preferable electron transfer from anatase particles, as a major component absorbing light, to rutile particles, which has often been claimed in the literature, since the conduction-bottom position of rutile is slightly lower than that of anatase [75]. Zeta potential analysis (Table S3) showed that rutile NPs (RUT) were positively
charge, whereas anatase NPs were negatively charged, confirming the first hypothesis, i.e., the favorable adsorption of chloroplatinate anions on rutile than on anatase.
4. Conclusions It was found that platinized rutile was more active than platinized anatase and platinized HomoP25, probably due to ability of photoabsorption of more photons (narrower band gap). Therefore, proposed in literature an electron transfer between two polymorphic forms of titania as the main reason of high photocatalytic activity of P25 has been discarded. Moreover, it was found that in P25 sample, rutile was platinized first, probably due to positively charged surface allowing easy adsorption of chloroplatinate ions. Although, there are many reports claiming interparticle electron transfer (IPET) in heterogeneous photocatalytic systems, especially preferable transfer from anatase to rutile particles in P25, which seems to be consistent with the experimental results, probably there is no direct evidence for IPET. The authors' group has suggested that for P25 photocatalysis no synergetic effect (due to IPET between anatase and rutile) is required for interpretation of high photocatalytic activity of P25 [1]. The results shown in the present study suggest that IPET may occur not only between anatase and rutile (hetero-IPET), but also between single anatase particles and single rutile particles when isolated ANA and RUT are heated at relatively low temperature to form aggregates/agglomerates (homo-IPET). This may be the first evidence for IPET of titania particles. Furthermore, it is suggested that careful discussion is needed for understanding photocatalytic-reaction mechanism considering the possible hetero and homo-IPET.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2017.08.046. 332
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Interparticle electron transfer in methanol dehydrogenation on platinumloaded titania particles prepared from P25 Kunlei Wanga, Zhishun Weib, Bunsho Ohtania,b, Ewa Kowalskaa,b, a b
T
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Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Action spectra Interparticle electron transfer Platinum nanoparticles P25 Photocatalytic activity Titania
Commercial titania sample (P25) was homogenized first (HomoP25), and then crystalline phases (anatase and rutile) were isolated from it by chemical dissolution methods followed by samples’ purification by washing and annealing. The photocatalytic activities were tested for anaerobic methanol dehydrogenation in two reaction systems: (1) under UV/vis polychromatic irradiation with in situ platinum (Pt) deposition, and (2) under UV/vis monochromatic irradiation (action spectra) for ex situ platinum deposition. The properties of bare and modified titania samples were investigated by XRD, DRS, STEM, BET and zeta potential measurements. It was found that platinized rutile was more active than platinized anatase and platinized HomoP25, due to ability of photoabsorption of more photons (narrower band gap). Moreover, in HomoP25 sample, rutile was platinized first, probably due to positively charged surface allowing favorable adsorption of chloroplatinate ions. More than 10 times lower content of Pt (< 0.1 wt%) deposited on annealed samples (forming aggregates with an increased interface between titania NPs) than that on HomoP25 (2 wt%) resulted in similar level of photocatalytic activity, suggesting an interparticle electron transfer (IPET) between titania NPs inside one aggregates (one Pt NP was sufficient for one aggregate).
1. Introduction P25 (also known as AEROXIDE® TiO2 P 25, Evonik P25 or Degussa P25) is a commercial titania sample, produced by Nippon Aerosil Co., Ltd., and composed of two crystalline forms of titania: anatase and rutile, and amorphous phase [1]. P25 has been widely used as photocatalyst due to high photocatalytic activity in various photocatalytic reaction systems [1,2], such as decomposition of organic (e.g., phenols (nitro-, chloro-), dyes, acids (formic, oxalic, salicylic, hydroxybenzoic and humic), herbicides (isoproturon, simazine and propazine), pharmaceuticals, chloromethane, catechol, dioxane, aromatic aminocompounds [3–7]) and inorganic (arsenic [8]) compounds in water and wastewater, degradation of gas-phase pollutants (carbon tetrachloride, carbon monoxide, acetylene, ethanol, 2-propanol, acetone, trichloroethylence, NOx [9–13]), purification of indoor environment (mite allergens [14]), inactivation of microorganisms [15–18], selfcleaning surfaces (cotton textiles, cementitious and cellulose materials [19–21]), and for solar energy conversion [22]. Complete degradation (100%) of various compounds (often resulting in their mineralization) has been frequently reported, e.g., (i) textile dyes (Reactive Yellow 17 (RY17), Reactive Red 2 (RR2) and Reactive Blue 4 (RB4)) were
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completely decomposed (mineralized) on suspended P25 (1 g/L) during 3–12 h of UV irradiation, and during longer time (> 12 h) of solar radiation [3], ii) pharmaceutical drugs were more efficiently decomposed and mineralized on P25 than on Merck anatase under UV irradiation reaching complete mineralization for 22 h and 28 h, respectively [23], and (iii) P25 showed the highest photocatalytic activities among forty commercial titania photocatalysts for oxidative decomposition of acetic acid [24]. It should be pointed that P25 does not have the best surface properties (as shown in Table 1) among various titania photocatalysts [24], but usually it has better photocatalytic activities (for both oxidation and reduction reactions) than other photocatalysts even those with better surface properties (larger specific surface area, smaller crystallites, larger crystallinity, better morphology, e.g., faceted nanoparticles [25,26]). Therefore, high photocatalytic activity of P25 could not be simply explained by surface properties, and various reasons have been already proposed, such as (i) co-presence of some impurity in the crystal lattice of titania, e.g., Fe3+, which may retard charge carriers’ recombination [27], (ii) enrichment of titania surface with OH groups resulting in enhanced formation of hydroxyl radicals under irradiation [28–30] (iii), presence of amorphous titania (usually inactive form of titania due to fast recombination of charge carriers), which could
Corresponding author at: Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan. E-mail address: [email protected] (E. Kowalska).
http://dx.doi.org/10.1016/j.cattod.2017.08.046 Received 16 June 2017; Received in revised form 18 August 2017; Accepted 22 August 2017 Available online 26 August 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.
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Table 1 The characterization of isolated samples and HomoP25. Sample
HomoP25 ANA RUT
Crystal composition (XRD)
Crystallite size (XRD)
SSA (BET)
Particle size (STEM)
anatase (%)
rutile (%)
NC part (%)
anatase/nm
rutile/nm
/m2g−1
average/nm
mean/nm
78 86 0
14 1 98
8 13 2
25.3 23.7 –
39.6 – 39.5
52 61 28
27.5 23.3 40.2
22.5 19.5 32.4
NC–non-crystal, SSA–specific surface area.
2. Experimental
participate in transfer of charge carriers between different phases (anatase ↔ amorphous ↔ rutile (various combination of electrons and holes migrations between two or three phases have been proposed [31]), due to the existence of localized Anderson states at the band edges of the amorphous phase creating tails into the bandgap [31]), (iv) co-existence of anatase and rutile (e.g., the overlayer of rutile on an anatase crystal [31]) resulting in electron transfer between them, and thus retardation of charge carriers’ recombination. The last reason is the most often used for explanation of the high photocatalytic activity of P25 [2,31–33], where usually the transfer of electrons from anatase to rutile has been proposed, due to more negative potential of conduction band (CB) of anatase (larger bandgap) than rutile [34–41]. In contrast, some reports proposed opposite direction of electron transfer from rutile to anatase, e.g., migration of holes from anatase core to the rutile overlayer and adverse transfer of electrons [31], and transfer of electrons from rutile to anatase lattice trapping sites [2,42]. Although, the results of some investigations suggest the electron transfer between anatase and rutile in P25 (e.g., by EPR spectroscopy [2,42]), the reference samples of anatase and rutile have not been the same as those in P25. Therefore, it is impossible to definitely conclude that high photocatalytic activity of P25 is caused by transfer of charge carriers between crystalline phases. In our previous study, anatase and rutile crystalline particles were isolated from P25 and re-build P25 sample (R-P25) was prepared with the same anatase to rutile ratio as in P25 (anatase/rutile = 5.6). It was found that the activity of isolated-anatase was higher than that of P25 and R-P25 for oxidative decomposition of acetic acid and acetaldehyde; whereas the activity of isolated-rutile was the highest for water oxidation (oxygen liberation in the presence of silver sulfate) [1], suggesting that single crystalline phases (anatase and rutile) were responsible for high photocatalytic activity of P25, but not charge carriers’ migration. However, during methanol dehydrogenation (with in-situ platinum (Pt) photodeposition), the P25 and R-P25 showed higher photocatalytic activity than that of isolated-anatase and isolatedrutile, which could support the hypothesis of charge carriers’ transfer between anatase and rutile. Nevertheless, it must be pointed that in this reaction, Pt nanoparticles (NPs) are formed in situ as an electron collector for the hydrogen liberation [43–45]. It is thought that in this reaction not only the intrinsic properties of titania, but also the properties of Pt NPs are crucial for overall photocatalytic activity. For example, it was shown that efficiency of methanol dehydrogenation on octahedral anatase particles (OAPs, with one of the highest photocatalytic activities among other titania samples for oxidative decomposition of organic compounds and medium activities for methanol dehydrogenation) was significantly enhanced when distribution of Pt NPs on OAPs was improved (inhibition of Pt NPs aggregation with simultaneous decrease in the content of bare OAPs (without Pt NPs)) [46]. Therefore, methanol dehydrogenation on P25 and isolated crystalline phases was investigated in detail in the present study (with special emphasis on Pt-loading) to confirm or reject the possibility of charge carriers’ migration between anatase and rutile as the main reason of high photocatalytic activity of P25.
2.1. Sample preparation Since P25 is highly heterogeneous and sampling it (at different positions in the package) results in receiving various data of its composition (the content of anatase (73–85%), rutile (14–17%) and noncrystalline phase (0–13) [1]), P25 sample (AEROXIDE® TiO2 P 25 produced by Nippon Aerosil Co., Ltd) was homogenized first (HomoP25), i.e., 20 g of P25 was suspended in 0.2 L of Milli-Q water, mechanical stirred for 24 h and freeze-dried for 24 h (< 193 K and < 10 Pa; details in SI) [47,48]. Anatase particles (isolated-anatase) and rutile particles (isolated-rutile) were prepared from HomoP25 by procedures proposed in literature [49,50] with slight modifications (for sample purification). In brief, isolated-anatase sample (ANA) was obtained by suspending HomoP25 (6.0 g) in a mixture of hydrogen peroxide (H2O2, 30%, 200 mL) and ammonia (NH3, 25%, 6 mL), stirring for 15 h at 298 K, washing with water, freeze-drying and aerobic annealing for 2.5 h at 473 K. The isolated-rutile sample (RUT) was obtained from HomoP25 (9.0 g) by its suspending in an aqueous solution of hydrofluoric acid (10%, 200 mL), stirring for 15 h, washing with water and with alkaline solution (NaOH, 1 M), freeze-drying and aerobic annealing for 2.5 h at 473 K. Reference sample of HomoP25 annealed at 473 K (in the presence of air) was also prepared, and was named as HomoP25-200. For action spectrum (AS) analysis, Pt NPs were first deposited on HomoP25, ANA and RUT samples by photodeposition method. Pt photodeposition was carried out under UV/vis irradiation (400-W highpressure mercury arc, λ > 290 nm) on 500 mg of titania suspended in aqueous methanol (50 vol%, 25 mL). Aqueous solution of chloroplatinic acid (H2PtCl6) was added to the test tube (55-mL pyrex glass containing the suspension of titania) under continuous stirring (to enable uniform distribution of Pt NPs on titania). Then, air from the tube was purged by argon bubbling (ca. 15 min), the tube was sealed with a rubber septum, and the absence of oxygen was checked by gas-chromatography (TCDGC, Shimadzu GC-8A-IT). The suspension was irradiated under continuous stirring (1000 rpm) at 298 K in a thermostated water bath [51], and the amount of evolved hydrogen was checked each 15 min by TCDGC. After irradiation, the sample was centrifuged, washed with methanol (3 times) and water (5 times), and freeze-dried. 2.2. Characterization of samples Absorption properties of samples were measured by diffuse reflectance spectroscopy (DRS). The measurements were carried out on Jasco V-670 spectrophotometer equipped with a PIN-757 integrating sphere. The baseline was recorded using BaSO4 as a reference. The crystalline composition was investigated by XRD analysis on a Rigaku Intelligent X-ray diffraction system SmartLab equipped with: a sealed tube X-ray generator (a copper target; operated at 40 kV and 30 mA), a D/teX high-speed position sensitive detector system and an ASC-10 automatic sample changer. Data acquisition conditions were as follows: range: 10–90°, scan speed: 1.00° min−1 and scan step 0.008°. The obtained XRD patterns were analyzed by Rigaku PDXL, a crystal structure analysis package including Rietveld analysis, installed in a 328
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computer controlling the diffractometer. The crystallinity of the samples was evaluated by the internal standard method, where highly crystalline nickel oxide (NiO) was used as the standard [24]. In brief, 40 mg of NiO (20.0 wt%) was mixed thoroughly with a 160 mg of titania sample (80.0 wt%) by grinding in an agate mortar. The specific surface area (SSA) was measured by nitrogen adsorption at 77 K on titania (100–150 mg) using a NOVA-1200e analyzer by the Brunauer-Emmett-Teller (BET) equation. During the pretreatment (degasification), the samples were heated at 423 K for 1.5 h. Scanning transmission electron microscopy (STEM) was used for morphology examination. Titania powders were suspended in water by ultrasounds for a few minutes, several drops of the suspension were placed on a copper grid (Onken, type A, 150 μm), and then the grid was dried under vacuum overnight. More than 500 NPs of titania and 300 NPs of Pt were counted for evaluation of distribution of particles’ sizes. It should be pointed that for evaluation of Pt NPs’ size, only the width of single NP was considered, but not the length of formed aggregates (as shown in Fig. 5).
Fig. 1. XRD patterns of RUT, ANA and HomoP25.
The intensity of irradiation, measured by a Hioki 3664 optical power meter, was in the range of 6.19–7.63 mW. During the irradiation, a portion (0.2 mL) of the gas phase from the reaction mixture was withdrawn with a syringe and injected into a gas-chromatograph. Apparent quantum yield (Φapp) was calculated as the ration of the rate of electron consumption from the rate of hydrogen generation to the flux of incident photons, assuming that two photons are required.
2.3. Photocatalytic activity test The photocatalytic activities of samples were evaluated for methanol dehydrogenation on platinized titania samples by measurement of the amount of liberated hydrogen in two reaction systems, i.e., (1) under polychromatic irradiation (UV/vis), and (2) under monochromatic irradiation (action spectrum (AS) analysis). For polychromatic irradiation (1), the Pt NPs were photodeposited in situ under anaerobic conditions. The procedure was the same as that described for deposition of Pt on titania (point 2.1.), with the exception of: i) the volume of testing tubes (35 mL), ii) the volume of methanol (5 mL), iii) the amount of weighted titania (50 mg). For AS analysis (2), first Pt was deposited on titania (as described in point 2.1.) and then apparent quantum yield was measured, as follows: 30 mg of photocatalyst was suspended in 3 mL of aq. methanol (50 vol %), air was removed from a quartz cell by argon bubbling (ca. 15 min), and a cell was sealed with a rubber septum. Then, a cell was irradiated for 1 h using a diffraction grating type illuminator (JASCO CRM-FD) equipped with a 300-W xenon lamp (Hamamatsu Photonics C2578-02).
3. Results and discussion The homogenization of P25 sample (Table S1) was efficient and resulted in preparation of HomoP25 sample of uniform composition with the content of anatase, rutile and non-crystalline phases amounted to 78%, 14% and 8%, respectively, independently on the sampling position, as shown in Table S2. The isolation processes resulted in preparation of almost pure anatase and rutile samples, as shown in Fig. 1 and Table 1. The disappearance of anatase in XRD pattern of RUT confirmed that anatase was not stable in the HF solution, as first proposed by T. Ohno et al. [1,40,49,52]. Anatase disappearance was also observed by a decrease in specific surface area (SSA) from 52 m2/g (HomoP25) to 28 m2/g (RUT), because of its smaller particles/crystallites than those in rutile (Table 1, Fig. 2). Moreover, the isolation process did not change the original Fig. 2. STEM (SE mode) images of HomoP25 (A), RUT (B) and ANA (C) with distribution of NPs’ sizes (D).
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Fig. 3. (A) Dependence of photocatalytic activity on the amount of Pt loading, and (B) relative DRS of HomoP25, ANA and RUT.
overall activity in this reaction system. Therefore, the influence of Ptloading amount (0.005–2 wt%) on the photocatalytic activity was investigated, and the obtained data are shown in Fig. 3A. It was found that the amount of platinum highly influenced the photocatalytic activity. Obviously, with an increase in Pt-loading amount, the photocatalytic activities increased for all tested samples, confirming that Pt NPs are necessary for efficient hydrogen evolution. RUT sample showed the highest photocatalytic activity independently on loading amount of platinum, probably due to broader photoabsorption than that of anatase (narrower band-gap), as shown in Fig. 3B. Although, the lower level of conduction band (CB) of rutile has been reported as the main reason of its lower photocatalytic activity than that of anatase (poor reduction ability of photogenerated electrons) [70–72], the presence of Pt NPs (as a co-catalyst and an electron pool) compensates it, and probably the broader photoabsorption results in enhanced overall photocatalytic activity. For larger amount of loaded-platinum (2 wt%), the activities of all samples seemed to saturate (were almost the same), and slight decrease at the higher loadings are attributable to the shielding of light by Pt deposits. The saturation can be interpreted by the expectation that at least one Pt NP was deposited on each titania particle (all titania particles were modified), on the basis of assumption that only Pt-deposited particles were active for photocatalytic hydrogen evolution and the activity did not depend on the number of Pt NPs if at least one Pt NP was deposited, as was suggested previously [45]. In the lower Pt content range (0.005–0.2 wt%), the photocatalytic activities of RUT and ANA are significantly (few times) larger than that of HomoP25. It should be reminded that isolated samples were annealed at 473 K (for sample purification). Although this relatively low temperature should not cause NPs sintering (what was confirmed by STEM observations where only a few larger rutile NPs could be observed (Fig. 2)), it is proposed that the interface between NPs was increased allowing enhanced electron transfer between two (or more) titania NPs. In this regard, much lower amount of Pt could be sufficient for high photocatalytic activity, i.e., one Pt NP on one titania aggregate (as shown in Fig. 4). In other words, interparticle electron transfer (IPET) must occur within those aggregates/agglomerates formed by relatively low-temperature heating. Slight decrease in photocatalytic activity for larger than 0.2 wt% content of Pt for ANA and RUT is possibly caused by Pt aggregation (increase in Pt size with loading amount was observed, e.g., as shown for HomoP25 sample in Fig. 5) or “inner filter effect” (as Pt NPs are dark limiting photon absorption by titania). To confirm that enhanced photocatalytic activity could be obtained by titania NPs aggregation by annealing, HomoP25 was also annealed at the same temperature as ANA and RUT (473 K), and obtained data for this sample (HomoP25-200) are shown also in Fig. 3A. It was found, that lower amount of Pt was sufficient to cause the same photocatalytic activity for HomoP25-200, e.g., the same photocatalytic activities for both samples were obtained for 0.2 wt% (HomoP25-200) and 0.5 wt% (HomoP25) of Pt loading. By the annealing at 473 K in air, the activity of HomoP25-200 at the lower region of Pt-loading amount was slightly
rutile NPs present in HomoP25 since the full width at half maximum (FWHM) of rutile peaks in RUT was almost the same as that in HomoP25 (ca. 40-nm rutile crystallites). Similarly, the isolation of anatase was very efficient, suggesting that almost all rutile crystallites were dissolved in the mixture of hydrogen peroxide and ammonia. It was reported that titania treatment with alkaline solution of hydrogen peroxide resulted in formation of yellow Ti4+-H2O2 complex for both crystalline forms of titania, but dissolution of rutile seemed to be faster than that of anatase [50]. The partial dissolution of also anatase was confirmed by yellow coloration of isolated-anatase sample (before annealing, which was successfully used for sample purification) and slight decrease in crystallite size (Table 1). Although it was proved that single crystalline phases could be obtained efficiently, non-crystal part (NC) could not be removed completely (2% in RUT), and even an increase in its content was observed for ANA sample (from 8% in HomoP25 to 13%). It is proposed that titania crystallites dissolved during isolation process could re-form into amorphous titania particles (The possibility of other compounds/impurities was discarded by XPS analysis.). Nevertheless, amorphous titania should not influence the overall photocatalytic activity significantly due to the small content of NC in the final products (ANA, RUT), and its low photocatalytic activity (fast charge-carriers’ recombination) [53–55]. The microscopic observations confirmed that rutile NPs are larger than anatase NPs (Fig. 2). Moreover, the small content of larger NPs in HomoP25 suggests that all of them are composed of rutile phase (Fig. 2, Table 1). Interestingly, the average size of anatase (23.3 nm) in ANA sample and the average size of rutile (40.2 nm) in RUT sample are almost the same as respective crystallite sizes (23.7 nm and 39.5 nm). In addition, the average size of titania NPs in HomoP25 (27.5 nm) is practically the same as weighted average size of anatase and rutile crystallites (27.4 nm). Therefore, it has been concluded that multiphase particles (anatase-rutile, anatase-amorphous, rutile-amorphous) are not present in P25 in contrast to some previous reports (e.g., anatase with rutile overlayer [31]). Of course, an existence of several multiphase particles could not be completely rejected, but their content in the overall composition is negligible. It should be pointed that isolated samples possessed also some larger aggregates, which were not observed in HomoP25 sample, because of possible NPs sintering during annealing, which was used as post-treatment operations for sample purification. The photocatalytic activities of P25 and isolated samples were examined for methanol dehydrogenation. It is known that bare titania samples are practically inactive for hydrogen generation (Although the position of conduction band of titania is negative enough to reduce proton, the high activation overpotential for hydrogen evolution results in negligible activity.). Therefore, the metallic co-catalysts have been usually used for efficient hydrogen evolution, e.g. platinum [43,45,56,57], gold [58–62], silver [63–66], and copper [67–69]. It is thought that not only the properties of titania, but also the amount and the properties of Pt NPs (size and distribution) should influence the 330
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explanation of mechanism needs further study, which is now in progress. One of the most valuable methods to investigate the mechanism of photocatalytic reactions is action spectrum analysis. Action spectra of methanol dehydrogenation on bare and platinizedsamples (for 0.2 wt% of Pt) were investigated, and obtained data are shown in Fig. 6A. The necessity of Pt NPs’ presence was confirmed since bare ANA and bare RUT sample were practically inactive in this reaction system. At short UV range (350–365 nm), the apparent quantum efficiency (Φapp) of platinized samples correlated with the overall photocatalytic activity of those samples (Fig. 3A), i.e., Pt@RUT > Pt@ANA > Pt@HomoP25. Much lower photocatalytic activity of Pt@ HomoP25 was caused by insufficient amount of Pt in that sample (nonaggregated titania NPs), as has been discussed above. The shape of action spectra of Pt@ANA and Pt@RUT resembled respective absorption spectra (Fig. 3B and Fig. 6C). Interestingly, the photocatalytic activity of Pt@HomoP25 was higher than that of Pt@ANA at UV range of 380–395 nm. It should be reminded that at this UV range mainly rutile absorbs light (especially at 395 nm). Therefore, higher photocatalytic activity of Pt@HomoP25 than that of Pt@ANA indicates that rutile was platinized first, especially taking into consideration much lower content of rutile than anatase in HomoP25 (14% vs. 78%). To confirm this assumption, various composition of samples (physical mixtures) were prepared and tested, i.e., (a) 15% Pt(0.2 wt%)/RUT and 85% ANA; (b) 50% Pt(0.2 wt%)/RUT and 50% ANA; (c) 15% RUT and 85% Pt(0.2 wt %)/ANA; (d) 15% RUT and 85% Pt(0.1 wt%)/ANA; (e) 15% Pt(0.2 wt %)/RUT and 85% Pt(0.2 wt%)/ANA; (f) 15% Pt(0.1 wt%)/RUT and 85% Pt(0.1 wt%)/ANA. Firstly, larger content of platinized sample in the mixture resulted in enhanced photocatalytic activity, i.e., (b) > (a), (c) > (d), and (e) > (f). At short wavelengths, the mixtures of both platinized phases ((e) and (f)) showed higher photocatalytic activities than that of mixtures composed of one bare and one modified phase, which is not surprising since both platinized phases showed high photocatalytic activities (as shown in Fig. 6A). It should be pointed that all those mixtures had lower activities than that of Pt@
Fig. 4. Schematic drawing showing interparticle electron transfer in aggregated singlephase titania particles (Pt@ANA and Pt@RUT).
improved compared with that of P25, i.e., the amount of Pt deposits required for activation of titania particles was reduced by annealing, but was lower than those of ANA and RUT. This may be also explained by the aggregation/agglomeration of titania particles by the relatively low-temperature heating to enable photoexcited electrons transfer within aggregates/agglomerates (IPET), and the effective size of HomoP25-200 aggregates/agglomerates for IPET is smaller than those of ANA and RUT (Note that even though the heating temperature at the final stage of isolation of ANA and RUT was similar to that of HomoP25, ANA and RUT isolation processes could modify their surface structure affecting the size and connection of aggregate/agglomerates.). The
Fig. 5. (A-B) STEM observation of HomoP25 modified with 2 wt% of Pt NPs: (A) SEmode, and (B) TEM-mode, (C) distribution of Pt NPs’ sizes, and (D) XRD patterns for bare and modified titania.
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Fig. 6. Action spectra (A, B, D) and absorption spectra (C, D) of samples: (a) 15% Pt(0.2 wt%)/RUT and 85% ANA; (b) 50% Pt (0.2 wt%)/RUT and 50% ANA; (c) 15% RUT and 85% Pt(0.2 wt%)/ANA; (d) 15% RUT and 85% Pt(0.1 wt%)/ANA; (e) 15% Pt (0.2 wt%)/RUT and 85% Pt(0.2 wt%)/ANA; (f) 15% Pt(0.1 wt%)/RUT and 85% Pt (0.1 wt%)/ANA.
RUT. However, the Φapp of (e) sample (Φapp = 73%, containing platinized anatase and rutile) was higher than that of Pt@ANA (Φapp = 69%) confirming that rutile was more active phase in this system. The most interesting is the shape of action spectra of (a) and (b) samples containing only platinized rutile with the sharp peak at 395 nm. The explanation is similar to that used for explanation of higher photocatalytic activity of Pt@HomoP25 than Pt@ANA (Fig. 6A), i.e., at this wavelength only rutile can absorb light, whereas at shorter wavelength (380 nm) both phases absorb light. Therefore, a decrease in Φapp (at 380 nm) was caused by the competition between two phases: active (Pt@RUT) and inactive (ANA), about photon absorption. Interestingly, re-estimated absorption spectrum (from the DRS measurements of ANA and RUT (details in SI)) shows characteristic peak (Fig. 6C) observed at action spectrum. The characteristic peak in the spectrum is clearly caused by the difference in photoabsorption properties between anatase and rutile, due to the difference in the bandgap of rutile (3.0 eV) and anatase (3.2 eV) [41], i.e., i) at the wavelengths shorter than ca. 390 nm both anatase and rutile can absorb photons; ii) at 390–400 nm the absorption properties of anatase are decreasing, and iii) at the wavelength longer than ca. 400 nm only rutile can absorb photons. In addition, HomoP25 with larger content of platinum (0.5 wt %) was also tested and characteristic peak at 395 nm was obtained (Fig. 6D; the same as for (a) and (b) samples), confirming that rutile phase was platinized first. Therefore, it was shown that rutile is more active form of titania for methanol dehydrogenation in the presence of platinum as a co-catalyst. Moreover, at first rutile was platinized, despite larger content of anatase in P25. One of the possible reasons is preferable adsorption of Pt precursor (hexachloroplatinum(IV) ions) on rutile particles in P25. As a general trend, it has been often reported that rutile particles tend to exhibit higher photocatalytic activity for deposition of metal such as silver than anatase particles [73,74]. Another possible reason is preferable electron transfer from anatase particles, as a major component absorbing light, to rutile particles, which has often been claimed in the literature, since the conduction-bottom position of rutile is slightly lower than that of anatase [75]. Zeta potential analysis (Table S3) showed that rutile NPs (RUT) were positively
charge, whereas anatase NPs were negatively charged, confirming the first hypothesis, i.e., the favorable adsorption of chloroplatinate anions on rutile than on anatase.
4. Conclusions It was found that platinized rutile was more active than platinized anatase and platinized HomoP25, probably due to ability of photoabsorption of more photons (narrower band gap). Therefore, proposed in literature an electron transfer between two polymorphic forms of titania as the main reason of high photocatalytic activity of P25 has been discarded. Moreover, it was found that in P25 sample, rutile was platinized first, probably due to positively charged surface allowing easy adsorption of chloroplatinate ions. Although, there are many reports claiming interparticle electron transfer (IPET) in heterogeneous photocatalytic systems, especially preferable transfer from anatase to rutile particles in P25, which seems to be consistent with the experimental results, probably there is no direct evidence for IPET. The authors' group has suggested that for P25 photocatalysis no synergetic effect (due to IPET between anatase and rutile) is required for interpretation of high photocatalytic activity of P25 [1]. The results shown in the present study suggest that IPET may occur not only between anatase and rutile (hetero-IPET), but also between single anatase particles and single rutile particles when isolated ANA and RUT are heated at relatively low temperature to form aggregates/agglomerates (homo-IPET). This may be the first evidence for IPET of titania particles. Furthermore, it is suggested that careful discussion is needed for understanding photocatalytic-reaction mechanism considering the possible hetero and homo-IPET.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2017.08.046. 332
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