CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201301140
Production of Hydrogen by Glycerol Photoreforming Using Binary Nitrogen–Metal-Promoted N-M-TiO2 Photocatalysts Sean Taylor, Mihir Mehta, and Alexander Samokhvalov*[a] The need for renewable energy focuses attention on hydrogen obtained by using sustainable and green methods. The sustainable compound glycerol can be used for hydrogen production by heterogeneous photocatalysis. A novel approach involves the promotion of the TiO2 photocatalyst with a binary combination of nitrogen and transition metal. We report the synthesis and spectroscopic characterization of the new N-MTiO2 photocatalysts (M = none, Cr, Co, Ni, Cu), and the photoca-
talytic reforming of glycerol to hydrogen under ambient conditions and near-UV or visible light versus benchmark P25 TiO2. In units of activity mmol m2 h1, N-Ni-TiO2 is five-fold more active than P25, and N-Cu-TiO2 is 44-fold more active. The photocatalytic activity of N-M-TiO2 increases from Cr to Co and Ni, whereas the photoluminescence decreases; the change in activity is due to the modulation of charge recombination.
1. Introduction The need for sustainable, renewable, green sources of energy of the 21st century has caused an increased interest in the production of hydrogen. Among the various methods for the production of hydrogen,[1] those which consume sustainable resources, such as light, water, or biomass are preferred. The photocatalytic generation of hydrogen from water on a semiconductor photoelectrode was first demonstrated by Fujishima and Honda[2] in 1972. Thousands of research papers have been published since, and TiO2 remains a benchmark photocatalyst. Titanium dioxide Degussa P25, recently rebranded as P25 Evonik[3] or Aeroxide,[4] is a standard heterogeneous photocatalyst that consists of approximately 80 % anatase and 20 % rutile, has a surface area of about 30–50 m2 g1, and contains crystalline TiO2 typically around 20 nm in size. Major advantages of this TiO2 photocatalyst are its high stability to photochemical and chemical corrosion, high activity under UV light, low toxicity, and low cost. TiO2 can be used for photocatalysis under near-UV light in either the liquid[5] or the gas phase;[6] examples include the recently proposed combination of photocatalysis and adsorption.[7] Both photocatalytic oxidation[8] and reduction[9] reactions can be conducted with TiO2. However, the drawbacks of TiO2 are its low absorption of light with wavelengths greater than about 400 nm and low activity under visible light. Several approaches have been developed for the effective utilization of visible light (l > 400 nm) by TiO2-based photocatalysts. One way to increase the absorption of visible light is to promote the TiO2 by using nanoparticles (NPs) of a noble metal such as gold.[10] Another possibility is to promote the [a] S. Taylor, M. Mehta, Dr. A. Samokhvalov Department of Chemistry Rutgers University 315 Penn St., Camden, NJ 08102 (USA) Fax: (+ 1) 856-225-6506 E-mail: [email protected]
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TiO2 with cations of transition metals that have a high absorption in the visible part of the solar spectrum, such as Cr.[11] However, many transition metal promoters cause only a marginal improvement or a deterioration[11] of the photocatalytic activity of TiO2. This is also known for photocatalysis with other metal oxide semiconductors.[12] The reason for this deterioration is that the substitutional site formed on introduction of the transition metal promoter leads to an oxygen vacancy, V(O), that serves as a charge recombination center.[13] Yet another way to improve the absorption of visible light by TiO2 is to promote TiO2 with nonmetals, and nitrogen is one of the most popular nonmetal promoters.[6a] Nitrogen atoms substitute the oxygen atoms in TiO2 ; this creates electronic states within the bandgap just above the valence band (VB).[14] It was noted in a recent review paper that both metal and nonmetal promoters create lattice defects, which serve as charge recombination centers that can decrease photocatalytic activity.[11] Recently, the use of pairs of metal/nonmetal promoters for the formation of binary promoted TiO2 was proposed.[15] Although significant success has recently been achieved in computational studies of binary promoted TiO2 photocatalysts,[16] experimental reports are scarce.[17] DFT calculations have shown that Cr and N promoters form a strong coupling inside Cr-N-TiO2. This coupling is believed to cause a ten-fold increase in photocatalytic activity in the decomposition of methylene blue under UV irradiation[18] compared to the photocatalytic activity of Cr-TiO2 or N-TiO2. The V-N-TiO2 photocatalyst, prepared through nitridation[19] of V-doped V-TiO2, has a narrower band gap (2.76 eV) than either V-TiO2 (2.91 eV) or N-TiO2 (2.92 eV), and shows enhanced activity over both compounds in the photocatalytic degradation of methylene blue under visible light. The N-Fe-TiO2 photocatalyst, formed from TiO2 doped simultaneously with N and Fe using sol-gel synthesis,[20] shows an improvement in the photocatalytic degradation of rhodamine B in water under visible and UV irradiation ChemPhysChem 2014, 15, 942 – 949
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CHEMPHYSCHEM ARTICLES versus pure TiO2. However, codoping of rutile (110) with Fe and N red-shifts the bandgap into the visible range, but the sample is not active in the decomposition of adsorbed trimethyl acetate.[21] The Cu-TiO2 catalyst shows enhanced bacterial inactivation in water due to photocatalytic oxidation versus TiO2, whereas binary promoted Cu-N-TiO2 does not.[22] It is important to pay attention to both the high absorption of light and the minimization of electron-hole recombination in promoted TiO2 photocatalysts. In order to minimize charge recombination, organic or inorganic sacrificial electron donors are used: photogenerated holes in the VB oxidize the sacrificial donor instead of oxidizing water. In this way, the photogenerated electrons in the conduction band (CB) do not recombine with holes, but instead reduce protons to hydrogen. For sustainable heterogeneous photocatalysis, it is advantageous to use organic waste as the sacrificial donor. Glycerol, C3H8O3, is a major byproduct of the trans-esterification of vegetable oils to biodiesel.[23] Only one paper[24] reports hydrogen evolution from glycerol in water using binary metal–nonmetal-promoted TiO2. Specifically, TiO2 nanotubes were sequentially doped with platinum and nitrogen by photodeposition and impregnation methods, respectively.[24] To the best of our knowledge, the photocatalytic production of hydrogen from glycerol in water using binary nitrogen and transition metal promoted N-M-TiO2 photocatalysts has not been reported. We have reported the oxidation, spin, and coordination states of the following transition metal promoters in ZnO/SiO2 sorbents: copper,[25] iron, and manganese.[26] Most recently, we reported the combined experimental and DFT computational study of the CuII promoter sites in the Cu-ZnO/SiO2 sorbent.[27] We have also published several papers on the preparation, characterization, and performance of a new family of promoted Ag-TiO2 functional materials for adsorption in the liquid phase.[28] We report here 1) the preparation of several binary nitrogen/transition metal promoted N-M-TiO2 photocatalysts (M = none, Cr, Co, Ni, Cu), 2) their characterization by UV/Vis diffuse reflectance spectroscopy (UV/Vis DRS), Raman spectroscopy, Brunauer–Emmett–Teller (BET) surface area measurements, and photoluminescence (PL) spectroscopy, and 3) photocatalytic tests of hydrogen production from a solution of the sustainable sacrificial donor glycerol in water under ambient conditions, and under near-UV and visible light. The novel binary promoted N-M-TiO2 photocatalysts show the following advantages versus P25 TiO2 : 1) stronger absorption of light in the near-UV and visible ranges, 2) weaker charge recombination, and 3) up to 44-fold higher rates of production of hydrogen.
2. Results and Discussion 2.1. Photocatalyst Characterization by UV/Vis DRS Figure 1 shows the normalized UV/Vis DRS spectra of P25 TiO2, N-N-TiO2 (no metal), N-Cr-TiO2, N-Co-TiO2, N-Ni-TiO2, and N-CuTiO2 photocatalysts in the spectral range 360–890 nm. One can see that P25 TiO2 has a rather sharp absorption cutoff at about 400 nm (3.1 eV), which is consistent with the well-known 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. UV/Vis DRS spectra of (1) P25 TiO2, (2) N-N-TiO2 (no metal), (3) N-CrTiO2, (4) N-Co-TiO2, (5) N-Ni-TiO2, and (6) N-Cu-TiO2 photocatalysts.
bandgap of TiO2 at about 3.2 eV for anatase and 3.0 eV for rutile. The quantum size effect, namely the changes in the bandgap as a function of particle size, is very small for both anatase and rutile. The excitation radii for titania NPs[29] are between 7.5 and 19 . Thus, quantum confinement size effects for TiO2 NPs were observed only for NPs smaller than 2 nm,[30] and the blueshift of the band gap was less than 0.1–0.2 eV. This indicates that the differences among our N-M-TiO2 photocatalysts in the UV/Vis DRS spectra between 360 and 450 nm (Figure 1) are not due to the size of the TiO2 NPs in the different photocatalysts, but due to the metal and N promoters. Our photocatalysts consist mainly of anatase (Figure 3), and the wavelength of 360 nm (3.45 eV) on the left edge of the UV/Vis DRS spectra (Figure 1) corresponds to the energy difference between the X1a and X1b electronic bands of nanocrystalline anatase.[31] Thus, we normalized Kubelka–Munk functions (Figure 1) for the different photocatalysts at 360 nm. All the binary promoted photocatalysts N-M-TiO2 and N-NTiO2 (no metal) show a redshift in the absorption onset from 400 nm (observed in P25 TiO2) to about 500 nm (Figure 1). Such a redshift is consistent with the reported spectra of both N-promoted TiO2[19] and the binary nitrogen-metal-promoted photocatalyst Cr-N-TiO2.[18] This redshift is due to photoexcitation from the filled 2p states of the N atom in the TiO2 lattice, and is consistent with the chemical composition of all our NM-TiO2 and N-N-TiO2 photocatalysts. Therefore, all our samples have a stronger absorption in the near-UV and visible range than the reference P25 TiO2. Based on this consideration alone, all our samples were expected to show a higher hydrogen evolution rate than P25 in photocatalytic tests; we test this assumption below. Further, all our N-M-TiO2 photocatalysts except N-Ni-TiO2 show distinct absorption bands in the visible range (Figure 1). Specifically, the UV/Vis DRS spectrum of N-CrTiO2 shows a broad band at approximately 600–800 nm with the spectral maximum at 710 nm as determined by multi-Gaussian fitting.[32] A similar absorption band was reported in the UV/Vis DRS spectrum of Cr-N-TiO2 that was synthesized from CrIII acetylacetonate using a sol-gel method followed by calcination. The band at about 700 nm was due to CrV as confirmed by the electron paramagnetic resonance (EPR) spectra.[15] The band at 712 nm observed by us (Figure 1) is clearly different ChemPhysChem 2014, 15, 942 – 949
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CHEMPHYSCHEM ARTICLES from the band reported for Cr-N-TiO2 with absorption maximum at 550 nm. This band was determined to be due to CrIII, based on X-ray photoelectron spectroscopy (XPS) results.[18] We conclude that our N-Cr-TiO2 photocatalyst most likely contains the Cr promoter in the CrV state. To our knowledge, the UV/Vis DRS spectrum of the binary promoted N-Co-TiO2 photocatalyst has never been reported. The UV/Vis DRS spectrum of the N-Co-TiO2 photocatalyst shows a broad band at around 500–750 nm with a maximum at 615 nm determined by multi-Gaussian fitting (Figure 2). The
www.chemphyschem.org at 630 nm with a maximum at 800 nm, which is consistent with the visually observed bluish color of this material. In the UV/Vis DRS spectrum of the Cu-TiO2 photocatalyst,[34] the broad absorption band at 600–800 nm is due to the CuII in an octahedral coordination environment. Our assignment of the UV/Vis DRS peak at 630–890 nm in Figure 1 to CuII is also consistent with our recent report on the UV/Vis DRS spectrum of Cu-ZnO/SiO2 sorbents calcined in air.[27] Specifically, the absorption band observed at greater than 600 nm for Cu-ZnO/SiO2 was assigned to CuII in an octahedral environment, which is consistent with the EPR spectra reported by us.[27] Thus, we believe that our N-Cu-TiO2 photocatalyst (Figure 1) contains a CuII promoter. However, the CuII present in the N-Cu-TiO2 photocatalyst may have another absorption peak at 210–270 nm due to the ligand to metal charge transfer (LMCT) from the O 2p to the CuII 3d orbitals as reported for the Cu-TiO2 photocatalyst.[34] This peak is outside the spectral range in Figure 1. Therefore, the redshift of the absorption onset for the N-Cu-TiO2 photocatalyst from 400 nm to about 500 nm (Figure 1) is not due to the LMCT band of CuII, but due to the absorption of light by the nitrogen promoter in the N-Cu-TiO2 catalyst. 2.2. Photocatalyst Characterization by Raman Spectroscopy
Figure 2. UV/Vis DRS spectrum of the N-Co-TiO2 photocatalyst and its multiGaussian curve fitting. Thick solid line: UV/Vis DRS spectrum; dotted line: four-Gaussian curve fitting; thin solid lines: individual Gaussian peaks obtained by curve fitting.
absorption edge at 360 nm in Figure 2 corresponds to the top of the VB in TiO2. The VB of TiO2 is known to be quite broad (a few eV), and absorbance by the whole VB is outside of the range of our spectrometer. We performed the multi-Gaussian peak fitting in the range 300–900 nm. It is important to keep in mind that the Gaussian peak between 300 and approximately 380 nm in Figure 2 does not reflect the density of electronic states that absorb photons. However, each of the fitted peaks at longer wavelengths corresponds to the optical absorbance due to certain chromophoric species in the N-Co-TiO2 photocatalyst. One can see in Figure 2 the change of the slope of the spectrum at about 380 nm that forms the spectral shoulder. The fitted peak in Figure 2 centered at 386 nm corresponds to the well-known energy of the bandgap of TiO2.[31a] The Gaussian peak in Figure 2 centered at 446 nm corresponds to the well-known optical absorbance by the nitrogen promoter.[6a] The Gaussian peak centered at 615 nm corresponds to the Co promoter, as a similar absorption band centered at roughly 600 nm was reported in the spectra of the Co-TiO2 photocatalysts and was assigned to the CoII state.[33] Therefore, the absorption band at around 500–750 nm and centered at 615 nm (Figures 1 and 2) is likely to be due to the CoII promoter in our N-Co-TiO2 photocatalyst. Further, we believe that the UV/Vis DRS spectrum of the binary promoted N-Cu-TiO2 photocatalyst has not yet been reported. The UV/Vis DRS spectrum of the NCu-TiO2 photocatalyst shows a broad absorption band starting 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Raman spectroscopy is known to be a very sensitive characterization tool for phase analysis of nanocrystalline TiO2 and is superior to X-ray diffraction.[35, 36] We could not measure Raman spectra of our samples at room temperature, as a result of both strong photoluminescence and thermal photodecomposition of the samples. Figure 3 shows the Raman spectrum of the N-Cu-TiO2 photocatalyst obtained in a frozen ice matrix at 77 K (see the Experimental Section).
Figure 3. Raman spectrum of N-Cu-TiO2 photocatalyst in ice matrix at 77 K.
The main spectral features are the narrow peaks at 388 cm1 [with full width half maximum (FWHM) of 11 cm1], at 512 cm1 (FWHM of 13 cm1), and 635 cm1 (FWHM of 14 cm1). These are the typical Raman bands of anatase B1g (388 cm1), A1g (512 cm1), and Eg (635 cm1). Our data are consistent with the reported Raman bands of anatase at 395, 514, and 638 cm1 in TiO2 photocatalysts obtained using sol-gel and calcination synthesis.[36] The Raman spectra of other N-MTiO2 photocatalysts are similar to that of the N-Cu-TiO2 photoChemPhysChem 2014, 15, 942 – 949
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CHEMPHYSCHEM ARTICLES catalyst, and the Raman peaks corresponding to rutile[36] at 233, 447, and 610 cm1 were not detected.
2.3. Photoreforming Glycerol to Hydrogen Using Binary Promoted Photocatalysts The dependence of the yield of hydrogen upon the initial concentration of the sacrificial electron donor glycerol in water solution was reported[37] for the Cu-TiO2 photocatalyst. The amount of hydrogen produced increases as the concentration of glycerol increases from 0 to 4 vol %, and slowly decreases at higher concentrations. We chose to use 10 vol % glycerol in water in our photocatalytic experiments, and we used 20 mg photocatalyst per 10 mL liquid phase. Such a solid/liquid ratio in the photocatalytic suspension is within the range of 0.5– 3.0 g L1, which was recommended in the recent critical note.[38] Figure 4 shows the spectrum of the light used in our photocatalytic experiments. This spectrum was calculated from the emission spectrum of the medium pressure mercury lamp pro-
www.chemphyschem.org mental Section) monitors the changes of the concentration of evolving hydrogen continuously, as opposed to the periodic sampling in the off-line analysis by gas chromatography.[41] In photocatalytic reforming of glycerol in water with the TiO2based photocatalysts,[23, 24, 42] the only gaseous products are H2 and CO2. The hydrogen sensor used by us was not cross-sensitive to carbon dioxide or water vapors, so selective determination of hydrogen was possible. Figure 5 shows a typical trace of photocatalytic hydrogen evolution: the volumetric concentration of hydrogen (in ppmv) obtained with the N-Cu-TiO2 photocatalyst versus the time of
Figure 5. Typical hydrogen evolution trace in the photocatalytic experiments.
Figure 4. Spectrum of light used in the photocatalytic experiments.
vided by the manufacturer after correction for the optical transmission curve of the photocatalytic vessel made of Pyrex. For convenience, the same spectral range is shown in Figure 4 as in the UV/Vis DRS spectra of our photocatalysts in Figure 1. Major emission lines in Figure 4 are at 366, 405, and 436 nm (the UV and blue part of the visible spectrum), and 546 and 578 nm (the green part of the visible spectrum). Assuming that photocatalytic activity depends mostly on optical absorption, all our photocatalysts were expected to be more effective than P25 TiO2 ; we test this assumption further below. It has been reported that low concentrations of H2S can be quantitatively determined by inline continuous measurement using a commercial H2S sensor.[39] To our knowledge, there are no reports on the inline continuous measurement of hydrogen produced in photocatalytic experiments using a hydrogen sensor. In our photocatalytic experiments, we continuously purged the photocatalytic suspension with ultra-high purity argon as described elsewhere,[40] and continuously analyzed the evolving gas mixture. Our inline sampling system with a commercial electrochemical hydrogen sensor (see the Experi 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
illumination. The arrows indicate the time when the optical shield protecting the photocatalytic vessel from the excitation light was removed (light ON) and inserted (light OFF). Upon removing the shield (light ON), evolution of H2 starts, and hydrogen concentration [H2] reaches a steady-state value; upon inserting the shield (light OFF), evolution of H2 decreases and stops. The hydrogen evolution trace in Figure 5 is not symmetric in time relative to the ON/OFF events. The rather long time to return to zero concentration is likely due to a gradual desorption of the remaining hydrogen from the surface of the photocatalyst. In emerging applications, hydrogen mixed with an inert gas, such as that obtained in photocatalytic glycerol reforming, can be used as a fuel in certain types of fuel cells.[43] There is no commonly accepted unit for the measure of reaction rates in heterogeneous photocatalysis.[38] Reported units of activity are mmol h1,[44] mmol per experiment,[45] molar ratio of hydrogen/sacrificial donor,[46] x times the activity compared to P25 TiO2,[47] mmol h1 m2,[48] and others. Measuring photocatalytic activities with the P25 reference material allows a comparison of activities observed with different samples and reported by different authors. Table 1 shows the hydrogen production rates obtained with our N-M-TiO2 photocatalysts (M = Cu, Ni, Co, Cr) and P25 TiO2 expressed in various units of measure: ppmv H2 , mmol H2 (g photocatalyst)1 h1, and mmol H2 (total area of photocatalyst in m2)1 h1, and the BET total surface areas of these photocatalysts. The rate of hydrogen production for N-N-TiO2 (no metal) is about the same as that for N-Cr-TiO2 (32 ppmv), and both are ChemPhysChem 2014, 15, 942 – 949
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Table 1. Hydrogen production rates and BET surface areas of N-M-TiO2 photocatalysts. Photocatalyst
[H2] [ppmv]
H2 rate [mmol g1 h1]
S(BET) area [m2 g1]
H2 rate [mmol m2 h1]
N-Cu-TiO2 N-Ni-TiO2 N-Co-TiO2 N-Cr-TiO2 P25 TiO2
400 125 80 32 64
1615 506 313 126 252
7.3 19.9 24.9 39.7 50.0
221 25 13 3 5
less than the value for reference P25 TiO2 (64 ppmv). For all other N-M-TiO2 photocatalysts (M = Co, Ni, Cu), the hydrogen production rate is higher than that of P25 TiO2. The reported N-Cr-TiO2 catalyst prepared by the sol-gel method[18] is more active than P25 TiO2 under both UV (254 nm) and visible light excitation. Thus, it is somewhat surprising that our N-Cr-TiO2 photocatalyst shows lower activity than P25. Such a difference is most likely due to the different oxidation states of the Cr promoter: CrIII in the reported photocatalyst[18] and CrV in our samples. The reported[15] potential photocatalyst Cr-N-TiO2 with Cr in the CrV state has not been tested in photocatalytic experiments. One would expect that the higher optical absorption would result in a higher hydrogen evolution rate in at least some of the multiple units of measure: ppmv, mmol g1 h1, or mmol m2 h1 (Table 1); however, this is not the case. For instance, our N-Ni-TiO2 photocatalyst has a weaker optical absorption than N-Cr-TiO2 throughout the whole optical range (Figure 1), yet its hydrogen evolution rate is higher in all the units of measure (Table 1). Therefore, one needs to consider the rate of charge recombination, which is detrimental for photocatalytic activity. It is known to be difficult to quantify the nonradiative charge recombination rate, but it is relatively straightforward to quantify the radiative charge recombination rate. The latter is achieved by measurement of the photoluminescence (PL) spectra of the photocatalysts.
Figure 6. Photoluminescence spectra of N-Co-TiO2 with lexc = 260 nm (solid line) and 280 nm (dashed line).
photocatalysts and P25 TiO2 show the same spectral shape, although the intensity varies. Table 2 shows hydrogen production rates of the N-M-TiO2 photocatalysts (M = Cu, Ni, Co, Cr) and P25 TiO2, as well as the PL intensity measured at 360 nm with lexc = 260 nm. This PL is due to a major direct transition, X1b !X1a, between the CB and VB in nanocrystalline anatase.[31a]
Table 2. Hydrogen production rates, BET surface areas, and photoluminescence of N-M-TiO2 photocatalysts. Photocatalyst
H2 rate [mmol g1 h1]
S(BET) area [m2 g1]
H2 rate [mmol m2 h1]
PL intensity of X1b !X1a transition [a.u.]
N-Cu-TiO2 N-Ni-TiO2 N-Co-TiO2 N-Cr-TiO2 P25 TiO2
1615 506 313 126 252
7.3 19.9 24.9 39.7 50.0
221 25 13 3 5
30 10 20 28 17
2.4. Photocatalyst Characterization by PL Spectroscopy Figure 6 shows the PL spectra of the N-Co-TiO2 photocatalyst obtained with lexc = 260 and 280 nm; the PL spectra are vertically offset for clarity. The PL peaks at 360 and 380 nm observed in the spectrum with lexc = 280 nm are consistent with the PL peaks reported for the Fe-TiO2 photocatalyst.[20] In our experiments, we measured the PL spectra of the suspension of the photocatalyst in water using an angular fluorescence accessory (see the Experimental Section). By using an angular accessory in the PL experiment spectral artefacts due to secondary absorption and emission of light are minimized.[49] Therefore, the PL peaks in Figure 6 are narrow and well-resolved. This is in contrast with the PL spectra in other reports,[31] in which the energies of the PL transitions had to be determined by complicated multi-peak fitting of a single broad (several tens of nm) spectral band. In Figure 6, the spectral range greater than 490 nm is not shown due to the fringe of the second order diffraction peak,[49] which has the center at 520 nm for lexc = 260 nm. All of the other studied N-M-TiO2 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The N-Cr-TiO2, which shows the lowest hydrogen production rate (Table 2), has the strongest PL peak among all the N-MTiO2 photocatalysts except N-Cu-TiO2. In a reference experiment, we measured the PL spectrum of the pellet of the neat N-Cr-TiO2 photocatalyst in air, without using a fluorescence cuvette. As expected, the PL spectrum of the pellet of neat NCr-TiO2 has the same peaks that are observed in the PL spectrum of N-Cr-TiO2 suspended in water within the fluorescence cuvette (see the Experimental Section). A recent paper[41] reports the PL spectra of Ni-TiO2, Fe-TiO2, and binary metal-metal promoted Ni-Fe-TiO2. PL spectra similar to those in Figure 6 with several distinct peaks were obtained, but the PL transitions were not assigned.[41] We assigned the PL peaks in Figure 6 based on the reported energy levels in nanocrystalline anatase.[31] Figure 7 shows the PL transitions of anatase NPs: emission from the CB to the VB[31a] and emission from shallow trap levels due to oxygen vacancies, V(O), to the VB.[31b] ChemPhysChem 2014, 15, 942 – 949
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Figure 7. Photoluminescence transitions in nanocrystalline anatase.
The following peaks in Figure 6 represent the transitions from the CB to the VB (Figure 7): the peak at 360 nm is due to the X1b !X1a transition, the peak at 393 nm is due to the X1b ! G3 transition, the peak at 422 nm is due to the G1b !X2b transition, and the peak at 435 nm is due to the G1b !X1a transition. The following peaks represent the transitions from the shallow trap states due to oxygen vacancies, V(O), to the VB of anatase: the peak at 445 nm is the V1(O)!G3 transition, the peak at 459 nm is the V2(O)!G3 transition, and the peak at 485 nm is the V3(O)!G3 transition. We conclude that the N and M promoters in our N-M-TiO2 photocatalysts do not create new N or M-induced fluorescent energy levels, but rather increase or decrease the energy of a radiative electron-hole recombination in anatase (Figures 6 and 7), depending on the position of the metal in the first row of transition metals. The N-Cu-TiO2 photocatalyst shows the highest hydrogen evolution rate among all the photocatalysts studied by us (Table 1) in units of ppmv, mmol g1 h1, or mmol m2 h1. It was reported[37] that the photocatalytic activity of Cu-TiO2 depends on the oxidation state of the Cu promoter; CuII, CuI or metallic copper. During our photocatalytic tests, we observed a change in the color of the N-Cu-TiO2 photocatalyst. This observation could indicate the in situ reduction of the as-prepared CuII promoter state of the pre-photocatalyst to the photocatalytically active CuI or Cu0 state of the true photocatalyst. Experiments are in progress to determine the oxidation state of the Cu promoter in the N-Cu-TiO2 photocatalyst in situ, during the photocatalytic reaction. Notably, emission lines in the visible range of the spectrum of the mercury lamp used (at 546 and 578 nm, Figure 4) do not correspond to the absorption maxima in the UV/Vis DRS spectrum of the N-Cu-TiO2 photocatalyst. Therefore, it is not the absorption of visible light by the CuII promoter that defines the rate of hydrogen evolution. Further, by comparing the UV/Vis DRS spectrum (Figure 1) of the second most active photocatalyst, N-Ni-TiO2, with the spectrum of our light source (Figure 4), one can conclude that our N-Ni-TiO2 photocatalyst does not absorb photons in the visible range at 405, 436, 546, and 578 nm more strongly than the other photocatalysts, yet its photocatalytic activity is the highest of all the N-M-TiO2 catalysts except N-Cu-TiO2. Hence, the enhanced activity of the N-Ni-TiO2 photocatalyst is not due the enhanced absorption of visible light, but due to the decreased radiative 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org charge recombination (observed in the photoluminescence measurements). Computational studies of the energy of transition metal (V, Cr, Mn, Fe, Co, and Ni) induced impurity levels in anatase[50] and rutile[51] are available. The Cr promoter creates electronic states in the middle of the bandgap of anatase, but Co and Ni promoters[50] create electronic states slightly above the top of the VB. Further, the DFT computations suggest that NiO clusters adsorb strongly onto rutile and anatase in the Ni-TiO2 photocatalyst; this creates new chemical bonds with the surface of TiO2.[52] As a result, the top of the VB of Ni-promoted rutile and anatase originates from the electronic states of oxygen in the NiO cluster. However, the Ni 3d states of the supported NiO cluster contribute to the CB of anatase.[52] We explain the observed high photocatalytic activity and weak photoluminescence of the binary promoted N-Ni-TiO2 photocatalyst through the modification of the electronic structure of the CB (initial state for fluorescence transition) by the electronic states of the supported Ni promoter. Specifically, we propose that in our NNi-TiO2 photocatalysts there are triplet states just below the bottom of the CB due to mixing with the d–d states of the supported transition metal, Ni. Thus, the probability of photoluminescence from the CB to the VB in the binary promoted N-Ni-TiO2 would decrease, and result in higher photocatalytic activity. Figure 8 shows the proposed energy level diagrams of the binary promoted N-M-TiO2 photocatalysts: N-Ni-TiO2 (left) and N-Cr-TiO2 (right).
Figure 8. Proposed energy levels in binary promoted N-M-TiO2 photocatalysts.
The rather low photocatalytic activity of our N-Cr-TiO2 photocatalyst despite its high optical absorption both in the nearUV and visible range is due to the electronic decoupling of the Cr cation from the VB and CB of the TiO2. The mechanism of action of the Ni promoter in the binary promoted TiO2 photocatalyst is expected to be the quenching of the photoluminescence from the CB to the VB; work is in progress to test this hypothesis in a broader context. Our findings can be used to predict the photocatalytic activity of the other binary promoted N-M-TiO2 photocatalysts from experimentally measured PL spectra and published energies of electronic levels due to the M promoter.
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CHEMPHYSCHEM ARTICLES 3. Conclusion We describe the synthesis of a previously unreported family of new N-M-TiO2 photocatalysts (M = none, Cr, Co, Ni, Cu) by simultaneous introduction of N and M promoters, the structural and spectroscopic characterization of these N-M-TiO2 photocatalysts, and photocatalytic reforming of glycerol in water to hydrogen under ambient conditions, near-UV, or visible light. The M and N promoters do not form new electronic states with radiative charge recombination, but rather modulate the photoluminescence from the nanocrystalline TiO2 host. The Co, Ni, and Cu promoter cations in the binary promoted N-M-TiO2 photocatalysts significantly enhance the rate of evolution of hydrogen upon photoreforming glycerol in water under nearUV or visible light compared to P25 TiO2. In units of activity of heterogeneous photocatalysts of mmol m2 h1, the N-Ni-TiO2 photocatalyst is five times more active than P25 TiO2, and NCu-TiO2 is 44 times more active. We used a systematic approach to measure both photocatalytic activity and photoluminescence in suspension in water at room temperature. It is not a strong absorption of light, but a low rate of radiative charge recombination that increases the photocatalytic activity of binary promoted N-M-TiO2 photocatalysts in the near-UV or visible range.
Experimental Section Chemicals Precursors for the synthesis of N-M-TiO2 (titanium(IV) isopropoxide, ethylenediamine, nitrates of CrIII, CoII, NiII, CuII, and ammonium, absolute ethanol), and glycerol were purchased from Sigma, and P25 TiO2 Evonik was purchased from Acros Organics. The N-M-TiO2 photocatalysts (M = Cr, Co, Ni, Cu, none) were prepared by the modified sol-gel method.[53] Specifically, a solution of the respective metal nitrate in deionized water (0.33 m, 0.75 mL) was added dropwise to a solution of titanium(IV) isopropoxide (2.870 g) and ethylenediamine (0.300 g) in absolute ethanol (25 mL) under strong stirring at 75 8C, until formation of a gel occurred. The amount of M added corresponds to a ratio of 0.025 mole M per mole Ti. The resultant gel was dried under vacuum at room temperature to constant weight, then was calcined in flowing air (flow rate 0.5 L min1) at 450 8C for 4 h. In the synthesis of the N-N-TiO2 catalyst (without metal promoter), we used the same molar amount of ethylenediamine as in the synthesis of N-M-TiO2, but instead of metal nitrate, we used the same molar amount of ammonium nitrate (the gel does not form without ammonium nitrate).
Instrumentation and Characterization UV/Vis DRS was measured with a UV/Vis spectrometer from Ocean Optics using a xenon LS-1 lamp as the light source, a fiber-optical reflection probe, and a commercial WS-1 diffuse reflectance standard. Raman spectra were measured at 77 K using a MicroRaman spectrometer from Avantes at l = 785 nm; the sample was a suspension of the N-M-TiO2 photocatalyst in distilled water at a solid/ liquid ratio of about 1:3, frozen at 77 K in liquid nitrogen using a custom-built sample holder in a Dewar vessel. The total surface area of the photocatalysts was measured with a BET Nova 2000e (Quantachrome Instrument Corp., USA) using nitrogen adsorption/ desorption isotherms at 77 K. Before measuring the total surface 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org area, the samples were degassed for 3 h at 120 8C, and the surface area S(BET) was calculated with the BET equation using the program supplied with the BET apparatus. PL spectra of the photocatalysts were recorded using a Cary Eclipse fluorescence spectrometer equipped with the angular accessory (Agilent, Inc.) to minimize primary and secondary absorption of light[49] in the suspension of the photocatalyst. We used the default setting of the angular accessory at 308 and excitation wavelength lexc = 260 or 280 nm. The quartz fluorescence cuvette was filled with the suspension of photocatalyst (40 mg) in deionized water (0.3 mL), degassed by sonication, closed with a Teflon stopper, and sealed with parafilm tape. The PL spectra were measured after the solid photocatalyst had fully settled on the bottom of the fluorescence cuvette. The position of the cuvette was adjusted[54] until the spatial profile of the excitation light coincided with the layer of the photocatalyst on the bottom of the cuvette, and the highest PL signal was obtained.
Photocatalytic Tests In the photocatalytic tests, a medium pressure 450 W mercury lamp from Ace Glass was used as the light source. The photocatalyst (20 mg) was suspended in a solution of glycerol in water (10 vol %, 10 mL) in the Pyrex reaction vessel. Prior to the test, the suspension was degassed by ultrasonication for 10 min to remove dissolved oxygen. During the photocatalytic test, the suspension was continuously purged with 99.999 % Ar at a constant flow rate of 30 sccm (standard cubic centimeter per minute). The whole photocatalytic setup, including the light source and Pyrex reaction vessel containing the suspension of the photocatalyst, was immersed into a water circulation thermostat (Neslab Inc.) maintained at 30 8C. A mixture of water and ethylene glycol in a circulation thermostat absorbs the infrared radiation emitted by the mercury lamp, but transmits both near-UV and visible light. The concentration of evolving hydrogen was continuously measured with an inline electrochemical hydrogen sensor (model 4101-07, from Sierra Monitor Inc.) in parts per million by volume (ppmv) units. Analog data from the sensor were acquired with the LabView program written in-house.
Acknowledgements A.S. sincerely thanks Research Corporation for Science Advancement (RCSA) for his Cottrell College Science grant #20241, and Rutgers University for Research Council Grant #202221. S.T. sincerely thanks the Chemistry Department for his John C. Collier undergraduate Research Grant in Chemistry, and Rutgers University—Camden for his Dean’s undergraduate award. Keywords: glycerol · hydrogen · photocatalysis · titanium dioxide · transition metals [1] K. Liu, C. Song, V. Subramani, Hydrogen and Syngas Production and Purification Technologies, Wiley, Hoboken, 2010. [2] A. Fujishima, K. Honda, Nature 1972, 238, 37 – 38. [3] A. Samokhvalov, Catal. Rev. Sci. Eng. 2012, 54, 281 – 343. [4] B. Ohtani, O. O. Prieto-Mahaney, D. Li, R. Abe, J. Photochem. Photobiol. A 2010, 216, 179 – 182. [5] P. V. Kamat, J. Phys. Chem. Lett. 2012, 3, 663 – 672. [6] a) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 2001, 293, 269 – 271; b) C. Ampelli, R. Passalacqua, C. Genovese, S. Perathoner, G.
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Received: November 30, 2013 Revised: January 25, 2014 Published online on February 25, 2014
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Production of Hydrogen by Glycerol Photoreforming Using Binary Nitrogen–Metal-Promoted N-M-TiO2 Photocatalysts Sean Taylor, Mihir Mehta, and Alexander Samokhvalov*[a] The need for renewable energy focuses attention on hydrogen obtained by using sustainable and green methods. The sustainable compound glycerol can be used for hydrogen production by heterogeneous photocatalysis. A novel approach involves the promotion of the TiO2 photocatalyst with a binary combination of nitrogen and transition metal. We report the synthesis and spectroscopic characterization of the new N-MTiO2 photocatalysts (M = none, Cr, Co, Ni, Cu), and the photoca-
talytic reforming of glycerol to hydrogen under ambient conditions and near-UV or visible light versus benchmark P25 TiO2. In units of activity mmol m2 h1, N-Ni-TiO2 is five-fold more active than P25, and N-Cu-TiO2 is 44-fold more active. The photocatalytic activity of N-M-TiO2 increases from Cr to Co and Ni, whereas the photoluminescence decreases; the change in activity is due to the modulation of charge recombination.
1. Introduction The need for sustainable, renewable, green sources of energy of the 21st century has caused an increased interest in the production of hydrogen. Among the various methods for the production of hydrogen,[1] those which consume sustainable resources, such as light, water, or biomass are preferred. The photocatalytic generation of hydrogen from water on a semiconductor photoelectrode was first demonstrated by Fujishima and Honda[2] in 1972. Thousands of research papers have been published since, and TiO2 remains a benchmark photocatalyst. Titanium dioxide Degussa P25, recently rebranded as P25 Evonik[3] or Aeroxide,[4] is a standard heterogeneous photocatalyst that consists of approximately 80 % anatase and 20 % rutile, has a surface area of about 30–50 m2 g1, and contains crystalline TiO2 typically around 20 nm in size. Major advantages of this TiO2 photocatalyst are its high stability to photochemical and chemical corrosion, high activity under UV light, low toxicity, and low cost. TiO2 can be used for photocatalysis under near-UV light in either the liquid[5] or the gas phase;[6] examples include the recently proposed combination of photocatalysis and adsorption.[7] Both photocatalytic oxidation[8] and reduction[9] reactions can be conducted with TiO2. However, the drawbacks of TiO2 are its low absorption of light with wavelengths greater than about 400 nm and low activity under visible light. Several approaches have been developed for the effective utilization of visible light (l > 400 nm) by TiO2-based photocatalysts. One way to increase the absorption of visible light is to promote the TiO2 by using nanoparticles (NPs) of a noble metal such as gold.[10] Another possibility is to promote the [a] S. Taylor, M. Mehta, Dr. A. Samokhvalov Department of Chemistry Rutgers University 315 Penn St., Camden, NJ 08102 (USA) Fax: (+ 1) 856-225-6506 E-mail: [email protected]
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TiO2 with cations of transition metals that have a high absorption in the visible part of the solar spectrum, such as Cr.[11] However, many transition metal promoters cause only a marginal improvement or a deterioration[11] of the photocatalytic activity of TiO2. This is also known for photocatalysis with other metal oxide semiconductors.[12] The reason for this deterioration is that the substitutional site formed on introduction of the transition metal promoter leads to an oxygen vacancy, V(O), that serves as a charge recombination center.[13] Yet another way to improve the absorption of visible light by TiO2 is to promote TiO2 with nonmetals, and nitrogen is one of the most popular nonmetal promoters.[6a] Nitrogen atoms substitute the oxygen atoms in TiO2 ; this creates electronic states within the bandgap just above the valence band (VB).[14] It was noted in a recent review paper that both metal and nonmetal promoters create lattice defects, which serve as charge recombination centers that can decrease photocatalytic activity.[11] Recently, the use of pairs of metal/nonmetal promoters for the formation of binary promoted TiO2 was proposed.[15] Although significant success has recently been achieved in computational studies of binary promoted TiO2 photocatalysts,[16] experimental reports are scarce.[17] DFT calculations have shown that Cr and N promoters form a strong coupling inside Cr-N-TiO2. This coupling is believed to cause a ten-fold increase in photocatalytic activity in the decomposition of methylene blue under UV irradiation[18] compared to the photocatalytic activity of Cr-TiO2 or N-TiO2. The V-N-TiO2 photocatalyst, prepared through nitridation[19] of V-doped V-TiO2, has a narrower band gap (2.76 eV) than either V-TiO2 (2.91 eV) or N-TiO2 (2.92 eV), and shows enhanced activity over both compounds in the photocatalytic degradation of methylene blue under visible light. The N-Fe-TiO2 photocatalyst, formed from TiO2 doped simultaneously with N and Fe using sol-gel synthesis,[20] shows an improvement in the photocatalytic degradation of rhodamine B in water under visible and UV irradiation ChemPhysChem 2014, 15, 942 – 949
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CHEMPHYSCHEM ARTICLES versus pure TiO2. However, codoping of rutile (110) with Fe and N red-shifts the bandgap into the visible range, but the sample is not active in the decomposition of adsorbed trimethyl acetate.[21] The Cu-TiO2 catalyst shows enhanced bacterial inactivation in water due to photocatalytic oxidation versus TiO2, whereas binary promoted Cu-N-TiO2 does not.[22] It is important to pay attention to both the high absorption of light and the minimization of electron-hole recombination in promoted TiO2 photocatalysts. In order to minimize charge recombination, organic or inorganic sacrificial electron donors are used: photogenerated holes in the VB oxidize the sacrificial donor instead of oxidizing water. In this way, the photogenerated electrons in the conduction band (CB) do not recombine with holes, but instead reduce protons to hydrogen. For sustainable heterogeneous photocatalysis, it is advantageous to use organic waste as the sacrificial donor. Glycerol, C3H8O3, is a major byproduct of the trans-esterification of vegetable oils to biodiesel.[23] Only one paper[24] reports hydrogen evolution from glycerol in water using binary metal–nonmetal-promoted TiO2. Specifically, TiO2 nanotubes were sequentially doped with platinum and nitrogen by photodeposition and impregnation methods, respectively.[24] To the best of our knowledge, the photocatalytic production of hydrogen from glycerol in water using binary nitrogen and transition metal promoted N-M-TiO2 photocatalysts has not been reported. We have reported the oxidation, spin, and coordination states of the following transition metal promoters in ZnO/SiO2 sorbents: copper,[25] iron, and manganese.[26] Most recently, we reported the combined experimental and DFT computational study of the CuII promoter sites in the Cu-ZnO/SiO2 sorbent.[27] We have also published several papers on the preparation, characterization, and performance of a new family of promoted Ag-TiO2 functional materials for adsorption in the liquid phase.[28] We report here 1) the preparation of several binary nitrogen/transition metal promoted N-M-TiO2 photocatalysts (M = none, Cr, Co, Ni, Cu), 2) their characterization by UV/Vis diffuse reflectance spectroscopy (UV/Vis DRS), Raman spectroscopy, Brunauer–Emmett–Teller (BET) surface area measurements, and photoluminescence (PL) spectroscopy, and 3) photocatalytic tests of hydrogen production from a solution of the sustainable sacrificial donor glycerol in water under ambient conditions, and under near-UV and visible light. The novel binary promoted N-M-TiO2 photocatalysts show the following advantages versus P25 TiO2 : 1) stronger absorption of light in the near-UV and visible ranges, 2) weaker charge recombination, and 3) up to 44-fold higher rates of production of hydrogen.
2. Results and Discussion 2.1. Photocatalyst Characterization by UV/Vis DRS Figure 1 shows the normalized UV/Vis DRS spectra of P25 TiO2, N-N-TiO2 (no metal), N-Cr-TiO2, N-Co-TiO2, N-Ni-TiO2, and N-CuTiO2 photocatalysts in the spectral range 360–890 nm. One can see that P25 TiO2 has a rather sharp absorption cutoff at about 400 nm (3.1 eV), which is consistent with the well-known 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. UV/Vis DRS spectra of (1) P25 TiO2, (2) N-N-TiO2 (no metal), (3) N-CrTiO2, (4) N-Co-TiO2, (5) N-Ni-TiO2, and (6) N-Cu-TiO2 photocatalysts.
bandgap of TiO2 at about 3.2 eV for anatase and 3.0 eV for rutile. The quantum size effect, namely the changes in the bandgap as a function of particle size, is very small for both anatase and rutile. The excitation radii for titania NPs[29] are between 7.5 and 19 . Thus, quantum confinement size effects for TiO2 NPs were observed only for NPs smaller than 2 nm,[30] and the blueshift of the band gap was less than 0.1–0.2 eV. This indicates that the differences among our N-M-TiO2 photocatalysts in the UV/Vis DRS spectra between 360 and 450 nm (Figure 1) are not due to the size of the TiO2 NPs in the different photocatalysts, but due to the metal and N promoters. Our photocatalysts consist mainly of anatase (Figure 3), and the wavelength of 360 nm (3.45 eV) on the left edge of the UV/Vis DRS spectra (Figure 1) corresponds to the energy difference between the X1a and X1b electronic bands of nanocrystalline anatase.[31] Thus, we normalized Kubelka–Munk functions (Figure 1) for the different photocatalysts at 360 nm. All the binary promoted photocatalysts N-M-TiO2 and N-NTiO2 (no metal) show a redshift in the absorption onset from 400 nm (observed in P25 TiO2) to about 500 nm (Figure 1). Such a redshift is consistent with the reported spectra of both N-promoted TiO2[19] and the binary nitrogen-metal-promoted photocatalyst Cr-N-TiO2.[18] This redshift is due to photoexcitation from the filled 2p states of the N atom in the TiO2 lattice, and is consistent with the chemical composition of all our NM-TiO2 and N-N-TiO2 photocatalysts. Therefore, all our samples have a stronger absorption in the near-UV and visible range than the reference P25 TiO2. Based on this consideration alone, all our samples were expected to show a higher hydrogen evolution rate than P25 in photocatalytic tests; we test this assumption below. Further, all our N-M-TiO2 photocatalysts except N-Ni-TiO2 show distinct absorption bands in the visible range (Figure 1). Specifically, the UV/Vis DRS spectrum of N-CrTiO2 shows a broad band at approximately 600–800 nm with the spectral maximum at 710 nm as determined by multi-Gaussian fitting.[32] A similar absorption band was reported in the UV/Vis DRS spectrum of Cr-N-TiO2 that was synthesized from CrIII acetylacetonate using a sol-gel method followed by calcination. The band at about 700 nm was due to CrV as confirmed by the electron paramagnetic resonance (EPR) spectra.[15] The band at 712 nm observed by us (Figure 1) is clearly different ChemPhysChem 2014, 15, 942 – 949
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CHEMPHYSCHEM ARTICLES from the band reported for Cr-N-TiO2 with absorption maximum at 550 nm. This band was determined to be due to CrIII, based on X-ray photoelectron spectroscopy (XPS) results.[18] We conclude that our N-Cr-TiO2 photocatalyst most likely contains the Cr promoter in the CrV state. To our knowledge, the UV/Vis DRS spectrum of the binary promoted N-Co-TiO2 photocatalyst has never been reported. The UV/Vis DRS spectrum of the N-Co-TiO2 photocatalyst shows a broad band at around 500–750 nm with a maximum at 615 nm determined by multi-Gaussian fitting (Figure 2). The
www.chemphyschem.org at 630 nm with a maximum at 800 nm, which is consistent with the visually observed bluish color of this material. In the UV/Vis DRS spectrum of the Cu-TiO2 photocatalyst,[34] the broad absorption band at 600–800 nm is due to the CuII in an octahedral coordination environment. Our assignment of the UV/Vis DRS peak at 630–890 nm in Figure 1 to CuII is also consistent with our recent report on the UV/Vis DRS spectrum of Cu-ZnO/SiO2 sorbents calcined in air.[27] Specifically, the absorption band observed at greater than 600 nm for Cu-ZnO/SiO2 was assigned to CuII in an octahedral environment, which is consistent with the EPR spectra reported by us.[27] Thus, we believe that our N-Cu-TiO2 photocatalyst (Figure 1) contains a CuII promoter. However, the CuII present in the N-Cu-TiO2 photocatalyst may have another absorption peak at 210–270 nm due to the ligand to metal charge transfer (LMCT) from the O 2p to the CuII 3d orbitals as reported for the Cu-TiO2 photocatalyst.[34] This peak is outside the spectral range in Figure 1. Therefore, the redshift of the absorption onset for the N-Cu-TiO2 photocatalyst from 400 nm to about 500 nm (Figure 1) is not due to the LMCT band of CuII, but due to the absorption of light by the nitrogen promoter in the N-Cu-TiO2 catalyst. 2.2. Photocatalyst Characterization by Raman Spectroscopy
Figure 2. UV/Vis DRS spectrum of the N-Co-TiO2 photocatalyst and its multiGaussian curve fitting. Thick solid line: UV/Vis DRS spectrum; dotted line: four-Gaussian curve fitting; thin solid lines: individual Gaussian peaks obtained by curve fitting.
absorption edge at 360 nm in Figure 2 corresponds to the top of the VB in TiO2. The VB of TiO2 is known to be quite broad (a few eV), and absorbance by the whole VB is outside of the range of our spectrometer. We performed the multi-Gaussian peak fitting in the range 300–900 nm. It is important to keep in mind that the Gaussian peak between 300 and approximately 380 nm in Figure 2 does not reflect the density of electronic states that absorb photons. However, each of the fitted peaks at longer wavelengths corresponds to the optical absorbance due to certain chromophoric species in the N-Co-TiO2 photocatalyst. One can see in Figure 2 the change of the slope of the spectrum at about 380 nm that forms the spectral shoulder. The fitted peak in Figure 2 centered at 386 nm corresponds to the well-known energy of the bandgap of TiO2.[31a] The Gaussian peak in Figure 2 centered at 446 nm corresponds to the well-known optical absorbance by the nitrogen promoter.[6a] The Gaussian peak centered at 615 nm corresponds to the Co promoter, as a similar absorption band centered at roughly 600 nm was reported in the spectra of the Co-TiO2 photocatalysts and was assigned to the CoII state.[33] Therefore, the absorption band at around 500–750 nm and centered at 615 nm (Figures 1 and 2) is likely to be due to the CoII promoter in our N-Co-TiO2 photocatalyst. Further, we believe that the UV/Vis DRS spectrum of the binary promoted N-Cu-TiO2 photocatalyst has not yet been reported. The UV/Vis DRS spectrum of the NCu-TiO2 photocatalyst shows a broad absorption band starting 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Raman spectroscopy is known to be a very sensitive characterization tool for phase analysis of nanocrystalline TiO2 and is superior to X-ray diffraction.[35, 36] We could not measure Raman spectra of our samples at room temperature, as a result of both strong photoluminescence and thermal photodecomposition of the samples. Figure 3 shows the Raman spectrum of the N-Cu-TiO2 photocatalyst obtained in a frozen ice matrix at 77 K (see the Experimental Section).
Figure 3. Raman spectrum of N-Cu-TiO2 photocatalyst in ice matrix at 77 K.
The main spectral features are the narrow peaks at 388 cm1 [with full width half maximum (FWHM) of 11 cm1], at 512 cm1 (FWHM of 13 cm1), and 635 cm1 (FWHM of 14 cm1). These are the typical Raman bands of anatase B1g (388 cm1), A1g (512 cm1), and Eg (635 cm1). Our data are consistent with the reported Raman bands of anatase at 395, 514, and 638 cm1 in TiO2 photocatalysts obtained using sol-gel and calcination synthesis.[36] The Raman spectra of other N-MTiO2 photocatalysts are similar to that of the N-Cu-TiO2 photoChemPhysChem 2014, 15, 942 – 949
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2.3. Photoreforming Glycerol to Hydrogen Using Binary Promoted Photocatalysts The dependence of the yield of hydrogen upon the initial concentration of the sacrificial electron donor glycerol in water solution was reported[37] for the Cu-TiO2 photocatalyst. The amount of hydrogen produced increases as the concentration of glycerol increases from 0 to 4 vol %, and slowly decreases at higher concentrations. We chose to use 10 vol % glycerol in water in our photocatalytic experiments, and we used 20 mg photocatalyst per 10 mL liquid phase. Such a solid/liquid ratio in the photocatalytic suspension is within the range of 0.5– 3.0 g L1, which was recommended in the recent critical note.[38] Figure 4 shows the spectrum of the light used in our photocatalytic experiments. This spectrum was calculated from the emission spectrum of the medium pressure mercury lamp pro-
www.chemphyschem.org mental Section) monitors the changes of the concentration of evolving hydrogen continuously, as opposed to the periodic sampling in the off-line analysis by gas chromatography.[41] In photocatalytic reforming of glycerol in water with the TiO2based photocatalysts,[23, 24, 42] the only gaseous products are H2 and CO2. The hydrogen sensor used by us was not cross-sensitive to carbon dioxide or water vapors, so selective determination of hydrogen was possible. Figure 5 shows a typical trace of photocatalytic hydrogen evolution: the volumetric concentration of hydrogen (in ppmv) obtained with the N-Cu-TiO2 photocatalyst versus the time of
Figure 5. Typical hydrogen evolution trace in the photocatalytic experiments.
Figure 4. Spectrum of light used in the photocatalytic experiments.
vided by the manufacturer after correction for the optical transmission curve of the photocatalytic vessel made of Pyrex. For convenience, the same spectral range is shown in Figure 4 as in the UV/Vis DRS spectra of our photocatalysts in Figure 1. Major emission lines in Figure 4 are at 366, 405, and 436 nm (the UV and blue part of the visible spectrum), and 546 and 578 nm (the green part of the visible spectrum). Assuming that photocatalytic activity depends mostly on optical absorption, all our photocatalysts were expected to be more effective than P25 TiO2 ; we test this assumption further below. It has been reported that low concentrations of H2S can be quantitatively determined by inline continuous measurement using a commercial H2S sensor.[39] To our knowledge, there are no reports on the inline continuous measurement of hydrogen produced in photocatalytic experiments using a hydrogen sensor. In our photocatalytic experiments, we continuously purged the photocatalytic suspension with ultra-high purity argon as described elsewhere,[40] and continuously analyzed the evolving gas mixture. Our inline sampling system with a commercial electrochemical hydrogen sensor (see the Experi 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
illumination. The arrows indicate the time when the optical shield protecting the photocatalytic vessel from the excitation light was removed (light ON) and inserted (light OFF). Upon removing the shield (light ON), evolution of H2 starts, and hydrogen concentration [H2] reaches a steady-state value; upon inserting the shield (light OFF), evolution of H2 decreases and stops. The hydrogen evolution trace in Figure 5 is not symmetric in time relative to the ON/OFF events. The rather long time to return to zero concentration is likely due to a gradual desorption of the remaining hydrogen from the surface of the photocatalyst. In emerging applications, hydrogen mixed with an inert gas, such as that obtained in photocatalytic glycerol reforming, can be used as a fuel in certain types of fuel cells.[43] There is no commonly accepted unit for the measure of reaction rates in heterogeneous photocatalysis.[38] Reported units of activity are mmol h1,[44] mmol per experiment,[45] molar ratio of hydrogen/sacrificial donor,[46] x times the activity compared to P25 TiO2,[47] mmol h1 m2,[48] and others. Measuring photocatalytic activities with the P25 reference material allows a comparison of activities observed with different samples and reported by different authors. Table 1 shows the hydrogen production rates obtained with our N-M-TiO2 photocatalysts (M = Cu, Ni, Co, Cr) and P25 TiO2 expressed in various units of measure: ppmv H2 , mmol H2 (g photocatalyst)1 h1, and mmol H2 (total area of photocatalyst in m2)1 h1, and the BET total surface areas of these photocatalysts. The rate of hydrogen production for N-N-TiO2 (no metal) is about the same as that for N-Cr-TiO2 (32 ppmv), and both are ChemPhysChem 2014, 15, 942 – 949
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Table 1. Hydrogen production rates and BET surface areas of N-M-TiO2 photocatalysts. Photocatalyst
[H2] [ppmv]
H2 rate [mmol g1 h1]
S(BET) area [m2 g1]
H2 rate [mmol m2 h1]
N-Cu-TiO2 N-Ni-TiO2 N-Co-TiO2 N-Cr-TiO2 P25 TiO2
400 125 80 32 64
1615 506 313 126 252
7.3 19.9 24.9 39.7 50.0
221 25 13 3 5
less than the value for reference P25 TiO2 (64 ppmv). For all other N-M-TiO2 photocatalysts (M = Co, Ni, Cu), the hydrogen production rate is higher than that of P25 TiO2. The reported N-Cr-TiO2 catalyst prepared by the sol-gel method[18] is more active than P25 TiO2 under both UV (254 nm) and visible light excitation. Thus, it is somewhat surprising that our N-Cr-TiO2 photocatalyst shows lower activity than P25. Such a difference is most likely due to the different oxidation states of the Cr promoter: CrIII in the reported photocatalyst[18] and CrV in our samples. The reported[15] potential photocatalyst Cr-N-TiO2 with Cr in the CrV state has not been tested in photocatalytic experiments. One would expect that the higher optical absorption would result in a higher hydrogen evolution rate in at least some of the multiple units of measure: ppmv, mmol g1 h1, or mmol m2 h1 (Table 1); however, this is not the case. For instance, our N-Ni-TiO2 photocatalyst has a weaker optical absorption than N-Cr-TiO2 throughout the whole optical range (Figure 1), yet its hydrogen evolution rate is higher in all the units of measure (Table 1). Therefore, one needs to consider the rate of charge recombination, which is detrimental for photocatalytic activity. It is known to be difficult to quantify the nonradiative charge recombination rate, but it is relatively straightforward to quantify the radiative charge recombination rate. The latter is achieved by measurement of the photoluminescence (PL) spectra of the photocatalysts.
Figure 6. Photoluminescence spectra of N-Co-TiO2 with lexc = 260 nm (solid line) and 280 nm (dashed line).
photocatalysts and P25 TiO2 show the same spectral shape, although the intensity varies. Table 2 shows hydrogen production rates of the N-M-TiO2 photocatalysts (M = Cu, Ni, Co, Cr) and P25 TiO2, as well as the PL intensity measured at 360 nm with lexc = 260 nm. This PL is due to a major direct transition, X1b !X1a, between the CB and VB in nanocrystalline anatase.[31a]
Table 2. Hydrogen production rates, BET surface areas, and photoluminescence of N-M-TiO2 photocatalysts. Photocatalyst
H2 rate [mmol g1 h1]
S(BET) area [m2 g1]
H2 rate [mmol m2 h1]
PL intensity of X1b !X1a transition [a.u.]
N-Cu-TiO2 N-Ni-TiO2 N-Co-TiO2 N-Cr-TiO2 P25 TiO2
1615 506 313 126 252
7.3 19.9 24.9 39.7 50.0
221 25 13 3 5
30 10 20 28 17
2.4. Photocatalyst Characterization by PL Spectroscopy Figure 6 shows the PL spectra of the N-Co-TiO2 photocatalyst obtained with lexc = 260 and 280 nm; the PL spectra are vertically offset for clarity. The PL peaks at 360 and 380 nm observed in the spectrum with lexc = 280 nm are consistent with the PL peaks reported for the Fe-TiO2 photocatalyst.[20] In our experiments, we measured the PL spectra of the suspension of the photocatalyst in water using an angular fluorescence accessory (see the Experimental Section). By using an angular accessory in the PL experiment spectral artefacts due to secondary absorption and emission of light are minimized.[49] Therefore, the PL peaks in Figure 6 are narrow and well-resolved. This is in contrast with the PL spectra in other reports,[31] in which the energies of the PL transitions had to be determined by complicated multi-peak fitting of a single broad (several tens of nm) spectral band. In Figure 6, the spectral range greater than 490 nm is not shown due to the fringe of the second order diffraction peak,[49] which has the center at 520 nm for lexc = 260 nm. All of the other studied N-M-TiO2 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The N-Cr-TiO2, which shows the lowest hydrogen production rate (Table 2), has the strongest PL peak among all the N-MTiO2 photocatalysts except N-Cu-TiO2. In a reference experiment, we measured the PL spectrum of the pellet of the neat N-Cr-TiO2 photocatalyst in air, without using a fluorescence cuvette. As expected, the PL spectrum of the pellet of neat NCr-TiO2 has the same peaks that are observed in the PL spectrum of N-Cr-TiO2 suspended in water within the fluorescence cuvette (see the Experimental Section). A recent paper[41] reports the PL spectra of Ni-TiO2, Fe-TiO2, and binary metal-metal promoted Ni-Fe-TiO2. PL spectra similar to those in Figure 6 with several distinct peaks were obtained, but the PL transitions were not assigned.[41] We assigned the PL peaks in Figure 6 based on the reported energy levels in nanocrystalline anatase.[31] Figure 7 shows the PL transitions of anatase NPs: emission from the CB to the VB[31a] and emission from shallow trap levels due to oxygen vacancies, V(O), to the VB.[31b] ChemPhysChem 2014, 15, 942 – 949
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Figure 7. Photoluminescence transitions in nanocrystalline anatase.
The following peaks in Figure 6 represent the transitions from the CB to the VB (Figure 7): the peak at 360 nm is due to the X1b !X1a transition, the peak at 393 nm is due to the X1b ! G3 transition, the peak at 422 nm is due to the G1b !X2b transition, and the peak at 435 nm is due to the G1b !X1a transition. The following peaks represent the transitions from the shallow trap states due to oxygen vacancies, V(O), to the VB of anatase: the peak at 445 nm is the V1(O)!G3 transition, the peak at 459 nm is the V2(O)!G3 transition, and the peak at 485 nm is the V3(O)!G3 transition. We conclude that the N and M promoters in our N-M-TiO2 photocatalysts do not create new N or M-induced fluorescent energy levels, but rather increase or decrease the energy of a radiative electron-hole recombination in anatase (Figures 6 and 7), depending on the position of the metal in the first row of transition metals. The N-Cu-TiO2 photocatalyst shows the highest hydrogen evolution rate among all the photocatalysts studied by us (Table 1) in units of ppmv, mmol g1 h1, or mmol m2 h1. It was reported[37] that the photocatalytic activity of Cu-TiO2 depends on the oxidation state of the Cu promoter; CuII, CuI or metallic copper. During our photocatalytic tests, we observed a change in the color of the N-Cu-TiO2 photocatalyst. This observation could indicate the in situ reduction of the as-prepared CuII promoter state of the pre-photocatalyst to the photocatalytically active CuI or Cu0 state of the true photocatalyst. Experiments are in progress to determine the oxidation state of the Cu promoter in the N-Cu-TiO2 photocatalyst in situ, during the photocatalytic reaction. Notably, emission lines in the visible range of the spectrum of the mercury lamp used (at 546 and 578 nm, Figure 4) do not correspond to the absorption maxima in the UV/Vis DRS spectrum of the N-Cu-TiO2 photocatalyst. Therefore, it is not the absorption of visible light by the CuII promoter that defines the rate of hydrogen evolution. Further, by comparing the UV/Vis DRS spectrum (Figure 1) of the second most active photocatalyst, N-Ni-TiO2, with the spectrum of our light source (Figure 4), one can conclude that our N-Ni-TiO2 photocatalyst does not absorb photons in the visible range at 405, 436, 546, and 578 nm more strongly than the other photocatalysts, yet its photocatalytic activity is the highest of all the N-M-TiO2 catalysts except N-Cu-TiO2. Hence, the enhanced activity of the N-Ni-TiO2 photocatalyst is not due the enhanced absorption of visible light, but due to the decreased radiative 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org charge recombination (observed in the photoluminescence measurements). Computational studies of the energy of transition metal (V, Cr, Mn, Fe, Co, and Ni) induced impurity levels in anatase[50] and rutile[51] are available. The Cr promoter creates electronic states in the middle of the bandgap of anatase, but Co and Ni promoters[50] create electronic states slightly above the top of the VB. Further, the DFT computations suggest that NiO clusters adsorb strongly onto rutile and anatase in the Ni-TiO2 photocatalyst; this creates new chemical bonds with the surface of TiO2.[52] As a result, the top of the VB of Ni-promoted rutile and anatase originates from the electronic states of oxygen in the NiO cluster. However, the Ni 3d states of the supported NiO cluster contribute to the CB of anatase.[52] We explain the observed high photocatalytic activity and weak photoluminescence of the binary promoted N-Ni-TiO2 photocatalyst through the modification of the electronic structure of the CB (initial state for fluorescence transition) by the electronic states of the supported Ni promoter. Specifically, we propose that in our NNi-TiO2 photocatalysts there are triplet states just below the bottom of the CB due to mixing with the d–d states of the supported transition metal, Ni. Thus, the probability of photoluminescence from the CB to the VB in the binary promoted N-Ni-TiO2 would decrease, and result in higher photocatalytic activity. Figure 8 shows the proposed energy level diagrams of the binary promoted N-M-TiO2 photocatalysts: N-Ni-TiO2 (left) and N-Cr-TiO2 (right).
Figure 8. Proposed energy levels in binary promoted N-M-TiO2 photocatalysts.
The rather low photocatalytic activity of our N-Cr-TiO2 photocatalyst despite its high optical absorption both in the nearUV and visible range is due to the electronic decoupling of the Cr cation from the VB and CB of the TiO2. The mechanism of action of the Ni promoter in the binary promoted TiO2 photocatalyst is expected to be the quenching of the photoluminescence from the CB to the VB; work is in progress to test this hypothesis in a broader context. Our findings can be used to predict the photocatalytic activity of the other binary promoted N-M-TiO2 photocatalysts from experimentally measured PL spectra and published energies of electronic levels due to the M promoter.
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CHEMPHYSCHEM ARTICLES 3. Conclusion We describe the synthesis of a previously unreported family of new N-M-TiO2 photocatalysts (M = none, Cr, Co, Ni, Cu) by simultaneous introduction of N and M promoters, the structural and spectroscopic characterization of these N-M-TiO2 photocatalysts, and photocatalytic reforming of glycerol in water to hydrogen under ambient conditions, near-UV, or visible light. The M and N promoters do not form new electronic states with radiative charge recombination, but rather modulate the photoluminescence from the nanocrystalline TiO2 host. The Co, Ni, and Cu promoter cations in the binary promoted N-M-TiO2 photocatalysts significantly enhance the rate of evolution of hydrogen upon photoreforming glycerol in water under nearUV or visible light compared to P25 TiO2. In units of activity of heterogeneous photocatalysts of mmol m2 h1, the N-Ni-TiO2 photocatalyst is five times more active than P25 TiO2, and NCu-TiO2 is 44 times more active. We used a systematic approach to measure both photocatalytic activity and photoluminescence in suspension in water at room temperature. It is not a strong absorption of light, but a low rate of radiative charge recombination that increases the photocatalytic activity of binary promoted N-M-TiO2 photocatalysts in the near-UV or visible range.
Experimental Section Chemicals Precursors for the synthesis of N-M-TiO2 (titanium(IV) isopropoxide, ethylenediamine, nitrates of CrIII, CoII, NiII, CuII, and ammonium, absolute ethanol), and glycerol were purchased from Sigma, and P25 TiO2 Evonik was purchased from Acros Organics. The N-M-TiO2 photocatalysts (M = Cr, Co, Ni, Cu, none) were prepared by the modified sol-gel method.[53] Specifically, a solution of the respective metal nitrate in deionized water (0.33 m, 0.75 mL) was added dropwise to a solution of titanium(IV) isopropoxide (2.870 g) and ethylenediamine (0.300 g) in absolute ethanol (25 mL) under strong stirring at 75 8C, until formation of a gel occurred. The amount of M added corresponds to a ratio of 0.025 mole M per mole Ti. The resultant gel was dried under vacuum at room temperature to constant weight, then was calcined in flowing air (flow rate 0.5 L min1) at 450 8C for 4 h. In the synthesis of the N-N-TiO2 catalyst (without metal promoter), we used the same molar amount of ethylenediamine as in the synthesis of N-M-TiO2, but instead of metal nitrate, we used the same molar amount of ammonium nitrate (the gel does not form without ammonium nitrate).
Instrumentation and Characterization UV/Vis DRS was measured with a UV/Vis spectrometer from Ocean Optics using a xenon LS-1 lamp as the light source, a fiber-optical reflection probe, and a commercial WS-1 diffuse reflectance standard. Raman spectra were measured at 77 K using a MicroRaman spectrometer from Avantes at l = 785 nm; the sample was a suspension of the N-M-TiO2 photocatalyst in distilled water at a solid/ liquid ratio of about 1:3, frozen at 77 K in liquid nitrogen using a custom-built sample holder in a Dewar vessel. The total surface area of the photocatalysts was measured with a BET Nova 2000e (Quantachrome Instrument Corp., USA) using nitrogen adsorption/ desorption isotherms at 77 K. Before measuring the total surface 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org area, the samples were degassed for 3 h at 120 8C, and the surface area S(BET) was calculated with the BET equation using the program supplied with the BET apparatus. PL spectra of the photocatalysts were recorded using a Cary Eclipse fluorescence spectrometer equipped with the angular accessory (Agilent, Inc.) to minimize primary and secondary absorption of light[49] in the suspension of the photocatalyst. We used the default setting of the angular accessory at 308 and excitation wavelength lexc = 260 or 280 nm. The quartz fluorescence cuvette was filled with the suspension of photocatalyst (40 mg) in deionized water (0.3 mL), degassed by sonication, closed with a Teflon stopper, and sealed with parafilm tape. The PL spectra were measured after the solid photocatalyst had fully settled on the bottom of the fluorescence cuvette. The position of the cuvette was adjusted[54] until the spatial profile of the excitation light coincided with the layer of the photocatalyst on the bottom of the cuvette, and the highest PL signal was obtained.
Photocatalytic Tests In the photocatalytic tests, a medium pressure 450 W mercury lamp from Ace Glass was used as the light source. The photocatalyst (20 mg) was suspended in a solution of glycerol in water (10 vol %, 10 mL) in the Pyrex reaction vessel. Prior to the test, the suspension was degassed by ultrasonication for 10 min to remove dissolved oxygen. During the photocatalytic test, the suspension was continuously purged with 99.999 % Ar at a constant flow rate of 30 sccm (standard cubic centimeter per minute). The whole photocatalytic setup, including the light source and Pyrex reaction vessel containing the suspension of the photocatalyst, was immersed into a water circulation thermostat (Neslab Inc.) maintained at 30 8C. A mixture of water and ethylene glycol in a circulation thermostat absorbs the infrared radiation emitted by the mercury lamp, but transmits both near-UV and visible light. The concentration of evolving hydrogen was continuously measured with an inline electrochemical hydrogen sensor (model 4101-07, from Sierra Monitor Inc.) in parts per million by volume (ppmv) units. Analog data from the sensor were acquired with the LabView program written in-house.
Acknowledgements A.S. sincerely thanks Research Corporation for Science Advancement (RCSA) for his Cottrell College Science grant #20241, and Rutgers University for Research Council Grant #202221. S.T. sincerely thanks the Chemistry Department for his John C. Collier undergraduate Research Grant in Chemistry, and Rutgers University—Camden for his Dean’s undergraduate award. Keywords: glycerol · hydrogen · photocatalysis · titanium dioxide · transition metals [1] K. Liu, C. Song, V. Subramani, Hydrogen and Syngas Production and Purification Technologies, Wiley, Hoboken, 2010. [2] A. Fujishima, K. Honda, Nature 1972, 238, 37 – 38. [3] A. Samokhvalov, Catal. Rev. Sci. Eng. 2012, 54, 281 – 343. [4] B. Ohtani, O. O. Prieto-Mahaney, D. Li, R. Abe, J. Photochem. Photobiol. A 2010, 216, 179 – 182. [5] P. V. Kamat, J. Phys. Chem. Lett. 2012, 3, 663 – 672. [6] a) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 2001, 293, 269 – 271; b) C. Ampelli, R. Passalacqua, C. Genovese, S. Perathoner, G.
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Received: November 30, 2013 Revised: January 25, 2014 Published online on February 25, 2014
ChemPhysChem 2014, 15, 942 – 949
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