Photodehydrogenation of Ethanol over Cu2O/TiO2 Heterostructures (original) (raw)
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RSC Advances, 2013
CuO x /TiO 2 nanocomposites prepared by copper photodeposition (1.0 and 2.5 wt% copper loading) on TiO 2 (synthesized by three different routes) are studied in the ethanol photo-dehydrogenation in gasand liquid-phase operations, and characterized in terms of surface area, phase composition by XRD, morphology and copper-oxide nanoparticle size distribution, and copper species by UV-visible diffuse reflectance spectroscopy. Cu 2+ ions partially enter into the titania structure leading to the creation of oxygen vacancies responsible for the shift in the band gap, but also the creation of traps for photogenerated holes and electrons. While the band gap shifts to lower energies with the copper content, a maximum photocatalytic activity is shown for the intermediate copper loading. Gas-phase operations allow a higher H 2 productivity with respect to liquid-phase operations, and especially a higher selectivity (about 92-93%) to acetaldehyde. It is remarked that the route of photodehydrogenation of ethanol to H 2 and acetaldehyde has an economic value about 3.0-3.5 times higher than the alternative route of photoreforming to produce H 2. Gas-phase operations would be preferable for the photo-dehydrogenation of ethanol.
CuO x −TiO 2 Photocatalysts for H 2 Production from Ethanol and Glycerol Solutions †
The Journal of Physical Chemistry A, 2010
Hydrogen production by photocatalytic reforming of aqueous solutions of ethanol and glycerol was studied with the use of impregnated and embedded CuO x /TiO 2 photocatalysts. Embedded CuO x @TiO 2 was prepared by a water-in-oil microemulsion method, which consists in the formation of Cu nanoparticles in the microemulsion followed by controlled hydrolysis and condensation of tetraisopropyl orthotitanate with the aim of covering the protected metal particles with a surrounding layer of porous titanium oxyhydroxide. Mild calcination leads to the complete removal of the organic residues, the crystallization of TiO 2 , and an unavoidable oxidation of copper. Two reference samples were prepared by classical wet impregnation of preformed TiO 2 with different ratios of anatase, rutile, and brookite polymorphs. The two supports were prepared by sol-gel (TiO 2 -SG) and microemulsion (TiO 2 -ME) methods. Superior performances have been observed for the embedded system, which shows higher hydrogen production rates with respect to the impregnated systems using either ethanol or glycerol as sacrificial molecules. Deep structural characterization of the materials has been performed by coupling high resolution transmission electron microscopy (HRTEM), high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), X-ray absorption fine structure (XAFS), and electron paramagnetic resonance (EPR) techniques. Correlation between copper oxidation state and its dispersion and reactivity has been attempted. Finally, the stability of the CuO x /TiO 2 catalysts was also studied with respect to carbonaceous deposits and copper leaching. † Part of the special issue "Green Chemistry in Energy Production Symposium".
Solar Energy Materials and Solar Cells, 2016
A series of Cu doped titania, Cu x Ti 1 À x O 2 À δ (x¼0.0, 0.02, 0.06) and copper oxide-titanium oxide nanocomposites, xCuO-yTiO 2 (x:y¼ 1:9, 2:8, 5:5) were synthesized by sol-gel method and characterized by relevant techniques. The role of Cu ions in enhancement of photocatalytic evolution of H 2 from H 2 Omethanol/glycerol mixtures in both sunlight and UV-visible irradiation over Cu/Ti oxides was investigated. X-ray absorption fine structure (XAFS), supported by X-ray diffraction and Raman studies, revealed that Cu substitution in TiO 2 stabilized anatase lattice, lengthened Ti-O bonds, decreased the coordination number around Cu ions, and induced oxygen ion vacancies and distortion (σ) in lattice. Distorted structures are more open and flexible with improved charge carrier dynamics and favourable photocatalytic properties. Nanosized Cu-Ti-O powders with enhanced N 2-BET surface area and microporosity exhibited improved photocatalytic properties. Reduction of CuO to photocatalytically more active Cu 2 O by photo-generated eon the surface of composites was evident by the absence of Cu 2 þ peak in the XPS spectra of the composite sample after exposure to light. The most active formulations for sunlight assisted photocatalytic H 2 generation were Cu 0.02 Ti 0.98 O 2 for the doped samples, referred to as CuTi(2), and xCuO-yTiO 2 (x:y ¼2:8) for the composite samples. Performance of the most active, CuTi(2), was monitored in a up-scaled photoreactor in order to investigate the influence of illumination area, catalyst concentration, form of catalyst (powder/films) and different sacrificial agents on H 2 yield. With an aim to identify practical materials for pilot plants, 6 mg of CuTi(2) was dispersed on (30 cm  0.7 cm) ITO/PET films that exhibited enhanced efficiency (3.06%) as compared to same amount of CuTi(2) powder (1.41%). The results showed that the utilization of the CuTi(2) photocatalyst (without costly cocatalysts) with the proper selection of optimum operational conditions under sunlight in a up-scaled photoreactor, generated H 2 yield of 1.167 L/h/m 2 with apparent quantum efficiency, AQE, of 7.5% and solar to fuel efficiency, SFE, of 3.9% for the photocatalytic hydrogen evolution reaction (HER). Our results suggest that with this efficiency, H 2 at 1 L/h would be evolved photocatalytically over 0.9 m 2 of CuTi(2) photocatalyst, exposed to sunlight. Inputs obtained from the present study will be useful for further scale up of sunlight driven photocatalytic hydrogen production over low cost and efficient Cu modified TiO 2 .
Several studies have shown that combining TiO2 and Cu2O enhances the photocatalytic activity of the material by generating a heterojunction capable of extending the light absorption in the visible and reducing the electron-hole recombination rate. Ball milling has been chosen as an alternative methodology for photocatalyst preparation, among the several techniques documented in the literature review. The results of a previously reported investigation enabled the identification of the most effective photocatalyst that can be prepared for hydrogen generation by combining Cu2O and TiO2 (i.e., 1%wt Cu2O in TiO2 photocatalyst prepared by ball-milling method at 200 rpm and 1 min milling time). To optimize photocatalytic hydrogen generation in the presence of the greatest photocatalyst, the effects of (i) sacrificial species and their concentration, (ii) temperature, and (iii) pH of the system are taken into account, resulting in a light-to-chemical energy efficiency of 8% under the best-t...
Highly stable CuO incorporated TiO 2 catalyst for photocatalytic hydrogen production from H 2 O
A CuO incorporated TiO 2 catalyst was found to be an active photocatalyst for the reduction of H 2 O under sacrificial conditions. The catalytic activity originates from the photogeneration of excited electrons in the conduction bands of both TiO 2 and CuO resulting in a build-up of excess electrons in the conduction band of CuO. Consequently, the accumulation of excess electrons in CuO causes a negative shift in the Fermi level of CuO. The efficient interparticle charge transfer leads to a higher catalytic activity and the formation of highly reduced states of TiO 2 /CuO, which are stable even under oxygen saturated condition. Negative shift in the Fermi level of CuO of the catalyst TiO 2 /CuO gains the required overvoltage necessary for efficient water reduction reaction. The function of CuO is to help the charge separation and to act as a water reduction site. The amount of CuO and crystalline structure were found to be crucial for the catalytic activity and the optimum CuO loading was ca. ∼5-10% (w/w). P h o t o c h e m . P h o t o b i o l . S c i . , 2 0 0 5 , 4 , 8 5 7 -8 6 1
Active Cu species in Cu/TiO2 for photocatalytic hydrogen evolution
In this work highly active Cu/TiO 2 was prepared by an impregnation method for enhancing photocatalytic hydrogen production. The samples were characterised by hydrogen temperature-programmed reduction (H 2 -TPR), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller specific surface area (BET). At the optimum Cu loading (0.5 wt%), H 2 -TPR analysis indicated the presence of readily reducible Cu species. Literature suggests these are isolated Cu atoms coordinated to oxygen atoms of the TiO 2 surface, and small Cu 2 O clusters on the defect sites of the TiO 2 surface. At higher Cu loadings (1 and 2 wt%), a second H 2 -TPR peak appeared which, according to literature, suggests the additional presence of bulk Cu 2 O particles on the TiO 2 surface. Calcination conditions were found to influence performance with the best activity achieved at a temperature of 300 °C. Optimised conditions gave a Cu/TiO 2 hydrogen production yield 3.5 times greater than bare TiO 2 . This significant enhancement was mainly attributed to the presence of fine Cu 2 O clusters and highly dispersed Cu on the TiO 2 surface.
Cu-doped TiO 2 with varying amounts of Cu (0.2, 0.3, 0.5, 1, 2, and 5) are prepared by impregnation method and calcined at 350 and 450°C for 5 h. These catalysts are characterized by X-ray diffraction, diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy energy-dispersive X-ray spectroscopy (EDAX), and transmission electron microscopy (TEM). The DRS studies are clearly showing the expanded photo response of TiO 2 into the visible region on impregnation of copper ions. TEM images are depicting the fine dispersion of Cu particles on TiO 2 surface. XPS studies are showing change in the binding energy values of Ti 2p, O 1s, and Cu 2p, indicating that copper ions are in interaction with TiO 2 . XPS results are also confirming that the oxidation state of copper is +2 in samples calcined at 350°C and +1 in samples calcined at 450°C. EDAX analysis supports the presence of copper species on the surface layers of TiO 2 . Photocatalytic hydrogen production activity studies are conducted over CuO/TiO 2 and Cu 2 O/ TiO 2 catalysts in pure water and glycerol:water mixtures under solar irradiation. Maximum hydrogen production of 265 and 290 µmol h -1 is observed over 2 wt % CuO/TiO 2 and Cu 2 O/TiO 2 catalysts in pure water. A significant improvement in hydrogen production is observed in glycerol:water mixtures and maximum hydrogen production of 16,500 and 20,060 µmol h -1 is obtained over 0.5 wt % CuO/TiO 2 and Cu 2 O/TiO 2 catalysts in 5% glycerol aqueous solutions. No hydrogen production activity is observed on reduced catalysts under solar irradiation. Furthermore, when these catalysts are studied under UV irradiation, 2-3 fold increase in activity is observed on calcined catalysts, and the same level of activity is observed on reduced catalysts, but under these conditions the activity is limited by the dissolution of Cu ions into the solution. However, under solar irradiation a continuous and stable activity is observed over Cu 2 O/TiO 2 catalyst. On the basis of the characterization and hydrogen production activity results, finely dispersed Cu in +1 oxidation state that is in interaction with TiO 2 is proposed as a promising visible sensitive photocatalyst for the continuous production of hydrogen from glycerol:water mixtures.