Microsoft Word - 476hernandez.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 43, 2015 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Chief Editors: Sauro Pierucci, Jiří J. Klemeš Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-34-1; ISSN 2283-9216 Steam Reduction of CO2 in a Photocatalytic Fluidized Bed Reactor Vincenzo Vaiano*, Diana Sannino, Paolo Ciambelli Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano (SA), Italy. vvaiano@unisa.it Unlike traditional catalysts that drive chemical reactions by thermal energy, photocatalysts can induce chemical reactions by light activation. It is well known that greenhouse gases, such as CO2, are the primary causes of global warming. From an environmental point of view, it is interesting to transform CO2 into hydrocarbons, such as CH4. Since this transformation has high energy duty, a photocatalytic process can be an effective way. There are few examples in literature concerning the use of photocatalytic fluidized bed photoreactor for the reduction of CO2 into CH4. The aim of this work is to investigate the performances of a high efficiency two-dimensional fluidized bed catalytic photoreactor with Cu/TiO2, Ru/TiO2 and Pd/TiO2. CH4 was the main product with very few amounts of CO. No deactivation phenomena were observed. Pd/TiO2 photocatalysts showed the best performances. At Pd load of 1 wt. %, CH4 photoproduction was 64 μmol g-1 h- 1, against 15 μmol g-1 h-1 achieved with bare TiO2. The photoreactivity reached with Pd/TiO2 is significantly higher than that reported in the current literature on gas-solid photocatalytic systems for the photoreduction of CO2. 1. Introduction Transition and noble metals supported on titania (TiO2) have been extensively studied as photocatalysts in several chemical reactions. Heterogeneous photocatalysis can be an effective alternative to remove bacteria (Rizzo et al., 2013) and organic pollutants such as methyl-ethyl ketone (Hajaghazadeh et al., 2014), cyclohexane (Murcia et al., 2013), methylene blue (Vaiano et al., 2014a), atrazine (Sacco et al., 2015), spiramycin (Vaiano et al., 2014b), highly polluted wastewater (Vaiano et al., 2014c) and NOx (Sannino et al., 2013b), or obtain partial oxidation products in mild conditions such as acetaldehyde (Sannino et al., 2013a) and benzene (Vaiano et al., 2014d). Unlike traditional catalysts that drive chemical reactions by thermal energy, photocatalysts can induce chemical reactions by solar energy. It is known that greenhouse gases, such as CO2, are the primary causes of global warming. The advantage of transforming CO2 into hydrocarbons, such as CH4, via photocatalytic reduction is to utilize UV or UV-visible light at low temperature and pressure. Promising photocatalysts for CO2 photoreduction are Pt/TiO2 (Zhang et al., 2009), Ru-TiO2/SiO2 (Sasirekha et al., 2006) and Ag/TiO2 (Koci et al., 2014). These photocatalysts were tested in fixed and/or batch photo-reactor, obtaining very low CH4 production rates. There are no examples in literature concerning the use of a catalytic fluidized bed photoreactor for testing the reduction of CO2 into CH4. In this work we have investigated the performances of suach a photoreactor with different photocatalyst formulations. 2. Experimental 2.1 Catalysts preparation and characterization Me/TiO2 (Me=Cu, Ru, Pd) catalysts were prepared by wet impregnation of anatase titania (PC500, Crystal Global) with solutions of different precursor salts, followed by drying at 120°C and calcination in air at 450°C for 2 h. In particular, (C5H8O2)3Ru, CuN2O6, Pd(NH3)4(NO3)2 were used for Ru/TiO2, Cu/TiO2 and Pd/TiO2, DOI: 10.3303/CET1543168 Please cite this article as: Vaiano V., Sannino D., Ciambelli P., 2015, Steam reduction of co2 in a photocatalytic fluidized bed reactor, Chemical Engineering Transactions, 43, 1003-1008 DOI: 10.3303/CET1543168 1003 respectively. The catalysts were named xMe, where x is the nominal metal loading and Me is the metal (Cu, Ru or Pd). Physico-chemical characterisation of catalysts has been performed with different techniques. Laser Raman spectra were obtained at room temperature with a Dispersive MicroRaman (Invia, Renishaw), equipped with 633 nm diode-laser, in the range 100-2500 cm-1 Raman shift. UV-Vis reflectance spectra were recorded with a Perkin Elmer spectrometer Lambda 35. Equivalent band gap (Ebg) determinations were obtained from Kubelka-Munk theory by plotting [F(R∞)*hν]2 vs hν and calculating the x intercept of a line through 0.5 < F(R∞) < 0.8. X-ray diffraction (XRD) was carried out using an X-ray microdiffractometer Rigaku D-max-RAPID, using Cu-Kα radiation and a cylindrical imaging plate detector. Diffraction data from 0 to 204 degree horizontally and from -45 to 45 degree vertically were collected. The incident beam collimators enable different spot sizes to be projected onto the sample. Mass titration method was used to estimate the acidity of sample powders useful to measure the PZC (point zero charge) of the photocatalysts. The mass titration experiments were performed using procedures described elsewhere (Noh and Schwarz, 1989). Shorter stabilization times after each powder addition (2 h) were used to minimize possible dissolution of sample powders. 2.2 Photocatalytic tests Photocatalytic tests were carried out at 140°C and atmospheric pressure, feeding 30 (stp)L/h He stream containing 1 vol. % CO2, with H2O/CO2 feeding ratio in the range 0.4-4. The fluidized bed reactor used in the work was designed for working with a gas flow rate in the range 20–70 (stp)L/h and a Sauter average diameter in the particles size range 50–100 μm, assuring optimal fluidization. It was a two dimensional reactor with 40 mm X 6 mm cross section, 230 mm height. Pyrex glass walls, and a bronze filter (mean pore size 5 μm) to provide a uniform distribution of fed gas. In order to decrease the amount of transported particles, an expanding section (50 mm X 50 mm cross-section at the top) and a cyclone, specifically designed are located on the top and at the outlet of the reactor, respectively. The reactor was illuminated by two UVA-LEDs modules (80X 50 mm) positioned in front of the reactor pyrex windows (UV light intensity: 90 mW cm-2). The catalyst weight was 2.2 g, diluted with of 20 g of glass spheres (grain size: 70–110 μm) (Lampugnani Sandblasting HI-TECH) to make easier the fluidization and to avoid a too large light absorption by the photocatalyst. In these conditions, the bed expansion is about 20%. The outlet gas composition was continuously measured by an on-line quadrupole mass detector and a continuous CO-CO2-CH4 NDIR analyser. In this way, CH4, CO2 and CO were mainly detected, although other compounds were also followed in order to test the possible formation of other intermediates. 3. Results and discussion 3.1 Catalyst characterization The Me/TiO2 photocatalysts are listed Table 1. In the same Table the metal nominal loading, equivalent band gap energy, TiO2 average crystallite size, and PZC values are also reported. Table 1: Me/TiO2 photocatalysts and their characteristics Catalyst CuO wt % RuO2 wt % Pd wt % Ebg eV TiO2 average crystallites size nm PZC pH unit TiO2 - - - 3.4 7 6.6 2Cu 2 - - 3.4 17 6.1 5Cu 5 - - 3.0 16 5.8 10Cu 10 - - 2.5 18 5.3 0.7Ru - 0.7 - 2.4 13 4.4 1.7Ru - 1.7 - 2.0 14 4.5 3.7Ru - 3.7 - 1.9 13 4.1 0.5Pd - - 0.5 2.3 21 3.5 1Pd - - 1 2.2 25 3.4 1.5Pd - - 1.5 2.2 23 3.3 While TiO2 nanoparticles absorb light of wavelength lower than 365 nm , after metal deposition the absorption wavelength of Me/TiO2 catalysts extends to the visible region. The phenomenon becomes more evident the higher is the active phase loading. These differences in the absorption properties corresponded to a decrease in the equivalent band gap energies with respect to bare TiO2, as shown in Table 1. Crystal phase composition of the materials was determined by XRD measurements (Figure 1). Anatase was the only crystalline phase of TiO2 identified in all samples. 1004 No signals attributed to CuO or PdO phases were detected for 2Cu, 5Cu, 0.5Pd and 1Pd samples, due to the high dispersion and low metal content present in the materials. The catalysts with the highest Cu or Pd content showed also a small peak at about 36 and 33°, attributed to the reflection of CuO (Etefagh et al., 2013) and PdO (Mohajeri et al., 2010), respectively. In the case of Ru/TiO2 RuO2 peaks at about 28° and 35° (Debecker et al., 2014) are visible for the samples 1.7Ru and 3.7Ru. Anatase crystallite size of the catalysts was evaluated from XRD analysis, using the Scherrer equation (Table 1). For bare TiO2 the anatase crystallite size was about 7 nm and increased after the active phase deposition. 20 30 40 50 60 70 80 2 θ(degree) In te ns ity (a .u .) 20 30 40 50 60 70 80 2 θ(degree) In te ns ity ( a. u. ) 20 30 40 50 60 70 80 2 θ(degree) In te ns ity (a .u .) Figure 1: XRD spectra for Cu/TiO2 (a), Ru/TiO2 (b) and Pd/TiO2 (c) CuO RuO2 RuO2 PdO TiO2 2Cu 5Cu 10Cu TiO2 0.7Ru 1.7Ru 3.7Ru TiO2 0.5Pd 1Pd 1.5Pd a) b) c) 1005 3.2 Photocatalytic results Tests carried out in the absence of UV light evidenced that no reaction occurred in dark conditions. The UV-irradiation of photocatalysts in the presence of a mixture of CO2 and H2O led to the evolution of CH4 as main product, as well as trace amounts of CO. No deactivation phenomena were observed during the irra- diation time. The influence of H2O/CO2 feeding ratio on methane formation rate is reported in Figure 2. 0 2 4 6 8 10 12 14 16 18 0 0.5 1 1.5 2 2.5 3 3.5 4 H2O/CO2 C H 4 pr od uc tio n (µ m ol g -1 h -1 ) 2Cu 5Cu 10Cu 0 2 4 6 8 10 12 0 0.5 1 1.5 2 2.5 3 3.5 4 H2O/CO2 C H 4 pr od uc tio n (µ m ol g -1 h -1 ) 0.7Ru 1.7Ru 3.7Ru 0 20 40 60 80 0 0.5 1 1.5 2 2.5 3 3.5 4 H2O/CO2 C H 4 pr od uc tio n (µ m ol g -1 h -1 ) 0.5Pd 1Pd 1.5Pd Figure 2: Influence of H2O/CO2 feeding ratio on methane production for Cu/TiO2 (a), Ru/TiO2 (b) and Pd/TiO2 (c) a) b) c) 1006 0 10 20 30 40 50 60 70 C H 4 fo rm at io n r at e, µ m o l g -1 • h -1 2C u 5C u 10 C u 0. 7R u 1. 7R u 3. 7R u 0. 5P d 1P d 1. 5P d Ti O 2 Figure 3: CH4 formation rate over different photocatalysts; H2O/CO2=4 For all the tested catalysts (except for 0.5Pd sample) CH4 production increased with the increase of H2O/CO2 feeding ratio. At fixed H2O/CO2 ratio photocatalytic activity decreased by increasing the metal loading. This last result could be explained considering that the formation of CuO, RuO2 and PdO crystallites segregated on TiO2 surface occurred with the increase of active phase loading, indicating a worsening of metal dispersion, as shown by XRD results (Figure 1). This phenomenon is responsible for the lowering of photocatalytic activity, as previously observed in other gas-phase photocatalytic reactions (Ciambelli et al., 2008). Figure 2 shows hat the catalyst containing 1 wt% Pd yields the highest CH4 production rate with respect to the others Me/TiO2 samples. The specific photocatalytic activities for the formation of CH4 in the steady state conditions, for H2O/CO2=4, are summarized in Figure 3. In the case of Cu/TiO2 and Ru/TiO2 samples the formation of CH4 was lower than TiO2 alone. Moreover photo-catalytic activity decreased by increasing Cu or Ru content (as previously observed in Figure 2).The behavior was completely different when Pd was used as active phase. In this case, it is evident the existence of an optimal Pd loading. In the case of 1Pd and 1.5Pd CH4 formation rate was higher than that obtained on bare TiO2, whereas on 0.5Pd photocatalyst a value very similar to TiO2 was achieved. A marked increase in the CH4 formation rate, up to a value of about 64 μmol g-1 h-1, was found, when Pd load was equal to 1 wt.%, which is therefore the optimal active Pd loading. The photoreactivity reached on 1Pd sample is also significantly higher than that found in the current literature on gas-solid photocatalytic systems for the photoreduction of CO2 (Anpo et al., 1995). 4. Conclusions Several photocatalysts active in the photocatalytic reduction of CO2 to methane, were prepared and tested. Operating conditions were optimized to obtain the maximum photoreactivity in a photocatalytic fluidized bed reactor with high illumination efficiency. It was identified a class of photocatalysts based on the use of Pd supported on titania. 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