Format And Type Fonts CCHHEEMMIICCAALL EENNGGIINNEEEERRIINNGG TTRRAANNSSAACCTTIIOONNSS VOL. 45, 2015 A publication of The Italian Association of Chemical Engineering www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Sharifah Rafidah Wan Alwi, Jun Yow Yong, Xia Liu Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-36-5; ISSN 2283-9216 DOI: 10.3303/CET1545044 Please cite this article as: Tahir B., Tahir M., Saidina Amin N.A., 2015, Carbon dioxide reduction with hydrogen in a continuous catalytic monolith photoreactor, Chemical Engineering Transactions, 45, 259-264 DOI:10.3303/CET1545044 259 Carbon Dioxide Reduction with Hydrogen in a Continuous Catalytic Monolith Photoreactor Beenish Tahir , Muhammad Tahir , Nor A. S. Amin* Chemical Reaction Engineering Group/ Low Carbon Energy Group, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor Baharu, Johor, Malaysia. noraishah@cheme.utm.my Photocatalytic CO2 reduction with H2 as a reductant over gold (Au)-doped TiO2 nanocatalysts in a continuous monolith photoreactor has been investigated. The nanocatalysts were characterized by XRD, SEM, N2 adsorption-desorption and UV-Visible spectroscopy. Crystalline nanoparticles of anatase phase TiO2 were obtained in the Au-doped TiO2 samples. CO was the major product over 0.5 wt. % Au-doped TiO2 with the yield rate of 12,305 ppm g-catal. -1 h -1 , 318 times higher than un-doped TiO2 catalyst. Significantly higher photoactivity of Au-doped TiO2 was obviously due to fast electron transfer with hindered recombination rates and larger illuminated surface area inside the monolith channels. The CO production rate was gradually reduced with increasing the space velocity. The stability of the reused catalysts for CO production sustained at cyclic runs. It is evident Au-doped TiO2 nanocatalyst supported over monolith channels is highly potential for continuous CO2 photoreduction to CO and hydrocarbons. 1. Introduction Photocatalytic reduction of carbon dioxide (CO2) to useful chemicals has grown into an intense area of research owing to global warming and energy crises (Wang et al., 2015). The reduction of CO2 to CO, CH4, HCOOH, HCHO, and CH3OH, with water as the reducing agent, has been reported for the first time more than three decades ago (Inoue et al., 1979). The photocatalytic CO2 reduction with water is a challenging task as H2O is a weak reductant and is hardly reducible. However, photoreduction of CO2 with H2 through reverse water gas shift (RWGS) reaction is more effective to produce fuels (Tahir et al., 2015). Among the semiconductors materials, TiO2 is a promising photocatalyst due to numerous advantages such as strong oxidative-reductive potential, cheaper, abundantly available, and chemically/thermally stable (Ruzmanova et al., 2013). However, TiO2 photoactivity is relatively lower due to the fast recombination rate of electron-holes pairs, which can be improved by modifying its structure with noble metals (Sacco et al., 2015). Among the noble metals, gold (Au) nanoparticles doping into TiO2 can efficiently enhance photoactivity. The purposes of Au-doping or depositing into TiO2 are: (i) to modify TiO2 surface morphologies, (ii) to improve e-/h+ pair’s separation by acting as electron trap and, (iii) to increase the surface electron activity by localized surface plasma resonance (Sellappan et al., 2013). Au-doped TiO2 catalysts, investigated for CO2 photoreduction with H2O to hydrocarbons, registered higher TiO2 photoactivity with Au-metal (Mei et al., 2013). Therefore, it is envisaged that gold-doped TiO2 system would efficiently reduce CO2 through RWGS reaction. Among the structured supports, monolith containing parallel straight channels takes advantages of distinctive structure, higher surface area to volume ratio, and efficient light harvesting. Monolith substrate provides up to 100 times higher specific surface area than other types of catalyst supports having the same outer dimensions (Liou et al., 2011). We reported monolith photoreactor for CO2 reduction and found good CO2 conversion efficiency and CO selectivity (Tahir and Amin, 2015b). However, monolith photoreactor was investigated in a batch mode of operation. Recently, enhanced photocatalytic CO2 reduction to CH4 via steam reforming over metal-doped TiO2 in a photocatalytic fluidized bed reactor has 260 been reported (Vaiano et al., 2015). Therefore, it is anticipated that RWGS reaction in a continuous monolith photoreactor over Au-doped TiO2 would be appreciable to further enhance the photocatalysis process efficiency. The objective of this study is to test the performance of a continuous monolith photoreactor and photoactivity of Au-doped TiO2 nanoparticles for the reduction of CO2 by H2 through RWGS reaction. 2. Experimental 2.1 Catalyst preparation and characterization The gold-doped TiO2 catalysts were synthesized using modified sol-gel method as reported previously (Tahir and Amin, 2015a). Typically, a mixture of 7 mL acetic acid and 10 mL isopropanol was added to a mixture of 10 mL titanium tetra isopropoxide in 30 mL isopropanol under vigorous stirring. The stirring was continued for 24 h until a clear sol was produced. Next, calculated amount of gold chloride (HAuCl2. 3H2O), dissolved in isopropanol, was added to the titanium solution and stirred for the next 6h until clear and thick sol was obtained. The monoliths were dip-coated in the resulting Au/TiO2 sol for a few seconds and excess sol was blown off using compressed hot air. The Au/TiO2 monoliths and the remaining sol were dried at 80 ° C for 12 h and finally calcined at a rate of 5 ° C min -1 up to 500 ° C and held for 5 h. Similarly, TiO2 samples were prepared using the same procedure. Powder X-ray diffraction (XRD) was performed on Bruker D8 advance diffractometer. Cu- Kα radiation was used, operating at 40 kV and 40 mA with a scan range of 10-80 degree (2θ), a scan speed of 1.2 degree (2θ) per min and wavelength (λ) of 1.54A o. The morphology of the nanocatalysts was estimated using scanning electron microscopy (SEM) carried out with JEOL JSM6390 LV SEM instrument. N2 adsorption– desorption isotherms was performed at -196 o C using a Micrometrics ASAP 2020 Surface Area and Porosity Analyser. Ultraviolet-Visible (UV-Vis) diffuse reflectance absorbance spectra of the samples were determined using Agilent, Cary 100 UV-Vis spectrophotometer equipped with an integrated sphere. 2.2 Photoactivity test The photocatalytic CO2 reduction with H2 as a reducing agent was conducted in a cylindrical stainless steel reactor of continuous flow with a total volume of 150 cm 3 as depicted in Figure 1 (a). The chamber was equipped with a quartz window for passing light irradiations from the reflector lamp. The catalyst coated ceramic monoliths with channels per square inch (CPSI) 200 were inserted in the middle of the chamber. The light source used was a 200 W Hg lamp for UV irradiation source. The light intensity of 150 mW cm -2 was measured with an online optical process monitor ILT OPM-1D and a SED008/W sensor. All the experiments were carried out in a continuous mode using different flow rates at fixed CO2/H2 molar feed ratios of 1.0. The compressed CO2 and H2 flow rates were regulated by mass flow controllers (MFC). The products were analyzed using an on-line gas chromatograph (GC-Agilent Technologies 6890 N, USA) equipped with a thermal conductivity detector (TCD) and a flame ionized detector (FID). 3. Results and discussion 3.1 Catalyst characterization The XRD spectra of TiO2 and Au-doped TiO2 samples are presented in Figure 1 (b). The XRD peaks of pure TiO2 and Au-doped TiO2 revealed a pure crystalline and anatase phase TiO2. The peaks pertaining to Au-doping into TiO2 were not detected. In the case of 0.3 wt. % Au, the TiO2 crystallite size slightly decreased but has no significant effect in 0.5% Au-loaded TiO2 (Table 1). This revealed a value of around 17±1 nm was found for all the samples with no appropriate change with the introduction of gold. Similar observations are reported previously (Hidalgo et al., 2011). The SEM images of the nanoparticles catalyst and coated over the monolith channels are presented in Figure 2. The morphology of the catalyst coated monolith is shown in Figure 2 (a-b). It is obvious that the catalyst was entirely coated over the channel surface with no broken layer observed. The TiO2 nanoparticles are obvious in Figure 2 (c), identified as spherical and mesoporous TiO2 structure. Similarly, Au-doped TiO2 nanoparticles are shown in Figure 2 (d). Obviously, gold-doped TiO2 nanoparticles are uniform in size with higher mesoporosity. Figure 3 (a) exhibits the N2 adsorption-desorption isotherms of TiO2 and Au-doped TiO2 nanoparticles. All the isotherms have close resemblance with type IV curve with obvious hysteresis loops, thus conforming TiO2 and Au- doped TiO2 as mesoporous materials. The monolayer-multilayer adsorption on the internal surface is obvious in the initial part of the isotherms (at low P/Po). The capillary condensation could be seen in the upper part of isotherms at higher P/Po, where the steep increment in the adsorption volume is observed as the pores were saturated with the liquid. (Tahir and Amin, 2015a). The specific surface area, pore volume and pore sizes of all the samples are reported in Table 1. The BET surface area of TiO2 was 261 43 m 2 /g closer to BET surface area of Au-doped TiO2. Thus in the case of Au-doped-TiO2 samples, the increase in the surface area was not significant. It is evident that Au-doping in TiO2 samples could not alter the BET surface area, confirming no significant effect on TiO2 morphology. However, the larger BJH surface area of all the Au-doped TiO2 samples are attributed to the suppression of TiO2 crystal growth by Au- doping, thus enhancing mesoporosity. Similarly, BJH adsorption pore volume increased with Au- doping. Thus, the increased in the BJH surface area and pore volume was obviously due to the controlled crystal growth in the Au-doped TiO2 samples. Figure 1: (a) Schematic of experimental setup for CO2 reduction with H2 in monolith photoreactor, (b) XRD spectra of pureTiO2 and Au-doped TiO2 samples with different metals concentrations The optical properties of the pure TiO2 and Au-doped TiO2 were studied by measuring the absorbance spectra of wavelengths ranging from 200 to 800 nm as presented in Figure 3 (b). All the samples of Au- doped TiO2 show higher absorbance intensities than un-doped TiO2. The presence of Au metal improves the absorbance spectra towards visible light due to their optical absorption spectrum ranging between 500 to 700 nm, associated with surface plasmon resonance absorption peak of Au-metal. These results confirmed that the Au-metal could enhance TiO2 photoactivity in the visible region due to plasmonic response. The band gap energies were determined using Tauc plot i.e. (ahv) 2 versus (hv) by extrapolating the linear region of the plot to the intercept of the photon energy axis. The obtained band energy values of TiO2 and Au-doped TiO2 catalysts are reported in Table 1. The band gap energy of pure TiO2 was 3.12eV, reduced to 3.03 and 2.93 eV for 0.3 and 0.5 wt. % Au-doped TiO2 samples, respectively. This shows Au impurity level is beneficial for extending the absorption spectrum wavelength towards the visible region. Figure 2: SEM images of TiO2 and Au-doped TiO2 coated over monolith channels: (a-b) front and cross section of monolith channels coated with catalysts, (c) TiO2 nanoparticles (d) Au/TiO2 nanoparticles. 262 Figure 3: (a) N2 adsorption-desorption isotherms of TO2 and Au-doped TiO2 samples, (b) UV–Vis diffuse reflectance absorbance spectra of TiO2 and Au-doped TiO2 samples. Table 1: Summary of physiochemical characteristics of TiO2 and Au/TiO2 samples Catalysts BET surface area (m 2 /g) BJH adsorption surface area (m 2 /g) BJH pore volume (cm 3 /g) Crystallite size (nm) Band gap energy (eV) TiO2 43 52 0.134 19 3.12 0.3 wt.% Au- TiO2 46 58 0.23 17 3.03 0.5 wt.% Au-TiO2 47 74 0.24 18 2.93 3.2 Photoactivity test of CO2 reduction with H2 Preliminary investigations of CO2 photo-reduction with H2 was performed in a continuous flow monolith photoreactor at 100 ° C and feed flow rate 20 mL/min. The carbon containing compounds was not detected in the cases of (a) catalyst coated monolith without reactants under UV-irradiation, (b) reactants with catalyst coated monoliths without UV-irradiation. This confirmed catalysts and monoliths did not degrade and any carbon containing compounds produced were derived from CO2 through photocatalysis. The effects of Au-loading into TiO2 for photocatalytic CO2 reduction with H2 to CO through RWGS reaction using different irradiation times at 100 ° C, CO2/H2 ratio 1.0 and flow rate 20 mL/min are presented in Figure 4 (a). At the start of the reaction, the photocatalytic CO2 reduction into CO was significantly higher in all Au-doped TiO2 catalysts. The maximum CO production was observed initially and then gradually reduced over the irradiation time. Using un-doped TiO2 poor photoactivity is registered attributed to higher electron– hole pair’s recombination rate over the TiO2 surface. By doping Au-metal into TiO2, there was a substantial increase in CO2 reduction to CO as the major reduction product. A 0.5 wt. % Au-doped TiO2 catalyst was optimum, then the yield rates gradually decreased with higher Au-doping. The maximum production of CO over 0.5 wt. % Au-doped TiO2 was 12,305 ppm g-catal. -1 h -1 , 318 times higher than un-doped TiO2 catalysts calculated based on 2h irradiation time. Significantly higher TiO2 photoactivity in the presence of Au-metal was obviously due to the plasmonic effects with hindered recombination of charges over the Au- doped TiO2 surface and efficient light distribution inside monolith channels. Furthermore, in continuous monolith photoreactor, there were efficient adsorption-desorption processes, more production of electron- hole (e-/h+) pairs and their mobility over Au-doped TiO2 catalysts, resulting in much higher CO production. This development confirmed higher performance of monolith photoreactor in continuous flow over Au- doped TiO2 catalysts. Among the hydrocarbons, CH4 was found to be the major product in continuous monolith photoreactor at different irradiation time (0-10 h) as presented in Figure 4 (b). Using lower Au-doped TiO2, higher CH4 yield rate was observed, and then gradually decreased at more Au-loading. Besides, initially, CH4 yield increased and then decreased at an elongated irradiation time until reaching to steady state. The decreased in CH4 production by the irradiation time was probably due to the following: Firstly, when the reaction start-up, there was higher photoactivity of catalysts, thus more production of electrons, resulting in 263 higher CO2 photoreduction to CH4. At prolonged irradiation times, CH4 production decreased, possibly due to decrease in catalyst photoactivity. Secondly, at the start-up of reaction time, there was adsorbed CO2 and H2, which efficiently converted to CO and CH4 in the presence of higher mobility of electrons. However, at a prolonged reaction time, there was the efficient desorption of CO with the feed stream, resulting lower possibility of its conversion to CH4. Third possibility may be the photo-oxidation of CH4 with oxygen at a prolonged irradiation time in the presence of efficient monolithic catalysts, resulting in decreased of CH4 production. However, the further investigation is required to find out the actual possibly way in the reduction of CH4 production. The hydrocarbon products observed were C2H4,C2H6,C3H6 and C3H8. The production of higher hydrocarbons confirmed higher performance of Au-doped TiO2 monolithic catalysts compared to un-doped TiO2. Figure 4: Effects of Au-loading and irradiation time on CO2 reduction with H2 at CO2/H2 ratio 1.0, molar flow rate 20 mL/min, and temperature 100 °C; (a) CO production, (b) CH4 production. Figure 5: (a) Effects of space velocity on CO2 reduction to CO at CO2/H2 ratio 1.0; (b) Stability test of reused Au/TiO2 for CO2 reduction with H2 to CO at 100 o C, CO2/H2 ratio 1.0 and flow rate 20 mL/min. The effects of space velocity on the performance of Au-doped TiO2 catalyst for photocatalytic CO2 reduction with H2 in a monolith photoreactor are presented in Figure 5 (a). The reactant feed rates were changed to vary the space velocity at fixed CO2/H2 molar ratio and keeping all other parameters constant. With the increase in space velocity, production of CO and hydrocarbons gradually decreased. It indicates, at the same reaction conditions, a lower space velocity results in higher CO2 photoreduction. This is because a lower space velocity gives the reactant gases longer residence time over the catalyst surface in a monolith photoreactor (Tahir et al., 2015). In order to examine the stability of the photocatalyst, 264 experiments were repeated for certain times on the recycled catalyst coated monoliths. After each cycle, the monolith was removed from the reactor and placed in open air for 24 h before starting the next run. The stability test for photocatalytic CO2 reduction with H2 over Au-doped TiO2 catalyst is illustrated in Figure 5 (b). It is noticeable CO yield was higher in the first run; gradually decreased in the second photocatalytic cyclic run, but a slight decrease in CO yield was observed after third run. Therefore, the Au- doped TiO2 catalyst partially lost its activity in cyclic runs for photocatalytic CO2 reduction with H2 to CO. In the case of hydrocarbons, in first cyclic run, catalyst exhibits much higher CH4 yield which gradually decreased in second and third cyclic runs. This was possibly catalyst lost partial photoactivity for CH4 productions as explained previously. Similarly, hydrocarbons production also decreases after every run, confirming no coke production over the catalyst surface. These results exhibited better initial activity of Au- doped TiO2 for the production of CH4 and hydrocarbons. The colour of Au-doped catalyst during photocatalytic CO2 reduction remained un-changed during every cyclic run when catalyst coated monolith was exposed to open air. These results confirmed higher stability of Au- doped TiO2 catalyst, with supressed coke production. 4. Conclusions The performance of monolith photoreactor is tested for continuous CO2 reduction through RWGS reaction over Au/TiO2 catalyst. CO was observed as the main product with yield rate 12,305 ppm g-catal. -1 hr -1 , a 318 times higher than TiO2 with selectivity 99.5 % over 0.5 wt. % Au/TiO2. The higher efficiency of the monolith photoreactor was obviously due to a larger illuminated surface area, higher photon energy consumption and better utilization of reactor volume. The stability test revealed higher stability of Au-doped TiO2 catalyst. 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