- Email: [email protected]
Turning CO2 into Valuable Chemicals
Available online at www.sciencedirect.com
Energy Procedia 37 (2013) 6704 – 6709
GHGT-11
Turning CO2 into valuable chemicals Oluwafunmilola Olaa*, M. Mercedes Maroto-Valera, Sarah Mackintosha a
Centre for Innovation in Carbon Capture and Storage (CICCS), School of Engineering and Physical Sciences & Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh, EH14 4AS, United Kingdom.
Abstract The transformation of CO2 using semiconductor photocatalysts is a promising alternative for developing sustainable energy sources. Nickel (Ni) based TiO2 photocatalysts immobilized onto quartz plate was assembled in a unique reactor configuration to provide a high ratio of illuminated catalyst surface area for the reduction of CO2 to fuels under visible light. The improved conversion efficiency is ascribed to the presence of the metal, Ni, which served as electrons traps that suppressed recombination, resulting in effective charge separation and CO 2 reduction. More importantly, the quantum efficiency is higher than those reported in the literature that used similar reactor configurations. 2013The TheAuthors. Authors. Published Elsevier ©©2013 Published byby Elsevier Ltd.Ltd. Selection and/or peer-review under responsibility of GHGT Selection and/or peer-review under responsibility of GHGT Keywords: CO2 Photoreduction; Titanium dioxide; Photocatalysis; Photoreactor; Sol-gel; Nickel
1. Introduction As carbon dioxide capture and storage (CCS) is still in the demonstration phase, other complimentary alternatives have been developed for utilizing carbon dioxide (CO2) by converting it into chemicals and fuels [1]. Roy et al [2], Koci et al [3] and Usubharatana et al [4] pointed out alternatives; including photocatalysis, direct photolysis, and electrochemical reduction to utilize CO 2 as opposed to geological storage. The captured CO2 from CCS plants will serve as a relatively pure and inexpensive feedstock for these conversion methods to synthesize products such as methane, methanol, ethanol etc that can be used as chemicals, feedstock in fuel cells or hydrogen sources for electricity. Mikkelsen et al [5] have highlighted the difference between photocatalysis and electrochemical reduction as the source of electrons which is obtained from irradiating semiconductors under light in the * Corresponding author. Tel.: +44 131 451 3342 E-mail address: [email protected].
1876-6102 © 2013 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.06.603
Oluwafunmilola Ola et al. / Energy Procedia 37 (2013) 6704 – 6709
former and the application of an applied current in the latter. Although electrochemical cells can convert CO2, Yano et al. [6, 7] reported the deactivation of electrodes within a short time as a major drawback linked with its usage. This is due to the deposition of poisoning species i.e. adsorbed organic compounds on the electrode. Aresta [8] and Olah et al. [9] also reported that electrochemical reduction was energy intensive with slow kinetics. Creutz and Fujita [10] identified the need for an inexpensive hydrogen source in direct hydrogenation while Wu et al [11] reported high energy photons in direct photolysis as drawbacks to their utilization. Conversely, photocatalysis (an artificial form of photosynthesis) is more energy efficient compared to other reduction methods since it has the potential to utilize solar energy, a cheap and abundant non-fossil energy source for the conversion of CO2 into profitable products such as methanol, methane and other chemicals [12, 13]. Usubharatana et al. [4] and Palmisano et al [14] also noted the low energy input needed by photocatalysis as compared to other reduction methods that require higher energy consumption. Photocatalysis is, therefore, highly regarded for its low energy input, profitable products, and use of renewable energy. Amongst photocatalysts, titanium dioxide (TiO2) is particularly noteworthy due to its unique properties. However, its use is limited due to its large band gap; as it can only be activated by ultraviolet light which represents 2-5% of sunlight [15]. Since visible light accounts for 45% of the solar spectrum, there is a need to develop titania based photocatalysts which are active under the visible light spectrum. Attempts to improve the efficiency of this catalyst for solar fuel production is limited to the overall process efficiency being largely dependent on two factors - the reactor configuration and physicochemical properties of the catalyst. Accordingly, the loading of metals on the surface of titania can tailor its band width towards the visible light [15]. An investigation conducted by Ola et al [16] demonstrates that C1-C2 hydrocarbons can be produced from immobilizing nickel (Ni) loaded TiO2 nanoparticles onto quartz plates for CO2 reduction. However, the photocatalytic investigation and product selectivity of these materials has not been fully exploited. Accordingly, this work investigates the photocatalytic effect of increased Ni concentration on CO2 reduction and correlates the characterization with its performance for the generation of other C1-C2 compounds. 2. Experimental 2.1. Preparation of Ni loaded TiO2 nanoparticles The preparation of pure TiO2 and nickel loaded TiO2 nanoparticles used during the experimental phase was synthesized by the improved sol-gel method followed by drying and calcination, as described elsewhere [16]. Under controlled conditions, the appropriate amount of metal precursor was introduced within the hydrolysis and polycondensation phase depending on the weight percent loading calculated from the amount of precursors used in the synthesis. The calculated amount of nickel (II) nitrate hexahydrate (Ni (NO3)2.6H2O) was then dissolved in 14ml of acetic acid and stirred for 12 hours. Titanium butoxide was added to n-butanol with a volume ratio of 1:4. The subsequent solutions were stirred for 6 hours at room temperature. The homogeneous mixture was dried at 150oC for 3 hours and calcined at 500oC in a chamber furnace (Carbolite CWF 1100) to burn off organic compounds and complete the crystallite formation. The samples were then crushed into powder form to obtain both pure TiO2 and the metal loaded TiO2 nanoparticles with loading ratios within the range of 0.5wt% - 2.0wt%.
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2.2. Characterization of metal loaded catalysts Ultraviolet-visible spectroscopy (UV-Vis, Varian Cary 300) was used for the measurement of the threshold wavelength, band gap and the absorbance of ultraviolet light as a function of the transmittance. The morphology and elemental composition of the nanoparticles were evaluated by transmission electron microscopy (TEM, JEOL 2100F), while analysis of the crystallographic structure of the catalyst was done by the X-ray diffraction (XRD) using the Hiltonbrooks X-ray powder diffractometer. The crystalline size in the catalysts was detected by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra) using a -ray source (1486.6eV) typically operated at 12kV anode potential. The main C mono1s peak = 285eV was used for charge correcting the high resolution scans. CASAXPS software with Kratos sensitivity factors was used in analyzing the elements present. 2.3. Photocatalytic reduction of CO2 The quartz plate photoreactor with dimensions of 5.5cm X 11cm (diameter and height), with ports for sampling the gaseous by-products, measuring the pressure and gas (CO2) inlet (Figure 1) was used for CO2 reduction. The average light intensity measured in front of the quartz window using the radiometer (Omnicure R2000) was 68.35mW/cm2 for the visible light source (500W Halogen floodlight lamp).
Fig. 1. Schematic of the quartz plate photoreactor
Approximately 0.2g of catalyst was evenly dispersed on the quartz plate with dimensions of 4.0cm X 9.7cm to provide a high ratio of illuminated surface area of catalyst in the reactor. The coated quartz plate was inclined at an angle of 15o facing the light rays emitted through the quartz window to maximize light utilization efficiency. The photoreactor was covered with a gloved box to exclude any external light source and maximize light energy within the reactor. Leak test was performed by purging the photoreactor with helium. If leak-proof, the reactor was then purged with this gas for 1 hour, then the gas flow was changed to ultra pure CO 2 gas saturated with water vapor for 1 hour. After 1 hour, the light source was switched on and the curtains of the gloved box shut to enable photoreduction take place. Readings were taken after 4 hours with extracted gas samples injected
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Oluwafunmilola Ola et al. / Energy Procedia 37 (2013) 6704 – 6709
into the mass spectrometer (MS, Hiden Analytical) equipped with capillary, quadruple mass analyzer (HAL 201-RC) and Faraday/Secondary electron multiplier (SEM). 3. Results and discussion 3.1. Catalyst Characterization Anatase was the only crystalline phase observed after calcination at 500 oC from the XRD diffraction pattern. No diffraction peak for the metal or oxide of Ni was observed, probably due to the low loading and high dispersion of Ni in the TiO2 structure. Peak broadening was observed in the main anatase peak with increased Ni loading, due to the decrease in crystallite size. Since the ionic radii of Ni2+ (0.69 Å) and Ti4+ (0.68 Å) ions [17] are similar, Ni can be easily incorporated into the TiO2 lattice. The band gap energy of TiO2 decreased upon the addition of Ni, causing a slight shift towards the visible light region. The 2wt% Ni-TiO2 sample had the lowest bandwidth of 2.72eV. The presence of Ni was found to modify the band gap energy of TiO2 towards visible light. The change in optical absorption can be attributed to the formation of defect centers generated from the substitution of Ti 4+ by Ni2+ ions. The X-ray photoelectron spectroscopy (XPS) spectra of the Ni-based samples shows two individual peaks at 854.1 eV and 859.0 eV which can be assigned to the Ni 2p spin orbit splitting into 2p 3/2 and 2p1/2 peaks. The peaks correspond to primary NiO species on the surface of TiO 2 [18]. 3.2. Photocatalytic reduction of CO2 Following 4 hours of light irradiation, the production of fuels such as methanol, ethanol and acetaldehyde was observed from the photocatalytic reduction of vapor-phase CO2 using water as a reductant under visible light (Figure 2). Methanol Ethanol Acetaldehyde
Product yield ( mol/gcath)
15 12 9 6 3 0
A
B
C
D
E
Fig. 2. Production rate over Ni based TiO2 nanoparticles (A TiO2, B 0.5wt% Ni-TiO2, C 1.5wt% Ni-TiO2 and E 2wt% Ni-TiO2) using the quartz plate photoreactor system
1wt% Ni-TiO2, D
The presence of Ni was found to increase the fuel production rate compared to TiO2. As shown in Figure 2, fuel production rate steadily increases with an increase in loading ratio to give an optimal ratio
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Oluwafunmilola Ola et al. / Energy Procedia 37 (2013) 6704 – 6709
of 1wt%Ni-TiO2 after which reduced product yield was observed for the subsequent loading ratios. The quantum efficiency of methanol (4.38%) and acetaldehyde (6.32%) production for 1wt%Ni-TiO2 catalyst was estimated based on the total photon energy absorbed by the catalyst during the reaction. The production rate and quantum efficiency was found to be higher than the reported value of 2.38% where a similar reactor configuration was used [16]. This improved conversion efficiency is due to the synergy obtained from the combined use of Ni, which possesses a band edge suitable for CO2 reduction to C1-C2 compounds, and the unique reactor configuration providing improved light distribution and contact efficiency for these photocatalysts.
4. Conclusions A series of Ni-based TiO2 photocatalysts were successfully synthesized by the sol-gel method and immobilized onto quartz plates. The experimental investigation conducted indicates that Ni loading plays a critical role in the photocatalytic activity of TiO2, due to Ni modifying the optical and electronic properties of TiO2 towards visible light. The improved conversion efficiency is probably ascribed to the presence of the metal, Ni, which served as electrons traps that suppressed recombination, resulting in effective charge separation and CO2 reduction under visible light. More importantly, the quantum efficiency was significantly improved indicating that light energy was effectively utilized in the quartz plate reactor, compared with similar reactor configurations reported in the literature. Acknowledgements The authors thank the financial support provided by the School of Engineering and Physical Sciences and the Centre for Innovation in Carbon Capture and Storage (EPRSC grant EP/F012098/1) at HeriotWatt University. References [1] Van der Geer J, Hanraads JAJ, Lupton RA. The art of writing a scientific article. J Sci Commun 2000;163:51 9. 1] Centi, G, Perathoner S. Opportunities and prospects in the chemical recycling of carbon dioxide to fuels. Catal Today 2009;148:191-205 [2] Roy SC, Varghese OK, Paulose M, Grimes CA. Toward solar fuels: photocatalytic conversion of carbon dioxide to hydrocarbons. ACS Nano 2010;4:1259-1278. [3] Koci K., Obalova L, Lacny Z. Photocatalytic reduction of CO2 over TiO2 based catalysts. Chem Pap 2008;62:1-9. [4] Usubharatana P, Mcmartin D, Veawab A, Tontiwachwuthikul P. Photocatalytic process for CO2 emission reduction from industrial flue gas streams. Ind Eng Chem Res 2006;45:2558-2568. [5] Mikkelsen M, Jorgensen M, Krebs FC. The teraton challenge: a review of fixation and transformation of carbon dioxide. Energ Environ Sci 2010;3:43-81. [6] Yano H, Shirai F, Nakayama M, Ogura K. Efficient electrochemical conversion of CO2 to CO, C2H4 and CH4 at a threephase interface on a Cu net electrode in acidic solution. J Electroanal Chem 2002;519:93-100.
Oluwafunmilola Ola et al. / Energy Procedia 37 (2013) 6704 – 6709
[7] Yano J, Morita T, Shimano K, Nagami Y, Yamasaki S. Selective ethylene formation by pulse-mode electrochemical reduction of carbon dioxide using copper and copper-oxide electrodes. J Solid State Electr 2007;11:554-557. [8] Aresta M. Carbon Dioxide as Chemical Feedstock. Weinheim: Wiley-VCH; 2000. [9] Olah GA, Goeppert A, Prakash GKS. Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons. J Org Chem 2009;74:487-498. [10] Creutz C, Fujita E. Carbon dioxide as a feedstock, Washington: National Academy Press; 2001, p. 83-92. [11] Wu JCS. Developments and innovation in carbon dioxide (CO2) capture and storage technology: carbon dioxide (CO2) storage and utilization, London: Taylor & Francis; 2010, p. 463-495. [12] Indrakanti VP, Kubicki JD, Schobert HH. Photoinduced activation of CO2 on Ti-based heterogeneous catalysts: Current state, chemical physics-based insights and outlook. Energ Environ Sci 2009;2:745-758. [13] Jiang Z, Xiao T, Kuznetsov V, Edwards P. Turning carbon dioxide into fuel, Philos T Roy Soc A 2010;368: 3343-3364. [14] Palmisano G, Augugliaro V, Pagliaro M, Palmisano L. Photocatalysis: a promising route for 21st century organic chemistry. Chem Commun 2007;33:3425-3437. [15] Ola O, Maroto-Valer M, Mackintosh S, Liu D, Lee C, Wu J. Performance comparison of CO2 conversion in slurry and monolith photoreactors using Pd and Rh-TiO2 catalyst under ultraviolet irradiation. Appl Catal B Environ 2012;126: 172-179. [16] Ola O, Maroto-Valer M. Solar fuel production using Nickel based nanoparticles. Prepr Pap Am Chem Soc Div Energy Fuels Chem 2012;57:238-240. [17] Ahrens LH, The use of ionization potentials. Part I: Ionic radii of elements. Geochim Cosmochim Ac 1952;2:155-169. [18] Moulder JF, Stickle W, Sobol E, Bomben D. Handbook of X-ray photoelectron spectroscopy a reference book of standard spectra for identification and interpretation of XPS data. In: Moulder JF, Chastain J, editors. Physical Electronics: Eden Prairie; 1995, p. 50-223.
6709
Energy Procedia 37 (2013) 6704 – 6709
GHGT-11
Turning CO2 into valuable chemicals Oluwafunmilola Olaa*, M. Mercedes Maroto-Valera, Sarah Mackintosha a
Centre for Innovation in Carbon Capture and Storage (CICCS), School of Engineering and Physical Sciences & Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh, EH14 4AS, United Kingdom.
Abstract The transformation of CO2 using semiconductor photocatalysts is a promising alternative for developing sustainable energy sources. Nickel (Ni) based TiO2 photocatalysts immobilized onto quartz plate was assembled in a unique reactor configuration to provide a high ratio of illuminated catalyst surface area for the reduction of CO2 to fuels under visible light. The improved conversion efficiency is ascribed to the presence of the metal, Ni, which served as electrons traps that suppressed recombination, resulting in effective charge separation and CO 2 reduction. More importantly, the quantum efficiency is higher than those reported in the literature that used similar reactor configurations. 2013The TheAuthors. Authors. Published Elsevier ©©2013 Published byby Elsevier Ltd.Ltd. Selection and/or peer-review under responsibility of GHGT Selection and/or peer-review under responsibility of GHGT Keywords: CO2 Photoreduction; Titanium dioxide; Photocatalysis; Photoreactor; Sol-gel; Nickel
1. Introduction As carbon dioxide capture and storage (CCS) is still in the demonstration phase, other complimentary alternatives have been developed for utilizing carbon dioxide (CO2) by converting it into chemicals and fuels [1]. Roy et al [2], Koci et al [3] and Usubharatana et al [4] pointed out alternatives; including photocatalysis, direct photolysis, and electrochemical reduction to utilize CO 2 as opposed to geological storage. The captured CO2 from CCS plants will serve as a relatively pure and inexpensive feedstock for these conversion methods to synthesize products such as methane, methanol, ethanol etc that can be used as chemicals, feedstock in fuel cells or hydrogen sources for electricity. Mikkelsen et al [5] have highlighted the difference between photocatalysis and electrochemical reduction as the source of electrons which is obtained from irradiating semiconductors under light in the * Corresponding author. Tel.: +44 131 451 3342 E-mail address: [email protected].
1876-6102 © 2013 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.06.603
Oluwafunmilola Ola et al. / Energy Procedia 37 (2013) 6704 – 6709
former and the application of an applied current in the latter. Although electrochemical cells can convert CO2, Yano et al. [6, 7] reported the deactivation of electrodes within a short time as a major drawback linked with its usage. This is due to the deposition of poisoning species i.e. adsorbed organic compounds on the electrode. Aresta [8] and Olah et al. [9] also reported that electrochemical reduction was energy intensive with slow kinetics. Creutz and Fujita [10] identified the need for an inexpensive hydrogen source in direct hydrogenation while Wu et al [11] reported high energy photons in direct photolysis as drawbacks to their utilization. Conversely, photocatalysis (an artificial form of photosynthesis) is more energy efficient compared to other reduction methods since it has the potential to utilize solar energy, a cheap and abundant non-fossil energy source for the conversion of CO2 into profitable products such as methanol, methane and other chemicals [12, 13]. Usubharatana et al. [4] and Palmisano et al [14] also noted the low energy input needed by photocatalysis as compared to other reduction methods that require higher energy consumption. Photocatalysis is, therefore, highly regarded for its low energy input, profitable products, and use of renewable energy. Amongst photocatalysts, titanium dioxide (TiO2) is particularly noteworthy due to its unique properties. However, its use is limited due to its large band gap; as it can only be activated by ultraviolet light which represents 2-5% of sunlight [15]. Since visible light accounts for 45% of the solar spectrum, there is a need to develop titania based photocatalysts which are active under the visible light spectrum. Attempts to improve the efficiency of this catalyst for solar fuel production is limited to the overall process efficiency being largely dependent on two factors - the reactor configuration and physicochemical properties of the catalyst. Accordingly, the loading of metals on the surface of titania can tailor its band width towards the visible light [15]. An investigation conducted by Ola et al [16] demonstrates that C1-C2 hydrocarbons can be produced from immobilizing nickel (Ni) loaded TiO2 nanoparticles onto quartz plates for CO2 reduction. However, the photocatalytic investigation and product selectivity of these materials has not been fully exploited. Accordingly, this work investigates the photocatalytic effect of increased Ni concentration on CO2 reduction and correlates the characterization with its performance for the generation of other C1-C2 compounds. 2. Experimental 2.1. Preparation of Ni loaded TiO2 nanoparticles The preparation of pure TiO2 and nickel loaded TiO2 nanoparticles used during the experimental phase was synthesized by the improved sol-gel method followed by drying and calcination, as described elsewhere [16]. Under controlled conditions, the appropriate amount of metal precursor was introduced within the hydrolysis and polycondensation phase depending on the weight percent loading calculated from the amount of precursors used in the synthesis. The calculated amount of nickel (II) nitrate hexahydrate (Ni (NO3)2.6H2O) was then dissolved in 14ml of acetic acid and stirred for 12 hours. Titanium butoxide was added to n-butanol with a volume ratio of 1:4. The subsequent solutions were stirred for 6 hours at room temperature. The homogeneous mixture was dried at 150oC for 3 hours and calcined at 500oC in a chamber furnace (Carbolite CWF 1100) to burn off organic compounds and complete the crystallite formation. The samples were then crushed into powder form to obtain both pure TiO2 and the metal loaded TiO2 nanoparticles with loading ratios within the range of 0.5wt% - 2.0wt%.
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Oluwafunmilola Ola et al. / Energy Procedia 37 (2013) 6704 – 6709
2.2. Characterization of metal loaded catalysts Ultraviolet-visible spectroscopy (UV-Vis, Varian Cary 300) was used for the measurement of the threshold wavelength, band gap and the absorbance of ultraviolet light as a function of the transmittance. The morphology and elemental composition of the nanoparticles were evaluated by transmission electron microscopy (TEM, JEOL 2100F), while analysis of the crystallographic structure of the catalyst was done by the X-ray diffraction (XRD) using the Hiltonbrooks X-ray powder diffractometer. The crystalline size in the catalysts was detected by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra) using a -ray source (1486.6eV) typically operated at 12kV anode potential. The main C mono1s peak = 285eV was used for charge correcting the high resolution scans. CASAXPS software with Kratos sensitivity factors was used in analyzing the elements present. 2.3. Photocatalytic reduction of CO2 The quartz plate photoreactor with dimensions of 5.5cm X 11cm (diameter and height), with ports for sampling the gaseous by-products, measuring the pressure and gas (CO2) inlet (Figure 1) was used for CO2 reduction. The average light intensity measured in front of the quartz window using the radiometer (Omnicure R2000) was 68.35mW/cm2 for the visible light source (500W Halogen floodlight lamp).
Fig. 1. Schematic of the quartz plate photoreactor
Approximately 0.2g of catalyst was evenly dispersed on the quartz plate with dimensions of 4.0cm X 9.7cm to provide a high ratio of illuminated surface area of catalyst in the reactor. The coated quartz plate was inclined at an angle of 15o facing the light rays emitted through the quartz window to maximize light utilization efficiency. The photoreactor was covered with a gloved box to exclude any external light source and maximize light energy within the reactor. Leak test was performed by purging the photoreactor with helium. If leak-proof, the reactor was then purged with this gas for 1 hour, then the gas flow was changed to ultra pure CO 2 gas saturated with water vapor for 1 hour. After 1 hour, the light source was switched on and the curtains of the gloved box shut to enable photoreduction take place. Readings were taken after 4 hours with extracted gas samples injected
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Oluwafunmilola Ola et al. / Energy Procedia 37 (2013) 6704 – 6709
into the mass spectrometer (MS, Hiden Analytical) equipped with capillary, quadruple mass analyzer (HAL 201-RC) and Faraday/Secondary electron multiplier (SEM). 3. Results and discussion 3.1. Catalyst Characterization Anatase was the only crystalline phase observed after calcination at 500 oC from the XRD diffraction pattern. No diffraction peak for the metal or oxide of Ni was observed, probably due to the low loading and high dispersion of Ni in the TiO2 structure. Peak broadening was observed in the main anatase peak with increased Ni loading, due to the decrease in crystallite size. Since the ionic radii of Ni2+ (0.69 Å) and Ti4+ (0.68 Å) ions [17] are similar, Ni can be easily incorporated into the TiO2 lattice. The band gap energy of TiO2 decreased upon the addition of Ni, causing a slight shift towards the visible light region. The 2wt% Ni-TiO2 sample had the lowest bandwidth of 2.72eV. The presence of Ni was found to modify the band gap energy of TiO2 towards visible light. The change in optical absorption can be attributed to the formation of defect centers generated from the substitution of Ti 4+ by Ni2+ ions. The X-ray photoelectron spectroscopy (XPS) spectra of the Ni-based samples shows two individual peaks at 854.1 eV and 859.0 eV which can be assigned to the Ni 2p spin orbit splitting into 2p 3/2 and 2p1/2 peaks. The peaks correspond to primary NiO species on the surface of TiO 2 [18]. 3.2. Photocatalytic reduction of CO2 Following 4 hours of light irradiation, the production of fuels such as methanol, ethanol and acetaldehyde was observed from the photocatalytic reduction of vapor-phase CO2 using water as a reductant under visible light (Figure 2). Methanol Ethanol Acetaldehyde
Product yield ( mol/gcath)
15 12 9 6 3 0
A
B
C
D
E
Fig. 2. Production rate over Ni based TiO2 nanoparticles (A TiO2, B 0.5wt% Ni-TiO2, C 1.5wt% Ni-TiO2 and E 2wt% Ni-TiO2) using the quartz plate photoreactor system
1wt% Ni-TiO2, D
The presence of Ni was found to increase the fuel production rate compared to TiO2. As shown in Figure 2, fuel production rate steadily increases with an increase in loading ratio to give an optimal ratio
6708
Oluwafunmilola Ola et al. / Energy Procedia 37 (2013) 6704 – 6709
of 1wt%Ni-TiO2 after which reduced product yield was observed for the subsequent loading ratios. The quantum efficiency of methanol (4.38%) and acetaldehyde (6.32%) production for 1wt%Ni-TiO2 catalyst was estimated based on the total photon energy absorbed by the catalyst during the reaction. The production rate and quantum efficiency was found to be higher than the reported value of 2.38% where a similar reactor configuration was used [16]. This improved conversion efficiency is due to the synergy obtained from the combined use of Ni, which possesses a band edge suitable for CO2 reduction to C1-C2 compounds, and the unique reactor configuration providing improved light distribution and contact efficiency for these photocatalysts.
4. Conclusions A series of Ni-based TiO2 photocatalysts were successfully synthesized by the sol-gel method and immobilized onto quartz plates. The experimental investigation conducted indicates that Ni loading plays a critical role in the photocatalytic activity of TiO2, due to Ni modifying the optical and electronic properties of TiO2 towards visible light. The improved conversion efficiency is probably ascribed to the presence of the metal, Ni, which served as electrons traps that suppressed recombination, resulting in effective charge separation and CO2 reduction under visible light. More importantly, the quantum efficiency was significantly improved indicating that light energy was effectively utilized in the quartz plate reactor, compared with similar reactor configurations reported in the literature. Acknowledgements The authors thank the financial support provided by the School of Engineering and Physical Sciences and the Centre for Innovation in Carbon Capture and Storage (EPRSC grant EP/F012098/1) at HeriotWatt University. References [1] Van der Geer J, Hanraads JAJ, Lupton RA. The art of writing a scientific article. J Sci Commun 2000;163:51 9. 1] Centi, G, Perathoner S. Opportunities and prospects in the chemical recycling of carbon dioxide to fuels. Catal Today 2009;148:191-205 [2] Roy SC, Varghese OK, Paulose M, Grimes CA. Toward solar fuels: photocatalytic conversion of carbon dioxide to hydrocarbons. ACS Nano 2010;4:1259-1278. [3] Koci K., Obalova L, Lacny Z. Photocatalytic reduction of CO2 over TiO2 based catalysts. Chem Pap 2008;62:1-9. [4] Usubharatana P, Mcmartin D, Veawab A, Tontiwachwuthikul P. Photocatalytic process for CO2 emission reduction from industrial flue gas streams. Ind Eng Chem Res 2006;45:2558-2568. [5] Mikkelsen M, Jorgensen M, Krebs FC. The teraton challenge: a review of fixation and transformation of carbon dioxide. Energ Environ Sci 2010;3:43-81. [6] Yano H, Shirai F, Nakayama M, Ogura K. Efficient electrochemical conversion of CO2 to CO, C2H4 and CH4 at a threephase interface on a Cu net electrode in acidic solution. J Electroanal Chem 2002;519:93-100.
Oluwafunmilola Ola et al. / Energy Procedia 37 (2013) 6704 – 6709
[7] Yano J, Morita T, Shimano K, Nagami Y, Yamasaki S. Selective ethylene formation by pulse-mode electrochemical reduction of carbon dioxide using copper and copper-oxide electrodes. J Solid State Electr 2007;11:554-557. [8] Aresta M. Carbon Dioxide as Chemical Feedstock. Weinheim: Wiley-VCH; 2000. [9] Olah GA, Goeppert A, Prakash GKS. Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons. J Org Chem 2009;74:487-498. [10] Creutz C, Fujita E. Carbon dioxide as a feedstock, Washington: National Academy Press; 2001, p. 83-92. [11] Wu JCS. Developments and innovation in carbon dioxide (CO2) capture and storage technology: carbon dioxide (CO2) storage and utilization, London: Taylor & Francis; 2010, p. 463-495. [12] Indrakanti VP, Kubicki JD, Schobert HH. Photoinduced activation of CO2 on Ti-based heterogeneous catalysts: Current state, chemical physics-based insights and outlook. Energ Environ Sci 2009;2:745-758. [13] Jiang Z, Xiao T, Kuznetsov V, Edwards P. Turning carbon dioxide into fuel, Philos T Roy Soc A 2010;368: 3343-3364. [14] Palmisano G, Augugliaro V, Pagliaro M, Palmisano L. Photocatalysis: a promising route for 21st century organic chemistry. Chem Commun 2007;33:3425-3437. [15] Ola O, Maroto-Valer M, Mackintosh S, Liu D, Lee C, Wu J. Performance comparison of CO2 conversion in slurry and monolith photoreactors using Pd and Rh-TiO2 catalyst under ultraviolet irradiation. Appl Catal B Environ 2012;126: 172-179. [16] Ola O, Maroto-Valer M. Solar fuel production using Nickel based nanoparticles. Prepr Pap Am Chem Soc Div Energy Fuels Chem 2012;57:238-240. [17] Ahrens LH, The use of ionization potentials. Part I: Ionic radii of elements. Geochim Cosmochim Ac 1952;2:155-169. [18] Moulder JF, Stickle W, Sobol E, Bomben D. Handbook of X-ray photoelectron spectroscopy a reference book of standard spectra for identification and interpretation of XPS data. In: Moulder JF, Chastain J, editors. Physical Electronics: Eden Prairie; 1995, p. 50-223.
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