carbon nanotube solar-energy-material: Artificial photosynthesis

Journal of CO2 Utilization 18 (2017) 89–97

Contents lists available at ScienceDirect

Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou

Photocatalytic back-conversion of CO2 into oxygenate fuels using an efficient ZnO/CuO/carbon nanotube solar-energy-material: Artificial photosynthesis Mohsen Lashgaria,b,* , Sanaz Soodia , Parisa Zeinalkhania a b

Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran Center for Research in Climate Change and Global Warming: Hydrogen and solar division, Zanjan 45137-66731, Iran

A R T I C L E I N F O

Article history: Received 6 July 2016 Received in revised form 1 January 2017 Accepted 23 January 2017 Available online xxx Keywords: CO2 back-conversion Artificial photosynthesis Carbon-based solar-fuels Nanocomposite photocatalysts Photo water splitting and hydrogenation phenomena

A B S T R A C T

Fuel shortage, energy crisis and boundless dumping of greenhouse gases into the atmosphere, are some challenging issues of human societies. To overcome these energy-related/environmental problems, solar conversion of CO2 to carbon-based fuels is a promising route, which is achievable through the atomistic hydrogenation of CO2 molecules inside semiconductor-assisted water-photosplitting reactors. In this paper, using a facile hydrothermal method, an eco-friendly, low-price, nanocomposite solar-energymaterial was synthesized in the absence and presence of carbon nanotube (CNT), and applied in aqueous media for the photochemical synthesis of ethanol, oxalic acid and formaldehyde. The enhancing power of CNT on the photocatalyst performance was explained in detail in terms of its ability to harvest more incident photons, temporarily store H atoms, increase the surface area and improve the charge separation phenomenon. Based on atomistic hydrogenation hypothesis, a reaction scheme was finally proposed for each photochemically synthesized product. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction A broad spectrum of C/H/O based fuels/food including alcohols, aldehydes, carboxylic acids, carbohydrates, hydrocarbons, and . . . , can be represented by this general formula: ClHmOn (l,m, n = 0, 1, 2, . . . ). The energy stored in these materials is released through the oxidation/respiration processes inside biological cells or abiotic reactors, and thereby the energy of bio-globe (earth and its living organisms) becomes chemically supplied [1]. The byproducts of this oxidation reaction (Eq. (1)) are carbon dioxide (CO2) and water (H2O) molecules, which are normally emitted to the environment:  m n m C l Hm On þ l þ  O2 ! lCO2 þ H2 O þ energy ð1Þ 4 2 2 The reverse reaction [2,3], i.e. the conversion of CO2 to oxygenate fuels/food occurs routinely in the Nature’s bioreactors–the green plants and photosynthetic bacteria, and the energy of this CO2-fixation process is supplied by sunlight [4]. By doing this

* Corresponding author at: Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran. E-mail address: [email protected] (M. Lashgari). http://dx.doi.org/10.1016/j.jcou.2017.01.017 2212-9820/© 2017 Elsevier Ltd. All rights reserved.

back-conversion process, known as photosynthesis, the carbon cycle is completed and CO2 is naturally converted to its origins [5]. Concerning this cycle, it is obvious that the level of CO2 in the atmosphere is dictated by the balance between the rates of CO2 production and its consumption [1]. Therefore, the boundless dumping of CO2 gas into the atmosphere (resulting by the burning of carbon-based fuels; see Eq. (1)) alters the balance and increases CO2 amount in the environment; this fact is considered as the main cause of global warming and climate change phenomena [6,7]. To combat these challenging issues, a straightforward route is to reduce the carbon emission or accelerate the back-conversion process (CO2 transformation into oxygenate fuels). Since CO2 is a stable molecule [8,9], its conversion to oxygenate fuels is energy-consuming and needs proper catalyst materials. Industrially, CO2 conversion to alcohols is carried out under high temperature and pressure conditions through a reduction reaction with H2 molecules upon Cu/Zn based catalysts [9–11]. During this industrial process, hydrogen atoms are transiently generated from the dissociative adsorption of H2 molecules on the catalyst surface [12,13]. These atoms are very reactive and could attach to CO2 molecules and reduce (hydrogenate) them to oxygenate products. Instead of supplying H2 molecules and carrying out their dissociative adsorption on the catalyst surface, H atoms could be directly generated via water photosplitting [14,15], through proton

90

M. Lashgari et al. / Journal of CO2 Utilization 18 (2017) 89–97

photoreduction reaction upon appropriate semiconductor materials (refer to Section 3.4, Eq. (7)). During this process, atomistic hydrogenation of CO2 molecules could take place and oxygenate compounds become synthesized [16,11]. In this work, we have attempted to do artificial photosynthesis in aqueous media, and convert CO2 molecules to oxygenate (C/H/O based) fuels. The photocatalysts under study are ZnO/CuO (abbreviated as ZOC) and its composite with carbon nanotube (ZOC-CNT). We utilized CuO, because it is a low-cost, narrowbandgap (1.7 eV) p-type semiconductor [17]. This material is widely applied as the main component of industrial catalysts for alcohols synthesis using CO2 and H2 gaseous feed [10,11]. Similarly, the other component, i.e. ZnO is inexpensive and usually utilized as the catalyst support for the CO2 hydrogenation process [10,18,13]. This material is able to adsorb CO2 molecules and enhance the catalyst performance [19]. Like TiO2 [20–23], ZnO is an n-type wide-bandgap semiconductor (3.2 eV) with a good photo-stability [24], but in contrast, it has a higher electron mobility [25]. Since the resulting composite, i.e. ZOC is a p-n junction, an improved charge separation [26–29] and hence a high photocatalytic performance is expected for this binary composite. The remaining component, i.e. CNT is a black conductive material with a strong absorption in the whole spectral region [30,31]. This nanostructured component is also able to facilitate the charge mobility, serve as H-atom reservoir and increase the surface area of the resulting nanocomposite material [32–34]. Therefore, a superior performance is expected in the presence of CNT for the photocatalytic hydrogenation of CO2 to oxygenate fuels. Concerning the effect of CNT or CuO amount on the photocatalyst performance, it is worth noting that since both components are photoactive, their presence in the photocatalyst material could enhance the absorption of incident light. Regarding this fact, it should be however emphasized that extra addition of CNT, is not recommended, because it causes a shadow and prevents photons from reaching to the semiconductor surface [35,36]. In respect of CuO, moreover, we should remark that by increasing the quantity of this solar-energy-material, although the absorption of photons is boosted, it does not necessarily result in a better photocatalytic activity. This is because the ability of the solar-energy-material to absorb more photons is not the sole determining factor; the role of photocatalyst support (ZnO), as mentioned previously, should also be taken into account [by increasing the ratio of CuO to ZnO, since the quantity of ZnO decreases in the composite material, its interaction with the reactant species is reduced and the less photocatalytic activity is therefore rationalized]. Finally, based on physicochemical concepts of the water photosplitting, the formation of transient H-atoms and the hydrogenation of CO2 molecules on the photocatalyst surface, some reaction pathways will be proposed for the observed photosynthesized products. 2. Experimental 2.1. Synthesis of nanocomposite photocatalysts The binary nanocomposite photocatalyst (zinc oxide/copper oxide; ZOC) was hydrothermally synthesized according to the literature [37]. To prepare the ternary photocatalyst (ZOC-CNT), we also used a similar procedure with some modifications. Here, 0.014 g of multiwall CNT functionalized with carboxylic groups (d  8 nm, ‘  30 mm; Neutrino Company) was ultrasonically dispersed in 20 ml de-ionized water. Then, 50 ml aqueous solution containing 0.235 g Cu(CH3COO)2H2O (Fluka; 99%) and 0.700 g Zn (NO3)26H2O (Merck; 99%) was prepared and added to the original mixture. The pH of medium was adjusted to about 11 by adding

0.5 M sodium hydroxide solution. The obtained mixture was thereafter poured into a handmade stainless steel (SS 316) autoclave with an internal reaction vessel made of polytetrafluoroethylene (PTFE), and was heated for 24 h at 453 K. After decanting, the precipitate was washed several times with ethanol and distilled water, and finally dried for 12 h at 333 K. The photocatalyst composites synthesized here, i.e. ZOC and ZOC-CNT were gray and dark gray powders, respectively. 2.2. Characterization X-ray diffraction (XRD) pattern of the nanocomposite photocatalysts was determined using a Philips X’Pert Pro Xray powder diffractometer (l = 1.54 Å; Cu Ka beam). The optical absorbance spectra of the photocatalysts were measured in the diffuse reflectance mode through a Varian Cary 5 UV–vis-NIR spectrometer (BaSO4 was used as the blank). To record photoluminescence (PL) spectra of the photocatalyst materials, a Varian Cary Eclipse Fluorescence Spectrophotometer was utilized (lex = 335 nm). N2 adsorption-desorption (BET isotherm) and porosimetry experiments of the photocatalyst powders were conducted at 77 K on a BELSORB-max (BEL, Japan) instrument. Raman spectra were recorded at room temperature using an upright Teksan Raman microscope/spectrometer (APUS model), equipped with a DPSS Nd: YAG laser source (lex = 532 nm) and a CCD Array detector. The field emission scanning electron microscopy (FE-SEM) images of the nanocomposite powders were taken by a ZEISS SIGMA VP electron microscope. Transmission electron microscopy (TEM) images were also observed by using a ZEISS EM10C 100 kV electron microscope. X-ray photoelectron spectra (XPS) were recorded via an 8025BesTec (Germany) spectrometer with monochromatic Al Ka radiation source (1486.6 eV). 2.3. Photo-reactor and catalytic conversion of CO2 to fuel The artificial photosynthesis experiments were carried out in a homemade photoreactor–consisting of a 500-W power-tunable Xe-lamp (Ushio Xenon Short Arc Lamp) and a double-walled cylindrical glass vessel (capacity: 50 ml, irradiated surface area: 25 cm2) equipped with a temperature-controlling bath circulator (WCR-P6). We applied xenon light source, because its spectrum is close to that of Sun. The intensity of light was also set at 100 mW/ cm2, similar to that coming directly from the sun to the earth’s surface under non-cloudy (ideal) conditions [4]. The reaction mixture was 50 ml distilled water containing 0.05 g suspended photocatalyst material [38] and dissolved CO2 gas. To increase the solubility of gas as well as the photocatalyst performance, using a sodium bicarbonate solution, the pH of medium was kept at 8.5 [39]. During the test, the added photocatalyst was continuously dispersed using a magnetic stirrer, and a pure CO2 gas (99.9%) was bubbled through the reaction medium. The products were finally analyzed by the procedures described below, after 2 h operation of the photoreactor. The experiments were repeated at least three times and the mean values were reported as final data; the error bars were also plotted using the standard deviation of these independent measurements [40]. 2.4. Product analysis The reaction products in aqueous medium were analyzed using different chromatography and spectroscopy methods. A Knauer HPLC system equipped with Eurokat H column (300  8 mm, 10 mm) and a four channel K2600 UV–vis. detector was applied in the analysis of carboxylic acid products (Eluent: 0.05 M H2SO4,

M. Lashgari et al. / Journal of CO2 Utilization 18 (2017) 89–97

Temp: 75  C, Flow: 0.6 ml/min). For the case of alcohols, we used an Aligent gas chromatograph (6890, Propack Q, FID, Carrier gas: N2, Column Temp: 160  C, Flow: 20 ml/min). Formaldehyde was spectrofluorimetrically measured through Nash procedure [41], with a Varian Cary Eclipse fluorescence spectrometer at 411 nm excitation and 507 nm emission wavelengths. 3. Results and discussion 3.1. Synthesis confirmation (XRD, XPS and Raman spectra) XRD patterns of the binary (ZOC) and ternary (ZOC-CNT) nanocomposite photocatalysts synthesized here are depicted in Fig. 1. This figure shows that the XRD patterns of both photocatalyst materials are very close together. In the case of ZOC-CNT, the observation of a broad low-intensity peak around 20 to 30  (see the inset) is due to the presence of CNT component. By comparing the XRD pattern of ZOC with those of CuO (JCPDS card no. 80-0075) and ZnO (JCPDS card no. 80-1917) components, the formation of ZOC (as well as ZOC-CNT) is approved. The synthesis of photocatalyst was also approved by XPS analysis (Fig. 2). The spectra obtained here are similar to those reported by Wang et al., for ZnOCuO nanoparticles deposited on a reduced graphene oxide substrate [37]. Therefore, the presence of Cu2+, Zn2+ and O2 ions in the nanocomposite material and hence the synthesis of photocatalyst are likewise verified [for a detailed explanation, see Ref. [37]]. The presence of CNT as well as ZnO and CuO components in the composite solar-energy-materials was also confirmed by Raman spectra; see Fig. 3. Here, the peaks around 1580 and 1350 cm1, observed for the ternary composite material, are due to G-band and D-band phonon modes of CNT, respectively [42]. Moreover, the peaks near 1100 and 300 cm1 are respectively ascribed to 2LO mode of ZnO [37] and A1g mode of CuO [43], which approve the synthesis of these compounds in both solar-energy-materials. The remaining (relatively strong) peaks around 544 and 410 cm1 correspond to E2 and E1 modes of ZnO, respectively [44]. Finally, we should declare that the attenuation of Raman signals recognized in the spectrum of CNT-containing composite, has also been witnessed elsewhere [37], for ZnO-CuO nanoparticles in the presence of graphene oxide nanosheets.

91

3.2. Optical response and BET adsorption Diffuse reflectance spectra of the photocatalyst materials under investigation are plotted in Fig. 4. This figure indicates that both binary and ternary photocatalysts are able to absorb the incident photons throughout the spectral region. In these spectra, the absorption in the visible region is related to the presence of CuO (Eg = 1.7 eV [17]), whereas the UV portion is due to ZnO component (Eg = 3.2 eV [24]). Moreover, Fig. 4 reveals that CNT has an enhancing effect on the photon absorption. Concerning the absorption of photons by the photocatalyst materials, it is worth noting that during the photoexcitation process, excitons (bound electron-hole pairs) are transiently generated [45,46]. Since these photogenerated quasi-particles are unstable, they may radiatively annihilate and release their energy as a PL emission [46,47]. Alternatively, these photogenerated entities (e/h+) can be utilized in a redox process itheir constituent charges become effectively separated [48]. Under this condition, by impeding the annihilation of excitons, the intensity of PL-emission decreases and the photocatalyst performance increases correspondingly [49]. Fig. 5 confirms this expectation and demonstrates a PL quench in the presence of CNT. Here, CNT can act as a good electronic conductor and facilitate the charge transport/separation phenomenon, which in turn results in a decrease in the PL intensity [49]. This alternatively means that ZOC CNT is able to utilize a greater portion of the energy of absorbed photons. Therefore, our expectation is that this photocatalyst could exhibit a superior activity to effectively harvest the energy of incident photons and produce greater quantity of oxygenate fuels [this claim will be confirmed later; see Table 2 and Fig. 9]. The adsorption-desorption isotherms of the binary and ternary photocatalysts are plotted in Fig. 6 and the results are summarized in Table 1. This table indicates that in the presence of CNT, the photocatalyst surface area increases significantly and more reactant molecules could be hence adsorbed or trapped in the nanopores of ZOC-CNT photocatalyst. This alternatively means that for the ternary photocatalyst material, a greater number of reaction centers (adsorbing sites) are available and thus its catalytic activity should be high.

Fig. 1. X-ray diffraction (XRD) pattern of the binary nanocomposite in the absence and presence of carbon nanotube (the inset is a vertically expanded version of XRD pattern re-plotted for ZOC-CNT in the range of 20–30  ). The filled and unfilled circles correspond to the peaks of zinc oxide and copper oxide components, respectively.

92

M. Lashgari et al. / Journal of CO2 Utilization 18 (2017) 89–97

Fig. 2. XPS diagrams of the nanocomposite photocatalyst (with CNT): (a) survey scan, (b–e) are detailed spectra, recorded at C 1s, O 1s, Cu 2p and Zn 2p spectral regions, respectively.

3.3. SEM, TEM and the photocatalysts morphology The morphology of nanocomposite photocatalysts synthesized here is shown in Fig. 7. This figure illustrates that the binary nanocomposite material, i.e. ZOC consists of relatively uniform nanoparticles. In the case of ternary nanocomposite, moreover, CNTs are distributed somewhat uniformly throughout the photocatalyst material (see Fig. 7b). As we have already seen through BET and PL studies, this conductive network of CNTs (Fig. 8) could serve as a proper medium to adsorb/trap more reactant species and facilitate the separation of e/h pairs, through the formation of a Schottky barrier between CNT conductors and semiconductor nanoparticles [32]. These evidences justify why a greater performance is expected for the ternary nanocomposite (ZOC-CNT) as compared with that of binary one (without CNT).

3.4. Product analysis, energy efficiency and mechanistic description of the phenomenon As mentioned in the Introduction, during the combustion process, every oxygenate material is oxidized to CO2 and H2O (see Eq. (1)). By doing the reverse reaction and applying an appropriate energy source, CO2 molecules could revert to their origins and hence a mixture of compounds is synthesized [50]. Therefore, the product analysis of these systems is often rather complicated. To bypass this difficulty, a straightforward strategy is to make a limitation on the product diversity [51]. This can be achieved by proper selection of the photocatalyst components [52–54,50], and noticing the physicochemical phenomena which occur during the photosynthesis process. In the present work, the reaction between CO2 and H2O molecules was carried out in the aqueous solution, and the product

M. Lashgari et al. / Journal of CO2 Utilization 18 (2017) 89–97

93

Fig. 3. Raman spectra of the solar-energy-material in the presence (gray) and absence (red) of CNT; data are depicted in two Raman shift domains: (a) 200–1250 cm1 and (b) 1250–1750 cm1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Diffuse reflectance spectra of the nanocomposite photocatalysts under study.

40

ZOC

emission intensity

ZOC-CNT 30

20

10

0

350

370

390

410

430

450

wavelength (nm) Fig. 5. Photoluminescence (PL) spectra of the photocatalyst materials under investigation.

analysis was performed in the same medium. The results are depicted in Fig. 9, indicating that the yield of products is substantially increased by adding CNT to ZOC photocatalyst. Moreover, in the presence of ZOC-CNT, ethanol is photosynthesized

as a new by-product. The production of ethanol is obviously resulting from the presence of CNT in the photocatalyst material. This product seems to be the characteristic of CNT-containing photocatalysts, which has been also observed elsewhere for a titania-based CNT nanocomposite [55]. As mentioned previously,

94

M. Lashgari et al. / Journal of CO2 Utilization 18 (2017) 89–97

Table 1 BET data of the nanocomposite photocatalyst materials under study. Photocatalysts

Sa [m2 gr1]

db [nm]

Vp c [cm3 gr1]

Vm d [cm3 (STP) gr1]

ZOC ZOC-CNT

29.82 96.63

14.07 5.89

0.10 0.14

6.85 22.20

a

Surface area. Mean pore diameter. c Total pore volume. d Total volume of monolayer gas, adsorbed at the standard temperature and pressure (STP) condition. b

80

(a)

V /[cm3 (STP) g-1]

60 40 20 0 0

0.2

0.4

0.6

0.8

1

p/p0

100

(b)

V [cm3 (STP) g-1]

80 60 Fig. 7. FE-SEM image of the nanocomposite photocatalysts applied for the conversion of CO2 to fuel: (a) ZOC and (b) ZOC-CNT.

40

To determine the percent of photonic energy which is harvested by the photocatalyst material for the photosynthesis of a particular product, we utilized the following energy efficiency formula [56]:

20 0 0

0.2

0.4

p/p0

0.6

0.8

1

Fig. 6. Nitrogen adsorption-desorption isotherms (BET diagrams) of the nanocomposite photocatalyst in the absence (a) and presence (b) of carbon nanotube (the filled and unfilled circles are standing for adsorption and desorption data, respectively).

the superiority of ZOC-CNT is justifiable in terms of the ability of CNT to (a) increase the surface area and trap more reactant species in the photocatalyst nanopores (see Table 1), (b) store hydrogen atoms [33,34] and serve as H source for the reaction progress, (c) enhance the photon absorption and (d) facilitate the charge separation via formation of a pseudo-Schottky junction between ZOC and CNT constituents [32]. Table 2 Energy efficiency (%) data obtained for the oxygenate fuels under study. Photocatalyst

Formaldehyde

Ethanol

Oxalic acid

ZOC ZOC-CNT

0.03 0.10

– 10.19

0.43 2.22

eð%Þ ¼

Q n

f

 100

ð2Þ

Where Q is the heat of combustion (being calculated using the enthalpy data [57]), n the mole of photosynthesized product and f. the energy of incident light (equal to intensity  area). The energy efficiency data were calculated for each oxygenate product; see Table 2. Here, the maximum energy performance was expectedly obtained for ethanol photosynthesis upon ZOC-CNT photocatalyst. We also observed that in the presence of CNT, the photoconversion efficiency (e %) is boosted about 3 and 4 times for formaldehyde and oxalic acid, respectively. Concerning the photoconversion of CO2 to oxygenate compounds, we should declare that under lht irradiation, e/h pairs are photogenerated at the conduction (CB) and valence (VB) bands of the semiconductor (SC) photocatalyst, respectively (Eq. (3)). In the absence of hole-scavenger additives (the case of this study, we did not use any external hole-scavenger), water molecules are oxidized þ by the photogenerated holes (hVB ) and hence protons are produced at the photocatalyst surface [14]: þ

hn þ SCˆ hVB þ e CB

ð3Þ

M. Lashgari et al. / Journal of CO2 Utilization 18 (2017) 89–97

95

þ HCO 3 þ H ! CO2 þ H2 O

ð6Þ

Corresponding to the water oxidation reaction (Eq. (4)), to satisfy the charge neutrality (e + h+ = 0), the photogenerated electrons must also be consumed via a reductive process. In aqueous media, these electrons are usually used up through the proton reduction reaction (Eq. (7)) and hence hydrogen atoms (radicals) become transiently generated at the photocatalyst surface [15]. Alternatively, these photogenerated electrons could be utilized by CO2 molecules [15,8], which are already produced (Eq. (6)) on the photocatalyst surface. The result of this electrontransport phenomenon (Eq. (8)) is the formation of a bent  CO 2 radical-anion attached to the catalyst surface via C atom [59,60]. þ e CB þ H ! H



ð7Þ

  e CB þ CO2 ! CO2

ð8Þ

The hydrogen atoms (radicals)eg generated during the proton photoreduction reaction (Eq. (7)), are highly reactive;hey can attach to CO2 molecules (Eq. (9)) and produce carboxyl (CO2H) radicals on the photocatalyst surface [16,58–62]. Alternatively, these radicals could be produced through the reaction between the + already photogenerated  CO 2 . and H species (Eqs. (4) and (8)) on the photocatalyst surface [8]; see Eq. (10): 

CO2 þ H !  CO2 H



Fig. 8. TEM images of the CNT-containing nanocomposite taken at different zones: a) CNT network with photocatalyst nanoparticles, b) CNT fibers, and c) their junction with the nanoparticles.

þ

2hVB þ H2 O ! 2Hþ þ 1=2O2

ð4Þ

With the generation of prot pH decreases at the photocatalyst/ solution interface. Thus, according to Eq. (5) through (6), carbonate anions (the stable form of CO2 in basic media; the case of present work) are locally transformed to carbon dioxide [58]:  Hþ þ CO2 3 ! HCO3

ð5Þ

ð9Þ

þ  CO 2 þ H ! CO2 H

ð10Þ

By the photocatalytic transformation of CO2 into carboxyl radical (CO2H), it could undergo further radical reactions, and the synthesis of oxalic acid, formaldehyde (methanediol) and ethanol becomes rationalized as follows. 3.4.1. Oxalic acid This is the predominant product (see Fig. 9), which could be easily synthesized through the recombination (dimerization) of carboxyl radicals at the initial stages of the process: 2ð CO2 HÞ ! HO2 C  CO2 H

ð11Þ

3.4.2. Formaldehyde/methandiol Besides the dimerization (Eq. (11)), carboxyl radicals can undergo further hydrogenation processes by the reaction with the hydrogen atoms (Ha) – which are transiently photogenerated

H

C

OH

O +Ha

C

H

O

O

H

H +2Ha H

H H

Fig. 9. Oxygenate fuels photo-produced after 2 h operation of the photoreactor (50 mg photocatalyst in 50 ml water).

H

+

O

H O

C

O

C

O

H

Scheme 1. A possible reaction pathway for the formation of formaldehyde/ methanediol, sketched based on consecutive hydrogenation of the surface-bound  COOH radicals with the transiently photogenerated hydrogen atoms (Ha) at the photocatalyst surface.

96

M. Lashgari et al. / Journal of CO2 Utilization 18 (2017) 89–97

H

C

Acknowledgements

H H

O

+Ha

H C

The authors would like to acknowledge the research council of IASBS for financial support. A special thank is also due to Forensic Medicine Organization of Zanjan for providing GC facilities. We should finally acknowledge the anonymous referees for their useful comments to enhance the quality of work.

OH

+Ha -H2O

H

H C

+Ha

Appendix A. Supplementary data

H

H C

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcou.2017.01.017.

H

References

Scheme 2. Formaldehyde conversion to hydroxymethyl and methyl radicals: a consecutive hydrogenation pathway.

(Eq. (7)) and are abundantly available on the photocatalyst surface; see Scheme 1: According to the scheme presented here, the synthesis of formaldehyde becomes possible through the breaking of C O bond or more likely, it could be synthesized indirectly via the formation of methanediol [8,62], which is in equilibrium with formaldehyde in aqueous media [63]. The photocatalytic synthesis of formaldehyde has also been reported by others, and seems to be the intrinsic ability of ZnO-containing photocatalysts [64,61]. 3.4.3. Ethanol In comparison to oxalic acid and formaldehyde, the reaction pathway for the synthesis of ethanol is more complex, and the generation of hydroxymethyl (CH2OH) and methyl (CH3) radicals seems to be essential [65]. The electron paramagnetic resonance (EPR) investigations which have been carried out elsewhere confirm the generation of these radical species during the CO2 photoconversion processes [66]. Moreover, according to other udies [61,62], it is conceived that hydroxymethyl and methyl radicals could be produced through a hydrogenation pathway using the formaldehyde precursor (Scheme 2). By the formation of hydroxymethyl and methyl radicals, they could simply recombine and produce ethanol: 

CH2 OH þ  CH3 ! CH3 CH2 OH

ð11Þ

4. Conclusion Using a facile hydrothermal procedure, two nanocomposite zinc oxide/copper oxide (ZOC) solar-energy-materials were synthesized in the absence and presence of carbon nanotube (CNT) and applied as an efficient photocatalyst for the photochemical conversion of CO2 molecules to ethanol, oxalic acid and formaldehyde. In the presence of CNT not only the surface area increases but more molecules may be trapped in the pores of the photocatalyst material. In this system, the charge transport phenomenon could also be enhanced by the formation of a pseudo-Schottky junction between CNT and semiconductor nanoparticles. The ability of CNT to sorb H-atoms is another interesting matter, justifying the superiority of the CNT-containing photocatalyst to temporarily store and deliver the photogenerated H atoms for use in the subsequent hydrogenation processes. For each photosynthetic product, a reaction scheme was proposed on the basis of physicochemical phenomena occurring during the water photosplitting process in the presence of dissolved CO2 molecules.

[1] M. Lashgari, Use of solar and alternative energy to reduce emissions, U.S.-Iran Symposium on Climate Change: Impacts and Mitigation, Irvine, California, 2015. [2] V. Singh, I.J.C. Beltran, J.C. Ribot, P. Nagpal, Photocatalysis deconstructed: design of a new selective catalyst for artificial photosynthesis, Nano Lett. 14 (2014) 597–603. [3] S.C. Roy, O.K. Varghese, M. Paulose, C.A. Grimes, Toward solar fuels: photocatalytic conversion of carbon dioxide to hydrocarbons, ACS Nano 4 (2010) 1259–1278. [4] M. Lashgari, M. Ghanimati, Efficient mesoporous/nanostructured Ag-doped alloy semiconductor for solar hydrogen generation, J. Photon. Energy 4 (2014) 044099, doi:http://dx.doi.org/10.1117/1.JPE.4.044099. [5] G.A. Olah, G.K.S. Prakash, A. Goeppert, Anthropogenic chemical carbon cycle for a sustainable future, J. Am. Chem. Soc. 133 (2011) 12881–12898. [6] A.E. Creaner, B. Gao, Carbon Dioxide Capture: An Effective Way to Combat Global Warming, Springer, Heidelberg, 2015. [7] P. Matson, T. Dietz, Advancing the Science of Climate Change, The National Academies Press, Washington, DC, 2010. [8] S.N. Habisreutinger, L. Schmidt-Mende, J.K. Stolarczyk, Photocatalytic reduction of CO2 on TiO2 and other semiconductors, Angew. Chem. Int. Ed. 52 (2013) 7372–7408. [9] V.A.P. O’shea, D.P. Serrano, J.M. Coronado, Current challenges of CO2 photocatalytic reduction over semiconductors using sunlight, in: E.A. Rozhkova, K. Ariga (Eds.), From Molecules to Materials, Springer International Publishing, Switzerland, 2015. [10] V.N. Nguyen, L. Blum, Syngas and synfuels from H2O and CO2: current status, Chem. Ing. Tech. 87 (2015) 1–23. [11] M.M. Gui, L.L. Tan, W.-J. Ong, S.-P. Chai, A.R. Mohamed, CO2 photocatalytic reduction: photocatalyst choice and product selectivity, in: E. Lichtfouse, J. Schwarzbauer, D. Robert (Eds.), CO2 Sequestration, Biofuels and Depollution, Environmental Chemistry for a Sustainable World 5, Springer International Publishing, Switzerland, 2015. [12] W. Wang, S. Wang, X. Ma, J. Gong, Recent advances in catalytic hydrogenation of carbon dioxide, Chem. Soc. Rev. 40 (2011) 3703–3727. [13] M.S. Spencer, The role of zinc oxide in Cu/ZnO catalysts for methanol synthesis and the water–gas shift reaction, Top. Catal. 8 (1999) 259–266. [14] M. Lashgari, M. Ghanimati, A highly efficient nanostructured quinary photocatalyst for hydrogen production, Int. J. Energy Res. 39 (2015) 516–523. [15] P. Usubharatana, D. McMartin, A. Veawab, P. Tontiwachwuthikul, Photocatalytic process for CO2 emission reduction from industrial flue gas streams, Ind. Eng. Chem. Res. 45 (2006) 2558–2568. [16] E.V. Kondratenko, G. Mul, J. Baltrusaitis, G.O. Larrazábalc, J. Pérez-Ramírez, Status and perspectives of CO2 conversion into fuels and chemicals by catalytic, photocatalytic and electrocatalytic processes, Energy Environ. Sci. 6 (2013) 3112–3135. [17] J.Y. Zheng, G. Song, C.W. Kim, Y.S. Kang, Facile preparation of p-CuO and p-CuO/ n-CuWO4 junction thin films and their photoelectrochemical properties, Electrochim. Acta 69 (2012) 340–344. [18] Y.H. Wang, W.G. Gao, H. Wang, Y.E. Zheng, K.Z. Li, R.G. Ma, Morphology and activity relationships of macroporous CuO–ZnO–ZrO2 catalysts for methanol synthesis from CO2 hydrogenation, Rare Metal (2015), doi:http://dx.doi.org/ 10.1007/s12598-015-0520-7. [19] D. Gouvêa, S.V. Ushakov, A. Navrotsky, Energetics of CO2 and H2O adsorption on zinc oxide, Langmuir 30 (2014) 9091–9097. [20] P. Reñones, A. Moya, F. Fresno, L. Collado, J.J. Vilatela, V.A.P. O’shea, Hierarchical TiO2 nanofibres as photocatalyst for CO2 reduction: influence of morphology and phase composition on catalytic activity, J. CO2 Util. 15 (2016) 24–31. [21] A. Nikokavoura, C. Trapalis, Alternative photocatalysts to TiO2 for the photocatalytic reduction of CO2, Appl. Surf. Sci. 391 (2017) 149–174. [22] M.S. Akple, J. Low, S. Liu, B. Cheng, J. Yu, W. Ho, Fabrication and enhanced CO2 reduction performance of N-self-doped TiO2 microsheet photocatalyst by bicocatalyst modification, J. CO2 Util. 16 (2016) 442–449. [23] M.S. Akple, J. Low, Z. Qin, S. Wageh, A.A. Al-Ghamdi, J. Yu, S. Liu, Nitrogendoped TiO2 microsheets with enhanced visible light photocatalytic activity for CO2 reduction, Chin. J. Catal. 36 (2015) 2127–2134. [24] M. Grätzel, Photoelectrochemical cells, Nature 414 (2001) 338–344.

M. Lashgari et al. / Journal of CO2 Utilization 18 (2017) 89–97 [25] W. Yu, J. Zhang, T. Peng, New insight into the enhanced photocatalytic activity of N-, C- and S-doped ZnO photocatalyst, Appl. Catal. B 181 (2016) 220–227. [26] W.N. Wang, F. Wu, Y. Myung, D.M. Niedzwiedzki, H.S. Im, J. Park, P. Banerjee, P. Biswas, Surface engineered CuO nanowires with ZnO islands for CO2 photoreduction, ACS Appl. Mater. Interfaces 7 (2015) 5685–5692. [27] R. Marschall, Semiconductor composites: strategies for enhancing charge carrier separation to improve photocatalytic activity, Adv. Funct. Mater. 24 (2014) 2421–2440. [28] J. Low, B. Cheng, J. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Appl. Surf. Sci. 392 (2017) 658–686. [29] S.G. Kumar, K.S.R.K. Rao, Comparison of modification strategies towards enhanced charge carrier separation and photocatalytic degradation activity of metal oxide semiconductors (TiO2, WO3 and ZnO), Appl. Surf. Sci. 391 (2017) 124–148. [30] Z.P. Yang, L. Ci, J.A. Bur, S.Y. Lin, P.M. Ajayan, Experimental observation of an extremely dark material made by a low-density nanotube array, Nano Lett. 8 (2008) 446–451. [31] G. An, W. Ma, Z. Sun, Z. Liu, B. Han, S. Miao, Z. Miao, K. Ding, Preparation of titania/carbon nanotube composites using supercritical ethanol and their photocatalytic activity for phenol degradation under visible light irradiation, Carbon 45 (2007) 1795–1801. [32] H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu, X. Wang, Semiconductor heterojunction photocatalysts: design construction, and photocatalytic performances, Chem. Soc. Rev. 43 (2014) 5234–5244. [33] E. David, An overview of advanced materials for hydrogen storage, J. Mater. Process Technol. 162–163 (2005) 169–177. [34] A. Nikitin, X. Li, Z. Zhang, H. Ogasawara, H. Dai, A. Nilsson, Hydrogen storage in carbon nanotubes through the formation of stable C-H bonds, Nano Lett. 8 (2008) 162–167. [35] H. Sun, S. Wang, Research advances in the synthesis of nanocarbon-based photocatalysts and their applications for photocatalytic conversion of carbon dioxide to hydrocarbon fuels, Energy Fuels 28 (2014) 22–36. [36] Z. Li, V.P. Kunets, V. Saini, Y. Xu, E. Dervishi, G.J. Salamo, A.R. Biris, A.S. Biris, Light-harvesting using high density p-type single wall carbon nanotube/ntype silicon heterojunctions, ACS Nano 3 (2009) 1407–1414. [37] C. Wang, J. Zhu, S. Liang, H. Bi, Q. Han, X. Liu, X. Wang, Reduced graphene oxide decorated with cuO-ZnO hetero-junctions: towards high selective gas-sensing property to acetone, J. Mater. Chem. A 2 (2014) 18635–18643. [38] F. Gonell, A.V. Puga, B. Julián-López, H. García, A. Corma, Copper-doped titania photocatalysts for simultaneous reduction of CO2 and production of H2 from aqueous sulfide, Appl. Catal. B 180 (2016) 263–270. [39] Z. Fang, S. Li, Y. Gong, W. Liao, S. Tian, C. Shan, C. He, Comparison of catalytic activity of carbon-based AgBr nanocomposites for conversion of CO2 under visible light, J. Saudi Chem. Soc. 18 (2014) 299–307. [40] M. Ali, F. Zhou, K. Chen, C. Kotzur, C. Xiao, L. Bourgeois, X. Zhang, D.R. MacFarlane, Nanostructured photoelectrochemical solar cell for nitrogen reduction using plasmon-enhanced black silicon, Nat. Commun. 7 (2016) 11335, doi:http://dx.doi.org/10.1038/ncomms11335. [41] T. Nash, The colorimetric estimation of formaldehyde by means of the Hantzsch reaction, J. Biochem. 55 (1953) 416–421. [42] J. Yu, B. Yang, B. Cheng, Noble-metal-free carbon nanotube-Cd0.1Zn0.9S composites for high visible light photocatalytic H2-production performance, Nanoscale 4 (2012) 2670–2677. [43] S. Pal, S. Maiti, U.N. Maiti, K.K. Chattopadhyay, Low temperature solution processed ZnO/CuO heterojunction photocatalyst for visible light induced photo-degradation of organic pollutants, CrystEngComm 17 (2015) 1464– 1476. [44] L. Kong, X. Yin, M. Han, L. Zhang, L. Cheng, Carbon nanotubes modified with ZnO nanoparticles: high-efficiency electromagnetic wave absorption at hightemperatures, Ceram. Int. 41 (2015) 4906–4915.

97

[45] G.D. Scholes, G. Rumbles, Excitons in nanoscale systems, Nat. Mater. 5 (2006) 683–696. [46] S.W. Koch, M. Kira, G. Khitrova, H.M. Gibb, Semiconductor excitons in new light, Nat. Mater. 5 (2006) 523–531. [47] C. Hariharan, Photocatalytic degradation of organic contaminants in water by ZnO nanoparticles: revisited, Appl. Catal. A 304 (2006) 55–61. [48] Y. Zhang, J. Mu, One-pot synthesis photoluminescence, and photocatalysis of Ag/ZnO composites, J. Colloid Interface Sci. 309 (2007) 478–484. [49] J. Liu, H. Bai, Y. Wang, Z. Liu, X. Zhang, D.D. Sun, Self-assembling TiO2 nanorods on large graphene oxide sheets at a two-phase interface and their antirecombination in photocatalytic applications, Adv. Funct. Mater. 20 (2010) 4175–4181. [50] W. Yu, D. Xu, T. Peng, Enhanced photocatalytic activity of g-C3N4 for selective CO2 reduction to CH3OH via facile coupling of ZnO: a direct Z-scheme mechanism, J. Mater. Chem. A 3 (2015) 19936–19947. [51] E. Karamian, S. Sharifnia, On the general mechanism of photocatalytic reduction of CO2, J. CO2 Util. 16 (2016) 194–203. [52] B. Michalkiewicz, J. Majewska, G. Ka˛dziołka, K. Bubacz, S. Mozia, A.W. Morawski, Reduction of CO2 by adsorption and reaction on surface of TiO2nitrogen modified photocatalyst, J. CO2 Util. 5 (2014) 47–52. [53] J. Low, B. Cheng, J. Yu, M. Jaroniec, Carbon-based two-dimensional layered materials for photocatalytic CO2 reduction to solar fuels, Energy Storage Mater. 3 (2016) 24–35. [54] Y. Li, W. Zhang, X. Shen, P. Peng, L. Xiong, Y. Yu, Octahedral Cu2O-modified TiO2 nanotube arrays for efficient photocatalytic reduction of CO2, Chin. J. Catal. 36 (2015) 2229–2236. [55] X.-H. Xia, Z.-J. Jia, Y. Yu, Y. Liang, Z. Wang, L. Ma, Preparation of multi-walled carbon nanotube supported TiO2 and its photocatalytic activity in the reduction of CO2 with H2O, Carbon 45 (2007) 717–721. [56] D.-S. Lee, Y.-W. Chen, Photocatalytic reduction of carbon dioxide with water on InVO4 with NiO cocatalysts, J. CO2 Util. 10 (2015) 1–6. [57] P. Atkins, J. Paula, Physical Chemistry, 8th edition, W.H Freeman and Company, NY, 2006. [58] M. Gattrell, N. Gupta, A. Co, A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper, J. Electroanal. Chem. 594 (2006) 1–19. [59] J. Shen, R. Kortlever, R. Kas, Y.Y. Birdja, O. Diaz-Morales, Y. Kwon, I. LedezmaYanez, K.J.P. Schouten, G. Mul, M.T.M. Koper, Electrocatalytic reduction of carbon dioxide to carbon monoxide and methane at an immobilized cobalt protoporphyrin, Nat. Commun. 6 (2015) 8177, doi:http://dx.doi.org/10.1038/ ncomms9177. [60] Q. Lu, F. Jiao, Electrochemical CO2 reduction: electrocatalyst, reaction mechanism, and process engineering, Nano Energy 29 (2016) 439–456, doi: http://dx.doi.org/10.1016/j.nanoen.2016.04.009. [61] M. Subrahmanyam, S. Kaneco, N. Alonso-Vante, A screening for the photo reduction of carbon dioxide supported on metal oxide catalysts for C1–C3 selectivity, Appl. Catal. B 23 (1999) 169–174. [62] L.C. Grabow, M. Mavrikakis, Mechanism of methanol synthesis on Cu through CO2 and CO hydrogenation, ACS Catal. 1 (2011) 365–384. [63] N. Matubayasi, S. Morooka, M. Nakahara, H. Takahashi, Chemical equilibrium of formaldehyde and methanediol in hot water: free-energy analysis of the solvent effect, J. Mol. Liq. 134 (2007) 58–63. [64] N. Ahmed, M. Morikawa, Y. Izumi, Photocatalytic conversion of carbon dioxide into fuels, in: S.L. Suib (Ed.), New and Future Developments in Catalysis: Activation of Carbon Dioxide, Elsevier Amsterdam, 2013. [65] J. Gao, X. Mo, J.G. Goodwin, V. La, and Fe promotion of Rh/SiO2 for CO hydrogenation: detailed analysis of kinetics and mechanism, J. Catal. 268 (2009) 142–149. [66] I.A. Shkrob, T.W. Marin, H. He, P. Zapol, Photoredox reactions and the catalytic cycle for carbon dioxide fixation and methanogenesis on metal oxides, J. Phys. Chem. C 116 (2012) 9450–9460.