Photocatalytic Conversion in a Continuous Membrane Reactor

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CO2 to liquid fuels: photocatalytic conversion in a continuous membrane reactor Francesca Rita Pomilla, Adele Brunetti, Giuseppe Marci, Elisa Isabel GarciaLopez, Enrica Fontananova, Leonardo Palmisano, and Giuseppe Barbieri ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01073 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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ACS Sustainable Chemistry & Engineering

CO2 to liquid fuels: photocatalytic conversion in a continuous membrane reactor

Francesca Rita Pomilla,1,2,3 Adele Brunetti,1 Giuseppe Marcì,3 Elisa Isabel Garcia-Lopez,3 Enrica Fontananova,1 Leonardo Palmisano,3 Giuseppe Barbieri1,*

1

Institute on Membrane Technology (ITM-CNR), National Research Council

c/o the University of Calabria, Cubo 17C, Via Pietro Bucci, 87036 Rende CS, Italy. 2

Department of Environmental and Chemical Engineering, the University of Calabria Via Pietro Bucci, 87036 Rende CS, Italy

3

“Schiavello-Grillone” Photocatalysis Group. Dipartimento di Energia, Ingegneria dell’informazione e modelli Matematici (DEIM), Università di Palermo Viale delle Scienze, 90128 Palermo, Italy

Author whom correspondence should be addressed: Giuseppe BARBIERI, [email protected]

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Abstract

The photocatalytic reduction of CO2 into added-value chemicals using sunlight is a promising approach to promote energy bearing products, mitigating the adverse effects of anthropogenic CO2 emissions. In this work, exfoliated C3N4 was incorporated into Nafion matrix and used in a continuous photocatalytic reactor for converting CO2 into liquid fuels. Comprehensive structural and morphological DRS, FT-IR, ATR-IR, SEM measurements were performed for C3N4 loaded Nafion membrane and then compared with those of a Nafion membrane without of any catalyst. The synergic effect of C3N4 organic-catalyst embedded in a Nafion dense matrix and a continuous operating mode of the photo-reactor was successfully accomplished for the first time, as yet absent in the literature, analyzing the reactor performance as a function of key parameters such as contact time and H2O/CO2 feed molar ratio. The reactor performance was analyzed under UV-Vis light in terms of productivity, selectivity and converted carbon. Alcohols (MeOH + EtOH) production was 32.8 µmol gcatalyst-1 h-1 corresponding to 47.6 µmol gcatalyst-1 h-1 of total converted carbon per gram of catalyst at the best operating conditions. So far, this value results to be higher than most of the literature values reported up to date. Moreover, the membrane reactor converted at least 10 times more carbon than the batch system, as a result of the catalyst embedding in a Nafion matrix.

Keywords CO2 photoreduction, Carbon Nitride, Photocatalytic membrane reactor, continuous operation.


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Introduction The recent SPIRE initiatives [1] developed in the framework of H2020 calls define CO2 utilization as one of the most important pillars to boost sustainability, making the chemical industry competitive [2]. Although the reduction of CO2 emissions is a desirable solution but not achievable in the short term, a more realistic approach is the reuse of emitted CO2 as a raw feedstock to promote energy bearing products, mitigating the adverse effects of anthropogenic CO2 emissions. Among the various ways of converting CO2 [2], photocatalytic reduction whereby CO2 produces value-added products such as formic acid or fuels through mimicking the artificial photosynthesis is an important utilization pathway [3,4]. To date, titanium dioxide remains the most widely studied photo-catalyst material used for this purpose owing to its wide availability, low cost, nontoxicity, long-term stability, etc. [5]. TiO2 can be used in its pure form, or doped with different metals and non-metals, or even in a highly dispersed form within mesoporous silica or carbon materials to enhance its own photo-activity. However, the rapid recombination of the photo-generated electron-holes across the microstructure of TiO2 photo-catalysts, as well as their low adsorption capacity for CO2, severely compromise their catalytic efficiency in CO2 conversion. Consequently, if on the one hand there is an intense effort to develop alternative catalytic materials [6,7], with improved charge separation kinetics, on the other hand, the exploration of new technological approaches [8,9] might reveal some of the ways of enhancing the photocatalytic conversion of CO2. A metal free polymeric catalyst, graphitic carbon nitride, has been recently explored for the capture and activation of CO2 via thermal catalysis in the presence of sacrificial hydrocarbons [10,11]. The unique electric, optical, structural and physicochemical properties, make graphitic-C3N4-based materials a new class of multifunctional nano-platforms for electronic, catalytic and energy applications [12]. Especially, graphitic C3N4-based photo catalysts have attracted increasing interest worldwide, since Wang and his co-workers first discovered the photocatalytic H2 and O2 evolution over C3N4 in 2009 [13]. This photocatalyst is active under UV-Vis light with a band gap energy of about 2.7 eV and absorption of light at about 455 nm [14,15,16]. Recently, several groups have extended the photocatalytic application of graphitic C3N4 to the reduction of CO2 [17,18,19,20,21,22,23,24] and CO2 capture [25], because of its unique semiconductor and surface characteristics that allow the creation of charge carriers for the reduction of CO2 preadsorbed/activated molecules by surface basic sites on nitrogen-rich carbon nitride frameworks with tunable texture and surface functions. If photocatalyst is a key player, an equally important role is covered by catalyst configuration during the photochemical reaction [26]. The mass transfer rate of CO2 and the catalyst surface area are two other important parameters which must be controlled to 3 ACS Paragon Plus Environment


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improve the photocatalytic process. Problems related to the low surface-area-to-volume ratio, nonuniform light distribution and poor light utilization efficiency are usually encountered in existing photo-reactors, limiting products yield [27]. The catalyst immobilization into polymeric membrane supports and, thus the use of a membrane reactor for this reaction type, can be an interesting and valid solution to adopt. Its use offers several advantages such as a better catalyst exposition to UV light to carry out the reaction, the tuning of contact between reactants and catalyst, the reduction of catalyst aggregate formation, an easier recovery of the catalyst which can be simply reused, and a better control of fluid-dynamics. Nafion is the most used polymer in polymer electrolyte membrane fuel cells, which usually operate for really long-time (tens thousands hours) up to 80°C, although other measures were carried up to 140°C [28]. That confirms the long term stability of Nafion under heating. Similar resistance is expected in the photocatalytic applications under visible/UV light irradiation. In our previous work [8], home-made dense mixed matrix TiO2-based Nafion membranes were used in a photocatalytic reactor operated in continuous for CO2 photoreduction, obtaining a performance higher than most literature data reported up to now. A key role was played by the continuous operating mode of the photocatalytic reactor which allowed a continuous removal of products from the reaction volume, promoting conversion and depleting secondary and oxidation reactions. Inspired by these findings, this work reports on the reduction of CO2 to fuels in a continuous photocatalytic membrane reactor equipped with a Nafion membrane with embedded exfoliated C3N4. More reaction products which can be different from those detected in a batch system and higher CO2 conversion are expected from the use of a catalytic Nafion membrane loaded with exfoliated C3N4. The catalytic Nafion membrane loaded with exfoliated C3N4 converted CO2 in a set of products, that is, MeOH, EtOH, HCHO and acetone; the distribution of which strongly depends on H2O/CO2 feed molar ratio and residence time. Both these variables can be “easily” hand laded in a continuous system such as that proposed in this work. To enhance CO2 adsorption onto the catalyst surface and relative activity, exfoliated C3N4 was prepared obtaining a material with a higher specific surface area with respect to the graphitic C3N4 type. After a preliminary verification of its activity in a batch reactor, exfoliated C3N4 was embedded in a Nafion matrix to increase the catalyst performance. Firstly, structural and morphological properties of C3N4, alone and embedded in the dense Nafion membrane were characterized confirming the subsistence of physical and chemical C3N4 properties also when embedded in a polymeric matrix. Then, continuing the same way as the previous work [8], a 4 ACS Paragon Plus Environment

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correlation between the contact time and the products distribution of a photocatalytic membrane reactor for CO2 reduction was established for the first time. This parameter, in fact, plays a key role in determining reactor performance, allowing the opportune modulation of products removal from the reaction volume, depleting undesired secondary reactions. In addition, the membrane reactor performance was explored as a function of an excess, a defect and a stoichiometric feed of H2O, considering that H2O/CO2 feed molar ratio has a primary role in the attainment of a much more efficient and selective conversion process. In summary, this work, couples the assets offered by the continuous operating mode with the use of a recent and promising catalyst (C3N4) directly embedded in a dense polymer matrix, focusing not only on the materials but on the process itself. To the best of the authors’ knowledge, this is the first time in the literature where this synergic approach is proposed for CO2 photoreduction. Carbon dioxide is the largest emitted green-house gas and any action, such as the one proposed in this paper, for reducing its emission is eco-friendly and green by definition. The CO2 conversion into liquid fuels such as methanol and ethanol was demonstrated by using a catalytic membrane. It is a green process because of the possibility to use sunlight as an energy source. The photo-reduction is a right way for CO2, although further investigation is necessary for increasing its conversion. Therefore, the novel catalytic membrane continuous system, showing results among the best available in the open literature, appears to be a promising process based on green chemistry, which is suitable to perform CO2 reduction by sunlight.

Materials and Method Nafion™ 5 wt.% solution was purchased from Quintech e.K. – Brennstoffzellen Technologie, Germany. Melamine (C3H6N6, 99.0%), methanol (anhydrous 99.8%), absolute ethanol (reagent ISO, reagent Pharmacopoeia European, ≥99.8%), acetone (analytical standard) and formaldehyde (absolute, reagent ISO, reagent Pharmacopoeia European ≥99.8%) were obtained from SigmaAldrich. All chemicals were used as received without any further purification. Water used throughout the experiments was purified by means of a Milli-Q system. CO2 was purchased from Sapio srl (Italy). CO2 purity was analytically verified, it being the feed of the membrane reactor.



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Preparation, characterization and batch test of photocatalyst Carbon nitride (C3N4) was prepared by heating directly at 2°C min-1 melamine (10 g) up to 520°C, temperature kept for 2 h, in a covered crucible exposed in static air. The solid was pounded and then heated again, without coverage, at 3°C min-1 up to 500°C keeping this temperature for 4 h. Bulk and surface were characterized for defining the physicochemical properties of the powder. Their crystalline phase structures were analyzed at room temperature by powder X-ray diffraction by using a Panalytical Empyrean, equipped with CuKα radiation and PixCel-1D (tm) detector. The Brunauer-Emmett-Teller SSA of the synthesized catalyst was measured by nitrogen adsorption– desorption isotherms using a Micromeritics Asap. Infrared spectra of the samples in KBr (Aldrich) pellets were obtained with an FTIR-8400 Shimadzu spectrophotometer and recorded with 4 cm-1 resolution and 256 scans. The diffuse reflectance spectra were measured in air at room temperature in 250-800 nm wavelength range using a Shimadzu UV-2401 PC spectrophotometer, with BaSO4 as reference material. A 350 mg C3N4 sample was placed in a cylindrical batch reactor (130 mL, Pyrex) for photocatalytic CO2 reduction experiments. The reactor was firstly cleaned under light with humid He and after [29], in dark condition, CO2 filled for 30 minute. 3.6 µL of distillate water was introduced into the reactor by means of a syringe. Afterwards, the batch reactor was placed in a Solar Box (1500 W, Xe lamp) and the experiments were carried out for 7 h. The species concentrations were analyzed by a Shimadzu gas-chromatograph equipped with FID and a Hewlett Packard GC with TCD, injecting in each a 250 µL sample by means of gastight syringe.

Preparation and characterization of photocatalytic membranes Nafion and catalytic Nafion membranes were prepared following the same procedure reported in [8]. Nafion™ 5 wt.% solution (20 g) was dried at ca. 60-70 °C under stirring; then, Nafion was dissolved in EtOH:H2O=50:50 wt./wt. at room temperature under stirring for 1 h to obtain the polymeric solution. Two types of flat sheet membranes were prepared: a Nafion membrane (not including catalyst) and a photocatalytic Nafion™ membrane (Table 1). For catalytic membrane, after a complete polymer dissolution, the catalyst (C3N4) was added to the obtained solution and the resulting dispersion was left under stirring for 1 h more. Then it was sonicated for 3 h more at room temperature in order to favor the homogenization of the dispersion. The catalyst-polymer dispersion obtained was then casted in a Petri dish (diameter=7.1 cm) and the solvent evaporation was carried out in a climatic chamber (60±4°C and 12±5% as relative humidity) for about 20 h. The membrane obtained was treated thermally, heating it at 10°C min-1 up to 100°C, then at 0.5°C min-1 up to 6 ACS Paragon Plus Environment

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120°C, and kept at this temperature for 4 h. No specific investigation on C3N4 loading content was carried out and the chosen C3N4 percentage of 1.23 wt% was the same as before for TiO2 [8]. In the case of the Nafion membrane, the same procedure was followed but without catalyst dispersion.

Table 1. Membranes solution compositions Membrane Photocatalytic Nafion (bare) (C3N4-Nafion) Polymeric solution Solvent (EtOH:H2O (50:50 wt.%)) wt.% 97.8 97.827 Polymer, wt.% 2.173 2.173 Catalyst addition Catalyst, wt.% 0.027 0 Resulting membrane Catalyst in membrane, wt.% 1.23 0 Polymer in membrane, wt.% 98.77 100 Then, the flat sheet membranes obtained were detached from the Petri dishes with a small amount of water and then dried at room temperature. The two membrane sides were distinguished as the casting plate-facing and air-facing surfaces. The membranes were characterized by different techniques. The morphologies of the membrane surfaces (air- and plate-facing) and cross section were examined by an EVO MA10 Zeiss scanning electron microscope (SEM). The samples were sputtered with gold prior to SEM analysis. A Perkin Elmer Spectrum One was utilized for Fourier transform infrared (FT-IR) spectroscopy analyses in attenuated total reflectance mode of both air-facing and casting plate-facing membrane surfaces. Diffuse reflectance spectroscopy and FT-IR analysis were carried out in three different points for both casting sides. Permeability was also measured to evaluate the membranes mass transport properties and integrity. The final step of membrane preparation was the membrane cleaning. In this procedure carried out using the membrane reactor set up described in the next section, all the membranes underwent “blank reaction” measurements, where, together with H2O, an argon stream was fed into the reaction module instead of CO2, in continuous for 8-24 h, at the same operating conditions chosen for the catalytic experiments including UV-Vis irradiation. The aim of this procedure was to clean the membrane from any residuals of solvent and other low molecular weight organics eventually present in the polymer solution that could be released during the reaction measurements invalidating the results. 7 ACS Paragon Plus Environment


1.7 1.6 1.5

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Nafion (bare) membrane HCHO

0.08 0.06

MeOH

0.04 EtOH

0.02 0

Acetone 0

5

10 15 20 25 Time-on-stream, h

30

Figure 1. Membrane cleaning (removal of any residuals potentially present) before reaction. C3N4 loaded Nafion membrane (active area and thickness respectively 20.4 cm2 and 59.54 ± 0.18 µm) and bare Nafion membrane (active area and thickness respectively 4.1 cm2 and 81.3 ± 4.3 µm).

Figure 1 shows the species distribution as a function of time on stream. MeOH, EtOH, HCHO and C3H6O (acetone) were found in the first sample after 2 h testing at the reactor exit. Their flow rate suddenly decreased along with the time and after 7 h no species were detected at the reactor outlet. This treatment was further continued up to 10-12 h. The continuous operating mode of the membrane reactor, implied a continuous removal of residual species from the reactor volume; therefore, their decreasing trend confirmed their nature as residues. The zero values reached by each species profile clearly indicated that the membranes no longer contained any compound; therefore, the species measured in reaction experiments could only be the result of the photocatalytic activity of the C3N4-Nafion membrane. The presence of the same compounds (Figure 1, right side) in a bare Nafion membrane confirms the previous hypothesis. The difference in the profile evolution along with time could be attributed to the different membrane thickness and the filler (catalyst) presence.

Photocatalytic reaction experiments Photocatalytic membrane was utilized for CO2 photoreduction with H2O as reducing agent. A medium–high mercury vapor pressure lamp (Zs lamp, Helios Ital quartz, Milan) with emittance from 360 nm (UV-Vis) to 600 nm was used for irradiating the membrane. The flat sheet membrane


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was assembled in a stainless steel module equipped with a quartz window allowing UV-Vis irradiation of the catalytic membrane surface. Once placed in UV-Vis exposure chamber, the membrane module was fed continuously with CO2 and H2O by means a mass flow controller and an HPLC pump, respectively. Figure 2 shows the experimental apparatus set up. The membrane reactor mainly consists of three parts: the feed/retentate chamber, the permeate volume and the catalyst loaded membrane. The two reactor chambers can be considered as lumped parameter systems since reaction takes place in the membrane layer and no concentration gradient of any chemical species is expected, also owing to the low conversion of this specific reaction. Thus, the composition of these two volumes is the same as the related reactor exiting streams, which are retentate and permeate. The module was placed in vertical position to facilitate permeate and retentate sampling, as well as avoiding stagnant zones. Table 2 summarizes the operating conditions explored. The trans-membrane pressure difference of 2 bar was regulated by a back pressure controller. The gaseous fraction of retentate and permeate flows was measured by bubble soap flow-meters, after cooling streams outgoing from the reactor with apposite ice traps. The composition of these streams was measured by an Agilent Technologies 7890A gas chromatograph with TCD (HP-PLOT and Molsieve columns). The condensate fraction of the retentate and permeate, periodically sampled, was analyzed by an Agilent 6890N gas chromatograph with FID and an HP-5 column. It has to be highlighted that all the measurements, lasting 25-30 h, were performed in steady state conditions, which were reached within 5 h for all of the species. The catalytic membrane was used in the continuous photocatalytic membrane reactor for some weeks changing feed molar ratio, feed pressure, feed flow rate, etc. During the whole period, C3N4 piling out was not evident since no loss in reactor performance and/or selectivity was measured. Anyhow, the catalytic activity owing to C3N4, even though expected to be long, needs specific measures.

Table 2. Operating conditions for reaction measurements CO2 flow rate, mL(STP*) min-1 H2O/CO2 feed molar ratio Feed Pressure, bar Permeate Pressure, bar Contact time, s

11-107 0.5; 2; 5 2 1 2; 9.8; 18.7

*

STP represent the standard temperature and pressure condition equivalent to 25°C and 1 bar.



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Figure 2. Scheme of Photocatalytic membrane reactor setup.

Particular attention was devoted to analyzing the effect of contact time on membrane reactor performance. It was defined as the ratio of the catalyst weight dispersed in the membrane to CO2 feed flow rate, considered as the limiting reagent (eq. 1). No difference in CO2 exiting the reactor from the feed one could be measured practically; therefore, CO2 conversion did not represent an appreciable value to be considered significant in order to evaluate the reactor performance. Instead, it was evaluated through the produced species flow rate/catalyst weight (eq. 2), the total converted carbon/catalyst weight ratio (eq. 3), and reaction selectivity (eq. 4).

&#' () "#

#2 #

! " "#

#)#' "#

,

, % (eq. 1)

+#

*#' #" ) ,-.-/01.

∑ # "# "# )#' ) *#' #" )

8) "#

4 5 67 ∑9 :

) "#

,

+# ,-.-/01.

(eq. 2)

(eq. 3)

, ; (eq. 4)


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Results and discussion Photocatalysts Characterization Figure 3a shows XRD analysis of C3N4 powder in which two characteristic peaks at 27.3° and 13.1° are clearly defined, in good agreement with patterns reported in the open literature [30]. The highangle peak at 27.3° is characteristic of an interlayer stacking of conjugated aromatic systems, which is indexed to the (0 0 2) peak corresponding to the average interlayer distance of 0.326 nm [14,31]. The lower diffraction peak at 13.1° is indexed to the (1 0 0) plane and assigned to the in-plane structural packing of aromatic systems with an average distance of 0.675 nm, characteristic of heptazine heterocyclic ring (C6N7) units [14].

a)

27.3 b)

Trasmittance, %

30000

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20000

10000

0 10

13.1

20 30 2θ, degree

40

d)

c)

Figure 3. (a) XRD spectra of C3N4 (red line) measured for powder and (black line) as reported by Wen et al. [12]. b) FT-IR spectrum of C3N4 powder (red line) and (black line) as reported by Liao et al. [32]. c-d) SEM image of C3N4 powder at two different magnitude, at c) 5µm and at d) 30 µm, respectively. 11 ACS Paragon Plus Environment


The chemical structure is also confirmed by FT-IR whose peaks (Figure 3b) at 890, 1250, 1324, 1405, 1454, 1571, and 1636 cm-1 correspond to the typical stretching vibration modes of heptazine heterocyclic ring units [32]. The peak at 804 cm−1 belongs to the characteristic breathing mode of triazine units representing the condensed CN heterocycles [33]. The SEM images (Figure 3c-d) shows the graphitic structure and the partial exfoliation of C3N4 powder caused by the second thermal treatment to which it was subjected. The diffuse reflectance spectra were recorded for investigating the electronic band structure of C3N4 sample which was determined by the intersection (red spot) of the tangent lines (Figure 4, blue lines) of the KubelkaMunck modified function, [F(R’∞)hν]1/2, versus the energy of the exciting light. The band gap identified of 2.8 eV (Figure 4) falls in the region of visible light absorption of the photocatalyst corresponding at ca. 443 nm. The discrepancy with the theoretical value of 2.7 eV of the bulk materials is ascribable to the intrinsic superiority of 2D structures of the exfoliated C3N4 [34].

3 Band gap energy 2.8 eV

[F(R'∞)h ]½ , eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

1

0 1.6

2

2.4

h

2.8

3.2

3.6

eV

Figure 4. Kubelka-Munck modified function vs. the energy irradiating the C3N4 sample. The BET analysis of the exfoliated powder sample gave a specific surface area of 154 m2 g-1. The related adsorption/desorption isotherm and the specific surface area of the bulk g-C3N4 of 7 m2 g-1 are reported in [35]. Zhang et al. [36] measured a specific area of 160 m2 g-1.This relative high value can be attributed to the two thermal treatments undergone by the melamine, in agreement with Papailias et al. [37] who stated that structural and optical properties highly depend on the process temperature and, thus, poly-condensation degree [38]. To evaluate the photocatalytic activity, the reaction measurements were carried out on C3N4 powder in batch conditions. The batch reactor conversion was followed in time for an initial H2O/CO2 feed molar equal to 0.5, obtaining, after 6 h of irradiation, 0.6 micromol gcatalyst-1 h-1 of converted carbon, 12 ACS Paragon Plus Environment

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with CH4 and CO as the most abundant products, accordingly to literature data of gas phase CO2 photoreduction [21, 39].

Membranes morphological and chemical characterization Figure 5a shows FT-IR/ATR representative spectra relative to the photocatalytic membrane for both air-facing and casting plate-facing surfaces. Firstly, the analyses confirmed the presence of C3N4 catalyst in the membrane matrix as the characteristic chemical structure was maintained by peak presence between ca. from 1630 to 1320 cm-1, in agreement with FT-IR analysis of C3N4 powder (Figure 3, right side). All the signals in the range between 970 and 1400 cm-1 belong to the Nafion structure. In particular, the bands appearing at 970-983 cm-1 are attributed to C ̶ O ̶ C stretching vibrations; the band at ∼1066 cm-1 can be related to the symmetric stretching of ̶ SO3 ̶ . In the range between 1400 and 1100 cm-1 the asymmetric stretching bands of ̶ SO3 ̶ should be found, but they are covered from the more intense bands of ̶ CF2 stretching, visible in the spectra [40]. Moreover, interactions between the Nafion matrix and the loaded C3N4 occur as already reported in the literature by Wu et al. [41]. The C3N4 nanosheets consist of a two-dimensional material that through ammine groups establish crosslinking interactions with the hydrophilic -SO3- groups present in the Nafion matrix. FT-IR/ATR analysis of C3N4 Nafion membrane shows, with respect to Nafion membrane, a blue-shift of SO3-characteristic asymmetric vibration from 1054 cm-1 to 1063 cm-1 (Figure 5a, inset). The g-C3N4 nanosheets incorporated in the membrane during membrane formation turns the Nafion original “spherical structure” into a new “lamellar structure” increasing the amorphous phase [41]. OH band intensity of the casting plate-facing surface spectrum was higher than the air-facing surface one, most likely owing to the higher catalyst concentration related to its segregation on this membrane side, as also confirmed by SEM images. Indeed, from the perusal of Figure 5b), the air-facing surface of the C3N4-loaded Nafion membrane appears dense in agreement with the expectations for a membrane prepared by solvent evaporation method. On the contrary, the casting plate-facing surface presents some porosities owing to the slow solvent evaporation from the more internal layers of the cast solution that can give rise to local phase separation processes (Figure 5c). Moreover, the SEM image of the casting plate-facing surface highlights a partial segregation of the C3N4 partially deposited on the bottom part of the membrane. The SEM images of the cross section (Figure 5 d and e) confirmed the dense nature of the membrane and the formation of catalyst micro-aggregates, present in the whole cross section, but more concentrated in the bottom part of the same. The integrity of catalyst when embedded in membrane matrix was also confirmed by UV–Vis diffuse reflectance spectroscopy analyses that showed a band gap of 2.8 eV for the membranes in dry and wet conditions as for C3N4 catalyst powder. Moreover, Nafion matrix was proved to be 13 ACS Paragon Plus Environment


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transparent to UV-visible radiation, as the spectrum was recorded analyzing the air-side membrane surface.

1.2 a) segregation of C3N4 catalyst on glass side surface

0.8

0.8 1063

1054

0.4

0.4

0 1080

1040

1000

0 4000

3500

3000 2500 2000 1500 Wave number, cm -1

1000

500

Figure 5. a) FT-IR (ATR) spectra of (black line) air-facing and (red line) casting plate-facing membrane surfaces of C3N4–loaded Nafion membrane (in the inset, the blue shift of -SO3-groups in the Nafion owing to the presence of the catalyst). SEM of (b) air-facing (magnification 5kX), (c) casting plate-facing surfaces (5 kX), (d) cross-section (3 kX), (e) detail (15 kX), of cross section of C3N4–loaded Nafion membrane

Photocatalytic membrane reactor measurements As aforementioned, photocatalytic reaction measurements were carried out analyzing the effect of the feed molar ratio and contact time (eq. 1) on the membrane reactor performance. At a contact time of 2 s, MeOH and EtOH flow rates increased with H2O/CO2 feed molar ratio (Figure 6), reaching 17.9 14 ACS Paragon Plus Environment

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and 14.9 µmol gcatalyst-1 h-1, respectively, at a feed molar ratio equal to 5 against 4 and 1.7 µmol gcatalyst1

h-1 obtained at a feed molar ratio of 0.5. Contrarily, to the water defect corresponds a larger HCHO

production; therefore, at H2O/CO2 feed molar ratio of 0.5, HCHO was the most abundant product, reaching a flow rate of 27 µmol gcatalyst-1 h-1 (Figure 6). This trend can be explained through the reactions stoichiometry (eqs 5-7).

CO2 + 2 H2O = CH3OH + 3/2 O2

eq. 5

2 CO2 + 3 H2O = CH3CH2OH + 3 O2

eq. 6

CO2 + H2O = HCHO + O2

eq. 7

A water excess (as in the case of H2O/CO2 feed molar ratio=5) leads to MeOH and EtOH as the main products whereas a water defect (as in the case of H2O/CO2 feed molar ratio=0.5) leads to HCHO as the main one. Traces of acetone were also found at a feed molar ratio of 0.5, most probably owing to the occurrence of secondary reactions of intermediates not desorbed from the catalytic site, as reported in the literature [42,43].

C3N4 weight/CO2 feed flow rate=2.0 s

30

20 HCHO

10

MeOH EtOH

0

Acetone

0

1 2 3 4 5 H2O/CO2 feed molar ratio, -

Figure 6. Flow rate/catalyst weight of the various species formed as a function of H2O/CO2 feed molar ratio at a contact time of 2 s



12

Page 16 of 30

C3N4 weight/CO2 feed flow rate=9.8 s

10 8

HCHO

6

MeOH

4

EtOH

2

Acetone

0 0

1 2 3 4 5 H2O/CO2 feed molar ratio, -

Figure 7. Flow rate/catalyst weight of the various species formed as a function of H2O/CO2 feed molar ratio at a contact time of 9.8 s.

Similar trends were obtained at a contact time of 9.8 s (Figure 7), where MeOH and EtOH were again the favored products at a high feed molar ratio, even though their flow rates were ca. half of that obtained at a contact time of 2 s. This effect can be attributed to the time that the reacting species remain in contact with the catalyst under UV-Vis light. The high contact time, or, in other terms, the slower removal of the reaction mixture from reaction volume can induce MeOH and EtOH to be partially oxidized producing formaldehyde (eq. 8) or fully oxidized to CO2 as reaction product. On the contrary, a low contact time implies a fast removal of products from the reaction volume exposed to UV light and, thus, less promotion of secondary reactions. CH3OH +1/2 O2 = HCHO+ H2O

eq. 8

Analogously, at a feed molar ratio of 0.5, HCHO flow rate/catalyst weight decreased from 27 (at 2 s) to 9 µmol gcatalyst-1 h-1 (at 9.8 s) reaching zero at a feed molar ratio equal to 5. Acetone production instead increased as a function of the feed molar ratio up to stable value of ca. 2 µmol gcatalyst-1 h-1.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Species flow rate/C3N4 weight, -1 mol g catalyst h-1

Page 17 of 30

Figure 8. Flow rate/catalyst weight of the various species formed as a function of C3N4 weight/CO2 feed flow rate at H2O/CO2 feed molar ratio equal to 2.

Figure 8 and Figure 9 show the effect of the contact time. MeOH and EtOH were preferential products at the lowest value of contact time. As it increased, their flow rate decreased down to a constant value, whereas HCHO and acetone increased as a result of oxidation reaction and secondary reaction between intermediates, respectively. At a feed molar ratio of 5, which means in a water excess, no HCHO was detected (Figure 9). In these conditions, even though MeOH and EtOH decreased along with contact time, they remained the most abundant products in the whole range of contact time considered in the experiments.

Figure 9. Flow rate/catalyst weight of the various species formed as a function of C3N4 weight/CO2 feed flow rate at H2O/CO2 feed molar ratio equal to 5. 17 ACS Paragon Plus Environment


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Figure 10. Reaction selectivity as a function of C3N4 weight/CO2 feed flow rate (left side) and H2O/CO2 feed molar ratio (right side).

The reaction selectivity (eq. 4) is an important parameter to define the products distribution. The highest selectivity was observed in alcohols production at a low value of contact time and high H2O/CO2 feed molar ratio (Figure 10). The best MeOH and EtOH selectivities were, in fact, 54.6% and 45.4 % respectively, at H2O/CO2 feed molar ratio equal to 5 and contact time of 2 s, representing 100% of carbon-containing products. Globally, alcohols selectivity tended to increase with feed molar ratio. This behavior in the CO2 reduction reaction was previously explained as usual outcome when in the presence of water excess (eqs. 6-7). Indeed, it followed a decreasing trend with contact time, confirming how the slow removal of products from the reaction volume promoted secondary reactions, favoring HCHO and acetone formation. The capability of the system to convert CO2 into other carbon-containing species was quantified in terms of total converted carbon/catalyst weight (eq. 3) and the results are summarized in Figure 11. The highest conversion (47.6 µmol gcatalyst-1 h-1) occurred at the highest H2O/CO2 feed molar ratio and lowest contact time, with an amount of converted carbon 8 times greater than that measured in the worst conditions (H2O/CO2 feed molar ratio=5 and contact time=18.7 s). Comparing the results obtained by the photocatalytic membrane reactor operated in continuous mode with that of the batch system, the membrane reactor converted at least 10 times more carbon than the batch, also in unfavorable conditions. The dispersion in the Nafion matrix increases the active surface of the catalyst implying an easier achievement of the catalytic sites by light and reagents. Moreover, the continuous removals of products from the reaction volume not only favors the reaction, but also promotes the species desorption and the release of active sites, making them available for further conversion. 18 ACS Paragon Plus Environment

-1 g catalyst h-1

mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Converted Carbon/C 3N4 weight,

Page 19 of 30

Figure 11. Total Converted Carbon as function of H2O/CO2 feed molar ratio and contact time.

Table 3 compares the photocatalytic membranes reactor performance with other results available for C3N4 in the literature. The same table includes a previous comparison [8] made for TiO2 based catalytic membranes. Notably, the MeOH flow rate/catalyst weight of the photocatalytic membrane reactor presented in this work is, under some experimental conditions, significantly higher than the ones obtained with other reaction systems. In comparison with our previous work [8] where a TiO2based catalyst was embedded in a Nafion matrix, a wider products distribution resulted, with comparable MeOH production. By comparing this work with the data reported for batch systems, the advantage offered by the continuous system is confirmed. The MeOH production is, in fact, higher than most of the ones obtained by using simple C3N4 as catalyst and higher or, at least, comparable when it is used as co-catalyst in batch conditions. Moreover, it is worth noticing that, as already observed in our previous work [8] and differently from what can be found in most of the literature for batch reactors, neither CH4 nor CO were detected, alcohols being the main products. The synergic effect of the use of a photocatalytic membrane and a continuous flow mode reactor allows the substrate to undergo a lower degree of reduction as fresh CO2 is continuously fed into the system and the produced species are continuously removed from the catalytic sites, reducing the possibility of having over-oxidation [44].



Page 20 of 30

Remarks on possible reactions involved A pioneering work of Inoue et al. [45] reported the following reduction reactions: CO2 + 2H+ + 2 e- = HCOOH

eq. 9

HCOOH + 2H+ + 2 e- = HCHO+H2O

eq. 10

HCHO + 2H+ + 2 e- = CH3OH

eq. 11

CH3OH + 2H+ + 2 e- = CH4 + H2O

eq. 12

and, the oxidation reaction: H2O + 2h+ = ½ O2 + 2 H+

eq. 13

Each of the hypothesized reduction reactions involves two electrons and the various reactions occur in sequence; thus, 8-electrons are required per each CO2 molecule to obtain CH4. Ethanol and acetone, also experimentally detected in this work, were not included among the above reactions. For ethanol, the below reactions could be: CH3OH + HCHO + 2H+ + 2 e- = C2H5OH + H2O +

CH4 + CH3OH = C2H5OH + 2H + 2 e

-

CH4 + HCHO = C2H5OH

eq. 14 eq. 15 eq. 16

To obtain ethanol, two more electrons are needed (12 electrons in total for the reduction of two CO2 molecules, 6 of which are necessary to obtain one molecule of CH3OH, 4 for one molecule of HCHO and 2 electrons in eq. 14) or the presence of CH4. However in our system, no CH4 was detected; therefore, the first hypothesis is more likely.

Similarly for acetone, the reactions which could occur on the irradiated catalyst surface are. C2H5OH + CH3OH = CH3COCH3 + H2O + 2 H+ +2 eC2H5OH+ HCHO = CH3COCH3 + H2O

eq.17 eq. 18

Equations 17 and 18 do not involve any additional electrons. Other solutions can be hypothesized and, in addition, mechanisms based on elementary steps would be useful for understanding the reaction path and then the species produced. However, it is worth to tentatively correlate the Inoue schemes with some variables used in the present work such as residence time and feed molar ratio. 20 ACS Paragon Plus Environment

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During CO2 reduction, the chemical species move from one (feed) membrane face to the permeate one. Along this path, accordingly to the schemes of Inoue et al. [45] and the others reported above, CO2 conversion to HCOOH, HCOOH to HCHO, HCHO to CH3OH, CH3OH and HCHO to C2H5OH and acetone take place. CO2 is always inside the membrane, independent from the feed molar ratio; thus, the first, 2-electrons step, could occur everywhere in the catalytic membrane, although no HCOOH was detected in the exiting stream. The lower water content (low feed molar ratio) does not favor the oxidative reaction (eq. 13) and hence the H+ production is low. Therefore, the schemes implying four electrons are favored also for the small availability of this ion (Figure 6, HCHO production). The higher feed molar ratio favors the larger-electrons schemes leading, e.g., to MeOH formation owing to H+ large availability. The feed molar ratio plays a significant role: its lower value favors the low-electrons schemes (e.g., HCHO production, four-electrons), whilst a higher value leads to high-electrons scheme (e.g., MeOH, six-electrons). Therefore, the whole process is a combination of the reactions (2, 4, 6, etc. electrons) since these schemes describe a set of reactions in series and as such, at the end, all the reactants and products of each reaction are in principle expected in the exiting stream.


ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 22 of 30

Table 3. Comparison with results reported in literature Catalyst

C3N4 catalyst embedded in a Nafion membrane C3N4 C3N4* C3N4 catalyst placed in glass fiber C3N4 nanotubes C3N4 C3N4 C3N4 C3N4 3 MEA-C3N4 AgBr/C3N4 C3N4 3%Cu/TiO2* 2 Au-C3N4 catalyst placed in glass fiber C3N4/ZIF 8 (nanocluster on C3N4 nanotubes) CdIn2S4/C3N4 20% wt. 6 SO/0.12B-0.2P-C3N4 Ag-N doped C3N4-G-50 TiO2 catalyst embedded in a Nafion membrane TiO2 nanoparticles in porous cavities of commercial Nafion membranes Membrane microreactor TiO2/Y-zeolite anchored on Vycor glass Ti-mesoporous zeolite Ti-b(OH) and Ti-b(F) TiO2 and 2% Cu/TiO2 in NaOH solution Cu/TiO2 film supported on optical-fiber Cu/TiO2 and Ag/TiO2 film supported on opticalfiber TiO2 anatase TiO2 polymorphs TiO2 polymorphs phtalocyanines/TiO2

Reactor configuration

Continuous Batch Batch Continuous Batch Batch Batch Batch Continuous Batch Batch Batch Continuous Batch Continuous Batch Batch Continuous Continuous

MeOH

Flow rate/catalyst weight, µmol gcatalyst-1 h-1 EtOH HCHO Acetone HCOOH

0.9-17.9

0.3-14.9

0-27

0-1.8

25

0

CH4

CO

0 5

0

47

5.2

50

0.62-1 traces

51 52

12

53

102

51

0.34 21

2

46 47

1118 4.3

6.5

53 50

6.25-7.5 50 0

(supercritical CO2 as feed)

111 5 3.5 5.9 0.78 0.45

Continuous

4.12

Batch Continuous Continuous Batch

0.075

48 49

0.75 42.7

Continuous Batch Batch Batch Batch Continuous

48 49

0.28

20 12.6-45 56

This work 46

1250 0.5

0.26 10 Traces 0.28

Ref.

52

0

0

traces

0

0

8 54

38

55

8 7.5 1

56 57 58 59 60 61

3.15 2.1 26

0.40 2.13

62 63 64 65

*under UV light 22

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Conclusions In this work, photocatalytic CO2 conversion was carried out, as yet not in the literature, in a

continuous photocatalytic reactor with an exfoliated C3N4-based membrane irradiated by UV light. The effect of H2O/CO2 feed molar ratio and contact time on species production, reaction selectivity and converted carbon were investigated. Total converted carbon/catalyst weight ratio varied between 5 to 47.6 µmol gcatalyst-1 h-1, with the latter value obtained at an H2O/CO2 feed molar ratio of 5 and 2 s as contact time with alcohols as prevailing product. The membrane reactor converted at least 10 times more carbon than the batch system, as a result of the better dispersion of catalyst which is embedded in the Nafion matrix. Overall, alcohols production was promoted by a low contact time as a result of the fast removal of the reaction mixture from the reaction volume exposed to UV light and, thus, with less promotion of oxidation and/or secondary reactions. On the contrary, the slow removal caused a partial oxidation of MeOH and EtOH, favoring HCHO production. In all the cases, a water defect corresponded to a larger HCHO production, reaching a flow rate of 27 µmol gcatalyst-1 h-1 at an H2O/CO2 feed molar ratio equal to 0.5. The highest MeOH and EtOH selectivities were 54.6% and 45.4 % respectively, at H2O/CO2 feed molar ratio equal to 5 and contact time of 2 s, representing 100% carbon-containing products. A really interesting alcohols production rate of 32.8 µmol gcatalyst-1 h-1 was obtained at the best operating conditions.

Acknowledgements The research project PON 01_02257 “FotoRiduCO2 - Photoconversion of CO2 to methanol fuel”, cofunded by MiUR (Ministry of University Research of Italy) with Decreto 930/RIC 09-11-2011 in the framework the PON “Ricerca e competitività 2007-2013”, is gratefully acknowledged.



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Synopsis The CO2 photocatalytic reduction in a membrane reactor, a green chemistry process, promotes renewable energy production, mitigating the anthropogenic CO2 emissions.


Photocatalytic Conversion in a Continuous Membrane Reactor

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