graphene oxide nanocomposites

Chemical Engineering Journal 263 (2015) 374–384

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Post-combustion CO2 capture using mesoporous TiO2/graphene oxide nanocomposites Shamik Chowdhury, Ganesh K. Parshetti, Rajasekhar Balasubramanian ⇑ Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576, Republic of Singapore

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 TiO2/GO nanocomposites with

different GO content prepared by colloidal blending.  Synthesized nanocomposites systematically evaluated as adsorbent for CO2 capture.  TiO2/GO have CO2 adsorption capacity of up to 1.88 mmol g1 at 25 °C.  TiO2/GO feature fast CO2 uptake kinetics and high CO2/N2 selectivity.  The nanocomposite also exhibit low energy consumption for regeneration.

a r t i c l e

i n f o

Article history: Received 19 August 2014 Received in revised form 3 November 2014 Accepted 5 November 2014 Available online 13 November 2014 Keywords: CO2 capture Graphene oxide Titanium dioxide Nanocomposite Adsorption equilibrium Selectivity

a b s t r a c t The development of robust adsorbents with high adsorption capacity and high selectivity to separate CO2 from flue gas streams, a major greenhouse gas contributing to global warming, is a topic of current global interest. In this study, a series of mesoporous titanium dioxide/graphene oxide (TiO2/GO) nanocomposites, with different GO to TiO2 mass ratios, were synthesized using a simple colloidal blending process and systematically investigated for the first time as potential CO2 adsorbent materials. The synthesized composites were characterized by diffraction, spectroscopy, and microscopy techniques. Pure component adsorption isotherms of CO2 were measured at 0, 25, and 50 °C and pressures up to 100 kPa. Analysis of adsorption results showed that the CO2 adsorption isotherms could be well fitted to the temperature dependent Toth model. The adsorption kinetic data were well-represented by the Avrami model. TiO2/ GO exhibited a CO2 uptake capacity of 1.88 mmol g1 at room temperature, which is much higher than many other commonly used adsorbents. Moreover, TiO2/GO also demonstrated a low heat of adsorption and remarkably high CO2/N2 selectivity and hence merits further consideration for capturing CO2 from dry flue gas. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Carbon capture and storage (CCS) can significantly reduce the amount of anthropogenic energy-related CO2 emissions, and

⇑ Corresponding author. Tel.: +65 65165135; fax: +65 67744202. E-mail address: [email protected] (R. Balasubramanian). http://dx.doi.org/10.1016/j.cej.2014.11.037 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

contribute to global climate change mitigation. Therefore, the deployment of CCS-based technology on fossil-fueled power plants, the single largest point source of CO2 emissions, is of major interest [1]. Currently, three different strategies are being considered to capture CO2 from coal-fired plants and large industrial sources: post-combustion capture (capturing CO2 from flue gas following normal combustion); pre-combustion capture (removing CO2 from fossil fuels before combustion, e.g. following gasification

S. Chowdhury et al. / Chemical Engineering Journal 263 (2015) 374–384

of solid fuel); and oxyfuel combustion systems (adjusting the combustion conditions to generate flue gas with easily separable CO2) [2]. Among the different CCS methods available, post-combustion capture appears to be the most feasible approach as it can be retrofitted to existing power plants without upgrading or modifying the existing systems [3]. Accordingly, much effort has been expended to develop various physical and chemical methods for post-combustion CO2 capture, including chemical absorption, membrane separation, cryogenic fractionation, and pressure swing adsorption (PSA) [3]. Among these techniques, PSA has been receiving increasing attention since both chemical absorption and cryogenic separation have high energy requirements, while most membrane-based separation methods are still in their infancy. In addition, PSA is highly energy efficient and offers several competitive advantages [4]. Although many different types of adsorbents, including zeolites, activated carbons, metal–organic frameworks (MOFs), organic–inorganic hybrids, calcium oxides, and hydrotalcites, have been comprehensively studied for CO2 capture [5], each of these materials has its own intrinsic limitations. Therefore, the development of robust adsorbents with high adsorption capacity, rapid uptake, high selectivity for CO2 over other components in flue gas, and minimal energy penalty for regeneration, remains a significant challenge. Recently, mesoporous titanium dioxide (TiO2) has attracted tremendous research interests worldwide as an effective adsorbent for a wide range of applications. Previous studies indicated that TiO2 has a high adsorption affinity toward potentially toxic elements (e.g. As, Cd, Cr, etc.), volatile organic compounds, and certain gases (e.g. H2, CO, CO2, NH3, etc.) [6–9]. Because of its unsaturated surface atoms, TiO2 exhibits very high adsorption capacity and size-selective adsorption of molecules. In addition, the appreciable surface area and very tight pore size distribution of TiO2 result in achieving quantitative adsorption in short time [7]. Moreover, the uniform mesopore channels of TiO2 augment the density of active sites with high accessibility and also facilitate the diffusion of atoms, thereby ensuring fast adsorption uptake [10]. Progress in materials science and engineering suggests that further improvements in the adsorption characteristics of TiO2 can be achieved by the immobilization of TiO2 particles onto nanostructured carbon materials (e.g. C60, carbon nanotubes, and graphene materials) because of their unique morphology, nanosized scale and intriguing physico-chemical properties [11]. In particular, graphene oxide (GO), a perfectly functionalized graphene, is a promising candidate for supporting metal or metal oxide nanoparticles due to the significant population of oxygen-functionalities on its graphitic backbone [12,13]. It is, therefore, of great interest to develop a suitable new adsorbent for CO2 removal from post-combustion flue by combining TiO2 with GO. In the present work, mesoporous TiO2/GO nanocomposites were synthesized by a simple one-step colloidal blending method and examined as potential low cost adsorbents for CO2 capture. Equilibrium adsorption of CO2 was measured volumetrically as a function of temperature and pressure and analyzed using suitable isotherm models. The effect of GO content on the CO2 uptake was studied by altering the GO to TiO2 mass ratio in the TiO2/GO hybrid. Additionally, the isosteric heat of adsorption and Henry’s law constant was evaluated to characterize the adsorption behavior and understand the affinity of the adsorbent for the adsorbate. Finally, the kinetics of CO2 uptake was investigated to evaluate the potential of TiO2/GO composites for postcombustion CO2 capture. To the best of our knowledge, this is the first systematic investigation of CO2 adsorption by TiO2/GO hybrids.

375

2. Experimental 2.1. Materials Graphite powder (<20 lm) was purchased from Sigma–Aldrich and used as received. Sulfuric acid (98 wt.%, H2SO4, Merck), phosphoric acid (85 wt.%, H3PO4, J.T. Baker), potassium permanganate (KMnO4, Acros Organics), hydrogen peroxide (30 wt.%, H2O2, Sigma–Aldrich), and titanium(IV) oxide (TiO2, Acros Organics) were used as obtained from the supplier without any further purification. Deionized water was used during the experimental process. 2.2. Synthesis of GO GO was prepared from natural graphite powder using an improved Hummer’s method [14]. The detailed procedure is presented in the Supplementary information. 2.3. Synthesis of TiO2/GO nanocomposites TiO2/GO nanocomposites were prepared by a one-step colloidal blending method [15]. In a typical procedure, a weighed amount of GO was dispersed into 50 mL of deionized water by ultrasonication for 0.5 h, and then 1 g TiO2 was added to the as-prepared GO suspension. The mixture was then sonicated for 1.5 h and further stirred at room temperature for 12 h to produce TiO2/GO homogeneous dispersion. The product was filtered and dried in air at room temperature. By changing the weight of GO into 1 g TiO2, a series of TiO2/GO composites denoted as ‘‘TiO2/GO-X’’ (where X represents the GO to TiO2 mass ratio = 0.10, 0.20 and 0.30) were prepared. 2.4. Materials characterization The structure and surface morphology of the synthesized materials was investigated by field-emission scanning electron microscopy (FE-SEM), Raman spectroscopy, wide angle X-ray diffraction (XRD), and N2 adsorption–desorption measurements using the Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halenda (BJH) methods. The surface chemistry of the materials was analyzed by X-ray photoelectron spectroscopy (XPS). Detailed information about the characterization techniques can be found in the Supplementary information. 2.5. CO2 adsorption measurements CO2 adsorption equilibrium and kinetics of the synthesized samples were measured volumetrically in a Micromeritics ASAP 2020 adsorption apparatus. The adsorption isotherms were obtained at three different temperatures (0, 25 and 50 °C) and pressures up to 100 kPa. The adsorption temperature was controlled by using a Dewar bottle with a circulating jacket connected to a thermostatic bath utilizing water as coolant. About 100 mg of adsorbent sample was used for the CO2 adsorption studies. Before each adsorption experiment, all the samples were degassed at room temperature under vacuum for 24 h to desorb any moisture and organics. Ultra high purity (UHP) grade CO2 gas (99.9%) was used during the study. The CO2 adsorption kinetics (adsorption amount as a function of time) was also measured in the Micromeritics ASAP 2020 system using a built-in function (‘‘Rate of Adsorption’’) at the same time when the adsorption equilibrium data were collected. The change in gas pressure and adsorption volume with time after the CO2 reservoir was connected to the sample chamber

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3.1. Materials characterization FE-SEM of GO prepared by the improved Hummer’s method revealed a randomly distributed and loosely stacked structure with a crumpled appearance (Fig. 1a and b). The EDX elemental microanalysis identified C and O as the only elements contained in GO, which was consistent with the molecular structure of GO. TEM images were also captured to further investigate the morphological features of the GO nanosheets (see Supplementary information Fig. S1). It was observed that the prepared GO nanosheets were transparent with a well-defined single layered structure. The GO nanosheets were also folded and wrinkled which might have originated due to various oxygen-functional groups, including carboxyl, hydroxyl and epoxy groups, introduced during the oxidation process [16]. It appears that the wrinkled morphology can reduce the surface energy of GO nanosheets and thereby make these 2D nanosheets stable [17]. The XRD profile of GO showed a distinct peak from (0 0 2) diffraction at 10.8° corresponding to a d-spacing of 8.1 Å compared with graphite’s major peak from (0 0 2) at 26.5° corresponding to a d-spacing of 3.3 Å (Fig. 2a). The large interlayer spacing of GO might be attributed to the intercalation of oxygen functional groups and water molecules during the Hummer’s process [18]. Fig. 3a illustrates the XPS wide-scan survey spectrum of the as-prepared GO. Only C1s and O1s peaks were detected at 285 and 533 eV, respectively. The high resolution C1s XPS spectrum of GO presented a well-defined double-peak feature (Fig. 3e), indicating a high oxidation level of GO [19]. The peak

(b)

(a)

1000 nm m

1 µm (d)

(c)

GO 1 µm (e)

GO

1 µµm (f) GO

1 µm

1 µm

Fig. 1. SEM images of GO (a and b), pure TiO2 (c), TiO2/GO-0.10 (d), TiO2/GO-0.20 (e) and TiO2/GO-0.30 (f).

(0 0 2)

(0 0 2) Intensity (a.u.)

3. Results and discussion

(a)

Intensity (a.u.)

was recorded and then converted into transient adsorption uptakes to generate the adsorption kinetics. The adsorption equilibrium amount was considered as the final adsorption amount at the terminal pressure and temperature.

(1 0 1)

(0 0 4)

(1 0 0)

10

20

30

40

50

60

70

2θ (degree)

10

20

30

40

50

60

70

2θ (degree)

(b)

A (1 0 1)

A (0 0 4) A (1 0 5) A (2 1 1)

R (1 1 0)

TiO2/GO − 0.30

Intensity (a.u.)

376

R (1 0 1)

A (2 0 0) A (0 0 2)

TiO2/GO − 0.20

TiO2/GO − 0.10

Pure TiO2

10

20

30

40

50

60

70

2θ (degree) Fig. 2. XRD patterns of GO (a), pure TiO2, TiO2/GO-0.10, TiO2/GO-0.20, and TiO2/GO0.30 (b). The inset in (a) shows the XRD pattern of pure graphite. [A = Anatase; R = Rutile].

at 285.0 eV could be ascribed to CAC, C@C, and CAH bonds on the graphene frame while the peak at 287.1 eV could be assigned to C@O bonds. The minor peak at 288.7 eV might be due to OAC@O bonds. Raman spectrum of the synthesized GO showed two prominent absorption bands (see Supplementary information, Fig. S2): the G-band at 1596 cm1 due to the first order scattering of the E2g phonon of sp2 C atoms and the D-band at 1350 cm1 originating from the breathing mode of j-point photons of A1g symmetry [20]. Moreover, the intensity ratio of the D band to G band (ID/IG = 0.77) was identical to that reported in the literature [21]. All these results demonstrated the successful synthesis of GO nanosheets. The measured specific surface area of GO was about 32 m2 g1. This value was much less than the theoretical value of an individual graphene sheet (2630 m2 g1), which can be attributed to the strong hydrogen bonding that causes tight sheet associations [22,23]. The effective deposition of TiO2 on GO nanosheets, via colloidal blending to yield TiO2/GO-X nanocomposites, was further investigated. The representative FE-SEM images of pure TiO2, TiO2/GO0.10, TiO2/GO-0.20, and TiO2/GO-0.30 are shown in Fig. 1c–f. A good distribution of TiO2 on GO nanosheets seems to have occurred during preparation of the nanocomposites. The TiO2 particles formed large agglomerates on the top of GO nanosheets, with bigger and rigid agglomerates in the composite with the highest GO content (i.e. TiO2/GO-0.30). The EDX spectra of TiO2/GO-0.10, TiO2/GO-0.20 and TiO2/GO-0.30 showed the coexistence of C, O and Ti in the synthesized nanocomposites, further corroborating the successful deposition of TiO2 on GO sheets. The XRD pattern

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(a)

(1) GO (2) TiO2

(b)

O1s

(1) TiO2/GO − 0.10 (2) TiO2/GO − 0.20

O1s

Ti2p

(2)

C1s (1)

1200

1000

800

600

400

200

Ti2p

(3)

Intensity (cps)

Intensity (cps)

(3) TiO2/GO − 0.30

C1s (2)

(1)

0

1200

1000

Binding Energy (eV)

(c)

(1) GO (2) TiO2

800

600

400

200

0

Binding Energy (eV)

(d)

458.90

(1) TiO2/GO − 0.10

458.90

(2) TiO2/GO − 0.20

Intensity (cps)

Intensity (cps)

(3) TiO2/GO − 0.30

464.60 (2)

(2)

(1)

(1)

468

464.60 (3)

466

464

462

460

458

456

468

466

464

460

458

456

Binding Energy (eV)

Binding Energy (eV)

(e)

462

(f)

C=O

C-C

Intensity (cps)

Intensity (cps)

C-C

O=C-O

290

288

286

284

282

Binding Energy (eV)

C=O O=C-O

290

288

286

284

282

Binding Energy (eV)

Fig. 3. XPS survey scan spectra of GO, TiO2, TiO2/GO-0.10, TiO2/GO-0.20, and TiO2/GO-0.30 (a and b), Ti 2p XPS spectra of GO, TiO2, TiO2/GO-0.10, TiO2/GO-0.20, and TiO2/GO0.30 (c and d), and high resolution C1s XPS spectra of GO (e) and TiO2/GO-0.10 (f).

of the composites exhibited crystal composition similar to that of pure TiO2, consisting primarily of the anatase and rutile TiO2 phases (Fig. 2b). The peaks at 25.4°, 37.8°, 48.3°, 54.2°, 55.3°, and 62.8° could be assigned to the diffractions of the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), and (0 0 2) crystal planes of the anatase phase in TiO2 (JCPDS card No. 21-1272), and the characteristic diffraction peaks observed at 27.6° and 36.3° might be due to the (1 1 0) and (1 0 1) faces of rutile phase in TiO2 (JCPDS card No. 21-1276).1 However, the characteristic diffraction peak of GO was absent, mostly because of the stack disorder caused by the intercalation of TiO2

1

Joint Committee on Powder Diffraction Standards (JCPDS).

particles into stacked GO layers [24]. The XPS measurements of pure TiO2, TiO2/GO-0.10, TiO2/GO0.20, and TiO2/GO-0.30 were further carried out to study the chemical state of elements in the synthesized composites, and the results are shown in Fig. 3. The survey spectra clearly showed the existence of C, O and Ti in the TiO2/GO-X hybrid materials (Fig. 3b). The deconvoluted Ti2p spectra revealed the Ti2p3/2 and Ti2p1/2 spin–orbital splitting photoelectrons of Ti4+, identical to those of pure TiO2, at binding energies of 458.9 and 464.6 eV, respectively (Fig. 3c and d). The splitting between these bands was 5.7 eV, implying the presence of the normal state of TiO2 in the prepared TiO2/GO nanocomposites [23]. Nevertheless, the non-appearance of representative peaks centered at 466 and

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(a) Quantity Adsorbed (mmol/g)

18

Table 1 Specific surface area, pore volume and the average pore size of pure TiO2 and TiO2/ GO-X composites, estimated from N2 adsorption–desorption measurements using BET and BJH methods.

Adsorption Desorption

16 14

Sample

SBET (m2 g1)a

Vt (cm3 g1)b

Dp (nm)c

12

TiO2 TiO2/GO-0.10 TiO2/GO-0.20 TiO2/GO-0.30

45.684 99.536 87.767 83.121

0.158 0.382 0.297 0.269

18.188 30.856 24.909 21.973

10 8 a

6

b c

4

BET specific surface area. Total pore volume. Average pore diameter.

2 0

2.5

0.2

0.4

0.6

0.8

1.0 0.8 0.6 0.4

TiO2/GO − 0.10 TiO2/GO − 0.20 TiO2/GO − 0.30

1.5

1.0

0.5

0.0 o

0.2 0.0 0

10

20

30

40

50

60

70

80

Pore Width (nm) Fig. 4. (a) N2 adsorption–desorption isotherms for TiO2/GO-0.10 at 196.15 °C, (b) pore size distribution of TiO2/GO-0.10 using NLDFT.

460 eV suggested the absence of TiAC bond in the samples. Further peak deconvolution of the C1s spectra showed that the contribution of oxygen-containing bands (C@O and OAC@O) in C1s core levels significantly decreased after combination with TiO2 (Fig. 3f). Such variations in the chemical state of GO in TiO2/ GO systems have been previously reported [24,25]. N2 adsorption–desorption data were also recorded for textural analysis of the synthesized TiO2/GO-X nanocomposites. According to IUPAC classification, the TiO2/GO-0.10, TiO2/GO-0.20, and TiO2/ GO-0.30 samples generated a Type IV isotherm, typical of mesoporous compounds (Fig. 4a). In addition, the composites did not exhibit any limiting adsorption at high P/Po, which is typical of a Type H3 hysteresis loop often associated with the presence of aggregates of plate-like particles forming slit-shaped pores [26]. In this case, it could be correlated to GO nanosheets coated with TiO2, as clearly observed in the FE-SEM images (Fig. 1). The BET specific surface area of the nanocomposites was considerably larger compared to that of pure TiO2 powder, suggesting that GO could be considered as an excellent 2D support for the immobilization of TiO2 (Table 1). Also, the deposition of TiO2 on GO nanosheets resulted in a noticeable increase of the total pore volume, suggesting that enhanced porosity was created when TiO2 was in contact with GO sheets. Fig. 4b depicts the pore size distribution obtained by applying the NLDFT method,2 which suggests that the pore volume in the composites was mainly due to the presence

2

2.0

25 C o 50 C

2.0

Non-Local Density Functional Theory.

CO2 Adsorbed (mmol/g)

Differential Pore Volume (cm3/g)

Relative Pressure (P/P0)

(b)

GO Pure TiO2

1.0

CO2 Adsorbed (mmol/g)

0.0

1.5

1.0

0.5

0.0 0

20

40

60

80

100

Pressure (kPa) Fig. 5. CO2 adsorption isotherms for GO, TiO2, TiO2/GO-0.10, TiO2/GO-0.20 and TiO2/GO-0.30 at 0 °C (up) and CO2 adsorption isotherms at 25 and 50 °C for TiO2/ GO-0.10 (below).

of mesopores. These findings are consistent with those from previous studies [27,28]. However, a gradual increase in the GO content led to deterioration of the textural properties of the composites which might be due to agglomeration of GO at higher concentrations [29]. Therefore, the molar ratio of GO to TiO2 powder is a critical factor to regulate the content of GO and TiO2 in the final composites. Accordingly, TiO2/GO-0.10 resulted in the highest specific surface area and maximum pore volume. 3.2. CO2 adsorption isotherms Low pressure static CO2 adsorption experiments were initially conducted at 0 °C. Fig. 5 presents the CO2 adsorption isotherms of GO, pure TiO2, TiO2/GO-0.10, TiO2/GO-0.20, and TiO2/GO-0.30. In general, the CO2 adsorption capacity of the TiO2/GO nanocomposite systems was noticeably higher than that of both GO and TiO2 over the whole pressure range. The better adsorption properties of the TiO2/GO-X composites could be attributed to a synergistic effect between the GO nanosheets and TiO2 particles,

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resulting in increase in the specific surface area as well as the mesopore volume, which ultimately leads to significant enhancement in the adsorption of CO2. Meanwhile, the amount of CO2 adsorbed on the TiO2/GO-X nanocomposites increased in the order: TiO2/GO0.30 < TiO2/GO-0.20 < TiO2/GO-0.10. This trend might be explained by the higher specific surface area and larger pore volume of TiO2/ GO-0.10 compared to the other samples (Table 1). With increasing pressure, the textural features of the adsorbent become progressively critical for CO2 uptake, because high surface area implies that there are many active sites available for adsorption and large pore volume represents adequate space for storage of CO2 [30]. In addition, the high uptake capacity of TiO2/GO-0.10 could also be due to the condensation of CO2 in the space available within the mesopores. TiO2/GO-0.10 was further tested for CO2 adsorption at high temperature conditions of up to 50 °C and the adsorption isotherms are shown in Fig. 5. The amount of CO2 adsorbed on TiO2/ GO-0.10 decreased with increasing temperature which was due to increase in the thermal energy of CO2 molecules at elevated temperatures, leading to lesser adsorption [31]. To examine the CO2 adsorption uptake of competing adsorbents, the CO2 uptake capacity of TiO2/GO-0.10 and other adsorbent materials reported in literature was compared (Table 2). It should be noted that the maximum amount of CO2 uptake by various sorbents varies as a function of experimental conditions. Both the temperature and pressure have a profound effect on the amount of CO2 uptake per unit sorbent. Therefore, for a direct and meaningful comparison, the maximum amount of CO2 adsorbed on TiO2/GO-0.10 in this study was compared to that of other reported sorbents under similar experimental conditions. TiO2/GO-0.10 showed appreciable CO2 adsorption performance in comparison to zeolites, activated carbons, some MOFs and mesoporous alumina. 3.3. Correlation of isotherms Adsorption equilibrium models can accurately simulate the experimental data over a wide range of temperature and pressure conditions. Many such models are available, and the traditional single component Langmuir isotherm with two parameters and the Toth model with three parameters were adopted to correlate the adsorption equilibrium of CO2 on TiO2/GO-0.10 at low pressures. The Langmuir isotherm assumes that there is only one type of binding site with equal affinity and has the following formulation:

q¼

qm K L P 1 þ KLP

ð1Þ

where q (mmol g1) is the amount of CO2 adsorbed, qm (mmol g1) is the maximum adsorption capacity, P (kPa) is the CO2 equilibrium pressure, and KL (kPa mmol1) is the Langmuir equilibrium constant. The Toth isotherm is commonly applied to describe the adsorption equilibria of gases and vapors because of its correct behavior at both low (Henry-type limit) and high (saturation limit) pressure ranges, and is expressed as:

qs bP

q¼

ð2Þ

t 1=t

½1 þ ðbPÞ 

where qs (mmol g1) is the saturation loading, b (kPa1) describes the adsorption affinity, and t is a measure of adsorbent heterogeneity. The Toth isotherm parameters qs, b and t are temperaturedependent according to the following equations:

qs ¼ qs;0 exp

b ¼ b0 exp

   T v 1 T ref

ð3Þ

   DHads T ref 1 Rg T ref T

ð4Þ

  T ref t ¼ t0 þ a 1  T

ð5Þ

where qs,0, b0 and t0 are the corresponding parameters at a selected reference temperature Tref (K), DHads (kJ mol1) is a measure of the heat of adsorption, Rg is the ideal gas constant (8.314 J mol1 K1), and a and v are the fitting parameters. The fitting of the models to the experimental equilibrium data was performed by a nonlinear regression analysis using the software Origin Pro 8.0 (OriginLab, Northampton, MA) (Fig. 6). The model constants and correlation coefficients (R2) thus obtained are listed in Table 3. Taking Tref = 298 K, the temperature-dependent Toth parameters were estimated via a least square analysis and are also given in Table 3. The results suggest that both Langmuir and Toth models provided a good agreement with the experimental equilibrium data. However, the Toth isotherm showed a more accurate correlation than the Langmuir model due to its adjustable parameter and good simulation behavior over the entire temperature and pressure range [43]. Moreover, the Langmuir isotherm does not take into consideration the heterogeneity of binding sites, which appears to be practically incorrect, particularly for heterogeneous materials like graphene-based nanocomposites. The applicability of the Toth model to the adsorption isotherm measurements for pure CO2 on TiO2/GO-0.10 indicated the existence of multi-molecular layer adsorption.

Table 2 Comparison of the CO2 adsorption capacity of TiO2/GO-0.10 with different adsorbents. Material

T (°C)

P (bar)

CO2 capture capacity (mmol g1)

Method

Reference

Acid treated bentonitic clay Cu-MOF c-Alumina Zeolite-like MOF (sodalite) Mesocarbon microbeads Activated graphite fibers Wood based activated carbon Ammonia treated activated carbon b-Zeolite Mesoporous MgO Activated carbon from olive stones Microwave activated carbon N-doped activated carbon TiO2/GO-0.10

25 25 25 25 25 25 25 25 30 25 25 25 25 25

1 1 1 1 1 1 1 1 1 1 1 1 1 1

0.58 0.65 0.68 1.20 1.22 1.3 1.5 1.7 1.76 1.8 2.0 2.1 2.3 1.88

Volumetric analysis Volumetric analysis Volumetric analysis Volumetric analysis Thermogravimetric analysis Volumetric analysis Thermogravimetric analysis Thermogravimetric analysis Volumetric analysis Thermogravimetric analysis Thermogravimetric analysis Volumetric analysis Thermogravimetric analysis Volumetric analysis

Venaruzzo et al. [32] Bao et al. [33] Chen and Ahn [30] Chen et al. [34] Peng et al. [35] Meng and Park [36] Pevida et al. [37] Pevida et al. [37] Xu et al. [38] Bhagiyalakshmi et al. [39] Plaza et al. [40] Yi et al. [41] Pevida et al. [42] This study

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As discussed in Section 3.2, the Toth equation showed an excellent fit to the equilibrium isotherm data. Therefore by applying Eq. (6) to the Toth model, KH can be calculated as follows:

2.5

CO2 Adsorbed (mmol/g)

o

0 C o 25 C o 50 C Langmuir Toth

2.0

1.5

KH ¼

qm b

ð7Þ

1=t

Nevertheless, the empirical element of such equations often introduces a singularity which makes them behave improperly in the limit as P ? 0, thereby overestimating the Henry’s constant [46]. Hence the Virial model (Eq. (8)) was considered to estimate the Henry’s law constant of CO2 on TiO2/GO-0.10 [47].

1.0

0.5

  P 1 3 4 ¼ exp 2A1 q þ A2 q2 þ A3 q3 þ    q KH 2 3

0.0 0

20

40

60

80

100

Pressure (kPa) Fig. 6. Nonlinear fit of Langmuir and Toth isotherm models to the experimental CO2 adsorption equilibrium data of TiO2/GO-0.10 at different temperatures.

The magnitude of the Toth parameter t describes the system heterogeneity as well as the interactions and mobility (immobility) of the molecules adsorbed. If t equals 1, the Toth equation is transformed to the Langmuir model. If t deviates significantly from unity, the gas–solid adsorption system is heterogeneous. On the other hand, if lateral interactions between the adsorbed molecules are stronger than the adsorptive potential, t is greater than unity [44]. In the present study, the t values greatly deviated from unity, which indicated a strong degree of surface heterogeneity for the adsorption of CO2 on TiO2/GO-0.10. The greater heterogeneity increased the affinity of binding sites for target molecules, with higher adsorption affinity at lower temperatures (see Table 3). 3.4. Henry’s law constant The interaction between the adsorbent surface and the adsorbed gas molecules is characterized by the Henry’s law constant, since at low pressure molecule-surface forces are predominant, and is usually calculated from the slope of the adsorption isotherm at the limit of zero loading i.e. the Henry’s law region [45]:

  q dq K H ¼ lim ¼ lim P!0 P P!0 dP

ð6Þ

where KH (mmol g1 kPa1) is the Henry’s law constant and q (mmol g1) is the loading. A higher value of KH indicates a stronger interaction between the adsorbate-adsorbent pair.

ð8Þ

where A1, A2, A3 are the Virial coefficients. A plot of ln (P/q) vs q should approach the axis linearly as q ? 0 with slope 2A1 and intercept ln (KH). The Virial plot method is a reliable technique to compute the Henry’s law constant with high accuracy. In addition, it omits the constraint of the saturation concentration. The Henry’s constant for CO2 adsorption on TiO2/GO-0.10, determined from the Virial plots, was found to be 5.1  102, 4.3  102, and 2.9  102 mmol g1 kPa1 at 0, 25 and 50 °C respectively. The high KH values could be reasonably attributed to the strong equilibrium affinity of CO2 for TiO2/GO-0.10 at low coverage; the gas molecules interact with the adsorbent surface but interactions between adsorbed molecules are negligible. In addition, KH decreased substantially with increasing adsorption temperature, which was obvious. It seems that adsorption at low pressures is sensitive to temperature leading to considerable changes in CO2 uptake characteristics of TiO2/GO-0.10. 3.5. Kinetics of CO2 adsorption Efficient CO2 capture from flue gases and other gas mixtures requires an adsorbent that demonstrates not only a large CO2 uptake but also fast adsorption kinetics. Therefore the CO2 uptake kinetics of TiO2/GO-0.10 was also evaluated. Fig. 7 presents the CO2 adsorption kinetic data of TiO2/GO-0.10 at 0, 25 and 50 °C. The rate of adsorption was very fast, and the adsorbent attained saturation levels in about 3 min. This finding implies that CO2 can be effectively separated while operating with short adsorption cycle times, which would be highly advantageous for practical industrial applications. However, a decrease in the rate of CO2 adsorption on TiO2/ GO-0.10 was noted with increasing temperature. The observed trend was consistent with the equilibrium isotherm results. In order to quantitatively describe the CO2 adsorption kinetics and identify the possible adsorption mechanism, the Avrami model

Table 3 Isotherm parameters and kinetic constants for adsorption of CO2 on TiO2/GO-0.10 at different temperatures. T (°C)

Langmuir qm (mmol g1)

0 25 50

3.964 3.398 2.265

Toth

0.011 0.011 0.008

Temperature dependent Toth isotherm parameters Tref (K) qs,0 (mmol g1) 298 3.825 Avrami model constants T (°C) Ce,exp (mmol g1) 0 25 50

R2

KL (kPa mmol1)

2.193 1.820 1.253

0.993 0.991 0.991

qs (mmol g1)

b (kPa1) 2

5.063 3.825 2.472

2.918  10 1.409  102 1.045  102

t

R2

0.686 0.665 0.581

0.999 0.999 0.999

v

b0 (kPa1)

DHads (kJ mol1)

t0

a

3.061

1.409  102

19.603

0.665

0.233

Ce,cal (mmol g1)

kA (s1)

nA

R2

2.251 1.859 1.315

0.028 0.033 0.024

0.539 0.557 0.512

0.995 0.994 0.994

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S. Chowdhury et al. / Chemical Engineering Journal 263 (2015) 374–384

-20

2.0

-16

Qst (kJ/mol)

CO2 Adsorbed (mmol/g)

2.5

1.5

1.0 o

0 C o 25 C o 50 C Avrami

0.5

-12

-8

0.0 0

50

100

150

200

250

-4 0.0

300

Time (s)

was used for fitting the experimental data since it considers both physical and chemical interactions between CO2 and the adsorbent, which is particularly important for functionalized adsorbent materials such as TiO2/GO-0.10. The Avrami equation is expressed as follows [48]: nA

C t ¼ C e ½1  eðkA tÞ 

ð9Þ

1

where Ct (mmol g ) is the amount of CO2 adsorbed at time t, Ce (mmol g1) is the amount of CO2 adsorbed at equilibrium, kA (s1) is the Avrami kinetic constant and nA is the Avrami exponent. The estimated model parameters and the corresponding correlation coefficients are given in Table 3. The exceedingly high R2 values confirmed that the CO2 adsorption kinetic data could be well represented by the Avrami model over the entire adsorption period (Fig. 7). Also, the calculated Ce,cal values showed a good agreement with the experimental Ce,exp values, further demonstrating the applicability of this model (Table 3). The kinetic constant at 0 and 25 °C was higher than the one at 50 °C, which might be due to the fact that physisorption was predominant at low temperatures. Moreover, the decrease of Avrami exponent with temperature indicated that the adsorption of CO2 on TiO2/GO-0.10 was less contact time dependent at elevated temperature. 3.6. Energetics of CO2 adsorption The isosteric heat of adsorption (Qst, kJ mol1), which reflects the nature of adsorbate-adsorbent and adsorbate–adsorbate interactions, is extensively used for evaluating the energetic heterogeneity of the adsorption system. Estimation of the isosteric heat of adsorption is also very important for designing PSA systems [31]. Therefore, by using the temperature-dependent Toth isotherm parameters, the isosteric heat of adsorption was determined from the van’t Hoff equation as follows:

Q st ¼ Q 0;st ð¼ DHads Þ 

aRg T ref t

ln

q

ðqts  qt Þ1=t

0.4

0.6

0.8

1.0

CO2 Fractional Loading (q /qs)

Fig. 7. CO2 adsorption kinetics at different temperatures on TiO2/GO-0.10 and Avrami model fit to the experimental data.

( "

0.2

#

" 

lnðq=qs Þ

#)

1  ðq=qs Þ1=t ð10Þ

From Eq. (10), it appears that the isosteric heat is equal to the heat of adsorption at zero fractional loading (i.e. q/qs = 0). Isosteric heats for CO2 at 25 °C on TiO2/GO-0.10 are shown as a function of fractional loading in Fig. 8. The Qst values were all negative and tend to be less negative with increasing surface loading (q), indicating the exothermic nature of adsorption as well as

Fig. 8. Isosteric heat of adsorption of pure CO2 on TiO2/GO-0.10 as a function of fractional loading.

decrease in the strength of CO2-TiO2/GO-0.10 interaction with q. It could be explained that adsorption sites with the highest affinity for CO2 are occupied first while sites with weaker affinities are occupied at higher loadings. At zero loading, the isosteric heat was approximately 19.6 kJ mol1. In comparison with many other competitive adsorbents such as zeolites and MOFs, which have heats of adsorption between 20 and 50 kJ mol1 [49,50], the zero coverage isosteric heat of TiO2/GO-0.10 is remarkably lower, which is likely to result in a lower energy penalty for adsorbent regeneration and desorption of the captured CO2 [51]. 3.7. Adsorbent selectivity Apart from attaining a high adsorption capacity and fast uptake kinetics, an effective adsorbent that can be used within CO2 capture systems must also have high selectivity so that the CO2 component of the polluted gas stream is totally removed for subsequent compression with no or minimal interference from other major gaseous species, especially N2. Moreover, the development of a CO2 selective adsorbent is preferable since pure CO2 can be extracted from the adsorbent and utilized as another carbon source for industry. This would make the CCS process economically feasible. The most basic approach for examining the adsorptive selectivity of a material for CO2 from a gas mixture is the estimation of the selectivity factor, defined as the molar ratio of the adsorption quantities at the relevant partial pressure of the gases [51]. In post-combustion CO2 capture systems, the partial pressures of CO2 and N2 are 15 and 75 kPa, respectively. The selectivity factor is normalized to the composition of the gas mixture according to Eq. (11) [51]:

S¼

q1 =q2 P1 =P2

ð11Þ

where S is the selectivity factor, qi is the quantity of component i adsorbed, and Pi represents the partial pressure of component i. In this study, the selectivity of TiO2/GO-0.10 for CO2 over N2 was determined by recording single-component isotherms of CO2 and N2 at 25 °C (using an identical procedure as outlined in the Experimental Section) and then dividing the mass of CO2 adsorbed at 15 kPa by the mass of N2 adsorbed at 75 kPa according to Eq. (11). TiO2/GO-0.10 showed an appreciable SCO2 =N2 of 22 under these conditions. Table 4 lists the equilibrium CO2/N2 selectivity of various adsorbent materials estimated from pure component

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Table 4 Comparison of the CO2/N2 selectivity of different adsorbent materials with TiO2/GO-0.10 calculated from the pure component adsorption isotherms. Material

T (°C)

P CO2 (bar)

P N2 (bar)

SCO2 =N2

Reference

Siliceous zeolites (BEA) Porous synthetic hectorites Carbonized macadamia nut shells Zeolite-imidazolate framework (ZIF-8) Activated carbon from biosludge Zeolite beta (K+-exchanged) Crystalline porous materials (CPM-5) MOF-5 MOF-177 BrightBlack™ TiO2/GO-0.10

30 30 25 25 0 25 25 25 25 20 25

1 1 1 30 10 1 0.15 1 1 1 0.15

1 1 1 30 10 1 0.75 1 1 1 0.75

10 12.1 13 13.15 14 14.7 16.1 17.48 17.73 18.9 22

Pham et al. [52] Sethia et al. [53] Bae and Su [54] Zhang et al. [55] Hao et al. [56] Yang et al. [57] Sabouni et al. [31] Saha et al. [58] Saha et al. [58] Hornbostel et al. [59] This study

Fig. 9. Different chemisorption modes of CO2 onto TiO2/GO-0.10 (Labels 1 M or 2 M refer to the number of metal atoms involved in the adsorption).

the other hand, as an acidic species, CO2 could bind to the oxygen functionalities from the surface of GO (O-C coordinated, Fig. 9). However, in order to confirm this mechanism, crystallographic characterization (via XRD, IR or NMR spectroscopy)3 of CO2-loaded composite adsorbent is essential for direct observations of the location of the CO2 molecules within the structure, which in turn would facilitate the understanding of the interactions within the material giving rise to the observed adsorptive behavior. 4. Conclusion

adsorption isotherms. Although it is difficult to make a direct comparison of the SCO2 =N2 for TiO2/GO with the literature values because of the different experimental conditions and calculation techniques adopted to estimate the selectivity, we can still make a general comparison and note that the SCO2 =N2 of TiO2/GO-0.10 was remarkably higher than those recently reported for other adsorbent materials. The selective separation of gases depends on the different physical properties of the gas molecules, such as polarizability or quadrupole moment, leading to a higher enthalpy of adsorption of certain molecules over others [49]. Since the polarizability (CO2, 29.1  1025 cm3; N2, 17.4  1025 cm3) and quadrupole moment (CO2, 13.4  1040 C m2; N2, 4.7  1040 C m2) of CO2 are higher than those of N2 [60], it is expected that a higher affinity of the pore surface of TiO2/GO-0.10 for CO2 leads to a higher selectivity [61]. However, it must be noted that the selectivity factor determined in this study does not take into account the competition of gas molecules for the adsorption sites on the pore surface since it is estimated from single-component adsorption isotherms and hence does not depict the selectivity that would be obtained from dosing a gas mixture. Nevertheless, it gives an idea to quantify the performance of the adsorbent in real CO2 capture systems. 3.8. CO2 adsorption mechanism In the current study, the adsorption kinetic data could be well-represented by the Avrami model suggesting that CO2 can be adsorbed onto TiO2/GO-0.10 in two ways: physisorption and chemisorption. While physisorption occurs through the formation of intermolecular electrostatic interactions, such as London dispersion forces, or van der Waals forces, chemisorption involves the transfer of electrons between the adsorbent and the adsorbate with the formation of chemical bonds between the two species causing adhesion of the adsorbate molecules. Now, CO2 is an amphoteric molecule: the carbon atom is acidic while the oxygen atoms are weakly basic [62]. When CO2 is adsorbed as a base, the binding mode can involve one or two oxygen atoms of CO2 (monodentate or bidentate) and one or two Ti centers (Fig. 9). On

In this study, CO2 adsorption potential of TiO2/GO hybrid materials, synthesized by a simple one-step colloidal blending process, was experimentally investigated. Pure component CO2 adsorption increased with decreasing temperature and increasing adsorbate concentration. TiO2/GO-0.10 with the lowest GO to TiO2 mass ratio demonstrated the highest adsorption rate because of its large specific surface area and total pore volume. Maximum CO2 adsorption achieved was 1.88 mmol g1 at 25 °C and 1 bar, a value comparatively higher than other solid CO2 adsorbents, including zeolite, activated carbon and some MOFs. Adsorption isotherms were successfully modeled and the kinetic properties of CO2 uptake were also studied. The selectivity for CO2 over N2, computed from single component isotherms at conditions pertinent to post-combustion applications, was also much higher than that of many other previously reported adsorbents. Moreover, the isosteric heat of adsorption at zero fractional loading, computed from the van’t Hoff equation using the temperature dependent Toth isotherm parameters, revealed the possibility of desorbing the gas and regenerating the adsorbent at a much lower energy penalty. Overall, TiO2/GO-0.10 represents a very promising candidate for energy-efficient CO2 capture from post-fossil fuel combustion processes. Acknowledgements One of the authors, Shamik Chowdhury, gratefully acknowledges the financial support provided by the National University of Singapore for his Doctoral study. The authors also thank A⁄STAR’s Institute of Materials Research and Engineering (IMRE), Singapore and A⁄STAR’s Data Storage Institute (DSI), Singapore for technical assistance. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.11.037. 3

Infrared Spectroscopy (IR); Nuclear Magnetic Resonance Spectroscopy (NMR).

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