Materials Research Bulletin 63 (2015) 13–23

Photocatalytic CO2 reduction by CH4 over montmorillonite modifiedTiO2 nanocomposites in a continuous monolith photoreactor

Muhammad Tahir a,b, Beenish Tahir a, NorAishah Saidina Amin a,*aChemical Reaction Engineering Group (CREG)/Low Carbon Energy Group, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM,Skudai, Johor Baharu, Johor, MalaysiabDepartment of Chemical Engineering, COMSATS Institute of Information Technology Lahore, Punjab, Pakistan

A R T I C L E I N F O

Article history:Received 12 August 2014Received in revised form 13 November 2014Accepted 21 November 2014Available online 23 November 2014

Keywords:Layered compoundsSemiconductorsPhotocatalysisNanostructureCatalytic properties

A B S T R A C T

In this study, the performance of montmorillonite (MMT) modified TiO2 nanocomposites forphotocatalytic CO2 reduction with CH4 in a continuous monolith photoreactor has been investigated.The MMT modified TiO2 nanocomposites were dip-coated over monolith channels and werecharacterized by XRD, SEM, TEM, XPS, N2-adsorption–desorption and UV–vis spectroscopy. The MMTproduced anatase phase of TiO2 and reduced TiO2 crystallite size from 19 nm to 13 nm. CO was the majorreduction product with a yield rate of 237.5 mmol g-catal.�1 h�1 over 10 wt.% MMT-loaded TiO2 at 100 �C,and CO2/CH4 feed ratio 1.0. The photoactivity of MMT-loaded TiO2 monolithic catalyst was 2.52 timeshigher than bare TiO2. Likewise, low concentrations of C2H6, CH3OH, C3H6 and C3H8 were detected in theproducts mixture. These results inferred MMT modified TiO2 and monolith photoreactor were beneficialfor enhancing photocatalysis process with appreciable productivity. The stability test revealedphotoactivity of MMT-loaded TiO2 nanocomposites partially diminished in recycle runs.

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1. Introduction

Greenhouse gases, primarily carbon dioxide (CO2) and methane(CH4), emitted by human activities contribute to global warming [1].CO2 reforming of CH4, mainly termed as dry reforming of CH4, is oneof the significant research topics in recent years. Both CO2 and CH4

are stable molecules, which are not easy to be reduced to otherchemicals at mild reaction conditions. The use of phototechnologywould break the thermodynamic barrier of endothermic reactions[2,3]. Nevertheless, photocatalytic CO2 reduction with CH4 is apromising sustainable pathway that not only reduces CO2 emissionbut also recycled them back to fuels. However, efficient and selectivephotocatalytic systems for CO2 photoreduction to value-addedchemicals are vigorously sought [4,5].

Among various semiconductors, titanium dioxide (TiO2) as aphotocatalyst has been researched excessively over the pastdecades due to its advantages such as relatively cheaper, availablein excess, chemically/thermally and biologically stable, non-toxic,and possesses higher oxidative potentials [6,7]. TiO2 is able togenerate electrons and holes, which are good reductant andpowerful oxidants for initiating redox reactions. But TiO2 is mainlyfunctional under UV-light and has a lower CO2 photo-reduction

* Corresponding author. Tel.: +60 7 553 5579; fax: +60 7 558 8166.E-mail address: noraishah@cheme.utm.my (N.S. Amin).

http://dx.doi.org/10.1016/j.materresbull.2014.11.0420025-5408/ã 2014 Elsevier Ltd. All rights reserved.

activity due to immediate recombination of photogeneratedcharges (e�/h+). On the contrary, TiO2 photoactivity depends onits crystalline structure, while the modification of its structurewith dopants or charge trapping materials could enhanceproductivity and selectivity [8,9].

Recently, mesoporous materials are under consideration toimprove TiO2 photoactivity because of the larger surface area withgreater charge trapping capabilities. The most common materialsto modify TiO2 structure include clay minerals, activated carbon,graphene, carbon nanotubes, and zeolites [10–12]. However,abundantly available natural clays and among them pillaredclays are deliberated to be more proficient to enhance TiO2

photoactivity. The clay particles are used as support over whichTiO2 nanoparticles are fixed to the surface of a suitable matrix withparticle size in the order of micrometers. Moreover, nanoclaymaterials are low cost, environment friendly and possess goodadsorption capacity [13]. Among pillared clays, montmorillonite(MMT) is a type of natural multilayered clay classified as crystallattice of 2:1 layered silicates. In MMT, silicates layers are carriedtogether by relatively weak forces, thus water or any otherrelatively polar molecule can get in between the unit layers,triggering its lattice to expand. These layers are piled together witha regular van der Waals gap between them called interlayer orgallery. Therefore, silicate layers could disperse during theintercalation process fostering unique intercalation/exfoliationcharacteristics. The clay–TiO2 heterojunction makes operation

14 M. Tahir et al. / Materials Research Bulletin 63 (2015) 13–23

with photocatalytic material easier for trapping the photogen-erated charge particles, thus can improve TiO2 photoreductionefficiency [14,15].

In order to increase TiO2 photocatalytic activity, it ispostulated that MMT shortens the length of charge transferand can improve diffusions of ions over TiO2. The MMT supportedTiO2 nanocomposites can adsorb substances (e.g., CO2, CH4) on itsexternal surfaces or within its interlayers by substitution orinteraction. Likewise, during TiO2 intercalation process, MMTcould function as a barrier, thus inhibiting the formation ofcrystalline network. In this manner, the dispersed TiO2 over MMTlayers improved the surface characteristics with appreciablecations exchange capacity [16,17].

Among photocatalytic reactors, structured ones such asmonolith reactor with microchannels are considered veryefficient for photocatalytic applications. The advantagesof monolith include high illuminated surface area to volume ofthe reactor, high flow rates, lower pressure drop, more catalystloading and efficient light utilization/distribution inside channels[18]. Moreover, the unique structure of a monolith provides asurface area to volume ratio 10–100 times higher than other typesof catalyst supports with the same outer dimensions [19,20]. Themobility of charges on TiO2 surfaces supported over monolithchannels and to some extent vacant d-orbits of transition metalsover MMT galleries could promote CO2 reduction with CH4.

For the first time, photocatalytic CO2 reduction with CH4 asreductant over MMT modified TiO2 in a continuous monolithphotoreactor is reported in this paper. The objective of this workis to examine the effectiveness of montmorillonite modified TiO2

catalysts supported over monolith channels for photocatalyticCO2 reduction with CH4. The amounts of MMT-loading into TiO2

were 5, 10 and 15 wt.%, examined at different irradiation times.The catalysts were characterized by XRD, SEM, TEM, N2

adsorption–desorption, XPS and UV–vis spectroscopy. In addi-tion, the stability tests of reused catalyst were investigated to

Fig. 1. Schematic of experimental setup for photocatalytic CO2

comprehend catalyst deactivation in photocatalytic CO2

reforming of CH4 applications.

2. Experimental

2.1. Synthesis of nanocomposites

Montmorillonite modified TiO2 nanocomposites were synthe-sized using modified sol–gel method. The precursors employed weretitanium tetraisopropoxide (TTIP) (�98%, Merck), isopropanol(99.7%, Merck), acetic acid (100%, Merck), and montmorillonite(1.44P, Nanocor). The titanium solution was prepared using molarratios; Ti (C3H7O)4: 15C3H8O: 2CH3COOH (1 M). Typically, 15 mL ofTTIP was dispersed in 45 mL of isopropanol. The hydrolysis processwas initiated byadding drop wise 10 mL of acetic acid (1 M) dissolvedin 15 mL isopropanol and stirred overnight at room temperature.Next, MMT dispersed in isopropanol was added drop wise intotitanium sol. The gelation process was continued by stirring themixture foranother6 h until thick sol wasproduced.The sol obtainedwas poured into a glass container for monolith coating.

The dimension of cylindrical monolith is 60 mm diameter and20 mm long. The cell density of the cordierite ceramic monolithswith square channels, purchased from the Pingxiang MeitaoCompany China, is 200CPSI (cells per square inch). The monolithswere initially washed with isopropanol to remove any organicmaterials. Next, the monoliths were dried at 80 �C for 12 h and theirweights recorded before they were immersed in titanium sol anddipped for a specified time. This process was duplicated for thesecond coating to enhance catalyst loading. The catalyst-loadedmonoliths were dried at 80 �C overnight under air flow and finallycalcined at a rate of 5 �C min�1 up to 500 �C and held for 5 h. Aftercalcination, the weight of the coated monolith was also recorded.Using this procedure, 5, 10 and 15 wt.% MMT-loadedTiO2 monolithic catalysts were prepared. For comparison, TiO2

nanoparticles were synthesized using the same process as above.

reduction with CH4 in a continuous monolith photoreactor.

Fig. 2. XRD spectra of un-doped TiO2, MMT and MMT-loaded TiO2 nanocompositeswith different MMT concentrations: (a) TiO2, (b) MMT, (c) 5 wt.% MMT/TiO2, (d)10 wt.% MMT/TiO2, and (e) 15 wt.% MMT/TiO2.

Table 1Cell parameters and crystallite sizes of bare TiO2 and MMT-loaded TiO2 samples.

Sample Cell parameter(Å)

Cell volume(Å3)

Crystallite sizea

(nm)

a = b c

TiO2 3.764 9.486 134.39 195% MMT–TiO2 3.783 9.503 135.97 1710% MMT–TiO2 3.784 9.506 136.13 1415% MMT–TiO2 3.782 9.494 135.78 13

a Crystallite sizes calculated using Scherrer equation.

M. Tahir et al. / Materials Research Bulletin 63 (2015) 13–23 15

2.2. Characterization of catalysts

Powder X-ray diffraction (XRD) of catalysts was performed onBruker D8 advance diffractometer (Cu-Ka radiation, wavelengthl = 1.54 Å, operated at 40 kV and 40 mA). The scanning rate was1.2� min�1 from 1.3 to 75� of 2u. The Scherrer equation was appliedto calculate the crystallite size of photocatalysts. The scanningelectron microscopy (SEM) was taken out with JEOL JSM6390 LVSEM instrument. The particle size and lattice structure of the singlecrystals were visualized using a high resolution transmissionelectron microscope (HRTEM) (FEI-Tecni G2). The Brunauer–Emmett–Teller (BET) specific surface area and pore size of thecatalysts were measured by N2 adsorption–desorption isotherms at77 K using a Micrometrics ASAP 2020 surface area and porosityanalyzer. The XPS measurement was performed using Omicron DAR400 analyzer. The photocatalyst was fixed to the sample holderusing carbon tape. The pass energy used was 20 eV, while theinstrument was operated at 15 kV. The binding energies werecalibrated against the C 1s signal (284.6 eV) as the internal standard.Ultraviolet–visible (UV–vis) diffuse reflectance absorbance spectraof the samples were determined using Agilent, Cary 100 UV–visspectrophotometer equipped with an integrated sphere. Initially,blank runs were conducted to correct the base line. The absorbancespectra were analyzed at ambient temperature in the wavelengthrange of 200–800 nm. The band gap energies of the photocatalystswere determined from the extrapolation of Tauc plot to the abscissaof photon energy.

2.3. Photocatalytic activity test

The schematic of the continuous monolith photoreactor systemfor CO2 reduction with CH4 is demonstrated in Fig. 1. The reactorconsisted of a stainless steel cylindrical vessel with 5.5 cm lengthand a total volume 150 cm3

. It is fitted with a quartz window and areflector lamp located above the reactor. The coated monolithswere placed at the center of the reactor chamber. The light sourcefor UV irradiation was a 200 W Hg reflector lamp equipped withcooling fans for removing lamp heat. The straight square cordieritemonolith channels were illuminated with a light flux coming fromthe reflected light source. An optical process monitor ILT OPM-1Dand a SED008/W sensor were placed above the upper surface ofthe monolith to measure the intensity of the lights enteringthe channels. The irradiation intensity passing through the top ofthe reactor was 150 mW cm�2. The reactor was covered withaluminum foil to ensure the lights for the reactions came onlythrough the quartz window.

Prior to feeding the reactants, the reactor was purged for 2 husing helium (He) flow to remove air as well as to find out forleakage less than 2 bar pressure. The compressed CO2 and CH4 flowwere regulated by mass flow controllers (MFC) through the reactorchamber. The temperature was controlled using a heating sourceand pressure was kept constant at 1 atm. In continuous monolithphotoreactor, CO2/CH4 feed ratios of 1.0 was adjusted at a feed rateof 20 mL/min and the products were continuously analyzed overthe entire irradiation time.

The on-line gas chromatograph (GC-Agilent Technologies6890 N, USA) equipped with a thermal conductivity detector(TCD) and a flame ionized detector (FID) was used to analyze theproducts. FID detector was connected with an HP-PLOT Qcapillary column (Agilent, length 30 m. ID 0.53 mm, film40 mm) for separation of C1–C6 hydrocarbons, alcohols andoxygenated compounds. The TCD detector was connected toUCW982, DC-200, Porapak Q and Mol Sieve 13 X columns. TheUCW-982 was used to back flush and reversed flow to ensure C6

and higher compounds could be detected earlier in thechromatogram. Meanwhile, C1–C2, C3–C5 compounds and light

gasses (H2, O2, N2, CO) were separated using Porapak Q, DC-200,and HP MS, respectively.

3. Results and discussion

3.1. Characterization of catalysts

Fig. 2 presents the XRD patterns of MMT, TiO2 and MMT-loadedTiO2 catalysts. The peaks of TiO2 revealed a pure crystalline andanatase phase. With MMT loading, TiO2 remained originalreflections of anatase crystalline phase with no additional peak.However, diffraction peak around 2u = 25� (10 1) of MMT modifiedTiO2 nanocomposites became weaker and wider and similar trendscould be determined for all MMT-loaded TiO2 anatase peaks.However, in case of MMT loading, the diffraction peaks were widerand weaker than those of catalysts prepared without MMT. TheXRD pattern of MMT showed a broadened basal (0 0 1) reflectionaround 2u = 3.70�, due to the preferred orientation of platy shapedparticles and stacking disorder of MMT layers [21]. However, MMTloaded TiO2 nanocomposites have dissimilar XRD patterns than thepure MMT. The eminent MMT peak at around 2u = 3.70�, due tolayered clays, has disappeared for all TiO2 samples containingMMT. This brought out the layered structure of MMT destroyed andproduced MMT/TiO2 nanocomposites [22]. Therefore, layeredstructure of silicate in MMT has an effect on crystal lattice ofMMT containing TiO2 nanocomposites. The addition of MMT in thesystem allowed the silicate layer of MMT to behave as a barrier,which prevented natural crystallization of TiO2.

The Scherrer equation based on XRD (10 1) peaks was used tocalculate the size of the crystallites as presented in Table 1. Thecrystallite sizes calculated were 19, 17, 14, and 13 nm for TiO2 and 5,10 and 15 wt.% MMT loaded TiO2 nanocomposites, respectively.Furthermore, the cell parameters and cell volumes of TiO2 and

16 M. Tahir et al. / Materials Research Bulletin 63 (2015) 13–23

MMT-loaded TiO2 crystallites are summarized in Table 1. The cellparameters and cell volumes are similar to those reported inJCPDS-ICSD (89-4921) standards for anatase TiO2 i.e., a = b = 3.777Å, c = 9.501, and v = 135.54 Å3. These reflections have confirmedfully developed tetragonal crystal shape of anatase TiO2. Moreimportantly, MMT-loading did not affect the lattice parametersand the cell structure of TiO2.

The morphology and EDX analysis of MMT, TiO2 and MMT/TiO2

nanocomposites were investigated using scanning electronmicroscopy (SEM) as illustrated in Fig. 3(a–j). The SEM imagesof MMT plates are shown in Fig. 3(a–b). MMT consists of disorderedstacked sheets with a high aspect ratio. The MMT also consists ofsmaller layers of very crusty and flaky pieces with segmentedclusters instead of flat and uniform layers. The EDX mapping ofMMT elemental composition is presented in Fig. 3(c–d). Thenanoparticles of TiO2 with mesoporous structure could beobserved in Fig. 3(e). The EDX mapping analysis in Fig. 3(f–g)shows the presence of Ti and O as the major components. SEMimages of MMT/TiO2 nanocomposites are presented in Fig. 3(h),which indicates the disappearance of MMT plates while TiO2

nanoparticles are uniformly distributed throughout the MMT. TheEDX mapping analysis in Fig. 3(i) revealed even distributed of TiO2

over MMT. The EDX analysis in Fig. 3(j) shows the presence of Ti, Si,Al, Mg, O, C and F as the major components. The presence of F and Cwas possibly due to organic solution, which was used to deposit

Fig. 3. SEM micrographs of MMT modified TiO2 nanocomposites: (a–b) MMT layered, (c–TiO2 nanocomposites, (i–j) EDX mapping of MMT/TiO2.

catalyst over the sample holder. The presence of Pt was due toplatinum coating before FESEM analysis.

The transmission electron microscopy (TEM) of MMT loadedTiO2 nanocomposite is illustrated in Fig. 4(a–f). The mesoporousstructures of MMT/TiO2 nanoparticles are obvious as presented inFig. 4(a). The particle size distribution was in the range of 8–18 nmwith the average particle size having the diameter 13.50 nm, closerto the crystallite size estimated by Scherrer equation (Fig. 4(b)).The TiO2 nanoparticles are well dispersed with MMT and can beclearly observed from Fig. 4(c–e). It is obvious that the orderlystructure of MMT layers is completely destroyed due to efficientintercalation process and TiO2 nanoparticles are distributed overthe MMT surface during the hydrolysis process. One can see similarobservations in the SEM and XRD analysis. The electron diffractionwas performed to identify the crystalline structure of TiO2 asillustrated in Fig. 4(f). The lattice fringe spacing of TiO2 has a valueof about 0.35 nm that clearly relates to anatase phase of TiO2.

The adsorption isotherms and pore size distributions of TiO2

photocatalysts are depicted in Fig. 5. Fig. 5(a) exhibits the N2

adsorption–desorption isotherms of un-loaded TiO2 and 5 and10 wt.% MMT-loaded TiO2 catalysts. The well-defined hysteresisloops with a steep desorption branches and less steep adsorptionbranches for all materials belong to type IV hysteresis loops,indicating mesoporous structure of particles. Although, MMT isnon-porous, the mesoporous characteristics of MMT/TiO2

d) EDX mapping of MMT, (e) TiO2 nanoparticles (f–g) EDX mapping of TiO2, (h) MMT/

Fig. 4. TEM images of MMT-loaded TiO2 nanocomposite: (a) mesoporous MMT/TiO2, (b) Size distribution of MMT/TiO2 nanoparticles, (c–e) TiO2 dispersion over MMT (f) d-spacing of TiO2 nanoparticles and dispersion of electron in anatase TiO2.

Fig. 5. Adsorption–desorption isotherms and pore size distribution of TiO2 and MMT-loaded TiO2 nanocomposites; (a) adsorption–desorption isotherms, (b) BJH pore sizedistributions.

M. Tahir et al. / Materials Research Bulletin 63 (2015) 13–23 17

Table 2Summary of N2 adsorption–desorption characteristics of TiO2 and MMT-modified TiO2 samples.

Type ofcatalyst

Surface area(m2/g)

Pore volume(cm3/g) Pore width

(nm)BET surface area t-Plot external surface area BJH adsorption surface area t-Plot

micropore volumeBJH adsorption pore volume BJH pore diameter

TiO2 43 34 52 0.0039 0.134 10.335% MMT–TiO2 44 44 47 0.000 0.119 10.1310% MMT–TiO2 42 48 48 0.000 0.102 8.56

18 M. Tahir et al. / Materials Research Bulletin 63 (2015) 13–23

nanocomposites may be attributed to the lower MMT loading anduniform dispersion of mesoporous TiO2 into the MMT structure.The initial part of isotherms (at the lower P/Po) attributed tomonolayer-multilayer adsorption on the internal surface ofmaterials. The steep increment at higher P/Po is due to capillarycondensation within pores followed by saturation as the pores

Fig. 6. XPS spectra of 10% MMT/TiO2 nanocomposite (a) spectra of Ti 2p, (b

filled with liquid. The monolayer–multilayer is more dominant inTiO2 in which capillary action started at P/Po = 0.55. However,capillary condensation action was more prominent in the MMTloaded TiO2 composite eminent at P/Po of 0.51 and 0.45 for 5 and10 wt.% MMT, respectively. Fig. 5(b) portrays the BJH pore sizedistribution of TiO2 and MMT-loaded TiO2 nanoparticles. The pore

) O 1s, (c) C 1s, (d) Na 1s, (e) Si 2p, (f) Al 2p, (g) Mn 2p, and (h) Mg 2p.

M. Tahir et al. / Materials Research Bulletin 63 (2015) 13–23 19

sizes distribution curves of all samples were determined from aBJH adsorption branch of the isotherm, which displays singledistribution peak. The TiO2 pore size distribution peak is in therange of 3.4–26 nm, while the peaks of MMT modified TiO2 exist inthe range of 3.4–23 nm and 2.8–16 nm for 5 and 10 wt.% MMTloaded TiO2, respectively. Obviously, the pore size distributionnarrowed after TiO2 surface modification with MMT. The silicatelayer of MMT can behave as a barrier, which prevented theagglomeration and natural crystallization of TiO2 sol.

The surface area, pore volume and pore size of all samples aresummarized in Table 2. The BET surface area of 43, 44, and42 m2g�1 was obtained for bare TiO2, 5 and 10 wt.% MMT loadedTiO2, respectively. Apparently, the BET surface area of TiO2 andMMT/TiO2 samples was almost the same. The minor difference inthe surface areas was perhaps due to measurement error. However,the t-plot external and BJH adsorption surface area somewhatincreased with MMT loading, approving higher mesoporosity inMMT-loaded TiO2 nanocomposites than in bare-TiO2 samples. Onthe other hand, both t-plot micropores and BJH pore volumegradually decreased with MMT-loading, possibly due to TiO2

nanoparticles distributed over the MMT layers, having lower porevolumes. More importantly, the pore diameter of MMT-loaded TiO2

nanocomposites decreased, confirming MMT hindered TiO2 crystalgrowth. Thus, the decrease in pore volume and pore diameter waspossibly due to the dispersion of TiO2 over MMT.

The X-ray photoelectron spectroscopy (XPS) measurementswere carried out to investigate the chemical states of all theelements in modified TiO2 samples. The XPS spectra of MMT-loaded TiO2 nanocomposites are depicted in Fig. 6. The Ti 2p XPSspectra in Fig. 6(a) indicate peaks appear at 458.73 (Ti 2p3/2) and464.45 (Ti 2p1/2) both of which correspond to Ti4+ oxidation statesand pure anatase phase of TiO2 [14]. Therefore, no obvious Ti peakposition change in MMT modified TiO2 was observed. Oxygenspectra in Fig. 6(b) showed O 1s peak located around 529.82 eVassigned to lattice oxygen (O2�) in anatase TiO2, while peaksaround 530.58 eV reflect the presence of OH group over the surfaceof TiO2. The oxygen peak at around 532.415 eV is possibly relatedwith the adsorbed H2O over the surface [23]. Fig. 6(c) shows thecarbon peaks (C 2p) located at 284.6 eV assigned to elementalcarbon (C—C) while the one at 586.1 eV is assigned to C—O. Tracesof these impurities may be ascribed to the carbon residues from theorganic precursor or due to carbon tap [15]. The peaks related to Na1s spectra in Fig. 6(d) are located at bending energies of 1068.21and 1073.19 eV. The appearance of Na 1s peaks were probably due

Fig. 7. (a) Absorption spectra of MMT, TiO2 and MMT modified TiO2 samples measured

samples.

to the formation of sodium titanate because of the presence ofsodium inside the MMT galleries [24]. The binding energies of theSi 2p peaks were located at 102.5 and 99.53 eV, which reflects theirpresence in MMT as shown in Fig. 6(e). Similarly, Al 2p peakslocated at 74.4 eV was probably due to the interaction betweenSi/Al in Fig. 6(f). On the other hand, the binding energy peaks of Mn2p and Mg 2p were unclear, possibly due to their minute presencein the MMT galleries (Fig. 6(g–h)). In general, all the possibleelements presented in MMT are detected through XPS analysis.

The UV–vis diffuse reflectance absorbance spectra of TiO2

nanoparticles and MMT modified TiO2 nanocomposites aredepicted in Fig 7(a). The absorption band edge of TiO2 appearedaround 397 nm, which has a red shift compared with intrinsicanatase TiO2 (energy gap 3.20 eV). The absorption band edge ofMMT modified TiO2 samples were located at 392, 396 and 397 nmfor 5, 10 and 15 wt.% MMT loaded TiO2, respectively. The lightabsorption of purified MMT increased to become almost transpar-ent in the wavelength longer than 250 nm. Similar trends for lightabsorption are reported in literature over MMT-loaded TiO2 [25].The band gap energies were determined using Tauc plot i.e., (ahn)2

versus (hv) (where, a is the absorbance, h is Planks constant and nis light frequency), by extrapolating the linear region of the plot tothe intercept of the photon energy axis (Fig. 7(b)). The Ebgestimated were 3.11, 3.14, 3.12 and 3.11 eV for TiO2, 5, 10 and15 wt.% MMT loaded TiO2 samples, respectively. These resultsrevealed MMT-loading into TiO2 has no effect in shifting TiO2 bandgap.

3.2. Photoreduction of CO2 with CH4

Firstly, preliminary experiments were executed to confirm anycarbon containing compounds originated during photocatalyticCO2 reduction with CH4. The first trial was conducted forphotocatalytic CO2 reduction with CH4 in the absence of anycatalysts. Gas chromatographic analysis showed no carboncontaining compounds was found inferring the photocatalysisprocess could not proceed without catalyst. In the second test,purging helium gas was used for the cases of (1) emptyreactor + helium, (2) uncoated monolith + helium, (3) MMT/TiO2

and TiO2 coated monolith + helium. Once again, productions ofcarbon containing compounds were not observed in each case. Thisproved that the carbon containing compounds were not producedfrom any organic residues. Thus, any carbon containing com-pounds produced were derived from CO2 photoreduction with

by a diffuse reflectance method; (b) band gap energy calculations of corresponding

Fig. 8. Effects of time dependence on production of CO over various MMT modifiedTiO2 catalysts in a continuous monolith photoreactor at 100 �C, CO2/CH4 mole ratio1.0 and feed flow rate 20 mL/min.

Fig.10. The yield of methanol over MMT loaded TiO2 samples during photocatalyticCO2 reduction with CH4 in a continuous monolith photoreactor.

20 M. Tahir et al. / Materials Research Bulletin 63 (2015) 13–23

CH4. CO, C2H6 and CH3OH were found to be major CO2 reductionproducts in all the experiments. All the experiments were repeatedin triplicate and average values have been reported.

Fig. 8shows continuous production of CO as a function ofirradiation time for photocatalytic CO2 reduction with CH4 usingvarious MMT loadings into TiO2 at 100 �C, CO2/CH4 ratio 1.0 and flowrate 20 mL/min. Using bare TiO2, smaller amount of CO wasproduced, while MMT-loaded TiO2 samples improved yield ratesignificantly. The higher yield rate is evidence that TiO2photoactivitycan be enhanced by dispersing TiO2 into MMT. The optimum loadingof MMT was 10 wt.% beyond which CO2 and CH4 photoreduction wasreduced. Loading MMT above 10 wt.% reduced TiO2 photoactivitybecause of shading effect. Therefore, 10 wt.% MMT loaded-TiO2

samplewasthemostactive,overwhichthecontinuousproductionofCO was 237.5 mmol g-catal.�1 h�1, 2.52 times higher than un-loadedTiO2 catalyst. Moreover, the photocatalytic activity difference inTiO2

and MMT-loaded TiO2 was mainly due to effective CO2 and CH4

adsorption and efficient charges transport over highly dispersedMMT/TiO2 nanocomposites.

Apart from CO, ethane (C2H6) and methanol (CH3OH) were alsoobserved during the photocatalytic CO2 reduction with CH4 asshown in Figs. 9 and 10. In Fig. 9, among TiO2 and MMT-loaded TiO2

Fig. 9. The yield of C2H6 over MMT loaded TiO2 samples during photocatalytic CO2

reduction with CH4 in a continuous monolith photoreactor.

Fise

monolithic catalysts 10 wt.% MMT-loaded TiO2 was more efficientfor C2H6 production. On the other hand, 15 wt.% MMT-loaded TiO2

favored CH3OH production as illustrated in Fig. 10. Initially, muchhigher C2H6 and CH3OH yield rates were observed, but thengradually decreased at elongated irradiation time. Since,the continuous flow mode of monolith photoreactor was employedthe production rate of C2H6 and CH3OH reached to a maximumvalue, then gradually decreased. The decrement in production ratesuggests gradual deactivation of monolithic catalysts. Similarobservations have been reported in literature during photo-catalytic CO2 reduction studies [26]. The selectivity of differentproducts over MMT-loaded TiO2 photocatalysts is presented inFig. 11. The selectivity of CO over TiO2 increased from 76% to 81%over 10% MMT-loaded TiO2. The products selectivity over 10 wt.%MMT/TiO2 was in the order of CO (81%) > C2H6 (19%) > CH3OH(0.19%).

The results from this study are compared to the work reportedby various researchers. Yuliati et al. [2] reported photocatalytic CO2

reduction with CH4 over Ga2O3 distributed at the bottom of celltype photoreactor. The main products observed were 1.04 mmol ofC2H6 and 2.48 mmol of H2 with traces of CO at 473 K. The loweryield rates were due to higher stability of CO2 and CH4 molecules.

g. 11. The effects of MMT-loaded TiO2 photocatalysts on CO, C2H6 and CH3OHlectivity.

M. Tahir et al. / Materials Research Bulletin 63 (2015) 13–23 21

Similarly, Ko9cí et al. [27] reported photocatalytic CO2 reductionwith H2O vapors over kaolinite/TiO2 nanocomposites in a slurrytype photoreactor. The CO2 reduction products were CH4 andCH3OH with a yield of 7.6 and 4.5 mmol g-catal.�1 at a 24 hirradiation time. Recently, ZnS/MMT nanocomposites wereinvestigated for photocatalytic CO2 reduction with H2O in a stirredbatch annular reactor with a suspended catalyst. The reactionproducts observed were CO, CH4 and H2 with yield rates 2.95, 27.9and 300 mmol g-catal.�1 for 24 h irradiation time [28]. Theseresults confirmed MMT performed better than kaolinite for CO2

photoreduction. However, in the present study, higher photo-activity of MMT/TiO2 nanocomposites and larger CO yield rate withappreciable amount of other hydrocarbons have been observedusing continuous monolith photoreactor. The results presentedhere shows thermodynamically stable CO2 and CH4 molecules canefficiently and continuously be reduced to valuable products usingMMT/TiO2 nanocomposites and monolith photoreactor.

A comparison between previous results and the present studyrevealed that the monolithic geometry facilitated in improvingproduct formation and light distribution. In general, the higherproduction of CO during photocatalytic CO2 reduction with CH4 inmonolith photoreactor was possibly due to a larger illuminatedsurface area, higher photonic efficiency and efficient adsorption–desorption processes inside monolith channels. Likewise, inMMT-loaded TiO2 nanocomposites, TiO2 is highly dispersed overMMT, resulting in smaller particle size and higher mobilityof charges. Similarly, the presence of metals in MMT/TiO2

nanocomposites trapped electrons during TiO2 photocatalysisand enhanced its photocatalytic activity [17].

3.3. Stability test of recycled catalysts

In order to examine the stability of photocatalyst, theexperiments were repeated for three times on the recycledcatalyst coated monoliths. For each round, the monolith wasremoved from the reactor and placed in open air overnight beforethe next run began. The stability test for photocatalytic CO2

reduction with CH4 to CO over MMT-loaded TiO2 catalyst isillustrated in Fig. 12. Obviously, yield of CO in every run increasedup to the irradiation time of 1 h and then gradually diminished.Nevertheless, it is noticeable that CO yield was more eminent inthe first run; slightly decreased in the second and third cyclic

Fig. 12. Stability test of reused MMT-loaded TiO2 photocatalyst for CO2 reductionwith CH4 to CO at 100 �C, CO2/H2 ratios 1.0 and flow rate 20 mL/min.

runs. However, the initial increase in CO production was probablydue to the followings: (a) in first cyclic run, possible CO2 and CH4

adsorption over the catalyst surface, which was efficientlyproduced CO in second and third cyclic runs, (b) in first cyclicrun, some of CO remained un-desorbed over the catalyst surface,but desorbed during the next cyclic run, thus increased the overallCO evolution during the initial irradiation time and (c) possiblecoke deposited over monolithic catalyst, which was effectivelyconverted to CO at the startup of irradiation time. After an initialirradiation time, all the possible CO production from previouscyclic runs disappeared, and the catalyst started to functionnormally. Therefore, MMT-modified TiO2 partially lost its activityin the second and third runs during photocatalytic CO2 reductionwith CH4 in a continuous monolith photoreactor. During photo-catalytic CO2 reduction with CH4, large amount of coke wasdeposited over the catalyst surface. Nevertheless, the stability testdemonstrated only partial loss of catalyst photoactivity even up to10 h of irradiation times in each and every run investigated forthree consecutive runs. Therefore, it is evident that MMTprovided stability to TiO2 to prolong its photoactivity.

Similarly, the stability of MMT-loaded TiO2 catalyst forC2H6 production is presented in Fig. 13. Obviously, in the firstcyclic run, the catalyst exhibited much higher C2H6 yield,which gradually decreased in second and third cyclic runs.This was perhaps due to the loss of catalyst photoactivity forC2H6 production as explained previously. The production ofCH3OH and other hydrocarbons also decreased gradually aftereach cyclic run, confirming partially deactivated MMT-loadedTiO2 catalyst. These results exhibited better initial activity ofMMT-loaded TiO2 for the production of C2H6, CH3OH andhydrocarbons. The color of MMT-loaded catalyst duringphotocatalytic CO2 reduction with CH4 slightly turned blackduring dry reforming of CH4 in photocatalysis process.The darkening of catalyst color may be an indication ofthe formation of intermediate carbon species adsorbed overthe catalyst surface.

3.4. Reaction mechanism of CO2 photoreduction with CH4

During photocatalytic CO2 reduction with CH4 over MMT-modified TiO2, CO, C2H6 and CH3OH are the potential products

Fig. 13. Stability test of reused MMT-loaded TiO2 photocatalyst for CO2 reductionwith CH4 to C2H6 at 100 �C, CO2/H2 ratios 1.0 and flow rate 20 mL/min.

Fig. 14. Schematic presentation of photocatalytic CO2 reduction with CH4 over MMT/TiO2 in a monolith photoreactor.

22 M. Tahir et al. / Materials Research Bulletin 63 (2015) 13–23

observed during the reaction of two molecules (CO2, CH4).Therefore, a possible reaction mechanism is illustrated in Eqs.(1)–(8).

TiO2 + hn ! h+ + e� (1)

MMT (metals) + e�! MMT (metals) � e� (2)

CH4+ h+!�CH3 + H+ (3)

CO2 + e�!�CO2� (4)

�CO2� + H++ e�! CO + OH� (5)

�CH3 + �CH3! C2H6 (6)

�CH3 + OH+! CH3OH (7)

�CH3 + H++ e�! CH4 (8)

Upon absorption of UV-light irradiations, the electron–hole pairswere generated on the TiO2 as explained in Eq. (1). The photo-generated electrons are trapped by metals if any presented in MMT(Eq. (2)) and/or charges can directly participate in reduction andoxidation processes. After the generation of electrons and holes,CH4 is converted to �CH3 during the oxidation process with therelease of H+ as explained in Eq. (3). During reduction process,electrons undergo CO2 reduction to �CO2

� radical (Eq. (4)), which isfinally converted to CO in Eq. (5). The ethane was produced bycoupling two �CH3 species (Eq. (6)), while methanol was producedby reacting �CH3 with OH+ (Eq. (7)). In this photocatalytic system,both CH4 and CO2 are firstly adsorbed over the catalyst surface andthen converted to �CH3 and �CO2

� species. As discussed previously,CO was the main products with much higher yield rate andselectivity compared to C2H6 and CH3OH. Therefore, it is envisagedsome of the �CH3 species may convert back to CH4 as explained inEq. (8). Based on the above discussed observations and chemicalreactions, the schematic photocatalytic CO2 reduction with CH4

over MMT-loaded TiO2 nanocomposites in a monolith photo-reactor is portrayed in Fig. 14. Obviously, TiO2 nanoparticles were

evenly distributed over MMT, enhancing reduction of CO2

and oxidation of CH4, thus finally producing CO, CH3OH andhydrocarbons.

4. Conclusions

In this study, photocatalytic CO2 reduction with CH4 overmontmorillonite (MMT) modified TiO2 nanocomposites in acontinuous monolith photoreactor was investigated. During theintercalation process, layered structure of MMT destroyed, produc-ing MMT/TiO2 nanocomposites. Modification of TiO2 with MMTcontrolled crystal growth and produced anatase phase of TiO2. Theexperimental results revealed efficient CO2photoreductionwith CH4

to CO, C2H6, CH3OH and C3H8 over MMT modified TiO2

nanocomposites. The continuous production of CO as the mainproduct was 237.5 mmol g-catal.�1 h�1 over MMT-modified TiO2

monolithic catalyst, 2.52 times higher than TiO2 based monolithiccatalyst. The increased in the yield rate indicated higher efficiency ofMMT-loaded TiO2 nanocomposites due to hindered recombinationof photogenerated charges in MMT and efficient use of photonenergyinmonolithphotoreactor. The electronswere moremobile onthe catalyst surface which led to effective desorption of products,resulting in higher product yield rates. Meanwhile, stability testrevealed yield rates of CO, CH3OH and hydrocarbons graduallydecreased in each recycle run confirming MMT-loaded TiO2 catalystwas partially deactivated in each cyclic run.

Acknowledgements

The authors would like to extend their deepest appreciation tothe Ministry of Higher Education (MOHE), Malaysia and UniversitiTeknologi Malaysia for the financial support of this research underLRGS (Long-term Research Grant Scheme, Vot 4L800), RUG(Research University Grant, Vot 02G14) and FRGS (FundamentalResearch Grant Scheme, Vot 4F404).

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