TiO2 heterojunction in a monolith photoreactor

Journal of CO₂ Utilization 38 (2020) 99–112

Contents lists available at ScienceDirect

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

Enhanced photocatalytic CO2 reduction to fuels through bireforming of methane over structured 3D MAX Ti3AlC2/TiO2 heterojunction in a monolith photoreactor

T

Muhammad Tahir Chemical Reaction Engineering Group (CREG), Department of Chemical Engineering, School of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 UTM, Johor Bahru, Johor, Malaysia

A R T I C LE I N FO

A B S T R A C T

Keywords: Structured 3D Ti3AlC2 MAX Dry reforming of methane (DRM) Bireforming of methane (BRM) Fixed-bed reactor Monolith reactor

Design and fabrication of three dimensional Ti3AlC2 MAX/TiO2 composite immobilized over monolithic support was obtained through sol-gel approach. With partial oxidation and incorporation of Ti3AlC2 essentially promotes light absorption, charge transfer and extends photo-induced charge carrier lifetime. The highest CO yield of 1566 μmol g-cat−1 was obtained over Ti3AlC2 MAX/TiO2, being 6.8 folds higher than pure TiO2 NPs. Performance of structured composite tested in methane steam reforming (MSR), methane dry reforming (MDR) and methane bi-reforming (MBR) reveals 1.2 and 1.6 folds higher activity in MBR than using MDR and MSR, respectively. Similarly, quantum yield in a monolith photoreactor was 3.5 folds higher than using a fixed-bed system. This divulges that MBR gave proficient oxidation and reduction reactions in electron-rich 3D MAX structure, whereas, monolith photoreactor provides larger photon-energy consumption with improved sorption process to boost production of CO and H2 with enhanced stability. Thus, this work demonstrated 3D Ti3AlC2 MAX/TiO2 a promising catalyst and monolith photoreactor an efficient photon flux harvesting system for boosting hydrogen rich syngas production.

1. Introduction Global warming due to excessive emission of greenhouse gases (CO2, CH4) and energy crises due to high energy demand for daily human activities are the major challenges modern society is facing [1,2]. Thus, conversion of greenhouse gas to valuable chemicals is highly demanding [3]. Among the different alternatives, dry reforming of methane (DRM) is an effective approach for utilizing both CO2 and CH4 as a feedstock in the production of syngas (CO, H2) according to the reaction in Eq. (1) [4,5]. DRM offers numerous advantages such as mitigation of both gases and direct production of syngas [6,7]. However, converting stable CO2 and CH4 molecules is a challenging task due to endothermic reaction at elevated temperature with excessive energy supply [8,9]. In addition, using bireforming of methane (BRM) process with the help of CO2 and H2O, higher H2/CO syngas ratio can be achieved as illustrated in Eq. (2) [10]. Although, BRM has advantages such as hydrogen rich syngas, but it also demands excessive energy input in addition of coking and instability of catalyst [11].

CH 4 + CO2 → 2CO + 2H2 ΔH298K = + 247 kJ/mol ΔG298K = + 170 kJ/mol

(1)

3CH 4 + CO2 + 2H2 O→ 4CO + 8H2 ΔH298K = + 220 kJ/mol ΔG298K = + 151 kJ/mol

(2)

Using photo-technology, hydrogen rich syngas gas can be produced though CH4/H2O oxidation with CO2 reduction under light irradiation during BRM process. However, light energy assisted limited reported are available in DRM and BRM in CO2 reduction applications. For instance, Shi et al. [12] pioneered photocatalytic DRM process with the production of oxygenated compounds over Cu/CdS supported TiO2SiO2 catalyst. In their work, fixed bed was used at 373 K but uncontrolled selectivity and productivity was obtained. Similarly, photocatalytic methane dry reforming was tested in a fixed bed system using Ga2O3 catalyst under UV-light and 314–673 K reaction temperature. The catalyst was favorable for the production of CO and H2 with C2-C4 hydrocarbons in smaller amounts [13]. In another development, CO2 reduction with H2/CH4 was tested under UV light over MgO catalyst [14]. Noble metal loaded Pt/TiO2 for photocatalytic DRM for the

E-mail address: [email protected]. https://doi.org/10.1016/j.jcou.2020.01.009 Received 19 October 2019; Received in revised form 6 January 2020; Accepted 6 January 2020 2212-9820/ © 2020 Elsevier Ltd. All rights reserved.

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

selective production of syngas was investigated using fixed bed system [15]. In the recent development titanium nanotubes loaded with Au/Rh has been tested for photocatalytic DRM under high reaction temperature [16]. Previously we reported photocatalytic DRM process over TiO2 and g-C3N4 based catalysts [17,18]. Obviously, TiO2 has been used in reforming reaction due to high stability, however, existing photocatalysts suffer from lower charge separation and poor light absorption [19–21]. Thus, seeking highly efficient, selective, photo-stable and costeffective photocatalyst remains key for stimulating photocatalytic reforming of CO2 to selective fuels [22]. Recently, two and three dimensional layered ternary transition metal carbide compounds, also known as “MAX”, have become popular in hetero-junction formation due to their excellent electron transport characteristics [23]. MAX has formula Mn+1AXn with M early transition metal, A is an A-group element and X = C or N with n = 1, 2, or 3. They combined the merits of both metals and ceramics and are highly conductive to transport charges. In addition, they are low cost materials with high chemical/thermal stability. In this family, Ti3AlC2 has attracted much consideration due to high electrical conductivity and it has graphite like structure. 2D layered structure of Ti3AlC2 has a unique combination of metallic and ceramic properties and Fermi level is dominated with D-d orbitals of M element. Numerous studies have been conducted on the use of Ti3AlC2 in different applications such as electrical, thermal and catalytic properties of MAX phases [24,25]. However, to the best of our knowledge, so far no photocatalytic study has been reported with the use of MAX Ti3AlC2 as a support or mediator in CO2 reduction applications. Although, structured catalysts provide improved photoactivity and selectivity, however, higher photon flux consumption can be attained using the monolith photoreactor system [26,27]. This was obviously due to a larger illuminated surface area, improved sorption process and larger catalyst loading [26,28]. In another work, monolithic Pd/gC3N4/reduced graphene oxide for selective CO2 hydrogenation has been reported [29]. Similarly, we reported dynamic H2 evolution in a monolith photoreactor [30]. Therefore, the performance of Ti3AlC2 MAX based TiO2 composite for the selective CO2 conversion to fuels using CH4 and H2O reducing agents would be boosted using a monolith photoreactor. According to available literature, Ti3AlC2 MAX based TiO2 composite has never been reported in photocatalytic CO2 reduction application, whereas, monolith photoreactor and bireforming of methane would be new innovation in this field. Herein, we designed an innovative and non-noble metals based structured multi-component 3D Ti3AlC2 MAX dispersed TiO2 nanoparticles towards enhanced photocatalytic CO2 reduction. The assembly of TiO2 with Ti3AlC2 as a solid electron mediator was successfully synthesized for superior photocatalytic CO2 reduction. It was further investigated that bulk Ti3AlC2 has lower mediator activity compared to exfoliated partially oxidized sheets. The activity of newly developed catalyst was tested in different reforming systems which include methane steam reforming (MSR), methane dry reforming (MDR) and methane bi-reforming (MBR). The performance of a fixed bed and a monolith photoreactor was further compared to investigate quantum energy utilization. The stability analysis was further conducted to understand coke formation in MBR process with the role of MAX in the composite sample. This innovative approach provides an alternative way to manipulate the transfer of electrons for boosting photoactivity in a monolith photoreactor for energy applications.

Pingxiang Meitao Company, China. The size of the monolith was 20 mm diameter and 60 mm length with cordierite structure and cylindrical shape. All the chemicals and materials were used as received without further purification. 2.2. Preparation of Ti3AlC2/TiO2 composite catalyst Ti3AlC2 MAX based TiO2 composite was synthesized using a ultrasonic assisted sol-gel approach. First, the bulk Ti3AlC2 was partially oxidized using a hydrothermal approach. For this purpose, 1 g Ti3AlC2 placed in a crucible was heated for 2 h at 500 °C and resulting sample was named as partially oxidized Ti3AlC2 or O-Ti3AlC2 to develop TiO2 NPs over the layered 3D surface. Next, Ti3AlC2/TiO2 composite was synthesized using modified sol-gel method. Briefly, 10 mL titanium precursor (Titanium tetraorthotitanate) was dispersed in a 20 mL isopropanol and was hydrolysed with a mixture of 10 mL (1 M) acetic acid and 10 mL isopropanol. The acetic acid solution was added dropwise into the titanium mixture under continuous stirring to get controlled growth of TiO2 nanoparticles. The slurry was constantly stirred for next 12 h until a clear titanium sol was obtained. Next, typical amount of bulk Ti3AlC2 dispersed in isopropanol was added into the titanium sol and continuously stirred for next 12 h to get a homogenous dispersion of titanium with 3D Ti3AlC2 sheets. Finally, the slurry was dried at 100 °C for 12 h and was calcined at 500 °C for 2 h to get Ti3AlC2/TiO2 composite. For the synthesis of pure TiO2, a similar method was employed but without adding Ti3AlC2. 2.3. Preparation of Ti3AlC2/TiO2 loaded over monolithic support Monolithic Ti3AlC2/TiO2 composite was synthesized using a modified sol-gel dip-coating method according to our previous work [31]. Initially monoliths were washed with alcohol and acetone to remove impurities and then dried in oven at 100 °C overnight under airflow. In the next step, slurry of Ti3AlC2/TiO2 composite prepared in the previous section without drying was stored in a glass container for monolith coating. The dried monoliths initial weight was calculated and dipped into catalyst sol in a glass container for 30 s and then taken out. The excess catalyst over the monolith channels was blown off using hot compressed air. The monolith was dried using hot air before coating for the second time to confirm uniform and required amount of catalyst loading. After blowing excess sol, monolith was placed in oven at 100 °C overnight, calcined at 500 °C for 2 h and cooled to room temperature. Finally, weight of coated catalyst over the monolithic support was calculated by subtracting final weight of monolith from the initial weight of monolith and named as monolithic Ti3AlC2/TiO2 composite catalyst. Scheme for the synthesis of 3D Ti3AlC2/TiO2 composite and their loading over the monolithic support is demonstrated in Fig. 1. 2.4. Characterization of materials The crystalline phase of catalysts were analysed using X-ray diffraction (XRD) on a D8 advance X-ray diffractometer (Bruke, Germany) operated at 40 kV and 40 mA with Cu Kα radiation source. The morphology and structure was investigated using field emission scanning electron microscopy (FESEM) with Hitachi SU8020 microscope. Similarly, transmission electron microscopy (TEM) images were obtained using HITACHI HT7700 machine. The elemental state of samples was obtained using X-ray photoelectron spectroscopy (XPS) with Axis Ultra DLD instrument. All the binding energies were calibrated by C1 s at 284.6 eV as the internal standard. Ultraviolet-visible (UV–vis) diffuse reflectance spectra were measured using a Cary 100 Agilent spectrophotometer equipped with integrated spheres for loading powder samples. Photoluminescence (PL) spectra were obtained on a HORIBA spectrometer under wavelength 325 nm. Raman analysis were conducted using spectrophotometer (HORIBA Scientific) operated at wavelength 532 nm as an excitation source.

2. Experimental 2.1. Materials Titanium tetraorthotitanate (Merck), isopropanol (Merck) and acetic acid (Merck) were used for the synthesis of TiO2 NPs. Titanium aluminum carbide (Ti3AlC2) with high purity (99.99 %) was purchased from China Company. Similarly, monoliths were purchased from the 100

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

Fig. 1. Scheme for the synthesis of 3D Ti3AlC2/TiO2 composite and their loading over the monolithic support.

2.5. Photoactivity test The performance of catalysts was investigated for photo-catalytic steam reforming of methane (CH4, H2O), dry reforming of methane (CO2−CH4) and bireforming of methane (CO2-CH4-H2O) under UVlight in a fixed bed and monolith photoreactor as illustrated in our previous work [18,32]. The reactor consists of a square vessel, mass flow controllers (MFC), reflector lamp (150 mW cm−2), cooling fans and water saturator. The reactor was fixed with a 10 mm thickness glass window at the top for passing light irradiations. The lamp was located 5 cm above the glass window and cooling fans were provided to remove heat. In addition, feed gases such as CO2 and CH4 were regulated by mass flow controllers (MFC). For investigating SRM and BRM, feed gases were passed through the water saturator to carry moisture. All the experiments were conducted without providing external heat and under atmospheric pressure. In a typical procedure, powder photo-catalyst (0.15 g) was distributed at the reactor bottom for fixed bed reactor, however, for a monolith system, coated monoliths were placed at the centre of the reactor chamber. The feed gases were passed through the reactor for 30 min to remove air and to saturate the catalyst surface. The CO2/CH4 gas mixture with 1.0 feed ratio were constantly passed through the photo-reactor during the entire irradiation time. During photo-catalytic bi-reforming activity test, a mixture of CO2 and CH4 with feed ratio 1.0 was passed through the water saturator to carry moisture. The lamp was turned-on to initiate the reaction and products were analysed using online gas chromatography. GC-TCD/FID (GC-Agilent Technologies 6890 N, USA) was used for the analysis of reaction products. In addition, a HP-PLOT Q capillary was equipped with FID detector for separation of C1-C3 hydrocarbons. Similarly, TCD detector was equipped with Porapak Q and Mol Sieve MS 5A columns for the analysis of CO and H2. 3. Results and discussion Fig. 2. (a) XRD analysis of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 composite samples; (b) Raman spectra of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 composite samples.

3.1. Characterization analysis Fig. 2 (a) presents XRD patterns of Ti3AlC2, TiO2 and Ti3AlC2/TiO2 samples. For the pristine Ti3AlC2, diffraction peaks appeared at 2θ of 9.42°, 19.08°, 33.95°, 35.90°, 38.72°, 41.75°, 48.37°, 52.39°, 56.41°, 60.31° and 70.30° indexed to (002), (004), (101), (103), (104) and (105), (107), (108), (109), (110) and (118), respectively, corresponds

to MAX phase Ti3AlC2 crystal structure (Card #01-074-8806). Similarly, TiO2 diffraction peaks appeared at 2θ of 25.58°, 38.07°, 48.37°, 54.14°, 55.43° and 63.14° indexed to (101), (004), (200), (211), (105) and (204), respectively, confirming TiO2 existence as anatase phase 101

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

(Card# 01-075-2545). After TiO2 dispersed over 3D Ti3AlC2 layered structure, all the peaks of both Ti3AlC2 and TiO2 were appeared. However, TiO2 peak become wider with the Ti3AlC2, resulting in reduced crystallite sizes. Fig. 2 (b) shows Raman spectra of pristine TiO2, Ti3AlC2 and Ti3AlC2/TiO2 composite samples. The bands at 142, 197, 394, 512 and 635 cm−1 are assigned to Eg(1), Eg(2), B1g, A1g+B1g and Eg(3) modes of TiO2. Similarly, Ti3AlC2 bands appeared at ca. 150, 204, 264, 412 and 605 cm−1, attributed to ω1, ω2, ω3, ω4 and ω5 characteristic modes of MAX Ti3AlC2 phase. All the modes are associated to Ti and Al in Ti3AlC2 sample. In the literature similar observations have been reported for the characteristic bands of Ti3AlC2 [33]. No obvious characteristic bands of Ti3AlC2 found in the Raman Spectra of Ti3AlC2/TiO2 composite, because of their overlapping with TiO2 bands. This further confirms that TiO2 bands have higher frequencies compared to Al. However, it is worth noted that the band of TiO2 at 142 cm−1 was shifted towards 154 cm−1 after loading Ti3AlC2, confirming it affects the Raman vibrational frequencies of TiO2. This indicates well distribution and a good interaction of Ti3AlC2 with TiO2 to develop Ti3AlC2/TiO2 composite. The morphology of pure TiO2, pristine Ti3AlC2, oxidized Ti3AlC2 and Ti3AlC2/TiO2 composite was investigated using FESEM and results are presented in Fig. 3. Fig. 3 (a) shows spherical and uniform size TiO2 NPs, produced during controlled sol-gel approach. Fig. 3 (b) shows untreated Ti3AlC2 samples with all the sheets stacked together. When Ti3AlC2 were heated to 500 °C, it was partially oxidized, producing TiO2 NPs over Ti3AlC2 surface as presented in Fig. 3 (c–d). However, when Ti3AlC2 was dispersed into TiO2, a good dispersion of TiO2 NPs over 3D Ti3AlC2 obtained, mainly attributed to multi-layered structure as shown in Fig. 3 (e). Fig. 3 (f) further confirms uniformly distributed TiO2 NPs over the Ti3AlC2 layers. Thus, SEM images of Ti3AlC2/TiO2 composite confirms an obvious distributed TiO2 NPs over 3D layered structure to provide more interface for conducting electrons. This is consistent with XRD patterns and Raman spectra results, thus further confirms the formation of Ti3AlC2/TiO2 composites. The structure and morphology of TiO2 and Ti3AlC2/TiO2 composite was further investigated using TEM analysis as demonstrated in Fig. 4. Fig. 4 (a) presents pure TiO2 images with uniform size TiO2 NPs. The dspacing for TiO2 (101) of 0.35 nm was obtained, corresponds to pure anatase phase TiO2 as shown in Fig. 4 (b). The TEM analysis of Ti3AlC2/ TiO2 has been presented in Fig. 4 (c–d). It could be seen that Ti3AlC2 dark color sheets are obvious indicating the existence of exfoliated nanostructure. The presence of TiO2 NPs with their uniform distribution over the Ti3AlC2 sheets are also obvious. The lattice fringe d-spacing were identified for crystalline TiO2 and Ti3AlC2 structures [34]. The lattice spacing for TiO2 of 0.35 nm was obtained corresponds to (101) crystal plane of TiO2. Similarly, lattice fringes of 0.21 and 0.46 nm were identified corresponding to Ti3AlC2 and in agreement with XRD peaks. The surface chemical states of the TiO2, Ti3AlC2 and Ti3AlC2/TiO2 samples were investigated using XPS and results are presented in Fig. 5. Fig. 5 (a) shows Ti 2p spectra of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 samples. The Ti 2p spectrum of TiO2 displays two peaks with binding energies 458.5 and 464.2 eV, corresponds to Ti 2p3/2 and Ti 2p1/2, respectively, and can be assigned to titanium as Ti+4. However, Ti 2p spectrum of Ti3AlC2 are deconvoluted into four peaks, in which 453.8 and 459.9 eV can be assigned to Ti-C and 457.5 and 463.9 eV belonging to Ti-O in Ti3AlC2 sample. Compared to Ti3AlC2 and TiO2, the binding energies of all the Ti-related peaks in the Ti3AlC2/TiO2 exhibits a positive shift. The Ti 2p binding energies in Ti3AlC2/TiO2 appeared at 458.6 and 464.3 eV relating to Ti 2p3/2 and Ti 2p1/2, respectively, and can be assigned to titanium as Ti4+ or TiO2. Similarly, additional peaks appeared at 455.7 and 461.1 eV can be assigned to titanium as Ti-C in Ti3AlC2 [35]. The high resolution C 1s spectra of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 samples have been demonstrated in Fig. 5 (b). The C 1s spectrum of TiO2 reflects two peaks with binding energies 284.6 and 286.7 eV, which are attributed to C-C and C = O, respectively. Ti3AlC2

presents four peaks with binding energies 281.0, 284.6, 286.4 and 288.5 eV, corresponds to C-Ti, C-C, C-O-C and C = O, respectively, attributing to Ti3AlC2 and adventurous carbon. Similarly, C 1s binding energies of Ti3AlC2/TiO2 appeared at 282.8, 284.6, 286.5 and 288.5 eV, corresponds to C-Ti, C-C, C-O-C and C = O, respectively [33,36]. This reveals that pristine Ti3AlC2 and Ti3AlC2/TiO2 samples have similar reflection and the presence of carbon was obviously due to Ti3AlC2. Furthermore, no obvious Ti-C peak detected in Ti3AlC2/TiO2 sample, because of the limited content of Ti3AlC2 or due to its good dispersion with TiO2 in addition of surface covered by TiO2 NPs. Fig. 5 (c–d) demonstrates XPS spectra of Al 2p for Ti3AlC2 and Ti3AlC2/TiO2 samples. The Al 2p spectrum of Ti3AlC2 appeared at 70.4 and 73.1 eV, corresponds to Al in MAX and Al-Ti-O, respectively [5]. Similarly, Al 2p in Ti3AlC2/TiO2 with binding energy 74.1 and 75.8 eV, attributed to Al as Al-Ti-O and Al oxide in the composite sample, respectively [24]. This was obviously due to exfoliated Ti3AlC2 layers with good interaction with TiO2 to develop heterojunction. Fig. 6 (a) demonstrates FTIR analysis of TiO2, Ti3AlC2 and Ti3AlC2/ TiO2 composite samples. Pure TiO2 shows band at 673 cm−1 relating to Ti-O-Ti vibrations, 1650 and ∼ 3400 cm−1 relating to OH vibrations. The presence of water was probably due to the adsorbed H2O molecules over the catalyst surface. In FTIR spectrum of Ti3AlC2, bands were appeared between 2100−2400 cm−1 assigned to AleOH stretching vibration, bands between 1200−1500 cm−1 ascribed to CeC and CeO vibrations modes while bands between 400−600 cm−1 present AleO, TieO and TieC vibration modes in Ti3AlC2 sample. Besides, 1650 and 3752 cm−1 bands attributed to OH vibrational modes. Furthermore, composite of Ti3AlC2/TiO2 reflects the similar patterns of vibrational modes. Thus, successful fabrication of Ti3AlC2/TiO2 with their good interaction was obtained using the sol-gel method. The light harvesting efficiency of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 composite was investigated by UV–vis diffuse reflectance spectra and the results are presented in Fig. 6 (b). Obviously, Ti3AlC2 shows efficient light absorption because of its dark color, while TiO2 can only absorb UV-light, due to its wide band gap and similarly reported previously [37]. The light absorption of Ti3AlC2/TiO2 is obviously increased in the visible region probably having its dark color. This was also due to TiO2 dispersed homogeneously over the 3D Ti3AlC2 layered structure as evidenced by SEM analysis. The band gap energies were calculated from the plot of (αhv)2 vs. photon energy (eV) as depicted in Fig. 6 (c). The calculated band gap energies were 3.20 and 3.07 eV for TiO2 and Ti3AlC2/TiO2 composite samples, respectively. The decreased in band gap energy of TiO2 with Ti3AlC2 loading was probably due to reduced crystallite sizes of TiO2 NPs. The impact of Ti3AlC2 loading on the interfacial charges separation in TiO2 and Ti3AlC2/TiO2 was further investigated by photoluminescence (PL) measurement and the results are presented in Fig. 6 (d). Generally, lower the PL intensity, lower the electron hole pairs (e−/h+) recombination rate. Intense PL emission peak of TiO2 indicates higher recombination of photo-excited charge carriers (e−/h+). With Ti3AlC2 coupling with TiO2 NPs, PL intensity was decreased, which demonstrates positive effect on the separation of charge carrier in the composite samples. This significantly lower PL intensity in Ti3AlC2/ TiO2 suggesting that heterojunction is successfully developed between Ti3AlC2 and TiO2 which effectively suppresses charge recombination rate. This is also because of strong conductive properties of Ti3AlC2 and electrons would be migrated before being consumed for CO2 reduction. In addition, lower PL intensity of Ti3AlC2/TiO2 composite would also be due to Ti3AlC2 PL intensity closer to zero because of its dark color as similarly reported in UV–vis analysis. Previously, g-C3N4/Ti3C2 composite has been synthesized and reporting the similar results [38]. Likewise, very strong PL intensity of Ag2WO4 was obtained which become weaker in Ti3C2/Ag2WO4 composite sample [39]. All these findings further confirm that MAX based materials are promising for proficient separation of photo-excited charge carriers.

102

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

Fig. 3. FESEM analysis of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 composite; (a) TiO2, (b–c) Ti3AlC2, (d) O-Ti3AlC2, (e–f) TiO2 loaded Ti3AlC2.

to the formation of TiO2 over 3D structure of Ti3AlC2 resulting in faster charge carrier separation. As can be seen that CO yield is distinctly increased by coupling Ti3AlC2 with TiO2 NPs. Highest CO evolution of 1566 μmol g-cat−1 was obtained with Ti3AlC2/TiO2 composite, which is 3.85, 6.81 and 9.91 times higher compared to O-Ti3AlC2, TiO2 and bulk Ti3AlC2 samples, respectively. Similarly, using Ti3AlC2/TiO2 composite, H2 evolution rate was 1.2, 1.47, and 2.78 folds higher than using OTi3AlC2, TiO2 and Ti3AlC2 samples, respectively. This significantly improved activity with the loading of Ti3AlC2 into TiO2 was due to its conductive properties and working as a mediator for transporting charge carrier [40]. Although, coupling Ti3AlC2 is beneficial for transporting electrons, however, an excess quantity should have adverse effects on the photocatalytic performance for CO2 reduction. An excess loading of Ti3AlC2 lead to the light shielding effect. For example, any excess loading of Ti3AlC2 would reduce the amount of TiO2 resulting in minimizing absorption of UV-light irradiations, thus suppressing the photocatalytic activity of TiO2 to generate electrons.

3.2. Photocatalytic CO2 reduction to fuels 3.2.1. Effect of partial oxidation of Ti3AlC2 and TiO2 loading The photocatalytic conversion of CO2 using methane and water via dry and bireforming of methane was conducted in a gas phase fixed bed and monolith photoreactors under UV-light irradiations in a continuous flow system. Initially, qualitative experiments were conducted to ensure CO and H2 were obtained during photocatalysis process under light irradiation. The photoactivity of catalysts for photocatalytic H2 and CO production has been displayed in Fig. 7. Using all the catalysts, CO was generated effectively with appreciable amounts of H2 evolution. Using pristine Ti3AlC2, lower amounts of CO and H2 were produced. Similarly, using pristine TiO2, CO and H2 were the main products with higher amount of CO evolution. The efficiency of Ti3AlC2 was lower compared to TiO2, as Ti3AlC2 has conductive properties and TiO2 works as a semiconductor for oxidation and reduction process. When Ti3AlC2 was partially oxidized, its efficiency was obviously enhanced. This was due 103

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

Fig. 4. TEM analysis of TiO2 and Ti3AlC2/TiO2 composite samples; (a) morphology of TiO2 NPs, (b) d-spacing of TiO2, (c) TEM images of Ti3AlC2/TiO2, (d) d-spacing analysis of Ti3AlC2 and TiO2 NPs.

while production of C2H6 and H2 were 125 and 128 μmole g-cat−1 after 2 h of irradiation time. These findings reveal selective production of CO during DRM process. This significantly enhanced CO production during the DRM process was due to selective reverse water gas shift (RWGS) reaction, resulting in lower H2 evolution. According to DRM, equal moles of CO and H2 would be produced, however, currently, CO to H2 molar ratio of 12 is obtained. This revealed that selective CO production, obviously, due to higher separation and transportation of charge carrier and proficient sorption process over Ti3AlC2/TiO2 composite. Previously in literature, MgO was employed for photocatalytic DRM for CO and H2 evolution of 3.6 and 0.5 μmole with CO/H2 ratio of 7.2 under UV-light irradiation [43]. Similarly, Ga2O3 was tested for photocatalytic DRM with C2H6, CO and H2 yield rate of 1.2, 3.82 and 10.53μmole g-cat-1 h-1, respectively [44]. Recently, Au/TNTs with CO, H2, C2H6 and CH3OH production of 11.9, 104, 11.4 and 0.95 μmole gcat-1 h-1, respectively, were reported during the DRM process under UVlight [16]. Previously, we reported enhanced photocatalytic DRM process over Ni-MMT/TiO2 under UV-light, however, catalyst lost photoactivity over irradiation time [18]. Photocatalytic bireforming of methane for the production of CO, H2 and C2H6 is presented in Fig. 8 (c). The highest amount of CO of 1860 μmole g-cat−1 was achieved after 2 h irradiation time, which is 4.69 and 6.87 folds higher than production of H2 and C2H6, respectively. Obviously, CO was highest and H2 production was apparently increased compared to SRM and DRM process. This confirms oxidation and reduction process over the Ti3AlC2/TiO2 composite was further increased by adding water to CO2/CH4 mixture. This was probably due to more production of protons which promoted H2 evolution due to more consumption of protons. Previously, we tested DRM over La/TiO2 and observed enhanced CO and H2 production under UV-light irradiation in which smaller amount of H2 was detected [32]. In other work, we reported the use of Cu/g-C3N4 for photocatalytic CO2 reduction by H2O

3.2.2. Performance comparison of steam, dry and bireforming of methane The photocatalytic activity of the Ti3AlC2/TiO2 composite for the production of CO, H2 and C2H6 was further evaluated through steam reforming of methane (SRM), dry reforming of methane (DRM) and bireforming of methane (BRM) in a fixed bed reactor under UV-light irradiation. Fig. 8 (a) presents SRM for the production of CO, H2 and C2H6 at different irradiation time. The Ti3AlC2/TiO2 composite presents best photocatalyst performance with irradiation time 2 h during the SRM process. Remarkably, CO and C2H6 were observed as the main products with a significant amount of H2 evolution. However, photoactivity was somewhat declined after 2 h of irradiation time. This was probably due to the catalyst surface saturated with the side products such as hydrocarbons that were not detected during the analysis [17,41]. The CO amount of 1188 μmole g-cat−1 was obtained after 2 h which is 1.16 times higher than the production of C2H6 and 9 fold more than the yield of H2 obtained. Obviously, production of CO and C2H6 were much closer, confirming single step production of hydrocarbons using photocatalytic SRM due to proficient charges separation and utilization with appropriate redox potential for the production of hydrocarbons. Previously, photocatalytic steam reforming of methane was reported over Ag/g-C3N4 with the production of CO and H2 as the main products [2]. Direct production of H2 from photocatalytic water and methane conversion over Pt/TiO2 has been reported. The other products identified were CO2 and C2H6 under UV-light of wavelength 254 nm [42]. This confirms that adsorption of methane was proficient due to 3D layered structure of Ti3AlC2 with surface vacancies, resulting in more coupling of ethane during methane oxidation process. The production of CO, H2 and C2H6 during photocatalytic DRM process are presented in Fig. 8 (b). During the DRM process, the results were different than using SRM, in which CO and H2 were obtained as the major CO2 and CH4 conversion products. The CO yield of 1566 μmole g-cat−1 was produced using DRM process over Ti3AlC2/TiO2, 104

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

Fig. 5. XPS analysis of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 composite samples; (a) Ti2p of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 samples, (b) C 1 s of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 samples, (c) Al 2p of Ti3AlC2, and (d) Al 2p of Ti3AlC2/TiO2 sample.

presented in Fig. 9. Using both photoreactors, CO, H2 and C2H6 were obtained as the major products and their yield rate is totally dependent on the reactor types. Fig. 9 (a) shows CO production at different irradiation times in a fixed bed and monolith photoreactor. Obviously, continuous production of CO was obtained in both reactors, confirming the high stability of the catalyst for CO production during the BRM process. The CO production of 2791 μmol g-cat−1 was obtained after 5 h in a fixed bed reactor, which is 1.4 folds higher than using monolith photoreactor. This could be due to prominent RWGS reaction for CO production in a fixed bed reactor. The production of H2 over Ti3AlC2/ TiO2 composite in a fixed bed and in a monolith system during BRM has been presented in Fig. 9 (b). Obviously, production of H2 was highest in a monolith reactor than using fixed and production trends were consistent throughout the irradiation time. The amount of H2 evolution of 1561 μmol g-cat−1 was obtained in a monolith photoreactor which is 3.13 folds more that using fixed-bed photoreactor under the same

with the production of CO, H2 and CH3OH but in smaller amounts under visible light irradiation [45]. Comparing all three processes, CO production of 1188, 1566 and 1860 μmole g-cat−1 was obtained during photocatalytic SRM, DRM and BRM processes, respectively under UV irradiations using fixed-bed reactor. Similarly, production of H2 was 132, 125 and 375 μmole g-cat−1 through SRM, DRM and BRM processes under the same operating conditions. This confirms higher CO and H2 would be produced through photocatalytic bireforming of methane in a continuous flow photoreactor system over 3D MAX based Ti3AlC2/TiO2 composite under UVlight irradiation. 3.2.3. Performance comparison of powder catalyst with monolithic support The photocatalytic performance of 3D Ti3AlC2/TiO2 was further estimated for CO, H2 and C2H6 production in a fixed bed and monolith photoreactor systems under UV-light irradiations and results are 105

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

Fig. 6. (a) FTIR analysis of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 samples; (b) UV–vis analysis of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 composite samples; (c) Plot of (αhv)2 versus photon energy (eV) for band gap calculation; (d) PL analysis of TiO2, Ti3AlC2 and Ti3AlC2/TiO2 composite samples.

in a monolith reactor, but it was produced in a significant amount using fixed-bed reactor as presented in Fig. 9 (c). The production of C2H6 in a fixed bed reactor was declined after 4 h of irradiation time, however, a continuous production of C2H6 was achieved using monolith photoreactor. This reveals higher stability of catalyst loaded over monolithic support than it was employed as a bulk powder bed. The C2H6 yield of 413 μmol g-cat−1 after 4 h was obtained, which is 7.6 folds higher than using monolith photoreactor. Selective production of CO, H2 and C2H6 over Ti3AlC2/TiO2 in a fixed bed and monolith photoreactor are demonstrated in Fig. 9 (d–e). It could be seen that photon flux only strikes at the small portion of catalysts, resulting in less production of charge carrier during photocatalysis process. However, using monolith reactors, photon flux is penetrating through the monolith channels, thus activating entire exposed catalyst surface area, resulting in surplus production of charges for oxidation and reduction process. Furthermore, in a fixed bed reactor, there are mass transfer limitations due to the small portion of catalyst interfacial contact with gas molecules. Comparatively, in a monolith photoreactor, gas mixture is passing through the channels, resulting in effective diffusion and transfer of species towards the catalyst surface. Using fixed bed reactor, the highest amount of CO was obtained, probably due to parallel BRM and RWGS reaction. Comparatively, highest H2 was generated in a monolith photoreactor, perhaps, due to parallel BRM and WGS reaction under the same operating conditions. Therefore, using monolith photo-reactor system, higher photon flux utilization with a larger contact area of light with feed and catalyst and enhanced space volume would stimulate H2 production under the same operating conditions. This demonstrated that the

Fig. 7. Effect of Ti3AlC2 loading on the performance of TiO2 for photocatalytic dry reforming of methane for the production of CO, H2 and C2H6 in a fixed bed photoreactor.

reaction conditions. This selective H2 evolution in a monolith photoreactor was obviously due to larger illuminated active surface area with efficient adsorption-desorption process and minimum mass transfer limitations. On the other hand, smaller amount of C2H6 was produced

106

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

under different photon flux according to Eq. (3) [46].

Quantum

Yield,

QY (%) =

n × production rate (μmole / s ) photon flux (μmole / s )

(3)

QY was calculated using 2, 2 and 14 electrons for H2, CO and C2H6 productions with light incident area 36 cm2 and light intensity 100 mW cm−1 with wavelength 254 nm. The summary of QY for CO, H2 and C2H6 production in a fixed bed and monolith photoreactor is presented in Table 1. During the SRM process, a QY of 2.22 % was obtained for C2H6 production in a fixed bed which is 5.15 and 44.4 % higher than QY for CO and H2 production. This confirms that methane steam reforming effectively produces ethane in addition of CO and H2 due to proficient oxidation process. This also confirms effective adsorption of water and methane over the catalyst surface for the production of above products. However, DRM gave highest QY for CO production of 0.568 %, which is 2.07 and 12.34 folds higher than QY for C2H6 and H2 production in a fixed bed reactor. A further increase in QY for CO, C2H6 and H2 was obtained when H2O was added into DRM system. The QY for CO production with BRM in a fixed bed reactor was 1.14 and 4.96 folds higher than C2H6 and H2 production, respectively. Furthermore, CO QY using BRM and fixed bed reactor was 1.18 and 1.56 folds higher compared to DRM and SRM, respectively. Similarly, QY for H2 evolution with BRM was 2.95 and 2.72 folds more than using DRM and SRM reforming systems. Comparatively, QY for C2H6 was highest using SRM with fixed bed reactor which was 3.75 and 8.10 folds higher than using BRM and DRM systems. By comparing performance of the reactors, highest QY was obtained using the BRM reaction system for CO production in a fixed bed reactor while monolith photoreactor promoted QY for H2 production. The QY for H2 production in a monolith was 3.52 folds higher than using fixed bed reactor during the BRM process. This confirms hydrogen rich syngas production with the same catalyst and photon flux using monolith photoreactor. This was obviously due to proficient utilization of photon energy with more production of electrons and faster sorption process in a monolith photoreactor. There are limited reports available on photocatalytic CO2 reduction through SRM, DRM and BRM processes and in most of the reports, QY has not been reported. A QY of 0.567 and 0.184 % for CO and H2 has been reported during photocatalytic dry reforming of methane over Agmodified g-C3N4 catalyst under UV-light irradiation of intensity 150 mW cm−2 [2]. During photocatalytic CO2 reduction with H2O, a QY of 0.057 % has been reported for acetaldehyde production using NiO/InTaO4 in an internally illuminated monolith photoreactor system [47]. Similarly, a QY of 0.015 % was reported during photocatalytic CO2 reduction with H2O in a monolith photoreactor in the presence of Ru-Pd/TiO2 catalyst [48]. Previously, we reported photocatalytic dry reforming of methane over La/TiO2 with CO and H2 as the main products [32]. However, catalyst activity was declined over the irradiation time in a fixed-bed photocatalytic reactor. The significantly enhanced QY in the current study was probably due to the followings: (a) effective transfer of photo-generated electrons from the conduction (CB) of TiO2 on the surface of 3D layered Ti3AlC2 due to large interfacial contact area and good interaction due to growth of TiO2 NPs over Ti3AlC2 employing sol-gel approach; (b) aAs the catalyst is more proficient for CO2 adsorption, thus introducing water into CO2/CH4 systems, enables efficient adsorption of all the reactants and provides equal opportunities for oxidation and reduction of adsorbed molecules, (c) with the addition of the Ti3AlC2 into TiO2 increases the specific surface area and enhances the adsorption ability of photocatalytic system, (d) monolithic support provides larger illuminated surface area with higher photon flux consumption for generating photo-induced electrons, (f) proficient adsorption and desorption process and reactive species over the structured catalyst and inside monolithic system and surface reactions with minimum mass transfer limitations.

Fig. 8. Performance of Ti3AlC2/TiO2 for CO, H2 and C2H6 production in different reforming processes: (a) SRM; (b) DRM and (c) BRM process.

engineering approach through the monolith reactor is an efficient system for photocatalytic BRM because of large exposed catalyst film to light flux in a larger volume of the reactor. 3.2.4. Performance comparison with literature Performance comparison of catalysts and reactors can be conducted by calculating product yield rates, however, this is not an appropriate approach in photocatalytic reaction systems due to using different catalyst loading and light intensity. Therefore, ideal method is to calculate quantum yield (QY) of different reactors and catalysts employed 107

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

Fig. 9. Performance comparison of fixed bed and monolith photoreactor for photocatalytic BRM process over Ti3AlC2/TiO2 composite for CO, H2 and C2H6 production: (a) CO production; (b) H2 production; (c) C2H6 production; (d) Schematic of fixed bed reactor process; (e) Schematic of monolith photoreactor system.

CO was continuously evolved. This reveals that RWGS reaction was activated and consuming H2 for CO production. In addition, there was no obvious declined in photo-activity for CO, H2 and C2H6 production, confirming the high stability of 3D Ti3AlC2 MAX based TiO2 composite. This would also be due to faster sorption process with larger illuminated surface area, thus minimizing carbon formation. According to literature, an obvious declined in photo-activity was observed during photocatalytic DRM and BRM processes. For example, Ag-La modified g-C3N4 catalyst was tested for photocatalytic CO2 reduction through DRM and BRM process in a monolith photoreactor but catalyst lost activity over irradiation time [49]. Similarly, a declined in photo-activity of Ag/pg-C3N4 catalyst has been reported during photocatalytic BRM process [50]. In another development, La/TiO2 catalyst for photocatalytic DRM was tested and reported declined catalyst activity in cycles [32]. To understand high stability in the current work, analysis of the spent catalyst was further conducted. Fig. 10 (b) shows PL analysis of Ti3AlC2/TiO2 before and after the photocatalytic bireforming of methane reaction. After stability experiments, PL intensity was slightly increased, probably due to distortion of TiO2 NPs over the Ti3AlC2 surface. However, it did not affect the catalyst activity and continuous and stable production of CO, H2 and C2H6 were obtained. Fig. 10 (c) presents Raman spectra of fresh and spent Ti3AlC2/TiO2 composite catalysts after three cycles. Obviously, all the Raman peaks positioned

Table 1 Summary of production rate and Quantum Yield (%) for CO2 reduction with CH4/H2O in a fixed-bed and monolith photoreactor. Feed

CH4-H2O CO2-CH4 CO2-CH4-H2O CO2-CH4-H2O

Type of reactor

Fixed-bed Fixed-bed Fixed-bed Monolith

Production Rate (μmole/g-cat. h)

Quantum Yield, QY (%)

CO

H2

C2H6

CO

H2

C2H6

594 783 930 575

66 64 188 660

511 63 136 13

0.431 0.568 0.675 0.417

0.05 0.046 0.136 0.479

2.22 0.274 0.592 0.056

3.2.5. Stability analysis The stability and recyclability analysis of catalyst is very important to understand the performance of catalysts for practical applications. The experiments were conducted in three cycles for the duration of 20 h. After every cycle, the lamp was turned off while feed gases were constantly flowing through the reactor. Fig. 10 (a) shows stability of composite catalyst for photocatalytic bireforming of methane over Ti3AlC2/TiO2 composite in a continuous flow monolith photoreactor system. In all the three cycles, CO and H2 were identified as the main bireforming products and they were constantly produced with irradiation times. More important, initially, highest amount of H2 was produced, however, after 2 h, its production reached to steady state, while 108

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

Fig. 10. (A) stability analysis of Ti3AlC2/TiO2 composite in a monolith reactor; (B) PL analysis of fresh and spent catalyst; (C) Raman analysis of fresh and spent samples; (D) SEM images of fresh and spent samples.

CH 4 → C + 2H2

(4)

CO + H2 → C+ H2 O

(5)

CO2 + 2H2 → C+ 2H2 O

(6)

(7)

2CO → C+ CO2

at the same place in both the fresh and spent catalyst samples. No significant changes before and after the reaction were observed, suggesting crystal structure and interaction among the components remained unchanged after the photocatalytic reaction. Besides, no additional peak appeared relating to the amorphous carbon formation, further confirmed higher stability of catalyst samples. The morphology of spent catalyst was further investigated using SEM analysis as shown in Fig. 10 (d). There was no obvious effect on catalyst morphology, confirming good interaction of TiO2 particles with Ti3AlC2 sheets. This further confirms higher activity and stability of composite catalyst due to 3D Ti3AlC2 layered structure. It is well established that methane generates coke over the catalyst surface during cracking reaction as shown in Eq. (4). Similarly, series of coke formation reactions like CO and CO2 reduction reactions in Eq. (5) and (6) and boudouard reaction in Eq. (7) have probability to contribute in coke formation. According to spent catalysts analysis, there was no coke formation over the catalyst surface even after three cycles. This confirms all these reactions were not activated, probably due to the presence of Ti3AlC2 MAX and using monolithic support with a lower operating temperature.

3.2.6. Proposed mechanism for CO2 reforming of methane It is well-established that faster separation and transforming charge carriers (e_/h+) within the catalyst surface are accountable for promoting photo-activity. Similarly, if the exposed catalyst surface area with light irradiation is increased such as using monolith photoreactor, there would be more production of charges due to higher photon flux utilization for activating photocatalyst. Therefore, possible transforming of charge carriers and photocatalytic reaction mechanism for CO2 reduction with CH4/H2O through bireforming towards selective production of H2 and CO under UV-light is presented in Fig. 11. A good interface heterojunction was developed between the Ti3AlC2 and TiO2, resulting in faster transfer and separation of charges due to higher conductive properties of 3D Ti3AlC2 MAX. The photo-catalysis is a multi-step reaction with involvement of holes for the oxidation of CH4/ H2O and electrons for the reduction of CO2 to get final products such as CO, H2 and C2H6 in several steps according to reactions in Eqs. (8)–(17) [51]. A: Photoactivation of catalyst hv

TiO2 ⟶TiO2 (e−) + TiO2 (h+) Ti3AlC2 + TiO2

(e−)

→ Ti3AlC2

B: Oxidation process 109

(8)

(e−)

+ TiO2

(9)

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

Fig. 11. (a) Morphological scheme of catalyst activation; (b) schematic of BRM process over Ti3AlC2/TiO2 composite, (c) Steps in BRM process for CO, H2 and C2H6 production.

CH 4 + h+ → CH•3 + H+

(10)

H2 O+h+ → OH−+2H+

(11)

OH−+h+ → OH•

(12)

CH•3+OH• → CO + 4H+

(13)

2CH•3

(14)

→ C2 H 6

(-0.41 V), thus, thermodynamically, these products have the potential to be produced using composite catalyst. Furthermore, compared to fixed-bed, monolith photoreactor was more efficient due to efficient oxidation and reduction reactions over a larger illuminated surface area, enabling more production of CO and H2 from the stable CO2 and CH4 molecules. Besides, fixed bed reactor has a slow sorption process, and there would be effective RWGS reaction, promoting CO production. Compared to DRM, when H2O is added to CO2 and CH4 reaction system, it produces more proton and hydroxyl ions, enabling more H2 and CO generation. Using monolithic support, catalyst exposed surface area to light irradiation is much higher compared to fixed bed, resulting in more production of charges and their effective separation to get proficient reforming photocatalytic activity.

C: Reduction process

CO2 + e− → CO•2−

(15)

CO•2−

(16)

+ H• → CO + H2 O

2H+ + 2e− → H2

(17) 4. Conclusions

When light strikes over the catalyst surface, charge carrier (e-) and (h+) are produced as illustrated in Eq. (8). These charge carriers will recombine with the release of unproductive heat, if there is no reactant (CO2, CH4, and H2O) adsorbed over the catalyst surface or there is no sink to trap electrons. In this system, electrons were effectively separated due to excellent conductive properties of Ti3AlC2 as illustrated in Eq. (9). The oxidation of adsorbed CH4 and H2O occurs through utilizing holes as illustrated in Eqs. (10) & (11). The intermediate products were further converted to CO and C2H6 through further steps as explained in Eqs. (12)–(14). Similarly, electrons (e−) are consumed in the reduction of CO2 with the production of CO and H2 as illustrated in Eqs. (15)–(17) [18]. According to experimental results, CO and H2 were identified as the main products during photocatalytic bireforming of methane over 3D MAX Ti3AlC2/TiO2 composite catalyst in a monolith photoreactor which confirms selective oxidation and reduction process for yielding these products. The conduction band of TiO2 (CB= ∼ -0.50 V) calculated in our previous work [52] is at a higher position than the reduction potential of CO and H2 e.g., CO2/CO (-0.48) and H+/H2

3D Ti3AlC2 MAX based TiO2 composite was designed and fabricated via sol-gel approach. Benefiting from the conductive properties of Ti3AlC2, photo-generated charge carrier of TiO2 can be effectively separated and transferred. The highest photoactivity was achieved with Ti3AlC2/TiO2 with CO, H2 and C2H6 production rate of 930, 188 and 136 μmol g-cat−1 h-1 in a fixed-bed reactor under UV-light irradiations. By comparing different reforming processes, efficiency of BRM with Ti3AlC2/TiO2 for CO production were 1.18 and 1.56 folds higher than using DRM and SRM, respectively. This was obviously due to more utilization of charge carriers for oxidation and reduction process over electron rich Ti3AlC2/TiO2 catalyst. More importantly, Ti3AlC2/TiO2 catalyst performance was further improved when used in a monolith photoreactor with the selective production of H2 and CO. The yield rate of H2 as the main product in a monolith photoreactor was 3.5 folds higher than using fixed bed reactor. In addition, QY in a monolith photoreactor for CO and H2 production of 0.417 and 0.479 % were obtained. The H2/CO ratio in fixed bed of 0.20 was increased to 1.15 110

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

which is 5.67 folds higher during photocatalytic BRM process. The enhanced and selective CO2 reduction to CO and H2 was due to larger illuminated surface area and faster sorption process. The stability analysis further confirms continuous activity in cycles without obvious catalyst deactivation. This work also promotes further understanding and mechanism during photocatalytic dry and bireforming for the selective production of CO and H2 under UV-light. The findings from this work provide new idea for the design and development of new materials in recycling greenhouse gases to valuable chemicals.

(DRM) to fuels, Appl. Surf. Sci. 504 (2020) 144177. [18] M. Tahir, B. Tahir, Z.Y. Zakaria, A. Muhammad, Enhanced photocatalytic carbon dioxide reforming of methane to fuels over nickel and montmorillonite supported TiO2 nanocomposite under UV-light using monolith photoreactor, J. Cleaner Prod. 213 (2019) 451–461. [19] H. Wang, D. Wu, C. Yang, H. Lu, Z. Gao, F. Xu, K. Jiang, Multi-functional amorphous TiO2 layer on ZIF-67 for enhanced CO2 photoreduction performances under visible light, J. CO2 Util. 34 (2019) 411–421. [20] S. Wan, M. Chen, M. Ou, Q. Zhong, Plasmonic Ag nanoparticles decorated SrTiO3 nanocubes for enhanced photocatalytic CO2 reduction and H2 evolution under visible light irradiation, J. CO2 Util. 33 (2019) 357–364. [21] D.O. Adekoya, M. Tahir, N.A.S. Amin, g-C3N4/(Cu/TiO2) nanocomposite for enhanced photoreduction of CO2 to CH3 OH and HCOOH under UV/visible light, J. CO2 Util. 18 (2017) 261–274. [22] C. Yang, Q. Li, Y. Xia, K. Lv, M. Li, Enhanced visible-light photocatalytic CO2 reduction performance of Znln2S4 microspheres by using CeO2 as cocatalyst, Appl. Surf. Sci. 464 (2019) 388–395. [23] C. Peng, X. Yang, Y. Li, H. Yu, H. Wang, F. Peng, Hybrids of two-dimensional Ti3C2 and TiO2 exposing {001} facets toward enhanced photocatalytic activity, ACS Appl. Mater. Interfaces 8 (9) (2016) 6051–6060. [24] W.H.K. Ng, E.S. Gnanakumar, E. Batyrev, S.K. Sharma, P.K. Pujari, H.F. Greer, W. Zhou, R. Sakidja, G. Rothenberg, M.W. Barsoum, N.R. Shiju, The Ti3AlC2 MAX phase as an efficient catalyst for oxidative dehydrogenation of n-Butane, Angew. Chem. Int. Ed. Engl. 57 (6) (2018) 1485–1490. [25] M. Liu, J. Chen, H. Cui, X. Sun, S. Liu, M. Xie, Ag/Ti3AlC2 composites with high hardness, high strength and high conductivity, Mater. Lett. 213 (2018) 269–273. [26] K. Yuan, L. Yang, X. Du, Y. Yang, Performance analysis of photocatalytic CO2 reduction in optical fiber monolith reactor with multiple inverse lights, Energy Convers. Manage. 81 (2014) 98–105. [27] R. Chen, X. Cheng, X. Zhu, Q. Liao, L. An, D.D. Ye, X.F. He, Z.B. Wang, Highperformance optofluidic membrane microreactor with a mesoporous CdS/TiO2/ SBA-15@carbon paper composite membrane for the CO2 photoreduction, Chem. Eng. J. 316 (2017) 911–918. [28] T. Wang, L. Yang, X. Du, Y. Yang, Numerical investigation on CO2 photocatalytic reduction in optical fiber monolith reactor, Energy Convers. Manage. 65 (2013) 299–307. [29] R. Zhang, Z. Huang, C. Li, Y. Zuo, Y. Zhou, Monolithic g-C3N4/reduced graphene oxide aerogel with in situ embedding of Pd nanoparticles for hydrogenation of CO2 to CH4, Appl. Surf. Sci. 475 (2019) 953–960. [30] N. Fajrina, M. Tahir, Engineering approach in stimulating photocatalytic H2 production in a slurry and monolithic photoreactor systems using Ag-bridged Z-scheme pCN/TiO2 nanocomposite, Chem. Eng. J. 374 (2019) 1076–1095. [31] M. Tahir, Photocatalytic carbon dioxide reduction to fuels in continuous flow monolith photoreactor using montmorillonite dispersed Fe/TiO2 nanocatalyst, J. Cleaner Prod. 170 (2018) 242–250. [32] B. Tahir, M. Tahir, N.A.S. Amin, Tailoring performance of La-modified TiO2 nanocatalyst for continuous photocatalytic CO2 reforming of CH4 to fuels in the presence of H2O, Energy Convers. Manage. 159 (2018) 284–298. [33] J. Low, L. Zhang, T. Tong, B. Shen, J. Yu, TiO2/MXene Ti3C2 composite with excellent photocatalytic CO2 reduction activity, J. Catal. 361 (2018) 255–266. [34] X. Yu, W. Yin, T. Wang, Y. Zhang, Decorating g-C3N4 nanosheets with Ti3C2 MXene nanoparticles for efficient oxygen reduction reaction, Langmuir 35 (8) (2019) 2909–2916. [35] W. Lian, Y. Mai, J. Wang, L. Zhang, C. Liu, X. Jie, Fabrication of graphene oxideTi3AlC2 synergistically reinforced copper matrix composites with enhanced tribological performance, Ceram. Int. 45 (2019) 18592–18598. [36] L. Pan, J. Zhang, X. Jia, Y.-H. Ma, X. Zhang, L. Wang, J.-J. Zou, Highly efficient Zscheme WO3–x quantum dots/TiO2 for photocatalytic hydrogen generation, Chin. J. Catal. 38 (2) (2017) 253–259. [37] Y. Li, Z. Yin, G. Ji, Z. Liang, Y. Xue, Y. Guo, J. Tian, X. Wang, H. Cui, 2D/2D/2D heterojunction of Ti3C2 MXene/MoS2 nanosheets/TiO2 nanosheets with exposed (001) facets toward enhanced photocatalytic hydrogen production activity, ppl, Catal. B: Environ. 246 (2019) 12–20. [38] N. Liu, N. Lu, Y. Su, P. Wang, X. Quan, Fabrication of g-C3N4/Ti3C2 composite and its visible-light photocatalytic capability for ciprofloxacin degradation, Sep. Purif. Technol. 211 (2019) 782–789. [39] Y. Fang, Y. Cao, Q. Chen, Synthesis of an Ag2WO4/Ti3C2 Schottky composite by electrostatic traction and its photocatalytic activity, Ceram. Int. 45 (2019) 22298–22307. [40] A. Bafaqeer, M. Tahir, A. Ali Khan, N.A. Saidina Amin, Indirect Z-scheme assembly of 2D ZnV2O6/RGO/g-C3N4 nanosheets with RGO/pCN as solid-state Electron mediators toward visible-light-enhanced CO2 reduction, Ind. Eng. Chem. Res. 58 (2019) 8612–8624. [41] B. Tahir, M. Tahir, N.A.S. Amin, Ag-La loaded protonated carbon nitrides nanotubes (pCNNT) with improved charge separation in a monolithic honeycomb photoreactor for enhanced bireforming of methane (BRM) to fuels, Appl. Catal. B: Environ. 248 (2019) 167–183. [42] L. Yu, Y. Shao, D. Li, Direct combination of hydrogen evolution from water and methane conversion in a photocatalytic system over Pt/TiO2, Appl. Catal. B: Environ. 204 (2017) 216–223. [43] K. Teramura, T. Tanaka, H. Ishikawa, Y. Kohno, T. Funabiki, Photocatalytic reduction of CO2 to CO in the presence of H2 or CH4 as a reductant over MgO, J. Phys. Chem. B 108 (1) (2004) 346–354. [44] L. Yuliati, H. Itoh, H. Yoshida, Photocatalytic conversion of methane and carbon dioxide over gallium oxide, Chem. Phys. Lett. 452 (2008) 178–182. [45] B. Tahir, M. Tahir, N.A.S. Amin, Photo-induced CO2 reduction by CH4/H2O to fuels

Declaration of interest statement The authors declared no potential conflicts of interest with respect to the research, authorship and publication of this article. CRediT authorship contribution statement Muhammad Tahir: Methodology, Writing - original draft, Data curation, Formal analysis, Conceptualization. Acknowledgements This work is fully sponsored by Universiti Sains Malaysia (USM), Malaysia and Ministry of Education (MOE), Malaysia through Long Term Research Grant Scheme (LRGS NanoMITE, 203/PJKIMIA/ 6720009). References [1] Z. Fu, Q. Yang, Z. Liu, F. Chen, F. Yao, T. Xie, Y. Zhong, D. Wang, J. Li, X. Li, G. Zeng, Photocatalytic conversion of carbon dioxide: from products to design the catalysts, J. CO2 Util. 34 (2019) 63–73. [2] A. Bafaqeer, M. Tahir, N.A.S. Amin, Well-designed ZnV2O6/g-C3N4 2D/2D nanosheets heterojunction with faster charges separation via pCN as mediator towards enhanced photocatalytic reduction of CO2 to fuels, Appl. Catal. B: Environ. 242 (2019) 312–326. [3] A.A. Khan, M. Tahir, Recent advancements in engineering approach towards design of photo-reactors for selective photocatalytic CO2 reduction to renewable fuels, J. CO2 Util. 29 (2019) 205–239. [4] A. Movasati, S.M. Alavi, G. Mazloom, Dry reforming of methane over CeO2-ZnAl2O4 supported Ni and Ni-Co nano-catalysts, Fuel 236 (2019) 1254–1262. [5] W.-C. Chung, Y.-E. Lee, M.-B. Chang, Syngas production via plasma photocatalytic reforming of methane with carbon dioxide, Int. J. Hydrogen Energy 44 (35) (2019) 19153–19161. [6] J. Goscianska, R. Pietrzak, J. Matos, Catalytic performance of ordered mesoporous carbons modified with lanthanides in dry methane reforming, Catal. Today 301 (2018) 204–216. [7] Y. Lu, S. Jiang, S. Wang, Y. Zhao, X. Ma, Effect of the addition of Ce and Zr over a flower-like NiO-MgO (111) solid solution for CO2 reforming of methane, J. CO2 Util. 26 (2018) 123–132. [8] L. Xu, F. Wang, M. Chen, X. Fan, H. Yang, D. Nie, L. Qi, Alkaline-promoted Co-Ni bimetal ordered mesoporous catalysts with enhanced coke-resistant performance toward CO2 reforming of CH4, J. CO2 Util. 18 (2017) 1–14. [9] A.H. Khoja, M. Tahir, N.A.S. Amin, Dry reforming of methane using different dielectric materials and DBD plasma reactor configurations, Energy Convers. Manage. 144 (2017) 262–274. [10] T. Stroud, T.J. Smith, E. Le Saché, J.L. Santos, M.A. Centeno, H. Arellano-Garcia, J.A. Odriozola, T.R. Reina, Chemical CO2 recycling via dry and bi reforming of methane using Ni-Sn/Al2O3 and Ni-Sn/CeO2-Al2O3 catalysts, Appl. Catal. B: Environ. 224 (2018) 125–135. [11] E.H. Yang, Y.S. Noh, G.H. Hong, D.J. Moon, Combined steam and CO2 reforming of methane over La1-xSrxNiO3 perovskite oxides, Catal. Today 299 (2018) 242–250. [12] D. Shi, Y. Feng, S. Zhong, Photocatalytic conversion of CH4 and CO2 to oxygenated compounds over Cu/CdS-TiO2/SiO2 catalyst, Catal. Today 98 (4) (2004) 505–509. [13] L. Yuliati, H. Itoh, H. Yoshida, Photocatalytic conversion of methane and carbon dioxide over gallium oxide, Chem. Phys. Lett. 452 (1-3) (2008) 178–182. [14] K. Teramura, T. Tanaka, H. Ishikawa, Y. Kohno, T. Funabiki, Photocatalytic reduction of CO2 to CO in the presence of H2 or CH4 as a reductant over MgO, J. Phys. Chem. B 108 (2004) 346–354. [15] B. Han, W. Wei, L. Chang, P. Cheng, Y.H. Hu, Efficient visible light photocatalytic CO2 reforming of CH4, ACS Catal. 6 (2) (2015) 494–497. [16] B. László, K. Baán, E. Varga, A. Oszkó, A. Erdőhelyi, Z. Kónya, J. Kiss, Photo-induced reactions in the CO2-methane system on titanate nanotubes modified with Au and Rh nanoparticles, Appl. Catal. B: Environ. 199 (2016) 473–484. [17] A. Muhammad, M. Tahir, S.S. Al-Shahrani, A. Mahmood Ali, S.U. Rather, Template free synthesis of graphitic carbon nitride nanotubes mediated by lanthanum (La/gCNT) for selective photocatalytic CO2 reduction via dry reforming of methane

111

Journal of CO₂ Utilization 38 (2020) 99–112

M. Tahir

[46]

[47]

[48]

[49]

reduction of carbon dioxide to methanol, Renew. Sustain. Energy Rev. 116 (2019) 109389. [50] B. Tahir, M. Tahir, N.A.S. Amin, Silver loaded protonated graphitic carbon nitride (Ag/pg-C3N4) nanosheets for stimulating CO2 reduction to fuels via photocatalytic bi-reforming of methane, Appl. Surf. Sci. 493 (2019) 18–31. [51] F. Pan, X. Xiang, Z. Du, E. Sarnello, T. Li, Y. Li, Integrating photocatalysis and thermocatalysis to enable efficient CO2 reforming of methane on Pt supported CeO2 with Zn doping and atomic layer deposited MgO overcoating, Appl. Catal. B: Environ. 260 (2020). [52] N. Shehzad, M. Tahir, K. Johari, T. Murugesan, M. Hussain, Improved interfacial bonding of graphene-TiO2 with enhanced photocatalytic reduction of CO2 into solar fuel, J. Environ. Chem. Eng. 6 (6) (2018) 6947–6957.

over Cu-modified g-C3N4 nanorods under simulated solar energy, Appl. Surf. Sci. 419 (2017) 875–885. P. Ganguly, M. Harb, Z. Cao, L. Cavallo, A. Breen, S. Dervin, D.D. Dionysiou, S.C. Pillai, 2D nanomaterials for photocatalytic hydrogen production, CS Energy Lett. 4 (7) (2019) 1687–1709. P.-Y. Liou, S.-C. Chen, J.C.S. Wu, D. Liu, S. Mackintosh, M. Maroto-Valer, R. Linforth, Photocatalytic CO2 reduction using an internally illuminated monolith photoreactor, Energy Environ. Sci. 4 (4) (2011) 1487. O. Ola, M. Maroto-Valer, D. Liu, S. Mackintosh, C.-W. Lee, J.C.S. Wu, Performance comparison of CO2 conversion in slurry and monolith photoreactors using Pd and Rh-TiO2 catalyst under ultraviolet irradiation, Appl. Catal. B: Environ. 126 (2012) 172–179. D. Adekoya, M. Tahir, N.A.S. Amin, Recent trends in photocatalytic materials for

112