Applied Catalysis A: General

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1 Applied Catalysis A: General 493 (215) 9 12 Contents lists available at ScienceDirect Applied Catalysis A: General jou rn al hom epage: Photocatalytic CO 2 reduction with H 2 as reductant over copper and indium co-doped TiO 2 nanocatalysts in a monolith photoreactor Muhammad Tahir 1, NorAishah Saidina Amin Chemical Reaction Engineering Group/Low Carbon Energy Group, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 8131 UTM, Skudai, Johor Baharu, Johor, Malaysia a r t i c l e i n f o Article history: Received 24 September 214 Received in revised form 24 December 214 Accepted 24 December 214 Available online 12 January 215 Keywords: Photocatalysis CO 2 reduction H 2 reductant Metal-doped TiO 2 Monolithic support Catalyst stability a b s t r a c t The photocatalytic CO 2 reduction with H 2 over copper (Cu) and indium (In) co-doped TiO 2 nanocatalysts in a monolith photoreactor has been investigated. The catalysts, prepared via modified sol gel method, were dip-coated onto the monolith channels. The structure and properties of nanocatalysts with various metal and co-metal doping levels were characterized by XRD, SEM, TEM, N 2 adsorption desorption, XPS, and UV vis spectroscopy. The anatase-phase mesoporous TiO 2, with Cu and In deposited as Cu + and In 3+ ions over TiO 2, suppressed photogenerated electron hole pair recombination. CO was the major photoreduction product with a maximum yield rate of 654 mol g 1 h 1 at 99.27% selectivity and 9.57% CO 2 conversion over 1. wt% Cu 3.5 wt% In co-doped TiO 2 at 12 C and CO 2 /H 2 ratio of 1.5. The photoactivity of Cu In co-doped TiO 2 monolithic catalyst for CO production was 3.23 times higher than a single ion (In)-doped TiO 2 and 113 times higher than un-doped TiO 2. The performance of the monolith photoreactor for CO production over Cu In co-doped TiO 2 catalyst was 12-fold higher than the cell-type photoreactor. More importantly, the quantum efficiency of the monolith photoreactor was significantly improved over Cu In co-doped TiO 2 nanocatalyst using H 2 as a reductant. The stability of the monolithic Cu In co-doped TiO 2 catalyst for CO partially reduced after the third run, but retained for hydrocarbons. 215 Elsevier B.V. All rights reserved. 1. Introduction In recent years, the increasing level of carbon footprint has become a severe global environmental issue because of global warming and energy source depletion. Among various alternatives for energy, photocatalytic CO 2 reduction into renewable fuels is a promising way to generate reproducible chemical energy at relatively low temperature and atmospheric pressure [1,2]. In 1979, Inoue et al. [3] pioneered photocatalytic CO 2 reduction with H 2 O to chemicals and fuels using semiconductor materials such as TiO 2, ZnO, WO 3, SiC, CdS, and GaP. Soon after, CO 2 photoreduction using water as a reductant to various products such as CO, CH 4, CH 3 OH, HCHO, and HCOOH attracted considerable interests [4 1]. However, H 2 O is a weak reductant and is hardly reducible, thus photoreduction of H 2 O to H 2 proceeds preferably through water splitting as compared to that of CO 2. Therefore, CO 2 Corresponding author. Tel.: ; fax: addresses: bttahir@yahoo.com, mtahir@ciitlahore.edu.pk (M. Tahir), noraishah@cheme.utm.my (N.S. Amin). 1 Permanent address: Department of Chemical Engineering, COMSATS Institute of Information Technology, Lahore, Punjab, Pakistan. photoreduction with H 2 through reverse water gas shift (RWGS) reaction is more viable to produce fuels at higher yield rates [11 13]. The large amount of H 2 would be produced through water splitting under sunlight irradiations. In this context, utilization of H 2 as a reducing agent with the development of new photocatalysts for CO 2 reduction is relevant. Among the various photocatalysts, titanium dioxide (TiO 2 ) has drawn considerable attention. It is low-cost, stable, nontoxic, widely available, and has higher oxidation potential [14]. But TiO 2 photoactivity is relatively low due to fast recombination of excited electron (e ) and hole (h ) and it is only functional under UV light irradiations. TiO 2 photoactivity can be amended by modifying its structure with metals, which can function as electron traps [15 17]. Among the metals, indium (In) has been investigated as an efficient metal to enhance TiO 2 photoactivity owing to effective capturing of photo-generated charges [18]. In-metal is relatively cheap, has lower toxicity, and has multiple oxidation states, which can improve trapping and charge mobility over the TiO 2 surface. Recently, efficient photocatalytic CO 2 reduction with H 2 O vapors to CH 4 over In-doped TiO 2 nanocatalysts was reported, obviously due to hindered recombination of charges by In-metal [19]. Co-doping TiO 2 with two different metal atoms has attracted considerable interests, since it can increase photoactivity compared X/ 215 Elsevier B.V. All rights reserved.

2 M. Tahir, N.S. Amin / Applied Catalysis A: General 493 (215) with single-element TiO 2 doping. Photocatalytic water splitting for H 2 generation over N In co-doped TiO 2 Pd nanocomposites, investigated by Sasikala et al. [2] and reported higher performance in the presence of In and N metals. Similarly, Ru In 2 O 3 /TiO 2 co-doped system was utilized for water splitting with higher H 2 production in the presence of In and Ru metals [21]. The incorporation of Cu-metal ions (Cu, Cu +, Cu 2+ ) into TiO 2 allows the formation of electron trapping sites and promotes charge transfer from TiO 2 to metal ions. In this perspective, copper and iodine co-doped TiO 2 photocatalyst for photocatalytic CO 2 reduction with H 2 O vapor was reported by Zhang et al. [22]. Cu and I metals served as efficient acceptors of electrons that were transferred from the TiO 2 conductance band. The co-doping of copper with indium has not been investigated in CO 2 reduction applications. Therefore, Cu In co-doped TiO 2 could be explored further through RWGS reaction. The transformation and uniform distribution of light energy are crucial factors in the design of photocatalytic reactors. Several heterogeneous supports have been explored, namely glass support, quartz rods, optical fibers, and monoliths [23]. Among the structured supports, multiple-channel monoliths are by far the most efficient to maximize illuminated surface area and light utilizations [24]. Monolith has some eminent benefits over other reactors: specifically larger illuminated surface area per reactor volume, efficient adsorption desorption process, and effective light energy distribution over the catalyst surface [25]. In this perspective, Liou et al. [26] investigated honeycomb monolith photoreactor for CO 2 reduction with H 2 O vapors over NiO/InTaO 4 catalyst and reported methanol yield of.3 mol g-catal 1 h 1 and quantum efficiency of.12%. Similarly, copper-based TiO 2 honeycomb monolith was tested by Ola and Maroto-Valer [27] for CO 2 reduction with H 2 O vapors. The monolithic structure significantly increased the amount of catalyst loading and overall process efficiency. Recently, cordierite structure monolith has been used as a catalyst support. The discern advantages of monolith structure include lower pressure drop, good mass transfer, and efficient light penetration inside the microchannels [28,29]. It is envisaged that higher CO 2 reduction through RWGS reaction can be achieved by doping TiO 2 photocatalyst with Cu and In electron acceptors. The objective of this study is to investigate copper- and indium-based TiO 2 nanocatalysts for photocatalytic CO 2 reduction with H 2 as a reductant in a monolith photoreactor. The performances of the monolithic catalysts were further evaluated using different operating parameters such as different metal-doping levels, feed ratios, reaction temperature, and irradiation time. The reaction mechanism and stability of Cu In co-doped TiO 2 monolithic catalyst for RWGS reaction were also deliberated. 2. Experimental 2.1. Catalyst preparation and dip-coating The TiO 2 and co-doped TiO 2 monolithic catalysts were prepared using modified sol gel according to the method described in Fig. S1. Precisely, 1 ml of titanium tetraisopropoxide (Merck, 98%) was added to 3 ml of isopropanol and stirred for 3 min. A mixture of 7 ml of 1 M acetic acid and 1 ml of isopropanol was then added to the solution under constant stirring for 24 h. For metal doping, In(NO 3 ) 3 3H 2 O dissolved in isopropanol was added to the titanium solution and stirred for 6 h. Similarly, Cu(NO 3 ) 2 3H 2 O in isopropanol was added to the mixture under vigorous stirring for the next 6 h until clear sol was produced. The sol obtained was transferred into a glass container for monolith dip-coating. All the monolithic supports (Pingxiang Meitao Company, China) used were cylindrical in shape, having dimensions of 6 mm in diameter and 2 mm in length. They were cordierite straight channel monoliths with a square cell density of 2 CPSI (cells per square inch). Before coating, the monoliths were washed with acetone and isopropanol to eliminate any organic materials and then dried at 8 C for 12 h. The dried monoliths were immersed into sol at a specific speed and time for catalyst loading. Excess sol was blown off using compressed hot air. The coating procedure was repeated to reach the desired catalyst loading. The coated monolith was dried at 8 C for 12 h, calcined at a rate of 5 C min 1 up to 5 C and held for 5 h. Catalyst loading was calculated by subtracting coated monolith weight from the initial bare monolith weight. For every coating, three readings were noted for bare monolith as well as catalyst-coated monolith and their average values were reported. Different In and Cu doping levels were prepared by varying the quantity of In(NO 3 ) 3 3H 2 O and Cu(NO 3 ) 2 3H 2 O. The amounts of In used were 2, 3.5, 5, and 7 wt%. For co-doping, copper doping levels were.5, 1, and 1.5 wt% at fixed In doping (3.5 wt%). For comparison, un-doped TiO 2 was prepared according to the same procedure Catalyst characterization The structure and morphology of the nanoparticles were characterized using XRD, SEM, HRTEM, and N 2 adsorption. Powder X-ray diffraction (XRD) of catalysts was performed on Bruker D8 advance diffractometer (Cu K radiation, wavelength = 1.54 Å, operated at 4 kv and 4 ma). The scanning rate was 1.2 min 1 from 15 to 7 of 2. The crystalline phases were identified by reference of powder diffraction data (JCPDS-ICSD). TiO 2 crystal size was calculated using Scherrer equation. The scanning electron microscopy (SEM) was carried out with JEOL JSM639 LV SEM instrument. The particle size and lattice structure of the individual crystals was visualized using a high-resolution transmission electron microscope (HRTEM) (FEI-Tecni G2). Meanwhile, the Brunauer Emmett Teller (BET) specific surface area and pore size of the catalysts were measured by N 2 adsorption desorption isotherms at 77 K using a Micrometrics ASAP 22 Surface Area and Porosity Analyzer. The XPS measurement was performed using Omicron DAR 4 analyzer. The photocatalyst was fixed to the sample holder using a carbon tape. The pass energy used was 2 ev while the instrument was operated at 15 kv. The survey spectra were recorded in the range of 14 ev. The binding energies were calibrated against the C 1s signal (284.6 ev) as the internal standard. Ultraviolet visible (UV vis) diffuse reflectance absorbance spectra of the samples were determined using Agilent, Cary 1 UV vis spectrophotometer equipped with an integrated sphere. Initially, blank runs were conducted to correct the baseline. The absorbance spectra were analyzed at ambient temperature in the wavelength range 2 8 nm. The band gap energies of the photocatalysts were determined from the extrapolation of Tauc plot to the abscissa of photon energy Photocatalytic activity test The schematic of the monolith photoreactor system for photocatalytic CO 2 reduction with H 2 is illustrated in Fig. S2. The reactor consisted of a stainless steel cylindrical vessel with 5.5 cm in length and a total volume of 15 cm 3. It is fitted with a quartz window of thickness 1 mm. The catalyst-coated cordierite monolith was placed inside the cylindrical stainless steel chamber illuminated by a reflected light source. The light source was a 2 W Hg lamp for UV irradiation, equipped with a cooling fan at the top and sides to remove lamp heat. The light intensity was measured with an online optical process monitor ILT OPM-1D and a SED8/W sensor. The total light influx passing at the top of the reactor was measured to be 15 mw cm 2. The reactor was covered with aluminum foil to ensure lights for the reactions came through the quartz window

3 92 M. Tahir, N.S. Amin / Applied Catalysis A: General 493 (215) 9 12 Intensity (a.u) Theta (degree) TiO 2 2% In/TiO 2 3.5% In/TiO 2 5% In/TiO 2.5% Cu-3.5% In/TiO 2 1.% Cu-3.5% In/TiO 2 1.5% Cu-3.5% In/TiO 2 Fig. 1. XRD patterns of TiO 2, In-doped TiO 2, and Cu In co-doped TiO 2 catalysts. only. Meanwhile, the temperature was controlled using a heating source. In the case of the cell-type photoreactor, the reactor chamber was the same as that of the monolith reactor (L = 5.5 cm, V = 15 cm 3 ) as depicted in Fig. S3. Likewise, the same source of light was used for the photocatalytic activity test. However, 5 mg of nanocatalyst powder was suspended uniformly at the bottom of the cell reactor to ensure efficient light distribution over the catalyst surface. Before feeding, the reactor was purged with helium (He). A mixture of gases (CO 2, H 2, and He, purity = 99.99%) was passed through the reactor for a specific time to saturate the catalysts. All the experiments were carried out in a batch mode. The H 2 concentration was fixed at 2 mol.%, while CO 2 concentrations of 1, 2, 3, and 4 mol.% were used to control the CO 2 /H 2 ratios at.5, 1., 1.5, and 2., respectively. He gas was used to culminate the mole balance to 1 mol.%. The pressure inside the reactor was set to.4 bar using a pressure controller. The CO 2 photoreduction products were determined using an on-line gas chromatograph (GC-Agilent Technologies 689N, USA) equipped with a thermal conductivity detector (TCD) and a flameionized detector (FID). The FID detector was connected with a HP-PLOT Q capillary column (Agilent, length 3 m, ID.53 mm, film 4 m) for separation of C 1 C 6 paraffin and olefins hydrocarbons, alcohols, and oxygenated compounds. The TCD detector was connected to UCW982, DC-2, Porapak Q and Mol Sieve 13X columns. C 1 C 2, C 3 C 5 compounds and light gasses (H 2, O 2, N 2, CO) were separated using Porapak Q, DC-2 and MS-13X column, respectively. 3. Results and discussion 3.1. Characterization of nanocatalysts Fig. 1 presents the XRD patterns of TiO 2, In-doped TiO 2, and Cu In co-doped TiO 2 nanocatalysts. The patterns reveal that all the samples are good crystalline materials and exist in anatase phase of TiO 2. XRD patterns of Cu In co-doped TiO 2 coincided with that of pure TiO 2 and In-doped TiO 2 samples. The spectrum of Cu In codoped TiO 2 sample exhibits anatase phase of TiO 2 with 2 peaks at , 38.2, 48.87, 54.23, 55.37, and compared with JCPDS-ICSD standards for anatase ( ). All these peaks were consistent with (1 1), ( 4), (2 ), (1 5), (2 1 1), and (2 4) planes that confirmed tetragonal anatase TiO 2. The diffraction peaks of Cu and In were not detected in TiO 2 samples, suggesting that these species were highly dispersed over TiO 2 surface or probably due to lower metal contents. However, with the addition of Cu and In into TiO 2, the diffraction peaks became wider as the crystallite size and crystallinity increased. More prominently, all the peaks coincided, hence no peak shifting was observed. Numerous reports have discussed the incorporation of metal ions into TiO 2, but no distortion of the peaks and the presence of metal peaks were observed [15,2]. The crystallite sizes, as calculated from the width of the XRD peaks using Scherrer equation, are given in Table 1. Apparently, the crystallite size decreased with Cu and In co-doping into TiO 2. It can be seen that the crystallite size is around 8 19 nm for all the samples. The cell parameters and cell volumes of TiO 2 and co-doped TiO 2 nanocatalysts are listed in Table 1. Both cell parameters and cell volumes are comparable with those reported in JCPDS-ICSD ( ) standards for anatase TiO 2, i.e. a = b = Å, c = 9.51 Å, and v = Å 3. Obviously, Cu and In doping do not alter the cell structure of the fully developed tetragonal anatase TiO 2 crystal. Fig. 2 exhibits high-magnification SEM images and EDX mappings of un-doped TiO 2 and metal-doped TiO 2 catalysts. The uniform and spherical TiO 2 nanoparticles with mesoporous structure are obvious in Fig. 2a. The EDX mapping analysis in Fig. 2b and c divulges the presence of Ti and O elements in TiO 2. Meanwhile, it can be observed that 1.% Cu and 3.5% In co-doped TiO 2 nanoparticles have smaller particle size, more mesoporous structure compared with un-doped TiO 2 as shown in Fig. 2d g. However, in Cu- and In-doped TiO 2 samples, agglomerate formations can be seen. The EDX mapping analysis in Fig. 2e and f revealed uniform distribution of In-metal in the TiO 2 texture. The EDX analysis of 1.% Cu 3.5% In co-doped TiO 2 sample is presented in Fig. 2h and i, confirming the presence of Cu and In metals in TiO 2. Noticeably, both metals (Cu and In) were evenly spread over the TiO 2 surface. The elemental compositions of all the metals are presented in Table 1. Fig. S4 presents the SEM micrographs of bare monolith and catalyst coated over monolith channels. The structure of the monolith channels is obvious in Fig. S4a, while Fig. S4b elucidates the rough surface of uncoated monolith channels. The pore morphology of the samples coated over the monolith is illustrated in Fig. S4c. Obviously, nanocatalyst particles, investigated at 1 m of SEM magnification, are entirely coated over the channel surface with a smooth and thoroughly distributed uniform layer. At a magnification of 1 nm, it is evident that the nanoparticles are in mesoporous structure as shown in Fig. S4d. Fig. 3 exhibits the TEM micrographs of the mesoporous 1.% Cu 3.5% In co-doped TiO 2 catalyst coated over the monolith channels. In Fig. 3a, the TEM image reveals the uniform distribution of Cu In co-doped TiO 2 nanoparticles with mesoporous structure. Fig. 3b displays the particle size distribution of Cu In/TiO 2 catalyst in the range 7 18 nm with a mean value of 13.6 nm. Therefore, distinct nanoparticle size and mesoporosity prevailed in Cu In codoped TiO 2 catalyst. Fig. 3c and d exhibits the lattice fringe spacing of TiO 2 nanoparticle and electron dispersion having value of about.35 nm that clearly relates to anatase phase of TiO 2. The electron dispersion confirms anatase phase of TiO 2 crystal structure. Fig. S5 displays the N 2 adsorption desorption isotherms of TiO 2, In-doped TiO 2, and Cu In co-doped TiO 2 samples. All the isotherms are typical type IV curve hysteresis loops, characteristics of mesoporous materials, and associated with capillary condensation. The capillary condensation behaviors of In/TiO 2 samples were the same as TiO 2. By doping Cu into In/TiO 2 sample, similar trends were observed, indicating mesoporous materials with capillary condensation process. The surface area, pore volume, and pore size of all samples are summarized in Table 2. The BET surface area and BJH external surface of TiO 2 were 43 and 52 m 2 g 1, respectively, confirming mesoporous TiO 2. In-doping into TiO 2 increased the surface area and pore volume, possibly due to suppression of TiO 2 crystal growth by In-metal. The results agreed well with previous reports [29]. The surface area of the sample slightly decreased with Cudoping. The decrease in surface area can be attributed to larger ionic radius of Cu when compared with that of TiO 2 as similarly

4 M. Tahir, N.S. Amin / Applied Catalysis A: General 493 (215) Table 1 Cell parameters and crystallite sizes of bare TiO 2 and modified TiO 2 samples and EDX analysis. Sample Cell parameter (Å) Cell volume (Å) Crystallite size a (nm) EDX analysis b (wt%) a = b c In Cu TiO % In TiO % In TiO % In TiO % Cu 3.5% In TiO % Cu 3.5% In TiO % Cu 3.5% In TiO a Crystallite sizes calculated using Scherrer equation and 1 1 peak of TiO 2. b Elemental compositions were determined through EDX mapping. Fig. 2. SEM micrographs of catalyst samples: (a) SEM image of TiO 2, (b, c) EDX mapping of TiO 2 and elemental analysis, (d) SEM image of 3.5% In/TiO 2, (e, f) EDX mapping of 3.5% In/TiO 2 and elemental analysis, (g) SEM image of 1.% Cu 3.5% In/TiO 2, (h, i) EDX mapping and elemental compositions of 1.% Cu 3.5% In/TiO 2 catalyst. observed in the literature [3]. On the other hand, for all samples, i.e. In/TiO 2 and Cu In/TiO 2, the pore volumes are higher compared to un-doped TiO 2 nanoparticles. Therefore, higher mesoporosity due to the smaller particle diameter and larger pore volume could enhance molecular transportation rates of reactants and products to increase the photoactivity. The chemical states of the component elements of the 1.% Cu 3.5% In co-doped TiO 2 catalyst was analyzed by X-ray Table 2 Summary of physiochemical characteristics of TiO 2 and modified TiO 2 samples. Catalyst type Surface area (m 2 g 1 ) Pore volume (cm 3 g 1 ) Pore width (nm) S BET BJH surface area t-plot micropore volume BJH adsorption pore volume BJH pore width TiO % In TiO % In TiO % In TiO % Cu 3.5% In TiO

5 94 M. Tahir, N.S. Amin / Applied Catalysis A: General 493 (215) 9 12 Fig. 3. TEM images of 1.% Cu 3.5% In co-doped TiO 2 catalyst coated over monolith channels; (a) mesoporous Cu and In co-doped TiO 2 nanoparticles; (b) particle size distribution of Cu In/TiO 2 nanoparticles; (c) HRTEM image of Cu In/TiO 2 for lattice fringes calculation; (d) electron dispersion of the corresponding sample. Fig. 4. XPS spectra of 1.% Cu 3.5% In co-doped TiO 2 sample: (a) wide spectra, (b) Ti 2p, (c) In 3d, (d) Cu 2p, (d, e) O 1s, and (f) C 1s.

6 M. Tahir, N.S. Amin / Applied Catalysis A: General 493 (215) (a) (b) Absorbance (a.u) ( ahv ) Wavelength (nm) Photon energy (hv) Fig. 5. (a) UV vis diffuse reflectance absorbance spectra of un-doped TiO 2 and Cu In co-doped TiO 2 samples; (b) band gap energy calculation of corresponding samples. photoelectron spectroscopy (XPS). The XPS survey spectrum, presented in Fig. 4a indicates the presence of the In, Cu, O, and C species. The Ti 2p spectrum in Fig. 4b indicates the peaks with binding energies (BE) located at ev (2p 3/2 ) and ev (2p 1/2 ), both of which attributed to Ti 4+ oxidation state or TiO 2. Fig. 4c illustrates the In 3d spectrum peaks of In 3d 5/2 and In 3d 3/2 with BE region centered at and ev, respectively, indicating indium as In 3+ specie or In 2 O 3. The XPS spectrum of Cu 2p is given in Fig. 4d. The binding energy of the Cu 2p 3/2 peak is ev and the binding energy of Cu 2p 1/2 is ev, which are characteristics of Cu 2 O. Hence, Cu 2 O is the primary Cu species in the Cu In TiO 2 sample, indicating that the dominant Cu specie is Cu (1) on Cu In co-doped TiO 2 sample [31,32]. The spectrum of O 1s, shown in Fig. 4e, reveals three peaks centered at 53.8, 531, and ev. The BE value of 53.8 ev is attributed to the lattice oxygen O 2 in TiO 2, In 2 O 3, and Cu 2 O and is in agreement with the reported values [33]. The oxygen peaks at around and ev are possibly due to H 2 O or the free hydroxyl group (O H) on the surface. The C 1s spectrum of all the carbon peaks is shown in Fig. 4f. The peak with a binding energy located at ev is assigned to elemental carbon (C C), while the one at ev corresponds to C O. The presence of carbon may be recognized as the carbon from carbon tap used for sample analysis. Fig. 5a depicts the UV vis diffuse reflectance absorbance spectra of TiO 2, In/TiO 2, and Cu In/TiO 2 samples. The absorption band edge shifted toward longer wavelength with the addition of In into TiO 2 nanoparticles. However, when Cu was co-doped with In into TiO 2, the band edge slightly reduced. With increasing Cu content, the absorption band edge moved again toward longer wavelength. The E bg values for all samples were measured from a plot of ( hv) 2 versus photon energy (hv) as shown in Fig. 5b. The band gap values of all samples are listed in Table S1. The band gap energies calculated were 3.12, 3.19, 3.2, 3.22, and 3.24 ev for TiO 2 and 2, 3.5, 5, and 7 wt% In-doped TiO 2 samples, respectively. The increase in the TiO 2 band gap energy can be attributed to the higher indium oxide (In 2 O 3 ) band gap energy (E bg = 3.7 ev) [34]. The shift in TiO 2 band gap energy toward UV region has been reported in the literature for In-doped TiO 2 nanoparticles. Therefore, doping In into TiO 2 allows the formation of a UV semiconductor, which could also improve the lifetime of photo-generated charge particles. Similarly, E bg measured from the intercept of the tangents for Cu and In-co-doped TiO 2 samples were 3.14, 3.9, and 3.13 ev for.5, 1., and 1.5% Cu co-doped with 3.5 wt% In TiO 2 catalysts, respectively. Obviously, there was a gradual decrease in the band gap energy with Cudoping up to 1%. Larger band energy with higher Cu-loading was possibly due to Cu 2 O shading over the TiO 2 surface. The summary of the band gap energies is presented in Table S Photoreduction of CO 2 with H 2 Initially, a series of preliminary tests were conducted to confirm any carbon-containing compounds originated from CO 2 during photocatalytic reactions. Two sets of experiments were conducted at 12 C using purging and reacting gases with and without light irradiations and the results are summarized in Table S2. The first test was conducted in the absence of light irradiations for cases of: bare monolith/catalyst-coated monolith + He, catalyst-coated monolith + He + H 2, and catalyst-coated monolith + He + H 2 + CO 2. In each case, carbon-containing compounds were not detected, confirming CO 2 was not reduced thermally. In the second set, catalyst-coated monolith + He and catalyst-coated monolith + He + H 2 were tested in the presence of light irradiations. In this case, smaller amount of methane was detected in the product mixture. The trace amount of methane produced was perhaps from the breaking of organic residues left in the catalyst. However, when the CO 2 reduction was conducted in the presence of H 2 and light irradiations, significant amount of CO and hydrocarbons was produced. This initial analysis confirmed that CO was only produced from CO 2 during its reduction under light irradiations. The effect of In-doping on the photoactivity of TiO 2 for photocatalytic CO 2 reduction with H 2 under UV light irradiations is presented in Fig. S6. CO and CH 4 were found as major CO 2 reduction products confirming H 2 functions as an efficient reducing agent for photocatalytic CO 2 reduction. The yield of CO production increased remarkably with In-doped TiO 2 up to an optimum of 3.5 wt% In and then decreased gradually. The amount of CO produced was 3837 mol g-catal 1 over 3.5 wt% In-doped TiO 2, considerably higher than un-doped TiO 2 (173 mol g-catal 1 ). However, a surge in CO production could be seen over Cu In co-doped TiO 2 catalysts as illustrated in Fig. 6. Among the three Cu 3.5 wt% In TiO 2 samples (.5, 1., and 1.5 wt% Cu), 1 wt% Cu co-modified with 3.5 wt% In/TiO 2 samples appeared to be

7 96 M. Tahir, N.S. Amin / Applied Catalysis A: General 493 (215) 9 12 Yield of CO (µmole g-catal. -1 ) TiO 2 3.5% In-TiO 2.5% Cu-3.5% In-TiO 2 1.% Cu-3.5% In-TiO 2 1.5% Cu-3.5% In-TiO Irradiation time (min) Fig. 6. Effect of Cu and In co-doped TiO 2-monolithic catalysts on production of CO at CO 2/H 2 ratio 1. and temperature 1 C. Yield of CO (µmole g-catal. -1 ) CO 2 /H 2 =.5 CO 2 /H 2 =1. CO 2 /H 2 =1.5 CO 2 /H 2 = Irradiation time (min) Fig. 7. Effect of CO 2/H 2 molar feed ratio on the photoactivity of 1.% Cu 3.5% In co-doped TiO 2 for the evolution of CO during photocatalytic CO 2 reduction with H 2 at temperature 1 C optimum having CO production rate 3,243 mol g-catal 1 calculated for 4 h irradiations, eight times higher than In/TiO 2 catalyst. The significant increase in CO production was probably due to efficient production of electron hole pairs and inhibited recombination rate over co-doped TiO 2 catalyst coated over the monolith microchannels. Using Cu In co-doped TiO 2 catalyst, Cu + sites can serve as electron traps in addition to In-metal (In 3+ ), thus suppressing photo-generated electron hole pairs recombination. Therefore, the product rate increases significantly with the Cu concentration to give optimum Cu-doping of 1. wt% in 3.5 wt% In-doped TiO 2, after which decreased CO rate was observed at higher doping levels. The decrease in CO evolution at higher Cu loading could be possibly due to the shading effect, which reduced the TiO 2 photoexcitation capacity. In addition, higher Cu content (above 1. wt%) may lead to the formation of recombination hubs, specifically when there is the largest production of electron hole pairs using co-doped metals [35]. From the above demonstration, it is obvious that copper is the most active dopant to enhance TiO 2 photoactivity. The XPS analysis reveals copper is presented as Cu 2 O over TiO 2. Based on thermodynamics, the electron trapped by metal (M n+ ) within the photocatalyst (e.g. TiO 2 ) is more feasible if the reduction potential of metal (M n+ ) is more positive than the TiO 2 conductance band. The potential redox values of Cu + and Cu 2+ are given in Eqs. (1) and (2) [36]. As Cu 2 O has the highest positive potential redox value of Cu + (+.52 V) than the reduction potential of TiO 2 (.5 V) [12], Cu 2 O dopant should perform as an efficient electron trapper to hinder electron hole pair recombination. Cu + + e Cu (E =.52 V) (1) Cu e Cu (E =.34 V) (2) The dependence of CO 2 /H 2 molar ratios for photocatalytic CO 2 reduction with H 2 to CO over 1.% Cu 3.5% In-co-doped TiO 2 catalyst at different irradiation time is illustrated in Fig. 7. The production of CO gradually increased with increasing initial CO 2 concentration up to 3% CO 2 at a fixed H 2 concentration of 2% (CO 2 /H 2 ratio 1.5). Since the amount of adsorbed CO 2 increases with an increase in the initial CO 2 concentration, CO production also escalates. However, maximum CO was observed at an optimum CO 2 /H 2 ratio of 1.5, and then gradually reduced. At lower CO 2 concentrations, large amount of H 2 molecules could adsorb over the catalyst surface to react with CO 2 triggering lower CO 2 photoreduction. However, the higher CO 2 concentration covered maximum active sites and H 2 would have to compete with CO 2 for the active sites, thus reducing reaction rate. Therefore, the concentration of both reactants would be optimized for maximum conversion. Similar findings have been reported in literature during photocatalytic CO 2 with H 2 O vapors [37]. Fig. S7 depicts the yield of CH 4 at different CO 2 /H 2 molar feed ratios and irradiation times. A considerable amount of CH 4 was produced in a CO 2 /H 2 feed ratio of.5, but decreased at higher initial CO 2 /H 2 molar feed ratios. The increase in the CH 4 yield rate at a lower CO 2 /H 2 feed ratio (higher H 2 concentration) was apparently due to higher H 2 adsorption over catalyst coated over monolith channels, resulting in an efficient conversion of CO 2 to CO and finally to CH 4. On the other hand, at a higher CO 2 concentration (high CO 2 /H 2 feed ratios), more CO 2 was adsorbed producing more CO. This phenomenon is likely due to adsorption competition between CO 2 and H 2 molecules to the active catalyst sites during photoreduction process. The hydrocarbon productions over 1.% Cu 3.5% In co-doped TiO 2 catalyst at different initial CO 2 /H 2 molar feed ratios are illustrated in Fig. S8. The hydrocarbons observed were C 2 H 4, C 2 H 6, C 3 H 6, and C 3 H 8 using CO 2 /H 2 feed ratios of.5, 1., 1.5, and 2.. Obviously much higher C 2 H 6 and C 2 H 4 yields were observed at CO 2 /H 2 feed ratio of.5, and then reduced intensely at higher initial CO 2 /H 2 molar feed ratios. Besides, trace distribution of C 3 H 6 and C 3 H 8 over all the initial feed ratios with time were also observed in the product mixture. The much higher C 2 H 4 and C 2 H 6 production at a lower CO 2 /H 2 feed ratio was possibly due to more H 2 adsorption over the catalyst surface, resulting in the superfluous production of intermediate CH 3 and, which possibly reacted with each other to produce smaller amount of paraffin and olefin hydrocarbons. However, CO production as the major product confirmed efficient photocatalytic reverse water gas shift reaction over Cu In co-doped TiO 2 monolithic nanocatalysts. RWGS reaction is an endothermic reaction and is only favorable at higher temperatures in the presence of catalyst [38]. The effect of temperature on photocatalytic performance of RWGS reaction over 1% Cu 3.5% In co-doped TiO 2 is presented in Fig. 8. The minimum temperature of 7 C was selected as this was the temperature measured inside the reactor without providing any input heat. A significant increase in CO yield and selectivity could be observed at elevated temperature, exhibiting higher CO 2 photoreduction. Similarly, CH 4 production increased with increasing temperature. The optimum temperatures of 1 and 12 C were observed for CH 4 and CO production, respectively, beyond which yield decreased rapidly. No products were observed when the reaction was

8 M. Tahir, N.S. Amin / Applied Catalysis A: General 493 (215) CO CH Yield of CO (umole g-catal. -1 ) Yield of CH 4 (umole g-catal. -1 ) Reaction temperature ( o C) Fig. 8. Effect of reaction temperature on the yields of CO and CH 4 over 1.% Cu 3.5% In co-doped TiO 2 nanocatalysts. Yield of CO (µmole g-catal. -1 ) TiO 2 3.5% In-TiO 2 1.% Cu-3.5% In-TiO Irradiation time (h) 3.23 fold high 35 fold high fold high Fig. 9. Effect of irradiation time on the photoactivity of un-doped TiO 2, 3.5% In-doped TiO 2, and 1.% Cu 3.5% In co-doped TiO 2 monolithic catalysts for the evolution of CO during CO 2 reduction at 12 C and CO 2/H 2 ratio 1.5. conducted without light irradiations at a reaction temperature of 12 and 15 C under the same reaction conditions. Therefore, it is confirmed that CO 2 reduction process is photochemical and is dependent on efficient adsorption desorption of reactants and products over the photocatalyst surface [39]. At lower temperature, surface coverage is higher and products do not easily desorb. Therefore, yield of CO and CH 4 increased with temperature, possibly due to efficient desorption of products [36]. This makes possible enhanced collisions between the charge transfer excited-state species and reactant molecules [4]. The decrease in CO yield at elevated temperature (above 12 C) can be ascribed to the desorption of reactants (CO 2 and H 2 ) from the catalyst surface. Conversely, at higher temperature, the adsorption of reactants becomes more important as the surface is sparsely covered. Recently, similar observations have been reported in the literature [41]. It is also observed that the CH 4 yield decreased beyond 1 C. This decrease in CH 4 production at elevated temperature was probably due to readily desorption of photogenerated CO, thus limited its possibility to be converted into CH 4. The effect of irradiation time on CO production over un-doped TiO 2 and co-metal-doped TiO 2 samples at irradiation times of 8 h is shown in Fig. 9. Using un-doped TiO 2, the amount of CO was very small, but increased to 35 times higher with In-doping. A 7 8 Fig. 1. Photoreduction of CO 2 to CH 4 over un-doped and metal-doped TiO 2 monolithic catalysts at different irradiation times at 12 C and CO 2/H 2 ratio 1.5. significantly higher CO production was observed with 1.% Cu 3.5% In co-doped TiO 2 catalyst. The evolution of CO over 1.% Cu 3.5% In co-doped TiO 2 monolithic catalyst was 3.23 times higher than 3.5 wt% In-doped TiO 2 and 113 times higher than un-doped TiO 2. These results confirmed Cu and In co-doping monolithic catalysts are more efficient than In-doped TiO 2 and TiO 2 -based catalysts. Therefore, Cu metal plays an important role in transferring electrons from the TiO 2 conductance sites for the reduction of CO 2, resulting in higher photoactivity. Fig. 1 illustrates the effect of irradiation time on photocatalytic CO 2 reduction to CH 4. Apparently, the CH 4 yield increased rapidly over TiO 2 catalyst and decreased with co-doped TiO 2 catalysts. CH 4 production over TiO 2 increased continuously with irradiation time due to slower CO desorption process over the catalyst surface. Over 1.% Cu 3.5% In co-doped TiO 2, a significant reduction in CH 4 yield could be attributed to higher production and electron mobility. This was also possibly due to efficient desorption of CO and thermodynamic reduction potential of CO 2 /CO [31]. Therefore, it could be generalized that the presence of In ions over the surface of TiO 2 and dispersion of Cu-species imparted adverse effect on CH 4 production during photocatalytic CO 2 reduction with H 2. Moreover, due to higher mobility of electrons and efficient desorption process over the catalyst surface supported over the monolith microchannels, a series of higher hydrocarbons (C 2 and C 3 ) was produced during photocatalytic CO 2 reduction with H 2 over different catalysts (Fig. 11). At the beginning of the photoreduction process, intermediate carbon-based species were initially adsorbed on the catalyst surface and then photoreduced to various carbonbased products, resulting in higher hydrocarbon yields. However, as the hydrocarbon concentration increased, so did adsorption over the catalyst surface which led to reversible photochemical reactions, gradually decreasing hydrocarbon yield over the irradiation time. The hydrocarbons identified were C 2 H 4, C 2 H 6, C 3 H 6, and C 3 H 8 over un-doped and metal-doped TiO 2 catalysts. However, a significant amount of C 2 H 6 as major product was observed over TiO 2, In/TiO 2, and Cu In/TiO 2 catalyst at 8 h of irradiation time. Table 3 provides the yield rates and selectivity of different products over various TiO 2 -based catalysts during photocatalytic CO 2 reduction with H 2 as reducing agent under optimum operating conditions. The products observed were CO, CH 4, C 2 H 4, C 2 H 6, C 3 H 6, and C 3 H 8 over TiO 2, 3.5% In/TiO 2, and 1.% Cu 3.5% In/TiO 2 catalysts. The photocatalytic activity for CO production was higher in the order corresponding to Cu In/TiO 2 > In/TiO 2 > TiO 2. On the other hand, the photoactivity for CH 4 production was in the order

9 98 M. Tahir, N.S. Amin / Applied Catalysis A: General 493 (215) 9 12 Table 3 Summary of the products produced during photocatalytic CO 2 reduction with H 2 using TiO 2 and metal modified TiO 2 catalysts coated over monolith channels. Catalysts Conversion a (%) Yield rates b ( mol g-catal 1 h 1 ) Selectivity c (%) CO 2 H 2 CO CH 4 C 2H 4 C 2H 6 C 3H 6 C 3H 8 CO CH 4 TiO % In/TiO % Cu 3.5% In/TiO a Conversion of CO 2 or H 2 (%) = moles of CO 2 or H 2 in feed moles of CO 2 or H 2 in product 1. moles of CO 2 or H 2 in feed b Yield rates calculated on a 8-h irradiation basis. Operating conditions: CO 2/H 2 molar ratio 1.5, reaction temperature 12 C. c Selectivity of product C i (%) = moles of C i in product mixture 1, C Total moles of C produced i is the mole of carbon species i (CO, CH 4, C 2H 4, C 2H 6, C 3H 6, and C 3H 8) in the products mixture and C is the mole of total carbon compounds produced. -1 Yield of hydrocarbons (µmole g-catal. ) Irradiation time (h) Fig. 11. Evolution of hydrocarbons at different irradiation time during photoreduction of CO 2 at 12 C and CO 2/H 2 ratio 1.5 over TiO 2, 3.5% In-doped TiO 2, and 1.% Cu 3.5% In co-doped TiO 2 monolithic catalysts H 2 conversion (%) (b) TiO 2 (2.3%) I In/TiO 2 (5.48%) II Cu-In/TiO 2 (13.5%) III Type of systems Cu-In/TiO 2 (26.5%) Fig. 12. (a) Profile of H 2 conversion using different type of monolithic systems at 12 C under UV light irradiation: (I) TiO 2 + CO 2 + H 2, (II) 3.5% In/TiO 2 + CO 2 + H 2, (III) 1.% Cu 3.5% In/TiO 2 + H 2, (IV) 1.% Cu 3.5% In/TiO 2 + CO 2 + H 2; (b) profile of CO 2 conversion over different monolithic catalysts. (a) IV of TiO 2 > Cu In/TiO 2 > In/TiO 2. Similarly, photoactivity of catalysts for C 2 H 6 production was in the order of TiO 2 > Cu In/TiO 2 > In/TiO 2. It is evident that Cu and In co-doped TiO 2 catalyst was very efficient for CO production with appreciable amounts of hydrocarbons. Furthermore, photocatalyst efficiencies for CO 2 conversion was higher in the order of 1.% Cu 3.5% In/TiO 2 (9.6%) > 3.5% In/TiO 2 (6.4%) > TiO 2 (1.7%), while catalysts efficiencies for H 2 conversion was higher in the order of 1.% Cu 3.5% In/TiO 2 (26.5%) > 3.5% In/TiO 2 (5.48%) > TiO 2 (2.3%). It is interesting to note conversion of H 2 is much higher. According to stoichiometric reactions in Eqs. (3) and (4), 1 and 4 mol of H 2 are required to produce 1 mol of CO and 1 mol of CH 4, respectively. light, Cu In/TiO CO 2 + H 2 2 CO + H 2 O (3) light, Cu In/TiO CO 2 + 4H 2 2 CH 4 + 2H 2 O (4) As the selectivity of CO is about 99.27% (Table 3), thus Eq. (3) is more favorable, indicating consumption of H 2 and CO 2 would be approximately much closer. Thus, conversion of H 2 should be approximately closer to CO 2 conversion. However, a much higher H 2 conversion was registered possibly due to higher consumption of protons by Cu metal during its reduction as described in Eqs. (5) (9) [36,41]. UV-light, heat Cu 2 O+H 2 2Cu + H 2 O (5) Cu+h + Cu + +TiO 2 (6) 2Cu + Cu + Cu 2+ (7) Cu + /Cu 2+ +e /2e Cu (8) Cu I O e Cu O 2 /H+ Cu I O (9) Eq. (5) presents the reduction of Cu 2 O with H 2 to produce Cu. During photocatalytic CO 2 reduction over Cu In/TiO 2 catalyst, generation of Cu ions takes place through the oxidation of Cu metal by photogenerated holes (Eq. (6)). Eqs. (7) (9) illustrate the redox cycle of Cu 2+ /Cu + that may be played over TiO 2 matrix. The Cu species could be oxidized to Cu + by ions of H + and/or O 2 present in the system. Due to rapid transfer of excited electrons to the Cu particles, the separations between electron and hole pairs are enhanced resulting in higher photocatalytic activity for CO 2 reduction. The temperature-programmed reduction (TPR) of 3.5% In 2 O 3 /TiO 2 and 1.% Cu 2 O 3.5% In 2 O 3 /TiO 2 catalysts were investigated to confirm the possibility of H 2 consumption for CO 2 reduction. A mixture of 1% H 2 gas diluted in argon (Ar) was used for the catalyst reduction in a temperature range of 5 6 C. The TPR analysis report of photocatalysts is shown in Fig. S9. The amount of H 2 consumed in the reduction of different metals is tabulated in Table S3. During In 2 O 3 /TiO 2 catalyst reduction, H 2 consumption peak did not appear at 12 C, confirming H 2 was not consumed for In 2 O 3 reduction under the experimental conditions. On the other hand, H 2 consumption peak is obvious for Cu 2 O reduction at temperature 124 C. These results confirmed that H 2 was being consumed by Cu 2 O reduction during CO 2 reduction at 12 C and under UV light irradiations. However, the actual amount of H 2 consumption for CO 2 reduction was determined by conducting experiments in the absence of CO 2 as explained in Eq. (5). The summary of H 2 and CO 2 consumptions over different type of nanocatalysts and monolithic systems is presented in Fig. 12. Obviously, conversion of H 2 was apparently closer to CO 2 conversion in the case of TiO 2 and In/TiO 2 monolithic systems. However, it was very motivating to perceive from Fig. 12 that 13.5% H 2 was directly consumed without CO 2 reduction under

10 M. Tahir, N.S. Amin / Applied Catalysis A: General 493 (215) Yield rate of CO (µmole g-catal. -1 h -1 ) CO CH4 a (288) 12 fold high b Type of system (654) Fig. 13. Performance analysis of monolith photoreactor for CO 2 reduction with H 2 over 1.% Cu 3.5% In/TiO 2 catalyst at 12 C, CO 2/H 2 ratio 1.5, and irradiation time 8 h: (a) CO 2 reduction in monolith reactor without light irradiation, (b) CO 2 reduction in cell-type reactor under light irradiation, (c) CO 2 reduction in monolith reactor under light irradiations. UV light irradiations at 12 C. This amount of H 2 was probably consumed for Cu 2 O reduction without its being consumed for CO 2. Therefore, much higher H 2 consumption was due to its utilization for CO 2 and Cu 2 O reduction Performance analysis of microchannel monolith photoreactor The performance of the microchannel monolith photoreactor was compared with the cell type in the presence of light irradiations over 1.% Cu 3.5% In-co-doped TiO 2 catalyst and the results are presented in Fig. 13. The yield rates of CO and CH 4 observed were 288 and mol g-catal 1 h 1, respectively, over a Cu In/TiO 2 catalyst at CO 2 /H 2 ratio of 1.5 and temperature of 12 C in the celltype photoreactor. The performance comparison of the monolith and cell-type photoreactor revealed a 12-fold higher productivity of the monolith than the cell-type reactor under identical reaction conditions. Therefore, it can be concluded that significantly higher efficiency of monolith photoreactor was obviously due to a larger illuminated surface area, efficient light distribution, and utilization over the catalyst surface and enhanced adsorption desorption process [25]. The performance of photoreactors is usually evaluated based on quantum efficiency. The quantum efficiency was calculated for each experiment, as the ratio of the product rate (mol/s) with photonic flux (mol/s). Two electrons are needed to transform CO 2 into CO. If one electron hole pair is to be generated by one photon activation, then the quantum efficiency of CO 2 photoreduction to CO can be defined as the ratio of production rate (mol/s) with photonic flux (mol/s) as shown in Eq. (1) [42]. The detailed calculation method is reported in the supplementary materials. Quantum efficiency (%) = 2 moles of CO production rate (mol/s) moles of photon flux (mol/s) 1 (1) The quantum efficiency can be estimated by the CO production rate and moles of photon input energy from the UV light. The summary of quantum efficiency for CO production in a monolith photoreactor over 3.5% In/TiO 2 and 1.% Cu 3.5% In/TiO 2 is presented in Table 4. Using 1.% Cu 3.5% In/TiO 2 in a monolith photoreactor, the quantum efficiency for CO production was.65%, much higher than the quantum efficiency (.18%) over 3.5% In/TiO 2 c Yield rate of CH 4 (µmole g-catal. -1 h -1 ) catalyst. Previously, we reported photocatalytic CO 2 reduction with H 2 O vapors to CO over 1% In/TiO 2 catalyst in a monolith photoreactor with quantum efficiency calculated as.1% [29]. Obviously, higher quantum efficiency was obtained using H 2 as the reductant compared to H 2 O vapors. Much lower quantum efficiency was observed in the cell-type reactor. Higher monolith efficiency can be attributed to higher photons absorption and utilization inside the microchannels due to larger illuminated active surface area Reaction mechanism of CO 2 photoreduction with H 2 In this study, CO and CH 4 were identified as the main photoreduction products during photocatalytic CO 2 reduction with H 2 over In-doped TiO 2 and Cu In co-doped TiO 2 monolithic catalysts. Photocatalytic CO 2 reduction is a multi-electron process, which involves photo-excited electron (e ) and hole (h + ) generation over the semiconductor surface. In the photoreduction of CO 2 with H 2 as reductant, reactions in Eqs. (11) (22) may express CO 2 photoreduction to CO and hydrocarbons [19,4,43]. TiO 2 h e cb + h + vb (11) In 3+ + e cb In 3+ e cb Cu + /Cu 2+ + e cb /2e cb Cu + /Cu 2+ e cb /2e cb (12) (13) H 2 + 2h + 2H + (14) CO 2 + e O C O (15) O C O + H + CO + OH (16) CO e CO H+ +e C + OH (17) C H+ +e CH H+ +e CH 2 H + +e CH 3 H + +e CH 4 (18) 2 CH 2 C 2 H 4 (19) 2 CH 3 C 2 H 6 (2) CH 3 + CH 2 + CH C 3 H 6 (21) CH 3 + CH 3 + CH 2 C 3 H 8 (22) Eqs. (11) (13) reveal photo-excited electron hole pair production and their trapping by Cu and In metals, thus increasing the lifetime of charges to precede oxidation and reduction processes. CO 2 reduction by electrons and H 2 oxidation with holes is illustrated in Eqs. (14) and (15). CO was possibly produced according to Eq. (16), while it can be further reduced to C species (Eq. (17)). Eq. (18) divulges CH 4 production through utilization of H + ions and electrons. There are many possible routes of C 2 C 3 hydrocarbons production and the most appropriates are illustrated in Eqs. (19) (22) [29]. The reaction pathways for photocatalytic CO 2 reduction with H 2 as reductant in monolith photoreactor is presented in Fig. 14. The electron and holes (e, h + ) generated over TiO 2 surface reacted with CO 2 and H 2 molecules to form CO and H 2 O as RWGS reaction products. However, some of the CO and H 2 O possibly reacted with photogenerated charges which further generated intermediate species which were finally converted into hydrocarbons such as CH 4, C 2 H 4, C 2 H 6, C 3 H 6, and C 3 H 8. The significantly higher amount of CO was possibly due to efficient trapping of electrons by metal and co-metal ions. The efficient adsorption desorption process over the mesoporous Cu In codoped catalyst coated in monolith microchannels also contributes to maximizing process efficiency. In addition, CH 4 produced as the second major product was obviously due to CH 3 reacted with H at higher rate to produce CH 4 rather than CH 3 and CH 2 species reaction to produce higher hydrocarbons [44]. Therefore, higher amount of CO at selectivity 99.27% confirmed efficient RWGS reaction over Cu In co-doped TiO 2 in a monolith photoreactor.

11 1 M. Tahir, N.S. Amin / Applied Catalysis A: General 493 (215) 9 12 Table 4 Summary of operating parameters and calculated quantum efficiencies. System CR a, this research MMR b [29] MMR b, this research Light source Mercury lamp 2 W, UVA light, I = 15 mw cm 2 Mercury lamp 2 W, UVA light, I = 15 mw cm 2 Mercury lamp 2 W, UVA light, I = 15 mw cm 2 Mercury lamp 2 W, UVA light, I = 15 mw cm 2 Temperature ( C) Pressure (bar) Catalyst 1% Cu 3.5% In/TiO 2,.5 g 1% In/TiO 2,.552 g 3.5% In/TiO 2,.49 g 1% Cu 3.5% In/TiO 2,.55 g Reducing agent H 2 H 2O H 2 H 2 Main product CO CO CO CO Specific rate ( mol g 1 h 1 ) Quantum efficiency (%) c a Cell-type reactor. b Microchannel monolith reactor. c Quantum efficiency = (number of electrons moles of production rate)/(moles of UV photons flux). Fig. 14. Reaction schemes for CO 2 photoreduction with H 2 in a monolith photoreactor: (a) oxidation reduction process; (b) adsorption desorption process Stability analysis of reused catalyst The stability of the catalyst in photocatalysis especially for CO 2 photoreduction applications is important. In order to investigate the stability of catalysts, experiments under UV light irradiations were repeated for three times on the recycled catalyst-coated monoliths. In the case of TiO 2 and 3.5% In/TiO 2 -coated monoliths, after two cycles of photocatalysis reactions, all the samples retained their initial activity. However, after the third run, In-doped TiO 2 catalyst partially lost its activity, production of CO decreased while CH 4 and higher hydrocarbons increased. The stability test for photocatalytic CO 2 reduction with H 2 to CO over 1.% Cu 3.5% In co-doped TiO 2 catalyst is shown in Fig. 15. After each cycle, the monolith was removed from the reactor and placed in open air for some time before starting the next run. Apparently, yield of CO was much higher in the first run; slightly decreased in the second photocatalytic cyclic run, but a significant decrease in CO yield was observed after the third run. Conversely, 1.% Cu 3.5% In/TiO 2 catalyst exhibited much higher CH 4 yield in the third run as shown in Fig. 16. This was possibly due to a photochemical reaction between carbon species Yield of CO (µmole g-catal. -1 ) Run-1 Run-2 Run Irradiation time (min) Fig. 15. Stability test on photoactivity of 1.% Cu 3.5% In co-doped TiO 2 catalyst for CO 2 photoreduction to CO at 12 C and CO 2/H 2 ratio 1.5. CO

12 M. Tahir, N.S. Amin / Applied Catalysis A: General 493 (215) for CO production was 3.23 times higher than 3.5% In/TiO 2 and 113 times higher than un-doped TiO 2 in monolith photoreactor. The monolithic catalysts also demonstrated better performance in the presence of H 2 reducing agent for photocatalytic CO 2 reduction. The quantum efficiency in microchannel monolith photoreactor was significantly higher for CO production over 1.% Cu 3.5% In co-doped TiO 2 nanocatalysts than the cell-type photoreactor. More importantly, the enhanced conversion efficiency was probably due to improved charge separation in TiO 2 -supported monolith microchannels and the presence of Cu and In species served as electron traps which suppressed electron hole recombination. The stability test revealed yield rates and selectivity of CO partially decreased, but higher CH 4 and hydrocarbons were observed with more cycles. Acknowledgements Fig. 16. Stability test of 1.% Cu 3.5% In co-doped TiO 2 monolithic catalyst for the production of CH 4 during CO 2 photoreduction at 12 C and CO 2/H 2 ratio 1.5. adsorbed over the catalyst and H 2 as explained previously. Similarly, much higher C 2 C 3 paraffin and olefin hydrocarbons yield rates were observed during the third recycle of Cu In/TiO 2 catalyst as portrayed in Fig. S1. The hydrocarbon yields were in the order of CH 4 > C 3 H 6 > C 3 H 8 > C 2 H 6 > C 2 H 4. Besides, an appreciable amount of C 4 C 6 hydrocarbons (iso-butane, n-butane, 1-butene, iso-pentane, n-pentane, n-hexane, and iso-hexane) were detected when Cu In/TiO 2 was recycled for the third run inferring better initial activity of Cu In/TiO 2 for the production of CH 4 and hydrocarbons. The increase in hydrocarbons yields were probably due to carbon deposition over the catalyst surface as H 2 preferably reacted with active carbon species instead of CO 2. To verify this hypothesis, a series of experiments were conducted in the absence of CO 2 using reused monoliths for the cases of (a) helium gas and (b) helium gas with H 2 and the results are presented in Table S4. Interestingly, significant amounts of hydrocarbons were produced in the presence of helium gas only. This was perhaps due to the reactions between adsorbed hydrogen, water, and carbon species over the catalyst surface. On the other hand, large amounts of CO and hydrocarbons were produced using hydrogen as a reductant. These results confirmed that adsorbed carbon species reacted with hydrogen and a series of reactions took place, resulting in higher hydrocarbon formation as explained in Eqs. (18) (21) in the reaction mechanism section. The color change of Cu and In co-doped TiO 2 monolithic catalysts is depicted in Fig. S11. The color of Cu-loaded catalyst turned black during photocatalytic CO 2 reduction, but was gradually restored to its original color when catalyst-coated monolith was exposed to open air at normal temperature. The darkening in catalyst color may be an indication of the formation of intermediate carbon species absorbed over the catalyst surface. 4. Conclusions The photocatalytic CO 2 reduction with H 2 as a reducing agent using Cu In co-doped TiO 2 catalysts supported over monolith channels was conducted under UV light irradiations. The presence of Cu and In species over TiO 2 modified the crystalline and optical properties of TiO 2. Both Cu and In co-doped to TiO 2 acted as efficient photocatalysts for CO 2 reduction through reverse water gas shift reaction. At various CO 2 /H 2 ratios and reaction temperature, different photoactivity and selectivity on the formation of CO and hydrocarbons, namely CH 4, C 2 H 4, C 2 H 6, C 3 H 6, and C 3 H 8 were exhibited. Performance of 1.% Cu 3.5% In co-doped TiO 2 catalyst The authors would like to extend their deepest appreciation to the Ministry of Education (MOE), Malaysia, and Universiti Teknologi Malaysia for the financial support of this research under LRGS (Long-term Research Grant Scheme, Vot 4L8), RUG (Research University Grant, Vot 2G14), and FRGS (Fundamental Research Grant Scheme, Vot 4F44). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at References [1] J. Yu, K. Wang, W. Xiao, B. Cheng, Phys. Chem. Chem. Phys. 16 (214) [2] M. Tahir, N.S. Amin, Renew. Sustain. Energy Rev. 25 (213) [3] T. Inoue, A. Fujishima, K. Satoshi, K. Honda, Nature 277 (1979) [4] M. Halmann, M. Ulman, B.A. Blajeni, Sol. Energy 31 (1983) [5] M. Anpo, H. Yamashita, K. Ikeue, Y. Fujii, S.G. Zhang, Y. Ichihashi, D.R. Park, Y. Suzuki, K. Koyano, T. Tatsumi, Catal. Today 44 (1998) [6] K. Mori, H. Yamashita, M. Anpo, RSC Adv. 2 (212) [7] Q. Zhang, Y. 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