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Inorganic nanoparticles/metal organic framework hybrid membrane reactors for efficient photocatalytic conversion of CO2 James Wainaina Maina, Jurg Schutz, Luke Grundy, Elise Des Ligneris, Zhifeng Yi, Lingxue Kong, Cristina Pozo-Gonzalo, Mihail Ionescu, and Ludovic F Dumée ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11150 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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ACS Applied Materials & Interfaces

Inorganic nanoparticles/metal organic framework hybrid membrane reactors for efficient photocatalytic conversion of CO2

Authors James W. Maina*a,b, Jürg A. Schützc, Luke Grundyc, Elise Des Lignerisa, Zhifeng Yia, Lingxue Konga, Cristina Pozo-Gonzaloa, Mihail Ionescub, Ludovic F. Duméea

Affiliations a

Deakin University, Geelong, Institute for Frontier Materials, 75 Pigdons Road, Waurn Ponds, Vic

3216, Australia, b

Australian Nuclear Science and Technology Organization, New Illawara Road, Lucas Height, NSW,

Australia c

Commonwealth Scientific and Industrial Research Organization (CSIRO), 75 Pigdons Road, Waurn

Ponds Vic 3216, Australia *Corresponding author email: [email protected]; +61 415 411 649

ACS Paragon Plus Environment


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Abstract Photocatalytic conversion of carbon dioxide (CO2) to useful products has potential to address the adverse environmental impact of global warming. However, most photocatalysts used to date exhibit limited catalytic performance, due to poor CO2 adsorption capacity, inability to efficiently generate photoexcited electrons, and/or poor transfer of the photo-generated electrons to CO2 molecules adsorbed on the catalyst surface. The integration of inorganic semiconductor nanoparticles across metal organic framework (MOF) materials has potential to yield new hybrid materials, combining high CO2 adsorption capacity of MOF and the ability of the semiconductor nanoparticles to generate photoexcited electrons. Herein, controlled encapsulation of TiO2 and Cu-TiO2 nanoparticles within zeolitic imidazolate framework (ZIF-8) membranes was successfully accomplished, using rapid thermal deposition (RTD), and their photocatalytic efficiency towards CO2 conversion investigated under UV irradiation. Methanol and carbon monoxide (CO) were found to be the only products of the CO2 reduction, with yields strongly dependent upon the content and composition of the dopant semiconductor particles. CuTiO2 nanoparticles doped membranes exhibited the best photocatalytic performance, with 7 µg of the semiconductor nanoparticle enhancing CO yield of the pristine ZIF-8 membrane by 233 %, and methanol yield by 70%. This work opens new routes for the fabrication of hybrid membranes containing inorganic nanoparticles and MOFs, with potential application not only in catalysis but also in electrochemical, separation and sensing applications. Keywords Metal organic frameworks, hybrid membranes reactors, CO2 conversion, photo-catalysis, inorganic semiconductor nanoparticles


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1.0. Introduction The utilization of CO2 as a raw material to produce valuable chemicals and fuels, is increasingly gaining attention as a sustainable solution to mitigate the adverse effect of the anthropogenic emission of CO2.1-6 One promising utilization pathway to generate fuels or reactive chemicals from CO2 is photocatalytic conversion, whereby the greenhouse gas is converted to useful products, such as carbon monoxide, formic acid, or fuels, including methanol and methane, in the presence of a suitable photocatalytic material.2 To date, titanium dioxide (TiO2) remains the most widely studied photocatalyst material due to its wide availability, low cost, non-toxicity and long-term stability.2,

7

However, the rapid

recombination of the photo-generated electron-holes across the microstructure of TiO2 photocatalysts, as well as their low adsorption capacity for CO2, severely compromise their catalytic efficiency towards CO2 conversion.2 Consequently, there is an intense effort to develop alternative catalytic materials, with improved charge separation kinetics while providing higher adsorption capacity for CO2, to facilitate a much more efficient conversion processes. Metal organic frameworks (MOFs), a class of hybrid materials composed of metal ions/clusters coordinated to organic ligands,8 have recently gained attention as potential alternatives to TiO2 catalysts, due to their high porosity and surface area, which endow them with extreme adsorption capacities for CO2.3, 9-11 The catalytic properties of MOF materials are as varied as the available chemistries, and may be readily controlled via judicious selection of MOF building blocks, or through doping or modification with photoactive materials.12-14 Furthermore, MOFs such as ZIF-6715 and Ru-MOF16 have been shown to exhibit significantly longer charge separation time, as compared to TiO2 based catalysts17-18, highlighting their high potential in photocatalysis. These catalytic properties combined with



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the high adsorption capacity for CO2, make MOF especially promising materials for a onepot capture and conversion of CO2.12 Although a number of MOFs have been demonstrated to be active under visible light, the products yield of most materials remain very low, and further research is required to greatly enhance their catalytic efficiency.3 In addition, most studies on MOF photo-catalysts investigated are powder based systems, which might face engineering challenges associated with light scattering and post-reaction particles recovery, during scale-up to an industrial scale.19 For this reason, membrane reactor based catalytic systems are more attractive than powder based systems for one pot CO2 capture and photocatalytic conversion, due to their facile scale-up, higher interfacial surface area and their ease of recovery following the photocatalytic reactions.19-20 Although different techniques for the growth of MOF membranes and films across porous supports have been developed,21-23 the incorporation of functional nanoparticles within MOF films still remain a challenge due to the complicated synthesis routes that are typically employed to make the films and membranes.24-25 In this study, rapid thermal deposition (RTD) has been employed for the first time to achieve a controllable encapsulation of TiO2 and Cu-TiO2 nanoparticles within ZIF-8 membranes, for efficient photocatalytic conversion of CO2. RTD is a highly promising route for the fabrication of MOF membranes in a time efficient manner (typically in less than 1 h),26 and consumes only a fraction of the chemicals that would be required for solvothermal and hydrothermal synthesis approaches. The fabrication route is a two-step process, where a pristine ZIF-8 membrane layer is first deposited across a graphene oxide (GO) coated stainless steel substrate, after which a secondary thin film layer consisting of the semiconductor nanoparticles dispersed within ZIF-8 matrix is deposited.

Photocatalytic

conversion of CO2 using the hybrid membrane reactor produced methanol and carbon monoxide as the only products, with the yields strongly dependent on the composition and


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the content of the dopant semiconductor nanoparticles. To the best of our knowledge, this is the first report in the use of continuous MOF based membrane reactors for photo-catalytic conversion of CO2.



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2.0. Results and discussions Semiconductor nanoparticles doped ZIF-8 membrane reactors were successfully fabricated using rapid thermal deposition (RTD), and their photocatalytic efficiency towards CO2 conversion were investigated under UV light. The fabrication process involves the deposition of a primary pristine ZIF-8 membrane layer across a GO coated stainless steel substrate, after which a secondary thin film layer consisting of the semiconductor nanoparticles dispersed within ZIF-8 matrix is deposited. A schematic illustration for the fabrication process is shown in Scheme 1.

Scheme 1. Schematic illustration of the assembly of semiconductor nanoparticles doped MOF membranes.

The porous stainless steel substrate was selected due to its high void volume, which is necessary to allow efficient transport of CO2 to the catalytic sites, as well as efficient removal of catalytic products from the catalyst surface. The surface of the stainless steel substrates was first modified with graphene oxide (GO) via electroplating to introduce carboxylic and hydroxyl functional groups, which are known to interact with metal ions during MOF growth,21, 27 and to enhance nucleation of MOF crystals across porous supports. Graphene electroplating is a highly promising route for the surface modification of metallic materials, due to its high time efficiency and simplicity.28 In addition to introducing carboxylate (COOH) and hydroxyl (OH) functional groups across the surface of the metal substrate, GO was also selected due to its excellent chemical stability29 and its demonstrated ability to protect metal substrates from potentially corrosive environments.30-31 Furthermore GO


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stability during photocatalytic reactions has been previously demonstrated, where GO functional groups were shown not to participate in the CO2 conversion product formation.32 Successful deposition of GO was confirmed by visual observation, as the color of the substrate changed from light silver-grey to black following electroplating for 1 min, and by scanning electron microscopy (SEM) (Figure 1 b), where GO wrinkles could be clearly observed from the surface of the metal fibre after plating. The surface modification led to a well homogenous nucleation and growth of ZIF-8 nano-crystals following rapid thermal deposition (Figure 1 c), suggesting a favourable interaction between the GO coated surfaces with the MOF building blocks. This homogenous nucleation was, however, not observed when MOF crystals were deposited on non-modified substrate (Figure S1), and the deposited crystals could be clearly seen delaminating from the metal fibre, highlighting the critical role of the surface modification on binding MOF on to the metal support.

Figure 1. SEM images of (a) Porous stainless steel substrate (b) GO coated stainless steel fiber (c) nucleation of ZIF-8 nano crystals on the surface of GO coated metal fiber, (d)



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Surface morphology of pristine ZIF-8 membrane without nanoparticles doping, (e) surface morphology of the ZIF-8 membrane containing 7 µg of TiO2 nanoparticles (f) surface morphology of the hybrid membrane containing 14 µg of TiO2 nanoparticles Figure 1 d to f show that the surface morphology of ZIF-8 membranes was maintained following doping with the inorganic nanoparticles, suggesting that the particles did not compromise the crystallization process. The dispersibility of the inorganic nanoparticles across the MOF matrix was assessed by sectioning the top surface of the hybrid membrane containing 14 µg of TiO2 nanoparticles prior to TEM imaging. Figure 2(b) and (c), demonstrate there was no major aggregate exceeding the size of the primary particles (~25 nm, Figure 2a), suggesting good dispersibility across the top layer of the MOF membranes. It is noteworthy to consider that most previous work in the encapsulation of inorganic nanoparticles within MOF crystals required surface modification with a polymer, to prevent the nanoparticles agglomeration.33-34 However, the surface coating of the inorganic nanoparticles may limit the interfacial contact between the catalytic nanoparticles and the MOF, and thus inhibit charge transfer during photocatalysis.35 Excellent dispersibility were achieved in the current work without the need of any surface modification, highlighting the high potential of the RTD approach in the synthesis of inorganic nanoparticles/MOF hybrid materials for catalytic applications. The improved dispersibility is attributed to the rapid crystallization of the MOFs, which denies the dispersed inorganic nanoparticles sufficient time to agglomerate.


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Figure 2. TEM images of (a) TiO2 nanoparticles, (b) TiO2/ZIF-8 hybrid membrane, (c) HRTEM image of the TiO2/ZIF-8 hybrid membrane. The presence of the TiO2 particles within the MOF matrix is highlighted by the red squares.

Figure 3 shows gas permeation and selectivities of the ZIF-8 and the ZIF-8/TiO2 hybrid membrane reactors containing 14 µg of the TiO2. The ZIF-8 membranes exhibited high gas permeation for He, N2, Ar and CO2, with He diffusing at the highest rate of 2.38 ± 0.004 mol.m-2.s-1Pa-1, while Ar and CO2 had the lowest diffusion rate of 0.81 ± 0.015 and 0.88 ± 0.017 mol.m-2.s-1Pa-1 respectively. The membrane also exhibited Knudsen selectivities for He, Ar, and CO2 relative to nitrogen, suggesting the membrane were almost defect free.36 Knudsen selectivities are typically observed during separation of small gas molecules using ZIF-8 membranes,21, 36 and the lack of a sharp cut-off point between CO2 and N2 is typically attributed to the flexible pore structure of ZIF-8.21 The Knudsen selectivities were maintained even for the TiO2 nanoparticles doped ZIF-8 membranes, suggesting the catalytic nanoparticles did not have noticeable negative impact on the structural integrity of the MOF membranes. However, a decrease in gas permeance is noted across all gas tested, with permeance for helium decreasing from 2.38 ± 0.004 to 1.23 ±0.18 x10-6 mol.m-2.s-1Pa-1, while permeance for CO2 decreased from 0.88 ± 0.017 to 0.438 ± 0.006 x10-6 mol.m-2.s-1Pa-1. This decrease in permeance may be attributed to an increased thickness following the introduction of the TiO2



nanoparticles containing secondary layer, as well as an increased resistance to gas flow due to the presence of the inorganic nanoparticles. This decrease in permeance may potentially be mitigated by using a MOF with larger pore channels to form the secondary layer. 3.0

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Figure 3. (a) Gases permeance across pristine ZIF-8 membrane (b) calculated gas selectivity relative to N2 (c) Gases permeance across TiO2/ ZIF-8 hybrid membrane (d) calculated gas selectivity relative to N2 The photocatalytic efficiency of the hybrid membrane reactors towards CO2 conversion were evaluated under the irradiation of UV light, in the presence of dimethylacetamide (DMAC) as the solvent. DMAC has been previously shown to be a suitable solvent to facilitate efficient photocatalytic conversion of CO2, due to its higher solubilisation capacity for CO2 and stability under the irradiation of UV light.37 Methanol and CO were found to be the only products when either pristine ZIF-8 membranes or inorganic


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nanoparticles modified ZIF-8 membranes were used as the catalyst (Figure 4). The pristine ZIF-8 membrane produced 10.3 ±1.3 ppm of CO and 20.8 ± 1.9 ppm of methanol, following a 6 h photocatalytic reaction cycle. The yields were however enhanced after doping with inorganic semiconductor nanoparticles, with 14 µg of TiO2 doping producing 16.3 ± 0.7 ppm of CO and 28.8 ± 2.6 ppm of methanol, while similar amount of Cu-TiO2 nanoparticles resulted in 29.7 ± 1.3 ppm of CO and 31.3 ±3.4 ppm of methanol, corresponding to 188 and 50% increase for CO and methanol respectively, as compared to the amount generated using the pristine ZIF-8 membrane alone. The dramatic increase in the product yields highlights the synergistic effects arising from the ability of the semiconductor nanoparticles to generate photo-excited electrons upon irradiation with light, and the high CO2 adsorption capacity of MOF.13 Li et al.13 previously demonstrated using ultrafast spectroscopy that electrons generated by semiconductor nanoparticles can be transferred to MOF matrix, and participate in photocatalytic reactions. 40

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The products yield was also found to be dependent on the content of the Cu-TiO2 nanoparticles loaded on to the MOF membranes (Figure 4b). While only a slight decrease in products yield is noted as the content of nanoparticles is increased from 7 to 28 µg a further increase resulted in a more significant decrease, with 42 µg on nanoparticles loading producing 11.3 ± 0.7 ppm and 14.9 ± 0.5 ppm of CO and MeOH respectively. The highest yields were obtained with the 7 µg of the Cu-TiO2 nanoparticles loading, and corresponds to 2,170 ppm of CO and 2,238 ppm for methanol per gram of catalyst, which is higher than most of the previously reported yields using MOF based catalysts.3, 38-39 These excellent performance are attributed not only to the synergistic effect between the semiconductor particles and MOF, but also to the use of DMAC, which exhibit higher CO2 solubility as compared to most commonly used solvent such as water. The role of the solvent was better highlighted after a control experiment with ZIF-8 nanoparticles (Figure S2) exhibited significantly higher catalytic activity in the presence of DMAC as compared to the same amount of catalyst in the presence of MECN. In the presence of DMAC, the ZIF-8 nanoparticles produced 11,000 ppm/g cat. CO and 6,600 ppm/g cat. MeOH. However, when the same amount of catalyst was tested in the presence of acetonitrile (MECN) solvent, the CO generation reduced by almost 70% to 3,333 ppm/g cat., while no methanol generation was detected. In comparison to MECN, DMAC is a stronger Lewis base, with Lewis basicity of 0.73 BKT, while MECN has a lower Lewis basicity of 0.23 BKT. 40 Consequently DMAC is expected to have a more intimate interaction with CO2 which is a Lewis acid, thus contributing favourably to the photo-conversion process. The photocatalytic activity of ZIF-8 has been previously attributed to the generation of photoactive hydroxyl radicals (OH●) upon irradiation of ZIF-8 with UV.41 The presence of defect sites across ZIF-8 framework has also been demonstrated to give catalytic activity to ZIF-8 material.42-43 Under the irradiation of


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visible light, however, pure ZIF-8 powder was shown to be photocatalytically inactive towards CO2 conversion.38 The catalytic reactivity of native TiO2 and CuTiO2 powders was also investigated when dispersed directly in the DMAC solvent, to assess the performance of the dopants alone. The commercial TiO2 nanoparticles (P25) yielded 2,600 ppm of CO and 4,596 ppm methanol per gram of catalyst, while the Cu-TiO2 nanoparticles, as expected,2, 44 exhibited higher product yields with 11,467 ppm/g cat. CO, 12,000 ppm/g cat. H2 and 4,157 ppm/g cat. of methanol. While, the yields of the nanoparticles are higher than that obtained from the membranes reactor systems under similar reaction conditions, the relatively better performance is likely to be lost with scale up, due to the light scattering typical with nanoparticles dispersions, severely compromising the utilization of UV light.19 In addition, the requirement for centrifugation to recover the particles from the reaction medium is a major obstacle that hampers large scale utilization at an industrial scale. Membrane reactor based system have the potential to address this challenge since the membranes can be easily recovered from a reaction medium, while scale up can be directly accomplished by increasing the surface area of the membrane.19 Furthermore, the high CO2 adsorption capacity of MOF materials make the hybrid membrane reactors extremely promising, for simultaneous capture of CO2 and catalytic conversion. To investigate long-term stability of the hybrid membrane reactors under the photocatalytic condition, the membrane doped with 14 µg of Cu-TiO2 nanoparticles was recycled for two consecutive cycles and change in product yields and membrane properties monitored. A progressive decrease in both CO and MeOH generation was noted upon recycling (Figure 4 a). The CO generation for the membrane reactor containing 14 µg of CuTiO2 nanoparticles was reduced to 9.3 ppm after the 3rd cycle, while MeOH generation was reduced to 19 ppm, corresponding to a 69 % and 39 % decrease respectively. Further



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characterization of the membrane recovered after the catalytic cycles were therefore carried out, to investigate the cause of the decrease in the catalytic performance. SEM imaging (Figure 5) revealed no significant changes in membrane morphology after the reactions, suggesting that the membrane material was stable during the three catalytic cycles. This was further confirmed by XRD analysis (Figure 6) which shows the characteristic diffraction patterns of ZIF-8 were preserved.

Figure 5. SEM images of the CuTiO2/ZIF-8 hybrid membrane before catalysis (a), (b) and after catalysis (c,) (d).


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Figure 6 XRD analysis of the CuTiO2/ZIF-8 hybrid membrane, (a) before catalysis (b) after three catalytic cycles.

TGA analysis (Figure S3) also revealed that the membranes recovered after the catalytic reactions exhibited a degradation profile similar to the one obtained before catalysis, while N2 adsorption showed only a slight decrease in BET surface area, from 138 m2/g to 114 m2/g. (Figure 7 (a). Interestingly, the CO2 adsorption (Figure 7 (b)) studies showed a significant decrease in adsorption capacity, from 10.8 cm3/g to 3.4 cm3/g corresponding to 69 % decrease in the adsorption capacity. This cause for the decrease in CO2 adsorption capacity, may be related to strong adsorption of reaction intermediates such as CO, –CH3 and –OCH3 45 across the catalytic sites present within the membrane reactors. Strong adsorption of the reaction intermediates has been previously shown to retard catalytic performance of ZIF-8/Zn2GeO4 hybrid catalysts,38 which highlights the need for a higher temperature activation to liberate the catalytic sites following photocatalytic reactions. Further work will explore potential regeneration pathways including thorough membrane wash with a solvent, followed by high temperature activation.



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Figure 7. (a) N2 adsorption on the CuTiO2/ZIF-8 hybrid membrane before and after catalysis, (b) CO2 adsorption before and after catalysis


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3.0 Conclusions In summary, we have demonstrated an efficient route for the fabrication of membrane reactor composed of semiconductor nanoparticles and MOF, and demonstrated their catalytic efficiency towards CO2 conversion. Although, MOF membranes have been widely studied for diverse separation applications, incorporation of functional nanoparticles within the membranes matrix is difficult due to the complicated synthesis processes typically employed to make MOF membranes. Here, we show by using RTD, a thin film composed of catalytic semiconductor nanoparticles dispersed within MOF matrix can be deposited on a preformed MOF membrane, in just about 15 min. As a proof of concept, we have demonstrated that TiO2 and Cu-TiO2 doped ZIF-8 membrane exhibit higher catalytic efficiency towards CO2 conversion as compared to the pristine ZIF-8 membrane, and the product yield can be controlled depending on the composition and the dosage of the semiconductor nanoparticles. Given that membrane based systems are considered advantageous as compared to powder based system for large scale photocatalytic applications, this work opens route for the fabrication of highly active membrane reactors for simultaneous capture of CO2 from a point source and photocatalytic conversion. Supporting information SEM image of MOF deposited on surface unmodified stainless steel substrate (Figure S 1), TEM image of ZIF-8 nanoparticles (Figure S2,) TGA analysis of Cu-TiO2 doped ZIF-8 membrane before and after 3 catalytic cycles (Figure S3).

Acknowledgments The

authors acknowledge

the Australian

Institute

of

Nuclear

Science

and

Engineering (AINSE) for financial support, through Mr. James Maina AINSE postgraduate



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research award (PGRA). They also thank Deakin University for Dr. Dumee’s Alfred Deakin Fellowship and Prof. Hodgson for his advice and support.


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4.0. Experimental Section 4.1. Materials 2-methyl imidazole (Hmim, 99 %), zinc chloride (ZnCl2, >98 %), sodium, N, Ndimethylacetamide (DMAC, 99.8 %), and zinc acetate dihydrate (Zn(OAc)2.2H2O, ≥ 99 %), Zinc nitrate hexahydrate (Zn(NO3)2.6H2O, ≥98%), methanol (99.8%), TiO2 (P25), copper (II) nitrate trihydrate (Cu(NO3)23H2O, ≥ 99 %), natural graphite (particle size < 45 µm), sulphuric acid (H2SO4, 98%), potassium permanganate (KMnO4, 99%), and hydrogen peroxide (H2O2) were purchased from Sigma Aldrich, and used without further purifications. The porous stainless steel substrates (SS316 grade) was obtained from Anping DeChengli Hardware products Co., Ltd (China). 4.2.Synthesis of graphene oxide (GO) GO was synthesized from graphite powder according to a modified Hummers method.4647

Briefly, 1 g of graphite was added into 30 mL of sulphuric acid (98%) containing 0.5 g of

NaNO3 mixture under vigorous stirring for 1 h. 5 g KMnO4 was then added gradually, while the temperature of the mixture was maintained below 10 oC. The mixture was then heated to 38 °C and stirred until it became light brown, after which 50 mL of deionized water was slowly added to the solution and temperature increased to 95 °C in a water bath and stirred for 15 min. An additional 50 mL of deionized water was then added to stop the reaction, followed by 5 mL of 30 % hydrogen peroxide. The sample was then ﬁltered on a porous anodized aluminium oxide membrane (pore size, 0.1 µm) (Whatman, Little Chalfont, UK) with a vacuum filtration system and washed with 10 v/v % HCl solution followed by rinsinig with excess deionized water. The solid was subsequently redispersed in deionized water and ultra-sonicated for 30 mins at 200 W with a water bath ultrasonic machine (FS300b, Decon).



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4.3. Electroplating of GO on the surface of the stainless steel substrate Electroplating was carried out from a solution containing 0.5 mg/mL of GO in water. The stainless steel substrate (3 x 3.5 cm) was immersed in ethanol and sonicated for about 15 minutes, after which it was connected to anode, while a titanium foil of similar dimension was used as the cathode, with the distance between the two electrodes fixed at 1 cm. A voltage of 10 V was then applied across the electrode for exactly 1 min, after which the GO coated substrate was removed from the solution, and dried using a blow drier. 4.4. Preparation of Cu doped TiO2 nanoparticles Cu doped TiO2 (P25) nanoparticles were prepared by wet impregnation method, as previously published in the literature.48 0.015 g of copper (II) nitrate trihydrate was dissolved in 20 ml of deionized water, after which 0.2 g of the TiO2 nanoparticles was added, and the mixture stirred for 1 h. The solvent was then evaporated slowly in a water bath, at 80 oC and the obtained powder were subsequently dried in an oven at 120 oC, and then calcined at 500 o

C for 30 min, in air. 4.5. Fabrication of inorganic nanoparticles doped MOF membranes 0.66 g of Zn(OAc)2.2H2O and 0.5 g of Hmim were separately dissolved in 7.5 mL of a

solvent containing water and DMAC at the ratio of 1:2. The Hmim solution was then added to the Zinc acetate solution in a dropwise manner, followed by stirring for 1 min. The surface modified substrate (3.5 x 3 cm), was then placed in a crucible, and 200 µL of the MOF precursor solution applied gently on the surface. The sample was then transferred to pre-heated oven at 200 oC for 15 min, to initiate the crystallization of ZIF-8 across the stainless steel substrate. After 15 min, the membrane was allowed to cool to room


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temperature, after which the procedure is repeated 2 more times, to obtain a dense ZIF-8 membrane across the porous substrate. At the final stage, a given amount of inorganic nanoparticles dispersion (0.34 mg/mL) in water were added to the MOF solutions and stirred for one minute, after which 200 µl of the resultant solution was gently applied on the surface of the membrane, prior to heating at 200 oC for 15 min . 4.6. Synthesis of ZIF-8 nanoparticles ZIF-8 nanoparticles (~ 40 nm) were prepared following a previously published procedure.49 1.47 g of Zn(NO3)2.6H2O and 3.25 g of Hmim were separately dissolved in 100 mL of methanol, after which the zinc nitrate solution was rapidly poured into the Hmim solution, and vigorously stirred for 1 h, at room temperature. The particles were then separated under centrifugation, and washed three times with deionized water.

4.7. Characterization techniques Scanning electron microscopy (SEM) was performed on Zeiss Supra 55VP FEG SEM, under a beam voltage of 5 keV and a working distance of 10 mm. Transmission electron microscopy imaging was conducted using JEOL TEM FEG 2100F, operating at an acceleration voltage of 200 kV and beam current of 130 µA. N2 adsorption and CO2 adsorption studies were carried out using micrometric TriStar 3000, at 77 K and 273 K respectively. The samples were degassed for 6 h at 150 oC prior to measurement. XRD measurement were conducted using X’Pert Pro MRD XL, with Cu Kα anode, operating at 40 kV and 30 mA, while thermogravimetric analysis were carried out using Q50 Thermal Gravimetric Analyzer, by heating at the rate of 20 oC/min from room temperature to 800 oC under nitrogen atmosphere.



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4.8. Photocatalytic conversion of CO2 Photocatalytic experiments were carried out in a custom made Teflon lined stainless steel autoclave (100 mL), fitted with a quartz window, an inlet and out let valve for gas. The reaction was carried in the presence of DMAC, with TEOA amine as sacrificial agent. 5 ml of DMAC/TEOA (20:1 v/v) was placed in beaker, after which CO2 was purged through the solution for 30 min, to saturate the solution with CO2. A hybrid membrane, measuring 3.5 x 1.5 cm, was then placed in the photocatalytic reactor with the nanoparticles modified surface facing upwards, after which the reactor was evacuated. The reactor was then loaded with CO2 and left for 30 min to allow CO2 to adsorb on the membrane. After the 30 min, CO2 was released and the CO2 saturated DMAC solution was loaded. The reactor was then sealed and filled with CO2 to the pressure of 120 kPa, after which it was placed under UV lamp and irradiated for 6 h, with a cut off filter of wavelength between 320 – 480 nm. Liquid products were analysed using a Varian gas chromatograph fitted with FID detector and H-PLOT capillary column. Gaseous product were detected using a diffusive MultiRAE detector from RAE Systems fitted with electrochemical H2, CO and CH4 sensors.


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TOC/Abstract Graphic


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