Titania Photocatalyst for CO2 Reduction Based on

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Fabrication of Gold/Titania Photocatalyst for CO2 Reduction based on pyrolytic Conversion of the Metal-Organic Framework NH2-MIL-125(Ti) loaded with Gold Nanoparticles. Kira Khaletskaya, Anna Pougin, Raghavender Medishetty, Christoph Rösler, Christian Wiktor, Jennifer Strunk, and Roland A. Fischer Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03017 • Publication Date (Web): 08 Oct 2015 Downloaded from http://pubs.acs.org on October 11, 2015

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Chemistry of Materials

Fabrication of Gold/Titania Photocatalyst for CO2 Reduction based on pyrolytic Conversion of the Metal-Organic Framework NH2-MIL-125(Ti) loaded with Gold Nanoparticles Kira Khaletskaya,1 Anna Pougin,2 Raghavender Medishetty,1 Christoph Rösler,1 Christian Wiktor,1 Jennifer Strunk*,3 and Roland A. Fischer*,1 1Department 2Laboratory

of Inorganic Chemistry II, Ruhr University Bochum, Universitätsstraße 150, 44780 Bochum, Germany

of Industrial Chemistry, Ruhr University Bochum, Universitätsstraße 150, 44780 Bochum, Germany

3MPI

CEC, Stiftstraße 34 - 36, 45470 Mülheim an der Ruhr, Germany and Center for Nanointegration Duisburg-Essen (CENIDE), 47057 Duisburg, Germany

ABSTRACT: Titania exhibits unique photo-physical and -chemical properties and can be used for potential applications in the field of photocatalysis. The control of TiO2 in terms of phase, shape, morphology, and especially nano-scale synthesis of TiO2 particles still remains a challenge. Ti-containing metal-organic frameworks (MOFs), such as MIL-125, can be used as sacrificial precursors to obtain TiO2 materials with diverse phase compositions, morphologies, sizes and surface areas. MIL-125 is composed of Ti/O clusters as the secondary building units (SBUs) bridged by 1,4-benzene-dicarboxylate (bdc). In this study, pre-formed and surfactant stabilized gold nanoparticles (GNPs) were deposited onto the surface of amino functionalized NH2-MIL-125 during solvothermal synthesis. Targeted gold/titania nano composites, GNP/TiO2, were fabricated through the pyrolysis of GNP/NH2-MIL-125 nanocrystals. The modification of TiO2 with GNPs significantly increased the photocatalytic activity of the MOF derived TiO2 material for the reduction of CO2 to CH4 as compared to TiO2 reference samples such as P-25 and Aurolite (Au/TiO2). The new materials GNP/TiO2 and TiO2 derived by the MOF precursor route were thoroughly characterized by PXRD, FTIR and RAMAN, TEM and N 2 adsorption studies.

INTRODUCTION In recent years, the synthesis and characterization of transition-metal oxides with controlled size and shape as well as uniform dimension have attracted huge interest due to their unique physical and chemical properties and potential applications in the fields of catalysis and photoelectronic devices.1-6 Among transition metal oxides like ZnO or CuO, TiO2 is of extraordinary importance, owing to its special properties such as photocatalytic activity. Among numerous methods for the synthesis of TiO2, the sol-gel route remains favorable, because of its low cost and simple nature of processing.7-9 The process normally proceeds via an acid-catalyzed hydrolysis step of titanium (IV) alkoxide followed by the condensation. Different synthetic conditions result in TiO2 particles with different morphologies and diverse properties.10 Nevertheless, the control of TiO2 in terms of phase, shape, morphology, and especially nano-scale synthesis of TiO2 particles from a simple and controllable method remains a challenge. Meanwhile, the solid-state pyrolysis of metal-organic frameworks (MOFs) has emerged as an alternative route to synthesize metal oxides with diverse morphologies.11-16 MOFs are a class of crystalline micro- and mesoporous materials composed of metal ions or metal ion clusters connected to each other by organic ligands.17,18 Advantages of MOFs over other precursors for solid state synthesis of metal oxides are its porosity and long-range ordering that offer unique opportunity to synthesize unusual metal oxide morphologies while using the porous MOFs as a host matrix for loading with additional components. Spongy CuO has been achieved by direct pyrolysis of [Cu3(btc)2] (btc = 1,3,5-benzene-tricarboxylate) framework.11 A method for synthesizing porous cubicshaped ZnO particles a few tens of micrometers in size on the

basis of a pyrolytic conversion of [Zn4O(bdc)3] (bdc = 1,4-benzene-dicarboxylate) has also been described recently.13 Powell et al. reported fabrication of porous Mn 2O3, starting from a 3D coordination network of a coordination complex built up from a mixed-valent MnII/MnIII cluster [{MnIII12MnII9 (µ4-O)8 (glycH)12(µ-1,1-N3)6(OH2)6(N3)1.5}{MnII(µ-1,3-N3)4.5 (OH2)1.5}]Cl4 · ca.7.5 H2O, where (glycH)2- is the dianion of propan-1,2,3-triol.16 Pyrolysis of this compound resulted in the formation of a sponge like morphology of α-Mn2O3. In order to obtain TiO2 material, Ti-containing MOFs previously reported in the literature can be used as sacrificial precursors. 19-22 Up to now, only two reports are known which demonstrate the pyrolysis of Ti-containing MOFs.23,24 One work reported on the pyrolysis of Ti-modified IRMOF-3 by means of post synthetic modification of a Zn-based MOF with an amine functionality. IRMOF-3 was loaded with titanium isopropoxide followed by its carbothermal pyrolysis. The carbothermal pyrolysis entailed the carbonization of a post synthetically modified MOF while performing a carbothermal reduction of a coordinated metal compound.23 Consequently, the MOF acted as a template for the formation of nanoporous carbon during the carbonization procedure and resulted in the formation of amorphous carbon supported TiO2 nanoparticles. However, due to the less efficient postsynthetic modification of the MOF precursor with titanium isopropoxide, the Ti content in the final product after pyrolysis was only 4.3 wt%, which greatly limited its photocatalytic activity. Recently, Zhao et al. reported the facile synthesis of robust TiOx/C composites using Ti-containing MOFs as the sacrificial precursors.24 Such TiOx/C composites showed high photocatalytic activities towards the degradation of methylene blue.

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During the last decades TiO2 based photocatalysts have attracted great interest due to excellent chemical stability, nontoxicity and low costs.25-30 TiO2 material is widely used as a photocatalyst for the CO2 reduction.31 CO2 emissions, caused by the utilization of fossil fuels, increased drastically in the atmosphere during the last years. Such fast-growing CO2 content leads to the climate change and consequently, become one of the greatest environmental problems of our time. Efficient capture of CO2 is one possibility to solve the global-warming problem. The photocatalytic conversion of CO2 into hydrocarbons by applying solar light and water is an ideal avenue, because the decrease of CO2 concentration in the atmosphere meets the purpose of environmental protection and at the same time generates solar fuels and/or raw materials for the chemical industry.32 Many reports have shown that CO2 can be reduced by water vapor or solvent in the presence of photocatalysts.33-38 For the vast majority of systems studied so far, irradiation of the photocatalyst with UV light was necessary. However, TiO2 based materials often exhibit very low efficiencies for CO2 reduction.39,40 One of the key reasons is the inefficient CO2 adsorption on TiO2 surfaces.20,41 Modification of the TiO2 through surface decoration with noble metal nanostructures is increasingly being considered to maximize its photocatalytic efficiency.42-44 Noble metal co-catalyst species favor electron-hole separation and induce interfacial electron transfer or allow visible light absorption by their plasmon. Recently, the potential of gold nanoparticles (GNPs) in photocatalysis has received considerable attention. The improved photocatalytic performance was reported to be caused by the visible light absorption of the gold surface plasmon and electron storage capacities of Au.45-47 One of the most studied composites is Au/TiO2 material, which can be obtained by deposition-precipitation, colloidal synthesis or photodeposition of Au.48-52 Recently, the reduction of CO2 to formate has been studied using Au and Pt loaded MOFs.53 Herein, the goal of this study is to combine the GNPmodification of TiO2 with the possible advantages of using a Tibased MOF as sacrificial precursor for the TiO2 component while also acting as support matrix for the dispersion and stabilization of small GNPs. Thus, Ti-MOFs containing GNPs can be pyrolyzed to yield a GNP/TiO2 nanocomposite material of controllable microstructures. We selected Ti-based MOF with the sum formula [Ti8O8(OH)4(C6H3C2O4NH2)6] abbreviated as NH2-MIL-125(Ti).19 It is isostructural to the parent system MIL-125(Ti) and is constructed from titanium-oxo-hydroxo clusters and amino-terephtalate linkers (bdc-NH2).54 Our choice was motivated by the nicely reproducible synthesis and the higher water stability in comparison to parent MIL-125(Ti). The pyrolytic conversion of NH2-MIL-125(Ti) with GNPs mainly anchored at the outer surface yielded the GNP/TiO2 composites, which turned out to be excellent photocatalyst for the UV-light promoted reduction of CO2 to methane.

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higher relative amount of MeOH over DMF (DMF:MeOH = 1:1 (v/v)) compared to the synthesis of the parent framework, MIL125 (DMF: MeOH = 9:1 (v/v)). The mixture was filled into a teflon-lined autoclave, heated to 423 K and kept at this temperature for 15 h. This solvothermal route yielded a green-yellow GNP/NH2-MIL-125 powder. The reference sample NH2-MIL125(Ti) was synthesized without addition of GNPs by the same approach.

Scheme 1. Schematic illustration of the encapsulation of preformed gold nanoparticles into/on NH2-MIL-125 framework. The good agreement between the simulated XRD pattern of MIL-125 (black trace) reported by the Férey group,19 NH2MIL-125 (blue trace) and GNP/NH2-MIL-125 (green trace) indicates the formation of the NH2-MIL-125 phase (Figure 1). Thus, the presence of NH2 groups in the organic linkers did not influence the structure of the parent MIL-125 framework. The expected (111) and (200) reflexes of Au were not found in the diffractogram of GNP/NH2-MIL-125 (green trace), most probably due to the low concentration or small size of the GNPs. Thus, the diffraction pattern of GNP/NH2-MIL-125 (green trace) remained nearly unchanged in comparison to the reference sample NH2-MIL-125 (blue trace), confirming that the presence of GNPs did not influence the structure of the framework. Additionally, no characteristic peaks corresponding to bulk TiO2 phases, such as anatase and rutile, were observed which indicates the absence of bulky aggregated TiO2 species. Nitrogen (N2) sorption isotherms at 77 K were measured to evaluate the pore structures and specific surface areas of the two samples, gold-free NH2-MIL-125 and GNP/NH2-MIL-125, (Figure S3 in SI). Prior to the measurement, the samples were activated in dynamic vacuum (p = 10-7 mbar) at 473 K overnight to remove the guest molecules from the pores. The BET surface area was determined to be 1599 for NH2-MIL-125 reference framework and 1212 m2/g for the loaded material by using BET-method-based calculations on N2 adsorption isotherm data. The isotherms exhibit a type I sorption behavior typical for microporous materials. The BET specific surface area of NH2-MIL-125 (blue trace, Figure S3 in SI) is slightly higher than the one previously reported (1203 m2/g)19 and lower than the theoretical one of the parent MIL-125 (2140 m2/g).54 The specific surface area of GNP/NH2-MIL-125 decreased slightly after GNPs deposition due to the higher relative weight of the sample and potential blockage of the pores by the nanoparticles.

RESULTS AND DISCUSSION Fabrication of GNPs/NH2-MIL-125 hybrid material and microstructure characterization. Scheme 1 demonstrates the fabrication of GNP/NH2-MIL125(Ti) hybrid materials. In the first step, pre-formed GNPs, amino-terephtalic acid (H2bdc-NH2) and titanium (IV) isopropoxide as the Ti-precursor were dissolved in a mixture of N,N-Dimethylformamide (DMF) and methanol (MeOH). The solvent used in the synthesis process requires a significantly


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Figure 1. PXRD patterns of Au (red; calculated), MIL-125 (black; calculated), NH2-MIL-125 (blue; experimental) and GNP/NH2-MIL-125 (green; experimental).

Figure 2. BFTEM images of a) NH2-MIL-125 reference sample and of b-d) GNP/NH2-MIL-125 hybrid composites.

Before conversion of GNP/NH2-MIL-125 into GNP/TiO2 composites, it was fundamentally important to examine its thermal decomposition behavior at elevated temperatures. The thermo gravimetric analysis (TGA) data of GNP/NH2-MIL-125 and NH2-MIL-125 (Figure S4 in SI) showed a gradual weight loss (20.5 % for NH2-MIL-125 with GNPs and 29 % for NH2MIL-125) up to 200 °C, which can be attributed to the evaporation of solvents (CH3OH and DMF) that were incorporated into the framework during the synthesis. A small weight loss (11 % for NH2-MIL-125 with GNPs and 10 % for NH2-MIL-125) occurs at ~330 °C which is ascribed to the decomposition of the organic components and the crystallization of TiO2. Accordingly, the temperature for the pyrolytic decomposition of GNP/NH2-MIL-125 and NH2-MIL-125 to GNP/TiO2 and TiO2 materials was set to 450 °C. The samples were pyrolyzed for 2 h under O2 and yielded a dark grey powder for GNP/NH2-MIL125 and a light grey powder for the reference sample. The PXRD patterns of GNP/TiO2 (green trace) and TiO2 (blue trace) are shown in Figure 3. According to the PXRD data, the crystallinity of NH2-MIL-125 framework was completely lost in both cases. TGAs of GNP/TiO2 and TiO2 (Figure S5 in SI) support this finding since no weight loss steps could be found which could be related to the degradation of the framework. There is a perfect match for the 2θ values between the calculated pattern of rutile (red trace) and GNP/TiO2 (green trace). Sharp reflexes (in red) at 27.3, 36.1, 39 41.2, 44, 54.3, 56.6° etc. belong to the (110), (101), (200), (111), (210), (211), (220) etc. lattice planes of rutile and confirm the formation of TiO2 crystallites. TiO2 fabricated from the pure NH2-MIL-125 (blue trace) exhibited (101) and (200) anatase reflections (in black) at 25.8 and 48.7° in addition to the rutile reflections. Apparently, the selective formation of the rutile phase was based on the presence of GNPs on the NH2-MIL-125 framework. Thermodynamically, rutile is the most stable polymorph of TiO2 at all temperatures, exhibiting a lower total free energy than metastable phases of anatase or brookite. The anatase to rutile phase transformation is known to start from 600 °C.57 Herein, the selective formation of rutile phase at lower temperatures of 450 °C was observed and is attributed to the thermal conductivity of GNPs.58 The GNPs act as hot spots which increase the local temperature at specific sites at the NH2-MIL-125 framework and therefore, enable the selective formation of rutile nuclei from the possibly initially formed anatase nuclei during the pyrolysis process.

The presence of GNPs in GNP/NH2-MIL-125 hybrid composites was also indicated by a Bright Field TEM (BFTEM) measurement (Figure 2).55 The BFTEM image in (a) shows NH2-MIL-125 nanocrystals without GNPs in a size range 300450 nm. The nanoparticles in GNP/NH2-MIL-125 material (bd) exhibit a broader size distribution (e.g. particles with diameters of 3.8, 7.3 or 20 nm) than the pre-formed, highly monodisperse GNPs before synthesis (2.1 ± 0.8 nm). Their size partially increased during the solvothermal treatment during the MOF synthesis at comparably high temperature.56 The increased size indicated a preferential deposition of the GNPs on the outer surface of the growing MOF crystals rather than their incorporation and stabilization in the bulk of the host matrix. Furthermore, some aggregations of GNPs can be observed on the outer surface of the MOF crystals. GNP/TiO2 nanocomposite synthesis and microstructure characterization.

Figure 3. PXRD data of GNP/TiO2 (green) and TiO2 (blue) compared to the calculated pattern of anatase (black) and rutile (red).



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Figure 4. DRIFT spectra of GNP/TiO2 (purple) and TiO2 (petrol) reference sample measured at room temperature. To estimate the specific surface area of GNP/TiO2 hybrid nano composites, N2 sorption isotherms at 77 K were measured (Figure S8 in SI). The isotherm exhibited the type III sorption behavior typical for the non-porous materials. A comparably low BET specific surface area of 19.8 m2/g confirmed the nonporosity of GNP/TiO2 composites.

Figure 5. BFTEM images of GNP/TiO2 nano composites. The FT Raman spectra of GNP/TiO2 and TiO2 (Figure S6 in SI) were complementary to the PXRD data in Figure 3. The spectrum of GNP/TiO2 exhibited the bands at 150, 250, 428 and 605 cm-1 which were in good agreement with the spectra of TiO2 rutile single crystals from literature.59-61 Since the TiO2 reference sample obtained by the pyrolysis of NH2-MIL-125 without GNP loading turned out to be a mixture of rutile and anatase (by XRD), a scanline measurement (details in SI) was performed over 200 µm of the sample in order to confirm the presence of both TiO2 phases. The bands at 146, 249, 437 and 610 cm -1 could be attributed to the rutile crystallites, while the bands at 155, 200, 397, 513 and 626 cm-1 were a clear indication for the

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simultaneous formation of anatase phase.60 The IR spectra of GNP/TiO2 and TiO2 in the region of 4000-1500 cm-1 measured at room temperature are shown in Figure 4. The spectrum of GNP/TiO2 (purple trace) exhibited two absorption bands for OH stretching modes representing rutile at 3652 and 3410 cm-1 respectively.62-67 Two bands at 2829 and 2930 cm-1 were assigned to the symmetric and asymmetric ν(CH2) vibrations of the carbon surface contaminations. The spectrum of TiO 2 (petrol trace) displayed, additionally to the rutile band at 3410 cm 1 , contributions of anatase in the material. The absorptions at 3670 cm-1 and 3650 cm-1 were assigned to vibrations of hydroxyl groups on the surface of anatase.63-65 In the range of 2400-2000 cm-1 both spectra consisted of the main contributions which were attributed to the CO2 and CO adsorptions during the pyrolysis process. The band at 2040 cm-1 found in the spectrum of GNP/TiO2 composites could be assigned to CO adsorbed on very small gold clusters as reported for the electrooxidation of CO at negative potentials on gold electrodes.66,68 Typically, physisorbed CO on titania exhibits bands around 21752210 cm-1.67 CO adsorbed on the Ti4+ cations in both samples can be attributed to the band at 2208 cm-1.64,66 Additionally, in case of TiO2, the band at 2045 cm-1 arose from chemisorbed CO species on Ti3+ sites.67 CO2 molecules linearly adsorbed from a gas-phase on the surface cations were visible as a very strong band at 2342 cm-1.69 The permanence of CO2 and CO adsorptions was evidenced by the temperature stability DRIFT measurements (Figure S7 in SI) by applying a linear heating ramp of 5 °C/min to a maximum temperature of 600 °C in continuous Ar flow. Even after heating to 600 °C, the vibration bands of the adsorbed species remained unchanged.

Figure 6. HAADF-STEM images of a GNP/TiO2 composite. The bright contrast features are GNPs or their agglomerations. The red markers indicate the beam positions for the EDX measurements. a) left: HAADF-STEM image of GNP/TiO2 and right: corresponding EDX spectrum. b) left: HAADF-STEM image of GNP/TiO2 and right: corresponding EDX spectrum.

Interestingly, the fabricated TiO2 nanocrystals resembled the morphology and size of the initial MOF crystals. The TiO 2 phase can be described as nanocrystalline, but yet with a welldefined shape with a size of about 300-450 nm. BFTEM images showed GNP/TiO2 composites with GNPs on the surface (Figure 5). Due to the repeated high temperature treatment, the size of the particles slightly increased again. Scanning transmission electron microscopy (STEM) was combined with energy dis-


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persive x-ray spectroscopy (EDX) to analyze the local composition of a GNP/TiO2 composite (Figure 6). The according high angle annular dark field (HAADF)-STEM images showed the position of the EDX measurements. In (a), the beam was positioned directly on one of the gold nanoparticles visible as bright contrasts features. In (b) the beam was placed on TiO2 free of deposited GNPs. Both spectra displayed signals corresponding to the titanium in the GNP/TiO2. The EDX spectrum in (a) also showed signals corresponding to gold, which proves the presence of GNPs. Photocatalytic CO2 Reduction to CH4 at GNPs/TiO2 As mentioned above, the doping with noble metals like gold is beneficial for TiO2 catalysts. The improved photocatalytic performance can be based on the visible light absorption of the gold surface plasmon and/or the electron storage capacities of Au.45-47 The catalytic activities the samples GNP/TiO2 and TiO2 (50 mg each) fabricated by pyrolysis of NH2-MIL-101(Ti) were tested at the solid gas interface (photocatalytic CO2 reduction) performed in a homemade photoreactor and compared to the activity of 50 mg of P-25 (commercial TiO2 composed of mixed anatase/rutile crystallites). The catalyst amount of 50 mg was chosen, because it homogeneously covered the sample cup inside the reactor as a thin layer in order to allow full exposure of the catalyst to the illumination. Prior to all activity tests, the samples were cleaned inside the reactor to remove contaminants from the catalyst surface by the well-established cleaning method developed by Mul et al. (experimental details regarding the photoreactor set-up and sample cleaning can be found in SI).70 The methane formation under photo-illumination or photo-irradiation with a 200 W Hg/Xe (200-750 nm) lamp was monitored during the period of 6 hours. The reactor was operated in the batch mode with 1.5 % of CO2 and 0.6 % of H2O in He at room temperature and a pressure of 1.1 to 1.5 bar. A higher CO2 concentration was chosen in order to balance the unfavorable adsorption capacity of CO2 on TiO2 in comparison to H2O adsorption. The introduction of H2O into the reactor was realized with a metal-sealed double saturator system installed between gas-supply and reactor to load the reactant gas stream with H2O. With the help of cooling liquid around the saturator cups, the loading of H2O in the gas-phase was controlled to be 0.6 %. By running the reaction in the batch mode, the transport of molecules between the solid-phase catalyst and gas-phase reactant depends on diffusion. The inside volume of the reactor was estimated to be around 26 mL. In general, two subsequent CO2 reduction experiments were performed to confirm the stability of the catalytic performance. The samples were not subjected to a cleaning procedure between the subsequently performed experiments, but in the case of commercial samples only one cycle has been performed as they tend to show relatively constant reactivity. The methane formation during two photocatalytic CO2 reduction experiments within 6 hours of illumination for GNP/TiO2 and TiO2 is shown in Figure 7, which confirms that both samples are active photocatalysts for the CH4 formation. The data was obtained by the integration of online GC (GC = Gas Chromatography) detected by a flame ionization detector (FID). GC samplings were performed through a pressure balancing mechanism between the evacuated sample loop and the reactor under pressure between 1.1 to 1.5 bar. Consequently, the GC sampling did not involve the opening of the system to the outside atmosphere. Obviously, in both reaction

runs the yield of CH4 was significantly higher for the GNP/TiO2 sample which is in good agreement with observations from the literature.45-47,61 In the first reaction run, there is a steep increase of almost 50 % for the CH4 formation for both samples, if looking at the development between the 2nd and the 4th hour from nil at 0 h. However, only a small methane yield increase of only 14 % was observed for both samples between the 4th and 6th hour. By contrast, the CH4 yield increased linearly during the whole measurement time for both samples in the second reaction run. Concluding from these observations, the formation of methane in the second run was more efficient, confirming possible reusability and stability of the catalytic performance for both samples. Note, such second catalytic runs as comparisons were not performed for the commercial reference samples of TiO2 (P25) and Aurolite (Au/TiO2), as this data is well known in the literature and the respective characterization of catalyst stability was not of interest in this work.

Figure 7. CO2 reduction experiments and methane evolution observed within 6 hours of UV-lamp illumination/photo-irradiation measured after 2, 4 and 6 h for (i) GNP/TiO2 (purple) and (ii) as-synthesized TiO2 (anatase/rutile mixed phase) obtained by pyrolysis of NH2-MIL-125(Ti) (petrol) during two subsequent (run 1 and run 2) and the commercial reference samples (iii) P-25 (blue, pure TiO2) and (iv) Aurolite (magenta, Au/TiO2) (run 1). The samples were not cleaned from possible CxHy species between two subsequent measurements. The reactor was just purged with inert gas overnight. Note, at 0 h and without UV illumination/photo-irradiation there was no CH4 detected. Interestingly, the yield of CH4 for TiO2 was increased by a factor 3 after two hours of the second run in comparison with the product yield observed after the 2 nd hour of the first run. In case of GNP/TiO2 material, the product amount after the 2nd hour remained the same in both reaction runs. Strunk et al. observed a specific formation of a carbon pool for the Ti/SBA-15 catalyst in comparison to Au/Ti/SBA-15 material.46 According to their report, the carbon pool could be built up during the first reaction run on the TiO2 catalyst, which cannot be removed by the evacuation. Thus, un-hydrated or un-hydrogenated CxHyOz intermediate species are adsorbed to the catalyst surface and participated in the product formation at the beginning of the second reaction run. Those species were identified to be formalde-



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hydes or paraformaldehydes by IR spectroscopy. 46 Consequently, no time for the accumulation of intermediates was required in the second run for TiO2 and the product formation started earlier yielding a higher amount of methane. On the other hand, such carbon accumulation, either due to their removal by evacuation, a faster hydration/hydrogenation or less of reactivity in the subsequent reaction runs, seemed not to happen when GNP/TiO2 was used as the catalyst. The comparative data of Figure 7 provide evidence that the presence of GNPs in TiO2 (rutile phase) were responsible for the higher activity in the photoreduction of CO2 compared to the as-synthesized TiO2 (anatase/rutile mixed phase) obtained after the pyrolysis of NH2-MIL-125(Ti) (Table S1 describes the quantum yield values of these photoconversion reaction of CO2 to CH4). Besides methane formation, a very small amount of ethane and propane were observed for both samples (Figure S9 in SI). The yields for these additional components rarely exceeded 1.8 ppm. Therefore, C2H6 and C3H8 were the side products of the photocatalytic reduction reaction, which could have been formed due to CO coupling and protonation or the oligomerization of CH x. Their formation was likely independent from photocatalytic CO2 reduction. If they arise from CHx, which is clearly an advanced intermediate of CH4, the trend in yield amounts should resemble the one from Figure 7. Most likely, ethane and propane formation occurs like coupling of CO on TiO2 sites since CO is believed to be the first intermediate in the photocatalytic CO2 reduction.70 The fact that the presence of GNPs did not significantly affect the ethane and propane formation confirms this assumption. In general, the maximum amount of 62 ppm of CH4 was produced when GNP/TiO2 sample was used as a photocatalyst and only 14 ppm of the product was formed with the MOF derived TiO2 catalyst without deposited GNPs. Due to the phenomenon of carbon pool formation, only run 1 is considered to be relevant for the product formation evaluation in case of commercial TiO2. The ppm values for methane formation in case of 50 mg GNP/TiO2 can be compared to methane amount produced in the presence of the same amount of P-25, which is commonly used as the benchmark photocatalyst. The amount of methane formed during the photocatalytic CO2 reduction within 6 hours of illumination for P-25 was only 15.8 ppm (Figure S10 in SI). This CH4 amount was in a good accordance to the CH4 amount produced in case of the reference TiO2 material (after pyrolysis of NH2-MIL-125(Ti)) without GNPs. Therefore the GNPs significantly increased the photocatalytic activity of TiO2. Moreover, we also compared the photo-reactivity of our compound with Aurolite which also showed that the GNP/TiO2 after pyrolysis of MOF showed better photocatalysis (Figure 7). However, no oxidative photo-products such as O2 were observed during this photocatalytic reaction. As indicated by Stolarcyk, the OH· radicals formation at the surface as the hole consumption reaction could have avoided the formation of oxidative products or the storage of O2 in the pores of TiO2 which avoids the evolution and detection of O2 from GC.31 After photocatalytic CO2 reduction reactions, GNP/TiO2 and TiO2 samples were characterized by DRIFTS and PXRD and compared to the pristine reference materials. No significant changes were observed by IR (Figure S11 in SI) and diffraction experiments (Figure S12 in SI) confirming the reusability and stability of both photocatalysts. Our novel GNP/TiO2 nano composite exhibits several benefits in comparison to previously reported Au/TiO2 materials used for the heterogeneous CO2/H2O reduction to CH4. Clearly, it was more active in photocatalytic CO2 reduction than P-25,

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Aurolite (commercial Au/TiO2) and it did not show any signs of deactivation in the second reaction run. Considering the material being phase-pure, non-porous rutile, the surface area of almost 20 m2/g is rather high. This is likely caused by the uniform, small crystallite size and the presence of well-separated primary particles without much tendency to agglomerate to secondary particles. As a consequence, gold is mostly deposited on the outer surface of the material, where it is best accessible to light and reactants. In comparison of reference Au/TiO2 materials, where the gold was introduced, for example, by deposition-precipitation on a sol-gel prepared TiO2 or P-25, a considerable amount of the gold might be deposited in pores or contact areas between primary particles, where it might be shielded from light and/or reactants. Furthermore, the well-developed rutile structure of our material is beneficial for photocatalysis: fewer recombination centers for photogenerated electrons and holes, such as lattice defects, are expected to be present, which typically reduce the efficiency of the process.48,71 It needs to be pointed out that the well-developed rutile crystal structure was obtained at relatively mild conditions, so that a rather high surface area of the material was preserved. It is possible that the TiO2 material derived by MOF pyrolysis may contain C and N dopants, which will change the photophysical characteristics, e.g. light absorption shifted to the visible range.72-76 However, we did not study this effects herein and focused on UV illumination/photo-irradiation exclusively. Note, comparison of TiO2 with P-25 (Figure 7) indicate that such kind of doping is less important than the modification with GNPs.

CONCLUSION In summary, a new strategy for the fabrication of photocatalytically active GNP/TiO2 hybrid nano composites was developed. Ti-containing MOF, previously reported by Kim et al.54 was used as a sacrificial precursor to obtain TiO 2. The photocatalytic activity of NH2-MIL-125 framework itself has already been demonstrated for CO2 reduction20 or water splitting.77 However, the utilization of Ti-containing MOFs as photocatalysts carries the risk of framework decomposition, especially in aqueous environments where most photocatalytic reactions are carried out. Consequently, NH 2-MIL-125 was pyrolyzed into TiO2. To enhance the photocatalytic performance, TiO2 was doped with GNPs. The deposition of GNPs occurred on the outer surface of the initial MOF yielding GNP/NH2-MIL-125 nano composites with a plenty of well-dispersed GNPs attached to the crystal surfaces. The pyrolytic decomposition of such composites resulted in the selective rutile-phase formation of GNP/TiO2 with the retained size and morphology of the original MOF crystals. This approach enriched the conventional techniques of GNP/TiO2 preparation and enabled the controlled nano-scale synthesis of TiO2 in terms of phase, shape and morphology. The catalytic activities of 50 mg of GNP/TiO2, TiO2, P-25 and Aurolite reference materials were tested by means of photocatalytic CO2 reduction performed in a homemade gasphase photoreactor. The yield of CH4 was significantly higher for GNP/TiO2 due to the presence of GNPs in the material. Further studies may be directed towards a more uniform distribution of the GNPs and further downscaling and tailoring the primary particle size of the TiO2. The elucidation of the effects of light heteroelement doping (C, N) together with the GNPs loading and CO2 reduction under visible light conditions will be another direction of improving the GNP/TiO2 materials presented


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herein which were derived by Ti-MOF based sacrificial precursor chemistry.

EXPERIMENTAL SECTION Materials: All reagents and solvents were commercially available and used without further purification. Gold (III) chloride trihydrate (HAuCl4∙3H2O), Polyvinylpyrrolidone (PVP, Mw~55000) and 2-Aminoterephthalic acid (H2BDC-NH2) were purchased from Aldrich Chemicals (Germany). Sodium borohydride (NaBH4) was delivered by VWR International (Germany). Titanium (IV) isopropoxide (98 %) was delivered by Acros Organics (U. S. A.). N,N-Dimethylformamide and Diethylether were obtained by Fisher Chemicals (Germany). Methanol and Ethanol were purchased from Sigma Aldrich (Germany). AUROlite was purchased from Stream chemicals. In all experiments deionized water was used. Synthesis of PVP-modified Gold Nanoparticles: PVPstabilized gold nanoparticles were prepared using the synthetic procedure reported by our group previously. 55 Following this approach, 1-3 nm GNPs were prepared by the reduction of gold precursor with sodium borohydride. HAuCl 4 (39.1 mg, 0.1 mmol) was mixed with polymer PVP (111 mg, 1 mmol) in 98 ml of H2O. After both components were dissolved, the mixture was cooled in ice bath to 0 °C. Then, NaBH 4 (38 mg, 1 mmol) dissolved in 2 ml of H2O, was added under vigorous stirring to the cold solution of HAuCl4 and PVP and the mixture was stirred for 30 minutes. The resulting deep red color of the solution indicated the successful reduction of gold precursor to GNPs.78,79 The solution was reheated to room temperature and the PVP-stabilized GNPs were collected by centrifugation. After the washing with water and ethanol, GNPs were dried in vacuum (p = 10-3 mbar) for 12 h to yield a dark red solid. The PXRD patterns of the GNPs (Figure S1 in SI) showed good match for the 38.1 and 44.5 2 values compared with the calculated pattern for fcc Au. These broad (111) and (200) reflections unambiguously reveal the formation of very small GNPs. The average size of GNPs (2.1 ± 0.8 nm) was confirmed by the BFTEM measurements (Figure S2 in SI). Synthesis of NH2-MIL-125: Ti-based MOF, Ti8O8(OH)4(bdc)6 (MIL-125), was originally reported (bdc = benzene-1,4-dicarboxylate) by Férey et al.19 Ti-based MOF with amino function (Ti8O8(OH)4(bdc-NH2)6 (NH2-MIL-125) was synthesized by procedure reported by Kim and co-workers.54 A solution of titanium (IV) isopropoxide (0,177 ml, 0,6 mmol) and 2-aminoterephthalic acid (217 mg, 1,2 mmol) in a 10 ml mixture of DMF and CH3OH (1:1, v/v) was prepared. The substrate mixture was transferred to a 20 ml teflon-lined autoclave and heated at 423 K for 15 h in a convection oven. The mixture was cooled to room temperature after the solvothermal reaction, and a yellow solid powder was recovered by filtration, washed twice with DMF to remove the unreacted organic ligand, and then washed again with CH3OH to exchange DMF. Finally, NH2-MIL-125 was dried at 100 °C in the oven. Synthesis of GNP/NH2-MIL-125: PVP-stabilized GNPs (10 mg) were dissolved in a 10 ml mixture of DMF and CH 3OH (1:1, v/v) in a sonicator. Afterwards, 217 mg of H 2BDC-NH2 were added to the mixture and stirred for 10 minutes. Titanium (IV) isopropoxide (0,177 ml) was added dropwise to the mixture of the GNPs and the linker under vigorous stirring. The mixture was put into a 20 ml teflon-lined autoclave and heated at 423 K for 15 h in an oven. The green-yellow precipitate was collected by centrifugation, washed twice with DMF and then with CH3OH and dried at 80 °C overnight.

Note: For controlling the size and distribution of GNPs, a number of attempts have been performed by decreasing the reaction temperature to 373 K and increasing the time to 60 h. This resulted in better distribution along with size control of GNPs. However, the MOF particle size has been increased in parallel to several micrometers which is likely to provide larger TiO2 particles with undesired lower surface area which could reduce the catalytic activity. These samples were not further investigated, rather the product from synthesis at 423 K has been used for proof-of-concept and further experiments to yield GNP/TiO2 and the respective catalytic studies. Pyrolytic Conversion of GNP/NH2-MIL-125 to GNP/TiO2: The powder of GNP/NH2-MIL-125 (500 mg) was placed in a glass combustion boat, inserted within a quartz tube, connected to a gas bottle. The moisture-free O2 was passed through the sample in order to conduct the thermal decomposition of the framework under oxygen. The tube was heated up to 450 °C, kept for 2 h, and then cooled to room temperature. The product appeared as a gray powder. The reference sample TiO2 was prepared by the same synthetic approach from NH2-MIL-125 without GNPs and resulted in light gray powder. Activation Procedure of GNP/NH2-MIL-125 and GNP/TiO2 prior to N2 Measurements: Gas adsorption studies were done to confirm the permanent porosity of NH2-MIL-125 as well as to prove the fact that GNPs were adsorbed onto the MOF surface. The samples were activated in dynamic vacuum (p = 10-7 mbar) at 473 K for 12 h. GNP/TiO2 powder was kept under vacuum for 2 h at 423 K to remove water and air molecules. Nitrogen sorption measurements were performed at 77 K. Photocatalytic CO2 Reduction. CO2 reduction experiments were performed under static conditions. A gas cylinder with a prepared mixture of 1.5 % CO2 in He was used to allow a gas flow of 26 Nml/min. The gas was flown through two metalsealed, temperature-controlled saturators to enrich the gas phase with H2O at 5 °C resulting in a H2O concentration of 0.6 %. This gas mixture was trapped inside the photoreactor after the initial pressure was adjusted to 1500 mbar. Consecutively, the reactor content was sampled with a GC measurement and the lamp was started. To check the progress of the reaction, the GC measurement was performed every 2 h. The reaction was completed after 6 h. The analogous consecutive reaction run of 6 h was performed in order to test the stability of the catalytic performance or to identify the presence of the stable intermediates that were already described as a carbon pool by Strunk and co-workers.46 Between two measurement runs, the sample was not cleaned from possible CxHy species. After the first reaction run, the reactor was pumped down to 5∙10-2 mbar and flushed with high purity He. In all photocatalytic experiments 50 mg of catalyst (GNP/TiO2, TiO2, P-25 and Aurolite) was used.

ASSOCIATED CONTENT Characterization of GNPs, TGA data, sorption data, Raman spectra and photocatalytic procedures can be found in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *Prof. Dr. Roland. A. Fischer ([email protected]) *Dr. Jennifer Strunk ([email protected])

Author Contributions



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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. KK and RM performed the synthesis and general sample characterization. AP investigated the catalytic properties. CR and CW did the TEM based characterizations. JS and RAF designed and guided the project.

ACKNOWLEDGMENT This work was supported by the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft (DFG). Part of this work was funded by the German Ministry of Education and Research (BMBF) within the program “Technologies for sustainability and climate protection: Chemical Processes and Use of CO2”, Project-No. 033RC1007A “PhotoKat” and EU 7th FP FMP 4GPhotocat (grant 309636). RM acknowledges funding by Alexander von Humboldt foundation.

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Titania Photocatalyst for CO2 Reduction Based on

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