Catalysis Communications 59 (2015) 55–60 Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom Short Communication Pt nanoparticles loaded titanium picolinate framework for photocatalytic hydrogen generation Quang Duc Truong a,b,⁎, Huu Thu Hoa b, Thanh Son Le b a b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Faculty of Chemistry, VNU University of Science, Hanoi 10000, Vietnam a r t i c l e i n f o Article history: Received 17 June 2014 Received in revised form 22 September 2014 Accepted 25 September 2014 Available online October 2014 Keywords: Inorganic-organic hybrids Titanium picolinate Hydrogen evolution Solar energy Photocatalytic activity a b s t r a c t Pt nanoparticles have been embedded into layers of a titanium picolinate framework by a photodeposition method TEM images show the embedded and adsorbed Pt nanoparticles in the space between the layers with size of c.a nm The obtained Pt loaded titanium picolinate framework functions as an efficient photocatalyst for hydrogen evolution from methanol/water solution upon irradiation at wavelength longer than 300 nm (159.3 μmol h−1/0.1 g TiPF catalyst) © 2014 Elsevier B.V All rights reserved Introduction The development of alternative and renewable energy source has gained increasing attention at global level Solar energy conversion into stored chemical offers intriguing opportunities to harvest green abundant energy for projected demands Because hydrogen is one of most promising alternative carbon-free fuels, the transformation of solar energy into hydrogen storage represents enormous promise of achieving solar fuels [1,2] For instance, considerable research efforts have been made to explore novel phototcatalysts for water splitting into hydrogen A variety of photocatalytic systems have been developed for hydrogen evolution such as inorganic semiconductors (titanates [3], tantalates [4], niobates [5], oxynitrides [6], titanium oxides [7–9]), metal-based complexes [10,11] and polymer-like semiconductor [12] Recently, crystalline metal organic hybrids have proved to be promising photocatalysts for degradation of organic pollutants and solar energy conversion [13,14] The unique property of crystalline metal organic hybrids is that the material properties could be intergraded from functions of both constituting organic linkers and inorganic metal-oxo clusters The organic linkers can be easily modified in order to precisely tune the layered spacing, pore size and surface properties as well as optical property of the target materials and the metal-oxo clusters response for redox activity and photocatalytic properties The development of photoactive metal organic hybrids offers unique opportunity to study insight into photocatalytic process at a molecular level ⁎ Corresponding author E-mail address: tqduc@mail.tagen.tohoku.ac.jp (Q.D Truong) http://dx.doi.org/10.1016/j.catcom.2014.09.045 1566-7367/© 2014 Elsevier B.V All rights reserved It has been demonstrated that electron transfer takes place from the photoexcited organic linker to the metal-oxo cluster within a metal organic hybrids, namely MOF-5 [14] Inspired by this finding, several works have developed metal organic hybrid compounds for photocatalytic hydrogen production [15,16,18] Garcia et al used zirconium telephthalate and zirconium 2-amino-telephthalate for photocatalytic hydrogen generation in methanol or water/methanol upon irradiation at wavelength longer than 300 nm [15] However, the photocatalytic activity of the zirconium telephthalate (UiO-66) under visible light irradiation was not observed presumably due to that fact that the edge of conduction band (CB) energy of Zr-oxo clusters is lower that that of organic counterparts In order to increase the ability of UiO-66 to generate hydrogen, an analogous MOF material using aminoterephthalate instead of terephthalate as linker was synthesized The amino group the aromatic ring provides the lone pair of nitrogen for interaction with the π-orbitals of the benzene ring, donating electron density to the antibonding orbitals, resulting in a higher HOMO level and absorption to the visible region [19] As a result, UiO-66(NH2) shows enhanced activity of times for hydrogen generation in water/methanol mixtures [15] Alternatively, Matsuoka et al used a Ti-based framework for the hydrogen production reaction under visible light irradiation which provides efficient charge transfer from the excited state of the organic linker to metal-oxo clusters due to the higher energy edge of CB of titaniumoxo clusters [16] At the same time, Li et al also reported the photocatalytic reduction of CO2 under visible light irradiation by tuning of the absorption of a Ti-based MOF with a visible-light-active organic linker, 2-amino-telephthalate [17] Very recently, Lin et al have attained the visible-light-promoted photocatalytic hydrogen production by using 56 Q.D Truong et al / Catalysis Communications 59 (2015) 55–60 Ir-Zr-based MOFs in which the long-live state of excited Ir-organic moiety photosensitizer in a rigid MOF framework is beneficial for improved hydrogen evolution rate [18] Clearly, metal–organic hybrids offer a powerful platform to integrate functions for photocatalytic reaction as well as turning the reaction rate for solar energy harvesting In this communication, we report the incorporation of Pt nanoparticles into titanium picolinate framework (TiPF) which functions as a photocatalyst for hydrogen evolution from methanol/water solution The titanium picolinate framework was formed hydrothermally by refluxing titanium picolinato complex at 100 °C Pt nanoparticles as cocatalysts were embedded into TiPF via an in situ photodeposition process (Pt/TiPF) Pt/TiPF exhibited efficient photocatalytic activities for hydrogen production from an aqueous methanol solution upon irradiation at wavelength longer than 300 nm (159.3 μmol h− 1/0.1 g TiPF catalyst) Experimental section 2.1 Synthesis method The Ti-based frameworks were synthesized by refluxing the solution of titanium picolinato complex at 100 °C Briefly, the yellow transparent peroxo-titanic acid solution was obtained by adding excess amount of ammonia solution (10 cm3, 28%, J T Baker, Germany) and hydrogen peroxide solution (50 cm3, 30%, J T Baker, Germany) to purified titanium metal (10 mmol, Alfa Aesar, America) The identified amount of picolinic acid (30 mmol, Merk, Germany) was added to the yellowish solution of peroxo titanic acid, producing stable red complex solution The mole of picolinic acid should be at least times (R = 3) that of titanium to ensure the complex formation with a formation of stable transparent solution An aging step at 60 °C is necessary to remove any excess reagent A solution of complex with excess amount of ligand (20 mmol, R = 5) was adjusted to 200 cm3 of volume (0.05 M) The 300 cm3 beaker was then covered and heated in a water bath at 100 °C, and was held for 24 h with continuous stirring at 500 rpm to obtain precipitates The resultant precipitates were separated by centrifugation and washed with distilled water by sonication for several times; the obtained specimens were dried at 60 °C to dryness The obtained particles were then allowed to adsorb water from moisture air for one day before the characterization was carried out Pt nanoparticles as cocatalysts were deposited on TiPF by a photodeposition method in a closed system Typically, a suspension containing TiPF (100 mg), μmol H2PtCl6, in methanol solution (50 mL/50 mL) was irradiated with light from a 500 W Xe lamp for h with stirring Generally, it is accepted that the limited amount of H2PtCl6 will deposited completely on the catalyst upon the light irradiation The obtained precipitate was separated by centrifugation, washed repeatedly with fresh methanol, and dried in air at room-temperature overnight Finally, the obtained powder was dried at 60 °C for 12 h, yielding Pt/TiPF spectra of solid samples were evaluated in the range of 200–800 nm with a spectrophotometer instrument Shimadzu, UV 2450 2.3 Photocatalytic production of hydrogen The photocatalytic hydrogen evolution was carried out in a closed gas circulation system using Pyrex reactor Each sample contained 50 mL of aqueous methanol solution of 8/2 water/methanol (v/v) Typically, 0.1 g of catalyst was suspended in water by sonication for a minute Then, methanol was used to fill to 50 mL and the reactor was sealed with rubber septum Prior to photocatalytic H2 evolution, the solution was degassed by a circulation and evacuation for 30 to remove dissolved oxygen 500 W Xenon arc lamp was employed as light source delivering from the top of the cell through a Pyrex window (light intensity, 2.5 mW cm−2) The wavelength of light was N300 nm and the reaction temperature was kept at 19 °C using a cooling water bath In some cases, a filter (HOYA; L42) was used to cut off entire UV light, passing light with the wavelength N 420 nm The amount of hydrogen evolved was measured using an online gas chromatograph (GC-14C, MS-5A column, Shimadzu, Japan) equipped with a thermal conductivity detector (TCD) Results and discussion 3.1 Characterization of TiPF and Pt/TiPF Refluxing the titanium picolinato complex at 100 °C for 24 h resulted in the formation of white crystalline solid The obtained samples were characterized by powder XRD in the 2θ range of 5–70° Fig 1a shows the XRD pattern of the obtained powder by refluxing the complex at 100 °C for 24 h TiPF exhibits the same diffraction pattern as lamellar titanium picolinate synthesized by hydrothermal method in our previous report [20,21] This result indicates that the titanium picolinate framework has been produced Particularly, the low-angle reflection at 2θ = 6.05° reveals the presence of ordered layered structures This peak position corresponds to a layer spacing of 14.60 Å The XRD pattern also exhibits the reflection of multiorder at 11.849°, 17.79° (see Supplementary Information, Fig S1a) We noted that the XRD peak intensity of Pt/TiPF is much weaker than that of TiPF This may be due to the fact that the embedded Pt nanoparticles disturb the long-range ordered structure of the lamellar TiPF A typical N2 sorption–desorption isotherm taken at 77 K of the obtained lamellar TiPF by refluxing method is shown in Fig S1b It has a Type I isotherm, indicating the microporous structures The surface area is 265 m2 g−1 2.2 Characterization of the synthesized particles The crystalline phase of the samples was characterized using powder X-ray diffraction (XRD; Rigaku RINV-2200, 40 kV and 30 mA) with CuKα radiation (λ = 1.5406 Å) Data were collected in the 2θ–θ scanning mode with a scan speed of 4° min−1 and a step size of 0.02° The morphology of particles was observed using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4700) at an accelerating voltage of kV Transmission electron microscopy (TEM JEOL 2010, 100 kV) was conducted using specimens dispersed in ethanol and then dropped onto Cu microgrid coated with a holey carbon film, followed by the evaporation of ethanol N2 adsorption and desorption isotherms were measured at 77 K (Micromeritics ASAP 2010) to evaluate the Brunauer–Emmett–Teller (BET) specific surface area DRS b a Fig XRD patterns of (a) TiPF synthesized by refluxing method and (b) Pt/TiPF Q.D Truong et al / Catalysis Communications 59 (2015) 55–60 with 175 m2 g−1 as internal micropores and a micropore volume of 0.078 cm3 g−1 as determined by the t-plot micropore method The obtained lamellar TiPF in this work shows lower specific surface area compared to that synthesized by hydrothermal method in our previous report [20,21] Upon Pt loading, the color of the TiPF powders changed from white to brown due to the plasmonic absorption of Pt nanoparticles The Pt/TiPF obtained after photodeposition of Pt shows the diffraction pattern matching well with that of Ti-MOF (Fig 1b), revealing that the framework was remained after the photodeposition process Notably, the low-angle reflection was shifted from 2θ = 6.05° to 2θ = 5.75°, corresponding with an increased layer spacing of 0.76 Å This result indicates that Pt nanoparticles have been encapsulated between layers together with the deposition on the surface of particles Moreover, the Pt/TiPF shows a BET specific surface area of 237 m2 g− which is lower than that of pristine sample (265 m2 g−1) Fig shows the TEM images of TiPF and Pt/TiPF obtained after the photodeposition reaction It can be seen that the synthesized TiPF particles comprise nanoplates in 2D size of 500 nm × 500 nm TEM image of the particles obtained after the photodeposition reaction shows the homogeneous distribution of tiny dark contrast, indicating the presence of Pt nanoparticles The TEM images reveal the successful production of Pt nanoparticles High magnification TEM images of synthesized particles are shown in Fig 2c, d It can be seen from Fig 2c that the TiPF particles consist of equally spaced parallel lamellae with very different electron contrast The dark layers indicate the presence of the strongest scatterers, whereas the organic material stays practically invisible between those layers From these TEM image, the layer spacing is estimated of about 1.46 nm which is consistent with the result in our previous report [20,21] More importantly, high magnification TEM image of Pt/TiPF in 500 nm 57 Fig 2d clearly shows the embedded and adsorbed Pt nanoparticles in the space between the layers as strongest dark contrasts which are not observed in TEM image of TiPF (see HRTEM images in Fig S2 for clarify) The observed Pt particles in the space between the layers are consistent with the result from XRD analysis The Pt nanoparticles are in size of c.a nm in accordance with the layer spacing of TiPF (14.60 Å) It is worth noting that the Pt nanoparticles synthesized by photodeposition in this study are much smaller than other nanoparticles incorporated in the metal organic frameworks [22–27] It was also found that the uniform Pt particles were also homogeneously distributed entire layers Fig displays the UV–vis DR spectrum of TiPF It can be seen that TiPF that has a band at wavelength at around 370 nm original from titanium-oxo clusters and tailing beyond 400 nm The observed spectrum is consistent with the white color appearance of the crystalline solid We assume that the absorption band tailing beyond 400 nm which is not observed in pristine titania structure, i.e anatase TiO2 ST01 (Fig S3), arises from the picolinate counterparts Although picolinic acid absorbs light with wavelength shorter than 320 nm (Fig S4), optical transitions in TiPF arise from ligand-to-metal charge transfer (LMCT) Particularly, organic linkers and metal-oxo clusters are parts of one entity and the picolinate linker may promote ligandto-metal charge transfer in the TiPF, resulting in the red shift of the absorption band 3.2 Photocatalytic production of hydrogen In an attempt to explore the potential photocatalytic activity of TiPF, the catalyst deposited with Pt was used for hydrogen evolution reaction from an aqueous solution of methanol as a sacrificial electron donor b a 500 nm c d 1.46 nm Pt 20 nm 20 nm Fig TEM images of (a) TiPF and (b) Pt/TiPF after the photodeposition reaction High magnification TEM images of (c) TiPF and (d) Pt/TiPF obtained after the photodeposition reaction Q.D Truong et al / Catalysis Communications 59 (2015) 55–60 Kubelka-Munk (a.u) 58 200 a 300 400 b 500 600 Wavelength (nm) Fig Ultraviolet–visible diffuse reflectance spectrum of TiPF The inset shows photograph image of suspensions (a) TiPF and (b) Pt/TiPF photocatalyst under irradiation of Pyrex filtered UV light (λ N 300 nm) and visiblelight longer than 420 nm The blank test was carried out in the dark Hydrogen was not detected for the test; under light irradiation without TiPF catalyst, however, photoproduction of H2 did not take place These blank tests indicate that light irradiation and catalyst are crucial for the hydrogen evolution reaction Furthermore, the reaction carried out in the absence of methanol did not generate detectable H2 amounts Thus, sacrificial electron donor is required to provide electrons for the reduction semi-reaction of water [15] A typical time course of photocatalytic hydrogen production under Pyrex filtered UV light irradiation (λ N 300 nm) over Pt/TiPF is displayed in Fig 4a In order to study the robustness and stability of the catalyst, the reaction was allowed to react for 7.5 h, with intermittent evacuation every 2.5 h The intermittent evacuation of gas was usually carried out as a simple way to test the stability of the catalyst [12] The system steadily generated hydrogen with a linear increase in the amount of H2 of an evolution rate of average 159.3 μmol h−1 with the total amount of hydrogen evolved after 7.5 h irradiation of 1195 μmol The hydrogen evolution rate was slightly reduced cycle by cycle as a result of the decrease in the amount of sacrificial agent (methanol), the partly degradation of the photocatalyst, poisoning of by-products derived from methanol oxidation and the possible partial deterioration of TiPF evac a structure during the reaction Furthermore, TiPF, without co-catalyst Pt nanoparticles, also exhibits the photocatalytic activity for hydrogen production with an evolution rate of 22.8 μmol h−1 The lower evolution rate on TiPF is compared to that on Pt/TiPF (Fig S5), suggesting that the co-catalyst plays an important role as electron trap to improve the charge separation and final hydrogen production rate These results clearly indicate that Pt/TiPF functions as an efficient photocatalyst for hydrogen production For direct comparison, the photoproduction rate in this work (159.3 μmol h− with 100 mg catalyst) is much higher than that on Pt/Ti-MOF-NH2 (11.7 μmol h− with 10 mg catalyst, Table S1) [16], but much lower than that on Pt/TiO2 (Table S2) The time course of photocatalytic hydrogen generation from aqueous methanol solution over synthesized particles under visible-light irradiation (λ N 420 nm) was shown in Fig 4b The optical filters were used to monitor the wavelength of the broad-band visible-light source by cutting of the entire UV light Notably, the photocatalytic generation of H2 was observed with the total amount of hydrogen evoluted after h irradiation of 2.55 μmol The photoactivity of TiPF under visible light irradiation is reasonable as the optical spectrum of TiPF shows an absorption bands beyond 420 nm (Fig S3) However, the hydrogen evolution rate is much lower than that on Pt/Ti-MOF-NH2 (Table S1) [16] The photocatalytic activity of the Ti-based framework materials has been studied previously [16,17] On the basis of investigation results, the reaction mechanism was speculated as follows Generally, Pt was used as co-catalyst which plays a role as electron trap to prevent the electron–hole combination Upon the irradiation of UV light, the direct excitation of Ti-oxo clusters occurred Electron in valence band (VB) of Ti-oxo clusters is excited to conduction band (CB) which can be transferred directly to deposited Pt [16,17] Consequently, photo-excited electron and hole were driven to react with adsorbed agent leading the photocatalytic reaction Methanol was used as electron donor or sacrificial reagent to consume the photogenerated hole on the Tioxo clusters In particular, the photogenerated holes can transfer directly to adsorbed methanol molecules [28–30] On the other side, the photogenerated electron will be trapped at the Pt islands followed by the reduction of a proton from water or/and from methanol producing adsorbed H• radicals [31] Finally, methanol is photooxidized to carbon dioxide via the formation of the stable intermediates formaldehyde and formic acid and hydrogen is evolved on the Pt islands (Fig 5) At the same time, the observed photocatalytic activity under visible light irradiation suggests that the organic counterparts were also beneficial to the photocatalytic H2 production reaction It was proved that evac b 450 400 2.5 Hydrogen / µmol Hydrogen / µmol 350 300 250 200 150 1.5 100 0.5 50 0 2.5 Time / h 7.5 Time / h Fig (a) Time course of photocatalytic hydrogen production from water containing 20 vol.% methanol as an electron donor under irradiation of light (300-W Xe lamp, wavelength longer than 300 nm) by 1.0 wt.% Pt-deposited TiPF photocatalyst The reaction was continued for 7.5 h, with evacuation every 2.5 h (dashed line) (b) Time course of photocatalytic hydrogen production from water containing 20 vol.% methanol as an electron donor under irradiation of visible light (of wavelength longer than 420 nm) by 1.0 wt.% Pt-deposited TiPF photocatalyst Q.D Truong et al / Catalysis Communications 59 (2015) 55–60 59 H+ H2 Pt Ti C O N H ee- eh+ • CH OH + H+ CH3OH h+ h+ CH3OH • CH OH + H+ Fig Schematic illustration of photocatalytic hydrogen production over Pt-embedded TiPF upon light absorption in the LMCT band, a long-lived excited charge separation state occurs by transferring an electron from an organic ligand to Ti4 + [15,17] Thus, upon the illumination of light, the photogenerated electrons from excited organic linker were also transferred to the CB of titanium-oxo clusters for the reduction of photon from water/methanol Recently, Gascon et al demonstrated that the photogenerated charges in MIL-125(Ti) framework are not mobile; thus, the charge transport in the framework is different in a semiconductor The thermally activated hopping is main charge transport mechanism due to the isolation of Ti-oxo clusters by organic linkers [32] The experimental analysis of final oxidation products has proved difficult in this study due to the limitation of current analytic instrument The ongoing efforts are carried out to determine the formation of CO2 and formaldehyde to elucidate clearly insight into LMCT mechanism References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] Conclusions [12] [13] In conclusion, we have successfully incorporated Pt nanoparticles into layers of a titanium picolinate framework by a photodeposition method TEM images show the embedded and adsorbed Pt nanoparticles in the space between the layers with size of c.a nm The obtained Pt loaded titanium picolinate framework can function as an efficient photocatalyst for hydrogen evolution from methanol/water solution Our study thus provides a novel framework to integrate metal nanoparticles for functional applications, particularly for solar-driven fuel production [14] [15] [16] [17] [18] [19] [20] Acknowledgments This work was partly supported by Grant-in-Aid for Basic Research from Vietnam National University [21] [22] [23] Appendix A Supplementary data [24] Supplementary data to this article can be found online at http://dx 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The obtained Pt loaded titanium picolinate framework can function as an efficient photocatalyst for hydrogen evolution from methanol/water solution Our study thus provides a novel framework to... have successfully incorporated Pt nanoparticles into layers of a titanium picolinate framework by a photodeposition method TEM images show the embedded and adsorbed Pt nanoparticles in the space