Plasmonic Au@Pd Nanoparticles Supported on a Basic Metal

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Letter

Plasmonic Au@Pd Nanoparticles Supported on Basic Metal-Organic Framework: Synergic Effect for Bosting H Production from Formic Acid 2

Meicheng Wen, Kohsuke Mori, Yasutaka Kuwahara, and Hiromi Yamashita ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00558 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 23, 2016

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ACS Energy Letters

Plasmonic Au@Pd Nanoparticles Supported on Basic Metal-Organic Framework: Synergic Boosting H2 Production from Formic Acid Meicheng Wen†, Kohsuke Mori†,‡,§,*, Yasutaka Kuwahara†,‡, and Hiromi Yamashita†,‡,* † Graduate

School of Engineering, Osaka University, Suita, 565-0871, Japan

‡

Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Kyoto, Japan

§

JST, PRESTO, 4-1-8 HonCho, Kawaguchi, Saitama 332-0012, Japan

Corresponding Author *Hiromi Yamashita [email protected]

ACS Paragon Plus Environment

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ABSTRACT: We report a synergistic catalysis of plasmonic Au@Pd nanoparticles supported on titanium doped zirconium based amine-functionalized MOFs (UiO-66(Zr100-xTix)) for boosting room-temperature hydrogen production from formic acid (HCOOH) under visible light irradiation. Our results revealed that the electronically promoted Pd sites by the LSPR effect of Au as well as the doping of amine-functionality in the MOFs with titanium ions play crucial roles in achieving exceptional catalytic performance. Remarkably, a high H2 production rate of 42000 mL h-1 g-1 (Pd) with high TOF 200 h-1 based on Pd was obtained under visible light irradiation. The KIE measurements demonstrated that the dissociations of O-H and C-H bonds of formic acid, which are two important steps for hydrogen production from HCOOH, are individually facilitated by the assist of amine groups within MOFs and active electron rich Pd sites induced by LSPR effect under visible light irradiation.

TOC GRAPHICS


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The worldwide problem of air pollution and energy crises caused by a dramatic increasing use of limited fossil fuels has become a serious issue. It is clear that exploring sustainable and benign energy sources is prerequisite to meet environmental and socio-economic concerns. Hydrogen has received significant attention in industry as a candidate for renewable energy, in consideration of high energy density, non-toxicity, and efficient energy conversion when it is combined with proton exchange membrane full cell (PEMFC) technology.1-3 However, the efficient and safe storage, handling and distribution of hydrogen are still major challenges. A feasible strategy is to store hydrogen in liquid phase under ambient conditions, which can release hydrogen in situ at certain condition for direct use in fuel cells. Formic acid (FA, HCOOH), which is the major products formed during biomass processing, is identified as one of the most promising hydrogen carriers for fuel cells designed in terms of simplicity, nontoxicity and a high hydrogen content (4.4 wt %).4 FA decomposition can follow two different pathways, through either dehydrogenation, yielding H2 and CO2 or the dehydration into CO and H2O. The expected dehydrogenation pathway (△G = -32.8 KJ mol-1) is thermodynamically favored, but a competitive pathway of dehydration (△G = -20.7 KJ mol-1) producing toxic CO usually occurs, which is not desirable.5,6 Looking for suitable catalysts that enable selective H2 production from FA dehydrogenation is highly desirable. To date, the selective dehydrogenation of FA in homogeneous reactions with organometallic complexes at ambient temperatures has been reported.7-9 In particular, the catalytic activity and selectivity are enhanced significantly in the presence of different additives, such as amines,10 and phosphine ligand6. In spite of their outstanding progress,5,

11-14

there are some inherent

shortcomings, such as harsh reaction conditions, poor selectivity, separation issues, and the addition of extra additives which may cause severe difficulties in device fabrication. In this


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context, heterogeneous catalysts are widely applied in FA decomposition because it is facile to be controlled and retrieved.15-20 Current research has suggested that Pd is the most practical heterogeneous catalyst metal for the FA decomposition.21,22 Unfortunately, the monometallic Pd catalysts still suffer as a result of a need for additives, poor selectivity toward dehydrogenation pathway, and easily be poisoned by CO byproduct. Recent study has demonstrated that the activity of Pd based catalyst is highly dependent on the surface electron density of Pd.23 A promising strategy is to tailor electronic property of Pd nanoparticle and geometric structure through alloying or forming a core-shell structure with other metal to enhance the catalytic activity and selectivity of Pd catalysts.24,25 For example, Au@Pd,26 AgPd,27 PdCu24 alloy nanoparticles and Cu@Pd28 core shell structure have been developed. The electronic Pd species promoted by the charge transfer arising from the difference in work function of two metals, plays a crucial role in achieving a high catalytic activity for dehydrogenation of FA. It is well known that nanostructured Au can strongly absorb visible light efficiently owing to their localized surface plasmon resonance (LSPR).29,30 The free conduction electrons confined in Au nanoparticles resonated with the electromagnetic field of the incident light. As a result, energetic hot electrons triggered by LSPR can be transferred either to the neighboring absorbent or come back to the ground state by releasing the excess energy to the surrounding environment.31-35 In the case of plasmonic Au@Pd nanoparticles, plasmonically excited hot electron will transfer across the Au and Pd interface due to the lower electronegativity of Pd (2.20) than that of Au (2.54), resulting in an increase of the surface electron density of Pd,36 which is useful to enhance the catalytic activity for H2 production by dehydrogenation of FA. MOFs are an attractive class of hybrid organic-inorganic porous crystalline materials, built from metal ions connecting to organic moieties. These porous materials offer significant


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chemical diversity because the linker of MOFs can be modified by functional groups and the nodes of MOFs can be partially replaced by foreign metal ions. It is also evident that partial substitution with suitable metal ions as an electron mediator in the cluster can efficiently facilitate the charge separation by oxo-bridged hetero-metallic assemblies.37 In this paper, we have demonstrated that the monodispersed Au@Pd nanoparticles combined with the titanium doped zirconium based amine-functionalized metal-organic frameworks (UIO-66(TixZr100-x)) exhibited a dramatic enhancement in the selective hydrogen production from FA decomposition under visible light irradiation. Such an improvement can be attributed to not only the positive combination effect of Pd and Au, but also the cooperative action between active metal nanoparticles and -NH2 groups within the framework of photoactive Ti-doped MOFs, and the electronically promoted Pd sites by the LSPR effect. The reported synthetic protocol is a promising candidate for FA dehydrogenation because it has the following advantages: (i) without addition of additives, (ii) superior catalytic activity under visible light irradiation compared with thermal reaction, (iii) no CO contamination from FA in an aqueous phase. It is undoubted that the electronically promoted Pd by the charge transfer arising from the difference in work function of the Pd and Au metals and the LSPR effect of Au, plays a crucial role in achieving highly catalytic activity for dehydrogenation of FA. X-ray diffraction (XRD) was used to characterize the phase structure and crystalline size of the as-synthesized MOFs. As shown in Figure S1, the main XRD peaks of the Ti ions doped MOFs correspond well to those of MOFs without Ti doping and no additional new reflection was observed, indicating that the doping of Ti ions within UiO-66 did not change the framework structure and no new phase of Ti compounds was formed. The presence of Au@Pd nanoparticle has a slight influence on homogeneity and crystallinity in the XRD patterns due to the strong


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reducibility of NaBH4. The absence of the characteristic Pd and Au peak was due to the small nanoparticle size and low loading weight of Pd and Au. The porosity of samples was analyzed by N2 sorption experiments as shown in Figure S2. The completely reversible isotherms of pure UiO-66(Zr100) and UiO-66(Zr85Ti15) exhibited type-I behavior, which is typical feature of microporous materials. In comparison with pure UiO-66(Zr100), a hysteresis loop in the relative pressure (P/P0) around 0.9 was observed, suggesting the formation of mesopore structure after doping with Ti ions. Table S1 summarizes the Brunauer-Emmett-Teller surface area (SBET) and pore volume (Vp) calculated from N2 adsorption-desorption isotherms. The sample doped with Ti ions showed a slight decrease in surface area and pore volume compared with that of pure UiO66(Zr100). After loading with Au@Pd nanoparticles, the surface area and pore volume of UiO66(Zr100) and UiO-66(Zr85Ti15) decreased, suggesting that the internal cavities were occupied or blocked by Au@Pd nanoparticles. 4 UiO-66(Zr100) UiO-66(Zr85Ti15)

KM-function / a.u.

Au@Pd/UiO-66(Zr100)

3

2

1

300

400

500

600

700

Wavelength / nm

0 800

-2 -1

200

Au@Pd/UiO-66(Zr85Ti15)

Enhanced H2 amount / % (mW cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 1. UV-vis spectra of samples UiO-66(Zr100), UiO-66(Zr85Ti15), Au@Pd/UiO66(Zr100), Au@Pd/UiO-66(Zr85Ti15), and wavelength dependence of the enhancement of catalytic performance of Au@Pd /UiO-66(Zr85Ti15) under irradiation by monochromatic


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light (λ = 400 nm, 6 mW cm-2 and 480 nm, 6 mW cm-2) and a LED lamp (λ = 530 nm, 10 mW cm-2) irradiation. Figure 1 shows the UV-vis spectra of the samples. As it can be seen, all samples displayed intense absorption bands in the high energy region (λ < 300 nm), which can be assigned to the π→π∗ transition of the linker, and moderately intense low-energy absorption bands starting from 300 nm to 450 nm, which corresponds to the chromophore on the linker of MOFs.37 In contrast, after doping with Ti ions, the absorption edge of MOFs displayed a slight red shift compared with the sample without Ti doping, because several energy bands are generated between the LUMO and HOMO state of UiO-66(Zr85Ti15).38 This result indicated the successful substitution of Ti ions into the framework of UiO--66. Obviously, strong peaks located at approximately 540 nm associated with LSPR of Au were clearly observed in the spectra of Au@Pd /UiO-66(Zr100) and Au@Pd/UiO-66(Zr85Ti15). Similar results have also been reported that an intense Au plasmonic absorption peak centered at around 540 nm was observed on Au nanoparticles with size smaller than 10 nm.39-41 It is worth noting that the substitution of Ti ions within the MOFs greatly increases the intensity of the plasmonic peak with a red shift of the band. It is suggested that the increase of the electron density on the surface of Au nanoparticles would lead to the enhancement of their plasmonic absorption.42 As shown in Figure S3, the Au-Au interatomic distance of Au@Pd/UiO-66(Zr85Ti15) is longer than that of Au@Pd/UiO-66(Zr100), indicating that Ti dopants enhance the electron density of Au. Possible explanations for red shift is that the dielectronic-constant of MOFs changed after doping with Ti species in MOFs as the plasmonic absorption position of Au is highly depended on the dielectronic-constant of support.43 To determine whether Ti ions are successfully doped in the nodes of MOFs, the local structures of the Ti-oxide species were investigated by spectroscopic methods. Figure S4 shows


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Ti K-edge XANES spectra of UiO-66(Zr85Ti15) and reference TiO2. TiO2 displayed well-defined pre-edge peaks from 4960 to 4975 eV attributed to the transitions from the 1s core level of the Ti atom to three different kinds of molecular orbitals (1t1g, 2t2g, and 3eg) of TiO2 having octahedral geometry. In the case of UiO-66(Zr85Ti15), a characteristic and sharp pre-edge peak located at around 4970 eV was observed, which can be assigned to the surrounded by oxygen atoms, indicating that the Ti-oxide moieties have tetrahedral coordination in the nodes of MOFs. In the Fourier transforms of EXAFS data, TiO2 showed an intense peak at around 1.8 Å and a broad peak ranging from 2.0 to 3.0 Å, which corresponds to neighboring oxygen atoms (Ti-O) and a contiguous Ti-O-Ti bond, respectively. On the other hand, only a strong peak at around 1.8 Å was observed, which is assignable to neighboring oxygen atoms (Ti-O). No broad peak ranging from 2.0 to 3.0 Å was observed for UiO-66(Zr85Ti15), indicating the absence of a contiguous TiO-Ti bond for the sample of UiO-66(Zr85Ti15). These results clearly revealed that titanium oxide species exist in an isolated and tetrahedrally-coordinated state in the nodes of MOFs, without forming aggregated TiO2. Figure 2a presents low-magnification TEM images of Au@Pd/UiO-66(Zr85Ti15). Au@Pd nanoparticles with a mean size of 7.3 nm can be clearly observed on MOFs without significant agglomeration, suggesting that Au@Pd nanoparticles were well distributed on surface of MOFs, which is due to the large surface area of MOFs manipulating the nucleation and growth of metal nanoparticles. Figure 2c displays the HAADF-STEM image of an individual Au@Pd nanoparticle, in which the dark shell and bright core suggested the formation of core shell structure. It should be noted that two types of crystal lattice were observed on the metal nanoparticle; the shell interplanar fringes were determined to be 2.78 Å and 1.58 Å, which is ascribed to the crystal lattice of the (101) planes and (21-1) planes of Pd. Therefore, the exposed


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facet of analyzed nanoparticles was measured to be Pd {1-3-1}. The core interplanar fringes of nanoparticle were determined to be 2.01 Å, which is ascribed to the crystal lattice of the (200) planes of Au. This result indicates that Au core is covered by Pd shell. In order to further determine the distribution of Pd and Au elements in an individual Au@Pd nanoparticle, HAADF image of Au@Pd nanoparticle and corresponding element mapping images are shown in Figure 2f-g, the Pd-L signals are weak in the core part and strong in the shell part, whereas the signal of Au-L in center part is strong than that in the periphery part, which clearly suggests that the Pd mainly existed in the shell part. EDX line scan profile also suggests that the core shell structure of metal nanoparticle, where the Pd signal is detected on the surface of nanoparticle and Au signal is detected in the core of nanoparticle. The presence of uniform Pd and Au in this individual nanoparticle was consistent with the formation of proposed core shell structure.


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Figure 2. (a) Low-magnification TEM image of the Au@Pd /UiO-66(Zr85Ti15), (b) size distribution diagram of Au@Pd /UiO-66(Zr85Ti15), (c) HAADF-STEM image of an individual Au@Pd nanoparticle, (d) and (e) the magnified cross-sections of image, (g) and (h) highmagnification EDX spectra of Pd and Au for an individual Au@Pd nanoparticle. Cross-sectional EDX line profile on a single Au@Pd core shell nanoparticle.


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Catalytic activity was tested by the H2 production through FA dehydrogenation. The reaction was carried out by stirring the catalyst (0.01 g) in 0.2 mL of FA and 4.8 mL of water under Ar atmosphere. Figure 3 summarizes the produced H2 amount during 2 h of reaction in the dark (black bars) or under visible light irradiation with a Xe lamp (gray bars). No significant reaction occurred over Au/UiO-66(Zr85Ti15) without Pd nanoparticles even under light irradiation. Little reaction was observed over Pd/UiO-66(Zr85Ti15) under dark condition, and enhancement of activity under light irradiation is slight, suggesting that the reaction is driven by Pd nanoparticles. As shown in Figure 3, all Au@Pd supported MOFs exhibited much higher activities than monometallic nanoparticles supported MOFs owing to the synergistic effect of Au and Pd. The optimized Pd loading amount was found to be 0.6 wt % as displayed in Figure S6. Increase of Pd loading amount from 0.2 to 0.6 wt % significantly increased catalytic activity, while further increase to 1.0 wt % resulted in the low turnover number. This might be attributed to the thicker Pd shell prevents the charge redistribution from Au to the surface of Pd shell. This result is in good agreement with the reported work.23 However, the physical mixture of Pd/UiO-66(Zr85Ti15) and Au/UiO-66(Zr85Ti15) scarcely exhibited reaction enhancement. Moderate enhancement was observed in the AuPd alloy supported UiO-66(Zr85Ti15), but it was still lower than that of Au@Pd/ UiO-66(Zr85Ti15), which can be attributed to the different adsorption mode of formic acid on Pd nanoparticles.23 On the basis of above results, it is evident that the presence of Au@Pd nanoparticles with core-shell structure is essentially important for promoting the catalytic activity. The enhanced activity of Au@Pd nanoparticle supported sample is due to the electronically promoted Pd by the charge transfer arising from the difference in work function of Au and Pd. The electron rich Pd nanoparticle has positive effect on adsorption and activation of formic acid, further enhancing the rate of H2 production. This phenomenon is well consistent


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with those observed in previous reports.20,23 Furthermore, the increase in the amount of Ti doping enhances the activity, and Au@Pd/UiO-66(Zr85Ti15) produces the largest amount of H2, because Ti dopants can not only induce the electron rich metal nanoparticles, but also enhance visible light absorption of MOFs and plasmonic absorption efficiency of Au, leading to the high catalytic activity of Pd. Further increase of the doping amount of Ti caused significant decrease of activity, which can be attributed to the lower surface area (Table S1) and lower crystallinity (Figure S5) of UiO-66(Zr80Ti20) compared with other samples. Moreover, the present catalytic system efficiently suppressed unfavorable CO impurity, as the produced H2 and CO2 amount was around 1 : 1 ratio and no CO was detected by gas chromatograph equipped with a methanizer, which meets the requirement of PEMFC standard (10 ppm). It is noteworthy that the activity of all Au@Pd supported samples was greatly enhanced under visible light irradiation as shown in Figure 3, and the Au@Pd/UiO-66(Zr85Ti15) produced the largest amount of H2, which was nearly 1.5 times compared with that in the dark. The enhanced activity of Au@Pd/UiO-66(Zr85Ti15) can be assigned to the synergistic effect between support and Au@Pd in promoting the activation of formic acid and preventing the CO adsorption upon light irradiation. Remarkably, the open system reaction equipped with a gas burette at 30 oC gave a high H2 production rate of 42000 mL h-1 g-1 (Pd) with high TOF 200 h-1 based on Pd under visible light irradiation as shown in Figure S7, which was much higher than that in the dark condition (H2 production rate of 34000 mL h-1 g-1 (Pd) with TOF 161 h-1 based on Pd) and those reported in the literature for different active catalytic systems, such as Au@Pd/N-mrGO (89-1, 25 oC),44 Ag@Pd (156 h-1, 35 oC),23 and Pd/HBETA (59.2 h-1, 50 oC).45 Au@Pd/ED-MIL-101(106 h-1, 90 oC)18. In addition, the Au@Pd/UiO66(Zr85Ti15) shows considerable stability and can be recycled at least 3 times with a slightly decrease in activity for the H2 production from formic acid under visible light irradiation (Figure


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S8). The structural information of sample after three repeated cycles was measured and shown in Figure S9. The crystallinity of reused sample decreased, which might be the main reason for the decreased in catalytic activity during recycling experiments under visible light irradiation.

Au+Pd/UiO-66(Zr85Ti15) AuPd/UiO-66(Zr85Ti15) Au@Pd/UiO-66(Zr80Ti20) Au@Pd/UiO-66(Zr85Ti15) Au@Pd/UiO-66(Zr90Ti10) Au@Pd/UiO-66(Zr95Ti5) Au@Pd/UiO-66(Zr100) k r a D

Pd/UiO-66(Zr85Ti15)

t h g i L

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Au/UiO-66(Zr85Ti15)

0

10

20

30

40

50

60

70

H2 amount / mmol Figure 3. Amount of H2 produced through FA dehydrogenation in the dark (black bars) or under visible light irradiation (gray bars, λ > 420 nm, 320 mW cm-2) over different catalysts.

The light emitted from Xe lamp at λ > 420 nm contains not only visible light but also a part of infrared light. In fact, the temperature of the suspension of Au@Pd/UiO-66(Zr85Ti15) slightly heated up (about 32 oC) under infrared light irradiation. In order to investigate the effect of infrared thermal heating, the thermal reaction at 30, and 32 oC was performed in the dark with Au@Pd/UiO-66(Zr85Ti15). As displayed in Figure S10, The catalytic activity of H2 production from formic acid in the dark at 32 oC was a little higher than the production at 30 oC, much lower


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than that under light irradiation. Therefore, the H2 production enhancement is essentially enhanced by synergistic effect between support and Au@Pd under visible light irradiation. Table 1. KIE in the decomposition of formic acid. Catalyst

Formic acid

Au@Pd/UiO-

HCOOH

0.96

-

HCOOD

0.96

1.00

DCOOH

0.2

4.8

HCOOH

1.82

-

HCOOD

1.79

1.02

DCOOH

0.55

3.31

66(Zr85Ti15)a Au@Pd/UiO66(Zr85Ti15)b

Reaction rate

kH/kD

Reaction condition: 0.02 g catalyst, 5 mL H2O, 0.15 mL FA, Ar atmosphere, [a] in the dark condition. [b] under light irradiation.

To verify this synergistic effect, wavelength-dependent catalytic reactions were also performed by using monochromatic light (λ = 400 nm or 480 nm) and a LED lamp (λ = 530 nm) irradiation. As presented in Figure 1, monochromatic light (λ = 400 nm or 480 nm) illumination improved H2 production rate of Au@Pd/UiO-66(Zr85Ti15) in contrast to dark condition. The increasing rates of catalytic performance with light intensities were well consistent with the light absorption of Au@Pd/UiO-66(Zr85Ti15). The largest enhancement of 2.0 % (mW cm-2)-1 was found with monochromatic light (λ = 400 nm), which can be assigned to the absorption of linker. As well known, the photoactive MOFs have been employed in many photocatalytic reactions as semiconductors. In the case of Ti doped UiO-66, the linker of MOF acts as a chlorophyll. The incident visible light absorption by the organic linker produces the photo-excited electrons, which transfer through a linker-to-cluster charge-transfer mechanism (LCCT),46 and then to Pd nanoparticles, resulting in the formation of activated Pd species. It was noted that the large


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enhancement of 1.5 % (mW cm-2)-1 was obtained with LED lamp (λ = 530 nm) irradiation, nearly approaching the LSPR wavelength (544 nm) of sample. This wavelength dependence clearly confirms the positive effect of LSPR component in enhancing the catalytic performance. After absorption of visible light by Au, an energetic conduction electron is formed. This energetic electron transfers from Au to Pd due to the charge heterogeneity existed in the abrupt atomic interface between Au and Pd, promoting the formation of electron rich Pd.36,40,47 Reducing the abrupt atomic interface by physically mixed Au/UiO-66(Zr85Ti15) and Pd/UiO66(Zr85Ti15) precludes the electron transfer between Au and Pd, leading to the small enhancement of catalytic activity as shown in Figure 3. Therefore, integration of Au@Pd nanoparticle and photo-reactive Ti doped MOFs can allow the light energy to be harvested by the plasmonic component and photo-reactive support to be utilized by the catalytic component.

Scheme 1. A possible reaction pathway for the visible light enhanced H2 production from FA decomposition by Au@Pd/UiO-66(Zr85Ti15) was proposed under visible light irradiation.


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The promotional role of weakly basic –NH2 groups within the support in achieving high catalytic activity has previously been reported in both homogeneous and heterogeneous systems, in which the -NH2 groups cooperatively involve in the reaction pathway.15,25,48 Table 1 summarized the results for kinetic isotope effect (KIE) by using HCOOD, DCOOH, and HCOOH, and Scheme 1 proposes the possible reaction pathway for the visible light enhanced H2 production from formic acid by using Au@Pd/UiO-66(Zr85Ti15). The kH/kD values using HCOOH and HCOOD was equal to 1 both under dark condition and visible light irradiation, which clearly suggests that a large amount of weakly basic –NH2 groups have a positive effect on O-H bond dissociation, in which the weakly basic -NH2 groups acts as a proton scavenger to form -+HNH2 and a Pd-fomate intermediate (step 1). Pd-fomate species then undergoes C-H bond dissociation to afford CO2 and a Pd-hydride (Pd-[H]-) species (step 2). Finally, the reaction of hydride species with -+HNH2 produces molecular hydrogen, following the regeneration of the metal species (step 3). The kH/kD values obtained from competitive reaction of HCOOH and DCOOH under visible light irradiation was smaller than that obtained under dark condition, which suggested that the rate-determining C-H cleavage from the Pd-formate intermediate to release H2 is facilitated by the electronic Pd. After the visible light absorption by plasmonic Au@Pd nanoparticle and photoactive MOFs, the photo-generated electron migrate to Pd active site. Such electron rich Pd species significantly facilitates C-H bond cleavage from the Pdformate intermediate.15,23,49,50 Meanwhile, formic acid serves as an electron donor,51,52 which donate electrons to the electron-deficient organic linkers and Au to continue the photo-assisted H2 production from Formic acid. On the basis of KIE study, it is reasonable to conclude that the synergistic effect of the weakly basic amine groups within metal-organic frameworks and the


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LSPR effect of Au@Pd nanoparticles and photoactive MOFs induced electronically promoted Pd species which accounts for its high catalytic activity. In summary, the integration of plasmonic Au@Pd nanoparticle and Ti-doped MOFs bearing – NH2 functional groups affords highly effective light energy harvesting system to be used for boosting room-temperature hydrogen production from FA dehydrogenation under visible light irradiation without any additives. As expected, plasmonic Au@Pd supported MOFs showed much better catalytic performance under visible light irradiation as compared to dark condition. The change in the charge density of the Pd surface caused by charge separation derived from LSPR and photoactive MOFs plays important roles in promoting the dissociation of C-H bond. On the other hand, the basic –NH2 groups within the nanopores of MOFs have a positive effect on the O-H bond dissociation, and the cooperation of the amine functionality within MOFs structure and electron rich Pd is responsible for the high catalytic activity. This study supplies a platform to design other plasmonic materials based on MOFs for efficient catalytic reactions under various light environments.

ASSOCIATED CONTENT Supporting Information. XRD patterns, N2 sorption isotherms, XANES and EXAFS of samples, time course of reaction in H2 production from Formic acid, recycle study and catalytic reaction at different condition. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This research was supported by JST, PRESTO (J1510B2533). The present work was also partially supported by Grants-in-Aid for Scientific Research (Nos. 26220911, 25289289, and 26630409, 26620194) from the Japan Society for the Promotion of Science (JSPS) and MEXT. KM, YK and HY thank MEXT program “Elements Strategy Initiative to Form Core Research Center”. The authors appreciate assistance from Ms. Pei-Hsuan Liu at the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan for TEM measurement.

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