Size-Dependent Catalytic Activity of Palladium Nanoparticles

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Size-Dependent Catalytic Activity of Palladium Nanoparticles Fabricated in Porous Organic Polymers for Alkene Hydrogenation at Room Temperature John Mondal, Quang Thang Trinh, Avijit Jana, Wilson Kwok Hung Ng, Parijat Borah, Hajime Hirao, and Yanli Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03127 • Publication Date (Web): 03 Jun 2016 Downloaded from http://pubs.acs.org on June 4, 2016

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utilized for transfer hydrogenation of alkenes involving microwave irradiation and high temperature.

Scheme 1: Synthesis of Pd@TRIA nanocatalysts. Herein, a new POP material (TRIA) was developed through non-aqueous polymerization of 2,4,6-triallyoxy-1,3,5triazine monomer, and uniform fabrication of Pd NPs with two different particle diameters (8 nm and 3 nm) was successfully achieved in the POP to deliver two different types of new catalysts Pd@TRIA-1 and Pd@TRIA-2, respectively (Scheme 1). The triazine organic framework could serve as a connecting unit for various functional entities and as a metal-coordinating group owing to the presence of enriched N atoms. The developed TRIA, Pd@TRIA-1 and Pd@TRIA-2 were thoroughly characterized using several characterization techniques such as powder X-ray diffraction (XRD), transmittance electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), elemental mapping, N2 sorption measurements, X-ray photoelectron spectroscopy (XPS), FT-IR and 13C CP MAS NMR. Pd@TRIA enhances the dehydrogenation of formic acid and thereby assists catalytic transfer hydrogenation of alkenes at room temperature. Pd@TRIA-1 (with 8 nm Pd NP size) exhibits higher catalytic performance for catalytic transfer hydrogenation of alkenes than that of Pd@TRIA-2 (with 3 nm Pd NP size), with the former showing significant recyclability for ten catalytic cycles and efficient H2 generation by dehydrogenation of formic acid. To the best of our knowledge, this is the first example of using porous polymer supported Pd NPs with different sizes as nanocatalysts for performing sizedependent alkene hydrogenation at room temperature using formic acid as sustainable H2 source. formic acid shows high energy density, excellent stability, no obvious toxicity, and easy handling capability by avoiding flammable H2 gas at high pressure and temperature. Density functional theory (DFT) calculations were also employed to gain insight into the reactivity trends. Using DFT calculations, two possible reasons for higher catalytic efficiency of the 8nm Pd NPs were identified. The bibenzyl product binds to the 3 nm Pd NPs more strongly, thereby making desorption of the product more difficult and reducing the overall efficiency of the catalyst. In addition, the first hydrogenation step in the reaction on the 3nm Pd NPs has higher activation barrier than that for the 8 nm NPs, which also renders the reaction on the former more sluggish.

polymerization of 2,4,6-triallyoxy-1,3,5-triazine with cross linker divinyl benzene under hydrothermal conditions. Azobisisobutyronitrile (AIBN) was used as a radical initiator for this reaction. The white color TRIA-POP is insoluble in water and common organic solvents. Thermal stability of the polymer was supported by TGA analysis (Figure S1 in the Supporting Information (SI)). The reaction of nanoporous polymer TRIA and Pd(OAc)2 in methanol under refluxing condition resulted in the formation of the black color Pd@TRIA-1. Here, methanol acts not only as a solvent but also a reducing agent to reduce Pd(II) to Pd(0) NPs with 8 nm diameter. The Pd(0) NPs were encapsulated on the exterior surface of the TRIA POP. The formation of Pd(0) NPs in methanol was confirmed by UV-Vis spectra (Figure S2A). The Pd@TRIA-2 material was developed using a strong reducing agent (NaBH4), and this agent was necessary in this case to nucleate small particles. Pd(II) was successfully coordinated with the N and O donor sites of the TRIA POP by vigorous stirring in methanol to yield yellow color Pd(II)-TRIA composite. The appearance of weak signals at 26.5 and 179.1 ppm from 13C MAS NMR spectrum of Pd(II)-TRIA material (Figure S2B) could be attributed to the carbonyl and methyl carbons of the incorporated Pd(OAc)2, respectively. Then, NaBH4 was used to reduce Pd(II) to Pd(0) NPs with 3 nm particle size. In this case, NaOAc used plays a dual role, namely, aiding the solubilization of Pd-POP and inhibiting the aggregation of NPs. The Pd content in Pd@TRIA-1 and Pd@TRIA-2 was 0.7659 mmol/g and 0.8598 mmol/g respectively, as determined by inductively coupled plasma mass spectrometry (ICP-MS). Elemental (C, H and N) analysis data are provided in Table S1. The diffused peak at 2θ = 20.2o for the TRIA polymer in the wide angle powder XRD pattern (Figure 1Aa) can be attributed to the amorphous polymeric frameworks. The wide angle powder XRD patterns of Pd@TRIA-1 and Pd@TRIA-2 (Figure 1Ab,1Ac) displayed three characteristic diffraction peaks with 2θ values of 40.1, 45.9 and 67.2o, which can be readily indexed to the (111), (200) and (220) crystalline planes corresponding to the face centered cubic (fcc) arrangement of palladium NPs (JCPDS No. 46-1043), respectively. A little hump at 2θ = 19.9o refers to the presence of an amorphous framework of the POP. This very distinct XRD pattern, revealing that the Pd NPs were dispersed on the external surface of the TRIA polymer.18,19

2. RESULTS AND DISCUSSION 2.1 Catalyst synthesis and characterization Following the synthetic route outlined in Scheme 1, nanoporous organic polymer TRIA was derived by non-aqueous 2

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Figure 1. (A) Wide angle powder XRD patterns, (B) N2 adsorption/desorption isotherms, (C) 13C CP MAS NMR spectra and (D) FT-IR spectra of TRIA (a), Pd@TRIA-1 (b) and Pd@TRIA-2 (c).

The porosity in the TRIA POP, Pd@TRIA-1 and Pd@TRIA-2 materials was determined employing N2 adsorption/desorption analysis at 77K (Figure 1B). The TRIA material shows typical type II isotherm with a very small hysteresis loop and a sharp nitrogen uptake in the high P/P0 pressure region corresponding to the existence of interparticle void space (Figure 1Ba). The BET (Brunauer-Emmett-Teller) surface area of the TRIA material is 620 m2g-1. The small hysteresis loop in the isotherm is generated by the swelling of the polymeric network upon the gas adsorption or due to mesoporosity originated from the interparticle void space. Pd@TRIA1 material shows typical type II isotherm with the sharp increase in nitrogen uptake at a high-pressure region corresponding to the interparticle void space (Figure 1Bb). The BET surface area for the Pd@TRIA-1 material is 573 m2g-1. The preservation of N2 adsorption/desorption isotherms indicates that the pore systems were not blocked or altered substantially by Pd NPs.20,21 The Pd@TRIA-2 material also showed typical type II isotherm (Figure 1Bc) with a BET surface area of 459 m2g-1. Appreciable decrease in the BET surface area compared with the TRIA material suggests the occupation of small palladium NPs inside the nanoporous channel of the polymer. The pore volumes for the TRIA, Pd@TRIA-1 and Pd@TRIA2 are 0.51 ccg-1, 0.46 ccg-1 and 0.32 ccg-1, respectively. Gradual decrease in the pore volumes signifies that palladium NPs are successfully anchored with the N and O donor sites of the triallyloxy-triazine organic ligand. Pore size distributions of the TRIA, Pd@TRIA-1 and Pd@TRIA-2 materials are given in Figure S3. No remarkable change in the pore size distribution for the Pd@TRIA-1 material as compared with TRIA reveals the preservation of pores after immobilization of large Pd NPs in the external surface area. In the case of the Pd@TRIA-2 material, significant decrease in the pore dimension suggests that the Pd NPs are successfully accommodated inside the nanoporous channel of the polymer.22,23

Figure 2. (A) XPS survey, (B) N 1s and (C) O 1s of (a) TRIA, (b) Pd@TRIA-1, and (c) Pd@TRIA-2. (D) Pd 3d spectra of (a) Pd@TRIA-1 and (b) Pd@TRIA-2. 13

C CP MAS NMR spectra of the TRIA, Pd@TRIA-1 and Pd@TRIA-2 materials are given in Figure 1C. For the 13C CP

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MAS NMR spectrum of TRIA (Figure 1Ca), the broad signals centered at 112.0, 126.8 and 136.7 ppm are attributed to the characteristic aromatic carbon atoms of the porous polymer (overlap of C7-9). The characteristic peak at C1 (144.1 ppm) can be assigned to triazine carbon of the melamine unit, which is consistent with previous analogous reports.24,25 The spectrum also exhibits strong signals at 28 and 39.7 ppm in the aliphatic region, which can be assigned to different aliphatic carbons (overlap of C3-6) in the bridging units.26 The small peak at C2 (68.3 ppm, Figure 1Ca) likely represents the carbon atom of the -OCH2 bridging unit. In the 13C CP MAS NMR spectra of the Pd@TRIA-1 and Pd@TRIA-2 materials (Figure 1Cb and 1Cc), the signals for the aromatic carbons, aliphatic carbons and central carbon of triazine unit are similar to the NMR signals of nanoporous polymer TRIA, thus confirming the structural integrity of the polymer after the incorporation of the NPs. FT-IR spectra of the TRIA, Pd@TRIA-1 and Pd@TRIA-2 materials are given in Figure 1D. The absorption bands centered at 1572 cm-1, 1350 cm-1 and 807 cm-1 are suggestive of the characteristic triazine framework in all the materials.27 For Pd@TRIA-1 and Pd@TRIA-2, only subtle shifts in the peak positions under these characteristic regions were observed (Figure 1Db and 1Dc), which can be attributed to the chelation of Pd(0) with the triazine framework through the lone pair of electrons on the N atom. The bands at 2927 cm-1 and 2854 cm-1 can be assigned to the stretching vibrations of the CH2 and CH groups in the aliphatic moiety.28 EDX patterns and elemental mapping images of C, N, O, and Pd elements as well as FE-SEM images of the Pd@TRIA-1 and Pd@TRIA-2 materials are given in Figures S4 and S5 respectively, which clearly depict the presence of constituting elements. XPS spectra of TRIA, Pd@TRIA-1 and Pd@TRIA-2 in Figure 2A show three peaks centered at around 284.9, 397.8 and 530.8 eV that can be attributed to C 1s, N 1s and O 1s, respectively. For Pd@TRIA-1 and Pd@TRIA-2, two new peaks were generated at 338.4 and 339.3 eV in the survey spectra (Figure 2Ab and 2Ac), corresponding to the Pd 3d binding energies. The N 1s spectra of all the three materials are given in Figure 2B. The N 1s binding energy peak of the TRIA material centered at 397.8 eV can be ascribed to a single type nitrogen atom of the triazine unit. In order to illustrate the interaction between Pd0 NPs and the triazine unit of the framework, N 1s spectra analysis for both the Pd@TRIA-1 and Pd@TRIA-2 materials was carried out. Insignificant upshifted N 1s binding energy at 398.0 eV for the Pd@TRIA-1 material (Figure 2Bb) suggests the weak coordination of N atoms from the triazine unit to Pd.29,30 However, for the Pd@TRIA-2 material, the binding energy peak of the N 1s spectrum (Figure 2Bc) remarkably shifted to 399.0 eV, which indicates strong interactions of small palladium NPs inside the nanoporous channel of the POP. Higher binding energy shifts in O 1s spectra (Figure 2C) at 532.0 and 531.9 eV as compared with that of TRIA at 530.8 eV suggest strong interactions between oxygen atoms of the allyloxy unit of the polymeric framework and the Pd NPs. This result indicates that O atoms are more strongly coordinated to Pd in Pd@TRIA-1 because N has a negligible coordination. In order to evaluate the oxidation state of Pd, XPS analysis of Pd 3d binding energies of the Pd@TRIA-1 and Pd@TRIA-2 materials was performed (Figure 2D). Two sets of spin-orbit doublet could be established from Pd (3d5/2,3/2) peaks. By considering spin-orbit separation of 5.2 eV, Pd 3d XPS spectrum was decomposed into two doublets with a ratio of intensities at 3:2. The low-

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binding-energy doublets of Pd 3d5/2 and Pd 3d3/2 (Figure 2D) centered at 335.5 and 335.6 eV as well as 340.3 and 340.6 eV for Pd@TRIA-1 and Pd@TRIA-2 respectively are attributed to the Pd(0) species in the hybrids,31 while the peaks of Pd 3d5/2 binding energies at 337.27 eV and 337.19 eV can be recognized to the Pd2+ in the Pd@TRIA-1 and Pd@TRIA-2 materials respectively.32,33 Figure S6B shows the Pd 3d spectrum of Pd@C material. The ratios of Pd0/Pd2+ in Pd@TRIA-1 and Pd@TRIA-2 are 0.52 and 0.54, respectively. It is evident that Pd 3d XPS spectra of all Pd@POP samples present a significant concentration of Pd(II) species in spite of performed reduction. The presence of this oxidized Pd form in the Pd@POP catalysts could be attributed to highly dispersed single site isolated Pd species strongly interacted with the allyloxy triazine moiety through O or N atoms. Very recently, Arrigo et al. studied N-Pd covalent bond formation between pyridinic N atoms and Pd, which was consistent with the shift of the binding energy of Pd 3d close to the Pd(II) state. They observed that the reduction became complicated after the coordination of Pd(II) species to N atoms of the support.29 The bonding state and relative abundance of Pd in the presence cases were also investigated by high resolution XPS spectra analysis (Figure S6C).

Figure 3. (A-C and E) TEM images of Pd@TRIA-1, (D) SAED pattern and (F) HR-TEM image of Pd@TRIA-1, (G-I) TEM images of Pd@TRIA-2.

TEM images of these materials are given in Figure 3 and Figure S7. Pd NPs are embedded on the external surface of the nanoporous polymer due to the coordination ability of 1,3,5triazine organic backbone. The Pd NPs with the average sizes of 8-10 nm in Pd@TRIA-1 cannot be accommodated in the interior cavities of nanoporous polymer TRIA. The formation of large Pd NPs in Pd@TRIA-1 may be due to the hydrophobic nature of nanoporous polymer, and methanol cannot enter proficiently into the pores by capillary force, resulting in the diffusion of palladium precursors from the pores to the external surface of the polymer (Figure 3 A-C and E). This assump-

tion was supported by previously reported literature.10 Selected area electron diffraction pattern (SAED) of Pd@TRIA-1 is presented in the Figure 3D. HR-TEM image of Pd@TRIA-1 (Figure 3F) was employed to calculate interplanar spacing of 1D lattice fringes for Pd NPs, which is 0.2014 nm corresponding to the (111) lattice spacing of face centered-cubic arrangement of Pd(0) species. TEM images of Pd@TRIA-2 (Figure 3G-I) exhibit a dual size distribution, where relatively small NPs with an average diameter of 2.5 nm are located in the interior pores and large NPs with an average diameter of 3.6 nm are distributed on the external surface.34 The average size distributions of the Pd NPs in the two Pd catalysts are provided in the inset of Figure 3A and 3I.

2.2 Catalytic transfer alkene hydrogenation with Pd@TRIA catalysts The newly developed Pd0 nanocatalysts were employed for formic acid assisted catalytic transfer hydrogenation of alkenes.35 We started our study with catalytic transfer hydrogenation of trans-stilbene 1a (0.25 mmol) in EtOH (5 mL) with the Pd@TRIA-1 catalyst (5 mg) in a sealed tube at room temperature. In the absence of any additive (external hydrogen source), no catalytic conversion of 1a took place (Table 1, entry 1). Then, we employed HCOOH as a hydrogen source for the catalytic reduction of trans-stilbene 1a (Table 1, entry 2), and we achieved only 20% conversion at 16 h, and no improvement of catalytic conversion could be observed later. The addition of an equivalent amount of Et3N to the catalytic system (Table 1, entry 3) drove the reaction to completion within 4 h with 99% conversion of 2a. On the other hand, by employing Pd@TRIA-2 (5 mg) in the catalytic transfer hydrogenation reaction under identical reaction conditions, the conversion of 2a could be achieved to 99% at 10 h (Table 1, entry 4). The highest turn over frequency (TOF) of 16.1 h-1 and 5.7 h-1 for Pd@TRIA-1 and Pd@TRIA-2 catalysts respectively was achieved when the Et3N/HCOOH molar ratio was 1:1 (Figure 4a). A decrease in TOF values at 11.8 h-1 and 3.8 h-1 took place when the molar ratio of Et3N/HCOOH was 1.6:1. When HCOONa was used under refluxing condition (with oil bath at 100 oC) employing the Pd@TRIA-1 catalyst, no conversion of 2a took place (Table 1, entry 5). Isopropyl alcohol is well known for its excellent hydrogen donating ability in the transfer hydrogenation of alkenes. In our case, however, it can be regarded as a poor hydrogen transfer agent that provides no conversion of 2a (Table 1, entry 6). Surprisingly, for the transfer hydrogenation in MeOH, 97% conversion at 12 h was achieved (Table 1, entry 7). Upon running the identical catalytic reaction in THF, 90% conversion was attained in 12 h (Table 1, entry 8). In a non-polar solvent like toluene, 99% conversion was obtained after 16 h (Table 1, entry 9). DMF is a poor solvent for the catalytic transfer hydrogenation of 1a (Table 1, entry 10). No conversion of 2a from 1a took place in the absence of Pd (Table 1, entry 11), suggesting that integrated palladium has a decisive role to play in situ hydrogen generation from the HCOOH/Et3N reaction system for the reduction of olefins. The newly developed methodology is also effective for the large scale reaction with consistent product conversion (Table 1, entry 12). To better clarify the role of the designed porous polymer, we performed control experiments with the Pd@C, Pd@SiO2 and Pd@TiO2 catalysts. The Pd loading in Pd@C, Pd@SiO2 and Pd@TiO2 was maintained in the same manner, and TEM images of the respective materials are provided in Figure S8. 4



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Pd loading in Pd@C, Pd@SiO2 and Pd@TiO2 catalysts are 0.827 mmol/g, 0.815 mmol/g and 0.806 mmol/g respectively, as determined by ICP-MS analysis. The Pd@C, Pd@SiO2 and Pd@TiO2 catalysts (Table 1, entries 13-15) provided 100%, 20% and 16% conversion after 12 h, 8 h and 12 h respectively, suggesting that our newly developed Pd@TRIA-1 nanocatalyst is superior to others. The high performance can be explained from π−π interactions between aromatic alkene and aromatic framework of POPs along with the coordination of alkene with Pd NPs on external surface.33 Bulusheva and coworkers recently proposed that single isolated Pd atoms supported on N-doped carbon are the active sites for hydrogen production from formic acid.30 Similar finding regarding high catalytic performance in acetylene hydrogenation employing N-doped carbon nanotubes immobilized with Pd owing to NPd interaction was reported by Arrigo et al.29 Following these observations, we can emphasize the impact of N-Pd interaction in high catalytic performance.

Figure 4. (a) TOF as a function of the Et3N/HCOOH ratio using Pd@TRIA-1 and Pd@TRIA-2 catalysts with reaction time of 4 h and 10 h, respectively; (b) Catalytic transfer hydrogenation of trans-stilbene (0.25 mmol) as a function of time (h) in EtOH (5 mL) at 25 oC for Pd@TRIA-1, Pd@TRIA-2, Pd@C, Pd@SiO2 and Pd@TiO2 catalysts, respectively. For the case of catalyst removed, at t = 2 h, the catalyst was removed by filtration; (c) TOF (h-1) for transfer hydrogenation of various alkenes with Pd@TRIA-1, Pd@TRIA-2 and Pd@C catalysts; (d) Recyclability of Pd@TRIA-1, Pd@TRIA-2 and Pd@C catalysts for catalytic transfer hydrogenation of trans-stilbene.

Very recently, Verho et al. performed transfer hydrogenation employing Pd0-Amp-MCF catalyst (Table 1, entry 16) under microwave condition at 100 oC to achieve 18% conversion after 15 min.16 Ni/Ru/Pt/Au heteroquatermetallic NPs (Table 1, entry 17) have been utilized by Uozumi et al. to furnish 98% conversion in 2-propanol at 100 oC after 24 h.36 Transfer hydrogenation was performed employing Cu/DH catalyst (Table 1, entry 18) by Dhakshinamoorthy et al. to achieve 99% conversion under heating condition at 80 oC in EtOH after 8 h.15 Our group has designed Fe3O4@GO nanocatalyst (Table 1, entry 19) to carry out transfer hydrogenation

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of alkene, affording 80% conversion after 20 h.17 Basset and co-workers have developed Pd/KCC-1-NH2 catalyst (Table 1, entry 21) to show 99% conversion of trans-stilbene under microwave condition at 100 °C after 12 h.37 These comparison studies in the recent literature survey for transfer alkene hydrogenation make our newly designed Pd@TRIA nanocatalyst more promising in nature. Pd/TRIA, prepared by employing a physical mixture of TRIA polymer and Pd NPs, was also tested for this transfer hydrogenation reaction (Table 1, entry 20), but only 30% product conversion was achieved after 20 h. The greater catalytic efficiency of the Pd@TRIA-1 material than Pd/TRIA and other reported catalysts is due to the synergetic effect of nanoporous confinement and electron donation of TRIA POPs in the former. The allyloxy triazine organic backbone (either with O or N atoms) would coordinate to Pd through π-electron donations, making the Pd surface more active for efficient dehydrogenation of HCOOH to generate H2, thereby facilitating the transfer hydrogenation reaction. The results including the comparison in catalytic performance with various Pd based nanocatalysts (Pd@C, Pd@SiO2 and Pd@TiO2 catalysts) for trans-stilbene hydrogenation timeon-stream conversion (%)are provided in Figure 4b, demonstrating that Pd@TRIA-1 exhibited the highest catalytic performance among the tested five catalysts. Nearly twofold increase in TOF (TOF value calculated excluding diffusion limitation)38 value for trans-stilbene hydrogenation employing Pd@TRIA-1 catalyst (TOF of 16.5 h-1) was achieved as compared with the conventional Pd@C catalyst (TOF of 7.55 h-1). A negligible diffusion limitation for the present catalyst was considered, and this assumption can only be applicable for the small NPs under rapid stirring rates as suggested by Vannice.38 In the case of 9-vinyl carbazole, Pd@TRIA-1 and Pd@TRIA2 afforded TOF values of 9.32 h-1 and 6.81 h-1 respectively, which are significantly higher than conventional Pd@C catalyst displaying a TOF of 2.24 h-1. Two- and three-fold decreases in TOF values for Pd@TRIA-2 catalyst (TOF 6.29 h-1) and conventional Pd@C catalyst (TOF 3.22 h-1) as compared with the Pd@TRIA-1 catalyst (TOF 10.7 h-1) in phenyl styrene hydrogenation were noticed. We also investigated catalytic performance with time-on-stream profile in catalytic transfer hydrogenation of phenyl styrene and α-methyl-trans-stilbene (Figure 5) under the optimized reaction conditions by employing various Pd catalysts, suggesting that the reactivity is pronouncedly affected by Pd NP size fabricated in POP and the nature of framework supports used for the immobilization of Pd NPs. This comparison study using different Pd nanocatalysts in the catalytic transfer hydrogenation reaction of different aromatic alkenes at room temperature proves significantly enhanced catalytic performance of our catalysts, since special 3D-network structure of nanoporous triazine POP enables high dispersion and well accessibility of enriching electron density on the metallic Pd and accelerates the hydrogenation reaction as experimentally evidenced by XPS spectra in Pd-3d region (Figure 2D) and N2 adsorption/desorption isotherms (Figure 1B). The adsorption of H assisted from the cleavage of C-H bond is greatly favored with electron-enriched metallic palladium nanoclusters as reported by Tsang et al.39 Higher catalytic activity of Pd@TRIA-1 than Pd@TRIA-2 is presumably due to the fact that the face atoms of Pd NPs are exposed more extensively in the former, and thus the hydrogenation preferentially takes place on the face atoms by direct access of Pd NPs deposited on the external surface of TRIA POP. Here, negligible diffusion limitation is applicable for small particles

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with rapid stirring rates in catalytic reactions.40 A rate enhancement of formic acid decomposition assisted catalytic transfer hydrogenation was observed with the increase of (a) mole fraction of formic acid (Figure 4a) and (b) relative number of surface Pd atoms corresponding to the Pd mass fraction, signifying that more accessible Pd(111) crystalline facets could facilitate the dehydrogenation of formic acid (Figure S9) in a similar way to a previous report by Mullins et al.41 Table 1. Screening of different catalysts for catalytic transfer hydrogenation of trans-stilbene at room temperature via dehydrogenation of HCOOH/Et3N Adducta Pd-Catalyst 1a

2a

Reaction conditions

a

Reaction conditions: trans-stilbene (0.25 mmol, 0.045g), catalyst (5 mg), HCOOH (10 equivalent, 2.5 mmol, 115 mg), Et3N EtOH EtOH EtOH EtOH H2O i PrOH MeOH THF Ph-CH3 DMF EtOH EtOH EtOH EtOH EtOH EtOH

Time (h) 24 16 4 10 12 15 12 12 16 16 24 4 12 8 12 0.25

Convb (%) 0 20 99 99 0 0 97 90 99 12 0 97 100 20 16 18

EtOH EtOH EtOH EtOH MeOH

24 8 20 20 12

98 99 80 30 99

Entry

H-Donor

Catalyst

Solvent

1 2 3 4 5c 6 7 8 9 10 11 12d 13 14 15 1616

No-additive HCOOH HCOOH/NEt3 HCOOH/NEt3 HCOONa i PrOH HCOOH/NEt3 HCOOH/NEt3 HCOOH/NEt3 HCOOH/NEt3 HCOOH/NEt3 HCOOH/NEt3 HCOOH/NEt3 HCOOH/NEt3 HCOOH/NEt3 HCOOH/NEt3

Pd@TRIA-1 Pd@TRIA-1 Pd@TRIA-1 Pd@TRIA-2 Pd@TRIA-1 Pd@TRIA-1 Pd@TRIA-1 Pd@TRIA-1 Pd@TRIA-1 Pd@TRIA-1 TRIA Pd@TRIA-1 Pd@C Pd@SiO2 Pd@TiO2 Pd0-AmPMCF Ni/Ru/Pt/Au Cu/DH Fe3O4@GO Pd/TRIA Pd/KCC-1NH2

1736 1815 1917 20e 2137

i

PrOH N2H4 N2H4 HCOOH/NEt3 HCOOH

constant of 3.94 Å, which is in good agreement with the experimental value of 3.88 Å.47 Two topmost layers and the adsorbates were allowed to fully relax, while the atoms in the two bottom layers were kept fixed. The constraint on the bottom layers was adopted to reduce the computational cost while still maintaining high accuracy of the simulations.48-52 Note that the energy and geometry of the system do not change significantly when the bottom layers are fully relaxed.53 The Brillouin zone was sampled with a 2×2×1 Monkhorst-Pack grid, and repeated slabs were separated by 20 Å to minimize their interactions. Increasing the slab thickness to 5 layers, the inter-slab spacing to 25 Å, the cut-off energy to 600 eV or the grid to (4×4×1) could change the calculated binding energies by less than 1 kcal/mol. The lattice optimization and the convergence test to choose appropriate structural and simulation parameters are presented in the SI (Tables S2 and S3). The Pd(111) has been known as the most stable surface among all different facets of Pd.49-51 Therefore, the exploitation of a periodic slab is a reasonable approach to modeling the surface of 8 nm Pd NP, since the real (111) facet of the 8 nm truncated octahedral particle constitutes a very large surface including as many as 493 Pd atoms (equivalent to at least a 13×13 slab). We also evaluated the adsorption energy of the reactant (stilbene) on bigger periodic slab size of p(6×6) and the product (bibenzyl) on the periodic slab size of p(7×10), and the calculated adsorption energy of those molecules differs only by 1 kcal/mol from the corresponding values on the p(5×5) slab.

(10 equivalent, 2.5 mmol, 252 mg), solvent (5 mL), room temperature (25 oC); bDetermined by GC analysis using dodecane as GC internal standard; all the products were confirmed by GC-MS analysis; cReaction was carried out at 100 °C; dReaction was conducted with 10 mmol of trans-stilbene. Conv = Conversion; 15Ref. 15: 100 °C, µw; 16Ref. 16: 60 °C; 17 Ref. 17: 80 °C; 36Ref. 36: 100 °C; 37Ref. 37: 100 °C; e Pd@TRIA-1 was prepared by a physical mixing method. To gain theoretical insights into the observed different activities of the different-sized Pd NPs in POP, we determined the reaction energy profiles by performing DFT calculations. All calculations were performed using the Perdew-BurkeEnzerhof functional (PBE),42 a plane-wave basis set with a cut-off kinetic energy of 400 eV, and the projector-augmented wave (PAW) method43,44 as implemented in the Vienna Abinitio simulation package (VASP).44,46 Two different models were used in the study (Figure 6). To model the 8 nm particle, we used a slab approach (Figure 6a). The Pd(111) surface was modeled as a 4-layer p(5×5) slab with an optimized lattice

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Figure 5. Time-on-stream profiles for catalytic transfer hydrogenation of (A) phenyl styrene and (B) α-methyl-trans-stilbene catalyzed by Pd based catalysts.

To model the 3 nm NPs, we used a cluster approach (Figure 6b,6c). A plausible structure of the 3 nm NPs has a truncated octahedral shape54 and thus involves eight (111) facets and six (100) facets. The particle contains 1289 atoms in total (Figure 6b), and calculations on this real model are prohibitive. However, since the surface area of the (111) facet accounts for ~78% of the total surface of the 3 nm Pd NPs, it is deemed reasonable to simulate the reaction only on the (111) facet. In this study, to simulate the reaction on 3 nm Pd NPs, we used a Pd cluster that includes 3 topmost layers from the (111) facet of the real 3 nm NPs in the unit cell of 25×25×25 Å (Figure 6c). Only the Γ point was used to sample the Brillouin zone. The adsorption energy of the reactant on the thicker cluster (including 4 layers, the Supporting Information) is almost identical to that on the 3-layer cluster used herein. Transition states were located using the Climbing-Image Nudged Elastic Band (CI-NEB) method55 until the force on each relaxed atom became smaller than 0.02eV/Å. The transition state was subsequently re-optimized, and frequency calculations confirmed the nature of the transition states.

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7). There are several adsorption configurations of stilbene on the Pd surfaces, and the most stable configurations are presented in Figure 7. The most stable configuration is shown in Figure 7c, which is 6.1 and 4.7 kcal/mol more stable than the structures in Fig. 6a and 6b, respectively. These results are consistent with the trend reported in earlier theoretical studies that showed stronger binding of benzene in the “bridge configuration” on transition metals.56,57 In structure 6c, the aromatic rings are coordinated parallel to the surface with a separation of 2.2 Å, and the ethylene group -CH=CH- is bound to the surface via the di-σ configuration over the bridge site of two surface Pd atoms. The adsorption energy of stilbene in its most stable configuration on the 8 nm Pd NPs is -57.6 kcal/mol, whereas it is -62.7 kcal/mol on the 3 nm Pd NPs. The stronger binding energy of the adsorbate on a smaller particle is consistent with literature reports.58,59 Similar to the adsorption of stilbene, the product (bibenzyl) also bound more strongly on the 3 nm NPs than on the 8 nm NPs (the adsorbed structure is shown as inserts in Figure 8 and Figure S11 in the SI). The binding energies of bibenzyl on the 3 nm and 8 nm NPs are -39.4 and -36.1 kcal/mol, respectively. The stronger binding of the product on the smaller NPs makes desorption more difficult, thus reducing the overall efficiency. Since the content of steps should increase when the particle size decreases, we also evaluated the binding energy of bibenzyl on the step sites of the (211) slab and on the truncated octahedral Pd201 cluster (equivalent to a smaller 1.6 nm Pd NP, details of the calculations and structures are presented in the SI). The computed binding energy of bibenzyl on step sites of the Pd(211) slab is -43.1 kcal/mol, which is 7.0 kcal/mol larger in magnitude than on the Pd(111) terrace sites. The larger adsorption energy of bibenzyl on the (211) facet implies that an increase in the ratio of step sites as a result of a decrease in the size of the NP makes the desorption of the product even more difficult. We also examined the binding on a smaller 1.6 nm Pd particle (with the truncated octahedral cluster Pd201). The calculated binding energy of bibenzyl on the terrace of (111) facet of the 1.6 nm particle is -40.6 kcal/mol. On the step edge of (111) and (100) facets, it is -48.6 kcal/mol (Figures S12S14 in the SI), which again validates the trend that bibenzyl binds more strongly on a smaller particle. Our argument is consistent with the study by Bao et al.,60 who demonstrated that the efficiency of CO production is diminished as a result of sluggish CO removal on edge and terrace sites, which occurs when the size of the NP is decreased.

Figure 6. (a) Slab model used to simulate the reaction on the 8 nm Pd NPs; (b) Real model of the 3 nm Pd NPs with truncated octahedral shape, which include 1289 Pd atoms per NP; c) Cluster model used to simulate the reaction on the 3 nm Pd NPs.

The relative energy of the surface intermediate C14H12+x* along the reaction profile was computed by assuming that the total energy of the reactant in the gas phase, clean surface and surface hydrogen is the reference energy for the reaction: C14H12 (gas) + (1-x)* + xH* C14H12+x*, where * and H* denote the total energy of the clean surface and the total energy of chemisorbed hydrogen, respectively. Firstly, we examined the adsorption of the reactant (stilbene) on the Pd surfaces. Aromatic compounds can adapt two stable adsorption configurations, depending on the coordinated position of the ring to the surface, namely, the “hollow” and “bridge” sites (Figure

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ACS Applied Materials & Interfaces surface in the reaction on the 3 nm NPs, which means that the product is more reluctant to dissociate in this case. These computationally derived trends explain the experimental observation that the reaction was slower on the 3 nm NPs than on the 8 nm NPs. Table 2. Dehydrogenation of formic acid assisted transfer hydrogenations of various alkenes catalyzed by Pd@TRIA-1 Entrya

Alkene

Alkane

p-ClC6H4 1

H

Time (hr)

Conb(%)

H3C

p-ClC6H4

5

97

H3C

p-FC6H4

5

99

H3C

p-FC6H4

6

99

H3C

p-OCH3C6H4

6

98

6

99

4

99

7

95

6

99

H H p-FC6H4

2

H

H H

3

p-CH3C6H4 H

H H

4

p-OCH3C6H4 H

H H

5

Ph

Ph

H

H3C

Ph

Ph

H 6

Ph H

Ph

H

Ph

Ph H 7

Ph

Ph

COPh

COPh

H 8

p-C(CH3)3C6H4 H

H3C

p-C(CH3)3C6H4

H H p-BrC6H4

9

H

H3C

p-BrC6H4

12

20

H3C

p-NH2C6H4

5

95

H3C

p-B(OH)2C6H4

6

98

8

97

10

85

7

99

7

100

6

96

15

55

H H p-NH2C6H4

10

H

H H

11

p-B(OH)2C6H4 H

H H

12

Ph

Ph H3C

H3C

H

Ph

Ph 13

Figure 7. Different adsorbed configuration of stilbene including (a) Hollow, (b) Bridge-1, and (c) Bidge-2 on the Pd(111) surface. Inserted values are relative energies. Structures (7b) and (7c) differ by the position of the benzene ring with respect to the bridge site, indicated by the dash line connecting two atoms in the figures.

14

Since hydrogenation and dehydrogenation are considered to be insensitive to the surface structure in the literature,61,62 and the terrace accounts for 78% of the total active surface area,54 we only evaluated the reaction energy profile on the terrace. The reaction energy profiles of the stilbene hydrogenation by a surface chemisorbed H on the terraces of the 8 nm and 3 nm Pd NPs are shown in Figure 8. The activation energy barriers for the first and second hydrogenation on the 8 nm NPs are 26.3 and 25.8 kcal/mol, respectively. However, on the 3 nm Pd NPs, the corresponding activation barriers are higher for both of the two steps: the first hydrogenation on the 3 nm NPs can only be facilitated with the activation barrier of 39.2 kcal/mol, and the barrier energy for the second hydrogenation is 38.6 kcal/mol. Although these computed barrier heights appear to be too large and thus may be overestimated, a comparison of the barriers for the different sized NPs suggests that the hydrogenation of stilbene on the 3 nm NPs is slower. Furthermore, the product binds more strongly to the

17

H CH3

H H 15 H N 16

N

Ph

CH3

H H

CH2OH

Ph

CH2OH

a

Reaction conditions: aromatic alkene (0.25 mmol), Pd@TRIA-1 (5 mg), HCOOH (10 equivalent, 2.5 mmol, 115 mg), Et3N (10 equivalent, 2.5 mmol, 252 mg), EtOH (5 mL), room temperature (25 oC); bDetermined by GC analysis using dodecane as GC internal standard; all the products were confirmed by GC-MS analysis. We then examined the substrate scope of the catalytic transfer hydrogenation reaction for various olefins employing the Pd@TRIA-1 nanocatalyst under the optimized reaction conditions. Table 2 signifies that the synthesized Pd@TRIA-1 nanocatalyst is highly effective to reduce mono- and disubstituted alkenes with the catalytic transfer hydrogen from the HCOOH/Et3N reaction mixture. Our protocol was tolerant in the presence of chloro (-Cl) and fluoro (-F) functional groups,

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affording excellent conversions of the corresponding saturated alkanes after 5 h (Table 2, entries 1,2). It was observed that electron-donating styrenes required long reaction times to provide the corresponding substituted ethylbenzenes. The catalytic transfer hydrogenation reactions proceeded well when methyl, methoxy groups and tert-butyl (p-Me, p-OMe and tBu) were installed on the styrenes, delivering the corresponding desired products in 99% conversion (Table 2, entries 3, 4 and 8), and the reaction finished within 6 h. Vinyl substituted polycyclic aromatic hydrocarbon, 2-vinyl naphthalene, could easily participate in the reaction to give the 99% product conversion after 7 h (Table 2, entry 14). Successful catalytic transfer hydrogenation was conducted for di-substituted alkenes. α-Phenylstyrene underwent the transfer hydrogenation reaction smoothly to give 99% product conversion after 6 h (Table 2, entry 5). Similarly, sterically hindered α-methyltrans-stilbene (Table 2, entry 12) exhibited 97% conversion after 8 h. The Pd-catalyst was also successful in the hydrogenation of 9-vinylcarbazole (Table 2, entry 15) with 100% conversion after 7 h. Chemoselective reduction of the C=C bond of α,βunsaturated ketones was fruitfully achieved employing the newly developed methodology. Chalcone and cinnamyl alcohol (Table 2, entries 7 and 16) gave 95% and 96% product conversion of the corresponding saturated alkanes after 7 h and 6 h, respectively. Surprisingly, very poor conversion was obtained for Br-substituted styrene (Table 2, entry 9). Only 20% product conversion was obtained after 20 h of the catalytic reaction. It was assumed that the Br functional group somehow got activated with the Pd0 catalyst, possibly inhibiting the progress of the reaction. When –NH2 and –B(OH)2 functional groups were attached to styrene, the reactions completed after 5 h and 6 h to afford the corresponding products with 95% and 98% conversions, respectively (Table 2, entries 10 and 11). Norbornene, a cyclic olefin, reached 85% product conversion after 10 h (Table 2, entry 13). Surprisingly, very poor conversion was achieved for cis-cyclooctene (Table 2, entry 17), which may be due to the presence of almost orthogonal allylic C-H bonds. Although widespread mechanistic study was not yet performed, the decomposition of the transient Pd-formate species (as proposed in Scheme S1) indicating the formic acid degradation may be considered as the rate-determining step, and NEt3 may play a crucial role in facilitating the reaction progress (Figure 4a) with the dehydrogenation of proton from formic acid. Cao et al. reported that exclusive amine promoted formic acid decomposition reaction employing Au-ZrO2 catalyst was unimolecular in nature with preliminary mechanistic studies.62 From the proposed mechanism in Scheme S1, the alkene-Pd intermediate can undergo a reductive elimination to liberate alkane by the surface hydrogen on Pd.

2.3 Reusability of Pd@TRIA-1 nanocatalyst for catalytic transfer alkene hydrogenation Pd@TRIA-1 catalyst presents tremendous recyclability with a consistent conversion for trans-stilbene hydrogenation at least 10th competitive catalytic cycles without significant loss of catalytic activity. Detailed procedure of reusability test is provided in the SI. As shown in Figure 4d, Pd@TRIA-2 catalyst could be reused for at least five times. When Pd@C was used, the conversion of trans-stilbene decreased to 42% and 18% in the 3rd and 4th catalytic runs, respectively. To confirm that the Pd@TRIA-1 nanocatalyst is indeed heterogeneous in nature, we conducted leaching test and Hg(0)-poisoning

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test (See Experimental Section) considering catalytic transfer hydrogenation of trans-stilbene in the presence of HCOOH and Et3N reaction mixture as a model reaction. This experimental result clearly signifies that our Pd@TRIA-1 nanocatalyst is truly heterogeneous in nature and Pd(0) NPs are strongly anchored with the allyloxy triazine organic backbone in such a way that the no obvious leaching of Pd metal in the reaction mixture took place during the course of reaction. AAS (atomic absorption spectroscopy) analysis result suggests that Pd content in the filtrate is below the detection limit of our instrument and the filtrate is completely colorless. Negligible change in Pd content in each catalytic cycle was observed (Table S4 in the SI). After the 10th catalytic run, the Pd content in Pd@TRIA-1 nanocatalyst was found to be 0.7648 mmol/g, which is still comparable with that of the fresh catalyst (0.7659 mmol/g). No leaching of Pd (Table S4) further illustrates that PdPOP catalysts did not show significant deactivation during the ten consecutive catalytic runs. After the 10th catalytic run, only a decrease in the product conversion in 12th catalytic run (65%) was observed, which may be due to the blockage of some active sites by the accumulation of intermediate products.63 TEM images (Figure S15) after the 4th, 6th and 10th cycles predict that Pd(0) NPs are strongly anchored and well dispersed on the external surface of TRIA-POP by maintaining size within 7-8 nm with no obvious aggregation. The characterization of reused Pd@TRIA-1 catalyst after the 12th catalytic run (Figures S15 and S16) predicts that the prepared catalyst is very stable and sustainable over multiple reuses and could withstand the catalytic reaction conditions. The BET surface area of the reused Pd@TRIA-1 catalyst is 510 m2/g, which is comparable with the fresh catalyst (573 m2/g). The preservation of BET surface area of the reused Pd@TRIA-1 catalyst signifies high recyclability of the catalyst, revealing no sign of pore wall destruction and pores clogging (Figure S16D in the SI). The BET surface area of reused Pd@TRIA-2 catalyst was significantly decreased from 459 m2/g to 232 m2/g (Figure S17 in the SI), indicating the destruction of pore wall with the loss of porosity to hinder the accessibility of catalyst sites. This obervation led to a hypothesis for the reduction of catalytic activity for the Pd@TRIA-2 catalyst. A similar phenomenon regarding the low recyclability of POP based catalyst was also reported by Nguyen et al.64

3. CONCLUSION In conclusion, we have successfully achieved triazine functionalized POP employing one-pot facile non-aqueous polymerization technique, and facile fabrication of Pd NPs with two different sizes (8 nm and 3 nm). A simple and successful strategy has been discovered for catalytic transfer hydrogenation of alkenes at room temperature with efficient H2 generation by dehydrogenation of formic acid in the presence of Pd@TRIA catalyst. Pd-nanocatalyst (8 nm particle size) exhibits better catalytic performance compared with the Pdnanocatalyst (3 nm particle size). Enhancement in catalytic performance for large size Pd NPs is due to the increase in binding energy of reactant and the decrease in binding of product, which is postulated by density functional theory computational model studies. Reusability, no leaching of Pd and no use of flammable H2 gas at high pressure and temperature make the present catalyst and protocol a prospective candidate in heterogeneous catalysis.

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Figure 8. Energy profile for stilbene hydrogenation on the 8 nm and 3 nm Pd NPs. Inserted figures represent the structure of intermediate on the p(5×5) model of the 8 nm particle (structures on the 3 nm cluster are similar). Inserted values are computed activation energies in kcal/mol. I1: Reservoir of stilbene (gas) + clean surface + surface H; I2: adsorbed stilbene; I3: initial state for the first hydrogenation; I4: Transition state for the first hydrogenation (TS1); I5: Intermediate 1; I6: initial state for the second hydrogenation; I7: transition state for the second hydrogenation (TS2); I8: adsorbed product bibenzyl; I9: products (gas) + clean surface.

4. EXPERIMENTAL SECTION Synthesis of nanoporous organic polymer TRIA: In a typical synthesis procedure, divinylbenzene (5.99 mmol, 781 mg), 2,4,6-triallyoxy-1,3,5-triazine (1.49 mmol, 373 mg) and AIBN (0.152 mmol, 25 mg) were mixed together in a round bottomed flask containing acetone (15 mL) and the resulted mixture was allowed to stir at room temperature under nitrogen atmosphere for 6 h. Then, the resultant mixture was hydrothermally treated in an autoclave at 120 °C under static condition for 24 h. The final white color solid material TRIA was isolated and dried in air. Synthesis of Pd@TRIA-1 material: In a typical synthesis procedure, polymer TRIA (0.150 g) was dispersed in methanol (20 mL) by sonication. Then Pd(OAc)2 (0.04 g) was added in the solution, and the resulting mixture was allowed to reflux at 65 oC under N2 atmosphere for 6 h. After that the black color solid material was isolated by simple filtration and washed with methanol for 2-3 times. Then, black color material was dried in air and designated as Pd@TRIA-1. Synthesis of Pd@TRIA-2 material: To a solution of TRIA polymer (0.150 g) in methanol (50 mL), Pd(OAc)2 (0.04 g) was added and stirred for 24 h at room temperature. After that yellow color precipitate was isolated by simple filtration technique, the recovered yellow Pd-polymer composite was vigorously dispersed in water (100 mL) at room temperature for 2

h. Sodium acetate (100 mg ) was added into the solution followed by addition of NaBH4 (1 mL 1(M) ) solution, and the mixture was stirred for 15 minutes. After that blackish grey material was obtained by simple filtration, which is denoted as Pd@TRIA-2. The residual acetate was removed from the composite material by through washing with water followed by methanol. Room temperature catalytic alkene hydrogenation with dehydrogenation of HCOOH catalyzed by Pd@TRIA catalyst: In a typical catalytic alkene hydrogenation procedure, the respective alkene (0.25 mmol) with Pd@TRIA nanocatalyst (5 mg) in absolute ethanol (5 mL) was placed in a dry 25 mL sealed tube. Then, the Pd-catalyst was dispersed in the mixture by sonication thoroughly. A mixture of HCOOH (10 equivalent, 2.5 mmol, 115 mg) and Et3N (10 equivalent, 2.5 mmol, 252 mg) was added dropwise into the prepared reaction mixture. The tube was capped tightly and the reaction mixture was allowed to stir at room temperature (25 oC) for the time referred. The reaction was monitored periodically by analyzing the reaction mixture with GC-MS until the full conversion of substrate. The products were confirmed by employing GC-MS analysis technique. The conversions (%) of the corresponding alkanes products were determined by using dodecane as the internal standard. Leaching test: In a typical catalytic transfer hydrogenation reaction, a mixture of trans-stilbene (0.25 mmol, 0.045 g),

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Pd@TRIA-1 catalyst (5 mg), HCOOH (2.5 mmol, 115 mg), and Et3N (2.5 mmol, 252 mg) in ethanol (5 mL) was stirred at room temperature in a sealed tube. After 2 h of the reaction, only 60% product conversion was achieved after 2 h, as determined by GC analysis using dodecane as internal standard. The reaction mixture was removed quickly from the solid Pd catalyst by centrifugation, and then transferred to another reaction vessel under the identical conditions. No additional increase in the conversion was observed during 10 h continues reaction in the absence of Pd@TRIA-1 (Figure 4b). Hg(0)-poisoning test: we conducted Hg-poisoning test for significant inhibition of the catalytic transfer hydrogenation reaction in order to indicate that the active catalytic Pd-species is heterogeneous in nature. Two sets of reactions for catalytic transfer hydrogenation using trans-stilbene (0.25 mmol, 0.045 g), Pd@TRIA-1 catalyst (5 mg), HCOOH (2.5 mmol, 115 mg), and Et3N (2.5 mmol, 252 mg) in ethanol (5 mL) were allowed to stir at room temperature, and one of them was regarded as a control experiment. After 2 h, 60% and 62% conversion of trans-stilbene were measured by GC-FID. After that, excess elemental mercury (100 mg) was introduced in one reaction setup and the reactions were continued for additional 2 h. After 2 h consequent analysis by GC-FID, it was found that the quenching of hydrogenation reaction in Hg added reaction took place, affording 62% trans-stilbene conversion owing to the formation of Pd-Hg amalgam on the Pd-NP surface by the introduction of Hg(0). On the other hand, the reaction mixture without added elemental Hg achieved 100% conversion of trans-stilbene after additional 2 h continuation of reaction. Supporting Information. TGA data of all materials, elemental analysis data, UV-Vis spectra, pore size distributions, EDX spectra and elemental mapping, FE-SEM images, TEM images of TRIA polymer, wide angle powder XRD patterns, details and results of the DFT calculations, reused catalyst characterization, and test for heterogeneous nature of catalyst. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected], [email protected]

Notes The authors declare no competing financial interest.

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This research was supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) Programme-Singapore Peking University Research Centre for a Sustainable Low-Carbon Future, and the NTU-A*Star Silicon Technologies Centre of Excellence under grant no. 11235100003. HH is grateful for a JST-PRESTO grant and a Nanyang Assistant Professorship. QTT, WKHN and HH thank the High-Performance Computing Centre at Nanyang Technological University for com-

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puter resources. JM and AJ thank to Department of Science & Technology (DST), India for DST-INSPIRE Faculty Research project grant (GAP-0522) in CSIR-IICT, Hyderabad.

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(64) Totten, R. K.; Kim, Y.S.; Weston, M. H.; Farha, O. K.; Hupp, J. T.; Nguyen, S. T. Enhanced Catalytic Activity through the Tuning

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of Micropore Environment and Supercritical CO2 Processing: Al(Porphyrin)-Based Porous Organic Polymers for the Degradation of a Nerve Agent Simulant. J. Am. Chem. Soc. 2013, 135, 11720-11723.

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TOC Figure

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Size-Dependent Catalytic Activity of Palladium Nanoparticles

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