Metal-Nanoparticle-Catalyzed Hydrogen ... - ACS Publications

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Metal-Nanoparticle-Catalyzed Hydrogen Generation from Formic Acid Zhangpeng Li† and Qiang Xu*,†,‡ †

Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan ‡ AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), Yoshida, Sakyo-ku, Kyoto 606-8501, Japan CONSPECTUS: To meet the ever-increasing energy demand, the development of effective, renewable, and environmentally friendly sources of alternative energy is imperative. Hydrogen (H2) is a renewable, clean energy carrier, which exhibits a threefold energy density compared to gasoline; H2 is considered one of the most promising alternative energy carriers for enabling a secure, clean energy future. However, the realization of a hydrogen economy is restricted by several unresolved issues. Particularly, one of the most difficult challenges is the development of a safe, efficient hydrogen storage and delivery system. To this end, hydrogen storage techniques based on liquid-phase chemical hydrogen storage materials have become an attractive choice. Formic acid (FA) with a high volumetric capacity of 53 g H2/L demonstrates promise as a safe, convenient liquid hydrogen carrier. However, generating H2 from FA in a controlled manner at ambient temperature is still challenging, which primarily depends on the catalyst used. Hence, for practical purposes, it is imperative to develop high-performance heterogeneous catalysts for the dehydrogenation of FA. Ultrasmall metal NPs with a high surface-to-volume ratio and “clean” surface, and hence a high density of active sites exposed to reactants, are of significance for heterogeneous catalysis. However, the size of these “clean” ultrasmall metal NPs inevitably increase, and these particles undergo aggregation during synthesis and catalysis because of their high surface energy. The immobilization of metal NPs into appropriate support materials affords considerable advantages for catalytic applications, which not only offers spatial confinement to control the nucleation and growth of particles, but also prevents them from aggregation; hence, catalytic performance is significantly enhanced. In addition, the functionalization of the support with electron-rich groups is beneficial to the formation of intermediates for FA dehydrogenation, which in turn promotes the catalytic performance. In this Account, studies of hydrogen generation from FA using heterogeneous catalysts were reviewed, mainly focusing on the results reported by our group. By varying support materials (metal−organic frameworks, silica, graphene, and porous carbons) and synthetic strategies, a wide range of highly active metal NP catalysts for efficient H2 generation from FA under mild conditions were developed. In addition, the design and synthetic strategies were described, by which the size and composition of the NPs, as well as the well-defined NPs-support interactions, can be controlled for the enhancement of catalytic performance for the FA dehydrogenation. Furthermore, the performance of the prepared catalysts for the effective release of H2 from FA for the purpose of liquid-phase chemical hydrogen storage was discussed. Finally, the challenges, expected improvements, and future opportunities in this research area were summarized.

1. INTRODUCTION

content (4.4 wt %), nontoxicity, and easy storage and transportation, and it is regarded as a potential carrier for the production and storage of hydrogen.4−8 Hydrogen can be generated from FA via a dehydrogenation pathway (HCOOH → CO2 + H2). The dehydration pathway (HCOOH → CO + H2O) producing CO as the impurity, which is toxic for fuel cell catalysts, is an undesired side reaction and must be strictly controlled.9,10 The selectivity and reactivity of FA decomposing to CO2 and H2 are crucial for FA-based hydrogen storage, in which CO2 as the only product, besides H2, can be recycled to

Hydrogen, the most abundant element in the universe, has attracted increasing attention as one of the most promising energy carriers because of its high gravimetric energy density (nearly three times greater than that of gasoline), and environmentally benign nature.1 Currently, the effective, safe storage and delivery of hydrogen are still key challenges for the implementation of a hydrogen economy. Liquid organic compounds, also known as liquid organic hydrogen carriers (LOHC), are the most popular potential candidates for hydrogen storage because of their convenient transportation, refueling, and handling.2,3 Among LOHC materials, formic acid (FA, HCOOH) is one of the major products from biomass processing. FA exhibits characteristics of a high hydrogen © 2017 American Chemical Society

Received: March 19, 2017 Published: May 19, 2017 1449

DOI: 10.1021/acs.accounts.7b00132 Acc. Chem. Res. 2017, 50, 1449−1458

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Accounts of Chemical Research Scheme 1. Summary of Recent Developments of Heterogeneous Catalysts for the Dehydrogenation of FAa

a

The image of the Au@SiO2_AP is adapted with permission from ref 44. Copyright 2012 Royal Society of Chemistry. The image of the Pd/C_m is adapted with permission from ref 51. Copyright 2015 American Chemical Society.

render a carbon-neutral cycle,11−13 and they are strongly dependent on the catalysts used. Thus far, both homogeneous and heterogeneous catalysts for the selective liquid-phase dehydrogenation of FA have been developed.14−25 Heterogeneous catalysts have attracted particular interest because their use allows for facile separation, recycling, and low operating temperatures. Ultrasmall metal nanoparticles (NPs) typically exhibit superior catalytic performance compared to their larger counterparts for the dehydrogenation of FA. However, ultrasmall NPs are thermodynamically unstable because of their high surface energies; thus, to stabilize these NPs, capping agents are frequently used during NP synthesis. Inevitably, the NPs surfaces typically suffer from contamination by the capping molecules, leading to the significant loss of catalytic activity. The immobilization of highly dispersed metal NPs to well-designed supports is a promising approach to produce stable ultrasmall NPs with clean surfaces (uncapped) and ensure optimum catalyst utilization. Over the years, numerous support materials, e.g., metal−organic frameworks (MOFs),26−30 carbon,31,32 metal oxides,33 and zeolites,15,34 have been used to immobilize metal NPs to improve catalytic performance. Each support exhibits its own advantages, but all materials exhibit the same general properties, e.g., high surface area, well-defined porosity, and strong support-NP interactions. Aiming at the efficient production of CO-free H2 from the dehydrogenation of FA, our group has developed a series of heterogeneous catalysts involving the immobilization of highly active catalysts into appropriate supports utilizing various synthetic strategies (Scheme 1). In this account, the recent advances in the preparation of metal NPs were emphasized, and their catalytic performance for FA dehydrogenation for the purpose of efficient hydrogen storage and delivery, as well as the challenges, expected improvements, and future outlook in this research area, was discussed.

MOFs exhibit high surface area, porosity, and chemical tenability, the loading of metal NPs into their pores allows for the control of the particle nucleation and growth in a nanosize region, affording enhanced catalytic performance.37−39 In 2011, our group reported the use of bimetallic Au−Pd NPs as efficient catalysts for the generation of hydrogen from FA. These catalysts are immobilized into a mesoporous MOF, MIL101(Cr), which is prepared by simple liquid impregnation (Figure 1A).26 MIL-101(Cr) with window sizes of ∼1.2 and 1.6 nm and two types of mesopores (∼2.9 and 3.4 nm) demonstrate advantages as an instinct support for facilitating the encapsulation of metal NPs and the adsorption of the catalytic substrate inside the pores.37 Moreover, to improve the interactions between the metal precursors and MIL-101(Cr) support, electron-rich ethylenediamine (ED) was grafted onto coordinatively unsaturated Cr(III) centers to form ED-MIL101; ED-MIL-101 exhibits enhanced ability for the immobilization of small metal NPs. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images indicated that the sizes of the bimetallic Au−Pd NPs immobilized into ED-MIL-101 and MIL-101 are in the ranges of 2−8 and 2−15 nm, respectively (Figure 1B,C). The prepared Au−Pd/ED-MIL-101 catalyst (Au−Pd loading: 20.4 wt %; Au:Pd = 2.46) exhibits superior catalytic performance compared to those of other counterparts, over which 3 mmol of FA can be completely converted to H2 and CO2 in 65 min (nAuPd/nFA = 0.0085) at 90 °C (Figure 1D). The enhanced activity is related to the smaller particle sizes of the Au−Pd/ ED-MIL-101 catalyst by the introduction of ED into MIL-101 and the bimetallic synergy between Au and Pd.26 The prepared catalysts represent the first highly active metal catalyst immobilized into the MOF for the complete conversion of FA to high-quality H2 at a convenient temperature for the purpose of liquid chemical hydrogen storage. This study sheds light on a new route to immobilize metal NPs for the complete dehydrogenation of FA. Thus far, metal NPs supported on various MOFs, including NH2-MIL-125,27 MIL-100(Fe),28 ZIF-8,29 and NH2−UiO-66,30,40 for the dehydrogenation of FA have been widely investigated, which largely extend the functions of MOF-based catalysts.

2. METAL NANOPARTICLES IMMOBILIZED INTO MOF FOR THE DEHYDROGENATION OF FORMIC ACID MOFs, also known as porous coordination polymers, are compounds composed of metal ions or metal clusters coordinated to organic ligands. MOFs have emerged as a new class of promising hybrid functional materials.35,36 Because 1450


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Figure 1. (A) Schematic illustration of immobilization of Au−Pd NPs to MIL-101. HAADF-STEM images of (B) Au−Pd/ED-MIL-101 and (C) Au−Pd/MIL-101. (D) Volume of the generated H2 versus time for hydrogen generation from FA in the presence of 20 mg of different metal NP catalysts, 140 mg of FA, 70 mg of sodium formate (SF), and 1.0 mL of water at 90 °C. Adapted with permission from ref 26. Copyright 2011 American Chemical Society.

Figure 2. (A) Schematic illustration for syntheses of (a) Au@SiO2, (b) Au@SiO2_EN, and (c) Au@SiO2_AP. (B) Volume of the generated gas (CO2 + H2) versus time for hydrogen generation from the aqueous solution (1.0 mL) of FA (3.0 M) and SF (1.0 M) in the presence of different Au NP catalysts (60 mg, 2 wt % Au) at 90 °C. Adapted with permission from ref 44. Copyright 2012 Royal Society of Chemistry.

3. METAL NANOPARTICLES IMMOBILIZED INTO SILICA FOR THE DEHYDROGENATION OF FORMIC ACID Nanostructured silica (SiO2) with striking characteristics of cost-effectiveness, easy synthesis, high specific surface area, interconnected porosity, and tunable pore size is a particularly promising as the support for the immobilization of metal NPs for a wide range of reactions. The silanol groups located on the silica surface facilitate their functionalization with organic functional groups, such as amino or thiol groups, via covalent grafting,41−43 which can act as strong ligands to anchor metal ions onto the silica surface and then stabilize the resulting metal NPs. Our group has synthesized a high-performance nanoreactor composed of amine-functionalized Au NPs encapsulating within silica nanospheres, using HAuCl4 as the precursor in a polyoxyethylene-nonylphenyl ether/cyclohexane reversed micelle system followed by NaBH4 reduction (Figure 2A).44 Au@ SiO2 prepared under different conditions exhibits distinct catalytic performance for the decomposition of aqueous FA. The presence of amine in the silica sphere leads to the high activity of Au NPs and selectivity for the decomposition of FA to H2 and CO2, while the Au NPs encapsulated in SiO2 without amine functionalization are inactive for this reaction (Figure 2B). At 90 °C, 3.0 mmol of FA is completely converted to H2 and CO2 in 360 and 240 min in the presence of 60 mg of Au@

SiO2_EN and Au@SiO2_AP (AP: 3-aminopropyltrimethoxysilane), respectively (nAu/nFA = 0.002). Notably, all Au NPs supported on the outer SiO2 surface are inactive irrespective of whether the outer surface is functionalized with amine, indicating that the encapsulation of Au NPs inside the aminefunctionalized SiO2 nanospheres is crucial for achieving a suitable environment around the Au NPs for the effective catalytic dehydrogenation of FA (Figure 2B). Interestingly, the effects from FA and reaction temperature are confirmed to lead to an activity improvement during the first run, after which enhanced reaction rates are observed in the subsequent run over the Au@SiO2_EN or Au@SiO2_AP catalysts (Figure 2B).44 By utilizing a similar strategy, Pd NPs encapsulated within hollow silica nanospheres (Pd@SiO 2 ) are synthesized, exhibiting high catalytic activities for the decomposition of FA at convenient temperatures.45 Recently, Zhang and co-workers have used Schiff basemodified SiO2 (Schiff-SiO2) as the support to encapsulate Au NPs by in situ reduction.46 The prepared Au@Schiff-SiO2 catalyst exhibits good performance for the generation of H2 by FA dehydrogenation, related to the synergistic mechanism between the electronegative Au NPs and the protonated Schiff 1451


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Figure 3. (A) Schematic illustration for Pd/MSC-30 nanocatalyst preparation via a NaOH-assisted reduction approach. (B) TEM (inset SAED) and (C) HAADF-STEM images of the Pd/MSC-30 prepared with NaOH. (D) TEM image of the Pd/MSC-30 prepared without NaOH. (E) Volume of the generated gas (CO2 + H2) versus time for gas evolution from the aqueous FA-SF (1:1) solution over Pd/MSC-30 prepared (a) with and (b) without 0.5 mL of 2.0 M NaOH added (nPd/nFA = 0.01, 50 °C), and (c) the hydrolytic dehydrogenation of SF over the Pd/MSC-30 prepared with NaOH (nPd/nSF = 0.01, 50 °C). (F) Volume of the generated gas (CO2 + H2) versus time for the dehydrogenation of FA-SF (1:1) at different temperatures over the Pd/MSC-30 prepared with NaOH (nPd/nFA = 0.01). Inset of (F): Arrhenius plot (ln(TOF) vs 1/T). Adapted with permission from ref 49. Copyright 2014 Royal Society of Chemistry.

4.1. Metal Nanoparticles Immobilized to Commercially Available Carbons

base at the interface for C−H activation in FA, as well as the encapsulating structure of Au NPs in the support.

Active carbons (ACs) with large surface areas and excellent adsorption capacities have been widely employed as supports in several important heterogeneous catalysis reactions. In particular, porous AC is the most preferred one because the additional confinement effect can effectively prevent NPs from overgrowth and aggregation. In 2014, highly dispersed Pd NPs are successfully immobilized into nanoporous carbon, Maxsorb MSC-30, by a sodium hydroxide (NaOH)-assisted reduction approach (Figure 3A).49 NaOH serves as an efficient dispersing agent to control the size distribution of Pd NPs during synthesis. TEM and HAADF-STEM images of Pd/MSC-30 prepared with NaOH indicated that the Pd NPs are highly dispersed into the MSC-30 framework, with an average size of 2.3 ± 0.4 nm (Figure 3B,C). In sharp contrast, a larger average particle size of 3.6 ± 0.6 nm is observed for Pd NPs loaded without NaOH (Figure 3D), confirming that NaOH plays a crucial role in controlling the growth of small-sized Pd NPs.49 In addition, the amount of N2 adsorption appreciable decreases after the loading of Pd NPs, indicating that Pd NPs are

4. METAL NANOPARTICLES IMMOBILIZED INTO CARBON FOR THE DEHYDROGENATION OF FORMIC ACID Carbon with unique properties of a large specific surface area, high porosity, low density, and excellent chemical stability has been advocated as a leading material in several important fields. By the fine engineering of its structure and modification of its surface, carbon materials with high surface areas and large pore volumes have been widely used as catalyst supports to improve the dispersibility and stability of NPs, thereby promoting catalytic performance.47,48 In the following sections, carbon supports have been classified into three types, i.e., commercial carbon, graphene, and N-doped carbon, respectively, and the syntheses of metal NPs immobilized by these carbon materials and catalytic performance for FA dehydrogenation have been discussed. 1452


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Figure 4. (A) Schematic illustration for the preparation of Pd/C_m catalyst via a methanol-mediated WCGA. (B) TEM and (C) HAADF-STEM images of the Pd/C_m catalyst. (D) Volume of the generated gas (CO2 + H2) versus time for the dehydrogenation of FA-SF at different temperatures over the Pd/C_m catalyst (FA:SF = 1:1, nPd/nFA = 0.006). (E) TEM image of the Pd/C_m catalyst recycled from the reaction solution. (F) XPS spectra of Pd element in the as-prepared and recycled Pd/C_m, and Pd2+/C. (G) XRD patterns of carbon support, as-prepared and recycled Pd/C_m. Adapted with permission from ref 51. Copyright 2015 American Chemical Society.

successfully dispersed into the pores of the MSC-30 host,50 similar to the case of MOF-loaded metal NPs.26 The turnover frequency (TOF) for FA dehydrogenation reaches 2623 h−1 at 50 °C over the Pd/MSC-30 catalyst prepared with NaOH; this value is considerably greater than that over the catalyst prepared without NaOH (Figure 3E). Even at 25 °C, a high TOF of 750 h−1 is obtained for the complete dehydrogenation of FA (Figure 3F). Notably, the loss of catalytic activity and selectivity is negligible after 5 cycles at 50 °C for the Pd/MSC-30 catalyst, indicating that this catalyst exhibits high stability during FA decomposition. This result is further confirmed by XRD and TEM observations.49 More recently, our group has developed a facile methanolmediated weakly capping growth approach (WCGA) using anhydrous methanol as a mild reductant and a weak capping agent for immobilizing Pd NPs onto Vulcan XC-72R carbon nanospheres (Pd/C_m) (Figure 4A).51 TEM and HAADFSTEM images of the as-prepared Pd/C_m catalyst revealed that the highly dispersed Pd NPs with an average size of 1.4 ± 0.3 nm are immobilized on carbon supports (Figure 4B,C). Remarkably, the Pd NPs exhibit an exceedingly high catalytic activity for the completely selective dehydrogenation of FA at ambient temperatures, over which a TOF of 7256 h−1 is obtained at 60 °C, as well as an average H2 generation rate of up to 43 L H2 gPd−1 min−1 (Figure 4D). The TEM image of the

catalyst collected after the reaction suggested that the high dispersion of the Pd NPs is maintained during the reaction (Figure 4E). The XPS and XRD results for the catalysts suggested that the surface electronic structure (Figure 4F) and crystallinity (Figure 4G) of Pd(0) in Pd/C_m are maintained during catalysis.51 This facile, effective methanol-mediated WCGA, as well as the excellent performance of the catalysts prepared using this method, highlights new opportunities for the effective utilization of FA for hydrogen storage. 4.2. Metal Nanoparticles Immobilized onto Graphene-Based Materials

Compared to other carbon materials, graphene and its derivatives have demonstrated significantly enhanced catalytic performance for several reactions because of the large specific surface area, unique interaction with active particles, and excellent chemical tolerance of graphene.52 Among its derivatives, graphene oxide (GO) with abundant oxygencontaining functional groups, e.g., hydroxyl, carbonyl, and carboxyl groups, can offer numerous perspectives for anchoring and dispersing metal ions and metal NPs, making it extremely attractive as supports for heterogeneous catalysts. Despite these advantages, there is a lack of studies reporting the syntheses of highly dispersed noble metal NPs with high catalytic activity on graphene-based supports via new synthetic strategies. 1453


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Figure 5. (A) Schematic illustration of immobilization of AgPd NPs on rGO by a non-noble metal sacrificial approach (NNMSA). (B) Volume of the generated gas (CO2 + H2) versus time for the dehydrogenation of FA over (Co6)Ag0.1Pd0.9/rGO at different temperatures (nAgPd/nFA = 0.02, nSF/nFA = 2.5). TEM images of the as-prepared (C) Ag0.1Pd0.9/rGO, and (D, E) (Co6)Ag0.1Pd0.9/rGO catalysts. Adapted with permission from ref 53. Copyright 2015 American Chemical Society.

Figure 6. (A) Schematic illustration for preparation of Pd/PDA-rGO nanocatalyst. (B) TEM and (C) HAADF-STEM images of Pd/PDA-rGO. (D) Volume of the generated gas (CO2 + H2) versus time for the dehydrogenation of (a) FA/SF (1:1) at 50 °C and (b, c) pure FA at 50 and 25 °C, respectively, over Pd/PDA-rGO and (d) the dehydrogenation of FA/SF (1:1) at 50 °C over Pd/rGO (nPd/nFA = 0.015). Adapted with permission from ref 55. Copyright 2015 American Chemical Society.

NPs with a size of 2.0−4.5 nm on rGO are observed for (Co6)Ag0.1Pd0.9/rGO (Figure 5D,E), demonstrating that the sacrificial Co3(BO3)2 prevents the aggregation of primary AgPd particles (Figure 5A). As a continuation to this study, by employing the NNMSA, highly dispersed bimetallic Au0.6Pd0.4 NPs are successfully immobilized onto rGO, which exhibit a record-breaking activity at 50 °C with a high TOF of 4840 h−1 for hydrogen generation without CO impurity from the FA/SF (1:2.5) system.54 Meanwhile, the rational modification of GO with electronrich functional groups may offer new opportunities for tailoring the alkalinity or acidity and electronic properties of GO and facilitate the dispersion of the metal precursors on the support surfaces as well as control the size during the growth of metal NPs for the performance optimization of the resultant

In 2015, our group has developed a non-noble-metal sacrificial approach (NNMSA) to successfully immobilize highly dispersed AgPd NPs onto reduced GO (rGO), whereby Co3(BO3)2 coprecipitates with AgPd NPs and is subsequently sacrificed by acid etching (Figure 5A).53 A series of (Cox)AgyPd1‑y/rGO catalysts, where x represents the Co/(Ag + Pd) molar ratio and y represents the molar percentage of Ag in AgPd, is synthesized and tested for the dehydrogenation of FA in an FA-SF system.53 Among them, the (Co6)Ag0.1Pd0.9/rGO catalyst exhibits extremely high activity (TOF = 2739 h−1 at 50 °C) for the complete dehydrogenation of FA to generate COfree H2 (Figure 5B). The exclusive generation of H2 and CO2 without CO formation is confirmed by gas chromatography analyses. In contrast to the severe aggregation observed for Ag0.1Pd0.9 on the rGO surface (Figure 5C), highly dispersed 1454


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Accounts of Chemical Research catalysts.47 For the first time, an efficient strategy via the effective alkalization of rGO with 1,4-phenylenediamine (PDA) has been explored.55 Owing to the coordination effects between the metal ions and amine groups ligated to rGO, monodispersed Pd NPs are readily anchored onto diamine-alkalized rGO (PDA-rGO) (Figure 6A). TEM and HAADF-STEM images of Pd/PDA-rGO indicated that Pd NPs are homogeneously dispersed on rGO with an average size of 1.5 nm (Figure 6B,C), whereas Pd NPs are severely aggregated with a considerably larger average particle size (10 nm) on rGO in the absence of PDA, demonstrating that the diamine groups play a crucial role in controlling the growth of Pd NPs on rGO.55 The catalytic performance of the prepared Pd/PDA-rGO for FA dehydrogenation was evaluated. The reaction is completed in 1.05 min to generate 282 mL of gas (H2 + CO2), affording a high TOF of 3810 h−1 at 50 °C (nFA:nSF = 1:1; FA, 6 mmol; nPd/nFA = 0.015). Remarkably, the dehydrogenation of pure FA at 50 and 25 °C (nPd/nFA = 0.015) is completed in 2.67 and 18.5 min, affording high TOF values of 1500 and 216 h−1, respectively (Figure 6D). In contrast, Pd/rGO exhibits low activity for the FA dehydrogenation under the same conditions (Figure 6D), highlighting that diamine is critical for the enhanced catalytic performance. The alkaline −NH2 group, serving as a proton scavenger, is beneficial for the cleavage of the O−H bond in FA, affording a Pd-formate intermediate and a −[H2NH]+ group in the initial reaction step. The Pd-formate species subsequently undergoes β-hydride elimination, affording CO2 and a Pd-hydride species. Finally, the −[H2NH]+ group reacts with the Pd-hydride species to generate H2.55 4.3. Metal Nanoparticles Immobilized into N-Doped Carbon

As described above, N-containing functional groups can facilitate the anchoring of metal NPs, functioning as basic coordination sites, to stabilize the NPs so as to achieve high activity. Cao and co-workers have reported that the efficiency of Pd NPs for H2 generation is significantly boosted by the electronic modulation of the carbon support surface using pyridinic-N group.17 Most recently, we have developed a tandem nitrogen functionalization approach (TNFA) to prepare N-functionalized porous carbon, which served as a distinct support for immobilizing Pd nanoclusters (NCs) (Figure 7A). As shown in the HAADF-STEM images of the prepared Pd/N-MSC-30two-175 catalyst, Pd NCs are homogeneously distributed throughout the support with a mean size of 1.4 nm (Figure 7B,C).56 For the selective dehydrogenation of FA to H2, the best catalytic activity is observed for the Pd/N-MSC-30-two175 catalyst, and the reaction is completed in 0.35 min to generate 144 mL of gas (H2 + CO2) from an aqueous FA-SF system (nFA:nSF = 1:2.5; FA, 3 mmol; nPd/nFA = 0.02) at 60 °C (Figure 7D). Considering the total metal amount used in the reaction, a TOF of 8414 h−1 is obtained at 60 °C. This value is considerably greater than those obtained over the Pd/N-MSC30-one and Pd/MSC-30 counterparts. Results obtained from XRD, HAADF-STEM, and elemental analysis indicated that a better dispersibility of Pd NCs and a higher content of N in the Pd/N-MSC-30-two-175 catalyst are achieved via TNFA.56 Notably, recycled Pd/N-MSC-30-two-175 exhibits a particle size distribution similar to that observed before the catalytic reaction (Figure 7E), indicating that the as-prepared catalyst

Figure 7. (A) Schematic illustration of synthesis of Pd/N-MSC-30two-175 via a tandem nitrogen-functionalization approach. (B, C) HAADF-STEM images of the prepared Pd/N-MSC-30-two-175 catalyst. (D) Volume of the released gas (H2 + CO2) versus time for the dehydrogenation of FA-SF solution at 60 °C over different catalysts: (1) Pd/MSC-30, (2) Pd/N-MSC-one, and (3) Pd/N-MSC30-two-175 (nFA:nSF = 1:2.5; FA, 3 mmol; nPd/nFA = 0.02). Inset: Corresponding TOF values. (E) HAADF-STEM image of recycled Pd/N-MSC-30-two-175 catalyst. Adapted with permission from ref 56. Copyright 2017 American Chemical Society.

exhibits excellent durability or recyclability toward FA decomposition.

5. CONCLUSIONS AND PERSPECTIVES In this Account, recent advances of the metal-nanoparticlecatalyzed dehydrogenation of FA for CO-free H2 production were discussed. By using various support materials and synthetic strategies, a wide range of highly active metal NP catalysts were developed, which demonstrate considerable potential for the clean, efficient generation of H2 from FA under convenient conditions (Table 1). The size and composition of the NPs, as well as the NP-support interactions, can be used for determining the catalytic performance for this reaction. The rational functionalization of the supports with electron-rich functional groups provided a powerful platform to 1455


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Accounts of Chemical Research Author Contributions

Table 1. Catalytic Activities for the Dehydrogenation of FA Catalyzed by Heterogeneous Catalysts −1

catalyst

temp (°C)

additive

TOF (h )

ref

Au−Pd/ED-MIL-101 Pd/NH2-MIL-125 AgPd@MIL-100(Fe) Ag18Pd82@ZIF-8 Ag1Pd4/NH2−UiO-66 Au@SiO2_AP Au@Schiff-SiO2 Pd/MSC-30 Pd/C_m (Co6)Ag0.1Pd0.9/rGO (Co3)EAu0.6Pd0.4/rGO Pd/PDA-rGO Pd/N-MSC-30-two-175 Pd/CN0.25

90 32 25 80 80 90 50 50 60 50 50 50 60 25

HCOONa HCOONa none HCOONa none HCOONa none HCOONa HCOONa HCOONa HCOONa HCOONa HCOONa none

26 27 28 29 40 44 46 49 51 53 54 55 56 17

Pd−B/C AuPd/C Pd/S-1-in-K

30 50 50

HCOONa HCOONa HCOONa

106 214 58a 580 893a 123 4368a 2623 7256a 2739 4840 3810 8414 752a 5530b 1184a 2972a 3027

Q.X. proposed the topic of the review. Z.L. investigated the literature and wrote the manuscript. Q.X. and Z.L. discussed and revised the manuscript. Notes

The authors declare no competing financial interest. Biographies Zhangpeng Li received his Ph.D. degree in Physical Chemistry from the Lanzhou Institute of Chemical Physics, CAS, in 2012. Currently, he is a Postdoctoral Researcher at the National Institute of Advanced Industrial Science and Technology (AIST, Japan) in Prof. Qiang Xu’s group. His research interests include the design and synthesis of nanocatalysts and their applications in liquid-phase chemical hydrogen storage. Qiang Xu received his Ph.D. degree in Physical Chemistry in 1994 from Osaka University. He is the Director of AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEMOIL), Prime Senior Researcher of National Institute of Advanced Industrial Science and Technology (AIST, Japan), Adjunct Professor at Kobe University and at Kyoto University, and a Distinguished Honorary Professor at The Hong Kong Polytechnic University. He received the Thomson Reuters Research Front Award in 2012 and was recognized as among the highly cited researchers (2014−2016) in both Chemistry and Engineering by Thomson Reuters. His research interests include the chemistry of nanostructured materials and their applications, especially for energy. He is a member of the European Academy of Sciences (EURASC).

18 31 15

a

Initial TOF value calculated based on total Pd atoms. bInitial TOF value calculated based on surface Pd sites.

prepared highly dispersed metal NPs. These functional groups are beneficial to the formation of the intermediates for FA dehydrogenation, resulting optimized catalytic performance. Despite the remarkable advances achieved in this field, there are still some issues to be addressed. In term of the synthesis, more active, stable, and well-dispersed metal NP catalysts with “clean” surfaces, as well as controllable structure and composition, are urgently needed. Compared to intensive studies conducted on size and composition, the effect of particle shape or morphology on the catalytic dehydrogenation performance of FA has rarely been explored. Rationally, systematically designed metal NPs with distinctive nanostructures for FA dehydrogenation represent an interesting research direction in the future. The mechanism for the nucleation and growth of NPs on the supports, as well as the interaction between NPs and the supports, should be explored in detail for optimizing catalytic performance. From the viewpoint of practical applications, further studies are required for the scalable production of stable NP-based catalysts for catalytic H2 generation from FA. It is imperative to substitute noble-metal catalysts by nonprecious metal catalysts from economic and ecological considerations. A deep understanding of the catalytic mechanisms is needed, which would benefit the judicious design and synthesis of catalysts with desired properties suitable for practical applications. In conclusion, over the past decade, remarkable progress has been made with respect to liquid chemical storage materials for hydrogen storage and delivery. With continuous efforts in this research area, practical applications of NP-based catalysts are expected.

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ACKNOWLEDGMENTS The authors thank the Editor for kind invitation and METI and AIST for financial support. The authors are pleased to acknowledge the fine work of the talented and dedicated graduate students, postdoctoral fellows, and colleagues who have worked with us in this area and whose names can be found in the references.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qiang Xu: 0000-0001-5385-9650 1456


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