PdAg Nanoparticles Supported on Functionalized Mesoporous Carbon

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PdAg Nanoparticles Supported on Functionalized Mesoporous Carbon: Promotional Effect of Surface Amine Groups in Reversible Hydrogen Delivery/Storage Mediated by Formic Acid/CO 2

Shinya Masuda, Kohsuke Mori, Yuya Futamura, and Hiromi Yamashita ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04099 • Publication Date (Web): 02 Feb 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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PdAg Nanoparticles Supported on Functionalized Mesoporous Carbon: Promotional Effect of Surface Amine Groups in Reversible Hydrogen Delivery/Storage Mediated by Formic Acid/CO2 Shinya Masuda,[a] Kohsuke Mori,*[a,b,c] Yuya Futamura[a] and Hiromi Yamashita*[a,c] a

Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka

University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan b

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

c

Elements Strategy Initiative for Catalysts Batteries (ESICB), Kyoto University, Katsura, Kyoto

615-8520, Japan

ACS Paragon Plus Environment

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ABSTRACT: Highly dispersed PdAg nanoparticles supported on phenylamine-functionalized mesoporous carbon were synthesized and assessed as bifunctional heterogeneous catalysts for the interconversion of formic acid (FA) and CO2 in chemical hydrogen storage systems. A high turnover frequency (TOF) of 5638 h-1 for evolved H2 based on the total amount of Pd in the sample was obtained during the dehydrogenation of FA, corresponding to a TOF value of 21,686 h-1 based on the quantity of surface Pd atoms. In addition, this material promoted the hydrogenation of CO2 to FA with a turnover number (TON) of 839 over 24 h for produced FA based on the total amount of Pd, corresponding to a TON of 3227 based on the quantity of surface Pd atoms. Both the synergistic alloying effect and the surface functionalization with weakly basic phenylamine molecules were vital aspects of these high catalytic activities. Experimental and theoretical studies revealed that the cooperative action of the phenylamine groups in the vicinity of active PdAg NPs significantly affected the O–H dissociation of FA as well as the CO2 adsorption capacity of the catalyst, which ultimately boosted the catalytic activity for both reactions. This work also represents the demonstration of the catalyst that promotes the reversible interconversion between FA and CO2 under heterogeneous conditions and is recyclable at least three cycles without loss of activity. Additionally, the present catalyst performs well even in flow chemistry experiments using a fixed-bed reactor, indicating the potential for various industrial applications.

KEYWORDS. formic acid • CO2 • phenylamine • PdAg • mesoporous carbon


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INTRODUCTION In recent years, the use of hydrogen as a clean and sustainable energy source in place of nonrenewable fossil fuels has become of increasing interest.[1][2] This is primarily because the use of hydrogen in proton exchange membrane fuel cells can release a significant amount of chemical energy (based on the combustion reaction 2H2 + O2 → 2H2O, 284 kJ/mol) and generates only water as a byproduct.[3] However, the safe, economical and controlled storage and release of hydrogen remains challenging. For these reasons, hydrogen storage materials have received much attention.

[4-7]

Formic acid (FA, HCOOH) has emerged as one of the most promising

hydrogen storage compounds because it has a high gravimetric capacity for hydrogen (4.4 wt%), is relatively nontoxic and is a nonflammable liquid under ambient conditions.[8] Additionally, the use of FA could allow economical CO2-mediated hydrogen storage energy cycling via the regeneration of FA through the hydrogenation of CO2. As a result of the above, considerable effort has been directed toward the search for highly active catalysts for the dehydrogenation of FA, which has resulted in relatively sophisticated systems intended for industrial applications.[9-14] The decomposition of FA by selective dehydrogenation (HCOOH → H2 + CO2) is thermodynamically favored (∆G = -48.4 kJ mol-1). It is desirable to completely suppress the side reaction HCOOH → CO + H2O (∆G = -28.5 kJ mol1

), because the generated CO is both highly toxic and capable of poisoning Pt fuel cell

catalysts.[15,16] While selective and highly active dehydrogenation has already been demonstrated under ambient conditions utilizing homogeneous catalysts, heterogeneous systems are more practical in on-board applications.[13,14] Previous papers have reported that Au, Rh and Pd nanoparticles (NPs) are catalytically active for the dehydrogenation of FA.[17,18] As well, bimetallic NPs, especially those based on Pd in conjunction with a coinage metal such as Au or


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Ag, exhibit superior activity compared to those of monometallic catalysts. This improved catalytic activity and selectivity is attributed to the electronic activation of Pd species by charge transfer resulting from the difference in the work functions of the two metals.[4,17,19] In addition to dehydrogenation catalysts, CO2 hydrogenation catalysts for the regeneration of FA must be researched in order to develop economical CO2-mediated hydrogen energy systems. The gas phase hydrogenation of CO2 has a positive free energy change (∆G = +33 kJ mol-1), while the same reaction in aqueous solution proceeds more readily because of the relatively low activation energy (∆G =-4 kJ mol-1).[20] To date, there has been significant progress in the research of homogeneous transition metal complexes that function in aqueous media. Unfortunately, the development of heterogeneous catalysts has not proceeded to the same extent, primarily because these materials frequently require high catalyst concentrations, extremely high pressures and the use of organic solvents.[21-24] Moreover, the design of advanced reaction systems that enable reversible H2 storage and release under practical conditions and in a portable manner remains a challenging task. Despite the importance of this goal, only a few catalytic systems have been reported so far, even including homogeneous catalytic processes.[24] The first reversible hydrogen storage system incorporating FA was based on the use of a homogeneous proton-switchable Ir complex in aqueous media.[24] Since then, an ammonium bicarbonate/formate redox cycle involving a Pd nanocatalyst has been proposed that allows hydrogen storage and delivery without a CO2 supply.[25] A potassium formate/bicarbonate rechargeable H2 battery driven by Pd-supported reduced graphene oxide (rGO) or nitrogen-doped mesoporous carbon (MSC) has also been proposed.[26,27] Pd-supported graphitic carbon nitride (Pd/g-C3N4) has been demonstrated to be


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active for the interconversion of FA and CO2, although the reversibility of this process has not been achieved.[28] More recently, our group reported a PdAg-supported amine-functionalized mesoporous silica (SBA-15) as a dual heterogeneous catalyst for the interconversion of FA and CO2.[29] A detailed investigation of the effects of surface modification with different amine functionalities showed that weakly basic phenylamine molecules on this material effectively promoted both reactions (to a greater extent than strongly basic functional groups) because the moderate interaction between amine groups and FA resulted in optimal activity. In the course of our ongoing quest to develop more powerful catalysts for the interconversion of FA and CO2, we determined that PdAg NPs supported on phenylamine-functionalized MSC can act as an efficient heterogeneous catalyst with improved activity and durability for the targeted reactions. This catalyst is capable of promoting both reactions and can be recycled at least three times without loss of activity. In the work reported herein, the role of amine functional groups in the vicinity of active PdAg NPs was also studied, based on density functional theory (DFT) calculations. Additionally, other interesting aspects of this material, including potential applications to flow chemistry experiments using a fixed-bed reactor, are discussed.

RESULTS AND DISCUSSION Characterization The procedure for the synthesis of PdAg-supported phenylamine-functionalized MSC catalysts (PdAg/amine-MSC) is summarized in Scheme 1. The MSC (SBET (BET surface area) = 209 m2·g1

, Vp (all pore volume) = 0.40 cm3·g-1) was treated with an aqueous nitric acid solution, after


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which the surface was functionalized with p-phenylenediamine to produce amine-MSC. This process resulted in the reaction of oxyfunctional groups (–COOH and –OH) on the MSC with amine groups to generate amide and C–N bonds. The concentration of grafted amine functional groups on

the amine-MSC was determined to be approximately 0.57 mmol g-1 based on CHN elemental analysis (Table S1). This value closely corresponds to the result of 0.53 mmol g-1 obtained from thermogravimetric (TG) analysis (Figure S1). Pd and Ag were deposited by mixing the amineMSC support material with aqueous solutions containing Pd(NO3)2 and AgNO3. In the final step, the samples were reduced using NaBH4, affording PdAg/amine-MSC. Monometallic (Pd) and bimetallic (PdAg) catalysts supported on unmodified MSC (denoted as Pd/MSC and PdAg/MSC) and a monometallic catalyst on amine-MSC (Pd and Ag/amine-MSC) were also obtained using the same general process. The high-angle X-ray diffraction (XRD) pattern of the PdAg/amine-MSC exhibits a broad, intense peak at a 2θ value of approximately 26°, corresponding to amorphous MSC. No significant changes in this pattern were observed after functionalization with the amine or deposition of the metals (Figure S2). The presence of Pd or PdAg peaks could not be confirmed as these peaks were very weak. All samples generated characteristic type IV curves with sharp capillary condensation steps and H1-type hysteresis loops over the relative pressure (p/p0) range of 0.6–0.9 in the N2 adsorption desorption isotherms, indicating highly ordered and well-preserved cylindrical mesopores (Figure S3). Table S2 summarizes the Brunauer–Emmett–Teller (BET) surface areas (SBET) and pore volumes (Vp) calculated from the N2 adsorption desorption isotherms of each sample. After amine functionalization to generate the amine-MSC specimen (SBET = 130 m2g-1, Vp = 0.34 cm3g-1), both SBET and Vp were reduced compared to the values for the original MSC


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(SBET = 209 m2g-1, Vp = 0.40 cm3g-1), while no further significant changes were observed after the deposition of Pd and Ag (SBET = 156 m2g-1, Vp = 0.36 cm3g-1). These results suggest that the amine-MSC maintained its original surface area and pore volume despite the presence of the modifier and metal within the mesopores. Figure 1 presents a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image together with Pd and Ag energy dispersive X-ray spectroscopy (EDX) maps of the PdAg/amine-MSC. Here, Pd and Ag atoms appear as yellow and red, respectively. The PdAg NPs were evidently well dispersed and had a narrow size distribution, with an average diameter of 1.2 nm. The elemental distribution along a single NP provides evidence for the formation of bimetallic alloy structure with homogeneous distribution of the Pd and Ag. EDX analysis also confirmed that the average Pd:Ag molar ratio in a single NP was approximately 85:15 (Table S3). TEM analysis further indicated that the average NP diameters in the other catalyst specimens were similar to that in the PdAg/amine-MSC (Pd/MSC = 2.6 nm, Pd/amineMSC = 1.8 nm and PdAg/MSC = 2.3 nm, Figures S4-S6). However, a comparison of the particle sizes clearly demonstrates that amine functionalization resulted in the formation of smaller NPs. The electronic state of each metal was investigated by X-ray photoelectron spectroscopy (XPS) analysis (Figures S7 and S8). The Pd 3d peaks of the Pd/amine-MSC were shifted to higher binding energies than those of Pd on unmodified MSC, and a similar tendency was observed in the case of the Ag 3d peaks of PdAg/amine-MSC compared with PdAg/MSC. These results confirmed that the surface amine groups were situated in close proximity to the PdAg NPs and affected their electronic states. In contrast, the Pd 3d peaks generated by the PdAg/amine-MSC and PdAg/MSC were shifted to lower binding energies than those of the corresponding monometallic Pd catalysts. This outcome is attributed to the ability of Ag atoms to


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donate electrons to Pd atoms after alloying, based on their electronegativity values of 1.93 and 2.20.

X-ray absorption fine structure (XAFS) data were acquired to investigate the effects of alloying and of modification with amine groups on the local structures of the Pd and Ag species. The Pd K-edge X-ray absorption near-edge structure (XANES) spectra of the unmodified materials Pd/MSC and PdAg/MSC were found to resemble that of Pd foil (Figure 2 (A)). In addition, the Pd K-edge Fourier transform-extended X-ray absorption fine structure (FT-EXAFS) spectra contained a single sharp peak associated with Pd-Pd bonds at approximately 2.5 Å that is also suggestive of metallic Pd (Figure 2 (B)). The Pd−Pd distance in the PdAg/MSC, however, was found to be slightly longer compared with the values for the Pd/MSC and for Pd foil, indicating the presence of heteroatomic Pd−Ag bonds. The Ag K-edge XANES spectrum of PdAg/MSC resembles that of Ag foil and its FT-EXAFS spectrum shows a single intense peak ascribed to contiguous Ag-Ag bonds at approximately 2.6-2.8 Å (Figure 3 (A), (B)). This peak is shifted to a slightly shorter interatomic distance compared with that of pure Ag foil, again indicating Pd−Ag bonds. These results present clear evidence for the formation of PdAg alloy nanoparticles in the PdAg/MSC. In contrast, the XANES spectra of the Pd/amine-MSC and PdAg/amine-MSC differ from that of Pd foil and instead resemble that of PdO (Figure 2 (A)). The FT-EXAFS spectra of both materials exhibit two peaks associated with Pd−Pd and Pd−O(N) bonds in the vicinity of 1.5 Å (Figure 2 (B)), with no shift in the Pd−Pd distance for either sample. The Ag K-edge FT-EXAFS spectrum of PdAg/amine-MSC shows an intense singlet peak ascribed to nearest metal−metal


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bonding without any shift. Similar results were observed in the XPS spectra of these specimens (Figures S7, S8). Based on these data, a reasonable structural model for PdAg/amine-MSC has the Ag atoms preferentially located in the core region, with the Pd atoms situated in the shell. This configuration is based on the EXAFS analysis showing that the Pd atoms in PdAg/amineMSC are more highly oxidized than those in PdAg/MSC, either because of their exposure to the surface or coordination with N atoms in the surface amine groups. It is therefore apparent that the addition of amine groups affects the nucleation of the metals as well as the NP growth process, ultimately producing PdAg NPs with a different structure. Dehydrogenation of formic acid The dehydrogenation of FA was performed using an aqueous solution of HCOOH/HCOONa (FA:SF = 9:1, molar ratio, pH=2.5) and the evolved gas volume was measured using a gas burette (Scheme S1). The time courses of the evolved gas volumes and the turnover frequency (TOF) values calculated at 5 min for the various specimens are summarized in Figures S9 and 4, respectively. The PdAg/amine-MSC catalyst exhibited the highest catalytic activity, with a TOF of 5638 h-1 for evolved H2 based on the total amount of Pd in the sample (as determined by inductively coupled plasma (ICP) spectroscopy, Table S4), in conjunction with a H2 production rate of close to 1,070,000 mL·h-1·gPd–1. These values are approximately 25 times those obtained from PdAg/MSC without amine modification. Additionally, the dispersion of Pd species in the PdAg/amine-MSC catalyst was determined to be 26 % from CO adsorption measurements (Table S5). Thus, a maximum TOF value of 21,686 h-1 was obtained from the PdAg/amine-MSC based on the quantity of surface Pd atoms. The dehydrogenation activity of the PdAg/amine-MSC was equal to or greater than those recently reported for other heterogeneous catalyst systems, including Pd/S-1-in-k (TOF = 3027 h-1, FA/SF = 1, 50 °C),[9] Pd/CN0.25 (TOF = 752 h-1, SF = 0,


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25 °C),[10] Pd/C_m (TOF = 7256 h-1, FA/SF = 1, 60 °C),[11] Pd/PDA-rGO (TOF = 3810 h-1, FA/SF = 1, 50 °C)[30] and PdAg/SBA-15-phenylamine (TOF = 822 h-1, FA/SF = 9, 75 °C).[29] No induction period was observed in the initial stage of the reaction, and the H2 to CO2 ratio during the course of the reaction remained close to 1. It should be noted that the present catalytic system can suppress unfavorable CO contamination from the dehydration pathway (HCOOH → H2O + CO) less than detection limit by GC (2 ppm). This excellent selectivity meets the criteria of the PEM fuel cells standard, which is a CO concentration lower than 10 ppm. Figure 4 also shows the average diameters of the NPs as determined by TEM analysis. The PdAg NPs on the amine-MSC support were highly dispersed with an average diameter of 1.2 nm, and were the smallest among the various specimens. This result suggests one possible explanation for the high catalytic activity of this material. The Ag catalyst did not show any activity (data not shown), while the catalytic performance of the PdAg was higher than that of the monometallic Pd analogue. The beneficial effects of alloying Pd with Ag with regard to the dehydrogenation of FA have been discussed in detail elsewhere.[17,19] As can be seen from the time courses of the catalytic activities, the PdAg/amine-MSC catalyst was more stable than the Pd/amine-MSC (Figure S9). The formation of electron-rich Pd species (as determined from XPS analysis) upon the addition of Ag is known to suppress the undesirable dehydration reaction (HCOOH → CO + H2O) as well as the adsorption of CO, both of which may enhance the stability of the PdAg/amine-MSC.[19] As noted, the amine functionalization plays a crucial role in determining the elementary steps in the catalytic dehydrogenation of FA. To better understand the role of the amine functional groups, potential energy profiles in the absence and presence of phenylamine molecules were generated using DFT calculations, employing a Pd11Ag11 cluster as a model for alloy NPs. In the


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absence of the amine molecules (Figure 5 (A)), the trans-AgO–PdH(O)-bridged configuration (1a) was the lowest energy HCOOH adsorption structure. In this structure, the carbonyl oxygen and the acidic hydrogen are coordinated to Ag and Pd atoms, respectively. The adsorbed HCOOH undergoes O–H bond dissociation via transition state TS1a/1b with a barrier of 11.9 kcal/mol to produce the formate (HCOO–) intermediate (1b). This species subsequently isomerizes to a trans-AgH–Pd(O)-bridged configuration (1c) via TS1b/1c with a barrier of 13.9 kcal/mol. Following this, C–H bond scission occurs via TS1c/1d to form CO2 and Pd–H species. Finally, the catalytic cycle is completed with the release of H2 via TS1d/1e with a barrier of 23.0 kcal/mol. From the energetic point of view, the final H2 desorption is the rate-determining step. In the presence of phenylamine (Figure 5 (B)), the formation of HCOOH–PhNH2 appears reasonable as a result of the acid-base interaction. In this process, the incoming HCOOH molecule encounters a basic amine group in the vicinity of NPs prior to adsorption on the surface of the catalyst. The optimized structure of the acid–base pair is the cis-AgO–PdO(C)-bridged configuration 2a, in which the carbonyl oxygen and non-carbonyl oxygen interact with Ag and Pd atoms, respectively. The O–H bond distance is increased from 0.985 Å in free trans-HCOOH to 1.075 Å in 2a, suggesting that HCOO– is spontaneously generated to form 2a′ without any transition state when an incoming HCOOH molecule is coupled with a phenylamine group. Starting from 2a*, isomerization via TS2a′/2b (which may be the rate-determining step) and C–H bond dissociation via TS2b/2c form CO2 and hydride species, with barriers of 14.2 and 9.3 kcal/mol, respectively. These activation energies are similar to those without the amine functional groups. Finally, H2 production occurs via TS2c/2d with a barrier of 7.8 kcal/mol, which is substantially lower than the value of 23.0 kcal/mol calculated in the absence of the amine. It


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can be concluded that both the O–H bond dissociation and the H2 desorption steps are enhanced by the presence of neighboring amine functional groups.

Formic acid synthesis via the hydrogenation of CO2 with H2 The activities of these materials during the hydrogenation of CO2 with H2 to give FA were assessed in an autoclave using a 1.0 M aqueous NaHCO3 solution at pH=8.5 under a total pressure of 2.0 MPa (H2:CO2 = 1:1, volume ratio) at 100 °C. Figure 6 graphs the TON values at 24 h, together with the average diameters of the NPs as determined by TEM analysis. The modification with phenylamine evidently increased the catalytic performances of both the monometallic Pd and bimetallic PdAg catalysts. The formation of smaller NPs is one possible reason for this improved catalytic activity. The reaction did not proceed using only the monometallic Ag catalyst (data not shown), while the PdAg/amine-MSC showed the highest catalytic activity together with > 99% selectivity and a TON and reaction rate of 839 and 328 mmol·h-1·gPd–1 for produced FA based on the total amount of Pd in the sample (as determined by inductively coupled plasma (ICP) spectroscopy, Table S4), indicating the synergic alloying effect between Pd and Ag. Additionally, the TON value based on the number of surface Pd atoms calculated from CO adsorption measurements reached 3227. Among the bases examined, NaHCO3 showed the highest activity, while NaOH and trimethylamine were less effective, whose TON were 354 and 393 respectively. The TON value generated by the PdAg/amine-MSC catalyst under relatively mild conditions was superior to those reported for other heterogeneous catalysts, including Pd/mpg-C3N4 (TON = 85 (24 h) at 100 °C, 4 MPa in H2O/NEt3),[28] Ru/LDH (TON = 698 (24 h) at 100 °C, 2 MPa in NaOH (1.0 M)),[31] Au/TiO2 (TON = 215 (20 h) at 70


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°C, 4 MPa in EtOH/NEt3),[32] Ir-PN-PEI@TNT(Na+) (TON = 1012 (20 h) at 140 °C, 2 MPa in NaOH (0.1 M))[33] and PdAg/SBA-15-phenylamine (TON = 874 (24 h) at 100 °C, 2 MPa in NaHCO3 (1.0 M)).[29] It has been reported that the strong electron-donating ability of PNP pincer-type ligands or Nheterocyclic carbenes in Ir(III) complexes are responsible for facilitating the hydrogenation of CO2.[22,34] Moreover, we previously identified a correlation between the hydrogenation rate and the Ru 3p binding energy as determined by XPS analysis for a series of Ru-based supported layered double hydroxide (LDH) catalysts. That is, the TON based on Ru content increased as the binding energy decreased.[31] Considering these findings, the improved catalytic activity of the PdAg/amine-MSC compared with that of the Pd/amine-MSC can possibly be ascribed to the formation of electron-rich Pd species as a result of charge transfer from Ag, as evidenced by the XPS analysis (Figure S7). The hydrogenation of CO2 is initiated by the dissociation of H2 to produce metal-hydride species. This is followed by the adsorption of HCO3– generated under basic conditions, which undergoes hydrogenation to afford a formate intermediate.[29] Energetically, the reduction of HCO3– is more likely to proceed if active H attacks the C atom of HCO3– rather than the O atoms.[35] Finally, the generation of formic acid together with H2O regenerates the initial active species. In an effort to elucidate the role of the grafted phenylamine groups during the CO2 hydrogenation reaction, potential energy profiles were constructed using a Pd11Ag11 cluster model in the absence and presence of phenylamine molecules. In the absence of the amine molecules (Figure 7 (A)), the dissociation of H2 occurs at Pd sites via TS3a/3b with a barrier of 11.9 kcal/mol. Next, HCO3– is adsorbed on the Pd, followed by


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reduction through the attack of a hydride via TS3c/3d with a barrier of 56.5 kcal/mol. This represents the rate-determining step in this catalytic cycle. In the presence of phenylamine (Figure 7 (B)), the dissociation of H2 occurs via TS4a/4b with a barrier of 13.2 kcal/mol (similar to that in the absence of the amine). However, the activation energy for the reduction of HCO3– via TS4c/4d were calculated to be 32.1 kcal/mol, which are substantially lower than those in the absence of the amine. These results demonstrate that the interaction between the HCO3– and the phenylamine molecules through N–H···O hydrogen bonding plays a pivotal role in the stabilization of the reaction intermediate, thus affecting the reduction reactivity. Another important function of the grafted amine groups is related to their ability to adsorb CO2, as a result of their basicity. In a separate experiment, the CO2 adsorption capacity and adsorption rate of the amine-MSC were assessed by TG and found to be 1.00 mmol g-1 (3 h) and 0.34 mmol g-1 h-1. The latter value is 1.5 times greater than that obtained for the untreated MSC (Table S6). This characteristic of the amine-MSC tends to concentrate CO2 around the periphery of the active NPs, thus increasing the catalytic activity.

Reversible activity during the interconversion of formic acid and CO2 The reversibility of the PdAg/amine-MSC catalyst during the interconversion of FA and CO2 was also examined. A catalyst sample was initially used for the dehydrogenation of FA over a span of 15 min and then collected by centrifugation after the reaction. The recovered catalyst was subsequently redispersed in an aqueous NaHCO3 solution (1.0 M) and employed for the hydrogenation reaction to produce FA. After 24 h, the catalyst was again collected by centrifugation and dispersed in distilled water, then once again used for the dehydrogenation of


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FA. As shown in Scheme 2 and Figure S10, the PdAg/amine-MSC functioned as a reversible heterogeneous catalyst for the interconversion of FA and CO2 without loss of its inherent catalytic activity over at least three repeated cycles. To the best of our knowledge, this is the first demonstration of a reversible and recyclable heterogeneous catalyst allowing the interconversion of FA and CO2 (Scheme 2). Importantly, the XRD pattern of the recovered catalyst after the third cycle showed only peaks associated with an amorphous MSC phase (Figure S11). XPS analysis also demonstrated no significant changes, suggesting no variations in the electronic state of the material (Figure S12). A TEM image also confirmed that the particle sizes remained essentially unchanged after the reversible reaction, with no agglomeration of the NPs (with an average diameter of 1.3 nm after the third cycle, Figure S13). Moreover, no metal leaching was observed in the filtrate by ICP analysis. These results indicate that the amine-functionalized MSC was thermally and mechanically stable and thus also stabilized highly-dispersed PdAg NPs within a unique microenvironment.

Flow chemistry applied to the dehydrogenation of FA by PdAg/amine-MSC The applicability of the present catalyst was also highlighted by trials using a flow reaction system in conjunction with a fixed-bed reactor. The PdAg/amine-MSC was employed in a simple continual-flow reactor by placing the catalyst in a glass tube in which the dehydrogenation of FA took place. The experimental setup is shown in Figure 8. An aqueous FA solution (1.0 M) was pumped into the heated glass reactor (flowing rate : 1 mL min-1) and the evolved gases were trapped in a burette. The reaction time course data are plotted in Figure 8. The reaction proceeded smoothly without a substantial induction period or any evidence of deactivation. The


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initial TOF values (calculated at 5 min) were 2280, 1340 and 257 at 75, 50 and 25 °C, respectively. Notably, the present system exhibited approximately half the level of activity observed when using the batch reactor. We expect that more enhanced activity would be attained by screening the detail reaction conditions. Further experiment is now under investigation in our laboratory. CONCLUSIONS Highly dispersed PdAg bimetallic NPs supported on phenylamine-functionalized MSC were synthesized. This material was intended to act as a bifunctional heterogeneous catalyst for the interconversion of FA and CO2, thus allowing chemical hydrogen storage. The addition of phenylamine was found to lead to the formation of highly-dispersed PdAg NPs, to modify the electronic state of the Pd and to increase the CO2 adsorption capacity. Moreover, the energetics of the amine-functionalized catalyst promote both the dehydrogenation of FA and the hydrogenation of CO2 to produce FA, as confirmed by DFT calculations. The grafted phenylamine group have a positive effect on the O−H bond dissociation in the dehydrogenation of FA, while interaction between the HCO3– and the phenylamine molecules plays a pivotal role in the stabilization of the reaction intermediate in the hydrogenation of CO2. This catalyst showed reversibility and recyclability during the interconversion of FA and CO2 under heterogeneous conditions, without any reduction in its catalytic activity during repeated usage. This material also functions efficiently under flow chemistry conditions in a fixed-bed reactor along with retention of its inherent activity. It is therefore likely that this catalyst can be applied to various industrial-scale processes. This study supplies a platform to design future enhanced catalysts not only for the dehydrogenation of FA but also for hydrogenation of CO2 to produce FA.


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EXPERIMANTAL SECTION Materials. Acetone, nitric acid (HNO3), p-phenylenediamine, silver nitrate, sodium hydroxide (NaOH), sodium borohydride (NaBH4), Formic acid (FA) and Sodium formate (SF) were purchased from Nakalai Tesque, Inc. Pd(II) nitrate solution was obtained from Wako Pure Chemical Ind. Co., Ltd. Mesoporous carbon (average pore diameter 100 Å ± 10 Å) was obtained from Aldrich Chemical Co. Synthesis of acid treated mesoporous carbon. Acid treated mesoporous carbon (Acid treated-MSC) was prepared by the typical procedure.[36,37] The MSC (1.0 g) was mixed with the high concentration aqueous nitric acid solution (70 %, 50 mL) and the solution was heated up to 80 °C in an oil bath. After stirring with


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10 h, the sample was collected by filtration and washed with distilled water until the pH was adjusted to 7. Then, it was dried under vacuum overnight. Synthesis of Amine functionalized MSC (amine-MSC). The synthesis of amine functionalized carbon support was referred to the previously reported method. [30] An acid treated MSC (1.0 g) was added into 500 mL of distilled water and sonicated for 2 h. After sonication, the aqueous solution was mixed with acetone (80 mL) that contained excess amount of p-phenylenediamine (PDA, 3.24 g) and stirred at RT for 48 h. The resultant amine-MSC precipitation was separated by filtration and washed with acetone and distilled water until no color was confirmed in the filtrate, followed by drying under vacuum for overnight. Synthesis of PdAg supported amine-MSC catalyst (PdAg/amine-MSC). The amine-MSC (0.3 g) was mixed with an aqueous solution (80 mL) that contained Pd(NO3)2 (0.606 mL, Pd 47mM) and AgNO3 (0.285 mL, 100mM). After mixed, NaOH aqueous solution (1M) was added to adjust the pH around 10 and stirred for 30 min. Subsequently, the sample was reduced by NaBH4 and collected by filtration, then, dried under vacuum overnight. Pd and Ag supported on amine-MSC or MSC catalyst were also prepared by same method. Characterization. Powder X-ray diffraction (XRD) patterns were recorded using a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ=1.54056 Å). Transmission electron microscope (TEM) images were obtained with a Hitachi HF-2000 FE-TEM operated at 200 kV. Nitrogen adsorption-desorption isotherms were measured at -196 °C using BELSORP-max system (MicrotracBEL Corp.). Samples were degassed at 150 °C for 3 h under vacuum to vaporize the


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physisorbed water. Specific surface area was calculated by BET (Brunauer-Emmett-Teller) method using nitrogen adsorption data ranging from p/p0=0.05 to 0.35. Quantification of loading amount of amine functionality in samples was performed by two methods. One is the CHN elemental analysis using a MICRO CORDER JM-10 (J-SCIENCE LAB CO., Ltd.). Another is thermogravimetric (TG) analysis using a Rigaku Thermo plus EVO2 TG8121 system from room temperature to 700 °C at a heating rate of 10 °C min-1 in air flow (10 mL min-1). X-ray photoelectron spectroscopy (XPS) was performed with Shimadzu ESCA-3400 system. Mg Kα X-ray radiation (hν=1253.6 eV) was used as the excitation source. The binding energy of the spectra was calibrated using C 1s core level for the contaminant at 284.5 eV. Pd content in samples was determined by inductively coupled plasma atomic emission spectroscopy (ICP-ES) measurements with a Nippon Jarrell-Ash ICAP-575 Mark II instrument. Scanning transmission electron microscopy (STEM) images and elemental mapping were obtained using JEOL-ARM 200F equipped with a Kvex energy-dispersive X-ray detector (JED-2300T) operated at 200 kV. Pd and Ag K-edge XAFS spectra were recorded by using a fluorescenceyield collection technique at the beam line 01B1 station with an attached Si(111) monochromator at SPring-8, JASRI, Harima, Japan (prop. No. 2017B1081, 2017B1084). The data reduction was performed by using the REX2000 program (Rigaku). Measurement of CO2 adsorption capacity. CO2 gas adsorption experiments were performed under dry conditions by thermogravimetric (TG) analysis using TG-DTA system (Rigaku, Thermo plus EVO2). The sample (10 mg) was loaded into a platinum pan and heated at 140 °C for 180 min with a heating rate of 10 °C min-1 under a N2 flow (100 mL min-1) to remove the physisorbed water. Then, the temperature was lowered to 30 °C and kept for 60 min in a flow of N2 (100 mL min-1) to stabilize the sample


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weight and temperature. The amount of adsorbed CO2 was monitored for 3 h under a flow of 10 % CO2 containing N2 (total 100 mL min-1). [38,39] Catalytic dehydrogenation of formic acid. The catalyst was placed in a reaction vessel (30 mL) equipped with a reflux condenser and a gas burette (Scheme S1). After purging with N2, FA:SF = 9:1 aqueous solution (0.3 M, 10 mL) was added to the reaction vessel and reacted at 75 °C with magnetic stirring. TOF values [h-1] were determined according to Equation TOF=PatmVH2 / RTNPdt in which Patm is the atmospheric pressure, VH2 is the generated volume of H2, R is the gas constant, T is room temperature, NPd is the mol number of Pd or surface exposed Pd, and t is the reaction time. Catalytic hydrogenation of CO2. CO2 hydrogenation to FA was conducted with a batch reactor system (stainless autoclave (60 mL)) using a catalyst (50 mg) and 1.0 M aqueous NaHCO3 solution (10 mL) under a total pressure of 2.0 MPa (H2:CO2 = 1:1, volume ratio) at 100 °C. After 24 h, disodium succinate (external standard) was added and the yield of formic acid was determined by HPLC using a Shimadzu HPLC instrument equipped with a Bio-Rad Aminex HPX-87H Ion Exclusion Column (300 mm×7.8 mm). 5 mm H2SO4 (0.500 mL min-1) was used as a mobile phase. TON values were determined according to Equation. TOF=[produced FA after 24 h]/[ mol number of total Pd or surface exposed Pd] DFT calculation.


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All DFT calculations were performed with the DMol3 program in Materials Studio 17.2.[40,41] The generalized gradient approximation (GGA) exchange-correlation functional proposed by Perdew, Burke, and Ernzerhof (PBE) was combined with the double numerical basis set plus polarization functions (DNP). A Pd11Ag11 cluster was chosen as a model of alloy NPs, where the bottom two layers were fixed at the corresponding bulk position and the top layer was allowed to relax during geometry optimizations. Reversible interconversion of formic acid and CO2. The reversible interconversion of FA and CO2 was conducted using the same methods above. First, the catalyst was used for dehydrogenation of formic acid and collected by centrifugation. Then, the recovered catalyst was redispersed to aqueous NaHCO3 solution (1.0 M) and used for the hydrogenation reaction to produce FA. After the hydrogenation reaction (24 h), the catalyst was collected by centrifugation and dispersed to distilled water, which was subjected to the dehydrogenation of FA again. Then, it was repeated 3 times. Flow chemistry in the dehydrogenation of formic acid. The dehydrogenation of FA was conducted in flow chemistry to investigate the continual-flow property in a simple continual-flow reactor. The appropriate catalyst (0.1 g) was mixed with SiO2 (3 ㎛, 0.2 g) and filled in the reactor. The detail setup system is illustrated in Figure 8. An aqueous FA solution (1.0 M) was pumped into the glass reactor (flowing rate : 1 mL min-1) heated from outside in various temperature, and the evolved gas was trapped into a gas burette.

AUTHOR INFORMATION


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Corresponding Author *K.M.: tel and fax, +81-6-6879-7460; e-mail, [email protected]. osaka-u.ac.jp. *H.Y.: tel and fax, +81-6-6879-7457; e-mail, yamashita@mat. eng.osaka-u.ac.jp. ACKNOWLEDGMENT The present work was supported by JST-PRESTO (JPMJPR1544). The part of this work was supported by Grants-in-Aid for Scientific Research (Nos. 26220911, 25289289, 26630409, and 26620194) from the Japan Society for the Promotion of Science (JSPS) and MEXT. Supporting Information Available: elemental analysis, TG, XRD, N2 adsorption desorption isotherms, TEM, XPS, CO adsorption. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES [1]

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Scheme 1. An illustration summarizing the syntheses of Pd or PdAg-supported amine or amine-MSC catalysts.


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Figure 1. (A) HAADF-STEM image and size distribution plot for PdAg/amine-MSC. (B) HAADFSTEM image of the region indicated in (A). The values in parentheses are the atomic ratios of Pd to Ag in selected NPs as determined by EDX analysis using the Cliff-Lorimer method. EDX maps of (C) Pd and (D) Ag over the region shown in (B).


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Figure 2. Pd K-edge (A) XANES and (B) FT-EXAFS spectra of (a) PdAg/amine-MSC, (b) Pd/amineMSC, (c) PdAg/MSC, (d) Pd/MSC, (e) PdO and (f) Pd foil specimens.


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Figure 3. Ag K-edge (A) XANES and (B) FT-EXAFS spectra of (a) PdAg/amine-MSC, (b) PdAg/MSC and (c) Ag foil specimens.


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Figure 4. TOF values at 5 min for evolved H2 based on the total amount of Pd during the dehydrogenation of FA and average diamter of the NPs over (a) Pd/MSC, (b) PdAg/MSC, (c) Pd/amineMSC and (d) PdAg/amine-MSC. Reaction conditions: catalyst (0.05 mg), HCOOH/HCOONa = 9/1 (molar ratio) in aqueous solution (0.3 M (total molar), 10 mL) at 348 K for 15 min under N2. The Pd loadings on the Pd/amine-MSC and PdAg/amine-MSC were determined to be 0.93 and 0.91 wt%, respectively, based on ICP data.


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Figure 5. Potential energy profiles for the dehydrogenation of formic acid in the (A) absence and (B) presence of phenylamine.


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Figure 6. TON values at 24 h for produced FA based on the total amount of Pd during the hydrogenation of CO2 and average diamter of the NPs over (a) Pd/MSC, (b) PdAg/MSC, (c) Pd/amine-MSC and (d) PdAg/amine-MSC. Reaction conditions: catalyst (50 mg), NaHCO3 in aqueous solution (1.0 M, 10 mL) and total pressure of 2.0 MPa (CO2:H2 = 1:1, volume ratio) at 100 °C for 24 h.


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ACS Catalysis

Figure 7. Potential energy profiles for the hydrogenation of CO2 in the (A) absence and (B) presence of phenylamine.


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Scheme 2. The FA/CO2-mediated hydrogen energy cycle over a PdAg/amine-MSC catalyst.


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ACS Catalysis

Figure 8. A liquid flow reaction system based on FA decomposition over a PdAg/amine-MSC catalyst and the resulting data.


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Graphic Abstract


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PdAg Nanoparticles Supported on Functionalized Mesoporous Carbon

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