Highly Efficient Supported Palladium-Gold Alloy Catalysts for

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Highly Efficient Supported Palladium-Gold Alloy Catalysts for Hydrogen Storage based on Ammonium Bicarbonate/Formate Redox Cycle Kengo Nakajima, Mitsuhiro Tominaga, Moe Waseda, Hiroki Miura, and Tetsuya Shishido ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04698 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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ACS Sustainable Chemistry & Engineering

Highly Efficient Supported Palladium-Gold Alloy Catalysts for Hydrogen Storage based on Ammonium Bicarbonate/Formate Redox Cycle Kengo Nakajimaa, Mitsuhiro Tominagaa, Moe Wasedaa, Hiroki Miuraa,b,d, Tetsuya Shishidoa,b,c,d*

a

Department of Applied Chemistry for Environment, Graduate School of Urban Environmental

Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan b

Research Center for Hydrogen Energy-based Society, Tokyo Metropolitan University, Tokyo

192-0397, Japan c Research

Center for Gold Chemistry, Tokyo Metropolitan University, Tokyo 192-0397, Japan

d Elements

Strategy Initiative for Catalysts & Batteries, Kyoto University, Kyoto 615-8520, Japan

* Corresponding author: Tel: +81-42-677-2850, Fax: +81-42-677-2850 (T. Shishido) E-mail address: [email protected] (T. Shishido)

ACS Paragon Plus Environment

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

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ABSTRACT

We have developed supported Pd–Au alloy catalysts for reversible hydrogen storage-release process with highly efficiency based on the reversible reactions involving ammonium formate and bicarbonate. Supported Pd–Au random alloy catalysts exhibited a superior activity than supported monometallic Pd and Au catalysts in both bicarbonate hydrogenation and formate dehydrogenation under ambient conditions even without additives such as acids or bases. We used a series of PdAu alloy NPs supported on active carbon (Pd-Au/AC) with various Au/Pd ratios ranging from 0.1 to 10 to investigate the effects of the Au/Pd ratio. Turn over frequency (TOF) of bicarbonate hydrogenation increased monotonically with the Au/Pd ratio. Conversely, we found a volcano type relationship between the TOF of formate dehydrogenation and Au/Pd ratio and 1Au1Pd/AC catalyst exhibited the highest TOF value. On the basis of our structural and kinetic analysis, we discuss the correlation between the activity and the state of Pd and Au in supported Pd–Au alloy NPs.

KEYWORD

formate dehydrogenation, bicarbonate hydrogenation, hydrogen storage and release, Pd-Au alloy nanoparticles,


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Introduction Hydrogen (H2) has attracted attention as a chemical energy carrier in a sustainable energy economy because hydrogen possesses environmentally benign nature with high gravimetric energy density compared with liquid fuels.1-4 However, the current inability to generate H2 from renewable materials and related storage and transportation issues interferes with the development of a sustainable H2 economy.5-6 Therefore, there is currently great interest in developing new methods for using hydrogen-based energy and stable and high-efficient storage. These issues are associated with the capture and on-demand evolution of hydrogen under ambient conditions.7-10 Recently, various solid carriers including simple metal hydrides, metal hydride alloys, 1113

metal borohydrides,14 and ammonia borane,15-17 have been proposed. However, quantitative

capture and release of hydrogen consumes additional energy. Conversely, liquid organic hydrogen carriers (LOHCs) including ammonia, amine boranes, formic acid and alcohols have received attention because LOHCs can be operated at ambient conditions18-26. Recently, formic acid (FA) and formates have drawn interest. FA and formates are non-toxic, non-flammable and stable.23, 27 Becuse formates are non-irritating and non-corrosive. Moreover, FA and formates are also suitable for easy transportation, handling and storage. Efficient dehydrogenation of FA and formates to generate hydrogen (H2) and reversible hydrogenation of CO2 and bicarbonate to form FA and formates can be achieved with the use of appropriate catalysts. Beller and co-workers have designed a reversible hydrogen storage system using bicarbonate/formate redox cycle (Scheme 1) with a Ru-based homogeneous catalyst.28 They demonstrated that the two reactions (i.e., bicarbonate hydrogenation and formate dehydrogenation) could be combined to make a closed carbon cycle for hydrogen storage without any additives. In


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1986, Sasson et al. proposed that sodium formate salts could be used as chemical carriers for hydrogen storage based on Pd/C catalyst. However, the efficiency of hydrogenation of sodium bicarbonate to corresponding formate was rather low.29 Su et al. reported that hydrogenation of ammonium bicarbonate to formates and hydrogen and the dehydrogenation of ammonium formate were catalyzed by Pd/AC catalyst.30 Recently, Xu and co-workers reported that a bimetallic Pd−Au nanoparticles (NPs) supported on a p-phenylenediamine-functionalized reduced graphene oxide (Pd−Au/PDA−rGO) worked as efficient catalyst for the interconversion of bicarbonate/formate to storage hydrogen (H2) under basic conditions.31 However, the correlation between the structure and electronic state of bimetallic Pd−Au NPs and the activity for the interconversion is still unclear. Moreover, there is a need to further improve the efficiency of heterogeneous catalysts at ambient conditions. Herein, we prepared a series of supported Pd-Au alloy catalysts with various Au/Pd ratios. We performed detailed characterization and kinetic analysis to examine the bicarbonate hydrogenation and formate dehydrogenation and gain insight into the effect of local structure and electronic states of Pd−Au alloy NPs on the activity of supported Pd−Au alloy catalyst. The atomic ratio of Au/Pd affected their catalytic activities in reversible reactions between ammonium bicarbonate/ammonium formate under mild reaction conditions.


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H2 Release

HCO2Formate

HCO3Bicarbonate

H2 Storage Scheme 1. Hydrogen storage and release by redox cycle between bicarbonate and formate.


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Experimental Section Materials. HAuCl4·3H2O, PdCl2 and Poly(vinyl alcohol) (PVA, Mw 9000-10000, 80 % hydrolyzed) were purchased from Wako Chemicals, Furuya Metal Co. Ltd, and Sigma-Aldrich, respectively. Active carbon (Ketjen black carbon ECP) was used as support. Analytical grade chemicals as received without further purification were used as substrates and solvents. Preparation of Supported Pd, Au and Pd-Au alloy Catalysts. Supported Pd–Au alloy catalysts were prepared by a colloid immobilization method32-33. PVA and NaBH4 aqueous solution were used as protecting and reducing agents, respectively. More detailed descriptions of the preparation methods are in the Supporting Information. The prepared catalysts were denoted xAuyPd/AC, where x and y indicate the molar ratio of Au to Pd. The total loading amount of metal was 5 wt%. Bicarbonate Hydrogenation. The low temperature bicarbonate hydrogenation was performed in a 50-mL glass-lined stainless-steel reactor. Appropriate amounts of ammonium bicarbonate and catalyst were added to distilled water (20 mL). The reactor was sealed and purged with high purity Ar three times, followed by charged with 5 MPa of H2. Then, the reactor was held at the set temperature. During the reaction, the solution was stirred at 300 rpm. Formate Dehydrogenation. The formate dehydrogenation was performed in a 30-mL twonecked round bottom flask. The catalyst (100 mg) was added to 4 mL of distilled water. The reactor was sealed and purged with N2 three times. The flask was heated to the 313 K with an oil bath. The reaction was started by the injection of 1 mL of an aqueous solution of ammonium formate (5 mmol). During the reaction, the solution was stirred at 300 rpm.


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Aqueous-Phase Product Analysis. After the reaction, the mixture was cooled to room temperature and then the gas pressure was recorded and was gradually released. The solid catalyst was separated by using a 0.45 μm polytetrafluoroethylene (PTFE) filter (Millipore Millex LH). The obtained solution was analyzed by high performance liquid chromatography (HPLC) (JASCO HPLC system equipped with a refractive index detector (JASCO RI-2031)). Aminex 87-H column (Bio-Rad) at a column temperature of 313 K was used for separation of the organic acids and reaction intermediates. 0.1 M H2SO4 was used as the mobile phase at 0.2 mL min-1 flow Gas-Phase Product Analysis. The volume of evolved gas was determined by a gas burette. The released gas was analyzed by TCD-GC (Shimadzu GC-8A, Porpapak-Q and MS5A columns). CO was detected with FID-GC (Shimadzu GC-14B, Porapak-Q column) equipped with a methanizer. Catalyst Characterization. The supported catalysts were characterized by N2-adsorption, X-ray diffraction(XRD), transmission electron microscope(TEM) imaging, X-ray photoelectron spectroscopy(XPS), and X-ray absorption fine structure (XAFS) techniques. The Brunauer– Emmett–Teller (BET) specific surface area was estimated from N2 isotherms obtained using a BELSORP-mini II (BEL Japan, Osaka, Japan) at 77 K. XRD patterns were recorded using Cu K radiation and a one-dimensional X-ray detector (XRD: SmartLab, RIGAKU). The samples were scanned from 2Ө=35° to 50° at a scanning rate of 0.05° s-1 and a resolution of 0.005°. High angle annular dark field-scanning TEM (HADDF-STEM) images were recorded on a JEOL JEM3200FS. X-ray photoelectron spectroscopy (XPS) was performed on a JEOL JPS-9010 MX instrument. The spectra were measured with MgKα radiation (15 kV, 400 W) in a chamber at a base pressure of ~10-7 Pa. All spectra were calibrated against the C1s peak (284.6 eV) as a reference. The adsorbed amount of CO on the catalyst was measured by the CO pulse method at room temperature using an Okura BP-2 instrument (Okura Riken, Japan) with a thermal conductivity


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detector (TCD). The catalyst precursor was treated with helium at 473 K for 1 h. Au L3-edge and Pd K-edge XAFS spectra were measured at the BL01B1 beam line (SPring-8) with a Si(311) twocrystal monochromator. XAFS spectra were obtained at room temperature, and were analyzed with the REX2000 version 2.5 (Rigaku). The loading amounts of palladium and gold in Pd-Au/AC catalysts were determined by atomic emission spectroscopic analysis with a Shimadzu AA-6200.

Results and Discussion Figure 1 shows the results of ammonium bicarbonate hydrogenation over Pd-Au/AC catalysts with various Au/Pd ratios. The Pd-Au/AC catalysts showed a higher production rate of ammonium formate (HCO2NH4) per weight of Pd than that of the Pd/AC catalyst regardless of the Au/Pd ratio. The Au/AC catalyst showed no activity. These results indicate that the active site for ammonium bicarbonate hydrogenation is Pd and its activity was enhanced by the addition of Au. The production rate of HCO2NH4 increased with the Au/Pd ratio up to Au/Pd = 5. The turnover number frequency (TOF) increased with the Au/Pd ratio monotonically and 10Au1Pd/AC catalyst showed the highest TOF value (5820 h-1) among the AC-supported catalysts. Figure 2 shows the result of ammonium formate dehydrogenation over supported Pd-Au/AC catalysts with various Au/Pd ratio. Pd-Au/AC catalysts showed a higher production rate of hydrogen (H2) per weight of Pd than that of Pd/AC catalyst except for 10Au1Pd/AC catalyst. No hydrogen formed on Au/AC under these reaction conditions, indicating that Pd operated as an active site for ammonium formate dehydrogenation. Both the production rate of H2 and TOF increased as the Au/Pd ratio increased to Au/Pd = 1 and then decreased. The 1Au1Pd/AC catalyst showed the highest TOF value (4200 h-1) among the AC-supported catalysts.


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The H2 production rates via dehydrogenation of ammonium formate by using 1Au1Pd/AC catalyst at 313 K was compared with a commercial 5wt% Pd/AC (Sigma-Aldlich). The H2 production rate of dehydrogenation of ammonium formate over 5wt% Pd/AC (Sigma-Aldrich) was 1326 mmol h-1 g(Pd)-1 and TOF was 606 h-1. These values are much lower than those on 1Au1Pd/AC catalyst (5136 mmol h-1 g(Pd)-1 and 4200 h-1). The formation rate of ammonium formate (HCO2NH4) via hydrogenation of ammonium bicarbonate with 5 MPa H2 (333 K) by using commercial 5wt% Pd/AC (Sigma-Aldrich) was 837 mmol h-1 g(Pd)-1 and TOF was 382 h-1. These values are much lower than those on 1Au1Pd/AC catalyst (2144 mmol h-1 g(Pd)-1 and 1756 h-1). We also compared the H2 production rates via dehydrogenation of formates (Na and K salts) by using 1Au1Pd/AC catalyst at 313 K (Table S2). The H2 production rate of dehydrogenation of ammonium formate was much faster than those of K and Na salts. It was reported that the volumetric hydrogen density is related to the solubility of formate salts in bicarbonate/formate hydrogen storage system.30 The solubility of ammonium formate (ca. 22 mol L-1) is nearly double that of sodium formate (ca. 12 mol L-1) at room temperature. Thus, we focused on the ammonium bicarbonate/formate system.


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2500

6000 5000

2000

4000 1500 3000 1000

TOF / h-1

Production rate of HCO2NH4 / mmol g(Pd)-1 h-1

2000 500

1000

n.d.

0

P u1 d 0 1A P d u5 1A P d u3 1A P d u1 3A P d u1 5A P d 10 u1P Au d 1P d Au

0

1A

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

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Figure 1. Production rate of ammonium formate over Pd-Au/AC, Au/AC and Pd/AC catalysts. Conditions: 1 mol L-1 NH4HCO3 20 mL, 5 MPa H2, 0.1 g catalyst, 333 K. TOF values were calculated based on the number of surface Pd atoms, which was determined from the amount of adsorbed CO (Figure S1 and Table S1). No CO adsorbed on the Au/AC catalyst.


10

6000

4500 4000

5000

Production rate of H2 / mmol g(Pd)-1h-1

3500

4000

3000 2500

3000

2000

2000

1500 1000

1000

500 0

Au

0

1A P u1 d 0 1A Pd u5 1A Pd u3 1A Pd u1 3A Pd u1 5A Pd u 10 1P Au d 1P d

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


TOF / h-1

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Figure 2. Production rate of hydrogen over Pd-Au/AC, Au/AC and Pd/AC catalysts. Conditions: 1 mol L-1 NH4HCO2 4 mL, 0.1 g catalyst, 313 K. TOF values were calculated based on the number of surface Pd atoms, which was determined from the amount of adsorbed CO (Figure S1 and Table S1). No CO adsorbed on the Au/AC catalyst.


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To get insight into the significant effects of the Pd/Au ratio on the activities in both ammonium bicarbonate hydrogenation and ammonium formate dehydrogenation, the local structure and electronic states of supported Pd–Au alloy NPs were analyzed by various spectroscopic techniques. Analysis by atomic absorption spectroscopy (AAS) indicated that Au and Pd were successfully loaded on the AC support regardless of the Au/Pd ratio (Table 1). The morphologies of the Pd–Au alloy NPs were identified by HAADF-STEM. Figure 3 shows HAADF-STEM images of Pd–Au/AC catalysts with various Au/Pd ratio. The mean particle sizes were estimated to be ca. 3 nm. We observed no notable differences in the mean size, suggesting that the remarkable differences in the catalytic activities of the Pd–Au/AC catalysts with different Au/Pd ratios in ammonium bicarbonate hydrogenation/ammonium formate dehydrogenation were not caused by differences in the size of the Pd–Au alloy NPs supported on AC.


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Table 1. Au and Pd loadings and Au/Pd molar ratios of the as synthesized catalysts determined by AAS analysis.

Catalyst

Pd (wt%)

Au (wt%)

Pd+Au (wt%)

Pd:Au (mol/mol)

Au:Pd (mol/mol)

1Au10Pd

4.21

0.74

4.95

10.59

0.094

1Au5Pd

3.64

1.30

4.94

5.16

0.19

1Au3Pd

3.08

1.84

4.92

3.11

0.32

1Au1Pd

1.72

3.19

4.91

1.00

1.00

3Au1Pd

0.75

4.18

4.93

0.33

3.00

5Au1Pd

0.48

4.43

4.91

0.20

5.02

10Au1Pd

0.24

4.66

4.90

0.096

10.39


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(a1)

(b1)

10 nm

10 nm

10 nm

10 nm

(f1)

10 nm

10 nm

(h1)

(g1)

10 nm

(c1)

(e1)

(d1)

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10 nm


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120

120

2.9±0.8

3.2±1.1

60

1

2

3

4

5

6

7

8

0

9

Diameter / nm

(d2)

120

(e2)

90

Counts

Counts

90

60

60

30

30

2

3

4

5

6

7

8

0

9

Diameter / nm

(g2)

60

30

1 2 3 4 5 6 7 8 9

Diameter / nm

120

(f2) 3.0±1.0

2.9±0.9

90

1 2 3 4 5 6 7 8 9

Diameter / nm

120

2.9±0.8

Counts

0

1 2 3 4 5 6 7 8 9

Diameter / nm

120

1

60

30

30

0

120

0

1 2 3 4 5 6 7 8 9

Diameter / nm

(h2)

2.7±1.0

2.5±0.8

90

Counts

90

60

30

0

3.1±1.0

Counts

Counts

Counts

30

(c2)

90

90

60

0

120

(b2)

(a2)

90

Counts

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


60

30

1

2

3

4

5

6

Diameter / nm

7

8

9

0

1

2

3

4

5

6

7

8

9

Diameter / nm

Figure 3. HADDF-STEM images of Pd-Au/AC catalysts with various Au/Pd ratios. (a1)Pd, (b1)1Au10Pd, (c1)1Au5Pd, (d1)1Au3Pd, (e1)1Au1Pd, (f1)3Au1Pd, (g1)5Au1Pd, (h1)10Au1Pd, and (a2-h2) are histograms of particle size distribution.


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We performed XPS measurements of Pd–Au/AC catalysts to investigate the electronic states of Au4f orbitals of Au and Pd on AC support. Figure 4 shows the Au 4f XP spectra of Pd–Au/AC catalysts with various Pd/Au ratios. For 10Au1Pd/AC, Au 4f7/2 and 4f5/2 peaks appeared at 84.1 and 87.6 eV, respectively. These peaks shifted to higher binding energies as the Au/Pd ratio was increased. This result clearly indicates that electrons transferred from Pd to Au in Pd-Au alloy NPs. This electron transfer is consistent with the order of Pauling electronegativity of Pd (2.20) and Au (2.54). Generally, peaks attributed to Pd 3d5/2 and 3d3/2 appear at 335 and 340 eV, respectively (Figure S1)34. The peaks of 1Au3Pd/AC appeared at a higher binding energy than those of other Pd–Au/AC catalysts. For 10Au1Pd/AC, it is too difficult to analyze owing to quite low intensity. It was also difficult to confirm the peak position because of their low intensity (because of low Pd loading, For 3Au1Pd, 5Au1Pd, 10Au1Pd, 0.75, 0.48, and 0.24 wt%, respectively. see Table 1) and overlap of the peaks with the Au 4d5/2 features.


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Au4f5/2

Au4f7/2

(h) (g)

Intensity / cps

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


200

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(f) (e) (d) (c) (b) (a) 90

88 86 84 82 Binding energy / eV

80

Figure 4. Au 4f XP spectra of Au–Pd/AC catalysts. (a) Pd, (b) 1Au10Pd, (c) 1Au5Pd, (d) 1Au3Pd, (e) 1Au1Pd, (f) 3Au1Pd, (g) 5Au1Pd, and (h) 10Au1Pd.


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Figure 5 shows normalized Au L3-edge XANES spectra of Pd–Au/AC catalysts and Au foil. The shape and absorption edge energy of Au foil are similar to those of Pd–Au/AC catalysts regardless of Au/Pd ratios. This result suggests that the Au in the Pd-Au alloy NPs is present in a metallic state. It is well known that the intensity of white line at 11921 eV, which is due to electronic transitions from 2p3/2 core-level states, directly reflects the electronic states of the vacant d orbitals of gold35-36. The L3-edge reflects the final vacant d states of both 5d3/2 and 5d5/2 levels. The whiteline intensity of Pd–Au/AC catalysts was lower than that for Au foil. The intensity decreased as the Au/Pd ratio in the Pd-Au alloy NPs decreased. The lower intensity of the white-line observed for the Pd–Au/AC catalysts compared with the Au foil implies charge-transfer from Pd to Au and Pd–Au alloy formation on Pd–Au/AC catalysts. Pd K-edge XANES spectra of Au–Pd/AC catalysts are shown in Figure S2. The white-line intensity at 24360 eV gradually decreased with a decrease in the Au/Pd ratio and the spectrum of the Pd foil showed the lowest intensity. These results clearly indicate that electron-deficient Pd species in the Pd–Au NPs with a high Au/Pd ratio was generated by charge transfer from Pd to Au. The results of XPS and XAFS are consistent each other.


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(B)

Normalized absorption

0.5

0.05

(A)

Normalized absorption

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


(h) (g) (f) (e) (d)

Au foil 10Au1Pd 5Au1Pd 3Au1Pd 1Au1Pd 1Au3Pd 1Au5Pd 1Au10Pd

(c) (b) (a) 11900

11920

11940

11960

Photon energy / eV

11980

11918

11920

11922

11924

Photon energy / eV

Figure 5. Au L3-edge XANES spectra (A) and their magnification around the white line (B) of PdAu/AC catalysts. (a) 1Au10Pd, (b) 1Au5Pd, (c) 1Au3Pd, (d) 1Au1Pd, (e) 3Au1Pd, (f) 5Au1Pd, (g) 10Au1Pd, and (h) Au foil.


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The local structures of Pd–Au alloy NPs supported on AC was analyzed by curve-fitting analyses of the Au L3 and Pd K-edge EXAFS spectra of Pd–Au/AC catalysts37. Figure 6 shows the change in the coordination numbers (Au–Pd, Au–Au, Pd–Au, and Pd–Pd bonds) against the Au content in Pd–Au/AC catalysts. The coordination numbers of Au–Pd and Pd–Pd bonds monotonically decreased with the Au/Pd ratio, in other words, Au content in the Pd–Au alloy NPs. On the contrary, the coordination numbers of Au–Au and Pd–Au increased as the Au/Pd ratio increased. On the basis of on these results, we propose that random Pd–Au alloy NPs were dispersed on the AC (Scheme 2). The configuration of Au and Pd atoms on the surface of the Pd–Au alloy NPs varied as the Au/Pd ratio was changed. For the Pd–Au alloy NPs with a high Au/Pd atomic ratio, isolated single Pd atoms surrounded by Au atoms were mainly present. For the 10Au/1Pd/AC catalyst, which exhibited the highest catalytic activity in bicarbonate hydrogenation, the coordination numbers of Au–Pd and Pd–Pd bonds were quite small. These results suggest that isolated single Pd atoms surrounded by Au atoms in the Pd–Au alloy NPs showed high activity in bicarbonate hydrogenation. For the 1Au1Pd/AC catalyst, which exhibited the highest catalytic activity in formate dehydrogenation, the coordination numbers of Au–Pd and Pd–Pd bonds were almost equivalent, suggesting that Pd atoms surrounded by equivalent numbers of Pd and Au atoms in the Pd–Au alloy showed high activity in formate dehydrogenation.


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10

Au-Au Au-Pd

C. N.

8

(A)

6 4 2 0 0

20

40

60

80

100

Au content (%) 10

(B)

Pd-Au Pd-Pd

8

C. N.

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


6 4 2 0 0

20

40

60

80

100

Au content (%) Figure 6. The change in coordination numbers of (A) Au–Au and Au–Pd, and (B) Pd–Au and Pd– Pd bonds against Au content in Pd–Au/AC catalysts.


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Pd

Au

1Au3Pd

1Au1Pd

3Au1Pd

5Au1Pd

10Au1Pd

Scheme 2. Proposed structure of Pd–Au alloy NPs with various Au/Pd ratios.


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On the basis of previous studies and our structural characterization of Pd–Au alloy NPs, we propose catalytic cycles for bicarbonate hydrogenation and formate dehydrogenation at the Au– Pd alloy NPs in aqueous media in Scheme 3. In the bicarbonate hydrogenation [Scheme 3 (A), bicarbonate is adsorbed on gold [step(i)] and hydrogen adsorbed dissociatively on palladium and gold [step(ii)]. The nucleophilic hydrogen atoms formed on Pd then undergo nucleophilic addition to the carbonyl group adsorbed on Au, forming and then eliminating formate [step(iii)]. Finally, the hydroxyl group reacts with hydrogens on Au to form H2O, which is eliminated [step(iv)]. In the formate dehydrogenation [Scheme 3 (B)], formate ion adsorbs on Au [step(i)]. Then, nucleophilic addition of water to the carbonyl groups adsorbed to Au forms an intermediate [step(ii)]. Cleavage of the C-H bond in the intermediate leads to bicarbonate formation and elimination [step(iii)]. Finally, the combination of hydrogens on Pd and Au proceeds to form a hydrogen molecule [step(iv)].


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Scheme 3. Possible reaction pathway for (A) bicarbonate hydrogenation and (B) formate dehydrogenation over Au–Pd alloy catalysts.


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The reaction mechanism and the role of Pd and Au species in the catalytic cycle were discussed based on a kinetic study of the Pd–Au/AC and Pd/AC catalysts for both ammonium bicarbonate hydrogenation (Figures S4 and S5) and ammonium formate dehydrogenation (Figure S6). In the case of ammonium bicarbonate hydrogenation, the reaction orders of bicarbonate and hydrogen were positive on both Pd/AC and Au–Pd/AC (Table 2). Table 3 summarized the the initial reaction rates by using H2 and D2 with each catalyst. The kinetic isotope effect (KIE) (kH/kD) values based on H2 and D2 were 2.2 and 1.5 for Pd/AC and Pd–Au/AC. On the basis of these results, the ratedetermining step was considered to involve nucleophilic addition of hydrogen to the carbonyl group in bicarbonate adsorbed on gold. The surface of Au NPs is generally accepted to exhibit a Lewis acidic nature.38 Structural analysis reveals that Au in Au–Pd alloys becomes more electronpoor with an increase in Au content. The stronger Lewis acidity of the Au atom would allow the Au atom to withdraw lone pair electrons from the carbonyl oxygen in adsorbed bicarbonate more strongly, facilitating the nucleophilic addition of nucleophilic hydrogens on Pd. As a result, the TOF value increased with an increase in the Au/Pd ratio.


25


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Table 2. Reaction order in ammonium bicarbonate hydrogenation.

Catalyst

[HCO3-]

p[H2]

Pd

0.6

0.9

1Au1Pd

0.5

1.1

Conditions: 0.3–1 mol L−1 NH4HCO3 20 mL, 5 MPa H2, 0.1 g catalyst, 333 K. Rate: Production rate of NH4HCO2 / mol h−1 cat(Pd)−1

Table 3. KIE in ammonium bicarbonate hydrogenation.

Catalyst Pd 1Au1Pd

Hydrogen

Rate / mol h-1 g(Pd)-1

H2

0.15

D2

0.07

H2

0.42

D2

0.27

kH/ kD 2.2 1.5

Conditions: 1 mol L−1 NH4HCO3 20 mL, 1 MPa H2 (D2), 0.1 g catalyst, 333 K. Rate: Production rate of NH4HCO2 / mol h−1 cat(Pd)−1


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In the case of ammonium formate dehydrogenation, the reaction orders of formate were 0.9 and 0.8 for Pd/AC and Au–Pd/AC, respectively. We also examined the kinetic isotope effect (KIE) by using protiated and deuterated water (H2O and D2O) and deuterated sodium formate (HCOONa and DCOONa) (Table 4). The KIE (kH/kD) values for H2O and D2O were 2.2 and 1.8 for Pd/AC and Pd–Au/AC, respectively. Conversely, the KIE (kH/kD) values for HCOO− and DCOO− were 1.1 and 1.4 for Pd/AC and Pd–Au/AC, respectively. These KIE values suggest that insertion of H2O is the rate-determining step and this step is accelerated by alloying. Conversely, C-H bond cleavage via β-hydrogen elimination [step(iii)] is inhibited by alloying. Similar to the hydrogenation described above, the stronger Lewis acidity of the Au atom, the stronger attraction of the lone pair of electrons on the oxygen of the carbonyl group in the substrate formate. This makes it easier for nucleophilic addition of nucleophilic hydrogen on Pd to occur. Therefore, the process of step(ii) appears to be activated as the Au/Pd ratio increases. Conversely, the KIE based on sodium deuterated formate suggests that cleavage of the C-H bond and the β-hydrogen elimination process [step(iii)] from the intermediate were suppressed. In general, electron-rich Pd is effective for β-hydrogen elimination.39-41. The structural analysis results of the catalyst described above indicate that Pd is in an electron-deficient state owing to its alloying with Au. Therefore, the process of step(iii) may be suppressed by increasing the Au/Pd ratios. The trade-off between acceleration and inhibition of the two processes through alloying suggests that the TOF value of formate dehydrogenation had the highest value at Au/Pd = 1.

Table 4. KIE in the sodium formate dehydrogenation.


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Production rate of H2 /mol・g(Pd)-1・h-1

Catalyst Pd 1Au1Pd

H 2O D 2O H 2O D 2O

Production rate of H2 /mol・g(Pd)-1・h-1

Catalyst Pd 1Au1Pd

0.648 0.293 1.157 0.657

HCO2Na DCO2Na HCO2Na DCO2Na

0.648 0.566 1.157 0.854

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kH / kD 2.2 1.8

kH / kD 1.1 1.4

Conditions: 1 mol L−1 HCO2Na (DCO2Na) 20 mL, 1 MPa H2O (D2O), 0.1 g catalyst, 313 K.


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Conclusion We developed highly active Pd–Au/AC catalysts for the catalytic interconversion of bicarbonate (HCO3−) and formate (HCOO−) for hydrogen storage and release system. Although the supported monometallic Au catalyst was totally ineffective the supported monometallic Pd catalyst showed a low activity and Au–Pd/AC catalysts exhibited high catalytic activity for hydrogenation of HCO3− and dehydrogenation of formate under mild reaction conditions. Detailed structural analysis clarified the formation of random Pd–Au alloy NPs with a uniform size of approximately 3 nm. The electronic state and configuration of Au and Pd atoms in the Pd–Au alloy NPs was varied by changing the Au/Pd ratio. The TOF of the hydrogenation of HCO3− increased with the Au/Pd ratio monotonically. For Pd–Au alloy catalysts with a high Au/Pd atomic ratio, electrondeficient and isolated single Pd atoms were present in the random Pd–Au alloy, which acted as the main catalytic active site for efficient hydrogenation of HCO3−, because the nucleophilic addition of hydrogen formed on Pd to the carbonyl group of bicarbonate adsorbed on Au was accelerated by Lewis acidic nature of Au. A volcano type relationship between TOF of dehydrogenation of HCOO− and Au/Pd ratio was obtained and 1Au1Pd/AC catalyst showed the highest TOF. We attributed this result to a trade-off in the acceleration of nucleophilic addition of nucleophilic hydrogen on Pd by Au and inhibition of C-H bond cleavage through β-hydrogen elimination at Pd.


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ASSOCIATED CONTET Supporting information Preparation method of Pd-Au/AC, Au/AC and Pd/AC catalysts, Amount of CO adsorption on AuPd/AC catalysts, dehydrogenation of formates (NH4, K and Na salts) over Pd-Au/AC catalyst, Kinetic results, Pd 3d XP spectra, Pd-K edge XANES spectra, Au L3-egde and Pd K-edge EXAFS analysis

Author information

Acknowledgement This study was supported in part by the Program for Element Strategy Initiative for Catalysts & Batteries (ESICB), Platform for Technology and Industry. A part of this work was also supported in part by Grants-in-Aid for Scientific Research (B) (Grant

17H03459)

and

Scientific

Research

on

Innovative

Areas

(Grant 17H06443) commissioned by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The XAFS experiments at SPring–8 were performed under the approval (proposal No. 2017B1245 and 2017B1326) of the Japan Synchrotron Radiation Research Institute (JASRI). We thank Mr. Eichi Watanabe of Tokyo Metropolitan University for providing technical support of HAADF-TEM observations. We thank Andrew Jackson, PhD, from Edanz Group (www.edanzeditting.com/ac) for editing a draft of this manuscript.


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For Table of Contents Use Only

Supported Pd–Au random alloy NPs work as highly efficient catalyst in reversible hydrogen storage-release process under mild conditions


35

Highly Efficient Supported Palladium-Gold Alloy Catalysts for

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