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Screening of carbon-supported PdAg nanoparticles in the hydrogen production from formic acid Miriam Navlani-García, Kohsuke Mori, Ai Nozaki, Yasutaka Kuwahara, and Hiromi Yamashita Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01635 • Publication Date (Web): 24 Jun 2016 Downloaded from http://pubs.acs.org on June 29, 2016

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Industrial & Engineering Chemistry Research

Screening of carbon-supported PdAg nanoparticles in the hydrogen production from formic acid

Miriam Navlani-García,1 Kohsuke Mori,* 1,2,3 Ai Nozaki,1 Yasutaka Kuwahara,1,2 and Hiromi Yamashita*1,2

1

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

Osaka University, 2-1 Yamada-oka, Suita, 565-0871 Japan. 2

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

Katsura, Kyoto 615-8520, Japan. 3

PRESTO, JST, Honcho, Kawaguchi, Saitama 332-0012, Japan

Corresponding authors: *[email protected] *[email protected]

KEYWORDS: H2 production, formic acid, palladium, silver, bimetallic NPs, alloy.

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ABSTRACT

A screening of carbon-supported PdAg nanoparticles (NPs) (PdAg/C) in the H2 production from formic acid (FA) dehydrogenation was carried out by using uniform ~3-5 nm PdAg alloy NPs with a wide compositional range in terms of PVP/Metal and Pd/Ag ratios. The evaluation of the catalytic performance combined with the detailed characterization indicated the beneficial effect of the Ag incorporation and also revealed that the best performance achieved by Pd1Ag2/C (PVP/metal = 1) can be ascribed to its optimum composition and electronic features.


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1. Introduction Driven by depletion of fossil fuel, increasing worldwide energy needs, and environmental concerns, there is an increasing demand for safe and renewable energy carriers for transportation and other energy-related applications 1,2. Molecular hydrogen (H2), associated with the proton exchange membranes (PEM) technology, is currently claimed as a viable and advantageous option for delivering high-quality energy services in an efficient, clean, and safe way 3,4. However, in the present social and technological context, there are still barriers to be overcome towards the H2 economy implantation, mainly related to the lack of methods for the efficient and safe H2 storage and transportation 3,5-8. Important efforts have been dedicated to the development and study of H2 storage materials, either physical or chemical storage. Among them, liquid-phase chemical H2 storage shows important advantages with respect to other technologies. Several liquid H2 carriers, such as aqueous boron and nitrogen-based compounds (Li/NaBH4 and NH3BH3) 9-15 and hydrous hydrazine (H2NNH2·H2O) 16-18, as well as alcohols 19-22 have received much attention. Within these liquid H2 carriers, FA, recognized as a potential H2 storage system in 1978

23

, is nowadays considered as one of the most outstanding

materials for this application 1. At first glance, this fact could not be ascribed to its relatively low H2 content (4.4 wt.%), but advantageous properties such as its lowtoxicity and suitability for easy transportation, handling, and safe storage, as well as its useable/net capacity (defined as the effective H2 content that can be recovered in the form of H2 from a chemical H2 storage system numerous studies

24

) make this molecule deserving

1,25-32

. In this line, many investigations deal with the preparation of

highly active and robust homogeneous catalysts that selectively produce H2 and CO2 from FA under mild conditions 4,33-36 while the development of heterogeneous catalysts 3 ACS Paragon Plus Environment


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achieving competitive activity at low temperature and high selectivity is still a challenging task 8,37, even though mayor achievements were recently reported 38. Most of the heterogeneous catalysts are based on noble metal NPs

39,40

, especially

Pd, as it has been addressed to be one of the most selective towards the H2 production as well as catalytically active under mild conditions, being both features the main focus of comparison with the usually best-performing homogeneous systems

25-27,41-45

.

Unfortunately, monometallic Pd catalysts are deactivated quickly due to the poisoning intermediates, and recent studies found that Pd-based bimetallic or multimetallic catalysts may overcome the poisoning and improve the selectivity to enhance the H2 production from FA decomposition

46,47

. A wide variety of bi/tri-metallic Pd-based

catalysts are already reported in the literature (PdCu 48, PdCo49, PdNi 50, PdAu 43,46,51,52, AgAuPd

53

, NiAuPd

54

and so forth), but those bimetallic catalysts consisting of PdAg

have been claimed as one of the most attractive approaches for this application

26,55-58

.

The enhancement in the catalytic performance of PdAg bimetallic NPs has been attributed to several factors. For instance, Tedsree et al.

56

found that the enhanced

catalytic activity of the Ag–Pd core–shell nanocatalyst was due to the Pd terrace sites in the shell, which was electronically promoted by the Ag core. A theoretical study reported by Cho et al.

59

suggested that the H2 selectivity of the bimetallic Pd/Ag

catalysts strongly depends on the Pd atomic layer thickness at near surface and the thinnest Pd monolayer is responsible for enhancing the H2 selectivity by reducing the surface binding strength of specific intermediates, such as HCOO and HCO. Meanwhile, a recent work developed in our research group

26

addressed that the

enhancement in the H2 production of core−shell Pd−Ag nanocatalysts was attributed to the charge transfer from Ag core to the Pd shell owing to the net difference in work


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function, which strengthens the adsorption of formate species through the strong backdonation. Herein we report a screening of carbon supported PVP-capped PdAg catalysts in the H2 production from the FA dehydrogenation reaction. Parameters such as Pd/Ag and PVP/Metal molar ratios were systematically tuned in order to optimize the final catalytic performance.

2. Experimental 2.1 Materials and reagents Palladium (II) acetate, silver nitrate, polyvinylpyrrolidone K-30, ethylene glycol and 1,4-dioxane were used for the NPs synthesis. Commercial SHIRASAGI M (Osaka Gas Chemicals Co. Ltd.) carbon was used as support. Formic acid (98-100 %) and sodium formate were used to conduct the catalytic tests. All reagents were supplied by Nacalai Tesque and used as received.

2.2. PdAg NPs synthesis and purification Colloidal bimetallic NPs were prepared by using the polyol method in ethylene glycol as solvent and reducing agent, and polyvinylpyrrolidone (PVP) as capping agent 60,61

. In order to synthesize NPs with different features, various compositions in Pd/Ag

and PVP/Metal molar ratios were used. The amount of palladium in the NPs was fixed, while both Ag and PVP amounts were changed with Pd/Ag of 1/0.5, 1/1, 1/2 and 1/4 and PVP/Metal of 1, 5 and 10, for each bimetallic composition. According to their



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compositions, the colloids were denoted as PdxAgy(z), where “x” and “y” are related to the PdAg molar ratio and “z” indicates the PVP/Metal ratio. The syntheses were carried out in an Ar atmosphere by means of a Schlenk system and the experimental steps were as follows. Two solutions were prepared for each colloidal composition. For solution 1, PVP (different amounts depending on the target PVP/Metal ratio) was added to 15 mL of ethylene glycol in a three necked round-bottomed flask and the solution was stirred at 80 °C. Once PVP was completely dissolved, the desired amount of Ag precursor (silver nitrate) was added and temperature and stirring were kept for 2 h. Solution 2 was prepared using a two necked round-bottomed flask, where 0.125 mmol of Pd precursor (palladium (II) acetate) were dissolved in 6.25 mL of dioxane by stirring for 2 h at room temperature. Then solution 1 was cooled to 0 ºC with an ice bath and NaOH 1 M was added under stirring in order to adjust the pH of the resulting mixture to 9-10. Then, solution 2 was poured into solution 1 under vigorous stirring to ensure homogenization and the final mixture was heated at 100 ºC for 2 h, while purging with Ar to ensure inert atmosphere during the reduction reaction of metal ions. The addition of the base in solution 1 containing Ag precursor, as well as the faster reduction rate of Ag ions as compared with Pd

62

, might favor the formation of

bimetallic PdAg NPs with an inhomogeneous structure, Ag-rich in the core and Pd-rich in the surface of the NPs. Afterwards, the NPs were purified as described elsewhere

25,27

. To this end, each as-

prepared colloid was pouring into a glass bottle together with an excess of acetone and the solution was shaken, which caused the extraction of PVP to the acetone phase and flocculation of the metallic NPs. PdAg NPs were subsequently separated by either 6 ACS Paragon Plus Environment

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decantation or centrifugation, depending on the composition, and the NPs were dispersed in a known amount of MeOH, so as to have the desired metal concentration in the final colloids. According to this procedure, a screening of 12 colloids with various Pd/Ag and Pd/Metal molar ratios was prepared: Pd1Ag0.5(1, 5 and 10), Pd1Ag1(1, 5 and 10), Pd1Ag2(1, 5 and 10) and Pd1Ag4(1, 5 and 10).

2.3. Catalysts preparation In this study, 12 PdAg/C catalysts were prepared by using the metallic PdAg NPs colloids and the commercial SHIRASAGI M (Osaka Gas Chemicals Co. Ltd.) carbon support. The bimetallic NPs were loaded on the carbon material by using the impregnation method by mixing the carbon support with the adequate amount of metal colloid so as to have a target 0.5 wt.% Pd loading. These mixtures were vigorously stirred for 48 h using a magnetic stirrer and then the samples were dried at 60 ºC to remove the methanol solvent, washed several times with a cold mixture of H2O/EtOH (50:50, v/v) and dried overnight at 60 ºC. The as-prepared catalysts were denoted as PdxAgy/C(z), according to the nomenclature used for the counterpart colloid.

2.4. Catalytic tests of FA dehydrogenation The catalytic performance of the PdAg/C catalysts was evaluated by using a closed liquid-phase system. 50 mg of each powder sample and 9.6 mL of water were mixed into a Pyrex reaction vessel (30 mL) which was sealed with a rubber septum and the sample was treated in an ultrasound bath for 15 min to disperse the catalyst. After that, it was bubbled with Ar gas for 30 min to ensure inert atmosphere. To start the reaction, 7 ACS Paragon Plus Environment


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0.39 mL of FA 98-100 % (which resulted in a final concentration of 1 M) was added into the vessel and it was placed in an oil bath at 30 ºC with magnetic stirring for 3 h. The gas phase was sampled with gas tight syringe periodically and the H2 evolution was analyzed using a Shimadzu GC14B equipped with MS5 A column. Control experiments were also performed to confirm the no evolution of H2 gas while using the bare carbon support. In addition, the catalytic activity of the best-performing PdAg/C catalyst was assessed by using an open system. In this case, the catalyst was placed into a reaction vessel with a reflux condenser and equipped with a gas burette. The sample was purged with inert gas and then an aqueous solution containing HCOOH and HCOONa (HCOOH/HCOONa= 9/1; 1 M; 10 mL) was added to the reaction vessel and the H2 generation at 75 ºC was monitored. It should be mentioned at this point that the addition of HCOONa is expected to have a beneficial effect in the overall catalytic performance 26,63

, as the H2 production undergoes without generation of the undesired CO, avoiding

the subsequent catalyst poisoning, by the following reaction pathway: + → + (1) + → + + (2) → + (3)

2.5. Catalysts characterization UV-vis diffuse reflectance spectra of the metallic colloids were collected using a Shimadzu UV-2450 spectrophotometer. Transmission Electron Microscopy (TEM) micrographs were recorded using a Hitachi H-800 electron microscope equipped with an energy-dispersive X-ray (EDX) detector, operated at 200 kV. This characterization


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technique was used to confirm the NPs size and distribution in the different catalysts. The average NPs diameters (dTEM) were calculated by counting more than 100 NPs. In addition, Pd K-edge and Ag K-edge XAFS spectra were recorded using a fluorescence-yield collection technique at the beam line 01B1 station with an attached Si (111) monochromator at SPring-8, JASRI, Harima, Japan (prop. No. 2015B1068 and 2016A1095). The EXAFS data were normalized by fitting the background absorption coefficient, around the energy region higher than the edge of about 35–50 eV, with the smoothed absorption of an isolated atom. The EXAFS data were examined using the Rigaku EXAFS analysis program. Pd 3d and Ag 3d X-ray photoelectron spectroscopy (XPS) data were collected using an ESCA 3400 Electron Spectrometer and the quantitative analyses were done from the integrated intensities of the spectra. The surface Pd/Ag ratio present in the samples was calculated from the contribution of both oxidized and metallic state of Pd and Ag and their contribution to the total area (Pd +Ag) for each sample.

3. Results and discussion 3.1. Catalytic tests of FA dehydrogenation The catalytic performance of the PdAg/C catalysts in the FA decomposition reaction was evaluated by monitoring the H2 output in a closed liquid-phase system during 3 h at 30 ºC. It is worth highlighting that the selectivity towards the H2 production was almost 100% for all samples, indicating that the PdAg catalysts boost only the FA dehydrogenation (HCOOH ↔ H2 + CO2), avoiding therefore the undesired production of CO through the FA dehydration reaction (HCOOH ↔ CO + H2O). The H2 evolution 9 ACS Paragon Plus Environment


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profiles with the time (results not shown here) indicated that the FA dehydrogenation proceeded without induction period for all samples.

Figure 1. H2 output after 3 h of reaction at 30 ºC for all the studied PdAg/C catalysts.

Figure 1 shows a screening of the H2 output after 3 h of reaction at 30 ºC for all the studied PdAg/C catalysts as a function of the NPs composition, in terms of Pd/Ag and PVP/M ratios. As it can be seen, the catalytic activities were significantly influenced by both Pd/Ag and PVP/M ratios. In general, the H2 output increased gradually with the Ag content until reaching an optimum Pd/Ag ratio, which appeared to be Pd/Ag=1/2 almost regardless the PVP/M ratios. Further incorporation of Ag resulted in a drastic decay of the catalytic activity for all the PVP/M ratios investigated. Among the samples with the optimum composition (Pd/Ag = 1/2), the highest H2 production (266 µmol) was achieved with the catalyst prepared by using the colloid with PVP/M = 1. It should be noticed that the catalytic performance of the counterpart monometallic Pd-based catalyst prepared with a PVP/M ratio of 1 was poorer than the bimetallic PdAg catalysts, 10 ACS Paragon Plus Environment

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producing as much as 142 µmol of H2 after 3 h under the same experimental conditions (See the H2 evolution profile in Figure S1), indicating the beneficial effect of the Ag addition. The applicability of the best-performing catalyst was highlighted by the open system reaction conducted at 75 ºC and using an aqueous solution with HCOOH/HCOONa = 9/1. The H2 evolution profile (See Figure S2) indicated that FA decomposition proceeded without induction time and the reaction rate reached 3000 mL min−1g−1 (Pd) with a TOF value of 855 h−1 (notice that the TOF value, defined as the mole of H2 produced per mole of metal, was calculated on the basis of Pd and by using the amount of H2 generated after 4 min of reaction). The suitability of the present catalytic system is pointed out by the comparison with other previously reported PdAg-based catalysts, such as Ag/Pd alloy (144 h-1, 20 ºC), Ag@Pd (626 h-1, 90 ºC)

56

, Ag1Pd3 alloy foam

(35.1 h-1, 25 ºC) 64, Ag42Pd58/C (382 h-1, 50 ºC) and Ag0.1–Pd0.9/rGO (105 h-1, 25 ºC) 65

. The results plotted in Figure 1 also enclose the PVP/M ratio effect and it can be

observed that the amount of H2 generated decreased as the PVP/M ratios increased for all the Pd/Ag ratios studied. This fact could be firstly ascribed to a partial poisoning or blocking of the catalytic active sites by the polymer molecules 66, which would explain the poorer activity of samples with PVP/M=10 as compared with PVP/M=1 and PVP/M=5 counterparts. Nevertheless, the NPs purification step as well as the repeated washing with H2O/EtOH mixture in the deposition step of NPs onto the carbon support might weaken this argument. Then, the better performance attained by using lower PVP amount might be tentatively ascribed to either NPs size or NPs surface composition and electronic effect induced by the capping agent molecules. In order to elucidate these possible effects, further characterization results are included in the next section. 11 ACS Paragon Plus Environment


3.2. Characterization of the bimetallic colloid

a) Pd1Ag0.5 PVP/M=1 PVP/M=5 PVP/M=10

200

300

400 500 600 Wavelength (nm)

700

300


300

d)


700

700

Pd1Ag4

Absorbance (a.u.)

4 3.5 3 2.5 2 1.5 1 0.5 0

PVP/M=1 PVP/M=5 PVP/M=10

200

Pd1Ag1 PVP/M=1 PVP/M=5 PVP/M=10

200

c) Pd1Ag2

3.5 3 2.5 2 1.5 1 0.5 0

b)

4 3.5 3 2.5 2 1.5 1 0.5 0

Absorbance (a.u.)

Absorbance (a.u.)

4 3.5 3 2.5 2 1.5 1 0.5 0

Absorbance (a.u.)

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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200

300


700

Figure 2. UV-vis spectra of the bimetallic colloids for the three PVP/M ratios and different composition: a) Pd1Ag0.5; b) Pd1Ag1; c) Pd1Ag2; d) Pd1Ag4.

The analysis of the optical properties of Pd–Ag colloids was reported to be an useful tool to estimate qualitatively the presence of allowed structures 67. The UV-vis spectra of the as-prepared bimetallic colloids are shown in Figure 2. Regardless the colloid composition, all the metallic colloids showed UV absorption at λmax=250–300 nm, which, according to theoretical calculation, was ascribed to the surface plasmon resonance (SPR) absorption of spherical Pd NPs

68

. On the contrary, Ag has been

reported to produce an intense, sharp and surface-sensitive plasmon absorption at around 420 nm

69

. The lack of intense absorption in this region suggested the Pd rich

surface morphology in the PdAg NPs. Nevertheless, it should be noticed that, in some cases, a broad band centered at around 400 nm can be observed, which could be 12 ACS Paragon Plus Environment

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attributed to the presence of Ag atoms on the NPs surface. The significant shift with respect to the Ag absorption peak (at 420 nm) was probably due to the formation of a Ag/PVP-based bimetallic structure. A similar observation was already reported by Wang and coworkers in a study where Ag/Pd NPs stabilized by PVP were prepared as catalyst for electroless copper deposition 62. The same authors also reported the effect of PVP in the synthesis of AgPd bimetallic NPs 70. According to the electrochemical measurements, they concluded that Ag+ ions are favored to be reduced in the initial stage of the bimetallic NPs formation. The reduction of Ag+ ions is additionally promoted by the addition of NaOH in the Ag precursor solution in our case. They reported that steric hindrance induced by PVP makes the diffusion of Pd2+ more difficult than Ag+ through polymeric obstacles, which led to the reduction of Pd2+ on the Ag-rich nuclei to form Pd-rich surface NPs. The relatively higher absorption intensity attributed to the presence of Ag cluster on the NPs surface (400 nm) in the PVP/M=10, compared to those with lower PVP/M ratios, and especially observed in the Pd1Ag2 and Pd1Ag4 colloids (Figure 2c and 2d), might be related to the hindered diffusion of Pd2+ together with the high Ag+ content in the reaction media.

3.3. Characterization of the PdAg/C catalysts



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Figure 3. TEM micrographs for the PdAg/C catalysts with different Pd/Ag and PVP/M ratios: a) Pd1Ag0.5/C(1); b) Pd1Ag0.5/C(5); c) Pd1Ag0.5/C(10); d) Pd1Ag1/C(1); e) Pd1Ag1/C(5); f) Pd1Ag1/C(10); g) Pd1Ag2/C(1); h) Pd1Ag2/C(5); i) Pd1Ag2/C(10); j) Pd1Ag4/C(1); k) Pd1Ag5/C(5); l) Pd1Ag4/C(10).


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Figure 3 shows the TEM micrographs and size distribution diagrams for all the carbon-supported PdAg catalysts. The average NP size, determined by counting ~100 NPs, together with the metal dispersion is shown in Table 1. As it can be seen in TEM micrographs, all the catalysts showed homogeneous distribution of individual spherical PdAg NPs on the support.

Table 1. Particle size (dTEM) and dispersion (DTEM). SAMPLE

dTEM (nm) DTEM (%)

SAMPLE

dTEM (nm) DTEM (%)

Pd1Ag0.5/C(1)

3.9 ± 0.8

23.1

Pd1Ag2/C(1)

3.5 ± 0.9

25.7

Pd1Ag0.5/C(5)

3.4 ± 0.7

26.5

Pd1Ag2/C(5)

3.0 ±0.6

30.0

Pd1Ag0.5/C(10)

3.4 ± 0.6

26.5

Pd1Ag2/C(10)

3.1 ± 0.8

29.0

Pd1Ag1/C(1)

3.6 ± 0.9

25.0

Pd1Ag4/C(1)

5.0 ± 3.0

18.0

Pd1Ag1/C(5)

3.3 ± 0.6

27.3

Pd1Ag4/C(5)

4.0 ± 1.1

22.5

Pd1Ag1/C(10)

3.4 ± 0.6

26.5

Pd1Ag4/C(10)*

3.6 ± 1.0

25.0

(*) Notice that the heavy aggregates found in sample Pd1Ag4/C(10) were not considered in the determination of the average NP size, as they are not representative of the sample itself.

PdAg colloidal NPs with various compositions were prepared by using solutions with different concentration of Ag+, while the concentration of Pd2+was fixed for all samples. The average NP sizes were similar for all the catalysts prepared by using colloids obtained by dissolving relatively low concentration of cations in the solution (Pd1Ag0.5, Pd1Ag1 and Pd1Ag2). In these cases, the average NPs size ranged from 3.0 to 3.9 nm, and showed narrow size distribution regardless the PVP/M ratio. In 15 ACS Paragon Plus Environment


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comparison, monometallic Pd NPs prepared using PVP/Pd ratios of 1 and 10 under identical experimental conditions showed average NP size of 5.5 and 3.9 nm, respectively (See Figure S3), which are considerably larger than those of PdAg NPs, especially when low amount of PVP was used in the colloidal synthesis (for PVP/M=1; 3.9, 3.6 and 3.5 nm, for Pd1Ag0.5/C(1), Pd1Ag1/C(1) and Pd1Ag2/C(1), respectively, compared to 5.5 nm of monometallic Pd/C catalyst; and for PVP/M=10; 3.4, 3.4 and 3.1 nm, for Pd1Ag0.5/C(10), Pd1Ag1/C(10) and Pd1Ag2/C(10), respectively, compared to 3.9 nm of monometallic Pd/C catalyst). This fact indicated that the addition of Ag tends to reduce the NPs size with respect to the Pd monometallic counterpart, which might be related to the reduction rate in the colloidal media

71

. The reduction rate of Pd ions is

favored by the presence of Ag, which leads to the formation of more seeds in the nucleation step and ultimately produces smaller size of NPs than the monometallic counterparts. It should also be noticed that the effect of PVP seems to be weakened respect to the preparation of monometallic NPs in the case of these three NPs compositions (Pd1Ag0.5, Pd1Ag1 and Pd1Ag2), as similar average size was observed for each composition regardless the PVP/M ratio. Samples prepared using colloids synthesized by dissolving higher cation concentration in the initial solution (Pd1Ag4) resulted in larger average NPs size, especially when lower PVP contents were used (5.0 and 4.0 nm, for Pd1Ag4/C(1) and Pd1Ag4/C(5), respectively). On the other hand, relatively smaller (3.6 nm) NPs can be observed in Pd1Ag4/C(10), however, heavy aggregation was observed in some regions (See inset in Figure 3 l), which might be due to the high concentration of metal precursors and PVP in the synthesis of the counterpart colloid.


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2.5

a)

b) Ag K-edge XANES

Pd K-edge XANES Absorption (a.u.)

Absorption (a.u.)

5.2

2

4.2

1.5

3.2

1

2.2

Pd foil PVP/M=1 PVP/M=5 PVP/M=10

0.5

0

Ag foil PVP/M=1 PVP/M=5 PVP/M=10

1.2

0.2

25450 25500 25550 25600 25650

24300 24350 24400 24450 24500

Energy (eV)

Energy (eV)

Pd FT-EXAFS

d) Ag FT-EXAFSAg foil

Pd foil PVP/M=1 PVP/M=5 PVP/M=10

450 400 350

450


400

Magnitude (a.u)

c) Magnitude (a.u)

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


350

300

300

250

250

200

200

150

150

100

100

50

50

0

0

0

1

2

3

4

5

6

0

Interatomic distance (Å)

1

2

3

4

5

6

Interatomic distance (Å)

Figure 4. Pd and Ag K-edge XANES and FT-EXAFS spectra of samples Pd1Ag2/C prepared using colloid with different PVP/M ratios: Foils (references); Sample Pd1Ag2/C(1); Sample Pd1Ag2/C(5); Sample Pd1Ag2/C(10). a) Pd K-edge XANES; b) Ag K-edge XANES; c) Pd FT-EXAFS; d) Ag FT-EXAFS.

XAFS analysis was performed in order to elucidate the structure of the NPs with the best-performing composition (Pd1Ag2). X-ray absorption near-edge structure (XANES) spectra of samples Pd1Ag2/C(1), Pd1Ag2/C(5) and Pd1Ag2/C(10), together with Pd



foil as reference material, are shown in Figure 4a. These four spectra displayed a similar shape, representing similar electronic structures and the presence of Pd(0) species in the samples. The presence of metallic Ag species was also demonstrated by the shape of the counterpart spectra for Ag (Figure 4b). In the Fourier transformation (FT) of k3weighted extended X-ray absorption fine structure (EXAFS) spectra, Pd and Ag foils showed an intense singlet peak at around 2.5 and 2.7 Å, respectively (Figure 4c and 4d), which are ascribed to the contiguous Pd-Pd and Ag-Ag metal-metal bonding. These peaks were also displayed in three Pd1Ag2/C catalysts with different PVP/M ratios, confirming the existence of the metallic Pd and Ag species. However, these peaks were slightly shifted toward longer or shorter interatomic distances for Pd and Ag, respectively, compared with the corresponding pure foils. This result clearly confirms the existence of heteroatomic Pd−Ag bonding in the studied catalysts 26,69.

a) Pd 3d

b) Ag 3d

3000

3000 Pd1Ag2/C(1)

2000 1500

Pd1Ag2/C(5)

1000 500

Pd1Ag2/C(10)

0

2500

Counts (a.u)

2500

Counts (a.u)

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Pd1Ag2/C(1)

2000 1500

Pd1Ag2/C(5)

1000 500

Pd1Ag2/C(10)

0

332 334 336 338 340 342 344 346 348

364 366 368 370 372 374 376 378 380

Binding Energy (eV)

Binding Energy (eV)

Figure 5. XPS spectra of samples Pd1Ag2/C(1), Pd1Ag2/C(5) and Pd1Ag2/C(10). a) Pd 3d spectra; b) Ag 3d spectra.

To gain further insight into the electronic features of Pd and Ag in the Pd1Ag2/C, Pd 3d and Ag 3d XPS spectra were recorded and the results are plotted in Figure 5. Both 18 ACS Paragon Plus Environment

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set of spectra showed the 3d5/2 and 3d3/2 electronic transitions peaks. Each of those transitions can be deconvoluted into two different peaks, associated to electronically different Pd and Ag species

58,61,72

. The peaks at lower binding energies (in black) are

ascribed to the presence of M(0), while the peaks at higher binding energies (in red) are due to the presence of M(X+). The analysis of both set of spectra revealed that Pd and Ag were mainly in the metallic form, but slight contribution of the oxidized forms was detected for both elements (See Table 2). Table 2. Results of the XPS analysis for Pd and Ag in Pd1Ag2/C samples. Pd(0) Pd(II) Ag(0) Ag(I) Pd/Ag (atomic %) (atomic %) (atomic %) (atomic %) Pd1Ag2/C(1) 86.2 13.8 82.7 17.3 1.26 Pd1Ag2/C(5) 85.8 14.2 87.4 12.6 1.20 Pd1Ag2/C(10) 82.8 17.2 87.2 12.8 0.78 Sample

It was previously reported that the presence of oxidized Pd species in the catalysts is a consequence of the PVP-M interaction.

61,73

This interaction consists of the electron-

withdrawing property of PVP through the C=O groups, which lead to electron deficient metal surfaces, confirmed in the XPS spectra as a partially oxidized element (M(X+)) 74. Since PVP interacts with both Pd and Ag as a capping polymer, it significantly influences on the presence of Pd(II) and Ag(I) in the samples. The results in Table 2 indicated that the relative proportion of Pd and Ag in their metallic form depends on the PVP/M ratios used in the NP synthesis. It can be expected that the Pd(II) content increases gradually with increasing the PVP contents, because higher PVP content enhances the electron withdrawing from Pd surface. Nevertheless, a different tendency was observed for Ag. This tendency observed for Ag(I) present in the samples might indicate that not only the PVP effect should be considered when analyzing the electronic features of NPs, but also the charge transfer from Ag to Pd originated from 19 ACS Paragon Plus Environment


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the different ionization potentials of Pd and Ag (8.34 and 7.58 eV, respectively)

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26

.

Upon consideration of the results in Table 2, it seems that the charge transfer from Ag to Pd is favored at lower PVP, because higher content of Ag(I) was observed in Pd1Ag2/C(1) compared with those of higher PVP/M ratios (17.3, 12.6 and 12.8 atomic %, for PVP/M of 1, 5 and 10, respectively). Thus, Ag is highly capped by the polymer chains at higher PVP contents, and the charge transfer from the Ag rich cores to the Pd rich shell in the NPs is hampered. The surface Pd/Ag ratio present in the samples is other important aspect to be considered. It should be noticed that the surface atomic Pd/Ag ratios for the three samples are substantially higher than the theoretical Pd/Ag= 0.5, confirming the formation of NPs with Pd rich surface. As it can be observed in Table 2, there is also a relationship between PVP content and surface composition of NPs; the Pd/Ag surface ratio increased with decreasing the amount of PVP. This result correlates well with the enriched Ag surface observed in the UV-vis spectra for the colloid used to prepare Pd1Ag2/C(10) compared to the colloids with lower PVP contents (See Figure 2c)), which might be related to the relative interaction strength between both elements and PVP and the steric hindrance exerted by the polymer molecules. In addition, the allowed structure of the PdAg NPs was confirmed by HR-TEM. To this end, a representative colloid (PdAg2(1)) was analyzed and the results revealed that the lattice space was 0.23 nm, which is between the (111) lattice spacing of facecentered cubic (fcc) Ag (0.235 nm) and Pd (0.224 nm) 75.

3.4. Discussion


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The enhancement effect of the Ag incorporation in monometallic Pd catalysts in the H2 production from FA has been faced by other authors. For instance, Lu

71

demonstrated that the H2 production ability of Pd/C catalysts was significantly improved by the addition of Ag as a consequence of the effective inhibition of CO and H2 adsorption on Pd surface. The outstanding catalytic behavior of a PdAg catalyst was also pointed out by Zahmakiran and coworkers, where a sample composed of PdAg alloy and MnOx NPs supported on amine-grafted silica facilitated the H2 production at room temperature

55

. The activity of core–shell AgPd@Pd nanocatalysts on TiO2 was

also assessed by Tsuji et al. 57. In that case, it was found that the H2 production ability was strongly dependent on the degree of alloying, being optimum for the Ag93Pd7@Pd/TiO2 sample. The effect of the Pd/Ag composition in bimetallic catalyst was the goal of the study conducted by Dai and coworkers 76. To this end, they reported that the highly dispersed AgPd NPs on ZIF-8 showed significant composition dependence toward the FA dehydrogenation. Among investigated, Ag18Pd82@ZIF-8 displayed highest activity compared with other bimetallic compositions as well as the monometallic NPs. Against this background, herein we performed a systematic study where PVP-capped PdAg NPs with different compositions were prepared by the simple polyol method and subsequently supported on carbon in order to assess the NPs features in the final catalytic behavior. The H2 evolution results revealed that the catalytic activity is strongly dependent on the NPs composition, as well as on the PVP amount used to prepare the colloidal NPs. The amount of evolved H2 slightly increased when the composition changed from Pd1Ag0.5 to Pd1Ag1, but a sharp increase was observed by further Ag incorporation in



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the bimetallic system (Pd1Ag2). The dramatic decrease of the catalytic activity with higher Ag content (Pd1Ag4) indicated that there is an optimum Pd/Ag ratio, which affords the best-performing catalysts for the FA dehydrogenation, among investigated. Furthermore, apart from the composition view point, the poor behavior of samples based on Pd1Ag4 NPs might also be related to their large average NPs size, which resulted in heavy aggregation in the case of sample Pd1Ag4/C(10) (notice that the H2 output achieved with this sample is almost negligible). Nevertheless, due to the similar average NP sizes of the other three compositions (Pd1Ag0.5, Pd1Ag1 and Pd1Ag2), the different catalytic performances displayed by these samples cannot be explained based on their NPs size. To gain insight into the effect of NPs characteristics in the catalytic performances in the H2 production from the FA, the set of best performing catalysts (Pd1Ag2/C) was further analyzed by means of XPS. The electronic properties of these samples revealed that both Pd and Ag are mainly in the metallic form, but contribution of their oxidized species was also observed. Interestingly, the relative proportion of M(0) and M(X+) was strongly dependent on the PVP/M ratios used in the preparation of the counterpart colloids. The higher relative proportion on Pd(0) and Ag(I) observed in the lower PVP/M ratio cannot be explained only in terms of the electron-withdrawing effect of the PVP from the metal surface, but also considering the charge transfer from Ag to Pd due to their different ionization potentials, which might be favored at lower polymeric coverage degree of the metal surface. In addition, the NP surface compositions (Pd/Ag ratios) were also dependent on the amount of PVP, being higher in the case of sample Pd1Ag2/C(1).


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Then, the best catalytic performance achieved by Pd1Ag2/C(1) as compared to those with higher PVP/M ratios can be ascribed to a higher electronic density in the Pd atoms as well as a higher surface Pd enrichment, which ultimately result in a catalyst with optimum features for the H2 production from the FA dehydrogenation.

4. Conclusions. The polyol method was used to produce uniform ~3-4 nm PdAg alloy NPs across a wide compositional range in terms of PVP/M and Pd/Ag ratios. They were subsequently loaded on a carbon support and tested in the FA dehydrogenation. The catalytic activity displayed by these samples was strongly dependent on the NPs composition (Pd/Ag ratios) as well as on the PVP amount used for the colloidal synthesis. Samples with composition of Pd1Ag2 showed higher H2 production as compared to those with lower or higher Ag loadings, pointing out the optimum relative proportion of both elements in the allowed NPs. Among Pd1Ag2/C-based samples, the best catalytic performance was attained by the sample prepared from the colloid with lower PVP amount (Pd1Ag2/C(1)), which brings a higher relative proportion of Pd in the metallic state, as well as higher surface Pd/Ag ratio. The high initial TOF value of 855 h−1 attained by this sample confirms the suitability of the present catalytic system for the target application. The screening of the catalytic behavior achieved by PdAg/C reported herein increases the likelihood of future tailored features of Pd-based bimetallic systems towards the development of highly efficient catalysts for the H2 generation from the FA dehydrogenation reaction.



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Supporting Information. Additional results of the characterization of the catalysts as well as the catalytic tests are included as Supporting Information.

Acknowledgments The present work was partially supported by Grants-in-Aid for Scientific Research (Nos. 26220911, 25289289, and 26630409, 26620194) from the Japan Society for the Promotion of Science (JSPS) and MEXT. We acknowledge Dr. Eiji Taguchi and Prof. H. Yasuda at the Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, for their assistance with the TEM measurements. KM, YK and HY thank MEXT program “Elements Strategy Initiative to Form Core Research Center”.

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Screening of Carbon-Supported PdAg ... - ACS Publications

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