Surface-Engineered Polydopamine Particles as an Efficient Support

Subscriber access provided by UNIV OF REGINA

Article

Surface-engineered polydopamine particles as efficient support for catalytic applications Yanhong Liu, Guozhu Li, Runze Qin, and Danlei Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03340 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 6, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37

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

Langmuir

1 2

Surface-engineered polydopamine particles as efficient

3

support for catalytic applications

4

Yanhong Liu, Guozhu Li,* Runze Qin , Danlei Chen

5 6 7

Key Laboratory for Green Chemical Technology of Ministry of Education,

8

Collaborative Innovative Center of Chemical Science and Engineering (Tianjin),

9

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,

10

China

11 12

Email: [email protected]

13

polydopamine

particle,

14

Keywords

15

hydrogenation, kinetic, active energy

surface

chemistry,

16 17

1

ACS Paragon Plus Environment

support,

palladium,

Langmuir

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

1

Page 2 of 37

ABSTRACT

2

Mussel-inspired polydopamine (PDA) particles with the size of ~270 nm are

3

employed as support of Pd nanoparticles (NPs) for catalyst preparation. Surface

4

morphology of the PDA particle has been modified via corrosion of CF3COOH.

5

Surface chemistry of the obtained PDA particle has been engineered by the formation

6

of

7

monolayer-modified PDA (SAM-PDA) particles are used to load Pd NPs by simply

8

adding H2PdCl4 solution to a suspension of SAM-PDA particles at room temperature.

9

TEM, EDX mapping, DLS, XRD, XPS, UV-vis and FTIR are employed to

10

characterize the catalyst and investigate the process. Uniform Pd NPs (2-3 nm) have

11

been well dispersed on SAM-PDA particles via controllable surface engineering.

12

Surface charge and interactions with metal ion are regulated by the monolayer of

13

carboxylic acid. The surface chemistry of PDA particles has been finely engineered

14

for efficient loading of noble metal NPs. The obtained Pd/SAM-PDA catalyst has

15

shown greatly increased activity and good reusability compared with Pd/PDA in the

16

reduction of 4-nitrophenol (4-NP) by sodium borohydride or H2. The kinetic data of

17

4-NP

18

Langmuir-Hinshelwood (L-H) model, and the calculated apparent activation energy of

19

this process is 40.77 kJ mol-1.

carboxylic

acid

hydrogenation

terminated

catalyzed

alkanethiol

by

monolayer.

Pd/SAM-PDA

20

2


are

The

fitted

obtained

to

a

Page 3 of 37

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

Langmuir

1

■ INTRODUCTION

2

Supported noble metal has been studied extensively due to its potential application

3

as catalyst in industry. The metal-support interaction plays a key role in catalyst

4

preparation which should be considered for designing and fabricating excellent

5

catalysts.1 Support can facilitate metal dispersion and prevent loss in solution. Surface

6

modification of the support to generate a suitable environment for controllable metal

7

loading is one of the most efficient ways for catalyst development.2 Particles with

8

submicrometer/nanometer sizes are good candidates of support owing to their high

9

specific surface area and easily accessible functional sites on the outside surface.3

10

Various particles have been selected and investigated as support of noble metal

11

nanoparticles (NPs). Inorganic particles, including silica4, iron oxide5-7 and carbon

12

beads,8-11 have been studied. For instance, Xin et al. loaded Pt NPs onto bayberry

13

tannin grafted silica beads for hydrogenation.12 For better immobilization of metal

14

NPs, many works were carried out based on surface modification of the inorganic

15

particles by organic ligands.13, 14 For example, Dragu and coworkers incorporated

16

PVP or TPP to functionalize the spherical mesoporous carbon beads for the

17

stabilization of Pd NPs.15 Recently, organic particles used as support have recently

18

attracted a lot of attention, since these well-defined macromolecular structures enable

19

the construction of precisely controlled catalyst structures.16 Cellulose,17 chitosan,18

20

dendrimer,19, 20 and biopolymer 21 have been employed to support noble metal NPs.

21

Melanin-like particles of PDA have been synthesized with size control by Lee

22

group.22 These free-standing PDA particles can be utilized as a reactive platform to 3


Langmuir

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

1

template the loading and dispersion of noble metal nanoparticles (NPs). The unique

2

advantages of PDA particles come from: (i) high surface area can promote heat and

3

mass transfer. The adsorption and dispersion of metal salt for metal loading can be

4

facilitated, and exposure of a greater percentage of active sites will be achieved;23 (ii)

5

beneficial near-homogeneous catalytic properties might be realized due to the small

6

(100-300 nm), uniform and well-defined carrier particles;24 (iii) the small PDA

7

particles are more fixable to be handled and characterized.25 In addition, the organic

8

PDA particles can be easily modified or removed as needed.26

9

In this regard, the present study deals with the system of Pd NPs supported by PDA

10

particles (Pd/PDA) in an attempt to look into the influence of surface chemistry of

11

PDA on their catalytic performance for hydrogenation. Bare or surface-modified PDA

12

particles have been synthesized and used to prepare palladium catalysts, as shown in

13

Scheme 1. The synthesis of highly active Pd/PDA catalysts with well dispersed Pd

14

NPs has been realized by controllable surface engineering.

4


Page 4 of 37

Page 5 of 37

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

Langmuir

1 2 3

Scheme 1. Schematic illustration of the preparation of Pd-supported PDA particles

4

without and with surface regulation: multipatched Pd/PDA particle (up), uniform,

5

small and well-dispersed Pd NPs loaded on monolayer-grafted PDA particle

6

(Pd/SAM-PDA) (down).

7 8

■ EXPERIMENTAL SECTION

9

Synthesis of polydopamine (PDA) particles

10

The preparation of polydopamine nanospheres was carried out using dopamine

11

hydrochloride in NaOH solution, following a method used previously. We dissolved

12

40.5 mg of dopamine hydrochloride (Aladdin Industrial Inc.) in 10 mL of deionized

13

water. Under vigorous stirring, 378 µL of 0.556 mol/L NaOH solution was added to a

14

dopamine hydrochloride solution at 50 °C. The color of the solution turned to pale 5


Langmuir

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

1

yellow as soon as NaOH was added and gradually changed to dark brown. After aging

2

for 5 h, melanin-like PDA particles were retrieved and selected as a dispersed solution

3

by centrifugation. All large-size materials were removed by low-speed centrifugation

4

(2000 rpm). Small-sized materials were removed by high-speed centrifugation (8000

5

rpm) and washed with deionized water several times. The obtained NPs were

6

redispersed in 10ml water.

7

Preparation of carboxylic acid monolayer on the surface of PDA particles

8

11-Mercaptoundecanoic acid [HS(CH2)11COOH] was purchased from J&K

9

scientific Ltd.. Monolayer of HS(CH2)11COOH was formed on the surface of PDA

10

particles using a method reported previously.27 CF3COOH (0.2 ml) and 100 mM

11

ethanolic solution of HS(CH2)11COOH (0.05ml) was added into the as-synthesized

12

solution of PDA particles with stirring. The COOH monolayers were formed by

13

overnight stirring with N2 protection. The obtained particles (SAM-PDA) were

14

retrieved by centrifugation (8000 rpm) and redispersed in 10 ml deionized water.

15

Catalyst preparation

16

Pd(II) ions were introduced onto the surface of PDA particles and reduced to Pd(0)

17

simultaneously by simply mixing. Specifically, 89 µl H2PdCl4 (10g/L), 2 ml of

18

SAM-PDA solution and 3 ml water was mixed, and stirred for 30min. The metal salts

19

were subsequently reduced by polydopamine, which resulted in the formation of

20

Pd/SAM-PDA.

21

Catalytic characterizations

22

The morphologies and microstructures of the PDA NPs, Pd/PDA NPs, and 6


Page 6 of 37

Page 7 of 37

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

Langmuir

1

Pd/SAM-PDA NPs were characterized using a Tecnai G2 F20 transmission electron

2

microscope (TEM FEI, Netherlands). X-ray diffraction (XRD) was recorded on a

3

D/MAX-2500 Advance X-ray diffractometer with Cu Ka1 radiation (λ=1.5406 Å)

4

(Rigaku, Japan). Fourier transform infrared (FTIR) spectra were performed on a

5

Vertex 70 FTIR spectrometer (Bruker, Germany). X-ray photoelectron spectroscopy

6

(XPS) measurements were performed on an ESCALAB 250Xi spectrometer with

7

monochromatic Al Ka (hν = 1486.6 eV) (Thermo Fisher, America.). All XPS spectra

8

were corrected according to the C 1s line at 284.8 eV. Measurements of the particle

9

size and zeta potential were conducted in Malvern Beijing Lab using a Zetasizer Nano

10

ZS (Malvern, United Kingdom).

11

Catalytic reactions

12

The obtained catalysts were used to catalyze two model reactions, the reduction of

13

4-nitrophenol (4-NP) by NaBH4 and hydrogenation of 4-NP by H2, to evaluate their

14

catalytic performance.

15

4-NP reduction by NaBH4: The reduction of 4-NP by the supported metal catalyst

16

in the presence of NaBH4 was carried out to examine its catalytic activity and

17

recyclability. 3 mL of 0.1 mM 4-NP and 10 µL of 3 M NaBH4 solutions were added

18

into a quartz cuvette followed by addition of catalyst solution to the mixture. For the

19

Pd/PDA (or Pd/SAM-PDA) catalyst, the final concentration of noble metal (Pd) is

20

3.35 × 10-6 mol/L based on the amount of Pd added into the solution without counting

21

the lost. The color of the solution changed gradually from yellow to transparent as the

22

reaction proceeded. UV-Vis spectrometry was used to record the change in 7


Langmuir

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

1

absorbance at a time interval of 1s at λ=400nm. Therefore, a U3010

2

spectrophotometer was employed to monitor the conversion progress of 4-NP to

3

4-aminophenol (4-AP) at ambient temperature. The catalyst (Pd/PDA or

4

Pd/SAM-PDA) was recovered from the reaction mixture by high-speed centrifugation

5

and reused in sequential runs.

6

Hydrogenation of 4-NP: The hydrogenation reaction was carried out in an 80 mL

7

custom designed stainless autoclave with a Teflon inner layer at room temperature. In

8

a typical procedure, certain amount of catalyst was dispersed in 60 mL of 0.1 mM

9

4-NP. The final concentration of noble metal (Pd) is 2.37 × 10-6 mol/L based on the

10

amount of Pd added into the solution without counting the lost. The reactor was

11

sealed, purged with hydrogen and pressurized to 0.4 MPa. To eliminate the resistance

12

of diffusion as much as possible, all of the experiments were conducted at an agitation

13

speed of 1200 rpm. A constant pressure of 0.4 MPa was maintained throughout the

14

reaction. During the reaction, the mixture was withdrawn and sampled every 5 or 10

15

minutes from the reactor via sampling valve. The reactants were analyzed by UV-Vis

16

spectrometry. The average turnover frequency (TOFave) is defined as mol

17

hydrogenated 4-NP per mol catalyst per minute (period of that 90% of 4-NP was

18

hydrogenated to 4-AP or period of 60 min).

19

Kinetic model.

20

In the kinetically controlled regime, hydrogenation of 4-NP can be described by a

21

Langmuir-Hinshelwood model.28 The Langmuir-Hinshelwood (L-H) type model used

22

in this study is listed below. 8


Page 8 of 37

Page 9 of 37

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

Langmuir

‫ݎ‬ଵ =

݀‫ܥ‬஺ ݇‫ܭ‬஺ ‫ܭ‬஻ ଷ ‫ܥ‬஺ ‫ܥ‬஻ ଷ = ଺ ݀‫ݐ‬ （1 + ‫ܭ‬஺ ‫ܥ‬஺ ） × （1 + ‫ܭ‬஻ ଵ/ଶ ‫ܥ‬஻ଵ/ଶ ）

1

Where r1 is the rate of reaction, CA is the concentration of 4-NP, CB is the pressure

2

of hydrogen, k is the rate constant of the reaction, KA is the adsorption constant of

3

4-NP, and KB is the adsorption constant of hydrogen.

4

In this model, on-surface reaction is the rate-controlled step and a dual-site

5

adsorption happened on the catalyst with molecular adsorption of 4-NP and

6

dissociative adsorption of hydrogen.28,

7

analysis was carried out for the L-H equation to obtain the best fitted parameters (k,

8

KA, KB) using a program written in MATLAB.

9

■ RESULTS AND DISCUSSION

29

A non-linear multivariable regression

10

Melanin-like PDA particles were synthesized following the method reported by Lee

11

and coworkers.22 As shown in Figure 1(a-b), uniform PDA particles have been

12

obtained through neutralization of dopamine hydrochloride with NaOH, followed by

13

spontaneous air oxidation of dopamine. The average size of PDA particles measured

14

from TEM images is 272.9 ± 17.5 nm, which is in good agreement with the result

15

from Dynamic Light Scattering (DLS). As shown in Figure S1, the particle size of

16

PDA is 245.2 nm measured by DLS.

17

These organic particles were employed as support to load Pd NPs for catalyst

18

preparation. As reported previously, sorption of PdCl42- can be driven by the affinity

19

of PDA for highly polarizable and weakly hydrated anions.30 Moreover, PDA can

20

reduce noble metal ions and grow metal NPs on the surface following its tenaciously

21

retaining of metal ion.31-33 Therefore, Pd NPs have been loaded onto the 9


Langmuir

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

1

as-synthesized PDA particle by simply stirring the solution of metal salt and PDA

2

particles at room temperature. The obtained suspension of PDA particles were washed

3

and separated by centrifugation to eliminate the influence of the residual free

4

dopamine. Because Wang et al. reported that both PDA particles and the residual

5

solution (free dopamine and its slightly oxidized derivative, dopa-quinone) can reduce

6

metal ion.34

7

The deposition of Pd NPs on the surface of PDA particles was validated by TEM,

8

as displayed in Figure 1(c-d). Multipatched Pd/PDA particles have been obtained.

9

The PDA particles decorated with a number of irregular Pd NPs with the size of ~50

10

nm, and their arrangement is distinguishable (Figure 1). Pd NPs with wide size

11

distribution indicate that the formation of Pd NPs on PDA surface is out of control.

12

On the surface of PDA, PdCl42- is adsorbed and subsequently reduced to form Pd NPs.

13

The obtaining of multipatched particles is in consistent with Wang’s results34.

10


Page 10 of 37

Page 11 of 37

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

Langmuir

1 2

Figure 1. TEM images of synthesized PDA particles (a, b) and Pd/PDA particles

3

without surface regulation (c, d).

4 5

As reported in previous works, modulation of surface chemistry should be one of

6

the most effective approaches for activity improvement of the catalyst.35, 36 We expect

7

that surface chemistry, especially surface charge, of the support (PDA particles)

8

exhibits an outstanding role in catalyst preparation. Therefore, we tried to graft a

9

monolayer of carboxylic acid on the surface of PDA particle using a facile and

10

efficient method reported by Jiang group.27 After simple immersion of PDA particles 11


Langmuir

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

1

into an alkanethiol-containing solution (HS(CH2)11COOH and CF3COOH), a

2

monolayer of carboxylic acid alkanethiol was supposed to be spontaneously formed

3

on the corroded PDA surface through thiol-catechol/quinone adduct formation. This

4

improved approach for monolayer preparation is in a manner analogous to the

5

reaction between thiols and noble metals in the formation of conventional

6

self-assembled monolayers (SAMs). The as-synthesized SAM-PDA particles are also

7

used to support Pd NPs via simply mixing of support suspension with H2PdCl4

8

solution. The obtained catalyst (Pd/SAM-PDA) has been characterized by TEM and

9

EDX, and results are displayed in Figure 2(a-f). Well distributed Pd NPs with the

10

diameter of 2-3 nm on PDA particles have been obtained. As measured by EDX, the

11

atomic ratio of S/Pd in Pd/SAM-PDA is 0.13. HAADF-STEM images and EDX

12

mapping were also collected for an individual particle of Pd/SAM-PDA catalyst

13

(Figure 2g). S element is finely distributed on the surface of PDA particle, indicating

14

the successful grafting of HS(CH2)11COOH monolayer on PDA surface. Mapping

15

result of Pd element further provides an evidence of successful loading and fine

16

dispersion of Pd NPs on SAM-PDA particles.

17

Surface chemistry (surface morphology and surface charge) of PDA particle has a

18

significant effect on the resulting metal dispersion of catalysts. Surface roughness of

19

PDA particle has been increased by the corrosion of CF3COOH, and surface

20

chemistry of PDA particle has been engineered by the formation of anionic monolayer

21

on the polymer surface.37 A small amount of CF3COOH is added for two reasons.

22

Firstly, the roughness of the surface can be increased due to mild corrosion of PDA. 12


Page 12 of 37

Page 13 of 37

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

Langmuir

1

Then a more suitable environment can be created for nanoparticle immobilization.

2

Secondly, the formation of interplane hydrogen bonds between carboxylic acid groups

3

of bound thiolate on PDA surface and free thiols in the bulk can be disrupted for a

4

monolayer with better quality.27 Therefore, SAM-PDA particles with carboxylic acid

5

monolayers and high surface roughness have been prepared successfully.

6 7

Figure 2. TEM images (a-e), EDX spectrum (f), HAADF-STEM images and EDX

8

mapping images of the Pd/SAM-PDA catalyst.

9 10

Surface charges (zeta-potentials) affected by solution pH were measured, which is 13


Langmuir

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

1

one of the most important characteristics for noble metal immobilization. The

2

zeta-potentials of both PDA particles and SAM-PDA particles decrease with

3

increasing pH.34 But their isoelectric points (IEPs) are dramatically different, which

4

occur at pH of 3.15 and 5.66, respectively (Figure 3a). 38

5

Below the IEP, the surfaces of the particles are positively charged. The pH of the

6

solution during Pd immobilization is 2~3. Therefore, SAM-PDA particles are more

7

positively charged than bare PDA particles under acidic environment of the solution

8

due to its higher isoelectric point. Weaker electrostatic attraction between PdCl42- and

9

PDA particle leads to its decoration by a number of irregular Pd NPs. In comparison,

10

Carboxylic acid monolayer grafted on rough PDA particle provides a suitable

11

nanoenvironment for the formation of uniform and small Pd NPs. During the growth

12

of Pd NPs, discrete distribution of carboxylic acid groups on PDA particle guarantees

13

improved electrostatic interaction between the support and metal ions. SAM-PDA

14

particles with more positive charges possess stronger electrostatic attraction of anionic

15

PdCl42- to effectively load Pd NPs (Figure 3b). But just the reason of surface charge

16

cannot guarantee the formation of well-dispersed Pd NPs on SAM-PDA particles. We

17

suppose other interactions exist during this process. The mechanism of detailed

18

interactions in this process is still not clear. Therefore, more characterizations of the

19

catalyst-preparation process were conducted in latter section for a deeper

20

investigation.

14


Page 14 of 37

Page 15 of 37

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

Langmuir

1 2

Figure 3 Zeta-potentials of PDA and SAM-PDA (a) and Pd/PDA and Pd/SAM-PDA

3

(b) at different pH.

4 5

X-ray photoelectron spectroscopy (XPS) was used to characterize the surface of

6

Pd/SAM-PDA NPs. Figure S2 shows the XPS survey spectra of Pd/SAM-PDA. The

7

atomic ratio of S/Pd calculated from XPS results is 0.87 (Table S1). In comparison,

8

the ratio of S/Pd from EDX results is only 0.13. It can be explained by the fact that Pd

9

was deposited on the monolayer of HS(CH2)11COOH. Therefore, more S element can

10

be detected by XPS due to its higher sampling depth than that of EDX.

11

XPS core level regions of C 1s, N 1s, S 2p, and Pd 3d are displayed in Figure 4.

12

The major component at binding energy (BE) of 284.8 eV is assigned to C1s, in

13

conformity with the standard value of carbon. The peak in the N 1s spectrum at BE =

14

400.14 eV is the diagnostic peak of indole and indoline structures existing in PDA.

15

The monolayer of HS(CH2)11COOH is characterized by a minor component at 163.5

16

eV BE (FWHM = 2.62 eV) in the S 2p XPS spectrum, which provides evidence for

17

successful anchoring of HS(CH2)11COOH onto the surfaces of PDA NPs. The state of

18

Pd on SAM-PDA particle in 30min during preparation was also characterized. The 15


Langmuir

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

1

two peaks at 337.8 and 343.0 eV can be assigned to Pd2+, and the peaks at 335.7 and

2

340.9 eV are attributed to Pd0. Around 10% of Pd is in the state of Pd0 in the initial

3

30min estimated from the area of the respective XPS peaks. The result suggests that

4

PdCl42- is only partially reduced by SAM-PDA and a large ratio of Pd2+ in the state of

5

anionic PdCl42- is present in the early stage.

6 7

Figure 4. XPS of (a) C 1s, (b) N 1s, (c) S 2p, (d) Pd 3d for Pd/SAM-PDA prepared in

8

30min.

9 10

The interaction of H2PdCl4 with PDA and that with SAM-PDA in catalyst

11

preparation were compared and investigated by UV-vis and IR. As monitored by

12

UV-vis in Figure 5(a-b), interaction between PdCl42- and the monolayer-modified 16


Page 16 of 37

Page 17 of 37

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

Langmuir

1

PDA surface in the local environment is much different from that of the bare PDA

2

particle. No obvious adsorption peak has been detected during the reaction of H2PdCl4

3

with PDA (Figure 5a). While a strong peak initially appears at 207.8 nm and then

4

gradually declines in Figure 5b. The peak at 207.8 nm confirms the presence of

5

metal-ligand interaction.39-41 It indicates that the monolayer of HS(CH2)11COOH

6

interacts with H2PdCl4, and a metal-ligand transition exists when the H2PdCl4

7

precursor is initially added to the SAM-PDA solution. Moreover, the intensity of the

8

peak decreased gradually with the increase of time. It can be explained by the

9

reduction of the metal ions (PdCl42-) to the metal NPs (Pd0).

10

Dopamine, PDA NPs, SAM-PDA NPs, and Pd/SAM-PDA NPs were characterized

11

by FTIR, as summarized in Figure 5c. The two weak peaks at approximately 1603

12

cm-1 and 1506 cm-1 of PDA NPs are the characteristic peaks of indole and indoline

13

structures.

14

SAM-PDA sample indicates the existence of the carboxylic acid groups, which further

15

proves the successful grafting of carboxylic monolayer on PDA. In the spectrum of

16

Pd/SAM-PDA, the peaks at approximately 638.0 cm-1 and 500.8 cm-1 from the

17

stretching vibrations of the Pd-O bonds indicate the interaction between Pd and

18

carboxylic acid of SAM-PDA based on the results of UV-vis 44, 45.

42, 43

The appearance of two IR peaks at 1676 cm-1 and 1194 cm-1 for

19

The crystallinity and phase composition of the resulting Pd/SAM-PDA were

20

investigated by XRD. The obtained XRD pattern is shown in Figure 5d. The

21

diffraction peaks at 2θ values of 40.1º, 46.7º, 68.1º, 82.1º, and 86.6º can be indexed

22

to the (111), (200), (220), (311), and (222) crystal planes of Pd, respectively. The 17


Langmuir

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

1

result further verifies the formation of Pd on the surface of SAM-PDA NPs. The

2

broadening of reflection peaks indicates good dispersion of ultrafine particles, which

3

is in good agreement with the TEM results.

4 5

Figure 5 UV-vis spectra of the solutions during catalyst preparation using PDA

6

particles (a) and SAM-PDA particles (b) as support at different deposition time of Pd.

7

FT-IR spectra of Pd/SAM-PDA, PDA-SAM particles, PDA particles and dopamine

8

hydrochloride (c). XRD spectrum of Pd/SAM-PDA and the standard bar graph of Pd

9

(d).

10 11

Both catalysts (Pd/PDA and Pd/SAM-PDA) have been employed to catalyze two

12

model reactions, including the reduction of 4-NP by NaBH4 and the hydrogenation of

13

4-NP by H2. Catalytic reduction of 4-NP by borohydride ions is the mostly used 18


Page 18 of 37

Page 19 of 37

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

Langmuir

1

reaction to quickly test the catalytic activity of metal NPs in aqueous solution.46 It

2

takes place at room temperature and can be accurately monitored by UV-Vis

3

spectroscopy. Hydrogenation of 4-NP in water by H2 is a practical reaction to evaluate

4

catalysts for potential real applications.47

5

For the reduction of 4-NP by borohydride ions, pseudo-first-order kinetic with

6

respect to 4-NP is used to evaluate the catalytic rate due to the high concentration of

7

NaBH4.29 Then the rate constant k of the reaction can be used to evaluate the activity

8

of catalyst. Typical plot of -ln(A/A0) against the reaction time for the calculation of k

9

is displayed in Figure S3. As shown in Figure 6a, the k value of reaction catalyzed by

10

Pd/SAM-PDA (0.073±0.020 s-1) is 6 times of that catalyzed by Pd/PDA (0.012±0.008

11

s-1) and 36 times of unsupported Pd NPs (0.002±0.001 s-1), indicating outstanding

12

activity of Pd/SAM-PDA. It can be deduced that the activity of catalyst can be greatly

13

improved through surface modification of PDA particles.

14

The reuse property of the catalysts has also been tested, and data are summarized in

15

Figure 6b. 100% conversion of 4-NP can be achieved in all these recycle experiments,

16

but the rate of the reaction catalyzed by Pd/PDA decreases sharply after 2-times

17

recycles. The rate constant k for the 4th-cycle reaction is only 28% of the first-time

18

reaction. In comparison, the as-prepared Pd/SAM-PDA still possesses good catalytic

19

performance even after 6 times’ recycles. No obvious drop of the reaction rate was

20

observed. Therefore, metal leaching can be suppressed by the grafting of carboxylic

21

acid monolayers on PDA surface via electrostatic attraction and COO- chelation.

19


Langmuir

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

1 2

Figure 6. (a) Catalytic activity of unsupported, PDA-supported and SAM-PDA

3

supported Pd NPs for the reduction of 4-NP by NaBH4 (n=3). (b) The values of the

4

rate constant k for each cycle using Pd/PDA (blue) or Pd/SAM-PDA (red) as catalyst

5

(100% conversion of 4-NP is achieved in all these reactions in less than 2 min). The

6

reaction conditions are as follow: [4-NP] = 0.1 mmol L-1, [NaBH4] = 10 mmol L-1,

7

[Pd] = 3.35 × 10-6 mol L-1, and room temperature.

8 9

The activities of the catalysts obtained here (Pd/PDA and Pd/SAM-PDA) are also

10

compared with that reported previously using rate constant k normalized to the

11

concentration of noble metal (g L-1). The k values of 4-NP reduction by NaBH4

12

catalyzed by various Pd catalysts reported in the literature were summarized in Table

13

1. The k value of Pd/SAM-PDA (2.04×102 s-1g-1L) is 29 times and 6 times of that of

14

Pd NPs and Pd/PDA, respectively. The outstanding k value of Pd/SAM-PDA further

15

confirms the advantage of surface-engineered PDA particle as a support.

16 17

Table 1 Comparison of the rate constants (k) of various catalysts for the reduction of

18

4-NP by NaBH4 at room temperature. 20


Page 20 of 37

Page 21 of 37

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

Langmuir

1

Catalyst

Temperature °C

Concentration of 4-NP mM

k (s g L)

Pd NPs Pd/PDA Pd/SAM-PDA Ni@Pd/KCC-1 GPDAP GP Pd/CNs Pd/C(amorphous carbon) Pd/SBA-15 Pd/SPB-PS Pd/microgel-PS Pd/Al2O3 Pd/FG Pd/PiHP Pd/Fe3O4@SiO2@KCC-1 Pd/PBCG055 CNT-Pd Pd/P. pastoris Pd/Al-SBA-15 AuPd/C Fe3O4@PDA-Au6 graphene/PDA-Au2

RT RT RT RT RT RT RT RT 20 15 15 RT RT RT RT RT RT 30 6 RT RT RT

0.1 0.1 0.1 0.1 0.067 0.067 0.12 1.67 0.1 0.1 0.1 0.1 0.1 0.12 0.1 0.06 1.33 0.09 0.162 0.09 0.33 0.033

6.94±0.30 (3.36±2.25)×10 (2.04±0.56)×102 1.53a 1.79×10a 7.54a 6.70×10 3.46×10 1.77a 1.13×102 6.56 1.02×10 2.35 2.09 2.61×10 5.40 9.59 0.93 2.18 0.086 1.25×10a 2.81a

a

-1 -1

Reference This work This work This work 48 49 49 50 51 52 53 53 54 55 56 57 58 59 60 61 62 63 64

Based on ICP analysis.

2 3

Superior activity of Pd/SAM-PDA has also been found in hydrogenation of 4-NP

4

by H2 in comparison to Pd/PDA. Typical spectra of the reaction solution catalyzed by

5

Pd/PDA and Pd/SAM-PDA were recorded and summarized in Figure S4.

6

Room-temperature hydrogenation of 4-NP to 4-AP in water catalyzed by various

7

catalysts is illustrated in Figure 7. When Pd NPs and Pd/PDA were used as catalyst,

8

the conversion of 4-NP reached only 38% and 81% in 30 min. While the reaction

9

catalyzed by Pd/SAM-PDA gave 100% conversion of 4-NP in 30 min. Corresponding

10

TOFs were also calculated and displayed in Figure S5 for comparison. 21


Langmuir

100

80

Conversition (%)

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

Page 22 of 37

Pd/SAM-PDA Pd/PDA Pd NPs

60

40

20

0 0

10

20

30

40

50

60

Time (min)

1 2

Figure 7. Hydrogenation of 4-NP catalyzed by unsupported Pd (blue triangle),

3

Pd/PDA (red circle) or Pd/SAM-PDA catalyst (black square) at room temperature.

4

The reaction conditions are as follow: [4-NP] = 0.1 mmol L-1, [Pd] = 2.37 × 10-6 mol

5

L-1, and at 0.4 MPa of H2 and room temperature.

6 7

The preparation of catalyst has been further investigated to figure out the effects of

8

reaction conditions. The effects of functional group and chain length of the monolayer

9

on catalytic activity of Pd/SAM-PDA are shown in Figure 8a. Results show that

10

monolayers of alkyl carboxylic acid are superior facilitators to improve catalytic

11

performance of supported Pd. It indicates that both the chain flexibility of monolayer

12

and specific complexation of carboxylic acid with Pd play key roles in the formation

13

of catalyst. As displayed in Figure 8b, solution with lower pH is more suitable for the

14

immobilization of Pd on SAM-PDA particles. It can be explained by the mechanism

15

deduced from the results of zeta potential. Under acidic condition, more positively

16

charged SAM-PDA particles can facilitate their interactions with anionic PdCl42-. In 22


Page 23 of 37

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

Langmuir

1

addition, an optimal SAM concentration is needed to prepare monolayer with suitable

2

density to load Pd NPs. As shown in Figure 8c, 0.95 µmol/L of HS-C11-COOH is the

3

best concentration for catalyst preparation.

4

In order to figure out whether this surface-engineered process is only specific for

5

Pd, other noble metals (Pt, Au and Ag) were also employed for the preparation of

6

SAM-PDA supported catalysts for hydrogenation. Promotions of the catalytic activity

7

for all noble metals have been achieved by the formation of carboxylic acid

8

monolayers on PDA (Figure 8d). When surface-engineered SAM-PDA is used to

9

support noble metal NPs, the TOFs are 3.0 times, 1.7 times, 2.1 times and 2.1 times of

10

the corresponding ones supported by bare PDA for Pd, Pt, Au and Ag, respectively.

11

Results of catalytic activity for hydrogenation obtained here and reported

12

previously are summarized in Table 2. Superior catalytic activity of Pd/SAM-PDA

13

has been obtained for the hydrogenation of 4-NP in water at room temperature.

23


Langmuir

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

Page 24 of 37

1 2

Figure 8. TOFs of various supported noble metal catalysts for the hydrogenation of

3

4-NP under 0.4 MPa H2 at room temperature. (a) Pd catalysts supported on various

4

monolayer-modified PDA particles, including bare PDA and PDA modified by

5

2-mercaptothanesulfonic acid sodium Salt (HS-C2-SO3-Na); 3-mercaptopropionic

6

acid

7

11-mercaptoundecanoic acid (HS-C11-COOH); (b) Pd/SAM-PDA catalysts prepared

8

at different pH values; (c) Pd/SAM-PDA catalysts prepared by different concentration

9

of HS-C11-COOH; (d) various noble metal catalysts supported by PDA and

10

(HS-C3-COOH),

4-mercaptobenzoic

acid

(HS-C6H4-COOH),

and

SAM-PDA particles.

11 12

Table 2. Comparison of the TOFave values of various catalysts for the hydrogenation 24


Page 25 of 37

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

Langmuir

1

2

of 4-NP at low temperature and pressure.

a

Catalyst

Solvent

Temperature °C

Pressure MPa

TOFave min−1

Pd/PDA Pd/SAM-PDA SiO2-bisILs[PF6]-Pd0 Fe3O4-SH-Pd HMMS-NH2-Pd Fe3O4-NH2-Pd Pd/Fe3O4 NAP-Mg-Pd(0) SiliaCat Pd0 MEFO-Pd Pd@NAC-800 Pd/Fe3O4 Pd/C c HMMS-salpr-Pd

Water Water Water Ethanol Ethanol Ethanol THF THF Methanol Methanol Ethanol Water Water Ethanol

RT RT 30 RT RT RT RT RT RT 30 RT RT RT RT

0.4 0.4 0.1 0.1 0.1 0.1 0.1 atmospheric 0.1 0.1 0.1 0.1 0.1 0.1

0.83±0.12 2.51±0.75 4.67 1.30 a 1.63 1.40 b 0.26 0.74 1.67 1.93 1.67 7.85±0.38 1.03 1.59

Reference This work This work 65 66 67 68 69 70 71 72 73 74 74 75

ICP analysis; b based on atomic absorption spectroscopic (AAS) analysis; c Commercial Pd/C.

3 4

A series of experiments were carried out for 4-NP hydrogenation catalyzed by

5

Pd/SAM-PDA at various temperatures (26 °C, 50 °C, 52 °C, 75 °C, 80 °C) and

6

various initial 4-NP concentrations (0.1 mM, 1 mM and 4 mM) under 0.4 MPa of H2.

7

The kinetic data were collected, analyzed, and fitted by an L-H model, as shown in

8

Figure 9. The residual plots of the fitting results are displayed in Figure S6. Detailed

9

reaction conditions and values of R2 for all L-H fittings are summarized in Table S2.

10

Results show that the selected L-H model gives a good fit of the experimental data.

11

The catalysis process involves a dual-site adsorption with molecular adsorption of

12

4-NP and dissociative adsorption of H2, and on-surface reaction is the rate control

13

step.

14

The rate constant (k), the adsorption constant of 4-NP (KA) and the adsorption 25


Langmuir

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

Page 26 of 37

1

constant of hydrogen (KB) are determined and summarized in Table 3. The k values of

2

Pd/SAM-PDA range from 0.1391 s-1 to 1.7179 s-1 at 26 °C and 80 °C, respectively. To

3

obtain the apparent activation energy (Ea) for the hydrogenation catalyzed by

4

Pd/SAM-PDA, the natural log of k was plotted as a function of inverse temperature

5

(an Arrhenius plot), as displayed in Figure 10. From this analysis, the activation

6

energy of 4-NP hydrogenation catalyzed by Pd/SAM-PDA is 40.83 kJ mol-1. Yao et

7

al.76 reported that the apparent activation energy of 4-NP over colloidal Pd drops from

8

63.60 kJ mol-1 to 6.28 kJ mol-1 when the rate determining step is changed from the

9

surface reaction to the transfer of hydrogen from the gas phase into the liquid. When a

10

reaction is controlled by external mass transport (either gas-liquid or liquid-solid mass

11

transport), it has an activation energy of less than about 25 kJ mol-1.76,

12

relatively high value of observed activation energy on Pd/SAM-PDA further suggest

13

the negligibility of both gas-liquid and liquid-solid mass transport in this study.

14

Therefore, it can be verified that the rate-controlled step is on-surface reaction.

15

Moreover, the apparent activation energy of Pd/SAM-PDA is much lower than that of

16

colloidal Pd (63.60 kJ mol-1) reported by Yao et al., indicating a higher catalytic

17

activity of Pd NPs supported by surface-engineered PDA particles. These results are

18

also in good agreement with our previous work on shaped Pt NPs, where the values of

19

Ea were calculated to be 74.23 kJ mol-1 and 40.98 kJ mol-1 for cubic Pt NPs and

20

tetrahedral Pt NPs, respectively.29 Herein, we attribute the higher catalytic activity of

21

Pd/SAM-PDA to the suitable environment of SAM-PDA particles for Pd

22

immobilization. 26


77

The

Page 27 of 37

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

Langmuir

1 2

Figure 9. The reaction kinetic data and L-H fitting results of 4-NP hydrogenation

3

catalyzed by Pd/SAM-PDA at 26 °C (○), 50 °C (), 52 °C (□), 75 °C (△), 80 °C (┼),

4

and various initial 4-NP concentrations of 0.1 mM (a), 1 mM (b, d) and 4 mM (c). The

5

concentration of the catalyst in the reaction volume was calculated to: [Pd] = 4.75 ×

6

10-8 mol L-1 for (a) and [Pd] = 2.37 × 10-6 mol L-1 for (b-d).

7 8

Table 3. Parameters of the L-H model obtained at various temperatures under 0.4

9

MPa of H2 and the corresponding apparent activation energy for Pd/SAM-PDA. T/K 299.15a 323.15 325.15a 348.15 353.15

k1/s-1 0.1391 0.4692 0.5364 1.4000 1.7179

KA/L mol-1 0.0126 0.0108 0.0071 0.0068 0.0160

KB/MPa-1 8.5346 7.4080 5.6618 4.2500 9.3949

10

27


Ea/kJ mol-1

40.77

Langmuir

0.5

Pd/SAM-PDA

0.0 -0.5

lnk

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

Page 28 of 37

-1.0

y=-4903.2x+14.4 R2=0.99962

-1.5 -2.0

0.0028 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034

1/T 1 2

Figure 10 Arrhenius plotting for 4-NP hydrogenation over Pd/SAM-PDA under 0.4

3

MPa

4 5

■ CONCLUSIONS

6

In this study, PDA particles (272.9 ± 17.5 nm) are employed as support for the

7

preparation of noble-metal catalysts. Noble metal NPs can be spontaneously loaded

8

onto the surface of bare PDA particles but without size & dispersion control. Then,

9

surface morphology of PDA particle is modified by the corrosion of CF3COOH, and

10

surface chemistry of the obtained particle is engineered via the formation of

11

carboxylic acid terminated alkanethiol monolayer. The well-defined SAM-PDA

12

particles can load Pd NPs with small size (2-3 nm) and uniform distribution due to

13

increased surface roughness and suitable interactions among PDA, alkanethiol, and

14

metal NPs.

15

When the surface-engineered catalyst (Pd/SAM-PDA) was used to catalyze the 28


Page 29 of 37

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

Langmuir

1

reduction of 4-NP by NaBH4, the rate constant (k, s-1) increased 6 times in comparison

2

to Pd/PDA without any surface modification. In addition, the Pd/SAM-PDA catalyst

3

can be recycled easily and effectively. Experimental results indicate that the catalyst

4

still shows good catalytic performance without any decrease of rate constant k after

5

recovery and reuse for 6 times. Moreover, Pd/SAM-PDA can efficiently convert 4-NP

6

to 4-AP using H2 in 30 min in water at room temperature with a TOFave of 2.51 min-1.

7

The promotion effect of carboxylic acid monolayer on PDA particle is also valid for

8

other noble metals (Pt, Au and Ag). The kinetic data of 4-NP hydrogenation catalyzed

9

by

Pd/SAM-PDA

were

well

fitted

to

an

L-H

model

involving

a

10

surface-reaction-control mechanism. The apparent activation energy of 4-NP

11

hydrogenation over Pd/SAM-PDA is 40.77 kJ mol−1.

12

This work validates that surface science of support, e.g. surface morphology and

13

surface charge, has great influence on the activity of noble metal NPs via control of

14

their size and dispersion. Moreover, metal leaching can also be efficiently suppressed

15

by proper control of surface chemistry. In the future, surface modification of supports

16

should be deliberately investigated and optimized to improve metal dispersion for the

17

development of more active catalysts.

18 19

■ AUTHOR INFORMATION

20

Corresponding Author

21

*(G. L.) E-mail: [email protected]

22

Notes 29


Langmuir

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

1

The authors declare no competing financial interest.

2

■ ACKNOWLEDGMENTS

3

This work was supported by the research funds of the National Natural Science

4

Foundation of China (No. 21306132) and the Doctoral Program of Higher Education

5

(No. 20120032120008).

6

■ REFERENCES

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

(1) Ahmadi, M.; Mistry, H.; Roldan Cuenya, B., Tailoring the Catalytic Properties of Metal Nanoparticles via Support Interactions. J. Phys. Chem. Lett. 2016, 7(17), 3519-3533. (2) Yan, W.; Mahurin, S. M.; Overbury, S. H.; Dai, S., Nanoengineering catalyst supports via layer-by-layer surface functionalization. Top. Catal. 2006, 39(3-4), 199-212. (3) Fan, J.; Gao, Y., Nanoparticle-supported catalysts and catalytic reactions - a mini-review. J. Exp. Nanosci. 2006, 1(1-4), 457-475. (4) Du, X.; He, J., Spherical silica micro/nanomaterials with hierarchical structures: synthesis and applications. Nanoscale 2011, 3(10), 3984-4002. (5) Bagheri, S.; Julkapli, N. M., Modified iron oxide nanomaterials: Functionalization and application. J. Magn. Magn. Mater. 2016, 416, 117-133. (6) Dalpozzo, R., Magnetic nanoparticle supports for asymmetric catalysts. Green Chem. 2015, 17(7), 3671-3686. (7) Gawande, M. B.; Branco, P. S.; Varma, R. S., Nano-magnetite (Fe3O4) as a support for recyclable catalysts in the development of sustainable methodologies. Chem. Soc. Rev. 2013, 42(8), 3371-3393. (8) Ehrburger, P.; Walker, P. L., Carbon as a support for catalysts : II. Size distribution of platinum particles on carbons of different heterogeneity before and after sintering. J. Catal. 1978, 55(1), 63–70. (9) Ehrburger, P.; Mahajan, O. P.; Walker, P. L., Carbon as a support for catalysts : I. Effect of surface heterogeneity of carbon on dispersion of platinum. J. Catal. 1976, 43(1–3), 61-67. (10) Moreno-Castilla, C.; Mahajan, O. P.; Walker, P. L.; Jung, H. J.; Vannice, M. A., Carbon as a support for catalysts—III glassy carbon as a support for iron. Carbon 1980, 18(4), 271-276. (11) Auer, E.; Freund, A.; Pietsch, J.; Tacke, T., Carbons as supports for industrial precious metal 30


Page 30 of 37

Page 31 of 37

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

Langmuir

1

catalysts. Appl. Catal. A-Gen. 1998, 173(2), 259-271.

2

(12) Xin, H.; Li, L.; Liao, X.; Bi, S.; Xin, H.; Li, L.; Liao, X.; Bi, S., Preparation of platinum nanoparticles

3 4

supported on bayberry tannin grafted silica bead and its catalytic properties in hydrogenation. J. Mol.

5

(13) Kunz, S., Supported, Ligand-Functionalized Nanoparticles: An Attempt to Rationalize the

6 7 8

Catal. A-Chem. 2010, 320(1-2), 40-46.

Application and Potential of Ligands in Heterogeneous Catalysis. Top. Catal. 2016, Ahead of Print. (14) Costa, N. J. S.; Rossi, L. M., Synthesis of supported metal nanoparticle catalysts using ligand assisted methods. Nanoscale 2012, 4(19), 5826-5834.

9

(15) Dragu, A.; Kinayyigit, S.; García-Suárez, E. J.; Florea, M.; Stepan, E.; Velea, S.; Tanase, L.; Collière,

10 11 12

V.; Philippot, K.; Granger, P., Deoxygenation of oleic acid: Influence of the synthesis route of

13

(16) Karimi, B.; Behzadnia, H.; Farhangi, E.; Jafari, E.; Zamani, A., Recent application of

14 15

polymer-supported metal nanoparticles in Heck, Suzuki, and Sonogashira coupling reactions. Curr. Org.

16

(17) Kaushik, M.; Moores, A., Review: nanocelluloses as versatile supports for metal nanoparticles

17 18 19 20 21 22 23 24 25 26 27 28

Pd/mesoporous carbon nanocatalysts onto their activity and selectivity. Appl. Catal. A: Gen. 2015, 87(3), 100-111.

Synth. 2010, 7(6), 543-567.

and their applications in catalysis. Green Chem. 2016, 18(3), 622-637. (18) El, K. A., Chitosan as a sustainable organocatalyst: a concise overview. ChemSusChem 2015, 8(2), 217-44. (19) Kainz, Q. M.; Reiser, O., Polymer- and Dendrimer-Coated Magnetic Nanoparticles as Versatile Supports for Catalysts, Scavengers, and Reagents. Acc. Chem. Res. 2014, 47(2), 667-677. (20) Heerbeek, R. v.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Reek, J. N. H., Dendrimers as Support for Recoverable Catalysts and Reagents. Chem. Rev. 2002, 102(10), 3717-3756. (21) Kumbhar, A.; Salunkhe, R., Recent Advances in Biopolymer Supported Palladium in Organic Synthesis. Curr. Org. Chem. 2015, 19(21), 2075-2121. (22) Ju, K.-Y.; Lee, Y.; Lee, S.; Park, S. B.; Lee, J.-K., Bioinspired polymerization of dopamine to generate

melanin-like

nanoparticles

having

an

excellent

free-radical-scavenging

property.

Biomacromolecules 2011, 12(3), 625-632.

29

(23) Yang, X.; Chen, D.; Liao, S.; Song, H.; Li, Y.; Fu, Z.; Su, Y., High-performance Pd–Au bimetallic

30 31

catalyst with mesoporous silica nanoparticles as support and its catalysis of cinnamaldehyde

32

(24) Nietzel, S.; Joe, D.; Krumpfer, J. W.; Schellenberger, F.; Alsaygh, A. A.; Fink, G.; Klapper, M.;

hydrogenation. J. Catal. 2012, 291(7), 36-43.

31


Langmuir

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

1 2

Müllen, K., Organic nanoparticles as fragmentable support for Ziegler–Natta catalysts. Journal of

3

(25) Zang, L.; Qiu, J.; Wu, X.; Zhang, W.; Sakai, E.; Yi, W., Preparation of Magnetic Chitosan

4 5

Nanoparticles As Support for Cellulase Immobilization. Industrial & Engineering Chemistry Research

6

(26) Ye, Q.; Zhou, F.; Liu, W., Bioinspired catecholic chemistry for surface modification. Chemical

7 8 9 10 11 12 13

Polymer Science Part A: Polymer Chemistry 2015, 53(1), 15-22.

2014, 53(9), 3448-3454.

Society Reviews 2011, 40(7), 4244-4258. (27) Wang, H.; Chen, S.; Li, L.; Jiang, S., Improved method for the preparation of carboxylic acid and amine terminated self-assembled monolayers of alkanethiolates. Langmuir 2005, 21(7), 2633-2636. (28) Bawane, S. P.; Sawant, S. B., Hydrogenation of p-nitrophenol to metol using Raney nickel catalyst: Reaction kinetics. Applied Catalysis A General 2005, 293(1), 162-170. (29) Li, X.; Li, G.; Zang, W.; Wang, L.; Zhang, X., Catalytic activity of shaped platinum nanoparticles for hydrogenation: a kinetic study. Catalysis Science & Technology 2014, 4(9), 3290-3297.

14

(30) Marcus, Y., Ion Solvation. John Wiley & Sons Ltd: New York, 1986.

15

(31) Fei, B.; Qian, B.; Yang, Z.; Wang, R.; Liu, W. C.; Mak, C. L.; Xin, J. H., Coating carbon nanotubes by

16

spontaneous oxidative polymerization of dopamine. Carbon 2008, 46(13), 1795-1797.

17

(32) Guo, L.; Liu, Q.; Li, G.; Shi, J.; Liu, J.; Wanga, T.; Jiang, G., A mussel-inspired polydopamine coating

18 19

as a versatile platform for the in situ synthesis of graphene-based nanocomposites. Nanoscale 2012, 4,

20

(33) Jiang, Y.; Li, G.; Li, X.; Lu, S.; Wang, L.; Zhang, X., Water-dispersible Fe3O4 nanowires as efficient

21 22

supports for noble-metal catalysed aqueous reactions. Journal of Materials Chemistry A 2014, 2(13),

23

(34) Xu, H.; Liu, X.; Su, G.; Zhang, B.; Wang, D., Electrostatic repulsion-controlled formation of

24

5864-5867.

4779-4787.

polydopamine–gold janus particles. Langmuir 2012, 28(36), 13060-13065.

25

(35) Kim, B. G.; Kim, H.-J.; Back, S.; Nam, K. W.; Jung, Y.; Han, Y.-K.; Choi, J. W., Improved reversibility

26 27

in lithium-oxygen battery: Understanding elementary reactions and surface charge engineering of

28

(36) Xie, X.; Xue, Y.; Li, L.; Chen, S.; Nie, Y.; Ding, W.; Wei, Z., Surface Al leached Ti3AlC2 as a substitute

29 30

for carbon for use as a catalyst support in a harsh corrosive electrochemical system. Nanoscale 2014,

31

(37) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B., Mussel-inspired surface chemistry for

metal alloy catalyst. Scientific Reports 2014, 4, 4225.

6(19), 11035-11040.

32


Page 32 of 37

Page 33 of 37

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

Langmuir

1 2

multifunctional coatings. Science 2007, 318(5849), 426-430. (38)

Gao,

H.;

Sun,

Y.;

Zhou,

J.;

Xu,

R.;

Duan,

H.,

Mussel-inspired

synthesis

of

3 4

polydopamine-functionalized graphene hydrogel as reusable adsorbents for water purification. ACS

5

(39) Duff, D. G.; Edwards, P. P.; Johnson, B. F. G., Formation of a Polymer-Protected Platinum Sol: A

6 7

New Understanding of the Parameters Controlling Morphology. The Journal of Physical Chemistry

8

(40) Yonezawa, T.; Toshima, N., Mechanistic consideration of formation of polymer-protected

9 10

nanoscopic bimetallic clusters. Journal of the Chemical Society, Faraday Transactions 1995, 91(22),

11

(41) Tu, W.; Liu, H., Rapid synthesis of nanoscale colloidal metal clusters by microwave irradiation.

12

applied materials & interfaces 2013, 5(2), 425-432.

1995, 99(43), 15934-15944.

4111-4119.

Journal of Materials Chemistry 2000, 10(9), 2207-2211.

13

(42) Zhang, Q.-L.; Xu, T.-Q.; Wei, J.; Chen, J.-R.; Wang, A.-J.; Feng, J.-J., Facile synthesis of uniform Pt

14 15

nanoparticles on polydopamine-reduced graphene oxide and their electrochemical sensing.

16

(43) Nishizawa, N.; Kawamura, A.; Kohri, M.; Nakamura, Y.; Fujii, S., Polydopamine particle as a

17 18 19

Electrochimica Acta 2013, 112, 127-132.

particulate emulsifier. Polymers 2016, 8(3), 62. (44) Weng, X.; Ren, H.; Chen, M.; Wan, H., Effect of surface oxygen on the activation of methane on palladium and platinum surfaces. ACS Catalysis 2014, 4(8), 2598-2604.

20

(45) Kilic, A.; Kilinc, D.; Tas, E.; Yilmaz, I.; Durgun, M.; Ozdemir, I.; Yasar, S., The orthopalladation

21 22 23

dinuclear [Pd(L1)(μ-OAc)]2, [Pd(L2)(μ-OAc)]2 and mononuclear [Pd(L3)2] complexes with [N, C, O] or

24

(46) Herves, P.; Pérez-Lorenzo, M.; Liz-Marzán, L. M.; Dzubiella, J.; Lu, Y.; Ballauff, M., Catalysis by

25 26

metallic nanoparticles in aqueous solution: model reactions. Chemical Society Reviews 2012, 41(17),

27

(47) Wanting Zang; Guozhu Li; Li Wang; Zhang, X., Catalytic hydrogenation by noble-metal

28

nanocrystals with well-defined facets: a review. Catalysis Science & Technology 2015, 5(5), 2532-2553.

29

(48) Dong, Z.; Le, X.; Dong, C.; Wei, Z.; Li, X.; Ma, J., Ni@Pd core–shell nanoparticles modified fibrous

30 31

silica nanospheres as highly efficient and recoverable catalyst for reduction of 4-nitrophenol and

32

(49) Ma, J. X.; Yang, H.; Li, S.; Ren, R.; Li, J.; Zhang, X.; Ma, J., Well-dispersed

[N, O] containing ligands: Synthesis, spectral characterization, electrochemistry and catalytic properties. Journal of Organometallic Chemistry 2010, 695(5), 697-706.

5577-5587.

hydrodechlorination of 4-chlorophenol. Applied Catalysis B Environmental 2015, 162(162), 372–380.

33


Langmuir

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

1 2

graphene-polydopamine-Pd hybrid with enhanced catalytic performance. RSC Advances 2015, 5(118),

3

(50) Wu, X.; Lu, C.; Zhang, W.; Yuan, G.; Xiong, R.; Zhang, X., A novel reagentless approach for

4 5

synthesizing cellulose nanocrystal-supported palladium nanoparticles with enhanced catalytic

6

(51) Fang, Y.; Wang, E., Simple and direct synthesis of oxygenous carbon supported palladium

7

97520-97527.

performance. Journal of Materials Chemistry A 2013, 1(30), 8645-8652.

nanoparticles with high catalytic activity. Nanoscale 2013, 5(5), 1843-8.

8

(52) Morère, J.; Tenorio, M. J.; Torralvo, M. J.; Pando, C.; Renuncio, J. A. R.; Cabañas, A., Deposition of

9 10

Pd into mesoporous silica SBA-15 using supercritical carbon dioxide. Journal of Supercritical Fluids the

11

(53) Yu, M.; Yan, L.; Polzer, F.; Ballauff, M.; Drechsler, M., Catalytic Activity of Palladium Nanoparticles

12 13

Encapsulated in Spherical Polyelectrolyte Brushes and Core−Shell Microgels. Chemistry of Materials

14

(54) Arora, S.; Kapoor, P.; Singla, M. L., Catalytic studies of palladium nanoparticles immobilized on

15 16

alumina synthesized by a simple physical precipitation method. Reaction Kinetics Mechanisms &

17

(55) Wang, Z.; Xu, C.; Gao, G.; Li, X., Facile synthesis of well-dispersed Pd-graphene nanohybrids and

18

2011, 56(2), 213-222.

2007, 19(5), 1062-1069.

Catalysis 2010, 99(1), 157-165.

their catalytic properties in 4-nitrophenol reduction. Rsc Advances 2014, 4(26), 13644-13651.

19

(56) Li, H.; Han, L.; Cooperwhite, J.; Kim, I., Palladium nanoparticles decorated carbon nanotubes:

20 21

facile synthesis and their applications as highly efficient catalysts for the reduction of 4-nitrophenol.

22

(57) Le, X.; Dong, Z.; Liu, Y.; Jin, Z.; Huy, T. D.; Le, M.; Ma, J., Palladium nanoparticles immobilized on

23 24 25

core–shell magnetic fibers as a highly efficient and recyclable heterogeneous catalyst for the reduction

26

(58) Cho, K. Y.; Seo, H. Y.; Yong, S. Y.; Kumar, P.; Lee, A. S.; Baek, K. Y.; Yoon, H. G., Stable 2D-structured

27 28 29

supports incorporating ionic block copolymer-wrapped carbon nanotubes with graphene oxide toward

30

(59) Ji, D. K.; Choi, M. Y.; Choi, H. C., Catalyst activity of carbon nanotube supported Pd catalysts for

31

Green Chemistry 2012, 14(3), 586-591.

of 4-nitrophenol and Suzuki coupling reactions. Journal of Materials Chemistry A 2014, 2(46), 19696-19706.

compact decoration of metal nanoparticles and high-performance nano-catalysis. Carbon 2016, 105, 340-352.

the hydrogenation of nitroarenes. Materials Chemistry & Physics 2016, 173, 404-411.

32

(60) Chen, H.; Huang, D.; Lin, L.; Odoom-Wubah, T.; Huang, J.; Sun, D.; Li, Q., Facile fabrication of Pd

33

nanoparticle/ Pichia pastoris catalysts through adsorption–reduction method: A study into effect of 34


Page 34 of 37

Page 35 of 37

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

Langmuir

1 2 3

chemical pretreatment. Journal of Colloid & Interface Science 2014, 433(433), 204–210. (61) Zeng, Z., Naturally nano: synthesis of versatile bio-inpired monodisperse microspheres from Bacillus spores and their applications. Green Chemistry 2015, 18(1), 186-196.

4

(62) Yang, L.; Liu, X.; Lu, Q.; Huang, N.; Liu, M.; Zhang, Y.; Yao, S., Catalytic and Peroxidase-Like

5 6

Activity of Carbon Based-AuPd Bimetallic Nanocomposite Produced Using Carbon Dots as the

7

(63) Zeng, T.; Zhang, X.-l.; Niu, H.-y.; Ma, Y.-r.; Li, W.-h.; Cai, Y.-q., In situ growth of gold nanoparticles

8 9

Reductant. Analytica Chimica Acta 2016, 930, 23-30.

onto polydopamine-encapsulated magnetic microspheres for catalytic reduction of nitrobenzene. Applied Catalysis B: Environmental 2013, 134–135, 26-33.

10

(64) Luo, J.; Zhang, N.; Liu, R.; Liu, X., In situ green synthesis of Au nanoparticles onto

11 12

polydopamine-functionalized graphene for catalytic reduction of nitrophenol. RSC Advances 2014,

13

(65) Li, J.; Shi, X. Y.; Bi, Y. Y.; Wei, J. F.; Chen, Z. G., Pd nanoparticles in ionic liquid brush: A highly

14 15

active and reusable heterogeneous catalytic assembly for solvent-free or on-water hydrogenation of

16

(66) Wang, H. B.; Zhang, Y. H.; Zhang, Y. B.; Zhang, F. W.; Niu, J. R.; Yang, H. L.; Li, R.; Ma, J. T., Pd

17 18

immobilized on thiol-modified magnetic nanoparticles: A complete magnetically recoverable and

19

(67) Wang, P.; Zhang, F.; Long, Y.; Xie, M.; Li, R.; Ma, J., Stabilizing Pd on the surface of hollow

20 21

magnetic mesoporous spheres: a highly active and recyclable catalyst for hydrogenation and Suzuki

22

(68) Zhang, F.; Jin, J.; Zhong, X.; Li, S.; Niu, J.; Li, R.; Ma, J., Pd immobilized on amine-functionalized

23 24

magnetite nanoparticles: a novel and highly active catalyst for hydrogenation and Heck reactions.

25

(69) Zhang; Rongzhao; Jianming; Fuwei; Wang; Shoufeng; Chungu, Magnetically separable and

26 27

versatile Pd/Fe3O4 catalyst for efficient Suzuki cross-couplingr and selective hydrogenation of

28

(70) Kantam, M. L.; Chakravarti, R.; Pal, U.; Sreedhar, B.; Bhargava, S., Nanocrystalline magnesium

29 30

oxide-stabilized palladium(0): An efficient and reusable catalyst for selective reduction of nitro

31

(71) Pandarus, V.; Ciriminna, R.; Béland, F.; Pagliaro, M., Selective hydrogenation of functionalized

32 33

4(110), 64816-64824.

nitroarene under mild conditions. Acs Catalysis 2011, 1(6), 657-664.

highly active catalyst for hydrogenation reactions. Solid State Sciences 2012, 14(9), 1256-1262.

coupling reactions. Catalysis Science & Technology 2013, 3(6), 1618-1624.

Green Chemistry 2011, 13(37), 1238-1243.

nitroarenes. Chinese Journal of Chemistry 2011, 29(3), 525–530.

compounds. Advanced Synthesis & Catalysis 2008, 350(6), 822-827.

nitroarenes under mild conditions. Catalysis Science & Technology 2011, 1(9), 1616-1623. (72) Xu, S.; He, J.; Cao, S., Catalytic behaviors of the palladium complex of MgO-supported 35


Langmuir

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

1 2

melamino-formaldehyde polymer for hydrogenations of different substrates. Journal of Molecular

3

(73) Li, Z.; Li, J.; Liu, J.; Zhao, Z.; Xia, C.; Li, F., Palladium nanoparticles supported on

4 5

nitrogen-functionalized active carbon: A stable and highly efficient catalyst for the selective

6

(74) Zhang, D.; Chen, L.; Ge, G., A green approach for efficient p -nitrophenol hydrogenation

7

Catalysis A Chemical 1999, 147(1–2), 155-158.

hydrogenation of nitroarenes. Chemcatchem 2014, 6(5), 1333–1339.

catalyzed by a Pd-based nanocatalyst. Catalysis Communications 2015, 66, 95-99.

8

(75) Liu, H.; Wang, P.; Yang, H.; Niu, J.; Ma, J., Palladium supported on hollow magnetic mesoporous

9 10

spheres: a recoverable catalyst for hydrogenation and Suzuki reaction. New Journal of Chemistry 2015,

11

(76) Yao, H. C.; Emmett, P. H., Kinetics of catalytic liquid phase hydrogenation. I. the Hydrogenation of

12 13

aromatic nitrocompounds over colloidal rhodium and palladium. J. Am. Chem. Soc. 1959, 17(16),

14

(77) Rode, C. V.; Vaidya, M. J.; Jaganathan, R.; Chaudhari, R. V., Hydrogenation of nitrobenzene to p

15 16

-aminophenol in a four-phase reactor: reaction kinetics and mass transfer effects. Chemical

39(6), 4343-4350.

4125-4132.

Engineering Science 2001, 56(4), 1299-1304.

17 18

36


Page 36 of 37

Page 37 of 37

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

Langmuir

1

2 3

37


Surface-Engineered Polydopamine Particles as an Efficient Support

Recommend Documents