Elucidating interaction between palladium and N ... - ACS Publications

Subscriber access provided by AUT Library

Article

Elucidating interaction between palladium and N-doped carbon nanotubes: Effect of electronic property on activity for nitrobenzene hydrogenation Zhiyan He, Baoqiang Dong, Wenli Wang, Guangxing Yang, Yonghai Cao, HongJuan Wang, Yanhui Yang, Qiang Wang, Feng Peng, and Hao Yu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03965 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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 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 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.

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 32 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

ACS Catalysis

Elucidating interaction between palladium and N-doped carbon nanotubes: Effect of electronic property on activity for nitrobenzene hydrogenation Zhiyan Hea,‡, Baoqiang Donga,‡, Wenli Wanga, Guangxing Yanga, Yonghai Caoa, Hongjuan Wanga, Yanhui Yangb, Qiang Wangb,*, Feng Pengc, Hao Yua, * a School

of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. China

b Department

of Applied Chemistry, College of Chemistry and Molecular

Engineering, Nanjing Tech University, Nanjing 211816, P. R. China c

School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, P. R. China

1

ACS Paragon Plus Environment

ACS Catalysis 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 2 of 32

Keywords: palladium; nitrogen-doped carbon nanotube; metal-support interaction; nitrobenzene hydrogenation; electron transfer

Abstract: Nitrogen dopants of carbon materials remarkably improve the stability and tune catalytic performance of supported metal nanoparticles. However, it is still controversial how the Pd-N metal-support-interaction (MSI) influences the catalysis. Herein, density function theory (DFT) calculation and X-ray photoelectron spectroscopy (XPS) were combined to rationalize the Pd-N MSI. DFT calculations suggested that Pd adsorb on N-doped carbon nanotubes (N@CNTs) and donate electrons to pyridinic nitrogen. It was further experimentally proved using XPS through a titration method by gradually increasing Pd content or changing the N content of support by a post-heat-treatment. The Pd catalysts display electron-deficiency depending on the intensity of MSI between Pd and pyridinic nitrogen, measured by Pd3d binding energy. It paves the way to the rational synthesis of Pd catalysts with tunable electronic state for the targeted catalytic reaction. Using the hydrogenation of nitrobenzene as probe reaction, it was revealed that the reaction activity can be facilely tuned by the Pd-N MSI, due to the strong adsorption of nitro-groups on electrondeficient Pd nanoparticles.

2


Page 3 of 32 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

ACS Catalysis

INTRODUCTION Carbon-supported palladium catalysts (Pd/C) are widely used in industrial hydrogenation 1, 2, oxidation 3 and electrocatalysis for fuel cells 4, 5. It is always highly desired to reduce the loading and enhance the catalytic efficiency of Pd by either exposing more active surfaces or elevating intrinsic activity, because of the limited resource and the high price. To this end, the surfaces and interfaces of Pd/C catalysts need to be optimized to maximize the dispersion and activity. Carbon nanotubes (CNTs) have been employed as an appropriate support for noble metals affording unique catalytic behaviors due to their electronic properties modified by topological defects, surface functionalities and nanosized curvature6, 7. Recently, heteroatom dopants in CNTs, including either single- (N8, 9, S10, B11, P12) or dual-dopants (S-N13, B-N14), have been found effective to offer more sites stabilizing Pd nanoparticles to reach higher exposure of metal surfaces. More importantly, the dopants may modify the electronic structure of carbons and significantly promote the electron transfer between metal and support 15-19. Nitrogen-doped CNTs (NCNTs) show good performance because of the small difference between the electronegativities of N and C (N: 3.04, C: 2.55) and their similar atom diameter (N: 0.075 nm, C: 0.077 nm). NCNTs have displayed promise as catalyst support in hydrogenation20, 21, ammonia decomposition22 and oxygen evolution reactions23, because they provide more nucleation domains for metal nanoparticles around the N-rich sites24, 25. Li et al.26 fabricated ultrafine Pd nanoparticles (1.2 nm) 3


ACS Catalysis 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 4 of 32

supported on N-doped ordered mesoporous carbon showing high catalytic activity and selectivity for the hydrogenation of phenol to produce cyclohexanone. Dong et al.

27

have deposited N-doped carbon layers on CNTs, and the activity of NCNT supported Pd for nitrobenzene hydrogenation was 3.85 times higher than CNTs. Chen et al.20 proposed that NCNTs acted as an electron donor to Pd prepared by a gel method, evidenced by the lower binding energy of Pd compared with that on the oxidized CNTs. Bi et al.28 have engineered pyridinic-N for tuning Pd on carbon materials for the formic acid dehydrogenation. They observed that Pd was negatively charged, therefore suggested that electrons transferred from N-doped carbons to Pd nanoparticles, and pyridinic-N was the main contributor to the electronic property of Pd. Wang et al.29 observed electron-rich Pd nanoparticles supported on N-doped mesoporous carbons as well, emphasizing the impact of pyridinic N on the electronic properties of Pd. Conversely, Nie et al.30 synthesized highly active electron-deficient Pd clusters on Ndoped carbons for aromatic ring hydrogenation, where the nitrogen functionalities was regarded as Lewis basic sites. These controversial results suggest the complexity of Pd-N interaction. It is worthy of mentioning that the chemical nature of N dopants is not homogeneous on N-doped carbons. There is great diversity of nitrogen groups on carbon materials, represented by pyridinic nitrogen (denoted as NP) and graphitic nitrogen (denoted as NG), which are usually the most abundant on N-doped carbons31. Theoretical studies have shown that the NP and NG may act as electron acceptor and donor, respectively, as anchoring metal 4


Page 5 of 32 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

ACS Catalysis

nanoparticles32, 33. Our previous study on Pt supported on NCNTs34 showed that Pt interacts with NCNTs that act as electron donors to increase the electron density of Pt nanoparticles prepared by the polyol reduction method. A detailed XPS study suggested that Pt binds preferentially to NG sites. These electron-rich Pt nanoparticles exhibited excellent catalytic activity in CO electro-oxidation, as well as in glycerol oxidation. Furthermore, we demonstrate that the electronic property of Pt catalysts can be regulated by controlling the anchoring site of Pt on NCNTs, since the direction of electron transfer is opposite on NP and NG35. However, it is not clear yet whether there is any preferential interaction between palladium and NP or NG, and what is the influence on catalytic activity in industry-relevant hydrogenation reactions. The aim of this work is to rationalize the interaction between Pd nanoparticles and Ndoped CNTs. We theoretically investigated the binding strength and electronic interaction of Pd on NP or NG by DFT calculations. A titration methodology, involving measuring the occupation of NCNT surfaces by changing the amount of Pd nanoparticles (denoted by ωPd/N@CNT, where ω represents for the actual loading of Pd) and nitrogen content of support (denoted by ωPd/N@CNT-HT-T, where T represents for the heat treatment temperature of support N@CNT), was employed to experimentally determine the selective anchorage of Pd with different types of nitrogen sites36, 37. By doing so, we unambiguously demonstrated a strong interaction between NP on NCNTs and Pd nanoparticles, which resulted in electron-deficient Pd catalysts. This result may lay the foundation for the rapidly developing subject area regarding the 5


ACS Catalysis 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 6 of 32

metal-nitrogen interaction. Based on the insight, the rational design of Pd on N-doped carbons for the hydrogenation of nitrobenzene, an important large-volume chemical process, may be feasible through tuning the Pd-N interaction. RESULTS AND DISCUSSION 2.1 DFT calculations for Pd-N interactions Theoretical calculations were firstly conducted to verify if a palladium atom/cluster can be stabilized by nitrogen sites on a graphene sheet. The DFT calculations allow us separately to predict the adsorption of a palladium atom/cluster on a NP or NG site, and analyze the charge distribution under the stable configuration, which is hard to be experimentally studied because of the difficulty synthesizing N-doped carbons containing a single type of nitrogen functionality38. Fig. 1 lists the optimized structures of a Pd single atom and Pd4 cluster on a graphene layer doped by a NG atom or a vacant site with three NP atoms.

6


Page 7 of 32 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

ACS Catalysis

Figure 1 Schematic optimal structures and Bader charges of (a-b) Pd single atom or (cd) Pd4 cluster on a graphene sheet doped by a NG atom (a, c) or a vacant site with three NP atoms (b, d). The bond lengths are showed in angstroms. When a Pd atom is adsorbed near NG, the optimized configuration shows that the Pd atom is bounded with and stabilized by the two C atoms next to the N atom, as illustrated in Fig. 1(a), while no chemical bond formed between Pd and N. This is similar to the results obtained by Tian et al. 4 in the case of Pt interacted with nitrogendoped graphene. On the vacant site with three NP atoms (Fig. 1(b)), the Pd atom is chemically bonded with the three N atoms but is outside of the graphene plane due to the steric effect. The adsorption energies for the above cases are -1.11 eV and -2.42 eV, 7


ACS Catalysis 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 8 of 32

respectively, indicating that the adsorption of Pd atom on the NP site is stronger than that on NG. Analysis of the charge distribution shows that the Pd atom is both positively charged with +0.18 e and +0.56 e on NG and NP. This result implies that electrons transfer from Pd to support, leading to electron-deficiency of Pd. The similar conclusion can be drawn in the case of Pd cluster consisting of four atoms. In the optimal configuration, the Pd4 cluster is stabilized like an inverted pyramid on either NG or NP. The overall adsorption energy of Pd4 is -1.03 eV and -2.80 eV on NG and NP, respectively, indicating the stronger adsorption on pyridinic vacancy as well. The charge analysis indicated the heterogeneity of charge distribution among the four atoms. Although the Pd atom directly bounded on graphene is positively charged as well, the outmost three Pd atoms are slightly negatively charged by -0.01 e to -0.06 e. Nevertheless, the overall charges of Pd4 cluster near the NG site and the NP vacancy are -0.01 e and +0.49 e, respectively. This result ascertains the stronger electronic interaction between Pd and NP and the strong electron-deficiency of Pd clusters on NP, and the weak electron-enrichment of Pd on NG. It should be noted that the Pd-N interaction is quite different from the Pt-N interaction reported in our previous works 34, 35,

where Pt/NCNTs always exhibit higher electron density and the NCNTs act as

electron donor. DFT calculations indicate that the NG-stabilized Pt atom is negatively charged, meanwhile the preferential anchorage of Pt on NG may enhance the electron enrichment of Pt. However, regardless of the kind of nitrogen group, atomic Pd always

8


Page 9 of 32 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

ACS Catalysis

exhibits an electron-deficient state, which could result in very different catalytic behavior of Pd/NCNT catalysts. 2.2 Effect of Pd content on the Metal-Support Interaction To experimentally verify the Pd-N interaction revealed by DFT calculations, different contents of solvated Pd cations were reduced by NaBH4 and deposited onto N@CNTs and CNTs. By doing so, we attempted to observe the preferential interaction with nitrogen sites at low Pd loadings through the variation of binding energy of NP and NG and their content ratio, representing the selective occupation of nitrogen site 34. Table 1 Properties of Pd/N@CNT and Pd/CNT catalysts

Pd (wt.%)

N@CNT

CNT

d

(nm)

TEM

D

a TEM

(%)

D

b CO

(%)

0.42

2.30.2

48.7

30.2

0.67

2.50.2

44.8

29.6

1.65

2.50.6

44.8

27.4

2.27

2.70.4

41.5

25.3

4.30

2.60.2

43.1

19.6

0.51

2.40.2

46.3

/

2.54

3.10.2

35.8

33.9

4.40

3.30.2

33.5

/

a. DTEM (%) = 5.6/r, where r is radius of Pd particle (Å). 𝑄𝐶𝑂 × 𝐴𝑟𝑃𝑑

b. 𝐷𝐶𝑂(%) = 2 × 𝑞𝑒 × 𝑁𝐴 × 𝑚 × 𝑤𝑃𝑑, where QCO is the electric quantity for CO oxidation, ArPd is 106.42, qe is 1.6×10-19, NA stands for Avogadro's constant, which equals to 6.02×1023, m is the mass of catalyst, wPd is the actual Pd loading determined by ICP.

9


ACS Catalysis 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 10 of 32

TEM observations showed that 0.42 wt.% to 4.3 wt.% Pd nanoparticles were evenly dispersed on N@CNTs (Fig. S1 and S2). The Sauter mean diameters of Pd nanoparticles were calculated from TEM images. The particle sizes of Pd nanoparticles were summarized in Table 1. Compared to their counterparts on CNTs, the Pd nanoparticles are smaller on N@CNTs at similar Pd loadings, demonstrating the advantages of N@CNTs to increase the number of exposed Pd atoms on surface, which is consistent with the literature reports 15, 39. The dispersion of Pd was calculated either by particle sizes or by electrochemically CO-stripping experiments, and compared with Pd/CNTs. Figure S3 shows the CV curves for CO-stripping over the Pd/N@CNT catalysts. As shown in Table 1, the dispersion calculated by the geometrical surface area of Pd nanoparticles decreases with increasing Pd loading, while the dispersion by CO-stripping is very low at low Pd contents, despite the smaller particle sizes. The abnormal low CO adsorption at low Pd loadings may be caused by the weak adsorption of CO on sub-nanometer Pd clusters, which has been reported by Bulushev et al. for Ndoped carbon supported Pd catalysts 40, 41.

10


Page 11 of 32 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

ACS Catalysis

Figure 2 Pd3d XPS spectra of (a) Pd/N@CNT and (b) Pd/CNT with varied Pd. (c) and (d) show the dependence of Pd03d5/2 binding energy and Pd2+ fraction on the Pd content, respectively. The Pd/N@CNT catalysts were subjected to XPS measurement to characterize the interaction between N groups and Pd nanoparticles. The Pd 3d spectra (Figure 2(a)) are deconvoluted by two doublets with 5.5 eV splitting distance and intensity ratio of 3:2, namely Pd03d5/2, Pd03d3/2 and Pd2+3d5/2, Pd2+3d3/2, allowing for extracting the binding energy of Pd0 and the fraction of Pd2+ to evaluate the electronic property. In order to investigate the influence of N-doping on the electronic structure of Pd, the Pd3d XPS spectra of undoped CNTs supported Pd nanoparticles with nominal loadings of 1, 3, and 5 wt.% were measured and compared (Fig. 2(b)). The effects of N-doping on the electronic structure of Pd are summarized in Fig. 2(c) and (d). As shown in Fig. 2(c), the binding energy of Pd on N@CNTs is distinctly 11


ACS Catalysis 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 12 of 32

higher than that on CNTs. Considering the size effect42, the binding energy was compared at different Pd loading with varied Pd NP sizes. It can be seen that the Pd binding energy is always higher on N@CNTs within the range of Pd content investigated in this work. The higher binding energy can be attributed to the metalsupport interaction (MSI) between N@CNTs and Pd nanoparticles, suggesting that electrons transfer from Pd nanoparticles to N@CNTs leading to the electron deficiency. It can be further supported by the proportion of Pd2+ on these two supports. As is shown in Figure 2(d), the Pd2+ content of Pd/N@CNTs is significantly higher than that of Pd/CNTs, regardless of the loading, despite the larger variation of the ratio for Pd/CNT than that for Pd/N@CNT, probably due to the different initial electronic state of Pd in different size ranges. It is consistent with the electron-deficient state revealed by the Pd binding energy. The experimental results above agree well with aforementioned DFT calculations, which indicates that, regardless of the type of nitrogen sites, Pd atoms donate electrons to support to exhibit the electron deficiency. It should be noticed that the Pd binding energy depends on the Pd loading on both of the supports. It may be caused by the size effect of Pd in part, evidenced by the slight decrease of binding energy with Pd content of Pd/CNTs, where the Pd particle size increased from 2.4 nm to 3.3 nm. It may be stemmed from the different electronic interaction between Pd nanoparticles and NP or NG, as is revealed by DFT calculations. Because of the higher adsorption energy, Pd nanoparticles preferentially interacted with NP sites at low Pd contents, donating electrons to them, which resulted in the increase 12


Page 13 of 32 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

ACS Catalysis

of Pd binding energy as the Pd content was lower than 1.65 wt.%. At higher Pd contents, other nitrogen sites, e.g. NG, also anchored Pd nanoparticles and modulated the electronic property of Pd. Hence, the binding energy of Pd declined due to the weak electron-withdrawing ability of NG.

Figure 3 (a) N1s XPS spectra of N@CNTs and Pd/N@CNTs with varied Pd contents. (b) Dependence of NP to NG ratio on Pd content, (c) Dependences of binding energy of NG and NP species on Pd content. Considering that the binding energy of Pd may be affected by the particle size effect of Pd nanoparticles, we conducted the analysis of N1s XPS spectra. Since the electronic interaction is mutual, it could be expected that the N1s binding energy changes in a way opposite to that of Pd. Fig. 3(a) shows N1s XPS spectra of Pd/N@CNTs with different 13


ACS Catalysis 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 14 of 32

Pd contents. These spectra are deconvoluted into five nitrogen groups, i.e. pyridinic, pyrrolic, graphitic, N-oxide and chemisorbed nitrogen, as summarized in Tables S1 and S2. In this work, pyrrolic nitrogen accounts ca. 10 at.% which is much less than NP (ca. 35 at.%). In addition, the electronegativity of pyrrolic nitrogen is weaker than that of NP, resulting in weak interaction with Pd43. Therefore, we focus on the most abundant NP and NG sites in this work. To verify the preferential anchorage of Pd on NP, we compared the content ratio of NP to NG as the Pd content increasing. As is shown in Fig. 3(b), when the Pd content increases from 0 to 1.65 wt.%, the NP/NG ratio declines from 0.76 to 0.59, which clearly indicates that the NP sites are preferentially occupied by Pd. The ratio then increases slightly, because Pd nanoparticles have to occupy NG after the NP sites are occupied completely. Correspondingly, the N1s binding energy of NP declines from 398.72 eV (without Pd) to 398.40 eV (with 1.65 wt.% Pd), because the nitrogen sites accept electrons from Pd nanoparticles (Fig. 3(c)). This result is in complete agreement with the characteristics of Pd3d XPS spectra, in which the highest Pd binding energy is reached on the catalyst with 1.65 wt.% Pd (Fig. 2(c)). Meanwhile, the N1s binding energy of NG has little change as the Pd content increase from 0.42 wt.% to 4.30 wt.%, indicating that the electronic interaction between them is weak. When the Pd content is higher than 1.65 wt.%, the binding energy of NP increased slightly, probably due to the weaker electron donation of Pd nanoparticles with larger particle size 44.

14


Page 15 of 32

2.3 Effect of Nitrogen content on the Metal-Support Interaction Aforementioned XPS results clearly revealed the electronic interaction between Pd and nitrogen dopants. N@CNTs, especially the NP sties, acted as acceptor of electrons from Pd, resulting in the electron-deficiency of Pd catalysts. By the titration methodology with changing Pd loading, the preferential occupation of NP sites by Pd nanoparticles was revealed. To further confirm this, we attempted to explore the Pd-N MSI via changing the nitrogen content of N@CNTs. NP

NG

N@CNTs

(a)

N@CNTs-HT-900

Intensity (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

ACS Catalysis

(b)

N@CNTs-HT-1100

(c)

N@CNTs-HT-1300

(d)

396

398

400

402

404

406

Binding Energy (eV)

Figure 4 N1s XPS spectra of (a) N@CNTs and the N@CNTs annealed at (b) 900 oC, (c) 1100 oC, and (d) 1300 oC. We changed the nitrogen content and the relative proportion of NP and NG by a post heat treatment at 900-1300 oC. The N@CNT-HT-T samples were characterized by 15


ACS Catalysis 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 16 of 32

Raman spectroscopy (Fig. S4) and XPS (Fig. 4). The Raman study (Fig. S4) indicated that the ID/IG ratio gradually decreased with the increasing heat treatment temperature, because the graphitization degree of N@CNTs was improved at high temperatures. As is shown in Fig. 4, the N1s XPS spectra displayed that the heat treatment at 900 -1300 oC

reduced the total nitrogen content of N@CNTs from 7.08 % to 1.84 %, in the

meantime the NP/NG ratio decreased from 0.76 to 0.45 (Table S3), since the N-dopants, especially NP, are thermodynamically unstable at high temperatures 45. Table 2 Properties of Pd/N@CNTs-T catalysts with 3 wt.% nominal loading DTEMa

DCOb

(%)

(%)

2.70.4

41.5

25.3

2.52

2.90.3

38.6

26.9

2.92

1.90

2.90.1

38.6

26.3

1.84

2.11

2.80.2

40.0

27.8

N/(N+C)

Pd

(at. %)

(wt. %)

Pd/N@CNTs

7.08

2.27

Pd/N@CNTs-900

5.25

Pd/N@CNTs-1100 Pd/N@CNTs-1300

Catalyst

dTEM (nm)

a. DTEM (%) = 5.6/r, where is radius of Pd particle (Å). 𝑄𝐶𝑂 × 𝐴𝑟𝑃𝑑

b. 𝐷𝐶𝑂(%) = 2 × 𝑞𝑒 × 𝑁𝐴 × 𝑚 × 𝑤𝑃𝑑, where QCO is the electric quantity for CO oxidation, ArPd is 106.42, qe is 1.6×10-19, NA stands for Avogadro's constant, which equals to 6.02×1023, m is the mass of catalyst, wPd is the actual Pd loading determined by ICP.

We fixed the nominal Pd loading at 3 wt.% to investigate the effect of N@CNTs. The Pd/N@CNT-T catalysts were characterized by TEM (Fig. S5) and electrochemical CO stripping (Fig. S6) to determine the particle size and dispersion of Pd. The properties of Pd/N@CNT-T are summarized in Table 2. The Pd/N@CNT-T catalysts were 16


Page 17 of 32 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

ACS Catalysis

subjected to XPS measurement to extract the electronic interaction as well. The Pd3d and N1s spectra are shown in Supporting Information (Fig. S7), and the detailed peak fitting parameters are summarized in Table S4.

Figure 5 Effects of N content of N@CNTs on (a) Pd03d5/2 binding energy, (b) the ratio of NP/NG, (c) the binding energy of NP and (d) NG. Fig. 5(a) shows the effect of the N content of N@CNT on the Pd03d5/2 binding energy. It was ascertained again that the Pd binding energy is significantly higher on N@CNTs than that on undoped CNTs, because of the electron donation from Pd to N-dopants. 17


ACS Catalysis 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 18 of 32

The Pd03d5/2 binding energy almost linearly increases with the increasing N content on the annealed N@CNTs, with the expectation on the N@CNTs without the heat treatment. The expectation is probably because N@CNTs synthesized at 760 oC without any heat treatment are more defective, evidenced by the Raman analysis shown in Fig. S4, which may hinder the electron transfer because of the lower electron mobility 46. The heat treatment significantly modified the ratio of NP/NG. As shown in Fig. 5(b), the NP/NG ratio of N@CNT support decreases from 0.77 to 0.45 with the increasing annealing temperature from 900 to 1300 oC, along with the N content decreasing from 4.86 to 1.28 wt.%. Nevertheless, the NP/NG ratio significantly reduced after supporting Pd, indicating that Pd preferentially interacts with NP then nucleates around it, consistent with previous results. (Fig. 2(c)) It should be noted that the extent of decrease of NP/NG ratio diminished at low N contents (high annealing temperatures). On the N@CNT-HT-1300, the ratio remained unchanged after loading 3 wt.% Pd. It can be explained by the saturation of nitrogen sites by Pd nanoparticles at very low loadings, at which there is not preference to the anchorage of Pd nanoparticles because all the sites would be occupied. The different interaction priority and strength of Pd on NP and NG can be further confirmed by the shift of N1s binding energy before and after loading Pd nanoparticles on N@CNTs. As shown in Fig. 5(c), regardless of the change of N content, the binding energy of NP significantly decreases by 0.2-0.3 eV after loading Pd nanoparticles, 18


Page 19 of 32 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

ACS Catalysis

revealing the distinct electron transfer from Pd to NP. The relatively higher pyridinic N1s binding energy of 3Pd/N@CNTs may be caused by the same reason as the exceptional point in Fig. 5(a) as aforementioned. Fig. 5(d) shows the effect of Pd loading on the binding energy of NG. Compared with NP, the binding energy of NG declined slightly by less than 0.1 eV after Pd loading (Fig. 5(d)), which means the weak interaction between Pd and NG. Combining these results, it can be concluded that Ndopants, represented by NP and NG, accept electrons from Pd nanoparticles resulting in electron-deficiency of Pd. The electron transfer occurs mainly between Pd and NP sites, the amount of which controls the electronic property of Pd catalysts, thereby the catalytic performance may be tuned via this MSI. 2.4 Catalytic activity for nitrobenzene hydrogenation Nitrobenzene hydrogenation reaction was used as a model reaction to study the effect of Pd-N interaction on catalytic activity. Our previous study 27 indicated that the N@CNT supported Pd catalysts were superior for this reaction to those supported on undoped CNTs. The relative activity of para-substituted NB has been investigated in this work (Fig. S8).

19


ACS Catalysis 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

Figure 6 (a) Dependence of the TOF of nitrobenzene hydrogenation on Pd03d5/2 binding energy of Pd/N@CNT catalysts. Reaction conditions: 2 mL NB, 4 mg catalyst, 18 mL EtOH, 318 K, 0.5 MPa H2, toluene as internal standard. (b1), (c1) and (d1) display the optimized structure of NB adsorbed on a Pd atom supported on pristine graphene, graphene doped with a NG and a vacant site with three NP, respectively. The catalyst models are same as those in Fig. 1. The adsorption energies are marked correspondingly. The bond lengths are in angstroms. (b2), (c2) and (d2) show the corresponding Baber charge density difference plots of (b1), (c1) and (d1), respectively. The green regions represent the accumulation of electronic charges, while the red regions indicate the depletion of electronic charges. The effect of electronic property of Pd on the hydrogenation activity is shown in Fig. 6, as is illustrated by the relationship between Pd03d5/2 binding energy and NB hydrogenation turnover frequency (TOF). In general, the TOF of 3Pd/N@CNTs-HT-T increases with the Pd03d5/2 binding energy, indicating that the electron deficiency of Pd 20


Page 20 of 32

Page 21 of 32 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

ACS Catalysis

is beneficial for the NB hydrogenation. An exception was observed again for the Pd/N@CNTs without annealing, probably due to the effect of defects as aforementioned in Fig. 5a. Besides, Pd/N@CNT is stable evidenced by the TEM images after use, as shown in Fig. S9. The MSI between Pd nanoparticles and the nitrogen centers may change the adsorption property of metals, which has been suggested by Chen and coworkers47 in the case of Pt supported on CNTs with defects and oxygen groups. For the hydrogenation of NB, the adsorption and subsequent hydrogenation reaction may occur at nitro group or aromatic ring. Because of the strong electron-withdrawing effect of nitro group, the aromatic ring in NB is electron deficient 48.

Hence, a NB molecule tends to be adsorbed via the coordination with the nitro group

on metal catalysts, for the favorable adsorption through nitro group49, instead of the aromatic ring, under the hydrogenation reaction conditions, resulting in the slower kinetics of aromatic ring hydrogenation reaction50. It is reasonable to conclude that the adsorption of electron-rich nitro groups could be enhanced on the electron-deficient Pd nanoparticles, thus the reactivity of NB to aniline is improved. Furthermore, the formation of aniline would reduce the adsorption because of the weak electronwithdrawing effect of amine group, which guarantees the good selectivity to aniline. This scenario is verified through DFT calculation. As shown in Fig. 6(b-c), the Pd atom of Pd/graphene or Pd/NG/P@graphene directly donates electrons to two O atoms, and N atom of NB also back donates electrons from their σ-bonds, weakening the O-N bonds of NB. As illustrated in Fig. 6(b2-c2), the charge density between Pd and O atoms 21


ACS Catalysis 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 32

increase significantly in the order from Pd/graphene, Pd/NG@graphene to Pd/NP@graphene, while the charge density between N and O atoms decreases significantly from Pd/graphene, Pd/NG@graphene to Pd/NP@graphene. Furthermore, the adsorption energies of NB on the Pd/graphene, Pd/NG@graphene to Pd/NP@graphene substrates decrease from -0.87, -1.19 to -1.29 eV with the increase of charge transfer between Pd and O atoms (from 0.38, 0.42 to 0.85 e). Hence, it is reasonable to conclude that the adsorption of electron-rich nitro groups could be enhanced on the electron-deficient Pd nanoparticles, which can be further supported by the higher reactivity of NB with electron-donating para-substituent group (Fig. S8). In this work, the electronic properties of Pd were modified by the MSI between Pd and N@CNTs-T. It suggests that adjusting the nitrogen content in support is a powerful tool to affect the electronic properties of Pd, thereby improve the catalytic reaction. According to the understanding to the preferential interaction between Pd and NP, the rational design and synthesis of high performance Pd catalysts could be achieved by harnessing the interaction to reach a tunable electron-deficiency of Pd surfaces. This approach may play a crucial role in the design of supported Pd catalysts for industryrelevant hydrogenation reactions. CONCLUSIONS The MSI between Pd and N-doped CNTs was theoretically and experimentally investigated. DFT calculations indicate that the adsorption of Pd on pyridinic N is more favorable than that on graphitic N. Meanwhile, the MSI results in the electron donation 22


Page 23 of 32 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

ACS Catalysis

from single Pd atom or a Pd4 cluster to pyridinic N, leading to the electron-deficient Pd. These results were verified on a series of Pd/N@CNTs catalysts with varied Pd loading and N content prepared by the NaBH4 reduction method. XPS measurements revealed the electron-deficiency of Pd nanoparticles on N@CNTs compared to those on undoped CNTs. The stronger interaction between Pd and pyridinic N was demonstrated as well. Our results suggested the heterogeneity of MSI on N@CNTs, where the pyridinic N preferentially anchors Pd nanoparticles. Hence, the electronic property of Pd is determined either by nitrogen content or by the distribution of nitrogen groups. Nitrobenzene reduction was used as a model reaction to show how powerful the MSI is to harness the catalytic activity of Pd catalysts. It was clearly revealed that the electron-deficiency of Pd caused by the MSI was beneficial to the activity of Pd/N@CNTs for the hydrogenation of nitrobenzene to aniline. These results rationalize the principles of the Pd-N MSI and provide the insights helping the rational design of noble metal catalysts supported on N-doped carbon materials. MATERIALS AND METHODS Chemicals Analytic PdCl2, NaBH4 and KOH were used as-received without further purification. CNTs were purchased from Shenzhen Nanotech Port Limited Company. CNTs were stirred in concentrated HCl (12 mol/L) at room temperature for 12 hours before use, and then thoroughly washed with deionized water until neutral pH. The resulting solids were dried in vacuum overnight and designated as purified CNTs. 23


ACS Catalysis 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 32

Preparation of N@CNTs N-doped carbon layer was deposited on pristine CNTs from pyridine by a chemical vapor deposition of pyridine following a literature report51. Briefly, CNTs were placed in a quartz tube. It was ramped from room temperature to 760 oC at a heating rate of 10 oC/min

in a horizontal tubular furnace at atmosphere pressure under flowing Ar (200

Ncm3 min-1). Pyridine was fed into the reactor by a syringe pump at 1.5 ml h-1 to conduct the deposition reaction. After the reaction for an anticipated duration (ca. 1.5 h), the furnace was cooled down to room temperature in Ar atmosphere. The asprepared products are denoted as N@CNTs. Post-heat-treatment was carried out to alter the content of nitrogen. 500 mg of N@CNTs were uniformly spread on a corundum boat placed in a horizontal corundum tube. The tube was firstly vacuumed for 15 min, and then argon was fed at a flowrate of 50 mL/min. The heat treatment was carried out at a ramping rate of 10 oC /min below 1000 oC and 5 oC/min above 1000 oC. As the desired temperature reached, the sample was annealed for 2 h and then was cooled to room temperature. The annealed sample are denoted as N@CNT-T, where T represents the heat treatment temperature. Preparation of Pd catalysts 0.2 g carbon materials (CNTs, N@CNTs or N@CNTs-HT-T) were added in 20 mL deionized water, and sonicated until the suspension became homogeneous. Corresponding to a nominal Pd content, a certain amount of the H2PdCl4 solution (6.09 mg/mL Pd2+) was dropped into the suspension with stirring. Then KOH aqueous 24


Page 25 of 32 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

ACS Catalysis

solution (1 mol/L) was used to adjust pH to 8.5. After that, excess NaBH4 aqueous solution was added drop wise into the suspension in an ice bath. Finally, the slurry was filtered and washed with deionized water, absolute ethanol several times till filtrate turned neutrality, then the filtrated cake was dried overnight in vacuum oven at 75 oC. The synthesized catalysts are donated as Pd/N@CNT-T, where T represent for the heat treatment temperature of support. Characterization Inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer ICP8300) was used to analyze the content of Pd. Before measurement, the samples were fully combusted at 800 oC and the residuals were dissolved in aqua regia. Raman spectra were measured in a LabRAM Aramis micro Raman spectrometer with an excitation wavelength of 632.8 nm with 2 μm spot size. X-ray photoelectron spectroscopy (XPS) was performed in a Kratos Axis ultra (DLD) spectrometer equipped with an Al Kα Xray source. The binding energies were referenced to the C1s peak at 284.6 eV. The morphology and microstructures of the catalysts were observed in a JEOL JEM2010 microscope operated at 200 kV. By TEM statistics, the average particle diameter is calculated as Σnidi3/Σnidi2, where ni is the number of particles having a characteristic diameter of di. The catalysts were characterized electrochemically using thin-film electrodes applied to the 5 mm glassy carbon disk of a rotating ring-disk electrode (Pine Instrument Company). Prior to electrode preparation, the glassy carbon disk was carefully polished 25


ACS Catalysis 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 32

in terms of the polishing process in the manual. The catalyst ink was made of 10 mg catalyst and 5 ml DI water over sonification, then 10 uL slurry was dropcasted on to the electrode which was dried under argon. Until it was dry, 10 uL of Nafion solution (5% v/v) was added on top of the electrode. Electrochemical test was performed in 1.0 M potassium hydroxide solution in a three-electrode cell with a Hg/HgO reference electrode with 1.0 M KOH. A graphite rod was used as counter electrode. The cyclic voltagramms were obtained from -1.05 V to 0.16 V at a scan rate of 50 mV/s. The electrolyte was saturated with argon to remove dissolved oxygen. CO-stripping voltammetry was performed between -1.05 and 0.16 V at a sweep rate of 50 mV/s after CO electroadsorption (10 %CO+90% He) at 50 mV. Prior to CO-stripping, excess CO was removed from the electrolyte by flushing with Ar for 25 min, confirmed by the second CV curve without CO electrooxidation peak after stripping. To confirm the maximum coverage of CO on Pd surfaces, the H/H+ (desorption/oxidation) region should be completely prohibited at low potential prior to the CO oxidation peak. Taking commercial Pd/C as an example, the current from -1.05 to -0.5 V was prohibited after CO adsorption, otherwise there is hydrogen oxidation peak as the dashed curve after CO stripping (Figure S10). Nitrobenzene hydrogenation Nitrobenzene hydrogenation was carried out in a high-pressure autoclave under magnetic stirring at 1000 rpm in order to exclude the mass transfer effect through evaluating the change of reaction conversion with stirring rate. The catalyst was firstly 26


Page 27 of 32 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

ACS Catalysis

added to the reaction vessel, then the solvent ethanol, the substrate nitrobenzene and the internal standard toluene were added in turn. After that, nitrogen was purged several times to vent the air. When the system was heated to a given temperature (measured by a thermocouple), hydrogen was purged and pressure of the system kept 0.5 MPa. After the reaction, product was taken out and analyzed by gas chromatography (Agilent GC7890B) which equipped with an HP-5 capillary column and a flame ionization detector. The concentration of product was calculated through standard curves established with authentic samples normalized to internal standard toluene. Computation method All spin-polarized computations were performed with Vienna Ab initio simulation package (VASP) which based on density functional theory.

52, 53

The exchange-

correlation interaction used the general gradient approximation (GGA) formulated by Perdew-Burke-Ernzerhof (PBE). 53 Electron interactions were described with projector augmented wave (PAW) pseudo potentials. The plane-wave basis set energy cutoff was restricted to 400 eV, and an 11×11×1 k-point mesh was used for the interaction of the Brillouin-zone. All structures were fully relaxed without any symmetry constrains. The electronic self-consistency criterion was set to 10-6 eV. The global transferred charges were calculated by the atomic Bader charge analysis. 53 54, 55 The 6x6 periodic slab models of N-doped graphene (N@graphene) have been adopted as simplified models of N@CNTs systems, which include at least 12 Å vacuum intervals to avoid interaction with their own images. The adsorption energy of a Pd 27


ACS Catalysis 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 32

single atom/Pd4 cluster on the N@graphene doped by a NG atom or a vacant site with three NP atoms is calculated by ∆Eads = (EPd/N@graphene –(EN@graphene + EPd))/N, where N is the number of palladium atoms in each N@graphene layer per super cell. EPd, EN@graphene, and EPd/N@graphene are the total energies per super cell of the Pd/Pd4, N@graphene, and Pd/N@graphene systems, respectively. ASSOCIATED CONTENT Supporting Information The supplemented Figures and Tables are available free of charge on the ACS Publications Website at DOI: AUTHOR INFORMATION Corresponding Authors * [email protected]; * [email protected]. ORCIDid: Yu Hao: 0000-0003-2862-8054 Wang Qiang: 0000-0001-8558-8226 Yang Guangxing: 0000-0002-9416-4780 He Zhiyan: 0000-0003-4699-9625 Author Contributions 28


Page 29 of 32 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

ACS Catalysis

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (No. 21676100), the Guangdong Natural Science Foundation (No. 2017A030312005), Science and Technology Program of Guangzhou City (No. 201707010058). We thank Prof. Fengying Dai for ICP analysis.

29


ACS Catalysis 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 30 of 32

REFERENCES (1) Li, Z.; Liu, J.; Xia, C.; Li, F., ACS Catalysis 2013, 3 (11), 2440-2448. (2) Kim, J. D.; Choi, M. Y.; Choi, H. C., Materials Chemistry and Physics 2016, 173, 404-411. (3) Shinde, V. M.; Skupien, E.; Makkee, M., Catalysis Science & Technology 2015, 5 (8), 4144-4153. (4) Gavidia, L. M. R.; Garcia, G.; Anaya, D.; Querejeta, A.; Alcaide, F.; Pastor, E., Applied Catalysis B-Environmental 2016, 184, 12-19. (5) Li, H.; Zhang, Y. Y.; Wan, Q. J.; Li, Y. W.; Yang, N. J., Carbon 2018, 131, 111119. (6) Charlier, J. C., Accounts of Chemical Research 2002, 35 (12), 1063-1069. (7) Ago, H.; Kugler, T.; Cacialli, F.; Salaneck, W. R.; Shaffer, M. S. P.; Windle, A. H.; Friend, R. H., Journal of Physical Chemistry B 1999, 103 (38), 8116-8121. (8) Xu, X.; Li, Y.; Gong, Y. T.; Zhang, P. F.; Li, H. R.; Wang, Y., Journal of the American Chemical Society 2012, 134 (41), 16987-16990. (9) Perini, L.; Durante, C.; Favaro, M.; Perazzolo, V.; Agnoli, S.; Schneider, O.; Granozzi, G.; Gennaro, A., Acs Applied Materials & Interfaces 2015, 7 (2), 1170-1179. (10)Louisia, S.; Contreras, R. C.; Heitzmann, M.; Axet, M. R.; Jacques, P. A.; Serp, P., Catalysis Communications 2018, 109, 65-70. (11)Ravat, V.; Nongwe, I.; Meijboom, R.; Bepete, G.; Coville, N. J., Journal of Catalysis 2013, 305, 36-45. (12)Silva, J. C. M.; de Freitas, I. C.; Neto, A. O.; Spinace, E. V.; Ribeiro, V. A., Ionics 2018, 24 (4), 1111-1119. (13)Dong, X.; Chao, S.; Wan, F.; Guan, Q.; Wang, G.; Li, W., Journal of Catalysis 2018, 359, 161-170. (14)Dai, B.; Chen, K.; Wang, Y.; Kang, L.; Zhu, M., ACS Catalysis 2015, 5 (4), 25412547. (15)Bae, G.; Youn, D. H.; Han, S.; Lee, J. S., Carbon 2013, 51, 274-281. (16)Liu, Z.; Shi, Q.; Zhang, R.; Wang, Q.; Kang, G.; Peng, F., Journal of Power Sources 2014, 268, 171-175. (17)Li, R.; Wei, Z.; Gou, X., ACS Catalysis 2015, 5 (7), 4133-4142. (18)Shi, J.; Zhao, M.; Wang, Y.; Fu, J.; Lu, X.; Hou, Z., Journal of Materials Chemistry A 2016, 4 (16), 5842-5848. (19)Perazzolo, V.; Brandiele, R.; Durante, C.; Zerbetto, M.; Causin, V.; Rizzi, G. A.; Cerri, I.; Granozzi, G.; Gennaro, A., Acs Catalysis 2018, 8 (2), 1122-1137. (20)Chen, P. R.; Chew, L. M.; Kostka, A.; Muhler, M.; Xia, W., Catalysis Science & Technology 2013, 3 (8), 1964-1971. (21)Bian, S.-W.; Ma, Z.; Song, W.-G., Journal of Physical Chemistry C 2009, 113 (20), 8668-8672. (22)García-García, F. R.; Álvarez-Rodríguez, J.; Rodríguez-Ramos, I.; Guerrero-Ruiz, A., Carbon 2010, 48 (1), 267-276. 30


Page 31 of 32 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

ACS Catalysis

(23)Xie, K. P.; Xia, W.; Masa, J.; Yang, F.; Weide, P.; Schuhmann, W.; Muhler, M., Journal of Energy Chemistry 2016, 25 (2), 282-288. (24)Chizari, K.; Janowska, I.; Houllé, M.; Florea, I.; Ersen, O.; Romero, T.; Bernhardt, P.; Ledoux, M. J.; Pham-Huu, C., Applied Catalysis, A: General 2010, 380 (1), 72-80. (25)Lepró, X.; Terrés, E.; Vega-Cantú, Y.; Rodríguez-Macías, F. J.; Muramatsu, H.; Kim, Y. A.; Hayahsi, T.; Endo, M.; Torres R, M.; Terrones, M., Chemical Physics Letters 2008, 463 (1), 124-129. (26)Li, Z.; Liu, J. H.; Huang, Z. W.; Yang, Y.; Xia, C. G.; Li, F. W., ACS Catalysis 2013, 3 (5), 839-845. (27)Dong, B. Q.; Li, Y. H.; Ning, X. M.; Wang, H. J.; Yu, H.; Peng, F., Applied Catalysis, A: General 2017, 545, 54-63. (28)Bi, Q.-Y.; Lin, J.-D.; Liu, Y.-M.; He, H.-Y.; Huang, F.-Q.; Cao, Y., Angewandte Chemie, International Edition 2016, 55 (39), 11849-11853. (29)Wang, F.; Xu, J.; Shao, X.; Su, X.; Huang, Y.; Zhang, T., ChemSusChem 2016, 9 (3), 246-251. (30)Nie, R.; Jiang, H.; Lu, X.; Zhou, D.; Xia, Q., Catalysis Science & Technology 2016, 6 (6), 1913-1920. (31)Arrigo, R.; Havecker, M.; Schlogl, R.; Su, D. S., Chemical Communications 2008, (40), 4891-4893. (32)Schiros, T.; Nordlund, D.; Palova, L.; Prezzi, D.; Zhao, L. Y.; Kim, K. S.; Wurstbauer, U.; Gutierrez, C.; Delongchamp, D.; Jaye, C.; Fischer, D.; Ogasawara, H.; Pettersson, L. G. M.; Reichman, D. R.; Kim, P.; Hybertsen, M. S.; Pasupathy, A. N., Nano Letters 2012, 12 (8), 4025-4031. (33)Zhao, M. W.; Xia, Y. Y.; Lewis, J. P.; Zhang, R. Q., Journal of Applied Physics 2003, 94 (4), 2398-2402. (34)Ning, X.; Yu, H.; Peng, F.; Wang, H., Journal of Catalysis 2015, 325, 136-144. (35)Ning, X. M.; Li, Y. H.; Dong, B. Q.; Wang, H. J.; Yu, H.; Peng, F.; Yang, Y. H., Journal of Catalysis 2017, 348, 100-109. (36)Scardamaglia, M.; Struzzi, C.; Aparicio Rebollo, F. J.; De Marco, P.; Mudimela, P. R.; Colomer, J.-F.; Amati, M.; Gregoratti, L.; Petaccia, L.; Snyders, R.; Bittencourt, C., Carbon 2015, 83, 118-127. (37)Ejaz, A.; Jeon, S., International Journal of Hydrogen Energy 2018, 43 (11), 56905702. (38)Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J., Science 2016, 351 (6271), 361-365. (39)Zelong, L.; Jinlei, L.; Jianhua, L.; Zelun, Z.; Chungu, X.; Fuwei, L., ChemCatChem 2014, 6 (5), 1333-1339. (40)Bulushev, D. A.; Zacharska, M.; Lisitsyn, A. S.; Podyacheva, O. Y.; Hage, F. S.; Ramasse, Q. M.; Bangert, U.; Bulusheva, L. G., ACS Catalysis 2016, 6 (6), 3442-3451. (41)Bulushev, D. A.; Zacharska, M.; Shlyakhova, E. V.; Chuvilin, A. L.; Guo, Y.; Beloshapkin, S.; Okotrub, A. V.; Bulusheva, L. G., ACS Catalysis 2016, 6 (2), 681691. 31


ACS Catalysis 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 32 of 32

(42)Henry, C. R.; Chapon, C.; Giorgio, S.; Goyhenex, C., Springer Netherlands: 1997; 117-152. (43)Won Jun, L.; Joonwon, L.; Sang Ouk, K., Small Methods 2017, 1 (1-2), 1600014 (44)Tsud, N.; Johanek, V.; Stara, I.; Veltruska, K.; Matolin, V., Surface Science 2000, 467 (1-3), 169-176. (45)Rao, R. G.; Blume, R.; Hansen, T. W.; Fuentes, E.; Dreyer, K.; Moldovan, S.; Ersen, O.; Hibbitts, D. D.; Chabal, Y. J.; Schlögl, R., Nature Communications 2017, 8 (1). (46)Snow, E. S.; Campbell, P. M.; Ancona, M. G.; Novak, J. P., Applied Physics Letters 2005, 86 (3), 911. (47)Chen, W.; Ji, J.; Duan, X.; Qian, G.; Li, P.; Zhou, X.; Chen, D.; Yuan, W., Chemical Communications 2014, 50 (17), 2142-2144. (48)Lipkowitz, K. B.; Chang, C. P., Journal of the American Chemical Society 1982, 104 (9), 2647-2648. (49)Boronat, M.; Concepción, P.; Corma, A.; González, S.; Illas, F.; Serna, P., Journal of the American Chemical Society 2007, 129 (51), 16230-16237. (50)Tomkins, P.; Gebauer-Henke, E.; Leitner, W.; Müller, T. E., ACS Catalysis 2015, 5 (1), 203-209. (51)Gui‐Li, T.; Qiang, Z.; Bingsen, Z.; Yu‐Guang, J.; Jia‐Qi, H.; Sheng, S. D.; Fei, W., Advanced Functional Materials 2014, 24 (38), 5956-5961. (52)Liu, X.; Fu, L.; Liu, N.; Gao, T.; Zhang, Y.; Liao, L.; Liu, Z., Journal of Physical Chemistry C 2011, 115 (24), 11976-11982. (53)Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G., Journal of Computational Chemistry 2007, 28 (5), 899-908. (54)Henkelman, G.; Arnaldsson, A.; Jónsson, H., Comput. Mater. Sci. 2006, 36 (3), 354-360. (55)Grimme, S., Journal of Computational Chemistry 2006, 27 (15), 1787-1799.

32


Elucidating interaction between palladium and N ... - ACS Publications

Recommend Documents