Structure of Nanodiamond in Direct Dehydrogenation - ACS Publications

Apr 13, 2017 - School of Materials Science and Engineering, University of Science and Technology of ... University of Chinese Academy of Sciences, Shi...
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Revealing the Role of sp2@sp3 Structure of Nanodiamond in Direct Dehydrogenation: Insight from DFT study TianFu Liu,†,‡ Sajjad Ali,†,§ Bo Li,*,† and Dang Sheng Su*,† †

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, Liaoning, People’s Republic of China ‡ School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, Anhui, People’s Republic of China § University of Chinese Academy of Sciences, Shijingshan District, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: To understand the superior performance of nanodiamond (ND) catalyst in dehydrogenation reactions in comparison with other nanostructured carbon catalysts, firstprinciples calculations are performed to study the direct dehydrogenation of isobutane catalyzed by ND catalyst. The NDs form a unique sp2@sp3 core−shell structure because of the diminishment of the surface dangling bond. The calculations show that, in comparison to carbon nanotubes (CNTs), NDs have a much lower activation barrier of the first C−H bond activation, which is the rate-limiting step in the reaction. Moreover, the complete reaction pathways revealed from the calculations and the adsorption of isobutene further verify the better activity and selectivity of ND catalyst. The investigation of different morphologies, ND sizes, and the presence of surface hydrogens indicates that the sp2@sp3 core−shell structure is crucial for the observed excellent reactivity. The Bader charge analysis shows that the oxygen functional groups on ND have less charge than those on CNT, which favors the homolytic cleavage of the C−H bond of isobutane. Moreover, the carbon atoms on ND could accept more charge than their counterparts on CNT, revealing the active role of surface carbon during the C−H bond activation. The current work establishes the relation between the structures of ND catalyst and the catalytic performance of dehydrogenation reactions, which paves the way for further optimization. KEYWORDS: nanodiamond, dehydrogenation, isobutane, DFT, sp2@sp3 core−shell

1. INTRODUCTION

reactions. Among them, NDs and derived OLCs with the advantages of nanoscale size (1−5 nm), large surface to volume ratio, sp2@sp3 hybrid structure, and tunable surface functionalities exhibit superior activity and selectivity for the dehydrogenation of alkane. Zhang et al. reported ND as a novel catalyst with unique sp2@sp3 hybrid structure for direct dehydrogenation of ethylbenzene under steam-free conditions, with excellent activity (up to 14 times), better selectivity (97% higher), and stability free from coke in comparison to other carbon catalysts.12 Liu et al. reported that the hybrid nanocarbon formed from ND exhibits a much higher selectivity (56%) for oxidative dehydrogenation of n-butane in comparison to single-walled CNTs (12%) and multiwalled CNTs (20%) after 10 h of reaction, which is attributed to the sp2@sp3 hybrid structure with the strongly curved and strained graphitic

The direct dehydrogenation reaction of alkane to alkene has been applied to the production of important feedstocks in chemical industry such as styrene, propene, and butene. The traditional metal oxide catalysts for dehydrogenation used in industry are limited by low selectivity and coke formation.1−5 Recently, nanostructured carbon materials have been reported to show great potential as metal-free catalysts for dehydrogenation processes, which is environmentally and economically attractive.6−8 In particular, Su et al. reported that carbon nanotubes (CNTs) with modified surface functionality have been shown better selectivity in comparison to the contemporary best metal oxide catalyst, V/MgO.9 This work opened up the possibility of application of carbon catalysts in the area of dehydrogenation of light alkanes. Recently, various nanostructured carbon materials, such as CNTs,10 carbon nanofibers, 11 nanodiamonds (NDs), 12,13 onion-like carbons (OLCs),14 few-layered graphene,15 and fullerene-like carbons,16 have been reported as active catalysts for dehydrogenation © 2017 American Chemical Society

Received: December 21, 2016 Revised: March 17, 2017 Published: April 13, 2017 3779

DOI: 10.1021/acscatal.6b03619 ACS Catal. 2017, 7, 3779−3785

Research Article

ACS Catalysis surface formed from the UDD precursor.13 Wang et al. reported that a hybrid nanocarbon with a variable sp2/sp3 ratio obtained from the annealing treatment of ND, which combines the intrinsic core properties of diamond with surface reactivity, exhibits a 10.6% steady-state conversion and 90% propene selectivity in comparison with CNT in the direct dehydrogenation of propane.17 ND has also been used to synthesize composite materials, such as ND/CNT-SiC monolith,18 NDgraphene,19 and ND/carbon nitride,20 which have also shown synergy effects for dehydrogenation reactions. In the literature, the impressive performance of NDs in dehydrogenation reactions is proposed to be due to its special sp2@sp3 core−shell structure.12−14,17 However, the exact relation between the sp2@sp3 core−shell structure and the observed reactivity has still not been established. In our previous works, several important aspects of the nanostructured carbon catalysts in dehydrogenation reactions have been carefully studied, such as the selectivity of alkene on nanocarbon catalysts,21 doping effects of heteroatoms on dehydrogenation of ethane,22 the descriptor of active sites,23 and the application of the Brønsted−Evans−Polanyi (BEP) relation.24 In current work, we report first-principles calculations on the direct dehydrogenation of isobutane catalyzed by ND. For the first time, the reaction pathway of dehydrogenation reactions on ND catalysts is revealed. The related properties along the pathway are analyzed and compared with those of the CNT catalyst. The ND sizes, selectivity for isobutene, and hydrogens on the surface are studied to gain more knowledge of the ND catalyst for dehydrogenation reactions. Moreover, charge analysis is performed to explore the origin of the reactivity. In the end, the results suggest that the sp2@sp3 core−shell structure is decisive in the observed superior reactivity of ND. Overall, the current study deepens the understanding of the reactivity of ND in dehydrogenation reactions and establishes the structure−performance relation, which is useful for the further optimization of the ND catalysts.

Figure 1. Octahedral (A and B), cuboctahedral (C and D), and cubic (E and F), before (A, C, and E) and after (B, D and F) structural optimization. After structure optimizations, the carbon atoms are separated and form a core−shell structure. The carbon atoms in the core region are in orange, and carbons at the surface are in gray.

because of the absence of dangling bonds.28,29 The transition from bulk diamond to carbon onions leads to the elongation of surface to core, which is the consequence of the flattening of surface corrugation in order to form π bonds from dangling orbitals.29 According to this principle, the transformation from sp3 to sp2 bonding could result from the improved stability by reducing the number of dangling bonds. For the cuboctahedral ND with a surface area of 40% (111) and 60% (100), the bond length of the (111) surface atoms is around 1.44 Å, and the bond length of the (100) surface atoms is around 1.54 Å. On the basis of the criterion mentioned above, the (111) surface atoms undergo transformation from sp3 to sp2 bonding, while the (100) surface atoms reconstructed to increase the (111) surface area initially, followed by a reorientation to curved graphite-like cages. The core−shell distance is around 1.69 Å, indicating that a core−shell separation is present. In the case of a cube with the (100) surface, the core−shell separation is not observed, but surface atoms also exhibit reconstruction after relaxations. The bond length of surface carbon atoms is around 1.52 Å, shorter than the sp3 bond length (1.54 Å) but longer than the sp2 bond length (1.40 Å). The results suggest that (100) is more resistant to re-formation. For the identification of active sites of dehydrogenation on carbon catalysts, the consensus in previous studies has been that the ketonic CO group functionalized in pretreatment or generated during the reaction is responsible for the catalytic performance.10−15 Furthermore, Mao et al. computationally studied the screening of active sites, suggesting that the dissociation of ethane on quinone and diketone group is exothermic, which is thermodynamically more favorable than the other oxygen functional groups. 22 Therefore, the dehydrogenation of isobutane has been studied on the quinone and diketone functional groups. Several different active sites are considered which include the octahedral with quinone (Octa-q) and with diketone (Octa-di), the cuboctahedral with quinone (Cubo-q) and with diketone (Cubo-di), and the cubic with quinone (Cubic-q) and with diketone (Cubic-di). The optimized structures are shown in Figure S1 in the Supporting Information, and the bond distance between carbon and oxygen atoms is around 1.2 Å, indicating that a CO bond is formed.

2. RESULTS AND DISCUSSION 2.1. ND Core−Shell Hybrid Structure. The most common crystal surfaces of ND are (100) and (111). For the geometry optimization, three morphologies are chosen, which are octahedral, cubic, and cuboctahedral, terminated by (111), (100), and 40% (111) and 60% (100), respectively.25−27 As shown in Figure 1, the transition of carbon atoms from sp3 hybridization to sp2 hybridization is the most significant phenomenon during the optimization. For the octahedral model which contains 165 carbon atoms, the bond length of the surface atoms is significantly shortened to approximately 1.45 Å from 1.54 Å in the bulk diamond after optimization. Winter and Ree suggested that an indication of changes in the C−C bonding can be obtained by comparing the bond lengths before and after optimization.28 The typical lengths of sp3 and sp2 carbon bonds are 1.54 and 1.40 Å, respectively. According to this criterion, the surface carbon atoms have transformed from sp3 to sp2 bonding after optimization. The sp2-bonded atoms at surface form a rounded shell, which is separated from the atoms in the core (the orange atoms in Figure 1B). The distance between the shell and core carbon atoms is approximately 2.25 Å. Previous semiexperimental and theoretical studies suggested that despite the strain energy associated with the curved surfaces, small closed molecules such as fullerene and carbon onion gain more stability over planar graphite with a large number of unpaired surface electrons, 3780

DOI: 10.1021/acscatal.6b03619 ACS Catal. 2017, 7, 3779−3785

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ACS Catalysis 2.2. C−H Bond Activation on ND. The desired product of the dehydrogenation of butane is the corresponding butene, which needs to abstract two hydrogen atoms. The reaction mechanism of the direct dehydrogenation reaction can be described as (i) the reduction of the oxygen atoms on the carbon by the isobutane and (ii) the subsequent generation of a hydrogen molecule from the hydrogenated oxygen group. It has been reported that the dissociation of the first C−H bond is the rate-limiting step.15,21−23 Thus, the investigation of the first C− H bond dissociation of isobutane could provide key information on the reactivity of ND. Two pathways are possible for the first C−H bond dissociation: either primary hydrogen or tertiary hydrogen abstraction. On the ND catalyst (Octa-q), the calculated barriers are 1.05 and 1.37 eV for tertiary and primary hydrogens, respectively. Obviously, the activation of tertiary hydrogen is more favorable than that of the primary hydrogen. On the reaction pathway shown in Figure 2, the C−H bond is

with that on CNTs are shown in Figure 3. Specifically, activation barriers on CNT are 25.0%, 20.0%, 17.9%, 9.3%,

Figure 3. Activation barrier for the first C−H bond activation on CNTs and NDs. The details of the reaction pathway and optimized structures are shown in Figure S2 in the Supporting Information.

34.3%, and 83.6% higher than the barriers of Octa-q, Octa-di, Cubic-q, Cubic-di, Cubo-q, and Cubo-di, respectively. The calculations clearly indicate that ND is much more active than CNT for the breaking of the first C−H bond. It is worth mentioning that the cubic ND has a higher activation barrier than octahedral and cuboctahedral ND. Cubic-q and Cubic-di have activation barriers of 1.15 and 1.27 eV, respectively, while Octa-q, Octa-di, Cubo-q, and Cubo-di have activation barriers of 1.05, 1.12, 0.92, and 0.23 eV, respectively. On the other hand, the octahedral and cuboctahedral NDs form the sp2@sp3 core−shell structures as shown in Figure 1, while cubic ND only undergoes a reorientation of surface atoms without the formation of the core−shell structure. Therefore, it is suggested that the sp2@sp3 core−shell structure leads to the better C−H bond activation reactivity. 2.3. Size Effect. The size of the catalyst is one of the most important parameters in controlling the catalytic performance. It is reported that ND has a size range of 1−5 nm.28,29 The reactivity of the dehydrogenation reaction could be sensitive to the ND size, which is explored by using cuboctahedral NDs with approximate diameters of 4.6, 10.1, and 14.6 Å, as shown in Figure 4. The activation barriers for first C−H bond activation are 0.83, 0.69, and 0.23 eV on the ND catalysts with diameters of 4.6, 10.1, and 14.6 Å, respectively. With an increase in the cluster size, the activation barrier shows a monotonic decrease. The surface atoms of the ND with diameters of 4.6, 10.1, and 14.6 Å have bond lengths of around 1.48, 1.47, and 1.44 Å, respectively, which indicates that with the size increase, the sp2 bonding contents are also increasing. Meanwhile, the core−shell structure is more evident with an increase in size. This observation also suggests that the sp2@sp3 hybrid core−shell structure is a determining factor for ND catalysts in dehydrogenation reactions. 2.4. Complete Reaction Pathway. To fulfill the whole catalytic cycle, the reaction pathway after the first C−H bond breaking is explored and shown in Figure 5. The calculations are performed for both CNT and the octahedral ND, which is one of the most predominant morphologies in NDs produced

Figure 2. Reaction pathway and barrier of (A) tertiary hydrogen abstraction and (B) primary hydrogen abstraction on ND (Octa-q). Color code: carbon is gray, hydrogen is white, and oxygen is red.

elongated to around 1.4 Å from 1.1 Å in TS1 during the tertiary hydrogen activation. In the final state, the tertiary hydrogen is attached to O with a distance of around 0.99 Å and the isobutane molecule becomes a radical in the gas phase. Dathar et al. also reported that the difference in activation barriers between primary and tertiary hydrogens is close to 0.38 eV on graphene catalyst, which indicates that the tertiary C−H bond dissociation of isobutane is easier than the primary C−H dissociation.15 Therefore, the isobutane dehydrogenation reaction is initiated by the tertiary C−H bond breaking, which is also considered as the rate-limiting step. The activation barriers of the first C−H bond (tertiary hydrogen) on different active sites on ND catalysts together 3781

DOI: 10.1021/acscatal.6b03619 ACS Catal. 2017, 7, 3779−3785

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ACS Catalysis

barriers on CNT for the first and second hydrogen abstractions are higher than their counterparts on ND. Thus, ND has a much better reactivity toward C−H bond activations than CNT, which is in good agreement with the experimental observations.9,13 To complete te catalytic cycle, the active sites which have been converted to hydroxyls must be regenerated, which proceeds by the formation of a hydrogen molecule from the hydrogenated dicarbonyl group. As shown in Figure 6, the

Figure 4. Activation barriers of the first C−H bond breaking in an isobutane molecule on cuboctahedral NDs with diameters of 4.6, 10.1, and 14.6 Å, respectively.

Figure 6. Energy profile for regeneration of a hydrogenated quinone group at (A) CNT and (B) ND. Color code: carbon is gray, hydrogen is white, and oxygen is red.

activation barrier on ND is 2.78 eV, which is higher than that of CNT (2.22 eV) for the regeneration process. However, the activation barriers on CNT and ND are both much higher than the activation barrier of the hydrogen abstraction of isobutane, which indicates that the regeneration of active sites could be the bottleneck of the reaction. In the experimental study, increased hydrogen coverage on the used catalyst has been observed, which results in a decrease in the active sites and drop in the initial activity.12 Indeed, the direct dehydrogenation is an endothermic process, requiring a temperature higher than that for oxidative dehydrogenation, which is also partially attributed to the difficulty of regeneration of active sites.32 The introduction of oxygen into the system has been proposed to decrease the energy barrier of the regeneration.13,15 The DFT calculations revealed that the oxygen molecule can readily dissociate into oxygen atoms at ketonic sites with an energy barrier of 0.38 eV.33 The active site regeneration with the hydroxyl oxidation by the oxygen atom is investigated. As shown in Figure S6 in the Supporting Information, the two hydroxyl groups are easily oxidized by the oxygen atom without any perceivable barrier. The introduction of an oxidant such as oxygen not only changes the thermodynamics of the dehydrogenation reaction but also promotes regeneration of the active sites. 2.5. Selectivity for Isobutene. It is experimentally observed that the selectivity of the butane on ND is much higher than that of CNT.12 The interactions between the

Figure 5. Reaction pathway of the dehydrogenation of isobutane at a quinone group on (A) CNT and (B) ND. Color code: carbon is gray, hydrogen is white, and oxygen is red.

by detonation methods.30,31 As shown in Figure 5, for the first C−H bond breaking, at the TS1a for CNT and TS1b for ND, the C−H bond length in the isobutane molecule is elongated, from around 1.1 Å to 1.4 Å. The barriers for the first C−H bond breaking are estimated to be 1.39 and 1.05 eV for CNT and ND, respectively. After TS1a and TS1b, an intermediate state is reached on the reaction pathway, IM1a and IM1b, respectively. In the intermediate state, the isobutane molecule becomes an isobutyl radical in the gas phase, and one of the oxygen becomes a hydroxyl group after hydrogen addition. In the following step, one of the primary hydrogens is seized by the other oxygen atom, leading to an increase in length of the C−H bond in isobutane from around 1.1 Å to 1.4 Å, as shown in TS2a and TS2b, and the barriers are calculated to be 0.84 and 0.45 eV for CNT and ND, respectively. In the end, isobutene is formed in the gas phase and the two oxygen groups are converted to two hydroxyls. In general, the activation 3782

DOI: 10.1021/acscatal.6b03619 ACS Catal. 2017, 7, 3779−3785

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ACS Catalysis product isobutene molecule and the active site have an important effect on the selectivity. The weak interaction will facilitate the desorption of the products and prevent them from undergoing deep dehydrogenation to form coke.21 Therefore, a low adsorption energy of isobutene could be beneficial for the selectivity of alkane. According to eq S1 shown in the Supporting Information, the binding energies of isobutene (Eads) for the CNT and ND (octahedral quinone) are calculated to be −1.14 and −0.11 eV, respectively. Therefore, the binding of isobutene on CNT is much stronger than that on ND catalyst, which can lead to better selectivity toward isobutene on ND catalysts. Liu et al. reported that, for the dehydrogenation of n-butane, 56% selectivity for C4 alkene was observed on ND catalyst, while the single-walled CNT and multiwalled CNT only provided 12% and 20% selectivities for C4 alkene, respectively.13 The calculation results above are in accordance with experimental studies. 2.6. Effect of Hydrogen on the Surface. Hydrogen atoms or other functional groups are inevitably found on ND surfaces. It has been pointed out in the literatue that the surface of nanodiamonds could be terminated with hydrogen or various functional groups.28,29 Hydrogen on the surface of NDs could serve to stabilize the nanocrystal bulk diamondlike structure and symmetry by eliminating the dangling bond.34 It is important to explore the effects of surface hydrogen on the dehydrogenation of isobutane on ND. The calculations show that ND fully terminated with hydrogen (ND-H) retains the diamond structure after optimization (shown in Figure S7 in the Supporting Information), which means that the sp3 carbon atoms on the surface have been stabilized by hydrogen atoms. This observation indicates that not only the morphology but also the surface groups play a role in the stability of ND by influencing the surface reconstruction and the formation of the sp2 carbon shell. Subsequently, the first C−H bond dissociation of isobutane on ND-H is performed, and the result is compared to bare ND. The barrier on ND-H is calculated to be 2.24 eV, while it is only 1.05 eV when there is no hydrogen coverage, as shown in Figure S8 in the Supporting Information. It could be deduced that the reconstruction of sp2@sp3 hybrid core−shell structure is important for the decreased activation barrier. Without the core−shell morphology, the activation barrier increases significantly in comparison to the bare ND with sp2@sp3 core−shell morphology. 2.7. Electronic Structure Analysis. From the above results, the activation of the C−H bond shows strong preference for the ND over that of CNT. For the different morphologies of ND, octahedral and cuboctahedral forms exhibit lower activation barriers in comparison to cubic. To understand the different catalytic performance, the atomic charges on the surface atoms are analyzed using Bader charge analysis.35 It is found that charges on the oxygen atoms are −1.06, −1.02, −1.03, −1.04, −1.05, −0.97, and −0.85 e for the oxygens on CNT, Octa-q, Octa-di, Cubic-q, Cubic-di, Cubo-q, and Cubo-di, respectively, as shown in Figure 7. The charge distribution reflects the electrophilic ability of the oxygen atom. As the oxygen atom on CNT has greater charge than that on ND, it has more difficulty in accommodating more electrons for CNT in comparison to the oxygen on NDs, which influences the ability of the active site to break a C−H bond.36 Moreover, the trend of charge on oxygen on NDs is consistent with the activation barrier, i.e. Cubic-di > Cubic-q > Octa-di > Octa-q > Cubo-q > Cubo-di, which validates that the charge on oxygen

Figure 7. Atomic charges on oxygen atoms on various ND catalysts from a Bader charge analysis.

atoms is correlated with the catalytic ability of abstraction of hydrogen in isobutane. The charge analysis has also been carried out on the reaction intermediates of the C−H bond activation, as given in Table 1, Table 1. Bader Charge Difference of the Oxygen and Carbon Atoms in the CO Functional Group between the Initial State (IS1, As Shown in Figure 5) to the Intermediate State (IM1 As Shown in Figure 5) during the First C−H bond Activationa change in Bader charge of the CO functional group structure

O

C

CNT octahedral quinone octahedral diketone cubic quinone cubic diketone cuboctahedral quinone cuboctahedral diketone

−0.03 −0.11 −0.16 −0.11 −0.02 −0.10 −0.34

−0.39 −0.51 −0.41 −0.39 −0.46 −0.40 −0.44

a

The negative values mean that the charge is increased after C−H bond activation.

which could also reveal the difference between CNT and ND. The CO group that takes part in the first C−H bond dissociation is investigated. From Table 1, it is very interesting to note that the more greatly reduced atoms are carbons instead of oxygens for both CNT and ND in C−H bond activation, as the carbon obtained more charge. This indicates that for carbon catalyst the carbon atom in the CO group is a good electron acceptor. The observation once again reveals that carbon atoms are actively involved during C−H bond activation instead of being a spectator, which was also discovered in our previous work.23 It is also noted that the carbon on ND obtained greater charge than that on CNT in the formation of hydrogenated carbonyl groups, which implies a better reactivity toward C−H bond activation. On the basis of this charge analysis, it could be concluded that ND with the sp2@sp3 core−shell structure has a higher ability to accommodate extra electrons in comparison to CNT, which facilitates the charge transfer in the reactant and active sites and leads to a better reactivity. 2.8. Connection to the Experimental Observations. It has been well documented that the ND catalysts show 3783

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ACS Catalysis performance much improved from that of the CNT catalysts in dehydrogenation reactions.12,13,17 The importance of the formation of an sp2@sp3 core−shell structure for ND catalysts is also observed experimentally. For example, pristine ND does not have impressive dehydrogenation performance. However, once the sp2@sp3 hybrid structure is formed during the reaction, the performance of the butane dehydrogenation is significantly improved.13 Furthermore, NDs annealed at different temperatures show different dehydrogenation reaction performances, which reveals that NDs containing only pure sp2 carbon or sp3 carbon do not have a good reactivity in comparison to the sp2@sp3 hybrid ND.17 From the current work, the role of the sp2@sp3 hybrid structure for ND catalysts is revealed. The calculations indicate that the sp2@sp3 core− shell structure causes the ND catalysts to have a lower C−H bond activation barrier and weaker isobutene adsorption energy. Furthermore, an electronic structure analysis explains the origin of the superior performance from the sp2@sp3 core− shell structure. Overall, a deeper understanding of the catalytic role of the sp2@sp3 core−shell structure of ND is obtained.

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AUTHOR INFORMATION

Corresponding Authors

*B.L.: e-mail, [email protected]; tel, 86-24-83970027; fax, 86-2483970019. *D.S.S.: e-mail, [email protected]; tel, 86-24-23971577; fax, 8624-83970019. ORCID

Bo Li: 0000-0001-8895-2054 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21573255, 21133010, 51221264, 21261160487) and the “Strategic Priority Research Program” of the Chinese Academy of Sciences, Grant No. XDA09030103. B.L. acknowledges a financial grant from the Institute of Metal Research (Y3NBA211A1). This work is also supported by State Key Laboratory of Catalytic Materials and Reaction Engineering (RIPP, SINOPEC). The computations are also supported by National Supercomputer Center in Guangzhou (NSCC-GZ) and the Special Program for Applied Research on Super Computation of the NSFC Guangdong Joint Fund (the second phase).

3. CONCLUSIONS The direct dehydrogenation of isobutane on ND catalyst has been investigated by first-principles calculations, which include the first C−H bond activation, complete pathway, size effects, selectivity for isobutene, surface hydrogen effects, and electronic structure analysis. The main conclusions are summarized as follows. (1) The structural optimization shows that, for octahedral and cuboctahedral NDs, sp2@sp3 core−shell hybrid structures are formed, while cubic ND only shows a reorientation of surface atoms. (2) Calculations of the direct dehydrogenation of isobutane indicate that the activation barriers of NDs are significantly lower than those of CNT for the C−H bond dissociation of isobutane. The adsorption of the product, isobutene, on the active sites is stronger on CNT than on ND, which could prevent deep dehydrogenation on NDs. Therefore, NDs show improvements over CNT catalysts in both activity and selectivity. (3) Comparisons of activation barriers of different structures, ND sizes, and presence of surface hydrogen atoms reach the conclusion that the core−shell sp2@sp3 hybrid structure is crucial for the reactivity of ND. The Bader charge analysis shows that the oxygen on ND is more reducible than that on CNT, which benefits homolytic C−H bond dissociation. The carbon atom in the CO group is actively participating in the reaction as an electron acceptor during C−H bond activation. Moreover, the unique core−shell structure of NDs leads to a better accommodation of electrons in the formation of hydrogenated carbonyl groups.

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oxygen atoms, and the structure and energy profile of bare ND and ND-H (PDF)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b03619. Computational setup, equation for adsorption energy (Eads), optimized structures of NDs with active sites, activation barriers of the first C−H bond on NDs, imaginary frequencies of transition states, effect of van der Waals correction, regeneration of active sites by 3784

DOI: 10.1021/acscatal.6b03619 ACS Catal. 2017, 7, 3779−3785

Research Article

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DOI: 10.1021/acscatal.6b03619 ACS Catal. 2017, 7, 3779−3785