The Theoretical Investigation of On-Purpose Propane

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

The Theoretical Investigation of On-Purpose Propane Dehydrogenation over the Two Dimensional Ru-Pc Framework Kaifeng Niu, Zhenhua Qi, Youyong Li, Haiping Lin, and Lifeng Chi J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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The Journal of Physical Chemistry

The Theoretical Investigation of On-purpose Propane Dehydrogenation over the Two Dimensional Ru-Pc Framework Kaifeng Niu†, Zhenhua Qi†, Youyong Li, Haiping Lin* and Lifeng Chi* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren'ai Road, Suzhou, 215123, Jiangsu, PR China. † These authors contributed equally to this work.

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ABSTRACT

The on-purpose dehydrogenation of light alkanes has received considerable interests for producing olefins from abundant and cheap feedstock. The prevalent catalysts have been limited to the Pt-Sn/Al2O3 and Cr2O3/Al2O3. In this work, we demonstrate by density functional theory that the two-dimensional Ru-phthalocyanine (2D Ru-Pc) is a potential single-atom-catalyst for selective alkene production via direct C-H bond activations. By analyzing the reaction pathways and the electronic structures of the transition states, we show that the catalytic origin of 2D Ru-Pc in C-H bond activations is the highly exposed d orbitals of the single Ru atom. While the alkene selectivity can be attributed to the weak Ru-π binding interaction between the produced alkene and the 2D Ru-Pc catalyst. The distinct catalytic selectivity of such 2D Ru-Pc may inspire further investigations of selective dehydrogenation of light alkanes on single metal catalysts.

INTRODUCTION

In recent decades, the global trade markets have been registering a rapid growth in demand of light olefins, which are the key feedstock in the chemical industry.1-4 Producing these olefins from abundant and cheap alkanes has been considered as a profitable strategy to close the


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growing “olefin gap” and has attracted extensive research interests.5 In this field, the onpurpose synthesis of the propylene, also known as the propane dehydrogenation (PDH), has become a focus of researches due to the fast-growing demand of propylene.6-7 Mechanistically, the challenges of PDH are two-fold: (i) propane is a chemically unreactive molecule, highly active catalysts are required to break the C-H bond at reasonable temperatures, (ii) on the other hand, the catalysts should not strongly bond to the propylene molecules, so that the desired products can leave the catalysts before being over-dehydrogenated.8 The first challenge can usually be tackled by using precious metal catalysts e.g. Pt. The second challenge regarding to the propylene selectivity, however, remains a central issue in PDH due to the following reasons. Thermodynamically, the over-dehydrogenation of propylene is feasible, because the weakest C-H bond in propane (4.31 eV) is stronger than that in propylene (3.34 eV).9 Kinetically, the propylene desorption from metal catalysts is slow due to the strong di-σ interaction between the olefin and metal surfaces, which significantly promotes the overhydrogenation.10 For instance, Somorjai et. al. have reported that the propane dehydrogenations were observed at the room temperature on the Pt(111) surface, producing both propylene and coke.11-12 In recent years, alloying Pt with a second metal e.g. Sn and Cu have been widely studied to increase the propylene selectivity by reducing the propylene adsorption on catalysts.10 Examples are, Guo et. al. reported that propylene adsorption on Pt could be inhibited by the Cu-Pt interaction. As a result, the selectivity of propylene was significantly increased to 90.8% when Pt catalysts were alloyed with 0.5 wt% Cu.13 Yang et.

al. investigated the commercial Pt-Sn catalysts in PDH with first-principles calculations, and ACS Paragon Plus Environment

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reported that the catalytic selectivity on Pt-Sn alloys could be attributed to the weakened propylene adsorption on the Pt2Sn/Pt(111), in comparison with that on the Pt(111).14 Despite the successful examples of alloy catalysts, the industrial PDH still suffer from the overhydrogenation of propylene and fast deactivation of the Pt-Sn. Mainly because that the selective generation of desired active sites in alloy surfaces is practically very difficult.15 Inspired by the previous studies, we propose that single transition metal atoms may serve as efficient catalysts in PDH, because the highly localized d orbitals may not only lower the activation barriers for C-H bond scissions, but also inhibit the di-σ binding mode of propylene on the catalysts and thus decrease the adsorption energy.16-17 Despite that the single atom catalysts (SAC) have been reported in a wide range of reactions,18-22 the PDH driven by SAC has not yet been addressed. Taking into account that the SAC produced on supporting materials e.g. metal oxides, are often very active and easily agglomerated at the operation temperatures of PDH, the calculations in this study have been focused on the two-dimensional transition metal phthalocyanine (2D TM-Pc) frameworks (M = Fe, Mn, Ni, Cu).23-26 The 2D TM-Pc contains one transition metal atom per unit cell, and provides new candidates for the heterogeneous catalysis due to their stable structures and the uniformly distributed singleatom active sties.27-28 For instance, Sun et. al. and Chen et. al. have reported 2D Cr-Pc and 2D Fe-Pc as active SACs toward the CO oxidation and the oxygen reduction reaction (ORR).29-30 Recently, the Du et. al. have successfully synthesized the 2D Ni-Pc and explored its outstanding activity toward the oxygen evolution reaction (OER) with an overpotential less than 0.25 V.31 Herein, the catalytic activity and propylene selectivity of 2D TM-Pc frameworks


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(TM = Ag, Co, Cr, Cu, Fe, Ni, Pd, Pt, Rh, Ru) in PDH have been screened with first-principles calculations. The 2D Ru-Pc shows the lowest reaction of propylene formation and weakest binding energy of propylene on Ru atom, indicating a promising SAC for PDH reactions. Subsequent Climbing-Image Nudged Elastic Band (CI-NEB) calculations reveal that the 2D Ru-Pc exhibit outstanding catalytic activity and selectivity of propylene production via successive dehydrogenations of propane. By analyzing the electronic structures of the transition states in the elementary reactions, we show that the propylene selectivity can be ascribed to (i) the site-selective dehydrogenation at the C2 carbon atom in the formed 1-proxyl group, due to the formation of chemically stable Ru-π species, and (ii) the weak propylene adsorption (-0.55 eV) on the 2D Ru-Pc comparing to the high activation barriers (1.28 eV) for over-dehydrogenations. Moreover, we show that the 2D Ru-Pc is a general SAC catalyst for producing other olefins via selective dehydrogenation of alkanes.

COMPUTATIONAL DETAILS AND METHODS

The spin-polarized Density Functional Theory (DFT) calculations were performed with the Vienna Ab initio Simulation Package (VASP), where the Projector Augmented-Wave (PAW) potentials were employed to describe the electron-ion interactions.32-33 The Perdew-BurkeEnzerhof (PBE) of Generalized Gradient Approximation (GGA) pseudopotentials were used to describe the exchange-correlation interactions.34 The van der Waals interactions were described with the vdW-D3 method.35 The cutoff energy of plane wave basis was set as 400


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eV. The structures were optimized until the atomic force was less than 0.01 eV per Angstrom. A vacuum layer of 20 Å was adopted to avoid the periodic image interactions. The searches of transition states in PDH were firstly calculated with the CI-NEB method, in which seven structural images were inserted between the initial and final states.36-38 The central images of CI-NEB were then used as the input structures of the subsequent Dimer calculations.39-40 The Brillouin zone of reciprocal space was modeled based on the Γ centered Monkhorst-Pack scheme, where a 4 × 4 × 1 gird was adopted in geometry optimizations, searches of the transition states and calculations of electronic properties.41 All the 2D TM-Pc frameworks were modeled with the periodic monolayer of 2 × 2 supercells composed of four primitive cells, each of which is consisted of 20 C atoms, 8 N atoms, 4 H atoms and 1 transition metal atom. The thermal stability of the 2D TM-Pc surfaces was studied by the Ab-initio Molecular Dynamics (AIMD) calculations at 500 K and 1000 K for 2000 fs. In order to assess the effects of the reaction temperature and the pressure, the PDH mechanism was investigated in two scenarios.42 In the first scenario, the potential energies of the PDH reactions were calculated to investigate the reaction kinetics.43 In the second scenario, the Gibbs free energies of reactive systems at the reaction temperature were investigated, which were defined according to eq. (1): ∆G(T,p) = GS(n + 1)(T) ― GSn(T)

(1)


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The GS(n + 1)(T) and GSn(T) denoted the free energies at the state S(n+1) and Sn, respectively. The temperature T and the pressure p were set as 623.15 K and 1.01 ×105 Pa, respectively.10 The Gibbs free energy was calculated by: G = Epot + EZPE – TS

(2)

in which the Epot, EZPE and S were referred to as the potential energy, the zero-point energy and the entropy, respectively. The entropy was calculated by: hνi

S(T) = kB∑i(kBT

1

exp

(

hνi kBT

)

― ln (1 ― exp

―1

(

hνi k BT

)

― 1 ))

(3)

where the kB, h and νi are referred to as the Boltzmann’s constant, the Planck’s constant and the vibrational frequency, respectively.43 The zero-point energy was calculated by considering the vibrational frequencies over all normal modes: 1

EZPE = 2∑ℏν

(4)

where the ν was the vibrational frequency of the normal mode, the ℏ was the reduced Planck constant (detail definition can be found in the Supporting Information).44 RESULTS AND DISCUSSION The catalytic performance of PDH can be evaluated by the propane activations and propylene selectivity. Based on the Brønsted-Evans-Polanyi (BEP) relationship, the catalytic activity for breaking the C-H bond of propane can be predicted by the potential energies for reactions (∆E) between the final states and the initial states of the C-H bond activations.45-46 Experimentally, it has been reported that the dehydrogenations of the linear alkane is initiated from C-H bond


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activations at the terminal methyl groups, and the reaction energies of this elementary step can be employed as the theoretical descriptor for estimating the catalytic activity of 2D TMPc.47 The reaction energy is calculated according to eq. (5): ∆E = EC3H7 + H + TM ― Pc ― EC3H8 + TM ― Pc

(5)

in which the EC3H8 + TM ― Pc are the potential energy of a physisorbed propane on 2D TM-Pc monolayers and the EC3H7 + H + TM ― Pc are the potential energy of the co-adsorption of the 1proxyl radicals and H atoms on the 2D TM-Pc.10,48 As has been reported previously, the propylene adsorption may serve as a vital criteria for estimating the propylene selectivity of PDH, i.e. the catalytic selectivity increases as the propylene adsorption energy decreases.14 In this work, the adsorption energy of the propylene on 2D TM-Pc is calculated according to eq. (6): Ead = EC3H6 + TM ― Pc ― ETM ― Pc ― EC3H6

(6)

where EC3H6 + TM ― Pc, ETM ― Pc and EC3H6 are the potential energy of the propylene adsorbed on the 2D TM-Pc, the clean 2D TM-Pc framework, and an isolated propylene molecule, respectively. The reaction energies of the first C-H bond activations with respect to the adsorption energies of propylene on 2D TM-Pc frameworks are shown in Figure 1. The Pt2Sn, which has been reported as the active sites for PDH in commercial Pt-Sn catalysts is selected as a reference (reaction energy = 1.22 eV and propene adsorption energy = -0.42 eV) in Figure 1.14 The detailed adsorption configurations of 1-proxyl radicals and H atoms on the 2D TM-Pc and corresponding potential energies are summarized in Figure S1 and Table S1. Calculations


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show that the 1-proxyl radicals are weakly bonded to 2D Cr-Pc, 2D Ni-Pc and 2D Ag-Pc, where large metal-carbon distances are observed (3.97 Å, 3.65 Å and 3.93 Å, respectively). Such large distances indicate that the 2D Cr-Pc, 2D Ni-Pc and 2D Ag-Pc do not exhibit considerable interactions with the C radicals. As a result, the potential energies of the C-H bond activations are relative high. On the other hand, however, the bonding characteristics of the Cu and Pt atom with the C radical lead to a strong structural distortions, which results in high potential energies of C-H bond activations. By contrast, the 2D Ru-Pc and the 2D Rh-Pc frameworks show not only low reaction energies, agreeing well with the activities of the corresponding surface catalysts,49-50 but also weak propylene adsorption energies. In particular, the 2D Ru-Pc framework exhibits the lowest potential energy of C-H bond activations (1.01 eV) and a comparable propylene adsorption energy (-0.55 eV) with respect to the Pt2Sn catalyst (-0.42 eV).14 Therefore, subsequent calculations have been focused on the 2D Ru-Pc.


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Figure 1. The scatter plot of the catalytic performance of the 2D TM-Pc framework towards PDH reactions. The dashed horizontal line (potential energy = 1.22 eV) and the dashed vertical line (propylene adsorption energy = -0.42 eV) are selected to indicated the corresponding values of the Pt2Sn catalyst in Ref. [14].

The intrinsic electronic structure and the thermal stability of the 2D Ru-Pc framework are investigated before the PDH mechanics. As seen in Figure 2a, the 2D Ru-Pc framework possesses a planar structure. The lattice constant of the primitive cell (labeled in dashed red square) is 10.72 Å. Figure 2b shows the spin-polarized density of states (DOS) projected onto

d electrons of the Ru in the 2D Ru-Pc, a Ru atom in the Ru(0001) surface and an isolated Ru atom in the vacuum. As expected, the localization of d states in the 2D Ru-Pc (red line) is in


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between the discrete atomic orbitals in an isolated Ru atom (blue) and the d bands in metallic Ru surfaces (black). The featured peaks in the energy window from -2.0 eV to 2.0 eV indicate that the 2D Ru-Pc may exhibit a higher catalytic activity than the Ru surfaces. As the typical PDH requires temperatures well above 500 K, AIMD calculations are carried out to evaluate the thermal stability of the 2D Ru-Pc. As shown in Figure S2, the maximum root mean square deviation (RMSD) is only about 0.40 Å at 1000 K, indicating that the 2D Ru-Pc is a very stable structure at high temperatures.10,51 In addition, the oxidization of Ru-Pc can be excluded by large increase of Gibbs free energy (ΔG) during the formation of RuO2, which requires an oxygen partial pressure significantly higher than the ambient pressure (Figure S3). Based on the calculations described above, successive dehydrogenation processes are investigated to explore the catalytic activity and selectivity of 2D Ru-Pc as potential PDH catalysts.

Figure 2. (a) The optimized structure of the 2D Ru-Pc framework. The C, H, N and Ru atoms are labeled in the grey, white, blue and dark cyan circles, respectively. (b) The spin-polarized


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DOS projected onto d orbital of the Ru atom in the 2D Ru-Pc framework (red), the Ru(0001) surface atom (black) and the Ru atom in the vacuum (blue). The Fermi level is set as 0.00 eV.

Mechanistically, the propylene production via PDH is consisted with two successive and selective dehydrogenation steps, and of great importance, the subsequent fast propylene desorption to avoid over-dehydrogenations.8 Figure 3 shows possible elementary steps of C-H bond and C-C bond activations in the process of PDH. The detailed energy profiles of these reactions are listed in Table 1.

Figure 3. The possible elementary reactions in the PDH. The desired reaction path is shown in red.

Table 1. The potential energy and the Gibbs free energy profiles of the PDH on the 2D Ru-Pc framework. Reactio n No.

Reaction

ΔE (eV)

Ea (eV)

ΔG (eV)

Ga (eV)


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1 4 5 7 8 9 10 11

C3H8* → C3H7* + H* C3H7*+ H*→ C3H6* + H2 C3H7*+ H*→ CH3CH=CH2* + H2 CH3CH=CH2*→ CH3CH=CH* + H* CH3CH=CH2*→ CH3C=CH2* + H* CH3CH=CH* + H*→ CH3CH ≡ C* + H2 CH3CH=CH* + H*→ CH3C ≡ CH* + H2 CH3C=CH2* + H*→ CH3C ≡ CH* + H2

1.01 0.64 0.45 0.90 0.99 1.11 0.66 0.66

1.08 1.65 1.02 1.28 1.17 2.77 1.95 1.48

0.48 -0.68 -0.52 0.89 0.77 -0.64 -0.19 -0.18

0.79 1.21 0.54 1.13 0.87 1.39 1.72 1.08

ΔE, Ea refer to the potential energies and activation barriers for the reactions, respectively. ΔG is the difference of the Gibbs free energies between the final and initial states. Ga refers to the Gibbs free energy of the transition states.

Before the PDH, the propane is physisorbed on the 2D Ru-Pc framework, with an adsorption energy of -0.53 eV and a C-Ru distance of 3.35 Å. As shown in the Step I of Figure 3, the reaction may proceed via three possible reaction paths: dehydrogenation on a terminal methyl group (reaction 1), dehydrogenation on the methylene group (reaction 2) and the demethylation (reaction 3). All these reactions result in carbon radicals that are chemically bonded to the Ru atom. The numerical values of reaction energies and C-Ru distances are shown in Figure S4. As listed in Table 1, the reaction 1 exhibits the lowest potential energy (1.01 eV) among the reaction 1, 2 and 3 in Step I. In addition, the numerical value of the energy barrier for the reaction 1 (Ea-1 = 1.08 eV) is lower than the potential energies of the reaction 2 (4.13 eV) and 3 (1.59 eV). Therefore, the corresponding energy barriers for the reaction 2 and 3 are presumably higher than that of the reaction 1. This indicates that the PDH will proceed with reaction 1 on the 2D Ru-Pc, agreeing well with the previous study.52 Furthermore, the adsorption energy of the H atom on the 2D Ru-Pc (-0.25 eV) indicates that the dissociated H atoms would easily desorb from the Ru active site. Starting from the final state of reaction 1,


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possible subsequent reaction paths include the dehydrogenations and C-C bond activation of methylene groups (reaction 4-6 in Figure 3). DFT calculations show that the C-C bond cleavage of the 1-proxyl radicals on the 2D Ru-Pc requires a high activation barrier of 3.30 eV, and thus is not likely to occur (Figure S5). Moreover, the propylene is only weakly adsorbed on the 2D Ru-Pc (-0.55 eV). Thus, the propylene selectivity can be determined by the activation barriers of the dehydrogenations of the 1-proxyl radical (reaction 4 and 5), which results in the overdehydrogenation and the desired propylene, respectively. As shown in Figure 4, the 1propylene radical (CHCH2CH3) is generated at the final state of reaction 4 yielding a C-Ru distance of 1.85 Å and an activation barrier of 1.65 eV (Table 1), while the reaction 5 is energetically much more favorable due to the smaller energy barrier (1.02 eV). The selectivity may be interpreted by the stable “propylene π” configuration at the transition state of reaction 5 (TS-5), in which the first two C atoms are coordinated to the Ru active site.53 In addition, our results suggest that the 2D Ru-Pc framework exhibits a satisfactory selectivity with the catalysts reported previously (Table S2).5,14,54-55 However, the deep dehydrogenations (reaction 7 to 11) need to be inhibited until the propylene molecules desorb from the catalysts.10 The energy profiles and optimized geometries from propylene to propyne via reaction 7 to 11 are therefore considered (Figure S6). As listed in Table 1, the dehydrogenation barriers of the propylene at both the terminal methylene group and the penultimate homo-methyl (-CH) group (reaction 7 and 8) are higher than the reaction barrier in the synthesis of the propylene (Ea-7 = 1.28 eV > Ea-8 = 1.17 eV > Ea-1 = 1.08 eV). Despite that the reaction 7 and reaction 8 might take place after the synthesis of the propylene due to their relative low activation


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barriers (0.20 eV higher than that of reaction 1), the inversed reactions of the reaction 7 and 8 to re-generate the propylene possess much lower activation barriers in comparing with those of losing more hydrogen atoms to produce over-dehydrogenated products (reaction 9 to 11, Figure S6). Therefore, the inversed reactions of the reaction 7 and 8 are kinetically more favorable, so that the over-dehydrogenations can be effectively prohibited. In addition, the effect of the entropy is investigated by the Gibbs free energy. Under conditions of T = 623.15 K and the ambient pressure,56 the Gibbs free energy profile (dashed lines in Figure 4) shows that the increase of the temperature would not only promote the catalytic activity but also maintain the propylene selectivity of the 2D Ru-Pc. As listed in Table 1, the Gibbs free energy of the transition state for the reaction 1 is 0.79 eV, indicating the catalytic activity is promoted with the increase of the temperature. Moreover, the order of Ga-5 > Ga-1 > Ga-4 in Step I and Step II remains unchanged at T= 623.15 K, indicating that the increase of temperature will not significantly affect the propylene selectivity.

Furthermore, the

relationship between barriers of deep dehydrogenations and the synthesis of the propylene remains as Ga-7 > Ga-8 > Ga-1 (Figure S6). Therefore, the deep dehydrogenations can be effectively prohibited on the 2D Ru-Pc.


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Figure 4. The reaction pathways of the synthesis of the propylene, in which the pathway labeled in red results in the formation of the propylene (step I and step II). The solid and dashed lines represent the potential energies and the Gibbs free energies at 623.15 K and 1.01 × 105 Pa, respectively. The insert figures are the corresponding configurations. Hydrogen atoms in the adsorbed molecules are represented with white circles. Carbon atoms are presented with brown circles. The Ru atom and hydrogen atoms adsorbed on Ru active site are drawn as dark cyan and red circles, respectively.

In addition to the energy profiles, the propylene selectivity can be further clarified by the bonding analysis of the transition states of reaction 4 and 5, as the activation barriers and stability of transition states are closely dependent on the adsorbate-surface interactions.57-59


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Figure 5 shows the spin-polarized DOS of the C and H atoms, and the DOS of Ru active sites projected onto d orbital of the transition state in reaction 4 (TS-4) and reaction 5 (TS-5), respectively. As seen in Figure 5a, the DOS of C1 strongly overlaps with the Ru active sites at the TS-4. By performing the band decomposed charge density analysis, the bonding orbitals of C1-Ru bond are observed at the energy window of [-7.00, -4.00] eV, while the anti-bonding orbitals locate at [1.00, 3.00] eV, referring to the formation of C1-Ru σ bond. However, an obvious mismatch between the DOS of C2 and the Ru active sites is observed from -7.00 eV to the Fermi level, suggesting no chemical bonds is formed. Such bonding characteristics can be confirmed by the charge difference density analysis. As illustrated in Figure 5a (3), the electrons accumulate between the C1 atom to the Ru active site, while the charge density near the C2 atom remains unchanged, indicating that only C1 is chemically interacted with the Ru atom. Despite that the same reaction on Ru(0001) results in a similar transition configuration, a more detailed analysis shows that the TS-4 on the 2D Ru-Pc suffers a structural strain as its C1-Ru-C2 angle (139.14º) is significantly different from the relaxed configuration of that (119.28º) on the Ru(0001) surface (Figure S7).60 Consequently, the TS-4 complex on the 2D Ru-Pc is much less stable than on Ru(0001). By contrast, at the TS-5 on 2D Ru-Pc, the 4d orbital of Ru active site overlap with both C1 and C2 atoms, indicating the formation of the C1-Ru and C2-Ru bonds. The bonding orbitals of the C1-Ru and C2-Ru bonds can be recognized at the [-6.00, -4.00] eV and [-3.50, -2.00] eV, respectively. The anti-bonding orbitals are in the energy window of [3.00, 5.00] eV. As displayed in Figure 5b (4), the electrons are accumulated in between the C1, C2 and Ru atoms, confirming the bonding characteristics


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discussed above. Note that an electron depletion can be observed between C1 and C2 atoms, referring to the characteristic of a π bond.51 Correspondingly, the TS-5 can be stabilized by such Ru-π interaction and therefore exhibits a lower activation barrier.

Figure 5. The spin polarized PDOS of C, H atoms in the 1-proxyl radicals and the Ru active sites on the 4d orbital in (a) the transition state of the terminal dehydrogenation (TS-4) and (b) transition state for penultimate dehydrogenation in Step II (TS-5). The Fermi level is set as 0.00 eV. The insert figures in (a) are the band composed charge densities in the energy range of (1) [-7.00, -4.00] eV, (2) [1.00, 3.00] eV and (3) charge difference density. The isosurface values are set as ±0.20 e/Å3, ±0.10 e/Å3 and ±0.05 e/Å3. The insert figures in (b) are the band composed charge densities in the energy range of (1) [-6.00, -4.00] eV, (2) [-3.50, -2.00] eV, (3) [3.00, 5.00] eV and (4) charge density difference analysis, respectively. The charge density


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difference is calculated by ∆ρ = ρad ― ρmol ― ρsurf, where ρad, ρmol, ρsurf are the total charge densities of propylene adsorbed surface, a transition state complex and the clean surface. The yellow and blue contours denote electron accumulation and depletion. The isosurface values are set as ±0.30 e/Å3, ±0.10 e/Å3, ±0.10 e/Å3 and ±0.04 e/Å3, respectively. Carbon atoms are presented with brown circles, the hydrogen atoms in the adsorbed molecules are represented with white circles, Ru atom and hydrogen atoms adsorbed on Ru active site are drawn as dark cyan and red circles, respectively.

Furthermore, the propylene selectivity of PDH catalysts is inversely proportional to the strength of propylene adsorption, in which weak adsorptions result in high selectivity. Several propylene adsorption configurations (e.g. di-σ, propylene π and propylidyne) have been reported on metal catalysts, in which the di-σ mode is the most stable mode.53 As shown in Figure 6d, the propylene prefers to bind to the Ru(0001) surface with the di-σ configuration, similar to that on Pt(111).10 As seen in Figure 6c, The DOS of C1 and C2 overlap with Ru1 and Ru2 in a wide energy range from -6.00 eV to the Fermi level. Partial charge densities and differential charge densities show that two σ bonds between C and Ru atoms can be recognized at the energy range of [-6.00, -5.00] eV and [-4.00, -2.00] eV. The corresponding anti-bonding orbital is observed in the energy window of [0.00, 2.00] eV. As a result, two independent σ bonds (C1-Ru1 and C2-Ru2) are observed after the propylene adsorption on the Ru(0001) surface. Such bonding nature between the propylene and the Ru surface atoms leads to a strong


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exothermic adsorption with an adsorption energy of -1.78 eV, which agrees to the previous study.14 However, such di-σ adsorption configuration is prohibited on the 2D Ru-Pc. As seen in Figure 6b, the C1 and C2 atoms are bonded to the same Ru atom of the 2D Ru-Pc, with an adsorption energy of -0.55 eV and a Gibbs free energy of -0.44 eV. The C1-Ru and C2-Ru distances is 2.48 Å and 2.64 Å, respectively. Such configuration agrees very well to the “propylene π” adsorption mode (Figure 6b).53 The strong overlap of the states of C1 and C2 atoms with the Ru atom in the energy windows of [-5.00, -3.00] eV and [2.00, 3.00] eV can be assigned as the bonding and anti-bonding orbitals, respectively (Figure 6a). Moreover, the differential charge density analysis shows a clear charge accumulation between the C=C double bond and the Ru atom, and confirms the “propylene π” interaction. Taking into consideration of the energy barriers in PDH is higher than 1.00 eV, such mild propylene adsorption energy would effectively prevent the deep dehydrogenations. Thus, the propylene selectivity on 2D Ru-Pc can be assured by (i) the large activation barriers for deep hydrogenations, and (ii) the weak propylene adsorption.


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Figure 6. (a) The spin-polarized PDOS, band decomposed charge density and charge difference analysis of the propylene adsorption on the 2D Ru-Pc framework. Insert figures (1) and (2) are the charge density at the energy range of the [-5.00, -3.00] eV and [2.00, 3.00] eV, respectively. (3) The charge density difference of the propylene adsorption. The isosurface values are ±0.10 e/Å3, ±0.10 e/Å3 and ±0.03 e/Å3, respectively. (b) The adsorption configuration of the propylene on the 2D Ru-Pc framework. (c) The spin-polarized PDOS, band decomposed charge density and charge difference analysis of the propylene adsorption on the Ru(0001) surface. Insert figures (1) to (3) are the charge densities at the energy range of the [-6.00, -5.00] eV, [-4.00, 2.00] eV and [0.00, 2.00] eV, respectively. (4) The charge density difference. The charge density difference is calculated by ∆ρ = ρad ― ρmol ― ρsurf, where ρad, ρmol, ρsurf are the total


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charge densities of propylene adsorbed surface, a propylene and the clean surface. The yellow and blue contours denote electron accumulation and depletion, respectively. Isosurface values are ±0.10 e/Å3, ±0.05 e/Å3, ±0.05 e/Å3 and ±0.10 e/Å3, respectively. The Fermi level is set as 0.00 eV. (d) The propylene adsorption on the Ru(0001) surface (di-σ configuration). The C, H and Ru atoms are represented by brown, white and dark cyan circles, respectively.

In addition, as previous studies have revealed that the length of alkane chain may possibly affect the selectivity of the C-H bond activations, the dehydrogenation of hexane have also been investigated on 2D Ru-Pc.61-62 The detailed reaction pathway and the energy profiles can be found in Figure S8 and Table S3. As shown, the 2D Ru-Pc exhibits not only high activity towards the dehydrogenations of the hexane, but also distinct 1-hexene selectivity. The first C-H bond activation barrier is 1.46 eV (Ea-S1), which is slightly higher than that of propane. The subsequent dehydrogenation at the penultimate methylene group in the 1-hexyl radical (reaction S5) exhibits lower energy barrier than that at terminal methylene group (reaction S4) as well as the C-C bond cleavage of the 1-hexyl radicals (reaction S6). Note that the activation barriers of the dehydrogenations is: Ea-S4 > Ea-S1 > Ea-S5, indicating the 1-hexene would be selectively generated (Figure S9). Calculations of Gibbs free energies show that such selectivity is maintained under the reaction conditions. Subsequent electronic structure analysis shows that the catalytic origins are similar with those in propane dehydrogenations. More detailed information can be found in Figure S10. Furthermore, the rate-limiting barrier in the synthesis of the 1-hexene is lower than any of over dehydrogenations (Figure S9b).


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Taking into account that the 1-hexene adsorption energy is only -0.87 eV, it can be deduced that the 1-hexene would desorb from the catalyst in advance of over-dehydrogenations. Such results agree well with the reaction paths in PDH, indicating that the length of carbon chain would not significantly affect the activity and the selectivity of the 2D Ru-Pc framework. CONCLUSIONS In summary, DFT calculations have been performed to investigate the catalytic performance of the 2D TM-Pc frameworks in PDH. The 2D Ru-Pc is selected as a candidate catalyst as it shows the lowest reaction energy of C-H bond activation and the propylene adsorption. Subsequent NEB and dimer calculations show that the 2D Ru-Pc exhibits distinct catalytic activity and propylene selectivity. The deep dehydrogenations can be inhibited by not only the high activation barriers, but also the weak propylene-catalyst interactions. The analysis of density of states reveal that the enhanced propylene selectivity compared with Ru(0001) can be attributed to the different bonding characteristics between Ru active sites and the key transition state complexes. On the 2D Ru-Pc, the 1-proxyl has strong tendency in forming π characteristics (reaction 5), rather than the enhancement of the C-Ru σ bond (reaction 4). Moreover, we show that chain length of alkanes would not significantly affect the activity and the selectivity, indicating that the 2D Ru-Pc frameworks can be used as a general catalyst for selective dehydrogenation of alkane molecules. We believe that such 2D Ru-Pc would extend the applications of single atom catalysis to selective dehydrogenation of alkanes, and inspire further catalyst design in future heterogeneous catalysis.


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ASSOCIATED CONTENT Supporting Information. Supplementary DFT calculations of the PDH and the selective dehydrogenation of the hexane (PDF). AUTHOR INFORMATION Corresponding Authors *Lifeng Chi: [email protected] *Haiping Lin: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT We acknowledge the Collaborative Innovation Centre of Suzhou Nano Science &Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the 111 Project. This work was supported by the National Natural Science Foundation of China (NSFC, 21790053, 21771134, 91227201, 91545127, 21622306 and 21403149), the National Major State Basic Research Development Program of China (2017YFA0205002,


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The Theoretical Investigation of On-Purpose Propane

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