Theoretical Insight into Catalytic Propane Dehydrogenation on Ni(111

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Theoretical Insight into Catalytic Propane Dehydrogenation on Ni(111) Tinnakorn Saelee,† Supawadee Namuangruk,§ Nawee Kungwan,*,†,‡ and Anchalee Junkaew*,§ †

Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Center of Excellence in Materials Science and Technology, Chiang Mai University, Chiang Mai 50200, Thailand § National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), Pathum Thani 12120, Thailand Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on June 19, 2018 at 08:33:35 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Here, propane dehydrogenation (PDH) to propylene and side reactions, namely, cracking and deep dehydrogenation on Ni(111) surface, have been theoretically investigated by density functional theory calculation. On the basis of adsorption energies, propane is physisorbed on Ni(111) surface, whereas propylene exhibits chemisorption supported by electronic charge results. In the PDH reaction, possible pathways can occur via two possible intermediates, i.e., 1-propyl and 2-propyl. Our results suggest that PDH reaction through 1-propyl intermediate is both kinetically and thermodynamically more favorable than another pathway. The C−C bond cracking during PDH process is more difficult to occur than the C−H activation reaction because of higher energy barrier of the C−C bond cracking. However, deep dehydrogenation is the preferable process after PDH, owing to the strong adsorption of propylene on Ni(111) surface, resulting in low selectivity of propylene production. This work suggests that Ni(111) has superior activity toward PDH; however, the enhancement of propylene desorption is required to improve its selectivity. The understanding in molecular level from this work is useful for designing and developing better Ni-based catalysts in terms of activity and selectivity for propane conversion to propylene. PDH, can take place.12−14 Moreover, another side reaction, namely, the deep dehydrogenation of propylene to 1-propenyl (CH3CHCH*) and 2-propenyl (CH*2 CCH3) via steps 3D and 3E leading to coke formation, is also possible resulting in deactivation on the active site of the catalyst surface. Thus, high effective catalysts with high selectivity of propylene are very essential for PDH reaction. On the other hand, good selectivity of propylene can be achieved by inhibiting the side reactions of C−C bond cracking and deep dehydrogenation on the surface of catalyst. Many catalysts have been proposed in the literature for PDH reaction, including (1) metals as nonoxidative dehydrogenation catalysts, such as platinum (Pt)15−19 and nickel (Ni);20,21 (2) metal oxides as oxidative catalysts, such as chromium oxide (CrOx),22−25 vanadium oxide (VOx),26−29 molybdenum oxide (MoOx),29,30 and gallium oxide (GaOx);23,31−33 (3) carbonbased materials, such as carbon nanofibers34 and mesoporous carbon;35 (4) zeolites;36−38 and so on. Among metal-based catalysts, Pt has been widely employed in industries due to its high reactivity on PDH and minimum cracking, leading to high conversion of propane.7,8,12 However, the poisoning of the

1. INTRODUCTION Propylene is an important monomer to produce polypropylene, which is a useful polymer for automotive part, textile, furniture, clothing, tube, and packaging applications.1,2 Moreover, it is a crucial chemical feedstock to synthesize other important chemicals, such as propylene oxide, acrylic acid, acrylonitrile, cumene, and alcohols. Those chemicals are precursors in many products for painting, coating, and automotive parts.3,4 Typically, propylene can be produced from steam cracking of liquid feedstocks,5 catalytic cracking refinery units,5,6 and propane dehydrogenation (PDH) reaction.7 Among those processes, PDH is a simple approach to generate propylene by direct elimination of two H atoms from propane using effective heterogeneous catalysts.8−10 The PDH mechanisms and side reactions, including C−C bond cleavage and deep dehydrogenation on catalyst surface, have been proposed as illustrated in Scheme 1.11 The PDH mechanisms can be classified into two possible pathways based on the order of eliminated H position. One is through 1-propyl (*CH2CH2CH3) intermediate (Int) in pathway A, where one H of the primary carbon is eliminated first. Another is via 2-propyl (CH3*CHCH3) intermediate in pathway B, where one H of the secondary carbon is eliminated first. However, the side reactions of the C−C bond cracking of propane, propyl intermediates, and propylene, which are described in steps 1C, 2C, 2C′ during PDH, and 3C after © XXXX American Chemical Society

Received: April 26, 2018 Revised: May 27, 2018

A

DOI: 10.1021/acs.jpcc.8b03939 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Proposed Mechanisms of PDH and Side Reactions on Catalyst Surface

has been reported as a candidate for the PDH process due to its high reactivity for hydrocarbon reaction, eco-friendliness, and lower cost compared to Pt-based catalysts.20,43−52 Yan et al.20 experimentally studied PDH reaction on Ni supported by SiO2 (Ni/SiO2). They found that the reaction rate of propane conversion is very high (about 1 × 1014 molecule/s at 523 K). The high propane conversion refers to high reactivity of Ni/ SiO2. The reactivity of Ni-based catalysts for methane conversion is higher than that of Pt-based catalysts.53−55 Although the reactivity of propane conversion on Ni/SiO2 is very high, CH4 was detected as a main product from the C−C bond cracking. This predominant side reaction results in low selectivity of PDH in this catalyst.20 Moreover, the deactivation of Ni/SiO2 from coke formation induced by deep dehydrogenation resulting in low propylene production was also reported.20 The low selectivity of PDH in Ni was also reported in other experimental studies. For instance, McKee56 proposed that the selectivity of propylene production might be interrupted by C−C bond cracking of propane during the PDH process due to the high reactivity of Ni toward the cracking reaction of alkane. Moreover, Ni-based catalysts have been reported as highly reactive catalysts for deep dehydrogenation leading to the creation of coking on Ni surface.57,58 From the two possible side reactions of C−C bond cracking and deep dehydrogenation, the PDH mechanisms on Ni surface are still controversial at present. Understanding these reactions is necessary to develop better Ni-based catalysts for PDH. Although the reaction mechanisms of PDH reaction and competitive side reactions are intensively studied on Pt catalysts, detailed reaction mechanisms on Ni-based catalyst have not been well understood yet. The following are three important questions: What are the factors that suppress the selectivity of PDH reaction on Ni? Are PDH and side reactions on Ni-based catalyst similar to those on Pt-based catalysts? Are Ni-based catalysts more reactive and selective than Pt-based catalysts? To answer these questions, PDH and side reactions of C−C bond cracking of C3 intermediate and deep dehydrogenation on Ni(111) catalyst have been explored and compared to those on Pt(111) surface by a DFT method. The interactions between propane/propylene and Ni surface have been elucidated by calculating the adsorption energy as well as

active site from coke formation at high temperatures can cause deactivation of catalyst, which is the main limitation of using Pt for PDH process.15,39 To improve the catalytic efficiency of PDH, understanding of reaction mechanisms and nature of catalysts is decisive. However, the catalytic PDH reaction in molecular level is not easily accessible by a conventional experiment. To attain the insights of those processes in molecular level, a density functional theory (DFT) method can be used as a tool to clarify the nature of active sites, configurations of adsorbates on catalysts, etc. Moreover, it can also uncover the reaction mechanism through transition-state (TS) calculations.40 In the literature, PDH reaction mechanisms in Pt-based catalysts have been investigated by using DFT calculations.11,12,41,42 For example, Yang et al.11,41 theoretically studied the activity and selectivity of PDH reaction on different facets of Pt, including Pt(111) and Pt(100), and Pt(211), which represent flat and step surfaces of Pt catalyst, respectively. Among the different shapes of Pt surfaces, the flat surface of Pt(111) is less active for PDH reaction than Pt(100) and Pt(211) facets. However, the selectivity of propane conversion to form propylene on Pt(111) surface is the highest by suppressing C−C bond cracking. Furthermore, the PDH mechanism and side reactions of C−C bond cracking and deep dehydrogenation reactions have been studied on Pt(111).13,57 During the PDH process, the selectivity of C−H activation was found to be better than C− C cracking reaction on Pt(111) supported by lower energy barrier. However, after PDH, propylene was more difficult to desorb than undergoing deep dehydrogenation. This aspect causes coke formation, resulting in low selectivity of propylene production on Pt(111) surface. In addition, expensiveness of Pt is another limitation to apply it in large-scale production. Thereby, development of cheaper metal-based catalysts with comparable/higher performance in terms of reactivity and selectivity than Pt-based catalysts is necessary for PDH process. Among various choices of reactive catalysts, Ni-based catalyst has been widely used in various catalytic applications, such as stream reforming processes,43,44 water−gas shift reaction,45,46 and biomass gasification.47 Ni-based catalyst has also been reported as a good catalyst for hydrogenation 48 and dehydrogenation of small alkanes.49−51 Recently, this catalyst B


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The Journal of Physical Chemistry C

Figure 1. Structure of Ni(111) slab projected along the (a) (010) direction; (b) (100) direction; and (c) possible active sites on the Ni(111) surface.

Figure 2. Top and side views of the most stable configurations of (a) propane and (b) propylene on Ni(111) surface.

i.e., C−C bond cracking and deep dehydrogenation on Ni(111), the TS calculation was systematically performed using climbing image nudge elastic band68 and DIMER methods.69−71 The force tolerance of 0.05 eV/Å and the energy convergence of 1.0 × 10−7 eV/atom were used as the criteria for searching TS in each elementary step. The attained TS structures were confirmed by single imaginary frequency in vibrational analysis. For a slab model, a 6 × 4 Ni(111) slab with a dimension of 14.73 Å × 8.51 Å × 21.02 Å was constructed. As illustrated in Figure 1a,b, 4 Ni layers consisting of 107 Ni atoms are separated by ∼15 Å of vacuum region along the z-axis to avoid interactions between periodic images. The two bottom layers of Ni(111) slab were fixed to their bulk lattice positions, whereas the two top layers and adsorbed species were allowed to fully relax during the calculation. The possible active sites of the Ni(111) slab are exhibited in Figure 1c; there are atop site of Ni atom (atop site), bridge site between Ni−Ni atoms (bridge site), and 3-fold hollow sites (hexagonal close-packed (hcp) and face-centered cubic (fcc) sites).

electronic charge analyses. This information is useful for improving the efficiency of PDH in Ni-based catalysts.

2. THEORETICAL METHODS All periodic boundary calculations were performed by a planewave-based density functional theory (DFT)59 using the Vienna ab initio simulation package (VASP).60,61 The projector augmented wave (PAW) method62,63 with the generalized gradient approximation refined by Perdew, Burke, and Ernzernhof (PBE)64 was employed in this work. The geometry optimization was obtained by minimizing the Hellmann− Feynman forces with the conjugate-gradient algorithm until the force between the atoms is less than 0.01 eV/Å. The criterion for electronic self-consistent field iteration is 1.0 × 10−6 eV/ atom. The k-point grid of 3 × 5 × 1 was generated using the Monkhorst−Pack scheme,65 and the cutoff value for plane wave basis set was set to 400 eV. The electron occupancies were determined using the Methfessel−Paxton scheme66 with a smearing energy of 0.2 eV. van der Waals dispersion correction was applied using the DFT-D3 method proposed by Grimme et al.67 To explore the PDH mechanism as well as side reactions, C


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Table 1. Adsorption Energies (Eads) in Electronvolt and Interatomic Distances (d) in Angstrom of Propane and Propylene on Ni(111) Surface adsorbate

Eads (eV)

dC−Ni (Å)

favored site

propane

−0.61

atop

propylene

−1.39

fcc and atop

dH−Ni (Å)

C ···Ni = 3.23 C2···Ni2 = 3.15 C3···Ni3 = 3.25 C1−Ni1 = 2.23 C1−Ni2 = 2.17 C1−Ni3 = 2.11 C2−Ni3 = 1.99 1

1

H ···Ni H2···Ni2 H3···Ni3 H1···Ni1 H2···Ni2 1

1

= = = = =

2.34 2.14 2.36 2.00 1.89

dC−C (Å)

dC−H (Å)

C −C = 1.52 C2−C3 = 1.52 1

2

C1−C2 = 1.44 C2−C3 = 1.50

C −H1 C2−H2 C3−H3 C1−H1 C1−H2 1

= = = = =

1.11 1.12 1.11 1.12 1.13

Figure 3. Charge density differences of (a) propane and (b) propylene adsorption on the Ni(111) surface with isovalue of ±0.003 e/Å3. Charge accumulation and depletion are represented by green and red regions, respectively.

illustrated in Figure 2b. The C1 atom of propylene binds to Ni1, Ni2, and Ni3 atoms at the hollow site of Ni(111), whereas the C2 of propylene binds to atop of Ni3. The adsorption height of propylene from Ni(111) calculated from the bond length between C2 and Ni3 atoms is 1.89 Å. The strong bond formation between propylene and Ni(111) surface causes the elongation of the C1C2 bond of propylene from 1.34 to 1.44 Å. The Eads value of −1.39 eV and the distorted structure of the adsorbed propylene molecule signify that the interaction between propylene and Ni(111) surface is chemisorption. 3.2. Electronic Charge Properties. 3.2.1. Charge Density Difference and Bader Charge Analysis. To understand the interaction between propane/propylene and Ni(111) during the adsorption process, the electronic charge properties are elucidated herein. As a result, the charge density differences of the most stable adsorption configurations of propane and propylene on Ni(111) are displayed in Figure 3. Charge accumulation and charge depletion are represented by green and red regions, respectively. In addition, charge transfer between adsorbed molecules (i.e., propane and propylene) and Ni(111) surface was quantitatively analyzed by performing Bader charge analysis.72,73 The number of valence electrons of neutral atom/molecule is used as a reference for calculating the Bader charge change. More information about Bader charge of individual atoms on the top layer of Ni(111) surface and adsorbates, such as Ni1, Ni2, and Ni3 atoms of Ni(111) and C and H atoms of propane and propylene, are reported in Table S3 of SI. As a result, charge density differences of propane-adsorbed Ni(111) and propylene-adsorbed Ni(111) are illustrated in Figure 3. For propane adsorption (see Figure 3a), the interaction can be observed from charge accumulation and depletion between H2 of propane and Ni2 of the substrate. In addition, the Bader charge analysis signifies electron transfer from Ni substrate to propane approximately 0.26 |e| during the adsorption process. Figure 3b shows the charge density difference of propylene-adsorbed Ni(111). There are four chemical bonds between propylene and Ni(111) surface, including three bonds of C1 atom to Ni1, Ni2, and Ni3 and

The adsorption energy (Eads) of each adsorbed molecule (i.e., propane and propylene) on Ni(111) surface is calculated as follows Eads = Emolecule/Ni(111) − E Ni(111) − Emolecule

where Emolecule/Ni(111) is the total energy of the adsorbate molecule−substrate complex, ENi(111) is the total energy of a clean Ni(111) surface, and Emolecule is the total energy of an isolated molecule in vacuum. A negative Eads indicates energetically favorable adsorption.

3. RESULTS AND DISCUSSION 3.1. Adsorption of Propane and Propylene on Ni(111). The interaction of a reactant and a product of PDH reaction on Ni(111) surface is discussed in this subsection. The possible adsorption configurations of propane and propylene were optimized by varying the location of the adsorbates (i.e., propane and propylene), which are atop, bridge, and hollow (fcc and hcp) sites of Ni(111) (see Figure 1c). The optimized structures of possible configurations are given in Tables S1 and S2 in the Supporting Information (SI). The most stable configurations of propane and propylene on Ni(111) surface are shown in Figure 2a,b, respectively. The calculated Eads values and selected interatomic distances of propane and propylene on Ni(111) are summarized in Table 1. For propane adsorption, propane prefers to adsorb on atop sites of Ni(111) surface, as shown in Figure 2a. The adsorbed propane interacts with Ni(111) surface by H1−Ni1, H2−Ni2, and H3−Ni3 interactions. The equilibrium adsorption height of propane from the Ni(111) surface is 2.14 Å measured from the nearest distance between H2 atom of propane and atop of Ni2. The adsorption energy of propane on Ni(111) surface is −0.61 eV. The undistorted structure of the adsorbed propane and the low Eads value indicate the physisorption of propane and Ni(111). It is worth mentioning that this value is more negative than that on Pt(111) (−0.42 eV),12 suggesting the stronger adsorption of propane on Ni surface than Pt surface. For propylene adsorption, propylene prefers to adsorb on Ni(111) surface on two adsorption sites, fcc and atop sites, as D


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Figure 4. Comparison projected density of states (PDOSs) of (a) propane−Ni(111) and (b) propylene−Ni(111) systems. Panel I illustrate PDOSs of isolated propane and isolated propylene. Panel II present PDOSs of propane and propylene during adsorption on Ni(111) surface. Panel III depict PDOS of Ni(111) surface before and during propane and propylene adsorption.

one C2···Ni3 bond. Similar to propane adsorption, the Bader charge result indicates the charge transfer from Ni(111) to adsorbed propylene approximately 0.55 |e|. At the same isosurface value of charge density difference, the electron density of the propylene-adsorbed Ni(111) system is more obviously changed compared to the propane-adsorbed Ni(111) system. Moreover, more number of electrons are transferred from the substrate to propylene than propane. These evidences support that propylene adsorption is stronger than propane adsorption. This aspect corresponds well with the adsorption energy results in the previous section. 3.2.2. Density of State (DOS) Analysis. The interactions of propane/propylene and Ni(111) were investigated in detail by projected density of states (PDOSs). The PDOSs of propane, propylene, and Ni(111) before and during adsorption processes are plotted in Figure 4a,b. The Fermi level (EF) is set at 0 eV. The bonding and antibonding states are represented in the lower and higher regions from EF, respectively. The density of states (DOSs) of C, H, and Ni atoms are projected into the pstate (p-DOS), s-state (s-DOS), and d-state (d-DOS), respectively. In Figure 4, panel I present the PDOSs of isolated propane and propylene, panel II present the PDOSs of adsorbates when they are adsorbed on the Ni surface, and panel III present PDOSs of Ni(111) of before and during adsorption. For the preadsorption condition, the overlapping of p-DOS peaks of C atoms and s-DOS peak of H atoms expresses the C−H bonds in propane and propylene molecules (see panel I of Figure 4a,b). For bare Ni(111) surface, the d-DOS of Ni atoms are demonstrated in panel III (blue peaks). The d-band center (εd) of bare Ni(111) surface is −1.43 eV, which is similar to the result reported by Hammer and Nørskov (−1.48 eV).74 For propane adsorption, PDOS peaks of adsorbed propane are shown in panel II of Figure 4a. The interactions between H atoms of propane and Ni atoms of Ni(111) surface can be explained by the s-DOS of H atoms of adsorbed propane and d-

DOS of Ni atoms of Ni(111) surface. The PDOS peaks from H and C atoms of adsorbed propane are shifted and broadened compared to those of isolated propane in panel I of Figure 4a. However, no significant change of d-DOS of Ni(111) surface can be observed when comparing the red and blue peaks in panel III of Figure 4a. This aspect signifies the weak physisorption of propane on Ni(111) surface. For the propylene adsorption, the PDOS peaks of adsorbed propylene are depicted in panel II of Figure 4b. As discussed in the adsorption part, propylene binds on the Ni(111) surface via four chemical bonds (see Figure 2b). These interactions of propylene on the Ni(111) surface can be described by the overlapping of the p-DOS of propylene (C1 and C2 atoms) and d-DOS of Ni(111) surface (Ni1, Ni2, and Ni3 atoms). The redistribution of p-DOS signal of adsorbed propylene indicates bond formations of C1 and C2 atoms of propylene to Ni atoms on Ni(111) surface. The d-DOS curve of Ni(111) surface obviously changes after propylene adsorption (red peaks in panel III of Figure 4b). The strong overlapping of d-DOS of the Ni(111) surface with p-DOS of C1 and C2 of adsorbed propylene in the range of −8.4 to −4.5 eV indicates d−p hybridization of C atoms of propylene and Ni atoms of Ni(111) surface. In summary, PDOS results describe the strong chemical bonding between propylene and Ni(111) and the weak interaction between propane and Ni(111), which correspond very well with the Eads, charge density difference, and Bader charge results. 3.3. Propane Dehydrogenation (PDH) Reaction on Ni(111). Detailed mechanisms of PDH on Ni(111) are discussed in this subsection. The mechanism of PDH reaction of propane to propylene involves two dehydrogenation steps: the first step is the conversion of propane (C3H8) to propyl species (C3H7) and the second step is the conversion of propyl species (C3H7) to propylene (C3H6), as described in Scheme 1. The propyl species from the first step can be produced from two possible pathways: pathways A and B. These two possible E


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Figure 5. Geometries of the initial state (IS), transition state (TS), intermediate (Int), and final state (FS) structures for PDH reaction in (a) pathway A and (b) pathway B.

Figure 6. Energy profiles of PDH for pathway A and pathway B. The relative energies are displayed in brackets, and the Ea barriers are displayed in square brackets.

F


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Table 2. Activation Energies (Ea) in Electronvolt for PDH, C−C Cracking, and Deep Dehydrogenation Reactions on Ni(111) and Pt(111) Surfaces Ea (eV) reaction PDH

propylene desorption C−C cracking

deep dehydrogenation a

elementary step step step step step

1A 1B 2A 2B

step step step step step step

1C 2C 2C′ 3C 3D 3E

CH3CH2CH3 → CH3CH2CH2* + H* CH3CH2CH3 → CH3CHCH3* + H* CH3CH2CH*2 → CH3CHCH*2 + H* CH3CHCH*3 → CH3CHCH*2 + H* CH3CHCH2* → CH3CHCH2(g) CH3CH2CH3 → CH3CH2* + CH3* CH3CH2CH*2 → CH3CH*2 + CH*2 CH3CHCH*3 → CH3CH* + CH*3 CH3CHCH2* → CH3CH* + CH2* CH3CHCH2* → CH3CHCH* + H* CH3CHCH*2 → CH3CCH*2 + H*

Ni(111) (this work)

Pt(111)11,a

0.69 0.79 0.09 0.05 1.16 2.39 1.45 0.90 1.14 0.84 0.85

0.69 0.70 0.73 0.70 0.97 2.44 1.69 1.81 2.00 0.76 0.77

PDH on 3 × 3 Pt(111) surface calculated by PAW−PBE method implemented in VASP.

dehydrogenation for the C1−H1 bond activation is 0.05 eV. Similar to step 2A, the second dehydrogenation of pathway B occurs easily. Next, the detached H1 atom is adsorbed on the hcp site of the Ni(111) surface before moving to the more stable fcc site on the Ni(111) surface with a diffusion energy of 0.14 eV (Int B-1 to FS). The higher energy barrier of TSB1 than TSB2 in pathway B indicates that the first dehydrogenation is also the rate-determining step for PDH reaction, which is the same as in pathway A. In addition, PDH on Ni(111) from this work is compared to that on Pt(111) reported in the literature. The energy barriers of the first and second dehydrogenations on Ni(111) and Pt(111) are given in Table 2. For the first dehydrogenation, the Ea of propane to produce 1-propyl on Ni(111) in pathway 1A is the same as that of Pt(111) with an Ea of 0.69 eV. Meanwhile, the Ea of the first dehydrogenation of propane on Ni(111) in pathway 1B is 0.79 eV, which is slightly higher than the Ea of 0.70 eV in Pt(111). For the second dehydrogenation, the energy barrier of 1-propyl dehydrogenation to form propylene in pathway 2A is 0.09 eV, which is lower than that of Pt(111) (0.70 eV). Meanwhile, the Ea of 2-propyl dehydrogenation in pathway 2B is only 0.05 eV, which is also lower than that of Pt(111) (0.68 eV). These results indicate that Ni(111) is a more active catalyst for propyl species (1-propyl and 2-propyl) dehydrogenation than Pt(111) surface. From PES in Figure 6, pathway A is more thermodynamically favorable than pathway B by comparing the stabilities of intermediates. The relative energy of Int A is −0.55 eV, which is more thermodynamically stable than that of Int B (−0.23 eV). By considering the energy barriers of the rate-determining step of C−H activation in both pathway A and pathway B (see Figure 6), the first dehydrogenation of propane in pathway A is slightly lower than that of pathway B. To indicate the kinetically favorable pathway for the main PDH in the present study, we have compared the reaction rates of IS → TSA1 (kTSA1) and IS → TSB1 (kTSB1), which are the initial/competitive steps of pathways A and B. The details of calculations are provided in the SI. As a result, the calculated kTSA1/kTSB1 values in the range of 298−900 K signify that pathway A is more kinetically preferred than pathway B in the order of 104−102 in the temperature range of 298−900 K (see Table S4). Therefore, the PDH reaction in pathway A is a major contribution to the formation of adsorbed propylene on Ni(111) surface (FS). Finally, desorption of propylene after PDH process has a positive effect on the selectivity toward propylene production.

pathways start from the same initial structure of adsorbed propane (initial state, IS) to yield the same final structure of adsorbed propylene (final state, FS). As a result, the geometries of the initial state (IS), transition states (TSA1, TSA2, TSA-diff, TSB1, TSB2, and TSB-diff), intermediates (Int A-1, Int A, and Int B), and the final state (FS) of pathways A and B are displayed in Figure 5a,b, respectively. The energy profiles of both pathway A (black line) and pathway B (blue line) are presented in Figure 6. Their corresponding activation energies (Ea) and relative energies of each state of reaction on the Ni(111) surface are summarized and compared to PDH on Pt(111) surface11,41 in Table 2. The summation of the total energies of isolated propane and bare Ni(111) surface was set as the reference value for relative-energy calculation. For pathway A (Figure 5a), the first dehydrogenation (step 1A) of adsorbed propane (CH3CH2CH3), denoted as IS, to form 1-propyl (CH3CH2CH*2 ) intermediate, denoted as Int A, starts from the C1−H1 bond stretching of propane from 1.11 to 1.63 Å. At the TSA1 state, H1 is dissociated to the hcp site of Ni(111) to form Int A-1. The Ea of the first dehydrogenation is 0.69 eV. Then, the detached H1 atom diffuses from hcp site to more stable fcc site of Ni(111) surface (Int A). The Ea of H1 atom diffusion via TSA-diff is only 0.19 eV. The Ea of H atom diffusion from hcp to fcc site on the Ni(111) surface in this study is in good agreement with the energy barrier of 0.15 eV reported by Kristinsdóttir et al.75 For the second dehydrogenation (step 2A), Int A can be activated to form propylene (CH3CHCH2*), denoted as FS, by elongation of the C2−H2 bond from 1.16 to 1.43 Å, leading to the strong adsorption of H2 atom on the hcp site of the Ni(111) surface. The Ea of the second dehydrogenation via TSA2 is 0.09 eV. This second step is a facile process. Therefore, the first dehydrogenation is the rate-determining step for PDH reaction in pathway A. For pathway B (Figure 5b), the first dehydrogenation (step 1B) of propane (IS) starts from C2−H2 bond activation of adsorbed propane at the secondary C atom to produce H2 adsorbed on the fcc site of Ni(111) surface and form 2-propyl (CH3CH*2 CH3) intermediate (see Int B in Figure 5b). Next, the activated C2−H2 bond is elongated from 1.12 to 1.55 Å at the TSB1 state, requiring energy of 0.79 eV to form Int B. The H2 atom of adsorbed Int B is strongly adsorbed on the preferred fcc site of Ni(111) surface, so there is no diffusion to other sites. For the second dehydrogenation (step 2B), Int B is continuously dehydrogenated by elongation of C1−H1 bond from 1.12 to 1.32 Å at the TSB2 state. The Ea of the second G


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Figure 7. Geometries of C−C bond cracking reactions of propane (step 1C), 1-propyl (step 2C), 2-propyl (step 2C′), and propylene (step 3C).

Figure 8. The obtained energy profiles are compared in Figure 9. The activation energies of C−C bond cracking and deep dehydrogenation are summarized in Table 2. 3.4.1. C−C Bond Cracking Mechanism on Ni(111) Surface. As presented in Scheme 1, the first side reaction of steps 1A and 1B is the C−C cracking of propane represented by step 1C. In this C−C cracking step, the C1−C2 bond of propane can be cleaved to form methyl (CH3*) and ethyl (CH3CH2*) adsorbed on fcc and atop sites of Ni(111), respectively (see Figure 7). The C1−C2 bond is prolonged from 1.52 to 2.21 Å in the TSC1. From Table 2 and Figure 9, the Ea of C1−C2 breaking is 2.39 eV, which is much higher than that of the first dehydrogenation of propane (about 0.70 eV). The high Ea value corresponds to the steric effect of H that obstructs the contact points between C of propane and Ni atoms on the surface. This extremely high Ea indicates that this cracking requires extremely high temperature. Therefore, the first dehydrogenation of propane, step 1A, can proceed as the preferable process. The next possible cracking is the C−C bond cracking of 1proply (step 2C), and the cracking starts with the elongation of the C1−C2 bond of Int A from 1.53 to 2.17 Å in TSC2 to form stable methylidene (CH*2 ) and ethylene (CH3CH*2 ) species on hcp and atop sites of the Ni(111) surface (FSC2), respectively. The C1−C2 activation energy of 1-propyl on Ni(111) is 1.45 eV, which is higher than the energy barrier of the second

The energy barrier of propylene desorption (ΔEdes) from Ni(111) surface after PDH reaction from FS is calculated as ΔEdes = E FS − E2H/Ni(111) − Epropylene

where EFS is the total energy of the adsorbed propylene and two H atoms, which are detached from the propane structure during the PDH process on the Ni(111) surface, E2H/Ni(111) is the total energy of the Ni(111) surface with two detached H atoms without propylene, and Epropylene is the total energy of isolated propylene molecule. The energy barrier of propylene desorption from Ni(111) surface is 1.16 eV. 3.4. Side Reactions on Ni(111) Surface. As mentioned earlier, the C−C bond cracking and deep dehydrogenation (i.e., dehydrogenation of adsorbed propylene on surface catalyst) as side reactions can affect the selectivity of propane conversion to form propylene on Ni(111) surface. Hence, these two side reactions are investigated and discussed in this subsection. The proposed mechanisms of C−C bond cracking and deep dehydrogenation are described in Scheme 1. The possible C−C bond cracking reactions of propane, 1-propyl, 2-propyl, and propylene are denoted as steps 1C, 2C, 2C′, and 3C, respectively. As a result, the structures of C−C bond cracking reactions are displayed in Figure 7. The deep dehydrogenation can be triggered through 3D and 3E to produce 1-propenyl (CH3CHCH*) and 2-propenyl (CH3CCH*2 ), as displayed in H


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Figure 8. Geometries of deep dehydrogenation to produce 1-propenyl (step 3D) and 2-propenyl (step 3E) on Ni(111) surface.

Figure 9. Energy profiles of dehydrogenation and side reactions, including cracking reaction and deep dehydrogenation of propylene. The relative energies are displayed in brackets, and the Ea barriers are denoted in square brackets.

B from 1.52 to 2.08 Å in TSC′2 to generate methyl (CH*3 ) and ethylidene (CH3CH*), which are adsorbed on fcc and hcp sites of the Ni(111) surface (FSC′2), respectively. In Table 2, the Ea of 2-propyl cracking is 0.90 eV, which is much higher than the energy barrier of the second dehydrogenation (step 2B) on the Ni(111) surface (0.05 eV). For the C−C bond cracking of propylene (3C), the C1−C2 bond of propylene can be activated

dehydrogenation (step 2A) on Ni(111) surface (0.09 eV). The C−C cracking along 2C is less preferable than the 1-propyl dehydrogenation (step 2A). Another possible side reaction is the C−C bond cracking of 2-propyl (step 2C′ in Scheme 1). This C−C cracking through 2C′ is the side reaction of 2-propyl dehydrogenation (step 2B). As shown in Figure 7, the C−C bond cleavage begins with stretching of the C2−C3 bond of Int I


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The Journal of Physical Chemistry C to form methylidene (CH2*) and ethylidene (CH3CH*) adsorbed on the hcp sites of the Ni(111) surface (FSC3). The activated C1−C2 bond of FS is prolonged from 1.43 to 2.05 Å with the activation energy of 1.16 eV. From the overall C−C bond cracking reactions along pathway A, the activation energies of C−C bond cracking decrease in this following order: CH3CH2CH3 > CH3CH2CH*2 > CH3CHCH*2 , which is in accordance with the fact that the removal of hydrogen can enhance the interaction between C and Ni atoms due to the less steric effect. This feature leads to weakening of the C−C bond of adsorbed 1-propyl and propylene on the Ni(111) surface resulting in lower cracking barrier along the PDH process.76,77 Although the energy barriers of the C−C bond cracking tend to decrease along the PDH reaction to produce propylene, the lowest-energy-barrier C−C bond cracking, which is located as the competitive reaction with desorption and deep dehydrogenation of propylene, is still high (around 1.14 eV). Hence, this work can confirm that the C−C bond cracking cannot take place during the PDH process. This result is in good agreement with the C−C bond cracking on Pt(111) surface.11 The C−C bond cracking in Pt(111) is more favorable than the deep hydrogenation after propyne formation. 3.4.2. Deep Dehydrogenation Mechanism on Ni(111) Surface. For the deep dehydrogenation, the strong adsorption of propylene on the Ni(111) surface causes the further reactions of deep dehydrogenation to form 1-propenyl (CH3CHCH*) and 2-propenyl (CH3CCH*2 ) via step 3D and step 3E, respectively. The first deep dehydrogenation of propylene (FS) initiates from C1−H1′ bond activation to form 1-propenyl (step 3D in Scheme 1). As illustrated in Figure 8, the activated C1−H1′ bond of TSD3 is elongated from 1.43 to 1.76 Å, requiring activation energy of 0.84 eV. The detached H′1 atom is adsorbed on fcc site on the Ni(111) surface (FSD3). Another deep dehydrogenation starts with C1−H2′ bond activation of propylene to form 2-propenyl on the Ni(111) surface (step 3E). The activated C1−H2′ bond of TSE3 is elongated from 1.43 to 1.62 Å with the Ea of 0.85 eV. The detached H2′ atom is also adsorbed on the fcc site of the Ni(111) surface (FSE3). Both deep dehydrogenations via step 3D and step 3E are more preferable than propylene desorption due to their Ea values. According to our results, steps 1A and 2A are kinetically and thermodynamically favorable for PDH reaction on Ni(111), as summarized in Table 2. This PDH pathway is also more preferable than the cracking side reaction via TSC1 and TSC2. However, the Ea of C−C cracking (1.14 eV) is comparable to the Ea of the product desorption (1.16 eV) after PDH process. Moreover, the energy barriers of deep dehydrogenation via steps 3D and 3E are 0.84 and 0.85 eV, respectively, which are lower than those of propylene desorption and C−C cracking. Therefore, the adsorbed propylene is more preferable undergoing the deep dehydrogenation than desorption to desired propylene. These results describe the low selectivity of propylene production using Ni(111) surface reported in previous experimental studies.20 In the Pt(111) surface, the Ea of propylene desorption is 0.97 eV, which is higher than that of deep dehydrogenation (about 0.76 eV) but lower than that of propylene cracking (2.00 eV).11 In summary, the PDH reactions on Ni(111) and Pt(111) can be compared as follows:

(i) Similar adsorption behaviors of propane and propylene adsorption on Ni(111) and Pt(111) can be observed. For instance, propane shows the physisorption interaction, while propylene forms strong chemical interaction with those catalyst surfaces. (ii) For the main PDH reaction, the C−H cleavage via 1propyl, pathway A, is the most kinetically and thermodynamically preferable path in Ni(111). After the first H dissociates, the second dehydrogenation occurs easily with low Ea. In contrast with PDH on Ni(111), the activation barriers of the first and second dehydrogenation on Pt(111) are in the range of 0.65− 0.75 eV in both 1-propyl and 2-propyl pathways proposed by Yang et al.11 (iii) The C−C bond cracking is less preferable than the main dehydrogenation in both Ni(111) and Pt(111). It should be noted that the C−C cleaving processes (i.e., steps 1C, 2C, 2C′, and 3C) on Ni(111) in this work require less activation energies than those in Pt(111), as compared in Table 2.11 (iv) After propylene formation, deep hydrogenation is the most preferable process compared to propylene desorption and C−C cracking in both Ni(111) and Pt(111). This feature results in low selectivity of propylene production in both catalysts. (v) Zha et al.78 proposed that the first C−H activation is the rate-controlling step on Pt-based catalysts. They also suggested that propylene desorption plays an important role in the activity and selectivity of PDH on Pt-based catalysts. The findings of this work also suggested a similar conclusion that the first C−H activation and enhancement of propylene adsorption are the key factors affecting the activity and selectivity of PDH in Ni. Therefore, suppressing propylene−Ni(111) interaction can enhance the selectivity of propylene production. In the literature, increment in the hydrogen pressure/coverage on the catalyst surface has been suggested to enhance propylene desorption and also decrease further deep hydrogenation on Pt(111).79 Moreover, improvement of PDH in Pt can be achieved by alloying Pt with other elements, such as In and Sn.78 In Ni catalyst, improvement of propylene production was observed in Ni−Au.80 The proposed methods in Pt(111) provide good guidance for increasing the efficiency of Ni-based catalysts for PDH.

4. CONCLUSIONS The reaction mechanisms of propane dehydrogenation to propylene as well as the side reactions of C−C bond cracking and deep dehydrogenation reaction on Ni(111) catalyst were attained by DFT calculations. Adsorption of propane on Ni(111) is physisorption, whereas adsorption of propylene on Ni(111) surface is chemisorption. These adsorption behaviors can be confirmed by electronic charge results, such as the charge density difference and Bader charge change. The DOS analysis also reveals that the strong interaction between propylene and Ni(111) surface is due to the d−p hybridization of the formed C−Ni bond on the Ni(111) surface. For PDH mechanism, two possible pathways are proposed. Dehydrogenation of propane to form 1-propyl intermediate (pathway A) is more kinetically and thermodynamically favorable than pathway B. The C−C cracking is difficult to occur during PDH process due to high energy barrier. The J


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activity of the PDH reaction on Ni(111) surface is higher than that on the Pt(111) surface because the energy barriers of the first and second dehydrogenation on Ni(111) surface are lower than that of the Pt(111) surface. However, the selectivity of propylene production on Ni(111) is poor due to the less favorable propylene desorption compared to deep hydrogenation after the main PDH process. Enhancement of propylene adsorption is a key factor for improving the selectivity of PDH in Ni. In summary, this study shows the intrinsic nature of Ni(111) surface and provides an insight into the PDH reaction on this catalyst. The result of this work is essential for designing and developing highly selective Ni catalyst by enhancing propylene desorption and inhibiting side reactions, such as deep dehydrogenation and C−C bond cracking reactions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b03939. Additional results of propane and propylene adsorption on Ni(111), electronic charge analysis, and kinetic analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.K.). *E-mail: [email protected] (A.J.). ORCID

Nawee Kungwan: 0000-0002-2960-6853 Anchalee Junkaew: 0000-0002-3193-1110 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from Thailand Graduate Institute of Science and Technology (TGIST, Contract No. SCA-CO-2559-2292-TH) and the Graduate School, Chiang Mai University. S.N. and A.J. acknowledge the Thailand Research Fund (RSA6180080). N.K. acknowledges the Thailand Research Fund (RSA6180044) and the Center of Excellence in Materials Science and Technology, Chiang Mai University. Computational resources are provided by the National Nanotechnology Center (NANOTEC) and the National e-Science Infrastructure Consortium. The support from Computational Chemistry Laboratory, Department of Chemistry, Faculty of Science, Chiang Mai University, is also acknowledged.

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REFERENCES

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Theoretical Insight into Catalytic Propane Dehydrogenation on Ni(111

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