Tailoring catalytic properties of V2O3 to propane dehydrogenation through single-atom doping: A DFT study

Journal Pre-proof Tailoring Catalytic Properties of V2 O3 to Propane Dehydrogenation through Single-Atom Doping: A DFT Study Jun Zhang (Investigation) (Writing - original draft), Rui-Jia Zhou (Formal analysis), Qing-Yu Chang (Software), Zhi-Jun Sui (Validation), Xing-Gui Zhou (Supervision), De Chen (Resources), Yi-An Zhu (Project administration) (Writing - review and editing)

PII:

S0920-5861(20)30075-4

DOI:

https://doi.org/10.1016/j.cattod.2020.02.023

Reference:

CATTOD 12691

To appear in:

Catalysis Today

Received Date:

20 October 2019

Revised Date:

11 January 2020

Accepted Date:

19 February 2020

Please cite this article as: Zhang J, Zhou R-Jia, Chang Q-Yu, Sui Z-Jun, Zhou X-Gui, Chen D, Zhu Y-An, Tailoring Catalytic Properties of V2 O3 to Propane Dehydrogenation through Single-Atom Doping: A DFT Study, Catalysis Today (2020), doi: https://doi.org/10.1016/j.cattod.2020.02.023

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Tailoring Catalytic Properties of V2O3 to Propane Dehydrogenation through Single-Atom Doping: A DFT Study

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Jun Zhang,† Rui-Jia Zhou,† Qing-Yu Chang,† Zhi-Jun Sui,† Xing-Gui Zhou,† De Chen,‡ Yi-An Zhu*,†

United Chemical Reaction Engineering Research Institute (UNILAB), State Key Laboratory of

Department of Chemical Engineering, Norwegian University of Science and Technology, N-7491

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Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

Corresponding author: [email protected] (Yi-An Zhu)

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Trondheim, Norway

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Graphical abstract

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Highlights The electronic structure of V2O3 upon single-atom doping is examined.

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A weak Lewis acid-base interaction is found to occur on the pristine surface.

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The first dehydrogenation step is identified as the rate-limiting step for PDH.

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Mn1-V2O3 is suggested to be a good catalyst candidate for PDH.

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ABSTRACT

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Vanadium-oxide-based catalysts have recently been found very promising for the catalytic dehydrogenation of propane. In this work, self-consistent density functional theory calculations

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have been performed to examine how the electronic structure of the V2O3(0001) surface is modified by single-atom doping and how the catalytic properties can be tailored to propane dehydrogenation.

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The structural stability of single-atom-doped V2O3(0001) surfaces is assessed by comparing the

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adsorption energies of single atoms with the cohesive energies of bulk metals. A weak Lewis acid-base interaction is found to occur on the pristine surface, which can be strengthened and weakened by substitution of single atoms for V and O, respectively. On these two types of oxide surfaces, single atoms act as promoters and active sites. The first dehydrogenation step is identified as the rate-limiting step by microkinetic analysis. On all the single-atom-doped surfaces, the activation energy for water formation is higher than that for hydrogen recombination, implying that 2

reduction of the oxide surfaces is difficult to take place during the course of the reaction. If a compromise between the catalytic activity and catalyst selectivity is made, Mn1-V2O3 is suggested to be a good candidate as the catalyst for propane dehydrogenation.

Key words: Single-atom doping; PDH; Density functional theory; Lewis acid-base interaction;

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V2O3 catalyst

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1. Introduction

Propylene, which is traditionally produced from steam cracking and fluid catalytic cracking (FCC) of naphtha and other byproducts of oil, can no longer meet the demand that growing rapidly in the past few decades [1, 2]. For this reason, much attention has been given to the on-purpose propane dehydrogenation to produce propylene [3] through either the catalytic (PDH) or the oxidative dehydrogenation (ODH) technology [4, 5]. The PDH reaction is highly endothermic and has one

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molecule of reactant consumed for every two molecules of product produced. Therefore, high

temperature and low partial pressure of the feed stocks are needed to achieve a reasonable propane conversion. On the other hand, although the ODH process is not limited by thermodynamic

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equilibrium and can operate at a low temperature, it suffers from low propylene selectivity which is

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due primarily to the deep oxidation to carbon oxides. As a consequence, only the PDH process has

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been successfully commercialized for the on-purpose propylene production, in which the Al2O3-supported PtSn and CrOx catalysts are employed [6]. However, the selectivity towards

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propylene and catalyst stability over PtSn/Al2O3 catalysts are still unsatisfactory [7, 8] and CrOx/Al2O3 catalysts suffer from fast deactivation and environmental problems [9-11]. Recently,

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many metal-oxide catalysts, such as Ga2O3 [12-16], Al2O3 [17], VxOy [18-22], ZnO [23], PdO [24] and CeO2 [25], prove to be active for PDH, among which VxOy is found to be very promising [18,

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26].

The coordinatively unsaturated oxygen on V2O5 is proposed to be the active center for the

oxidative dehydrogenation of propane [27], which may lead to the formation of propanal or acetone [28, 29]. Very recently, Xiong et al. [26] reported that the PDH reaction taking place on V2O5 can be divided into two periods, namely, the oxidative dehydrogenation period and non-oxidative

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dehydrogenation period. In the initial induction period, the ODH process dominates and V2O5 is gradually reduced to V2O3 by propane. At the steady state, the PDH process dominates and the O ion serves as the active site. However, despite this information, the detailed reaction mechanism for PDH on vanadium oxides remains elusive. Single-atom catalyst (SAC) has recently received much attention in heterogeneous catalysis [30-33]. Examples include M1-ZnO (M = Mn, Fe, Co, and Ni) for water-gas shift reaction [34],

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M1-Al2O3 (M = Pd, Fe, Co, and Ni) for CO oxidation [35], Pt1-MoC for the hydrogenation of nitrobenzene derivatives [36]. Considerable progress has been made with the synthesis of SACs [37-39] and In-situ/Operando characterization techniques [40]. However, understanding of how

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single atoms (SAs) are anchored on metal-oxide surfaces and how the electronic structure of the catalyst surface is modified by SAs is still very limited. In this regard, DFT calculations can be

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carried out to study the geometrical and electronic structures of SACs at an atomic level. For

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instance, Xiong et al. [41] reported that electrons are transferred from Pt to Sn on Pt1-Sn1-CeO2 by using a combined DFT and experimental approach, which causes the selectivity toward propylene

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to be higher on Pt1-Sn1-CeO2 than on Pt1-CeO2.

In this contribution, DFT calculations are first carried out to assess the structural stability of the

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V2O3 surfaces doped with single atoms from 13 transition-metal elements including Mn, Fe, Co, Ni,

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Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au. Then, the electronic structure of the doped V2O3(0001) is analyzed by performing the Bader charge and density of states analysis and by computing charge density difference upon single-atom doping. Next, the adsorption behavior of simple species participating in the PDH reaction is examined by considering the effect of the Lewis Acid-Base interaction. After that, the dominant reaction pathway and rate-determining step are identified by using microkinetic analysis. Finally, we conclude by discussing the implication of our results for 5

rational screening of single-atom-doped V2O3 catalysts for PDH.

2. Computational Details

2.1 DFT calculation All spin-polarized DFT calculations were carried out using the Vienna Ab initio Simulation Package (VASP) [42-44]. The BEEF-vdW functional [45] was employed to treat the exchange and

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correlation of the Kohn-Sham theory. Because standard exchange-correlation functionals suffer from the “self-interaction error” that is connected to the spurious interaction of an electron with itself, an additional Hubbard-type term was applied in the rotationally invariant DFT+U method.

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The effective U value for V (Ueff = 2.5) was obtained by fitting the calculated formation enthalpies

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of transition-metal oxides to available experimental data [46-48]. The projector-augmented wave (PAW) method was used to describe the interactions between ion cores and valence electrons.

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Energy cutoff of up to 600 eV was applied to converge the total energy per atom in the slab to within 1 meV. Partial occupancies of orbitals were determined by the Gaussian smearing method

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with an energy smearing of 0.1 eV. Transition states were located by either the dimer method [49] or the climbing-image nudged elastic band (CI-NEB) method [50, 51]. Microkinetic analysis was

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performed by using the CatMAP code [52] at 850 K and 0.35 bar of C3H8. Corrections to the

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entropy and enthalpy of gas-phase molecules and adsorbed species were made in the ideal-gas limit and the harmonic limit, respectively. Campbell’s method [53, 54] was employed to calculate the degree of rate control. 2.2 Structural model XRD patterns obtained by Xiong et al [26]. showed that V2O5 can be gradually reduced to V2O3 by propane, and V2O3 is quiet stable under the PDH reaction conditions, in complete accord with 6

our calculations that the formation of H2O on V2O3(0001) is kinetically much more hindered than the formation of H2 (see Fig. S1). It was found that V2O3 undergoes a transformation from the antiferromagnetic insulating phase with a monoclinic structure to the paramagnetic metallic crystal adopting a corundum structure at around 160 K [55]. Fully relaxed lattice constants obtained from DFT calculations are a = 5.133 Å, b = 5.133 Å, c = 14.097 Å, α = 90°, β = 90° and γ = 120°, which agree well with the experimentally measured values [56]. V2O3(0001) proves to be the most stable

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surface from a thermodynamic perspective, which was represented as a four-unit-layer slab with a p(2 × 2) supercell [see Fig. 1(b)] and a 12 Å vacuum spacing was used to separate periodic slabs. The bottom two layers were fixed to their crystal lattice positions and the remaining layers as well

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as adsorbates were allowed to fully relax. Brillouin zone sampling was performed using a 3 × 3 × 1 Monkhorst-Pack grid.

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In this work, the V2O3(0001) surface was doped with atomically dispersed atoms from 13

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transition-metal elements, including Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au, because they are among the most widely used heterogeneous catalysts in chemical and biochemical

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applications. Single atoms could be adsorbed on the defect-free metal-oxide surfaces or located in the anion [57] and cation [38] vacancies. To assess the structural stability of the single-atom-doped

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surfaces, the binding energies (∆Ebinding) of the single atoms on the pristine [M1-V2O3(0001)],

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vanadium-deficient [M1-(Vvac)-V2O3(0001)], and oxygen-deficient [M1-(Ovac)-V2O3(0001)] surfaces were calculated and compared with the calculated cohesive energies (Ecohesive), as illustrated in Fig. 1(a). Considering the inherent uncertainty of DFT calculations (0.2 eV), only the catalysts that have single-atom binding energy more negative than the cohesive energy are believed to be thermodynamically stable. As can be seen in the figure, the atoms that are initially placed on the defect-free surface tend to aggregate into clusters, regardless of their chemical identity. The 7

deposition of the 3d metals in the vanadium vacancies is energetically most favorable while the noble metals including Pd, Pt, and Au prefer to stay in the oxygen vacancies. Thus, the V2O3(0001), M1-(Vvac)-V2O3(0001) (M = Mn, Fe, Co, Ni, Cu), and M1-(Ovac)-V2O3 (0001) (M = Pd, Pt, Au) surfaces [see Fig. 1(b-d)] are finally constructed to study the effect of the doped M on the physical

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and chemical properties of the vanadium-oxide catalyst.

Fig. 1. (a) Comparison between the adsorption energies of single atoms at different sites with the cohesive energies of the corresponding bulk metals, top and side views of (b) pristine V2O3(0001), (c) M1-(Vvac)-V2O3(0001) (M = Mn, Fe, Co, Ni, Cu), (d) M1-(Ovac)-V2O3(0001) (M = Pd, Pt, Au), where V-O(o) denotes the ortho-V-O site and V-O(p) denotes the para-V-O site.

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3. Results and Discussion

3.1. Electronic structure of single-atom-doped V2O3(0001) Table 1. Effective Bader charges on M and electrons transferred on the single-atom-doped surfaces M

pristine V2O3

Mn Fe Co Ni Cu Pd Pt Au

M1-(Vvac)-V2O3 (0001)

M1-(Ovac)-V2O3 (0001)

Δq |e|a V 0.01+ 0.14+ 0.14+ 0.14+ 0.01+ 0.240.180.13-

O 0.01+ 0.02+ 0.06+ 0.07+ 0.11+ 0.01+ 0.02+ 0.01+

The positive and negative values signify an electron-loss and an elecrtron-gain process,

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Effective Bader charge on M (|e|) 1.72+ 1.38+ 1.30+ 1.14+ 1.03+ 0.87+ 0.850.660.54-

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Surface

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respectively.

It was reported that single atoms on oxide surfaces can serve as either promoters [58] or active

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sites [59] for a specific reaction. In either case, the electronic structure of the catalyst surfaces would be markedly modified by the presence of the doped atoms, which in turn gives rise to

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adsorption and catalytic properties that are distinctly different from those of the pristine surface.

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Thus, the Bader charge analysis was performed to examine the charge redistribution upon M doping. From Table 1, one can see that there are fewer electrons transferred from M to the surrounding

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oxygen ions as we move from left to right across the first transition series in the periodic table, giving rise to less negatively charged O ions adjacent to M. Interestingly, the charges on the V ions on Mn1-(Vvac)-V2O3(0001) and Cu1-(Vvac)-V2O3(0001) compare closely to those on the pristine surface, while on Fe1-(Vvac)-V2O3(0001), Co1-(Vvac)-V2O3(0001), and Ni1-(Vvac)-V2O3(0001), the charges on the V ions differ greatly from those on the pristine surface, where more electrons (0.14|e|) are accommodated by one of the V ions, indicating that the introduction of M may have 9

different effects on the electronic structure of its nearest neighbor V ions. On the M1-(Ovac)-V2O3(0001) surfaces, M are negatively charged, in contrast to those on M1-(Vvac)-V2O3(0001). The V ions coordinated to the M are less positively charged than those on the pristine surface while the charges on the nearest neighbor O ions remain almost constant, indicating the modification of the electronic structure of the oxide surface by doped M is localized

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around adjacent V ions.

Fig. 2. Density of states projected onto the d states of (a) M, (b) V ion, (c) the p states of O ion, and (d) charge density difference for Fe adsorption at the vanadium vacancy and Pt adsorption at the 10

oxygen vacancy. Charge accumulation and depletion are colored yellow and cyan, respectively, with the isosurface value being 0.08 e/Å3. The density of states (DOS) projected onto the d states of M on M1-(Vvac)-V2O3(0001) and M1-(Ovac)-V2O3(0001) is then calculated and shown in Fig. 2(a). From the figure, one can see that the energy of the d band is shifted farther below the Fermi level as we move from left to right across the first transition series of the periodic table and proceed in the order of Pd, Pt, and Au. In

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particular, the peaks of the Au 5d orbitals appear well below the Fermi level, suggesting the interaction between Au ion and the adsorbate may be weaker than that for the other SACs. As shown in Fig. 2(b), the 3d orbitals of the V ion on Mn1-(Vvac)-V2O3(0001) and

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Cu1-(Vvac)-V2O3(0001) resemble that of the V ion on the pristine surface, implying that the

introduction of Mn or Cu may have a minor impact on the electronic structure of the V ions nearby.

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However, the d states projected onto V ions on M1-(Vvac)-V2O3(0001) (M = Fe, Co, and Ni) are

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different from those of the pristine surface, suggesting the electronic structure of the V ion on these surfaces is modified by the doped M. Interestingly, the influence exercised by M is actually

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different among the V ions in the outermost layer (see the calculated DOSs shown in Fig. S2). As shown in Fig. 2(c), the calculated DOSs projected onto the O 2p states are quite similar. The p-band

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center of the O ion is shifted to a higher energy upon replacement of V with Mn, Fe, Co, and Ni. On

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the contrary, the p-band center of the O ion is downshifted upon substitution of Cu for V or replacement of O with Pd, Pt, and Au. In general, the modification of the electronic structure of the surface by the 3d metals can spread over several V-O bond lengths away from the dopant on M1-(Vvac)-V2O3(0001), while the effects introduced by the doped Pd, Pt, and Au on M1-(Ovac)-V2O3(0001) are localized around adjacent V ions, which is confirmed by the computed charge density difference shown in Fig. 2(d). 11

3.2. Adsorption on single-atom-doped surfaces 3.2.1. Adsorption of propane and propylene Table 2. Calculated adsorption energies of propane and propylene at the favorable site on pristine V2O3(0001), M1-(Vvac)-V2O3(0001), and M1-(Ovac)-V2O3(0001), the shortest distance between H atoms in propane and the oxide surfaces, and the C=C bond length in the adsorbed propylene

V2O3(0001)

Mn Fe M1-(Vvac)-V2O3(0001) Co Ni Cu Pd M1-(Ovac)-V2O3(0001) Pt Au

dH-surf (Å) 2.183 2.160 2.168 2.166 2.172 2.174 1.864 1.899 1.672

Propylene ΔEads (eV) -0.74 -0.93 -0.68 -1.01 -0.64 -0.80 -0.82 -0.57 -0.57

dC=C (Å) 1.361 1.355 1.387 1.376 1.394 1.383 1.476 1.444 1.409

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Propane ΔEads (eV) -0.29 -0.44 -0.38 -0.36 -0.35 -0.30 -0.29 -0.30 -0.41

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It has previously been suggested that the activation of physisorbed propane is the rate-limiting

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step for the overall reaction [6], and the energy difference between propylene desorption and propylene dehydrogenation measures the selectivity toward propylene [7]. Therefore, it is of central

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importance to understand the general trends in adsorption energies of propane and propylene from one catalyst to the next. The adsorption energy of the simple species ( Eads ) was calculated as

Eads  Esurf adsorbate  Esurf  Eadsorbate

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(1)

where Esurf adsorbate , Esurf , and Eadsorbate are the total energies of surface with adsorbate, surface,

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and gas-phase adsorbate, respectively, and a negative Eads value indicates an energy-gain process. From Fig. S3, one can see that propane is weakly physisorbed on the surface without forming a chemical bond, which is reflected in the calculated adsorption energies and the shortest distance between H atom in propane and the oxide surface that fall within the typical range of physisorption (see Table 2). In addition, the calculated DOSs projected onto the physisorbed propane on the 12

different surfaces do not change with respect to that onto gaseous propane, as shown in Fig. S4. The Bader charge analysis shows that only trace amount of electrons (< 0.1) are transferred between physisorbed propane and the surfaces. As suggested in our previous study [60], propylene can be adsorbed on oxide surfaces in either the di-σ or the π mode. On the pristine and M1-(Vvac)-V2O3(0001) surfaces, the adsorption of propylene adopts the π mode, and the favorable sites for propylene on the pristine and

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M1-(Vvac)-V2O3(0001) surfaces are the V and M sites, respectively (see Table S1). However, on the M1-(Ovac)-V2O3(0001) (M = Pd, Pt) surfaces, propylene tends to stick to the surface by adopting the di-σ mode. Taking the structure of the chemisorbed propylene on Pt1-(Ovac)-V2O3(0001) as an

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example (see Fig. S5), we can see that chemical bonds are formed between the carbon atoms and surface metal ions with the C=C bond significantly stretched, and the adsorption energies fall within

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the range of -0.26 ~ -0.57 eV (see Table S2). Interestingly, the π mode dominates when propylene is

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adsorbed on Au1-(Vvac)-V2O3(0001) because of the presence of the noble gold atom that has the

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d-band well below the Fermi level.

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Fig. 3. Contour plots of charge density difference for propylene adsorption on (a) pristine V2O3(0001), (b) Fe1-(Vvac)-V2O3(0001), and (c) Pt1-(Ovac)-V2O3(0001). Charge accumulation and depletion are colored red and blue, respectively, with the isosurface value being 0.03 e/Å3; (d) schematic representations of HOMO on the surface V ion and LUMO on propylene; (e) density of states projected onto gas-phase and adsorbed propylene on V2O3(0001), Fe1-(Vvac)-V2O3(0001), and Pt1-(Ovac)-V2O3(0001).

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To further shed light on the adsorption behavior of propylene on different surfaces, electronic structure analysis is also performed and the results are summarized in Fig. 3. From the figure, we can see that charges on propylene are 0.01-, 0.12-, and 0.27- when it is adsorbed on V2O3(0001),

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Fe1-(Vvac)-V2O3(0001), and Pt1-(Ovac)-V2O3(0001), respectively. The calculated DOSs shown in Fig. 3(e) indicate a trace amount of antibonding states are shifted below the Fermi level (-2 ~ 0 eV)

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when propylene is adsorbed on the pristine surface. For the adsorption of propylene on

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Fe1-(Vvac)-V2O3(0001), the antibonding states are shifted below the Fermi level more significantly than on the pristine surface, indicating a stronger interaction between propylene and the surface. On

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the Pt1-(Ovac)-V2O3(0001) surface, the formation of chemical bonds between propylene and the substrate gives rise to more occupation of the antibonding states, which means the interaction

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between the chemisorbed propylene and the Pt1-(Ovac)-V2O3(0001) surface is much stronger than

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that on the pristine surface. In addition, the observed trend by DOS analysis agrees well with the finding by using the Bader charge analysis in the sense that the interaction between propylene and surfaces varies in the order V2O3(0001) < Fe1-(Vvac)-V2O3(0001) < Pt1-(Ovac)-V2O3(0001). In addition, the C=C bond in the propylene molecule is weakened and elongated from 1.335 Å in the gas phase to 1.361 Å on the pristine surface, to 1.387 Å on Fe1-(Vvac)-V2O3(0001), and to 1.444 Å on Pt1-(Ovac)-V2O3(0001) (see Table 2), which follows the same ordering of the binding strength of 14

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propylene to surfaces.

Fig. 4. (a) Plot of the adsorption energy of propylene at the M site against d-band center of M, (b)

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plot of the bonding energy of propylene at the M site against d-band center of M, and (c)

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decomposition of adsorption energies of propylene on V2O3(0001), M1-(Vvac)-V2O3(0001), and M1-(Ovac)-V2O3(0001).

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The adsorption energies of propylene on the metal-oxide surfaces are summarized in Table 2. As can be seen in Fig. 4(a), plotting these data against the d-band center of surface metal ions does not

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give a straight line. To gain a better understanding of the dominant factor determining the trend in the binding strength, the adsorption energy is decomposed by using the method proposed by Yang et al. [61]:

Eads  Edistortion,surf  Edistortion,ads  Ebonding

(2)

where Edistortion, surf and Edistortion,ads are calculated as the energy difference of surface and

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adsorbate before and after adsorption, respectively, and Ebonding is the bonding energy that measures the strength of direct interaction between distorted adsorbate and surface. Then, the bonding energy is plotted against the d-band center of the 3d metal ions [see Fig. 4(b)], where a higher d-band center signifies a weaker interaction between propylene and the metal ions. This general trend is often taken to be an indication that a greater amount of electrons are transferred to the antibonding orbital of propylene when more electrons are filled in the d bands of the 3d metals.

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The reason the adsorption energy of propylene does not scale with the d-band center of metal ion can be traced to the varied degrees of the surface and propylene distortions. The calculated energy components on the metal-oxide surfaces are shown in Fig. 4(c). One can see from the figure that

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upon chemisorption of propylene Edistortion, surf and Edistortion,ads are highly positive at the Pd-V(o) site on Pd1-(Ovac)-V2O3(0001) and Pt-V(p) site on Pt1-(Ovac)-V2O3(0001), indicating that both the

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surface and adsorbate are greatly distorted, which has a negative effect on the chemisorption of

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propylene. On Au1-(Ovac)-V2O3(0001), propylene is coordinated to the V site in the π mode because the Au d-band appear well below the Fermi level.

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3.2.2. Adsorption of H and 2-propyl

On the V2O3(0001) both the V and O sites are considered to be able to bind H and 2-propyl while

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on the M1-(Vvac)-V2O3(0001) and M1-(Ovac)-V2O3(0001) surfaces there are four sites that can

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accommodate the reaction intermediates. The calculated adsorption energies are listed in Table 3. From the table, it can be seen that atomic H prefers to bind to the V site with an adsorption energy of -2.41 eV on the pristine surface. On the M1-(Vvac)-V2O3(0001) surfaces, atomic H favors either the V site (M = Mn, Cu) or the O site (M = Fe, Co, Ni) that is nearest to M. The reasoning behind this fact is as follows. On the one hand, Mn and Cu have relatively weak impact on the p-band center of O, but the d-band center of V is shifted to a higher energy by the doped M. On the other 16

hand, Fe, Co, and Ni donate fewer electrons than the pristine surface to the nearest O ions, which in turn shifts the p-band center of the O ions to a higher energy but simultaneously shifts the d-band center of the V ion to a lower energy. Thus, atomic H is readily chemisorbed at the V site on Mn1-(Vvac)-V2O3(0001) and Cu1-(Vvac)-V2O3(0001) and the O site on M1-(Vvac)-V2O3(0001) (M = Fe, Co, and Ni). Since the degree of the surface distortion upon H chemisorption at the O site is higher than that when H is chemisorbed at the V site, the correlation between the adsorption energy

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of H at the O site and the p-band center of the O ion does not give a straight line [see Fig. 5(a)]. After dividing the adsorption energy into three terms, as in Eq. 2, the bonding energies of atomic H at the O site scale linearly with p-band center of O ions [Fig. 5(b)], suggesting that the higher

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p-band center of the O ion would lead to a stronger bonding of the atomic H to the O site. On the M1-(Ovac)-V2O3(0001) surfaces, the adsorption energies of H at the M site on Pd1-(Ovac)-V2O3(0001)

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and Pt1-(Ovac)-V2O3(0001) are calculated to be more negative than those on the other surfaces,

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which can be traced to the different geometrical and electronic structures of the different SACs. Since the Au 5d orbitals appear well below the Fermi level, exception occurs on the

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Au1-(Ovac)-V2O3(0001) surface where the atomic H cannot be stably chemisorbed at the Au site but relaxed to the Au-V bridge site, as shown in Fig. S6.

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Table 3. Calculated adsorption energies (in eV) of H, 2-propyl, H&H, and 2-propyl&H on

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V2O3(0001), M1-(Vvac)-V2O3(0001), and M1-(Ovac)-V2O3(0001) Surface V2O3 Mn Fe Co Ni Cu Pd Pt

H M -1.97 -1.90 -1.91 -1.58 -1.49 -2.72 -2.80

V -2.41 -2.57 -2.25 -2.18 -2.30 -2.38 -2.09 -2.40

O -2.03 -2.45 -2.35 -2.62 -2.64 -2.31 -1.72 -2.04

2-propyl M V -1.88 -1.53 -2.13 -1.61 -1.93 -1.64 -2.02 -1.07 -1.98 -0.93 -1.77 -0.90 -1.47 -1.08 -1.57 17

H&H V-O(o) -4.74 -4.73 -4.84 -4.86 -4.78 -4.87 -4.44 -4.50

V-O(p) -4.30 -4.97 -5.04 -5.09 -5.10 -5.08 -4.15 -4.46

2-propyl&H V-O(o) V-O(p) -4.13 -3.92 -4.23 -4.53 -4.11 -4.36 -4.24 -4.52 -4.31 -4.52 -4.26 -4.47 -3.76 -3.66 -3.57 -3.59

Au

-

-2.41

-1.99

-0.92

-1.71

-4.53

-4.03

-3.90

-3.93

Upon chemisorption, 2-propyl favors the V site on all the surfaces because the steric hindrance at the O site is more pronounced than that at the V site, and the adsorption energy of 2-propyl at the metal site scale linearly with that of H at the same site, as indicated in Fig. 5(c). The explanation is that, both 2-propyl and H act as the Lewis base to donate its electrons to the surface if they are placed at the O site. Conversely, they act as the Lewis acid to withdraw electrons donated by metal

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ions when bound to the metal ion.

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Fig. 5. Plot of the adsorption energy of H at the O site against p-band center of O, (b) scaling relation between the bonding energy of H on at the O site and p-band center of O, and (c) scaling

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relation between the adsorption energies of H and 2-propyl at the metal site. 3.2.3. Adsorption of H&H and 2-propyl&H

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On the pristine surface, the coadsorption of H and H (denoted H&H) at the V-O(o) site is energetically more favorable than that at the V-O(p) site, and the adsorption of 2-propyl&H follows

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the same trend. On the M1-(Vvac)-V2O3(0001) surfaces, the binding strength varies in the order V-O(p) > V-O(o) > M-O(o) > M-O(p) (see Table 3 and Table S3). On the M1-(Ovac)-V2O3(0001) surfaces, the most stable site for the adsorption of H&H and 2-propyl&H is at the V-M sites, and the adsorption energies (see Table S4) are much more negative than those on the pristine surface. Table 4. Calculated effective Bader charges on H and coadsorbed H&Ha Surface

M

H (|e|) 18

H&H (|e|)

V2O3(0001)

Mn Fe M1-(Vvac)-V2O3(0001) Co Ni Cu Pd M1-(Ovac)-V2O3(0001) Pt Au a

V 0.430.370.370.370.370.420.450.440.52-

O 0.53+ 0.54+ 0.55+ 0.55+ 0.55+ 0.56+ 0.53+ 0.54+ 0.54+

V 0.530.520.520.520.520.520.520.520.51-

O 0.54+ 0.54+ 0.54+ 0.54+ 0.54+ 0.54+ 0.53+ 0.53+ 0.54+

The positive and negative values signify loss and gain of electrons of the adsorbed species,

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respectively. There is evidence that the Lewis acid-base interaction has a positive effect on the C-H bond activation over metal oxides [60] and the doped M may influence the strength of the Lewis

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acid-base interaction [62-64]. The adsorption of a Lewis base at the O site may enhance the ability

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of the oxide surfaces to donate electrons to the Lewis acid at the V site, giving rise to a stronger chemical bonding between them. Thus, it is of crucial importance to understand the interaction that

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occurs when a pair of amphoteric species is coadsorbed at adjacent sites. Here the Bader charge analysis is used to characterize the Lewis acid-base interaction on the different oxide surfaces. The

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results given in Table 4 show that a weak Lewis acid-base interaction is present at the V and O ion pairs on the pristine surface and at the V-O(o) site on M1-(Vvac)-V2O3(0001), while this interaction

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does not occur on the other metal and oxygen ion pairs because the charge transfer accompanying

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the Lewis acid-base interaction is less than 0.1 electrons (see Table S5 - S7). The Lewis acid-base interaction at the V-O(o) site on M1-(Vvac)-V2O3(0001) is strengthened by

substituting M for V (see Table 4). On the one hand, the substitution causes the oxide surface to be electron-deficient, and, consequently, fewer electrons are transferred from the V ion to the adsorbed H. On the other hand, the electronic structure of V would be modified by binding H to the adjacent O site, which can be seen clearly in Fig. S7. If another H is chemisorbed at the O site, more 19

electrons will be gained by the pre-adsorbed H on M1-(Vvac)-V2O3(0001) than on the pristine surface, and the pre-adsorbed H becomes more negatively charged. In contrast, the Lewis acid-base interaction at the V-O(o) site on M1-(Ovac)-V2O3(0001) is weakened by substituting M for O (see Table 4). Because more electrons are accommodated by the V ion that is nearest to the M on M1-(Ovac)-V2O3(0001) and the ability of atomic H to gain electrons does not change very much, more electrons would be transferred from V to the individually

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chemisorbed H. Therefore, when another H is chemisorbed at an adjacent O site, fewer electrons would be further transferred to the pre-adsorbed H than on the pristine surface; that is, the Lewis acid-base interaction on M1-(Ovac)-V2O3(0001) is weakened.

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3.3. Mechanism of propane dehydrogenation

The main reaction of the PDH process can be simply described by two dehydrogenation steps,

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along with desorption of propylene and H2. Starting from gas-phase propane, the abstraction of H

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can proceed via the heterolytic mechanism to form 1-propyl and 2-propyl. However, the activation of the C-H bond at the methylene group to form 2-propyl is more likely [65], and 2-propyl is

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therefore thought of as the initial state for the second dehydrogenation step. 3.3.1. Mechanism of PDH on the pristine surface

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The energy profile for PDH on V2O3(0001) is shown in Fig. 6(a). The energy barriers for the

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activation of the C-H bond in propane are predicted to be 1.12 and 1.62 eV at the V-O(o) and V-O(p) sites, respectively, which are quite close to the results on Cr2O3(0001) (1.40 eV) that were obtained by using the same method [60]. Interestingly, these values are much lower than the result by Xiong et al. [26] who used a different functional and considered a radical mechanism. For the second dehydrogenation step, the activation energies at the V-O(o) and V-O(p) sites are 1.58 eV and 1.73 eV, respectively, and the V-O(o) site is also the main active site. As discussed earlier, the strength of 20

the Lewis acid-base interaction is comparable at the V-O(o) and V-O(p) sites, so the site preference cannot arise from this interaction. To better understand how the topological structure of the active sites may affect the transition state energy, the adsorption energy of coadsorbed species ( Ecoads ) involved in the activated complex is decomposed into six contributions: constrained constrained Ecoads  Eint  Edistortion, surf  Edistortion, A  Edistortion, B  Ebonding , A  Ebonding , B

(3)

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constrained constrained where Eint is the interaction energy between adsorbed species, Ebonding (or Ebonding ,A , B ) is

the bonding energy that measures the strength of direct interaction between distorted species A (or B) and distorted surface, and Edistortion, surf , Edistortion, A , and Edistortion, B are the distortion energies

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of the surface, A, and B, respectively. The derivation of this equation can be found in Sec. S1 in the Supporting Information. The energy components presented in Fig. S8 indicate that the bonding

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energy of 2-propyl to the V-O(o) site are more negative than that to the V-O(p) site on the distorted

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surface while the other terms in Eq. 3 compare closely to those at the V-O(p) site, which means that the interaction between 2-propyl in the TS1 and the surface is stronger than the interaction between

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2-propyl in the TS3 and the surface. In addition, as shown in Fig. 6, the bond length of C-V (2.264 Å) in the TS1 is shorter than that (2.316 Å) in the TS3. Thus, the lower energy barrier for the

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dehydrogenation at the V-O(o) site is a consequence of the formation of an additional C-V bond.

21

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Fig. 6. Energy profiles for PDH on (a) V2O3(0001), (b) Fe1-(Vvac)-V2O3(0001), and (c)

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Pt1-(Ovac)-V2O3(0001), and the geometries of the transition states. All the potential energies are

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referenced to gas-phase propane and bare surface and bond lengths are given in Å. 3.3.2. Mechanism of PDH on Fe1-(Vvac)-V2O3(0001) Among the four sites on Fe1-(Vvac)-V2O3(0001), as illustrated in Fig. 1(c), the V-O(o) site is most

active for the first dehydrogenation step. The activation energy is calculated to be 1.03 eV, which is 0.09 eV lower than that on the pristine surface. For the second dehydrogenation step, the transition state energies at the four sites are quite close, while the initial states at the V and O ion pairs are 22

more stable than those at the Fe and O ion pairs. Thus, the V-O(o) site is identified as the active site for PDH on Fe1-(Vvac)-V2O3(0001), where the doped Fe acts as a promoter. 3.3.3. Mechanism of PDH on Pt1-(Ovac)-V2O3(0001) From Fig. 6(c), one can see that both the first and second dehydrogenation steps readily take place at the Pt-V(p) site on Pt1-(Ovac)-V2O3(0001), which is identified as the active site. On the other hand, the energy barriers for the first and second dehydrogenation steps (0.83 and 0.73 eV,

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respectively) are much lower than those on the pristine (1.12 and 1.58 eV, respectively) and M1-(Vvac)-V2O3(0001) surfaces. From Fig. S8, one can see that the bonding energies of H and

2-propyl on the Pt1-(Ovac)-V2O3(0001) surface are more negative than those on the V2O3(0001) and

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Fe1-(Vvac)-V2O3(0001) surfaces. Hence, the lower energy barriers can be attributed to the stronger binding strength between the activated complex and the surface. In addition, because the Pt and V

3.3.4 Microkinetic analysis

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explain the lower activation energy.

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ion pairs are located slightly above the oxide surface, the decreased steric hindrance also helps

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Microkinetic analysis is then performed to identify the rate-determining step along the reaction pathway. The calculated degrees of rate control suggest that the first dehydrogenation step governs

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the overall rate for PDH on all the surfaces of interest. On V2O3(0001), Fe1-(Vvac)-V2O3(0001), and

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Pt1-(Ovac)-V2O3(0001), for example, the values for the transition states for the first dehydrogenation step are predicted to be 0.99, 0.83, and 1.00, respectively, which take the most positive value on the respective surface. At the active V-O(o) site on the pristine surface, the turnover frequency (TOF) at 850K and 0.35 bar of C3H8 is calculated to be 7.03 × 10-3 s-1, which is 112 times faster than our previous work on Cr2O3(0001) (6.28 × 10-5 s-1) under the same reaction conditions [60]. At the Pt-V site on Pt1-(Ovac)-V2O3(0001), the TOF is 0.52 s-1, which is 74 times faster than that on the pristine 23

surface. Thus, the superior catalytic performance of the Pt1-(Ovac)-V2O3 catalyst for PDH could be attained by doping trace amounts of Pt on V2O3.

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3.3.5 Mechanism of PDH on single-atom-doped V2O3 catalysts

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Fig. 7. (a) Energy profiles for PDH at the kinetically most favorable site on different SACs; (b) the geometries of the transition states for the first and second dehydrogenation steps over

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Mn1-(Vvac)-V2O3(0001). The energies of intermediates and transition states are referenced to gaseous propane and bare surfaces and are given in eV and bond lengths are given in Å.

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Based on the information given by microkinetic analysis, the first dehydrogenation step is

identified as the rate-limiting step. Thus, the activation of C-H bond in propane plays a key role in PDH and the corresponding activation energies can be used to predict the catalytic activity of the catalysts. The energy profiles at the most active site on the different catalysts are shown in Fig. 7(a). Among the M1-(Vvac)-V2O3 catalysts, the most active one is predicted to be Mn1-(Vvac)-V2O3, which

24

lowers the activation energy for the first dehydrogenation by 0.28 eV with respect to the pristine surface, and the geometries of the transition states for the first and second dehydrogenation steps at the different sites on Mn1-(Vvac)-V2O3(0001) are shown in Fig. 7(b). The TOF at V-O(o) site on this surface is calculated to be 0.117 S-1, which is 17 times faster than that at the active site on the pristine surface. The DOSs projected onto the V and O ions on Mn1-(Vvac)-V2O3(0001) are similar to those on the pristine surface, though the d-band center of V and p-band center of O at the V-O(o)

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site are slightly shifted to a higher energy. In addition, Mn1-(Vvac)-V2O3(0001) has the highest d-band center of V and p-band center of O among the M1-(Vvac)-V2O3(0001) surfaces. For the other M1-(Vvac)-V2O3(0001) surfaces, the energy barriers of the first dehydrogenation steps are also lower

-p

than those on the pristine surface, which would facilitate the PDH reaction, as can be seen in Table S8. We have also calculated the activation energies for the second dehydrogenation step and the

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listed in Table S9 and Table S10, respectively.

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deep dehydrogenation step at the four different sites on M1-(Vvac)-V2O3(0001), and the data are

For the PDH on Pd1-(Ovac)-V2O3(0001) and Au1-(Ovac)-V2O3(0001), because the effect of the

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doped M is localized on the surfaces [see Fig. 2(d)], only elementary steps that take place on the M and V ion pairs are calculated. The first dehydrogenation step proceeds more readily on

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Pd1-(Ovac)-V2O3(0001) than on the other surfaces. Au1-(Ovac)-V2O3(0001) is still noble for the

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activation of propane because the Au 5d orbitals appear well below the Fermi level, which in turn changes the active site to the Au-V bridge site. 3.3.6. H surface diffusion Upon dehydrogenation of propane, the occupation of the active sites by atomic H and its diffusion on the oxide surface would play an important role in the PDH kinetics. Fig. S9 shows that the migration of atomic H between the V and O ions on the pristine surface is strongly inhibited by 25

energy barriers of 1.96 eV at the V-O(o) site and 2.38 eV at the V-O(p) site, respectively. However, the energy barrier for H migration between the oxygens ions is calculated to be only 0.83 eV, which indicates the migration between the oxygens ions dominates the H surface diffusion. For H diffusion on M1-(Vvac)-V2O3(0001), although the introduction of M lowers the energy of transition states at some sites, the migration of atomic H between the metal and the oxygen sites is still not kinetically favorable. On the M1-(Ovac)-V2O3(0001) surfaces, atomic H diffuses between the M and

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V sites quite readily with energy barriers lower than 1 eV (see Fig. S10). However, because the effect of Pd, Pt, and Au is localized around the surrounding V ions, the H diffusion barriers between the V and O ions are close to that on the pristine surface.

-p

3.3.7. Hydrogen recombination and water formation

Atomic H chemisorbed on the surface may recombine to form gaseous H2 or combine with lattice

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O to produce H2O. The competition between these two reactions is closely related to the structural

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stability of the oxide catalysts. A low activation energy for the formation of H2O makes it possible to create oxygen vacancies on the oxide surface and may finally lead to the collapse of the catalyst.

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As mentioned above, the formation of H2 is much more likely than that of H2O on the pristine surface. On the M1-(Vvac)-V2O3(0001) (M = Mn, Co, Ni, and Cu) surfaces, the formation of H2 is

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kinetically more favorable than that on the pristine surface, as shown in Fig. 8(a). In addition, the

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selectivity toward H2 on M1-(Vvac)-V2O3(0001) (M = Co and Ni) is also higher than that on the pristine surface. On the M1-(Ovac)-V2O3(0001) surfaces, the activation barriers for H combination at the M ion and V ion pairs and for the formation of H2O at M-O site are higher than those on the pristine and M1-(Vvac)-V2O3(0001) surfaces. Most importantly, on all the single-atom-doped surfaces concerned, the activation energy difference between water formation and hydrogen recombination is predicted to be positive (see Fig. 8a), implying a good structural stability during 26

the course of the reaction. The activation energies for the hydrogenation recombination at the

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different sites on M1-(Vvac)-V2O3(0001) are also given in Table S11.

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Fig. 8. (a) Calculated activation energies for formation of H2O and H2 and the activation energy

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differences; (b) calculated activation energies for deep dehydrogenation and desorption of propylene and the activation energy differences.

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3.3.8. Catalyst selectivity

Both experimental [66] and theoretical studies [61] indicate that a greater energy difference

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between propylene dehydrogenation and propylene desorption leading to a higher selectivity toward propylene. The desorption energy of propylene, the activation energy of deep dehydrogenation, and the energy difference between them are presented in Fig. 8(b). From the figure, one can see that, on the pristine surface, the energy barriers for the deep dehydrogenation are calculated to be 1.53 eV and 1.57 eV at the V-O(o) and the V-O(p) sites, respectively, which are much higher than that for

27

propylene desorption (0.74 eV), indicating that the physisorbed propylene is more readily released from the surface rather than undergoing dehydrogenation to yield coke. By comparison, the energy barrier difference between dehydrogenation and desorption at the active site on V2O3(0001) is less positive than that on Cr2O3(0001) and ZnO (1010) [60]. For the M1-(Vvac)-V2O3(0001) surfaces, Co1-(Vvac)-V2O3(0001) exhibits the highest selectivity towards propylene because of its highest deep dehydrogenation barrier and comparable desorption energy of propylene. For the other

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M1-(Vvac)-V2O3(0001) surfaces, the predicted selectivity toward propylene is lower than that on the pristine surface. On the M1-(Ovac)-V2O3(0001) surfaces, the selectivity toward propylene is lower than that on the pristine surface and increases in the order Pd < Pt < Au. It is worth noting that the

-p

propylene dehydrogenation is kinetically more favorable than propylene desorption on

Pd1-(Ovac)-V2O3(0001), implying it is not a good catalyst candidate. As for Pt1-(Ovac)-V2O3(0001),

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although the calculated TOF for PDH is higher on than that on Mn1-(Vvac)-V2O3(0001), its

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selectivity toward propylene is lower; moreover, replacement of the precious Pt with Mn would lower the cost of the catalyst, suggesting that Mn1-(Vvac)-V2O3 could be a more promising catalyst

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for PDH.

In general, the trend in the activation energy difference between propylene dehydrogenation and

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desorption follows the empirical observation that a higher selectivity can only be attained at the

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expense of a lower catalytic activity, and Mn1-(Vvac)-V2O3 shows the best catalytic performance among the SACs of interest if a compromise is made between the catalytic activity and catalyst selectivity.

4. Conclusions In this contribution, plane wave DFT calculations have been carried out to explore the kinetics of the PDH reaction over the V2O3 catalysts that are doped with single atoms from 13 transition-metal 28

elements including Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au. By comparing the adsorption energies of the single atoms with the cohesive energies of the bulk metals, the M1-(Vvac)-V2O3(0001) (M = Mn, Fe, Co, Ni, and Cu) and M1-(Ovac)-V2O3(0001) (M = Pd, Pt, and Au) surfaces are predicted to be thermodynamically stable. Electronic structure analysis shows that the modification of the electronic structure of the surface by the 3d metals can spread over several V-O bond lengths away from the dopant on M1-(Vvac)-V2O3(0001), while the effects introduced by

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the doped Pd, Pt, and Au on M1-(Vvac)-V2O3(0001) are localized around adjacent V ions. The Lewis acid-base interaction occurs at the V and O ion pairs on the pristine surface, which can be strengthened and weakened by substitution of single atoms for V and O, respectively. On

-p

M1-(Vvac)-V2O3(0001), the active site for PDH is V-O(o) and M acts as the promoter, while on

M1-(Ovac)-V2O3(0001) the M and V ion pairs act as the active site. The first dehydrogenation step is

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identified as the rate-limiting step by microkinetic analysis. H surface diffusion is strongly inhibited

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between the V and O ions, but it is kinetically feasible for atomic H to diffuse between the M and V sites on M1-(Ovac)-V2O3(0001) and between the O and O sites on M1-(Vvac)-V2O3(0001). On all the

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single-atom-doped surfaces the activation energy difference between water formation and hydrogen recombination is predicted to be positive, implying a good structural stability during the course of

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the reaction. Among the catalysts of interest, Mn1-(Vvac)-V2O3(0001) exhibits the best catalytic

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performance for PDH if a compromise is made between the catalytic activity and catalyst selectivity.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi: … .

29

Credit_Author_Statement

Jun Zhang: Investigation; Writing - original draft Rui-Jia Zhou: Formal analysis Qing-Yu Chang: Software Zhi-Jun Sui: Validation Xing-Gui Zhou: Supervision De Chen: Resources Yi-An Zhu: Project administration; Writing - review & editing

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgement

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This work is supported by the Natural Science Foundation of China (91645122, 21473053, and

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U1663221), the National Key Research and Development Program of China (2018YFB0604700), and the Fundamental Research Funds for the Central Universities (222201718003). The

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computational time provided by the Notur project is highly acknowledged.

30

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34

Supporting Information Tailoring Catalytic Properties of V2O3 to Propane Dehydrogenation through Single-Atom Doping: A DFT Study

†

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Jun Zhang,† Rui-Jia Zhou,† Qing-Yu Chang,† Zhi-Jun Sui,† Xing-Gui Zhou,† De Chen,‡ Yi-An Zhu*,†

United Chemical Reaction Engineering Research Institute (UNILAB), State Key Laboratory of

Department of Chemical Engineering, Norwegian University of Science and Technology, N-7491

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‡

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Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

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Trondheim, Norway

*

Corresponding author: [email protected] (Yi-An Zhu)

S1

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Fig. S1. (a) Energy profile for formation of H2 and H2O on V2O3(0001) and (b) geometries of the

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transition states.

S2

ro of -p

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Fig. S2. Density of states projected onto V on M1-(Vvac)-V2O3(0001).

S3

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Fig. S3. Configurations of physisorbed propane on (a) pristine V2O3(0001), (b)

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Fe1-(Vvac)-V2O3(0001), and (c) Pt1-(Ovac)-V2O3(0001).

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Fig. S4. Density of states projected onto gas-phase propane, physisorbed propane on V2O3(0001),

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Fe1-(Vvac)-V2O3(0001), and Pt1-(Ovac)-V2O3(0001).

S5

Table S1. Adsorption energies of propylene and C=C bond length Surface

M

M1-(Vvac)-V2O3(0001)

dC=C (Å) 1.360 1.360 1.362 1.360 1.361 1.358 1.360 1.372

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M1-(Ovac)-V2O3(0001)

Mn Fe Co Ni Cu Pd Pt Au

Above V ΔEads (eV) -0.76 -0.62 -0.76 -0.64 -0.75 -0.35 -0.45 -0.62

S6

Table S2. Chemisorption energies of propylene on M1-(Ovac)-V2O3(0001) M

M1-(Ovac)V2O3(0001)

Pd Pt Au

dC=C (Å) 1.476 1.499 -

M-V(p) ΔEads (eV) -0.80 -0.57 -0.57

dC=C (Å) 1.495 1.431 1.409

V-M(o) ΔEads (eV) -0.61 -0.26 -

dC=C (Å) 1.473 1.490 -

V-M(p) ΔEads (eV) -0.55 -0.57 -0.54

dC=C (Å) 1.478 1.444 1.408

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Surface

M-V(o) ΔEads (eV) -0.82 -0.50 -

S7

Fig. S5. Configurations of propylene adsorbed on (a) V2O3(0001), (b) M1-(Vvac)-V2O3(0001), and (c)

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M1-(Ovac)-V2O3(0001).

S8

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Fig. S6. (a) Top and (b) side views of the chemisorption configuration of atomic H at the Au-V

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bridge site on Au1-(Ovac)-V2O3(0001).

S9

Table S3. Adsorption energies (in eV) of 2-propyl&H and H&H at the M-O site on M1-(Vvac)-V2O3(0001) M

H&H M-O(o) -4.12 -4.00 -4.24 -3.73 -3.60

M-O(p) -3.83 -3.54 -3.89 -3.40 -2.91

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Mn Fe Co Ni Cu

2-propyl&H M-O(o) M-O(p) -4.71 -4.63 -4.64 -4.27 -4.64 -3.83 -4.32 -4.10 -4.30 -3.96

S10

Table S4. Adsorption energies (in eV) of 2-propyl&H and H&H at the M-V sites on M1-(Ovac)-V2O3(0001) M

H&H M-V ortho -4.40 -4.70 -4.25

M-V para -4.79 -4.81 -3.45

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Pd Pt Au

2-propyl&H M-V M-V ortho para -5.25 -4.44 -5.27 -4.50 -4.65 -4.53

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Fig. S7. Charge density difference for chemisorption of atomic H at (a) the V site on V2O3(0001), (b)

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the Fe site on Fe1-(Vvac)-V2O3(0001), (c) the Pt site on Pt1-(Ovac)-V2O3(0001), (d) the O site on pristine V2O3(0001), (e) the O site on Fe1-(Vvac)-V2O3(0001), and (f) the O site on

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Pt1-(Ovac)-V2O3(0001). Charge accumulation and depletion are colored yellow and cyan respectively,

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with the isosurface value being 0.1 e/Å3.

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Table S5. Bader charge analysis for adsorption of H&H at the V-O(p) site Surface

M

Mn Fe M1-(Vvac)-V2O3(0001) Co Ni Cu

O 0.55+ 0.55+ 0.54+ 0.54+ 0.54+ 0.54+

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V2O3(0001)

H&H (|e|) V 0.530.440.440.430.430.43-

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Table S6. Bader charge analysis for adsorption of H&H at the M-O(o) site Surface

M

O 0.53+ 0.54+ 0.53+ 0.54+ 0.53+

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Mn Fe M1-(Vvac)-V2O3(0001) Co Ni Cu

H&H (|e|) M 0.400.440.280.330.34-

S14

Table S7. Bader charge analysis for adsorption of H&H at the M-O(p) site Surface

M

O 0.56+ 0.56+ 0.55+ 0.56+ 0.56+

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Mn Fe M1-(Vvac)-V2O3(0001) Co Ni Cu

H&H (|e|) M 0.390.440.280.390.35-

S15

Table S8. Activation energies for the first dehydrogenation step Surface

M

Mn Fe M1-(Vvac)-V2O3(0001) Co Ni Cu

M-O(p) 1.10 1.48 2.02 1.50 1.53

V-O(o) 1.12 0.84 1.03 1.11 1.07 1.23

V-O(p) 1.62 1.07 1.16 1.68 1.14 1.11

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V2O3(0001)

ΔEact (eV) M-O(o) 1.08 1.45 1.44 1.45 1.33

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Table S9. Activation energies for the second dehydrogenation step Surface

M

Mn Fe M1-(Vvac)-V2O3(0001) Co Ni Cu

M-O(p) 1.31 1.20 1.27 0.85 1.26

V-O(o) 1.58 1.90 1.57 2.00 1.63 1.62

V-O(p) 1.73 1.36 1.70 1.41 1.49 1.12

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V2O3(0001)

ΔEact (eV) M-O(o) 1.21 1.20 1.56 0.79 1.04

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Table S10. Activation energies for the deep dehydrogenation step Surface

M

Mn Fe M1-(Vvac)-V2O3(0001) Co Ni Cu

M-O(p) 1.84 1.79 2.20 1.76 2.14

V-O(o) 1.53 2.34 1.62 1.65 1.42 1.54

V-O(p) 1.57 1.19 1.28 1.54 1.29 1.25

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V2O3(0001)

ΔEact (eV) M-O(o) 1.64 1.64 2.10 1.81 2.01

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Table S11. Activation energies for the formation of H2 Surface

M

Mn Fe M1-(Vvac)-V2O3(0001) Co Ni Cu

M-O(p) 0.18 0.44 0.14 0.16 0.06

V-O(o) 0.66 0.39 0.51 0.85 0.63 0.76

V-O(p) 0.43 0.60 0.53 0.77 0.95 0.87

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V2O3(0001)

ΔEact (eV) M-O(o) 0.46 0.90 0.54 0.66 0.37

S19

S1. Derivation of Eq. 3 for the decomposition of the adsorption energy of coadsorbed species The adsorption energy of a coadsorbed species can be divided in to six terms (see Equation 3): constrained constrained Ecoads  Eint  Edistortion,surf  Edistortion, A  Edistortion, B  Ebonding , A  Ebonding , B

where Ecoads is the adsorption energy of a coadsorbed species, which is defined as Ecoads =Esurface A B  Esurf  EA  EB

where Esurface A B , Esurf , E A , and EB are the total energies of surface with coadsorbed species, surface, gas phase adsorbate A and gas phase adsorbate B, respectively. It is worth noting that these

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values can be obtained from geometry optimization calculations by the DFT method.

constrained constrained Ebonding (or Ebonding ,A , B ) is the bonding energy that measures the strength of direct interaction

between distorted species A (or B) and distorted surface, which is defined as

-p

constrained constrained constrained Ebonding  EAconstrained , A  Esurface  A  Esurface

re

constrained constrained where Esurface , and EAconstrained are the total energies of constrained surface with  A , Esurface

adsorbate A, constrained surface, and constrained adsorbate A, respectively. These energies can be

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obtained from single-point-energy calculations, and the constrained structures are obtained from

na

optimized structure of the surface with species A and species B coadsorbed on the surface. For example, one can obtain the total energy of constrained surface with species A by removing

ur

the species B from the structure of the surface with species A and species B coadsorbed on the surface, and then calculating the single point energy of this structure.

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Edistortion,surf , Edistortion, A , and Edistortion, B are the distortion energies of the surface, A, and B,

respectively. Edistortion, surf is defined as constrained Edistortion,surf  Esurface  Esurface constrained where Esurface and Esurface are the total energies of constrained surface and relaxed surface,

respectively.

S20

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Eint is the interaction energy between adsorbed species, which can be calculated from Equation 3.

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Fig. S8. Decomposition of the adsorption energy of the activated complex for the first

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dehydrogenation step.

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ro of -p re lP na ur Jo Fig. S9. Energy profiles for H migration (a) on V2O3(0001) and (b) - (f) M1-(Vvac)-V2O3(0001). The energies are referenced to gaseous 0.5H2 and bare surface and are given in eV.

S23

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Fig. S10. Energy profiles for H diffusion on (a) Pd1-(Ovac)-V2O3(0001), (b) Pt1-(Ovac)-V2O3(0001), and (c) Au1-(Ovac)-V2O3(0001), and (d) the definition of the sites. The energies are referenced to

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gaseous 0.5H2 and bare surface and are given in eV.

S24