Coverage Effect on the Activity of the Acetylene Semihydrogenation

Article Cite This: J. Phys. Chem. C 2018, 122, 6005−6013

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Coverage Effect on the Activity of the Acetylene Semihydrogenation over Pd−Sn Catalysts: A Density Functional Theory Study Jiubing Zhao, Shenjun Zha, Rentao Mu, Zhi-Jian Zhao,* and Jinlong Gong* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China

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S Supporting Information *

ABSTRACT: The existence of acetylene impurities in ethylene feedstock is harmful to downstream polymerization reactions. The removal of acetylene can be achieved via semihydrogenation reaction, which is normally catalyzed by Pd-based catalysts. This paper describes the coverage effect on the activity of acetylene hydrogenation reactions over Pd and PdSn alloy surfaces. High-coverage models are presented to construct coverage-dependent adsorption energies of C2H2, C2H4, and H2 on Pd(111) and Pd3Sn(111) surfaces. It has been validated that the downshift of d-band center caused by preadsorbed molecules makes the adsorption weaker along with the increase of coverage, and the geometric effect can be neglected. An iterative method has been applied to predict surface coverages of reaction intermediates. Previous calculations with low-coverage models indicate that alloying Pd with late or post-transition metals, in general, enhances ethylene selectivity, accompanied with lower hydrogenation activity. However, by applying a high-coverage model, we show that the predicted hydrogenation barriers are comparable over Pd(111) and Pd3Sn(111) surfaces. single-site heterogeneous catalysts.18,19 For example, the Lindlar catalyst is widely employed in industries for hydrogenation reactions, where lead or sulfur was used to tune the hydrogenation ability of Pd. Currently, the most commonly used industrial catalyst for acetylene semihydrogenation is Agmodified Pd catalyst.20,21 Other Pd alloy catalysts such as PdCu,22 PdGa,23 and PdNi24 have also shown improved catalytic properties in acetylene semihydrogenation compared with pure Pd. In most cases, these alloying components are late and post-transition metals, which shift down the d-band center of Pd in the alloy, thus enhance the desorption of the desired product ethylene, and prevent its overhydrogenation.25−27 Actually, the difference between the hydrogenation barrier and the desorption barrier of ethylene is defined as the crucial factor influencing the selectivity.28,29 During the last two decades, density functional theory (DFT) has become a powerful tool for studying heterogeneous catalytic processes.30−32 Normally, the atomic scale analysis of elementary steps offered by DFT studies is very hard to be performed experimentally.33 In addition, the mechanistic understanding offered by DFT calculations served as guidelines for designing new catalysts with higher activity and/or selectivity. Using DFT calculations, Nørskov and co-workers

1. INTRODUCTION Ethylene is one of the most important feedstocks in the chemical industry.1 In 2016, the global production of ethylene was over 150 million tones, with most of it going to the plastic industry by polymerization reactions.2 Normally, the production of polyethylene requires an acetylene impurity content in the ethylene feedstock below 5 ppm by volume to prevent catalyst poisoning and ensure the purity of the produced polyethylene.3 However, ethylene feedstock produced by cracking can contain more than 2 vol % acetylene. Thus, it must be removed before polymerization processes.4 Most of the existing and under-construction ethylene plants use catalytic hydrogenation method to refine the cracking gas to produce the desired polymer-grade ethylene product.5 Pd-based catalysts have been recognized to selectively hydrogenate acetylene to ethylene.6,7 Previous studies have proposed that hydrogenation occurs via a Horiuti−Polanyi mechanism.8−10 Namely, H2 is dissociated on Pd,11 and the hydrogen adatom then diffuses toward the adsorbed acetylene on Pd, forming new C−H bonds. In addition, a concerted hydrogenation mechanism has also been reported for sequential hydrogenation,12 which we did not take into account. It has been reported that there were two major issues in this process:6,13−17 overhydrogenation to form ethane and oligomerization to generate green oil. The selectivity of hydrogenation reactions can be tuned by alloying Pd with another metal component or by formation of © 2018 American Chemical Society

Received: November 19, 2017 Revised: February 25, 2018 Published: March 1, 2018 6005

DOI: 10.1021/acs.jpcc.7b11394 J. Phys. Chem. C 2018, 122, 6005−6013

Article

The Journal of Physical Chemistry C screened about 70 different bimetallic compounds for acetylene semihydrogenation reactions.28 The optimal catalyst suggested in this study is considered to be located within a window with a suitable binding strength of the adsorbates,34−36 which is equivalent to the well-known Sabatier principle.37 When the adsorption of acetylene is too weak, the activity is low because of the poor activation of the reactants. However, the product ethylene easily desorbs from catalyst surface to increase the selectivity. As we know, too strong adsorption is easy to activate the reactants accompanying with high activity but too strong adsorption normally activates a series of side reactions, for example, overhydrogenation of ethylene, leading to low selectivity. Indeed, previous calculations have indicated that Pd alloy catalysts, for example, PdAg,38 PdFe,39 PdGa,40 and PdIn,41 usually improve the selectivity to ethylene simultaneously suppress the activity of catalysts compared with pure Pd.39,41−44 However, recent studies by Esmaeili et al. reported that the addition of Sn into Pd enhances both activity and selectivity for acetylene semihydrogenation reaction.45,46 To gain a deeper understanding on this phenomenon, we performed DFT calculations over a series of PdSn alloy surfaces, including (111) of Pd and Pd3Sn surfaces as well as PdSn(010) surface. By applying the coverage-dependent adsorption energy together with Langmuir adsorption isotherm, the adsorbate coverages have been predicted and corresponding barriers at this coverage have been calculated. Our calculation indicates that a high-coverage environment is a key factor to correctly describe the relative activity over calculated PdSn alloy surfaces. Herein, our study shed light on how the coverage effect influences the reaction kinetics.

Figure 1. Top and side views of (a) Pd(111), (b) Pd3Sn(111), and (c) PdSn(010) model surfaces. The blue and pink balls denote Pd and Sn atoms, respectively. This definition is used throughout this paper.

Sads = 0.7Sgas − 3.3R

(1)

where Sads is the entropy of the adsorbate; Sgas is the entropy of the corresponding gas-phase species, obtained from thermodynamic tables;54 and R is the ideal gas constant. We assume no entropy change for surface reaction because the harmonic approximation is poor at determining entropy in low-frequency vibrational modes. It has been observed that the adsorption energy can be strongly affected by the presence of other coadsorbates on the surface,55 which has been partially considered in our work. To investigate the coverage effect on adsorption, differential adsorption energies of the reactants C2H2, C2H4, and H are calculated at coverages of 1/16, 1/8, 3/16, 1/4, 5/16, 3/8, 7/ 16, and 1/2 ML at adsorption sites, which are performed on a (4 × 4) model slab. The differential adsorption free energy of acetylene, ethylene, and hydrogen is defined as

2. MODELS AND COMPUTATIONAL DETAILS Periodic DFT calculations were carried out with the Vienna Ab initio Simulation Package.47 The calculations employed the generalized-gradient approximation (GGA) in the form of the Bayesian error estimation functional with van der Waals corrections.48,49 The interactions between the atomic cores and electrons were described by the projector augmented wave method.50 The valence wave functions were expanded by a plane wave with a cutoff energy of 400 eV. A Methfessel− Paxton smearing with a 0.15 eV width was employed to speed up the convergence, and the total energies were evaluated by extrapolating to zero broadening.51 The three slab models employed in this study are sketched in Figure 1. For Pd and Pd3Sn, we chose the flat surface (111) as the sufficient evaluated surface to investigate the properties of catalysts. For Pd/Sn alloy at a 1:1 ratio, we calculated its surface energies and found that (010) is the relative stable and ordered surface among other low-miller index surfaces (see Table S1 in the Supporting Information). Meanwhile, the segregation energies of PdSn catalysts were calculated (Table S2), and no segregation case was found from PdSn catalysts. The thickness of the employed slabs is four layers (except for five layers of PdSn(010)), with top two layers relaxed on each surface. Optimized geometries were found when the force on each relaxed atom was less than 0.02 eV/Å. A k-point mesh of 3 × 3 × 1 was used in a 4 × 4 unit cell of each model slab. Entropic contributions and zero-point energies were taken into account, which converts the DFT-calculated energy into free energy.52 The entropy of the adsorbate was calculated according the Campbell’s method with the following equation:53

Gads(θx) = Gtotal, N − Gtotal, N − 1 − Ggas

(2)

where Gtotal,N and Gtotal,N−1 are the total Gibbs free energy after adsorption of N and N − 1 adsorbates, respectively, and Ggas is the Gibbs free energy of the adsorbate in the gas phase. On PdSn(010), the adsorption free energies are always positive for both kinds of adsorbates even at the lowest coverage (1/16 ML) considered in our calculations. Thus, we just consider the coverage effect on Pd and Pd3Sn surfaces. All kinetic and thermodynamic analysis are carried out at 373 K with the 1, 0.1, and 8.9 atm partial pressures for H2, C2H2, and C2H4, respectively, which are close to the experimental conditions.23

3. RESULTS AND DISCUSSION 3.1. Adsorption of Acetylene and Ethylene on Pd/Sn Surfaces at a Low Coverage. The adsorption energies for C2H2 and C2H4 and their adsorption geometries on Pd(111), Pd3Sn(111), and PdSn(010) are shown in Table 1 and Figure 2, respectively. The adsorption energies and Gibbs free Table 1. Adsorption Energies (Eads/eV) and Gibbs Free Adsorption Energies (Gads/eV) of C2H2 and C2H4 on Surfaces Pd(111) C2H2 C2H4 6006

Pd3Sn(111)

PdSn(010)

Eads

Gads

Eads

Gads

Eads

Gads

−1.83 −0.81

−1.42 −0.40

−1.08 −0.47

−0.68 −0.05

−0.18 −0.27

0.19 0.15


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The Journal of Physical Chemistry C

Table 2. Single-Point Energy of Gas-Phase Acetylene on the (111) Surface of Pd and Pd3Sn and the Ratio of the SinglePoint Energy of N Molecular Acetylene to One Molecular Acetylene Pd(111) coverage/ML

Esingle/eV

1/16 2/16 3/16 4/16 5/16 6/16

−18.43 −36.88 −54.74 −72.96 −91.93 −109.99

Pd3Sn(111) ratio

coverage/ML

Esingle/eV

ratio

2.00 2.97 3.96 4.99 5.97

1/16 2/16 3/16 4/16 5/16 6/16

−18.93 −37.76 −56.60 −75.42 −94.21 −113.83

2.00 2.99 3.98 4.98 6.01

Figure 2. Top and side views of C2H2 and C2H4 adsorption geometries on Pd(111), Pd3Sn(111), and PdSn(010) model surfaces.

Figure 4. Plots of the Gibbs adsorption energy of acetylene on Pd(111) and Pd3Sn(111) against the d-band centers. The coverage of each point is performed in the plot.

Scheme 1. Iterative Method To Identify the Coverage of C2H2, C2H4 and H

Figure 3. (a,b) Trend of the changes in differential adsorption free energies of C2H2 and C2H4 with the coverage on Pd(111) and Pd3Sn(111) surfaces. The coverage dependence of the differential adsorption free energy of C2H2 with one molecular C2H4 coadsorbing on Pd(111) surface is illustrated by the dashed red line in (a). The relationship between the coverage and the differential adsorption free energy of C2H4 with xC2H2 coadsorbing on Pd(111) surface is shown by the blue dashed line.

adsorption energies of C2H2 and C2H4 on all sites are listed in Table S2. Under ultrahigh vacuum (UHV) conditions, acetylene preferentially adsorbs at 3-fold hollow sites on Pd(111),56 which was also found to be energetically most favorable in this study. While the most stable site for C2H4 adsorption is bridge, with two newly formed C−Pd bonds. At 6007


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Figure 5. (a,b) Trend of changes in the differential adsorption free energies of H with the coverage on Pd(111) (6C2H2 coadsorbed) and Pd3Sn(111) (3C2H2 and 1C2H4 coadsorbed). (c,d) Trend of the changes in differential adsorption free energies of C2H2 and C2H4 with the coverage on Pd(111) (2H coadsorbed) and Pd3Sn(111) (3H coadsorbed) surfaces.

isolated Pd atoms, further weakening the binding strength of adsorbates by forcing them to interact with less Pd atoms. 3.2. Coverage-Dependent Adsorption Energy for Single Adsorbate. As shown in Table 1, the calculated adsorption energies of C2H2 and C2H4 are very strong according to the low-coverage model. If we simply apply the Langmuir adsorption model with a constant adsorption energy predicted by a low-coverage model, that is, over 1 eV of acetylene over Pd surfaces, all active sites will be occupied by the strong binding C2H2, which is much higher than the reported saturated coverage of acetylene, 0.46 ML over Pd(111) measured by the low-energy electron diffraction method.59 Because the adsorption energy, in general, depends on the coverage of adsorbates, understanding the interactions between adsorbed acetylene over Pd and PdSn alloy surfaces is important to gain a correct description of surface species population under reaction conditions. The calculated coverage-dependent differential adsorption energies are shown in Figure 3a,b by solid lines. The differential adsorption free energy is observed to be positively correlated with the coverage above a threshold and a constant when the coverage is below this threshold. The point where the differential adsorption free energy is 0 eV indicates thermodynamic equilibration between adsorbed and gas-phase species, corresponding to the saturation coverage. On the Pd(111) surface, the calculated saturation coverage of acetylene is about 0.40 ML, in agreement with experimental results of 0.46 ML.59 It is also found that the saturation coverage of ethylene is about 0.27 ML in this work, which is also very close to the previous experimental result of 0.25 ML.60 Note that the pressure used in our calculation is much higher than those of the surface science studies performed in refs 59 and 60. Therefore, the

Figure 6. Coverage of reactants on the (111) surface of Pd and Pd3Sn.

1/16 and 1/4 ML coverages, the calculated binding energies are −0.81 and −0.74 eV, respectively. Previous temperatureprogrammed desorption experiment measured the adsorption energy of ethylene under UHV conditions to be −0.61 eV on Pd(111) at a 1/4 ML coverage, which is slightly weaker than our DFT result at the same coverage,57 due to the well-known tendency of GGA method to overestimate binding energies.58 With the addition of Sn atoms to the Pd catalyst, the binding strength of adsorbates becomes weaker. The introduction of Sn tunes the electronic structure of Pd by downshifting its d-band center, which weakens the adsorption of adsorbates. Indeed, linear correlation between d-band center and adsorption energy has been observed (Figure S1 in the Supporting Information) when the acetylene binds with the same π adsorption mode on these terrace surfaces. Moreover, the surface-existing Sn atoms divided the complete Pd surface into small ensembles or 6008


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might be caused by the following two factors: (a) geometric effect due to the increased steric hindrance and (b) electronic effect due to the shifts of the d-band center caused by an adsorbed molecule. Because the steric hindrance mainly comes from the repulsion between adsorbates, single-point calculations were done for the adsorbates only, that is, without a metal slab, with the optimized geometry over metal surfaces, which ignores the electronic effect between adsorbate and metal slab. The calculated single-point energies are shown in Table 2. Clearly, the ratio of the single-point energy of N acetylene molecules to one acetylene molecule is nearly N, indicating nearly no interactions between adsorbates; thus, the geometric effect can be neglected. Test calculations further indicate that there are no interactions for two molecular C2H2 with each other until the distance between them is less than 2.13 Å and the nearest adsorbates on the 0.44 ML model are 3.61 Å apart from each other. Further analysis indicates that the weakened binding strength is mainly caused by the electronic effect. A perfect linear relationship has been observed between the d-band center of the Pd atoms, which will host the next adsorbate, and its corresponding differential binding energy, as shown in Figure 4. Therefore, the change of adsorption energy with the coverage of adsorbates is mainly due to the downshift of the d-band center caused by the preadsorbed molecule, making the adsorption of the next adsorbate weaker. 3.3. Competing Adsorption of H, C2H2, and C2H4. Under reaction conditions, both acetylene and ethylene and other surface intermediates coadsorbed over catalyst surfaces. To identify the cross-interactions between different adsorbates, we first set up the high-coverage model, which accounts for the competing adsorption of C2H2 and C2H4 on the Pd(111) surface, as shown in Figure 3a by dashed lines. The dashed blue line shows the trend of the differential adsorption energy of C2H4 with coadsorbed C2H2 or vice versa against the coverage of C2H4. It almost coincides with the solid blue line as well as the case of the red dashed line, which suggests that the crossinteraction between C2H2 and C2H4 is nearly the same as the C2Hx (x = 2 or 4) self-interaction. This result is not surprising because the repulsive interaction mainly induced by the downshift of the d-band center via similar C−Pd interactions because of the existence of the neighboring adsorbates, either C2H4 or C2H2. Therefore, it is safe for us to employ the interaction models built via single-type adsorbates to describe the case of multiadsorbate systems, even for the environmental coverages, which are not covered by our explicit DFT calculations. On the basis of the relationship obtained in section 3.2, we can use the Langmuir adsorption model to estimate the coverage of acetylene and ethylene on Pd(111) and Pd3Sn(111) surfaces before any reaction occurs. First, we consider the

Figure 7. (a) Gibbs free energy profiles of acetylene hydrogenation to ethane on Pd(111) and Pd3Sn(111) with high-coverage models and PdSn(010). (b) Reaction pathways on Pd(111) and Pd3Sn(111) with a low-coverage model. (c) Comparison of the reaction barrier of first acetylene hydrogenation between high-coverage (0.35 ML C2H2 and 0.07 ML H on Pd(111) and 0.19 ML C2H2 and 0.13 ML H on Pd3Sn(111)) and low-coverage issue.

agreement might also be due to the accidental error cancellation caused by the pressure difference, DFT errors, and the approximation on the treatment of desorption entropy change. As expected, the differential adsorption free energy becomes less negative with increasing the coverage of adsorbates on Pd(111) and Pd3Sn(111) surfaces, as shown in Figure 3, which

Table 3. Free Energy Barrier (ΔG⧧/eV) and Reaction Free Energy (ΔG/eV) of Hydrogenation Reactions on Surfaces

Pd(111) Pd3Sn(111) PdSn(010)

θhigh θlow θhigh θlow θ

TS1

TS2

TS3

TS4

C2H2 + H

C2H3 + H

C2H4 + H

C2H5 + H

ΔG⧧

ΔG

ΔG⧧

ΔG

ΔG⧧

ΔG

ΔG⧧

ΔG

0.92 1.06 0.88 1.27 0.63

0.25 0.31 −0.18 0.24 −0.96

0.36 1.01 0.46 0.77 0.53

−0.67 −0.12 −0.71 −0.40 −0.95

0.77 0.98 0.75 1.09 0.85

−0.05 0.43 −0.01 0.44 0.07

0.77 0.90 0.64 0.74 0.32

−0.91 −0.49 −1.29 −0.83 −1.74

6009


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coadsorbed H, that is, 0.07 ML, to identify whether the coadsorbed H would influence the coverage effect on C2H2 adsorption (coverage of C2H4 is too low to be calculated). As illustrated in Figure 5c, a linear relationship between adsorption energy and coverage (for the range larger than 0.19 ML) of C2H2 or C2H4 is established, and the function is shown in section S3 in the Supporting Information. The coverages of H, C2H2, and C2H4 based on multi-adsorbed species model are 0.07, 0.35, and 0.00 ML, respectively, on Pd(111), as shown in Figure 6. Note that the newly predicted coverage is very close to the input values, and these converged values are used in section 3.4. A similar procedure can be done to obtain the coverage over Pd3Sn(111) (Figure 5b,d). 3.4. Acetylene Hydrogenation Mechanism with Coverage Effect on Pd and PdxSny Alloy Surfaces. It has been suggested that a stronger C binding energy is expected in the case of a higher d-band center.30 After carefully analyzing the origin of coverage effect in section 3.2, the d-band center was affected not only by metal30,61 but also by the coadsorbed species on surfaces. Furthermore, the local environment around the reaction active sites and reactant coverage will have a meaningful impact on the intrinsic hydrogenation kinetics.21 Therefore, the predicted coverage of reactants gained in section 3.3 was applied in the analysis of acetylene hydrogenation reactions. On Pd(111), five C2H2 were preadsorbed as a reaction environment, and two C2H2 and one H were preadsorbed on Pd3Sn(111). Here, we follow the Horiuti−Polanyi mechanism for the acetylene hydrogenation. The reaction begins with the adsorption of acetylene and hydrogen, which subsequently combine to form vinyl. Vinyl reacts with a second adsorbed hydrogen atom to form ethylene. Adsorbed ethylene may either desorb to the gas phase or react further to form ethane. The high-coverage model was constructed by placing a number of coadsorbed CHCH, CH2CH2, and H in the unit cell, which is close to the coverage predicted in section 3.3. Under reaction conditions, other C2Hx species, for example, CCH2, have been reported to coexist during the reaction.62 We tested the influence of coadsorbed CCH2 on the corresponding hydrogenation barrier by replacing all coadsorbed CHCH by CCH2. The obtained CHCH hydrogenation barrier with coadsorbed CCH2 is 0.87 eV, very close to the value (0.92 eV) with coadsorbed CHCH over Pd(111). This is not surprising because the adsorbate−adsorbate interaction was mainly induced by the electronic effect, as discussed in section 3.2, and the binding mode is similar for CHCH and CCH2 over a 3-fold hollow site. Therefore, we did not explicitly include CCH2 in our high-coverage model. For CHCH2, we expect that its coverage should be low because of the following fact: the first hydrogenation step is expected to be the rate-determining step because of its high barrier. Therefore, the second hydrogenation step should be in quasi-equilibrium. Because the second hydrogenation step is strongly exothermic (Figure 7a), the coverage of the reactant (CHCH2*) should be much lower than the coverage of the product (CH2CH2*). Note that CH2CH*2 coverage is already low according to our analysis in section 3.3 (Figure 6). The corresponding energy profiles based on the coverage of reactants on the (111) surfaces of Pd and Pd3Sn and PdSn(010) are shown in Figure 7a, and the energy profiles without coverage effect are shown in Figure 7b. All the reaction free energies and free energy barriers are listed in Table 3. The weak adsorption of acetylene on PdSn(010) induces low

competing adsorption between C2H2 and C2H4. The coverages of C2H2 and C2H4 were calculated according to the following equations: 1 θ = * 1+K ( θ ) p C2H 2 * C2H 2 + K C2H4(θ*)pC2H4 θC2H2 =

θC2H4 =

(3)

K C2H2(θ )pC H * 2 2 1 + K C2H2(θ )pC H + K C2H4(θ )pC H * 2 2 * 2 4

(4)

K C2H4(θ )pC H * 2 4 1 + K C2H2(θ )pC H + K C2H4(θ )pC H * 2 2 * 2 4

(5)

θ* is the free site coverage; θC2H2 and θC2H4 are the coverages of C2H2 and C2H4, respectively, px is the partial pressure of the adsorbate x (x = C2H2 and C2H4), and Kx(θ*) is the coveragedependent equilibrium constant of x (x = C2H2 and C2H4) adsorption, which can be related to the differential adsorption free energies of adsorbates: ⎡ Gads,C H (θ ) ⎤ 2 2 * ⎥ K C2H2(θ ) = exp⎢ − * ⎢⎣ ⎥⎦ RT

(6)

⎡ Gads,C H (θ ) ⎤ 2 4 * ⎥ K C2H4(θ ) = exp⎢ − * ⎢⎣ ⎥⎦ RT

(7)

where R is the ideal gas constant and T is the reaction temperature. Gads,C2H2(θ*) and Gads,C2H4(θ*) are the differential adsorption free energies of C2H2 and C2H4, respectively, against free site coverage, which were fitted by the values shown in Figure 3. The corresponding fitted functions of Gads,C2H2(θ*) and Gads,C2H4(θ*) of Pd(111) and Pd3Sn(111) surfaces are shown in section S3 in the Supporting Information. By applying the coverage-dependent adsorption energy (see details in Supporting Information section S3), we obtained the coverage of C2H2 (0.43 ML) and C2H4 (0.00 ML) on Pd(111) and C2H2 (0.20 ML) and C2H4 (0.07 ML) on Pd3Sn(111). Because the reactants of acetylene hydrogenation contain H, we should also consider the coverage of H on each surface. An iterative strategy has been proposed in this study to calculate the coverages of C2H2, C2H4, and H, by constructing a highcoverage model of H or C2Hx with coexistence of fixed C2Hx or H coverage (the extrinsic coverage gained by last iteration). The detail of the iterative method is shown in Scheme 1. Our iterative method starts from the results of coverages of C2H2 and C2H4 on the (111) surfaces of Pd and Pd3Sn because of their strong binding nature over Pd and Pd3Sn. On Pd(111), we first carried out the adsorption of six molecular C2H2 (0.375 ML) on the surface as the reference of hydrogen adsorption (reference models are shown in Figure 5a). As shown in Figure 5a, we obtain the H adsorption energy as a function of its coverage, with six coadsorbed C2H2 (see section S3 in the Supporting Information). Combining the above relationship of C2H2 and C2H4, the predicted coverages of H, C2H2, and C2H4 are 0.07, 0.37, and 0.00 ML on Pd(111). Recall that the coverage-dependent adsorption energy was built under conditions of θH = 0 ML, θC2H2 = 0.43 ML, and θC2H4 = 0.00 ML, which are different from the final conditions. Because a non-negligible H coverage has been predicted, in the next step, the interaction models were developed with 6010



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surface coverage, and our (4 × 4) unit cell, corresponding to 1/ 16 ML coverage, is suitable to describe it. Comparing the reaction barrier of each step in acetylene hydrogenation at high coverage and low coverage, hydrogenation reaction becomes much easier with weakened adsorption of reactants at high coverage. Note that kinetic Monte Carlo simulations have also found the decrease of hydrogenation barrier with increasing coverage.21 Previous calculations indicated that alloying late or posttransition metals, for example, PtSn,42 PtGa,63 PdAg,38 and PdIn,41 in general, lowers the d-band center of Pd or Pt and thus suppresses its hydrogenation/dehydrogenation activity. Our calculation at a low coverage is consistent with previous findings (Figure 7b).64 However, this is not the case in our high-coverage calculations. The barrier of the rate-determining step on Pd(111) at a high coverage was 0.92 eV, only 0.14 eV lower than the value calculated at a low coverage. However, the high-coverage barrier on Pd3Sn(111) is only 0.88 eV, much lower than the low-coverage case (1.27 eV). Clearly, at a high coverage, the first hydrogenation barrier becomes comparable on both Pd(111) and Pd3Sn(111). Because higher reaction barrier reflects lower rate constants, the qualitative activity trends in our work considering the coverage effect show slightly increasing activity with Sn addition, consistent with the experimental observations.45 According to Figure 7b,c, lowcoverage calculations, which have been applied for most of the previous DFT studies,38,65 could not capture this important feature because of the relatively high hydrogenation barrier over Pd3Sn(111). Our high-coverage model also predicts similar barriers for the hydrogenation of ethylene to ethane on Pd(111) and Pd3Sn(111) (Figure S3 in the Supporting Information).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11394. Surface energy data; adsorption properties of acetylene and ethylene on each surface; fitted coverage-dependent adsorption energies; high-coverage data; coverage effect on Pd(100) and Pd3Sn(100); additional discussion of reaction barrier on each surface; and coverage effect on Pd(100) and Pd3Sn(100) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.-J.Z.). *E-mail: [email protected] (J.G.). ORCID

Zhi-Jian Zhao: 0000-0002-8856-5078 Jinlong Gong: 0000-0001-7263-318X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of China (nos. 21506149, 21525626, and 21676181) and the Program of Introducing Talents of Discipline to Universities (no. B06006).

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4. CONCLUSIONS First-principles calculations were used to understand the effects of alloying Pd with Sn on the binding energies of various C2Hx intermediates. Three models with different PdSn mole ratios are constructed to represent PdSn alloy surfaces. The addition of Sn to Pd weakens the binding energies of all C2 intermediates studied herein, which is due to both electronic and geometric effects. Through the electronic structure calculation, it has been found that the d-band center of Pd downshifts with alloying of Pd with Sn. According to the geometric effects, Pd atoms were separated by Sn, which forces the adsorbates to interact with less Pd atoms. Therefore, the binding energies become progressively weaker with the increase of Sn composition. By investigating the coverage effect on adsorption energy, we found that the adsorption energy decreases with the increase of coverage. While analyzing the origin of coverage effect, geometric effect is found to be minimum. The electronic effect is mainly caused by the downshift of the d-band center with the increase of coverage. On the basis of the function of adsorption energy of reactants, we further obtained the surface coverages and reaction barriers of the hydrogenation reaction calculated in high coverage of reactants, which was found to be lower than that in low coverage. Moreover, the rate-limiting step becomes more feasible on Pd3Sn(111) than Pd(111) taking the coverage effect into consideration, which is opposite to the results in which the coverage effect was not considered. Our calculations suggest that a high-coverage model is essential to obtain a more accurate description of the reaction kinetics, when strong binding species exist over catalyst surfaces. 6011


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Coverage Effect on the Activity of the Acetylene Semihydrogenation

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