Coverage Effect on the Activity of the Acetylene Semihydrogenation

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Coverage Effect on the Activity of the Acetylene Semi-Hydrogenation over Pd-Sn Catalysts: A Density Functional Theory Study Jiubing Zhao, Shenjun Zha, Rentao Mu, Zhi-Jian Zhao, and Jinlong Gong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11394 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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The Journal of Physical Chemistry

Coverage Effect on the Activity of the Acetylene Semi-Hydrogenation over Pd-Sn Catalysts: A Density Functional Theory Study Jiubing Zhao, Shenjun Zha, Rentao Mu, Zhi-Jian Zhao*, 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 Abstract The existence of acetylene impurity in ethylene feedstock is harmful to the downstream polymerization reactions. The removal of acetylene can be achieved via semi-hydrogenation reaction, which normally catalyzed by Pd-based catalyst. 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 the 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 adsorption weaker along with the increasing 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 model indicate alloying Pd with late or post transition metals in general enhances ethylene selectivity, accompanied with lower hydrogenation activity. However, by applying the high coverage model, we show that the predicted hydrogenation barriers are comparable over Pd(111) and Pd3Sn(111) surfaces.

* Email: [email protected] and [email protected] 1

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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 the acetylene impurity content in the ethylene feedstock below 5 ppm by volume in order to prevent catalysts poisoning and ensure the purity of the produced polyethylene.3 However, ethylene feedstock produced by cracking can contain more than 2 vol % acetylene, thus 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 in order 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 the hydrogenation occurs via a Horiuti-Polanyi mechanism.8-10 Namely, H2 is dissociated on Pd,11 and the hydrogen adatom then diffuses towards 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 in to account. It has been reported that there were two major issues in this process:6, 13-17 over-hydrogenation 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 single-site heterogeneous catalysts.18, 19 For example, the Lindlar catalyst is widely employed in industry for hydrogenation reactions, where lead or sulphur was used to tune hydrogenation ability of Pd. Currently, the most commonly used industrial catalyst for acetylene semi-hydrogenation is based on Ag modified Pd catalyst.20, 21 Other Pd-alloy catalysts such as PdCu,22 PdGa,23 and PdNi24 have also emerged the improved catalytic properties in acetylene semi-hydrogenation compared with pure Pd. In most cases, these alloying components are late and post transition metals, which shifts down the d-band center of the Pd in the alloy, thus, enhances the desorption of desired product ethylene and prevents its over hydrogenation.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, 2


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Nørskov and co-workers screened about 70 different bimetallic compounds for acetylene semi-hydrogenation reactions.28 The optimal catalyst suggested in this study considered to be located within a window with 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 due to the poor activation of the reactants, however, the product ethylene is easily desorbed to increase selectivity. As we know, too strong adsorption is easy to activate the reactants accompanying with high activity but too strong adsorption induces high property of side reaction, e.g. over hydrogenation of ethylene, leading to low selectivity. Indeed previous calculations have indicated that Pd-alloy catalysts, e.g. PdAg,38 PdFe,39 PdGa,40 and PdIn,41usually 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 semi-hydrogenation reaction.45,

46

In

order 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 high coverage environment is a key factor to correctly describe relative activity over calculated PdSn alloy surfaces. Herein, our study shed lights on how the coverage effect influences the reaction kinetics. 2. Models and computational details Periodic DFT calculations were carried out with the Vienna Ab-initio Simulation Package (VASP).47 The calculations employed the generalized-gradient approximation (GGA) in the form of the Bayesian error estimation functional with van der Waals corrections (BEEF-vdW).48, 49 The interaction between the atomic cores and electrons were described by the projector augmented wave (PAW) method.50 The valence wave functions were expanded by plane-wave with a cutoff energy of 400eV. A Methfessel-Paxton smearing with 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 1:1 ratio, we calculated its surface energies and 3



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find that (010) is the relative stable and ordered surface among other low miller index surfaces (See Table S1 in SI (Supporting Information)). Meanwhile, the segregation energies of PdSn catalysts were calculated (Table S2) and no segregation case were found from PdSn catalysts. The thickness of the employed slabs are four layers (except for 5 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. The k-points mesh of 3×3×1 was used in a 4×4 unit cell of each model slab. Entropic contributions and zero point energies (ZPE) were taken into account, which converts DFT calculated energy into free energy.52 The entropy of adsorbate was calculated according the Campbell’s method with the following equation:53

Sads = 0.7Sgas − 3.3R

(1)

where Sads is the entropy of 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 co-adsorbates on the surface,55 which has been partially considered in our work. In order to investigate the coverage effect on adsorption, differential adsorption energies of reactants C2H2, C2H4 and H are calculated at coverage of 1/16 ML, 1/8 ML, 3/16 ML, 1/4 ML, 5/16 ML, 3/8 ML, 7/16 ML, and 1/2 ML at adsorption sites, which are performed on (4 ×4) model slab. The differential adsorption free energy of acetylene, ethylene and hydrogen, which is defined as

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, and Ggas is the Gibbs free energy of adsorbate in 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 the kinetic and thermodynamic analysis are carried out at 373K with the 1 atm, 0.1 atm and 8.9 atm partial pressures for H2, C2H2 and C2H4, respectively, which are close to the experimental conditions.23

4


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3. Results and discussion 3.1. Adsorption of acetylene and ethylene on Pd/Sn surfaces at 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 adsorption energies of C2H2 and C2H4 on all the sites are listed in Table S2. Under UHV (Ultra-High Vacuum) 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 1/16 and 1/4 ML coverage, the calculated binding energy is -0.81 eV and -0.74 eV, respectively. Previous TPD experiment measured the adsorption energy of ethylene under UHV conditions to be -0.61eV on Pd(111) at 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 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 adsorption of adsorbates. Indeed, linear correlation between d-band center and adsorption energy has been observed (Figure S1 in SI) when the acetylene binds with same π adsorption mode on these terrace surface. Moreover, the surface existing Sn atoms divided the complete Pd surface into small ensembles or isolated Pd atoms, further weakening the adsorbate binding strength 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 energy of C2H2 and C2H4 is very strong according to the low coverage model. If we simply apply Langmuir adsorption model with constant adsorption energy predicted by low coverage model, i.e. over 1 eV of acetylene over Pd surfaces, all active site 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 low energy electron diffraction (LEED) 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 were shown in Figure 3(a)-(b) by solid lines. The differential adsorption free energy is observed to be 5



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positively correlated with 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 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 results of 0.25 ML.60 Note that the pressure used in our calculation is much higher than the surface science studies performed in Ref 59, 60. Therefore, the agreement might also due to the accidently error cancellation caused by the pressure difference, DFT errors as well as 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 shown in Figure 3, which might be caused by the following two factors: a) geometric effect due to the increased steric hindrance; 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 adsorbate, single point calculations were done for the adsorbates only, i.e. without 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 is no interactions for two molecular C2H2 with each other until distance between them are less than 2.13 Å and the nearest adsorbates on 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 downshift the d-band center caused by the preadsorbed molecule, making the adsorption of next adsorbate weaker. 3.3 Competing adsorption of H, C2H2 and C2H4 Under reaction conditions, both acetylene, ethylene and other surface intermediates co-adsorbed over catalyst surfaces. In order to identify the cross-interactions between 6


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different adsorbates, we first set up the high coverage model which account for the competing adsorption of C2H2 and C2H4 on Pd(111) surface, as shown in Figure 3(a) by dashed lines. The dashed blue line shows the trend of the differential adsorption energy of C2H4 with co-adsorbed C2H2 or vice versa against coverage of C2H4. It almost coincides with the solid blue line, as well as the case of red dashed line, which suggests the cross-interaction between C2H2 and C2H4 is nearly the same as the C2Hx (x = 2 or 4) self-interaction. This result is not surprising since the repulsive interaction mainly induced by the downshift of d band center via similar C-Pd interactions, due to 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 adsorbate to describe the case of multi-adsorbate systems, even for the environmental coverages which are not covered by our explicitly DFT calculations. Based on the relationship obtained in Section 3.2, we can use 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 competing adsorption between C2H2 and C2H4. The coverage of C2H2 and C2H4 were calculated according to the following equation:

θ* =

1 1 + KC2 H 2 (θ* ) pC2 H 2 + K C2 H 4 (θ* ) pC2 H 4

θC H = 2

2

θC H = 2

4

K C 2 H 2 (θ * ) pC 2 H 2 1 + K C 2 H 2 (θ * ) pC 2 H 2 + K C 2 H 4 (θ * ) pC 2 H 4 K C 2 H 4 (θ * ) pC 2 H 4 1 + K C 2 H 2 (θ * ) pC 2 H 2 + K C 2 H 4 (θ * ) pC 2 H 4

(3)

(4)

(5)

θ* is the free site coverage; θ C H and θ C H are the coverage of C2H2 and C2H4, px is the 2

2

2

4

partial pressure of the adsorbate x (x = C2H2 and C2H4), K x (θ* ) is the coverage dependent equilibrium constant of x (x = C2H2 and C2H4) adsorption, which can be related to the differential adsorption free energies of adsorbates:  G ads ,C 2 H 2 (θ * )  K C2 H 2 (θ * ) = exp  −  RT  

(6)

 G ads ,C 2 H 4 (θ * )  K C2 H 4 (θ * ) = exp  −  RT  

(7)

where R is the ideal gas constant and T is the reaction temperature. Gads ,C2 H 2 (θ* ) and Gads ,C2 H 4 (θ* ) are the differential adsorption free energies of C2H2 and C2H4 against free site 7



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coverage, which were fitted by the values shown in Figure 3. The corresponding fitted function of Gads ,C2 H 2 (θ* ) and Gads ,C2 H 4 (θ* ) of Pd(111) and Pd3Sn(111) surfaces are shown in Section S3 in SI. By applying the coverage dependent adsorption energy (see details in SI 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 coverage of C2H2, C2H4 and H, by constructing high coverage model of H or C2Hx with coexistence of fixed C2Hx or H coverage (the extrinsic coverage gained by last iteration). The detail of iterative method is shown in Scheme 1. Our iterative method starts from the results of coverage of C2H2 and C2H4 on (111) surfaces of Pd and Pd3Sn, due to their strong binding nature over Pd and Pd3Sn. On Pd(111), we first adsorbed 6 molecular C2H2 (0.375 ML) on the surface as the reference of hydrogen adsorption (reference models are shown in Figure 5 (a)). As shown in Figure 5(a), we obtain the H adsorption energy as a function of its coverage, with 6 co-adsorbed C2H2 (see Section S3 in SI). Combining the above relationship of C2H2 and C2H4, the predicted coverage of H, C2H2 and C2H4 are 0.07 ML, 0.37 ML 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 is different from the final conditions. Because a non-negligible H coverage has been predicted, in the next step, the interaction models were developed with co-adsorbed H, i.e. 0.07 ML, to identify whether the co-adsorbed H would influence the coverage effect on C2H2 adsorption (coverage of C2H4 is too low to be calculated). As illustrated in Figure 5(c), a linear relationship between adsorption energy and coverage (for the range larger than 0.19ML) of C2H2 or C2H4 is established, and the function is shown in Section S3 in SI. The coverage of H, C2H2 and C2H4 based on multi-adsorbed species model are 0.07 ML, 0.35 ML and 0.00 ML 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. Similar procedure can be done to obtain the coverage over Pd3Sn(111) (Figure 5(b), (d)). 3.4. Acetylene hydrogenation mechanism with coverage effect on Pd and PdxSny alloy Surfaces It has been suggested that stronger C binding energy is expected in case of a higher 8


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d-band center.30 After carefully analyzing the origin of coverage effect in Section 3.2, d-band center was not only affected by metal,30, 61 but also the co-adsorbed species on surfaces. Furthermore, the local environment around the reaction active sites and reactant coverage will have a meaningfully 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 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, and subsequently combines 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 co-adsorbed CHCH, CH2CH2 and H in the unitcell which is close to the coverage predicted in Section 3.3. Under reaction conditions, other C2Hx species, e.g. CCH2, have been reported to be co-exist during the reaction.62 We tested the influence of co-adsorbed CCH2 on the corresponding hydrogenation barrier by replacing all the co-adsorbed CHCH by CCH2. The obtained CHCH hydrogenation barrier with co-adsorbed CCH2 is 0.87 eV, very close to the value (0.92 eV) with co-adsorbed 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 3-fold hollow site. Therefore, we did not explicitly include CCH2 in our high coverage model. For CHCH2, we expect its coverage should be low, due to the following facts: the first hydrogenation step is expected to be rate-determining step because its high barrier. Therefore, the second hydrogenation step should be in quasi-equilibrium. Since 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 CH2CH2* coverage is already low according to our analysis in Section 3.3 (Figure 6). The corresponding energy profiles basing on the coverage of reactants on (111) surfaces of Pd and Pd3Sn and PdSn(010) are shown in Figure 7(a) and the energy profiles without coverage effect was shown in Figure 7(b). The weak adsorption of acetylene on PdSn(010) induces low 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 steps 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 9



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Carlo simulations have also found the decrease of hydrogenation barrier with coverage increasing.21 Previous calculations indicated that alloying late or post transition metals, e.g. PtSn,42 PtGa,63 PdAg,38 PdIn,41 in general lower the d-band center of Pd or Pt, thus suppresses its hydrogenation/dehydrogenation activity. Our calculation at low coverage consistent with previous finds (Figure 7(b)).64 However, this is not the case in our high coverage calculations. The barrier of rate-determining step on Pd(111) at high coverage was 0.92 eV, only 0.14 eV lower than value calculated at low coverage. But that high coverage barrier on Pd3Sn(111) is only 0.88 eV, much lower than the low coverage case (1.27 eV). Clearly, at 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 coverage effect show slightly increasing activity with Sn addition, consistent with the experimental observations.45 According to Figure 7(b,c), low coverage calculations, which have been applied for most of the previous DFT studies,38, 65 could not capture this important feature, due to 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 SI). 4 Conclusion First-principle 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 the C2 intermediates studied herein, which is due to both electronic and geometric effects. Through the electronic structure calculation, it has been found that 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 adsorbate to interact with less Pd atoms. Therefore, the binding energies becomes progressively weaker with the increase of Sn composition. By investigating the coverage effect on adsorption energy, we found 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 d-band center with the increase of coverage. Based on 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 10


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found to be lower than that in low coverage. Moreover, the rate-limiting step becomes more feasible on Pd3Sn(111) than Pd(111) taking coverage effect into consideration, which is opposite to the results that coverage effect was not considered. Our calculations suggest that high coverage model is essential to obtain a more accurate description of the reaction kinetics, when strong binding species exist over catalyst surfaces.

Supporting Information Surface energy date; High coverage date; Coverage effect on Pd(100) and Pd3Sn(100).

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

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Page 18 of 28

List of Table and Figure Captions Table 1. Adsorption energies (Eads/eV) and Gibbs free adsorption energies (Gads/eV) of C2H2 and C2H4 on surfaces. Table 2. Single point energy of gas phase acetylene on (111) surface of Pd and Pd3Sn and the ratio of the single point energy of N molecular acetylene to one molecular acetylene. Table 3. Free energy barrier (△G‡/eV) and reaction free energy (△G/eV) of hydrogenation reactions on surfaces. Scheme1. Iterative method to identify coverage of C2H2, C2H4 and H. 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. Figure 2. Top and side views of C2H2 and C2H4 adsorption geometries on Pd(111), Pd3Sn(111) and PdSn(010) model surfaces. Figure 3. (a) and (b) show the trend of the differential adsorption free energies of C2H2 and C2H4 changes 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 co-adsorbing on Pd(111) surface is illustrated by the dashed red line in (a). The relationship between the coverage and differential adsorption free energy of C2H4 with xC2H2 co-adsorbing on Pd(111) surface is shown by the dashed blue line. 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. Figure 5. (a) and (b) show the the trend of the differential adsorption free energies of H changes with the coverage on Pd(111) (6 C2H2 co-adsorbed) and Pd3Sn(111) (3 C2H2 and 1 C2H4 co-adsorbed). (c) and (d) show the trend of the differential adsorption free energies of C2H2 and C2H4 changes with the coverage that on Pd(111) (2 H co-adsorbed) and Pd3Sn(111) (3 H co-adsorbed) surfaces. Figure 6. The coverage of reactants on (111) surface of Pd and Pd3Sn. Figure 7. (a) Gibbs free energy profiles of acetylene hydrogenation to ethane on Pd(111), Pd3Sn(111) with high coverage models and PdSn(010). (b) Reaction pathways on Pd(111) and

Pd3Sn(111) with 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.

18


Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Adsorption energies (Eads/eV) and Gibbs free adsorption energies (Gads/eV) of C2H2 and C2H4 on surfaces. Pd(111)

Pd3Sn(111)

PdSn(010)

Eads

Gads

Eads

Gads

Eads

Gads

C2H2

-1.83

-1.42

-1.08

-0.68

-0.18

0.19

C2H4

-0.81

-0.40

-0.47

-0.05

-0.27

0.15

Table 2. Single point energy of gas phase acetylene on (111) surface of Pd and Pd3Sn and the ratio of the single point energy of N molecular acetylene to one molecular acetylene. Pd(111)

Pd3Sn(111)

Coverage / ML

Esingle / eV

1/16

-18.43

2/16

-36.88

3/16

Ratio

Coverage / ML

Esingle / eV

Ratio

1/16

-18.93

2.00

2/16

-37.76

2.00

-54.74

2.97

3/16

-56.60

2.99

4/16

-72.96

3.96

4/16

-75.42

3.98

5/16

-91.93

4.99

5/16

-94.21

4.98

6/16

-109.99

5.97

6/16

-113.83

6.01

Table 3. Free energy barrier (△G‡/eV) and reaction free energy (△G/eV) of hydrogenation reactions on surfaces.

Pd(111)

Pd3Sn(111)

PdSn(010)

TS1

TS2

TS3

TS4

C2H2+H

C2H3+H

C2H4+H

C2H5+H

‡

△G

△G

△G

θhigh

0.92

0.25

θlow

1.06

θhigh

‡

‡

△G

△G

△G

△G‡

△G

0.36

-0.67

0.77

-0.05

0.77

-0.91

0.31

1.01

-0.12

0.98

0.43

0.90

-0.49

0.88

-0.18

0.46

-0.71

0.75

-0.01

0.64

-1.29

θlow

1.27

0.24

0.77

-0.40

1.09

0.44

0.74

-0.83

θ

0.63

-0.96

0.53

-0.95

0.85

0.07

0.32

-1.74 19



Page 20 of 28

Scheme1. Iterative method to identify coverage of C2H2, C2H4 and H.

20


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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.

21



Page 22 of 28

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

22


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Figure 3. (a) and (b) show the trend of the differential adsorption free energies of C2H2 and C2H4 changes 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 co-adsorbing on Pd(111) surface is illustrated by the dashed red line in (a). The relationship between the coverage and differential adsorption free energy of C2H4 with xC2H2 co-adsorbing on Pd(111) surface is shown by the dashed blue line.

23



Page 24 of 28

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.

24


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Figure 5. (a) and (b) show the the trend of the differential adsorption free energies of H changes with the coverage on Pd(111) (6 C2H2 co-adsorbed) and Pd3Sn(111) (3 C2H2 and 1 C2H4 co-adsorbed). (c) and (d) show the trend of the differential adsorption free energies of C2H2 and C2H4 changes with the coverage that on Pd(111) (2 H co-adsorbed) and Pd3Sn(111) (3 H co-adsorbed) surfaces.

25



Page 26 of 28

Figure 6. The coverage of reactants on (111) surface of Pd and Pd3Sn.

26


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Figure 7. (a) Gibbs free energy profiles of acetylene hydrogenation to ethane on Pd(111), Pd3Sn(111) with high coverage models and PdSn(010). (b) Reaction pathways on Pd(111) and

Pd3Sn(111) with 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.

27



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TOC Graphic

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Coverage Effect on the Activity of the Acetylene Semihydrogenation

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