Selective Hydrogenation of Acetylene over PdâBoron Catalysts: A

Article pubs.acs.org/JPCC

Selective Hydrogenation of Acetylene over Pd−Boron Catalysts: A Density Functional Theory Study Bo Yang,† Robbie Burch,† Christopher Hardacre,*,† P. Hu,*,† and Philip Hughes‡ †

CenTACat, School of Chemistry & Chemical Engineering, The Queen’s University of Belfast, Belfast BT9 5AG, United Kingdom Johnson Matthey Catalysts, P.O. Box 1, Billingham, Teesside TS23 1LB, United Kingdom

‡

ABSTRACT: Boron-modified Pd catalysts have shown excellent performance for the selective hydrogenation of alkynes experimentally. In the current work, we investigated the hydrogenation of acetylene on boron-modified Pd(111) and Pd(211) surfaces, utilizing density functional theory calculations. The activity of acetylene hydrogenation has been studied by estimating the effective barrier of the whole process. The selectivity of ethylene formation is investigated from a comparison between the desorption and the hydrogenation of ethylene as well as comparison between the ethylene and the 1,3-butadiene formation. Formation of subsurface carbon and hydrogen on both boron-modified Pd(111) and Pd(211) surfaces has also been evaluated, since these have been reported to affect both the activity and the selectivity of acetylene hydrogenation to produce ethylene on Pd surfaces. Our results provide some important insights into the Pd−B catalysts for selective hydrogenation of acetylene and also for more complex hydrogenation systems, such as stereoselective hydrogenation of longer chain alkynes and selective hydrogenation of vegetable oil. systems.1 These considerations account for the fact that Pd is utilized as the dominant catalyst for selective removal of acetylene industrially. However, Pd alone is not selective enough for ethylene production with over hydrogenation to ethane being a major drawback. Our recent results have shown that the low coordination sites in Pd are less selective for production of ethylene, leading to over hydrogenation.6 Hence, Pd−Ag catalysts were developed and are currently widely used.7−16 Our previous theoretical studies have also revealed that the main role of Ag in the improvement of pure Pd catalyst is by both blocking the low coordination sites and also modifying the flat surfaces. This results in an enhancement in the hydrogenation selectivity of ethylene at both the step sites and the close-packed faces.6 A series of intermetallic Pd−Ga catalysts were developed recently by Schlögl and co-workers. It was found that, compared with Pd/Al 2O3 and Pd20Ag80 catalysts, Pd−Ga catalysts exhibited a similar activity per surface area but higher selectivity and stability.17−19 Therein, it was suggested that the superior catalytic properties of Pd-Ga catalysts could be attributed to isolation of active Pd sites in the crystallographic structure of Pd−Ga. The presence of subsurface carbon and hydrogen atoms has also been reported to influence the activity and selectivity of acetylene hydrogenation on Pd surfaces to produce ethylene.6,20−25 The presence of subsurface hydrogen atoms was

1. INTRODUCTION In the olefin industry, selective removal of acetylene from ethylene feeds is crucial, since the acetylene acts as a poisoning species for the downstream catalyst used for polymerization of ethylene.1,2 The most common method used to remove the alkyne is to selectively hydrogenate acetylene to ethylene, which reduces the amount of acetylene and increases production of ethylene. However, both acetylene and ethylene are unsaturated hydrocarbons that can be hydrogenated to form ethane, and the catalysts employed for hydrogenation need, therefore, to be highly selective. Moreover, as the final concentration of acetylene in the feed for polymerization needs to be as low as a few parts per million, the catalyst used should also be very active. These criteria must be met while also reducing the issue of acetylene polymerization which can occur during the selective hydrogenation process, leading to formation of “green oil”, a species which will not desorb from the surface and, therefore, inhibits the reactant diffusion process and reduces the reaction rates. Pd catalysts have been identified to be both selective and active to hydrogenate acetylene in ethylene feed. In addition, while other metals including Ni, Cu, Ag, and Au have also been found to be highly selective, they are all less active than Pd.1,3,4 Our recent work showed that the low activity of Ni toward acetylene hydrogenation is due to the strong adsorption of the reactants, while it is the weak adsorption of the reactants on Au that leads to low hydrogenation activity.5 Pt and Ir can also show high activity for hydrogenation of acetylene, but the selectivity to the desired product, ethylene, is very low in these © 2014 American Chemical Society

Received: December 15, 2013 Revised: January 18, 2014 Published: January 21, 2014 3664

dx.doi.org/10.1021/jp412255a | J. Phys. Chem. C 2014, 118, 3664−3671

The Journal of Physical Chemistry C

Article

where Etotal is the energy of the system after adsorption, Eg is the energy of the gas-phase adsorbent, and Eslab is the energy of slab.

found to be more active than the surface hydrogen atoms, leading to over hydrogenation of ethylene on Pd hydride phases which are formed under high hydrogen pressures. In the presence of subsurface carbon, it was found that the pathway for subsurface and bulk hydrogen diffusion to the surface sites or the direct hydrogenation route to ethylene will be blocked, resulting in a decrease in the selectivity for ethylene. Nørskov and co-workers26 suggested that the underlying physical principles of subsurface carbon were identical to the effect of alloying Pd with Ag that the adsorption of both acetylene and ethylene would be weakened and the selectivity would be increased. More importantly, the above observations also provide a possible strategy to modify the catalyst performance by introducing some light atoms and, therefore, changing the subsurface chemistry of the catalysts. For example, Tsang and co-workers recently reported that the subsurface site of Pd catalyst can be modified with carbon atoms to obtain high stereoselectivity in the hydrogenation of alkynes to cis-alkenes in the liquid phase.27,28 Moreover, excellent double-bond and stereoselectivity in semihydrogenation of alkynes forming the corresponding cis-alkene products has also been reported by Krawczyk et al. when Pd−B/SiO2 catalyst was employed.29 The effect of subsurface carbon has been a subject of many studies as introduced above, but regarding the effect of boron, one may ask the following questions: (i) What role does the boron play in the selective alkyne hydrogenation system? (ii) What is the difference between the effect of subsurface carbon and boron? In order to answer these questions, we hereby report a density functional theory (DFT) study on the effects of the presence of subsurface boron atoms at both flat and step sites on the activity and selectivity of Pd for acetylene hydrogenation for production of ethylene. Some other important issues in this area, including formation of 1,3-butadiene and subsurface carbon and hydrogen, will also be investigated to obtain some insight into the activity−selectivity−stability relationship on boron-modified Pd surfaces.

3. RESULTS AND DISCUSSION 3.1. Adsorption of Boron on Pd Surfaces. Adsorption of the boron atom at the surface sites (i.e., fcc and hcp sites) and subsurface sites (i.e., tetrahedron and octahedron sites) of Pd(111) and Pd(211) were studied, and the relative adsorption energies are listed in Table 1. Similar to the calculations Table 1. Binding Energies of Boron (Ead,B) on the Surface and Subsurface Sites of Pd(111) and Pd(211) Pd(111) Ead,B/eV

Ead,B/eV

hcp −6.48

tetra −7.66 Pd(211)

octa −7.79

fcc

hcp

4-fold

tetra

octa

−7.79

−6.56

−7.67

−7.79

−7.79

undertaken for adsorption of carbon atoms on Pd(211),6 the corresponding adsorption of boron at the 4-fold (B5) site is also calculated in the current work, and the adsorption energy is also listed in Table 1. The favored adsorption sites of boron at the surface and subsurface of Pd(111) are found to be the hcp and octahedral sites, respectively. This is analogous to the results from carbon adsorption on Pd(111).6 However, the adsorption energies of boron at the fcc, subsurface tetrahedral and octahedral sites are equivalent on the Pd(211) surface and higher than those at the other sites. In fact, it is found that the boron atom can diffuse from the surface fcc sites and subsurface tetrahedral sites to the octahedral sites after optimization at the step of the Pd(211) surface, indicating adsorption of boron at the former two sites is quite unstable. From the above results for the adsorption of boron atoms on the Pd(111) and Pd(211) surfaces, one can see that the occupation of the octahedral sites will be preferred in both cases. Furthermore, the adsorption energies of boron with respect to the energy of (B2H6 − 3H2)/2 at these sites over both the Pd(111) and the Pd(211) surfaces are −0.92 eV, indicating that these boron atoms are quite stable on these sites and it is likely that it will not be removed from the surface in the form of borane during hydrogenation. It has been reported experimentally that the equilibrated surface coverage of boron on Pd is ∼0.3 monolayer (ML).40 Therefore, Pd(111) and Pd(211) surfaces with subsurface boron coverages of 0.25 and 0.33 ML, respectively, were used as model catalysts to investigate the hydrogenation of acetylene as a function of the structure of the boron-modified Pd surface. The configurations of these structures are shown in Figure 1, in which all boron atoms are present in the most stable subsurface sites, i.e., the octahedral sites, on both surfaces. These surfaces are defined as Pd(111)−B and Pd(211)−B, respectively. 3.2. Acetylene Hydrogenation on Pd−Boron Surfaces. 3.2.1. Activity of Ethylene formation. Flat Surface. The adsorption energies of C2H2, C2H3, and C2H4 on Pd(111)−B are listed in Table 2. The adsorption geometries of these C2 species on Pd(111)−B are represented in Figures 2 and 3 showing a high degree of similarity as found on the clean Pd(111) surface.6,41 C2H2 and C2H3 are found to adsorb at the hollow sites, and C2H4 adsorbs in a 2 − σ mode. However,

2. COMPUTATIONAL DETAILS The density functional calculations shown in this work were performed with the Vienna Ab-initio Simulation Package (VASP) in slab models.30−33 The exchange-correlation functional PW91 was used to calculate the electronic structure with the generalized gradient approximation (GGA). 34 The projector-augmented wave (PAW) method was employed to describe the interaction between atomic cores and electrons.35,36 For boron-modified Pd(111) surfaces, four layer 4 × 4 slabs with the upmost two Pd layers relaxed during optimization were used to model the adsorption and reaction processes. A 2 × 2 × 1 k-point sampling in the surface Brillouin zone was used for flat surfaces. For boron-modified Pd(211) surfaces, 12-layer slabs with 1 × 4 surface supercells were employed with a 4 × 2 × 1 k-point grid and the upmost 6 layers were relaxed with the surface adsorbates as well as the doped boron species. The vacuum was set to be more than 12 Å to make sure those processes take place on one side of the slabs in all of the models. An energy cutoff of 500 eV and the converge criteria of the force on each relaxed atoms below 0.05 eV/ Å were used in this work. Transition states were located with a constrained minimization method.37−39 Adsorption energies were defined as follows Ead = Etotal − (Eg + Eslab)

fcc −6.40

(1) 3665



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Figure 1. Illustration of the boron-modified Pd(111) (a and b) and Pd(211) (c and d) surfaces, which were defined as Pd(111)−B and Pd(211)−B in the text. Blue and red balls are palladium and boron atoms, respectively, and this notation is used throughout the paper.

Table 2. Adsorption Energies (eV) of C2H2, C2H3, C2H4, and C4H6 on Boron-Modified Pd(111) and Pd(211) Surfaces Pd(111) −B Pd(211) −B

C2H2

C2H3

C2H4

C4H6

−1.54 −1.68

−2.21 −2.46

−0.68 −0.73

−1.10 /

Figure 3. Energy profile of C2H2 hydrogenation on Pd(111)−B surface. (g) and (ad) stands for the gaseous and adsorption states, respectively, of the species investigated. Configurations of TS1, MS, and TS2 are also shown.

barrier of 1.01 eV for C2H2 hydrogenation on this surface. This was obtained using the two-step model for the effective barrier estimation reported by our group recently for acetylene hydrogenations.6 Stepped Surface. Two adsorption sites of C2H2 on Pd(211)−B were studied, namely, the 4-fold sites beneath the step edges (B5 sites) and the hollow sites adjacent to the step edge (fcc and hcp sites). It is found that the 4-fold site is favored with an adsorption energy of −1.68 eV compared with −1.23 and −1.02 eV for adsorption at the fcc and hcp hollow sites, respectively. It should be noted that this adsorption (−1.68 eV) is much weaker than at the same sites of unmodified Pd(211) surface (−2.26 eV), although in both cases the adsorption geometries are the same. The same trend is found for adsorption of C2H4 on the Pd(211)−B and Pd(211) surfaces, i.e., the adsorption of C2H4 on Pd(211)−B (−0.73 eV) being much weaker than that on Pd(211) (−1.17 eV). This coupled with the trend of the adsorption energies on Pd(111)−B can be attributed to the shift of the projected density of d states away from the Fermi level of the surface Pd atoms on addition of B, as shown in Figure 5.43 The energy profile of C2H2 hydrogenation to form C2H4 on Pd(211)−B is shown in Figure 4, with the transition state structures of C2H2 and C2H3 hydrogenation. The hydrogenation barriers of C2H2 and C2H3 were calculated to be 1.16 and 0.47 eV, respectively. As found for the Pd(111) surface, the rate-limiting step was also found to be the first hydrogenation step, and the effective barrier was estimated to be 1.16 eV. In our previous work, we investigated the hydrogenation of C2H2 on the Pd(111) and Pd(211) surfaces in the presence of subsurface carbon atoms. Therein, the effective barriers of C2H2 hydrogenation on Pd(111)−0.25C (Pd(111) surface with the subsurface C coverage of 0.25 ML) and Pd(211)−C surfaces were calculated to be 1.00 and 1.10 eV, respectively. Therefore, the activities of the boron-modified Pd surfaces are both slightly

Figure 2. Adsorption configurations of C2H2 and C2H4 on Pd(111)−B and Pd(211)−B surfaces. Gray and white balls are carbon and hydrogen atoms, respectively.

adsorption of C2H2 and C2H4 on Pd(111)−B is much weaker than that on the unmodified Pd(111) surface, indicating that, similar to the case of the carbon-modified Pd(111) surface, the electronic structure of surface Pd atoms may be changed upon boron doping. The activation barriers for hydrogenation of C2H2 and C2H3 over Pd(111)−B were calculated to be 1.01 and 0.71 eV, respectively. The energy profile for C2H2 hydrogenation on Pd(111)−B is shown in Figure 3 together with the corresponding transition state structures. It is found that hydrogen atoms bind to the same Pd atoms with the carbon atoms being attacked, which is consistent with our previously reported results for hydrogenation reactions.5,42 The ratelimiting step of the whole process on Pd(111)−B is found to be hydrogenation of C2H2, i.e., C2H2 (ad) + H (ad) → C2H3 (ad) (where ad denotes the surface-adsorbed state), with an effective 3666



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3.2.2. Ethylene Selectivity on Pd−Boron Surfaces. The selectivity toward ethylene formation over the different surfaces may be compared by calculating the difference between the hydrogenation barriers and the desorption barriers of ethylene, defined as ΔEa. The desorption barriers are estimated with the absolute value of the adsorption energies according to the approximation made in previous studies.5,6,44−48 Namely, one can obtain ΔEa = Ea,hydr − Ea,des = Ea,hydr − |Ead|

(2)

where Ea,hydr and Ea,des are the hydrogenation and desorption barriers of ethylene, respectively. This equation indicates that the more positive the ΔEa is, the more selective the catalyst will be for production of ethylene compared with ethane formation. The adsorption energies of C2H4 on Pd(111)−B and Pd(211)−B are listed in Table 2. The hydrogenation barriers of C2H4 over the boron-modified surfaces were calculated to be 1.02 eV for Pd(111)−B and 0.86 eV for Pd(211)−B. Therefore, the corresponding ΔEa on Pd(111)−B and Pd(211)−B are 0.34 and 0.13 eV, respectively, both of which are very similar to those on carbon-modified surfaces (0.33 and 0.18 eV, respectively), and are all higher than those on the unmodified Pd(111) and Pd(211) surfaces (0.06 and −0.45 eV, respectively). More importantly, the favored process on Pd(211) is the over hydrogenation of ethylene to form ethane, while desorption of ethylene is favored on both Pd(211)−B and Pd(211)−C. This would indicate a significant increase in ethylene selectivity on modification of the Pd(211) with boron or carbon. Previously, we reported that the activity of acetylene hydrogenation was dependent on the adsorption energy of C2H2 on the catalyst surface.5 In the current work, the effect of the adsorption energy of C2H2 on the hydrogenation activity is weaker than that of the adsorption energy of C2H4 on the selectivity of Pd catalysts following boron/carbon doping. 3.3. Butadiene Formation on Pd−Boron Surfaces. Our previous work investigated the formation of 1,3-butadiene, a possible precursor identified for green oil formation, in an acetylene hydrogenation system with three possible pathways on several Pd-based surfaces, as shown in Scheme 1.49 In the

Figure 4. Potential energy profile of C2H2 hydrogenation on Pd(211)−B surface. Definitions of (g) and (ad) are the same with those in Figure 3. Corresponding configurations of TS1, MS, and TS2 on this surface are also shown.

Scheme 1. Three Possible Pathways Containing Coupling Reactions and Hydrogenation Reactions of Acetylene To Produce 1,3-Butadiene on Pd Surface

current work, these pathways are also employed to study the formation mechanism of 1,3-butadiene on Pd(111)−B and the effect of boron on its formation, since the flat Pd(111) surface was found to be dominant in 1,3-butadiene formation in our previous work.49 The transition state structures of coupling reactions of C2H2 + C2H2, C2H2 + C2H3, and C2H3 + C2H3 on Pd(111)−B are shown in Figure 6. The corresponding adsorption geometries of the products, C4H4, C4H5, and C4H6, are also shown in Figure 6. As found for the transition state structures of the coupling reactions on the Pd(111) surface, the C2H2 and C2H3 species adsorb at two adjoining fcc and hcp sites without boron beneath the surface. One can also see from Figure 6 that the

Figure 5. d-Projected density of states (PDOS) of the Pd atoms in clean and boron-modified Pd(111) and Pd(211) surfaces. Pd atoms projected are those C2 species, i.e., C2H2, C2H3, and C2H4, adsorb on. Fermi level (Ef) has been set to be zero.

lower than those of carbon-modified surfaces with the same boron/carbon coverage, but the difference is small. 3667



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The reaction barriers of the coupling and hydrogenation steps, according to Scheme 1, are listed in Table 4, and the C− Table 4. Reaction Barriers (Ea) and C−C and C−H Distances at the Transition States of the Hydrogenation and Coupling Reactions on Pd(111)−B, According to Scheme 1 pathway I

DC−C/Å

Ea/eV

DC−H/Å

Ea/eV

1.97

0.60

1.93

0.60

Pd(111) pathway III Pd(111)

Eeff a,hydr

a

C3

C4

0.81 1.02

0.64 0.60

TS2

TS3

C2H2 + H

C2H2 + C2H3

C4H5 + H

Ea/eV

DC−H/Å

Ea/eV

DC−C/Å

Ea/eV

1.01

1.65

0.79

2.06

0.60

DC−H/Å 1.85

TS1

TS2

TS3

C2H2 + H

C2H2 + H

C2H3 + C2H3

Ea/eV

DC−H/Å

Ea/eV

DC−H/Å

Ea/eV

DC−C/Å

1.01

1.65

1.01

1.65

0.35

2.31

(3)

Eeff a,but

where and are the effective barriers of acetylene hydrogenation to produce ethylene and formation of 1,3butadiene, respectively. Therefore, the higher the ΔEeff a the higher the selectivity for 1,3-butadiene formation. The effective barriers of acetylene hydrogenation on Pd(111), Pd(111)−0.25C, and Pd(111)−B are 1.07, 1.00, and 1.01 eV, respectively. The corresponding ΔEeff a on Pd(111), Pd(111)−0.25C, and Pd(111)−B are calculated to be 0.10, 0.00, and 0.00 eV, respectively. Therefore, Pd(111) shows the highest selectivity for 1,3-butadiene formation, while Pd(111)− B is the same with Pd(111)−C. This indicates that formation of green oil may be less favored over Pd(111)−B than over Pd(111). 3.4. Carbide and Hydride Formation on Pd−Boron Surfaces. Formation of carbide and hydride at the subsurface sites of Pd(111)−B and Pd(211)−B has also been investigated in the current work, since the presence of these species will influence both the activity and the selectivity of acetylene hydrogenation on clean Pd surfaces. Previously, we have shown6 that the presence of subsurface carbon will increase the activity of the Pd(111) surface but decrease that of Pd(211), while the selectivity is increased on both surfaces. It should be noted that typically the subsurface carbon atoms are formed from decomposition of C2 species in the feed, i.e., there will be some C2 loss during the reaction, although their formation will

Table 3. Reaction Barriers (eV) of the Hydrogenation Reactions of C4H4 and C4H5 on Pd(111)−Ba C2

1.85

TS1

eff eff ΔEaeff = Ea,hydr − Ea,but

adsorption geometries of C4H4, C4H5, and C4H6 on Pd(111)− B are all similar to those on the Pd(111) surface. The hydrogenation steps of C4H4 and C4H5 on different carbon atoms, which are marked as C1, C2, C3, and C4 in Figure 6 at different parts of the adsorbed radicals, are also studied on the Pd(111)−B surface, and the hydrogenation barriers are listed in Table 3. One can see from Table 3 that the

1.02 1.18

DC−H/Å

C and C−H distances at the transition states are also listed. The energy profiles of these three pathways on Pd(111)−B are shown in Figure 6. The effective barriers of pathways I, II, and III are calculated to be 1.12, 1.04, and 1.01 eV, respectively.6,52 From these calculations, pathways II and III are the relatively favored ones on the Pd(111)−B surface. When comparing the effective barriers of 1,3-butadiene formation on the unmodified Pd(111) (0.97 eV) and Pd(111)−0.25C (1.00 eV) with Pd(111)−B (1.01 eV), it is clear that formation of 1,3-butadiene on Pd(111)−B is suppressed, albeit the differences in the barriers are small. Furthermore, the effective barriers for acetylene hydrogenation to produce ethylene and formation of 1,3-butadiene can be compared by examining ΔEeff a , which is defined as

Figure 6. Energy profile of 1,3-butadiene (C4H6) formation from 2C2H2 + 2H on Pd(111)−B surface. Definition of Pathways I, II, and III is in accordance with those in Scheme 1. Transition state structures of relative coupling reactions of C2H2 + C2H2, C2H2 + C2H3, and C2H3 + C2H3 (from left to right) on Pd(111)−B surface (first panel). Adsorption structures of the produced C4H4, C4H5, and C4H6 are also shown (second panel). Different carbon atoms are marked as 1, 2, 3, and 4.

C1

TS3 C4H5 + H

1.12

pathway II

0.60 0.74

TS2 C4H4 + H

Ea/eV Pd(111)

C4H4 C4H5

TS1 C2H2 + C2H2

The definitions of C1, C2, C3, and C4 are shown in Figure 6.

hydrogenation of the terminal C1 in C4H4 is favored on Pd(111)−B and the hydrogenation of terminal C4 in C4H5 is favored, readily forming 1,3-butadiene. The adsorption energy of C4H6 on Pd(111)−B is found to be −1.10 eV, which is much weaker than that on Pd(111) (−1.64 eV) or Pd(211) (−1.94 eV). It should be mentioned that the favored adsorption geometry of 1,3-butadiene on Pd(111)−B is the same as that over a clean Pd(111) surface reported previously.49−51 3668



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subsurface boron was not calculated in this study, since the preferred adsorption sites of carbon and boron are the same. Adsorption of hydrogen atoms at the subsurface sites of Pd(111) is only slightly weakened in the presence of boron atoms, but the adsorption processes become almost energetically neutral. The decrease of the adsorption energy of hydrogen at 1 ML coverage is caused by one-half of the hydrogen atoms moving from the subsurface sites to the surface sites after geometrical optimization. The same trend is also observed on Pd(211)−B surface, which can be seen from the second panel of Figure 7. From the above results of the adsorption of carbon and hydrogen atoms at the subsurface sites of the Pd(111)−B and Pd(211)−B surfaces, one can see that formation of subsurface carbon and hydrogen is largely suppressed on boron-modified Pd surfaces, which will give rise to two benefits: first, loss of the feed C2 species during reaction will be minimized; second, the selectivity to ethylene formation will not be reduced by preventing formation of subsurface hydrogen.

to some extent increase the selectivity/activity. In the presence of subsurface hydrogen atoms, the activity is increased but, in addition, the over hydrogenation of ethylene is also promoted due to the high activity of the subsurface hydrogen atoms. Therefore, the presence of subsurface hydrogen atoms reduces the effectiveness of the system. The trends for the adsorption energies of carbon and hydrogen as a function of their coverage at the subsurface sites of Pd(111)−B and Pd(211)−B were calculated using eq 1 and are shown in Figure 7. The adsorption energies of carbon are

4. CONCLUSIONS In this work, we systematically investigated the activity of boron-modified Pd catalysts for selective hydrogenation of acetylene, ethylene selectivity, 1,3-butadiene formation, and subsurface carbon and hydrogen formation using DFT calculations. The following conclusions can be drawn. (1) The activity of acetylene hydrogenation on Pd(111)−B and Pd(211)−B is similar to those on carbon-modified Pd surfaces with the same carbon coverage. The Pd(111) surface modified by boron atoms shows higher activity than clean Pd(111), while the boron-modified Pd(211) displays lower activity than clean Pd(211). (2) When the ethylene desorption and hydrogenation are compared, the selectivity of ethylene formation on Pd(111)−B and Pd(211)−B are both higher than those on clean Pd(111) and Pd(211) and similar to those on carbon-modified Pd(111) and Pd(211) surfaces. (3) 1,3-Butadiene formation on Pd(111)−B is slightly suppressed when compared with those on clean Pd(111) and carbon-modified Pd(111) surfaces. Furthermore, hydrogenation of acetylene is favored over 1,3butadiene formation on Pd(111)−B. (4) Formation of subsurface carbon and hydrogen on Pd(111) and Pd(211) is prevented in the presence of boron atoms. From the above results, it is clear that boron-modified Pd surfaces show similar properties with carbon-modified ones, regarding the activity and selectivity, but will display better performance than clean Pd surfaces. This gives rise to an alternative way to increase the performance of Pd catalysts. These results also possess some important insights into the further utilization of Pd−B catalysts for other hydrogenation reactions.

Figure 7. Adsorption energy (Ead) of carbon and hydrogen at the subsurface sites of boron-modified Pd(111) (above) and Pd(211) (below) surfaces to the coverage of subsurface carbon/hydrogen atoms. Adsorption energies of carbon are reported with respect to the energy of 1/2C2H2 − 1/2H2 according to the equation of 1/2C2H2 → C + 1/2H2. Adsorption energies of hydrogen are calculated with the energy of 1/2H2.

reported with respect to the energy of 1/2C2H2 − 1/2H2 according to the equation 1/2C2H2 → C + 1/2H2. The adsorption energies of hydrogen are calculated with respect to the gaseous energy of 1/2H2. Figure 7 also shows the trends of the adsorption energies of carbon and hydrogen at the subsurface sites over the unmodified Pd(111) and Pd(211) surfaces to make a comparison. From Figure 7 it is clear that the presence of subsurface boron atoms results in a weaker bonding of carbon atoms at the subsurface sites of Pd(111)−B than clean Pd(111). More importantly, adsorption of carbon becomes endothermic even if its coverage on the subsurface sites is as low as 0.25 ML, indicating that formation of subsurface carbon species is prevented on this surface. It should also be noted that the adsorption energy of carbon at the subsurface with 1 ML of

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The authors declare no competing financial interest. 3669



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ACKNOWLEDGMENTS This work was financially supported by EPSRC and Johnson Matthey through the CASTech programme (EP/G011397/1). The authors would like to thank The Queen’s University of Belfast for computing time. B.Y. also acknowledges the financial support of the Dorothy Hodgkin Postgraduate Award (DHPA) studentship jointly funded by EPSRC and Johnson Matthey.

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