Identification of the Active and Selective Sites over Single Pt Atom

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Identification of the Active and Selective Sites over Single Pt Atom Alloyed Cu Catalyst for the Hydrogenation of 1,3Butadiene: A Combined DFT and Microkinetic Modeling Study Kunran Yang, and Bo Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01980 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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The Journal of Physical Chemistry

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Identification of the Active and Selective Sites over Single Pt Atom Alloyed Cu

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Catalyst for the Hydrogenation of 1,3-Butadiene: A Combined DFT and

3

Microkinetic Modeling Study

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Kunran Yang and Bo Yang*

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School of Physical Science and Technology, ShanghaiTech University, 393 Middle

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Huaxia Road, Shanghai 201210, China

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E-mail: [email protected]

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Abstract

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Selective hydrogenation of butadiene to butenes is an important industrial process,

12

and single Pt atom alloyed with Cu(111) surface shows superior activity and

13

selectivity for this reaction. By utilizing density functional theory (DFT) calculations

14

combined with microkinetic modeling, herein we systematically studied the

15

hydrogenation of butadiene over the Pt/Cu(111) SAA catalyst, and identified the

16

active sites and probed the product selectivity at different sites under reaction

17

conditions. Although the structure of the single-atom alloy is found stable in vacuum,

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it is likely that aggregation of surface Pt atoms could be induced upon butadiene

19

adsorption and the aggregated structure shows lower activity than the single Pt site. In

20

addition, we found that Cu site shows almost identical hydrogenation activity with the

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Pt site, while considering the concentration of the surface Pt sites, which gives a good

22

explanation on the experimental observations reported previously that the activity of

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the Pt/Cu(111) SAA catalyst was unaffected by the occupation of CO at Pt sites.

24

Furthermore, all butene isomers produced would preferably desorb rather than being

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further hydrogenated to butane at the surface sites considered. Although the selectivity

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between butene isomers over the single Pt sites is different from that over the Cu sites,

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the overall SAA catalyst gives the same selectivity trend with the single Pt sites. Our

28

work shows, at the molecular level, how different sites over the Pt/Cu(111) SAA

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catalyst contribute to the hydrogenation activity and product selectivity. 1 ACS Paragon Plus Environment

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1. Introduction

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Alkenes are valuable raw materials in industry, and are mainly produced through the

4

thermal cracking process. However, the cracking gas obtained always contains a

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minor amount of highly unsaturated hydrocarbon species, e.g. butadiene (C4H6) and

6

acetylene (C2H2). These species will act as poisons to the catalysts used for the

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subsequent polymerization of alkenes.1 Therefore, it is of significant importance to

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remove these C4H6 and C2H2 from the alkenes feed. Typically this can be achieved

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through the selective hydrogenation of these highly unsaturated hydrocarbons to

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corresponding alkenes, e.g. butenes (C4H8) and ethylene (C2H4). Since the alkenes in

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the feed are also unsaturated and could be further hydrogenated to alkanes, the

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catalysts used for this process should be highly selective.

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Platinum-group metals (PGMs) are commonly used in hydrogenation reactions, and a

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number of studies reported their high activity for C4H6 hydrogenation.2-3 However, it

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was also found experimentally that pristine PGM catalysts always give 15-30%

16

selectivity to the by-product butane (C4H10), which limits the applications of these

17

catalysts in the industrial processes.4-5 In contrast, inert Group IB metals such as Cu

18

were utilized for the selective hydrogenation of C4H6 to C4H8, with low C4H10

19

formation.6-7 However, it was also found that the activation of H2 might be

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rate-limiting in the hydrogenation processes over these inert metals, which results in

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lower activity compared with the PGM catalysts.8

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The modifications of Cu catalysts have been widely studied and some Cu-based

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bimetallic catalysts were developed for hydrogenation reactions.9-11 Among them,

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single-atom alloy (SAA) catalysts, where an individual, isolated active metal atom is

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embedded in Cu(111), show unique properties in C4H6 hydrogenation reactions as

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reported by Sykes and co-workers.12 The authors found that C4H6 is readily

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hydrogenated over the Pt/Cu(111) SAA catalyst, where the surface composition of the

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single Pt atom is ~0.02 monolayer (ML) in Cu(111). It was found that the selectivity

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of C4H8 formation is nearly 100% at temperatures lower than 240 K. Sykes’ group 2 ACS Paragon Plus Environment

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further reported applications of Cu-based SAA catalysts in the selective

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hydrogenation of some other unsaturated hydrocarbons.12-14 The high activity of these

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Cu-based SAA catalysts was considered originated from two aspects. On one hand, H2

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dissociation is feasible over the doped single atom, as suggested by previous

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computational studies,15-17 and the hydrogen atoms at the single atom site could

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further spill over onto Cu.18-19 On the other hand, the hydrogen atoms over bare Cu

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surface are weakly bound and are efficient for the hydrogenation reactions. Therefore,

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the single-atom site and Cu surface facilitate different steps in C4H6 hydrogenation

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reactions, leading to excellent activity and selectivity.12, 14

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In order to determine the active sites for the reaction, Sykes and co-workers

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conducted the hydrogenation of C4H6 on a treated Pt/Cu(111) surface, where there

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were pre-covered hydrogen atoms and the single Pt sites were blocked by CO.12

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Temperature programmed desorption (TPD) results suggested that desorption of C4H8

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at ~240 K was unaffected by the occupation of CO at Pt sites, and the desorption

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temperature of CO is 350 K. Hence the authors concluded that the hydrogenation

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reactions dominantly take place at Cu sites, and the hydrogenation mechanism of

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C4H6 over Pt/Cu(111) should be similar with that over pristine Cu surface. However,

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from the paper we found that the product distribution of C4H6 hydrogenation over

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Pt/Cu(111) at 433 K follows 1-butene > trans-2-butene > cis-2-butene, and it is

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interesting to see that this order is different from that over pristine Cu, which is

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1-butene > cis-2-butene > trans-2-butene as reported by many groups,7,

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indicating that it would be necessary to further identify the active and selective sites

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over the Pt/Cu(111) SAA catalysts.

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In our previous study, we found that surface aggregation of the single atoms over SAA

25

surfaces could be induced by strongly adsorbed acetylene.21 Similarly, C4H6 may also

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be able to induce such surface aggregation over Pt/Cu(111) SAA catalyst, changing

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the hydrogenation activity. In the current work, we first study the stability of single-Pt

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sites, aggregated-Pt sites under C4H6 hydrogenation conditions utilizing density

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functional theory (DFT) methods to gain insight into the active sites and 3 ACS Paragon Plus Environment

9-10, 20


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hydrogenation mechanisms. The hydrogenation of C4H6 over Cu(111) is also studied

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in order to obtain the activity and selectivity of the Cu sites over the Pt/Cu(111) SAA

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catalysts. The activity and selectivity of different sites are compared based on the

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microkinetic modeling results obtained under different reaction conditions considered.

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2. Computational Details

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Vienna Ab initio Simulation Package (VASP) code was utilized to conduct the DFT

8

calculations with periodic slab models.22-25 Perdew-Burke-Ernzerhof (PBE) functional

9

with the generalized gradient approximation (GGA) was used to describe the

10

electronic structures, and projector augmented wave (PAW) method was applied to

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calculate the interaction between electrons and atomic cores. All the calculations used

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a cut-off energy of 500 eV with the force threshold of 0.05 eV Å-1. The four-layer

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Cu(111) slab with a 4×4 supercell was built and the vacuum was set to be higher than

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12 Å. By substituting 1, 2, 3 and 4 Cu atom(s) with Pt atom(s) at the upmost layer of

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Cu(111), the corresponding Pt1/Cu(111), Pt2/Cu(111) Pt3/Cu(111) and Pt4/Cu(111)

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structures could be generated, and the subscript of Pt denotes the number of

17

aggregated Pt atoms. Pt1 means no aggregation and stands for the single Pt atom in

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Cu(111). For all the calculations performed, metal atoms located at the bottom two

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layers were fixed to simulate bulk structure and the remaining atoms were fully

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relaxed. The surface Brillouin zone was described with 3×3×1 k-point grid. Transition

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state structures were optimized using the constrained minimization method.26-31

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Vibrational frequencies were calculated to characterize all of the transition states

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structures with only one imaginary frequency. The adsorption energies were

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calculated as: Eads = Eadsorbate+slab - (Eadsorbate + Eslab), where Eslab+adsorbate is the energy of

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the slab and the adsorbate, Eadsorbate is the energy of the gas-phase adsorbate, and Eslab

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is the energy of the slab.

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3. Results

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3.1 Scheme of C4H6 hydrogenation to C4H10 4 ACS Paragon Plus Environment

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The reactant C4H6 (H2C=CH-CH=CH2) possesses two conformations, i.e. trans-C4H6

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(trans-H2C=CH-CH=CH2) and cis-C4H6 (cis-H2C=CH-CH=CH2), and the overall

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reaction pathways of C4H6 hydrogenation to C4H10 are shown in Scheme 1. The

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hydrogenation reactions are considered following the Horiuti-Polanyi mechanism,32-33

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in which H2 is firstly dissociated to two hydrogen atoms on the surface and then the

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hydrogen atoms are added to the adsorbed C4 species sequentially, due to the low

7

hydrogen dissociation barrier reported in the literature.14, 16-17 The first step of C4H6

8

hydrogenation is commonly considered to take place at the terminal carbon atom,34

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and H2C=CH-CH-CH3 (C4H7) with three unsaturated carbon atoms could be generated

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on the surface. Subsequently, C4H7 may undergo hydrogenation at different carbon

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atoms producing three butene (C4H8) isomers, i.e. 1-butene (H2C=CH-CH2-CH3,

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1-C4H8), trans-2-butene (trans-H3C-CH=CH-CH3, t-2-C4H8) and cis-2-butene

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(cis-H3C-CH=CH-CH3, c-2-C4H8). The further hydrogenation of these three C4H8

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could give H2C-CH2-CH2-CH3 (1-C4H9) or H3C-CH-CH2-CH3 (2-C4H9). Finally,

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C4H9 could be fully hydrogenated to C4H10 and then desorbs from the catalyst surface.

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3.2 Aggregation of single Pt atoms over Cu(111)

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In this part we investigate the stability of Pt1/Cu(111) SAA structure and the

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possibility of Pt atoms aggregation under realistic conditions. As shown in Figure 1,

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we hereby consider four structures: the Pt1/Cu(111) SAA structure where one Pt atom

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is embedded in Cu(111) and the aggregated-SAA (A-SAA) structures with 2, 3 or 4

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doped Pt atoms in Cu(111), i.e. Pt2/Cu(111), Pt3/Cu(111) and Pt4/Cu(111). The energy

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cost for the aggeregation of SAA structures to the A-SAA counterparts is shown in

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Figure 2. As introduced in our recent work,21 we assume that the A-SAA structures are

25

generated from the aggregation of Pt atoms in one or more Pt1/Cu(111), and hence the

26

energy of the SAA structure is set to be the energy reference in Figure 2. One can see

27

in Figure 2 that, when there are no adsorbates on the surface, Pt1/Cu(111) is the most

28

stable and the aggregation of Pt atoms in Cu(111) surface is energy-consuming. The

29

stability of the Pt1/Cu(111) structure is also proved by the experimental and 5 ACS Paragon Plus Environment


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theoretical results reported in the literature.17, 35

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In Figure 2, we also show the energy cost for the aggregation from SAA to A-SAA

3

structures upon the adsorption of reactants and semi-hydrogenated products in the

4

pathways of C4H6 hydrogenation. It should be noted that Pt2/Cu(111) structure is more

5

stable than Pt1/Cu(111) upon the adsorption of t-C4H6 and c-C4H6, indicating that

6

surface aggregation of two Pt atoms is likely to be induced upon the adsorption of

7

C4H6. However, the aggregation of 3 or 4 Pt atoms in Cu(111) surface is unfavorable

8

even with adsorbates on the surface, and hence these structures may not be generated

9

during C4H6 hydrogenation. Consequently, C4H6 species are capable of inducing

10

surface aggregation over Pt/Cu(111) surface to form the Pt2/Cu(111) structure, and we

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only consider the hydrogenation reactions taking place over the SAA Pt1/Cu(111) and

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the A-SAA Pt2/Cu(111) structures.

13 14

3.3 Hydrogenation of C4H6 to C4H10

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As mentioned above in the introduction part, it was suggested by Sykes and

16

co-workers that Cu sites are the possible active sites over the Pt/Cu(111) SAA catalyst,

17

we hereby also consider the hydrogenation reactions occurring over Cu(111). The

18

most stable adsorption structures of C4 species and the corresponding transition state

19

structures during butadiene hydrogenation over Pt1/Cu(111), Pt2/Cu(111) and Cu(111)

20

are presented in Figures 3, 4 and 5, respectively. The corresponding activation barriers

21

and reaction energies of the elementary steps in the reaction pathways of t/c-C4H6

22

hydrogenation are listed in Table 1.

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One can see from Figure 3 that t/c-C4H6 adsorbs at the top site of one Pt and one Cu

24

with a di-π-bonded configuration over Pt1/Cu(111). In Table 1, the adsorption energies

25

of t-C4H6 and c-C4H6 over Pt1/Cu(111) are -0.57 and -0.63 eV, respectively, which are

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much weaker than those reported over Pt(111) (typically stronger than -1 eV)

27

according to the literature.36 The hydrogenation of C4H6 preferably takes place at the

28

terminal C atom above the Cu atom as shown in Figure 3, and the distance between H

29

and the C atom is 1.579 Å and 1.665 Å for t-C4H6 and c-C4H6, respectively. As shown 6 ACS Paragon Plus Environment

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in Scheme 1, the addition of H to C4H7 would happen at two different carbon atoms,

2

and producing different C4H8. It is found that all of the C4H8 products would follow

3

the π adsorption configuration at the Pt atom on Pt1/Cu(111).

4

As one can see from Figure 4 and 5, most of the intermediate and transition state

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structures of the elementary steps of C4H6 hydrogenation to C4H10 over Pt2/Cu(111)

6

and Cu(111) are similar to those over Pt1/Cu(111). For example, the adsorption

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structures of C4H6 over Pt1/Cu(111), Pt2/Cu(111) and Cu(111) are the same that they

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all follow a di-π-bonded configuration. However, the binding strength of the reactants,

9

intermediates and products are different over these three surfaces. Taking the

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adsorption energies of C4H6 as an example, one can see from Table 1 that the

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adsorption over Pt1/Cu(111) is stronger than that over Cu(111), which is consistent

12

with the experimental observations that the desorption temperature of C4H6 over the

13

single Pt site is higher than that over Cu terraces.12

14 15

4. Discussion on the reaction kinetics

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Microkinetic analysis is carried out with the CatMAP module developed by Nørskov

17

and co-workers.37-39 Detailed information and parameters of the microkinetic

18

modeling are listed in the Supporting Information (SI) file, which are similar to those

19

we used recently.40 It should be mentioned that, the coverage effect can be neglected

20

due to the low concentration (~2%) of active Pt sites on the catalyst surface, and it has

21

been found experimentally that these Pt sites are well-isolated.12

22 23

4.1 Reaction rates of C4H6 hydrogenation at different surface sites

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The kinetics of the adsorption/desorption process are quite complex under the reaction

25

conditions, and may be influenced by many factors, such as the catalyst pore size, the

26

reaction temperature and pressure. It is still rather difficult to accurately measure the

27

adsorption/desorption barriers with DFT calculations. However, we notice that the

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order of selectivity between butenes obtained experimentally, i.e. 1-butene >

29

trans-2-butene > cis-2-butene,12 is quite different from the thermodynamical stability 7 ACS Paragon Plus Environment


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1

trend trans-2-butene > cis-2-butene > 1-butene, indicating that the rate-determining

2

steps should be surface hydrogenation steps instead of the adsorption/desorption step

3

of butadiene. Therefore, in our microkinetic models, the adsorption/desorption

4

process are considered equilibrated. Moreover, previous results suggested that the

5

spillover of H from Pt site to Cu site over Pt/Cu(111) catalyst is also readily,16-17

6

resulting in H adatoms at the Cu sites, therefore, we also study the kinetics of C4H6

7

hydrogenation over hydrogen pre-covered Cu(111).

8

The

9

rtotal = r1-C 4 H 8 + rt-2-C 4 H 8 + rc-2-C 4 H 8 + rC 4 H10 , where the four rates at the right side of the

10

equation denote the formation rates of corresponding species. Figure 6a shows the

11

trend of rtotal varying with temperature at 1 bar over Pt1/Cu(111), Pt2/Cu(111) and

12

Cu(111). One can see that Pt1/Cu(111) shows the highest activity compared with the

13

other two structures within 300-400 K. Surprisingly, the rtotal over Pt2/Cu(111) is

14

lower than that over Pt1/Cu(111), indicating that the surface-aggregated sites are

15

unfavorable for C4H6 hydrogenation compared with the SAA sites. This is possibly

16

due to the fact that, at steady state, the surface coverage of C4 species is close to 1 ML

17

over Pt2/Cu(111) at low reaction temperature, indicating that the free Pt2 sites left for

18

the reactions are limited, whilst the Pt1/Cu(111) still possesses a large amount of free

19

sites, as shown in Figure S1. This is consistent with the much stronger adsorption of

20

the reactants and reaction intermediates on Pt2/Cu(111), as one can see from Table 1.

21

In addition, the rtotal of C4H6 hydrogenation over Pt1/Cu(111) and Cu(111) increases

22

with temperature in the range of 300-340 K in Figure 6a, and above 340 K the rtotal

23

over these two surfaces stays constant or slightly decrease. The obtained result is

24

consistent with the experimental results that the conversion of C4H8 increases with

25

temperatures up to ~330 K over the Pt/Cu(111) SAA catalyst and then 100%

26

conversion of C4H6 is reached.12

27

It should be noted that the above kinetic model we built over Cu(111) is based on the

28

assumption that the Cu sites of a real Pt/Cu(111) SAA catalyst would be covered with

29

hydrogen diffused from the Pt sites, and therefore the dissociation barrier of H2 over 8

total

rates

of

C4H6

hydrogenation

(rtotal)

ACS Paragon Plus Environment

can

be

written

as:

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Cu(111) is ignored. However, it is widely recognized that the dissociation of H2 would

2

be the rate-determining step for the hydrogenation reactions taking place over Cu(111).

3

Therefore, we further calculate the barrier of the dissociative adsorption of H2 over

4

Cu(111), which is 0.49 eV. By taking the H2 dissociation barriers into consideration

5

while running the microkinetic modeling over Cu(111), we obtained the rtotal at

6

different temperatures and the trend is illustrated in Figure S2. One can see that clean

7

Cu(111) maintains lower rtotal than that with pre-covered hydrogen on the surface,

8

proving that H2 dissociation limits the rate of C4H6 hydrogenation over Cu catalysts.

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4.2 Active sites of the Pt/Cu(111) SAA catalyst

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One can see from Figure 6a that the rtotal over Pt1/Cu(111) is much higher than that

12

over hydrogen pre-covered Cu(111). However, the activity of the single Pt site

13

measured would be limited by its low surface concentration, and experiments show

14

that the single Pt site accounts for ~0.02 ML in Cu(111) surface of a reported

15

Pt/Cu(111) SAA catalyst.12 To investigate the activity between Cu and single Pt sites

16

of the Pt/Cu(111) SAA catalyst, we compared the hydrogenation activity of the two

17

sites by considering their surface concentrations, i.e. 0.02 ML for single Pt and 0.98

18

ML for Cu. The activity contribution of the two sites is shown in Figure 6b, and it is

19

clearly shown here that Cu sites maintain almost identical rtotal with the single Pt site

20

in the temperature range studied. Interestingly, experimental results reported in the

21

literature showed that the formation of C4H8 over the Pt/Cu(111) SAA catalyst is

22

unaffected when the single Pt sites are blocked,12 which can be explained by our

23

microkinetic results.

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4.3 Selectivity between C4H8 and C4H10

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The formation rates of C4H8 and C4H10 over Pt1/Cu(111), Pt2/Cu(111) and Cu(111)

27

structures within 300-400 K and 1-100 bar are shown in Figures S3-S5. It can be

28

found that the rates of C4H10 formation are lower than those of C4H8 formation by

29

several orders of magnitude over all of the three structures investigated. This is 9 ACS Paragon Plus Environment


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consistent with the experiments that no detectable C4H10 is observed during C4H6

2

hydrogenation over the Pt/Cu(111) SAA catalyst. It should be mentioned here that the

3

selectivity trend obtained from microkinetic calculations is consistent with that

4

obtained by calculating the difference between Ea,C4H8 and |Ead,C4H8|, where Ea,C4H8 is

5

the activation energy of C4H8 hydrogenation and |Ead,C4H8| denotes the absolute value

6

of the adsorption energy of C4H8, a descriptor we developed before to determine the

7

selectivity between further hydrogenation and desorption.21, 29-31, 41-47 The values of

8

the descriptor over different surfaces are listed in Table S1, showing that all of the

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three C4H8 would favorably desorb from Pt1/Cu(111), Pt2/Cu(111) and Cu(111). It

10

should also be noted that, although C4H10 is unfavorable to be generated compared

11

with C4H8, the formation rate of C4H10 increases with temperatures, as shown in

12

Figure 7.

13 14

4.4 Selectivity between the C4H8 isomers

15

The selectivity trends of 1-C4H8, t-2-C4H8 and c-2-C4H8 formation over different

16

surface structures are shown in Figure 8. One can see that the trends of the selectivity

17

among C4H8 isomers at varied temperatures are different over Pt1/Cu(111), Pt2/Cu(111)

18

and Cu(111). For example, the selectivity of 1-C4H8 decreases by around 20%, 10%

19

and 2% from 300 K to 400 K for Pt1/Cu(111), Pt2/Cu(111) and Cu(111) structures.

20

However, the selectivity of both 2-butenes (t-2-C4H8 and c-2-C4H8) increases within

21

the same temperature range over these three structures.

22

While comparing the selectivity of butenes over different structures, one can find the

23

following two trends,

24

(i)

all the surfaces sites studied prefers the production of 1-C4H8;

25

(ii)

regarding the selectivity between 2-butenes, t-2-C4H8 production is favored

26

over Pt sites whilst the Cu sites will give rise to the formation of c-2-C4H8.

27

It should be mentioned that the higher selectivity of c-2-C4H8 than t-2-C4H8 over Cu

28

sites is consistent with the results reported in the literature. Since we mentioned above

29

that the concentration of Pt site over the Pt/Cu(111) SAA catalyst reported in the 10 ACS Paragon Plus Environment

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1

literature is around 0.02 ML, we hereby carry out microkinetic simulations over a

2

surface that possess 0.02 ML of single Pt site and 0.98 ML of Cu site. The calculated

3

selectivity of different C4H8 are shown in Figure 8d, from which one can see that the

4

selectivity is following the order of 1-butenes > t-2-C4H8 > c-2-C4H8, and is the same

5

as the selectivity trend obtained by experiments over a Pt/Cu(111) SAA catalyst.12

6

This indicates that, although the activity of the Pt sites and Cu sites over the

7

Pt/Cu(111) SAA catalyst are almost identical, the selectivity of the whole catalyst is

8

similar to that at the Pt sites.

9 10

5. Conclusions

11

In summary, we investigated the hydrogenation of C4H6 at different surface sites over

12

the Pt/Cu(111) SAA catalyst with DFT calculations and microkinetic simulations. The

13

following conclusions are obtained.

14

(i)

Although the single Pt atom in Cu(111) surface is stable in vacuum, it is likely

15

that aggregation of the surface Pt to Pt2/Cu(111) could be induced upon the

16

adsorption of butadiene. However, Pt2/Cu(111) structure shows lower activity

17

for C4H6 hydrogenation than Pt1/Cu(111).

18

(ii)

While considering the concentration of different surface sites, Cu sites possess

19

almost identical activity with the single Pt site for the hydrogenation of C4H6,

20

which gives an explanation on the experimental observation that the formation

21

of C4H8 is unaffected when the single Pt sites are blocked.

22

(iii)

Butenes prefer to desorb from all of the considered structures rather than to be

23

further hydrogenated to butane. Although the selectivity between butene

24

isomers over the single Pt sites is different from that over the Cu sites, the

25

overall SAA catalyst gives the same selectivity trend with the single Pt sites.

26

Our work provides molecular level understandings on the selective hydrogenation of

27

C4H6 through DFT calculations combined with microkinetic simulations, which could

28

be further utilized to facilitate rational design and applications of SAA catalysts for

29

other systems. 11 ACS Paragon Plus Environment


1 2

Acknowledgements

3

This work is financially supported by the National Science Foundation of China

4

(21603142) and ShanghaiTech University. We thank the HPC Platform of

5

ShanghaiTech University for computing time.

6 7

Supporting Information

8

Detailed information and parameters of the microkinetic modeling, and the formation

9

rates obtained.

10 11

References

12

1.

13

Raw Materials Produced by Steam Cracking. Elsevier: 1986; Vol. Volume 27.

14

2.

15

hydrogenation of 1,3-butadiene over Pt/SiO2: effect of Pt dispersion and kinetic

16

analysis. Catal. Sci. Technol. 2017, 7 (13), 2717-2728.

17

3.

18

butadiene hydrogenation catalysed by the transition metals. Appl. Catal., A 2002, 229

19

(1), 251-259.

20

4.

21

platinum surfaces of different structures. Catal. Lett. 1997, 46 (1-2), 37-41.

22

5.

23

of Pd in Hydrogenation of 1,3-Butadiene over Co-Pd Catalysts. J. Catal. 1995, 157

24

(1), 179-189.

25

6.

26

Selective hydrogenation of C4-alkynes over a copper on silica catalyst. Appl. Catal., A

27

1994, 120 (1), 163-177.

28

7.

29

preparation of Cu/TiO2 catalysts by deposition–precipitation with urea for selective

Derrien, M. L., Selective Hydrogenation Applied to the Refining of Petrochemical

Hu, C.; Sun, J.; Kang, D.; Zhu, Q.; Yang, Y., Mechanistic insights into complete

Moyes, R. B.; Wells, P. B.; Grant, J.; Salman, N. Y., Electronic effects in

Yoon, C.; Yang, M. X.; Somorjai, G. A., Hydrogenation of 1,3-butadiene on

Sarkany, A.; Zsoldos, Z.; Stefler, G.; Hightower, J. W.; Guczi, L., Promoter Effect

Koeppel, R. A.; Wehrli, J. T.; Wainwright, M. S.; Trimma, D. L.; Cant, N. W.,

Wang, Z.; Brouri, D.; Casale, S.; Delannoy, L.; Louis, C., Exploration of the

12 ACS Paragon Plus Environment

Page 12 of 28

Page 13 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


1

hydrogenation of unsaturated hydrocarbons. J. Catal. 2016, 340, 95-106.

2

8.

3

1995, 376 (6537), 238-240.

4

9.

5

Cu-Zn/TiO2 catalysts for selective hydrogenation of butadiene. J. Catal. 2017, 347,

6

185-196.

7

10. Delannoy, L.; Thrimurthulu, G.; Reddy, P. S.; Methivier, C.; Nelayah, J.; Reddy,

8

B. M.; Ricolleau, C.; Louis, C., Selective hydrogenation of butadiene over TiO2

9

supported copper, gold and gold-copper catalysts prepared by deposition-precipitation.

Hammer, B.; Norskov, J. K., Why gold is the noblest of all the metals. Nature

Wang, Z.; Wang, G.; Louis, C.; Delannoy, L., Novel non-noble bimetallic

10

Phys. Chem. Chem. Phys. 2014, 16 (48), 26514-26527.

11

11. Guczi, L.; Schay, Z.; Stefler, G.; Liotta, L. F.; Deganello, G.; Venezia, A. M.,

12

Pumice-Supported Cu–Pd Catalysts: Influence of Copper on the Activity and

13

Selectivity of Palladium in the Hydrogenation of Phenylacetylene and But-1-ene. J.

14

Catal. 1999, 182 (2), 456-462.

15

12. Lucci, F. R.; Liu, J.; Marcinkowski, M. D.; Yang, M.; Allard, L. F.;

16

Flytzani-Stephanopoulos, M.; Sykes, E. C. H., Selective hydrogenation of

17

1,3-butadiene on platinum-copper alloys at the single-atom limit. Nat. Commun. 2015,

18

6, 8550

19

13. Lucci, F. R.; Lawton, T. J.; Pronschinske, A.; Sykes, E. C. H., Atomic Scale

20

Surface Structure of Pt/Cu(111) Surface Alloys. J. Phys. Chem. C 2014, 118 (6),

21

3015-3022.

22

14. Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.; Lawton, T. J.; Baber,

23

A. E.; Tierney, H. L.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H., Isolated Metal

24

Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations. Science

25

2012, 335 (6073), 1209-1212.

26

15. Fu, Q.; Luo, Y., Active Sites of Pd-Doped Flat and Stepped Cu(111) Surfaces for

27

H2Dissociation in Heterogeneous Catalytic Hydrogenation. ACS Catal. 2013, 3 (6),

28

1245-1252.

29

16. Fu, Q.; Luo, Y., Catalytic Activity of Single Transition-Metal Atom Doped in 13 ACS Paragon Plus Environment


1

Cu(111) Surface for Heterogeneous Hydrogenation. J. Phys. Chem. C 2013, 117 (28),

2

14618-14624.

3

17. Lucci, F. R.; Marcinkowski, M. D.; Lawton, T. J.; Sykes, E. C. H., H-2 Activation

4

and Spillover on Catalytically Relevant Pt-Cu Single Atom Alloys. J. Phys. Chem. C

5

2015, 119 (43), 24351-24357.

6

18. Tierney, H. L.; Baber, A. E.; Kitchin, J. R.; Sykes, E. C. H., Hydrogen

7

Dissociation and Spillover on Individual Isolated Palladium Atoms. Phys. Rev. Lett.

8

2009, 103 (24), 246102.

9

19. Lucci, F. R.; Darby, M. T.; Mattera, M. F. G.; Ivimey, C. J.; Therrien, A. J.;

10

Michaelides, A.; Stamatakis, M.; Sykes, E. C. H., Controlling Hydrogen Activation,

11

Spillover, and Desorption with Pd-Au Single-Atom Alloys. J. Phys. Chem. Lett. 2016,

12

7 (3), 480-485.

13

20. Phillipson, J. J.; Wells, P. B.; Wilson, G. R., The hydrogenation of alkadienes.

14

Part III. The hydrogenation of buta-1,3-diene catalysed by iron, cobalt, nickel, and

15

copper. J. Chem. Soc. A 1969, 0 (0), 1351-1363.

16

21. Yang, K.; Yang, B., Surface Restructuring of Cu-based Single-atom Alloy

17

Catalysts under Reaction Conditions: The Essential Role of Adsorbates. Phys. Chem.

18

Chem. Phys. 2017, 19 (27), 18010-18017.

19

22. Kresse, G.; Hafner, J., Ab initio molecular dynamics for liquid metals. Phys. Rev.

20

B 1993, 47 (1), 558-561.

21

23. Kresse, G.; Hafner, J., Ab initio molecular-dynamics simulation of the

22

liquid-metal-amorphous-semiconductor transition in germanium. Phys. Rev. B 1994,

23

49 (20), 14251-14269.

24

24. Kresse, G.; Furthmüller, J., Efficiency of ab-initio total energy calculations for

25

metals and semiconductors using a plane-wave basis set. Comp. Mater. Sci. 1996, 6

26

(1), 15-50.

27

25. Kresse, G.; Furthmüller, J., Efficient iterative schemes for \textit{ab initio}

28

total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54 (16),

29

11169-11186. 14 ACS Paragon Plus Environment

Page 14 of 28

Page 15 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


1

26. Alavi, A.; Hu, P.; Deutsch, T.; Silvestrelli, P. L.; Hutter, J., CO Oxidation on

2

Pt(111): An \textit{Ab Initio} Density Functional Theory Study. Phys. Rev. Lett. 1998,

3

80 (16), 3650-3653.

4

27. Michaelides, A.; Liu, Z. P.; Zhang, C. J.; Alavi, A.; King, D. A.; Hu, P.,

5

Identification of General Linear Relationships between Activation Energies and

6

Enthalpy Changes for Dissociation Reactions at Surfaces. J. Am. Chem. Soc. 2003,

7

125 (13), 3704-3705.

8

28. Liu, Z.-P.; Hu, P., General Rules for Predicting Where a Catalytic Reaction

9

Should Occur on Metal Surfaces: A Density Functional Theory Study of C−H and

10

C−O Bond Breaking/Making on Flat, Stepped, and Kinked Metal Surfaces. J. Am.

11

Chem. Soc. 2003, 125 (7), 1958-1967.

12

29. Yang, B.; Gong, X.-Q.; Wang, H.-F.; Cao, X.-M.; Rooney, J. J.; Hu, P., Evidence

13

To Challenge the Universality of the Horiuti-Polanyi Mechanism for Hydrogenation

14

in

15

Non-Horiuti-Polanyi Mechanism. J. Am. Chem. Soc. 2013, 135 (40), 15244-15250.

16

30. Yang, B.; Burch, R.; Hardacre, C.; Hu, P.; Hughes, P., Mechanistic Study of

17

1,3-Butadiene Formation in Acetylene Hydrogenation over the Pd-Based Catalysts

18

Using Density Functional Calculations. J. Phys. Chem. C 2014, 118 (3), 1560-1567.

19

31. Yang, B.; Burch, R.; Hardacre, C.; Hu, P.; Hughes, P., Importance of surface

20

carbide formation on the activity and selectivity of Pd surfaces in the selective

21

hydrogenation of acetylene. Surf. Sci. 2016, 646, 45-49.

22

32. Horiuti, J.; Polanyi, M., Catalysed Reaction of Hydrogen with Water and the

23

Nature of Over-voltage. Nature 1933, 132, 931-931.

24

33. Horiuti, J.; Polanyi, M., Catalytic Interchange of Hydrogen between Water and

25

Ethylene and between Water and Benzene. Nature 1934, 134, 377-378.

26

34. Joice, B. J.; Rooney, J. J.; Wells, P. B.; Wilson, G. R., Nature and reactivity of

27

intermediates in hydrogenation of buta-1,3-diene catalyzed by cobalt and

28

palladium-gold alloys. Discuss. Faraday Soc. 1966, 41 (0), 223-236.

29

35. Liu, J.; Lucci, F. R.; Yang, M.; Lee, S.; Marcinkowski, M. D.; Therrien, A. J.;

Heterogeneous Catalysis: Origin and Trend


of the Preference

of a


1

Williams, C. T.; Sykes, E. C. H.; Flytzani-Stephanopoulos, M., Tackling CO

2

Poisoning with Single-Atom Alloy Catalysts. J. Am. Chem. Soc. 2016, 138 (20),

3

6396-6399.

4

36. Valcárcel, A.; Clotet, A.; Ricart, J. M.; Delbecq, F.; Sautet, P., Comparative DFT

5

study of the adsorption of 1,3-butadiene, 1-butene and 2-cis/trans-butenes on the

6

Pt(111) and Pd(111) surfaces. Surf. Sci. 2004, 549 (2), 121-133.

7

37. Medford, A. J.; Shi, C.; Hoffmann, M. J.; Lausche, A. C.; Fitzgibbon, S. R.;

8

Bligaard, T.; Nørskov, J. K., CatMAP: A Software Package for Descriptor-Based

9

Microkinetic Mapping of Catalytic Trends. Catal. Lett. 2015, 145 (3), 794-807.

10

38. Gu, Y.; Zhao, Y. H.; Wu, P. P.; Yang, B.; Yang, N. T.; Zhu, Y., Bimetallic PtxCoy

11

nanoparticles with curved faces for highly efficient hydrogenation of cinnamaldehyde.

12

Nanoscale 2016, 8 (21), 10896-10901.

13

39. Latimer, A. A.; Kulkarni, A. R.; Aljama, H.; Montoya, J. H.; Yoo, J. S.; Tsai, C.;

14

Abild-Pedersen, F.; Studt, F.; Nørskov, J. K., Understanding trends in C-H bond

15

activation in heterogeneous catalysis. Nat. Mater. 2017, 16 (2), 225-229.

16

40. Wu, P.; Yang, B., Significance of Surface Formate Coverage on the Reaction

17

Kinetics of Methanol Synthesis from CO2 Hydrogenation over Cu. ACS Catal. 2017,

18

7187-7195.

19

41. Yang, B.; Wang, D.; Gong, X.-Q.; Hu, P., Acrolein hydrogenation on Pt(211) and

20

Au(211) surfaces: a density functional theory study. Phys. Chem. Chem. Phys. 2011,

21

13 (47), 21146-21152.

22

42. Yang, B.; Burch, R.; Hardacre, C.; Headdock, G.; Hu, P., Origin of the Increase of

23

Activity and Selectivity of Nickel Doped by Au, Ag, and Cu for Acetylene

24

Hydrogenation. ACS Catal. 2012, 2 (6), 1027-1032.

25

43. Yang, B.; Cao, X.-M.; Gong, X.-Q.; Hu, P., A density functional theory study of

26

hydrogen dissociation and diffusion at the perimeter sites of Au/TiO2. Phys. Chem.

27

Chem. Phys. 2012, 14 (11), 3741-3745.

28

44. Yang, B.; Burch, R.; Hardacre, C.; Headdock, G.; Hu, P., Influence of surface

29

structures, subsurface carbon and hydrogen, and surface alloying on the activity and 16 ACS Paragon Plus Environment

Page 16 of 28

Page 17 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


1

selectivity of acetylene hydrogenation on Pd surfaces: A density functional theory

2

study. J. Catal. 2013, 305, 264-276.

3

45. Yang, B.; Burch, R.; Hardacre, C.; Headdock, G.; Hu, P., Understanding the

4

Optimal Adsorption Energies for Catalyst Screening in Heterogeneous Catalysis. ACS

5

Catal. 2014, 4 (1), 182-186.

6

46. Yang, B.; Burch, R.; Hardacre, C.; Hu, P.; Hughes, P., Selective Hydrogenation of

7

Acetylene over Pd-Boron Catalysts: A Density Functional Theory Study. J. Phys.

8

Chem. C 2014, 118 (7), 3664-3671.

9

47. Yang, B.; Burch, R.; Hardacre, C.; Hu, P.; Hughes, P., Selective hydrogenation of

10

acetylene over Cu(211), Ag(211) and Au(211): Horiuti-Polanyi mechanism vs.

11

non-Horiuti-Polanyi mechanism. Catal. Sci. Technol. 2017, 7, 1508-1514.

12 13 14



1 2

Scheme 1. Considered hydrogenation pathways of butadiene to butane, taking butenes

3

as partially hydrogenated intermediates.

4


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1

Table 1. ZPE-corrected activation energies (Ea) and reaction energies (∆E) for the

2

elementary steps within butadiene hydrogenation reactions over Pt1/Cu(111),

3

Pt2/Cu(111) and Cu(111) surfaces. All the energies are in eV. Pt1/Cu(111)

Pt2/Cu(111)

Ea

∆E

Ea

∆E

Ea

∆E

Cu(111)

Elementary step

I

t-C4H6(g) + * -> t-C4H6*

\

-0.57

\

-0.78

\

-0.34

II

c-C4H6(g) + * -> c-C4H6*

\

-0.63

\

-0.87

\

-0.42

III

1-C4H8(g) + * -> 1-C4H8*

\

-0.51

\

-0.49

\

-0.12

IV

t-2-C4H8(g) + * -> t-2-C4H8*

\

-0.28

\

-0.25

\

-0.08

V

c-2-C4H8(g) + * -> c-2-C4H8*

\

-0.39

\

-0.32

\

-0.11

VI

t-C4H6* + H* -> t-C4H7* + *

0.64

-0.07

0.76

-0.26

0.54

-0.09

VII

c-C4H6* + H* -> c-C4H7* + *

0.68

-0.12

0.81

-0.24

0.59

-0.12

VIII

t-C4H7* + H* -> 1-C4H8* + *

0.63

-0.33

0.86

0.08

0.57

-0.26

IX

t-C4H7* + H* -> t-2-C4H8* + *

0.68

-0.34

0.90

0.09

0.67

-0.45

X

c-C4H7* + H* -> 1-C4H8* + *

0.62

-0.37

0.80

0.01

0.55

-0.29

XI

c-C4H7* + H* -> c-2-C4H8* + *

0.70

-0.41

0.87

0.02

0.60

-0.45

XII

1-C4H8* + H* -> 1-C4H9* + *

0.74

0.08

0.71

0.09

0.64

0.05

XIII

t-2-C4H8* + H* -> 2-C4H9* + *

0.83

-0.08

0.74

0.16

0.95

0.40

XIV

c-2-C4H8* + H* -> 2-C4H9* +*

0.87

0.23

0.86

0.15

0.89

0.35

XV

1-C4H9* + H* -> C4H10(g) + 2*

0.52

-0.47

0.60

-0.49

0.55

-0.92

XVI

2-C4H9* + H* -> C4H10(g) + 2*

0.59

-0.56

0.59

-0.57

0.49

-1.07

XVII

2H* -> H2 (g) + 2*

0.32

0.54

0.50

0.62

0.93

0.44

4 5



1 2

Figure 1. Adsorption configurations of t-C4H6 (left) and c-C4H6 (right) over the SAA

3

and A-SAA structures, i.e. Pt1/Cu(111), Pt2/Cu(111), Pt3/Cu(111) and Pt4/Cu(111),

4

where the subscript of Pt denotes the numbers of aggregated Pt atoms in Cu(111). Pt,

5

Cu, C and H atoms are noted by blue, red, grey and white balls, respectively, and this

6

notation is utilized throughout the paper.

7


Page 20 of 28

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1 2

Figure 2. Energy cost for the aggregation of the SAA Pt1/Cu(111) structure to the

3

A-SAA Pt2/Cu(111), Pt3/Cu(111) and Pt4/Cu(111) structures under different conditions.

4

The “Clean surface” coordinate denotes the energies of a surface without any

5

adsorbates, whilst the remaining coordinates present the corresponding adsorbates

6

considered to induce the aggregation. All the energies are ZPE-corrected.

7



1 2

Figure 3. Adsorption configurations of C4H6, C4H7, C4H8, C4H9 and their

3

corresponding hydrogenation transition states over Pt1/Cu(111).

4


Page 22 of 28

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1 2


3

corresponding hydrogenation transition states over Pt2/Cu(111).

4



1 2


3

corresponding hydrogenation transition states over Cu(111).

4


Page 24 of 28

Page 25 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


1 2

Figure 6. Total rate of C4H6 hydrogenation plotted as a function of temperature over

3

(a) Pt1/Cu(111) Pt2/Cu(111) and Cu(111) structures, and (b) the Pt site and Cu site of

4

the SAA catalyst surface where the surface concentration of the single Pt site is 0.02

5

ML and Cu site is 0.98 ML. The pressure is fixed at 1 bar.

6 7



1 2

Figure 7. Formation rate of C4H10 plotted as a function of temperature over

3

Pt1/Cu(111), Pt2/Cu(111) and Cu(111) structures at 1 bar.

4 5


Page 26 of 28

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1 2

Figure 8. Selectivity of different butene isomers plotted as a function of temperature

3

over (a) Pt1/Cu(111), (b) Pt2/Cu(111) and (c) Cu(111) structures. (d) shows the

4

selectivity obtained by considering the concentration of surface single Pt atoms, i.e.

5

0.02 ML, while running the microkinetic modeling. The pressure is fixed at 1 bar.

6



1

TOC Graphic

2 3

4 5


Page 28 of 28

Identification of the Active and Selective Sites over Single Pt Atom

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