Heterogeneous Single Cluster Catalysts for Selective Semi

Subscriber access provided by IDAHO STATE UNIV

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Heterogeneous Single Cluster Catalysts for Selective SemiHydrogenation of Acetylene with Graphdiyne-Supported Triatomic Clusters Deng-Hui Xing, Cong-Qiao Xu, Yang-Gang Wang, and Jun Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02029 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17 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

The Journal of Physical Chemistry

Heterogeneous Single Cluster Catalysts for Selective Semi-Hydrogenation of Acetylene with Graphdiyne-Supported Triatomic Clusters Deng-Hui Xing,a Cong-Qiao Xu,a Yang-Gang Wang,b* Jun Lia,b* Dedicated to Professor Xintao Wu for his 80th birthday

a

Department of Chemistry & Key Laboratory of Organic Optoelectronics, and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China

b Department

of Chemistry, Southern University of Science and Technology, Shenzhen 518000, China Email: [email protected] (J.L.); [email protected] (Y.G.W.)

ABSTRACT: Abstract: Graphdiyne (GDY) is a newly discovered, novel carbon material highly promising for chemical, physical, material, and industrial applications. The unique yet natural pores of GDY provide novel prospect to design stable metal-cluster catalyst strongly bonded to its carbon framework. We report here a theoretical study using density functional theory (DFT) on a heterogeneous surface single-cluster catalyst (SCC) supported by GDY. This SCC features a triatomic metal cluster strongly anchored on the 18-member-ring hexagon of GDY support, MxM’3-x/GDY (M, M’= Ru, Os), and can hydrogenate acetylene to ethylene with significant selectivity. This feature is attributed to a synergistic effect among the metal atoms in the cluster and the role of GDY as a charge buffer. The state-of-the-art chemical bonding and micro-kinetics analyses are performed to understand the mechanism and catalytic performance. Our work highlights the importance of stably anchored surface isolated metal clusters in heterogeneous catalysis and the synergy among metal atoms in cluster and the peripheral metal-support interfacial atoms.

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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. INTRODUCTION Maximizing the atomic utilization of a noble-metal catalyst is a key principle for reducing the cost in industrial catalysis. The single atom catalysts (SACs),1-3 with singly dispersed metal atoms on heterogeneous surface, have achieved the highest efficiency of utilizing catalytic metal atoms and the ultimate limit of this principle, because of its promising properties in high selectivity, stability, activity, and atom efficiency. However, as noted already,2,3 SAC is not always advantageous for complex catalytic processes where the multi-atom active sites or complicated multi-step (e.g. redox) reactions are necessary for activating and catalyzing the reaction species. It is therefore ideal to design well-defined stable metal cluster catalyst with specific active sites that can synergistically catalyze a target reaction. For this purpose we have recently extended the SAC idea and proposed a concept of heterogeneous surface single-cluster catalyst (SCC). The oxide surface anchored SCC have shown exceptional benefits for the N2-to-NH3 conversion.4,5 Because of the naturally existed pores, GDY (Scheme 1a) clearly provides a characteristic substrate for anchoring metal atoms and clusters via strong covalent metal-ligand bonding. We have therefore designed specific triatomic SCC structure, MxM’3-x (M, M’= Ru, Os), to explore the possibility of using GDY as support by taking advantage of its special frame structure and electronic properties. These SCCs of ruthenium and osmium are shown to exhibit excellent catalytic reactivity in selective hydrogenation of acetylene by synergy of the three metal atoms in the cluster anchored on the carbon framework. Carbon allotropes, such as fullerene, carbon nanotube, and graphene have been known since 1980s.6,7 Notably, a landmark discovery in carbon materials is the recent preparation of graphdiyne (GDY), a new planar carbon allotrope composed of sp and sp2 carbon atoms and exhibiting a highly delocalized π-π conjugation system all over the planar frame.8-10 As shown in Scheme 1, the structure of GDY can be divided into carbon six-member ring (6MR) and 18-member ring (18MR) hexagon as the basic geometry elements. There are extensively studies of GDY in lithium storage, electrode material, catalysis, and so on.11-19 The 18MR pores supply natural frames to clamp metal clusters to form stable and isolated structure through robust metal-carbon covalent bonding. For example, Wu and his coworkers find that GDY is an efficient support to cobalt nanoparticle for oxygen evolution reaction (OER).17 A theoretical study also reveals that GDY-supported Au38 cluster has good performance for CO oxidation.20 Counter electrodes of GDY supported Pt nanoparticles improve the power conversion 2


Page 2 of 17

Page 3 of 17 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


efficiency of dye-sensitized solar cells.21 Single metal atom doped GDY is also shown to be a versatile catalyst for dehydrogenation of light metal hydrides and CO2 capture.22 Given the characteristic pores in GDY, metal atoms and clusters with such support seem to be ideal candidate for hydrogenation of unsaturated hydrocarbon as well.

Scheme 1. (a) The local geometry structure and primitive cell (the rhomboid with dashed lines) of GDY. Side length of a-b and c-d are 1.43 and 6.61 Å, respectively. (b) The side view and top view of the structure of MxM’3-x /GDY. Carbon and MxM’3-x cluster are shown in gray and blue color, respectively. Mb and Mt denote the metal atom inside and above the carbon frame. In industrial production of polyethylene, elimination of acetylene from ethylene is often achieved by selective hydrogenation that converts acetylene to ethylene without further reduction to ethane, which is the so-called semi-hydrogenation.23-26 The selective hydrogenation of acetylene is achieved on, for example, oxide supported palladium,27,28 as well as silver,29etc. Alternative catalysts such as palladium supported on carbon are also explored.30,31 On the other hand, isolated single-atom sites in intermetallic nanostructures are also shown to have high catalytic selectivity for semi-hydrogenation of alkynes.32,33 In this article, we designed a series of GDY-embedded metal clusters MxM’3-x (Mx, M’= Ru, Os; x = 0-3) by first-principles calculations for selective hydrogenation of acetylene to achieve the efficient utilization of each metal atom and overcome the limitation of SAC in multiple active-site involved reactions, as shown in Scheme 1b. We find that, in this new type of SCC, two of metal atoms mainly 3



activating the hydrocarbon and the third one dissociating H2 molecule, which is reminiscent of bi-metallic clusters in oxides.34 This synergistic effect arises from the size mismatch between metal cluster and carbon frame, causing metal atoms bathed in different chemical environment from the isolated cluster. All the four catalysts (Ru3, Os1Ru2, Ru1Os2, Os3 supported on GDY) show robust stability and activity. Our theoretical study predicts that a selectivity of ethylene against ethane under 420 K can be achieved.

2. COMPUTATIONAL DETAILS Calculations were carried out by using density functional theory (DFT) as implemented in Vienna ab initio simulation package (VASP).35 The spin-polarized Kohn-Sham formalism with gradient-corrected exchange and correlation functional of Perdew-Burke-Ernzerhof (PBE) was adopted.36,37 The dispersion correction with Becke-Johnson damping was included to account for the van der Waals interaction.38,39 The projector augmented wave (PAW) potentials were utilized to replace the pseudo-potential of inner core electrons of each atom.40,41 The cut-off of the kinetic energy for the plane-wave basis sets was set to 400 eV. The convergence criterion of the force was set to be 0.03 eV/Å for geometry optimization and transition state searching.  point-only (1×1×1) scheme was adopted for the k-point sampling of the Brillouin zone for the geometry optimizations and a scheme of 4×4×1 was applied for static energy calculations. The climbing-image nudged elastic band (CI-NEB) method was used to locate the transition states.42,43 Vibrational frequencies were calculated to identify the transitions states with only one imaginary frequency. GDY-supported Ru3, Os1Ru2, Ru1Os2, and Os3 clusters are designed to model SCC (Scheme 1). A 2×2×1 hexagonal (a = b = 18.93 Å, c = 28.30 Å) supercell of the primitive cell was selected as the structure of GDY substrate, with a vacuum layer (28.30 Å) above the planar GDY. The binding energy and adsorption/desorption energy are defined following the thermodynamic convention. That is, the binding energy of M3 on GDY was calculated as Eb = EM3/GDY – (EM3 + EGDY) and X…M3/GDY adsorption or desorption energy was calculated as Ead = EX*@M3/GDY–Egas-phase X–EM3/GDY and Ed = - Ead . Micro-kinetic modelling was carried out by numerically solving the differential equations that describe the coverage of each surface intermediates using the CatMAP code.44,45. We noticed that the TOF map relies on the sampling points in scaling, however, as it shows four points with good linear scaling relationship will also provide inspiring insights.46 All parameters were derived from DFT 4


Page 4 of 17

Page 5 of 17 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


calculations and the rate constants were calculated using harmonic transition state theory with ZPE corrections and harmonic entropy contribution included (see details in SI). Periodic natural population analysis (NPA) was applied to evaluate the atomic charges in the SCCs.47 Periodic energy decomposition analysis combined with the natural orbitals for chemical valence (EDA-NOCV) was further performed by using BAND code of the Amsterdam Density Functional package (ADF) to evaluate the electronic interactions between the triatomic clusters and GDY support.48-50 Calculations by using ADF-BAND package were performed with TZP Slater-type basis set and a scheme of 1×1×1 k-point sampling. We also calculated Mayer51 and Nalewajski-Mrozek52-56 bond order indices using ADF package with a model cluster of M3/GDY, where the first neighboring carbon rings of GDY 18MR were saturated by hydrogen atoms, to assess the covalent metal-support interaction. 3. RESULTS AND DISCUSSION 3.1. Binding of Clusters on GDY Since the metal-support interaction is critical in controlling the stability and catalytic activity in heterogeneous catalysis, we first consider the interaction between MxM’3-x cluster and GDY prior to examining the catalytic performance. As shown in Table S1, the calculated binding energies are more than -7.0 eV, suggesting that these MxM’3-x clusters (Ru3, Os1Ru2, Ru1Os2, Os3) can strongly bind with GDY and are highly stable. The periodic natural population analysis results show that GDY gains electron density upon coordination to the metal clusters, and the natural population charges (see Table S3) of Ost and Osb in Os3/GDY are +0.17 |e-| and +0.14 |e-|, and those of Rut and Rub in Ru3/GDY are +0.18 |e-| and +0.06 |e-|, respectively. These charge differences indicate that the metal atoms in cluster are soaked in unequal charge environment. That is, the structural distortion caused by the size mismatch in the 18MR hexagon differs three atoms in Ru3 and Os3, thus providing opportunity to synergistically tune the catalytic behaviour of the hydrogenation reactions. For the GDY-supported bimetallic clusters MxM’3-x (M, M’ = Ru, Os), the large binding energies (Table S1) also suggest strong covalent metal-support interaction (CMSI)57,58 between MxM’3-x cluster and GDY, implying high stability of these supported structures. For comparison, we also calculated Pd3 and Pt3 cluster on GDY and the calculated binding energies are only -4.14 and -5.16 eV, respectively, indicating that the traditional Pd/Pt catalyst for acetylene hydrogenation may not be able to form SCC with high stability, compared to Os and Ru. 5



Figure 1. The NOCV difference pair of the Os3/GDY. Electron migrates from blue to yellow pair. Occ is the natural orbital occupation number, Eoi is the orbital interaction energy in eV, and %Eoi is the percentage of energy contribution of one NOCV pair in the total orbital interaction energy, EToi (see Table S4). (a) and (b) are the two main components in NOCV pairs. The isovalue of natural orbital is set to be 0.03 a.u.

To understand the CMSI in these structures, EDA-NOCV analysis and bond order calculations were performed (see Tables S4 and S5 of SI).48-56 Here, we take Os3/GDY as an example for simplicity. The intrinsic bond energy for binding of Os3 on GDY is -13.04 eV (~300 kcal/mol), suggesting the enormous covalent interactions between Os3 and GDY. Orbital correlation analysis shows that Os3 of Os3/GDY formally loses two electrons while GDY gains two electrons in this system through a D3h symmetry model in Figure S3. From the periodic EDA-NOCV analysis, the first two natural orbital pairs with the largest occupation number for Os3/GDY are depicted in Figure 1. Clearly, charge migrates from Os 5d orbitals to Os-C bond to help the cluster anchored in the frame. On the other hand, the two types of bond order indices calculated for Os-C of the model clusters (Table S5) both lie in the ranges of 0.2 ~ 0.8. The significant bond orders and the large percentage of energy contribution of the NOCV pairs in the total orbital interaction energy reveal that there is significant covalent bonding between metal and support C atoms. Therefore, we can conclude that the size mismatch effect distorts the triatomic cluster while the CMSI between metal and GDY conserves the stability.

3.2. Reaction Mechanism of Acetylene Hydrogenation on MxM’3-x /GDY

6


Page 6 of 17

Page 7 of 17 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


Since the triatomic clusters are rather stably anchored in the 18MR pore of GDY, we now investigate the reaction mechanism for acetylene hydrogenation on this SCC model. The catalytic cycle is found to follow the Langmuir-Hinshelwood (L-H) mechanism in accordance with the previous studies,59 and can be written as following (* means the adsorbed state on GDY): Step 1: 𝐶2𝐻2∗ + 𝐻2∗ ⟶𝐶2𝐻3∗ + 𝐻 ∗

(1)

Step 2: C2H3∗ + H ∗ ⟶C2H4∗

(2)

Step 3: C2H4∗ + H2∗ ⟶C2H5∗ + H ∗

(3)

Step 4: C2H5∗ + H ∗ ⟶C2H6

(4)

where step 1 and 2 are the first semi-hydrogenation of acetylene, and step 3 and 4 are the second hydrogenation of acetylene.

Figure 2. The reaction energy profile of hydrogen addition steps of acetylene and ethylene, with the energy of Os3/GDY as a reference. Ed is desorption energy of ethylene in eV. Color code: C (gray), Os (blue), H in C2Hx (white), and H in H2 (brown). In Figure 2, we show the reaction paths for acetylene hydrogenation catalysed by Os3/GDY as an example. Threefold Os3 cluster binds two reactants and triggers the reactions, consistent with the previous study on three-fold PdGa.60 Our calculations show that acetylene binds on the two Osb atoms via two  bonds with Os and an additional d- interaction (see Figure S7), and the dissociation of H2 7



Page 8 of 17

is nearly barrierless on Ost site, forming two isolated H atoms (see Figure S2) with the distance of H…H being 1.80 Å. For adsorbed acetylene, the C-C distance of 1.45 Å lies between that of ethylene (1.34 Å) and ethane (1.51 Å) in gas phase, indicating that acetylene is significantly activated on Os3/GDY. The barriers of the first two-step hydrogenations of acetylene to form C2H4* are 0.41 and 0.08 eV, respectively. However, ethylene hydrogenation has relatively high energy barriers (0.74 and 1.53 eV for step 3 and 4), preventing further reaction at low temperature. Though -bonded ethylene is more favourable for further hydrogenation,61 it is the di- bonded ethylene adsorbed on catalyst of Os3/GDY because of the strong CMSI (see Figure S7). When H2 and C2H4 are co-adsorbed and activated on Os3/GDY, one H atom will migrate to the carbon in C2H4* to yield a C2H5* and the hydrocarbon binds to the catalyst with a single d- bond. After that, another H* dissociates from Ost and comes to the hydrocarbon and form a gas phase ethane. During the hydrogenation of ethylene, produced from acetylene, the barrier of the rate-determining step is 1.53 eV, significantly higher than previous steps. Competition between the ethylene desorption and further reaction is the apparent factor that affects the selective hydrogenation of acetylene. It is shown in Figure 2 that the desorption energy Ed of C2H4* is only 0.65 eV, much lower than the barriers for further hydrogenation. Hence, the reaction can be controlled to have the formed C2H4* diffused to gas phase so as to accomplish a selective hydrogenation of acetylene. Table 1 lists the energy barriers during acetylene hydrogenation on all four MxM’3-x /GDY catalysts. The di- bond between C-Mb can be found in both C2H2* and C2H4*. Nevertheless, the d- bond can only be located in C2H2* with additional assistance of the Mt. Thus, the d- interaction is the prominent factor that accounts for the fact that the adsorption energy of acetylene is markedly larger than ethylene on MxM’3-x/GDY, as shown in Table 1. These bonding pattern in C2H2* and C2H4* on Os3/GDY are shown in Figure S7. In Table 1, the barriers of the first two steps corresponding to the hydrogenation of acetylene are lower than those in further hydrogenation of ethylene. Hence, the two kinds of metal atoms at the SCC active center provide the synergistic sites for activating H2 and activation of hydrocarbon. Table 1. Adsorption Energies & Energy Barriers from DFT Calculations on MxM’3-x/GDY in eV. Ead-C2H2

Ead-C2H4

Reaction Barrier Step 1

8


Step 2

Step 3

Step 4

Page 9 of 17 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


Ru3/GDY -2.64 Os1Ru2/GDY -2.54 Ru1Os2/GDY -2.45 Os3/GDY -2.89 * All energies are in eV.

-1.21 -0.82 -0.58 -0.65

0.30 0.64 0.52 0.41

0.61 0.21 0.32 0.08

0.51 0.72 0.67 0.74

0.65 1.13 0.45 1.53

We evaluated the turnover frequencies (TOFs) as the production rate with the micro-kinetic modeling under the 420 K and the partial pressures, PC2H2=1/3 bar and PH2= 2/3 bar. We consider the log10(TOFC2H4/TOFC2H6) as an indicator of the selectivity of this MxM’3-x/GDY system and a volcano surface of generation rate of ethylene are shown in Figure 3. All the four SCC perfectly locate in the region where the TOF from 0.1 to 105 s-1 ， indicating that the reaction is highly observable under realistic condition. In Figure 3(a), a combination of both large EC2H4 and EH2 does not promise a satisfying TOF of ethylene, because it leads to a strong binding of C2H4 and significantly decrease the desorption rate of C2H4. So does a combination of both small EC2H4 and EH2 since the hydrogenation rate of *C2H2 to *C2H4 is largely decreased.

Figure 3. The simulated TOF of (a) ethylene and (b) ethane through the micro-kinetic modeling and (c) the selectivity of generating ethylene vs. ethane. TOFC2H4 and TOFC2H6 are the yielding rate of gas-phase hydrocarbon. (d)The TOF of C2H4 and C2H6 in the same map. Solid color map is the TOF surface of C2H4 and transparent color surface is that of C2H6. TOFs below 10-10s-1 is not shown in the map. 9



The calculated selectivity map is shown in Figure 3(c). The calculated log10(TOFC2H4/TOFC2H6) values for Os3, Ru1Os2, Os1Ru2 and Ru3 SCC are 11.5, 10.6, 10.58, and 4.7, respectively, indicating that that SCC models give high selectivities for C2H4 formation and making them good candidate catalysts for the hydrogenation of acetylene. The high selectivity in the region of positive EH2 comes from the almost zero generation rate of C2H4 and C2H6,TOF

Heterogeneous Single Cluster Catalysts for Selective Semi

Recommend Documents