RESEARCH PAPER

Single-layer graphdiyne-covered Pt(111) surface: improvedcatalysis confined under two-dimensional overlayer

Xi Chen & Zheng-Zhe Lin

Received: 19 January 2018 /Accepted: 23 April 2018# Springer Science+Business Media B.V., part of Springer Nature 2018

Abstract In recent years, two-dimensional confinedcatalysis, i.e., the enhanced catalytic reactions in con-fined space between metal surface and two-dimensionaloverlayer, makes a hit and opens up a new way toenhance the performance of catalysts. In this work,graphdiyne overlayer was proposed as a more excellentmaterial than graphene or hexagonal boron nitride fortwo-dimensional confined catalysis on Pt(111) surface.Density functional theory calculations revealed the su-periority of graphdiyne overlayer originates from thesteric hindrance effect which increases the catalyticability and lowers the reaction barriers. Moreover, withthe big triangle holes as natural gas tunnels, graphdiynepossesses higher efficiency for the transit of gaseousreactants and products than graphene or hexagonal bo-ron nitride. The results in this work would benefit futuredevelopment of two-dimensional confined catalysis.

Keywords Graphdiyne . Two-dimensional cover .

Confined catalysis . Nanostructured catalysts .Modelingand simulation

Introduction

Since the birth of graphene (Geim and Novoselov 2007;Novoselov et al. 2004, 2005a), two-dimensional atomiccrystals are of considerable interest because of theirunique structural and electronic properties. Possible ap-plications of two-dimensional atomic crystals in cataly-sis are also paid close attention (Novoselov et al.2005b). People have tried to exploit the specialty ofgraphene for heterogeneous catalysis (Machado andSerp 2012), electrocatalysis (Antolini 2012; Hur andPark 2013), and photocatalysis (An and Yu 2011). How-ever, graphene is conventionally considered as chemi-cally inert due to its saturated C–C bonds. So, vacancies,impurities, and chemical modifications are widely con-sidered for enhancing the catalytic ability of graphene(Deng et al. 2011; Jia et al. 2014; Liu et al. 2010; Su andLoh 2013; Tang et al. 2012, 2014; Yan et al. 2015; Yuet al. 2011) and other two-dimensional materials (Anet al. 2017; Ataca and Ciraci 2012; Deng et al. 2015b;Lee et al. 2014; Li et al. 2013; Lin 2016; Lin and Chen2016; Ma et al. 2015; Yu et al. 2014). In chemicallymodified systems, low-coordinated transition metalatoms are more active than saturated two-dimensionalmaterial surfaces to act as catalytic sites. In recent years,scientists keep pursuing the ultimate activity of transi-tion metal catalysts, i.e., the single-atom catalysts an-chored to special substrates, which exhibit more superi-or catalytic ability than conventional metal nanoparti-cles (Lin et al. 2013; Qiao et al. 2011; Wei et al. 2014;Yang et al. 2013). For example, single Pt or Ir atomembedded in FeOx surface has shown their ability to

J Nanopart Res (2018) 20:136 https://doi.org/10.1007/s11051-018-4236-0

PACS 82.65.+r Surface and interface chemistry;heterogeneous catalysis at surfaces PACS 82.20.KhPotentialenergy surfaces for chemical reactions

X. Chen : Z.<Z. Lin (*)Department of Applied Physics, School of Physics andOptoelectronic Engineering, Xidian University, Xi’an 710071,Chinae-mail: linzhengzhe@hotmail.com

transform CO into other molecules (Lin et al. 2013;Qiao et al. 2011). Single Pd atom embedded in graphenehas shown remarkable performance in selective hydro-genation of 1, 3-butadiene (Yan et al. 2015). Single Featom embedded in graphene has been demonstrated ashighly efficient catalyst for benzene oxidation (Denget al. 2015a).

To achieve the synthesis of single-atom catalysts,state-of-the-art techniques have to be employed, whichlimits the widespread application of single-atom cata-lysts. People have also explored the opposite side ofultimate catalyst size, i.e., combining two-dimensionalmaterials with Blarge^ catalysts. It is well establishedthat catalysts in confined space can have enhancedactivity (Pan and Bao 2008; Smit and Maesen 2008).In recent years, two-dimensional confined catalysis (Muet al. 2012; Yao et al. 2014; Zhang et al. 2015; Zhouet al. 2016), which utilizes the confinement of two-dimensional overlayer to enhance the catalytic abilityof metal surface, has made a hit. Much effort wasdevoted to study two-dimensional graphene or hexago-nal boron nitride-covered Pt(111) surface (Mu et al.2012; Yao et al. 2014; Zhang et al. 2015). It has beenfound that gaseous molecules can readily intercalate thetwo-dimensional overlayer, and the confined space be-tween the overlayer and the underlying metal acts asnano reactors. Such novel idea will open a new era ofcatalysis.

With the rapid development of two-dimensional ma-terials, more and more candidates for two-dimensionalconfined catalysis have been born. Several decades ago,graphyne and its family (graphdiyne, graphyne-3 etc.)were predicted (Baughman et al. 1987; Narita et al.1998, 2000) to be two-dimensional-layered C allotropeswith acetylenic C≡C bonds. In 2010, graphdiyne wassuccessfully synthesized on copper surface via a cross-coupling reaction using hexaethynylbenzene (Li et al.2010). Recently, large segments of graphyne andgraphdiyne films were successfully synthesized (Haley2008; Johnson II et al. 2007; Kehoe et al. 2000; Li et al.2010, 2014; Matsuoka et al. 2017; Tahara et al. 2007;Zhou et al. 2015). From the view of structure, the bigtriangle holes in graphdiyne could accommodate smallmolecules and act as gas tunnels. It can be inferred thatwith the natural gas tunnels, graphdiyne overlayer maybe more suitable than graphene or hexagonal boronnitride for two-dimensional confined catalysis. Theoret-ical research has confirmed the possibility of smallmolecules passing through the triangle holes of

graphdiyne (Jiao et al. 2011; Zhang et al. 2012). Inaddition, recent reports suggested graphdiyne as catalystfor CO oxidation (Wu et al. 2014) and excellent catalystsupport (Qi et al. 2015). Comprehensively consideringthe above situation, further research on graphdiyne-covered systems could benefit future development oftwo-dimensional confined catalysis.

In this work, density functional theory (DFT) calcu-lations were carried out to investigate two-dimensionalconfined catalysis on graphdiyne-covered Pt(111) sur-face. Catalytic CO oxidation on graphdiyne-coveredPt(111) surface was systematically studied and com-pared with pristine Pt(111) surface. The reaction mech-anism of CO oxidation was analyzed to reveal the two-dimensional confined catalytic effect of graphdiyne andthe advantage of graphdiyne cover to graphene. Due tothe steric hindrance of graphdiyne, the barriers, and freeenergy changes of CO oxidation substeps ongraphdiyne-covered Pt(111) surface are corresponding-ly lower than on pristine Pt(111). The catalytic reactionon graphdiyne-covered Pt(111) surface is more thermo-dynamically favorable than on pristine Pt(111) andcould be easily proceeded at room temperature. Thiswork primarily explored the advantage of porous two-dimensional material to confined catalysis and providedbeneficial information for future development of two-dimensional confined catalysis.

Computational details

DFT calculations were performed using the Vienna abinitio simulation package (Kresse and Furthmüller1996a, b; Kresse and Hafner 1994; Kresse 1993). Theprojector-augmented wave method (Blöchl 1994;Kresse and Joubert 1999) was usedwith a kinetic energycutoff of 400 eV. The generalized gradient approxima-tion of Perdew-Burke-Ernzerhof (Perdew et al. 1996)was employed as the exchange-correlation functional.Grimme’s DFT-D2 correction (Grimme 2006) wasemployed to account for van der Waals interactions(with C6 = 24.67 J nm6mol−1 and RvdW = 1.75Å chosenfor Pt (Yao et al. 2014)). The Brillouin-zone integrationwas performed with 2 × 2 × 1 Monkhorst-Pack grid(Monkhorst and Pack 1976) and a Gaussian smearingof σ = 0.05 eV. The convergence of total energy wasconsidered to be achieved until the energy difference oftwo iterated steps was less than 10−6 eV. Geometrieswere fully relaxed without any symmetric constrains

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until the Hellmann-Feynman forces were below0.001 eV/Å. To verify the accuracy of energy calcula-tion, we tried to enhance the kinetic energy cutoff to600 eV, which resulted in a change of total energy lessthan 0.2%. For a same molecule (CO, O, or O2), thedifference between the energies of adsorption configu-rations is kept in an accuracy of 0.001 eV. So, theaccuracy of our calculations is tested. To save compu-tation time, we used a kinetic energy cutoff of 400 eVthroughout the paper.

The surface of Pt bulk or nanoparticles is modeled asPt(111) surface. Graphdiyne-covered Pt(111) surfacewas simulated by a repeated slab model in which (1 ×1) graphdiyne layer was placed on top of a four-layered

(2ffiffiffi

3p

× 2ffiffiffi

3p

) Pt slab with the bottom two layers fixed.The replicas of simulation system were separated by avacuum layer of at least 12 Å in the direction perpen-dicular to the Pt(111) surface. The size of the two fixedbottom layers was set according to the optimized latticeconstant a = 3.97 Å of Pt bulk, which is about 1% largerthan the experimental value a = 3.92 Å. The graphdiynelattice constant was adapted accordingly, resulting in astrain of 3%.

The search of reaction paths and transition states wasperformed using the climbing image nudged elasticband (CINEB) method (Henkelman et al. 2000; Millsand Jónsson 1994; Mills et al. 1995), with linear inter-polation between the coordinates of reactant and productas initial guess of reaction paths. Seven images wereinserted between two stable states. The reaction pathswere relaxed by minimizing the residual forces withquasi-Newton algorithm. The geometries of reactants,products, and transition states were verified by means offrequency calculations. In free energy calculations, thezero-point energy (ZPE) and entropic corrections havebeen included. At 0 K, the free energy of a species iscalculated according to

G0 ¼ EDFT þ EZPE; ð1Þwhere EDFT is the relaxed DFT total energy and EZPE isthe ZPE. At T = 300 K and pressure p = 1 atm, the freeenergy of a species is calculated according to

G ¼ EDFT þ EZPE þ ∫T0CpdT−TS; ð2Þ

where ∫T0C pdT is the integrated heat capacity, T is thetemperature, and S is the entropy. Here, ZPE is calcu-lated with the vibrational frequencies as calculated with-in DFT. For solid phase species (Pt bulk and

graphdiyne), the volume is treated as fixed because ofsmall thermal expansion coefficient (e.g.,

αPt ¼ 1V

1V

� �

p ¼ 9:0� 10−6K−1). Then, we have ∫T0Cpd

T ¼ ∂lnZ∂ 1=kTð Þ þ PΔV≈ ∂lnZ

∂ 1=kTð Þ and S ¼ 1T

1T þ klnZ, and

then the term ∫T0CpdT−TS≈-kTlnZ. Here, the partition

function Z ¼ ∏i

1−e−ℏωi=kT� �−1

is calculated via har-

monic approximation with canonical frequencies ωi.For gas phase species (CO, O2 and CO2), the integrated

heat capacity ∫T0CpdT and entropy S are obtained fromstandard tables of thermodynamic data (Atkins and dePaula 2006; Gao et al. 2015), in which the effect of gasvolume expansion following the increase of temperatureis included in Cp(T) and S(T).

For the adsorption of species A on another species B,the binding energy is defined as

Eb ¼ G0 Að Þ þ G0 Bð Þ−G0 A−Bð Þð Þ; ð3ÞwhereG0(A),G0(B), andG0(A-B) are the free energy ofA, B, and A-B complex at 0 K, respectively, includingZPE corrections. For a reaction, the potential barrier iscalculated according to

Ea ¼ G0≠−G0 reactant; ð4Þ

where G0 reactant is the sum of free energies of thereactants at 0 K, and G0

≠ is the free energy of transitionstate at 0 K. The standard free energy of activation reads

ΔG≠ ¼ G≠−Greactant; ð5ÞwhereGreactant is the sum of free energies of the reactantsand G≠ is the free energy of transition state at T = 300 Kand pressure p = 1 atm. The standard free energy changeof a reaction reads

ΔG ¼ Gproduct−Greactant; ð6ÞwhereGproduct is the sum of free energies of the productsat T = 300 K and pressure p = 1 atm.

Results and discussion

Graphdiyne-covered Pt(111) surface

To obtain the most stable structure of graphdiyne-covered Pt(111) surface, we considered high-symmetryT, B, F, and H configurations, in which the center ofgraphdiyne hexagonal is positioned above the surface Pt

J Nanopart Res (2018) 20:136 Page 3 of 12 136

atom, the bridge of the surface Pt-Pt bond, the FCChollow site, and the HCP hollow site, respectively. Thebinding energy per C atom of graphdiyne on Pt(111)surface is defined as Eb/N = (G0(Pt) +G0(G) −G0(Pt-G))/N, where G0(Pt), G0(G), and G0(Pt-G) are the freeenergy of clean Pt slab, pristine graphdiyne sheet, andgraphdiyne-covered Pt slab at 0 K, respectively, andN = 18 is the number of C atoms in the graphdiyne sheet.The T configuration (Fig. 1) was found to be the moststable, with a binding energy Eb/N = 0.091 eV/atom (thebinding energies of B, F and H configurations are 0.086,0.090, and 0.088 eV/atom, respectively) and a distanced = 3.1 Å from the Pt(111) surface to the graphdiynesheet. The calculated values are close to Ref. (Pan et al.2015) (Eb/N = 0.11 eV/atom and d = 2.88 Å). Comparedwith the binding energy of graphene on Pt(0.042~0.084 eV/atom) (Olsen and Thygesen 2013),graphdiyne is bound more strongly on Pt. It is worthnoting that the graphdiyne-Pt distance is close tographene-Pt and hexagonal-boron-nitride-Pt distance(> 3 Å) (Olsen and Thygesen 2013; Yao et al. 2014;Zhang et al. 2015), providing appropriate environmentfor two-dimensional confined catalysis underneathgraphdiyne.

O2 adsorption and dissociation on graphdiyne-coveredPt(111) surface

Before investigating O2 adsorption and dissociation ongraphdiyne-covered Pt(111) surface, we first consideredO2 adsorption and dissociation on pristine Pt(111) sur-face for comparison. Figure 2a exhibits the high-symmetry FCC hollow site, the HCP hollow site, andthe BRIDGE site for O2 on Pt(111). We also consideredthe ATOP site where O2 is positioned on top of thesurface Pt atom, but the O2 molecule gradually movesto nearby locations during geometry relaxation. Thebinding energies of O2 on FCC/HCP/BRIDGE siteswere found to be Eb = 0.68/0.45/0.69 eV. For the moststable binding on the BRIDGE site, the calculated bind-ing energy is close to reported values in Ref. (Kattel andWang 2014), (Li et al. 2015) and (Li et al. 2016) (0.69,0.62 and 0.81 eV, respectively). In the dissociationprocess of O2 on Pt(111) (i.e., the O–O bond breakage),the O–O bond gradually rotates to the x direction,reaching the transition state with Ea = 0.38 eV andΔG≠ = 0.33 eV via a configuration near the FCC site.This calculated reaction path is in agreement with re-ported in Ref. (Li et al. 2016). The standard free energy

change of the dissociation reaction O2→ 2O isΔG = −1.62 eV. The binding energies of single dissociated Oatom on the FCC/HCP hollow sites are Eb = 4.50/3.96 eV, respectively. Note that on the most stableFCC site, the binding energy of O atom is in agreementwith Ref. (Bleakley and Hu 1999; Kattel and Wang2014; Li et al. 2015, 2016) (4.0~4.4 eV). The abovecalculations reproduced O2 binding energy and dissoci-ation barrier those are similar to previous reports, indi-cating the reliability of our calculation method.

Before investigating O2 adsorption on graphdiyne-covered Pt(111) surface, we checked the penetration ofO2 and CO through graphdiyne layer. Previous workshave proved that the barriers for small molecules (CO,O2, N2, and H2) passing through the hole of free-standing graphdiyne are less than 0.3 eV (Jiao et al.2011; Zhang et al. 2012). For the graphdiyne-coveredPt(111) system, our CINEB calculations proved that O2

and COmolecule could pass through the triangular holeof graphdiyne without potential barrier and arrive at Ptsurface. This makes the possibility of O2 and COreacting beneath graphdiyne cover.

Then, the O2 adsorption beneath graphdiyne coverwas investigated. The potential energy map of O2 mol-ecule under graphdiyne cover was plotted in the irre-ducible area shown by the hexagonal dotted lines in Fig.2b. Geometry relaxations and calculations of bindingenergy Eb (including ZPE corrections) were performedwith the centroid of O2 fixed on every FCC/HCP/BRIDGE/ATOP site underneath the graphdiyne cover.All other positions outside can be mapped into theirreducible area according to the symmetry. In the irre-ducible area, the potential energy surface (PES) wasplotted using data interpolation of binding energies atall FCC, HCP, BRIDGE, and ATOP sites. On everyadsorption site, geometry optimizations were performedfor different O2 initial orientations (i.e., vertical, parallelor other directions to the Pt(111) surface) to find thelowest-energy configuration. We found that on top of asame adsorption site, different initial orientations of O2

did not affect the result of geometry optimization, andthen we got the adsorption energies on every position ofPt(111) surface. The color map of O2 PES underneaththe graphdiyne cover is shown in the right of Fig. 2b.The BRIDGE site in the triangular C acetylenic ring wasfound to be the most stable adsorption site (see the leftpanel of Fig. 2b) with a binding energy ofEb = 1.142 eV,and the neighboring FCC site is the second most stablewith a binding energy of Eb = 1.138 eV (the red area in

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the right panel of Fig. 2b).With largest binding energies,the BRIDGE and FCC sites in the acetylenic triangle arethermodynamically favorable for O2 adsorption. Here,the calculated binding energy of O2 on graphdiyne-covered Pt(111) surface (1.14 eV) is much larger thanon pristine Pt(111) surface (0.69 eV). When O2 getsclose to the C skeleton of graphdiyne, the repulsion

between graphdiyne and O2 causes the decrease ofbinding energy. In the right panel of Fig. 2b, we cansee a large area with lower binding energy (about Eb =0.6~0.7 eV, the blue area in the right panel of Fig. 2b)due to the steric hindrance of graphdiyne sheet. Thisarea is energetically unfavorable for O2 molecule toaccess. Furthermore, to see if there exists other possible

Fig. 1 The top and side view ofthe T configuration ofgraphdiyne-covered Pt(111) slab.The simulation cell is enclosed bydashed lines. Pt and C atoms arerepresented in blue and gray,respectively

Fig. 2 a Schematic of FCC, HCP, BRIDGE, and ATOP adsorp-tion sites of O2 on Pt(111) surface. The left panel of b, d shows themost stable site for O2/O underneath graphdiyne layer, and thecolor map represents the PES of O2/O in the irreducible area(enclosed by dashed lines), respectively. The color bars show the

binding energies in eV. In b, the minimal-energy path of O2 isplotted along the path shown by the arrow. c O2 dissociation ongraphdiyne-covered Pt(111) surface. Pt, C, and O atoms are rep-resented in blue, gray, and red, respectively

J Nanopart Res (2018) 20:136 Page 5 of 12 136

stable positions between the above-mentioned sites, aminimal-energy path along BRIDGE-ATOP-BRIDGE-ATOP (shown by the arrow in Fig. 2b) was plotted usingCINEB method. According to the energy profile (thetop-right of Fig. 2b), we can see no potential valleysbeyond the two stable BDIRGE sites (position A and C)and the two unstable ATOP sites (position B and D).Such calculation could partially exclude the existence ofother metastable sites beyond FCC/HCP/BRIDGE sites.According to the above results, the O2 adsorption main-ly happens in the separated area surrounded bygraphdiyne acetylenic rings. The binding energy of O2

on graphdiyne-covered Pt(111) surface is larger than onpristine Pt(111) surface, which is advantageous to thepre-adsorption of O2.

The barrier and standard free energy of activation ofO2 dissociation on graphdiyne-covered Pt(111) surfacewere found to be Ea = 0.34 eV and ΔG≠ = 0.28 eV,respectively, which are lower than Ea = 0.38 eV andΔG≠ = 0.33 eV of the dissociation on pristine Pt(111)surface. At T = 300 K and pressure p = 1 atm, the rateconstant of O2 dissociation on graphdiyne-coveredP t ( 1 1 1 ) s u r f a c e i s r ¼ kT

h exp −ΔG≠=kT� �

¼ 1:2� 108s‐1, which is about seven times of r =1.8 × 107 s−1 on pristine Pt(111) surface. In the primarystep of O2 dissociation (Fig. 2c), one of the O atomsgoes across the neighboring HCP site and reaches thenearby FCC site with elongating O–O bond length. Thestandard free energy change from O2 (the initialconfiguration in Fig. 2c) to two O atoms located atnearest FCC sites (the final configuration in Fig. 2c),i.e., the coadsorption state of O atoms, is ΔG = −1.70 eV. Then, two O atoms may migrate and becometotally separated. The standard free energy changeΔG =− 1.91 eV from O2 to two totally separated O atoms islarger than on pristine Pt(111) surface (ΔG = −1.62 eV). Overall, the above results sufficiently demon-strate that the O2 adsorption and dissociation ongraphdiyne-covered Pt(111) surface is more thermody-namically favorable than on pristine Pt(111) surface,which provides a prerequisite for CO oxidation.

To plot the PES of O atom on graphdiyne-coveredPt(111) surface, geometry relaxations and binding ener-gy calculations (including ZPE corrections) were per-formed with the O atom put on every FCC/HCP/BRIDGE/ATOP site. The interpolated PES is shown inthe right panel of Fig. 2d. According to the results, thebinding energy of O atom on graphdiyne-coveredPt(111) surface (3.5~4.3 eV) is lower than that on

pristine Pt(111) surface (4.0~4.5 eV). The FCC site inthe triangular C acetylenic ring was found to be the moststable adsorption site (see the left panel of Fig. 2d) witha binding energy of Eb = 4.34 eV. The binding energieson FCC sites (4.11~4.34 eV) are generally higher thanon HCP sites (3.51~3.76 eV). Nevertheless, the bindingenergies on the FCC sites underneath the C skeleton ofgraphdiyne (4.11~4.16 eV) are lower than on the moststable FCC site (4.34 eV) due to the steric hindrance.Further calculations on O migration paths indicate thatthe migration barriers of O atom on graphdiyne-coveredPt(111) surface are below 0.8 eV, and the rate calcula-tions indicate that the migration of O atom ongraphdiyne-covered Pt(111) surface is possible at roomtemperature. The details about O migration will bediscussed in the BCO and O migration on graphdiyne-covered Pt(111) surface^ section.

By summarizing the results of the above calculations,the O2 adsorption and dissociation on Pt(111) is remark-ably enhanced by the use of graphdiyne. The free energydifference of O2 dissociation on Pt(111), as well as thedissociation barrier, is greatly reduced by the spatialconfinement effect of graphdiyne. At room temperature,The O2 dissociate rate on graphdiyne-covered Pt(111)surface is about ten times than on pristine Pt(111) sur-face. After dissociation, the O atoms on graphdiyne-covered Pt(111) surface could move without the re-straint of graphdiyne (low migration barrier). Overall,graphdiyne-covered Pt(111) surface provides a favor-able condition for O2 dissociation.

CO adsorption on graphdiyne-covered Pt(111) surface

To investigate CO adsorption on graphdiyne-coveredPt(111) surface, geometry relaxations and calculationsof binding energy (including ZPE corrections) wereperformed with CO fixed on every FCC/HCP/BRIDGE/ATOP site, and the PES of CO was plottedin the irreducible area shown by the hexagonal dottedlines in Fig. 3a. CO prefers to be perpendicular to the Ptsurface, with the C atom down. On graphdiyne-coveredPt(111), the O atom of standing CO molecule gets closeto the graphdiyne sheet, causing more steric hindrancethan the lying O2 molecule. In the region of triangular Cacetylenic ring, CO suffers less hindrance and obtainslarger binding energy. The ATOP site in the middle oftriangular C acetylenic ring (see the left panel and thebox in Fig. 3a) was found to be the most stable adsorp-tion site with a binding energy Eb = 1.56 eV, which is

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obviously lower than Eb = 1.89 eV on pristine Pt(111)surface. The binding energies on FCC sites are muchlower, which are in the range of 0.38~0.45 eV. On otheradsorption sites, the binding energies are even below0.35 eV. So, the center ATOP site is much more favor-able than other sites in thermodynamics. At T = 300 Kand pressure p = 1 atm, the standard free energy changeof CO adsorption isΔG = − 1.04 eV. The energy differ-ence between the most stable ATOP site and the neigh-boring FCC sites is 1.11 eV, and the energy differencesbetween all the neighboring FCC and HCP sites are inthe range of 0.08~0.15 eV. Further details about COmigration rate on graphdiyne-covered Pt(111) surfacewill be discussed in the BCO and O migration ongraphdiyne-covered Pt(111) surface^ section.

CO and O migration on graphdiyne-covered Pt(111)surface

To further demonstrate the existence of prerequisite forCO oxidation, we investigated the migration of CO andO on graphdiyne-covered Pt(111) surface. Potential bar-rier calculations indicate that CO and O could migrateand encounter each other at room temperature. Since theCO migration process on graphdiyne-covered Pt(111)surface is complex with many different adsorption sitesand migration steps, we only investigated some typicalsteps in the calculations. The arrows of 1~4 in Fig. 3bpresents the shortest COmigration path from one ATOPsite to another, with corresponding potential barriersshown in Table 1. Step 1 from the most stable ATOPsite to the neighboring FCC site has the highest barrierEa = 1.17 eV. These following steps 2~4 are of muchlower barriers than step 1, indicating that step 1 is therate-determining step. At 300 K, the standard free ener-gy of activation of step 1 is ΔG≠ = 1.11 eV, and the

corresponding rate constant is r ¼ kTh exp −ΔG≠=kT

� �

¼ 1:4� 10‐6s‐1. Such rate is too low for CO to migrateat room temperature. At T = 400 K, the rate constant ofstep 1 is r = 6.5 × 10−2 s−1, which is passable for thereaction to proceed.

Via step 1, CO arrives at the neighboring FCC site.Then, the CO molecule may go across the HCP siteunder the acetylenic chain via step 2~4 and arrive atanother ATOP site, with corresponding potential bar-riers Ea below 0.3 eV (see Table 1). CO may also returnto the ATOP site via the inverse process of step 1 or go toother HCP sites. The potential barrier of step 2 (the

inverse process of step 1) is Ea = 0.26 (0.06) eV, respec-tively. Since the barriers are very low, these processescould easily happen at room temperature. Besides step2~4, other paths (e.g., step 5) may be also thermody-namically possible. Because of the complexity of calcu-lating all the migration paths, here, we only investigatethe shortest route from one ATOP site to another, show-ing the possibility of CO migration.

Summing up the above analysis, CO migration ongraphdiyne-covered Pt(111) surface is difficult at roomtemperature due to too slow the rate-determining step 1.At T = 400 K or above, the rate of step 1 is passable andCO migration becomes possible. Once overcoming thebarrier of step 1, the barriers following steps are muchlower and CO would move from one ATOP site toanother.

Then, the migration of O on graphdiyne-coveredPt(111) surface was investigated, finding lower barriersthan CO migration. We inferred that O migration ispossible at room temperature. The arrows of 1~4 inFig. 3c presents the shortest O migration path fromone most stable FCC site to another, with correspondingpotential barriers shown in Table 2. With the highestbarrier Ea = 0.73 eV, step 3 was considered as the rate-determining step. At 300 K, the standard free energy ofactivation of step 1 is ΔG≠ = 0.67 eV, and the corre-

sponding rate constant is r ¼ kTh exp −ΔG≠=kT

� �

¼ 3:4 s‐1, which is large enough for step 3 to proceedat room temperature. The following steps 2~4 are withlower barriers (below 0.4 eV, see Table 2). Anothermigration path, i.e., steps 5 and 6, are less possiblebecause the barrier Ea = 0.81 eVof step 5 is higher thanstep 1.

Overall, the about results indicates that at room tem-perature, O atoms could freely migrate on graphdiyne-covered Pt(111) surface with low barriers (i.e., highrate), while COmigration is hindered by the graphdiyneskeleton. On graphdiyne-covered Pt(111) surface, wan-dering O atoms have possibility to meet adsorbed COmolecules. This mechanism provides opportunities forO to encounter CO and perform the oxidation reaction.

CO oxidation on graphdiyne-covered Pt(111) surface

In this section, the reaction paths and mechanism of COoxidation on graphdiyne-covered Pt(111) surface arestudied both thermodynamically and kinetically, andthe free energy chart of the total reaction process is

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presented to reveal the advantage of graphdiyne-covered Pt(111) surface to catalyze CO oxidation. Withadsorbed CO molecules and dissociated O atoms ongraphdiyne-covered Pt(111) surface, the reaction O +CO→CO2 happens when O atom migrates and meetsCO molecule. As discussed in the BCO and O migrationon graphdiyne-covered Pt(111) surface^ section, themigration barrier of O atom (Ea = 0.66 eV) is muchlower than CO (Ea = 1.17 eV), and thus the migrationof O atom on graphdiyne-covered Pt(111) surface iseasier than CO. Therefore, the process of one migratingO atom attacking a standing CO molecule, which iskinetically favorable, is studied in the following text.

First, one migrating O atommay approach a standingCO molecule (located at the most stable ATOP site) andarrives at the HCP (or FCC) site closest to CO under-neath the C acetylenic chain. Then, an O-HCP (or O-FCC) coadsorption state forms (see the left panel ofFig. 4a). At T = 300 K, the standard free energy changeof forming the O-HCP (O-FCC) state is ΔG = 0.82(0.22) eV, respectively. Second, the O atom climbs overa barrier, binds with CO and then a CO2molecule forms.Starting from the O-HCP (O-FCC) state, the standardfree energy of activation of O + CO reaction is ΔG≠ =0.22 (2.24) eV. The right panel of Fig. 4a portrays thefree energy chart of the two different reaction paths. Thehigh barrier of O-FCC route indicates that this route isvery unfavorable. In the kinetically favorable O-HCProute, the process of forming O-HCP coadsorption stateis the rate-determining step, and the total free energybarrier ΔG≠ = 0.82 + 0.22 = 1.04 eV is lower than thetotal free energy barrier ΔG≠ = 1.09 eV on pristine

Fig. 3 a The top/side views of the most stable site for CO ongraphdiyne-covered Pt(111) surface are shown in the left panel/thebox, respectively. The color map represents the PES of CO in theirreducible area (enclosed by dashed lines). b Migration paths of

CO on graphdiyne-covered Pt(111) surface. cMigration paths of Oon graphdiyne-covered Pt(111) surface. Pt, C, and O atoms arerepresented in blue, gray, and red, respectively

Table 1 The potential barriers of CO migration steps 1~5 in Fig.3b

CO migration step 1 2 3 4 5

Potential barrier Ea (eV) 1.17 0.26 0.10 0.06 0.05

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Pt(111). In Fig. 4b, the initial, transition, and final statesof O-HCP route are shown. Finally, the formed CO2

desorbs from graphdiyne-covered Pt(111) surface, witha desorption free energy changeΔG = −0.117 eVat T =300 K and pressure p = 1 atm. The very weak bindingenergy of CO2 (Eb = 0.10 eV) and the entropy increaseof CO2 into gas phase leads to negative ΔG, which isadvantageous for CO2 desorption.

Overall, the above results indicate the enhancementof reaction O + CO→ CO2 by graphdiyne cover onPt(111) surface. On graphdiyne-covered Pt(111) sur-face, the standard free energy change of the reaction iscalculated to be ΔG = −1.01 eV, which is much lowerthan ΔG = −0.52 eV on pristine Pt(111) surface. Inaddition, the overall reaction barrier is slightly decreasedby the use of graphdiyne on Pt(111). Summarizing theabove results, the reaction O + CO → CO2 ongraphdiyne-covered Pt(111) surface is both thermody-namically and kinetically favorable at room tempera-ture. The graphdiyne cover on Pt(111) remarkably en-hances the reactivity and benefits the production of CO2.

Summary

According to the above discussion about O2 dissociation(Sec. 3.2), O migration (Sec. 3.4) and O + CO reaction,the total reaction path on graphdiyne-covered Pt(111)surface is more energetically favorable than on pristinePt(111). The free energy landscape from O2 dissocia-tion, CO oxidation, to CO2 production is plotted inFig. 4c. It can be seen that the reaction path ongraphdiyne-covered Pt(111) surface generally has lowerfree energy than on pristine Pt(111). According to thecalculations in the BGraphdiyne-covered Pt(111) sur-face,^ BO2 adsorption and dissociation on graphdiyne-covered covered Pt(111) surface,^ BCO adsorption ongraphdiyne-covered Pt(111) surface,^ BCO and O mi-gration on graphdiyne-covered Pt(111) surface,^ andBCO oxidation on graphdiyne-covered Pt(111) surface^sections, we summarized the free energy changes of keysteps (shown by the arrows and data in Fig. 4c) toexhibit the thermodynamic and kinetic advantage ofgraphdiyne-covered Pt(111) surface to CO oxidation.

First, the free energy change of O2 adsorption ongraphdiyne-covered Pt(111) surface is more negativethan on pristine Pt(111) surface. Second, the dissocia-tion barrier of O2 on graphdiyne-covered Pt(111) sur-face is lower than on pristine Pt(111) surface, and thedissociation free energy of O2 on graphdiyne-coveredPt(111) surface is more negative than on pristine Pt(111)surface. The lower ΔG of O2 adsorption and dissocia-tion on graphdiyne-covered Pt(111) surface promotesthe reaction towards the positive direction better thanon pristine Pt(111). Finally, the reaction barrier of O +CO→CO2 on graphdiyne-covered Pt(111) surface islower than on pristine Pt(111) surface, and the reactionfree energy change of O + CO→CO2 on graphdiyne-covered Pt(111) surface is more negative than on pris-tine Pt(111) surface. Overall, graphdiyne-coveredPt(111) surface has the advantage to catalyze COoxidation.

In addition, the O + CO→ CO2 step includescoadsorption and reaction substeps. On graphdiyne-covered Pt(111) surface, the O + CO coadsorptionsubstep is rate-determining, with ΔG = 0.82 eV largerthan the reaction substep with ΔG≠ = 0.22 eV. On pris-tine Pt(111), the O + CO reaction substep is rate-deter-mining, with ΔG≠ = 0.92 eV larger than thecoadsorption substep with ΔG = 0.17 eV. By contrast,the free energy barrier of rate-determining substep ongraphdiyne-covered Pt(111) surface is lower than onpristine Pt(111) surface. This is also a benefit for COoxidation performing on graphdiyne-covered Pt(111)surface.

Conclusions

In this work, graphdiyne-covered Pt(111) surface wastaken as a model to study the superiority of graphdiynein two-dimensional confined catalysis and unveil thecatalysis mechanism. Graphdiyne-covered Pt(111) sur-face was proved to be excellent catalyst to promote COoxidation at room temperature. The comparison of cal-culation results exhibited the better catalytic ability ofgraphdiyne-covered Pt(111) than pristine Pt(111). The

Table 2 The potential barriers of O migration steps 1~6 in Fig. 3c

O migration step 1 2 3 4 5 6

Potential barrier Ea (eV) 0.66 0.39 0.73 0.08 0.81 0.23

J Nanopart Res (2018) 20:136 Page 9 of 12 136

CO oxidation includes CO and O2 adsorption, O2 dis-sociation, O atom migration and O + CO→CO2 step.On graphdiyne-covered Pt(111) surface, the adsorbedCO and O2 molecules are limited in the C acetylenicring due to the steric hindrance. With less hindrance andlower migration barriers, the dissociated O atoms arefree to move on graphdiyne-covered Pt(111) surface atroom temperature. The wandering O atom meets COand forms CO2. Finally, with very low binding energy,CO2 is easy to leave the Pt surface.

Using free energy calculations, the free energy land-scape of the whole reaction was presented to demonstratethe superiority of graphdiyne-covered Pt(111) to pristinePt(111). At room temperature, the free energy changes ofO2 dissociation and the O + CO→ CO2 step ongraphdiyne-covered Pt(111) are more negative than onpristine Pt(111). Furthermore, the barriers of O2

dissociat ion and the O + CO → CO2 step ongraphdiyne-covered Pt(111) are lower than on pristinePt(111). The advantages in free energy changes andpotential barriers of the key reaction steps makegraphdiyne-covered Pt(111) superior to pristine Pt(111)in catalysis. In addition, the acetylenic triangle pores ofgraphdiyne play a role of natural CO and O2 entrance andCO2 exit. By contrast with graphene overlayer usingfractures as molecular tunnels (Yao et al. 2014),graphdiyne overlayer could be more excellent in two-dimensional confined catalysis and would improve theheterogeneous catalysis ability of transitionmetal surface.

Funding This work was supported by the National NaturalScience Foundation of China (Grant No. 11304239), the Funda-mental Research Funds for the Central Universities (No.JB180513), and the 111 Project (B17035).

Fig. 4 a The left panel presents the O-HCP and O-FCCcoadsorption states of O and CO. The dashed circles mark the Oatoms underneath the C acetylenic chain. The arrows show theattack directions of O atoms. The right panel shows the free energychart of two different reaction paths for O + CO→ CO2 ongraphdiyne-covered Pt(111) surface. b The O-HCP path of CO+

O→CO2 on graphdiyne-covered Pt(111) surface, with the topand side views of initial, transition, and final states shown. Pt, C,and O atoms are represented in blue, gray, and red, respectively. cThe free energy chart of total reaction paths on pristine (green) andgraphdiyne-covered (blue) Pt(111)

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Compliance with ethical standards

Conflict of interest The authors declare that they have no con-flict of interest.

References

An XQ, Yu JC (2011) Graphene-based photocatalytic composites.RSC Adv 1:1426

An Y-R, Fan X-L, Luo Z-F, Lau W-M (2017) Nanopolygons ofmonolayer MS2: best morphology and size for HER cataly-sis. Nano Lett 17(1):368–376

Antolini E (2012) Graphene as a new carbon support for low-temperature fuel cell catalysts. Appl Catal B 123:52

Ataca C, Ciraci S (2012) Dissociation of H2O at the vacancies ofsingle-layer MoS2. Phys Rev B 85(19)

Atkins P, de Paula J (2006) Physical chemistry. Oxford UniversityPress, Oxford

Baughman RH, Eckhardt H, Kertesz M (1987) Structure-propertypredictions for new planar forms of carbon: layered phasescontaining sp2 and sp atoms. J Chem Phys 87(11):6687–6699

Bleakley K, Hu P (1999) A density functional theory study of theinteraction between CO and O on a Pt surface: CO/Pt(111),O/Pt(111), and CO/O/Pt(111). J Am Chem Soc 121:7644–7652

Blöchl PE (1994) Projector augmented-wave method. Phys Rev B50:17953–17979

Deng D, Chen X, Yu L, Wu X, Liu Q, Liu Y, Yang H, Tian H, HuY, Du P, Si R, Wang J, Cui X, Li H, Xiao J, Xu T, Deng J,Yang F, Duchesne PN, Zhang P, Zhou J, Sun L, Li J, Pan X,Bao X (2015a) A single iron site confined in a graphenematrix for the catalytic oxidation of benzene at room temper-ature. Sci Adv 1(11):e1500462

Deng D, Yu L, Pan X, Wang S, Chen X, Hu P, Sun L, Bao X(2011) Size effect of graphene on electrocatalytic activationof oxygen. Chem Commun 47:10016–10018

Deng J, Li H, Xiao J, Tu Y, Deng D, Yang H, Tian H, Li J, Ren P,Bao X (2015b) Triggering the electrocatalytic hydrogen evo-lution activity of the inert two-dimensional MoS2 surface viasingle-atom metal doping. Energy Environ Sci 8(5):1594–1601

Gao D, ZhouH,Wang J,Miao S, Yang F,Wang G,Wang J, Bao X(2015) Size-dependent electrocatalytic reduction of CO2 overPd nanoparticles. J Am Chem Soc 137(13):4288–4291

Geim AK, Novoselov KS (2007) The rise of graphene. Nat Mater6:183–191

Grimme S (2006) Semiempirical GGA-type density functionalconstructed with a long-range dispersion correction. JComp Chem 27:1787–1799

Haley MM (2008) Synthesis and properties of annulenic subunitsof graphyne and graphdiyne nanoarchitectures. Pure ApplChem 80(3):519–532

Henkelman G, Uberuaga BP, Jónsson H (2000) A climbing imagenudged elastic band method for finding saddle points andminimum energy paths. J Chem Phys 113:9901–9904

Hur SH, Park JN (2013) Graphene and its application in fuel cellcatalysis: a review. Asia Pac J Chem Eng 8:218–233

Jia T-T, Lu C-H, Zhang Y-F, Chen W-K (2014) A comparativestudy of CO catalytic oxidation on Pd-anchored grapheneoxide and Pd-embedded vacancy graphene. J Nanopart Res16:2206

Jiao Y, Du A, Hankel M, Zhu Z, Rudolph V, Smith SC (2011)Graphdiyne: a versatile nanomaterial for electronics and hy-drogen purification. Chem Commun 47(43):11843–11845

Johnson CA II, Lu Y, Haley MM (2007) Synthesis of graphynesubstructures via directed alkyne metathesis. Org Lett 9:3725–3728

Kattel S,Wang G (2014) Beneficial compressive strain for oxygenreduction reaction on Pt (111) surface. J Chem Phys 141:124713

Kehoe JM, Kiley JH, English JJ, Johnson CA, Petersen RC, HaleyMM ( 2 0 0 0 ) C a r b o n n e t w o r k s b a s e d o ndehydrobenzoannulenes. 3. Synthesis of graphyne substruc-tures. Org Lett 2:969–972

Kresse G, Furthmüller J (1996a) Efficiency of ab-initio totalenergy calculations for metals and semiconductors using aplanewave basis set. Comput Mater Sci 6:15–50

Kresse G, Furthmüller J (1996b) Efficient iterative schemes for abinitio total-energy calculations using a plane-wave basis set.Phys Rev B 54:11169–11186

Kresse G, Hafner J (1994) Ab initio molecular-dynamics simula-tion of the liquid-metal–amorphous-semiconductor transitionin germanium. Phys Rev B 49:14251–14269

Kresse G, Joubert D (1999) From ultrasoft pseudopotentials to theprojector augmented-wave method. Phys Rev B 59:1758–1775

Kresse GH (1993) Ab initiomolecular dynamics for liquid metals.Phys Rev B 47:558–561

Lee JH, Jang WS, Han SW, Baik HK (2014) Efficient hydrogenevolution by mechanically strained MoS2 nanosheets.Langmuir 30(32):9866–9873

Li C, Yang S, Li S-S, Xia J-B, Li J (2013) Au-decorated silicene:design of a high-activity catalyst toward CO oxidation. JPhys Chem C 117(1):483–488

Li G, Li Y, Liu H, Guo Y, Lia Y, Zhua D (2010) Architecture ofgraphdiyne nanoscale films. Chem Commun 46:3256–3258

Li K, Li Y, Wang Y, He F, Jiao M, Tang H, Wu Z (2015) Oxygenreduction reaction on Pt(111) and Pt(100) surfaces substitutedby the subsurface cu: a theoretical perspective. JMat ChemA3(21):11444–11452

Li R, Li H, Liu J (2016) First principles study of O2 dissociation onPt(111) surface: stepwise mechanism. Int J Quantum Chem116:908–914

Li Y, Xu L, Liu H, Li Y (2014) Graphdiyne and graphyne: fromtheoretical predictions to practical construction. Chem SocRev 43(8):2572–2586

Lin J,Wang A, Qiao B, Liu X, Yang X, Wang X, Liang J, Li J, LiuJ, Zhang T (2013) Remarkable performance of Ir1/FeOxsingle-atom catalyst in water gas shift reaction. J Am ChemSoc 135:15314–15317

Lin Z-Z (2016) Graphdiyne-supported single-atom Sc and Ticatalysts for high-efficient CO oxidation. Carbon 108:343–350

Lin Z-Z, Chen X (2016) Transition-metal-decorated germanene aspromising catalyst for removing CO contamination in H-2.Materials & Design 107:82–89

J Nanopart Res (2018) 20:136 Page 11 of 12 136

Ma DW, Li T, Wang Q, Yang G, He C, Ma B, Lu Z (2015)Graphyne as a promising substrate for the noble-metal sin-gle-atom catalysts. Carbon 95:756–765

Machado BF, Serp P (2012) Graphene-based materials for cataly-sis. Catal Sci Technol 2:54–75

Matsuoka R, Sakamoto R, Hoshiko K, Sasaki S, Masunaga H,Nagashio K, Nishihara H (2017) Crystalline graphdiynenanosheets produced at a gas/liquid or liquid/liquid interface.J Am Chem Soc 139(8):3145–3152

Mills G, Jónsson H (1994) Quantum and thermal effects in H2

dissociative adsorption: evaluation of free energy barriers inmultidimensional quantum systems. Phys Rev Lett 72:1124–1127

Mills G, Jónsson H, Schenter GK (1995) Reversible work transi-tion state theory: application to dissociative adsorption ofhydrogen. Surf Sci 324:305–337

Monkhorst HJ, Pack JD (1976) Special points for Brillouin-zoneintegrations. Phys Rev B 13:5188–5192

Mu R, Fu Q, Jin L, Yu L, Fang G, Tan D, Bao X (2012)Visualizing chemical reactions confined under graphene.Angew Chem Int Ed 51(20):4856–4859

Narita N, Nagai S, Suzuki S, Nakao K (2000) Electronic structureof three-dimensional graphyne. Phys Rev B 62:11146–11151

Narita N, Nagai S, Suzuki S, Nakao K (1998) Optimized geome-tries and electronic structures of graphyne and its family.Phys Rev B 58:11009–11014

Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI,Grigorieva IV, Dubonos SV, Firsov AA (2005a) Two dimen-sional gas of massless dirac fermions in graphene. Nature438:197–200

Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y,Dubonos SV, Grigorieva IV, Firsov AA (2004) Electric fieldeffect in atomically thin carbon films. Science 306:666–669

Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich V,Morozov SV, Geim AK (2005b) Two-dimensional atomiccrystals. Proc Natl Acad Sci U S A 102:10451–10453

Olsen T, Thygesen KS (2013) Random phase approximationapplied to solids, molecules, and graphene-metal interfaces:from van der Waals to covalent bonding. Phys Rev B 87:075111

Pan X, Bao X (2008) Reactions over catalysts confined in carbonnanotubes. Chem Commun 47(47):6271–6281

Pan Y,WangY,Wang L, Zhong H,Quhe R,Ni Z, YeM,MeiW-N,Shi J, Guo W, Yang J, Lu J (2015) Graphdiyne–metal con-tacts and graphdiyne transistors. Nano 7(5):2116

Perdew JP, Burke K, Ernzerhof M (1996) Generalized gradientapproximation made simple. Phys Rev Lett 77:3865–3868

Qi H, Yu P, Wang Y, Han G, Liu H, Yi Y, Li Y, Mao L (2015)Graphdiyne oxides as excellent substrate for electroless de-position of Pd clusters with high catalytic activity. J AmChem Soc 137:5260–5263

Qiao B, Wang A, Yang X, Allard LF, Jiang Z, Cui Y, Liu J, Li J,Zhang T (2011) Single-atom catalysis of CO oxidation usingPt1/FeOx. Nat Chem 3:634–641

Qu LT, Liu Y, Baek JB, Dai LM (2010) Nitrogen-doped grapheneas efficient metal-free electrocatalyst for oxygen reduction infuel cells. ACS Nano 4:1321

Smit B, Maesen TLM (2008) Towards a molecular understandingof shape selectivity. Nature 451(7179):671–678

Su CL, Loh KP (2013) Carbocatalysts: graphene oxide and itsderivatives. Acc Chem Res 46:2275

Tahara K, Yoshimura T, Ohno M, Sonoda M, Tobe Y (2007)Syntheses and Photophysical properties of boomerang-shaped Bis(dehydrobenzo[12]annulene) and trapezoid-shaped Tris(dehydrobenzo[12]annulene). Chem Lett 36(7):838–839

Tang Y, Liu Z, Dai X, Yang Z, Chen W, Ma D, Lu Z (2014)Theoretical study on the Si-doped graphene as an efficientmetal-freecatalyst for CO oxidation. Appl Surf Sci 308:402–407

Tang Y, Yang Z, Dai X (2012) A theoretical simulation on thecatalytic oxidation of CO on Pt/graphene. Phys Chem ChemPhys 14:16566–16572

Wei H, Liu X,Wang A, Zhang L, Qiao B, Yang X, Huang Y, MiaoS, Liu J, Zhang T (2014) FeOx-supported platinum single-atom and pseudo-single-atom catalysts for chemoselectivehydrogenation of functionalized nitroarenes. Nat Commun5:5634

Wu P, Du P, Zhang H, Cai C (2014) Graphdiyne as a metal-freecatalyst for low-temperature CO oxidation. Phys ChemChem Phys 16:5640

Yan H, Cheng H, Yi H, Yao T, Wang C, Li J, Wei S, Lu J (2015)Single-atom pd1/graphene catalyst achieved by atomic layerdeposition: remarkable performance in selective hydrogena-tion of 1,3-butadiene. J Am Chem Soc 137:10484

Yang X-F, Wang A, Qiao B, Li J, Liu J, Zhang T (2013) Single-atom catalysts: a new frontier in heterogeneous catalysis.Accounts Chem Research 46:1740–1748

Yao Y, Fu Q, Zhang YY, Weng X, Li H, Chen M, Jin L, Dong A,Mu R, Jiang P, Liu L, Bluhm H, Liu Z, Zhang SB, Bao X(2014) Graphene cover-promoted metal-catalyzed reactions.Proc Natl Acad Sci U S A 111(48):17023–17028

Yu L, Pan X, Cao X, Hu P, Bao X (2011) Oxygen reductionreaction mechanism on nitrogen-doped graphene: a densityfunctional theory study. J Catal 282:183–190

Yu Y, Huang S-Y, Li Y, Steinmann SN, Yang W, Cao L (2014)Layer-dependent electrocatalysis of MoS2 for hydrogen evo-lution. Nano Lett 14(2):553–558

Zhang H, He X, Zhao M, Zhang M, Zhao L, Feng X, Luo Y(2012) Tunable hydrogen separation in sp-sp(2) hybridizedcarbon membranes: a first-principles prediction. J PhysChem C 116(31):16634–16638

Zhang Y, Weng X, Li H, Li H, Wei M, Xiao J, Liu Z, Chen M, FuQ, Bao X (2015) Hexagonal boron nitride cover on Pt(111): anew route to tune molecule—metal interaction and metal-catalyzed reactions. Nano Lett 15:3616–3623

Zhou J, Gao X, Liu R, Xie Z, Yang J, Zhang S, Zhang G, Liu H, LiY, Zhang J, Liu Z (2015) Synthesis of graphdiyne nanowallsusing acetylenic coupling reaction. J Am Chem Soc 137(24):7596–7599

Zhou Y, ChenW, Cui P, Zeng J, Lin Z, Kaxiras E, Zhang Z (2016)Enhancing the hydrogen activation reactivity of nonpreciousmetal substrates via confined catalysis underneath graphene.Nano Lett 16(10):6058–6063

136 Page 12 of 12 J Nanopart Res (2018) 20:136