Chemical Physics Letters

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Chemicl Physics Letters 536 (2012) 9 13 Contents lists vilble t SciVerse ScienceDirect Chemicl Physics Letters journl homepge: www.elsevier.

Curtiss c Deprtment of Chemistry, University of Illinois t Chicgo, Chicgo, IL 60607, United Sttes b Deprtment of Physics, University of Illinois t Chicgo, Chicgo, IL 60607, United Sttes c Mterils

1 Chemicl Physics Letters 536 (2012) 9 13 Contents lists vilble t SciVerse ScienceDirect Chemicl Physics Letters journl homepge: Methne bond ctivtion by Pt nd Pd subnnometer clusters supported on grphene nd crbon nnotubes John Russell,c, Peter Zpol c,, Petr Král,b, Lrry A. Curtiss c Deprtment of Chemistry, University of Illinois t Chicgo, Chicgo, IL 60607, United Sttes b Deprtment of Physics, University of Illinois t Chicgo, Chicgo, IL 60607, United Sttes c Mterils Science Division, Argonne Ntionl Lbortory, Argonne, IL 60439, United Sttes rticle info bstrct Article history: Received 3 Jnury 2012 In finl form 22 Mrch 2012 Avilble online 31 Mrch 2012 Activtion of the C H bond is importnt for hydrocrbon ctlysis with pplictions in energy technology such s production of synthetic fuels. Subnnometer clusters such s Pt 4 show gret promise for ctlytic ctivities potentilly much greter thn monolith. Using density functionl theory, we investigted C H bond breking, n importnt step of the conversion rection of methne to liquid fuels, on Pt nd Pd subnnometer clusters with grphene nd crbon nnotubes s supports. Our results show tht CNT supports cn be selected by size nd chirlity to provide stble support for subnnometer Pt nd Pd clusters nd tilor their ctlytic ctivity. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction 2. Methods The ctivtion of lknes is n importnt chllenge due to limited fossil fuel reserves nd incresing globl demnd for petrochemicls [1]. Renewble energy sources in the future my still require tht energy be stored s liquid fuels for trnsporttion. Highly selective, efficient nd robust ctlysts re therefore importnt to meet this chllenge, nd re n ctive re of reserch. C H bond ctivtion in CH 4 provides good model chemistry for similr bond ctivtion in other lknes. Subnnometer Pt clusters on Al 2 O 3 show gret promise with ctlytic ctivities much greter thn Pt monolith [2] due to undercoordintion of Pt toms nd high surfce re to volume rtio in the clusters. Crbon nnotubes (CNTs) nd grphene hve lso been proposed s supports for trnsition metl ctlysts [3 6], including plldium [7] nd pltinum tetrmers [8], due to their high surfce re nd cpbility to stbilize these clusters [9]. Recent experimentl dvnces in the seprtion of single chirlity CNTs [10,11] rise the possibility of mtching ctlytic mterils such s subnnometer clusters to select CNT supports to improve ctlytic potentil of the combined system. A key question is whether the dsorption on support will modify the ctlytic ctivity of subnnometer clusters due to the formtion of cluster support bonds which increses coordintion of the cluster toms. We investigte the role of CNT curvture nd chirlity for methne C H bond ctivtion on CNT-supported Pt nd Pd tetrmers using first principles methods. Corresponding uthor. E-mil ddress: zpol@nl.gov (P. Zpol). Methne ctivtion by supported Pt nd Pd tetrmers ws investigted with first-principles pproch using density functionl theory (DFT) with the B3LYP hybrid functionl [12,13] s implemented in the GAUSSIAN09 softwre pckge [14]. A generlized (5d, 7f) 6-31G bsis set ws used for crbon nd hydrogen toms in the methne nd support structure, while Stuttgrt SDD effective core potentils nd corresponding bsis sets were used for Pt nd Pd toms of the clusters. The support ws represented by 66-crbon tom grphene model with edges terminted by 22 hydrogen toms, either flt or bent to CNT curvture, the ltter being shown in Figure 1 for (7,0) CNT. We investigted nine support structures including grphene, (10-10), (15-0), (8-8), (10-0), (5-5), (8-0), (7-0) nd (4-4) CNTs in order of incresing curvture. The support structure for given curvture r 1 ws produced by liner trnsformtion which mpped the plnr grphene to cylinder of rdius r such tht x 2 ¼ r cos x r ; y2 ¼ y 1 ; nd z 2 ¼ r sin x r. The rdius r is given s, pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3ðn r 0 ¼ r 2 þ nm þ m 2 Þ ; ð1þ 2p where n nd m re the lttice vectors nd r is the crbon crbon covlent bond length (r = 1.41 Å) [15]. We studied CNTs of both zigzg (n,0) nd rmchir (n,n) chirlities, where the grphene is rotted by 90 before bending. The geometry ws then optimized with the hydrogen toms fixed to hold the curvture, nd the crbon toms were llowed to relx. The optimized structures were clculted with unrestricted spin, however the lowest energy spin configurtions were closed shell singlet in ll cses. The energy /$ - see front mtter Ó 2012 Elsevier B.V. All rights reserved.

2 10 J. Russell et l. / Chemicl Physics Letters 536 (2012) 9 13 where E M TS is the totl energy in the trnsition stte. Negtive EM energies re possible provided tht E M TS > EM MA, which indictes no pprent brrier. 3. Results 3.1. Cluster nnotube binding Figure 1. Clculted structures of methne molecule recting with plldium tetrmer (top row), supported on (7,0) CNT represented by finite model: () moleculr dsorption, (b) trnsition stte, (c) dissocitive dsorption. A methne molecule recting with pltinum tetrmer using the sme model (bottom row, truncted) shows hydrogen dsorption on single Pt tom rther thn occupying bridge site s for the Pd tetrmer. required to bend the support to the shpe of the CNT hs qudrtic dependence on r, s expected. The reltive energy of bending where DE ¼ E b E flt ðevþ cn be fit such tht DE ¼ 127:666 r 2 ðå 2 Þ with RMS devition of ev. The optimized support structure ws then combined with the metl clusters to find the binding energies of the cluster to the support. All cluster support structures were optimized in both singlet nd in triplet sttes. Cluster stbility on the CNT support ws evluted by clculting the binding energy, E B : E M B ¼ E ðm 4 CNTÞ E M4 E CNT ; ð2þ where metl M is Pd or Pt, E ðm4 CNTÞ is the totl energy of the combined cluster support system, E M4 is the energy of the (triplet) cluster, nd E CNT is the energy of the (singlet) support. Negtive E B energies re exothermic. Next, methne ws dded to the lowest-energy singlet nd triplet configurtions of the cluster support systems nd then optimized to find both moleculr dsorption (MA) nd dissocitive dsorption (DA) of the methne on the supported cluster. The energy of dsorption, E MA(DA), ws clculted by, E M MAðDAÞ ¼ EMAðDAÞ ðch 4 M 4 CNTÞ E ðm 4 CNTÞ E CH4 ; where E MAðDAÞ ðch 4 M 4 CNTÞ is the totl energy of the combined methne cluster support system in the configurtion corresponding to either MA or DA, nd E CH4 is the totl energy of the methne. Negtive E MAðDAÞ energies re exothermic. Finlly, the trnsition stte (TS) ws found by optimiztion using the Berny Algorithm [16]. The pprent brrier, E M is given s, ð3þ E M ¼ EM TS E ðm 4 CNTÞ E CH4 ; ð4þ The cluster binding energies to the support, E B, re shown in Figure 2. The trend is tht E B increses with CNT curvture for both Pd nd Pt. Additionlly, optimized geometries show trend in which the number of cluster support bonds increses with curvture. This is not surprising due to the greter rectivity of smll CNTs which re highly strined [17,18]. Our results show tht for triplet Pd 4 CNT systems, clusters bind more strongly to zigzg CNT supports thn to rmchir CNT supports with the exception of the (10,0) CNT. The opposite result is obtined with singlet Pd 4 CNT systems. In contrst, in triplet Pt 4 CNT systems, clusters bind more strongly to rmchir CNT supports thn to zigzg supports except t very high curvture of r 1 > 0.3 Å 1. But in singlet Pt 4 CNT systems, clusters bind more strongly to zigzg CNT supports thn to rmchir CNT supports. This shows tht electronic effects of the support ply role in the binding properties of the cluster s rmchir CNTs re metllic nd zigzg CNTs re mostly semiconducting [19]. Our results using the B3LYP hybrid functionl show tht Pd 4 nd Pt 4 cluster CNT systems hve stronger binding energies in the triplet stte thn in the singlet stte. These results re in greement with previous DFT studies with the GGA-PBE functionl [8] tht showed Pt 4 stbility on (10,0) CNTs with binding of 2.31 ev nd on grphene with binding of 1.35 ev, both in singlet sttes. The lowest energy Pt 4 binding configurtion clculted with the GGA-PBE functionl hs three surfce bonds to either (10,0) CNT or grphene supports, which is similr to tht found with the B3LYP hybrid functionl Methne ctivtion by unsupported clusters We use the gs phse Pd 4 nd Pt 4 clusters s reference for comprison with the clusters supported on CNTs shown in Figure 1. The comprison is pproprite to determine the effect of the support on ctlytic ctivity becuse the dditionl bonds from the cluster to the support my reduce the undercoordintion tht is chrcteristic of subnnometer cluster ctivity. The lowest energy configurtions of unsupported Pd 4 nd Pt 4 gs phse clusters re trigonl pyrmids in triplet sttes. In Figure 3, we show the spin density of Pd 4 recting with methne. The rection strts with methne nd the cluster seprted, nd proceeds from MA, to the C H bond insertion TS, nd finlly to DA. The unpired spin density resides on the cluster throughout the rection with very little contribution from the crbon tom. The C H bond breking rection is endothermic on Pd 4 clusters but is exothermic on Pt 4 cluster s shown in Figure 3. The pprent brrier for breking the C H bond in methne dsorbed on Pd 4 is 1.0 ev, nd the dissocition is endothermic, by 0.9 ev. Conversely, the pprent brrier to C H bond insertion on Pt 4 is much lower t 0.1 ev, nd the dissocition is exothermic, by 0.56 ev. In comprison, other studies hve found brriers for C H bond breking in methne to be 0.66 ev on Pd (111) surfce [20] nd 0.74 ev on Pt (111) surfce [21,2]. Overll, this shows tht Pt 4 hs higher ctivity thn Pd 4 for C H bond breking of methne Rection energies, islnds of stbility nd instbility The size nd chirlity of the support cn strongly effect the dsorption nd dissocition of methne on subnnometer clusters.

3 J. Russell et l. / Chemicl Physics Letters 536 (2012) Figure 2. Clculted Pd 4 /Pt 4 cluster binding energy E M B to the CNT support s function of CNT curvture nd chirlity: () rmchir nd (b) zigzg. The number of surfce bonds from the cluster to the support re shown next to ech point. Inset: Pd 4 cluster with one surfce bond to grphene (left) nd Pt 4 cluster with two surfce bonds to grphene (right). Figure 3. Adsorption energies nd rection brriers of C H bond breking which proceeds from the seprted methne nd unsupported Pd (Pt) tetrmers, to moleculr dsorption (MA) of the methne, to the trnsition stte (TS) of C H bond insertion, nd finlly to dissocitive dsorption (DA). Inset spin density plots of the Pd tetrmer shows very little contribution of spin density from the methne throughout the rection. All energies re reltive to M 4 þ CH 4 nd spin density ( b) isosurfce contour vlues re 0:003 e = 3 0. The energies of dsorption, E M MA nd EM DA, re shown s function of curvture in Figure 4, for methne dsorbed on Pd 4 nd Pt 4 clusters supported by rmchir nd zigzg CNTs nd grphene. In ll cses the rection on Pd is endothermic becuse E Pd DA > EPd MA. However, the trend of the rection energy is less endothermic for smll dimeter CNTs for both zigzg nd rmchir chirlities s the difference between E Pd DA nd EPd MA decreses with curvture, though not monotoniclly. In contrst, the rection on Pt is exothermic for grphene nd CNTs of lrge dimeter where E Pt DA < EPt MA. The trends in dsorption energies, E M MA nd EM DA, for Pd nd Pt clusters re relted to the cluster binding energy to the support, E M B. Cluster binding energy EPd B rnges from 1.1 to 1.9 nd E Pt B rnges from 1.3 to 2.7 ev for grphene to (7,0) CNT supports s shown in Figure 2. The lest endothermic Pd 4 rection occur

4 12 J. Russell et l. / Chemicl Physics Letters 536 (2012) 9 13 Rection energies on the concve (10,10) model CNT support re similr to those found on the convex side. Moleculr nd dissocitive dsorption energies nd the dsorption energies for triplet Pt 4 re 0.14 nd 0.40 ev, respectively. Therefore, the rection energies for C H bond breking on triplet concve-(10,10) CNT supported Pt 4 cluster is 0.27 ev, slightly more exothermic thn the rection on the outer side of the CNT wll. Overll this shows tht the Pt clusters would likely be more stble on the outer CNT surfce nd less stble on the inside of CNT support. The rection energies for methne dissocition re comprble for clusters on the inner nd outer surfces of the CNT. A possible explntion for the weker cluster binding to the concve CNT wll is tht steric strin of the CNT cretes greter electron density on the CNT exterior surfce compred to the interior surfce s the crbon toms gin more sp 3 chrcter with incresing CNT curvture C H bond ctivtion in methne Figure 4. Energies of moleculr dsorption (MA), E M MA, nd dissocitive dsorption (DA), E M DA, for tetrmer clusters where M is Pd (stright) or Pt (dshed), s function of support curvture nd chirlity, including grphene (r 1 ¼ 0:0 Å), zigzg (open circles) nd rmchir (filled circles) CNTs. The C H bond insertion rection is exothermic if E M DA < EM MA. when the Pd cluster is strongly bound ( 1.5 to 1.75 ev) to smll dimeter CNT support. Conversely, the most exothermic Pt 4 rection occurs when the Pt cluster is wekly bound ( 1.7 nd 1.8 ev) to lrge dimeter CNT support. Moreover, the rection is more exothermic on CNTs of prticulr sizes nd chirlities, which might be understood s islnds of stbility. For Pd, the lest endothermic rection is for sizes tht re tightly curved (5,5) rmchir nd (7,0) zigzg CNTs. Likewise for Pt, the most fvorble sizes re (10,10) nd (8,8) rmchir CNTs. The lest fvorble size nd chirlity, n islnd of instbility, for methne C H bond insertion rections on Pt re (15,0) CNTs. We postulte tht the reson some supports hve lower or higher dsorption energies is due to geometric distortion nd electronic effects of the support. The bond energy of the metl CNT bond increses with curvture. However, the incresed metl CNT bond energy is off-set by distortion of the CNT nd the cluster wy from n optiml geometry in order to fit multiple binding sites on the support. We define the distortion energy s the difference in energy between the support (cluster) optimized in the rection steps nd the initil relxed configurtion. For both Pd 4 nd Pt 4 clusters, the distortion energy for the support is inversely proportionl to the support rdius. Distortion energies rnged from to ev for triplet Pd 4 brriers nd from to ev for triplet Pt 4 brriers, with grphene hving the lest distortion nd (7,0) CNTs hving the gretest distortion. A qulittive description of the clculted distortion energies is tht cluster nd CNT distortion is greter for dsorption on zigzg CNTs thn on rmchir CNTs. The pprent brriers, E M, for methne C H bond insertion on supported Pd 4 nd Pt 4 clusters re shown s function of support curvture nd chirlity in Figure 5. The trends in the pprent brriers, E M, cn be compred to those of the cluster support binding energies, E M B, s shown in Figure 2. For Pd, the pprent brrier EPd hs, pproximtely, direct reltionship to the binding energy E Pd B. A strongly bound ( 1.9 ev) Pd 4 on high curvture (4,4) CNT hs n pprent brrier of 0.5 ev, which is hlf the pprent brrier of n unsupported Pd 4 cluster s shown in Figure 3. The trend in Pd pprent brrier, E Pd, with chirlity is less cler. Armchir CNTs hve lower brriers t curvture below 0.18 Å 1 nd zigzg CNTs hve lower pprent brriers otherwise. In contrst, the trend in the Pt pprent brriers, E Pt with curvture is roughly opposite with brrier energies E Pt decresing with binding energies E Pt B. However, the trend is not liner nd the Pt 4 cluster on lrge curvture (8,8) CNT with reltively weker binding ( 1.8 ev) hs the lowest pprent brrier of 0.1 ev, which is better thn the unsupported Pt 4 cluster with n pprent brrier of 0.1 ev. Apprent brriers for Pt C H bond ctivtion E Pt, re lower for rmchir CNTs thn for zigzg CNTs except t very high curvture bove 0.3 Å 1. The trends for rection brriers with chirlity re similr to the trends in dsorption energies, E Pt MA nd EPt DA with chirlity Methne ctivtion by clusters on concve support We explore the rectivity nd stbility of ctlysts on the inside of CNTs becuse of interest in using CNTs s nnorectors, lthough our model is limited to lrge dimeter CNTs. First, we compre Pt cluster binding to the concve side of (10,10) model support with our results for the convex surfce. A singlet Pt 4 binds with 0.5 ev to the concve support, compred to 0.7 ev to the convex side. A triplet Pt 4 binding is much weker to the concve side with binding energy of 0.9 ev, compred to binding energy of 1.7 ev on the convex side. Figure 5. Clculted pprent brriers, E M, for CH bonds with () M = Pd 4 nd (b) M =Pt 4 ctlysts on supports by curvture nd chirlity, including grphene (r 1 = 0.0 Å), rmchir, nd zigzg CNTs. Brriers re reduced for Pd clusters by CNT supports nd incresed for Pt clusters reltive to gs phse (shown in Figure 3).

5 J. Russell et l. / Chemicl Physics Letters 536 (2012) Conclusions Both Pt nd Pd subnnometer clusters re stble on CNT supports with binding energies for Pt higher thn for Pd. In the triplet stte, Pt nd Pd hve opposite ordering of binding energies reltive to the chirlity of the support, with rmchir supports hving lowest energy for Pt 4 nd with zigzg supports hving lowest energy for Pd 4. This pttern is reversed in the higher energy singlet stte. Rection energies for CH 4 dissocition on supported Pd clusters re more exothermic reltive to gs phse Pd 4 while rection energies on supported Pt clusters re less exothermic reltive to gs phse Pt 4. When compred to n initil stte of bound cluster support system with seprted methne, ll rections on supported Pd 4 re endothermic nd ll rections on supported Pt 4 re exothermic except for highly curved rmchir supports. In ll cses, triplet sttes for products re preferred. For Pd clusters, brriers for C H bond ctivtion decrese with incresing CNT curvture, while the trend is opposite for Pt clusters. Certin CNT sizes nd chirlities result in lower brriers thn the overll trend. Our results show tht CNT supports cn be selected by size nd chirlity to provide stble support for subnnometer Pt nd Pd clusters nd tilor their ctlytic ctivity. Acknowledgments The uthors cknowledge the use of Argonne LCRC nd the Center for Nnoscle Mterils computer resources. Work ws supported by the US Deprtment of Energy, Office of Science, Office of Bsic Energy Sciences, under Contrct No. DE-AC02-06CH References [1] J. Lbinger, J. Bercw, Nture 417 (2002) 507. [2] S. Vjd et l., Nt. Mter. 8 (2009) 213. [3] A. Titov, P. Zpol, P. Král, D.J. Liu, H. Iddir, K. Bishy, L. Curtiss, J. Phys. Chem. C 113 (2009) [4] N. Cuong, A. Fujiwr, T. Mitni, D. Chi, Comput. Mter. Sci. 44 (2008) 163. [5] S. Kim, Y. Jung, S-J. Prk, Colloids Surf. A (2008) 189. [6] S.R. Stoynov, A. Titov, P. Král, Coor. Chem. Rev. 253 (2009) [7] T. Prsomsri, D. Shi, D.E. Ressco, Chem. Phys. Lett. 497 (2010) 103. [8] N. Cuong, A. Sugiym, A. Fujiwr, T. Mitni, D.H. Chi, Phys. Rev. B 79 (2009) [9] T.W. Odom, J.H. Hfner, C.M. Lieber, Top. Appl. Phys. 80 (2001) 173. [10] A.A. Green, M.C. Hersm, Adv. Mter. 23 (2011) [11] M.C. Hersm, Nt. Nnotechnol. 3 (2008) 387. [12] A.D. Becke, J. Chem. Phys. 98 (1993) [13] C. Lee, W. Yng, R. Prr, Phys. Rev. B 37 (1988) 785. [14] M.J. Frisch et l., GAUSSIAN09, Revision A.1, GAUSSIAN, Inc., Wllingford CT, [15] B.J. Cox, J.M. Hill, Crbon 45 (2007) [16] B. Schlegel, J. Comput. Chem. 3 (1982) 214. [17] S. Prk, D. Srivstv, K. Cho, Nno Lett. 3 (2003) [18] D.A. Horner, P.C. Redfern, M. Sternberg, P. Zpol, L.A. Curtiss, Chem. Phys. Lett. 450 (2007) 71. [19] X. Lu, Z. Chen, Chem. Rev. 105 (2005) [20] Z.-P. Liu, P. Hu, J. Am. Chem. Soc. 125 (2003) [21] A. Michelides, P. Hu, J. Am. Chem. Soc. 122 (2000) 9866.

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