A mechanistic study of H 2 S decomposition on Ni- and Cu-based anode surfaces in a solid oxide fuel cell

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1 Chemicl Physics Letters 421 (2006) A mechnistic study of H 2 S decomposition on Ni- nd Cu-bsed node surfces in solid oxide fuel cell Y.M. Choi, Chrles Compson, M.C. Lin b, Meilin Liu, * Center for Innovtive Fuel Cell nd Bttery Technologies, School of Mterils Science nd Engineering, 771 Ferst Drive, N.W. Georgi Institute of Technology, 0245 Atlnt, GA 30332, United Sttes b Deprtment of Chemistry, Emory University, 1515 Dickey Drive, Atlnt, GA 30322, United Sttes Received 8 August 2005; in finl form 6 Jnury 2006 Avilble online 10 Februry 2006 Abstrct The mechnisms of interction between H 2 S nd Ni- or Cu-bsed node surfces in solid oxide fuel cell were elucidted by density functionl slb model clcultions. Two rection pthwys vi moleculr nd dissocitive dsorption processes were mpped out following minimum energy pths. The energy for H 2 S dsorption t the top site of Ni(111) lying prllel to the surfce is predicted to be 0.55 ev, while tht for the dissocitive dsorption is 1.75 ev. In contrst, the formtion of initil moleculr complexes on Cu surfce is energeticlly unfvorble ( 0.0 ev), suggesting tht Cu is more sulfur-tolernt thn Ni. Ó 2006 Elsevier B.V. All rights reserved. 1. Introduction One unique dvntge of solid oxide fuel cells (SOFCs) over other types of fuel cells is the potentil for direct utiliztion of wide rnge of fuels, including nturl gs, liquid fuels, gsified col, nd biofuels [1 3]. Mny of these fuels, however, come with contminnts such s sulfurcontining compounds, which re difficult to remove economiclly below ppm level nd gretly degrde fuel cell performnce. Thus, it is impertive to develop novel node mterils for SOFCs tht cn tolerte low concentrtion of sulfur contminnts. While Cu-bsed node mterils hve been extensively studied due to their better sulfur tolernce thn Ni-bsed nodes [1,2], little high level computtions hve been performed [4]. Recently, direct comprison of sulfur tolernce on Ni nd Cu surfces in terms of dsorption energies t the sme level of periodic density functionl theory (DFT) ws reported [5]. A relible mechnistic study for hydrogen sulfide (H 2 S) decomposition on the node surfces t the moleculr level my increse the understnding of incipient sulfur formtion, which cn be * Corresponding uthor. Fx: E-mil ddress: meilin.liu@mse.gtech.edu (M. Liu). pplied to guide the development of new sulfur-tolernt mterils. In this study, we report the mechnisms of interction between H 2 S nd the most densely pcked Ni nd Cu{1 1 1} surfces using periodic DFT clcultions. In ddition, we hve estimted vibrtionl frequencies with vrious optimized structures of dsorbed H 2 S nd HS, together with the detiled potentil energy surfces for ll low-lying rection pthwys from H 2 StoS. 2. Computtionl methods Electronic structure clcultions by solving the Kohn Shm equtions were performed using the VASP (Vienn b initio simultion pckge) softwre [6,7] with the projected ugmented wve method (PAW) [8]. Surfce models were constructed with periodic boundry conditions to mimic infinite surfces. Generlized grdient pproximtion (GGA) using the Perdew Wng (PW91) functionl [9] ws utilized to describe the exchnge nd correltion energies. The Brillouin zone ws smpled t (5 5 1) k- point with the Monkhorst Pck method [10]. A kinetic energy cut-off of 400 ev ws used for ll the clcultions. The second order Methfessel Pxton method [11] ws used for the Fermi surfce smering with 0.2 ev width. In this /$ - see front mtter Ó 2006 Elsevier B.V. All rights reserved. doi: /j.cplett

2 180 Y.M. Choi et l. / Chemicl Physics Letters 421 (2006) study, ll slbs were seprted by vcuum spcing greter thn 10 Å. As described in our previous work [5], fivelyer supercell model ws constructed. Ech lyer of this model hs four toms to represent p(2 2) unit cell tht corresponds to surfce coverge of 0.25 monolyer (ML). The clcultions were first crried out by fixing ll lyers to locte stble intermedites nd then by relxing the three uppermost lyers to re-optimize energeticlly fvorble intermedites for mechnistic studies. All the unrelxed lyers were set to estimted bulk prmeters. Moreover, vibrtionl frequencies were clculted, providing scientific bsis for investigtions into sulfur-tolernt node mterils using vibrtionl spectroscopy [5]. In ddition to the periodic DFT method, the structures nd vibrtionl frequencies of gs-phse H 2 S nd HS were clculted by using the PW91PW91/6-31+G(d) level of theory s employed in the GAUSSIAN 03 pckge [12]. In this study, we clculted the reltive energies ccording to the following eqution. DE ¼ E½slb þ dsorbteš E½slbŠ E½dsorbteŠ where E[slb + dsorbte], E[slb], E[dsorbte] re the clculted electronic energies of dsorbed species on metl surfce, bre metl surfce, nd gs-phse molecule. It should be noted tht ll surfce clcultions were crried out with non-spin-polriztion methods since the mgnetic effect for Ni(1 1 1) using spin-polriztion clcultions is insignificnt for dsorption energies nd geometries [13,14]. 3. Results nd discussion 3.1. Clcultions of bulk Ni(1 11) nd Cu(1 11) surfces nd gs-phse H 2 S nd HS Estimted bulk lttice constnts for Ni(1 1 1) nd Cu(111) re 3.53 nd 3.64 Å, respectively, which re in close greement with the experimentl vlues of 3.52 nd 3.61 Å [15]. The optiml Ni Ni bond distnce vries from to Å, while tht of Cu Cu vries from to Å. Chnging the pseudopotentils from GGA-PAW to ultrsoft pseudopotentils hs no effect on the lttice constnts. As summrized in Tble 1, predicted geometricl prmeters nd vibrtionl frequencies of gs-phse H 2 S nd HS in 10 Å cubic box re in line with vilble experimentl nd theoreticl dt. Tble 1 Geometricl prmeters nd vibrtionl frequencies of gs-phse H 2 S nd HS clculted t the GGA PAW method Symmetry H 2 S HS C 2V C 1V Clc Experiment b Clc Experiment c r(s H) (Å) [1.357] [1.363] h(h S H) ( ) 92.1 [92.3] 91.6 v sym (cm 1 ) 2669 [2660] [2622] 2660 v sym (cm 1 ) 2658 [2636] 2615 v bend (cm 1 ) 1226 [1207] 1183 Vlues in brckets were predicted t the PW91PW91/6-31+G(d) level of theory using the GAUSSIAN 03 code [12]. b From Ref. [16]. c From Ref. [17] Moleculr dsorption In order to initilly locte fvorble geometries of vrious species, clcultions were performed on n unrelxed Ni surfce, in which only dsorbed H 2 S species were fully optimized. An H 2 S molecule ws plced on four ctive sites, i.e., top, bridge, nd threefold hcp nd fcc [5], producing four types of locl intermedites s shown in Fig. 1. Tble 2 lists dsorption energies nd moleculr prmeters of dsorbed H 2 S species on the Ni(111) surfce. The top-site mtop-prllel lying prllel to the surfce is the most stble with n dsorption energy of 0.48 ev on the unrelxed surfce similr to the interctions of H 2 S Pd(111) [18] nd H 2 O Cu(111) [19], while mtop-up with the sulfur tom towrd the surfce is wekly bound with n dsorption energy of 0.14 ev. The bridge-site mbrg minimum is lso stble with n dsorption energy of 0.42 ev. Interestingly, n initil geometry optimiztion with prllel orienttion t the bridge site produces the dissocitive dbrg species tht will be discussed in the following section. The mhcp nd mfcc intermedites on 3.2. Rection mechnisms The decomposition of H 2 S on metllic node surfce (M) my proceed by stepwise moleculr nd dissocitive dsorptions ccording to the following scheme. I. moleculr dsorption M(s) + H 2 S(g) M-SH 2 (s) M-SH(s) + H(s) M-S(s) + 2H(s) II. dissocitive dsorption M-SH(s)* + H(s) where, s, g, nd sterisk stnd for surfce, gs, nd different dsorbed intermedite. Fig. 1. Schemtic illustrtion of intermedites for the moleculr dsorption optimized on four ctive sites, i.e., top, bridge, nd threefold hcp nd fcc. The vlues re for clculted dsorption energies in ev on the unrelxed surfce. Ni, S, nd H toms re in gry, yellow, nd light blue, respectively (For interprettion of the references in colour in this figure legend, the reder is referred to the web version of this rticle).

3 Y.M. Choi et l. / Chemicl Physics Letters 421 (2006) Tble 2 Adsorption energies nd moleculr prmeters, i.e., bond length in (Å) nd ngle in ( ), of dsorbed H 2 S species on Ni(111) Active site top bridge b hcp fcc Species mtop-up mtop-prllel mbrg mhcp mfcc [ 0.55] 0.42 [ 0.48] r(s H) [1.367] [1.373] [1.371] [1.375] h(h S H) [91.3] 92.0 [93.7] d(s S) c [2.170] [1.753] c(ni H S H) [102.4] [136.6] d d d [134.2] e f g Adsorption energies in ev reltive to unrelxed or relxed Ni(111) nd gs-phse H 2 S molecule. b In brckets, the dsorption energy nd prmeters were clculted on relxed Ni(111), otherwise on unrelxed Ni(111). c Verticl distnce between surfce nd sulfur tom. d g c(ni 1 H S H)c(Ni 4 H S H), nd c(ni 2 H S H), respectively, re dihedrl ngles. (see mfcc in Fig. 1 for numbering). Fig. 2. Schemtic energy profiles for H 2 S Ni(111) interctions vi the moleculr nd dissocitive dsorption on the surfce by relxing the three uppermost lyers. Reltive energies re in ev. threefold hcp nd fcc sites, respectively, hve dsorption energies of 0.39 nd 0.35 ev, respectively. Their sulfur toms re insignificntly shifted from the center Ni toms (see Fig. 1). The top-site mtop-prllel nd bridge-site mbrg intermedites, summrized in Tble 2, re pplied for mechnistic study of the moleculr dsorption since they re energeticlly the most fvorble. All surfce clcultions for the mechnistic study were crried out on the surfce by relxing the three uppermost lyers s mentioned before. For the moleculr dsorption pthwy shown in Fig. 2, the first step is the brrierless formtion of the moleculr precursor mtop-prllel nd mbrg with exothermicities of 0.55 nd 0.48 ev, respectively. These vlues re close to the exothermicities of the H 2 S moleculr complex on pyrite (FeS 2 ) [20], Fe(1 00) [21], nd Fe(110) [22] (0.48, 0.46, nd 0.50 ev, respectively), but re slightly different from tht on Pd(1 11) with 0.22 ML coverge (0.66 ev) [18]. For rection pthwy vi the mtop-prllel intermedite, it produces mlm1, Ni SH + H, with the exothermicty of 1.64 ev fter overcoming rection brrier of 0.07 ev vi TS1, while the dsorbed sulfide on mbrg cn decompose vi TS3 lying 0.04 ev below the rectnts (or 0.44 ev bove mbrg) to give mlm2 with the exothermicity of 1.67 ev, ssuming the H toms re totlly dissocited. The mlm1 nd mlm2 intermedites cn further frgment to give Ni S + 2H with n exothermicity of 2.17 ev fter overcoming smll rection brrier of TS2 nd TS4, respectively. Then, dissocited hydrogen tom diffuses on the surfce. In this study, we restricted optimiztion processes within the Ni(1 1 1) supercell model to void ny unnecessry interctions. To hve better picture of the surfce nd the distnces between toms in the neighboring cells, we depict lrger surfce frgment corresponding to p(4 4) unit cell with finl products, Ni S + 2H. (see Fig. 3) A short distnce found between

4 182 Y.M. Choi et l. / Chemicl Physics Letters 421 (2006) H toms in neighboring cells, Å (1.492 Å for the dissocitive pthwy), my be indictive of further H 2 forming process under fuel-cell operting conditions. In ddition, we exmined the effect of surfce coverge on dsorption energies. For the clcultions, unrelxed three-lyer surfce model similr to Alfonso nd coworkers [18] ws used to sve CPU resources. For the top site mtop-prllel intermedite, decresing the coverge from 0.25 to 0.11 ML corresponding to p(2 2) nd p(3 3) super cells, respectively, leds to slight chnge in dsorption energy from 0.58 nd 0.62 ev (8%). The surfce coverge effects on energetics for other locl minim, nd trnsition sttes will be reported in subsequent communictions. Under fuel-cell operting conditions, we ssume the surfce products my further rect with oxygen nion (O 2 ) forming H 2 O nd SO 2 or interct with ech other producing H 2 molecules vi the pprently short hydrogen bond. Furthermore, the dsorbed sulfur tom my diffuse to the other ctive sites, i.e., top, bridge, nd hcp, s described elsewhere [5]. It is interesting to note tht the predicted dsorption energies for the moleculr dsorption of H 2 S on n unrelxed Cu(111) surfce t top, bridge, nd threefold hcp nd fcc sites re 0.04 nd 0.09, 0.02, 0.00, 0.01 ev, respectively, supporting mechnisticlly the fct tht Cubsed nodes re more sulfur-tolernt s mentioned in Section 1. Their energetics nd geometricl prmeters re compiled in Tble 3. Fig. 3. Geometricl illustrtion of surfce dsorbed products on Ni(111) vi the moleculr nd dissocitive dsorption. Given in prentheses re the bond distnces of the finl products for the dissocitive dsorption. Bond lengths re in Å. Tble 3 Adsorption energies nd moleculr prmeters, i.e., bond length in (Å) nd ngle in ( ), of dsorbed H 2 S species on Cu(111) without surfce relxtion Active site top bridge hcp fcc up prllel r(s H) h(h S H) d(s S) b c(ni H S H) c c c d e f Adsorption energies in ev reltive to unrelxed Cu(111) nd gsphse H 2 S molecule. b Verticl distnce between surfce nd sulfur tom. c f c(cu 1 H S H), c(cu 4 H S H), c(cu 2 H S H), nd c(cu 3 H S H), respectively, re dihedrl ngles. Fig. 4. Schemtic illustrtion of intermedites for the dissocitive dsorption optimized on three ctive sites, i.e., top, top-to-top, nd hcp-to-fcc, respectively. The vlues re clculted dsorption energies in ev. Ni, S, nd H toms re in gry, yellow, nd ple blue, respectively (For interprettion of the references in colour in this figure legend, the reder is referred to the web version of this rticle) Dissocitive dsorption Similr to the moleculr dsorption process, n initil geometry ws optimized on the unrelxed Ni surfce by plcing hydrogen tom on the top site s shown in Fig. 4, producing not moleculrly dsorbed intermedite but dissocited intermedite in decomposed HS nd H. In ddition to the top site, the two hydrogen toms of the H 2 S molecule were locted on top-to-top (Ni 1 Ni 4 ) nd hcp-to-fcc, resulting in dbrg nd dhcp-fcc, respectively. Their energetics nd geometricl dt re summrized in Tble 4. Similr to the moleculr dsorption, we re-optimized dtop, due to its highest stbility, by relxing the three uppermost lyers to crry out mechnistic study for the dissocitive dsorption. Shown in Fig. 2 is potentil energy profile for the dissocitive dsorption on the Ni(111) surfce. As n H 2 S molecule without brrier pproches the Ni surfce presented, the totl energy Tble 4 Adsorption energies nd moleculr prmeters, i.e., bond length in (Å) nd ngle in ( ), of dsorbed HS species on Ni(111) vi the dissocitive dsorption Active site top b bridge hcp-fcc Species dtop dbrg dhcp-fcc 1.69 [ 1.75] r(s H) [1.416] h(ni S Ni) 68.5 [69.3] d(s S) c [1.738] c(ni Ni S H) d [100.6] Adsorption energies in ev reltive to unrelxed or relxed Ni(111) nd gs-phse H 2 S molecule. b The dsorption energy nd prmeters in brckets were clculted on relxed Ni(111). c Verticl distnce between surfce nd sulfur tom. d Dihedrl ngle.

5 Y.M. Choi et l. / Chemicl Physics Letters 421 (2006) Tble 5 Clculted vibrtionl frequencies in cm 1 of dsorbed H 2 S nd HS species for the moleculr nd dissocitive dsorptions on Ni(111) Moleculr dsorption b Dissocitive dsorption c v i mtop-up mtop-prllel mbrg mhcp mfcc dtop dbrg dhcp-fcc v [2426] 2390 [2372] [2057] v [2380] 2343 [2339] [1238] v [1093] 1143 [1135] The vibrtionl modes in brckets were clculted on relxed Ni(111), otherwise on the unrelxed Ni(111) surfce. b v 1 nd v 2 nd v 3 correspond to two S H stretching nd H S H bending modes, respectively. c v 1 nd v 2 correspond to S H nd Ni H stretching modes, respectively. decreses, forming dtop with n exothermicity of 1.75 ev. After overcoming TS5 with smll 0.12 ev brrier, the H tom of the surfce HS species dissocites, producing similr products s in the cse of moleculr dsorption (see Fig. 3), in which sulfur tom is locted on the fcc site. The exothermicity of the finl products is the sme s tht of the moleculr dsorption. Similr to the moleculr dsorption, further H 2 forming process cn occur through the dissocitive dsorption due to the short H H bond distnce of Å (see Fig. 3). In ddition to the mechnistic studies, s summrized in Tble 5, to guide surfce vibrtionl spectroscopy, the vibrtionl frequencies for the dsorbed H 2 S nd HS species on the Ni(111) surfces were estimted by displcing dsorbed species. Since the dsorbed sulfur toms re strongly bound to the surfce, predicted vibrtionl frequencies for the surfce-s stretching mode my be close to tht of sulfur tom solely dsorbed t the fcc site of Ni(111) (441 cm 1 ) [5]. 4. Conclusion The interctions of H 2 S with Ni- nd Cu-bsed nodes for SOFCs hve been exmined by periodic DFT clcultions to elucidte the rection mechnisms of H 2 S dsorption nd decompostion. Two rection pthwys vi moleculr nd dissocitive dsorption were studied bsed on energeticlly fvorble species. Compring dsorption energies of dsorbed H 2 S sepcies on Ni(111) nd Cu(1 11), Cu is much more surfur-tolernt thn Ni due to its wek binding energy ( 0.0 ev), implying no further interction on the Cu surfce. We pln to exmine the further intertions of the dissoctied hydrogen nd/or sulfur toms with oxygen ions (O 2 ) electrochemiclly pumped through n electrolyte such s yittri-doped zirconi or gdoli-doped ceri under fuel-cell operting conditions. Acknowledgements The uthors re grteful for the finncil support of this work provided by DOE-NETL-SECA core technology progrm (Grnt No. DE-FC26-04NT42219) directed by Dr. Lne Wilson. The uthors lso pprecite the reviewer s criticl comments tht mde the mnuscript more meningful nd informtive. We cknowledge the use of CPU s from Ntionl Center for High-performnce Computing, Tiwn. MCL cknowledges the support from Tiwn Ntionl Science Council for Distinguished Visiting Professorship t the Center for Interdisciplinry Moleculr Science, Chio Tung University, Hsinchu, Tiwn. References [1] H. Kim, J.M. Vohs, R.J. Gorte, Chem. Commun. (2001) [2] L.G. Mrinowski, G.L. Anderson, E.H. Cmr, US Ptent (1988). [3] L. Aguilr, S. Zh, Z. Cheng, J. Winnick, M. Liu, J. Power Source 135 (2004) 17. [4] S.P. Wlch, W.A. Goddrd III, Surf. Sci. 72 (1978) 645. [5] Y.M. Choi, C. Compson, M.C. Lin, M. Liu, J. Solid Stte Chem., submitted for publiction. [6] G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) [7] G. Kresse, J. Hfner, Phys. Rev. B 47 (1993) 558. [8] P. Blochl, Phys. Rev. B 17 (1994) 953. [9] J.P. Perdew, Y. Wng, Phys. Rev. B 45 (1992) [10] H.J. Monkhorst, J.D. Pck, Phys. Rev. B 13 (1976) [11] M. Methfessel, A.T. Pxton, Phys. Rev. B 40 (1989) [12] M.J. Frisch et l., GAUSSIAN 03, Revision C.01, Gussin Inc., Wllingford, CT, [13] G. Kresse, J. Hfner, Surf. Sci. 459 (2000) 287. [14] F. Mittendorfer, J. Hfner, Surf. Sci. 472 (2001) 133. [15] CRC Hndbook of Chemistry nd Physics, CRC Press, New York, [16] G. Herzberg, Moleculr Spectr nd Moleculr StructureElectronic Spectr nd Electronic Structure of Polytomic Molecules, vol. III, Krieger, Mlbr, FL, [17] K.P. Huber, G. Herzberg, Moleculr Spectr nd Moleculr StructureConstnts of Ditomic Molecules, vol. IV, Vn Nostrnd, New York, [18] D.R. Alfonso, A.V. Cugini, D.C. Sorescu, Ctl. Tody 99 (2005) 315. [19] H. Ruusk, T.A. Pkknen, R.L. Rowley, J. Phys. Chem. B 108 (2004) [20] A. Stirling, M. Bernsconi, M. Prrinello, J. Chem. Phys. 119 (2003) [21] D.E. Jing, E.A. Crter, J. Phys. Chem. B 108 (2004) [22] D.E. Jing, E.A. Cter, Surf. Sci. 583 (2005) 60.