35nd ACS National Meeting April 6-10, 2008 New Orleans, Louisiana Organizers : Umit S. Ozkan Jingguang G Chen Presiding: Thursday April 10, 2008, 11:45.

35nd ACS National Meeting April 6-10, 2008 New Orleans, Louisiana Organizers : Umit S. Ozkan Jingguang G Chen Presiding: Thursday April 10, 2008, 11:45 am. -12:15 PM. Morial Convention Center, Room: Rm. 208 N.Galea, D.Knapp, E.Kadantsev, M.Shiskin, T.Ziegler Department of Chemistry University of Calgary,Alberta, Canada T2N 1N4 New Orleans National Meeting Studying SOFC anode activity with DFT: Suggestions for coke reduction and the effects of hydrogen sulfide adsorption Symposium on Roles of Catalysis in Fuel Cells Division for Petrochemistry

Solid Oxide Fuel Cell – CH 4 *Most common SOFC material *Ni-YSZ Temp. 800 – 1000 o C V + - AnodeCathodeElectrolyte *YSZ  CH 4 + 4O 2-  2H 2 O + CO 2 + 8e - (Direct Oxidation,coaking)  CH 4 + H 2 O  CO + 3H 2 (Steam Reforming Reaction)  H 2 /CO + O 2-  H 2 O/CO 2 + 2e - (Oxidation Reaction) Molecular hydrogen or methane gas is typical anode fuel. CH 4 adsorbs on Ni anode surface and decomposes, blocking adsorption sites with graphene, most stable form of carbon. The problem of coking

Activation on Ni H 2 --> 2H* CH 4 --> xH*+CH 4-x Triple Phase Boundary (TPB) Reactions Anode Electrolyte Cathode 2O 2- Nickel/YSZ YSZ Nickel YSZ O 2 (g) Oxygen rich YSZ 4e - Pre-activation on Ni Burning on oxygen rich YSZ 2H+ O 2 ----> H 2 O +2e- CH 4-x + +(8-x)/2 O 2- ---> CO 2 +(4-x)/2H 2 O+(8-x)e- +C(Coke)

Surface Calculations – CH 4 Two classes of active adsorption sites. Stepped surfaces more reactive than planar surfaces. Supercell; 3 layers, 2x2(planar) or 2x3(stepped) surface. Planar (111) - *C Stepped (211) - *C Steps and Terraces

Calculations – CH 4 Vienna Ab Initio Package (VASP). ADF BAND Projector augmented wave (PAW) method. Frozen core (BAND) Generalized gradient approximation (GGA) functional PBE96. Planar (111) Surfaces: 2x2 unit cell, with 3 layers. Stepped (211) Surfaces: 3x3 unit cell, with 3 layers. Theoretical equilibrium bulk lattice constants, a O (Ni) is 3.52Ǻ and a O (Cu) is 3.61Ǻ. 10Ǻ vacuum region between slabs. Cu(111): 5 x 5 x 1 Monkhorst-Pack k-point mesh. Other Surfaces: 4 x 4 x 1 Monkhorst-Pack k-point mesh. Kinetic energy (wave function) cutoff energy is 25Ry = 340eV. Charge density (augmentation) cutoff energy is 50Ry = 680eV. Energies converged to 10 -3 eV. TS and reaction barriers calculated using the nudged-elastic band (NEB) method. MatLab mathematical software package. Computational Details N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33

Ni(111) and Ni(211) Surfaces : Adsorption and Decomposition of CH 4 Theoretical literature – Nørskov. Planar surface implies that coking should not occur. Stepped surface energies illustrating final exothermic dissociation reaction is driving force of coke formation. (a) (b) Decomposition of CH 4 on steps and terraces of Ni Graphene

Ni(111) & Ni(211) Decomposition of CH 4 on steps and terraces of Ni N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33

Graphene Carbon is adsorbed at step base, resulting in formation of graphene (coke) layer on (111) terrace. Ni and hexagonally structured carbon atoms lie parallel to one another. Graphene island of finite size is required for stability. Blocking all step sites is NOT needed to prevent formation. Sparse covering of promoter atoms (e.g. gold, sulfur, alkali) or replacing Ni with Cu can hinder coke formation. (Pictorial representation of surface) Graphen formation

Cu(111) and Cu(211) Surfaces : Adsorption and Decomposition of CH 4 Activity of copper in the dissociation of methane will be poor. Carbon cokes will not form on copper surfaces. Consistent with experimental SOFC observations. Galea et al. Journal of Catalysis 247 (2007) 20-33 Decomposition of CH 4 on steps and terraces of Cu N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33

Cu(111) & Cu(211) Galea et al. Journal of Catalysis 247 (2007) 20-33 Decomposition of CH 4 on steps and terraces of Cu N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33

Step Edge - Cu-Ni(211) : Adsorption and Decomposition of CH 4 Cu surface segregation occurs as Cu has a lower surface energy than Ni. Likely that Ni steps that nucleate *C formation are blocked by Cu atoms, exposed terrace Ni sites contribute to activity. Endothermic *C production on alloy, with reasonable activity. (a) Copper Galea et al. Journal of Catalysis 247 (2007) 20-33 Decomposition of CH 4 on Cu-steps and Ni-terraces

S-Ni(211) Galea et al. Journal of Catalysis 247 (2007) 20-33 Decomposition of CH 4 on S-steps and Ni-terraces N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33

100% Step – Au/S-Ni(211) : Adsorption and Decomposition of CH 4 Small amounts of sulfur / gold can discourage the adsorption of carbon at the step by blocking edge sites, mimicking the nature of the planar nickel surface. (a) Sulfur or Gold Galea et al. Journal of Catalysis 247 (2007) 20-33 Decomposition of CH 4 on (S,Au,S) steps and Ni-terraces N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33

A. Conclusions – CH 4 Our research theoretically studies methods used experimentally to block step sites and reduce graphitic carbon formation. Propensity to coking of Ni surface explained by strong adsorption of *C atoms at step edge, followed by graphene growth over terrace sites. Thermodynamic energies and kinetic barriers of methane ads. n and dis. n on Cu surfaces are high, explaining poor activity and lack of coke. Cu-Ni alloys, where Cu blocks step sites, the catalyst retains activity due to Ni, while *C formation remains endothermic due to Cu. S-Ni stepped surface (and Au) demonstrates that step blocking renders step sites inactive to methane dis. n and forces ads. n onto terrace sites. Galea, N.M.; Knapp, D.; Ziegler, T. J. Catal. 2007, 247, 20.

Activation on Ni H 2 --> 2H* CH 4 --> xH*+CH 4-x Triple Phase Boundary (TPB) Reactions Anode Electrolyte Cathode 2O 2- Nickel/YSZ YSZ Nickel YSZ O 2 (g) Oxygen rich YSZ 4e - Pre-activation on Ni Burning on oxygen rich YSZ 2H+ O 2 ----> H 2 O +2e- CH 4-x + +(8-x)/2 O 2- ---> CO 2 +(4-x)/2H 2 O+(8-x)e- M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress

Triple Phase Boundary (TPB) Reactions Anode Electrolyte Cathode 2O 2- Nickel/YSZ YSZ Nickel YSZ O 2 (g) Oxygen rich YSZ 4e - Activation on YSZ Activation and burning on oxygen rich YSZ H 2 +O 2- ----> H 2 O +2e- CH 4 +4 O 2- ---> CO 2 +2H 2 O+8e - M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress

Triple Phase Boundary (TPB) Reactions Anode Electrolyte Cathode 2O 2- Nickel/YSZ YSZ Nickel YSZ O 2 (g) Oxygen rich YSZ 4e - Activation on YSZ 9%-YSZ Zr O Y M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress

Molecular Hydrogen Adsorption on Oxygen Rich YSZ Initial adsorption of H 2 (g) on 9%-YSZ is energetically more favourable than on nickel. TS energy barriers all < +5 kcal/mol. M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress

Methane adsorption on Oxygen rich YSZ: initial stage. M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress

Methane adsorption on Oxygen rich YSZ: Second stage.

Third stage: formaldehyde decomposition on oxygen enriched YSZ surface.

Methane adsorption on oxygen deficient YSZ surface.

B. Conclusions – CH 4 It might be possible to construct anodes of inactive conductors and electrolytes that can oxydize fuels. M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress

Activation on Ni H 2 --> 2H* CH 4 --> xH*+CH 4-x Solid Oxide Fuel Cell – H 2 S Anode Electrolyte Cathode 2O 2- Nickel/YSZ YSZ Nickel YSZ O 2 (g) Oxygen rich YSZ 4e - Pre-activation on Ni with sulfur deposition Burning on oxygen rich YSZ 2H+ O 2 ----> H 2 O +2e- CH 4-x + +(8-x)/2 O 2- ---> CO 2 +(4-x)/2H 2 O+(8-x)e- H2S --> S*+H2(g)

Calculations – H 2 S Vienna Ab Initio Package (VASP). Projector augmented wave (PAW) method. Generalized gradient approximation (GGA) functional PBE96. Orthorhombic 2x2 unit cell, with 3 layers. Theoretical equilibrium bulk lattice constant, a O, is 3.52Ǻ. 10Ǻ vacuum region between slabs. 5 x 5 x 1 Monkhorst-Pack k-point mesh. Kinetic energy (wave function) cutoff energy is 400eV. Charge density (augmentation) cutoff energy is 800eV. Energies converged to 10 -3 eV. TS and reaction barriers calculated using the nudged-elastic band (NEB) method. MatLab mathematical software package. Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C 2007, 111, 14457.

Surface Calculations – H 2 S Hydrogen (pairs) Surface Coverage,  2H, is ratio between number of adsorbed hydrogen atom pairs and number of Ni surface atoms. i.e. 2H:Ni = 1:4,  2H = 0.25ML. Repeated supercell; 3 layers, 2x2 surface. Sulfur Surface Coverage,  S, is ratio between number of adsorbed sulfur atoms and number of Ni surface atoms. i.e. S:Ni = 1:4,  S = 0.25ML. Steps and Terraces

Maximum Adsorption of H 2 S(g) On the basis of thermodynamic energy, the most stable sulfur surface coverage is  S = 0.50ML. Concurs with experimental coverage of 0.50-0.60 ML. Natural S ads. n cutoff point explains decreased exp. activity. Surface + 4H 2 S(g)  4*S-Surface + 4H 2 (g) “S” “S__S” “S_S_S” “S_S_S_S” Surface+4H 2 S(g) 4S*-surface+ 4H 2 S(g)

Hydrogen Sulfide Adsorption  S = 0-0.25 ML : H 2 S adsorption is an exothermic reaction.  S = 0.25-0.50 ML : H 2 S adsorption is endothermic. Overall difference in energy is due to steric interactions on the surface. n*S-Surface + H 2 S(g)  (n+1)*S-Surface + H 2 (g) (c) (a) (d) (b) nS*-Surface+H 2 S(g) (n+1)S*-surface+ H 2 (g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C 2007, 111, 14457.

Hydrogen Sulfide Adsorption  S = 0-0.25 ML : H 2 S adsorption is an exothermic reaction.  S = 0.25-0.50 ML : H 2 S adsorption is endothermic. Overall difference in energy is due to steric interactions on the surface. n*S-Surface + H 2 S(g)  (n+1)*S-Surface + H 2 (g) (c) (a) (d) (b) nS*-Surface+H 2 S(g) (n+1)S*-surface+ H 2 (g)

Adsorption Energies Adsorbed*S-Surface,AdsorptionNi-S Bond Species Final  S. Energy, E Ads.Distance (Ǻ) *SH 2 102.18 *SH0.25ML772.18(x2) *S1162.15(x3) (*H)(64) *SH 2 72.30 *SH0.50ML532.24(x2) *S902.22, 2.19(x2) (*H)(61)- E Ads (kcal/mol) = E Surface + E Gas - E AdsorbedSpecies Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C 2007, 111, 14457.

Molecular Hydrogen Adsorption 0S :  S = 0.00ML, max.  2H = 0.50ML : Ads. n strongly exothermic. 1S :  S = 0.25ML, max.  2H = 0.25ML : Adsorption exothermic. 2S :  S = 0.50ML, max.  2H = 0.25ML : Adsorption endothermic. Presence of surface sulfur reduces hydrogen adsorption by half. n*S-Surface + xH 2 (g)  2x*H-n*S-Surface nS*-Surface+xH 2 (g) 2xH*-nS*-Surface

Multiple H 2 S(g) Adsorptions at 800 o C Surface + 2H 2 S(g)  2*S-Surface + 2H 2 (g) Point A : Despite large TS barriers, exothermic/exergonic nature of overall reaction produces a  S = 0.50ML surface. Point B : Removal of H 2 S from the anode fuel feed allows the partial removal of surface sulfur, due to small difference in energy between species “S__S” and “S”. Surface+2H 2 S(g) 2S*-Surface+ 2H 2 (g)

CSTR Kinetic Model Continually Stirred Tank Reactor (CSTR) model.  Reactor described by a ‘box’ (mimicking the anode), with a specific volume and maintained at a particular temperature.  The ‘surface’ within the box (mimicking the anode surface) has a specific reactive surface and vacant adsorption site concentration.  Gaseous fuel continually flows into CSTR (anode fuel feed) and gaseous products or unused fuel continually flow out with a specific flowrate.  Gaseous species can adsorb/desorb on the surface, and adsorbed species can react with each other.  Sulfur surface coverage and surface steric interactions are considered by dissecting the surface into equally sized sections (2x2) and considering each section as a vacant site. Determining Rate of Reactions :  T  S = T.  S (translational/rotational).  H 2 S(g)/800 o C, T  S = 53 kcal/mol,  H 2 (g)/800 o C, T  S = 34 kcal/mol. Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C 2007, 111, 14457.

Rate of Formation of Individual Species Individual rate constants, k, used to determine time-dependant rate of formation of each species in reaction scheme. Example reaction mechanism : Integration over time : Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C 2007, 111, 14457.

Point A – Surface Sulfur Formation : Initial Adsorption on  S = 0ML Surface A  S =0.25ML surface (a 100% CSTR surface coverage of *S) is initially formed via H 2 S(g) adsorption and H 2 (g) desorption. Anode Fuel at 800 o C pH 2 = ~1atm, pH 2 S = 1x10 -5 atm = 10ppm. Initial Surface,  S = 0.00ML. Surface + H 2 S(g)  *S-*H-*H *S-*H-*H  *S + H 2 (g) *S + H 2 S(g)  *S-*S-*H-*H *S-*S-*H-*H  *S--*S + H 2 (g) Surface + 2H 2 (g)  4*H Further H 2 S(g)/H 2 (g) adsorption/desorption results in a 100% CSTR surface coverage of 2*S, a  S =0.50ML surface.

Point B - Surface Sulfur Removal : Initial Adsorption on  S = 0.50ML Surface Equilibrium is reached upon the production of a  S =0.25ML surface (a 100% CSTR surface coverage of *S). Anode Fuel at 800 o C pH 2 = 1atm, (No H 2 S(g) in fuel). Initial Surface,  S = 0.50ML. *S--*S + H 2 (g)  *S-*S-*H-*H *S-*S-*H-*H  *S + H 2 S(g) *S + H 2 (g)  *S-*H-*H *S-*H-*H  Surface + H 2 S(g) Surface + 2H 2 (g)  4*H Model mimics experimental attempts to purge sulfur from surface by eliminating H 2 S from anode fuel feed.

A. Conclusions – H 2 S Our research studies the affects of consecutive adsorption and dissociation of H 2 S and subsequent desorption of H 2 on Ni surfaces. Failure of S-based pollutants in anode fuel to cause completely inoperable conditions within SOFC anode is due to inability of planar Ni to favourably adsorb H 2 S at a S coverage greater than 50%. The endergonic nature of H 2 S ads. n at  S >0.50ML causes cutoff point. Complete irreversibility of H 2 S ads. n caused by large endothermic/ endergonic energy difference between  S = 0 and 0.25 (*S) ML. A  2H = 0.50ML is achieved without the presence of surface sulfur. At  S = 0.25 and 0.50 ML, only a  2H = 0.25ML coverage is formed. Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C 2007, 111, 14457.

Removal of Remaining Sulfur by O 2 Treatment 1S*-Surface+O 2 (g) --> “Clean surface +SO 2 (g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C, accepted.

Removal of Sulfur by O 2 Treatment 1S*-Surface+O 2 (g) --> “Clean surface +SO 2 (g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C, accepted.

Removal of Sulfur by O 2 Treatment 2S*-Surface+O 2 (g) --> “1S* surface +SO 2 (g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C, accepted.

Removal of Sulfur by O 2 Treatment Surface coverage of selected species determined by kinetic CSTR model at 800 0 C of O2 exposure to  S = 0.50ML surface. Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C, accepted.

B Conclusions – H 2 S Sulfur with coverage  S = 0.25 ML can be removed by O 2 Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C accepted.

Acknowledgements Thank You! Financial support was provided by the Alberta Energy Research Institute and the Western Economic Diversification Department. Calculations were carried out on WestGrid computing resources, funded in part by the Canadian Foundation for Innovation, Alberta Innovation and Science, BC Advanced Education, and the participating research institutions.

35nd ACS National Meeting April 6-10, 2008 New Orleans, Louisiana Organizers : Umit S. Ozkan Jingguang G Chen Presiding: Thursday April 10, 2008, 11:45.

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35nd ACS National Meeting April 6-10, 2008 New Orleans, Louisiana Organizers : Umit S. Ozkan Jingguang G Chen Presiding: Thursday April 10, 2008, 11:45.

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Presentation on theme: "35nd ACS National Meeting April 6-10, 2008 New Orleans, Louisiana Organizers : Umit S. Ozkan Jingguang G Chen Presiding: Thursday April 10, 2008, 11:45."— Presentation transcript:

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