Transcript Slide 1
New Orleans National Meeting 35nd ACS National Meeting April 6-10, 2008 New Orleans, Louisiana Symposium on Roles of Catalysis in Fuel Cells Division for Petrochemistry 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 Studying SOFC anode activity with DFT: Suggestions for coke reduction and the effects of hydrogen sulfide adsorption Solid Oxide Fuel Cell – CH4 The problem of coking H2 + O + V 2- *Ni-YSZ Anode H 2O + 2e - *Most common SOFC material O2 + 4 e - Electrolyte Cathode *YSZ 2O Temp. 800 – 1000 oC 2- 2- O CH4 + 4O2- 2H2O + CO2 + 8e- (Direct Oxidation,coaking) CH4 + H2O CO + 3H2 (Steam Reforming Reaction) H2/CO + O2- H2O/CO2 + 2e- (Oxidation Reaction) Molecular hydrogen or methane gas is typical anode fuel. CH4 adsorbs on Ni anode surface and decomposes, blocking adsorption sites with graphene, most stable form of carbon. Triple Phase Boundary (TPB) Reactions Pre-activation on Ni Nickel/YSZ Anode YSZ Electrolyte 2O2- Cathode 2O2O2(g) 2O24e- Activation on Ni H2 --> 2H* CH4 --> xH*+CH4-x +C(Coke) Burning on oxygen rich YSZ 2H+ O2 ----> H2O +2eCH4-x + +(8-x)/2 O2- ---> CO2+(4-x)/2H2O+(8-x)e- Nickel YSZ Oxygen rich YSZ Surface Calculations – CH4 Steps and Terraces Planar (111) - *C Stepped (211) - *C Two classes of active adsorption sites. Stepped surfaces more reactive than planar surfaces. Supercell; 3 layers, 2x2(planar) or 2x3(stepped) surface. Calculations – CH4 Computational Details 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, aO(Ni) is 3.52Ǻ and aO(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 10-3eV. TS and reaction barriers calculated using the nudged-elastic band (NEB) method. MatLab mathematical software package. N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33 Ni(111) and Ni(211) Surfaces : Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Ni 10 CH4(g) Ni(111) (a) Relative Energy (kcal/mol) 5 0 -5 (a) -10 (b) -15 -20 Ni(211) (b) -25 Surface *CH3, *H *CH2, 2*H *CH, 3*H Theoretical literature – Nørskov. *C, 4*H Graphene Planar surface implies that coking should not occur. Stepped surface energies illustrating final exothermic dissociation reaction is driving force of coke formation. Ni(111) & Ni(211) Decomposition of CH4 on steps and terraces of Ni 45 Bengaard et al. J. Catal. 2002, 209 , 365-384. 40 Ni(111) : Planar Surface Ni(211) : Stepped Surface H H 35 C H 31 Relative Energy (kCalmol -1) Ni 30 Ni Ni 3-fold 25 24 24 20 22 20 H H Ni 0 -5 H Ni Ni Ni Ni Ni Ni Ni H 2-fold@edge Ni Ni C *C,4*H Ni -10 Ni Ni 1-fold@edge Ni Ni Ni Ni Ni 5 coordinate site -15 N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33 Ni Ni -5 Ni Ni Ni Ni Ni Ni Ni H H C CH4(g) + Surface Ni Ni C *CH,3*H H Ni Ni Ni Ni 4 C Ni Ni Ni H Ni Ni Ni Ni Ni Ni H Ni Ni Ni Ni *CH2,2*H *CH3,*H Ni 11 11 10 10 Ni 23 23 17 C H 25 13 H 15 5 41 Graphene Graphene Graphen formation 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. Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Graphene island of finite size Ni is required for stability. Blocking all step sites is NOT needed to prevent formation. Ni Ni Ni Ni Ni Ni Ni Sparse covering of promoter atoms (e.g. gold, sulfur, alkali) or replacing Ni with Cu can hinder coke formation. Ni Ni (Pictorial representation of surface) Ni Ni Cu(111) and Cu(211) Surfaces : Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Cu 110 Relative Energy (kcal/mol) 100 90 Cu(111) 80 70 60 50 CH4(g) Cu(211) 40 30 20 10 0 Surface TS *CH3, *H TS *CH2, 2*H TS *CH, 3*H TS *C, 4*H Activity of copper in the dissociation of methane will be poor. Carbon cokes will not form on copper surfaces. Consistent with experimental SOFC observations. N.Galea,D.Knapp,T.Ziegler Galea et al. Journal of Catalysis Journal 247of (2007) Catalysis 20-33247 (2007) 20-33 Cu(111) and Cu(211) Surfaces : Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Cu 110 Relative Energy (kcal/mol) 100 90 Cu(111) 80 70 60 50 CH4(g) Cu(211) 40 30 20 10 0 Surface TS *CH3, *H TS *CH2, 2*H TS *CH, 3*H TS *C, 4*H Activity of copper in the dissociation of methane will be poor. Carbon cokes will not form on copper surfaces. Consistent with experimental SOFC observations. N.Galea,D.Knapp,T.Ziegler Galea et al. Journal of Catalysis Journal 247 of(2007) Catalysis 20-33 247 (2007) 20-33 Cu(111) and Cu(211) Surfaces : Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Cu 110 Relative Energy (kcal/mol) 100 90 Cu(111) 80 70 60 50 CH4(g) Cu(211) 40 30 20 10 0 Surface TS *CH3, *H TS *CH2, 2*H TS *CH, 3*H TS *C, 4*H Activity of copper in the dissociation of methane will be poor. Carbon cokes will not form on copper surfaces. Consistent with experimental SOFC observations. N.Galea,D.Knapp,T.Ziegler Galea et al. Journal of Catalysis Journal 247 (2007) of Catalysis 20-33 247 (2007) 20-33 Cu(111) and Cu(211) Surfaces : Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Cu 110 Relative Energy (kcal/mol) 100 90 Cu(111) 80 70 60 50 CH4(g) Cu(211) 40 30 20 10 0 Surface TS *CH3, *H TS *CH2, 2*H TS *CH, 3*H TS *C, 4*H Activity of copper in the dissociation of methane will be poor. Carbon cokes will not form on copper surfaces. Consistent with experimental SOFC observations. N.Galea,D.Knapp,T.Ziegler Galea et al. Journal of Catalysis Journal 247of (2007) Catalysis 20-33247 (2007) 20-33 Cu(111) and Cu(211) Surfaces : Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Cu 110 Relative Energy (kcal/mol) 100 90 Cu(111) 80 70 60 50 CH4(g) Cu(211) 40 30 20 10 0 Surface TS *CH3, *H TS *CH2, 2*H TS *CH, 3*H TS *C, 4*H Activity of copper in the dissociation of methane will be poor. Carbon cokes will not form on copper surfaces. Consistent with experimental SOFC observations. N.Galea,D.Knapp,T.Ziegler Galea et al. Journal of Catalysis Journal 247 (2007) of Catalysis 20-33 247 (2007) 20-33 Cu(111) & Cu(211) 110 100 Relative Energy (kCal/mol) 90 80 70 Decomposition of CH4 on steps and terraces of Cu Adsorption Energy Cu(111) & Cu(211) Supercell surface unit cells =(2x2)& (3x3). Layers =3 & 9. Lat t ice Const anta, = 3.615 Angst roms. Lat t ice Vectors (Angst roms) :x = 4.316 & 6.261. y = 4.984 & 7.668. z = 14.069& 16.588. GGA = P BE, ut ilizing P AW Met hod. Cut off Energies (Ry) :Wavefunct ion =37 & 25. 70.69 Charge Densit y =74 & 50. k-points : Monkhorst -pack3x2x1 = & 4x4x1. 105.35 (eV) *CH3 : 1.30 & 1.78 *CH2 : 3.91 & 3.33 *CH : 4.69 & 5.47 *C : 4.50 & 5.81 *H : 2.42 & 2.58 86.79 72.30 72.03 Cu 70.50 60 Cu Cu H C H H H Cu H Cu Cu 50 56.09 C Cu 44.54 40.10 H 44.70 H 40 C H Cu Cu Cu 48.19 Cu Cu Cu Cu Cu Cu 30 20 10 0 Cu H CH H Cu Cu Cu 33.24 H Cu 23.33 Cu Cu Cu Cu Cu Cu Cu Cu 27.11 *C = 5-coordination site 30.49 *H = 3-fold@edge2f *CH = 5-coordination site *CH2 = 3-fold@edge2f 7.75 Cu(111) Cu(211) Cu CH4(g) = 2-fold@Edge CH4(g) + Surface *CH3 = 2-fold@Edge *CH3 + *H *CH2 + 2*H *CH + 3*H N.Galea,D.Knapp,T.Ziegler Journal of Catalysis Galea et al. Journal of Catalysis 247 (2007) 20-33 247 (2007) 20-33 *C + 4*H Step Edge - Cu-Ni(211) : Adsorption and Decomposition of CH4 Decomposition of CH4 on Cu-steps and Ni-terraces Galea et al. Journal of Catalysis 247 (2007) 20-33 50 Cu(211) Copper (a) Relative Energy (kcal/mol) 40 30 CH4(g) 20 10 Cu-Ni(211) (a) 0 -10 -20 Ni(211) -30 Surface *CH3, *H *CH2, 2*H *CH, 3*H 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. *C, 4*H S-Ni(211) Decomposition of CH4 on S-steps and Ni-terraces 40 35 Relative Energy (kCal/mol) 30 NiS(211) Supercell surface unit cells = (2x3). Layers = 9. Lat tice Constanta, = 3.615 Angstroms. Lat tice Vect ors (Angst roms) :x = 6.1005, y = 4.9796, z = 16.000. GGA = P BE, utilizing P AW Method. Cut off Energies (Ry) :Wavefunct ion = 25. Charge Densit y = 50. k-point s : Monkhorst -pack = 4x4x1.H C Ni C Ni Ni 28.81 Ni C (35) S Ni Ni Ni Ni Ni S Ni Ni Ni *Ni(111) = 30 *Ni(211) = 19 22.52 Ni S Ni Ni Ni Ni H H S S H H H Ni Ni C H H H Ni H 25.66 25 38.85 H S Ni 20 15 Ni Ni Ni S (19) S (26) Ni Ni Ni Ni Ni Ni *Ni(111) = 18 *Ni(211) = 12 *Ni(111) = 24 *Ni(211) = 20 Ni Ni (8) *Ni(111) = 7 *Ni(211) = 12 17.78 *C = 3-fold@edge2f *H = 3-fold@edge2f 14.35 *CH2 = 3-fold@edge2f 10 10.05 *CH3 = 3-fold@edge2f 5 *Bengaard et al. J. Catal. 2002, 209 , 365-384. 0 CH4(g) + Surface *CH3 + *H *CH2 + 2*H 4.05 *CH = 3-fold@edge2f *CH + 3*H N.Galea,D.Knapp,T.Ziegler Journal of Catalysis Galea et al. Journal of Catalysis 247 (2007) 20-33 247 (2007) 20-33 *C + 4*H 100% Step – Au/S-Ni(211) : Adsorption and Decomposition of CH4 Decomposition of CH4 on (S,Au,S) steps and Ni-terraces 30 Sulfur or Gold Relative Energy (kcal/mol) 20 (a) Au100%-Ni(211) CH4(g) S100%-Ni(211) 10 Ni(111) (a) 0 -10 -20 Ni(211) -30 Surface *CH3, *H *CH2, 2*H *CH, 3*H *C, 4*H 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. N.Galea,D.Knapp,T.Ziegler Journal Catalysis Galea et al. Journal of Catalysis 247 of (2007) 20-33 247 (2007) 20-33 *C", 4*H A. Conclusions – CH4 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. Triple Phase Boundary (TPB) Reactions Pre-activation on Ni Nickel/YSZ Anode YSZ Electrolyte 2O2- Cathode 2O2O2(g) 2O24e- Activation on Ni H2 --> 2H* CH4 --> xH*+CH4-x Burning on oxygen rich YSZ 2H+ O2 ----> H2O +2eCH4-x + +(8-x)/2 O2- ---> CO2+(4-x)/2H2O+(8-x)e- M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress Nickel YSZ Oxygen rich YSZ Triple Phase Boundary (TPB) Reactions Activation on YSZ Nickel/YSZ Anode YSZ Electrolyte 2O2- Cathode 2O2O2(g) 2O24e- Activation and burning on oxygen rich YSZ H2+O2- ----> H2O +2eCH4 +4 O2- ---> CO2+2H2O+8eM.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress Nickel YSZ Oxygen rich YSZ Triple Phase Boundary (TPB) Reactions Activation on YSZ Nickel/YSZ Anode YSZ Electrolyte 2O2- Cathode 2O2O2(g) 2O24e- Zr 9%-YSZ O Y Nickel YSZ Oxygen rich YSZ M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress Molecular Hydrogen Adsorption on Oxygen Rich YSZ 0 H2(g) Relative Energy (kcal/mol) -20 Initial adsorption of -40 -60 H2O(g) -80 -100 -120 -140 O"-Surface 2*OH *OH2 M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress O'-Surface H2(g) on 9%-YSZ is energetically more favourable than on nickel. TS energy barriers all < +5 kcal/mol. Methane adsorption on Oxygen rich YSZ: initial stage. 0 -10 -20 CH 3OH V E (kcal/mol) -30 -40 -50 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. -60 -70 -80 -90 *CH 3 *H -100 CH 4 *CH 4 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress Methane adsorption on Oxygen rich YSZ: Second stage. *CH *H H 2 10 -10 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. E (kcal/mol) -30 -50 CH 2O H 2 V -70 -90 -110 *CH 2 H 2 *CH 3 *H QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Third stage: formaldehyde decomposition on oxygen enriched YSZ surface. 0 Energy (kcal/mol) -20 -40 -60 -80 -100 -120 -140 *CHO H 2 CH 2O *CH 2O QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. *CHO*H QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Methane adsorption on oxygen deficient YSZ surface. CH 3OH V 120 100 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. E (kcal/mol) 80 60 *CH 3 *H 40 20 0 CH 4 *CH 4 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. B. Conclusions – CH4 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 Solid Oxide Fuel Cell – H2S Pre-activation on Ni with sulfur deposition Nickel/YSZ Anode YSZ Electrolyte 2O2- Cathode 2O2O2(g) 2O24e- Activation on Ni H2 --> 2H* CH4 --> xH*+CH4-x H2S --> S*+H2(g) Burning on oxygen rich YSZ 2H+ O2 ----> H2O +2eCH4-x + +(8-x)/2 O2- ---> CO2+(4-x)/2H2O+(8-x)e- Nickel YSZ Oxygen rich YSZ Calculations – H2S 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, aO, 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 10-3eV. 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 – H2S Steps and Terraces 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. 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. Maximum Adsorption of H2S(g) Surface + 4H 4S*-surface+ 4*S-Surface4H + 24H Surface+4H <--> S(g) 2S(g) 2(g) 2S(g) “S” “S_S_S_S” “S__S” 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. “S_S_S” Hydrogen Sulfide Adsorption n*S-Surface +2S(g) H2S(g) (n+1)*S-Surface + H2(g) nS*-Surface+H <-->(n+1)S*-surface+ H2(g) (d) (c) (a) (b) S = 0-0.25 ML : H2S adsorption is an exothermic reaction. S = 0.25-0.50 ML : H2S adsorption is endothermic. Overall difference in energy is due to steric interactions on the surface. Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C 2007, 111, 14457. Hydrogen Sulfide Adsorption nS*-Surface+H S(g) <--> (n+1)S*-surface+ H (g) n*S-Surface + 2H 2S(g) (n+1)*S-Surface 2+ H2(g) (d) (c) (a) (b) S = 0-0.25 ML : H2S adsorption is an exothermic reaction. S = 0.25-0.50 ML : H2S adsorption is endothermic. Overall difference in energy is due to steric interactions on the surface. Hydrogen Sulfide Adsorption nS*-Surface+H <-->(n+1)S*-surface+ H2(g) n*S-Surface +2S(g) H2S(g) (n+1)*S-Surface + H2(g) (d) (c) (a) (b) S = 0-0.25 ML : H2S adsorption is an exothermic reaction. S = 0.25-0.50 ML : H2S adsorption is endothermic. Overall difference in energy is due to steric interactions on the surface. Hydrogen Sulfide Adsorption nS*-Surface+H <--> (n+1)S*-surface+ H2(g) n*S-Surface + H2S(g) (n+1)*S-Surface + H2(g) 2S(g) (d) (c) (a) (b) S = 0-0.25 ML : H2S adsorption is an exothermic reaction. S = 0.25-0.50 ML : H2S adsorption is endothermic. Overall difference in energy is due to steric interactions on the surface. Hydrogen Sulfide Adsorption nS*-Surface+H <-->(n+1)S*-surface+ H2(g) n*S-Surface +2S(g) H2S(g) (n+1)*S-Surface + H2(g) (d) (c) (a) (b) S = 0-0.25 ML : H2S adsorption is an exothermic reaction. S = 0.25-0.50 ML : H2S adsorption is endothermic. Overall difference in energy is due to steric interactions on the surface. Adsorption Energies Adsorbed *S-Surface, Adsorption Ni-S Bond Energy, EAds. Distance (Ǻ) 10 2.18 77 2.18(x2) *S 116 2.15(x3) (*H) (64) *SH2 7 2.30 53 2.24(x2) 90 2.22, 2.19(x2) (61) - Species Final S. *SH2 *SH *SH *S (*H) 0.25ML 0.50ML EAds (kcal/mol) = ESurface + EGas - EAdsorbedSpecies Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C 2007, 111, 14457. Molecular Hydrogen Adsorption nS*-Surface+xH <--> 2xH*-nS*-Surface n*S-Surface + xH 2(g) 2(g) 2x*H-n*S-Surface 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. Molecular Hydrogen Adsorption nS*-Surface+xH <--> 2xH*-nS*-Surface n*S-Surface + xH 2(g) 2(g) 2x*H-n*S-Surface 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. Molecular Hydrogen Adsorption nS*-Surface+xH <--> 2xH*-nS*-Surface n*S-Surface + xH 2(g) 2(g) 2x*H-n*S-Surface 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. Multiple H2S(g) Adsorptions at 800oC Surface+2H <--> 2S*-Surface+ 2H2(g) (g) 2S(g) Surface + 2H 2S(g) 2*S-Surface + 2H 2 Point A : Despite large TS barriers, exothermic/exergonic nature of overall reaction produces a S = 0.50ML surface. Point B : Removal of H2S from the anode fuel feed allows the partial removal of surface sulfur, due to small difference in energy between species “S__S” and “S”. CSTR Kinetic Model Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C 2007, 111, 14457. 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 : TS = T.S(translational/rotational). H2S(g)/800oC, TS = 53 kcal/mol, H2(g)/800oC, TS = 34 kcal/mol. G H T S G k BT k exp RT h 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 : k1 k2 A B C D k1 k2 Integration over time : d A k1AB k1C dt d B k1AB k1C dt d C k1AB k1C k2 C k2 D dt d D k2 C k2 D dt 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 Anode Fuel at 800oC pH2 = ~1atm, pH2S = 1x10-5atm = 10ppm. Initial Surface, S = 0.00ML. Surface + H2S(g) *S-*H-*H *S-*H-*H *S + H2(g) *S + H2S(g) *S-*S-*H-*H *S-*S-*H-*H *S--*S + H2(g) A S=0.25ML surface (a 100% CSTR surface coverage of *S) is initially formed via H2S(g) adsorption and H2(g) desorption. Surface + 2H2(g) 4*H Further H2S(g)/H2(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 Anode Fuel at 800oC pH2 = 1atm, (No H2S(g) in fuel). Initial Surface, S = 0.50ML. *S--*S + H2(g) *S-*S-*H-*H *S-*S-*H-*H *S + H2S(g) *S + H2(g) *S-*H-*H *S-*H-*H Surface + H2S(g) Model mimics experimental attempts to Surface + 2H2(g) 4*H purge sulfur from surface by eliminating H2S from anode fuel feed. Equilibrium is reached upon the production of a S=0.25ML surface (a 100% CSTR surface coverage of *S). A. Conclusions – H2S Our research studies the affects of consecutive adsorption and dissociation of H2S and subsequent desorption of H2 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 H2S at a S coverage greater than 50%. The endergonic nature of H2S ads.n at S >0.50ML causes cutoff point. Complete irreversibility of H2S 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 O2 Treatment 1S*-Surface+O2(g) --> “Clean surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment 1S*-Surface+O2(g) --> “Clean surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment 1S*-Surface+O2(g) --> “Clean surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment 1S*-Surface+O2(g) --> “Clean surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment 1S*-Surface+O2(g) --> “Clean surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment 1S*-Surface+O2(g) --> “Clean surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment 1S*-Surface+O2(g) --> “Clean surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment 1S*-Surface+O2(g) --> “Clean surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment 2S*-Surface+O2(g) --> “1S* surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment 2S*-Surface+O2(g) --> “1S* surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment 2S*-Surface+O2(g) --> “1S* surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment 2S*-Surface+O2(g) --> “1S* surface +SO2(g) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. Removal of Sulfur by O2 Treatment Surface coverage of selected species determined by kinetic CSTR model at 8000 C of O2 exposure to S = 0.50ML surface. Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C , accepted. B Conclusions – H2S Sulfur with coverage S = 0.25 ML can be removed by O2 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.