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35nd ACS National Meeting April 6-10, 2008 New Orleans, Louisiana

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.

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35nd ACS National Meeting April 6-10, 2008 New Orleans, Louisiana

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  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

  2. - + V *Most common SOFC material Temp. 800 – 1000 oC Anode Electrolyte Cathode *Ni-YSZ *YSZ Solid Oxide Fuel Cell – CH4 The problem of coking • 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.

  3. Triple Phase Boundary (TPB) Reactions Pre-activation on Ni Nickel/YSZ YSZ Electrolyte Anode Cathode 2O2- 2O2- O2(g) 2O2- 4e- Burning on oxygen rich YSZ Activation on Ni Nickel 2H+ O2 ----> H2O +2e- H2 --> 2H* YSZ CH4-x ++(8-x)/2O2- ---> CO2+(4-x)/2H2O+(8-x)e- CH4 --> xH*+CH4-x Oxygen rich YSZ +C(Coke)

  4. Surface Calculations – CH4 Steps and Terraces Stepped (211) - *C Planar (111) - *C • Two classes of active adsorption sites. • Stepped surfaces more reactive than planar surfaces. • Supercell; 3 layers, 2x2(planar) or 2x3(stepped) surface.

  5. 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

  6. Ni(111) and Ni(211) Surfaces :Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Ni • 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. (b) (a) Graphene

  7. Ni(111) & Ni(211) Decomposition of CH4 on steps and terraces of Ni N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33

  8. 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. • 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)

  9. Cu(111) and Cu(211) Surfaces :Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Cu • 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 Journal of Catalysis 247 (2007) 20-33 Galea et al. Journal of Catalysis 247 (2007) 20-33

  10. Cu(111) and Cu(211) Surfaces :Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Cu • 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 Journal of Catalysis 247 (2007) 20-33 Galea et al. Journal of Catalysis 247 (2007) 20-33

  11. Cu(111) and Cu(211) Surfaces :Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Cu • 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 Journal of Catalysis 247 (2007) 20-33 Galea et al. Journal of Catalysis 247 (2007) 20-33

  12. Cu(111) and Cu(211) Surfaces :Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Cu • 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 Journal of Catalysis 247 (2007) 20-33 Galea et al. Journal of Catalysis 247 (2007) 20-33

  13. Cu(111) and Cu(211) Surfaces :Adsorption and Decomposition of CH4 Decomposition of CH4 on steps and terraces of Cu • 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 Journal of Catalysis 247 (2007) 20-33 Galea et al. Journal of Catalysis 247 (2007) 20-33

  14. Cu(111) & Cu(211) Decomposition of CH4 on steps and terraces of Cu N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33 Galea et al. Journal of Catalysis 247 (2007) 20-33

  15. 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 • 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. Copper (a)

  16. S-Ni(211) Decomposition of CH4 on S-steps and Ni-terraces N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33 Galea et al. Journal of Catalysis 247 (2007) 20-33

  17. 100% Step – Au/S-Ni(211) : Adsorption and Decomposition of CH4 • 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. Decomposition of CH4 on (S,Au,S) steps and Ni-terraces Sulfur or Gold (a) N.Galea,D.Knapp,T.Ziegler Journal of Catalysis 247 (2007) 20-33 Galea et al. Journal of Catalysis 247 (2007) 20-33

  18. 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.

  19. Triple Phase Boundary (TPB) Reactions Pre-activation on Ni Nickel/YSZ YSZ Electrolyte Anode Cathode 2O2- 2O2- O2(g) 2O2- 4e- Burning on oxygen rich YSZ Activation on Ni Nickel 2H+ O2 ----> H2O +2e- H2 --> 2H* YSZ CH4-x ++(8-x)/2O2- ---> CO2+(4-x)/2H2O+(8-x)e- CH4 --> xH*+CH4-x Oxygen rich YSZ M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress

  20. Triple Phase Boundary (TPB) Reactions Activation on YSZ Nickel/YSZ YSZ Electrolyte Anode Cathode 2O2- 2O2- O2(g) 2O2- 4e- Activation and burning on oxygen rich YSZ Nickel YSZ H2+O2- ----> H2O +2e- Oxygen rich YSZ CH4 +4O2- ---> CO2+2H2O+8e- M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress

  21. Triple Phase Boundary (TPB) Reactions Activation on YSZ Nickel/YSZ YSZ Electrolyte Anode Cathode 2O2- 2O2- O2(g) 2O2- 4e- Zr 9%-YSZ Nickel O YSZ Oxygen rich YSZ Y M.Shishkin N.Galea,D.Knapp,T.Ziegler, work in progress

  22. Molecular Hydrogen Adsorption onOxygen Rich YSZ • Initial adsorption of H2(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

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

  24. Methane adsorption on Oxygen rich YSZ: Second stage.

  25. Third stage: formaldehyde decomposition on oxygen enriched YSZ surface.

  26. Methane adsorption on oxygen deficient YSZ surface.

  27. 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

  28. Solid Oxide Fuel Cell – H2S Pre-activation on Ni with sulfur deposition Nickel/YSZ YSZ Electrolyte Anode Cathode 2O2- 2O2- O2(g) 2O2- 4e- Burning on oxygen rich YSZ Activation on Ni Nickel 2H+ O2 ----> H2O +2e- H2 --> 2H* YSZ CH4-x ++(8-x)/2O2- ---> CO2+(4-x)/2H2O+(8-x)e- CH4 --> xH*+CH4-x Oxygen rich YSZ H2S --> S*+H2(g)

  29. 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.

  30. Surface Calculations – H2S Steps and Terraces • 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. • 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. • Repeated supercell; 3 layers, 2x2 surface.

  31. Maximum Adsorption of H2S(g) Surface + 4H2S(g)  4*S-Surface + 4H2(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+4H2S(g) <--> 4S*-surface+ 4H2S(g) “S” “S_S_S_S” “S__S” “S_S_S”

  32. Hydrogen Sulfide Adsorption n*S-Surface + H2S(g)  (n+1)*S-Surface + H2(g) • 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. nS*-Surface+H2S(g) <--> (n+1)S*-surface+ H2(g) (d) (c) (a) (b) Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C 2007, 111, 14457.

  33. Hydrogen Sulfide Adsorption n*S-Surface + H2S(g)  (n+1)*S-Surface + H2(g) nS*-Surface+H2S(g) <--> (n+1)S*-surface+ H2(g) • 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. (d) (c) (a) (b)

  34. Hydrogen Sulfide Adsorption n*S-Surface + H2S(g)  (n+1)*S-Surface + H2(g) nS*-Surface+H2S(g) <--> (n+1)S*-surface+ H2(g) • 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. (d) (c) (a) (b)

  35. Hydrogen Sulfide Adsorption nS*-Surface+H2S(g) <--> (n+1)S*-surface+ H2(g) n*S-Surface + H2S(g)  (n+1)*S-Surface + H2(g) • 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. (d) (c) (a) (b)

  36. Hydrogen Sulfide Adsorption n*S-Surface + H2S(g)  (n+1)*S-Surface + H2(g) nS*-Surface+H2S(g) <--> (n+1)S*-surface+ H2(g) • 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. (d) (c) (a) (b)

  37. Adsorption Energies EAds (kcal/mol) = ESurface + EGas - EAdsorbedSpecies Galea, N.M.; Kadantsev, E.S.; Ziegler, T. J. Phys. Chem. C 2007, 111, 14457.

  38. Molecular Hydrogen Adsorption n*S-Surface + xH2(g)  2x*H-n*S-Surface nS*-Surface+xH2(g) <--> 2xH*-nS*-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.

  39. Molecular Hydrogen Adsorption n*S-Surface + xH2(g)  2x*H-n*S-Surface nS*-Surface+xH2(g) <--> 2xH*-nS*-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.

  40. Molecular Hydrogen Adsorption n*S-Surface + xH2(g)  2x*H-n*S-Surface nS*-Surface+xH2(g) <--> 2xH*-nS*-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.

  41. Multiple H2S(g) Adsorptions at 800oC Surface+2H2S(g) <--> 2S*-Surface+ 2H2(g) Surface + 2H2S(g)  2*S-Surface + 2H2(g) • 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”.

  42. 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 : • TS = T.S(translational/rotational). • H2S(g)/800oC, TS = 53 kcal/mol, • H2(g)/800oC, TS = 34 kcal/mol.

  43. 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.

  44. 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) Surface + 2H2(g)  4*H • A S=0.25ML surface (a 100% CSTR surface coverage of *S) is initially formed via H2S(g) adsorption and H2(g) desorption. • Further H2S(g)/H2(g) adsorption/desorption results in a 100% CSTR surface coverage of 2*S, a S=0.50ML surface .

  45. 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) Surface + 2H2(g)  4*H • Equilibrium is reached upon the production of a S=0.25ML surface (a 100% CSTR surface coverage of *S). • Model mimics experimental attempts to purge sulfur from surface by eliminating H2S from anode fuel feed.

  46. 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.

  47. 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.

  48. 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.

  49. 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.

  50. 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.

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