1 / 61

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.

Télécharger la présentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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.

More Related