Development of Pd Cu Membranes for Hydrogen Separation

Transcription

1 Development of Pd Cu Membranes for Hydrogen Separation Shahrouz Nayebossadri, John Speight, David Book School of Metallurgy & Materials University of Birmingham The Hydrogen & Fuel Cell Researcher conference 16 th 18 th Dec University of Birmingham

2 Why Hydrogen Separation? More than 80% of hydrogen available on an industrial scale is produced by steam reforming of natural gas.* Reforming of natural gas ( C, 3-25 bar, and Ni catalyst) CH 4 +H 2 O CO+3H 2 (1) Shift reaction (Water Gas Shift) CO+H 2 O H 2 +CO 2 (2) The product gas stream contains 74% H 2, 18CO 2, 7% CH 4, 1% CO, H 2 S and other by-products, which can be further purified (99.999%) by different methods, such as Pressure Swing Adsorption (PSA). Many applications require H 2 with higher purity (including PEM fuel cells). Dense metallic membranes offer an effective method to produce ultra pure hydrogen. * F. Gallucci, et al. Chemical Engineering Science, 92, 2013, 40 2

3 What is a Membrane? P1 P P2 A membrane is a barrier between two phases It can be used to separate mixture (A&B) if one component (A) permeates through the membrane Pressure differential between the feed gas and permeate side is required for the gas permeation 3

4 Palladium Membrane Advantages: Separation based on the solutiondiffusion mechanism Production of high purity hydrogen ( %) Acceptable hydrogen permeability Problems: Hydrogen embrittlement (α β transition below 2MPa & 298 o C ) Surface poisoning (CO, H 2 S,...) High cost of Pd Hydrogen flux needs to be improved (1) Adsorption (2) Dissociation (3) Ionization (4) Diffusion (5) Desorption (6)Recombination 4

5 Alloying Permeability (mol m -1 s -1 Pa -0.5 ) Alloying Pd with other metallic elements such as Ag, Cu, Fe, Ni, Pt, Y, and... Identical specimens of palladium (left) and palladiumsilver (right) after 30 thermal cycles in hydrogen Avoid α β phase transition Improve mechanical property Improve hydrogen permeability Increase resistance to the surface poisoning G.J. Grashoff et al, Platinum Metals Rev., 27 (4), 1983, x x x x x10-8 Pd Pd-Ag 24 Pd-Cu 53 Pd-Y 8 1.0x Temperature ( o C) 5

6 Pd-Cu System Temperature ( o C) permeability (mol m -1 s -1 Pa -0.5 ) at 350 o C 650 (Cu,Pd) x (bcc+fcc) bcc (bcc+fcc) fcc + ) Phase) + ) Phase) 1.0x x Palladium (at. %) P.R. Subramanian et al. J. Phase Equilib. 1991, 12, (2), 231 Pd 47 Cu 53 with bcc structure shows higher permeability than Pd at 350 o C. bcc phase is not thermally stable and susceptible to the surface poisoning. fcc phase alloys show some resistance to the surface poisoning. 6

7 Pd-Cu-Ag Ternary Alloys Hydrogen Diffusivity & solubility 6.0x x10-5 Diffusion coefficient (cm 2 s -1 ) Diffusion coefficient (cm 2 s -1 ) 1.0x x x x x x x x x x10-5 (3) Pd 45.8 Cu 51.9 Ag 2.3 (bcc) (1) Pd 46.6 Cu 53.4 (bcc) (2) Pd 53.1 Cu 46.9 (bcc+fcc) (4) Pd 45.1 Cu 51 Ag 3.9 (bcc+fcc) 9.0x x x x x x x x x x x x10-5 Solubility constant (Pa -0.5 ) Solubility constant (Pa -0.5 ) Temperature ( o C) Temperature ( o C) 0.0 Ag addition decreases the hydrogen diffusivity in both bcc and (bcc+fcc) phases. Ag addition increases the hydrogen solubility in both bcc and (bcc+fcc) phases. 7

8 Effect of Ag Addition on Hydrogen Diffusion In the bcc phase hydrogen diffusion takes place through a series of jumps between Tetrahedral Sites (T site) via PdCu 2 /Pd 2 Cu windows i.e., Transition States (TS) H atom Cu Pd Ag replacement for Pd or Cu in the PdCu 2 /Pd 2 Cu TS Increases the activation energy for the hydrogen diffusion Transition State T sites E a (Pd 47 Cu 53 )=5.82 kj mol -1 ( o C) E a (Pd 45.8 Cu 51.9 Ag 2.3 )=10 kj mol -1 ( o C) P. Kamakoti et al. J. Memb. Sci. 2003, 225,

9 Effect of Ag Addition on Hydrogen Solubility Lattice constant (nm) Ag expand the lattice parameter therefore, higher solubility can be achieved (4)Pd 45.1 Cu 51 Ag (2)Pd 45.8 Cu 51.9 Ag 2.3 Metal-H pair interaction Ag favours interstitial hydrogen by decreasing the binding energies of most stable O sites (3)Pd 53.1 Cu (1)Pd 46.6 Cu Pd concentration (at.%) Ag-H interaction in Pd-Ag-H: -36 kj mol -1 Cu-H interaction in Pd-Cu-H: +6.5 kj mol -1 Geometrical factor (Available free space to be occupied by hydrogen). Binding energies in O sites predicted by CE model C. Ling et al. J. Memb. Sci. 2011, 371, 189 9

10 Effect of Ag Addition on Hydrogen Permeability Permeability (mol m -1 s -1 Pa -0.5 ) 2.0x x x x10-8 (1) Pd 46.6 Cu 53.4 (bcc) (2) Pd 45.8 Cu 51.9 Ag 2.3 (bcc) (4) Pd 45.1 Cu 51 Ag 3.9 (bcc+fcc) (3) Pd 53.1 Cu 46.9 (bcc+fcc) 1.2x x x x x x Temperature ( o C) Hydrogen permeability is mainly controlled by hydrogen diffusion in the bcc phase.* Hydrogen Permeability can be improved by solubility enhancement within the fcc phase.* * S. Nayebossadri, J. Speight, D. Book, Journal of Membrane Science, 451, (2014),

11 (111) (200) (220) (311) (222) (111) (200) (220) (311) (222) Intensity (a.u.) Pd-Cu-M membranes (M:Y, Ti, Zr, V, Nb, and Ni) Structural Analysis As-rolled (1) Pd-Cu o C-96h (1) Annealed (2) Pd-Cu-Ti (2) Annealed (3) Pd-Cu-Nb (3) Annealed (4) Pd-Cu-Ni (4) Annealed (5) Pd-Cu-V (5) Annealed (6) Pd-Cu-Y (6) Annealed fcc (7) Pd-Cu-Zr fcc (7) Annealed (deg) 2 (deg) Pd-Cu-Zr sample shows structural stability after annealing. 11

12 Pd-Cu-M membranes (M:Y, Ti, Zr, V, Nb, and Ni) H/M gradients Hydrogen Solubility Whilst, Ti, Nb, and V additions have no noticeable effect on hydrogen solubility, Ni addition reduces hydrogen solubility at temperatures higher than 200 o C compared to the binary Pd-Cu alloy. Hydrogen solubility in Pd-Cu-Y and Pd-Cu-Zr alloys are almost doubled compared to the binary Pd-Cu alloy at temperatures higher than 200 o C (1) Pd-Cu (2) Pd-Cu-Ti (3) Pd-Cu-Nb (4) Pd-Cu-Ni (5) Pd-Cu-V (6) Pd-Cu-Y (7) Pd-Cu-Zr * S. Nayebossadri, J. Speight, D. Book, To be published Temperature ( o C) 12

13 Diffusion coefficient (cm 2 s -1 ) Pd-Cu-M membranes (M:Y, Ti, Zr, V, Nb, and Ni) Hydrogen Diffusivity 6.0x x x x x10-5 (4) (1) (5) (3) (2) (6) (7) (4) Pd-Cu-Ni (1) Pd-Cu (7) Pd-Cu-Zr (6) Pd-Cu-Y (5) Pd-Cu-V (3) Pd-Cu-Nb (2) Pd-Cu-Ti Linear fit 1.0x /T (K) Hydrogen diffusivity in all Pd-Cu-M (M: Y, Ti, Zr, V, Nb, and Ni) alloys is lower than binary Pd-Cu alloy. * S. Nayebossadri, J. Speight, D. Book, To be published 13

14 Pd-Cu-M membranes (M:Y, Ti, Zr, V, Nb, and Ni) Permeability (mol m -1 s -1 Pa -0.5 ) Hydrogen 350 o C 7.0x x x x x10-9 Reference data (1) Pd-Cu (2) Pd-Cu-Ti (3) Pd-Cu-Nb (4) Pd-Cu-Ni (5) Pd-Cu-V (6) Pd-Cu-Y (7) Pd-Cu-Zr 2.0x10-9 Pd-Cu-Zr Pd-Cu-Y 1.0x Pd concentration (at.%) Pd-Cu-Y and Pd-Cu-Zr samples show almost similar hydrogen permeability to their binary alloys, despite their higher thickness (100 µm vs. 25µm). * S. Nayebossadri, J. Speight, D. Book, To be published 14

15 Relative hydrogen flux (%) Pd-Cu-M membranes (M:Y, Ti, Zr, V, Nb, and Ni) Exposure to 1000ppm H 2 S+H 450 o C Hydrogen flux decreases to ~70% and ~50% of its original value for Pd-Cu- Zr and Pd-Cu alloys respectively after 8h of exposure. Resistance to sulphur poisoning is improved by Zr addition. 100 (1) Pd-Cu (2) Pd-Cu-Ti 90 (4) Pd-Cu-Ni 80 (7) Pd-Cu-Zr (3) Pd-Cu-Nb 70 (5) Pd-Cu-V (6) Pd-Cu-Y Time (h) * S. Nayebossadri, J. Speight, D. Book, To be published 15

16 Intensity (a.u.) Pd-Cu-M membranes (M:Y, Ti, Zr, V, Nb, and Ni) Post Poisoning Structural Analysis Pd-Cu-Zr sample may change the local atomic composition on the surface leading to a less favourable S-surface interaction Structural stability of Pd- Cu-Zr sample significantly slow down Cu segregation leading to a slower kinetics for bulk sulfidation. Pd 4 S PdCu CU 2 S Cu 0.93 Ti 0.77 Cu 2 S, low (1) Pd-Cu CU 4 Pd (2) Pd-Cu-Ti (3) Pd-Cu-Nb (4) Pd-Cu-Ni (5) Pd-Cu-V (6) Pd-Cu-Y (7) Pd-Cu-Zr * S. Nayebossadri, J. Speight, D. Book, To be published 2 (deg) 16

17 Summary Hydrogen permeability in the bcc Pd-Cu alloys is mainly dominated by hydrogen diffusion. Hydrogen permeability in the fcc Pd-Cu alloys can be significantly improved by enhancing hydrogen solubility. Hydrogen solubility in the fcc Pd-Cu alloys can be significantly improved by addition of small amount (>2 at.%) of Y and Zr. Structural stability achieved as a result of Zr addition significantly improves the resistance to the sulphur poisoning We propose Pd-Cu-Zr as a new potential alloy membrane with improved permeability, thermal stability and sulphur poisoning resistance. 17

18 Acknowledgment Prof. Rex Harris Dr. John Speight Dr. David Book UK Sustainable Hydrogen Energy Consortium ( , ) Delivery of Sustainable Hydrogen ( ) Hydrogen and Fuel Cell Research Hub ( ) Birmingham Science City - Hydrogen Energy ( ) TSB Hydrogen Permeable Novel Membranes (HYPNOMEM) ( ) 18

19 Thank you 19

20 Membrane Test Rig 20

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22 Intensity (a.u.) (a) (b) RT RT 600 o C 600 o C 500 o C 500 o C 400 o C 400 o C 300 o C 300 o C 200 o C 200 o C 100 o C 100 o C RT RT (degree) 2 (degree) 22