Near-Surface Alloys for Improved Catalysis Manos Mavrikakis

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1 Near-Surface Alloys for Improved Catalysis Manos Mavrikakis Department of Chemical and Biological Engineering University of Wisconsin Madison Madison, Wisconsin 53706

2 Acknowledgements Ratko Adzic (BNL) J. Chen (U of Delaware) Jeff Greeley, Ye Xu, Anand Nilekar Jens Nørskov and CAMP Denmark DoE (BES-Chemical Sciences) NSF - CTS NPACI, DOE-NERSC, PNNL (supercomputing resources)

3 Outline H 2 catalytic chemistry: Identifying Promising Catalysts from 1 st Principles: H 2 and H on Bimetallic Near Surface Alloys (NSAs) Oxygen Reduction Reaction (ORR) Improved catalysts with ML structures Further improvements with mixed-metal ML structures

4 Hydrogen on NSA s H 2 /Rh(111) 1 Ni/Pt(111) 1. J. Kitchen, N. Khan, M. Barteau, J. Chen, B. Yashhinskiy, and T. Madey, Surf. Sci. 544 (2003) R. Schennach, G. Krenn, B. Klötzer, K. Rendulic, Surf. Sci. 540 (2003) V/Rh(111 )

5 Methods Density Functional Theory DACAPO total energy code 1,2 Periodic self-consistent PW91-GGA 3 Ultra-soft Vanderbilt pseudo-potentials 4 Plane wave basis sets with 25-Ry kinetic energy cut-off Spin polarization as needed Four-metal-layer slabs; (2x2) unit cell; top two layers relaxed First Brillouin zone sampled at 18 k-points Nudged Elastic Band method for reaction paths 5 1. B. Hammer, L. B. Hansen, J. K. Nørskov, Phys. Rev. B 59, 1999, J. Greeley, J. K. Nørskov, M. Mavrikakis, Annu. Rev. Phys. Chem. 53, 2002, J. P. Perdew et al., Phys. Rev. B 46, 1992, D. H. Vanderbilt, Phys. Rev. B 41, 1990, G. Henkelman, H. Jónsson, J. Chem. Phys. 113, 2000, 9978.

6 Ideal Bimetallic Near Surface Alloys Segregation properties of two metals are critical Consider two special classes: Overlayers Subsurface Alloys Overlayers* Subsurface Alloys

7 Stability of NSA s with Respect to Hydrogen-induced Segregation B.E. H sol - B.E. H host Cu*/Ru Pd*/Ru Pt subsurface alloys Pd subsurface alloys Ir subsurface alloys Ru subsurface alloys Rh subsurface alloys Ru-based overlayers Re-based overlayers Pt*/RuPd*/Re Cu*/Re Ni*/Ru Fe*/Ru Mo*/Re Co*/Ru Ni*/Re Pt*/Re Au*/Pt Rh*/Re Mo/Ru H-induced segregation Ta/Pt W/Pt Ta/Rh V/Pd W/Rh Mo/PtTa/Pd Re/Pt Fe/Pt Ru/Pt W/Ru Ru/Rh Mo/Rh Re/Rh Mo/Pd Co/Pt Ru/Pd Ni/Pt Rh/Pt Fe/Pd Ir/Rh Ir/Pt Ir/Pd Cu/Pt V/Pt E seg

8 B.E. H on Close-Packed Metal Surfaces Overlayers* Subsurface Alloys Ni V Ta W Mo Re Fe Ru Co Pd Rh Ir Pt Cu Au Ni*/Ru Pd*/Ru Rh*/Re Ni*/Re Cu*/Re Pd*/Re Pt*/Re Cu*/Ru Pt*/Ru Ru/Rh Ir/Rh Mo/Rh Re/Rh Mo/Pd Ir/Pd Ru/Pd Re/Pd Fe/Pd W/Pd V/Pd Ta/Pd Cu/Pt Ir/Pt Rh/Pt Ni/Pt Ru/Pt Re/Pt Co/Pt Fe/Pt W/Pt Mo/Pt V/Pt Ta/Pt B.E. H (ev)

9 Correlation of B.E. H with Clean Surface -1.8 Properties Pt subsurface alloys Pd subsurface alloys B.E. H (ev) Rh subsurface alloys ε d (ev)

10 O 2 dissociation: Does E b TS follow E b FS? 1.5 Eb TS (ev, PW91) E FS b (ev, PW91) Au +10% Ag Pt Cu Ir Pt -2% Pt 3 Co skin Pt 3 Co Pt 3 Fe skin Pt 3 Fe Y. Xu, A. V. Ruban, M. Mavrikakis, JACS 126, 4717 (2004).

11 BEP Plot for H 2 Dissociation onnsa s J. Greeley, M. Mavrikakis, Nature Materials 3, 810 (2004) Noble metals Au 1.0 Cu 0.8 E TS (ev) Pd-terminated alloys Ta/Pd V/Pd Re/Pd Mo/Pt Ta/Pt Pd*/Re Ru/Pt Re/Pt Pt-terminated alloys Hydrogen B.E. (ev)

12 Metal monolayer deposition by galvanic displacement of a less noble metal monolayer deposited at underpotentials Electroless (spontaneous) deposition of one metal on another metal Pt 2+ Cu 2+ Pt Cu Pt Pd Pd Cu UPD /Pd + Pt 2+ Pt/Pd + Cu 2+ Ru Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Surf. Sci. 2001, 474, L173 Zhang, J.; Vukmirovic, M.; Xu, Y.; Mavrikakis, M.; Adzic, R. R. Angew. Chem. Int. Ed. 2005, 44, 2132 Brankovic, S. R.; McBreen, J.; Adzic, R. R. J. Electroanal. Chem. 2001, 503, 99 Sasaki, K.; Wang, J. X.; Balasubramanian, M.; McBreen, J.; Uribe, F.; Adzic, R. R. Electrochim. Acta 2004, 49, 3873

13 A new kind of Minority/Defect site for TM Catalysis J. Greeley, M. Mavrikakis, Catalysis Today 111, 52 (2006) Pt V Isolated metal hetero-atoms near metal surfaces can be viewed as generating an alternative type of defect/minority surface sites, associated with a different kind of near-surface impurity. These sites could be more poisonresistant and possess better catalytic kinetics than the rest of the surface sites.

14 NSA s - Summary First-Principles Methods can help with identifying promising bimetallic NSAs with interesting catalytic properties Example: 1. H and H 2 on NSA s: Fine-tuning BE H is possible 2. Some NSA s: Activate H 2 easily AND bind atomic H weakly useful for highly selective low T H- transfer reactions Developing Catalyst Preparation Techniques with Layer-by- Layer control of metal deposition (ALD-like) is critical for making the desired NSAs New type of NSA - defect site

15 Low Temperature Fuel cells Proton Exchange Membrane Anode (PEM) fuel cell Cathode Representative Catalysis Anode: Cathode: 2H 2 4 H e - O 2 2 O - 2O - +2H + +2e - 2OH - 2OH - + 2H + 2H 2 O Oxygen Reduction Reaction (ORR)- Very slow kinetics

16 Pt monolayers on transition metals Pt monolayers on Ru(0001), Ir(111), Rh(111), Au(111) and Pd(111) Ru(0001), Ir(111) and Rh(111) lead to compression of Pt overlayer Au(111) leads to expansion of Pt overlayer Pt monolayer Substrate (Ru, Ir, Rh, Pd, Au) Pd(111) has almost same lattice constant as Pt(111)

17 O 2 Dissociation Barrier 1.30 Pt ML /Ir(111) 1.10 Compressive TS E b E b TS / ev /ev Pt(111) Pt ML /Pd(111) strain 0.50 Pt ML /Au(111) Expansive strain O E O b /ev / ev J Zhang, MB Vukmirovic, Y Xu, M Mavrikakis, RR Adzic, Angew. Chemie, Int. Ed., 44, 2132 (2005)

18 OH Formation Barrier Pt ML /Au(111) Expansive TS E b /ev E b TS / ev Pt ML /Pd(111) Pt(111) strain Compressive 0.50 Pt ML /Ir(111) strain O E /ev b / ev J Zhang, MB Vukmirovic, Y Xu, M Mavrikakis, RR Adzic, Angew. Chemie, Int. Ed., 44, 2132 (2005)

19 The best catalyst performs a Balancing Act - j K /ma cm Pd Pt ML /Pd(111) 15 Pt(111) Au Pt Pd Ir Rh Ru O Eb /ev E a /ev E a for O 2 dissociation (filled red circles) E a for OH formation (open circles) Kinetic currents (j K ; blue squares) j k value for Pt(111) (black square) obtained from Markovic et.al., J. Electroanal. Chem. 1999, 467, Pt ML /Ru(0001) 2--Pt ML /Ir(111) 3--Pt ML /Rh(111) 4--Pt ML /Au(111) 5--Pt(111) 6--Pt ML /Pd(111) J Zhang, MB Vukmirovic, Y Xu, M Mavrikakis, RR Adzic, Angew. Chemie, Int. Ed., 44, 2132 (2005)

20 Further Decrease of Pt -loading: The role of OH poisoning in ORR Pt binds OH strongly OH blocks active sites on Pt Replace part of the Pt ML on Pd(111) with another transition metal M Hypothesis M will attract initial OH and induce repulsion on neighbouring Pt-OH decrease OH binding and OH coverage on Pt OH Pt Pd O M Pd

21 ORR Experiments on (M x Pt 1-x ) ML /Pd(111): Current.vs. Surface Composition Effect of addition of different amount of M: Ir & Ru in Pt ML /Pd(111) Around 20% M gives highest ORR activity J Zhang, MB Vukmirovic, K. Sasaki, A. U. Nilekar, M Mavrikakis, RR Adzic, JACS, 127, (2005) -j k / macm -2 at 0.85V RHE Pt molar percentage % (Ir x Pt 1-x ) ML / Pd (111) (Ru x Pt 1-x ) ML / Pd (111) M (Ru or Ir) molar percentage %

22 Mixed metal Pt monolayer Modeling these systems with DFT: (Pt 3 M) ML /Pd(111) to get θ M = 0.25 ML Calculate: OH+OH or O+OH repulsion with M being: Au Os Pd Re Pt Rh Ru Pt monolayer Pd Substrate Other metal (M)

23 OH+OH or O+OH optimal structures M with reactivity less or equal compared to Pt in Pt ML /Pd(111): Au, Pd, Pt 1 st OH on Pt-top and 2 nd OH on Pt-Pt-bridge site M with reactivity higher than Pt in Pt ML /Pd(111): Rh, Ru, Ir 1 st OH on M-top and 2 nd OH on Pt-top M with much higher reactivity than Pt in Pt ML /Pd(111): Os, Re (break the OH bond to form H 2 O and O) O on M-top and 1 st OH on Pt-Pt-bridge site

24 Comparison of theory with experiment R 2 = 0.99 Ir Re Os (Pt 3 M) ML /Pd(111) -jk at 0.8V / ma.cm Au Pt Pd Ru Rh OH-OH or OH-O repulsion relative to OH-OH repulsion on Pt ML /Pd(111) /ev M 400% increase in ORR activity for (Pt 3 Re) ML /Pd(111) compared to Pt(111) J Zhang, MB Vukmirovic, K. Sasaki, A. U. Nilekar, M Mavrikakis, RR Adzic, JACS, 127, (2005)

25 ORR - Summary: A combination of first-principles studies with experiments can identify key ORR reactivity descriptors: Kinetics of O-O Bond-breaking Kinetics of O-H Bond-making BE O OH-poisoning Use less Pt: Pt ML /Pd(111) 30% more current [compared to Pt(111)] Use even less Pt: Pt 3 M/Pd(111) up to 400% increase in current [compared to Pt(111)] First-principles can help with further improvements of ORR catalysts, by identifying materials which are: Cheaper (low Pt loading) More active Increased stability of Pt against oxidation

26 Dr. Jeff Greeley Dr. Ye Xu Department of Chemical and Biological Engineering University of Wisconsin-Madison Anand Nilekar

27 Acknowledgements Ratko Adzic (BNL) J. Chen (U of Delaware) Jeff Greeley, Ye Xu, Anand Nilekar Jens Nørskov and CAMP Denmark DoE (BES-Chemical Sciences) NSF - CTS NPACI, DOE-NERSC, PNNL (supercomputing resources)