Density functional modeling of catalytic materials and adsorbents for potential industrial applications University of Sofia Petko Petkov, Hristiyan Aleksandrov, Georgi N. Vayssilov University of Sofia, Bulgaria

2  Purification of hydrogen for fuel cell applications by transition metal exchanged zeolites  Other modeled systems Cerium dioxide nanoparticles: Platinum clusters on CeO 2 Surface carbonates Hydrogenated transition metal clusters Metal-organic frameworks (MOF) ‏ Outline

Purification of hydrogen for fuel cell applications by transition metal exchanged zeolites – DFT study 3  Complexes of small molecules with transition metal ions  Purification from CO  Purification from H 2 S and NH 3

4 Hydrogen for fuel cell applications  Proton-exchange membrane (PEM) FCs require highly purified H 2 feed due to poisoning of the noble metal catalyst on the electrode by CO  Desired CO concentration in the feed: below 10 ppm  Strategies for purification: Selective catalytic oxidation of the CO in H 2 rich feed (PROX) on different supported transition metal catalysts Highly selective CO adsorption from the hydrogen feed at ambient conditions  Determination of adsorbents that allow to produce ultrapure of hydrogen using thermodynamic data derived from computational modeling Aleksandrov, Petkov, Vayssilov, Energy Environ. Sci., 2011, 4, 1879.

Model and computational details 5  monoclinic unit cell a = b = Å, c = Å  double cell: a = b = Å, c = Å Unit cell  Periodic calculations (VASP) ‏ DFT: PW91 Ultrasoft pseudopotentials Γ point Energy cutoff 400 eV spin-polarized calculations for Ir +, Co +, and Ni + force on each atom less than 210 4 eV/pm MOR structure

 CO – along O z -M bond: Co +, Rh + and Ir + (more stable) ‏  CO – perpendicular to the surface: Ni +, Cu + and Ag +  Entropy contribution for adsorption - always negative 6 M(CO) + complexes Cu Ni Co Ir Ag Rh + ΔGΔG ΔSΔS ΔHΔH All values are in kJ/mol, Δ G at 298 K Ag(CO) + Co(CO) + Ag O Si Al C Co O Si Al C

 Ag + - thermodynamically unstable (ΔG>0 at 298 K)  Cu + forms tetrahedral dicarbonyl complexes  All other cations form planar M(CO) 2 + complexes  Rh(CO) 2 + and Ir(CO) 2 + → more stable than the monocarbonyls 7 M(CO) 2 + complexes xΔ G xΔ H Cu Ni Co Ir Ag Rh + ΔGΔH Cu(CO) 2 + Ir(CO) 2 + M(CO) + M(CO) 2 + site-specific dicarbonyls complex-specific dicarbonyls All values are in kJ/mol, Δ G at 298 K

8 Purification of H 2 from CO  Minimal CO concentration that can be achieved if most of the cations participate in M(CO) 2 and/or M(CO)  With dicarbonyl: Rh + : 2.6 Co + : 5.910 -9 Ir + : 4.810 -8  With monocarbonyls: Ni + : 7.7 Co + : 1.9 Cu + : 3.9  Additional advantage of Cu, Co and Ni – lower cost M(CO) 2 (dotted lines) ‏ M(CO) 2 + M(CO) (solid lines) ‏ 1 – {[M(CO) 2 ] + [M(CO)]} < < of the metal centers to be involved in complexes other than that with the impurity

9 Summary  Recommended adsorbent - Cu exchanged zeolite: H 2 purification down to CO concentrations of Purifies H 2 also from H 2 S and NH 3 Lower price of copper compared to the other metals  For even deeper purification - Co or Ni exchanged zeolite to reach CO concentrations below  For zeolite with Si:Al ratio of 47 and feed with initial concentration of CO in hydrogen of 100 ppm, 1.00 kg of adsorbent (Co +, Ni +, Cu + ) will purify 76 m 3 H 2 Aleksandrov, Petkov, Vayssilov, Energy Environ. Sci., 2011, 4, 1879.

Modeling of cerium dioxide nanoparticles 10  Platinum cluster supported on ceria  Carbonate species on ceria nanoparticle  CeO 2 - key component and support in heterogeneous catalysts: automotive catalysts, WGS, preferential CO oxidation (PROX)..  Main activity - oxygen storage and release The release of O 2 is accompanied by reduction of part of Ce 4+ ions Ce n O 2n → Ce n O 2n-1 + ½ O 2 (nCe 4+ → 2 Ce 3+ + (n-2)Ce 4+ ) ‏

Platinum of cerium oxide nanoparticles  Pt 8 cluster on the CeO 2 - formation of Ce 3+ due to electron transfer  Oxygen spillover from CeO 2 to Pt 8 cluster favored on nanoparticles but disfavored on regular CeO 2 (111) ‏ generates large fraction of Ce 3+ ions Vayssilov, Lykhach, Migani, Staudt, Petrova, Tsud, Skála, Bruix, Illas, Prince, Matolín, Neyman, Libuda, Nature Mater. 10, 310 (2011).

Experimental confirmation Measurement of the temperature dependence of Ce 3+ /Ce 4+ enhancement ration for Pt on nanostructured CeO 2 by resonant photoelectron spectroscopy (RPES) Ce 3+ due to electron transfer from Pt to CeO 2 Ce 3+ due to O transfer from CeO 2 to Pt referent system

Carbonates on CeO 2 nanoparticles  Computational modeling resulted in:  new assignment of the vibrational bands in the complex IR spectra of surface carbonates on ceria  reliable detection of the surface species on ceria surface, which is critical for clarification of the mechanisms of the rich variety surface processes on ceria

Hydrogen spillover on transition metal clusters in zeolites 14  Confirmed for Rh 6 /zeolite  MD simulation of the process Н+Н+ Н-Н- Bare adsorbed clusterHydrogenated cluster Rh 6 H 3 /zeo Rh 6 /zeo(3H) ‏ ±4Experiment Rh-Oz Vayssilov, Gates, Rösch Angew. Chem. Int. Ed. 42 (2003) 1391 E RS = -120 kJ/mol per transferred H Oxidation of the metal moiety: q(Rh 6 )=~2.0 e

15 10ps MD run

16 Modeling of metal-organic frameworks  Search of materials for hydrogen storage  Clarification of the catalytic active centers e.g. in Au-functionalized MOFs  Understanding the structure and chemical/sorption behavior of defects in MOFs

17 Other computationally demanding problems  Dynamics of noble (Au, Pt) metal nanowires in cavities or on surfaces  Simulation of hydrogen production from water on ceria  Dynamical behavior and stability of defects in MOFs ……..

18 Acknowledgments  Bulgarian Supercomputing Center  Center of Excellence “Supercomputing Applications”  HPC Europa2 at Barcelona Supercomputing Center  National Center of Excellence on Advanced materials UNION

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