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Espoo 17.9.2009 Kari Laasonen, Department of Chemistry Chemistry on Fe 55 nanoparticle Kari Laasonen, Giorgio Lanzani, University of Oulu Large part of.

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Presentation on theme: "Espoo 17.9.2009 Kari Laasonen, Department of Chemistry Chemistry on Fe 55 nanoparticle Kari Laasonen, Giorgio Lanzani, University of Oulu Large part of."— Presentation transcript:

1 Espoo Kari Laasonen, Department of Chemistry Chemistry on Fe 55 nanoparticle Kari Laasonen, Giorgio Lanzani, University of Oulu Large part of catalysis happen on nanosize particles in car exhaust catalysts the particle size is 4-10 nm. Typical material Pt, Pd, Rh need for cheaper catalysts (materials) Metal catalysts are “almost” everywhere Palasantzas et al. Adv. Eng. Mater. 7, 21 (2005)

2 Espoo Kari Laasonen, Department of Chemistry Chemistry on Fe 55 nanoparticle the carbon nanotubes (CNT) are catalysed by particles of size 1.5 – 4 nm New science: so far all catalytic studied has been done either on flat or stepped surfaces. Here we can study chemical reactions on a real nanocluster Why the nanoclusters are so good catalysts. What is the role of the structure of the nanoparticle. This cannot be modelled with stepped surfaces. Lanzani et al. J.Phys.Chem. C, 113, (2009) Lanzani et al. Nano Research, 2, 660 (2009)

3 Espoo Kari Laasonen, Department of Chemistry Chemistry on nanoparticles Structure of a ca atom (ca. 3 nm) Ru cluster, with several different active sites Gavnholt and Schiotz, Phys. Rev. B, 77, (2008), See also Honkala et al. Science, 307, 555 (2005) (ammonia synthesis over modelled nanoparticle)

4 Espoo Kari Laasonen, Department of Chemistry here I will focus to the TKK’s (Esko Kauppinen’s group) aerosol reactor used for CNT growth the reaction happen on 1+ nm size Fe (or Ni) particles. The nanoparticle is in the gas phase. The nanoparticles are created in the reactor. CO is a common carbon source (also HCCH is widely used) reaction to get the carbon: CO(g) + CO(g) -> CO(s) + CO(s) -> C(s) + CO 2 (g) Calculations: VASP, PBE functional, PAW pseudopotentials, non-collinear magnetism

5 Espoo Kari Laasonen, Department of Chemistry THE NANOPARTICLE: Fe 55 The most convenient system for our study is Fe 55 in a super cell of 21 Å * 21 Å * 21 Å.  Formation energy = eV/atom;  Icosahedral geometry with size (largest Fe- Fe distance) of 9.68 Å and hcp-hcp distance = 2.50 Å;  Dipole moment = ( -0.05, -0.03, 0.04) el Å;  Magnetic moment: μ = (2.33, 0.48, 1.06) μ B /atom, Icosahedral symmetry; Non-collinear behavior is result of competing ferromagnetic and anti ferromagnetic interactions. We have studied the stability of Fe clusters of different size and geometry (from 7 to 55 atoms).

6 Espoo Kari Laasonen, Department of Chemistry STUDIED SYSTEM First principle DFT calculations (VASP code) has been used to study CO, H 2, atomic C and O adsorption and decomposition on icosahedral Fe 55. The geometry optimization has been started from the high symmetry adsorption sites of one the 20 triangular face of the Fe 55 cluster that are resulted identical for symmetry. E D G B A F C A => on plane hcp site B => on plane almost bridge site C => on plane bridge site D => on edge bridge site E => on vertice top site F => on plane hcp site G => on edge top site

7 Espoo Kari Laasonen, Department of Chemistry A,B,C,F,G G D, EE The perpendicular adsorption is not favourable when the oxygen is toward the surface (╧ OC). During the relaxations the molecule started with the molecule adsorbed throught the carbon (╧ CO), this spontaneously moved, to the E and G (top) sites of the surface. E D G B A F C BE = eV BE = eV

8 Espoo Kari Laasonen, Department of Chemistry Carbon and oxygen atoms have several stable adsorption sites OC A eV (hollow)A eV D eV (top)D eV E eV (hollow)E eV F eV (top)F eV (ref O 2 ) (ref C atom)

9 Espoo Kari Laasonen, Department of Chemistry ATOMIC O AND C ON Fe 55 All the possible combinations were considered, but only 6 geometries had exothermic dissociative chemisorptions (ΔE = E(C and O on Fe 55 )-E(CO)-E(Fe 55 )). C_O_2 ΔE = eV (O on F, C carbide-like on F)‏ C_O_1 ΔE = eV (O on A, C on F)‏ C_O_3 ΔE = eV (O on A, C carbide like on A)‏ C_O_8 ΔE = eV (O on F, C on F)‏ C_O_11 ΔE = eV (O on D, C on F)‏ C_O_9 ΔE = eV (O on F, C on F)‏

10 Espoo Kari Laasonen, Department of Chemistry CO dissociation on flat part of the cluster Barrier 0.99 eV, reaction energy eV

11 Espoo Kari Laasonen, Department of Chemistry CO dissociation over the edge of the cluster Barrier 0.77 eV, reaction energy eV

12 Espoo Kari Laasonen, Department of Chemistry C and O on the same face C and O on different face All the studied barrier From 2 starting geom and 6 end geom.

13 Espoo Kari Laasonen, Department of Chemistry CO dissociation CO dissociation over the edge of the cluster has lower barrier than on the facet, (barrier 0.77 eV, vs eV) the geometry of the nanocluster has a big role on flat Fe(110) surface the barrier is 1.52 eV ( Jiang and Carter, Surf. Sci. 570, 167, (2004) ) the lowest barrier found is with a stepped Fe(211) surface 0.78 eV ( Borthwick et al., Surf. Sci. 620, 2325, (2008), PBE functional ) the edge in the Fe 55 is much smaller perturbation than the atomistic step the Fe 55 is an unusually stable cluster so it is very likely less reactive than many of the other clusters.

14 Espoo Kari Laasonen, Department of Chemistry CO 2 formation over the edge of the cluster (CO is on the edge !) Barrier 1.13 eV, reaction energy eV

15 Espoo Kari Laasonen, Department of Chemistry CO 2 formation over the edge of the cluster Barrier 1.08 eV, reaction energy 0.88 eV

16 Espoo Kari Laasonen, Department of Chemistry NH 3 dissociation on Fe 55 Our aim is to provide theoretical understanding of ammonia decomposition on iron nanoparticles catalyst in the H 2 fuel processing system.

17 Espoo Kari Laasonen, Department of Chemistry REACTIVITY HYPHOTESIS The reaction mechanism for NH 3 on the small iron nanoparticle surface has not been completely established and we would like to suggest a dissociative reaction that proceed as inverse process of the ammonia synthesis on Ru(0001) surface. (1) N 2 +2* → 2N*, (2) H 2 +2* → 2H*, (3) N*+H* → NH*+*, (4) NH*+H* → NH 2 *+*, (5) NH 2 *+H* → NH 3 *+*, (6) NH 3 * → NH 3 +* Where * stands for an empty site on the surface. This reactivity is already well studied on the flat surfaces. It has been shown that the first reaction is rate-determining step.

18 Espoo Kari Laasonen, Department of Chemistry REACTIVITY STUDY

19 Espoo Kari Laasonen, Department of Chemistry NH 3 ADSORPTION ON Fe 55 Adsorption of NH 3 on high symmetry sites of the cluster has been studied: the top (D and F) are the only stable sites and from the bridge and hollow sites, the adsorbed migrates on the nearest top site: B.E. are reported above. Fe 55 NH 3 structure (top site) were also previously observed (table on the left) for smaller cluster Fe 13 NH Fe 7 NH >>- 0.4 NH 3 on Fe(hkl) Fe 4 NH FeNH 3 B.E. (eV)

20 Espoo Kari Laasonen, Department of Chemistry (110) (100) (111) revPBE PW91 Fe(hkl)/B.E. (eV) ATOMIC N ON Fe 55 These results (table on the left) are in agreement with the absorption results on the hcp sites of the Fe flat surfaces. In particular, the calculated value for the E site (on Fe 55 ) (-1.01 eV) is quite close to the hcp site on the (111) surface (-1.39 eV) which geometry is similar to the one on the cluster. B => E D => E F => A Atomic binding energies (B.E.): A = eV C => E E = eV

21 Espoo Kari Laasonen, Department of Chemistry ATOMIC H ON Fe 55 Atomic binding energies (B.E.): A => E C => eV E = eV These results are in agreement with the previous obtained for the absorption on the Fe flat surfaces. B => E D => C F => E

22 Espoo Kari Laasonen, Department of Chemistry NH 3 DECOMPOSITION

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25 Espoo Kari Laasonen, Department of Chemistry CONCLUSIONS - NH3 For NH 3, only the interaction N-Fe is favourable, and only the top are the stable sites (-0.38 eV

26 Espoo Kari Laasonen, Department of Chemistry Conclusions Reactions on nanometer size clusters can be studied the barriers are lower than on flat surface. The facet edge seem to be very reactive many of the binding energies on Fe55 will differ from the results on flat Fe surfaces – the nano is different. more reaction studies are needed for the true nanoclusters. Here we have studied only one cluster. Larger clusters and different metal should be studied. we looked the H 2 dissociation - it breaks very easily.

27 Espoo Kari Laasonen, Department of Chemistry Other fun things beside comp chem Thank you Funding: EU 6 FP, STREPS project BNC tubes, NMP4-CT

28 Espoo Kari Laasonen, Department of Chemistry


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