espoo 17.9.2009 kari laasonen, department of chemistry chemistry on fe 55 nanoparticle kari...

Espoo 17.9.2009Kari Laasonen, Department of Chemistry

Chemistry on Fe55 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)


Chemistry on Fe55 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, 12939 (2009) Lanzani et al. Nano Research, 2, 660 (2009)


Chemistry on nanoparticles

Structure of a ca. 2000 atom (ca. 3 nm) Ru cluster, with several different active sites

Gavnholt and Schiotz, Phys. Rev. B, 77, 035404 (2008),See also Honkala et al. Science, 307, 555 (2005) (ammonia synthesis over modelled nanoparticle)


• 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) + CO2(g)

• Calculations: VASP, PBE functional, PAW pseudopotentials, non-collinear magnetism


THE NANOPARTICLE: Fe55

The most convenient system for our study is Fe55 in a super cell of 21 Å * 21 Å * 21 Å.Formation energy = -3.87 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) μB /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).


STUDIED SYSTEM

First principle DFT calculations (VASP code) has been used to study CO, H2,

atomic C and O adsorption and decomposition on icosahedral Fe55

. 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.

ED

G

BA F

CA => on plane hcp siteB => on plane almost bridge siteC => on plane bridge siteD => on edge bridge siteE => on vertice top site F => on plane hcp site G => on edge top site


A,B,C,F,G G

D, E E

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.

ED

G

B AF

C

BE = -1.43 eV

BE = -1.81 eV


Carbon and oxygen atoms have several stable adsorption sites

O CA -3.87 eV (hollow) A -7.91 eV D -3.87 eV (top) D -5.83 eVE -4.27 eV (hollow) E -7.96 eVF -4.37 eV (top) F -6.44 eV (ref O2) (ref C atom)


ATOMIC O AND C ON Fe55

All the possible combinations were considered, but only 6 geometries had exothermic dissociative chemisorptions (ΔE = E(C and O on Fe55)-E(CO)-E(Fe55)).

C_O_2ΔE = -2.44 eV(O on F, C carbide-like on F)C_O_1

ΔE = - 0.32 eV(O on A, C on F)

C_O_3ΔE = -2.50 eV(O on A, C carbide like on A)

C_O_8ΔE = -1.97 eV

(O on F, C on F)

C_O_11ΔE = - 0.99 eV

(O on D, C on F)

C_O_9ΔE = - 1.78 eV (O on F, C on F)


• CO dissociation on flat part of the cluster• Barrier 0.99 eV, reaction energy -0.69 eV


• CO dissociation over the edge of the cluster• Barrier 0.77 eV, reaction energy -0.16 eV


C_O_1 C_O_2 C_O_3 C_O_8 C_O_9 C_O_11G 01 ΔE =+1.52 eV ΔE =-0.63 eV ΔE =-0.62 eV ΔE =-0.16 eV ΔE = 0.03 eV ΔE = 0.82 eV

Bar.= 1.00 eV Bar.= 0.93 eV Bar.= 1.24 eV Bar.= 0.77 eV Bar.= 2.71 eV Bar.= 2.86 eV

E01 ΔE =+1.43 eV ΔE =-0.69 eV Work in ΔE =-0.25 eV Work in Work in

Bar.= 1.81 eV Bar.= 3.27 eV progress Bar.= 2.89 eV progress progress

C and O on the same face C and O on different face

All the studied barrierFrom 2 starting geom and 6 end geom.


CO dissociation

• CO dissociation over the edge of the cluster has lower barrier than on the facet, (barrier 0.77 eV, vs. 0.99 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 Fe55 is much smaller perturbation than the atomistic step

• the Fe55 is an unusually stable cluster so it is very likely less reactive than many of the other clusters.


• CO2 formation over the edge of the cluster (CO is on the edge !)• Barrier 1.13 eV, reaction energy -0.37 eV


• CO2 formation over the edge of the cluster• Barrier 1.08 eV, reaction energy 0.88 eV


NH3 dissociation on Fe55

Our aim is to provide theoretical understanding of ammonia decomposition on iron nanoparticles catalyst in the H2 fuel processing system.


REACTIVITY HYPHOTESIS

The reaction mechanism for NH3 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) N2+2* → 2N*,

(2) H2+2* → 2H*,(3) N*+H* → NH*+*,(4) NH*+H* → NH2*+*,

(5) NH2*+H* → NH3*+*,

(6) NH3* → NH3+*

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.


REACTIVITY STUDY


NH3 ADSORPTION ON Fe

55

Adsorption of NH3 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. Fe55NH3 structure (top site) were also previously observed (table on the left) for smaller cluster.

•- 0.91•Fe13NH3

•- 0.93•Fe7NH3

•-0.7 >>- 0.4

• NH3 on Fe(hkl)

•- 1.06•Fe4NH3

•- 1.01•FeNH3

•B.E. (eV)


•- 1.45•- 2.22•(110)

•- 2.43•- 3.16•(100)

•- 1.39•- 2.18•(111)

•revPBE•PW91•Fe(hkl)/B.E. (eV)

ATOMIC N ON Fe55

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 Fe55) (-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 => ED => EF => A

Atomic binding energies (B.E.):A = - 0.81 eVC => EE = - 1.01 eV


ATOMIC H ON Fe55

Atomic binding energies (B.E.):A => EC => -0.36 eVE = -0.49 eV

These results are in agreement with the previous obtained for theabsorption on the Fe flat surfaces.

B => ED => CF => E


NH3 DECOMPOSITION


CONCLUSIONS - NH3

• For NH3, only the interaction N-Fe is favourable, and only the top are the stable sites (-0.38 eV<B.E.(NH3)<-0.24eV).

• Fe3N conformations are the only stable for the atomic adsorption of nitrogen (-1.01 eV < B.E.(N) < -0.81 eV).

• Fe3N conformations are observed also for the atomic absorption on nitrogen the Fe flat surfaces: (-2.43 eV < B.E.(N)revPBE < -1.39 eV).

• Fe3H and Fe2H conformations are also observed (-0.49 eV < B.E.(N) < -0.36 eV). These results are in agreement with the previous obtained for the absorption on the Fe flat surfaces.

• A dissociation paths for NH3 are identified. The complete dissociation reaction, to atomic nitrogen and hydrogen involve three steps: (I) NH3 NH2+H; (II) NH2 NH+H; (III) NH N+H. The reaction barrier for the overall process is 1.48 eV. Please consider that in order to get so low value for a flat surface, it's necessary to use quite expensive metal as Ru .


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 H2 dissociation - it breaks very easily.


Other fun things beside comp chem

Thank you

Funding: EU 6 FP, STREPS project BNC tubes, NMP4-CT-2006-03350

espoo 17.9.2009 kari laasonen, department of chemistry chemistry on fe 55 nanoparticle kari...

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