Transcript PowerPoint-esitys

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)
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
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)
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
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)
Espoo 17.9.2009
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) +
CO2(g)
• Calculations: VASP, PBE functional,
PAW pseudopotentials, non-collinear
magnetism
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
THE NANOPARTICLE: Fe55
We have studied the stability of
Fe clusters of different size and geometry (from
7 to 55 atoms).
 Icosahedral
symmetry;
 Non-collinear
behavior is result
of
competing
ferromagnetic and
anti ferromagnetic
interactions.
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
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 FeFe 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,
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 Fe55 cluster that are resulted identical for
symmetry.
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
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
E
D
G C
A F
B
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.
D, E
E
BE = -1.43 eV
A,B,C,F,G
G
BE = -1.81 eV
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
D
G C
A
B
F
Carbon and oxygen atoms have several stable adsorption sites
A
D
E
F
O
-3.87 eV
-3.87 eV
-4.27 eV
-4.37 eV
(ref O2)
(hollow)
(top)
(hollow)
(top)
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
C
A
-7.91 eV
D
-5.83 eV
E
-7.96 eV
F
-6.44 eV
(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_1
ΔE = - 0.32 eV
(O on A, C on F)
C_O_2
ΔE = -2.44 eV
(O on F, C carbide-like on F)
C_O_9
ΔE = - 1.78 eV
(O on F, C on F)
C_O_8
ΔE = -1.97 eV
(O on F, C on F)
C_O_3
ΔE = -2.50 eV
(O on A, C carbide like on A)
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
C_O_11
ΔE = - 0.99 eV
(O on D, C on F)
• CO dissociation on flat part of the cluster
• Barrier 0.99 eV, reaction energy -0.69 eV
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
• CO dissociation over the edge of the cluster
• Barrier 0.77 eV, reaction energy -0.16 eV
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
All the studied barrier
From 2 starting geom and 6 end geom.
C_O_1
G 01
C_O_2
C_O_3 C_O_8 C_O_9 C_O_11
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
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
C and O on different face
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.
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
• CO2 formation over the edge of the cluster (CO is on the edge !)
• Barrier 1.13 eV, reaction energy -0.37 eV
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
• CO2 formation over the edge of the cluster
• Barrier 1.08 eV, reaction energy 0.88 eV
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
NH3 dissociation on Fe55
Our aim is to provide theoretical understanding of
ammonia decomposition on iron nanoparticles
catalyst in the H2 fuel processing system.
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
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.
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
REACTIVITY STUDY
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
NH3 ADSORPTION ON Fe55
•B.E. (eV)
•FeNH3
•- 1.01
•Fe4NH3
•- 1.06
•Fe7NH3
•- 0.93
•Fe13NH3
•- 0.91
• NH3 on Fe(hkl)
•-0.7 >>- 0.4
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
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.
ATOMIC N ON Fe55
Atomic binding energies (B.E.):
B => E
A = - 0.81 eV
D => E
C => E
F => A
E = - 1.01 eV
•Fe(hkl)/B.E. (eV)
•(111)
•PW91
•- 2.18
•revPBE
•- 1.39
•(100)
•(110)
•- 3.16
•- 2.22
•- 2.43
•- 1.45
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
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.
ATOMIC H ON Fe55
Atomic binding energies (B.E.):
A => E
C => -0.36 eV
E = -0.49 eV
B => E
D => C
F => E
These results are in agreement with the previous obtained for the
absorption on the Fe flat surfaces.
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
NH3 DECOMPOSITION
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
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 .
Espoo 17.9.2009
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 H2 dissociation - it breaks very easily.
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry
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
Espoo 17.9.2009
Kari Laasonen, Department of Chemistry