Diapositiva 1

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Dipartimento di Chimica Inorganica e Analitica
"Stanislao Cannizzaro" Università di
Palermo Viale delle Scienze, Parco d'Orleans II,
I-90128 Palermo, Italy
SERA A NEW ALGORITHM TO EXPLAIN THE METALLIC
ASE Giampaolo Barone, Rosario Troia, Dario
Duca http//www.unipa.it/dduca
SERA is a time dependent Monte Carlo algorithm,
considering the Stefan-Boltzmann and the Fourier
laws to simulate the local energy exchange inside
a given metal and between the metal and the
environment. Its kernel tries to re-balance,
after the occurrence of each event, the surface
energy distribution. Along the re-balancing time,
the energy changes determine different ASE
values, causing variable local reactivity on the
crystallite surface. Random number generators
drive the elementary events whereas an internal
clock allows us to follow the dynamics. By using
just physical constants, without the help of fit
procedures, we were able to mimic experimental
surface findings and to find out a
self-consistent interpretation of the metallic
ASE on simple physical-chemistry bases
SERA, Surface Energy Reassessment Algorithm, is
suggested as a tool to explain phenomena
connected with the Available Surface Energy
(ASE), earlier hypothesised and parameterised in
studying elementary processes on metal
crystallites and catalysis. Within such a
hypothesis, confined energy fluctuations should
control the local ASE values of the active sites.
These fluctuations, able to change the local
reactivity of a material, would take place as a
consequence of simple surface events, occurring
in the neighbourhoods of the same sites. By using
SERA, ad-sorption, diffusion, de-sorption and
reactive events as well as their effects on the
catalyst particles, on the whole reaction and on
the apparent changes of the associated energetics
could be easily rationalised.
Snapshot of the energy distribution on a
crystallite surface the energy values increase,
from blue to red, following the rainbow spectrum.
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Simulated process and some examples of the
elementary events involved. Note that event A
mimics the helicopter rotation, which influences
the steric hindrance of CO.
The home made SERA code, implemented in Visual
F90, whose flow-chart is reported in the 7th
box is designed to work with complex
heterogeneous catalytic system. However, to test
its ability in managing surface processes, it was
firstly employed to mimic the adsorption
de-sorption phenomena of CO on Ni, Pd and Pt
surfaces, box 4th, (W.A. Brown, R. Kose, D.A.
King, Chem. Rev. 1997, 98, 797 Y.Y. Yeo, L.
Vattuone, D.A. King, J. Chem. Phys. 1996, 104,
3810). The results are reported in the following
box. In passing, it has to be noticed that the
simulated crystallites contained a significant
number of atoms (4000 10000) and have to take
account of non-linear differential equation
number into the same order of magnitude. Fitting
parameters were not introduced and just
fundamental physical laws were considered. The
steric hindrance determining the probability of
residence, ??(?), of CO on the surface, which is
referable to the helicopter rotation of the CO
molecules (box 12-15),was the only arbitrary
concept introduced.
Helicopter model to mimic the rotation, under
different angles with respect to given metal (Ni,
Pd and Pt) surfaces, of the CO molecules. The h
values were determined by DFT-PEC analyses
whereas the inclination and rotation angles were
monitored by DFT-PES studies, as reported in the
following. The results are in agreement with the
steric hindrance hence with the residence
probabilities showed by the bridged ?2-CO
molecules considered into the SERA simulations.
CO n ? COn
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CO /Ni, /Pd and /Pt sticking probability
surface population plotsST vs. ? black and
white circles represent experimental and
simulated points, the red ellipses show the
residence probabilities, ??(?), considered in the
corresponding ranges of ?.T 300 K, pCO
5.5x10-5 Pa, metal crystallite surface 2.x10-6
m2, metal crystallite thickness 2.x10-7 m.
Inclination angle, theta /,
rotation angle , phi /, B3LYP/LANL2DZ_6-31G
(d,p) PES /kJmol-1 of ?2-CO/Ni. The metallic
cluster was characterized by 8 atoms, the height
of CO with respect to the metallic surface (6/8
atoms) was previously determined by a PEC
performed at the same level of calculation.
Bi-phase reactor scheme (?) (A) thermostat, (B)
gas-flow, (?) support, (?) metal crystallite
crystallite details (?) (?) surface unit-cell,
(?) wall unit-cell, (?) level unit-cell, (?) bulk
unit-cell
unit-cell details (?). --------------------------
---- Frequency of hitting and diffusion are
calculated following the collision and the
transition-state theory, respectively. -----------
------------------- De-sorption, as the surface
population, is automatically accounted for, along
all the simulation, by the code.
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Inclination angle, theta /,
rotation angle , phi /, B3LYP/LANL2DZ_6-31G
(d,p) PES /kJmol-1 of ?2-CO/Pd. The metallic
cluster was characterized by 8 atoms, the height
of CO with respect to the metallic surface (6/8
atoms) was previously determined by a PEC
performed at the same level of calculation.
Energy exchange occurring among the neighbouring
cells. Following the Fourier law As is the
surface area between two unit-cell, dQ the
thermal energy exchanged in the unit-time dt
whereas ?(T) the thermal conductivity and dT and
dx the cell temperature difference and the cell
inter-centers distance, respectively.
SERA algorithm is an implementation of the ASE
concept. Its sound ability to reproduce the
complex experimental results of the CO/metal
interactions is consistent with the physical
meaning of the basis idea. Moreover, the steric
hindrance hence the probability of residence, the
weak argument of the model, was already
introduced and justified and employed in
mimicking surface events, e.g. D. Duca, G. La
Manna, M.R. Russo, Phys. Chem. Chem. Phys. 1999,
1, 1375 D. Duca, G. La Manna, Zs. Varga, T.
Vidóczy, Theor. Chem. Acc. 2000, 104, 302 D.
Duca, G. Barone, Zs. Varga, G. La Manna, J. Mol.
Struct. (Theochem) 2001, 542, 207 G. La Manna,
G. Barone, Zs. Varga, D. Duca, J. Mol. Struct.
(Theochem) 2001, 548, 173 G. Barone, D. Duca, G.
La Manna, Recent Research Development in Quantum
Chemistry (vol 2), S.G. Pandalai (ed.),
Transworld Research Network, Trivandrum, 2001 D.
Duca, G. Barone, Zs. Varga, Catal. Letters 2001,
72, 17 G. Barone, D. Duca, J. Catal. 2002, 211,
296. In the next box significant information by
this work is fixed.
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CO /Ni (triangle), /Pd (circle) and /Pt (square)
calculated de-sorption activation energy
surface population plots (Ea? vs. ?) and
temperature (black) and surface population (blue)
dynamic evolution plots T, ? vs. tempo. In the
first plots the simulated conditions are those of
the 9th box in the second ones is changed the
starting temperature, 350 K.
SERA
Inclination angle, theta /,
rotation angle , phi /, B3LYP/LANL2DZ_6-31G(
d,p) PES /kJmol-1 of ?2-CO/Pt. The metallic
cluster was characterized by 8 atoms, the height
of CO with respect to the metallic surface (6/8
atoms) was previously determined by a PEC
performed at the same level of calculation.
The first curves show the ASE and the steric
effects on the activation energy of the processes
here studied, the second ones the ability of SERA
to manage steady-state and big-bang conditions.
flow -chart
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EUROPACAT-6 INNSBRUCK, 31st AUGUST 4th
SEPTEMBER 2003