vietnam ecat cinetique reelle 2
TRANSCRIPT
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Ecole Franco-Vietnamienne 2005 de Catalyse
Cintique et Raffinage
IRCInstitut de Recherches
sur la Catalyse
Module Cintique et Catalyse htrogne
Dr. Michel VRINAT
PART 2
Real kinetic
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Real kinetic
W y kinetic studies?
Use of a catalyst to increase t e rate of a reaction and (or) to modify t edirection of t e transformation (selectivity).
All catalysts development need a kinetic study in order to quantify and to
understand t e effects of experimental parameters over t e reaction.
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Real kinetic
Power rate law
Reactor (T)R1(P1)
R2 (P2)Products
...22
1
1
2
2
1
1
/ FFEE
PPrr
RTE PPPPekv !
Useful for engineer in the building of a unit
No information on the mechanism of the reaction
Need of models
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Real kinetic
UNIMOLECULAR REACTIONS
k1
a) A ' A B
k1
k2
Stationary state : [ A ] = constant
rate of formation = rate of disappearance
k1
[ A ] [ S ] = k1
[ A ] + k2
[ A ]
equation of conservation of catalytic sites:
[ S ]0 = [ S ] + [ A ]
resolution : ? A? A ? A ? A_ aASAkAkk ! 012'1 ? A ? A? A ? AAkkk
SAkA1
'12
01
!
? AAkv 2! ? A ? A? AAKAK
Skv'
'
21
!2
'1
1'
kk
kK
!with
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Real kinetic
b) A ' A B ' B
k1
k1
k2 k3
k3
Stationary state for A: [ A ] = constante
? A? A ? A ? AAkAkSAk '121 ! ? A ? A? ASAkk
kA
2'1
1
!
? A ? A? A ? ABkSBkAk '332 ! ? A ? A? A ? A? ASBkk
SA
kk
k
k
kB
'3
3
2'1
1
'3
2
!
Stationary state for B: [ B ] = constante
Equation of conservation of t e catalytic sites
? A ? A ? A ? ASBAS !0
? A ? A ? A ? A ? A
! Bk
kA
kk
k
k
kA
kk
kSS
'3
3
2'1
1
'3
2
2'1
10 1
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Real kinetic
? AAkv 2!? A? A
? A ? ABKAKSAKk
v
BA
A
!
"
0'
2
1
With2'1
1'
kk
kKA
!
!
'3
2'"1
k
kKK AA
Rate of reaction:
and
Remark: if the surface reaction is slow as compared to the adsorption-desorption
steps, we have
et
'12 kk and
'3k
'1
1"'
k
kKKK AAA !!!
? A ? A? A ? ABKAK
AKSkv
BA
A
!
1
02and
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Real kinetic
BIMOLECULAR REACTIONS
1) Langmuir-Hins elwood model:
reaction between two species adsorbed on the catalyst surface
a) only one type of catalytic sites
CA
(
B
+
(
C
(
A
B kSurface equilibrium adsorption established and slow surface reaction
? A? A? ASA
AKA ! BK, CK
? A ? A ? A ? A ? ACBASS !0
? A? ABAkv ! ? A ? A ? A? A ? A ? A_ a2
20
1 CKBKAK
SBKAKkv
CBA
BA
!
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Real kinetic
Remarks:
1. if KC[ C] is low (few adsorption of t e product)
? A ? A? A? A ? A_ a22
0
1 BKAKSBKAKkv
BA
BA
!
2. adsorption of C important its an in ibitor
Ex: H2S produced during t e HDS of sulfur compounds
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Real kinetic
b) two type of catalytic sites
CAS1
(
B
+(
A
BS2k
? A? A21 SS
BAkv !
? A? A? A1
1
SA
AK
S
A ! ? A ? A_ aAKSS A! 1101 ? A ? A ? A? A? AAKSAK
AAA
AS
!!1
01
1
!BK .
? A? A
? A? A
? A ? A0201
11SS
BK
BK
AK
AKkv
B
B
A
A
!
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Real kinetic
c) dissociative adsorption of a reactant
C2A
(+2S
B
+(
C
(
A2
B
? A? A? A2
2
SA
AKA ! , KB , KC
? A ? A? AAKSA A! ? A ? A ? A ? A ? A_ aCKBKAKSS CBA ! 10
? A Afv !
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Real kinetic
Exemple 1: Study of a Pt-R /Al2O t ree ways catalyst
In a detail kinetic study of a Pt-Rh/Al2O3 catalyst used for CO and NO
reduction, Lecomte proposed that the reaction proceeds according to
the following mechanisms:
Determine the rate equations corresponding to such mechanisms, and
discriminate between both proposals using experimental values given
in Table 1.
J. Lecomte , Thse Lille
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Real kineticTable 1:Results of kinetic study at 300C
Mechanism A:
KCO
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Real kinetic
Exemple 2: CO met anation over Ni/Al2O
CO + H2 CH4 + H2O Berty reactor, N2+ CO + H2 ; 47 -62 K
r = A exp(E/RT)PH2xPCO
y
Bartholomew etal., Appl. Catal., 1982
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Real kinetic
Exemple 2
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Real kinetic
Exemple 2
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Real kinetic
Exemple 2
Rate determining step could change with the temperature
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Real kinetic
Exemple : Hydrodesulfurization of sulfur compounds
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Real kinetic
S
R R
S
H RR
S
R R
S
R R R R
R R
DDS
SH
R R
HYD
H+
B-
Exemple
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Real kinetic
HDS: new approac
S
R R
S
H R
R
S
R R
S
R R R R
R R
DDS
SH
R R
HYD
H+
B-
Exemple
S. Kasztelan , D. Guillaume, I.E.C. Res. 1994
E.Olguin M. Vrinat, Appl. Catal., 1998
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Real kinetic
Exemple
S S
S
DBT-DH
DBT-TH DBT-HH
BP
S
DBT
R kD H D H H S
D H H S
!
.( ) ( )
( )
E E E E E
E E E
2 2 2
2 2
1
1
12
12
2
R k H HD H
H H S H D H D H H D H
!
.( ( ) ( ) )
E E
E E E E E E E
2
2 2 2 21 2
Rds= addition ofH- (and afterH+)
Under lowH2S partial pressure
Rds = elimination reaction E2, under high H2S pressure
= iPi
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Real kinetic
2- Eley-Rideal model:
reaction between one species adsorbed on the catalyst surface and
one molecule of the gas phase
CA
B
+(
C
(B
k
? A? ABAkv !? A ? A? A
? A ? ACKBKSBKA
kvCB
B ! 1 0
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Real kinetic
Exemple : V2O5/TiO2 catalyst for Catalytic Selective Reduction of NO
OHNONONH 222 6444 p
DeNOx Process by reduction of NO using NH3 has been performed
over a V2O5/TiO2 and the solid characterized by TPD and FTIR.
a) IR studies indicated adsorbed NH3 species, but no NO adsorbed species,suggesting an Eley-Rideal mechanism:
MNH 3 ' 3NH
3NH-M + NO p 2k Produits + M (slow)
-M (fast)
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12
NHNH
NHNHNO
PK
PKPkv
vv!Rate equation could be written:
Dumesic etal., ICC1992
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Real kinetic
b) Such an equation could not fit the experimental values. Thereforethe authors proposed a second model in which NH3 adsorption would be slow,
according to:
k1
NH3 + M NH3-M (slow)
NH3-M+ NO Produits + M (slow)
Rate equation:
p n
1k
p
2k
NONONH
NONHNO
PK
PPK
PPKPkv
21
1
2
3
3
1 vv!
212211 ; kkKkkK !!
Good fit with experimental kinetic results
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Real kinetic
C) Previous mechanism does not agree with TPD and FTIR studies which indicatea rapid adsorption of NH3 and adsorption of this reactant over2 type of sites.
Therefore the authors proposed a new mechanism:
NH3-M+ S
p n
2k NH3-S+ M (slow)
MNH 3 ' 3NH -M (fast)
k2
NH3-S+ NO p 3
k Produits + S (slow)
Corresponding to the rate equation:
333
3
21
13
1 NHNHNONONH
NONHNO
PKPKPPK
PPKPkv
v
vvv!
322321 ;3 kkKkkKK NH !!with
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Real kineticHow to differenciate L-H and E-R ?
? A ? A? A? A ? ACKBK
SBKAkv
CB
B
!1
0? A ? A? A? A ? A_ a2
20
1 BKAK
SBKAKkv
BA
BA
!
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Real kinetic
3- Stationary state Model:Mechanism in which two reactions are r.d.s.
Introduced by Mars and Van Krevelen for catalytic mild oxidation
Hydrocarbon + cata oxidic state Produit + reduced catalyst
Reduced cata. + oxygene cata in oxidic stateoxyd
UAPkv 11 ! (U : O2coverage )
nOPkv 2122 U!
21 vv ! n
OA
n
OA
PkPk
PkPkv
2
2
21
21
!n
OA PKPkv 221
111!
ki , n
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Real kinetic
Exemple: production of tetra ydrofuran
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Real kinetic
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Real kinetic