The selective catalytic reduction (SCR) of NO by ammonia Structure

The selective catalytic reduction (SCR) of NOx by ammonia
SCR:
4 NO + 4 NH3 + O2
In diesel
vehicles:
4 N2 + 6 H2O
NOx
N2
(NH2)2CO + H2O → CO2 + 2 NH3
H2O
SCR mechanism over Fe-ZSM-5:
2 NO + O2
2 NO2ads (metal ions)
2 NO + 2 NO2ads + 4 NH3
rate limiting step
4 N2 + 6 H2O
S. Brandenberger, October 2008
Structure of iron sites in Fe-ZSM-5
ZSM-5 zeolite with channel structure:
S i z e
Exchange of H+ with Fe: a large variety
of Fe species are formed in Fe-ZSM-5:
Fe2O3 particles
FexOy clusters
HO
Fe
O
OH
Fe
OH
Which one
is the active
species?
3+
Fe
S. Brandenberger, October 2008
Problem: Identification of the SCR active sites in Fe-ZSM-5
Reason: No method (e.g. UV/VIS, IR, EPR, EXAFS, XPS, XRD) can definitively distinguish between
isolated iron species and iron oxide clusters of different nuclearity in the same sample
Solution: A statistical approach
Idea: At least two iron ions, located on Al -sites, must be present within a sphere with r = 6.5 Å to form an
oxygen bridged Fe-O-Fe species
6.5 A°
Procedure: Calculating of the averaged number of Al sites bearing an iron ion = NFe ; NFe = f(Si/Al,Fe/Al)
Assumption: Al Atoms are statistical distributed and the variation of the local Si/Al ratio can
x
N Fe ⋅ e − N
be described by a Poisson distribution:
P ( x; N Fe ) =
Fe
x!
Rice, M.J., Chakraborty, A.K., and Bell, A.T., J. Catal. 186 (1999) 222.
Goodman, B.R., Hass, K.C., Schneider, W.F., and Adams, J.B., Catal. Lett. 68 (2000) 85.
S. Brandenberger, October 2008
Si/Al: 14
1
ies
pec
s
ed
ster
u
l
c
0.6
N ⋅ e − N Fe
1 − P( x = 0; N Fe ) = 1 − Fe
= 1 − e − N Fe
0!
0
0.4
0.2
dimeric spec
ies
0
0
0.2
0.4
0.6
N Fe ⋅ e − N Fe − N Fe
⋅e
1!
1
P=
0.8
1
Fe/Al
100
Fe/Al
90
0.74
0.45
0.41
0.3
0.15
0.09
0.04
0.02
H-ZSM-5
80
70
Correlation of the measured
SCR activity with the calculated
concentration of different species
DeNOx [%]
Probability
0.8
60
50
40
30
20
10
0
200
300
400
500
600
700
Temperature [°C]
S. Brandenberger, October 2008
SCR activity of the different iron species
Temperature
Isolated species
OH
HO
3+
Fe
200-300 °C
FexOy
Fe2O3
-
-
-
Active
-
-
Active
-
Active
Active
Fe
O
s-1
s-1
Fe
OH
(250 °C)
(350 °C)
Active
TOF ≈ 100
≥ 450 °C
s-1
Active
TOF ≈ 46
≈ 400 °C
Particles
Active
TOF ≈ 14
≈ 350 °C
Oligomeric species
Dimeric species
(400 °C)
Active
TOF ≈ 120
s-1
(350 °C)
Active
TOF ≈ 170
s-1
(400 °C)
Active
TOF ≈ 27 s-1
(500 °C)
S. Brandenberger, October 2008

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