Transcript No Slide Title

Chemisorption: Active Sites
A chemical reaction takes place: chemical bonds are formed
Driving force: minimisation of Gibbs Free Energy (50 - 400 kJ/mol)
Chemisorption important for:





Adsorption process
Heterogeneous catalysis
Reduction of ores
Chemical Vapour Deposition
Coal gasification and
combustion
 Corrosion
Catalysis and Catalysts - Chemisorption: Active Sites
Characterisation of surfaces:





Specific surface area of phases
Types of active sites
Number of active sites
Reactivity of active sites
Stability of active sites
Metal Dispersion D
nS
D
nT
 Dispersion:
ns = number of surface atoms
nT = total number of atoms
 Chemisorption: titration of surface sites
number of moles in monolayer
nads
Stoichiometry ??
p
Catalysis and Catalysts - Chemisorption: Active Sites
ns
Various Modes of Adsorption
O
C
a.
O
O
O
O
C
C
C
C
b.
c.
d.
C
O
e.
a. linear or terminal (X = 1)
b. bridged (X = 0.5)
c. bridged (X = 0.67)
d. valley or triple (X = 0.33)
e. dissociative adsorption (X = 0.5)
Catalysis and Catalysts - Chemisorption: Active Sites
X = average number of
adsorbed molecules per
active site
Crystallographic Planes
Coordination number:
FACE 100
FACE 110
FACE 111
c.f.c.
c.f.c.
c.f.c.
8
7
Catalysis and Catalysts - Chemisorption: Active Sites
9
Adsorption Stoichiometry
Metal
N2O/Me
Pt
H/Me
CO/Me
1
1
Cu
0.5
poor H2 dissociation
catalyst
1
Ni
0.67
1
carbonyl formation!
1
2
1
Rh d > 2 nm
Rh d < 2 nm
Catalysis and Catalysts - Chemisorption: Active Sites
Volume-Surface Mean Diameter dVS
15
dVS  6 
D
VA 1

SA D
nS
nT
dVS
(nm)
10
5
Pt
Ni
0
0.0
0.5
1.0
D
D
most fundamental parameter
dVS
most convenient for measuring directly (XRD, EM)
Catalysis and Catalysts - Chemisorption: Active Sites
Dispersion D versus Diameter d
 Diameter d can be measured or calculated from several
techniques
– Electron Microscopy
– X-ray Diffraction, X-ray Photoelectron Spectroscopy
 Relation D and d (for spheres) ??
4
R 3
Volume
R d
3

 
2
Surface Area 4R
3 6
Volume  nTVA
Surface Area  nSSA
Catalysis and Catalysts - Chemisorption: Active Sites

d nTVA
V

D A
6 nSSA
SA
Number of Surface Atoms per Unit Area
part. size ca 5 nm
33% (111) plane
33% (100) plane
33% (110) plane
(atoms.nm-2)
part. size ca 15 nm
70% (111) plane
25% (100) plane
5% (110) plane
(atoms.nm-2)
Co
15.1
-
Ni
15.4
17.5
Pt
12.5
14.2
Pd
12.7
14.5
Ru
16.3
-
Rh
13.3
15.5
Cu
14.7
16.7
Metal
Catalysis and Catalysts - Chemisorption: Active Sites
Dependent on particle size
Shapes of Catalyst Particles on a Support
a.
b.
Spherical
poisoned part of
surface
Hemispherical
c.
Crystallite
Catalysis and Catalysts - Chemisorption: Active Sites
d.
Complete wetting
Chemisorption - Experimental Techniques
 Gravimetry
 Volumetry
 Spectroscopy (Infrared,
Raman)
 Pulse techniques
 Temperature Programmed
Desorption (TPD)
Gravimetry
S
Catalysis and Catalysts - Chemisorption: Active Sites
R
Chemisorption of N2O on Cu Catalyst
2 Cu(s) + N2O
Cu2O(s) + N2
 W (mg O/g Cu)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
50
100
150
200
Time (min)
Catalysis and Catalysts - Chemisorption: Active Sites
250
300
H2 Chemisorption on Ni/Al2O3 (323 K)
0.3
SNi = 12 m2/g
nad (mmol/g)
Total chemisorption
after high-T evacuation
D Ni = 18 %
0.2
Strong chemisorption
Irreversible
0.1
Weak chemisorption
Reversible
after subsequent low-T evacuation
0
0
20
40
60
p (kPa)
Catalysis and Catalysts - Chemisorption: Active Sites
80
100
Results of Barometric H2 Chemisorption
Sample
6 wt% Ni/Al2O3a
10 wt% Cu/Al2O3
10 wt% Ni/Al2O3
1 wt% Pt/Al2O3
a
Monolithic catalyst.
Catalysis and Catalysts - Chemisorption: Active Sites
Smetal
m2/g
0.76
0.6
11.9
1.54
Metal Dispersion
%
1.8
0.8
17.9
55
Pulse-Response Method
Catalyst
Detector
CO
Pulse
Response
Example: Ptsurface + CO
Pt-CO
Difference in total peak area
nsurface
Catalysis and Catalysts - Chemisorption: Active Sites
Pulse-Response Method
CO chemisorption on reduced 5wt% Pt/Al2O3
TCD signals after CO pulses
Detector signal
1.0
0.0
0
Catalysis and Catalysts - Chemisorption: Active Sites
Time of analysis
1
Pulse-Response Method
CO chemisorption on reduced 5wt% Pt/Al2O3
Cumulative amount of chemisorbed CO
n ad (mmol/g)
0.08
0.06
2/g
2
SPt
= =3 3mm
S Pt
/g
0.04
Monolayer capacity:
0.06 mmol / g Pt
DPt
24 %
D=
Pt = 24 %
0.02
0.00
0
0.5
1
Pulsed volume (ml)
Catalysis and Catalysts - Chemisorption: Active Sites
1.5
Step-Response Method
Example:
2 Cu(s) + N2O
Catalyst
Cu2O(s) + N2
Mass
Spectrometer
N2O
N 2O
N2
t
Step
Catalysis and Catalysts - Chemisorption: Active Sites
Response
Step-Response Method
Quantification of metallic Cu surface area of reduced
Cu-ZnO/Al2O3 catalyst by N2O chemisorption:
Concentration of gas-phase species
(% atm.)
2 Cu(s) + N2O
Cu2O(s) + N2
N2
N2 O
6
4
2
Dead
time
0
Flow 0
switch in
t0
300
600
Time (s)
Catalysis and Catalysts - Chemisorption: Active Sites
900
Flow
switch out
Temperature Programmed Desorption (TPD)
Example: NH3 Desorption from H-ZSM-5
Weak acid sites
Strong acid sites
Catalysis and Catalysts - Chemisorption: Active Sites
dndes
dt
Silicalite
400
Catalysis and Catalysts - Chemisorption: Active Sites
600
800
T (K)
1000
dndes
dt
Silicalite
400
Catalysis and Catalysts - Chemisorption: Active Sites
600
800
T (K)
1000
Spectroscopy
Example: IR Absorption of Pyridine on Zeolite HY
L
L
B
B
1700
1600
1500
 (cm-1)
Catalysis and Catalysts - Chemisorption: Active Sites
1400
Chemisorption Techniques
 Most techniques are rather “blind” but quantitative
 Transient techniques also give kinetic information
– pulse response, step response
– TPD (NH3 also acid strength)
 “In-situ” techniques also give qualitative information:
– in-situ infrared spectroscopy
– simultaneous measurement of heat effects
Catalysis and Catalysts - Chemisorption: Active Sites