Solid Catalysts

1
Solid Catalysts
What do we need for making a good Catalyst ?

• High activity per unit of volume in the eventual
  reactor
• High selectivity
• Sufficiently long life time with respect to
  deactivation
• Possibility to regenerate, particularly if it
  deactivates fast
•  Reproducible preparation
• Sufficient thermal stability against sintering,
  structural
• changes or volatilization
• High mechanical strength with respect to crushing
  (e.g. under
• the weight of the catalyst bed or during the
  shaping processes)
• High attrition resistance (mechanical wear)
• .

2
The Structure of Metals and their Surfaces
fcc
bcc
hcp
3
Crystal structure

• A metal has a crystal structure and a lattice
  constant which minimize the energy

bcc hcp fcc
4
Crystal structures in the periodic table
5
Surface Crystallography
Surface density fcc, (111)gt100gt110 fcc,
(110)gt100gt111
Stepped surfaces
fcc (755)
6
Adsorbate sites
7
The two-dimensional lattice
(2x2)S-Ni(100)
S-Ni(100)
8
Surface Structures
Clock reconstruction C-Ni(100)
9
Alloys
10
Oxides and Sulfides
11
Oxides and Sulfides
12
Sulfides MoS2 (CoMOS)
Sulfur
Molybdenum
Cobalt
13
Hydrodesulfurization Catalysis

• Large Environmental Impact
• New Technological Challenges

SO2 emissions from fuels
New S regulations world wide
EU
Catalyst MoCoS
14
Mo Deposition on Au(111)

• Mo coverage 10
• High degree of dispersion.
• Self-assembled Mo nanoclusters, size 20-30Å.
• Well-defined nanometer spacing.

500 Å
Helveg, Lauritsen, Lægsgaard, Stensgaard,
Nørskov, Clausen, Topsøe and Besenbacher. Phys.
Rev. Lett. 84, 951 (2000)
4035 x 4090 Å2,, insert 890 x 920 Å2
15
Sulfiding Mo on Au(111)

• High degree of dispersion
• MoS2 nanoclusters,size 20-30Å
• Triangular morphology

Good model system for HDS catalysts
Mo deposited in H2S (10-6 mbar), Postannealing to
673K
Helveg, Lauritsen, Lægsgaard, Stensgaard,
Nørskov, Clausen, Topsøe and Besenbacher. Phys.
Rev. Lett. 84, 951 (2000)
807 x 818 Å2
16
STM Image of MoS2 Nanocluster

• Triangular shape
• Hexagonal lattice, sulfur atoms
• a3.15 0.05Å
• Single layer (0001) MoS2

41Å ? 42Å
Helveg, Lauritsen, Lægsgaard, Stensgaard,
Nørskov, Clausen, Topsøe and Besenbacher. Phys.
Rev. Lett. 84, 951 (2000)
17
Understanding the nanocluster
a) STM image of a triangular MoS2 nanocluster.
b) A cobalt-promoted MoS2 nanocluster
c) DFT calculation
c)
Bollinger, Lauritsen, Jakobsen, Nørskov, Helveg,
Besenbacher Phys. Rev. Lett. 87 196803 (2001)
Metallic state
18
Surface Free Energy

• Surfaces are always covered by the component or
  structure that
• lowers surface free energy of the system.

• Clean, polycrystalline metals expose mostly
  their most densely
• packed surface.

• Open surfaces such as fcc (110) often
  reconstruct to a geometry in
• which the number of neighbors of a surface
  atom is maximized.

• In alloys, the component with the lower surface
  free energy
• segregates to the surface.

• Impurities in metals, such as C, O, or S,
  segregate to the surface.

• Small metal particles on an oxidic support
  sinter at elevated tempe-
• ratures because loss of surface area means a
  lower total energy.

• In oxidic systems, however, the surface free
  energy provides a
• driving force for spreading over the
  surface if the active phase has a
• lower surface free energy than the support.

19
Characteristics of Small Particles and Porous
Material
1) The surface energy for each surface (h,k,l)
is plotted in a polar plot so the length of
the vector is proportional to the surface energy.
2) At the end of the vector a surface plane is
defined orthogonal to the vector.
3) The inner envelope of these surfaces defines
the equilibrium shape
20
Free particle
g111 1.97 J/m2 g100 2.2 J/m2 g110 2.1 J/m2

• Simulation for free fcc particle with (100) facet
  up

21
The Wulff construction
is the adhesion energy at the surface
22
Overview

• gs ginterface gsubstrat

(100)
gs
-1.0 0.8 0.0 1.0
Top and contact planes are (100), calculated Cu
energies
23
Adsorbed particle

• Simulation for adsorbed fcc particle with (100)
  facet up

g111 1.97 J/m2 g100 2.2 J/m2 g110 2.1 J/m2 (no
interaction)
24
Examples of particles on surfaces
25
Model support Thin Al2O3 film on NiAl

• 5 Å thick film
• Thin enough to allow tunneling
• Thick enough to develop Al2O3 nature, e.g. a band
  gap
• Structure model
• (111) termination of g -Al2O3
• Oxygen terminated
• Two Al - O sandwich layers

26
STM of metals on Al2O3 /AlNi
Evaporate Pd on the Al2O3
A general trend
Metals form clusters preferentially along the
edges
2 ML Pd 650?650 Å2
Besenbacher et al.
27
Atomic resolution on Pd nanocrystals on Al2O3
300?300 Å2
28
Adhesion energy
Wadh g100 gsubstrat ginterface
The energy per unit area needed to pull the
system into its constituents
29
Nanocrystal shape and adhesion energy
30
Reversibility of shape changes
Synthesis from H2CO2CO
The methanol catalyst undergoes strong changes by
changes in gas composition
31
Dynamical changes in structure of nano
crystalline catalysts
Changes in apparent coordination number
determined with in situ EXAFS Grunwaldt et al.
(2000) Jour. Cat. 194, 452-460
Changes in particle shape with gas
composition. Methanol catalyst Cu/ZnO, In-situ
TEM at Haldor Topsøe A/S Hansen et al.HTAS
(2002), Science 295, 2053-2055
H2 (red.) H2H2O (ox.) H2CO
(red.)
32
The pore system
A good support offers

• controlled surface area and porosity
• thermal stability
• high mechanical strength against crushing and
  attrition

Micropore pore width ? 2 nm Mesopore pore
width 2 50 nm Zeolittes
33
Measuring the surface area

• Dynamic equilibrium between adsorbate and
  adsorptive
• the rate of adsorption and desorption in any
  layer are equal

• In the first layer, molecules adsorb on
  equivalent adsorption sites.

• Molecules in the first layer constitute the
  adsorption sites for molecules
• in the second layer, and so on for higher
  layers.

• Adsorbate-adsorbate interactions are ignored

• The adsorption-desorption conditions are the
  same for all layers
• but the first.

• The adsorption energy for molecules in the 2nd
  layer and higher equals
• the condensation energy.

• The multilayer grows to infinite thickness at
  saturation pressure
• (P P0).

34
The Surface Area (the BET model)
BET Brunauer, Emmet, Teller.
35
BET
The total coverage is then
36
BET
should be eliminated
Leading to
37
BET
P0 is the equilibrium pressure over multilayer
of the fluid or solid
What is it for N2 at 77K?
38
BET
At T75 K the equilibrium pressure for N2 is
P0750 mbar.
We will expres the amount of gas adsorbed Va in
terms of the amount adsorbed in one monolayer V0
Substitute this into the previous expression for
qt
39
Example of BET area
 
V01616ml leading to 32 m2g-1 if we assume
AN216Å2
40
Simple BET
41
Capillary condensation
42
Both Physisorption and Pores
43
Supported Catalysts
Good support materials are
Al2O3 MgO SiO2 Carbon
High area and very stable
High area but less stable
Good support materials have areas from 1-200-
1000m2/g
The surface energy of good support materials are
in general low Why are such materials very
brittle? How are high area materials made?
44
Catalyst Shaping
Dictated by strength and transport limitations
45
Catalyst Preparation
It is an integral part of courses Nanostructured
materials for heterogeneous catalysis
26510 (both synthesis, test, individual
projects) Catalysis, spectroscopy and structure
26330
46
Catalysts testing
Idealized plug flow reactor
Is not a simple task due to heat and mass
transport. Just minor errors in temperature can
lead to erroneous results. Is in particular a
problem for strongly exothermic or
endothermic processes Measurements should be
performed in the zero conversion limit and
without transport limitations
47
The 10 commandments for testing

• What is the purpose?
• What is the strategy
• What type of reactor PFR or CSTR
• PFR is it ideal?
• Isothermal conditions
• Limit transport effects and gradients
• TOF, selectivity, space velocity i.e. meaningfull
  units. Measure activity in the zero conversion
  limit.
• Stability?
• Reproducibility
• State all experimental details

In the following we shall see what happens if we
for example are not careful about the mass
transportation limitations.
48
Thiele diffusion modulus
Ficks 1st and 2. laws of diffusion. Notice D may
be considerably smaller than normal gas diffusion
(Knudsen diffusion).
The reaction rate
We are only interested in steady State solutions
49
Thiele diffusion modulus
50
Thiele diffusion modulus
The continuum law
51
Thiele diffusion modulus
For a first order reaction n1 we get a simple
form
52
Thiele diffusion modulus
53
Thiele diffusion modulus
54
The apparent activation energy
55
Optimization of catalysts
The higher the temperature the better (except for
exothermic processes) The lower the catalyst
loading the better (consider the price of Pt)
This gives some guidelines how to design and
operate the catalyst
56
Transport limitations
Steady state solution
57
Transport limitations
We make a guess
Resulting in the solution
58
Transport limitations
We now introduce an efficiency factor rate
with/rate without transport
Limits D,R large and L, k small, transport has
no influence D,R small and L, k
large, transport limitations
59
Transport limitations
Typically
This gives some guidelines how to design and
operate the catalyst