The role of ceria in catalysis

1
The role of ceria in catalysis
B. Murugan National Centre for Catalysis
Research IITM, Chennai-36. 18-12-2007
2
Rare earths 15 lanthanide elements divided
into two groups First four elements ceric (or)
light rare-earths Remaining elements yttric
(or) heavy rare-earths Bastnasite, Monazite and
Loparite principle cerium ores Monazite most
abundant Ce two stable valence states Ce4
and Ce3 Ce is the unique rare-earth for which
dioxide is the normal stable phase contrary to
the others for which Ln2O3 is the normal
stoichiometry.
3
Why do we need to talk about ceria? Owing to
number of application catalysis, chemicals,
glass and ceramics, phosphors and metallurgy The
applications of ceria based materials are related
to a potential redox chemistry involving Ce(III)
and Ce(IV), high affinity of the element for
oxygen and sulfur and absorption/excitation
energy bands associated with its electronic
structure.
4

• Applications of cerium in catalysis and chemicals
• Fluid Catalytic Cracking huge amounts consumed
  for refinery operations convert crude oil to
  lower molecular weight fractions.
• TWC major technological application vehicle
  emission control to remove pollutants from
  vehicle (auto-exhaust) emissions significant
  portion of cerium consumed annually.
• Oxidizing agent potential use as additives to
  aid combustion to reduce the particle emissions
  from Diesel engine.
• SOx control agent.
• Eletrode material in SOFC.
• EB dehydrogenation ceria addition improves
  activity for styrene formation.
• Supports the ammoxidation of propylene to produce
  acrylonitriles.

5
Crystal Structure of ceria The Fluorite structure
Fluorite has a very simple structure space
group Fm3m The structure can be viewed as a
face-centered cubic array of Cerium (green) ions
with the oxygen (purple) ions residing in the
tetrahedral holes.
6
Consider the stoichiometry of single unit cell.
Each of the corner cerium ions is 1/8 inside the
cell since there are eight corners these add up
to one ion inside the cell. There are six faces
to a single cell, each with a cerium ion one-half
inside the cell. Therefore a single cell
contains four cerium ions. A single cell also
contains eight oxygen ions, each one located
entirely within the unit cell. Since there are
four cerium ions and eight oxygen ions inside the
cell, the 12 stoichiometry is maintained.
7

• We can also view the structure as a simple cubic
  array of oxygen with a cerium in the center of
  alternate cubes.
• Considered that way, there are obviously diagonal
  planes of cubes containing no cations.
• These planes will obviously be planes of
  weakness, accounting for fluorite's excellent
  octahedral cleavage.

8
Octahedral Holes Regardless of whether hexagonal
layers are stacked in an AB or ABC fashion, there
exist two types of spaces or holes between the
layers. One type of space is called an
octahedral hole, and is formed between three
atoms in one layer and three atoms in the layer
immediately above or underneath. Although it
takes six spheres to form an octahedron, the name
is derived from the fact that the resulting shape
has eight sides.
9
Tetrahedral Holes A second type of space which
can exist between stacked hexagonal layers is
called a tetrahedral hole. Tetrahedral holes are
formed between three atoms in one layer and a
single atom immediately above or underneath.
10
Octahedral Holes in the Fluorite Structure In
the fluorite structure, the fluoride ions reside
within the tetrahedral holes formed by the
face-centered cubic array of calcium ions, and
the octahedral holes are vacant. In this
illustration the green cylinders outline eight of
the vacant octahedral holes. This illustration
shows the vacant octahedral holes in the fluorite
structure, outlined by the green spheres, as seen
from the top.
11
Tetrahedral Holes in the Fluorite Structure This
illustration shows the location of the
tetrahedral holes in the fluorite structure.
12
Why the fluoride ions would reside in the
tetrahedral holes rather than the octahedral
holes? The most obvious answer to this question
is, of course, stoichiometry. There are two
oxygen atoms for every one cerium atom, and since
an array of N atoms results in the formation of N
octahedral holes, there would simply not be
enough spaces for all oxygen atoms. If the ions
were reversed, with the oxygen ions forming the
face-centered cubic array, there would be enough
cerium ions to fill only 1/4 of the tetrahedral
holes or 1/2 of the octahedral holes this would
be terribly inefficient.
13
Technically, the descriptions of the fluorite
structure given above are inaccurate in the sense
that because the oxygen ions are in fact larger
than the cerium ions, they therefore do not "fit
inside" the tetrahedral holes. As can be seen
here, the cerium ions form a sort of "expanded"
face-centered cubic structure and do not
physically touch each other. Nevertheless this
does represent the most efficient packing
arrangement.
14
Defect Structure of Ceria

• Defects in ceria intrinsic or extrinsic
• Intrinsic defects due to thermal disorder or by
  the redox process
• Extrinsic defects by impurities or by the
  introduction of aliovalent dopents.
• Three possible thermally generated intrinsic
  disorder in ceria
• CeCe 2 OO ? VCe 2VÖ CeO2
  ?E 3.53 eV Schottky
• CeCe ? Cei VCe ?E 3.53
  eV Frenkel
• OO ? OI VÖ
  ?E 3.20 eV
  Frenkel

From variation in ?E, it is evident that the
predominant defect category is the anion
Frenkel-type. Results obtained from X-ray,
neutron diffraction and combined dilatometric and
X-ray lattice parameter measurements proved that
the predominant defects in ceria are anion
vacancies.
15
Faber et al. examined the electron density
distribution using XRD and concluded that the
amount of interstitial Ce is less than 0.1 of
the total defect concentration in CeO1.91. The
process of ceria reduction may be written as Oo
2CeCe Vo 2CeCe 1/2O2 (gas) In the
case of H2 reduction Oo 2CeCe H2 (gas)
Vo 2CeCe H2O (gas) Oxide vacancies may
also be introduced by doping with oxides of
metals with lower valencies, e.g. dissolution of
CaO and Gd2O3 CaO CaCe Vo Oo Gd2O3
2GdCe Vo 3Oo Already existing oxide
vacancies may be removed by doping with oxides of
higher valency than 4 Nb2O5 Vo 2NbCe Oo
16
Electrical behavior of ceria Ceria can be
classified as mixed conductor showing both
electronic and ionic conduction. Its electrical
properties are strongly dependent upon T, oxygen
partial pressure and presence of impurities or
dopents. For general case in CeO2-x the total
conductivity is given by ?t CeCee?e
h?e?h VÖ2e?Ö At high temperatures and low
oxygen partial pressures, ceria behaves as an
n-type semiconductor and electrons liberated
following the reduction are the primary charge
carriers. Oo ? VÖ 2e- 0.5O2 (g) Transition
from n-type to p-type conduction is observed at
lower temperatures and higher oxygen partial
pressures near stoichiometric composition, where
electronic conductivity arises from holes
introduced by impurities IO ? ICe VÖ Oo VÖ
0.5O2 ? Oo 2 h? h? indicates an electron hole
17
Ionic conductivity due to the mobility of oxide
ion vacancy It is always much lower than the
electronic conductivity in pure reduced
ceria. However, the situation is different in
ceria doped with oxides of two or three-valent
metals due to the introduction of oxide ion
vacancy. The electronic conductivity in air may
be very low and the doped ceria under these
conditions are excellent electrolytes. The
conductivity mechanism is the hopping of oxide
ions to the vacant sites and the ionic
conductivity ?i may be expressed as ?i (?o /T)
exp (-EH/kT), EH is the activation energy for
small polaron hopping. The ionic conductivity
increases with increasing ionic radius, from Yb
to Sm, but decreased at rdopant gt 0.109 nm. The
most important parameter for ionic conductivity
in fluorites is the cation match with the
critical radius, rc. Highest conductivity
ionic radius of the dopant is as close to rc as
possible
18
Lattice Defects and Oxygen Storage Capacity of
Nanocrystalline Ceria and Ceria-Zirconia

• Ceria-based oxides - automotive exhaust emission
  control systems as catalyst supports and oxygen
  promoters.
• Three-way automotive catalytic converters -
  oxidize CO and hydrocarbons and at the same time
  reduce nitrogen oxides.
• A high rate of simultaneous conversion of all the
  pollutants can only be achieved within a narrow
  operating window near the stoichiometric
  air-to-fuel ratio.
• CO-NOx conversions are strongly affected by the
  local oxygen partial pressure at the catalyst
  surface.
• At high oxygen partial pressures (under lean
  conditions), the NOx conversions drop off
  precipitously, whereas at low oxygen partial
  pressures (under rich conditions), the CO
  conversions are low.

19

• The role of ceria, and more recently
  ceria-zirconia, is to act as an oxygen
  storage-and-release component to stabilize the
  local oxygen partial pressure at the catalyst
  surface even when the air-to-fuel ratio in the
  engine exhaust fluctuates with time.
• Pure ceria has a serious problem of degradation
  in performance with time at elevated
  temperatures.
• Traditionally, this degradation has been
  attributed to decrease in its surface area and in
  turn its oxygen storage capacity (OSC).

20

• However, recent experimental observations on pure
  ceria suggest that the surface area may not be
  the only parameter that determines the
  effectiveness of ceria.
• It has been proposed that in pure ceria "active"
  weakly bound oxygen species are present, which
  belong to the bulk rather than to the surface.
• It is likely that these weakly bound oxygen
  species undergo fast exchange with the
  environment and provide OSC. Such "active" oxygen
  species become deactivated following a
  high-temperature treatment.

21

• Pulsed neutron diffraction data both in the
  reciprocal space by the Rietveld refinement and
  in the real space by the atomic pair-distribution
  function (PDF) analysis - presence of the
  vacancy-interstitial (Frenkel-type) oxygen
  defects in CeO2.
• These defects were found to disappear following a
  high-temperature treatment of 1073 K (800 C). It
  is possible that the interstitial oxygen ions are
  the "active" species that provide necessary
  oxygen mobility crucial in the functioning of
  ceria as a catalyst support
• Decreasing concentration of the Frenkel-type
  oxygen defects at high temperatures contributes
  to deterioration of the oxygen storage properties
  in thermally aged ceria.

22

• Zirconia is known to alleviate partially the
  degradation of ceria at high temperatures. The
  beneficial effect of doping ceria with zirconia
  is believed to be due to stabilizing the surface
  area by suppressing thermal sintering.
• However, it has been observed that ceria-zirconia
  mixed oxides with low surface area still maintain
  a high oxygen storage capacity compared to
  undoped ceria, and therefore other mechanisms
  must be present.
• Zirconia keeps ceria slightly reduced, and
  preserves oxygen defects up to high temperatures.
  
• The enhanced stability of oxygen defects in
  ceria-zirconia accounts for the improved oxygen
  storage capacity and thermal stability of
  ceria-zirconia systems.

23
Temperature dependence of the neutron diffraction
patterns
24
Temperature dependence of the crystallite size in
ceria (filled circles) and ceria-zirconia (open
diamonds)
25

• Perfect fluorite structure. All the Td sites are
  filled by oxygen ions, and all the Oh sites are
  empty.
• (b) Oxygen defects in fluorite structure. Some
  oxygen ions (filled circle) occupy the
  interstitial Oh sites, leaving vacancies in the
  Td sites (not shown). The interstitial oxygen
  ions are displaced from the centers of the
  interstitial Oh sites in the lt110gt directions.

O
Ce
In the general case, the concentration of
vacancies may exceed that of interstitial ions,
resulting in oxygen non-stoichiomety.
26
Temperature dependence of the oxygen defect
concentration. Filled circles oxygen
interstitial ions, open circles oxygen vacancies.
27
The EPR spectra obtained from the as prepared
samples at 77 K
28
CO2 output profiles in the temperature-programmed
reduction experiment using CO.
29
Temperature dependence of defect concentration in
ceria and ceria-zirconia

• High temperature treatment Ceria exhibits a
  dramatic drop in the concentrations of vacancies
  and interstitial ions, these concentrations
  remain virtually constant in ceria-zirconia.
• Interstitial oxygen ions in ceria-containing
  compounds are likely to form during sample
  processing.
• When oxygen-deficient material is oxidized to
  CeO2 or (Ce,Zr)O2, absorbed oxygen ions may at
  first enter the roomier octahedral sites, rather
  than fill the spatially tight tetrahedral sites.
• If annealing temperature is not high enough they
  may not be able to overcome a potential barrier
  to get into the regular tetrahedral sites, and
  remain in the octahedral sites.

30

• Only when the sample is treated at sufficiently
  high temperature thermally activated interstitial
  ions may enter regular tetrahedral sites and
  recombine with vacancies.
• Because of the smaller ionic radius of zirconium
  ions, mixing zirconia with ceria will reduce the
  lattice constant and produce the atomic-level
  pressure at the smaller tetrahedral sites, making
  them even more difficult to reach for the
  interstitial oxygen ions than in pure ceria.
• This may explain the enhanced stability of oxygen
  defects against thermal aging in ceria-zirconia,
  where the recombination of interstitial ions with
  vacancies may be expected to occur at higher
  temperatures compared to pure ceria.

31

• The interstitial oxygen ions are the "active"
  ions that provide necessary mobility crucial to
  the function of ceria as an oxygen storage
  medium.
• Apart from decreasing surface area the
  annihilation of the oxygen Frenkel-type defects
  might contribute to deterioration of the oxygen
  storage capacity in thermally aged automotive
  catalyst supports.
• Doping ceria with zirconia may improve the oxygen
  storage properties of ceria at three different
  levels. At the level of the microstructure, it
  inhibits surface diffusion and in turn the loss
  of surface area at high temperatures. At the
  mesoscopic level, substantial doping may result
  in the formation of an interface structure that
  facilitates the oxygen transport from bulk to the
  surface. Besides, as demonstrated by the above
  study, at the atomic-level, it stabilizes the
  oxygen defective structure.

32
Activation energy for oxygen migration as a
function of the composition
33

• Computer simulation studies further proved that,
• Ce4/Ce3 reduction energy is significantly
  reduced even by small amounts of zirconia this
  effect is magnified when the association between
  Ce3 ions and oxygen vacancies is taken into
  account, resulting in the bulk reduction energies
  becoming comparable with values calculated for
  pure ceria surfaces.
• Activation energy for oxygen migration in the
  bulk is found to be low and decreases almost
  monotonically with the zirconia content this
  indicates facile oxygen diffusion through the
  bulk catalyst.

34

• Ceria based fuel electrodes for SOFC
• The electrolyte in SOFC must consist of a good
  ion conductor and no electronic conductivity
  often YSZ is used.
• Electrodes must possess good electron
  conductivity in order to facilitate the
  electrochemical reaction and to collect the
  current from the cell.
• Anodic oxidation of the fuel (H2 or CO) can take
  place in the vicinity of the three-phase
  boundary, where oxide ions, gas molecule and
  electrons are present.
• TPB should therefore be extended.
• One way is to use mixed ionic and electronic
  conductor partially reduced ceria can be used
  as part of the SOFC anode.
• Ceria based anodes have important advantages over
  conventional Ni-based anodes ability to endure
  repetitive redoxing and ability to avoid (or
  tolerate) carbon deposition from hydrocarbon
  fuels.

35
Problems associated with ceria as anode in SOFC
and ways to overcome

• In the temp. range 700-1000 oC ceria undergoes a
  change of volume when the oxygen partial pressure
  is varied from air to that of the operating SOFC
  anode.
• The electronic conductivity of doped ceria is not
  sufficient to take care of the current collection
  in an SOFC stack.
• Sintering of doped ceria anode on YSZ electrolyte
  limits the oxide ion conductivity due to the
  radii misfit of Ce4 and Zr4.

36

• Diesel Soot Abatement Technology
• Diesel engine exhaust ? Particulate matter (soot)
  NOx
• Pt Ce fuel additives with Pt treated filter ?
  lowest temp. activity (595 K)
• The oxidation of soot with NO2 is catalyzed by
  cerium present in the activated soot and not by
  Cu (or) Fe-activated soot.
• Pt Ce
• O2 2NO ? 2NO2 soot ? 2NO CO2
• Continuously Regenerating Diesel Particulate
  Filter (CR-DPF).
• When Pt and Ce additives are applied, there is a
  synergistic effect resulting in a high oxidation
  rate.

37

• This synergy can enhance the use of the proposed
  oxidation cycle because the reactions involving
  NO are kinetically coupled.
• If the rate at which NO2 oxidize soot is high,
  the NO2 concentration is lowered, which
  facilitates the formation of NO2 from NO. At high
  NO2 concentrations, this formation is limited by
  thermodynamics.
• The resulting ash from the cerium does not plug
  the filter, in contrast to copper, where serious
  filter plugging are reported.
• When 25 ppm of Ce additive is used for a typical
  heavy duty truck, the filter will be 50 filled
  after 75,000 to 150,000 miles.
• Cu deteriote ceramic fibre-wound filters.
• Cu-regeneration problem high temp. required.

38

• Ceria based Wet-Oxidation catalyst
• Mn-CeO2 composites and Ru/CeO2 best catalysts
• The function of the wet-oxidation catalysts
  should be confined to
• Activation of O2
• Direct electron transfer with the reactants
  (redox reaction) in the first step of the
  reaction.
• Ceria seems to effectively contribute to both
  factors
• The very mobile nature of the oxygen on CeO2 is
  one of the critical causes for the high
  performance of ceria-containing wet-oxidation
  catalysts.
• The sole function of the wet-oxidation catalyst
  is to produce active radicals via interaction
  with the pollutants in the first step of the
  reaction.
• This rxn. involves free radical mechanism.

39
Ceria in catalytic combustion Noble metal
associated with ceria and ceria-zirconia are used
as catalysts Several studies showed clearly the
participation of oxygen atoms from the bulk of
ceria for both combustion of CO and HC. Ceria
stabilizes noble metal in high oxidation states
leading to the superior interaction in the case
of O-Pt-O-Ce- There are some surface oxygen
anionic vacancies. These vacancies induce the
formation of surface oxygen peroxide or
superoxide close to the metal-ceria interface and
might be the true active species. So the role of
the metal might be only that of donor/acceptor of
electrons.
40

• Fluid Catalytic Cracking
• Heavy hydrocarbons to gasoline-range hydrocarbons
• Catalyst mixture of zeolite and SiO2-Al2O3
  fast coke formation on catalyst regeneration
  required.
• If the feed contains higher sulfur content then
  part of (lt 10) sulfur remains trapped in the
  coke which builds up on the catalyst.
• This sulfur is to be oxidized to SO2/SO3 in the
  regeneration step.
• A highly effective and less costly approach is
  incorporation of SOx adsorption/reduction
  additive
• The function of this additive is to transform SOx
  back to H2S which will be treated in Claus plant.
• Commercial catalytic system Ceria/Mg-aluminate
  spinel-MgO solid solution.
• This catalyst contains basic site for SOx
  adsorption, active site for oxidation of SO2 to
  SO3 and redox properties for the conversion of
  sulfates to H2S under reducing atmosphere.
• The role of ceria in this catalytic formulation
  derives from its basic/redox character.

41
A mechanism proposed for the action of CeO2-MgO
based catalyst in the treatment pf SO2 in FCC
plants
Ceria can also have an important role in the
reduction of sulfates to give H2S Under FCC
conditions, ceria also reduce NOx emissions from
cracking unit. Here the role of ceria is to
provide oxygen vacancy for the reduction of NO to
N2.
42
de-SOx processes Ceria with its double
functionality (redox material with basic sites)
represents a more versatile solution CeO2 SO2
? Ce2O2S SO2 ? S2 (elemental sulfur) CeO2
CeO2 SO2 ? sulfated CeO2 CO (or) CH4 ? H2S
Ce2O2S 2CeO2 H2S H2 ? Ce2O2S
2H2O Ce2O2S SO2 ? 2CeO2 S2 The presence of
Cu and Ni in ceria based catalyst significantly
increases the performance at low
temperature. This may be attributed to the
promotional effect of metal on the redox activity
of ceria. Moreover the presence of metal favors
the decomposition of sulfate species and
decreases the breakthrough temperature of the
reaction. Cu is selective to S2 whereas, Ni
favors H2S.
43
Syn-gas production Reforming reaction
application in fuel cell technology Alternative
process for syn-gas production CH4 CO2 ? 2CO
2H2 CH4 0.5O2 ? CO 2H2 Ceria-zirconia based
catalysts high reducibility and oxygen storage
capacity Two pathway mechanism HC/CH4
decomposition to carbon then the carbon atom
react with oxygen from ceria based
support. Oxygen replenished by dissociation of
CO2 in dry reforming or by H2O in steam reforming
CO 2H2
CH4
H2O/O2/CO2
O
M
M
M
M-Ce-ZrO2
O2-
Ce-ZrO2