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The role of ceria in catalysis
B. Murugan
National Centre for Catalysis Research
IITM, Chennai-36.
18-12-2007
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.
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.
Applications of cerium in catalysis and chemicals
1. Fluid Catalytic Cracking – huge amounts consumed for refinery
operations – convert crude oil to lower molecular weight fractions.
2. TWC – major technological application – vehicle emission control – to
remove pollutants from vehicle (auto-exhaust) emissions – significant
portion of cerium consumed annually.
3. Oxidizing agent – potential use as additives to aid combustion – to
reduce the particle emissions from Diesel engine.
4. SOx control agent.
5. Eletrode material in SOFC.
6. EB dehydrogenation – ceria addition improves activity for styrene
formation.
7. Supports the ammoxidation of propylene to produce acrylonitriles.
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.
Crystal Structure of ceria: The Fluorite structure
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 1:2 stoichiometry is maintained.
1. We can also view the structure as a simple cubic array of
oxygen with a cerium in the center of alternate cubes.
2. Considered that way, there are obviously diagonal planes of
cubes containing no cations.
3. These planes will obviously be planes of weakness, accounting
for fluorite's excellent octahedral cleavage.
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.
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.
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.
Tetrahedral Holes in the Fluorite Structure
This illustration shows the location of the tetrahedral holes in the
fluorite structure.
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.
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.
Defect Structure of Ceria
1. Defects in ceria – intrinsic or extrinsic
2. Intrinsic defects – due to thermal disorder or by the redox process
3. Extrinsic defects – by impurities or by the introduction of aliovalent
dopents.
Three possible thermally generated intrinsic disorder in ceria
CeCe + 2 OO V’’’’Ce + 2VÖ + CeO2 E = 3.53 eV Schottky
CeCe Cei•••• + V’’’’Ce E = 3.53 eV Frenkel
OO O’’I + 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.
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 = V••o + 2Ce’Ce + 1/2O2 (gas)
In the case of H2 reduction:
Oo + 2CeCe + H2 (gas) = V••o + 2Ce’Ce + 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 = Ca’’Ce + V••o + Oo
Gd2O3 = 2Gd’Ce + V••o + 3Oo
Already existing oxide vacancies may be removed by doping with
oxides of higher valency than 4
Nb2O5 + V••o = 2Nb•
Ce+ Oo
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 = [Ce’Ce]ee + [h]eh + [VÖ]2eÖ
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 I’Ce + VÖ + Oo
VÖ + 0.5O2 Oo + 2 h h indicates an electron hole
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 > 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
Lattice Defects and Oxygen Storage Capacity of Nanocrystalline
Ceria and Ceria-Zirconia
1. Ceria-based oxides - automotive exhaust emission control systems as
catalyst supports and oxygen promoters.
2. Three-way automotive catalytic converters - oxidize CO and
hydrocarbons and at the same time reduce nitrogen oxides.
3. 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.
4. CO-NOx conversions are strongly affected by the local oxygen partial
pressure at the catalyst surface.
5. 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.
1. 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.
2. Pure ceria has a serious problem of degradation in
performance with time at elevated temperatures.
3. Traditionally, this degradation has been attributed to decrease
in its surface area and in turn its oxygen storage capacity
(OSC).
1. However, recent experimental observations on pure ceria
suggest that the surface area may not be the only parameter
that determines the effectiveness of ceria.
2. 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.
3. 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.
1. 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.
2. 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
3. Decreasing concentration of the Frenkel-type oxygen defects
at high temperatures contributes to deterioration of the
oxygen storage properties in thermally aged ceria.
1. 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.
2. 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.
3. Zirconia keeps ceria slightly reduced, and preserves oxygen
defects up to high temperatures.
4. The enhanced stability of oxygen defects in ceria-zirconia
accounts for the improved oxygen storage capacity and
thermal stability of ceria-zirconia systems.
Temperature dependence of the neutron diffraction patterns
Temperature dependence of the crystallite size in ceria
(filled circles) and ceria-zirconia (open diamonds)
(a) 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 <110>
directions.
In the general case, the concentration of vacancies may exceed that
of interstitial ions, resulting in oxygen non-stoichiomety.
O
Ce
Temperature dependence of
the oxygen defect
concentration. Filled circles:
oxygen interstitial ions, open
circles: oxygen vacancies.
The EPR spectra obtained from the as prepared samples at 77 K
CO2 output profiles in the
temperature-programmed
reduction experiment using CO.
Temperature dependence of defect concentration
in ceria and ceria-zirconia
1. High temperature treatment: Ceria exhibits a dramatic drop in
the concentrations of vacancies and interstitial ions, these
concentrations remain virtually constant in ceria-zirconia.
2. Interstitial oxygen ions in ceria-containing compounds are likely
to form during sample processing.
3. 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.
4. 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.
5. Only when the sample is treated at sufficiently high
temperature thermally activated interstitial ions may enter
regular tetrahedral sites and recombine with vacancies.
6. 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.
7. 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.
8. The interstitial oxygen ions are the "active" ions that provide
necessary mobility crucial to the function of ceria as an oxygen
storage medium.
9. 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.
10. 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.
Activation energy for oxygen migration as a function of the
composition
Computer simulation studies further proved that,
1. 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.
2. 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.
Ceria based fuel electrodes for SOFC
1. The electrolyte in SOFC must consist of a good ion conductor and no
electronic conductivity – often YSZ is used.
2. Electrodes must possess good electron conductivity in order to
facilitate the electrochemical reaction and to collect the current from
the cell.
3. 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.
4. TPB should therefore be extended.
5. One way is to use mixed ionic and electronic conductor – partially
reduced ceria can be used as part of the SOFC anode.
6. 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.
1. 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.
2. The electronic conductivity of doped ceria is not sufficient to take care of
the current collection in an SOFC stack.
3. Sintering of doped ceria anode on YSZ electrolyte – limits the oxide ion
conductivity due to the radii misfit of Ce4+ and Zr4+.
Problems associated with ceria as anode in SOFC and ways to overcome
Current collector
Ceria
YSZ-scales YSZ
Provides sufficient adhesion
Ceria thin layer – governs volume instability during redoxing
Maintains high electronic conductivity
Diesel Soot Abatement Technology
1. Diesel engine exhaust Particulate matter (soot) + NOx
2. Pt + Ce fuel additives with Pt treated filter lowest temp. activity
(595 K)
3. 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.
5. This synergy can enhance the use of the proposed oxidation cycle
because the reactions involving NO are kinetically coupled.
6. 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.
7. The resulting ash from the cerium does not plug the filter, in
contrast to copper, where serious filter plugging are reported.
8. 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.
9. Cu deteriote ceramic fibre-wound filters.
10. Cu-regeneration problem – high temp. required.
Ceria based Wet-Oxidation catalyst
Mn-CeO2 composites and Ru/CeO2 – best catalysts
The function of the wet-oxidation catalysts should be confined to
1. Activation of O2
2. 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.
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.
Fluid Catalytic Cracking
1. Heavy hydrocarbons to gasoline-range hydrocarbons
2. Catalyst: mixture of zeolite and SiO2-Al2O3 – fast coke formation on catalyst
– regeneration required.
3. If the feed contains higher sulfur content then part of (< 10%) sulfur
remains trapped in the coke which builds up on the catalyst.
4. This sulfur is to be oxidized to SO2/SO3 in the regeneration step.
5. A highly effective and less costly approach is incorporation of SOx
adsorption/reduction additive
6. The function of this additive is to transform SOx back to H2S which will be
treated in Claus plant.
7. Commercial catalytic system : Ceria/Mg-aluminate spinel-MgO solid
solution.
8. 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.
9. The role of ceria in this catalytic formulation derives from its basic/redox
character.
MgO CeO2
CeO2/Mg2Al2O5
Ce2O3
MgO CeO2
O2
MgSO4 Ce2(SO4)3
H2S
H2
SO3
SO2
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.
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.
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
M-Ce-ZrO2
Ce-ZrO2
M M M
CH4 H2O/O2/CO2
O2-
CO + 2H2
O*