Copy (2) of Ceria

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The role of ceria in catalysis

B. Murugan

National Centre for Catalysis Research

IITM, Chennai-36.

18-12-2007


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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.


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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 apotential 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.


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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 toreduce the

particle emissions from Diesel engine.

4. SOxcontrol 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.


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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


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Consider the stoichiometry of single unit cell.

Each of the corner cerium ions is 1/8 inside the cell; since there areeight 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.


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1. We can also view the structure as a simple cubic array of oxygenwith 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.


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Octahedral Holes

Regardless of whether hexagonal layers arestacked 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 eightsides.


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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.


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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 vacantoctahedral holes in the fluorite

structure, outlined by the green

spheres, as seen from the top.


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Tetrahedral Holes in the Fluorite Structure

This illustration shows the location of the tetrahedral holes in the

fluorite structure.


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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.


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Technically, the descriptions of the fluorite structure

given above are inaccurate in the sense that because theoxygen 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.


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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 OOVCe + 2V + CeO2 E = 3.53 eV Schottky

CeCe Cei+ VCe E = 3.53 eV Frenkel

OOOI + V E = 3.20 eV Frenkel

From variation in E, it is evident that the predominant defect category isthe 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.


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Faber et al. examined the electron density distribution using XRD andconcluded that the amount of interstitial Ce is less than 0.1% of the totaldefect 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 withlower valencies, e.g. dissolution of CaO and Gd2O3

CaO = CaCe + Vo + Oo

Gd2O3 = 2GdCe + V

o + 3Oo

Already existing oxide vacancies may be removed by doping with oxides ofhigher valency than 4

Nb2O5 + Vo = 2NbCe+ Oo


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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 = [CeCe]ee + [h]eh + [V]2e

At high temperatures and low oxygen partial pressures, ceria behaves as an n-typesemiconductor and electrons liberated following the reduction are the primary

charge carriers.

Oo V + 2e- + 0.5O2 (g)

Transition from n-type to p-type conduction is observed at lower temperatures andhigher oxygen partial pressures near stoichiometric composition, where electronicconductivity arises from holes introduced by impurities

IO ICe + V + Oo

V + 0.5O2 Oo + 2 h

h

indicates an electron hole


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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


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Lattice Defects and Oxygen Storage Capacity of Nanocrystalline

Ceria and Ceria-Zirconia

1. Ceria-based oxides - automotive exhaust emission control systems as catalystsupports 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-NOxconversions are strongly affected by the local oxygen partial pressureat 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.


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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).


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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 fastexchange with the environment and provide OSC. Such "active"

oxygen species become deactivated following a high-temperature

treatment.


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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 theinterstitial 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.


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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 bysuppressing 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.


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Temperature dependence of the neutron diffraction patterns


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Temperature dependence of the crystallite size in ceria (filled

circles) and ceria-zirconia (open diamonds)


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(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

directions.

In the general case, the concentration of vacancies may exceed that of

interstitial ions, resulting in oxygen non-stoichiomety.

O

Ce


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Temperature dependence of the

oxygen defect concentration.

Filled circles: oxygen interstitial

ions, open circles: oxygen

vacancies.


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The EPR spectra obtained from the as prepared samples at 77 K


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CO2 output profiles in the

temperature-programmed reduction

experiment using CO.


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Temperature dependence of defect concentration in

ceria and ceria-zirconia

1. High temperature treatment: Ceria exhibits a dramatic drop in theconcentrations 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.


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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.


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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.


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Activation energy for oxygen migration as a function of the

composition


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Computer simulation studies further proved that,

1. Ce4+/Ce3+ reduction energy is significantly reduced even by smallamounts 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.


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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 electronsare 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.


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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

Providessufficientadhesion

Ceria thinlayer governsvolumeinstabilityduringredoxing

Maintains highelectronicconductivity


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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.


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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 islowered, 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.


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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.


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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


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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 SOxadsorption/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.


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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.


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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.


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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

CH4H2O/O2/CO2

O2-

CO + 2H2

O*

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Copy (2) of Ceria