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Surface chemistry and microstructural analysis ofCexZr1xO2y model catalyst surfaces

$

Alan E. Nelsona, Kirk H. Schulzb,*

aDepartment of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta., Canada T6G 2G6

bDave C. Swalm School of Chemical Engineering, Mississippi State University, P.O. Box 9595, Mississippi State, MS 39762, USA

Received 30 May 2002; received in revised form 30 May 2002; accepted 27 January 2003

Abstract

Cerium-zirconium mixed metal oxides are widely used as promoters in automotive emissions control catalyst systems (three-

way catalysts). The addition of zirconium in the cubic lattice of ceria improves the redox properties and the thermal stability,

thereby increasing the catalyst efficiency and longevity. The surface composition and availability of surface oxygen of model

ceria-zirconia catalyst promoters was considered to develop a reference for future catalytic reactivity studies. The microstructure

was characterized with X-ray diffraction (XRD) to determine the effect of zirconium substitution on crystalline structure and

grain size. Additionally, the Ce/Zr surface atomic ratio and existence of Ce3 defect sites were examined with X-ray

photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) for samples with different zirconium concentrations.

The surface composition of the model systems with respect to cerium and zirconium concentration is representative of the bulk,

indicating no appreciable surface species segregation during model catalyst preparation or exposure to ultrahigh vacuumconditions and analysis techniques. Additionally, the concentration of Ce3 defect sites was constant and independent of

composition. The quantity of surface oxygen was unaffected by electron bombardment or prolonged exposure to ultrahigh

vacuum conditions. Additionally, XRD analysis did not indicate the presence of additional crystalline phases beyond the cubic

structure for compositions from 100 to 25 at.% cerium, although additional phases may be present in undetectable quantities.

This analysis is an important initial step for determining surface reactions and pathways for the development of efficient and

sulfur-tolerant automotive emissions control catalysts.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Cerium; Zirconium; Oxide; Model; Catalyst; Emissions; Automotive

1. Introduction

Cerium oxide (CeO2) is used in automotive emis-

sions control catalysts to regulate the partial pressure

of oxygen near the catalyst surface [1,2]. Despite its

widespread use and application, pure cerium dioxide

has poor thermal stability and is known to sinter at

1123 K [3]. In order to increase its thermal stability

and ability to store and release oxygen during opera-

tion, zirconium is substituted into the cubic structure

of ceria. The addition of zirconium to the cubic

structure of ceria is reported [46] to increase the

oxygen storage capacity of the system while enhan-

cing the thermal stability under high temperatures [7],

Applied Surface Science 210 (2003) 206221

$A portion of this work was performed at Department of

Chemical Engineering, Michigan Technological University, 1400

Townsend Drive, Houghton, MI 49931, USA.* Corresponding author. Tel.: 1-662-325-2480;

fax: 1-662-325-2482.

E-mail address: [email protected] (K.H. Schulz).

0169-4332/03/$ see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0169-4332(03)00157-0


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as compared to pure ceria. Indeed, a 10 at.% zirconium

substitution in cerium oxide markedly increases the

oxygen storage capacity compared to ceria-only sys-

tems [3]. Ceria-zirconia solidsolutions are also reportedto have three to five times the oxygen storage capacity

(per gram of catalyst) than ceria-only systems [8].

Bulk reduction of pure ceria is reported to occur at

approximately 1173 K, while ceria-zirconia reduction

begins at lower temperatures near 853 K. Vliac et al.

[9] suggest the enhanced oxygen storage property of

the ceria-zirconia system is the result of enhanced

diffusion of bulk oxygen anions to the surface.

Additionally, Raman spectroscopy, X-ray diffraction

(XRD), and EXAFS analysis indicates zirconium has

a lower coordination number within the ceria lattice,

and consequently increases bulk oxygen mobility [9].

The near-neighbor zirconium oxygen atoms are not

observed in ceria-zirconia cubic matrices and their

absence is attributed to a high degree of structural

disorder. The oxygen atoms are positioned at a weak

bonding distance, resulting with increased oxygen

mobility. This is thought to be a mechanism for release

of the stress generated by the insertion of a smaller

zirconium ion into the lattice [9]. By moving the

oxygen ions to a non-bonding distance, mobile oxygen

anions are generated within the lattice.

In order to improve ceria-zirconia low-temperatureperformance and resistance to deactivation (SO2), a

fundamental understanding of the structure and sur-

face chemistry is required. This paper presents infor-

mation on the characterization of cerium-zirconium

mixed metal oxide powders and model catalysts pre-

pared via co-precipitation routines. The surface seg-

regation of cerium and zirconium, as well as the

oxidation state of the metals, was investigated with

Auger electron spectroscopy (AES) and X-ray photo-

electron spectroscopy (XPS). This information will be

useful for comparing reaction and deactivationmechanisms under oxidizing and reducing conditions

in subsequent investigations.

2. Experimental

2.1. Materials and synthesis

Several cerium-zirconium solid solution preparation

methods have been reported which produce medium

surface area mixed oxide powders suitable for auto-

motive catalystemulation, includinghydroxide, acetate,

and surfactant-assisted chloride precipitation [8,10]. In

this study, a complete range metastable cerium-zirco-nium mixed metal oxide powders (CexZr1xO2y,

1 ! x ! 0) were prepared through a hydroxide

precipitation technique reported by Hori et al. [8].

Predetermined quantities of cerium(IV) ammonium

nitrate (Alfa Aesar, CAS #16774-21-3) and zirconium

oxynitrate (Alfa Aesar, CAS #13826-66-9) precursors

are completely dissolved in deionized water with mild

heat. Upon dissolution of the precursors, excess ammo-

nium hydroxide ($100 vol.%) is added to precipitate

the cerium-zirconium mixed metal oxide powder.

The resultant precipitate is filtered in a vacuum funnel

and thoroughly washed with excess distilled water

($2 l/10 g precipitate). The ceria-zirconia powder is

allowed to dry in the hood overnight and is subse-

quently annealed at atmospheric conditions. Sample

annealing stabilizes the metal oxides and eliminates

carbonates and nitrate compounds remaining from the

preparation technique [7]. The annealed powders are

milled and stored in a dry environment. A sufficient

quantity of ceria-zirconia powder was initially pre-

pared and utilized through this research to eliminate

compositional inconsistencies that arise from multiple

preparations. The cerium-zirconium mixed oxideswere formed into model wafers using a standard

13 mm diameter FT-IR pellet die and hydraulic press.

The oxide powders (approximately 100 mg) are loaded

into the die and subjected to a pressure of 670 MPa for

a duration of 10 min. The prepared samples have a

thickness of approximately 100150 mm, depending on

the initial quantity of oxide power used.

2.2. Equipment and analysis

Surface compositional analysis was performed withAES and XPS. Auger electron spectroscopic analysis

was performed with a Physical Electronics 545

Scanning Auger Microprobe (SAM). The Auger spec-

trometer was operated with an electron gun filament

(tungsten) current of 2.0 nA and an incident electron

energy of 2.0 keV with a corresponding resolution of

0.6%. The spectrometer is operated at a background

pressure of approximately 2 1010 Torr obtained

with a 220 l/s ion-pump and titanium sublimation

pump. Sample positioning was performed with a

A.E. Nelson, K.H. Schulz / Applied Surface Science 210 (2003) 206221 207


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xyz-rotary ultrahigh vacuum sample manipulator and a

custom UHV terminal [11]. XPS was performed with

a Physical Electronics 1600 XPS Surface Analysis

System located in a dedicated stainless steel chamberused exclusively for XPS surface characterization.

The analyzer is fitted with an Omni Focus III

small-area lens that produces 800 um diameter surface

analysis area. The XPS spectra were obtained by using

an incident achromatic Mg Ka X-ray source

(1253.6 eV) operated at 300 W with a corresponding

voltage of 15 kV. Survey spectra are an average of 10

scans over a range from 0 to 1100 eV with a pass

energy of 26.95 eV. The high-resolution XPS spectra

are a composite average of 15 scans with a pass

energy of 23.5 eV at an incident sample angle of

458. The background pressure during XPS analysis

was approximately 8 109 Torr. Additionally, the

microstructure of the ceria-zirconia model catalysts

was characterized with XRD and scanning electron

microscopy (SEM).

3. Results and discussion

3.1. Effects of catalyst annealing

Several methods of model catalyst preparation wereconsidered to produce a model catalyst suitable for

ultrahigh vacuum spectroscopic characterization,

while maintaining attributes similar to automotive

catalyst systems. Specifically, the effect of annealing

conditions on the final properties of the two-dimen-

sional model catalysts was investigated. The ceria-

zirconia samples require annealing to eliminate car-

bonates and nitrates remaining from the preparation

technique [8]. However, excessive annealing could

result in particle sintering and undesirable crystallite

migration [12]. Published research of ceria-zirconiasuggests particle sintering occurs at temperatures in

excess of 773 K at prolonged exposure times. As a

result, annealing conditions of 1 h at 773 K in atmo-

sphere were used throughout the experiments. Two

distinct preparation routines were investigated to dis-

cern the effects of annealing on the final two-dimen-

sional model catalyst properties. The first set of model

catalysts was prepared from the dry cerium-zirconium

mixed oxide powder followed by wafer formation

and subsequent sample annealing. The second set of

samples was prepared from annealed oxide powder,

which was formed into the model catalyst systems.

The SEM micrographs of each preparation method

are located in Fig. 1. The images clearly indicate thesamples prepared with annealed oxide powder directly

formed into the model catalysts have less surface

roughness and irregularity (Fig. 1b), compared to

samples prepared with dry mixed oxide powder fol-

lowed by wafer formation and subsequent annealing

(Fig. 1a). As a result of the SEM analysis, the pre-

paration method of annealing the oxide powders at

773 K for 1 h followed by subsequent milling and two-

dimensional wafer formation was adopted. Addition-

ally, the apparent bulk density of the model catalysts

was characterized with a modified Archimedes density

measurement method to ensure adequate density and

validate the model catalyst systems further (Table 1).

The measured density of CeO2 model systems is 95%

of theoretical compared to a ZrO2 density of 85% of

theoretical. The global average for all prepared and

analyzed samples is 89% of theoretical. Considering

alumina with a bulk density of 97% of theoretical is

sufficient for ultrahigh vacuum applications [13], the

densities of the model systems are adequate to limit

the extent of diffusional processes which arise in

desorption spectrocscopies.

3.2. Crystalline structure and grain size

The phase diagram for intermediate cerium-zirco-

nium oxides is the subject of debate due to reported

phases of metastable tetragonal crystallinity [14,15].

Indeed, several publications have indicated the

presence of three tetragonal phases, including t

(2040 at.% cerium), t0 (4065 at.% cerium), and t00

(6580 at.% cerium) over the intermediate composi-

tional range [5,1618]. However, the t00 phase exhibits

no tetragonality and the t0

phase is a metastable structureformed through a diffusionless transition [18]. Hori

etal. [8] reported cerium-zirconium mixed metal oxides

in excess of 50 at.% cerium prepared via similar

co-precipitation routines and aged at 1273 K exist as

a cubic (fluorite) solid solution. They also report the

presence of a separate tetragonal phase (zirconium rich)

for compositions less than 49 at.% cerium.

The XRD patterns of the prepared cerium-zirco-

nium oxides are shown in Fig. 2 for comparison. The

reference pattern (PDF 34-0394) for a standard cubic

208 A.E. Nelson, K.H. Schulz / Applied Surface Science 210 (2003) 206221


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CeO2 sample, shown as vertical dotted lines, is super-

imposed on the XRD patterns. The pattern of the

100 at.% CeO2 sample matches with the reference

data, verifying the cubic crystalline structure (CaF2)

for the sample. As the amount of CeO2 in the mixedoxide decreases, the positions of the peaks of spectra

shift farther away from those of pure CeO2 and the

peaks also broaden (FWHM) as a function of decreased

cerium loading. Duwez and Odell [14] and other

researchers have also reported similar results. Another

important observation from the XRD patterns is that a

cubic solid solution is still maintained up to 25 at.% of

cerium oxide (75 at.% zirconium oxide). There is no

indication from XRD analysis of a separate tetragonal

ZrO2 phase under any composition. In previous studies,

mixed oxides prepared via the same route could form asolid cubic solution up to only 50 at.% cerium [8].

These results could be attributed to the zirconium oxide

precursor used in the preparation of the mixed oxides

and the preparation method itself, as the crystalline

structure of the final mixed oxide is highly sensitive to

these two factors. The pure zirconium oxide pattern is

Table 1

Bulk density and XRD

Bulk density X-ray diffraction (XRD)

Theoretical

(g cm3

)

Archimedes

(g cm3

) 0.08

Cubic lattice

parameter (A) 0.005

Scherrer grain

size (A) 0.05

Ce1.0Zr0.0O2y 7.13 6.75 5.413 109.2

Ce0.9Zr0.1O2y 7.00 6.36 5.390 70.62

Ce0.8Zr0.2O2y 6.86 6.15 5.371 61.28

Ce0.7Zr0.3O2y 6.71 5.89 5.353 50.89

Ce0.6Zr0.4O2y 6.57 5.80 5.328 51.32

Ce0.5Zr0.5O2y 6.42 5.68 5.301 46.82

Ce0.25Zr0.75O2y 6.02 5.23 5.219 58.70

Ce0.0Zr1.0O2y 5.60 4.83 Monoclinic 148.8

Fig. 1. SEM micrographs of prepared CeO2 model catalyst surfaces. The micrographs represent two evaluated preparation methods, including

preparation with dry mixed oxide powder followed by wafer formation and subsequent calcination (a), and preparation with calcined oxide

powder directly formed into the model catalysts (b). The samples were imaged at 1000 magnification.



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identified as the monoclinic ZrO2 spectra. This is in

agreement with the reported phase diagrams [14,15]

for a cerium-zirconium mixed oxide system at ambient

conditions. The shifted peak position as a function of

composition indicates a change in the cubic lattice

parameter of the system [8,19]. As a result, X-ray

powder diffraction characterization of the fresh oxide

powders indicates a cubic solid solution from 100 to

25 at.% cerium and a monoclinic structure for pure

zirconia. While our XRD analysis indicates a cubic

structure over the intermediate composition range

(25100 at.% cerium), we acknowledge the limitations

of XRD and the possibility of intermediate tetragonal

phases at high zirconium concentrations.

The lattice parameters and grain sizes were calcu-

lated [19] from the diffraction patterns as a function of

sample composition (Table 1). The data indicates the

lattice parameter, a0, follows a downward trend with

increasing amount of zirconium incorporated into

the mixed oxides. This agrees well with Vegards

Fig. 2. XRD patterns of cerium-zirconium oxides. The reference pattern (PDF 34-0394) for a standard CeO2 cubic structure is superimposed

on the XRD pattern as vertical lines. The data indicates a cubic solid solution from 100 to 25 at.% cerium and a monoclinic structure for pure

zirconia.



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law, which states that the lattice parameter of a solid

solution is directly proportional to the atomic percent

solute present [19]. Zirconium is reported to have an

atomic radius ratio (rcation/ranion) of 0.59 compared to0.68 for cerium [20]. This implies that zirconium has a

smaller ionic radius and would result in a reduction

of the lattice parameter of the crystalline system as

Ce4 ions are being substituted by Zr4 ions. This

observation is consistent with the calculated cubic

lattice parameters. The lattice parameter of pure cer-

ium oxide, CeO2, isfound to be 5.413 A, a value which

is comparable to that reported in a previous study [14].

The data also indicates the grain size decreases gra-

dually with increasing amounts of zirconium.

3.3. AES analysis of surface composition

The chemical composition of the prepared model

catalysts was analyzed with AES to characterize sur-

face species segregation compared with the bulk

composition [21]. Initially, the effects of ultrahigh

vacuum conditions and electron beam bombardment

were examined to determine the potential impacts on

surface oxygen and surface reduction. A 100 at.%

cerium oxide sample was degassed under high vacuum

conditions for 1 h and subsequently transferred to the

ultrahigh vacuum chamber for Auger spectroscopiccharacterization. Following the collection of an initial

Auger spectrum (1 h), the sample remained is position

for an additional 3 h, as to bombard the surface with

the incident electron source. The sample was again

analyzed at intervals of 28 and 96 h of ultrahigh

vacuum exposure. The specimen was not exposed

to additional electron bombardment except for sample

alignment and data collection. The collected Auger

spectra were normalized with respect to the cerium

MNN transition and the amplitude of the oxygen KLL

transition was determined (Table 2). Based on the

quantification of the oxygen KLL transition ampli-

tude, it is evident that surface oxygen remains constant

for excess exposures to ultrahigh vacuum conditions.

Additionally, no appreciable reduction is surfaceoxygen content occurs as a result of electron bombard-

ment. If any surface oxygen is removed as a result of

beam bombardment or ultrahigh vacuum exposure, it

occurs during initial degassing or sample positioning.

The zirconium MNN transition amplitudes for each

composition were numerically averaged (n 7) and

graphed as a function of bulk composition (Fig. 3).

Qualitative analysis of the zirconium peak amplitudes

indicates a linear correlation with respect to bulk

composition (R2 0:9924). This regressed slope

value indicates that cerium is more sensitive to the

Auger electron process than zirconium for the com-

positions considered and analyzer used. This observa-

tion is in agreement with published sensitivity factors

for Auger spectroscopic analysis [22]. The Auger

analysis of the mixed oxides indicates the surface

composition is representative of the bulk regardless

of cerium-zirconium concentrations. In addition to

zirconium MNN peak intensity correlation, additional

numerical analysis was performed using the oxygen

KLL transition. Similar to the zirconium peak inten-

sity analysis, the amplitude of the oxygen KLL

(513 eV) transition was quantified and correlated tobulk mixed oxide composition (Fig. 4). The oxygen

KLL peak intensity analysis indicates a non-linear

relationship with composition, decreasing with increas-

ing zirconium concentration. This decreasing trend

in surface oxygen concentration with increasing zirco-

nium substitution is suggested to result from surface

contamination, as opposed to surface oxidation state

and oxygen diffusion.

3.4. XPS analysis of surface composition

The catalyst systems were further analyzed with

XPS to verify surface composition and elemental

oxidation states. Due to observed charging effects

during XPS analysis, the binding energy scale was

calibrated using adventitious carbon (285.4 eV) [23].

This resulted in a binding energy correction of 6 to

10 eV for each spectrum. The spectral features were

fitted with Gaussian distributions and the peak posi-

tions and areas were determined (Table 3). The Ce 3d

Gaussian peak fits corresponding to Ce3 and Ce4

Table 2

Effect of ultrahigh vacuum on ceria surface oxygen

Time (h) Oxygen KLL (513 eV) transition amplitude

1 5570 50

4a 5500 50

28 5520 50

96 5530 50

a Sample was bombarded with incident electron beam for 3 h.



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states are based on published CeO2 XPS analysis by

Burroughs et al. [24] and Pfau and Schierbaum [25].

However, due to the complex electronic structure,

the absolute distinction between 3d94f2Vn1 and

3d94f2Vn2 final states for Ce3 (v0, u0) and Ce4

(v, u) could not be resolved. As a result, the high-

resolution spectra for the Ce 3d3/2 and Ce 3d5/2 ioni-

zation features were numerically fitted with eightGaussian distributions representing the initial and final

states in Ce 3d core level X-ray photoelectron spectra

(Fig. 5). A composite spectrum of the Ce 3d3/2 and Ce

3d5/2 ionization features as a function of composition

is shown in Fig. 6.

The bands labeled v collectively represent the Ce

3d5/2 ionization, while bands labeled u represent the

Ce 3d3/2 ionization. The bands with unprimed labels

represent the primary Ce 3d5/2 and Ce 3d3/2 transi-

tions, while the primed labels represent ionization

satellite features. Specifically, the band (u0, u) located

at (901.0901.4 eV) is the Ce 3d3/2 ionization and

the band (v0, v) located at (882.5882.8 eV) is the Ce

3d5/2 ionization for Ce3 and Ce4 [2426]. The bands

labeled v0 (885.5885.8 eV), v00 (889.0889.3 eV) and

v000 (898.3898.6 eV) are satellites arising from the Ce

3d5/2 ionization, while bands u0 (904.0904.3 eV), u00

(907.4907.7 eV), u000

(916.7916.9 eV) are satellitesarising from the Ce 3d3/2 ionization [24,25]. The Ce

3d3/2 and Ce 3d5/2 peak areas and amplitudes increased

as a function of increasing cerium concentration.

This is attributed to increasing cerium concentration,

as the amplitude and area of XPS photoemission

features are proportional to surface composition

[27]. The difference in Ce 3d3/2 and Ce 3d5/2 binding

energies is also in agreement with an expected value

of 18.6 eV [24]. Examination of the Ce 3d3/2 and

Ce 3d5/2 photoemission features indicates a slight

Fig. 3. Zirconium AES peak intensity analysis. The zirconium MNN transition amplitudes for each composition were numerically averagedover seven data points. Linear regression analysis (zero constant) performed with the zirconium peak amplitude as a function of composition

produces a correlation with an R2 value of 0.9924.



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chemical shift to a higher binding energy (0.30.4 eV)

from 100 to 25 at.% cerium. Although a slight binding

energy shift is observed with changing composition,

the shift is suggested to arise from the changing

surface electronic structure independent of oxidation

state.

To verify the surface oxidation state as a function

of composition, the bands representative of Ce3 and

Fig. 4. Oxygen AES peak intensity analysis. The amplitude of the oxygen KLL (513 eV) transition was averaged (n 7) for eachcomposition and correlated to bulk mixed oxide composition. The oxygen KLL peak intensity analysis indicates a non-linear relationship with

composition, decreasing with increasing zirconium concentration.

Table 3

Summary of principle XPS binding energies

Photoemission binding energy (eV)

Ce 3d5/2 Ce 3d3/2 Zr 3d5/2 Zr 3d3/2 O 1s C 1s

Ce1.0Zr0.0O2y 882.5 901.0 529.6 285.4

Ce0.9Zr0.1O2y 882.6 901.0 181.5 184.0 529.6 285.4

Ce0.8Zr0.2O2y 882.7 901.2 182.4 184.9 529.8 285.4

Ce0.7Zr0.3O2y 882.6 901.1 182.4 184.8 529.7 285.4

Ce0.6Zr0.4O2y 882.7 901.3 182.4 184.8 529.9 285.4

Ce0.5Zr0.5O2y 882.7 901.2 182.3 184.8 529.9 285.4

Ce0.25Zr0.75O2y 882.8 901.4 182.4 184.8 530.0 285.4

Ce0.0Zr1.0O2y 182.4 184.9 530.1 285.4



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Ce4 ions were further studied. The bands labeled u0

and v0 represent the 3d104f1 initial electronic state

corresponding to Ce3, while the peaks labeled u000

and v000 represent the 3d104f0 state of Ce4 ions [25].

Analysis of the peak areas associated with the repre-sentative Ce3 (u0, v0) and Ce4 (u000, v000) features

indicates a constant concentration of Ce3 defect sites

on the surface independent of zirconium substitution.

Because the surface composition is representative of

the bulk, this suggests a constant cerium surface

oxidation state independent of composition. Addition-

ally, the intensities of the Ce3 emissions (u0, v0) are

relatively small compared to Ce4 emissions (u000, v000),

which are in relative proportion to highly oxidized

cerium oxide. Considering previously published ceria

XPS spectra [25], the intensities of the Ce4 emissions

are consistent with surface oxygen stoichometry of

(y 0:00:08) for CexZr1xO2y.

The narrow scan spectra of the Zr 3d3/2 and Zr 3d5/2

features were deconvoluted with two Gaussian dis-tributions representing the primary Zr 3d5/2 and Zr3d3/

2 features. A composite spectrum of the Zr 3d3/2 and

Zr 3d3/2 ionization features as a function of composi-

tion is shown in Fig. 7. The ionization features are

distinct and uninfluenced by satellite features. The

Zr 3d5/2 ionization feature increases from 181.5 to

182.4 eV, while the Zr 3d3/2 ionization feature

increases from 184.0 to 184.9 eV as zirconium con-

centration increases. However, unlike the gradual

increase in binding energy observed with cerium

Fig. 5. Ce 3d photoemission Gaussian peakfit for CeO2. The analyzed region of cerium illustrates the complex satellite structure arising from

a mixture of multielectron and multiplet interactions. The bands labeled v collectively represent the Ce 3d5/2 ionization, while bands labeled u

represent the Ce 3d3/2 ionization. Additionally, the bands with unprimed labels represent the primary Ce 3d 5/2 and Ce 3d3/2 transitions, while

the primed labels represent ionization satellite features.



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ratio expected on the basis of J multiplicities [24,27].

The peak areas and amplitudes increase as a function

of increasing zirconium (decreasing cerium) concen-

tration.

The high-resolution spectrum for the O 1s ioniza-

tion feature was numerically fitted with three Gaussian

features representing the primary O 1s ionization

feature and chemically shifted O 1s features from

chemisorbed surface species [23,28,29]. The analysis

of the O 1s photoemission feature results in the

deconvolution of three O 1s ionization binding ener-

gies, as supported by independent analysis of zirco-

nium oxide [23]. The O 1s Gaussian peak fit analysis

for pure ZrO2 is shown in Fig. 8, and the O 1s feature

as a function of composition is located in Fig. 9. The

primary band (529.6530.1 eV) represents the O 1s

ionization for oxygen associated with the cerium-

zirconium complex [23,28,29], while the additional

Fig. 7. Zr 3d high resolution XPS analysis. The spectra for Zr 3d photoemission includes two distinct features corresponding to Zr 3d3/2 and

Zr 3d5/2 ionization bands. The analysis indicates an initial shift to a higher zirconium binding energy between 90 and 80 at.% cerium, followed

by a region of constant binding energy.



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accounted for by the changing surface electronic

structure and the binding energies associated with

adsorbed surface water and CO species, respectively.

Additional analysis of the O 1s feature also suggests

a low concentration of Ce3 surface defect sites,

further supporting the conclusion of a highly oxidized

surface. The presence of Ce3 surface defect sites

results in an additional O 1s band with a core level

shift of2.4 eV [25]. The absence of an O 1s band

2.4 eV higher than the binding energy for lattice

oxygen atoms (ca. 529.8 eV) indicates a relatively

low concentration of Ce3 ions, suggesting a near-

stoichometric surface.

3.5. XPS peak area correlation

The peak areas for the dominant photoemission

features were calculated using an adjusted baseline

relative to the signal background. In this analysis, the

primary photoemission features included the Ce 3d3/2,

Fig. 9. O 1s high resolution XPS spectra. The O 1s photoemission features are represented as a function of composition. The spectra suggest a

slight compositional shift in binding energy, and also indicate secondary O 1s bands suggested to arise from surface (OH) and (CO) species.



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Zr 3d3/2, O 1s and C 1s bands. Surface atomic con-

centrations and atomic ratios calculated from these

core level transitions are located in Table 4. The

cerium to zirconium atomic ratio is in agreement

with the expected ratios calculated form the bulk

composition, as indicated by the values in parentheses.

Additional qualitative information regarding the sur-

face oxidation state was also extracted from the XPS

atomic concentration analysis. The ratio of the oxygen

concentration to the summation of the cerium and

zirconium concentrations was calculated and com-

pared to the expected ratio from the bulk composition.

The value ranged from 2.2 to 2.7, with a value of 2indicating a fully oxidized surface and a correspond-

ing oxidation state of Ce4. Analysis clearly indicates

an excess quantity of surface oxygen with regards to

stoichometric cerium and zirconium concentrations. A

similar observation of excess surface oxygen was also

reported in an independent study of ceria-zirconia

systems [32] and is attributed to the high concentration

of surface oxygen as an adsorbed layer of oxidized

carbon species (CO, CO2) or water. This is further

supported in this analysis by the detection of surface

carbon species.The area of the Zr 3d3/2 peak was also considered to

estimate the surface composition of the model catalyst

systems. The calculated Zr 3d3/2 peak areas were

linearly regressed as a function of zirconium concen-

tration to correlate zirconium photoemission peak

areas to bulk composition. The analysis produced

a linear relationship with bulk composition, as indi-

cated by an R2 value of 0.9852. The analysis provides

excellent correlation between XPS analysis of surface

compositions and the actual bulk composition.

3.6. Effect of electron bombardment on

surface composition

The effect of electron bombardment on ceria-

zirconia model catalysts was examined to determine

impact on surface oxygen and surface reduction. A

100 at.% cerium oxide sample was initially degassed

under high vacuum for 1 h and transferred to the

ultrahigh vacuum chamber for XPS characterization.

Following the collection of initial XPS data, the

sample was bombarded with an electron source for

2 h and subsequently analyzed with XPS. The sample

was bombarded for an additional 16 h before a final setof XPS data was collected.

Two sets of XPS data were collected at different

spectrometer angles to discern between surface and

bulk species. The data included a 458 and 158 acquisi-

tion angle to determine elemental compositions to

depths of approximately 2535 and 1015 A, respec-

tively. The 458 acquisition angle data is more repre-

sentative of the bulk composition, while the 158

acquisition angle is limited to surface compositions.

The data obtained at each spectrometer angle were

analyzed with peakfit routines and the surface atomicconcentrations were calculated (Table 5). Based on the

quantification of the oxygen and carbon photoemis-

sion features, it is evident that electron bombardment

is responsible for removing adsorbed surface carbon

contaminants. This is clearly evidenced by the reduc-

tion of surface carbon at a 458 analysis angle, and

enhanced at a 158 spectrometer angle. The decrease in

the carbon to oxygen and carbon to cerium ratios as

a function of electron beam exposure also supports

the observation of a reduction of adsorbed surface

Table 4

XPS elemental surface concentrations

Atomic concentrations ( 0.5) Atomic ratios

Ce (at.%) Zr (at.%) O (at.%) C (at.%) Ce/Zr O/(CeZr)

Ce1.0Zr0.0O2y 19.4 0.0 49.1 31.5 2.5

Ce0.9Zr0.1O2y 17.1 1.6 49.6 31.6 10.7 (9.0) 2.7

Ce0.8Zr0.2O2y 14.2 3.1 46.5 36.1 4.6 (4.0) 2.7

Ce0.7Zr0.3O2y 16.8 4.9 48.3 30.0 3.4 (2.3) 2.2

Ce0.6Zr0.4O2y 12.8 8.3 50.2 28.6 1.5 (1.5) 2.4

Ce0.5Zr0.5O2y 10.3 10.2 50.2 29.3 1.0 (1.0) 2.4

Ce0.25Zr0.75O2y 8.0 15.0 51.3 25.6 0.5 (0.3) 2.2

Ce0.0Zr1.0O2y 0.0 24.6 56.8 18.6 0.0 (0.0) 2.3



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carbon contaminants. The ratio of oxygen to cerium is

relatively constant as a function of electron beam

exposure at 458, while 158 angle analysis indicates

a decrease as a function of electron beam bombard-

ment. This might suggest a slight cerium reduction at

the gas phase boundary, although the 458 acquisition

indicates a relatively constant near surface oxygen to

cerium ratio. A constant cerium oxidation state as a

function of electron beam exposure is also supported

by the analysis of the Ce3d3/2 binding energy. As a

result, it is suggested the surface composition with

respect to cerium oxidation state is uninfluenced by

electron bombardment, while the removal of adsorbed

surface carbon species is evidenced.

4. Conclusions

The chemical composition and availability of sur-

face oxygen of ceria-zirconia model automotive emis-

sions control catalyst promoters were considered to

develop a reference for future catalytic reactivity ana-

lysis. The XPS and AES analysis of ceria-zirconia

model catalysts clearly indicate (1) no significant sur-

face species segregation occurs, confirming the surface

composition is representative of the bulk; (2) theconcentration of Ce3 surface defect sites is constant

and independent of composition, suggesting incorpora-

tion of zirconium into the ceria lattice does not inher-

ently create additional defect sites; (3) the surface

chemistry of the oxides is unaffected by prolonged

exposure to ultrahigh vacuum or electron bombard-

ment; (4) the model catalyst surface stoichometry is

consistent with (y 0:00:08) for CexZr1xO2y.

Additionally, XRD analysis did not indicate the pre-

sence of additional crystalline phases beyond the cubic

structure for compositions from 100 to 25 at.% cerium,

although the existence of an additional phase is pos-

sible. This analysis is an important initial step for

determining surface reactions and pathways for the

development of efficient and sulfur-tolerant automo-

tive emissions control catalysts.

Acknowledgements

The authors gratefully acknowledge the assistance

of Melissa K. Graves in the Department of Chemical

Engineering at Mississippi State University for XPS

analysis. The authors would also like to acknowledge

financial support from the Ford Motor Company.Additionally, A.E. Nelson would like to acknowledge

additional support from the Interdisciplinary Center for

Advanced Propulsion (ICAP) at Michigan Technolo-

gical University through the Department of Energy

(DOE) Graduate Automotive Education Technology

(GATE) program.

References

[1] K.C. Taylor, Catal. Rev. Sci. Eng. 35 (1993) 457.[2] A. Trovarelli, Catal. Rev. Sci. Eng. 38 (1996) 439.

[3] J.-P. Cuif, S. Deutsch, M. Marczi, H.-W. Jen, G.W. Graham,

W. Chun, R.W. McCabe, SAE Technical Paper Series 980668,

1998.

[4] J.R. Gonzalez-Velasco, M.A. Gutierrez-Ortiz, J.-L. Marc, J.A.

Botas, M.P. Gonzalez-Marcos, G. Blanchard, Appl. Catal. B.

Environ. 22 (1999) 167.

[5] P. Fornasiero, G. Balducci, R. Di Monte, J. Kaspar, V. Sergo,

G. Gubitosa, A. Ferrero,M.J.Graziani, J. Catal. 164 (1996)173.

[6] H.-W. Jen, G.W. Graham, W. Chun, R.W. McCabe, J. Culf, S.

Deutsch, M. Marczi, SAE Technical Paper Series 980668,

1998.

Table 5

Effect of electron bombardment of surface species

Atomic concentrations ( 0.5) Atomic ratios

Ce (at.%) O (at.%) C (at.%) O/Ce C/O C/Ce

458 As prepared 19.1 50.5 30.4 2.6 0.60 1.59

2 (h) 17.8 58.0 24.2 2.1 0.42 1.34

16(h) 23.2 54.6 22.2 2.3 0.41 0.97

158 As prepared 10.9 50.1 39.0 4.6 0.78 3.58

2 (h) 22.5 55.4 22.2 2.5 0.40 0.99

16 (h) 32.1 54.6 13.4 1.7 0.25 0.42



16/16

[7] J.R. Gonzalez-Velasco, M.A. Gutierrez-Ortiz, J.-L. Marc, J.A.

Botas, M.P. Gonzalez-Marcos, G. Blanchard, Appl. Catal. B.

Environ. 35 (2000) 19.

[8] C.E. Hori, H. Permana, K.Y.S. Ng, A. Brenner, K. More,

K.M. Rahmoeller, D. Belton, Appl. Catal. B. Environ. 16(1998) 105.

[9] G. Vliac, R. Di Monte, P. Fornasiero, E. Fonda, J. Kaspar, M.

Graziani, Studies in Surface Science and Catalysis: Catalysis

and Automotive Pollution Control IV, vol. 116, Elsevier,

Amsterdam, 1998.

[10] D. Terribile, A. Trovarelli, J. Llorca, C. de Leitenburg, G.

Dolcetti, Catal. Today 43 (1998) 79.

[11] S.L. Peterson, K.H. Schulz, C.A. Schulz Jr., J.M. Vohs, Rev.

Sci. Instrum. 66 (1995) 3048.

[12] C.H. Bartholomew, Chem. Eng. 11 (1984) 96.

[13] Accuratus Ceramic Technical Bulletin, Accuratus Ceramic

Corporation, Washington, NJ, 1999.

[14] P. Duwez, F. Odell, J. Am. Ceram. Soc. 33 (1950) 274.

[15] E. Tani, M. Yoshimura, S. Somiya, J. Am. Ceram. Soc. 66

(1983) 506.

[16] M. Yashima, H. Arashi, M. Kakihana, M. Yoshimura, J. Am.

Ceram. Soc. 77 (1994) 1067.

[17] M. Yashima, K. Morimoto, N. Ishizawa, M. Yoshimura, J.

Am. Ceram. Soc. 76 (1993) 1745.

[18] M. Yashima, K. Morimoto, N. Ishizawa, M. Yoshimura, J.

Am. Ceram. Soc. 76 (1993) 2865.

[19] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley,

Reading, MA, 1956.

[20] R.P. Ingel, P. Lewis, B.A. Bender, R.W. Rice, Adv. Ceram. 12

(1984) 408.

[21] A.E. Nelson, K.H. Schulz, Surf. Sci. Spectra 7 (2000) 281.

[22] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach,

R.E. Weber, Handbook of Auger Electron Spectroscopy,Perkin-Elmer Corporation, Eden Prairie, MN, 1978.

[23] B.V. Crist, XPS Handbook of the Elements and Native

Oxides, XPS International, Ames, IA, 1999.

[24] P. Burroughs, A. Hamnett, A. Orchard, G. Thornton, J. Chem.

Soc., Dalton Trans. (1976) 1686.

[25] A. Pfau, K.D. Schierbaum, Surf. Sci. 321 (1994) 71.

[26] T.L. Barr, C.E. Fries, F. Cariati, J.C.J. Bart, N. Giordano, J.

Chem. Soc., Dalton Trans. (1983) 1825.

[27] D. Briggs, M.P. Seah, Practical Surface Analysis by Auger

and X-ray Photoelectron Spectroscopy, Wiley, New York, 1983.

[28] J. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, in: J.

Chastain (Ed.), Handbook of X-ray Photoelectron Spectro-

scopy, second ed., Perkin-Elmer Corporation, Eden Prairie,

MN, 1992.

[29] A. Platau, L.I. Johansson, A.L. Hagstrom, S.-E. Karlsson,

S.B.M. Hagstrom, Surf. Sci. 63 (1977) 153.

[30] C.D. Wagner, D.A. Zatko, R.H. Raymond, Anal. Chem. 52

(1980) 1445.

[31] D. Nordfors, A. Nilsson, N. Martensson, S. Svensson, U.

Gelius, H. Agren, J. Electron Spectrosc. Relat. Phenom. 56

(1991) 117.

[32] A. Galtayries, R. Sporken, J. Riga, G. Blanchard, R. Caudano,

J. Electon Spectrosc. Relat. Phenom. 88 (1998) 951.


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