An X-ray absorption spectroscopic investigation of aged automotive catalysts

A. Frennet and J.-M. Bastin (Eds.) Catalysis and Automotive Pollution Control III Studies in Surface Science and Catalysis, Vol. 96 1995 Elsevier Science B.V.

749

AN X-RAY ABSORPTION SPECTROSCOPIC INVESTIGATION OF AGED AUTOMOTIVE CATALYSTS

F. Mairea, M. Capelleb, G. Meunierb, J.F. Beziaub, D. Bazina, H. Dexperta, F. Garinc, J.L. Schmitte and G. Mairec

aLURE, bdt. 209D, Paris Sud, 91405 Orsay-cedex bpSA, Centre Technique de Belchamp, 25420 Voujeaucourt et PSA Etudes et Recherches, 78140 Vdlizy-Villacoublay cLERCSI, URA1498, CNRS-ULP-EHICS, 4 rue Blaise Pascal, 67070 Strasbourg, France

ABSTRACT

By XAS investigations of model and automotive catalysts (Pt, Rh / CeO2 / A1203) associated with analytical electron microscopy, it is shown that for both model and aged catalysts treated in air at high temperatures, true alloyed phases were formed. Despite sintering of the alloyed aggregates, the ECE Test Procedure indicated that the commercial aged catalysts were still active and gave emissions below the standard limits.

1. I N T R O D U C T I O N

A bimetallic catalyst system of industrial importance presently is the threeway catalyst (T.W.C.) used for the simultaneous conversion of carbon monoxide, hydrocarbons,and nitrogen oxides in automobile exhaust. Typical commercial T.W.C.'s contain platinum and rhodium associated with ceria and alumina. Platinum is an effective catalyst for the oxidation of CO and "HC". However, for the reduction of nitric oxide this metal is less effective [1,2]. Rhodium is the essential element in the T.W.C. for the conversion of NO into N2. Ceria promotes platinum and rhodium by preventing sintering of the catalyst particles, increasing the dispersion of the catalyst [3], providing oxygen storage by shifting between

750 Ce203 and CeO 2 when passing from fuel-rich to fuel lean conditions [4], by enhancing the water-gas shift reaction [5,6], and stabilizing the alumina support [7].During ageing the T.W.C. displays a general loss of activity. The thermally induced sintering due to elevated temperatures, and various compositions of the exhaust gas, results in the support sintering and modifications (A1203, CeO2) , in the noble metal particles coalescence or migration of oxides on the surface or in the bulk in oxidative conditions [8,9,10]. Deactivation may also occur via chemically induced poisoning, which affects active metal sites due to strong deposition or chemisorption of contaminants on the catalyst such as P, S, Pb, Zn, Ca, Si, etc. [11]. Here too, the temperature and composition of the exhaust gas affect the degree of deactivation. Over the last decade, several investigations of the deactivation processes were dedicated to the studies of noble metal particles present in "in-use" catalysts. Some studies indicate that the particles are clearly bimetallic and not separate clusters of Rh and Pt [12]. Reports of beneficial synergistic reactivity of Pt-Rh bimetallics toward catalyzing automotive exhaust reactions has motivated further research involving Ptx-Rhl_x alloys [ 13]. Studies on model surfaces of PtRh alloys (single crystals) confirmed synergies for different reactions [14,15]. The major investigations published on aged automotive catalysts were obtained for metals deposited on AI203 in the abscence of CeO2, or with simple laboratory feedstreams and accelerated catalyst ageing schedules [13,16]. Extrapolation of results obtained from laboratory reactor experiments to the actual exhaust environment should be done with caution. Commercial T.W.C.'s contain significant amounts of CeO 2. In addition to the Pt-Rh interactions, the interactions between base metal additives and the noble metals can play an important role in determining the performance and durability of T.W.C.'s. To understand the general loss of activity during ageing of automotive exhaust catalysts we used X-Ray Absorption Spectroscopy (XANES and EXAFS) allowing "in-situ" measurements to characterize flesh and vehicle-aged commercial catalysts. Complementary techniques were necessary 9 XRD, TEM, STEM, XPS in association with catalytic tests performed in a laboratory reactor working under transient conditions [16], along with various cycles done at Belchamp (PSA) under more realistic conditions. We already have made preliminary investigations by X-Ray Absorption Spectroscopy on model catalysts with high loadings of Pt and Rh/A1203, treated under elevated temperatures and various compositions of gases, showing major changes in microstructure and microchemistry" increasing particle size, presence of bimetallics or alloys, interactions between Pt, Rh and CeO2 or A1203, catalytic deactivation [17, 18]. We present in this paper results obtained for vehicle-aged, commercial

751 automotive exhaust catalysts which have been studied using the aforementioned techniques. 2. EXPERIMENTAL

2.1. catalysts Two sets of catalysts were selected for this study, "model" catalysts with high contents of noble metals supported on alumina (already discussed in reference 17) and the "commercial" catalysts furnished by PSA containing Pt, Rh and CeO2 / A1203 (fresh and aged). Model catalysts: Four supported catalysts were employed : (a) 1%Rh/y-Al203 calcined in air at 600~ (b) 10%Pt/7-AI203 reduced at 300~ in H 2, (c) 5%Pt-l%Rh/7-A1203 calcined in air at 600~ (d) 5%Pt-l%Rh/ y-Al203 calcined in air at 1600~ The catalysts were prepared by co-impregnation of 7-alumina from Jolmson Matthey with solutions of platinum sponge dissolved in HC1 and of RhC13 in H20. Calcination at 600~ did not affect greatly the structure of 7-A120 3 despite sintering, whereas at 1600~ in air the A1203 was exclusively of the alpha type as detennined by XRD. Commercial formulations: These catalysts contained Pt and Rh supported on alumina deposited in monoliths which also contained ceria. The two vehicle-aged samples A 2 and B 2 were collected after 65,000 and 103,000 customer-operated kilometers respectively, both aged catalysts contained approximately equivalent amounts of precious metals and contaminants, as listed

Table 1"Characterization data (in wt. %)for fresh and aged catalysts Catalyst

Pt

Rh

Ce

P

S

Pb

Zn

9

0.70 0.72

0.16 0.13

16.3 7.6

0.01 0.82

0.05 0.14

0.05 0.29

0.02 0.32

40 13

0.60 0.68

0.12 0.13

13.7 12.7

0.01 1.19

0.03 0.17

0.01 0.17

0.02 0.37

gw~ific area

m2/g BET A1 Fresh A2 Aged (103,000

km) B 1 Fresh B2 Aged (65,000 km)

752 in Table 1, data for the fresh catalysts A 1 and B1 are included for comparison. The powders, A 1, A 2, B1, B2, were extracted from monoliths using an "ad-hoc" tool to scratch the channels of the monoliths. The amount of catalyst needed for XAS analysis were 800 mg for the transmission mode and 2000 mg for the fluorescence mode. The samples A1, A 2 and B 1, B 2 were not obtained from the same monolith. The main difference concerned the aged A2 sample having a much lower content of Ce due to erosion of the wash-coat at the entrance of the monolith. The ECE Test Procedure indicated that both A 2 and B 2 aged catalysts were still active and trader the standard limits.

2.2 X-Ray Absorption Spectroscopy (XANES and EXAFS) X-Ray Absorption Spectroscopy is a technique in which the ejected photoelectron acts as a probe of the surrounding environment in a manner similar to electron scattering. Since the absorption edges of different elements are well separated in energy (which is the case for the LII I Pt edge (11,564 eV) and the K Rh edge (23,220 eV)) the teclmique is element specific and able to examine the surroundings of Rh or Pt in the presence of the support [19,20]. We used X-Ray Absorption Spectroscopy combining X-Ray absorption near edge structure (XANES) and extended X-Ray absorption fine structure (EXAFS) to extensively characterize our fresh and aged commercial catalysts. XANES is sensitive to the electronic data (oxidation state) and EXAFS data can provide information on bond distances, coordination numbers, disorder, and types of ligand for the first few coordination spheres [21 ]. We used the synchrotron radiation facilities of LURE (ORSAY) from the DCI storage ring running at 1.85 GeV with an average current of 250 mA. The XAS data (XANES and EXAFS) were collected on the EXAFS-4 station using a conventional step-by-step set up with a channel cut mono-chromator Si (111) for Pt and Si (311) for Rh and two ion chambers as detectors. In this work, all X-Ray absorption spectra were obtained at 25~ with the transmission detection configuration. Consequently all the catalysts listed above were studied at the LII I Pt edge but only the model catalysts were studied at the K Rh edge. The EXAFS spectra were analyzed in a conventional manner [22]. The amplitude and phase shift parameters associated with the backscattering process were extracted from reference compounds : Pt mid Rh polycrystalline foils ; PtO2 ; Rh203 ; RhC13,nH20 ; H2PtC16,6H20. The structural parameters associated with each shell in the reference compounds were obtained from crystallographic data. For consistency, the phase and amplitude functions obtained from the reference compounds were compared with those generated by Mc Kale et al. [23] using curved wave models for scatterers of the same identity.

753 We measured the variation of eri namely the Debye-Waller factor, arising from both static and dynamic disorder, compared to its value in the standard. Finally, we used a two-shell least-squares fitting procedure to extract the values of Nj(coordination numbers), Rj(bond distances) and At~i. The photoelectron mean free path F has been fixed to its value in the standard. The XAS spectra recorded with the commercial formulations correspond to an averaging along the length of the monolith.

2.3. Analytical electron microscopy Only the commercial formulations, fresh and aged, respectively A1, B1 and A2, B 2 have been characterized. For the TEM measurements, ethanol was added to a few finely ground grains of the catalyst and the resultant suspension sonicated in an ultrasound bath before deposition of a few drops on a 200 mesh copper grid coated with a holey carbon film. Bright field micrographs were recorded at accelerating voltages of 200 kV using a JEOL 2010 insmnnent equipped with a spectrometer for energy dispersion Z-MAX from TRACOR (available at the I.F.P. in Rueil-Malmaison) to differentiate Pt, Rh, Pt/Rh aggregates from CeO2, to determine the ratio Pt/Rh in the bimetallies and to localize other oxides (eg. Ba, Zr, La). Metallic particles are not visualized under 7 A.

3. RESULTS The results from different techniques will be presented in the following sections and then discussed together 3.1 XAS ANALYSIS Two kinds of information were extracted from the data. Qualitative information is gained from the change in the white line intensity. Quantitative results are extracted from the EXAFS analysis, and a common way to visualize the change of the metal environment when comparing different catalysts is to plot the magnitude of the Fourier transform (FFT) of the EXAFS oscillations. For the supported model catalysts studied it resulted that for all catalysts treated in air at 600 ~ only well dispersed oxidic phases were observed on the LIII edge of Pt and the K edge of Rh as illustrated in Figure 1 for the 5%Pt]A1203, 1%Rh]A1203 and the 5%Pt-l%Rh/A1203 catalysts treated in air at 600 ~ [17,18]. The Fourier transform curves are compared with the references PtO2 and Rh203. It appears that the contributions of the second and third shells are very weak compared to bulk PtO2 or Rh203 in agreement with the recent work of Beck et al. [24] for Rh/A1203 catalysts after treatment in high temperature oxidizing environments. The 5%Pt-1%Rh/7-A1203 catalyst treated in air at 1600 ~ reveals the formation

754 of an alloy Pt51Rh49 with a mean particle size _<10 A as deduced from EXAFS, data in agreement with XRD measurements (Pt52Rh48).The composition of the alloy determined by EXAFS and XRD seems to disagree with the microanalysis of the 5%Pt-l%Rh/AI20 3 catalyst treated in air at 1600~ for which a composition of Pt73Rh27 was expected. To account for the coincidence of EXAFS, XRD with microanalysis it is necessary to consider monometalllic phases almost atomically dispersed. In Table 2 we report quantitative EXAFS analysis data for the model catalysts indicating Pt-O and Rh-O distances higher than for the reference, materials, PtO 2 and Rh203, in the case of the catalyst treated at 1600~ in air and ascribed to metal-support interface interactions for the highly dispersed particles.

Table 2: Quantitative EXAFS analysis data for the model catalysts. Catalysts

Cxxxdin~on number 6.0

Distance

Pt LIII

Neighbour s O

Rh K

O

6.0

2.00

Pt LIII RhK

0 0

5.4 6.4

1.98 2.00

Pt LIII 5%Pt1%Rh/A120 3 air 1600 ~

O Pt Rh

1.2 2.2 2.9

2.10 2.73 2.73

RhK

O Rh Pt

1.7 2.0 3.0

2.15 2.73 2.73

Pt foil PtO 2 Rh foil Rh20 3

Pt LIII Pt LIII Rh K RhK

Pt O Rh O

12.0 6.0 12.0 6.0

2.77 2.04 2.69 2.05

5%PffA120 3 air 600 ~ 1%Rh/AI20 3 air 600 ~ 5%Pt1%Rh/AI203 air 600 ~

Edge

A 2.04

Furthermore the 5%Pt-l%Rh/AI20 3 catalyst treated in air at 1600 ~ reveals a very strong metallic character as deduced from quantitative EXAFS data and qualitative XANES spectra on both Pt LII I and K Rh edges. The Figure 2 shows that on the Rh K edge, the white line changes after normalization for Rh

755

I

10.8

r

,

10.C

5%Pt/A1203

o

'Pt02

~"

air 600~

x qa

,

i~~'.

;

i

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DISTANCE ~ x 10-1

DISTANCE A x 10-1 10.6

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1%Rh / A i 2 0 3 air 600~

X r "0

t 9.2

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Rh203

> r = =

.=

e~

r f

J

r

o

0.00

20.00

40.00

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DISTANCE ~ x 10-1

7

10.8

40.00

DISTANCE /~ x 10-1 5.9

5%Pt-1%Rh / Ai20 3

x

20.00

air 600~

5%Pt-1%Rh / A120 3 air 1600~

"o ..,= =1

r "O

0.00

20.00

40.00

DISTANCE /~ x 10-1

0.00

20.00 40.00 D I S T A N C E /~ x 10 -1

Figure 1 Model catalysts : different FFT moduli are shown. Curve (1) corresponds to the 5%Pt/A120 3 treated m air at 600 ~ (Pt LIII edge). Curve (2) to bulk Pt02. Curve (3) to the l%Rh/Al20 3 treated m air at 600 ~ (Rh K edge). Curve (4) to Rh20 3. Curve (5) to the 5%Pt-l%Rh/A120 3 treated m air at 1600 ~ (Pt L iIiedge).

metal, Rh203,

l%Rh/7-A1203 and two model catalysts. The 5%Pt-l%Rh/?A1203 catalyst treated in air at 600~ is constituted of PtOx and RhOx entities only.

756 15.00

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,,,-

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(2)

=: 0

r~

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.<

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0.00 .,

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ENERGY (eV) Figure 2 Model catalysts. Changes m the XANES (7~ edge K) as a function of the various samples : Curve (1) Rh20 3 reference. Curve (2) 1%Rh/AI203 air 600 ~ Curve (3) 5%Pt-1%Rh/AI20 3 air 600 ~ and curve (4) 5%Pt-l%Rh/AI20 3 air 1600 ~ Curve (5) Rh metal reference For the commercial formulations both the flesh and aged automotive exhaust catalysts were studied by XAS. Important differences were observed between the fresh and the aged catalysts as shown in Figure 3 where the different Fourier transform moduli are represented. Here too the A 1 and B 1 flesh catalysts are well dispersed in oxidic forms. The aged catalysts A2 and B 2 reveal great similarities, firstly an increase in the average particle size (d=20-25 A) and secondly the presence of the same alloy PtsoRh20 probably inhomogeneous and different in composition from the previous model PtslRh49 catalyst. A 2 and B 2 are constituted of bimetallic (alloy) clusters mainly in the metallic state as deduced from EXAFS and XANES. Neither a contribution from oxygen, nor cerium, atoms was necessary to improve the EXAFS fit. Complementary EXAFS data, obtained for the A 2 aged catalyst by "in situ" measurements under hydrogen at atmospheric pressure at 430~ for 4 hours, are similar to the A 2 catalyst. The quantitative EXAFS data obtained on the Pt LIII edge are indicated in Table 3.

757

~ 6.7

.

.

~, ~o.o{-

x

,

'",

9

9

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~

x

A2 A g e d

0.00

2k00

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40.00

0.00

=

DISTANCE /~ x I0 -I r

_.

20.00

40.00 DISTANCE ~ x 10-I

I

J i

;g

B2 A g e d

BI Fresh

I

[ 1 1

,

f i

L

0.00

20.00 40.00 DISTANCE ~ x I0 -1

0.00

20.00

40.00

DISTANCE /~ x 10-I

Figure 3 Commercial vehicle aged automotive exhaust catalysts ('Figure 3). FFT moduli 9 Curve (1) corresponds to the A1 fresh catalyst. Curve (2) to the A2 aged 103,000 kin. Curve (3) to the B1 fresh catalyst and Curve (4) to the B2 aged 65, 000 km.

758

Ce02 Pt/Rh

,

."

3

Figure 4 B2 aged 65, 000 km Particles of noble metals associated with CeO2G = 250k Table 3" EXAFS data for commercial formulations

Catalysts

Neighbours

A1 fresh A2 aged

O Pt Rh O Pt Rh

B1 fresh B 2 aged

Coordination number 2.5 7.0 2.0 4.2 6.0 2.0

Distance A 2.06 2.76 2.76 2.02 2.75 2.76

3.2 Electron m i c r o s c o p y analysis

The alumina supports of the flesh commercial catalysts were originally yA120 3 which became more crystallized during the ageing process 9 (8 and 0)-

759 A1203. From complementary XRD and XPS measurements CePO4 and CeA10 3 phases were observed for the aged catalysts. The sintering of the ceria particles observed for the aged catallysts A2 and B2 were similar : mean particle sizes of ceria (A) : Al=130 ; A2=320 ; BI=60 ; B2=350. Both vehicle aged catalysts display average metal particle sizes (d(A)) which are larger than those of the fi'esh catalysts, but still moderate in agreement with their catalytic activity. d(A) A 1=20 ; B I=10 (after reduction in H 2 at 300~ befor TEM measurements); A2=80 ; B2=70. For sample B2, bimetallic particles of Pt and Rh (alloy) between 30 and 300 A, have been shown by EDS to always be associated with the presence of Ce. The atomic ratio PffRh increases with the size of the particles. The average being around 1.6 for a mean particle size of 70 A. Here again as for the EXAFS data, to explain the divergence with the expected stoichiometry, one needs to assume the presence of bimodal distributions of particles containing very small particles and bigger ones (12).

4. DISCUSSIONAND CONCLUSION From the results presented in this publication three points have to be underlined" 1. In both the case of the model catalysts treated in air at high temperature, and of the commercial aged catalysts, the formation of alloyed phases are shown. 2. Some discrepancies seem to exist between XAS data (EXAFS) and TEM-EDS data concerning essentially with the size of the noble metal particles. 3. Is it necessary to establish a special relationship between the mono or bimetallic phases (Pt, Rh, Pt-Rh) and the CeOx phases in order to improve the resistance to the general loss of activity during ageing which results from the thermally induced sintering due to elevated temperatures, and under various compositions of the exhaust gas ? Considering first the formation of alloys during the ageing of the T.W.C. it is clear, in our case, that these alloy phases observed in the aged commercial catalysts A2 and B2 represent the majority of the noble metal particles even if some monometallic Pt or Rh particles are present, meaning that the remaining catalytic activity, must be attributed to the alloy phases. Thus implying that alloying is benefical in practical catalysts. But if so, the questions remain, when are the effects most noticeable?, what are the conditions? and what are the metal ratios in the alloy, as already mentioned by Joyner (25)? The answer can only come from extended work on model Pt-Rh alloys. Studies on model surfaces of Ptx Rhl_ x alloy single crystals have already confirmed synergies for different

760 reactions (14,15). Furthermore, it is worthwhile to underline the relative low sintering of these alloy particles around 30 A (EXAFS data) - 80 A (TEM data). Concerning the estimates of the mean particle size of noble metals determined from XRD, EXAFS or TEM it is necessary to consider that the various techniques used have intrinsic strengths and weaknesses as pointed out recently (26). XRD gives no indication of the range of particles sizes present in the catalyst. Very small particles are likely to produce diffraction peaks which are too broad to be observed. EXAFS is not phase-specific, but is element specific. As with XRD, the EXAFS method is insensitive to the range of particle sizes. However, XAS is well adapted for "in-situ" measurements, and provides physical and chemical information on local environments even if averaged. Electron microscopy is neither element nor phase specific, but provides a much more direct measurement of particle sizes. But it only involves the study of a relatively small proportion of the whole catalyst particle (and the limitation in the size determination is 10 A for TEM and 100 A for STEM). We consider in our case that XAS and analytical electron microscopy are complementary and that the tendancies observed with both techniques are in good agreement. Concerning any relationship between the mono-or bimetallic phases, and the CeOx phases present in the T.W.C., it is clear from the literature that ceria improves the resistance to the coalescence of the noble metal particles [28,29]. From our results, it appears for the commercial T.W.C. A2 and B2, that the noble metal particles (alloys) are always associated with CeOx as deduced from TEMEDS data (Figure 4). On the other hand it is impossible, by EXAFS, to correlate the alloy particles with the wash coat via an interface constituted of Pt-Ce or RhCe bonds which does not exclude an interface between the alloy particle and ceria via Pt-O and Rh-O bonds with oxygen from the CeOx support. In this case, due to the size of the alloy particle, EXAFS being more an average volume technique, is unable to provide evidence for the interaction with the substrate CeOx. Some results obtained recently in our laboratory by "in-situ" EXAFS measurements on fresh Pt, Rh/CeO2/A120 3 catalysts show the epitaxy of the noble metal particles on CeOx entities in agreement with recent work of Bernal et al. on ceria-supported Rh catalysts [27]. The structural nature of such epitaxial relationship might well be interpreted as a kind of SMSI [18].

ACKNOWLEDGEMENTS

The authors (M.C.b) would like to thank the staff members who contributed to the use of the TEM-EDS facilities at the Institut Fran~ais du P6trole, RueilMalmaison.

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