Chapter 8 The role of cerium-based oxides used as oxygen storage materials in DeNOx catalysis

Chapter 8 THE ROLE OF CERIUM-BASED OXIDES USED AS OXYGEN STORAGE MATERIALS IN DeNOx CATALYSIS X. Courtois, N. B ion, P. Marrcot and D. Duprez* LACCO, Laboratoire de Catalyse en Chimie Organique, CNRS & University of Poitiers, 40 Av. Recteur Pineau 86022 Poitiers Cedex, France *Corresponding author: LACCO, Laboratoire de Catalyse en Chimie Organique, CNRS & University of Poitiers, 40 Av. Recteur Pineau 86022 Poitiers Cedex, France. TeL: +33 5 49 45 39 98, Fax.: +33 5 49 45 34 99, E-mail: daniel.duprez@ univ-poitiers.fr

Abstract Materials with high 'oxygen storage capacity' (OSC) are now widely used in automotive converter catalysts. They are mainly composed of Ce-based oxides (CeZrOx, CeZrPrOx, etc.) having both multiple cationic valencies and oxygen vacancies. These properties allow the catalyst to store active O species (O2-, superoxide, etc.) in 02 excess and to release them when the 02 concentration in gas phase decreases or becomes nil. After having briefly examined the main properties of these OSC materials and the methods employed for their characterization, their impact in automotive catalysis will be reviewed, with a special insight in DeNO x catalysis: (1) in three-way catalysis (2) in automotive catalysis under lean conditions (lean-burn spark ignition engine and diesel).

1. I N T R O D U C T I O N Use of oxygen storage components in three-way catalysis (TWC) was proposed by Ghandi et aL in 1976 [1] and implemented in the real exhaust catalysts at the beginning of the 1980s. Since the pioneer's work of Yao and Yu Yao [2] and of Su et al. [3,4], numerous studies were devoted to a better knowledge of oxygen storage capacity (OSC) properties of ceria-based compounds [5,6] and, specially, to CexZrl_xO 2 mixed oxides [7-10]. OSC compounds are known to enlarge the 'operating window' of TWC, i.e. the A range within which CO and HC oxidation as well as NOx reduction can occur at the optimal rate (higher than 90% conversion [ 11 ]). Although it can be inferred that OSC materials exhibit the highest impact under transient conditions, numerous investigations were carried out at the stationary state. However, kinetic studies carried out at various concentrations may give an idea of what can occur in transient regime.

Past and Present in DeNO x Catalysis

P. Granger and V.L Pdlrvulescu (Editors)

9 2007 Published by Elsevier B.V.

236

2. O S C M E A S U R E M E N T S

Past and Present in D e N O x Catalysis

AND OXYGEN

MOBILITY

2.1. OSC measurements Oxygen storage capacity is generally measured in a pulse chromatographic reactor using CO [12-16] or, to a lesser extent H 2 [17] as reducers. The procedure is depicted in Figure 8.1. Pulses of CO (or H2) are injected over the pre-oxidized sample. According to the nomenclature of Yao and Yu Yao, the CO2 produced upon the first CO pulse allows to calculate the OSC while the total amount of CO2 formed upon several CO pulses (typically 10 pulses) gives the 'oxygen storage capacity, complete' (OSCC). In most cases, only OSC is retained for characterizing TW catalysts [ 18]. Figure 8.1 shows the typical behavior of a ceria-based sample. As a rule: (1) The reduction phase (phase 1) is slower than the re-oxidation one (phase 2). The CO2 formation decreases regularly upon each CO pulse while the re-oxidation is achieved upon the first pulse of 02. This is a rather general phenomenon in catalysis. Oxides (like rare-earth oxides) reduced more slowly than their suboxides may be re-oxidized. It is interesting to note that the reverse phenomenon can be observed with the metals (Pt, Rh and Pd). Their oxides are reduced at a much lower temperature than the metal can be re-oxidized [19-21] even though the nature of support and the metal particle size may change the redox properties significantly [20,22,23]. (2) Depending on the temperature, there may be a carbon deficit in the mass balance upon individual pulses, i.e. CO consumption may be higher than CO 2 formation. A certain amount of carbon may be stored in the catalyst and released upon the following CO pulses or upon the first pulses of 02. In this case, some CO2 appears in phase 2. A detailed investigation of C and O mass balance during OSC measurements has been made by Holmgren et al. [24]. (3) In most cases, there is a good agreement between the OSC measured upon the first pulse of CO (phase 1), and the OSC deduced from alternate pulses CO and 02 (phase 3), which tends to prove that the catalysts show stable redox properties all along the procedure of measurement [25]. Several reaction steps may occur in the course of the oxygen storage process [24,26]: Step 1: Oxygen adsorption, which may be dissociative (Eqn. 1) or not (Eqn. 2): 1/202 -+- ?s -+ Os

(1)

02 + 92-'s "+ 0~-

(2)

where ?s represents a free vacant site, which may be negatively charged as in Eqn. 2. Dioxygen species, such as superoxides or peroxides, were evidenced by Fourier transform infrared (FTIR) studies [27-29]. Step 2: Reaction between COg and surface oxygen or dioxygen species to give CO2 (Eqn. 3) or carbonates (Eqn. 4) COg + 0 s ~ C02g .ql_?

(3)

COg +O~s --+ CO~-

(4)

237

Cerium-Based Oxides Used as Oxygen Storage Materials Phase 1 CO saturation

Phase 2

Phase 3

0 2 saturation

CO/O 2 alternately

i'i: i ." } } oai'[" :{{["

c%

1 2 3 4 5 6 7 8 910

qp

:

:

:

:

:

"

:i

::

:

:

!iiii

'

ii!ii

'

12345

1

ql

2

3

OSC

)SC(

Figure 8.1. General procedure for OSC measurements over TWC catalysts. CO and

CO 2 are

separated over a small Porapak column inserted between the sample and the TC detector. The first CO pulse is injected over pre-oxidized sample. CO can be replaced by H 2.

Surface carbonites, carbonates, inorganic carboxylates and sometimes formates (specially at low temperature) were identified by Fourier transform infrared (FTIR) [30,31 ]. Step 3" Oxygen diffusion from the bulk sites to the surface Ob .hi._.9s ~

Os Jr- .9b

(5)

Step 4: Side reaction such as CO dissociation (Eqn. 6), the Boudouard reaction (Eqn. 7) or the water gas shift (WGS) reaction (Eqn. 8, with surface OH species) may also occur: COg "Jr-2? s --+ C~ + Os

(6)

2COg -t- ?s ----> Cs -'1-CO 2

(7)

COg + OH s --+ C O 2 + 1/2H2 + .9s

(8)

On ceria or ceria-zirconia, OH s species are generally associated to the formation of CeOOH groups with a correlative reduction of the surface [32,33].

2.2. OSC of ceria and CexZr~_xO z oxides Selected OSC values are reported in Table 8.1 for ceria and cerium-zirconium mixed oxides. These results confirm that the isomorphous substitutioI,, of Ce 4+ by Zr 4+ ions clearly improves the catalyst stability. BET (Brunauer, Emmett, Teller) area of ceria treated at 900~ is close to 20 m 2 g-1 while it amounts to 35-45 m 2 g-~ for most mixed

238

Past and Present in D e N O x Catalysis

Table 8.1. The OSC values of ceria and CexZrl_xO 2 oxides Catalyst

Tox (*C) BET area (m 2 g-l)

O storage Red. (*C)

CeO 2

900 (6 h)

22

CO

400

71

3.2

Ce0.6Zr0.402

900 (6 h)

52

CO

400

232

4.5

CeO 2 NP 1

900 (4 h)

13

CO

400

64

4.9

Ce0.gZr0.102 NP l

900 (4 h)

18

CO

400

125

6.9

Ce0.v5Zr0.2502 NP 1

900 (4 h)

19

CO

400

112

5.9

Ce0.9Zr0.102 SG 2

900 (4 h)

43

CO

400

252

5.9

Ce0.v5Zr0.2502 SG 2

900 (4 h)

32

CO

400

217

6.8

Pt/CeO 2

500 (1 h)

49

CO

600

250

Pt/Ce0.v5Zr0.2502

500 (1 h)

72

CO

600

723

10

OSC itmol O g-1 m-2

5.1

Pt/CeO2

1000 (1 h)

2

CO

600

150

75

Pt/Ceo.75Zro.2502

1000 (1 h)

14

CO

600

522

37

CeO~'4

377 (2 h)

5-18

H2

377

70

Ce0.9Zr0.10~'4

377

(2 h)

20-25

H2

377

280

12.4

360

22.5

Ceo.65Zro.350~'4

377 (2 h)

14-18

H2

377

CeO~'5

377 (2 h)

5-18

H2

377

Ce0.9Zr0.10~'5

377 (2 h)

20-25

H2

377

Ceo.65Zro.350~'5

377 (2h)

14-18

H2

377

1.2 13 7.5

6.1

0.10 0.58 0.47

Reference

[9] [9] [10] [10] [10] [10] [10] [13] [13] [13] [13] [17] [17] [17] [17] [17] [17]

1prepared by nitrate precipitation 2prepared by a sol-gel technique (Zr propoxide) 3prepared by ball milling of oxides 4Total OSC (value close to OSCC) 5Dynamic OSC (pulse of H2) oxides prepared by coprecipitation or sol-gel methods. Theoretical OSC values may be calculated according to the following assumptions" 9 Only Ce 4+ is reduced into Ce 3+ in the OSC process. Zr 4+ virtually cannot be reduced. 9 One oxygen atom out of four (among those, which are associated with Ce ions) is available in the storage process. The theoretical surface density of oxygen ions was evaluated by Madier et al. for different crystallographic planes of CeO 2 and CexZrl_xO 2 oxides [14]. For ceria, the theoretical O density would be of 13.7, 9.7 and 15.8at.Onm -2 for (100), (110) and (111) surfaces respectively, which gives a mean surface density of 13.1 at.O nm -2 if one assumes an equidistribution of the three crystallographic planes. This figure leads to a theoretical OSC of 5.4 txmol O m -2. The hypothesis of equidistribution may be not valid in all cases, which can explain some difference in the reported results. Note that the (111) surface is thermodynamically the most stable [34,35]. Due to the decrease of the lattice parameter upon Zr substitution, O surface density increases with the Zr content in the materials (see Ref. [36] for a review on the

239

Cerium-Based Oxides Used as Oxygen Storage Materials

micro- and nanostructures of CexZrl_xO 2 mixed oxides). The mean oxygen surface density may be approximated by the following equation: So(at.O nm -2) = 13.07 + 1.28 (1 - x)

(9)

which leads to a theoretical OSC of: OSCth (ls

Om -2) : 5.43x + 0.531x(1 - x)

(10)

The values of OSCth for the compounds listed in Table 8.1 are given in Table 8.2. If data of Table 8.1 is compared with the theoretical OSC values given in Table 8.2 (for one O layer), it appears that: 9 The main effect of substituting Ce by Zr is to increase the thermal stability of the materials in significant proportion. 9 OSC of CexZr~_xO2 mixed oxides is higher than that of pure ceria. It may also be higher than the theoretical OSC, which proves that several oxygen layers can be involved in the storage process. Comparison of OSC evolutions with temperature for CeO 2 (25m2g -~) and

Ce0.63Zr0.3702 oxide (43m 2 g-l) was reported by Bedrane et al. [15,37]. Figure 8.2 summarizes the specific behavior of these compounds in the oxygen storage process. Up to 300~ there is virtually no difference between OSC properties of ceria or of Ce0.63Zr0.370 2. By contrast, above 300~ OSC of the mixed oxide dramatically increases while that of pure ceria remains practically constant.

Table 8.2. Theoretical OSC values of CexZrl_xO2 mixed oxides corresponding to the reduction of one surface layer x

OSfth (txmolOm -2)

1

0.9

0.75

0.65

0.60

5.43

4.93

4.17

3.65

3.39

500 450 400 "-~ 350 0 0 300 |

R

o 250 E 200 0 co 150 0 100 50 J- ~

200

I

250

I

300 350 400 Temperature (~

I

I

450

Figure 8.2. Evolution of the OSC for CeO 2 (~) and Ceo,63Zro,370 2 (ll).

500

240

Past and Present in DeNO x Catalysis

Table 8.3. Comparison of OSC of ceria and Ce0.63Zr0.3702 at 400 and 500~ Oxide

OSfth

(m-~) CeO2 Ce0.63Zr0.370 2

5.43 3.54

OSC 400~ (ttmol)

OSC 500~ (ttmol)

g-1

m-2

n

g-1

m-2

n

49 167

1.96 3.88

0.36 1.10

64 480

2.56 11.2

0.47 3.2

The OSC values at 400 and 500~ for the two samples are compared in Table 8.3. At these temperatures, the layer number n involved in the reduction process is significantly higher for the mixed oxide than for ceria. At 500~ more than three cerium ion layers (i.e. almost two cells) are reduced upon the first pulse of CO.

2.3. Correlation with oxygen mobility Oxygen diffusion (Eqn. 5) being a crucial step in the global oxygen storage process, OSC measurements have been tentatively correlated to oxygen mobility measured by 180/160 isotopic exchange. The principle has been described in detail elsewhere [14,38-40]. It is based on the exchange between gaseous 1802 and 160 species of the support via the metal particles (Figure 8.3). The reaction is carried out in close-loop reactor connected to a mass spectrometer for 1802, 180160 and 1602 analyses as a function of time [38]. The gases should be in equilibrium with the metallic surface (fast adsorption/desorption steps: 1 and 1'). If the bulk diffusion is slow (step 6) and the direct exchange (step 5) does occur at a negligible rate, coefficients of surface diffusion D s can be calculated from the simple relationship between the number of exchanged atoms Ne and ~/7 given by the model of circular sources developed by Kramer and Andre [41]: 2

N e =

(11)

--~C18Iox/~t

where I0 is the length of the metal/support interface, i.e. the total perimeter of the metal particles per m 2 of catalyst, and C~m8 is the surface concentration (at. m -2) of 180 atoms on the metal. The specific particle perimeter I 0 is a function of the metal loading x m% and the dispersion D%: (12)

I o -- flXm D2

180IA601802 16.02T160 I r~

mo~ility Surface

[_1802 ~60~

Figure 8.3. Measurement of surface diffusion by isotopic exchange.

241

Cerium-Based Oxides Used as Oxygen Storage Materials

Table 8.4. Relative oxygen surface mobility for some oxides at 400~ (base 10 for alumina) Oxides

CeO2 MgO ZrO2 CeO2/A1203 A1203 SiO2

BET area (m 2 g-l)

Relative oxygen mobility at 400~

60 150 40 100 100 200

2300 50 28 18 10 0.1

The coefficient of surface diffusion for alumina is 2 x 10-18 m2s-1

where/3 is a parameter depending on nature of metal and particle shape. For most metal, /3 is comprised between 2 x 105 and 106m g-1 [40,42] so that the specific perimeter of particles can be greater than 108 m g-'. This huge length allows understanding the critical role played by the metal/surface junction in diffusion processes and in catalysis. Equation 11 is generally applied in the first seconds of exchange: C~m8 is then close to the metal surface concentration, all adsorbed O atoms being 180 atoms. Some results obtained by Martin and Duprez [38] with this isotopic exchange technique are reported in Table 8.4. Ceria shows the highest coefficient of diffusion in the range of 10-'5-10 -~6 m 2 s-', which is coherent with the high OSC values obtained with this oxide. When surface diffusion is the only process of exchange, ag tends to an equilibrium value a* at t --+ oo. In most cases, after a rapid step of surface diffusion, it can be observed that ag continues slowly decreasing. This phenomenon corresponds to a slow step of bulk diffusion (coefficient Db). A model of bulk diffusion in spherical grains was developed by Kakioka et al. which led to the following equation [43]: 2 p~gg A V/-~b t -Ln~.g = q/-~

(13)

where A is the BET area of the oxide used as support and p its density. Equations 11 and 13 can be applied for oxides showing a relatively low surface diffusion step. Measurement of D b could be carried out with alumina and zirconia and was found in the range of 10-22-10 -23 m 2 s -1 [38]. Interestingly, it was noticed that the bulk O diffusion was an order of magnitude higher in zirconia than in alumina. With ceria, measurement of D b by Eqn. 13 is not possible because of the very fast surface diffusion process, which did not allow discriminating the two regimes of diffusion. The problem is still more complex with ceria-zirconia samples for which both surface and bulk diffusion occur at a very high rate. A spatio-temporal 3D model was then developed in which all the physical steps (adsorption, desorption, surface and bulk diffusion) involved in the exchange process are taken into account simultaneously [44,45]. A program based on this model was developed and used to determine coefficients of surface and bulk diffusion in CeZrOx-supported metal catalysts. Some results are reported in Table 8.5 for three Pt/CeZrOx samples [46]. CZ-O is a conventional mixed oxide prepared by hydrolysis of ZrO(NO3) 2 with an aqueous ammonia solution in the presence of a fine ceria powder. CZ-R was obtained by a reducing treatment in CO at 1200~ while CZ-D was prepared by high-energy ball

242

Past and Present in DeNO x Catalysis

Table 8.5. Coefficients of diffusion of Pt/CeZrOx catalysts [46] Catalyst BET area (m 2 g-l) Metal particle size (nm) Temperature (~ of exchange Surface diffusion D s (• 10-20 m2 s-l) Bulk diffusion D b (• 10-23 m 2 s- 1)

Pt/CZ-O

Pt/CZ-R

Pt/CZ-D

104 5.2 323 109 4.5

3 59 332 1450 36

37 2 334 1.7 0.34

milling of CeO 2 and ZrO 2 powders. This study confirmed that a high temperature of reduction can induce an extremely high mobility in CeZrOx oxides [36]. 180/~60 isotopic exchange can detect the presence of binuclear oxygen species (superoxide O 2, peroxide O2-). When 02 adsorption leads to the formation of such species, exchange proceeds via a 'multiple exchange' mechanism in which two atoms at once are exchanged:

1802g -1L[160 @@160]s --->16 02g -t- [180 @@180]s

(14)

While 160180 is the primary product of exchange in the simple mechanism (exchange atom per atom), 160 2 is the primary product when exchange occurs via a multiple mechanism [14,40,47]. This type of mechanism could be linked to the presence of superoxides species, which can be detected by FTIR (bands at 1126cm -1) [48,49]. At the beginning of exchange, the ratio P32/P34 is close to 1 on ceria, which means that both mechanisms (simple and multiple) occur with an equal probability. On ceria-zirconia, this ratio is much higher than unity, which implies that the multiple mechanism is predominant [ 14]. In parallel, exchange of superoxide species can be followed by FTIR. It was shown that [160--160]- species exchange directly into [180-180]- ones without intermediary formation of [180-160]- (Figure 8.4). Finally, a clear correlation was observed between OSC and intensity of the band at 1126 cm -~ .

160160 (1126 cm -1) A

180180(1062 cm -1)

0.005

11'oo

lO'SO

Wavenurnber (crn -1) Figure 8.4. The FTIR spectra of adsorbed superoxide species during exchange of pre-oxidized Ceo.63Zro.370 2 with 180 2 [14].

243

Cerium-Based Oxides Used as Oxygen Storage Materials

3. IMPACT OF OSC MATERIALS IN THREE-WAY CATALYSTS The OSC materials are able to increase the efficiency by enlarging the operating window of TW catalysts [50,51]. NOx reduction in TW catalysts is closely coupled to oxidation reactions since there is a competition between NOx and O2 reaction on the reducers present in the exhaust gases [52]. We will first examine the effect of these OSC materials on the oxidation efficiency of TW catalysts.

3.1. Impact of OSC on oxidation activity- steam effects Ceria-based OSC compounds may have an impact on oxidation reactions especially when the catalysts are working around the stoichiometry (as this is the case under TW conditions). One of the first systematic studies was reported by Yu Yao [53,54]. Most results were obtained in 02 excess (0.5% CO +0.5% 02 or 0.1% HC + 1% 02). Several series of Pt, Pd and Rh/A1203 of various dispersion, as well as metal foils, were investigated in CO, alkane and alkene oxidation. The effect of metal dispersion in CO and the propane oxidation are shown in Figure 8.5. In every case, large particles of metal are more active in oxidation than the smallest ones. CO oxidation is moderately structure-sensitive (less than one order of magnitude between metal foil and much dispersed catalysts). By contrast, propane oxidation (and in general oxidation of small alkanes) are strongly structure-sensitive (two orders of magnitude between large and small particles). Rate equations were also expressed as

I

CO ox 250~ 9 Rh 9 Pd

10

v

(1) .m O) m

E

0.1

i1) t~ rr

O.Ol

o

ox 250~ 0 Rh [] Pd ........ [] .... ~ et

-

'[3.. .... ....

0.001

-

~

9 9176

"o.. 9 1 7 6 1 4 9 ~ 1 7. .6. . . . . .

9149 ........

0.0001 0

I 20

I 40

9 9 1 4 9 1 4 9. . . . .

9

I 60

I 80

I 1 O0

Metal dispersion %

Figure 8.5. Effect of metal dispersion on turnover frequencies in CO and propane oxidation (M/AleO3 catalysts). Adapted from the data of Yu Yao [53,54].

244

Past and Present in DeNO x Catalysis

Table 8.6. Kinetic parameters of CO, propane and propene oxidation over M/AI203 and M/CeO2-A1203 catalysts (M = Pt, Pd or Rh) CO

Pt/A1 Pt/Ce-A1 Pd/A1 Pd/Ce-A1 Rh/A1 Rh/Ce-A1 r/Pt r/Pd 'qRh

C3H 8

E

m

n

E

104-125 84 108-133 50 92-113 104 2 (250~ 1 (250~ 5 (250~

+1.0 +0.5 +0.9 0 +1.0 0

-0.9 +0.3 -0.9 +1.0 -0.8 +0.2

84-105 96 66-96 63 100 84 0.5 (250~ 0.2 (350~ 3 (400~

C3H 6

m

n

-1.0 -1.0 +0.1 +0.1 0 +0.1

+2.0 +2.0 +0.6 +0.6 +0.5 +0.4

E

67-125 80 63-117 63 67-92 92 3 (300~ 0.5 (250~ 2 (300~

m

n

+2.0 +1.5 +1.5 +0.7 -0.8 0

-1.0 -0.6 -0.5 -0.3 +0.9 +0.5

a function of activation energy, E, and kinetic orders with respect to Oe (m) and

to CO or HC (n):

P~)2P~7O(HC)

r = k0 e x p - ~ - ~

(15)

A summary of the results is given in Table 8.6. Also reported in the Table are the ratios r/between the specific activity of M/CeO2-A120 3 and M/A120 3 catalysts. Kinetic orders in CO oxidation on M/A120 3 can be explained by the classical Langmuir-Hinshelwood expression for the rate equation, as a function of the rate constant k, the adsorption constants K and the partial pressures P: r = k

K~176176176

(16)

(1 + KoPo + KcoP~o) ~ On alumina, there is a competition between CO and 0 2 adsorption on the same metal sites. As CO is much more strongly adsorbed than O2 on Pt, Pd and Rh, one has: 1 + K o P o < < KcoPco and Eqn. 16 reduces to: r = k K ~ 1 7 6 , i.e., an order of 1 in 0 2 and of 1 in CO.

(17)

KcoPco Equations 16 and 17 imply that 0 2 adsorption is not dissociative, which is coherent with the kinetic data. However, 0 2 should be dissociated in further steps of the surface reaction. On ceria, new sites for 0 2 activation are created at the metal/support interface or in the vicinity of metal particles. As CO and 02 do not compete with the same sites, the rate equation becomes: r

=

k

K~176

1 +KoPo

•

Kco/~

(18)

1 +Kco&o

with orders between 0 and 1 for CO and 02, which is again coherent with data of Table 8.6. Numerous studies on CO oxidation have confirmed these conclusions, most

245

Cerium-Based Oxides Used as Oxygen Storage Materials

of them differing by the intimate nature of the site for 0 2 activation as well as by the chemical state of the metals in the reaction. For Pt/CeOz-AI203, Serre et al. [55,56] have described a mixed site P t - O - C e , specially created in reducing conditions. The catalyst deactivates after a prolonged exposition in oxidative medium. The presence of specific sites for O 2 adsorption on ceria was elegantly demonstrated by Johansson et al. on Pt/CeOx and Pt/SiO 2 catalysts prepared by electron beam lithography [57]. On both samples, a kinetic bistability depending on the gas mixing ratio,/3 = Pco/(Pco + Po2), can be observed. The bistable region is shifted considerably to much higher/3 values on Pt/CeOx than on Pt/SiO2. This could be simulated in kinetic modeling by introducing an oxygen reactant supply via the CeOx support, which maintains a high CO conversion rate even in CO excess. Similar rate expressions can be derived for propane oxidation. The first step would be the dehydrogenating adsorption of propane on metals giving a CxHy fragment (coverage: 0c). Contrary to CO, propane is much less strongly adsorbed on the metals than O2. The order 2 in HC and 1 in O2 on Pt could be explained by a surface reaction between OZads and two CxHy species. On Rh and Pd, O2 shows a moderate inhibitory effect much less significant than on Pt, kinetic orders being all comprised between 0 and 1. Clearly, OSC materials improve CO and to a lesser extent HC oxidation in O2 substoichiometry. OSC materials can also promote reactions of steam with CO (water-gas shift) or HC (steam reforming) [58,59]. Both reactions produce hydrogen (Eqns 19 and 20): CO-+- H 20

~

CO 2 +

(19)

H2

CnHm+nH20--+ nCO-+- (n-k- 2 ) H 2

(20)

The promoter role of ceria in WGS reaction is demonstrated in Figure 8.6. Temperature-programmed conversion of CO was carried out in an oxygen-deficient medium. The oxygen content was adjusted to obtain a maximum conversion of 40% by

100

~Ot'tv~"

..r

90 8o o~

9

#

70

r

._o 60 > cO

50

o

40

O 0

30

-.-,/

20 10

,.

9"

--++ ~ - + ~ ' * ~ 50

100

g

~,-

~+'

+ +++'~ + ~ + + t ++ 150

.

.

.

200 250 300 350 Temperature (~

II.-.+. -.

CeA I

400

450

.

iI i 500

Figure 8.6. Temperature-programmed reaction of 0.8% CO in 0.16% 0 2 + 10% H20 over 1% Pt or 0.2% Rh catalysts supported on alumina (A) or 12% CeO2-A1203 (CeA) [58].

246

Past and Present in D e N O x Catalysis

oxidation. The catalysts were 1% Pt and 0.2% Rh deposited over alumina (A) or 12% CeO2-A1203 (CeA). The CO conversion is not limited by thermodynamic. It is higher than 99% over the entire range of temperature. Two regions can be seen on the reaction profiles: the low-temperature domain corresponding to CO oxidation (limited at a 40% conversion) and the high temperature one where the CO having not been oxidized react with water (WGS domain). For every reaction, Pt is the best metal and ceria acts as a promoter. However, an exceptional increase of conversion can be observed in WGS when the metals are deposited on CeA. Similar experiments were carried out using propane instead of CO as reducer [58]. It was shown that Pt was the best metal for C3H 8 oxidation, while Rh was the most active one in propane steam reforming. Introduction of 12% CeO2 in the alumina support confirms that CeO 2 rather inhibits C3H 8 oxidation over Pt while it acts as a promoter on Rh. In both cases, especially with Rh, CeO2 increases the rate of propane steam reforming.

3.2. Impact of OSC on NO conversion The role of OSC materials in NO conversion is more complex. Most metals, specially Rh, are able to decompose NO, but this reaction is rapidly inhibited by O species resulting from this decomposition [60]. On Rh, for instance, R h - O species are replaced by R h - N O + ones in which NO is no longer dissociated [61]. O species may react with adsorbed species of the reducer (CO, HC) to form CO 2. The first role of the OSC materials could be to liberate metal sites by accepting O species. Indirect effects can also occur, with the Ce3+/Ce 4+ redox system being able to regulate the metal state (zero-valent or ionic) during richness oscillations.

3.2.1. Reduction of NO by CO The reaction may lead to N 2 (Eqn. 21) or N20 (Eqn. 22). NO -t- CO ~

1/2N2 -}- C O 2

2NO + CO --+ N 20 -+-C O 2

(21) (22)

For environmental reasons, reaction (Eqn. 21) (NO --+ N2) should be promoted, N20 having a dramatic greenhouse gas effect. The different steps of reaction (Eqn. 23) have been investigated in detail, mainly by FTIR spectroscopy [61-63]. One of the possible intermediate is isocyanate. NCO species could be formed on the metal and migrate on the support, which may explain the large differences observed when Rh is supported on different oxides (alumina, silica, zirconia, ceria-alumina, etc.). However, the main step should be the dissociative adsorption of NO: NOg -+-2, ~ N , + O ,

(23)

This adsorption reaction has been extensively studied on most noble metals, especially on Rh by NO thermodesorption [64-66]. On Rh/ZrO2 [65], it was shown that N 2 left the surface from two separate features: a sharp fll peak at 170~ due to N 2 desorption

247

Cerium-Based Oxides Used as Oxygen Storage Materials

by reaction (Eqn. 24) and a broad peak to N 2 recombination (Eqn. 25):

(/~2) between

180~ and 430~

corresponding

NOg nt- N , --+ N2g nt- O,

(24)

N . + N . --+ N2g

(25)

These reactions are very sensitive to particle size of rhodium. On big particles (10 nm), both reactions can occur while, on the smallest ones (3-6 nm), only the recombination feature was observed. One of the first systematic studies of the NO reduction by CO was reported by Taylor and Kobylinski in 1974 [67]. The reaction was carried out in large excess of CO (0.5% NO + 2% CO), which favors the formation of N 2. The light-off temperature 7'5o (50% conversion of NO) allowed ranking the metals (0.5% M/A1203) by increased activity: Ru (205~

> Rh (296~

> Pd (43 I~

> Pt (471~

Except Ru (not usable in TWC because of the volatility of its oxide [68]), the most active metal is the rhodium. This has been largely confirmed by further studies so that Rh may be considered as a key-component of TWC for NO reduction [69,70]. As far as Pd is concerned, it seems that the active site is composed of pdn+-pd ~ pairs, which may explain the higher activity of Pd in N O + C O + O 2 mixture (Ts0 ~, 200~ [71]. A detailed kinetic study by Pande and Bell on Rh catalysts has evidenced a significant support effect [72]. The kinetic data were represented by a conventional power law expression: RNo -- kNoP~oP~o

(26)

where RNO is the reaction rate per second and per metal site, P, the pressure in atm and kNo the kinetic constant in atm -(~+/3) s -1. The main results are reported in Table 8.7. The different catalysts may be ranked according to their relative activity: TiO 2 (9.5) > La203 (5.8) > SiO 2 (5.2) > MgO (3.6) > A1203 (1) Between 100 and 200~ the selectivity to N20 is very high (70-85%) and decreases with the temperature in accordance with most of the studies on the reduction of NO by CO. The kinetics of this reaction (1.5% N O + 3% CO, 200~ on a 1% Rh/SiO 2 catalyst promoted by Mo, Ce and Nb (1-10 wt-%) was studied by Hecker et al. [73]. A significant particle size effect was observed: the biggest particles of Rh are the most active per Rh site (Figure 8.7). On silica, the promoter effects are mainly due to particle size effects, certain promoters, especially ceria being able to stabilize Rh particles. The presence of Mo tends to slow down the reaction so that Mo appears as an inhibitor of the reaction. Kinetic parameters obtained by Hecker et al. are close to those reported by Pande and Bell: orders slightly negative in NO ( - 0 . 2 to -0.5, exceptionally - 1 . 2 for Mo) and orders nil or slightly positive with respect to CO (0 to 0.4); activation energies between 117 and 139 kJ mo1-1.

248

Past and Present in DeNO x Catalysis

Table 8.7. Kinetics of the CO + NO reaction at 210~ over Rh catalysts. Gas composition is 1.3%NO+ 3.9%CO [72] Catalyst

4.6% Rh]SiO 2 4.2% Rh]AI203 4.6% Rh/MgO 4.3% Rh]TiO 2 4.2% Rh/La203

Order/NO

Order/CO

Ea(kJmo1-1)

kNo 10 -3

55

-0.2

+0.1

140

4.30

175

64

-0.4

"~0

101

0.82

100

46

-0.2

+0.1

108

2.92

50

26

+0.1

-0.2

82

7.75

14

10

-0.2

'~'0

126

4.75

BET area (m 2 g-l)

Dispersion

250

(%)

0.020

>, 0.015 O r

o" 0.010

r

L_ L= >

o

t!__

0.005

F-

o

I--I O A 1

Mo series Ce series Nb series Unmodified

0.000 1

2

3

4

5

6

7

Rh particle size (nm)

Figure 8.7. Effect of the dispersion of Rh on the NO reduction by CO at 200~ over promoted Rh/SiO 2 catalysts (1.5% NO+ 3% CO) [73].

The reduction of NO by CO was investigated in detail by Oh et al. in closer conditions than those encountered in automotive converters" weaker NO and CO concentrations, close to the stoichiometry, more realistic metal content (< 1%) [74,75]. The main conclusions of Oh's works are: 9 The high sensitivity of the reaction to particle size of Rh is confirmed: at 230~ in a mixture of 0.5% N O + 1% CO, the turnover frequency increases from 0.017 s -1 for a highly dispersed catalyst to 0.74 s -~ for a catalyst dispersed at 1.7%, the activity per metal site on unsupported Rh catalysts being still much higher. 9 Ceria remarkably increases the activity of Rh/A1203:2-9 wt-% CeO 2 added to a 0.014% Rh/A1203 catalyst increases its activity by a factor 5 by 250~ However, ceria decreases the activation energy (120kJmo1-1 on Rh/A1203: instead of 80 on Rh/CeO2-A1203) so that the two catalysts have nearly the same activity around 300-310~

249

Cerium-Based Oxides Used as Oxygen Storage Materials

9 The reaction CO + NO may be seen as an oxidation of CO by NO, which competes with the oxidation of CO by 02. On Rh, CO oxidation by dioxygen is almost two orders of magnitude faster than CO oxidation by NO. Nevertheless, if the three reactants are present together, the rate of CO oxidation by 02 dramatically decreases while that of CO by NO increases. As a consequence, both reactions occur practically at the same rate [76]. Most of the kinetic observations can be interpreted as follows: 9 The key-step is the NO decomposition (Eqn. 23): the global reaction rate depends for a great part of the rate on this step. 9 At low temperature, N 2 is mainly formed by the reaction of NOg with adsorbed N* species (Eqn. 24) or by the same reaction with adsorbed NO (Eqn. 27):

(27)

N O , + N , --+ N 2 + O* + * while N20 is formed by a similar reaction (Eqn. 28):

(28)

N O , + N , ~ N 2 0 , -+-9

At high temperature, NO is almost totally dissociated: N 2 0 cannot then be formed and N2 stems essentially from recombination of adsorbed N* species (Eqn. 25). The selectivity to N 2 increases with the temperature and virtually reaches 100% around 300~ (Figure 8.8). The chemical state of the metal can play a decisive role on the reaction mechanism. In TWC, Rh is thought to remain in the zero-valent state, which favors NO dissociation [77,78]. However, the role of the OSC materials is complex, and it is not inert with respect to NO activation. Ranga Rao et al. [79] showed that, when bulk oxygen vacancies are formed in a reduced Ce0.6Zr0.402 solid solution, NO was efficiently decomposed on the support to give N20 and N 2. Further studies by the same group

200

U.A.

CO 150

NO 100 N2

_J,-==--IHl'ld

50 N20

50

100

150

200

250

300

350

400

Temperature (~

Figure 8.8. Temperature-programmed reaction of CO (1%)+ NO (1%) over a pre-oxidized 0.2% Rh/CeO2-A1203 catalyst (Pirault and Mar6cot, unpublished results).

250

Past and Present in D e N O x Catalysis

Table 8.8. Effect of the reducing pretreatment of 0.5% Rh catalysts on the NO + CO reaction rate (1% NO+ 1% CO) [80,81] Support

Rh precursor

Reaction rate at 200"C after pretreatment in H2, for 2 h at 200~ (mole NO g-1 S-1 X 10 9)

Reaction rate at 200"C after pretreatment in CO + NO from 200 to 500"C (mole NO g-1 S-1 X 109)

A1203

Chloride Chloride Chloride Chloride Nitrate

70 331 336 1120 16301

32 90 46 83 15901

Ce0.4Zr0.602

Ce0.6Zr0.402 CeO2 Ce0.6Zr0.4 0 2

1Reaction rate at 160~ using different CexZrl_xO 2 composition and different metals confirmed this prominent property of supports containing reduced ceria [80,81]. Selected results of these studies are reported in Table 8.8. A significant increase of activity can be observed on Rh catalysts supported on reducible oxides. Activity exaltation is severely annihilated when the catalysts are treated in the reaction mixture. Nevertheless, the presence of chlorine largely upset the results. C1 probably slows down the reduction of the support, particularly in the CO + NO mixture. By surface science techniques, Mullins and Overbury showed that the presence of reduced ceria might create new active sites for NO dissociation [82]. The degree of decomposition is increased and the onset temperature for decomposition is reduced when Rh is supported on reduced ceria (Rh/CeOx) compared to Rh on oxidized ceria (Rh/CeO2) NO dissociation being self-inhibiting. The promotion by reduced ceria could be due to a spillover phenomenon of O species from metal to support. In fact, this is not sufficient to explain all the results of Mullins and Overbury. An exposure of the Rh/CeO x surface to water leads to a re-oxidation accompanied by a hydroxylation of the support while the metal surface is left unchanged. In fact, it seems that preferential orientation of Rh surface on reduced ceria may also explain the specific role of CeOx surface. This is consistent with the fact that NO dissociation occurs at lower temperatures on Rh (110) and on Rh (100) than on Rh (111) [83,84]. However, reduced ceria is able, alone, to dissociate NO. Martinez-Arias et al. [85] have first investigated by electron paramagnetic resonance (EPR) and FTIR spectroscopies NO reaction on ceria pre-outgassed at different temperatures and showed the role of superoxides differentially coordinated in the formation of hyponitrites species further decomposed into N20. Later Haneda et al. [86] have demonstrated that reduced ceria and reduced praseodymium oxide dissociate NO even though the presence of a noble metal (Pt) significantly increases the formation of N 2 or N20. The main results of this study are summarized in Table 8.9. Lanthanum and samarium show virtually no NO dissociation activity even in the presence of Pt. These supports are not reducible and have no OSC property. The intrinsic NO dissociation activity of platinum is very weak, probably in reason of the low metal dispersion. The behavior of terbium oxide is more surprising. Although it is reducible in H 2, it is unable to dissociate NO except in the presence of Pt.

251

Cerium-Based Oxides Used as Oxygen Storage Materials

Table 8.9. NO dissociation over reduced rare-earth oxides and over 1% Pt catalysts deposited on these oxides. Gas Prior to NO dissociation (970 ppm NO), the samples are reduced for 1 h in H2 at 500~ [86] BET area

(m2 g-l)

H 2 temperatureprogrammed reduction

(25-500~

N 2 (N20) formed (mmol g-l)

Ixmol H 2 g-1 at 200 ~C

at 400 ~

RE oxide

La203 CeO 2 Pr6Oll

7.5 55 9.5

Sm203

Tb407

2.5 75 773

0.0 (0.0) 14.7 (2.6) 19.6(4.6)

1.5

0.0 (0.0)

8.0

22

1008

0.0 (0.0)

Pt/RE oxides

Pt/La203 Pt/CeO 2 Pt/Pr601, Pt/Sm203 Pt]Tb407

17 271 1823 41 1356

0.0 (0.0) 50.0 (1.1) 225 (226) 0.0 (0.0) 27.2 (8.1)

1.8 (0.2) 52.3 (0.0) 431 (10.5) 5.5 (0.2) 248 (0.0)

The specific role of OSC materials in NO activation and NO dissociation has largely been confirmed by many authors over P t - R h [87,88] and Pd catalysts [89,90] or even over bare OSC oxides [91]. By EPR, Lecomte et al. [87] evidence the presence of O~- superoxide species over a Pt-Rh/AI203 catalyst modified by ceria. The formation of these species could be closely related to the performance of the Pt-Rh/CeOz-AI203 catalyst in CO + NO reaction. When the ceria-containing catalyst is pre-reduced before reaction, a typical temperature-programmed reaction profile can be observed (Figure 8.9). While 100 90

e..

%

80

~ tO

E

7o t Pt.Rh/CeO2 -...

60 J

>

O z

| o9

9

50-

9

9

40

oil= ...

; 9

50

loo

9

1;o

o o

_

30

20 10

o

o9 ,

9

0

o o

!._

tO O

-

[]

200

Pt-

~

o oo~176 o

28o

oO

a;o

a;o

400

450

Temperature (~ Figure 8.9. Effect of ceria on the NO reduction by CO (gas composition: 0.5% NO + 0.5% CO,

25000h-1). After Ref. [87].

Past and Present in DeNOx Catalysis

252

prereducing, the catalyst has virtually no effect on Pt-Rh/A120 3, a significant activity peak can be observed at low temperature on the ceria-based catalyst. This peak disappears upon calcination, and a profile like that of Figure 8.9 can then be recorded. It is not systematically recorded in bench tests with complex synthetic mixtures [92]. The behavior of CeO2-A120 3 support is rather general and can be observed with other ceria-containing catalysts such as Rh/CeO2-ZrO2 catalysts [80,81] or Pd/CeO2-ZrO 2 [93]. It has also been shown that pre-reduced ceria-zirconia supports present a noticeable activity in NO + CO in the absence of metal. This activity totally disappears when the support is calcined [88].

3.2.2. Reduction o f N O by H 2 a n d hydrocarbons Compared to CO, these reactions were much less studied over TW catalysts. Kobylinski and Taylor [67] have compared the NO reduction by CO and by H 2. Their main results are summarized in Tables 8.10 (light-off activity) and 8.11 (selectivity). Pt and Pd are by far the most active metals in NO reduction by H 2 while Rh and Ru present the highest activity in NO reduction by CO. However, when the two reducers are injected together (last column), CO tends to impose its behavior in NO reduction. This is due to a strong adsorption of CO, which inhibits the reduction by H2. Although CO seemed to inhibit the reduction by H2, this reducer still maintains a very significant activity. The CO inhibition is largely compensated by the high temperature of working. Interestingly, Table 8.11 confirms the good selectivity of Rh (and Ru) to N 2

Table 8.10. Temperatures (T~

for 50% NO conversion as a function of the reducing agent. Catalysts: 0.5% metal on alumina. Space velocity: 24 000 h-1 Catalyst

Pt Pd Rh Ru

NO + H2

NO + CO

NO + CO + H2

121 106 163 237

471 431 296 205

398 330 275 210

Table 8.11. Product selectivity (% NO converted in N2 and NH3) and reaction selectivity (% NO

converted by NO + CO and NO + H2). Gas composition 1.5% NO + 4.5% CO + 4.5% H2 Catalyst

T (~

NO conv.

Selectivity

% N 2 from NO+CO

(%) Pt Pd Rh Ru

515 515 482 432

94 94 100 100

NO --+ N 2

NO --+ NH 3

NO + CO

NO + H2

23 26 67 92

77 74 33 8

8 9 20 29

92 91 80 71

40 38 29 31

Cerium-Based Oxides Used as Oxygen Storage Materials

253

while Pt and Pd lead to a significant formation of ammonia. Maunula et al. [94] have confirmed the superiority of Pt over Rh in the reduction of NO by H 2. They also found that N20 would be an intermediate in the formation of N2. Relatively, few studies were devoted to the reduction of NO by HC in TW conditions. Platinum seems to be a good reducer of NO by light alkanes, specially the methane [95]: CH 4 + 4NO --+ 2N 2 + C O 2 + 2H 20

(29)

However, some contradictory results were obtained in several studies. For instance, in the C H 4 - N O reaction, some authors have reported that N20 was the primary product [95] while others found that ammonia was first produced [96]. The presence of water can play a decisive role since H20 allows generating H2 by WGS or steam reforming [59]. Olefins generally show a higher activity than alkanes. Propene for instance has been found more reactive than propane. Some exceptions should be quoted, ethylene has been found less reactive than C H 4 in NO reduction at stoichiometry [97]. The role of ceria is practically not evoked in these previous studies. More recently, Prrez-Hem~indez et al. have compared the catalytic behavior of Pt/ZrO2, Pt/CeO2 and Pt/CeO2-ZrO2 in NO reduction by CH 4 or CO [98]. All the catalysts were found more active and more selective to N 2 in NO reduction by CH 4. However, the cerium content plays a decisive role in decreasing the differences of NO light-off activity between CH 4 + NO and CO + NO. Rather different results were obtained by Bera et al. [99] who found that Pt/CeO2 is more active in CO + NO (100% conversion achieved at 270~ than in N O + C H 4 (100% conversion at 350~ However, the superiority of the ceria support with respect to alumina in NO reduction was demonstrated both for Pt and Pd catalysts. As said before, ceria may also modify the catalyst behavior by increasing the H 2 content in rich conditions, which may induce a higher capacity of the catalyst to reduce NO. A close parallelism could be established between the OSC of aged catalysts and their light-off activity in CO, HC and NOx conversion [100]. Different evolutions of both the OSC and the catalytic activity were observed depending on the method of ageing (laboratory ageing or engine bench ageing) and on the type of catalyst (PtRh/CeO2-A1203 or PdRh/CeO2-A1203). In every case, the conversion of NOx is the most sensitive to catalyst ageing. This has been confirmed by Muraki and Zhang who compared CO, HC and NO conversions over aged Rh catalysts supported on ceria and ceria-zirconia [70]. The conversions were significantly higher over ceria-zirconia supported Rh catalysts, much more stable than those supported on pure ceria. Ageing the catalyst, especially in rich/lean oscillation, may also lead to heterogeneity in the Ce local concentration. Finally, possible migration of C e 4+ ions was observed by Fan et al. in Pt/Ce0.67Zr0.330 2 catalyst in rich/lean oscillations. Cerium ions may then decorate Pt particles and deeply modified their behavior in NO reduction [101 ].

4. IMPACT OF OSC MATERIALS IN LEAN-BURN AND DIESEL CATALYSIS The impact of oxygen storage in DeNOx catalysis in 02 excess is more complex and strongly depends on the process used for NOx reduction. The impact of OSC materials will be examined on two processes: the selective reduction by HC (HC-SCR) and the NOx-trap process.

254

Past and Present in D e N O x Catalysis

4.1. Impact of OSC materials in HC-SCR In the 1990s, most investigations were devoted to passive DeNOx, i.e. NO x reduction by the HC present in the exhaust gas. Even in lean conditions, there remains Ce 3+ cations and oxygen vacancies, which may participate in NO or HC activation. In 2000, Djega-Mariadassou [77] proposed the following model for NO adsorption in Rh/CeO 2 where Rh is partially oxidized while there remain reduced sites of ceria on the support (Figure 8.10) However, there are also evidences that Ce 3+ cations could participate in NO activation and dissociation [85,102,103]. The model of Figure 8.10 was later modified to take into account this possibility [ 104]. It seems that the behavior of Pd/CeO2-ZrO2 can be explained by similar models of NO and HC activation [105]. NO would be adsorbed on reduced sites of ceria-zirconia while the hydrocarbon (propylene in this study) would be partially oxidized into CxHyO z intermediates over PdOx sites. For the reduction of NO in oxygen excess, it seems, however, that conventional catalysts, even doped with OSC materials, cannot adsorb the HC's to reduce the NO x efficiently. Engineers of Toyota have proposed a system composed of a dual bed in which the conventional Pt catalyst is mixed with different zeolites [ 106]. A relatively close contact between Pt and the zeolite, which stores the HC's is required, but the advantage of this technique is that the zeolite as well as the presence of an OSC materials prevent the poisoning of Pt by heavy HC's. Some experiments were carried out with Pt in the zeolite but the most interesting system would the multi-component catalyst schematized in Figure 8.11. The idea to insert an OSC component such as Ce ions in zeolite has received much attention. Cordoba et al. [107] have shown that H-ZSM5 promoted both by Ce and Pd has excellent properties in lean-DeNOx by dodecane (Figure 8.12). As in the previous

/

Rh+

Figure 8.10. Model of adsorption site for NO in the presence of O2 (adapted from Ref. [77]). The cubes represent oxygen vacancies. Rh+a would be the site for HC (or CO) activation.

NO

N2

HG

Figure 8.11. Cooperative effect of an OSC component and a zeolite in SCR-HC of NO in 02 excess.

Cerium-Based Oxides Used as Oxygen Storage Materials

255

80 HMOR --12t-- Pd-HMOR

o~ 60

Ce-HMOR

t.O m

> tO O

0 z

--

PdCe-HMOR

40

20

0

100

200

300

!

!

400

500

Temperature (~

Figure8.12. Cooperative effect of Pd and Ce in lean-DeNOx by dodecane (900 ppm NO-+-100 ppm NO 2 +

30ppm N20 +400ppm C12H26 -I-6% 0 2 [107].

example (Figure 8.11) the zeolite should help storing and probably transforming the dodecane molecule into highly reducing species. Thanks to their redox properties, Ce ions could adsorb NO and maintain Pd in the better chemical state to reduce NO. Very few studies were devoted to NO reduction by H 2 in lean conditions because of the high reactivity of H 2 with 02. Costa et al. [108,109] discovered a new low loaded Pt catalyst able to perform H2-DeNO x with a good selectivity. It consists of 0.1% Pt supported on La0.sCe0.sMnO 3. This catalyst was found very active (74% NO conversion at 140~ in a mixture, 0.25% NO, 1% H 2, 5% 02, 5% H20/He, GHSV, and 80000h -1) and more selective to N 2 (87% at 140~ than a conventional 0.1% Pt/alumina catalyst [ 108]. By transient isotopic exchange reaction, Costa and Efstathiou showed that the interface metal/support played a significant role in NO reduction. Two kinds of active NOx species were evidenced: M - N O 2+ located on the La0.sCe0.sMnO 3 support and M - O - ( N O ) - O - M at the interface metal/support, while on a non-selective Pt catalyst, virtually all the active species are on Pt ( P t - N O ~+ or Pt-nitrate) [109].

4.2. Impact of OSC materials on NOx-tra p systems One of the most promising processes is the active DeNOx based on NOx-tra p materials. It has been developed for lean-burn gasoline engines. Cerium compounds are thought to intervene in different steps of the whole process: (1) NO oxidation, (2) NOx storage, (3) Nitrate desorption and NOx reduction. Most probably, the main role of OSC materials is to accelerate HC partial oxidation during rich-spikes (giving CO and H 2 as NO x reducers). However, this beneficial effect of OSC compounds competes with a detrimental reaction, i.e. the reduction of the OSC materials itself that may delay HC decomposition and thus NO x reduction [110]. Nakatsuji et al. [111], in cooperation with researchers of Isuzu, have studied the effect of OSC materials on simplified catalysts in as much as Rh was directly deposited on oxides known for their OSC properties (Ce, Ce-Zr, Ce-Pr, C e - N d - P r , C e - G d - Z r oxides). These catalysts were submitted to periodic lean (55 s)/rich (5 s) excursions as in the conventional NOx-trap system. Compared to non-OSC supports, ceria allows maintaining a high DeNOx activity even in large 02 excess during the lean period (Figure 8.13).

256

Past and Present in D e N O x Catalysis 100

80 tO "~ i,,..

60 ~

> t" 0

40

o

2o

z

0

1%Rh/alumina @ 300~

; Oxygen concentration (%)

Figure 8.13. Effect of 02 concentration over Rh-loaded catalysts at the temperature of their maximum activity (lean/rich spans: 55/5 s; 50000 h - l ) .

Even in absence of the classical Ba component, OSC materials may play a role in the NOx reduction using severely lean/rich conditions. Most of other studies implying OSC materials were devoted to the chemical interaction between the NOx-tra p materials (Ba) and the OSC oxides. The order of introduction of the different functions (metal = Pd, OSC and NOx-trap) was studied by Kolli et al. [112]. Although the aim of this work was to study the catalysts in TWC conditions, some interesting results were obtained, which may be generalized to other conditions. It was shown that the best catalysts consisted of P d - B a or B a - P d deposited over OSC-A1203 support, i.e. the OSC materials should be impregnated first. Although their catalyst was not optimized, Liotta et al. [ 113] have investigated a Pt-OSC/Ba-A1203 catalyst in NOxtrap conditions. Interestingly, they showed that Ba ions migrated through the OSC layer (CeZrOx). This property could allow a better control of the Ba dispersion as well as an increased resistance to SO2 poisoning. However, the presence of Ba is not essential for the NOx-trap when OSC material is used as support. Eberhardt et al. [ 114] and Philipp et al. [115] have compared the NO x trap behavior of Ba/A1203 and Ba/CeO 2 materials. Ba/CeO2 show higher NO~ storage efficiency than Ba/A1203 within the temperature range of 200-400~ No solid/solid reaction between Ba and ceria was observed below 780~ while Ba reacted at lower temperature with A1203 to form inactive Ba aluminate. In these previous studies, however, no noble metal was impregnated on the support, which may bias the results in real catalysts. Corbos et al. have compared the NO~ storage capacities of Pt/CeZrOx with a classical Pt/Ba/A1203 and with Pt/Ba/CeZrOx. The values given in Table 8.12 show a better capacity for Pt/CeZrOx at certain temperatures [ 116].

Table 8.12. NOx storage capacities (ixmolg-1) calculated for the first 100s. The catalysts were stabilized at 700~ under 02, H20 and N2; storage mixture: 350 ppm NO, 10% O2, 10% H20, 10% CO2 and N2. (Ref. [116])

NOx storage capacities (Ixmolg-l) Storage temperature (~ Pt/Ba/A1 (129m 2 g-l) Pt/CeZr (61 m 2 g - l ) Pt/Ba/CeZr (47 m 2 g - l )

200 13.1 17.1 9.4

300 13.7 14.6 11.8

400 18.3 15.0 23.3

Cerium-Based Oxides Used as Oxygen Storage Materials

257

Similar results were obtained by Lin et al. [117] who investigated the effect of La and Ce on NOx storage properties of Pt/Ba-A120 3. Substituting La for Ce increased the NO x storage capacity from 341 Ixmol g-1 in Pt2.sLa30.sBa33.aAl100 to 10201xmolg -1 for the Pt2.sCe30.sBa33.nAl100 catalyst. Finally, unconventional OSC supports have also been investigated. Machida et al., for instance, studied a Pd/MnOx-CeO2 as NOx-storage materials [118]. This catalyst was tested in lean/rich conditions using H2 as reducer with, however, an unusual period (9 min. lean/3 min. rich). The catalyst showed both excellent activity and selectivity (almost 100% to N2).

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

Ghandi, H.S., Piken, A.G., Shelef, M. et al. (1976) SAE Paper 760201. Yao, H.C. and Yu Yao, Y.F. (1984) J. Catal., 86, 254. Su, E.C., Montreuil, C.N. and Rothschild, W.G. (1985) Appl. Catal., 17, 75. Su, E.C. and Rothschild, W.G. (1986) J. Catal., 99, 506. Engler, B., Koberstein, E. and Schubert, P. (1989) Appl. Catal., 48, 71. Miki, T., Ogawa, T., Haneda, M. et al. (1990) J. Phys. Chem., 94, 6464. Ozawa, M., Kimura, M. and Isogai, A. (1993) J. Alloys Comp., 193, 73. Fornasiero, P., Di Monte, R., Ranga Rao, G.J. et al. (1995) J. Catal., 151, 168. Cuif, J.P., Blanchard, G., Touret, O. et al. (1997) SAE Paper Nr 970463. Rossignol, S., Grrard, F. and Duprez, D. (1999) J. Mater Chem., 9, 1615. Lox, E.S.J. and Engler, B.H. (1999) In Environmental Catalysis (G. Ertl et al. eds.), Wiley-VCH, New York, p. 1. Kacimi, S., Barbier, J. Jr., Taha, R. et al. (1993) Catal. Lett., 22, 343. Hori, C.E., Permana, H., Simon Ng, K.Y. et al. (1998) Appl. Catal. B Environ., 16, 105. Madier, Y., Descorme, C., LeGovic, A.M. et al. (1999) J. Phys. Chem. B, 103, 10, 999. Bedrane, S., Descorme, C. and Duprez, D. (2002) Catal. Today, 73, 233. Rohart, E., Larcher, O., Hrdouin, C. et al. (2004) SAE Paper 04-01-1274. Trovarelli, A., Zamar, F., Llorca, J. et al. (1997) J. Catal., 169, 490. Duprez, D. and Descorme, C. (2002) In Catalysis by Ceria and Related Materials (A. Trovarelli ed.), Chapter 7. Imperial College Press, London, pp. 243-80. Barbier, J., Bahloul, D. and Marrcot, P. (1992) J. Catal., 137, 377. Martin, D. and Duprez, D. (1995) Appl. Catal. A Gen., 131, 297. Pisanu, A.M. and Gigola, C.E. (1996) Appl. Catal. B Environ., 11, L37. Douidah, A., Marrcot, P., Szabo, S. et al. (2002) Appl. Catal. A Gen., 225, 21. Yoshida, H., Yazawa, Y. and Hattori, T. (2003) Catal. Today, 87, 19. Holmgren, A., Andersson, B. and Duprez, D. (1999) Appl. Catal. B Environ., 22, 215. Rossignol, S., Descorme, C., Kappenstein, C. et al. (2001) J. Mater. Chem., 11, 2587. Bedrane, S., Descorme, C. and Duprez, D. (2002) CataL Today, 75, 401. Sofia, J., Martinez-Arias, A. and Conesa, J. (1995) J. Chem. Soc. Faraday Trans., 91, 1669. Li, C., Domen, K., Maruya, T. et al. (1989) J. Am. Chem. Soc., 111, 7683. Descorme, C., Madier, Y. and Duprez, D. (2000) J. Catal., 196, 167. Li, C., Sakata, Y., Arai, T. et al. (1989) J. Chem. Soc. Faraday Trans., 1, 85, 929; Li, C., Sakata, Y., Arai, T. et al. (1989)J. Chem. Soc. Faraday Trans., 1 85, 1451. Binet, C., Badri, A., Boutonnet-Kizling, M. et al. (1994) J. Chem. Soc. Faraday Trans., 90, 1023. Daturi, M., Finnochio, E., Binet, C. et al. (1999) J. Phys. Chem. B, 103, 4884. Norman, A. and Perrichon, V. (2003) Phys. Chem. Chem. Phys., 5, 3557.

258 [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61 ] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73]

[74] [75] [76] [77]

Past and Present in DeNO x Catalysis

Sayle, T.X.T., Parker, S.C. and Catlow, C.R.A. (1994) Surf. Sci., 316, 329. Conesa, J.C. (1995) Surf. Sci., 339, 337. Di Monte, R. and Ka~par, J. (2005) J. Mater. Chem., 15, 633. Bedrane, S. (2002) Ph.D dissertation, Poitiers University, France. Martin, D. and Duprez, D. (1996) J. Phys. Chem., 100, 9429. Holmgren, A., Duprez, D. and Andersson, B. (1999) J. Catal., 182, 441. Duprez, D. (2006) In Isotopes in Catalysis (J. Hargreaves et al. eds.), Chapter 7. Imperial College Press, London, 2006. Kramer, R. and Andre, M. (1979) J. Catal., 58, 287. Duprez, D., Pereira, P., Miloudi, A. et al. (1982) J. Catal., 75, 151. Kakioka, H., Ducarme, V. and Teichner, S.J. (1971) J. Chim. Phys., 68, 1715. Galdikas, A., Descorme, C. and Duprez, D. (2004) Solid State lonics, 166, 147. Galdikas, A., Descorme, C. and Duprez, D. (2004) Appl. Surf. Sci., 236, 342. Galdikas, A., Descorme, C., Duprez, D. et al. (2004) Top. Catal., 30/31, 405. Duprez, D. (1997) Stud. Surf. Sci. Catal., 112, 13. Li, C., Domen, K., Maruya, K. et al. (1989) J. Am. Chem. Soc., 111, 7683. Li, C., Domen, K., Maruya, K. et al. (1990) J. Catal., 123, 436. Nunan, J.G., Silver, R.G. and Bradley, S.A. (1992) In Catalytic Control of Air pollution (R.G. Silver, J.E. Sawyer, J.C. Summers eds.), ACS Symposium Series, Vol. 495, Chapter 7. p. 83. Ozawa, M. (1998) J. Alloys Comp., 275-277, 886. P~rvulescu, V.I., Grange, P. and Delmon, B. (1998) Catal. Today, 46, 233. Yu Yao, Y.F. (1980) Ind. Eng. Chem. Prod. Res. Dev., 19, 293. Yu Yao, Y.F. (1984) J. Catal., 87, 152. Serre, C., Garin, F., Belot, G. et al. (1993) J. Catal., 141, 1. Serre, C., Garin, F., Belot, G. et al. (1993) J. Catal., 141, 9. Johansson, S., 0sterlund, L. and Kasemo, B. (2001) J. Catal., 201, 275. Barbier, J. Jr. and Duprez, D. (1993) Appl. Catal. B Environ., 3, 61. Barbier, J. Jr. and Duprez, D. (1994) Appl. Catal. B Environ., 4, 105. Dubois, L.H., Hansma, P.K. and Somorjai, G. (1980) J. Catal., 65, 318. Dictor, R. (1988) J. CataL, 109, 89. Solymosi, F., Volgyesi, L. and Rasko, J. (1980) Z. Phys. Chem., 120, 79. Solymosi, F., B~ins~igi, T. and Nov~ik, E. (1988) J. Catal., 112, 183. Cho, B.K., Shanks, B.H. and Bailey, J.E. (1989) J. Catal., 115, 486. Zafiris, G.S. and Gorte, R.J. (1991) J. CataL, 132, 275. Kasp~r, J., de Leitenburg, C., Fornasiero, P. et al. (1994) J. Catal., 146, 136. Kobylinski, T.P. and Taylor, B.W. (1974) J. Catal., 33, 376. Taylor, K. (1984) In Catalysis, Science and Technology (J.R. Anderson, M. Boudart eds.), Vol. 5. Springer Verlag, Berlin, p. 119. Leclercq, C., Dathy, C., Mabilon, G. et al. (1991) In Catalysis and Automotive Pollution Control CaPoC 2 (A. Crucq, ed.), Stud. Surf. Sci. Catal., 71, 181. Muraki, H. and Zhang, G. (2000) Catal. Today, 63, 337. Duplan, J.L. and Praliaud, H. (1991) In Catalysis and Automotive Pollution Control CaPoC 2. (A. Crucq ed.), Stud. Surf. Sci. Catal., 71, 667. Pande, N.K. and Bell, A.T. (1986) J. Catal., 98, 7. Hecker, W.C., Wardinsky, N.D., Clemmer, P.G. et al. (1992) In Progress in Catalysis. Proceedings of 12th Canadian Symposium on Catalysis. (K.J. Smith, E.C. Sanford eds.), Stud. Surf. Sci. Catal. 73, 211. Oh, S.H. (1990) J. Catal., 124, 477. Oh, S.H. and Eickel, C.C. (1991) J. Catal., 128, 526. Oh, S.H. and Carpenter, J.E. (1986) J. Catal., 101, 114. Djega-Mariadassou, G., Fajardie, F., Temp~re, J.-F. et al. (2000) J. Mol. Catal., 161, 179.

Cerium-Based Oxides Used as Oxygen Storage Materials [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116] [117] [118]

259

Djega-Mariadassou, G. (2004) Catal. Today, 90, 27. Ranga Rao, G., Fomasiero, P., Di Monte, R. et al. (1996) J. Catal., 162 1. Di Monte, R., Fomasiero, P., Graziani, M. et al. (1998) J. Alloys Comp., 275-277, 877. Fomasiero, P., Ranga Rao, G., Kaspar, J. et al. (1998) J. Catal., 175, 269. Mullins, D.R. and Overbury, S.H. (2002) Surf. Sci., 511, L293. Schmatloch, V., Jirka, I. and Kruse, N. (1994) J. Chem. Phys., 100, 8471. Hopstaken, M.J.P. and Niemantsverdriet, J.W. (2000) J. Phys. Chem., 104, 3058. Martinez-Arias, A., Sofia, J. and Conesa, J.C. et al. (1995) J. Chem. Soc. Faraday Trans., 91, 1679. Haneda, M., Kintaichi, Y. and Hamada, H. (2002) Phys. Chem. Chem. Phys., 4, 3146. Lecomte, J.J., Granger, P., Leclercq, L. et al. (1999) Colloids Surf. A, 158, 241. Granger, P., Delannoy, L., Lecomte, J.J. et al. (2002) J. Catal., 207, 202. Noronha, F.B., Baldanza, M.A.S., Monteiro, R.S. et al. (2001) Appl. Catal. A, 210, 275. Kolli, T., Rahkamaa-Tolonen, K., Lassi, U. et al. (2005) Catal. Today, 100, 297. Gonzalez-Velasco, J.R., Guttierez-Ortiz, M.A., Marc, J.L. et al. (2001) Appl. Catal. B Environ., 33, 303. Zu, Z., Wan, C.Z., Lui, Y.K. et al. (1996) Catal. Today, 30, 83. Martinez-Arias, A., Femfindez-Garcia, M., Iglesias-Juez, A. et al. (2001) Appl. Catal. B Environ., 31, 51. Maunula, T., Ahola, J., Salmi, T. et al. (1997) Appl. Catal. B Environ., 12, 287. Harrison, B., Wyatt, M. and Gough, K.C. (1982) In Catalysis (C. Kemball, D.A. Dowden eds.), Vol. 5. The Royal Society of Chemistry, London, p. 127. Vartui, J.C. and Gonzalez, R.D. (1973) Ind. Eng. Chem. Proc. Res. Dev., 12, 265. Ault, J.W. and Ayen, R.J. (1971) AIChE J., 17, 265. P6rez-Hemfindez, R., Aguilar, F., G6mez-Cortbs, A. et al. (2005) Catal. Today, 107-108, 175. Bera, P., Patil, K.C. and Jayaram, V. (2000) J. Catal., 196, 293. Taha, R., Duprez, D., Mouaddib-Moral, N. et al. (1998) In Catalysis and Pollution Control CaPoC 4. (N. Kruse, A. Frennet, J.M. Bastin eds.), Stud. Surf. Sci. Catal., 116, 549. Fan, J., Wu, X., Ran, R. et al. (2005) Appl. Surf. Sci., 245, 162. Overbury, S.H., Mullins, D.R., Huntley, D.R. et al. (1999) J. Catal., 186, 296. Daturi, M., Bion, N., Saussey, J. et al. (2001) Phys. Chem. Chem. Phys., 3, 252. Thomas, C., Gorce, O., Villain, F. et al. (2006) J. Mol. Cat. A, 249, 71. Thomas, C., Gorce, O., Fontaine, C. et al. (2006) Appl. Catal. B, 63, 201. Tanabe, T., Hatanaka, M., Tsuji, R. et al. (2002) R&D Review of Toyota CRDL, 37, 1. Cordoba, L.F., Flytzani-Stephanopoulos, M. and Montes de Correa, C. (2001) Appl. Catal. B Environ., 33, 25. Costa, C.N., Stathopoulos, V.N., Belessi, V.C. et al. (2001) J. Catal., 197, 350. Costa, C.N. and Efstathiou, A.M. (2004) J. Phys. Chem. B., 108, 2620. Epling, W.S., Campbell, L.E., Yezerets, A. et al. (2004) Catal. Rev., 46, 163. Nakatsuji, T., Ruotoistenmaki, J., Komppa, V. et al. (2002) Appl. Catal. B Environ., 38, 101. Kolli, T., Lassi, U., Rahkamaa-Tolonen, K. et al. (2006) Appl. Catal. A Gen., 298, 65. Liotta, L.F., Macaluso, A., Arena, G.E. et al. (2002) Catal. Today, 75, 439. Eberhardt, M., Riedel, R., G6bel, U. et al. (2004) Top. Catal., 30/31, 135. Philipp, S., Drochner, A., Kunert, J. et al. (2004) Top. Catal., 30/31, 235. Corbos, E.C., Elbouazzaoui, S., Courtois, X. et al. (2007) Top. Catal., 42/43, 9. Lin, H.-Y., Wu, C.-J., Chen, Y.-W. et al. (2006) Ind. Eng. Chem. Res., 45, 134. Machida, M., Kurogi, D. and Kijima, T. (2003) Catal. Today, 84, 201.