Palladium-substituted lanthanum cuprates: application to automotive exhaust purification

Applied Catalysis B: Environmental 10 (1996) 325-344

Palladium-substituted lanthanum cuprates: application to automotive exhaust purification

Nolven Guilhaume * , Stefan D. Peter, Michel Primet Laboratoire d’Application de la Chin& L? I’Enuironnement, Unite’ Mixte CNRS - Uniuersite’ Claude Bernard

Lyon I, 43 Boulevard du II Nouembre 1918, 69622 Villeurbanne Cedex, France

Received 30 December 1995; revised 15 February 1996; accepted 15 February 1996

Abstract

A series of palladium-substituted La,CuO,, corresponding to the formula La,Cu I _ x Pd,O, (X = O-0.2) were prepared by metal nitrate decomposition in a polyacrylamide gel. This method allows an easy incorporation of palladium in the mixed-oxides, which are formed at moderate temperature with rather high specific areas (13-17 m*/g). The partial substitution of copper for palladium allows a strong improvement of the three-way catalytic activity, in particular for NO reduction. The light-off temperatures for the conversions of CO, NO and C,H, decreased markedly when increasing the palladium content, the activity of catalysts La,Cu,,,Pd,,,O, and La,Cu,,Pdu,,O, being comparable to that of a Pt-Rh/CeO,-AlaO, catalyst for NO reduction, and higher for CO and C,H, oxidation.

All the La&u 1 .Pd,O, catalysts are activated under reacting conditions. This activation corresponds to the destruction of the mixed-oxide structure, with formation of reduced Pd” ions atomically dispersed, surrounded by Cut and Cu*’ species on a lanthanum oxycarbonate matrix. This high dispersion state of the two transition metals in various oxidation states is supposed to originate from the initial La,Cu 1 _ * Pd,O, structure.

Keywords; Three-way catalysis; Exhaust gas purification; Sol-gel method; Lanthanum cuprate; Perovskite-related oxides; Palladium; Platinum substitution; Rhodium substitution

1. Introduction

Perovskites ABO, and related oxides have recently received much attention as catalysts for various reactions [ 11. The diversity of physicochemical properties and catalytic activities of perovskites and related oxides are derived from the fact that they exhibit a wide range of oxygen non-stoichiometry (oxygen excess

* Corresponding author. Fax: (+ 33-78) 941995.

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326 N. Guilhaume et al./Applied Catalysis B: Enuironmental 10 (1996) 325-344

as well as deficiency), and that their composition can be varied extensively in the form of AA’BB’O, compounds, allowing control of the valence state of metal ions by an appropriate choice of substituents [2].

Lanthanum cuprate, La,CuO,, has a K,NiF,-type structure consisting of alternate layers of ABO, perovskite and A0 salt [3]. The catalytic activity of La,CuO, and substituted-La,CuO, has been investigated for various reactions such as N,O [4-81 and NO [9] decomposition, methanol [lo] and CO [ 1 l-151 oxidation, H,O, decomposition [ 161, and NO reduction by CO [ 17- 191.

At present, three-way catalysts (TWC) used for automotive exhaust gas purification consist mainly of platinum and rhodium metals deposited on a CeO,-y-Al,O, washcoat. They insure CO and hydrocarbon total oxidation simultaneously with NO, reduction. Because of the limited supply and high cost, the replacement of noble metals by lower-cost active phases is highly desirable. Transition metal oxides were early candidates for automotive catalytic converters, but their low resistance to sulphur poisoning prohibited their use in place of noble metals. However, the progress in clean fuel production via hydrodesulfurization reactions now allows the use of very low sulphur-content fuels, and this problem has thus become less crucial.

We report here the preparation and catalytic activity of palladium-substituted lanthanum cuprates, La,Cu, _XPd,O,, as TWC. In order to increase the specific area and to improve the activity of the La,Cu,_,Pd,O, catalysts, the samples were prepared using a polyacrylamide (PAA) gel method [20]. This process is well adapted to the elaboration of highly divided and homogeneous powders as precursors for metal oxides. The solids were tested in conditions approaching the real working conditions of TWC, by the mean of cycling light-off experiments, in which the influence of alternatively oxidising and reducing feed streams were investigated, and by tests in the presence of large amounts of water vapour.

2. Experimental

2.1. Preparation of catalysts

All catalysts were prepared from water-soluble metal salts, complexed by citric acid, and embedded in a polyacrylamide gel formed by in-situ polymerization.

2.1.1. Typical preparation of L.u,CuO, La(NO,), .5H,O (99%, Aldrich) and Cu(NO,), . 3H,O (99.5%, Merck)

were used as precursors. The required amounts of metal salts (lop2 mol lanthanum nitrate, 5 . 1O-3 mol copper nitrate) were dissolved in 100 ml water and complexed by citric acid (one mole/cation positive charge). The pH of the

N. Guilhaume et al. /Applied Catalysis B: Environmental 10 (1996) 325-344 321

solution was adjusted to 6-7 by addition of concentrated ammonia. 6 g of acrylamide and 0.5 g of N,N’-methylenebisacrylamide (reticulating agent) were added, and the resulting solution was heated at 90-95°C. Polymerization of the organic monomer was initiated by adding 50 mg of azo-bis-isobutyronitrile (AIBN) dissolved in 2 ml ethanol, and 0.05 ml of N,N,N’,N’-tetramethylethyl- enediamide (TEMED) which acts as a radical transfer agent. Polymerization occurs generally within a few minutes.

2. I .2. Pd-containing catalysts In the case of the preparation of the palladium-substituted lanthanum cuprates,

the required amounts of Pd(NH,),(NO,), (Strem Chemicals) were dissolved in the aqueous solution together with the lanthanum and copper nitrates.

2.1.3. Calcination of the organic gels: The wet gels were calcined first at 450°C in a muffle furnace in air (heating

rate 2”C/min) for 2 h. The resulting meringue, very light and crumbly, was ground in a mortar, and the powder was calcined again at 700°C (heating rate 2”C/min) for 3 h in flowing air.

2.2. Physicochemical characterizations

Specific surface areas were measured by nitrogen adsorption at - 196°C on samples previously evacuated under vacuum (5 . 10e5 ton-) at 300°C.

Powder XRD patterns were recorded with a D 500 Siemens diffractometer using monochromatized CuKa radiation. The patterns were recorded at 3 G “(28) < 70 with a scan rate of 1.2” min- ‘. The patterns were compared with I.C.D.D. reference data for phase identification. The ‘Indexing’ program, 1.61 version, was used for the indexation of the tetragonal La,CuO, phase.

Elemental analyses were obtained from the ‘Service Central d’Analyses du CNRS’. Chemical analysis (wt.-% found (theoretically)): La,CuO,: Cu, 16.2% (15.68); La,Cu,,,,Pd,.,,O,: Pd, 0.35% (0.26), Cu, 14.9% (15.50); La,Cu,.,,Pd,,,,O,: Pd, 1.24% (1.311, Cu, 14.7% (14.82); La,Cu,,gPdO.,O,: Pd, 2.41% (2.601, Cu, 13.9% (13.96); La,Cu,.,PdO,,O,: Pd, 4.73% (5.14), Cu, 12.2% (12.28). The percentage of residual carbon was less than 1% in all samples.

Infrared spectroscopic study of CO adsorption was performed with a Nicolet Magna 550 FT-IR spectrometer. The spectral range observed was 4000-400 cm- ‘, with a resolution set to 4 cm-‘. The sample was pressed into a thin disk and introduced into a cell allowing in-situ treatment. The sample was outgassed under vacuum at 300°C then cooled at room temperature and CO (10 ton-s) was introduced. The solid was treated under CO at increasing temperatures (200, 300 and 400°C) and the spectra (32 scans) were recorded after cooling at room temperature.

328 N. Guilhaume et al. /Applied Catalysis B: Enuironmental 10 (1996) 325-344

X.P.S. and X-ray induced Auger measurements were carried out with a VG-type ESCA III spectrometer, using Al-Ka radiation (1486.6 eV). The binding energies were calibrated with respect to the C 1s energy of pollution graphitic carbon at 284.6 eV. When necessary, the spectra were deconvoluted using a computer program in order to separate the components of the experimental curve.

2.3. Catalytic activity measurements

Three-way catalysts insure both total oxidation (of CO and hydrocarbons) and NO reduction simultaneously. Because of the NO removal via a reduction reaction, excess of oxygen tends to be detrimental to NO conversion. Automo- tive systems have thus been designed to maintain a air/fuel ratio at or very near a stoichiometric mixture, by using an oxygen detector which detects oxygen imbalance and initiates appropriate corrections. As a consequence, the catalyst has to operate in the presence of a reacting mixture which composition fluctuates between fuel-rich and fuel-lean approximately once a second.

The catalytic activity of the solids was evaluated in both stationary and cycling conditions. The compositions can be varied from single component to complex mixtures including CO, NO, C,H,, 0, and H,O. Automotive exhaust contains various hydrocarbon compounds, but propene is often chosen as model hydrocarbon for more complex mixtures like alkenes which are the most abundant species in exhaust [21]. In cycling experiments, two feed streams with rich and lean compositions were sent periodically on the catalyst at a chosen frequency.

The laboratory system for activity tests [22] includes two independent feed stream blending sections equipped with two sets of five mass flow meters. The supply gases are 0, (5 vol.-%), CO (5 vol.-%), NO (1 vol.-%) and C,H, (1 vol.-%) diluted in nitrogen. The total flow rate of each stream is adjusted to 10 1. h- ’ by an additional flow of pure nitrogen. Cycling between the two streams is obtained by switching two electrovalves prior to the reactor section. The frequency of the oscillations can be varied from 0.075 to 1 Hz. The analytical train is composed by five infrared analysers (CO, CO,, NO, N,O, C,H,) and one paramagnetism analyser (0,). Before flowing through the analysers, the streams are homogenised in a mixing volume in order to analyse an average composition. The apparatus is connected to a computer which drives the different sections and collects the data.

The catalysts (0.20 g of powder) are loaded in a Pyrex plug flow reactor (internal diameter 10 mm).

Table 1 presents the simulated exhaust compositions chosen in stationary and cycling light-off tests. In the case of stationary light-off tests (feed stream A), the oxidants/reducers ratio is stoichiometric (S = 11, as defined by S = (2 0, + NO)/(CO + 9 C,H,). In cycling tests, the feed streams B and C were

N. Guilhaume et al. /Applied Catalysis B: Erwironmental 10 (1996) 325-344 329

Table 1 Gas compositions of simulated exhaust used in stationary and cycling light-off tests. Stoichiometric factor S=(2 0, +NO)/(CO+9 C,H,)

Composition (vpm)

Stationary Cycled

Feed stream A Feed stream B Feed stream C

02 co NO C,H,

N, Hz0 (when present) Stoichiometry (S)

5600 3317 7x23 6200 10781 1619 1000 1000 1000 667 667 667

balance balance balance _ 10% 10% Stoichiometric 1 ‘Rich’/reducing 0.462 ‘Lean’/oxidizing 2.184

sent periodically over the catalyst at a frequency of 0.1 Hz. The average composition of these two gas mixtures is the same as that in stationary conditions. We chose a frequency of 0.1 Hz, smaller than the oscillation frequency in an engine (around 1 Hz), because the design of our apparatus and the flow rate are such that at 1 Hz the two streams blend together and the catalyst receives an average composition. When cycling at 0.1 Hz, the compositions reaching the catalytic bed correspond to 80-85% of the two individual feed streams, as shown by tests with a catharometer in place of the catalytic bed. In the case of activity measurements in the presence of steam, the two feed streams flow through thermostated water saturators.

Each catalyst was evaluated for light-off performance in three reactions: . stationary light-off (heating rate 2°C min-‘1, . transient light-off (heating rate 5°C min- ’ >, - transient light-off in the presence of 10 vol.-% steam (heating rate 5°C

mine1 >, for the most active catalysts. The solids were cooled down under nitrogen between each activity test. A Pt-Rh/CeO,-y-Al,O, catalyst (1.13 wt.-% Pt, 0.19 wt.-% Rh, 19.3

wt.-% Ce) was used as reference TWC. It was prepared by successive graftings of y-alumina (107 m*/g) with Ce(acac),, followed by calcination at 400°C. This support was impregnated with aqueous solutions of H,PtCl, and RhCl,, dried at 110°C then calcined at 500°C under flowing nitrogen for 2 h. Platinum and rhodium were reduced under H, at 500°C during 2 h.

3. Results and discussion

3.1. Preparation and characterization of catalysts

La,CuO, can be easily prepared by decomposition and calcination of lanthanum and copper nitrates or oxides. However, in these conditions high

330 N. Guilhaume et al./Applied Catalysis B: Environmental 10 (19961325-344

Table 2 Specific surface areas of the La,&, _ .Pd,O, (x = O-0.2) catalysts prepared by the PAA gel method

Samples Specific areas (m* g- ‘1

La,CuO,

La,% ,,Pdo olO, LaKuo.~~Pdo.& La&u0 9Pdo.I0, La&u,.,Pdo 2%

13 17 14 15.8 17

temperature (1000°C) solid-solid reactions are required to form the desired product in a single phase, which lead to a solid with a very low specific area.

We improved the preparation method by using a polyacrylamide (PAA) gel process [20], which is intermediate between the ‘amorphous citrate’ and a sol-gel process: the metal cations are complexed by citric acid to obtain a stable aqueous solution, which is gelled by radical polymerization of two organic monomers. The complexation of metal cations by citric acid is important because polymerization of the organic radicals does not occur in the presence of cu2+ ions if they are not complexed, probably because of the coordination of the bidentate diamine TEMED which cannot act as a radical transfer agent.

This method is not a real sol-gel process in the sense that there is no building of an oxide or hydroxide network like in the case of hydrolysis/condensation reactions of metal alkoxides. But it allows to obtain powders with very homogeneous compositions, because the homogeneity of the salts in solution, which is blocked by the organic gel, is preserved upon calcination of this gel. The surface areas of the solids prepared by this method are also subsequently larger than those of solids prepared by ceramic methods: the specific areas are in the 13-17 m2 g-’ range (Table 2), because a calcination temperature of 700°C was high enough to insure the formation of the mixed-oxide.

X-ray diffraction patterns of the La,Cu, _xPd,O, solids prepared by the PAA method, after calcination at 700°C are presented in Fig. 1; only three spectra are presented, corresponding to x = 0, 0.05 and 0.2, because all samples present similar patterns. The diffraction lines are broad, and the samples crystallize as a mixture of orthorhombic and tetragonal La,CuO, phases. The pattern of La,CuO, also shows two small peaks which can be attributed to the main diffraction lines of CuO. It is known that the Ln,CuO, (Ln = Pr-Gd) compounds have a tetragonal K,NiF,-type structure [23], while La,CuO, adopts an orthorhombic structure, which is a distorted K,NiF,-type structure [3]. In our case a mixture of tetragonal and orthorhombic La,CuO, is observed.

Further calcination of La,CuO, at 1000°C allowed to obtain a sample of better crystallinity, whose pattern (Fig. 2) presents only the diffraction lines of purely orthorhombic La,CuO,, together with small lines corresponding to the most intense lines of Nd,O,. Chemical analysis confirmed that all the samples contain about 1.1 wt.-% Nd, present as an impurity in the lanthanum nitrate

N. Guilhaume et al./Applied Catalysis B: Erwironmental 10 (1996) 325-344 331

Fig. 1. X-ray diffraction patterns of the LaCu, ,Pd YO, catalysts after calcination at 700°C (A: x = 0, B: x = 0.05, C: x = 0.2). 0: orthorhombic La,CuO,: T: tetragonal La2Cu0,; v: CuO.

precursor. This suggests that the presence of this small amount of neodymium in La,CuO, can favour a tetragonal structure together with the orthorhombic one. After a calcination at lOOO”C, neodymium is expulsed from the structure and

Fig. 2. X-ray diffraction pattern of LaCuO, after calcination at 1000°C. v : Nd,O,.

332 N. Guilhaume et al. /Applied Catalysis B: Environmental 10 (19961 325-344

only orthorhombic La,CuO, is obtained. Mizuno et al. [24] studied the influence of lanthanum partial substitution by Ce or Sr, and of copper partial substitution by Zr or Al in La,CuO,. They also observed that in some cases this substitution can promote a different crystallization from orthorhombic to tetragonal: the structures of La,CuO,, La,,9Ce,,,Cu0,, La,.,Ce,,,CuO, and La,Cu,.,Zr,,,O, were orthorhombic, while those of La,.,Sr,.,CuO,, La,,,Sr,&uO, and La,Cq,,Al,,O, were tetragonal.

All the palladium-containing solids also crystallize as a mixture of tetragonal and orthorhombic phases (Fig. 1). No diffraction lines corresponding to palladium oxide or metal can be observed, suggesting that this metal is incorporated into the La,CuO, structure. However, the formation of very small PdO particles, which could not be detected by XRD, cannot be excluded. Infrared study of CO adsorption on these solids did not allow us to check this point, because the disks of self-supported catalysts are not transparent to IR.

3.2. Catalytic activity measurements

The activity of La,Cu, _xPd,O, catalysts (x = O-0.2) was evaluated first in a series of light-off experiments, in stationary conditions in the presence of the stoichiometric feed stream A defined in Table 1. Successive tests in identical conditions showed that in all cases the activity of the solids was not stable: the temperatures of isoconversions (i.e. 50 and 80%) of the three pollutants CO, NO and C,H,, decrease regularly in each test, suggesting a slow activation of the catalysts in the reaction conditions. The activity reached a stable and optimal level after 4 or 5 successive tests. This optimal activity could be attained directly by activation of the catalysts under CO at 500°C for 1 h. The results described hereafter and in Table 3 were obtained after this activation treatment of the solids. The activities of the catalysts La,Cu 1 _xPd,O, (x = O-0.2) are compared

Table 3 Temperatures PC) corresponding to 50 and 80% conversion of CO, NO and C,H,, in stationary and cycling light-off tests

Catalyst T (% conversion) Stationary Cycling

La,CuO,

LaD,.,Pd,.&~

T50 T80 T50 T80

La2Cuod’4.&4

La2Cuo Pdo.P,

La,C%aPdo.zO,

Pt-Rh/CeO, -Al,O,

T50 T80 T50 T80 T50 T80 T50 T80

co NO GH, co NO C,H, 315 415 295 320 460 290 385 445 320 400 > 500 320 250 355 255 255 360 255 285 425 265 310 420 285 220 340 260 220 355 270 240 400 280 260 410 300 205 320 245 210 280 250 215 340 270 230 370 280 195 320 250 170 300 270 205 345 275 210 365 305 250 310 300 255 290 295 300 335 340 300 315 315

N. Guilhaume et al. /Applied Catalysis B: Environmental IO (1996) 325-344 333

6000

5000

4000 ?

e

5

3000 i

8

s

2000 8

1000

0

150 200 250 300 350 400 450 500

TEMPERATURE (“C)

Fig. 3. Activity of catalyst La&O, for CO, NO and C,H, conversions in the presence of feed stream A, in stationary conditions.

with those of the reference catalyst Pt-Rh/CeO,-Al,O,, tested in the same conditions.

3.2.1. Activity of unsubstituted catalyst La,CuO, La,CuO, is known to be active in the reduction of NO by CO [ 17-191, which

is often considered as a model reaction in three-way catalysis. This solid was tested in the presence of the more complex feed stream A, containing CO, NO, 0, and C,H, in a stoichiometric composition (Fig. 3). The oxidation reactions (of CO and C,H,) are rather easy, but NO reduction is strongly inhibited in the presence of oxygen. The NO conversion curve shows clearly two different behaviours: in the temperature range 255-400°C the conversion starts slowly and reaches about 35% at 400°C; it corresponds to a gas mixture in which all oxygen is not consumed. At higher temperatures, when no more oxygen remains, the slope of the curve increases suddenly between 400 and 500°C; but at this temperature NO and C,H, are still incompletely converted.

3.2.2. Activity of Pd-containing catalysts in stationary conditions The palladium-substituted catalysts La,Cu, _XPd,O, (x = 0.01-0.2) were

tested in light-off tests, in the same conditions as La,CuO,. Fig. 4A-C shows the conversions of CO, NO and C,H, in stationary composition conditions. For comparison, the results obtained with a Pt-Rh/CeO,-Al,O, reference catalyst are included. Table 3 summarizes the light-off temperatures T50 and T80,

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100

90

80

70

$ 60

E B 50 8

g 40 6

30

20

10

0

150 200 250 300 350 400 450

TEMPERATURE (“c,

150 200 250 300 350 400 450

TEMPERATURE (“C)

N. Guilhaume et al. /Applied Catalysis B: Environmental 10 (1996) 325-344 335

corresponding to 50 and 80% conversion of the component for each catalyst. The introduction of palladium is clearly beneficial to the activity of the La,Cu, _XPd,O, catalysts: the conversion temperatures of palladium-containing solids are significantly lower than those of La,CuO,. Total conversions of the three pollutants are reached with all La,Cu, _XPd,O, (x # 0) catalysts at temperatures below 400°C.

Comparison of the catalysts La,Cu, _*Pd,O, (x = O-0.2) with the reference catalyst Pt-Rh/CeO,-Al,O, shows their high activity for oxidation reactions. Propene oxidation (Fig. 4C) is only slightly influenced by the introduction of palladium, since the four Pd-substituted solids have nearly the same activity, which is always superior to that of the Pt-Rh catalyst. The activity of La,CuO, itself is comparable to that of reference catalyst. This suggests that copper ions are responsible for the oxidation of propene. Copper oxide is well known for its high activity in methane [REF] and various hydrocarbon [REF] combustion; Cu2+ ions in the La,CuO, structure also present this good activity.

The strong effect of palladium on the activity can be noticed on Fig. 4A and 4B: as expected, Pd*+ ions seem to promote the CO + NO reaction. The activity of the two catalysts La,Cu,,, Pd,,,O, and La,Cu,,Pd,,,O, is comparable to that of reference catalyst for NO reduction, while CO oxidation is performed at lower temperatures. It can be noticed that a high Pd substitution (20 at.-% Pd relative to Cu> does not improve the activity of the catalyst. The optimum activity is reached with the composition of La,Cu,,,Pd,,,O,.

3.2.3. Effect of cycling between reducing/oxidizing feed streams The catalysts were tested in cycling conditions between rich and lean

compositions, chosen to be very reducing (S = 0.46, feed stream B) and very oxidant (S = 2.18, feed stream Cl, at a low frequency which insures large amplitude oscillations of the feed streams on the catalytic bed (superior to 80% of the individual feed streams). The oscillation of the reacting mixture does not have a conclusive effect on the catalyst activities; on Fig. 5 the differences in the T50 temperatures between stationary and cycling measurements are presented for the catalysts La,Cu, _.\-Pd,O,. The activity of palladium-containing solids is not always modified in the same way; cycling may be a penalty for the conversion of one component while it favours the conversion of another. On the whole, the activity is not much modified in cycling experiments.

An interesting feature of the La,Cu, _xPd,O, catalysts can be noticed with the analysis of N,O formation during cycling light-off tests (Fig. 6). N,O is a by-product of NO uncompleted reduction, and its formation has to be avoided as it contributes to the greenhouse effect. As can be seen on that figure,

Fig. 4. Conversions versus temperature on LaCu, _ ., Pd IO, (x = O-0.2) and reference catalysts, in stationary conditions; Pd ‘i corresponds to LaCu, _,Pd VO,. and Ref. to Pt-Rh/CeO,-Al,O,. (A) CO conversion; (B) NO conversion; (C) C,H, conversion.

336 N. Guilhaume et al./Applied Catalysis B: Environmental IO (1996) 325-344

250

200

150

lfl NO Station. I

110 NO Cycling

@ C3H6 Station. /

??C3H6 Cycling :

PdO PdO.01 Pd0.05 PdO.1 Pd0.2

Fig. 5. Comparison of the T50 temperatures, in stationary and cycling conditions for La,&, _,Pd,O, catalysts (x = o-0.2); Pd, corresponds to LaCu, _,Pd,O, catalyst.

La,Cu,_,Pd,O, catalysts produce much less N,O than reference Pt-Rh catalysts. The selectivity in N,O (relative to transformed NO) at the maximum of N,O formation is 18% for the reference Pt-Rh catalyst, but reaches only 7% at highest in the case of the La,Cu,,,Pd,,,O, catalyst.

140 I 130

120

110 i

PdO.1 Pd0.05

4

150 200 250 300 350 400 450 500

TEMPERATURE (“C)

Fig. 6. N,O formation (ppm) during cycling light-off tests. Pd, corresponds to the LaCu, _xPd,O, catalyst, and Ref. to Pt-Rh/CeO, -Al,O,.

N. Guilhaume et al/Applied Catalysis B: Environmental 10 (1996) 325-344 331

Table 4 Temperatures (“Cl of 50 and 80% conversions with catalysts La,Cu, _xPd,O, (x = 0.05-0.2), in the absence or in presence of 10 vol.-% steam, in cycling conditions

Catalyst T (% conversion) Cycling Cycling + 10 vol.-% Ha0

CO NO C,H, CO NO C,H,

LaKuO.ajPdO& T50 220 355 210 225 415 275 T80 260 410 300 250 485 305

La,Cu,,,Pd,,Q T50 210 280 250 195 390 260 T80 230 370 280 210 465 290

LaJua.aPda a04 T50 170 300 270 165 290 250 T80 210 365 305 175 370 270

Pt-Rh/CeO, -Al,O, T50 255 290 295 < 150 260 250 T80 300 315 315 175 280 270

3.2.4. Effect of steam in cycling experiments The most active catalysts, containing 5 to 20 at.-% Pd (relative to Cu), were

tested in cycling conditions in the presence of 10% H,O. The comparison of conversion temperatures in the absence or in the presence of steam is presented in Table 4. If CO and C,H, oxidation reactions are almost unaffected by the presence of steam, NO reduction is strongly inhibited, at least in the case of the less Pd-substituted catalysts (x = 0.05 and 0.1). Only the La,Cu,,,Pd,,O, solid keeps an activity unchanged for NO reduction when steam is present (Fig. 7). On the contrary, the reference catalyst Pt-Rh/CeO,-Al,O, is activated when

100 1

1

150 200 250 300 350 400 450 500

TEMPERATURE (“C)

Fig. 7. Comparison of the conversions of NO in the absence or in the presence of 10% steam, for LaCu, asPd a,asO,, LaCu,,Pde ,O, and LaCu, sPd,,O, catalysts, in cycling conditions. Pd, corresponds to the LaCu, rPd,O, catalyst.

338 N. Guilhaume et al./Applied Catalysis B: Environmental 10 (1996) 325-344

water is present (Table 41, as can be evidenced by the lower T50 and T80 temperatures.

An explanation to the inhibition of NO reduction in the presence of steam could be the activity of these three catalysts in water gas shift (W.G.S.) and steam reforming reactions; additional experiments showed that they can catalyse carbon monoxide and propene oxidations by steam, at moderate temperatures (300-450°C). Classical Pt-Rh-based three-way catalysts are also active for these reactions, but the hydrogen produced can be dissociated on the metal particles and reduce NO. On oxide catalysts, H, is not dissociated, and cannot be useful to NO reduction, while the other reductants, in particular CO, are consumed by water. Furthermore, a strong adsorption of water at the surface of these catalysts, which would poison the sites for NO adsorption, can also be responsible for the inhibition of NO reduction.

3.3. EfSect of activation: characterizations qf catalysts after catalytic activity measurements and after activation under CO

X-ray diffraction studies of catalysts after activity tests reveal that the initial solids are deeply modified after reactions: the La,Cu, _ ,Pd,O, structure is decomposed in all cases and is not observed anymore in the diffraction pattern, which is that of lanthanum oxycarbonate La,CO, (Fig. 8). The formation of La,CO, can be due either to carbonation of La,O, by CO, produced during the reactions, or simply by contact with atmospheric CO, after tests, as no special precautions were taken to keep the samples under inert atmosphere. However, the strong affinity of La,O, for CO, and the high stability of the oxycarbonate La,CO, (up to 1000°C under 1 atm CO, [25]) suggest that it is formed as soon as the La,Cu, _-I Pd,O, structure collapses, and could even be the driving force of this collapse. The La,CuO, structure consists in (Lao)+ sheets intercalated between the (LaCuOJ perovskite structure. These sheets of lanthanum oxide may react with CO,, leading to the destruction of the K,NiF, structure.

Only three weak diffraction lines in the XRD patterns cannot be attributed to La,CO,. Their assignation to copper or palladium species is uncertain. Two of

Fig. 8. X-ray diffraction pattern of LaCu, 8 Pd, 204 after catalytic tests; v : La2C0,; + Cu,O (?)

N. Guilhaume et al. /Applied Catalysis B: Enuironmental 10 (1996) 325-344 339

them could correspond to Cu,O, but the pattern is too noisy to attribute these very weak lines precisely. No Pd-containing phase is observed, though the amounts of palladium contained in catalysts La,Cu,.,Pd,,,O, and La,Cu,,,Pd,,,O, (2.41 and 4.73 wt.-%, respectively) should be enough to allow its detection by XRD, subject to sufficient crystallization of the particles.

The palladium phase formed by treatment under CO was better characterized by IT-IR study of CO adsorption on La,Cu,,Pd,.,O,; as mentioned before, the fresh catalyst is not transparent to IR, and no absorption bands can be observed. However, upon thermal treatment of the catalyst at 400°C under CO, the sample becomes transparent, together with the appearance of very strong carbonate bands (1350-1550 cm-‘, saturated). Two vC0 bands are observed at 2115 and 2064 cm- ‘. The first one can be assigned to CO coordinated linearly on Cu”+ (Cu’ or Cu2+) [26]. Its position could also correspond to CO coordinated on Pd2+, but palladium ions are expected to be reduced by CO at 400°C and their presence is very unlikely. The second band at 2064 cm-’ is typical for CO linearly adsorbed on Pd” [27]. The activation of the catalysts leads to the reduction of Pd2+ ions into the metallic state. It is interesting to note that no bands corresponding to CO bridging two Pd atoms are observed in the 1920- 1960 cm-’ region. This suggests that reduced palladium atoms are very closely associated with CL?‘+ species, and in a certain way ‘diluted’ on the copper surface. This intimate association between palladium and copper, originating from the initial mixed-oxide structure, could be the reason for the high activity of these catalysts.

XPS spectra of the most Pd-loaded catalyst La,Cu,,,Pd,,,O,, in fresh state and after activity measurements, were recorded in the La 3d, Cu 2p, Pd 3d, 0 1s and C 1s regions. The corresponding binding energies are shown in Table 5.

The XPS spectra of the Cu 2p region present an intense satellite peak on the high binding energy sides of the Cu 2p,,, and Cu 2p,,, peaks (Fig. 9A). Such a peak was also observed by Mizuno et al., who studied the reduction of NO by CO on solids La ,_,A,Cu,_,B;O, (A’ = St-, Ce; B’ = Al, Zr) [24]. The presence of this satellite peak is characteristic for Cu2+ species, while the Cu+ peak appears at nearly the same binding energy but with no satellite peak. The

Table 5 Binding energies of La,&, ,Pdo 204 in fresh state and after activity measurements

Binding energy (eV)

La 3d,,, Cu 2P,,, Pd 3d,,,

0 1s

Fresh sample

835.6 935.4 331.2

529.3 531.9

After catalytic tests

836.8 935.2 335.7 338.9 530.0 532.5

340 N. Guilhaume et al. /Applied Crrfalysis B: En~~ironmenfd 10 (1996) 325-344

957 937 Binding Energy I eV

929.2 909.2

Kinetic Energy I eV

Fig. 9. XPS (A) and Auger (B) spectra of catalyst La?Cu,, Pd,,,O, in fresh state (I). and after activity measurements (2), in the Cu 2p regions.

intensity ratio Z,,,/Z,,,;,i, can therefore be a measure of the oxidation state of copper. In the case of catalyst La,CuO,, they observed that the ratio Zsat/Zmain decreased when the solid had been used for the CO + NO reaction, which was attributed to the formation of Cu+ species. The Auger spectra of copper also presented a Cut peak, in addition to the Cu’+ Initial peak, when catalyst had been submitted to reacting conditions. The ratio Zsat/Zn,ain in the XPS spectra of catalyst La,Cu,,, Pd,,,O, after tests also decreases when compared to the fresh solid, and the characteristic Cut Auger peak is also present (Fig. 9B). After activity tests the catalyst contains now Cut and Cu2+ ions.

The position of the Pd 3d doublet in the spectrum of a fresh catalyst (Fig. 101 is typical for a Pd2+ species, but in a surrounding different from palladium oxide, and can reasonably be assigned to Pd*+ ions in the La,&, _lPd,O, structure. After catalytic tests, the spectrum presents in addition a new doublet at a lower B.E., typical for PdO (6.5% of the total Pd). It can be noticed that still about 35% of the palladium remains in an oxidized form, which was not revealed by CO adsorption. This suggests that the Pd” obtained after reduction is

N. G~till~aume et al. /Applied Catalysis B: Environmental 10 (19961 325-344 341

345 343 341 339 337 335

Binding Energy / eV

Fig. 10. XPS spectra of catalyst LazCu,,,, Pd, ?04 in fresh state (I ), and after activity measurements (2), in the Pd 3d regions. In spectrum 2. the full line is the sum of the two doublets (dashed lines).

located on the surface of the catalyst particles, and therefore responsible for the activity, while unreduced Pd2+ ions remain in the bulk and are not accessible to co.

The La 3d,,, doublets are ill defined in the spectrum of the fresh catalyst; on the opposite, they are well resolved and displaced in the spectrum of an used catalyst. The same phenomenon was observed in the La 3d XPS spectra of oxidized and HZ-reduced perovskite LaCoO, [28], where the ill-resolved doublet was assigned to La engaged into the perovskite structure and the well-resolved one to La,O,. The high B.E. measured for catalyst after tests corresponds well to that of lanthanum carbonate [29].

The presence of surface carbonates in the fresh catalyst, which can also be observed in the infrared spectra (KBr pellets), and in higher amounts in the used solid is also confirmed by the 0 1 s spectra. Two 0 1 s peaks are observed in the spectra of fresh and after tests-catalysts, which can be attributed to lattice oxygen (529.3 and 530.0 eV, respectively) and oxygen of carbonates (531.9 and 532.5 eV, respectively). Their relative percentages are 40/60% in the fresh catalyst, and 26/74% in used catalyst, and confirm the higher amounts of carbonates in the catalyst after tests. The formation of a surface carbonate layer was also shown on freshly prepared LaCoO, [28]. In the case of catalyst LazCu,,PdO.,O,, which was prepared in an organic gel, the large amount of surface carbonate in the fresh catalyst can be found by comparison of atomic ratios measured by XPS and chemical analysis (Table 6). The surface and bulk

342 N. Guilhaume et al/Applied Catalysis B: Enuironmentai 10 (1996) 325-344

Table 6 Surface and bulk atomic ratios (relative to La) deduced from XPS and chemical analysis, for catalyst

LaKu,.,Pd&~

La cu Pd 0

Fresh catalyst

Surface analysis (XPS)

1 0.284 0.087 2.75 1

Chemical analysis

1 0.41 0.095 2.32

Catalyst after tests

Surface analysis (XPS)

1 0.254 0.058 3.014

atomic ratios are very different, the surface being obviously enriched in lanthanum and oxygen, due to the formation of a surface lanthanum oxycarbonate.

The activation of the catalysts, which was observed under reacting conditions, or achieved more quickly by thermal treatment of the catalysts under CO, corresponds to the collapse of the La,Cu, _xPd,O, structure, together with partial reduction of Pd2+ and Cu2+, leading to the formation of Pd” and Cu+ highly dispersed particles, in addition to the presence of Pd2+ and Cu2+. Previous studies of perovskite-type solids LaMO, (M = Fe, Ni, Co, Rh) have shown that their reduction by hydrogen leads to the zerovalent state of the transition metal, which is also obtained in a highly dispersed state in the La,O, matrix, the metal particle size being in the range 2.5-4 nm according to the nature of the metal, and not detected by XRD [30].

The active state of catalysts La,Cu, _xPd,O, ( x = O-0.2) must be considered as Pd” atoms in intimate association with Cu,O and CuO, highly dispersed on the lanthanum oxycarbonate as support. The role of the initial mixed-oxide structure should be restricted to the formation of highly dispersed active phases in various oxidation states. These results have to be related to the work of Tanaka et al. [31] who designed new potential three-way catalysts composed of palladium deposited on Co/Fe-perovskite oxides. Lin et al. also studied [32] complex catalysts based on copper or cobalt metal oxides promoted with La, Ce, Sr, Zr and small amounts of Pt and Rh, deposited on alumina. They also observed that a significant improvement of the activity, in particular for the reduction of NO, was obtained when the catalysts were doped with Pt and Rh.

4. Conclusion

Preparation of Pd-substituted La,CuO, by a PAA gel method allows the easy substitution of copper by palladium, the Pd2+ ions being incorporated in the La,CuO, structure. The mixed-oxide phases were formed at moderate temperatures with rather large surface areas.

The partial substitution of copper by palladium has a strong effect on the catalytic properties of these solids. The tests in the presence of complex

N. Guilhaume et al. /Applied Catalysis B: Enuironmental 10 11996) 325-344 343

mixtures of CO, NO, 0, and C,H,, including cycling between strongly oxidizing/reducing gas mixtures, showed that the solids La,Cu,,Pd,~,O, and La,Cu,,,Pd,,,O, are highly active for the simultaneous conversion of the three pollutants. Their activity can be compared to that of a noble metal-based Pt-Rh/CeO,-Al,O, catalyst. However, in the presence of large amounts of additional steam, the reduction of NO is strongly inhibited, and only the most Pd-substituted catalyst keeps the same activity for this reaction as in the absence of steam.

Restructuration of these catalysts occurs under reacting conditions, leading to a more active state. This activation corresponds to destruction of the initial mixed-oxide structure, with formation of highly-dispersed metallic palladium and Cu,O particles. The initial structure of the single mixed-oxide phase may be the key to the formation of this highly dispersed state.

Acknowledgements

The authors thank Mr. Pierre Delichere, from the Institut de Recherches sur la Catalyse, for XPS measurements and interpretation.

References

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