Effect of Alkaline Addition on Hydrocarbon Oxidation Activities of Palladium Three-way Catalyst

CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLI'V Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.

83

Effect of Alkaline Addition on Hydrocarbon Oxidation Activities of Palladium Three-way Catalyst H. Shinjoh, N. Isomura, H. Sobukawa, and M. Sugiura TOYOTA CENTRAL R&D LABS., INC. Nagakute-cho, Aichi-gun, Aichi-ken, 480-11, Japan ABSTRACT The effect of alkaline addition on hydrocarbon oxidation activities of Pd catalyst supported on ,/-alumina was investigated using simulated automotive exhaust gases. The hydrocarbon oxidation activity of the Pd catalyst with alkaline earth metal such as Mg, Ca, Sr, and Ba is higher than that with nothing added. On the other hand, the activity of the Pd catalyst with alkali metal such as K and Cs is lower than that with nothing added. From the results of the partial reaction orders in C3H6 oxidation, TPR, and XPS, it was concluded that the alkaline addition to the Pd catalyst increased electron density of Pd on the catalyst and weakened the adsorption strength of Pd with hydrocarbons. The addition of alkaline earth metal suppressed the hydrocarbon chemisorption on the Pd catalyst and therefore allowed the catalytic reaction to proceed smoothly. On the other hand, the addition of alkali metal caused such a strong oxygen adsorption on Pd that rejected the hydrocarbon adsorption and suppressed the reaction. 1. INTRODUCTION Automotive three-way catalysts consist of noble metals, supports with a large specific surface and some additives. Many kinds of additives, such as cerium oxide, nickel, and other compounds, are used to supplement the activity of noble metals and to improve durability of automotive three-way catalysts. Cerium oxide is generally added to store oxygen under oxidizing conditions and to release the stored oxygen under reducing conditions(1-8). Nickel compound is often added as a scavenger of hydrogen sulfide and its addition also improves catalytic activity(9-12). Recently, barium(Ba) compound has been added to some kind of three-way catalysts, for example, palladium(Pd) only three-way catalyst and NOx storage reduction three-way catalyst(13-15). In spite of the fact that automotive three-way catalysts containing such alkaline compounds are already in practical use, the effect of alkaline addition on the catalytic activities of the catalysts are not yet clear. The authors have been studying the effect of alkaline addition on the catalytic activity of automotive three-way catalysts. We have found that the addition of Ba to Pd or platinum(Pt) three-way catalysts is effective for improvement of catalytic activity under reducing conditions, and that the suppression of hydrocarbon(HC) chemisorption on the catalysts by the addition of Ba allowed the catalytic reaction to proceed smoothly (16,17). This paper systematically reports the effect of alkaline addition, that is, alkali metals and alkaline earth metals, on hydrocarbon oxidation activity of Pd three-way catalyst.

84 2. EXPERIMENTAL

2.1. Catalysts The Pd catalyst was prepared impregnating y-alumina powder (a BET area of 200m2/g) with aqueous solution of palladium nitrate. The powder was dried overnight at 110~ in air, followed by calcination at 600~ for 5h in air, pressed, crushed, and sieved into 0.5 to 1.0 mm particles. The Pd/X catalysts (X=Li, Na, K, Cs, Mg, Ca, Sr, and Ba) were prepared impregnating the Pd catalyst powder with aqueous solution of each nitrates. The powder was dried, calcined, followed by pressing, crushing, and then sieved into particles in the same procedures mentioned above. Pd and alkaline loading amounts were 0.024 and 0.17 mole to one molar y-alumina, respectively.

2.2. Catalytic activity measurements The laboratory reaction system used was a conventional flow system with a tubular fixedbed reactor as described elsewhere(18). The characteristic feature of this system is its ability to simulate various air to fuel ratios (A/F) of automotive exhaust gases using eight mass flow controllers. In this study, catalytic activity on the catalysts in simulated automotive exhaust gases was measured as a function of E, which is a normalized value of A/F by a stoichiometric -l one in the simulated exhaust gas, at 300~ and 420,000 h space velocity. The compositions of the simulated exhaust gases for each ~, are shown in Table 1. Catalytic activity was expressed as percent conversions of NOx(NO+NO2), CO, and HC. C3H6 oxidation activity was also measured to decide the kinetic parameters on the catalysts using the same laboratory reaction system. The compositions of C3H6 and O 2 were changed from 0.017 to 0.133 vol% and from 0.1 to 1.3 vol%, respectively, and space velocity was the same as that mentioned above.

2.3. Temperature programmed reduction(TPR) measurement

The TPR measurement was performed using a flow system with a fixed-bed tubular reactor as described elsewhere(19). The 5 vol% H2/Ar was used in this measurement. Space -1

velocity was 7,000 h and heating rate was a linear rate of 50~ from -30 up to 300~ The effluent from the reactor was analyzed by both a thermal conductivity detector and a quadrupole mass spectrometer.

2.4. X-ray photoelectric spectroscopy (XPS) measurement The XPS measurement was performed using PHI-5500MC with MgK~ radiation as -9 incident beam. The base pressure of the instrument was 1x 10 Tort. Before conducting the XPS analysis, a catalyst was heated in pretreatment chamber connected to XPS chamber under 10 Tort of 10vol% C3H6/N2 at 400~ and then the pretreatment chamber was evacuated to 10 Tort and the catalyst transferred into the spectrometer without exposure to air. The electronbinding energy scale was calibrated by assigning 74.2 eV to A1 2p peak position.

85 Table 1 Compositions of simulated exhaust gases (N2 balance). #:[HJCO]=I/3

~. H2/CO#

C3H6

NO

(%)

02

C02 H20

0.960

2.00

0.062

0.12

0.41

10.0

3

0.967

1.73

0.060

0.12

0.41

10.0

3

0.974

1.49

0.058

0.12

0.43

10.0

3

0.980

1.33

0.057

0.12

0.46

10.0

3

0.985

1.20

0.056

0.12

0.49

10.0

3

0.990

1.00

0.055

0.12

0.54

10.0

3

0.992

1.05

0.055

0.12

0.56

10.0

3

0.994

1.01

0.054

0.12

0.57

10.0

3

0.996

0.99

0.054

0.12

0.60

10.0

3

0.998

0.96

0.054

0.12

0.62

10.0

3

0.999

0.95

0.053

0.12

0.63

10.0

3

1.000

0.93

0.053

0.12

0.65

10.0

3

1.001

0.92

0.053

0.12

0.66

10.0

3

1.002

0.91

0.053

0.12

0.67

10.0

3

1.004

0.88

0.053

0.12

0.70

10.0

3

1.006

0.85

0.052

0.12

0.72

10.0

3

1.008

0.84

0.052

0.12

0.75

10.0

3

1.010

0.83

0.052

0.12

0.78

10.0

3

1.015

0.77

0.051

0.12

0.85

10.0

3

1.020

0.73

0.050

0.12

0.92

10.0

3

1.027

0.69

0.050

0.12

1.03

10.0

3

1.034

0.65

0.049

0.12

1.13

10.0

3

1.040

0.60

0.049

0.12

1.21

10.0

3

3. R E S U L T S A N D D I S C U S S I O N

The conversions of HC, CO, and NOx on the Pd and Pd/Sr catalysts plotted as a function of ~. in simulated exhaust gases at 300~ are shown in Figs.1 and 2, respectively. The catalytic activity on the Pd/Sr catalyst was superior to that on the Pd catalyst, in particular, under reducing conditions defined as ~.<1. The conversion of HC, as representation of hydrocarbon oxidation activity, on the catalysts with alkaline earth metals and that with alkali metals plotted as a function of ~. in simulated exhaust gases are shown in Figs.3 and 4,

86 respectively, in comparison with the Pd catalyst with no such additives. The hydrocarbon oxidation activity in the simulated exhaust gases under reducing conditions for the additives followed the order of Ba > Sr, Ca > Mg > none > Li > Na > K > Cs The alkaline earth metal addition to the Pd catalyst improved the hydrocarbon oxidation activity. Similar phenomena have been observed on Pd/Ba and Pd/La catalysts, and it is concluded that the suppression of hydrocarbon chemisorption on Pd by the addition of Ba or La allows the catalytic reaction to proceed smoothly under reducing conditions(16,20). On the other hand, the alkali metal addition, especially K or Cs, to the Pd catalyst deteriorated the hydrocarbon oxidation activity. In C3H6-O 2 reaction system, the hydrocarbon oxidation activity on the Pd catalysts with alkaline compound was measured to get further information. The rate of carbon dioxide formation V(CO2) in the reaction of C3H 6 with 02 is given by the following equation (1). (1)

V(CO2)=kxP(C3H6)mx P(o2)nxexp(-AE/RT)

where P(C3H6) and P(02) are the partial pressures of C3H6 and 02, m and n are the partial reaction orders of CBH6 and O 2, respectively. The values of m and n were determined from a conventional log-log relationship between V(CO2), obtained

i00

l'"

NOx

80cC3 O3 C_ [13 > CO C_3

60-

40-

/ I

20 --

I,

0.96

I

'

'

' i

/ /

H C/" ./

/

/

J

'1

/

/

CO

!

~

1.

0.98

!

_

1

i.O0

I

......

!.

.

.

1.02

.

1

, !

1.04

A Fig.1 Conversion efficiencies as a function of X in simulated exhaust gases at 300~ catalyst.

for Pd

87

100

c

0

/'

NOx

80

60

/

~

a

/

-r-t

r._. > o

rj

/ H C

40

/

/

/

/

/

/

/CO

20

0

_l

. . . . .

t

0.96

.....

I

I

t

o.g8

_

t

I

1.00

!

t.02

.,. f

,

1.04

A

Fig.2 Conversion efficiencies as a function of X in simulated exhaust gases at 300~ catalyst.

I00

....

,

'

80 C o .r,,4

60

r_

:> c o

40

I

!

.

.

.

.

.

.

.

.

for Pd/Sr

w

none

.Z/,//"


/

(.3 rj :2=

2O --

9

!

0.96

! ....

!

0.98

I

,

t

.....

1.00

t

.....

I,

1.02

t

....... f

1.04

A Fig.3 The effect of alkaline earth metal addition: HC conversion as a function of X in simulated exhaust gases at 300~ on the Pd catalyst.

88

t00

- i

I

1

"

1

'

'

I

.

.

.

.

I

"

i

.......

~

I

"-

I--

--

80 C o .r-,,t

50

no

lkl C 1:3

""

40-

""~'"~"

/ / i /..///

20-

.....

I

~

0.96

~

.

I

0.98

l

I

1.00

__

1

_l

1.02

_

!

f

,

1.04

A Fig.4 The effect of alkali metal addition: HC conversion as a function of X in simulated exhaust gases at 300~ on the Pd catalyst. under conditions of low conversion of usually less than 30%, and partial pressures of respective species. The partial reaction orders in the C3H6-O2 reaction system determined on the Pd and Pd/X catalysts (X=Li, Na, K, Cs, Mg, Ca, Sr, and Ba) are summarized in Table 2. The partial reaction order m in the equation (1), was a large negative order(-1.39) on the Pd catalyst. The m values on the Pd/X catalysts with the alkaline components were all larger than that on the Pd catalyst, even showing positive orders for the catalyst with K or Cs. On the other hand, the partial reaction order n was positive on all the catalysts except for the catalysts with K and Cs. However, it was nearly zero or a negative order on the catalyst with K or Cs, respectively. The relationship between the partial reaction order m and the hydrocarbon oxidation activity in the simulated reducing automotive exhaust gas at k=0.98 are shown in Fig. 5. It was found that the maximum order of m was a certain negative one(-0.6), that is, the hydrocarbon oxidation activity increased with increasing the order of m from -1.39 to -0.6 and decreased with increasing the order of m further than -0.6. The intensity of Pd 3d peak by XPS of Pd, Pd/Ba, and Pd/K catalysts plotted as a function of binding energy is shown in Fig. 6. The dashed line shows the position of Pd 3ds/2 peak at 335.0 eV, which is in very good agreement with that in other studies (21,22). The value of Pd 3d peak on the Pd/Ba catalyst was a little smaller than that for the Pd catalyst. The shift value was approximately -0.2 eV. On the other hand, The value of Pd 3d peak on the Pd/K catalyst was much lower than that on the Pd catalyst. These data indicates that electron density of Pd increases because of electron transfer from alkaline compounds to Pd on the catalyst. The shift of Pd 3d peak on the catalysts with alkaline earth metals was almost the same as that on the Pd/Ba catalyst, and that on the catalyst with Na or Cs were larger than those on the catalysts with alkaline earth metal.

89 Table 2 Partial reaction orders in C3H 6 oxidation on Pd catalysts. [V(CO2)=kxP(C3H6)mx P(o2)nxexp(-AE/RT)] m

iO0

--

Pd

-1.39

1.10

Pd/Li

-0.46

0.71

Pd/Na

0.00

0.45

Pd/K

1.21

-0.70

Pd/Cs

3.64

0.01

Pd/Mg

-0.81

0.85

Pd/Ca

-0.45

0.91

Pd/Sr

-0.93

0.96

Pd/Ba

-0.73

0.61

~-

-

Ba

Sr ~0 Ca

80 co s cD

150 n o n e ~ ~ O Mo Na

>

40

U

20

c o u

"1"

0

K

0 __

-2

9

-1

I

i

....

i

2

,1

,

i

3

Fig. 5 The relationship between m, as the partial reaction order with respect to C3H6, and the HC conversion for ~=0.98 in simulated exhaust gases at 300~ for the Pd catalysts with alkaline compounds.

90 The H 2 uptake on the Pd and Pd/K catalysts in flowing 5 vol% H2/Ar plotted as a function of temperature obtained by TPR measurement using the quadrupole mass spectrometer are shown in Fig.7. The oxygen adsorbed on Pd of the Pd catalyst was reduced below 100~ On the other hand, that of the Pd/K catalyst was harder to reduce than that of the Pd catalyst. The molar ratio of H 2 uptake to Pd on the Pd and Pd/K catalysts were 0.5 and 2.0, respectively. It suggests that H 2 reduces not only Pd oxides but also potassium compound, perhaps potassium carbonate. ; .

._~.__~.~ '

.

.

.

.

.

.

.

'

. . . .

i

I

"

. . . .

~

. . . . . . .

Pd/K

I

*f-"l

C 4J C H

_

346

|

. . . . . . . .

342

|

,

,

334

338

Binding

330

Energy

326

(eV)

Fig. 6 XPS spectra of Pd 3d for Pd, Pd/Ba, and Pd/K catalysts. The dashed line shows the position of Pd 3d5/2 peak at 335.0 eV.

~

o

~

c~

Pd/K

Cq

.

I

tO0

.

.

.

.

.

.

!

....

200

'I'emp e ra ture (~) Fig. 7 H 2 uptake on the Pd and Pd/K catalysts obtained by TPR measurement.

300

91 4. CONCLUSIONS The effect of alkaline addition on the hydrocarbon oxidation activity of Pd catalyst loaded on ~,-alumina was investigated using simulated exhaust gases. The hydrocarbon oxidation activity of Pd catalyst with alkaline earth metal such as Mg, Ca, Sr, and Ba is higher than that with nothing added. On the other hand, the activity of Pd catalyst with alkali metal such as K and Cs is lower than that with nothing added. From the partial reaction orders in the C3H6-O2 reaction system and characterization by XPS and TPR on the catalysts, it was concluded that the alkaline addition to the Pd three-way catalyst weakened the adsorption strength of hydrocarbons on Pd. The addition of alkaline earth metal suppressed the hydrocarbon chemisorption on the Pd catalyst and therefore allowed the catalytic reaction to proceed smoothly. On the other hand, the addition of alkali metals, in particular K or Cs, caused such a strong oxygen adsorption on Pd that rejected the hydrocarbon adsorption and therefore suppressed the reaction. It was considered that the effect of the alkaline addition to the strength of adsorbed hydrocarbons on Pd was caused by the increase of electron density of Pd. REFERENCES 1. 2. 3. 4. 5.

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