1990 Catalyst Characterization by a Probe Reaction the Number of Active Hydrogen

8/10/2019 1990 Catalyst Characterization by a Probe Reaction the Number of Active Hydrogen

1/13

JOURNAL OF CATALYSIS 12 6, 388--400 (1990)

Catalyst C ha racter izat ion by a Probe Reaction The Num ber of

Act ive Hydrogen

E D U A R D O C H O R E N , J o s i~ O . H E R N A N D E Z , A R N E D O A R T E A G A ,

G E O M A R A R T E A G A , H E L l L U G O , M A R C O S A R R A E Z , A N D RI ~S P A R R A ,

A N D J O R G E S ,~ N C H E Z

Laboratorio de Superficies Escuela de lngenieria Quimica Universidad del Zulia Apartado 526

Maracaibo 4001 Venezuela

R e c e i v e d J a n u a r y 1 9 , 1 9 8 9 ; r e v i s e d M a r c h 2 7 , 1 9 9 0

H y d r o g e n t i t r a ti o n b y e t h y l e n e p u ls e s a t ro o m t e m p e r a t u r e i s p r o p o s e d f o r m e a s u r i n g th e n u m b e r

o f a c ti v e h y d r o g e n a t o m s ( N A H ) o n m e t a l - su p p o r t e d c a t a l y st s . T h e r e a c t i o n p r o d u c t , e t h a n e ,

d e s o r b s q u a n t i t a t iv e l y a n d c a n b e m e a s u r e d d i r e c t ly b y g a s c h r o m a t o g r a p h y . O n P t /A i 2 0 3 c a t a l y s ts

t h e d e t e r m i n a t i o n o f h y d r o g e n c h e m i s o r p t i o n b y t i t r a ti o n o f a d s o r b e d e t h y l e n e w i t h h y d r o g e n p u l s e s

a t r o o m t e m p e r a t u r e g i v e s v a lu e s i d e n t ic a l to t h o s e d e t e r m i n e d b y d i r e c t c h e m i s o r p t i o n o n c l e a n

c a t a l y s t s . T h e m e t h o d w a s t e s t e d o n P t / A I 2 03 a n d P t /S i O E c a t a l y s t s . R e s u l t s s h o w t h a t f o r P t /A I 2 0 3

t h e n u m b e r o f a c ti v e h y d r o g e n c a n e x c e e d s e v e r a l t i m e s th e n u m b e r o f m e t a l a t o m s o n t h e c a t a l y s t,

a n d i n c r e a s e s w i t h t h e t im e o f t r e a t m e n t w i t h h y d r o g e n a t h i g h te m p e r a t u r e . T h e i n it ia l t u r n o v e r

r a t e fo r e t h y l e n e h y d r o g e n a t i o n i s c o n s t a n t w h e n r e f e r r e d t o N A H , b u t n o t w h e n r e f e r r e d t o th e

p e r c e n t a g e o f e x p o s e d m e t a l a t o m s , a s d e t e r m i n e d b y h y d r o g e n c h e m i s o r p t io n . F o r P t /S i O z c at a -

l y st s , m e a s u r e m e n t s s u g g e st t h a t t h e n u m b e r o f a c ti v e h y d r o g e n a t o m s i s le s s t h a n t h a t i n d i c a te d

b y t h e H C d e t e r m i n a t i o n p e r f o r m e d b y d i r e c t a d s o r p t i o n o f h y d r o g e n . 1990Academic Press, In c.

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

Numerous attempts have been made to

relate the catalytic activity for olefin hydro-

genation with the fraction of exposed metal

1-18). As a new approach to the problem

distinct from the chemisorption techniques

described in the papers cited above ethyl-

ene hydrogenation is proposed as a probe

reaction for counting the number of active

hydrogen atoms on the catalyst surface. In

this work ethylene hydrogenation is carried

out using pulses of ethylene at room temper-

ature on a catalyst saturated with hydrogen.

The reaction product ethane is quantita-

tively desorbed and measured. Conversely

adsorbed ethylene is then titrated with hy-

drogen pulses.

Previously Bond and Sermon 19) pro-

posed a method of olefin titration by a flow

react ion at 373 K using a low partial pressure

of 1-pentene in nitrogen. This procedure dif-

fers from ours in three ways: First the oper-

0021- 9517 / 90 3 . 00

Copyright 1990by Academic Press, Inc.

All rights of reproduction n any fo rm reserved.

ation at 373 K reduced through desorption

the amount of hydrogen to be titrated as

shown in TPD spectra

11, 20, 21).

Addition-

ally secondary reactions may occur at 373

K 19) but not at room temperature. Third

the large amoun t of olefin passing over the

catalyst in a flow reaction may cause the

formation of carbonaceous deposits that

modify the state of the catalyst. With a pulse

technique this effect is minimized due to

the small quantity of olefin which is sent

sequentially to the surface. Our method is

similar to the method developed by August-

ine and Warner 22, 23) since it uses pulse-

wise hydrogena tion of butene.

E X P E R I M E N T A L

Catalysts

The catalys ts were prepared by the incipi-

ent wetness impregnation method. The main

characteristics and pretreatments of sup-

ports are summarized in Table 1. Aluminas

and silica gel were calcined in air at 873 K

388


2/13

CATALYST CHARACTERIZATION BY A PROBE REACTION

TABLE 1

Characteristics and Pretr eatments of Catalysts

389

Catalyst wt% Pt Support Impregnation Support

complex pretreatment

1 0.28 y-Al203 (NH4)2PtCI 4 NH 3 gas flow at room

temperature on dried catalyst

2 0.53 3,-A1203 H2PtCI 6 Support suspended in a HCI

solution before impregnation

3 0.57 y-AI203 H2PtCI 6 None

4 0.42 r/-Al203 (NH4)2PtCI4 HN 3 gas flow at room

temperature on dried catalyst

5 0.47 y-AI,O3 H2PtCI6 Dry support

6 0.52 0-A1203 H2PtCI 6 Dry support

7 0.6 SiO 2 H2PtCI6

8 a 0.5 AI203

9 b 6.3 SiO 2

Note Support surface areas: 160 m2/g for T-AI203 , 155 m2/g for 0-AI203 ; 265 m2/g for silica.

Sample II from the Committee of Reference Catalysts of the Catalysis Society of Japan.

b EUROPT-1.

and sieved to particle sizes from 0.15 to 0.25

mm (60-80 mesh). The platinum content

was determined spectrophotometrically by

the stannous chloride technique

24)

and X-

ray fluorescence. The catalysts were all

stored in glass flasks with plastic snap caps

after drying.

Gases

Argon (Linde UHP, >99.999 ) and he-

lium (Linde Chromatographic , >99.9999 ),

purified by passing them through alumina

traps, were used as carrier gases. The alum-

ina trap was dr ied in situ at 573 K. Hydrogen

(Linde UHP, >99.999 ) or H 2 produced by

an Eihygen generator was used for catalyst

treatments and titrations. Ethylene (Linde

CP, >99.5 ), ethane (Matheson CP,

>99.0 ), and oxygen (Linde UHP,

>99.99 ) were used as received.

Determination of the Hydrogen Number

HN) and Hydrogen Chemisorption by

Titration of Ethylene with Hydrogen

HC E))

The experiments were carried out in a

stainless steel apparatus designed to work

by either pulse or continuous flow. The reac-

tor used was a 6-mm-o.d. U-shaped Vycor

tube. Switching of gases was accomplished

without mixing or access of air. The reaction

products of each pulse were collected at the

reactor exit in a cool trap for 8 min. The t rap

was a 40-cm U tube of stainless steel, -~-

in. o.d., filled in the middle with 20 cm of

Chromosorb 102. The trap was cooled to

195 K when argon was used as carrier gas

and to 77 K when the carrier gas was helium.

The trapped gases were then analyzed with

a gas chromatograph, equipped with a col-

umn of Chromosorb 102 kept at 303 K. A

loop of 0.202 cm 3 (0.185 cm 3, NTP) was used

as a calibrated volume for the gas pulses.

The reactor was charged with 250 or 500

mg of cata lyst , dried fo r 2 h at 393 K in Ar,

heated to 673 K (I0 K/min) in carrier gas,

and then treated in H2 through the desired

time (10 min, 1 h, 5 h, overnight, etc.). The

catalyst was cooled to room temperature

and then swept for 15 rain with carrier gas.

From this point on, the measurements fol-

lowed the next sequence (notation is defined

in Table 2):

Step 1.

Hydrogen adsorbed on the cata-

lyst was titrated with ethylene pulses until

the amount of ethane formed was negligible

(El, total amount of ethane produced in this

step). The first ethylene pulses produced


3/13

390 CHOREN ET AL.

TABLE 2

Sequence of Titrations and Notation

Ptt Total amount of platinum in sample,/zmol

Step 1: Fir st titration

H~a~ + C2H4 ~ C2H6 C2H41a~

E~ Amount f

ethane ormed

in the titration by ethylene of adsorbed H, at room temperature, after the catalyst

treatment with hydrogen at high temperature,/xmol

HN Stoichiometric ratio of adsorbed hydrogen titrated in Step 1; HN = 2Ei/Pt . For Pt/AI203 catalysts, HN

is also called NAH: number of active hydrogen atoms see text).

E

HT

HC E)

H/Pt

Step 2: Second titration

C2H4~ol + H2

~

C2H6 + Hio~

Amount of ethane formed in the titration by H~ of ethylene adsorbed at the end of Step 1, tzmol

Amount of H2

consumed

in the second titration,/zmol

Amount of H2

adsorbed

in the second titration: HC E) = HT - E2,/zmol

Stoichiometric ratio of hydrogen adsorbed at the end of the second titration: H/Pt = 2HC E)/Pt

Note.

Subscript a) means adsorbed, and the equations are descriptive only, not stoichiometric.

most o f the ethan e in the titration. T he stoi-

chiometric amount of hydrogen atoms t i-

trated in this step per total platinum atom is

called the hydr ogen numbe r HN): HN =

2E1/Ptt.

S t e p 2 .

Following step 1, the adsorbed

ethylene was titrated with pulses o f hydro-

gen until the hy drog en peak area in succes-

sive chromatograms was constant. The

amount of hydrogen chemisorbed HC E) is

given by the difference between the amoun t

of hydrogen consumed, HT, and the amount

of ethane fo rme d in the titration, E2; that is,

HC E) = HT - E 2.

The ratio of hydro gen atoms p er platinum

ato m H/Pt) is given by H/Pt = 2 HC E)/

Ptt.

S t e p 3 .

To repeat step 2, the catalyst from

step 2 was treated with ethylene pulses at

room temperature until ethane was not de-

tected.

Steps 2 and 3 were repeated two or three

times to verify reprodu cibility of the results.

D i r e c t H y d r o g e n C h e m i s o r p t io n H C )

The amount of hydrogen chemisorbed on

clean catalyst, HC, was measured by the

method of hydrogen pulses at room temper-

ature

4, 25,

26). The procedure was the

following: The catalyst treated with hydro-

gen was cooled at room temperature in an

Ar stream. Then it was heated again to 673

K at a rate of 10 K/min. At this temp era ture

the furnace was turned off and the reac tor

was maintained overnight in the inert gas

stream. After that, the reactor was recon-

nected to the gas stream and subjected to

another TPD. After the catalyst was cooled

to 298 K, pulses o f 8.25/xmol of hydr ogen

were sent on the catalyst until a constant

peak area was measured.

In some cases the catalyst remains after

the reduction in a highly metastable state;

under these condi tions neither HC E) n or

HC gives reproducible results. The catalyst

must be annealed by successive OT- HT cy-

cles, as indicated by Prasad et al. 26), be-

fore proceeding with the measurements.

S t u d i e s o f C a t a l y t ic A c t i v i t y

Initial rates for ethylene hydrogenation

were mea sured in the reaction system fitted

to carry out flow reactions. The rea ctor was

loaded with 10 mg of Pt catalys t mixed with

190 mg of Vycor glass. The catalyst was

dried i n s i tu at 390-3 95 K in a N 2 stre am 40

ml/min) for 2 h, heated to 673 K, and then

red uce d in a H 2 stream at this tem per atur e


4/13

C A T A L Y S T

CHARACTERIZATION

B Y A P R O B E R E A C T I O N

TABLE 3

Ethylene and Hydrogen Titrations over Catalysts 5, 6, and 7

391

Ej HT E 2 HC(E) HN H/P t HT/ E 2

(/xmol) (/~mol) (/~mol) (/~mol) (NAH)

Catalyst 5 (0.47% Pt/T-AI203)

I 1.34 ~ 2.25

13.02 b 10.20 3.57 6.63 2.59 1.32 2.9

Catalyst 6 (0.52%

Pt / q -AIz03)

17.49' 10.28 3.82 6.46 2.33 0.84 2.7

11.15 3.77 7.38 0.96 3.0

11.05 3.75 7.30 0.95 3.0

11.15 3.74 7.31 0.95 3.0

18.07 11.34 3.62 7.72 2.35 1.00

10.66 3.67 6.99 0.90

Catalyst 7 (0.6% Pt/SiO2)

4.26 d 2.15 0.8 1.35 1.11 0.35 2.7

2.08 0.71 1.37 0.36 2.9

First ethylene titration. Hydrogen reduction at 673 K for 13.5 h.

b Second ethylene t itration, after 10 min of H2 retreatment at 298 K.

c The same treatmen t as on catalys t 5, but after the reduc tion s tep in H2 at 673 K, the catalyst was cooled down

in Hz flow instead of He and swept with carrier gas for 15 min.

a H2 reduction at 673 K for 10 min.

f o r l 0 m i n. N e x t , t h e r e a c t o r w a s c o o l e d t o

t h e r e a c t i o n t e m p e r a t u r e , 2 7 3 K , i n a N 2

s t r e a m . G a s s a m p l e s t a k e n d u r i n g t h e f l ow

e x p e r i m e n t s w e r e a n a l y z e d u s in g th e o n li n e

g a s c h r o m a t o g r a p h .

A t t h e e n d o f t h e f ir s t r u n , t h e c a t a l y s t

w a s a g a i n h e a t e d t o 6 73 K i n a N 2 s t r e a m a n d

s u b j e c t e d t o o n e o f t h e f o l l o w i n g t r e a t m e n t s :

( I ) c a t a l y s t r e d u c t i o n i n a H 2 s t r e a m a t 6 7 3

K f o r 1 h o r il l ) e x p o s u r e t o a n o x y g e n

s t r e a m f o r 1 h a t 6 73 K a n d t h e n r e d u c t i o n

i n a H 2 s t r e a m a t 6 7 3 K f o r 1 h . A f t e r H 2

t r e a t m e n t , t h e c a t a l y s t w a s c o o l e d t o t h e

r e a c t i o n t e m p e r a t u r e in a n i n er t g a s s t r e a m

a n d t h e r e a c t i o n w a s a g a i n c a r r ie d o u t . S i m i-

l ar p r o c e d u r e s w e r e f o l l o w e d f o r th e l a s t

s t e p , i . e . , a f t e r 5 h o f h y d r o g e n t r e a t m e n t .

RESULTS

T a b l e 3 s h o w s t h e r e s u l ts o b t a i n e d o n c a t -

a l y s t s 5 , 6 , a n d 7 a s t y p i c a l o n e s . A s a m p l e

o f c a t a l y s t 5 , c o n t a i n i n g 10 .1 / ~ m o l o f P t ,

w a s d r i e d i n a s t r e a m o f c a r r i e r g a s a t 3 93 K

f o r 2 h . I t w a s t h e n h e a t e d t o 6 73 K i n a H 2

s t r e a m a t a h e a t i n g r a te o f 1 0 K / r a i n a n d

k e p t a t t h i s t e m p e r a t u r e f o r 1 3 . 5 h . A f t e r

t h is t r e a t m e n t , t h e H 2 f l o w w a s c h a n g e d t o

H e c a r r i e r ga s a n d t h e c a t a l y s t w a s c o o l e d

t o r o o m t e m p e r a t u r e . A t th is t e m p e r a t u r e ,

t h e s u r f a c e h y d r o g e n w a s t i t r a t e d w i t h

p u l se s o f e th y l e n e . O n l y e t h y l e n e a n d e t h-

a n e w e r e d e t e c t e d . A H N v a l u e o f 2 .2 5 w a s

o b t a i n e d .

I n o r d e r t o m e a s u r e t h e h y d r o g e n l o s s

d u r i n g t h e c o o l i n g in H e f lo w , a s o u r c e o f

e r r o r , t h e c a t a l y s t w a s t r e a t e d w i t h a H 2

s t r e a m f o r 1 0 m i n , f o l l o w e d b y a H e s w e e p

f o r 1 0 m i n . T h e e t h a n e p r o d u c e d d u r i n g t h is

e t h y l e n e t i tr a t i o n , E I , w h i c h e q u a l s t h e H N

v a l u e , i n c r e a s e d b y 1 5 . R i g h t a f t e r t h is

s e c o n d e t h y l e n e t i t r a t i o n t h e c a t a l y s t w a s

t i t ra t e d w i t h p u l s e s o f h y d r o g e n , H T . A t o t a l

o f 1 0 . 2 / ~ m o l o f H : p r o d u c e d 3 . 57 p .m o l o f

e t h a n e w h i l e 6 . 6 3 / . t m o l o f a d s o r b e d H 2 r e -

m a i n e d o n t h e s u rf a c e , H C ( E ) , w h i c h c o r -

r e s p o n d s t o a

H Pt

r a t i o o f 1 . 3 2 .

R e s u l t s o n c a t a l y s t 6 a r e a l s o s h o w n i n

T a b l e 3 . T h i s c a t a l y s t w a s s u b j e c t e d t o t h e


5/13

392

CHOREN ET AL.

TABLE 4

Dependence of HN and H/Pt Values on Hydrogen Reduction Time at 673 K

Reduction Catalyst 7 Catalyst 8 Catalyst 9

time (h) (0.6 Pt /Si O 2) (0.5 Pt/A1203) (6.3 Pt/SiO2)

HN H/Pt HN H/Pt HN H/Pt

0.25 1.10 0.37 1.2 0.83 1.85 0.9 i

1.00 - - - - 2.6 0.84 1.86 0.90

5.00 1.17 0.35 . . . .

10.00 -- - - 4.3 0.73 -- --

same treatment as catalyst 5 but it was

cooled in a H2 stream instead of He after the

reduction step in H2 at 673 K was carried

out. The E 1 value was 17.49 txmol C2H6,

which corresponds to a HN value of 2.33.

Three successive determinations of HT and

E2 show the high reproducibility of HC(E)

obtained by ethylene titration with hydro-

gen pulses. A second run on a fresh sample

of the same catalyst shows the reproducibil-

ity of the HC(E) values.

Table 4 shows the effect of increasing time

in H2 treatment on the HN value and the

H/Pt ratio on catalys ts 7 (0.5 Pt/SiO2), 8

(0.5 Pt/AI203), and 9 (6.3 Pt/SiO2). It is

observed that for alumina-supported cata-

lysts, HN values increase with reduction

time while the H/Pt ratioqobtained from

HC(E) values--remains constant and that

HN values do not change with the hydrogen

treatment time for silica-supported cata-

lysts.

Table 5 shows two values of hydrogen

chemisorption, HC(E) and HC, for a set of

catalysts. The hydrogen chemisorption dur-

ing hydrogen titration of adsorbed ethylene,

HC(E), was calculated by the difference HT

- E2, and HC was measured by direct che-

misorption by pulse, as described above.

To ensure that the HC and HC(E) values

corresponded to the same catalyst state,

both were de termined in the same run: First,

the direct chemisorption of hydrogen, HC,

was carried out on the reduced and cleaned

catalyst; a titration with ethylene removed

the adsorbed hydrogen and saturated the

surface with olefin; and then the procedure

followed from step 2 of the sequence de-

scribed in Table 2. The values of HC(E)

reported are averages after several cycles

of hydrogen and ethylene titrations. It is

observed that both HC and HC(E) values

are very similar for alumina-supported cata-

lysts. In contrast, for the silica-supported

catalysts the HC(E) value is less than HC.

After the alumina-supported catalysts

were reduced a t high temperature (673 K, in

this work), subsequent treatment s by ethyl-

ene, hydrogen, or oxygen titrations at room

temperature did not significantly change the

value of HC(E). For instance, catalyst 3 ti-

trated repeatedly with ethylene and hydro-

gen, including TPD of hydrogen up to 723

K, showed only a drop of the HC(E) value

from 8.6 to 7.1. The presence of oxygen at

high temperatures was avoided to evaluate

the poisoning role of carbonaceous res-

idues.

Table 6 and Figures 1, 2, and 3 show the

effect of the reduct ion time in a H z stream

on the initial activity for ethylene hydroge-

nation measured in a flow reac tor at 273 K.

This effect was studied on catalysts 5, 6,

and 7 using treatments I and II as described

above. The activity decreased steadily dur-

ing the reaction. The initial points of the

curves follow the deactivation law N =

Noe- ~t (27); the initial activi ty was then cal-

culated for t = 0. The turnover frequencies

were calculated twice, taking HC(E) and

HN as measures of the number of active

sites.


6/13

CATALYST C H A R A C T E R I Z A T I O N BY A PROBE REACTION

TABLE 5

Hydrogen Titrations (Step 2)

393

C a t a l y s t % P t H T E 2 H C ( E ) H C H / P t H T / E 2

( p~ mol ) ( / . t mo l ) ( / ~ mol ) ( p .mol )

1 0 . 2 8 7 . 7 2 . 7 5 . 0 4 . 9 1 .5 2 . 9

2 0 . 5 3 1 4 . 3 4 . 9 9 . 4 9 . 4 1 . 4 2 . 9

3 0 . 5 7 1 3 .1 4 . 5 8 . 6 8 . 6 1 . 3 2 . 9

3 ~ 0 . 5 7 1 1 . 3 4 , 2 7 . 1 N D 1 .1 2 . 7

4 0 . 4 2 5 . 9 2 , 2 3 . 7 3 . 3 0 . 7 2 . 7

5 0 . 4 7 4 . 2 7 1 . 5 4 2 .7 3 2 . 7 8 0 . 9 2 . 8

6 0 . 5 2 1 . 1 2 3 . 7 5 7 . 3 7 7 . 4 0 0 , 9 5 3 . 0

7 0 . 6 0 2 . 0 0 . 6 7 1 . 33 3 . 1 2 0 . 3 5 d 3 . 0

8 0 . 5 0 , 8

9 b 6 , 3 6 . 0 0 . 7 5

9 ~ 14 .4 1 .8

Note N D , n o t d e t e r m i n e d .

u C a t a l y s t 3 a f t e r 6 d a y s o f u s e . A l a r g e n u m b e r o f e t h y l e n e a n d h y d r o g e n t i t r a t io n s , t e m p e r a t u r e - p r o g r a m m e d

d e s o r p t i o n s ( T P D ) , a n d r e r e d u c t i o n s w e r e p e r f o r m e d o n c a t a l y s t 3 . T h e p r e s e n c e o f o x y g e n a n d t e m p e r a t u r e s

h i g h e r t h a n 7 2 0 K w e r e a v o i d e d t h r o u g h o u t t h i s p e r io d .

b 5 0 m g o f c a t a l y s t i n h y d r o g e n a t 6 7 3 K , l 0 m i n ; 5 h in A r a t 6 7 3 K .

' S a m e s a m p l e , T P D ( 10 K / m i n ) u p t o 6 73 K . C o n f i n e d o v e r n i g h t in A r a n d t h e n a n o t h e r T P D u p t o 67 3 K .

d T h e H / P t r a ti o c a l c u l a t e d f r o m d i r e c t h y d r o g e n c h e m i s o r p t i o n ( H C ) g i v e s a v a l u e o f 0 .8 .

D I S C U S S I O N

T h e H C D e t e r m i n a ti o n

Table 5 shows that on Pt/AI203 catalysts

the values of chemisorbed hydrogen deter-

mined by ethylene titration with hydrogen

agree very well with those directly deter-

mined by hydrogen adsorption on clean cat-

alysts. On Pt/SiO2, HC(E) is markedly less

than HC. Catalyst 7 shows a H/Pt ratio of

0.80 when determined by direct hydrogen

chemisorption; nevertheless, when ethyl-

ene left on the surface by the first set of

pulses is titrated with hydrogen pulses, the

corresponding H/Pt ratio is only 0.35. Simi-

lar considerations can be applied to catalyst

9 (Tables 4 and 5). The reason for the differ-

ence is that ethylene is irreversibly and inac-

tively adsorbed on a fraction of the catalyst

surface. Morrow and Sheppard

28)

de-

tected these residues spectroscopically. The

high reproducibility found for the repetition

step 3-s tep 2 cycle (Table 3) shows that this

ethylene acts as a true poison and that from

the very beginning only a fraction of the

catalyst surface will be active for olefin hy-

drogenation.

The advantage of measuring the active

hydrogen by the reaction itself after the cat-

alyst reduction at high temperature cannot

be overestimated. The state of the so-called

clean surface of a supported metal cata-

lyst is far from known. A good example is

what happened with EUROPT-I, as seen

in Table 5. When the authors of this paper

applied the cleaning method used by most

of the laboratories participating in Part 4 of

Ref. (17)-5 h at 500C in flowing car-

tier--the value found for the HC deter-

mined by direct adsorption of hydrogen

agreed very well with the values of Table 1

of the quoted reference, but when the cata-

lyst was left confined overnight and then

swept with carrier and heated to 673 K be-

fore cooling to room temperature, the HC

value climbed to more than twice the for-

mer. Th is was discussed previously by

Tsuchiya e t a l . in their work on platinum

black 11) . They found that to obtain repro-

ducible results, they had to use a four-step


7/13

394 CHOR EN ET AL.

TABLE 6

Effect of the H2 Reduction Time at 673 K on the Initial Catalytic Activity for Ethylene Hydrogenation

Catalyst Treatment Reduction H/Pt HN Turnover a Turnover b

time (h) frequency frequency

(s -l) (s- b

5 1 0.17 0.72 0.87 7.1 5.8

1 0 0 71 1 55

10.2 4.9

5.0 0,71 2.05 12.1 4. I

5 II 0.17 0.75 0.84 7.1 6.3

1.0 0.76 1.95 13.6 5.3

5,0 0.76 2.68 c __

6 I 0.17 0.91 0,90 6.8 6.9

1.0 0.86 1.80 9.5 4.5

5.0 0.85 2.20 12.4 4.7

6 II 0.17 0.90 0.94 6.8 6.9

1.0 0.87 2,35 14.3 5.4

5.0 0.86 2.96 c __

7 0.17 0.33 1,13 7.0 2.0

5.0 0.33 1.20 6.9 1.0

Reaction conditions Catalyst 5 and 6 Catalyst 7

Ethylene partial press ure

Hydrogen partial pres sure

Nitrogen partial pressur e

Flow of mixture = 100 cm3/min

Reaction temperature = 273 K

7.33 kPa 6.40 kPa

51.33 kPa 10.0 kPa

42.66 kPa 84.93 kPa

a HC was taken as the measure of the number of active sites.

b HN was taken as the measure of the number of active sites.

Initial conversion = 100 .

26 CATALYST PRETREATMENT

Hz ol 6/3K)

Fl

TIME

20 ~ O lT h

) L \ O l h o )

~ A 5 h

freshsample)

~ ~ o ~

2 ; , 9 5 r 6 5 .

TIME min.}

26

2

~ t

~

C T LYST PRETRE TMENT

H z 0f

6 7 3 K

TIME

e 0 1 7 h

o 1 h b)

A S h

9 17 25 33 41 49

57 65 7

TIME mi , )

FIG, 1. Catalytic activity as a function of time for ethylene hydrogenat ion to ethane in a flow microreac-

tor over catalyst 5. Reaction temperature: 273 K. (a) Treatment I. (b) Treatment II (see text).


8/13

CATALYST CHARACTERIZATION BY A PROBE REACTION 395

26

A CATALYST PRETREATMENT

al 673 K )H2

2C ~ 0,17 h ~ 20

\

o

I h

o )

i \ , , s h = '

h

f r e s h s a m p le )

~ ~ ~, ~,~

1 9 17

25 33 41

4 9 5 7 6 5 7 3

26

CATALYST PRETREAT~ENT

H at 673 K )

TIME

17 h

o I h b)

a 5 h

TIME min) TIME min.)

F IG . 2 . C a t a l y t i c a c t i v i t y a s a f u n c t i o n o f t i m e f o r e t h y l e n e h y d r o g e n a t i o n t o e t h a n e i n a f l ow m i c r o r e a c -

t o r o v e r c a t a l y s t 6 . R e a c t i o n t e m p e r a t u r e : 2 7 3 K . a ) T r e a t m e n t I . b ) T r e a t m e n t II s e e t e x t ) .

procedure, like that used in this work; 1)

using a TPD up to 500C; 2) main tain ing the

catalyst at this temperature in a nitrogen

stream for 20 min before cooling it; 3) con-

fining the cata lyst overnight at room temper-

ature; and 4) before the next run, evacuat-

ing the cata lyst to less than 10 -5 Tort for 30

min, at room temperature.

An additional advantage in using hydro-

gen and ethylene titrations is that the reac-

I0.0

90

e . o

>- 70

6 0

?

5

> 40

o

3.0

2 0

I0

o lo

r i i i i i

CATALYST PRETREATUENT

Ha ol 6T3 K I

TIME

0 1 7 h

~ Sh

l I I l I I I I

20 30

4 0 5 0 6 0 7 0 8 0 9 0 / 0 0

TIME min,)

F I G . 3 . C a t a l y t i c a c t i v i t y a s a f u n c t i o n o f t i m e f o r

e t h y l e n e h y d r o g e n a t i o n t o e t h a n e i n a fl o w m i c r o r e a c -

t o r o v e r c a t a l y s t 7. R e a c t i o n t e m p e r a t u r e : 2 73 K .

tion product, ethane, is collected and mea-

sured quantitatively.

T h e H T E 2 R a t i o

Tables 3 and 5 show that this ratio, i.e.,

the ratio between the total hydrogen amount

necessary to produce ethane from the ad-

sorbed ethylene and that needed to saturate

the surface, and the ethane produced by the

ethylene actively adsorbed is constant

throughout and its value is about 3, in agree-

ment with the reaction: 3H2 g + C 2 H 4 s ) =

C2H6 g) + 4H s).

That is, one actively adsorbed ethylene

molecule statistically blocks four hydrogen

sites.

S o u r c e s o f E r r o r

The hydrogen loss and the ethylene self-

hydrogenation were explored as sources of

error in the HN and HC E) determinations.

The amounts of ethane produced in the

first ethylene titration, Ej, were lower when

the catalysts were cooled in argon than

when they were cooled in hydrogen. This is

an indication that a fraction of the H 2 active

for the hydrogenation desorbs during the

cooling in argon. To avoid this desorption,

the catalysts must be cooled in Hz.

To evaluate the effect of ethylene self-


9/13

396 CHOREN ET AL.

hydrogenation, a sample of clean catalyst

was saturated with ethylene. The reaction

showed only traces of ethane, which indi-

cates that ethylene self-hydrogenation is

negligible at room temperature. This fact

has already been pointed out by Sermon and

Bond for temperature of 370 K 29).

The high reproducibility and consistency

of HC E) and HC Table 5) and that of the

last column of Table 6 show that any o ther

source o f error can be considered negligible.

M echan ism o f the E thy lene

Hydrogena t ion on Suppor ted

Plat inum Catalys ts

Ethylene hydrogenation catalyzed by

platinum is a fast reaction 30-32). To obtain

kinetic information it is necessary to carry

out the reaction at temperatures near or be-

low 273 K. The reaction order is 1 for hydro-

gen and between 0 and -0.5 for ethylene.

This range can be attributed to the experi-

mental conditions.

Recently, Soma 33) found a maximum

value in the reaction rate at 203 K for in-

creasing partial pressure of ethylene at a

constant pressure of hydrogen. This sug-

gests that a high partial pressure of ethylene

should play an inhibitory role.

From analysis of the work of Horiuti and

Polanyi 34), the consensus is that the reac-

tion occurs by a Langmuir-Hinshelwood

mechanism, that is, with the two chemi-

sorbed species:

C 2 H 4 s ) + 2H s) = C 2 H 6 g ) . 1)

Soma measured the coverage degree of

the catalyst surface by using the 2120 cm-i

infrared band for the reversibily adsorbed

hydrogen atoms and by using the 1205 cm -l

band of the 7r complex of ethylene adsorbed

on the metal 33, 3 5). Soma also found that

the reaction ceased after evacuation of the

gas phase, even in those cases in which the

surface was still saturated with hydrogen

and ethylene. This fact was verified by us.

A pulse of 8.3 /xmol of ethylene was in-

jected over a charge of Pt/AI203 catalyst

containing 12.5/zmol of Pt and 22.8/xmol of

react ive hydrogen atoms HN = 1.8). The

only product collected was ethane, 5.8

/xmol. The cata lyst re tained 11.2 tzmol of

hydrogen atoms and 2.5/xmol o f ethylene.

After the analysis was over ca. 10 min) the

refrigerant bath was replaced and the de-

sorbing products from the reactor were col-

lected during 1 h. Analysis of the trap con-

tent gave only 1.1 /zmol of ethane.

Immediately a new pulse of ethylene was

injected on the catalyst; the products were

ethylene and 1.2/xmol of ethane. The rate of

desorption of hydrogen at room temperature

was measured in a separate experiment. The

same amount of catalyst was pretreated as

the previous one and left 1 h in an argon flow

of 30 ml/min. The desorbed hydrogen 1.1

/.Lmol) was measured by resaturating the sur-

face with hydrogen pulses. This value must

be considered a lower limit of the amount o f

desorbed hydrogen because hydrogen

pulses do not restore the totality of the ad-

sorbed hydrogen coming from reduction.

These results strongly suggest that the 1.2

/zmol of ethane collected af ter 1 h of trapping

was produced by the desorbed hydrogen

coming from the gas phase.

Titration of the surface saturated in ethyl-

ene with hydrogen pulses may be described

by the reaction

C2Hn s) + H2 g) = C2H6 g) 2)

and the titration of the surface saturated in

hydrogen with ethylene pulses may be de-

scribed by the reaction

2H s) + C2H4 g) = C2H6 g). 3)

Rienacker, studying the hydrogenation of

butadiene on nickel film, as described by

Panchenkov and Lebedev

36),

concluded

that the absence of a growth in the electrical

resistance of the film during the course of

the reaction indicates that the butadiene is

not adsorbed in the presence of hydrogen,

and that the reaction proceeds between the

adsorbed hydrogen atoms and the molecules

of butadiene approaching them from the gas-

eous phase.

All these facts suggest that the global re-


10/13

CATALYST CHARACTERIZATION BY A PROBE REACTION 397

action occurs by coparticipation of reac-

tions (2) and (3) . The H2(g) and CzHa(g)

terms do not exclude the possibility that

both species are in some adsorbed state be-

fore they react. The reaction occurs if ad-

sorbed ethylene and hydrogen gas or ad-

sorbed hydrogen and ethylene gas are

present, but it stops in the absence of react-

ants in the gas phase. Even if the hypothesis

of an Eley-Ri deal mechanism is neglected,

it appears that there is not a single reaction

route for ethylene hydrogenation. The in-

crease in initial activity for Pt/AI203 cata-

lysts with the time of treatment of hydrogen

may be attributed to the increase in the num-

ber of sites on which reaction (3) occurs.

T h e N u m b e r o f A c t i ve H y d r o g e n

1 . C l e a n c a t a l y s t s .

On the Pt/AI203 cata-

lysts the HN value reaches a maximum fixed

value. After about 5 h of treatment in hydro-

gen at 673 K, the HN value agrees within

experimental error with that obtained after

an overnight treatment. The question that

arises is the physical meaning which we call

the Number of Active Hydrogen (NAH).

The results of the flow reaction measure-

ments (Table 6 and Figs. 1 and 2) show that

the quantity kinetically significant is HN and

not the H/Pt ratio calculated from HC. For

alumina-supported catalysts, the NAH is

equal to HN.

The activity of Pt/SiO2 cata lysts may be

due to only a fraction of the surface platinum

atoms because the greater part of them are

irreversibly inhibited by the olefin from the

very beginning of the reaction. The activity

does not change with the time of catalyst

reduction. In this case the si tuation is not as

clear as the alumina support, but we believe

that for SiO2-supported catalysts the NAH

is represented by H/Pt measured by titration

of adsorbed ethylene with hydrogen gas,

HC(E), because this quantity does not

change after successive cycles of ethyl-

ene-hydrogen titrations, showing that the

primary inhibition is fast and fixed.

2 . P o i s o n e d c a t a l y s t s .

Leclercq and

Boudart

37) ,

studying the catalytic hydro-

genation of cyclohexene on supported plati-

num, found that for clean and sulfur-poi-

soned catalysts, only the HC values among

the H2-O 2 adsorption and titration methods

correlated well with activity on clean and

poisoned catalysts. Very probably the pres-

ence of oxygen during the adsorptions and

titrations modified the catalyst state. With

ethylene titrations these effects are avoided

because the olefin is one of the participants

in the process, as proved by Leclercq

et al .

38) .

T h e H y d r o g e n N u m b e r s a n d t h e N a t u r e

o f th e A c t i o e S i t e s

In the pulsewise hydrogenation of l-bu-

tene at room temperature over catalysts of

platinum supported on porous glass, Au-

gustine and Warner 22) adopted the crite-

rion of simple action for the hydrogen

atoms. Augustine

e t a l . 23)

recently found

much higher values of HN than those ob-

tained ,. n this work. Twenty- four successive

pulses of 1-butene had total conversion, that

is, 10.7/zmol for 1.1/zmol of total Pt in the

reactor, as shown in Fig. 3 of Ref. 23) .

The conversion started to fall from the 25th

pulse, but Augustine et al . still continued

titrating large amounts of hydrogen, without

hydrogen in the gas phase. They explained

these results by a fast return of the hydrogen

to the metal f a s t r e o e r s e s p i l lo o e r ) . Augus-

tine and Warner

22)

classified the surface

atoms as a function of the degree of coordi-

native unsaturation in three groups based on

the structure of Pt complexes. These are:

IM (step), eM (edge), and 3M (kink). They

also evaluated the proportion of each type

of surface Pt atoms on the catalyst . Defining

a platinum atom as the active site, they pro-

pose that the only type of Pt that is able to

hydrogenate the olefin complete ly is the 3M

type, and that hydrogen migration between

the different types does not take place. This

last conclusion was based on their results

and those from Tsuchiya

et al . 39) .

From

these hypotheses and the results of the pres-

ent work, it is very difficult to explain how

1.1 /xmol of total platinum containing only


11/13

3 98 C H O R E N E T A L .

0 .163 /zmol o f

M

s i t e s c a n h y d r o g e n a t e

0 . 4 4 6 / ~ m o l o f o le f in p e r p u l s e .

T h e c o m p l e x i t y o f t h e s u r f a c e s t r u c t u r e

o f th e a l u m i n a , t h e h i g h l y d i s p e r s e d s t a te

o f t h e m e t a l , t h e p o s s i b i li t y o f s tr o n g

m e t a l - s u p p o r t i n t e r a c ti o n e f f e c t s , a n d t h e

l a c k o f e n o u g h s p e c t r o s c o p i c i n f o r m a t i o n

a b o u t a d s o r b e d h y d r o g e n s t at e s m a k e it ne c -

e s s a r y t o c o n s i d e r a n y m o d e l w i t h e x t r e m e

c a u t i o n . N e v e r t h e l e s s , t h e r e i s a c o r p u s o f

e x p e r i m e n t a l e v i d e n c e t h a t c a n g i v e s o m e

c l u e s o n t h e m a t t e r :

- - T h e r e a c t i o n s t o p s i n t h e a b s e n c e o f

e t h y l e n e o r h y d r o g e n i n t h e g a s p h a s e b e -

c a u s e a t ro o m a n d l o w e r t e m p e r a t u r e s h y -

d ro g e n m i g ra t i o n i s n o t k i n e t i c a l l y s i g -

nif icant .

- - T P D s p e c t r a I1, 20, 21) s h o w t h a t r e -

c o m b i n a t i o n a n d d e s o r p t i o n o f h y d r o g e n a r e

v e r y l o w a t r o o m a n d l o w e r t em p e r a t u r e s .

- - V a l u e s o f H N g r e a t e r t h a n 2 i n P t/A 1 20 3

i n d i c a t e t h a t t h e s u p p o r t p l a y s a n a c t i v e

r o le , w h a t e v e r t h e s u p p o s e d s t o ic h i o m e t r y .

- - T h e s i n g u l a r c o n d i t i o n s t h a t a r e n e c e s -

s a ry t o m a k e a c t i v e s i t e s c r e a t e d b y s p i l l -

o v e r o n th e s u p p o r t i t s e lf i .e . , h ig h r e a c t io n

t e m p e r a t u r e , l o n g i n d u c t i o n t i m e ) a r e n o t

p r e s e n t i n t h i s w o r k .

I t i s w o r t h e x a m i n i n g t h e p o s s i b i l i t y o f

r e v e r s e s p i l l o v e r . S u p p o s e t h a t a l l t h e h y -

d ro g e n a t o m s t h a t e n h a n c e t h e i n i t i a l c a t a -

l y s t a c t i v i t y w e re s p i l t o v e r d u r i n g t h e c a t a -

l y s t p r e t r e a t m e n t . I f t h e m i l d e st c a s e i s

e x a m i n e d , i . e . , c a t a l y s t 5 w i t h 0 . 1 7 h o f h y -

d r o g e n r e d u c t i o n F i g. l a ) , it c a n b e s e e n

t h a t it r e a c h e s t h e s t e a d y c o n d i t i o n a t a b o u t

2 5 m i n t u r n o v e r f r e q u e n c y , 2 s - 1 ). T h e a r e a

o f t h e t r ia n g l e d e f i n e d a b o v e t h e l i n e 2 s -1

c o r r e s p o n d s t o a b o u t 3 0 0 0 /x m o l o f e th a n e /

/ xm o l o f P t. T h i s o v e r p r o d u c t i o n i s n o t p o s -

s i bl e w i t h o u t a s o u r c e o f h y d r o g e n , k e e p i n g

i n m i n d t h a t t h e t i t ra t io n o f h y d r o g e n w i t h

e t h y l e n e H N = 0 . 87 ) l a s t s a t l e a s t I h .

S u p p o s e n o w t h a t d u r in g t h e f lo w r e a c t i o n

t h e m e t a l a t o m s p e r fo rm a t r i p l e f u n c t i o n :

h y d r o g e n a t e e t h y l e n e , d i s s o c ia t e h y d r o g e n ,

a n d , s p i l l i t o v e r t h e s u p p o r t . T h i s s p i l l o v e r

h y d r o g e n w i ll n e v e r r e t u r n t o m e t a l c l u s t e r s,

b e c a u s e t h e s u r f a c e c o n c o n t r a t io n o f h y d ro -

g e n a r o u n d p l a t i n u m c l u s t e r s w o u l d b e t h e

g r e a t e s t . C o n s e q u e n t l y r e v e r s e s p i l l o v e r , i f

i t e x i s t s a t r o o m t e m p e r a t u r e i n P t / A I 2 0 3

c a t a l y s t s , i s n o t k i n e t i c a U y s i g n if i ca n t .

W e n o t e t h at t h is w o r k c o n c e r n s c a t a ly s t s

i n t h e i r i n i t i a l s t a t e i n o rd e r t o s t u d y c a t a -

lys t s in it ia l ac t iv i ty ) . O ur goa l i s the e luc i -

d a t i o n o f t h e s t a t e o f t h e m e t a l a n d i ts i n te r -

a c t i o n s w i t h t h e s u p p o r t . F o r c a t a l y s t s

o p e r a t i n g a t s t e a d y s t a t e , d e t e r m i n a t i o n o f

t h e n a t u r e o f th e a c t i v e s i t e s is a f a r m o r e

c o m p l e x t a s k a s s u g g e s t e d b y Z a e r a a n d S o -

m o r j a i

40)

a n d o t h e r w o r k e r s . T h e l a r g e

d e c r e a s e s i n a c t i v i t y t h a t c a n b e s e e n i n

F i g s . 1 , 2 , a n d 3 s t r o n g l y s u g g e s t t h a t H N ,

H C , O T , H T , a d s o r p t i o n o f C O , a n d a n y

o t h e r m e t h o d a p p l i e d o n c l e a n c a t a l y s t s a r e

n o t t h e p r o p e r m e t h o d s f o r m e a s u r i n g a c t i v e

s i t e s i n s t e a d y c a t a l y s t s .

ON LUSIONS

P r o b e r e a c t i o n s a r e s i m p l e a n d u s e f u l

t o o l s t o s t u d y t h e a c t i v e s u r f a c e o f c a t a l y s t s .

T h e e t h y l e n e h y d r o g e n a t i o n b y p u l s e s o n

P t /A I 2 0 3 a n d P t / S i O 2 r e v e a l e d s o m e i n te r -

e s t in g f e a t u r e s o f t h e s e c a t a l y s t s a n d t h e

reac t ion i t s e l f :

Pt/Al203 Catalysts

- - T h e i n i t i a l c a t a l y s t r e a c t i v i t y i n c r e a s e s

w i t h t h e t i m e o f c a t a l y s t h y d r o g e n a t i o n a t

h ig h te m p e r a t u r e . A m e a s u r e o f t h a t r e ac t i v -

i t y i s o b t a i n e d b y t i t r a t i o n w i t h e t h y l e n e

p u l s e s o f t h e a c t i v e h y d r o g e n a t r o o m t e m -

p e r a t u r e N A H ) . C a t a l y s t r e a c t i v i ty i s p r o -

p o r ti o n a l to N A H v a l u e s a n d n o t to t h e H /

P t r a t i o c a l c u l a t e d f r o m H C .

- - N A H v a l u e s g r e a t e r t h a n 2 s u g g es t c o -

p a r t i ci p a t i o n o f t h e s u p p o r t .

- - A H T / E 2 r a t io o f a b o u t 3 i n d ic a te s t h a t

o n e e t h y l e n e m o l e c u l e a c t i v e l y a d s o r b e d i n-

v o l v e s f o u r h y d r o g e n s i t e s .

- - T h e r e a c ti o n d o e s n o t o c c u r at a k i ne t-

i c a l l y s i g n i f i c a n t r a t e w h e n b o t h o l e f i n a n d

h y d r o g e n a r e i r r e v e r s i b l y a d s o r b e d . I t i s

s u p p o s e d t h a t t h e g l o b a l r e a c t i o n t a k e s

p l a c e b y c o p a r t i c i p a t i o n o f t w o r e a c t i o n s

w i t h a t l e a s t o n e o f t h e r e a c t a n t s i n t h e g a s

p h a s e w i t h r e a c t i o n 2 ) s e e t e x t ) t h e p r e v a i l -


12/13

CAT AL YST CHARACT E RIZ AT ION B Y A PROB E RE ACT ION 399

i n g o n e , a s i n d i c a t e d b y t h e z e r o o r d e r i n

e thy l ene . T he i ncrease i n the i n i t i a l ca ta l ys t

reac t i v i ty m ay b e a t tr i buted to a greater con-

t r i but ion o f reac t i on 3 ) t o the g l oba l re -

ac t i on .

Pt Si02 Catalysts

T h e a c t i v i t y o f P t/ S iO 2 is due t o on l y

a

f r a c t io n o f th e s u r f a c e p l a ti n u m a t o m s b e -

cau se a subs tant ia l par t o f them are irrevers -

i b l y i nhi b i t ed by the o l e f i n f rom the very

b e g i n n i n g o f t h e r e a c ti o n . T h e N A H i s p r o b -

a b l y r e p r e s e n t e d b y H / P t m e a s u r e d b y t itr a-

t io n o f a d s o r b e d e t h y l e n e w i t h h y d r o g e n

H C E ) .

- - T h e p r o c e d u r e f o r c l ea n i n g t h e ca t a ly s t

s u r f a c e i s o f g r e at i m p o r t a n c e t o t h e H C

v a l u e d e t e r m i n e d b y d i r e c t c h e m i s o r p t i o n

o f h y d r o g en . T h e H C v a l u e c a n c h a n g e b y

a f a c t o r o f 2 o r m o r e d e p e n d i n g o n t h e c l e a n -

i n g p r o c e d u r e .

A C K N O W L E D G M E N T S

P a r t i a l f i na nc i al suppo r t by t he C o nse jo N a c iona l de

l n v e s t i g a c i o n e s C i e n t f f i c a s y T 6 c n i c a s d e V e n e z u e l a

( C o n v e n i o C O N I C 1 T - L U Z F 4 ) a n d t h e C o n s e j o d e D e -

sa r r o l lo C ie n f ff i c o y H um a nf s t i c o o f t he U n ive r s ida d

d e l Z u l i a ( C O N D E S ) i s g r a t e f u l l y a c k n o w l e d g e d . W e

a l s o a p p r e c i a t e t h e a i d o f th e I n s t i t u t o V e n e z o l a n o d e

lnve s t i ga t i one s C ie n t i f i c a s ( I V I C ) i n de t e r m in ing the

p l a ti n u m c o n t e n t o f s o m e o f t h e c a t a l y s t s u s e d i n t h i s

w o r k . P r o f e s s o r K a t a n ( U n i v e r s i d a d C e n t r a l d e V e n e -

z u e la ) k in d l y g a v e u s t h e E U R O P O T - I s a m p l e , a s d i d

P r o f e s s o r M ur a ka m i w i th s a m p le 11 ( c a t a ly s t 8 i n th i s

w o r k ) f r o m t h e C o m m i t t e e o f R e f e r e n c e C a t a ly s t s o f

t h e C a t a l y s ts S o c i e t y o f J a p a n .

R E F E R E N C E S

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