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