Journal of Catalysis 186, 433–441 (1999)
Article ID jcat.1999.2557, available online at http://www.idealibrary.com on
Sintering of Chlorinated Pt/
γ
-Al
2
O
3
Catalysts: An
In Situ
Study
by X-Ray Absorption Spectroscopy
A. Borgna,
∗
,1
T. F. Garetto,
∗
C. R. Apestegu´ıa,
∗
F. Le Normand,†
,2
and B. Moraweck‡
∗
INCAPE (UNL-CONICET), Santiago del Estero 2654, (3000) Santa Fe, Argentina; †LERSCI, Universit
´
e Louis Pasteur, 4 Rue Blaise Pascal, F 67070
Strasbourg, France; and ‡Institut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, F 69626 Villeurbanne, France
Received March 24, 1998; revised December 11, 1998; accepted May 13, 1999
In situEXAFS spectroscopy was used for identifying the surface
species involved in the sintering of alumina-supported Pt catalysts
underdriedoxidizingatmospheres.APt/γ-Al
2
O
3
catalyst(0.62wt%
Pt, 0.88 wt% Cl) was heated in a 2% O
2
/N
2
gaseous mixture from
300 to 525
◦
C for about 120 min and then kept at this temperature
for up to 720 min. The main observation, which is in good agree-
ment with ex situ TPR experiments, was that chlorine is always
present in the surrounding of platinum during the oxidizing treat-
ment. The metal sintering process involved three successive steps
duringwhichthechlorineandoxygencoordinationspassedthrough
amaximum, whereasplatinum coordinationexhibited aminimum.
Formationof Pt(OH)
4
Cl
2
specieswas detectedat theend of thefirst
step, i.e., when the temperature reached 500
◦
C. After about 4 h
of treatment, we deduced that platinum species are made up of a
metal platinum core surrounded by a double coating of oxychlori-
nated species. More precisely, EXAFS experiments suggested that
surface platinum oxide is made of rigid PtO
6
octahedra, but their
assembly led to a largely disordered structure. The absence of a
long-range order allows the location of residual chlorine species ei-
ther between the PtO
6
octahedra or at the Pt-oxidized surface shell.
c
1999 Academic Press
KeyWords:sintering;EXAFS;reformingcatalysts;chlorine;oxy-
chlorinated species; “cherry” model.
1. INTRODUCTION
Under industrial conditions, sintering of Pt-based naph-
tha reforming catalysts occurs mainly during the in situ
catalyst regeneration, which is performed under oxidizing
atmospheres. When coke burning is carried out, both the
highexothermicityof thereactionand the formationofwa-
ter cause the platinum crystallites to grow. The low oxygen
pressure used to avoid a dangerous increase in the reactor
temperaturefavorsthesinteringofthemetallicfraction(1).
1
To whom correspondence should be addressed. Fax: +(54)-42-
(531068). E-mail: aborgna@fiqus.unl.edu.ar.
2
Present address: IPCMS-GSI, UMR 1046 du CNRS, Bat. 69, 23, rue
du Loess, 67037, Strasbourg, France.
Toprevent platinum particle growth,chloridedcompounds
are usually added to the oxidizing mixture (2). The chemi-
cal nature of surface intermediate species plays a key role
in determining the kinetics of platinum sintering (3). How-
ever, very few studies have employed in situ characteriza-
tion techniques,whicharerequiredtofollowtheformation
kinetics of intermediate oxy- or hydroxychlorided species
during metal sintering. Knowledge on the exact chemical
nature of these species and on their formation mechanism,
especially under industrial conditions, is still lacking. It is
therefore of both fundamental and practical interest to un-
dertake in situ studies using modern spectroscopy to iden-
tify the surface species responsible for sintering.
Extended X-ray absorption fine structure (EXAFS)
spectroscopyisapowerfultooltocharacterizemetallicsup-
portedcatalysts,beingespeciallyusefulfor determiningthe
local environment around metal atoms and the evolution
of the local structure of chemical species formed during a
dynamic process. In a previous paper (4), we investigated
thesintering ofunchlorinated Pt/Al
2
O
3
catalystsin oxygen-
containing atmospheres using EXAFS spectroscopy. We
observed the existence of Pt–Pt metal distances after the
oxidizing treatments and proposed that the oxidation of Pt
particles only involves the outermost metal shells in a dis-
torted PtO
2
arrangement. Recently, in an in situ EXAFS
study (5), we investigated the redispersion of sintered
Pt/Al
2
O
3
catalysts, at 500
◦
C and under HCl/H
2
O/O
2
/N
2
at-
mospheres, and found that [Pt(OH)
4
Cl
2
]
2−
species, which
are formed by the attack of surface chlorided species on
partially oxidized Pt particles, are responsible for Pt redis-
persion. The EXAFS analysis indicated that, at the end of
the redispersion process, small raftsof Ptcontaining Cl and
O in an octahedral environment and Pt particles consisting
ofametalliccorecoatedby [Pt(OH)
4
Cl
2
]
2−
speciescoexist.
In this paper, we use in situ EXAFS spectroscopy to inves-
tigate the sintering kinetics of a fresh chlorinated Pt/Al
2
O
3
catalyst in a 2% O
2
/N
2
gaseous mixture. The aim was to
identifythechemicalnatureofsurfaceintermediatespecies
formed during the metal sintering process.
433
0021-9517/99 $30.00
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c
1999 by Academic Press
All rights of reproduction in any form reserved.