K. Kili et al., Modification by lanthanide (La, Ce) promotion of catalytic properties of palladium: Characterization of the catalysts, PCCP PHYS C, 1(7), 1999, pp. 1623-1631
Pd/gamma-Al2O3 catalysts modified by cerium or lanthanum oxide were prepare
d according to a standard procedure. The nature of the interaction between
the metallic particles and the promoted support was addressed. The effects
of changing the following parameters were studied: (i) precursor palladium
salt (chloride or nitrate), (ii) content of lanthanide (from the unpromoted
catalyst to the pure lanthanide oxide) and (iii) nature of the lanthanide
(La or Ce). This paper focuses on the study of the transition metal, wherea
s the lanthanide redox properties have been reported in an earlier paper (J
. Phys. Chem., 1988, 92, 2561). Both structural [transmission electron micr
oscopy (TEM), and extended X-ray absorption fine structure (EXAFS) at the P
d K edge] and spectroscopic investigations (XPS at the Pd 3d core level) em
phasize the role of the precursor palladium salt. When derived from a palla
dium chloride precursor and after a standard treatment including calcinatio
n and reduction, the chloride ion is quantitatively trapped by the support
and the metal-support interaction is strong. This is thought to be due to t
he preferential localization of the chlorine atoms in bridge positions betw
een the metal and the lanthanide. When exposed to an air atmosphere, the pa
lladium atoms at the interface are partially chlorided and cerium is in the
+4 oxidation state. After a mild H-2 treatment at 150 degrees C, palladium
is fully reduced, except when the cerium content is within 1-5% where the
reduction is only partial. Cerium is converted into the trivalent oxidation
state either as a chlorinated or an oxychlorinated form. It is thought tha
t this metal-support interaction at the interface, which does not exist in
the nitrate precursor catalysts, might explain the catalytic behaviour of s
uch catalysts by the creation of new active sites or a promoting effect (J.
Mel. Catal. A, in the press). The strength of this interaction depends on
the cation of the support at the interface (Ce3+ > La3+ > Al3+) and is stro
nger for a cerium content between 1 and 5%.