Application of microemulsion method for development of methanol steam reforming Pd/ZnO catalysts
J Therm Anal Calorim
Application of microemulsion method for development of methanol steam reforming Pd/ZnO catalysts
Justyna Pawlonka 0 1
Wojciech Gac 0 1
Magdalena Greluk 0 1
Grzegorz Słowik 0 1
0 Department of Chemical Technology, Faculty of Chemistry, Maria Curie-Sklodowska University , 3 M. Curie-Sklodowska Sq., 20-031 Lublin , Poland
1 & Wojciech Gac
Thermal decomposition of palladium acetylacetonate adsorbed on zinc oxide (ZnO), and the formation of palladium oxide (PdO) and palladium zinc alloy (PdZn) phases were studied. Two types of ZnO supports were prepared by the microemulsion method using different surfactants, i.e. hexadecyltrimethylammonium bromide (CTAB) and (Bis(2-ethylhexyl) sulfosuccinate sodium salt, AOT). The nanoparticles of ZnO synthesized in the presence of CTAB surfactant showed higher specific surface area, smaller crystallite size and more irregular shape. Palladium was loaded on the surface of obtained supports by the impregnation method from the acetone solution of palladium acetylacetonate. Thermogravimetric studies indicated that palladium precursor loaded on CTAB-modified ZnO support was less stable and simultaneously decomposed in broader range of temperatures. Slight differences between the forms of precursors adsorbed on the supports were demonstrated by the diffuse reflectance infrared Fourier transform spectroscopy studies. Thermal decomposition of palladium acetylacetonate precursors in the air led to the formation of PdO species. The influence of ZnO morphology on the metal-oxygen bonds strength in PdO and formation of active phases were observed. Strongly dispersed PdZn crystallites on ZnO supports were formed upon reduction at 350 C. Smaller crystallites of the size equal to 6.5 nm were detected in the Pd/ZnO-AOT catalysts.
Palladium acetylacetonate; Zinc oxide; TG; DRIFTS; TPR; Decomposition
Introduction
Supported palladium catalysts have been widely used for
oxidation and hydrogenation reactions of numerous
chemical compounds [
1, 2
]. In recent years, a lot of
attention has been paid to the development of supported
palladium alloy catalysts, such as PdZn, PdIn or PdGa,
showing their superior activity and selectivity in the
hydrogenation of carbon oxides to methanol, water gas
shift reaction, methanol steam reforming or hydrogenation
of alkenes and alkadienes [
3–7
]. The impregnation method
from aqueous solutions is one of simplest ways of the
preparation of catalysts and has been often used for the
synthesis of alumina-, silica- or carbon-supported
palladium catalysts. It is well-accepted opinion that the support
plays the crucial role in the formation of small crystallites
and thus influences the activity, selectivity and durability
of catalysts [8]. The formation of active sites in Pd/ZnO
catalysts is a complex process and includes (1) adsorption
of metal precursor on the support surface, (2) thermal
decomposition of the metal precursor to oxide form (or
directly to metallic form) and (3) reduction of oxide species
to metallic palladium. The surface or bulk PdZn alloy
phases are formed at elevated temperatures as a result of
enhanced reduction of ZnO in the presence of metallic
palladium. It has been often claimed that the size of PdO
and Pd particles, PdO stability, interactions with the
support and also catalytic performance of catalysts can be
influenced by nature of palladium precursors [
9–11
].
Palladium nitrate Pd(NO3)2, tetraamminepalladium(II) nitrate
Pd(NH3)4(NO3)2, tetraamminepalladium(II) chloride
Pd(NH3)4Cl2 as well as palladium chloride PdCl2 belong to
the most popular precursors used for preparation of
catalysts, and their application in the impregnation method has
been mostly determined by the isoelectric point (IEP) of
the supports. Several papers demonstrated beneficial
properties of palladium catalysts obtained by impregnation
of the supports from nonaqueous solutions of
organometallic complexes, such as palladium
acetylacetonate (Pd(acac)2) [
12, 13
]. The use of aqueous solution may
lead to the uncontrolled growth of palladium oxide species
during drying and precursor decomposition. The way of
adsorption of Pd(acac)2 on the oxide supports and
transformation of the complex to palladium crystallites are still
under debate. Palladium in the planar Pd(acac)2 complex is
coordinated with two bidentate acetylacetonate ligands.
bdiketonate ligands may interact with hydroxyl groups or
defect sites on the supports, leading to the formation of
hydrogen bonds or disruption of Pd-acetylacetonate ligand
bonds with ligand exchange [
14, 15
]. There are a few
works related to bulk and supported Pd(acac)2
decomposition [
16–18
]. Such phenomena have not been well
documented for Pd/ZnO systems.
The properties of metal oxide supports can be modified
by the changes of precipitation conditions, thermal
treatment or introduction of secondary elements. The promising
preparation method of ZnO with suitable structural and
surface properties is p (...truncated)