Supported ruthenium catalysts are widely used in a variety of important
commercial reactions. These reactions over supported ruthenium catalysts include
synthesis of ammonia, selective hydrogenation of benzene to cyclohexane, steamreforming
reactions, catalytic wet air oxidation of several organic compounds, methanation
of Cox and Fischer-Tropsch reactions. The catalysts are employed in environmental
applications such as DeNOx and oxidation of CO. These catalysts are even employed in
many organic transformations including oxidation of alcohols to aldehydes and nitriles. In
heterogeneous catalysis often the active metal component are carried out on the support
materials. This improves the mechanical strength, thermal stability and dispersion of
active component on the support surface. The few of such effects would be due to the
crystallographic defects and the strong metal support interaction (SMSI). Therefore,
besides the active component and the promoter the support is also considered as the
integral part of the catalytic system. There are many reasons to employ supported
metal/metal oxides system instead of bulk metal oxides in the catalytic reactions. One of
the reasons would be to increase the dispersion and metal surface are of the active
component. Often the metal/metal oxide systems are prepared by the calcination of the
catalysts samples at high temperatures wherein the possibility of sintering of the catalyst
leading to decrease in the surface area. Thus use of suitable support will prevent the active
component from sintering. Sometimes support plays the role of heat conduction media.
The traditional view of the catalyst support is that it is an inert material, which only
provides the surface area for the active component. However, the physical and chemical
properties of the support have also been recognized as major contributions to the resultant
catalytic activity.
In order to understand the structural aspects of the active component and reactivity
of supported ruthenium catalysts, a detailed systematic investigation is undertaken in the
present work. Two model reactions have been chosen to compare the catalytic properties
on different supports and to characterize the catalysts by various methods to understand the
structure of the active ruthenium species. The reactions carried out are hydrogenation of
nitrobenzene to aniline and hydrodechlorination of 1,2,4-trichlorobenzene. The physicochemical
characterization of ruthenium catalysts supported on various supports like Nb2O5,
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Al2O3, Activated carbon, CeO2, ZrO2, CeO2-ZrO2, SiO2, MCM-41 and SBA-15 have been
investigated. The catalysts are found to be active for both hydrogenation of nitrobenzene
and hydrodechlorination of 1,2,4-trichlorobenzene. The investigation employed the
COchemisorption
technique to determine the dispersion, metal area of the ruthenium catalysts.
The effect of support and loading of ruthenium on the nature of dispersed phase has been
studied. The following are summary of the results reported in the present thesis.
The catalysts supports like Nb2O5, Al2O3, Activated carbon, SiO2, are taken from
commercial origin. CeO2, ZrO2, and CeO2-ZrO2 supports are prepared by precipitation and
co-precipitation method. MCM-41 and SBA-15 have been synthesized by hydrolysis
method using TEOS as silica source and P123 triblock co-polymer as structure directing
agent. Ruthenium catalysts are prepared by impregnation method using RuCl3 as a
precursor. The catalysts have been characterized by using an array of adsorption and
several spectroscopic techniques like BET surface are, pore size distribution, X-ray
diffraction, X-ray photoelectron spectroscopy, Transmission electron microscopy. The
dispersion, metal area and crystallite size of ruthenium are determined using CO
chemisorption. A down flow fixed bed reactor is used to evaluate the catalytic properties
for the hydrogenation and hydrodechlorination reactions.
For a given supported ruthenium catalysts the BET surface areas decreases with
increasing ruthenium loading on the support due to pore blockage of the support surface by
the deposition of ruthenium particles. Pore size distribution results are also support these
findings.
Supports like activated carbon, SiO2 are found to be X-ray amorphous. However
the supports like SBA-15 and MCM-41 show their mesoporous hexagonal ordering in the
low-angle X-ray diffractograms. ZrO2 shows the presence of both monoclinic and
tetragonal phase, CeO2 exists in cubic phase and CeO2-ZrO2 shows a solid solution with
cubic phase. Nb2O5 exists in TT phase. The XRD results of supported ruthenium catalysts
give an idea of structure of ruthenium species present on the surface of the catalysts. In all
the catalysts ruthenium is found to be in higher dispersed form at lower loadings (> 3 wt%
Ru)) and crystalline at higher ruthenium loadings.
The morphology and the crystallite size of ruthenium is observed using
transmission electron microscopy. From the TEM images it is clear that the ruthenium
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204
particles are present in highly dispersed form at lower loadings and formation of large
crystallites of ruthenium at higher loadings.
Dispersion, metal area, crystallite size and active sites of ruthenium are determined
using the CO chemisorption method. The dispersion was found to decrease with ruthenium
loading. This might be due to decrease in the distance between the metal particles with
loading which would lead to metal-metal interactions and agglomerization of particles. The
metal areas of the catalysts were found to increase with ruthenium loading up to 3.0 wt%
and tend to decrease/remain constant at higher loadings. The crystallite size of ruthenium
found to increase with ruthenium loading. These findings are in good agreement with that
of the XRD and TEM analysis.
X-ray photoelectron spectroscopy is used to determine the exact oxidation state of
ruthenium present on the surface of the support. XPS reveals the presence of Ru0 in almost
all the catalysts. This clearly indicates the ruthenium chloride precursor is completely
reduced in the experimental conditions.
The TPR profiles of supported ruthenium catalysts exhibit mainly two peaks one at
low temperature and second at high temperature. The peak low temperature is due to the
reduction of RuCl3 precursor and the high temperature is due to the reduction of oxides of
ruthenium formed during the preparation of the catalysts. In almost all the catalysts we
could observe a shift in Tmax postions to higher temperature due to the formation of large
crystallites of ruthenium (or) the decrease in the metal support interaction between the
metal and the support.
The rate of hydrogenation activity of supported ruthenium catalysts found to
increase with ruthenium loading up to 3.0 wt% in case of SiO2, ZrO2, CeO2, CeO2-ZrO2
and Al2O3. In the systems like SBA-15, MCM-41 activated carbon and Nb2O5 the
hydrogenation activity increase up to 4.5 wt%. The selectivity for the formation of aniline
is almost above 90 % in all the catalysts. Benzene is formed as byproduct in systems like
CeO2, CeO2-ZrO2. The results of CO-chemisorption, TPR and XRD further support the
hydrogenation activity of the catalyst.
The activity for the hydrodechlorination of 1,2,4-trichlorobenzene is found to
increase with ruthenium loading up to 3.0 wt% and levels off at higher loadings due to
increase in the crystallinity of ruthenium. The selectivity for the formation of benzene is
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also found to increase with ruthenium loading up to 3.0 wt % and leveled off at higher
loadings. Dichlorobenzene, chlorobenzene are formed as byproducts in the reaction. It is
known that the catalyst with more number of available active sites exhibit high activity
than other catalysts. Table 1 shows the comparison of various supported ruthenium
catalysts. The catalytic activity is measured under similar experimental condition for each
of the reaction. As observed from the Table 1 the activated carbon catalysts show better
activity for the hydrodechlorination reactions and SBA-15 for the hydrogenation activity.