Nanoscale design in heterogeneous oxidation catalysis

1
Nano-scale design in heterogeneous oxidation
catalysis
Rick B. Watson and Umit S. OzkanDepartment of
Chemical and Biomolecular Engineering The Ohio
State University
Catalysis has always been a nano-scale science
because the surface reactions studied and the
active sites on which they take place are on the
nano, if not atomic, scale. In this poster we
highlight some of the design methods we use to
control the nano-structure (composition, size,
bonding) of catalytic materials and the
analytical tools we use to characterize these
properties for past and ongoing research topics
in the field of heterogeneous oxidation
catalysis.
The accompanying increase in activity and/or
selectivity in partial oxidation reactions with
the use of alkali (Li, Na, K, Rb, and Cs) doping
has been widely studied and offers a way to
adjust the red-ox and acidic properties of
supported transition metal oxides. The positive
effects of alkali promotion arise from the
alkalis ability to alter oxidation/reduction
behavior, affect surface acidity, and/or cause a
synergism between alkali and transition metal
oxide phases. Work from our laboratory has
examined the structural changes associated with
alkali promotion at low levels and found that the
presence of potassium significantly alters the
electronic structure of the surface MoOx domains
supported over the binary oxide of silica/titania
at the atomic scale.
We have learned from our research that the
nano-structure of an oxide support can play a
crucial role in determining the type and
reactivity of supported molybdenum oxide surface
species. With this in mind, we have begun to
collaborate with Colloid and Interface Research
Group (Dr. Jim Rathman) from our department.

• advanced materials
• nanofabrication
• functional nanostructures

- precision - structure control -
understanding of weak intermolecular forces
molecular self-assembly
Using a bottom-up design process, the colloid
group can prepare nano-structured materials based
on the use of surfactant self-assembly. We have
begun to explore the potential of these
techniques as preparation routes for advanced
catalytic supports. Initial results are presented
here comparing our SiTi 11 support and 10Mo
catalysts to those prepared implementing the use
of the surfactant cetyltrimethylammonium chloride
(CTAC).
Cross Polarization or Polarization Transfer is a
modification to traditional NMR to observe very
insensitive nuclei coupled to proton, such as the
hydroxyl groups (OH) in close proximity of the Si
nuclei. CP-MAS 29Si NMR spectroscopy can be used
to characterize the structure of silica networks
and has been used by our group to characterize
SiO2TiO2 mixed oxides.
To the left, X-ray diffraction patterns were used
to determine the crystallite size of anatase
titania in 10Mo/SiTi 11 catalysts with
increasing levels of potassium promotion.
Molybdenum oxide-based catalysts constitute an
important class of metal oxides used in
catalyticcoxidation. Supported molybdenum oxide
can exist as 2 and 3D domains possessing several
structures (MoO, Mo-O-Mo, or Mo-O-support
bonds). The nature of the active oxygen species
and red-ox properties will play a critical role
in catalyst performance and will certainly depend
upon transition metal loading, dispersion, and
the properties of the oxide support.
Possible structures resulting from surfactant
self-assembly
Subsequent analysis will show the chemical nature
of the support and metal-support interaction is
also changing with the addition of potassium.
Self-assembled structures can be used as reaction
templates for the synthesis of solids with
uniform pore geometries and pore diameters in the
2-100 nm range. Our data show that we can prepare
catalysts with increased surface area and pore
volume by using the surfactant templates during
the sol-gel preparation.
For an optimum combination of activity and
selectivity in selective oxidation there should
exist a balance between the activation of the
hydrocarbon and the ease of removal of oxygen
from the catalyst surface. Oxygen that is too
tightly bound will result in low activity, while
a catalyst with oxygen that is too labile will be
very active, but not selective. Here we highlight
the techniques we use to study the nature of
these oxygen species.
Contributions from the silicon nuclei show
Gaussian peaks at approximately -109 ppm (Q4),
-101 ppm (Q3), -92 ppm (Q2), -84 ppm (Q1). The
relative contributions of these structural sites
were calculated from deconvoluted, integrated
areas. The NMR data show that with the K/Mo molar
ratio the nature of the silica surface changes.
The change in the silica surface arises from the
increasing interaction of MoOx with the surface
hydroxyl groups.
TiO2 (anatase) crystallite size as determined
from X-ray Diffraction line broadening on
K-promoted 10wt.Mo/SiO2TiO2 11 catalysts
Electron microscopy techniques are commonly used
by our group to determine crystallite size.
Internal structure can also be examined through
diffraction experiments, and composition data
collected by characteristic X-ray emission.
Raman Spectroscopy Laser radiation scattered
from a sample contains photons identical to
incident photons, but also contains photons that
shift to a different energy level. These energy
levels are associated with the vibrational energy
levels of the sample. We apply this technique to
study the structure of supported metal oxides.
Results from TEM analysis agree with the XRD
patterns showing how TiOx domains that are
initially evenly dispersed over silica increase
in size and crystallinity with the K/Mo molar
ratio. At the alkali level of K/Mo0.3, a very
large segregation of titania is observed. Within
experimental error, the bulk characterization
provided by EDX analysis did not provide evidence
that molybdenum is preferentially located over
silica-rich, titania-rich, or Si/Ti interfacial
regions of the support on any of the samples
studied. Thus, the analysis suggests that any
preferential interaction of Mo with TiO2 or SiO2
that may exist would have to be a surface effect.

Pore size distributions of SiTi 11 supports and
10Mo catalysts
High-resolution IR spectra can also be used to
ascertain the presence and relative quantity of
the same isolated and geminal hydroxyl groups
that were detected in the 1H-29Si CP-MAS NMR
spectra. These data are plotted together below,
showing a minimum of surface hydroxyls near
K/Mo0.07.
Raman spectroscopy was performed on the 10Mo
samples to determine differences in the types of
supported MoOx species. To the left, data
indicate that the samples prepared using CTAC
possesses stronger MoO bonds, characterized by a
shift to lower wavenumber. The constant position
of the anatase titania band is present at
642cm-1. Research is underway to determine the
nature of Si and Ti domains within the CTAC
samples.
Species observed in CP-MAS 29Si-NMR
To the left, Raman spectra is used to identify
the surface-supported MoOx species over
Mo/Al2O3 catalysts. At low loading levels, 1Mo
atom per square nm, isolated species exist
exhibiting a single band from MoO species. As
loading is increased, bands arising from 2
dimensional structures appear. Strong bands
associated with 3D crystalline MoO3 are detected
at higher loadings when the support is no longer
able to accommodate a monolayer coverage of
supported species.
Raman spectra of 10Mo/SiTi 11 catalysts
Temperature-Programmed Reduction (TPR) determines
the number of reducible species present on the
catalyst surface and reveals the temperature at
which the reduction of each species occurs. The
data show that the CTAC sample certainly
possesses stronger Mo-O bonds that are reduced at
a higher temperature. Studies are underway in
our laboratory to determine the reactivity in
selective oxidation.
TiO2
Raman spectra of dehydrated Mo/Al2O3 catalysts
(Dilor spectrometer, 514.5nm Ar Laser, 5mW)
Comparison of NMR and DRIFTS ratios of
free/geminal hydroxyls on 10 Mo/Si-Ti 11
catalysts with differing K/Mo molar ratios.
TEM micrograph of 40-nm TiO2 particle in
10(K/Mo0.03)/SiTi 11.
TEM micrograph of 10 (K/Mo0.3)/SiTi 11.
TEM micrograph of 10 (K/Mo0.07)/Si-Ti 11.
CP-MAS 29Si-NMR spectra of K-promoted
10Mo/SiTi 11 catalysts
One way to adjust the nano-scale structure of a
partial oxidation catalyst is to support the
active metal oxide on a binary oxide support,
which may provide additional support linkages
that can adjust the reactivity of the supported
species. The unique interaction of MoOx
structures with a SiO2/TiO2 1/1 mixed oxide
support is shown below.
TPR 10Mo/SiTi 11 catalysts
Electron Paramagnetic Resonance (EPR) or Electron
Spin Resonance (ESR) is a spectroscopic technique
which detects species that have unpaired
electrons, generally meaning that it must be a
free radical, if it is an organic molecule, or
that it has transition metal ions if it is an
inorganic complex. The basic physical concepts of
the technique are analogous to those of NMR, but
instead of the spins of the atom's nuclei,
electron spins are excited. ESR can be used to
determine the oxidation state and coordination of
certain transition metals. The technique can
also be applied to detect and monitor the
formation of surface intermediates, such as O2-
(superoxide ion).
The main MoO Raman frequency remains essentially
unchanged with weight loading (1004-1006 cm-1)
until crystalline MoO3 appears at 20. Small,
yet discernable, shoulders are observed in the
main MoO Raman band for 2-15 wt. loading and
indicate that MoOx species attached to titania
(Raman bands present 1004-1006 cm-1) and species
attached to silica (Raman bands present as
shoulders in the range 977-998 cm-1) co-exist.
Dr. Jim Rathman Jared Archer Chang Liu Matt Woods

The g values for different coordination
environments of Mo(V) are reported in the
literature as follows six-coordinate Mo(V),
g1.944 and gII1.892 five-coordinate Mo(V),
g1.957 and gII1.866 and four-coordinate
Mo(V), g 1.926 and gII1.755. Because the
perpendicular component is expected to be the
most sensitive to the coordination of the
isolated MoOx species, the data indicate that
with the addition of potassium there is a change
in the coordination sphere (six-coordinate
distorted toward five-coordinate) of the Mo(V)
species that reaches a maximum at K/Mo ) 0.07.
This change is the result of an increased number
of Si-O- ligands attached to the MoOx-supported
species, as would be in agreement with the NMR
and DRIFTS results.
Sponsors National Science FoundationU.S.
Department of EnergyOhio Department of
Development
ESR spectra of 10Mo/SiTi 11 catalysts with
differing K/Mo molar ratios under dehydrated
conditions, with corresponding variation of g
(perpendicular) and normalized signal intensity
with K/Mo molar ratio.
Raman spectra of dehydrated Mo/SiO2TiO2 11
catalysts with increasing weight loading (Dilor
spectrometer, 514.5nm Ar Laser, 5mW)