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Final report
Title: Coupling Thermal and Photocatalysis in Novel Metal Oxides
for CO2 to Fuels
PI: Justin Notestein (ChBE), Kimberly Gray (EnvE)
Supported Student: Kevin Schwartzenberg
Introduction: Photocatalytic CO2 reduction (PCCR) is an
attractive alternative to sequestration or natural photosynthesis
for mitigating the impacts of CO2 emissions. However, human-made
solar fuel from CO2 is a recognized ‘grand challenge’ in catalysis
that requires both fundamental research and technological
development. Specifically, the US Department of Energy has
identified the lack of efficient and controllable photocatalysts as
a primary concern. Over one year we synthesized, characterized, and
tested novel catalytic nano-architectures consisting of
redox-active oxide clusters on photoactive supports (e.g.
MnOx-TiO2). The novel proposal is that combining both heat and
light will more readily generate oxygen vacancies and turnover a
CO2 reduction cycle.
Data collected from this project has led to a multi-year,
multi-PI, international collaboration sponsored by the US National
Science Foundation and the governments of Northern Ireland and the
Republic of Ireland.
Figure 1 gives the overarching design hypothesis for this work,
wherein a light harvesting material is further modified by
reducable oxide clusters, here manganese oxides on titania. The
construction of this system was proposed to tune the bandgap of the
titania, the oxidation state of the manganese oxide (and therefore
its reactivity with CO2) and the reducibility of the manganese
oxide (and therefore its ability to transit a complete
reduction-oxidation cycle with minimal heat input). Understanding
these functions first required a materials synthesis campaign and a
testing regime, described below. Additional characterization and
materials testing is being continued under the auspices of the
US-Ireland R&D partnership.
Results: For this study, we examined the effects of 1) manganese
precursor (acetate or nitrate), 2) the manganese loading on the
titania particles, and 3) the crystal structure of the titania
(anatase or rutile). Samples were prepared by incipient wetness
impregnation, a scaleable synthesis technique used widely by
industry for catalyst preparation with minimum of waste products
and
Figure 1. Proposed Photo-‐ and
Thermal Catalytic Cycle on
Nanostructured Metal Oxide Composites.
A catalytic cycle for the
reduction of CO2 on MnOx clusters
supported on semiconductor
nanoparticles of TiO2. Reduced metal
centers and oxygen deficiencies
are sites of CO2 adsorption and
activation. C-‐O bonds are proposed
to be cleaved by input of
light energy and the driving
force to reoxidize the reduced
MnOx. Thus, key design needs
are a solar-‐matched light
absorber and a MoOx cluster that
is readily re-‐reduced.
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a high level of control. Titania powders are wetted with minimum
volume of an aqueous solution of the Mn precursor, stirred, dried,
then heated in static air to remove the ligands and bring the
manganese oxide to its final state, denoted as MnOx. Different
amounts of Mn salt are added such that the surface density of MnOx
in the final material ranges between 0.25 Mn atoms.nm-2, to 8 Mn
atoms.nm-2. For reference, complete coverage of the titania surface
should occur, geometrically, near 6 Mn atoms.nm-2.
Figure 2 shows the clear differences in the composite oxide
optical properties when changing the surface loading of the added
MnOx. Figure 3 shows that the dark color is attributable to a new
MnOx feature below 400 nm, but also on anatase, to a red shift in
the band gap of the titania support from 3.15 eV to as low as 2.2
eV. In contrast, the optical spectra of the rutile materials are
indistinguishable from a mechanical combination of the two oxides.
Thus, a key first benchmark was reached, demonstrating that the
underlying band gap could be shifted into the solar range and that
there could be substantial increases in total light absorption, by
surface modifications only.
The second key structural requirement is that the surface of the
catalyst be reducible at relatively mild conditions, and that a
large number of reduced sites be generated. This is required to
provide a driving force for CO2 adsorption and C-O bond cleavage.
Samples were reduced in H2, such as would be generated on TiO2
photocatalyst surfaces from splitting of water. Loss of surface O
was measured directly by thermogravimetric analysis as a function
of temperature. Select results are shown in Figure 4 for the Mn
nitrate-derived materials on anatase, which was a sample that
shifted the TiO2 bandgap. At low levels of Mn addition, the new
cation actually eliminates the tendency of the anatase to form
vacancies at moderate temperature. However, when the MnOx coverage
approaches a complete monolayer (4 Mn.nm-2 and higher), high
temperature reduction features characteristic of extended manganese
(IV) oxides appear at greater than 300°C, but there is also a new,
lower temperature feature at ~200°C. X-ray absorption spectroscopy
at Argonne National Lab Advanced Light Source also shows increases
in as-synthesized oxidation state with increasing loading for this
sample, and in general, a
Figure 2. Optical micrographs of
as-‐prepared MnOx-‐TiO2 powders, showing
clear differences with respect to
Mn loading, crystal structre,
and Mn precursor. Differences are
quantified in Figure 3.
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dependence of oxidation state on the synthesis conditions,
precursor, and type of titania used. A
anatase
MnOx-anatase from nitrate
MnOx-anatase from acetate
Figure 3. Diffuse reflectance
UV-‐visible spectra of MnOx-‐TiO2
composite materials as a function
of Mn precursor, loading, and
titania crystal form. On anatase,
strong MnOx-‐based absorptions are
formed below 400 nm.
Extrapolating down the band gap
of the TiO2 domains (dashed
line shown for one sample) gives
a significant red shift with
increasing MnOx loading. At high
loading, there is a pronounced
effect of the precursor used.
Interactions on rutile are
negligible, and data on a
complementary set on rutile not
shown.
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high resting oxidation state and a low reduction temperature are
desirable, as such materials are more able to accommodate
reversible loss of framework O atoms as required by this catalytic
cycle. Initial testing of photocatalytic reduction of CO2 at room
temperature with these materials shows no reactivity beyond that
expected by the parent TiO2. However, this is expected, as the
materials do not begin to reduce significantly until ~200°C,
generating reactive sites. A new reactor is under construction to
carry out this testing.
Outlook: Overall, these materials, especially Mn nitrate-derived
MnOx on anatase at Mn surface densities approaching complete
monolayer coverage, have been demonstrated to be prime candidates
for testing hypotheses on the heat-assisted photocatalytic
reduction of CO2.
The results from this work are being compiled for publication in
the near future. Most importantly, they served as the foundation
for a successful grant proposal to the US National Science
Foundation. With lead PI Kimberly Gray, it also includes Justin
Notestein and Eric Weitz. Additional researchers and funding come
from the governments of Northern Ireland and the Republic of
Ireland, forming the core of the US-Ireland R&D partnership.
Details for this funded project are given below. In addition to the
development of many more materials, activities of this new project
will include:
Additional experiments to understand the reducibility of the
surface with CO, and the propensity of the surface to oxidize CO
with O2. (Northwestern)
• In situ FTIR spectroscopy in the presence of CO2 to understand
surface defects and their role in activating CO2.
(Northwestern)
• Custom reactors to carry out photochemical reactions at
elevated temperature (Northwestern and Ulster)
• Photoelectrochemical testing and characterization (Ulster)
• Planned experiments in pilot scale solar photoreactors at the
“Plataforma Solar de Almeria” in Spain via pre-existing memorandia
of understanding (Ulster)
• Computational screening and investigation of materials
properties (e.g. Figure 5, Tyndall Institute).
New Project: SusChem: Using theory-driven
design to tailor novel nanocomposite oxides for solar fuel
production.
Location: Northwestern University Sponsor: U.S.
NSF/CBET/Catalysis and
Figure 4. Rate of loss of
framework O atoms in flowing H2
as a function of temperature
and Mn loading for the
nitrate-‐derived precursor on anatase.
Low-‐temperature reduction features at
~200°C for the highest-‐loaded
samples indicate promise for CO2
photoreduction.
Figure 5. Example of periodic
DFT calculations (Tyndall Institute)
for a Mn2O3 cluster on
anatase TiO2 after reduction, showing
formation of reduced centers in
both oxide components.
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Biocatalysis Award Period: 9/1/2014 to 8/31/2017 Total Funding:
$550,000 (component for U.S. researchers)
