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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)