(197f) Mesoporous Metal Oxide Aerogels for Sustainable Fuels Generation

Ashley M. Pennington,^1,2 Catherine L. Pitman,^1,2 Paul A. DeSario,¹ Jeremy J. Pietron,¹ Todd H. Brintlinger,³ Debra R. Rolison¹

¹ Chemistry Division, U.S. Naval Research Laboratory

² National Research Council Postdoctoral Associate

³ Materials Science and Technology Division, U.S. Naval Research Laboratory

Renewable and sustainable production of fuels for civilian, industrial, and military use is an important objective for achieving both national and environmental security. Our approach is to synthesize, characterize, and test catalytically active mesoporous metal||metal oxide aerogels (M||TiO₂ and M||CeO₂) for fuel generation and processing chemistries. These mesoporous expressions, a covalently bonded pore–solid network of TiO₂ or CeO₂ nanoparticles, permit unimpeded mass transport of reactive species to a high density of catalytic sites dispersed in three dimensions throughout the high surface–area, ultraporous catalytic architecture. By the addition of a nanoscopic metal co-catalyst (Cu or Au), we increase the catalytic activity and selectivity of these metal oxide architectures. Both copper and gold also feature visible light surface plasmon resonances (SPR) that sensitize the metal||metal oxide composite to solar radiation, enabling generation of electron–hole pairs that can initiate solar fuel production. Variations of the catalysts tested include Cu or Au nanoparticles (NPs) supported on the metal oxide aerogels (Cu/CeO₂, Cu/TiO₂, Au/TiO₂), or gold metal NPs entrained in the metal oxide nanoscopic network (Au–TiO₂).

Carbon monoxide (CO) oxidation is an important reaction relating to energy conversion, energy storage, cleaning of exhaust, and other industrial applications. Catalytic activity of our metal||metal oxide catalysts for CO oxidation is monitored by a packed bed reactor with an inline GC and by using in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS). Both reactors run either dark or illuminated reactions, allowing us to characterize catalytic and photocatalytic activity of these metal||metal oxide aerogels under various relative humidities. We relate the CO oxidation performance of our catalytic aerogel compositions to their physical, chemical, and electronic attributes, as characterized by X-ray diffraction (XRD), N₂ porosimetry, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) with tomographic reconstruction, electrochemical impedance spectroscopy (EIS), and diffuse-reflectance UV–visible spectroscopy (UV–Vis).

Our earlier SPR-driven photoelectrochemical water splitting studies^[1] showed different activity over the Au–TiO₂ and Au/TiO₂ aerogels for the same weight loading and particle size. We bridge from liquid-phase photoelectrochemical water splitting to gas-phase CO oxidation in order to further investigate the effects of the nature of the Au||TiO₂ interface on thermocatalytic (dark) and photocatalytic (visible) heterogeneous catalysis. Unlike what was noted for SPR-driven photoelectrochemical water splitting at these catalytic platforms, the method of gold NP incorporation into the oxide aerogel is inconsequential to thermocatalytic CO oxidation—activities of the Au–TiO₂ and Au/TiO₂ aerogels under dark conditions are indistinguishable when comparing at the same Au weight loadings. Visible light–driven photocatalytic CO oxidation over Au–TiO₂ and Au/TiO₂ will be conducted to investigate the effect of the 3D structure of the Au||TiO₂ interface on the photocatalytic activity. Tomographic reconstruction of transmission electron micrographs provides a nanoscale, 3D picture of the Au||TiO₂ interfaces. In Au–TiO₂ aerogels, the TiO₂ contacts the gold NP from multiple directions similar to a spheroid joint, whereas the Au||TiO₂ interfaces in Au/TiO₂ aerogels tend to be conformal on one side of the gold NP. The Au||TiO₂ contact line is known to be active for CO oxidation and it is expected that the difference in the structure of the Au||TiO₂ interface in Au–TiO­₂ and Au/TiO₂ will play a crucial role in photocatalytic activity.

We also test the catalytic and photocatalytic activity of copper on titania (Cu/TiO₂) and copper on ceria (Cu/CeO₂) aerogels. Copper, a more abundant less expensive alternative to gold, would be an attractive co-catalyst if Cu-containing composite aerogels could demonstrate high activity, selectivity, and durability. The oxidation state of copper plays a crucial role in its ability to catalytically oxidize CO at low temperatures (<100 °C). The co-continuous covalently bonded solid network of metal oxide nanoparticles stabilizes low-oxidation state copper (Cu¹⁺, Cu⁰) by increasing the degree of Cu||MO_x interfacial contact. More catalytically active than Cu²⁺, the incorporation of lower oxidation state copper (Cu¹⁺, Cu⁰) on the metal oxide aerogels results in lower activation energy accompanied with a higher CO conversion when compared to copper supported on commercial metal oxides, which occurs mainly as Cu²⁺. UV–visible spectroscopy reveals a strong, persistent, and stable SPR characteristic of metallic copper (Cu⁰), as well as a band feature for Cu₂O (Cu¹⁺) indicating that the copper NPs are stabilized against oxidation. Additionally, the SPR demonstrates that the composite aerogel is sensitized (via the SPR) for visible-light photocatalytic applications.^[2] Cu/TiO₂ aerogels boast a higher activity as well as a lower light off temperature (~70 °C) compared to copper NPs supported on commercial anatase TiO₂ powders (~100 °C).^[3] Additionally, newly-synthesized Cu/CeO₂ aerogels show promise as they outperform Cu/TiO₂ aerogels during low temperature CO oxidation. Multiple regenerations of each catalyst were conducted to confirm catalytic stability—a necessity for implementation in commercial applications. In our work, we demonstrate how exploiting the design of nanoscale interfacial materials can create active metal||metal oxide catalysts that are stable under practical conditions, bringing the dream of economical, environmentally friendly, and efficient catalysts and photocatalysts one step closer to commercialization.

References:

^[1] P.A. DeSario, J.J. Pietron, D.E. DeVantier, T.H. Brintlinger, R.M. Stroud, D.R. Rolison, Plasmonic Enhancement of Visible-Light Water Splitting with Au–TiO₂ Composite Aerogels. Nanoscale, 2013, 5, 8073–8083.

^[2] P.A. DeSario, J.J. Pietron, T.H. Brintlinger, M. McEntee, J.F. Parker, O. Baturina, R.M. Stroud, D.R. Rolison, Oxidation-Stable Plasmonic Copper Nanoparticles in Photocatalytic TiO₂ Nanoarchitectures. Nanoscale, 2017, 9, 11720–11729.

^[3] P.A. DeSario, C. L. Pitman, D. J. Delia, D. M. Driscoll, A. J. Maynes, J. R. Morris, A.M. Pennington, T.H. Brintlinger, D.R. Rolison, J.J. Pietron, Low-temperature CO Oxidation at Persistent Low-valent Cu Nanoparticles on TiO₂ Aerogels. Applied Catalysis B, in press.