61st Annual Report on Research 2016

Overview of project. This new research direction funding supports synthesis of a select series of photocatalytically active porous monolithic metal oxides (e.g., TiO₂, ZnO, Ga₂O₃) that will subsequently coated with co-catalysts chosen based on their recognized methane photo-oxidation catalytic activity (e.g., Pt, Ni, CeO₂). Of primary interest is identifying hybrid catalysts for electrochemical or photochemical transformation of methane at moderate temperatures. This work utilizes botanically-templated light-gathering structures comprised of catalytically active oxides that, when decorated with metal or non-metal catalysts, will enable new photo-thermal chemical reactions. Two students from under-represented groups were funded to work on this project during the past year (Anthony Montoya, Hispanic-American, and Nathaniel Coleman, Jr., African-American) and Coleman successfully defended his PhD thesis in late summer 2015 and started a postdoctoral position in Kansas State University in January 2016. In January 2016, Montoya was awarded a UI GAANN (Graduate Assistance in Areas of National Need) fellowship to support his research studies.

Templated materials synthesis. During this past year, Nathan Black, a second year graduate student, has made significant materials synthesis progress in utilization of sensitive porous botanical templates from living leaf structures (ZZ and Jade plants). His improvements on our DMP dehydration process create leaves that retain their complex cellular and vascular structure with ~5-10% mass of the original template. The dehydrated cellulose/lignin scaffolds are stable and robust and absorb solvents. Black has recently extended our botanical range to other templates with clear directed pore structures, such as a celery stalk that retains its columnar structure upon DMP drying and he has converted it into anatase titania monoliths that mirror the striated celery structure. In addition, he has identified conditions where the DMP dehydration solution can be utilized to directly react with hydrated metal oxide precursors such as titanium alkoxides, to rapidly produce gel-like beads of the metal oxide/hydroxide in rapid fashion, which may serve as an additional avenue for porous oxide growth.

To date, Black has been successful in producing many botanically templated crystalline macroporous metal oxides at moderate ~500 - 800 °C temperatures in air (e.g., TiO₂, CeO₂, Ga₂O₃, ZnO, and WO₃) from our more well-studied ZZ and Jade succulent templates. He has also branched into several first-row transition metal oxides as many of them have shown either electrochemical redox activity or more specifically oxidation abilities that could make them key components of hybrid catalyst structures for methane oxidation. So far, Black has used metal halide and acetate solution infiltration to grow MnO_x, Co₃O₄, NiO, Ni_1-xCo_xO, and CuO macroporous structures. He observes porous structures that reflect the original botanical template. In some cases, the cellular walls show micrometer thicknesses and the wall itself appears to be formed by fusion of very small particles that are 10’s of nanometers in diameter (e.g., for TiO₂ from alkoxide precursors) to large faceted crystallites as large as a few micrometers in diameter. While the macrostructures are similar, the porous structure’s walls clearly vary with precursor, oxide target, infiltration methods, and thermolysis temperature. For example, NiO grown from metal acetates and heated to higher 800 °C temperatures leads to larger crystallite surface structures while CeO₂ shows much smoother walls with domains under 100 nm (TOC graphic). WO₃ and CeO₂ grown from the Jade template both show well-defined large and open cell foam-like morphologies (TOC graphic).

SEM-EDS and XRD analysis of products and air-pyrolyzed templates reveals the presence of inorganic impurities that arise from plant mineral ions present in the leaf templates, specifically major Ca components with minor and varying degrees of Na, Mg, and K. These ions may incorporate into some of the growing oxide structures or form secondary oxides at particle interfaces. We have begun XPS surface analysis of several metal oxide surfaces and initial results show low but detectable Ca and K on the Co₃O₄ surfaces and Ca, K, and Mg on NiO surfaces. In the case of nickel, there also appears to be more than one Ni chemical environment, which may result from surface calcium reaction with the NiO particles.

Photocatalysis and electrocatalysis studies. As was noted in last year’s report, Nate Black has been trained to affix his templated metal oxides on graphite-wax electrodes for examination of their oxidation ability using the oxygen evolution reaction (OER) in basic (0.1M KOH) solutions. He has examined his NiO and Co₃O₄ products versus a RuO₂ commercial standard and commercial free-standing nanoparticles of NiO and Co₃O₄. The templated Co₃O₄ shows an OER onset potential (~1.4-1.5 V vs. RHE) that is lower both commercial RuO₂ powder and Co₃O₄ nanoparticles. The templated NiO have onset potentials of ~1.6 V that is near the RuO₂ standard and at slightly higher potentials than commercial NiO nanoparticles. These electrochemical studies help inform our related photochemical studies using these metal oxides in composite structures with templated photocatalysts for oxidative photocatalysis of methane and methanol. To date, our photochemical activation experiments with the templated single metal oxides have not led to selective CH₄ oxidation versus observing CO₂/H₂O formation. In photochemical reactions, we have begun photooxidation investigations using organic dye and methanol oxidation in conjunction with peroxide activation in an effort to better gauge the potential utility of our templated oxides in selective oxidation reaction.

Other group researchers have interfaced with Nate Black’s work by providing access to new composite structures using photocatalytically active carbon nitrides (Montoya) or electrochemically active metal phosphides (Coleman) and also direct carbon formation for electrically conducting porous monoliths embedded with nanoscale Co and Ni metal particles. Separately, several of these co-catalysts have shown activity in photochemical and electrochemical reduction reactions, particularly hydrogen evolution. In the coming year, we will continue to target photo-electrochemical hybrid structures using botanically templated oxide scaffolds for selective oxidation reactions related to methane/methanol conversions.