60th Annual Report on Research 2015

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 catalytic activity (e.g., Pt, Ni, CeO₂). The proposed new macroporous/nanoparticulate dual catalyst structures are being investigated for both reduction and oxidation centered photocatalysis. Of primary interest is identifying hybrid photo-thermal catalysts for chemical transformations of methane at moderate temperatures. This work specifically seeks to synthesize botanically-templated light-gathering photoactive oxides that, when decorated with metal or non-metal catalysts, will enable new photo-thermal chemical transformations of thermodynamically stable methane. Two students from under-represented groups were funded to work on this new directions project.

Templated materials synthesis. During this project period, Nate Black, a new graduate student, has made significant materials synthesis progress in the reproducible production of sensitive porous botanical templates from living leaf structures (ZZ and Jade plants). He improved on our previously published dimethoxypropane (DMP) chemical dehydration method that had some materials inconsistencies. His revised DMP dehydration process creates leaves that retain their complex cellular and vascular structure, but have masses just 5-10% of their original fresh/wet masses. The dehydrated cellulose/lignin scaffolds are stable and robust for at least a year without apparent degradation. Nate also successfully designed precursor infiltration strategies (typically using metal halides or acetates in methanol solutions) that yield inorganic oxides after air pyrolysis of the template/precursor composite (see TOC graphic). Specifically he has grown macroporous TiO₂, NiO, Co₃O₄, Ni_1-xCo_xO, and CuO transition-metal oxides at 500 – 800 °C and characterized their porous structures by XRD and SEM-EDS. The Ni_1-xCo_xO materials followed a Vegard’s Law relationship between precursor composition, product composition and change in cubic lattice parameter. Several of these oxide materials show well-defined nanoscale compacted particle architectures on their replicated botanical cell walls, which may provide benefits of nanostructures catalyst function with the interconnected nature of a bulk solid. In addition to these transition metal oxides, Nate Black has been successful in botanically templated crystalline and potentially photoactive porous metal oxides including CeO₂, Ga₂O₃, ZnO, and WO₃. We are currently characterizing the physical and optical properties of these structures and have begun photocatalysis experiments with them.

Photocatalysis and electrocatalysis studies. A co-advised graduate student (with Prof. Leddy UI electrochemist), Matt Lovander, has determined that the NiO and Co₃O₄ materials are stable in strongly basic solutions and show electrochemical charge uptake (capacitance – Faradaic and double layer) that is comparable to commercial nanoparticle powders. Of particular interest to this current study, the Co₃O₄ templated product also shows early onset for the oxygen evolution reaction (OER), comparable to the Co₃O₄ nanoparticles near 500 mV (vs. SCE) much lower than the carbon wax background. In the coming year, we will be further examining these metal oxides in conjunction with photoactive structures (e.g., templated TiO₂ with Ni/Co oxide surface coatings) for oxidative photocatalysis of methane and methanol.

Tony Montoya, a senior graduate student, has studied several porous templated metal oxides (e.g., CeO₂, Ga₂O₃, TiO₂, WO₃) photocatalysts with gaseous methane with and without O₂ in an IR gas cell and larger 50 ml flask reactor. Several experiments have been conducted using a high energy 450 W UV mercury lamp irradiation for several hours. The gas mixture was analyzed non-invasively by IR and directly by RGA-MS. Both analyses show that the methane remains essentially intact and resists oxidation or dehydrogenation. Tony is currently expanding our studies to include external heating to ~100 °C along with UV irradiation. He will be examine these photocatalysts for methane photooxidation under photo-thermal combined conditions and is also beginning catalytic experiments in aqueous solution with and without H₂O₂ to examine effects of dissolved methane and hydroxide radical formation by several photocatalysts. Tony has also synthesized several photoactive solids including WO₃ and carbon nitride polymer solids that can be coated with Ag, Pd, or Pt particles through solution photoreduction (using methanol oxidation as a sacrificial reagent). The semiconducting carbon nitride powders without metal co-catalysts are active in the UV photooxidation of methyl orange dye using atmospheric O₂ and when coated with Pt nanoparticles show H₂ evolution from aqueous solutions. Our synthetic methods for carbon nitrides makes them amenable to coating formation on Nate Black’s templated metal oxides for composite catalyst formation.

A recent Ph.D. graduate, Nate Coleman, has grown several photoactive metal phosphide materials on photoactive TiO₂ in an effort to produce catalysts that may be active for methane oxidation without using expensive noble metal catalysts. Matt Lovander determined that some of Nate’s phosphorus-rich metal phosphides (e.g., NiP₂ and CuP₂) are electrocatalytically active for hydrogen formation under applied potential. Matt found that under acidic 0.5 M H₂SO₄ conditions, the MP₂ materials only require moderate applied potentials of less than ~500 mV (vs. RHE) to show clear H₂ evolution. This may open up an additional avenue for methane activation in the year ahead where we can introduce photoelectrochemical reactions to complement the proposal’s thermal-photocatalytic studies.