Kelly L. Materna

Electrochemical CO2 Reduction to Hydrocarbons on a Heterogeneous Molecular Cu Catalyst in Aqueous Solution

Exploration of heterogeneous molecular catalysts combining the atomic-level tunability of molecular structures and the practical handling advantages of heterogeneous catalysts represents an attractive approach to developing high-performance catalysts for important and challenging chemical reactions such as electrochemical carbon dioxide reduction which holds the promise for converting emissions back to fuels utilizing renewable energy. Thus, far, efficient and selective electroreduction of CO2 to deeply reduced products such as hydrocarbons remains a big challenge. Here, we report a molecular copper-porphyrin complex (copper(II)-5,10,15,20-tetrakis(2,6-dihydroxyphenyl)porphyrin) that can be used as a heterogeneous electrocatalyst with high activity and selectivity for reducing CO2 to hydrocarbons in aqueous media. At -0.976 V vs the reversible hydrogen electrode, the catalyst is able to drive partial current densities of 13.2 and 8.4 mA cm(-2) for methane and ethylene production from CO2 reduction, corresponding to turnover frequencies of 4.3 and 1.8 molecules·site(-1)·s(-1) for methane and ethylene, respectively. This represents the highest catalytic activity to date for hydrocarbon production over a molecular CO2 reduction electrocatalyst. The unprecedented catalytic performance is attributed to the built-in hydroxyl groups in the porphyrin structure and the reactivity of the copper(I) metal center.

New Ir Bis-Carbonyl Precursor for Water Oxidation Catalysis

This paper introduces Ir(I)(CO)2(pyalc) (pyalc = (2-pyridyl)-2-propanoate) as an atom-efficient precursor for Ir-based homogeneous oxidation catalysis. This compound was chosen to simplify analysis of the water oxidation catalyst species formed by the previously reported Cp*Ir(III)(pyalc)OH water oxidation precatalyst. Here, we present a comparative study on the chemical and catalytic properties of these two precursors. Previous studies show that oxidative activation of Cp*Ir-based precursors with NaIO4 results in formation of a blue Ir(IV) species. This activation is concomitant with the loss of the placeholder Cp* ligand which oxidatively degrades to form acetic acid, iodate, and other obligatory byproducts. The activation process requires substantial amounts of primary oxidant, and the degradation products complicate analysis of the resulting Ir(IV) species. The species formed from oxidation of the Ir(CO)2(pyalc) precursor, on the other hand, lacks these degradation products (the CO ligands are easily lost upon oxidation) which allows for more detailed examination of the resulting Ir(pyalc) active species both catalytically and spectroscopically, although complete structural analysis is still elusive. Once Ir(CO)2(pyalc) is activated, the system requires acetic acid or acetate to prevent the formation of nanoparticles. Investigation of the activated bis-carbonyl complex also suggests several Ir(pyalc) isomers may exist in solution. By (1)H NMR, activated Ir(CO)2(pyalc) has fewer isomers than activated Cp*Ir complexes, allowing for advanced characterization. Future research in this direction is expected to contribute to a better structural understanding of the active species. A diol crystallization agent was needed for the structure determination of 3.

Silatranes for binding inorganic complexes to metal oxide surfaces

A ruthenium complex containing silatrane functional groups has been synthesized and covalently bound to a conductive metal oxide film composed of nanoparticulate ITO (nanoITO). The silatrane-derived siloxane surface anchors were found to be stable in the examined range of pH 2 to 11 in aqueous phosphate buffer, and the ruthenium complex was found to have stable electrochemical features with repeated electrochemical cycling. The non-coordinating properties of the silatrane group to metals, which facilitates synthesis of silatrane-labeled coordination complexes, together with the facile surface-binding procedure, robustness of the surface linkages, and stability of the electrochemical properties suggest that incorporating silatrane motifs into ligands for inorganic complexes provides superior properties for attachment of catalysts to metal oxide surfaces under aqueous conditions.

Poly(ethylene oxide)-poly(butylene oxide)-poly(ethylene oxide)-templated synthesis of mesoporous alumina: effect of triblock copolymer and acid concentration.

Mesoporous alumina was synthesized via a one-pot self-assembly of aluminum isopropoxide and poly(ethylene oxide)-poly(butylene oxide)-poly(ethylene oxide) triblock copolymer in an acidic ethanol solution. The effects of the polymer concentration and nitric acid concentration, independently, on the adsorption properties (such as surface area, pore volume, microporosity, mesoporosity, and pore width) were studied. An increase in the specific surface area and the pore volume was seen for the samples containing a polymer/aluminum isopropoxide wt. ratio up to 0.71 and a polymer/acid wt ratio of 0.88. Titania isopropoxide was also added to the synthesis to illustrate the extension of this approach to alumina-based mixed metal oxides.

Stable iridium dinuclear heterogeneous catalysts supported on metal-oxide substrate for solar water oxidation.

Significance While research on single-atom catalysts (SACs) is arguably mature, rare work has been done on atomically dispersed catalyst featuring two atoms. We synthesized Ir dinuclear heterogeneous catalyst in a facile photochemical way. It exhibits outstanding stability and high activity toward water oxidation. The significance of this work can also be appreciated from the catalysis perspective. A grand challenge in heterogeneous catalysis is how to understand the detailed mechanisms at the molecular level, because the most active heterogeneous catalysts are often poorly defined in their atomic structures. Our finding is built upon recent advances aimed at studying SACs but takes a crucial step forward. It provides a material platform to study reactions that would require more than one active site. Atomically dispersed catalysts refer to substrate-supported heterogeneous catalysts featuring one or a few active metal atoms that are separated from one another. They represent an important class of materials ranging from single-atom catalysts (SACs) and nanoparticles (NPs). While SACs and NPs have been extensively reported, catalysts featuring a few atoms with well-defined structures are poorly studied. The difficulty in synthesizing such structures has been a critical challenge. Here we report a facile photochemical method that produces catalytic centers consisting of two Ir metal cations, bridged by O and stably bound to a support. Direct evidence unambiguously supporting the dinuclear nature of the catalysts anchored on α-Fe2O3 is obtained by aberration-corrected scanning transmission electron microscopy (AC-STEM). Experimental and computational results further reveal that the threefold hollow binding sites on the OH-terminated surface of α-Fe2O3 anchor the catalysts to provide outstanding stability against detachment or aggregation. The resulting catalysts exhibit high activities toward H2O photooxidation.

Optimization of Photoanodes for Photocatalytic Water Oxidation by Combining a Heterogenized Iridium Water-Oxidation Catalyst with a High-Potential Porphyrin Photosensitizer

The development of water-splitting dye-sensitized photoelectrochemical cells has gained interest owing to their ability to generate renewable fuels from solar energy. In this study, photoanodes were assembled from a SnO2 film sensitized with a combination of a high-potential CF3 -substituted porphyrin dye with a tetrahydropyranyl-protected hydroxamic acid surface-anchoring group and a Cp*Ir (Cp*=pentamethylcyclopentadienyl) water-oxidation catalyst containing a silatrane anchoring group. The dye/catalyst ratios were varied from 2:1 to 32:1 to optimize the photocatalytic water oxidation. Photoelectrochemical measurements showed not only more stable and reproducible photocurrents for lower dye/catalyst ratios but also improved photostability. O2 production was confirmed in real time over a 20 h period with a Clark electrode. Photoanodes prepared from 2:1 and 8:1 dye/catalyst sensitization solutions provided the most active electrodes for photocatalytic water oxidation and performed approximately 30-35 turnovers in 20 h.

Antimony Complexes for Electrocatalysis: Activity of a Main-Group Element in Proton Reduction

Main-group complexes are shown to be viable electrocatalysts for the H2 -evolution reaction (HER) from acid. A series of antimony porphyrins with varying axial ligands were synthesized for electrocatalysis applications. The proton-reduction catalytic properties of TPSb(OH)2 (TP=5,10,15,20-tetra(p-tolyl)porphyrin) with two axial hydroxy ligands were studied in detail, demonstrating catalytic H2 production. Experiments, in conjunction with quantum chemistry calculations, show that the catalytic cycle is driven via the redox activity of both the porphyrin ligand and the Sb center. This study brings insight into main group catalysis and the role of redox-active ligands during catalysis.

End-On Bound Iridium Dinuclear Heterogeneous Catalysts on WO3 for Solar Water Oxidation

Heterogeneous catalysts with atomically defined active centers hold great promise for high-performance applications. Among them, catalysts featuring active moieties with more than one metal atom are important for chemical reactions that require synergistic effects but are rarer than single atom catalysts (SACs). The difficulty in synthesizing such catalysts has been a key challenge. Recent progress in preparing dinuclear heterogeneous catalysts (DHCs) from homogeneous molecular precursors has provided an effective route to address this challenge. Nevertheless, only side-on bound DHCs, where both metal atoms are affixed to the supporting substrate, have been reported. The competing end-on binding mode, where only one metal atom is attached to the substrate and the other metal atom is dangling, has been missing. Here, we report the first observation that end-on binding is indeed possible for Ir DHCs supported on WO3. Unambiguous evidence supporting the binding mode was obtained by in situ diffuse reflectance infrared Fourier transform spectroscopy and high-angle annular dark-field scanning transmission electron microscopy. Density functional theory calculations provide additional support for the binding mode, as well as insights into how end-on bound DHCs may be beneficial for solar water oxidation reactions. The results have important implications for future studies of highly effective heterogeneous catalysts for complex chemical reactions.

Silatrane Anchors for Metal Oxide Surfaces: Optimization for Potential Photocatalytic and Electrocatalytic Applications

Silatrane surface anchors are protected siloxanes that are known to bond firmly (from pH 2-11) to metal oxide electrodes under heating. However, these conditions are not always compatible with the other functionality present. A silatrane-containing porphyrin molecule and a silatrane-containing ruthenium complex have now been designed, synthesized and optimized conditions have been identified for surface binding. Two mild, room-temperature surface binding methods were explored: binding with or without an acidic pretreatment; these methods were compared to the traditional, harsher binding conditions involving strong heating. We find that a preacidified electrode gave comparable surface loadings at room temperature compared to sensitization by using the previous strong heating method. This was also true on TiO2, SnO2, and nanoITO electrodes and thus may be generalizable. The new, milder binding methods also resulted in excellent aqueous and electrochemical stability from pH 2-11. Using a water-insoluble porphyrin with a silatrane anchor further increased the aqueous stability of the deposit, aided by the insolubility of the porphyrin. Finally, X-ray photoelectron spectroscopy (XPS) data confirmed for the first time that the triethanolamine released from the silatrane on deprotection/binding in turn binds to TiO2, SnO2, and nanoITO electrodes. This undesired triethanolamine deposit was easily removed from the surface by electrochemical voltage cycling or with an aqueous acidic wash for 1 h.