William A. Maza

Cooperative electrochemical water oxidation by Zr nodes and Ni–porphyrin linkers of a PCN-224 MOF thin film

Here, we demonstrate a new strategy for cooperative catalysis and proton abstraction via the incorporation of independent species competent in the desired reactivity into a metal–organic framework (MOF) thin film. The highly porous MOF, designated as PCN-224-Ni, is constructed by Zr–oxo nodes and nickel(II) porphyrin linkers. Films of PCN-224-Ni were grown in situ on FTO and were found to electrochemically facilitate the water oxidation reaction at near neutral pH.

Study of Electrocatalytic Properties of Metal–Organic Framework PCN-223 for the Oxygen Reduction Reaction

A highly robust metal-organic framework (MOF) constructed from Zr6 oxo clusters and Fe(III) porphyrin linkers, PCN-223-Fe was investigated as a heterogeneous catalyst for oxygen reduction reaction (ORR). Films of the framework were grown on a conductive FTO substrate and showed a high catalytic current upon application of cathodic potentials and achieved high H2O/H2O2 selectivity. In addition, the effect of the proton source on the catalytic performance was also investigated.

Proton-Coupled Electron Transport in Anthraquinone-Based Zirconium Metal–Organic Frameworks

The ditopic ligands 2,6-dicarboxy-9,10-anthraquinone and 1,4-dicarboxy-9,10-anthraquinone were used to synthesize two new UiO-type metal-organic frameworks (MOFs; namely, 2,6-Zr-AQ-MOF and 1,4-Zr-AQ-MOF, respectively). The Pourbaix diagrams (E vs pH) of the MOFs and their ligands were constructed using cyclic voltammetry in aqueous buffered media. The MOFs exhibit chemical stability and undergo diverse electrochemical processes, where the number of electrons and protons transferred was tailored in a Nernstian manner by the pH of the media. Both the 2,6-Zr-AQ-MOF and its ligand reveal a similar electrochemical pKa value (7.56 and 7.35, respectively) for the transition between a two-electron, two-proton transfer (at pH < pKa) and a two-electron, one-proton transfer (at pH > pKa). In contrast, the position of the quinone moiety with respect to the zirconium node, the effect of hydrogen bonding, and the amount of defects in 1,4-Zr-AQ-MOF lead to the transition from a two-electron, three-proton transfer to a two-electron, one-proton transfer. The pKa of this framework (5.18) is analogous to one of the three electrochemical pKa values displayed by its ligand (3.91, 5.46, and 8.80), which also showed intramolecular hydrogen bonding. The ability of the MOFs to tailor discrete numbers of protons and electrons suggests their application as charge carriers in electronic devices.

Chapter 9:Photocatalytic CO2 Conversion to Fuels by Novel Green Photocatalytic Materials

In this chapter the necessity of solar energy storage is explained first, followed by a calculation of the photocatalytic rates necessary for practical application of artificial photosynthesis systems to achieve effective solar energy storage. Next, we introduce TiO2 as an active CO2 and H2O conversion photocatalyst, including the current understanding of CO2 sorption modes, and mechanistic details of the conversion of H2O and CO2 to hydrocarbons. Subsequently, various modifications of TiO2 are described that enhance photocatalytic performance, with the focus on earth-abundant elements, including copper, iron, and nickel, in the form of metal or metal oxide nanoparticles and composites. Next, the creation of isolated Ti sites in mesoporous silica framework materials is discussed, followed by a description of the mechanism of CO2 conversion to hydrocarbons in these materials. Several attempts to create visible light-sensitive MMCT binuclear sites are also addressed. For effective CO2 reduction, water oxidation activity is essential. We will discuss some issues related to methodology to determine water oxidation efficacy, and discuss the mechanism of activity of IrOx and CoOx sites when present in mesoporous silica-based materials. Some attention is also paid to the prevention of the reverse reaction (hydrocarbons and oxygen to form CO2 and water), which would significantly decrease conversion efficiency. We conclude the overview of TiO2-based systems by discussing composites with graphene and metal organic framework materials. Finally, the currently achieved activity data are reflected upon, and implications of for avenues of future research activity are identified.