Scientists at the Helmholtz Center for Materials and Energy (HZB) in collaboration with the School of Chemistry and ARC Centre of Excellence for Electromaterials Science at Monash University, Australia, have precisely characterized the electronic states of a manganese (Mn) water-splitting catalyst for artificial photosynthesis.
The team led by Professor Emad Aziz, head of the HZB Institute “Methods for Material Development“ and Professor Leone Spiccia from Monash University investigated the changes in the local electronic structure of the Mn 3d orbitals of a Mn catalyst derived from a dinuclear MnIII complex during the water oxidation cycle using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) analyses.
At HBZ, Emad Aziz is doing research on artificial water-splitting catalysts with the goal of getting them to perform at the level of the oxygen evolution center of natural photosynthesis. Top catalyst candidates are manganese complexes embedded in a Nafion matrix, a teflon-like polymer.
Leone Spiccia’s lab developed and provided the samples. The manganese complexes produce nanoparticles of manganese oxides within the Nafion matrix. When exposed to light and biased simultaneously, these oxides promote water oxidation, a key reaction associated with the splitting water into oxygen and hydrogen. The hydrogen can be stored as an energy carrier, Spiccia explained.
The next step, said Munirah Khan of the Freie Universität Berlin, the scientist in charge of the experiments, was to figure out which of the potential manganese complexes in nafion yields the best manganese oxides. Khan studied the formation of manganese oxides and their catalytic effect using X-ray light at BESSY II, the HZB’s synchrotron radiation source. In her doctorate research work, Khan used the RIXS method, which allowed her to select and further investigate the manganese species involved in catalytic processes with high precision.
Of the various manganese complexes, one in particular—designated Mn(III)—turned out to be the one that most efficiently formed manganese oxides. Among the team’s findings, reported in a new paper in the journal ChemSusChem, was that enhanced catalytic activity (water oxidation) originated from the narrowing of the local HOMO–LUMO gap when electrical voltage and visible light illumination were applied simultaneously to the Mn catalytic system.
(HOMO is the highest occupied molecular orbital; LUMO, the lowest un-occupied molecular orbital. The HOMO is the highest energy MO that has any electrons in it. The LUMO is the next highest energy orbital (it will be empty). The LUMO is the lowest energy place to put or excite an electron. The energy difference between the HOMO and LUMO—the HOMO-LUMO gap—is generally the lowest energy electronic excitation that is possible in a molecule. The energy of the HOMO-LUMO gap can indicate what wavelengths the compound can absorb.)
We are developing our methods to construct multi-dimension catalytic pathways for such novel materials in the energy and time scales. Our goal is to provide synthetic chemists with a full picture of the catalytic process under real test conditions in order to enhance their work on the function of these materials, and figure out if and under what conditions it might be used for technological application in converting light to chemical energy. If we succeed, it could mean we’re well on our way towards a continuous, environmentally-friendly, and cost-effective storage form of solar energy.—Emad Aziz
Jie Xiao, Munirah Khan, Archana Singh, Edlira Suljoti, Leone Spiccia, and Emad F. Aziz (2015) “Enhancing Catalytic Activity by Narrowing Local Energy Gaps—X-Ray Studies of a Manganese Water Oxidation Catalyst” ChemSusChem doi: 10.1002/cssc.201403219
Munirah Khan, Edlira Suljoti, Archana Singh, Shannon A. Bonke, Tim Brandenburg, Kaan Atak, Ronny Golnak, Leone Spiccia and Emad F. Aziz (2014) “Electronic structural insights into efficient MnOx catalysts” J. Mater. Chem. A, 2, 18199-18203 doi: 10.1039/C4TA04185B