A less-expensive way to duplicate the complicated steps of photosynthesis in making fuel

January 23, 2014

Currently, the most efficient methods we have for making fuel from sunlight and water involve rare and expensive metal catalysts, such as platinum (credit: Argonne National Laboratory)

Argonne National Laboratory researchers have found a new, more efficient, less-expensive way to make fuel — principally, hydrogen — from sunlight and water: linking a synthetic cobalt-containing catalyst to an organic light-sensitive molecule called a chromophore.

Chromophore molecules, such as chlorophyll, are involved in capturing light for photosynthesis.

Currently, the most efficient methods we have for  making fuel involve rare and expensive metal catalysts, such as platinum. Although cobalt is significantly less efficient than platinum when it comes to light-induced hydrogen generation, the drastic price difference between the two metals makes cobalt the obvious choice as the foundation for a synthetic catalyst, said Argonne chemist Karen Mulfort.

The Argonne study wasn’t the first to look at cobalt as a potential catalytic material; however, a paper by the researchers in Physical Chemistry Chemical Physic identified a new mechanism to link the chromophore with the catalyst.

Previous experiments with cobalt attempted to connect the chromophore directly with the cobalt atom within the larger compound, but this eventually caused the hydrogen generation process to break down. Instead, the Argonne researchers connected the chromophore to part of a larger organic ring that surrounded the cobalt atom, which allowed the reaction to continue significantly longer.

Future studies in this arena could involve nickel- and iron-based catalysts — metals that are even more naturally abundant than cobalt, although they are not quite as effective natural catalysts.

The research was supported by DOE’s Office of Science.

Abstract of Physical Chemistry Chemical Physics paper

We have designed two new supramolecular assemblies based on Co(II)-templated coordination of Ru(bpy)32+ (bpy = 2,2′-bipyridyl) analogues as photosensitizers and electron donors to a cobaloxime macrocycle, which are of interest as proton reduction catalysts. The self-assembled photocatalyst precursors were structurally characterized by Co K-edge X-ray absorption spectroscopy and solution-phase X-ray scattering. Visible light excitation of one of the assemblies has yielded instantaneous electron transfer and charge separation to form a transient Co(I) state which persists for 26 ps. The development of a linked photosensitizer–cobaloxime architecture supporting efficient Co(I) charge transfer is significant since it is mechanistically critical as the first photo-induced electron transfer step for hydrogen production, and has not been detected in previous photosensitizer–cobaloxime linked dyad assemblies. X-band EPR spectroscopy has revealed that the Co(II) centres of both assemblies are high spin, in contrast to most previously described cobaloximes, and likely plays an important role in facilitating photoinduced charge separation. Based on the results obtained from ultrafast and nanosecond transient absorption optical spectroscopies, we propose that charge recombination occurs through multiple ligand states present within the photosensitizer modules. The studies presented here will enhance our understanding of supramolecular photocatalyst assembly and direct new designs for artificial photosynthesis.