Artificial photosynthesis part 2: Scientific advances in the imitation of the oxygen-evolving complex

Imitating the function of plants, algae, and cyanobacteria which provide us with oxygen and carbohydrates through photosynthesis is an ongoing challenge in materials science. In this article, I will explain how researchers have addressed the difficult task of replicating the molecular structure of the natural oxygen-evolving organometallic complex (OEC).

The natural OEC is a biological catalyst that has a deformed cube-shaped molecular structure with four manganese atoms, five oxygens, and one calcium [1]. The two main difficulties in replicating it lie in the incorporation of calcium because it has a larger atomic radius (231 picometers) than manganese (161 pm) and oxygen (48 pm). The other difficult part is the formation of an unstable oxo-manganese bridge (O-Mn-O bond) outside the cube structure.

The active centers of performing the water oxidation reaction are attributed to the oxo bonds that form the manganese atoms. Calcium, on the other hand, has the function of stabilizing (modulating) and completing this catalytic reaction, as well as of adsorbing water molecules [2]. Besides, it is established in several publications that manganese presents oxidation states of III and IV, which facilitates the release of molecular oxygen [3].

Plants are known to contain small traces of elements such as manganese and iron. Even once we analyzed the chemical composition of a scorpion's stinger by energy dispersive X-ray spectroscopy (EDS) and found a high manganese percentage in it. This manganese is probably coming from the plants that consume scorpions, and it gives them the mechanical properties to its stinger.

In 2012, Dr. Nocera's research group employed material science to fabricate an artificial leaf with the ability to dissociate water molecules at neutral pH [4]. What they called artificial leaf was the combination of layers of semiconductor materials bonded to catalysts on a steel plate. A catalyst can transform one product into another; in this case it splits water into hydrogen and oxygen.

On one side of the steel plate is deposited a catalyst containing NiMoZn to produce hydrogen, which replaced the use of the commonly used metal (platinum). On the other side of the plate, a cobalt-based catalyst with a structural arrangement similar to that of the natural OEC. This synthetic Co-OEC consists of a cube with cobalt and oxygen atoms. In Figure 1, the presence of an M (moderator) element completes the cube, and it is probably made of alkaline metal ions, as in the case of cobaltates [4].

In my point of view, cobalt has been extensively studied for the water splitting because of to its high oxidation potential of 1.82 V. Understanding that the oxidation potential is the ability of a material to gain or remove electrons from a nearby compound. I would like to point out that taking into account the oxidation or reduction potentials of the elements helps us to design more efficient catalytic materials in a given reaction.

In an interview, Dr. Nocera demonstrates one of the potential applications of artificial leaves, video in youtube [5]. Using a lamp, a small solar cell and a mini fuel cell, you can turn on a fan. This seems expensive at first glance, but they are simple containers to hold liquids and gases that can be made from low-cost materials. In terms of operation, the solar cell captures energy from the lamp to transmit it to the artificial leaf through wires, which in turn dissociates water molecules. Subsequently, the hydrogen obtained is used to generate electricity in the fuel cell, which allows the fan to operate.

Another approach to obtain the structure of the natural OEC was to use the chemistry of organometallic compounds. Zhang and collaborators (2015) synthesized a molecule containing manganese and calcium very similar to the desired structure, see Figure 1. However, it was not possible to obtain one of the oxo bonds of manganese out of the cube. Its artificial OEC contains proteins anchored to the molecular structure that resembles the natural OEC and is the most similar so far [6].

Despite great scientific advances on this subject, it remains a challenge to obtain an identical structure to that of the natural OEC. We trust in the scientific community that has given these brushstrokes in the way of imitating nature to take the next step.

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@ritch

References:

[1] J. Yano, J. Kern, K. Sauer, M.J. Latimer, Y. Pushkar, J. Biesiadka, B. Loll, W. Saenger, J. Messinger, A. Zouni, V.K. Yachandra. Where Water Is Oxidized to Dioxygen: Structure of the Photosynthetic Mn4Ca Cluster. Science, 314:821–825, 2006.
[2] J.P. McEvoy, G.W. Brudvig. Water-Splitting Chemistry of Photosystem II. Chemical Reviews 106:4455–4483, 2006.
[3] J. Yano, V. Yachandra. Mn4Ca Cluster in Photosynthesis: Where and How Water is Oxidized to Dioxygen. Chemical Reviews 114:4175–4205, 2014.
[4] D.G. Nocera. The artificial leaf. Accounts of Chemical Research 45:767-776, 2012.
[5] Adam Shaw [BBC Studios]. (June 17, 2013). Artificial leaves replicate photosynthesis? [Video file]. Recovered from [youtube].
[6] C. Zhang, C. Chen, H. Dong, J.R. Shen, H.Dau, J. Zhao. A synthetic Mn4Ca-cluster mimicking the oxygen-evolving center of photosynthesis. Science 348:690-693, 2015.

You may be interested in reading the first part of this article:
Artificial photosynthesis part 1: Understanding the structure of the oxygen-evolving complex.