Photocatalysis in Seawater

H2O2 production from a sustainable catalyst using light and seawater

If there is one thing on earth that we have enough of, it is seawater. (And unfortunately, we have a lot more of it each and every day). In fact, 97% of the water on earth is saltwater, 3 % freshwater (broken down as 2% glaciers and ice, less than 1% from groundwater, lakes, and streams). For our purposes, water is a green solvent, but it’s also expensive to purify and we can usually find better uses for potable water than running photocatalysis.

So, seawater. There’s a lot of it. Also, it’s kind of messy. Disregarding whatever microorganisms, biomass, organic and inorganic pollutants (and microplastics ), might be present, saltwater also consists of ~3-5% of salts. Ions that include Na+ Ca²+, K+, Mg²+, Cl- and SO4²- but also many other trace metals. The sort of mixture that most people aren’t too excited about throwing into their favorite reaction just for fun. But in a recent paper by the Shoubnik Das (Antwerp) and Adam Slabom (Stockholm) groups in JACS entitled, “Lignin-Supported Heterogeneous Photocatalyst for the Direct Generation of H2O2 from Seawater”, they did just that (Ref 1). Well not just for fun, they had their reasons.

The authors are looking to make H2O2 from water, specifically seawater with visible-light photocatalysts. The benefit and needs are simple, as H2O2 is a high-energy oxidant that is used everywhere in chemical synthesis, industrial applications, and mining with potential as a green fuel source. While H2O2 is a green oxidant, its synthesis is often not. H2O2 synthesis uses a multistep process involving hydrogenation and oxidation of an alkylanthraquinone, organic solvents and liquid-liquid extraction generating a significant amount of waste and wastewater. As such, a modern approach involves generating H2O2 from water. To make H2O2 with photochemistry is not a new idea. (In fact, the authors cite ~30 papers using catalysts of every shape, size, and color of homogeneous, heterogenous, organic and inorganic catalysts, materials and macromolecules capable of generating H2O2. However, the authors claim most use either pure water, or water/alcohol systems and are deactivated by salts (we’ll take their word for it.) An alternative approach involves electrocatalysis.

For this work, the author up the sustainability angle of their work by preparing their photocatalysts on lignin frameworks (another raw material of which we have plenty). Lignin is actually a great support for heterogenous catalysts due to its high carbon content and stability. It isn’t innocent either, as it can enhance photocatalysts by adjusting band gaps and eliminating recombination of holes. If you are going to the trouble of using seawater, it wouldn’t make sense to have an expensive rare metal photocatalyst. For the catalyst, the authors selected BiOBr nanostructures which are hydrothermally grown under alkaline conditions on hydrolysis lignin, stable in seawater and can be recycled more than five times (a material they abbreviate as LBOB).

So, with the goal of generating a sustainable heterogenous photocatalyst that can generate H2O2 using cheap (sustainable) starting materials in seawater, the authors set out to characterize their catalyst. First, they determined that LBOB, has a band gap in the visible region of 2.9 eV -> 427 nm and a conduction band at 0.03 vs RHE suggesting suitability for direct 2e- oxygen reduction and indirect 1e- pathways. In addition, the authors looked at the BiOBr system on lignin with chitosan (CBOB) and graphene (GBOB) for comparison.

Pathways for generation of H2O2:

Direct Oxygen Reduction Reaction:
O2 + 2 e- +2 H+ -> H2O2
Indirect Oxygen Reduction Reaction:
O2 + e- -> O2
O2-· + e- +2 H+ -> H2O2

Then the authors started “dirtying up” their system. First adding 0.6 M NaCl to pure water, with LBOB outperforming the others. With LBOB, an increase in H2O2 continued after 6 hours (2100 µM H2O2). Then a series of experiments involved adding individual salts such as CaCl2, MgCl2, KCl, or Na2SO4(with an increase in conversion with KCl) and sacrificial acids, bases, and alcohols (formic acid the highest conversion). Finally, further experiments were performed in seawater with LBOB generating 4000 µM H2O2 after 48 hours. The catalyst was highly stable in seawater, with similar conversions after 5 cycles of recovery. Remarkably, the highest conversions were observed for the system in open air (4085 µM) in 6 hours compared to a closed system containing O2 (1710 µM) suggesting efficient absorption/desorption of O2 on the catalysts.

More experiments in seawater with acid, base, and various metals follow with characterization of the catalyst structures and reaction mechanism with various spectroscopic techniques (lots of IR, Raman, XPS and XFAS, TEM and HR-TEM, WTFs, and PXRDS. The main take away being that the LBOB catalyst is highly stable, efficient, and recyclable in saltwater.

Is this a lot of H2O2? How does this compare to every other H2O2 production method out there? We’re sure there is a great review out there that can explain all that. For us, we are drawn to the idea of technology using reagents (like seawater) that are essentially free to generate important raw materials. Stability and transport of concentrated H2O2 is problematic. Can we think of a few ideas where generating H2O2 in situ in remote areas would be beneficial? Water treatment for one and mining for another. As shipping becomes more fraught, we might expect an increase in importance in on site generation of important feed stocks.

References:

(1) Gopakumar, A.; Ren, P.; Chen, J.; Manzolli Rodrigues, B. V.; Vincent Ching, H. Y.; Jaworski, A.; Doorslaer, S. Van; Rokicińska, A.; Kuśtrowski, P.; Barcaro, G.; Monti, S.; Slabon, A.; Das, S. Lignin-Supported Heterogeneous Photocatalyst for the Direct Generation of H2O2 from Seawater. J. Am. Chem. Soc. 2022, 144 (6), 2603–2613. https://doi.org/10.1021/jacs.1c10786.

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