A Green New World of Photocatalysis
Most chemical reactions involving the breaking and making of carbon–carbon (C–C) or carbon–nitrogen, carbon–oxygen (C–N, C–O) bonds cost a lot of energy. The introduction of a catalyst lowers the energy required to complete a catalytic cycle and speed up the rate of reaction. The catalyst, typically an organic molecule or thin film with a central or bridging transition metal (organometallic catalyst), interacts with a molecular reactant to gain or lose electron(s), becoming reduced or oxidized, respectively). The reduced or oxidized reactant may react with another molecule to give a final product that disassociates from the catalyst, returning the catalyst to its original form so it can drive the same reaction again and again. Catalysts that break and make molecular bonds are central to industrial N2 fixation, water splitting, CO2 reduction, polymer synthesis, and depolymerization.
Compared to traditional catalysis, photocatalysis involves a catalyst bound to a molecule that absorbs light, allowing the catalytic system to access new reaction pathways and lower the reaction’s overall energy requirement.1 Photocatalysts are widely popular in industrial settings, particularly in fields of medicinal and pharmaceutical chemistry. The production of challenging molecular products and novel drug discovery is made possible by photocatalysts that allow otherwise inaccessible C–C and C–N cross coupling reactions to occur.2
Ideal photocatalytic systems efficiently absorb light over a broad spectrum—from low energy microwaves that heat your food to higher energy ultraviolet (UV) light that can cause sun burns—and transport that energy to the catalyst so it can drive a reaction. Economically and environmentally speaking, ideal photocatalysts are cheap, scalable, tunable, and contain metals that require less energy to mine or dispose of than the catalytic system saves by lowering energetic barriers to drive chemical reactions.
Ruthenium (Ru)-, iridium (Ir)-, palladium (Pd)- and platinum (Pt)-based complexes are the current photocatalytic standards for their overall efficiency in industrial settings. Their efficiency and absence of policy that directs industry to adopt more environmentally friendly alternatives overshadows other transition metal-based photocatalysts that are cheaper, have a smaller burden on the planet, and could be as efficient driving the same reactions.
Take Pd and Pt, for example. These metals are often found in catalysts that refine petroleum and are used in catalytic converters that convert hydrocarbons, carbon monoxide, and nitrogen oxide (greenhouse gases) from vehicle exhaust to water, carbon dioxide, and nitrogen.3 Nickel (Ni) is not only a low-cost replacement to Pd and Pt, but also has more diverse uses than these heavy hitters because of its facile oxidative addition and ready access to multiple oxidation states.4
Pt, Pd, and Ni are all mined and extracted from ore – a natural rock that contains valuable minerals. According to a 2015 report from Argonne National Lab, it takes about 426 kJ of energy to process one ton of Pd or Pt. Compare that to Ni, which only requires 91 kJ of energy to process the same amount.5 The same report also states that in 2014, a total of 97.4 tons of Pd and Pt were mined, which is significantly less than ~32,000 tons of Ni mined the same year. Besides increased energy costs to mine less material, Pd and Pt respectively cost about 2,000 and 10,000 times more than Ni on a mole-for-mole basis.4 Once placed at the centers of organometallic catalysts, Ni-based catalytic reactions more often go through a radical intermediate (the in-between species of a catalytic cycle), whereas Pd and Pt catalysts do not.
The energetic, environmental, availability, and financial benefits of Ni over current catalytic “standards” make Ni photocatalysts highly desirable. In order to optimize Ni photocatalysts and present them as an earth friendly alternative to current standards, we must close the knowledge gap between the two catalyst classes and better understand the fundamental chemistry of the Ni-based catalytic cycle. There’s one glaring hurdle to overcome with studies that attempt to close this gap, however. Catalytic intermediates are extremely short-lived...lasting only one billionth of a second on average. This makes it difficult to understand the mechanism by which Ni catalysts efficiently drive chemical reactions.
Researchers within the Bioinspired Light-Escalated Chemistry (BioLEC) EFRC used pulse radiolysis, a relatively new analytical method in this field of study, to observe short-lived Ni(I)-based catalytic intermediates.2 This process occurs in a series of steps as outlined in Figure 1: (1) Ni(II) catalysts are dissolved in solvent while a rapid pulse of electrons is delivered to a submerged cathode to reduce, or give negative charge to, the Ni(II) species in the solvent. (2) All the Ni(II) that that come into contact with the cathode during the short pulse are reduced Ni(0). (3) The small amount of Ni(0) species produced interacts with the abundance of Ni(II) still in solution to produce two Ni(I) intermediate species.
When the solvent is observed over time with an appropriate light source and detector, the researchers are able to visualize the generated catalytic intermediates and their evolution over time. Tracking the catalytic intermediate population over time enables clear kinetic analysis to a reaction that has eluded scientists for years.
What this means for the future.
This study—in its significant kinetic analysis and novel methodology in observing Ni catalytic intermediates—will advance development of efficient Ni photocatalysts that could provide cost-effective and environmentally friendly alternatives to Pd and Pt photocatalysts. Our increased understanding of Ni(II) intermediates could shift our attention away from standard Pd/Pt photocatalysts and toward Ni-based systems, offering a much broader and complementary set of catalytic reactions capable of C–C, C–O, and C–N bond transformations. The growing impacts of climate change and great demand for photocatalysts from academic- to industrial-level settings require that we optimize Ni-based catalysts to reduce our burden on the planet down to the molecular level.
1. Sayre, H. J.; Tian, L.; Son, M.; Hart, S. M.; Liu, X.; Arias-Rotondo, D. M.; Rand, B. P.; Schlau-Cohen, G. S.; Scholes, G. D., Solar Fuels and Feedstocks: The Quest for Renewable Black Gold. Energy Environ. Sci. 2021, 14, 1402-1419. https://doi.org/10.1039/D0EE03300F
2. Till, N. A.; Oh, S.; MacMillan, D. W. C.; Bird, M. J., The Application of Pulse Radiolysis to the Study of Ni(I) Intermediates in Ni-Catalyzed Cross-Coupling Reactions. J. Am. Chem. Soc. 2021, 143 (25), 9332-9337. https://doi.org/10.1021/jacs.1c04652
4. Tasker, S. Z.; Standley, E. A.; Jamison, T. F., Recent Advances in Homogenous Nickel Catalysts. Nature 2014, 509, 299-309. https://doi.org/10.1038/nature13274
3. Britannica, Palladium. Encyclopedia Britannica. 2021.
5. Benavides, P. T.; Cronauer, D. C.; Adom, F.; Wang, Z.; Dunn, J. B., The Influence of Catalysts on Biofuel Life Cycle Analysis (LCA). Argonne National Laboratory 2015. https://doi.org/10.1016/j.susmat.2017.01.002
Sayre et al.: Prof. Robert Knowles and Dr. Victoria Cleave for editing assistance, Drs. Braddock Sandoval and Bryan Kudisch for editing the Enzyme Catalysis section, and Ian Perry for TBADT structure. This work was supported as part of. BioLEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0019370.
Tasker et al.: This work was supported by the NIGMS and the NSF.
Till et al.: BioLEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0019370. Use of the Laser Electron Accelerator Facility (LEAF) of the BNL Accelerator Center for Energy Research (ACER).