Many of us are taught in secondary school about catalysts: “substances that speed up a chemical reaction without being consumed.” In the world around us, catalysis is present in many forms, from heterogeneous catalysts used in the processing of fossil fuels, to electrocatalysts in hydrogen fuel cells, to the enzymes in our own bodies. Despite the fact that all of these processes operate on similar principles, it can sometimes feel like the different disciplines within the umbrella of “catalysis” can be siloed from one another—with research communities even speaking in different dialects. Recent research by the Center for Molecular Electrocatalysis (CME) has bridged between these various genres of catalysis, providing perspective on just how universal some conclusions about the field might be.
The CME, led by Pacific Northwest National Laboratory, was formed in 2009 with the goal of controlling chemical transformations for energy applications. Populated mostly by chemists and chemical engineers, the primary focus of this EFRC is to study the interconversion of electrical energy and the energy stored in chemical bonds. These electron-driven chemical transformations, when achieved with catalysts, are referred to by the titular term “electrocatalysis.” One example of an electrocatalytic system is a hydrogen fuel cell, which now powers some cars with electricity generated from the oxidation of hydrogen rather than gasoline or diesel. Unlike a battery, a fuel cell is fed with streams of fuel and oxidant in order to release energy, but it can sustain that process as long as those feeds are supplied, making a fuel cell similar to a combustion engine. However, unlike a combustion engine, the reactions can take place without a rapid and sudden release of energy; in fact, they occur in two parts, known as “half-reactions,” that are physically separated from one another. In a fuel cell, hydrogen gas (the fuel) and oxygen gas from the air (the oxidant) are fed into a system loaded with catalysts, which are often powders of noble metals such as platinum supported on a sheet of carbon. Hydrogen is split into protons and electrons at the anode; the electrons in this process can flow through an external circuit to do work, while the protons move across the cell—often through a semipermeable membrane—and are combined with oxygen gas and electrons at the cathode to produce water. A single anode–cathode pair, a “cell” in this process, produces a voltage of a little more than 1 V; however, many of these cells can be connected in series to form a “stack” to increase the supplied voltage, which is usually in the 300–800 V range for today’s electric motors.
The CME has historically focused on reactions involving H, O, and N bonds, with a particular emphasis on the study of proton (H+) transfers. Proton transfers are extremely common in electrocatalytic reactions. They are also quite common in enzymatic/biological catalysis. Biological catalysts are highly selective, offering researchers inspiration for synthetic catalyst design as well as the possibility of direct integration into biological fuel cells. One of the ways in which the CME has bridged the electrocatalysis and biocatalysis communities—beyond collaboration with other EFRCs working in enzyme catalysis research directly1—is through the study of how electrochemically induced proton and electron transfers could lead to the long-range conduction of protons, or “shuttling.”2 CME researchers provided theoretical support for experimental researchers who designed “proton wires,” which are long, thin molecules that contain 1–4 nitrogen– nitrogen or nitrogen–oxygen sites where protons could sit, stabilized by both covalent and hydrogen bonding. By applying an electrical potential, the experimentalists either removed or added an electron to the molecule. This induced a chain reaction of physical shifts in the atoms to which the protons were connected. They found that as the molecular chain grew in length, increasing the number of proton transfers occurring within the molecule for a given electron transfer, that electron transfer also occurred at a less oxidative potential, in a way that was predictable. That is to say, the electron transfer became easier and less energy intensive. Because similar mechanisms are employed by enzymes in biocatalysis, this effort illustrates how different fields of catalysis can feed back into one another and lead to insights into chemical mechanisms.
CME researchers have also built a bridge between homogeneous and heterogeneous catalysts, which facilitate reactions in a single phase and at a solid-reactant phase interface, respectively. One illustration of this is the CME’s investigation of catalysts for the oxygen reduction reaction, the cathodic reaction in most fuel cells. In the heterogeneous electrocatalysis literature, a type of catalyst known as an M-N-C (metal-nitrogen-carbon) material has performed well for this reaction, offering a cheap alternative to platinum—the industry standard—as an electrode material. While this material performs well, it has been difficult for researchers to understand why, or to attempt to rationally improve the catalyst. Various groups in the CME have turned to molecular catalysts such as metal porphyrins, which act as very small-scale and well-defined analogs of these bulk M-N-C materials, to answer questions about the mechanism of the oxygen reduction reaction on M-N-C’s.3–10 Through catalyst design, the researchers have been able to alter the reaction mechanism at certain molecular catalysts known as porphyrins, causing the reaction rate to scale more favorably with applied electrical potential, granting insight into the likely identity of active sites on the M-N-C’s that account for the greatest reactivity. More recently, these insights were used to design a new type of molecular catalyst—with ligands referred to as phen2N2—that, when characterized electrochemically and spectroscopically, behaves more similarly to M-N-C catalyst than any other molecular catalyst to date, giving researchers a much better molecular picture of how M-N-C’s work.10 In addition to this work, other CME researchers are interested in studying colloidal nanoparticles, a form of catalyst that is intermediate between small molecular catalysts and large catalytic surfaces.11
A critical component of achieving a fundamental understanding of these chemical reactions hinges on thermodynamics. In 2010, CME researchers published a review on how to use thermochemical data—data on the energetics of chemical processes—to map reaction pathways for proton and electron transfers.12,13 Depending on the reactant molecule and the solvent it is in, the relative easiness of reaction steps such as removing a proton or removing an electron might be different. Electrical potential is the variable that quantifies the difficulty of these electrochemical reaction steps: for a reductive (electron-gaining) reaction, operating at more reductive potential requires a greater energy input. The opposite is true for oxidative (electron-losing) reactions, which require greater energy input at less reductive—that is, more oxidative—potentials. The CME has broadly contributed to the scientific understanding of how to define electrical potentials in different solvent systems, especially systems other than water, for many energy-relevant transformations, including the production and breakdown of hydrogen in acetonitrile,14 the reduction of oxygen and carbon dioxide in acetonitrile and DMF,15 and hydrogen atom abstraction in any arbitrary solvent.16
Finally, CME researchers are now pursuing connections between electrocatalysis and heterogeneous thermal catalysis – the type of catalysis used in industrial processes such as oil refining and the production of fertilizer. In addition to designing novel systems that integrate electrocatalysis with thermal catalysis,17 recent work measuring the electrical potential of thermal catalysts during operation has shown that even for catalysis that does not rely on the explicit transfer of electrons, this electrical potential can be a strong indicator of thermal reaction rates.18 The researchers measured the rate of a liquid-phase catalytic reaction using platinum catalysts suspended in solution but disconnected from any electrodes. By measuring the electrical potential of a platinum wire immersed in the same solution, the researchers discovered that across a wide range of solvents and reaction conditions, this potential was the variable most correlated to reaction rate. This suggests that, even if practitioners of thermal catalysis are unaware of electric fields produced at their catalysts, these fields do exist, and they go hand-in-hand with outcomes of thermal catalytic experiments. It is possible that as a result, researchers studying liquid-phase catalysis might not even need to monitor their reaction of interest; perhaps all they need to do in order to screen catalytic conditions is measure electrical potentials in their solution.
As it turns out, the lines between the worlds of enzyme catalysis, heterogeneous catalysis, homogeneous catalysis, thermal catalysis, and electrocatalysis are not so distinct. As research from the CME shows, by building more bridges between our disciplines, we just might realize the potential for great discoveries.
