Chemical reactions for the energy transition | MIT News
A challenge in decarbonization energy system means knowing how to manage new types of fuel. Traditional fuels such as natural gas and petroleum can be combined with other materials and then heated to high temperatures so that they react chemically to produce other fuels or useful substances, or even energy to perform a work. But new materials such as biofuels cannot absorb so much heat without breaking down.
A key ingredient in such chemical reactions is a specially designed solid catalyst which is added to promote the reaction, but which is not itself consumed in the process. With traditional materials, the solid catalyst generally interacts with a gas; but with biomass-derived fuels, for example, the catalyst must operate with a liquid — a particular challenge for those who design catalysts.
For nearly a decade, Yogesh Surendranath, associate professor of chemistry at MIT, has focused on chemical reactions between solid catalysts and liquids, but in a different situation: rather than using heat to cause reactions , he and his team inject electricity from a battery or a renewable source such as the wind or the sun to give chemically inactive molecules more energy to react. And the key to their research is the design and manufacture of solid catalysts that work well for reactions involving liquids.
Recognizing the need to use biomass to develop sustainable liquid fuels, Surendranath wondered if he and his team could take the principles they learned about designing catalysts to drive liquid-solid reactions with electricity and apply them to reactions that occur at liquid-solid interfaces without any input of electricity.
To their surprise, they found that their knowledge was directly relevant. Why? “What we found – surprisingly – is that even when you don’t connect wires to your catalyst, there are tiny internal ‘wires’ that do the reaction,” Surendranath explains. “So the reactions that people generally think work without any current flow actually involve electrons shuttling from one place to another.” And that means Surendranath and his team can use the powerful techniques of electrochemistry to solve the problem of designing catalysts for sustainable fuels.
A new hypothesis
Their work focused on a class of chemical reactions important in the energy transition that involve the addition of oxygen to small organic (carbon-containing) molecules such as ethanol, methanol and formic acid. The conventional assumption is that the reactant and oxygen react chemically to form product plus water. And a solid catalyst – often a combination of metals – is present to provide sites at which the reactant and oxygen can interact.
But Surendranath offered a different take on what is happening. In the usual configuration, two catalysts, each composed of many nanoparticles, are mounted on a conductive carbon substrate and immersed in water. In this arrangement, negatively charged electrons can flow easily through carbon, while positively charged protons can flow easily through water.
Surendranath’s hypothesis was that the conversion of reactant to product progresses by means of two distinct “half-reactions” on the two catalysts. On a catalyst, the reactant turns into a product, in the process sending electrons into the carbon substrate and protons into the water. These electrons and protons are captured by the other catalyst, where they drive the conversion of oxygen into water. Thus, instead of a single reaction, two separate but coordinated half-reactions together achieve the net conversion of reactant to product.
As a result, the overall reaction actually involves no net production or consumption of electrons. This is a standard “thermal” reaction resulting from the energy contained in the molecules and perhaps a little extra heat. The conventional approach to designing a catalyst for such a reaction would focus on increasing the rate of conversion of this reactant to product. And the best catalyst for that kind of reaction might turn out to be, say, gold or palladium or some other expensive precious metal.
However, if this reaction actually involves two half-reactions, as Surendranath proposed, there is a flow of electric charge (electrons and protons) between them. So Surendranath and others in the field could instead use electrochemical techniques to design not a single catalyst for the overall reaction, but rather two separate catalysts – one to speed up a half-reaction and one to speed up the reaction. another half-reaction. “This means that we don’t need to design a single catalyst to do all the heavy lifting of accelerating the entire reaction,” Surendranath explains. “We might be able to pair two inexpensive, earth-abundant catalysts, each of which does half the reaction well, and together they do the overall transformation quickly and efficiently.”
But there is another consideration: electrons can flow through the entire composite catalyst, which encompasses the catalyst particle(s) and the carbon substrate. For the chemical conversion to occur as quickly as possible, the rate at which electrons are introduced into the catalyst composite must exactly match the rate at which they are removed. Focusing only on electrons, if the conversion of the reaction to product on the first catalyst sends the same number of electrons per second into the “electron bath” of the composite catalyst as the conversion of oxygen to water on the second catalyst takes out, the two half-reactions will be balanced, and the flow of electrons – and the rate of the combined reaction – will be rapid. The trick is to find good catalysts for each of the half-reactions that match perfectly in terms of incoming and outgoing electrons.
“A good catalyst or pair of catalysts can maintain an electrical potential – essentially a voltage – at which the two half-reactions are fast and balanced,” says Jaeyune Ryu PhD ’21, a former member of the Surendranath lab and lead author of the study; Ryu is now a postdoctoral fellow at Harvard University. “The rates of the reactions are equal and the voltage in the catalyst composite will not change during the overall thermal reaction.”
Rely on electrochemistry
Based on their new understanding, Surendranath, Ryu and their colleagues turned to electrochemical techniques to identify a good catalyst for each half-reaction that would also team up to work well together. Their analytical framework to guide the development of catalysts for systems that combine two half-reactions is based on a theory that has been used to understand corrosion for nearly 100 years, but has rarely been applied to understand or design catalysts for reactions involving small molecules important for the energy transition.
Key to their work is a potentiostat, a type of voltmeter that can either passively measure the voltage of a system or actively change the voltage to cause a feedback. In their experiments, Surendranath and his team use the potentiostat to measure catalyst voltage in real time, monitoring its change from millisecond to millisecond. They then correlate these voltage measurements with simultaneous but separate measurements of the overall rate of catalysis to understand the reaction pathway.
For their study of the conversion of energy-bound small molecules, they first tested a series of catalysts to find good ones for each half-reaction – one to convert reactant to product, producing electrons and protons , and another to convert oxygen. to water, consuming electrons and protons. In each case, a promising candidate would produce a fast reaction, that is, a rapid flow of electrons and protons inwards or outwards.
To help identify an efficient catalyst to perform the first half-reaction, the researchers used their potentiostat to input carefully controlled voltages and measured the resulting current flowing through the catalyst. A good catalyst will generate a lot of current for little voltage applied; a bad catalyst will require a high applied voltage to get the same amount of current. The team then followed the same procedure to identify a good catalyst for the second half-reaction.
To speed up the overall reaction, the researchers had to find two catalysts that matched well – where the amount of current at a given applied voltage was high for each of them, ensuring that one produced a fast flow of electrons and protons, the other consumed them at the same rate.
To test promising pairs, the researchers used the potentiostat to measure the voltage of the composite catalyst during net catalysis – without changing the voltage as before, but now simply measuring it from tiny samples. In each test, the voltage will naturally stabilize at a certain level, and the goal is for this to happen when the rate of both reactions is high.
Validate your hypothesis and look to the future
By testing both half-reactions, the researchers were able to measure how the reaction rate of each varied with changes in applied voltage. From these measurements, they could predict the voltage at which the full reaction would proceed most rapidly. The measurements of the complete response matched their predictions, supporting their hypothesis.
The team’s new approach of using electrochemical techniques to examine reactions thought to be strictly thermal in nature provides new insights into the detailed steps by which these reactions occur and thus how to design catalysts for them. accelerate. “Now we can use a divide and conquer strategy,” Ryu says. “We know that the net thermal reaction in our study occurs through two ‘hidden’ but coupled half-reactions, so we can aim to optimize one half-reaction at a time” – possibly using low-cost catalyst materials to one or both.
Surendranath adds: “One of the things that excites us about this study is that the result is not definitive in itself. This has really launched a whole new area of action in our research program, including new ways to engineer catalysts for the production and processing of renewable fuels and chemicals.
This research was funded primarily by the Air Force Office of Scientific Research. Jaeyune Ryu PhD ’21 was supported by a Samsung Fellowship. Additional support was provided by a Senior Research Fellowship from the National Science Foundation.
This article originally appeared in the Fall 2021 issue of Energy Futuresthe magazine of the MIT Energy Initiative.