Concevoir des catalyseurs pour des carburants durables à l’aide de l’électrochimie

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Concept de catalyseur chimique

Yogesh Surendranath et son équipe apportent de puissantes techniques d’électrochimie pour résoudre le problème de la conception de catalyseurs pour des carburants durables.

L’un des défis de la décarbonation du système énergétique est de savoir comment gérer les nouveaux types de carburants. Les combustibles traditionnels tels que le gaz naturel et le pétrole peuvent être combinés avec d’autres matériaux, puis chauffés à des températures élevées afin qu’ils réagissent chimiquement pour produire d’autres combustibles ou substances utiles, voire de l’énergie pour effectuer un travail. Mais les nouveaux matériaux tels que les biocarburants ne peuvent pas absorber autant de chaleur sans se décomposer.

Un ingrédient clé de ces réactions chimiques est un catalyseur solide spécialement conçu qui est ajouté pour favoriser la réaction, mais qui n’est pas lui-même consommé dans le processus. Avec les matériaux traditionnels, le catalyseur solide interagit généralement avec un gaz ; mais avec des carburants dérivés de la biomasse, par exemple, le catalyseur doit fonctionner avec un liquide – un défi particulier pour ceux qui conçoivent des catalyseurs.

Pendant près d’une décennie, Yogesh Surendranath, professeur agrégé de chimie à[{” attribute=””>MIT, has been focusing on chemical reactions between solid catalysts and liquids, but in a different situation: rather than using heat to drive reactions, he and his team input electricity from a battery or a renewable source such as wind or solar to give chemically inactive molecules more energy so they react. And key to their research is designing and fabricating solid catalysts that work well for reactions involving liquids.

Recognizing the need to use biomass to develop sustainable liquid fuels, Surendranath wondered whether he and his team could take the principles they have 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 is directly relevant. Why? “What we found — amazingly — is that even when you don’t hook up wires to your catalyst, there are tiny internal ‘wires’ that do the reaction,” says Surendranath. “So, reactions that people generally think operate without any flow of current actually do involve electrons shuttling from one place to another.” And that means that Surendranath and his team can bring the powerful techniques of electrochemistry to bear on the problem of designing catalysts for sustainable fuels.

Catalyst Reaction Electrochemistry

This figure presents two views of the chemical reactions for producing renewable fuels and chemicals. The top equation represents the conversion of the reactant (R) plus oxygen (O2) to a product (P) plus water (H2O). The diagram below illustrates researchers’ hypothesis that the overall reaction is the result of two coordinated half-reactions occurring on separate catalyst materials, here represented by gray structures. On the left-hand catalyst, the reactant turns into a product, sending electrons (e-) into the carbon support material (black) and protons (H+) into water (blue). On the right-hand catalyst, electrons and protons are consumed as they drive the reaction of oxygen to water.
Credit: Image courtesy of the researchers.

A novel hypothesis

Their work has focused on a class of chemical reactions important in the energy transition that involve adding oxygen to small organic (carbon-containing) molecules such as ethanol, methanol, and formic Voltage Catalyst Composite

In this diagram, the two “hidden” half-reactions responsible for the observed catalysis are depicted on opposite sides of a box in which the voltage level of the catalyst composite (the catalysts plus the carbon substrate) is indicated as pink. The conversion of reactant to product is on the left, and the conversion of oxygen to water is on the right. With a well-matched pair of catalysts, the reaction at the left will release electrons at the same rate as the reaction at the right picks them up, and the voltage will be constant. The goal is for that matching to occur when both reaction rates are high.
Credit: Image courtesy of the researchers.

Drawing on electrochemistry

Based on their new understanding, Surendranath, Ryu, and their colleagues turned to electrochemistry techniques to identify a good catalyst for each half-reaction that would also pair up to work well together. Their analytical framework for guiding catalyst development for systems that combine two half-reactions is based on a theory that has been used to understand corrosion for almost 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 reaction to occur. In their experiments, Surendranath and his team use the potentiostat to measure the voltage of the catalyst in real time, monitoring how it changes millisecond to millisecond. They then correlate those 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 small, energy-related molecules, they first tested a series of catalysts to find good ones for each half-reaction — one to convert the reactant to product, producing electrons and protons, and another to convert the oxygen to water, consuming electrons and protons. In each case, a promising candidate would yield a rapid reaction — that is, a fast flow of electrons and protons out or in.

To help identify an effective catalyst for performing the first half-reaction, the researchers used their potentiostat to input carefully controlled voltages and measured the resulting current that flowed through the catalyst. A good catalyst will generate lots of current for little applied voltage; a poor catalyst will require 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 expedite the overall reaction, the researchers needed 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 as one produced a rapid flow of electrons and protons, the other one consumed them at the same rate.

To test promising pairs, the researchers used the potentiostat to measure the voltage of the catalyst composite during net catalysis — not changing the voltage as before, but now just measuring it from tiny samples. In each test, the voltage will naturally settle at a certain level, and the goal is for that to happen when the rate of both reactions is high.

Validating their hypothesis and looking ahead

By testing the two half-reactions, the researchers could measure how the reaction rate for each one varied with changes in the applied voltage. From those measurements, they could predict the voltage at which the full reaction would proceed fastest. Measurements of the full reaction matched their predictions, supporting their hypothesis.

The team’s novel approach of using electrochemistry techniques to examine reactions thought to be strictly thermal in nature provides new insights into the detailed steps by which those reactions occur and therefore into how to design catalysts to speed them up. “We can now use a divide-and-conquer strategy,” says Ryu. “We know that the net thermal reaction in our study happens 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 for one or both.

Adds Surendranath, “One of the things that we’re excited about in this study is that the result is not final in and of itself. It has really seeded a brand-new thrust area in our research program, including new ways to design catalysts for the production and transformation of renewable fuels and chemicals.”

This research was supported primarily by the Air Force Office of Scientific Research. Jaeyune Ryu PhD ’21 was supported by a Samsung Scholarship. Additional support was provided by a National Science Foundation Graduate Research Fellowship.

This article appears in the Autumn 2021 issue of Energy Futures, the magazine of the MIT Energy Initiative.