A group of UWM chemists has pioneered a new method to measure the energetics of chemical reactions. Their work was published in May in ChemCatChem, the European Society Journal for Catalysis. The publication is a top-level chemistry catalysis journal in Europe, and their article was featured as its cover.
This new method, pioneered by chemistry PhD student Robert Bavisotto, UWM laboratory manager Quintus Owen, chemistry PhD student Nicholas Hopper, and distinguished professor of chemistry Wilfred Tysoe, has the potential to change how researchers measure kinetic processes – possibly leading to breakthroughs in material science, battery production, fuel efficiency, computing, and more.
How does it work, and why is this method so groundbreaking? Here are the four things you need to know.
1. These are the basics.
Bavisotto, the lead author of the paper, is a physical chemist. He works at the intersection of physics and chemistry, studying the interactions between energy and matter. Specifically, he is interested in ultra-high vacuum surface science. Any chemical reaction that a scientist investigates could be altered by even small amounts of contamination, which is why ultrahigh vacuum science is important to study fundamental chemistry. Ultrahigh vacuum equipment provides environments with extremely low pressures by pumping air molecules out of the vacuum chamber, like the one shown right, thus removing any contaminants and providing a “model” system to study.
Bavisotto currently focuses on lateral interactions in catalysis. Catalysis traditionally deals with accelerating chemical reactions, although recent focus has been on understanding the interactions molecules can have with each other on the surface of a catalyst. It’s been recently discovered that these lateral interactions can influence a variety of aspects during a chemical reaction.
“We think that if you understand those interactions, you can intelligently design better catalysts for producing more of you want, producing it quicker, or producing more pure samples of what you want,” Bavisotto said.
Studying surface interactions between molecules can lead to the development of better fuel additives for cars, for example, or better lubricants that let moving parts function longer in trains, cars, or airplanes.
A crucial aspect to understanding surface reactions is knowing how to measure their energetics – in particular, the activation barrier. The activation barrier is the amount of energy a molecule needs to have available to undergo a specific chemical reaction.
2. The old method is imprecise and doesn’t study all reactions.
“Before our paper, there’s really only one method that can give you the energetics of reactions on surfaces. It’s called temperature programmed desorption (TPD),” Bavisotto said. In laymen’s terms, “You put stuff on the surface, and then you heat it up.”
When a chemical interaction occurs on a surface inside of a vacuum chamber, scientists can use a mass spectrometer to measure the gas byproducts from the reaction. They measure the temperature at which the reaction occurred, as well as the rate at which they achieved that temperature. Those measurements give scientists information about the activation barrier that they can use to test their calculations pertaining to their experiment. Often, researchers will rely on computers to calculate energetics values and use TPD to back up their computations.
The problem is that TPD can only measure processes that produce a gas product. There is no experimental technique that can tell you what’s occurring on the surface, Bavisotto said.
“So, any scientist who is working in the field of material science or chemistry, whether it’s looking for better batteries or looking for a better catalyst, can calculate these energetic values, but they have no method to check the values that the computational scientists are providing,” he explained.
The values calculated from computation chemistry are often used to model real chemical reactions, but the rates found in experiments don’t always match the computationally derived values. This generated significant frustration. For example, Bavisotto and his team were researching how to modify a compound called furfural in order to use it as a fuel additive, but when they were trying to computationally model their experimental reactions rates, large discrepancies occurred.
“The computed values were not accurate enough to reproduce actual experimental results… Were we doing the experiment wrong? Or was their calculation not done well enough?” Bavisotto said. “I was finding that a lot of research groups were performing quantum chemical calculations and providing values that worked for their simulations, but there’s no method for experimentally testing them.”
3. This method eliminates the guesswork.
So, Bavisotto and his colleagues came up with a new method to check their numbers. It’s called temperature program spectroscopy, or TPS. “It does everything TPD does, but it doesn’t need to form a gas phase product,” Bavisotto said. “It can look at things on the surface and it can check that values that are computationally derived. That’s huge.”
Instead of waiting for a reaction to occur and then measuring gas byproducts and temperature, scientists using TPS slowly and linearly vary the temperature inside of the vacuum where the reaction is occurring while continuously measuring spectroscopic data of the reaction. One method of TPS, called the TPS-Redhead Analysis Mode (TPS-RAM), named after a Canadian physicist who pioneered the mathematics used, continuously collects spectra while heating and measures the rate at which a reaction occurs.
“This method can directly measure the kinetic values for the chemical process occurring ,” Bavisotto said. “It can experimentally measure the energetic values instead of just computationally calculating them.”
A different mode of TPS, called the Van’t Hoff Analysis Mode (TPS-VHAM), named after the Dutch physicist who first introduced the mathematical framework, analyzes the ratio between two compounds as the temperature slowly and linearly increases. Imagine you have a mixture of two substances that are in equilibrium. Here, substance A is being interconverted into substance B via a chemical reaction. This method can give you the energetics of the conversion between the two substances by comparing the relative amount of each substance at various times during the reaction.
The third mode of TPS, the Arrhenius Analysis Mode (TPS-AAM), named after the Swedish physicist who laid the groundwork with his mathematics, differs a bit from the previous two modes. Instead of increasing the temperature linearly as the experiment occurs, scientists raise the temperature to a certain point and then perform the experiment at that selected temperature. Then, the experiment is repeated at multiple different temperatures. This method shows the rate of the reaction as a function of temperature, which can help scientists determine the reaction’s activation barrier with a greater level of precision.
4. This new method can make a big impact.
The TPS method can serve as a way to check computational models of surface reactions. “This will allow all chemists to have reliable kinetic numbers used to predict chemical selectivity or reaction rates… It’s useful for battery science. It’s useful for material science. It’s useful for so many different fields,” Bavisotto said.
“That’s one of the reasons why we think that this will have such a profound impact. … There are a lot of people that have existing spectroscopic techniques, but they aren’t using it with this method,” Bavisotto said. “We were able to design this technique with spare parts. This technique is likely going to be adopted very fast because it costs some research groups almost no additional money to do it.”
In fact, Bavisotto said, scientists in research labs in the U.S., Argentina, and Germany have already expressed interest in incorporating the TPS method into their labs.
It’s no wonder; computational science is taking off. Using computers to simulate and calculate things like the activation barrier of certain chemical reactions is much faster than doing all experiments by hand, but computers don’t always give the correct answer. TPS gives scientists a way to check those calculations instead of blindly trusting the computer’s numbers.
And that, in turn, can lead to many more scientific breakthroughs.
By Sarah Vickery, College of Letters & Science