“The beginning of the end” of the fossil fuel era is here, according to the UN Climate Change Executive Secretary speaking after the COP28 summit in 2023. The summit agreed on the first ‘global stocktake’ of climate actions that will help countries document their 2025 climate action plans in line with the Paris Agreement.
However, while much of the focus is on the energy transition, fossil fuels remain an integral part of manufacturing the petrochemicals needed for countless everyday products, from plastics to pharmaceuticals. Replacing this reliance on fossil-derived feedstocks will require a complete overhaul of major industrial processes.
“There are many ways in which to do that, but new catalysts will be at the forefront of it all,” says Dr Christian Reece, a research group leader at the Rowland Institute, Harvard University. “In my opinion, the only real framework that works right now to move away from fossil fuels is changing the feedstock to CO2, biomass, pyrolysis or other sources.”
These potential feedstocks are available in abundance and utilising them will help to reduce their environmental impact. Capturing CO2 from emissions has the double benefit of limiting climate change and reducing fossil fuel reliance.
Existing petrochemical reactions have been carefully refined over decades to achieve improved performance. However, the urgent need to drastically limit carbon emissions means that industrial processes will need to develop the next generation of technologies quickly. To optimise these processes successfully in a short timeframe will involve a new approach that builds on deep scientific understanding.
The shift to new feedstocks to meet the industrial demand for hydrocarbons will require a major investment in finding new and improved catalysts. However, this discovery process has traditionally relied on a trial-and-error approach to get empirical evidence for improving efficiency. This can hamper innovation in the field and make it difficult to take significant leaps forward. Today, after decades of fundamental research into how catalysts function, catalysis researchers and engineers could soon provide the key to supporting the global efforts to phase out fossil fuels.
Importance of characterising catalysts
The key to accelerating the process of developing and refining new catalysts lies in a better understanding of their structures.
“The holy grail of catalysis is getting what we call structure-function relationships,” says Reece. “Researchers have been looking into this for the last 40 years, but getting a structure-function relationship on a real catalyst is quite tough because there are lots of components. Characterisation is important for understanding what is responsible for the activity.”
Characterisation studies in the lab rely on small-scale flow reactors, perhaps only a few millilitres, to mimic industrial reactors that are many orders of magnitude larger. The low dead-volume of such small cells allows users to efficiently introduce and remove gases. By comparison, real-world industrial processes cannot be controlled precisely enough to detect the subtle changes in catalyst behaviour that can provide insight into the structure-function relationship. However, the lab studies are becoming ever closer to the reality and yielding new information.
“There’s a renaissance in the field right now about this combination of fundamental understanding and application at the same time,” says Reece. “The structural information may not yet be directly applicable to the industry, but it will at least provide an insight into how the material evolves as a function of coverage or temperature, and how that influences the activity.”
Probing structure and kinetics with robust reaction cells
Infrared spectroscopy (IR) is critical to understanding the structure and function of a catalyst. It allows users to follow the course of a reaction as the reactants bind to the catalyst and even subtle changes in the binding properties can be detected in the spectra.
In particular, DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) allows researchers to study powdered catalyst materials in realistic settings like a packed bed. “That’s the big benefit of DRIFTS over other traditional techniques. It’s the closest we’ll get to the real world of what’s going on in a reactor,” says Reece. “A couple of years ago, we started heavily using diffuse reflectance spectroscopy and pushing it to see how far we can drive it forward.”
Reece takes a ‘bottom-up’ approach to research, trying to develop ways of understanding the structure-function relationship in a well-studied test system, like carbon monoxide oxidation.
“It’s a useful tool for characterisation using spectroscopy because carbon monoxide has such finely resolved spectral features that are very dependent on the structure,” he says. “Carbon monoxide is a great probe molecule, but it also has the added benefit of being a future feedstock for making hydrocarbons. So, learning how and where it binds and how that changes the structure of materials is really important.”
IR spectroscopy provides a valuable way to study catalysts to get either structural or kinetic insights. However, proving the relationship between the two is a real challenge. This requires analysing the reaction in different ways for evidence that the structure and activity are truly affecting one another, rather than the results being based on correlation.
“We’ve been working with Harrick Scientific to design reaction cells where we do pulsed experiments or ultra-high vacuum experiments inside these cells, so that we can get both the structural information from these probes and also the kinetic information,” says Reece. “The cells are fantastic – they’re very simple to set up and easy to use, but they are also very robust. So, you can do a lot of fun things with the Harrick cells!” The team at Harrick have been brilliant. They are always willing to engage collaboratively with us when we are trying to push their cells to their limits. They are also great at engaging with us to try and adapt their cells to match our needs.
This approach still requires capturing the structure and kinetics separately and then aligning them afterwards, which takes a lot of work. Reece’s goal is to combine the techniques to do them simultaneously, he says, “That would be a big breakthrough.” Such a tool would be a powerful way to get reliable insights into structure-function relationships of catalysts. However, building this up to the complexity of real-world catalysts is another challenge. This is where computational models may help.
Experimental data needed to drive machine learning
Reece started out his research career as a theorist for his PhD in 2013 and gradually moved over to experimental studies. However, he still sees both aspects as being crucial to achieving the long-term goal of making the leap from understanding model systems to industrial applications.
“We’re making good progress. Experimental techniques are advancing, and theoretical tools are going way beyond what I could have imagined. When I started doing theory, the concept of modelling hundreds of atoms at DFT accuracy was insane, and now I’ve heard of teams going over a billion atoms,” he says.
While models can be infinitely precise down to an atomic level, they need experimental evidence to verify their accuracy. The power of machine learning (ML) is driving rapid advances in modelling, but it is still based on assumptions about the existing structure.
“We just have to get the theoretical models to a point where they’re good enough, and then we can refine further. But to get them to this point depends on fundamental data that’s well defined and robust enough to train a model on,” says Reece. “If we can take a few good measurements of a really well-defined structure, I hope that can provide enough of an anchor in the training dataset.”
Advancing sustainability through fundamental insights
This grand unification of theory and practice could be at least ten years away. However, as the petrochemical industry looks to reinvent many of its systems, this deep understanding may make all the difference.
“When it comes to industrial processes, catalysts work really well, and they work with high efficiency. So, finding dramatic changes is going to take significant rethinking on how we run these processes,” says Reece. “Small incremental changes will help reduce CO2 emissions, but if we need a complete reshaping of the industry, then it’s only going to come from fundamental insight.”
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