Decoding Catalytic Processes: Advancing FTIR Research with Iso-Potential Operando Spectroscopy

Thought leadersProf. Dr. Raimund HornHead of the Institute of Chemical Reaction EngineeringHamburg University of Technology

Join Bruker to explore the challenges and methodology underlying the use of FTIR tools with iso-potential operando spectroscopy

What are the primary challenges associated with using spectroscopy to study industrial catalytic reactors, and how does the iso-potential operando DRIFTS technique overcome these challenges?

Using spectroscopy to study catalysts inside industrial catalytic reactors involves several significant challenges. These reactors are often large, constructed from steel, and operate under extreme conditions such as high temperatures, high pressures, and sometimes toxic chemical environments.

These conditions make direct spectroscopic measurements difficult. Traditional methods, including miniaturized operando cells, often force compromises. These cells may not replicate reactor conditions accurately and might not provide the sensitivity or precision needed for detailed spectroscopic studies.

The iso-potential operando DRIFTS method addresses these challenges by decoupling the catalytic reaction from spectroscopic measurements. We use a spatial sampling system that extracts the reaction stream at specific points in the reactor and feeds it into a spectroscopic cell.

The key is replicating the reactor's conditions—temperature, pressure, and chemical composition—inside the spectroscopic cell. This setup allows us to study catalyst surface species and structural dynamics in a way that accurately reflects the reactor's environment.

Could you elaborate on the key differences between the dissociative and associative mechanisms in CO₂ methanation, and what insights did iso-potential DRIFTS provide to resolve this debate?

In CO₂ methanation, two main reaction pathways have been proposed. The dissociative mechanism suggests CO₂ adsorbs onto the metal catalyst, mostly Ni, breaking into carbon and oxygen atoms. These intermediates then react with hydrogen to form methane.

Conversely, the associative mechanism involves CO₂ adsorption onto the catalyst support, e.g. gamma-alumina, forming intermediates such as formates or bicarbonates. These intermediates are reduced step-by-step to methane via hydrogen spillover from the metal surface.

Using iso-potential DRIFTS, we provided evidence supporting the associative mechanism. We observed formate as a key surface intermediate, correlating strongly with the rate of CO₂ conversion.

Interestingly, adsorbed CO, which many assumed to be active in the reaction, turned out to be a spectator species, present on the surface but not directly participating in the reaction.

This insight not only clarifies the reaction pathway but also highlights opportunities for catalyst design. For instance, we can now focus on optimizing catalyst supports to enhance formate formation and stability, potentially improving the efficiency of CO₂ methanation processes.

How do you ensure identical temperature, pressure, and gas composition between the reactor and the spectroscopic cell?

Achieving identical conditions—or iso-potentiality—between the reactor and the spectroscopic cell is critical to the method’s success. We start by equipping the reactor with a spatial sampling probe to extract small amounts of the reaction mixture at precise locations. This sampled gas is immediately transferred to the spectroscopic cell, minimizing pressure drops and ensuring the chemical composition remains unchanged.

Temperature synchronization is another essential component. We measure the reactor’s local temperature using a pyrometer or thermocouple and replicate it in the spectroscopic cell with high precision. Pressure consistency is maintained by designing the sampling line to minimize fluctuations, typically keeping pressure differences within a few millibars.

We also use highly diluted catalysts in the spectroscopic cell to minimize reaction rates and ensure the catalyst behaves as it would in the reactor.

In your CO oxidation study, what role do surface species like adsorbed CO on terras versus under-coordinated platinum sites play in determining catalytic behavior?

Surface species, particularly adsorbed CO, play a critical role in determining the behavior of platinum catalysts in CO oxidation. Using iso-potential DRIFTS, we distinguished between CO molecules adsorbed on well-coordinated platinum terrace sites and those on under-coordinated sites, such as edges and corners.

Our findings showed that CO on terrace sites desorbs more readily due to weaker binding, freeing up active sites for oxygen adsorption and subsequent reaction. In contrast, CO on under-coordinated sites binds more strongly, remaining on the surface even as the temperature increases.

This difference explains why catalytic activity increases dramatically when the temperature rises enough to desorb CO from terrace sites, allowing the reaction to proceed more efficiently.

Given the dynamic nature of catalysts, how can the iso-potential DRIFTS technique contribute to understanding catalyst deactivation and structural changes under reaction conditions?

Catalysts are highly dynamic and can undergo structural and electronic changes during reactions, leading to phenomena like deactivation. Iso-potential DRIFTS is particularly well-suited to studying these changes because it provides real-time insights into surface species and structural dynamics under operating conditions.

For instance, our studies observed how temperature and concentration gradients within the reactor influence catalyst properties like oxidation states, active site distributions, and even physical structure. By tracking these changes, we can identify the causes of deactivation—such as coking, sintering, or poisoning—and develop strategies to mitigate them.

This dynamic perspective also aids in designing more robust catalysts. For example, understanding how a catalyst cokes over time allows us to create materials more resistant to this process or optimize operating conditions to reduce its impact. Iso-potential DRIFTS enables a deeper understanding of these processes, bridging the gap between fundamental research and industrial application.

How adaptable is the iso-potential operando DRIFTS method to other spectroscopy or catalytic processes, such as liquid-phase reactions or reactions producing complex hydrocarbon mixtures?

While we have focused on gas-phase reactions and infrared spectroscopy, iso-potentiality principles can be extended to other spectroscopic techniques, such as Raman spectroscopy, X-ray diffraction, and even electron microscopy. In fact, we have already demonstrated its compatibility with synchrotron-based X-ray diffraction.

The method is also adaptable to other types of catalytic processes. For example, extending it to liquid-phase reactions would require adjustments to handle phase changes and ensure chemical potential consistency. Similarly, for reactions involving complex hydrocarbon mixtures, the spectroscopic setup could be tailored to address specific challenges, such as identifying non-volatile intermediates.

The versatility of this method makes it a powerful tool for exploring a wide range of catalytic systems. From studying environmental processes like CO₂ reduction to optimizing sustainable fuel production, iso-potential DRIFTS offers unique opportunities to advance scientific understanding and industrial innovation.

About the Speaker

Prof. Dr. Raimund Horn is a distinguished expert in chemical reaction engineering at the Hamburg University of Technology. He has been the head of the Institute of Chemical Reaction Engineering since 2013. His research focuses on catalytic processes, reactor modeling, and the investigation of kinetics and dynamics in chemical reactors. Prof. Horn has significantly contributed to the understanding of catalytic oxidation processes and spatial profiling of chemical reactions within reactors.

This information has been sourced, reviewed and adapted from materials provided by Bruker Optics.

For more information on this source, please visit Bruker Optics.