Unlocking Catalytic Insights with UV–vis–NIR Absorption Spectroscopy

Diagram of a catalysis process, illustrating how a catalyst speeds up a chemical reaction without being consumed © Thirawat - stock.adobe.com

A comprehensive review published in Chemical Reviews delves into the growing role of in situ ultraviolet–visible–near infrared (UV–vis–NIR) absorption spectroscopy in catalytic research. This powerful method allows researchers to probe catalysts and catalytic reactions under operational conditions, offering real-time insights into reaction mechanisms, catalyst behavior, and material properties. The review, authored by Max L. Bols, Jing Ma, Fatima Rammal, and others from KU Leuven and Stanford University, consolidates key advances in using this technique across various catalytic fields, from enzymatic to heterogeneous and photocatalysis (1).

Read More: Catalysis in Spectroscopy

Spectroscopic Methods Described

UV–vis–NIR absorption spectroscopy encompasses a broad range of photon energies, with wavelengths spanning from 200 to 2500 nm, which corresponds to the ultraviolet (UV), visible (vis), and near-infrared (NIR) regions of the electromagnetic spectrum. Each region of this spectrum provides information into different electronic and vibrational transitions in molecules and materials. In the UV region (200–400 nm), intense transitions like ligand-to-metal charge transfer (LMCT) and metal-to-ligand charge transfer (MLCT) are commonly observed, especially in transition metal ion complexes, providing structural and electronic information. The visible range (400–700 nm) often captures weaker d-d transitions, typical of transition metal ions, which offer insights into the coordination environment of metal centers in catalysts. In the NIR region (700–2500 nm), overtones and combination bands of fundamental vibrations in O–H, C–H, and N–H groups are detected, helping to probe structural defects and molecular interactions. These transitions make UV–vis–NIR spectroscopy particularly useful in catalysis, where electronic excitations reveal oxidation states, band gaps, and adsorbed species on catalyst surfaces. Different techniques such as transmission, diffuse reflectance/reflection spectroscopy (DRS), and fiber optics are used depending on the nature of the sample (solid, liquid, or gas), with each mode providing different angles of information. Additionally, combining UV–vis–NIR with other spectroscopic methods like magnetic circular dichroism (MCD) or resonance Raman spectroscopy (rR) enhances the capability to probe complex catalytic systems, offering complementary data on electronic structure and catalytic mechanisms (1,2).

Detailed Findings

UV–vis–NIR spectroscopy absorption range is well-suited to study a wide array of chemical and electronic transitions in materials. This range captures critical excitations in organic molecules and transition metal ions (TMIs), making it invaluable for catalysis research, where understanding electron movements and material defects is key. One of the review’s highlights is the application of UV–vis–NIR spectroscopy in the study of transition metals like copper and iron, which are central to many industrial catalysts, especially in zeolites (1,2).

In the field of heterogeneous catalysis, this technique shines when studying metal-exchanged zeolites. These materials, which house transition metals in well-defined crystallographic sites, allow researchers to collect fingerprint spectra that reveal the active sites crucial to catalytic reactions. This capability enables not only structural insights but also kinetic measurements that can inform the design of more efficient catalysts (1,2).

Photocatalysis, another area that benefits from UV–vis–NIR spectroscopy, leverages transient absorption techniques to track excited states. Ultrafast measurements—ranging from femtoseconds to nanoseconds—capture the dynamic behavior of catalysts as they absorb light and promote chemical reactions. The study of these rapid events can lead to breakthroughs in developing more efficient solar cells and light-driven chemical processes (1,2).

Techniques and Applications

Beyond structural characterization, the review article emphasizes how UV–vis–NIR absorption can monitor changes in oxidation states, surface interactions, and electronic properties in real time. This technique is particularly useful for heterogeneous catalysts, such as transition metal oxides, where it tracks shifts in oxidation states, oxygen vacancies, and band gaps during reactions. For electrocatalysis, which powers devices like fuel cells, UV–vis–NIR spectroscopy is used to measure these changes under varying electrochemical conditions, offering a window into the electronic processes that drive catalytic efficiency (1).

However, UV–vis–NIR absorption spectroscopy also presents challenges. The broad absorption bands typical of this range can complicate the interpretation of spectra, especially at high temperatures where signals can be obscured by vibronic coupling. Advanced data analysis methods have helped overcome these hurdles, particularly in the study of coking reactions on acid zeolites (1).

Combining UV–vis–NIR spectroscopy with other methods, such as magnetic circular dichroism (MCD) and resonance Raman spectroscopy (rR), enhances its utility. These complementary techniques provide a more comprehensive picture of catalytic processes, offering insights into both the electronic structure and the magnetic properties of transition metal catalysts. By integrating multiple spectroscopic approaches, researchers can achieve a deeper understanding of catalytic systems and their underlying mechanisms (1).

Broader Impacts

While UV–vis–NIR absorption spectroscopy is widely used in catalysis, the authors argue that its full potential remains underexplored. Complex catalysts, particularly in heterogeneous systems, often present spectra that are difficult to interpret due to overlapping signals. However, with advances in optical equipment, statistical analysis, and the integration of databases, the resolution and usability of this technique are expected to improve.

Looking ahead, the review anticipates that further developments in transient absorption equipment will push the boundaries of the technique, allowing researchers to probe even faster processes. Additionally, the growing use of UV–vis–NIR spectroscopy in fields such as battery research and sensor technology signals its broader applicability beyond traditional catalysis. For example, sulfur-based batteries and semiconductor sensors have properties that are ideally suited for analysis in the UV–vis–NIR range, highlighting the method’s versatility (1).

The review by Bols and colleagues underscores the significant contributions UV–vis–NIR absorption spectroscopy has made to understanding catalytic processes. Its ability to provide detailed insights into the electronic and structural properties of catalysts has opened new avenues for research, particularly in fields like heterogeneous catalysis and photocatalysis. With ongoing advancements in data processing and experimental techniques, UV–vis–NIR spectroscopy is poised to become an even more powerful tool in the study of catalytic systems (1).

References

(1) Bols, M. L.; Ma, J.; Rammal, F.; Plessers, D.; Wu, X.; Navarro-Jaén, S.; Heyer, A. J.; Sels, B. F.; Solomon, E. I.; Schoonheydt, R. A. In Situ UV–Vis–NIR Absorption Spectroscopy and Catalysis. Chem. Rev. 2024, 124, 5, 2352–2418. DOI: 10.1021/acs.chemrev.3c00602

(2) Jentoft, F. C. Ultraviolet–Visible–Near Infrared Spectroscopy in Catalysis: Theory, Experiment, Analysis, and Application Under Reaction Conditions. Advances in Catalysis 2009, 52, 129–211. DOI: 10.1016/S0360-0564(08)00003-5