New Advances Of Raman Spectroscopy - Characterization of Catalysts and Catalytic Reactions

Raman Spectroscopy

Raman Spectroscopy is a kind of non-destructive technique that offers detailed information regarding chemical structure, polymorphy, phase, molecular interactions and crystallinity through Raman shift. Raman imaging and Raman analysis are performed and the technique is in accordance with the light interaction with the chemical bonds within a material.

This write-up brings light on the latest applications of Raman spectroscopy in the field of catalysis and catalytic reactions.

Aspects of Raman Spectroscopy in Catalysis

Mode of operation of heterogeneous catalysis is of great interest scientifically and economically. Raman spectroscopy has the potentiality to act as a powerful vibrational spectroscopic method for a primary and molecule-level characterization of catalytic reactions and catalysts.

Raman spectra reveal specific information on the catalysts’ structure (defect) in the bulk and surface. It also displays the presence of reaction intermediates, adsorbates and surface depositions. Thus Raman spectroscopy provides significant insights into reaction mechanisms.

Major advances have happened in the usage of Raman spectroscopy for the characterization of heterogeneous catalysts during the previous decade. These are inclusive of the development of new methods and probable directions of research for application of Raman spectroscopy to working catalysts.

The main focus is on gas-solid catalytic reactions and photocatalytic reactions in the liquid phase. Theoretical calculations are utilized for facilitating band assignments and description of resonance Raman effects.

Modern Raman spectroscopy possessing a single-stage Raman spectrometer permits high throughput and flexibility in design of in situ and operando for the studies of working catalysts. 

Operando Raman Spectroscopy

Catalysis accounts for 85% of chemicals and fuels produced. Anyhow, catalysis is lengthy and a costlier process. Raman spectroscopy is used in monitoring catalytic reactions. Operando Raman spectroscopy has induced attention of researchers due to the following reasons:

• Raman spectroscopy is sensitive to a minute modification of the structure
• The Raman signal is least affected by the presence of gases and water
• Raman spectroscopy is able to be operated in a wide pressure range 0.1-10 MPa and temperature range 25-1000°C.

Operando means the measurement of spectroscopy operated under realistic reaction conditions. An approach, which combines simultaneous kinetic measurement and in situ spectroscopy measurements have been reported. Operando measurements are cutting-edge techniques, which allow researchers to obtain deep insight into activation, deactivation and function, establishing a structure-performance association for a catalytic system. It is essential for comprehension of the origin of reaction pathways and active sites. It can be used for reduction of inconsistency in reproducibility due to separated measurements.

There is still a long way to go for the deep understanding of catalytic reactions:

• Operando techniques can improve catalytic processes through the determination of catalyst structure and surface species under read industrial situations.
• Operando techniques can be used in the design of the next generation of industrial catalysts.
• Rational design of active sites as a dreamful methodology can come true.

Other Modern Developments

Major developments have taken place in the enhancement of Raman signal of heterogeneous catalysis through the use of surface enhanced Raman spectroscopy (SERS), shell-isolated nanoparticle surface-enhanced Raman spectroscopy (SHINERS) and UV resonance Raman spectroscopy. Time-solved Raman studies can be applied to kinetic and structural characterization. The usage of tip-enhanced Raman spectroscopy (TERS) and coherent anti-Stokes Raman spectroscopy (CARS) have led to the recent developments in spatially resolved Raman analysis of catalytic processes and catalysis.

Probing Single Atom Catalysts

One of the applications of Raman spectroscopy imaging is probing of single atom catalysts (SAC) and catalytic reaction process. It is useful in characterizing the structure of SAC and in-situ monitoring of the catalytic reaction on them through shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). It is a valuable tool for study of SAC at solid-liquid interfaces. Palladium SACs on different supports have been recognized by Raman spectroscopy and nucleation conditions of palladium species from single atoms to nanoparticles.The process of hydrogenation of nitro compounds in palladium SACs is monitored in-situ and to uncover the unique catalytic properties of SAC, molecular insights are obtained.

SERS & TERS

SERS and TERS possess highly localized chemical sensitivity, which form them ideal for studies of chemical reactions and processes on catalytic surfaces. Since adsorbates, reaction intermediates and catalyst structures  are obtained in lower quantities, electromagnetic fields through Raman spectroscopy offer ample chances to illustrate reaction mechanisms. Insufficient signal enhancement and substrate instability limit the application of SERS and TERS in catalysis. Through sophisticated colloidal synthesis methods and  SHINERS, the challenges can be overcome. Using SERS, submonolayer sensitivity is achieved. Using SERS, adsorption and desorption phenomena of amines, which are significant for dehydroamination reactions on copper surfaces. 

For complex reactions like carbon dioxide reduction, rational design and understanding of heterogeneous catalysts needs knowledge of elementary steps and chemical species prevalent on the surface of the catalyst under operating conditions. Through in-situ SERS, surface of silver nanoparticles during plasmon-excitation-driven carbon dioxide reduction in water. Rich array of C1-C4 compounds have been found on the photocatalytically active surface due to surface sensitivity and high spatiotemporal resolution. Abundant presence of multi-carbon compounds like butanol indicates the favorability of C-C coupling on the photoexcited silver surface. Isotope labelling can be used in nanoscale probing, which confirms that detected components are intermediates and products of the catalytic reaction in preference to spurious contaminants. This approach brings out the surface chemical knowledge, which will be useful in the modelling and engineering of catalysis.

Ultraviolet Raman Spectroscopy

For characterization of solid catalysts, ultraviolet Raman spectroscopy is an efficient tool. In UV Raman spectroscopy, when the excitation wavelength is less than 260 nm, the Raman spectrum obscures fluorescence and appears at shorter wavelengths. This is advantageous for characterization of zeolite materials, which are difficult to be characterized using conventional Raman spectroscopy. Resonance enhancement offers the chance to probe specific compounds in a heterogeneous mixture. It also enhances the intensities of bands above the detection limit for weak signals. Spectra of adsorbed benzene manifest the ability of resonance Raman spectroscopy for the detection of subtle distortions in the structure of benzene. UV Raman spectra obtained during catalysis of hydrocarbon conversions can diagnose the topology of the coke.

UV Raman spectroscopy possesses a short wavelength laser source and resonance Raman effect. It helps in increasing the intensity of Raman scattering. It is helpful for the monitoring of surface phase transformation of metal oxide photocatalysts. It is able to offer the information regarding the phase and active sites and their mechanisms of assembling, which are beneficial for the knowledge of heterogeneous catalysis and rational design of selective and highly active catalysts.

Conclusion

A short introduction to Raman spectroscopy has been provided. Features of Raman spectroscopy in catalysis have been pointed out. Application of Operando Raman spectroscopy in catalysis has been detailed. Other modern establishments and probing of SAC have been discussed shortly. Usage of SERS and TERS have been explained. Finally, UV Raman spectroscopy usage in catalysis has been briefed.