Experimental methods in chemical engineering: Raman spectroscopy

Abstract

When photons impinge on a substrate, most scatter with the same frequency (elastic scattering or Rayleigh dispersion) and only 10 −7 scatter with a different energy (inelastic scattering). This inelastic interaction (Raman scattering) exchanges energy in the region of molecular vibrational transitions for crystalline and amorphous materials. Raman bands in a spectra represent vibrational transitions, like infrared, however the selection rules are different. Typically, the vibrations that are intense in Raman are weak in infrared and vice versa. A remarkable feature of the Raman effect is that it is highly sensitive to nanocrystals, even below 4 nm, which are too small to generate XRD patterns. Plasmonic enhancement, like surface-enhanced Raman spectroscopy (SERS) boost the Raman signal by 10 4 , providing single-molecule detection capability. Glass, quartz, and sapphire are transparent to Raman effect (depending on the energy of the incident excitation radiation), which makes it ideal to examine materials under reaction conditions (in-situ cells and operando reactors that operate over a broad range of temperature, pressures, and environments). Raman spectroscopy emerged in the 1930s; however, infrared spectrometry displaced it. With the advent of powerful lasers in the 1970s, more researchers began to apply Raman routinely. In 2019, the Web of Science indexed 20 400 articles mentioning Raman against 50 000 articles mentioning infrared. Chemical engineers apply Raman less frequently than in material science, physical chemistry, and applied physics, with 887 articles vs 6250, 3700, and 3510 for the other disciplines. A bibliometric analysis identified four research clusters centred on thin films and optics, graphene and nanocomposites, nanoparticles and SERS, and photocatalyst. K E Y W O R D S operando, Raman, Rayleigh, Stokes bands, surface enhanced Raman spectroscopy 1 | INTRODUCTION Raman spectroscopy has become an important and popular analytical tool since it is non-destructive and identifies molecular structure properties and how they change under reactive environments. The technique is based on the Raman effect that C.V. Raman described in 1928 [1] : when electromagnetic radiation (typically monochromatic laser light; near ultraviolet, visible, or near infrared) interacts with a materials molecular vibrations, the incident energy of the photons are scattered with a predominantly lower energy (inelastic scattering). However,

Experimental methods in chemical engineering: Density functional theory

“…Determining elementary reaction rates and identifying molecular species on catalyst surfaces is fast becoming reality. [ 124 ] Spectroscopic methods provide valuable molecular‐level data, although they can barely provide a full picture of reaction pathways for a complicated reaction. DFT, on the other hand, is capable of capturing each reaction step and generates reaction energy diagrams of reaction pathways (red cluster).…”

On the Origins of Some Spectroscopic Properties of “Purple Iron” (the Tetraoxoferrate(VI) Ion) and Its Pourbaix Safe-Space

“…With Fourier Transform Infrared (FTIR) Spectroscopy [ 28 ], two related fields of interest appeared, namely, “aqueous solutions” and “wastewater”. With Raman Spectroscopy [ 29 ], the umbrella term “physical chemistry” appeared in the keywords. Other instrumental methods of chemical analysis contained in the series of reports mentioned above include Electron Paramagnetic Resonance (EPR/ESR) Spectroscopy [ 30 ], Nuclear Magnetic Resonance (NMR), Mass Spectrometry (MS), X-Ray Photoelectron Spectroscopy (XPS), amongst others.…”

Experimental methods in chemical engineering: Mössbauer spectroscopy

“…Chemical engineering was tenth with 120 articles. Compared to Raman and Fourier transform infrared spectroscopy (FTIR), which grew from 1000 articles per year in the late 1980s and early 1990s to over 20 000 in 2020, [ 18,19 ] MÖS has stagnated at 1 250 100 () since 1991. These three techniques address the molecular level changes and reactivity (bottom rows of Figures 9 and 10), but MÖS has been less successful integrating operando versus the other two.…”