Operando spectroscopy
Operando spectroscopy is an analytical methodology wherein the spectroscopic characterization of materials undergoing reaction is coupled simultaneously with measurement of catalytic activity and selectivity.
[2] In the context of organometallic catalysis, an in situ reaction involves the real-time measurement of a catalytic process using techniques such as mass spectrometry, NMR, infrared spectroscopy, and gas chromatography to help gain insight into functionality of the catalyst.
[3] Understanding the catalytic mechanism and active site is crucial to creating catalysts with optimal efficiency and maximal product yield.
[1] It was coined by Miguel A. Bañares, who sought to name the methodology in a way that captured the idea of observing a functional material — in this case a catalyst — under actual working, i.e. device operation, conditions.
The first international congress on operando spectroscopy took place in Lunteren, Netherlands, in March 2003,[3] followed by further conferences in 2006 (Toledo, Spain),[5]2009 (Rostock, Germany), 2012 (Brookhaven, USA), and 2015 (Deauville, France).
[3] The analytical principle of measuring the structure, property and function of a material, a component disassembled or as part of a device simultaneously under operation conditions is not restricted to catalysis and catalysts.
Catalyst scientists would ideally like to have a "motion picture" of each catalytic cycle, whereby the precise bond-making or bond-breaking events taking place at the active site are known;[7] this would allow a visual model of the mechanism to be constructed.
Complications arise, for example, for gas phase reactions which require large void volumes, which make it difficult to homogenize heat and mass within the cell.
[7] Time-resolved spectroscopy theoretically monitor the formation and disappearance of intermediate species at the active site of the catalyst as bond are made and broken in real time.
[8] Most industrial catalysis reactions require excessive pressure and temperature conditions which subsequently degrades the quality of the spectra by lowering the resolution of signals.
Continuing development of operando reaction-cell design is in line with working towards minimizing the need for compromise between optimal catalysis conditions and spectroscopy.
[11] Also, Meunier reports that when using DRIFTS, there is a noticeable temperature difference (on the order of hundreds of degrees) between the crucible core and the exposed surface of the catalyst due to losses caused by the IR-transparent windows necessary for analysis.
[10] Raman spectroscopy is one of the easiest methods to integrate into a heterogeneous operando experiment, as these reactions typically occur in the gas phase, so there is very low litter interference and good data can be obtained for the species on the catalytic surface.
Operando confocal Raman micro-spectroscopy has been applied to the study of fuel cell catalytic layers with flowing reactant streams and controlled temperature.
A case study by Beale et al. involved preparation of iron phosphates and bismuth molybdate catalysts from an amorphous precursor gel.
The redox dynamics of sulfur with Ni/GDC[clarification needed] anode during solid oxide fuel cell (SOFC) operation at mid- and low-range temperatures in an operando S K-edge XANES have been studied.
[19] Fixed energy methods (FEXRAV) have been developed and applied to the study of the catalytic cycle for the oxygen evolution reaction on iridium oxide.
It allows to obtain a rapid screening of several systems under different experimental conditions (e.g., nature of the electrolyte, potential window), preliminary to deeper XAS experiments.
Nanotechnology, used in materials science, involves active catalytic sites on a reagent surface with at least one dimension in the nano-scale of approximately 1–100 nm.
[24] The reduced scale of these reactions affords several opportunities while presenting unique challenges; for example, due to the very small size of the crystals (sometimes <5 nm), any X-ray crystallography diffraction signal may be very weak.
A recent study from laboratory of Günther Rupprechter shows that operando spectroscopy can also be used to investigate the performance of VOC sensing semiconductor nanomaterials.
