In situ/operando spectroscopy and applications Flashcards
Why is it desirable to perform in situ/operando catalytic studies?
Gain an understanding of what a working catalyst looks like.
What is the difference between in situ and in operando characterisation?
In operando usually implies measurement of rate or selectivity during the characterisation to confirm it is acting in the expected manner for the conditions.
In situ is characterisation under the same conditions or observed under realistic conditions.
What are the key challenges with in situ measurements?
Technique cannot rely on vacuum and may need to penetrate inside a reaction vessel.
Need to be sure that the catalyst isn’t bypassed.
Need to ensure characterisation doesn’t change the sample, e.g. UV-light activated processes
If a species is observed, still need to understand the role. Spectators and non-catalytic cycle species likely to be longer lived and easier to see.
What are the different types of IR spectroscopy?
Transmission (TIR), Diffuse-Reflectance (DRIFTS), attenuated total reflection (ATR), reflection-absorption (RAIRS)
How can DRIFTS be used to characterise catalysts in situ?
Build in windows to allow entry and exit of IR, otherwise pretty standard IR set-up. DRIFTS allows characterisation of the surface/top part of the catalyst bed. May be problematic as not entirely sure of the temperature at this point?
What changes need to be made to RAIRS to measure catalysts?
NB. RAIRS good for single crystals.
Polarization modulation RAIRS helps to mitigate gas phase absorption.
Example: Able to tell the different between atop, hollow and bridging sites for CO on Pd (111).
How can in situ microscopy be used to measure catalysts?
STM: Sample of singlet crystal catalyst can be seen. For CO on Pt (557) can see reversible restructuring as the pressure is increased.
TEM: Generally done using lithography. Gas flow through sample, electrons do not have long path in gas to traverse to prevent scattering. Study formation of catalysts in situ.
How can X-ray absorption spectroscopy be used to measure catalysts?
(XAS) NEXAFS - can get information about atomic chemical environment, molecular orbital information and oxidation state/charge distribution
EXAFS - can get information about near neighbour environment, number/type of neighbours and inter-atomic distances
Energy absorption is very element specific. With all this information able to see structure changes, and build up idea of functional performance.
How can we XANES to measure catalysts in situ?
Compare to reference spectra. Peaks around ionisation threshold will indicate orbitals where electrons can be excited. Commonly there are more for oxidised - this can be seen with additional peaks “WHITE LINE”. Oxidation can also be indicated by a shift in energy of peak, e.g. sulfur.
How does EXAFS work?
Diffraction-type experiment, near-by atoms can scatter the wave-like electron and then interference to produce interference around a singlet point. By Fourier transform, can obtain bond lengths and rough coordination numbers from this data.
How can we use in situ pXRD to characterise catalysts?
X-rays readily penetrate sample environments so can see reduction processes and phases present. Sharp thin reflections indicate larger molecules and broader give smaller molecules due to increased edge effects. This way, we can monitor crystal structure changes through a reaction.
What is the biorefinery concept?
Concept of a processing plant where biomass feedstocks are converted and extracted into a spectrum of valuable products. NB. Biomass feedstocks are commonly highly oxygenated, switching from oxygen poor to oxygen rich feeds.
