UV-VIS-NIR spectroscopy and microscopy of heterogeneous catalysts

Abstract

This critical review article discusses the characterization of heterogeneous catalysts by UV-VIS-NIR spectroscopy and microscopy with special emphasis on transition metal ion containing catalysts. A review is given of the transitions, that can be observed in the UV-VIS-NIR region and the peculiarities of catalytic solids that have to be taken into account. This is followed by a short discussion of the techniques that have been developed over the years: diffuse reflectance spectroscopy, UV-VIS microscopy, in situ or operando spectroscopy, the combination of UV-VIS spectroscopy with other spectroscopic techniques, with chemometrics and with quantum chemistry. In the third part of this paper four successes of UV-VIS-NIR spectroscopy and microscopy are discussed; (1) coordination of transition metal ions to surface oxygens; (2) quantitative determination of the oxidation states of transition metal ions; (3) characterization of active sites and (4) study of the distribution of transition metal ions and carbocations in catalytic bodies, particles and crystals (104 references).

Silica-supported Ti chloride tetrahydrofuranates, precursors of Ziegler–Natta catalysts

“…The samples were measured in the form of self supporting pellets 30 inside a quartz cell in controlled atmosphere. UV-Vis-NIR spectra were collected in Diffuse Reflectance mode on a Cary5000 Varian spectrophotometer.…”

“…7 When the grafting 25 procedure is performed on a highly dehydroxylated silica, the grafted metal species can assume a single-site character. [2][3][4]8 Several heterogeneous catalysts for olefin polymerization are supported on amorphous silica; the silica types involved are porous, with high specific surface areas and potentially reactive 30 surface hydroxyl groups almost entirely upon the pore walls. 9 The first developed polyolefin catalyst based on silica was the Phillips catalyst for ethylene polymerization, where the active sites are diluted chromium centres.…”

“…However, it is difficult to verify whether thf is still attached to the Ti sites or is adsorbed on the silica support. In this regard, it should be noticed that independent 30 FT-IR experiments of CO adsorbed at 100 K provide an evidence that the majority of grafted Ti sites do not have coordination vacancies available for CO insertion (Section S4 and Figure S3), suggesting that they retain the six-fold coordination geometry characteristic of the starting complexes. As a consequence, at 35 least a fraction of the thf should be coordinated to the Ti sites.…”

Binder Effects in SiO2‐ and Al2O3‐Bound Zeolite ZSM‐5‐Based Extrudates as Studied by Microspectroscopy

“…20, 21 Given the recent enhancements in chemical imaging methods,22, 23 such as single‐molecule and confocal fluorescence microscopy,21, 24–27 UV/Vis microspectroscopy,28, 29 IR and Raman microscopy,30–32 transmission soft and hard X‐ray microscopy,33–36 X‐ray diffraction computed tomography37 and focused‐ion‐beam scanning electron microscopy,38 coupled with the improved control of these commercial catalysts for modern‐day processes,39 the time is ripe for a shift in academic focus towards understanding these more complex multi‐component and hierarchically structured catalyst materials.…”

Bimetallic Catalysts Containing Gold and Palladium for Environmentally Important Reactions

“…The electronic structure of bimetallic nanoparticles can be characterized using the UV-vis method which reveals the electronic absorption in the UV and visible regions [69,70]. The basis of this technology is that electron conductors absorb near the UV and visible region whilst semiconductors absorb in the UV and visible region so this method is mostly used for powder samples, providing information about the electronic properties in the coordination and oxidation states.…”