Raman and IR studies of surface metal oxide species on oxide supports: Supported metal oxide catalysts

https://www.lehigh.edu/operando/Publications/2005%20Recent%20Conceptual%20Advances%20in%20Catalysis.pdf

Recent conceptual advances in the catalysis science of mixed metal oxide catalytic materials

The molecular aspect of the Raman vibrational selection rules allows for the molecular structural and reactivity determinations of metal oxide catalytic active sites in all types of oxide catalyst systems (supported metal oxides, zeolites, layered hydroxides, polyoxometalates (POMs), bulk pure metal oxides, bulk mixed oxides and mixed oxide solid solutions). The molecular structural and reactivity determinations of metal oxide catalytic active sites are greatly facilitated by the use of isotopically labeled molecules. The ability of Raman spectroscopy to (1) operate in all phases (liquid, solid, gas and their mixtures), (2) operate over a very wide temperature (−273 to >1000 °C) and pressure (UHV to ≫100 atm) range, and (3) provide molecular level information about metal oxides makes Raman spectroscopy the most informative characterization technique for understanding the molecular structure and surface chemistry of the catalytic active sites present in metal oxide heterogeneous catalysts. The recent use of hyphenated Raman spectroscopy instrumentation (e.g., Raman–IR, Raman–UV-vis, Raman–EPR) and the operando Raman spectroscopy methodology (e.g., Raman–MS and Raman–GC) is allowing for the establishment of direct structure–activity/selectivity relationships that will have a significant impact on catalysis science in this decade. Consequently, this critical review will show the growth in the use of Raman spectroscopy in heterogeneous catalysis research, for metal oxides as well as metals, is poised to continue to exponentially grow in the coming years (173 references).

Monitoring surface metal oxide catalytic active sites with Raman spectroscopy

Evolution of isomorphously substituted iron zeolites during activation: comparison of Fe-beta and Fe-ZSM-5

IR spectroscopy of probe molecules (pyridine, 2,6-dimethylpyridine, and CO) as well as high-resolution solid state NMR and especially double-resonance experiments give a new insight into the acidic sites of amorphous silica alumina (ASA). ASA samples are heterogeneous compounds that contain a silica alumina mixed phase as well as aluminum clusters and pure silica zones. The distribution of various forms depends both on the preparation method and on the Si/Al ratio. Formation of mixed phase leads to the creation of acidic hydroxyl groups of various strength, up to that present in dealuminated HY zeolite. Detailed spectroscopic analyses show that these acidic OH groups correspond to the silanol groups located in close vicinity to an Al atom in tetrahedral environment. The strength of the acidity of the OH species of ASA could be modified both by the location of the vicinal Al atom on the surface or in the bulk and by the number of aluminum atoms in the vicinity of silanol group. Cogelification of high silica-containing ASA appears as the best mean to prepare homogeneous amorphous aluminosilicate, which exhibits the strongest Bronsted acidity.

Nature, structure and strength of the acidic sites of amorphous silica alumina: an IR and NMR study.

Structure and activity of fluorinated alumina. 1. Determination of the number of protonic sites by an infrared study of adsorbed pyridines

Laser-Raman spectroscopy of the alumina-supported rhenium oxide metathesis catalyst

Structure and activity of fluorinated alumina. 2. Nature of the active site for 2-methylpropene oligomerization

The binding energies of the F(1s) and Al(2p) electrons in fluorine containing alumina catalysts are reported. The X-ray photoelectron peaks confirm the formation of a bulk phase of AlF3 at higher fluorine contents, whereas the fluorine signals at lower percentages originate from surface groups which are substituted by fluorine. By comparing X-ray photoelectron spectroscopy with X-ray diffraction we conclude that the combination of these techniques is a potential tool in detecting crystals on a catalyst surface which have a size under the detection limit of X-ray diffraction.

X-ray photoelectron (ESCA) spectra of some fluorine containing aluminas

We have studied the nature of the interaction between carrier and promoter for a number of catalysts. It is concluded that surface compounds are formed as well on silica as on γ-alumina. The interaction of alumina with the promoter is stronger, resulting in better dispersion, while on silica the formation of crystallites is observed at low promoter content.

F.P.J.M. Kerkhof

Papers

Structure and activity of fluorinated alumina. 1. Determination of the number of protonic sites by an infrared study of adsorbed pyridines

Laser-Raman spectroscopy of the alumina-supported rhenium oxide metathesis catalyst

Structure and activity of fluorinated alumina. 2. Nature of the active site for 2-methylpropene oligomerization

X-ray photoelectron (ESCA) spectra of some fluorine containing aluminas

Dispersion and Compound Formation in Some Metathesis Catalysts and Fluorine Containing Alumina's, Studied by XPS and Laser-Raman Spectroscopy