Isolated Vanadium Surface Complexes on Aluminum Fluoride ‐ A Model for the Relevance of Oxygen Atoms of Aluminum Oxide Supports in Catalytic Oxidation Reactions

Abstract

The redox properties and reactivity of oxidation catalysts of isolated mononuclear oxovanadium complexes supported on aluminum fluoride, AlF3, and on aluminum oxide, Al2O3, were compared. AlF3 is an interesting model for a non‐oxide support to study the role of oxygen atoms of nonreducible oxide supports during catalytic oxidation with supported vanadium oxide catalysts. Solid‐state 1H NMR indicate the presence of reactive FH and OH groups on the surface of AlF3 and the presence of reactive OH groups on the surface of Al2O3. Oxovanadium(V) triisopropoxide, VO(OiPr)3, was grafted onto the surfaces of both supports. Immobilization led to mononuclear vanadium complexes with oxidation states of +V (major) and +IV (minor). In contrast to alumina, the vanadium surface species were neither reducible nor oxidizable on AlF3. Redox cycles were studied by electron paramagnetic resonance of vanadium(IV). In situ IR spectroscopic investigations showed high and comparable initial stoichiometric reactivity to propane for both catalysts. Reactions on AlF3 stopped after a short initial period as a result of the absence of surface oxygen of the support. The results indicate the relevance of surface oxygen of the alumina support in oxidation reactions. In contrast to vanadium on alumina, VOx/AlF3 is catalytically inactive in the oxidative dehydrogenation of propane. Only after partial oxidation of AlF3 to Al2O3 above 500 °C was catalytic activity observed. The investigations support a recent report contradicting common oxygen‐exchange models, which assume the involvement of only surface vanadium oxide layers, and thus it appears as though a more complex process is in operation.

Lewis Acid Sites Stabilized Nickel Catalysts for Dry (CO2) Reforming of Methane

“…Although AlF 3 is not clearly observed in the XRD pattern, the presence of AlF 3 on the surface of KF‐modified Al 2 O 3 is proven by high‐resolution (HR) TEM images and X‐ray photoelectron spectroscopy (XPS) analysis below. The apparent absence in the XRD pattern probably owes to the low concentration of AlF 3 , because part of AlF 3 may be oxidized and converted back to Al 2 O 3 after calcination in air at high temperature . Further increasing the amount of KF up to 10 wt.% resulted in the strong peaks of K 12.5 Al 21.83 O 38 at the expense of the Al 6 KO 9.5 phase.…”

Single-Site VOx Moieties Generated on Silica by Surface Organometallic Chemistry: A Way To Enhance the Catalytic Activity in the Oxidative Dehydrogenation of Propane

“…Therefore, a deeper insight on the structure of catalysts 3 and IWI-0.98 is required. Although some authors have proposed the exclusive existence of isolated terminal vanadyl complexes on the surface of dehydrated silica up to the monolayer coverage, ,, it is generally accepted that vanadium catalysts prepared by traditional preparation techniques present a mixture of isolated vanadyls and bidimensional polymeric vanadia species. ,,,− ,, This has been proposed even for V densities in the 0.6–0.7 V/nm 2 range. , Systematic attempts have been directed at the individuation of such polymeric structures by Raman and UV–vis spectroscopy, , but the general applicability of these methods is still a matter of debate. ,− , Therefore, in spite of the very similar spectroscopic features revealed in the 51 V MAS NMR, Raman, and UV–vis spectra of catalyst 3 and of IWI-0.98, the application of these techniques might not be adequate to estimate the isolation or polymerization degree of vanadia in these catalysts and their pristine structures could differ. Remarkably, in a recent study on the grafting of VOCl 3 on dehydroxylated silica surfaces, a comparable XAS analysis as the one presented here, was able to clearly identify polymeric structures for a V density of ca.…”

“…Opposing views exist on the catalytic relevance of the isolated/polymeric vanadia species. On the basis of the comparison of the observed catalytic turnover frequencies (TOF), a number of authors have inferred the lack of a specific influence of the degree of polymerization of the VO x species on catalyst activity. , In many of these cases, the choice of the support has been regarded as the key feature able to influence the catalytic activity by tuning the electronic properties of the vanadium center. ,, Using silica as a support, high vanadia dispersion has been often observed to play a crucial role in order to achieve higher catalytic activity and selectivity . According to these observations, vanadyl and V–O-support bonds should play the major role in this kind of catalysis.…”

Catalytic Effect of Potassium Compounds in Soot Oxidation

“…Temperatures were monitored by using an ES DVT2 temperature controller (Jeol Ltd., Japan) with a calibrated thermocouple. A standard consisting of Mn 2+ ( I =5/2) embedded in MgO was used to determine g values (fourth low field line: g =1.981) . The measured EPR spectra were quantified by numerical double integration of the peak areas and normalized with respect to the sample masses.…”

“…Soot samples of 3-4 mg were sampled thermophoretically and transferred to EPR tubes (Wilmad-Labglass, SP Industries, Inc.,U SA). EPR spectra were recorded by using aJ ES-RE2X (Jeol Ltd.,J apan) at Xband frequency at RT with slightly different measurement conditions than described by Haeßner et al [102] (microwave frequency: 9.26 GHz;m icrowave power:5mW;m odulation amplitude:0 .4 mT; sweep time:4 min;s weep width:3 0mT; time constant:0 .1 s; modulation frequency:1 00 kHz). Microwave frequencies were measured by using aR 5372 microwave frequency counter (Advantest Corp.,J apan).…”