Characterization of Iron Promoter in Tungstated Zirconia Catalysts by Mössbauer Spectroscopy at Very Low Temperatures

Abstract

A tungstated zirconia (WZ) catalyst with iron promoter used for the conversion of n-pentane into isopentane has been characterized by Mössbauer spectroscopy. The Mössbauer spectra have been recorded in zero magnetic field in the temperature range 0.05-295 K and with a magnetic field up to 7 T between 4.2 and 50 K. Both the recording of Mössbauer spectra with an applied magnetic field and at extremely low temperature allowed for the demonstration that iron is present in the catalysts as (i) hematite (alpha-Fe2O3) particles a few 10 nm in size, (ii) very small oligomeric Fe(III) species, probably in solid solution in zirconia, and (iii) Fe(III) oxide clusters showing magnetic ordering, probably embedded in the first surface layer and thus forming "rafts". These latter clusters form two ensembles with quite different sizes: one with diameters of about 3 nm, the other with diameters larger than 30 nm. These results are in agreement with those recently obtained by X-ray absorption spectroscopy and electron paramagnetic resonance.

On the Synergetic Catalytic Effect in Heterogeneous Nanocomposite Catalysts

“…As can be found in the literature and also the present review, the most prevailing synergetic catalytic effects can be attributed to type I: One component activation by the other. The activation of components can be achived either by doping heterogeneous ions or valence change of one component (Figures 9,10,12,15,and 17), by promoted/sustained electrohole separation (Figures 19 and 21), or by metal particle charging via the electron donation from the oxide components (Figures 13 and 19). In spite of all the differences in the synergetic processes, typically a charge transfer process between the components is usually included in type I of the synergetic effect.…”

“…To achieve more satisfactory catalytic performance for enhanced activity and selectivity and reduced environmentally-unfriendly side effects, multicomponent composite catalysts are the natural choices. − In fact, bicomponent or multicomponent composite catalysts have attracted great attention recently in the heterogeneous catalysis field. These heterogeneous composite catalysts are generally composed of one or more catalytically active components and a functional support, in which the interaction between the catalytic components and the support materials can possibly endow the composite catalysts with much improved catalytic properties, such as significantly enhanced catalytic activity, selectivity for target product(s), chemical stability, and prolonged lifetime.…”

Study of the surface chemistry and morphology of single walled carbon nanotube–magnetite composites

“…Mössbauer spectrometry (MS) is an important research method for materials characterization [13][14][15][16][17][18][19][20][21][22]. However, it has not been, in comparison to other methods, extensively applied in the study of the surface chemistry of materials.…”

“…This, comparatively, minor use of this method in catalysis studies is due to the fact that it application relevance is limited to phases containing those elements that exhibits the Mössbauer effect. In this regard, studies of catalytically active phases containing: iron [12][13][14][18][19][20][21], gold [17], ruthenium and tin [15] in its framework had been studied by Mössbauer spectrometry. It is accepted that correctly interpreted Mössbauer spectra (MS) offers very useful information on: phase identification, determination of the oxidation state, structural information, particle size, kinetics of bulk transformation and lattice vibrations [12,16].…”

Mössbauer Spectroscopy in Heterogeneous Catalysis

“…With the development of nanoscaled catalysts, several recent studies have pointed out the great interest of recording Mo¨ssbauer spectra below 4.2 K up to 0.055 K (100,203,204); it allowed the identification at the surface of various support such as mesoporous silica or ZrO 2 , the formation of nanometric iron oxide clusters; this identification was not possible in classical low temperature studies conducted above 4.2 K, which concluded to the presence of larger particles. The analysis by Mo¨ssbauer spectroscopy performed at lower temperature enabled to show that these larger particles were agglomerates of nanometric iron clusters and allowed to reach another level of resolution of ferric particles structures (204).…”