K. Lázár

Spectroscopic and catalytic study on metal carbonyl clusters supported on Cab-O-Sil. I. Impregnation and decomposition of Fe3(CO)12

Impregnation of Fe3(CO)12 on Cab-O-Sil has been studied by ir spectroscopy, Mossbauer spectroscopy, and mass spectrometry. Isotope exchange between CO ligands of the impregnated sample and gas phase CO molecules was also investigated. On impregnation, two types of interaction can be distinguished: (i) interaction of the type FeCO…HOSi and CO…HOSi, shown by the shift to lower and higher ir frequencies for bridged and for terminal CO, respectively, and (ii) interaction between the metal framework and the support revealed in the oxidation of iron to form very small iron oxide particles. On impregnation a small amount of CO is evolved as a result of the interaction. CO exchange occurs faster with alumina-supported clusters than with silica-supported samples. On decomposition up to 370 K, the metal framework is retained and the cluster structure can be partly restored in a CO atmosphere. Above 420 K, Fe3(CO)12 is decomposed to form Fe2+ oxide on the surface. A possible mechanism for impregnation is discussed in terms of electron donation from the support oxygen to the iron d-bands as a result of which the metal-carbon bond strength is influenced. On decomposition the metallic iron interacts with the support OH groups causing oxidation and Fe2+ formation.

Characterisation and activity in n-hexane rearrangement reactions of metallic phases on Pt–Sn/Al2O3 catalysts of different preparations

The platinum–tin interactions in Pt–Sn/Al2O3 catalysts were followed through several characterisation methods and modified by using two preparation procedures (1.5 wt% Pt, Sn:Pt=1:1): conventional coimpregnation with H2PtCl6 and SnCl4 (T sample) or by use of the bimetallic precursor [Pt(NH3)4]SnCl6, which was synthesised in the support porosity (N sample). The effects of these interactions on catalytic properties were displayed by the activity and selectivity in n-hexane rearrangement reactions. For both samples platinum and tin are reduced, but they have very different platinum dispersions which are related to different temperature-programmed reduction profiles: 52% for sample T and 4% for sample N. Insitu tin Mossbauer spectroscopy confirms that the majority of tin is reduced, and a minority remains as SnII; air treatment leads to a partial reoxidation of SnII to SnIV, sample N retaining more tin as alloy. X-Ray diffraction displays the simultaneous presence of PtSn, Pt3Sn and Pt with more alloys on sample N; the co-impregnated sample, which has a greater platinum phase, shows a better dispersion of tin (XPS data), in accordance with a high interaction with alumina. The catalytic activity was controlled by the platinum phase; for sample T, the influence of the addition of tin is restricted, whereas the catalyst prepared from the bimetallic precursor exhibits particular properties, attributable to the stabilisation of platinum in smaller ensembles, and the modifying effect of tin was clearly evidenced. The catalytic properties are explained by the distribution and morphology of Pt ensembles present on various faces of Pt–Sn alloys. The lower amount of alloys in sample T can be related to a higher initial activity in C5 ring closure whereas the higher amount of these phases on catalyst N is in accord with a higher turnover frequency, and a good selectivity for the formation of olefins which are transformed into C6 saturated skeletal isomers in longer runs. The results are supplemented by thermodynamic data on the reduction of tin oxides and by the geometric properties of the low-index faces of PtSn and Pt3Sn alloys.

Double promotion of palladium/silica catalysts by iron and magnesium oxide in the synthesis of methanol from carbon monoxide and hydrogen

Abstract The structure and catalytic properties of 2 wt.-% Pd/SiO2 catalysts promoted with 2 wt.-% MgO and 0.2-5 wt.- % iron have been investigated. Catalysts were characterized by TPR, hydrogen chemisorption, and Mossbauer spectroscopy and their catalytic performance in methanol formation was also measured. At 16 at.-% iron a sharp maximum was observed in the activity which can be interpreted as being due to simultaneous promotion with iron and MgO. Independent mechanisms of promotion were found for the two modifiers: iron influenced the metallic component by forming bimetallic PdFe particles while MgO changed the properties of silica in a favourable way.

In situ Mössbauer study of framework substituted (Fe)ZSM-5 zeolites

The coordination states, stability, and reducibility of isomorphically substituted framework Fe(III) were studied in ZSM-5 zeolites (Si/Fe = 22) by in situ Mossbauer spectroscopy. Supplementary methods as X-ray diffraction and temperature-programmed reduction were used as well, and the catalytic activity of (Fe)ZSM-5 was tested in CO hydrogenation. The effects of exchange of charge neutralizing protons for Li + were also studied. The measurements revealed that the symmetry of coordination of the framework Fe(III) depends considerably on the nature of charge compensation. It can be suggested that Bronsted acidic bridged hydrogen induces greater asymmetry around the framework Fe(III) than that generated by cationic (e.g., Li + H 3 O + , or PrNH 3 + ) compensation. The framework Fe(III) withstands hydrogen reduction and CO + H 2 treatment at 570 K, but a part of these substituted ions can be reduced to Fe 2+ state at 670 K. Li + ion exchange decreases the reducibility of the framework Fe(III).

Surface characterization and catalytic CO + H2 reaction on Fe82.2B17.8 amorphous alloy

Abstract Fe82.2B17.8 amorphous ribbon has been used as a catalyst for the Fischer-Tropsch-type reaction of CO+H2. Specific activity has been found to be at least an order of magnitude higher than that of either the crystallized ribbon of identical composition or the supported iron catalyst. Before and after the catalytic tests the ribbons were characterized by XRD, XPS, UPS and Mossbauer spectroscopy in transmission and in conversion electron modes. Conversion electron Mossbauer spectroscopy and UPS proved that the surface of the amorphous ribbons is being partially crystallized during 8000 min reaction time at a maximum reaction temperature of 560 K. The superior catalytic activity has been explained by stabilization of the small iron particles and Fe2O3 by boron atoms at the surface and by suppressed carbide formation.

Isomorphously substituted Fe-ZSM-5 zeolites as catalysts: Causes of catalyst ageing as revealed by X-band EPR, Mössbauer and 29Si MAS NMR spectra

Abstract An unconstrained curve fitting/parameter estimation program adapted to PC was applied for the deconvolution of X-band EPR spectra of Fe(III) in ZSM-5 (MFI) zeolites. The sub-spectra of framework (FW) and extra-framework (EFW) Fe(III) ions sited in environments of different (ligand) symmetry could be identified. The EPR transition probability, r , of Fe(III) in the little oxide clusters (diads, triads) was 13.75 times larger than that incorporated into the lattice. Increase of the cluster size and ordering of the random structure led to reduction of r . When the co-ordination of Fe(III) ions in the oxide clusters approached the cubic symmetry of Fe(III) in the lattice, r converged to 1.0. The state: r =1.0 is equivalent to the development of a crystalline phase. The experimental determination of r revealed important details of catalyst action. The initial activity in the direct oxidation of benzene to phenol (oxidant: N 2 O) is due to oxide clusters of random structure (known as “ferrihydrite”). Partial and eventually (almost) complete loss of activity occurs when the clusters get ordered to hematite (or in reducing atmosphere to magnetite, as well). Contrary to fully inactive hematite, magnetite retains about 50% of the original activity because it exposes [Fe(III)] Th –O–[FeIII)] Oh linkages supposed to be responsible for oxidation activity.

Spectroscopic and catalytic studies on metal carbonyl clusters supported on Cab-O-Sil. II. Impregnation and decomposition of Ru3(CO)12 and the mixture of Ru3(CO)12 and Fe3(CO)12

Abstract The behavior of Ru 3 (CO) 12 (I), H 2 Ru 3 Fe(CO) 12 (II), a 1:1 Ru 3 (CO) 12 and Fe 3 (CO) 12 mixture (III), RuFe 2 (CO) 12 (IV), and Fe 3 (CO) 12 (V) deposited on Cab-O-Sil HS-5 has been compared. (III) and (V) have been studied by Mossbauer spectroscopy and by ir-spectroscopy, and (I)-(V) by temperatureprogrammed decomposition (TPDC) and temperature-programmed reduction (TPR). Decomposition, which is faster in hydrogen than in helium or in vacuum, and is reversible below 400 K, is normally faster for (V) than for (I). At low temperature, CO ligands leave the metal carbonyl cluster (MCC) in one step for (V), whereas they are decomposed stepwise via the formation of subcarbonyl species for (I). In this range the formation of Ru 3 (CO) 3 species has been verified. On decomposition of (V), there is some CO adsorption, as indicated by ir spectroscopy and low catalytic activity. This increases when decomposition occurs in helium, and is attributed to the smaller particles stabilized by the metal-carbon species, formed from CO during the decomposition. For (I), decomposition results in a slight oxidation, indicated by weak ir bands in the range of 2100–2140 cm −1 . Interaction between Fe and Ru in (III) does not occur in the impregnated phase, but develops during the decomposition, which starts with Fe 3 (CO) 12 decomposition and thereby influences the decomposition of Ru 3 (CO) 12 . However, reduction of iron is also facilitated by the presence of ruthenium, as indicated by Mossbauer spectroscopy. The general feature revealed during decomposition in helium, i.e., the increase of surface carbon, is also operative here, and thus the dispersion of the metal is higher than for decomposition in hydrogen. The mechanism of the decomposition is discussed in terms of the formation of subcarbonyl species for Ru-containing samples and the formation of surface carbon is also considered. The mechanism and possible reaction pathways are given.

Direct evidence for the correlation between surface carbon and CO + H2 selectivity on iron and iron-Ruthenium catalysts prepared from metal carbonyl clusters

Abstract In situ Mossbauer studies and simultaneous catalytic reactions of CO + H2 mixtures at 1 and 20 bars pressure to form olefins were carried out on Fe and Fe—Ru bimetallic catalysts prepared by impregnation of Cab-O-Sil® with a hexane solution of Fe3(CO)12 and a mixture of Fe3(CO)12 and Ru3(CO)12. Under reaction conditions the main component of the Mossbauer spectra(I.S. = 0.0 mm s−1 and Q.S. = 0.5 mm s–) can be assigned to the reacting species of surface carbon, which can be distinguished from χ-carbide which has also formed during the reaction. In the presence of ruthenium, the spectra are very similar to those measured on iron, but a large χ-carbide signal with hyperfine splitting cannot be detected. From the kinetic and in situ Mossbauer experiments carried out under different conditions, direct evidence was found for the formation of mobile, reactive carbon which participates in the production of olefins and higher hydrocarbons. It can be transformed either into other types of carbon, one of which is responsible for methane formation, or into that species responsible for catalyst deactivation. A possible mechanism is discussed in this paper.

Attempts to produce uniform Fe(III) siting in various Fe-content ZSM-5 zeolites: Determination of framework/extra-framework ratio of Fe(III) in zeolites by EPR and Mössbauer spectroscopy

Abstract Fe(III)-content ZSM-5 catalysts play important role in selective redox reactions, like biomimetic oxidations, or selective catalytic reactions (SCR) of pollutants, dangerous for the environment. There is a dispute in the literature whether the catalytic activity of these materials is due to isomorphously substituting framework (FW) iron, or extra-framework (EFW) oxide/hydroxide material of high dispersity, not incorporated into the lattice during the synthesis, or became ejected from the framework following the post-synthesis treatments (calcination, heat-treatment). In either case it is a must to know the FW/EFW ratio in order to correlate the catalytic activity with the number of active sites, or active surface area of the catalyst ingredient. In the case of EFW iron only (introduced, e.g. by ion-exchange) the chemical identity, particle size, location and thereby the probability of attainment from the fluid phase(s) are uncertain. Isomorphic substitution is more complicated, because in addition to these even the FW/EFW ratio is questionable. From both practical and academic interest it would be advantageous (e.g. from the point of view of above correlation) to have uniform FW, or EFW Fe(III) siting, or when it is mixed, to know the FW/EFW ratio. The authors approach this problem from several directions: they elaborate various synthesis methods to minimise EFW iron; extract unwanted EFW iron by reducing/complexing agents (SO 2 , hydroxylamine), or, if necessary, reintroduce EFW iron by (liquid and solid) ion-exchange and monitor the changes thus produced in the FW and EFW iron contents by X-band EPR and Mossbauer spectroscopy. Using mathematical methods and sophisticated analysis of the sub-spectra (intensity dependence on the iron content and temperature), the EPR spectrum-deconvolution made possible to reinterpret the X-band EPR Fe(III) spectra in weak ligand-field media, like zeolites. If the total iron contents of samples are known (e.g. from XRF spectra) the FW/EFW ratio can be computed on the basis of the deconvoluted EPR spectra alone. These values are in very good agreement with similar estimates obtained from the Mossbauer spectra, proving that the Mossbauer “loudness” for the various iron components is nearly equal.

Oxygen transfer centers in Fe-FER and Fe-MFI zeolites: redox behavior and Debye temperature derived from in situ Mössbauer spectra

Abstract77 and 300 K in situ Mössbauer spectra recorded after redox treatments of Fe-FER and Fe-MFI ferrisilicates are compared. Similar behavior is found for the two catalysts; reversible redox processes can be detected. Oxidation and coordination states are identified after various treatments, and the value of the average Debye temperature is estimated (260 < ΘD < 300 K). Analysis and comparison of data support the assignment of redox centers to Feframework–O–Feextra-framework pairs.