Gerhard Mestl

The Catalytic Use of Onion-like Carbon Materials for Styrene Synthesis by Oxidative Dehydrogenation of Ethylbenzene

-hybridizednanostructuredcarbonhasreceivedincreasingattention both from a fundamental point of view and forpotential applications. A large variety of new fullerene-related materials (giant fullerenes, nanotubes, nanospheres,nanocones, nanofolders, nanobundles, onion-like carbons(OLCs)) have been synthesized.

Carbon Nanofilaments in Heterogeneous Catalysis: An Industrial Application for New Carbon Materials?

Special carbon! Carbon nanofilaments differ from graphite and soot catalysts in their high stability during the oxidative dehydrogenation of ethylbenzene to styrene. The high yields of styrene achieved suggest that a first industrial application of carbon nanofilaments in catalysis is possible.

High temperature partial oxidation reactions over silver catalysts

The exceptional catalytic activity of silver for a number of partial oxidation reactions has been known for nearly a century. Despite the wid espread use of silver in heterogene ous catalysis, there still remain unresolved questions about the mechanistic details of reaction. The ethylene epoxidation and formaldehyde synthesis reactions are the two industrially-relevant reactions which have received, by far, the most attention. The importance of these reactions cannot be underestimated. Both ethylene epoxide and formaldehyde serve as primary chemicals for a wide variety of materials which find use in an enormous number of products. There is, therefore, a great scientific and economi c motivation for understanding th

Thermally and Chemically Induced Structural Transformations of Keggin-Type Heteropoly Acid Catalysts

Abstract Raman characterization revealed that the Keggin anion structure of H 4 PVMo 11 O 40 is inherently unstable upon heat treatment and loss of water. Vanadyl and molybdenyl species are expelled from the Keggin cage and defective Keggin structures are formed. These defective structures further disintegrate to presumably Mo 3 O 13 triads of the former Keggin. These Keggin fragments oligomerize at later stages to molybdenum oxygen clusters comparable to hepta- or octamolybdates. The final disintegration and structural reorganization product is MoO 3 . This disintegration and recondensation process seems to be strongly affected by the heating rate and hence the presence of water in the sample. Only partial expulsion of V occurred under moderate dehydration conditions. The absence of water during heat treatments stabilizes the intermediate defective structures. Raman spectroscopy proved that free polyacids are unstable under catalytic partial oxidation conditions. Therefore, it can be suggested that intact Keggin anions are not the active species within an operating partial oxidation catalyst. From this Raman spectroscopy study it may be inferred that the structurally reorganized intermediates are relevant for the catalytic action. The Raman investigations of the HPA decomposition additionally revealed a dependency of the decomposition process on the reactive atmosphere and the presence of Cs. The presence of Cs led to a partial stabilization of the structural disintegration products of PVMo 11 and to the formation of the thermodynamically stable, but catalytically inactive Cs 3 -salt. Cs also inhibited the condensation of MoO 3 -type oxides. O 2 present in the gas phase also led to stabilization of the structural reorganization intermediates. Importantly, the presence of water did not lead to a stabilization of the intact Keggin structure. In contrast, hydrolysis of the Keggin anions seemed to be enhanced compared to the water-free situation. This observation is of high importance because water is added to the feed in industrial partial oxidation reactions. Hence, under industrial conditions, HPA-derived catalysts are inherently unstable and cannot contain intact Keggin anions at their active surface. Catalytic partial oxidation conditions even led to a more pronounced structural reorganization and amorphous suboxides of the MoO 3− x type seemed to be formed. Hence, heteropolyacids have to be understood only as defined molecular precursor compound.

Large scale synthesis of carbon nanofibers by catalytic decomposition of ethane on nickel nanoclusters decorating carbon nanotubes

A large scale synthesis of carbon nanofibers with a controlled diameter of about 50 nm has been achieved at relatively low temperatures (550–650 °C) by the decomposition of ethane on a carbon nanotube supported nickel catalyst. The carbon nanofibers can be used as a catalyst or a catalyst support without subsequent purification, due to the use of carbon nanotubes as support, the high nanofiber yields, and the purity obtained.

Mixed molybdenum oxide based partial oxidation catalyst 2. Combined X-ray diffraction, electron microscopy and Raman investigation of the phase stability of (MoVW)5O14-type oxides

Thermal activation of a nanocrystalline Mo5O14-type Mo0.64V0.25W0.09Ox catalyst leads to enhanced catalytic performance in the partial oxidation of methanol, propylene and acrolein. This thermal activation process was invest igated by X-ray diffraction, transmission electron microscopy and Raman microspectroscopy. Ther mal activation of the nanocrystalline Mo0.64V0.25W0.09Ox precursor oxide in inert atmospheres induces partial crystallization of a Mo5O14-type oxide only in a narrow temperature range ending at 818 K. The Raman spectrum of the crystalline Mo5O14 oxide was identified by statistical analysis and by comparison with XRD and TEM results. The observed Raman bands in the M=O stretching mode regime were attributed to the different Mo=O bond distances in Mo 5O15. A fraction of the precursor oxide remains nanocrystalline after activation as shown by Raman spectroscopy. HRTEM identified amorphous surface layers on top crystalline cores. Above 818 K, the Mo5O14-type structure disproportionates into the stable phases MoO 2 and MoO3. This disproportionation occurs via an intermediate state which is formed by bundles of molybdenum oxide chains exhibiting structural order in only one dimension as shown by HRTEM. These results from the combined structural analysis suggest that the improvement of the catalytic performance of the MoVW oxide catalyst in the partial oxidation of methanol is related to the formation of the Mo5O14 type mixed oxide.

In situ Raman spectroscopy — a valuable tool to understand operating catalysts

Abstract Laser Raman spectroscopy (LRS) is one of the most powerful tools for the in situ study of catalytic materials and surfaces under working conditions. Raman characterizations can be carried out at temperatures as high as 1000 K and in controlled atmospheres. Modern high light-throughput spectrometers permit the recording of the whole spectral range from 100 to 4000 cm −1 at once and time resolutions in the subsecond regime for materials with high Raman cross-sections. Transient temperature or pressure response studies, e.g. pulse experiments with isotope labels, are thus possible, and kinetic and spectroscopic characteristics can be related. Modern quartz fiber optics render possible easy spectroscopic access to catalytic reactors of defined and well characterized operation conditions. Quantitative relation of real catalytic steady state operation, e.g. catalytic activity and selectivity, to changes in the catalyst structure is thus made possible. Several in situ LRS studies are discussed including the characterization of supported and unsupported Mo-based catalysts, confocal Raman microspectroscopy of mixed MoVW oxide catalysts, oxygen exchange in Sb 2 O 3 /MoO 3 oxide physical mixtures elucidating the catalytic synergy effects, and active surface intermediates during oxidative coupling of methane, and NO and N 2 O decomposition over Ba/MgO catalysts related to the catalytic reaction via transient pressure step experiments.

Molybdenum oxide based partial oxidation catalyst: 1. Thermally induced oxygen deficiency, elemental and structural heterogeneity and the relation to catalytic performance

Abstract A mixed oxide catalyst containing Mo, V, and W was used for the partial oxidation of methanol. The relation between the structure and the degree of reduction of this mixed oxide catalyst and its catalytic performance was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), Rutherford backscattering (RBS), XPS, ion scattering spectroscopy (ISS), UPS, X-ray diffraction (XRD) and Raman microspectroscopy. Thermal activation of the MoVW mixed oxide led to an increase in the methanol conversion by a factor of 3 and an increase in selectivity to formaldehyde from 66% to 80%. SEM–EDX characterization of the untreated catalyst revealed the presence of at least two different phases in the sample on a micron range, one with a high V concentration, and another with all three metals present. TEM–EDX showed a homogeneous element distribution in the submicron regime. The thermally activated mixed oxide revealed an inhomogeneous element distribution in the micron and submicron regime as shown by SEM–EDX and TEM–EDX. The activation led to a reduction of the bulk oxide as determined by RBS and its surface as shown by XPS, ISS, and UPS. The formation of Mo 4+ and V 4+ centers was detected at the catalyst surface upon thermal activation. XRD of the starting material showed the presence of nanocrystalline material which was identified as being a mixture of a majority of Mo 5 O 14 and a minority of MoO 3 -type MoVW oxides. Confocal Raman microspectroscopy confirmed the presence of two different components. The major component could be identified as amorphous Mo 5 O 14 -type MoVW mixed oxide. The second, minor component was similar to an amorphous MoO 3 -type MoVW oxide. XRD showed that the thermally activated mixed oxide consisted of a mixture of a majority of crystalline Mo 5 O 14 -type oxide and of small amounts of crystalline MoO 3 -type and MoO 2 -type oxides. The Raman spectrum of the Mo 5 O 14 -type phase could be identified by statistical data evaluation of 1000 spectra and by comparison with the XRD result. Raman microscopy confirmed the presence of a minority of MoO 3 - and MoO 2 -type oxide. The formation of Mo 5 O 14 -type oxide upon loss of oxygen is discussed with respect to the remarkable increase in the catalytic activity and selectivity.

Reaction of NO with carbonaceous materials: 2. Effect of oxygen on the reaction of NO with ashless carbon black

The reaction rate in the reaction of NO with an ashless carbon black was considerably enhanced in the presence of oxygen. A similar effect has also been observed in the reaction of NO with the oxidized carbon black. The creation of more active sites on the carbon surface in the reaction with oxygen and NO2 (produced by the reaction of NO with O2) is proposed to be responsible for this enhancement. A lower apparent activation energy was observed in the reaction of NO with the oxidized carbon black than with the unmodified carbon black. It was found that the adsorption of NO is restrained to some extent on the carbon black surface mostly covered by carbon–oxygen (C(O)) complexes. However, the adsorption of NO and the formation of products at ca. 40°C were enhanced only when C(O) complexes of higher thermal stability were left on the surface. Hence, the more thermally stable C(O) complexes are assumed to play an important role in the enhancement of the reaction rate by activating the neighboring carbon atoms.

Reaction of NO with carbonaceous materials: 1. reaction and adsorption of NO on ashless carbon black

Abstract The mechanism of the reaction of NO with ashless carbon black was studied in detail. The differences between the amounts of oxygen and nitrogen measured and the calculated concentrations of oxygen and nitrogen suggest the formation of surface carbon–oxygen complexes C(O) and carbon–nitrogen complexes C(N). The dependence of the specific rate of reaction at its initial stage on temperature is different from that at steady state, pointing to different rate-determining steps for the two reaction regimes. The decomposition of surface complexes is suggested to be the rate-determining step for the reaction under steady-state conditions. This explains very well the measured zero reaction order with respect to NO. NO is proposed to adsorb parallel to the carbon black surface. The dissociation of the N–O bond leads to the formation of C(O) and C(N) complexes which are heterogeneous in structure. The adsorbed NO molecules are not stable on the carbon black surface at temperatures above 300°C. The reaction of NO with carbon at higher temperatures results in the formation of more surface complexes with higher thermal stability. The surface C(O) complexes involved in the CO formation differ in structure from those involved in the CO 2 formation. The mechanism of the NO–carbon reaction at low temperatures appears to be different from that at high temperatures (>750°C).