Olaf Timpe

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.

Nanosizing Intermetallic Compounds Onto Carbon Nanotubes: Active and Selective Hydrogenation Catalysts**

Therefore, nanosizing andsupporting the annealed metal products remain challenges.Another difficulty is in directly preparing supportedcatalysts while simultaneously obtaining good crystallite sizecontrol. A good catalyst support should be capable ofinhibiting sintering and loss of the catalyst during reaction.Fabrication of supported intermetallics catalysts in nanoscaledimensionsrequiresareliablemethodthatfacilitatesnotonlysize control but a thermally stable phase under reactionconditions. Since the work of Iijima in 1991,

Surface chemistry of carbon: activation of molecular oxygen

Three different industrial carbon blacks were characterized in their surface chemistry with respect to their relative abilities to activate molecular oxygen. A variety of techniques was used including x-ray diffraction, gasification experiments, electron spectroscopy as XPS and UPS, helium ion scattering spectroscopy, thermal desorption spectroscopy and the catalytic oxidation of aqueous SO2 as a “chemical probe” for activated oxygen. The activation properties for molecular oxygen at high temperatures were probed by analysing the gasification characteristics in 5 vol % oxygen-inert gas mixtures; at low temperatures the activity in the oxidation of SO2 to sulfuric acid with molecular oxygen was taken as indicator. For this reaction the carbons had to be activated by ammonia treatment at elevated temperatures. Thermal desorption spectroscopy and valence band photoemission provided data for the identification of reaction intermediates in the activation process of oxygen. The important role of surface chemica...

In situ analysis of metal-oxide systems used for selective oxidation catalysis: how essential is chemical complexity?

The mode of operation of selective oxidation reactions is described by a series of chemical rules defining the catalyst and some reaction intermediates. In contrast to catalytic processes over metallic elements, little is known, however, about the atomistic details of selective oxidation. In particular, the participation of the subsurface region of the catalyst in the kinetically relevant elementary steps (Mars–van Krevelen mechanism) is not positively verified. Using in situ X-ray absorption techniques to study binary and ternary molybdenum oxides the present contribution shows that it is possible to tackle some of the problems in selective oxidation by direct experimental observation. The modification of the Mo–O local bonding interaction upon thermal reduction of MoO3to MoO3-xis illustrated. This was also found for mixed Mo–V oxides in which the chemical state of the vanadium seemed unaffected by the reaction but the surface Mo : V ratio varied substantially with the gas phase composition. It is further shown that the solid-state phase transformation between reduced and oxidised forms of molybdenum oxides occur so rapidly, that possibly relevant suboxide cannot be identified by ex situ phase analysis. Observation of the time-law of redox transformations showed that lattice oxygen is only available for selective oxidation if the associated solid-state transformation occurs in the kinetic regime of reaction control and not in that of diffusion control.

The structure of molybdenum-heteropoly acids under conditions of gas-phase selective oxidation catalysis: a multi-method in situ study

Abstract The present study focuses on the evidence about the existence of Keggin ions under various reactive conditions. The stability of the hydrated parent heteropoly acid (HPA) phases is probed in water, by thermal methods in the gas phase, by in situ X-ray diffraction and in situ EXAFS. An extensive analysis of the in situ optical spectra as UV-Vis-near-IR (NIR) in diffuse reflectance yields detailed information about the activated species that are clearly different from Keggin ions but are also clearly no fragments of binary oxides in crystalline or amorphous form. Infrared spectroscopy with CO as probe molecule is used to investigate active sites for their acidity. Besides OH groups evidence for electron-rich Lewis acid sites was found in activated HPA. All information fit into a picture of a metastable defective polyoxometallate anion that is oligomerised to prevent crystallisation of binary oxides as the true nature of the “active HPA” catalyst. The as-synthesized HPA crystal is thus a precatalyst and the precursor oxide mixture is the final deactivated state of the catalyst.

Structural and chemical characterization of N-doped nanocarbons

Abstract Fullerene blacks with 0.7–4.5 at% of nitrogen incorporated into their carbon framework were produced by feeding N2 gas into the center of a carbon arc through a hollow graphite electrode. The formation of toluene-soluble fullerenes is effectively suppressed under these conditions. Optical emission spectroscopy of the arc plasma revealed the presence of N atoms and CN radicals. Chemical composition and stability of the N-doped fullerene blacks were examined and compared to their N-free counterparts produced in pure helium atmosphere by X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy and temperature-programmed oxidation with mass-spectrometric detection of the product gases. The nitrogen content was not diminished when N-doped fullerene black was heated to 500 °C in an in situ XPS experiment. Graphitic onions could be formed from n-doped fullerene black by intense electron irradiation in the electron microscope without loss of the nitrogen content. Structural and morphological properties were studied by powder X-ray diffraction, high-resolution transmission electron microscopy and the determination of BET (Brunauen Emme Teller) specific surface areas. The performance of N-doped and undoped fullerene blacks in the removal of ionic contaminants from water was examined in a dynamic adsorption scheme using diammonium chromate(VI) as a test compound.

Catalytic methanol oxidation over copper: observation of reaction-induced nanoscale restructuring by means of in situ time-resolved X-ray absorption spectroscopy

The catalytically active copper phase for the partial oxidation of methanol is studied by means of time-resolved extended X-ray absorption fine structure (EXAFS) spectroscopy combined with the detection of the catalytic turnover. It is found that the active form of the copper is a strained nanocrystalline form of the metal. The metal is no longer made up from large crystallites but contains a defect structure in which oxygen is already intercalated.

Stacking sequence and interlayer coupling in few-layer graphene revealed by in situ imaging

In the transition from graphene to graphite, the addition of each individual graphene layer modifies the electronic structure and produces a different material with unique properties. Controlled growth of few-layer graphene is therefore of fundamental interest and will provide access to materials with engineered electronic structure. Here we combine isothermal growth and etching experiments with in situ scanning electron microscopy to reveal the stacking sequence and interlayer coupling strength in few-layer graphene. The observed layer-dependent etching rates reveal the relative strength of the graphene–graphene and graphene–substrate interaction and the resulting mode of adlayer growth. Scanning tunnelling microscopy and density functional theory calculations confirm a strong coupling between graphene edge atoms and platinum. Simulated etching confirms that etching can be viewed as reversed growth. This work demonstrates that real-time imaging under controlled atmosphere is a powerful method for designing synthesis protocols for sp2 carbon nanostructures in between graphene and graphite.

On the CO-oxidation over oxygenated Ruthenium

Abstract The oxidation of carbon monoxide over polycrystalline ruthenium dioxide (RuO2) powder was studied in a packed-bed reactor and by bulk and surface analytical methods. Activity data were correlated with bulk phases in an in-situ X-ray diffraction (XRD) setup at atmospheric pressure. Ruthenium dioxide was pre-calcined in pure oxygen at 1073 K. At this stage RuO2 is completely inactive in the oxidation of CO. After a long induction period in the feed at 503 K RuO2 becomes active with 100% conversion, while in-situ XRD reveals no changes in the RuO2 diffraction pattern. At this stage selective roughening of apical RuO2 facets was observed by scanning electron microscopy (SEM). Seldom also single lateral facets are roughened. EDX indicated higher oxygen content in the following order: flat lateral facets > rough lateral facets > rough apical facets. Further, experiments in the packed bed reactor indicated oscillations in the CO2 formation rate. At even higher temperatures in reducing feed (533–543 K) the sample reduces to ruthenium metal according to XRD. The reduced particles exhibiting lower ignition temperature are very rough with cracks and deep star-shaped holes. An Arrhenius plot of the CO2 formation rate below the ignition temperature reveals the reduced samples to be significantly more active based on mass unit and shows lower apparent activation energy than the activated oxidized sample. Micro-spot X-ray photoelectron spectroscopy (XPS) and XPS microscopy experiments were carried out on a Ru(0001) single crystal exposed to oxygen at different temperature. Although low energy electron diffraction (LEED) images show a strong 1×1 pattern, the XPS data indicated a wide lateral inhomogeneity with different degree of oxygen dissolved in the subsurface layers. All these and the literature data are discussed in the context of different active states and transport issues, and the metastable nature of a phase mixture under conditions of high catalytic activity.

The Origin of the Positive Effect of Cadmium Acetate on the Action of Supported Palladium Catalysts

An unusually strong promoter effect on Pd catalysts is obtained from the complex [CdPd(CH3COO)4⋅CH3COOH]2 (depicted on the right), which could be isolated from a solution of Pd(OAc)2 and Cd(OAc)2 in acetic acid. In the total oxidation of ethylene, supported catalysts prepared from this precursor complex exhibited a higher catalytic activity than catalysts prepared from binary palladium acetate.