Annette Trunschke

In Situ Generation of Active Sites in Olefin Metathesis

The depth of our understanding in catalysis is governed by the information we have about the number of active sites and their molecular structure. The nature of an active center on the surface of a working heterogeneous catalyst is, however, extremely difficult to identify and precise quantification of active species is generally missing. In metathesis of propene over dispersed molybdenum oxide supported on silica, only 1.5% of all Mo atoms in the catalyst are captured to form the active centers. Here we combine infrared spectroscopy in operando with microcalorimetry and reactivity studies using isotopic labeling to monitor catalyst formation. We show that the active Mo(VI)-alkylidene moieties are generated in situ by surface reaction of grafted molybdenum oxide precursor species with the substrate molecule itself gaining insight into the pathways limiting the number of active centers on the surface of a heterogeneous catalyst. The active site formation involves sequential steps requiring multiple catalyst functions: protonation of propene to surface Mo(VI)-isopropoxide species driven by surface Brønsted acid sites, subsequent oxidation of isopropoxide to acetone in the adsorbed state owing to the red-ox capability of molybdenum leaving naked Mo(IV) sites after desorption of acetone, and oxidative addition of another propene molecule yielding finally the active Mo(VI)-alkylidene species. This view is quite different from the one-step mechanism, which has been accepted in the community for three decades, however, fully consistent with the empirically recognized importance of acidity, reducibility, and strict dehydration of the catalyst. The knowledge acquired in the present work has been successfully implemented for catalyst improvement. Simple heat treatment after the initial propene adsorption doubled the catalytic activity by accelerating the oxidation and desorption-capturing steps, demonstrating the merit of knowledge-based strategies in heterogeneous catalysis. Molecular structure of active Mo(VI)-alkylidene sites derived from surface molybdena is discussed in the context of similarity to the highly active Schrock-type homogeneous catalysts.

First principles calculations of the structure and V L-edge X-ray absorption spectra of V2O5 using local pair natural orbital coupled cluster theory and spin-orbit coupled configuration interaction approaches

A detailed study of the electronic and geometric structure of V2O5 and its X-ray spectroscopic properties is presented. Cluster models of increasing size were constructed in order to represent the surface and the bulk environment of V2O5. The models were terminated with hydrogen atoms at the edges or embedded in a Madelung field. The structure and interlayer binding energies were studied with dispersion-corrected local, hybrid and double hybrid density functional theory as well as the local pair natural orbital coupled cluster method (LPNO-CCSD). Convergence of the results with respect to cluster size was achieved by extending the model to up to 20 vanadium centers. The O K-edge and the V L2,3-edge NEXAFS spectra of V2O5 were calculated on the basis of the newly developed Restricted Open shell Configuration Interaction with Singles (DFT-ROCIS) method. In this study the applicability of the method is extended to the field of solid-state catalysis. For the first time excellent agreement between theoretically predicted and experimentally measured vanadium L-edge NEXAFS spectra of V2O5 was achieved. At the same time the agreement between experimental and theoretical oxygen K-edge spectra is also excellent. Importantly, the intensity distribution between the oxygen K-edge and vanadium L-edge spectra is correctly reproduced, thus indicating that the covalency of the metal-ligand bonds is correctly described by the calculations. The origin of the spectral features is discussed in terms of the electronic structure using both quasi-atomic jj coupling and molecular LS coupling schemes. The effects of the bulk environment driven by weak interlayer interactions were also studied, demonstrating that large clusters are important in order to correctly calculate core level absorption spectra in solids.

Sites for Methane Activation on Lithium‐Doped Magnesium Oxide Surfaces

Density functional calculations yield energy barriers for H abstraction by oxygen radical sites in Li-doped MgO that are much smaller (12±6 kJ mol(-1)) than the barriers inferred from different experimental studies (80-160 kJ mol(-1)). This raises further doubts that the Li(+)O(˙-) site is the active site as postulated by Lunsford. From temperature-programmed oxidative coupling reactions of methane (OCM), we conclude that the same sites are responsible for the activation of CH4 on both Li-doped MgO and pure MgO catalysts. For a MgO catalyst prepared by sol-gel synthesis, the activity proved to be very different in the initial phase of the OCM reaction and in the steady state. This was accompanied by substantial morphological changes and restructuring of the terminations as transmission electron microscopy revealed. Further calculations on cluster models showed that CH4 binds heterolytically on Mg(2+)O(2-) sites at steps and corners, and that the homolytic release of methyl radicals into the gas phase will happen only in the presence of O2.

How Strain Affects the Reactivity of Surface Metal Oxide Catalysts

Highly dispersed molybdenum oxide supported on mesoporous silica SBA-15 has been prepared by anion exchange resulting in a series of catalysts with changing Mo densities (0.2-2.5 Mo atoms nm(-2) ). X-ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO4 units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temperature-programmed reduction and a steep increase in the catalytic activity observed in metathesis of propene and oxidative dehydrogenation of propane at 8 % of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calculations, NEXAFS spectra at the O-K-edge at high Mo loadings are explained by distorted MoO4 complexes. Limited availability of anchor silanol groups at high loadings forces the MoO4 groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.

The Potential of Microstructural Optimization in Metal/Oxide Catalysts: Higher Intrinsic Activity of Copper by Partial Embedding of Copper Nanoparticles

Optimal dispersion of the active metal usually is the main goal in preparation of supported metal catalysts. Whereas the oxide support often can be considered as inert, in many systems it is known to chemically affect the catalytic material under reaction conditions exceeding the simple role as a physical carrier of metal particles. Strong metal support interaction (SMSI) is an example, which was first detected for Pt/TiO2 [1] and was also found in Cu/ZnO. Cu/ZnO/(Al2O3) catalysts are active in industrial methanol synthesis and a Cu/ZnO synergy is discussed in literature, the nature of which is still under debate. However, it seems clear that synergetic effects should have their origin in the Cu–ZnO interface region. The catalytic properties are, thus, not only determined by the amount of Cu surface area (SCu) accessible to gas but also by the nature of the Cu–ZnO interface contacts. As a result of the latter effect, deviations from linearity of SCu–activity correlations can be observed. Usually, the value of SCu is regarded as the dominating effect and a catalyst of considerably lower SCu would be estimated to exhibit inferior activity compared to a high SCu sample of the same Cu loading. The potential of intrinsic or synergistic effects, which are responsible for different specific activities of Cu, is not easily determined because surface and interface area are interrelated by morphology and microstructure, which are difficult to control during preparation of applied Cu/ZnO/(Al2O3) systems. Today, the Cu/ZnO/Al2O3 system has been industrially employed for more than 40 years. During this period of time the preparation of successful catalysts has been extensively studied in academic and industrial laboratories and a considerable optimization regarding activity and stability could be achieved. The resulting technically relevant synthesis route is based on preparation of Cu, Zn basic carbonate precursors by coprecipitation, for example, from metal nitrate solutions and sodium carbonate as precipitating agent. The precipitate is aged, recovered, and calcined, which results in an intimate mixture of the oxides. Finally, the CuO component is reduced to obtain active catalysts with the typical atomic Cu/Zn ratio near 70:30 and a molar Al content of 10–20 %. Catalysts prepared according to this procedure usually possess high values of SCu and are highly active due to their efficient mesoand nanostructuring during synthesis. Whereas the optimization of Cu dispersion can be regarded as widely advanced in these systems, optimization of the Cu–oxide contact interface, that is, the microstructural arrangement of the components, presents further unexplored room for improvement. Herein, we present a novel type of Cu/Zn/Al2O3 catalyst that was prepared according to the technically applied synthesis route described above with two important modifications: 1) Instead of pure sodium carbonate solution and Cu, Zn, Al–nitrate solution, as used in conventional catalyst preparation, a combination of sodium aluminate and carbonate solution was used as precipitating agent for a Cu, Zn–nitrate solution during coprecipitation; 2) the ageing period was suppressed by realizing a continuous process with direct spray-drying of the fresh precipitate suspension. Structural and catalytic characteristics of this sample, denoted catalyst A, are compared to a conventionally prepared catalyst, referred to as catalyst B. The latter sample showed an activity similar to a commercial methanol synthesis catalyst, provided by S d-Chemie AG. As revealed by X-ray diffraction (XRD), the washed precipitate consisted of crystalline hydroxy carbonate phases for catalyst B, whereas the precursor of catalyst A was completely X-ray amorphous. The precursors were subjected to the same treatments during washing, calcination, and reduction. It is important to note that both samples exhibit exactly the same industrially relevant metal composition of Cu/Zn/Al =60:25:15 and, thus, experimental differences can be related to a different microstructure. Investigation of the microstructure of the reduced catalysts was carried out by means of high resolution transmission electron microscopy (HRTEM). Recently, a comprehensive characterization of the microstructure of a series of industrially relevant Cu/ZnO/Al2O3 samples was presented by some of the authors. These bulk catalysts were quite different from classical supported systems and Cu particles of an average diameter(<10 nm) were detected to be separated by small ZnO particles, which prevented them from sintering, and forming a porous framework of individual particles. This type of morphology was also found in catalyst B and is represented in Figure 1, right. Unlike catalyst B, individual separated oxide particles are hardly detected and, consequently, the porous Cu/ZnO particle arrangement is absent in the novel material. The Cu particles are partly embedded in the oxide matrix re[a] Dr. M. Behrens, Dr. A. Furche, Dr. I. Kasatkin, Dr. A. Trunschke, Prof. Dr. R. Schlçgl Fritz Haber Institute of the Max Planck Society Department of Inorganic Chemistry, Faradayweg 4–6, 14195 Berlin (Germany) Fax: (+ 49) 30-8413-4405 E-mail : behrens@fhi-berlin.mpg.de [b] Dr. W. Busser, Prof. Dr. M. Muhler Ruhr University Bochum, Industrial Chemistry Universit tsstraße 150, 44801 Bochum (Germany) [c] Dr. B. Kniep, Dr. R. Fischer S d-Chemie AG, Research & Development Catalysts Waldheimer Straße 13, 83052 Bruckm hl (Germany)

In situ FTIR studies of high-temperature adsorption of hydrogen on zirconia

The interaction of hydrogen with ZrO2 at high temperatures (673–873 K) has been studied by using in situ FTIR spectroscopy. OH species are formed with O—H stretching frequencies at 3650 cm–1 on the surface of monoclinic and at 3730 cm–1 on the surface of tetragonal zirconia. The role of these hydroxy groups in the mechanism of hydrogen uptake and desorption at high temperatures, previously observed on monoclinic ZrO2, is discussed. According to our proposal, the nature of the hydrogen uptake of monoclinic zirconia consists of a heterolytic dissociation of molecular hydrogen and the simultaneous formation of OH species and hydridic species at adjacent O2– and Zr3+ surface sites.

Methane Coupling over Magnesium Oxide: How Doping Can Work

Electronic doping of magnesium oxide catalysts has an effect on the oxidative coupling of methane. Highly active sites can be created by co-modification of MgO with iron and gold in ppm quantities.

Towards the "pressure and materials gap": Hydrogenation of acrolein using silver catalysts

Abstract The hydrogenation of acrolein has been studied over various Ag/SiO2 catalysts in a pressure range from 50mbar to 20bar. Increasing partial pressures of acrolein and/or hydrogen lead to increasing selectivities of allyl alcohol. The selectivity towards allyl alcohol also depends on the structural features of the Ag/SiO2 catalyst. Larger particles seem to favour the production of propanal. This observation is discussed in view of the different plane-to-edge-ratio of the different catalysts. The interaction of hydrogen with silver samples has been studied with TAP at very low pressures as well as with calorimetry at ambient pressures. Both methods indicate, that also the hydrogen adsorption is structure-sensitive. No interaction of hydrogen with electrolytic silver or the support material alone was observed. IR spectroscopy has been used to elucidate the interaction of acrolein with Ag/SiO2 catalysts. A strong interaction with the support material was found, hindering the observation of probable silver-acrolein interaction.

Acetylation and benzoylation of various aromatics on sulfated zirconia

Abstract Sulfated zirconia (SZ) has been found to have a high performance in the benzoylation of anisole. Thus, several aromatics were reacted with benzoic anhydride, benzoyl chloride, and acetic anhydride as acylating agents on SZ, to give the corresponding benzophenones and acetophenones. The rate of the acylation reactions is dependent on the nature of the respective aromatic. The reactivity decreases in the following order: anisole>mesitylene > 3-chloroanisole ∼ m-xylene ∼ 2-chloroanisole > toluene for benzoylations on SZ. The application of SZ as a catalyst for the acetylation of aromatics was only successful in case of anisole.

Synthesis of MoVTeNb Oxide Catalysts with Tunable Particle Dimensions

Reliable procedures for the controlled synthesis of phase‐pure MoVTeNb mixed oxides with M1 structure (ICSD 55097) and tunable crystal dimensions were developed to study the structure sensitivity of the selective oxidation of propane to acrylic acid. A series of powdered M1 catalysts was successfully prepared on a gram scale by using a hydrothermal‐based route, purification of biphasic M1‐M2 (M2 phase – ICSD 55098) oxide systems, and an innovative approach utilizing a superheated water vapor treatment of calcined precursors. The influence of the preparation technique on the particle morphology and the size is discussed. Detailed experimental studies highlight that the as‐derived catalytic materials were indeed phase‐pure and compositionally uniform MoVTeNb oxide M1 powders, composed of single‐crystalline and structural defect‐free crystals grown along the c axis. The morphologically different catalysts were studied in the selective oxidation of propane to acrylic acid, revealing that active sites appear on the entire M1 surface and illustrating the high sensitivity of catalyst performance on the catalyst synthesis method.