Nanostructured Catalysts in Activation of Light Alkanes

Transcription

1 Nanostructured Catalysts in Activation of Light Alkanes The project is focussed on fundamental studies dealing with the activation of light alkanes in oxidation reactions at the surface of solid oxide catalysts. Prospective raw material changes in the chemical industry require alternative approaches including the direct activation of light alkane molecules, like methane, ethane, propane, and butane. The use of short- chain alkanes derived from natural gas or renewable resources instead of traditional crude oil- based building blocks such as olefins and aromatics in petro- chemistry is a chemically and technologically challenging task. The difficulty in catalytic oxidation arises from the chemical reactivity of reaction intermediates and/or the desired unsaturated or oxygenated target product, which is generally higher than that of the hydrocarbon substrate. Selectivity, i.e., avoiding the formation of the undesired thermodynamically most favorable product CO2, is, consequently, the major challenge.[1, 2] Fundamental principles of oxidation catalysis have been proposed, essentially based on comprehensive experimental studies performed in the last thirty years on the oxidation of olefins, in particular propylene. The concept highlights (i) lattice oxygen, (ii) metal oxygen bond strength, (iii) host structure, (iv) redox properties, (v) multifunctionality of active sites, (vi) site isolation, and (vii) phase cooperation as important pillars.[3] However, the predictive power of these guidelines is not satisfying in point of oxidation of saturated short- chain hydrocarbons. New experimental approaches are needed that go beyond apparent relations between crystallographic phase composition of structurally very complex catalysts and reactivity. Selectivity is affected by an array of parameters resulting from reaction conditions (feed composition, temperature, pressure), solid- state properties, and surface dynamics of the heterogeneous catalyst. The interplay of these thermodynamic and kinetic parameters is reflected in the abundance of different active oxygen species at the surface, the occurrence of acid- base sites that may influence the reaction network, and the presence of surface as well as gas- phase radical species. To understand better which factors are critical in terms of selectivity, kinetic studies under relevant conditions (high productivity of the catalyst) in combination with spectroscopic investigations of the working catalyst are indispensable. Comprehensive characterization of the fresh and used catalysts provides supplemental information regarding the catalyst precursor and the deactivated state. The working schedule in the project includes controlled synthesis of well- defined oxide materials,[4-7] kinetic analysis of complex reaction networks,[8-10] and the investigation of solid- state and surface properties of the catalyst under operation.[8, 11-17] We reduce complexity by studying only pure phases that represent functional models of well- performing catalysts including alkaline earth oxides in the oxidative coupling of methane,[18-21] the M1 phase of MoVTeNb oxide in ethane and propane oxidation,[4, 8-11, 15] and vanadyl pyrophosphate (VO)2P2O7 in oxidation of n- butane.[12, 13, 17] The bulk catalysts are compared with binary oxides and metal oxide monolayers supported for example on meso- structured silica as inert carrier,[22-25] as well as molecular complexes that contain differently coordinated oxygen atoms.[26] Highlights of our research will be outlined in the following. Pure MgO as Model Catalyst in Oxidative Coupling of Methane The role of surface structure and defects in the oxidative coupling of methane (OCM) was studied over magnesium oxide as a model catalyst. Pure, nano- structured MgO catalysts with varying primary particle size, shape and specific surface area were prepared by sol- gel synthesis, oxidation of metallic magnesium, and hydrothermal post treatments.[19] The initial activity of MgO in the OCM reaction is clearly structure- sensitive. Kinetic studies reveal the occurrence of two parallel reaction mechanisms and 1

2 a change in the contribution of these pathways to the overall performance of the catalysts with time on stream. The initial performance of freshly calcined MgO is governed by a surface- mediated coupling mechanism involving direct electron transfer between methane and oxygen at structural defects (steps) on the surface of MgO. The water formed in the OCM reaction causes sintering of the MgO particles and loss of active sites by degradation of structural defects, which is reflected in decreasing activity of MgO with time on stream. At the same time, gas- phase chemistry becomes more important, which includes formation of ethane by coupling of methyl radicals formed at the surface and the partial oxidation of C2H6. The catalysts in the dehydroxylated state before the reaction and after catalysis have been studied by infrared and photoluminescence spectroscopy.[18] The abundance of structural defects, in particular mono- atomic steps, on the dehydroxylated MgO surface characterized by a band in the FTIR spectrum of adsorbed CO at 2146 cm - 1 and Lewis acid/base pairs probed by co- adsorption of CO and CH4 correlate with the initial rates of both methane consumption and C2+ hydrocarbon formation (Figure 1). Infrared spectroscopy evidences strong polarization of C- H bonds due to adsorption of methane on dehydroxylated MgO surfaces that contain a high number of mono- atomic steps. It is postulated that these sites effectively promote intermolecular charge transfer between adsorbed methane and weakly adsorbed oxygen that leads to the dissociation of one C- H bond in the methane molecule and simultaneous formation of a superoxide species. Heterolytic splitting of C- H bonds in the presence of oxygen at the surface of dehydroxylated MgO already at room temperature has been proven by the appearance of an EPR signal associated with superoxide species that are located in close vicinity to a proton. With time on stream, MgO sinters and loses activity. The deactivation process involves the depletion of mono- atomic steps in particular due to the interaction with water as co- product of the reaction and the reconstruction of the MgO termination under formation of polar and faceted surfaces as evidenced by electron microscopy, FTIR, and photoluminescence spectroscopy. Figure 1. Rate of methane consumption and C 2+ formation at t=0 measured at T=1023 K, applying a feed composition of CH 4/O 2/N 2=3/1/1, and a contact time of g s ml - 1 plotted versus the integrated area of the CO adsorption peak at 2146 cm - 1 that indicates the abundance of mono- atomic steps. 2

3 Since water is an unavoidable reaction product, the stabilization of mono- atomic steps is not trivial. Therefore, alternative strategies should also be considered in the design of stable catalysts for OCM. Recently, the formation of strongly bound O2 - species, the precursors of dissociatively adsorbed O2, has been observed on single- crystal CaO films doped with Mo 2+ ions.[27] Such an electronic doping does not require any surface structural defects. The concept, which is also applicable to powder catalysts,[21] provides another promising approach to active and stable OCM catalysts. We currently pursue this idea by studying doped CaO and MgO in OCM in collaboration with the TU Berlin. Oxidation of C3- C4 Alkanes over Mixed Oxides One promising catalyst for the direct oxidation of propane to acrylic acid with high selectivity is mixed MoVTeNb oxide that crystallizes in the so- called M1 structure (ICSD 55097).[8, 10, 28] Investigation of MoVTeNb M1 oxide in non- oxidative reactions, such as olefin metathesis demonstrates the diversity of the M1 phase as a catalyst, which is based on its surface dynamics and structural stability.[9] MoVTeNb M1 oxide is available via different synthesis routes.[29-32] Hydrothermal synthesis benefits from kinetic control that facilitates formation of the metastable phase. Yet, proper choice and control of the reaction conditions is quite crucial for targeted synthesis of the phase including the desired micro- structure. Spectroscopic and analytical studies are particularly important to understand the mechanisms behind the complex reactions under hydrothermal conditions and to rationally design experiments for the synthesis of new phases exhibiting designated properties. In this context we have developed an analytical autoclave that enables the synthesis of comparatively large batches of oxides according to a new concept that implies a guided synthesis based on spectroscopic information obtained online.[5, 7] The synthesis is performed automated according to a pre- assigned sequence of reaction steps. The reaction parameters, such as temperature, pressure, power intake of the stirrer, and ph are measured and recorded. Reactants can be fed into the autoclave at reaction temperature by using HPLC pumps. In situ Raman spectroscopy was applied to investigate the speciation of molybdates under hydrothermal conditions in aqueous solutions in the temperature range between 20 C and 200 C and at ph values between 7 and 1 in this autoclave.[5] The nature and abundance of molecular and supra- molecular species differs significantly compared to the distribution of species under ambient conditions at the same ph. Whereas heptamolybdate [Mo7O24] 6 dominates under ambient pressure at 25 C in the ph range between 6 and 5, at C, chain- like or molecular structures of dimolybdates [Mo2O7] 2 and trimolybdates [Mo3O10] 2 are preferentially formed. In acidic solutions (ph<2), supramolecular species, like [Mo36O11] 28, which generally predominate at 25 C, do not occur at T>100 C. Instead, β- [Mo8O26] 4 is the final molecular precursor of precipitation reactions that was detectable by Raman spectroscopy. The structural type of the solid phase formed through addition of vanadyl sulfate under hydrothermal conditions is sensitively controlled by the nature of the molecular precursor, which is adjusted by the ph. In acidic medium, hexagonal MoO3 (ICSD 80290) is formed, while at ph = 5.8 nano- crystalline M1 (ICSD 55096) was obtained. The results clearly demonstrate the power of in situ spectroscopy as a tool to improve our understanding of the inorganic reactions occurring under hydrothermal conditions. Elucidation of nucleation and growth of solid particles requires first of all knowledge about the molecular and supra- molecular precursors occurring under the applied conditions in 3

4 solution. The existing broad knowledge concerning speciation of polyoxometalates in aqueous solution under ambient conditions cannot be transferred to the situation inside an autoclave at high temperatures. The synthesis of mixed MoV oxides starting from different molecular precursors illustrates that based on in situ spectroscopy, an interactive synthesis is possible that opens up new prospects in controlled fabrication of nano- structured materials.[5, 7] MoVTeNb oxide catalysts exclusively composed of the M1 phase have been studied in the direct oxidation of propane to acrylic acid applying a broad range of reaction conditions with respect to temperature ( K), propane concentration (1-3%), O2 concentration ( %), steam concentration (0 40%), and contact time ( s. gcat. Nml - 1 ). Model experiments were performed to study the reactivity of possible intermediates propene, acrolein, and CO.[8, 28] The reactivity of MoVTeNb M1 oxide was compared with the reactivity of a M1 phase that contains only Mo and V at the metal positions of the crystal structure.[4] Analysis of the three- dimensional experimental parameter field measured in fixed bed reactors revealed that the complexity of the reaction network in propane oxidation over MoVTeNb M1 oxide is reduced compared to less- defined catalysts due to phase purity and homogeneity.[8] The oxidative dehydrogenation of propane to propene followed by allylic oxidation of propene comprises the main route to acrylic acid. The oxygen partial pressure was identified as an important process parameter that controls the activity in propane oxidation over phase- pure M1 without negative implications on the selectivity. High O2 concentration in the feed keeps the catalyst in a high oxidation state, which provides an increased number of active sites for propane activation. Auxiliary steam increases activity and selectivity of M1 by changing the chemical nature of the active sites and by facilitating acrylic acid desorption. The M1 phase that contains only Mo and V produces acrylic acid, but shows low selectivity.[10] In situ X- ray diffraction has been carried out to explore the structural stability of the MoVTeNb M1 phase under stoichiometric, oxidizing, and reducing reaction conditions. Phase purity apparently accomplishes absolute stability in terms of the crystalline bulk structure and the catalytic performance over month even under extreme reaction conditions.[8] In contrast, the catalyst surface changes dynamically and reversibly when the feed composition is varied, but only in the outermost surface layer in a depth of around one nanometer.[8, 11, 12, 15, 33] Synchrotron based near ambient pressure X- ray photoelectron spectroscopy (NAP- XPS) has demonstrated that MoVTeNb M1 oxide is a self- supported monolayer catalyst. The addition of steam causes enrichment in V and Te on the surface at the expense of Mo. In dry feed the surface is enriched in Mo. Switching on and off the steam changes the surface composition reversibly. In addition, surface vanadium becomes more oxidized in presence of steam. These changes correlate with the abundance of acrylic acid detected in the in situ photoelectron spectroscopy experiment. Such a dynamic redox behavior was not observed for a M1 phase that contains only Mo and V at the metal positions. MoV M1 oxide produces acrylic acid as well, but the selectivity to undesired products CO and CO2 is much higher compared to MoVTeNb M1 oxide. MoV M1 oxide shows an average V oxidation state between 4.40 and 4.45 with only little fluctuations depending on the feed composition. High amounts of vanadium (ca. 50 at%) are irreversibly enriched at the surface. The high V concentration might be responsible for consecutive reactions in which the desired product acrylic acid is largely further oxidized to carbon oxides. 4

5 In summary, our results clearly show an impact of the crystalline bulk structure on surface composition and reactivity of Mo- based mixed oxides in selective oxidation of propane. Under working conditions, the oxide surface is significantly reconstructed and differs from the bulk in terms of composition and oxidation state of V. The selective MoVTeNb oxide catalyst features dynamic site isolation due to segregation of Te, and is characterized by improved redox dynamics compared to the unselective catalysts composed of the same crystal structure. The experiments illustrate critical relations between solid- state and surface dynamics in catalysis. The electronic structure of the selective oxidation catalysts MoVTeNb M1 oxide and vanadyl pyrophosphate has been investigated by conductivity measurements applying the microwave cavity perturbation technique in combination with NAP- XPS, soft X- ray absorption, and resonant photoelectron spectroscopy under operation.[11-13, 15, 17, 34-36] The experiments show that steam decreases the electrical conductivity of MoVTeNb M1 oxide, decreases the work function, and modifies the valence band structure.[11, 15] The catalyst forms a subsurface space charge region depleted in electrons under reaction conditions. These changes in the electronic structure seem to be related to the surface dynamics and the increase in the concentration of V 5+ and Te 4+ in presence of steam. The presence of band bending and hence the formation of a high surface potential barrier, which electrons have to overcome on their way from the bulk to the surface, could be necessary to terminate the bulk surface charge transfer, limit the formation of active oxygen species on the surface and finally promote high selectivity to acrylic acid, while suppressing its total oxidation to CO2. The surface potential barrier and the limited bulk surface charge transfer would also explain the extraordinary stability of the MoVTeNb M1 oxide bulk phase under reaction conditions. A comparative investigation of the oxidation of n- butane to maleic anhydride on the highly selective catalyst vanadyl pyrophosphate and the moderately selective MoVTeNb M1 oxide supports this concept and shows that the catalysts act like semiconducting gas sensors with a dynamic charge transfer between the bulk and the surface, as indicated by the gas- phase- dependent response of the work function, electron affinity, and the surface potential barrier determined by NAP- XPS.[12] A relationship between the surface barrier due to band bending and the vanadium oxidation state is experimentally supported by the change in the average vanadium oxidation state measured by XPS and the intensity of the V 3d valence state induced by variation in the feed composition. The surface states could correspond to a V 4+ /V 5+ redox couple as a part of the self- supported monolayer. The surface potential barrier height is the difference between the hypothetical Fermi level of the catalyst without surface states and the Fermi level of the catalyst with surface states. A high V 5+ /V 4+ ratio can increase the surface barrier height to such an extent, that the bulk surface electron transport is impeded with implications on the reduction of gas phase oxygen and selectivity to the desired product as discussed above. To verify this hypothesis, further in situ investigations of binary and complex oxides and the measurement of charge carrier densities using the microwave Hall effect are in progress. Mono- and sub- monolayers of vanadium oxide species supported on an inert support, such as silica, may serve as models for the thin film observed at the surface of complex mixed oxides.[22-25, 37, 38] Therefore, we are dealing with the synthesis and characterization of supported vanadium oxides using mesoporous silica SBA- 15 as carrier. Although progress has been made in recent years in characterization of vanadium oxide supported on SiO2 (including MCM- 41 and SBA- 15), Al2O3, TiO2, and 5

6 ZrO2 under different environments through spectroscopic studies, the molecular structure of the supported vanadium oxide species is not yet clear. Important information about the coordination of vanadium atoms can be provided by UV- vis spectroscopy. Models of different vanadia, titania and vanadia- titania clusters located on the surface of silica have been optimized using density functional theory (DFT).[23] The comparison of calculated and experimental apparent absorption spectra reveals that titania species generally show a higher nuclearity compared to vanadia species at similar loadings. SBA- 15 based catalysts loaded with both vanadia and titania are supposed to contain two types of species: species in which the V ions are anchored to the titania ones and those in which V and Ti ions alternate and are mainly coupled to the support through M- O- Si (M = V, Ti) bridges. The latter provide the major contribution to the apparent absorption spectra at low Ti loadings. Vanadium oxo peroxo complexes are found to be valuable functional models for the vanadium haloperoxidase enzyme which is involved, among others, in the oxidation of alcohols, the production of halogenated organics, as well as in sulfoxidation processes. These reactions involve always activated oxygen species bound to the catalyst. Since vanadium oxo peroxo complexes contain oxygen of different geometies and coordination, suggesting different chemical behavior, it is very important to know which of these oxygen species are involved in a particular reaction step. Geometric and electronic properties of the K[VO(O2)Hheida] complex, as well as of the molecular ions (VO(O2)Hheida), and (VO(O)Hheida) have been evaluated using density functional theory (DFT).[26] The vibrational modes of peroxo and vanadyl oxygen in K[VO(O2)Hheida] are strongly coupled, and the calculated excitation energies can explain details of the experimental infrared and Raman spectra. The theoretical O 1s core ionization potentials (IP) vary between the different oxygen species and are consistent with results from X- ray photoemission (XPS). Theoretical O 1s core excitation spectra are confirmed by results from O K- edge NEXAFS measurements for crystalline K[VO(O2)Hheida] 2(H2O) under oxygen and helium pressure. The present theoretical and experimental results can be substantiated further by examining other model compounds which are interesting in connection with haloperoxidase enzymes, such as K[VO(O2)ada] and VO(O2)bpg. Studies along these lines are currently under way. References [1] R. Schlögl, Active Sites for Propane Oxidation: Some Generic Considerations, Topics in Catalysis, 54 (2011) [2] R. Schlögl, Heterogeneous Catalysis, Angewandte Chemie International Edition, 54 (2015) [3] R.K. Grasselli, Fundamental Principles of Selective Heterogeneous Oxidation Catalysis, Topics in Catalysis, 21 (2002) [4] T. Lunkenbein, F. Girgsdies, A. Wernbacher, J. Noack, G. Auffermann, A. Yasuhara, A. Klein- Hoffmann, W. Ueda, M. Eichelbaum, A. Trunschke, R. Schlögl, M.G. Willinger, Direct Imaging of Octahedral Distortion in a Complex Molybdenum Vanadium Mixed Oxide, Angewandte Chemie International Edition, 54 (2015) [5] J. Noack, F. Rosowski, R. Schlögl, A. Trunschke, Speciation of Molybdates under Hydrothermal Conditions, Zeitschrift für anorganische und allgemeine Chemie, 640 (2014) [6] A. Trunschke, Synthesis of Solid Catalysts, in: R. Schlögl (Ed.) Chemical Energy Storage, Walter de Gruyter GmbH, Berlin/Boston, 2013, pp

7 [7] M. Sanchez Sanchez, F. Girgsdies, M. Jastak, P. Kube, R. Schlögl, A. Trunschke, Aiding the Self- Assembly of Supramolecular Polyoxometalates under Hydrothermal Conditions To Give Precursors of Complex Functional Oxides, Angewandte Chemie International Edition, 51 (2012) [8] R. Naumann D'Alnoncourt, L.I. Csepei, M. Hävecker, F. Girgsdies, M.E. Schuster, R. Schlögl, A. Trunschke, The reaction network in propane oxidation over phase- pure MoVTeNb M1 oxide catalysts, Journal of Catalysis, 311 (2014) [9] K. Amakawa, Y.V. Kolen'ko, R. Schlögl, A. Trunschke, The M1 Phase of MoVTeNbO as a Catalyst for Olefin Metathesis and Isomerization, ChemCatChem, 6 (2014) [10] K. Amakawa, Y.V. Kolen'ko, A. Villa, M.E. Schuster, L.- I. Csepei, G. Weinberg, S. Wrabetz, R. Naumann d'alnoncourt, F. Girgsdies, L. Prati, R. Schlögl, A. Trunschke, Multifunctionality of Crystalline MoV(TeNb) M1 Oxide Catalysts in Selective Oxidation of Propane and Benzyl Alcohol, ACS Catalysis, 3 (2013) [11] C. Heine, M. Havecker, A. Trunschke, R. Schlogl, M. Eichelbaum, The impact of steam on the electronic structure of the selective propane oxidation catalyst MoVTeNb oxide (orthorhombic M1 phase), Physical Chemistry Chemical Physics, 17 (2015) [12] M. Eichelbaum, M. Hävecker, C. Heine, A.M. Wernbacher, F. Rosowski, A. Trunschke, R. Schlögl, The Electronic Factor in Alkane Oxidation Catalysis, Angewandte Chemie International Edition, 54 (2015) [13] C. Heine, M. Hävecker, E. Stotz, F. Rosowski, A. Knop- Gericke, A. Trunschke, M. Eichelbaum, R. Schlögl, Ambient- Pressure Soft X- ray Absorption Spectroscopy of a Catalyst Surface in Action: Closing the Pressure Gap in the Selective n- Butane Oxidation over Vanadyl Pyrophosphate, The Journal of Physical Chemistry C, 118 (2014) [14] D. Maganas, M. Roemelt, M. Hävecker, A. Trunschke, A. Knop- Gericke, R. Schlögl, F. Neese, 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, Physical Chemistry Chemical Physics, 15 (2013) [15] C. Heine, M. Hävecker, M. Sanchez- Sanchez, A. Trunschke, R. Schlögl, M. Eichelbaum, Work Function, Band Bending, and Microwave Conductivity Studies on the Selective Alkane Oxidation Catalyst MoVTeNb Oxide (Orthorhombic M1 Phase) under Operation Conditions, The Journal of Physical Chemistry C, 117 (2013) [16] C. Heine, F. Girgsdies, A. Trunschke, R. Schlögl, M. Eichelbaum, The model oxidation catalyst α- V2O5: insights from contactless in situ microwave permittivity and conductivity measurements, Appl. Phys. A, 112 (2013) [17] M. Eichelbaum, R. Glaum, M. Hävecker, K. Wittich, C. Heine, H. Schwarz, C.- K. Dobner, C. Welker- Nieuwoudt, A. Trunschke, R. Schlögl, Towards Physical Descriptors of Active and Selective Catalysts for the Oxidation of n- Butane to Maleic Anhydride, ChemCatChem, 5 (2013) [18] P. Schwach, N. Hamilton, M. Eichelbaum, L. Thum, T. Lunkenbein, R. Schlögl, A. Trunschke, Structure sensitivity of the oxidative activation of methane over MgO model catalysts: II. Nature of active sites and reaction mechanism, Journal of Catalysis, doi: /j.jcat (2015). [19] P. Schwach, W. Frandsen, M.- G. Willinger, R. Schlögl, A. Trunschke, Structure sensitivity of the oxidative activation of methane over MgO model catalysts: I. Kinetic study, Journal of Catalysis, doi: /j.jcat (2015). 7

8 [20] K. Kwapien, J. Paier, J. Sauer, M. Geske, U. Zavyalova, R. Horn, P. Schwach, A. Trunschke, R. Schlögl, Sites for Methane Activation on Lithium- Doped Magnesium Oxide Surfaces, Angewandte Chemie International Edition, 53 (2014) [21] P. Schwach, M.G. Willinger, A. Trunschke, R. Schlögl, Methane Coupling over Magnesium Oxide: How Doping Can Work, Angewandte Chemie International Edition, 52 (2013) [22] M.A. Smith, A. Zoelle, Y. Yang, R.M. Rioux, N.G. Hamilton, K. Amakawa, P.K. Nielsen, A. Trunschke, Surface roughness effects in the catalytic behavior of vanadia supported on SBA- 15, Journal of Catalysis, 312 (2014) [23] S. Klokishner, O. Reu, G. Tzolova- Müller, R. Schlögl, A. Trunschke, Apparent Absorption Spectra of Silica Supported Vanadium Titanium Oxide Catalysts: Experimental Study and Modeling, The Journal of Physical Chemistry C, 118 (2014) [24] C. Carrero, M. Kauer, A. Dinse, T. Wolfram, N. Hamilton, A. Trunschke, R. Schlogl, R. Schomacker, High performance (VOx)n- (TiOx)m/SBA- 15 catalysts for the oxidative dehydrogenation of propane, Catalysis Science & Technology, 4 (2014) [25] K. Amakawa, L. Sun, C. Guo, M. Hävecker, P. Kube, I.E. Wachs, S. Lwin, A.I. Frenkel, A. Patlolla, K. Hermann, R. Schlögl, A. Trunschke, How Strain Affects the Reactivity of Surface Metal Oxide Catalysts, Angewandte Chemie International Edition, 52 (2013) [26] L.L. Sun, K.E. Hermann, J. Noack, O. Timpe, D. Teschner, M. Hävecker, A. Trunschke, R. Schlögl, DFT Studies and Experiments on Biocatalytic Centers: Structure, Vibrations, and Core Excitations of the K[VO(O2)Hheida] Complex, The Journal of Physical Chemistry C, 118 (2014) [27] Y. Cui, X. Shao, M. Baldofski, J. Sauer, N. Nilius, H.- J. Freund, Adsorption, Activation, and Dissociation of Oxygen on Doped Oxides, Angewandte Chemie International Edition, 52 (2013) [28] R. Naumann d Alnoncourt, Y.V. Kolen ko, R. Schlögl, A. Trunschke, A new way of probing reaction networks: analyzing multidimensional parameter space, Combinatorial Chemistry and High Throughput Screening, 15 (2012) [29] Y.V. Kolen'ko, K. Amakawa, R.N. d'alnoncourt, F. Girgsdies, G. Weinberg, R. Schlögl, A. Trunschke, Unusual Phase Evolution in MoVTeNb Oxide Catalysts Prepared by a Novel Acrylamide- Gelation Route, ChemCatChem, 4 (2012) [30] Y.V. Kolen'ko, W. Zhang, R.N. d'alnoncourt, F. Girgsdies, T.W. Hansen, T. Wolfram, R. Schlögl, A. Trunschke, Synthesis of MoVTeNb Oxide Catalysts with Tunable Particle Dimensions, ChemCatChem, 3 (2011) [31] A. Celaya Sanfiz, T.W. Hansen, F. Girgsdies, O. Timpe, E. Rödel, T. Ressler, A. Trunschke, R. Schlögl, Preparation of Phase- Pure M1 MoVTeNb Oxide Catalysts by Hydrothermal Synthesis- Influence of Reaction Parameters on Structure and Morphology, Topics in Catalysis, 50 (2008) [32] P. Beato, A. Blume, F. Girgsdies, R.E. Jentoft, R. Schlögl, O. Timpe, A. Trunschke, G. Weinberg, Q. Basher, F.A. Hamid, S.B.A. Hamid, E. Omar, L. Mohd Salim, Analysis of structural transformations during the synthesis of a MoVTeNb mixed oxide catalyst, Applied Catalysis, A: General, 307 (2006) [33] A. Celaya Sanfiz, T.W. Hansen, D. Teschner, P. Schnörch, F. Girgsdies, A. Trunschke, R. Schlögl, M.H. Looi, S.B.A. Hamid, Dynamics of the MoVTeNb Oxide M1 Phase in Propane Oxidation, The Journal of Physical Chemistry C, 114 (2010)

9 [34] R. Glaum, C. Welker- Nieuwoudt, C.- K. Dobner, M. Eichelbaum, F. Gruchow, C. Heine, A. Karpov, R. Kniep, F. Rosowski, R. Schlögl, S.A. Schunk, S. Titlbach, A. Trunschke, Resource- Efficient Alkane Selective Oxidation on New Crystalline Solids: Searching for Novel Catalyst Materials, Chemie Ingenieur Technik, 84 (2012) [35] M. Eichelbaum, R. Stößer, A. Karpov, C.- K. Dobner, F. Rosowski, A. Trunschke, R. Schlögl, The microwave cavity perturbation technique for contact- free and in situ electrical conductivity measurements in catalysis and materials science, Physical Chemistry Chemical Physics, 14 (2012) [36] M. Eichelbaum, M. Hävecker, C. Heine, A. Karpov, C.- K. Dobner, F. Rosowski, A. Trunschke, R. Schlögl, The Intimate Relationship between Bulk Electronic Conductivity and Selectivity in the Catalytic Oxidation of n- Butane, Angewandte Chemie International Edition, 51 (2012) [37] N. Hamilton, T. Wolfram, G. Tzolova Müller, M. Hävecker, J. Kröhnert, C. Carrero, R. Schomäcker, A. Trunschke, R. Schlögl, Topology of silica supported vanadium- titanium oxide catalysts for oxidative dehydrogenation of propane, Catalysis Science & Technology, 2 (2012) [38] P. Gruene, T. Wolfram, K. Pelzer, R. Schloegl, A. Trunschke, Role of dispersion of vanadia on SBA- 15 in the oxidative dehydrogenation of propane, Catalysis Today, 157 (2010)