Co-reporter:S. Kühn;Dr. D. Weber; M. Lerch ; T. Ressler
ChemCatChem 2016 Volume 8( Issue 4) pp:758-766
Publication Date(Web):
DOI:10.1002/cctc.201501076
Abstract
Molybdenum-based mixed oxides represent well-known model catalysts for the selective oxidation of light alkenes. Here, the anion lattice of (Mo,V)5O14 was for the first time modified by substituting oxygen ions with nitrogen ions. Investigations by XRD analysis, X-ray absorption spectroscopy, and FTIR spectroscopy revealed that the incorporation of nitrogen in the structure of Mo5O14 proceeded without changing the average valence of metal centers. Additionally, impedance spectroscopy confirmed the formation of oxygen vacancies. Significant changes in conductivities remained after the removal of nitrogen. Temperature-programmed reduction measurements were performed to investigate oxygen mobility. The enhanced reducibility of oxide nitrides correlated with the increased conductivity. Catalytic performance in selective propene oxidation was determined by online mass spectrometry und gas chromatography at different temperatures. Selectivity towards acrolein increased with increasing conductivity whereas the formation of total oxidation products COx decreased.
Co-reporter:A. Müller;G. Koch;D. Weber;M. Lerch
Reaction Kinetics, Mechanisms and Catalysis 2016 Volume 119( Issue 2) pp:429-444
Publication Date(Web):2016 December
DOI:10.1007/s11144-016-1055-0
The solid-state kinetics of the reduction of VO2 to corundum-type V2O3 were investigated by temperature-programmed measurements. Experimental parameters were chosen to be similar to those used in conventional testing of heterogeneous catalysts. In situ X-ray diffraction experiments showed that the oxidation of V2O3 to corundum-type VO2 during propene oxidation and subsequent reduction of VO2 to V2O3 with propene were single step processes with no intermediate vanadium oxides detectable. Here, H2 consumption profiles obtained for the non-isothermal reduction of VO2 to corundum-type V2O3 were transformed to reduction degree α traces. Extracted α traces were analyzed by the model-free Ozawa, Flynn and Wall (OFW) method, the Kissinger method, and the model depending Coats–Redfern analysis method, respectively. Relevant kinetic characteristics (i.e. apparent activation energy Ea, and solid-state reaction model g(α)) was determined for the reduction of VO2 to corundum-type V2O3. The Kissinger method yielded a value of Ea ~83 kJ/mol for the rate determining step of the reduction of VO2 to corundum-type V2O3. Evolution of apparent activation energy Eaα obtained from a model-free OFW analysis exhibited a constant value of Eaα ~104 kJ/mol in the reduction degree α range between 0.05 and 0.45. In the α range from 0.55–0.95 the apparent activation energy Eaα increased significantly to ~133 kJ/mol. A model dependent Coats–Redfern analysis yielded a nucleation model without growth restriction [Power law (P4)] as best candidate to describe a suitable reaction model for the reduction of VO2.
Co-reporter:R. Zubrzycki, T. Ressler
Microporous and Mesoporous Materials 2015 Volume 214() pp:8-14
Publication Date(Web):15 September 2015
DOI:10.1016/j.micromeso.2015.04.022
•Supporting heteropolyoxo molybdates on SBA-15 resulted in regular Keggin ions.•PMo12-SBA-15 (10, 14, 19 nm) formed a mixture of mostly [MoO4] and [MoO6] units.•The resulting [MoxOy] structures depended on the pore size of the SBA-15.•The stability of the Keggin ions depended mostly on the nature of the support.•Tailoring the pore radius of SBA-15 permitted to prepare Mo oxide model systems.Large pore SBA-15 was successfully synthesized and used as support material for molybdenum based oxidation catalysts. H3[PMo12O40] was supported on SBA-15 with modified pore radii (10, 14, 19 nm). All samples were prepared with a similar surface coverage of 1 Keggin ion per 13 nm2 independent of the pore radii. Structural evolution and catalytic activity of H3[PMo12O40] supported on SBA-15 (PMo12-SBA-15) with different pore radii (10, 14, 19 nm) were investigated under selective propene oxidation conditions by in situ X-ray absorption spectroscopy investigations. PMo12-SBA-15 (10, 14, 19 nm) formed a mixture of mostly tetrahedral [MoO4] and octahedral [MoO6] units during thermal treatment in propene oxidation conditions. A higher concentration of octahedral [MoO6] units and higher oligomerized [MoxOy] units were detected for act. PMo12-SBA-15 (10 nm) compared to act. PMo12-SBA-15 (14, 19 nm). The higher concentration of [MoxOy] units present in act. PMo12-SBA-15 (10 nm) resulted in an increased catalytic activity compared to activated PMo12-SBA-15 (14, 19 nm) with a lower concentration of [MoxOy] units. Selectivities towards oxidation products during propene oxidation were comparable and largely independent of the pore radii of act. PMo12-SBA-15 (10, 14, 19 nm). Apparently, tailoring the pore radius of silica SBA-15 permitted to prepare Mo oxide model systems to investigate correlation between activity and structure of characteristic oxide species at similar surface coverage.
Co-reporter:Rafael Zubrzycki;Dr. Jan Dirk Epping ; Thorsten Ressler
ChemCatChem 2015 Volume 7( Issue 7) pp:1112-1121
Publication Date(Web):
DOI:10.1002/cctc.201402970
Abstract
Vanadium-containing Keggin-type heteropolyoxo molybdate ([PV2Mo10O40]5−) was supported on silica SBA-15 (denoted as PV2Mo10-SBA-15). The structural evolution and catalytic activity of PV2Mo10-SBA-15 and a suitable reference V2Mo10Ox-SBA-15 were investigated under selective propene oxidation conditions by using in situ X-ray absorption spectroscopy. 31P MAS NMR measurements of supported PV2Mo10-SBA-15 and reference H3PO4-SBA-15 were performed after the catalytic reaction. PV2Mo10-SBA-15 formed a mixture of mainly tetrahedral [MoOx] and [VOx] units during thermal treatment under propene oxidation conditions. Changes in the average local structure around V centers coincided with the changes in the average local structure around Mo centers and the onset of catalytic activity. In addition, mainly tetrahedral [MoOx] and [VOx] units seemed to be in close proximity and interacted under catalytic conditions. Conversely, in the reference material V2Mo10Ox-SBA-15 synthesized with individual V and Mo source precursors, Mo and V centers appeared to be more separated from each other. The structural environment of P in PV2Mo10-SBA-15 under catalytic conditions corresponded to a mixture of various species. P was connected to both the support material SBA-15 via POSi bonds and [MoOx] or [VOx] units. Apparently, the proximity of V and Mo in Keggin precursors was a prerequisite for obtaining (Mo,V) oxide species on the support material.
Co-reporter:J. Scholz, A. Walter, A.H.P. Hahn, T. Ressler
Microporous and Mesoporous Materials 2013 180() pp: 130-140
Publication Date(Web):
DOI:10.1016/j.micromeso.2013.05.032
Co-reporter:F. Girgsdies, W.-S. Dong, J.K. Bartley, G.J. Hutchings, R. Schlögl, T. Ressler
Solid State Sciences 2006 Volume 8(Issue 7) pp:807-812
Publication Date(Web):July 2006
DOI:10.1016/j.solidstatesciences.2006.04.008
The crystal structure of ε-VOPO4 was determined in the space group Cc from X-ray powder diffraction data using a rigid body approach. The resulting structure is compared to a recently published, slightly different structure model (space group P21/nP21/n) using Rietveld refinement. It was found that the new Cc model consistently yields a better fit to the observed data and exhibits a less distorted, more stable geometry. The crystal structure of ε-VOPO4 is discussed in comparison to β-VOPO4, monoclinic VPO4⋅H2O, and other related structures.
Co-reporter:J. Scholz, A. Walter, T. Ressler
Journal of Catalysis (January 2014) Volume 309() pp:105-114
Publication Date(Web):1 January 2014
DOI:10.1016/j.jcat.2013.08.031
•Highly dispersed V oxide tetrahedra formed on alkaline, high-surface area support.•Polymerization degree of V oxide units increased with increasing surface coverage.•Investigation of structure–activity correlations by DR-UV–Vis, XAS, and GC analysis.•Acrolein formation rate exhibited a “vulcano curve” as a function of surface coverage.•Maximum in catalytic performance at an average of one to two VOV bonds.Vanadium oxides on an alkaline, high-surface-area MgO/SBA-15 support are introduced as a model system for investigating structure–activity correlations in selective oxidation reactions. The supported vanadium oxide catalysts were investigated by X-ray diffraction, N2 physisorption, X-ray absorption spectroscopy, and diffuse reflectance UV–vis spectroscopy. The vanadium oxide species consisted of tetrahedral units, while the oligomerization degree depended on catalyst loading and vanadium oxide dispersion. Low-oligomeric [VO4] tetrahedra could be stabilized at comparably high coverage due to the alkaline surface of the oxide support. The structure was compared to higher-oligomeric vanadium oxide species present on acidic supports such as SBA-15. Catalytic performance of the oxide catalysts in selective propene oxidation was investigated by gas chromatography. Acrolein turnover frequency as a function of surface coverage exhibited volcano-type behavior. At optimal surface coverage, the catalyst structure consisted mainly of dimeric [V2O7] units.Graphical abstractDownload high-res image (67KB)Download full-size image
Co-reporter:T. Ressler, A. Walter, J. Scholz, J.-P. Tessonnier, D.S. Su
Journal of Catalysis (4 May 2010) Volume 271(Issue 2) pp:305-314
Publication Date(Web):4 May 2010
DOI:10.1016/j.jcat.2010.02.009
In situ X-ray absorption spectroscopy (XAS) under reaction conditions of selective propene oxidation was employed to elucidate the local structure of as-prepared and activated molybdenum oxide supported on hollow vapor-grown carbon nanofibers (VGCNF). The local structure of as-prepared MoxOy-VGCNF was very similar to that of hexagonal MoO3. During heat treatment in propene- and oxygen-containing atmosphere, as-prepared MoxOy-VGCNF transforms into activated MoxOy-VGCNF above 623 K. The local structure around the Mo centers in activated MoxOy-VGCNF is similar to that of α-MoO3. Temperature- and time-dependent XAS measurements showed a rapid transformation from hex-MoO3 to α-MoO3 supported on VGCNF under reaction conditions. Subsequently, the resulting activated MoxOy-VGCNF catalyst exhibited a slowly increasing average oxidation state. The latter coincided with the formation of acrylic acid, which is hardly detectable during catalysis on regular, binary α-MoO3. Moreover, activated MoxOy-VGCNF is much more active in the selective oxidation of propene compared to α-MoO3. The correlation between catalytic selectivity and average oxidation state as a result of suitable reduction–oxidation kinetics corroborates the importance of structural complexity rather than chemical complexity.Molybdenum oxide supported on hollow vapor-grown carbon nanofibers is active in oxidation of propene to acrylic acid. In situ XAS under reaction conditions showed structural transformation from hex-MoO3 to α-MoO3.Download high-res image (103KB)Download full-size image
Co-reporter:T. Ressler, U. Dorn, A. Walter, S. Schwarz, A.H.P. Hahn
Journal of Catalysis (30 September 2010) Volume 275(Issue 1) pp:1-10
Publication Date(Web):30 September 2010
DOI:10.1016/j.jcat.2010.07.001
Investigations into structure and properties of heteropolyoxomolybdates (HPOM) supported on nanostructured silica SBA-15 for the selective oxidation of propene are presented. H4[PVMo11O40] Keggin ions supported on SBA-15 (PVMo11-SBA-15) were prepared by incipient wetness of silica SBA-15 with suitable precursor solutions. Structure and properties of the resulting material were studied ex situ by X-ray diffraction and nitrogen physisorption measurements and in situ by combined X-ray absorption spectroscopy (XAS) and mass spectrometry under various reaction conditions. The characteristic structure of PVMo11-SBA-15 and its evolution under reactive gas atmosphere are compared to that of bulk H4[PVMo11O40]⋅xH2O. Structural investigations of as-prepared PVMo11-SBA-15 have shown that the respective Keggin ions can be readily supported on silica SBA-15. In situ XAS measurements under reducing or oxidizing conditions revealed a pronounced support interaction effect. This effect resulted in a further decreased thermal stability of the supported Keggin ions compared to bulk H4[PVMo11O40]⋅xH2O. Apparently, no stable HPOM Keggin ions could be obtained on silica SBA-15 under reaction conditions. However, in spite of their low thermal stability, HPOM supported on SBA-15 remain suitable well-defined precursors for preparing mixed oxide model catalysts for selective oxidation reactions.Keggin type molybdates supported on silica SBA-15 exhibited a decreased stability under selective propene oxidation conditions. The resulting active catalysts consisted of a mixture of tetrahedrally and octahedrally coordinated Mo centers.Download high-res image (166KB)Download full-size image
Co-reporter:T. Ressler
Catalysis Today (30 July 2009) Volume 145(Issues 3–4) pp:258-266
Publication Date(Web):30 July 2009
DOI:10.1016/j.cattod.2009.02.011
Time-resolved measurements are required to elucidate time-dependencies of the electronic and geometric structure of a catalyst under changing reaction conditions. Monitoring the evolution of the bulk structure of a catalyst under changing conditions reveals the solid-state kinetics of the corresponding reaction. X-ray absorption spectroscopy (XAS) permits to reveal quantitative phase composition and average valence together with the evolution of the local structure. Hence, combining time-resolved XAS with simultaneous catalysis measurements may elucidate correlations between catalytic performance, the catalyst state under reaction conditions, and its solid-state kinetics. Here, results from time-resolved in situ XAS investigations of various molybdenum-based selective oxidation catalysts are compared and discussed. Model systems (i.e. α-MoO3, hexagonal MoO3 supported on SBA-15, and H4[PVMo11O40]) suitable to distinguish structural effects and promotion by additional metal centers have been studied under changing reaction conditions. Correlations between reduction and oxidation solid-state kinetics and catalytic performance reveal the dependence of the selectivity of the catalyst on its electronic structure. In particular the re-oxidation kinetics and the average valence under reaction conditions appear to be determined by the defect structure of the underlying catalyst bulk.
Co-reporter:Thorsten Ressler, Olaf Timpe
Journal of Catalysis (25 April 2007) Volume 247(Issue 2) pp:231-237
Publication Date(Web):25 April 2007
DOI:10.1016/j.jcat.2007.01.017
Time-resolved in situ X-ray absorption spectroscopy studies on an activated H5[PV2Mo10O40] oxidation catalyst were performed to obtain correlations between the dynamic structure and the catalytic selectivity of the material. Both the geometric and electronic structures of the vanadium and molybdenum metal centers of the catalyst change dynamically under the reaction conditions used. Moreover, the selectivity of the catalyst exhibits a pronounced correlation with the degree of reduction and the solid-state kinetics of the reoxidation process. The corresponding extent of reoxidation curve can be simulated with a solid-state kinetic model assuming three-dimensional diffusion as the rate-limiting step. Thus, the partially reduced catalyst exhibits a rate constant of the bulk-diffusion limited reoxidation, coinciding with the temporal evolution of the selectivity of the catalyst.
Co-reporter:Patrick Kurr, Igor Kasatkin, Frank Girgsdies, Annette Trunschke, Robert Schlögl, Thorsten Ressler
Applied Catalysis A: General (15 October 2008) Volume 348(Issue 2) pp:
Publication Date(Web):15 October 2008
DOI:10.1016/j.apcata.2008.06.020
Microstructural characteristics of various real Cu/ZnO/Al2O3 catalysts for methanol steam reforming (MSR) were investigated by in situ X-ray diffraction (XRD), in situ X-ray absorption spectroscopy (XAS), temperature programmed reduction (TPR) and electron microscopy (TEM). Structure–activity correlations of binary Cu/ZnO model catalysts were compared to microstructural properties of the ternary catalysts obtained from in situ experiments under MSR conditions. Similar to the binary system, in addition to a high specific copper surface area the catalytic activity of Cu/ZnO/Al2O3 catalysts is determined by defects in the bulk structure. The presence of lattice strain in the copper particles as the result of an advanced Cu–ZnO interface was detected only for the most active Cu/ZnO/Al2O3 catalyst in this study. Complementarily, a highly defect rich nature of both Cu and ZnO has been found in the short-range order structure (XAS). Conventional TPR and TEM investigations confirm a homogeneous microstructure of Cu and ZnO particles with a narrow particle size distribution. Conversely, a heterogeneous microstructure with large copper particles and a pronounced bimodal particle size distribution was identified for the less active catalysts. Apparently, lattice strain in the copper nanoparticles is an indicator for a homogeneous microstructure of superior Cu/ZnO/Al2O3 catalyst for methanol chemistry.The microstructure of Cu/ZnO/Al2O3 catalysts for methanol steam reforming was investigated and compared to structure–activity correlations of binary Cu/ZnO model catalysts. Similar to the binary system, characteristic defects (e.g. lattice strain) in the Cu phase of Cu/ZnO/Al2O3 catalysts are indicative of a homogeneous microstructure and superior catalytic performance.
Co-reporter:H. Soerijanto, C. Rödel, U. Wild, M. Lerch, R. Schomäcker, R. Schlögl, T. Ressler
Journal of Catalysis (15 August 2007) Volume 250(Issue 1) pp:19-24
Publication Date(Web):15 August 2007
DOI:10.1016/j.jcat.2007.04.024
A zirconium oxynitride catalyst was used for the decomposition of ammonia to hydrogen and nitrogen. The onset of catalytic activity at ∼550 °C coincided with the onset of nitrogen ion mobility in the material and a phase change from the initial β′ phase (∼Zr7O11N2) to the nitrogen-rich β″ ZrON phase (∼Zr7O9,5N3). No hydrazine formation during an extended time on stream was detectable. Moreover, the onset of activity was also correlated to a rapid change in the electronic structure of the surface accompanying formation of the more active β″ ZrON phase. The results presented here show for the first time a direct correlation among the onset of ion conductivity as a bulk property, a modified electronic structure of the surface, and the catalytic performance of a heterogeneous catalyst.
Co-reporter:T. Ressler, A. Walter, Z.-D. Huang, W. Bensch
Journal of Catalysis (10 March 2008) Volume 254(Issue 2) pp:170-179
Publication Date(Web):10 March 2008
DOI:10.1016/j.jcat.2007.12.012
Local structure and catalytic properties of molybdenum oxide supported on nanostructured SiO2 (SBA-15) were studied under reaction conditions by combined X-ray absorption spectroscopy (XAFS) and mass spectrometry. MoO3 supported on SBA-15 exhibits stability and catalytic properties different from those of binary bulk oxides. The interaction between support and molybdenum oxide stabilizes particular hexagonal MoO3 structure that is highly active and selective. The hex-MoO3–SBA-15 is stable under reducing (propene) and oxidizing reaction conditions in the temperature range from 20 to 500 °C. In contrast to bulk hex-MoO3, the onset of activity at about 250 °C is not accompanied by a transformation to α-MoO3. Moreover, in contrast to α-MoO3, hex-MoO3–SBA-15 is capable of directly oxidizing propene to acrylic acid without additional metal sites.
Co-reporter:T. Ressler, A. Walter, Z.-D. Huang, W. Bensch
Journal of Catalysis (10 March 2008) Volume 254(Issue 2) pp:170-179
Publication Date(Web):10 March 2008
DOI:10.1016/j.jcat.2007.12.012
Local structure and catalytic properties of molybdenum oxide supported on nanostructured SiO2 (SBA-15) were studied under reaction conditions by combined X-ray absorption spectroscopy (XAFS) and mass spectrometry. MoO3 supported on SBA-15 exhibits stability and catalytic properties different from those of binary bulk oxides. The interaction between support and molybdenum oxide stabilizes particular hexagonal MoO3 structure that is highly active and selective. The hex-MoO3–SBA-15 is stable under reducing (propene) and oxidizing reaction conditions in the temperature range from 20 to 500 °C. In contrast to bulk hex-MoO3, the onset of activity at about 250 °C is not accompanied by a transformation to α-MoO3. Moreover, in contrast to α-MoO3, hex-MoO3–SBA-15 is capable of directly oxidizing propene to acrylic acid without additional metal sites.
