Cotter, T.

Schlögl, R.

Trunschke, A.

English

MPG.PuRe

. CxHyO2(H2O)CxHyCOxH2OH2MMO surface with high oxygen potentialMMO surface with low oxygen potentialMoO2Mo2Ch-MoO3TPR-CarburisationSynthesis and reactivity of molybdenum-based transition metal carbides in the dehydrogenation of propaneThomas Cotter, Annette Trunschke and Robert SchlöglFritz-Haber-Insitut der Max-Planck-Gesellschaft, Berlin400In situ powder X-ray diffraction ACFHI400300-399°C400-499°C500-599°CIntroduction and MotivationSelective or partial oxidation reactions of light alkanes and oxygenates are regularly carried out over transition metal oxide surfaces. Such reactions in which total oxidation of the substrate molecule is undesired are uniquely problematic and the rigorous conditions under which the catalyst operates often confound comprehensive understanding. Molybdenum-based mixed metal oxides (MMOs) constitute an important class of such catalysts and in particular the oxyfunctionalisation of propane to acrylic acid relies upon the action of a novel class of MoVTe catalysts. Here we present molybdenum carbide (Mo2C) as a model for a highly reduced oxide surface under conditions of total oxygen conversion in a carbon-enriched atmosphere. The activation of propane under the influence of various feed conditions is studied in conjunction with the observed surface properties of the catalyst.Objective 1: Develop and characterise a MMO system to compare the kinetics of reduction-carburisationAmmonium heptamolybdate (AHM)HNO3 (aq)  @ 75°CObjective 2: Understand and compare the reactivity of molybdenum-based carbide materials in the activation of propane810ConversionNote 1: Synthesis and characterisation of a mixed Mo-V oxide series80100S(CH4)S(C2H4)2θ22 24 26 28 30 32 34 36 38 40 4202002θ- Mo4O11 (orthorhombic)h-MoO3→ (Mo4O11)→ MoO2 → Mo2C h-Mo(V)O3→ Mo(V)O2→ Mo(V)Cx22 24 26 28 30 32 34 36 38 40 420200600-699°C700°C750°C (hold)50°Ch-MoO3 h-(Mo,V)O3[+ Ammonium metavanadate(AMV)]pH = 1.0Synthesis of the complex hexagonal molybdate (Mo(1-x)VxOy.xNH4) is carried out by controlled precipitation in acidic media followed by aging (2h)Mo2CMoO2Increasing V contentα-MoO3 precursor (ref.)h-MoO3precursor h-Mo(V)O3precursorh-Mo(V)O3precursor2θ02460:00 3:00 6:00 9:00 12:00 15:00Yield C3H6Mo2C pretreated @950K1 hour; 80:20 H2/CH410 mL/min C3H820 mL/min H28 mL/min N2450 mg cat. (#9450)5000 h-1 @773K• Initially high activity towards hydrogenolysisGreater than 2:1 ratio of CH4:C2H6Fast deactivation by C deposition•Steady, but low dehydrogenation observedInitial conversion at 550°C051015200:00 0:28 0:57 1:26 1:558486844784788425Time (hh:mm)Increasing V contentNH4+7000 8000 9000 10000 11000 12000600610620630640650660670680Time / sTemperature / °C0.00.10.20.30.40.50.6n(CO) / A.U. 7000 8000 9000 10000 11000 12000600610620630640650660670680Time / sTemperature / °C0.00.20.40.60.81.0α / (a+b)/b0100200300400500600700Temperature (°C)Time abs. (hours)Temperature profile1°C/min0.2°C/min0 6 2412 18Scale up reaction to multi-gram quantities in rotating furnace – transfer to gloveboxoptimise temperature ramp, gas flow and concentration: 360sccm H2, 90sccm CH4Sample 8425 8478 8447 8486V/(Mo+V) 0.0 0.03 ±0.010.08 ±0.010.11 ±0.01Carbon (%w/w)5.16 5.21 5.05 5.34BET (m2/g)70 110 106 97607080volume [cc/g]# 8425120140160180volume [cc/g]# 8447100120140160volume [cc/g]# 8478120140160volume [cc/g] # 848670 m2/g 110 m2/g 106 m2/g 97 m2/gV/(Mo+V) = 0 V/(Mo+V) = 0.03 V/(Mo+V) = 0.075Mo2C0153045600:00 0:28 0:57 1:26 1:558486844784788425Initial conversion at 500°CTime (hh:mm)%Increasing V contentMo2C Mo0.9V0.1CxNote 2: Synthesis and characterisation of mixed Mo-V carbidesTime (hh:mm)%%02040600:00 3:00 6:00 9:00 12:00 15:00S(C3H6)S(C2H6)C bal.%A series of Mo/V oxide precursors were synthesised with a common crystalline phase and progressively substituted V content (0-11%) 0:48 2:13 3:39 5:04 6:3002468101214161820300350400450500550TIme (h:mm)1:14 3:22 5:31 7:39 9:4702468101214161820300350400450500550TIme (h:mm)Comparison of propane activation on freshly prepared carbide surfacesActivation of precursor oxide materials in C3H8/H2/N2  - Temperature programmed reduction-carburisation under reaction conditions% %Temperature /°CTemperature /°Ch-MoO3 h-Mo0.9V0.1O3Conversion (C3H8)Yield (C3H6)Conversion (C3H8)Yield (C3H6)•Increasing vanadium content → higher initial activity•Higher temperature → lower activity (v. fast deactivation?)•Increasing vanadium content → higher overall activity – mostly hydrogenolysis •Increasing vanadium content → higher stability to reduction/activationIn situ synthesis of Mo(V)Cx materials was carried out in a STOE XRD equipped with a Parr XRK900 cell to determine the influence of vanadium on the reaction kinetics.Increasing vanadium was found to stabilisethe initial reduction MoO3 -> MoO2 and additionally it was observed that the vanadium influenced the nanostructure of the as produced carbides (see below and inset). 450 mg h-Mo(V)Ox loaded into reactor, ramp 50°C steps under H 2/C3H8/N2 (20/10/8 sccm) to 500°CMo2C cf. Mo(V)CxInfluence of vanadium on MoO2 -> Mo2CTPR-Carburisation of 500mg h-Mo(V)O3 under 200 sccm H2/CH4 (80:20)To observe differences in the kinetics of carburisation, the evolution of CO during the carburisation step was observed to give the reaction coordinate α vs. time.V/(Mo+V)00.030.050.080.11In situ TPR-Carburisation of 100-200mg h-Mo(V)O3 under 100 sccm He/H2/CH4 (50:40:10)ACKNOWLEDGEMENTSInternational Max Planck Research School for Complex Surfaces in Material ScienceM. Hävecker, D. Teschner, R. Naumann, W. ZhangA. Trunschke, R. SchlöglREFERENCESR.B. Levy, and M. Boudart, Science 181 (1973) 547-549.O. Mougin, J.-L. Dubois, F. Mathieu, and A. Rousset, Journal of Solid State Chemistry 152 (2000) 353-360.T. Cotter, F. Girgsdies, W. Zhang, A. Trunschke, R. Schlögl; in preparation0.0 0.2 0.4 0.6 0.8 1.01020304050volume [cc/g]relative pressure p/p00.0 0.2 0.4 0.6 0.8 1.020406080100volume [cc/g]relative pressure p/p00.0 0.2 0.4 0.6 0.8 1.0020406080volume [cc/g]relative pressure p/p00.0 0.2 0.4 0.6 0.8 1.0020406080100volume [cc/g]relative pressure p/p0V/(Mo+V) = 0.11Conditions:50mg wafer (Mo2C) at 500°C, 0.3 mbar; gas flow: C3H8 – 2.5 sccmH2 – 1.25 sccmHe – 1.25 sccmMo 3d core level spectra240             236             232             228             224Objective 3: Compare and correlate catalytic activity with observable surface chemistryIn situ XPS at BESSY beamlineMo2C under C3H8/H2/HeIn situ Raman study of surface carbon species1200 1400 1600 18000100200Intensity A.U.Raman shift (cm-1)00.20.40.60.811.20:00:00 6:00:00 12:00:00 18:00:00CH4C2H4C2H6C3H60h2h4h6.5h8.5h11h13h15h19hTime (hh:mm:ss)Pressed wafer, 200 mg (#10000)Re-carburised @950K for1 hour; 80:20 H2/CH45 mL/min C3H8 4 mL/min CO210 mL/min H2 1 mL/min N2Relative reaction products over timeRaman spectra recorded in situ SiO2 cell through 2mm window 50X ULWD objective λ=633 nmScans: 6Scan time: 300sBE / eV00:00 02:24 04:48 07:12 09:36 12:000.00.20.40.60.81.0 C2H6 C2H4 C3H6 C4H8Relative conc. A.U.Time (hh:mm)00:00 02:24 04:48 07:12 09:36 12:0001020304050Relative surface conc (%)Time (hh:mm) Mo (0) Mo (II) Mo (IV) Mo (IVB) Mo (V)Normalised product conc. over timeMo oxidation state over time A series of bimetallic carbides (Mo/V) were synthesised on a large scale (~10g) with high surface area and transferred under inert conditions for catalytic testing and characterisation• Vanadium substitution is observed to stabilise the reduction-carburisation of the precursor oxides• The influence of vanadium substitution also served to modify the nanostructure and resulting mesoporosity of the product carbides• Vanadium substitution results in a higher conversion due to increased hydrogenolysis activity• The oxycarbide appears to be more active precursor for dehydrogenation site• Oxygen is observed to migrate to the catalyst surface from traces in the bulk• Adventitious carbon may be removed but at expense of over reduction• Mo(IV) – Mo (V) species show dehydrogenation• Indication that the Mo(0) and Mo(IV) phases are correlated• Hydrogenolysis appears to be on highly active sites and is deactivated via C deposition• Dehydrogenation activity is not acutely deactivated by adventitious carbon

Synthesis and reactivity of molybdenum-based transition metal carbides in the dehydrogenation of propane

http://hdl.handle.net/11858/00-001M-0000-0012-0A28-A

Synthesis and reactivity of molybdenum-based transition metal carbides in the dehydrogenation of propane

Abstract

Similar works

Full text

Available Versions

MPG.PuRe

MPG.PuRe