In situ molecular beam mass spectrometry for high temperature catalysis research: Heterogeneous Homogeneous Catalytic Oxidations

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1 In situ molecular beam mass spectrometry for high temperature catalysis research: Heterogeneous Homogeneous Catalytic Oxidations Katrin Pelzer

2 Co-workers MBMS group

3 Catalytic Oxidations Enormous industrial importance High temperatures > 1000 C e.g. HCN synthesis Chemical transformations occur on the catalyst surface Described by surface reaction steps only Exothermic surface reactions rise the temperature of surrounding gas phase Surface and gas phase reaction steps take place simultaneously Heterogeneoushomogeneous mechanism Handbook of Heterogeneous Catalysis, Vol. 1, p. 21

4 Heat transport in wall Reactor Modeling with Detailed Chemistry Homogeneous reaction pathways become feasible: Transport of energy, species Gas phase reactions Adsorption surface reactions desorption Deutschmann O., Interactions between Transport and Chemistry in Catalytic Reactors, Habilitation. Thermal radiation Diffusion Exothermic surface reactions Energetic coupling Desorption of heterogeneously formed intermediates Substancial coupling key intermediates: radicals

5 Homogeneous-Heterogeneous Mechanism Surface Gas Phase Interaction: Energetic + Substantial Coupling oxidative coupling of CH 4 on strong basic oxides or Pt 2 CH 4 + ½ O 2 CH 3 CH 3 + H 2 O Radical formation: By heat exchange CH 4,s + O - s CH 3,s + OH- CH 3,s CH 3,g CH 3,g + CH 3,g CH 3 CH 3,g Pt or MgO By surface desorption Lunsford J. H. Angew. Chem. Int. Edit. Engl. 1995, 34,

6 Homogeneous Contributions Dehydrogenation of ethane to ethene over Pt in the gas phase C 2 H 6,g C 2 H 4,g + H 2,g Δ r H = +137 kj/mol Heat from surface oxidation: C 2 H 6 + 7/2 O 2 2 CO H 2 O Δ r H = kj/mol (energetical coupling) Huff, Androulakis, Sinfelt, J. Catal. 2000, 191, Oxidative coupling of methane to ethylene and water 2 CH 4 + O 2 C 2 H H 2 O Catalysts produces CH 3 radicals: gas phase coupling to ethane and dehydrogenation to ethylene (substantial coupling) Mims, Mauti, Dean, Rose, J. Phys. Chem. 1994, 98,

7 Mechanistic details Elementary steps Intermediates

8 Motivation In-situ investigation of the gas phase above a catalyst working under technical conditions Detection of reactive gas phase intermediates indication of homogeneous reaction steps Knowledge for optimization or development of new high temperature processes Understanding of mechanistic details of heterogeneoushomogeneous reactions

9 Motivation CH 4 is the main component of natural gas substitution of crude oil as chemical feedstock in the future??? conventional way: steam reforming CH 4 + H 2 O CO + 3 H 2 ΔH r = +206 kj/mol (Ni catalyst, τ ~ 1s)

10 Motivation Functionalization and upgrading of small hydrocarbons to olefins or oxygenates e.g. Oxidative coupling of methane to C 2 HCs (MgO, Pt) 2 CH 4 + ½ O 2 C 2 H 6 + H 2 O Δ r H = -221 kj/mol J.H. Lunsford, Angew. Chem. Int. Edit. Engl. 1995, 34, Production of syngas from methane via CPO over different metal catalysts alternatively to highly endothermic steam reforming process e.g. over Rh CH 4 + ½ O 2 CO + 2 H 2 Δ r H = -36 kj/mol n CO + (2n+1) H 2 C n H (2n+2) + n H 2 O (Co, Fe) D. A. Hickman, L. D. Schmidt, Science, 1993, 259,

11 Target Reaction alternative way: Catalytic Partial Oxidation (CPO) CH 4 + ½ O 2 CO + 2 H 2 ΔH r = -36 kj/mol (Ni, Pt, Rh τ ~ 1ms) direct mechanism indirect mechanism A. P. E. York, T. Xiao, M. L. H. Green, Topics in Catalysis 1993, 22 (3-4),

12 Target Reaction: CPO Methane Analysis of products methane CPO reactor group of L. D. Schmidt University of Minneapolis, USA

13 Molecular Beam Mass Spectrometer Mass spectrometer Collimator chamber Skimmer chamber Pyrometer scanner Reactor chamber Turbomolecular pump

14 Wall Reactor Setup Pt/Rh tube 90/10 w%, 10 mm, 5.0 mm OD, 0.3 mm wall thickness Fabeckstr. Room 3032 tiny orifice

15 Catalytic Wall Reactor to MS adiabatic expansion quenching of all gas phase species 1. Reaction tube 2. Nozzle position 3. Tube clamps 4. Water cooling 5. Mounting rods 6. Insulation bushings 7. Springs 8. Windows 9. Positioning rings 10. Reactor holder 11. Electrical contacts 12. Gas in 13. Gas out Horn et al., Review of Scientific Instruments (2006), 77(5), / /9.

16 Reactor Setup High temperature catalytic wall reactor installed in MBMS chamber

17 Molecular Beam Formation Reacting gases expand through the nozzle into the vacuum background: free jet Investigation of the reaction composition on a timescale of milliseconds Nozzle in Pt wall reactor p [mbar] 2*10-7 2*10-5 1* µm

18 Pyrometer setup with scanning mirror 310 mm Reading point: ~ 3-4 mm Increment: 4 mm 22 measurements on tube length 80 mm wall reactor Pyrometer 300 mm Pyrometer scanner cross section Rotating mirror (0.45 / step )

19 Temperature Profiles Pyrometer for reaction controlling and temperature profile measurements Pyrometer setup Controller Box Scanning Mirror Scanning-Dot on Pt-Tube

20 Appearance Potential MS Discrimination between species with the same mass numbers e - + CH 3 CH e - m/z = IP(CH 3+ ) = 9.84eV e - + CH 4 CH 3+ + H + 2e - 15 amu AP(CH 3+ ) = 14.30eV e - + C 2 H 6 CH 3+ + CH 3 + 2e - AP(CH 3+ ) = 13.46eV Ionization potential (IP) of an atom or molecule is the energy required to remove completely an electron Minimum energy that must be imparted to an atom or molecule to produce a specified ion is called appearance potential (AP) X can be selectively detected at m/z (X z+ ) if IP(X) < electron energy < AP (X z+ /XY) Identification of the reactive gas phase species by their IP/AP potentials threshold ionization method

21 Threshold Ionization Discrimination between radicals and fragments of stable molecules c/s Ionization energy: 10 ev Peak formation at 15 amu 400 ilament CH 4 CH CH 3 CH 3 e - e - e - e amu CH 3+?? c/s Ionization energy: 12 ev No peak overlap Focus to MS amu

22 Threshold Ionization CO in N 2 as model system for CH 3 radical detection CH 3. in methane CPO on Pt CO in N 2 Expected radical concentration: ppm Measured concentration range: ppm 12 C 1 H 3+ / 12 C 1 H 3 at m/z = 15amu CH 3 + e - CH e - IP = 9.84 ev 12 C 1 H 3+ / 12 C 1 H 4 at m/z = 15amu CH 4 + e - CH 3+ + H + 2 e - AP = ev 14 N 2+ / 14 N 2 at m/z = 28amu N 2 + e - N e - IP = ev 12 C 16 O + / 12 C 16 O at m/z = 28amu CO + e - CO + 2 e - IP = ev

23 Threshold Ionization CO in N 2 as model for CH 3 detection: Linear calibration 14 N 2+ / 14 N 2 at m/z = 28 amu N 2 + e - N e - Detection Limit: 230 ppm IP = ev 12 C 16 O + / 12 C 16 O at m/z = 28 amu CO + e - CO e - IP = ev Measured concentration range: ppm

24 Simulated Results Calculated temperatures and radical concentrations Temperature calculation within the Pt-tube Surface (Pt) and gas phase mechanisms available for simulation Mechanism requires experimental data for validation Zerkle, Allendorf, Wolf, Deutschmann J. Catal. 2000, 196, Mims, Mauti, Dean, Rose, J. Phys. Chem. 1994, 98 (50), Radical concentration for the same example Max. radical concentration: 1250 ppm 1173 K, 200 cm/s, C/O = 1 (simulation with ChemKin) Max. T: 1573 K CH 4 /O 2 : 500/450 ml/min K. Williams

25 Ionization Efficiency Curves Experiment offers Ionization Efficiency curves: Intensity of an ion as a function of the energy of the ionizing electron i = f(v) Variation of the electron energy: 4eV-150eV Steps: 0.1eV Electron bombardment ionization Ionization probability p: p(e) (E-Ei)n Simple ionization n = 1, double ionization n = 2... Linear ascent of the intensity of the mass from the corresponding IP Problem: thermal energy spread of electrons (Maxwell-Boltzmann) dn N 2 E = 3/ 2 π ( kt ) e E kt de

26 Experimental Approach catalytic wall reactor (Pt, T max = 1300 C, atmospheric pressure) coupling to a QMS via molecular beam sampling interface QMS with electron impact source & threshold ionization capability principle discrimination of interfering ions (same nominal m/z value) by means of their ionization- and appearance potentials Inhomogeneous electrons determination of shape and width of the electron energy spread function

27 Energy Spread and Offset Energetically inhomogeneous electrons from the source: Filament contaminations Thermal energy spread (Maxwell-Boltzmann) Potential drop along the filament Potential gradients inside the source N 2 at 28 amu IP N2 = 15.6 ev energy offset 1 ev energy spread: σ = 0.49eV IP ± 2σ = IP ± 0.5eV 1 ev Gaussian i( V ) C = σ 2π IP e ( E V ) 2 2σ 2 ( E IP) de

28 Homogeneous-Heterogeneous Mechanism surface gas phase interaction oxidation reactions start at the surface heat of reactions (ΔrH << 0) increase gas phase temperature heat generation much faster than heat removal reactor light-off, reactor runs autothermally surpass of homogeneous reaction barriers gas phase reactions possible F. Cavani, F. Trifirò, Catal. Today, 1999, 51, T. A. Garibyan, L. Y. Margolis, Catal. Rev.-Sci. Eng. 1989, 31, starting sequence 300 ml/min CH 4, 240ml/min O 2, C/O = 0.6

29 Temperature Profile Reaction ignition ~ C T [ C] CH 4 = 600 ml/min O 2 = 500 ml/min He = 200 ml/min C/O = tube position [mm] heating cold reaction Gas flow cold heating start heating reaction start reaction

30 Spatially Resolved Measurements Sliding the reaction zone along the nozzle by increasing the flow rate (adding He) V & & < & 1 < V2 V3 V & 1 V & 2 V & 3

31 Reaction-Zone-Shifting Access to different reaction zones by variation of temperature during methane CPO He: 2000 ml/min CH 4 : 200 ml/min O 2 : 150 ml/min C/O = 0.7 T Zone : 681 C Only educts detectable T 1 H 2 He CH 4 H 2 O CO CO 2 O 2

32 Reaction-Zone-Shifting T Zone : 925 C mainly total oxidation products T 2 H 2 He CH 4 H 2 O CO CO 2 O 2

33 Reaction-Zone-Shifting T Zone : 1103 C partial oxidation products T 3 H 2 He CH 4 H 2 O CO O 2 CO 2

34 GC analysis Products of radical recombination: C 2 hydrocarbons during catalytic partial oxidation of methane Recombination of CH 3 fragments signal [v] ethylene 0.43 % ethane 0.23 % CH 3. + CH3. acetylene 0.18 % C 2 H 6 -x H C 2 H 2 6 C 2 H 2, C 2 H time [min]

35 Detection of intermediates CH 3 in oxidative coupling of methane on Pt Expected radical concentration: ppm 12 C 1 H 3+ / 12 C 1 H 3 at m/z = 15 amu CH 3 + e - CH e - IP = 9.84 ev 12 C 1 H 3+ / 12 C 1 H 4 at m/z = 15 amu CH 4 + e - CH 3+ + H + 2 e - AP = ev CH 4 : 600 ml/min O 2 : 500 ml/min He: 200 ml/min T max : 1520 K

36 Detection of CH 3 Horn et al., Review of Scientific Instruments (2006), 77(5), / /9.

37 IE curves: Radical concentrations Radical formation in the gas phase??? 7000 peak area [c] CH 3 + from CH ev ev offset CH 3 + from CH ev ev offset > 1300 C ~ 1250 C ~ 1100 C energy [ev] Background not heated Background heated CPO 80%,40A CPO 0%, 0A CPO 50%, 30A CPO new, 80%, 40A

38 Product Compositions Compound Conc C Conc. > 1300 C CO CO H O CH C 2 H C 2 H C 2 H carbon monoxide and dioxide nearly unaffected more hydrogen production of C 2 compounds oxygen nearly completely consumed observation of benzene Radicals are involved in the mechanism!!!

39 Radical reactions Anders Holmen, Ola Olsvik, O. A. Rokstad, Fuel Processing Technology 42, (1995). Radicals are formed in gas phase or at catalyst surface at high temperatures Started radical chain reactions lead to the desired products Gas Phase Reaction started

40 Surface Investigations Tube surface after usage L 3 b d a c a b wie 2a 4 5 a b a a b c DSC00979 DSC00981 Gas in 5 mm Light microscope image Document archive: ID 9340

41 Catalyst Aging 0 1c 2b 3 4b 5c 1350 temperature[ C] Temperature [ C] tube position [mm]

42 Morphological transformations hottest reaction zone CO 2 and H 2 O thermodynamically favored in the inlet region of the tube exothermicity of the total oxidation reaction creates the highest temperature altered by thermal etching pit formation and faceting catalytically etched with the generation of grain-like structures (Pt/Rh)

43 Surface Studies SEM image of area 4b hot outgas zone: -Pt/Rhcrystals - Carbon coverage Carbon formation Pt and Rh transported via the gas phase In colder regions Pt is strongly enriched pure Pt particles downstream of the reactor shown by EDX

44 Take home messages First in-situ observation of radicals in methane partial oxidation under reaction conditions Radicals are directly involved in reaction mechanism Gas Phase reaction starts at temperatures > 1200 C Strong increase in C 2 production and benzene formation Outlook: Quantification of radicals Usage of pure Pt tube ID 11028

45 Thank you for the attention!!!