In situ molecular beam mass spectrometry for high

Molecular Beam Mass Spectroscopy 20/11/2006Katrin Pelzer, Dept. Inorg. Chemistry, FHI-MPG, Berlin

In situ molecular beam massspectrometry for high temperature

catalysis research:Heterogeneous – Homogeneous

Catalytic OxidationsKatrin Pelzer


Co-workers

• MBMS group

http://images.google.de/imgres?imgurl=http://www.ipter.bashedu.ru/ipter_www/globus.JPG&imgrefurl=http://www.ipter.bashedu.ru/ipter_www/intproject%25201.htm&h=1103&w=1133&sz=173&tbnid=oh4Na_C6IAN8zM:&tbnh=146&tbnw=150&hl=de&start=51&prev=/images%3Fq%3Dglobus%26start%3D40%26svnum%3D10%26hl%3Dde%26lr%3D%26sa%3DN


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

Heterogeneous-homogeneous mechanism

Surface and gas phase reactionsteps take place simultaneously

Handbook of Heterogeneous Catalysis, Vol. 1, p. 21


Reactor Modeling with Detailed Chemistry

Homogeneous reaction pathways become feasible:

• Exothermic surface reactions– Energetic coupling

• Desorption of heterogeneously formed intermediates– Substancial coupling

Transport of energy, species

Heat transport in wall

Gas phasereactions

Thermalradiation Diffusion

Adsorptionsurface reactionsdesorption

Deutschmann O., Interactions between Transport and Chemistry in Catalytic Reactors, Habilitation.

key intermediates: radicals


Homogeneous-Heterogeneous Mechanism

• Surface – Gas Phase Interaction: Energetic + Substantial Coupling

Pt or MgO

CH4,s + O-s → CH3,s⋅ + OH-

CH3,s⋅ → CH3,g⋅

CH3,g⋅ + CH3,g⋅ → CH3CH3,g

Radical formation:By heat exchange

By surface desorptionLunsford J. H. Angew. Chem. Int. Edit. Engl. 1995, 34, 970-980.

oxidative coupling of CH4 on strongbasic oxides or Pt

2 CH4 + ½ O2 → CH3CH3 + H2O


Homogeneous Contributions

• Dehydrogenation of ethane to ethene over Pt in the gas phaseC2H6,g → C2H4,g + H2,g ΔrH° = +137 kJ/mol

Heat from surface oxidation: C2H6 + 7/2 O2 → 2 CO2 + 3 H2O ΔrH° = -1560 kJ/mol

(energetical coupling)

Huff, Androulakis, Sinfelt, J. Catal. 2000, 191, 46-54.

• Oxidative coupling of methane to ethylene and water2 CH4 + O2 →C2H4 + 2 H2O

Catalysts produces CH3 radicals: gas phase coupling to ethane anddehydrogenation to ethylene(substantial coupling)

Mims, Mauti, Dean, Rose, J. Phys. Chem. 1994, 98, 13357-13372.


Mechanistic details

Elementary steps

Intermediates


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 heterogeneous-homogeneous reactions


Motivation

• CH4 is the main component of natural gas

• substitution of crude oil as chemical feedstock in the future???• conventional way: steam reforming

CH4 + H2O → CO + 3 H2 ΔHr = +206 kJ/mol (Ni catalyst, τ ~ 1s)


Motivation

• Functionalization and upgrading of small hydrocarbons to olefins or oxygenates– e.g. Oxidative coupling of methane to C2 HCs (MgO, Pt)

2 CH4 + ½ O2 → C2H6 + H2O ΔrH° = -221 kJ/mol

J.H. Lunsford, Angew. Chem. Int. Edit. Engl. 1995, 34, 970-980.

• Production of syngas from methane via CPO over different metal catalysts alternatively to highly endothermic steam reforming process– e.g. over Rh CH4 + ½ O2 → CO + 2 H2 ΔrH° = -36 kJ/mol

– n CO + (2n+1) H2 → CnH(2n+2) + n H2O (Co, Fe)…

D. A. Hickman, L. D. Schmidt, Science, 1993, 259, 343-346.


Target Reaction

indirect

mechanism

direct mechanism

• alternative way: Catalytic Partial Oxidation (CPO)CH4 + ½ O2 → CO + 2 H2 ΔHr = -36 kJ/mol (Ni, Pt, Rh τ ~ 1ms)

A. P. E. York, T. Xiao, M. L. H. Green, Topics in Catalysis 1993, 22 (3-4), 345-358.


Target Reaction: CPO Methane

methane CPO reactor

group of L. D. Schmidt

University of Minneapolis, USA

• Analysis of products


Molecular Beam Mass Spectrometer

Mass spectrometer

Skimmer chamber

Reactor chamber

Pyrometer scanner

Turbomolecular pump

Collimator chamber


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


Catalytic Wall Reactor

1. Reaction tube2. Nozzle position3. Tube clamps4. Water cooling5. Mounting rods6. Insulation bushings7. Springs8. Windows9. Positioning rings10. Reactor holder11. Electrical contacts12. Gas in13. Gas out

to MS adiabatic expansion quenching of all gas phase

species

Horn et al., Review of Scientific Instruments (2006), 77(5), 054102/1-054102/9.


Reactor Setup

High temperature catalytic wall reactor installed in MBMS chamber


Molecular Beam Formation

125 µm

Nozzle in Pt wall reactor

• Reacting gases expand through the nozzle into the vacuum background: “free jet”

• Investigation of the reaction composition on a timescale of milliseconds

2*10-7

2*10-5

1*10-4

p [mbar]


Pyrometer setup with scanning mirror

Rotating mirror (0.45 °/ step )

wall reactor

Pyrometer

Pyrometer scanner cross section

300 mm

80 mm

310 mm • Reading point: ~ 3 - 4 mm• Increment: 4 mm• 22 measurements on tube length


Temperature Profiles

Pyrometer for reaction controlling and temperature profile measurements

Pyrometer setup

Controller Box

Scanning Mirror

Scanning-Dot onPt-Tube


Appearance Potential MS

• Discrimination between species with the same mass numbers

– e- + CH3⋅ → CH3+ + 2e- m/z = IP(CH3

+) = 9.84eV– e- + CH4 → CH3

+ + H⋅ + 2e- 15 amu AP(CH3+) = 14.30eV

– e- + C2H6 → CH3+ + CH3⋅ + 2e- AP(CH3

+) = 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 (Xz+) if IP(X) < electron energy < AP (Xz+/XY)

Identification of the reactive gas phase species by their IP/AP potentials

threshold ionization method


Threshold Ionization

Discrimination between radicals and fragments of stable molecules

e-

e-

e-e-

CH4

CH3.

CH3CH3

CH3+??

to MS

Filament

Focus

Ionization energy: 12 eV

Ionization energy: 10 eV Peak formation at 15 amu

No peak overlap

400

200

11 12 13 14 15 16 17 18

c/s

amu

11 12 13 14 15 16 17 18

c/s

amu

1000500

15002000

25003000



CO in N2 as model system for CH3⋅ radical detection

Expected radical concentration:

102 – 103 ppm

CH3. in methane CPO on Pt CO in N2

Measured concentration range:

2260 – 20960 ppm

CH3⋅ + e- → CH3+ + 2 e-

12C1H3+ /12C1H3⋅ at m/z = 15amu

IP = 9.84 eV

CH4 + e- → CH3+ + H⋅ + 2 e-

12C1H3+ /12C1H4 at m/z = 15amu

AP = 14.01 eV

N2 + e- → N2+ + 2 e-

14N2+ /14N2 at m/z = 28amu

IP = 15.58 eV

CO + e- → CO + 2 e-

12C16O+ /12C16O at m/z = 28amu

IP = 14.014 eV



N2 + e- → N2+ + 2 e-

14N2+ /14N2 at m/z = 28 amu

IP = 15.58 eV

CO + e- → CO+ + 2 e-

12C16O+ /12C16O at m/z = 28 amu

IP = 14.014 eV

Measured concentration range:2260 – 20960 ppm

CO in N2 as model for CH3• detection: Linear calibration

Detection Limit:230 ppm


1173 K, 200 cm/s, C/O = 1 (simulation with ChemKin)

Max. T: 1573 K CH4/O2 : 500/450 ml/min

K. Williams

• Calculated temperatures and radical concentrations

Simulated Results

Temperature calculation within the Pt-tube

Radical concentration for the same example

• Surface (Pt) and gas phase mechanisms available for simulation

• Mechanism requires experimental data for validation

Zerkle, Allendorf, Wolf, Deutschmann J. Catal. 2000, 196, 18-39.Mims, Mauti, Dean, Rose, J. Phys. Chem. 1994, 98 (50), 13357.

Max. radical concentration:1250 ppm


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)

( )dEe

kTE

NdN kT

E−

= 2/32π


Experimental Approach

• catalytic wall reactor (Pt, Tmax= 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


Energy Spread and Offset

• N2 at 28 amu• IP N2 = 15.6 eV energy offset ≈ 1 eV

Energetically inhomogeneous electrons from the source:• Filament contaminations• Thermal energy spread (Maxwell-Boltzmann)• Potential drop along the filament• Potential gradients inside the source

energy spread: σ = 0.49eV IP ± 2σ = IP ± 0.5eV ≈ 1 eVGaussian

∫∞ −

−−

⋅=

IP

VE

dEIPEeCVi 127.12)(

)(2

)( 2

2

σ

πσ


starting sequence

300 ml/min CH4, 240ml/min O2, C/O = 0.6

surface ⇔ gas phase interaction

Homogeneous-Heterogeneous Mechanism

F. Cavani, F. Trifirò, Catal. Today, 1999, 51, 561-580. T. A. Garibyan, L. Y. Margolis, Catal. Rev.-Sci. Eng. 1989, 31, 355-384.

• 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


Temperature Profile

Gas flow

8.4

7.6

6.8 6

5.2

4.4

3.6

2.8

2

1.2

0.4

coldheating

reaction

0

200

400

600

800

1000

1200

1400

tube position [mm]

T [°

C]

cold heating start heating reaction start reaction

Reaction ignition ~ 600 – 700 °C

CH4 = 600 ml/minO2 = 500 ml/minHe = 200 ml/minC/O = 0.6


Spatially Resolved Measurements• Sliding the reaction zone along the nozzle by increasing the flow rate (adding He)

321 VVV &&& <<

1V& 2V& 3V&


Reaction-Zone-Shifting• Access to different reaction zones by variation of temperature

during methane CPO

He CH4H2OH2 CO CO2O2

T1

TZone: 681 °C

Only educts detectable

He: 2000 ml/minCH4: 200 ml/minO2: 150 ml/minC/O = 0.7


Reaction-Zone-Shifting

TZone: 925 °C

mainly total oxidation products

He CH4 H2OH2 CO CO2O2

T2


Reaction-Zone-Shifting

TZone: 1103 °C

partial oxidation products

He CH4 H2OH2 CO CO2O2

T3


GC analysis• Products of radical recombination:

– C2 hydrocarbons during catalytic partial oxidation of methane– Recombination of CH3 fragments

0.3

0.35

0.4

0.45

0.5

0.55

0.6

28 29 30 31 32 33 34 35

time [min]

sign

al [v

]

ethylene0.43 %

ethane0.23 %

acetylene0.18 %

CH3. + CH3

. C2H6

C2H6 C2H2, C2H4- x H2


Detection of intermediates• CH3· in oxidative coupling of methane on Pt• Expected radical concentration: 100 – 1000 ppm

CH3· + e- → CH3+ + 2 e-

12C1H3+ /12C1H3· at m/z = 15 amu

IP = 9.84 eVCH4 + e- → CH3

+ + H· + 2 e-

12C1H3+ /12C1H4 at m/z = 15 amu

AP = 14.01 eV

CH4: 600 ml/minO2: 500 ml/minHe: 200 ml/min

Tmax: 1520 K


Detection of CH3·

Horn et al., Review of Scientific Instruments (2006), 77(5), 054102/1-054102/9.


0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

9 10 11 12 13 14 15 16energy [eV]

peak

are

a [c

]

Background not heated Background heated CPO 80%,40ACPO 0%, 0A CPO 50%, 30A CPO new, 80%, 40A

> 1300°CCH3

+ from CH3·9.84 eV + 0.6 eV offset

IE curves: Radical concentrations

CH3+ from CH4

14.01 eV + 0.6 eV offset

Radical formation in the gas phase ???

~ 1250°C

~ 1100°C


Compound Conc. [Vol%] @ 1100°C Conc. [Vol%] @ > 1300°CCO 20 27.7CO2 3.8 4.8H2 5 37O2 4 0.5

CH4 30 8.6C2H2 0.01 5C2H4 0.4 0.32C2H6 0.3 -

• carbon monoxide and dioxide nearly unaffected• more hydrogen• production of C2 compounds• oxygen nearly completely consumed• observation of benzene

Radicals are involved in the mechanism!!!

Product Compositions


Radical reactions

• Radicals are formed in gas phase or at catalyst surface at high temperatures• Started radical chain reactions lead to the desired products

Anders Holmen, Ola Olsvik, O. A. Rokstad, Fuel Processing Technology 42, 249-267 (1995).

Gas Phase Reaction started


Surface Investigations

Tube surface after usage

Document archive: ID 9340Light microscope image

5 mm

DSC00981DSC00979

ab

cd

a bwie 2a

0 1 2 3 3La b ca ba

4 5

Gas in


5c4b32b1c0

Catalyst Aging

350

550

750

950

1150

1350

0 1 2 3 4 5 6 7 8 9

tube position [mm]

tempe

rature [°C]

Tem

pera

ture

[°C

]


Morphological transformations

• altered by thermal etching– pit formation and faceting

• catalytically etched– with the generation of grain-like

structures (Pt/Rh)

• hottest reaction zone– CO2 and H2O thermodynamically favored in the inlet region of the tube– exothermicity of the total oxidation reaction creates the highest temperature


Surface Studies

• SEM image of area 4b – hot outgas zone:- Pt/Rh crystals- Carbon coverage

• 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

Carbon formation


Take home messages

• Gas Phase reaction starts at temperatures> 1200 °C

• Strong increase in C2 production and benzene formation

• Outlook:– Quantification of radicals– Usage of pure Pt tube

First in-situ observation of radicals in methane

partial oxidationunder reaction conditions

ID 11028

Radicals are directly involved in reaction

mechanism


Thank you for the attention!!!

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In situ molecular beam mass spectrometry for high