High Temperature Catalysis

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1 Fritz-Haber Haber-Institute, Max-Planc Planc-Society, Inorganic Chemistry Deartment High Temerature Catalysis Raimund Horn Lecture Series Modern Methods in Heterogeneous Catalysis 3..9

2 Outline. What is High Temerature Catalysis. Why can solid bodies glow and how can I measure their temerature? 3. High Temerature Catalytic Processes in Industry and Research 4. Surface and Gas Phase Reaction inetics 5. Physical Transort Processes of Momentum, Heat and Mass 6. Interaction of Chemistry and Transort 7. Numerical Simulation of High Temerature Catalytic Reactions 8. Research Examle: utothermal Methane Oxidation on Rh and Pt

3 . What is High Temerature Catalysis ΔG RT r e r > # o r g

4 High Temerature Catalysis Catalysis on Glowing Catalysts 55 C T 3 C

5 High Temerature Catalysis Catalysis on Glowing Catalysts 55 C T 3 C

6 . What is High Temerature Catalysis

7 . Why can solid bodies glow? o when a solid body is heated its surface emits radiation of wavelengths in the range.-µm called thermal radiation o heating raises some of the atoms and molecules of which the solid body consists of to higher energy levels from which they return sontaneously to lower energy states emitting electromagnetic radiation o electromagnetic radiation can be either thought off as waves with a wavelength λc/ν or as a bunch of hotons with an energy εhν o absortivity and emissivity of an oaque body are defined as follows: a q a ν ν e i ν qν q q e ν i bν o q ν a dν and q ν i dν are the absorbed and incident radiation energy er unit area er unit time in the frequency range ν to νdν

8 . Why can solid bodies glow? a q a ν ν e i ν qν q q e ν i bν Some comments on that: o for any real body, a ν will be less than unity and will vary considerably with ν and temerature o a hyothetical body for which a ν will be less than unity but indeendent of ν and temerature is called a gray body o the limiting case of a gray body with a ν defines a blac body o the emissivity e ν is also a quantity less than unity for real, nonfluorescing surfaces and is equal to unity for blac bodies

9 . Why can solid bodies glow? Material Material Material T T T Material T T q q cav e b

10 . Why can solid bodies glow? Material number of modes inside a cavity of volume V 3 8πν dn N ν V ν 3c V dν Traditional Physics Rayleigh-Jeans 95 8πν 3 3 n c n T T q q cav e b E ex [ E / T ] de E T ex[ E / T ] de E dn 8πν ρν ν E T 3 V dν c Ultraviolet-Catastrohe n ρ ν n nhν ex n ν ex E Quantum Physics Max Planc 9 [ nhν / T ] [ nhν / T ] V dn 8πν 3 dν c n ex hν hν / T hν ex hν / T

11 . Why can solid bodies glow? Wien s Dislacement Law: Stefan-oltzmann Law: λ max q e b T. 884 cm 4 σ T W m T 4

12 . Temerature Measurement by Pyrometrie Material Material T T T q q cav e b aq cav emissivities of different materials T q q e e q a e b e

. Temerature Measurement by Pyrometrie Radiance of a blac body P T σ T

σ T ε λ ε λ 4 4 4 4 cylindrical grahite body cavity yielding

.. total ower of energy radiated from P an object ε... emissivity σ.

13 . Temerature Measurement by Pyrometrie Radiance of a blac body P T σ T Radiance of a real body P T ε λ σ T Two color yrometer P P ε λ σ T ε λ σ T ε λ ε λ cylindrical grahite body cavity yielding homogeneous temerature distribution blac body P... total ower of energy radiated from P an object ε... emissivity σ... Stefan oltzmann constant... surface area observed T... absolute temerature carbon calibration 6 C

14 . Temerature Measurment by Pyrometrie

15 3. High Temerature Catalytic Processes in Industry and Research high temerature catalysis lays an imortant role in industry industrial high temerature rocesses 4NH 3 5O 4NO 6H O 9 C, Pt/Rh NH 3 CH 4.5O HCN 3H O C, Pt/Rh NH 3 CH 4 HCN 3H C, Pt/Rh CH 4 H O CO 3H 7- C, Ni CH 3 OH ½ O HCHO H O 6-7 C, g catalytic combustion noble metals fuel reforming noble metals solid oxide fuel cells 8- C, Ni cermet / YSZ / La,SrMnO 3

16 3. High Temerature Catalytic Processes in Industry and Research high temerature rocesses at the research stage CH 4 ½ O CO H > C, e.g. Rh CH 4 O C H 4 H O 6-8 C, e.g. Li/MgO CH 4 ½ O HCHO H O >6 C, e.g. VO x C H 6 ½ O C H 4 H O 7- C, e.g. Pt/Sn C H 6 ½ O CH 3 CHO H O >5 C, VO x

4. Surface and Gas Phase Reaction inetics 3. ooeeing examle: corrosion of coer domains: e.g. gaseous domain 3D, bul domain 3D, interhase domain D hases: o gas hase usually er domain o surface hase or more er domain e.

17 4. Surface and Gas Phase Reaction inetics 3. ooeeing examle: corrosion of coer domains: e.g. gaseous domain 3D, bul domain 3D, interhase domain D hases: o gas hase usually er domain o surface hase or more er domain e.g. stes, terraces o bul hase or more er domain, e.g. CuO, Cu S secies: o gas secies f g - gl, surface secies sf n- sl n, bul secies bf n- bl n

$.. l g in mol 3 m o the comosition of surface hases is usually secified in terms of site fractions l s n f s Z n n n N f s,.$

18 4. Surface and Gas Phase Reaction inetics 4. Concentration within hases imortant for catalysis: o for gas hase secies 3D domain the molar concentration is written Y ρ [ X ] W f g,... l g in mol 3 m o the comosition of surface hases is usually secified in terms of site fractions l s n f s Z n n n N f s,..., N l s Z n Γ σ n mol m o for inetics we need the surface molar n [ X ] in concentration of a secies

19 4. Surface and Gas Phase Reaction inetics 4.3 Surface reaction inetics: o classic exressions for adsortion Langmuir adsortion, cometitive adsortion, dissociative adsortion and surface reaction rates Langmuir Hinshelwood Hougen Watson can be used to describe surface inetics in high temerature catalytic reactions o these tye of rate exressions are comarably ease to determine exerimentally but every catalyst will have a unique rate exression o for examle, Pt on alumina catalysts for CO oxidation may have a similar general form of the rate exression by the numerical values of the constants will vary among formulations. Even catalysts with the same comosition may have different rate exressions due to differences in the manufacturing method o if the mechanism changes with exerimental conditions, e.g. change of rate limiting ste at changing temeratures, the general form of the rate law might change o elementary ste mechanisms do not have these drawbacs but are difficult to determine

20 examle : Langmuir adsortion isotherm for a single comonent o the usual form of the Langmuir adsortion isotherm is [ ] / / / / / / ] [ ] [ ] [ ] [ ] [ ] ][ [ ] [ ] [ ] [ ] [ ] ][ [, RT RT RT RT s s s s O s s O dt s d s s O a a c c Γ Γ Γ θ θ b b θ o this general form is readily derived from an elementary ste mechanism alying mass action inetics Surface and Gas Phase Reaction Surface and Gas Phase Reaction inetics inetics

21 examle : cometitive adsortion of two secies and o emirical exression / / / ] [ ] [ ] [ / / / ] [ ] [ ] [,,,,,,,,,,,,,, s s O s s O c c c c c c θ θ θ θ o an analogues treatment of the following elementary ste mechanism using mass action inetics as for the Langmuir adsortion leads to Surface and Gas Phase Reaction Surface and Gas Phase Reaction inetics inetics

22 4. Surface and Gas Phase Reaction inetics examle 3: Langmuir-Hinshelwood Rate Exressions o Langmuir-Hinshelwood Hougen Watson rate exressions are often used to describe surface reactions such as e.g. ss Cs o in this mechanism it is assumed that and adsorb cometitively onto the surface and undergo a bimolecular surface reaction to C, O s s, O s s rxn s s C O s θ Γ c, θ Γ [ s ] [ s ] d[ C] dt rxn c, [ s][ s] c,γ[ ] [ ] c,[ ] c,γ[ ] [ ] c,[ ] rxn c, c,γ [ ][ ] [ ] [ ] c, c,

23 4. Surface and Gas Phase Reaction inetics examle 3: literature results on CO oxidation on suorted Pt catalysts Can. J. Chem. Eng

24 4. Surface and Gas Phase Reaction inetics examle 4: elementary ste inetic model: q i i i T i ß [ X ] i ν i ex Ei RT

25 o gas hase reactions are tyically strongly ressure deendent and the inetics are highly nonliner 4. Surface and Gas Phase Reaction inetics 4.4 Gas hase reaction inetics: o gas hase reactions roceed via radicals, often in chain reactions o radical chain reactions consist of initiation, branching and termination reactions o gas hase reactions are tyically strongly ressure deendent and the inetics are highly nonliner examle: H O H O Initiation ranching Termination H O H HO Proagation H O M HO M H O O H O H OH H O OH H OH O OH OH H O H H O HO HO M OH HO H, O, OH, HO HO H H O O O M O wall inert

26 Pressure deendence of gas hase reactions: o for surface reactions is a function of T o for gas hase reactions, can be a function of T and o a simle exlaination follows from the Lindemann mechanism Frederic Lindemann 9 [ ] [ ][ ] [ ] [ ][ ] [ ] [ ][ ] [ ] [ ] [ ][ ] [ ][ ][ ] [ ] [ ][ ] [ ] [ ] ~,, M f M M M M M C dt C d M C M C C dt C d M C M C C C observed observed Surface and Gas Phase Reaction Surface and Gas Phase Reaction inetics inetics

27 Limiting cases: 4. Surface and Gas Phase Reaction inetics observed observed observed [ ] M f [ M ] for [ M ] [ ] M for [ M ] M ~ Examle: rate constant for the association reaction CCl 3 O CCl O

28 4. Surface and Gas Phase Reaction inetics examle for non-linear behaviour and ressure deendence of gas hase reactions exlosion diagram for H /O system HO H H O H H H O O O OH M HO M H, O, OH, HO wall inert

29 5. Physical Transort Processes of Momentum, Heat and Mass 5. Momentum transort o air of arralel lates in rest with area searated by distance Y o t lower late is set in motion in ositive x direction with Vconstant F F V μ Y dv τ yx μ dy Newton s law of viscosity x τ yx flux of x momentum in y direction in g m/s/m /sn/m o the x-momentum flows from a region of high momentum to a region of low momentum similar to heat and mass o the roortionality constant µ is called the dynamic viscosity, has the unit Pa sg/m/s and is material secific air.8e-5 Pa s, glycerol Pa s

30 5. Physical Transort Processes of Momentum, Heat and Mass 5. Heat transort: Fourier s law of heat conduction o qheat flux in z direction in J/m /s o heat flows from a region of high temerature to low temerature o the roortionality constant λ is called the thermal conductivity, has the unit W/m/s and is material secific air.5, stainless steel6, l5

31 5. Physical Transort Processes of Momentum, Heat and Mass 5.3 Mass transort: Fic s law of diffusion o Jmolar flux in z direction in mol/m /s o mass atoms, molecules flow from a region with high concentration to a region with low concentration o the roortionality constant D is called the diffusion coefficient, has the unit m /s and is material secific H in N 7.79E-5

32 6. Interaction of Chemistry and Transort Examle: instantaneous reaction e.g. dimerization o catalyst surrounded by a stagnant film through which has to diffuse to reach the catalyst surface o at the surface, react instanteneously

33 6. Interaction of Chemistry and Transort combined molar flux : N steady state and stoichiometry require : N mass balance on over a thin slab gives : boundary condition : at z r cd z / δ inserting into the combined molar flux gives : N d dx ln x dz dz xa boundary condition : at z x x x x z δ x N z dx dz z dn C r dz x z N N z N z C cd δ z z z cd x ln ln x dx dz z ln

34 7. Numerical Simulation of High Temerature Catalytic Reactions Gas Surface Secies Thermodynamics ri Gas Secies Transort Data fi ci inetic Model of Surface Chemistry inetic Model of Gas Phase Chemistry Mathematical Model of Momentum, Heat and Mass Transort Reactor Model

35 7. Numerical Simulation of High Temerature Catalytic Reactions Examle: Plug Flow Model Secies alance ρu c dy dz & ω W P s& c W Examle: Plug Flow Model Energy alance ρu c c dt dz g g c &ω Wh P s& W h hp ˆ T T w system of couled differential algebraic equations since in catalysis s& s and Z

36 7. Numerical Simulation of High Temerature Catalytic Reactions o gas-hase and surface chemical rate exressions: ν χ i ν χ i i,...,i ν ν ν i i i gas concentration mol/m 3 ω& I i ν i q i q i fi ν i [ X ] [ X ] ri ν i surface concentration mol/m s& I i ν i q i q i fi ν i [ X ] [ X ] ri ν i

37 7. Numerical Simulation of High Temerature Catalytic Reactions o tyical rrhenius exression for the forward rate constant: fi T i β E i ex RT i o modification for coverage deendent rate constants ossible E RT θ, θ... βi i fi it ex f θ o rate constants for adsortion reactions are calculated from sticing coefficients fi γ m Γ π tot i RT W

38 7. Numerical Simulation of High Temerature Catalytic Reactions o in first aroximation thermodynamic roerties of secies regardless of hase are functions of temerature only o thermodynamic data are neeeded to calculate the reverse rate constant for gas hase reactions even if no energy balance is solved C M m a m T m H T M H C dt H RT m a m T m m a M, T S T C S dt S T 98 M m m a lnt am, R m m a T

39 7. Numerical Simulation of High Temerature Catalytic Reactions calculation of the reverse rate constant from the forward rate constant and the equilibrium constant ΔS R i ν i S R ΔH i RT ν i H RT i ci ΔSi ΔH i ex R RT ν i atm i RT fi ri ci if thermodynamic information for surface secies are not available which is usually the case, reverse rate arameters have to be secified

40 Glowing Thans For Your ttention!!!

dn i where we have used the Gibbs equation for the Gibbs energy and the definition of chemical potential

dn i where we have used the Gibbs equation for the Gibbs energy and the definition of chemical potential Chem 467 Sulement to Lectures 33 Phase Equilibrium Chemical Potential Revisited We introduced the chemical otential as the conjugate variable to amount. Briefly reviewing, the total Gibbs energy of a system