tap temporal analysis of products - fhi · • for porous solids, the relative pore volume ε b and...
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Lecture Series at Fritz-Haber-Institute Berlin„Modern Methods in Heterogeneous Catalysis“
24.11.2006
TAPTemporal Analysis of Products
Cornelia BreitkopfInstitut für Technische Chemie
Universität Leipzig
Transient methods are a powerful tool for gaining insights into the mechanisms of
complex catalytic reactions.
Dimensions in Heterogeneous Catalysis
Macrokinetics and Microkinetics
Kinetic measurements
Complexity of heterogeneously catalyzed reactions –Macrokinetics and Microkinetics
O. Deutschmann, XXXIV. Jahrestreffen Deutscher Katalytiker/ Fachtreffen Reaktionstechnik, Weimar 21.-23.3.2001
Kinetic measurements in practise
Tasks of lab investigations
o catalyst preparation
o catalyst screening
o activity
o selectivity
o stability
o scale-up
o process optimization
Laboratory reactors
o microcatalytic pulse reactor
o gaschromatographic reactor
o single pellet diffusion reactor
o catalytic fixed bed reactor
o recycle reactor
o TAP
Questions
• What distinguishes TAP from other lab reactors ?
• Which information do we need to understand TAP ?
• How can TAP applied in heterogeneous chemistry?
Outline
• Steady state and unsteady state experiments
• Diffusion and mathematics
• TAP-Method and modeling
• Applications
Recommended literature and books
• Gleaves JT et al. Appl. Cat. A 160 (1997) 55-88.• Phanawadee P. PhD thesis Washington 1997.• Chen S. PhD thesis Washington 1996.• Yablonsky GS et al. J. Catal. 216 (2003) 120-134.• Gleaves JT et al. Cat. Rev.-Sci. Eng. 30 (1988) 49-116.• Dumesic JA et al (Ed) „The Microkinetics of Heterogeneous Catalysis“ ACS, Washington 1993.• Stoltze P. Progress in Surface Science 65 (2000) 65-150.• Kobayashi H et al.. Cat. Rev.-Sci. Eng. 10 (1974) 139-176.• Engel T, Ert G. Adv. Catal. 28 (1979) 1-78.• Ertl G. The dynamics of interactions between molecules and surfaces. Berichte Bunsengesellschaft
für Physikalische Chemie 99 (1995) 1282.• Creten G et al. J. Catal. 154 (1995) 151-152.• Haber J. 1983. Concepts in catalysis by transition metal oxides. Bonelle JP et al. (Ed.) D. Reidel
Publishing Company. Surface Properties and Catalysis by Nonmetals. 45, 1-45.• Hinrichsen O. DECHEMA-Kurs „Angewandte Heterogene Katalyse“, Bochum 2001.• Dewaele O, Froment GF. J. Catal. 184 (1999) 499-513.• Christoffel EG „Laboratory Studies of Heterogeneous Catalytic Processes“ Stud.Surf.Sci.Catal. 42. • Müller-Erlwein E. Chemische Reaktionstechnik, Teubner Stuttgart-Leipzig, 1998, 237.
…continued
Jens Hagen
- Emig G., Klemm E.. Technische Chemie. Springer 2005.- Baerns M., Behr A., Brehm A., Gmehling J., Hofmann H., Onken U., Renken A.. Technische Chemie, Viley VCH 2006.
Steady state and unsteady state experiments
Kinetic investigations
Constructing of a mathematical model
Evaluation of thereaction mechanism
Parameter estimation
How to do…?
• Steady-state or unsteady-state experiment ?• Quantitative evaluation of kinetic data ?
Steady-state experiments
• Most common reaction technique used in heterogeneous catalysis
• Achieved by operation such that temperature, pressure, concentration, and flow rate at anypoint in the reactor is time invariant
• Access to activity, selectivity, reaction order, activation energy
Steady-state experiments
• Advantages:- Easy to build and operate- Results can be described with mathematical
models based on algebraic equations- Most industrial processes are operated under
steady-state conditions
• Disadvantages:- Provide global kinetic parameters, limited
information on individual reaction steps- Interpretations often based on „simple“
assumptions
Unsteady-state experiments
• Transient techniques provide information on
- Reaction intermediates (pulse response) (Gleaves 1988)
- Reaction sequence in a multistep reaction(Kobayashi 1975)
- Rate constants of elementary steps(Ertl 1979, Creten 1995)
Unsteady-state experiments
• Transient techniques provide information on
- Investigation of complex kinetic phenomena(oscillating chemical reactions, hysteresis) that are not observable under steady-stateconditions
- Probe of catalyst surfaces that are not easilyobserved under steady-state conditions(oxidation catalysis) (Haber 1983)
Unsteady-state experiments
• Disadvantages
- Not easy to build up, expensive
- Main problem: theory is very complex
Steady-state and transient methods
Steady-state methods
• measure overall performance
• give integrated picture of reaction system
• have minimum reactor residence time of 1 s
Transient methods
• give information on individual steps
• operate in millisecond time regime; resolution increase
Pulse methods
• response to pertubations describe mathematically thetransient behaviour
• evaluation of rate parameters from responsemeasurements, such as mass transfer coefficients, diffusivities, and chemical kinetic constants
• use of: - fixed-bed (column) chromatography- isotope technique- slurry adsorber- single-pellet
Pulse methods
Diffusion and mathematics
Diffusion in porous solids
Characteristic value to characterize the influence of internal transport phenomena of heterogeneous reactionson the surface between a fluid phase and a porous solid is DaII (see textbooks)
DaII = f (k, cexternal, lcharacteristic, Deff)
if fluid is gaseous: diffusion in pores depends ondimensions of pore system
Diffusion mechanisms
Transport mechanisms in porous solids
Pore diffusion dependingon pore diameter
Molecular diffusion
Atkins
Molecular diffusion
• mixture of two components A and B,concentration gradient (in one dimension y): under steady-state conditions the diffusional flow of onecomponent is described by 1. Fick law
• DAB ...binary molecular diffusion coefficient of component A diffusing through B
• DAB = f( molecular properties of A and B, T, c or p)
⎟⎟⎠
⎞⎜⎜⎝
⎛−=
dydcDJ A
ABA
General view to transport equations for gases
Transport
Mass Heat Energy
Flux of mass density
Flux ofheat density
Flux of momentum
⎟⎠⎞
⎜⎝⎛−=
dzdcDj in ⎟
⎠⎞
⎜⎝⎛−=
dzdTjQ λ ⎟⎟
⎠
⎞⎜⎜⎝
⎛−=
dyduj z
P η
Fourier´s Law Newton´s Law(viscosity)
Fick´s Law
General view to transport equations for gases
Atkins
Molecular diffusion coefficient for gases
• Hirschfelder/Curtiss/Bird oder Chapman-Enskog relationfor gases at low T, p (± 10% accuracy; extrapolation possible)
122
,,
,,3
,2
001834.0 −
Ω
+
= smp
MMMM
TD
DAB
BrAr
BrAr
gsAB σ
constantsforcetorespectwithintegralCollisionΩBandAgasesoffactorcollisionAverageσ
BandAmoleculesofmassesmolecularRelativeM,M[bar] Pressure p
[K]eTemperaturT
D
2AB
Br,Ar,
Ω = f(kB*T / εAB) kB…Boltzmann constantεAB..force constant(see tables in textbooks)
Molecular diffusion coefficient for gases
Ω = f( kB*T / εAB)
Knudsen diffusion
• under low pressure conditions and/or for small pores:collision of gas with pore wall > collision of gas with gas
• mean free path length of molecule > pore diameterΛ …mean free path lengthσ2…molecular cross-sectionV… gas molar volume at p
• NA/V at 298 K : cges ≈ 3.1019 *p (molecules/cm3) (dimension of p 105 Pa)
• mean free path length with typical σ (9-20*10-16 cm2)
ANV
221πσ
=Λ
)(102
nmp
≅Λ
Knudsen diffusion
)(102
nmp
≅Λ
• Conditions for Knudsen diffusion
dpore [nm] <1000 <100 <10 <2p [bar] 0.1 1 10 50
Knudsen diffusion
• Knudsen flow through one zylindrical pore
• for porous solids, the relative pore volume εb and thetortuousity factor τK have to be considered
• estimation of τ is a complex procedure (see literature)
)01.0,293(1083
26
, MPaKats
mMRTd
D piK
−≈=π
MRTd
D p
K
PiK
eff
πτε 8
3, = effective diffusion coefficient
Effective diffusion coefficients
• diffusional flow in the pores may be described byan effective diffusion coefficient
• relation to surface: surface of pore mouths is representingonly a part of the outer surface of a particle
• pores are not ideally cylindrical
• pores are connected by a network
ε
τ
Characterization of porous structures
• Porosity
• Pore radius, pore diameter• Pore radius distribution, -density
• Specific surface area• Tortuousity factor
⎥⎦
⎤⎢⎣
⎡⎥⎦
⎤⎢⎣
⎡kgm
mm 3
3
3
,mass- volume,total
volumepore
p
ppp
r
rpp dr
rHdrhrdrhrH
p
p
)()(;)()(
min,
== ∫
Transition region of diffusion
• for heterogeneous reactions in a porous solid, theconditions of pressure or pore diameter may be such that the system is between Knudsen and moleculardiffusion
• mean free path length ≈ pore diameter
• both equations for DM and DKn apply
KnM* D
1D1
D1
+=
• for porous solids experimental evaluation of diffusioncoefficients
Knudsen and molecular diffusion
Emig, Klemm
Background - Mathematics
• Delta function f(x) = δ (x-a)
- functional which acts on elements of a function
- integral
- multiplication with a function f(x)
yxforayxa
yaxdtat
y
x
<<<<<
⎩⎨⎧
=−∫ ,01
)(δ
∫+∞
∞−
=− )()()( afdxaxxf δ
Background - Mathematics
• Delta function
- graphical representation as „Sprungfunktion“ (limes)
- graphical representation as „Pulsfunktion“
x
u(x-a)
1
a
)(sin)(
1lim)( axnax
axn
−−
=−∞→ π
δ
)()( axdx
axdu−=
− δ
1
x
u(t-a)
a
∫⎩⎨⎧
=−01
)( dtatδ
Background - Mathematics
• Laplace transformation- aim: transformation of differential and integral
expressions into algebraic expressions- definition
- examplesdttFesf ts )()(
0∫∞
−=
[ ]s
es
dteLsf tsts 111)(00
=−===∞
−∞
−∫
[ ]s
dteeeLsf ttst
−=== ∫
∞−
ααα 1)(
0
TAP – method and modeling
Transient experiments
• Transients are introduced into a system byvarying one or more state variables (p, T, c, flow)
• Examples:
- Molecular beam experimentspressure change
- Temperature-programmed experimentstemperature change
- Step change experimentsconcentration change
TAP – A transient technique
• The key feature which distinguishes it from otherpulse experiments is that no carrier gas is usedand, gas transport is the result of a pressuregradient.
• At low pulse intensities the total gas pressure isvery small, and gas transport occurs via Knudsen diffusion only.
• Pulse residence time under vacuum conditions ismuch shorter than in conventional pulse experiments. Thus a high time resolution isachievable.
TAP – Features
• Extraction of kinetic parameters differs compared to steady-state and surface science experiments
- Steady-state: kinetic information is extracted fromthe transport-kinetics data by experimentallyeliminating effects of transport
- Surface science: gas phase is eliminated
- In TAP pulse experiments gas transport is noteliminated. The pulse response data provides in-formation on the transport and kinetic parameters.
TAP – Features
• TAP pulse experiments are state-defining
- Typical pulse contains ≈ 1013 – 1014 moleculesor 1010 - 109 moles
- Example:
- Sulfated zirconia: 100 m2/g with 3 wt-% S,5*1018 S/m2
- 1 pulse of n-butane: 1*1014 molecules- per pulse 1/50000 of surface addressed
TAP – System hardware
Simplified schematic of a TAP pulse response experiment
DetectorGas pulseReaction zone
• Injection of a narrow gas pulse into an evacuated microreactor
• Gas pulses travel through the reactor• Gas molecules (reactant and product)
are monitored as a function of time and produce a transient response at the MS
TAP – System hardware
TAP – System hardware
TAP – System hardware
1) High speed pulse valve2) Pulse valve manifold3) Microreactor4) Mass spectrometer5) Vacuum valve6) Manual flow valve7) Mosfet switch
TAP – Response curves
Typical pulse response experimental outputs
TAP – Multipulse experiment
• Key features of input:- typical pulse intensity rangefrom 1013 to 1017
molecules/pulse- pulse width 150-250 µs- pulse rates 1-50 s-1
• Key features of output:- different products havedifferent responses- individual product responsecan change with pulse number
TAP – Pump probe experiment
• Key features of input:- different reaction mixturesare introduced sequentiallyfrom separate pulse valves
• Key features of output:- output transient responsespectrum coincides withboth valve inputs
TAP – Transient response data
In contrast to traditional kinetic methods, that measureconcentrations, the observable quantity in TAP pulse response experiments is the time dependent gas flowescaping from the outlet of the microreactor.
The outlet flow is measured with a MS (QMS) that detectsindividual components of the flow with great sensitivity.
The composition of the flow provides information on thetypes of chemical transformation in the microreactor.
The time dependence of the flow contains information on gas transport and kinetics.
TAP – Theory
Goal
• Interpretation of pulse response data- determine typical processes- find parameters for these processes- develop a model
• Analyzation of experiments that provideparameters of diffusion, irreversible adsorption or reaction and reversible reaction
TAP – General models
Currently there are three basic models in applicationbased on partial-differential equations.
- One-zone-model- Three-zone-model- Thin-zone-model
The mathematical framework for the one-zone-modelwas first published in 1988 (Gleaves).
Basic assumptions of one-zone-model:- catalyst and inert particle bed is uniform- no radial gradient of concentration in the bed- no temperature gradient (axial or radial)- diffusivity of each gas is constant
TAP – Gas transport model
The gas transport is the result of Knudsen diffusion.
An important characteristic of this tranport process is thatthe diffusivities of the individual components of a gas mixture are independent of the presssure or the compositionof the mixture.
De,i …effective Knudsen diffusivityMi …molecular weightTi …temperature1,2 …gas 1, gas 2
2
22
1
11 T
MD
TM
D ee =
TAP – Transport model
(1) Diffusion only case –Mass balance for a non-reacting gas A transported byKnudsen diffusion
CA … concentration of gas A (mol/cm3)DeA … effective Knudsen diffusivity of
gas A (cm2/s)t … time (s)z … axial coordinate (cm)εb … fractional voidage of the packed
bed in the reactor
2
2
zCD
tC A
eAA
b ∂∂
=∂∂ε
TAP – Transport model
The equation can be solved using different sets of initial and boundary conditions that correspond to different physical situations.
initial conditions: 0 ≤ z ≤ L , t = 0 , A
NC
b
pAzA ε
δ=
0=∂∂
zCAboundary conditions: z = 0 ,
z = L , 0=AC
NpA … number of moles of gas A in one pulseA … cross-sectional area of the reactor (cm2)L … length of the reactor (cm)
TAP – Transport model
The gas flow at the reactor exit FA (mol/s) is described by
LzA
eAA zCDAF =∂∂
=
and the gas flux (mol/cm2 s) by
AFA=AFlux
To solve for the gas flow it is useful to express initialand boundary conditions in terms of dimensionlessparameters.
TAP - Transport model
Use of dimensionless parameters
… dimensionless axial coordinate
… dimensionless concentration
… dimensionless time
Lz
=ξ
LANCC
bPA
AA ε=
2LDt
b
eA
ετ =
TAP – Transport model
Dimensionless form of mass balance
2
2
ξτ ∂∂
=∂∂ AA CC initial conditions: 0 ≤ ξ ≤ 1 , t = 0 , ξδ=AC
boundary conditions: ξ = 0 ,ξ = 1 ,
0=∂∂ξ
AC
0=AC
Solution:- analytical a) method of separation of variables
b) Laplace transformation- numerical for more complex problems
TAP – Transport model
Method of separation of variables:
solution for dimensionless concentration
( ) ( )τππξτξ 22
0
)5.0(exp)5.0(cos2),( +−+= ∑∞
=
nnCn
A
solution for dimensionless flow rate
( ) ( )τππξπξτξτξ 22
0)5.0(exp)5.0(sin)12(),(),( +−++=
∂∂
−= ∑∞
=
nnnCFn
AA
dimensionless flow rate at the exit (ξ=1)
( )τππ 22
0)5.0(exp)12()1( +−+−= ∑
∞
=
nnFn
nA
TAP – Transport model
( )τππ 22
0)5.0(exp)12()1( +−+−= ∑
∞
=
nnFn
nA
Expression for the dimensionless exit flow rate as a function of dimensionless time
Standard diffusion curve
For any TAP pulse response experiment that involves onlygas transport, the plot of the dimensionless exit flow rateversus dimensionless time will give the same curveregardless the gas, reactor length, particle size, or reactortemperature.
TAP – Transport model
Due to initial condition the surface area under SDC is equal to unity – flow rate in dimensional form
⎟⎟⎠
⎞⎜⎜⎝
⎛+−+−= ∑
∞
=2
22
02 )5.0(exp)12()1(
LDtnn
LD
NF
b
eA
n
n
b
eA
pA
A
επ
επ
An important property of this dimensional dependenceis that its shape is independent on the pulse intensity ifthe process occurs in the Knudsen regime.
TAP – Transport model
⎟⎟⎠
⎞⎜⎜⎝
⎛+−+−= ∑
∞
=2
22
02 )5.0(exp)12()1(
LDtnn
LD
NF
b
eA
n
n
b
eA
pA
A
επ
επ
Characteristics of standard diffusion curve
… peak maximum
… corresponding height
… „fingerprint“ for Knudsen regime
eA
bpp D
Lt2
61,
61 ετ ==
2, 85.1,85.1L
DHFb
eAppA ε==
31.0, ≈= ppppA tHF τ
TAP – Transport model
a) Standard diffusion curve showing key time characteristics and the criterion for Knudsen diffusion
b) Comparison of standard curve with experimental inert gas curveover inert packed bed
TAP – Transport + adsorption model
(2) Diffusion + irreversible adsorption
(adsorption is first order in gas concentration)
AabvsA
eAA
b CkSazCD
tC )1(2
2
εε −−∂∂
=∂∂
AaA Ck
t=
∂∂θ
as … surface concentration of activesites (mol/cm2 of catalyst)
ka … adsorption rate constant(cm3 of gas/mol s)
Sv … surface area of catalyst per volume of catalyst (cm-1)
θA … fractional surface coverage of A
TAP - Transport + adsorption model
(2) Diffusion + irreversible adsorption
Flow rate FA at reactor exit
⎟⎟⎠
⎞⎜⎜⎝
⎛+−+−−= ∑
∞
=2
22
0
´2 )5.0(exp)12()1()exp(
LDtnntk
LD
NF
b
eA
n
na
b
eA
pA
A
επ
επ
withb
abvsa
kSakε
ε )1(´ −=
Exit flow curve for diffusion+irreversible adsorption- is smaller than SDC by a factor of exp(-ka´t)- is always placed „inside“ the SDC (fingerprint)
TAP - Transport + adsorption model
Exit flow curve for the diffusion+irreversible adsorption case
Comparison of irreversible adsorptioncurves with standard diffusion curve(A) ka=0 (SDC), (B) ka=3 (C) ka=10
TAP – Transport and adsorption model
(3) Diffusion + reversible adsorption
Mass balances- for component A in gas phase
)()1(2
2
AdAabvsA
eAA
b kCkSazCD
tC θεε −−−
∂∂
=∂∂
- for component A on the catalyst surface
AdAaA kCk
tθθ
−=∂∂
kd …desorption rate constant (s-1)
TAP - Transport + adsorption model
Exit flow curve for the diffusion+reversible adsorption case
Comparison of reversible adsorptioncurves with standard diffusion curve(A) ka=0 (SDC), (B) ka=20, kd=20 (C) ka=20, kd=5
TAP – Transport, adsorption, reaction
(3) Diffusion + reversible adsorption + irreversible reactionA(gas) B(gas)
A(ads) B(ads)kr
)()1(2
2
AdAAaAbvsA
eAA
b kCkSazCD
tC θεε −−−
∂∂
=∂∂
ArAdAAaAA kkCk
tθθθ
−−=∂∂
)()1(2
2
BdBBaBbvsB
eBB
b kCkSazCD
tC θεε −−−
∂∂
=∂∂
BdBBdBArB kCkkt
θθθ+−=
∂∂
TAP – Transport, adsorption, reaction
(3) Diffusion + reversible adsorption + irreversible reaction
The initial and boundary conditions used in thetransport-only case can be applied to thetransport+reaction case as well.
Initial condition t = 0 , CA = 0
Inlet boundary condition )(,0 1,1, t
zC
Dz AeA δ=
∂∂
−=
TAP – Transport, adsorption, reaction
(3) Diffusion + reversible adsorption + irreversible reaction
)(1,1, t
zC
D AeA δ=
∂∂
−
The inlet flux is respresented by a delta function. With this combination, analytical solutions in theLaplace-domain can be easily derived when theset of differential equations that describe the modelis linear.
TAP – Characteristic values
Use of dimensionless parameters
eA
baa D
Lkk2
´ ε= dimensionless apparent adsorptionrate constant
b
abvsa
kSakε
ε )1(´ −=with
ka´ contains:
- the kinetic characteristics of the active site (ka)- the structural characteristics of the wholecatalytic system
b
bvs Saε
ε )1( −
TAP – Extended models
Three-zone-model
Additional boundary conditions between the different zones have to be applied
catalystinert inert
zone 1 zone 2 zone 3
pulse outcome to QMS
Evaluation of curve shapes and „fingerprints“ arebasicly the same as for the one-zone-model
TAP – Theory
Moment-based quantitative descriptionof TAP-experiments
• idea behind: observed TAP data consist of a set of exitflow rates versus time dependencies
• analytical solutions in integral form can be usuallyobtained
• analysis of some integral characteristics (moments) of theexit flow rate
• moments reflect important primary features of theobservations
TAP – Theory - Moments
• moment Mn of the exit flow rate (i.e. not of concentration)
n…order of moment
• representation in dimensionless form
dttFtMt
nn )(
0∫=
)/(with 2
0 eAbA
nn DL
tdFmε
τττ == ∫∞
TAP – Theory - Moments
Application to TAP experiments:- for irreversible adsorption / reaction
DaI … Damköhler number I
- mean dimensionless residence time
eA
baI
I DLkDa
DaXm
2´
0 withcosh
11 ε==−=
[ ]Dif
n I
n Ires
DanBnADanB
nA
mm ττ
∑
∑∞
=
∞
=
+
+==
0
0
0
1
)()(
)()(
)12()1()( +−= nnnA22)5.0()( π+= nnB
Ae
bDif D
L
,
2ετ =
Applications
TAP - Examples
Mechanism and Kinetics of the Adsorption and Combustionof Methane on Ni/Al2O3 and NiO/Al2O3 (Dewaele et al. 1999)
- Supported Ni catalysts for steam reforming of natural gasto synthesis gas
- Combination of TPR with single pulse experiments
TAP - Examples
Selective oxidation of n-butane to maleic anhydride (MA) over vanadium phosphorous oxide (VPO)
(Gleaves et al. 1997)
• important heterogeneous selective oxidation• complex reaction (electron transfer, fission of C-H-bonds, addition of oxygen, ring closure)
• literature: (VO)2P2O7-lattice as active-selective phase, but itis not clear how the lattice supplies the oxygen
- possible involvation of oxygen adspecies adsorbed on vanadium surface sites
- possible involvation of bulk oxygen- possibly other crystal phases are active
TAP - Examples
Selective oxidation of n-butane to maleic anhydride (MA) over vanadium phosphorous oxide (VPO)
• Steady-state kinetic studies yielded different rate expressions and kinetic parameters – this may be relatedto differences in c. composition, c. structure, c. oxydationstate, etc. no unique information
• Aim of TAP investigations:Relate changes in kinetic dependencies to changes inthe catalyst
• How to do ? Systematic alteration of the state of thecatalyst and measuring kinetic dependencies
TAP - Examples
Selective oxidation of n-butane to maleic anhydride (MA) overvanadium phosphorous oxide (VPO)
Kinetics of n-butane conversion over VPO in the absenceof gas phase oxygen
- n-butane curve lies completelywithin the generated diffusioncurve irreversible process
- initial step is irreversible (fissureof CC or CH bond)
TAP - Examples
Selective oxidation of n-butane to maleic anhydride (MA) overvanadium phosphorous oxide (VPO)
Kinetics of n-butane conversion on oxygen-treated VPO
- values of apparent activationenergy is strongly dependenton the VPO oxidation state
- decreasing number of activecenters
- TAP + Raman: V4+ V5+Arrhenius plots for pulsed n-butane conversionover an oxygen-treated VPO catalyst sampleas a function of oxidation state
TAP – sulfated zirconia (SZ)
TAP investiagtions on sulfated zirconias
C. Breitkopf. J. Mol. Catal. A: Chem. 226 (2005) 269.
X. Li, K. Nagaoka, L. Simon, J.A. Lercher, S. Wrabetz, F.C. Jentoft, C. Breitkopf, S. Matysik, H. Papp. J. Catal. 230 (2005) 214-225.
C. Breitkopf, S. Matysik, H. Papp. Appl. Catal. A: Gen. 301 (2006) 1.
C. Breitkopf, H. Papp. Proceedings of The 1st International Conference on Diffusion in Solids and Liquids DSL-2005, July 6-8, 2005, Aveiro, Portugal, 67. (oral presentation)
TAP – Investigation of poisoned samples
Comparison of active sample Kat1 with samples of Na-modified surfaces
3 % of active centres poisoned40 % of active centres poisonedall centres poisoned
Sulfation Step: 5.7x10-3 mol (NH4)2SO4 Kat1
5x10-5 mol Na2SO4 + 5.7x10-3 mol (NH4)2SO4 SZ-Na15x10-4 mol Na2SO4 + 5.2x10-3 mol (NH4)2SO4 SZ-Na25x10-3 mol Na2SO4 + 0.7x10-3 mol (NH4)2SO4 SZ-Na3
Calcination @ 873 K for 3 h
Characterization
Sample Kat1 SZ-Na1 SZ-Na2 SZ-Na3
BET [m2/g] 103 76 45 10
Pore diameter [Å] 35 50 94 135
Pore volume[cm3/g]
0.08 0.11 0.11 0.04
SO4 content w.% 9.0 9.9 7.5 5.1
S atoms/m2 5.3*1018 7.8*1018 1.0*1019 2.9*1019
Na+ ions/m2 - 2.2*1017 3.7*1018 1.6*1020
S/Na+ - 35.4 2.7 0.29
Catalysis and TPD
Activation: 2 h at 600°C in air, 30 ml/minReaction: 5 % n-butane (99.95 % purity) in N2, 20 ml/min
Poisoning of 3 % of active Brønsted centres led to significant decrease in activity, loss of Brønsted centres led to inactive samples
0 50 100 150 200 2500
5
10
15
20
SZ-Na1 SZ-Na2 SZ-Na3 Kat1
150 °C
yiel
d i-b
utan
e / %
time on stream / min
TPD
100 200 300 400 500 600
Kat1
Na2
Temperatur [°C]
DRIFTS
Sample L/B Ratio (by pyrads.) at 323 K
Surface ContentNa [%]
S=O mode [cm-1] at 673 K
Kat1 0.9 - 1402SZ-Na1 1.1 not detectable 1397SZ-Na2 1.7 3.5 1389SZ-Na3 13.0 8 -
L/B ratio determined according to Platon A, Thomas WJ. Ind. Engin. Chem. Res. 24 (2003) 5988.
TAP – n-butane single pulses
SZ-Na1 SZ-Na2Kat1
0 1 2 3 4 5
0,0
0,2
0,4
0,6
0,8
1,0
H
öhen
norm
. Int
m/e
43
[w.E
.] 448 K 423 K 398 K 373 K 348 K
Zeit [s]0,0 0,2 0,4 0,6 0,8 1,0
0,0
0,2
0,4
0,6
0,8
1,0
Höh
enno
rm. I
nt. m
/e 4
3 [w
.E.] 448 K
423 K 398 K 373 K 348 K
Zeit [s]0,0 0,2 0,4 0,6 0,8 1,0
0,0
0,2
0,4
0,6
0,8
1,0 448 K 423 K 398 K 373 K 348 K
Höh
enno
rm. I
nt. m
/e 4
3 [w
.E.]
Zeit [s]
τ (348 K) = 0.47 sτ (423 K) = 0.16 s
τ (348 K) = 1.58 sτ (423 K) = 0.49 sτ … residence time
τ (348 K) = 0.20 sτ (423 K) = 0.11 s
Interaction of n-butane single pulse with surface decreaseswith increasing amount of sodium
Kinetic modeling
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