L18-1
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
1. Mass transfer of A to surface
2. Diffusion of A from pore mouth to
internal catalytic surface
3. Adsorption of A onto catalytic surface
4. Reaction on surface
5. Desorption of product B from surface
6. Diffusion of B from pellet interior to pore mouth
7. Diffusion of B from external surface to the bulk fluid (external diffusion)
Review: Steps in a Heterogeneous Catalytic Reaction
Ch 10 assumes steps 1,2,6 & 7 are fast, so only steps 3, 4, and 5 need to be considered
L18-2
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Assume the total # of active sites remains constant (no catalyst deactivation occurs):
Review: Adsorption StepA(g) + S ⇌ A·S
S: open (vacant) surface site A·S: A bound to a surface site
The adsorption of A (gas phase) on an active site S is represented by:AI
-S-S-S-
Rate of adsorption = rate of attachment – rate of detachmentAD A A v A A Sr k P C k C
partial pressure of A Molar conc of vacant sites on surface
A
-S-S-S-
Using adsorption equilibrium constant (KA)
AA
A
kKk
A S
AD A A vA
Cr k P C
KEquation I
Surface
Vacant active siteA B
Cv is not measurable, but the total # of sites, Ct can be measured
Review: Site Balance
Ct = Cv + CA·S + CB·SSite balance: Use to express Cv in terms of measurable species
L18-3
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Review: Surface Reaction StepAfter the molecule is adsorbed onto the surface, it can react by a few different mechanisms1. Singe site mechanism: Only the site to which the reactant is absorbed is
involved in the reactionAI
-S-⇌
BI
-S-
A·S ⇌ B·S
B S
S S A SS
Cr k C
KS
SS
kwhere K
k
2. Dual site mechanism: Adsorbed reactant interacts with another vacant site to form the product
AI
-S-S-S⇌
B I
-S-S-S-
A·S + S ⇌ S + B·S
B S v
S S A S vS
C Cr k C C
K
Equation IIa
Equation IIb
3. Eley-Rideal mechanism: reaction between adsorbed reactant and a molecule in the gas phase
⇌ C I
-S-S-S-
A·S + B(g) ⇌ C·S
C S
S S A S BS
Cr k C P
K Equation IIcAI
-S-S-S
B
L18-4
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Review: Desorption StepProducts are desorbed into the gas phase
C I
-S-S-S-⇌
C
-S-S-S-
C·S ⇌ C + S
C vD,C D C S
D,C
P Cr k C
KD
D,CD
kwhere Kk
Equation III
Note that the desorption of C is the reverse of the adsorption of C
D,C AD,Cr r
Also the desorption equilibrium constant KD,C is the reciprocal of the adsorption equilibrium constant KC
D,CC
1KK
D,C D C S C C vr k C K P C
Substituting 1/KC for KD,C in the rate equation for product desorption gives:
L18-5
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Review: Evaluating a Catalytic Reaction Mechanism
• Collect experimental data from test reactor• Derive a rate law
• Select among types of adsorption, surface reaction, and desorption• Write rate laws for each individual step, assuming all are reversible• Postulate which step is rate limiting
• Surface reaction step is rate limiting ~70% of the time!• Use non-rate-limiting steps to eliminate the surface concentration terms
that cannot be measured• Assume PSSH (rate of ads = rate of surface rxn = rate of desorp)
• No accumulation of species on the surface or near interface• Each species adsorbed on surface is a reactive intermediate• Net rate of formation of species i adsorbed on the surface is 0,
ri·S=0• See if rate law is consistent with data• If not, then try other surface mechanism (i.e., dual-site adsorption or Eley-
Rideal) or choose a different rate-limiting step (adsorption or desorption)
L18-6
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
L18: CVD & Catalyst Deactivation• Chemical vapor deposition (CVD)
• Important process in the formation of microcircuits (electrically interconnected films ICs), microprocessors & solar cells
• Used to deposit thin films of material, such as Si, SiO2, & germanium (Ge)• Mechanism of CVD is similar to those of heterogeneous catalysis except
that site concentration (CV) is replaced w/ fraction of surface coverage (fV)
Si Si Si Si
SiHHsilicon hydride adsorption
Si Si Si Si
SiHH
Si Si Si Si
Si
Surface reactionH2
No desorption occurs, the product, Si, remains attached to the surface, forms a new surface
L18-7
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Growth of Silicon Film by CVD
Write out elementary reactions and assume a rate-limiting step 2 2SiH g S SiH S ⇌1. Adsorption
Rate of adsorption = rate of attachment – rate of detachment
AD SiH SiH v SiH SiH2 2 2 2r k P f k f SiH2AD SiH SiH v2 2
SiH2
fr k P f
K
2. Surface reaction: 2 2SiH S Si S H g ⇌
S S SiH S Si H v2 2r k f k C P f Si H v2S S SiH2
S
C C fr k f
K
Si Si Si Si
SiHH
Si Si Si Si
SiHH
Si Si Si Si
Si
adsorptionSurface reaction
fv & fSiH2: fraction of the surface covered by vacant sites or SiH2, respectively
Surface coverage is in terms of fraction of surface, not conc of active sites
L18-8
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
What is the rate of Ge deposition if the surface reaction is rate limiting?a) r”Dep=kdisPGeCl4-k-disPGeCl2PCl2
b) r"Dep=kAPGeCl2fv -k-AfGeCl2
c) r”Dep=kHPH2fv2
-k-HfH2
d) r”Dep=kSfGeCl2fH2 -k-S CGePHCl
2fv2
e) r”Dep=kSfGeCl2fH2
Gas-phase dissociationkdis
4(g) 2(g) 2(g)k disGeCl GeCl Cl
Adsorption (1)kA
2(g) 2k AGeCl S GeCl S
Adsorption (2)kH
2(g) k HH 2S 2H S
Surface reaction kS2GeCl S 2H S Ge s 2HCl g 2S
Surface reaction is believed to be the rate-limiting step
Growth of Germanium Films by CVDGermanium films have applications in microelectronics & solar cell fabrication
L18-9
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Gas-phase dissociation
kdis
4(g) 2(g) 2(g)k disGeCl GeCl Cl
Adsorption (1)kA
2(g) 2k AGeCl S GeCl S
Adsorption (2)kH
2(g) k HH 2S 2H S
Surface reaction kS2GeCl S 2H S Ge s 2HCl g 2S
Surface reaction is believed to be the rate-limiting step:
ks: surface specific reaction rate (nm/s)
fH2: fraction on the surface occupied by H2
fGeCl2: fraction of the surface covered by GeCl2
Growth of Germanium Films by CVDGermanium films have applications in microelectronics & solar cell fabrication
" 2Dep S GeCl H2r k f fRate of Ge deposition (nm/s):
*Surface coverage is in terms of fraction of surface, not conc of active sites
L18-10
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Catalyst Deactivation
Thus far, we assumed the total conc. of active sites on the surface was constant, which means the catalyst’s activity is constant throughout its lifetime
In reality, there is a gradual loss of catalytic activity (active sites on surface of the catalyst) as the reaction takes place
•Main types of catalyst deactivation•Sintering (aging): loss of active surface due to high temperature•Coking or fouling: carbonaceous material (coke) deposits on surface
•Poisoning: molecules irreversibly bind to the active site
We will evaluate the kinetics of general catalyst deactivation and these specific types
L18-11
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Catalyst Deactivation Kinetics• Adjustments for catalyst decay need to be made in the design of reactors• Catalyst activity a(t) is used as a quantitative specification
'A'A t 0
r (t)
ra tCatalyst activity at time t:
Reaction rate for catalyst used for time t
Reaction rate for fresh, unused catalyst
For fresh, unused catalyst, t 0a 1 0 a t 1
A,A Bfn C C ,..r .' k Ta t .etcRate of consumption of reactant A on catalyst used for time t is:
a(t): time-dependent catalyst activity k(T): T-dependent specific rate constantfn(CA, CB…etc): function of gas-phase conc. of reactants, products & contaminants
Rate of catalyst decay: d d A Bdar p a t k T h C ,C ,...,etcdt
Function of activity
Temperature-dependent specific decay constant
Functionality of rd on reacting species conc. h=1: no conc dependence; h=Cj: linearly dependent on concentration
L18-12
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Sintering (Aging)• Loss of active surface area resulting from the prolonged exposure to
high gas-phase temperatures• Active surface area is lost by
• Crystal agglomeration and growth of metals deposited on support• Narrowing or closing of pores inside the catalyst pellet• Surface recrystallization• Elimination of surface defects (active sites)
• Sintering is usually negligible at temperatures below 40% of the melting temperature of the solid
• Second-order decay of reaction rate with respect to present activity: 2
d dr k a
d
1a t1 k t
Catalyst activity at time t:
dd d 0
0
E 1 1k k T expR T T
Sintering decay constant:
L18-13
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Coking (Fouling)• Common to reactions involving hydrocarbons• A carbonaceous (coke) material is deposited on surface of catalyst
nCC AtConcentration of carbon on surface (g/m2):
• Coking can be reduced by running at high pressure & hydrogen-rich feeds
• Catalyst deactivated by coking is often regenerated by burning off the carbon
m1a t
1 k ' t
Catalyst activity at time t:
A & n are fouling parameters
(one of many different expressions for a(t))m is a fouling parameter
L18-14
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Poisoning• Poisoning molecule is irreversibly chemisorbed to active sites• Reduces number of active sites available for reaction• Catalyst can be poisoned by reactants, products, and impurities
'kdpoisoning by impurity: P S P S
d d Pdar a t k ' Cdt
'kdpoisoning by reactant: A S A S 'kdposioning by product: B S B S
A S B S For the overall reaction:
a(t): time-dependent catalyst activitykd: specific decay constantCP: concentration of the poison
L18-15
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
•Reactant & catalyst enter at top of reactor•Reactant & catalyst flow down the length of the reactor together as a plug
•Product and spent catalyst (black) flow out of reactor outlet
•Spent catalyst is regenerated by passing it through a separate regeneration unit, and newly regenerated catalyst is fed back into the top of the reactor
Moving-Bed Reactor• When catalyst decay occurs at a significant rate, they require frequent
regeneration or replacement of the catalyst• Moving-bed reactor enables continuous regeneration of spent catalyst• Operates in the steady state, like a PBR
L18-16
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
Moving-Bed Reactor DesignReactantsu0 (dm3/s)
fresh catalyst US (g/s)
ZZ + DZ
WW + DW
Products & coked catalyst
Catalyst flow US << reactant flow u0
As far as the reactants are concerned, reactor acts like a PBR:
AA0 A
dXF r 'dW
ndAssume the decay rate da k a
d la s
tw i :
Decay rate must be in terms of W (da/dW) not t (da/dt) US relates W to t→ use US to convert da/dt to da/dW
Relate t to US:S
WtU
S
dWdtU
Multiply dt/dW by -da/dt to get –da/dW:
S
dt 1dW U
S
nd
da k a 1d UdtdtW
nd
S
kda adW U
If the rate of consumption of A for catalyst used for time t: jA a W' k T fn Cr
AA
A0
rX 'ddW F
A
A0
jk fnCTa WdXdW F
j
X WA A0 A
0 0fa W
F dXd
TW
k nC
Catalyst activity vs position in PBR
Integrate to get a(W)
SWU t
L18-17
Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois at Urbana-Champaign.
A→B Given constant T, pure A in feed, and 1st order kinetics for reaction & catalyst deactivation, find XA(t) in a fluidized batch reactor of constant volume
d dr kddat
a Rate of deactivation:Rate of rxn: A Ar Cak
Solve the batch reactor design eq for XA. The batch reactor design eq must be combined with the rate eq. The rate eq contains ‘a’, so we need to use the rate of deactivation to find how ‘a’ varies w/ time. We will determine how ‘a’ varies with time by integrating the rate of deactivation eq & solving for ‘a’:
d ddar k adt
a td
a 1 0
da k dta
dlna k t k tda e
Ak t
AdkC erInsert a into the rate eq: A0 A
dA
k tC 1 eXkr
Batch reactor design eq: AA0 A
dXN W
dtr ' A
kA tdA 0 A0 C 1
dXN ek X W
dt
Integrate & solve for XA
X tA k tA0A d
A A00 0
C WdXk e dt
1 X N
tk tA0 d
AA0 d 0
C W kln 1 X eN k
k tA0 dA
d A0
C Wln 1 X k 1 e
k N C WA0 k tdk 1 ek Nd A0
A1 X e
C WA0 k tdk 1 ek Nd A0AX 1 e