catalysis che 481 581 spring 2015 complete
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Gene and Linda Voiland School of Chemical Engineering and Bioengineering
CatalysisFromFundamentals to
ApplicationsProfessor Norbert Kruse
Prerequisites:Undergraduate courses of chemical reaction kinetics and
engineering
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CHE 481/581: Catalysis (Spring 2015)
Prof. Norbert Kruse
Office: Wegner 155A
Email:[email protected]
Office Phone: (509) 335 6601
Office Hours: Fridays 11:00 am1 pm, with an appointment
Tanya Stewart, Secretary Senior
Office: Wegner 155C
Email:[email protected] Phone: (509) 335 1256
Course:
a) Aim: get acquainted with the fundamentals of (heterogeneous) catalysis. Think in terms of
kinetics and mechanisms and use the surface science approach for doing so. Get an overview
on the major large-scale applications of catalysis.
b) Textbooks: there is no unique textbook treating all the topics covered by the course. A
copy of the slides will be provided.
for Surface Chemistry: Gary Attard and Colin Barnes, Surfaces, Oxford Chemistry Primers
for Spectroscopy: J. W. Niemantsverdriet, Spectroscopy in Catalysisan Introduction,Wiley-VC
for Theory in Surface Chemistry: Roald Hoffman, Solids and Surfaces: A Chemists View of
Bonding in Extended Structures, VCHc) Schedule: Terrell Lib. 24. Tu and Th 12pm to 1:15pm until May 1st. No classes on January23rd, March 25thand 27th.
d) Methodology: Avoid a monologue, but rather try to engage a discussion when appropriate.
Slides provide a scaffold. They are to be completed by the students according to theindividual needs. Instructor jumps back to the basics when necessary. More detailed
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected] -
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considerations will be either developed at the blackboard or defined as homework. Try toavoid lengthy mathematical derivation of formulas homework
Grading:
Oral (30 to 40 min each student): 70%. Project presentation: 30%
Projects will be assigned according to a list of subjects provided until end of March. A
subject may range from a hot topic in catalysis to the presentation of a large-scale catalyticprocess not subject to the course. Every student has to present his/her project in PPT format
during 12 min., followed by questions during 6 min. Presentations take place on 22nd, 24th,30thApril and 1stMay. Room TBD.
Homework will be defined occasionally and is intended to digest and deepen certainaspects of a subject. No written homework is required, however, there is expectation of
background knowledge to support the new knowledge provided. The oral exam may includequestions on such homework issues.
Oral exams will take in my office at Wegner 155. Oral exam and PPT presentation will beweighted as defined as above.
Grading Scale:
90-100% A 77-79% B-
87-89% A- 73-76% C+
83-86% B+ 70-72% C
80-82% B 0-69% F
Course Website
The contents of this course will be available online on Angel:http://lms.wsu.eduor via email.
http://lms.wsu.edu/http://lms.wsu.edu/http://lms.wsu.edu/ -
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Students with Disabilities
Reasonable accommodations are available for students with a documented disability. If youhave a disability and need accommodations to fully participate in this class, please either visitor call the Access Center (Washington Bldg 217; 509-335-3417) to schedule an appointment
with an Access Advisor. All accommodations MUST be approved through the AccessCenter.
Academic Integrity
I encourage you to work with classmates on assignments. However, each student must turn in
original work. No copying will be accepted. Students who violate WSUs Standards of
Conduct for Students will receive an F as a final grade in this course, will not have the optionto withdraw from the course and will be reported to the Office of Student Standards and
Accountability. Cheating is defined in the Standards for Student Conduct WAC 50-26-010(3). It is strongly suggested that you read and understand these definitions:http://apps.leg.wa.gov/wac/default.aspx?cite=504-26-010
Safety
Washington State University is committed to maintaining a safe environment for its faculty,
staff, and students. Safety is the responsibility of every member of the campus communityand individuals should know the appropriate actions to take when an emergency arises. Insupport of our commitment to the safety of the campus community the University has
developed a Campus Safety Plan,http://safetyplan.wsu.edu/.It is highly recommended that
you visit this web site as well as the University Emergency Management web site athttp://oem.wsu.edu/ to become familiar with information provided.
Caveat
The schedule and procedures outlined in this syllabus are subject to change in the event ofcircumstances beyond the instructors control or in response to ongoing assessment oflearning.
http://apps.leg.wa.gov/wac/default.aspx?cite=504-26-010http://safetyplan.wsu.edu/http://oem.wsu.edu/http://oem.wsu.edu/http://safetyplan.wsu.edu/http://apps.leg.wa.gov/wac/default.aspx?cite=504-26-010 -
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Catalysis Kinetic Phenomenon
Some early definitions:
A catalyst increases the rate of a reaction without being consumed by a reactionWilhelm Ostwald (1853-1932)
A chemical reaction has to be thermodynamically feasible in order to beaccelerated Alfred Mittasch (1869-1953) *
A catalyst doesnt appear in the stoichometric equation.
However: Catalysts undergo structural or chemical changes during the catalyticreaction (mainly in heterogeneous reactions).
The catalyst is formed by the catalytic reaction.
Agatha Christie (1891-1976)
(The Mysterious Mr. Quinn, 1930): Do you happen to know anything about
catalysis? The young man stared at him. Never heard of it. What is it? Mr.Satterthwaite quoted gravely, A chemical reaction depending for its success onthe presence of a certain substance which itself remains unchanged.
(Curtain, Poirots last case, 1940): So we get the curious result that we have herea case of catalysisa reaction between two substances that takes place only inthe presence of a third substance, apparently taking no part in the reaction andremaining unchanged. That is the position. It means that where X was present,
crimes took placebut X did not actively take part in these crimes.
_____________________________________________________________________________________________
* non-catalytic
catalytic
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Some general comments on the importance of catalysis:
About 80% of the industrial production is based on the application of the catalytic
process at least in one intermediate stage.
High economic importance
About 90% of all catalytic processes are heterogeneous in nature.
Incidental remark: homogeneous -heterogeneous catalysis
Reactants and catalyst in the same phase not in the same phase
However: there is no general theory of catalysisBlack Magic
Two examples for the importance of empirical research:
Ammonia synthesis:
N2+ 3H22NH3 About 20,000 different catalysts were tested
Fischer-Tropsch Reaction:
CO +H2 2O About 15,000 different catalysts were testedEq. not equilibrated!
In both cases, metals are used to catalyze the reaction (mainly nano-sized
particles on an oxidic support.)
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A historical example
Formation of water:
2H2+ O22H2O Dbereiner (1822)
First construction of a lighter:
The lever eallows contacting a piece of zinc with
sulfuric acid in a glass vessel: Zn + 2H+ Zn2+ + H2
H2is produced and mounts in the jack so as to mix up with O 2. Sponge-like Pt
is placed inside the nozzle fand allows ignition of the reaction.
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For a catalyst with a pore structure (texture) diffusion processes into the pores
and out of them have to be added.
b)Homogeneous Catalysis:
Catalytic Cycle :
A physical separation (unit operations like distillation, extraction, etc.) is
necessary in order to obtain the pure product.
Two examples involving multiple intermediate steps:
Ammonia Synthesis (heterogeneous catalysis):
All intermediate steps were clarified in a Nobel Prize winning work by G. Ertl
(Nobel Prize 2007)
Reaction Advancement
PotentialEnergy
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Hydroformylation (Homogeneous catalysis)
Note that the catalytic cycle above includes changes in the number of metal
valence electrons between 18 and 16, back and forth.
Catalysts are coordination compounds of Rh and Co
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Isokinetic temperature
Kinetic compensationreality or fiction?
We can always find a temperature for which k0has the samenumerical value, no matter which catalyst for a givenreaction is considered.
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Example: methanation
CO + 3 H2 CH4+ H2O
Compensation effect for the methanation of CO for a number of metals, both base
and noble.
kJ/molEA
10
20
60 80 100 120
.
.
.
.
.
. .
.
Ru
CoNi
Rh
Fe
Pd
Pt
Ir
ln k0
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Conversion of crotonicaldehyde: Butyraldehyde selectivity:
Note the distinction between:
TON is frequently used in homogeneous catalysis:
Processes start to become economic when TON > 20,000
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Some statistical thermodynamics
Comparing reaction rates of heterogeneous catalysis with those of non-catalytic
reactions according to the Transition State Theory (TST)
Consider a bimolecular reaction (non-catalytic)
AB is the activated complex in homogeneous phase.
The same bimolecular reaction under catalytic conditions reads:
AB*2
is the activated complex bound to the surface of the catalyst.
The reaction rate according to TST then is:
n
nn
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in general:
k: Boltzmann constant
h: Planck constant
:Partition function of component iEo
: Difference in energy of initial energy T=0 K
(
)
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Heterogeneous Catalysis occurs at the surface of a solid
Transmission Electron Microscopy (TME) of metal
particles on a support (Ag/Al2O3).
Metal particles are well separated from each other
and the pore structure of the catalyst becomes visible(texture).
Image blurring indicates carbon deposition
High Resolution Transmission
Electron Microscopy (HRTEM)revealing the morphology of Rh
nanosized particles on a TiO2
support.
10nm
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Ball model of a single metal grain to visualize the different sites exposed.
In white are: low coordination atoms have empty valence orbitals which are
supposed to be preferred binding sides for adsorbing atoms and molecules
(gasses or liquid molecules).
Field Ion Microscopy (FIM)
of a single Rh particle, in top
view, atom by atom. Not all atoms areseen; only those in low coordination.
Miller indexes of individual surface
facets are also given.
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Metal particles on a support should be nanosized so as to create as large asurface area as possible.
n n n
r ~ N
Number of atoms located along the particle radius.
Ns
= 2 N2
Number of atoms at the surfaceNt = 2/3 N3
Total number of atoms in the particle
Surface fraction is a function ofradius for a hemispherical particleon a flat support.
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Some comments on the nature of catalysts and their supports
Supports should provide high surface area so as to optimize the dispersion of the
catalytically active phase. For metal-based catalysts, frequently the followingsupports are used.
Al2O3, SiO2, TiO2, ZnO, MgO, C
Some of these materials can be prepared with specific surface areas as large as1,000 m2/g (equivalent to the size of a football field).
The catalytically active metal phase is frequently supplied by impregnation using
an aqueous solution of a suitable precursor compound such as Me-nitrate.
Impregnation can be performed wet using immersion techniques or by incipient
wetness. In the latter case, the volume of the aqueous precursor solution matchesthe volume of the support pores so the solid doesnt appear wet. The preparationin water enables solvated metal cations to bind to surface hydroxyl of the support.
The details of the binding mechanism depend on the pH conditions and the point
of zero charge (PZC) of the support.
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pH > PZC cationic adsorption
pH < PZC anionic adsorption
A different type of catalyst are zeolites whose primary construction units are
tetrahedric [SiO4]4-and [AlO4]5- .These materials are highly crystalline withspecific surface areas sometimes exceeding 1,000 m2/g. They have acidic
properties and may or may not contain metallic cations.
[SiO4]4-and [AlO4]
5- units share oxygen atoms so as to form Si-O-Al bridges.
Starting from a 3D SiO2network the replacement of Si by Al creates localized
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charges that have to be compensated in order to maintain electroneutrality (per
Al-either one H+or one Me+is needed).
General formula:
x [ (Me+, Me2+) AlO2]. y SiO2
. z H2O
The occurrence of protons causes Brnsted acidity which allows hydrocarbon
cracking in petrochemical industry.
Heat treatment allows Brnsted acidity to turn into Lewis acidity.
Similar to the construction of secondary building blocks in silica highly
symmetric polyhedral units can be formed to build zeolites.
Heating
Lewis Center
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a) Sodalite
b)Zeolite A
c) Faujasite (zeolites X and Y)
Windows vary between 2.6 in sodalite and 7.4 in faujasite.
The number of basic zeolites can be constructed from sodalite units in which
for reasons of simplicity, the bent Si-O-Al bridges are considered as straight
lines. Corners contain either Si or Al.
Sodalite unit
Sodalite unit Sodalite unit
Large cavity
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More recent developments aim at synthesizing ordered silicas so as to create
mesopores molecular sieves.
1992 - Scientists of Mobil Oil Corporation (USA) applied self-assembledtemplate (micelles of CTAB surfactant) and synthesized a family of ordered
mesoporous silicas named MCM (Mobil Composition of Matters) 41 and 48*
The CTAB molecule (cetyltrimethylammoniumbromide) consists of a long
(cetyl) hydrocarbon skeleton causing hydrophobic properties (tail) and a terminalionic group causing hydrophilic properties (head).
CTAB micelle Silicate self-assembling around the micelle
*Kresge C.T., Leonowicz M.E., Roth W.J., Vartuli J.C., Beck J.S., Nature, 1992, 359, 710
712
Template-
directed
Inorganic
precursor
Template
Template
removalCondensation
MesoporousTemplate-oxide
N(CH3)3
+
Br-
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MCM-41 is characterized by a unique pore size distribution. Pores usually have
diameters between 2-5 nm with hexagonal structure. The total specific surface
area may range from 900-1,500 m2/g.
TEM image and model of MCM-41
V. Meynen et al. / Microporous and Mesoporous Materials 125 (2009) 170223
Micelles can be shaped by use of proper surfactants.
g = V/al
gpacking parameterVvolume of the hydrophobic part ofsurfactant
(including solubilized compounds)aeffective area of the hydrophilic head
(depends also on counter-ions)
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Micelle shape and type versus g
More recent developments have lead to new silicates like SBA-15 or KIT-6
KIT-6 pore size vsHTT temperature Model of KIT-6 doublegyroid Mesostructure
Y. Doi et al, Chem. Commun., 2010, 46, 63656367
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Physisorption and Chemisorption
Physisorption Chemisorption
Forces dispersion valence or electrostatic forcescreate covalent and/or ionic
bonds
van der Waals
a)E Keesom
b) E Debye
c) E London
HphysHcond ~5-20 kJ/mol Hchem 54kJ/mol
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multilayers monolayer limit
Reactants all gasses below reactive gasses
the critical temperature
Reversibility yes yes, in many cases,
but also irreversible.
Dependence on decreases with increasing maybe complicated in
temperature temperature case of an activated
process
Coverage
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Potential diagrams
The above diagram applies to the case of physisorption on top of a chemisorbed
layer. The turnover from the physisorbed state to the chemisorbed state is non-
activated for the observer but can only occur as long as empty chemisorption sites
are available. A diffusion process in the physisorbed state is possible and occurs
with activation energy of Edif.
Chem
Edif
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H chem
The above potential diagram applies to the case of H2adsorption on a Cu surface.
H chem = 34 kJ/mol
EA = 21 kJ/mol
E (*Cu - H) = 233 kJ/mol
Generally: H chem = 2E (M - H) - E(H - H) = 34 kJ/molwhich is low for metals with a closed d-shell.
H chem
*Chemisorption site
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The turnover from the physisorbed state into the chemisorbed state is activated
for H2/Cu. In other cases, like CO on open d-shell metals the dissociation is non-
activated and leads to the deposition of carbon and oxygen. H chem can be
considerably larger in this case (between 100-160 kJ/mol). On the other hand, CO
chemisorption on Cu occurs without dissociation and is weak.
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Dynamics of adsorption
Calculation of the surface residence time of molecules before thermoadsorption
For kinetic for order process:
surface residence time (lifetime)
6 . 1012s-1at 300K
Homework:
Calculate the surface lifetimes at 300K and 600K for the activation energies Ed=4, 20, 40, 80, 160 kJ/mol.
Calculation of the concentration (surface coverage) of adsorbed species:
For physisorption:
phys = J ..wphys wphys probability ofphysisorption
For chemisorption:
chem = J ..s J impingement rate[ L-2.T-1]
s sticking probability
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calculation of the impingement rate:
< =
n mean velocity (m s-1)n number of molecules per m3m molecular mass in kg per molecule
with:
and n J = 2.64 .1024p / (M . T) ( m 2 .s 1) m molecular mass in
atomic mass units
Homework:
Calculate the impingement rate per surface site for nitrogen molecules at 1 barand 273K. Assume the surface site to have a size of 10 2. Calculate the numberof multiple layers assuming Ed= 40 kJ/mol and a probability of physisorptionwphys = 1.
Calculation of the rate of chemisorption:
For a non-activated process: Ra
= J .s
This allows the sticking probability to be defined as:
S = Ra
/ J > tM
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tM= characteristic time of the measurement (molecules must be on the surface tobe measured)
S0 Sticking probability at zero coverage. It can be anticipated that values of
S0are influenced by the surface structure and the temperature.
Example : N2/Fe
S0 7 . 10-8 for the (110) surface (densely packed)
S0 4 . 10-6 for the(111) surface (open structure)
It is interesting to see that the sticking probabilities for N2chemisorption on Fevary in the same manner as the reaction rate of the ammonia synthesis
N2+ 3H22NH3
For comparison, the sticking probability S0 of the CO molecule on transitionmetal surfaces variesbetween 1 and 0.1.
The Langmuir Isotherm:
with (NA
) . Ed
= Hchem
n (Hchem/ RT)
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at constant temperature:
n .pModel assumptions made by Langmuir
1. all surface sites are treated in the same manner: no difference ismade between terrace sites, steps or kinks
2. adsorbed species do not interact laterally3. incoming molecules hitting an occupied site are being reflected
without energy loss
The last argument leads to the following functionaldependence of s vs.coverage :
Assuming every incoming molecule hitting an empty site will get adsorbed,
s0= 1, we will then receive:
with
For application purposes: V / Vs
Vs gas volume giving rise tomonolayer formation
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The experiment consists of introducing a known volume of gas from a calibratedreservoir into a reactor containing the catalyst sample and measuring the volumeconsumed due to adsorption.
An equivalent derivation of the Langmuir equation would be to consider a
dynamic equilibrium between adsorption and thermal desorption.
)[ ] = kd. [ ] [ ] concentration of sites perunit surfaca area
1/p
1/V
1/Vs
tg = 1/ Vs.kL
linearization leads to:
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= ka /kd adsorption coefficient (equilibrium constant of adsorption-desorption), will be denoted later as KA
1
0
p
T3 < T2 < T1T1
T2
T3
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Evaluation of the isosteric heat of adsorption:
dln p /dT| =const
= -Hchem/RT2
analog:ClausiusClapeyron
ln p = f (T) Hchem isosteric heat of adsorption
isosters (only equilibrium states are
considered)
qstHchem
p
V / Vs
p1 p2 p3
T1
T2
T3
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Homework:How does the relative sticking probability s/s0depend on thecoverage in case physisorption on top of a chemisorbed layer is taken intoaccount?
Langmuir Model for the co-adsorption of two species (competitiveadsorption for the same surface sites):
Adsorption and desorption rates for A species:
Ra= ka . pA(1 - -).[ ]Rd= kd
.
Dynamic equilibrium:
Ra = Rd= >
Analog for B:
= =
Homework: Calculate the coverage ratio A/Bassuming both species have thesame pressure pA= pBfor adsorption at room temperature. Species A isconsidered to adsorb by 40kJ/mol stronger than B:HAHB= 40 kJ/mol. Recall that the adsorption enthalpy is given by thedifference of the activation energies for adsorption and desorption.
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Chemisorption of atoms and molecules on metal surfaces: simple
theoretical concepts
Formation of electron bands in solids:
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Adsorption of an atom on the surface of a metal
Free atom
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Moving the free atom to the surface would cause a broadening of the originallysharp electron levels due to a resonance effect. Filled states above the Fermi levelmay lose charge towards the metal.
Donation effect Acceptor effect
Example: Alkalines adsorption Halogens adsorption
on transition metal
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back donation
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d-bands are less broad than s-bands. Therefore, the interaction with d bands givesrise to localized bonding. For the chemisorption of a hydrogen molecule, both theoccupied bonding molecular orbital of H2and the unoccupied molecular orbitalhave to be correlated with the metal d-band. The partial occupation of theantibonding MO by charge transfer from the metal to the H2gives rise toweakening of the H-H bond.
back donationjellium
Consideration of the d-band effect:
1s
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To dissociate the CO molecule the relative position of the Fermi level isimportant. Charge transfer into antibonding MO of the CO molecule causes bondweakening which is the first step to dissociation. As a consequence the COmolecule will tilt to allow its oxygen atom to contact the metal surface. This
process will finally lead to bond breaking with oxygen and carbon atoms beingdeposited into next nearest neighbor sites of the catalyst surface.
Appropriate orbitals: dz, dyz, dxz
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Metallic OrbitalsCO molecular orbitals
5-dz2 provides bonding
dxzdyz-2 * provides bonding
dxzdyz
dz2
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Symmetry of metallic surface planes: Miller indices
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Basic planes of the cubic face-centered system:
Plane (323)
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The direction in a crystal is defined by brackets: [uvw], expressed by the smallestset of integers of a collinear vector of the indicated direction, such that hu+kv+lw= 0
Ordered overlayer structures
a) E.A. WoodJ. Appl. Phys. 35 (1964) 1306
Elementary vectors of the surface a1, a2Elementary vectors of the adsorbate b1, b2
(|b1| / |a1| x |b2| / |a2|) + angle
b) R.L. Park, H.H. MaddenSurface Sci. 11 (1968) 188
b1= m11 a1+ m12 a2
b2= m21a1+ m22a2 M = fcc (100), (110), (111) M =
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Some examples:
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Area of the elementary unit cell
General classification of overlayer structures:1. mij are integers simple structure (M integer)
Adsorbed species are in well-defined local positions.
2. relation between (a1 a2) and (b1b2) given by rational numbers (M fractionalnumber)
two periodic lattices with 3b1 = 4a1or b1= 4/3a1(incommensurate)
3. incoherent structure: irrational numbers between a and b (M irrational number)
|b1|
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Experimental evidence for ordered overlayer structures
Low Energy Electron Diffraction (LEED)
What means low?
Electron wave length : acceleration of electrons by
application of a potential difference
e: elementary charge
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for U = 100 V
electron microscope: U = 105 V 4
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k = (2/) s
s = unit vector in the samedirection as k
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Electron diffraction at a one-dimensional grating of atoms
Constructive interference is obtained for wavelets propagating along the surfaceof the cones. Diffraction only occurs for certain angles which define the orderof diffraction. For a surface, a second series of cones has to be constructed. For arectangular lattice, constructive interference is obtained where the two sets ofcones intersect. The screen therefore contains a periodic arrangement of spots.
a1 a2 Bragg condition
electron gun
Spherical screen
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ai aj
* = ij(i, j = 1,2)
r: radius of screen curvature in the center of which the sample is placed.
Since distances between spots on the screen are proportional to the reciprocalof distances in real space, the reciprocal lattice can be designed as follows:
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Construction of the reciprocal lattice and the respective lattice vector:
a1*= 1/ a1 sin anglebetween vectors
a2*= 1/ a2 sin
a1a2* = 0 a1a2* a1*a2a2a1* = 0
a1a1* = 1
a2a2* = 1
Nodes of the reciprocal lattice
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Periodic surface structures and ordered overlayers in real and reciprocal space:
It can be shown using matrix calculates that:
(M*is the matrix of M inversely transposed, )
a2*a2
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An example:reciprocal lattice
real space lattice
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Kinetic parameters for elementary reaction steps: desorption energy
for thermal desorption
Evaluation of the data is based on the Polanyi-Wigner equation:
n - is order of the desorptionprocess
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, Ed and n can all be evaluated from dataThe experiment consists in heating the sample according to a temperature
program, which in most cases, is linear.
T = T0+t
Determination of the temperature for which the pressure in the reaction chamberreaches a maximum (the chamber is continuously evacuated)
{
}
Hypothesis:
{n } for the maximum
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Substitution :
n
1storder kinetics:
2ndorder kinetics:
Result: Tm f() for the 1storder process anda constant activation energy
Tmif for a 2ndorder process withconstant activation energy: orfor a 1st order process withcoverage dependence of theactivation energy{Ed= g ()}
n n
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lineshape analysis: for a 1storder process, the desorption trace isasymmetric around the maximumfor a 2ndorder process, we receive a symmetric curvearound Tm
determination of Ed:
1storder: a) hypothesis b) variation of
n n n
n
Differentiation > 2nd order: (symmetrical peak)
n( ) n
verification of the kinetic order:
n n nn
problem:Edand frequently depend on extrapolation 0for increasing temperature T
Best data treatment: simulation of the thermal desorption spectrum by fitting withthe Polanyi-Wigner equation
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Some examples for Temperature Programmed Desorption experiments:
area
H2/ Ni (100)
Coverage (atoms/cm )(a) 4.6 x 1013
(b) 8.8 x 1013
(c) 1.0 x 1014
(d) 1.7 x 1014
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multiple states appear notalways resolved separately butoverlapping, so a deconvolutionhas to be made. The appearanceof different states may dependon the coverage.
CO/ Ni (100)
Desorption spectra of carbon monoxide from clean Ni(100) followingadsorption at 137 K. Coverages were 7.0 x 1013molecules/cm2(I)and 2.6x 1014molecules /cm2(II)
(I)
(II)
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Back to real catalysis: determining the specific surface area of
materials
Adsorption in multiple layers
i is the fraction of the surface covered with i layers. is then the fraction of thesurface remaining empty. Similar to the Langmuir model, the surface isconsidered homogeneous.
i = i0
0 = =
1iii
0 = 1 -
1ii
Under dynamic equilibrium conditionsi values remain constant
i= i0 is i = Ji wi
i0 =Ji wi
surface
3
1
2
4
0
couche 3
couche 2
couche 1
layer 3
layer 2
layer 1
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i =i1 heat of physisorption remains constant in all layers following thefirst one (molecules in direct contact with the surface are assumed to have adifferent heat of physisorption and therefore, a different mean lifetime).
With:Wi= i-1 Ji1i-1= i 0
i i-2=
i =
0
i =
0 i = i-1 =i-2 =1
1 0
we can now express the overall coverage of the surface as:
c i xi
with: x = J1 / 0 andc= 0 / 1
1 - = 1c
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The above formula makes use of the following series expansion:
= (1 + x + x2 i) -1=
xi = x(1+2x +3x2i-1) = using:
and with:
=
we obtain:
V/Vs if pq (condensation on the surface)
then we can identify qp0 as the saturation vapor pressure
literature: S. Brunauer, P.H. Emmett, E. Teller, J. Amer.Chem.Soc. 60 (1938) 309
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one of the advantages of the BET equation is that it can be linearized:
a b
determination of the constant c:
Hphysis usually larger
than Hcondsince the molecules interact more strongly
with the surface than with themselves in the multilayer. The larger the difference,the steeper the slope in the above figure.
range of application:
0.05 p/p00.35
p/p0
p/V(p0-p)
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if c >> 1 we receive:
Vs= V (1 - p/ p0)
For pressures far below the saturation pressure, that is p/p0
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Some examples of BET measurements:
V
p/p0
Similar to Langmuir:indicates a microporousmaterial
p/p0
V
H2O/graphite
Br2/SiO2
The catalyst surface is hydrophobicand the formation of the first layer isnot visible.
H2O/graphiteBr2/SiO2
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Some criticism as to the BET equation: Surfaces are energetically uniform. Lateral interactions between physisorbed molecules do not exist. Condensed molecules are treated identically to the liquid phase of these
molecules.
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Catalytic reaction kinetics
For reasons of simplicity, we shall consider a sequence of steps involvingadsorption, reaction, and desorption. Each of these steps will follow first order
kinetics. The scheme therefore reads:
k1 k2 k3A Aad Bad B
k -1 k -2 k -3
It is assumed that the concentration of active sites is much smaller than theconcentration of reactants and products. The steady state kinetics of the reactionwill then be determined by the surface reaction step (the concentration of activesites being the bottle-neck of the overall speed).
R = RiRi = R2R-2 = R3 R-3= (k2Ak-2 .B) [ ] using:
quasi-equilibrium
rate determining for theoverall reaction
quasi-equilibrium
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At the beginning of the reaction, with little product formation:
(1)
pA+ pB= p
()
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Five different case studies:
1) weak adsorption (A + B) facilitates equation (1)
R = k2KA PA[ ] 1st order kinetics with regard to A
Attention: HAis negative!
2) strong adsorption (A)
KB pB> 1
R = k2 .[ ] zero kinetic order with regard to A and B
Bad
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3) strong adsorption (B)
KB pB >> KA pA
positive 1storder kinetics with regard to A, negative 1storder kinetics for B
the apparent activation energy is now larger than in case 1 (the desorption energyis positive and equal to the adsorpton enthalpy).
4) adsorption determines the reaction rate
R = k1.pA.[ ] surface coverage is low
5) desorption determines the reaction rate
R = k3.B.[ ] KB . pB >> 1
1/T
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What changes for bimolecular reactions?
DAssuming that the surface reaction is the slowest step (C and D are weaklyadsorbed) we receive:
both HA and HBare negative!z is the number of next-nearest neighbor sites
for strong adsorption of A we receive:
; reaction follows 1storder kinetics in A and negative 1storder kinetics in B ; theoverall reaction order is zero.
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Mechanistic alternative for reactions on the surface of oxidic
catalysts:
example: SO2+ O2 SO3 catalyst: V2O5
redox mechanism:
CatO + A Cat- + AO
Cat+ O2 CatO_____________________________
A + O2 AO sum reaction
Rred = R oxyd for steady state conditions
kr.pA(1-)
.[ ] = ko.pO2
..[ ] fraction of the reducedsurface
a) if kr pA>>k0pO2 is close to 1 oxidation islimiting
zero reaction order for A
reduction\
of catalyst
oxidation
/
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b) if isvery small compared to1 zero order reaction kineticsfor oxygen
R = kr . pA
The difference with respect to the Langmuir-Hinshelwood type mechanism onmetals is that according to the above Mars-Krevelen mechanism is a fullyreversible chemical alteration of the catalyst surface takes place while for L-Hthis does not occur at all.
Examples with industrial application:
a. oxidation of hydrocarbons on mixed oxides of V, Mo
b.oxidation of ortho-xylene to phtalic anhydride on V2O5/SiC