Kinetics of Catalytic Reactions

256 Pages·2005·1.67 MB·English

by  M. Albert Vannice

Kinetics of Catalytic Reactions
M. Albert Vannice
Kinetics of Catalytic
Reactions
With 48 Illustrations
M.AlbertVannice
WilliamH.JoyceChairedProfessor
DepartmentofChemicalEngineering
ThePennsylvaniaStateUniversity
UniversityPark,PA16802
[email protected]
LibraryofCongressControlNumber:2005923777
ISBN-10:0-387-24649-5 e-ISBN:0-387-25972-4 Printedonacid-freepaper.
ISBN-13:978-0387-24649-9
(cid:1)2005SpringerScienceþBusinessMedia,Inc.
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Foreword
Heterogeneous catalysis has shaped our past and will shape our future.
Already involved in a trillion dollar’s worth of gross domestic product,
catalysisholdsthekeytoneartermimpactareassuchasimprovedchemical
process efficiency, environmental remediation, development of new energy
sources,andnewmaterials.Furthermore,recentadvancesinunderstanding
and computing chemical reactivity at the quantum level are opening new
pathways that will accelerate the design of catalysts for specific functions.
This enormous potential will ultimately be turned into reality in laboratory
reactors and have its impact on society and the economy in the industrial
reactorsthatlieattheheartofallchemicalprocesses.Becausethequantita-
tivemeasureofcatalystperformanceisthereactionrate,itsmeasurementis
central toprogress in catalysis.
The pages that follow are a comprehensive guide to success for reaction
rate measurements and analysis in catalytic systems. The topics chosen, the
clarityofpresentation,andtheliberaluseofspecificexamplesilluminatethe
full slate of issues that must be mastered to produce reliable kinetic results.
The unique combination of characterization techniques, thorough discus-
sion of how to test for and eliminate heat and mass transfer artifacts,
evaluation of and validity tests for rate parameters, and justification of the
uniformsurfaceapproximation,alongwiththemorestandardidealreactor
analysesanddevelopmentofrateexpressionsfromsequencesofelementary
steps, will enrich readers from both science and engineering backgrounds.
Well-explained real examples and problems that use experimental data will
help students and working professionals from diverse disciplines gain oper-
ational knowledge.
Thisbookcapturesthecareerlearningofanoutstandingcatalytickineti-
cist. Drawing on experience that began with a paper showing the power of
physical chemical and thermodynamic constraints for eliminating incorrect
rateformulationsandincludesacitationclassicpaperontheCOhydrogen-
ation reaction over group VIII metals plus the development of rate expres-
sions for a wide variety of catalyst systems, Vannice captures not only the
theory of the Boudart school of chemical kinetics, but also its practical
v
vi Foreword
application. He has created a resource that will help the next generation of
catalytic scientists and engineers provide the validated kinetic analyses that
will be critical to thedevelopment of nano,micro,andmacroscale catalytic
systems ofthe future.
W. NicholasDelgass
Purdue University
January2005
Preface
The field of catalysis, especially heterogeneous catalysis, involves the util-
ization of knowledge from various disciplines, including chemical engineer-
ing,chemistry,physics,andmaterialsscience.After more thantwodecades
ofteachingagraduatecourseincatalysis,whosemembershipwascomprised
primarilyofstudentsfromtheaboveprograms,andconsultingwithnumer-
ous industrial researchers, it became apparent to me that a book would be
useful that focused on the proper acquisition, evaluation and reporting of
rate data in addition to the derivation and verification of rate equations
based on reaction models associated with both uniform and nonuniform
surfaces.Suchabookshouldfamiliarizeandprovideitsreaderwithenough
background information to feel comfortable in measuring and modeling
heterogeneous catalytic reactions. For a single individual to attempt such
anundertakingisalmostaguaranteethatsomeissueswillbeaddressedless
adequately than others; regardless, I hope that these latter topics will meet
minimum standards! My goal is a text that will be self-contained and will
provide a convenience forthe practitioner incatalysis.
I would first like to acknowledge here the people who have been most
influential in my life and have inspired me to this point in my career,
whereupon I have been willing to undertake the effort to write this book
(which is bound to reveal some of my deficiencies, I am sure!). Clearly, I
mustthankmyparentswho,duringthetimeIattendedasmallhighschool
of 19 students in Nebraska, always had me oriented towards a college
education. Second, I gratefully acknowledge my graduate school mentor,
ProfessorMichel Boudart,whocreatedmyinterestinkineticsandcatalysis
andinstilledinmethenecessityofaccurate,reproducibledata.Third,Inote
my friends from graduate school, especially Professor Nicholas Delgass,
who have continued to educate me during the past three decades. (They
have upheld the old adage that one should never stop learning.) Fourth, I
mustmentionmygraduatestudents,whohaveprovidedmemuchpleasure,
not only in their accomplishments as we worked together, but also in the
successes they have subsequently achieved. In particular, I would like to
express special appreciation to one of them, Dr. Paul E. Fanning, who
vii
viii Preface
graciouslyvolunteeredhistimetoverycarefullyproofreadthistextbookand
offer valuable suggestions. Also, I would like to acknowledge the review of
Chapter6.3andthecommentsofferedbyDr.EvgenyShustorovich,bothof
which were greatly appreciated. Next, I would like to thank my secretary,
KathyPeters,forherpatienceandpersistenceduringthetimeshetypedthe
drafts of these chapters as they traveled, at times uncertainly, via air mail
betweenAlicante,SpainandPennState.Imustalsoexpressmygratitudeto
ProfessorFranciscoRodriguez-ReinosoforhishospitalityattheUniversity
ofAlicanteduringmyone-yearsabbaticalstaytheretowritethisbook.Last,
butcertainlynotleast,Isincerelythankmywife,BetteAnn,forherpatience
andunderstandingduringthemanydaysandnightsIwasabsentduringthe
pastthree decadeswhileworkingin thelabto establish aresearch program
or attendingnecessary scientific meetings (invariably on herbirthday!).She
was also kindenough tooffer hersecretarialskillsandproofread this book
from a grammatical perspective.
Contents
Foreword....................................................... v
Preface ....................................................... vii
List ofSymbols ............................................... xiii
1. Regular Symbols ...........................................xiii
2. Greek Symbols............................................. xv
3. Subscripts .................................................xvi
1. Introduction................................................. 1
2. Definitionsand Concepts ...................................... 5
2.1 Stoichiometric Coefficients .............................. 5
2.2 Extent of Reaction ..................................... 5
2.3 Rate ofReaction....................................... 6
2.4 TurnoverFrequency or Specific Activity................... 6
2.5 Selectivity............................................. 8
2.6 Structure-SensitiveandStructure-Insensitive Reactions ...... 8
2.7 ElementaryStep and RateDeterminingStep (RDS)......... 8
2.8 Reaction Pathway orCatalytic Cycle..................... 10
2.9 Most Abundant Reaction Intermediate (MARI)........... 11
2.10 Chain Reactions ...................................... 11
2.11 Reaction Rates in Reactors............................. 11
2.12 Metal Dispersion (Fraction Exposed) .................... 12
2.13 Metal-SupportInteractions(MSI) ....................... 12
References................................................. 13
3. Catalyst Characterization .................................... 14
3.1 Total (BET) Surface Area ............................... 15
3.2 Pore Volume and PoreSize Distribution................... 17
3.2.1 HgPorosimetry Method .......................... 17
3.2.2 N Desorption Method ........................... 18
2
3.2.3 Overall PoreSize Distribution ..................... 18
3.3 Metal Surface Area, CrystalliteSize, and Dispersion ........ 19
ix
x Contents
3.3.1 Transmission ElectronMicroscopy(TEM)........... 19
3.3.2 X-Ray Techniques ............................... 20
3.3.2.1 Line Broadeningof X-Ray Diffraction
(XRD) Peaks............................ 20
3.3.2.2 Extended X-RayAbsorption Fine
Structure(EXAFS)....................... 22
3.3.3 Magnetic Measurements .......................... 23
3.3.4 Chemisorption Methods .......................... 23
3.3.4.1 H Chemisorption........................ 23
2
3.3.4.2 CO Chemisorption ....................... 28
3.3.4.3 O Chemisorption........................ 30
2
3.3.4.4 H(cid:1)O Titration Techniques ............... 31
2 2
3.3.5 Relationships Between Metal Dispersion,
Surface Area, and CrystalliteSize .................. 32
References................................................. 33
Problems.................................................. 35
4. Acquisition and Evaluation ofReaction Rate Data................ 38
4.1 Typesof Reactors...................................... 38
4.1.1 Batch Reactor................................... 38
4.1.2 Semi-Batch Reactor.............................. 42
4.1.3 Plug-FlowReactor (PFR)......................... 42
4.1.4 Continuous Flow Stirred-Tank Reactor(CSTR)...... 49
4.2 Heat and Mass Transfer Effects.......................... 51
4.2.1 Interphase (External) Gradients
(Damko¨hler Number) ............................ 52
4.2.1.1 Isothermal Conditions .................... 52
4.2.1.2 NonisothermalConditions................. 55
4.2.2 Intraphase (Internal)Gradients (Thiele Modulus)..... 56
4.2.2.1 Isothermal Conditions .................... 56
4.2.2.2 NonisothermalConditions................. 61
4.2.2.3 Determining an Intraphase(Internal)
Effectiveness Factor from a
Thiele Modulus.......................... 61
4.2.3 Intraphase Gradients(Weisz-Prater Criterion)........ 63
4.2.3.1 Gas-Phase orVapor-PhaseReactions ....... 63
4.2.3.2 Liquid-Phase Reactions ................... 67
4.2.4 OtherCriteriato Verify the Absence ofMassand
Heat Transfer Limitations
(The Madon-BoudartMethod) .................... 77
4.2.5 Summary ofTestsforMassand Heat
Transfer Effects ................................. 80
References................................................. 82
Problems.................................................. 84
Contents xi
5. Adsorption and DesorptionProcesses........................... 87
5.1 Adsorption Rate....................................... 87
5.2 Desorption Rate ....................................... 90
5.3 Adsorption Equilibrium on Uniform (Ideal)
Surfaces-LangmuirIsotherms............................ 91
5.3.1 Single-Site(Nondissociative) Adsorption ............ 92
5.3.2 Dual-Site(Dissociative)Adsorption ................ 92
5.3.3 Derivation ofthe LangmuirIsothermby
Other Approaches................................ 95
5.3.4 Competitive Adsorption .......................... 97
5.4 Adsorption Equilibrium on Nonuniform(Nonideal)
Surfaces .............................................. 98
5.4.1 The Freundlich Isotherm.......................... 98
5.4.2 The Temkin Isotherm............................. 99
5.5 ActivatedAdsorption.................................. 101
References................................................ 103
Problems................................................. 104
6. Kinetic Data Analysisand Evaluation ofModel Parameters
for Uniform (Ideal) Surfaces................................. 106
6.1. Transition-State Theory (TST) orAbsolute Rate Theory ... 107
6.2 The Steady-State Approximation (SSA) .................. 113
6.3 Heats ofAdsorption and Activation Barrierson
Metal Surfaces:BOC-MP/UBI-QEP Method.............. 117
6.3.1 Basic BOC-MP/UBI-QEP Assumptions ............ 118
6.3.2 Heats ofAtomicChemisorption................... 120
6.3.3 Heats ofMolecular Chemisorption ................ 121
6.3.4 Activation Barriers forDissociation and
Recombination on Metal Surfaces................. 130
6.4 Use of aRate Determining Step(RDS)and/or
aMostAbundantReaction Intermediate (MARI) ......... 133
6.5 Evaluation ofParameter Consistency inRate
Expressions forIdeal Surfaces........................... 134
References................................................ 136
Problems................................................. 137
7. Modeling Reactionson Uniform (Ideal) Surfaces ................ 141
7.1 Reaction Models with aRDS – Unimolecular
Surface Reactions..................................... 142
7.2 Reaction Models with aRDS – Bimolecular
Surface Reactions..................................... 156
7.3 Reaction Models with aRDS – Reactions
between an Adsorbed Species and a Gas-PhaseSpecies ..... 170
7.4 Reaction Models with no RDS.......................... 171
7.4.1 ASeries of Irreversible Steps – General Approach ... 175