Heterogeneous Catalysis of Mixed Oxides: Perovskite and Heteropoly Catalysts

179 Pages·2013·3.896 MB·English

by  Makoto Misono (Eds.)

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Preface
Catalystsareutilizedinmostchemicalprocessesfortheproductionofmaterials
andgoodsusedinourdailylife,andalsofortheeliminationofhazardouscom-
poundstomaintainahealthyenvironment.Catalystshavethusbeenimportant
components of technology in the past and will be so in the future as well. At
thesametime,themechanismsofthecatalyticfunctionshave attracted much
attentionfromthesciencecommunity,andmanyeffortshavebeenmadetoelu-
cidatethesemechanisms.Theseeffortshavehelpedthedevelopmentofpracti-
cal catalysts. However, the author believes that catalyst materials should be
called“catalysts”onlywhentheusefulnessisprovedintheirpracticalapplica-
tions.Thismeansthatthescienceofcatalysisismeaningfulonlywhenthesci-
ence is useful for the development of practical catalysts. This book is written
from this standpoint and intends to provide the fundamental concepts and
knowledgeforthedesignofusefulcatalysts.
Catalystsareclassifiedintosolidcatalysts,molecularcatalysts,andbiocata-
lysts.Mainsolidcatalystsaremetallicandmixedoxidecatalysts.Inaddition,
therearesulfides,halides,carbons,organicpolymers,etc.Molecularcatalysts
are coordination compounds and organic molecules. Biocatalysts include
enzymesandmicroorganisms.Thesubjectofthisbookisthemixedoxidecat-
alystswhicharewidelyusedinindustrialprocessesfortheproductionofmate-
rialsandtoabatehazardouscompounds.
In thisbook,“catalyst design” isdefined as the rational and efficientpro-
cedure for the development of useful catalysts. It provides the guidelines for
effective trials (and errors) to obtain practically usable catalysts. For many
years the author has attempted to establish the guidelines on the basis of the
hypothesesthattheyareacquiredfromthestudiesonmixedoxidessatisfying
thefollowingconditions:(1)Thestructuresareregular(crystalline)sothatthe
surface can be estimated reliably from the bulk structure (that is, well-
characterized mixed oxides); (2) important chemical properties of catalysts,
namely, acid–base and reduction–oxidation (redox) properties, can be
changed widely and systematically while keeping the fundamental structures;
and (3) the catalytic performances of the “well-characterized mixed oxides”
are not far from the level required for practical applications. The last condi-
tion excludes the use of single crystals in our studies.
Thisbookiswrittenaccordingtotheideasandhypothesesdescribedabove.
Thesubstantialexamplesareadoptedmainlyfromtheworkoftheauthors’group
doneintheline,choosingperovskiteandheteropolycatalysts.Therefore,thisis
notanattempttocovercomprehensivelythecatalysisofmixed oxides,butthe
ix
x Preface
importantrelevantfindingsthatprecedetheauthor’sstudiesarereferredto.The
achievementsofothergroupsinlinewiththeaboveideasarenaturallyincluded.
But important mixed oxide catalysts like zeolites, clays, and supported metal
complexarenotincludedordescribedtoalimitedextent.
This book comprises five chapters. In Chapter 1, a concise introduction is
given to the fundamental knowledge of catalysis required to understand the
book, including the concept of catalyst design. In Chapter 2, the chemistry
of mixed oxides relevant to catalysis, that is, structures, acid–base, and redox
properties, is described. Here, the relationships of those properties with the
structure and composition of the solid bulk are emphasized and the roles of
solid bulk are stressed, although in some cases the surface different from
the solid bulk might be more important in catalysis.
Then, in Chapters 3 and 4, the chemistry and catalysis of perovskites and
heteropoly compounds are described from the viewpoint of catalyst design.
Perovskitesandheteropolycompoundsarechosenbecausethey,respectively,
representdoubleoxidesandsaltsofoxoacids, thetwomajorgroupsofmixed
oxides, and as described in these chapters, both mixed oxides satisfy the
above three conditions for the studies of the catalyst design. Chapter 5 is
abouttheuseofmixedoxidesforcatalystsupports.Afewinterestingfeatures
of mixed oxides when they are used for catalyst support are described.
Thus, this book mainly intends to provide the basic concepts of catalyst
design taking as examples perovskites and heteropoly compounds, but it is
also intended to provide a general view on the catalysis of mixed oxides. It
is further hoped that “designed catalysts” will play a significant role in
making the society and the environment greener and more sustainable.
Theauthordeeplyappreciatesextensivecollaborationsintheresearchonhet-
eropolyandperovskitecatalystsduringhiscareerattheUniversityofTokyo,with
the late T. Okuhara, N. Mizuno, K. Y. Lee, K. Inumaru, M. Hashimoto,
G.Koyano,K.Tabata,H.Tanaka,thelateY.Yoneda,J.Take,S.Nakata,J.M.
Dereppe,andE. A.Lombardo,aswellasmanypostdocsand studentswhoare
nowprofessorsandengineers.HealsothanksProfessorsY.Kamiya,H.Niiyama,
Y.Ono,Y.Izumi,N.Yamazoe,T.Yamase,K.Domen,K.Asakura,R.K.Grass-
elli, W. G. Klemperer, M. T. Pope, I. V. Kozhevnikov, A. Corma, the late J.
Haber,andthelateW.K.Hallforvaluablediscussionsandsuggestions.Showa
DenkoK.K.andToyotaMotorCorp.arehighlyappreciatedforthediscussion
onpracticalapplicationsofmixedoxidecatalysts.Thanksarealsoduetothestaff
ofElsevierwhohavepatientlyencouragedtheauthortocompletethisbook.
Inaddition,theauthorowesalottomanyeminentchemicalscientistsand
engineersintheworldfortheirinspirationandguidanceinthestudyofcatal-
ysis. Finally, the author expresses his sincere gratitude to his wife Yoshiko
and family for warmest supports.
Makoto Misono
Tokyo, November 2012
Chapter 1
Basis of Heterogeneous
Catalysis
Makoto Misono
Chapter Outline
1.1. CatalystandCatalysis 2 1.2.3. RateEquationof
1.1.1. RateandEquilibriumof CatalyticReaction 11
ChemicalReaction 1.2.4. ReactorTypeandRate
andRoleofCatalyst 2 Expression 14
1.1.2. ThreeEssential 1.2.5. ElucidationofReaction
FunctionsofCatalyst 3 Mechanism 15
1.1.3. EssenceofCatalytic 1.2.6. Mass andHeat
FunctionsBasedon Transfer 17
ReactionMechanism 4 1.2.7. Deactivationof
1.1.4. AShortHistoryof Catalyst 19
IndustrialCatalysts 5 1.2.8. Comparisonof
1.1.5. Classificationof Heterogeneous,
Catalysts 8 Homogeneous
1.1.6. PracticalApplications andBiocatalysis 20
ofCatalysts 9 1.3. CatalystDesign 20
1.1.7. Components andShape 1.4. Preparationand
ofIndustrialCatalysts 9 Characterizationof
1.2. RateofCatalyticReactionand Catalysts 22
ReactionMechanism 10 References 23
1.2.1. ReactionRate 10
1.2.2. AdsorptiononSolid
Surface;Rateand
Isotherm 11
Fundamental concepts and principles indispensable to understand the hetero-
geneous catalysis of mixed oxides are described in this chapter [1].
StudiesinSurfaceScienceandCatalysis,Vol.176.http://dx.doi.org/10.1016/B978-0-444-53833-8.00001-6
©2013ElsevierB.V.Allrightsreserved. 1
2 StudiesinSurfaceScienceandCatalysis
1.1 CATALYST AND CATALYSIS
Catalystisasubstancethatispresentinasmallamountinthereactionsystem
and accelerates the desired chemical reaction(s), but little changes during the
reaction. Catalysis is a general term for the function of catalyst. Almost all
materials and goods that are daily used are produced through catalytic pro-
cesses from various raw materials.
Many high-performance catalysts are used, in order to utilize efficiently
raw materials, including recycling of used materials, and to utilize efficiently
the existing as well as “new” energy sources. Catalysts are used not only for
chemicalsyntheticprocessesbutalsoforotherusessuchaselectrodesoffuel
cells and batteries. In addition, catalysts are used to protect and improve the
environment. The latter catalysts are called environmental catalysts, that is,
“KankyouShokubai”inJapanesewhichisanewwordfirstusedbytheauthor
in the late 1970s.
1.1.1 Rate and Equilibrium of Chemical Reaction
and Role of Catalyst
A catalyst changes (accelerates in most cases) the rate of chemical reactions
to approach the equilibrium but does not change the equilibrium itself. It is
importanttodistinguishbetweenthekinetics(rateofreaction)andthermody-
namics (equilibrium constant of reaction). The difference may be evident in
Fig. 1.1, which schematically shows the case of a simple reversible reaction
100
%)
A (

of
n
o
ati
ntr A B
e
c
n
Co Higher
activity
[A]
e
B A
0
Reaction time
FIGURE1.1 Timecourseofareversiblereaction,A$B.Theconcentrationreachesfasterthe
equilibriumconcentration[A] overacatalystwithahigheractivity,buttheequilibriumconcen-
e
trationisthesame.
Chapter 1 BasisofHeterogeneousCatalysis 3
of A$B. The rate to approach the equilibrium composition changes from a
catalysttoanother,buttheequilibriumfinally attainedisthesame.Ifthefor-
wardandreversereactionsarefirstorder,theratesareexpressedbyEq.(1.1)as
RateðforwardÞ¼k½A(cid:2), andrate ðreverseÞ¼k0½B(cid:2) (1.1)
Attheequilibrium,theratesofforwardandreversereactionsareidentical,
then,
k½A(cid:2)¼k0½B(cid:2) (1.2)
where the ratio of [B]/[A] is being determined by the equilibrium constant,
K¼k/k0. With an active catalyst, the rate to approach the equilibrium is fast,
andfor alessactivecatalystit isslower.Therefore,both forward andreverse
reactions are faster over a more active catalyst at the equilibrium, while both
rates are identical for each case.
1.1.2 Three Essential Functions of Catalyst
Catalyticactivity,selectivity,anddurability(¼catalystlife)arethethreemost
important functions of catalyst.
Activity is most fundamental. A reaction which does not occur in the
absence of catalyst could proceed in the presence of catalyst. With a more
active catalyst, the production rate per volume of reactor becomes larger
and the reactor volume can be made smaller.
Selectivityisthemostinterestingandattractivefunctionofcatalyst,which
selects one (or more) desirable reaction(s) among many reactions that would
possibly occur. Choosing a reaction which produces a thermodynamically
unfavorablebutvaluableproductisoneofthemostattractivefunctionsofcat-
alyst. A fascinating function of catalyst is stereoselectivity that produces one
of the two stereoisomers which have thermodynamically the same stability.
Theselectivitymaybedividedintotwocategories(Eq.1.3).Oneistoselect
oneproductamongseveralpossibleproductsstartingfromonereactingmolecule,
e.g.,choosingonlyAtoB(Eq.1.3a).Thismaybecalled(a)product-selectivity.
Anothertypeistoselectonereactantinamixtureofseveralreactantschoosing
A from a mixture of A and B (Eq. 1.3b). This is called (b) reactant-selectivity
(calledsubstratespecificityinenzymaticreactions):
(a) A B C, (b) A C
ð1:3Þ
D B D
Durability(orcatalystlife)isindispensableforcommercialcatalysts.Cat-
alyst is sometimes defined with a statement that catalyst and its performance
donotchangeduringthecatalyticreaction.But,inreality,itchanges(usually
itgraduallydeteriorates),andafteracertainperiod,ithastobesubstitutedbya
fresh catalyst or reactivated by an appropriate method. Most of commercial
4 StudiesinSurfaceScienceandCatalysis
catalysts are synthesized through many elaborate steps and often contain
expensiveelements,sothat,unlessthecatalystlifeislongenough,thecatalytic
processwouldbeverycostlyandneverbecommercialized.Commercialcata-
lysthasusuallyalifeofseveralmonthsto10years,andcontinuousreactivation
issometimesnecessaryasinthecaseoffluidizedcatalyticcracking.
1.1.3 Essence of Catalytic Functions Based on Reaction
Mechanism
Figure 1.2 illustrates schematically the reaction mechanism of oxidation of
carbon monoxide catalyzed by palladium (Eq. 1.4). This was demonstrated
for a palladium single crystal a long time ago by G. Ertl using techniques,
sophisticated at that time, under ultrahigh vacuum. Actual reaction mecha-
nismunderordinaryconditionsmaybedifferent,butthisdiagramissufficient
to understand the essence of catalytic process:
COþð1=2ÞO (cid:3)(cid:3)(cid:3)(cid:3)!CO (1.4)
2 2
Pd
Without catalyst
1
CO+––O
2 2
Gas phase
With catalyst
250 284
106
CO
O ~20 2
O O Gas phase
C O C
Pd Pd
CO and O adsorbed CO adsorbed
2
FIGURE1.2 EnergydiagramofCOþ(1/2)O !CO catalyzedbypalladium.Theinitialandfinal
2 2
statesdonotchangebythepresenceofcatalyst.ThenumbersaretheenergydifferencesinkJ(cid:3)1mol(cid:3)1.
Chapter 1 BasisofHeterogeneousCatalysis 5
In this figure, three essential points of catalytic functions are to be noted.
First, the oxygen molecule in which two oxygen atoms are strongly bonded
(494kJmol(cid:3)1)dissociatesintotwooxygenatomsonthesurfaceofpalladium
with a very low energy barrier. This is due to the formation of two oxygen-
palladium bonds that occurs simultaneously with the bond breaking of O .
2
Second, the oxygen atoms formed are reactive. If they are too stable, the
subsequent reactions would not take place. Third, after one cycle of catalytic
oxidation of CO to CO , as in Fig. 1.2, the catalyst surface returns to the ini-
2
tial state. Upon each cycle, one molecule of carbon dioxide is produced.
From the second point, the basic principle for the selection of a good cat-
alyst is deduced. As stated above, if the oxygen atom adsorbed on the cata-
lyst surface is too stable, the subsequent steps become endothermic and
would not take place. This is because the initial and final states of the reac-
tion (both in the gas phase) do not change by the presence of catalyst. If the
first step is too exothermic, the first step may occur very easily, but the fol-
lowing steps become inevitably very endothermic and hard to proceed. On
thecontrary,ifthefirststepisveryendothermic,thefirststepwouldnottake
place. Thus, the key intermediate, oxygen atom adsorbed in this case, must
have moderate thermodynamic stability, the energy level being not far either
from the initial or from the final state of the reaction. In other words, a good
catalyst must have moderate affinity to the key intermediate. In the case of
palladium, this condition is satisfied as seen in Fig. 1.2. The appropriate sta-
bility of the key intermediate is the first basic principle for selection of a
good catalyst.
Inalatersection(section2.1.5),theselectionofgoodmetaloxidecatalyst
for oxidation will be discussed on the basis of redox (or Mars–van Krevelen)
mechanism. The essential idea there is the same as described here.
1.1.4 A Short History of Industrial Catalysts
Catalysts have been used for large-scale productions of chemicals since the
beginning of the twentieth century. Examples are Pt for SO oxidation,
2
Fe for NH synthesis, and Zn–Cr oxide for methanol synthesis. In the case
3
of Fe catalyst, it took almost 10 years until the industrial production started
in 1913, after the Fe catalyst was discovered. During the 10 years, materials
andequipmentsnecessaryforthehigh-pressureprocessweredeveloped.This
example shows that, in order to industrialize a catalytic process, many exist-
ing as well as newly developed technologies have to be integrated. Another
goodexampleisso-calledthree-waycatalystfortheemissioncontrolofauto-
mobile. In this case, an elaborately synthesized multicomponent catalyst is
integrated to an emission control system, combined with a monolith support,
an oxygen sensor, and a computer-controlled device for fuel supply.
Table 1.1 collects important industrial catalysts which were commercia-
lized in the past.
6 StudiesinSurfaceScienceandCatalysis
TABLE1.1 MajorIndustrial Catalystsandthe Year ofInvention
Year Process(catalyst) Remarks
(cid:4)1890 SO oxidation(Pt) SubstitutionofNO catalystand
2 x
substitutedbyVoxidecatalyst
1913 NH synthesis(Fe) FixationofN ,high-pressure
3 2
technology
1916 Aceticacidfromacetylene(Hg) Viaacetaldehydeformedbyhydration
1925 Methanolsynthesis(Zn–Cr) Developmentofhigh-pressure
technology
1936 Crackingofoil(clay) Gasoline,solidacidcatalyst
1939 High-pressurepolyethylene(O ) Radicalpolymerization
2
(cid:4)1945 Hydroformylationofolefin Oxoprocess
(Cocomplex)
1949 Reformingofnaphtha(Pt–alumina) Gasoline,bifunctionalcatalysis
1954 Industrialproductionofzeolite Crystallinesolidacid
1955 Low-pressurepolyethylene Zieglercatalyst
(TiCl–AlR )
x 3
1957 Polypropylene(TiCl–AlR ) Stereoselectivepolymerization
x 3
1960 Aceticacidfromethylene Wackerprocess
(PdCl –CuCl )
2 2
1963 Ammoxidation(Mo–Bi–O) Allylicoxidationofpropylene,
SOHIOprocess
(cid:4)1965 Hydrotreatmentofheavyoil HDS,cleanfuel
(Mo–Co–S)
1967 Improvedreformingofnaphtha Bimetalliccatalysts
(Pt–Re–alumina)
(cid:4)1968 Shapeselectivity(zeolite) Isomerization,alkylation
1969 L-Aminoacid(fixedenzyme) Industrialuseofenzyme
(cid:4)1970 Improvedammoxidation Multi-componentMo-Bi-O
(Mo–Bi–Fe–O)
1973 Aceticacidfrommethanol Monsantoprocess
(Rhcomplex)
1974 L-DOPA Asymmetriccatalysisbycomplex
(cid:4)1975 SCR(V–Ti–O) Environmentalcatalyst
1976 MTGprocess(zeolite) Diversifiedsourceofhydrocarbons
Chapter 1 BasisofHeterogeneousCatalysis 7
TABLE1.1 MajorIndustrialCatalystsandtheYearofInvention—Cont’d
Year Process(catalyst) Remarks
(cid:4)1976 Emissioncontrolofautomobile Three-waycatalyst(TWC)
(Pt,Rh,Pd)
1976 Isopropanolfrompropylene FirstindustrialprocessbyHPA
1985 Acrylamide(bio-organism) Firstindustrialprocessbybiocatalyst
1997 Metallocenecatalystforpolyolefin Single-sitecatalyst
(cid:4)2002 Long-lifeautomotivecatalyst Improvedsupportandpreparation
2003 e-Capolactum(gasphase) Greencatalyst
(silicalite)
(cid:4)2003 High-performanceHDS Sulfur-freefuels
HDS,hydrodesulfurization,SCR,selectivecatalyticreductionofNOx,MTG,methanoltogasoline,
andHPA,heteropolyacids.
Starting molecule: acetylene ethylene methanol
Catalyst: Hg salt Pd salt Rh complex
Raw material: coal oil variable
(natural gas, oil, coal, biomass)
SCHEME1.1 Changeofprocessforaceticacidproductionwiththechangesinrawmaterialand
catalyst.
As seen from the table, many excellent catalysts have been introduced to
copewiththesocialneeds,suchaschangesinrawmaterials,desiresforbetter
materials, and environmental requirements. A good example may be the
production of acetic acid. It was produced from wood in early age, but along
with the change in the raw materials, new catalysts were developed (see
Scheme 1.1).
Another good example is solid acid catalyst. Catalytic cracking of oil to
gasoline was first carried out using acidic clay to replace thermal cracking,
in order to improve the yield of gasoline. Later synthetic amorphous silica–
alumina was introduced in place of clay catalyst, and then it was followed
by crystalline zeolites. Now new zeolite catalysts combined with amorphous
silica-alumina are commercially used in a fluidized-bed reactor. In the mean
time, catalytic performance and understanding of the nature of solid acids
made remarkable progress. This was in parallel with the development of oil
industry.