Pacific Northwest National LaboratoryPacific Northwest National LaboratoryDepartment of Energy, Office of ScienceDepartment of Energy, Office of Science
Chuck PedenAssociate Director
Institute for Interfacial CatalysisPacific Northwest National Laboratory
Richland, WA 99352
Surface Science Studies of Surface Science Studies of Catalytic Vehicle Emission Control Catalytic Vehicle Emission Control
Mt. St. Helens, October 5, 2004(Mount Ranier in Background)
July, 2003
PNNL is located near the Hanford Site in PNNL is located near the Hanford Site in Southeastern Washington StateSoutheastern Washington State
• 55 million gallons of wastes stored in 177 underground tanks
• 150-200 square miles of contaminated groundwater
• 1-4 cubic kilometers of contaminated soils• 9 reactors, 5 processing facilities, 1
plutonium finishing plant, 4 evaporators, and numerous smaller facilities
• 400+ million Curies of radioactive material
Major Environmental Issues
U.S. Department of Energy U.S. Department of Energy MultiprogramMultiprogram National LaboratoriesNational Laboratories
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Pacific Northwest
IdahoLawrence Berkeley
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Pacific Northwest National LaboratoryRichland, Washington
Pacific Northwest National LaboratoryRichland, Washington
• DOE Office of Science multiprogramlaboratory
• Established in 1965• ~4,200 staff• ~$730 million/yr.
budget• ~$15M/yr. for
catalysis research (60+ scientific staff)
PNNL staff delivered the most PNNL staff delivered the most presentations at the 19presentations at the 19thth Meeting of Meeting of the North American Catalysis Societythe North American Catalysis Society
0 5 10 15 20 25 30
Pacific Northwest National LaboratoryUniversity of Delaware
University of California at BerkeleyLehigh University
Oak Ridge National LaboratoryUtrecht University
Northwestern UniversityUniversity of Michigan
Purdue UniversityETH Zurich
Argonne National LaboratoryCompetence Centre for Catalysis, Chalmers
Sud-Chemie Inc.Delft University of Technology
Engelhard CorporationFritz-Haber-Institute
DegussaGeorgia Institute of Technology
KAISTPennsylvania State University
University of CincinnatiUniversity of Pennsylvania
University of SaskatchewanUniversity of South Carolina
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PostersTop 25
Institutions
Programs Span the Range from Basic to AppliedPrograms Span the Range from Basic to Applied
• Fundamental Catalysis Science• Catalytic Vehicle Emission Measurement and Control• Heterogeneous Catalysis of Bio-Based Feedstocks• Catalyst and Process Development Using Microchannel
Reactors• Catalyst Materials for Solid-Oxide Fuel Cells• Other ‘Miscellaneous’ Catalysis Projects including Solid
Acid Catalysis, Catalysis for Petroleum Refining, etc.
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Pacific Northwest lab forms Institute for Interfacial Catalysis, names director
PNNL to invest $8 million for a national center to study chemical transformations important for 'secure energy future'
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RICHLAND, Wash. -- Pacific Northwest National Laboratory today launched an $8 million Institute for Interfacial Catalysis to explore the fundamental chemical changes on surfaces where catalytic reactions take place. The Department of Energy lab also announced the appointment of University of Texas at Austin chemist John M. "Mike" White as the institute's director.
Understanding how chemicals interact at the surfaces of catalyst materials is seen as the key to converting energy into chemicals and vice versa, said White, who holds a joint appointment at PNNL while continuing as Robert A. Welch Chair of Materials Chemistry at the university. Catalysts, substances that modify chemical reaction rates and that remain unchanged afterward, also are crucial in a wide range of industrial and biological processes.
The Institute for Interfacial Catalysis at PNNLThe Institute for Interfacial Catalysis at PNNL
First Director:Professor John (Mike) WhiteUniversity Distinguished Teaching ProfessorDirector, Center for Materials ChemistryRobert A. Welch Chair in Materials Chemistry
• ACS Kendall Award (1990) • Guiseppe Paravanno Award, Michigan Catalysis Society
(1993) • Southwest Regional ACS Award (1999) • The Arthur W. Adamson Award for Distinguished
Services in the Advancement of Surface Chemistry (2001)
Presentation Presentation ‘‘GoalsGoals’’• Gain an appreciation for the past developments and
future challenges in catalytically controlling vehicle exhaust emissions.
• Introduce some ‘standard’ and ‘special’ surface science methods for model catalysis studies.
• “Three-way” catalysis on ‘precious’ metals.• Understanding the various roles of oxide materials in
vehicle exhaust emission control devices.
Two Parts:Two Parts:
Disclaimer!Disclaimer!• Will use examples drawn almost entirely for
studies performed in my laboratory.• A good reference for a more complete view
of the contributions of surface science to understanding vehicle emission control “3-way” catalysis is:
B.E. Nieuwenhuys, Advances in Catalysis 44 (2000) 259-328.
Part I OutlinePart I Outline• Introduction
– Emissions Standards– Current Catalyst Technology
• Catalytic Converters• Three-Way Catalysts
– Some experimental details • CO Oxidation on Single Crystal
‘Precious’ Metal Surfaces• NO Reduction on Rh Single
Crystal Surfaces
BRIEF HISTORY OF BRIEF HISTORY OF EMISSION STANDARDSEMISSION STANDARDS
• 1970 “Clean Air Act” enacted into law in the U.S.
• 1975-1980 Catalytic converters added for oxidizing CO and unburned hydrocarbons
– NOx control by ‘engine management’ (EGR)– Pt/Pd catalysts used as found to be vastly superior for
oxidation reactions than other catalyst materials.
See “Catalytic Air Pollution Control: Commercial Technology”, R.M. Heck and R.J. Farrauto, Van Nostrand Reinhold, 1995.
BRIEF HISTORY OF BRIEF HISTORY OF EMISSION STANDARDSEMISSION STANDARDS
• 1981-1990 Catalytically reduce NOx (NO and NO2) concurrent with CO and HC oxidation (“3-way” catalysts)
– Pt and Pd produce significant amount of NH3 rather than N2
– Ru excellent for NOx reduction but forms volatile RuO4
– Rh selective to N2 (and N2O) with good activity so it is included in 3-way catalyst formulations– Need for precise control of air/fuel ratio so O2 (aka ‘lambda’) sensor added to exhaust system– CeO2 addition significantly increased to ‘dampen’ oscillations in air/fuel ratios
• 1990 on Many phased-in changes (stricter standards: TLEV, LEV, ULEV, ZEV, SULEV)
– CeO2 stabilized and ‘oxygen storage’ properties improved by adding other oxides such as ZrO2 to form mixed oxides (solid solutions)
– Control of precious metal ‘location’ on the catalyst to minimize metal sintering– Pd-only formulations introduced (but not in widespread use)
See “Catalytic Air Pollution Control: Commercial Technology”, R.M. Heck and R.J. Farrauto, Van Nostrand Reinhold, 1995.
•• CO oxidationCO oxidation•• NO reductionNO reduction•• Oxidation of unburned fuelOxidation of unburned fuel
WashcoatWashcoat
AlAl22OO33 + Ceria + . . .+ Ceria + . . .+ + Precious Metal
Catalyst metalCatalyst metalPt, Pt, RhRh, and/or Pd, and/or Pd
ThreeThree--Way Exhaust CatalystsWay Exhaust Catalysts
Cordierite CeramicCordierite Ceramic(2MgO(2MgO··2Al2Al22OO33·· 5SiO5SiO22))
Catalytic Catalytic ““33--wayway”” Reactions All Operate Near Reactions All Operate Near 100% Conversion at the 100% Conversion at the ““StoichiometricStoichiometric PointPoint””
J.C. Schlatter, R.M. Sinkevitch, and P.J. Mitchell, Ind. Eng. Chem. Prod. Res. Dev. 22, 51 (1983).
HC
NOCO
S
100
80
60
40
20
015.415.215.014.814.614.414.214.0
Air/Fuel Ratio
% C
onve
rsio
n
““33--wayway”” reactions:reactions:•• NO reductionNO reduction•• CO oxidationCO oxidation•• Oxidation of unburned fuelOxidation of unburned fuel
The redox properties of the oxygen storage materials help maintain air/fuel ratios near the “stoichiometric” point.
““33--wayway”” catalysis includes perhaps the classic catalysis includes perhaps the classic ““structure sensitivestructure sensitive”” and and ““insensitiveinsensitive”” reactions reactions
•• CO oxidation CO oxidation –– CO + CO + ½½OO22 --> CO> CO22
•• NO reduction NO reduction –– NO + CO NO + CO --> > ½½NN22 + CO+ CO22
Incomplete reduction yieldsIncomplete reduction yields NN22OO
S.H. Oh and C.C. Eickel, J. Catal. 128, 526 (1991).
Selectivity is also Selectivity is also ‘‘structure sensitivestructure sensitive’’
S(NS(N22O)O)
5% Rh/SiO5% Rh/SiO22 80%80%<3% Rh/Al<3% Rh/Al22OO33 0%0%
““33--wayway”” reactions ideal for reactions ideal for establishing many fundamental establishing many fundamental
concepts in heterogeneous catalysisconcepts in heterogeneous catalysis1)1) Can we account for the origin of catalyst structure Can we account for the origin of catalyst structure
““insensitivityinsensitivity”” and and ““sensitivitysensitivity”” on a on a ‘‘molecularmolecular--levellevel’’??2)2) Does all chemistry occur via surface adsorbed species or Does all chemistry occur via surface adsorbed species or
can other mechanisms (can other mechanisms (e.g.e.g., so, so--called called EleyEley--RidealRideal) apply?) apply?3)3) For metalFor metal--catalyzed reactions, what roles are played by the catalyzed reactions, what roles are played by the
oxide support material?oxide support material?•• can portions of an overall catalytic reaction be carried out on can portions of an overall catalytic reaction be carried out on the the
surface of the oxide component?surface of the oxide component?•• what about participation by the surfaces of the what about participation by the surfaces of the ‘‘bulkbulk’’ oxide?oxide?
4) 4) For oxideFor oxide--catalyzed reactions, can catalyzed reactions, can ‘‘molecularmolecular--levellevel’’reaction mechanisms be established?reaction mechanisms be established?•• what about what about ‘‘structurestructure--sensitivitysensitivity’’??•• how do how do ‘‘impuritiesimpurities’’ effect oxideeffect oxide--catalyzed reactions?catalyzed reactions?
Catalytic Reactor/UHV Surface ScienceCatalytic Reactor/UHV Surface ScienceApparatus for Model Catalyst StudiesApparatus for Model Catalyst Studies
LEED Optics
Side View
Residual Gas Analyzer
FTIR AbsorptionSpectroscopy
High Pressure Catalytic Cell
Gas Chromatograph
Hemispherical Analyzer
Reactor for Testing Mechanisms Reactor for Testing Mechanisms on Model Catalystson Model Catalysts
Phenomenological kinetics are correlated with Phenomenological kinetics are correlated with inin--situsitu and and exex--situsitu spectroscopic measurements of reaction intermediates.spectroscopic measurements of reaction intermediates.
002
_110
002002
_110_110
110
001_
011
110
001001_
011_
011
011011
_
011
_
011
100
__
110
__
110
_
011
_
011
_101
_101
_121_
121
111
_110_
110
_ _101_ _101
Rh(100) Rh(111) Rh(110)
Geometry of Geometry of RhRh Single Crystal SurfacesSingle Crystal Surfaces
Reactions on Reactions on RhRh Single Crystal Surfaces ReproduceSingle Crystal Surfaces Reproduce‘‘Structure InsensitiveStructure Insensitive’’ and and ‘‘SensitiveSensitive’’ BehaviorBehavior
NO reductionNO reductionCO oxidationCO oxidation
Rh/AlRh/Al22OO33
Oh, et al.,J. Catal. 100(1986) 360.
Kinetic Modeling Quantitatively Accounts Kinetic Modeling Quantitatively Accounts for CO and Ofor CO and O22 Pressure DependencePressure Dependence
Oh, Fisher, Carpenter,
and Goodman, J. Catal. 100 (1986) 360.
Summary: Phenomenological Kinetics of CO Summary: Phenomenological Kinetics of CO Oxidation on Single Crystal Oxidation on Single Crystal RhRh (and Pt, Pd, and (and Pt, Pd, and
IrIr) Metal Surfaces Under Reducing or Mildly ) Metal Surfaces Under Reducing or Mildly Oxidizing Conditions are Consistent with a Oxidizing Conditions are Consistent with a
Classic Classic ‘‘LangmuirLangmuir--HinshelwoodHinshelwood’’ MechanismMechanism
ExEx--SituSitu Surface Analysis Verifies that Surface Analysis Verifies that Reaction is RateReaction is Rate--Limited by CO Desorption Limited by CO Desorption
LEED Optics
Side View
Residual Gas Analyzer
FTIR AbsorptionSpectroscopy
High Pressure Catalytic Cell
Gas Chromatograph
Hemispherical Analyzer
Reactor for Testing Mechanisms Reactor for Testing Mechanisms on Model Catalystson Model Catalysts
Rh(111)
Rh(111)
C.H.F. Peden and D.W. Goodman,J. Phys. Chem. 90 (1986) 1360-1365.
Reactivity on Ru(0001) Displays Reactivity on Ru(0001) Displays Significantly Different KineticsSignificantly Different Kinetics
Ru(0001)
Viewgraph…
More More ‘‘UnusualUnusual’’Kinetics Kinetics ……
Ru(0001)
FTIR is an FTIR is an ‘‘OperandoOperando’’ Spectroscopy that can Spectroscopy that can Identify Adsorbed Species Present on the Single Identify Adsorbed Species Present on the Single Crystal Metals Catalysts During Kinetic Studies Crystal Metals Catalysts During Kinetic Studies
Essentially no CO observed on catalyst surface by FT-IRAS during CO oxidation at the highest rates.Peden, C.H.F.; Goodman, D.W.; Weisel, M.D.; Hoffmann, F.M. "In-Situ FT-IRAS Study of the CO Oxidation Reaction over Ru(001): I. Evidence for an Eley-Rideal Mechanism at High Pressures?" Surface Science 253 (1991) 44-58.
Reactivity
CO Oxidation at CO Oxidation at ‘‘OptimumOptimum’’ Rates Occurs on Rates Occurs on Surface Oxides of Metals Other Than Surface Oxides of Metals Other Than RuRu??
Similar studies of NO reduction sheds light on the Similar studies of NO reduction sheds light on the origins of origins of ““structure sensitivitystructure sensitivity”” of this reactionof this reaction
•• CO oxidation CO oxidation –– CO + CO + ½½OO22 --> CO> CO22
•• NO reduction NO reduction –– NO + CO NO + CO --> > ½½NN22 + CO+ CO22
Incomplete reduction yieldsIncomplete reduction yields NN22OO
S.H. Oh and C.C. Eickel, J. Catal. 128, 526 (1991).
Selectivity is also Selectivity is also ‘‘structure sensitivestructure sensitive’’
S(NS(N22O)O)
5% Rh/SiO5% Rh/SiO22 80%80%<3% Rh/Al<3% Rh/Al22OO33 0%0%
Catalytic Reactor/UHV Surface ScienceCatalytic Reactor/UHV Surface ScienceApparatus for Model Catalyst StudiesApparatus for Model Catalyst Studies
LEED Optics
Side View
Residual Gas Analyzer
FTIR AbsorptionSpectroscopy
High Pressure Catalytic Cell
Gas Chromatograph
Hemispherical Analyzer
Reactor for Testing Mechanisms Reactor for Testing Mechanisms on Model Catalystson Model Catalysts
Phenomenological kinetics are correlated with Phenomenological kinetics are correlated with inin--situsitu and and exex--situsitu spectroscopic measurements of reaction intermediates.spectroscopic measurements of reaction intermediates.
100
80
60
40
20
0900800700600500
N O
Sel
ectiv
ity (%
)2
Temperature (K)
PNO = PCO = 8 Torr
Rh(100)
Rh(110)
Rh(111)
390395400405410
Binding Energy (eV)
Inte
nsity
(Arb
. Uni
ts)
N 1s
Rh(111)
NO(a)
N(a)
Rh(100)
Rh(110)
PCO = 8 Torr PNO = 8 Torr Trxn = 528 K
Post- Reaction XPS Analysis of NO-COon Rh(110), Rh(111) and Rh(100)
002
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002002
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110
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001001_
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_121_
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_110_
110
_ _101_ _101
Rh(100) Rh(111) Rh(110)
Geometry of Geometry of RhRh Single Crystal SurfacesSingle Crystal Surfaces
390395400405410
Binding Energy (eV)
Inte
nsity
(Arb
. Uni
ts)
N 1s
Rh(111)
NO(a)
N(a)
Rh(100)
Rh(110)
PCO = 8 Torr PNO = 8 Torr Trxn = 528 K
Post- Reaction XPS Analysis of NO-COon Rh(110), Rh(111) and Rh(100)N2O Selectivities versus temperature
over Rh(110), Rh(111) and Rh(100)100
80
60
40
20
0900800700600500
N O
Sel
ectiv
ity (%
)2
Temperature (K)
PNO = PCO = 8 Torr
Rh(100)
Rh(110)
Rh(111)
NN22O O selectivitiesselectivities vary with vary with RhRh surface structure surface structure and are correlated with composition of the and are correlated with composition of the
adsorbateadsorbate layer on the surface.layer on the surface.
‘‘SteadySteady--statestate’’ surface surface coveragescoverages of of adsorbed NO and Nadsorbed NO and N--atoms key to atoms key to differences in Ndifferences in N22O/NO/N22 selectivity.selectivity.
PROPOSED REACTION MECHANISM
CO(g) + S <=> CO(ad) (1)NO(g) + S <=> NO(ad) (2)NO(ad) + S -> N(ad) + O(ad) (3)CO(ad) + O(ad) -> CO2(g) + 2S (4)NO(ad) + N(ad) -> N2O(g) + 2S (5)N(ad) + N(ad) -> N2(g) + 2S (6)
Mechanism proposed on the basis of extensive phenomenological kinetics measurements
FTIR Measurements of NO surface coverage FTIR Measurements of NO surface coverage correlates with Ncorrelates with N22O O selectivitiesselectivities on Rh(111)on Rh(111)
20
30
40
50
60
70
80
0.04
0.05
0.06
0.07
0.08
0.090.1
510 540 570 600 630 660 690
Temperature Dependenceof NO Surface Coverages
Correlate with N 2O Selectivity
N2O
Sel
ectiv
ity (%
)
NO
FTIR Peak A
rea
Temperature (K)
In-situ FTIR spectra during NO-CO Reaction over Rh(111)
T(K)
(g) 673
(f) 648
(e) 623
(d) 598
(c) 573
(b) 548
(a) 523
2200 1800 1400
2045cm-1
1625 cm-1
1 x 10-3
Frequency (cm-1)
4.0 Torr CO0.8 Torr NO
Adsorbed NCO- species observed during in-situ reaction over Rh(110)
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
150020002500
Inte
nsity
(arb
itrar
y un
its)
Frequency (cm -1)
21752056
17741635
523K
548K
573K
598K
623K
648K
661K
673K
Accounting for NAccounting for N22O selectivity O selectivity changes on Rh(110) is difficultchanges on Rh(110) is difficult
Nitrogen atoms formed by NO dissociation react with adsorbed CO to form adsorbed isocyanate.
RhRh
ON N O
RhRh
OC N N
OC
+
What role is played by What role is played by isocyanateisocyanate in the reaction in the reaction
mechanism?mechanism?
Part I SummaryPart I SummaryThe introduction in 1975, and continuing improvements of the catalytic converter can be considered an “unqualified success” (Shelef and McCabe, Catalysis Today 62 (2000) 35-50) in providing a huge decrease in harmful emissions from transportation vehicles..
Understanding emission control catalytic reactions provides the means to understand a large number of fundamental scientific concepts in catalysis. Special ‘operando’ surface science techniques are especially useful in providing these scientific insights.
CO Oxidation, one of the simplest and most studied catalytic reaction (since Langmuir!!), still contains many poorly understood features of important practical importance. Notably, new catalysts for low (even room!) temperature CO oxidation needed for PEM fuel cells and new diesel engine operating modes.
Similarly, many mechanistic aspects of catalytic NO reduction on Rh(and alternate catalysts??) remain to be understood.
Model Surface Science Studies of Model Surface Science Studies of Oxide Materials in Vehicle Exhaust Oxide Materials in Vehicle Exhaust Emission Control DevicesEmission Control Devices
Part II OutlinePart II Outline• Introduction - Importance and difficulty
of studying oxides with surface science methods
• Ceria-zirconia ‘oxygen storage’materials
• Oxides for ‘lean-NOx’ reduction problem
Application AreasApplication Areas•• Environmental Environmental ‘‘Fate and TransportFate and Transport’’
•• Next generation microelectronic devicesNext generation microelectronic devices
•• Heterogeneous catalysis including Heterogeneous catalysis including photocatalysisphotocatalysis
•• Sensors, Fuel CellSensors, Fuel Cell
Research on Oxide Surfaces Research on Oxide Surfaces Has Broad Impact Has Broad Impact
O
O
O
O
O
Fe-O-Cr3+
Fe-OH
Al-OH2+-HCrO4
-
Fe-O-CrO3-
FeOFeOxx(s)(s) HH22OO(l)(l)
O
O
O
O
O
Fe-O-Cr3+
Fe-OH
Al-OH2+-HCrO4
-
Fe-O-CrO3-
O
O
O
O
O
Fe-O-Cr3+
Fe-OH
Al-OH2+-HCrO4
-
Fe-O-CrO3-
Fe-O-Cr3+
Fe-OH
Al-OH2+-HCrO4
-
Fe-O-CrO3-
FeOFeOxx(s)(s) HH22OO(l)(l)
SourceSourceImplantImplant
DrainDrainImplantImplantSiliconSilicon
SourceSource DrainDrainGateGate
Ld
C=C=εεεε0 0 A/dA/d
SourceSourceImplantImplant
DrainDrainImplantImplantSiliconSilicon
SourceSource DrainDrainGateGate
Ld
SourceSourceImplantImplant
DrainDrainImplantImplantSiliconSilicon
SourceSource DrainDrainGateGate
Ld
C=C=εεεε0 0 A/dA/d
““The metal oxides constitute a diverse and fascinating class The metal oxides constitute a diverse and fascinating class of materials whose properties cover the entire range from of materials whose properties cover the entire range from metals to semiconductors and insulators. Their surfaces play metals to semiconductors and insulators. Their surfaces play crucial roles in an extremely wide range of phenomena.crucial roles in an extremely wide range of phenomena.””
The Pleasure and Pain of Working The Pleasure and Pain of Working with Oxides and Insulatorswith Oxides and Insulators
Challenges and complications:1)1) Material ComplexityMaterial Complexity
-- more than one type of surface atommore than one type of surface atom-- large unit cellslarge unit cells-- electronic structure (somewhere between electronic structure (somewhere between
extended and localized)extended and localized)-- range of oxidation states (widely variable range of oxidation states (widely variable
stoichiometrystoichiometry))
2) 2) Practical problems with UHV surface Practical problems with UHV surface spectroscopiesspectroscopies-- sample mountingsample mounting-- sample heating and coolingsample heating and cooling-- sample conductivity when using charged sample conductivity when using charged
particle probesparticle probes
3)3) Difficult to obtain macroscopically large single crystals with rDifficult to obtain macroscopically large single crystals with reproducible structure and eproducible structure and compositioncomposition –– will discuss nextwill discuss next..
4)4) For catalysis, how does one normalize reaction rates; i.e., how For catalysis, how does one normalize reaction rates; i.e., how does one count does one count ““activeactive”” sites?sites?
OxygenOxygen--PlasmaPlasma--Assisted Molecular Beam Assisted Molecular Beam EpitaxyEpitaxy
S.A. Chambers, Surface Science Reports, Vol. 39, Nos. 5-6, pp. 105-180 (2000)
UPS/XPS/XPD/LEED
AFM/STM MBE/RHEED
UPS/XPS/XPD/LEED
AFM/STM MBE/RHEED
M g O / M g O (0 0 1 )
M g O / C r 0 .7 5 M o 0 . 2 5 /M g O (0 0 1 )
α -F e 2 O 3 /α -A l 2 O 3 (0 0 0 1 ) , ( 1 1 0 2 ) , & (1 1 2 0 )
F e 3 O 4 / M g O ( 0 0 1 ) & ( 0 11 )
γ -F e 2 O 3 /M g O (0 0 1 ) & (0 1 1 )
F e 3 O 4 (1 1 1 ) /α - A l 2 O 3 ( 0 0 0 1 )
β -M n O 2 /T iO 2 ( 11 0 )
R u O 2 & R u x T i1 - x O 2 / T iO 2 (1 1 0 )
CC ee OO 22 aa nn dd ZZ rrxx CC ee 11 -- xx OO 22 //SS rrTT iiOO 33 (( 00 00 11 ))
N b x T i 1 - x O 2 /T i O 2 (1 1 0 ) & (1 0 0 )
a n a t a s e T i O 2 /S rT iO 3 (0 0 1 )
C u 2 O / S r T iO 3 (0 0 1 )
(α -C r 2 O 3 ) n / (α - F e 2 O 3 ) m /α -A l2 O 3 (0 0 0 1 )s u p e r la t t i c e s
C o F e 2 O 4 / M g O ( 0 0 1 )
C o -d o p e d T i O 2 a n a t a s e/ S r T iO 3 ( 0 0 1 ) & L a A lO 3 (0 0 1 )
p ro to ty p i c a lo x id e
n o v e lp h o t o c a t a ly s t s
e n e rg ys t o ra g e
m a t e r i a ls
m o d e lm i n e r a ls
o x y g e ns to r a g e
m a t e r i a l s
s e m i c o n d u c t in g& c o n d u c t iv e
m a g n e ti co x id e s
Systems Grown and InvestigatedSystems Grown and Investigated
Substrates are chosen to provide the desired Substrates are chosen to provide the desired surface structure, as well as a good lattice surface structure, as well as a good lattice match with the structure of the ceria/match with the structure of the ceria/zirconiazirconiafilms.films.
InIn--situ (RHEED) and exsitu (RHEED) and ex--situ (LEED, XRD) situ (LEED, XRD) diffraction data demonstrate that the films diffraction data demonstrate that the films are very high quality single crystals with are very high quality single crystals with essentially unreconstructed surfaces.essentially unreconstructed surfaces.
High energy ion scattering (channeling) and High energy ion scattering (channeling) and xx--ray photoelectron diffraction (XPD) data ray photoelectron diffraction (XPD) data show that show that ZrZr substitutes for substitutes for CeCe in the lattice in the lattice of the film.of the film.
Cerium and zirconium are both in a +4 Cerium and zirconium are both in a +4 oxidation state as evidenced by XPS data.oxidation state as evidenced by XPS data.
High Quality CeHigh Quality Ce11--xxZrZrxxOO22 Films Have Been Films Have Been Successfully Grown on a Variety of SubstratesSuccessfully Grown on a Variety of Substrates
200 300 400 500 600 700 800 900 10000
1000
2000
3000
4000
5000
6000
16O
40Zr
140Ce
Aligned Spectrum Random Spectrum
Scat
terin
g Yi
eld
(Cou
nts)
Channel Number
Oxygen storage materials (ceria and ceria/Oxygen storage materials (ceria and ceria/zirconiazirconia) ) in in ““33--wayway”” automobile catalytic convertersautomobile catalytic converters
J.C. Schlatter, R.M. Sinkevitch, and P.J. Mitchell, Ind. Eng. Chem. Prod. Res. Dev. 22, 51 (1983).
HC
NOCO
S
100
80
60
40
20
015.415.215.014.814.614.414.214.0
Air/Fuel Ratio
% C
onve
rsio
n
““33--wayway”” reactions:reactions:•• NO reductionNO reduction•• CO oxidationCO oxidation•• Oxidation of unburned fuelOxidation of unburned fuel
The redox properties of the oxygen storage materials help maintain air/fuel ratios near the “stoichiometric” point.
Reduced air/fuel Reduced air/fuel ‘‘oscillations via improved oscillations via improved ‘‘engine engine managementmanagement’’ and oxygen storage materials and oxygen storage materials
1990 1990 CarsCars
1986 1986 CarsCars
Journal of Catalysis 216 (2003) 433Journal of Catalysis 216 (2003) 433--442442
Partial list of proposed Partial list of proposed ‘‘elementaryelementary’’reactions in 3reactions in 3--way catalytic convertersway catalytic converters
Reaction # Reaction Known?
1 H2(g) + 2Rh <=> 2H/Rh Yes 2 O2(g) + 2Rh --» 2O/Rh Yes 3 CO(g) + Rh <=> CO/Rh Yes 4 CO/Rh + O/Rh --» CO2(g) + 2Rh Yes 5 NO(g) + Rh <=> NO/Rh Yes 6 NO/Rh + Rh --» N/Rh + O/Rh Yes 7 NO/Rh + N/Rh --» N2O(g) + 2Rh Yes 8 N/Rh + N/Rh --» N2(g) + 2Rh Yes 9 N2O(g) + Rh --» O/Rh + N2(g) Yes 10 H2O(g) + Rh <=> H2O/Rh Yes 11 H2O/Rh + Rh <=> OH/Rh + H/Rh Poorly
12 OH/Rh + Rh <=> H/Rh + O/Rh Poorly 13 C3H6(g) + 2Rh <=> C3H5/Rh + H/Rh Poorly 14 C3H5/Rh + Rh --» C2H4/Rh + CH/Rh No 15 C2H4/Rh + 3Rh --» 2H/Rh + 2CH/Rh No
16 CH/Rh + O/Rh --» CO/Rh + H/Rh No
Reaction # Reaction Known?
17 SO2(g) + Rh <=> SO2/Rh Partially 18 SO2/Rh + O/Rh --» SO3(g) + 2Rh No 19 SO2/Rh + 2Rh <=> S/Rh + 2O/Rh Partially 20 S/Rh + 2H/Rh <=> H2S(g) + 3Rh Partially 21 3O/Rh + 2Rh(bulk) <=> Rh2O3(bulk) + 3Rh Partially 22 2Ce2O3 + O2(g) <=> 2Ce2O4 (i.e., 2CeO2) Poorly 23 O/Rh + Ce2O3 <=> Rh + Ce2O4 No 24 Ce2O4 + CO/Rh --» Ce2O3 + CO2(g) Partially
25 Ce2O4 + CO(g) --» Ce2O2/CO3 Poorly
26 Ce2O2/CO3 --» Ce2O3 + CO2(g) Poorly
27 Ce2O4 + 2H/Rh --» Ce2O2(OH)2 + 2Rh Poorly 28 Ce2O2(OH)2 <=> Ce2O3 + H2O(g) No 29 Ce2O3(H)2 <=> Ce2O3 + H2(g) No 30 H/Rh + Ce2O3(H) --» Rh + Ce2O3 + H2(g) No 31 Ce2O4 + SO2(g) <=> Ce2O2/SO4 Poorly
32 Ce2O2/SO4 --» Ce2O3 + SO3(g) No
NO ReductionNO Reduction2NO + 2CO 2NO + 2CO --> N> N22 + 2CO+ 2CO222NO + CO 2NO + CO --> N> N22O + COO + CO22
WaterWater--gas Shiftgas ShiftHH22O + CO O + CO --> H> H22 + CO+ CO22
““Oxygen StorageOxygen Storage””
Can single crystal model Can single crystal model ‘‘oxygenoxygen--storage materialsstorage materials’’
reproduce the behavior of the reproduce the behavior of the realistic systems?realistic systems?
CO TPD is a good qualitative measure of CO TPD is a good qualitative measure of the capability for the capability for ‘‘activeactive’’ oxygen storageoxygen storage
CeOCeO22(100)(100)
As grownAs grown‘‘CeOCeO22(100)(100)’’ ononαα--AlAl22OO33(0001)(0001)
AnnealedAnnealed‘‘CeOCeO22(100)(100)’’ ononαα--AlAl22OO33(0001)(0001)
CeOCeO22
RhRh
+ + COCO
TPDTPD
COCO22COCO
Gorte and coworkersGorte and coworkers
TPD following CO adsorption on Rh/CeTPD following CO adsorption on Rh/CexxZrZr11--xxOO22 films:films:COCO22 formation observed on Ceformation observed on Ce.86.86ZrZr.14.14OO22/Y/Y--ZrOZrO22 but not on Rh/CeObut not on Rh/CeO22/MgO/MgO
300 400 500 600 700
m/e=44
Temperature (K)
m/e=28
QM
S Si
gnal
(a.u
.)
300 400 500 600 700
m/e=44
Temperature (K)
m/e=28
QM
S Si
gnal
(a.u
.)
Rh/CeORh/CeO22 Rh/CeRh/Ce.86.86ZrZr.14.14OO22
COCO22 and CO TPD following sequential CO adsorptionand CO TPD following sequential CO adsorptionon Rh/Ceon Rh/Ce.86.86ZrZr.14.14OO22/Y/Y--ZrOZrO22
300 400 500 600 700
m/e=44
Temperature (K)
First
Fifth
Eighth
Tenth
QM
S Si
gnal
(a.u
.)
First
Fifth
Eighth
Tenth
m/e=28
QM
S Si
gnal
(a.u
.)300 400 500 600 700
Temperature (K)
Collaborators at Ford are studying stability of Collaborators at Ford are studying stability of metal particles on metal particles on zirconiazirconia--doped ceriadoped ceria
H.P. Sun, X.Q. Pan, University of Michigan
G.W. Graham, Ford Research Laboratory
C. H. F. Peden, S. Thevuthasan, PNNL
Summary and ConclusionsSummary and Conclusions1.1. High quality heteroepitaxial 500High quality heteroepitaxial 500--2000 2000 ÅÅ films of Cefilms of Ce11--xxZrZrxxOO22
can be grown on a variety of single crystal substrates.can be grown on a variety of single crystal substrates.2.2. COCO22 formation during TPD following adsorption of CO on formation during TPD following adsorption of CO on
Rh/CeRh/Ce11--xxZrZrxxOO22(111) is a good qualitative measure of (111) is a good qualitative measure of ‘‘activeactive’’ oxygen storage in these model ceria/oxygen storage in these model ceria/zirconiazirconia thin thin films.films.
3.3. CO/COCO/CO22 TPD results obtained on the CeTPD results obtained on the Ce11--xxZrZrxxOO22(111) thin (111) thin films clearly demonstrate the promotion of the films clearly demonstrate the promotion of the ‘‘oxygen oxygen storagestorage’’ capacity of ceria by capacity of ceria by zirconiazirconia. The mechanism by . The mechanism by which a second, less reducible oxide performs this which a second, less reducible oxide performs this promotion remains an active area of current and future promotion remains an active area of current and future study.study.
4.4. Model CeModel Ce11--xxZrZrxxOO22 films useful for studying metal sintering films useful for studying metal sintering on catalytic converter oxide supports.on catalytic converter oxide supports.
Oxide Materials for Oxide Materials for ‘‘Lean Lean NOxNOx’’ ReductionReduction
NOxNOx emission control is a emission control is a challenge in challenge in ““leanlean--burnburn”” enginesengines
• Hydrocarbon selective catalytic reduction.• Ammonia (urea) selective catalytic reduction.• Lean-NOx traps (LNTs – aka NOx Adsorbers or NOx
storage/reduction (NSR) catalysts).
Current “3-way”catalytic converters that use precious metal (Rh) for NOxreduction are ineffective for fuel-efficient ‘lean-burn’engines.
Some new catalyst technology options for Some new catalyst technology options for ‘‘leanlean--NOxNOx reductionreduction
~50% saving on fuel
NOx conv.~0%
S. Matsumoto, CATTECH Vol. 4, No.2, 102 (2000)S. Matsumoto, CATTECH Vol. 4, No.2, 102 (2000)
Biofuels industry battles to make its case State officials demand proof that fuel is eco-friendly
Unless the Texas biofuels industry can convince state officials its vegetable-based diesel fuel won't foul the state's air, it risks being forced from the huge Texas diesel market.
Texas is the nation's largest producer of biodiesel, a mix of regular diesel and vegetable oil. A decision to allow or forbid the fuel is expected before year's end.
The state's chief environmental regulatory agency, the Texas Commission on Environmental Quality, has told the fledgling industry it must prove the fuel is clean enough for Texas.
That position puts Texas in a class all its own; other states are embracing the fuel.
Nationwide, experts agree, biodiesel is a "clean fuel" because it is nontoxic and biodegradable, and because it drastically reduces emissions of hydrocarbons, sulfur, carbon monoxide and particulate pollutants.
TCEQ is concerned, however, that biodiesel may increase tailpipe emissions of one pollutant in particular, smog-producing nitrogen oxide, or NOx. The state must reduce NOx in urban areas to meet federal Clean Air rules.
http://msnbc.msn.com/id/14341290/
8/13/2006
DOE perception of dieselDOE perception of diesel•Heavy duty diesel trucks efficientlytransport goods in the US today, and they will continue to do so for the next 20+ years!
•The enormous popularity of inefficient gasoline powered light duty trucks is leading to alarmingly high levels of dependency on foreign oil and, correspondingly, high CO2 emissions. Diesel engines could provide large efficiency advantages!
Exhaust emissions are an obstacleExhaust emissions are an obstacle1995 2000
US Europe GroupUnregulated
EPA 94 Euro 1/2 1 NOx>6.5 g/bhphr, PM>0.25 g/bhphrEPA 98 Euro 3 NOx=3.5-6.5 g/bhphr, PM=0.1-0.25 g/bhphrEPA 02 Euro 4 NOx=2.0-3.4 g/bhphr, PM=0.1 g/bhphrEPA 07 ? NOx=1.18 g/bhphr, PM=0.01 g/bhprEPA 10 ? NOx=0.2 g/bhphr, PM=0.01 g/bhphr
2010 2005
2
1
1
1
2
2
1
2
3
2
2
2
1 3
3
1
4
3
3
3
4
4
2
2
2
2
012345
90% reductionby 2010
(particulate and NOx)
World Emission Levels for HD Automotive
NOxNOx emission control is a emission control is a challenge in challenge in ““leanlean--burnburn”” enginesengines
• Hydrocarbon selective catalytic reduction.• Ammonia (urea) selective catalytic reduction.• Lean-NOx traps (LNTs – aka NOx Adsorbers or NOx
storage/reduction (NSR) catalysts).
Current “3-way”catalytic converters that use precious metal (Rh) for NOxreduction are ineffective for fuel-efficient ‘lean-burn’engines.
Some new catalyst technology options for Some new catalyst technology options for ‘‘leanlean--NOxNOx reductionreduction
~50% saving on fuel
NOx conv.~0%
S. Matsumoto, CATTECH Vol. 4, No.2, 102 (2000)S. Matsumoto, CATTECH Vol. 4, No.2, 102 (2000)
Rh M etalRh M etal
OC
OOCC OO
O OCOO OC OOCC
OO OOOOOC OOCC
OO OO
N ONN OO
NO
NNOO
NN NN
N NNN NN
NOxNOx reduction reduction mechanism on mechanism on
oxideoxide--based based catalysts does catalysts does notnot
involve Ninvolve N--atom atom recombinationrecombination
•• For oxideFor oxide--based based NOxNOx emission emission control, the nature of the active control, the nature of the active adsorbed adsorbed NOxNOx species is species is important but difficult to important but difficult to determine.determine.
NO Adsorption on NO Adsorption on NONO22(ads)/Na(ads)/Na--Y,FAU at 300KY,FAU at 300K
0
0
0
0
0
1
1
1
1
120014001600180020002200
Wavenumber/cm-1
Abso
rban
ce
0.2
2158
2079
2021
1901
1566
1562
1419
1390
1304
1275
N2O3 N2O3N2O3
NO2+
N2O4+
NO2-; NO3
-
?? ??
““CooperativeCooperative”” chemisorptionchemisorption??
J. Szanyi*, J.H. Kwak, R.A. Moline, C.H.F. Peden, PCCP (2005).
Surface Science Studies of Surface Science Studies of ‘‘LeanLean--NOxNOx’’ Trap (LNT) Trap (LNT)
MaterialsMaterials
(aka (aka NOxNOx StorageStorage--Reduction (NSR) Reduction (NSR) and and NOxNOx AdsorberAdsorber Catalysts)Catalysts)
NONOxx Storage/Reduction CatalysisStorage/Reduction Catalysis
AlAl22OO33
BaOBaOPtPt
NO+ONO+O22 NONO22
NONOxxStorageStorage AlAl22OO33
BaOBaOPtPt
PtOPtOxx Ba(NOBa(NO33))22
+H+H22COCOHCHCss
NONOXX
Reduction
Reduction
AlAl22OO33
BaOBaOPtPt
NONOxx
NN22; CO; CO22
LeanLeanRichRich
BaOBaOPtPt
PtPt
Definition: Definition: NOxNOx ConversionConversion
He only30 min
200 ºC
Lean gases introduction
4 pulsesRich(1min)/Lean(4min)
15 min30min
050
100150
200250
300350
400450
0 500 1000 1500 2000 2500 3000Time (sec)
NO
x co
ncen
trat
ion
(ppm 4th pulse: Rich conversion
Lean conversion: 1min or 4 min
30 min Lean Conversion
1st lean –rich cycle
NOxNOx AdsorberAdsorber (LNT) Operation(LNT) Operation(See, e.g., (See, e.g., Fridell, et al., Catalysis Letters 66 (2000) 71.)
LeanLean RichRichNO +O2 NO2
Pt
Al2O3
PtBaO ?
Pt
Pt
Ba(NO3)2 RhPt
Al2O3
PtBa(NO3)2? BaO Pt
CO
Rh
NOx
N2Pt
H2, CO, HCN2
NO(g) NO2(g)
NO2(g) NOx(ad)
NOx(ad) NO2(g)
Pt + O2
BaCO3
Δ
NOx(ad) + ReductantNOx(g)
NOx(g) + ReductantN2+CO2+H2O
Δ
Catalyst
Metal
Characteristics: particle shrinks, with a linear shrinkage of ~ 31%
Theoretical value: 32%
2.61 μm
2.44 μm
Ba(NO3)2
1.80 μm
1.67 μm
BaO
Decomposition
Heating up to 800 °C in 1 torr0.8 N2 + 0.2 O2
In-situ TEM observation of morphological changes in Ba(NO3)2 upon heating
Wang, Kwak, Kim, Szanyi, Sharma, Thevuthasan, Peden, in press.
(c)
(a)
(d)11
1
400
20022
031122
2
BaO 111
Al2O3 10-10
BaO 200
BaO 200
BaO 111
BaO 200(d)11
1
400
20022
031122
2
111
400
20022
031122
2
BaO 111
Al2O3 10-10
BaO 200
BaO 200
BaO 111
BaO 200
(b)
Al2O3 10-10
111
400
20022
031122
2
BaO 111
BaO 111 BaO 200
BaO 200(b)
Al2O3 10-10
111
400
20022
031122
2
111
400
20022
031122
2
BaO 111
BaO 111 BaO 200
BaO 200
BaO maintains overall morphology of ‘precursor’Ba(NO3)2 but as a collection of small particles
Wang, Kwak, Kim, Szanyi, Sharma, Thevuthasan, Peden, in press.
TEM/EDS and Temperature-Programmed X-Ray Diffraction (TP-XRD) Studies of Ba/γ-Al2O3
2%-, 8%-, and 20%-BaO on high surface area Al2O3materials by standard ‘impregnation’ techniques using aqueous Ba(NO3)2 solutions.
Ba(NO3)2 γ–Al2O3
BaO/γ–Al2O3
Ba(NO3)2/γ–Al2O3
CatalystPreparation
Catalyst‘Activation’
NOx uptake +NO2 at RT or 300 °C NOx releaseat T > 600 °C
Calcined at 500 °C
‘Wet impregnation’ of γ–Al2O3 with an aqueous Ba(NO3)2 solutionDried at 125 °C
Summary of TP-XRD and TEM/EDX studies: Both ‘Monolayer’ and ‘Bulk’ Ba(NO3)2 morphologies present.
Can these be distinguished spectroscopically?
Heat
NO2 adsorptionat 300K
Heatin NO2
Large Ba(NO3)2crystallites
Al2O3
BaO nanoparticles
Heat Ba(NO3)2nanoparticles
Ba(NO3)2particles
+thin Ba(NO3)2
layer
Al2O3
Al2O3Al2O3
Szanyi, Kwak, Hanson, Wang, Szailer, Peden,J. Phys. Chem. B 109 (2005) 7339-7344.
Observed practical implications of the Ba-phase morphology.
• From TPD experiments, the “monolayer” morphology is found to decompose at lower temperature in vacuum and in a reducing atmosphere than “bulk”nitrates.
• “Monolayer” Ba-phase is also easier to ‘de-sulfate’.• Formation of a high-temperature (deactivating?)
BaAl2O4 phase requires BaO coverages above 1 monolayer.
• Morphology model at least partially explains relatively small use of Ba species (often <20%) in storing NOx during typical lean-rich cycling.
Distribution of NO and NODistribution of NO and NO22 Desorption Desorption Features Very Sensitive to Features Very Sensitive to BaOBaO LoadingLoading
400 600 800 1000
NO NO2
NO
x con
cent
ratio
n (p
pm)
Temperature (K)
2000
20 wt%
8 wt%
2 wt%
Al2O3
Ba(NO3)2 → BaO + 2 NO2 + 1/2 O2
Ba(NO3)2 → BaO+ 2 NO + 3/2 O2
Al2O3
8 wt% BaO/Al2O3
20 wt% BaO/Al2O3
2 wt% BaO/Al2O3
0 5 10 15 20
0
10
20
30
40
50
60
NO NO2
NOx
Inte
grat
ed a
rea
of N
Ox (A
.U.)
wt% barium oxide on Al2O3
FTIR after NOFTIR after NO22 adsorption on 2%,adsorption on 2%,8%8%--, and 20%, and 20%--BaO/AlBaO/Al22OO3 3 at 300Kat 300K
• Al2O3-bound nitrates (AN) decrease continuously with Bacoverage.
• Surface (“bidentate”– BN) and bulk (ionic – IN) nitrates are observed on BaO/Al2O3 catalysts. Their ratio (BN/IN) also decreases with BaO loading. 1800 1600 1400 1200
1254
12951315
1438
Abs
orba
nce
Wavenumbers (cm-1)
1582
20%
8%
2%
0.5
AN
BN
BNIN
AN
+NO
+NO
22
O OO O
NNBa(NOBa(NO33))22
Heat
Heat
Ba(NOBa(NO33))22NONO22
AlAl22OO33
AlAl22OO33AlAl22OO33
Heat
Heat
NO+NO+½½ OO22
Bridging Bridging NitratesNitrates(Surface)(Surface)
IonicIonicNitratesNitrates(bulk)(bulk)
==
OO
BaOBaO
Szanyi, Kwak, Hanson, Wang, Szailer, Peden,J. Phys. Chem. B 109 (2005) 7339-7344.
Heatin NO2
Preparation and Characterization of Model Preparation and Characterization of Model ‘‘LeanLean--NOxNOx’’ Trap Materials for UHV StudiesTrap Materials for UHV Studies
J. Szanyi et al., J. Szanyi et al., J.Phys.ChemJ.Phys.Chem. B 2005, . B 2005, 109,109, 7339.7339.
NO2 at RTHeat
Al2O3
Ba(NO3)2: monolayer+nanoparticles
Al2O3
Ba(NO3)2:monolayer+nanoparticles
BaO: monolayer+nanoparticles
Support
Base Metal Oxide on AlBase Metal Oxide on Al--Oxides:Oxides:A A Synthesis StrategySynthesis Strategy
NiAl (100) or (11O)
AlOx
θ-, or γ-Al2O3
BaO 1.1. Preparation of well Preparation of well ordered ultrathin ordered ultrathin support oxide filmsupport oxide film
2.2. Physical vapor Physical vapor deposition of deposition of BaBa on on support oxide filmsupport oxide film
Key issues:Key issues:-- nature of oxide supportnature of oxide support-- deposition conditionsdeposition conditions-- post deposition post deposition
treatmentstreatments
bcc NiAl(100) Crystal
O2-
Al3+
Al
Ni
Preparation of Preparation of θθ--AlAl22OO33 film on NiAl(100)film on NiAl(100)
P. P. GassmanGassman et al. et al. Surf. Surf. SciSci.. 19941994, 319, 95 & , 319, 95 & J. Elec. Spec. J. Elec. Spec. RelRel. . PhenPhen. . 19931993, 64/65, 315 , 64/65, 315 E. Ozensoy, C.H.F. Peden, E. Ozensoy, C.H.F. Peden, J.SzanyiJ.Szanyi, , J.Phys.ChemJ.Phys.Chem. B . B 2005,2005, 109109 3431. 3431.
I. Oxidation at 300K(~9,000 L O2)
Amorphous Al2O3 films on NiAl(100)
II. Annealing and ordering at 1200 K in UHV
θ-Al2O3/NiAl(100)
Ba Deposition on θ-Al2O3/NiAl(100)
Binding Energy (eV)Binding Energy (eV)
Ba 4dIn UHV at 300 K
XP
S In
tens
ity (a
.u.)
75.2
Al3+
72.6
Al0
67.1
Ni0
93.0
90.5
110 100 90 80 70 60
XP
S In
tens
ity (a
.u.)
Ba 4d
Al3+
In 10-7 Torr O2 at 800 K
Ni0
67.175.2
Al0
72.6
93.290.8
110 100 90 80 70 60
Al0, Al3+, Ni0: Ba: Al0, Ni0: Ba, Al3+:
Changes in XPS Intensities Ozensoy, et al., JPC B, in press.
NONOxx Surface ChemistrySurface Chemistry
Heatin NO2
NO2 at RTHeat
Al2O3
Ba(NO3)2:monolayer+nanoparticles
BaO: monolayer+nanoparticles
Al2O3
Support
Ba(NO3)2: monolayer+nanoparticles
J. Szanyi et al., J. Szanyi et al., J.Phys.ChemJ.Phys.Chem. B 2005, . B 2005, 109,109, 7339.7339.
390 393 396 399 402 405 408 411 414
What do we know now?What do we know now?XPS: NOXPS: NO22 Adsorption at Elevated PressuresAdsorption at Elevated Pressures
* The only NOx species
at 300K are
nitrates on both θ-Al2O3
and BaO surfaces.
* More nitrates on BaO
than on θ-Al2O3.
407.7
407.1
XPS
Int
ensi
ty (a
.u.)
Binding Energy (eV)
BaO/θ-Al2O3/NiAl(100)
θ-Al2O3/NiAl(100)
T = 300 K; PNO2=1.5TorrKey Points:
N 1s
300 400 500 600 700 800 900
BaO/θ-Al2O3/NiAl(100)
BaO (20 wt%) / γ-Al2O3795 K
780 K
660 K
620 K
Temperature (K)
QM
S In
tens
ity
(m/z
= 30
)Are these systems good models for Are these systems good models for
practical catalysts?practical catalysts?
Key point:
UHV TPD from
the model system is
consistent with the
high surface area
BaO on alumina
J.Szanyi et al., JPC B, 2005, 109, 27
Part II SummaryPart II SummaryReducible oxide materials, ceria and doped ceria, play an important role in modern “3-way” catalytic converters that has enabled these systems to meet increasingly stringent regulatory mandates on vehicle emissions..
A critical challenge for the catalysis community is the development of new emission control materials and processes for reducing NOxemissions from fuel-efficient ‘lean-burn’ engines such as diesels. By far, the new technologies showing the most promise to date involve oxide materials as the primary active component.
There is a great need to rapidly develop a fundamental understanding of oxide surface chemistry and physics to aid in the further development of these technologies. Creative surface science experiments that can include studies under realistic conditions, similar to those shown to be so successful for fundamental understanding of metal-catalyzed reactions, will play a critical role here.
