Structure-Function Studies. in Heterogeneous Catalysis

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1 Structure-Function Studies in Heterogeneous Catalysis Marcos Fernández-García Instituto de Catálisis y Petroleoquímica ICP-CSIC Energy-dispersive XAS Workshop ESRF, Grenoble, February, 2009

2 Overview Spatial-resolved XAS XAS-based Structure-Function Studies Focal spot nm; 1 x 1 μm (acquisition time; in-situ ) Time-dependent XAS Ejection, backscattering, and interference 1 fs Synchrotron incident X-ray pulse 100 ps Brown (J. Chem. Phys. 1999) Pump-probe XAS

3 XAS-based Structure-Function Studies Overview Spatial resolution in XAS Spatial Domain - Focal spot 50 nm (conventional) 1 μm (ED) 10 2 nm 2D, 3D Chemical /Structural mapping Optical Concentration/M Ox. State J. Synchr. Radiat. (2006) 13, Differential technique Femtometre-resolution x 100!!!! Nature (2005) 435, 78

4 XAS-based Structure-Function Studies Catalytic systems Representative examples; spatial domain Behavior of catalytic systems in industrial conditions Spatial-time resolved XAS studies Catal. Today. (2008) doi: /jcatod

5 XAS-based Structure-Function Studies Overview Time-resolved XAS Time domain analyzed ps/ns h/days nature of the phenomenon Experimental set-up and procedure - microscopic reversibility (aging) - S/N ratio Bressler, Chergi, Chem. Rev. 104 (2004) 1781

6 Catalysis: Time-resolved XAS Overview In-situ conditions (T, P; atmosphere) XAS set-up Conventional Quick Energy dispersive Time domain min to days 1 s to min 1 ms to 10 s ED-XAS Absence of movement - Constant energy scale - Stable focal point / spatial resolution - High time resolution Application - All catalysts (0.5 wt. % NM) - Suitable ref. materials - S/N ratio limited (2 nd shell EXAFS)

7 XAS-based Structure-Function Studies Catalytic systems Representative examples; time domain Minimize averaging / Instantaneous picture chemical species Similar chemical species (ox. state, local symmetry) Short existence (intermediate) High number of species (4,5) ; demanding systems Cu-Rh/Al 2 O 3 Unique choice Homogeneous catalysis: exchange of ligands Heterogeneous catalysis: Redox; T- Lambda TWCs

8 Cu-Rh bimetallic catalysts Main Characteristics TPR of Rh-Cu/Al 2 O 3 CATALYSTS Methane conversion (synthesis gas or higher HCs) CO 2 + CH 4 2CO + 2H 2 XCH 4 C X H 2X+2 + (x/2) H 2 Cu beneficial effect: - Interaction with the Support (Al 2 O 3 ) - Presence/Absence of Alloy - Oxidation State changes under reaction

9 D Cu/Al 2 O 3 XANES-TPR: FACTOR ANALYSIS Cu-Rh bimetallic catalysts Identify and follow Chemical Species during Reaction n { } = D C R + E i i i D = R C Data Matrix [D] [Z] = [D] T [D] Covariance Matrix [Z] Ph. reproduction Reproduction [Z] Diagonalization %SL n ppal factors J. Phys. Chem. (1995) 99, Real Ph. [R r ] [C r ] Needle search [T] o [T] -1 Abstract [R a ] [C a ]

10 D Cu-Rh bimetallic catalysts Cu/Al 2 O 3 ED-XANES-TPR: Factor Analysis [Rr] [Cr] 4 1,0 3 Normalized Absorbance (a.u.) 3 CuAl 2 O 4 -bulk 2 CuAl 2 O 4 -sup. 1 Fraction of pure Component 0,8 0,6 0,4 0, , Energy (ev) T (K) J. Catal. (1998), 178, 253 Two similar chemical phases CuAl 2 O 4 -sup CuAl 2 O 4 -bulk Strong (T,C) overlapping

11 D Cu/Al 2 O 3 ED-XANES-TPR H 2 consumption Cu-Rh bimetallic catalysts Intensity (a.u.) T (K) J. Catal. (1998), 178, 253 Correct simulation of phys-chem observables

12 Rh/Al 2 O 3 ED-XANES-TPR H 2 consumption Cu-Rh bimetallic catalysts Fraction of pure component Rh 3+ Rh H 2 consumption (a.u.) Rh/SiO 2 Rh/Al 2 O T (ºC) Caztal. Lett. (1997), 45, 163 Rh-O-Al bonds; competition for alumina surface

13 Rh-Cu/Al 2 O 3 ED-XANES-TPR Cu K-edge Chemical Species Cu-Rh bimetallic catalysts Rh/Cu = 0.14 Cu 3 Rh/Cu = 0.43 Cu 3 Normalized Absorbance (a.u.) CuAl 2 O 4 -bulk 2 CuAl 2 O 4 -sup. 1 CuAl 2 O 4 -bulk Energy (ev) Langmuir. (1999), 15, 5295 Rh/Cu > 0.15 CuAl 2 O 4 -sup absent Cu(I) intermediate

14 Rh-Cu/Al 2 O 3 ED-XANES-TPR Rh K-edge Chemical Species Cu-Rh bimetallic catalysts Rh/Cu= Rh Rh/Cu= Normalized Absorbance (a.u.) Rh 2 O 3 1 Rh 2 O Energy (ev) Rh/Cu < 0.15 formation disordered FCC alloy Rh positively charged Langmuir. (1999), 15, 5295 CO 2 Dry Reforming Activity x 3

15 Oxide Redox Catalysts M-Ce MIXED OXIDES Main Characteristics Chemistry dominated by interface effects between M / Ce species Mixed oxides; modulation redox activation Cu-Ce: Chemical activity WGS: CO + H 2 O CO 2 + H 2 CO-PROX: CO + O 2 (H 2 ) CO 2

16 Cu-Ce MIXED OXIDES Oxide Redox Catalysts Cu K-edge: Redox Chemistry Cu(II)-Ce(IV) Fluorite Network Cu(O)//CeO x Binary system J. Phys. Chem. B. (2005), 109, 19595

17 Cu-Ce MIXED OXIDES Oxide Redox Catalysts CO-PROX; Dependence on Cu Chemical State Cu-Ce Fluorite Network Cu(0)//CeO x Binary system fraction of pure component Cu(II) Cu(I) Cu(0) fraction of pure component Cu(0) Cu(II) CO CO 2 CO Intensity / a.u. O 2 Intensity / a.u Temp / ºC Temp / ºC Active System No Active System Cu(I)-Ce(IV) Interface Cu(0) J. Am. Chem. Soc. (2007), 129, 12054

18 Petrol engines working with stoichiometric A/F mixtures THREE WAY CATALYSTS (TWCs) CONTROL OF POLLUTANTS EMISIONS FROM AUTOMOBILES The main pollutants from automobile engines are CO, HC and NO RICH MIXTURE Power Automobile Catalysts The nature and amount of the emissions vary as a function of air-fuel (A/F) ratio in the engine. FUME COMBUSTION STOICHIOMETRIC MIXTURE NO x LEAN MIXTURE HC MOTOR FAIL NÇMOTOR OFF CO Consumption Air/Fuel Ratio λ λ = A/F A/F stoichiometric

19 Automobile Catalysts THREE WAY CATALYSTS (TWCs) Main Characteristics Zr-Ce Component (Zr,Ce)O x, Zr/Ce 1 higher OSC and durability. Pd-based system Substitution of Rh CO, HC oxidation (low temperatures) NO elimination Dynamic behavior and thermal degradation Temperature/Lambda Cycling MULTITECHNIQUE APPROACH Redox and structural behavior Conventional and Energy dispersive

20 Catalytic activity: stoichiometric-static conditions TWC C 3 H 6 +CO+NO+O 2 CO conv NO conv C 3 H 6 conv PdZC PdZCA PdCA PdA The presence of the HC diminishes the activity for CO and NO conversion PdZC converts CO and C 3 H 6 at lower temperatures PdZC also reaches 100% of NO conversion at slightly lower temperatures J. Catal. (2004), 221, 594.

21 Catalytic activity: stoichiometric-static conditions TWC C 3 H 6 +CO+NO+O 2 DRIFTS in situ K-M CO 2 (g) PdZC CO Pd CO (g) Wavenumber /cm [C 3 H + 5 ] H + [C 3 H 6 ] + C 3 H 6 O Pd 2+ O Pd 2+ O O Pd + O Pd + O

22 Catalytic activity: stoichiometric-static conditions C 3 H 6 +CO+NO+O 2 XANES in situ TWC Normalised Absorbance PdZC final intermediate inicial t CO+NO+O 2 Pd 0 Pd + Pd 2+ C 3 H 6 +CO+NO+O 2 Fraction of pure component Energy /ev Temperature /K HC presence affects Pd from RT HC-Pd interaction stabilises a π-allylic complex; the active species Evolution Pd-3D(Zr-Ce) to contacts Pd 0 contribute through toan oxycarbide - effectivelyintermediate eliminate the species, HC Pd of + the emissions from low temperatures by stabilizing partially oxidized Pd species - eliminate CO/ NO with the help of surface/bulk Pd 0 reduced species J. Catal. (2004), 221, 594.

23 TWCs LAMBDA OSCILLATIONS STOICHIOMETRIC MIXTURE RICH LEAN MIXTURE MIXTURE MOTOR FAIL Rich CO,HCmax.conc. FUME COMBUSTION NO x Power HC NÇMOTOR OFF Lean NO in presence of excess O 2 REDOX; STRUCTURAL CO Consumption Air/Fuel Ratio λ λ = A/F A/F stoichiometric

24 Study of the λ window: redox behavior TIME RESOLVED STUDY OF Pd REDOX BEHAVIOR UNDER OSCILLATING CONDITIONS In situ ED-XANES TWC Normalised Absorbance Pd K-edge results λ=1.01 Initial Pd 2+ PdZCA PdCA PdA Energy /ev λ=0.98 Final Pd Energy /ev ev 4f CR PdZCA PdCA PdA Energy position (ev) of the edge and 4f Continuum Resonance (CR) present in XANES spectra. Values relative to the Pd foil. Sample Series /Condition Edge 4f CR PdA PdCA PdZCA A/lean B/rich A/lean B/rich A/lean B/rich PdZC 4f CR small red shift: increase of the Pd Pd nearest distance most likely associated with the dissolution of C atoms in the Pd fcc structure Different Oxycarbide/Carbide Chemical Phases Chem. Commun. (2005) 2,

25 Study of the λ window: Structural behavior TIME RESOLVED STUDY OF Pd STRUCTURE UNDER λ FLUCTUATIONS TWC ED-EXAFS (Pd K-edge) in situ: CO + NO Pd Calcined CO, 673 K NO, 673 K PdO PdO Energy /ev R (A) Combination of Pd 0 PdO and size/shape variations Nature Mater. (2007) 6,

26 Study of the λ window: Structural behavior TWC TIME RESOLVED STUDY OF Pd STRUCTURE UNDER λ FLUCTUATIONS ED-EXAFS (Pd K-edge) in situ: CO + NO Strong and reversible structural modification under Lambda Oscillations Particles from 30 to 70 NM atoms

27 Study of the λ window: Structural behavior TWC TIME RESOLVED STUDY OF Pd STRUCTURE UNDER λ FLUCTUATIONS Synchronous ED-EXAFS and IR: CO + NO Dynamic surface-bulk NM changes during operation Nature Mater. (2007) 6,

28 Study of the λ window: Structural behavior TWC TIME RESOLVED STUDY OF Pd STRUCTURE UNDER λ FLUCTUATIONS ED-EXAFS (Pd K-edge) in situ: CO + NO + O 2 CN (Pd-Pd) CO+NO; CO+(NO+O 2 ) Strong and reversible structural changes under Lambda Oscillations O 2 enhanced phenomenon Aggregated state under CO: similar Dispersed state under Ox. Conditions: O 2 -promoted Time /s Angew. Chem. Int. Ed. (2007) 46,

29 Study of the λ window: Structural behavior TWC TIME RESOLVED STUDY OF Pd STRUCTURE UNDER λ FLUCTUATIONS ED-XAS (Pd K-edge XANES/EXAFS) in situ: CO + NO + O 2 CO+NO+O2 NO Normalized Absorbance NO+O Energy /ev Energy /ev Non-oxidative redispersion enhanced in presence of oxygen Angew. Chem. Int. Ed. (2007) 46,

30 Study of the λ window: Structural behavior SYNCHRONOUS TIME-RESOLVED MULTI-SPECTROSCOPIC STUDY High Energy XRD TWC Dynamic surface-bulk NM structural: size, shape, lattice Dynamic redox changes Catalytic Activity

31 Energy-dispersive XAS Future challenges Spatial Domain (time efficiency) - 2D, 3D Chemical /Structural nano-mapping - Angstrom/Sub-angstrom resolution Time Domain - Gas-solid - Light-solid Solid elemental process kinetics (μs) Opening Novel perspectives in Catalysis - Nucleation and growth of nano-phases - Dynamic of radiation-mater (photocatalysis) - Operando analysis of TON ( s -1 )

32 ACKNOWLEDGEMENTS Collaborators DRIFTS experiments XAS experiments -Dr. M. A. Newton -- Dr. M. Di Michel - Dr. A. Iglesias-Juez -Dr. A. Martínez-Arias -Dr. C. Belver - Dr. D. Gamarra - Dr. A. Kubacka - Prof. J. A. Anderson All staff at: Ce experiments - Dr. J.A. Rodríguez - Dr. J.C. Hanson -7.1, 9.2 and 9.3 at SRS - ID15, BM-29 and ID-24 at ESRF Financial sources EU program for Large Scale Installations Spain-MEC (CTQ ; CTQ )

33 Study of the λ window: redox behavior TWC Ce 4+ /Ce 3+ REDOX BEHAVIOR Oxygen Storage Capacity Absorbancia Normalizada Ce L III edge III calc 773 K λ = 1 λ = 0.99 λ = 0.98 λ = 0.97 λ = 0.96 A B 0 B 2 B 1 C PdZCA Reducción de Ce /% XANES in situ PdCA PdCA-e1273 PdZCA PdZCA-e1273 PdZCA-m x en CeO x Energía /ev λ 2.0 The presence of Zr increases the oxygen storage capacity This occurs in fresh and aged catalysts

34 Study of the λ window: redox behavior ED-XANES (Pd K-edge) in situ Oxidation % 80 PdZCA PdCA 60 PdA TIME RESOLVED STUDY OF Pd REDOX UNDER λ FLUCTUATIONS Time /s PdO Pd 0 transformation is delayed by effect of the promoter component, especially when Zr is present Zr increases the amount of oxygen transferred by the promoter oxide, limiting the loss of CO conversion in reducing conditions Gas fraction Gas fraction Gas fraction 0.0 C 3 H O 2 C 3 H 6 O 2 λ = 1.01 NO C 3 H 6 O 2 λ = 1.01 NO MS signals λ = 1.01 λ = 0.98 NO λ = 0.98 CO 723 K λ = 0.98 CO 723 K Time /s Time /s CO 723 K Time /s PdA TWC PdCA PdZCA

35 LAMBDA OSCILLATIONS: DYMANIC EFFECTS TWCs