Thermodynamic studies of oxidation and reduction of ceria and ceria mixed oxides

Transcription

1 Thermodynamic studies of oxidation and reduction of ceria and ceria mixed oxides R. J. Gorte Chemical & Biomolecular Engineering University of Pennsylvania Support: DOE-BES Collaborators: Paolo Fornasiero, John Vohs Parag Shah, & Gong Zhou

2 Ceria Catalysis: 1) Crucial for Oxygen Storage: ½O 2 + Ce 2 O 3 2CeO 2 2) Enhances WGS & SR rates by O transfer Rate (molecules/s.g cat) Pd/Al 2 O 3 Ceria Pd/Ceria CO + H 2 O = CO 2 + H 2 Proposed Mechanism 2CeO 2 + Pd = PdO + Ce 2 O 3 CO + PdO = CO 2 + Pd Ce 2 O 3 + H 2 O = 2 CeO 2 + H /T (K -1 ) 3) Pure ceria deactivates; zirconia is required for stabilization of ceria. 4) Catalysts often doped with other RE ions.

3 Note: Complete oxidation of Pd by ceria-zirconia has been observed in UHV at 150ºC by XPS. M. Y. Smirnov & G. W. Graham, Catal. Lett, 72 (2001) 39. Problem: Pd should not be oxidized by ceria at 150ºC! Reaction ΔH (kj/mol) Ce 2 O O 2 = 2 CeO ) Pd O 2 = PdO CeO 2 + Pd = PdO + Ce 2 O ) Note: for CeO (2-x/2) + x O 2 = CeO 2, ΔH/x ~ constant. M. Mogensen, Catalysis by Ceria and Related Materials", A. Trovarelli, ed

4 Question: Are the thermodynamic data for bulk ceria representative of catalytic ceria? Catalytic ceria is usually: a) in the form of nanoparticles. b) often doped with La, Sm, etc. c) often in the form of a solution with ZrO 2.

5 Needed: Thermodynamic data for catalytic forms of ceria. Assertions: 1) Calorimetric measurements of ΔH are difficult and often unreliable. 2) Equilibrium data provide ΔH, ΔS of oxidation more accurately. Concept: H 2 + ½O 2 = H 2 O 2CeO 2 = Ce 2 O 3 + ½O 2 P(O 2 ) ½ = P(H 2O) K *P(H 2 ) H 2O K = P(O 2 ) ½ CeO 2 ΔGº = -RT ln{ } ΔHº = -R δ ln{ } δ (1/T) ΔSº = (ΔHº - ΔGº)/T Note: P(O 2 ) is a fugacity established by H 2 -H 2 O equilibrium, not a true partial pressure.

6 Flow Titration: Get x (in CeO 2-x ) as a f(p ) O 2 PHO 2 P O = 2 K eq P H 2 2

7 Bulk Ceria by Flow Titration, (3-10 m 2 /g) O/Ce atomic ratio ºC 650ºC 700ºC 800ºC log P O2 (atm) 900ºC Observations: 1) Data identical to literature for bulk ceria (Bevan & Kordis, J. Inorg. Nucl. Chem, 26 (1964) 1509) 2) For T 700ºC, two ceria phases in equilibrium 3) ΔH ~ -760 kj/mol of O 2 4) At 700ºC, CeO 1.97 for 97% H 2 and 3% H 2 O. Limitation for flow the accessible range of P(O 2 ) for reasonable H 2 -H 2 O compositions is atm < P(O 2 ) < atm

8 Coulometric Titration: P(O 2 ) ref YSZ Electrolyte P(O 2 ) S YSZ electrolyte acts a) as an O 2 pump b) P(O 2 ) sensor V RT PO ( ) 2 ref = ln 4 F P( O2 ) atm < P(O 2 ) < 10-1 atm atm because PtZr 3 forms at ~10-20 atm; use of Ag electrodes allows lower P(O 2 ).

9 Validation of Coulometric Titration (10% Cu on SiO 2 ) Equilibrium at 973 K 1 : 2 Cu + ½ O 2 Cu 2 O P(O 2 ) eq = 3x10-11 atm Cu 2 O + ½ O 2 2 CuO P(O 2 ) eq = 5x10-5 atm Cu 2 O CuO O / Cu Cu Cu 2 O log [ P(O 2 ) (atm) ] 1 Equilibrium data from NIST Web-book, 2006

10 Bulk V 2 O 5 P(O 2 ) established by CO-CO Equilibrium at 873 K 3 : V 2 O 3 V 2 O 4 P(O 2 ) eq = 3x10-19 atm V 2 O 4 V 2 O 5 P(O 2 ) eq = 5x10-6 atm V ΔH lit = -260 kj/gmol-o 2 ΔH exp = -270 to -300 kj/gmol-o 2 O / V ΔH lit = -430 kj/gmol-o 2 ΔH exp = -385 kj/gmol-o K 873 K 923 K Log [ P(O 2 ) atm ] 3 Equilibrium data from CRC Handbook, Ed. 59, V +4 V +3

11 Surface versus Bulk CeO 2 : Problem: surface areas are not stable after redox cycles. Expt 1: Measure O:Ce ratio of samples having different surface areas, following reduction in 90% H 2-10% H 2 O at 600ºC. SA (m 2 /g) O:Ce Ratio % surface 7 17 Expt 2: Examine ceria supported on LA (10% La 2 O 3 /alumina) following reduction in 90% H 2-10% H 2 O at 700ºC. LA 15 wt% ceria 30 wt% ceria 50 wt% ceria SA (m 2 /g) O:Ce Ratio

12 30 wt% Ceria/LA versus Bulk CeO 2, ºC: K, 923K, 973K 1.95 O/Ce atomic ratio Bulk 30% ceria/la - ΔH (kj/mol-o 2 ) Bulk Supported log P O2 (atm) O/Ce atomic ratio

13 Ce x Zr (1-x) O 2 Solid Solutions 700ºC, Flow Titration: x = 0 x = 1 x= 0.92 x= 0.81 x= 0.59 x= 0.50 Observations: 1) Reducibility increases with increasing Zr content. 2) Surface area does not affect data (Δ=973 K; =1323 K) 3) Each sample shows a plateau region at high P(O 2 ); e.g. Ce 0.81 Zr 0.19 O 1.9. Note: Plateau due to pyrochlore clusters? Ce 0.81 Zr 0.19 O 1.9 ~ (CeO 2 ) 0.62 (Ce 2 Zr 2 O 7 ) 0.095

14 Ce 0.81 Zr 0.19 O 2-x Solid Solutions Isotherm shows two distinct regions: Ce 0.81 Zr 0.19 O 1.82 Ce 0.81 Zr 0.19 O 1.91 Ce 0.81 Zr 0.19 O 1.91 Ce 0.81 Zr 0.19 O O / M g 873 K 973 K 1073 K O:M =1.91, One lattice O 2- for every two Zr log [ P(O 2 ) (atm) ]

15 Note: XRD shows no evidence for compound formation Ce 0.81 Zr 0.19 O 1.91 reduction. Continuous shift in the lattice parameter.

16 High-Zirconia Solid Solutions Ce 0.25 Zr 0.75 O 2-x Ce 0.14 Zr 0.86 O O/M ratio Complete reduction to Ce +3 O/M ratio log [ P(O 2 ) (atm) ] log [ P(O 2 ) (atm) ] ( 873K, 973K, 1073K ) Ce 0.14 Zr 0.86 O 2 oxidizes more readily than Ce 0.25 Zr 0.75 O 2!

17 The oxidation enthalpies of Ce y Zr 1-y O 2 -ΔH (kj / gmol-o 2 ) y = 1 y = 0.81 y = 0.5 y = 0.25 y = K O/M in Ce y Zr 1-y O 2 ΔH for Ce y Zr 1-y O 2 is -525 kj/mol-o 2 (-760 kj/mol for CeO 2 ) 1. Independent of extent of reduction 2. Independent of composition 3. Similar to that reported for the pyrochlore (Ce 2 Zr 2 O 7 ) (H. Otobe, et al, J. Phys. Chem. Sol., 66, 329 (2005).)

18 Entropy affects the isotherm for Ce 0.81 Zr 0.19 O 2 : O/M ratio ºC 800ºC O:M =1.91, One lattice O 2- for every two Zr log [ P(O 2 ) (atm) ] -ΔS (J/gmol-O 2 /K) Ceria 300 Ce Ce O:M= O / M Shah, P. R., et. al. Chemistry of Materials, 2006, 18, Notes: 1) Large ΔS for CeO 2 ; all lattice oxygen equivalent. 2) ΔS for Ce 0.25 Zr 0.75 O 2 and for Ce 0.81 Zr 0.19 O 2 at low reduction similar to that for CeO 2. 3) Dramatic drop in ΔS for Ce 0.81 Zr 0.19 O 1.91.

19 Model for Ce y Zr 1-y O 2 reduction 1) Energetics related to local structure; O 2- removed from pyrochlore cluster. (Explains constant ΔH for all samples.) 2) For Ce 0.81 Zr 0.19 O 2 : - High ΔS for initial reduction Any lattice O 2- can be removed This defines Zr +4 ion can pair. Manyways to form (Zr +4 -Ce +3 )O cluster -Low ΔS for additional reduction Require removal of 2 cnd O 2- for each Zr +4 ion pair. Only one way to form (Zr +4 -Ce +3 )O cluster 3) For Ce 0.14 Zr 0.86 O 2 : -Low ΔS because two Ce +4 required for reduction

20 Effect of calcination on structure: 973 K v.s K Intensity (a.u.) (d) (c) Ce 0.14 Zr 0.86 Ce 0.33 Zr 0.86 (d) (c) (b) (b) Ce 0.59 Zr 0.41 (a) (a) Ce 0.81 Zr Thelta (degree) For y=0.81, 0.14, structure is stable: Ce 0.81 Zr 0.19 O 2 : Cubic Ce 0.14 Zr 0.86 O 2 : Tetragonal Intermediate phases undergo phase separation Ce 0.59 Zr 0.41 O 2 : becomes Ce 0.80 Zr 0.20 O 2 + Ce 0.14 Zr 0.86 O 2 : Zhang, et al, JACerS, 89, 1028 (2006)

21 Effect of calcination on 973-K isotherms: Ce 0.14 Zr 0.86 O K g-1323 K Ce 0.59 Zr 0.41 O K (a) O/M atomic ratio O/M atomic ratio Physical mixture of phases formed E-27 1E-26 1E-25 1E-24 1E-23 1E-22 1E-21 1E-20 P(O 2 ) atm 1.7 1E-27 1E-26 1E-25 1E-24 1E-23 1E-22 1E-21 1E-20 P(O 2 ) atm Ce 0.14 Zr 0.86 O 2 unaffected by calcination temperature Ce 0.59 Zr 0.41 O 2 looks like physical mixture of Ce 0.14 Zr 0.86 O 2 and Ce 0.8 Zr 0.2 O 2 Results depend only on the phases present.

22 ZrO 2 makes ceria more reducible also in absence of solid solution: CeO 2 / α-al 2 O 3 (0001) CeO 2 / ZrO 2 (100) α-al 2 O 3 (0001) CeO 2 ZrO 2 (100) CeO 2 CeO 2 XPS: Ce 3d CeO 2 XPS: Ce 3d 550 K 750 K 825 K 900 K 1000 K CeO 2 Ce 2 O K 600 K 750 K 825 K 900 K Binding Energy (ev) Binding Energy (ev)

23 CeO 2 /YSZ(100) - AFM Images 1.00 as deposited 4 nm CeO 2 film nm after annealing in air at 1273 K nm

24 CeO 2 /YSZ(100) - AFM Images Annealed in air at 1273 K Etched in nitric acid [010] 20 [001] nm nm

25 CeO 2 /YSZ(100) - AFM Images 1.00 after annealing in air at 1273 K nm after annealing in vacuum at 973 K nm

26 CeO 2 versus Ce 0.8 Sm 0.2 O O/Ce atomic ratio ºC 650ºC 700ºC 800ºC 900ºC O/Ce atomic ratio ºC 650ºC 700ºC 800ºC 900ºC log P O2 (atm) log P O2 (atm) Conclusion: RE dopants affect phase transition between 700 and 800ºC, but do not improve the reducibility of bulk ceria.

27 Effect of RE +3 dopants on oxidation rates: n-butane oxidation: Yb 0.2 Ce 0.8 O 1.9 CeO 2 Zr 0.2 Ce 0.8 O 1.9 La 0.2 Ce 0.8 O 1.9 Sm 0.2 Ce 0.8 O 1.9 Nb 0.1 Ce 0.9 O 2.05 Pr 0.2 Ce 0.8 O 2 Note: Differences in surface areas were negligible. S. Zhao and R. J. Gorte, Applied Catalysis A, 248 (2003) 9-18

28 Dopant effect depends on the particular hydrocarbon! No change with CH 4, C 2 H 6 ; large change with C 3 H 8, C 4 H 10. Rate (molecules/s.m 2 ) 1E+17 1E+16 Methane:O 2 = 1:3 Rate (molecules/s.m 2 ) 1E+17 1E+16 Ethane:O 2 = 1:5 Ceria SDC 1E /T (1/K) 1E /T (1/K) 1E+17 Propane:O 2 = 1:6 1E+17 Butane:O 2 = 1:8 Rate (molecules/s.m 2 ) 1E+16 Rate (molecules/s.m 2 ) 1E+16 1E+15 1E /T (1/K) 1E /T (1/K) Side point: If there is no correlation between oxidation of CH 4 and C 4 H 10, should one expect correlation with TPR (H 2 oxidation)?

29 Evidence for a 2 cnd rate process with n-butane over ceria: Light-off curves methane conversion % methane ceria SDC Butane Conversion % n-butane ceria SDC Temp (K) Temp (K)

30 Number of sites involved with low-temperature, n-butaneoxidation process corresponds to a partial monolayer. 140 Pulse studies: n-butane O CO2 Production umol/g ceria SDC 1 monolayer ~ 100 ol O/g Temp/K a) The surface of SDC is effectively reduced by Sm +3. b) Above 700 K, where CH 4 begins to react, bulk oxygen become accessible.

31 Conclusions: 1) Oxygen transfer important in WGS and steam reforming. 2) Bulk ceria is not very reducible e.g. CeO 1.97 in equilibrium with 90% H 2-10% H 2 O at 700 C. 3) Surface reduction of ceria easier than bulk 600 kj/mol-o 2 versus 760 kj/mol-o 2. 4) Ceria-zirconia more easily reduced than ceria. 525 kj/mol-o 2 versus 760 kj/mol-o 2. Entropy effects important. 5) ZrO 2 can affect redox properties and structure of supported ceria films. 6) Hydrocarbon oxidation activity of Ceria: Common dopants decrease rates Surface and bulk oxygen both important