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
Ceria Catalysis: 1) Crucial for Oxygen Storage: ½O 2 + Ce 2 O 3 2CeO 2 2) Enhances WGS & SR rates by O transfer. 10 19 Rate (molecules/s.g cat) 10 18 10 17 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 2 10 16 1.4 1.9 2.4 1000/T (K -1 ) 3) Pure ceria deactivates; zirconia is required for stabilization of ceria. 4) Catalysts often doped with other RE ions.
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 3 + 0.5 O 2 = 2 CeO 2-380 1) Pd + 0.5 O 2 = PdO -85 2 CeO 2 + Pd = PdO + Ce 2 O 3 +295 1) 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
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.
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.
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
Bulk Ceria by Flow Titration, (3-10 m 2 /g) O/Ce atomic ratio 2.00 1.95 1.90 1.85 1.80 1.75 600ºC 650ºC 700ºC 800ºC 1.70-30 -28-26 -24-22 -20-18 -16 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 titration: @700ºC, the accessible range of P(O 2 ) for reasonable H 2 -H 2 O compositions is 10-27 atm < P(O 2 ) < 10-21 atm
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 ) S @700ºC, 10-19 atm < P(O 2 ) < 10-1 atm 10-19 atm because PtZr 3 forms at ~10-20 atm; use of Ag electrodes allows lower P(O 2 ).
Validation of Coulometric Titration (10% Cu on SiO 2 ) 1 0.8 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 0.6 0.4 0.2 Cu Cu 2 O 0-18 -16-14 -12-10 -8-6 -4-2 log [ P(O 2 ) (atm) ] 1 Equilibrium data from NIST Web-book, 2006
Bulk V 2 O 5 P(O 2 ) established by CO-CO 2 2.5 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 +5 2.3 ΔH lit = -260 kj/gmol-o 2 ΔH exp = -270 to -300 kj/gmol-o 2 O / V 2.1 1.9 1.7 1.5 ΔH lit = -430 kj/gmol-o 2 ΔH exp = -385 kj/gmol-o 2 823 K 873 K 923 K -22-18 -14-10 -6-2 Log [ P(O 2 ) atm ] 3 Equilibrium data from CRC Handbook, Ed. 59, 1979. V +4 V +3
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) 3 25 35 89 O:Ce Ratio 1.98 1.95 1.93 1.92 % 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) 114 94 68 63 O:Ce Ratio - 1.55 1.75 1.95
30 wt% Ceria/LA versus Bulk CeO 2, 600-700ºC: 2.00 873K, 923K, 973K 1.95 O/Ce atomic ratio 1.90 1.85 1.80 Bulk 30% ceria/la - ΔH (kj/mol-o 2 ) 800 750 700 650 600 550 500 Bulk Supported 1.75-35 -30-25 -20-15 -10-5 log P O2 (atm) 450 1.75 1.80 1.85 1.90 1.95 2.00 O/Ce atomic ratio
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
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 2.0 2 O / M 1.96 1.92 1.88 1.84 1.8 g 873 K 973 K 1073 K O:M =1.91, One lattice O 2- for every two Zr +4 1.76-30 -26-22 -18-14 -10-6 -2 log [ P(O 2 ) (atm) ]
Note: XRD shows no evidence for compound formation Ce 0.81 Zr 0.19 O 1.91 reduction. Continuous shift in the lattice parameter.
High-Zirconia Solid Solutions 2 1.98 1.96 Ce 0.25 Zr 0.75 O 2-x Ce 0.14 Zr 0.86 O 2 2 1.98 O/M ratio 1.94 1.92 1.9 1.88 1.86 Complete reduction to Ce +3 O/M ratio 1.96 1.94 1.92 1.84-30 -26-22 -18-14 -10-6 -2 log [ P(O 2 ) (atm) ] 1.9-30 -28-26 -24-22 -20-18 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!
The oxidation enthalpies of Ce y Zr 1-y O 2 -ΔH (kj / gmol-o 2 ) 800 700 600 500 400 y = 1 y = 0.81 y = 0.5 y = 0.25 y = 0.14 973 K 1.78 1.83 1.88 1.93 1.98 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).)
Entropy affects the isotherm for Ce 0.81 Zr 0.19 O 2 : O/M ratio 2 1.96 1.92 1.88 1.84 1.8 1.76 600ºC 800ºC O:M =1.91, One lattice O 2- for every two Zr +4-30 -26-22 -18-14 -10-6 -2 log [ P(O 2 ) (atm) ] -ΔS (J/gmol-O 2 /K) 400 350 Ceria 300 Ce81 250 Ce25 200 150 100 50 0 O:M=1.91 1.76 1.8 1.84 1.88 1.92 1.96 2 O / M Shah, P. R., et. al. Chemistry of Materials, 2006, 18, 5363-5369. 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.
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
Effect of calcination on structure: 973 K v.s. 1323 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 0.19 47 49 51 20 30 40 50 60 70 80 2 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)
Effect of calcination on 973-K isotherms: Ce 0.14 Zr 0.86 O 2-973 K g-1323 K Ce 0.59 Zr 0.41 O 2 2 1.99 1.98 973 K 2 1.95 (a) O/M atomic ratio 1.97 1.96 1.95 1.94 O/M atomic ratio 1.9 1.85 1.8 1.93 1.92 1.75 Physical mixture of phases formed 1.91 1E-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.
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 3 450 K 600 K 750 K 825 K 900 K 940 930 920 910 900 890 880 870 940 930 920 910 900 890 880 870 Binding Energy (ev) Binding Energy (ev)
CeO 2 /YSZ(100) - AFM Images 1.00 as deposited 4 nm CeO 2 film 0.75 0.50 50 0 nm 0 0 0.25 0.50 0.75 1.00 0.25 0.2 0.4 0.6 0.8 0.8 0.6 0.4 0.2 1.00 after annealing in air at 1273 K 0 0 0.25 0.50 0.75 1.00 0.75 0.50 0.25 50 0 nm 0.2 0.4 0.6 0.8 0.8 0.6 0.4 0.2
CeO 2 /YSZ(100) - AFM Images Annealed in air at 1273 K Etched in nitric acid 5.00 5.00 2.50 2.50 [010] 20 [001] 0 2.50 5.00 0 0 2.50 5.00 20 10 10 nm nm 0 0-10 0 1 2 3 4 5-10 0 1 2 3 4 5
CeO 2 /YSZ(100) - AFM Images 1.00 after annealing in air at 1273 K 0 0 0.25 0.50 0.75 1.00 0.75 0.50 0.25 50 0 nm 0.2 0.4 0.6 0.8 0.8 0.6 0.4 0.2 1.00 after annealing in vacuum at 973 K 0.75 0.50 0.25 0 0.25 0.50 0.75 1.00 0 50 0 nm 0.2 0.4 0.6 0.8 0.4 0.2 0.8 0.6
CeO 2 versus Ce 0.8 Sm 0.2 O 1.9 1.90 2.00 1.95 O/Ce atomic ratio 1.85 1.80 1.75 600ºC 650ºC 700ºC 800ºC 900ºC 1.70-32 -30-28 -26-24 -22-20 -18-16 O/Ce atomic ratio 1.90 1.85 1.80 1.75 600ºC 650ºC 700ºC 800ºC 900ºC 1.70-30 -28-26 -24-22 -20-18 -16 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.
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
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+15 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 1000/T (1/K) 1E+15 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 1000/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+15 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 1000/T (1/K) 1E+14 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 1000/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)?
Evidence for a 2 cnd rate process with n-butane over ceria: Light-off curves methane conversion % 100 90 methane 80 70 60 50 40 ceria 30 20 SDC 10 0 400 500 600 700 800 900 1000 1100 Butane Conversion % 100 90 n-butane 80 70 60 50 ceria 40 30 20 SDC 10 0 400 500 600 700 800 900 1000 1100 Temp (K) Temp (K)
Number of sites involved with low-temperature, n-butaneoxidation process corresponds to a partial monolayer. 140 Pulse studies: n-butane O 2 120 CO2 Production umol/g 100 80 60 40 20 ceria SDC 1 monolayer ~ 100 ol O/g 0 400 500 600 700 800 900 1000 1100 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.
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