pubs.acs.org/cm Published on Web 08/27/2010 r 2010 American Chemical Society 5184 Chem. Mater. 2010, 22, 5184–5198 DOI:10.1021/cm101145d Correlation of Oxygen Storage Capacity and Structural Distortion in Transition-Metal-, Noble-Metal-, and Rare-Earth-Ion-Substituted CeO 2 from First Principles Calculation Asha Gupta, † U. V. Waghmare, ‡ and M. S. Hegde* ,§ † Materials Research Centre, and § Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India, and ‡ Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India Received April 23, 2010. Revised Manuscript Received August 6, 2010 Oxygen storage/release (OSC) capacity is an important feature common to all three-way catalysts to combat harmful exhaust emissions. To understand the mechanism of improved OSC for doped CeO 2 , we undertook the structural investigation by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H 2 -TPR (temperature-programmed hydrogen reduction) and density func- tional theoretical (DFT) calculations of transition-metal-, noble-metal-, and rare-earth (RE)-ion- substituted ceria. In this report, we present the relationship between the OSC and structural changes induced by the dopant ion in CeO 2 . Transition metal and noble metal ion substitution in ceria greatly enhances the reducibility of Ce 1-x M x O 2-δ (M = Mn, Fe, Co, Ni, Cu, Pd, Pt, Ru), whereas rare- earth-ion-substituted Ce 1-x A x O 2-δ (A = La, Y) have very little effect in improving the OSC. Our simulated optimized structure shows deviation in cation-oxygen bond length from ideal bond length of 2.34 A ˚ (for CeO 2 ). For example, our theoretical calculation for Ce 28 Mn 4 O 62 structure shows that Mn-O bonds are in 4 þ 2 coordination with average bond lengths of 2.0 and 3.06 A ˚ respectively. Although the four short Mn-O bond lengths spans the bond distance region of Mn 2 O 3 , the other two Mn-O bonds are moved to longer distances. The dopant transition and noble metal ions also affects Ce coordination shell and results in the formation of longer Ce-O bonds as well. Thus longer cation-oxygen bonds for both dopant and host ions results in enhanced synergistic reduction of the solid solution. With Pd ion substitution in Ce 1-x M x O 2-δ (M = Mn Fe, Co, Ni, Cu) further enhancement in OSC is observed in H 2 -TPR. This effect is reflected in our model calculations by the presence of still longer bonds compared to the model without Pd ion doping. The synergistic effect is therefore due to enhanced reducibility of both dopant and host ion induced due to structural distortion of fluorite lattice in presence of dopant ion. For RE ions (RE = Y, La), our calculations show very little deviation of bonds lengths from ideal fluorite structure. The absence of longer Y-O/La-O and Ce-O bonds make the structure much less susceptible to reduction. 1. Introduction Oxygen storage materials have become important after the introduction of catalytic treatment of automotive ex- haust in early 1970s. 1 Three-way catalysts that can eliminate CO, HCs (hydrocarbons), and NO x simultaneously have been used to control exhaust emissions. To widen the operating window of the catalyst for the fluctuating air- to-fuel ratio at the automobile converter, there was a need to develop a catalyst that can store oxygen in the oxygen-rich region and release oxygen in the oxygen-lean region. 2,3 Gandhi et al. first coined the term “oxygen storing” property for base metal oxides. 4 Yao and Yao 5 showed for the first time the oxygen storage capacity for CeO 2 and cited it as a suitable oxygen storage component for the three-way cata- lysis. Mechanism of OSC is given as CeO 2 þ δCO f CeO 2 - δ þ δCO 2 ; CeO 2 - δ þ δ=2O 2 f CeO 2 indicating high endurance of ceria toward fluctuating oxygen vacancies under oxygen excess and deficient conditions. Application of oxygen storage/release property of ceria for automobile exhaust catalytic treatment has led to the deve- lopment of ceria-based catalyst, and first among them is ceria-zirconia solid solution. 2,6-9 It is now well-established *Corresponding author. E-mail: mshegde@sscu.iisc.ernet.in. (1) Taylor, K. C. Catal. Rev.-Sci. Eng. 1993, 35, 25. (2) Kaspar, J.; Fornasiero, P.; Hickey, N. Catal. Today 2003, 77, 419. (3) Ozawa, M.; Kimura, M.; Isogai, A. J. Alloys Compd. 1993, 193, 73. (4) Gandhi, H. S.; Graham, G. W.; McCabe, R. W. J. Catal. 2003, 216, 433. (5) Yao, H. C.; Yao, Y. F. Y. J. Catal. 1984, 86, 254. (6) Fornasiero, P.; Balducci, G.; Kaspar, J.; Meriani, S.; di Monte, R.; Graziani, M. Catal. Today 1996, 29, 47. (7) Rao, G. R.; Ka  spar, J.; Meriani, S.; Di Monte, R.; Graziani, M. Catal. Lett. 1994, 24, 107–112. (8) Sugiura, M. Catal. Surv. Asia 2003, 7, 77–87. (9) Fornasiero, P.; Di Monte, R; Rao, G. R.; Kaspar, J.; Meriani, S.; Trovarelli, A.; Graziani, M. J. Catal. 1995, 151, 168–177.