Reducibility of Ce 1-x Zr x O 2 : Origin of Enhanced Oxygen Storage Capacity
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Catalysis Letters Vol. 108, Nos. 3–4, May 2006 (Ó 2006) DOI: 10.1007/s10562-006-0040-z
Reducibility of Ce1-xZrxO2: origin of enhanced oxygen storage capacity Gargi Dutta,a Umesh V. Waghmare,a Tinku Baidya,b M.S. Hegde,b,* K.R. Priolkar,c and P.R. Sarodec a
Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur Campus, Bangalore 560 064, India b Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India c Department of Physics, Goa University, Taleigao Plateau, Goa 403 206, India
Received 3 January 2006; accepted 25 January 2006
We combine first-principles calculations with EXAFS studies to investigate the origin of high oxygen storage capacity in ceriazirconia solid solution, prepared by solution combustion method. We find that nanocrystalline Ce0.5Zr0.5O2 can be reduced to Ce0.5Zr0.5O1.57 by H2 upto 850 °C with an OSC of 65 cc/gm which is extremely high. Calculated local atomic-scale structure reveals the presence of long and short bonds resulting in four-fold coordination of the cations, confirmed by the EXAFS studies. Bond valence analysis of the microscopic structure and energetics is used to evaluate the strength of binding of different oxide ions and vacancies. We find the presence of strongly and weakly bound oxygens, of which the latter are responsible for the higher oxygen storage capacity in the mixed oxides than in the pure CeO2. KEY WORDS: biomimetic oxidation catalysts; biotransformation; D-limonene; porphyrin; uv–vis spectroscopy; photooxidation.
1. Introduction Amount of oxygen that can be released under reducing condition and to uptake it under oxidizing condition is called oxygen storage capacity (OSC) [1,2]. Temperature programmed reduction (TPR) by H2 is generally employed to estimate OSC. The mechanism of CeO2–Ce2O3 transition associated with oxygen vacancy formation was explained on the basis of first-principles calculations as a possible reason behind the high OSC of ceria [3]. Even though ZrO2 cannot be reduced by CO or H2, OSC was enhanced in the Ce1-xZrxO2 solid solution [4–6]. This means, in the solid solution, Ce can be reduced lot more easily compared to pure CeO2. Because of its importance in auto exhaust catalysis, several studies exist in the literature to explain this observation [7–9]. It was suggested that the availability of free oxygen is increased by the transport of bulk oxygen to surface [10]. Later, movement of oxide ions from the tetrahedral sites to the vacant octahedral sites was suggested to be one of possibilities which can enhance OSC [11]. A common observation is that the X-ray diffraction lines of Ce1-xZrxO2 are much broader in the solid solution. This is generally attributed to the decrease in size of crystallites and therefore higher OSC was attributed to larger surface area of Ce1-xZrxO2. It is also possible that the local coordination around Zr in Ce1-xZrxO2 is different from the ideal eight-fold coordination of Ce in CeO2, which can lead to broadening of X-ray lines. EXAFS studies have reveale
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