High Oxygen Storage Capacity and High Rates of CO Oxidation and NO Reduction Catalytic Properties of Ce1-xSnxO2 and Ce0.78Sn0.2Pd0.02O2-delta

Ce1-xSnxO2 (x = 0.1-0.5) solid solution and its Pd substituted analogue have been prepared by a single step solution combustion method using tin oxalate precursor. The compounds were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and H-2/temperature programmed redution (TPR) studies. The cubic fluorite structure remained intact up to 50% of Sri substitution in CeO2, and the compounds were stable up to 700 C. Oxygen storage capacity of Ce1-xSnxO2 was found to be much higher than that of Ce1-xZrxO2 due to accessible Ce4+/Ce3+ and Sn4+/Sn2+ redox couples at temperatures between 200 and 400 C. Pd 21 ions in Ce0.78Sn0.2Pd0.02O2-delta are highly ionic, and the lattice oxygen of this catalyst is highly labile, leading to low temperature CO to CO2 conversion. The rate of CO oxidation was 2 mu mol g(-1) s(-1) at 50 degrees C. NO reduction by CO with 70% N-2 selectivity was observed at similar to 200 degrees C and 100% N-2 selectivity below 260 degrees C with 1000-5000 ppm NO. Thus, Pd2+ ion substituted Ce1-xSnxO2 is a superior catalyst compared to Pd2+ ions in CeO2, Ce1-xZrxO2, and Ce1-xTixO2 for low temperature exhaust applications due to the involvement of the Sn2+/Sn4+ redox couple along with Pd2+/Pd-0 and Ce4+/Ce3+ couples.

Ce1-xRuxO2-delta (x=0.05, 0.10): A New High Oxygen Storage Material and Pt, Pd-Free Three-Way Catalyst

Nanocrystalline Ce1-xRuxO2-delta (x = 0.05 and 0.10) of 8-10 nm sizes have been synthesized by hydrothermal method using melamine as complexing agent. Compounds have been characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray analysis (EDX) and their structures have been refined by the Rietveld method.The compounds crystallize in fluorite structure and the composition is Ce1-xRuxO2-x/2 where Ru is in +4 state and Ce is in mixed-valence (+3, +4) state. Substitution of Ru4+ ion in CeO2 activated the lattice oxygen. Ce1-xRuxO2-x/2 reversibly releases 0.22[O] and 0.42[O] for x = 0.05 and 0.10, respectively, which is higher than the maximumpossible OSC of 0.22 [O] observed for Ce0.50Zr0.50O2. Utilization of Higher OSC of Ce1-xRuxO2-delta (x = 0.05 and 0.10) is also reflected in terms of low-temperature CO oxidation with these catalysts, both in the presence and absence of feed oxygen. The Ru4+ ion acts as an active center for reducing molecules (CO, hydrocarbon ``HC'') and oxide ion vacancy acts as an active center for O-2 and NO, leading to low-temperature NO conversion to N-2. Thus due to Ru4+ ion, Ce1-xRuxO2-delta is not just a high oxygen storage material but also shows high activity toward CO, hydrocarbon ``HC'' oxidation, and NO reduction by CO at low temperature with high N-2 selectivity for three-way catalysis.

Structure of Ce1-xSnxO2 and its relation to oxygen storage property from first-principles analysis

CeO2-SnO2 solid solution has been reported to possess high oxygen storage/release property which possibly originates from local structural distortion. We have performed first-principles based density functional calculations of Ce1-xSnxO2 structure (x=0, 0.25, 0.5, 1) to understand its structural stability in fluorite in comparison to rutile structure of the other end-member SnO2, and studied the local structural distortion induced by the dopant Sn ion. Analysis of relative energies of fluorite and rutile phases of CeO2, SnO2, and Ce1-xSnxO2 indicates that fluorite structure is the most stable for Ce1-xSnxO2 solid solution. An analysis of local structural distortions reflected in phonon dispersion show that SnO2 in fluorite structure is highly unstable while CeO2 in rutile structure is only weakly unstable. Thus, Sn in Ce1-xSnxO2-fluorite structure is associated with high local structural distortion whereas Ce in Ce1-xSnxO2-rutile structure, if formed, will show only marginal local distortion. Determination of M-O (M=Ce or Sn) bond lengths and analysis of Born effective charges for the optimized structure of Ce1-xSnxO2 show that local coordination of these cations changes from ideal eightfold coordination expected of fluorite lattice to 4+4 coordination, leading to generation of long and short Ce-O and Sn-O bonds in the doped structure. Bond valence analyses for all ions show the presence of oxygen with bond valence similar to 1.84. These weakly bonded oxygen ions are relevant for enhanced oxygen storage/release properties observed in Ce1-xSnxO2 solid solution. (C) 2010 American Institute of Physics.

Reducibility of Ce1-xZrxO2: origin of enhanced oxygen storage capacity

We combine first-principles calculations with EXAFS studies to investigate the origin of high oxygen storage capacity in ceria-zirconia solid solution, prepared by solution combustion method. We find that nanocrystalline Ce0.5Zr0.5O2 can be reduced to Ce0.5Zr0.5O1.57 by H-2 upto 850 degrees 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.

Correlation of Oxygen Storage Capacity and Structural Distortion in Transition-Metal-, Noble-Metal-, and Rare-Earth-Ion-Substituted CeO2 from First Principles Calculation

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 CeO2, we undertook the structural investigation by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H-2-TPR (temperature-programmed hydrogen reduction) and density functional 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 CeO2. Transition metal and noble metal ion substitution in ceria greatly enhances the reducibility of Ce1-xMxO2-delta (M = Mn, Fe, Co, Ni, Cu, Pd, Pt, Ru), whereas rare-earth-ion-substituted Ce(1-x)A(x)O(2-delta) (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 angstrom (for CeO2). For example, our theoretical calculation for Ce28Mn4O62 structure shows that Mn-O bonds are in 4 + 2 coordination with average bond lengths of 2.0 and 3.06 angstrom respectively. Although the four short Mn-O bond lengths spans the bond distance region of Mn2O3, 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 Ce1-xMxO2-delta (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.

Sonochemical synthesis of Ce1-xFexO2-delta (0 <= x <= 0.45) and Ce0.65Fe0.33Pd0.02O2-delta nanocrystallites: oxygen storage material, CO oxidation and water gas shift catalyst

Nanocrystalline Ce1-xFexO2-delta (0 <= x <= 0.45) and Ce0.65Fe0.33Pd0.02O2-delta of similar to 4 nm sizes were synthesized by a sonochemical method using diethyletriamine (DETA) as a complexing agent. Compounds were characterized by powder X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and transmission electron microscopy (TEM). Ce1-xFexO2-delta (0 <= x <= 0.45) and Ce0.65Fe0.33Pd0.02O2-delta crystallize in fluorite structure where Fe is in +3, Ce is in +4 and Pd is in +2 oxidation state. Due to substitution of smaller Fe3+ ion in CeO2, lattice oxygen is activated and 33% Fe substituted CeO2 i.e. Ce0.67Fe0.33O1.835 reversibly releases 0.31O] up to 600 degrees C which is higher or comparable to the oxygen storage capacity of CeO2-ZrO2 based solid solutions (Catal. Today 2002, 74, 225-234). Due to interaction of redox potentials of Pd2+/0(0.89 V) and Fe3+/2+ (0.77 V) with Ce4+/3+ (1.61 V), Pd ion accelerates the electron transfer from Fe2+ to Ce4+ in Ce0.65Fe0.33Pd0.02O1.815, making it a high oxygen storage material as well as a highly active catalyst for CO oxidation and water gas shift reaction. The activation energy for CO oxidation with Ce0.65Fe0.33Pd0.02O1.815 is found to be as low as 38 kJ mol(-1). Ce0.67Fe0.33O1.835 and Ce0.65Fe0.33Pd0.02O1.815 have also shown high activity for the water gas shift reaction. CO conversion to CO2 is 100% H-2 specific with these catalysts and conversion rate was found to be as high 27.2 mu moles g(-1) s(-1) and the activation energy was found to be 46.4 kJ mol(-1) for Ce0.65Fe0.33Pd0.02O1.815.

A high performance oxygen storage material for chemical looping processes with CO2 capture

Chemical looping combustion (CLC) is a novel combustion technology that involves cyclic reduction and oxidation of oxygen storage materials to provide oxygen for the combustion of fuels to CO2 and H2O, whilst giving a pure stream of CO2 suitable for sequestration or utilisation. Here, we report a method for preparing of oxygen storage materials from layered double hydroxides (LDHs) precursors and demonstrate their applications in the CLC process. The LDHs precursor enables homogeneous mixing of elements at the molecular level, giving a high degree of dispersion and high-loading of active metal oxide in the support after calcination. Using a Cu-Al LDH precursor as a prototype, we demonstrate that rational design of oxygen storage materials by material chemistry significantly improved the reactivity and stability in the high temperature redox cycles. We discovered that the presence of sodium-containing species were effective in inhibiting the formation of copper aluminates (CuAl2O4 or CuAlO 2) and stabilising the copper phase in an amorphous support over multiple redox cycles. A representative nanostructured Cu-based oxygen storage material derived from the LDH precursor showed stable gaseous O2 release capacity (∼5 wt%), stable oxygen storage capacity (∼12 wt%), and stable reaction rates during reversible phase changes between CuO-Cu 2O-Cu at high temperatures (800-1000 °C). We anticipate that the strategy can be extended to manufacture a variety of metal oxide composites for applications in novel high temperature looping cycles for clean energy production and CO2 capture. © The Royal Society of Chemistry 2013.

Measurement of oxygen storage capacity in automotive catalysts

There is considerable disagreement in the literature on available oxygen storage capacity, and on the reaction rates associated with the storage process, for three-way automotive catalysts. This paper seeks to address the issue of oxygen storage capacity in a clear and precise manner. The work described involved a detailed investigation of oxygen storage capacity in typical samples of automotive catalysts. The capacity has also been precisely defined and estimates have been made of the specific capacity based on catalyst dimensions. A purpose-built miniature catalyst test rig has been assembled to allow measurement of the capacity and the experimental procedure has been developed to ensure accurate measurement. The measurements from the first series of experiments have been compared with the theoretical calculations and good agreement is seen. A second series of experiments allowed the effect of temperature on oxygen storage capacity to be investigated. This work shows very clearly the large variation of the capacity with temperature.

Maximizing the Localized Relaxation: The Origin of the Outstanding Oxygen Storage Capacity of kappa-Ce2Zr2O8

Pretty vacant: The excellent oxygen storage capacity (OSC) of ?-Ce2Zr2O8 (see picture; Ce gray, Zr green, O red) is shown to be a result of its unique structural features; after removing oxygen atoms, the structural relaxation is local (vacancy shown in brown), and both the localized structural relaxation and the number of localized structural relaxations are maximized.