Because of growing environmental concerns and increasingly stringent regulations governing auto emissions, new more efficient exhaust catalysts are needed to reduce the amount of pollutants released from internal combustion engines. To accomplish this goal, the major pollutants in exhaust-CO, NO(x), and unburned hydrocarbons-need to be fully converted to CO(2), N(2), and H(2)O. Most exhaust catalysts contain nanocrystalline noble metals (Pt, Pd, Rh) dispersed on oxide supports such as Al(2)O(3) or SiO(2) promoted by CeO(2). However, in conventional catalysts, only the surface atoms of the noble metal particles serve as adsorption sites, and even in 4-6 nm metal particles, only 1/4 to 1/5 of the total noble metal atoms are utilized for catalytic conversion. The complete dispersion of noble metals can be achieved only as ions within an oxide support. In this Account, we describe a novel solution to this dispersion problem: a new solution combustion method for synthesizing dispersed noble metal ionic catalysts. We have synthesized nanocrystalline, single-phase Ce(1-x)M(x)O(2-delta) and Ce(1-x-y)Ti(y)M(x)O(2-delta) (M = Pt, Pd, Rh; x = 0.01-0.02, delta approximately x, y = 0.15-0.25) oxides in fluorite structure. In these oxide catalysts, Pt(2+), Pd(2+), or Rh(3+) ions are substituted only to the extent of 1-2% of Ce(4+) ion. Lower-valent noble metal ion substitution in CeO(2) creates oxygen vacancies. Reducing molecules (CO, H(2), NH(3)) are adsorbed onto electron-deficient noble metal ions, while oxidizing (O(2), NO) molecules are absorbed onto electron-rich oxide ion vacancy sites. The rates of CO and hydrocarbon oxidation and NO(x) reduction (with >80% N(2) selectivity) are 15-30 times higher in the presence of these ionic catalysts than when the same amount of noble metal loaded on an oxide support is used. Catalysts with palladium ion dispersed in CeO(2) or Ce(1-x)Ti(x)O(2) were far superior to Pt or Rh ionic catalysts. Therefore, we have demonstrated that the more expensive Pt and Rh metals are not necessary in exhaust catalysts. We have also grown these nanocrystalline ionic catalysts on ceramic cordierite and have reproduced the results we observed in powder material on the honeycomb catalytic converter. Oxygen in a CeO(2) lattice is activated by the substitution of Ti ion, as well as noble metal ions. Because this substitution creates longer Ti-O and M-O bonds relative to the average Ce-O bond within the lattice, the materials facilitate high oxygen storage and release. The interaction among M(0)/M(n+), Ce(4+)/Ce(3+), and Ti(4+)/Ti(3+) redox couples leads to the promoting action of CeO(2), activation of lattice oxygen and high oxygen storage capacity, metal support interaction, and high rates of catalytic activity in exhaust catalysis.

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Pd/CeO2 (1 at. %) prepared by the solution-combustion method shows a higher catalytic activity for CO oxidation and NO reduction than Pd metal, PdO, and Pd dispersed over CeO2 by the conventional method. To understand the higher catalytic properties, the structure of 1 at. % Pd/CeO2 catalyst material has been investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. The diffraction lines corresponding to Pd or PdO are not observed in the high-resolution XRD pattern of 1 at. % Pd/CeO2. The structure of 1 at. % Pd/CeO2 could be refined for the composition of Ce0.99Pd0.01O1.90 in the fluorite structure with 5% oxide ion vacancy. Pd(3d) peaks in the XPS in 1 at. % Pd/CeO2 are shifted by 3 eV indicating that Pd is in a highly ionic +2 state. EXAFS studies show the average coordination number of 3 around Pd2+ ion in the first shell of 1 at. % Pd/CeO2 at a distance of 2.02 Å, instead of 4 as in PdO. The second shell at 2.72 Å is due to Pd−Pd correlation which is larger than 2.69 Å in PdO. The third shell at 3.31 Å having 7 coordination is absent either in Pd metal or PdO, which can be attributed to −Pd2+−Ce4+− correlation. Thus, 1 at. % Pd/CeO2 forms the Ce1 - x Pd x O2 - δ type of solid solution having −Pd2+−O2-−Ce4+− kinds of linkages.

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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), H2-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−x M x O2−δ (M = Mn, Fe, Co, Ni, Cu, Pd, Pt, Ru), whereas rare-earth-ion-substituted Ce1−x A x O2−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 Å (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 Å 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−x M x O2−δ (M = Mn Fe, Co, Ni, Cu) further enhancement in OSC is observed in H2−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.

The photocatalytic degradation of various organics such as phenol, p-nitrophenol, and salicylic acid was carried out with combustion-synthesized nano-TiO2 under UV and solar exposure. Under identical conditions of UV exposure, the initial degradation rate of phenol with combustion-synthesized TiO2 is 2 times higher than the initial degradation rate of phenol with commercial Degussa P-25 TiO2. The intermediates such as catechol (CC) and hydroquinone (HQ) were not detected during the degradation of phenol with combustion-synthesized TiO2, while both the intermediates were detected when phenol was degraded over Degussa P-25. This indicates that the rates of secondary photolysis of CC and HQ occur extremely faster than the rates at which they are formed from phenol and further implies that the primary hydroxylation step is rate limiting for the combustion-synthesized TiO2 aided photodegradation of phenol. The degradation rates of salicylic acid and p-nitrophenol were also investigated, and the rates were higher for combustion-synthesized titania compared to Degussa P-25 TiO2. Superior activity of combustion-synthesized TiO2 toward photodegradation of organic compounds can be attributed to crystallinity, higher surface area, more surface hydroxyl groups, and optical absorption at higher wavelength.

We determine chemical origins of increase in the reducibility of CeO 2 upon Ti substitution using a combination of experiments and first-principles density functional theory calculations. Ce 1-x Ti x O 2 (x ) 0.0-0.4) prepared by a single step solution combustion method crystallizes in a cubic fluorite structure, confirmed by Rietveld profile analysis. Ce 1-x Ti x O 2 can be reduced by hydrogen to a larger extent compared to CeO 2 or TiO 2 . Temperature programmed reduction of CeO 2 , TiO 2 , Ce 0.75 Ti 0.25 O 2 and Ce 0.6 Ti 0.4 O 2 up to 700 °C in H 2 gave CeO 1.96 , TiO 1.92 , Ce 0.75 Ti 0.25 O 1.81 , and Ce 0.6 Ti 0.4 O 1.73 , respectively. An extended X-ray absorption fine structure (EXAFS) study of mixed oxides at the Ti K-egde showed that the local coordination of Ti is 4:4, with Ti-O distances of 1.9 and 2.5 Å, respectively, which are also confirmed by our first-principles calculations. Bond valence analysis of the microscopic structure and energetics determined from first principles is used to evaluate the strength of binding of different oxygen atoms and vacancies. We find the presence of strongly and weakly bound oxygens in Ce 1-x Ti x O 2 , of which the latter are responsible for the higher oxygen storage capacity in the mixed oxides than in pure CeO 2 .

Flourite-type nanocrystalline Ce0.9Fe0.1O2−δ and Ce0.89Fe0.1Pd0.01O2−δ solid solutions have been synthesized by solution combustion method, which show higher oxygen storage/release property (OSC) compared to CeO2 and Ce0.8Zr0.2O2. Temperature programmed reduction and XPS study reveal that the presence of Pd ion in Ce0.9Fe0.1O2−δ facilitates complete reduction of Fe3+ to Fe2+ state and partial reduction of Ce4+ to Ce3+ state at temperatures as low as 105 °C compared to 400 °C for monometal-ionic Ce0.9Fe0.1O2−δ. Fe3+ ion is reduced to Fe2+ and not to Fe0 due to favorable redox potential for Ce4+ + Fe2+ → Ce3+ + Fe3+ reaction. Using first-principles density functional theory calculation we determine M−O (M = Pd, Fe, Ce) bond lengths, and find that bond lengths vary from shorter (2.16 Å) to longer (2.9 Å) bond distances compared to mean Ce−O bond distance of 2.34 Å for CeO2. Using these results in bond valence analysis, we show that oxygen with bond valences as low as −1.55 are created, leading to activation of lattice oxygen in the bimetal ionic catalyst. Temperatures of CO oxidation and NO reduction by CO/H2 are lower with the bimetal-ionic Ce0.89Fe0.1Pd0.01O2−δ catalyst compared to monometal-ionic Ce0.9Fe0.1O2−δ and Ce0.99Pd0.01O2−δ catalysts. From XPS studies of Pd impregnated on CeO2 and Fe2O3 oxides, we show that the synergism leading to low temperature activation of lattice oxygen in bimetal-ionic catalyst Ce0.89Fe0.1Pd0.01O2−δ is due to low-temperature reduction of Pd2+ to Pd0, followed by Pd0 + 2Fe3+ → Pd2+ + 2Fe2+, Pd0 + 2Ce4+ → Pd2+ + 2Ce3+ redox reaction.