Challenges in energy and the environment call for the development of highly active catalysts, allowing for a more efficient and cleaner use of energy supplies.[1] Catalytic combustion of methane is a leading technology in emission prevention and cleanup.[2] Its main advantage over traditional flame combustion is to stabilize complete oxidation of fuel at low temperature while simultaneously controlling NO x emissions. Catalysts yielding the highest activity at low temperatures consist of noble metals dispersed on high-surface-area oxide supports. PdO particles dispersed on oxide carriers are the most active methane combustion catalysts, but they still suffer from inadequate activity at low temperature (below 673 K) and deactivation at high temperature (above 973 K) owing to formation of metallic Pd from PdO particles.[3] This transformation is regulated by a complex dynamic of formation and decomposition of PdO to Pd under reaction conditions, which is affected by the temperature and the reaction mixture.[4] One possibility for avoiding this transformation is to disperse Pd already in the ionic form over an oxide support. Stabilization of precious metals as ionic moieties over reducible supports such as ceria (CeO 2 ) has been shown to be effective for several reactions, such as the water-gas shift reaction and total oxidation, [5] and the ability of ceria to stabilize Pd in a highly dispersed state is wellrecognized.[6] Insertion of the precious metal into the metal oxide lattice would lead to the highest degree of dispersion for a given metal loading, with important consequences in several catalytic applications. Isolated encapsulated Pd metal in ceria as a result of a strong metal-support interaction was reported in early studies of noble-metal / ceria systems. [6,7] Solid solutions based on PdO/CeO 2 of composition Ce 0.99 Pd 0.01 O 2Àd or Ce 0.76 Zr 0.19 Pd 0.05 O 2Àd were reported more recently and found to be active in CO/NO reaction and methane combustion; [8] this finding is also corroborated by recent density functional theory (DFT) calculations suggesting that insertion of Pd into CeO 2 surfaces provides a lower energy barrier for dissociative adsorption of methane.[9] However, stabilization of Pdsubstituted ceria is difficult, and Pd segregation out of the oxide to form PdO or metallic Pd crystallites is commonly observed at high temperatures.[8]Herein we report an ordered and stable Pd-O-Ce surface superstructure as revealed by DFT calculations on the basis of high-resolution (HR) TEM data. It results from a complex reconstruction of the (110) CeO 2 surface and leads to the opening of wide surface channels exposing highly undercoordinated oxygen atoms.We have prepared two Pd/CeO 2 catalysts by one-step solution combustion synthesis (SCS). The new catalysts contain between 1 and 1.71 wt % Pd and are denoted SCS1 and SCS2 (Table 1). We also prepared samples of conventional Pd/CeO 2 catalysts by incipient wetness impregnation (IWI). These catalysts were prepared from two different samples of commerc...

The structure and chemical nature of Pt in combustion-synthesized Pt/CeO2 catalysts have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and temperature-programmed reaction (TPR). Catalytic oxidation of CO over Pt/CeO2 is correlated with its structure. High-resolution XRD studies show that the structure could be refined for the composition of Ce1 - x Pt x O2 - δ in the fluorite structure with 6% oxide ion vacancy. TEM images show very few Pt particles on the CeO2 crystallite surface in as-prepared samples and a decrease in the density of Pt metal particles is observed on heating. XPS studies demonstrate that Pt is dispersed mostly in +2 (72%) and +4 (21%) oxidation states on CeO2, whereas only 7% is present as Pt metal particles. On heat treatment, Pt2+ species increase at the cost of Pt4+ ions. EXAFS studies show the average coordination number of 1.3 around the platinum ion in the first shell of 1% Pt/CeO2 at a distance of 1.98 Å, indicating oxide ion vacancy around the platinum ion. On heating, the average oxygen coordination of Pt and oxygen increases to 2.3. The second shell at 2.97 Å is due to Pt−Pt coordination, which is absent in PtO2 and PtO. The third shell at 3.28 Å is not observed either in Pt metal or any of the platinum oxides, which could be attributed to Pt2+−Ce4+ correlation. Thus, Pt/CeO2 forms a Ce1 - x Pt x O2 - δ type of solid solution having −□−Pt2+−O−Ce4+− kinds of linkages.

A 1% Pt/CeO2 catalyst prepared by the solution combustion method shows a higher catalytic activity for CO oxidation by O2 compared to Pt metal particles. At least six hydrogen atoms are taken up per Pt at −25 °C. The structure of 1% Pt/CeO2 catalyst has been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. Rietveld refinement shows that Pt ions are incorporated into the CeO2 matrix in the form of Ce1 - x Pt x O2 - δ solid solution. A decrease in oxygen content in 1% Pt/CeO2 is seen in relation to pure CeO2. TEM studies show that Pt is dispersed as atoms or ions and only a small amount as Pt metal particles. The Pt(4f) core level region in XPS shows that Pt is present mostly in the Pt2+ ionic state on CeO2 surface. FTIR of 1% Pt/CeO2 shows a strongly adsorbed CO peak at 2082 cm-1 corresponding to oxidized Pt. These structural studies show that Pt ions in the catalyst are substituted for Ce4+ ions in the form of Ce1 - x Pt x O2 - δ, creating oxide ion vacancies leading to a strong Pt2+−CeO2 interaction that is responsible for higher catalytic activity.

Ionically dispersed Rh over CeO2 in Rh/CeO2 catalysts prepared by a single step solution combustion method is shown to improve the redox property and catalytic activity. The H2/Rh ratio obtained from hydrogen uptake measurement was 5.4, 2.4, and 2.1, respectively in 0.5, 1, and 2% Rh/CeO2, indicating a significant contribution from the reduction of CeO2 in the presence of Rh. In 1% Rh/CeO2, the light-off temperature for CO oxidation is about 80 °C lower compared to Rh metal and 190 °C lower than that of Rh2O3. The enhanced redox property and CO oxidation activity of the catalyst has been correlated with the structure. The X-ray diffraction (XRD) pattern could be refined to the fluorite structure with Rh substituting in the Ce site. Transmission electron microscopy (TEM) images show only CeO2 crystallites of about 50 nm and no evidence of any metal particles up to 1 atom % Rh. X-ray photoelectron spectroscopy (XPS) studies demonstrate that Rh is dispersed in the +3 oxidation state on CeO2 with enhanced Rh ion concentration in the surface layers. An average coordination number of 2.5 at a distance of 2.05 Å in the first shell is obtained around Rh ions from extended X-ray absorption fine structure (EXAFS) spectroscopy, indicating an oxide ion vacancy around the Rh ion. The correlations at 2.72 and 3.16 Å correspond to Rh−Rh and Rh−Ce interactions, respectively. Thus, the enhanced catalytic activity of Rh/CeO2 is shown to be due to the formation of a Ce1 - x Rh x O2 - δ type of solid solution with −□−Rh3+−O−Ce4+− kind of linkages on the surface.

Nanocrystalline Ce(1)(-)(x)Ti(x)O(2) (0 < or = x < or = 0.4) and Ce(1-)(x)(-)(y)Ti(x)Pt(y)O(2)(-)(delta) (x = 0.15, y = 0.01, 0.02) solid solutions crystallizing in fluorite structure have been prepared by a single step solution combustion method. Temperature programmed reduction and XPS study of Ce(1)(-)(x)Ti(x)O(2) (x = 0.0-04) show complete reduction of Ti(4+) to Ti(3+) and reduction of approximately 20% Ce(4+) to Ce(3+) state compared to 8% Ce(4+) to Ce(3+) in the case of pure CeO(2) below 675 degrees C. The substitution of Ti ions in CeO(2) enhances the reducibility of CeO(2). Ce(0.84)Ti(0.15)Pt(0.01)O(2)(-)(delta) crystallizes in fluorite structure and Pt is ionically substituted with 2+ and 4+ oxidation states. The H/Pt atomic ratio at 30 degrees C over Ce(0.84)Ti(0.15)Pt(0.01)O(2)(-)(delta) is 5 and that over Ce(0.99)Pt(0.01)O(2)(-)(delta) is 4 against just 0.078 for 8 nm Pt metal particles. Carbon monoxide and hydrocarbon oxidation activity are much higher over Ce(1-)(x)(-)(y)Ti(x)Pt(y)O(2) (x = 0.15, y = 0.01, 0.02) compared to Ce(1)(-)(x)Pt(x)O(2) (x = 0.01, 0.02). Synergistic involvement of Pt(2+)/Pt degrees and Ti(4+)/Ti(3+) redox couples in addition to Ce(4+)/Ce(3+) due to the overlap of Pt(5d), Ti(3d), and Ce(4f) bands near E(F) is shown to be responsible for improved redox property and higher catalytic activity.

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