Ce Mixed Oxides Research Articles

Synergy of Fe xCe 1− xO 2 mixed oxides for N 2O decomposition

Fe–Ce mixed oxides prepared by coprecipitation showed considerable synergy in N 2O decomposition when compared with pure metal oxide counterparts. The mixed system also displayed higher stability in reaction at high temperature. Through characterisation by XRD, XPS and TPR, the activity–stability improvement is ascribed to the formation of Fe-doped ceria and Ce-doped hematite and also to increased surface area. The intimate interaction in the mixed phases results in easier reduction. Thus the release of oxygen, which is the rate-determining step in this N 2O decomposition, becomes more facile.

Spectroscopic Characterization of Heterogeneity and Redox Effects in Zirconium−Cerium (1:1) Mixed Oxides Prepared by Microemulsion Methods

Two Zr−Ce (1:1 molar ratio) mixed oxide specimens, made by the same microemulsion method from different cerium precursor salts (samples ZC1 and ZC2), have been studied by a combination of physicochemical techniques. After calcination at 773 K (high surface area materials HS: SBET = 96 ± 1 m2 g-1) both samples present similar characteristics in XRD, Raman and TEM (pseudocubic phase t‘ ‘), and XPS/Ar+-etching experiments. This latter evidence for those materials gives a similar moderate surface enrichment in cerium and a surface anion vacancy concentration (judged from O(1s) peak shifts) lower than in CeO2; the latter effect suggests an easy diffusion of vacancies to bulk or subsurface regions. Only EPR of adsorbed superoxide species detects a difference between both samples, evidencing in ZC2−HS the presence of small, more reducible Ce-rich bidimensional patches over a Zr-richer substrate. According to XPS/Ar+ etching profiles, by calcination at 1173 K (LS materials; SBET < 8 m2 g-1), surface segregation o...

New Pd/CexZr1−xO2/Al2O3 three-way catalysts prepared by microemulsion: Part 2. In situ analysis of CO oxidation and NO reduction under stoichiometric CO+NO+O2

The light-off behavior for CO oxidation and NO reduction under stoichiometric CO+NO+O2 gas mixtures of a series of palladium catalysts supported on ceria–zirconia, ceria–zirconia/alumina and alumina has been examined by means of catalytic activity tests, and by DRIFTS, XANES and EPR spectroscopies. The results show the promoting effect of the Zr–Ce mixed oxide on both reactions. The extent of promotion depends on the characteristics of the promoter entities present for each catalyst and on the nature of the reaction. As observed in the absence of NO, CO oxidation was mainly enhanced by contact between Pd and 3D aggregated promoter entities, although an inhibiting effect of NO on CO oxidation, attributed to passivation of the interface sites due to formation of oxidized states of Pd, was observed at low temperatures. In contrast, the higher activity for NO reduction over catalysts supported on ceria–zirconia/alumina suggests that most active sites for that reaction involve Pd interacting with dispersed promoter entities.

EPR study on oxygen handling properties of ceria, zirconia and Zr–Ce (1 : 1) mixed oxide samples

The characteristics of ceria, zirconia and Zr–Ce (1 : 1 molar ratio) samples prepared by using inverse microemulsions have been studied to obtain information on the modifications produced in the oxygen handling properties of the single oxides by the formation of the mixed oxide. The structural characteristics of the samples are compared on the basis of XRD and Raman spectroscopy data, which show that the Zr–Ce sample forms a homogeneous mixed oxide structure corresponding to the (pseudocubic) tetragonal phase t″ while CeO2 presents, as expected, a cubic fluorite structure and ZrO2 is formed mainly by a tetragonal phase with only traces of the thermodynamically most stable monoclinic phase. Evaluation by EPR of the type and intensity of superoxide radicals generated upon oxygen adsorption on the outgassed samples reveals a higher surface reducibility in the mixed oxide sample, associated vacancies defects being generated on it in higher amounts and at lower temperatures than on pure ceria. It is also shown that both Ce and Zr ions participate in the oxygen chemisorption process on the mixed oxide sample, actually giving rise to much larger amounts of O2 -–Zr4+ species than the pure ZrO2 sample; the latter species show a higher thermal stability than O2 -–Ce4+ species.

Electrocatalytic properties of Ru + Ti + Ce mixed oxide electrodes for the Cl 2 evolution reaction

A series of ternary oxides of nominal composition Ru 0.3Ti (0.7 − x) Ce x O 2 was prepared by thermal decomposition on Ti supports and used as electrodes to study the effect of substituting Ce for Ti on the kinetics and electrocatalysis of the Cl 2 evolution reaction. The surface properties were evaluated by cyclic voltammetry. Integration of the voltammetric curves provided the voltammetric charge, q ∗ , which is proportional to the active surface area. Kinetic data showed that Cl 2 evolution occurs with a single mechanism over the whole explored potential range and at all electrode compositions. Fractional reaction orders with respect to both Cl − and H + revealed double-layer effects attributed to the occurrence of Cl − specific adsorption. The apparent activity increased with CeO 2 content, whereas the true electrocatalytic properties, estimated by normalizing the current to unit surface charge, decreased in the presence of CeO 2 with a minimum of 10% CeO 2. The presence of CeO 2 also makes the oxide layers more prone to mechanical erosion by the evolving gas bubbles.

Kinetic and structural studies of oxygen availability of the mixed oxides Pr1–xMxOy (M = Ce, Zr)

One composition of Pr–Ce mixed oxide and a range of compositions of Pr–Zr mixed oxide were prepared by coprecipitation methods and characterized by x-ray powder diffraction, thermogravimetric analysis, and x-ray photoelectron spectroscopy. Based on phases formed, the PrOy—ZrO2 system in an oxygen-containing atmosphere at moderate temperatures (up to 800–1000 °C) is analogous to that of CeO2–ZrO2. Addition of either Ce or Zr to pure Pr oxide affects both the total amount of oxygen that can be reversibly exchanged between oxide and gas phase and the kinetics of the redox processes. Ce dramatically increases the amount (per Pr atom) and lowers the temperature of exchange, Zr slightly decreases the amount and also lowers the temperature of exchange, and both modifiers speed up the rate. These observations are rationalized in terms of bulk and surface structural features of the mixed oxides.