Over the past several years, cerium oxide and CeO2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO2-containing materials as oxidation and reduction catalysts is also presented.

Catalytic Properties of Ceria and CeO2-Containing Materials

Although technological practice should minimize environmental impact, this is not always economically feasible. During the past decade, for example, there has been increasing global concern over th...

CO2 Reforming of CH4

Superoxide dismutase mimetic properties exhibited by vacancy engineered ceria nanoparticles

Isotopic tracer and kinetic studies are used to probe the identity and reversibility of elementary steps required for H2O and CO2 reforming of CH4 on supported Pt clusters and to demonstrate a rigorous kinetic and mechanistic equivalence for CO2 and H2O reforming, CH4 decomposition, and water-gas shift reactions. Reforming rates are exclusively limited by C−H bond activation on essentially uncovered Pt crystallite surfaces and unaffected by the concentration or reactivity of co-reactants (H2O, CO2). Kinetic isotopic effects are consistent with the sole kinetic relevance of C−H bond activation (kH/kD = 1.58−1.77 at 873 K); these isotope effects and measured activation energies are similar for H2O reforming, CO2 reforming, and CH4 decomposition reactions. CH4/CD4 cross exchange rates are much smaller than the rate of methane chemical conversion in CO2 and H2O reforming reactions; thus, C−H bond activation steps are irreversible, except as required by the approach to equilibrium for the overall reforming rea...

http://iglesia.cchem.berkeley.edu/Publications/JPhysChemB_108_4094_2004.pdf

Mechanism and Site Requirements for Activation and Chemical Conversion of Methane on Supported Pt Clusters and Turnover Rate Comparisons among Noble Metals

The atomistic and electronic structures of oxygen vacancies on the (111) and (110) surfaces of ceria are studied by means of periodic density functional calculations. The removal of a neutral surface oxygen atom leaves back two excess electrons that are shown to localize on two cerium ions neighboring the defect. The resulting change of valency of these Ce ions (Ce4+ --> Ce3+) originates from populating tightly bound Ce 4f states and is modeled by adding a Hubbard U term to the traditional energy functionals. The calculated atomistic and electronic structures of the defect-free and reduced surfaces are shown to agree with spectroscopic and microscopic measurements. The preferential defect segregation and the different chemical reactivity of the (111) and (110) surfaces are discussed in terms of energetics and features in the electronic structure.

Electronic and Atomistic Structures of Clean and Reduced Ceria Surfaces

Direct conversion of methane into synthesis gas of H2/CO ratio of 2 has been demonstrated using CeO2 as an oxidant at 873–1073 K. The reaction was accelerated in the presence of Pt black (1 wt%). The reduced cerium oxide after the oxidation of methane can be used to convert carbon dioxide into carbon monoxide.

Partial Oxidation of Methane Using the Redox of Cerium Oxide

The oxidation of CH 4 with CeO 2 in the absence of gaseous oxygen (Step1) and subsequent reduction of CO 2 to CO or of H 2 O to H 2 (Step2) by the reduced cerium oxide, CeO 2−x , have been studied under atmospheric pressure at 673 to 1073K. The reaction of Step1 occurred at ≥873 K producing H 2 and CO with the ratio of 2. This synthesis gas was strongly suggested to be formed directly from CH 4 . Addition of Pt black (1 wt%) to CeO 2 remarkably enhanced this reaction. The reaction of Step2 proceeded at≥673K for both CO 2 and H 2 O. When the degree of reduction of the oxide in Step1 had been adjusted to ca. 10%, both CO 2 and H 2 O were stoichiometrically converted to CO and H 2 , respectively. If the degree of the reduction exceeded 10% in Step1, the carbon deposited on the CeO 2−x strongly retarded the formation of CO from CO 2 in Step2. However, the decomposition of H 2 O was not affected. Thus, it is suggested that the reoxidation of the reduced oxide (Step2) should be operated with H 2 O.

Tetsuya Ushiyama

Papers

Partial Oxidation of Methane Using the Redox of Cerium Oxide

The production of synthesis gas by the redox of cerium oxide

Electrocatalytic synthesis of propylene oxide during water electrolysis