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

Recent worldwide interest in building a decentralized, hydrogen-based energy economy has refocused attention on the solid oxide fuel cell (SOFC) as a potential source of efficient, environmentally friendly, fuel-versatile electric power. Due to its high operating temperature, the SOFC offers several potential advantages over polymer-based fuel cells, including reversible electrode reactions, low internal resistance, high tolerance to typical catalyst poisons, production of high-quality waste heat for (among other uses) reformation of hydrocarbon fuels, as well as the possibility of burning hydrocarbon fuels directly. Today, SOFCs are much closer to commercial reality than they were 20 years ago, due largely to technological advances in electrode material composition, microstructure control, thin-film ceramic fabrication, and stack and system design. These advances have led to dozens of active SOFC development programs in both stationary and mobile power and contributed to commercialization or development in a number of related technologies, including gas sensors,1 solid-state electrolysis devices,2 and iontransport membranes for gas separation and partial oxidation.3 Many reviews are available which summarize the technological advances made in SOFCs over the last 15-35 yearssreaders who are primarily interested in knowing the state-of-the art in materials, design, and fabrication (including the electrodes) are encouraged to consult these reviews.4-12 This review focuses on the factors governing SOFC cathode performancesadvances we have made over † Dedicated to Brian Steele, 1929-2003. Researcher, Entrepreneur, Consensus Seeker. 4791 Chem. Rev. 2004, 104, 4791−4843

Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes

Physical, chemical and electrochemical properties of pure and doped ceria

The direct electrochemical oxidation of dry hydrocarbon fuels to generate electrical power has the potential to accelerate substantially the use of fuel cells in transportation and distributed-power applications1. Most fuel-cell research has involved the use of hydrogen as the fuel, although the practical generation and storage of hydrogen remains an important technological hurdle2. Methane has been successfully oxidized electrochemically3,4,5,6, but the susceptibility to carbon formation from other hydrocarbons that may be present or poor power densities have prevented the application of this simple fuel in practical applications1. Here we report the direct, electrochemical oxidation of various hydrocarbons (methane, ethane, 1-butene, n-butane and toluene) using a solid-oxide fuel cell at 973 and 1,073 K with a composite anode of copper and ceria (or samaria-doped ceria). We demonstrate that the final products of the oxidation are CO2 and water, and that reasonable power densities can be achieved. The observation that a solid-oxide fuel cell can be operated on dry hydrocarbons, including liquid fuels, without reforming, suggests that this type of fuel cell could provide an alternative to hydrogen-based fuel-cell technologies.

Direct oxidation of hydrocarbons in a solid-oxide fuel cell

Cerium dioxide (CeO2, ceria) is becoming an ubiquitous constituent in catalytic systems for a variety of applications. 2016 sees the 40th anniversary since ceria was first employed by Ford Motor Company as an oxygen storage component in car converters, to become in the years since its inception an irreplaceable component in three-way catalysts (TWCs). Apart from this well-established use, ceria is looming as a catalyst component for a wide range of catalytic applications. For some of these, such as fuel cells, CeO2-based materials have almost reached the market stage, while for some other catalytic reactions, such as reforming processes, photocatalysis, water-gas shift reaction, thermochemical water splitting, and organic reactions, ceria is emerging as a unique material, holding great promise for future market breakthroughs. While much knowledge about the fundamental characteristics of CeO2-based materials has already been acquired, new characterization techniques and powerful theoretical methods are dee...

Fundamentals and Catalytic Applications of CeO2-Based Materials

Solid oxide fuel cells (SOFC) have much promise as efficient devices for the direct conversion of the energy stored in chemical fuels into electricity. The development of highly robust SOFC that can operate on a range of fuels, however, requires improvements in the electrodes, especially the anode, where nanoscale engineering of the structure is required in order to maximize the number of sites where the electrochemical reactions take place. In this article we review the approaches that are currently being used to improve anode performance and microstructure with a focus on new materials and synthesis techniques.

Nanostructured anodes for solid oxide fuel cells

Evidence for Oxidation of Ceria by CO2

Design parameters for temperature-programmed desorption from a packed bed

Differential steam-reforming rates were measured for methane, ethane, n -butane, n -hexane, 2,4-dimethylhexane, n -octane, cyclohexane, benzene, and toluene over 1 wt.% Pd/ceria between 620 and 770 K and were compared to rates observed on 1 wt.% Pd/alumina and 1 wt.% Pt/ceria. The H 2 O:C ratios at which stable rates were observed on Pd/ceria depended on the hydrocarbon, varying from less than 1:1 for methane to 3:1 for 2,4-dimethylhexane. In the series of alkanes from methane to n -hexane, for a H 2 O:C ratio of 2:1, the only products were CO, CO 2 , and H 2 O and the rate of formation of CO 2 and CO increased with carbon number. Benzene was also selectively converted to CO x and H 2 , but at lower rates than the alkanes with the exception of methane. For 2,4-dimethylhexane and n -octane, a significant amount of cyclohexane was observed in the initial products and cyclohexane was selectively converted to benzene at low conversions. Steam reforming of toluene also gave benzene as the initial product. FTIR measurements of Pd/ceria under methane-steam-reforming conditions indicate the formation of carbonates on the ceria. Pd/alumina showed significantly lower rates than Pd/ceria and much lower CO 2 :CO ratios in the initial products, implying ceria plays an important role in the reaction. Pt/ceria gave rates and selectivities that were essentially identical to that of Pd/ceria. The implications of these results for hydrocarbon fuel processing are discussed.

R.J. Gorte

Papers

Nanostructured anodes for solid oxide fuel cells

Evidence for Oxidation of Ceria by CO2

Design parameters for temperature-programmed desorption from a packed bed

A study of steam reforming of hydrocarbon fuels on Pd/ceria

Study of CO Oxidation Kinetics on Rh/Ceria