The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

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

Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.

Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

II. Structure of Perovskites 1982 A. Crystal Structure 1982 B. Nonstoichiometry in Perovskites 1983 1. Oxygen Nonstoichiometry 1983 2. Cation Nonstoichiometry 1984 C. Physical Properties 1985 D. Adsorption Properties 1986 1. CO and NO Adsorption 1986 2. Oxygen Adsorption 1987 E. Specific Surface and Porosity 1987 F. Thermal Stability in a Reducing Atmosphere 1989 III. Acid−Base and Redox Properties 1990 A. Acidity and Basicity 1990 B. Redox Processes 1991 1. Kinetics and Mechanisms 1992 2. Reduction−Oxidation Cycles 1993 C. Ion Mobility 1993 1. Oxygen Transport 1993 2. Cation Transport 1994 IV. Heterogeneous Catalysis 1995 A. Oxidation Reactions 1995 1. CO Oxidation 1995 2. Oxidation of Hydrocarbons 1996 B. Pollution Abatement 2001 1. NOx Decomposition 2001 2. Exhaust Treatment 2002 3. Stability 2004 C. Hydrogenation and Hydrogenolysis Reactions 2004 1. Hydrogenation of Carbon Oxides 2004 2. Hydrogenation and Hydrogenolysis Reactions 2006

Chemical structures and performance of perovskite oxides.

Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels.

The activity, long-term stability and the reasons for deactivation of a nanosize gold- supported Au/TiO2 in CO oxidation were investigated. Characterization of the catalyst sample was made by XPS, FTIR, TEM, BET, “depletive” oxidation and TPD instrumental methods. In spite of the experimentally proved very high activity at temperatures below 213 K, the catalyst exhibited a gradual decrease in initial activity. Two main reasons for the catalyst deactivation were found: (i) capability to adsorb CO and accumulate it as carbonates, this deactivation is reversible and after heating the catalyst surface is restored by CO2 evolution; (ii) agglomeration of Au particles, which causes irreversible, however weak deactivation.

Activity and deactivation of Au/TiO2 catalyst in CO oxidation

Preparation and characterization of a higher cobalt oxide

The catalytic activities of NiMnO3 and NiMn2O4 during heterogeneous catalytic decomposition of ozone and ozone-catalytic oxidation (OZCO) of benzene at low temperatures (20–80°C) have been investigated. The sensitivity of the two oxides towards strong catalytic poisons, such as nitrogen oxides, during the decomposition of ozone has also been estimated. On the basis of the experimental results obtained it is concluded that the NiMnO3 and NiMn2O4 obtained have a high activity with respect to the reactions of ozone decomposition and CO and CH oxidation in the presence of ozone at temperatures close to the room temperature. The sample with an ilmenite structure shows, in all cases, a higher catalytic activity. The surface oxygen of NiMnO3 is more reactive at room temperature than is the case of NiMn2O4. The hypothesis according to which when the two metal cations are in octahedral coordination the catalyst activity is higher and the stability towards catalytic poisons is enhanced has proved to be correct. It should be noted that a catalyst has been synthesized which is able to decompose ozone at room temperature and to activate the organic molecule to a degree permitting catalytic oxidation by ozone at room temperature. In addition, this catalyst shows a relatively high stability with respect to poisoning by nitrogen oxides.

Ozone decomposition, benzene and CO oxidation over NiMnO3-ilmenite and NiMn2O4-spinel catalysts

The kinetics of complete catalytic oxidation of benzene by ozone on MnO2 in gradientless conditions has been investigated. It was established that the decomposition of ozone and the oxidation of benzene on MnO2 proceed within the same temperature range (10–80 °C ), and the apparent activation energies are 32 kJ/mol and 30 kJ/mol, respectively. The complete oxidation of benzene by molecular oxygen takes place at temperatures above 160°C with an apparent activation energy of 88 kJ/mol. It has been assumed that the rate-determining step of complete oxidation by ozone is the ozone decomposition.

Complete oxidation of benzene on manganese dioxide by ozone

An alumina-supported cobalt oxide system with overstoichiometric oxygen (CoO x /Al 2 O 3 ) was investigated with respect to heterogeneous catalytic decomposition of ozone, complete oxidation of volatile organic compounds (VOCs) and oxidation of CO. The experiments were performed in the temperature range of −45 to 250 °C in an isothermal plug-flow reactor. The characterization of the CoO x /Al 2 O 3 catalyst was performed by chemical analysis, XPS, XRD, IR techniques, magnetic and adsorption measurements. A very high activity of the catalyst towards ozone decomposition was observed even at temperatures below −40 °C, the catalyst remaining active for a long time. The activity of the catalyst with respect to complete oxidation of VOCs and oxidation of carbon monoxide was studied in presence of different oxidizing agents (ozone or oxygen). A significant increase in the catalytic activity and decrease in the reaction temperature was observed using ozone as an oxidant. Two main reasons for this behaviour were found: (i) the high content of active and mobile oxygen obtained during the synthesis on the catalyst surface; (ii) the catalytically active complex of O − [Co 4+ ], formed during the reaction of ozone decomposition and able to oxidize VOCs at room temperature.

Dimitar Mehandjiev

Papers

Activity and deactivation of Au/TiO2 catalyst in CO oxidation

Preparation and characterization of a higher cobalt oxide

Ozone decomposition, benzene and CO oxidation over NiMnO3-ilmenite and NiMn2O4-spinel catalysts

Complete oxidation of benzene on manganese dioxide by ozone

Catalytic oxidation of VOCs and CO by ozone over alumina supported cobalt oxide