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

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

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

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

Cobalt and bimetallic Co-Pd systems are well-known Fischer-Tropsch catalysts. Compared to Co, the bimetallic systems exhibit an increased activity toward CO hydrogenation and methane conversion, attributed to resistance against oxidation. To study the oxidation behavior, model catalysts have been generated by depositing either Co or first Co and subsequently Pd onto a thin epitaxial alumina film grown on NiAl(110). Pure Co particles and bimetallic particles with a Co core and a Pd shell have been studied before and after exposure to oxygen and after thermal treatments, using X-ray photoelectron spectroscopy (XPS), temperatureprogrammed CO desorption (TPD), ferromagnetic resonance (FMR), and infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum. Large doses of O2 (1000 langmuirs; 1 langmuir ) 10 -6 Torr‚s) at 300 K lead to complete oxidation of Co particles. Upon annealing to temperatures above 530 K, XPS indicates that the cobalt oxide is mostly reduced by transfer of oxygen to the alumina support, resulting in its thickening. TPD, however, indicates the existence of persistent surface oxygen species, reducing the CO adsorption energy on the particles. Exposures to small doses of O2 (30-50 langmuirs) were also studied by a careful comparison of XPS, TPD, and FMR data. In this case, XPS indicates Co in a metallic state, whereas TPD and FMR indicate oxidic behavior. We conclude that small amounts of non-stoichiometric subsurface oxygen or subsurface and surface oxygen are present which are not detectable in the Co 2p XPS signal but have a pronounced effect on the surface chemistry and the magnetism, i.e., on certain bulk properties. In the case of bimetallic Co/Pd particles, an incomplete Pd shell on the Co particles even promotes oxygen uptake, while only a complete Pd shell inhibits oxidation.

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Oxidation of Alumina-Supported Co and Co−Pd Model Catalysts for the Fischer−Tropsch Reaction

The effect of chromium on the structural and catalytic properties was investigated for Pt-Cr/ZSM-5 system. From TPR measurements it is seen that the reduction profiles were modified by the addition of Cr to the Pt/ZSM-5. Chemissorption data showed that Cr promoted the Pt dispersion. The propane and cyclohexane reactions showed higher activity and aromatic selectivity for the bimetallic samples when compared to the single Pt/ZSM-5 system. Unlike the Pt/ZSM-5 catalyst at higher cyclohexane conversions the hydrogenolysis did not occur on the bimetallic system, and this behavior increases the efficiency by eliminating undesirable parallel reactions. The Pt–Cr interaction seems to be favored and not only prevents the migration of Pt particles to the external zeolite surface but also weakens the Pt interaction with reactant intermediates, inhibiting hydrogenolysis

Pt-Cr/ZSM-5 catalysts for propane and cyclohexane conversions

Catalytic combustion of methane over palladium alumina modified by niobia

Cr/SiO2 catalysts were prepared by impregnation of chromium acetate (1 and 3 wt.% Cr) and bis(triphenylsilyl)chromate (0.5 wt.% Cr) over commercial silica. These samples were analyzed by temperature programmed reduction (TPR) and infrared spectroscopy (FT-IR) of CO and C2H4 adsorbed in order to characterize the chromium species, mainly Cr2+ species distribution (Cr2+A, Cr2+B and Cr2+C) and polymerization aspects. The catalysts prepared with chromium acetate presented only Cr6+ after calcination, while the catalyst prepared with bis(triphenylsilyl)chromate presented Cr6+ and a fraction of Cr3+. After reduction with CO, the catalysts prepared with chromium acetate showed more Cr2+B than Cr2+A species. The distribution of Cr2+ species influenced the catalytic activity. The catalyst with 3 wt.% Cr from acetate presented the greatest amount of species Cr2+A and Cr2+B and the best polymerization activity. The behavior of isolated hydroxyls band during polymerization tests, characterized by FT-IR, suggests its participation in the reaction mechanism.

Characterization of Cr2+ and ethylene polymerization on Cr/SiO2 catalysts

The CO2 reforming of methane was studied over Pt supported on Al2O3, ZrO2, and x % ZrO2/Al2O3 (1 ≤ x ≤ 20 wt %). The Pt/Al2O3 deactivated very quickly during 20 h onstream at 1073 K and a CH4/CO2 ratio of 1:1, while the catalysts with a ZrO2 content above 5 wt % presented improved stability during 60 h. Temperature-programmed oxidation studies showed that the amount of carbon on Pt/Al2O3 is much larger than that on zirconia-containing catalysts. Deactivation is attributed to carbon formation surrounding the metal−support perimeter. The high stability of zirconia-based catalysts is probably due to strong Pt−Zrn+ interactions, which reduce carbon formation during the reaction by promoting CO2 dissociation. High ratios of CH4/CO2 were also used in an attempt to accelerate deactivation, but even under these severe deactivation conditions, the catalyst with 10 wt % ZrO2 exhibited excellent stability.

Martin Schmal

Papers

Oxidation of Alumina-Supported Co and Co−Pd Model Catalysts for the Fischer−Tropsch Reaction

Pt-Cr/ZSM-5 catalysts for propane and cyclohexane conversions

Catalytic combustion of methane over palladium alumina modified by niobia

Characterization of Cr2+ and ethylene polymerization on Cr/SiO2 catalysts

Coke formation on Pt/ZrO2/Al2O3 catalysts during CH4 reforming with CO2