In a time of growing need for catalysts, perovskites have been rediscovered as a family of catalysts of such great diversity that a broad spectrum of scientific disciplines have been brought to bear in their study and application. Because of the wide range of ions and valences which this simple structure can accommodate, the perovskites lend themselves to chemical tailoring. It is relatively simple to synthesize perovskites because of the flexibility of the structure to diverse chemistry. Many of the techniques of ceramic powder preparation are applicable to perovskite catalysts. In their own right, they are therefore of interest as a model system for the correlation of solid-state parameters and catalytic mechanisms. Such correlations [See figure in the PDF file] have recently been found between the rate and selectivity of oxidation-reduction reactions and the thermodynamic and electronic parameters of the solid. For commercial processes such as those mentioned in the introduction, perovskite catalysts have not yet proven to be practical. Much of the initial interest in these catalysts related to their use in automobile exhaust control. Current interest in this field centers on noble metalsubstituted perovskites resistant to S poisoning for single-bed, dual-bed, and three-way catalyst configurations. The formulations commercially tested to date have shown considerable promise, but long-term stability has not yet been achieved. A very large fraction of the elements that make up presently used commercial catalysts can be incorporated in the structure of perovskite oxides. Conversely, it is anticipated that perovskite oxides, appropriately formulated, will show catalytic activity for a large variety of chemical conversions. Even though this expectation is by no means a prediction of commercial success in the face of competition by existing catalyst systems, it makes these oxides attractive models in the study of catalytic chemical conversion. By appropriate formulation many desirable properties can be tailored, including the valence state of transition metal ions, the binding energy and diffusion of O in the lattice, the distance between active sites, and the magnetic and conductive properties of the solid. Only a very small fraction of possible perovskite formulations have been explored as catalysts. It is expected that further investigation will greatly expand the scope of perovskite catalysis, extend the understanding of solid-state parameters in catalysis, and contribute to the development of practical catalytic processes.

https://science.sciencemag.org/content/sci/195/4281/827.full.pdf

Perovskite oxides: materials science in catalysis.

Publisher Summary This chapter discusses the structure and reactivity of perovskite-type oxides. Perovskite-type oxides have the general formula ABO 3 (A, cation of larger size) and are structurally similar to CaTiO 3 , the mineral that gave its name to that group of compounds. These materials are first studied because of their important physical properties such as ferro-, piezo-, and pyroelectricity, magnetism and electrooptic effects. The most numerous and most interesting compounds with the perovskite structure are oxides. Some hydrides, carbides, halides, and nitrides also crystallize with this structure. The chapter reviews only the study of oxides and their behavior in the gas solid interface and in heterogeneous catalysis. An important characteristic of perovskites, mentioned in the chapter, is their susceptibility of partial substitution in both A and B positions. This provides a wealth of isomorphic compounds that can easily be synthesized. Given the extensive range of possibilities in the tailoring of their chemical and physical properties, there is no doubt that new reactions can be studied, where these oxides can participate as catalytic agents.

Structure and Reactivity of Perovskite-Type Oxides

Oxidation catalysis of perovskites --- relationships to bulk structure and composition (valency, defect, etc.)

Disclosed is a permanent magnet field-type rotating machine of a high efficiency having an internal magnet-type rotor. The rotating machine comprises an internal magnet-type rotor having a plurality of permanent magnets embedded in a rotor yoke, and a stator disposed in opposed, spaced relationship to the rotor, with an air gap formed between the rotor and the stator. The rotor yoke comprises a member in which ferromagnetic regions and non-magnetic regions coexist, and the non-magnetic regions are disposed respectively at those positions of the rotor yoke where leakage magnetic flux is liable to be generated.

Permanent magnet field-type rotating machine

Nonmagnetized permanent magnet material is heated in a pattern by a laser beam to a localized temperature above the Curie point or a temperature sufficient to reduce the coercivity sufficiently for an external field to magnetize the pattern in the direction of the field. Magnets so produced can have very high pole density, digital encoding and analog patterns having gradually varying local field strength. Alternating pole magnets of this type can be used for stepper motor magnets.

Magnetically coded device

Magnetically hard compositions having high values of coercivity, remanence and energy product contain rare earth elements, transition metal elements and boron in suitable proportions. The preferred rare earth elements are neodymium and praseodymium, and the preferred transition metal element is iron. The magnetic alloys have characteristic very finely crystalline microstructures.

High energy product rare earth-iron magnet alloys

This invention relates to permanent bonded magnets of very finely crystalline, melt-spun, rare earth-iron alloys. The compacts are magnetically isotropic.

Bonded rare-earth-iron-magnets.

In a preferred embodiment thin layers of rare earth metal-cobalt powder comprising an outer layer of RCo 5 and a discrete transitional layer of R 2 Co 17 are compressed against an adjacent thicker layer of iron powder to form a laminated green compact, with the rare earth-cobalt powdered material being magnetically aligned. The green laminate body is then sintered to densify the rare earth-cobalt material layer to a body wherein the pores are substantially noninterconnecting. The laminate structure is then magnetized. By this method a strong rare earth-cobalt (RCo 5 ) permanent magnet body is produced in which the rare earth-cobalt layer may be very thin (of the order of 1 to 2 millimeters) but of relatively large surface area and supported by a strong iron layer so as to be durable in handling, manufacturing and use. It is possible to press an RCo 5 layer directly onto a powdered iron layer when the oxygen content of the iron is suitably low. The method will also produce an R 2 Co 17 -iron composite.

Method of making a laminated rare earth metal-cobalt permanent magnet body

An amorphous rare-earth transition metal body is provided that has integral, predetermined regions of hard and soft magnetism. A method is provided for heating portions of an amorphous rare-earth transition metal alloy body, in situ, to transform predetermined portions from a state of low magnetic coercivity to a highly magnetically coercive state.

John J. Croat

Papers

High energy product rare earth-iron magnet alloys

Bonded rare-earth-iron-magnets.

Method of making a laminated rare earth metal-cobalt permanent magnet body

Selective magnetization of rare-earth transition metal alloys

Lanthanum Lead Manganite Catalyst for Carbon Monoxide and Propylene Oxidation