GLASS AND CERAMIC APPLICATIONS
The most efficient polishing agent, by far, for most glass compositions (particularly those produced commercially in large volume) is cerium oxide. This application consumes, either as a moderately pure oxide or as a cerium-oxide-dominated concentrate, a significant portion of the cerium products produced annually. Commercial glass polishes are based on cerium oxide powders with defined particle sizes and controlled dispersibility in aqueous systems. Polishing is the act of producing a sufficient degree of surface smoothness so that light, transmitted or reflected at that surface, is not disturbed by surface irregularities. The polishing mechanism is still poorly understood at the chemical/molecular level. Polishing results in glass removal and does show a dependence on chemical properties of the glass. The nature of the liquid present during polishing is crucial and only if active hydroxyl groups are present, in alcohols forexample but especially in water, does the polishing phenomenon happen. When a glass – typically an alkali silicate – is in contact with water, a complicated series of steps take place, ion exchange, dissolution of glass constituents and possible structural changes. A surface region of the glass is modified and it is this softer hydrated layer that is removed or reformed during glass polishing. Classic abrasives produce an improvement in surface finish but leave a fine but definite roughness, the scale of which relates to the grain size of abrasive used. Several not-so-hard oxides are reasonable polishing agents and can remove and/or reform the soft hydrated layer. In general, optimum polishing rate coincides approximately with Moh’s hardness for the polish of around 6.5, very close to the hardness of most glasses.
The best polishing agent – as regards rate of glass removal and ultimate surface finish – is cerium oxide slurried in water. This oxide contains the potential polyvalent cerium atom and redox reactions, due to C (IV)/Ce(III), may well be providing chemical assistance to the breaking up of the silicate lattice. In addition mobility within the hydrated layer around the ceric ion also plays a role. Certain substances can act as accelerators for the polishing process. The best appears to be Ce(OH)4, i.e. CeO2.2H2O, precipitated fresh, in situ, in the polishing slurry from a soluble Ce(IV)salt, that probably is involved in an equilibrium reaction.
SiO2 + Ce(OH)4 CeO2 + Si(OH)4
In addition, transient formation of [..Ce-O-Si..] complexed groupings has been suggested. Analytical techniques have identified Ce atoms incorporated below the final polished surface, perhaps from such intermediates. The breaking, and reforming, of Si-O bonds is probably aided by the transfer of OH groupings to incipient fracture sites by a transport mechanism using the relatively large and mobile coordination sphere around the oxophilic cerium atom.
The cerium concentrate (predominantly cerium oxide), derived from bastnasite, is an excellent polish base. In addition the oxide derived directly from the natural-ratio “rare-earth” chloride, as long as the cerium oxide content is near, or above, 50 %, provides an adequate glass polish; the polishing activity is better than the CeO2/LnO ratio would suggest. Materials, prepared prior to any Ln purification steps, are the source for the lowest cost polishes available that are used to treat TV face plates, mirrors and the like. For precision optical polishing the higher purity materials, based on separated cerium oxides, are preferred.
A major use for cerium compounds is the decolorization of glass. The dominant glass composition, measured by tonnage produced, is the soda-lime type made from cheap raw materials. The purity of these starting ingredients is crucial in determining finished color, or absence of color, in the finished product. There is unfortunately one common impurity, iron, particularly in silica sand, that can cause problems. Iron oxide is a moderately strong colorant in glass and as little as 0.01 % can be visually detected. The adverse contribution of iron to color is associated with the spectral absorption of the ferric and ferrous ions although the two iron ions do not contribute equally to discoloration, the ferrous component providing a blue-greenish tinge with nearly ten times as strong a discoloring power as the brown-yellow of the ferric ion. Glass can be decolorized, even if the total iron content cannot be changed, by keeping iron in the Fe3+ state by addition of Ce4+ to the glass bath.
Cerium (IV) is strong enough to oxidize iron (II) to iron (III) and cerium ions, either as (III) or (IV), are stable under the severe conditions of a molten silicate-glass bath. Furthermore, cerium itself has no absorption in the visible region. Economical additions of cerium, as the cheapest form of cerium – cerium concentrate, convert iron to the low-absorption ferric form and raw materials containing trace quantities of iron can thus be used more efficiently.
Most damage caused by light to exposed materials is due to u.v. radiation in the 300 – 400 nm range. The mechanism of photodegradation is complex and depends on material composition, impurities present etc. One approach to inhibit the degradation process is to try and screen out thedamaging radiation by incorporating some component that will absorb those wavelengths. Cerium provides strong absorption below 400 nm caused by charge transfer bands. Cerium(IV) in particular makes glass opaque to near u.v. radiation but shows no absorption in the visible and cerium (III) also shows u.v. absorbing behavior but somewhat less marked than for cerium(IV). The ability of cerium-doped glass to block damaging u.v. radiation has applications in several areas, e.g. medical glassware and display case glass windows. The photostability of pigments can be enhanced by surface additives that increase the provision of recombination centers for the photoproduced charge carriers that would otherwise cause chemical damage. The surface additive must be a one-electron redox couple and be present in both valence states. Cerium, through the Ce(III) / Ce(IV) interconversion, can provide this protective process as well as that attributable to direct u.v. absorption.
Cerium is used, for example, to provide pigments with light fastness and to prevent clear polymers from darkening in sunlight. The rate at which pigments, such as yellow lead chromates, darken on exposure to light can be reduced by producing a precipitated cerium salt coating on the pigment particles. Titanium dioxide, the dominant white pigment, can be used in two differentcrystalline forms, anatase and rutile. Both these can be given added u.v. stability by arranging for cerium oxide coatings on the TiO2 particles. This is particularly useful when otherwise the photosensitivity of the titania could initiate degradation of the carrying substrate.
Radiation -resistant Glass
Television glass faceplates are subjected to electron bombardment by high energy electrons, particularly with the high tube voltages needed for color displays. This bombardment, over time, tends to cause discoloration, or browning, of the glass due to the creation of color centers. This unwanted effect is suppressed by the addition of up to 1 % or so of cerium oxide to that glass. A similar suppression of gamma-ray induced discoloration is also possible and cerium-containing glasses are used in the construction of viewing windows in “hot-cells” in the nuclear industry. The suppression mechanism in both instances is believed to depend on the presence of both Ce3+ and Ce4+ ions within the glass lattice and the potential redox involvement of those ions in “mopping-up” both positive hole centers and free electrons.
This type of glass contains cerium and, on exposure to strong light, will develop a latent image that can, in a later step, be converted into a permanent structural or color change. The cerium ions can absorb ultra-violet radiation and release electrons into the glass matrix. Heat treatment causes these electrons to migrate to silver ions, also present, that initially form silver specks that in turn nucleate the crystal growth of other compounds within the glass. Highly detailed patterns can be produced that are not only decorative but also can help create masks, spacers and the like for electronic uses.
Cathode Ray Tubes
The cerium atom, upon excitation by energetic cathode-ray electrons, produces a characteristic emission (luminescence) that is in the blue to ultra-violet region, the precise wavelength depending on the symmetry and nature of the ions immediately surrounding the Ce atom in the host lattice. (In some crystalline lattices radiation of a longer wavelength is possible.) The Ce3+ emission is highly efficient, is broad-band in character and corresponds to a 5d – 4f transition. Because the transition is allowed, the emitting energy level has a very short lifetime (≈ 50ns) and the luminescence decays very rapidly. This property underlies the use of some cerium containing phosphors in specialized CRT applications.
Beam-indexing display tubes rely on a single electron beam to excite consecutively the color phosphors. The necessary feedback, to ensure that the beam excites the correct color phosphor, of the beam’s actual position to the control circuitry is supplied by beam-indexing phosphors that emit detectable ultra-violet when struck by the electron beam. Such a phosphor needs a high efficiencyand an extremely fast decay time; the preferred material is cerium-doped (≈2 atomic %) yttrium di-orthosilicate, Ce:Y2Si2O7, with a peak emission at 380nm.
A flying-spot scanner uses a phosphor to produce a point of light whereby a transparent piece of film or a slide can be imaged and the modulated transmitted- light image be converted to an electronic signal. The phosphor needs to cover the whole visible spectrum and be extremelyfast. By combining two cerium-containing phosphors, a garnet, Ce:Y3Al5O12, and a silicate, Ce:Y2SiO5, the first emitting in the range 500 – 650nm and the second over the range 370 – 500nm, the correct properties are achieved. In the garnet the very strong crystal field has shifted the emission color.
Particle and Radiation Detection
One method of detection of high-energy nuclear particles, or of high-energy radiation, requires capture in a material that will convert that energy to a visible luminescence. That luminescence can be supplied by Ce3+, whose emission, centered at 400nm or lower, closely matches the response of image intensifiers. Tracking chambers for neutron detectors, for example, can use fiber-optic face plates made from cerium-doped silicate glass that provides high resolution and fast response. As another example, crystalline cerium fluoride, CeF3, is a high density, 6.16 g1cm3, scintillator that provides a fast response, with 27ns decay constant, and is suitable forpositron emission tomography (PET). The material is resistant to radiation damage and is suitable
for high energy physics experimentation.
CATALYTIC AND CHEMICAL APPLICATIONS
The scale of the refinery operations needed to satisfy demand for gasoline is surprising. Within the U.S.A. the ≈120 operating fluid catalytic cracking (FCC) units consume ≈500 tons of catalyst (probably one-half of the world market) per day in converting crude oil to lower molecular-weight fractions, such as gasoline blending stock. An FCC unit has a lower temperature reactor and a higher temperature regenerator; the catalyst circulates between the two. Although essentially only one type of catalyst, ion-exchanged zeolite, is used there are a variety of catalyst compositions available many of which contain lanthanides including cerium.
FCC catalysts contain crystalline zeolites – the active component – and additives (e.g. see SOx control below) embedded in an inert matrix. The zeolite, an aluminosilicate faujasite Y-type with organic-molecule sized pores, requires cations within those pores for charge-neutrality, to give catalytic reactivity in the reactor and to provide thermal stability in the regenerator. Highly-charged ions, such as La3+ or Ce3+, bound within those zeolite cages to negatively charged -[AlO4]units, create a high electric field gradient strong enough to dissociate adsorbed water and provide a high surface acidity. Protonation of the organic molecules then produces the carbonium-ion intermediates that initiate the actual cracking reactions.
Lanthanide cations can be exchanged into the zeolites by immersion in mixed lanthanide salt solutions ; the Ln content can reach up 10 % by weight of the zeolite. Not all FCC catalysts contain Ln’s though and, when taken over all available compositions, the Ln content probably averages 2 %. Cerium, because of the potential availability of the Ce4+ state that tends to hydrolyze at the ion-exchange pH’s used, and because of the different catalyst production technologies practiced, is often partially removed from the precursor solutions. Nevertheless the production of FCC catalysts accounts for significant amounts of the world consumption of this element.
The Ln’s are used to give high cracking activity to FCC catalysts, especially to produce low-octane fuel from heavy crude-oil feedstocks. Consumption of lanthanides, and hence of cerium, in FCC catalysts has altered during the decade of the 1980’s because of increased demand for high-octane fuels, greater feedstock availability of lighter crudes, and changes in catalyst technology. FCC-Ln demand reached a peak in ≈1984 (one-third of all Ln consumption was in FCC) and has since fallen back.
This trend has influenced the supply and availability of cerium, particularly in comparison to the availability of lanthanum-rich cerium-poor materials. The inherent natural ratio of the two elements, La and Ce, in the source minerals imposes production constraints in trying to meet the market demand ratio. Put simply, you can’t make one without the other. The increase in Ln demand for FCC catalysts up to the mid ’80’s, together with the need to separate out cerium in order to make La-rich Ce poor compositions increasingly preferred, had led to a glut of Ce-based raw materials. However the subsequent drop in total FCC Ln requirement together with the growth of several markets for chemicals based on cerium alone has now reversed the picture. Currently, 1991, the La-rich Ce-poor portion of the raw material is in excess supply over demand and will probably remain so until a new major use for lanthanum-based materials is found.
Vehicle Emission Control
A major technological application of steadily growing importance for cerium is as one of the catalytically active components used to remove pollutants from vehicle (auto-exhaust) emissions. This market currently consumes a significant portion of the yearly production of= cerium derivatives. The active form of cerium is the oxide that can be formed in situ by calcination of a soluble salt such as nitrate or by deposition of slurried oxide.
The most widely used exhaust control device consists of a ceramic monolith with a thin-walled rectangular honeycomb structure that offers little resistance to the flow of gases. (Metallic monoliths are also possible.) The accessible surface of this monolith system is increased by applying a separate coating, a wash coat, of a high surface area material such as gamma-alumina with the catalytically active species impregnated into this washcoat. The catalyst needs
- to oxidize unburnt hydrocarbons,
- to convert CO to CO2,
- and to reduce NOX
and hence is termed a “three-way” catalyst. The whole system forms a “catalytic converter” that, suitably encased, is placed between the engine and the muffler/silencer unit.
In addition to platinum and other metals from the platinum group, the major active component in the current complex multi-functional systems is cerium oxide. Current catalytic converters contain ≈75 gms per converter of finely divided ceria dispersed within the washcoat. Most of the details of catalyst production are proprietary knowledge and patents only hint at the technology involved. Not only is the chemical nature of the catalytic participants crucial but so too is their spatial distribution with respect to one another. Elucidation of the detailed behavior of cerium is difficult and complicated by the presence of other additives, such as lanthanum oxide, that perform related functions. Ceria plays several roles, namely
- as a stabilizer for the high surface area alumina,
- as a promoter of the water-gas shift reaction,
- as an oxygen storage component, and
- as an enhancer of the NOX reduction capability of Rhodium.
The tendency of the high-surface -area gamma-alumina support to sinter and lose that crucial area during high-temperature operation is retarded by the intimate addition of several per-cent of cerium oxide to that alumina. (This function is also provided, probably more efficiently, by the periodic table precursor to cerium, lanthanum.) The mechanism is still under debate but may involve a surface Ln-aluminate species on the alumina.