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Reduction of metal oxides

Dear Reader,

One of my readers has asked an interesting question of what happens when lithium metal is heated with thorium dioxide. This is an interesting question as it relates to what may appear to be a solid solid reaction. Solid Solid reactions are interesting but this is not a case of such a reaction, but if you want to read about such reactions then go and look at Understanding solid/solid organic reactions by Colin Raston et. al., Journal of the American Chemical Society, 2001, volume 123, pages 8701-8708.

While I am a non specialist in the reduction of metals with lithium I am going to write about it from the point of an inorganic chemist, if any of my readers know the area better than me then please write in and correct me if I get something wrong.

David the radioactive Boy Scout had mixed up the thorium dioxide which he harvested from gas lantern mantles with lithium metal which he had obtained from electrical batteries. This mixture had been heated up to form thorium metal.

I hold the view that this step was not needed, as thorium dioxide makes a perfectly good target material for neutron bombardment, the oxygen has a small cross section for neutrons, the density of thorium dioxide is high and finally thorium dioxide is more chemically compatible with air, water and carbon dioxide than the metal. But I will concentrate here on the chemistry of the lithium / thorium dioxide reaction.

As the lithium metal has a lower melting point (180 oC) than the thorium dioxide (> 2000 oC), I expect that as the mixture is heated up the lithium will melt. I have melted lithium in the past with a normal household hot air gun in the following way.

Place some paraffin oil in a Pyrex flask, drop in a magnetic stir bar, add some chunks of lithium metal, then pump out the air and refill the flask with ARGON. Next switch on the stirrer and start to heat with the same type of heat gun as people use for stripping paint from wood surfaces. Heat until the lithium metals, then stop heating but leave the stirring on. Then go for lunch while the mixture is stirred while it cools. On a good day this makes very small balls of lithium metal, on a bad day when the stirring is not good you get big lumps.

On the other hand thorium dioxide has a melting point far above 1400 degrees centigrade; it will difficult to melt it even inside a quartz tube (the quartz tube will melt first). So I think it is safe to assume that the reaction will be between the solid thorium dioxide and liquid lithium metal.

What I think will occur is that at the interface between the solid and the liquid that electrons from the lithium atoms will be passed to the thorium dioxide. The electrical properties of the thorium dioxide will be important. I think that the thorium dioxide will be a semiconductor, this means that at room temperature that it will be a very good insulator (like a diamond) but as it is heated up the electrical conductivity of the solid will
increase. The conductivity of the thorium dioxide will also depend on the purity of the thorium dioxide.

I do not think that the thorium dioxide in a gas mantle will be very pure; I know that a typical gas mantle contains lanthanides which are dispersed in the solid thorium dioxide. This will increase the electrical conductivity of the solid. This mixture of solids is likely to be like the yttrium doped zirconia which is often used as a lambda (oxygen) sensor in the exhaust system of a modern car.

I modern car uses a computer controlled fuel injection system which uses at least one oxygen sensor in the exhaust system to make sure that the catalytic converter is fed with a sensible balance of gases (oxygen, unburnt fuel, nitrogen oxides and carbon monoxide) to allow it to clean up the exhaust gases. But I will return back to thorium dioxide and lithium metal. If anyone wants to read about the conductivity of doped thorium
and zirconium dioxides then I suggest that you look at

Electrochemical Studies on Partial Conductivity, Mobility and Concentration of Electrons and Defect Electrons in Doped Zirconium Dioxide and Thorium Dioxide, by L.D. Burke, H. Rickert and R. Steiner, Zeitschrift Fur Physikalische Chemie-Frankfurt, 1971, volume 74, pages 146 onwards.

Also I would suggest reading Identifying Doping Strategies To Optimize the Oxide Ion Conductivity in Ceria-Based Materials by C. Frayret et. al.,
Journal of Physical Chemistry C, 2010,  volume 114, pages 19062-19076.

Plus Mixed (oxygen ion and p-type) conductivity in yttria-stabilized zirconia containing terbia by P. Han and W.L. Worrell, Journal of the
Electrochemical Society
, 1995, volume 142, pages 4235 to 4246 might explain things a bit more.

When I was writing this I was on holiday and away from the university library so I made some educated guesses, I think that it will be best if I write about the two extremes.

The more mobile the electrons and oxygen anions are in the semiconducting thorium dioxide the deeper the reduction reaction will be in each grain of thorium dioxide. If the oxide anions (oxygen double minus atoms) and the electrons are mobile in the thorium dioxide then the following will occur.

Electrons from the lithium enters the gain and these can go to a thorium (IV) centre near the surface forming a thorium (III) centre. This thorium (III) centre can give up its electron reforming and thorium (IV) centre, and the electron can hop further into the grain to a new thorium (IV) centre. At the same time two maintain the electroneutrality of the grains of thorium dioxide the oxide anions will have to diffuse out of the

This will lead to gain loosing oxygen and gaining electrons, the thorium dioxide (thorium (IV) oxide) will be reduced first down to thorium (III) oxide. If the thorium (III) oxide is like some lanthanide (II) compounds then it will be an oxide which 2Th4+, 2e, 3O2- which will be very conductive. The transfer of electrons into the grain and oxide anions out will continue until the grain is one of thorium metal (melting point 1750 oC).

I doubt if David would have been able to heat his reaction mixture much above 1000 oC using a butane torch or some other common or garden gas flame so unless thorium dissolves in lithium metal then I would expect him to form fine grains of thorium. If he could have kept it hot and have excluded the air from the reaction mixture then he might have been able to sinter the thorium which would have increased the particle

The mobility of oxygen inside thorium metal will be an important thing to know, I know that for some metals oxygen is very able to diffuse into bulk of the metal from the surface.

Also depending on how much lithium David used it is possible that a thorium / lithium alloy will form. Here the thorium and the excess lithium will dissolve in each other to form an alloy. I have checked the inorganic crystallographic database and no crystalline lithium / actinide alloys have been reported but the absence of a solid in the database does not rule out the possibility that a lithium / thorium alloy exists.

At the same time the loss of electrons from the lithium and its replacement with oxide anions will lead to the formation of lithium oxide. Again the mobility of oxygen in lithium is important if you want to understand exactly what happens.

If we consider the opposite situation where either electrons or oxide anions are not mobile in the thorium dioxide then something else will occur.

What will occur is that the electrons from the lithium will reduce the thorium cations (positive ions) on the surface of the grain. The oxide anions from the surface will bind to lithium cations. This process will continue to form a thin layer of thorium metal on the surface of the thorium dioxide grains. If the thorium is soluble in the lithium then it will diffuse away from the grain into the bulk of the liquid lithium.


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