Barium carbonate, thermal stability

This latter point was stressed by some of us in a recent report studying NO storage and reduction on commercial LSR (lean storage-reduction) catalysts, in order to catch valuable information about the behaviour of typical NO storage materials in real application conditions. Nature, thermal stability and relative amounts of the surface species formed on a commercial catalyst upon NO and 02 adsorption in the presence and in the absence of water were analysed using a novel system consisting of a quartz infrared reactor. Operando IR plus MS measurements showed that carbonates present in the fresh catalyst are removed by replacement with barium nitrate species after the first nitration of the material. Nitrate species coordinated to different barium sites are the predominant surface species under dry and wet conditions. The difference in the species stabilities suggested that barium sites possess different basicity and, therefore, that they are able to stabilize nitrates at different temperatures. At temperatures below 523 K, nitrite species were observed. The presence of water at mild temperatures in the reactant flow makes unavailable for NO adsorption the alumina sites [181]. 

The effects of a hydrotalcite (magnesium/aluminium hydroxy carbonate) acid scavenger on the thermal stability of barium/zinc-stabilisedpoly(vittyl chloride) containing zinc pyrithione biocide in various proportions were investigated by heat stability, (fynamic thermal stabiUty and Brabender mastication experiments coupled with colour, anti-bacterial and anti-fungal measurements, and the results are discussed. 12 refs. 

Barium is a member of the alkaline-earth group of elements in Group 2 (IIA) of the period table. Calcium [7440-70-2], Ca, strontium [7440-24-6], Sr, and barium form a closely allied series in which the chemical and physical properties of the elements and their compounds vary systematically with increasing size, the ionic and electropositive nature being greatest for barium (see Calcium and calcium alloys Calcium compounds Strontium and STRONTIUM COMPOUNDS). As size increases, hydration tendencies of the crystalline salts increase solubilities of sulfates, nitrates, chlorides, etc, decrease (except fluorides) solubilities of halides in ethanol decrease thermal stabilities of carbonates, nitrates, and peroxides increase and the rates of reaction of the metals with hydrogen increase. 

The oxygen pressure at which the oxides of the metals Co, Fe(II), Zn, Mn, Mg, Na, K, Ca, Li, Ba(II) are still stable at 600 °C is very small [19] (< 10 atm). Hence, the important factor is the solubility of the oxide in the melt. The PCO2 values at which the oxides of alkali metals and barium precipitate from the melt are positive and large. Carbonates with these cations display good thermal stability while considerable pressures of carbon dioxide are required to guarantee stability for the other carbonates at 600 °C. Acid-base considerations make it also understandable [19] that the basic oxides have excellent properties as container material for carbonates. 

Catalysts suitable specifically for reduction of carbon-oxygen bonds are based on oxides of copper, zinc and chromium Adkins catalysts). The so-called copper chromite (which is not necessarily a stoichiometric compound) is prepared by thermal decomposition of ammonium chromate and copper nitrate [50]. Its activity and stability is improved if barium nitrate is added before the thermal decomposition [57]. Similarly prepared zinc chromite is suitable for reductions of unsaturated acids and esters to unsaturated alcohols [52]. These catalysts are used specifically for reduction of carbonyl- and carboxyl-containing compounds to alcohols. Aldehydes and ketones are reduced at 150-200° and 100-150 atm, whereas esters and acids require temperatures up to 300° and pressures up to 350 atm. Because such conditions require special equipment and because all reductions achievable with copper chromite catalysts can be accomplished by hydrides and complex hydrides the use of Adkins catalyst in the laboratory is very limited. 

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