No abstract

No abstract

A study has been made of the thermal decomposition of MgC03 . 3H20 (nesquehonite) and MgC03 . (NH&C03 . 4H20 using a variety of experimental techniques, including thermogravinietric analysis, differential thermal analysis and optical microscopy. The solids remaining at various stages during decomposition were characterized by measurement of their X-ray powder diagrams, surface areas, and particle densities. Nesquehonite decomposes first to crystalline MgC03. HzO, then to an amorphous magnesium carbonate of essentially zero surface area and finally to high area " active ¶ ¶ magnesia (-250 m2/g). Magnesium ammonium carbonate loses (NH&CO3 and most of its water below 100°C to give a second amorphous magnesium carbonate differing from that derived from nesquehonite in having a high surface area (-400 m2/g) and smaller COz content. Both of the amorphous carbonates have the pseudomorphic form of the parent crystal, and both recrystallize abruptly to magnesite on heating to 510°C in an atmosphere of C02.

No abstract

The differential heats of the adsorption of oxygen on copper have been measured calorimetrically as far as saturation. It is confirmed that oxidation takes place at 20" C and the results obtained by this approach agree with those obtained by Rhodin using a gravimetric method. The first quantities of oxygen, however, are adsorbed with a heat of 30 kcal in excess of the heat of oxidation of copper to cuprous oxide and there is evidence that this stage corresponds to the formation of only a fraction of a monolayer. The decomposition of nitrous oxide by copper has also been studied in a calorimeter and only that fraction of the surface which is capable of adsorbing oxygen with the high heat is able to effect the decomposition of NzO. Furthermore, measurements of the adsorption of carbon monoxide by copper have shown very close agreement between the amount of N2O which copper will decompose and the amount of CO which it will adsorb.The effect of heat treatment has been investigated. A reproducible surface may be maintained if heat treatment above 200" C is avoided. When, however, the surface area was decreased by 35 % as a result of sintering at 320" C, the proportion of sites active for the above phenomena decreased by 60 %. The results are discussed with reference to the work of Russell on the poisoning of copper catalysts and they are presented in like manner as giving evidence of the heterogeneity of metallic adsorbents.In 1935, Russell and Ghering,l using an ice calorimeter, made some measurements of the heat of adsorption of oxygen on copper and found that, with increasing amounts of oxygen taken up, the heat liberated fell steadily from 92 kcaljmole until at least 80 % saturation was reached. During recent years, as a result of the precise experiments of Rhodin2 and of Allen and Mitchell,3 it has been established that oxidation takes place under these conditions, and saturation corresponds to a state well beyond the formation of a monolayer. The continuous fall in the differential heats as layers of oxide are formed is therefore surprising, and for this reason it was of considerable interest to investigate the process in greater detail. In the course of this work it became clear that the process observed by Russell and Ghering is in fact a composite one, and under suitable experimental conditions the differential heat curve may be resolved at 20" C into three distinct stages : (i) limited adsorption with a heat greatly in excess of the heat of oxidation, (ii) true oxidation, and (iii) adsorption of oxygen to saturation on the oxide layer.The third stage has already been discussed in a series of papers from this laborat0ry.4~ 5 3 6 The heat of this adsorption is 56 kcal/mole and is independent of the thickness of the oxide layer at least as far as 170 A. The oxygen is adsorbed either as 0-or 0 2 -ions.The second stage, the oxidation of copper, has been the subject of numerous investigations during the last thirty years. In the temperature range above 700" C, it is well established that the parabolic law holds an...