On the Mechanism of Cu2O Oxidation at High Temperatures

Oxidation mechanism of nonstoichiometric cuprous oxide (Cu2 yO) has been studied as a function of temperature (973-1273 K) and oxygen pressure (1-105 Pa), using microthermogravimetric, marker and two-stage oxidation techniques. The last method consisted in using heavy oxygen isotope 18O in order to confirm the results, obtained with marker technique. It has been found that in early stages of reaction not exceeding 30 hours, CuO formation on the surface Cu2 yO follows approximately cubic rate law. In later stages, during long-term oxidation, lasting hundreds of hours, oxidation process follows strictly parabolic kinetics with the rate, being independent of Cu2 yO pre-treatment. Marker and two-stage oxidation experiments have shown that the growth process of CuO layer proceeds by the outward diffusion of cations, clearly indicating that cation sublattice of CuO is predominantly defected.

The application of singularity theory to isothermal autocatalytic open systems: the elementary scheme A + m B = ( m + 1)B

The stationary-state behaviour of the simple autocatalytic models: quadratic autocatalysis A + B → 2B; rate = k 1 ab ; cubic autocatalysis A + 2B → 3B; rate = k 1 ab 2 , in a well stirred, open system (isothermal c. s. t. r.) is determined via singularity theory methods. These methods allow all of the possible patterns for the dependence of the stationary-state extent of conversion x on the residence time Da (i. e. the x —Da bifurcation diagrams) to be identified and located. The cubic rate law has a cusp singularity, separating diagrams with a unique dependence of x on Da from those which display a simple S-shaped hysterisis loop with multiple stationary states. This behaviour is qualitatively similar to that shown by a simple exothermic reaction in an adiabatic c. s. t. r. (i. e. the two systems are contact equivalent). If the autocatalyst is not infinitely stable but instead undergoes a simple decay B → C; rate = k 2 b , a wider range of bifurcation diagrams is possible, with isolas or mushroom patterns. These arise as the system is ‘unfolded’ from its winged cusp singularity by varying parameters such as the catalyst lifetime and inflow concentration. It is shown that these are also the only patterns possible for a generalized order of autocatalysis, i. e. for a rate proportional to ab m , with m taking any value greater than one, integral or non-integral. The ranges of the above parameters over which the different responses are found are also given analytically for the general m . These parameters cannot, however, give a complete unfolding, so certain additional bifurcation diagrams that are found for the exothermic reaction in a non-isothermal, non-adiabatic reactor (which also has a winged-cusp singularity) are not found in the autocatalytic system.