Surface oxygen exchange kinetics

In the first place our understanding of factors that control and limit the interfacial kinetics is still rudimentary, and therefore should be a fruitful area for further investigation. The apparent correlation between the surface oxygen exchange coefficient and the tracer diffusion coefficient D for different classes of oxides, the fluorite-related and the perovskite-related oxides, as noted by Kilner et al. [73], clearly indicate the potential of isotopic exchange. 

However, a problem remains how to relate the observations (at equilibrium) from isotopic exchange to the conditions met during membrane operation. In chemical relaxation experiments, the oxide is studied after perturbation of the equilibrium state. These methods are thus complementary and probably their combined application, whenever possible together with spectroscopic techniques, such as FT-IR, UV and EPR, has a great capacity to elucidate the kinetics of surface oxygen exchange. 

The H2 production rate depends directiy on the rate of O2 removal from the H2O decomposition zone. This also depends on the O2 permeability of the membrane, which is a function of the electron and 02-ion conductivities, surface O2 exchange kinetics of the membrane, and oxygen partial pressure (PO2) gradient across the membrane (Balachandran et al., 2004 Ma, Balachandran, Chao, Park, Segre, 1997 Maiya et al., 1997). 

Upon exposure to higher p02 at SOFC temperatures, the cmiductivity decreases slowly [96]. This is observed in Fig. 3.19 as the significant decrease in conductivity for all of the samples as the /7O2 is increased above 10 " atm. The slow kinetics associated with this may be related to slow cation diffusion [101] and/or low oxygen mobility in these materials [103]. Slow reoxidation kinetics are beneficial to application in the SOFC anode where accidental and occasional exposure of the anode material to air may be expected. Of course, this assumes that a suitable and cost-effective cell synthesis procedure can be derived to initially form these highly conductive states. Furthermore, slow reoxidation may indicate slow surface oxygen exchange and low hydrocarbon oxidation activity. 

The surface exchange coefficient or oxygen transfer coefficient k, which describes the kinetics of surface oxygen exchange at the gas-solid interface. 

The chemical properties of oxide surfaces have been studied by several methods, including oxygen exchange. This method has been used to investigate the mechanisms of heterogeneous reactions for which oxides are active catalysts [36]. The dimerization step does not necessarily precede desorption and Malinin and Tolmachev [634], in one of the few reviews of decomposition kinetics of solid metal oxides, use this criterion to distinguish two alternative reaction mechanisms, examples being... 

Most of the kinetic results so far obtained upon oxygen exchange are summarized in Figs. 3 to 6, which also show the kinetics of the equilibration reaction plotted to the same scale. Before attempting an interpretation of these observations, it is desirable to consider first the nature and extent of the oxide surface and the adsorption of oxygen thereon and also to review briefly other experimental evidence regarding the interaction between gas and surface. 

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