Sintering, carbon monoxide, surface

Sol-gel technique has also been applied to modify the anode/electrolyte interface for SOFC running on hydrocarbon fuel [16]. ANiA SZ cermet anode was modified by coating with SDC sol within the pores of the anode. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the TPB area, which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode leads to higher stability of the cell in long-term operation due to the reduction of carbon deposition and nickel sintering. 

The degradation of the catalytic activity with reaction time or thermal sintering was analytically studied by the determination of the specific surface area of the exposed palladium particles, by the chemical adsorption of carbon monoxide the distribution of palladium particles, by an electron microscope and the crystal imperfection, by X-ray diifraction analysis, and so on. 

DoUimore and Tonge [15] ascribed the deceleratory decomposition of zinc formate in air (0 < nr < 0.3) to an initial instantaneous and extensive nucleation of reactant crystalhte surfaces with product zinc oxide and the operation of a contracting sphere mechanism. For 0.3 < nr < 0.8 the reaction rate is almost constant, probably as a result of reactant cracking. for both processes is 67 kJ mol". During the course of reaction the yields of hydrogen and carbon monoxide increased, while that of carbon dioxide decreased. This was attributed to a decrease in the catalytic activity of the product oxide, possibly as a result of sintering. The formation of higher molecular mass products was mentioned. 

A further decrease 1n the hydroxyl population was obtained when the silica was calcined, not in air or nitrogen, but in carbon monoxide. This is also shown in Figure 3. The curve representing the OH population in carbon monoxide split away from that in air at 600 C, and the separation increased up to sintering at 925 C. We believe that this is primarily due to a water gas shift mechanism in which surface moisture is removed by conversion to CO2 and H2- The CO could act through a direct attack on... 

The absence of water thus avoids the two problems of hydrolysis of the surface chromates and water-induced sintering of the support, both of which lead to a catalyst with lower activity. Reoxidation of the chromium after carbon monoxide reduction gives a different active site precursor, which produces a catalyst which gives a broader molecular weight distribution in the resulting polymer. Catalysts pre-reduced with carbon monoxide are more active than those reduced with ethylene in the reactor. 

On the other hand, while chlorides accumulate near the top of the catalyst, they are more mobile and can be detected in significant concentrations, up to 0.05%, at all levels in a deactivated bed. Although reasonable hves of at least two years can often be achieved in the presence of chloride there is more rapid movement of the peak in temperature profile, and the concentration of carbon monoxide in the outlet gas increases more rapidly. Surface chlorides, which are formed by reaction with zinc oxide, are mobile and sinter the catalyst surface. Chlorides are also soluble in condensed steam and can be washed down onto lower, more active catalyst layers. 

Another problem may arise when we wish to establish a component balance for a species that is both generated by a heterogeneous surface reaction and consumed by a homogeneous gas phase reaction viz. a carbon monoxide balance in sintering. Here the transfer of CO from the solid carbon would be described by the last term on the r.h.s. of Eq. (7.4.24), whereas the consumption of CO due to oxidation would be represented by the third term. 

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