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Characterisation of acid sites in catalysts by the determination of the irreversible heats of NH3 adsorption Background:
In this work we have been modelling aspects of automobile catalytic converter components from the perspective of adsorption and desorption on the surfaces of various powders.
Environmental concerns and legislation about the impact of automobile exhaust emissions have instigated research into improving the effectiveness of catalytic converters used in automobile exhaust systems. Since these technologies involve surface chemical and physical interactions a precise knowledge of the processes involved is crucial. The Microscal Flow Microcalorimeter is being applied to these surface chemical problems to help improve catalyst and support design.
A new model of Flow Microcalorimeter (FMC) for high pressure and high temperature gas–phase studies has been revealing surprising new aspects of catalyst activity. In this example of our recent studies, we have used the flow–through technology to measure both the heat and the matter transfer which occur when ammonia is preferentially adsorbed onto clay and zeolite surfaces from a pure nitrogen carrier.
This work has uncovered important differences between nominally identical clay catalysts. These are K10 catalyst materials from two different sources which are candidates for catalytic converters in car exhausts. These materials must perform their 'clean-up' function under a wide range of pressure and temperature regimes and in the presence of nitrogen, water, carbon dioxide, as well as smaller quantities of carbon monoxide, nitrous oxide and sulfurous compounds.
To simplify the comparison, all samples were subjected to a more simplified carrier/probe regime consisting of stabilization of the clay sample (about 20mg) in a flow of pure nitrogen, followed by adsorption of ammonia from a mixture of 5% ammonia in nitrogen, both at 1ml/min. Following cessation of the heat evolution and matter transfer effects (indicating that saturation of the surface had been achieved), the flow of mixture was switched back to a flow of pure nitrogen to measure any desorbed components of the interactions. Each experiment was performed at a constant temperature, i.e. isothermally.
Figure 1 shows the output traces for both the heat evolution profiles (top trace) and ammonia concentration change profiles (lower trace) for the adsorption and desorption cycles of 5% ammonia on Sample K10E at 80·7°C and atmospheric pressure. This view of the data shows the rates of heat evolution and matter transfer, i.e. the kinetics of the interactions. The areas under the enclosed peaks provide the integral quantities.
Figure 1
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Of particular interest is the fact that the first adsorption cycle (left side of the figure) clearly shows that the rate of heat evolution (top) is not in-step with the rate of ammonia adsorption (bottom). In fact the energy liberated on first exposure of sample K10E to ammonia indicates that the ammonia bonds on to the most accessible sites before starting to occupy higher energy sites. Across the same period, the matter uptake trace shows that the initially high rate of adsorption fell off at the same time as the higher energy sites became occupied by ammonia molecules. The result is that the molar enthalpy of first adsorption actually rises as the adsorption progresses and then falls off as the surface approaches full saturation.
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