Both thermocouples were connected to the

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Unformatted text preview: the control of the heating element of the furnace was placed near the heating element of the furnace. Both thermocouples were connected to the thermocontroller, which controlled the heating power of the furnace. Changes in the composition of the reaction gas mixture, corresponding to the reaction rate were measured by a thermal conductivity detector (Valco, USA). A personal computer continuously recorded the temperature in the reactor and the detector signal. A sample of 5 mg of each calcined catalyst was heated up in a linear temperature program of 5°C/min up to 950°C in the flow of the reduction mixture (8.5 ml/min). After the reduction the catalyst was cooled down to 40°C in the flow of the reduction mixture to avoid any possible oxidation of the catalyst. As soon as the temperature was below 40°C the reduced catalyst was passivated for 30 minutes in a flow of the oxidation 58 2. Experimental ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffifffffffffffffffffffffffffffffffffffff mixture. After the passivation the sample was reduced again up to 850°C at a constant heating rate of 10°C/min. Afterwards the catalyst was cooled down to room temperature in the flow of the reduction mixture and then heated up to 950°C in the flow of the oxidation mixture at a constant heating rate of 10°C/min. Before starting each measurement the baseline was allowed to stabilise and adjusted to zero. A stable baseline was also used as indicator for the absence of any leaks. For the TPR measurements a H2/Ar mixture was employed because of the large thermal conductivity differences between H2 and Ar. The amount of reducible Ni2+ ions was determined from the consumption of H2 from the 5 vol% H2/Ar mixture. Water formed during the reduction did not influence the detector signal because its thermal conductivity is very similar to that of Ar. For the TPOx an O2/He mixture was employed because of the large thermal conductivity differences between O2 and He. Th...
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