Positron emission profiling experiments

Abstract Zeolites are of prime importance to the petrochemical industry as catalysts for hydrocarbon conversion. In their molecule-sized micropores, hydrocarbon diffusion plays a pivotal role in the flnal catalytic performance. Here, we present the results of Positron Emission Profiling experiments with labeled hydrocarbons in zeolites with the MFI morphology. Single-component self-diffnsion coefficients of hexanes in silicalite-1 and its acidic connterpart H-ZSM-5 are determined. For the first time, self-diffnsion co-... 

SS, 3 X 2nun). A Nal detector continuously measures the passing activity through the sample loop. A GC run is remotely started at the moment that the sample loop is filled with [ NJ-NHs. The GC is equipped with a Haysep P column (SS, mesh 80-100, O.D. 1/8). The GC run is started at 90°C, and after 1 minute, the temperature is increased with 10°C/min to 130°C. The products are analysed by a thermal conductivity detector (TCD). A Nal detector, directed at the TCD, monitors labelled products. Behind the TCD detector, a heated electrical 3-way valve selects a small part from the [ NJ-NHg peak. Depending on the experiment, a pulse time of 2-10 s is used to inject the labelled gaseous ammonia into the reactant stream. The required specific activity of the radioactive ammonia pulse is 0.1 MBq/ml minimum to meet the statistical requirements of the positron emission profiling experiments. 

The temperature dependence of the conversion and product formation in the ammonia oxidation over a platinum sponge catalyst is investigated. Positron emission profiling experiments demonstrate that below 413K, the catalyst deactivates due to the poisoning of the catalyst surface, mainly by... 

The plug flow reactor is increasingly being used under transient conditions to obtain kinetic data by analysing the combined reactor and catalyst response upon a stimulus. Mostly used are a small reactant pulse (e.g. in temporal analysis of products (TAP) [16] and positron emission profiling (PEP) [17, 18]) or a concentration step change (in step-response measurements (SRE) [19]). Isotopically labeled compounds are used which allow operation under overall steady state conditions, but under transient conditions with respect to the labeled compound [18, 20-23]. In this type of experiments both time- and position-dependent concentration profiles will develop which are described by sets of coupled partial differential equations (PDEs). These include the concentrations of proposed intermediates at the catalyst. The mathematical treatment is more complex and more parameters are to be estimated [17]. Basically, kinetic studies consist of ... 

Figure 6 shows a schematic diagram of the reactor system used for tracer-exchange positron-emission profiling (TEX-PEP) experiments. During these tracer exchange experiments, a constant flow of unlabeled hydrocarbons in a hydrogen carrier stream is fed into the reactor. The -hexane/... 

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