de Am Arthur Jong

Low-Temperature Ammonia Oxidation Over Pt/γ-Alumina: The Influence of the Alumina Support

Deactivation of a Pt/γ-alumina catalyst used for low-temperature ammonia oxidation was studied using positron-emission profiling (PEP). Evidence is presented that irreversibly adsorbed nitrogen and oxygen species deactivate the catalyst. This deactivation is further accelerated by preadsorption of oxygen on the catalyst. Ammonia strongly interacts with the alumina support. A steady state of the ammonia oxidation reaction is reached after the alumina sites are saturated with ammonia. PEP experiments show that spillover of oxygen to the support is not significant during the ammonia oxidation.

Activation of ammonia dissociation by oxygen on platinum sponge studied with positron emission profiling

Positron emission profiling (PEP) is applied to study the adsorption and dissociation of ammonia on metallic and preoxidized platinum sponge in the temperature range 323–573 K. The results show that ammonia weakly interacts with platinum without dissociation. On the contrary, preoxidized platinum is able to dissociate ammonia. With increasing temperature more ammonia is converted into N2, N2O, and H2O. Adsorbed NO appears to be an important intermediate, while its formation strongly depends on the oxygen surface coverage. Temperature-programmed experiments are performed to characterize the adsorbed nitrogen species. Furthermore, H2, NH3, and NO are used to react with these adsorbed nitrogen species. These experiments indicate that mainly NHx species are present at the platinum surface.

Imaging of hydrocarbon reactions in zeolite packed-bed reactors using positron emission profiling

The ability of positron emission profiling (PEP) to measure concentration profiles of molecules labelled with positron-emitting nuclei, such as 11C, 13N, and 15O, inside chemical reactors has been demonstrated for the system n-hexane–Pt/H-zeolites under conditions typical of the hydroisomerization reaction. Data obtained in the absence of reaction were first measured and used to model mass transport processes in these biporous, packed-bed reactors. Images obtained under conditions where injected, labelled pulses underwent reaction revealed that the products did not exit the reactor and thus demonstrated the need for in situ measurement. Such experiments should provide a valuable new tool in the study of transient, initial phenomena so often of importance in heterogeneous catalysis, such as “preconditioning” and deactivation.

Positron Emission Profiling: a Study of Hydrocarbon Diffusivity in MFI Zeolites

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 final catalytic performance. Here, we present the results of Positron Emission Profiling experiments with labeled hydrocarbons in zeolites with the MFI morphology. Single-component self-diffusion coefficients of hexanes in silicalite-1 and its acidic counterpart H-ZSM-5 are determined. For the first time, self-diffusion coefficients of n-pentane and n-hexane in mixtures are studied. This shows that Positron Emission Profiling is a powerful technique for in situ investigations of the adsorptive and diffusive properties of hydrocarbons in zeolites. The diffusion of hydrocarbons in medium-pore zeolites is determined by a complex interplay of factors such as the loading, the temperature, the preference for certain pore locations, the interactions with other hydrocarbon molecules of the same type or of other types and the presence of acid protons. In the diffusion of mixtures, pore blockage by one of the components might occur, thus strongly decreasing the diffusivity of the faster diffusing hydrocarbon.

Positrons as in situ probes of transient phenomena

Techniques such as positron emission tomography (PET), positron emission particle tracking (PEPT) and positron emission profiling (PEP) are currently used to perform in situ studies of physical and chemical phenomena that occur within chemical reactors and other processing equipment under process conditions. PEPT is capable of measuring the flow patterns inside mixers and fluidized‐bed reactors by tracking a labelled particle. Currently a particle as small as 500 μm moving at 0.1 m/s can be continuously located with a resolution of 5 mm. The ability to perform such measurements greatly aids in the development and optimisation of such systems. PET and PEP are being employed to perform measurements of the concentration of molecules labelled by 11C, 13N and 15O inside packed‐bed reactors. The measured concentration distributions, as a function of position and time, are used as input for mathematical models based on the kinetics of the physical (e.g., diffusion, adsorption) or chemical processes occurring.

Adsorption and diffusion of alkanes and their mixtures in silicalite studied with positron emission profiling technique

Publisher Summary This chapter presents a study of adsorption and diffusion of alkanes and their mixtures in silicalite with positron emission profiling (PEP) technique. With the TEX-PEP technique experiments on the diffusion and adsorption of mixture of n -hexane/2-methylpentane in large silicalite-1 crystals are performed. By modeling the experimental, tracer exchange curves values of intracrystalline diffusion coefficient and adsorption constant are obtained. Slight preference for the adsorption of n -hexane is found. Diffusivity of n -hexane sharply decreases with the increasing fraction of its isomer because the last one occupies channel intersections, thus blocking zeolite network.