ИСТИНА

Fischer–Tropsch synthesis is a catalytic process of synthesis gas conversion into liquid hydrocarbons The process is relevant for the applied material and, even more, for environmental research, as it is particularly important for the biofuel production, city garbage utilization and natural gas monetization into alternative clean synthetic fuel. The ability of composite catalyst to remove the released heat is provided not only by its thermal conductivity, but also by availability of meso- and macropores [1], which provide intensified mass transfer and lower diffusion limitations. Mass transfer is essential for the process [2, 3]: notwithstanding that the reactants exist in the gas phase, catalyst pores are filled with liquid products. Normally efficiency of reaction is evaluated indirectly by real-time material balance and temperature-programmed sorption/desorption technique after reaction is over. There is no technique for in situ insight into intragrain/intrapore processes, which serve a genuine drive for catalytic efficiency. Preliminary µCT images of a used catalyst show that features with characteristic size more than 5 µm can be clearly distinguished and that debris can be seen. It is expected that upon filling with a liquid, contrast of a pore should change and it must be possible to investigate origination and diffusion of hydrocarbons. Fig. 1. CT compatible reaction cell (left panel). Reconstructed image of catalyst in the cell (right panel) Ultra-fast, absorption-based X-ray microtomography (µCT) is required to perform time lapse studies of the evolution of liquid hydrocarbons in the pore structure of catalyst. One would need the X-ray beam with mean energy of ~17 keV, relative energy bandwidth of 2-3%, homogeneous intensity distribution at the sample with the field of view of ~ 2×2 mm2. Experimental station must be equipped with a rapid CT stage. It is important that the number of photons at sample must be sufficient to acquire 3000 projections in 2–3 minutes, which demands flux density of about 1012 photons/sec/mm2. Measurements are to be organized as follows: catalyst is loaded in the reaction cell and is activated in a stream of pure hydrogen for one hour (total amount of gas ~10 ml) at T=400° C. Then the operator switches the gas stream to the CO+H2 mixture and reduces temperature to 180° C. Reaction begins at this point and continues for about 16 hours and CT snapshots of catalyst are acquired every 10–15 minutes over the entire duration of the chemical reaction. We have developed a specific container made of a single crystal diamond, which allows maintaining reaction conditions and compatible with tomographic imaging — see Fig. 1. This container has inlet and outlet to enable gas flow. It is integrated by means of flexible high quality PTFE gas pipes in a sealed gas supply system, which start with gas cylinder and ends with a bag collecting reaction waste. Heating is provided by means of hot-air blower device(s) with programmable settings. Samples are small cylindrically shaped rods with diameter of <2 mm and length <5 mm; they can be loaded in the container after it is disconnected from the gas pipes. We have done preliminary experiments in frame of ESRF collaboration program and showed that both pore system and liquid flow can be detected successfully — see Fig. 2. We believe that application of more advanced synchrotron facilities may bring quite exciting results revealing mechanisms of flow-and-diffusion control in heterogeneous catalysis.