Abstract
Iron-based Fischer–Tropsch synthesis (FTS) catalysts evolve in situ on exposure to synthesis gas (CO & H2) forming a mixture of iron oxides, iron carbides and carbonaceous deposits. Recently, the application of inelastic neutron scattering has shown the progressive formation of a hydrocarbonaceous overlayer during this catalyst conditioning period. The evolving nature of the catalyst alters the proportion of phases present within the catalyst, which may influence the transport of hydrogen within the reaction system. Preliminary quasi-elastic neutron scattering (QENS) measurements are used to investigate hydrogen diffusion within an un-promoted iron FTS catalyst that has experienced varying levels of time-on-stream (0, 12 and 24 h) of ambient pressure CO hydrogenation at 623 K. Measurements on the catalyst samples in the absence of hydrogen show the unreacted sample (t = 0 h) to exhibit little increase in motion over the temperature range studied, whereas the t = 12 and 24 h samples exhibit a pronounced change in motion with temperature. The contrast is attributed to the presence of the afore-mentioned hydrocarbonaceous overlayer. Measurements on the samples in the presence of liquid hydrogen show hydrogen diffusional characteristics to be modified as a function of the catalyst conditioning process but, due to the complexity of the evolving catalyst matrix, the hydrogen motion cannot be attributed to a particular phase or component of the catalyst. Problems in the use of hydrogen as a probe molecule in this instance are briefly considered. Coincident neutron diffraction studies undertaken alongside the QENS measurements confirm the transition from hematite pre-catalyst to that of Hägg carbide during the course of extended times-on-stream.
Highlights
The Fischer–Tropsch synthesis (FTS) reaction is a wellestablished, industrially significant, catalytic reaction involving the conversion of synthesis gas (CO and H 2) into a wide range of hydrocarbon products, which can be further processed to produce high value chemical intermediates utilised in the chemical manufacturing industry [1,2,3]
In order to test the hypothesis of moderated hydrogen diffusion throughout the iron based FTS catalyst conditioning period, here we report a preliminary investigation employing the technique of quasi-elastic neutron scattering (QENS) to examine the diffusion characteristics of an iron based FTS catalyst that has experienced different times-on-stream
With reference to an iron based un-promoted FTS catalyst, samples that have previously experienced 12 and 24 h time on stream in ambient pressure CO hydrogenation at 623 K were investigated for hydrogen diffusion by QENS
Summary
The Fischer–Tropsch synthesis (FTS) reaction is a wellestablished, industrially significant, catalytic reaction involving the conversion of synthesis gas (CO and H 2) into a wide range of hydrocarbon products, which can be further processed to produce high value chemical intermediates utilised in the chemical manufacturing industry [1,2,3]. Incoherent inelastic scattering provides information about uncorrelated atomic motion At higher energies this may represent molecular vibrations, but around the elastic peak this represents the stochastic diffusion processes occurring within a sample. In a system with a solid phase and a hydrogen-containing liquid the incoherent inelastic scattering will represent the stochastic motions of the liquid, whilst the coherent elastic scattering represents the structural order of the solid This simplification allows us to neglect the effects of correlated motions in the solid, and structure in the liquid, we should remember that this is an approximation that becomes decreasingly accurate with as the percentage of hydrogen in our sample reduces. The three samples have previously been analysed by INS by Warringham et al [8] This preliminary QENS study seeks to investigate whether this technique may provide any insight towards how the evolving hydrocarbonaceous overlayer affects reactant diffusion
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