Inelastic neutron scattering is used to examine an un-promoted iron based Fischer–Tropsch synthesis catalyst exposed to ambient pressure CO hydrogenation at 623 K for defined periods of time-on-stream (3, 6, 12 and 24 h).
Inelastic neutron scattering (INS) is employed to examine the evolution of a promoter-free iron-based Fischer−Tropsch synthesis catalyst (∼10 g catalyst charge) that is exposed to ambient pressure CO hydrogenation at 623 K for up to 10 days time-on-stream (T-o-S). The longer reaction time is selected to better understand how the formation of a previously described hydrocarbonaceous overlayer corresponds to the catalyst conditioning process. Although the onset of pseudo steady-state reactor performance is observed at approximately 9 h T-o-S, INS establishes that the intensity of the C−H stretching mode of the sp 3 -hybridized component of the hydrocarbonaceous overlayer saturates at about 24 h T-o-S, while the corresponding intensity of the C−H stretching mode of the sp 2 -hybridized component requires 100−200 h T-o-S to achieve saturation. This novel series of measurements reveal different aspects of the complex catalyst evolutionary process to be indirectly connected with catalytic turnover.
We have investigated a series of supported and unsupported nickel and cobalt catalysts, principally using neutron vibrational spectroscopy (inelastic neutron scattering, INS). For an alumina supported Ni catalyst we are able to detect hydrogen on the metal for the first time, all previous work has used Raney Ni. For an unsupported Ni foam catalyst, which has similar behaviour to Raney Ni but with a much lower density, the spectra show that there are approximately equal numbers of (100) and (111) sites, in contrast to Raney Ni that shows largely (111) sites. The observation of hydrogen on cobalt catalysts proved to be extremely challenging. In order to generate a cobalt metal surface, reduction in hydrogen at 250–300 °C is required. Lower temperatures result in a largely hydroxylated surface. The spectra show that on Raney Co (and probably also on a Co foam catalyst), hydrogen occupies a threefold hollow site, similar to that found on Co($$10\bar{1}0$$
10
1
¯
0
). The reduced surface is highly reactive: transfers between cells in a high quality glovebox were sufficient to re-hydroxylate the surface.
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