Water-wax behaviour in porous silica at low temperature Fischer-Tropsch conditions

© 2018 Water is a major product of Fischer-Tropsch synthesis, and hence the behaviour of water within Fischer-Tropsch synthesis catalysts and its potential influence on catalyst rate and selectivity are questions of long-standing interest. The present work applies three different magnetic resonance techniques to study how water interacts with a model wax, n-octacosane, within the pore space of a porous silica of mean pore size ∼18 nm. 1H magnetic resonance spectroscopy, spin-lattice relaxation time and pulsed-field gradient measurements were performed at 195 °C, and for water pressure in the range 3–13.6 bar, conditions relevant to low temperature Fischer-Tropsch synthesis. The uptake of water within this system is shown to be very similar to that observed for capillary condensation of water within the empty pore space of the same porous silica under the same experimental conditions; suggesting that capillary condensation of water within the wax-saturated pores is occurring. The behaviour of water is characterised by two regimes. At low water relative pressures of ∼0.3 ≤ P/P0 ≤ ∼0.8 water moves into the pore space, displacing wax from the pore surface and existing as a water-rich layer between the pore surface and an oil-rich phase in the centre of the pore; the strong interaction with the pore surface is evidenced by the short nuclear spin relaxation time values of water at the lowest pressures which then increase slightly as multi-layer adsorption at the pore surface occurs with increase in pressure. In the water relative pressure range ∼0.8 ≤ P/P0 ≤ ∼0.97, condensation of water within the pores is observed, characterised by increases in both spin-lattice relaxation time and molecular diffusivity. Analysis of the data suggests that as much as ∼40% of the pore surface is occupied by condensed water after condensation has occurred. It is suggested that these two regimes of water behaviour inside initially wax-filled pores might explain previously reported aspects of the behaviour of Fischer-Tropsch catalyst performance as a function of pore size and amount of co-fed water

Coherent long-distance displacement of individual electron spins

Controlling nanocircuits at the single electron spin level is a possible route for large-scale quantum information processing. In this context, individual electron spins have been identified as versatile quantum information carriers to interconnect different nodes of a spin-based semiconductor quantum circuit. Despite important experimental efforts to control the electron displacement over long distances, keeping the electron spin coherence after transfer remained up to now elusive. Here we demonstrate that individual electron spins can be displaced coherently over a distance of 5 micrometers. This displacement is realized on a closed path made of three tunnel-coupled lateral quantum dots. Using fast quantum dot control, the electrons tunnel from one dot to another at a speed approaching 100 m/s. We find that the spin coherence length is 8 times longer than expected from the electron spin coherence without displacement. Such an enhanced spin coherence points at a process similar to motional narrowing observed in nuclear magnetic resonance experiments6. The demonstrated coherent displacement will enable long-range interaction between distant spin-qubits and will open the route towards non-abelian and holonomic manipulation of a single electron spin.Comment: 16 pages, 4 figures, supplementary material

Operando magnetic resonance imaging of product distributions within the pores of catalyst pellets during Fischer–Tropsch synthesis

Optimisation of a heterogeneous catalytic process requires characterisation of the catalyst at industrially-relevant conditions and lengthscales. Here we use magnetic resonance imaging to gain insight into Fischer-Tropsch synthesis occurring in a pilot-scale fixed-bed reactor operating at 220 °C, 37 bar, and for three H2/CO feed ratios. Molecular diffusion and carbon number of hydrocarbon products are spatially-resolved within both the reactor and individual 1 wt% Ru/TiO2 catalyst pellets. These data highlight the importance of mass transfer, in addition to nanoscale catalyst activity, on catalyst performance. In particular, a start-up time of up to 3 weeks is required for steady-state to be achieved in the catalyst pores. Further, the average carbon number present in the pores can be as much as double that in the product wax. Operando characterisation of water and oxygenates present in the pores is also achieved. The presence of a water-rich liquid at the pore surface is confirmed