The Effect of Water on the Stability of Iron Oxide and Iron Carbide Nanoparticles in Hydrogen and Syngas Followed by in Situ X-ray Absorption Spectroscopy

Abstract

The effect of water on iron-based nanoparticles under hydrogen and syngas was investigated by in situ X-ray absorption spectroscopy. The iron oxide (γ-Fe2O3) nanoparticles, dispersed as a monolayer on flat silica surfaces, were readily converted into metallic iron in dry hydrogen at 350 °C and into iron carbide in dry syngas (H2/CO 2/1 vol/vol) at 325 °C. However, in the presence of water, the reduction did not proceed beyond magnetite (Fe3O4) up to 350 °C. Wustite (Fe(II)O or FeO(1–x)) was formed at 450 °C in wet syngas and 550 °C in wet hydrogen. Once formed, the iron carbide nanoparticles proved remarkably stable against oxidation in wet syngas at 350 °C. However, we observed the formation of a surface iron(II) oxide phase that increases with increasing H2O/CO ratio. This implies that the active surface of iron-based Fischer–Tropsch catalysts is covered by considerable amounts of adsorbed oxygen during the Fischer–Tropsch reaction. Reducing the temperature by only 20 K results in complete and irreversible oxidation to magnetite. We propose that the surface iron(II) oxide plays an important role during Fischer–Tropsch synthesis by regulating the relative rates of CO hydrogenation versus water gas shift and by stabilizing the iron carbide catalyst against irreversible deactivation by oxidation to magnetite.

Pore size effects in high-temperature Fischer–Tropsch synthesis over supported iron catalysts

“…This suggestion is in agreement with previous reports [1,3,[5][6][7]23]. Note however that Fischer-Tropsch rates and selectivities could be also influenced to some extent by residual iron oxide species which may control the water gas shift reaction and stabilize the supported iron carbide nanoparticles against oxidation [75].…”

“…Higher olefin to paraffin ratio was observed on larger iron particles. Similar results for carbon-supported catalysts were also obtained by Jung et al [75,76]. The TOF calculated from FT reaction rate and carbon monoxide adsorption increased with an increase in iron particle size.…”

“…The TOF calculated from FT reaction rate and carbon monoxide adsorption increased with an increase in iron particle size. In fact, most iron-supported catalysts investigated by Jung et al [75] had similar overall activity, because the lower TOF of highly dispersed catalysts was compensated by their high specific surface area. The influence of iron particles supported by carbon nanotubes on the FT catalysts performance was studied by Dalai et al [77,78].…”

Relating adatom emission to improved durability of Pt–Pd diesel oxidation catalysts

“…Therefore to allow comparison with supported catalysts, we performed our experiments at atmospheric pressure. To preserve the benefits of model catalysts while allowing heating in air at elevated temperatures, we used custom-made Si TEM grids with Si 3 N 4 windows [15,16]. Rapid heating was made possible by using a heating device [17] that provides heating and cooling rates over 100°C s À1 .…”

Sedimentary reservoir oxidation during geologic CO2 sequestration

“…Numerous experimental and computational efforts have been directed toward identifying and optimizing key FTS catalysis mechanisms on the surfaces of transition metals and metal oxides (Niemantsverdriet and van der Kraan, 1980;Dictor and Bell, 1986;Van Der Laan and Beenackers, 1999;de Smit and Weckhuysen, 2008;Gracia et al, 2009;Huo et al, 2009;Zhuo et al, 2009;Marshall and Medlin, 2011;Thü ne et al, 2012). Despite these efforts, the conditions under which FTS reactions can occur remain poorly constrained.…”