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A flash tempering process has been demonstrated for low-carbon low-alloy lath martensite, which includes heating the sample to 750 °C (Ac1: 752 °C), dwelling at 750 °C for 2 s, and quenching directly to room temperature. The high temperature accelerated the spread of the supersaturated carbon atoms in the lath martensite, activated the dislocation migrations and thus their combining and annihilation, and promoted the mass recrystallisation. The reduced dwelling time inhibited the coarsening of the martensitic laths and suppressed the growth of the recrystallized structures. The combination of higher temperature and the shorter dwelling period, so-called flash tempering, gives rise to the same microstructure and mechanical properties as those obtained by the conventional tempering process (i.e., dwelling at 600 °C for 1 h). The high efficient flash tempering, possessing significantly reduced energy and time consumptions, may have important consequence in industrialization of tempering process.

[Display omitted] Synthetic strategies in the preparation of embedded structure, impregnated, and precipitated catalysts have an important effect on the Fe-SiO 2 interaction, morphology, and catalytic performance. • Sintering of iron species is effectively inhibited by SiO 2 shell in Fe@Si catalyst. • Fe-SiO 2 interaction can be manipulated by preparation strategies. • The properties and catalytic performance of Fe-Si FTS catalysts are governed by the configuration and Fe-SiO 2 interaction. The physico-chemical properties of SiO 2 -promoted Fe-based Fischer-Tropsch synthesis catalysts were traditionally considered to be governed by Fe-SiO 2 interaction. Here we found that the configuration between SiO 2 and iron species also played a pivotal role in determining the structure and thus FTS performance of the catalysts. Fe@Si catalyst with embedded structure, impregnated Fe/Si catalyst as well as precipitated Fe-Si catalyst was fabricated by different methods respectively, and they were thoroughly characterized by multiple techniques. The results indicated that, despite the weaker Fe-SiO 2 interaction, the construction of SiO 2 shell outside the iron species core in Fe@Si catalyst strongly inhibited the reduction of iron oxides due to the confining effect of SiO 2 shell, which increased the diffusion resistance of H 2 O generated in the reduction process. However, the sintering of iron species was effectively hindered even during FTS reaction by the physical separation of SiO 2 shell. In contrast, iron species in impregnated Fe/Si and precipitated Fe-Si catalyst experienced aggregation with different degrees. The reduction behavior of these two catalysts were well correlated with the strength of Fe-SiO 2 interaction. The FTS performance showed that both Fe@Si and Fe/Si catalyst exhibited higher initial activity but deactivated gradually, while the activity of Fe-Si catalyst displayed an opposite trend. These phenomena were discussed in terms of the variation of iron carbides content at different stages, which was determined not only by Fe-SiO 2 interaction, but also by the manner that SiO 2 and iron species constructed. The present study contributed a new understanding of the structure-performance relationship for SiO 2 -promoted Fe-based FTS catalysts. [ABSTRACT FROM AUTHOR]

Highlights: [•] Most, medium and least stable Fe5C2(111), (−411) and (001) surfaces are used. [•] Adsorption of H2O molecules and O atoms was calculated at high coverage. [•] Surface stability correlates with the adsorption energies of H2O and O. [•] High coverage H2O prefers hydrogen bonding instead of adsorption on iron. [•] The number of surface O atoms depends on temperature and H2O/H2 ratio. [ABSTRACT FROM AUTHOR]

Abstract: Ab initio atomistic thermodynamics is utilized to achieve the understanding of the surface structure and stability of χ-Fe5C2 under CO and synthesis gas (H2/CO) pretreatment conditions in catalyst activation for Fischer–Tropsch synthesis (FTS). On the basis of the computed surface free energy (γ) as a function of the carbon chemical potential (μ C) by considering temperature, pressure, and H2/CO ratios, it is found that CO pretreatment favors stable carbon-rich facets, while small amount of H2 added into CO leads to a large decrease in μ C and thus stabilizing carbon-poor facets. The high activity of surface carbon toward hydrogenation might explain the enhanced initial activity of the FTS catalysts activated from CO pretreatment. Moreover, under both CO and H2/CO pretreatments, either low temperature or low pressure can lead to stable carbon-rich facets. [Copyright &y& Elsevier]

A theoretical chemistry approach combining Wulff construction, density functional theory, and microkinetics enables revealing the nature of facet-dependent key elementary reactions on single-phase iron carbide at single particle level, complementary to current limited understanding of this issue in experimental exploration of iron-catalyzed Fischer-Tropsch Synthesis. • Intrinsic catalysis of a single-phase iron carbide particle in FTS was studied. • Neither too strong nor too weak H adsorption is favorable for CH 4 formation. • Coupling of two H-deficient hydrocarbon intermediates is more facile. • The surface (111) and (10 1 -) are kinetically more viable for C 2 formation. • CH 4 formation is kinetically more facile on surface (010), (110) and (11 1 -). Elucidation of intrinsic working principle of single-phase iron carbide and its facet-dependent catalytic behavior remains a substantial challenge in iron-catalyzed Fischer-Tropsch synthesis. Here, we provided in-depth understanding of the iron carbide phase-dependent and facet-dependent properties on theoretically established Fe 5 C 2 particle model through an approach combining Wulff construction, density functional theory, and microkinetics. We studied two key probe reactions, C 2 formation via C 1 +C 1 coupling and CH 4 formation, by monitoring surface-dependent thermodynamics and microkinetics. Integration of results thereby allows us to assess the macroscopic catalytic properties at single particle level and evaluate contribution from individual exposed surface. The surface (111) and (10 1 -) are kinetically more viable for C 2 formation, whereas CH 4 formation is kinetically more facile on surface (010), (110) and (11 1 -). This study enriches the knowledge of the intrinsic working mechanism for single-phase iron carbide and provides fundamental insights into rational design of improved iron-based Fischer-Tropsch synthesis catalysts. [ABSTRACT FROM AUTHOR]