Studies of colloidal iron carbide nanoparticle Fischer-Tropsch catalysts: Characterizing adsorption sites and reactivity towards hydrogen atom transfers

Iron carbide catalysts have been used for nearly 100 years in the Fischer-Tropsch process (FTP), yet the atomic nature of the active site(s) for H2 and CO have not been fully characterized. The FTP has recently gained interest as a method for the sustainable production of aviation fuels; however, this process suffers from limited product selectivity. A better understanding of the active site(s) could allow for more rational design of iron carbide catalysts, where changes in active site structure(s) could be correlated with catalyst activity. Here, we present the synthesis, characterization, and catalysis by a well-defined dodecylamine-capped colloidal iron carbide (DDA-FexC) nanoparticle (NP) system. This colloidal NP system is amenable to solution phase reactivity studies, spectroscopic measurements, and catalysis towards olefin hydrogenation and carbon monoxide hydrogenation at mild conditions. The tandem use of x-ray and FTIR spectroscopies along with density functional theory (DFT) calculations and molecular dynamic simulations enabled the identification of the structures of adsorbed hydrogen (Hads) and carbon monoxide (COads) over these DDA-FexC NPs. 57Fe Nuclear resonant vibrational spectroscopy revealed a Fe-C vibration for COads consistent with terminally bound CO, as supported by DFT calculations. FTIR revealed a distribution of *C-D vibrations for NPs treated with D2, consistent with adsorption over surface carbide sites supported by DFT calculations. Extended x-ray absorption fine structure (EXAFS) measurements of DDA-FexC NPs treated with H2 and CO showed measurable increases in Fe-Fe and Fe-C bond lengths that varied with coverage. The experimentally measured vibrational energies are used to validate the active site structures generated by DFT calculations and molecular dynamics. These results demonstrate the powerful combination of experiment and theory to better understand an elusive catalytic system and may aid in the rational development of future iron carbide catalysts

Continuous Hydrothermal Synthesis of Inorganic Nanoparticles: Applications and Future Directions

Nanomaterials are at the leading edge of the emerging field of nanotechnology. Their unique and tunable size-dependent properties (in the range 1-100 nm) make these materials indispensable in many modern technological applications. In this Review, we summarize the state-of-art in the manufacture and applications of inorganic nanoparticles made using continuous hydrothermal flow synthesis (CHFS) processes. First, we introduce ideal requirements of any flow process for nanoceramics production, outline different approaches to CHFS, and introduce the pertinent properties of supercritical water and issues around mixing in flow, to generate nanoparticles. This Review then gives comprehensive coverage of the current application space for CHFS-made nanomaterials including optical, healthcare, electronics (including sensors, information, and communication technologies), catalysis, devices (including energy harvesting/conversion/fuels), and energy storage applications. Thereafter, topics of precursor chemistry and products, as well as materials or structures, are discussed (surface-functionalized hybrids, nanocomposites, nanograined coatings and monoliths, and metal-organic frameworks). Later, this Review focuses on some of the key apparatus innovations in the field, such as in situ flow/rapid heating systems (to investigate kinetics and mechanisms), approaches to high throughput flow syntheses (for nanomaterials discovery), as well as recent developments in scale-up of hydrothermal flow processes. Finally, this Review covers environmental considerations, future directions and capabilities, along with the conclusions and outlook