The reducing temperature and gas hourly space velocity were varied to obtain different Fe phase compositions for a fused-iron catalyst. These model catalysts were subject to the Fischer–Tropsch synthesis (FTS) and phase evolution was monitored. The BET surface area decreased as the reduction temperature was increased from 350 °C to 650 °C, and the pore diameter changed by the opposite trend. XRD and Mössbauer spectroscopy were used to quantitatively determine the phase composition. They indicated that a high reduction temperature and high GHSV (gas hourly space velocity) favored the transformation from Fe₃O₄ to α-Fe, although a higher reduction temperature led to larger α-Fe crystallite sizes. Furthermore, α-Fe gradually transforms to an iron carbide (χ-Fe₅C₂) phase in a period of 12 h when exposed to syngas (CO + H₂). After a 500 h run in the FTS, the partial carbide regenerated into Fe₃O₄ again. Magnetite had negligible catalytic activity for the FTS, whereas iron carbide was active. Nonetheless, the activity was proportional to the surface area of iron carbide. Higher reduction temperatures guaranteed lower methane selectivity and a higher olefin to paraffin ratio for C_2–4 hydrocarbons, and possibly high reducibility and large pore sizes as well.

Considerable interest has been paid in the last few years to the important environmentally benign catalytic hydrogenation reaction of dimethyl oxalate (DMO) to produce methyl glycolate (MG), ethylene glycol (EG) and ethanol (ETOH). SiO₂-supported noble metal elements, such as Au, Ag and Ru, have been nominated as efficient catalysts for the production of high MG selectivity. In this work, noble-metal-free Cu nanoparticles wrapped on reduced graphene oxide (RGO) nanosheets were synthesized using a green in situ sonochemical method in an aqueous medium and then applied without the calcination step as a robust catalyst in the DMO hydrogenation reaction. Moreover, the effects of ultrasound (US), ammonia (NH₄OH), a combination of both US and NH₄OH, and Cu loading (10, 25 and 45 wt%) on the morphology and catalytic performance of the produced Cu/RGO catalysts were studied in detail. Interestingly, transmission electron microscopy images revealed that a dramatic morphology evolution of the Cu catalysts from ultra small dots to elongated and leaf-like nanoparticles shapes occurred, depending on simple modification of the reaction parameters. Notably, the obtained catalytic results demonstrated that among the different produced six catalysts, the prepared Cu/RGO catalyst using 20 kHz of ultrasound and in the presence of ammonia with 25 wt% Cu loading displayed the highest MG selectivity (98.8%) at a relatively low reaction temperature (210 °C), while, the synthesized Cu/RGO catalyst with 45 wt% Cu loading exhibited the highest ETOH selectivity (94%) at 240 °C. Moreover, both catalysts showed long-term stability for at least 300 h while maintaining the high DMO conversion and selectivity ratio. The comprehensive characterization to the as-prepared, reduced and spent catalysts concluded that the synergistic ratio of Cu⁺/(Cu⁺ + Cu⁰), the high dispersion of Cu nanoparticles, the defective edges, sites and electron transfer provided from RGO were the main reasons for the obtained superior catalytic activity and long-term stability.

Silica-supported molybdenum carbide (Mo₂C/SiO₂) is found to be a highly active, selective and stable catalyst for the hydrogenation of dimethyl oxalate to ethanol at low temperatures (473 K).

Noble metal-free ultra-small and highly dispersed Cu/SiO ₂ catalyst with high Cu ⁺ /(Cu ⁺ + Cu ⁰ ) ratio synthesized by novel and rapid sonochemical method displayed excellent MG selectivity.

In this study, Fe3O4 nanocubes (NCs) decorated on RGO nanosheets (NSs) structures were successfully synthesized through an innovative and environmentally-friendly rapid sonochemical method. More importantly, iron(II) sulfate heptahydrate and GO were employed as precursors and water as reaction medium, meanwhile, NaOH within the generated free radicals from the high intensity ultrasound were sufficient as reducing and base agent in our clean synthesis. Moreover, the hydrothermal method as a conventional approach was employed to synthesize the same catalysts for the comparison with the ultrasonocation technique. The as-synthesized Fe3O4 and RGO/Fe3O4 NSs catalysts were exposed to industrially relevant Fischer-tropsch synthesis (FTS) conditions at various reaction temperatures (250-290 °C), and they subjected to fully characterization before and after FTS reaction using XRD, TEM, HRTEM, EDS mapping, XPS, FTIR, BET, H2-TPR, H2-TPD and CO-TPD to understand the structure-performance relationships. Notably, the catalysts produced using the sonochemical method had a better CO conversion rate [Fe3O4 (80%), RGO/Fe3O4 (82%)] than the hydrothermally synthesized catalysts. However, compared to the naked-Fe3O4 catalysts, the sonochemically and hydrothermally synthesized RGO-supported Fe3O4 catalysts had higher long chain hydrocarbon (C5+) selectivity values (72% and 67%) and C2-C4 olefin/paraffin selectivity ratio (3.2 and 2) and low CH4 selectivity values (6% and 8.5%), respectively. This can be attributed to their high surface area, the degree of reducibility, and content of Hägg iron carbide (χ-Fe5C2) as the most active phase of the FTS reaction. Proposed reaction mechanisms for the sonochemical and hydrothermal reaction synthesis of Fe3O4 and RGO/Fe3O4 nanoparticles are discussed. In conclusion, our developed surfactantless-sonochemical method holds promise for the eco-friendly synthesis of highly efficient catalysts materials for FTS reaction.