Mesoporous nitrogen-doped carbon nanoparticles with atomically dispersed iron sites (named mesoNC-Fe) are synthesized via high-temperature pyrolysis of an Fe containing ZIF-8 MOF. Hydrolysis of tetramethyl orthosilicate (TMOS) in the MOF framework prior to pyrolysis plays an essential role in maintaining a high surface area during the formation of the carbon structure, impeding the formation of iron (oxide) nanoparticles. To gain inside on the nature of the resulting atomically dispersed Fe moieties, HERFD-XANES, EXAFS and valence-to-core X-ray emission spectroscopies have been used. The experimental spectra (both XAS and XES) combined with theoretical calculations suggest that iron has a coordination sphere including a porphyrinic environment and OH/H₂O moieties responsible for the high activity in CO₂ electroreduction. DFT calculations demonstrate that CO formation is favored in these structures because the free energy barriers of *COOH formation are decreased and the adsorption of *H is impeded. The combination of such a unique coordination environment with a high surface area in the carbon structure of mesoNC-Fe makes more active sites accessible during catalysis and promotes CO₂ electroreduction.

Abstract: Mn and Li promoted Rh catalysts supported on SiO₂ with a thin TiO₂ layer were synthesized by stepwise incipient wetness impregnation approach. The thin TiO₂ layer on the surface of SiO₂ was proved to stabilize those small Rh nanoparticles and hinder their agglomeration. The reducibility of Rh on these catalysts depends on Rh particle size as well as the position of manganese oxide, and large Rh nanoparticles with MnO on Rh nanoparticles can be only reduced at an elevated temperature. Catalyst with large Rh particles exhibits a higher CO conversion and higher products selectivity towards long chain hydrocarbons and C2-oxygenates at the expense of decreasing methane formation than a similar catalyst with smaller Rh particles. This was attributed to the synergistic effect of Mn and Li promotion and molar ratio between Rh⁰ and Rh^δ+ sites on the surface of Rh nanoparticles. Moreover, Rh nanoparticles on MnO are proved to be more efficient in promoting hydrogenation of acetaldehyde to ethanol than its counterpart with MnO on Rh nanoparticles. Finally, in order to target high C2-oxygenates selectivity, low reaction temperature together with a low H₂/CO ratio in the feed is recommended. Graphic Abstract: [Figure not available: see fulltext.].

Iron carbides are unmistakably associated with the active phase for Fischer-Tropsch synthesis (FTS). The formation of these carbides is highly dependent on the catalyst formulation, the activation method and the operational conditions. Because of this highly dynamic behavior, studies on active phase performance often lack the direct correlation between catalyst performance and iron carbide phase. For the above reasons, an extensive in situ Mössbauer spectroscopy study on highly dispersed Fe on carbon catalysts (Fe@C) produced through pyrolysis of a Metal Organic Framework was coupled to their FTS performance testing. The preparation of Fe@C catalysts via this MOF mediated synthesis allows control over the active phase formation and therefore provides an ideal model system to study the performance of different iron carbides. Reduction of fresh Fe@C followed by low-temperature Fischer-Tropsch (LTFT) conditions resulted in the formation of the ε′-Fe_2.2C, whereas carburization of the fresh catalysts under high-temperature Fischer-Tropsch (HTFT) resulted in the formation of χ-Fe₅C₂. Furthermore, the different activation methods did not alter other important catalyst properties, as pre- and post-reaction transmission electron microscopy (TEM) characterization confirmed that the iron nanoparticle dispersion was preserved. The weight normalized activities (FTY) of χ-Fe₅C₂ and ε′-Fe_2.2C are virtually identical, whilst it is found that ε′-Fe_2.2C is a better hydrogenation catalyst than χ-Fe₅C₂. The absence of differences under subsequent HTFT experiments, where χ-Fe₅C₂ is the dominating phase, is a strong indication that the iron carbide phase is responsible for the differences in selectivity.

A nitrogen-doped carbon was synthesized through the pyrolysis of the well-known metal-organic framework ZIF-8, followed by a subsequent acid treatment, and has been applied as a catalyst in the electrochemical reduction of carbon dioxide. The resulting electrode shows Faradaic efficiencies to carbon monoxide as high as ∼78%, with hydrogen being the only byproduct. The pyrolysis temperature determines the amount and the accessibility of N species in the carbon electrode, in which pyridinic-N and quaternary-N species play key roles in the selective formation of carbon monoxide.

A supported cobalt catalyst with atomically dispersed Co-N_x sites (3.5 wt% Co) in a mesoporous N-doped carbon matrix (named Co@mesoNC) is synthesized by hydrolysis of tetramethyl orthosilicate (TMOS) in a Zn/Co bimetallic zeolitic imidazolate framework (BIMZIF(Co,Zn)), followed by high-temperature pyrolysis and SiO₂ leaching. A combination of TEM, XRD XPS and X-ray absorption spectroscopy studies confirm the absence of cobalt nanoparticles and indicate that these highly dispersed cobalt species are present in the form of Co-N_x. The exclusive formation of Co-N_x sites in the carbon matrix is attributed to the presence of a large amount of Zn and N in the BIMZIF precursor together with the presence of SiO₂ in the pore space of this framework, extending the initial spatial distance between cobalt atoms and thereby impeding their agglomeration. The presence of SiO₂ during high-temperature pyrolysis is proven crucial to create mesoporosity and a high BET area and pore volume in the N-doped carbon support (1780 m² g⁻¹, 1.54 cm³ g⁻¹). This heterogeneous Co@mesoNC catalyst displays high activity and selectivity (>99%) for the selective hydrogenation of nitrobenzene to aniline at mild conditions (0.5–3 MPa, 343–383 K). When more challenging substrates (functionalized nitroarenes) are hydrogenated, the catalyst Co@mesoNC displays an excellent chemoselectivity to the corresponding substituted anilines. The presence of mesoporosity improves mass transport of reactants and/or products and the accessibility of the active Co-N_x sites, and greatly reduces deactivation due to fouling.

The development of synthetic protocols for the preparation of highly loaded metal nanoparticle-supported catalysts has received a great deal of attention over the last few decades. Independently controlling metal loading, nanoparticle size, distribution, and accessibility has proven challenging because of the clear interdependence between these crucial performance parameters. Here we present a stepwise methodology that, making use of a cobalt-containing metal organic framework as hard template (ZIF-67), allows addressing this long-standing challenge. Condensation of silica in the Co-metal organic framework pore space followed by pyrolysis and subsequent calcination of these composites renders highly loaded cobalt nanocomposites (~ 50 wt.% Co), with cobalt oxide reducibility in the order of 80% and a good particle dispersion, that exhibit high activity, C5 + selectivity and stability in Fischer-Tropsch synthesis.

The effect of ruthenium particle size on Fischer-Tropsch synthesis (FTS) has been studied at 513 K, H₂/CO = 2 and 15 bar. Supported Ru catalysts with particle sizes ranging from 1.7 to 12 nm were prepared by using different Ru loadings and two different high surface area graphite (HSAG) supports to minimize the metal-support interaction. In addition, the effect of promotion with Cs is also evaluated. Microcalorimetric characterization during CO adsorption and XPS reveal a clear interaction between Ru and Cs. The FTS with Ru-based catalysts is, independent of the presence of promoter, highly structure-sensitive when the Ru particle size is under 7 nm. In this range the turnover frequency (TOF) for CO conversion increases with particle size, reaching a near constant value for Ru particles larger than 7 nm. Cs promoted catalysts display lower TOF values than the corresponding unpromoted samples. This somewhat reduced activity is attributed to the stronger CO adsorption on Cs promoted catalysts, as demonstrated by CO adsorption microcalorimetry. Product selectivity depends also on Ru particle size. Selectivity to C₅₊ hydrocarbons increases with increasing Ru particle size. For Cs-promoted catalysts, the olefin to paraffin ratio in the C₂-C₄ hydrocarbons range is independent of the Ru particle size, whereas it decreases for the unpromoted catalysts, showing the prevailing influence of the promoter.

The structure and catalytic performance of bifunctional 10 wt% Co/mesoHZSM-5 catalysts pretreated under different conditions, i.e. in stagnant air, or in a flow of air, N₂, or 1 vol% NO/Ar, were investigated for the Fischer-Tropsch synthesis (FTS) under fixed operating conditions of T = 513 K, P = 15 bar, H₂/CO = 1. The combination of acid sites and FTS functionality leads to the direct formation of gasoline range hydrocarbons and suppresses the formation of C20+ products. The highest activity, C5-C11 selectivity and lowest CH₄ selectivity were obtained for Co/mesoHZSM-5 catalyst pretreated in stagnant air. Pretreatment in gas flow resulted in a lower activity and C5-C11 selectivity, and in a higher CH₄ selectivity, in particular for samples pretreated with NO. Characterization shows that this underperformance is due to changes in the Co₃O₄ particle size distribution and cobalt reducibility, and is related to the cobalt loading relative to the mesopore area. Pretreatment in air or N₂ flow increased the number of small Co₃O₄ particles and increased cobalt reducibility by suppressing the formation of highly dispersed cobalt, e.g. cobalt silicates, in strong interaction with mesoHZSM-5. Pretreatment in a 1 vol% NO/Ar flow significantly increased cobalt dispersion further, decreasing the cobalt reducibility due to the strong interaction between cobalt and mesoHZSM-5. Based on both TEM and in situ DRIFTS studies, the optimum performance of Co/mesoHZSM-5 pretreated in stagnant air could be attributed to a lower fraction of small cobalt particles, known to promote the formation of CH₄via hydrogenolysis or direct methanation. Additionally, small cobalt particles are more susceptible to be oxidized under FT conditions, thereby decreasing FT activity and indirectly increasing CH₄ selectivity by increasing the H₂/CO ratio through the water gas shift reaction.