Construction of three-dimensional nitrogen-doped graphene aerogel (NGA) supported cobalt catalysts for Fischer-Tropsch synthesis

Is Carbon Heteroatom Doping the Key to Active and Stable Carbon Supported Cobalt Fischer-Tropsch Catalysts?

Carbon supports are an interesting alternative to established oxidic catalyst supports for Co-based Fischer-Tropsch synthesis (FTS) catalysts as they allow high Co reducibility and do not suffer from the formation of Co/support compounds. To optimize Co-based carbon-supported FTS catalysts, significant research has focused on doping carbon supports with heteroatoms, aiming to enhance both catalytic activity and stability. While improvements in FTS performance have been reported for N-doped carbon supports, the exact effects of heteroatom doping are still poorly understood, largely due to difficulties in directly comparing Co FTS catalysts supported on doped versus nondoped carbon materials. In this study, we synthesized a series of highly comparable N-, S-, and P-doped carbon nanofiber (CNF) model supports, which were combined with size-controlled, colloidal Co nanoparticles to create well-defined model FTS catalysts. Comprehensive characterization of these catalysts using in situ X-ray absorption spectroscopy (XAS), in situ X-ray diffraction (XRD), and in situ magnetometry revealed that the presence of dopants significantly altered the structure and properties of the catalytically active Co0 phase, affecting Co coordination numbers, crystal phase composition, and magnetic behavior. Challenging optimistic literature reports, our findings demonstrate that all the studied heteroatoms negatively impact either FTS activity or catalyst stability. Co on N-doped CNFs experienced rapid deactivation due to increased sintering as well as Co phase transformations, which were not observed for Co on nondoped CNFs. Co on S-doped CNF suffered from instability of carbon-bound S species in a hydrogen atmosphere, contributing to low FTS performance by S-poisoning. Finally, Co on P-doped CNFs displayed strong metal-support interactions that improved sintering stability, but FTS activity was hampered by low Co reducibility and the loss of active Co0 due to a complex sequence of cobalt phosphide formation and its subsequent decomposition into phosphorus oxides and cobalt oxide species under FTS conditions.

Plasma-Assisted Preparation of Reduced Graphene Oxide and Its Applications in Energy Storage

Reduced graphene oxide (rGO) exhibits mechanical, optoelectronic, and conductive properties comparable to pristine graphene, which has led to its widespread use as a method for producing graphene-like materials in bulk. This paper reviews the characteristics of graphene oxide and the evolution of traditional reduction methods, including chemical and thermal techniques. A comparative analysis reveals that these traditional methods encounter challenges, such as toxicity and high energy consumption, while plasma reduction offers advantages like enhanced controllability, the elimination of additional reducing agents, and reduced costs. However, plasma reduction is complex and significantly influenced by process parameters. This review highlights the latest advancements in plasma technology for reducing graphene oxide, examining its effectiveness across various gas environments. Inert gas plasmas, such as argon (Ar) and helium (He), demonstrate superior reduction efficiency, while mixed gases facilitate simultaneous impurity reduction. Additionally, carbon-based gases can aid in restoring defects in graphene oxide. This paper concludes by discussing the future prospects of plasma-reduced graphene and emphasizes the importance of understanding plasma parameters to manage energy and chemical footprints for effective reduction.

CO2 hydrogenation over Fe-Co bimetallic catalysts with tunable selectivity through a graphene fencing approach

Tuning CO2 hydrogenation product distribution to obtain high-selectivity target products is of great significance. However, due to the imprecise regulation of chain propagation and hydrogenation reactions, the oriented synthesis of a single product is challenging. Herein, we report an approach to controlling multiple sites with graphene fence engineering that enables direct conversion of CO2/H2 mixtures into different types of hydrocarbons. Fe-Co active sites on the graphene fence surface present 50.1% light olefin selectivity, while the spatial Fe-Co nanoparticles separated by graphene fences achieve liquefied petroleum gas of 43.6%. With the assistance of graphene fences, iron carbides and metallic cobalt can efficiently regulate C-C coupling and olefin secondary hydrogenation reactions to achieve product-selective switching between light olefins and liquefied petroleum gas. Furthermore, it also creates a precedent for CO2 direct hydrogenation to liquefied petroleum gas via a Fischer-Tropsch pathway with the highest space-time yields compared to other reported composite catalysts.

Hexamethyldisiloxane Removal from Biogas Using a Fe3O4-Urea-Modified Three-Dimensional Graphene Aerogel

Volatile methyl siloxanes (VMS), which are considered to be the most troublesome impurities in current biogas-cleaning technologies, need to be removed. In this study, we fabricated a series of Fe3O4-urea-modified reduced graphene-oxide aerogels (Fe3O4-urea-rGOAs) by using industrial-grade graphene oxide as the raw material. A fixed-bed dynamic adsorption setup was built, and the adsorption properties of the Fe3O4-urea-rGOAs for hexamethyldisiloxane (L2, as a VMS model pollutant) were studied. The properties of the as-prepared samples were investigated by employing various characterization techniques (SEM, TEM, FTIR, XRD, Raman spectroscopy, and N2 adsorption/desorption techniques). The results showed that the Fe3O4-urea-rGOA-0.4 had a high specific surface area (188 m2 g-1), large porous texture (0.77 cm3 g-1), and the theoretical maximum adsorption capacity for L2 (146.5 mg g-1). The adsorption capacity considerably increased with a decrease in the bed temperature of the adsorbents, as well as with an increase in the inlet concentration of L2. More importantly, the spent Fe3O4-urea-rGOA adsorbent could be readily regenerated and showed an excellent adsorption performance. Thus, the proposed Fe3O4-urea-rGOAs are promising adsorbents for removing the VMS in biogas.

Oxidative Cleavage of β-O-4 Linkage in Lignin via Co Nanoparticles Embedded in 3DNG as Catalyst

The cleavage of β-O-4 linkage in lignin is one of the key steps for oxidative conversion of lignin to low-molecular-weight aromatics. Herein, Co nanoparticles embedded in three-dimensional network of nitrogen-doped graphene (Co/NG@3DNG-X) were prepared through an immersion-pyrolysis procedure, in which X denotes the pyrolysis temperature. The detailed characterization of Co/NG@3DNG-X shows that the Co nanoparticles are coated with a few layers of nitrogen-doped graphene (NG) sheets that are further embedded in 3DNG matrix. The catalytic activities of the Co/NG@3DNG-X for the oxidative cleavage of β-O-4 linkage in lignin model compounds with O₂ as oxidant are explored. It is demonstrated that catalytic activities of as-prepared Co/NG@3DNG-X can be tuned by varying the pyrolysis condition, and the Co/NG@3DNG-900 shows the highest catalytic activity, which is attributed to the enriched Co-N_x species, the strong surface basicity, the high specific surface and the mesoporous motif of 3DNG network. More pronouncedly, the Co/NG@3DNG-900 can also effectively catalyze the oxidative cleavage of organosolv lignin, generating certain monomeric aromatics. Additionally, the intrinsic magnetic property of Co nanoparticles makes the Co/NG@3DNG-X be easily recovered from the reaction mixture, and the as-coated thin NG layer can protect Co nanoparticle from oxidation condition, which putting together afford the Co/NG@3DNG-X with good reusability and stability.

Vitamin C-Assisted Fabrication of Aerogels from Industrial Graphene Oxide for Gaseous Hexamethyldisiloxane Adsorption

Volatile methyl siloxanes (VMSs) as a trace impurity in biogas decreases its energy utilization, and thus need to be removed. In this paper, a one-step hydrothermal reduction was performed to produce three-dimensional reduced graphene oxide aerogels (rGOAs) using industrial-grade graphene oxide (IGGO) as raw material and vitamin C (VC) as a reductant to facilitate the fabrication of rGOAs. The synthesis of rGOAs was a simple, green, and energy-efficient process. The developed rGOAs were characterized using the Brunauer–Emmett–Teller method, Raman spectrometry, scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction measurements and contact angle. The results obtained showed that rGOA-1 with a VC/IGGO ratio of 1/1 (m/m) exhibited a hierarchical porous structure and super-hydrophobicity, yielding a high specific surface area (137.9 m2 g−1) and superior water contact angle (143.8°). The breakthrough adsorption capacity of rGOA-1 for hexamethyldisiloxane (L2, a VMS model) was 11 times higher than that of IGGO. Low inlet concentration and bed temperature were considered beneficial for the L2 adsorption. Interestingly, rGOA-1 was less sensitive to water, and it was readily regenerated for reuse by annealing at 80 °C. The rGOAs have been demonstrated to have great potential for the removal of siloxanes from biogas.