Size dependence of carbon-encapsulated iron-based nanocatalysts for Fischer–Trposch synthesis

The conversion from syngas derived from non-petroleum recourses to liquid fuels and chemicals via Fischer–Tropsch synthesis(FTS)is regarded as an alternative and potential route.Developing catalyst with controllable particle size and clarifying size effect are of significance to promote the process.Herein,we engineered carbon-encapsulation structure to restrict particle growth but avoid strong metal–support interactions.The prepared carbon-encapsulated nanoparticles(Fe@C)showed a superior catalytic activity compared with conventional carbon-supported nanoparticles(Fe/C).By tuning particle size from 3.0 to 9.1 nm,a volcano-like trend of iron time yield(FTY)peaked at 2659μmol·gFe^(−1)·s^(−1)is obtained with an optimum particle size of 5.3 nm.According to temperature-programmed reduction and desorption results,a linear relationship between apparent turnover frequency and CO dissociation capacity was established.The enhanced CO dissociative adsorption along with weakened H_(2)activation on larger nanoparticles resulted in higher C_(5+)selectivity.This study provides a strategy to synthesize carbon supported metal catalysts with controllable particle size and insight into size effect on Fe-based catalytic FTS.

Pyridinic-N doping carbon layers coupled with tensile strain of FeNi alloy for activating water and urea oxidation

Exploitation of oxygen evolution reaction(OER)and urea oxidation reaction(UOR)catalysts with high activity and stability at large current density is a major challenge for energy-saving H_(2) production in water electrolysis.Herein,we use the pyridinic-N doping carbon layers coupled with tensile strain of FeNi alloy activated by NiFe_(2)O_(4)(FeNi/NiFe_(2)O_(4)@NC)for efficiently increasing the performance of water and urea oxidation.Due to the tensile strain effect on FeNi/NiFe_(2)O_(4)@NC,it provides a favorable modulation on the electronic properties of the active center,thus enabling amazing OER(η_(100)=196 mV)and UOR(E_(10)=1.32 V)intrinsic activity.Besides,the carbon-coated layers can be used as armor to prevent FeNi alloy from being corroded by the electrolyte for enhancing the OER/UOR stability at large current density,showing high industrial practicability.This work thus provides a simple way to prepare high-efficiency catalyst for activating water and urea oxidation.

Strong electronic coupling of CoNi and N-doped-carbon for efficient urea-assisted H2 production at a large current density

Exploiting efficient urea oxidation reaction(UOR)and hydrogen evolution reaction(HER)catalysts are significant for energy-saving H2 production through urea-assisted water electrolysis,but it is still challenging.Herein,carbon-encapsulated CoNi coupled with CoNiMoO(CoNi@CN-CoNiMoO)is prepared by solvothermal method and calcination to enhance the activity/stability of urea-assisted water electrolysis at large current density.It exhibits good activity for UOR(E10/1,000=1.29/1.40 V)and HER(E-10/-1000=-45/-245 mV)in 1.0 M KOH+0.5 M urea solution.For the UOR||HER system,CoNi@CN-CoNiMoO only needs 1.58 V at 500 mA cm-2 and shows good stability.Density functional theory calculation suggests that the strong electronic interaction at the interface between NiCo alloy and N-doping-carbon layers can optimize the adsorption/desorption energy of UOR/HER intermediates and accelerate the water dissociation,which can expedite urea decomposition and Volmer step,thus increasing the UOR and HER activity,respectively.This work provides a new solution to design UOR/HER catalysts for H2 production through urea-assisted water electrolysis.

Modulating proton binding energy on the tungsten carbide nanowires surfaces for boosting hydrogen evolution in acid

ungsten carbides have attracted wide attentions as Pt substitute electrocatalysts for hydrogen evolution reaction (HER), due to their good stability in an acid environment and Pt-like behaviour in hydrolysis. However, quantum chemistry calculations predict that the strong tungsten-hydrogen bonding hinders hydrogen desorption and restricts the overall catalytic activity. Synergistic modulation of host and guest electronic interaction can change the local work function of a compound, and therefore, improve its electrocatalytic activity over either of the elements individually. Herein, we develop a creative and facile solid-state approach to synthesize self-supported carbon-encapsulated single-phase WC hybrid nanowires arrays (nanoarrays) as HER catalyst. The theoretical calculations reveal that carbon encapsulation modifies the Gibbs free energy of H* values for the WC adsorption sites, endowing a more favorable C@WC active site for HER. The experimental results exhibit that the hybrid WC nanoarrays possess remarkable Pt-like catalytic behavior, with superior activity and stability in an acidic media, which can be compared to the best non-noble metal catalysts reported to date for hydrogen evolution reaction. The present results and the facile synthesis method open up an exciting avenue for developing cost-effective catalysts with controllable morphology and functionality for scalable hydrogen generation and other carbide nanomaterials applicable to a range of electrocatalytic reactions.

Carbon composite support improving catalytic effect of NbC nanoparticles on the low-temperature hydrogen storage performance of MgH_(2)

Ultrafine carbon-based transition metal compounds have been widely investigated as efficient catalysts for enhancing the hydrogen storage performance of magnesium hydride.In this work,the carbon ther-mal shock method is applied to synthesize the ultrafine carbon-encapsulated NbC nanoparticles with an average grain size of 17.3 nm.The MgH_(2)-10 wt%NbC/C composites show excellent low-temperature hy-drogen storage performance with the onset dehydrogenation temperature of 196.1℃,which is 92.2℃ and 98℃ lower than that of MgH_(2)-10 wt%NbC and undoped MgH_(2),respectively.Specifically,MgH_(2)-10 wt%NbC/C can absorb 6.71 wt%H_(2) at 100℃ within 30 min around and retain almost 100%reversible hydrogen desorption capacity after 10 cycles.For the catalytic mechanism,the electron transfer process between multi-valence Nb cations of in-situ formed NbH x and Mg,H atoms can greatly improve the cyclic de/rehydrogenation kinetics of MgH_(2)-NbC/C.Besides,the enhancement of dehydrogenation kinetics can also be ascribed to MgH_(2) particle refinement by NbC nanoparticles,and destabilization of the Mg-H bond caused by carbon substrate.This investigation not only proves that carbon-encapsulated NbC nanoparti-cles can greatly enhance the hydrogen storage performance of MgH_(2) but provides an idea of preparing carbon-based transition metal carbides as effective catalysts for magnesium-based hydrogen storage ma-terials.

Fe_(3)C@C/C for catalytic ozonation of silicon-containing wastewater:Dual improvement of silicon resistance and catalytic effect

The improvement of catalysts’stability under harsh reaction conditions is vital for their practical applica-bility.Herein,iron carbide(Fe_(3)C)nanoparticles were encapsulated in graphitic carbon in situ and a carbon ball served as the carrier.The synthesized Fe_(3)C@C/C was first utilized to treat an m-cresol wastewater containing Si via catalytic ozonation.Compared with the commercial Fe/Al_(2)O_(3)catalyst,the resistance to Si of the Fe_(3)C@C/C was improved 22.68 times,while the TOC removal rate increased by a factor of 2.9,and it remained stable during 10 cycles and 12000 min of continuous reaction,which further demon-strated its potential for diverse applications.The catalyst exhibits improved resistance to Si because of the dual protection from the carbon-encapsulated structure and carbon carrier.Density functional theory calculations show that the encapsulation of Fe_(3)C using carbon significantly increases the resistance to adsorption of Si on its active sites.In addition,the activation of O_(3)is unimpeded on the Fe_(3)C adsorption sites by the protection from C,thus the generation of reactive oxygen species(ROS)by ozone is largely promoted.The mechanism associated with the resistance of the Fe_(3)C@C/C catalyst to Si and its elevated activity are also elucidated.