scholarly journals Structure of Active Sites of Fe-N-C Nano-Catalysts for Alkaline Exchange Membrane Fuel Cells

Nanomaterials ◽  
2018 ◽  
Vol 8 (12) ◽  
pp. 965 ◽  
Author(s):  
Hirofumi Kishi ◽  
Tomokazu Sakamoto ◽  
Koichiro Asazawa ◽  
Susumu Yamaguchi ◽  
Takeshi Kato ◽  
...  
Keyword(s):  
Active Sites ◽  
Density Functional ◽  
Carbon Nanomaterials ◽  
Catalyst Activity ◽  
Platinum Group Metal ◽  
Reduction Reaction ◽  
Ex Situ ◽  
Metallic Particles ◽  
Theoretical Approaches ◽  
Nano Catalysts

Platinum group metal-free (PGM-free) catalysts based on transition metal-nitrogen-carbon nanomaterials have been studied by a combination of ex situ and in situ synchrotron X-ray spectroscopy techniques; high-resolution Transmission Electron Microscope (TEM); Mößbauer spectroscopy combined with electrochemical methods and Density Functional Theory (DFT) modeling/theoretical approaches. The main objective of this study was to correlate the HO2− generation with the chemical nature and surface availability of active sites in iron-nitrogen-carbon (Fe-N-C) catalysts derived by sacrificial support method (SSM). These nanomaterials present a carbonaceous matrix with nitrogen-doped sites and atomically dispersed and; in some cases; iron and nanoparticles embedded in the carbonaceous matrix. Fe-N-C oxygen reduction reaction electrocatalysts were synthesized by varying several synthetic parameters to obtain nanomaterials with different composition and morphology. Combining spectroscopy, microscopy and electrochemical reactivity allowed the building of structure-to-properties correlations which demonstrate the contributions of these moieties to the catalyst activity, and mechanistically assign the active sites to individual reaction steps. Associated with Fe-Nx motive and the presence of Fe metallic particles in the electrocatalysts showed the clear differences in the variation of composition; processing and treatment conditions of SSM. From the results of material characterization; catalytic activity and theoretical studies; Fe metallic particles (coated with carbon) are main contributors into the HO2− generation.

scholarly journals The Challenge of Achieving a High Density of Fe-Based Active Sites in a Highly Graphitic Carbon Matrix

Catalysts ◽  
2019 ◽  
Vol 9 (2) ◽  
pp. 144 ◽  
Author(s):  
Jingkun Li ◽  
Qingying Jia ◽  
Sanjeev Mukerjee ◽  
Moulay-Tahar Sougrati ◽  
Goran Drazic ◽  
...  
Keyword(s):  
Porous Structure ◽  
Active Sites ◽  
Carbon Matrix ◽  
Platinum Group Metal ◽  
Reduction Reaction ◽  
High Specific Surface Area ◽  
High Density ◽  
Graphitic Carbon ◽  
High Degree ◽  
Degree Of Graphitization

As one of the most promising platinum group metal-free (PGM-free) catalysts for oxygen reduction reaction (ORR), Fe–N–C catalysts with a high density of FeNx moieties integrated in a highly graphitic carbon matrix with a proper porous structure have attracted extensive attention to combine the high activity, high stability and high accessibility of active sites. Herein, we investigated a ZnCl2/NaCl eutectic salts-assisted ionothermal carbonization method (ICM) to synthesize Fe–N–C catalysts with tailored porous structure, high specific surface area and a high degree of graphitization. However, it was found to be challenging to anchor a high density of FeNx sites onto highly graphitized carbon. Iron precursors with preexisting Fe–N coordination were required to form FeNx sites in the nitrogen-doped carbon with a high degree of graphitization, while individual Fe and N precursors led to a Fe–N–C catalyst with poor-ORR activity. This provides valuable insights into the synthesis-structure relationship. Moreover, the FeNx moieties were identified as the major active sites in acidic conditions, while both FeNx sites and Fe2O3 were found to be active in alkaline medium.

scholarly journals Graphene Supported Tungsten Carbide as Catalyst for Electrochemical Reduction of CO2

Catalysts ◽  
2019 ◽  
Vol 9 (7) ◽  
pp. 604
Author(s):  
Sahithi Ananthaneni ◽  
Zachery Smith ◽  
Rees B. Rankin
Keyword(s):  
Tungsten Carbide ◽  
Electrochemical Reduction ◽  
Density Functional ◽  
Low Cost ◽  
Co2 Reduction ◽  
Platinum Group Metal ◽  
Reduction Reaction ◽  
Depth Study ◽  
Reduction Of Co2 ◽  
Co2 Reduction Reaction

Electrochemical reduction of CO2 to useful chemical and fuels in an energy efficient way is currently an expensive and inefficient process. Recently, low-cost transition metal-carbides (TMCs) have been proven to exhibit similar electronic structure similarities to Platinum-Group-Metal (PGM) catalysts and hence, can be good substitutes for some important reduction reactions. In this work, we test graphene-supported WC (Tungsten Carbide) nanoclusters as an electrocatalyst for the CO2 reduction reaction. Specifically, we perform density functional theory (DFT) studies to understand various possible reaction mechanisms and determine the lowest thermodynamic energy landscape of CO2 reduction to various products, such as CO, HCOOH, CH3OH, and CH4. This in-depth study of reaction energetics could lead to improvements and development of more efficient electrocatalysts for CO2 reduction.

scholarly journals Highly Graphitized Fe-N-C Electrocatalysts Prepared from Chitosan Hydrogel Frameworks

2021 ◽  
Author(s):  
Giorgia Daniel ◽  
Tomasz Kosmala ◽  
Federico Brombin ◽  
Marco Mazzucato ◽  
Alessandro Facchin ◽  
...  
Keyword(s):  
Active Sites ◽  
Chemical Properties ◽  
Platinum Group Metal ◽  
Reduction Reaction ◽  
Point Of View ◽  
Thermochemical Conversion ◽  
Chitosan Hydrogel ◽  
Acidic Aqueous Solution ◽  
Before And After ◽  
Nitrogen Doped Carbon

The development of platinum group metal-free (PGM-free) electrocatalysts derived from cheap and environmentally friendly biomasses for oxygen reduction reaction (ORR) is a topic of relevant interest, particularly from the point of view of sustainability. Fe-nitrogen-doped carbon materials (Fe-N-C) have attracted particular interest as alternative to Pt-based materials, due to the high activity and selectivity of Fe-Nx active sites, the high availability and good tolerance to poisoning. Recently, many studies focused on developing synthetic strategies, which could transform N-containing biomasses into N-doped carbons. In this paper chitosan was employed as a suitable N-containing biomass for preparing Fe-N-C catalyst in virtue of its high N content (7.1%) and unique chemical structure. Moreover, the major application of chitosan is based on its ability to strongly coordinate metal ions, a precondition for the formation of Fe-Nx active sites. The synthesis of Fe-N-C consists in a double step thermochemical conversion of a dried chitosan hydrogel. In acidic aqueous solution, the preparation of physical cross-linked hydrogel allows to obtain sophisticated organization, which assure an optimal mesoporosity before and after the pyrolysis. After the second thermal treatment at 900 °C, a highly graphitized material was obtained, which has been fully characterized in term of textural, morphological and chemical properties. RRDE technique was used for understanding the activity and the selectivity of the material versus the ORR in 0.5 M H2SO4 electrolyte. Special attention was put in the determination of the active site density according to nitrite electrochemical reduction measurements. It was clearly established that the catalytic activity expressed as half wave potential linearly scales with the number of Fe-Nx sites. It was also established that the addition of the iron precursor after the first pyrolysis step leads to an increased activity because of both an increased number of active sites and of a hierarchical structure, which improves the access to active sites. At the same time, the increased graphitization degree, and a reduced density of pyrrolic nitrogen groups are helpful to increase the selectivity toward the 4e- ORR pathway.

scholarly journals High-performance fuel cell cathodes exclusively containing atomically dispersed iron active sites

2019 ◽  
Vol 12 (8) ◽  
pp. 2548-2558 ◽  
Author(s):  
Hanguang Zhang ◽  
Hoon T. Chung ◽  
David A. Cullen ◽  
Stephan Wagner ◽  
Ulrike I. Kramm ◽  
...  
Keyword(s):  
Fuel Cell ◽  
Active Sites ◽  
High Performance ◽  
Platinum Group Metal ◽  
Reduction Reaction ◽  
Polymer Electrolyte Fuel Cells ◽  
Metal Free ◽  
Fuel Cell Cathodes ◽  
Iron Active Sites ◽  
Cell Cathodes

Platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) with atomically dispersed FeN4 sites have emerged as a potential replacement for low-PGM catalysts in acidic polymer electrolyte fuel cells (PEFCs).

scholarly journals On the Acid Sites of Phosphorous Modified Zeosils

2021 ◽  
Author(s):  
Gaurav Kumar ◽  
Limin Ren ◽  
Yutong Pang ◽  
Xinyu Li ◽  
Han Chen ◽  
...  
Keyword(s):  
Acid Site ◽  
Active Sites ◽  
Density Functional ◽  
Phosphorous Acid ◽  
Density Functional Theory Calculations ◽  
Acid Sites ◽  
Ex Situ ◽  
Characterization Methods ◽  
Isopropanol Dehydration ◽  
Phosphorus Containing

The acid sites of phosphorus-containing zeosils were probed through a combination of solid acid characterization, density functional theory calculations, and kinetic interrogations, establishing their weakly Brønsted acidic character. Due to the disparity in acid-site strength, P-zeosils catalyzed the probe chemistry of isopropanol dehydration slower than aluminosilicate zeolites by an order of magnitude on an active site basis. Propene selectivity during isopropanol dehydration remained 20-30% higher than that of aluminosilicates, illustrating the distinct nature of the weakly acidic phosphorus active sites that favored unimolecular dehydration routes. Regardless of the confining siliceous environment, the nature of phosphorous active sites was unchanged, indicated by identical apparent uni- and bi-molecular dehydration energy barriers. Kinetic isotope experiments with deuterated isopropanol feeds implicated an E2-type elimination to propene formation on phosphorus-containing materials. Comparison of KIEs between phosphorus-containing zeosils and aluminosilicates pointed to an unchanged isopropanol dehydration mechanism, with changes in apparent energetic barriers attributed to weaker binding on phosphorous-active sites that lead to a relatively destabilized alcohol dimer adsorbate. Both ex-situ alkylamine Hofmann elimination and in-situ pyridine titration characterization methods exhibited phosphorous acid site counts dependent on probe molecules identity and/or concentration, underpinning the limitations of extending common characterization techniques for Brønsted-acid catalysis to weakly acidic materials.

scholarly journals On the Acid Sites of Phosphorous Modified Zeosils

2021 ◽  
Author(s):  
Gaurav Kumar ◽  
Limin Ren ◽  
Yutong Pang ◽  
Xinyu Li ◽  
Han Chen ◽  
...  
Keyword(s):  
Acid Site ◽  
Active Sites ◽  
Density Functional ◽  
Phosphorous Acid ◽  
Density Functional Theory Calculations ◽  
Acid Sites ◽  
Ex Situ ◽  
Characterization Methods ◽  
Isopropanol Dehydration ◽  
Phosphorus Containing

The acid sites of phosphorus-containing zeosils were probed through a combination of solid acid characterization, density functional theory calculations, and kinetic interrogations, establishing their weakly Brønsted acidic character. Due to the disparity in acid-site strength, P-zeosils catalyzed the probe chemistry of isopropanol dehydration slower than aluminosilicate zeolites by an order of magnitude on an active site basis. Propene selectivity during isopropanol dehydration remained 20-30% higher than that of aluminosilicates, illustrating the distinct nature of the weakly acidic phosphorus active sites that favored unimolecular dehydration routes. Regardless of the confining siliceous environment, the nature of phosphorous active sites was unchanged, indicated by identical apparent uni- and bi-molecular dehydration energy barriers. Kinetic isotope experiments with deuterated isopropanol feeds implicated an E2-type elimination to propene formation on phosphorus-containing materials. Comparison of KIEs between phosphorus-containing zeosils and aluminosilicates pointed to an unchanged isopropanol dehydration mechanism, with changes in apparent energetic barriers attributed to weaker binding on phosphorous-active sites that lead to a relatively destabilized alcohol dimer adsorbate. Both ex-situ alkylamine Hofmann elimination and in-situ pyridine titration characterization methods exhibited phosphorous acid site counts dependent on probe molecules identity and/or concentration, underpinning the limitations of extending common characterization techniques for Brønsted-acid catalysis to weakly acidic materials.

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