Biochar (BCH) is a carbon-based bio-material produced from thermochemical conversion of biomass. Several activation or functionalization methods are usually used to improve physicochemical and ...functional properties of BCHs. In the context of green and sustainable future development, activated and functionalized biochars with abundant surface functional groups and large surface area can act as effective catalysts or catalyst supports for chemical transformation of a range of bioproducts in biorefineries. Above the well-known BCH applications, their use as adsorbents to remove pollutants are the mostly discussed, although their potential as catalysts or catalyst supports for advanced (electro)catalytic processes has not been comprehensively explored. In this review, the production/activation/functionalization of metal-supported biochar (M-BCH) are scrutinized, giving special emphasis to the metal-functionalized biochar-based (electro)catalysts as promising catalysts for bioenergy and bioproducts production. Their performance in the fields of biorefinery processes, and energy storage and conversion as electrode materials for oxygen and hydrogen evolutions, oxygen reduction, and supercapacitors, are also reviewed and discussed.
The smart assembly of Co-containing polyoxometalates (POMs) and Co-based zeolitic imidazole frameworks (ZIF-67) is a promising tool to produce active electrocatalysts for the oxygen evolution ...reaction (OER). The encapsulation of SiW
11
Co inside ZIF-67 cavities
via
'
in situ
' synthesis produced two novel POM@ZIF-67 nanocomposites with low and high POM loadings-SiW
11
Co@ZIF-67 and SiW
11
Coh@ZIF-67, respectively. Other materials involving the analogue tri-substituted silicotungstate, SiW
9
Co
3
@ZIF-67 and SiW
9
Co
3
h@ZIF-67, have been prepared for comparison reasons. Regardless of the POM substitution degree, we demonstrated that the bidirectional synergistic POM-ZIF-67 interaction found for the already reported material, SiW
9
Co
3
@ZIF-67, is applicable to the other first-time prepared nanocomposites. Remarkably, this synergy is radically enhanced in the case of the two high POM loading samples, with these nanostructured materials being very active OER electrocatalysts, exhibiting 150-160 mV decreased overpotentials and much faster kinetics (∼35% decreased Tafel slopes) in relation to pristine ZIF-67. Moreover, they reveal intrinsic OER activities higher than those developed by the expensive commercial references (RuO
2
and IrO
2
powders) especially in the case of SiW
9
Co
3
h@ZIF-67. This important electrocatalytic improvement can be related to the higher POM loadings achieved, as well as to the formation of an open defect-enriched phase wrapping the 'standard' ZIF-67 framework in these nanomaterials.
Increasing the POM loadings in POM@ZIF-67 nanocomposites prepared
in situ
results in extremely modified framework electrocatalysts with highly enhanced OER performances.
•We explore the bulk-deposition of POM nanocrystals —Co4(PW9)2— on carbon supports.•Four MOF-74-derived nanocarbons Co@C, Co/Ni@C, N,S-Co@C and N,S-Co/Ni@C as supports.•The bulk-deposition of ...Co4(PW9)2 always improves the nanocarbon OER performances.•However, nanocarbon metal and doping features determine improvement magnitudes.•Co4(PW9)2↔Co/Ni@C synergy results in the highest intrinsic POM OER activity.
Ternary nanocomposites consisting of cobalt phosphotungstate, metal nanoparticles and a carbon matrix (Co4(PW9)2@NP@C) were synthesized by decorating four different MOF-74-derived nanocarbons with the sandwich-type polyoxometalate Co4(H2O)2(PW9O34)210−, Co4(PW9)2. An unprecedented strategy based on the “bulk” deposition of the POM salt nanocrystals has been persecuted on purpose, avoiding the homogeneous dispersion of POM clusters across the nanocarbon surfaces. Thereby, virtually unaltered Co4(PW9)2 nanocrystals were supported on three of the four nanocarbons, but unexpectedly the combination of bimetallic undoped Co/Ni@C support with the POM induces partial structural modifications in both materials. Consequently, the derived Co4(PW9)2@Co/Ni@C electrocatalyst undergoes a noteworthy electroactive surface area relative increase, but at the same time, its intrinsic OER activity declines. This is not an obstacle for this nanocomposite to exhibit (along with Co4(PW9)2@N,S-Co@C) very significant nominal OER performances: low overpotentials (ca. 400 mV to develop 10 mA cm−2 of current density), fast kinetics at intermediate overpotentials (Tafel slopes ≤67 mV dec−1) and remarkable stability levels. Regardless of the specific results, this preliminary study demonstrates for the first time the POM “bulk” deposition strategy as a useful tool to prepare highly active OER catalysts, and it shows up the dramatic effect that the nanocarbon doping and metal composition has on the resultant POM loading, electrocatalytically active area and OER behavior.
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A bidirectional synergy involving Co-based polyoxometalates (POMs) incorporated in the Co-containing zeolitic imidazolate framework, ZIF-67, that promoted a notable improvement in the oxygen ...evolution reaction (OER) performance was unveiled for the first time. Two POM@ZIF nanocomposite materialsSiW9Co3@ZIF-8 and SiW9Co3@ZIF-67were successfully prepared via a room temperature “in situ” approach, by the immobilization of POM anions SiW9Co3(H2O)3O3710– (SiW9Co3) in the cages of two isostructural ZIFs: ZIF-8 and ZIF-67, formed with Zn2+ and Co2+, respectively. Despite both SiW9Co3@ZIF-8 and SiW9Co3@ZIF-67 revealing similar and low occupancy degrees (ca. 6 POM units per 100 ZIF-cages), an intense synergy has been detected in the ZIF-67 based nanocomposite, while no synergistic interaction was found for POMs and ZIF-8. Due to this synergistic effect, the OER activity of SiW9Co3@ZIF-67 is significantly enhanced in comparison with that of ZIF-67: ∼110 mV decreased overpotential and double current density. Additionally, this nanocomposite exhibits excellent stability in the alkaline electrolyte. Insights into the nature and magnitude of the SiW9Co3–ZIF-67 synergy have shown up its interesting ZIF ↔ POM bidirectional nature that involves activation of the Co-based active sites in POM clusters as a consequence of a ZIF-67 → SiW9Co3 electron transfer, along with POM-induced generation of more active unsaturated Co-nodes in the ZIF-67 frameworks. These findings represent a promising “proof of concept” for the development of more efficient POM@MOF-based electrocatalysts in the future.
A guest@host POM@ZIF nanocomposite—PW11Co@ZIF-67—has been synthesized using an in situ strategy. This new nanocomposite exhibits (i) individually ZIF-67-cage-confined POM units, (ii) structural ...defects in the ZIF-67 host induced by the POM, and (iii) charge transfer from the ZIF-67 to the confined POM. In addition, it has served as a template to produce a set of derived samples by applying thermal treatment at various temperatures (200, 400, 500, 600, and 950 °C) under a N2 flow. We have used multiple characterization techniques, ICP-OES, CHNS analysis, XPS, ATR-IR, PXRD, Raman spectroscopy, N2/CO2 adsorption analysis, CV, and TEM/EDS, to fully assess the thermally-induced variation tendencies. The first two derivatives—D200 and D400—show the same nanoarrangement as the PW11Co@ZIF-67 precursor, although with incipient signs of both POM and ZIF-67 structural decompositions. The following samples—D500, D600, and D950—exhibit a carbonaceous nature consisting of C-embedded compositionally complex nanoparticles that involve Co and W present as diverse species, metallic/oxide/phosphate/phosphide. D500 presents the best intrinsic electrochemistry, probably due to the high proportion of pyridinic N moieties doping its C matrix combined with small-sized and highly dispersed Co-enriched nanoparticles. This study focuses on the need for a thorough physicochemical characterization of this class of highly nanostructured materials with a view to exploring their application in electrocatalysis.
This paper reports a new method to obtain ultra-small Pd and Pt nanoparticles (0.5-1 nm) supported on multi-walled carbon nanotubes (MWCNTs). Even at high loadings of both metals (22.3 and 31.5% ...(wt/wt) of Pd
2+
and Pt
2+
, respectively), very narrow and unimodal particle size distributions are achieved. The complexing capabilities of polyethyleneimine covalently attached to the surface of the tubes are optimal for the retention of Pd
2+
and Pt
2+
from solution. We have addressed the reduction of the retained ions by two approaches: a classical treatment with NaBH
4
in aqueous solution, and a novel method using hydrogen cold plasma to preserve the structural features of the material. Cold plasma produced degrees of reduction similar or even larger than NaBH
4
, supporting the advantage of hydrogen cold plasma as reducing agent as it is a simple, clean and fast (15 minutes) procedure. XPS analysis of the reduced materials show an increase in the electron density near the Fermi level. Pt/MWCNT materials have been tested as anode for methanol electrooxidation, showing a catalytic profile typical of that observed for platinum nanoparticles. The stability after 1000 cycles of the plasma-reduced materials is much larger than these reported for Pt/carbon materials, indicating the stability of the ultra-small nanoparticles.
This paper reports a new method to obtain ultra-small Pd and Pt nanoparticles (0.5-1 nm) supported on multi-walled carbon nanotubes (MWCNTs).
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•CO2 recycling via its electrocatalytic conversion into valuable products is a hot topic.•A plethora of diverse precursors (MOFs, biomass, polymers, etc.) is available to produce ...carbon electrocatalysts for CO2RR.•Carbonization is a very versatile tool to modulate the final structural and electrocatalytic properties of CO2RR catalysts.•Important efforts are required to improve catalysts selectivity toward the valuable C2+ products: ethanol, ethylene, etc.
The recycling of carbon dioxide via its electroreduction (CO2RR) is a key factor to stimulate the capture of this greenhouse gas, being a promising green strategy for achieving sustainable development. Nanostructured carbon materials play a protagonist role in the search of efficient and selective CO2RR electrocatalysts and, at the same time, pyrolysis is one of the most versatile tools to produce tailored nanocarbons. Nanocarbons involving a variety of dopants, metal single-atom sites/nanoparticles and textural/structural arrangements can be obtained through rational precursor/s selection and control of the carbonization conditions. Thus, this review aims to highlight the advances in CO2RR electrocatalysis using nanocarbons derived from MOFs, biomass, polymers, and other precursors, assessing the aspects to improve as well. Bearing in mind that selectivity toward valuable products is the ever-present obstacle, we summarize the nature and proportion of the produced compounds. With this, we provide a useful guide in which carbon synthetic conditions, structural features and electrocatalytic metrics are correlated to facilitate further progress in this boisterous field. After all, the main limitations detected in CO2RR research as well as potential approaches to address them are included.
The isomerization reaction of glucose to fructose was studied using five selected metal-organic frameworks (MOFs) as catalysts and a mixture of γ-valerolactone and 10% H
2
O as solvent. MOFs with ...different metal cations (Cr
3+
, Al
3+
, Cu
2+
, and Fe
3+
) were tested between 100 and 140 °C. The activity tests show that the MOF with chromium yields a higher amount of fructose. A comparison between MIL-101(Cr) and MIL-53(Cr) shows a higher yield of fructose with MIL-101(Cr) (23% at 140 °C) in a short reaction time, due to the higher pore size of the MOF structure. The stability of this catalyst was confirmed, and it could be recycled 5 times without a significant loss of activity and exhibited an excellent fructose yield of 23-35% after 1 h of the reaction. In this work, the superior results found are due to the large porous MIL-101(Cr) catalyst combined with aprotic solvents (γ-valerolactone−10% H
2
O).
High catalytic activity is found to be due to the combination of a chromium catalyst with large pores (MIL-101(Cr)) with aprotic solvents (γ-valerolactone−10% H
2
O).
The continuous and dramatic increase of global demand for energy resources makes it urgent to develop affordable nanostructured materials to act as efficient electrocatalysts (ECs) in the ...energy-related reactions. Metal–organic framework (MOF) template pyrolysis for the production of nanostructured carbon-based materials is a very promising methodology to produce carbon-based ECs. Herein, we report the preparation, characterization (XPS, CHNS analysis, ICP-OES, XRD, TEM, and SEM/EDS), and application as oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) ECs of a plethora of nanostructured carbon materials derived from RT-synthesized (RT, room temperature) MOF-74 with different metal compositions, Co, Ni and Co/Ni, and dopant heteroatoms, N–, S– and N/S-dual-doping. This has allowed the study of the aforementioned parameters’ influence on the OER and ORR electrocatalytic activity in alkaline medium. Highly synergetic effects have been detected in two cases: (1) when N/S-dual-doped carbon is produced from a monometallic Co-MOF-74 template, N,S–Co@C, and (2) when an undoped carbon is derived from bimetallic Co/Ni-MOF-74, Co/Ni@C. These two samples achieve OER performances with η10 = 0.41 and 0.44 V, respectively, along with Tafel slopes of 101 and 93 mV dec–1, being close to the state-of-art OER catalyst performance. In addition, ORR tests showed that the effect of heteroatom doping on ORR activity is always positive, regardless of metal composition.
The direct conversion reaction of glucose to 5‐hydroxymethylfurfural (HMF) is studied using metal organic framework (MOF) as Lewis‐acid catalysts and a polyoxometalate (POM), silicotungstic acid, as ...a Brønsted‐type acid with a mixture of 1% glucose solution in γ‐valerolactone (GVL)‐10% H2O at 140 °C. The study is carried out with two routes: one using MOF and POM tandem catalysts added independently and the other through the synthesis of a composite material denoted POM@MOF. The activity tests show that the profiles of the conversion and yield of HMF achieved in both routes are similar, with the reactions with MIL‐53(Al) and MIL‐101(Cr) catalysts producing the highest yield of HMF (40% after 8 h of reaction). Stability tests are performed on the POM@MOF catalysts based on MIL‐53(Al) and MIL‐101(Cr). MIL‐53(Al) and HSiW@MIL‐101(Cr) can be reused, showing a progressive loss in HMF yield due to the leaching of POM.
5‐Hydroxymethylfurfural (HMF) is synthesized in one step from glucose using tandem catlysts based on several metal organic framework catalysts and a polyoxometalate adding separately or by encapsulation. In both cases, the highest performance is obtained with MIL‐101(Cr) with high HMF yields of around 40%.
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