The efficiency of the two oxidation treatments studied seems to depend on the morphology (edge to basal plane ratio) of the carbon material.
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► Two carbon materials with different edge ...to basal plane ratio are oxidized by nitric acid or oxygen plasma. ► Wet oxidation attacks the C atoms at the edges and oxygen plasma is able to attack both C atoms at the edges and at the basal planes. ► Plasma oxidation seems more adequate for materials with lower edge to basal plane ratio as the CNFs. ► Modification of the oxidized materials with an amine molecule gives rise to materials probably holding ammonium carboxylate salt species.
Two graphitic carbon materials with different edge to basal plane ratio, high surface area graphite (HSAG) and graphitized carbon nanofibers (CNFs), were oxidized by two methods, aqueous-HNO
3 wet oxidation and oxygen plasma oxidation. Characterization of the materials by temperature-programmed desorption, thermogravimetry and X-ray photoelectron and Raman spectroscopies indicated that the amount and nature of oxygen surface groups introduced depended on the oxidation method and on the structure of the original material. While surface sites within the layers were only oxidized by oxygen plasma, surface sites at the edges of graphene layers were oxidized by both treatments being the wet oxidation more effective. Modification of the oxidized materials with a diamine or a triamine molecule resulted in the formation of ammonium carboxylate salt species on the carbon surface.
The development of efficient electrocatalysts for the energy-related reactions, based on earth-abundant elements, is extremely important for a sustainable energetic future. Herein, we report the ...application of Cu nanoparticles supported on undoped and N-doped graphene—Cu/GOE and Cu/GOE-u composites, respectively—as electrocatalysts for the oxygen reduction reaction (ORR). All the materials showed ORR electrocatalytic activities in alkaline medium. The Cu/GOE-u composite exhibited the most promising performance, with an onset potential of 0.84 V and a current density of
j
L
= − 4.4 mA cm
−2
(vs. 0.84 V and − 2.8 mA cm
−2
for Cu/GOE), which revealed the great influence of the created Cu–N
x
/C active sites on the ORR electrocatalytic activity. The pure GOE-u support showed worse performance than the GOE, demonstrating that the N-doping advantage is not linear and also depends on the type and amount of accessible active sites created. The N-doping allowed an increase in the selectivity for the 4-electron process, resulting in a % of H
2
O
2
produced < 25% for Cu/GOE-u (vs. almost 75% for Cu/GOE). Both nanocomposites revealed good tolerance to methanol crossover, and the Cu/GOE-u displayed a moderate long-term electrochemical stability, with current retention of 84% after 20,000 s.
Graphical abstract
1-butanol dehydration reaction has recently emerged as a sustainable route to produce butenes which can be further oligomerized to be applied as jet fuel. However, the high catalyst deactivation ...rates observed during this reaction due to coke deposition is still a pending matter. As promising catalysts for this reaction, we have supported two heteropolyacids (HPA), i.e. H4SiW12O40 (STA) and H3PW12O40 (TPA), on two commercial carbon materials: an activated carbon (AC) and a high surface area graphite (HSAG). Aiming to evaluate the role of HPA-support interactions, the STA was also dispersed over metallic oxides of different acidic nature, namely SiO2, Al2O3 and ZrO2. An exhaustive physicochemical characterization revealed that after the HPA dispersion thorough the support, the Keggin structure was maintained and an increase in the amount and strength of acid sites was provoked, but in different degree according to the HPA type and support’s nature. While the TPA-based catalysts developed less quantity of total acid sites, but higher strength than their STA-carbon counterparts, the STA/AC and TPA/AC samples exhibited a slight major amount of acid sites than STA/HSAG and TPA/HSAG. The HPA-support interactions have ultimately modulated to some extent the activity, selectivity, stability and regeneration ability of the synthesized catalysts, when applied in the gas phase butanol dehydration reaction at 275 °C. The higher STA decomposition temperature prompted by the graphitic support, among other factors, allowed the total regeneration of the highly active (39 mmolBuOH∙min−1∙ga.p) and n-butenes selective (>98 %) STA/HSAG catalyst by means of combustion of carbon deposits at 400 °C.
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•Two heteropolyacids, STA and TPA, are dispersed on supports of different nature.•The amount and strength of acid sites depend on the HPA type and support’s nature.•HPA-support interactions modulate the catalytic performance in butanol dehydration.•The optimal HPA-support interactions are observed in the STA/HSAG catalyst.•STA/HSAG is the only regenerable catalyst through a proper coke oxidation treatment.
Various carbon materials differing in surface area, nanostructure and grade of graphitization were used to support ruthenium catalysts for the steam reforming of glycerol. Activity measurements ...performed in a traditional reactor indicated that the reaction is structure sensitive and the initial activity for hydrogen production of the catalysts decreases with decreasing Ru particle size. Among the most active catalysts, stability in reaction was first related with the graphitization degree of the support. Ru-nanotubes was the most stable with the time on stream, whereas Ru-activated carbon underwent an abrupt deactivation due to carbon gasification. Ru-graphite, the other catalyst with highest initial activity, suffered from a slow deactivation, most likely due to coke formation derived from the olefins produced during reaction. The use of a membrane reactor prevents the formation of these undesired compounds, enhancing the hydrogen yield and rendering a very stable catalyst.
Excellent selectivities in the hydrogenation of cinnamaldehyde have been obtained with Ru(PPh
3)
3Cl
2 immobilised onto three different carbon materials: graphite, carbon nanofibers, and graphite ...oxide (GO).
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► Graphite, carbon nanofibers, and graphite oxide are functionalized. ► (Bis(2-aminoethyl)amine) and an alkoxide are employed for functionalization. ► Immobilization of Ru(PPh
3)
3Cl
2 on functionalized-carbon materials is achieved. ► Hybrid catalysts are active in the hydrogenation of cinnamaldehyde. ► Excellent selectivities to unsaturated alcohol are obtained.
The homogeneous complex Ru(PPh
3)
3Cl
2 has been immobilized onto three different carbon materials: graphite, carbon nanofibers, and graphite oxide (GO). All the supports were previously functionalized with (bis(2-aminoethyl)amine). For graphite oxide, a second functionalization strategy using N-3-(trimethoxysilyl)propyl-etylenediamine (TPEN) was also followed. XRD, NMR, TG, TPD, DRIFTS, and XPS techniques were applied for the characterization of the functionalized supports and the hybrid catalysts. The hybrid catalysts have been tested in the hydrogenation of cinnamaldehyde and, although less active, they showed selectivities comparables to that for the homogeneous complex. The different behavior observed for the samples after recycling and reusing can be related to the ligand-support interaction type. While activity and selectivity are preserved for the Ru grafted on the covalent-functionalized GO–TPEN, variations in activity and selectivity are observed for the amine-functionalized samples.
Ru catalysts were supported on two different carbon materials, multiwall carbon nanotubes and bamboo-like carbon nanotubes doped with nitrogen, which were synthesized by catalytic chemical vapour ...deposition of C
2H
2/H
2/N
2 or C
2H
2/NH
3/H
2/N
2, respectively, over Fe/SiO
2 catalyst. All the carbon supports and/or the prepared Ru catalysts were characterized by several techniques including transmission electron microscopy, X-ray photoelectron spectroscopy, N
2 adsorption isotherms and CO chemisorption. The Ru catalysts were tested in the catalytic ammonia decomposition reaction. High yields towards hydrogen production were achieved. Carbon nanotubes were heated in an inert atmosphere at temperatures up to 1773
K in order to study the effects of such support treatments on the ammonia decomposition reaction. The elimination of acidic groups from the surfaces, prior to catalyst preparation, and/or the surface graphitization of the materials produced a higher catalytic activity during the reaction. The catalytic activity of Ru particles was significantly improved when supported on carbon nanotubes doped with nitrogen.
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•Nanocarbons were modified in order to introduce alkyl chains on their surface.•Palladium catalysts have been prepared using these modified supports.•The catalysts were tested in the ...partial hydrogenation of 1,3-butadiene.•High selectivities to butenes were obtained.•Over-hydrogenation to butane took place over oxygen-containing functional groups.
Commercial carbon nanofibers with different graphitic structure and commercial multiwall carbon nanotubes (CNT) were chemically modified in order to introduce specific alkyl ligands on their surface. Palladium catalysts have been prepared using these modified supports and subsequently tested in the partial hydrogenation of 1,3-butadiene under conditions of excess hydrogen. Herein, we used thermogravimetry (TG), temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and nitrogen adsorption at 77K techniques in order to characterize both supports and catalysts. We focus on testing the effects of support surface functionalities either on morphology of supported palladium (Pd) nanoparticles (NPs) or on their catalytic performances. High selectivity to butenes was obtained with the catalysts prepared over supports containing alkyl chains, while over-hydrogenation to butane took place over oxygen-containing functional groups. Nicely the catalysts with modified supports minimize the secondary hydrogenation of butenes even at high conversions. Therefore, Pd NPs on modified nano-carbon catalysts may open up more opportunities to optimize the activity and the selectivity for partial hydrogenation reactions.
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•Cu-CeO2 catalytic system has been supported over carbon nanotubes by impregnation.•Li, Na, K and Cs have been employed as promoters.•Li, Na and K decreased the CeO2 particle size and ...improved the activity in the PROX reaction compared to undoped sample.•The Cu-CeO2 interaction was favored by K, while Cs showed a detrimental effect.•Both the Cu-CeO2 interaction and the CeO2 particle size influenced the catalytic performance.
The effect of alkali promotion (Li, Na, K and Cs) on the CO preferential oxidation (CO PROX) reaction has been studied over Cu-CeO2 catalyst supported on carbon nanotubes (CNT). The catalysts were prepared with 2.5wt.% Cu and 20wt.% CeO2 loadings, and alkali/Cu atomic ratios of 0.68. The catalytic performance and the characterization by powder X-ray diffraction (XRD), TEM-STEM, H2-temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) has been presented. TEM and XRD analyses showed that the addition of alkali reduces the size of ceria crystallites. It was found that in general the incorporation of alkali favored the conversion of CO at low temperatures, being more pronounced for the K-doped catalyst. The characterization showed that the addition of K favored the Cu-CeO2 interaction, the activity of the catalyst was enhanced and the opposite was observed for Cs-doped sample. The characterization results reveal that the observed increase in the Cu+ species proportion and in lattice oxygen are related to the better catalytic performance.
In this study, performance and feasibility of a hollow fibre membrane reactor (HFMR), consisting of a packed catalyst bed around a Pd coated Al2O3 hollow fibre membrane, has been studied and compared ...with both conventional Pd based tubular membrane reactor (TMR) fabricated from a stainless steel substrate and traditional fixed-bed reactor (FBR). The ceramic based HFMR presents several advantages over MRs such as the deposition of ultra thin Pd membranes and the possibility to scale up the whole multifunctional process by module configuration. The results obtained at 450°C during the methane dry reforming (MDR) reaction showed that, although CH4 conversion using the HFMR was almost the same as that of a catalytic TMR, the amount of Pd employed for the Pd layer deposition in the HFMR was fifteen times less than that in TMR. Moreover, the CH4 conversion using the HFMR was 72% higher than that in a traditional FBR and 34% higher than thermodynamic equilibrium. Also, a high purity COX-free H2 production (10.5ml/mgh) was achieved at 525°C using a sweep gas of 100ml/min in the lumen side of the HFMR.
► A new hollow fibre membrane reactor (HFMR) has been developed. ► The HFMR combines the processes of generating and separating H2 in a single step. ► Its performance was compared with that in a tubular membrane and fixed-bed reactors. ► The low-cost HFMR allows a possible large-scale COX free H2 production.
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