Macroscopic fibers of carbon nanotubes (CNT) have emerged as an ideal architecture to exploit the exceptional properties of CNT building blocks in applications ranging from energy storage to ...reinforcement in structural composites. Controlled synthesis and scalability are amongst the most pressing challenges to further materialize the potential of CNT fibers. This work shows that under floating catalyst chemical vapor conditions in the direct spinning method, used both in research and industry, the ceramic reactor tube plays an unsuspected active role in CNT growth, leading for example to doubling of reaction yield when mullite (Al
Si
O
(x ≈ 0:4)) is used instead of alumina (Al
O
), but without affecting CNT morphology in terms of number of layers, purity or degree of graphitization. This behaviour is confirmed for different carbon sources and when growing either predominantly single-walled or multi-walled CNTs by adjusting promotor concentration. Analysis of large Si-based impurities occasionally found in CNT fiber fabric samples, attributed to reactor tube fragments that end up trapped in the porous fibers, indicate that the role of the reactor tube is in catalyzing the thermal decomposition of hydrocarbons, which subsequently react with floating Fe catalyst nanoparticles and produce extrusion of the CNTs and formation of an aerogel. Reactor gas analysis confirms that extensive thermal decomposition of the carbon source occurs in the absence of Fe catalyst particles, and that the concentration of different carbon species (e.g. carbon dioxide and ethylene) is sensitive to the reactor tube type. These finding open new avenues for controlled synthesis of CNT fibers by decoupling precursor decomposition from CNT extrusion at the catalyst particle.
Display omitted
•Surface density affects crystallinity, varying CO2 photoreduction conversion and selectivity.•Optimum conversion results are obtained with a surface density close to 1g/m2.•An ...increase in CH3OH selectivity occurs with a decrease in the surface densities.•Peroxo and related carbonate surface species are detected as H2O oxidation products.•DC-magnetron sputtering is a sound tool to scale-up CO2 reduction photocatalysts.
Advancing in the photocatalyst scale-up is crucial for the development of highly efficient solar fuels production at industrial scale. Here, we report DC-magnetron sputtering as a suitable technique to produce photocatalytic TiO2 coatings for CO2 reduction with a view on process scalability. The crystallinity of the obtained TiO2 coatings varies with surface density, with amorphous or quasi-amorphous coatings obtained with very low densities, while UV light absorption coefficients show the opposite trend, which has been related to the proportionally higher abundace of surface defects and grain boundaries associated to the small crystal size and/or amorphicity of the lightest coatings. The as-prepared samples lead to the reduction of CO2 as demonstrated by 13C isotope tracing. An optimum catalyst area density of 1g/m2 (by geometric area) is obtained in terms of CO2 photoreduction production, which is ascribed to a compromise situation between crystallinity and absorption coefficient. Selectivity to the different reaction products also varies with the coating characteristics, with amorphous or quasi-amorphous light coatings favouring methanol formation, in contrast with the preferred CO evolution in heavier, crystalline ones. Raman spectroscopy reveals the formation of peroxo and peroxocarbonate species on the photocatalyst surface as oxidation products during the CO2 reduction, the accummulation of which is proposed to be related to the observed catalyst deactivation.
Display omitted
► Effect of silver over TiO2 and ZnO has been studied for Solar-Fuels production. ► Ag deposition decreases e−–h+ recombination rate and facilitates electron transfer. ► Enhancement ...of hydrocarbon production due to synergetic effect in Ag/TiO2 and Ag/ZnO. ► Deposition method and the silver loading have influence in CO2 photoreduction.
The effect of silver on the catalytic activity of Ag/TiO2 and Ag/ZnO systems for the CO2 photoreduction using water vapour as electron donor has been investigated. Catalysts prepared by different impregnation methods and different Ag loadings (1.5% and 3.0wt.%) were tested in this process. TEM results show that silver is well dispersed as small nanoparticles (2–3nm) on Ag/TiO2 samples, while it forms larger crystallites (150–300nm) on Ag/ZnO catalysts. Optical studies reveal that under UV light irradiation silver nanoparticles are reduced to their metallic state, due to the transfer of photoexcited electrons from the TiO2 conduction band. The formation of Ag0 leads to the appearance of surface plasmon resonance (SPR) effect. Regarding the photocatalytic activity, both semiconductors (TiO2 and ZnO) are active towards CO2 photoreduction although titania-based catalysts show higher performances. Further, silver incorporation leads to a significant increase in the hydrocarbon production, mainly attributed to lower electron–hole recombination rates. This may explain the observed enhancement of oligomerization processes, especially favoured by the synergetic effect between Ag nanoparticles and TiO2.
Mono- and bimetallic cobalt-iron silica supported catalysts were tested in a fixed-bed reactor and stirred-tank slurry reactor, for the Fischer–Tropsch reaction. For the fixed-bed reaction, the ...monometallic iron catalyst presented the highest activity. In the bimetallic catalysts an increase in alcohol selectivity is produced. In the slurry studies, CO conversion is greater for the cobalt catalyst and lower for bimetallic systems.
▪
Mono- and bimetallic cobalt and iron oxide nanoparticles deposited on the surface of a silica substrate were tested in a fixed-bed reactor and stirred-tank slurry reactor under different process conditions, for the Fischer–Tropsch reaction, this comparison is presented for the first time, being presented in this work a comparison of above systems in both reactor types. For the fixed-bed reaction, the monometallic iron-based catalyst presented the highest activity at low reaction temperatures. By contrast, the monometallic cobalt catalyst is the less active for all reaction temperatures. This catalyst records high values for the
α parameter, whereas the iron catalyst reveals major selectivity to alcohols. Moreover, the introduction of iron produces a decrease in hydrocarbon chain growth and an increase in alcohol selectivity. This behaviour might be due to the formation of a Co–Fe alloy, observed by several characterization techniques. In the slurry studies, CO conversion is greater for the cobalt-based catalyst and lower for bimetallic systems, whilst the iron catalyst deactivates in a few hours, which is explained by the formation of carbonaceous deposits. The bimetallic catalysts record a decrease in CO conversion and an enhancement in the selectivity of methane and light hydrocarbons with the increase in iron amount. The absence of alcohols in this type of reactor is explained by the greater solubility of hydrogen, as well as by the greater contact time of reactants, which decreases the possibility of non-dissociative CO insertion.
In this work, TiO
2
has been modified by treating it thermally together with different proportions (0.5–15 wt%) of La
2
O
3
. The resulting materials have been extensively characterized by XRD, TEM, ...N
2
adsorption isotherms, temperature-programmed CO
2
desorption, Raman, UV–Vis photoluminescence and X-ray photoelectron spectroscopies. The activity tests of these materials for the gas-phase photocatalytic reduction of carbon dioxide show that the main products of the reaction are in all cases CO and CH
4
, together with H
2
from the parallel reduction of water. After the preparation procedure, La phases are best described as oxycarbonates, and lead to improved activity with respect to TiO
2
with La contents up to 5 wt%. Higher loadings do not, however, lead to further enhanced activity. Retarded electron–hole recombination and enhanced CO
2
adsorption are invoked as the key factors contributing to this activity improvement, which is optimized in the case of 0.5 wt% La leading to higher productions of CO and CH
4
and increased quantum efficiency with respect to titania.
A FeII‐based 1D open framework that exhibits room‐temperature spin‐ crossover (SCO) is reported. This material shows a reversible structural, magnetic, and chromatic response to a sorption/desorption ...process, allowing the tuning of the spin‐transition properties by choosing the appropriate solvent (see figure). This feature represents the first example of a bifunctional material in which host–guest properties directly influence the SCO properties.
PDF
