Partial isomorphic substitution of Zn in IRMOF metal clusters by cobalt ions is described for the first time. Specifically, different numbers of Co2+ ions have been incorporated during solvothermal ...crystallization into the Zn-based MOF-5 (IRMOF-1) framework, which is one of the most studied MOF materials. The amount of Zn that can be substituted seems to be limited, being no more than 25% of total metal content, that is, no more than one Co atom inside every metal cluster formed by four transition-metal ions, on average. Several characterization techniques, including X-ray diffraction, DR UV−visible spectroscopy, N2 adsorption isotherms, and thermogravimetrical analysis, strongly support the effective incorporation of Co into the material framework. As-synthesized CoMOF-5 has cobalt ions in octahedral coordination, changing to tetrahedral by simple evacuation, presumably by the removal of two diethylformamide molecules per Co ion. Moreover, the H2, CH4, and CO2 uptake of MOF-5 materials systematically increases with the Co content, particularly at high pressure. Such an increase is moderate anyway, considering that Co is incorporated into unexposed metal sites that are less accessible to gas molecules.
The aim of this work was to assess the treatment of wastewater coming from a pharmaceutical plant through a continuous heterogeneous catalytic wet peroxide oxidation (CWPO) process using an Fe
2O
...3/SBA-15 nanocomposite catalyst. This catalyst was preliminary tested in a batch stirred tank reactor (STR), to elucidate the influence of significant parameters on the oxidation system, such as temperature, initial oxidant concentration and initial pH of the reaction medium. In that case, a temperature of 80
°C using an initial oxidant concentration corresponding to twice the theoretical stoichiometric amount for complete carbon depletion and initial pH of ca. 3 allow TOC degradation of around 50% after 200
min of contact time. Thereafter, the powder catalyst was extruded with bentonite to prepare pellets that could be used in a fixed bed reactor (FBR). Results in the up-flow FBR indicate that the catalyst shows high activity in terms of TOC mineralization (ca. 60% under steady-state conditions), with an excellent use of the oxidant and high stability of the supported iron species. The activity of the catalyst is kept constant, at least, for 55
h of reaction. Furthermore, the BOD
5/COD ratio is increased from 0.20 to 0.30, whereas the average oxidation stage (AOS) changed from 0.70 to 2.35. These two parameters show a high oxidation degree of organic compounds in the outlet effluent, which enhances its biodegradability, and favours the possibility of a subsequent coupling with a conventional biological treatment.
We studied the light emission of crystalline Si nanoparticles (SiNPs) with hydrogen and native oxide shell terminations using temperature- and time-dependent photoluminescence. We demonstrate that ...the broad emission normally observed for SiNPs after natural oxidation is in fact formed of two components, which originate from distinct recombination mechanisms that take place simultaneously in the same SiNPs sample. To identify the two spectral components, we exploited the different time scales associated with different emission mechanisms by carefully choosing the measurement time window at which only one of the emission mechanisms is active. Moreover, our experiments indicate that one of the emissions is due to recombination of photogenerated electrons and holes located in the crystalline core of the SiNPs (excitonic emission) whereas the other component originates from donor–acceptor recombination pairs involving states associated with the native oxide shell. These conclusions are supported from experiments carried out with the same SiNPs but where the surface-oxide shell is replaced by H termination. We conclude that both emission components are excited through electronic states of the SiNPs core, pointing out an effective core-to-shell energy/charge transfer. Furthermore, we show that the light emission quantum yield of SiNP ensembles is strongly affected by inter-NP charge transfer and therefore is not determined solely by the properties of the individual NPs. High quantum yields of up to 43%, observed for our surface-oxidized SiNP samples in solution, result from inhibition of inter-NP charge transfer.
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► The first comparative study between metals and carbons as catalysts in CH
4 decomposition to produce H
2. ► The most active catalyst was Ni mixed oxides from hydrotalcites (Ni–
ex ...LDHs). ► The most stable catalysts against deactivation were carbon blacks, mainly the CB-
vulcan. ► Carbon
vulcan produces 4 fold more H
2 than the most active and stable metal catalysts (Ni–
ex LDHs).
The CO
x
-free hydrogen production by decomposition of methane was carried out over metal-free carbons and bulk and supported metal catalysts. Catalysts based on Ni or Fe (oxides, spinels and
ex-hydrotalcite mixed oxides) and carbon-catalysts of different types (carbon black, activated carbon, carbon nanotubes and graphite) have been used and the performance of both different kinds of catalyst compared in the target reaction, focussing the comparative study on the initial activity and the resistance against deactivation. Catalytic results showed that activity of carbon catalysts is similar to that of the non-prereduced metal catalysts. Carbon blacks, and specifically
vulcan-type, were found to be most resistant catalysts against deactivation, while the most active ones were nickel catalysts derived from hydrotalcite-like precursors (
ex-LDH). It was also shown that pre-reduction treatment of nickel precursor to generate the metallic Ni
0 phase before the reaction decreases the threshold temperature to generate hydrogen by about 200
°C.
Along the course of the reaction, nanotubes and nanofibers are formed on the surface of metal catalysts. These carbon structures encapsulate metal particles and deactivate the catalysts. However, when carbons are used as catalysts, the nature of carbon deposits depends largely on the type of carbon material employed as catalyst in the methane decomposition. Thus, the use of carbon blacks and graphite yielded amorphous turbostratic carbon, whereas activated carbons mainly produced carbon, which showed the carbon black structure. Finally, the carbon by-product of the reaction, when carbon nanotubes were used bas catalysts, accumulates in the form of extra layers over the walls of carbon nanotubes thus increasing their wall thickness.
A wide variety of carbon materials (ordered mesoporous carbons, carbon blacks, activated carbon, carbon nanotubes, coke and graphite) have been investigated as catalysts for hydrogen production by ...methane decomposition, with the aims of identifying the carbon properties which control in a greater extension the catalytic activity and determine the nature of the active sites involved in the reaction.
The catalytic activity of the different carbon materials was determined and compared using temperature-programmed experiments in a thermobalance. The initial activity was followed through the threshold temperature, defined as the temperature at which hydrogen production starts being detected, whereas the average reaction rate was also calculated and compared. The lowest threshold temperature was observed with ordered mesoporous carbons (CMK materials), followed by activated carbon and carbon blacks. On the other hand, at long reaction times activated carbon was quickly deactivated yielding a relatively low average reaction rate. The deactivation process seems to be greatly linked to the presence of micropores while the long-term activity is retained in those materials with ordered mesoporosity (CMKs) or formed by nanoparticles (carbon blacks), which make them more resistant to deactivation by the formation of carbonaceous deposits.
Whereas no clear dependence is observed between the threshold temperature and the surface area neither with the presence of polar groups in the carbon catalysts, characterization of these materials by XPS shows that a direct relationship exists with the amount of defects present on the graphene layers. This fact strongly supports that these defects are the main active sites for methane decomposition over carbon catalysts.
Purpose
Previous studies in patients with Parkinson’s disease (PD) and impulse control disorders (ICDs) have produced heterogeneous results regarding striatal dopamine transporter (DaT) binding and ...activity in the mesocorticolimbic network. Our aim here was to study the relationship between striatal DaT availability and cortical metabolism, as well as motor, behavioural and cognitive features of PD patients with ICD.
Methods
In a group of PD patients with ICD (PD-ICD,
n
= 16) and 16 matched PD patients without ICD (PD-noICD,
n
= 16), DaT single-photon emission computed tomography (SPECT) imaging (DaTSCAN) was used to study DaT availability in predefined striatal volumes of interest (VOIs): putamen, caudate nucleus and ventral striatum (VS). In addition, the specific association of striatal DaT binding with cortical limbic and associative metabolic activity was evaluated by
18
F-fluorodeoxyglucose (FDG) positron emission tomography (PET) in PD-ICD patients and investigated using statistical parametric mapping (SPM8). Finally, associations between DaT availability and motor, behavioural and cognitive features were assessed.
Results
PD-ICD patients had a significantly lower DaT density in the VS than PD-noICD patients, which was inversely associated with ICD severity. Lower DaT availability in the VS was associated with lower FDG uptake in several cortical areas belonging to the limbic and associative circuits, and in other regions involved in reward and inhibition processes (
p
< 0.0001 uncorrected;
k
> 50 voxels). No significant results were observed using a higher conservative threshold (
p
< 0.05; FDR corrected). PD-ICD patients also displayed impairment in interference and attentional Stroop Task execution, and more anxiety, all associated with reduced DaT availability in the VS and caudate nucleus.
Conclusions
ICDs in PD patients are related to reduced DaT binding in the VS, which accounts for dysfunction in a complex cortico-subcortical network that involves areas of the mesolimbic and mesocortical systems, being associated with reward evaluation, salience attribution and inhibitory control processes.
The adsorption properties of different ordered mesoporous materials, for the removal of volatile organic compounds (VOCs) in air, have been studied by means of a thermal programmed desorption ...technique. Toluene, isopentane, and water have been used as adsorbates, whereas a variety of mesostructured materials (MCM-41, Al-MCM-41, and SBA-15) have been selected as adsorbents. As expected, Al-MCM-41 is a hydrophilic material, adsorbing high amounts of water due to the presence of acid sites. On the contrary, pure silica MCM-41 and SBA-15 samples exhibit hydrophobic properties, and they are capable of adsorbing toluene and isopentane in significant amounts. Among these materials, MCM-41 prepared by a sol−gel route MCM-41(H-sg) presents the best combination of hydrophobic character and hydrocarbon adsorption, being a potentially interesting adsorbent for the removal of VOCs from humid gases. This behavior has been related to the high structural disorder and low condensation degree of the silica walls present in this material.
In this work, a complete thermodynamic study of the first step of the Mn2O3/MnO thermochemical cycle for solar hydrogen production has been performed. The thermal reduction of Mn2O3 takes place ...through a sequential mechanism of two reaction steps. The first step (reduction of Mn2O3 to Mn3O4) takes place at teomperatures above 700°C, whereas the second reaction step (reduction of Mn3O4 to MnO) requires temperatures above 1350°C to achieve satisfactory reaction rates and conversions. Equilibrium can be displaced to lower temperatures by increasing the inert gas/Mn2O3 ratio or decreasing the pressure. The thermodynamic calculations have been validated by thermogravimetric experiments carried out in a high temperature tubular furnace. Experimental results corroborate the theoretical predictions although a dramatically influence of chemical kinetics and diffusion process has been also demonstrated, displacing the reactions to higher temperatures than those predicted by thermodynamics. Finally, this work demonstrates that the first step of the manganese oxide thermochemical cycle for hydrogen production can be carried out with total conversion at temperatures compatible with solar energy concentration devices. The range of required temperatures is lower than those commonly reported in literature for the manganese oxide cycle obtained from theoretical and thermodynamic studies.
► Thermodynamic simulation of Mn2O3 thermal reduction validated by experimental data. ► Two steps sequential mechanism without coexistence of the three Mn2O3, Mn3O4 and MnO phases. ► Experimental temperatures higher than predicted due to reaction kinetics and O2 diffusion. ► Successful prediction of the effect of pressure, gas/solid ratio and presence of O2. ► Total conversion feasible at temperatures below 1400°C, compatible with solar reactors.
This work reports the kinetic study of the first step of the Mn2O3/MnO thermochemical cycle for hydrogen production by water splitting. The reaction kinetics of Mn (III) oxide thermal reduction has ...been evaluated using dynamic thermogravimetric analysis at constant heating rate under nitrogen flow. This way the reaction rate can be described as a function of temperature and different kinetic models were fitted to the experimental data obtained from thermogravimetric experiments. A good fitting can be observed for each experiment, although a significant disparity in the values estimated for the Arrhenius parameters has been found (activation energies and pre-exponential factors). Unique values for the kinetic parameters have been calculated by application of a multivariate non-linear regression method for the simultaneous fitting of data from all the experiments carried out at different heating ramps. However, also in this case the values of the Arrhenius parameters are significantly different depending on the chosen kinetic equation. Optimal kinetic parameters have been finally calculated through the estimation of activation energy values by model-free isoconversional methods and using a rigorous multivariate nonlinear regression for the calculation of the model-dependant pre-exponential factors.
► Kinetic modelling of Mn2O3 thermal reduction from TG data at constant heating rates. ► Traditional model fitting leads to a high disparity in the kinetic parameters values. ► Simultaneous fitting leads to unique values but dependent on the model. ► Novel method based on the preliminary calculation of model-free activation energies. ► Arrhenius pre-exponential factor estimated by multivariate non-linear regression.
Methane decomposition offers an interesting route for the CO
2-free hydrogen production. The use of carbon catalysts, in addition to lowering the reaction temperature, presents a number of ...advantages, such as low cost, possibility of operating under autocatalytic conditions and feasibility of using the produced carbons in non-energy applications. In this work, a novel class of carbonaceous materials, having an ordered mesoporous structure (CMK-3 and CMK-5), has been checked as catalysts for methane decomposition, the results obtained being compared to those corresponding to a carbon black sample (CB-bp) and two activated carbons, presenting micro- (AC-mic) and mesoporosity (AC-mes), respectively. Ordered mesoporous carbons, and especially CMK-5, possess a remarkable activity and stability for the hydrogen production through that reaction. Under both temperature programmed and isothermal experiments, CMK-5 has shown to be a superior catalyst for methane decomposition than the AC-mic and CB-bp materials. Likewise, the catalytic activity of CMK-5 is superior to that of AC-mes in spite of the presence of mesoporosity and a high surface area in the latter. The remarkable stability of the CMK-5 catalyst is demonstrated by the high amount of carbon deposits that can be formed on this sample. This result has been assigned to the growth of the carbon deposits from methane decomposition towards the outer part of the catalyst particles, avoiding the blockage of the uniform mesopores present in CMK-5. Thus, up to 25
g of carbon deposits have been formed per gram of CMK-5, while the latter still retains a significant catalytic activity.
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