Simultaneous CO2 Reduction Research Articles

Unraveling the roles of atomically-dispersed Au in boosting photocatalytic CO2 reduction and aryl alcohol oxidation

Atomically-dispersed metal-based materials represent an emerging class of photocatalysts attributed to their high catalytic activity, abundant surface active sites, and efficient charge separation. Nevertheless, the roles of different forms of atomically-dispersed metals (i.e., single-atoms and atomic clusters) in photocatalytic reactions remain ambiguous. Herein, we developed an ethylenediamine (EDA)-assisted reduction method to controllably synthesize atomically dispersed Au in the forms of Au single atoms (AuSA), Au clusters (AuC), and a mixed-phase of AuSA and AuC (AuSA+C) on CdS. In addition, we elucidate the synergistic effect of AuSA and AuC in enhancing the photocatalytic performance of CdS substrates for simultaneous CO2 reduction and aryl alcohol oxidation. Specifically, AuSA can effectively lower the energy barrier for the CO2→*COOH conversion, while AuC can enhance the adsorption of alcohols and reduce the energy barrier for dehydrogenation. As a result, the AuSA and AuC co-loaded CdS show impressive overall photocatalytic CO2 conversion performance, achieving remarkable CO and BAD production rates of 4.43 and 4.71 mmol g−1 h−1, with the selectivities of 93% and 99%, respectively. More importantly, the solar-to-chemical conversion efficiency of AuSA+C/CdS reaches 0.57%, which is over fivefold higher than the typical solar-to-biomass conversion efficiency found in nature (ca. 0.1%). This study comprehensively describes the roles of different forms of atomically-dispersed metals and their synergistic effects in photocatalytic reactions, which is anticipated to pave a new avenue in energy and environmental applications.

A red-light-powered silicon nanowire biophotochemical diode for simultaneous CO2 reduction and glycerol valorization

A red-light-powered silicon nanowire biophotochemical diode for simultaneous CO2 reduction and glycerol valorization

NiCuAg: An electrochemically-synthesised trimetallic stack for CO2 reduction

Electrochemical CO2 reduction is a promising technique for the production of desirable hydrocarbons without the need to resort to fossil resources. However, high overpotentials and poor selectivity remain a challenge for CO2 electro-reduction, especially for deep reduction by more than two electrons. One apparently attractive approach for breaking the scaling relations caused by simultaneous CO2 reduction pathways and for achieving deeper reduction is the use of multi-metallic electrodes, where several promising metal catalysts are present in close proximity. Herein, noting the activity shown by Ni, Cu and Ag for CO2 electroreduction when used individually, we set out to synthesise a tri-metallic “stack” catalyst, NiCuAg, and then to test this for electrochemical CO2 reduction. The stack architecture was successfully generated and the trimetallic NiCuAg system did show improved Faradaic efficiency for the reduction of CO2 to formic acid when compared to the bare Ni and bimetallic NiCu controls under some select conditions. However, the two-layer NiCu stack and bare Ni exhibited consistently higher Faradaic efficiencies than NiCuAg for deeper CO2 electroreduction to methanol and ethanol, indicating that the combination of three individually promising metals does not necessarily translate into superior catalytic performance for deep carbon dioxide reduction. [Display omitted]

Photothermal-driven CO2 reduction over Nd2O3/TiO2 heterojunction catalysts in aqueous medium

Elevated carbon dioxide levels, a major environmental concern, pose challenges for causing many climate problems. Addressing this, conversion CO2 into valuable chemicals is highly important. In this work, we explore photothermal catalytic CO2 reduction, an attractive approach that combines photocatalysis and thermal catalysis. We developed an enhanced Nd2O3/TiO2 catalyst using the sol-gel method, which significantly improves the efficiency of converting CO2 to CO in the presence of water. This catalyst exhibited a CO production rate of 22 μmol·gcat−1·h−1 at 130 °C, markedly surpassing the performance of sole photocatalysis by 7.3 times. Through systematic characterizations, we discovered that Nd2O3 prevents TiO2’s transition from anatase to rutile, thereby increasing CO2 adsorption and creating more vacancies. The interface between Nd2O3 and TiO2 forms a Z-scheme heterojunction, crucial for the simultaneous reduction of CO2 and oxidation of water. This research introduces a promising catalyst for photothermal CO2 reduction, which also acts as a reducing reagent. The strategic engineering of dual metal oxide interfaces in this catalyst suggests its applicability in a range of photothermal catalytic processes, marking a significant advancement in the field of CO2 utilization.

Enhanced Microbial Protein Production from CO2 and Air by a MoS2 Catalyzed Bioelectrochemical System.

Carbon dioxide can be relatively easily reduced to organic matter in a bioelectrochemical system (BES). However, due to insufficient reduction force from in-situ hydrogen evolution, it is difficult for nitrogen reduction. In this study, MoS2 was firstly used as an electrocatalyst for the simultaneous reduction of CO2 and N2 to produce microbial protein (MP) in a BES. Cell dry weight (CDW) could reach 0.81±0.04 g/L after 14 d operation at -0.7 V (vs. RHE), which was 108±3 % higher than that from non-catalyst control group (0.39±0.01 g/L). The produced protein had a better amino acid profile in the BES than that in a direct hydrogen system (DHS), particularly for proline (Pro). Besides, MoS2 promoted the growth of bacterial cell on an electrode and improved the biofilm extracellular electron transfer (EET) by microscopic observation and electrochemical characterization of MoS2 biocathode. The composition of the microbial community and the relative abundance of functional enzymes revealed that MoS2 as an electrocatalyst was beneficial for enriching Xanthobacter and enhancing CO2 and N2 reduction by electrical energy. These results demonstrated that an efficient strategy to improve MP production of BES is to use MoS2 as an electrocatalyst to shift amino acid profile and microbial community.

Anodic oxidation of salicylic acid using multi-walled carbon nanotubes modified carbon felt with simultaneous CO2 reduction by electrocoagulated sludge derived MIL-53(Fe/Cu) metal-organic framework cathode decorated with CuFe2O4

Salicylic acid (SA) used to treat inflammation and swelling is frequently identified in water habitats. Exposure to SA can affect aquatic species, making it imperative to remediate SA-contaminated water. Besides, plastic waste and electrocoagulated sludge can be hazardous as unscientific management can result in severe environmental damage. Hence, converting them into electrocatalysts can be a feasible choice to combat environmental problems. Herein, a novel anodic oxidation (AO) coupled electrocatalytic reduction (ER) system for CO2 was developed to degrade SA in the presence of persulfate (PS) to convert SA into formic acid (HCOOH). The SA was mineralized into CO2 in AO using multi-walled carbon nanotubes (MWCNT) coated carbon felt (CF) anode, and the resulting CO2 was subsequently reduced to HCOOH at electrocoagulated sludge-based sFe-Cu@MOF cathode. The AO demonstrated 99.1±0.9 % breakdown of SA at 0.033 min−1 in 120 min with the MWCNT-CF + PS system, indicating 1.7-fold (58.0±1.5 %) higher degradation than CF + PS. In addition, the maximum yield of HCOOH was 0.024 mM in the AO-ER system. The stability tests of MWCNT-CF and sFe-Cu@MOF revealed SA degradation and yield of HCOOH were slightly dropped by 9 % and 6 %, respectively; thus, possess exceptional stability. Radical scavenging tests showed SO4•− as the primary radical involved in the degradation of SA. Phytotoxicity revealed that MWCNT-CF + PS significantly decreased the toxicity of SA towards Cicer arietinum L. Total operating cost of AO-ER was about 0.468 $/m3, suggesting it could be cost-effectively used to treat wastewater in practical field scenarios.

Strategic Use of Extremophilic Microalgae as a Carbon Source in the Thermo-Chemical Process

Microalgal biomass has a high CO2 fixation and growth rate when using CO2 as a carbon source. Moreover, biomass can also be employed as a carbon resource to produce biofuels and chemicals. As the growth rate of extremophilic microalgae remains unaffected by harsh conditions, the present study proposes that these microalgae (such as Galdieria sulphuraria) are a rapidly growing carbon resource for syngas production. Hence, two different experiments were performed as part of this study: (1) cultivation of G. sulphuraria under outdoor conditions and (2) conversion of G. sulphuraria into syngas. The productivity of G. sulphuraria under mixotrophic condition (0.82 g L–1 d–1) was about 1.6 faster than a widely cultivated Chlorella sp. HS2. Moreover, G. sulphuraria was converted into syngas using CO2 as a co-feedstock. The simultaneous reduction of CO2 and the oxidation of volatile matter (VM) from the thermolysis of G. sulphuraria promoted syngas formation. The chemical reaction was influenced by the molecular size of the VMs. In the presence of the Ni catalyst, low-molecular-weight VMs were formed owing to chemical bond scissions. Syngas formation under CO2 doubled compared with that under inert conditions. The findings suggest that G. sulphuraria is a feasible carbon source for CO2 fixation and chemical production.

Entropy engineering of La-based perovskite for simultaneous photocatalytic CO2 reduction and biomass oxidation.

Herein, the high-entropy perovskite, i.e. La(FeCoNiCrMn)O3, was prepared for simultaneous CO2 reduction and biomass upgrading. Based on the synergistic effect between the elements in the high-entropy material, an excellent CO evolution rate of 131.8 μmol g-1 h-1 and a xylonic acid yield of 63.9% were gained.

In-situ-derived self-selective electrocatalysts for solar formate production from simultaneous CO2 reduction and methanol oxidation

Solar-driven electrochemical CO2 reduction reaction (CO2RR) offers a promising route to achieve a carbon-neutral and energy-sustainable future. However, the anodic oxygen evolution reaction (OER) hinders the energy input utilization, and the added value of the product O2 is low. Here, through a combined CO2RR and selective methanol oxidation reaction (MOR), we report an efficient and unassisted solar-driven simultaneous cathodic and anodic production of formate on hydroxide-derived self-selective Cu-based electrocatalysts. Upon in situ treatments, Cu(OH)2-derived Cu (HOD-Cu) and CuO (HOD-CuO) electrocatalysts display efficient CO2RR and MOR performances at a wide potential range, respectively. The rational integration of the electrolyzer to a triple junction GaInP/GaAs/Ge photovoltaic cell could realize efficient solar-driven formate synthesis, leading to a solar-to-formate (STF) conversion efficiency of 3.63% and a production rate of 0.194 mmol h−1 cm−2. This work demonstrates a simultaneous formate generation by coupling CO2RR and MOR, providing new paths for solar-driven electrochemical synthesis.

Simultaneous CO2 reduction and NADH regeneration using formate and glycerol dehydrogenase enzymes co-immobilized on modified natural zeolite.

In this work, the co-immobilization of formate dehydrogenase (FDH) and glycerol dehydrogenase (GlyDH) enzymes is proposed to reduce CO2 into formic acid, an important chemical intermediate. The reduction of carbon dioxide is carried out by FDH to obtain formic acid, simultaneously, the GlyDH regenerated the nicotinamide cofactor in the reduced form (NADH) by the oxidation of glycerol into dihydroxyacetone. Natural zeolite was selected as immobilization support given its good properties and low cost. The natural zeolite was modified with subsequent acid-alkaline attacks to obtain a mesostructurization of the clinoptilolite. The two enzymes were co-immobilized on clinoptilolite, previously hetero-functionalized with amino and glyoxyl groups. The distribution of the enzymes was confirmed by fluorescence microscopy analysis. Furthermore, a great increase in the retained activity for the formate dehydrogenase enzyme was noted, passing from 18% to 89%, when the mesostructured clinoptilolite was used as support. The immobilization yield of formate dehydrogenase and glycerol dehydrogenase is around 100% with all the supports studied. The promising results suggest a possible development of this procedure in enzyme immobilization and biocatalysis. The biocatalysts were characterized to find the optimal pH and temperature. Furthermore, a thermal stability test at 50 °C was carried out on both enzymes, in free and immobilized forms. Finally, it was shown that the biocatalyst is effective in reducing CO2, both by using the cofactor in the reduced form (NADH) or the oxidized form (NAD+), obtaining NADH through the regeneration with glycerol in this latter case.

Photoelectrochemical hybrid cell for unbiased CO2 reduction coupled to alcohol oxidation

The reduction of CO2 to renewable fuels must be coupled to a sustainable oxidation process to devise a viable device that produces solar fuels. In photoelectrochemical cells, water oxidation to O2 is the predominant oxidation reaction and typically requires a pair of light absorbers or an applied bias voltage when coupled to CO2 reduction. Here, we report a bias-free photoelectrochemical device for simultaneous CO2 reduction to formate and alcohol oxidation to aldehyde in aqueous conditions. The photoanode is constructed by co-immobilization of a diketopyrrolopyrrole-based chromophore and a nitroxyl-based alcohol oxidation catalyst on a mesoporous TiO2 scaffold, which provides a precious-metal-free dye-sensitized photoanode. The photoanode is wired to a biohybrid cathode that consists of the CO2 reduction enzyme formate dehydrogenase integrated into a mesoporous indium tin oxide electrode. The bias-free cell delivers sustained photocurrents of up to 30 µA cm−2 under visible-light irradiation, which results in simultaneous aldehyde and formate production. Our results show that in the absence of an external bias, single light absorber photoelectrochemical cells can be used for parallel fuel production and chemical synthesis from CO2 and alcohol substrates.

A mathematical model for CO2 conversion of CH4-producing biocathodes in microbial electrosynthesis systems

Microbial electrosynthesis systems are a novel device for simultaneous CO2 reduction and CH4 production, and the CH4-producing biocathode in this system is the key component. This work presents a mathematical model to insight into the process of CH4 production on the biocathode. The model couples bioelectrochemical reaction, charge balance, mass transfer, dissolution of gaseous CO2 and interconversion of hydrated CO2, H2CO3, HCO3− and CO32−. To our knowledge, this is the first theoretical report for CH4-producing biocathodes in microbial electrosynthesis systems. This model is capable of predicting the response of CH4-producing biocathodes with operating time under various conditions and describing the effects of different parameters on CH4 production. The results suggest that the most important processes which influence the performance of biocathodes are the interconversions of hydrated CO2, H2CO3 and HCO3−.

Coupling CO2 reduction with ethane aromatization for enhancing catalytic stability of iron-modified ZSM-5

The shale gas revolution and the carbon-neutrality goal are motivating the landscape toward the synthesis of value-added chemicals or fuels from underutilized ethane with the assistance of greenhouse gas CO2. Combining ethane aromatization with CO2 reduction offers an opportunity to directly produce liquid products for facile separation, storage, and transportation. In the present work, Fe/ZSM-5 catalysts showed promise in the simultaneous CO2 reduction and ethane aromatization at atmospheric pressure and 873 K. The catalysts were further investigated using X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) measurements under in-situ conditions, indicating that most of Fe species existed in the form of Fe oxides and a portion of Fe was incorporated into the ZSM-5 framework generating Lewis acid sites. Both types of Fe species remained almost unchanged under reaction conditions, contributing to an enhanced aromatization activity of Fe/ZSM-5. The effects of CO2 and steam on the acid sites and in turn aromatization activity were also investigated by transient studies, which exhibited a reversible modification behavior. Moreover, CO2 was identified to be critical to enhance coke resistance and in turn catalyst stability. This work highlights the feasibility of using CO2 to assist the upgrading of abundant ethane from shale gas to aromatics over non-precious Fe-based zeolite catalysts.

Formate production from CO2 electroreduction in a salinity-gradient energy intensified microbial electrochemical system

The electricity production of microbial electrochemical system can be substantially strengthened by coupling with a reverse electrodialysis stack which extracts energy from salinity gradient, therefore provides a possible way for value-added products in cathode without external energy input. Here, a microbial reverse-electrodialysis CO2 reduction cell (MRECC) was developed and successfully utilized to drive CO2-to-formate conversion on a Bi/Cu cathode. Results confirmed the optimal anodic COD load and cathodic CO2 flow rate to be 1 g NaAc L−1 and 10 mL min−1. MRECC could yielded 143.5 ± 8.1 mg L−1 of formate with total energy efficiency of 4.6 ± 0.9% and coulombic efficiency of 46.4 ± 2.4%. Increasing or decreasing anode or cathode load impaired MRECC performance from economic and environmental viabilities. MRECC provided a promising platform for simultaneous CO2 reduction and value-added chemicals production by using sustainable energy from wastewaters.

Degradation of 4-nitrophenol by electrocatalysis and advanced oxidation processes using Co3O4@C anode coupled with simultaneous CO2 reduction via SnO2/CC cathode

Herein, we prepared novel three-dimensional (3D) gear-shaped Co3O4@C (Co3O4 modified by amorphous carbon) and sheet-like SnO2/CC (SnO2 grow on the carbon cloth) as anode and cathode to achieve efficient removal of 4-nitrophenol (4-NP) in the presence of peroxymonosulfate (PMS) and simultaneous electrocatalytic reduction of CO2, respectively. In this process, 4-NP was mineralized into CO2 by the Co3O4@C, and the generated CO2 was reduced into HCOOH by the sheet-like SnO2/CC cathode. Compared with the pure Co0.5 (Co3O4 was prepared using 0.5 g urea) with PMS (30 mg, 0.5 g/L), the degradation efficiency of 4-NP (60 mL, 10 mg/L) increased from 74.5%–85.1% in 60 min using the Co0.5 modified by amorphous carbon (Co0.5@C). Furthermore, when the voltage of 1.0 V was added in the anodic system of Co0.5@C with PMS (30 mg, 0.5 g/L), the degradation efficiency of 4-NP increased from 85.1%–99.1% when Pt was used as cathode. In the experiments of 4-NP degradation coupled with simultaneous electrocatalytic CO2 reduction, the degradation efficiency of 4-NP was 99.0% in the anodic system of Co0.5@C with addition of PMS (30 mg, 0.5 g/L), while the Faraday efficiency (FE) of HCOOH was 24.1 % at voltage of −1.3 V using the SnO2/CC as cathode. The results showed that the anode of Co3O4 modified by amorphous carbon can markedly improve the degradation efficiency of 4-NP, while the cathode of SnO2/CC can greatly improve the FE and selectivity of CO2 reduction to HCOOH and the stability of cathode. Finally, the promotion mechanism was proposed to explain the degradation of organic pollutants and reduction of CO2 into HCOOH in the process of electrocatalysis coupled with advanced oxidation processes (AOPs) and simultaneous CO2 reduction.

Efficient infrared light induced CO2 reduction with nearly 100% CO selectivity enabled by metallic CoN porous atomic layers

Conductors hold promise for applications in infrared-light CO2 reduction to fuels. However, their extremely high carrier densities unfortunately result in strong electron–hole recombination. Herein, an ultrathin conductor with porous structure is designed to prolong the lifetime of photoexcited electrons. Taking the synthetic metallic CoN porous atomic layers as an example, synchrotron-radiation photoelectron spectroscopy and UV-vis-NIR spectroscopy uncover they could realize simultaneous CO2 reduction and H2O oxidation under infrared-light irradiation. Ultrafast transient absorption spectroscopy first unveils the infrared-light excited electrons undergo sequential intraband relaxation and interband recombination processes, where the 9- and 1.6-fold increased time in these two processes verifies the Na2S solution dramatically prolongs the electron lifetime. The metallic CoN porous atomic layers exhibit infrared-light induced CO2 reduction with nearly 100% CO selectivity, while the CO evolution rate is increased by 50 times upon adding Na2S solution. This study affords possibilities for achieving high-efficiency infrared-light driven CO2 reduction performance.

MOF-253-Supported Ru Complex for Photocatalytic CO2 Reduction by Coupling with Semidehydrogenation of 1,2,3,4-Tetrahydroisoquinoline (THIQ).

MOF-253 (Al(OH)(dcbpy), dcbpy = 2,2'-bipyridine-5,5'-dicarboxylic acid) obtained via a microwave-assisted synthesis was used for the construction of a supported Ru complex containing dcbpy (MOF-253-Ru(dcbpy)2) by coordinating its open N,N'-chelating sites with Ru(II) in Ru(dcbpy)2Cl2. The as-obtained MOF-253-Ru(dcbpy)2 acts as a bifunctional photocatalyst for simultaneous CO2 reduction to produce formic acid and CO, as well as semidehydrogenation of 1,2,3,4-tetrahydroisoquinoline (THIQ) to obtain 3,4-dihydroisoquinoline (DHIQ). The performance over the surface-supported MOF-253-Ru(dcbpy)2 is superior to that over Ru-doped MOF-253 (Ru-MOF-253) obtained via a mix-and-match strategy, indicating that the use of open coordination sites in the MOFs for direct construction of a surface-supported complex is a superior strategy to obtain an MOF-supported homogeneous complex. This study shows the possibility of using an MOF as a platform for the construction of multifunctional heterogeneous photocatalytic systems. The coupling of photocatalytic CO2 reduction with the highly selective dehydrogenation of organics provides an economical and green strategy in photocatalytic CO2 reduction and production of valuable organics simultaneously.

Fabrication of Cu2O-RGO/BiVO4 nanocomposite for simultaneous photocatalytic CO2 reduction and benzyl alcohol oxidation under visible light

Cu2O-RGO/BiVO4 nanocomposite was obtained via a thermal treatment of Cu precursor in the presence of the pre-obtained RGO/BiVO4. The product was fully characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and UV–vis diffuse reflectance spectroscopy (DRS). It was found that olive-shaped BiVO4 of ca. 1.0–1.2 μm as well as small nanoparticles of Cu2O with a dimension of 40–50 nm were deposited on the surface of RGO. The as-formed Cu2O-RGO/BiVO4 nanocomposite exhibited superior photocatalytic activity for simultaneous CO2 reduction and benzyl alcohol oxidation under visible-light irradiation, as compared with bare Cu2O and RGO/BiVO4 nanocomposite. The superior photocatalytic performance observed over Cu2O-RGO/BiVO4 nanocomposite may be ascribed to the existence of the Z-Scheme charge transfer pathway in the nanocomposite, i.e., the photogenerated electrons transferred from BiVO4 to Cu2O to recombine with the photogenerated holes via RGO acting as a good electron mediator. The study demonstrates a high potential of using RGO to fabricate Z-Scheme photocatalytic systems for a variety of photocatalytic applications.

Theoretical and Experimental Investigation of Solar Methane Reforming through the Nonstoichiometric Ceria Redox Cycle

AbstractThe dry reforming of methane through the nonstoichiometric ceria (CeO2–CeO2−δ) redox cycle was examined theoretically and experimentally for converting high‐temperature, solar process heat to syngas. The aforementioned cycle is composed of: (1) endothermic reduction of ceria and simultaneous partial oxidation of methane and (2) exothermic oxidation of the reduced ceria and simultaneous reduction of CO2. In both steps, chemical equilibrium calculations indicate that isothermal operation is thermodynamically favorable under a wide range of conditions. The influence of the total amount of reactive gas, the operating temperature, and the inclusion of gas‐ or solid‐phase heat exchangers on the cycle performance was determined through a holistic process model. A theoretical solar‐to‐fuel conversion efficiency, defined as the ratio of the difference between the calorific value of syngas (H2 and CO) produced and methane converted to the total solar radiative input energy, of more than 45 % was predicted with no heat recuperation. Experimental validation was subsequently demonstrated in a packed‐bed‐type solar reactor using the high‐flux solar simulator at the University of Florida at three discrete isothermal temperatures, namely, 950, 1035, and 1120 °C. Upon the completion of the reduction at each temperature, the bed‐averaged oxygen nonstoichiometry equaled 0.07, 0.21, and 0.24, which yielded methane conversions of 9, 41, and 51 %, respectively. At 1120 °C, the extrapolated solar‐to‐fuel conversion efficiency was 9.82 %.

Chronopotentiometric Approach of CO2 Reduction in Molten Carbonates

Carbon capture and valorization is a new route increasingly discussed to reduce greenhouse gas emissions. One of the paths foreseen for CO2 valorization is its transformation into fuels by electrochemical reduction of carbon dioxide. In this approach, molten carbonates are of particular interest since they can be used to capture CO2 molecule because of its high solubility; hence, valorization by electrolysis is possible in molten carbonate media. To better understand CO2 reduction, chronopotentiometric and chronoamperometric techniques were used at gold and graphite electrodes immersed in four alkali carbonate eutectics, Li-Na, Li-K, Li-Na-K and Na-K. The results complemented a previous work based on cyclic voltammetry and confirmed the existence of a main CO2 reduction phenomena around −1.2 V vs Ag/Ag+ that involves two one-electron steps or a two-electron unique step. Transition time analysis showed that the reduction mechanism is either simultaneous reduction of CO2 adsorbed and diffusion species or rapid equilibrium between adsorbed and diffusing species. Furthermore, we identified another electrochemical system involving adsorbed CO2 and CO at higher potentials. In summary, we have succeeded in proving and précising some previous results, as well as identifying reduction mechanisms.