A Novel Sustainable Process for Multilayer Graphene Synthesis Using CO2 from Ambient Air

Graphene produced by different methods can present varying physicochemical properties and quality, resulting in a wide range of applications. The implementation of a novel method to synthesize graphene requires characterizations to determine the relevant physicochemical and functional properties for its tailored application. We present a novel method for multilayer graphene synthesis using atmospheric carbon dioxide with characterization. Synthesis begins with carbon dioxide sequestered from air by monoethanolamine dissolution and released into an enclosed vessel. Magnesium is ignited in the presence of the concentrated carbon dioxide, resulting in the formation of graphene flakes. These flakes are separated and enhanced by washing with hydrochloric acid and exfoliation by ammonium sulfate, which is then cycled through a tumble blender and filtrated. Raman spectroscopic characterization, FTIR spectroscopic characterization, XPS spectroscopic characterization, SEM imaging, and TEM imaging indicated that the graphene has fifteen layers with some remnant oxygen-possessing and nitrogen-possessing functional groups. The multilayer graphene flake possessed particle sizes ranging from 2 µm to 80 µm in diameter. BET analysis measured the surface area of the multilayer graphene particles as 330 m²/g, and the pore size distribution indicated about 51% of the pores as having diameters from 0.8 nm to 5 nm. This study demonstrates a novel and scalable method to synthesize multilayer graphene using CO₂ from ambient air at 1 g/kWh electricity, potentially allowing for multilayer graphene production by the ton. The approach creates opportunities to synthesize multilayer graphene particles with defined properties through a careful control of the synthesis parameters for tailored applications.     

Use of a Gemini-Surfactant Synthesized from the Mango Seed Oil as a CO2-Corrosion Inhibitor for X-120 Steel

A gemini surfactant imidazoline type, namely N-(3-(2-fatty-4,5-dihydro-1H-imidazol-1-yl) propyl) fatty amide, has been obtained from the fatty acids contained in the mango seed and used as a CO₂ corrosion inhibitor for API X-120 pipeline steel. Employed techniques involved potentiodynamic polarization curves, linear polarization resistance, and electrochemical impedance spectroscopy. These tests were supported by detailed scanning electronic microscopy (SEM) and Raman spectroscopy studies. It was found that obtained gemini surfactant greatly decreases the steel corrosion rate by retarding both anodic and cathodic electrochemical reactions, with an efficiency that increases with an increase in its concentration. Gemini surfactant inhibits the corrosion of steel by the adsorption mechanism, and it is adsorbed on to the steel surface according to a Langmuir model in a chemical type of adsorption. SEM and Raman results shown the presence of the inhibitor on the steel surface.     

Facile Fabrication of ZnO-ZnFe2O4 Hollow Nanostructure by a One-Needle Syringe Electrospinning Method for a High-Selective H2S Gas Sensor

Herein, a facile fabrication process of ZnO-ZnFe₂O₄ hollow nanofibers through one-needle syringe electrospinning and the following calcination process is presented. The various compositions of the ZnO-ZnFe₂O₄ nanofibers are simply created by controlling the metal precursor ratios of Zn and Fe. Moreover, the different diffusion rates of the metal oxides and metal precursors generate a hollow nanostructure during calcination. The hollow structure of the ZnO-ZnFe₂O₄ enables an enlarged surface area and increased gas sensing sites. In addition, the interface of ZnO and ZnFe₂O₄ forms a p-n junction to improve gas response and to lower operation temperature. The optimized ZnO-ZnFe₂O₄ has shown good H₂S gas sensing properties of 84.5 (S = R_a/R_g) at 10 ppm at 250 °C with excellent selectivity. This study shows the good potential of p-n junction ZnO-ZnFe₂O₄ on H₂S detection and affords a promising sensor design for a high-performance gas sensor.     

Effect of Ornamental Stone Waste Incorporation on the Rheology,Hydration, Microstructure,and CO2 Emissions of Ordinary Portland Cement

The ornamental stone industry generates large amounts of waste thus creating environmental and human health hazards. Thus, pastes with 0–30 wt.% ornamental stone waste (OSW) incorporated into ordinary Portland cement (OPC) were produced and their rheological properties, hydration kinetics, and mechanical properties were evaluated. The CO₂ equivalent emissions related to the pastes production were estimated for each composition. The results showed that the paste with 10 wt.% of OSW exhibited similar yield stress compared to the plain OPC paste, while pastes with 20 and 30 wt.% displayed reduced yield stresses up to 15%. OSW slightly enhanced the hydration kinetics compared to plain OPC, increasing the main heat flow peak and 90-h cumulative heat values. The incorporation of OSW reduced the 1-, 3-, and 28-days compressive strength of the pastes. Water absorption results agreed with the 28 days compressive strength results, indicating that OSW increased the volume of permeable voids. Finally, OSW incorporation progressively reduced the CO₂ emission per m³ of OPC paste, reaching a 31% reduction for the highest 30 wt.% OSW content. Overall, incorporating up to 10 wt.% with OSW led to pastes with comparable fresh and hardened properties as comported to plain OPC paste.     

Dense CO2 as a Solute,Co-Solute or Co-Solvent in Particle Formation Processes: A Review

The application of dense gases in particle formation processes has attracted great attention due to documented advantages over conventional technologies. In particular, the use of dense CO₂ in the process has been subject of many works and explored in a variety of different techniques. This article presents a review of the current available techniques in use in particle formation processes, focusing exclusively on those employing dense CO₂ as a solute, co-solute or co-solvent during the process, such as PGSS (Particles from gas-saturated solutions^®), CPF (Concentrated Powder Form^®), CPCSP (Continuous Powder Coating Spraying Process), CAN-BD (Carbon dioxide Assisted Nebulization with a Bubble Dryer^®), SEA (Supercritical Enhanced Atomization), SAA (Supercritical Fluid-Assisted Atomization), PGSS-Drying and DELOS (Depressurization of an Expanded Liquid Organic Solution). Special emphasis is given to modifications introduced in the different techniques, as well as the limitations that have been overcome.     

Efficient and selective molecular catalyst for the CO2-to-CO electrochemical conversion in water

Substitution of the four paraphenyl hydrogens of iron tetraphenylporphyrin by trimethylammonio groups provides a water-soluble molecule able to catalyze the electrochemical conversion of carbon dioxide into carbon monoxide. The reaction, performed in pH-neutral water, forms quasi-exclusively carbon monoxide with very little production of hydrogen, despite partial equilibration of CO₂ with carbonic acid—a low pK_a acid. This selective molecular catalyst is endowed with a good stability and a high turnover frequency. On this basis, prescribed composition of CO–H₂ mixtures can be obtained by adjusting the pH of the solution, optionally adding an electroinactive buffer. The development of these strategies will be greatly facilitated by the fact that one operates in water. The same applies for the association of the cathode compartment with a proton-producing anode by means of a suitable separator.One of the most important issues of contemporary energy and environmental challenges consists of reducing carbon dioxide into fuels by means of sunlight (1–3). One route toward this ultimate goal is to first convert solar energy into electricity, which will then be used to reduce CO₂ electrochemically. Direct electrochemical injection of an electron into the CO₂ molecule, forming the corresponding anion radical CO₂^.− requires a very high energy [the standard potential of the CO₂/ CO₂^.− couple is indeed −1.97 V vs. normal hydrogen electrode (NHE) in N,N′dimethylformamide (DMF)] (4, 5). Electrochemical conversion of CO₂ to any reaction product thus requires catalytic schemes that preferably avoid this intermediate. Carbon monoxide may be an interesting step en route to the desired fuels because it can be used as feedstock for the synthesis of alkanes through the classic Fischer–Tropsch process. A number of molecular catalysts for the homogeneous electrochemical CO₂-to-CO conversion have been proposed. They mainly derive from transition metal complexes by electrochemical generation of an appropriately reduced state, which is restored by the catalytic reaction. So far, nonaqueous aprotic solvents (mostly DMF and acetonitrile) have been used for this purpose (5–16). Brönsted acids have been shown to boost catalysis. However, they should not be too strong, at the risk of leading to H₂ formation at the expense of the CO. Trifluoroethanol and water (possibly in large amounts) have typically played the role of a weak acid in the purpose of boosting catalysis while avoiding hydrogen evolution.One of the most thoroughly investigated families of transition-metal complex catalysts of CO₂-to-CO conversion is that of iron porphyrins brought electrochemically to the oxidation degree 0. The importance of coupling electron transfer and introduction of CO₂ into the coordination sphere of iron with proton transfers required by the formation of CO, CO₂ + 2e⁻ + 2AH ↔ CO + H₂O + 2A⁻, appeared from the very beginning of these studies. Sustained formation of CO was indeed only achieved upon addition of weak Brönsted (17–19) and Lewis acids (20, 21). Such addition of Brönsted acids, however, opens the undesired possibility that the same catalyst that converts CO₂ into CO may also catalyze the reduction of the acid to dihydrogen. This is indeed what happens with Et₃NH⁺ as the acid, limiting the set of acids used for the CO₂-to-CO conversion to that of very weak acids such as propanol, water, and, the strongest one, trifluorethanol (22). Since then, the range of acidity has been extended to phenols, which proved compatible with CO faradic yields close to 100% (23, 24). Installation of phenol functionalities in the porphyrin molecules (Scheme 1) even allowed a considerable improvement of catalysis in terms of catalytic Tafel plots (turnover frequency vs. overpotential) with no degradation of the CO (vs. H₂) faradaic yield (25–27). The problem should, however, resurface upon going to stronger acids. That competition with hydrogen evolution is a general issue for molecular catalysis of the CO₂-to-CO conversion is confirmed by recent findings concerning catalysts derived from terpyridine complexes of first-row transition metals in (90:10, vol:vol) DMF/H₂O mixtures (28).Open in a separate windowScheme 1.Iron-porphyrin catalysts for CO₂-to-CO electrochemical conversion.The results thus obtained in nonaqueous or partially aqueous media enabled the discovery of remarkably efficient and selective catalysts of the CO₂-to-CO conversion. They were also the occasion of notable advances in terms of mechanisms and theory of concerted bond-breaking proton–electron transfer (29).It must, however, be recognized that, from the point of view of practical applications, the use of nonaqueous solvents is not the most exciting aspect of these results. One would rather like to use water as the solvent, which would render more viable the CO₂-to-CO half-cell reaction as well as its association with a water-oxidation anode through a proton-exchange membrane. Several approaches are conceivable in this respect. One of these consists of coating the electrode with a film, possibly making use of the water insolubility of the catalyst (see the three last entries of table 3 in ref. 5, refs. 30 and 31, and ref. 32 and references therein). One may also attempt to chemically attach catalyst molecules onto the electrode having in mind their use, or possible use, of the resulting films in water (33, 34).Our approach consisted of devising a catalyst fully soluble in water and able to convert CO₂ to CO selectively as well as efficiently with regard to overpotential and turnover frequency. Full solubility in water, indeed, allows an easy manipulation of pH and buffering of the system. It will also help the design of a full cell associating the cathode compartment with a proton-producing anode by means of a suitable separator (35). The challenges ahead in this endeavor were as follows. CO₂ is poorly soluble in water ([CO₂] = 0.0383 M) (36). Even more seriously, it is partially converted (K_hydration = 1.7 × 10⁻³) into carbonic acid, CO₃H₂, which has a first ionization pK_a of 3.6, that is, an apparent pK_a of 6.4 (36). These features make it a priori possible that the CO₂-to-CO conversion might be seriously challenged by H₂ evolution from reduction of carbonic acid and/or hydrated protons (37, 38). Indeed, a previous attempt showed poor CO₂/H₂ selectivity and practically no catalytic effect in cyclic voltammetry (37). In the same vein, Ni cyclam seems to be a selective and efficient catalyst of the CO₂-to-CO conversion in water under the condition of being operated with a mercury electrode, pointing to favorable specific interactions of the catalytic species with the mercury surface (38, 39), whereas the use of a carbon electrode leads to much less efficiency in terms of rate (maximal turnover frequency of 90 s⁻¹) and turnover (four cycles) (15).We have found that the iron tetraphenylporphyrin in which the four paraphenyl hydrogens have been substituted by trimethylammonio groups, hereafter designated as WSCAT (Scheme 1), fulfills these stringent requirements at neutral pHs.As seen in Fig. 1A, a very high catalytic current is observed in cyclic voltammetry of half a millimolar solution of WSCAT saturated with CO₂ at pH 6.7. In the absence of CO₂ (Fig. 1B) three successive waves are observed when starting from the Fe^III complex with Cl⁻ as counter ions and presumably as axial ligand. As expected, the shape of the first, Fe^III/II, wave reflects the strong axial ligation by Cl⁻ (40). The second wave is a standard reversible Fe^II/I wave. The third wave is the wave of interest for catalysis. It is irreversible, as opposed to what is observed in DMF (Fig. 1C), where all three waves are one-electron reversible waves, including the third Fe^I/0 wave, as with the simple FeTPP (Scheme 1) (see, e.g., refs. 23 and 24). The irreversibility and somewhat increased current observed here in water presumably reflects some catalysis of H₂ evolution from the reduction of water. The considerable increase in current observed in the Fe^I/0 potential region when CO₂ is introduced into the solution is a clear indication that catalysis is taking place. It is roughly similar to what has been observed in DMF under 1 atm CO₂ in the presence of a weak acid such as phenol (Fig. 1D and refs. 23 and 24).Open in a separate windowFig. 1.Cyclic voltammetry of 0.5 mM WSCAT at 0.1 V/s, temperature 21 °C. (A) In water + 0. 1 M KCl brought to pH 6.7 by addition of KOH under 1 atm CO₂. (B) Same as A but in the absence of CO₂. (C) Same as B but in DMF + 0.1 n-Bu₄NBF₄. (D) Same as C but under 1 atm CO₂, and presence of 3 M phenol.What is the nature of the catalysis observed in the Fe^I/0 potential region? Does it involve acid reduction (CO₃H₂ and/ or H⁺) or conversion of CO₂ into one of the reduction products? The answer is given by the results of preparative-scale electrolyses.A first series of preparative-scale electrolyses was performed at −0.97 V vs. NHE over electrolysis times between 1 and 4 h. The pH was brought to 6.7 by addition of KOH. The current density was ca. 0.1 mA/cm² in all cases. CO was found to be largely predominant with formation of only a very small amount of hydrogen. Over five of these experiments the average faradaic yields of detected products were as follows: CO, 90%; H₂, 7%; acetate, 1.4%; formate, 0.7%; and oxalate, 0.5%. The catalyst was quite stable during these periods of time. The variation of pH during electrolysis was small, passing from 6.7 to 6.9. The decrease in peak current registered before and after electrolysis was less than 5%.A longer-duration electrolysis (Fig. 2) was carried out at a somewhat less negative potential, −0.86 V vs. NHE, leading to a quasi-quantitative formation of CO (faradaic yield between 98% and 100%). After 72 h of electrolysis, the current was decreased by approximately half but CO continued to be the only reaction product.Open in a separate windowFig. 2.Electrolysis of a 0.5 mM WSCAT solution in water + 0.1 M KCl brought to pH 6.7 by addition of KOH under 1 atm CO₂ at −0.86 V vs. NHE. Temperature 21 °C. The reaction products were analyzed at the end of each day. Charge passed (Top); current density (Bottom).As an example of pH manipulation, the addition of a 0.1 M formic acid buffer at pH 3.7 resulted in the exclusive formation of hydrogen. With a 0.1 M phosphate buffer adjusted at the same pH, 6.7, as that where the electrolyses with no additional buffer were carried out, a 50–50 CO–H₂ mixture was obtained.Although deserving a precise kinetic analysis, a likely interpretation of the factors favorable to CO formation against H₂ evolution is summarized in Scheme 2. Despite Fe⁰ porphyrins’ being good catalysts of H₂ evolution (22), the fact that the reaction that converts CO₂ into CO₃H₂ is thermodynamically uphill and kinetically slow favors the CO-formation pathway, making it able to resist the fast H₂ evolution pathway in pH-neutral media. Another favorable factor is the wealth of H-bonding possibilities offered by water, a factor that has been shown to be quite important in stabilizing the primary Fe⁰–CO₂ adduct and therefore in boosting catalysis as transpires from the results obtained with OH-substituted tetraphenylporphyrins CAT and FCAT (Scheme 1) (25–27). Addition of a buffer, such as phosphate in 0.1 M concentration, opens an additional catalytic pathway for H₂ evolution able to compete with the CO-formation pathway even if the pH is kept at the same neutral value. Indeed, along this pathway, the delivery of protons (by PO₄H₂⁻) is not under a stringent kinetic limitation as it is in the pathway represented in Scheme 2. It is nevertheless clear that a future detailed mechanistic and kinetic investigation of the effect of pH changes in buffered and nonbuffered media is clearly warranted, based on cyclic voltammetry and determination of the CO and H₂ faradaic yields.Open in a separate windowScheme 2.Competition between CO formation and H₂ evolution.In view of the paucity of data concerning homogeneous molecular catalysis of the CO₂-to-CO conversion in water, benchmarking with other catalysts in terms of overpotential and turnover frequency does not seem possible at the moment. We may just note that a preliminary estimation of the maximal turnover frequency through the foot-of-the-wave analysis (discussed below) leads to the exceptionally high figure of 10⁷ s⁻¹ (i.e., a second-order rate constant of 2.5 × 10⁸ M⁻¹⋅s⁻¹).We are thus led to make the comparison with the characteristics of other catalysts obtained in an aprotic solvent such as DMF or acetonitrile, starting from the results shown in Fig. 1 C and D. The standard potential of the Fe^I/Fe⁰ couple in DMF (Fig. 1C) is −1.23 V vs. NHE. A systematic analysis of the wave obtained under 1 atm CO₂ and presence of 3 M phenol was carried out according to the foot-of-the wave approach, which aims at minimizing the effects of side phenomena that interfere at large catalytic currents. This technique has been previously described in detail and successfully applied in several instances (23–27). It was applied here, assuming that the reaction mechanism is of the same type as for FeTPP in the presence of phenol (Scheme 3) (24).Open in a separate windowScheme 3.Likely mechanism of the CO₂-to-CO conversion pathway.Combination of the foot-of-the wave analysis with increasing scan rates, which both minimize the effect of side phenomena (23–27), allowed the determination of the turnover frequency as a function of the overpotential, leading to the catalytic Tafel plot for the WSCAT catalyst shown as the red curve in Fig. 3. The turnover frequency, TOF, takes into account that the molecules that participate in catalysis are only those contained in the thin reaction-diffusion layer adjacent to the electrode surface in pure kinetic conditions (23, 24). The overpotential, η, is the difference between the standard potential of the reaction to be catalyzed and the electrode potential. Correlations between TOF and η provide catalytic Tafel plots that are able to benchmark the intrinsic properties of the catalyst independently of parameters such as cell configuration and size. Good catalysts appear in the upper left corner and bad catalysts in the bottom right corner of Fig. 3. These plots allow one to trade between the rapidity of the catalytic reaction and the energy required to run it. The other Tafel plots shown in Fig. 3 are simply a repetition of what has been established in detail in ref. 24.Open in a separate windowFig. 3.Catalytic Tafel plots in DMF or acetonitrile. See ref. 27 for details and references.It clearly appears that WSCAT is the best catalyst of the set of molecules represented in Fig. 3. It is expected that the electron-withdrawing properties of the para-N-trimethylammonium groups leads to a positive shift of the Fe^I/Fe⁰ couple, being thus a favorable factor in terms of overpotential (positive shifts of 200 mV vs. FeTPP, 100 mV vs. CAT, and 40 mV vs. FCAT). What is more surprising is that this effect, which tends to decrease the electron density on the iron and porphyrin ring at the oxidation state 0, does not slow down the catalytic reaction to a large extent. Systematic analysis of the factors that control the standard potential and the catalytic reactivity as a function of the substituents installed on the porphyrin ring is a clearly warranted future task.For the moment, we may conclude that substitution of the four parahydrogens of FeTPP by trimethylammonium groups has produced a water-soluble catalyst that is able, for the first time to our knowledge, to catalyze quantitatively the conversion of carbon dioxide into carbon monoxide in pH-neutral water with very little production of hydrogen. This seems a notable result in view of the hydration of CO₂, producing carbonic acid—a low pK_a acid—the catalytic reduction of which, and/or of the hydrated protons it may generate, into hydrogen might have been a serious competing pathway. Although a number of mechanistic details should be worked out, this notable result seemingly derives not only from the relatively small value of the hydration constant but also from the slowness of this reaction. As judged from its performances in DMF, this catalyst moreover seems particularly efficient in terms of catalytic Tafel plots relating the turnover frequency to the overpotential. Manipulation of the pH under unbuffered conditions or by introduction of an additional buffer should open the way to the production of CO–H₂ mixtures in prescribed proportions. The substitution by the four trimethylammonium groups ensured the water solubility of the catalyst. It may also be an adequate way of immobilizing it onto the electrode surface by means of a negatively charged polymer to be associated with a water-oxidation anode through a proton-exchange membrane.     

From the Cover: Ultraefficient homogeneous catalyst for the CO2-to-CO electrochemical conversion

A very efficient electrogenerated Fe⁰ porphyrin catalyst was obtained by substituting in tetraphenylporphyrin two of the opposite phenyl rings by ortho-, ortho''-phenol groups while the other two are perfluorinated. It proves to be an excellent catalyst of the CO₂-to-CO conversion as to selectivity (the CO faradaic yield is nearly quantitative), overpotential, and turnover frequency. Benchmarking with other catalysts, through catalytic Tafel plots, shows that it is the most efficient, to the best of our knowledge, homogeneous molecular catalyst of the CO₂-to-CO conversion at present. Comparison with another Fe⁰ tetraphenylporphyrin bearing eight ortho-, ortho''-phenol functionalities launches a general strategy where changes in substituents will be designed so as to optimize the operational combination of all catalyst elements of merit.The reductive conversion of CO₂ to CO is an important issue of contemporary energy and environmental challenges (1–10). Several low-oxidation-state transition metal complexes have been proposed to serve as homogeneous catalyst for this reaction in nonaqueous solvents such as N,N''-dimethylformamide (DMF) or acetonitrile (11–23).Among them, electrochemically generated Fe⁰ complexes have been shown to be good catalysts, provided they are used in the presence of Brönsted or Lewis acids (17–19). More recent investigations have extended the range of Brönsted acids able to boost the catalysis of the CO₂-to-CO conversion by electrogenerated Fe⁰TPP (Scheme 1) without degrading the selectivity of the reaction. They have also provided a detailed analysis of the reaction mechanism (24, 25).Open in a separate windowScheme 1.Iron-based catalysts for CO₂-to-CO reduction.This is notably the case with phenol, which gave rise to the idea of installing prepositioned phenol groups in the catalyst molecule as pictured in Scheme 1 under the heading “CAT.” The result was indeed a remarkably efficient and selective catalyst of the CO₂-to-CO conversion (26). At first blush, the comparison with the role of phenol in the case of FeTPP would entail attributing this considerable enhancement of catalysis to a local concentration of phenol much larger than can be achieved in solution. In fact, as analyzed in detail elsewhere (27), the role played by the internal phenol moieties is twofold. They indeed provide a very large local phenol concentration, favoring proton transfers, but they also considerably stabilize the initial Fe⁰–CO₂ adduct through H bonding. Although the favorable effect of pendant acid groups has been noted in several cases (see ref. 27 and references therein), this was, to our knowledge, the first time their exact role was deciphered. The difference in the role played by the phenol moieties takes place within the framework of two different mechanisms (see Scheme 2 for CAT and FCAT and Scheme 3 for FeTPP) (27). With FeTPP, the first step is, as with CAT and FCAT, the addition of CO₂ on the electrogenerated Fe⁰ complex (et₁ in Schemes 2 and ​and3).3). The strong stabilization of the Fe⁰–CO₂ adduct formed according to reaction 1 (in Schemes 2 and ​and3)3) in the latter cases compared with the first has a favorable effect on catalysis, but one consequence of this stabilization is that catalysis then required an additional proton (reactions 2₁ and 2₂ in Scheme 2), the final, catalytic loop-closing step being the cleavage of one of the C–O bonds of CO₂ concerted with both electron transfer from the electrode and proton transfer from one of the local phenol groups (et₂ in Scheme 2). In the FeTPP case, the C–O bond-breaking step (reaction 2 in Scheme 3) is different: it involves an intramolecular electron transfer concerted with proton transfer and cleavage of the C–O bond. The catalytic loop is closed by a homogeneous electron transfer step (et₂ in Scheme 3) that regenerates the initial Fe^I complex.Open in a separate windowScheme 2.Mechanism for the reduction of CO₂ with CAT and FCAT.Open in a separate windowScheme 3.Mechanism for the reduction of CO₂ with FeTPP.The object of the present contribution is to test the idea that introduction of different substituents on the periphery of the porphyrin ring may improve the efficiency of the catalysis of CO₂-to-CO conversion. In such a venture, we will have to take into account both the overpotential at which the reaction takes place and the catalytic rate expressed as the turnover frequency as detailed in the following sections. Taking these two aspects simultaneously into consideration is essential in view of the possibility that substitution may improve one of the two factors and degrade the other, or vice versa. As a first example, we examined the catalytic performances of the FCAT molecule (Scheme 1), in which four of the eight phenol groups have been preserved in the same ortho-, ortho''- positions on two of the opposite phenyl rings, while the two other phenyl rings have been perfluorinated (the synthesis and characterization of this molecule is described in SI Text). A query that first comes to mind is as follows. The inductive effect of the fluorine atoms is expected to ease the reduction of the molecule to the Fe⁰ oxidation state, and thus to be favorable to catalysis in terms of overpotential. However, will this benefit be blurred by a decrease of its reactivity toward CO₂? Indeed, the same inductive effect of the fluorine atoms tends to decrease the electronic density on the Fe⁰ complex and might therefore render the formation of the initial Fe⁰-CO₂ adduct less favorable. Change in the rates of the follow-up reactions of Scheme 1 may also interfere. A first encouraging indication that the fluorine substitution has a globally favorable effect on catalysis derives from the comparison of the cyclic voltammetric responses of FCAT and CAT as represented in Fig. 1: the peak potential is slightly more positive for FCAT [−1.55 V vs. normal hydrogen electrode (NHE)] than for CAT (−1.60 V vs. NHE), whereas the apparent number of electrons at the peak at 0.1 V/s is clearly larger in the first case than in the second (120 vs. 80) (26). However, a deeper analysis of the meaning of these figures in terms of effective catalysis is required. The mechanism of the reaction (Scheme 2) has been shown to be the same with FCAT as with CAT, and the various kinetic parameters indicated in Scheme 2, whose values are recalled in 26, 27). Comparison of the two catalysts may then be achieved more rationally based on the determination, in each case, of the catalytic Tafel plots, which relates the turnover frequency (TOF) to the overpotential (η). The latter is defined, in the present case of reductive processes, as the difference between the apparent standard potential of the CO₂/CO conversion, ECO2/CO0 and the electrode potential, E:η=E−ECO2/CO0.Large catalytic currents correspond to “pure kinetic conditions” in which a steady state is achieved by mutual compensation of catalyst transport and catalytic reactions. The cyclic voltammetry (CV) responses are then S-shaped independent of scan rate. They are the same with other techniques such as rotating disk electrode voltammetry and also during preparative-scale electrolyses. The fact that peaks instead of plateaus are observed at low scan rate, as in Fig. 1, derives from secondary phenomena related to the observed high catalytic efficiencies, such as substrate consumption, inhibition by products, and deactivation of the catalyst. These factors and the ways to go around their occurrence to finally obtain a full characterization of the mechanism and kinetics of the catalysis process are discussed in detail elsewhere (27). Under pure kinetic conditions, the active catalyst molecules are then confined within a thin reaction-diffusion layer adjacent to the electrode surface. During the time where the catalyst remains stable, the TOF is defined asTOF = N_product/N_active?cat, where N_product is the number of moles of the product, generated per unit of time, and N_active?cat is the maximal number of moles of the active form catalyst contained in the reaction–diffusion layer rather than in the whole electrochemical cell (for more information on the notions of pure kinetic conditions, reaction–diffusion layer, and on the correct definition of TOF, see refs. 23, 26, 28). For the present reaction mechanism (Scheme 2) as well as for all reaction schemes belonging to the same category, the TOF–η relationships are obtained from the following equations (28, 29), using the notations defined in Scheme 2 and the data listed in TOF=TOFmax{1+ipl2FSCcat0kf2nd ETexp(α2fE)+ipl2FSCcat0Dcatk1[CO2]exp[FRT(E−E10)]},withTOFmax=11k1[CO2]+1k2ap, k2ap=k21k22[PhOH]k−21+k22[PhOH].The S-shaped catalytic wave is characterized by a plateau current i_pl that may be expressed asipl=2FSDcatCcat0k1[CO2]1+k1[CO2]k2,ap(1+k2,apk1[CO2])[1+k2,apk2,2CZ0],[1]where S is electrode surface area; Ccat0 and D_cat are concentration and diffusion coefficient of the catalyst, respectively.Open in a separate windowFig. 1.CV of 1-mM FCAT (Lower Left) and CAT (Lower Right) in neat DMF + 0.1 M n-Bu₄NPF₆ at 0.1 Vs⁻¹. The same, Upper Left and Upper Right, respectively, in the presence of 0.23 M CO₂ and of 1 M PhOH. ip0, the peak current of the reversible Fe^II/Fe^I wave is a measure of a one-electron transfer.

Table 1.

Kinetic characteristics of the reactions in Scheme 2 from ref. 27

CO2 Separation and Capture Properties of Porous Carbonaceous Materials from Leather Residues

Carbonaceous porous materials derived from leather skin residues have been found to have excellent CO₂ adsorption properties, with interestingly high gas selectivities for CO₂ (α > 200 at a gas composition of 15% CO₂/85% N₂, 273K, 1 bar) and capacities (>2 mmol·g⁻¹ at 273 K). Both CO₂ isotherms and the high heat of adsorption pointed to the presence of strong binding sites for CO₂ which may be correlated with both: N content in the leather residues and ultrasmall pore sizes.     

A proposed potential role for increasing atmospheric CO2 as a promoter of weight gain and obesity

Human obesity has evolved into a global epidemic. Interestingly, a similar trend has been observed in many animal species, although diet composition, food availability and physical activity have essentially remained unchanged. This suggests a common factor—potentially an environmental factor affecting all species. Coinciding with the increase in obesity, atmospheric CO₂ concentration has increased more than 40%. Furthermore, in modern societies, we spend more time indoors, where CO₂ often reaches even higher concentrations. Increased CO₂ concentration in inhaled air decreases the pH of blood, which in turn spills over to cerebrospinal fluids. Nerve cells in the hypothalamus that regulate appetite and wakefulness have been shown to be extremely sensitive to pH, doubling their activity if pH decreases by 0.1 units. We hypothesize that an increased acidic load from atmospheric CO₂ may potentially lead to increased appetite and energy intake, and decreased energy expenditure, and thereby contribute to the current obesity epidemic.     

CO2 Adsorption on Activated Carbons Prepared from Molasses: A Comparison of Two and Three Parametric Models

Activated carbons with different textural characteristic were derived by the chemical activation of raw beet molasses with solid KOH, while the activation temperature was changed in the range 650 °C to 800 °C. The adsorption of CO₂ on activated carbons was investigated. Langmuir, Freundlich, Sips, Toth, Unilan, Fritz-Schlunder, Radke-Prausnitz, Temkin-Pyzhev, Dubinin-Radushkevich, and Jovanovich equations were selected to fit the experimental data of CO₂ adsorption. An error analysis (the sum of the squares of errors, the hybrid fractional error function, the average relative error, the Marquardt’s percent standard deviation, and the sum of the absolute errors) was conducted to examine the effect of using various error standards for the isotherm model parameter calculation. The best fit was observed to the Radke-Prausnitz model.     

Synthesis and Characterization of a Crystalline Imine-Based Covalent Organic Framework with Triazine Node and Biphenyl Linker and Its Fluorinated Derivate for CO2/CH4 Separation

A catalyst-free Schiff base reaction was applied to synthesize two imine-linked covalent organic frameworks (COFs). The condensation reaction of 1,3,5-tris-(4-aminophenyl)triazine (TAPT) with 4,4′-biphenyldicarboxaldehyde led to the structure of HHU-COF-1 (HHU = Heinrich-Heine University). The fluorinated analog HHU-COF-2 was obtained with 2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-biphenyldicarboxaldehyde. Solid-state NMR, infrared spectroscopy, X-ray photoelectron spectroscopy, and elemental analysis confirmed the successful formation of the two network structures. The crystalline materials are characterized by high Brunauer–Emmett–Teller surface areas of 2352 m²/g for HHU-COF-1 and 1356 m²/g for HHU-COF-2. The products of a larger-scale synthesis were applied to prepare mixed-matrix membranes (MMMs) with the polymer Matrimid. CO₂/CH₄ permeation tests revealed a moderate increase in CO₂ permeability at constant selectivity for HHU-COF-1 as a dispersed phase, whereas application of the fluorinated COF led to a CO₂/CH₄ selectivity increase from 42 for the pure Matrimid membrane to 51 for 8 wt% of HHU-COF-2 and a permeability increase from 6.8 to 13.0 Barrer for the 24 wt% MMM.     

High-energy resolution X-ray absorption and emission spectroscopy reveals insight into unique selectivity of La-based nanoparticles for CO2

The lanthanum-based materials, due to their layered structure and f-electron configuration, are relevant for electrochemical application. Particularly, La₂O₂CO₃ shows a prominent chemoresistive response to CO₂. However, surprisingly less is known about its atomic and electronic structure and electrochemically significant sites and therefore, its structure–functions relationships have yet to be established. Here we determine the position of the different constituents within the unit cell of monoclinic La₂O₂CO₃ and use this information to interpret in situ high-energy resolution fluorescence-detected (HERFD) X-ray absorption near-edge structure (XANES) and valence-to-core X-ray emission spectroscopy (vtc XES). Compared with La(OH)₃ or previously known hexagonal La₂O₂CO₃ structures, La in the monoclinic unit cell has a much lower number of neighboring oxygen atoms, which is manifested in the whiteline broadening in XANES spectra. Such a superior sensitivity to subtle changes is given by HERFD method, which is essential for in situ studying of the interaction with CO₂. Here, we study La₂O₂CO₃-based sensors in real operando conditions at 250 °C in the presence of oxygen and water vapors. We identify that the distribution of unoccupied La d-states and occupied O p- and La d-states changes during CO₂ chemoresistive sensing of La₂O₂CO₃. The correlation between these spectroscopic findings with electrical resistance measurements leads to a more comprehensive understanding of the selective adsorption at La site and may enable the design of new materials for CO₂ electrochemical applications.CO₂ has become a challenge for our society and we have to develop new materials for its photo/electrocatalysis, chemoresistive sensing, and storage (1–8). Particularly, for the variety of electrochemical applications the selective interaction of CO₂ and charge transfer with solids is in the foreground. At the same time, the interaction of CO₂ with solids in the electrochemical cell or sensing device is rather complex, thus it remains challenging to experimentally identify the key elements determining their selectivity and efficiency. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) provide complementary information on the electronic structure of materials (9, 10) and on the orbitals participating in the interaction with absorbing molecules (11). High-energy resolution fluorescence-detected (HERFD) XAS probes unoccupied states with a spectral resolution higher than regular XAS. Furthermore, with the same experimental setup XES can be measured, which allows one to probe the occupied states within the valence band (12). In situ HERFD XAS or XES experiments have been previously carried out to study the catalytic reaction at the surface of noble metals (11, 13–16), zeolites (17), and metal organic frameworks (18). Thus far, no such in situ experiments have been performed to directly track the changes of the electronic structure of a solid and its electrochemical activity toward CO₂. The rare-earth–based materials like perovskites and oxycarbonates, owing to their unique f-electron configuration of Ln (Ln = rare earth) and layered crystal structure, emerge as the most interesting for future photo- and electrochemical applications (3–8). Among rare-earth oxycarbonates (19, 20), particularly lanthanum strongly responds to CO₂ and shows up to 16-fold conductivity changes, not seen before for any metal oxides (21). This is very surprising because a direct injection of an electron into CO₂ molecule requires the activation energy of nearly 2 eV (22). To assess the origins of the unique CO₂ sensitivity of rare-earth oxycarbonate, it is essential to study in situ the interplay between the changes of the electronic structure of La-based nanoparticles upon CO₂ adsorption and changes of the macroscopic conductivity of a device.Here, to elucidate the underlying mechanism we first determine the structure and atomic positions of the lanthanum oxycarbonate. Using HERFD XAS and valence-to-core (vtc) XES results, we gain information about the electronic structure and band gap. Moreover, we combine in situ HERFD XAS and XES measurements with sensing performance tests to obtain the structure–function relationship. Finally, with all of the obtained information we discuss a mechanism of CO₂ adsorption on the La₂O₂CO₃ surface.     

Blending Wastes of Marble Powder and Dolomite Sorbents for Calcium-Looping CO2 Capture under Realistic Industrial Calcination Conditions

The use of wastes of marble powder (WMP) and dolomite as sorbents for CO₂ capture is extremely promising to make the Ca-looping (CaL) process a more sustainable and eco-friendly technology. For the downstream utilization of CO₂, it is more realistic to produce a concentrated CO₂ stream in the calcination step of the CaL process, so more severe conditions are required in the calciner, such as an atmosphere with high concentration of CO₂ (>70%), which implies higher calcination temperatures (>900 °C). In this work, experimental CaL tests were carried out in a fixed bed reactor using natural CaO-based sorbent precursors, such as WMP, dolomite and their blend, under mild (800 °C, N₂) and realistic (930 °C, 80% CO₂) calcination conditions, and the sorbents CO₂ carrying capacity along the cycles was compared. A blend of WMP with dolomite was tested as an approach to improve the CO₂ carrying capacity of WMP. As regards the realistic calcination under high CO₂ concentration at high temperature, there is a strong synergetic effect of inert MgO grains of calcined dolomite in the blended WMP + dolomite sorbent that leads to an improved stability along the cycles when compared with WMP used separately. Hence, it is a promising approach to tailor cheap waste-based blended sorbents with improved carrying capacity and stability along the cycles under realistic calcination conditions.     

Influence of Irradiation Parameters on Structure and Properties of Oak Wood Surface Engraved with a CO2 Laser

The work investigates the effects of CO₂ laser parameters (laser power and raster density) on wood mass loss in oak wood and impacts on its morphology, chemical structure, and surface properties (colour and hydrophilicity). The energy amount supplied onto the wood surface with a laser beam under different combinations of the irradiation parameters was expressed through a single variable—total irradiation dose. The mass loss was confirmed as linear-dependent on the irradiation dose. With the mass reduction, the roughness was enhanced. The roughness parameters Ra and Rz increased linearly with the mass loss associated with the increasing irradiation dose. The FTIR (Fourier transform infrared spectroscopy) spectroscopy also detected chemical changes in the main wood components, influencing primarily the wood colour space. Conspicuous discolouration of the engraved wood surface was observed, occurring just at the minimum laser power and raster density. The additional increasing of laser parameters caused a novel colour compared to the original one. The detected dependence of wood discolouration on the total irradiation dose enables us to perform targeted discolouration of the oak wood. The engraved surfaces manifested significantly better wettability with standard liquids, both polar and non-polar, and higher surface energy values. This guarantees appropriate adhesion of film-forming materials to wood. Identification of the changes in wood surface structure and properties, induced by specific CO₂ laser-treatments, is important for obtaining targeted discolouration of the wood surface as well as for the gluing or finishing of the surfaces treated in this way.     

Effect of Samples Size on the Water Removal and Shrinkage of Eucalyptus urophylla × E. grandis Wood during Supercritical CO2 Dewatering

Eucalyptus urophydis E. grandis green wood with different lengths were dewatered using CO₂ that was cyclically alternated between the supercritical fluid and gas phases. The results indicate that shorter specimens can be dewatered to below the fiber saturation point (FSP). There was no significant difference in the dewatering rate between the specimens of 20 and 50 mm in length. The dewatering was faster when the moisture content (MC) was over the FSP, leading to a greater gradient and a non-uniform distribution of moisture. The MC distributions in all specimens had no clear differences between in tangential and radial directions. Supercritical CO₂ dewatering generated a different moisture gradient than conventional kiln drying. Most water was dewatered from the end-grain section of the wood along the fiber direction, but a small amount of water was also removed in the transverse directions. There was no deformation in the specimens when the MC was above the FSP.     

The influence of feeding on gastric acid suppression in Helicobacter pylori-positive patients treated with a proton pump inhibitor or an H2-receptor antagonist after bleeding from a gastric ulcer

Background. This study investigated the influence of feeding on gastric acid suppression in Helicobacter pylori-positive patients treated with intravenous infusions of proton pump inhibitors (PPIs) or with H₂-receptor antagonists (H₂-RAs) after bleeding from a gastric ulcer. Methods. Forty-nine H. pylori-positive patients with bleeding gastric ulcers (44 men and 5 women) were divided into four groups: one group received an H₂-RA while fasting, one group received an H₂-RA while eating regularly, one group received a PPI while fasting, and one group received a PPI while eating regularly. Intragastric pH was monitored during fasting and nonfasting to calculate the pH 3 and pH 4 holding times and the mean pH. Results. During a 24-h fast, the pH 3 and pH 4 holding times and the mean pH were significantly higher in patients administered omeprazole (PPI; 93.2 ± 9.2%, 90.6 ± 11.1%, and 6.9 ± 0.6, respectively) than in those administered ranitidine (H₂-RA; 61.0 ± 27.5%, 55.8 ± 29.1%, and 4.8 ± 1.3, respectively; P < 0.001 for all). Results were similar during feeding (PPI meal, 98.9 ± 2.6%, 98.3 ± 3.7%, and 6.9 ± 0.3; H₂-RA meal, 59.8 ± 17.6%, 49.7 ± 18.0%, and 4.3 ± 0.7, respectively; P < 0.001 for all). In addition, the pH 3 and pH 4 holding times and the mean pH in the H₂-RA meal group were not significantly lower than those in the H₂-RA group (P = 0.999, P = 0.865, and P = 0.687, respectively). The values in the PPI and PPI meal groups were similar (P = 0.872, P = 0.777, and P > 0.999, respectively). Conclusions. Gastric acid suppression during the administration of an H₂-RA or a PPI soon after the cessation of gastric bleeding was scarcely affected by feeding. It may well be that H. pylori-positive patients with bleeding gastric ulcer can resume a regular diet and return to work soon after bleeding ceases.     

A Flexible and Wearable Nylon Fiber Sensor Modified by Reduced Graphene Oxide and ZnO Quantum Dots for Wide-Range NO2 Gas Detection at Room Temperature

Reduced graphene oxide (rGO) fiber as a carbon-based fiber sensor has aroused widespread interest in the field of gas sensing. However, the low response value and poor flexibility of the rGO fiber sensor severely limit its application in the field of flexible wearable electronics. In this paper, a flexible and wearable nylon fiber sensor modified by rGO and ZnO quantum dots (QDs) is proposed for wide-range NO₂ gas detection at room temperature. The response value of the nylon fiber sensor to 100 ppm NO₂ gas is as high as 0.4958, and the response time and recovery time are 216.2 s and 667.9 s, respectively. The relationship between the sensor’s response value and the NO₂ concentration value is linear in the range of 20–100 ppm, and the fitting coefficient is 0.998. In addition, the test results show that the sensor also has good repeatability, flexibility, and selectivity. Moreover, an early warning module was also designed and is proposed in this paper to realize the over-limit monitoring of NO₂ gas, and the flexible sensor was embedded in a mask, demonstrating its great application potential and value in the field of wearable electronics.