CeFeO3–CeO2–Fe2O3 Systems: Synthesis by Solution Combustion Method and Catalytic Performance in CO2 Hydrogenation

Rare-earth orthoferrites have found wide application in thermocatalytic reduction-oxidation processes. Much less attention has been paid, however, to the production of CeFeO₃, as well as to the study of its physicochemical and catalytic properties, in particular, in the promising process of CO₂ utilization by hydrogenation to CO and hydrocarbons. This study presents the results of a study on the synthesis of CeFeO₃ by solution combustion synthesis (SCS) using various fuels, fuel-to-oxidizer ratios, and additives. The SCS products were characterized by XRD, FTIR, N₂-physisorption, SEM, DTA–TGA, and H₂-TPR. It has been established that glycine provides the best yield of CeFeO₃, while the addition of NH₄NO₃ promotes an increase in the amount of CeFeO₃ by 7–12 wt%. In addition, the synthesis of CeFeO₃ with the participation of NH₄NO₃ makes it possible to surpass the activity of the CeO₂–Fe₂O₃ system at low temperatures (300–400 °C), as well as to increase selectivity to hydrocarbons. The observed effects are due to the increased gas evolution and ejection of reactive FeO_x nanoparticles on the surface of crystallites, and an increase in the surface defects. CeFeO₃ obtained in this study allows for achieving higher CO₂ conversion compared to LaFeO₃ at 600 °C.     

CO2 Methanation: Nickel–Alumina Catalyst Prepared by Solid-State Combustion

The development of solvent-free methods for the synthesis of catalysts is one of the main tasks of green chemistry. A nickel–alumina catalyst for CO₂ methanation was synthesized by solid-state combustion method using hexakis-(imidazole) nickel (II) nitrate complex. Using X-ray Powder Diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Hydrogen temperature-programmed reduction (H₂-TPR), it was shown that the synthesized catalyst is characterized by the localization of easily reduced nickel oxide on alumina surface. This provided low-temperature activation of the catalyst in the reaction mixture containing 4 vol% CO₂. In addition, the synthesized catalyst had higher activity in low-temperature CO₂ methanation compared to industrial NIAP-07-01 catalyst, which contained almost three times more hard-to-reduce nickel–aluminum spinel. Thus, the proposed approaches to the synthesis and activation of the catalyst make it possible to simplify the catalyst preparation procedure and to abandon the use of solvents, which must be disposed of later on.     

Physico-Chemical Modifications Affecting the Activity and Stability of Cu-Based Hybrid Catalysts during the Direct Hydrogenation of Carbon Dioxide into Dimethyl-Ether

The direct hydrogenation of CO₂ into dimethyl-ether (DME) has been studied in the presence of ferrierite-based CuZnZr hybrid catalysts. The samples were synthetized with three different techniques and two oxides/zeolite mass ratios. All the samples (calcined and spent) were properly characterized with different physico-chemical techniques for determining the textural and morphological nature of the catalytic surface. The experimental campaign was carried out in a fixed bed reactor at 2.5 MPa and stoichiometric H₂/CO₂ molar ratio, by varying both the reaction temperature (200–300 °C) and the spatial velocity (6.7–20.0 NL∙g_cat⁻¹∙h⁻¹). Activity tests evidenced a superior activity of catalysts at a higher oxides/zeolite weight ratio, with a maximum DME yield as high as 4.5% (58.9 mg_DME∙g_cat⁻¹∙h⁻¹) exhibited by the sample prepared by gel-oxalate coprecipitation. At lower oxide/zeolite mass ratios, the catalysts prepared by impregnation and coprecipitation exhibited comparable DME productivity, whereas the physically mixed sample showed a high activity in CO₂ hydrogenation but a low selectivity toward methanol and DME, ascribed to a minor synergy between the metal-oxide sites and the acid sites of the zeolite. Durability tests highlighted a progressive loss in activity with time on stream, mainly associated to the detrimental modifications under the adopted experimental conditions.     

High Performance Tunable Catalysts Prepared by Using 3D Printing

Honeycomb monoliths are the preferred supports in many industrial heterogeneous catalysis reactions, but current extrusion synthesis only allows obtaining parallel channels. Here, we demonstrate that 3D printing opens new design possibilities that outperform conventional catalysts. High performance carbon integral monoliths have been prepared with a complex network of interconnected channels and have been tested for carbon dioxide hydrogenation to methane after loading a Ni/CeO₂ active phase. CO₂ methanation rate is enhanced by 25% at 300 °C because the novel design forces turbulent flow into the channels network. The methodology and monoliths developed can be applied to other heterogeneous catalysis reactions, and open new synthesis options based on 3D printing to manufacture tailored heterogeneous catalysts.     

Catalysts for the Conversion of CO2 to Low Molecular Weight Olefins—A Review

There is a large worldwide demand for light olefins (C₂⁼–C₄⁼), which are needed for the production of high value-added chemicals and plastics. Light olefins can be produced by petroleum processing, direct/indirect conversion of synthesis gas (CO + H₂) and hydrogenation of CO₂. Among these methods, catalytic hydrogenation of CO₂ is the most recently studied because it could contribute to alleviating CO₂ emissions into the atmosphere. However, due to thermodynamic reasons, the design of catalysts for the selective production of light olefins from CO₂ presents different challenges. In this regard, the recent progress in the synthesis of nanomaterials with well-controlled morphologies and active phase dispersion has opened new perspectives for the production of light olefins. In this review, recent advances in catalyst design are presented, with emphasis on catalysts operating through the modified Fischer–Tropsch pathway. The advantages and disadvantages of olefin production from CO₂ via CO or methanol-mediated reaction routes were analyzed, as well as the prospects for the design of a single catalyst for direct olefin production. Conclusions were drawn on the prospect of a new catalyst design for the production of light olefins from CO₂.     

Fundamental Studies on CO2 Sequestration of Concrete Slurry Water Using Supercritical CO2

To prevent drastic climate change due to global warming, it is necessary to transition to a carbon-neutral society by reducing greenhouse gas emissions in all industrial sectors. This study aims to prepare measures to reduce the greenhouse gas in the cement industry, which is a large source of greenhouse gas emissions. The research uses supercritical CO₂ carbonation to develop a carbon utilization fixation technology that uses concrete slurry water generated via concrete production as a new CO₂ fixation source. Experiments were conducted using this concrete slurry water and supernatant water under different conditions of temperature (40 and 80 °C), pressure (100 and 150 bar), and reaction time (10 and 30 min). The results showed that reaction for 10 min was sufficient for complete carbonation at a sludge solids content of 5%. However, reaction products of supernatant water could not be identified due to the presence of Ca(HCO₃)₂ as an aqueous solution, warranting further research.     

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.     

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

Feasibility and safety of automated CO2 angiography in peripheral arterial interventions

Carbon dioxide (CO₂) gas is an established alternative to iodine contrast during angiography in patients with risk of postcontrast acute kidney injury and in those with history of iodine contrast allergy. Different CO₂ delivery systems during angiography are reported in literature, with automated delivery system being the latest. The aim of this study is to evaluate the safety, efficacy, and learning curve of an automated CO₂ injection system with controlled pressures in peripheral arterial interventions and also to study the patients’ tolerance to the system.From January 2018 to October 2019 peripheral arterial interventions were performed in 40 patients (median age-78 years, interquartile range: 69–84 years) using an automated CO₂ injection system with customized protocols, with conventional iodine contrast agent used only as a bailout option. The pain and tolerance during the CO₂ angiography were evaluated with a visual analog scale at the end of each procedure. The amount of CO₂, iodine contrast used, and radiation dose area product for the interventions were also systematically recorded for all procedures. These values were statistically compared in 2 groups, viz first 20 patients where a learning curve was expected vs the rest 20 patients.All procedures were successfully completed without complications. All patients tolerated the CO₂ angiography with a median total pain score of 3 (interquartile range: 3–4), with no statistical difference between the groups (P = .529). The 2 groups were statistically comparable in terms of comorbidities and the type of procedures performed (P = .807). The amount of iodine contrast agent used (24.60 ± 6.44 ml vs 32.70 ± 8.70 ml, P = .006) and the radiation dose area product associated were significantly lower in the second group (2160.74 ± 1181.52 μGym² vs 1531.62 ± 536.47 μGym², P = .043).Automated CO₂ angiography is technically feasible and safe for peripheral arterial interventions and is well tolerated by the patients. With the interventionalist becoming familiar with the technique, better diagnostic accuracy could be obtained using lower volumes of conventional iodine contrast agents and reduction of the radiation dose involved.     

CO2 enhancement of forest productivity constrained by limited nitrogen availability

Stimulation of terrestrial plant production by rising CO(2) concentration is projected to reduce the airborne fraction of anthropogenic CO(2) emissions. Coupled climate-carbon cycle models are sensitive to this negative feedback on atmospheric CO(2), but model projections are uncertain because of the expectation that feedbacks through the nitrogen (N) cycle will reduce this so-called CO(2) fertilization effect. We assessed whether N limitation caused a reduced stimulation of net primary productivity (NPP) by elevated atmospheric CO(2) concentration over 11 y in a free-air CO(2) enrichment (FACE) experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee. During the first 6 y of the experiment, NPP was significantly enhanced in forest plots exposed to 550 ppm CO(2) compared with NPP in plots in current ambient CO(2), and this was a consistent and sustained response. However, the enhancement of NPP under elevated CO(2) declined from 24% in 2001-2003 to 9% in 2008. Global analyses that assume a sustained CO(2) fertilization effect are no longer supported by this FACE experiment. N budget analysis supports the premise that N availability was limiting to tree growth and declining over time--an expected consequence of stand development, which was exacerbated by elevated CO(2). Leaf- and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to elevated CO(2); these observations are consistent with stand-level model projections. This FACE experiment provides strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.     

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.     

Spatial response of coastal marshes to increased atmospheric CO2

The elevation and extent of coastal marshes are dictated by the interplay between the rate of relative sea-level rise (RRSLR), surface accretion by inorganic sediment deposition, and organic soil production by plants. These accretion processes respond to changes in local and global forcings, such as sediment delivery to the coast, nutrient concentrations, and atmospheric CO₂, but their relative importance for marsh resilience to increasing RRSLR remains unclear. In particular, marshes up-take atmospheric CO₂ at high rates, thereby playing a major role in the global carbon cycle, but the morphologic expression of increasing atmospheric CO₂ concentration, an imminent aspect of climate change, has not yet been isolated and quantified. Using the available observational literature and a spatially explicit ecomorphodynamic model, we explore marsh responses to increased atmospheric CO₂, relative to changes in inorganic sediment availability and elevated nitrogen levels. We find that marsh vegetation response to foreseen elevated atmospheric CO₂ is similar in magnitude to the response induced by a varying inorganic sediment concentration, and that it increases the threshold RRSLR initiating marsh submergence by up to 60% in the range of forcings explored. Furthermore, we find that marsh responses are inherently spatially dependent, and cannot be adequately captured through 0-dimensional representations of marsh dynamics. Our results imply that coastal marshes, and the major carbon sink they represent, are significantly more resilient to foreseen climatic changes than previously thought.Coastal marsh extent and morphology are directly controlled by rate of relative sea-level rise (RRSLR) and the soil accretion rate, the latter associated with inorganic sediment deposition and organic soil production by plants. Previous studies observed that CO₂ fertilization increases marsh plant biomass productivity through increased water use efficiency and photosynthesis (1), and hypothesized that, as a consequence, marsh resilience should increase via increased organic accretion (2, 3). However, this hypothesis has not yet been tested, and the observed increased plant productivity in response to the CO₂ fertilization effect has not been translated into its actual geomorphic effects. In fact, direct CO₂ effects on vegetation and marsh accretion (as opposed to its indirect effects, e.g., via the increase in temperature) have not yet been incorporated into marsh models, and their importance relative to other leading forcings of marsh dynamics (e.g., inorganic deposition, RRSLR, nutrient levels) remains unknown. Here we use existing data and a 1D ecomorphodynamic model to assess the direct impacts of elevated CO₂ on marsh morphology, relative to ongoing [e.g., RRSLR, and suspended sediment concentration (SSC)] and emerging [nutrient levels (4–6)] environmental change.     

Effects of CO2 Concentration and the Uptake on Carbonation of Cement-Based Materials

Carbonation seriously deteriorates the durability of existing reinforced concrete structures. In this study, a thermodynamic model is used to investigate the carbonation reactions in cement-based materials. The effects of the concentration and amounts of CO₂ on the carbonation behaviors of mortar are discussed. The simulation results show that the mechanisms of the carbonation reaction of cement-based materials at different CO₂ concentrations may be different. Nearly all of the hydrate phases have a corresponding CO₂ concentration threshold, above which the corresponding carbonation reaction can be triggered. The thresholds of the C-S-H phases with different Ca/Si ratios are different. The calculation results also show that the phase assemblages in cement paste after being completely air-carbonated, primarily consist of a low-Ca/Si ratio C-S-H, strätlingite, CaCO₃ and CaSO₄. The pH of the pore solution exhibits a significant decrease when a higher Ca/Si ratio C-S-H phase is completely decalcified into a lower Ca/Si ratio C-S-H phase, by increasing the CO₂ uptake. Additionally, the experimental results and the previously published investigations are used to validate the simulation results.     

Efficient electrolyzer for CO2 splitting in neutral water using earth-abundant materials

Low-cost, efficient CO₂-to-CO+O₂ electrochemical splitting is a key step for liquid-fuel production for renewable energy storage and use of CO₂ as a feedstock for chemicals. Heterogeneous catalysts for cathodic CO₂-to-CO associated with an O₂-evolving anodic reaction in high-energy-efficiency cells are not yet available. An iron porphyrin immobilized into a conductive Nafion/carbon powder layer is a stable cathode producing CO in pH neutral water with 90% faradaic efficiency. It is coupled with a water oxidation phosphate cobalt oxide anode in a home-made electrolyzer by means of a Nafion membrane. Current densities of approximately 1 mA/cm² over 30-h electrolysis are achieved at a 2.5-V cell voltage, splitting CO₂ and H₂O into CO and O₂ with a 50% energy efficiency. Remarkably, CO₂ reduction outweighs the concurrent water reduction. The setup does not prevent high-efficiency proton transport through the Nafion membrane separator: The ohmic drop loss is only 0.1 V and the pH remains stable. These results demonstrate the possibility to set up an efficient, low-voltage, electrochemical cell that converts CO₂ into CO and O₂ by associating a cathodic-supported molecular catalyst based on an abundant transition metal with a cheap, easy-to-prepare anodic catalyst oxidizing water into O₂.The production of carbon-based fuels or chemicals using the most abundant carbon source (CO₂) requires designing efficient, cheap, selective, and sustainable processes able to convert CO₂ into useful products (1–7). Carbon monoxide production is an important step to fuels because it can be used as a feedstock in the Fischer–Tropsch process. Compared with water splitting, electrochemical reduction of CO₂ into CO is a greater challenge. This is particularly true when aiming to carry out this reaction selectively in friendly conditions, namely at neutral pHs, ambient temperature, and with abundant and cheap materials as catalysts as opposed to solid-state high-temperature electrolyzers (8).We recently discovered that substitution of the four paraphenyl hydrogens of iron tetraphenylporphyrin by trimethylammonio groups provides a water-soluble iron porphyrin (WSCAT) able to catalyze selectively the electrochemical conversion of CO₂ into CO in neutral water in homogeneous conditions (9). The next challenge was to efficiently immobilize this molecular catalyst onto the cathode and to set up an integrated electrochemical cell able to split CO₂ and H₂O into CO and O₂ according toCO2+2 H++2 e-⇄CO+H2O (ECO2/CO0=-0.11 V),H2O⇄½ O2+2 H++2 e- (EO2/H2O0=1.23 V),CO₂ ? CO + ½?O₂?(ΔG⁰ = 2.68?eV), [potentials referred to the standard hydrogen electrode (SHE)].Immobilization of the catalyst was achieved by preparation of a suspension containing Nafion, WSCAT, and carbon powder (Materials and Methods) (10). This solution was then sprayed onto a carbon support (glassy carbon electrode for cyclic voltammetry experiments and carbon felt or carbon Toray for electrolysis) and air-dried. Interactions between the positively charged catalyst and the negatively charged functionalities of the ionic polymer secure robust integration of the catalyst into the coated film. This was attested to by the absence of UV-vis signal corresponding to an iron porphyrin in a water solution in which the electrode was immersed for a few hours. The catalytic film consists of a thin coating of the electrode surface as confirmed by scanning electron microscopy (SEM) (Fig. 1).Open in a separate windowFig. 1.SEM images of the Nafion/carbon powder/WSCAT (Left) and CoP_i (Right) films. (A and B) Electrode structures. (C and D) Catalytic films on the supporting material (polymer-wrapped carbon microfiber and stainless steel microwire, respectively). (E and F) High-magnification images of the films.Conductive and catalytic properties for CO₂ reduction of the prepared electrode were characterized by cyclic voltammetry carried out in quasi-neutral water with no added buffer, thus preventing acid reduction (Fig. 2). Under argon, almost no capacitive current and no faradaic current were observed in the absence of carbon powder in the film, whereas the electrode exhibited a large capacitive current when carbon powder was incorporated in the coated film.Open in a separate windowFig. 2.Cyclic voltammetry (pH 6.7). Nafion/WSCAT film deposited on a 3-mm-diameter glassy carbon electrode, v = 0.1 Vs⁻¹, pH = 6.7. Light gray, film without carbon powder under argon. Blue, film plus carbon powder under argon. Red, film plus carbon powder under 1 atm of CO₂.This indicates that carbon powder renders the film conducting and thus makes addressable the molecular catalyst contained in the film. This is further confirmed by the observation of faradaic waves when started from the Fe^III complex. Although not being well defined, these waves may be assigned to the Fe^III/II, Fe^II/I redox couples. The slight increase of the current observed at more negative potentials (approximately −1.15 V vs. SHE) presumably reflects some catalysis of water reduction. In the presence of CO₂, a large increase of the current is seen, confirming the catalytic activity of WSCAT immobilized in the Nafion film toward CO₂ reduction. The onset of the catalytic wave is approximately −0.8 V vs. SHE corresponding to an overpotential ofη=ECO2/CO0-(RTln⁡10/F)×pH=390mV.An electrochemical membrane-separated cell was then assembled to couple the CO₂ reduction half-reaction to water oxidation. Although a significant overpotential is usually associated with the latter reaction, important progress has been recently made leading to a cheap and abundant material, easy to prepare, well characterized, and efficient at neutral pH, namely electrodeposited phosphate cobalt oxide (CoP_i), which could be used as water oxidation catalyst on the anodic side (6). A thin (50 mC/cm², i.e., approximately 330 nm) CoP_i film was thus deposited on a small stainless steel gauze (Fig. 1), with, as anodic electrolyte, a phosphate buffer (0.4 M, pH 7.3) ensuring a minimal anodic overpotential. The cell was constructed from flasks containing the electrolytes with a proton-permeable Nafion membrane and both electrodes clamped between the flasks (Fig. 3).Open in a separate windowFig. 3.Proton exchange membrane electrolysis cell configuration. Cathodic half-reaction is CO₂ reduction to CO on a carbon electrode coated with a Nafion/carbon powder/WSCAT film and anodic half-reaction is oxidation of H₂O to O₂ on a stainless steel gauze with a CoP_i electrodeposited film.A reference electrode was also introduced in the cathodic compartment so that both the cell voltage and the cathode potential electrode could be measured during electrolysis.The electrochemical characteristics of the cell were obtained by applying a potential scan to the cathode while simultaneously recording cell voltage, hence leading to the potential of both the cathode and the anode including ohmic drop vs. the reference electrode and the current (Fig. 4A). At pH 7.3, the onset of the current (at 1 mA/cm²) corresponds to a cell voltage of 2.3 V withηcathodic={ECO2/CO0-(RTln⁡10/F)×pH}-Ecathodic=320mVandηanodic+Ri=Ucell-Ecathodic-{EO2/H2O0-(RTln⁡10/F)×pH}=630mV.An independent measurement indicated a 10-Ω cell resistance, amounting to an ohmic drop contribution of 50 mV to the overpotential.Open in a separate windowFig. 4.Cell characterization and electrolysis at pH 7.3. (A) Current density as function of the electrode potential (vs. SHE) for both the cathode and the anode. (B) Entire cell overpotential as function of current density (dotted line corresponds to ohmic drop corrected data). (C) Charge transferred as function of time for an electrolysis at controlled cathodic potential, E_cathodic = −0.96 V vs. SHE (the dotted line corresponds to the partial charge for CO production). (D) Cell voltage as function of time for an electrolysis at controlled cathodic potential, E_cathodic = −0.96 V vs. SHE. (E) Product selectivity as function of time for an electrolysis at controlled cathodic potential, E_cathodic = −0.96 V vs. SHE. H₂ (o) and CO (red •). (F) Cell energy efficiency as function of time for an electrolysis at controlled cathodic potential, E_cathodic = −0.96 V vs. SHE.A first series of electrolysis was performed at controlled cathodic potential (E_cathodic = −0.86 V vs. SHE) for several hours, with an expansion vessel over the flasks to assess product selectivity and evaluate faradaic efficiency. The averaged current density was 0.7 mA/cm², the cell voltage 2.4 V (corresponding to 1.05-V overpotential), and gas chromatography analysis of gas in the headspace above solution leads to faradaic efficiencies of 90% for CO along with 10% for H₂ in the cathodic compartment and 99% for O₂ in the anodic compartment. It is worth noting that no hydrocarbons were detected in the cathodic compartment headspace. Ionic chromatography analysis of the cathodic electrolyte shows only traces of formate and oxalate. These results clearly indicate a high selectivity and a high faradaic efficiency of the cell for CO₂ and H₂O conversion to CO and O₂, respectively. A second series of electrolysis was performed at a controlled cathodic potential (E_cathodic = −0.96 V vs. SHE) for 30 h under CO₂ flow to evaluate cell stability. A linear increase of the charge passed vs. time was observed, corresponding to an averaged 1 mA/cm² current density (Fig. 4C); the cell voltage remained stable at a value of 2.5 V (Fig. 4D). Analysis of the gas mixture produced in the cathodic chamber over the course of the electrolysis showed an excellent stability of the cell selectivity (Fig. 4E). Note that the same experiment carried out with iron tetraphenylporphyrin instead of WSCAT led to a rapid decrease of the current to zero and a poor CO selectivity. The cell energy efficiency (EE) at 1 mA/cm² was evaluated to be 50% from the product of the faradaic yield for CO production (FY_CO = 0.9) and the ratio of the thermodynamics of the reaction over cell voltage: EE = FY_CO × {E⁰_O2/H2O − E⁰_CO2/CO}/U_cell (Fig. 4F). From the overpotential measured at the cathode (η_cathodic = 330 mV) and at the anode (η_anodic = 720 mV, ohmic drop being approximately 100 mV), the cathodic energy efficiency was evaluated asEEcathodic=FYCO×{EO2/H2O0-ECO2/CO0}/{EO2/H2O0-ECO2/CO0+ηcathodic}=72%,and the anodic energy efficiency asEEanodic=FYCO×{EO2/H2O0-ECO2/CO0}/{EO2/H2O0-ECO2/CO0+ηanodic}=63%.Further improvements of the overall cell energy efficiency concern both the anodic and cathodic catalytic films, and will allow running electrolysis at higher current densities. However, the results described here demonstrate that supported molecular catalyst based on the cheapest transition metal can be used to efficiently split CO₂ to CO and O₂ in neutral water and the performance of the low-cost, simply designed cell provides a robust basis to couple this electrolysis with a photovoltaic device producing electricity from solar energy (11–13), opening an avenue to CO₂-based solar fuels.     

CO2 insuffiation for potentially difficult colonoscopies： Efficacy when used by less experienced colonoscopists

AIM: To clarify the effectiveness of CO_2 insufflation in potentially difficult colonoscopy cases, particularly in relation to the experience level of colonoscopists. METHODS: One hundred twenty potentially difficult cases were included in this study, which involved females with a low body mass index and patients with earlier abdominal and/or pelvic open surgery or previously diagnosed left-side colon diverticulosis. Patients receiving colonoscopy examinations without sedation using a pediatric variable-stiffness colonoscope were divided into two groups based on either CO_2 or standard air insufflation. Both insufflation procedures were also evaluated according to the experience level of the respective colonoscopists who were divided into an experienced colonoscopist (EC) group and a less experienced colonoscopist (LEC) group. Study measurements included a 100-mm visual analogue scale (VAS) for patient pain during and after colonoscopy examinations, in addition to insertion to the cecum and withdrawal times. RESULTS: Examination times did not differ, however, VAS scores in the CO_2 group were significantly better than in the air group (P < 0.001, two-way ANOVA) from immediately after the procedure and up to 2 h later. There were no significant differences between either insufflation method in the EC group (P = 0.29), however, VAS scores for CO_2 insufflation were significantly better than air insufflation in the LEC group (P = 0.023) immediately after colonoscopies and up to 4 h afterwards. CONCLUSION: CO_2 insufflation reduced patient pain after colonoscopy in potentially difficult cases when performed by LECs.     

Experimental Study on Corrosion Performance of Oil Tubing Steel in HPHT Flowing Media Containing O2 and CO2

The high pressure and high temperature (HPHT) flow solution containing various gases and Cl⁻ ions is one of the corrosive environments in the use of oilfield tubing and casing. The changing external environment and complex reaction processes are the main factors restricting research into this type of corrosion. To study the corrosion mechanism in the coexistence of O₂ and CO₂ in a flowing medium, a HPHT flow experiment was used to simulate the corrosion process of N80 steel in a complex downhole environment. After the test, the material corrosion rate, surface morphology, micromorphology, and corrosion product composition were tested. Results showed that corrosion of tubing material in a coexisting environment was significantly affected by temperature and gas concentration. The addition of O₂ changes the structure of the original CO₂ corrosion product and the corrosion process, thereby affecting the corrosion law, especially at high temperatures. Meanwhile, the flowing boundary layer and temperature changed the gas concentration near the wall, which changed the corrosion priority and intermediate products on the metal surface. These high temperature corrosion conclusions can provide references for the anticorrosion construction work of downhole pipe strings.     

Structural Morphology and Optical Properties of Strontium-Doped Cobalt Aluminate Nanoparticles Synthesized by the Combustion Method

The study of structural morphology and the optical properties of nanoparticles produced by combustion methods are gaining significance due to their multifold applications. In this regard, in the present work, the strontium-doped cobalt aluminate nanoparticles were synthesized by utilizing Co_1−xSr_xAl₂O₄ (0 ≤ x ≤ 0.5) L-Alanine as a fuel in an ignition cycle. Subsequently, several characterization studies viz., X-ray diffraction (XRD), energy-dispersive X-ray (EDX) analysis, high-resolution scanning electron microscopy (HRSEM), Fourier transform infrared spectroscopy (FTIR), ultraviolet (UV) spectroscopy and vibrating sample magnetometry (VSM) were accomplished to study the properties of the materials. The XRD analysis confirmed the cubic spinel structure, and the average crystallite size was found to be in the range of 14 to 20 nm using the Debye–Scherrer equation. High-resolution scanning electron microscopy was utilized to inspect the morphology of the Co_1−xSr_xAl₂O₄ (0 ≤ x ≤ 0.5) nanoparticles. Further, EDS studies were accomplished to determine the chemical composition. Kubelka–Munk’s approach was used to determine the band gap, and the values were found to be in the range of 3.18–3.32 eV. The energy spectra for the nanoparticles were in the range of 560–1100 cm⁻¹, which is due to the spinel structure of Sr-doped CoAl₂O₄ nanoparticles. The behavior plots of magnetic induction (M) against the magnetic (H) loops depict the ferromagnetic behavior of the nanomaterials synthesized.     

Formation of Li2CO3 Nanostructures for Lithium-Ion Battery Anode Application by Nanotransfer Printing

Various high-performance anode and cathode materials, such as lithium carbonate, lithium titanate, cobalt oxides, silicon, graphite, germanium, and tin, have been widely investigated in an effort to enhance the energy density storage properties of lithium-ion batteries (LIBs). However, the structural manipulation of anode materials to improve the battery performance remains a challenging issue. In LIBs, optimization of the anode material is a key technology affecting not only the power density but also the lifetime of the device. Here, we introduce a novel method by which to obtain nanostructures for LIB anode application on various surfaces via nanotransfer printing (nTP) process. We used a spark plasma sintering (SPS) process to fabricate a sputter target made of Li₂CO₃, which is used as an anode material for LIBs. Using the nTP process, various Li₂CO₃ nanoscale patterns, such as line, wave, and dot patterns on a SiO₂/Si substrate, were successfully obtained. Furthermore, we show highly ordered Li₂CO₃ nanostructures on a variety of substrates, such as Al, Al₂O₃, flexible PET, and 2-Hydroxylethyl Methacrylate (HEMA) contact lens substrates. It is expected that the approach demonstrated here can provide new pathway to generate many other designable structures of various LIB anode materials.     

Thin Film Mixed Matrix Hollow Fiber Membrane Fabricated by Incorporation of Amine Functionalized Metal-Organic Framework for CO2/N2 Separation

Membrane separation technology can used to capture carbon dioxide from flue gas. However, plenty of research has been focused on the flat sheet mixed matrix membrane rather than the mixed matrix thin film hollow fiber membranes. In this work, mixed matrix thin film hollow fiber membranes were fabricated by incorporating amine functionalized UiO-66 nanoparticles into the Pebax^® 2533 thin selective layer on the polypropylene (PP) hollow fiber supports via dip-coating process. The attenuated total reflection-Fourier transform infrared (ATR-FTIR), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX) mapping analysis, and thermal analysis (TGA-DTA) were used to characterize the synthesized UiO-66-NH₂ nanoparticles. The morphology, surface chemistry, and the gas separation performance of the fabricated Pebax^® 2533-UiO-66-NH₂/PP mixed matrix thin film hollow fiber membranes were characterized by using SEM, ATR-FTIR, and gas permeance measurements, respectively. It was found that the surface morphology of the prepared membranes was influenced by the incorporation of UiO-66 nanoparticles. The CO₂ permeance increased along with an increase of UiO-66 nanoparticles content in the prepared membranes, while the CO₂/N₂ ideal gas selectively firstly increased then decreased due to the aggregation of UiO-66 nanoparticles. The Pebax^® 2533-UiO-66-NH₂/PP mixed matrix thin film hollow fiber membranes containing 10 wt% UiO-66 nanoparticles exhibited the CO₂ permeance of 26 GPU and CO₂/N₂ selectivity of 37.