Co2 and coronary flow regulation

Summary These data indicate that the observed change following an abrupt change in myocardial O₂ consumption, occurs too slowly to participate in the initial adjustment in metabolic coronary flow change, which is essentially complete within 30 sec. Also, the magnitude of response is insufficient to conclude that CO₂ can provide the major cause of flow regulation. However, CO₂ could act as a back-up regulator. These conclusions may be modified if further information becomes available.     

Biologically induced initiation of Neoproterozoic snowball-Earth events

The glaciations of the Neoproterozoic Era (1,000 to 542 MyBP) were preceded by dramatically light C isotopic excursions preserved in preglacial deposits. Standard explanations of these excursions involve remineralization of isotopically light organic matter and imply strong enhancement of atmospheric CO₂ greenhouse gas concentration, apparently inconsistent with the glaciations that followed. We examine a scenario in which the isotopic signal, as well as the global glaciation, result from enhanced export of organic matter from the upper ocean into anoxic subsurface waters and sediments. The organic matter undergoes anoxic remineralization at depth via either sulfate- or iron-reducing bacteria. In both cases, this can lead to changes in carbonate alkalinity and dissolved inorganic pool that efficiently lower the atmospheric CO₂ concentration, possibly plunging Earth into an ice age. This scenario predicts enhanced deposition of calcium carbonate, the formation of siderite, and an increase in ocean pH, all of which are consistent with recent observations. Late Neoproterozoic diversification of marine eukaryotes may have facilitated the episodic enhancement of export of organic matter from the upper ocean, by causing a greater proportion of organic matter to be partitioned as particulate aggregates that can sink more efficiently, via increased cell size, biomineralization or increased C∶N of eukaryotic phytoplankton. The scenario explains isotopic excursions that are correlated or uncorrelated with snowball initiation, and suggests that increasing atmospheric oxygen concentrations and a progressive oxygenation of the subsurface ocean helped to prevent snowball glaciation on the Phanerozoic Earth.     

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.     

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.     

Isoleucine 309 acts as a C4 catalytic switch that increases ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) carboxylation rate in Flaveria

Improving global yields of important agricultural crops is a complex challenge. Enhancing yield and resource use by engineering improvements to photosynthetic carbon assimilation is one potential solution. During the last 40 million years C(4) photosynthesis has evolved multiple times, enabling plants to evade the catalytic inadequacies of the CO(2)-fixing enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco). Compared with their C(3) ancestors, C(4) plants combine a faster rubisco with a biochemical CO(2)-concentrating mechanism, enabling more efficient use of water and nitrogen and enhanced yield. Here we show the versatility of plastome manipulation in tobacco for identifying sequences in C(4)-rubisco that can be transplanted into C(3)-rubisco to improve carboxylation rate (V(C)). Using transplastomic tobacco lines expressing native and mutated rubisco large subunits (L-subunits) from Flaveria pringlei (C(3)), Flaveria floridana (C(3)-C(4)), and Flaveria bidentis (C(4)), we reveal that Met-309-Ile substitutions in the L-subunit act as a catalytic switch between C(4) ((309)Ile; faster V(C), lower CO(2) affinity) and C(3) ((309)Met; slower V(C), higher CO(2) affinity) catalysis. Application of this transplastomic system permits further identification of other structural solutions selected by nature that can increase rubisco V(C) in C(3) crops. Coengineering a catalytically faster C(3) rubisco and a CO(2)-concentrating mechanism within C(3) crop species could enhance their efficiency in resource use and yield.     

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

Thermodynamic and achievable efficiencies for solar-driven electrochemical reduction of carbon dioxide to transportation fuels

Thermodynamic, achievable, and realistic efficiency limits of solar-driven electrochemical conversion of water and carbon dioxide to fuels are investigated as functions of light-absorber composition and configuration, and catalyst composition. The maximum thermodynamic efficiency at 1-sun illumination for adiabatic electrochemical synthesis of various solar fuels is in the range of 32–42%. Single-, double-, and triple-junction light absorbers are found to be optimal for electrochemical load ranges of 0–0.9 V, 0.9–1.95 V, and 1.95–3.5 V, respectively. Achievable solar-to-fuel (STF) efficiencies are determined using ideal double- and triple-junction light absorbers and the electrochemical load curves for CO₂ reduction on silver and copper cathodes, and water oxidation kinetics over iridium oxide. The maximum achievable STF efficiencies for synthesis gas (H₂ and CO) and Hythane (H₂ and CH₄) are 18.4% and 20.3%, respectively. Whereas the realistic STF efficiency of photoelectrochemical cells (PECs) can be as low as 0.8%, tandem PECs and photovoltaic (PV)-electrolyzers can operate at 7.2% under identical operating conditions. We show that the composition and energy content of solar fuels can also be adjusted by tuning the band-gaps of triple-junction light absorbers and/or the ratio of catalyst-to-PV area, and that the synthesis of liquid products and C₂H₄ have high profitability indices.The rapid changes in the global climate during the last century have been widely attributed to the anthropogenic emissions of carbon dioxide produced by combustion of fossil-based fuels (1). Today, the atmospheric concentration of CO₂ is increasing at a rate of ∼1.8 ppm/y, and this rate is expected to increase unless efforts are made to reduce the consumption of fossil energy fuels and to develop means for producing carbon-based fuels sustainably (2). One means for achieving the latter goal is artificial photosynthesis––a process in which solar radiation is used to drive the reduction of CO₂ to fuels (or fuel precursors) and chemicals (3, 4). In an artificial photosynthetic system one or more light absorbers are used to provide photogenerated electrons and holes for the photo/electrocatalytic reduction of carbon dioxide and water to a fuel, which is physically separated from the oxygen produced as a byproduct of water-splitting using an ion-conducting membrane. The overall efficiency with which such a system produces fuel depends on the identification, evaluation, and optimization of the components and system configuration.The efficiency of solar-driven, electrochemical reduction of CO₂ can be determined from the intersection of the current–voltage characteristics of the light absorber and the electrochemical load curve (5–7). This method has been used previously to calculate experimental and achievable solar-to-hydrogen (STH) efficiencies for water-splitting systems (8–10). The factors affecting the STH efficiency are the activities of the anode and cathode catalysts, the ohmic and Nernstian losses, and the semiconductor current–voltage characteristics (7, 11, 12). By contrast, the factors governing the efficiency of CO₂ reduction systems are not well explored and optimized and, therefore, the solar-to-fuel (STF) efficiencies of most systems are typically <7%. For example, the highest reported STF efficiency for formic acid synthesis is 1.8% using a photovoltaic (PV)-electrolyzer (13) and 4.6% using a photoelectrochemical cell (PEC, 14, 15); and the STF efficiency for CO synthesis is 2% using a PV-PEC (16) and 6.5% using PV-electrolyzer (17). The reasons for such low STF efficiencies are (i) higher kinetic overpotential and polarization losses for CO₂ reduction, and (ii) improper configuration of light absorbers to provide sufficient photovoltage and photocurrent density to drive CO₂ reduction. The factors affecting the STF efficiencies are (i) the catalyst used for the CO₂ reduction reaction (CO2RR), (ii) the catalyst used for the oxygen evolution reaction (OER), (iii) the electrolyte composition and concentration, (iv) the membrane or fuel separator, (v) the mechanism of CO₂ supply, and (vi) the current–voltage characteristic of the light absorber(s). The properties of each component and the operating conditions affect the cell voltage and the STF efficiency (18).The objectives of this study were to calculate the thermodynamic, achievable, and realistic STF efficiencies for CO₂ reduction to fuels; to determine optimal band-gaps for alternative light-absorber configurations required to achieve efficient CO₂ reduction; and to develop strategies for controlling the composition and energy density of solar fuels. The balance of this article is organized as follows. Theory describes the mathematical expressions used to determine the Shockley–Queisser (SQ) limits of multijunction light absorbers, the characteristics of electrochemical load curves for the OER and CO2RR, and how the properties of the light absorber(s) and catalysts are used to define the STF efficiency for CO₂ reduction. Results and Discussion presents thermodynamic, achievable, and realistic STF efficiencies for different CO2RR catalysts and device configurations. Conclusions and Perspectives presents conclusions and future directions to overcome present difficulties in making an efficient solar-driven electrochemical device for CO₂ reduction.