Gas-phase CO Research Articles

APEX observations of ortho-H2D+ towards dense cores in the Orion B9 filament

Context. Initial conditions and very early stages of star formation can be probed through spectroscopic observations of deuterated molecular species Aims. We aim to determine the ortho-H2D+ properties (e.g. column density and fractional abundance with respect to H2) in a sample of dense cores in the Orion B9 star-forming filament, and to compare those with the previously determined source characteristics, in particular with the gas kinetic temperature, [N2D+]/[N2H+] deuterium fractionation, and level of CO depletion. Methods. We used the Atacama Pathfinder EXperiment (APEX) telescope to observe the 372 GHz o-H2D+(JKa, Kc = 11, 0−11, 1) line towards three prestellar cores and three protostellar cores in Orion B9. We also employed our previous APEX observations of C17O, C18O, N2H+, and N2D+ line emission, and 870 μm dust continuum emission towards the target sources. Results. The o-H2D+(11, 0−11, 1) line was detected in all three prestellar cores, but in only one of the protostellar cores. The corresponding o-H2D+ abundances were derived to be ~ (12−30) × 10−11 and ~ 6 × 10−11. Two additional spectral lines, DCO+(5−4) and N2H+(4−3), were detected in the observed frequency bands with high detection rates of 100 and 83%, respectively. We did not find any significant correlations among the explored parameters, although our results are mostly consistent with theoretical expectations. Also, the Orion B9 cores were found to be consistent with the relationship between theo-H2D+ abundance and gas temperature obeyed by other low-mass dense cores. The o-H2D+ abundance was found to decrease as the core evolves. Conclusions. The o-H2D+ abundances in the Orion B9 cores are in line with those found in other low-mass dense cores and larger than derived for high-mass star-forming regions. The higher o-H2D+ abundance in prestellar cores compared to that in cores hosting protostars is to be expected from chemical reactions where higher concentrations of gas-phase CO and elevated gas temperature accelerate the destruction of H2D+. The validity of using the [o-H2D+]/[N2D+] abundance ratio as an evolutionary indicator, which has been proposed for massive clumps, remains to be determined when applied to these target cores. Similarly, the behaviour of the [o-H2D+]/[DCO+] ratio as the source evolves was found to be ambiguous. Still larger samples and observations of additional deuterated species are needed to explore these potential evolutionary indicators further. The low radial velocity of the line emission from one of the targeted prestellar cores, SMM 7 (~ 3.6 km s−1 versus the systemic Orion B9 velocity of ~ 9 km s−1), suggests that it is a chance superposition seen towards Orion B9. Overall, as located in a dynamic environment of the Orion B molecular cloud, the Orion B9 filament provides an interesting target system to investigate the deuterium-based chemistry, and further observations of species like para-H2D+ and D2H+ would be of particular interest.

Mass constraints for 15 protoplanetary discs from HD 1–0

Context. Hydrogen deuteride (HD) rotational line emission can provide reliable protoplanetary disc gas mass measurements, but this molecule is difficult to observe and detections have been limited to three T Tauri discs. No new data have been available since the Herschel Space Observatory mission ended in 2013. Aims. We set out to obtain new disc gas mass constraints by analysing upper limits on HD 1–0 emission in Herschel/PACS archival data from the DIGIT key programme. Methods. With a focus on the Herbig Ae/Be discs, whose stars are more luminous than T Tauris, we determined upper limits for HD in data previously analysed for its line detections. We studied the significance of these limits with a grid of models run with the DALI physical-chemical code, customised to include deuterium chemistry. Results. Nearly all the discs are constrained to Mgas ≤ 0.1 M⊙, ruling out global gravitational instability. A strong constraint is obtained for the HD 163296 disc mass, Mgas ≤ 0.067 M⊙, implying Δg/d ≤ 100. This HD-based mass limit is towards the low end of CO-based mass estimates for the disc, highlighting the large uncertainty in using only CO and suggesting that gas-phase CO depletion in HD 163296 is at most a factor of a few. The Mgas limits for HD 163296 and HD 100546, both bright discs with massive candidate protoplanetary systems, suggest disc-to-planet mass conversion efficiencies of Mp/(Mgas + Mp) ≈ 10–40% for present-day values. Near-future observations with SOFIA/HIRMES will be able to detect HD in the brightest Herbig Ae/Be discs within 150 pc with ≈ 10 h integration time.

CO2 valorisation via reverse water-gas shift reaction using promoted Fe/CeO2-Al2O3 catalysts: Showcasing the potential of advanced catalysts to explore new processes design

The RWGS reaction represents a direct approach for gas-phase CO2 upgrading. This work showcases the efficiency of Fe/CeO2-Al2O3 catalysts for this process, and the effect of selected transition metal promoters such as Cu, Ni and Mo. Our results demonstrated that both Ni and Cu remarkably improved the performance of the monometallic Fe-catalyst. The competition Reverse Water-Gas Shift (RWGS) reaction/CO2 methanation reaction was evident particularly for the Ni-catalyst, which displayed high selectivity to methane in the low-temperature range. Among the studied catalysts the Cu promoted sample represented the best choice, exhibiting the best activity/selectivity balance. In addition, the Cu-doped catalyst was very stable for long-term runs – an essential requisite for its implementation in flue gas upgrading units. This material can effectively catalyse the RWGS reaction at medium-low temperatures, providing the possibility to couple the RWGS reactor with a syngas conversion reaction. Such an integrated unit opens the horizons for a direct CO2 to fuels/chemicals approach.

Effects of 150–1000 eV Electron Impacts on Pure Carbon Monoxide Ices Using the Interstellar Energetic-Process System (IEPS)

Abstract Pure CO ice has been irradiated with electrons of energy in the range 150−1000 eV with the Interstellar Energetic-Process System. The main products of irradiation are carbon chains C n (n = 3, 5, 6, 8, 9, 10, 11, 12), suboxides, C n O (n = 2, 3, 4, 5, 6, 7), and C n O2 (n = 1, 3, 4, 5, 7) species. CO2 is by far the most abundant reaction product in all the experiments. The destruction cross section of CO peaks at about 250 eV, decreases with the energy of the electrons and is more than one order of magnitude higher than for gas-phase CO ionization. The production cross section of carbon dioxide has been also derived and is characterized by the competition between chemistry and desorption. Desorption of CO and of new species during the radiolysis follows the electron distribution in the ice. Low-energy electrons having short penetration depths induce significant desorption. Finally, as the ice thickness approaches the electron penetration depth the abundance of the products starts to saturate. Implications on the atmospheric photochemistry of cold planets hosting surface CO ices are also discussed.

Thermodynamic aspects of Cr<sub>2</sub>O<sub>3</sub> reduction by carbon

In order to save resources of chromium, technology of flux-cored wire surfacing is of great practical interest. In this case Cr2O3 chromium oxide and carbon as a reducing agent are used as fillers. Thermodynamic assessment of probability of 16 reactions between them under standard conditions and for certain reactions under conditions different from standard was carried out using tabulated thermodynamic data of reactants in temperature range of 1500 – 3500 K. The following states were considered as standard states for reactants: Cr(ref) (reference state, melting point 2130 K, boiling point 2952 K), Cr(liq), Cr(gas), Cr2O3 (cr, liq), Cr2O3 (gas), C(ref), and as possible reaction products and standard states for them CO(gas), CO2 (gas), Cr23C6 (сr), Cr7C3 (cr), Cr3C2 (cr). Probability of reactions was estimated using standard Gibbs energy and the Gibbs energy calculated using the Van Goff isotherm equation. Dissolution of chromium in metal of surfacing bath or probable partial pressures of CO and CO2 in gas phase was taken into account and was calculated from equilibrium of carbon gasification reaction. Presence of carbon in flux-cored wire with chromium oxide Cr2O3 as a reducing agent will necessarily lead to occurrence of reduction reactions with generation of chromium carbides, and possibly chromium itself. Generation of Cr7C3 (сr) carbide is likely. With longer life time of chromium oxide and carbon at a temperature above 2500 K, generation of chromium as a component of the surfacing bath is more thermodynamically probable than generation of its carbides. Chromium oxide has the highest reactivity in Cr2O3 (liq) state. Direct reduction is preferential. Generation of CO(gas) as a product of carbon oxidation is more probable. Dissolution of chromium in metal increases thermodynamic probability of reactions with its generation and further reduces probability of reactions in which chromium is the starting material.

Boron Doping in Tin Catalysts Towards Gas-Phase CO2 to Formic Acid/Formate Electroreduction with High Production Efficiency and Rate

Elemental doping is an effective approach to modify the local electronic structure of active sites. Herein, we tailored tin electrocatalysts with boron doping to tune and stabilize the valence state of Sn. The initial screening in the flow cell with 1 M KOH showed that the boron-doped Sn (Sn(B)) exhibited both higher formate production efficiency and rate than the pristine Sn particles in CO2 electroreduction. The optimal Sn(B) catalyst was translated to a solid-state electrolyzer incorporating solid electrolyte (either H+ or OH− solid conductor). The H+ solid conductor enabled direct production of formic acid, however, it sacrificed the anodic energy efficiency compared to formate production in the OH− solid conductor. H+ conductor cell produced a Faradic efficiency for formic acid of 88% with energy efficiency of 41% at cell voltage of 3.0 V and partial current density of 60.3 mA cm−2. Whereas, the Faradic efficiency of formate reached 87% at partial current density of 83 mA cm−2 and cell voltage of only 2.5 V at which the full-cell energy efficiency was attained 50% in the OH− conductor cell.

Photoelectrochemical reduction of CO2 with TiNT

In order to reduce CO2 emissions and utilise CO2 as a useful by-product, artificial photosynthesis is being explored for carbon capture and utilisation. Semiconductor photocatalysts excited by solar energy may be used to convert CO2 to fuels or useful chemicals, e.g. CO, CH4, CH3OH. The photocatalytic reduction of CO2 to useful products has been widely studied in order to overcome the greenhouse effect and the current energy necessities. However, this reaction has proved to be extremely low efficiency when compared to other processes. In order to improve these yields, photoelectrochemical reduction of CO2 has been considered, since it combines photocatalysis and electrocatalysis. To this end, a two compartment photoelectrochemical cell (PEC) has been designed and fabricated for the reduction of CO2. This custom built reactor consists of dual phases, where gas phase CO2 is feed into the cathode compartment and an aqueous phase in the anode compartment. The anode and cathodes for a sandwich were the anode perforated foil on exposing aligned titania nanotubes to electrolyte bonded to nafion; which forms a bridge to Pt deposited carbon cloth cathode electrode in gas phase compartment. CO2 reduction products were detected with GC connected to the reactor. The PEC reactor improved the yield and the formal quantum efficiency compared to our previous studies using photocatalytic reactors.

Measurements of CO2 molecular diffusion coefficients in crude oils from swelling-time curve and estimation using viscosity from the Stokes-Einstein formula

The CO2 diffusion coefficients in crude oils were measured from curves of oil-swelling vs. elapsed time with CO2 dissolution in the oils. The oil swelling was measured with CO2 up to a pressure of 10 MPa at 50 °C. The swelling coefficient and diffusion coefficient of CO2 and CH4 gases in the oil column were compared with CH4 gas. The diffusion coefficient of the heavy oil was evaluated as approximately 1.1–1.6% of that of bitumen. The swelling factors increased with pressure, and the diffusion coefficients in CO2 supercritical range were more than twice as high as those in the CO2 gas phase. An empirical equation to estimate gas solubility in crude oils vs. API gravity has been presented as well. Gas diffusion coefficients were shown to relate to oil viscosity based on the Stokes-Einstein formula and a new correlation between the two with the absolute average deviation (AAD%) of about 15.5%, derived.

The 3D Structure of CO Depletion in High-mass Prestellar Regions

Abstract Disentangling the different stages of the star formation process, in particular in the high-mass regime, is a challenge in astrophysics. Chemical clocks could help alleviate this problem, but their evolution strongly depends on many parameters, leading to degeneracy in the interpretation of the observational data. One of these uncertainties is the degree of CO depletion. We present here the first self-consistent magneto-hydrodynamic simulations of high-mass, star-forming regions at different scales, fully coupled with a nonequilibrium chemical network, which includes C–N–O bearing molecules. Depletion and desorption processes are treated time dependently. The results show that full CO depletion (i.e., all gas-phase CO frozen-out on the surface of dust grains) can be reached very quickly, in one-third or even smaller fractions of the freefall time, whether the collapse proceeds on slow or fast timescales. This leads to a high level of deuteration in a short time, both for typical tracers like N2H+, as well as for the main ion H 3 + , the latter being in general larger and more extended. N2 depletion is slightly less efficient, and no direct effects on N-bearing molecules and deuterium fractionation are observed. We show that CO depletion is not the only driver of deuteration, and that there is a strong impact on D frac when changing the grain size. We finally apply a two-dimensional Gaussian point-spread function to our results to mimic observations with single-dish and interferometers. Our findings suggest that the low-values observed in high-mass star-forming clumps are in reality masking a full-depletion stage in the inner 0.1 pc region.

Thermodynamic Aspects of Cr2O3 Reduction by Carbon

In order to save chromium resources, technology of flux-cored wire surfacing is of great practical interest. In this case, Cr2O3 chromium oxide and carbon as a reducing agent are used as fillers. The thermodynamic probability assessment of 16 reactions between them under standard conditions, as well as for certain reactions under different standard conditions, was carried out using tabulated thermodynamic data of reactants in the temperature range of 1500–3500 K. The following were considered as standard states for reactants: Cr(ref) (reference state, melting point 2130 K, boiling point 2952 K), Cr(liq), Cr(gas), Cr2O3(cr, liq), Cr2O3(gas), C(ref), and as possible reaction products and standard states for them CO(gas), CO2(gas), Cr23C6(сr), Cr7C3(cr), Cr3C2(cr). The reaction probability was estimated using standard Gibbs energy and was calculated using the Van Goff isotherm equation. The chromium dissolution in metal of surfacing bath or probable partial pressures of CO and CO2 in gas phase was considered and then calculated from the equilibrium of carbon gasification reaction. The carbon presence in flux-cored wire with chromium oxide Cr2O3 as a reducing agent will necessarily lead to the occurrence of reduction reactions with chromium carbide generation, and possibly chromium itself. The generation of Cr7C3(сr) carbide is likely. With a longer life span of chromium oxide and carbon at a temperature above 2500 K, chromium generation as a component of the surfacing bath is more thermodynamically probable than carbide generation. Chromium oxide has the highest reactivity in Cr2O3(liq) state. Direct reduction is preferential. The generation of CO(gas) as a carbon oxidation product is more probable. The chromium dissolution in metal increases the thermodynamic reaction probability with its generation, as well as further reduces the reaction probability in which chromium is the starting material.

Helium, inorganic and organic carbon isotopes of fluids and gases across the Costa Rica convergent margin

In 2017, fluid and gas samples were collected across the Costa Rican Arc. He and Ne isotopes, C isotopes as well as total organic and inorganic carbon concentrations were measured. The samples (n = 24) from 2017 are accompanied by (n = 17) samples collected in 2008, 2010 and 2012. He-isotopes ranged from arc-like (6.8 RA) to crustal (0.5 RA). Measured dissolved inorganic carbon (DIC) δ13CVPDB values varied from 3.55 to −21.57‰, with dissolved organic carbon (DOC) following the trends of DIC. Gas phase CO2 only occurs within ~20 km of the arc; δ13CVPDB values varied from −0.84 to −5.23‰. Onsite, pH, conductivity, temperature and dissolved oxygen (DO) were measured; pH ranged from 0.9–10.0, conductivity from 200–91,900 μS/cm, temperatures from 23–89 °C and DO from 2–84%. Data were used to develop a model which suggests that ~91 ± 4.0% of carbon released from the slab/mantle beneath the Costa Rican forearc is sequestered within the crust by calcite deposition with an additional 3.3 ± 1.3% incorporated into autotrophic biomass.

Rovibrational Spectral Analysis of CO3 and C2O3: Potential Sources for O2 Observed in Comet 67P/Churyumov–Gerasimenko

Abstract The recent ROSETTA mission to comet 67P/Churyumov–Gerasimenko detected surprisingly high levels of molecular oxygen (O2; hypervolatile species) in the coma. Current models predict that considerable levels of other hypervolatiles (such as molecular nitrogen, N2, methane, CH4, and Argon) should be found at similar levels, whereas they are more depleted. One explanation explored here is that larger (less volatile) parent molecules may have been formed during radiolysis of cometary ices and, upon sublimation, are subsequently broken down within the coma into smaller, more volatile fragments. In support of this hypothesis, this work employs reliable quantum chemical techniques to provide the spectral data necessary for the detection of two candidate precursor “parent” molecules, cyclic carbon trioxide (c-CO3), and cyclic dicarbon trioxide (c-C2O3). Benchmark computations performed for gas-phase CO2 give vibrational frequencies to within 1.5 cm−1 or better for the three fundamentals. Both c-CO3 and c-C2O3 have strong infrared features in the 4.5–5.5 μm (1800–2200 cm−1) range and other notable infrared features closer to 1100 cm−1 (9.10 μm). These molecules are both rotationally active, unlike CO2, and are therefore potentially observable and present new targets for radio telescope observations. Due to the stronger dipole moment, c-CO3 should be more easily detectable than the nearly non-polar c-C2O3. These data may help observations of these molecules and can provide insights as to how radiation-driven derivatization of CO/CO2 precursors could contribute to the generation of higher-mass parent species that subsequently degrade to produce more volatile species, such as O2, observed in cometary comae.

Engineering stable Pt nanoparticles and oxygen vacancies on defective TiO2 via introducing strong electronic metal-support interaction for efficient CO2 photoreduction

This work for the first time reports the promoting effect of strong electronic metal-support interaction (EMSI) in Pt/TiO2-VO (VO: oxygen vacancy) for gas-phase CO2 photoreduction. A novel in-situ surface hydrogenation was developed to prepare hydrogenated Pt/TiO2-VO in a continuous, high throughput diffusion flame aerosol reactor. The combined results of various characterization techniques confirmed the presence of EMSI between Pt and defective TiO2-VO resulted in the enhanced electronic density of Pt nanoparticles. Both the modulated electronic structure of Pt and surface oxygen vacancies simultaneously promoted the activation of surface adsorbed carbon intermediates and facilitated the separation of photogenerated charges, eventually boosting the photocatalytic activity of Pt/TiO2-VO. The optimized Pt/TiO2-VO demonstrated a high quantum yield of 1.49% with high CH4 selectivity (81%), which rendered 5.8- and 1.2-fold enhancements over its counterparts of TiO2-VO and Pt/TiO2. More significantly, the EMSI also played a critical role in preserving the surface metallic Pt0 and oxygen vacancies, and in sustaining high activity of the Pt/TiO2-VO, whereas rapid catalytic deactivation are observed for both TiO2-VO and Pt/TiO2.

Chlorella vulgaris cultivation in airlift photobioreactor with transparent draft tube: effect of hydrodynamics, light and carbon dioxide on biochemical profile particularly ω-6/ω-3 fatty acid ratio

Chlorella vulgaris is used for food and feed applications due to its nutraceutical, antioxidant and anticancer properties. An airlift photobioreactor comprising transparent draft tube was used for C. vulgaris cultivation. The effect of reactor parameters like hydrodynamics (0.3-1.5vvm), light intensity (85-400μmolm-2s-1), photoperiod (12-24h) and gas-phase carbon dioxide (CO2) concentration (5-15% v/v) were evaluated on microalgae and associated bacterial growth, biochemical profile; with special emphasis on ω-3, ω-6 fatty acids, and vitamin B12. The optimal growth of C. vulgaris without CO2 supplementation was observed at 1.2vvm, which was associated with higher algal productivity, chlorophyll, vitamin B12 content, and bacterial load along with 72% of nitrate removal. The higher light intensity (400μmolm-2s-1) and photoperiod (24:0) increased biomass productivity and ω-3 fatty acid content (in lipid) up to 2-3 fold. The elevated levels of gas-phase CO2 concentration (15% v/v) enhanced EPA content up to 7% and biomass productivity up to 171mgL-1day-1. However, the increase in CO2 concentration lowered vitamin B12 content (up to 30%) and bacterial load (2-3 log). Also, all the cultivation conditions favoured desirable ω-6/ω-3 ratio(in the range of 1-2).

Carbon stabilised saponite supported transition metal-alloy catalysts for chemical CO2 utilisation via reverse water-gas shift reaction

Chemical CO2 upgrading via reverse water gas shift (RWGS) represents an interesting route for gas phase CO2 conversion. Herein, nature inspired clay-based catalysts are used to design highly effective materials, which could make this route viable for practical applications. Ni and transition metal promoted Ni saponite clays has been developed as highly effective catalysts for the RWGS. Saponite supported NiCu catalyst displayed a remarkable preference for the formation of CO over CH4 across the entire temperature range compared to the saponite supported NiCo and Ni catalysts. The NiCu sample is also highly stable maintaining ∼ 55% CO2 conversion and ∼ 80% selectivity for CO for long terms runs. Very importantly, when compared with reference catalysts our materials display significantly higher levels of CO2 conversion and CO selectivity. This confirmed the suitability of these catalysts to upgrade CO2-rich streams under continuous operation conditions.

Molecular Oxygen Activation on Suspended Doped Cerium(IV) Oxide Particles in Molten Chloride Salts

Suspensions of cerium(IV) oxide (CeO2) particles in molten chloride salts were found to be active for molecular oxygen activation when doped with the larger Lanthanides, gadolinium (Gd), praseodymium, (Pr), and samarium, (Sm). The activity for O2 oxidation of gas phase CO on suspended doped ceria in molten mixtures of LiCl–LiBr–KBr and NaCl–KCl is significantly higher than undoped ceria or the salt mixtures alone in the temperature ranges of 350–600 °C, and 700–800 °C, respectively. Surface tension measurements show that the solid doped ceria particles suspended in the melt are contacting the gas–liquid interface of the bubbles. The activity for O2 oxidation of liquid phase MgCl2 in molten MgCl2–KCl to produce Cl2 and solid MgO was also observed to be significantly increased on suspended CeO2 doped with 5 wt% or 10 wt% Gd. The rates of oxidation were observed to increase with catalyst loading.

Towards Efficient Electrocatalytic Conversion of CO2 to Formate Via Novel Electrolyzer Configurations

Formate (HCOO-) has been proposed as an intermediate compound to store energy and carbon for microbial conversion to fuels and chemicals1. Formate can be generated by electrochemical reduction of CO2 and subsequently assimilated by microbes that are able to use it as a sole source of energy and carbon for growth and, as recently demonstrated, to produce higher-value chemicals like higher alcohols2. Using formate as an intermediate molecule could provide several advantages over direct conversion from gaseous feeds, such as utilizing renewable energy sources by uncoupling the power availability from the fluctuations inherent to renewable energy sources such as wind, solar, or tidal1. To enable formate as a foundation for the storage of carbon and energy and subsequent conversion to value-added fuels and chemicals, CO2 mass transport limitation issues need to be overcome. High current densities are required for CO2 electrolyzers to be relevant for industrial applications. Due to the solubility limit of CO2, current densities are restricted to about 30 mA/cm2 in a liquid/planar electrode configuration3. Proof of principle results have shown the mass transport issues can be overcome through utilization of gas diffusion electrodes (GDE) with high interphase (gas/liquid/solid) surface area4. Here we will discuss how higher current densities and faradaic efficiencies for the gas-phase CO2 to formate conversion process can be achieved through the utilization of novel bipolar membrane (BPM) architectures and reactor designs coupled with optimized reaction condition parameters. We will also show an introductory view of developed testing methods and equipment coupled with in-situ product analysis for electrocatalytic conversions not limited to formate production.

A feasibility study for detection thresholds of CO 2 at shallow depths at the CaMI Field Research Station, Newell County, Alberta, Canada

We present the results of a feasibility study for seismic monitoring using conventional surface seismic experiments at the CaMI Field Research Station, Alberta, Canada, where a small volume of gas-phase CO 2 is being injected into a sandstone reservoir at a depth of 300 m. We first apply a careful fluid substitution procedure to the results of reservoir gas saturation and pressure responses obtained from fluid flow simulations. We test different methods to compute the bulk modulus of the fluid for different fluid saturation models. Assuming a semi-patchy model and considering only the replacement of brine with a maximum saturation of 50% CO 2 , we estimate the reduction in P-wave velocity to be 20%. Adding an increase in pore pressure of 2.7 MPa increases the P-wave velocity reduction to 32%. After including a field-based signal-to-noise ratio of 5% to the synthetic seismic data, the time-lapse seismic anomaly should be detectable after one year of injection (266 tonnes of CO 2 ).

Exploration of the basic reactant in CO2 photoreduction: New insights from photophysics and photochemistry

Photoreduction of CO2 as an attractive path for CO2 conversion has drawn widespread interest. Experiments demonstrating the photoreduction of CO2 are conducted in water solution in the excited state, and solvent molecules play a major role in the reaction mechanism. Thus, a simple gas-phase CO2 model of the ground-state cannot fully demonstrate the characteristics of aqueous CO2 and the mechanisms of CO2 photoreduction in the excited state. A deep understanding of the characteristics of aqueous CO2 as the basic reactant is the cornerstone for the development of photocatalysts. In this study, a systemic investigation involving photophysics and photochemistry was performed to determine its existential status, kinetic and thermodynamic characteristics. In the excited state, H2CO3 as the main inorganic carbon in pure water should be considered in the photoreduction of CO2, and the development of photocatalysts should compatible with its characteristics.

SYNTHESIS, CHARACTERIZATION, ADSORPTION AND THERMODYNAMIC STUDIES OF PURE AND BINARY CO2-N2 MIXTURES ON OXYGEN ENRICHED NANOSTRUCTURED CARBON ADSORBENTS

Abstract Oxygen enriched porous carbons have been synthesized by a nanocasting technique using mesoporous zeolite as template and epoxy resin as precursor. Characterization results show the effect of the nanocasting technique on the development of heterogeneous surface, high basicity, and high surface area of 686.37 m2g-1, beneficial for CO2 adsorption. Pure component adsorption isotherms were correlated with Langmuir, Sips, and dual-site Langmuir (DSL) models and found that Sips and DSL isotherm models fitted well, indicating thre heterogeneous nature of the adsorbent surface. Dynamic breakthrough data for the binary system CO2-N2 were obtained using a fixed-bed column at different adsorption temperatures (30-100 °C) and CO2 feed concentrations (5-12.5% by volume). The developed adsorbent shows high adsorption capacity with complete regenerability over four adsorption/desorption cycles. Prediction of binary components (CO2-N2) was made by using extended Sips, extended DSL and IAST (ideal adsorbed solution theory) by utilizing pure component adsorption isotherm data. Experimental and predicted equilibrium data were compared with breakthrough curve data and it was found that the extended forms (Sips and DSL) indicated under-predicted CO2 adsorption equilibria because of differences in adsorptive strengths of CO2 and N2 molecules. Also, adsorption equilibria were closely predicted using IAST theory. Asymmetric x-y diagrams from Raoult’s law indicated positive deviation, implying that as the CO2 gas phase molar fraction increases, total adsorbed amounts increase. Negative values of molar Gibbs free energy change suggested feasibility of the adsorption process. Formation of a more ordered configuration of CO2 molecules on the adsorbent surface was seen as a higher heat of adsorption was exhibited for CO2 as compared to N2.