Bicarbonate Complex Research Articles

Exploring Rare Earth Element behavior in the Mount Etna volcanic aquifers (Sicily)

This study presents the first data on REY (Rare Earth Elements plus Yttrium) in the aquifer of Mount Etna (Sicily, Italy). Patterns normalized to chondrites indicate strong water–rock interaction, facilitated by a slightly acidic pH resulting from the dissolution of magma-derived CO2. REY patterns provide insights into the processes of both mineral dissolution and the formation of secondary phases. The relative abundance of light to heavy rare earth elements is compatible with the prevailing dissolution of ferromagnesian minerals (e.g., olivine or clinopyroxenes), reinforced by its strong correlation with other proxies of mineral dissolution (e.g., Mg contents). Pronounced negative Ce anomalies and positive Y anomalies demonstrate an oxidizing environment with continuous formation of secondary iron and/or manganese oxides and hydroxides. The Y/Ho fractionation is strongly influenced by metal complexation with bicarbonate complexes, a common process in C-rich waters. In the studied system, the measured REY contents are always below the limits proposed by Sneller et al. (2000, RIVM report, Issue 601,501, p. 66) for surface water and ensure a very low daily intake from drinking water.

Asymmetric urea, thiourea and squaramide receptors as minimal-anion binding motifs

ABSTRACT Asymmetric minimal receptors based on 4-nitrophenylurea, 1, and 4-nitrophenylthiourea, 2, were shown to bind a range of anions and are deprotonated by both fluoride and hydroxide to give a colorimetric response. A 4-trifluoromethylphenylurea receptor, 3, was deprotonated by fluoride but not hydroxide but was not discernible by the naked eye. Deprotonation of 1 by fluoride ultimately resulted in the formation of its bicarbonate complex as determined by X-ray crystallography in agreement with DFT calculations. Fluorescence spectroscopy revealed that emission of 2 reaches a maximum at two equivalents of anions except for fluoride where it continues to increase with increasing addition of TBAF allowing 2 to give a fluoride-specific response.

Enhanced activation of sulfite by the Co(II)-bicarbonate complexes for iohexol abatement: Role of free radicals versus high valent cobalt species

Sulfite activation with Co(II) (simplified as Co(II)/sulfite process) has been widely studied for the abatement of organic contaminants, yet the efficiency in the Co(II)/sulfite process is compromised by the slow redox cycling of Co(II)/Co(III). Regarding that the complexation effect between Co(II) and bicarbonate (HCO3−) can enhance the conversion between Co(II) and Co(III) in the advanced oxidation processes, a novel Co(II)/HCO3−/sulfite process is constructed in this study. The iohexol abatement efficiency in the Co(II)/HCO3−/sulfite process is >92 % within 10 min of reaction at trace Co(II) dosage, which is significantly higher that of Co(II)/sulfite process. Apart from the conventional radical generation in the Co(II)/sulfite process, it is unexpectedly found that high valence cobalt species (Co(IV)) are generated. HCO3− can enhance the generation of Co(IV), sulfate radical (SO4•−) and hydroxyl radical (HO•), which play the important role in iohexol abatement. The identification of Co(IV) and its contribution to iohexol abatement is elucidated in the Co(II)/HCO3−/sulfite process. The enhancement can be attributed to the complexation effect between Co(II) and HCO3−, in which the CoHCO3+ and CoCO3 species exhibit excellent ability on sulfite activation. According to electrochemical characterization and theoretical calculation, the presence of HCO3− can reduce the energy barrier in the reaction process, accelerating the Co(II)/Co(III) redox cycling. The Co(II)/HCO3−/sulfite process exhibits excellent practical application potential in the abatement of various pollutants in the water and wastewater treatment.

Tracing geochemical sources and health risk assessment of uranium in groundwater of arid zone of India

Water quality degradation and metal contamination in groundwater are serious concerns in an arid region with scanty water resources. This study aimed at evaluating the source of uranium (U) and potential health risk assessment in groundwater of the arid region of western Rajasthan and northern Gujarat. The probable source of vanadium (V) and fluorine (F) was also identified. U and trace metal concentration, along with physicochemical characteristics were determined for 265 groundwater samples collected from groundwater of duricrusts and palaeochannels of western Rajasthan and northern Gujarat. The U concentration ranged between 0.6 and 260 μg L−1 with a mean value of 24 μg L−1, and 30% of samples surpassed the World Health Organization’s limit for U (30 μg L−1). Speciation results suggested that dissolution of primary U mineral, carnotite [K2(UO2)2(VO4)2·3H2O] governs the enrichment. Water–rock interaction and evaporation are found the major hydrogeochemical processes controlling U mineralization. Groundwater zones having high U concentrations are characterized by Na–Cl hydrogeochemical facies and high total dissolved solids. It is inferred from geochemical modelling and principal component analysis that silicate weathering, bicarbonate complexation, carnotite dissolution, and ion exchange are principal factors controlling major solute ion chemistry. The annual ingestion doses of U for all the age groups are found to be safe and below the permissible limit in all samples. The health risk assessment with trace elements manifested high carcinogenic risks for children.

Carbonation Reaction Mechanisms of Portlandite Predicted from Enhanced Ab Initio Molecular Dynamics Simulations

Geological carbon capture and sequestration (CCS) is a promising technology for curbing the global warming crisis by reduction of the overall carbon footprint. Degradation of cement wellbore casings due to carbonation reactions in the underground CO2 storage environment is one of the central issues in assessing the long-term success of the CCS operations. However, the complexity of hydrated cement coupled with extreme subsurface environmental conditions makes it difficult to understand the carbonation reaction mechanisms leading to the loss of well integrity. In this work, we use biased ab initio molecular dynamics (AIMD) simulations to explore the reactivity of supercritical CO2 with the basal and edge surfaces of a model hydrated cement phase—portlandite—in dry scCO2 and water-rich conditions. Our simulations show that in dry scCO2 conditions, the undercoordinated edge surfaces of portlandite experience a fast barrierless reaction with CO2, while the fully hydroxylated basal surfaces suppress the formation of carbonate ions, resulting in a higher reactivity barrier. We deduce that the rate-limiting step in scCO2 conditions is the formation of the surface carbonate barrier which controls the diffusion of CO2 through the layer. The presence of water hinders direct interaction of CO2 with portlandite as H2O molecules form well-structured surface layers. In the water-rich environment, CO2 undergoes a concerted reaction with H2O and surface hydroxyl groups to form bicarbonate complexes. We relate the variation of the free-energy barriers in the formation of the bicarbonate complexes to the structure of the water layer at the interface which is, in turn, dictated by the surface chemistry and the degree of nanoconfinement.

Experimental determination of calcite solubility and the stability of aqueous Ca– and Na–carbonate and –bicarbonate complexes at 100–160 °C and 1–50 bar pCO2 using in situ pH measurements

The solubility of calcite was measured at 100, 120, 140 and 160 °C at 1–50 bar pCO2 in 10−3–0.1 mol·kg−1 NaCl solutions using a new experimental setup involving in situ pH measurements with high-temperature solid-contact H-selective glass and two types of reference electrodes: (i) Ag/AgCl in 3.5 M KCl, saturated AgCl placed in a Teflon extensible container with liquid junction, and (ii) solid-contact high-T Na-selective glass electrode in the cell without liquid junction. The stability constants of NaHCO3° and NaCO3− aqueous complexes formation were determined in NaCl-Na2CO3/NaHCO3 solutions in CO2-free media and under 10 bar pCO2 from 100 to 160 °C. These values allowed calculation of the pH of the calibration solution in the system NaCl-CO2-H2O used in the cell without liquid junction with Na+-selective electrode as a reference. This highly stable, low-cost electrode system can be recommended for routine pH measurements at 4 < pH < 10 in sodium-bearing solutions up to 160 °C and the critical point of CO2. The values of the stability constants of CaCO3° and CaHCO3+ aqueous complexes and calcite solubility product were generated at 100, 120, 140 and 160 °C allowing a description of the solubility of calcite in a wide range of pH and pCO2.

Bridges of Calcium Bicarbonate Tightly Couple Dipolar Lipid Membranes.

The interaction between lipid membranes and ions is associated with a range of key physiological processes. Most earlier studies have focused on the interaction of lipids with cations, while the specific effects of the anions have been largely overlooked. Owing to dissolved atmospheric carbon dioxide, bicarbonate is an important ubiquitous anion in aqueous media. In this paper, we report on the effect of bicarbonate anions on the interactions between dipolar lipid membranes in the presence of previously adsorbed calcium cations. Using a combination of solution X-ray scattering, osmotic stress, and molecular dynamics simulations, we followed the interactions between 1,2-didodecanoyl-sn-glycero-3-phosphocholine (DLPC) lipid membranes that were dialyzed against CaCl2 solutions in the presence and absence of bicarbonate anions. Calcium cations adsorbed onto DLPC membranes, charge them, and lead to their swelling. In the presence of bicarbonate anions, however, the calcium cations can tightly couple one dipolar DLPC membrane to the other and form a highly condensed and dehydrated lamellar phase with a repeat distance of 3.45 ± 0.02 nm. Similar tight condensation and dehydration has only been observed between charged membranes in the presence of multivalent counterions. Bridging between bilayers by calcium bicarbonate complexes induced this arrangement. Furthermore, in this condensed phase, lipid molecules and adsorbed ions were arranged in a two-dimensional oblique lattice.

Atomic-Scale Studies of Fe3 O4 (001) and TiO2 (110) Surfaces Following Immersion in CO2 -Acidified Water.

Difficulties associated with the integration of liquids into a UHV environment make surface‐science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV‐compatible dosing of ultrapure liquid water and studied its interaction with TiO2 and Fe3O4 surfaces. Herein, we describe a simple approach to vary the pH through the partial pressure of CO2 (pCO2 ) in the surrounding vacuum chamber and use this to study how these surfaces react to an acidic solution. The TiO2(110) surface is unaffected by the acidic solution, except for a small amount of carbonaceous contamination. The Fe3O4(001)‐(2 ×2 )R45° surface begins to dissolve at a pH 4.0–3.9 (pCO2 =0.8–1 bar) and, although it is significantly roughened, the atomic‐scale structure of the Fe3O4(001) surface layer remains visible in scanning tunneling microscopy (STM) images. X‐ray photoelectron spectroscopy (XPS) reveals that the surface is chemically reduced and contains a significant accumulation of bicarbonate (HCO3 −) species. These observations are consistent with Fe(II) being extracted by bicarbonate ions, leading to dissolved iron bicarbonate complexes (Fe(HCO3)2), which precipitate onto the surface when the water evaporates.

Evolutionary Loss of the Ability of the Photosystem I Primary Electron Donor for the Redox Interaction with Mn-Bicarbonate Complexes

The structure and functional organization of the photosystem I (PSI) reaction center (RC) donor side has a significant similarity to the reaction centers of purple bacteria (bRCs), despite the fact that they belong to different types of RCs. Moreover, the redox potential values of their primary electron donors are identical (~0.5 V). In our earlier reports [Khorobrykh et al. (2008) Phylos. Trans. R. Soc. B., 363, 1245-1251; Terentyev et al. (2011) Biochemistry (Moscow), 76, 1360-1366; Khorobrykh et al. (2018) ChemBioChem, 14, 1725-1731], we have demonstrated redox interaction of low-potential Mn2+-bicarbonate complexes with bRCs, which might have been one of the first steps in the evolutionary origin of Mn-cluster of the photosystem II water-oxidizing complex that occurred in the Archean (over 3 billion years ago). In this study, we investigated redox interactions between Mn2+-bicarbonate complexes and PSI. Such interactions were almost absent in the original PSI preparations and emerged only in preoxidized PSI preparations containing ~50% oxidized RCs. The interaction between Mn2+-bicarbonate complexes and PSI required increased Mn2+ concentrations, while its dependence on the HCO3- concentration indicated involvement of electroneutral low-potential [Mn(HCO3)2] complex in the process. Analysis of the PSI crystal structure revealed steric hindrances on the RC donor side, which could block the redox interaction between Mn2+-bicarbonate complexes and oxidized primary electron donor. Comparison of structures of RCs from the PSI and ancient RCs from heliobacteria belonging to the same type of RCs suggested that such hindrances should be absent in the primitive PSI in the Archean and allowed to explain their evolutionary origin as a consequence of PSI RCs into the united electron transport chain (ETC) of the photosynthetic membrane that was accompanied by the evolutionary loss of PSI capacity for the redox interaction with Mn2+-bicarbonate complexes.

Transforming atmospheric CO2 into alternative fuels: a metal-free approach under ambient conditions.

This work demonstrates the first-ever completely metal-free approach to the capture of CO2 from air followed by reduction to methoxyborane (which produces methanol on hydrolysis) or sodium formate (which produces formic acid on hydrolysis) under ambient conditions. This was accomplished using an abnormal N-heterocyclic carbene (aNHC)-borane adduct. The intermediate involved in CO2 capture (aNHC-H, HCOO, B(OH)3) was structurally characterized by single-crystal X-ray diffraction. Interestingly, the captured CO2 can be released by heating the intermediate, or by passing this compound through an ion-exchange resin. The capture of CO2 from air can even proceed in the solid state via the formation of a bicarbonate complex (aNHC-H, HCO3, B(OH)3), which was also structurally characterized. A detailed mechanism for this process is proposed based on tandem density functional theory calculations and experiments.

Quantum-mechanical process of carbonate complex formation and large-scale anisotropy in the adsorption energy of CO2 on anatase TiO2 (001) surface

Adsorption of CO$_2$ on a semiconductor surface is a prerequisite for its photocatalytic reduction. Owing to superior photocorrosion resistance, nontoxicity and suitable band edge positions, TiO$_2$ is considered to be the most efficient photocatalyst for facilitating redox reactions. However, due to the absence of adequate understanding of the mechanism of adsorption, the CO$_2$ conversion efficiency on TiO$_2$ surfaces has not been maximized. While anatase TiO$_2$ (101) is the most stable facet, the (001) surface is more reactive and it has been experimentally shown that the stability can be reversed and a larger percentage (up to ~ 89%) of the (001) facet can be synthesized in the presence fluorine ions. Therefore, through density functional calculations we have investigated the CO$_2$ adsorption on TiO$_2$ (001) surface. We have developed a three-state quantum-mechanical model that explains the mechanism of chemisorption, leading to the formation of a tridentate carbonate complex. The electronic structure analysis reveals that the CO$_2$-TiO$_2$ interaction at the surface is uniaxial and long ranged, which gives rise to anisotropy in binding energy (BE). It negates the widely perceived one-to-one correspondence between coverage and BE and infers that the spatial distribution of CO$_2$ primarily determines the BE. A conceptual experiment is devised where the CO$_2$ concentration and flow direction can be controlled to tune the BE within a large window of ~1.5 eV. The experiment also reveals that a maximum of 50% coverage can be achieved for chemisorption. In the presence of water, the activated carbonate complex forms a bicarbonate complex by overcoming a potential barrier of ~0.9 eV.

Identifying calcium-containing mineral species in the JEB Tailings Management Facility at McClean Lake, Saskatchewan

The JEB Tailings Management Facility (TMF) is central to reducing the environmental impact of the McClean Lake uranium mill facility that is operated by AREVA Resources Canada. This facility has been designed around the idea that elements of concern (e.g., U, As, Ni, Se, Mo) will be controlled through equilibrium with precipitants. Confirming the presence of calcium-containing carbonates in the JEB TMF is the first step in determining if gypsum (CaSO4·2H2O) controls the concentration of HCO3− (aq), limiting the formation of soluble uranyl bicarbonate complexes. A combination of X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), and microprobe X-ray fluorescence (XRF) mapping was used to analyze a series of tailings samples from the JEB TMF. Calcium carbonate in the form of calcite (CaCO3), aragonite (CaCO3), and dolomite (CaMg(CO3)2) were identified by analysing Ca K-edge μ-XANES spectra coupled with microprobe XRF mapping. This is the first observation of these phases in the JEB TMF. The combination of μ-XANES and XRF mapping provided a greater sensitivity to low concentration calcium species compared to the other techniques used, which were only sensitive to the major species present (e.g., gypsum).

RuIII(edta) catalyzed hydrogenation of bicarbonate to formate

Selective hydrogenation of bicarbonate to formate catalyzed by a ruthenium(III) complex, [RuIII(edta)] (edta4− = ethylenediaminetetraacetate), at moderate H2 pressure (2–8 atm) and temperature (30–40 °C) is reported. Formation of formate, the only reduction product, was identified by 13C NMR analysis of the resultant reaction mixture. Based on the spectral data, a working mechanism (admittedly speculative) involving the formation of ruthenium(III)-bicarbonate complex, [RuIII(edta)(HCO3)]2−, is proposed for the catalytic reaction.

The Nature of Ore‐forming Fluids of the Carlin‐type Gold Deposit in Southwest China: A Case from the Zimudang Gold Deposit

AbstractThe Zimudang gold deposit is a large Carlin‐type gold deposit in the Southwest Guizhou Province, China, with an average Au content of 6.2 g/t. Gold is mainly hosted in the fault zone and surrounding strata of the F1 fault and Permian Longtan Formation, and the ore bodies are strictly controlled by both the faults and strata. Detailed mineralogy and geochemistry studies are conducted to help judge the nature of ore‐forming fluids. The results indicate that the Au is generally rich in the sulfides of both ores and wall rocks in the deposit, and the arsenian pyrite and arsenopyrite are the main gold‐bearing sulfides. Four subtypes of arsenian pyrite are found in the deposit, including the euhedral and subhedral pyrite, framboidal pyrite, pyrite aggregates and pyrite veins. The euhedral and subhedral pyrite, which can take up about 80% of total pyrite grains, is the dominant type. Au distributed unevenly in the euhedral and subhedral pyrite, and the content of the Au in the rim is relatively higher than in the core. Au in the pyrite veins and pyrite aggregates is lower than the euhedral and subhedral pyrite. No Au has been detected in the points of framboidal pyrites in this study. An obvious highly enriched As rim exists in the X‐ray images of euhedral pyrites, implying the ore‐forming fluids may be rich in As. The relationship between Au and As reveals that the Au may host as a solid solution (Au+) and nanoparticles of native gold (Au0) in the sulfides. The high Co/Ni ratio (&gt;1) of sulfides and the enrichment of W in the ores all reflect that the gold‐bearing minerals and ore‐forming process were mainly related to the hydrothermal fluids, but the magmatic and volcanic activities cannot be neglected. The general existence of Au and As in the sulfides of both ores and wall rocks and the REE results suggest that the ore‐forming fluids may mainly be derived from the basin itself. The enrichment of Tl suggests that the ore‐forming fluids may be enriched in Cl. The Ce and Eu show slightly or apparently negative anomalies, which means the ore fluids were probably formed under reducing environment. The Y/Ho ratios of ore samples fluctuate around 28, implying the bicarbonate complexation and fluorine were both involved in the ore‐forming process. Combined with the previous studies and our results, we infer that the ore‐forming fluids enriched Au, As, HS− and halogen (F, Cl) were derived from the mixture of reducing basinal fluids and magmatic or volcanic hydrothermal fluids.

Characterization and reactivity of a terminal nickel(III)-oxygen adduct.

High-valent terminal metal-oxygen adducts are hypothesized to be the potent oxidizing reactants in late transition metal oxidation catalysis. In particular, examples of high-valent terminal nickel-oxygen adducts are scarce, meaning there is a dearth in the understanding of such oxidants. A monoanionic Ni(II)-bicarbonate complex has been found to react in a 1:1 ratio with the one-electron oxidant tris(4-bromophenyl)ammoniumyl hexachloroantimonate, yielding a thermally unstable intermediate in high yield (ca. 95%). Electronic absorption, electronic paramagnetic resonance, and X-ray absorption spectroscopies and density functional theory calculations confirm its description as a low-spin (S = 1/2), square planar Ni(III)-oxygen adduct. This rare example of a high-valent terminal nickel-oxygen complex performs oxidations of organic substrates, including 2,6-di-tert-butylphenol and triphenylphosphine, which are indicative of hydrogen atom abstraction and oxygen atom transfer reactivity, respectively.

Geochemistry of Trace Metals and Rare Earth Elements in Stream Water, Stream Sediments and Acid Mine Drainage from Darrehzar Copper Mine, Kerman, Iran

Darrehzar porphyry copper mine is located 10 km south of the famous Sarcheshmeh porphyry copper mine in Kerman province, Iran. Water and sediment samples in the Darrehzar porphyry copper mine were collected and analyzed in order to characterize the environmental effects of regional geology and mining activity. Stream affected by acid mine drainage (AMD) are \({\hbox {Ca}}{-}{\hbox {SO}}_{4}^{2-}\) in type, while unaffected streams are \({\hbox {Ca}}{-}{\hbox {HCO}}_{3}^{- }\) type. Acid drainage, with pH values as low as 2.5, is quite rich in dissolved copper (mean 306.53 mg/L) \(>\) iron (77.83 mg/L) \(>\) aluminium (48.62 mg/L) \(>\) manganese (6.90 mg/L) \(>\) zinc (2810.42 \(\upmu \)g/L) \(>\) cobalt (831.55 \(\upmu \)g/L) \(>\) nickel (298.47 \(\upmu \)g/L). The high sulfate concentration makes sulfate the major controlling factor of electrical conductivity. The strong correlation among the majority of the elements indicates either similar geochemical behavior or a common origin, (mostly sulfides). Calculation of Acid Mine Drainage Index (AMDI) indicates that Darrehzar mine is classified as dangerous in view of AMD hazards. In Darrehzar porphyry copper mine, total rare earth element content (\(\Sigma \)REE) in AMD (5.54–417.76 \(\upmu \hbox {g L}^{-1})\) is higher than natural water (REE in upstream and downstream water is 0.87–1.40 and 0.83–1.71 \(\upmu \hbox {g L}^{-1}\)respectively). The North American Shale Composite (NASC) normalized REE patterns of AMDs revealed strong enrichment of middle REE (MREE), and heavy REE (HREE) compared with light REE (LREE). Speciation modeling indicates the predominance of aqueous species i.e., \(\hbox {LnSO}_{4}^{+}\), \(\hbox {Ln} (\hbox {SO}_{4})_{2}^{-}, \hbox {Ln}^{3+}, \hbox {LnCO}_{3}^{+}, \hbox {LnHCO}_{3}^{2+}\) and \(\hbox {Ln} (\hbox {CO}_{3})_{2}^{-}\) (Ln refers to any lanthanide element). In upstream, where pH is 7.2 \(\le \) pH, REEs primarily exist as \(\hbox {LnCO}_{3}^{+}\), and HREE bicarbonate complexes (Ln \((\hbox {CO}_{3})_{2}^{-})\) exceed those of LREEs. Based on mean geoaccumulation indices (\(I_\mathrm{{geo}}\)), Darrehzar river sediments at mine site display the following trend: \(\hbox {Cu}>\hbox {Mo}>\hbox {Pb}>\hbox {Sb}>\) Cd. Calculated potential ecological risk index (RI) indicates higher risk at mine site. The mean potential ecological risk factors \((\hbox {E}_{\mathrm{i}})\) for sediment metals display the following trend: \(\hbox {Mo}>\hbox {Cd}>\hbox {Cu}>\) As \(>hbox{Pb}>\hbox {Ni}>\hbox {Cr}> \hbox {Zn}\).

Reduction of bicarbonate and carbonate to formate in molecular zinc complexes

The tris(2-pyridylthio)methylzinc bicarbonate complex, [κ4-Tptm]ZnOCO2H, is reduced by PhSiH3 to give the formate derivative, [κ4-Tptm]ZnO2CH. Isotopic labeling studies demonstrate that the generation of the formate moiety occurs via a sequence that involves release of CO2 followed by insertion into a zinc–hydride bond.

Reversible Reactions of Ni and Pd Hydroxo Pincer Complexes [(iPrPCP)M–OH] with CO2: Solid‐State Study of the Decarboxylation of the Monomeric Bicarbonate Complexes [(iPrPCP)M–OCOOH] (M = Ni, Pd)

AbstractMonomeric Ni and Pd hydroxides stabilized by the iPrPCP pincer ligand react with CO2 to give labile terminal bicarbonate complexes that readily lose CO2 and water to give binuclear carbonate complexes. Differential scanning calorimetry (DSC) has been used to monitor the decomposition of both bicarcabonates in the solid state. When the carbonate complexes are heated under reflux in thf in the presence of water, full decarboxylation takes place, restoring the starting hydroxides and demonstrating that CO2 insertion is a fully reversible process. The decarboxylation of the nickel carbonate complex is completed more readily, suggesting that the reaction of the Pd hydroxide with CO2 is more favourable than that of its nickel counterpart. This is supported by DFT calculations, which also shows that CO2 insertion takes place through a concerted Lipscomb‐type mechanism.

Adsorption of carbon dioxide on Al/Fe oxyhydroxide

The structure and reactivity of 0–70mol% Al/Fe iron oxyhydroxides (ferrihydrite in the absence and presence of Al) toward gaseous CO2 were investigated with X-ray photoelectron spectroscopy (XPS), atomic absorption (AA), scanning transmission electron microscopy with electron dispersive X-ray spectroscopy (STEM/EDS), X-ray diffraction (XRD), and attenuated total reflectance Fourier transform Infrared spectroscopy (ATR-FTIR) combined with density functional theory (DFT) calculations. Results showed that Al/Fe oxyhydroxide particles containing more than 20mol% Al consisted at least in part of Fe-oxyhydroxide with incorporated Al and a discrete AlOOH phase. Results from ATR-FTIR experiments and DFT calculations suggested that the bicarbonate complex formed by passing CO2 over the particles was accommodated on at least three distinct binding sites. At the lowest Al concentrations bicarbonate was bound to individual sites with primarily Fe or Al character. At the highest concentrations of Al (>20mol%) bicarbonate bound to discrete AlOOH phases became apparent. Results also suggested that the amount of CO2 adsorption for a given particle mass increased as the Al concentration was increased from 0 to 30%. This increase was likely due in large part to differences in the morphology of the particle aggregates that formed in the dry state, which would be expected to affect the amount of surface that was available to adsorb CO2.

Titanate nanotubes as support of solid base catalyst

Modified titanate nanotubes with enhanced basicity were synthesized by the incipient wetness impregnation of sodium titanate nanotubes (Na-TNT) with basic cations (Mg2+, K+, Ba2+, Zn2+). The essential morphology and mesoporosity of the nanotubes were maintained, as well as the tri-titanate structure, although in some cases (Mg2+) ion exchange and intercalation reactions brought about some contraction of the rolled titanate layers. Basic sites were probed with CO2 adsorption microcalorimetry and infrared spectroscopy, revealing strong (Qads>150kJmol−1), medium (150<Qads<120kJmol−1), and weak basic sites that were absent in Na-TNT. These sites could be identified correspondingly with IR bands for relevant carbonate and bicarbonate complexes. All modified TNT catalysts demonstrated high conversions for the COS and CS2 hydrolysis reactions at 553K, confirming the presence of important basic sites.