Atomic Force Microscopy and X‑ray Photoelectron Spectroscopy Study of NO₂ Reactions on CaCO₃ (101̅4) Surfaces in Humid Environments

In this study, alternating current (AC) mode atomic force microscopy (AFM) combined with phase imaging and X-ray photoelectron spectroscopy (XPS) were used to investigate the effect of nitrogen dioxide (NO₂) adsorption on calcium carbonate (CaCO₃) (101̅4) surfaces at 296 K in the presence of relative humidity (RH). At 70% RH, CaCO₃ (101̅4) surfaces undergo rapid formation of a metastable amorphous calcium carbonate layer, which in turn serves as a substrate for recrystallization of a nonhydrated calcite phase, presumably vaterite. The adsorption of nitrogen dioxide changes the surface properties of CaCO₃ (101̅4) and the mechanism for formation of new phases. In particular, the first calcite nucleation layer serves as a source of material for further island growth; when it is depleted, there is no change in total volume of nitrocalcite, Ca­(NO₃)₂, particles formed whereas the total number of particles decreases. This indicates that these particles are mobile and coalesce. Phase imaging combined with force curve measurements reveals areas of inhomogeneous energy dissipation during the process of water adsorption in relative humidity experiments, as well as during nitrocalcite particle formation. Potential origins of the different energy dissipation modes within the sample are discussed. Finally, XPS analysis confirms that NO₂ adsorbs on CaCO₃ (101̅4) in the form of nitrate (NO₃^–) regardless of environmental conditions or the pretreatment of the calcite surface at different relative humidity

Methane Conversion to Syngas for Gas-to-Liquids (GTL): Is Sustainable CO₂ Reuse via Dry Methane Reforming (DMR) Cost Competitive with SMR and ATR Processes?

Carbon dioxide is a greenhouse gas and is obtained as a waste via burning various forms of fuels. Syngas is an intermediate in large-scale long-chain hydrocarbon (C₁₀–C₂₀ alkanes and alcohols) production processes via Fischer–Tropsch (FT) synthesis, typically to obtain high quality fuels. Thus, it is of particular interest to engineer syngas production processes for FT that can consume various combustion process waste CO₂ in the process and thus partially contribute to the sustainable carbon neutral fuel synthesis. In this work, a quantitative economic comparison of five alternative processes is presented for the production of synthesis gas with a hydrogen-to-carbon monoxide ratio of 2, which is suitable for feeding to the Fischer–Tropsch gas-to-liquid process. Combinations of steam methane reforming (SMR), dry methane reforming (DMR), autothermal reforming (ATR) and reverse water gas shift (RWGS) are explored. An amine absorber/stripper system is used for carbon dioxide removal. The effects of the cost of natural gas and of liquid oxygen and the magnitude of a potential carbon tax are demonstrated. With current prices of raw materials, the configuration with the lowest total annual cost (TAC) features a system composed of both SMR and DMR reactors

Electronic Properties and Reactivity of Simulated Fe³⁺ and Cr³⁺ Substituted α‑Al₂O₃ (0001) Surface

Metal oxide-based minerals naturally contain transition metal impurities isomorphically substituted into the structure that can alter the structural and electronic properties as well as the reactivity of these metal oxides. Natural α-Al₂O₃ (corundum) can contain up to 9.17% (w/w) Fe₂O₃ and 1.81% (w/w) of Cr₂O_3. Here we report on changes in the structural and electronic properties of undoped and doped α-Al₂O₃ (0001) surfaces using periodic density functional theory (DFT) methods with spin unrestricted B3LYP functional and a local atomic basis set. Both structural and electronic properties are altered upon doping. Implications for doping effects on photochemical processes are discussed. As metal oxides are major components of the environment, including atmospheric mineral aerosol, DFT was also used to study the effect of transition metal impurities on gas/surface interactions of a model acidic atmospheric gas molecule, carbon monoxide (CO). The theoretical results indicated that the presence of Fe³⁺ and Cr³⁺ impurities substituted on the outer layer of natural corundum surfaces reduces the propensity toward CO adsorption relative to the undoped surface. However, CO–surface interactions resemble that of bulk α-Al₂O₃ when the impurity is substituted below the first surface layer. The presence and location of the mineral dopant were found to significantly alter the structural and electronic properties and gas/surface interactions studied here

Computational Studies of CO₂ Activation via Photochemical Reactions with Reduced Sulfur Compounds

Reactions between CO₂ and reduced sulfur compounds (RSC), H₂S and CH₃SH, were investigated using ground and excited state density functional theory (DFT) and coupled cluster (CC) methods to explore possible RSC oxidation mechanisms and CO₂ activation mechanisms in the atmospheric environment. Ground electronic state calculations at the CR-CC­(2,3)/6-311+G­(2df,2p)//CAM-B3LYP/6-311+G­(2df,2p) level show proton transfer as a limiting step in the reduction of CO₂ with activation energies of 49.64 and 47.70 kcal/mol, respectively, for H₂S and CH₃SH. On the first excited state surface, CR-EOMCC­(2,3)/6-311+G­(2df,2p)//CAM-B3LYP/6-311+G­(2df,2p) calculations reveal that energies of <250 nm are needed to form H₂S–CO₂ and CH₃SH–CO₂ complexes allowing facile hydrogen atom transfer. Once excited, all reaction intermediates and transition states are downhill energetically showing either C–H or C–S bond formation in the excited state whereas only C–S bond formation was found in the ground state. Environmental implications of these data are discussed with a focus on tropospheric reactions between CO₂ and RSC, as well as potential for carbon sequestration using photocatalysis

Dissolution of Hematite Nanoparticle Aggregates: Influence of Primary Particle Size, Dissolution Mechanism, and Solution pH

The size-dependent dissolution of nanoscale hematite (8 and 40 nm α-Fe₂O₃) was examined across a broad range of pH (pH 1–7) and mechanisms including proton- and ligand- (oxalate-) promoted dissolution and dark (ascorbic acid) and photochemical (oxalate) reductive dissolution. Empirical relationships between dissolution rate and pH revealed that suspensions of 8 nm hematite exhibit between 3.3- and 10-fold greater reactivity per unit mass than suspensions of 40 nm particles across all dissolution modes and pH, including circumneutral. Complementary suspension characterization (i.e., sedimentation studies and dynamic light scattering) indicated extensive aggregation, with steady-state aggregate sizes increasing with pH but being roughly equivalent for both primary particles. Thus, while the reactivity difference between 8 and 40 nm suspensions is generally greater than expected from specific surface areas measured via N₂–BET or estimated from primary particle geometry, loss of reactive surface area during aggregation limits the certainty of such comparisons. We propose that the relative reactivity of 8 and 40 nm hematite suspensions is best explained by differences in the fraction of aggregate surface area that is reactive. This scenario is consistent with TEM images revealing uniform dissolution of aggregated 8 nm particles, whereas 40 nm particles within aggregates undergo preferential etching at edges and structural defects. Ultimately, we show that comparably sized hematite aggregates can exhibit vastly different dissolution activity depending on the nature of the primary nanoparticles from which they are constructed, a result with wide-ranging implications for iron redox cycling

Combination of Argentophilic and Perfluorophenyl-Perfluorophenyl Interactions Supports a Head-to-Head [2 + 2] Photodimerization in the Solid State

Face-to-face perfluorophenyl–perfluorophenyl interactions (C₆F₅···C₆F₅) are achieved in a disilver metal–organic complex. The C₆F₅···C₆F₅ interactions along with argentophilic forces support <i>trans</i>-pentafluorostilbazole to undergo a head-to-head [2 + 2] photodimerization to form a cyclobutane that sustains a fluorinated two-dimensional metal–organic framework

Effect of Phosphate Salts (Na₃PO₄, Na₂HPO₄, and NaH₂PO₄) on Ag₃PO₄ Morphology for Photocatalytic Dye Degradation under Visible Light and Toxicity of the Degraded Dye Products

Ag₃PO₄ was synthesized by the precipitation method using three different types of phosphate salts (Na₃PO₄, Na₂HPO₄, and NaH₂PO₄) as a precipitating agent. Hydrolysis of each phosphate salt gave a specific pH that affected the purity and morphology of the prepared Ag₃PO₄. The Ag₃PO₄ prepared from Na₂HPO₄ showed the best photocatalytic activity induced by visible light to degrade methylene blue dye. During the photocatalytic process, Ag₃PO₄ decomposed and produced metallic Ag, and this evidence was confirmed by the X-ray diffraction technique and X-ray photoelectron spectroscopy. The photocatalytic efficiency decreased with the number of recycles used. This Ag₃PO₄ photocatalyst also degraded another cationic dye, rhodamine B, but did not degrade reactive orange, an anionic dye. The degraded products produced by the photocatalysis had lower toxicities than the untreated dyes using Chlorella vulgaris as a bioindicator

Heterogeneous Atmospheric Chemistry of Lead Oxide Particles with Nitrogen Dioxide Increases Lead Solubility: Environmental and Health Implications

Heterogeneous chemistry of nitrogen dioxide with lead-containing particles is investigated to better understand lead metal mobilization in the environment. In particular, PbO particles, a model lead-containing compound due to its widespread presence as a component of lead paint and as naturally occurring minerals, massicot, and litharge, are exposed to nitrogen dioxide at different relative humidity. X-ray photoelectron spectroscopy (XPS) shows that upon exposure to nitrogen dioxide the surface of PbO particles reacts to form adsorbed nitrates and lead nitrate thin films with the extent of nitrate formation relative humidity dependent. NO₂-exposed PbO particles are found to have an increase in the amount of lead that dissolves in aqueous suspensions at circumneutral pH compared to particles not exposed. These results point to the potential importance and impact that heterogeneous chemistry with trace atmospheric gases can have on increasing solubility and therefore the mobilization of heavy metals, such as lead, in the environment. This study also shows that surface intermediates that form, such as adsorbed lead nitrates, can yield higher concentrations of lead in water systems. These water systems can include drinking water, groundwater, estuaries, and lakes

Heterogeneous Uptake and Adsorption of Gas-Phase Formic Acid on Oxide and Clay Particle Surfaces: The Roles of Surface Hydroxyl Groups and Adsorbed Water in Formic Acid Adsorption and the Impact of Formic Acid Adsorption on Water Uptake

Organic acids in the atmosphere are ubiquitous and are often correlated with mineral dust aerosol. Heterogeneous chemistry and the uptake of organic acids on mineral dust particles can potentially alter the properties of the particle. In this study, heterogeneous uptake and reaction of formic acid, HCOOH, the most abundant carboxylic acid present in the atmosphere, on oxide and clays of the most abundant elements, Si and Al, present in the Earth’s crust are investigated under dry and humid conditions. In particular, quantitative adsorption measurements using a Quartz Crystal Microbalance (QCM) coupled with spectroscopic studies using Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy are combined to allow for both quantification of the amount of uptake and identification of distinct adsorbed species formed on silica, alumina, and kaolinite particle surfaces at 298 K. These oxides and clay particles show significant differences in the extent and speciation of adsorbed HCOOH due to inherent differences in surface −OH group reactivity. Adsorbed water, controlled by relative humidity, can increase the irreversible uptake of formic acid. Interestingly, the resulting layer of adsorbed formate on the particle surface decreases the particle hydrophilicity thereby decreasing the amount of water taken up by the surface as measured by QCM. Atmospheric implications of this study are discussed

Formation of Iron(III) (Hydr)oxides on Polyaspartate- and Alginate-Coated Substrates: Effects of Coating Hydrophilicity and Functional Group

To better understand the transport of contaminants in aqueous environments, we need more accurate information about heterogeneous and homogeneous nucleation of iron­(III) hydroxide nanoparticles in the presence of organics. We combined synchrotron-based grazing incidence small-angle X-ray scattering (GISAXS) and SAXS and other nanoparticle and substrate surface characterization techniques to observe iron­(III) (hydr)­oxide [10^–4 M Fe(NO₃)₃ in 10 mM NaNO₃] precipitation on quartz and on polyaspartate- and alginate-coated glass substrates and in solution (pH = 3.7 ± 0.2). Polyaspartate was determined to be the most negatively charged substrate and quartz the least; however, after 2 h, total nanoparticle volume calculationsfrom GISAXSindicate that positively charged precipitation on quartz is twice that of alginate and 10 times higher than on polyaspartate, implying that electrostatics do not govern iron­(III) (hydr)­oxide nucleation. On the basis of contact angle measurements and surface characterization, we concluded that the degree of hydrophilicity may control heterogeneous nucleation on quartz and organic-coated substrates. The arrangement of functional groups at the substrate surface (−OH and −COOH) may also contribute. These results provide new information for elucidating the effects of polymeric organic substrate coatings on the size, volume, and location of nucleating iron hydroxides, which will help predict nanoparticle interactions in natural and engineered systems