7 Search Results

Sequestration of CO ₂ within stable mineral carbonates (e.g., CaCO ₃ ) represents an attractive emission reduction strategy because it offers a leakage-free alternative to geological storage of CO ₂ in an environmentally benign form. However, the pH of aqueous streams equilibrated with gaseous streams containing CO ₂ (pH < 4) are typically lower than that which is required for carbonate precipitation (pH > 8). Traditionally, alkalinity is provided by a stoichiometric reagent (e.g., NaOH) which renders these processes environmentally hazardous and economically unfeasible. This work investigates the use of regenerable ion-exchange materials to induce alkalinity in CO ₂ -saturatedmore » aqueous solutions such that the pH shift required for mineralization occurs without the need for stoichiometric reagents. Na ⁺ -H ⁺ exchange isotherms (at [H ⁺ ] = 10 ⁻⁸ –10 ⁻¹  M) and rates were measured for 13X and 4A zeolites and TP-207 and TP-260 organic exchange resins in batch equilibrium and fixed-bed exchange experiments, respectively. At solutions equilibrated with CO ₂ at 1.0 atm (pH = 3.9), H ⁺ exchange capacities for the materials were similar (1.7–2.4 mmol H ⁺ /g material) and resulted in pH increases from 3.9 to greater than 8.0. Multi-component mixtures using Ca ²⁺ and Mg ²⁺ cations (at 10 ⁻³ –10 ⁻¹  M) in CO ₂ -saturated water were used to probe competitive ion exchange. The presence of divalent cations in solution inhibited H ⁺ exchange, reducing capacities to as low as 0.2 mmol H ⁺ /g for both resins and zeolites. Dynamic H ⁺ exchange capacities in fixed-bed ion exchange columns were similar to equilibrium values for resins (∼1.5 mmol/g) and zeolites (∼0.8 mmol/g) using inlet solutions that were equilibrated with gaseous streams of CO ₂ at 1.0 atm. However, exchange kinetics were limited by intraparticle diffusion as indicated by the increased rate parameters with increasing inlet flow rates (20–160 cm ³  min ⁻¹ ). Experimental calcite precipitation from mixing the alkaline CO ₃ ²⁻ -rich water solution obtained from the ion-exchange column with a simulated liquid waste stream solution achieved thermodynamic maximum yields. The results from these studies indicate that ion exchange processes can be used as an alternative to the addition of stoichiometric bases to induce alkalinity for the precipitation of CaCO ₃ , thereby opening a pathway toward sustainable and economic mineralization processes.« less

This work probes the effects of La on reactions involving CuO sorbents and H₂S in gaseous streams. La-CuO sorbents with nominal La/Cu atomic ratios of 1:2 achieved higher capacities (27 and 36 g H₂S per 100 g sorbent) compared to CuO sorbents (25 g H₂S per 100 g sorbent). Here, comparison of the La-CuO sorbents using energy dispersive x-ray spectroscopy and La L₃-edge XAS suggested that increased capacity resulted from more uniform dispersion of La within the CuO phase (3.3 wt.% vs. 1.0 wt.% La). X-Ray absorption near edge structure analysis suggested more electron-rich Cu species in La-CuO samples, whilemore » extended X-ray absorption fine structure modeling showed an increase in single-scattering path lengths for neighboring and near-neighbor atoms. These changes suggest that La induces strain in the Cu-O lattice, thereby increasing H₂S capacity by reducing energy barriers associated with surface reactions between oxygen atoms and adsorbed H₂S molecules.« less

An understanding of the fundamentals of the reaction between CuO with trace amounts of H₂S to form CuS products is critical for the optimal utilization of this process in sulfur removal applications. Unfortunately, CuS is a complex material, featuring various Cu_2-xS compounds (with 0 ≤ x ≤ 1), distorted crystal phases, and varying electronic structures and coordination environments of Cu and S ions. In this work, we combine ex situ and in situ X-ray absorption spectroscopy (XAS) at S and Cu K edges, fixed bed sorption experiments, DFT simulations, and other characterization techniques to speciate the CuS products formed atmore » different temperatures (298–383 K) and from CuO sorbents with different crystallite sizes (2.8–40 nm). The results of our analysis identify the formation of a distorted CuS layer at the surface of CuO crystals with disulfide groups with shorter Cu–S bonds and higher delocalization of the positive charge of the Cu center into (S^1–)2. This distorted CuS layer dominates the XAS signal at lower temperatures (298–323 K) and at the initial stages of sulfidation at higher temperatures (353 and 383 K) where conversion is low (<40%). First-principles atomistic simulations confirm the thermodynamic favorability of the formation of surface (S^1–)2 on both CuO (111) and ($$\bar{1}11$$) surfaces, providing further support for our experimental observations. Furthermore, these simulations reveal that the presence of disulfide bonds stabilized surface hydroxyl groups, leading to lower Gibbs Free Energies of their surface migration.« less

Developing fundamental insight for reactions between gas phase H₂S and solid phase CuO has the potential to lead to improved materials and processes for natural gas purification. However, this insight requires detailed knowledge of the atomistic characteristics of the solid and how these characteristics influence the reaction mechanism and kinetics. Herein, we use fixed bed reactors, X-ray absorption spectroscopy, and transmission X-ray microscopy to simultaneously probe the fundamental kinetics of the reaction of CuO with H₂S to form CuS, and thereby probe spatial-temporal chemical and structural changes of copper during this reaction. H₂S removal reaction kinetics show similar trends inmore » fixed bed reactors as in 10-20 µm sized particles. However, reaction fronts proceed through the entire diameter of particles heterogeneously, indicating the presence of pore diffusion resistance even at very small length scales. In addition, CuO sorbent samples with similar characteristics exhibit 3 times different sulfidation conversion with reaction rate constants that differ by a factor of 1.5. Furthermore, these differences in reaction kinetics and conversion indicate the critical impact of possible atomic scale differences and the formation of different copper sulfide products.« less