A Magnetically Recoverable Fe3O4-NH2-Pd Sorbent for Capture of Mercury from Coal Derived Fuel Gas

Review on magnetic adsorbents for removal of elemental mercury from coal combustion flue gas

Under the influence of human activities, atmospheric mercury (Hg) concentrations have increased by 450% compared with natural levels. In the context of the Minamata Convention on Mercury, which came into effect in August 2017, it is imperative to strengthen Hg emission controls. Existing Air Pollution Control Devices (APCDs) combined with collaborative control technology can effectively remove Hg2+ and Hgp; however, Hg0 removal is substandard. Compared with the catalytic oxidation method, Hg0 removal through adsorbent injection carries the risk of secondary release and is uneconomical. Magnetic adsorbents exhibit excellent recycling and Hg0 recovery performance and have recently attracted the attention of researchers. This review summarizes the existing magnetic materials for Hg0 adsorption and discusses the removal performances and mechanisms of iron, carbon, mineral-based, and magnetosphere materials. The effects of temperature and different flue gas components, including O2, NO, SO2, H2O, and HCl, on the adsorption performance of Hg0 are also summarized. Finally, different regeneration methods are discussed in detail. Although the research and development of magnetic adsorbents has progressed, significant challenges remain regarding their application. This review provides theoretical guidance for the improvement of existing and development of new magnetic adsorbents.

Insights into the Adsorption, Alloy Formation, and Poisoning Effects of Hg on Monometallic and Bimetallic Adsorbents

The removal of elemental mercury (Hg⁰) from coal-derived syngas at high temperatures is desired to improve the thermal efficiency of the coal-to-chemical processes. First-principles density functional theory (DFT) calculations for Hg⁰ adsorption are performed using different exchange correlation functionals (PBE, optPBE-vdW, and optB88-vdW). Gibbs free energy (ΔG) calculations are further performed to evaluate the feasibility of Hg⁰ adsorption on various exposed planes of metal nanoparticles and to obtain bimetallic compositions for Hg⁰ removal at various temperatures. Pd and Pt are shown to be suitable for Hg⁰ adsorption at high temperatures (473 K), whereas Rh and Ru are effective only until 373 K. The bimetallic adsorbents comprising Ag or Au along with Rh, Ru, Pd, or Pt are identified for Hg⁰ removal at high temperatures (473 K). The increase in Hg⁰ adsorption strength on various bimetallic surfaces is correlated to the upward shift in the d-band center. Further, calculations predict the tendency of Hg to segregate toward the surface of amalgams and disturb the perfect planar geometry of the Pd, Pt, Rh, Ru, Ir, Cu, Ag, and Au surfaces to form a noncrystalline Hg-rich amalgam surface. An analysis of the binding of various adsorbates (H, O, N, and S) shows that the adsorption becomes significantly weaker on various sites in close proximity to pre-adsorbed Hg. Moreover, for specific combinations of the adsorbate, surface composition, and the site location, the adsorption does not take place on the proximal sites. These results are complemented by the partial density of states calculations, which show changes in the electronic properties of the amalgam surface, thus explaining the poisoning effect of Hg on metallic catalysts.

Defining the Qualities of High-Quality Palladium on Carbon Catalysts for Hydrogenolysis

Palladium-catalyzed hydrogenolysis is often the final step in challenging natural product total syntheses and a key step in industrial processes producing fine chemicals. Here, we demonstrate that there is wide variability in the efficiency of commercial sources of palladium on carbon (Pd/C) resulting in significant differences in selectivity, reaction times, and yields. We identified the physicochemical properties of efficient catalysts for hydrogenolysis: (1) small Pd/PdO particle size (2) homogeneous distribution of Pd/PdO on the carbon support, and (3) palladium oxidation state are good predictors of catalytic efficiency. Now chemists can identify and predict a catalyst's efficiency prior to the use of valuable synthetic material and time.

Shaping Magnetite by Hydroxyl Group Numbers of Small Molecules

Despite numerous reports on magnetite formation with the assistance of various additives, the role of hydroxyl group (-OH) numbers in small polyol molecules has not yet been understood well. We selected small molecules containing different -OH numbers, such as ethanol, ethylene glycol, propanetriol, butanetetrol, pentitol, hexanehexol, and cyclohexanehexol, as additives in coprecipitation. By increasing the -OH number in these small polyol molecules, the formation of crystallization was slowed, and the size and shape of magnetite were regulated as well possibly due to the changed complexation strength and the stability of the precursor. The increase in temperature and the Fe²⁺/Fe³⁺ ratio can reduce the complexation strength. The nucleation and growth of magnetite proceed possibly through the aggregation of polyol-stabilized amorphous complexes and two-line ferrihydrite with low crystallinity based on the -OH numbers, suggesting a nonclassical pathway. The as-prepared magnetite showed a r₂/r₁ ratio after in vitro MRI measurement as follows: Fe₃O₄@He-6OH rod < Fe₃O₄@Pr-3OH sheet < Fe₃O₄@Pe-5OH cube. The Fe₃O₄@He-6OH rod and Fe₃O₄@Pr-3OH sheet displayed T₁-T₂ dual modal contrast ability, while the Fe₃O₄@Pe-5OH cube can be T₂-dominated. This research provides a simple but an essential approach for designing MRI contrast agents.

Effect of impregnation sequence of Pd/Ce/γ-Al2O3 sorbents on Hg0 removal from coal derived fuel gas

This study attempted to investigate the effect of impregnation sequence of the Pd/Ce/γ-Al₂O₃ sorbents on Hg⁰ removal. To this end, five kinds of sorbents were prepared and tested in simulated coal derived fuel gas (N₂-H₂-CO-H₂S-Hg), including Pd/γ-Al₂O₃, Ce/γ-Al₂O₃ and three kinds of Pd-based sorbents with Ce impregnation on γ-Al₂O₃ substrate. The tests were conducted at 250 and 300 °C respectively. According to the results, bimetallic Ce-Pd/γ-Al₂O₃ sorbent prepared by simultaneously impregnating Pd and Ce showed much higher and more stable removal efficiency of Hg⁰ than the other three kinds of sorbents. The Hg⁰ removal efficiency of Ce-Pd/γ-Al₂O₃ sorbent reached above 98% within 480 min at 250 °C and 91% within 200 min at 300 °C. Characterization results indicated that the sorbent Ce-Pd/γ-Al₂O₃ prepared by the co-impregnation method had bigger specific surface area (216.6 m²/g) than the other three kinds of Pd-based sorbents. The content Pd and Ce on the sorbent Ce-Pd/γ-Al₂O₃ surface is 0.21% and 0.61%, which proved higher than that of the other three kinds of Pd-based sorbents, and observation from STEM-XEDS maps showed it demonstrated the highest dispersion. It is found that Ce is likely to promote the dispersion of Pd on the support surface during the preparation of the sorbent under the co-impregnation method. Meanwhile, Ce enhanced the H₂S resistance of the sorbent. Thereby, Ce-Pd/γ-Al₂O₃ sorbent is found to have the optimal performance of mercury removal. In this study, the Hg⁰ removal mechanism of the Pd/Ce/γ-Al₂O₃ sorbents in the simulated coal derived fuel gas was also elaborated.

AMn2O4 (A=Cu, Ni and Zn) sorbents coupling high adsorption and regeneration performance for elemental mercury removal from syngas

AMn₂O₄ (A= Cu, Ni and Zn) spinel sorbents synthesized by a low-temperature sol-gel auto-combustion method were for the first time used to eliminate elemental mercury (Hg⁰) from syngas. CuMn₂O₄ sorbent exhibits the highest Hg⁰ adsorption performance under simulated syngas, higher than 95 % Hg⁰ capture efficiency is obtained at 200 °C. Adsorption-regeneration experiments demonstrate that the regenerability of CuMn₂O₄ is excellent. The influences of syngas compositions on Hg⁰ elimination over CuMn₂O₄ were examined. H₂S plays the most important role in Hg⁰ removal from syngas, which can be adsorbed on CuMn₂O₄ surface and transformed into active sulfur species to react with Hg⁰ to form surface-bonded HgS. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) experiments prove that HgS is formed on the spent sorbent. The surface Cu²⁺ cations and chemisorbed/lattice oxygens participate in Hg⁰ adsorption and transformation. HCl promotes Hg⁰ removal by forming surface active chlorine species. H₂, CO, and H₂O inhibit Hg⁰ removal under N₂ atmosphere. However, they exhibit no obvious effect on Hg⁰ removal with the assistance of H₂S. The excellent Hg⁰ capture performance, good regenerability and H₂O resistance of CuMn₂O₄ make it to be a very promising sorbent for Hg⁰ removal from syngas at higher temperature.

Heterogeneous photocatalysis and its potential applications in water and wastewater treatment: a review

There has been a considerable amount of research in the development of sustainable water treatment techniques capable of improving the quality of water. Unavailability of drinkable water is a crucial issue especially in regions where conventional drinking water treatment systems fail to eradicate aquatic pathogens, toxic metal ions and industrial waste. The research and development in this area have given rise to a new class of processes called advanced oxidation processes, particularly in the form of heterogeneous photocatalysis, which converts photon energy into chemical energy. Advances in nanotechnology have improved the ability to develop and specifically tailor the properties of photocatalytic materials used in this area. This paper discusses many of those photocatalytic nanomaterials, both metal-based and metal-free, which have been studied for water and waste water purification and treatment in recent years. It also discusses the design and performance of the recently studied photocatalytic reactors, along with the recent advancements in the visible-light photocatalysis. Additionally, the effects of the fundamental parameters such as temperature, pH, catalyst-loading and reaction time have also been reviewed. Moreover, different techniques that can increase the photocatalytic efficiency as well as recyclability have been systematically presented, followed by a discussion on the photocatalytic treatment of actual wastewater samples and the future challenges associated with it.