Pd/activated carbon sorbents for mid-temperature capture of mercury from coal-derived fuel gas

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.

Simultaneous Removal of Hg0 and H2S over a Regenerable Fe2O3/AC Catalyst

Simultaneous removal of Hg0 and H2S over a regenerable activated coke supported Fe2O3 catalyst (Fe2O3/AC) was studied in simulated coal-derived syngas. It was found that the Fe2O3/AC catalyst exhibited high capability for Hg0 and H2S removal, which was attributed to the catalytic oxidation activity of Fe2O3. Its capability for Hg0 and H2S removal increased with an increase of Fe2O3 loading amount, and the highest was at 150 °C for Hg0 removal. CO and H2 showed no obvious effect on Hg0 removal by Fe2O3/AC, while H2S had a promotion effect, which was due to S and FeSx produced by the H2S reaction on Fe2O3/AC. The results of SEM-EDX and the temperature programmed desorption experiment (TPD) revealed that Fe2O3 played a critical role in Hg0 oxidation, and HgS was generated upon the reaction of Hg0 with H2S on Fe2O3/AC. The used Fe2O3/AC catalyst after Hg0 and H2S removal could be effectively regenerated and still had high capability for Hg0 and H2S removal.

Role of NO and SO2 in mercury oxidation over a La2O3/Fe2O3 catalyst with high thermal stability

In this study, the thermal stability of a ferric oxide catalyst for mercury oxidation was found to be considerably promoted by doping with La₂O₃. The catalysts doped with La₂O₃ maintained a higher surface area when subjected to high-temperature calcination, with lower average pore size and a narrower pore size distribution. X-ray diffraction (XRD) results revealed that La₂O₃ doping hinders the growth of catalyst particles and crystallization of the material at high temperatures. Both NO and SO₂ inhibited Hg⁰ oxidation over the La₂O₃/Fe₂O₃ catalyst. Fourier transform infrared (FTIR) spectra revealed that SO₂ reacts with O₂ over the catalysts to form several species that are inert for mercury oxidation, such as SO₄^2-, HSO₄^-, or other related species; these inert species cover the catalyst surface and consequently decrease Hg⁰ oxidation capacity. In addition, NO or SO₂ competed with Hg⁰ for active sites on the La₂O₃/Fe₂O₃ catalyst and hindered the adsorption of mercury, thereby inhibiting subsequent Hg⁰ oxidation. Hg⁰ oxidation on the La₂O₃/Fe₂O₃ catalyst mainly followed the Eley-Rideal mechanism. Moreover, the inhibition effects of NO and SO₂ were at least partially reversible, and the catalytic activity was temporarily restored after eliminating NO or SO₂.

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.

Adsorption and desorption of SO2, NO and chlorobenzene on activated carbon

Activated carbon (AC) is very effective for multi-pollutant removal; however, the complicated components in flue gas can influence each other's adsorption. A series of adsorption experiments for multicomponents, including SO2, NO, chlorobenzene and H2O, on AC were performed in a fixed-bed reactor. For single-component adsorption, the adsorption amount for chlorobenzene was larger than for SO2 and NO on the AC. In the multi-component atmosphere, the adsorption amount decreased by 27.6% for chlorobenzene and decreased by 95.6% for NO, whereas it increased by a factor of two for SO2, demonstrating that a complex atmosphere is unfavorable for chlorobenzene adsorption and inhibits NO adsorption. In contrast, it is very beneficial for SO2 adsorption. The temperature-programmed desorption (TPD) results indicated that the binding strength between the gas adsorbates and the AC follows the order of SO2>chlorobenzene > NO. The adsorption amount is independent of the binding strength. The presence of H2O enhanced the component effects, while it weakened the binding force between the gas adsorbates and the AC. AC oxygen functional groups were analyzed using TPD and X-ray photoelectron spectroscopy (XPS) measurements. The results reveal the reason why the chlorobenzene adsorption is less affected by the presence of other components. Lactone groups partly transform into carbonyl and quinone groups after chlorobenzene desorption. The chlorobenzene adsorption increases the number of C=O groups, which explains the positive effect of chlorobenzene on SO2 adsorption and the strong NO adsorption.

Effect of the properties of MnOx/activated carbon and flue gas components on Hg0 removal at low temperature

Proposed removal mechanisms of Hg⁰ under a simulated multi-component gas over the Mn/AC sorbent.

Role of NO in Hg(0) oxidation over a commercial selective catalytic reduction catalyst V2O5-WO3/TiO2

Experiments were conducted in a fixed-bed reactor that contained a commercial catalyst, V2O5-WO3/TiO2, to investigate mercury oxidation in the presence of NO and O2. Mercury oxidation was improved by NO, and the efficiency was increased by simultaneously adding NO and O2. With NO and O2 pretreatment at 350°C, the catalyst exhibited higher catalytic activity for Hg(0) oxidation, whereas NO pretreatment did not exert a noticeable effect. Decreasing the reaction temperature boosted the performance of the catalyst treated with NO and O2. Although NO promoted Hg(0) oxidation at the very beginning, excessive NO counteracted this effect. The results show that NO plays different roles in Hg(0) oxidation; NO in the gaseous phase may directly react with the adsorbed Hg(0), but excessive NO hinders Hg(0) adsorption. The adsorbed NO was converted into active nitrogen species (e.g., NO2) with oxygen, which facilitated the adsorption and oxidation of Hg(0). Hg(0) was oxidized by NO mainly by the Eley-Rideal mechanism. The Hg(0) temperature-programmed desorption experiment showed that weakly adsorbed mercury species were converted to strongly bound ones in the presence of NO and O2.

Mechanism of Hg(0) oxidation in the presence of HCl over a commercial V2O5-WO3/TiO2 SCR catalyst

Experiments were conducted in a fixed-bed reactor containing a commercial V2O5/WO3/TiO2 catalyst to investigate mercury oxidation in the presence of HCl and O2. Mercury oxidation was improved significantly in the presence of HCl and O2, and the Hg(0) oxidation efficiencies decreased slowly as the temperature increased from 200 to 400°C. Upon pretreatment with HCl and O2 at 350°C, the catalyst demonstrated higher catalytic activity for Hg(0) oxidation. Notably, the effect of pretreatment with HCl alone was not obvious. For the catalyst treated with HCl and O2, better performance was observed with lower reaction temperatures. The results showed that both HCl and Hg(0) were first adsorbed onto the catalyst and then reacted with O2 following its adsorption, which indicates that the oxidation of Hg(0) over the commercial catalyst followed the Langmuir-Hinshelwood mechanism. Several characterization techniques, including Hg(0) temperature-programmed desorption (Hg-TPD) and X-ray photoelectron spectroscopy (XPS), were employed in this work. Hg-TPD profiles showed that weakly adsorbed mercury species were converted to strongly bound species in the presence of HCl and O2. XPS patterns indicated that new chemisorbed oxygen species were formed by the adsorption of HCl, which consequently facilitated the oxidation of mercury.

Mercury adsorption characteristics of HBr-modified fly ash in an entrained-flow reactor

In this study, the mercury adsorption characteristics of HBr-modified fly ash in an entrained-flow reactor were investigated through thermal decomposition methods. The results show that the mercury adsorption performance of the HBr-modified fly ash was enhanced significantly. The mercury species adsorbed by unmodified fly ash were HgCl2, HgS and HgO. The mercury adsorbed by HBr-modified fly ash, in the entrained-flow reactor, existed in two forms, HgBr2 and HgO, and the HBr was the dominant factor promoting oxidation of elemental mercury in the entrained-flow reactor. In the current study, the concentration of HgBr2 and HgO in ash from the fine ash vessel was 4.6 times greater than for ash from the coarse ash vessel. The fine ash had better mercury adsorption performance than coarse ash, which is most likely due to the higher specific surface area and longer residence time.