(128a) Experimental Investigations and Mechanistic Insights into the Sorptive Removal of Elemental Gas Phase Mercury (Hg0) from Syngas over Pt-Based Sorbents

Mercury, a toxic heavy metal, is a trace component in carbonaceous feedstocks, viz., coal, biomass, and municipal solid wastes, and gets released during their combustion and gasification in its elemental form (Hg⁰). The utilization of syngas derived from the gasification of carbonaceous feedstocks to produce valuable chemicals, demands the syngas to be free from Hg⁰ which can poison the sensitive catalyst (s) employed downstream. Hg⁰ is volatile and insoluble in water and cannot be trapped using conventional air pollution control equipment, due to which an alternative technology is required for its removal. Additionally, the capture of Hg⁰ from syngas is favored at elevated temperatures (>200 ºC) to improve the overall thermal efficiency of the interconnected gasification and chemical processes. However, the commercially available carbon and sulfur-based adsorbents are inefficient for the removal of Hg⁰ from syngas at high temperatures. DFT coupled ∆G calculations have shown Hg⁰ adsorption to be feasible on Pt surfaces at high temperatures (>200 ºC) [1].

In the present work, a series of Pt-based sorbents are synthesized using different support materials, and their efficacy in Hg⁰ removal at high temperatures is evaluated. XPS analysis of the fresh and spent sorbents is used to investigate the mechanism for Hg⁰ removal, and two different support-dependent pathways are proposed using results from XPS, XRD, and H₂-TPR analysis. Experiments are performed to elucidate the effect of temperature and the role of different syngas components on Hg⁰ removal. Further, the effect of syngas components H₂ and H₂O is shown to vary over different sorbents, and the negative effect of CO on Hg⁰ removal over all sorbents is explained using in-situ DRIFTS studies.

Methodology

The Pt-based monometallic adsorbents shortlisted from DFT calculations are prepared using the incipient wetness method using doped aluminas (SIRAL-40, SASOL), metal oxides (Ceria, zirconia, titania) and silica-based molecular sieve (SBA-15) as support materials. The performance of these sorbents is investigated using a bench-scale reactor system at high temperatures (200 to 350 ºC) using a Hg⁰ feed concentration of 25 ± 6 µg/m³ and by maintaining a constant gas hourly space velocity (GHSV) of 1.2×10⁵ h^-1. The physio-chemical properties of the sorbent are determined using BET), XRD, and H₂-TPR. XPS analysis of the spent sorbents exposed to Hg⁰ for 10 hours is performed to investigate the Hg⁰ removal mechanism. The effect of various syngas components viz., CO, CO₂, H₂, and H₂O on Hg⁰ removal performance at 270 ºC is shown, and the effect of CO on Hg⁰ removal performance is investigated using in-situ DRIFTS.

Results and Discussion

The narrow spectrum of Pt 4f for fresh and spent Pt/SBA-15 is given in Fig. 1 (a), wherein the two peaks at 71.8 eV and 75.1 eV indicate metallic Pt (Pt⁰). Following Hg⁰ adsorption, the Pt 4f spectra of spent Pt/SBA-15 exhibit a gradual shift towards lower binding energy, suggesting alterations in the Pt states. Furthermore, deconvolution of the XPS spectra reveals two new peaks at 71.1 eV and 74.4 eV on spent Pt/SBA-15, indicative of the formation of a Pt-Hg amalgam after Hg⁰ adsorption [2]. Similarly, the XPS analysis of the other Pt-based sorbents is performed and Hg⁰ removal over all sorbents is proposed to occur by Hg⁰ adsorption and formation of a Hg-Pt amalgam. From the XPS spectra of O 1s on fresh and spent Pt/ZrO₂, a decrease in the proportion of adsorbed oxygen (O_β) is observed after Hg⁰ adsorption (Fig. 1 (b)), based on which Hg⁰ oxidation is proposed to occur. Similar inferences are obtained for Pt/CeO₂ as well. Characterization results from XRD and H₂-TPR indicate the presence of Pt nanoparticles in the elemental state on Pt/SBA-15, Pt/S-40, and Pt/TiO₂, and the presence of Pt in the oxidized form on Pt/ZrO₂ and Pt/CeO₂. The correlation of these characterization techniques with the adsorption activity would be described in the full manuscript.

The initial Hg⁰ removal efficiency of various Pt-based sorbents is plotted in Fig. 2 (a). Pt/SBA-15, Pt/SIRAL-40 (S-40), and Pt/ZrO₂ demonstrate Hg⁰ removal efficiencies above 90% for temperatures up to 250 ºC. With an increase in the adsorption temperature to 270 ºC, the removal efficiency of Pt/S-40 and Pt/ZrO₂ declines to 91.0 % and 85.4 % respectively, whereas the efficiency of Pt/SBA-15 remains above 97.6 %. However, at a higher adsorption temperature of 350 ºC, the removal efficiency of Pt/ZrO₂ and Pt/CeO₂ is higher than Pt/SBA-15 which could be explained by the occurrence of Hg⁰ oxidation. Pt/SBA-15 is evaluated over varying temperatures for longer time durations, and the results are plotted in Fig. 2 (b). At 200 ºC, the efficiency of Hg⁰ removal declines below 95% after 55 h, whereas this drop occurs much earlier (within 0.68 h) at a higher adsorption temperature of 350 ºC. The decline in efficiency is attributed to the reduction in Hg⁰ solubility in Pt and the initiation of de-amalgamation. The Hg⁰ removal efficiency over Pt/SBA-15 at 270 ºC in N₂ is comparable to that reported for Pd-based sorbents [3], making it suitable for Hg⁰ removal from syngas at high temperatures. Further, a one-dimensional two-phase transient model is developed to predict the spatio-temporal features for Hg⁰ sorption over Pt/SBA-15 under isothermal conditions. The model considers the adsorption of Hg⁰ on the surface of Pt followed by amalgam formation. The model predicts Hg⁰ sorption over a range of temperatures and also the decrease in the adsorption capacity with an increase in temperature from 200 to 350 °C.

The Hg⁰ removal efficiency at 270 °C for various compositions of syngas is shown in Fig. 3. The adsorption efficiency over Pt/S-40, Pt/ZrO₂, Pt/TiO₂, and Pt/CeO₂ reduces significantly in the presence of syngas components CO, H₂ and H₂O thus showing their inhibitive effect towards Hg⁰ adsorption. In contrast, the adsorption efficiency of Pt/SBA-15 is retained above 90% in the presence of H₂ and H₂O. Thus, mesoporous and hydrophobic silica-based SBA-15 is highlighted as a promising support material for Pt-based sorbents in eliminating Hg⁰ from syngas at high temperatures in the presence of H₂O. In-situ DRIFTS experiments with CO in the feed are performed, which show the adsorption of CO and explain its competitive adsorption with Hg⁰ (Fig. 4 (a)). On stopping CO and purging N₂, the CO bands completely disappear indicating the complete desorption of CO. This makes the Pt⁰ sites available for Hg⁰ adsorption thereby explaining the restoration of Hg⁰ removal efficiency over Pt/SBA-15 after stopping the flow of CO (Fig. 4 (b)).

Significance of the work

Pt-based adsorbents for the removal of Hg⁰ at high temperatures exceeding 200 ^oC are developed in contrast to commercial adsorbents which work at much lower temperatures. A combination of reactor experiments and characterization (in-situ DRIFTS, XPS, XRD, and H₂-TPR) is used to propose a support and temperature-dependent Hg⁰ removal mechanism.

References

[1] D. Mhatre, D. Bhatia, Insights into the adsorption, alloy formation, and poisoning effects of Hg on monometallic and bimetallic adsorbents, Langmuir. 38 (2022) 6841–6859.

[2] G.W. Wu, S. Bin He, H.P. Peng, H.H. Deng, A.L. Liu, X.H. Lin, X.H. Xia, W. Chen, Citrate-capped platinum nanoparticle as a smart probe for ultrasensitive mercury sensing, Anal. Chem. 86 (2014) 10955–10960.

[3] L. Han, Q. Li, S. Chen, W. Xie, W. Bao, L. Chang, J. Wang, A magnetically recoverable Fe₃O₄-NH₂-Pd sorbent for capture of mercury from coal derived fuel gas, Sci. Rep. 7 (2017) 2–11.