Metal Sulfide Ion Exchangers: High Acid Stability of Na_2xMg_2y–xSn_4–yS₈ (NMS) and Topotactic Conversion to 2D Solid Acids with Semiconducting Character

Abstract

Metal sulfide ion exchange materials (MSIEs) are of interest for nuclear waste remediation applications. We report the high stability of two structurally related metal sulfide ion exchange materials, Na2x Mg2y–x Sn4–y S8 (Mg-NMS) and Na2SnS3 (Na-NMS), in strongly acid media, in addition to the preparation of Na2x Ni2y–x Sn4–y S8 (Ni-NMS). Their formation progress during synthesis is studied with in-situ methods, with the target phases appearing in <15 min, reaction completion in <12 h, and high yields (75–80%). Upon contact with nitric or hydrochloric acid, these materials topotactically exchange Na+ for H+, proceeding in a stepwise protonation pathway for Na5.33Sn2.67S8. Na-NMS is stable in 2 M HNO3 and Mg-NMS is stable in 4 M HNO3 for up to 4 h, while both NMS materials are stable in 6 M HCl for up to 4 days. However, the treatment of Mg-NMS and Na-NMS with 2–6 M H2SO4 reveals a much slower protonation process since after 4 h of contact both NMS and HMS are present in the solution. The resultant protonated materials, H2x Mg2y–x Sn4–y S8 and H4x [(H y Na y–1)1.33x Sn4––1.33x ]S8, are themselves solid acids and readily react with and intercalate a variety of organic amines, where the band gap of the resultant adduct is influenced by amine choice and can be tuned within the range of 1.88(5)–2.27(5) eV. The work function energy values for all materials were extracted from photoemission yield spectroscopy in air (PYSA) measurements and range from 5.47 (2) to 5.76 (2) eV, and the relative band alignments of the materials are discussed. DFT calculations suggest that the electronic structure of Na2MgSn3S8 and H2MgSn3S8 makes them indirect gap semiconductors with multi-valley band edges, with carriers confined to the [MgSn3S8]2– layers. Light electron effective masses indicate high electron mobilities.

Leveraging Hard–Soft Acid–Base Interactions for Effective Palladium Capture in Acidic Solutions

“…Recently, we highlighted the stability of two members of the NMS family�specifically, Na-NMS and Mg-NMS, demonstrating their robustness under highly acidic conditions for varying time durations, from several hours to a couple of days. 38 Herein, we report the efficient capture of Pd 2+ in extreme acidic solutions by using a layered potassium thiostannate, namely, K 2x Sn 4−x S 8−x (x = 0.65−1, KTS-3) (Figures 1 and 2). 39 The selection of KTS-3 is underpinned by its unique crystal structure and chemical properties.…”

“…The strong affinity of Lewis soft base site S 2– in the anionic frameworks for relatively soft Lewis acidic metal ions, along with the easily exchangeable cations located in the interlayer space or channels, and the flexibility of their structures make these materials excellent ion-exchange candidates for the capture of radioactive (e.g., Cs + , Sr 2+ , UO 2 2+ , Th 4+ , Ln 3+ ) and heavy metal ions (e.g., Ba 2+ , Co 2+ , Ni 2+ , Pb 2+ , Hg 2+ , Cd 2+ ). ,− Reports on MSIEs for Pd 2+ removal are quite limited, mainly focusing on metal sulfide nanoparticles (such as SnS, CuS, and MoS 2 ). ,, In addition, the sorption studies were conducted in hydrochloric acid solutions with higher pH values compared to the conditions that typically prevail in spent nuclear waste. ,, Moreover, although HLLWs generally contain high concentrations of nitric acid, there are rare examples in the literature of sorbents that can effectively remove Pd 2+ under such harsh conditions. Recently, we highlighted the stability of two members of the NMS familyspecifically, Na-NMS and Mg-NMS, demonstrating their robustness under highly acidic conditions for varying time durations, from several hours to a couple of days …”