An experimental study on the preparation of tochilinite-originated intercalation compounds comprised of Fe 1−xS host layers and various kinds of guest layers

Abstract

Tochilinite represents a mineral group of ordered mixed-layer structures containing alternating Fe 1− x S layers with mackinawite-like structure and metal hydroxide layers with Mg(OH) 2-like structure. In this article, we report the preparation of a series of tochilinite-originated (or Fe 1− x S-based) intercalation compounds (ICs). According to their preparation procedures, these ICs can be divided into four kinds. The first kind of IC was sodium tochilinite (Na-tochilinite), which was prepared by the hydrothermal reaction of metallic Fe particles with concentrated Na 2S·9H 2O aqueous solutions. The hydroxide layer of the Na-tochilinite was a mixed hydroxide of Na + ions along with a certain amount of Fe 2+ ions. When the hydroxide layer of the Na-tochilinite completely dissolved in aqueous solutions, a Fe-deficient mackinawite-like phase Fe 1− x S was obtained, which was probably an electron-deficient p-type conductor. The second kind of ICs was prepared by ‘low-temperature direct intercalation in aqueous solutions, using Na-tochilinite as a parental precursor. When the Na-tochilinite was ultrasonicated in aqueous solutions containing Lewis basic complexing agents (like NH 3, N 2H 4, 2,2′-bipyridine (bipy), and 1,10-phenanthroline (phen)), the Na + ions of the Na-tochilinite were removed and the Lewis basic complexing agents entered the hydroxide layer of the Na-tochilinite and became coordinated with the Fe 2+ ions, and the second kind of ICs was thus produced. The second kind of ICs includes NH 3 IC, N 2H 4 IC, N 2H 4-NH 3 IC, [Fe(bipy) 3] 2+-containing IC and [Fe(phen) 3] 2+-containing IC. The third kind of ICs, which includes NH 3 IC, N 2H 4–NH 3 IC and N 2H 4-LiOH (NaOH) IC, was prepared by the hydrothermal reaction of metallic Fe particles with (NH 4) 2S aqueous solution, S (elemental) + N 2H 4·H 2O aqueous solution, and S + N 2H 4·H 2O + LiOH (NaOH) aqueous solution, respectively. The third kind of ICs has a close relationship with the second kind of ICs both in composition and structure. The fourth kind of ICs was prepared by the oxidation and reduction of some of the N 2H 4-containing ICs mentioned above, which include N 2H 2 (diazene or diimide) IC, N 2 (dinitrogen) IC and NH 3 IC. The N 2H 2 IC was prepared by mild air oxidation of the N 2H 4-LiOH IC. The N 2 IC was prepared by strong air oxidation of the N 2H 4-LiOH IC, however, we have not been able to separate the pure phase N 2 IC. Hydrothermal reduction of the N 2H 4 IC made by the direct intercalation method in strong reducing environment by H 2S + Fe (metal) led to the production of the NH 3 IC of the fourth kind of ICs. The NH 3 ICs prepared by the three methods had similar compositions and structures. As almost all the ICs reported in this paper were extremely sensitive both to air and to the electron beam, they were mainly characterized by XRD. The properties and interrelationships (or mutual transformations) of the Fe 1− x S-based ICs revealed novel chemistry occurring in the sub-nanoscopic space between the micrometer- to nanometer-sized electron-deficient Fe 1− x S layers. An important finding of this novel chemistry was that the Fe 1− x S-based ICs tended to oxidize or reduce the intercalated species when the redox state of their environments varied. The results of our experiments potentially have many cosmochemical implications. The most important implication is that our experimental results, along with previous studies, strongly suggested that some of the ammonium salts, ammonia and carbonates existing in the matrix of the CM carbonaceous chondrites may have been formed by abiotic reactions employing molecular nitrogen as the nitrogen source and carbon monoxide as the carbon source and iron sulfide and/or iron hydroxide as catalysts.