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• Fe(OH) 3 and α-FeOOH show highest efficiency for H 2 S/O 2 removal among the 35 samples. • BET surface area and Bronsted acid sites drive adsorption of H 2 S and O 2. • Performance of α-FeOOH was substantiated in a pilot plant with raw biogas. This study deals with the development of iron oxide/hydroxide adsorbents for the alternating removal of H 2 S and O 2 from natural gas and biogas. The considered procedure initially implies the chemisorption of H 2 S. In the second step, O 2 is removed by reaction with the H 2 S-treated substrate accompanied by regeneration of the adsorbent. 35 different iron-based samples were physical-chemically characterized and tested towards H 2 S/O 2 break through behavior and adsorption capacity to evaluate structure-performance correlations. It was derived that high BET surface area, high proportion of meso and micro pores, small average pore diameter, low crystallinity and high number of Bronsted acid sites are crucial properties of the adsorbents. As a consequence, Fe(OH) 3 and α-FeOOH revealed highest efficiency for the successive adsorption of H 2 S and O 2 , while α-Fe 2 O 3 provided rather low performance. Mechanistic investigations were carried out with α -FeOOH and α -Fe 2 O 3 indicating rapid dissociation of H 2 S on α -FeOOH, while slow molecular adsorption prevailed on α -Fe 2 O 3. With proceeding H 2 S exposure, the adsorbents were partially transformed into sulfide and sulfate species with some formation of elemental sulfur. In subsequent reaction with O 2 , the H 2 S-exposed samples were re-oxidized under partial restructuring, whereas elemental sulfur and sulfate entities were additionally produced. Finally, the concept of the alternating adsorption of H 2 S and O 2 on best α -FeOOH substrate was technically evaluated in a biogas pilot plant. The investigations showed efficient conversion of H 2 S for 40–100 h, while subsequent O 2 conversion was achieved for ca. 5.5 h. [ABSTRACT FROM AUTHOR]

[Display omitted] • Alternating H 2 S and O 2 adsorption on Fe oxide/hydroxides was studied. • Bronsted acid sites were shown crucial for H 2 S uptake. • H 2 S adsorption led to strong transformation of the adsorbents into FeS and S 8. • O 2 exposure provided incomplete regeneration of the sulfurized adsorbents. Iron oxides are useful adsorbents for the alternating removal of H 2 S and O 2 from natural gas and biogas. In this context, the present paper aims to elucidate the mechanisms behind the consecutive adsorption of both gases. Three different types of iron oxides/hydroxides were investigated using in-situ infrared and Raman spectroscopy as well as ex-situ X-ray diffraction. The spectroscopic studies confirm that H 2 S is adsorbed on all kinds of surface sites, particularly at Brønsted and Lewis acid sites. However, on the Brønsted acid sites, H 2 S undergoes dissociation, leading to the formation of SH−, S2− and H 2 O. This mechanism substantiates the high H 2 S uptake capacity of the OH-containing adsorbents. Prolonged H 2 S adsorption provokes an increasing consumption of the adsorbent, yielding amorphous FeS and elemental sulfur, with hydrogen sulfate and sulfate species also forming as by-products. Furthermore, the subsequent O 2 adsorption on the sulfidized samples causes some re-formation of the initial adsorbent with the simultaneous production of elemental sulfur and sulfate entities. As a result of the mechanistic understanding gained in this study, it is concluded that the complete regeneration of the iron substrates is almost impossible due to their substantial degradation, associated with partial conversion into sulfates and the deposition of sulfur. [ABSTRACT FROM AUTHOR]