The intellectually and technically challenging pursuit of the emerging global environmentally “green” and energy-efficient infrastructure of the 21st century requires the development of a worldwide network of low- to medium-power fuel cell (FC) based portable electric power-generating devices and high-power biomass/clean coal “electric+chemical plants” with zero carbon footprint utilizing integrated coal gasification combined cycle with geologic carbon sequestration (IGCC-GCS) under energy-efficient low-temperature conditions. These emerging technologies require the deep and ultradeep desulfurization of gaseous feeds, since sulfur compounds, especially hydrogen sulfide H₂S are highly corrosive and poisonous to both technological processes and the environment. Therefore, it is of crucial importance for both academic and industrial research communities to have a solid understanding of the atomic-level structures of active sites and molecular-level mechanisms of surface chemical reactions of the novel deep and ultradeep desulfurization materials, especially desulfurization sorbents. This review critically analyzes the recent literature (last ∼20 years) on the experimental determination of molecular and atomic-level nature of adsorption sites, effects of desulfurization promoters, mechanisms of chemical reactions of H₂S, COS and CS₂ and physical processes during and upon regeneration of “spent” low-temperature H₂S sorbents based on ZnO that were developed for desulfurization of fuel reformates, syngas and similar streams. Recent trends in research on the ultradeep H₂S sorbents are discussed with an impetus on real-time in situ and Operando techniques of instrumental chemical analysis, and the challenges of direct determination of the structure of active sites and of the experimental mechanistic studies in general are described.

Fe- and Mn-promoted H₂S sorbents Fe_x–Mn_y–Zn_1−x−yO/SiO₂ (x, y = 0, 0.025) for desulfurization of model fuel reformates at room temperature were prepared, tested and characterized. Sulfur uptake capacity at 25 °C significantly exceeds that of both commercial unsupported ZnO sorbents and un-promoted supported ZnO/SiO₂ sorbents. Sulfur capacity and breakthrough characteristics remain satisfactory after multiple (∼10) cycles of adsorption/regeneration, with regeneration performed by a simple and robust heating in air. XRD shows that both “calcined” and “spent” sorbents contain nano-dispersed ZnO, and XPS confirms conversion of ZnO to ZnS. “Calcined” sorbent contains Fe³⁺ and Mn³⁺ that are reduced to Mn²⁺ upon reaction with H₂S, but not with H₂. Operando ESR is used for the first time to study dynamics of reduction of Mn³⁺ promoter sites simultaneously with measuring sulfidation dynamics of the Fe_x–Mn_y–Zn_1−x−yO/SiO₂ sorbent. Fe cations are believed to occupy the surface of supported ZnO nanocrystallites, while Mn cations are distributed within ZnO.

Ultra-deep desulfurization of liquid fuels is crucial for the environment, longer lifetime of combustion engines, and emerging “green,” sustainable, carbon-neutral fuels for fuel cell applications. Current interest is towards photocatalysis and photochemistry for production of clean fuels and valuable chemicals. This critical Review provides systematization and analysis of studies on photocatalytic, photosensitized, and photochemical desulfurization of liquid fuels in the recent approximately 20 years. Zeolites, single-site photocatalysts, TiO₂, and photosensitizers are discussed with their characterization and photocatalytic, photosensitized, and photochemical reaction networks. Pilot-scale photoreactors, kinetics of fuel desulfurization using sunlight, and uses of by-products of photodesulfurization of liquid fuels are covered (181 references).