Characterization of active sites, determination of mechanisms of H2S, COS and CS2 sorption and regeneration of ZnO low-temperature sorbents: past, current and perspectives

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 similar to 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.