Preparation Of Porous Nano ZnO Based Room Temperature Deep Desulfurizer And Its Structure-Activity Relationship

Coal-derived gas desulfurization is the key to realize the clean utilization of coal,and it plays a very important role in coal chemical industry.Zinc oxide（ZnO）is an important and commonly used desulfurizer,which is generally used for medium temperature desulfurization polishing in industrial application.But in fact,the reaction of ZnO with H₂S at room temperature has higher thermodynamic advantages and higher desulfurization efficiency,well meeting the strict requirements of emerging new technologies for the sulfur content in the feed gas,such as proton exchange membrane fuel cells.However,limited by reaction kinetics,the conventional ZnO-based adsorbents have a very low room temperature H₂S removal performance,this hampered their industrial application.Since nanostructured ZnO has nano scaled grains and exposes more active sites,it has a high room temperature H₂S removal activity in theory.Taking also the effect of mass transfer on desulfurization performance into consideration,this paper designed and prepared two kinds of porous nanostructured ZnO based adsorbents,namely activated carbon supported type and zinc-silicon composite type.Besides studying the effect of porosity,surface area and the grain size on the H₂S removal performance,the influence of chemical structural factors,such as the interaction between ZnO and support,surface basicity,phase composition,heterostructure and oxygen vacancy,on the room temperature H₂S removal performance of ZnO and their mechanisms are explored.The structure-activity relationships of the prepared adsorbents are also deeply investigated.In addition,in order to avoid being formed solid waste damaging the environment,this paper also explored the resource re-application of the exhausted adsorbents as hydrodesulfurization catalyst.The main research contents and the conclusions are as follows:（1）Soft nitriding method was used to modify the activated carbon（AC）support,and then the ZnO supported N modified AC adsorbents were prepared.N modification increased the surface basicity of adsorbents,and also modulated the interaction between ZnO and AC support,inducing the formation of lattice defect in ZnO and promoting the diffusion of S^2-in ZnO lattice.The adsorbents after N modification showed an optimal breakthrough sulfur capacity（BSC）two times that of without modification.Its high performance was attributed to the reactive adsorption of ZnO with H₂S and the catalytic oxidation of later on the carbon surface.The chemisorbed oxygen or oxygen-containing groups on the activated carbon surface provided oxygen source for H₂S catalytic oxidation.（2）Introduction of MgO into ZnO supported activated carbon adsorbents boosted the reactive adsorption and the catalytic oxidation of H₂S in desulfurization,resulting in a bifunctional ZnO-MgO/AC adsorbents with a much high BSC.The optimal BSC of adsorbents after introducing MgO was 106.7 mg/g,three times that of without.The promotion mechanism of MgO is based on its weak basicity and its inactivity toward H₂S at room temperature.In the moist conditions,MgO in water film could continuously provide dissociated HS^-for desulfurization,thus promoting the reaction of ZnO with H₂S and the catalytic oxidation of later on the carbon surface.In the dry conditions,MgO acted as basic sites and directly captured one H proton from H₂S,providing sufficient HS^-for abovementioned desulfurization reactions,which remarkably improved the H₂S removal performance of the adsorbent.（3）A new sol-gel strategy using metal nitrates-ethylene glycol（EG）-tetraethyl orthosilicate（TEOS）as precursors was developed to prepare porous nanostructured ZnO/Si O₂adsorbents with high ZnO loadings,high dispersion and high H₂S removal performance.Adopting airtight drying method and increasing the molar ratio of EG to zinc nitrate in the sol system could lead to the formation of a large amounts of hydrogen bonds between EG and Si-OH/H₂O during in gel drying process,which avoided EG being oxidized by zinc nitrates and inhibited the cross-linking condensation of Si-OH.The former prevented the generation of inactive monodentate ZnCO₃ species on the surface of ZnO,while the later promoted the formation of developed mesoporosity in the prepared adsorbents.In addition,EG also served as fuel to promote the decomposition of zinc nitrate during calcination and increased the adsorbents’surface area.The prepared adsorbents have the highest BSC of 108.9 mg/g.（4）Introduction of cobalt species into the abovementioned sol-gel system and the porous nanostructured zinc-cobalt-silicon ternary adsorbents with phase composition of Co²⁺doped ZnO/Si O₂,ZnO-Co₃O₄/Si O₂ and Zn²⁺doped Co₃O₄/Si O₂ were prepared.The adsorbent with molar ratio of Co/（Co+Zn）=0.3 performed the highest BSC of 180.8 mg/g.The high desulfurization performance was ascribed to three reasons:the introduced Co species improved the dispersion of ZnO and increased the surface area and the porosity of adsorbent;the formed ZnO and Co₃O₄ heterostructure supported the dissociation of H₂O and the diffusion of S^2-during desulfurization;the small Co₃O₄ grains have a higher reactivity towards H₂S and the variable valence of Co³⁺promoted the oxidation of H₂S to form elemental S and sulfates.The exhausted adsorbents could be used as COS hydrogenation catalyst,which provides an important reference for the re-utilization of the spent adsorbents.（5）ZnO/Al₂O₃ adsorbents enriched with oxygen vacancies were prepared using a sol-gel method by introducing nickel nitrate into the precursors.Although the Ni²⁺doping led to the growth of ZnO grains and decreased the surface area and the porosity of the adsorbents,it increased the concentrations of oxygen vacancy in the ZnO lattice,thus remarkably improving the room temperature H₂S removal performance of the adsorbent.The role of oxygen vacancy lies in promoting the surface hydroxylation of ZnO,endowing a weak alkaline environment for H₂S dissociation.Moreover,it also decreased the diffusion resistance of S^2-in ZnO lattice and increased its diffusion rate.