Theoretical Study Of Removal Of γ^A1₂O₃for Hydrogen Sulfide And Mercury

The sulfurized processes of H2S on dehydrated (100) and (110) aswell as hydrated3.0OH nm2(110) surfaces of γ-Al2O3have beeninvestigated by using periodic density functional theory method.Meanwhile, the removal of Hg is discussed on the clean and the metalcluster Pd, Pt and Cu laden γ-Al2O3surfaces. The simultaneous removalmechanisms and reactive states of H2S and Hg are systematicallyinvestigated to predicte the possibility of reaction. The main conclusionsobtained from this work are summarized as follows:(1) H2S is preferred to adsorb on the Al site along with S bond, andthe adsorption energies are–32.52,–114.38and–92.82kJ/mol on thedehydrated (100) and (110) as well as hydrated3.0OH nm2(110)surfaces of γ-Al2O3, respectively. Our results show that the bondingstrength of H2S on the hydrated3.0OH nm2(110) surface is weaker thanthat of on the dehydrated (110) surface. The second dissociation step of H2S is the rate determining step. The activation energy of the ratedetermining step are ranked in the following order: Ea(D110,77.31kJ/mol)>Ea(3.0OH nm2,62.81kJ/mol)>Ea(D100,51.60kJ/mol). Onthese three surfaces, the two H–S bonds cleavage processes present thefacile activation energies, which are facilitative to carry out thedesulfurization.(2) In unsupported metal γ-Al2O3surface, Hg exhibit strongabsorption on the dehydrated (100) surface, and the adsorption energy is–33.60kJ/mol. On the γ-Al2O3surface of load metal (Pd, Pt and Cu), Hgpreferentially adsorb on the metal (Pd, Pt and Cu) sites, the surfaceadsorption capacity for Hg changed from the original physical adsorptionto the chemical adsorption. In three different surfaces of the metal Pd andPt load, the bonding strengths of Hg on the metal laden dehydrated (110)surfaces are stronger than that of on the other.In different metal load ofthe same surface, the Pt laden surface shows stronger ability to take offthe mercury than the Pd laden surface. Comparing with the single solidmetal surfaces, the Hg adsorption is different, and it may be because theinteraction between metal clusters and the substrate can change theadsorptive property of metals.(3) The adsorption energies of HgS on dehydrated (100) and (110) aswell as hydrated3.0OH nm2(110) surfaces of γ-Al2O3are–150.63,–231.55and–254.32kJ/mol, respectively. Compared to the (100) surface, the (110) surfaces show enhanced adsorption capacity for HgS. On thedehydrated (100) surface, Hg takes part in the second dissociation step forH2S dissociation, and change the original HS individual activation energyand reaction energy. The adsorbed S and Hg in the gas phase can react toform the HgS through the E-R mechanism, and the activation energy ofthe reaction is78.15kJ/mol. On the dehydrated (110) surface, H2S canoccur dissociative adsorption, and activation energy of HS decompositionis78.15kJ/mol. In the process of forming HgS, the activation energy andreaction energy are86.83and–75.25kJ/mol, respectively. On thehydrated3.0OH nm2(110) surface, the adsorbed S and Hg in the gasphase can overcome a very small activation energy (6.75kJ/mol) to formHgS. Either the HS decomposition process or simple S adsorption process,the Hg can form HgS to be removed. Therefore, γ-Al2O3is a veryeffective adsorbent for simultaneous removal of H2S and Hg incoal-based syngas.