Effect Of Sulfur From Syngas On The CO Methanation Reaction On The Ni(111) Surface

Our country is rich in coal reserves, using coal resources to produce natural gas has important economic and environmental benefits. Metal Ni is commonly used as a major active constituent of catalyst in CO methanation reaction.However, because of a small amount of sulfur species existing in the syngas（mainly H₂ S gas）, the Ni based catalysts suffer severe poisoning effect, causing the loss of catalysis activity and change in selectivity. Until now, a plenty of research studies are still focused on performance testing of deactivated catalyst by sulfur poisoning and developing new type of catalyst for sulfur resistance,but the early stage of sulfur poisoning is still not clear. Thus, clarifying the mechanism of CO methenation in the early stage of sulfur poisoning is important for investigating the process of sulfur poisoning and the synthesize of new type of sulfur tolerant catalyst.In this paper, the mechanisms of CO methanation on Ni catalysts with different sulfur coverage have been systematically investigated using density functional theory（DFT） method together with the periodic slab models. First of all, the Ni（111） surfaces at 0.22 and 0.33 ML sulfur coverage were constructed;then the reactants and all possible intermediates involved in CO methanation on sulfur pre-covered Ni（111） surface were investigated; in the end, the optimal route in the CO methanation was selected by comparing the elementary reaction about the formation of CHx species. The main conclusion are as follows:（1） The influence of sulfur coverage on adsorption site of the reactants and possible intermediates in CO methanation was obtained. For H, C, O, CO, CH₃ O,OH, CH, CH₂, CH₃, HCO, CH₂ O and CH₂ OH, sulfur atoms on the Ni（111）surface changed asdsorption sites of these speceis from fcc on clear surface to hcp on surface with sulfur coverage of 0.22 and 0.33 ML. When the sulfur coverage turned form 0.22 ML to 0.33 ML, adsorption configurations of CH₂ O and CH₂ OH changed.（2） The influence of sulfur coverage on the adsorption energy of reactants and possible intermediates was obtained. With the increasing of sulfur coverage,sulfur atoms weakened adsorption of eactants and possible intermediates by physical blocking of site and changing electronic structure of Ni（111） surface.（3） The influence of sulfur coverage on the activation of CO molecule in CO methanation was obtained. On the Ni（111） surfaces at 0.22 and 0.33 ML sulfur coverage, energy barrier of CO dissociation and COH formation was greater than the energy barrier of HCO formation, this was the same as situation of clear Ni（111） surface. The formation of HCO specie was the major way in CO activation.（4） The influence of sulfur coverage on the optimal route of CO methanation and difficulty was obtained. With the increasing of sulfur coverage form 0 to 0.22 ML then to 0.33 ML, the optimal route of CO methanation transformed from CO+4H â†' CHOH+3H â†' CH+3H â†' CH₂+2H â†' CH₃+H â†'CH4 on clear surface to CO+4H â†' HCO+3H â†' CH₂O+2H â†' CH₃O+H â†'CH₃+H â†' CH4 on surface with sulfur coverage of 0.22 ML then to two competitive reaction routes（CO+5H â†' HCO+4H â†' CH₂O+3H â†' CH₂OH+2Hâ†' CH₂+2H â†' CH₃+H â†' CH4 and CO+4H â†' HCO+3H â†' CH₂O+2H â†'CH₂+2H â†' CH₃+H â†' CH4） on surface with sulfur coverage of 0.33 ML.Compare to the optimal route of CO methanation on clear Ni（111） surface, the increasing of sulfur atoms made the further reaction of HCO changed and raised the activation energy of rate-determining step in CO methanation.