| CO methanation catalyst,which takes metallic Ni as the main active component,is the core of synthesis methanation,which is the key process of coal to natural gas.Focusing on the problems of carbon deposition and inactivation of trace H2S on Ni-based catalysts for CO methanation,we used a quantum chemical calculation method to explore the reason at a electron-molecular level.The promoters of La,Zr and supporters of ZrO2,Al2O3and MoS2 were added to regulate the catalytic performance of Ni-based catalysts,to inhibit or eliminate the formation of C and the adsorption of S on Ni catalyst surfaces,to increase the stability of Ni-based catalysts and to enhance the activity of CO methanation and the selectivity of CH4.In this dissertation,the formation mechanism of CH4 was systematically investigated using the quantum chemical density functional theory on the terrace of Ni?111?surface with the highest exposure areas of Ni particles,the step of Ni?211?surface with higher activity,as well as the smaller particles Ni4 and the medium size Ni13 with rich corners;and do the activity of CO methanation and the selectivity of CH4 comparison with the results of the La@Ni?111?,Zr/Ni?211?and ZrNi3-Al2O3?110?surfaces.Meanwhile,the mechanism of CH4 formation was studied on the Mo-edge of MoS2?100?and Ni-Mo-S active sites formed by Ni-doped and S-adsorbed in the presence of trace H2S in syngas.In this work,the effects of formation,accumulation and elimination of C on the stability of Ni catalysts were investigated to improve the activity of CO methanation and the selectivity of CH4.The main conclusions are as follows:?1?The effects of morphologies and sizes on catalytic performances of Ni catalysts:the overall energy barriers for CH4 formation on Ni?111?,Ni?211?,Ni4-ZrO2?111?and Ni13-ZrO2?111?surfaces that only Ni as active component were close,the results showed that the different morphology single active Ni catalysts had little effect on the overall energy barrier of CH4 formation,and only the change of morphology and size cannot effectively improve the activity of CO methanation.Based on the overall energy barrier of the optimal pathways of CH4 and CH3OH formation,the reactions of CH4 and CH3OH formations on Ni?111?surface were competitive,while CH4 formation of was prior to CH3OH on Ni?211?,Ni4-ZrO2?111?and Ni13-ZrO2?111?surfaces,thus,the sequence of selectivity to CH4 was Ni?111?<Ni?211?<Ni4-ZrO2?111?<Ni13-ZrO2?111?,showing that the selectivity to CH4 can be enhanced by the Ni catalysts possessing abundant edges,horns,edges and folds.Especially the defect site in a B5 configuration on the stepped Ni?211?,the results of DFT calculation and Microkinetic modeling all show that the relative selectivity of CH4 is higher than that of CH3OH,that means that CO methanation is a structure sensitive reaction.?2?The effects of components and morphologies of Ni catalysts on the catalytic properties:the charge-transfer from La and Zr promoters to Ni surface increased the d-band electron density the of Ni atoms,this enhanced Ni reduction,activated the C–O bond,reduced the overall energy barrier of CH4formation and improved the overall energy barrier of CH3OH formation compared with Ni?111?,Ni?211?and Ni4-ZrO2?111?surfaces.The activity of CO methanation and the selectivity of CH4 were both enhanced on La@Ni?111?,Zr/Ni?211?and ZrNi3-Al2O3?110?surfaces because of the addition of La,Zr promoters.The synergistic catalytic ability of the promoter Zr with the Ni defect surfaces were stronger than La with the Ni terrance surfaces.?3?The effects of existence forms and modes of action of promoter Zr on catalytic performance:the Zr exists on Ni13-ZrO2?111?,Ni4-ZrO2?111?and ZrNi3-ZrO2?111?surfaces in the form of lattice Zr,interface Zr and promoter Zr,respectively.The lattice Zr in the ZrO2?111?supporter of Ni13-ZrO2?111?surface neither changed the reaction path nor promoted the reaction,which only played a supporting role;the interface Zr on Ni4-ZrO2?111?changed the reaction path and improved CH4 selectivity by stabilizing CH2O intermediates,this took on the interfacial effect;the electron of active Zr on ZrNi3-ZrO2?111?transferred to Ni,which enhanced the reducibility of Ni,activated the C–O bond,improved the activity of CO methanation and the selectivity of CH4,further the formation of CH4 was promoted,which attributed to synergistic effect.Therefore,the reactivity sequence of the three Zr forms on Ni4-ZrO2?111?,Ni13-ZrO2?111?and ZrNi3-Al2O3?110?was lattice Zr<interfacial Zr<promoter Zr,which was well consistent with the average energy of d-band on the corresponding surfaces.?4?The effect of promoter La on catalytic performance:Ni?111?surface with the most common exposed areas in Ni particles cannot easily occur carbon deposition,the 5d electron of La increased the 3d electron cloud density of Ni on LaNi?111?doped with a rich electronics agent La,and the overlap section of C2p–Ni3d and O2p–La3d for CO and C+O were larger on LaNi?111?,the chemical bond of C–Ni and O–La were stronger;the C–O bond was weakened;on one hand,the activated C–O bond would decrease the overall energy barrier of the optimal path of CH4 formation and simultaneously improve the activity of CH4formation;on the other hand,the activation energy of CO direct dissociation and CO disproportionation were decreased,leading to the formation of C.Thus,despite the synergistic effect of the promoter La with Ni enhanced the activity and selectivity of CO methanation,the formation of a large amount of C affected the catalytic performance of Ni-based catalysts.?5?The effect of of promoter Zr on catalytic performance:Ni?211?surface is the defect surface with higher activity in Ni particles,along with the formation of CH4,a large number of the surface C were produced.However,the surface C was preferentially hydrogenated to CH compared with C aggregated to produce C2 and C3,indicating that carbon deposition was not the main cause of Ni catalyst deactivation in the presence of reducing H.CH4 formation was prior to C formation on the ZrNi?211?surface,not only the activity and selectivity of CH4 formation were enhanced on the Zr-doped Ni?211?surface,but also carbon deposition could be inhibited,the stability of Ni catalysts would be improved.The synergistic effect between Zr and Ni mainly manifested in the charge transfer from Zr to Ni multifunctional activity site in Zr-Ni,the doped Zr atoms on the stepped Ni riched the electron density of the surface Ni atoms by providing electrons to the adjacent Ni,the reducibility of Ni was enhanced,the C–O bond was activated and the activity of CO methanation was increased;with the increasing number of H atoms on C and O,namely,the saturation of C and O increased,the activation of C–O bond was enhanced,and the hybridization between C2p and O2p was weakened,the activation energy barrier of C–O bond was reduced;until there was almost no overlap between C2p and O2pp in CH3OH,the C–O bond spontaneously dissociated,and CH3OH would not be formed on the ZrNi?211?surface.?6?The controlling of carbon deposition:carbon deposition easily occurred at the edges,angles and edges of the Ni catalysts particles,the surface wrinkle region was both the active site of forming CH4 and the surface C aggregated and inactivated.The keys whether the surface C occur carbon deposition included three points,firstly,the surface C is from direct dissociation or H-assistant of the CO,and the direct dissociation of the CO easily lead to the formation of the surface C;secondly,the CHx?x=1-3?weather occur the sequential hydrogenation into CH4 or the gradual dissociation into the surface C;thirdly,the generated surface C would be hydrogenated to produce CH or aggregate to produce C2.On the Zr-doped Ni?211?surface,the CH4 formation path was changed and the direct dissociation of CO was avoided,if the hydrogenation saturation of C and O was maxed and the C–O bond was weakened,the hydrogenation process of CHx would be shorten due to containing more H before the C–O breakage,so that the hydrogenation was easy rather than dissociation of CHx.At the same time,the dissociation process of H2 was low energy and strong reaction heat energy on the catalyst surfaces,which sufficiently provided sufficient H source for H-assisted CO dissociation and rapid hydrogenation of CHx as well as the surface C,and promoted the CH4formation and elimination of the surface C on the Zr Ni?211?surface.?7?The effect of Ni-doped and S-adsorption on formation of CH4 on the MoS2?100?surface:the overall energy barrier formation of CH4,CH3OH,the formation of C and adsorption S on the Mo S2?100?and S-Ni/MoS2?100?surfaces were studied,the results showed that the former two H2S molecular dissociation was strong exothermic and low energy barrier reaction,the dissociation of the third H2S was not conducive to the formation of CH4 in both thermodynamics and kinetics,namely,CO methanation was in preference to sulfide on the S-Ni/MoS2?100?surface,which confirmed that Ni/Mo catalysts microscopic model was reasonable in the forms of the adsorption of 2S on Ni/MoS2?100?surface to simulate the presence of trace H2S in CO methanation in syngas.The microenvironment in consistent of rich electronic Ni on"Ni-Mo-S"activity site"on the S-Ni/MoS2?100?surface,low coordination Mo and S electron hole was the activity center of CO methanation.On the sulfur-resistant MoS2?100?and S-Ni/MoS2?100?surfaces,the surface C was prior to be hydrogenated to CH compared with the C together generated C2,therefore,the CO methanation process on the sulfur-resistance MoS2?100?and S-Ni/MoS2?100?surfaces would not occur carbon deposition.Though the selectivity of CH4 was high on the MoS2?100?surface,the activity of CO methanation was low.Because of the lower barrier of CO methanation and the dissociation of CH3OH on the active centre of Ni-Mo-S,S-Ni/MoS2?100?showed the extremely activity and selectivity for CH4 formation. |
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