Structural Control Of Molybdenum Sulfides Applied In Hydrodesulfurization And Hydrogen Evolution Reaction

Present work utilized the small gas molecule of H2 as SDA to in-situ hydrothermal synthesis of Co-MoS2,which has not only expanded interlayer,but also substantial 1T phases and Co-Mo-S active sites.From the subsequent catalytic activity test of MoS2 and Co-MoS2,we found that the expanded 1T-Co-MoS2 displayed excellent HER and HDS activity.Based on this,present work investigated the effect of introduced H2 to the synthesis process on the interlayer structure of Co-MoS2,and the contribution of the expanded interlayer to activity enhancement through DFT calculations.Lastly,present work calculated the atomic 2H to 1T phase transition mechanism in MoS2,and the HDS reaction mechanism and microkinetic over various active sites of MoS2,in order to obtain the deep insight into the correlation between active sites and reactants.The main contents and conclusions of present work are listed below.1.This work is the first report of utilizing the small gas molecule of H2 as SDA to in-situ synthesize highly expanded Co-MoS2(9.2?11.1 (?)),which has substantial 1T phases(?75%)and Co-Mo-S active sites(40.9?91.3%).Among them,Co-MoS2-1.4 and Co-MoS2-2.0 exhibits superior HER and HDS activity,respectively.The enhancement of the related performance is attributed to such all-round engineering by tuning interlayer expansion,Co-doping and 2H?1T phase transition within MoS2.2.In light of DFT calculations,the thermodynamic role played by H2 on interlayer regulation of Co-MoS2 during synthesis process can be well illustrated.The interlayer distance of Co-MoS2 increases up to 10.3 A under the condition of T=448 K and pH2=1.4 MPa.The H2 could be dissociatively adsorbed on Co-MoS2,which thereby greatly affected the surface free energy of Co-MoS2.The most thermodynamically stable state is achieved by tuning the interlayer spacing of Co-MoS2,thus changing the surface energy,finally resulting in the interlayer distance of 10.3(?).3.Present work explained the different HER activity presented by different MoS2 and Co-MoS2 catalysts through calculating the Gibbs free energy of hydrogen adsorption by DFT calculations.Due to the fact that H2 is evolved from adsorbed proton on the surface,we investigated the surface H coverage(?H)before HER activity calculations.Firstly,the AIT(Ab Initial Thermodynamics)analysis was employed in this part to identify the numbers of stably adsorbed proton under the real HER reaction condition.Then,based on the DFT calculations,it can be found that the expanded interlayer spacing can greatly decrease the absolute value of ?GH for both edge sites and basal planes,optimizing the interaction between H and Co-MoS2 to improve the related HER activity;and Co-MoS2-1.4 with an interlayer spacing of 10.3 A exhibited the lowest absolute values of ?GH for both edge site and basal plane,indicating its highest HER activity.4.In the present work,phase transition mechanisms from semiconducting 2H phase to metallic 1T phase in MoS2 nanosheets were studied using DFT calculations.Various 2H?1T phase transition mechanisms that consist of nucleation and propagation steps,which were simulated by collective rotational and rotational/translational movements,single atom translational movement,as well as the gliding movement of one row for sulfur atoms,on both the basal planes and Mo-and S-edges with different S coverages were investigated.On the perfect basal plane,the 1T phase nucleation is unlikely due to the extremely high barrier of 2.25 eV/atom.Whereas the presence of defective S vacancies on the basal plane dramatically facilitate the 1T phase nucleation and propagation around the defective sites by the collective rotational movement of three S atoms.On the 2H phase basal plane with two S vacancies,the kinetic barriers for the 1T phase nucleation are as low as of 0.66?0.77 eV/atom.Similar to the promoting effect of S vacancies on the phase transition over the basal plane,DFT results also suggest that the S coverage on the Mo-and S-edges will affect the 1T phase nucleation and propagation.The 1T phase nucleation starting with the translational movement of single S atom on the bare Mo-edge and the gliding movement of an entire row of S atoms on the S-edge with 50%S coverage are kinetically favorable.While the 1T phase formation at the Mo-edge with 50%S coverage and the S-edge with 100%S coverage are unlikely.The present work not only confirms the important role of S vacancies/coverages in the 2H-1T phase transition,but also provides new insight into how and where the 2H-1T phase transition occurs at the atomic level,which also sheds light on the general phase transition mechanism for two-dimensional transition metal dichalcogenide materials.5.In the present work,thiophene hydrodesulphurization(HDS)over the Mo-edge,the S-edge,and the Mo-S connection edge of MoS2 catalyst with 50%sulfur coverage was studied using first-principles based microkinetic modeling.Two parallel HDS routes,i.e.,direct desulfurization(DDS)and hydrogenation(HYD)were taken into account.It has been found that the major reaction route of thiophene HDS on the Mo-and the Mo-S edges is temperature dependent.In the low temperature range of 500?600 K,the HYD route is dominant,leading to the C4H8 formation.As the temperature increases,the DDS route becomes competitive with the HYD route.At the temperature above 650 K,the DDS route will be the dominant HDS reaction route on the Moand the Mo-S edges.As for the S-edge,the DDS route leading to the formation of C4H6 constitutes the major thiophene HDS reaction route in the entire temperature range of 500?750 K.The microkinetic modeling results show the overall HDS activity on the S-edge is lower than that on the Mo-and the Mo-S edges.The Mo-S edge also provides a preferential reaction pathway,which facilitates 2-hydrothiophene migration from the Mo-edge to the S-edge,followed by remaining elementary steps with lower activation barriers in the DDS route.