Location-dependent effect of nickel on hydrogen dissociation and diffusion on Mg(0001)surface:Insights into hydrogen storage material design

Density functional theory(DFT)calculations have been performed to investigate the hydrogen dissociation and diffusion on Mg(0001)surface with Ni incorporating at various locations.The results show that Ni atom is preferentially located inside Mg matrix rather than in/over the topmost surface.Further calculations reveal that Ni atom locating in/over the topmost Mg(0001)surface exhibits excellent catalytic effect on hydrogen dissociation with an energy barrier of less than 0.05 eV.In these cases,the rate-limiting step has been converted from hydrogen dissociation to surface diffusion.In contrast,Ni doping inside Mg bulk not only does little help to hydrogen dissociation but also exhibits detrimental effect on hydrogen diffusion.Therefore,it is crucial to stabilize the Ni atom on the surface or in the topmost layer of Mg(0001)surface to maintain its catalytic effect.For all the case of Ni-incorporated Mg(0001)surfaces,the hydrogen atom prefers firstly immigrate along the surface and then penetrate into the bulk.It is expected that the theoretical findings in the present study could offer fundamental guidance to future designing on efficient Mg-based hydrogen storage materials.

Reactive ball-milling synthesis of Co-Fe bimetallic catalyst for efficient hydrogenation of carbon dioxide to value-added hydrocarbons

Catalytic hydrogenation of CO_(2) using renewable hydrogen not only reduces greenhouse gas emissions,but also provides industrial chemicals.Herein,a Co-Fe bimetallic catalyst was developed by a facile reactive ball-milling method for highly active and selective hydrogenation of CO_(2) to value-added hydrocarbons.When reacted at 320℃,1.0 MPa and 9600 mL h^(-1) g_(cat)^(-1),the selectivity to light olefin(C_(2)^(=)-C_(4)^(=)) and C_(5)+ species achieves 57.3% and 22.3%,respectively,at a CO_(2) co nversion of 31.4%,which is superior to previous Fe-based catalysts.The CO_(2) activation can be promoted by the CoFe phase formed by reactive ball milling of the Fe-Co_(3)O_(4) mixture,and the in-situ Co_(2)C and Fe_(5)C_(2) formed during hydrogenation are beneficial for the C-C coupling reaction.The initial C-C coupling is related to the combination of CO species with the surface carbon of Fe/Co carbides,and the sustained C-C coupling is maintained by self-recovery of defective carbides.This new strategy contributes to the development of efficient catalysts for the hydrogenation of CO_(2) to value-added hydrocarbons.