| The interactions between hydrogen and transition metal surfaces have been investigated extensively through decades,as they are fundamental steps that are of vital importance to a wide variety of chemical processes,including hydrogen storage,hydrogen-powered fuel cells,hydrogen embrittlement,hydrogen induced corrosion,hydrogenation/dehydrogenation reactions,and so on.The absorbed hydrogen impurities are found to considerably affect the physical,chemical and mechanical properties of the recipient metals.Although a combination of various experimental techniques has been applied,it can be scarcely possible to detect the specific surface states due to the small size of hydrogen atoms and the complexity of different reaction environments.Nowadays,atomistic modeling technique has become a powerful tool to probe the structure and properties of materials and the nature of chemical/electrochemical reactions.In this work,the behaviors of hydrogen on/in a series of transition metals and its influence on relevant reaction processes are studied using first-principle calculations.The following conclusions are drawn:(1).Through study of the adsorption and absorption of atomic hydrogen on and into close-packed fcc(111)surface of a series of transition metals under different coverages,we show that high enough hydrogen coverage(more than one monolayer)is needed to make H absorption much more feasibly.The pre-adsorbed H adatoms can modify H binding energies as well as H absorption energetics and can be felt very deeply in the 2nd-subsurface.It can significantly destabilize surface H while slightly stabilize 2nd-subsurface H.The kinetic activation energy for surface absorption is increased by pre-adsorbed H adatoms,while the relatively larger intrinsic barrier required for the second diffusion step is slightly decreased.As a result,surface penetration of atomic H and further diffusion into the bulk can be facilitated,while in turn the resurfacing process of absorbed H is blocked.(2).Electrochemical surroundings differ enormously from the low-pressure gas-phase environment.Both hydrogen surface adsorption and bulk absorption can be modified by the presence of an electrochemical double-layer formed between the metal-electrolyte interfaces.Under electrolyte conditions,the solvent effect is negligible,while the field effect can provide a minor thermodynamic driving force for H surface penetration via destabilizing surface H and stabilizing subsurface H simultaneously.(3).Through investigation of elementary steps involved in the initial stage of Cr(110)passivation in acid solutions(pH = 0),we predict how the passive monolayer forms and how it contributes to the electrode potential.Adsorption-driven passivation begins in the active region and a fully coated surface mainly with oxide is likely to be the starting point of the passive region.The calculated intrinsic limiting potential is in reasonable agreement with experimentally obtained passivation potentials.Then the intrusion of H impurities can stabilize all the surface intermediates at different degrees and weakens the metal-metal bond,by which it can have a negative influence on the limiting potential and facilitate breakdown of the passive layer.A higher hydrogen concentration makes more obvious effects.(4).Through a theoretical study of associative electrochemical ammonia synthesis over close-packed transition metal surfaces using a non-aqueous proton source,2,6-lutidinium(LutH+),which has been chosen in the Schrock Cycle,we demonstrate that LutH+ is a viable substitute for hydronium(H30+)in the electroreduction process,since this donor can suppress the rate of hydrogen evolution reaction(HER).We also show that the presence of 2,6-lutidinium can selectively stabilize the*NNH intermediate relative to*NH or*NH2 via formation of hydrogen bonds,so that the optimum limiting potential can be shifted upwards by 0.3 V.Therefore,this non-aqueous proton donor can be a very promising candidate for electrochemical N2 reduction. |
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