| With the gradual depletion of non-renewable energy resources and the worsening of the climate environment, the governments of different countries draw up various new-energy plan and encouragement. Therefore, people now turn their attention to new sustainable low-carbon energy, especially for hydrogen energy. Hydrogen is a kind of ideal clean energy due to the properties of high power density and renewable. While fuel-cell vehicles are appealing because they emit no pollutants, it’s been a challenge to find clean and affordable ways to produce, transport, and store hydrogen to fuel them. Hydrazine (N2H4) is a liquid at room temperature and has a hydrogen content as high as12.5wt%, which is used in the pharmaceutical and aerospace industry. Recent studies have shown that hydrazine can be decomposed in two ways: complete decomposition and incomplete decomposition. Therefore, hydrous hydrazine might be a promising hydrogen carrier for storage and transportation that has the distinct advantages of easyrecharging, the availability of the current infrastructure of liquidfuels for recharging, and the production of only nitrogen (whichdoes not need recycling) in addition to hydrogen. So, there are manystudies to develop effective catalysts for selective decompositionof hydrous hydrazine to hydrogen at mild conditions. However,compared with the experimental studies on the decomposition ofhydrazine, there are few theoretical works, such as the adsorptionof hydrazine on metal nanoparticles surfaces, the selectivity ofNi-based surface alloys toward hydrazine adsorption. Theseuncharted questions are of importance to further develop effectivecatalysts for selective decomposition of hydrazine.In recent years, computational chemistry has a fast developmentin hardware or software and the first principles method based onthe density functional theory has been widely applied in the studiesof surface catalysis. In order to understand the mechanism ofcatalytic decomposition of hydrazine at molecular level, in thispaper, theoretical investigations on hydrazine adsorption anddissociation on the surfaces of Rh(111) and Ni-M(111) alloys. Thestudies and obtained results are summarized as follows:(1) The interaction of hydrazine with an Rh(111) surface as amodel for adsorption to rhodium nanoparticles: Metal nanoparticles were found to be excellent catalysts for hydrogen generation fromhydrazine for chemical hydrogen storage. In order to gain a betterunderstanding of these catalytic systems, we have simulated theadsorption of hydrazine on rhodium nanoparticles surfaces bydensity functional theory (DFT) calculations with dispersioncorrection, DFT-D3in the method of Grimme. The rhodiumnanoparticles were modeled by the Rh(111) surface, in addition, theadsorptions at corners and edges sites of nanoparticles wereconsidered by using rhodium ad-atoms on the surfaces. Thecalculations showed that hydrazine binds most strongly to the edgeof nanoparticle with adsorption energy of-2.48eV, where thehydrazine bridges adatoms of edge with the molecule twisted toavoid a cis structure; similar adsorption energy was found at thecorner of nanoparticle, where the hydrazine bridges corner atomand surface atom with gauche configuration. However, we foundthat inclusion of the dispersion correction results in significantenhancement of molecule-substrate binding, thereby increasing theadsorption energy, especially the adsorption to the Rh(111) surface.The results demonstrate that the surface structure is a key factor todetermine the thermodynamics of adsorption, with low coordinatedatoms which providing sites of strong adsorption from the surface.(2) Selectivity of Ni-based surface alloys toward hydrazine adsorption: We use dispersion corrected DFT calculations (DFT+D3)to investigate the selectivity of Ni-based surface alloys towardhydrazine adsorption. A series of Ni-M (M=Fe, Pt, Ir, Pd and Rh) alloyfilms were investigated, namely Ni15/M1/Ni(111), Ni14/M2/Ni(111),Ni12/M4/Ni(111) and Ni8/M8/Ni(111). Our results show that the dopedatoms of Ir, Rh and Fe provide stronger adsorption sites than the Niatom on the Ni(111) surface, while the doped atoms of Pt and Pdprovide weaker adsorption sites. By analyzing the most favourableadsorption of hydrazine on Ni-M alloy surfaces we found thatNi8Fe8/Ni(111), Ni8Rh8/Ni(111), Ni15Ir1/Ni(111) and Ni14Ir2/Ni(111)present enhanced adsorption properties if compared to the pureNi(111) surface, and seem to be better candidates for hydrazinecatalysis, which are in agreement with that found by experiments.The correlation between d-band center position and adsorptionenergies of top modes in the Ni or doped atom has been calculatedat DFT+D3level to provide further insight into the Ni-based surfacealloy properties for hydrazine adsorption.(3) The Molecular Adsorption and Dissociation of Hydrazine onNi-Fe Alloy Surfaces: We have used density functional theory (DFT)with dispersion correction to investigate the adsorption and firstdissociation step of hydrazine on Fe3Ni(111), FeNi(111) andFeNi3(111) surfaces. The calculations have shown that the FeNi3(111) surface offers the strongest binding of molecularhydrazine adsorbed on the top of iron atom, while the strongestadsorption of NH2fragment bridging between iron atoms is obtainedon the Fe3Ni(111) surface, but both molecular hydrazine and NH2fragment can be adsorbed strongly on the FeNi(111) surface. It isalso found that the first dissociation step of hydrazine on theFeNi(111) surface is exothermic by-1.19eV, and presents anactivation energy barrier of only0.15eV. The similar energy barrieris found on the Fe3Ni(111) surface, but a higher reaction energy of-1.47eV is released on this surface. Furthermore, the electronicstructures of the molecular and dissociative adsorption arediscussed, which is intended to put some light into the bindingnature of the adsorption and stability among conformations of theadsorbed molecule on these surfaces. It is expected that our resultswould provide useful information for the development of catalyst forhydrazine dissociation. |
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