| Thorium dioxide has important applications in nuclear reactions,especially in light water and high-temperature gas cooled reactors.In comparison with uranium-based fuels,thorium-based fuels have more advantages in the physical properties,such as higher melting point,lower thermal expansion coefficients and higher corrosion resistivity.In this work,the configurations and electronic structures of ThO2(111),(110)and(100)pure surfaces,as well as the behaviors for the adsorptions of H2O and CO2 molecules on above three kinds of surfaces,were systematically investigated by using DFT+U method combined with a slab model.In addition,we also explored the effects of H2O coverage on the adsorption configurations and the high resolution electron energy loss spectroscopy(HREELS)for the corresponding adsorption configurations were further simulated by using the finite difference method.Our calculated results reveal that the(111)surface is most stable among three low index ThO2 surfaces,which is followed by the(110)surface,while the thermodynamic stability of the(100)surface is relatively poor.With respect to the ideal surface,the weak relaxation is observed for the(111)surface,however the relaxation of the(100)surface is somewhat obvious.The adsorptions of single water on three low index ThO2 surfaces have two models,which are corresponding to molecular adsorption and dissociative adsorption,respectively.For the(111)surface,the molecular adsorption and dissociative adsorption has similar energy stability,and the difference between adsorption energy is noly 0.05 eV.However,for the(110)and(100)surfaces,dissociative adsorption is significantly more stable than molecular adsorption.According to the simulated HREELS spectra of the adsorption of single water on different ThO2 surface,a strong peak at about 1000 cm-1 can be observed,which is corresponding to the rocking vibration of the water molecule.For the adsorption of more water molecules,three coverages of 0.5,0.75 and 1.0 ML are considered.Our results show that,on the(111)surface,the mixing of molecular and dissociative adsorption is predicted for three coverages.For the(110)surface,the dissociative adsorption of H2O is found at 0.5 ML,and in the high coverage(0.75 or 1.0 ML),the mixed geometries with a combination of molecular and dissociative adsorption are obtained.While on the(100)surface,only dissociative adsorptions are achieved for all three coverages.Furthremore,the results of vibrational calculation show that the stretching frequency of O-H bond of OH group has a good linear relationship with the corresponding O-H bond length.For the adsorption of CO2 molecule,on the ThO2(111)and(100)surface,the carbon atom forms a single bond with O atom on the surface,and two oxygen atoms of CO2 also bonds to the adjacent Th atoms on the surface.Interestingly,when CO2 is adsorbed on the(100)surface,there are five adsorption bonds,in which the carbon atom is attached to one surface oxygen,and meanwhile,four Th-0 adsorption bonds are formed since two oxygen of CO2 are at bridge sites between two adjacent Th atoms.Theorefore,our results indicate that the activation of CO2.becomes.more obvious on the ThO2(100)surface,and as a result with respect to the structure in the gas phase,the largest stretching of C-0 bond(about 0.13 A)is found when CO2 molecule is adsorbed on the(100)surface. |
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