Abstract
A first-principles calculation was applied to study the adsorption behavior of CO2 on δ-Pu(100) surface using a slab model within the framework of density functional theory. Results demonstrate that CO2 molecules are adsorbed on δ-Pu(100) surface at C-terminated bent state by multi-bond binding of C-Pu and O-Pu. The adsorption type belongs to strong chemical adsorption and the most stable adsorption configuration is H1-C4O4 with the adsorption energy of -6.430 eV. The adsorption stability order is hollow site>bridge site>top site. CO2 molecule mainly interacts with Pu surface atoms, while the interaction with other three Pu atoms is weak. The transfer of more electrons to the CO2 2πu orbital is beneficial to the bending and activation of C-O bonds. Moreover, the chemical bonding between Pu atoms and CO2 molecule is mainly ionic state and the reaction mechanism is that the C 2s, C 2p, O 2s and O 2p orbitals of CO2 molecule hybrid with Pu 6p, Pu 6d and Pu 5f orbitals, resulting in a new bond structure. The work function of the H1-C4O4 site changes the least, indicating that other electrons readily escape from the metal surface and the required energy is the smallest.
Science Press
Plutonium, one of the most complicated elements in the periodic table, is at the boundary between the light actinide elements (Th to Pu) with itinerant 5f electrons and heavy actinide elements (Am and later elements) with localized 5f electrons. Elemental Pu and its alloys and compounds are widely used in the nuclear engineering, e.g., the nuclear de-vices, nuclear reactor and aerospace fields. In principle, the more localized 5f electrons in Pu-based material
Eriksson et a
One can see that the theoretical researches on the surface properties and adsorption behavior of plutonium metal are still relatively few, and there are certain differences in the research results. Dmol3 program is mostly used in the research methods, so it is necessary to apply different methods to systematically study the above problems. At present, the theoretical calculation of CO2 molecule adsorption on plutonium surface by VASP program has seldom been reported publicly. To the best of our knowledge, mechanically, δ-phase plutonium is more ductile and readily-processing than its other allotrope, such as the room temperature α phase. Therefore, we plan to perform a first principles calculation for the adsorption of CO2 on a representative face of δ-phase plutonium, i.e., δ-Pu(100) surface. Specially, we focus on the stable adsorption configurations, the changes of electronic structure, the adsorption energies as well as the charge transfers induced by adsorption, which might give a new perspective on the corrosion and anti-corrosion mechanisms of CO2 with δ-Pu(100) surface.
All calculations were implemented within Vienna ab-initio Simulation Package (VASP5.4.4
CO2 molecule was packed in a box with side length of 1.5 nm when optimized, and the k-point mesh was set to 1×1×1. The ion position, cell shape and cell volume were changed when the δ-Pu cell was optimized with a 7×7×7 k-point mesh used. The lower two layers were fixed and the upper two layers were released for optimization with the shape and volume unchanged when the δ-Pu(100) 4-layer slab was optimized. We fixed the bottom 4, 3, and 2 layers, and released the CO2 molecule, CO2 and Pu surface atoms, CO2 and upper two layers Pu atoms for optimizing, and the results showed that the geometrical parameters change little and the Pu atoms move slightly. In order to improve calculation efficiency and save calculation time, 4 layers of Pu atoms were fixed and CO2 molecule on the optimized surface was released without changing its shape and volume when the CO2 adsorption system on δ-Pu(100) 4-layer slab surface was opti-mize
The surface energy, Esur, is then obtained by
| Esur=(Eslab-NEbulk)/2A | (1) |
where A is the cross-sectional area of the slab, Eslab is the total energy of the slab, N is the number of Pu atoms in the slab and Ebulk is the energy of a single Pu atom in the bulk.
The adsorption energy of CO2 molecule is given by
| Eads=E[CO/Pu(100)]-ECO-EPu(100) | (2) |
where E[CO/Pu(100)] is the total energy of the system after adsorption, ECO is the energy of the CO2 molecular, EPu(100) is the energy of the δ-Pu(100) surface before adsorption. The energy before and after adsorption is acquired under the same calculation method and accuracy. Furthermore, the energy of CO2 is the basic state energy under the same calculation method. For negative adsorption energy, the structural stability is increased by adsorption. Moreover, the less the adsorption energy, the more stable the structure.
The crystal structure of metallic δ-Pu is face centered cubic (fcc) with the space group code of Fm-3m, and the optimized Pu crystal lattice is 0.4772 nm, which is in good agreement with the experimental value of 0.4637 n
Fig.1 Crystal models of δ-Pu before (a) and after (b) optimization
Fig.2 Crystal models of CO2 molecule before (a) and after (b) opti-mization
From the test results of surface energy convergence and Ref.[
Fig.3 Structure model of δ-Pu(100) slab before (a) and after (b) optimization and top view of three adsorption sites on the surface (c) (surface Pu and other layers Pu atoms are in green and blue color, respectively)
A total of 9 adsorption configurations are established, as shown in
Fig. 4 Adsorption models of CO2 on δ-Pu(100) surface: (a) bridge parallel 1, (b) bridge parallel 2, (c) bridge vertical, (d) hollow parallel 1, (e) hollow parallel 2, (f) hollow vertical, (g) top parallel 1, (h) top parallel 2 and (i) top vertical (C, O, surface Pu and other layers Pu atoms are in grey, red, green and blue colors, respectively)
Nine kinds of highly symmetric adsorption models are optimized, and it is found that the optimization under the vertical adsorption models does not converge, which means that CO2 molecule in these models are not likely to adsorb on the δ-Pu(100) surface. For distinguishing, we define configu-rations in accordance with the adsorption position occupied by CO2 and the number of bonds between CO2 and Pu atoms adjacent to the surface layer. For instance, H1-C4O4 means that CO2 molecule is adsorbed on hollow parallel 1 and the C and O atoms are bonded to the adjacent 4 Pu atoms. As a result, six adsorption configurations are plotted in
Fig.5 Top and side views of the optimized structures for CO2 adsorption on δ-Pu(100) surface: (a) B1-C2O2, (b) B2-C2O2, (c) H1-C4O4, (d) H2-C4O3, (e) T1-C1O2 and (f) T2-C1O3 (B1: bridge 1 site, B2: bridge 2 site, H1: hollow 1 site, H2: hollow 2 site, T1: top 1 site, T2: top 2 site; C, O, surface Pu and other layers Pu atoms are in grey, red, green and blue color, respectively)
Note: dC-O1(2) i
The value of adsorption energy and C-O bond length can be taken as the judgment of CO2 adsorption strength for CO2/metal adsorption system, i.e., the smaller the adsorption energy and the larger the C-O bond length, the greater the CO2 adsorption strength and the more stable the adsorption.
According to
From the perspective of adsorption energy, except the adsorption energies of B1-C2O2 and T2-C1O3 configurations are positive, the adsorption energies of B2-C2O2, H1-C4O4, H2-C4O3 and T1-C1O2 adsorption configurations are -2.388, -6.430, -1.529 and -2.192 eV, respectively, which are significantly larger than 40 kJ/mol, showing that adsorption is strong chemical adsorption and the four adsorption configurations are stable. Furthermore, the order of adsorption stability is hollow site>bridge site>top site, which is consistent with the conclusion of C-O bond length analysis.
After adsorption, CO2 molecule is adsorbed on δ-Pu(100) surface by multi-bond binding of C-Pu and O-Pu. The C-Pu bond length of C atom in CO2 molecule and Pu surface atom is in the range of 0.242 52~0.274 12 nm, which approaches the C-Pu bond length in Pu carbide crystal (PuC: 0.246 20 nm, PuC2: 0.228 19 nm
From the height of C or O atoms away from Pu surface layer, hC-S in H1-C4O4, H2-C4O3, B2-C2O2 and T1-C1O2 adsorption configurations are 0.066 77, 0.134 93, 0.182 59 and 0.253 63 nm, respectively; hO1(2)-S are 0.143 17, 0.198 77, 0.233 62 and 0.285 81 nm, respectively. One can see that the smaller the hC-S and hO1(2)-S, the lower the height of C or O atoms from Pu surface layer, and the more stable the adsorption configuration. What's more, the stability order is hollow site>bridge site>top site, which has a good agreement with the previous analysis results.
In principle, the atom loses electrons and the valence is positive when the corresponding net charge is negative. On the contrary, the atom gains electrons and the valence is negative when the corresponding net charge is positive. Charge transfer occurs between C/O and Pu atoms originating from CO2 adsorption on the δ-Pu(100) surface. In order to clearly elucidate the interaction between CO2 molecule and Pu surface atoms, Bader charge distributio
| qatom=qBader–qvalence | (3) |
Note: q1st
where qatom, qBader and qvalence are the net charge, Bader charge and valence electron of atom, respectively.
Fig.6 Charge state of each atom after CO2 adsorption on δ-Pu(100) within the charge range of -0.20~0.20 e: (a) B1-C2O2, (b) B2-C2O2, (c) H1-C4O4, (d) H2-C4O3, (e) T1-C1O2 and (f) T2-C1O3 (red and blue mean positively and negatively charged, respectively)
As shown in
When CO2 are adsorbed to the hollow, bridge and top sites, the bonding numbers between C atom and the Pu surface adjacent atoms are 4, 2 and 1, respectively, and the average charge carried by CO2 molecules are 1.6947, 1.1442 and 0.5597 e, which show that the intensity of charge transfer from Pu atoms to CO2 molecule is hollow site>bridge site>top site. Moreover, CO2 molecule carries the most charge (2.0493 e) in H1-C4O4 configuration, indicating that the adsorption configuration is the most stable, which is consistent with the conclusion drawn from the changes in the intensity of C-O bond length and the value of adsorption energy. It also proves that hollow site has more the advantage in electron transfer number and energy than the top site and bridge site, demonstrating that the more the bonding number between C atoms and Pu surface atoms, the more stable the adsorption configuration. At the same time, the adsorption of hollow site shortens the distance between the nearest Pu-O atoms compared to other adsorption sites, which is beneficial to the interaction between Pu and O atoms, i.e., the hollow site is more stable.
As can be seen from
From the point of the microscopic mechanism, adsorption will result in the change of the electronic structure. For exploring the reaction mechanism between CO2 molecule and Pu surface atoms, the binding character between the CO2 molecule and Pu surface has been examined by the charge-density difference. For the fully relaxed, minimum total-energy configuration (H1-C4O4), the charge-density difference is given by
| Δρ=ρ[CO/δ-Pu(100)]–ρ[δ-Pu(100)]–ρ(CO) | (4) |
where ρCO/δ-Pu(100)], ρ[δ-Pu(100)] and ρ(CO) are charge densities of CO2/δ-Pu(100) adsorption system, δ-Pu(100) surface and CO2 molecule, respectively. The electronic density of states (DOS) of the most stable configuration (H1-C4O4) is calculated. The isosurfaces of charge-density difference in this site are also addressed, as shown in
Fig.8 Isosurfaces of charge-density difference for the most stable configuration (H1-C4O4) (yellow and blue color mean increase and decrease in charge density, respectively; isosurfaces level: 2 e/n
the bonding condition of ions in the adsorption system.
Fig.9 2D display of charge-density difference for H1-C4O4 configu-ration: (a) plane of Pu11, Pu12, Pu15 atoms and (b) plane of C1, O1 and O2 atoms
Fig.10 shows the total density of states (TDOS) and partial density of states (PDOS) of clean δ-Pu(100) surface and TDOS and PDOS of the most stable adsorption configuration (H1-C4O4) in the CO2/δ-Pu(100) adsorption system. Fig.11 shows the corresponding PDOS of H1-C4O4 configuration. From Fig.10a and 10b, one can see that the peak intensity, position and shape of TDOS on the δ-Pu(100) surface change remarkably induced from adsorption. The peak intensity increases in the range of -50 eV~-45 eV. The peak shape broadens and the area becomes larger from -25 eV to -15 eV. Furthermore, the peak shape broadens obviously, the area increases and the peak value decreases in the range of -12.5~5 eV. Secondly, as shown in Fig.10c, 10d and Fig.11, the C 2s, C 2p and O 2s orbitals overlap and hybrid with Pu 6p orbital in the low energy level range of -25 eV~-15 eV, and the C 2s, C 2p, O 2s, O 2p orbitals overlap and hybrid with Pu 6d, Pu 5f orbitals in the high energy level (from -7.5 eV to 2.5 eV). The intensity of Pu 5f and Pu 6d peaks near EF decreases slightly and the peak shape broadens, which indicates that the delocalization of Pu 5f and Pu 6d electrons becomes stronger and a small number of Pu 6d and Pu 5f electrons also participate in bonding. It is demonstrated that reaction
In order to clearly demonstrate the chemical state in H1-C4O4 configuration, we plot the 2D contour isosurfaces of the charge-density difference for Pu11, Pu12, Pu15 atoms and C1, O1, O2 atoms along the best planes of (001) and (010) in Fig.9a and Fig.9b, respectively. One can see that C/O atoms are surrounded by electrons, and there are no unpaired electrons between C/O and Pu atoms, demonstrating the ionic chemical bond, which is in good agreement with the 3D isosurfaces of the charge-density difference in Fig.8.
mechanism between CO2 molecule and surface Pu atoms is that the orbitals of CO2 molecule hybrid with Pu 6p, Pu 6d and Pu 5f orbitals, resulting in a new chemical bonding state.
Work function refers to the minimum energy that must be provided to enable an electron to escape from a solid surface immediately. It is the energy difference between the vacuum electrostatic potential and the Fermi level at infinity outside the metal, indicating the ability of electrons to escape from the metal surface. The calculation formula is as follows:
| Φ=Evacuum-EFermi | (5) |
where Φ, Evacuum and EFermi are the work function, vacuum level and the Fermi level, respectively. The change of electrostatic potential along the distance to z-axis in all configurations is shown in Fig.12, and the surface work function before and after adsorption is listed in Table 3.
Table 3 Surface work function changes of different adsorption configurations
Note: dC-O is the distance between the C and O atoms in CO2 molecule; q is the transfer charge between CO2 molecule and Pu atoms; Eads is the adsorption energy; Ф is the work function of the Pu surface.
At present, there is no experimental data of the work function of δ-Pu. The work function of its neighboring element (uranium) is in the range of 3.63~3.90 e
The calculation results in this work are compared with those in other references, as shown in
Note: ΔΦ means the change of the work function.
From
1) CO2 molecule are adsorbed on the δ-Pu(100) surface by multi-bonds binding of C-Pu and O-Pu. The adsorption type belongs to strong chemical adsorption. Furthermore, the most stable adsorption configuration is H1-C4O4 with the adsorption energy of -6.430 eV, and at this time, the C-O bond length is 0.141 81 nm and θO-C-O is 114.80°. The order of adsorption stability is hollow site>bridge site>top site, and the more the bond number of C-Pu and O-Pu, the lower the height of C or O atom from Pu surface layer, the more stable the configuration.
2) The adsorbed CO2 molecule are charged to different degrees (0.5365~2.0493 e) and CO2 molecules carry the most charge (2.0493 e) for H1-C4O4 configuration. CO2 molecule mainly interacts with the surface Pu atoms. The transferring intensity of charge from Pu atoms to CO2 molecule is hollow site>bridge site>top site, which is consistent with the conclusion of angle analysis of C-O bond length and adsorption energy. The chemical bonding between Pu atoms and C/O atom is mainly ionic state and the reaction mechanism between CO2 molecule and surface Pu atoms is that the C 2s, C 2p, O 2s and O 2p orbitals of CO2 molecule hybrid with Pu 6p, Pu 6d and Pu5f orbitals to produce new chemical bonding state.
3) The work function of the six configuration increases in the range of 0.8644~1.8723 eV after CO2 adsorption, and the work function of the H1-C4O4 site changes the least. Furthermore, the energy of the system is the smallest, which indicates that other electrons readily escape from the metal surface and the required energy is the least.
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