First-principles study of co-adsorption behavior of O2 and CO2 molecules on δ-Pu(100)surface

Chun-Bao Qi(戚春保), Tao Wang(王濤),?, Ru-Song Li(李如松),Jin-Tao Wang(王金濤), Ming-Ao Qin(秦銘澳), and Si-Hao Tao(陶思昊)

1Xi’an Research Institute of High Technology,Xi’an 710025,China

2Xi’an Jiaotong University,Xi’an 710049,China

Keywords: adsorption energy,density functional theory,electron density,reaction mechanism

1. Introduction

Plutonium, one of the most complex elements in the periodic table, which is on 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 devices, nuclear reactor, and aerospace fields. In principle, the more localized 5f electrons in Pu-based materials will produce many anomalous physical properties,[1,2]such as the three-peak structures in the photoemission spectra (PES), the negative thermal expansion coefficient in Pu–Ga alloy,and the abnormal magnetic properties,as compared with its neighboring element (U) in the periodic table. From the chemist’s viewpoint, additionally, Pu is very active in the atmosphere environment, and is readily to react with almost ordinary molecules in the ambient condition during purification,storage,and usage of Pu-based materials. Energetically,PuO, Pu2O3, and PuO2are the basic products of plutonium oxidation,[3–6]which poses a great challenge for the long-term storage and protection of plutonium-based materials. In fact,adsorption and dissociation behaviors of the ordinary gaseous molecules,including CO2and O2considered in this work,on plutonium surface are essential processes for hydrogenation and oxidation corrosion. As a consequence, it is significant to study the adsorption and dissociation of active gases on the plutonium surface in order to understand in depth the corrosion and anti-corrosion mechanisms of Pu-based materials.As a matter of fact,scare direct experimental observation is conducted on Pu-based materials due to its strong chemical activity,radioactivity,and toxicity.For this reason,theoretical work might be an appropriate candidate for studying the adsorption and dissociation behaviors of gas molecules on plutonium surface.

In recent years, there have also been some reports on theoretical researches. Eriksson and Cox[7]calculated the δ-Pu(100)surface under non-magnetic conditions by the surface linear muffin-tin orbit(FLMTO)method. The results showed that the 5f band narrowed, the bonding ability decreased and the surface lattice constant expanded.Huda and Ray[8–11]successively studied the adsorption behaviors of hydrogen, oxygen atoms, and hydrogen and oxygen molecules on the δ-Pu surface by using density functional theory (DFT) and Dmol3 program,showing that the 5f electron localization intensity of δ-Pu(111)surface was stronger than that of δ-Pu(100)surface.On the other hand,Li et al.[12,13]studied the surface geometry and electronic structure of δ-Pu monolayer by the full potential linear conjugate plane wave method(FLAPW).The results indicated the degree of localization of 5f electrons on (100)surface was stronger than that on (111) surface, which was exactly opposite to Huda’s result. Atta-Fynn and Ray[14,15]studied the adsorption of C, N, O atoms on δ-Pu(111) surface and the effect of surface relaxation on adsorption by the FLAPW method. Meng et al.[16]investigated the adsorption and dissociation behavior of CO2molecule on Pu(100) surface by the modified Perdew–Burke–Emzerh (PBE) method based on the generalized gradient density functional theory and Dmol3 program. The calculated dissociation energy barrier of CO2→CO+O was 0.66 eV and the dissociation adsorption energy was 2.65 eV.Luo et al.[17]studied the adsorption of CO molecule on Pu(100) surface by DFT and Dmol3 program. The results demonstrated that the dissociation energy barrier of CO molecule adsorbing on obliquely hollow was relatively small(0.280 eV).Wang and Ray[18]used GGADFT, DMOL3, and WIEN2k to study the adsorption of CO2on nano-layer of α-Pu(020)surface. The results revealed that the completely dissociated configuration (C+O+O) had the strongest surface binding (7.94 eV), followed by partial dissociated configuration (CO+O) and CO2molecular configuration (5.18 eV and 1.90 eV respectively). Atta-Fynn and Ray[19]used the ab initio calculation method within the framework of DFT and Dmol3 program to study the adsorption and dissociation behavior of CO2molecule on δ-Pu(111)surface. The results indicated that the fully dissociated configuration (C+O+O) showed the strongest binding to the surface (7.92 eV), followed by the partially dissociated configuration CO+O (5.08 eV), with the lowest binding energy of CO2molecule adsorption (2.35 eV). Wei et al.[20]used the GGA of periodic DFT and DMOL3 program to study the adsorption behavior of O atom on δ-Pu(111) surface. The results demonstrated that the adsorption of O atom on δ-Pu(111)surface belonged to strong chemical reaction. Guo et al.[21]studied the adsorption behavior of O2molecule on δ-Pu(100)surface by DMOL3 program, DFT and periodic plate model.The results showed that O dissociative adsorption was found to be energetically more favorable than molecular adsorption and the dissociative oxygen atom preferred adsorption on the center site. Li et al.[22]calculated the electronic structure of Pu compounds by using DFT.The results indicated that PuH2compound has the metallic character, and the Pu–H bonds in PuH3compound, Pu–O bond in PuO compound, Pu–N bond in PuN compound and Pu–S bond compound were essentially ionic in character. The Pu–C bond in PuC compound, Pu–O bond in Pu2O3compound and Pu–O bond in PuO2compound were covalent in character. One can see that the predecessors mostly used DMOL3 program to study the adsorption and dissociation of C, N, O, O2, and CO2on Pu surface and the results may be controversial,however,the VASP program has not been reported publicly to perform such researches. In the VASP program used is PAW pseudopotential, belonging to density functional method, which has the faster calculation speed and higher efficiency with the adjustable parameters than the DMOL3 program in which used is DSPP basic,thus, we select the VASP program to perform calculation in this paper.

To the best of our knowledge, the previous reports paid much attention to the adsorption of individual molecules,however, there have existed few researches on the co-adsorption process, such as CO2and O2co-adsorption addressed in this work, which might be more important from the viewpoint of practical applications. On the other hand, mechanically, δphase plutonium is more ductile and readily-processing than its another allotrope, such as the room-temperature α phase.Therefore,we plan to perform a first principles calculation for the co-adsorption of O2and CO2on a representative face of δphase plutonium,i.e.,δ-Pu(100)surface. Specially,we focus on the stable adsorption configuration,the change of electronic structure,the adsorption energy as well as the charge transfer induced by co-adsorption, which might give a new perspective of the corrosion and anti-corrosion mechanisms of O2and CO2with δ-Pu(100)surface.

2. Computational details

2.1. Methods

All calculations are implemented within Vienna ab-initio simulation package (VASP5.4.4)[23]on the basis of density functional theory (DFT).[24,25]It uses projection augmented wave(PAW)[26,27]pseudopotential method for first-principles calculation, which is an efficient plane wave pseudopotential package and widely used in the calculation of electronic structures.[28–30]Generalized gradient approximation (GGA)in the form of Perdew–Burke–Ernzerhof (PBE) exchange–correlation potential is used to approximate the electron exchange correlation interactions.[31,32]There are 16, 4, and 6 valence electrons in the outer shells for Pu ion(6s26p65f67s2),C ion(2s22p2),and O ion(2s22p4),respectively. Monkhorst–Pack (MP)[33]scheme is used to sample data inside the irreducible Brillouin-zone. Spin polarization effect is considered in all calculations.

The ion position,cell shape and cell volume are changed when the δ-Pu cell is optimized with a 7×7×7 k-point mesh used. The lower two layers are fixed and the upper two layers are released for optimization with the shape and volume unchanged when the δ-Pu(100)4-layer slab is optimized. The 4 layers of Pu atoms are fixed,and O2and CO2molecules[16]on the optimized surface are released when the O2and CO2adsorption system is optimized. The k-point mesh of δ-Pu(100)surface and adsorption system are both set to be 5×5×1. The proportional coefficient of acting force is 0.5 and the N-thorder Methfessel–Paxton method is applied to the optimization of the δ-Pu unit cell, its surface and adsorption system.For all optimization calculations,a plane-wave cut off energy of 450 eV is adopted to insure that the total energy of all structures is converged within 0.5 meV/atom, and the conjugate gradient (CG)[34]method of Hellman–Feynman force is applied to the ion optimization. The convergence criterion of atomic force is 10?2eV/?A and the energy convergence criterion is 10?4eV,respectively. The total energy is acquired by one-step self-consistent calculation on the basis of the optimized model. The wave function, charge density,work function and charge transfer of O2molecule, CO2molecule, δ-Pu(100) surface and adsorption system are also addressed in this paper.

The surface energy, Esur, is then obtained from the following equation:

where A is the cross-sectional area of the slab,Eslabis the total energy of the slab, N is the number of Pu atoms in the slab,and Ebulkis the energy of a single Pu atom in the bulk.

The co-adsorption energy of O2and CO2molecules on the δ-Pu(100)surface is given by

2.2. Models

2.2.1. Surface model

Fig.1. Structure model of δ-Pu(100)slab and top view of three adsorption sites on surface(Pu atoms on surface and on other three layers are green-and blue-colored, respectively): (a)before optimization, (b)after optimizations,and(c)top view;A:bridge site,B:hollow sites,and C:top site.

2.2.2. Co-adsorption models

The adsorption model contains sixteen Pu atoms and one O2and one CO2molecule with a total of 21 atoms. Moreover,the monolayer adsorption coverage(θ)of O2and CO2is 0.25.According to our previous work,[38,39]the best adsorption configuration of O2and CO2on δ-Pu(100)surface is bridge parallel 2 and hollow parallel 1, respectively. Consequently, the co-adsorption behavior of O2and CO2on δ-Pu(100) surface is investigated by studying the co-adsorption configuration of CO2and O2when they occupy the individual best adsorption configuration,and the following models are established.

Fig.2. Top and side views of different models for O2 adsorption on the δ-Pu(100)surface in CO2-based case(Pu on surface, Pu, C,and O atoms on other three layers are green-,blue-,grey-,and red-colored,respectively): (a)bridge parallel,(b)bridge vertical,(c)hollow parallel,(d)hollow vertical,(e)top parallels,and(f)top vertical.

Fig.3. Top and side views of different models for CO2 adsorption on δ-Pu(100)surface in O2-based case(Pu on surface,Pu,C,and O atoms on other three layers are green-,blue-,grey-,and red-colored,respectively): (a)bridge vertical 1,(b)bridge parallel 1,(c)bridge parallel 2,(d)bridge vertical 2,(e)hollow vertical 1,(f)hollow vertical 2,(g)hollow parallel,(h)top parallel 1,(i)top parallel 2,and(j)top vertical.

(i)O2molecules are placed at the top,bridge and hollow sites to establish a co-adsorption model when CO2is at the best adsorption configuration (denoted as CO2-based). Furthermore, each site involving with one of two cases: O2approaches parallelly or vertically to CO2,for this reason,there are six co-adsorption models as shown in Fig.2. (ii) Similarly, CO2molecules are placed at three sites to establish a co-adsorption model when O2molecule is in the best adsorption configuration(abbreviated as O2-based),and there are 10 adsorption models as shown in Fig.3.

3. Results and discussion

3.1. Adsorption configurations and energy

Six adsorption models in CO2-based case and ten coadsorption models in O2-based case are optimized. We define configurations in accordance with the adsorption positions occupied by O2and the bonding numbers of C and O atoms with Pu atoms adjacent to the surface layer in CO2-based case in order to distinguish them. For example, Bp-C4O6indicates that the adsorption state of O2is bridge parallel and C and O atoms are bonded with 4 and 6 Pu atoms adjacent to the surface layer, respectively. There are six adsorption configurations as shown in Fig.4. Similarly, we use the adsorption sites occupied by CO2and the bonding numbers of C and O atoms with Pu atoms adjacent to the surface layer to define the configuration in the O2-based case. For instance, Hv1-C4O8means that the adsorption state of CO2is hollow vertical 1 and C and O atoms are bonded with 4 and 8 Pu atoms adjacent to the surface layer,respectively. As a result,ten adsorption configurations are plotted in Fig.5. The optimized geometrical and energy parameters of O2and CO2co-adsorption on the δ-Pu(100)surface are listed in Table 1.

Fig.4. Top and side views of optimized structures for O2 adsorption on δ-Pu(100)surface in CO2-based case(Pu on surface,Pu,C,and O atoms on other three layers are green-,blue-,grey-,and red-colored,respectively): (a)Bp-C4O6,(b)Bv-C0O6,(c)Hp-C4O6,(d)Hv-C4O6,(e)Tp-C4O7,and(f)Tv-C4O7;Bp: bridge parallel,Bv: bridge vertical,Hp: hollow parallel,Hv: hollow vertical,Tp: top parallel,and Tv: top vertical.

Fig.5. Top and side views of optimized structures for CO2 adsorption on δ-Pu(100)surface in O2-based case(Pu on surface, Pu, C,and O atoms on other three layers are green-,blue-,grey-,and red-colored,respectively): (a)Bv1-C2O8,(b)Bp1-C1O9,(c)Bp2-C2O8,(d)Bv2-C1O9,(e)Hv1-C4O8,(f)Hv2-C2O7,(g)Hp-C2O6,(h)Tp1-C2O8,(i)Tp2-C1O9,and(j)Tv-C0O7;Bv1: bridge vertical 1,Bp1: bridge parallel 1,Bp2: bridge parallel 2,Bv2: bridge vertical 2,Hv1:hollow vertical 1,Hv2: hollow vertical 2,Hp: hollow parallel,Tp1: top parallel 1,Tp2: top parallel 2,and Tv: top vertical.

Table 1. Optimized geometrical and energy parameters of O2 and CO2 co-adsorption on δ-Pu(100)surface. d denotes bond length between C and O atoms,dC?Pu or dO?Pu is average bond distance between the adsorbed C or O atoms and neighbor surface Pu atoms,θO?C?O is OCO bond angle,hC?S or hO?S is average adsorption height of C or O atoms with respect to the first layer,and Eco?ads is co-adsorption energy.

3.2. Bader charge distribution

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 among C, O, and Pu atoms originating from O2and CO2co-adsorption on the δ-Pu(100)surface.In order to clearly elucidate the interaction among O2, CO2molecules and Pu surface atoms,Bader charge distribution[44]of(O2&CO2)/δ-Pu(100)co-adsorption system are calculated as listed in Table 2. The qsubstancein Table 2 is defined as follows:

where qatom, qBader, and qvalenceare the net charge, Bader charge, and valence electron of atom, respectively. Figures 6 and 7 show the charge distribution of ions in CO2-based and O2-based co-adsorption system,respectively.

As shown in Table 2, from the perspective of charge transfer, CO2molecule and two O atoms are in different charge states (in a range of 3.4654e–4.7718e) in the CO2-based case,and the q1stchanges obviously from ?0.4220e to?4.4978e–3.6621e,while q2ndchanges little from 0.5124e to?0.4217e–0.6702e.The q3rdchanges slightly from 0.2700e to?0.5124e–?0.4823e,the least change appears in the q4thcase(from ?0.3604e to ?0.4627e–?0.2643e).

Table 2. Net charge distribution of(O2 &CO2)/δ-Pu(100)co-adsorption system. qtotal is total net charge number of the C,O1,O2,O3,and O4 ions,q1st,q2nd,q3rd,and q4th represent total net charge numbers of the first layer to the fourth layer on the δ-Pu(100)surface,respectively. All q’s values are in units of e.

Fig.6. Charged state of each atom in CO2-based co-adsorption system,with red-and blue-colored spheres denoting positively and negatively charged atoms,respectively,charge ranging from ?0.20e to+0.20e. (a)Bp-C4O6,(b)Bv-C0O6,(c)Hp-C4O6,(d)Hv-C4O6,(e)Tp-C4O7,and(f)Tv-C4O7.

Fig.7. Charged state of each atom in O2-based co-adsorption system, with red-and blue-colored spheres denoting positively and negatively charged atoms, respectively, and charge ranging from ?0.20e to +0.20e. (a) Bv1-C2O8, (b) Bp1-C1O9, (c) Bp2-C2O8, (d) Bv2-C1O9, (e)Hv1-C4O8,(f)Hv2-C2O7,(g)Hp-C2O6,(h)Tp1-C2O8,(i)Tp2-C1O9,and(j)Tv-C0O7.

Similarly, in the O2-based case, the adsorbed C atoms and four O atoms are charged with 3.3860e–5.7961e, and qtotal value of Bp1-C1O9,Bv2-C1O9,Tp1-C2O8,and Tp2-C1O9sites are 5.7961e,5.4948e,5.5865e,and 5.4974e,respectively,in which the adsorbed C atoms are charged with 0.6743e,0.6012e, 0.6385e, and 0.5583e, respectively, indicating that the valences of C atoms are negative, however, in the other six configurations,the net charges of C atoms are negative and the valence are positive, demonstrating that these four sites react strongly with Pu surface atoms, and the adsorption energy values are ?15.768 eV, ?19.868 eV, ?23.131 eV, and?13.705 eV, respectively, which also establish the strong reaction in these configurations. The q1stchanges remarkably from ?0.4220e to ?5.1518e–?3.4615e,the q2ndchanges little from 0.5124e to ?0.8202e–0.1396e,the q3rdchanges slightly from 0.2700e to 0.0776e–0.5668e, the least change appears for the q4thcase(from ?0.3604e to 0.5067e–?0.2621e). All the above-mentioned results demonstrate that charge mainly transfer from surface Pu atoms to C and O atoms, namely, C and O atoms mainly interact with Pu surface atoms,while the interaction with other three layers of Pu atoms is weak.

The transfer charge is 4.7718e for the Tv-C4O7site,which is less than that(5.5865e)for Tp1-C2O8configuration,revealing that the charge transfer and reaction intensity of the latter are stronger than those for the Tp1-C2O8configuration.In the CO2-based case, the qtotalvalues of two top sites (Tp-C4O7and Tv-C4O7) are 4.5491e and 4.7718e, respectively,which are larger than those of bridge and hollow sites. Therefore,as compared with the hollow and bridge sites,top site is very favorable for charge transfer.

Figures 6 and 7 reflect the charge state of each atom after co-adsorption,one can see that O atoms always gain electrons and are negatively charged,which also hold the C atoms in Tv-C4O7, Bp1-C1O9, Bv2-C1O9, Tp1-C2O8, and Tp2-C1O9configurations, however, C atoms and Pu surface atoms lose electrons and are positively charged in the other 11 configurations,which are consistent with the results of Table 2.

3.3. Electronic structure analysis

From the viewpoint of the microscopic mechanism, coadsorption will result in the change of the electronic structure. For exploring the reaction mechanism among O2, CO2molecules, and Pu surface atoms, binding character among the O2, CO2molecules, and Pu surface has been examined by the charge–density difference. For the fully relaxed, minimum total-energy Tv-C4O7and Tp1-C2O8configurations in the CO2-based and O2-based case, respectively, the chargedensity difference is given by

Fig.8. Isosurfaces of charge–density difference for (a) Tv-C4O7 and (b)Tp1-C2O8 configurations,with yellow and blue colors denoting increase and decrease in charge density,respectively(Isosurfaces level: 0.005 e/?A3).

In order to clearly demonstrate the chemical state in Tv-C4O7configuration and Tp1-C2O8configuration, we plot the two-dimensional (2D) contour isosurfaces of the chargedensity difference for C1,O1,O2atoms and O1,O2,O3 atoms along the best planes of(?1 153.66 11.37)and(1 2.14 266.76)in Figs.9(a)and 9(b),respectively. One can see that C and O atoms are surrounded by electrons, and there is no unpaired electrons between C/O and Pu atoms,demonstrating the ionic chemical bond, which is in good agreement with the threedimensional(3D)isosurfaces of the charge–density difference in Fig.8.

Fig.9. 2D charge–density difference for (a) Tv-C4O7 and (b) Tp1-C2O8 configurations.

Figure 10 shows the total density of states (TDOS) and partial density of states (PDOS) of the clean δ-Pu(100) surface, and TDOS and PDOS of the most stable adsorption configuration(Tv-C4O7)in CO2-based co-adsorption system.Figure 12(a)shows the corresponding PDOS of Tv-C4O7configuration. From Figs. 10(a) and 10(b) one can see that the peak intensity, position and shape of TDOS change remarkably due to the co-adsorption. The intensity of the first peak increases and turns narrower, while the intensity of the second peak decreases and the peak shape broadens in a range of ?50 eV–42.5 eV.The number of peaks increases,the peak shape broadens and the area increases from ?25 eV to ?15 eV.In addition, the number and intensity of peaks increase in a range of ?12.5 eV–?2.5 eV.However,the peak intensity decreases and the peak shape broadens from ?2.5 eV to 2.5 eV.Secondly, as shown in Figs. 10(c), 10(d), and 12(a), the hybridization behavior occurs among C 2s, C 2p, O 2s orbitals,and Pu 6p orbital in a low energy range of ?25 eV–?15 eV,moreover, hybridization also occurs among C 2s, C 2p, O 2p orbitals, and Pu 6d, Pu 5f orbitals in a high energy range of?10 eV–?2.5 eV.The intensity of Pu 5f and Pu 6d peaks decreases evidently,and the peak shape broadens in the vicinity of Fermi level(EF),which indicates that the delocalization of Pu 5f and Pu 6d electrons become stronger,and a small number of Pu 6d and Pu 5f electrons participate in bonding as well.

Fig.10. (a) TDOS of clean δ-Pu (100) surface, (b) TDOS of the most stable configuration(Tv-C4O7)in CO2-based co-adsorption system,(c)PDOS of clean δ-Pu(100)surface, and(d)PDOS of the most stable configuration(Tv-C4O7)in CO2-based co-adsorption system.

Figure 11 displays the TDOS and PDOS of the clean δ-Pu(100)surface,and TDOS and PDOS of the most stable adsorption configuration(Tp1-C2O8)in O2-based co-adsorption system. Figure 12(b)shows the corresponding PDOS of Tp1-C2O8configuration. From Figs.11(a)and 11(b),one can see that the peak intensity, position and shape of TDOS change obviously after co-adsorption. The intensity of the first peak increases and the peak shape narrows, while the intensity of the second peak decreases and the peak shape broadens from?50 eV to ?42.5 eV. The peak intensity increases, the number of peaks increases, the peak shape broadens and the area increases in a range of ?25 eV–?15 eV.Moreover, the number, intensity and area of peaks increase from ?12.5 eV to?2.5 eV. The peak intensity decreases, the number of peaks increases,and the peak shape broadens remarkably in a range of ?2.5 eV–2.5 eV.As shown in Figs.11(c),11(d),and 12(b),the C 2s, C 2p, and O 2s orbitals overlap and hybridize with Pu 6p orbital in a low energy range of ?25 eV–?15 eV, furthermore,the C 2s,C 2p,O 2s,and O 2p orbitals overlap and hybridize with Pu 6d and Pu 5f orbitals in a high energy range of ?10 eV–2.5 eV.In the vicinity of EF,the intensity of Pu 5f peak decreases obviously, while the peak intensity decreases slightly and the Pu 6d peak shape broadens, demonstrating that the delocalization of Pu 5f and Pu 6d electrons becomes stronger,and Pu 6d and Pu 5f electrons also take part in bonding,in addition,the Pu 5f electrons make a main contribution.

Fig.11. (a) TDOS of clean δ-Pu (100) surface, (b) TDOS of the most stable configuration (Tp1-C2O8) in O2-based co-adsorption system, (c) PDOS of clean δ-Pu (100) surface and (d) PDOS of the most stable configuration(Tp1-C2O8)in O2-based co-adsorption system.

Fig.12. PDOS of the most stable configuration: (a)Tv-C4O7 for CO2-based system and(b)Tp1-C2O8 for O2-based system.

Furthermore,the change of the DOS in Tp1-C2O8configuration is greater than that in Tv-C4O7configuration,which indicates that the reaction intensity for Tp1-C2O8site is stronger than that for Tv-C4O7site,and the reaction mechanism among O2, CO2molecules and surface Pu atoms is that the C 2s, C 2p,O 2s,and O 2p orbitals overlap and hybrid with Pu 6p,Pu 6d, and Pu 5f orbitals, the delocalization of Pu 5f and Pu 6d electrons becomes stronger,resulting in a new chemical bonding state,and the O 2s,O 2p,and Pu 5f orbitals make a major contribution.

3.4. Surface work function analysis

Chemically,work function refers to the minimum energy that is required by 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,showing the ability of electrons to escape from the metal surface,which reads

where Φ, Evacuum, and EFermiare the work function, vacuum level,and Fermi level,respectively.The surface work function before and after co-adsorption are listed in Table 3.

Table 3. Surface work function changes (?Φ) of different adsorption configurations. All values are in units of eV.

At present, there are no data of experimental work function for δ-Pu. The work function of its neighboring element(uranium)is in a range of 3.63 eV–3.90 eV.[45]The work function of δ-Pu(100)exposed surface,calculated by Wei et al.,is 4.365 eV,[46]the work function of δ-Pu(111)slab, calculated by Atta-Fynn et al.,is 3.39 eV,[19]and the work function of δ-Pu(020) surface, calculated under non-spin polarization condition, is 3.53 eV.[18]In this work, the calculated work function of δ-Pu(100) surface is 2.9531 eV, and the surface work functions are in a range of 0.686 eV–2.245 eV of the six adsorption states for CO2-based co-adsorption. What is more,at Tv-C4O7site,the work function increases the least(0.686 eV)with an adsorption energy of ?17.296 eV,demonstrating that the energy is lowest and the site is most stable,for this reason,other electrons readily escape from the metal surface and the required energy is least.

The surface work functions of ten adsorption configurations increase from 0.146 eV to 1.721 eV for O2-based coadsorption. In addition, the work function of Tp1-C2O8configuration is smallest (3.125 eV), and the adsorption energy is also smallest (?23.131 eV), indicating that the energy is lowest and the structure is most stable,so that other electrons readily escape from the metal surface and the required energy is smallest.The increase of surface work function for Tv-C4O7configuration is 0.686 eV,which is larger than that(0.172 eV)for Tp1-C2O8site, revealing that the latter is more stable and the energy is lower, thus, the energy needed for electrons to escape from the solid surface is smaller.

4. Conclusions

Acknowledgment

All the work was performed on TianHe-2 platform,Lvliang Cloud Computing Center of China.