By the first-principles calculations, most studies indicated that the （1102）-CoO2 termination of LaCoO3 cannot be stabilized, which disagrees with the experimental observation. Besides the crystal structure, we foun...By the first-principles calculations, most studies indicated that the （1102）-CoO2 termination of LaCoO3 cannot be stabilized, which disagrees with the experimental observation. Besides the crystal structure, we found that the spin states of Co3＋ ions could affect surface stability, which previously were not well considered. By examining the different states of Co3＋ ions in hexagonal-phase LaCoO3, including low spin, intermediate spin, and high spin states, the surface grand potentials of these facets are calculated and compared. The results show that the spin states of Co3＋ ions have an important influence on stability of the LaCoO3 facets. Different from the previous results, the stability diagrams demonstrate that the （1102）- CoO2 termination can stably exist under O-rich condition, which can get an agreement with the experimental ones. Furthermore, the surface oxygen vacancy formation energies （Eov） of stable facets are computed in different spin states. The Eov of these possible exposed terminations strongly depend on the spin state of Co3＋ ions： in particular, the Eov of the HS states is lower than that of other spin states. This indicates that one can tune the properties of LaCoO3 by directly tuning the spin states of Co3＋ ions.展开更多
Metal-doped ceria catalysts have been applied in many important catalytic processes.In this work,we performed density functional theory calculations corrected by on-site Coulomb interactions to study the Pd-and Zr-dop...Metal-doped ceria catalysts have been applied in many important catalytic processes.In this work,we performed density functional theory calculations corrected by on-site Coulomb interactions to study the Pd-and Zr-doped CeO_(2)(111) surfaces with the dopant at different locations.The formation of oxygen vacancies and CO oxidation were systematically calculated on the various doped surfaces.We find that both Pd and Zr doping can activate the surface lattice O and reduce the energy barriers of CO oxidation.However,the promotion effect of the Zr dopant is limited to its existence in the first surface layer,while for the Pd dopant,the surface activity can be greatly enhanced even it occurs far below the surface.Besides,CO_(2) can be generated directly on the Pd-doped surfaces through reaction between CO and surface O,while the surface intermediate CO_(2)^(δ-) may readily form and restrict the releasing of CO_(2) by further oxidation to carbonates on the Zr-doped surfaces.Electronic analyses show that the doped Pd exists as Pd^(4+) and it has stronger electron affinity than other surface species during CO oxidation,contributing to the easy Pd^(4+) to Pd^(2+) transformation accompanied by direct CO_(2) formation at Pd-doped ceria.展开更多
The presence of chloride ions is the critical factor of passivity breakdown of the protective film and eventually leads to localized corrosion. However, the mechanism and the role of chlorides in these processes are s...The presence of chloride ions is the critical factor of passivity breakdown of the protective film and eventually leads to localized corrosion. However, the mechanism and the role of chlorides in these processes are still controversial. Hematite and chromia are generally believed to be the major components of outer and inner oxide layers on stainless steels. In the present paper, a comparative study of Cl ingress into pristine and defective α-Fe_(2)O_(3)(0001) surface, α-Cr_(2)O_(3)(0001) surface, along with their interface, was conducted using density functional theory. Vacancy formation energy calculation confirms good stability of α-Cr_(2)O_(3)and high reactive activity of the interface region. Cl inserts into an O vacancy is energetically more favorable than Fe vacancy and interstitial site, demonstrating Cl-induced degradation complies with the ion exchange model. Transition state search for Cl diffusion through O vacancies shows α-Cr_(2)O_(3)is more protective than α-Fe_(2)O_(3), while the interface region is the weak point of the duplex passive film.展开更多
The effects of Fe-C/N co-doping on the electronic and optical properties of NaTaO3 are studied with density func- tional theory. Our calculations indicate that mono-doped and co-doped sodium tantalate are both thermod...The effects of Fe-C/N co-doping on the electronic and optical properties of NaTaO3 are studied with density func- tional theory. Our calculations indicate that mono-doped and co-doped sodium tantalate are both thermodynamically stable. The co-doping sodium tantalate can reduce the energy band gap to a greater degree due to the synergistic effects of Fe and C （N） atoms than mono-doping sodium tantalate, and has a larger optical absorption of the whole visible spectrum. The band alignments for the doped NaTaO3 are well positioned for the feasibility of hydrogen production by water splitting. The Fe--C co-doping can enhance the absorption of the visible light and its photocatalytic activity more than Fe-N co-doping due to the different locations of impurity energy levels originating from their p-d hybridization effect.展开更多
Influence of doping and oxygen vacancy concentrations on oxygen ion or oxygen vacancy（V） migration energies of SmxCe1-xO2-δ（x = 0.0625, 0.125, 0.25 and δ = 0.0625, 0.125） systems using a GGA+U method are studied...Influence of doping and oxygen vacancy concentrations on oxygen ion or oxygen vacancy（V） migration energies of SmxCe1-xO2-δ（x = 0.0625, 0.125, 0.25 and δ = 0.0625, 0.125） systems using a GGA+U method are studied. Calculated results show that advantage migration types change from V?O2- to O2- ?V as x and δ increase. For V?O2- migrations of the Sm0.0625Ce0.9375O1.9375 and Sm0.125Ce0.875O1.9375 systems, electrostatic attractions between Sm3+ and V, defect associations between Ce3+ and V, and steric hindrances of Sm3+ affect the migration energies. For O2- ?V migrations of the Sm+（0.125）Ce0.875O1.875 and Sm0.25Ce0.75O1.875 systems, migration energies of O2- are affected by electrostatic repulsions between Sm3+ and O2- and defect associations between Ce3+ and V. Increases of the oxygen vacancy and Sm3+ doping concentrations benefit the oxygen ion and vacancy migrations, respectively.展开更多
An efficient and economical oxygen evolution reaction(OER)catalyst is critical to the widespread application of solar energy to fuel conversion.Among many potential OER catalysts,the metal oxyhydroxides,especially FeO...An efficient and economical oxygen evolution reaction(OER)catalyst is critical to the widespread application of solar energy to fuel conversion.Among many potential OER catalysts,the metal oxyhydroxides,especially FeOOH,show promising OER reactivity.In the present work,we performed a DFT+U study of the OER mechanism on theγ‐FeOOH(010)surface.In particular,we established the chemical potential of the OH?and hole pair and included the OH?anion in the reaction pathway,accounting to the alkaline conditions of anodic OER process.We then analyzed the OER pathways on the surface with OH‐,O‐and Fe‐terminations.On the surface with OH‐and O‐terminations,the O2molecule could form from either OH reacting with the surface oxygen species(-OH*and-O*)or the combination of two surface oxygen species.On the Fe‐terminated surface,O2can only form by adsorbing OH on the Fe sites first.The potential‐limiting step of the oxygen evolution with different surface terminations was determined by following the free‐energy change of the elementary steps along each pathway.Our results show that oxygen formation requires recreating the surface Fe sites,and consequently,the condition that favors the partially exposed Fe sites will promote oxygen formation.展开更多
First-principles calculations based on density functional theory corrected by Hubbard parameter U （DFT＋U） are applied to the study on the co-adsorption of O2 and H2O molecules to a-U（110） surface. The calculation...First-principles calculations based on density functional theory corrected by Hubbard parameter U （DFT＋U） are applied to the study on the co-adsorption of O2 and H2O molecules to a-U（110） surface. The calculation results show that DFT＋U method with Ueff = 1.5 eV can yield the experimental results of lattice constant and elastic modulus of a-uranium bulk well. Of all 7 low index surfaces of a-uranium, the （001） surface is the most stable with lowest surface energy while the （110） surface possesses the strongest activity with the highest surface energy. The adsorptions of O2 and H2O molecules are investigated separated. The O2 dissociates spontaneously in all initial configurations. For the adsorption of H2O molecule, both molecular and dissociative adsorptionsoccur. Through calculations of co-adsorption, it can be confirmed that the inhibition effect of O2 on the corrosion of uranium by water vapor originates from the preferential adsorption mechanism, while the consumption of H atoms by O atoms exerted little influence on the corrosion of uranium.展开更多
The density functional calculation is performed for centrosymmetric（La–Pm） GaO3 rare earth gallates, using a full potential linear augmented plane wave method with the LSDA and LSDA＋U exchange correlation to treat...The density functional calculation is performed for centrosymmetric（La–Pm） GaO3 rare earth gallates, using a full potential linear augmented plane wave method with the LSDA and LSDA＋U exchange correlation to treat highly correlated electrons due to the very localized 4f orbitals of rare earth elements, and explore the influence of U = 0.478 Ry on the magnetic phase stability and the densities of states. LSDA＋U calculation shows that the ferromagnetic（FM） state of RGaO3 is energetically more favorable than the anti-ferromagnetic（AFM） one, except for LaGaO3 where the NM state is the lowest in energy. The energy band gaps of RGaO3 are found to be in the range of 3.8–4.0 eV, indicating the semiconductor character with a large gap.展开更多
Hybrid density functional theory is employed to systematically investigate the structural,magnetic,vibrational,thermodynamic properties of plutonium monocarbide（Pu C and Pu C0.75）.For comparison,the results obtained...Hybrid density functional theory is employed to systematically investigate the structural,magnetic,vibrational,thermodynamic properties of plutonium monocarbide（Pu C and Pu C0.75）.For comparison,the results obtained by DFT,DFT ＋ U are also given.For Pu C and Pu C0.75,Fock-0.25 hybrid functional gives the best lattice constants and predicts the correct ground states of antiferromagnetic（AFM） structure.The calculated phonon spectra suggest that Pu C and Pu C0.75 are dynamically stable.Values of the Helmholtz free energy ？F,internal energy ？E,entropy S,and constant-volume specific heat Cv of Pu C and Pu C0.75 are given.The results are in good agreement with available experimental or theoretical data.As for the chemical bonding nature,the difference charge densities,the partial densities of states and the Bader charge analysis suggest that the Pu–C bonds of Pu C and Pu C0.75 have a mixture of covalent character and ionic character.The effect of carbon vacancy on the chemical bonding is also discussed in detail.We expect that our study can provide some useful reference for further experimental research on the phonon density of states,thermodynamic properties of the plutonium monocarbide.展开更多
The effects of cation ordering and surface compensating anions on the magnetic structure and catalytic properties of unilamellar Ni-Fe hydroxide nanosheets are studied by using the density functional theory （DFT） pl...The effects of cation ordering and surface compensating anions on the magnetic structure and catalytic properties of unilamellar Ni-Fe hydroxide nanosheets are studied by using the density functional theory （DFT） plus U method. Fe-segregation in the nanosheets yields magnetic domains with different spin alignments, while the surface compensating anions affect the local moments and valence states of the Fe atoms. The two conditions do not radically change the super-exchange nature of interactions between the paramagnetic metal centers, hut facilitate the formation of various magnetic superlattices in the nanosheets. The calculated free energy change of the intermediates shows that the most stable magnetic structure of Ni-Fe hydrox- ide nanosheets exhibits superior catalytic activity towards oxygen reduction/evolution reactions, which is indicative of magnetic catalyst. This is due to the cycle transition between Fe2＋ and Fe~~ ions in the reactions, which determines the sequence of cleavage of the O-H bond and the release of the OH group, controlling the rate-limiting steps of the reaction. The relationship of magnetism and catalytic activity of Ni-Fe hydroxide nanosheets is established by the valence state change of the Fe ions, which will be helpful to open the way for the design of hydroxide/layered double hydroxides （LDHs）-based magnetic catalysts.展开更多
CO oxidation at ceria surfaces has been studied for decades,and many efforts have been devoted to understanding the effect of surface reduction on the catalytic activity.In this work,we theoretically studied the CO ox...CO oxidation at ceria surfaces has been studied for decades,and many efforts have been devoted to understanding the effect of surface reduction on the catalytic activity.In this work,we theoretically studied the CO oxidation on the clean and reduced CeO_(2)(111)surfaces using different surface cells to dete rmine the relationships between the reduction degrees and calculated reaction energetics.It is found that the calculated barrier for the direct reaction between CO and surface lattice O drastically decreases with the increase of surface reduction degree.From electronic analysis,we found that the surface reduction can lead to the occurrence of localized electrons at the surface Ce,which affects the charge distribution at surface O.As the result,the surface O becomes more negatively charged and therefore more active in reacting with CO.This work then suggests that the localized 4 f electron reservoir of Ce can act as the"pseudo-anion"at reduced CeO_(2) surfaces to activate surface lattice O for catalytic oxidative reactions.展开更多
DFT+U is a widely used treatment in the density functional theory(DFT)to deal with correlated materials that contain open-shell elements,whereby the quantitative and sometimes even qualitative failures of local and se...DFT+U is a widely used treatment in the density functional theory(DFT)to deal with correlated materials that contain open-shell elements,whereby the quantitative and sometimes even qualitative failures of local and semi-local approximations can be corrected without much computational overhead.However,finding appropriate U parameters for a given system and structure is non-trivial and computationally intensive,because the U value has generally a strong chemical and structural dependence.In this work,we address this issue by building a machine learning(ML)model that enables the prediction of material-and structure-specific U values at nearly no computational cost.Using Mn–O system as an example,the ML model is trained by calibrating DFT+U electronic structures with the hybrid functional results of more than 3000 structures.The model allows us to determine an accurate U value(MAE=0.128 eV,R^(2)=0.97)for any given Mn–O structure.Further analysis reveals that M–O bond lengths are key local structural properties in determining the U value.This approach of the ML U model is universally applicable,to significantly expand and solidify the use of the DFT+U method.展开更多
Cl-containing cerium dioxide(Ce O2) catalysts have been found to exhibit unique catalytic activities. In the present work, using density functional theory calculations with the inclusion of on-site Coulomb correction,...Cl-containing cerium dioxide(Ce O2) catalysts have been found to exhibit unique catalytic activities. In the present work, using density functional theory calculations with the inclusion of on-site Coulomb correction, we systematically studied the effect of Cl on the physicochemical properties of Ce O2 surfaces by substituting one subsurface O with Cl. The calculated results show that substituting an O atom with a Cl atom results in structural distortion and the reduction of one surface Ce4+ cation to Ce3+. The protruding Ce3+ cation greatly improves the adsorption energy of O2 to produce an active O2- species, and maintains the catalytic oxidation cycle of CO on Ce O2(110). These results may help us obtain a better understanding of Cl-ceria interacting systems and provide some guidance for the design of effective Ce O2-based catalysts.展开更多
基金This work was supported by the National Natural Science Foundation of China （No.U1232118, No.21203099）, the National Basic Research Program （No.2014CB932403）, the Program of Introducing Talents of Disciplines to China Universities （No.B06006）, Research Program for Advanced and Applied Technology of Tianjin （No.13JCYBJC36800）, Doctoral Fund of Ministry of Education of China （No.20120031120033）, Fok Ying Tung Education Foundation （No.151008）, and Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund （the second phase）. We appreciate the supports from the National Super-Computing Center at Tianjin and Guangzhou.
文摘By the first-principles calculations, most studies indicated that the （1102）-CoO2 termination of LaCoO3 cannot be stabilized, which disagrees with the experimental observation. Besides the crystal structure, we found that the spin states of Co3＋ ions could affect surface stability, which previously were not well considered. By examining the different states of Co3＋ ions in hexagonal-phase LaCoO3, including low spin, intermediate spin, and high spin states, the surface grand potentials of these facets are calculated and compared. The results show that the spin states of Co3＋ ions have an important influence on stability of the LaCoO3 facets. Different from the previous results, the stability diagrams demonstrate that the （1102）- CoO2 termination can stably exist under O-rich condition, which can get an agreement with the experimental ones. Furthermore, the surface oxygen vacancy formation energies （Eov） of stable facets are computed in different spin states. The Eov of these possible exposed terminations strongly depend on the spin state of Co3＋ ions： in particular, the Eov of the HS states is lower than that of other spin states. This indicates that one can tune the properties of LaCoO3 by directly tuning the spin states of Co3＋ ions.
基金Project supported by National Key R&D Program of China(2018YFA0208602)National Natural Science Foundation of China (21825301,22003016,92045303)+2 种基金the Fundamental Research Funds for the Central Universities(222201717003)Shanghai Municipal Science and Technology Major Project(2018SHZDZX03)the Programme of Introducing Talents of Discipline to Universities (B16017)。
文摘Metal-doped ceria catalysts have been applied in many important catalytic processes.In this work,we performed density functional theory calculations corrected by on-site Coulomb interactions to study the Pd-and Zr-doped CeO_(2)(111) surfaces with the dopant at different locations.The formation of oxygen vacancies and CO oxidation were systematically calculated on the various doped surfaces.We find that both Pd and Zr doping can activate the surface lattice O and reduce the energy barriers of CO oxidation.However,the promotion effect of the Zr dopant is limited to its existence in the first surface layer,while for the Pd dopant,the surface activity can be greatly enhanced even it occurs far below the surface.Besides,CO_(2) can be generated directly on the Pd-doped surfaces through reaction between CO and surface O,while the surface intermediate CO_(2)^(δ-) may readily form and restrict the releasing of CO_(2) by further oxidation to carbonates on the Zr-doped surfaces.Electronic analyses show that the doped Pd exists as Pd^(4+) and it has stronger electron affinity than other surface species during CO oxidation,contributing to the easy Pd^(4+) to Pd^(2+) transformation accompanied by direct CO_(2) formation at Pd-doped ceria.
基金supported by the Key Research Program of Frontier Sciences,CAS(No.QYZDY-SSW-JSC012)the Aeronautical Science Foundation of China(No.20200038092002)。
文摘The presence of chloride ions is the critical factor of passivity breakdown of the protective film and eventually leads to localized corrosion. However, the mechanism and the role of chlorides in these processes are still controversial. Hematite and chromia are generally believed to be the major components of outer and inner oxide layers on stainless steels. In the present paper, a comparative study of Cl ingress into pristine and defective α-Fe_(2)O_(3)(0001) surface, α-Cr_(2)O_(3)(0001) surface, along with their interface, was conducted using density functional theory. Vacancy formation energy calculation confirms good stability of α-Cr_(2)O_(3)and high reactive activity of the interface region. Cl inserts into an O vacancy is energetically more favorable than Fe vacancy and interstitial site, demonstrating Cl-induced degradation complies with the ion exchange model. Transition state search for Cl diffusion through O vacancies shows α-Cr_(2)O_(3)is more protective than α-Fe_(2)O_(3), while the interface region is the weak point of the duplex passive film.
基金supported by the National Natural Science Foundation of China(Grant Nos.51572219 and 11204239)the Project of the Natural Science Foundation of Shaanxi Province,China(Grant Nos.2015JM1018,2013JQ1018,and 15JK1714)+1 种基金the Project of Natural Science Foundation of Department of Education of Shaanxi Province,China(Grant No.15JK1759)the Science Foundation of Northwest University of China(Grant No.12NW06)
文摘The effects of Fe-C/N co-doping on the electronic and optical properties of NaTaO3 are studied with density func- tional theory. Our calculations indicate that mono-doped and co-doped sodium tantalate are both thermodynamically stable. The co-doping sodium tantalate can reduce the energy band gap to a greater degree due to the synergistic effects of Fe and C （N） atoms than mono-doping sodium tantalate, and has a larger optical absorption of the whole visible spectrum. The band alignments for the doped NaTaO3 are well positioned for the feasibility of hydrogen production by water splitting. The Fe--C co-doping can enhance the absorption of the visible light and its photocatalytic activity more than Fe-N co-doping due to the different locations of impurity energy levels originating from their p-d hybridization effect.
基金Supported by the National Natural Science Foundation of China(No.51474133)Inner Mongolia Natural Science Foundation(No.2016MS0513)
文摘Influence of doping and oxygen vacancy concentrations on oxygen ion or oxygen vacancy（V） migration energies of SmxCe1-xO2-δ（x = 0.0625, 0.125, 0.25 and δ = 0.0625, 0.125） systems using a GGA+U method are studied. Calculated results show that advantage migration types change from V?O2- to O2- ?V as x and δ increase. For V?O2- migrations of the Sm0.0625Ce0.9375O1.9375 and Sm0.125Ce0.875O1.9375 systems, electrostatic attractions between Sm3+ and V, defect associations between Ce3+ and V, and steric hindrances of Sm3+ affect the migration energies. For O2- ?V migrations of the Sm+（0.125）Ce0.875O1.875 and Sm0.25Ce0.75O1.875 systems, migration energies of O2- are affected by electrostatic repulsions between Sm3+ and O2- and defect associations between Ce3+ and V. Increases of the oxygen vacancy and Sm3+ doping concentrations benefit the oxygen ion and vacancy migrations, respectively.
基金supported by the Chemical,Biological,Environmental,and Transport Systems(CBET)program of US National Science Foundation(CBET-1438440)~~
文摘An efficient and economical oxygen evolution reaction(OER)catalyst is critical to the widespread application of solar energy to fuel conversion.Among many potential OER catalysts,the metal oxyhydroxides,especially FeOOH,show promising OER reactivity.In the present work,we performed a DFT+U study of the OER mechanism on theγ‐FeOOH(010)surface.In particular,we established the chemical potential of the OH?and hole pair and included the OH?anion in the reaction pathway,accounting to the alkaline conditions of anodic OER process.We then analyzed the OER pathways on the surface with OH‐,O‐and Fe‐terminations.On the surface with OH‐and O‐terminations,the O2molecule could form from either OH reacting with the surface oxygen species(-OH*and-O*)or the combination of two surface oxygen species.On the Fe‐terminated surface,O2can only form by adsorbing OH on the Fe sites first.The potential‐limiting step of the oxygen evolution with different surface terminations was determined by following the free‐energy change of the elementary steps along each pathway.Our results show that oxygen formation requires recreating the surface Fe sites,and consequently,the condition that favors the partially exposed Fe sites will promote oxygen formation.
基金Project supported by the National Nature Science Foundation of China(Grant Nos.51401237,11474358,and 51271198)
文摘First-principles calculations based on density functional theory corrected by Hubbard parameter U （DFT＋U） are applied to the study on the co-adsorption of O2 and H2O molecules to a-U（110） surface. The calculation results show that DFT＋U method with Ueff = 1.5 eV can yield the experimental results of lattice constant and elastic modulus of a-uranium bulk well. Of all 7 low index surfaces of a-uranium, the （001） surface is the most stable with lowest surface energy while the （110） surface possesses the strongest activity with the highest surface energy. The adsorptions of O2 and H2O molecules are investigated separated. The O2 dissociates spontaneously in all initial configurations. For the adsorption of H2O molecule, both molecular and dissociative adsorptionsoccur. Through calculations of co-adsorption, it can be confirmed that the inhibition effect of O2 on the corrosion of uranium by water vapor originates from the preferential adsorption mechanism, while the consumption of H atoms by O atoms exerted little influence on the corrosion of uranium.
文摘The density functional calculation is performed for centrosymmetric（La–Pm） GaO3 rare earth gallates, using a full potential linear augmented plane wave method with the LSDA and LSDA＋U exchange correlation to treat highly correlated electrons due to the very localized 4f orbitals of rare earth elements, and explore the influence of U = 0.478 Ry on the magnetic phase stability and the densities of states. LSDA＋U calculation shows that the ferromagnetic（FM） state of RGaO3 is energetically more favorable than the anti-ferromagnetic（AFM） one, except for LaGaO3 where the NM state is the lowest in energy. The energy band gaps of RGaO3 are found to be in the range of 3.8–4.0 eV, indicating the semiconductor character with a large gap.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.21371160 and 21401173)
文摘Hybrid density functional theory is employed to systematically investigate the structural,magnetic,vibrational,thermodynamic properties of plutonium monocarbide（Pu C and Pu C0.75）.For comparison,the results obtained by DFT,DFT ＋ U are also given.For Pu C and Pu C0.75,Fock-0.25 hybrid functional gives the best lattice constants and predicts the correct ground states of antiferromagnetic（AFM） structure.The calculated phonon spectra suggest that Pu C and Pu C0.75 are dynamically stable.Values of the Helmholtz free energy ？F,internal energy ？E,entropy S,and constant-volume specific heat Cv of Pu C and Pu C0.75 are given.The results are in good agreement with available experimental or theoretical data.As for the chemical bonding nature,the difference charge densities,the partial densities of states and the Bader charge analysis suggest that the Pu–C bonds of Pu C and Pu C0.75 have a mixture of covalent character and ionic character.The effect of carbon vacancy on the chemical bonding is also discussed in detail.We expect that our study can provide some useful reference for further experimental research on the phonon density of states,thermodynamic properties of the plutonium monocarbide.
基金supported by the National Natural Science Foundation of China (21431003)the National Program on Key Basic Research Project (2014CB932101)the Fundamental Research Funds for the Central Universities (buctrc201514)
文摘The effects of cation ordering and surface compensating anions on the magnetic structure and catalytic properties of unilamellar Ni-Fe hydroxide nanosheets are studied by using the density functional theory （DFT） plus U method. Fe-segregation in the nanosheets yields magnetic domains with different spin alignments, while the surface compensating anions affect the local moments and valence states of the Fe atoms. The two conditions do not radically change the super-exchange nature of interactions between the paramagnetic metal centers, hut facilitate the formation of various magnetic superlattices in the nanosheets. The calculated free energy change of the intermediates shows that the most stable magnetic structure of Ni-Fe hydrox- ide nanosheets exhibits superior catalytic activity towards oxygen reduction/evolution reactions, which is indicative of magnetic catalyst. This is due to the cycle transition between Fe2＋ and Fe~~ ions in the reactions, which determines the sequence of cleavage of the O-H bond and the release of the OH group, controlling the rate-limiting steps of the reaction. The relationship of magnetism and catalytic activity of Ni-Fe hydroxide nanosheets is established by the valence state change of the Fe ions, which will be helpful to open the way for the design of hydroxide/layered double hydroxides （LDHs）-based magnetic catalysts.
基金financial support from the National Key R&D Program of China(No.2018YFA0208602)National Natural Science Foundation of China(No.21825301)。
文摘CO oxidation at ceria surfaces has been studied for decades,and many efforts have been devoted to understanding the effect of surface reduction on the catalytic activity.In this work,we theoretically studied the CO oxidation on the clean and reduced CeO_(2)(111)surfaces using different surface cells to dete rmine the relationships between the reduction degrees and calculated reaction energetics.It is found that the calculated barrier for the direct reaction between CO and surface lattice O drastically decreases with the increase of surface reduction degree.From electronic analysis,we found that the surface reduction can lead to the occurrence of localized electrons at the surface Ce,which affects the charge distribution at surface O.As the result,the surface O becomes more negatively charged and therefore more active in reacting with CO.This work then suggests that the localized 4 f electron reservoir of Ce can act as the"pseudo-anion"at reduced CeO_(2) surfaces to activate surface lattice O for catalytic oxidative reactions.
基金Chinese Academy of Sciences(Grant Nos.XDB33020000,CAS-WX2023SF-0101,ZDBS-LY-SLH007,and YSBR047)National Key R&D Program of China(2021YFA1400200,and 2021YFA0718700)National Natural Science Foundation of China(Grand Nos.12025407,12134012 and 12188101).
文摘DFT+U is a widely used treatment in the density functional theory(DFT)to deal with correlated materials that contain open-shell elements,whereby the quantitative and sometimes even qualitative failures of local and semi-local approximations can be corrected without much computational overhead.However,finding appropriate U parameters for a given system and structure is non-trivial and computationally intensive,because the U value has generally a strong chemical and structural dependence.In this work,we address this issue by building a machine learning(ML)model that enables the prediction of material-and structure-specific U values at nearly no computational cost.Using Mn–O system as an example,the ML model is trained by calibrating DFT+U electronic structures with the hybrid functional results of more than 3000 structures.The model allows us to determine an accurate U value(MAE=0.128 eV,R^(2)=0.97)for any given Mn–O structure.Further analysis reveals that M–O bond lengths are key local structural properties in determining the U value.This approach of the ML U model is universally applicable,to significantly expand and solidify the use of the DFT+U method.
基金supported by the National Basic Research Program of China(2011CB808505)the National Natural Science Foundation of China(21322307,21421004)+1 种基金the"Shu Guang"project of Shanghai Municipal Education CommissionShanghai Education Development Foundation(13SG30)for financial support
文摘Cl-containing cerium dioxide(Ce O2) catalysts have been found to exhibit unique catalytic activities. In the present work, using density functional theory calculations with the inclusion of on-site Coulomb correction, we systematically studied the effect of Cl on the physicochemical properties of Ce O2 surfaces by substituting one subsurface O with Cl. The calculated results show that substituting an O atom with a Cl atom results in structural distortion and the reduction of one surface Ce4+ cation to Ce3+. The protruding Ce3+ cation greatly improves the adsorption energy of O2 to produce an active O2- species, and maintains the catalytic oxidation cycle of CO on Ce O2(110). These results may help us obtain a better understanding of Cl-ceria interacting systems and provide some guidance for the design of effective Ce O2-based catalysts.
