Theoretical insights into the origin of highly efficient photocatalyst NiO/NaTaO3 for overall water splitting

NaTaO₃ loaded with NiO cocatalyst is one of the few photocatalysts for overall water splitting in UV region, which have attracted much attention. In this work, density functional theory calculations have been performed to investigate the interfacial geometries, electronic structures, charge transport, optical absorptions and band offsets of NiO(001)/NaTaO₃(001) slab models. By considering possible terminations of NaTaO₃(001) surface, two heterostructures denoted as NiO/TaO₂ and NiO/NaO have been constructed. Our results show that two kinds of contact are thermodynamically stable, and there is a stronger rumpling of atomic layers appearing in NiO/NaO than NiO/TaO₂. The calculated band structures reveal that NiO(001)/NaTaO₃(001) interfaces have indirect band gaps. The mobilities of photoinduced charge carriers in interfacial structures are faster than those in pure surfaces. NiO/TaO₂ has a higher mobility and lower recombination rate of photogenerated electrons and holes than NiO/NaO. Loading NiO on NaTaO₃ surface has a negligible effect on the extension of light absorption, which is consistent with experiments. Both heterostructures form a Type-II band alignment. The difference of electrostatic potentials around the interface as a driving force boosts the migration of electrons and holes to different domains of the interface, which is beneficial to extend the lifetime of photoinduced carriers and improve the photocatalytic activity of NaTaO₃ system. NiO/TaO₂ has the ability of overall splitting water with NiO as the oxidation cocatalyst, while in NiO/NaO, the photogenerated electrons and holes are accumulated on NiO and NaO side, respectively. Our results demonstrate that the function of NiO in NiO/NaTaO₃ photocatalytic system is determined by the termination property of NaTaO₃(001) surface, which may be one possible reason why it is difficult to ascertain whether NiO is a proton reduction cocatalyst or water oxidation cocatalyst experimentally.     

Theoretical screening of highly efficient single-atom catalysts for nitrogen reduction based on a defective C3N monolayer

Conversion of N₂ to NH₃ through electrochemical technology is one of the most attractive and promising alternatives to the traditional Haber-Bosch method. However, exploring the promising electrocatalysts with high stability, activity and selectivity for nitrogen reduction reaction (NRR) is still an important and long-standing challenge to accelerate the green production of NH₃. Herein, through the first-principles high-throughput screening, we systematically investigated the potentiality of single transition metal (TM) anchored on defective C₃N monolayer as TM-V_CC candidates for N₂ fixation. We carried out a comprehensive screening and systematical evaluation for stability, catalytic activity and selectivity toward NRR on TM-V_CC candidates. Our results reveal that, among 26 candidates, Mn-V_CC can significantly suppress HER and exhibit the outstanding NRR activity, with the most favorable limiting potential of ?0.75 V through the distal pathway, which is better than the currently stepped catalyst Ru (0001). More impressively, such a satisfactory NH₃ conversion is primarily ascribed to the strong back-donation interactions between d-electrons of Mn atom and the anti-orbitals of N₂ molecule, as well as efficient charge transfer of electrochemical process. Our findings not only broaden the development prospect of SACs for N₂ reduction but also pave a way for rational design and rapid screening of highly active C₃N-based catalysts for NRR.     

Graphene-based catalysts for carbon monoxide oxidation: Experimental and theoretical insights

Graphene-based catalysts are important in various applications ranging from hydrogen production to fuel cells and gas conversion reactions, due to their high surface area, low density, great mechanical properties, and rich electron density. Carbon monoxide oxidation (CO) is among the hottest research themes, due to its significant role in fundamental industrial, and environmental remediation applications. Graphene-based catalysts are highly promising for CO oxidation, due to their accessible surface area, low-cost, and ease of preparation. However, to the best of our knowledge, there are no review papers on graphene-based catalysts for CO oxidation. Therefore, it is important to provide a timely update on this area of research. This review provides a brief synopsis of the relevant milestones and highlights of graphene-based catalysts for experimental and theoretical CO oxidation reaction. This includes self-standing, doped, and metal nanoparticles supported graphene rooting from the preparation methods to the thermal CO oxidation and their related parameters and mechanisms. The CO oxidation reaction fundamentals (i.e., measurements, calculations, and mechanisms) and the effects of CO on the environment are also highlighted. Finally, the current challenges and drawbacks are discussed for further deployment of novel graphene-based catalysts for efficient CO oxidation under ambient conditions.     

Mechanistic insights into tandem amine-borane dehydrogenation and alkene hydrogenation catalyzed by [Pd(NHC)(PCy3)]

The mechanism of tandem dimethylamine-borane (NHMe₂BH₃, DMAB) dehydrogenation and alkene hydrogenation catalyzed by [Pd(NHC)(PMe₃)] are investigated by density functional theory (DFT) calculations [NHC = N,N′-bis(2,6-diisopropylphenyl) imidazole-2-ylidene]. Four possible DMAB dehydrogenation mechanisms have been carefully investigated involving concerted BH/NH activation, sequential BH/NH activation, sequential NH/BH activation, and proton transfer mechanism. DFT studies show that the NH proton transfers to ligated carbene carbon and sequential CH/BH activation is the most kinetically favorable pathway with the lowest activation barrier of 23.8 kcal/mol. For hydrogenation, it was found that a trans-dihydride Pd(II) complex, [Pd(H)₂(NHC)(PMe₃)], formed in the dehydrogenation process, serves as an effective catalyst for reduction of trans-stilbene.     

Enhanced optical absorption and photocatalytic H2 production activity of g-C3N4/TiO2 heterostructure by interfacial coupling: A DFT+U study

The electronic and optical properties of g-C₃N₄/TiO₂ heterostructure are investigated using spin-polarized DFT+U calculations. The equilibrium spacing (1.94 Å) and binding energy (24 meV/Ų) show that g-C₃N₄/TiO₂ is a van der Waals heterostructure. And the calculated band gap of g-C₃N₄/TiO₂ is significantly reduced compared with TiO₂. Therefore, the visible light response of g-C₃N₄/TiO₂ heterostructure is remarkably improved. Besides, the predicted type II band alignment would ensure that the electrons can migrate from g-C₃N₄ monolayer to anatase TiO₂ (101) surface, which leads oxidation and redox reactions can occur on g-C₃N₄ and TiO₂, respectively. Finally, a built-in electric field within the interface region will be set. Above processes can benefit the separation of photoexcited carriers and enhance the hydrogen-evolution activity. In addition, compared with TiO₂, g-C₃N₄/TiO₂ with higher conduction band minimum energy can effectively produce higher-energy electrons to reduce hydrogen ions. Moreover, the influence of composite distance and the number of g-C₃N₄ layers are also investigated systematically. The results indicate that the optical absorption is enhanced over the whole spectrum with the increase of the number of g-C₃N₄ layers. Similar visible light enhancing is also found when the composite distance is decreased.     

Confinement effects on structural,electronic properties and dehydrogenation thermodynamics of LiBH4

Confinement effect on the structural, electronic and thermodynamic properties of LiBH₄ is investigated by density functional theory. The thermodynamically and dynamically stable confinement structure is testified to be γ-LiBH₄@C₃₁Ti according to the adsorption energy and vibrational frequency calculations. The tridentate structure formed by [BH₄]⁻ and Li⁺ in the unconfined LiBH₄ changes into bidentate structure in γ-LiBH₄@C₃₁Ti. We observe that both the occupied and unoccupied states of H 1s, B 2s, B 2p, Li 2s, and Li 2p orbitals in the partial DOSs of γ-LiBH₄@C₃₁Ti shift to high energy level and the splits of DOS peaks occur at the states of H 1s, B 2p, and Li 2p orbitals. Different from the first-step decomposition reaction of LiBH₄, the one for γ-LiBH₄@C₃₁Ti changes into 2LiBH₄@C₃₁Ti → 2LiH + 2B@C₃₁Ti + 3H₂. Moreover, the reaction enthalpy for the first-step decomposition reaction of γ-LiBH₄@C₃₁Ti decreases to 5.864 eV, which is smaller than that (17.204 eV) of LiBH₄. According to the hydrogen removal energy calculations, we observe that the confinement effects make the removal of the first and second hydrogen atoms in γ-LiBH₄@C₃₁Ti easy.     

Properties of BaYO3 perovskite and hydrogen storage properties of BaYO3Hx

In this study, Density Functional Theory (DFT) calculations have been performed for BaYO₃ perovskite with the generalized gradient approximation (GGA) as implemented in Vienna Ab-initio Simulation Package (VASP). The structural optimization of BaYO₃ perovskite have been studied for the five possible phases: cubic, tetragonal, hexagonal, orthorhombic and rhombohedral to determine the most stable phase of BaYO₃ perovskite. It has been found that the cubic phase is the most stable one and electronic and mechanical properties of this phase have been investigated. Moreover, the elastic anisotropy has been visualized in detail by plotting the directional dependence of compressibility, Poisson ratio, Young's and Shear moduli for cubic phase. Then, hydrogen bonding to BaYO₃ perovskite has been conducted and hydrogen storage properties of BaYO₃H_x (x = 3 and 9) such as: formation energy, cohesive energy and gravimetric hydrogen storage capacity have been analyzed. Having no study about BaYO₃ perovskite and hydrogen bonding in the literature makes this study the first considerations of BaYO₃ perovskite. Hence, this work could enlighten the possible future studies.     

Computational investigation of hydrogen storage on B6Ti3+

The structures and hydrogen storage capacities of B₆Ti₃⁺ have been theoretically investigated using DFT with PBE exchange and correlation functional. It is found that the most stable B₆Ti₃⁺01 cluster can maximally adsorb ten hydrogen molecules, which corresponds to a gravimetric uptake capacity of 8.82 wt%. The uptake capacity exceeds the 2015 target set by US Department of Energy for vehicular application. Moreover, the HOMO-LUMO gap value of B₆Ti₃⁺01 (10H₂) is larger than that of B₆Ti₃⁺01, which manifests the B₆Ti₃⁺01 will be more stable after 10H₂ adsorbed. The hydrogen adsorption energies with Gibbs free energy correction are carried out to reveal whether adsorption of hydrogen on B₆Ti₃⁺ is favorable or not at different temperatures. The results indicate that the adsorption of ten hydrogen molecules on B₆Ti₃⁺01 is energetically favorable in a fairly wide temperature range. Therefore, B₆Ti₃⁺01 is considered to be a promising material for hydrogen storage.     

Electronic structure and optical properties of CuGaS2 and CuInS2 solar cell materials

We report energy bands, density of states and optical properties of CuGaS₂ and CuInS₂ chalcopyrites. The electronic structure has been computed using linear combination of atomic orbitals (LCAO) scheme within density functional theory (DFT) and full-potential linearised augmented plane wave method. The energy bands, density of states, components of dielectric tensors and absorption coefficients are compared with the available data. It is seen that the present LCAO-DFT calculations reproduce the electronic properties of both the chalcopyrites in a reasonable way. The optical properties show more absorption of solar radiations for CuGaS₂ chalcopyrite, depicting its more usefulness in the solar cells.     

Density functional and microkinetic study of CO2 hydrogenation to methanol on subnanometer Pd cluster doped by transition metal (M= Cu,Ni, Pt,Rh)

We study the CO₂ hydrogenation to methanol on subnanometer Pd₇ and transition metal doped Pd₆M (M = Cu, Ni, Pt, and Rh) clusters using a combination of density functional theory and microkinetic calculations. We find that, in general, the inclusion of transition metal dopants could decrease the activation energy of several important elementary reactions. This condition results in a significant improvement in the activity of the catalyst, especially for the Pd₆Ni cluster. We find that the Pd₆M clusters are more selective toward the formate pathway than the RWGS + CO hydrogenation pathway. We also compare the turnover frequency profiles of the clusters with that of the Cu(111) surface, representing the standard industrial catalyst. We find that the Pd₆Ni cluster can successfully overcome the TOF of Cu(111) surface, even at the low-pressure condition.     

First principles study of the ZrX2 (X = H,D and T) compounds

The Zr–H system is of particular importance for the solid state storage of hydrogen isotopes in the form of zirconium hydride. Here we report the structural, electronic, vibrational and thermodynamic properties of ZrH₂, ZrD₂ and ZrT₂ using density functional theory (DFT). The structural optimization was carried out by the plane-wave based pseudo-potential method under the generalized gradient approximation (GGA) scheme. The electronic structure of the ZrH₂ compound was illustrated explicitly. The vibrational, thermodynamic, and the effect of isotopes on ZrX₂ (X = H, D, T) compounds were evaluated by the frozen phonon method. Both the Raman and infrared active vibrational modes of ZrX₂ at Γ-point showed significant isotopic effect on ZrX₂ compounds. For example, the phonon energy gap between optical and acoustic modes reduces for ZrT₂ than ZrD₂ and ZrH₂. The formation energies of ZrX₂ compounds, including the ZPE contributions, were estimated to be −143.68, −147.87 and −150.01 kJ/(mole of compound) for X = H, D and T, respectively.     

Semiconducting ground-state of three polymorphs of Mg2NiH4 from first-principles calculations

Magnesium nickel hydrides (Mg₂NiH₄) are the prospective candidates for hydrogen storage and switchable mirror. The hydrides exist in two typical crystallographic forms, the low temperature (LT) phase in monoclinic structure, and the high temperature (HT) phase in cubic structure. LT has two modifications–untwinned (LT1) and microtwinned (LT2) structures. The electronic structures of the three polymorphs of Mg₂NiH₄ are investigated using ab initio calculations based on density functional theory. The calculated band gaps of LT1 and HT are in reasonable agreement with experimental observations and other theoretical predications, while the calculated band gap of LT2 is slightly lower than those of LT1 and HT. Electronic-structure analysis shows that strong interactions exist between Ni and H, whereas the interactions between Mg and H are negligible. The strong ionic character between Mg and NiH₄ complex can be viewed as the origin of the semiconducting ground-state.     

Modulation of spin effect in electronic and vibrational properties of terbium dihydride (TbH2): An ab-initio study

The interaction of hydrogen with terbium was correlated and the structural and electronic structure properties for the cubic rare earth terbium dihydride (TbH₂) using density functional theory (DFT) have been studied. The electronic band structure and density of states (DOS) for spin-up, spin-down and non-spin cases were studied and furthermore influence of higher pressure was also investigated. The charge contribution shows that TbH₂ has metallic characteristic with a mixed covalent and ionic bonding. The Fermi surfaces show hole type structure and provide a space where hydrogen can be absorbed and filled easily. The Löwdin population analysis show that the decrease in charge density of Tb atom which explain absorption of H atom by Tb. The charge density of TbH₂ also show decrease in differential charge density which verify the Löwdin population analysis. Furthermore, we have also analysed phonon dispersion curve at ambient and higher pressure and changes were studied. The TbH₂ can be a good alternative to develop a hydrogen storage device as mass ratio percentage is around 1.27%.     

Density functional study of adsorption and desorption dynamics of hydrogen in zirconium doped aluminium clusters

To reduce the greenhouse effect and as a fuel alternatives hydrogen is used as a secure and clean energy. But there are some challenges in the storage of hydrogen energy, the present accurate dynamics of adsorption and release of hydrogen. We report the adsorption and desorption of hydrogen atoms (up to eight) on the ZrAl_m (m = 3 to 7) clusters using density functional theory within B3PW91/LANL2DZ basis sets in the GAUSSIAN 09 package. Adsorption energy for per hydrogen atom in all clusters is found in the range of ?2.5 eV to ?3.3 eV, which shows the chemisorption of hydrogen on the ZrAl_m clusters. Change in the DOS with number of hydrogen is attributed to the charge transfer between the ZrAl_m clusters and hydrogen atoms. Highest HOMO-LUMO gap of 3.055 eV is found for the ZrAl₃H₇ cluster which indicates that the ZrAl₃H₇ cluster is chemically more stable. Work function values for hydrogen doped clusters are in the range of 3.5 eV–4.4 eV which suggests the low optical absorption in considered clusters. The calculated enthalpy difference further confirms the chemisorption of hydrogen on ZrAl_m clusters ZrAl₃H_n and ZrAl₇H_n(for n = 1 to 5 and 7) are more exothermic than other considered clusters. Desorption energies per hydrogen for these clusters are found lower than the some of the best catalytic clusters Pd₁₃ and Pt₁₃ indicating more catalytic active nature.     

Hydrogen evolution reaction and electronic structure calculation of two dimensional bismuth and its alloys

We present a systematic ab initio study of atomic hydrogen and oxygen adsorption on bismuthene monolayer and its alloys with arsenic and antimony through electronic structure calculations based on density functional theory within generalized gradient approximation. We systematically investigated the preferable adsorption site for hydrogen and oxygen atom on 2D Bi, BiAs and BiSb. It was found that the hydrogen atom prefers top site of bismuth atom and oxygen atom prefers to reside in the hexagonal ring of these 2D bismuth alloys. The free energy calculated from the individual adsorption energy for each monolayer subsequently guides us to predict the best suitable catalyst among the considered 2D monolayers. The 2D BiSb serves better for hydrogen evolution reaction (HER) with hydrogen adsorption energy as ?1.384 eV while 2D BiAs is suitable for oxygen evolution reaction (OER) with oxygen adsorption energy as ?1.092 eV. We further investigated the effects of the adsorbate atom on the electronic properties of 2D Bi, BiAs and BiSb. The adsorption of oxygen on 2D BiAs and BiSb was shown to reduce the bulk band gap by 40.56 and 67.79% respectively which will be advantageous for the observation of Quantum Spin Hall effect at ambient conditions.     

A density functional theory study of CO2 hydrogenation to methanol over Pd/TiO2 catalyst: The role of interfacial site

The interface between metal and support has a very significant influence on the activity and selectivity of the CO₂ hydrogenation to methanol, but there is still lack of investigation in understanding its role in the reaction process. In the current work, the synthesis of methanol through CO₂ hydrogenation on a model Pd/TiO₂ catalyst was studied based on the periodic density functional theory calculation, and the reaction mechanism and active sites were revealed after examining the possible routes. The charge density difference and Millikan charge analysis demonstrate that CO₂ adsorbed at the interfacial site is activated due to obtaining charge from the catalyst, and it is transformed into chemisorbed CO₂^δ−. It is found that interface is the active site for the subsequent hydrogenation process of CO₂ while metal Pd provides an active site to the dissociation of H₂. Moreover, there is a metal-support interaction, where the formed H at the Pd particles reacts with the CO₂ and intermediates adsorbed at interface by the spillover, and the methanol is produced on the support surface. In addition, the RWGS + CO-Hydro route is determined to be the dominant pathway for methanol synthesis, and CO hydrogenation to HCO is the rate-determining step.     

Exploration of carbon additives to the synthesis of Cu2Mo6S8 structures and their electrocatalytic activity in oxygen reduction reaction

Catalytic processes are contemplated as break point in generating alternative and sustainable energy platforms. The cathodic oxygen reduction reaction (ORR) is an important catalytic system, mainly finding practice in fuel cell and metal-air battery technologies. This work presents the synthesis, structural characterization and electrocatalytic properties of three different Cu₂Mo₆S₈ structures as alternative ORR electrocatalysts. The effect of different carbon additives during synthesis was studied and no positive influence of the carbon addition was indicated. Our findings show that only the bare Cu₂Mo₆S₈ enhances the ORR electro-performance to class with the state-of-the-art ORR catalysts. Excellent stability of 10,000 consecutive ORR cycles, a superior onset potential of 0.894 V and half-wave (E_1/2) potential of 0.641 V vs. reversible hydrogen electrode (RHE) increase the noteworthiness of the Cu₂Mo₆S₈ electrodes. Aside from experimental investigations, density functional theory calculations deliver profound knowledge on the structural and electronic properties (electronic band structure, partial density of states and electron density) of Cu₂Mo₆S₈.     

A comprehensive DFT study of CO2 catalytic conversion by H2 over Pt-doped Ni catalysts

PtNi bimetallic catalysts show superior performance for CO₂ catalytic conversion by hydrogen, but the underlying mechanism and the key elementary steps in controlling the activity and selectivity of CO₂ hydrogenation remain unclear. In present work, the complete reaction network for CO₂ hydrogenation has been investigated systematically over Pt/Ni (111) surface based on periodic density functional theory, and active sites and reaction mechanism have been determined. It is found that HCOOH is mainly produced by undergoing the HCOO pathways while synthesis of CH₃OH and CH₄ via RWGS+CO hydrogenation is the dominant reaction pathway, and their selectivity are determined by the competitive reaction between hydrogenation and CO bond scission of H₂COH species. The dissociation of COOH is regarded as the rate-determining step as it has the highest barrier (2.07 eV) in RWGS+CO hydrogenation. Moreover, it is observed that the doping of Pt on Ni surface can promote the transformation of CO₂ into chemisorbed CO₂^δ− and reduce the barrier in H₂ dissociation, which further facilitate the activation and hydrogenation of CO₂. More importantly, the doped Pt atom could promote H_xCO hydrogenation to H_xCOH, meanwhile, suppress H_xCOH dissociation into CH_x. Especially, the activation barrier and reaction energy for C formation is markedly enhanced, and the ability for C hydrogenation is promoted over Pt/Ni (111) surface, which could lower the possibility of coke formation. These results provide helpful information in understanding the process of CO₂ hydrogenation at atomic scale, and could benefit for the synthesis of Ni-based bimetallic catalysts.     

High dispersion and electrocatalytic activity of Pd/titanium dioxide nanotubes catalysts for hydrazine oxidation

Pd/titanium dioxide nanotubes (Pd/TiO₂-NTs) catalysts were prepared by a simple reduction method using TiO₂-NTs as support. The structure and morphology of the resulting Pd/TiO₂-NTs were characterized by transmission electron microscopy and X-ray diffraction. The results showed that Pd nanoparticles with a size range from 6 to 13 nm were well-dispersed on the surface of TiO₂-NTs. The electrocatalytic properties of Pd/TiO₂-NTs catalysts for hydrazine oxidation were also investigated by cyclic voltammetry. Compared to that of pure Pd particles and Pd/TiO₂ particles, Pd/TiO₂-NTs catalyst showed much higher electrochemical activity. This may be attributed to the uniform dispersion of Pd nanoparticles on TiO₂-NTs, smaller particle size and unique properties of TiO₂-NTs support. In addition, the mechanism of hydrazine electrochemical oxidation catalyzed by Pd/TiO₂-NTs are also investigated. The oxidation of hydrazine was an irreversible process, which might be controlled by diffusion process of hydrazine.     

Comparison of ethanol electro-oxidation on Pt/C and Pd/C catalysts in alkaline media

The ethanol electro-oxidation behaviors of Pt/C and Pd/C in alkaline media were compared using potentiodynamic and potentiostatic methods. Various ethanol and alkaline concentrations were studied. In addition, the temperature effect of ethanol oxidation was investigated. The Pd/C showed a higher activity toward ethanol oxidation than the Pt/C, especially in the potentiostatic measurement. This is mainly due to the higher oxyphilic characteristics of the Pd/C and the relatively inert nature of the Pd/C on C–C bond cleavage. The apparent activation energies of the ethanol oxidation on the Pd/C in alkaline media varied from 26.6 to 30.4 kJ mol⁻¹, depending on the potentials. The Tafel slopes could be divided into two parts on both catalysts: at low overpotentials, the Tafel slope on both the Pt/C and the Pd/C was close to 120 mV dec⁻¹ at all temperatures; at high overpotentials, the Tafel slope was ca. 260 mV dec⁻¹ on the Pd/C at all temperatures, but was much higher on the Pt/C, especially at high temperatures. Based on these results, it is concluded that Pd/C has less poisoning effect and higher activity than Pt/C for selective oxidation of ethanol to acetate.