DFT+U Study of the Electronic, Magnetic and Mechanical Properties of Co, CoO, and Co3O4

Cobalt nanoparticles play an important role as a catalyst in the Fischer-Tropsch synthesis. During the reaction process, cobalt nanoparticles can become  oxidized leading to the formation of two phases: CoO rock-salt and Co3O4 cubic spinel. Experimentally, it is possible to evaluate the phase change and  follow the catalyst degradation by measuring the magnetic moment, as each material presents a different magnetic structure. It is therefore important to  develop a fundamental description, at the atomic scale, of cobalt and its oxide phases which we have done here using density functional theory with  the Dudarev approach to account for the on-site Coulomb interactions (DFT+U). We have explored different Ueff values, ranging from 0 to 5 eV, and found  that Ueff = 3.0 eV describes most appropriately the mechanical properties, as well as the electronic and magnetic structures of Co, CoO and  Co3O4. We have considered a ferromagnetic ordering for the metallic phase and the antiferromagnetic structure for the oxide phases. Our results  support the interpretation of the catalytic performance of metallic cobalt as it transforms into its oxidized phases under experimental conditions.&nbsp

CO2 and H2 adsorption and reaction at Nin/YSZ(111) interfaces: a density functional theory study

To recycle CO2 into sustainable fuel and chemicals, co-electrolysis of CO2 and H2O can be achieved in solid oxide electrolysis cells, where the molecules are supplied to the Ni/YSZ electrode (YSZ = yttria-stabilized zirconia). Oxygen diffusion along the electrode has been identified as the critical step in the process, where YSZ is the common catalyst support. We have investigated the interaction of a CO2 molecule with the clean YSZ(111) surface and with Nin/YSZ(111) (n =1, 4-7, 10, 20) interfaces, using spin polarized density functional theory (DFT) and long-range dispersion correction. Here, we have considered up to six initial adsorption sites and two orientations for the CO2 molecule, which showed that the adsorption is stronger at the Nin/YSZ(111) (n =4-7, 10, 20) interface than on the clean YSZ(111) and Ni1/YSZ(111) systems. Additionally, we have determined that the preferential adsorption site of CO2 is at the interface between the Ni clusters and the YSZ(111) surface. We have observed a bending and stretching of the molecule, demonstrating its activation upon adsorption, due to charge transfer between the metal cluster and the molecule and a mixing between Ni orbitals and CO2 orbitals. In this work, we show that, although the electronic structure of the clusters depends on the cluster size, the interaction strength of CO2 with the interface is independent of the size of the supported nickel particle. Finally, we have considered the reverse water gas shift reaction and determined the hydrocarboxylic intermediate in the reaction mechanism over Ni5/YSZ(111)

CO2 and H2 Adsorption and Reaction at Nin/YSZ(111) Interfaces: A Density Functional Theory Study

To recycle CO2 into sustainable fuels and chemicals, coelectrolysis of CO2 and H2O can be achieved in solid oxide electrolysis cells, where the molecules are supplied to the Ni/YSZ electrode (YSZ = yttria-stabilized zirconia). Oxygen diffusion along the electrode has been identified as the critical step in the process, where YSZ is the common catalyst support. We have investigated the interaction of a CO2 molecule with the clean YSZ(111) surface and with Nin/YSZ(111) (n = 1, 4–7, 10, and 20) interfaces, using a spin-polarized density functional theory and a long-range dispersion correction. Here, we have considered up to six initial adsorption sites and two orientations for the CO2 molecule, which showed that the adsorption is stronger at the Nin/YSZ(111) (n = 4–7, 10, and 20) interface than on the clean YSZ(111) and Ni1/YSZ(111) systems. Additionally, we have determined that the preferential adsorption site of CO2 is at the interface between the Ni clusters and the YSZ(111) surface. We have observed a bending and stretching of the molecule, demonstrating its activation upon adsorption, because of charge transfer between the metal cluster and the molecule and a mixing between Ni orbitals and CO2 orbitals. In this work, we show that although the electronic structure of the clusters depends on the cluster size, the interaction strength of CO2 with the interface is independent of the size of the supported nickel particle. Finally, we have considered the reverse water gas shift reaction and determined the hydrocarboxylic intermediate in the reaction mechanism over Ni5/YSZ(111)

Stability and mobility of supported Nin (n = 1–10) clusters on ZrO2(111) and YSZ(111) surfaces: a density functional theory study

The performance of supported metal catalysts, such as nickel nanoparticles decorating yttria-stabilized zirconia (YSZ), depends on their microstructure and the metal–support interface. Here, we have used spin polarized density functional theory (DFT) to evaluate different Ni cluster geometries and determined the electronic structure of the most stable configurations. We have described the interaction of Nin (n = 1–10) clusters supported on the cubic ZrO2(111) and YSZ(111) surfaces, which show a preference for pyramidal shapes rather than flat structures wetting the surface. The interfacial interaction is characterized by charge transfer from the cluster to the surface. We also show how yttrium, present in YSZ, affects the Ni–Ni interaction. Through analysing the difference between the cohesive energy and the clustering energy, we show the preference of Ni–Ni bond formation over Ni-surface interaction; this energy difference decreases with the increase of the Nin cluster size. From the evaluation of the Ni atomic hopping rates on YSZ, we have demonstrated that under different temperature conditions, Ni atoms aggregate with other atoms and clusters, which affects the cluster size stability

Catalytic Conversion of CO and H2into Hydrocarbons on the Cobalt Co(111) Surface: Implications for the Fischer-Tropsch Process

The Fischer-Tropsch (FT) process consists of the reaction of a synthesis gas (syngas) mixture containing carbon monoxide (CO) and hydrogen (H2), which are polymerized into liquid hydrocarbon chains, often using a cobalt catalyst, although the mechanistic pathway is not yet fully understood. Here, we have employed unrestricted density functional theory calculations with a Hubbard Hamiltonian and long-range dispersion corrections [DFT+U−D3−(BJ)] to investigate the reaction of syngas and the selectivity toward the hydrocarbons formed on the cobalt Co(111) surface. The single CO and dissociated H2molecules prefer to adsorb at two different types of trigonal surface sites, and we discuss how the interatomic distances, fundamental vibrational modes, charge transfers, surface-free energies, and work functions are modified by the adsorbates. The coadsorption of the syngas molecules in close proximity provides enough energy for the system to cross the saddle points on the minimum energy pathway (MEP), leading to the catalytic hydrogenolysis of the C-O bond. The adsorbed CO, alongside the intermediates CH and OH, are further stabilized when the ratio of equilibrium coverage (C) isCH/CCO,CH,OH> 6:1 under the temperature conditions required for the FT process. We propose several mechanistic pathways to account for the formation of ethane (C2H6), as a model for long-chain hydrocarbons, as well as methane (CH4) which is an undesirable product. The MEPs for these processes show that the coupling of the C-C bond followed by hydrogenation is the most favorable process, which takes precedence over the production of CH4. The termination reaction suggests that water (H2O) remains weakly physisorbed to the surface, allowing the reutilization of its catalytic site. The simulated fundamental vibrational frequencies and scanning tunneling microscopy images of the surface-bound intermediates are in agreement with the available experimental data. Our findings are important in the interpretation of the elementary steps of the FT process on the Co(111) surface

Ni Deposition on Yttria-Stabilized ZrO₂(111) Surfaces: A Density Functional Theory Study

Nickel particles supported on yttria-stabilized zirconia (YSZ) play a significant role in the performance of solid oxide fuel cells (SOFC). We have investigated both pristine and doped ZrO₂ surfaces using spin polarized density functional theory (DFT) and also considering long-range dispersion forces. We have systematically studied Ni deposition on the bare ZrO₂(111) surface and on surfaces with two concentrations of Y, all at both high and low oxygen chemical potential. Among the several independent sites explored, the Ni adsorption preference is as follows: YSZ(111) without oxygen vacancy > YSZ(111) with oxygen vacancy > stoichiometric ZrO₂(111). For each surface, the adsorption site is similar: over the top oxygen. The evaluation of the geometric and electronic structure shows a mixing of Ni orbitals with surface atom orbitals. We have also investigated the influence of the yttrium atom on the Ni adsorption by considering up to 52 different configurations, which showed that Ni tends to adsorb away from the yttrium atom for any YSZ(111) surface, leading to a mixed electronic structure with enhanced charge transfer

Catalytic Conversion of CO and H2into Hydrocarbons on the Cobalt Co(111) Surface: Implications for the Fischer-Tropsch Process

The Fischer-Tropsch (FT) process consists of the reaction of a synthesis gas (syngas) mixture containing carbon monoxide (CO) and hydrogen (H2), which are polymerized into liquid hydrocarbon chains, often using a cobalt catalyst, although the mechanistic pathway is not yet fully understood. Here, we have employed unrestricted density functional theory calculations with a Hubbard Hamiltonian and long-range dispersion corrections [DFT+U−D3−(BJ)] to investigate the reaction of syngas and the selectivity toward the hydrocarbons formed on the cobalt Co(111) surface. The single CO and dissociated H2molecules prefer to adsorb at two different types of trigonal surface sites, and we discuss how the interatomic distances, fundamental vibrational modes, charge transfers, surface-free energies, and work functions are modified by the adsorbates. The coadsorption of the syngas molecules in close proximity provides enough energy for the system to cross the saddle points on the minimum energy pathway (MEP), leading to the catalytic hydrogenolysis of the C-O bond. The adsorbed CO, alongside the intermediates CH and OH, are further stabilized when the ratio of equilibrium coverage (C) isCH/CCO,CH,OH> 6:1 under the temperature conditions required for the FT process. We propose several mechanistic pathways to account for the formation of ethane (C2H6), as a model for long-chain hydrocarbons, as well as methane (CH4) which is an undesirable product. The MEPs for these processes show that the coupling of the C-C bond followed by hydrogenation is the most favorable process, which takes precedence over the production of CH4. The termination reaction suggests that water (H2O) remains weakly physisorbed to the surface, allowing the reutilization of its catalytic site. The simulated fundamental vibrational frequencies and scanning tunneling microscopy images of the surface-bound intermediates are in agreement with the available experimental data. Our findings are important in the interpretation of the elementary steps of the FT process on the Co(111) surface

Interaction of SO2 with the Platinum (001), (011), and (111) Surfaces: A DFT Study

Given the importance of SO2 as a pollutant species in the environment and its role in the hybrid sulphur (HyS) cycle for hydrogen production, we carried out a density functional theory study of its interaction with the Pt (001), (011), and (111) surfaces. First, we investigated the adsorption of a single SO2 molecule on the three Pt surfaces. On both the (001) and (111) surfaces, the SO2 had a S,O‐bonded geometry, while on the (011) surface, it had a co‐pyramidal and bridge geometry. The largest adsorption energy was obtained on the (001) surface (Eads = −2.47 eV), followed by the (011) surface (Eads = −2.39 and −2.28 eV for co‐pyramidal and bridge geometries, respectively) and the (111) surface (Eads = −1.85 eV). When the surface coverage was increased up to a monolayer, we noted an increase of Eads/SO2 for all the surfaces, but the (001) surface remained the most favourable overall for SO2 adsorption. On the (111) surface, we found that when the surface coverage was θ > 0.78, two neighbouring SO2 molecules reacted to form SO and SO3. Considering the experimental conditions, we observed that the highest coverage in terms of the number of SO2 molecules per metal surface area was (111) > (001) > (011). As expected, when the temperature increased, the surface coverage decreased on all the surfaces, and gradual desorption of SO2 would occur above 500 K. Total desorption occurred at temperatures higher than 700 K for the (011) and (111) surfaces. It was seen that at 0 and 800 K, only the (001) and (111) surfaces were expressed in the morphology, but at 298 and 400 K, the (011) surface was present as well. Taking into account these data and those from a previous paper on water adsorption on Pt, it was evident that at temperatures between 400 and 450 K, where the HyS cycle operates, most of the water would desorb from the surface, thereby increasing the SO2 concentration, which in turn may lead to sulphur poisoning of the catalyst

Interaction of SO2 with the Platinum (001), (011), and (111) Surfaces: A DFT Study

Given the importance of SO2 as a pollutant species in the environment and its role in the hybrid sulphur (HyS) cycle for hydrogen production, we carried out a density functional theory study of its interaction with the Pt (001), (011), and (111) surfaces. First, we investigated the adsorption of a single SO2 molecule on the three Pt surfaces. On both the (001) and (111) surfaces, the SO2 had a S,O‐bonded geometry, while on the (011) surface, it had a co‐pyramidal and bridge geometry. The largest adsorption energy was obtained on the (001) surface (Eads = −2.47 eV), followed by the (011) surface (Eads = −2.39 and −2.28 eV for co‐pyramidal and bridge geometries, respectively) and the (111) surface (Eads = −1.85 eV). When the surface coverage was increased up to a monolayer, we noted an increase of Eads/SO2 for all the surfaces, but the (001) surface remained the most favourable overall for SO2 adsorption. On the (111) surface, we found that when the surface coverage was θ > 0.78, two neighbouring SO2 molecules reacted to form SO and SO3. Considering the experimental conditions, we observed that the highest coverage in terms of the number of SO2 molecules per metal surface area was (111) > (001) > (011). As expected, when the temperature increased, the surface coverage decreased on all the surfaces, and gradual desorption of SO2 would occur above 500 K. Total desorption occurred at temperatures higher than 700 K for the (011) and (111) surfaces. It was seen that at 0 and 800 K, only the (001) and (111) surfaces were expressed in the morphology, but at 298 and 400 K, the (011) surface was present as well. Taking into account these data and those from a previous paper on water adsorption on Pt, it was evident that at temperatures between 400 and 450 K, where the HyS cycle operates, most of the water would desorb from the surface, thereby increasing the SO2 concentration, which in turn may lead to sulphur poisoning of the catalyst