Density Functional Theory Results Research Articles

Dynamical correlations in single-layer CrI3

Chromium triiodide is an intrinsically magnetic van der Waals material down to the single-layer limit. Here, we provide a first-principles description of finite-temperature magnetic and spectral properties of monolayer (ML) ${\mathrm{CrI}}_{3}$ based on fully charge self-consistent density functional theory (DFT) combined with dynamical mean-field theory, revealing a formation of local moments on Cr from strong local Coulomb interactions. We show that the presence of local dynamical correlations leads to a modification of the electronic structure of ferromagnetically ordered ${\mathrm{CrI}}_{3}$. In contrast to conventional $\mathrm{DFT}+U$ calculations, we find that the top of the valence band in ML ${\mathrm{CrI}}_{3}$ demonstrates essentially different orbital character for minority and majority spin states, which is closer to the standard DFT results. This leads to a strong spin polarization of the optical conductivity upon hole doping, which could be verified experimentally.

N-hydroxypyrazine-2-carboxamide as a new and green corrosion inhibitor for mild steel in acidic medium: experimental, surface morphological and theoretical approach

In this research work, N-hydroxypyrazine-2-carboxamide (NHP) is first introduced as a green, new and effective corrosion inhibitor to protect mild steel (MS) surface from corrosion destruction in a 1 M HCl aggressive solution. Experimental techniques such as gravimetric analysis and electron impedance spectroscopy (EIS) were used in the present analysis; the obtained results show that the inhibition efficiency for NHP was 93.51% at 200 ppm and 298K temperature. NHP molecules adsorbed on mild steel surface followed by Langmuir adsorption model. Similarly, the energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM) analyses were used to describe the changes in surface morphology and composition of MS surface, respectively. Quantum chemical analyses based on density functional theory (DFT) were also applied to calculate the frontier molecular orbitals, that is, E HOMO and E LUMO values for NHP inhibitor molecules; the DFT results are in good correlation with experimental results. Additionally, the studied inhibitor molecule does not produce any environmental threats and toxicity as it decomposed into amine and carbon dioxide and also pyrazine molecules are mainly used as drug precursors showing its green nature.

Syngas to ethanol on MoCu(2 1 1) surface: Effect of promoter Mo on C O bond breaking and C C bond formation

The mechanism of syngas to ethanol on MoCu(2 1 1) surface has been researched by density functional theory (DFT) calculation, and the effects of Mo as a promoter on C O bond breaking and C C bond formation have been discussed. Calculations show that Cu-Mo atoms constitute the active sites on MoCu(2 1 1) surface after Mo atom being served as a promoter of Cu catalyst. Compared with Cu(2 1 1), MoCu(2 1 1) has two improvements. Firstly, CH 3 is the most advantageous monomer on the MoCu(2 1 1) surface, which provides abundant CH 3 intermediate for syngas to ethanol. Secondly, the C C bond is formed mainly by inserting CHO into the abundant CH 3 , and the generated CH 3 CHO through multiple steps of hydrogenation to generate C 2 H 5 OH. The key of the promoter Mo on the MoCu(2 1 1) surface also has been verified by the analysis of its electronic properties. Differential charge density shows that the massive electron transfer from Mo to Cu, projected density of states ( p DOS) shows that the electron transfer from Mo to Cu makes the d-band center of MoCu(2 1 1) nearer to the Fermi level, these indicate that the MoCu(2 1 1) catalytic capacity increased. The addition of Mo in the Cu-based catalyst not only can effectively solve the problem of C O bond breaking, but also promote C C bond formation. About the influence of Mo content on C O bond breaking and C C bond formation, compared with MoCu(2 1 1), the DFT results and the d-band center of Mo 2 Cu(2 1 1) show that the increase of Mo content could not promote the synergistic effect of Cu/Mo on the generation of ethanol more effectively.

WITHDRAWN: Utility of Warped C80H30 nanographene for removal of toxic heavy metal ions from wastewater

The removal of heavy metal ions from wastewater has been a global issue as such ions pose significant health and environmental impacts. Drawing on a nanographene (NG), the present study employs density functional theory (DFT) to evaluate an effective heavy metal removal method for the purpose of removing Pb 2+ , Hg 2+ , and Cd 2+ ions from wastewater. NG was found to have high effectiveness and efficiency in capturing the heavy metals of aqueous media. Furthermore, the adsorption of Pb 2+ , Hg 2+ , and Cd 2+ ions was found to have the capability of diminishing the significant pristine NG (HOMO-LUMO) gap. The findings showed larger cation-NG charge transfers and greater binding energy in cation-NG systems that have larger transport rates of electrons. According to the time-dependent DFT results, the complexes under study were found to have “ligand-to-metal charge transfer” excitations. As a result, the theoretical findings of the present work could contribute to experimental works on identifying NG-based efficient and effective substances for the removal of contaminants from wastewater. • The first DFT study on the adsorption of pollutant Cd 2+ , Hg 2+ , and Pb 2+ ions on NG layer. • Detailed analysis of possible charge transfers between the cations and the NG. • TD-DFT analysis showed “ligand to metal charge transfer” excitations for the complexes.

Mechanistic understanding of methane combustion over H-SSZ-13 zeolite encapsulated palladium nanocluster catalysts

Catalytic methane (CH4) combustion to CO2 and H2O is of great practical significance and an important prototype catalytic reaction. Extensive experimental studies have suggested that zeolite supported palladium (Pd) catalysts are very active for catalytic CH4 oxidation, while the structure-performance relationship is still not clear. Herein, using H-SSZ-13 zeolite encapsulated Pd nanoclusters as a demonstration case, reaction mechanisms and kinetics of complete CH4 combustion were systematically investigated using first-principles density functional theory (DFT) calculations combined with atomistic thermodynamic analysis and the energetic span model (ESM). Four H-SSZ-13 zeolite encapsulated PdⅡxOmHn model catalysts, i.e., PdⅡ/H-SSZ-13, PdⅡO/H-SSZ-13, PdⅡ2O/H-SSZ-13, and PdⅡ3O3H/H-SSZ-13 were studied. The encapsulated [PdⅡ2O]2+ and [PdⅡ3O3H]+ nanoclusters were identified as the most stable binuclear and trinuclear Pd structures under experimental oxidative conditions. DFT calculations indicated that the most kinetically relevant step in the complete CH4 oxidation reaction over four Pd model catalysts is different. The first, third, third, and fourth C–H bond cleavage were identified as the most kinetically relevant steps over PdⅡ/H-SSZ-13, PdⅡO/H-SSZ-13, PdⅡ2O/H-SSZ-13, and PdⅡ3O3H/H-SSZ-13, respectively. Using the energetic span model, the relative turnover frequencies of CH4 oxidation over four PdⅡxOmHn/H-SSZ-13 model catalysts were calculated. Consistent with recent experimental observation, the encapsulated PdⅡ3O3H/H-SSZ-13 nanocluster was found to be the most active catalyst for complete CH4 oxidation. On the basis of DFT results and the ESM model analysis, it is noted that the reaction rate of complete CH4 oxidation over the PdⅡxOmHn/H-SSZ-13 catalysts is not completely dependent on one “highest” activation barrier, i.e., the barrier for the most kinetically relevant step. Neither one specific transition state nor one reaction step carried all the kinetics information that determines the catalytic performance of the PdⅡxOmHn/H-SSZ-13 catalysts. The present work sheds light on the structure-performance relationship for CH4 combustion over the zeolite supported noble metal catalysts.

Modeling the Structural Heterogeneity of Vicinal Silanols and Its Effects on TiCl4 Grafting onto Amorphous Silica

Molecular metal complexes such as MClx react readily with hydroxyl-terminated surfaces to produce some of the “single-atom” catalysts used in important large-scale commercial reactions, including olefin metathesis, polymerization, and epoxidation. While the local oxide environment can vary at each metal site, the manner and degree to which these differences impact the catalytic activities of individual sites are poorly understood. In this work, we develop a computational framework to model the grafting of metal complexes onto amorphous supports and apply the framework to examine TiCl4 grafting onto amorphous silica. We use density functional theory (DFT) to calculate free energies for TiCl4 reactions at vicinal silanol sites. The kinetics and thermodynamics depend on the dihedral angle between the silanols. Vicinal silanol pairs with small dihedral angles are predicted to yield bipodal [(≡SiO)2TiCl2] sites initially, but they react further with TiCl4 vapor to give vicinal pairs of monopodal sites, [ ≡ SiOTiCl3]. In contrast, silanol pairs with large dihedral angles yield vicinal pairs of monopodal Ti sites directly. The DFT results were used to construct a population balance model to describe the kinetics of TiCl4 grafting onto nonuniform sites of an atomistic model for amorphous silica. The solution of the population balance model predicts that the majority of the vicinal pairs graft as bipodal [(≡SiO)2TiCl2] sites first and then slowly convert to monopodal [≡SiOTiCl3] sites. The predictions provide a plausible explanation for the variability in the populations of monopodal and bipodal sites previously reported for TiCl4 grafting.

Photocatalytic Biorefinery to Lactic Acid: A Carbon Nitride Framework with O Atoms Replacing the Graphitic N Linkers Shows Fast Migration/Separation of Charge

AbstractThe rational regulation of the structure of photocatalysts with an enhanced visible‐light absorption property and rapid charge dynamics is promising and challenging. Here, a facile method was developed for preparing oxygen‐doped carbon nitride bundles with porous hollow trunk‐shaped structures (Ox@CNB) using melamine with assistance of hydrochloric acid. Experimental results and density functional theory (DFT) calculations indicated that Ox@CNB exhibited controllable bandgaps (2.20–2.60 eV) and a low photoluminescence intensity. Interestingly, Ox@CNB displayed excellent photocatalytic activity for the synthesis of lactic acid (81.4 %) and universality for the reforming of biomass‐derived monosaccharides to lactic acid. The oxidation active species of h+, ⋅OH, ⋅O2− and 1O2 all promoted the lactic acid production, and h+ played a major role. A possible reaction mechanism of lactic acid synthesis was proposed by combining the experimental with the DFT results. A one‐thousand‐fold up‐scaled experiment demonstrated the significant application potential of the system for the industrial production of lactic acid.

Moisture effect on the separation of CO2/CH4 mixtures with amine-functionalised porous silicas

The effect of minor amounts of water on the CO2 and CH4 adsorption on primary and secondary amine-functionalised mesoporous silicas (APTES@SBA-15 and DEAPTES@SBA15, respectively) was studied with a combination of high-pressure gas adsorption, solid state NMR of labelled 13CO2 and density functional theory (DFT) calculations. Known amounts of water were pre-adsorbed on the materials (0.047 to 0.157 mmol∙g−1) and the adsorption performance for CO2 and CH4 was compared to the performance of the dry samples. We observed that even when only minor amounts of water are present, the tertiary amine-functionalised material revealed a significant enhancement of the selectivity for CO2 (from ca. 5.8 to 208) while the one with primary amine maintained the same adsorption properties. This is related to the change in the adsorption mechanism in DEAPTES@SBA15 when water is present since water participates in the reaction of the tertiary amine with CO2 to produce bicarbonate (confirmed by NMR and DFT results). Such species was not observed in APTES@SBA-15 with water. From a broader perspective, the results presented in this work are relevant to confirm the suitability of this type of hybrid adsorbents in industrial applications related with CO2 adsorption, where minor amounts of water may be present in the process streams, such as during bio, landfill, or natural gas upgrading, or also in carbon capture applications.

Vertical Heterostructures between Transition‐Metal Dichalcogenides—A Theoretical Analysis of the NbS2/WSe2 Junction

AbstractLow‐dimensional metal‐semiconductor vertical heterostructures (VH) are promising candidates in the search of electronic devices at the extreme limits of miniaturization. Within this line of research, here a theoretical/computational study of the NbS2/WSe2 metal‐semiconductor vertical hetero‐junction using density functional theory (DFT) and conductance simulations is presented. First atomistic models of the NbS2/WSe2 VH considering all the five possible stacking orientations at the interface are constructed, and DFT and quantum‐mechanical (QM) scattering simulations are conducted to obtain information on band structure and transmission coefficients. Then an analysis of the QM results in terms of electrostatic potential, fragment decomposition, and band alignment is carried out. The behavior of transmission expected from this analysis is in excellent agreement with, and thus fully rationalizes, the DFT results, and the peculiar double‐peak profile of transmission. Finally, maximally localized Wannier functions, projected density of states, and a simple analytic formula to predict and explain quantitatively the differences in transport in the case of epitaxial misorientation are used. Within the class of Transition‐Metal Dichalcogenide systems, the NbS2/WSe2 VH exhibits a wide interval of finite transmission and a double‐peak profile, features that can be exploited in applications.

A density-functional benchmark of vibrational free-energy corrections for molecular crystal polymorphism.

Many crystal structure prediction protocols only concern themselves with the electronic energy of molecular crystals. However, vibrational contributions to the free energy (Fvib) can be significant in determining accurate stability rankings for crystal candidates. While force-field studies have been conducted to gauge the magnitude of these free-energy corrections, highly accurate results from quantum mechanical methods, such as density-functional theory (DFT), are desirable. Here, we introduce the PV17 set of 17 polymorphic pairs of organic molecular crystals, for which plane wave DFT is used to calculate the vibrational free energies and free-energy differences (ΔFvib) between each pair. Our DFT results confirm that the vibrational free-energy corrections are small, having a mean value of 1.0 kJ/mol and a maximum value of 2.3 kJ/mol for the PV17 set. Furthermore, we assess the accuracy of a series of lower-cost DFT, semi-empirical, and force-field models for computing ΔFvib that have been proposed in the literature. It is found that calculating Fvib using the Γ-point frequencies does not provide ΔFvib values of sufficiently high quality. In addition, ΔFvib values calculated using various approximate methods have mean absolute errors relative to our converged DFT results of equivalent or larger magnitude than the vibrational free-energy corrections themselves. Thus, we conclude that, in a crystal structure prediction protocol, it is preferable to forego the inclusion of vibrational free-energy corrections than to estimate them with any of the approximate methods considered here.

Single-wall carbon nanotube mechanical behavior using the modified embedded atom method with bond order (MEAM-BO)

We report the capability of the modified embedded atom method with bond order (MEAM-BO) potential to capture single-wall carbon nanotube (SWCNT) mechanical properties accurately by calculating the wavenumber of the radial breathing mode (RBM), elastic properties, and folding energy (ΔE), which are all associated with bond curvature. We find that the existing MEAM-BO potential gives results that correlate well with experimental and density functional theory (DFT) results with the exception of the folding energy. The MEAM-BO potential parameters are updated to produce a MEAM-BO* potential for the SWCNT system. Interestingly, including the SWCNT data base, improved the previous fit of the C-H binary system. Previous studies in the literature concluded that the CNTs diameter, d, was inversely proportional to the RBM wavenumber and folding energy. When comparing MEAM-BO*, MEAM-BO, REBO, and ReaxFF with DFT results, we found that MEAM-BO* gave the closest results to DFT for the RBM wavenumbers, folding energy, and SWCNT Young’s modulus, especially for small diameter SWCNTs. We conclude that MEAM-BO* captures SWCNT curvature effects and unsaturated hydrocarbons bond behavior.

Theoretical study on the interaction of flutamide anticancer drug with cucurbit[n]uril (n = 5–8) as a drug delivery system

AbstractThe supramolecular complexation of flutamide, an anti‐androgen drug with cucurbit[n]uril was studies using density functional theory (DFT). The structural and electronic analysis of the complexes was performed. The negative binding energy of the complexes show that complexation process is exothermic. The thermodynamic results reveal that the process of inclusion complex formation is spontaneous. To gain insightful knowledge about the nature of interactions present between the host‐guest molecules of the complexes, molecular electrostatic potential, non‐covalent interaction–reduced density gradient, and natural bond orbital analysis was done. These results suggest that electrostatic interactions and intermolecular hydrogen bond formation contribute to the stability of these complexes. The coupled‐cluster theory with single, double and perturbatively connected triple excitations calculations for the complexes support the DFT results. The quantitative decomposition of the interaction energies of the complexes was acquired by the local energy decomposition analysis. The atom‐centered density matrix propagation molecular dynamic analysis confirms the stability of the examined complexes.

Treatment of nitrate containing wastewater by adsorption process using polypyrrole-modified plastic-carbon: Characteristic and mechanism

Polypyrrole-modified plastic-carbon (PET-PPy) composite was prepared by using high porosity plastic-carbon materials and a special doping mechanism of polypyrrole to remove nitrate from water to achieve waste recycling. As a result, PET-PPy-500 showed remarkable nitrate adsorption in both acidic and alkaline wastewater. The pseudo-second-order kinetic and Langmuir isotherm models were fit for the nitrate adsorption by PET-PPy-500, and the maximum adsorption capacity predicted by the Langmuir model was 10.04 mg NO3–N/g (45.18 mg NO3−/g) at 30 °C. The ion exchange and electrostatic attraction were the main mechanisms of removing NO3− by PET-PPy-500, which was demonstrated by the interface characterization and theoretical calculation. The doped ions (Cl−) and/or other anions produced by charge transfer interaction were the main exchange ions in the process of NO3− adsorption. The main binding sites in the electrostatic adsorption process were nitrogen-containing functional groups, which can be confirmed by the results of XPS and density functional theory (DFT). Furthermore, DFT results also showed that the adsorption of nitrate by PET-PPy was a spontaneous exothermic process, and the adsorption energy at the nitrogen site was the lowest. The findings of this study provide a feasible strategy for the advanced treatment of nitrate containing wastewater.

Strengthened arsenic adsorption over Ni-modified γ-Al2O3 under different operation conditions: An experimental and simulation study

• Enhanced electron transfer among Al 2 O 3 , NiO and As 2 O 3 boosts adsorption performance. • NiO reduces TS energy barrier of physically adsorbed As 2 O 3 into chemisorption state. • Stronger attraction forces promote desorption temperatures of Ni-modified γ-Al 2 O 3 . • Bond stretch of Ni-O helps to capture more As 2 O 3 molecules at higher temperatures. • Experiments of arsenic adsorption are consistent with DFT/MD simulation results. Variations of operation conditions have negative effects on arsenic adsorption of γ-Al 2 O 3 . Therefore, it is necessary to modulate γ-Al 2 O 3 for better arsenic adsorption performances under complex industrial conditions. In this study, besides complete experiments used to characterize arsenic adsorption features of Ni-modified γ-Al 2 O 3 , density functional theory (DFT) calculations and molecular dynamics (MD) simulations are also wielded to investigate arsenic adsorption enhancement mechanisms in microscale and mesoscale, respectively. As to DFT calculations, it is revealed that the strengthened electron transfer between As 2 O 3 and the adsorbent is the principal factor for 24.59–63.14 % higher adsorption energies. What is more, transition state (TS) energy barriers of physisorption structures being transformed into chemisorption are curtailed by 24.93–54.23 % and negative effects of high temperatures on dissociation of adsorption structures are also partially impaired. As far as MD simulations are concerned, stronger attraction forces of the adsorbent to arsenic molecules and the As 2 O 3 aggregation effect are leading factors for capturing 26.75 % more As 2 O 3 , which counters adverse effects of variable operation conditions. Regarding experiments, it is manifest that the arsenic adsorption amounts over Ni-modified γ-Al 2 O 3 are averagely 24.28–32.55 % higher than those over pure γ-Al 2 O 3 , which is consistent with DFT and MD results. Analyses of above three aspects would provide significant information for Ni-modified γ-Al 2 O 3 being used in real industries to remove arsenic from flue gas.

Ligand assistance via solid-state coordination for promoting nickel dispersion over the Ni/Beta hydroisomerization catalyst

Hydroisomerization of alkanes is a vital process to improve cold flow properties of lubricant oil and increase octane number of gasoline in petrochemical industry. Non-noble metal catalysts used in this process always suffer from low metal dispersion due to weak interaction between metal species and support. Herein, density functional theory (DFT) calculations show that Ni2+ prefers to coordinate with two carboxyl oxygen of glycine in the absence of water. It makes the amino groups well exposed, forming strong interaction between the nickel-complex and aluminum groups over Beta zeolites. In contrast, Ni2+ coordinates with amino nitrogen and one of carboxyl oxygen of glycine in the presence of water. This structure has weaker interaction with support due to the absence of exposed amino groups. Based on the DFT results, a solid-state ligand assistance strategy was developed to increase metal dispersion over the Ni/Beta catalyst. UV-vis, XPS and FTIR characterization results match well with the DFT results and confirm the strong interaction between the nickel-complex and support in the absence of water. The Ni/Beta catalyst has smaller particle size of nickel oxide (0.8 nm) than that of the catalyst prepared by liquid-state ligand assistance method (5.1 nm). With increasing metal dispersion, the Ni/Beta outperforms the counterpart by liquid-state ligand assistance method and exhibits comparable isomers yield to commercial Pt catalysts in n-hexane hydroisomerization. This work provides a promising approach to boost metal dispersion on catalysts.

Metal paddlewheels as sensors for detection of SO2 gas: a DFT study

The sensing properties of the paddlewheel-type metal dimer (M-BTC: M = Fe, Ni, Cu, Zn, and Pd) toward SO2 gas were theoretically investigated using density functional theory (DFT) at the M06-L level of theory. Single-point calculations were carried at the M06 functional to correct the energetic properties and HOMO–LUMO energy gap. The O-bound adsorption complex of SO2 on the metal center of paddlewheels is found to be thermodynamically favorable than the S-bound ones. The trend of adsorption energy of SO2 on the metal center is in the order Pd-BTC < Cu-BTC < Ni-BTC < Zn-BTC < Fe-BTC. Among these five paddlewheels, the Ni-BTC and Zn-BTC paddlewheels are highly sensitive toward SO2 gas as compared to the other systems. The density of states reveals that the adsorption process significantly reduces the LUMO of the system to lower energies, enhancing the conductivity of the system. From DFT results, the energy gap of Ni-BTC and Zn-BTC is significantly reduced by 18.0 and 41.0% after the adsorption of SO2 on the metal center. These results suggest the great potential of Ni-, and Zn-BTC paddlewheels as SO2 sensors.

Engineering defect energy landscape of CoCrFeNi high-entropy alloys by the introduction of additional dopants

The concept of high-entropy alloys (HEAs) focusing on tuning the overall chemical complexity represents a novel alloy design strategy. In contrast, alloying of a metallic matrix with minor doping elements with limited and localized tunability has been a common practice to improve material performance. Combining the idea of globally engineering defect energy landscape in HEAs and the localized doping strategy in dilute alloys, in this work, we explore doping effects of minor elements in a HEA matrix to further enhance the overall and localized chemical tunability, aiming to improve its irradiation resistance. Specifically, we study the influence of minor Al, Cu, Ti, and Pd substitutional doping elements on defect energetics in a CoCrFeNi model HEA based on density-functional theory (DFT) calculations. The DFT results indicate that the formation and migration energies of vacancies can be strongly influenced when a dopant is introduced at the first nearest neighbor shells around a vacancy. On the other hand, interstitial energetics are only slightly affected. Among the four elements, Ti and Pd generally decrease vacancy formation energies and increase vacancy migration energies more significantly than Al and Cu. The doping effects become more pronounced when the concentration of the substitutional dopants increases. Based on the energy distributions obtained from DFT, we build a kinetic Monte Carlo (kMC) model to assess the impact of dopants on vacancy-mediated diffusivity in the doped HEAs. Our results suggest that Ti and Pd can lower the tracer diffusivity in the considered HEAs and act as trapping sites, whereas Cu may enhance the atomic transport. This work indicates that substitutional doping in HEAs is an effective strategy in metallurgy to further tune the defect and transport properties of complex alloys.

Synthesis and bader analyzed cobalt-phthalocyanine modified solar UV-blind β-Ga2O3 quadrilateral nanorods photocatalysts for wide-visible-light driven H2 evolution

In this work, an efficient wide solar light-driven photocatalyst based on solar UV-blind β-Ga2O3 quadrilateral nanorods (β-GOR) has been modified with cobalt phthalocyanine (CoPc). Guided by density functional theory (DFT) calculations and experimental results, H-bond strengthened interfacial connection was established to purchase wide range of solar radiations and applied as an efficient photocatalysts for H2 evolution from water. The optimized 3CoPc/β-GOR nanocomposite unveiled an unprecedented in the quantum efficiency (0.38%) at 660 nm excitation wavelength with outstanding structural stability compare to the pristine solar UV-blind β-GOR (0%). The much extended photoactivities are accredited to the formation of strong H-bond between CoPc and β-GOR, that accelerated charge transformation as confirmed from Bader charge/work function analysis, activation of O2, wide visible-light absorption, as well as joint DFT and experimental results. We believe that this system will also work for other metal phthalocyanines such as ZnPc, NiPc and HPc together with β-GOR towards next-generation efficient photocatalysts with wide-visible light response.

N2O Hydrogenation on Silver Doped Gold Catalysts, a DFT Study

In this study, the full reaction mechanism for N2O hydrogenation on silver doped Au(210) surfaces was investigated in order to clarify the experimental observations. Density functional theory (DFT) calculations were used to state the most favorable reaction paths for individual steps involved in the N2O hydrogenation. From the DFT results, the activation energy barriers, rate constants and reaction energies for the individual steps were determined, which made it possible to elucidate the most favorable reaction mechanism for the global catalytic process. It was found that the N2O dissociation occurs in surface regions where silver atoms are present, while hydrogen dissociation occurs in pure gold regions of the catalyst or in regions with a low silver content. Likewise, N2O dissociation is the rate determining step of the global process, while water formation from O adatoms double hydrogenation and N2 and H2O desorptions are reaction steps limited by low activation energy barriers, and therefore, the latter are easily carried out. Moreover, water formation occurs in the edges between the regions where hydrogen and N2O are dissociated. Interestingly, a good dispersion of the silver atoms in the surface is necessary to avoid catalyst poison by O adatoms accumulation, which are strongly adsorbed on the surface.

Ternary adsorption of Auramine-O, Rhodamine 6G, and Brilliant Green onto Arapaima gigas scales hydroxyapatite: Adsorption mechanism investigation using CCD and DFT studies

The simultaneous-competitive adsorption of Auramine-O (AO), Rhodamine 6G (R6G), and Brilliant Green (BG) dyes onto hydroxyapatite (HAP) produced from pirarucu (Arapaima gigas) scales was investigated by Fourier Transform Infrared (FTIR), Central Composite Design (CCD), and Density Functional Theory (DFT). Pirarucu scales were chosen as the precursor of the bioadsorbent owing to the large volume of this unutilized waste produced by the Brazilian pisciculture, which has increased the concern about solid disposal in the Amazon region. The CCD matrix presented each dye initial concentration as a factor influencing the removal of the other dyes. DFT was utilized to indicate the preferential order of adsorption. FTIR analyses of the dye-adsorbed HAP confirmed that the cationic dyes were removed from aqueous solution owing to interactions with anionic groups present on the HAP surface. High values of BG initial concentration negatively influenced the adsorption of AO, according to CCD, DFT, and FTIR results. Desirability analysis based on CCD results revealed the simultaneous optimum conditions of the ternary adsorption to be: HAP dosage 1.09 g.L−1; AO, R6G, and BG initial concentration 77, 29, and 39 mg.L−1, respectively, yielding percentage removals of AO, R6G, and BG as 93.78%, 23.94%, and 99.94%, respectively. The adsorption isotherms at 28, 38, and 48 °C suggested that the dyes in the ternary mixture were adsorbed in monolayer on the HAP surface (modified Langmuir model). The CCD, DFT, and FTIR studies effectively aimed to propose the possible multicomponent adsorption mechanism. This study shows that pirarucu scales is a residue biomass that can be easily processed to obtain an efficient, low-cost, and selective adsorbent regarding the studied dyes.